Abstract:
The present invention concerns, in part, a method for fabricating a silicon PIN detector component wherein three handle wafers are bonded to the wafer at varying points in the fabrication process. The utilization of three handle wafers during fabrication significantly ease handling concerns associated with what would otherwise be a relatively thin and fragile wafer, providing a stable and strong base for supporting those portions of the wafer that will constitute the PIN detector component. In a variant of the present invention, the third handle wafer comprises an optical element transparent in the wavelength of interest.

Description:
TECHNICAL FIELD  
       [0001]     The present invention generally concerns imaging sensors, and more particularly concerns fabrication methods and device structures for use with PIN (p-type/intrinsic/n-type) imaging sensors.  
       BACKGROUND  
       [0002]     In the field of photosensitive imaging devices there is a desire for increased resolving capability, particularly where the imaging devices are incorporated in, e.g., surveillance systems or high-performance professional digital cameras. A practical benefit associated with increased resolution in surveillance systems is the ability to recognize a feature having relatively smaller dimensions in a given field of view. A practical benefit associated with increased resolution in high-performance professional digital cameras is the ability to enlarge photographs without obvious artifacts like pixilation.  
         [0003]     One way of achieving improved resolution is to increase the number of picture elements (hereinafter “pixels”) used to sense a given field of view. The decision to increase pixels, however, is not cost-free. For example, if the overall dimensions of the sensor element are held constant, more pixels with less detecting area per pixel will be used to image a given field of view. A typical consequence of this is an increase in the susceptibility of the imaging system to artifacts associated with noise, e.g., the blotchy appearance of shadowy areas in images of scenes where in the actual view the shadowy areas were of relatively uniform illumination.  
         [0004]     Another approach which can maintain a desired noise performance is to hold the individual pixel size constant, but to increase the overall dimensions of the imaging sensor. This also has negative consequences. In particular, such an approach will require fabrication in semi-conductor materials of larger imaging sensors typically accompanied by lower yields. Thus the imaging sensor will be more expensive. In addition, if the imaging sensor is used in combination with optics, e.g., a zoom lens, the optical elements comprising the zoom lens, and the overall size of the zoom lens will have to be larger to maintain the same field of view.  
         [0005]     It is not surprising that given these constraints a hybrid approach is often pursued, i.e., the overall dimensions of the imaging sensor are increased while reducing somewhat the dimensions of the pixels. Nonetheless, even in compromise situations the same problems are encountered, e.g., susceptibility to noise and increased fabrication expense.  
         [0006]     Problems are also encountered in the selection of imaging sensor technology, e.g., PIN, active PN or CCD, as each of these have their own respective problems as the size of the pixel comprising the imaging sensor decreases. For example, in active PN devices a portion of the pixel actually comprises non-photo-sensitive control circuitry. As the size of pixels decrease for a constant imaging sensor dimension, the control circuitry becomes a larger percentage of the device area, eventually to the point where noise effects become intolerable.  
         [0007]     In environments where radiation events are possible, non-optical issues have to be taken into consideration. Different imaging sensor technologies have different radiation hardness properties, and these hardness properties, if desirable, should not be sacrificed if the pixel size is decreased in an effort to increase resolution.  
         [0008]     Designing the imaging sensor to have desirable radiation hardness properties may also cause unforeseen and unappreciated problems. For example, certain device features are more susceptible to radiation effects if their overall dimensions are larger, so there is a natural desire to decrease the dimensions of these device features. The decrease in device dimensions may, e.g., drive the sensor thickness to dimensions that are difficult to fabricate using conventional fabrication methods.  
         [0009]     Thus, those skilled in the art desire new imaging sensor designs that have improved resolution with desirable noise and radiation hardness properties. Those skilled in the art also desire fabrication methods that are capable of economically making new imaging sensors with desirable properties by achieving acceptable yield levels. Those skilled in the art further desire designs and methods that are particularly applicable to PIN imaging sensors.  
       SUMMARY OF THE PREFERRED EMBODIMENTS  
       [0010]     The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.  
