Abstract:
A microlens structure that includes a wedge formed to support and tilt the microlens is disclosed. The wedge results from heating a layer of patterned flowable material. The degree and direction of incline given to the wedge can be controlled in part by the type of patterning that is performed.

Description:
FIELD OF THE INVENTION 
     The invention relates to the fabrication of microlens structures for image capture or display systems, and more specifically to structures and methods of fabrication of microlens arrays for solid state imager systems. 
     BACKGROUND OF THE INVENTION 
     Solid state imagers, including charge coupled devices (CCD) and CMOS sensors, are commonly used in photo-imaging applications. A solid state imager includes a focal plane array of pixels. Each of the pixels includes a photovoltaic device for converting light energy to electrical signals. The photovoltaic device can be a photogate, photoconductor, or a photodiode having a doped region for accumulating photo-generated charge. 
     Microlenses are commonly placed in a corresponding array over the imager pixel(s). A microlens is used to focus light onto the initial charge accumulation region, for example. Conventional technology forms microlenses from photoresist material which is patterned into squares or circles provided respectively over the pixels. The patterned photoresist material is then heated during manufacturing to shape and cure the microlens. 
     Use of microlenses significantly improves the photosensitivity and efficiency of the imaging device by collecting light from a large light collecting area and focusing it on a small photosensitive area of the pixel. The ratio of the overall light collecting area to the photosensitive area of the pixel is known as the “fill factor.” 
     The use of microlens arrays is of increasing importance in imager applications. Imager applications are requiring imager arrays of smaller size and greater resolution. As pixel size decreases and pixel density increases, problems such as crosstalk between pixels become more pronounced. Also, pixels of reduced size have a smaller charge accumulation area. Reduced sizes of pixels result in smaller accumulated charges which are read out and processed by signal processing circuits. 
     As the size of imager arrays and photosensitive regions of pixels decreases, it becomes increasingly difficult to provide a microlens capable of focusing incident light rays onto the photosensitive regions. This problem is due in part to the increased difficulty in constructing a small enough microlens that has the optimal focal characteristics for the imager device process and that optimally adjusts for optical aberrations introduced as the light passes through the various device layers. Also, it is difficult to correct possible distortions created by multiple regions above the photosensitive area, which results in increased crosstalk between adjacent pixels. Crosstalk can result when off-axis light strikes a microlens at an obtuse angle. The off-axis light passes through planarization regions and a color filter, misses the intended photosensitive region and instead strikes an adjacent photosensitive region. 
     Microlens shaping and fabrication through heating and melting microlens materials also becomes increasingly difficult as microlens structures decrease in size. Previous approaches to control microlens shaping and fabrication do not provide sufficient control to ensure optical properties such as focal characteristics, radius of the microlens or other parameters needed to provide a desired focal effect for smaller microlens designs. Consequently, imagers with smaller sized microlenses have difficulty in achieving high color fidelity and signal-to-noise ratios. 
     BRIEF SUMMARY OF THE INVENTION 
     The various exemplary embodiments of the invention provide a variety of structures and methods used to adjust the shape, radius and/or height of a microlens for a pixel array. The embodiments use structures that affect volume and surface force parameters during microlens formation. Exemplary embodiments are directed to a microlens structure that includes a wedge formed to support and tilt the microlens to achieve desired focusing properties. The wedge results from heating a layer of flowable material. The flowable material is patterned such that a wedge is formed during reflow of the material. The degree and direction of incline given to the wedge can be controlled by the type of patterning that is done in the flowable material. 
     In one exemplary embodiment, a series of parallel strips, where each strip is successively smaller, is used as the wedge. When the patterned flowable material is reflowed, the larger strips at one end will become the thicker portion of the wedge. The smaller strips at the other end will become the narrower portion of the wedge. Each microlens can be patterned identically. Alternatively, pairs and other groupings can be patterned to form unlimited wedge arrangements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other advantages and features of the invention will become more apparent from the detailed description of the exemplary embodiments provided below with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates in plan view a reticle used to pattern photoresist material according to an exemplary embodiment of the invention; 
         FIG. 2  illustrates a cross-sectional view of photoresist material formed on a substrate having the reticle of  FIG. 1  patterned over it; 
         FIG. 3  is a cross-sectional view of the photoresist strips of  FIG. 2  after developing according to an exemplary embodiment of the invention; 
         FIG. 4  is a cross-sectional view of a solid resist wedge formed after reflow according to an exemplary embodiment of the invention; 
         FIG. 5  is a cross-sectional view of a microlens supported by the wedge according to an exemplary embodiment of the invention; 
         FIG. 6  is a plan view of a pair of adjacent microlens support areas with resist strips developed to form a complementary pattern according to an exemplary embodiment of the invention; 
         FIG. 7   a  is a cross-sectional view of a pair of adjacent microlenses supported by a pair of adjacent microlens support areas according to an embodiment of  FIG. 6 ; 
         FIG. 7   b  is a cross-sectional view of a pair of adjacent microlenses supported by a pair of adjacent microlens support areas sharing a pixel according to another embodiment of  FIG. 6 ; 
         FIG. 8  is a plan view of four adjacent microlens support areas with resist strips developed to form a complementary pattern according to an exemplary embodiment of the invention; 
         FIG. 9  is a schematic of an imaging device using a pixel having a microlens constructed in accordance with an embodiment of the invention; and 
         FIG. 10  illustrates a schematic of a processing system including the imaging device of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     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. The term “flow,” “flowing” or “reflowing” refers to a change in shape of a material which is heated and melts, thereby producing a material flow or shape alteration in the material caused by heating or other similar process. “Flow” is an initial melting and “reflow” is a subsequent melting of material that has been previously flowed. 
