A continuously cast copper ingot is made by a procedure in which turbulence is imparted to the metal/solid interface during the casting operation. The ingot is then hot worked to form a billet having a smaller average grain size and a larger diameter than possible in the past. The billet is especially useful for making electroplating anodes used in the damascene process for making copper interconnects in silicon wafers.

The present invention relates to making fine-grained electroplating anodes especially useful for producing copper interconnects in silicon semiconductor chips.
Copper interconnects in multi-layer silicon wafers and semiconductor chips are often made by the damascene process. This process is described in U.S. Pat. No. 4,789,648 and U.S. Pat. No. 5,539,255, the disclosures of which are incorporated herein by reference.
In this process, copper is selectively electrodeposited onto a silicon wafer from an electroplating anode made from copper or copper alloy. Before electrodeposition, an intricate circuit pattern of trenches is etched into the wafer to define the interconnects to be formed. The anode is then mounted in close proximity to, but not touching, the wafer. Both are immersed in an electrolytic bath, where copper from the anode is electrodeposited onto the wafer.
Typical electroplating anodes for use in the damascene process take the form of squat, cylindrical copper discs 200 to 300 mm in diameter and 2 to 6 cm thick. In some instances, the anode is formed with a hollow interior so that it is annular in configuration rather than cylindrical. In either case, the surface of the anode is machined very flat to provide uniform deposition over the entire silicon wafer. Uniform deposition is critical because the wafer will be sectioned to make several chips and each chip is intended to be identical to the next.
Electroplating anodes for the damascene process are produced commercially by sectioning copper rods and tubes and then machining the sections to the desired flatness on one face and to a mounting configuration on the other opposite face. The mounting configuration is dependent on the particular electrodeposition system in which the anode is used. These copper rods and tubes, in turn, are typically made by a multi-step step process including casting, hot working, cold working and annealing.
In order to achieve optimal performance in the damascene process, the average size of the copper grains in these anodes, and hence the rods and tubes used to make these anodes, should be no more than about 150 xcexcm. In addition, the grain size distribution should be fairly uniform throughout the cross section of the rod or tube and anode. A fine, uniform grain structure is important in maintaining smoothness (or, more accurately, xe2x80x9clocal flatnessxe2x80x9d) of the anode face. Moreover, a finer grain structure may be machined and polished to a smoother initial surface finish and, during deposition, the anodes will erode more uniformly and stay smooth for a longer time. A rough anode face is deleterious to uniform copper deposition.
Unfortunately, conventional manufacturing processes can only produce average grain sizes as small as 200 xcexcm in rods and tubes with diameters of 200 mm or more. Average grain size is often much larger. Moreover, grain size distributions in such rods and tubes are not particularly uniform. Furthermore, conventional billet manufacturing processes are inherently expensive, since they require multiple working steps including at least one cold working step.
In this connection, there are basically two different ways as a practical matter for reducing grain size of copper rods and tubes produced by conventional continuous casting procedures. The first is to hot work several times including reheating the billet between the hot working steps. The second, which is the technique normally used commercially, is to hot work and then cold work the billet followed by annealing. Both require a substantial amount of mechanical workingxe2x80x94on the order of 10 to 1 or more in terms of reduction in cross sectional area. Accordingly, these techniques can be very expensive. Furthermore, above a section thickness of about three inches or more, conventional cold working equipment cannot accomplish grain size reduction uniformly. In addition, cracks and other imperfections often occur during cold working leading to the production of large amounts of scrap and/or unacceptable product. As a practical matter, therefore, conventional manufacturing processes cannot consistently and reliably achieve average grain sizes as small as 200 xcexcm in copper rods and tubes with diameters of 200 mm or more.
Accordingly, there is a need for a new manufacturing process which can consistently and reliably produce copper rods and tubes having average grains sizes significantly less than 200 xcexcm, typically about 150 xcexcm or less, in rods and tubes with diameters of 200 to 300 mm or even more. In addition, it would also be desirable if such a process could provide rods and tubes with fairly uniform grain size distributions. And, it would be especially desirable if such a process could be done using less working steps than required in conventional processes, so that manufacturing costs could be reduced.
In accordance with the present invention, it has been found that copper ingots can be directly formed into rods and tubes having diameters of 200 mm or more and average grain sizes of 150 xcexcm and less by simple hot working, provided that the ingots are made by a continuous casting procedure in which turbulence is imparted to the metal/solid interface during the casting operation.
Accordingly, the present invention provides a new process for producing a copper or copper alloy billet comprising forming an ingot by a continuous casting procedure in which turbulence is imparted to the metal/solid interface in the casting die and thereafter hot working the ingot so formed to produce the billet.
In addition, the present invention also provides a new copper or copper alloy billet which is made by this process and which has a diameter of at least about 200 mm and an average grain size of about 180 xcexcm or less, preferably 150 xcexcm or less.
Similarly, the present invention also provides new electroplating anodes made from such rods and tubes.
In accordance with the present invention, copper and copper alloy rods and tubes having diameters of at least 200 mm and average grain sizes of 150 xcexcm or less are made by hot working ingots formed by a continuous casting procedure in which turbulence is imparted to the metal/solid interface during the casting operation.
Composition
The same coppers and copper alloys used to make conventional electroplating anodes for the damascene process and other plating processes can be used to make the rods and tubes and anodes of the present invention. Examples of such coppers and copper alloys are the deoxidized high phosphorous alloys (C12200, C12210 and C12220), the phosphorous deoxidized tellurium-bearing alloys (C14500, C14510 and C14520) and the phosphorous deoxidized sulfur-bearing alloys (C14700, C14710 and C14729).
In general, any copper or copper alloy can be used which does not contain ingredients, or amounts of ingredients, imparting an adverse impact on the silicon wafers and chips produced by the anodes of the present invention. The copper or copper alloy should also be compatible with the equipment used in the continuous casting process in the sense that no adverse interaction occurs between the two. For example, if a graphite mold is used, coppers or copper alloys which stick to graphite should be avoided.
Turbocasting
Conventional continuous casting is a well known technology in which molten metal flows through a vertically-arranged mold whose inlet is continuously fed with molten metal while frozen, solid metal is being withdrawn from the mold bottom. A cooler is provided to cool the mold and, thus, the metal passing through the mold. Pinch rollers or other withdrawal mechanisms are provided for controlling the rate at which the solidified billet passes out of the die while maintaining the liquid/solid interface of the metal being cast within the confines of the mold.
When copper and copper alloys are continuously cast following this general procedure, gross directional solidification occurs during transition of the metal from liquid to solid. This results in large, coarse, elongated crystals composed of dendrites being formed during solidification. This gross crystal structure imparts poor mechanical properties to the product ingot, and so it is customary to work the ingot to break up these large crystals and dendrites to a much smaller size.
To overcome this problem, U.S. Pat. No. 4,315,538 and U.S. Pat. No. 5,279,353, the disclosures of which are incorporated herein by reference, describe a modified continuous casting process in which turbulence is imparted to the molten metal immediately above the liquid/solid interface in the casting mold (hereinafter xe2x80x9cturbocastingxe2x80x9d). This can be done, for example, by feeding the molten metal into the casting mold through slots in a die cap or the side walls of the die, the slots being arranged to impart a cyclonic motion to the molten metal in the die. Alternatively, mechanical or magnetic mixers can be used to impart this turbulence. In addition, any other technique which will achieve the same turbulence in a continuous casting die can be used.
