Abstract:
Ellipse-shaped microlenses focus light onto unbalanced photosensitive areas, increase area coverage for a gapless layout of microlenses, and allow pair-wise or other individual shifts of the microlenses to account for asymmetrical pixels and pixel layout architectures. The microlenses may be fabricated in sets, with one set oriented differently from another set, and may be arranged in various patterns, for example, in a checkerboard pattern or radial pattern. The microlenses of at least one set may be substantially elliptical in shape. To fabricate a first set of microlenses, a first set of microlens material is patterned onto a support, reflowed under first reflow conditions, and cured. To fabricate a second set of microlenses, a second set of microlens material is patterned onto the support, reflowed under second reflow conditions, which may be different from the first conditions, and cured.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a Continuation In Part (C.I.P.) of application Ser. No. 10/681,308, filed Oct. 9, 2003, the entire disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to the field of semiconductor-based imager devices using microlenses, and more particularly to the fabrication of an array of microlenses.  
       BACKGROUND OF THE INVENTION  
       [0003]     The semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), CMOS active pixel sensors (APS), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others, that use an array of microlenses. Semiconductor-based displays using microlenses are also being developed.  
         [0004]     Use of microlens significantly improves the photosensitivity of the imaging device by collecting light from a large light collecting area and focusing it onto a small photosensitive area of a photosensor. As the size of imager arrays and photosensitive regions of pixels continue to decrease, it becomes increasingly difficult to provide a microlens capable of focusing incident light rays onto the photosensitive regions of the pixel. This problem is due in part to the increased difficulty in constructing a microlens that has the optimal focal characteristics for the increasingly smaller imager device. Microlens shaping during fabrication is important for optimizing the focal point for the microlens. This in turn increases the quantum efficiency for the underlying pixel array. Utilizing a spherical microlens shape is better for focusing incoming light onto a narrow focal point, which allows for the desired decrease in photosensor size. Spherical microlenses, however, suffer from gapping problems which are undesirable (described below).  
         [0005]     Microlenses may be formed through either a subtractive or an additive process. In the additive process, a lens material is formed on a substrate and subsequently is formed into a microlens shape.  
         [0006]     In conventional additive microlens fabrication, an intermediate material is deposited onto a substrate and formed into a microlens array using a reflow process. Each microlens is formed with a minimum distance, typically no less than 0.3 microns, between adjacent microlenses. Any closer than 0.3 microns may cause two neighboring microlenses to bridge during reflow. In the known process, each microlens is patterned as a single square with gaps around it. During the reflowing of the patterned square microlenses, a gel drop is formed in a partially spherical shape driven by the force equilibrium of surface tension and gravity. The microlenses then harden in this shape. If the gap between two adjacent gel drops is too narrow, the drops may touch and merge, or bridge, into one large drop. The effect of bridging is that it changes the shape of the lenses, which leads to a change in focal length or, more precisely, the energy distribution in the focal range. A change in the energy distribution in the focal range leads to a loss in quantum efficiency of, and enhanced cross-talk between, pixels. The gaps, however, allow unfocused photons through the empty spaces in the microlens array, leading to lower quantum efficiency and increased cross-talk between respective photosensors of adjacent pixels.  
         [0007]     It is desirable to form a microlens array having differently shaped microlenses. However, if the known techniques, which use a single reflow step, were used to form such microlenses, the differently shaped microlenses would have different focal characteristics, which would lead to poor focusing for certain photosensors and/or the need to modify the locations, shape, or symmetries of some photosensors.  
         [0008]     It is desirable to enhance the amount of light received from the microlenses and focused on the photosensors of an imager. It is also desirable to form a microlens array with varied sized and shaped microlenses, each having a focal length and focal position optimized for the color or wavelength of light it is detecting. It is also desirable to form a microlens array having minimized gapping between the microlenses without causing bridging during the microlens fabrication reflow process.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     Embodiments of the present invention provide an improved microlens array and methods of forming it. Ellipse-shaped microlenses are formed to focus fight onto unbalanced photosensitive areas and increase area coverage in a gapless layout of microlenses. The ellipse-shaped microlenses allow pair-wise or other individual shifts of the microlenses to account for asymmetries of shared pixel layout architectures. The microlenses may be fabricated in sets, with one set oriented differently from another set, and may be arranged in various patterns, for example, in a checkerboard or radial pattern. The microlenses of at least one set may be substantially elliptical in shape.  