     In addition, 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. Additional exemplary micro-optical devices which can employ the invention include other solid state imaging devices, e.g., CCD and others, and display devices where a pixel emits light. 
     Referring now the drawings, where like elements are designated by like reference numerals,  FIG. 1  illustrates a reticle  10  used to pattern a flowable material, for example, a photoresist material according to an exemplary embodiment of the invention. The reticle may be formed of a chrome material, for example. Stripes  11  on the recticle vary in size. In the exemplification shown, the reticle stripes  11  are shown decreasing in size from left to the right, such that the stripe widths are decreasing in a direction perpendicular to the longitudinal axis of the stripes. The reticle has openings  12  of about 0.3 microns to about 0.5 microns wide between the stripes  11 . The reticle  10  is placed over a substrate  5  having a layer of photoresist material  20  over a photosensitive region  6  of a pixel, as illustrated in  FIG. 2 . The photoresist material is a photo-sensitive transparent material  20 . For example, it could be the same material that is used for the formation of the microlens. In another embodiment, the material may be selected to control the phase of polarization. 
     Referring to  FIG. 3 , after development of the photoresist material  20 , a formation of photoresist strips  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37  remain on the substrate  5 . The photoresist strips  31 ,  32 , . . .  37  have widths W 1 , W 2 , . . . W 7  on the order of a few tenths of a micron. The widths W 1 , W 2 , . . . W 7  decrease in a direction perpendicular to the longitudinal axis of the strips. 
     Referring to  FIG. 4 , the photoresist strips  31 ,  32 , . . .  37  are subjected to reflow conditions to produce a wedge  15 . Comparing  FIG. 3  and  FIG. 4 , it can be seen that photoresist strips  31 - 37  have flowed together to generate the wedge  15 . The wedge  15  is thicker on the left side, where photoresist strips  31 ,  32 ,  33  were wider. The wedge  15  is thinner on the right side, where photoresist strips  35 ,  36 ,  37  were narrower. In other words, the wedge  15  has a first thickness D 1  on the side having the thickest photoresist strip  31  and a second thickness D 2  on the side having the thinnest photoresist strip  37 , wherein D 1  is greater than D 2 . Thus, the wedge  15  has a sloped upper surface having an angle, “α,” described by the tangent of the bumps of the upper surface  14  of the wedge  15  and the horizontal surface  4  of the substrate  5 . Angle α can be tailored to be any angle desired, but in an exemplary embodiment is typically less than about 10 degrees. 
     As illustrated in  FIG. 4 , the wedge  15  may not have a completely smooth upper surface  14 . The wedge may be smoothed out to have a flat surface by a smoothing process such as lithography. The degree to which the wedge is smoothed may depend on the chosen degree of resolution of the lithography tool and the flow properties of the flowable wedge material. The following discussion describes a wedge without a smooth surface for exemplary purposes only; however it should be noted that the wedge may have a smooth surface as well. 
     Referring to  FIG. 5 , the wedge  15  provides a support surface for a tilted microlens  25 , which is formed on and supported by the wedge  15 . Due to the slope of the wedge  15 , the microlens  25  is tilted such that its orientation allows its focal spot to shift to a target location, such as a photosensitive element  6 . This allows placement of a microlens off-center from the photosensitive element  6 . The microlens may be directly over, but not centered over the photosensitive element, or it may be adjacent to the photosensitive element; however the tilt angle of the wedge allows the microlens to direct incident light to the photosensitive element. In an array of microlenses formed according to this embodiment, all of the microlenses may have wedges with the same tilt angle such that the wedges are sloped in the same direction. 
     As described below in more detail, the focal characteristics of the microlens arrays are controlled by forming pattern structures of differing widths using a photoresist  25  and flowing the patterned structures to form a wedge to support and tilt the microlenses. The reticle used to pattern the structure has a series of parallel strips, each of which is successively smaller than the preceding strip, such that the structures formed by the smaller strips form the thinner side of the wedge. Subsequent processing, such as baking and packaging, takes place according to standard industry practice. 
