Abstract:
A method and apparatus for improving the planarity of a recessed color filter array when the recessed region or trench depth exceeds the thickness of the color filter film. The method includes the steps of coating the entire wafer with an additional coating material after applying the CFA, then planarizing that resist layer using CMP and then using a dry etch to transfer that planar surface down as far as required to achieve a planar color filter with a uniform thickness.

Description:
FIELD OF THE INVENTION 
   The invention relates generally to a solid state imaging device and more particularly to a method and apparatus to implement for reducing stack height of the solid state imaging device. 
   BACKGROUND OF THE INVENTION 
   There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCD&#39;s), photodiode arrays, charge injection devices (CID&#39;s), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems, and other image acquisition and processing systems. 
   CMOS imagers are well known. CMOS images are discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994); and are also disclosed in U.S. Pat. Nos. 6,140,630, 6,204,524, 6,310,366, 6,326,652, 6,333,205, and 6,326,868; assigned to Micron Technology, Inc., the entire disclosures of which are incorporated herein by reference. 
   Solid state imaging devices include an array of pixel cells, which converts light energy received, through an optical lens, into electrical signals. Each pixel cell contains a photosensor for converting a respective portion of a received image into an electrical signal. The electrical signals produced by the array of photosensors are processed to render a digital image. 
   In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion region accompanied by charge amplification; (4) resetting the floating diffusion region to a known state; (5) selection of a pixel cell for readout; and (6) output and amplification of a signal representing the pixel cell charge. Photo-charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion region. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower output transistor. 
   To detect color, the spectral components of incident light must be separated and collected. An absorptive color filter array (CFA) on top of an imager chip is currently the dominant technology for color detection in a solid state imager, for example, a CCD or CMOS imager. In a typical imager layout, a micro-lens and CFA is stacked as part of a pixel stack. In an effort to reduce the pixel stack height and bring the micro-lens and CFA closer to the photosensor, the CFA can be lowered into a recessed area within the imager. However, the problem exists that if the recess depth exceeds the thickness of the CFA film, the typical method of planarizing, i.e., chemical mechanical planarization (CMP), is no longer directly applicable to improve the planarity of the CFA on the pixel array. 
   There is needed, therefore, another method and apparatus providing a uniform color filter array within a recessed area in an imager for situations when the recessed area depth exceeds the thickness of the CFA film and improving the planarity of a recessed CFA. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention provides a method and apparatus for improving the planarity of a recessed CFA on the pixel array having a recessed region. The inventive method comprises the steps of forming a plurality of fabricated layers over a photo-conversion device; etching a trench into at least one of said plurality of fabricated layers; applying color filters into a portion of the trench; filling the remainder of the trench with a photoresist material; planarizing the photoresist material surface; and etching back the photoresist material and color filters until there is a uniform thickness in the color filter. If required, the etch can be masked by a suitable resist mask, protecting the passivation layer around the recessed region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages and features of the invention will become more apparent from the detailed description of exemplary embodiments provided below with reference to the accompanying drawings in which: 
       FIG. 1  shows a cross sectional view of the image sensor pixel array constructed in accordance with the exemplary embodiment of the invention; 
       FIG. 2  is a block diagram of the method containing the steps related to the color filter array forming process, according to an exemplary embodiment of the invention; 
       FIG. 2A  shows the color filter array structure in accordance with the first step of the exemplary embodiment of the invention; 
       FIG. 2B  shows the color filter array structure in accordance with the second step of the exemplary embodiment of the invention; 
       FIG. 2C  shows the color filter array structure in accordance with the third step of the exemplary embodiment of the invention; 
       FIG. 2D  shows the color filter array structure in accordance with the fourth step of the exemplary embodiment of the invention; 
       FIG. 3  shows a cross sectional view of the image sensor pixel constructed in accordance with another exemplary embodiment of the invention; 
       FIG. 4  shows a CMOS image sensor constructed in accordance with the exemplary embodiment of the invention; and 
       FIG. 5  shows a processor system incorporating at least one imager constructed in accordance with the exemplary embodiment of the invention. 
