BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention generally relates to processes for fabricating semiconductor devices. More particularly, this invention relates to a process of producing encapsulation die through a process that simultaneously forms cavities and through-holes in a semiconductor wafer and provides both quality and cost improvements over current techniques. 
   2. Description of the Related Art 
   Within the semiconductor industry, there are numerous applications that require encapsulating the surface of a semiconductor die. As an example, a microelectromechanical system (MEMS) device formed in or on a semi-conductor die (referred to herein as a device die) is often capped by a semiconductor or glass die (referred to herein as a cap die), forming a package that defines a cavity within which the MEMS device, such as a suspended diaphragm or mass, is enclosed and protected. Examples of MEMS devices protected in this manner include accelerometers, rate sensors, actuators, pressure sensors, etc. 
   For mass production, numerous MEMS devices are simultaneously fabricated in a device wafer that is bonded to an encapsulation (cap) wafer, after which the resulting wafer stack undergoes a dicing operation to singulate individual die packages from the wafers. The cavities are usually defined by recesses formed in the cap wafer by photolithographic/etch techniques. By the very nature of their operation, MEMS devices must be free to move to some degree, necessitating that the size and shape of the cavity within each package be adequate and consistently defined. However, current methods for processing cap wafers for MEMS devices are often plagued with yield loss and quality defects. 
   The difficulty in processing cap wafers revolves around the need to etch holes all the way through the cap wafer in order to provide access to bond pads on the device wafer. Existing processes utilize photolithographic/etch techniques to form the through-holes and recesses required by cap wafers. As is well known in the art, photolithography involves the use of light to effect the transfer of a pattern from a mask to a wafer, and using the pattern as a mask during etching of the wafer to define structural features, such as the holes and recesses in the cap wafer. Creating these through-holes using photolithographic/etch techniques requires processing both sides of the wafer and maintaining alignment of features on both sides. Inherently, photolithographical patterning of the wafers requires individual handling of the wafers through several steps to create the pattern on the wafer. If both surfaces of the cap wafer are being patterned, the patterned side of the wafer must be placed against wafer handling fixtures in order to process the opposite surface of the wafer, which further aggravates the quality control problem. As each wafer is subjected to additional processing steps requiring individual handling of the wafer, the chances of creating a defect increase. For example, photolithographical masking defects or scratches during the processing of the cap wafer are translated into the cap wafer during the through wafer etch, resulting in defects that can lead to the loss of individual die and even the entire wafer. Because processing of a wafer with an undesired through-hole typically cannot continue due to the adverse effect on equipment further along in the process, masking defects that result in the etching of such holes will result in the loss of the entire wafer, incurring a significant cost burden in the form of lost wafers, processing costs, and production time. Even small masking defects that do not create undesirable through-holes in the cap wafer can cause the loss of individual die. Because wafers are inspected by an automated system, the inspection process is likely to catch only those defects that are trained into the system, raising the potential that a defective die will pass the visual inspection. 
   In view of the above, any improvements in cap wafer processes over existing photolithographical techniques have the potential for providing quality, yield, and cost advantages. 
   SUMMARY OF INVENTION 
   The present invention is directed to a method of processing a wafer, and particularly a cap wafer configured for mating with a device wafer in the production of a die package. The method makes use of a semiconductor wafer having first and second oxide layers on oppositely-disposed first and second surfaces, respectively, thereof. First and second masking layers are then deposited on the first and second oxide layers, respectively, after which the first and second masking layers are etched using photolithographic/etch techniques to define first and second mask patterns, respectively. The masking layers are patterned such that the first and second mask patterns expose regions of the first and second oxide layers, including first and second regions of the first oxide layer and first regions of the second oxide layer. The first mask pattern masks third and fourth regions of the first oxide layer and the second mask pattern masks second regions of the second oxide layer, with the fourth regions of the first oxide layer being aligned with the second regions of the second oxide layer. 