Imparting turbulence to the molten metal in this way leads to greater uniformity in cooling than in conventional practice. In addition, it also leads to a shearing by high velocity molten metal of the primary dendrites which otherwise would form adjacent to the side wall of the die upon solidification. The net result is that a much better crystal structure is obtained in which the crystals are essentially equiaxed in configuration, finer in size, and distributed xe2x80x9cmore uniformlyxe2x80x9d than in ingots made by conventional practice. Because of this improved grain structure, the rods and tubes so obtained are amenable to hot and cold working, thereby eliminating production of large amounts of scrap and unacceptable product.
In accordance with the present invention, it has been found that continuously cast copper and copper alloy ingots made by turbocasting can be directly formed into large diameter rods and tubes having grain sizes of about 150 xcexcm or less by simple hot working to a reduction of cross sectional area of 6 to 1 or less. In particular, it has been found that the grain structure of copper ingots produced by turbocasting is fine enough and uniform enough so that simple hot working to a reduction of cross sectional area of 6 to 1 or less will achieve grain sizes of about 150 xcexcm and less, even in product rods and tubes having diameters of 200 to 300 mm or more. Accordingly, it is possible in accordance with the present invention to reduce the number of working steps (and the total amount of working) done on large diameter rods and tubes in traditional grain refinement processes while still achieving the smaller grain sizes accomplished by these processes.
Indeed, it has been found that the present invention will achieve average grain sizes significantly less than the 200 xcexcm minimum possible in conventional practice, thereby making it possible to produce products previously unavailable on an industrial scale. Thus, copper and copper alloy rods and tubes greater than 200 mm, greater than 250 mm and even greater than 300 mm, in diameter and further having average grain sizes of 175 xcexcm or less, more desirably 150 xcexcm or less, and even 100 xcexcm or less can be reliably and consistently produced by the present invention on an industrial scale, which is not possible with conventional technology.
Hot Working
As well understood, xe2x80x9cworkingxe2x80x9d refers to the significant, uniform mechanical deformation traditionally done to a metal or alloy to achieve a finer, more nearly-uniform grain structure. Working can either be done while the metal is above its solidus temperature, which is referred to as xe2x80x9chot working,xe2x80x9d or below its solidus temperature, which is referred to as xe2x80x9ccold working.xe2x80x9dNormally, hot working is done at temperatures above the midpoint of the range between 0xc2x0 C. and the melting or solidus temperature of the alloy, while cold working is normally done at or near room temperature. Since most metals are considerably softer at elevated temperatures, hot working can be performed over a larger range of cross-sections than cold working since less force is required.
Hot working can be done in accordance with the present invention using any technique which will accomplish the necessary uniform mechanical deformation. For example, forging or rolling can be employed. Normally however extrusion will be used, since the turbocast ingots to be deformed have a uniform or constant cross-sectional shape along their lengths.
Also, hot working in accordance with the present invention can be done in a single step or in multiple steps with or without intermediate heat treatments, as desired. In this connection, a significant feature of the present invention, as indicated above, is that significantly less working is required than in prior technology. In prior technology, area reductions of at least 10 to 1 are necessary to achieve the desired grain structure. Such large area reductions can only be accomplished with multiple working steps, either multiple hot working steps or hot working followed by cold working and subsequent anneal. In accordance with the present invention, however, desired grain structures can be achieved with much less working, e.g. area reductions of 6 to 1 or less, because turbocast billets are used. Such limited amounts of working can be achieved by a single hot working step, if desired, which is easier and less expensive to carry out. Furthermore, such limited amounts of working also translate to reduced production of waste due to ingot cracking and other similar phenomena. Multiple hot working steps and/or hot working followed by cold working can, of course, also be used if desired. However, even in this case the inventive technology is easier and less expensive to carry out because the overall amount of working needed to achieve the desired grain structure is considerably less.
The temperature at which hot working is done in accordance with the present invention is not critical. Normally, however, hot working will be done within 200xc2x0 F. of the solidus temperature of the particular metal being processed, since metal deformation is easier at these higher temperatures. In general, this means that hot working will normally be done at about 900xc2x0 F. to 1800xc2x0 F., more typically about 1000xc2x0 F. to 1300xc2x0 F. or even 1100xc2x0 F. to 1200xc2x0 F. Also, hot working can be done immediately after turbocasting, i.e. without cooling to cold working temperatures first, or alternatively after the ingot has been cooled to lower temperatures such as ambient temperature and then reheated to hot working conditions.
The amount of hot working done in carrying out the present invention should be sufficient to achieve the average grain size desired in the billet being produced. Normally, this means that hot working will be done by an amount of about 4 to 1 to about 6 to 1 in terms of area reduction, although amounts as little as 3.5 to 1 or even 3 to 1 are contemplated. Hot working by about 5 to 1 in terms of area reduction is typical. Hot working by amounts greater than about 6 to 1 are not normally necessary to achieve the desirable results of the present invention, although such large amounts of hot working may be advisable in limited instances.
In this connection, the amount of hot working needed to achieve the desirably small average grain sizes of the present invention varies considerably from case to case and depends on a variety of factors including the fineness of the cast microstructure, product diameter and composition of the ingot being processed as well as the manner in which hot working is carried out. With the above as a guide, however, the particular hot working conditions to be used in carrying out particular embodiments of the present invention can be easily determined by routine experimentation.
Billet Size
An important feature of the present invention is that finished products with large cross-sections can be produced. This is possible at least in part because much less working in terms of total area reduction is necessary to achieved the desired grain size relative to conventional technology. Thus, the present invention can eliminate the cold working step or subsequent hot working step of conventional technology, if desired. In any event because less area reduction is required in the inventive technology compared with conventional technology, less reduction in billet size is also achieved as a result of the working operation. The net effect is that product rods and tubes with larger diameters can be achieved by the present as compared with conventional practice when both start with ingots of the same size.
Thus, the present invention can easily provide cylindrical rods and tubes having diameters 200 to 350 mm, for example, by starting with turbocast ingots of 17 to 30 inches (about 430 to 760 mm), for example. Rods and tubes of this size, with the desired fine average grain structure, become very difficult if not impossible to produce by conventional technology, because the amount of working required dictates a starting ingot which is too big as a practical matter.
A further advantage of the present invention is that the product billets exhibit a greater degree of uniformity in grain structure from ingot center to surface than possible with prior technology. Significant non-uniformity in grain size distribution from ingot center to surface and gross ingredient segregation are the normal result when coppers and copper alloys are made using conventional continuous casting technology. This problem is only exacerbated when ingot diameters become large. This problem is largely eliminated by the present invention because the as-cast ingot produced by turbocasting already exhibits an improved grain size and grain size distribution.