         [0010]     To fabricate a first set of microlenses, a first set of microlens material is patterned onto a support, reflowed under first reflow conditions, and cured. To fabricate a second set of microlenses, a second set of microlens material is patterned onto the support, reflowed under second reflow conditions, which may be different from the first conditions, and cured.  
         [0011]     These and other features of the various embodiments of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a top view of an array of pixels having non-symmetrically shaped photo-sensitive areas.  
         [0013]      FIG. 2  is a top view of an array of microlenses constructed in accordance with an exemplary embodiment of the invention over the  FIG. 1  array of pixels.  
         [0014]      FIG. 3   a  is a cross-section taken along line X-X of  FIG. 2 .  
         [0015]      FIG. 3   b  is a cross-section taken along line Y-Y of  FIG. 2 .  
         [0016]      FIG. 4  is a top view of a step of fabricating the microlens array of  FIG. 2 .  
         [0017]      FIG. 5  is a top view of a subsequent step of fabricating the microlens array of  FIG. 2 .  
         [0018]      FIG. 6  is a top view of an array of microlenses constructed in accordance with another exemplary embodiment of the invention over the  FIG. 1  array of pixels.  
         [0019]      FIG. 7  is a top view of an array of microlenses constructed in accordance with another exemplary embodiment of the invention over the  FIG. 1  array of pixels.  
         [0020]      FIG. 8  is a top view of an array of microlenses constructed in accordance with another exemplary embodiment of the invention.  
         [0021]      FIG. 9  illustrates a process for fabricating a microlens array of the present invention.  
         [0022]      FIG. 10  is a schematic of an imaging device using a pixel having a microlens array constructed in accordance with an embodiment of the invention.  
         [0023]      FIG. 11  illustrates a schematic of a processing system including the imaging device of  FIG. 10 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof and show by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and 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 progression of processing steps described is exemplary of embodiments of the invention; however, the sequence of steps is not limited to that set forth herein and may be changed as is known in the art, with the exception of steps necessarily occurring in a certain order.  
         [0025]     The term “substrate,” as used herein, is to be understood as including silicon, silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, 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 or “substrate” in the following description, previous processing steps may have been utilized to form regions, junctions, or material layers in or over the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, germanium, gallium arsenide or other semiconductors.  
         [0026]     The term “pixel,” as used herein, refers to a photo-element unit cell containing a photosensor device and associated structures for converting photons to an electrical signal. For purposes of illustration, a single representative three-color pixel and its manner of formation is illustrated in the figures and description herein; however, typically fabrication of a plurality of like pixels proceeds simultaneously. Accordingly, the following detailed description is not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.  
         [0027]     Finally, while the invention is described with reference to a semiconductor-based imager, such as a CMOS imager, it should be appreciated that the invention may be applied in any micro-electronic or micro-optical device that requires high quality microlenses for optimized performance. Other exemplary micro-optical devices that can employ the invention include CCDs and display devices where pixels employ a photoemitter, as well as others.  
         [0028]     Referring now to  FIG. 1 , an array of imaging pixels  100  having asymmetrically shaped photosensitive areas  101 ,  102  is shown. In this depiction of an array of pixels  100 , microlenses and metal lines for communication with row and column drivers and other circuitry of an image sensor device have not yet been formed over the pixels. Because the photosensitive areas  101 ,  102  are asymmetrical, spherically shaped microlenses would be unable to direct light to some regions of the areas  101 ,  102 , while delivering most of the incident light to other regions of the photosensitive areas  101 ,  102 . Such a distribution of photons leads to loss in quantum efficiency.  
         [0029]     Referring now to  FIG. 2 , an array of microlenses  110  formed over pixel array  100  is shown. The array  110  includes a plurality of first microlenses  111  and a plurality of second microlenses  112 , each being formed over the pixel array  100 . The first microlenses  111  are formed for photosensitive areas  101  ( FIG. 1 ) and the second microlenses  112  are formed for photosensitive areas  102  ( FIG. 1 ). The first microlenses  111  are shown to have a substantially elliptical shape, while the second microlenses  112  are shown to have a substantially square shape. The edges of second microlenses  112  slightly overlap the edges of first microlenses  111 .  
