Abstract:
A method of forming a base for a color filter layer of an image sensor is disclosed. The image sensor includes an array of pixels formed in a substrate. The method comprises depositing a polymer layer over the substrate. The polymer layer is patterned to form gaps in the polymer layer, the gaps located between the pixels. Finally, a second polymer layer is deposited into the gaps.

Description:
TECHNICAL FIELD 
     The present invention relates to image sensors, and more particularly, towards color filter structure having reduced stress. 
     BACKGROUND 
     Image sensors are electronic integrated circuits that can be used to produce still or video images. Solid state image sensors can be either of the charge coupled device (CCD) type or the complimentary metal oxide semiconductor (CMOS) type. In either type of image sensor, a light gathering pixel is formed in a substrate and arranged in a two-dimensional array. Modern image sensors typically contain millions of pixels to provide a high resolution image. An important part of the image sensor are the color filters and micro-lens structures formed atop of the pixels. The color filters, as the name implies, are operative, in conjunction with signal processing, to provide a color image. The micro-lenses serve to focus the incident light onto the pixels, and thus to improve the fill factor of each pixel. 
     While the technology underlying the formation of color filters is relatively mature, there are still issues that may arise, particularly at higher integration densities. For example, U.S. Pat. Nos. 6,297,071, 6,362,513, and 6,271,900 show the current state of the color filter art. In the prior art, the color filter is typically formed from an organic material. It has been observed that when the color filter is formed atop of the substrate containing the pixel, physical cracking of the substrate may occur. It is believed that the interface of the organic color filter with the substrate causes stress, which in turn results in cracking. 
     The cracking of the substrate is undesirable insofar as it may cause defects in the image sensor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art cross sectional view of a portion of an image sensor. 
     FIGS. 2-6 are schematic cross-sections illustrating the method and structure of one embodiment of the present invention. 
     FIG. 7 is a top view of the structure of FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to a method for forming a color filter layer used in an image sensor. In the following description, numerous specific details are provided to provide a thorough understanding of the embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     FIG. 1 shows a prior art cross-sectional simplified diagram of an image sensor  101  having micro-lenses formed thereon. As seen in FIG. 1, the image sensor includes a plurality of pixels that have light detecting elements  103  formed in the substrate. The light detecting elements  103  may be one of several types, such as a photodiode, a photogate, or other solid state light sensitive element. Formed atop of each pixel is a micro-lens  105 . The micro-lens  105  focuses incident light onto the light detecting elements  103 . Micro-lenses are often formed by spin coating a layer of micro-lens material onto a planarized layer. The micro-lens material is then etched to form cylindrical or other shaped regions that are centered above each pixel. Then, the micro-lens material is heated and reflowed to form a convex hemispherical micro-lens. Moreover, in the region between the light detecting elements  103  and the micro-lens  105 , denoted by reference numeral  107 , there are various intervening layers that would typically include the color filter layers  109  and various metal conducting lines. It can be appreciated that the structure of FIG. 1 is merely one example of an image sensor structure and that the present invention is adaptable to any number of variants. Alternatively, the color filters  109  may be formed atop of the micro-lenses  105 . 
     The color filters  109  are typically a pigmented or dyed material that will only allow a narrow band of light to pass therethrough, for example, red, blue, or green. In other embodiments, the color filter may be cyan, yellow, or magenta. These are but example colors for the color filters  109 . While the use of pigmented or dyed color materials is the most prevalent form of color filters, other reflective type color filters may be used, such as a multilayer stack reflective material. The formation of color filters  109  is known in art and will not be described herein to avoid any unnecessary obscuration with the description of the present invention. 
     The present invention is directed towards the process of preparing (by deposition of stress relieving layers) the substrate prior to forming the color filters  109 . FIG. 2 shows a typical wafer substrate  201  that has pixels formed therein. The pixels include light detecting elements  203 , which in this case are photodiodes. However, it can be appreciated that other types of light detecting elements, now known (such as a photogate) or developed in the future, may be used. Further, the pixels will typically also include amplification and/or readout circuitry. For clarity, this circuitry is not shown in FIG.  2 . In one embodiment, the pixels may be active pixels, commonly known in the prior art. Details of forming the photodiode and other associated circuitry are known in the prior art and will not be repeated herein to avoid obscuring the present invention. Examples of the prior art may be seen in U.S. Pat. Nos. 5,904,493, and 6,320,617. 