Anode Manufacture
Electroplating anodes are made from the product rods and tubes of the present invention in the same way as conventional anodes. Thus, the hot worked rods and tubes are typically subdivided into sections normally about 10 to 50 times longer than the rods or tubes are thick, typically about 2 to 6 cm thick, and then machined to impart the desired flatness and mounting features. This produces anodes typically in the form of cylindrical discs 200 to 300 mm in diameter and, with a major face of the discs having a desired flat surface. Discs of different and even larger diameters and thickness can be produced. For example, discs with diameters of 250 mm or larger, 300 mm or larger, 325 mm or larger and even 350 mm or larger are contemplated, as are discs with thickness of 2.5 to 5 cm, 2 to 6 cm or even 1 to 10 cm. Indeed, the only constraint on the length of the tubes is the length of the rod or tube produced by hot working the turbocast billet.
Although similar in size and shape to conventional anodes, the anodes of the present invention differ from those produced by conventional technology in that they typically have average grain sizes of 175 xcexcm or less, 150 xcexcm or less, and even 100 xcexcm or less. This represents a significant advance over conventional anodes which have larger average grain sizes, as indicated above.
Other Billet Configurations
Although the invention has been described above in terms of producing rods and tubes and anodes with a cylindrical configuration, other product configurations are also contemplated. Thus, the present invention can be used to produce anodes and rods and tubes which have noncircular cross-sectional shapes such as squares, ovals, polygons, star patterns, and the like. These products can also be made to have the same minimum thickness dimensions (8 to 14 inches or more) and the same average grain sizes (xe2x89xa6175 xcexcm, xe2x89xa6150 xcexcm or even xe2x89xa6100 xcexcm) as the cylindrical products discussed above by following the present invention. Similarly, annular rods and tubes and anodes having outside diameters of about 8 to 14 inches (about 200 to 360 mm), inside diameters of about 5 to 9.5 inches (about 13 to 24 mm) and wall thicknesses on the order of about 1 to 3 inches (about 2.5 to 8 mm), more typically about 1.5 to 2.5 inches (about 4 to 6.5 mm) and even more specifically about 2 inches (about 5 mm), can be easily made in accordance with the present invention.
Optional Hot Working and Cold Working Steps
A desirable feature of the present invention is that the inventive rods and tubes can be produced without cold working, and without multiple hot working steps, as this reduces the overall cost of billet manufacture. On the other hand, the rods and tubes produced by the present invention can be subjected to cold working, before or after hot working, or multiple hot working steps, if desired. A significant advantage of the invention is that large diameter rods and tubes can be produced with smaller average grain sizes than possible before. This advantage will still be realized even if the billet is cold worked or subjected to multiple hot working steps in accordance with conventional technology.

Manufacture of fine-grained electroplating anodes


Negative photoimageable compositions are disclosed which contain a radiation sensitive component for producing an acid when subjected to radiation at wavelengths of 320 to 420 nanometers, a resin binder and a reactive oligomer. The compositions are useful in constructing printed circuits and integrated circuit packages.

1. Field of the Invention
This invention relates to i-line photoresist compositions for printed circuit applications which are imageable by exposure at i-line wavelengths of about 320 to 420 nanometers. In particular this invention is directed to a photoresist composition that will unexpectedly reduce line growth of the resist image of the printed circuit and unexpectedly has a longer shelf (storage) life than would have been expected.
2. Description of the Art
Photoresists are photosensitive films used to transfer images to a substrate. They are applied as liquid coatings or dry film compositions to a substrate to provide a photosensitive resist article. After forming the film on the substrate, the film is exposed through a patterned photomask to a source of activating energy such as light to form a latent image thereon after development. The photomask has areas opaque to activating radiation and other areas transparent to activating radiation. The pattern in the photomask of opaque and transparent areas defines a desired image that may be used to transfer the image to a substrate. A relief image is provided by development of the latent image pattern in the resist coating. The use of photoresist coatings is generally described, for example, by DeForest, Photoresist Materials and Processes, McGraw Hill book Company, New York (1975), and by Moreau, Semiconductor Lithography, Principals, Practices and Materials, Plenum Press, New York (1988) as well as in U.S. Pat. No. 5,366,846, the entire contents of this patent are incorporated herein by reference hereto. While the teachings of the above mentioned U.S. Pat. No. 5,366,846 are quite significant, it has been found that after development of the imaged photoresists, line growth of the resist had occurred. Therefore, there is still a need for better sidewall definition without substantial line growth during processing. The composition of the present invention provides a composition that even further reduces line growth of the resist sidewalls during development. Line growth produces etched lines which do not properly reproduce the desired geometries as defined by the mask.
Reference should also be had to the following for a greater understanding of the art: U.S. Pat. Nos. 5,034,304; 5,128,232; 4,189,323, 3,954,475 5,057,397, 5,340,697, 4,826,753, 5,180,653, 5,340,697, and the following patents or publications: Canadian Patent No. 2100,392, Japan 63075737A2, Japan 05,303,196; 05,281,727 and European Patent Application Nos. 0 458 325, 0 519 298, 0 621,509, 0 483,689, 0 483,693 and 0,672,954.
The present invention provides novel radiation sensitive coating compositions that are characterized in part as providing a cured coating layer that exhibits substantial flexibility as well as reducing line growth thereof during processing after a photoimaging step. In general, a composition of the invention comprises a resin binder, a reactive oligomer that comprises one or more internal crosslinking groups, and a radiation sensitive component (photoacid generator) that is photoactivatable at a wavelength of from about 320 to 420 nanometers. Such materials may be a substituted triazine compound or a substituted oxadiazole compound which produces an acid when activated by light energy at the wavelengths set forth above. As used herein, the term xe2x80x9creactive oligomerxe2x80x9d refers to a material that is a polymer molecule containing a number of repeating units, e.g., 2 to 50 and that is crosslinkable when in contact with photogenerated acid such as provided by triazines and oxadiazoles. Such reactive oligomers include materials such as the epoxidized butadiene family (See U.S. Pat. No. 5,366,846 incorporated herein by reference hereto), acetals, vinyl ethers and others as would be clear to those skilled in the art after reading this application.
While substantially non-reactive resin binders are suitably employed in the compositions of the invention, preferably the resin binder is a reactive polymer that can undergo photoactivated crosslinking with one or more components of the composition.
A variety of resin binders can be employed. Suitable reactive resins are those that contain a functional group that can react with one or more other components of the composition, for example a reactive hydrogen containing resin. Generally preferred reactive resins are phenolic resins. With respect to the crosslinker component, a variety of crosslinking agents may be employed. Preferred crosslinkers include an epoxy-containing materials and amine-based crosslinkers such as melamines, ureas, guanamines and mixtures thereof. Such crosslinkers will cure to form a polymerized network with the resin binder and reactive oligomer. Compositions of the invention are suitable for use as a liquid coating composition as well as a dry film.
The compositions of the invention have utility in a variety of applications. Hence, the invention includes processes employing the compositions of the invention, including processes for forming a relief image, processes for forming an imaged dielectric layer on a substrate, and processes for the manufacture of printed circuit boards, additive printed circuits, multilayer printed circuits, high density printed circuits, flexible circuits, surface mount devices, multichip modules, sequential build and other articles. The invention further provides novel articles of manufacture comprising substrates coated with the compositions of the invention.
The terms xe2x80x9ccrosslinkxe2x80x9d, xe2x80x9ccrosslinkingxe2x80x9d and other such terms used herein refer to any reaction of one or more of the components of a composition of the invention that results in reduced developer solubility of the composition.
