CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/842,513, filed Apr. 25, 2001, now U.S. Pat. No. 6,369,887, issued Apr. 9, 2002, which is a continuation of application Ser. No. 09/475,439, filed Dec. 30, 1999, now U.S. Pat. No. 6,256,094, issued Jul. 3, 2001, which is a divisional of and claims priority from application Ser. No. 08/964,451, filed Nov. 4, 1997, now U.S. Pat. No. 6,704,104, issued Mar. 9, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the manufacture of semiconductor wafers prepared by a method including applying a photo-resist layer, exposing the layer, and stripping the layer from the semiconductor wafer. More particularly, this invention pertains to a method for inspecting semiconductor wafers or other substrates to determine the presence of residual photo-resist material on the semiconductor wafer surface. 
     2. State of the Art 
     Semiconductor chips are produced in a multi-step process by which a plurality of identical electronic circuits is typically formed on a semiconductor substrate, such as a silicon wafer. The semiconductor substrate is then subdivided (diced) into individual chips which are further processed into semiconductor devices. 
     The electronic circuits are generally patterned into a semiconductor wafer by lithography. In this process, a resist material is coated onto the semiconductor wafer surface. As disclosed in commonly owned U.S. Pat. No. 5,350,236, issued Sep. 27, 1994, hereby incorporated herein by reference, the application of a material on a semiconductor substrate can be monitored by measuring light reflected from a surface of the semiconductor substrate. 
     After the resist material has been coated on the semiconductor wafer surface, it is selectively exposed to a radiation source, such as by the passage of radiation (i.e., light, e-beam, or X-rays) through a mask having the desired pattern. Some portions of the resist receive a high dosage of radiation while other portions receive little or no radiation, resulting in a difference in solubility from the resist portions. In a subsequent development step, a developer removes or etches portions of the resist coating from the semiconductor substrate at a rate higher than other portions. The selective removal results in a resist pattern which will become the electronic circuit pattern on the semiconductor substrate. Precision in the development time is critical for achieving complete removal of resist from some portions while leaving other portions substantially intact. Both insufficient development and excessive development will result in a lack of differentiation, forming a defective electronic circuit pattern on the semiconductor substrate. In addition, where the width of a conductor line(s) in the electronic circuit is critical, inadequate development results in an overly narrow line, and excessive development produces an overly wide line. Thus, precise endpoint detection (i.e., the moment at which precise development occurs) is a requirement for proper development. 
     Following the removal of the portions of the photo-resist material in the development process, the semiconductor wafer is subjected to further processing steps which may include doping, etching, and/or deposition of conductive materials in unprotected areas, i.e., areas devoid of photo-resist material. After one or more of these processing steps, the semiconductor wafer is subjected to a stripping step to remove the photo-resist material remaining on the semiconductor wafer. 
     After the removal of the photo-resist material, a subsequent processing step may include heating the semiconductor wafer in a diffusion furnace or applying a layer of material with a chemical vapor deposition system. Occasionally, a semiconductor wafer is inadvertently passed to a thermal furnace or vapor deposition system without removal or with only partial removal of the photo-resist material. The resulting damage to the processing equipment may be severe. For example, furnace diffusion tubes are irreparably damaged by vaporized hydrocarbons and carbon from the photo-resist material and, thus, the furnace diffusion tubes must be replaced. The replacement equipment and/or the downtime to repair the processing equipment is usually very costly. 
     Furthermore, the photo-resist carrying semiconductor wafer and one or more subsequent semiconductor wafers entering the processing equipment prior to shutdown of the equipment are usually also contaminated and must be discarded. At a late stage of manufacture, a semiconductor wafer may have a value between about $10,000 and $20,000. Thus, even an occasional loss is significant. 
     One method used in the industry to detect such residual photo-resist material is manual inspection with a microscope. However, manual inspection of semiconductor wafers to detect photo-resist materials has not been sufficiently effective. First, photo-resist is typically difficult to see using a conventional white light microscope, and even an experienced microscopist may inadvertently miss photo-resist on a wafer. Secondly, since manual inspection is laborious and time-consuming, it is generally not cost-effective to manually inspect more than a very small number of the semiconductor wafers (usually less than 10%). Thus, unstripped semiconductor wafers may still be missed by manual inspection. 
