Abstract:
A CMOS image sensor and method for making such a sensor includes a coating over the photosensing parts, wherein the coating performs a dual function. In fabrication, the coating prevents the formation of silicide, which is not optically opaque, on the photosensing parts. When the CMOS sensor is in use, the coating helps to couple light onto the photosensing parts, and therefore acts as an anti-reflective layer. The method of fabrication uses a self-aligning technique, which ensures pixel-to-pixel uniformity.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of semiconductor image sensors, and in particular, to CMOS image sensors.  
         BACKGROUND OF THE INVENTION  
         [0002]    CMOS image sensors typically comprise a matrix of pixels each containing a photosensing part, such as a photodiode, and other active or passive parts. CMOS image sensors are made with the same standard CMOS fabrication processes used in current high-volume wafer fabrication plants to produce IC devices, such as microprocessors, microcontrollers and DSPs. This means that signal processing and control circuits may be integrated on the same semiconductor material as the photosensing part and the other active or passive parts, thereby providing a low cost integrated imaging device. Also, CMOS image sensors can benefit from the advances made in the semiconductor industry.  
           [0003]    Increasingly, in what is called a salicide-type process, silicide is applied to IC devices as part of a CMOS fabrication process. The formed silicide has the effect of reducing parasitic resistance and improving switching speed. However, refractory metal silicide suppresses transmission of light and is therefore unsuitable for application to devices including photosensing parts.  
           [0004]    U.S. Pat. No. 6,160,282 discloses a CMOS image sensor pixel where the photosensing parts have been protected by silicon oxide from the formation of silicide. The sensor thereby gains from the improvements associated with silicide formation on some non-photosensing parts.  
           [0005]    U.S. Pat. No. 5,903,021 discloses how the performance of a CMOS image sensor pixel may be improved by partially pinning the photo-diode. Pinning involves covering part of the surface of the photo-diode with a layer of semiconductor material of the same type as the substrate. As a consequence, the potential of the surface is pinned to the potential of the substrate. This increases the quantum efficiency of the photo-diode in addition to reducing its dark current and improving its blue color response, which is normally the weakest of all the color responses. As a result, the pixel performs to a high standard.  
           [0006]    It has been shown by I. Murakami et al. in an article titled “Technologies to Improve Photo-Sensitivity and Reduce VOD Shutter Voltage for CCD Image Sensors” (IEEE Trans. Electron Devices, vol. 47, 2000, pp. 1566-1572), that quantum efficiency of a photodiode in an image sensor can be improved by applying an anti-reflective coating to increase light coupling.  
           [0007]    Photo response uniformity is an important parameter for image sensors. This parameter can be limited by the uniformity of the photo-diode capacitance from pixel-to-pixel within the sensor matrix. Variation of the patterning of the implants within the pixel can also cause a reduction in the matching of photo response between pixels. One technique used to improve this matching is a self-aligning technique where a single master layer is used to define the implant areas. Each of the different implant areas may then be selected by use of a lower tolerance select mask while maintaining the good matching achieved by use of the single master layer.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to provide a semiconductor image sensor comprising at least one pixel having a photosensing part, wherein the photosensing part has a coating which performs a dual function.  
           [0009]    One of the dual functions may be a fabrication function. The fabrication function preferably prevents the formation of silicide. Another one of the dual functions may be an in-use function, such as an anti-reflection function.  
           [0010]    The constituents and thickness of the coating may be optimized for maximum response at a particular wavelength. The photosensing part may comprise a photo-diode. The photodiode may be a pinned photodiode, or a partially pinned photodiode.  
           [0011]    Another aspect of the present invention is directed to a method for making a semiconductor image sensor comprising forming at least one pixel having a photosensing part, and coating the photosensing part with a coating which performs a dual function.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The invention will now be described, by way of example, with reference to the drawings, in which:  
         [0013]    [0013]FIG. 1 is a cross-sectional side view of a pixel in an image sensor according to the present invention.  
         [0014]    [0014]FIG. 2( a ) is a cross-sectional side view of a pixel in an image sensor with implant areas and silicon oxide islands formed and with a silicon dioxide layer and a silicon nitride layer according to the present invention.  
         [0015]    [0015]FIG. 2( b ) is a cross-sectional side view of a pixel in an image sensor as described in FIG. 2( a ) with a layer of photoresist applied thereto.  
         [0016]    [0016]FIG. 2( c ) is a cross-sectional side view of a pixel in an image sensor as described in FIG. 2( b ) after exposure to a suitable light source.  
         [0017]    [0017]FIG. 2( d ) is a cross-sectional side view of a pixel in an image sensor as described in FIG. 2( c ) after removal of the unexposed photoresist.  
         [0018]    [0018]FIG. 2( e ) is a cross-sectional side view of a pixel in an image sensor as described in FIG. 2( d ) after removal of the silicon nitride layer and silicon dioxide layer not below the exposed photoresist.  
         [0019]    [0019]FIG. 3( a ) is a cross-sectional side view of a pixel in an image sensor with a coating over the photosensitive area formed according to the present invention.  
         [0020]    [0020]FIG. 3( b ) is a cross-sectional side view of a pixel in an image sensor as described in FIG. 3( a ) with a layer of titanium deposited thereon.  