   Thereafter, an oxide mask for a subsequent silicon etch is formed on the exposed first and second regions of the first oxide layer and the exposed first regions of the second oxide layer, but is prevented from forming on the third and fourth regions of the first oxide layer and the second regions of the second oxide layer as a result of the presence of the first and second masking layers. The first and second masking layers are then removed to expose the third and fourth regions of the first oxide layer and the second regions of the second oxide layer, after which the third and fourth regions of the first oxide layer and the second regions of the second oxide layer are removed to expose first, second and third regions, respectively, of the wafer. The first, second and third regions of the wafer are then etched, wherein etching of the first regions of the wafer produces recesses in the first surface of the wafer and etching of the second and third regions of the wafer produces through-holes in the wafer. The oxide mask is then removed to yield a cap wafer with multiple through-holes and recesses. 
   In view of the above process, it can be seen that masking operations to define the through-holes and recesses in the wafer are not required after the oxide mask is formed, such that the oxide mask serves to protect the cap wafer throughout the handling and etching of the wafer. Furthermore, once the oxide mask is in place, further processing of the wafer can be limited to batch-type operations with automated transfers therebetween, thereby greatly reducing the chance of damage to the oxide mask during subsequent processing. Because reduced risk of damage to the oxide mask translates directly to fewer wafer defects and better quality, fewer wafers will be lost to gross defects and nondefective wafers will be of superior quality. 
   Additionally, most defects that might occur during and following application and patterning of the masking layers will be “self healing.” For example, if a scratch occurs in the mask patterns which is sufficiently deep to expose the underlying oxide layers of the wafer, the scratch will be filled by the oxide mask, so that the area under the scratch will be masked during the wafer etching step. Unless the feature created by the scratch is extremely large, etching of the wafer will not result in a defect being transferred to the wafer. For example, during an anisotropic etch, the wafer surface area beneath the oxide-filled scratch will be undercut so that the wafer surface area will still be removed as intended. On the other hand, if a defect in a mask pattern approaches the size of the features patterned in the mask pattern, the resulting feature etched in the cap wafer will have a smaller length and/or width dimension compared to the desired feature, and such a defect can be detected by an automated visual inspection. 
   Other objects and advantages of this invention will be better appreciated from the following detailed description. 


   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIGS. 1 through 6  are cross-sectional views of processing steps carried out to produce a cap wafer in accordance with a preferred embodiment of this invention. 
       FIG. 7  represents a MEMS device package incorporating a cap die singulated from the cap wafer produced by the steps of  FIGS. 1 through 6 . 
   


   DETAILED DESCRIPTION 
     FIG. 7  represents a MEMS device package  64  formed by bonding a device die  56  to a cap die  52 . The device die  56  is schematically represented as carrying a micromachined element  58  that is enclosed within a cavity  60  between the die  52  and  56 . The device die  56  is typically formed of a semiconductor material such as silicon, preferably monocrystallographic silicon, though it is foreseeable that other materials could be used. The cap die  52  is also preferably formed of a semiconductor material. The micromachined element  58  can be of any desired type, such as a proof mass, resonating structure, diaphragm or cantilever that relies on capacitive, piezoresistive and piezoelectric sensing elements to sense acceleration, motion, pressure, etc., all of which are known in the art. As is conventional, the micromachined element  58  is electrically interconnected to bond pads  62  on the device die  56 , such as with conductive runners (not shown) in the form of aluminum metallization. Through the bond pads  62 , the micromachined element  58  and its associated sensing elements can be electrically interconnected with appropriate signal conditioning circuitry (not shown). 
   The cap die  52  is shown as having a recess  50  that defines the cavity  60  and a through-hole  48  that provides access to the bond pads  62  on the device die  56 . According to a preferred aspect of this invention, the recess  50  and through-hole  48  are created by processing steps represented in  FIGS. 1 through 6 . Referring to  FIG. 1 , a wafer  10  is schematically represented as having two opposite surfaces  12  and  14 , each of which is covered with an oxide layer  16  and  18 . The wafer  10  is preferably a [ 100 ] p-type silicon wafer that has been is polished on both surfaces  12  and  14 . For convenience, the sides of the wafer  10  at which the surfaces  12  and  14  are located will be referred to as the frontside and backside, though other terminology could be used. The wafer  10  is preferably sufficiently thick to permit handling while the lateral dimensions of the wafer  10  are generally large enough such that it can be subsequently diced into a number of individual chips, e.g., the cap die  52  in  FIG. 7 . As an example, a suitable thickness for a wafer  10  having a diameter of about 125 millimeters is about 380 to about 625 micrometers. 