         [0011]     A first embodiment of the present invention comprises a method for fabricating a PIN focal plane imaging sensor detector component comprising: fabricating a process wafer comprising an n-type first handle layer; an intrinsic active layer; and a buried oxide layer between the n-type handle layer and the intrinsic active layer, wherein an exposed surface of the intrinsic active layer coincides with a first processing side of the process wafer; forming an N+ front side contact capture area and P+ detector circuitry in the intrinsic active layer on the first processing side of the process wafer and associated structures in a second oxide layer atop the intrinsic active layer of the process wafer; bonding a second handle wafer to the first processing side of the process-wafer by bonding the second handle wafer to the second oxide layer atop the intrinsic active layer; removing the first handle wafer layer to expose the buried oxide layer, wherein the surface of the buried oxide layer exposed by removal of the first handle wafer coincides with a second processing side of the process wafer; forming a backside contact region on the second processing side of the process wafer, during which the buried oxide layer and a portion of the intrinsic active layer above the N+ front side contact capture area is removed thereby exposing a surface of the active intrinsic layer and a portion of the N+ front side capture area; implanting N+ species in the exposed surface of the active intrinsic layer to form an N+ region, whereby the N+ region forms an electrical contact with the N+ front side contact capture area; bonding a third handle wafer to the second processing side of the process wafer; removing the second handle wafer to expose the second oxide layer, wherein the surface of the second oxide layer exposed by removal of the second handle wafer coincides with the first processing side of the process wafer; and forming metal contacts the first processing side of the process wafer with the N+ front side contact capture area; P+ detectors and associated structures.  
         [0012]     A second embodiment of the present invention comprises a PIN focal plane imaging sensor detector component created by a process comprising: fabricating a process wafer comprising an n-type first handle layer; an intrinsic active layer; and a buried oxide layer between the n-type handle layer and the intrinsic active layer, wherein an exposed surface of the intrinsic active layer coincides with a first processing side of the process wafer; forming an N+ front side contact capture area and P+ detector circuitry in the intrinsic active layer on the first processing side of the process wafer and associated structures in a second oxide layer atop the intrinsic active layer of the process wafer; bonding a second handle wafer to the first processing side of the process wafer by bonding the second handle wafer to the second oxide layer atop the intrinsic active layer; removing the first handle wafer layer to expose the buried oxide layer, wherein the surface of the buried oxide layer exposed by removal of the first handle wafer coincides with a second processing side of the process wafer; forming a backside contact region on the second processing side of the process wafer, during which the buried oxide layer and a portion of the intrinsic active layer above the N+ front side contact capture area is removed thereby exposing a surface of the active intrinsic layer and a portion of the N+ front side capture area; implanting N+ species in the exposed surface of the active intrinsic layer, whereby the N+ region forms an electrical contact with the N+ front side contact capture area; bonding a third handle wafer to the second processing side of the process wafer; removing the second handle wafer to expose the second oxide layer, wherein the surface of the second oxide layer exposed by removal of the second handle wafer coincides with the first processing side of the process wafer; and forming metal contacts on the first processing side of the process wafer with the N+ front side contact capture area; P+ detectors and associated structures; and wherein the combined overall thickness of the PIN imaging sensor is less than 30 microns.  
         [0013]     A third embodiment of the present invention comprises a method for processing a three-layer process wafer to form a PIN imaging device detector component using three handle wafers, the three-layer process wafer comprising a first handle wafer of N-type material, an intrinsic active layer, and a buried oxide layer positioned between the first handle wafer and the intrinsic active layer, the method comprising: implanting. P-type material though a photo-resistive mask to form a regular two-dimensional pattern of non-contiguous P+ detector sites in the intrinsic active layer of the process wafer; implanting N-type material through a photo-resistive mask to form an N+ front side contact capture area in the intrinsic active layer; growing a field oxide on the intrinsic active layer; etching the field oxide to expose regions where gate contact and control gates will be formed in the field oxide; forming the gate contact and control gates; forming an oxide layer on the field oxide layer, gate contact and control gates; processing the oxide layer formed on the field oxide layer, gate contact and control gates to a uniform thickness; bonding a second handle wafer to the oxide layer formed on the field oxide layer, gate contact and control gates; removing the first handle wafer to expose the buried oxide layer; etching a cavity in the buried oxide layer, the intrinsic active layer and the N+ front side contact capture area; removing the remaining buried oxide layer, thereby exposing a surface of the intrinsic. active layer; implanting N+ species in the exposed surface of the intrinsic active layer to form an N+ region, whereby the N+ region forms an electrical contact with the N+ front side contact capture area; bonding a third handle wafer to the process wafer; removing the second handle wafer to expose the field oxide layer; etching the field oxide layer through a photoresistive mask where metal contacts will be formed; and forming metal contacts with the N+ front side contact capture area; gate contacts and P+ detector sites in the etched areas.  