         [0030]     Each of the first microlenses  111  can be formed to have a first focal length in a longitudinal axis  115  and a second focal length in a lateral axis  116 , varied by adjusting the major and minor axes of the elliptical shape. Each of the second microlenses  112  can be formed to have a focal length of similar length as the first focal length of the first microlenses  111  or the second focal length of the first microlenses  111 . Alternatively, the second microlenses  112  can be formed to have a third focal length. In any case, the microlenses  111 ,  112  can be formed to have different focal lengths to cater to the shape and dimensions of the corresponding photosensitive areas  101 ,  102  in the semiconductor substrate  120 .  
         [0031]      FIG. 3   a  is a cross-section taken across line X-X of  FIG. 2 . Line X-X of  FIG. 2  is drawn along the longitudinal axis of the first microlens  111 .  FIG. 3   b  is a cross-section taken across line Y-Y of  FIG. 2 . Line Y-Y of  FIG. 2  is drawn along the lateral axis of the first microlens  111 . It should be noted that  FIGS. 3   a  and  3   b  are not actual cross-sections through an imager, but only schematical representations of the locational relationship of the microlenses to the photosensitive areas in an imager. For example, semiconductor substrate  120  represents several layers of an imager including, but not limited to, passivation layers, metallization layers, and color filter array layers.  
         [0032]     As can be seen from  FIGS. 2, 3   a,  and  3   b,  the first microlens  111  is longer in the longitudinal direction than it is in the lateral direction. Also, the first microlens  111  may be formed to focus light from an area outside of the first photosensitive region  101  to the first photosensitive region  101 .  
         [0033]     The second microlenses  112  are shaped differently than the first microlenses  111 , as shown, thereby maximizing the amount of space covered by a microlens. Since light transmitted without going through a microlens is not properly focused relative to any photosensitive areas, and may increase the incidence of cross-talk between pixels, there is great benefit to maximizing the amount of space covered by a microlens.  
         [0034]     The first microlenses  111  are formed from a first microlens material  11 . The first microlens material  11  ( FIG. 4 ) is deposited and patterned upon a support substrate  120 . The substrate  120  is formed of any suitable material which is transparent to electromagnetic radiation. Each deposition of the first microlens material  11  has an elongated hexagon-shaped configuration, which is substantially equal in size with the others. The first microlens material  11  is a material which, upon reflow, forms into a solidly cross-linked polymer impervious to subsequent reflow processes. During a reflow process conducted under reflow conditions, the elongated hexagon-shaped configuration of the first microlens material  11  is transformed into the first microlens  111 , which has an elliptically-shaped configuration with rounded edges and a curved top. The first microlenses  111 , which are transparent to electromagnetic radiation, will retain their shape even if a subsequent reflow process is performed.  
         [0035]     After patterning and reflowing the first microlens material  11  to form the first solidly cross-linked polymer microlenses  111 , a second microlens material  12  is patterned as shown in  FIG. 5 . The second microlens material  12 , patterned in a substantially square-shaped configuration, is positioned in some of the spaces between the first microlenses  111 . Additionally, portions of second microlens material  12  can be formed overlapping the first microlenses  111 , if desired.  
         [0036]     Reflowing and curing of the second microlens material  12  under reflow conditions, which may differ from the conditions of the reflow of the first microlens material  11 , forms second microlenses  112 , as shown in  FIG. 2 . The second microlenses  112  are impervious to subsequent reflow, just like the first microlenses  111 . The second microlenses  112 , which are of a different size and shape than the first microlenses  111 , specifically, a smaller size, are each somewhat square-shaped in configuration with rounded corners and a curved top. More particularly, the second microlenses  112  have a similar curvature and a smaller surface area than the first microlenses  111 . It should be appreciated that, in another embodiment, the second microlenses  112  may be larger than the first microlenses  111 .  
         [0037]     A microlens array  110  is thus formed, and includes any combination of two or more pluralities of microlenses  111 ,  112 . By fabricating the microlens array  110  by forming first microlenses  111  spaced apart, e.g., in a checkerboard fashion, and filling in the spaces with second microlenses  112  formed in a separate process, bridging between adjacent first microlenses  111  and between first and second microlenses  111 ,  112  is diminished. This is because the first microlenses  111  have already gone through a reflow process that has rendered them impervious to any subsequent reflow process. Thus, the subsequent reflow of the second microlens material  12  into the second microlenses  112  will not cause bridging between the first and second microlenses  111 ,  112  or between a pair of second microlenses  112 . The microlens array  110  is approximately space-less since the formed microlenses abut one another.  