Products such as these above are first photochemically set, the unwanted areas removed, and the remaining organic material is thermally fully set, i.e., hardened to its ultimate state.
A composition of the invention in general comprises a resin binder, an acid sensitive crosslinkable oligomer and a radiation activated component which produces an acid after radiation is directed thereon at wavelengths between about 320 to 420 nanometers. Preferably the compositions further include a crosslinking agent component in addition to the reactive oligomer.
The resin binder component may comprise a nonreactive resin or, preferably, the resin binder is a reactive resin that can undergo photoinitiated crosslinking with one or more components of the composition either photochemically or thermally. The resin binder preferably imparts aqueous alkaline developability to the composition. Thus, preferred resin binders include resins that contain polar functional groups, such as hydroxyl or carboxylate, that can impart aqueous alkaline developability. Further, the resin binder component is preferably employed in a composition in a concentration sufficient to render unexposed portions of a coating layer of the composition aqueous alkaline developable.
The term xe2x80x9cnonreactive resinxe2x80x9d as used herein refers to a resin that does not substantially polymerize with one or more of the components of the composition upon photoactivation of the same. Thus, upon curing of a composition, a nonreactive resin binder typically will be encapsulated within the polymerizable component(s) of the composition. Suitable nonreactive resins include, for example, urethanes, silicones, acrylates and the like. The resin binder component suitably may comprise both nonreactive and reactive resin binders.
A reactive resin binder is suitably any of a variety of materials that will undergo photoinitiated crosslinking with one or more components o the composition. Thus suitable resins include those that contain one or more reactive moieties, for example a functionality that contains a reactive hydrogen. Phenolic resins are particularly suitable reactive resins and are preferably employed in a concentration sufficient to render a coating layer of the composition developable with an aqueous alkaline solution or semi-aqueous alkaline solution. Suitable phenolic resins include, for example, phenol aldehyde condensates known in the art as the novolak resins, homo and copolymers of alkenyl phenols, partially hydrogenated novolak and poly(vinylphenol) resins, and homo and copolymers of N-hydroxyphenyl-maleimides.
Of the phenolic resins suitable as a reactive resin for the compositions of the invention, the phenol formaldehyde novolaks are preferred materials as the novolaks are able to form an aqueous alkaline developable, photoimageable coating composition. These resins are made following procedures known and disclosed in numerous publications such as DeForest, Photoresist Materials and Processes, McGraw-Hill Book Company, New York, Ch. 2, 1975; Moreau, Semiconductor Lithography Principles, Practices and Materials, Plenum Press, New York, Chs. 2 and 4, 1988; and Knop and Pilato, Phenolic Resins, Springer-Verlag, 1985, all said publications incorporated herein by reference for their teaching of making and using novolaks and other phenolic resins.
More particularly, novolak resins are the thermoplastic condensation product of a phenol and an aldehyde. Examples of suitable phenols for condensation with an aldehyde, especially formaldehyde, for the formation of novolak resins, include phenol; m-cresol; o-cresol; p-cresol; 2,4-xylenol; 2,5-xylenol; 3,4-xylenol; 3,5-xylenol; thymol and mixtures thereof. An acid catalyzed condensation reaction results in the formation of a suitable novolak resin which may vary in molecular weight from about 500 to 100,000 daltons. Preferred novolak resins include the cresol formaldehyde condensation products.
Another preferred reactive resin is a poly(vinylphenol) resin. Poly(vinylphenols) are thermoplastic materials that may be formed by block polymerization, emulsion polymerization or solution polymerization of corresponding monomers in the presence of a cationic catalyst. Vinylphenols used for production of poly(vinylphenol) resins may be prepared, for example, by hydrolysis of commercially available coumarins or substituted coumarins, followed by decarboxylation of the resulting hydroxy cinnamic acids. Useful vinyl phenols may also be prepared by dehydration of the corresponding hydroxyalkyl phenol or by decarboxylation of hydroxy cinnamic acids resulting from the reaction of substituted or non-substituted hydroxy benzaldehydes with malonic acid. Preferred poly(vinylphenol) resins prepared from such vinyl phenols have a molecular weight range of from about 2,000 to about 100,000 daltons. Procedures for the formation of poly(vinylphenol) resins also can be found in U.S. Pat. No. 4,439,516 incorporated herein by reference.
Other suitable reactive resin are polymers containing phenolic units and nonaromatic cyclic alcohol unites analogous in structure to the novolak resins and poly(vinylphenol) resins. Such copolymer resins are described in European Published Patent Application No. 0 401 499 having a publication date of Dec. 12, 1990 and incorporated herein by reference.
An additional class of suitable phenolic reactive resins include homo and copolymers of N-hydroxyphenyl maleimides. Such materials are disclosed in European Published Appln. No. 0,255,989 beginning on page 2, line 45 and continuing to page 5, line 51, incorporated herein by reference.
The concentration of the resin component of the compositions of the invention may vary within relatively broad ranges, and in general the resin component is between about 30 and 60 or more weight percent of total solids of the compositions. In general a reactive resin is employed in a composition in a concentration of from about 0 (where only nonreactive resin(s) are employed) to 60 or more weight percent of the total solids of the composition. As used herein, the term total solids of a composition refers to all components of a composition other than a solvent carrier.
Another component of the compositions of the invention is preferably a reactive oligomer such as an epoxidized polybutadiene that comprises one or more internal epoxide groups. The epoxidized polybutadiene suitably may also contain reactive groups in addition to one or more internal epoxy functionalities. For example, the butadiene may contain one or more vinyl groups, hydroxyl groups, carboxyl groups or pendant and/or terminal epoxy groups.
Particularly preferred epoxidized polybutadienes of the compositions of the invention are represented by the following formula (I):
where R and R1 are each independently selected from the group consisting of hydrogen, substituted and unsubstituted alkyl, substituted and unsubstituted aryl and a reactive group such as hydroxy, epoxy or an alkylene group, for example an alkylene group having from 2 to 10 carbon atoms, more typically having 2 carbon atoms. Suitable alkyl groups include those having from 1 to about 10 carbon atoms more typically from 1 to about 6 carbon atoms. Phenyl is a suitable aryl group. The alkyl and aryl groups may be suitably substituted by, for example, aryl, alkyl, alkylene, halo, alkoxy or hydroxy. The group R2 of formula (I) is preferably an alkylene carbon that forms a vinyl group with the carbon pendant to the butadiene backbone (i.e., R2 is CH2xe2x95x90), although a polybutadiene of formula (I) can be modified so that for one or more units of the butadiene the group R2 is other than an alkylene carbon. For example such an alkylene group can be saturated so that R2 is an alkyl carbon such as methyl. The value n of the above formula (I) is suitably 2 or greater, more preferably n is a value between about 10 and 25, still more preferably n is a value between about 20 and 25.
The polybutadiene of formula (I) above preferably has a molecular weight (weight average) of at least about 4000, preferably has a molecular weight of between about 4000 and 8000, still more preferably has a molecular weight between about 5000 and 6000.