         [0014]     A fourth embodiment of the present invention comprises a PIN imaging sensor detector component for imaging a field of view by detecting electromagnetic radiation emanating from the field of views the PIN imaging sensor comprising: an oxide layer having a first surface and a second surface; an intrinsic layer positioned atop the first surface of the oxide layer, wherein the intrinsic layer has a first surface in contact with the oxide layer, and a second surface facing the field of view, wherein electromagnetic radiation emanating from the field of view impinges on the second surface of the intrinsic layer, the intrinsic layer having at least one channel formed in the periphery of the imaging sensor wherein intrinsic material has been removed to expose a portion of the oxide layer, the at least one channel having sides; a plurality of non-contiguous P+ regions formed in the intrinsic layer at the first surface of the intrinsic layer, wherein the P+ regions extend partially into the intrinsic layer at a substantially uniform depth, the plurality of P+ regions arrayed in a substantially uniform two-dimensional array of rows and columns; a plurality of non-contiguous buried N+ regions formed in the intrinsic layer at the first surface of the intrinsic layer and adjacent to the channel, the N+ region comprising a portion of the side of the channel; an exposed N+ region formed in the second surface of the intrinsic layer and extending partially into the intrinsic layer, the exposed N+ region extending down the side of the channel and contacting the buried N+ regions, wherein the exposed N+ region, plurality of P+ regions and the intrinsic material between the P+ and exposed N+ regions together form a plurality of detector sites for detecting electromagnetic radiation emanating from the field of view; metal contacts extending through the oxide layer from the second surface of the oxide layer and contacting the buried N+ and P+ regions; and wherein the combined overall thickness of the PIN imaging sensor is less than 30 microns.  
         [0015]     Thus it is seen that the apparatus and method of the present invention provide a PIN imaging sensor detector component with improved characteristics. In particular, the method of the present invention enables PIN imaging sensors to be constructed with smaller detector sites while achieving desirable resistance to noise and radiation events. The desired resistance to noise and radiation is achieved, in part, by reducing the thickness of the PIN-type imaging sensor. The reduced thickness of the PIN-type imaging sensor is achieved by utilizing a fabrication method that uses three handle wafers at varying stages of the fabrication process. With this fabrication process, overall imaging sensor thickness can be reduced to 30 microns or less when operating as a photovoltaic detector or as an avalanche photodiode (APD). This decrease in imaging sensor thickness has the desirable effects of reducing the susceptibility of the imaging sensor to noise and radiation events.  