         [0038]     By forming the microlenses  111 ,  112  through separate reflow processes, the microlens array  110  can be formed to provide greater signal strength for pixels that would typically exhibit lower signal strength. For example, photosensitive areas  101  may tend to produce an inherently lower output signal for a given light intensity due to a blue color filter being provided over it. Thus, the microlens array  110  can be formed by creating larger microlenses, such as the first microlenses  111 , which will collect more light and help balance pixel signal strength for the different colored pixels of a pixel array. A balanced signal between colors assists the dynamic range of the photosensors in pixels because it avoids systematically sending pixels of one color into saturation while other pixels are only partially saturated.  
         [0039]     It should be appreciated that other shaped microlenses may be formed in any spaces on a substrate after the second plurality of microlenses. For example, after the second microlenses are formed, a third microlens material may be patterned in substantially square-, elliptical-, or other shapes in spaces between the first microlenses, the first and second microlenses, and/or the second microlenses, as necessary. Reflowing and curing of the third microlens material are performed under reflow conditions different from the first and second microlens material.  
         [0040]     It should also be appreciated that a third plurality of microlenses may be formed with the second plurality microlenses in a single reflow process. When the third microlenses and the second microlenses are spatially separated from one another, for example, with the first microlenses between them, bridging is not likely.  
         [0041]     Exemplary materials that may be used as microlens materials include, but are not limited to MFR-401 from JSR.  
         [0042]     In another exemplary embodiment of the invention, the microlenses may all be elliptically-shaped and rotated about an axis of the array, maximizing the benefits of the elliptical shape. Referring to  FIG. 6 , an array of microlenses  210  formed over pixel array  100  is shown. The array  210  includes a plurality of first microlens  211  and a plurality of second microlenses  212 , each being formed over the pixel array  100 . The first microlenses  211  are formed for the first photosensitive areas  101  and the second microlenses  212  are formed for the second photosensitive areas  102 . The first microlenses  211  are shown to have an elliptical shape, rotated about +45° from the vertical axis of the array  210 . The second microlenses  212  are also shown to have an elliptical shape, rotated about +45° from the vertical axis of the array  210 . The edges of second microlenses  212  slightly overlap the edges of first microlenses  211 .  
         [0043]     Each of the first microlenses  211  can be formed to have a first focal length in the longitudinal axis and a second focal length in the lateral axis, varied by adjusting the major and minor axes of the elliptical shape. Each of the second microlenses  212  can be formed to have a first focal length of similar length as the first focal length of the first microlenses  211  a second focal length of similar length as the second focal length of the first microlenses  211 . Alternatively, the second microlenses  212  can be formed to have different focal lengths. In any case, the microlenses  211 ,  212  can be formed to have different focal lengths to cater to the shape and dimensions of the corresponding photosensitive areas  101 ,  102  in the substrate  120 .  
         [0044]     In another exemplary embodiment, the orientation of the elliptically-shaped microlenses may be different from one another. Referring to  FIG. 7 , an array of microlenses  310  formed over pixel array  100  is shown. The array  310  includes a plurality of first microlens  311  and a plurality of second microlens  312 , each being formed over the pixel array  100 . The first microlenses  311  are formed for the first photosensitive areas  101  and the second microlens  312  are formed for the second photosensitive areas  102 . The first microlenses  311  are shown as having an elliptical shape, rotated about +45° from the vertical axis of the microlens array  310 . The second microlenses  312  are also shown to have an elliptical shape, but rotated about −45° from the vertical axis of the array  310 . The edges of second microlenses  312  slightly overlap the edges of first microlenses  311 .  
         [0045]     Each of the first microlenses  311  can be formed to have a first focal length in the longitudinal axis and a second focal length in the lateral axis, varied by adjusting the major and minor axes of the elliptical shape. Each of the second microlenses  312  can be formed to have a first focal length of similar length as the first focal length of the first microlenses  311  a second focal length of similar length as the second focal length of the first microlenses  311 . Alternatively, the second microlenses  312  can be formed to have different focal lengths. In any case, the microlenses  311 ,  312  can be formed to have different focal lengths to cater to the shape and dimensions of the corresponding photosensitive areas  101 ,  102  in the substrate  120 .  