     Moreover, other structures are present in the substrate  201 . One example are conductive structures  205  are polysilicon or metallic interconnects, such as those used to carry signals to or from the light detecting elements  203  or to or from other components in the pixels. Because these conductive structures  205  are usually formed atop of the substrate  201 , this causes an uneven topography characterized by valleys  207  and ridges  209 . Further, there is a need to insulate these conductive structures with an insulating dielectric. Typically, the insulating dielectric is an inorganic material, such as silicon dioxide or a nitride. As noted above, the interface between the organic color filter material and the inorganic substrate material may cause stress and cracking. 
     Turning to FIG. 3, according to one embodiment of the present invention, a polymer layer  301  is spin coated over the substrate  301 . The polymer layer  301  in one embodiment is polyglycidylmethylacrylate (PGMA). In one embodiment, the polymer layer  301  is between 0.01 to 0.5 microns. However, other types of materials may be substituted, but preferably materials that may be spun on and subsequently cured into a solid. 
     As will be seen below with respect to FIG. 4, the photoresist-type materials are advantageous in one embodiment since they can be directly patterned by exposure and development. Non-photoresist type materials, while still capable, would require a further etching step. Specifically, as seen in FIG. 4, the polymer layer  301  is patterned to include gaps  401  between each pixel. The gaps  401  are generally located between the light detecting elements  203  and are relatively narrow. Note that the illustration of FIG. 4 is not drawn to scale. Instead, the gaps  401  are generally significantly less in width than the width of each pixel. In some embodiments, the gaps are made to be very small relative to the pixel dimension, and perhaps even to the limit of resolution of the photolithography process. 
     Moreover, in one embodiment, the gaps  401  form a trench that surrounds each pixel. This can be more clearly seen in FIG. 7, where the polymer layer  31  form “islands” over the light detecting elements  203 . The gaps  401  extend around the periphery of the pixels. 
     Note also, that in one embodiment, the gaps  401  do not extend through the entire thickness of the polymer layer  301 . Instead, there remains a portion of the polymer layer  301  at the bottom of the gaps  401 . Typically, the gaps  401  would extend to nearly the underlying substrate. However, in other embodiments, the gaps  401  may extend completely through the polymer layer  301 . 
     In the case of a photoresist-type material, the patterning of the polymer layer  301  with the gaps  401  may be accomplished by exposing the polymer layer  301  to an exposing radiation from, for example, a stepper machine. Then, the polymer layer  301  can be developed to form the gaps  401 . In the case of a non-photoresist type material, the patterning of the polymer layer  301  may require the use of various masking and etching techniques. 
     Turning to FIG. 5, after the gaps  401  have been formed, a second polymer layer  501  is applied into the gaps  401  and over the polymer layer  301 . The second polymer layer  501  in one embodiment is an organic polyimide, such as polymethylmethylacrylate (PMMA). However, other types of materials may be substituted, but preferably materials that may be spun on and subsequently cured into a solid. Moreover, in one embodiment, the second polymer layer  501  has an index of refraction that is higher than the polymer layer  301 . While not necessary to address the cracking issue, by having the second polymer layer  501  have a higher index of refraction, this will further add in lowering the amount of cross-talk between adjacent pixels, as will as improve the amount of light incident on the pixels. This can be seen in FIG.  6 . 
     The polymer layer  301  and second polymer layer  501  form a base upon which the color filter layers can be formed. Thus, after the second polymer layer  501  is formed, the color filter layers can be formed using any number of conventional methods noted in the patents cited above. The result is seen in FIG.  6 . 
     The above method and structure will reduce stress in the image sensor. Further, the structure will increase light gathering efficiency and reduce cross-talk. 
     From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.