A polybutadiene of formula (I) can be prepared in several ways. For example, a commercially available polybutadiene can be selectively oxidized to provide internal epoxide groups. Typically poly-1,3-butadienes are employed. Polybutadienes, including polybutadiene that with varying terminal groups including reactive groups such as hydroxy, are available from vendors such as BF Goodrich, Nisso and Japan Synthetic Rubber. Under appropriate reaction conditions, an internal alkylene group will be more reactive to electrophilic substitution than a pendant vinyl group (i.e., where R2 above is an alkylene carbon). Thus, an internal carbon-carbon double bond of the polybutadiene can be converted to a halohydrin by reaction with a halogen such as Cl2 or Br2 in the presence of water and a substantially non-nucleophilic solvent such as tetrahydrofuran or an aromatic solvent such as toluene or xylene. The epoxide can then be formed by alkaline treatment of the halohydrin. Suitable bases for formation of the internal epoxide via a halohydrin include alkoxides such as potassium t-butoxide and hydroxides such as sodium hydroxide. In formation of the halohydrin, a stoichiometric equivalent of halogen should be employed to limit addition to the more reactive disubstituted internal vinyl groups of the polybutadiene.
It is also believed that direct oxidation of the internal alkylene group of the polybutadiene can be employed to form a epoxidized polybutadiene of the invention. For example, a commercially available polybutadiene can be admixed in a suitable solvent and reacted with a suitable oxidant to provide the epoxide group. Suitable oxidants will include peracids such as perbenzoic acid, meta-chlorobenzoic acid, peracetic acid and trifluoroperacetic acid; alkaline oxidants such as sodium hydroxide and hydrogen peroxide (30 wt. % in water); or a substantially neutral oxidation of bubbling O2 gas through the reaction mixture in the presence of a suitable catalyst such as Ag.
The internally epoxidized polybutadiene is preferably miscible with other components(s) of the composition both before and after curing. An epoxidized polybutadiene that is miscible with other components upon curing is believed to form a clear coating layer upon curing of the composition while a butadiene that is not miscible will provide an opaque cured coating layer, for example a milky-colored coating layer. While not wishing to be bound by theory, it is believed a clear, cured coating layer indicates the formation of a polymerized network comprising a substantially uniform distribution of the polymerizable components of the composition.
The concentration of the internally epoxidized butadiene component may vary within relatively broad ranges, and in general the internally epoxidized polybutadiene is employed in a concentration of at least about 10 weight percent of the total solids of a composition, more typically from about 10 to 60 or more weight percent of the total solids, still more typically from about 25 to 55 weight percent of total solids of a composition.
The compositions of the invention further includes radiation polyvinylphenol resin having a minor portion of its hydroxyl groups mesylated admixed with a novolak resin.
In addition to the resin and epoxidized polybutadiene components described above the photoresist contains a photoacid producing compound which undergoes photolysis with activating radiation at a wavelength of about 320 to 420 nanometers to yield a strong halogen acid. Suitable acid producing compounds halomethyl-s-triazines which conform to the following formulae II:
where P is a substituted or unsubstituted phenyl group or a substituted or unsubstituted naphthyl group and each X may be the same or different and each represents a halogen atom, preferably Cl or Br. Preferably P is the group:
when X is Cl or Br.
A preferred halomethyl-s-triazine conforms to the following formula III:
wherein Q is bromine or chlorine, P is NHR, xe2x80x94NR2, or xe2x80x94OR where R is phenyl or lower alkyl (lower alkyl meaning an alkyl group having 6 carbon atoms); n is an integer from 1 to 3; and W is an optionally substituted aromatic or heterocyclic nucleus or the compound of formula IV:
where Z is oxygen or sulfur and R1 is hydrogen, a lower alkyl or phenyl group. Of course, where W is an aromatic or heterocyclic nucleus, the ring may be optionally substituted. Without attempting an exhaustive listing of substituents, the following are noted as typical: chloro, bromo, phenyl, lower alkyl (an alkyl having no more than 6 carbon atoms), nitro phenoxy, alkoxy, acetoxy, acetyl, amino and alkyl amino.
The s-triazine compounds as described above generate free radicals when irradiated with actinic radiation of a wavelength from about 320 to about 420 nanometers. For this reason, the compounds are useful as photoinitiators in light sensitive compositions herein within the i-line wavelengths.
The s-triazine compounds are condensation reaction products of certain methyl-halomethyl-s-triazines and certain aldehydes or aldehyde derivatives. They may be prepared by the procedures disclosed in the above cited U.S. Pat. No. 3,954,475 and in accordance with the teachings of Wakabayashi et al., Bulletin of the Chemical Society of Japan, 42, 2924-30 (1969).
Specific examples of halogenated triazines suitable for use as photoacid generators in the invention include for example, 2-[1-(3,4-benzodioxolyl)]-4,6-bis(trichloromethyl)-1,2,5-triazine, 2-[1-(2,3-benzodioxolyl)]-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-[1-(3,4-benzodioxolyl)]-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-[1-(2,3-benzodioxolyl)]-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-(2-furfylethylidene)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-[2-(5-methylfuryl)ethylidene]-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-[2-(4-methylfuryl)ethylidene]-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-[2-(3-methylfuryl)ethylidene]-4,6-bis-(trichloromethyl)-1,3,5-triazine, 2-[2-(4,5-dimethylfuryl)ethylidene]-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-[2-(5-methoxyfuryl)ethylidene]-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-[2-(4-methoxyfuryl)ethylidene]-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-[2-(3-methoxyfuryl)ethylidene]-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-[2-(4,5-dimethoxy-furyl)ethylidene]-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(2-furfylethylidene)-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-[2-(5-methylfuryl)ethylidene]-4,6-bis(tribromoethyl)-1,3,5-triazine, 2-[2-(4-methylfuryl)-ethylidene]-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-[2-(3-methylfuryl)ethylidene]-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-[2-(4,5-dimethoxyfuryl)ethylidene]-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-[2-(5-methoxyfuryl)ethylidene]-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-[2-(4-methoxyfuryl)ethylidene]-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-[2-(3-methoxyfuryl)ethylidene]-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-[2-(4,5-dimethoxyfuryl)ethylidene]-4,6-bis(tribromomethyl)-1,3,5-triazine, 2,4,6-tris-(trichloromethyl)-1,3,5-triazine, 2,4,6-tris-(tribromomethyl)-1,3,5-triazine, 2-phenyl-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-phenyl-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-(4-methoxyphenyl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(4-methoxyphenyl)-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-(1-naphthyl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(1-naphthyl)-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-(4-methoxy-1-naphthyl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(4-methoxy-1-naphthyl)-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-(4-chlorophenyl)-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-styryl-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-styryl-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(4-methoxystyryl)-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-(3,4,5-trimethoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(3,4,5-trimethoxystyryl)-4,6-bis(tribromomethyl)-1,3,5-triazine, 2-(3-chloro-1-phenyl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(3-chlorophenyl)-4,6-bis(tribromomethyl)-1,3,5-triazine and the like.
Other photoacid generator activated at 320 to 420 nanometers suitable for use in this invention include substantial oxadiazols of the formula V
where X is Cl or Br and in particular where X is Cl.
Other triazine type photoacid producers that may be used for i-line exposure are disclosed in EP application 0,458,325 and taught in U.S. Pat. No. 5,366,846.
