Abstract:
A device and method for providing an optical trench structure for a pixel which guides incoming light onto the photosensor of the pixel. The optical trench structure has an optically reflecting barrier that substantially mitigates optical crosstalk. The optical trench structure is made of low dielectric constant material with an index of refraction that is less than the index of refraction of the material of surrounding layers (e.g., the substrate). This difference in refractive index causes an internal reflection into an optical path existing between a lens and pixel.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to solid state imaging devices and more particularly to a method and structure for optically isolating pixel regions to reduce optical crosstalk in a solid state image sensor. 
     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. 
     Semiconductor imaging devices include an array of pixels, which converts light energy received, through an optical lens, into electrical signals. Each pixel 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. 
     The amount of charge generated by the photosensor corresponds to the intensity of light impinging on the photosensor. Accordingly, it is important that all of the light directed to the photosensor impinges on the photosensor 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, a 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 imaging device. 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 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 imaging device. 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 photosensor target. 
       FIG. 1  illustrates the problem of optical crosstalk in a conventional backside illuminated imager. A conventional backside illuminated imager includes an array of photosensors  220 , for example, photodiodes, formed within a substrate  290 , a passivation layer  260 , a color filter array (CFA)  250  and an array of microlenses  240 . Ideally, incoming light  295  should stay within a photosensor optical path  223  when traveling through a microlens  240  to a respective photosensor  220 . However, light  295  can be reflected within the respective layers of the imager and at the junctions between these layers. Incoming light  295  can also enter the pixel at different angles, causing the light to be incident on a different photosensor. 
     Optical crosstalk particularly problematic when it occurs within the substrate itself. This can occur in situations where a substantial amount of light is passing through the substrate  290 , for example, in a backside illuminated pixel array or imager. For example, once light has passed the CFA layer  250 , even small amounts of crosstalk can distort an image because adjacent pixels rarely filter out the same color. That is, if one portion of the spectrum of the incoming light is especially intense, crosstalk below the CFA layer  250  will redirect filtered light to photosensors  220  designed to measure a different color. Transmission of light through the substrate also suffers from electrical interference which can distort the signal further. 
     Accordingly, there is a need and desire for an improved apparatus and method for reducing optical crosstalk and related electrical interference in imaging devices. There is also a need to more effectively and accurately increase overall pixel sensitivity and provide improved optical crosstalk immunity without adding complexity to the manufacturing process and/or increasing fabrication costs. 
     BRIEF SUMMARY OF THE INVENTION 
     Exemplary embodiments of the invention provide an optical trench structure for a pixel that guides incoming light onto the photosensor of the pixel. The optical trench structure has an optically reflecting barrier that substantially mitigates optical crosstalk. The optical trench structure is made of low dielectric constant material with an index of refraction that is less than the index of refraction of the material of surrounding layers (e.g., the substrate). This difference in refractive index causes an internal reflection into an optical path existing between a lens and pixel. 
     In other exemplary embodiments, materials with high reflectivity such as metals can be used to implement the optical trench structure. In yet another embodiment, to improve the difference in the index of refraction between the trench structure and the surrounding material, the surrounding layers may be formed with materials having a relatively high index of refraction Other embodiments include a method of forming the optical trench structure in a pixel, and employment of pixels containing the optical trench in imaging and display devices, and in systems including such devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other 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  illustrates a cross sectional view of a prior art pixel array; 
         FIG. 2  is a plan view of an image sensor pixel constructed in accordance with an exemplary embodiment of the invention; 
         FIG. 3  shows a cross sectional view of an image sensor pixel array containing the  FIG. 2  pixels constructed in accordance with the exemplary embodiment of the invention; 
         FIG. 4  shows the formation of the optical trench structure of  FIG. 3  constructed in accordance with the exemplary embodiment of the invention; 
         FIG. 5  shows the optical trench structure of  FIG. 3  constructed in accordance with another exemplary embodiment of the invention; 
         FIG. 6  shows the optical trench structure of  FIG. 3  constructed in accordance with another exemplary embodiment of the invention; 
         FIG. 7  shows a CMOS image sensor constructed in accordance with an embodiment of the invention; and 
         FIG. 8  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 over 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 one or more representative pixels is shown and described. Typically, fabrication of all pixels in an imager will proceed simultaneously in a similar fashion. 
     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 pixels arranged, for example, in rows and columns. 
     In addition, although the invention is described below with reference to a pixel for a CMOS imager, the invention has applicability to other solid-state imaging devices using pixels (e.g., a CCD or other solid state 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. 
     Referring to the  FIGS. 2 and 3 , embodiments of the invention provide a trench  350  within a substrate  290 , which is filled with a material  351  designed to provide internal reflection within an optical path  223 . The trench  350  and filling material  351  are formed around the optical path  223  in a substrate  290  for each pixel. The trench  350  and associated fill material  351  surround at least a portion of the optical path  223  between a backside illumination source  295  and a corresponding photosensor  220 , which corresponds to a lateral area defined by the photosensor  220 . The dimensions and location of the filled trench  350  can be tailored depending on the need; the trench  350  may extend from the substrate  290  to or through the color filter array  250 . The trench  350  can be etched by any method known in the art. 
     In a first embodiment of the invention, the fill material  351  is a low-dielectric constant material (low-k material), having a dielectric constant below 1.45 when the substrate is a silicon substrate. More generally, the low dielectric constant material within trench  350  has an index of refraction that is less than the index of refraction of the semiconductor material used for the surrounding substrate  290 . The low dielectric constant material may comprise, for example, carbon doped silicon dioxide or fluorinated silica glass oxide or any other material with a lower index of refraction than the substrate  290 , for example silicon. In other embodiments of the invention, the fill material  351  may comprise metals having high reflectivity, such as, for example, silver or copper. 