         [0016]     In conclusion, the foregoing summary of the embodiments of the present invention is exemplary and non-limiting. For example, one skilled in the art will understand that one or more aspects or steps from one embodiment can be combined with one or more aspects or steps from another embodiment of the present invention to create a new embodiment within the scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The foregoing and other aspects of these teachings are made more evident in the following Detailed Description of the Preferred Embodiments, when read in conjunction with the attached Drawing Figures, wherein:  
         [0018]      FIG. 1  depicts in a cross-sectional view a three-layer process wafer;  
         [0019]      FIG. 2  depicts in a cross-sectional view the process wafer of  FIG. 1  after steps in which a front side contact capture area for the detector backside contact, gate contact area and control gates have been formed in various layers of the wafer on a first processing side of the wafer, all in accordance with embodiments of the invention;  
         [0020]      FIG. 3  depicts the process wafer of  FIG. 2  after a second handle wafer has been bonded to a field oxide layer of the wafer on the first processing side of the wafer, all in accordance with embodiments of the invention;  
         [0021]      FIG. 4  depicts steps being performed on the first handle wafer of the process wafer depicted in  FIG. 3 , all in accordance with embodiments of the invention;  
         [0022]      FIG. 5  depicts the results of steps in which a backside contact profile is formed in a second processing side of the wafer of  FIG. 4 , all in accordance with embodiments of the invention;  
         [0023]      FIG. 6  depicts the results of steps in which a buried oxide layer is removed from the wafer of  FIG. 5  and the field oxide layer has been etched in the region of the backside contact, all in accordance with embodiments of the invention;  
         [0024]      FIG. 7  depicts the results of steps in which backside contact areas have been formed in the wafer of  FIG. 6  by implantation of N+ species, all in accordance with embodiments of the invention;  
         [0025]      FIG. 8  depicts the results of steps in which a bonding oxide layer has been formed in the process wafer depicted in  FIG. 7 , all in accordance with embodiments of the invention;  
         [0026]      FIG. 9  depicts the results of steps in which a third handle wafer has been bonded to the process wafer depicted in  FIG. 8 , all in accordance with embodiments of the invention;  
         [0027]      FIG. 10  depicts a step in which the second handle wafer is removed from the process wafer depicted in  FIG. 9 , all in accordance with embodiments of the invention;  
         [0028]      FIG. 11  depicts the results of steps in which metal contacts are formed in the field oxide layer on the first processing side of the process wafer, all in accordance with embodiments of the invention;  
         [0029]      FIG. 12  depicts the results of steps performed on the process wafer of  FIG. 11  during which the third handle wafer has been removed;  
         [0030]      FIG. 13  is a flowchart summarizing the fabrication steps depicted in  FIGS. 1-12 ; and  
         [0031]      FIG. 14  is a flowchart depicting an alternate method capable of forming a PIN imaging sensor in accordance with embodiments of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]      FIG. 1  depicts in cross-sectional view a three-layer process wafer  100  comprising a first handle wafer  110 ; a buried oxide layer  120  and an intrinsic active wafer layer  130 . The n-type handle wafer  110  is formed by a Czochralski process (CZ) or a magnetic field applied process (MCZ) and may be up to 700 microns thick. The intrinsic active layer  130  is approximately 10 to 200 microns thick after polishing to its final thickness, and can be formed by a float zone growth method process (FZ).  
         [0033]      FIG. 2  depicts the end result of several processing steps performed on the process wafer  100  depicted in  FIG. 1 . The detector sites  134  are formed by first oxidizing a portion of the intrinsic active wafer layer  130 . The oxide layer is then etched to reveal portions of the intrinsic active wafer layer where detector sites will be formed. The detector sites are then formed by ion implantation of boron or other P-type dopant materials such as aluminum or gallium. The front side capture area  132  for the backside contact is also formed by etching and then ion implantation using N-type material like arsenic. A field or second oxide layer  140  is then grown on intrinsic active layer  130 . The field oxide layer  140  is etched to expose regions where gate contacts and control gates will be formed. Next, the gate contact  142  and control gates  144  are formed in the etched regions. The gate contacts  142  and control gates can be implemented in metal or polysilicon. Gate contacts and control gate are optional and in alternate embodiments may not be used. Finally, another oxide layer is grown on the field oxide layer  140 , gate contact  142  and/or other control gates  144 . The resulting modified field oxide layer  140  is then processed to a uniform thickness.  
         [0034]      FIG. 3  depicts the bonding of the second handle wafer  150  to the oxide layer  140  using an SiO 2  bonding process, or other appropriate process.  FIG. 4  depicts the removal of the first handle wafer layer  110 . The first handle wafer layer  110  is ground to a thickness of 30-50 microns, and the remaining portion of the silicon is then etched to expose the buried oxide layer  120 .  