         [0046]     In another exemplary embodiment, the orientation of elliptically-shaped microlenses may be different from one another. For example, the microlens array may have a radial configuration around a center point C such that an extension of the longitudinal axes of all elliptically-shaped microlenses intersect at center point C. Referring to  FIG. 8 , an array of microlenses  410  is shown. The array  410  includes a plurality of first microlenses  411 , a plurality of second microlenses  412 , and a plurality of third microlenses  413 , each being formed over a pixel array formed in substrate  420 . The first microlenses  411  are shown to have an elliptical shape, rotated about 0°, +45°, −45°, or 90° from the vertical axis of the microlens array  410 . The second microlenses  412  are also shown to have an elliptical shape, rotated about 0°, +45°, −45°, or 90° from the vertical axis of the array  410 . The third microlenses  413  are shown to have a spherical shape or an elliptical shape, rotated +22.5°, −22.5°, +67.5°, or −67.5° from the vertical axis of the array  410 . The edges of second microlenses  412  slightly overlap the edges of first microlenses  411 , and the edges of third microlenses  413  slightly overlap the edges of first and second microlenses  411 ,  412 .  
         [0047]     Each of the first microlenses  411  can be formed to have a first focal length in the longitudinal axis and a second focal length in the lateral axis, varied by adjusting the major and minor axes of the elliptical shape. Each of the second microlenses  412  can be formed to have a first focal length of similar length as the first focal length of the first microlenses  411  a second focal length of similar length as the second focal length of the first microlenses  411 . Likewise, the third microlenses  413  can be formed to have similar focal lengths as the first microlenses  411  or the second microlenses  412 . Alternatively, the first, second and third microlenses  411 ,  412 ,  413  can be formed to have different focal lengths. In any case, the microlenses  311 ,  312  can be formed to have different focal lengths to cater to the shape and dimensions of the corresponding photosensitive areas in the substrate  420 .  
         [0048]     The formation of microlens array  410  is similar to the formation of microlens array  110 . By depositing a microlens material, patterning, reflowing, and curing to create each plurality of microlenses  411 ,  412 ,  413 , the first microlenses  411  are impervious to subsequent reflow during the formation of the second microlenses  412  and the second microlenses  412  are impervious to subsequent reflow during the formation of the third microlenses  413 . Fabricating the microlens array  410  by forming the first microlenses  411  spaced apart, e.g., in a radial fashion, filling in the spaces with the second microlenses  412  formed in a separate process, and filling in the spaces with the third microlenses  413  formed in another separate process, bridging between first, second, and third microlenses  411 ,  412 ,  413  is diminished. This is because the first microlenses  411  have already gone through a reflow process that has rendered them impervious to any subsequent reflow process. Thus, the subsequent reflow of the second microlens material into the second microlenses  412  will not cause bridging between the first and second microlenses  111 ,  112 . The microlens array  410  is approximately space-less since the formed microlenses abut one another.  
         [0049]     An example of reflow conditions is described next. The shape of the microlenses after being subjected to reflow conditions is defined by-several factors, including the thickness and type of material used to form the microlenses, the reflow temperature profile, and any pretreatment of the material that changes its glass transition temperature Tg. Examples of such pretreatments include ultraviolet light exposure or preheating the material to a temperature below the glass transition temperature Tg. An example of first reflow conditions may include providing first microlens material at a first thickness and from a first type of material, exposing the first microlens material with an ultraviolet light flood exposure of a specific dose, and reflowing at a first temperature ramp rate, followed by a cure. Second reflow conditions may include providing second microlens material of the first type of material at a second thickness and reflowing the second microlens material with the first temperature ramp rate, followed by a cure. Third reflow conditions may include providing a third microlens material of a second material type and of a third thickness, pre-heating the material to a temperature below the transition glass temperature Tg of the third microlens material for a set period of time, and then reflowing at a second temperature ramp rate, followed by a cure.  