     Accordingly, an object of the present invention is to provide an improved method for rapid automated detection of resist material on semiconductor wafers in order to reduce process downtime, material wastage, maintenance/repair expenses and production costs. 
     SUMMARY OF THE INVENTION 
     The present invention is an automated method and apparatus for determining the presence or absence of a photo-resist material on the surface of a semiconductor substrate by the detection of fluorescence, reflection, or absorption of light by the photo-resist material. 
     Photo-resist materials are generally organic polymers, such as phenol-formaldehyde, polyisoprene, poly-methyl methacrylate, poly-methyl isopropenyl ketone, poly-butene-1-sulfone, poly-trifuluorethyl chloroacrylate, and the like. Organic substances can generally fluoresce (luminescence that is caused by the absorption of radiation at one wavelength followed by nearly immediate re-radiation at a different wavelength) or will absorb or reflect light. Fluorescence of the material at a particular wavelength, or reflection/absorption by the material of light at a given wavelength, may be detected and measured, provided the material differs from the underlying semiconductor substrate in fluorescence or reflection/absorption at a selected wavelength or wavelengths. For example, a positive photo-resist generally fluoresces red or red-orange and a negative photo-resist generally fluoresces yellow. 
     In a particular application of the invention, the presence of photo-resist material on a semiconductor wafer surface may be rapidly and automatically determined, recorded, and used to drive an apparatus which separates semiconductor wafers based on the presence or absence (or quantity) of the photo-resist material. Thus, semiconductor wafers which have been incompletely stripped of photo-resist material (or not stripped at all) may be automatically detected and culled from a manufacture line of fully stripped semiconductor wafers and reworked. Thus, contamination of downstream processes by unstripped semiconductor wafers is avoided. 
     In this invention, the semiconductor wafer is irradiated with light which may be monochromatic, multichromatic, or white. In one version, the intensity of generated fluorescence peculiar to the photo-resist material at a given wavelength is measured. In another version, the intensity is measured at a wavelength which is largely or essentially fully absorbed by the photo-resist material. In a further variation, the intensity of reflected light is measured at a particular wavelength highly reflected by the photo-resist material but absorbed by the substrate. 
     The intensity of fluoresced or reflected light is measured by a sensing apparatus and the result is put to a logic circuit, e.g., a computer. The result may be recorded and used for a decision making step and control of a robotic device. The robot performs the semiconductor wafer handling tasks, such as transferring the semiconductor wafers from a semiconductor wafer cassette to an inspection stage, and transferring the inspected semiconductor wafers to a destination dependent upon the test results. 
     A permanent record of the test results may be automatically retained and printed, and semiconductor wafers identified as being partially or totally unstripped or otherwise abnormal or defective are separated for proper disposition. 
     The apparatus for conducting the detection test process is generally comprised of known components which in combination produce accurate results in a very short time without laborious manual inspection. A high test rate may be achieved in a continuous or semi-continuous manufacturing process, enabling all product units to be tested. The current laborious and time-consuming testing of a few random samples by manual microscopic inspection methods is eliminated. The test results are in electronic digital form and may be incorporated into a comprehensive automated manufacturing documentation/control system. 
     The test apparatus may comprise a stand-alone system through which individual substrate units are passed for a separate detection/measurement step. Thus, for example, following a stripping step, semiconductor wafers may be moved sequentially through the test apparatus for confirmation of full stripping, and for culling of non-stripped semiconductor wafers. 
     In another version of the invention, the test apparatus may be incorporated into a processing step such as embodied in a resist stripping device for in situ determination of residual resist material on semiconductor wafers undergoing stripping. The stripping end-point may be thus determined and may be used to activate automated transfer of the stripped wafers from the resist stripper to the following process step when stripping is complete. This embodiment is particularly adaptable to plasma and wet-stripping apparatuses. 