         [0035]      FIG. 5  shows the next processing step which forms a portion of the structure that will support the backside contact to the front side field plate. A portion of the intrinsic active wafer layer  130  is etched to reveal the oxide layer  140  formed in the previous step. KOH (potassium hydroxide) anisotropic etching, other anisotropic etching methods (using, for example, Tetra Methyl Ammonia Hydroxide), or plasma oxide etching may be used to form channel  160 . The sides of the channel  160  are sloped to accommodate an ion, implantation step in which N-type doping materials will be implanted into the sloped sides to accommodate the front side to backside contact. Other deep contact structures could be used, formed by an inductively coupled plasma or Bosch etch process for silicon through-via made conductive with implants, polysilicon or other means. Other materials may or may not require deep contacts depending on the resistivity of the detector bulk or substrate.  
         [0036]     In the next steps depicted in  FIG. 6 , the buried oxide layer  120  and a portion of the field oxide layer  140  in the channel  160  is removed.  FIG. 7  shows the results of one or more steps in which an N+ region  136  is formed by ion implantation in the surface of active intrinsic layer  130 . The N+ region extends across the surface of the intrinsic active layer  130  to the channel  160  and down the sloped sides of the channel  160  where it contacts the N+ front side contact capture area  132 .  
         [0037]     In the next processing step a third handle wafer  170  will be bonded to the second processing side of the process wafer  100 . Typically, a bonding oxide layer  138  is grown on the second processing side of the process wafer  100  as shown in  FIG. 8 . In alternate embodiments of the present invention, an anti-reflection coating (not shown) may be deposited on the intrinsic active layer before the bonding oxide layer is grown on the second processing side of the process wafer. In still further embodiments, the third handle wafer  170  may comprise an optical element; an optically transparent substrate (in the waveband of interest); or an opaque element.  FIG. 9  depicts the result of the processing step in which the third handle wafer  170  has been bonded to the second processing side of the process wafer  100 .  
         [0038]      FIG. 10  depicts the performance of a processing step in which the second handle wafer  150  is being removed. In one embodiment of the present invention, the second handle wafer  150  is ground to about 30-50 microns. Then the remaining portion of the second handle wafer is removed by etching.  
         [0039]      FIG. 11  depicts the result of several processing steps. In a first step, photoresist material is deposited and the oxide layer  140  is selectively etched down to the detector regions  134 , gate contact area  142  and the N+ front side contact capture area to form cavities  182 . Then metal  184  is deposited in the cavities  182  formed by etching, e.g., by sputtering. Then the photoresist mask is removed. Next another oxide layer is grown and selectively etched, and then bump formations  186  are achieved by depositing a bump metallization. The bump formations  186  are in contact with metal regions  184 . Other methods of forming contacts between the detector metal and sensor circuits or other components can be used.  
         [0040]      FIG. 12  depicts the results of additional processing steps. In a further processing step the third handle wafer  170  may be ground to 30-50 microns, and then etched to reveal the intrinsic active layer  130 ,.  
         [0041]     The steps depicted in  FIGS. 1-12  are generally summarized in the flow chart depicted in  FIG. 13 . In the method  300  depicted in  FIG. 13  at step  310  a process wafer is fabricated comprising an N-type first handle wafer; an intrinsic active layer; and a buried oxide layer between the N-type first handle wafer and the intrinsic active layer. In the method  300 , an exposed surface of the intrinsic active layer coincides with a first processing side of the process wafer. Next, at step  320  detector circuitry is formed in the intrinsic active layer on the first processing side of the process wafer. Associated structures (for example, gate contact areas and control gates are formed in a silicon dioxide layer grown atop the intrinsic active layer.  
         [0042]     Then, at step  330  a second handle wafer is bonded to the first processing side of the process wafer by bonding the second handle wafer to the second oxide layer formed atop the active intrinsic layer.  
         [0043]     Next steps are performed to form a backside contact region. This is accomplished by removing the first handle wafer to expose the buried oxide layer at step  340 . The surface of the buried oxide layer exposed by removal of the first handle wafer coincides with a second processing side of the process wafer. Then at step  350 , a backside contact region and associated circuitry are formed on the second processing side of the process wafer. During step  350  the buried oxide layer is removed. Next at step  360  a third handle wafer is bonded to the second processing side of the process wafer. Then, at step  370  metal contacts to the gate contact area, N+ front side contact capture area, and P+ detector sites are formed. Next, at step  380 , the third handle wafer is removed.  
