Abstract:
An image sensor is disclosed that has a concave micro-lens structure. The image sensor includes a plurality of pixels formed in a semiconductor substrate, each pixel including a light sensitive element. Further, a base material having a first index of refraction is formed over the pixels. Micro-lens cavities are formed in the base material over the light sensitive elements, the micro-lens cavity having a concave shape. Finally, a filler material is formed into the micro-lens cavities, the filler material having a second index of refraction that is higher than the first index of refraction.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to image sensors, and more particularly, towards an image sensor that has a concave-shaped micro-lens.  
         BACKGROUND  
         [0002]    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.  
           [0003]    Conventionally, micro-lenses are 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. 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 . 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 and various metal conducting lines. These components are excluded from the diagram in order to simplify the explanation herein and not to obscure the invention.  
           [0004]    It has been found that the convex shape of the micro-lenses will sometimes result in a greater likelihood of particle contamination, due to later processing steps. Furthermore, because of the particular processes used to form the micro-lenses, it is difficult to eliminate gaps between the micro-lenses  105 . Generally, it is desirable to minimize the gaps between the micro-lenses, since a larger micro-lens will result in a higher degree of light concentration. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is a prior art cross sectional view of a portion of an image sensor.  
         [0006]    [0006]FIG. 2 is a top view of an image sensor showing pixels arranged in a two dimensional array and with micro-lenses formed thereon.  
         [0007]    FIGS.  3 - 9  are cross sectional and corresponding top views of a semiconductor substrate illustrating one method for forming the apparatus of the present invention.  
         [0008]    [0008]FIG. 10 is an isometric view of an apparatus according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0009]    The present invention relates to a concave micro-lens structure for use with image sensors, either of the CMOS or CCD type. 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.  
         [0010]    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.  
         [0011]    [0011]FIG. 2 shows a top view of an image sensor  201  formed in accordance with the present invention. The image sensor  201  includes a plurality of pixels  203  typically arranged in a two dimensional array. In the example shown in FIG. 2, the image sensor shows a three by three array of pixels  203 , though it can be appreciated that an actual mage sensor  201  would have many more pixels, arranged in perhaps over a thousand rows and/or a thousand columns. Further, although FIG. 2 shows the pixels in ordered columns and rows, the pixels may be arranged in any type of ordered arrangement. For example, alternating rows may have their pixels slightly offset from each other laterally in a checkerboard format.  
         [0012]    The pixels  203  typically include a light sensitive element, such as a photodiode or a photogate as two examples. However, it can be appreciated that other types of light sensitive elements, now known or developed in the future, may be used. Further, the pixels  203  will also include amplification and/or readout circuitry. For clarity, this circuitry is not shown in FIG. 2. In one embodiment, the pixels  203  may be active pixels, commonly known in the prior art.  
         [0013]    Formed atop of each pixel  203  is a micro-lens  205 . The micro-lens  205  is concave in nature, in contrast to the prior art convex micro-lens. Because of the nature of the concave micro-lenses  205 , little or no gap exists between adjacent micro-lenses of adjacent pixels.  
         [0014]    Additionally, associated with each pixel  203  is a color filter  207 . The color filter  207  may be placed either between the micro-lens  205  and the light sensitive element, or alternatively, be formed atop of the micro-lens  205 . The color filter  207  is 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  207  and the present invention is meant to encompass a color filter  207  having any color. 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  207  is known in art and will not be described herein to avoid any unnecessary obscuration with the description of the present invention. For example, U.S. Pat. No. 6,297,071, U.S. Pat. No. 6,362,513, and U.S. Pat. No. 6,271,900 show the current state of the color filter art.  
         [0015]    FIGS.  3 - 9  are schematic cross sectional and top views of a semiconductor substrate illustrating one method for forming the structure of the present invention. Specifically, FIG. 3 is a cross-sectional view taken along line A-A of FIG. 2. A semiconductor substrate  301  has a plurality of light sensitive elements  303  (associated with the pixels  203  of FIG. 2) formed therein. FIG. 3 shows the light sensitive element  303  as a photodiode, though other substitutes and equivalents may be used. 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. However, examples of the prior art may be seen in U.S. Pat. No. 5,904,493 and U.S. Pat. No. 6,320,617.  
         [0016]    According to one embodiment, after the pixels  203  are formed in the substrate, an optically transparent (in at least a portion of the visible spectrum) base material  305  is formed over the substrate  301 . The base material  305  may be formed using a blanket deposition process, or alternatively, using a spin on method. In one embodiment, the base material is an epoxy or an acrylic. These materials are chosen because of their stability, each of handling, or appropriate index of refraction. As will be seen below, it is important that the base material have a relatively low index of refraction. For an epoxy or acrylic material, the index of refraction is in the 1.4 to 1.5 range. One example of a suitable material is polymethylmethacrylate (PMMA) or polyglycidylmethacrylate (PGMA). Alternatively, the base material may be an oxide.  
