Abstract:
A device and method to provide an optical guide of a pixel to guide incoming light onto a photosensor of the pixel and to improve the optical crosstalk immunity of an image sensor. The optical guide consists of an optically reflecting barrier formed as a trench that mitigates against optical crosstalk. The optical guide is made of an air-filled ring of spaced slots. In another embodiment, the optical guide structure can be filled with a low dielectric material with an index of refraction that is less than the index of refraction of the material used for the surrounding layers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a continuation of U.S. application Ser. No. 11/207,804, filed on Aug. 22, 2005, the disclosure of which is herewith incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates generally to solid state imaging devices and more particularly to a method and apparatus which optically isolates pixel regions to reduce optical crosstalk in a solid state image sensor.  
       BACKGROUND OF THE INVENTION  
       [0003]     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.  
         [0004]     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.  
         [0005]     These 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.  
         [0006]     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 as optical crosstalk.  
         [0007]     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 photosensor.  
         [0008]     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 a pixel cell in imager arrays increases, and as pixel cell size correspondingly decreases. The shrinking pixel cell sizes make it increasingly difficult to properly focus incoming light on the photosensor of each pixel cell without accompanying optical crosstalk.  
         [0009]     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 cell and 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 cell, providing more opportunities for the light to be reflected or refracted away from its intended photosensor target.  
         [0010]     One proposal to reduce optical crosstalk provides a continuous air-gap around the optical path to a photosensor. See Dun-Nian Yaung et al.,  Air - Gap Guard Ring for Pixel Sensitivity and Crosstalk Improvement in Deep Sub - micron CMOS Image Sensor , PROC. OF IEDM, 2003; see also T. H. Hsu et al.,  Light Guide for Pixel Cross Talk Improvement in Deep Submicron CMOS Image Sensor , IEEE E LECTRON  D EVICE  L ETTERS , vol. 25, no. 1, 2004, at 22-24.  FIG. 1  represents a cross sectional view of an imager showing two exemplary prior art techniques for dealing with optical crosstalk. The  FIG. 1  imager has an air-gap guard ring  221  surrounding a photosensor optical path  223  existing between a micro-lens  240  and a photosensor  220 . The air gap ring  221  is shown as being fabricated in the lower metallization layers M 2  of an imager. The air gap provides a refraction index difference between the air gap (n 2 =1) and the surrounding dielectric layers (n 1 =1.4˜1.6) and thus, the majority of incident light will be collected in the targeted pixel cell due to the total internal reflection in the air-gap/dielectric film interface. However, the presence of an air gap ring  221  is not ideally suited for solid state imagers. There are several reliability issues with the air gap such as its structural instability. Also, the color filter array (CFA) process, widely used in color imager fabrication, is known to have metallic and mobile ion contaminants that might easily diffuse through the air gaps and affect the devices and photosensor characteristics in the underlying pixel circuit.  
         [0011]     Alternatively as also shown in  FIG. 1 , planar metal-shielding  225  provided in an upper metallization layer M 4  has been used in an effort to reduce optical crosstalk, but these may degrade pixel sensitivity and/or are not suitable for use in zooming lens systems.  
         [0012]     Another method of reducing optical crosstalk uses optical waveguides. Optical waveguides are structures used for spatially confining and directing light onto the intended target. For instance, optical waveguides can be used to reduce the detrimental affects associated with light shields such as light piping and light shadowing. Optical waveguides, however, are not widely used to focus light directly onto the photosensor in imaging devices. Moreover, currently employed optical waveguide structures, require additional processing steps, adding to the complexity and costs of imager fabrication.  