     While the method and apparatus are particularly described herein as relating to the detection of photo-resist material in a lithographic process, they may also be used to detect the presence and quantity of any material on a semiconductor substrate, where the material and semiconductor substrate have differing fluorescing/absorbing properties at a given selected wavelength of radiation. The material may be an organic substance having naturally fluorescing properties under a particular spectrum of radiation, or may be a substance with little natural fluorescence, spiked with a material which fluoresces when irradiated with light of a particular wavelength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a diagrammatic view of an automated photo-resist material detection apparatus of the invention; 
     FIG. 2 is a graphical representation of exemplary results of detection tests conducted on a series of semiconductor wafers; 
     FIG. 3 is a diagrammatic view of a further embodiment of the automated photo-resist material detection apparatus of the invention; and 
     FIG. 4 is a diagrammatic view of an additional embodiment of the automated photo-resist material detection apparatus of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings, and particularly to FIG. 1, one embodiment of an automated photo-resist material detection apparatus  10  of the invention is shown. The illustrated components are generally not shown to scale. 
     An optical portion  12  of the photo-resist material detection apparatus  10  includes a light source  14  for generating a primary light beam  16  and a dichroic or dichromatic mirror  18  for directing at least some wavelengths of the primary light beam  16  onto a sample  20 , i.e., the semiconductor wafer, through a focusing lens  22 . An excitation filter  24 , such as a band pass filter, may be positioned in the path of primary light beam  16  for removing wavelengths from the primary light beam  16  which do not stimulate fluorescence, reflect, or absorb in the sample  20 . 
     As well known, the dichromatic mirror  18  reflects wavelengths of less than a given value, and passes wavelengths greater than the given value. 
     Where fluorescence of the sample  20  is desired, light source  14  is preferably a high energy lamp such as a mercury or xenon lamp which produces high intensity fluorescence-inducing illumination. 
     The sample  20  is preferably mounted on a stage  26  which is movable by motive means  28  to provide the desired positioning of the sample in the primary light beam  16 . A robotic device  30  loads the sample  20  onto the stage  26  and removes it after the test to another location for further processing or, alternatively, to a location for discard if the undesirable material is found on the sample  20 . 
     A secondary light beam  32  of fluoresced light and/or reflected light emanating from the sample  20  is shown passing through the dichromatic mirror  18  to a light intensity sensor  34 , such as a silicon diode sensor. The light intensity sensor  34  sends an electronic intensity signal  36  to a power meter  38 , which converts the electronic intensity signal  36  into an electronic numerical value signal  40  readable by a logic circuit  42  (such as a programmable computer circuit), preferably an analog to digital conversion in the power meter  38 . A small desktop computer may be used as the logic circuit  42 . 
     The sample  20  may be a substrate  44  having a layer or coating  46  of a material which differs from the substrate in fluorescing, absorption, and/or reflection properties at some wavelengths of incident light. The sample  20  may be a semiconductor wafer comprising a slice of crystalline silicon (silicon wafer) or may include various semiconductive material or material layers, including without limitation silicon wafers, silicon-on-insulative (SOI) structure, silicon-on-sappline (SOS) structure, gallium arsenide, or germanium upon which a layer of photo-resist material has been coated, processed and subsequently stripped. 
     Other lenses and filters, not shown, may be used to provide the desired light beam characteristics. As shown in FIG. 1, the secondary light beam  32  of fluoresced and/or reflected light from the sample  20  is passed through a suppression filter  48  to absorb non-fluoresced light or undesired reflected light and produce a filtered light beam  32 A substantially free of such undesired wavelengths. The filtered light beam  32 A may be further passed through a band pass filter  50  to produce a band pass filtered light beam  32 B having a narrow wavelength band of, for example, 700 nm ±80 nm. Such a wavelength is a characteristic fluorescing emission of commonly used positive photo-resist materials, as listed above. 
     The optical portion  12  of the photo-resist material detection apparatus  10  may comprise a microscope adapted for measurement of the fluorescent/reflected secondary light beam  32  from the sample  20 . 
     While the photo-resist material detection apparatus  10  may be used simply to determine the presence of a photo-resist material or other material on a substrate surface, its utility is enhanced by automation by which the samples  20  are moved to and from stage  26  by robotic device  30  as known in the art. Disposition of each sample  20  is determined by the test result therefor, and instructions  52  generated by a programmed logic circuit  42  are relayed to the robotic device  30  for proper control thereof. In a preferred embodiment, the stage  26  is moved along X-Y coordinates by instructions  54  from the logic circuit  42 , enabling testing at multiple locations, preferably nine or more, on the sample  20 . Because of the high rate at which the tests may be conducted, all wafers in a production line may be tested, greatly enhancing the detection of unstripped resist material. 