         [0017]    While specific examples are given above, it can be appreciated that the base material may be formed from any optically transparent material having a relatively low index of refraction. In the case where the base material  305  is applied using spin on techniques, the base material  305  has the advantage of being substantially planar. It is desirable to have the base material  305  have a top surface that is as planar and smooth as possible. Nevertheless, a blanket deposition, such as by chemical vapor deposition, may also be suitable.  
         [0018]    In one embodiment, the thickness of the base material  305  is on the order of 2 to 2.5 microns. However, thinner or thicker layers of the base material  305  may also be used, depending on various design parameters, such as desired focal length of the micro-lens.  
         [0019]    Still referring to FIG. 3, after the base material  305  has been deposited, a resin layer  307  is deposited. The resin layer  307  is also referred to as a sacrificial layer, and in one embodiment, is a phenyl resin. Because the resin layer  307  will be used as a sacrificial layer, again, there is some flexibility in the precise material used for the resin layer  307 .  
         [0020]    In one embodiment, the resin layer  307  will need to be patterned. Because of this, it is efficient to use a photoresist type material (such as a phenyl resin) as the resin layer  307 . In that way, the resin layer  307  can be “directly patterned” by simply the use of a photolithography apparatus and a developing process.  
         [0021]    Turning next to FIG. 4, the resin layer  307  is patterned and developed to remove portions  308  of the resin layer  307 . The portions  308  of the resin layer  307  that are removed are generally over the light sensitive elements  303  and are circular in shape. A top view of the resin layer  307  showing portions removed is shown in FIG. 5. The removed portions  308  of the resin layer  307  is related to the concave micro-lens to be eventually formed. Again, the specific shape and dimensions of the removed portions shown in FIGS. 4 and 5 is but one specific embodiment of the present invention. Other specific implementations are possible. For example, the size of the portions  308  shown in FIGS. 4 and 5 may be made smaller or larger depending upon the desired size of the micro-lenses to be formed. A larger size for the portion  308  will result in a larger micro-lens, and vice versa.  
         [0022]    Turning to FIG. 6, once the resin layer  307  has been developed (in the case of the resin layer  307  being a photoresist) or etched (in the case of a non-photoresist sacrificial layer), the remaining portions of the resin layer  307  are heated to a reflow temperature. This causes the resin layer  307  to adopt a minimum surface tension shape, which in many cases results in a spherical shape, as shown in FIG. 6.  
         [0023]    Once the reflow process has been finished, an anisotropic dry etch is performed using the reflowed resin layer  307  as an etching mask. In one embodiment, the etch is a reactive ion etch using O 2  as the primary gas and CH 3  as a secondary gas. In one embodiment, the etching ratio between the base material  305  and the resin layer  307  is on the order of 1.0 to 1.5. Thus, the underlying base material  305  is etched faster than the resin layer  307 . In one embodiment, the etching process is complete when the resin layer  307  is removed. Because of the reflowed shape of the resin layer  307 , the result of the dry etch is a hemispherical “pitting” of the base material  305  to form a concave micro-lens  701  over each light sensitive element  303 . The result is seen if FIG. 7 (cross-section) and FIG. 8 (top view).  
         [0024]    It should be noted that the spacing between adjacent micro-lenses can be varied by controlling the spacing of the removed portions  308  formed in the resin layer  307 . Small removed portions  308  that are spaced far apart from each other will result in relatively small micro-lenses and large gaps between the resultant micro-lenses. Large removed portions  308  will result in large resultant micro-lenses with small gaps. Moreover, by employing over-etching techniques during etching of the base material  305  and the resin layer  307 , the gaps between adjacent micro-lenses can be reduced to zero. It can be appreciated that the size of the removed portions  308 , the etching length, the composition of the base material  305  and resin layer  307 , and other process/design factors can be varied to achieve the desired result for the characteristics of the micro-lenses.  
         [0025]    Finally, turning to FIG. 9, a fill material  311  is formed over the base material  305  and micro-lens  701 . The fill material  311  should have a relatively high index refraction that is higher than the base material  305  such that bending and focusing of incident light onto the light sensitive elements  303  takes place. In one embodiment, the fill material  311  has an index of refraction of between 1.6 and 1.8. One example of the fill material  311  is an optically transparent polyimide. n one embodiment, the thickness of the polyimide layer  311  is on the order of 3 to 4 microns. The polyimide layer  311  is typically applied using spin coating. An isometric view of the completed structure in shown in FIG. 10.  
         [0026]    According to the present invention, a minimal spacing between micro-lenses can be more easily achieved. This improves the fill factor and efficiency in gathering light, thereby improving the sensitivity. Also, the concave shape of the micro-lenses provides advantages in available packaging techniques, which in turn can minimize particles and dust from interfering with the image sensor. Further, according to the present invention, the color filters can be formed using conventional methods either on top of the micro-lenses or in between the micro-lenses and the light sensitive elements  303 . Moreover, the use of a concave shaped micro-lens results in a relatively short focal length. This in turn allows for higher integration densities.  
         [0027]    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. Thus, regardless of the specific materials used, the present invention teaches the use of concave micro-lenses that are filled with the appropriate material to focus incident light. Accordingly, the invention is not limited except as by the appended claims.