The acid-generating agents compounds disclosed herein may be used alone or in admixture. The amount of acid generator may vary from about 0.01 to 5 parts by weight, preferably 0.1 to 3 parts by weight, per 100 parts by weight of the specific alkali soluble resin. When the amount of the acid-generator is less than 0.01 parts by weight, it is difficult to form a resist pattern and when the amount exceeds 5 parts by weight, the ability to strip the resist from the substrate becomes difficult.
In addition to the above a spectral photosensitizer component may also be included in the negative photoresist composition.
The preferred spectral photosensitizers are anthracene or substituted anthracene compounds such as diphenylanthracene, phenanthracene thioxanthones such as isopropyl thioxanthones of the structure:
A compound that contains at least two vinyl ether groups is a suitable crosslinker for compositions that comprise a photoacid generator compound as disclosed herein. Examples of compounds with at least two vinyl ether groups include divinyl ethers of aliphatic, cycloaliphatic, aromatic or araliphatic diols. Examples of such materials include divinyl ethers of aliphatic diols having from 1 to 12 carbon atoms, polyethylene glycols, propylene glycols, polybutylene glycols, dimethylcyclohexanes, etc. Specific examples include divinyl ethers of ethylene glycol, trimethylene-1,3-diol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, resorcinol, Bisphenol A, etc. A composition of the invention may also suitably comprise a melamine as a crosslinker or a composition may contain two or more different crosslinkers.
The concentration of the one or more crosslinkers in a composition may vary within a relatively wide range. As will be appreciated to those skilled in the art, suitable concentrations will vary with factors such as crosslinker reactivity and specific application of the composition. In general, a suitable concentration of one or more crosslinkers is about 5 to 30 weight percent of toal solids of the composition, preferably about 10 to 20 weight percent of total solids.
Other additives may be optionally included in the compositions of the invention such as dyes, fillers, wetting agents, fire retardants and the like. A suitable filler is the product sold under the name TALC by Cyprus Chemical and a suitable dye is Orasol Blue available from Ciba Geigy.
Thermal curing after development may be conducted at temperatures ranging from about 120xc2x0 to 180xc2x0 C., preferably 120xc2x0 to 140xc2x0 C., for a period of time between about 20 to 120 minutes. The thermal cure hardens unreacted components, promotes flexibility of the coating layer and can enhance adhesion of the coating layer to the substrate. The coating layer also may be post cured using RF or microwave energy by methods well known in the coatings industry.
The compositions of the invention will be highly useful as flexible photoimageable cover coats in the manufacture of flexible circuits. A suitable process for the manufacture of a flexible circuit provides for etching a flexible base material to form a circuit layer by standard procedures (e.g., print and etch), stripping of photoresist, followed by cleaning of the base material surface. A composition of the invention is then applied to the cleaned surface. The composition may be suitably applied to the base material by a variety of coating means, for example roller coating or screen coating. The applied coating layer is then imaged and developed to provide openings, e.g., for solder pads. The coated base material can then be processed and used in accordance with known procedures. For such flex circuit applications, the photoimageable composition preferably comprises a brominated phenolic resin binder to provide a more fire retardant composition coating layer. As is clear to those in the art, a brominated phenolic resin is a resin where at least a portion of the resin phenolic units are bromo substituted at one or more available ring positions.
Compositions of the invention will also be useful as dielectric innerlayers for multilayer printed circuit boards that are produced by a sequential layering process as described above and in U.S. Pat. No. 4,902,610. For example, a composition of the invention can be coated over a first circuit layer and then the coating layer imaged to provide openings that define interconnections, and then one or more other board layers sequentially formed over the imaged composition layer to form a circuit board having two or more layers of circuitry. The first circuit layer will be in electrical connection with other circuit layer(s) of the board by means of the imaged openings of the coating layer. A typical application of a dielectric innerlayer would comprise application of a two mil layer of a photoimageably composition of the invention over a pre-cleaned circuitry substrate having 6-mil wide conductors, formation of 3-mil diameter via apertures in the coating layer by means of exposure and developed as discussed above; and post-development curing of the photodielectric layer. The via apertures will be located at predetermined sites within the width of the underlying conductors. Thus the apertures can be formed simultaneously rather than individually drilling holes as is done in conventional multilayer board manufacturing processes. Further, the apertures can be significantly smaller in such a sequential fabrication process than can be done by drilling. For example, photoformed via apertures can be provided with diameters as small as 2-mil. Additionally, via apertures of any shape can be imaged and developed, including squares and slots. The vias can then be plated with electroless copper at the same time as the second layer of circuitry conductors are plated.
Other applications of the compositions of the invention include use as an outerlayer for surface pad mounts where, the pads interconnected to the underlying circuit layer of a printed circuit board by means of electrolessly plated photoformed vias. The compositions of the invention can also be used as a resist in full-build additive plating processes such as printed circuit manufacturing processes. For a full-build additive circuit manufacturing process, it is highly desirable to employ a permanent photoimageable dielectric mask as insulation between the electrolessly plated circuitry conductors. Further, the photoimageable mask should be capable of fine line resolution, aqueous developable and serve as a resist to full build high pH electroless copper plating solutions for many hours at elevated temperatures without unwanted extraneous copper deposition on the mask.
The following non-limiting Examples are illustrative of the invention.


A method, apparatus, system, and computer readable storage medium provide the ability to conduct global illumination. A 3D model of a scene is obtained in a computer graphics application. A camera location in the scene is identified and paths emitted from the camera location are traced into the scene (by recording hit points along the paths). The paths are filtered by selecting those paths that can receive light from a light source (i.e., paths whose next path segment hits the light source). Targets are identified based on the hit points. Photons are generated from the light source and follow a corresponding photon path. Each photon path is evaluated to determine if it intersects with a target, in which case it is recorded. A view of the scene is rendered based on the recorded photons.

1. Field of the Invention
The present invention relates generally to lighting a three-dimensional (3D) model, and in particular, to a method, apparatus, and article of manufacture for indirectly illuminating particular scene regions.
2. Description of the Related Art
Rendering algorithms often trace light particles from a scene's light sources as part of the scene's calculation of global illumination (photon mapping, instant radiosity, lightcuts, etc.). Generating these particle paths is a difficult problem because, in expansive cases, much of the scene is not even indirectly viewable from the camera. If a particle set could be restricted to only the regions affecting the camera, the set of stored photons becomes much smaller and later operations using these photons become more efficient. This increased efficiency translates into faster rendering times and increased rendering quality. The prior art fails to provide a mechanism for generating particle paths in such a way that the indirect illumination garnered from the paths is likely to contribute to the final image. Such problems may be better understood with a description of prior art lighting techniques.
Many applications attempt to perform simulated photography in a large scene. In other words, the application simulates the placement of a camera at a particular location in a scene and attempts to illuminate the scene (referred to as global illumination). Further, the light received at the camera's location must have a sufficient density to produce an accurate/acceptable rendering. As part of the illumination process, paths between the camera and the light source(s) are computed. If the path is complicated, there may not be sufficient density or enough paths relative to the entire scene (e.g., millions of paths in the scene but only a few that reach the camera location). For example, the camera may be located in a single room of a multi-room hotel looking out a window. Prior art global illumination techniques do not take the camera's location into account and may therefore insufficiently illuminate the hotel room where the camera is located (i.e., for simulated light). More specifically, the global illumination computation will consume processing resources by globally illuminating the scene without regard to a targeted area such as a camera's location. Accordingly, it is useful to label the camera's location in a scene and bias the scene illumination based on such a location. In other words, it is desirable to isolate a small important subset of the global illumination computation. However, prior art methodologies fail to perform global computations with such a bias. To better understand global illumination, a description of prior art global illumination computations is useful.