       FIG. 4  illustrates the  FIG. 3  structure prior to formation of passivation layer  260 . In this embodiment, the trench  350  extends into the substrate and not to or around the color filter array  250 . The trench  350  may be etched partially into the substrate  290 . The trench is then filled with the fill material  351  and planarized by chemical mechanical polishing (“CMP”), after which the passivation layer  260  and additional upper layers are formed thereon. The upper layers may also include, but are not limited to, a BPSG layer, an ILD layer, and an additional passivation layer between the substrate  290  and microlens array  240 . 
       FIG. 5  illustrates a modification of the embodiment shown in  FIG. 4  in which the trench contains multiple layers of fill material  351 ,  351 ′. Two fill material layers  351 ,  351 ′ deposited sequentially in trench  350  are shown in  FIG. 5 . These fill materials for layers  351 ,  351 ′ may include materials having indexes of refraction that differ from the surrounding substrate  290  and from each other, and may include metals having high reflectivity, or any combination thereof. 
       FIG. 6  illustrates a fill material formed of three layers  351 ,  351 ′,  351 ″ of different materials. Every successive layer adds another barrier against optical crosstalk because any light that passes a first layer  351  can still be reflected back toward the correct photosensor by successive layers  351 ′,  351 ″. The number of layers and of different fill materials that may be used to fill trench  350  is in no way limited by these examples. 
     It should be appreciated that in the exemplary embodiments, discussed above, the trench  350  has been described as extending into the substrate  290 . However, the trench  350  may be extended from or continue into additional layers of the imager. For example referring to  FIG. 3 , trench  350  may begin at the level of microlens layer  240 , or at the level of passivation layer  260 . In other words, the trench  350  may extend through any other layer included between the photosensor array  220  and the microlens layer  240 . The invention may be used in solid state imagers employing various kinds of photosensors formed on a substrate in photosensor layer  220 , including but not limited to photodiodes, photo transistors, photoconductors, and photogates. 
     In all of the described embodiments, there is a difference in refractive index between the surrounding substrate material (refractive index=n 1 ) and the material  351  used to fill the trench  350  (refractive index=n 2 ). If n 1  is greater than n 2 , there is total internal reflection for large angles of incidence of the incident light  295 , resulting in a considerable reduction in optical crosstalk. 
     In general, low dielectric constant materials will provide low refractive indexes. The various exemplary embodiments may use various materials alone ( FIG. 4 ), or in combination ( FIGS. 5 ,  6 ) as the fill material  351 ,  351 ′,  351 ″ such as those that have predominantly oxide properties such as SiO2/TEOS, Spin-cn-dielectric oxide (SOD), carbon doped silicon di-oxides, fluorinated silica glass oxide (FSG), etc. However, other commercially available materials can also be used such as SiLK, FLARE 2.0, Black Diamond Corel, PSiLK, Orion, LKD 5109 and XPX. It should be appreciated that this list of materials is in no way exhaustive of possible materials that can be used to fill the trench  350 , all that is required is that the index of refraction of the trench  350  fill material  351 ,  351 ′,  351 ″ be lower than the index of refraction of the material layers surrounding the trench  350  along the optical path  223 . 
     In another embodiment of the invention, fill materials with high light reflectivity such as metals may also be used to fill the trench  350 . Some metals have a very high light reflectivity such as aluminum, copper, silver and gold, and can effectively serve as an optical barrier material. It should be appreciated that the metals mentioned are in no way an exhaustive list of possible metals which can be used; moreover, metal alloys may also be used as the fill material  351 ,  351 ′,  351 ″. The metal fill material may be one or more of fill materials  351 ,  351 ′ and  351 ″ in the trench  350 , as shown in  FIGS. 4-6 . However, when using metals as fill materials, it is desirable to prevent the metals from diffusing into the active area of the substrate  290 . This is easily achieved by forming layer  351  as a barrier layer, for example a 50 angstrom layer of silicon nitride, to prevent metal layers  351 ′,  351 ″ from diffusing into the substrate  290  and interfering with the operation of the pixel. In addition, one or more reflective metal layers may be used in a layer of combination within trench  350  with layers of the non-metal materials discussed above. It should be appreciated that there are likely many alternatives for materials that may be suitably employed to fill the trench  350  for integrated image sensors including metals, polymers, semiconductors, and dielectric. 
       FIG. 7  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 any of the overlying optical structures of the invention. The CMOS pixel array circuitry is conventional and is only briefly described herein. Row lines of the array  1105  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  and row driver  1110 . 
     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 an analog-to-digital converter  1175  (ADC). The analog-to-digital converter  1175  supplies the digitized pixel signals to an image processor  1180  which forms and outputs a digital image. 
       FIG. 8  shows a system  1200 , a typical processor system which includes an imaging device  1210  (such as the imaging device  1100  illustrated in  FIG. 10 ) 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 imager. 
     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  1260  over a bus  1280 . Imaging device  1210  also communicates with the CPU  1220  over the bus  1280 . The processor-based 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. 
     The above described structure, system and fabrication methods can be applied to display devices employing photoemitters as well. For example, a pixel array similar to the array  1105  of  FIG. 7 , but employing photoemitters rather than photosensors, may be used in a display device to reduce crosstalk and to emit a more accurate signal. 
     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.