Abstract:
A structure and method for fabricating imagers that detect light from the backside of the wafer. The structure may have less complex focusing, reduced crosstalk, tighter pixel packing density, increased quantum efficiency, and wafer-level packaging. The fabrication of the imager includes forming an imaging device on a silicon wafer, adhering an interconnect wafer to the device wafer, forming interconnects on the interconnect wafer, etching away the substrate of the device wafer, and patterning additional layers such as nitrides, color filter arrays, and lenses on the backside of the device wafer.

Description:
FIELD OF THE INVENTION 
     Embodiments of the invention relate generally to solid state imaging devices and, more particularly, to a method of making an imaging structure that detects light from the backside of its associated semiconductor substrate. 
     BACKGROUND OF THE INVENTION 
     A CMOS imager includes a focal plane array of pixels, each pixel including a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for producing a photo-generated charge in a doped region of the substrate. A readout circuit is provided for each pixel and includes at least a source follower transistor and optionally, a row select transistor for coupling the source follower transistor to a column output line. The pixel also typically has a floating diffusion region, connected to the gate of the source follower transistor. Charge generated by the photosensor is sent to the floating diffusion region. The imager may also include a transistor for transferring charge from the photosensor to the floating diffusion region and another transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transference. 
     In a CMOS imager, the active elements of a pixel, for example a four transistor pixel, perform the necessary functions of (1) photon to charge conversion; (2) transfer of charge to the floating diffusion region; (3) resetting the floating diffusion region to a known state before the transfer of charge to it; (4) selection of a pixel for readout; and (5) output and amplification of a signal representing a reset voltage and a pixel signal voltage based on the photo converted charges. The charge at the floating diffusion region is converted to a pixel output voltage by a source follower output transistor. 
     A schematic diagram of a conventional CMOS four-transistor (4T) pixel  20  is illustrated in  FIG. 1 . The four transistors include a transfer transistor  22 , a reset transistor  23 , a source follower transistor  24 , and a row select transistor  25 . A photosensor  21 , e.g., a pinned photodiode, converts incident light into charge. A floating diffusion region  26  receives charge from the photosensor  21  through the transfer transistor  22  (when activated) and is also connected to the reset transistor  23  and the gate of the source follower transistor  24 . The source follower transistor  24  outputs a signal proportional to the charge accumulated in the floating diffusion region  26  to a sampling circuit when the row select transistor  25  is turned on. The reset transistor  23  resets the floating diffusion region  26  to a known potential prior to transfer of charge from the photosensor  21 . The photosensor  21  may be a photodiode (as shown in  FIG. 1 ), a photogate, or a photoconductor. If a photodiode is employed, the photodiode may be formed below a surface of the substrate and may be a p-n-p photodiode, an n-p-n photodiode, a p-n photodiode, or a n-p photodiode, among others. 
     CMOS semiconductor imaging devices include an array of pixels such as pixel  20  of  FIG. 1 , which convert light energy received, through optical lenses, into electrical signals. The electrical signals produced by the array of pixels are processed to render a digital image. 
     The amount of charge generated by the photosensor  21  corresponds to the intensity of light impinging on the photosensor  21 , for a given integration time. Accordingly, it is important that all of the light directed to the photosensor  21  impinges on the photosensor  21  rather than being reflected or refracted toward another photosensor (known as optical crosstalk). 
     For example, optical crosstalk may exist between neighboring photosensors in a pixel array. In an ideal imager, light enters only through the surface of the photosensor that directly receives the light stimulus. In reality, however, some light intended for one photosensor also impinges on another photosensor through the sides of the optical path existing between a lens and the photosensor. 
     Optical crosstalk can bring about undesirable results in the images produced by the imager. The undesirable results can become more pronounced as the density of pixels in the imager array increases, and as pixel size correspondingly decreases. The shrinking pixel sizes and greater pixel density make it increasingly difficult to properly focus incoming light on the photosensor of each pixel without accompanying optical crosstalk. 
     Optical crosstalk can cause a blurring or reduction in contrast in images produced by the imager. Optical crosstalk also degrades the spatial resolution, reduces overall sensitivity, causes color mixing, and leads to image noise after color correction. As noted above, image degradation can become more pronounced as pixel and related device sizes are reduced. Furthermore, degradation caused by optical crosstalk is more conspicuous at longer wavelengths of light. Light having longer wavelengths penetrates more deeply into the silicon structure of a pixel, providing more opportunities for the light to be reflected or refracted away from its intended target photosensor. 