   

   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 specific embodiments by which the invention may be practiced. It should be understood that like reference numerals represent like elements throughout the drawings. These exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. 
   The terms “wafer” and “substrate” are to be understood as including all forms of semiconductor wafers and substrates including, silicon, silicon-on-insulator (SOI), silicon-on-sapphire (SOS), 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 “wafer” or “substrate” in the following description, previous process steps may have been utilized to form regions or junctions in or above the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on other semiconductors, for example, silicon-germanium, germanium, or gallium arsenide. 
   The term “pixel” refers to a picture element unit cell containing circuitry including a photosensor and semiconductors for converting electromagnetic radiation to an electrical signal. For purposes of illustration, fabrication of a representative pixel is shown and described. Typically, fabrication of all pixels in an imager will proceed simultaneously in a similar fashion. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
   Although the invention is described herein with reference to the architecture and fabrication of one or a limited number of pixels, it should be understood that this is representative of a plurality of pixels as typically would be arranged in an imager array having pixel cells arranged in an array, for example, an array of pixel rows and columns. 
   In addition, although the invention is described below with reference to a pixel array for a CMOS imager, the invention has applicability to all solid-state imaging devices using pixels (e.g., a CCD imager). 
   The invention may also be employed in display devices where a pixel has a light emitter for emitting light. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
   Color filters in a trench recess are beneficial for reduced stack height and improved pixel optics. The recessed color filter array provides an improved acceptance angle range for incoming light, reducing optical crosstalk. The recessed color filter array essentially places the micro-lens and color filter substantially closer to the photosensor, thus reducing the amount of diffracted or misdirected light reaching neighboring pixels. However, if the color filter array thickness, while recessed, is less than the depth of the trench, effective planarization by CMP is not possible. 
     FIG. 1  shows a cross sectional view of an image sensor pixel array constructed in accordance with an exemplary embodiment of the invention. The image sensor  100  comprises a photo-conversion device  170 , a micro-lens  110 , and a plurality of fabricated layers between the photo-conversion device  170  and the micro-lens  110 . The photo-conversion device could include a photosensor, which receives light and generates an electrical signal or a photo emitter, which receives an electrical signal and generates light. The plurality of fabricated layers typically include a clear polymide planarization layer  120 , a color filter array layer  130 , a silicon nitride passivation layer  140 , a plurality of interlayer dielectric layers  150  and associated metallization layers, and a boron-phosphorous glass layer (BPSG)  160 . The color filter array  130  is recessed into a trench  190  in a passivation layer  140 . The color filter array  130  thickness is less than the depth of the trench  190 . 
     FIG. 2  is a flow chart of a method for forming a color filter array according to the invention.  FIGS. 2A-2D  illustrate the  FIG. 1  structure prior to formation of an upper planarization layer  120  and micro-lens layer  110 . 
     FIG. 2A  shows a photosensor  170  in a substrate  180 . Over that is BPSG layer  160 , which is below one or more interlayer dielectric layers  150  and associated metallization layers. Above the uppermost interlayer dielectric layer  150  can be a passivation layer  140 , e.g., a silicon nitride layer. According to an exemplary embodiment of the invention, in step  201 , and referring to  FIGS. 2 and 2A , a trench  190  is created in the passivation layer  140  above the photosensor  170 , which is filled partially with a color filter material  130 . The trench  190  may also be etched through passivation layer  140 , ILD and associated metallization layers  150  and partially into the BPSG layer  160 . The color filter array  130  thickness is less than the depth of the trench  190 . At this stage, the color filter array  130  will have imperfect planarity. The color filter array  130  can be any thickness between a thin layer above the surface of the bottom of the trench  190  and filling the depth of the trench  190  completely. 
   Next, referring also to  FIG. 2B , in step  202  any remaining trench above the color filter array  130  is filled with a fill material  125  such as a photoresist material. The photoresist material  125  can be a spin coated material but can be deposited as well. The photoresist material  125  fills the trench  190  until the material  125  exceeds the depth of the trench  190 . 
   Then in step  203 , referring also to  FIG. 2C , the resist material  125  surface is planarized to the top surface of the passivation layer  140 . The preferred method for planarizing the resist material  125  surface is CMP. However, any of a number of other methods for planarizing already known in the art can be used. 