     It is also, of course, understood that the primary light beam  16  can be a sheet beam having a width approximately the width of the sample  20 . The sample  20  can be passed through the sheet beam which will result in the inspection of the entire surface of the sample  20 . 
     In one embodiment of the photo-resist material detection apparatus  10 , the power meter  38  converts the electronic intensity signal  36  into a simple digital “0” or “1” value, depending upon whether the electronic intensity signal  36  is less than or more than a selected cutoff value. This is useful when the decision is simply one of acceptance or rejection. 
     In other embodiments of the photo-resist material detection apparatus  10 , the power meter  38  may produce an electronic numerical value signal  40  representative of (in proportion to) the measured light intensity. 
     The detection surface test area of the sample  20  which provides the fluoresced or reflected secondary light beam  32  for a test may vary, depending upon the desired resolution. Thus, for detecting the presence of photo-resist material on a narrow slot location of a wafer, the diameter of the measurement circle may be very small, e.g., less than a fraction of a mil. The measurement of light intensity from such small areas may require prior light amplification. However, for some applications, the measurement circle may be much larger, and light amplification may not even be required. 
     FIG. 2 shows the fluoresced light intensity output from the apparatus of FIG. 1, where tests were conducted on a series of twenty-two substrates  44  in the form of semiconductor wafers. Stripped slots were formed on all but five of the semiconductor wafers (numbers  1 ,  5 ,  10 ,  15  and  20 ) which remained unstripped. Three tests were conducted on each semiconductor wafer, the results averaged by computer and printed as a continuous line  58 . Light intensities are shown in watts, as determined by the power meter  38 . The unstripped semiconductor wafers produced light intensity values of about (1.2 to 1.4)×10 −0.08  watts, while intensity values were about (1.0 to 2.0)×10 −0.09  watts for the stripped semiconductor wafers. As shown, an intermediate cutoff value  60  of light intensity may be selected as the basis for acceptance/rejection of each semiconductor wafer  20  by the robotic device  30 . 
     Another version of the photo-resist material detection apparatus  10  of the invention is shown in FIG. 3. A primary beam  70  of high intensity radiation is generated by a lamp  72  and directed into a filter cube  74  to be reflected onto the sample  20  through a focusing lens  76 . As available commercially, filter cubes  74  comprise a plurality of optical light paths as exemplified by  78 A,  78 B, and  78 C, each with a dichroic mirror  80  for directing primary beam  70  optionally through optical filters  82  of differing characteristics, through the focusing lens  76  onto the surface  84  of the sample  20 . The filter cube  74  is rotatable about a vertical axis  77  for selectively aligning a desired optical light path  78 A-C with the high intensity lamp  72  and focusing lens  76 . The dichroic mirrors  80  in the selectable optical light paths  78 A-C may have different reflectance properties. The fluoresced and reflected light (output light)  86  from the sample  20  passes back through the focusing lens  76  and selected dichroic mirror  80  of the filter cube  74 , and through optional optical filter  88  to an output lens  90  normally used for observation. 
     As illustrated in FIG. 3, the output light  86  from the output lens  90  of the filter cube  74  is directed into a photo-multiplier tube (PMT)  92  which sends an electronic signal  94  to a computer  96  for recording, analysis and decision making. Signals  98  generated by computer  96 , programmed with appropriate software, control movement of the stage  100 . Signals  102  control robot  104  for sample movement onto the stage  100  and for disposition of the tested sample  20  from the stage. 
     The use of the filter cube  74  enables a rapid trial of various wavelengths of fluoresced/reflected light to determine the most advantageous output wavelength for production testing. 
     As shown in FIG. 4, the photo-resist material detection apparatus  10  (shown in FIG. 3) may be incorporated into a stripping tool  110  for in situ automated determination of the progress in stripping of the layer or coating  46  of material, such as resist, from the surface  112  of a semiconductor wafer  56 . Elements common between FIGS. 1-3 and FIG. 4 retain the same numeric designation. The stripping process may comprise wet- or dry-stripping performed in a stripping chamber  114 . The stripping chamber  114  is illustrated herein with a plasma generator  130 . The stripping chamber  114  has one or two entryways, not shown, for the introduction and removal of the semiconductor wafers  56  by a robot  116 . The semiconductor wafer  56  is shown on a movable stage  118  within the stripping chamber  114 . The movable stage  118  may be movable by one or wore stepper motors  120  or other motive means controlled by electronic signals  122  from a computer  124 . 