   The oxide layers  16  and  18  are preferably thin oxide layers deposited or thermally grown in a conventional manner on the surfaces  12  and  14  of the wafer  10 . Over-lying the oxide layers  16  and  18  are films  20  and  22  of a material that will serve as a maskant for a subsequent oxidation step. A preferred material for the films  20  and  22  is silicon nitride (Si 3 N 4 ), with a suitable thickness being about 1500 to about 2000 Angstroms. Other suitable materials for the films  20  and  22  include those that have a low oxidation rate relative to silicon and are compatible with semiconductor processing. A suitable process for depositing the films  20  and  22  is low pressure chemical vapor deposition (LPCVD). The oxide layers  16  and  18  between the silicon wafer surfaces  12  and  14  and the films  20  and  22  serve as barriers that allow the films  20  and  22  to be removed without removing any of the wafer  10 . For this purpose, suitable thicknesses for the oxide layers  16  and  18  are on the order of about 200 to about 750 Angstroms. 
   In  FIG. 2  the silicon nitride film  20  on the frontside of the wafer  10  has been patterned and etched, while  FIG. 3  shows the result of patterning and etching the silicon nitride film  22  on the backside of the wafer  10 . The nitride films  20  and  22  are preferably etched using a dry etch. The etching operation produces openings  24 ,  26  and  38  in the nitride films  20  and  22  that expose underlying regions  32 ,  34  and  40 , respectively, of the oxide layers  16  and  18 . The remaining portions of the nitride films  20  and  22  define islands  28 ,  30  and  36  on the frontside and backside of the wafer  10 . The islands  30  and  36  are aligned and correspond to the future locations of through-holes  48  in the wafer  10  ( FIG. 5 ), and therefore the through-hole  48  in the cap die  52  of  FIG. 7 . The islands  28  on the frontside of the wafer  10  correspond to the future locations of recesses  50  in the wafer  10  ( FIG. 5 ), and therefore the recess  50  in the cap die  52  of  FIG. 7 . 
   Following the patterning and etching steps of  FIGS. 2 and 3 , the wafer  10  undergoes oxidation to grow a thick field oxide  42  ( FIG. 4 ) on the regions  32 ,  34  and  40  of the oxide layers  16  and  18  exposed by the openings  24 ,  26  and  38  patterned and etched in the nitride films  20  and  22 . The field oxide  42  forms a continuous matrix that surrounds each of the individual islands  28 ,  30  and  36  of the nitride films  20  and  22 . The field oxide  42  will serve as an etch mask during etching of the through-holes  48  and recesses  50 , and as such needs to be sufficiently thick to protect the wafer  10  during the etch process. Suitable thicknesses will depend on the etch rate of oxide using a particular wet or dry etch process. For example, a thickness of at least 5000 Angstroms is desirable if using the preferred wet chemical anisotropic etchant tetramethyl ammonium hydroxide (TMAH). 
   Those skilled in the art will appreciate that the process of patterning and etching of the nitride films  20  and  22  to produce a mask for a subsequent thermally-grown oxide is a technique known as LOCOS (local oxidation of silicon), which is a standard MOS process. Certain processing parameters and techniques employed in LOCOS processes can be employed in the present invention, as long as they are not detrimental to the creation of the well-defined through-holes  48  and recesses  50  sought by the present invention. 