     In another embodiment, two tilted microlenses may be provided as part of a two-way shared pixel layout. By providing two tilted microlenses, it is possible to shift the focal point of each of the two microlenses in a desired manner. In other words, rather than having one microlens centered over one pixel, there may be more than one microlens over a single pixel or adjacent to the single pixel, each of which may focus incident light to that pixel. Thus, two microlenses may be formed over only one photosensitive element. Alternatively, two targeted devices can be placed closer together, allowing more pixel area elsewhere under the microlens for logic circuitry. Referring to  FIG. 6 , two reticles  50 ,  60  are oriented such that the wider reticle stripes  38 ,  39  are adjacent to each other. 
     The resulting wedges have their thicker portions adjacent to each other, such that both wedges  55 ,  65  would support microlenses that tilt away from their adjacent sides, as shown in  FIGS. 7   a  and  7   b . Referring to  FIG. 7   a , two tilted microlenses  75 ,  85  are used to shift their focal points such that two targeted photosensitive devices  56 ,  66  may be placed closer together in the substrate. Referring to  FIG. 7   b , two tilted microlenses are used to shift the focal points to a common photosensitive element  156 . In the example illustrated in  FIG. 7   b , wedge  165  has a larger angle β than angle α of wedge  155 . Since microlens  185  is not directly over the photosensitive element  156 , it must be tilted more (angle β must be greater than angle α) in order to direct incident light to the photosensitive element  156 . 
     Advantageously, by controlling the degree of tilt relative to a photosensitive element of the imager, more freedom in the design of photosensitive elements is permitted and the focal point of the tilted microlens can be shifted to where the photosensitive element is placed within the pixel. 
     Referring to  FIG. 8 , another embodiment is shown where four reticles  90 ,  100 ,  110 ,  120  are oriented diagonally such that the wider reticle stripes  91 ,  92 ,  93 ,  94  are closer to the center of the four reticles  90 ,  100 ,  110 ,  120  than other reticle stripes. In this embodiment, the wedges formed by reticles  90 ,  100 ,  110 ,  120  will result in four tilted microlenses provided as part of a four-way shared pixel layout. By providing four tilted microlenses, it is possible to shift the focal point of each of the four microlenses in a desired manner. Thus, the four microlenses may be formed over a single common photosensitive element. The resulting wedges supporting each microlens will have different angles, respectively chosen to direct incident light from its respective location to the common photosensitive element. Alternatively, the four microlenses may each be formed over one photosensitive element, but the four targeted devices (e.g., photosensitive devices) can be placed closer together, allowing more pixel area for logic circuitry if needed. 
     The orientation of the tilted microlens, such as the dimentions, shape, focal length and other focal characteristics are determined by one or more microlens and imager design parameters including: (1) the distance, width or size of the photosensor under the wedge where the microlens focuses light; (2) the viscosity of the microlens material used to form the microlenses during heating; (2) the dimensions and material of the wedge; (4) the alterations in flow behavior of the microlens material resulting from microlens structures affecting microlens material flow behavior during heating; (5) the effects of pre-heating or pre-flow treatment of wedge or microlens materials; (6) the approximate orientation of the microlense structure after heating of the microlens material is completed; and (7) the effects of the wedge material that may alter flow properties of the microlens material. 
       FIG. 9  illustrates an exemplary imaging device  200  that may utilize pixels having tilted microlenses constructed in accordance with the invention. The imaging device  200  has an imager pixel array  201  comprising pixels with microlens 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 . 
     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  210  which forms and outputs a digital image. 
       FIG. 10  shows system  900 , a typical processor system modified to include the imaging device  200  ( FIG. 9 ) of the invention. The processor-based system  900  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. 
     The processor-based system  900 , for example a camera system, generally comprises a central processing unit (CPU)  995 , such as a microprocessor, that communicates with an input/output (I/O) device  991  over a bus  993 . Imaging device sensor  200  also communicates with the CPU  995  over bus  993 . The processor-based system  900  also includes random access memory (RAM)  992 , and can include removable memory  994 , such as flash memory, which also communicate with CPU  995  over the bus  993 . Image sensor  800  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. 
     Although the above discussion describes the wedge as being formed of strips directly patterned using a reticle, it should be noted that the strips and their formation are not limited to such an embodiment. Other materials and methods may be used to form the series of strips that are flowed to form the wedge. For example, the strips may be formed of a microlens-forming material and may be formed using an etching process or lithography. 
     Various applications of the methods of the invention will become apparent to those of skill in the art as a result of this disclosure. Although certain advantages and embodiments have been described above, those skilled in the art will recognize that substitutions, additions, deletions, modifications and/or other changes may be made without departing from the spirit or scope of the invention. Accordingly, the invention is not limited by the foregoing description but is only limited by the scope of the appended claims.