Two different types of prior art are often utilized to compute global illumination—geometric based computations (e.g., progressive radiosity) and lighting based computations.
In geometric based prior art solutions, the geometric representations of areas not important in the scene are computed at a low quality or are actually modified to produce more/fewer vertices for use in the analysis. Thus, the geometry of a scene is analyzed and the rendering of non-important geometry may be performed at a reduced resolution/quality.
Most prior art systems that compute global illumination are lighting based. One such method utilizes virtual point lights (VPLs). A VPL is a point light source that is placed virtually in a scene location. A VPL is used as a source of light and may not be the originating light source. For example, if the light source is the sun that reflects or bounces off many objects in a scene (e.g., ground, window, wall, etc.), during a process of forward ray tracing (i.e. tracing rays from light sources) a VPL may be placed at any of the intersecting/reflecting locations (e.g., on the ground, window, wall, etc.). To use the VPLs, they must first be created. In a small scene, (e.g., a light source and a single room with a window), many VPLs can be projected into the scene with sufficient density to obtain a reasonable estimate of global illumination. However, many applications attempt to compute global illumination in a large scene (e.g., 1 mile by 1 mile with a camera in one room of a large house that has a window to the outside, or to a multi-room hotel). While millions of VPLs may be traced, the density in the room with the camera may be very low and insufficient. Thus, prior art methods fail to provide an efficient mechanism for computing global illumination that is biased in a particular location.
Another prior art approach is that of traditional ray tracing or backward ray tracing. Ray tracing is a technique that models the path taken by light by following rays of light as they interact with optical surfaces. In a 3D graphics environment, backward ray tracing follows rays from the camera eyepoint outward, rather than originating at a light source. Thus, visual information on the appearance of the scene is viewed from the point of view of the camera, and lighting conditions specified are interpreted to produce a shading value. The ray's reflection, refraction, or absorption are calculated when it intersects objects and media in the scene.
Building on ray tracing is another prior art technique called photon mapping. Photon mapping is noted for its ability to handle caustics (specular indirect effects) (e.g., rather than radiosity which is for diffuse indirect effects) as well as diffuse inter-reflection. Photon mapping uses forward ray tracing to deposit photons from the light sources into objects in the scene. The photons are stored in a binary space partitioning (BSP) tree data structure where neighbors can be quickly discovered and photons merged to constrain memory use. BSP is a method for recursively subdividing a space into convex sets by hyperplanes. The subdivision gives rise to a representation of the scene by means of a tree data structure referred to as the BSP tree. In the case of reflective or refractive objects, new photons are generated from the incoming set and further propagated through the scene, again using ray tracing, and the resulting photons are added to the tree. Each photon stored contains information including the direction the photon came from, where the photon hits a surface, and reflection properties at the surfaces.
A photon mapping algorithm usually proceeds in two phases. First a coarse illumination solution is prepared as described above. Second, the coarse illumination is "gathered," pixel by pixel, to produce a smooth final output. This gathering step requires many rays for quality results and is the subject of much research.
In many non-geometric based prior art solutions, global illumination is often analyzed in an unbiased context. Unbiased prior art techniques attempt to account for every path in a scene, a process that is computationally and processor intensive. Few prior art solutions use biased approaches to ensure relevant global illumination computations. One method that attempts to perform a bias based computation is that of "Importons". Importons are similar to "visual particles" that are emitted from the camera and bounce towards light. Importons move in the opposite direction to how photons travel but in contrast to photons, importons store color data that describes the factor with which an illumination at a certain location would contribute to the final image. Thus, with importons, the path is followed from the camera eyepoint, and importons are deposited at intersecting/bounce locations. Thereafter, photon mapping is performed. During the photon mapping (i.e., from the light source to the camera eyepoint), if a photon lands near an importons, it is stored, otherwise, the photon/location is ignored. Accordingly, since only particular photons are stored, the importons serve to bias the global illumination using photon mapping based on where importons are stored.
Global illumination may also be based on "daylight portals" to enable faster and better targeting for particle tracing. A daylight portal is a portal that is placed by the user (i.e., it is not automatically generated) and identifies a region of interest. These portals also present a major problem in scenes that are partially outdoors and partially indoors. Namely, using portals allows fast particle tracing into the indoor portion of the scene, but essentially disables global illumination due to the environment in outdoor portions of the scene. Portals can be disabled, allowing global illumination in all portions of the scene, but particle densities will be far from desirable (in large scenes, particles will be distributed over a large area). Moreover, even with portals, this density problem may occur, since portals may be located far from the camera, in other rooms, or even in other buildings. Thus, prior art solutions fail to allow particle densities to be relatively high in areas that actually contribute illumination to the final image.
In summary, the prior art global illumination computations have many deficiencies and problems. Geometric based prior art solutions modify scene geometry and are processor intensive. Unbiased lighting-based prior art solutions attempt to compute the global illumination for an entire scene and may reach the maximum capacity for memory/processor usage. The biased prior art computations focus on the final computation used during photon tracing/mapping and therefore also may exceed memory and processing limitations, or fail to have sufficient density of relevant data to compute a high quality global illumination solution.
In view of the above, one may note that all of the prior art solutions, whether geometric, lighting based, unbiased, or biased, fail to easily and efficiently illuminate a particular scene location without wasting precious memory and processing resources for illuminating an entire scene.

Directing indirect illumination to visibly influenced scene regions


A method for determining the rotational position of the drive shaft of a DC motor includes providing a period of a reference current ripple indicative of proper operation of the motor. The period of the reference current ripple is then compared with the period of each current ripple contained in a rotor current signal of the motor during operation of the motor. The current ripples of the rotor current signal of the motor having periods which differ from the period of the reference current ripple are then corrected. A corrected rotor current signal based on the corrected current ripples and uncorrected current ripples of the rotor current signal of the motor is then generated. The current ripples contained in the corrected rotor current signal are then counted to determine the position of the drive shaft of the motor and, consequently, the position of an element being driven by the motor.

The invention pertains to a method for determining the rotational position of the drive shaft of a direct current (DC) motor and especially determining the position of an element driven by the drive shaft within a predetermined path of movement by evaluating the current ripples contained in the rotor current signal.
The rotor current signal of a DC motor is made up of a DC component and an AC (analog current) component superimposed on the DC component. The AC component results from the interaction of the magnet (field), the rotor windings, and the commutator of the DC motor during operation of the DC motor. This manifests itself through transient changes in the induced voltage which produce ripples in the rotor current signal. When the rotor is rotating the current spikes, called xe2x80x9ccurrent ripplesxe2x80x9d and xe2x80x9ccurrent peaksxe2x80x9d in the following description, in the rotor current signal occur at a frequency corresponding to the number of collector plates. If the rotor has, e.g., 10 collector plates, then 10 current ripples are detected in the rotor current signal. Thus, counting the current ripples provides information on the current rotational position of the rotor of the DC motor and thus, provides information relative to the position of the driven element within its predetermined path of movement. For this purpose, the analog rotor current signal is digitized in order to count the current ripples.