         [0050]     A process for forming a microlens array with reference to  FIG. 9  is now described. At Step  150 , a first microlens material is patterned on the substrate. The patterning, as described above, can be a checkerboard pattern or a radial pattern, which includes spaces between portions of the first microlens material. A single reticle may be used to prepare each of the first microlens material patterns. In the patterning step, a thin film of microlens material of a first thickness is coated on the substrate, the material is exposed using a suitable mask, and it is developed to either dissolve the exposed microlens material (positive resist) or dissolve the unexposed microlens material (negative resist). At Step  155 , the first microlens material is reflowed at a first condition. Reflowing of the first microlens material turns the material into the first microlenses. At Step  160 , the first microlenses are cured, thus forming a checkerboard pattern of solidly cross-linked first microlenses.  
         [0051]     At Step  165 , the second microlens material is patterned on the substrate in some of the spaces between the first microlenses. A single reticle may be used to prepare each of the second microlens material depositions. If the second microlens material patterns are of the same size and orientation as the first microlens material patterns (as described with reference to  FIG. 6 ), the same reticle as was used for the pattern of the first microlens material patterns may be used for the pattern of the second microlens material patterns. For the pattern of the second microlens material, the reticle is shifted in the stepper job.  
         [0052]     At Step  170 , the second microlens material may be reflowed at a second condition to form the second microlenses. The second condition may differ from the first condition, for example, by varying the exposure and/or the dose of bleaching or the step baking temperature. By using different reflow conditions, the first microlenses and second microlenses can be formed having same or different focal lengths. At Step  175 , a second cure is, performed.  
         [0053]     In the formation of a microlens array having a third plurality of microlenses, Steps  180 - 190  are performed, otherwise the process is complete. At Step  180 , wherein third microlens material is patterned in remaining open spaces between the first and second microlenses. At Step  185 , the third microlens material may be reflowed at a third condition to form the third microlenses. The third condition may differ from the first and second conditions, for example, by varying the doses of exposing and/or bleaching or the step baking temperature. By using different reflow conditions, the third microlenses can be formed such that their focal lengths are the same or different than the focal lengths of the first and second microlenses. At Step.  190 , a third cure is performed.  
         [0054]      FIG. 10  illustrates an exemplary imaging device  200  that may utilize pixels having microlenses constructed in accordance with the invention. The imaging device  200  has an imager pixel array  100  comprising a plurality of pixels with microlenses constructed as described above. Row lines are selectively activated by a row driver  202  in response to row address decoder  203 . A column driver  204  and column address decoder  205  are also included in the imaging device  200 . The imaging device  200  is operated by the timing and control circuit  206 , which controls the address decoders  203 ,  205 . The control circuit  206  also controls the row and column driver circuitry  202 ,  204 .  
         [0055]     A sample and hold circuit  207  associated with the column driver  204  reads a pixel reset signal Vrst and a pixel image signal Vsig for selected pixels. A differential signal (Vrst-Vsig) is produced by differential amplifier  208  for each pixel and is digitized by analog-to-digital converter  209  (ADC). The analog-to-digital converter  209  supplies the digitized pixel signals to an image processor  213  which forms and outputs a digital image.  
         [0056]      FIG. 11  shows system  300 , a typical processor system modified to include the imaging device  200  ( FIG. 10 ) of the invention. The processor-based system  300  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, still or video camera system, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, and data compression system.  
         [0057]     The processor-based system  300 , for example a camera system, generally comprises a central processing unit (CPU)  395 , such as a microprocessor, that communicates with an input/output (I/O) device  391  over a bus  393 . Imaging device  200  also communicates with the CPU  395  over bus  393 . The processor-based system  300  also includes random access memory (RAM)  392 , and can include removable memory  394 , such as flash memory, which also communicate with CPU  395  over the bus  393 . Imaging device  200  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.  
         [0058]     While the invention has been described in detail in connection with exemplary embodiments known at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, while the microlens arrays are shown with microlenses abutting one another, since multiple reflow processes are performed, subsequently formed microlenses can overlap previously formed microlenses. Furthermore, while some microlenses are shown rotated at specific degrees of rotation (e.g., 0°, +45°, −45°, 90°, +22.5°, −22.5°, +67.5°, or −67.5°) from the vertical axis of the microlens arrays, the major axes of the elliptical microlenses may be turned in any suitable direction. Furthermore, instead of three reflow processes, only two reflow processes may be performed, or more than three reflow processes may be performed. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.