         [0021]    [0021]FIG. 3( c ) is a cross-sectional side view of a pixel in an image sensor as described in FIG. 3( b ) with a layer of silicide formed thereon except over the photosensitive area. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    With reference to FIG. 1, a pixel indicated generally at  101  is formed on a p-type substrate  118  with a photosensing part  102  and an active part  104 . The photosensing part  102  comprises a partially pinned photo-diode formed from an n-type well  116  within the substrate  118  and a p-type pinning layer  106  partially covering the n-type well  116 . The active part  104  comprises an NMOS transistor formed from two spaced apart, highly doped n-type implants  110  in a p-type well  114 . The n-type well  116  is positioned to connect the photo sensing and active parts  102 ,  104 . The p-type pinning layer  106  has an anti-reflection coating  130  of silicon nitride on silicon dioxide.  
         [0023]    The anti-reflection coating  130  increases light coupling to improve the photo-diode quantum efficiency. During fabrication of the pixel  101 , which involves the use of a salicide-type process, the anti-reflection coating  130  also prevents the formation of salicide over the photo-diode. Thus, the coating  130  has a dual function.  
         [0024]    With reference to FIG. 1, a pixel  101  is fabricated, prior to the creation of the implant regions  106  and  110  and application of the coating  130 , using a well known self-alignment technique. This technique involves creating lands or regions  108  of silicon oxide between parts of the pixel  101 . The regions  108  are formed on the surface of the pixel  101  by a process involving photolithography, while using a single master mask layer. The end result of this process is that the surface of the pixel  101  is blocked by silicon nitride everywhere apart from the areas where the regions  108  are to be formed. The pixel  101  is then heated in an oxygen atmosphere so that silicon oxide is formed in the unblocked regions. The silicon nitride blocking is subsequently removed, leaving the silicon oxide regions  108 .  
         [0025]    Creation of the regions  108  at this stage in the process allows the use of the edges of the regions  108  as reference axes. When each pixel  101  is created the width of the central region  108 , in particular, is kept constant, through use of the same master mask layer. This ensures accurate spacing between the N +  region  110  of the active part  104 , which is connected to the photosensitive part  102 , and the pinning layer  106 . Ensuring the accurate spacing between these parts is critical for pixel-to-pixel uniformity.  
         [0026]    Two separate masks (not shown) are used for the creation of the implant parts  106 ,  110 . Each of these masks covers the entire pixel surface except the specific implant part  106 ,  110 . The width of the central region  108  allows for some error in placement of the appropriate mask on the pixel without compromising the uniformity between pixels.  
         [0027]    The coating  130 , as shown in FIG. 1, is formed in a process as shown in FIG. 2. FIG. 2 a  shows a pixel  201  after the first step of the process, wherein a thin silicon dioxide layer  205  is formed over the exposed silicon. This silicon dioxide layer  205  may be formed by a number of different standard methods, including thermal oxidation or chemical vapor deposition. On top of the silicon dioxide layer  205 , a silicon nitride layer  207  is formed. Formation of the silicon nitride layer  207  is made by use of a chemical vapor deposition (CVD) process. This may be either thermal or plasma enhanced CVD. The silicon dioxide layer  205  and the silicon nitride layer  207  together form a coating  230 .  
         [0028]    The thickness of the coating  230 , over the photosensitive part  202 , is controlled by the length of time of the CVD deposition to give an optimum thickness of 300 Å (±50 Å) of the silicon nitride layer  207  and 250 Å (±50 Å) of the silicon dioxide layer  205 . However, because the coating has a dual function, the thickness of the coating  230  chosen is a balance between being thick enough to prevent silicide formation and being the correct optical path length to ensure an anti-reflective surface in the desired wavelength range. The peak transmission through the coating  230  is normally set to be a maximum at a wavelength of 450 nm. This acts to increase the quantum efficiency of the sensor to blue light, thereby improving color camera performance.  
         [0029]    A layer of photoresist  220  is then applied to the whole surface of the pixel  201  (FIG. 2 b ). A mask  224  is then placed over the pixel  201  such that the photosensitive part  202  is not covered (FIG. 2 c ). The pixel  201  is then illuminated through the mask  224  to expose the uncovered photoresist  228 . The light source  222  and the mask  224  are then removed as well as the unexposed photoresist  226 . An etching step is used to remove the coating  230  from the surface of the pixel  201 , everywhere apart from over the photosensitive part  202 , which is protected from the etching step by the exposed photoresist  228 . The photoresist  228  is then removed using a standard photoresist strip process, leaving a pixel  101  as shown in FIG. 1.  
         [0030]    The salicide type process involves forming silicide on the surface of the pixel. The silicide has the effect of reducing parasitic resistances, and is therefore desirable. However, silicide hampers light transmission and is unsuitable for application to a photosensing part.  
         [0031]    With reference to FIG. 3 a , a pixel  301  is shown which has gone through the process required to provide the coating  330 . A titanium layer  350  is deposited over the entire surface of the pixel  301  as shown in FIG. 3 b . In a thermal treatment the titanium reacts with exposed silicon to form silicide  352  but does not react with the silicon nitride layer  307  or silicon dioxide layer  308 . Unreacted titanium deposited on the silicon nitride coating and silicon dioxide may be removed in a wet processing step, as shown in FIG. 3 c , leaving the coating as an anti-reflective layer.