Such a method is known, e.g., from DE 195 11 307 C1. So that interference pulses superimposed on the rotor current signal are not included in the evaluation of the current ripple count, typically the analog rotor current signal undergoes conditioning before its digitization, e.g., it is subjected to frequency filtering. These previously known means and also other means are used so that a current ripple signal that is as free from interference pulses as much as possible can be supplied for digitization and subsequent count evaluation. In this context, interference pulses do not include pulses that are conditional to the commutator and that are superimposed on the rotor current signal.
However, during operation of a DC motor, particularly under a load, the current ripple contained in the rotor current signal can become distorted. Such a distortion can be evident by two current peaks. In the course of digitizing such a rotor current signal, instead of one current peak, two current ripples are indicated at the position of the distortion in the current ripple signal. Counting this double ripple leads to erroneous position determination of the driven element. Corresponding results apply for the absence or non-detection of a current ripple. These errors are conditional to the commutator and thus they can be eliminated without anything further through conditioning of the analog rotor current signal.
Such methods are used, e.g., in the automotive field, for controlling an adjustable drive, like those provided, e.g., for windows and sunroofs. An essential element in the determination of the position, e.g., of the window, is the element that can turn off the jamming protection when the window is being closed. Turning off this protection is necessary so that the window can travel completely into its upper block and into the seals arranged in this block without the increased load forcing the motor to be turned off. With erroneous counting of the current ripples for determining the position of the window the jamming protection may be turned off too early or too late.
Starting with the state of the art that has been discussed, the objective of the invention is based on refining a method according to the class mentioned above such that an exact current ripple count is guaranteed even when a current ripple contained in the rotor current signal is distorted or absent.
For the object of the invention, a correct, undistorted current ripple used as a calibration or reference variable is compared with other, possibly faulty current ripples. This alignment between a correctly defined current ripple and the other ripples is done by comparing each period with the others. For the reference current ripple, it is assumed that its period corresponds with sufficient precision to the detected period of the current ripple in the rotor current signal of the DC motor or from the period determined from this signal. A comparison of the periods of the reference current ripple with the period of each, actually measured current ripple in the current ripple signal leads to the correction of periods in the current ripple signal which do not exhibit sufficient agreement with the period of the reference current ripple.
For providing a reference current ripple, in one embodiment, the DC motor operates in connection with a desired movement of the driven element from a first position into a second position for a short time period without a load and the rotor current signal from this operating state of the DC motor is evaluated with reference to current ripples contained in the signal. This configuration of the invention assumes that erroneous current ripples which can be distorted or even missing occur at a higher frequency when the DC motor operates under a load and erroneous ripples do not occur or only occur at a negligible frequency when the DC motor operates without a load. Thus, the periods of the current ripple when the DC motor operates without a load can be used as reference current ripples.
The object of the invention uses the premise that a rapid change in the rotational speed of the DC motor, for instance a doubling or a halving resulting in a corresponding halved or doubled period as compared with the period of the reference current ripple, is not possible due to the moment of inertia of the moving parts of the DC motor. Conditional to the commutator, erroneous or distorted current ripples exhibit two maximums instead of one expected maximum in the analog rotor current signal. For instance, the detection of a rapid halving of the period for a current ripple relative to the period of the reference current ripple or also relative to the period of a previous current ripple allows the conclusion to be made that this period halving is not a result, for instance, of a rapid doubling of the rotational speed of the DC motor, but instead, it is a result of a distorted current ripple. The number of current-ripple periods recognized as erroneous in the current ripple signal is then corrected correspondingly.
The period of the reference current ripple can be determined advantageously from the rpm (revolutions per minute) or speed of the rotor of the DC motor. The rpm or speed of the rotor of the DC motor is determined from the present current and voltage values applied to the DC motor. Because erroneous periods can be noticed either through doubling or halving of the period of the reference current ripple, the factors that influence the actual rpm or speed of the rotor of the DC motor in addition to the current or voltage values, e.g., the motor inductivity or temperature influences, can be basically ignored. These factors can be ignored because the variables associated with these factors do not change enough to call attention to the fact that the period of the reference current ripple calculated from the detected rpm or speed is off by a factor of two.
However, it is advantageous for the period of the reference current ripple to have a tolerance range that is much smaller than the determined period of the reference current ripple. The period of the reference current ripple, with or without a tolerance range, can be adapted further depending on the actually measured period of the individual current ripples, so that in this way, other factors influencing the rpm and speed of the rotor are also considered in processing the period of the reference current ripple. This provides a self-teaching and somewhat self-calibrating system.
By comparing the period of a reference current ripple with each period of the current ripples contained in the current ripple signal, a correction is performed for each period recognized as erroneous without averaging a period over a certain amount of time. Therefore, for the method of the invention, the current ripple signal is corrected in its smallest unit, namely the period of each current ripple. This has the result that the rotational position of the drive shaft of the DC motor can be determined in a cycle corresponding to the period of the reference current ripple.
The number of current-ripple periods recognized as defective in the current ripple signal is corrected when a distorted current ripple occurs. For instance, two current ripples are contained in the current ripple signal instead of only one, if the detected period is halved relative to the period of the reference current ripple; a corresponding correction is performed that divides the number of detected current ripples with halved periods by a factor of two.
A corresponding process is performed for the inverse case when the period reproduced in the current ripple signal is, e.g., doubled by an apparent fusion of two current ripples. To correct this error, the number of such periods is doubled.
Therefore, with the method according to the invention, an exact position determination of the element driven by the DC motor is performed even when faulty current ripples are contained in the current ripple signal due to certain load conditions.
Fundamentally, a correction as described above can be performed at an arbitrary time within each period, in which the DC motor is operated for moving the driven element from a first position into a second position. In the scope of the previously described embodiment, in which a reference current ripple is provided by motor operation without a load, all that must be guaranteed for determining the reference current ripple is that the motor operates for a period of time without or essentially without a load before performing the adjustment movement for the driven element. This can be the case, e.g., at the motor start-up phase or also at the motor stop-down phase. Such an operating state, where the DC motor driving the driven element operates without a load, can be realized through a correspondingly conceived mechanical power transmission means from the DC motor to the driven element, for instance, a cable line. This can provide sufficient play in the power transmission between the driving of DC motor and the driven element in order to have a sufficient rotational movement of the DC motor without a load so that at least one reference current ripple can be detected.
The method according to the invention is suitable particularly for use in the scope of position detection for an adjustment device for opening and closing a window, a sunroof, or for adjusting a seat in a motor vehicle. Position determination, e.g., of the window, can then be performed just by evaluating the current ripple signal without the use of additional sensors. However, with such a use, the reference current ripple must be detected at the motor start and thus before movement of the window, such that simultaneous with the movement, the desired correction can be performed without having to store the entire current ripple signal at the beginning and without having to correct the performed movement relative to the position determination of the driven element at the end. Only in this way can the corresponding position of the window be detected with sufficient reliability to indicate whether the jamming protection can be turned off depending on other factors so that the window can be moved into the seals of the upper block.

Method for detecting the rotational position of the drive shaft a dc motor