     Electrical crosstalk may also occur when the photogenerated signals migrate through the silicon between pixels, and are collected at the wrong photodiode. Electrical crosstalk becomes more pronounced as pixel size decreases, and for longer wavelength light. 
       FIG. 2  illustrates the problem of optical and electrical crosstalk in a conventional frontside illuminated imager. A conventional frontside illuminated imager includes an array of pixels. For simplicity, a cross section of a single pixel  2  is illustrated. Pixel  2  has, for example, photodiodes, formed within a substrate  41 .  FIG. 2  also illustrates a metallization and interlayer dielectric layer  51  in contact with the substrate  41 . A nitride layer  91 , color filter array layer  96 , and microlens  97  are also provided. Ideally, incoming light  13  should stay within a photosensor optical path  12  when traveling through a microlens  97  to a respective photosensor of the pixel  2 . However, light  13  can be reflected within the respective layers of the imager and at the junctions between these layers. The incoming light  13  can also enter the pixel at different angles, causing the light to be incident on a different photosensor. Loss of the incident light  13  as it travels through the various layers also decreases the quantum efficiency of the device. 
     As noted, electrical crosstalk may also occur between pixels when photogenerated electrons migrate through the silicon layers. The thicker the silicon layers are, the greater space and opportunity for such migration to occur. However, thicker silicon layers provide greater overall structural stability to a device containing a pixel array. 
     Accordingly, there is a need and desire for an improved apparatus and method for reducing crosstalk and related electrical interference in imaging devices, without compromising structural stability. There is also a need to more effectively and accurately increase overall pixel sensitivity and provide improved crosstalk immunity without adding complexity to the manufacturing process and/or increasing fabrication costs. There is also a need to increase quantum efficiency. It would further be beneficial to provide an imager device having wafer level packaging. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a conventional CMOS four-transistor (4T) pixel. 
         FIG. 2  is a cross-section of a prior art backside illuminated wafer. 
         FIG. 3  is a cross-section of a portion of an embodiment of a pixel array in an initial stage of fabrication. 
         FIG. 4  is a cross-section of the embodiment of  FIG. 3  in a subsequent stage of fabrication. 
         FIG. 5  is a cross-section of the embodiment of  FIG. 4  in a subsequent stage of fabrication. 
         FIG. 6  is a cross-section of the embodiment of  FIG. 5  in a subsequent stage of fabrication. 
         FIG. 7  is a cross-section of the embodiment of  FIG. 6  in a subsequent stage of fabrication. 
         FIG. 8  is a cross-section of the embodiment of  FIG. 7  in a subsequent stage of fabrication. 
         FIG. 9  is a cross-section of the embodiment of  FIG. 8  in a subsequent stage of fabrication. 
         FIG. 10  is a block diagram of an imager employing the embodiment of  FIG. 9 . 
         FIG. 11  is a block diagram of a processor system employing the imager of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment of the invention. These embodiments are described in sufficient detail to enable those skilled in the art to practice them, and it is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made. 
     The term “substrate” is to be understood as a semiconductor-based material including silicon, silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a “substrate” in the following description, previous process steps may have been utilized to form regions or junctions in the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, germanium, or gallium arsenide. 
     The term “pixel” refers to a picture element unit cell containing a photosensor and transistors for converting light radiation to an electrical signal. For purposes of illustration, a representative pixel is illustrated in the figures and description herein and, typically, fabrication of all pixels in an imager pixel array will proceed simultaneously in a similar fashion. 
     Referring now to the drawings, where like elements are designated by like reference numerals,  FIG. 3  illustrates a cross-section of a portion of an embodiment of a pixel array in an initial stage of fabrication. A stack  5  is formed which comprises a device layer  10 , a buried oxide layer  30  and a substrate layer  40 . The device layer  10  has an imaging pixel  20  having a photodiode formed on one side. The device layer  10  may be a silicon layer. The pixel  20  may comprise a photodiode and associated transistors, but are represented herein as pixel  20 , for simplicity of illustration. The device layer  10  may have a thickness, t D , of at least 2.0 μm, depending on the desired sensitivity to red or infrared light. The thinner the device layer  10  is, the less sensitive the pixel  20  will be to red or infrared light. 
     On the opposite side of the device layer  10 , the buried oxide layer  30  is provided. The buried oxide layer  30  provides insulative properties to prevent crosstalk of photogenerated electrons from migrating laterally and prevent impurities from migrating into the device layer  10 . The buried oxide layer  30  may have a thickness, t B , in the range of about 0.5 μm to about 2.0 μm, depending on the desired quantum efficiency. The thinner the buried oxide layer  30 , the greater the quantum efficiency. 