         [0013]     Accordingly, there is a need and desire for an improved apparatus and method for reducing optical crosstalk 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  
       [0014]     Exemplary embodiments of the invention provide an optical guide structure for a pixel which guides incoming light onto the photosensor of the pixel. The optical guide structure consists of an optically reflecting barrier that mitigates against optical crosstalk. The optical guide structure is formed of a ring of spaced slots filled with air that provides an index of refraction that is less than the index of refraction of the material used for the surrounding dielectric. This difference in refractive index causes an internal reflection into an optical path existing between a lens and pixel. The spaced slots are configured to reflect light according to the wavelengths meant to be reflected and their widths, which are determined by the physical geometry distance of the respective wavelength of the light array. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     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:  
         [0016]      FIG. 1  illustrates a cross sectional view of a prior art pixel;  
         [0017]      FIG. 2  is a plan view of a image sensor pixel constructed in accordance with an exemplary embodiment of the invention;  
         [0018]      FIG. 3  shows a cross sectional view of the image sensor pixel of  FIG. 2 , constructed in accordance with the exemplary embodiment of the invention;  
         [0019]      FIG. 4  shows the optical guide structure in accordance with the exemplary embodiment of the invention;  
         [0020]      FIG. 5  shows the optical guide in accordance with another exemplary embodiment of the invention;  
         [0021]      FIG. 6  shows the optical guide structure in accordance with another exemplary embodiment of the invention;  
         [0022]      FIG. 7  shows a CMOS image sensor constructed in accordance with an embodiment of the invention; and  
         [0023]      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  
       [0024]     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.  
         [0025]     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.  
         [0026]     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.  
         [0027]     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, for example, in rows and columns.  
         [0028]     In addition, although the invention is described below with reference to a pixel for a CMOS imager, the invention has applicability to all solid-state imaging devices using pixels (e.g., a CCD imager).  
         [0029]     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.  
         [0030]     Referring to the  FIG. 2  top down view and the  FIG. 3  cross sectional view, embodiments of the invention provide a trench-like pattern  350  in an imager comprising a series of spaced trench-like etches surrounding the photosensor  220  filled with air  351 , leaving non-trench regions  370  throughout the pattern. The spacing of the trenches is designed to provide internal reflection within the optical path  223 . The trench pattern  350  is formed in optical path  223  between a photosensor layer  220  formed on a substrate and a micro lens layer  240  for each pixel cell. The trench pattern  350  surround the optical path between a lens and a corresponding photosensor of a pixel cell. The trench  350  is formed to surround a lateral area defined by the photosensor. The depth and width of the individual trenches in trench pattern  350  can be tailored depending on the need, and may extend from the upper layers of the image sensor, for example, beginning at the color filter array  250  level, to below a bottom ILD layer  210  into the protective layer  230 , formed typically of BPSG, provided above the active area of the photosensor layer  220 . The trench pattern can be etched by any method known in the art.  
         [0031]     In a first embodiment of the invention, the trench pattern  350  is left hollow with air  351 , forming air-gaps. The air gaps provide a refraction index difference between the air gap  351  (n 2 =1) and the surrounding material layers  230 ,  210 ,  260 ,  250  (n 1 =1.4˜1.6). In other words, the air-gaps  351  within the trench pattern  350  have an index of refraction that is less than the index of refraction of the material used for the surrounding imager layers shows as the BPSG layer  230 , ILD layers with associated metallization  210 , passivation TEOS layer  260 , and CFA layer  250 .  
         [0032]     The optical guide structure is designed to leave non-trench regions  370  within the trench pattern  350 . The width of the non-trench regions  370  are configured to reflect light according to the wavelengths commonly known in the art. The width of the non-trench regions  370  are determined by the physical geometry distance of the respective wavelength of the light array that is meant to be reflected. For example, for a wavelength of 0.5 nm, the width of the non-trench regions  370  between trenched regions would be a distance of approximately 500 A. In essence, as long as the non-trench regions are smaller than the designated wavelength, the respective light wavelength will not be able to detect the non-trench regions  370 . The optical guide structure, although containing non-trench regions, will function as a continuous trench-like pattern for optical purposes, while providing the optical guide structure with structural integrity due to the presence of the non-trenched areas in the trench pattern.  
         [0033]      FIG. 4  illustrates the  FIG. 3  structure prior to formation of an upper passivation layer  270  and micro-lens layer  240 . The trench pattern  350  may be etched through the CFA layer  250 , passivation layer  260 , ILD and associated metallization layers  210  and partially into the BPSG layer  230 . Then the passivation layer  270  and micro-lens layers are added.  