   With the field oxide  42  in place, the islands  28 ,  30  and  36  of the nitride films  20  and  22  are removed, such as with the use of phosphoric acid (H 3 PO 4 ), so as not to damage the field oxide  42 . Removal of the islands  28 ,  30  and  36  exposes underlying regions  44 ,  45  and  46  of the oxide layers  16  and  18  that coincide in size and shape to the islands  28 ,  30  and  36 , respectively. As such, the regions  45  and  46  are aligned and correspond to the future locations of the through-holes  48  in the wafer  10  ( FIG. 5 ). The regions  44 ,  45  and  46  are etched, such as through the use of a BOE (buffered oxide etch) dip, to expose underlying surface regions of the wafer  10 . The BOE dip removes some oxide from the surface of the field oxide  42  but not enough to destroy its usefulness as a mask. Using the field oxide  42  as a mask, the exposed wafer surface regions are etched to yield the structure represented in  FIG. 5 . As noted above, the wafer etch is preferably performed with the wet chemical anisotropic etchant TMAH. As a result of the wafer etch, the regions of the wafer  10  originally underlying the islands  30  and  36  of the nitride films  20  and  22  have been etched to form through-holes  48 , while the regions of the wafer  10  originally underlying the islands  28  of the nitride film  20  have been etched to form recesses  50  in the surface  12  of the wafer  10 . 
     FIG. 6  shows the result of stripping the field oxide  42  and the remaining regions  32 ,  34  and  40  of the oxide avers  16  and  18  from both sides of the wafer  10 , resulting in the wafer  10  comprising an array of cap die  52 . Each cap die  52  comprises a through-hole  48 , a recess  50 , and a land  54  that completely surrounds the recess  50 . The lands  54  subsequently serve as bonding surfaces for the cap wafer  10  when mated and bonded to a device wafer (not shown). When mated with the device wafer, the through-holes  48  can be aligned with bond pads (such as the bond pads  62  of  FIG. 7 ) or any other feature on the device wafer to which access is desired. Simultaneously, the recesses  50  are aligned with regions of the device wafer on which elements are present that are desired to be encapsulated (such as the micromachined element  58  of  FIG. 7 ). Thereafter, individual device packages (such as the package  64  in  FIG. 7 ) can be sawn from the wafer stack produced by the wafer bonding operation. As known in the art, if the operation of the micromachined element  58  requires or benefits from operating in a vacuum, the cap wafer  10  can be bonded to the device wafer so that each cavity  60  defined by the recesses  50  forms a reference vacuum chamber. 
   In view of the above, it can be seen that the above processing steps offer several advantages over prior practices in which photolithographic techniques are used to define the through-holes and recesses in cap wafers. One notable advantage is that this process moves all the single wafer patterning steps ahead of the step that forms the field oxide  42  used as the mask to define the through-holes  48  and recesses  50 . As such, the field oxide  42  is formed and then used without handling every wafer in the cassette. Another notable advantage is that the process is self healing to the extent that defects that might occur during patterning and etching of the nitride films  20  and  22  will not become etch defects in the wafer  10 . Instead, any region exposed by a defect in the nitride films  20  and  22  and not in the islands  28 ,  30 , and  36 , will be eliminated when the nitride is patterned, while any defects in the nitride films  20  and  22  within the islands  28 ,  30 , and  36  will become oxidized during growth of the field oxide  42 . The use of an anisotropic etchant such as TMAH will cause most oxide islands present in such defects to be undercut, such that the original defect in the nitride film  20  or  22  will not detrimentally effect the desired dimensions of the through-holes  48  and recesses  50 . On the other hand, only extremely large defects that are not undercut during the wafer etch are capable of leaving undesired silicon in the through-holes  48  and/or recess  50 , and will cause only the effected die to be nonfunctional. As such, the process of this invention has the potential for dramatically improving the quality of cap wafers while also decreasing the manufacturing costs associated with producing these wafers. 
   While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.
A method of processing a wafer, and particularly a cap wafer configured for mating with a device wafer in the production of a die package. Masking layers are deposited on oxide layers present on opposite surfaces of the wafer, after which the masking layers are etched to expose regions of the underlying oxide layers. Thereafter, an oxide mask is formed on the exposed regions of the oxide layers, but is prevented from forming on other regions of the oxide layers masked by the masking layers. The masking layers are then removed and the underlying regions of the oxide layers and the wafer are etched to simultaneously produce through-holes and recesses in the wafer. The oxide mask is then removed to allow mating of the cap wafer with a device wafer.