     Additionally, the device layer  10  may have graded doping, with a higher doping being near the buried oxide layer  30  interface. Graded doping may provide additional insulative properties against crosstalk. Furthermore, the device layer  10  may have an n-type or p-type doping, though a p-type doping is more likely when n-channel transistors are used in pixel  20 . 
     On the other side of the buried oxide layer  30 , the substrate layer  40  is provided. The substrate layer  40  may have any thickness and provides structural stability to the device layer  10  during initial stages of processing, including the formation of the pixel  20  circuitry and peripheral circuitry associated with a pixel array. One or more passivation layers, e.g. BPSG, may be provided over the pixel  20  circuitry to protect the pixel  20  circuitry. 
     Metallization and interlayer dielectric layers, represented collectively as ILD layer  50 , are provided over the device layer  10 , as shown in  FIG. 4 . Optionally, an additional epitaxial layer may be formed between the device layer  10  and the ILD layer  50 , in which the transistors for the pixel  20  may be formed. Such a structure would have greater fill factor since the footprint of the photodiode could be greater since the remaining pixel circuitry is in a different semiconductor layer. 
     The ILD layer  50  has a final metal layer containing metal bond pads  55 , that will connect to an interconnect wafer  70 , as shown in  FIG. 5 . The interconnect wafer  70  is adhered to the ILD layer  50 . In one embodiment, the interconnect wafer  70  may be adhered to the IDL layer  50  by bonding with an adhesive layer  60  such as epoxy. 
     The interconnect wafer  70  will provide structural support to the device layer  10  once the substrate layer  40  is etched away in subsequent steps, as described below. The interconnect wafer  70  also provides electrical signal paths into and out of the pixel array containing pixels  20  and peripheral circuitry associated with the array. The interconnect wafer  70  may also help provide a sealant between the device layer  10  and the outside environment, in the case of wafer level packaging, where a wafer contains an array of fabricated devices, each imager containing an array of pixels  20  and associated peripheral circuitry is bonded to a wafer having an interconnect wafer  70  extending over the device wafer. The interconnect wafer  70  may be made of silicon, or glass of another material. The interconnect wafer  70  may be unprocessed prior to bonding to the device layer  10 . However, the interconnect wafer  70  may be patterned prior to bonding with the device layer  10 , although this method requires additional alignment prior to bonding, as will be described below. 
     The adhesive, or epoxy, layer  60 , may be screen printed. Other methods for providing the adhesive layer  60  include anodic bonding, low temperature silicon bonding, or eutectic bonding. If the interconnect wafer  70  is patterned with conductors and external connections prior to bonding, the interconnect wafer  70  and device layer  10  must be aligned to ensure that the bond pads  55  are bonded to the corresponding electrical connection on the interconnect wafer  70 . The bonding may be followed by a cure to improve the bond strength and reduce outgassing during subsequent wafer processing steps. 
     As shown in  FIG. 6 , the interconnect wafer  70  is patterned to have via openings  75 . The patterning may be performed by anisotropic etch or by a laser ablation. The openings  75  are placed over the metal bond pads  55 , and the etch or laser patterning process stops on the metal bond pads  55 . 
     As shown in  FIG. 7 , the openings  75  are lined with a barrier metal  85  and filled with a metal plug  80 . Excess metal on the top of the interconnect wafer may then be polished away. 
     Once the processing of the interconnect wafer  70  is complete, the substrate layer  40  may then be etched away, as shown in  FIG. 8 . Substantially all of the substrate layer  40  may be etched away, since it is no longer needed to provided structural stability to the stack  5  with the interconnect wafer  70  in place. The substrate layer  40  may be etched away with an isotropic dry etch using, for example, SF 6  or XeF 2 . Alternatively, the substrate layer  40  may be etched away using known wet isoptroic etches. When this etch is performed, the sides of a wafer containing stack  5  and the top of the wafer containing the interconnect wafer  70  should be protected. 
     By removing the substrate layer  40 , the photodiode of pixel  20  will be placed closer to the source of incident light. Hence, requirements for focusing structures may be reduced. A microlens may not need to be precisely formed or may not be need at all. 