   Finally, referring to  FIG. 2D , in step  204  the resist material  125  and color filter array  130  are dry etched back to form a planarized CFA surface. After the etch process, the thickness of the color filter array  130  will be uniform and is less than the depth of the trench  190 . The resist material  125  and color filter array  130  can also be etched back by any method known in the art, e.g., wet etch. The preferred method is an unselective dry etch. It should be appreciated that the etch can be masked, if needed, by a suitable resist mask. By masking the etch, the passivation layer around the recessed area can be protected. After the etch, the optional upper planarization layer  120  and the micro-lens layer  110  are added. 
   By recessing the color filters in a trench, a reduced stack height can be obtained and the lens  110  can be located closer to the photo-conversion device  170 . The recessed color filter helps reduce optical crosstalk due to diffracted or misdirected light, effectively increasing the angular acceptance range for incoming light and reducing color artifacts. 
   It should be appreciated that in the exemplary embodiment discussed above the trench  190  has been described as recessed into the passivation layer  140 , however the trench  190  may be recessed from or continue into additional layers, i.e., a plurality of fabricated layers, e.g., layers  150 ,  160 . For example referring to  FIG. 3 , trench  190  may begin at the level of micro-lens layer  110 , or at the level of upper planarization layer  120  and continue downward through the passivation layer  140  into the interlayer dielectric layers  150  and associated metallization layers. In other words, the trench  190  may recess through any other layer included within the image sensor  100  between the photosensor layer  170  and the micro-lens layer  110 . The invention may be used in solid state imagers employing various kinds of photosensors formed on a substrate in photosensor layer, including but not limited to photodiodes, photo transistors, photoconductors, and photogates. 
     FIG. 4  illustrates an exemplary CMOS imager  1100  that may utilize the invention. The CMOS imager  1100  has a pixel array  1105  comprising pixels constructed to include the recessed color filter array in accordance with the invention. The CMOS pixel array  1105  circuitry are conventional and are only briefly described herein. Array row lines are selectively activated by a row driver  1110  in response to row address decoder  1120 . A column driver  1160  and column address decoder  1170  are also included in the imager  1100 . The imager  1100  is operated by the timing and control circuit  1150 , which controls the address decoders  1120 ,  1170 . 
   A sample and hold circuit  1161  associated with the column driver  1160  reads a pixel reset signal Vrst and a pixel image signal Vsig for selected pixels. A differential signal (Vrst-Vsig) is amplified by differential amplifier  1162  for each pixel and is digitized by analog-to-digital converter  1175  (ADC). The analog-to-digital converter  1175  supplies the digitized pixel signals to an image processor  1180  which forms a digital image. 
     FIG. 5  shows a processor system  1200  which includes an imaging device  1210  (such as the imaging device  1100  illustrated in  FIG. 3 ) of the invention. The processor system  1200  is exemplary of a system having digital circuits that could include image sensor devices. Without being limiting, such a system could include a computer system, 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 image sensor. 
   System  1200 , for example a camera system, generally comprises a central processing unit (CPU)  1220 , such as a microprocessor, that communicates with an input/output (I/O) device  1270  over a bus  1280 . Imaging device  1210  also communicates with the CPU  1220  over the bus  1280 . The processor system  1200  also includes random access memory (RAM)  1290 , and can include removable memory  1230 , such as flash memory, which also communicate with the CPU  1220  over the bus  1280 . The imaging device  1210  may be combined with a processor, such as a CPU, digital signal processor, or microprocessor, with or without memory storage on a single integrated circuit or on a different chip than the processor. 
   It should also be appreciated that the imager device  1100  of the claimed invention may also be used within display imager devices having light emitters fabricated on a substrate rather than photosensors. 
   The processes and devices described above illustrate preferred methods and typical devices of many that could be used and produced. The above description and drawings illustrate embodiments, which achieve the objects, features, and advantages of the present invention. However, it is not intended that the present invention be strictly limited to the above-described and illustrated embodiments. Any modification, though presently unforeseeable, of the present invention that comes within the spirit and scope of the following claims should be considered part of the present invention.