     Two optical ports  126 ,  128  are positioned in a wall  132  of the stripping chamber  114 . A primary high energy beam  134  of light from lamp  136  passes through a first optical port  126 , strikes the surface  112  of the semiconductor wafer  56  and is reflected as reflected beam  138  at an angle through the second optical port  128 . Fluoresced and/or reflected light produced by existing layers or coatings  46  of material on the surface  112  in response to the primary high energy beam  134  is also present in reflected beam  138 . The reflected beam  138  is passed through an optical band pass filter  140  and into a photo-multiplier tube  142  for generation of an electronic signal  144  indicative of the light intensity at the filtered light wavelength. The electronic signal  144  is received by a software program in the computer  124  and processed to provide instructions  146  to the robot  116  for removal of the wafer  56  from the stripping chamber  114 . Electronic signals  122  are also sent by computer  124  for controlling motion of the movable stage  118 . 
     The primary high energy beam  134  is shown in FIG. 4 as striking the wafer  56  at an angle of about 45 degrees. The angle  137  between primary high energy beam  134  and reflected beam  138  is preferably between 0 and 90 degrees. However, by using a dichromatic mirror as in FIGS. 1 and 3, primary high energy beam  134  and reflected beam  138  may both pass through the same optical port  126  or  128 , and angle  137  is 0 degrees. 
     The high energy lamp  136  is typically a mercury or xenon lamp, and the output may be filtered by a band pass filter  148  to provide the desired wavelengths for producing fluorescence, reflectance, and/or absorption in the particular resist material. 
     As indicated, the method depends upon a difference in fluorescence or light absorption/reflectance between the material to be detected, e.g., the photo-resist and the underlying substrate. A wavelength of incident illumination is typically chosen which maximizes the difference in fluorescence, absorption, or reflectance. It is preferred to use fluorescence as the measured output, but light absorbence may be used when the material to be detected strongly absorbs a particular wavelength of radiation while the substrate strongly reflects the same. 
     It should be understood that references herein to light of a particular “wavelength” encompass wavelength bands that are “about” a particular wavelength. In other words, the term “a particular wavelength” way include wavelengths both slightly longer and shorter than the “particular wavelength.” 
     The advantages of this method over prior resist inspection methods are substantial. 
     First, the test is rapid and automated, enabling all wafers to be tested. The inadvertent passage of unstripped wafers to downstream process equipment, with concomitant costly contamination and destruction of the equipment, may be virtually eliminated. 
     Second, laborious and time-consuming visual inspections for resist are eliminated. Such tests are less than adequate, in any case. 
     Third, the detection method is adaptable to any type of resist or other material which may be applied to a substrate surface. This is because the process may be based on the quantitative differences between the material and the substrate in fluoresced light, reflected light, or absorbed light. Particular wavelengths are chosen to accentuate these differences. 
     Fourth, the apparatus for conducting the automated resist detection tests comprises an assembly of readily available equipment items. 
     Fifth, the software program for controlling the robot and movable stage may be very simple and easy to construct. 
     Sixth, the process and equipment may be readily incorporated in a batch, continuous or semi-continuous manufacturing process for accurate in situ determination of the end-point of resist stripping. Such use enhances the accuracy of end-point determination. 
     Seventh, the automated test method and control thereof may be incorporated in a comprehensive manufacturing documentation and control system. 
     Eighth, the method may be used to determine the presence of a material in a very small area, or alternatively in a relatively large area, by using an appropriate optical lens. 
     Having thus described in detail preferred embodiments of the present invention, it is to be understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description, as many apparent variations thereof are possible without departing from the spirit or scope thereof.
A method and apparatus for detection of a particular material, such as photo-resist material, on a sample surface. A narrow beam of light is projected onto the sample surface and the fluoresced and/or reflected light intensity at a particular wavelength band is measured by a light detector. The light intensity is converted to a numerical value and transmitted electronically to a logic circuit which determines the proper disposition of the sample. The logic circuit controls a sample-handling robotic device which sequentially transfers samples to and from a stage for testing and subsequent disposition. The method is particularly useful for detecting photo-resist material on the surface of a semiconductor wafer.