     The stack  5  is then processed on the back side, as shown in  FIG. 9 , which shows the buried oxide layer  30  on top. A low temperature nitride layer  90  can be deposited to improve optical performance. A color filter array layer  95  may be provided over the nitride layer  90  and a microlens  100  layer may be formed over the color filter array layer  95 . The color filter layer  95  may include red, green, and blue filters in a Bayer pattern, or other filter colors and patterns known in the art. It should be noted that although the embodiment has been described as having a single nitride layer  90 , color filter array layer  95  and microlens  100  layer, the embodiments of the invention are not limited to having all of these layers and, optionally, one or more of these layers may be omitted, or other layers may be added. For example, a hardcoat may be provided over the microlens  100  layer to protect against reflow during subsequent development processes. 
     The resulting stack  5  shown in  FIG. 9  has a shorter distance between the pixel  20  and the microlens  100  than the distance between the pixel  2  and microlens  97  of the conventional frontside illuminated imager shown in  FIG. 2 . The stack  5  thus provides a shorter path for incident light to travel to the pixel  20  since the substrate layer  40  has been etched away. Therefore, quantum efficiency of the device is enhanced and incident light is more likely to stay within the optical path of the photosensor of pixel  20 . In addition, since the silicon layers are thinner, there is less space and less opportunity for the migration of photogenerated electrons and associated electrical crosstalk between pixel  20  and adjacent pixels. Another advantage to the stack  5  is that the interconnect wafer  70  may be part of a wafer which is already connected to a wafer containing the device layer  10 , which allows for wafer level packaging, and hence, smaller package size. 
       FIG. 10  illustrates a simplified block diagram of an imager  200 , for example a CMOS imager, employing a wafer structure having a layer of backside illuminated pixels constructed as described. Pixel array  201  comprises a plurality of pixels containing respective photosensors in a wafer, such as stack  5  of  FIG. 9 , arranged in a predetermined number of columns and rows. The row lines are selectively activated by the row driver  202  in response to row address decoder  203  and the column select lines are selectively activated by the column driver  204  in response to column address decoder  205 . Thus, a row and column address is provided for each pixel. The row and column lines may be formed in the ILD layer  50  of  FIG. 9 . 
     The CMOS imager  200  is operated by a timing and control circuit  206 , which controls decoders  203 ,  205  for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry  202 ,  204 , which apply driving voltages to the drive transistors of the selected row and column lines. The pixel signals, which typically include a pixel reset signal Vrst and a pixel image signal Vsig for each pixel are sampled by sample and hold circuitry  207  associated with the column driver  204 . A differential signal Vrst−Vsig is produced for each pixel, which is amplified by an amplifier  208  and digitized by analog-to-digital converter  209 . The analog to digital converter  209  converts the analog pixel signals to digital signals, which are fed to an image processor  210  which forms a digital image. 
       FIG. 11  shows in simplified form a typical processor system  300 , such as a digital camera, which includes an imaging device  200  ( FIG. 10 ) employing a pixel array on a wafer stack constructed as described above. The processor system  300  is exemplary of a system having digital circuits that could include imaging device  200 . Without being limiting, such a system could include a computer system, still or video camera system, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, and other systems employing an imaging device. 
     The processor system  300 , for example a digital still or video camera system, generally comprises a lens  396  for focusing an image on pixel array  201  when a shutter release button  397  is pressed, central processing unit (CPU)  395 , such as a microprocessor which controls camera and one or more image flow functions, which communicates with one or more input/output (I/O) devices  391  over a bus  393 . Imaging device  200  also communicates with the CPU  395  over bus  393 . The system  300  also includes random access memory (RAM)  392  and can include removable memory  394 , such as flash memory, which also communicates with CPU  395  over the bus  393 . Imaging device  200  may be combined with the CPU, with or without memory storage on a single integrated circuit or on a different chip. Although bus  393  is illustrated as a single bus, it may be one or more busses or bridges or other communication paths used to interconnect the system components. 
     While an embodiment has been described and illustrated above, it should be understood that it has been presented by way of example, and not limitation. For example, although the embodiment has been described and illustrated in conjunction with imager device wafers and a pixel array readout circuit associated with CMOS imagers, it is not so limited and may be employed with any solid state imager having a pixel array and an associated pixel array readout circuit. Furthermore, the embodiment is not limited to imaging devices and may be employed with any silicon wafer photosensitive device including an interconnect layer. In addition, although interconnect wafer  70  has been described as containing conductive interconnect structures, it may also include passive devices such as capacitors and inductors, and active devices such as transistors and diodes. It will be apparent that various changes in form and detail can be made to the described embodiment.