         [0034]     Referring to  FIG. 5 , in another embodiment, the trench-like pattern  350  can be filled with a low-dielectric constant material (low-k material), having a dielectric constant below 1.45. The low dielectric constant material  351 ′ within trench pattern  350  has an index of refraction that is less than the index of refraction of the material used for the surrounding imager layers.  
         [0035]     For example, the dielectric of the ILD layer  210  is typically implemented by depositing amorphous silicon dioxide, whose index of refraction is approximately between 1.45 and 1.54. Thus, in this example, a material with a lower index of refraction than 1.45 will fill the trench pattern  350 . Additionally, there are numerous other low-k polymers (discussed below) that can be used for the fill material  351 ′, as long as their respective reflective index is below that of the surrounding layers.  
         [0036]     It should be appreciated that in the exemplary embodiment discussed above the trench-like pattern  350  has been described as extending to and through the ILD layer  210 , passivation layer  260 , and CFA layer  250 , however it may be extended from or continue into additional layers. For example referring back to  FIG. 3 , trench pattern  350  may begin at the level of micro-lens layer  240 , or at the level of passivation layer  270 . In other words, the trench may extend through any other layer included within the image sensor between the photosensor layer  220  and the micro-lens layer  240 . The invention may also 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, photoconductors, photogates, etc.  
         [0037]     In all of the described embodiments, there is a difference in refractive index between the surrounding film material (refractive index=n 1 ) and the air  351  or the film material  351 ′ used to fill the trench region  350  (refractive index=n 2 ). If n 1  is greater than n 2 , for large angles of incidence of the light there would be total internal reflection, and a considerable reduction in optical crosstalk.  
         [0038]     In general, low dielectric constant materials will provide low refractive indexes. The various exemplary embodiments may use various materials as the fill material  351 ′ within the trench pattern  350  such as those that have predominantly oxide properties such as SiO 2 /TEOS, Spin-cn-dielectric oxide (SOD), carbon doped silicon di-oxides, fluorinated silica glass oxide (FSG), etc. It should be appreciated that this list of materials is in no way exhaustive of possible materials that can be used for filling the trench pattern  350  of the optical guide structure, as the important point is that the index of refraction of the trench pattern  350  fill material is lower than that of the material layer surrounding the trench pattern  350  along the optical path  223 .  
         [0039]     Additionally, the typical material used for the various layers  210 ,  260 ,  250 ,  270  is silicon dioxide, which has a low index of refraction. To improve the difference in the index of refraction between the fill material  351 ′ and the surrounding material, the surrounding layers, e.g.,  210 ,  260 ,  250 ,  270  may be formed with materials having a relatively high index of refraction, thus expanding the number of possible materials having a lower index of refraction, which can be used for the trench pattern  350  fill material.  
         [0040]     It should also be appreciated that in another exemplary embodiment the optical guide structure can be implemented above a raised photosensor structure, as described in U.S. patent application Ser. No. 10/857,876, filed on Jun. 2, 2004, entitled “Raised Photodiode Sensor”; assigned to Micron Technology, Inc., the entire disclosure of which is incorporated herein by reference. The optical guide  350  formed above a raised photosensor  220 ′ will have the same effect of isolating the photosensor. As shown in  FIG. 6 , a cross-sectional view of the raised photosensor  220 ′ within the image sensor, the optical guide structure  350  would be formed in the layers surrounding and above the raised photodiode, as described above.  
         [0041]      FIG. 7  illustrates an exemplary imager  1100  that may utilize any embodiment of the invention. The Imager  1100  has a pixel array  1105  comprising pixels constructed with the optical guide structure (i.e.,  FIG. 3 ) in accordance with an embodiment of the invention, or using other pixel architectures including the same. 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 .  
         [0042]     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.  
         [0043]      FIG. 8  shows a system  1200 , a typical processor system modified to include an imaging device  1210  (such as the imaging device  1100  illustrated in  FIG. 7 ) 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.  
         [0044]     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-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.  
         [0045]     It should be appreciated that there are likely many alternatives for materials that may be suitably employed to provide the optical guide for integrated image sensors including metals, polymers, semiconductors, and dielectric. This is especially true if a material other than amorphous silicon dioxide is used as the ILD. If the requirements of the optical guide cannot be met with a single material than a combination of materials can be used.  
         [0046]     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.