         [0044]     FIGS.  14 A-B depict an alternate method  400  capable of forming a PIN detector device in accordance with embodiments of the present invention. The method  400  operates on process wafer as depicted in  FIG. 1 . At step  410 , P-type material is implanted through a photo-resistive mask to form a regular two-dimensional pattern of non-contiguous P+ detector sites  134  in the intrinsic active layer  130  of the process wafer  100 . Next, at step  415  N-type material is implanted through a photo-resistive mask to form an N+ front side capture area  132  in the intrinsic active wafer layer. Then, at step  420  a field oxide layer is grown on the intrinsic active layer. Next, at step  425 , the field oxide is etched through a mask to expose regions where gate contact and control gates will be formed. Then, at step  430  the gate contact and control gates are formed. The gate contacts and control gates can be implemented in metal or polysilicon. Following this, at step  435  another oxide layer is formed on the field oxide layer, gate contact and control gates. Then, at step  440 , the oxide layer formed on the field oxide layer, gate contact and control gates is processed to a uniform thickness. The results of steps  420 - 440  are depicted in the process wafer of  FIG. 2 .  
         [0045]     Next, step  445  of method  400  is performed, whereby a second handle wafer is bonded to the oxide layer formed on the field oxide layer, gate contact and control gates. Then, at step  450  the first handle wafer is removed to expose the buried oxide layer  120 . Next, at step  455 , a cavity  160  is etched in the buried oxide layer, the intrinsic active layer, and the N+ front side contact capture area. Then, at step  460  the remaining buried oxide layer is removed, thereby exposing a surface of the intrinsic active layer. Next, at step  465  N+ species are implanted in the exposed surface of the intrinsic active layer to. form an N+ region. The N+ region forms an electrical contact with the N+ front side contact capture area. Then, at step  470  a third handle wafer is bonded to the process wafer. Next, at step  475 , the second handle wafer is removed to expose the field oxide layer. Then, at step  480 , the field oxide layer is etched through a photoresistive mask in regions where metal contacts will be formed. The regions coincide with the N+ front side contact capture area, the gate contact area and the detector sites. Next, at step  485  the metal contacts are formed in the etched regions with the N+ front side contact capture area, the gate contact area and the detector sites.  
         [0046]     It is also an embodiment of the present invention that the fabrication and removal processes can be stored on an electronic medium, such as RAM (random access memory) ROM (read only memory) or other non-volatile memory (NVM). Thus a computer may be used to implement the invention.  
         [0047]     The present invention has been described in terms of steps which may represent a single actual process step, or the combination of multiple process steps. One of ordinary skill in the art will appreciate that the steps of the method of the present invention can be performed in different combinations and sequences then as set forth herein. All such combinations and orderings of steps are within the scope of the present invention. Also, the disclosed sequence of steps is not critical and it is contemplated that one skilled in the art would be aware of modifications in the sequence of steps described herein. In other words, combining steps and modifying the sequence of steps described does not depart from the scope of the invention.  
         [0048]     Furthermore, while the present invention has been described in terms of particular materials there are other materials, which may be interchangeable, that should be understood by one skilled in the art to be equivalents. Indeed, the present invention may be accomplished with any combination of materials that achieve a similar result, including a variety of detector materials in place of the intrinsic silicon.  
         [0049]     Furthermore, while specific dimensions have been provided to describe the invention, it is contemplated that one of ordinary skill in the art may modify the dimensions without departing from the invention.  
         [0050]     Thus it is seen that the foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the best method and apparatus presently contemplated by the inventors for fabricating a triple-bonded PIN focal plane structure. One skilled in the art will appreciate that the various embodiments described herein can be practiced individually; in combination with one or more other embodiments described herein; or in combination with fabrications methods differing from those described herein. Further, one skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments; that these described embodiments are presented for the purposes of illustration and not of limitation; and that the present invention is therefore limited only by the claims which follow.