Patent Publication Number: US-2010123260-A1

Title: Stamp with mask pattern for discrete lens replication

Description:
FIELD OF THE INVENTION 
     The embodiments described herein relate to optical lenses and methods of making the same. 
     BACKGROUND OF THE INVENTION 
     Microelectronic imagers are used in a multitude of electronic devices. As microelectronic imagers have decreased in size and improvements have been made with respect to image quality and resolution, they have saturated commonplace devices including mobile telephones and personal digital assistants (PDAs) in addition to their traditional uses in digital cameras. 
     Microelectronic imagers include image sensors that typically use charged coupled device (CCD) systems and complementary metal-oxide semiconductor (CMOS) systems, as well as other systems. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are quickly becoming very popular because they have low production costs, high yields, and small sizes. 
     As shown in  FIG. 1 , microelectronic imager modules  150  are often fabricated at a wafer level. The imager module  150  includes an imager die  108 , which includes an imager array  106  and associated circuits (not shown). The imager array  106  may be a CCD or CMOS imager array, or any other type of solid state imager array. The imager module  150  may also includes a lens structure  112 , which includes a spacer  109  and at least one lens element  111  arranged on a lens carrier  110 . The spacer  109  maintains the lens element  111  at a proper distance from the imager array  106 , such that light striking a convex side of the lens element  111  is directed to the imager array  106 . The spacer  109  may be bonded to the imager die  108  by a bonding material  104  such as epoxy. Typically, the lens element  111  comprises an optically transmissive glass or plastic material configured to focus light radiation onto the imager array  106 . In addition, the lens structure  112  can include multiple lenses, or may be combined with another optically transmissive element, such as a package lid. The fabrication of one such imager module and associated lens support structure is discussed in co-owned U.S. patent application Ser. No. 11/605,131, filed on Nov. 28, 2006 and U.S. patent application Ser. No. 12/073,998, filed on Mar. 12, 2008. 
     In practice, imager modules  150  are fabricated in mass rather than individually. As shown in a top-down view in  FIG. 2A  and a cross-sectional view in  FIG. 2B , multiple imager dies  108   a - 108   d , each die including a respective imager array  106   a - 106   d , are fabricated on an imager wafer  210 . As shown in  FIGS. 3A and 3B , multiple lens elements  111   a - 111   d , corresponding in number and location to the imager arrays  106   a - 106   d  on the imager wafer  210 , may be fabricated on a lens wafer  220  using a replication process such as ultraviolet embossing is used to duplicate the surface topology of a master mold structure onto a thin film of an ultraviolet-curable epoxy resin applied to the lens wafer  220 . As shown in  FIG. 4A , the imager wafer  210  and lens wafer  220  are then assembled with the lens elements  111   a - 111   d  being optically aligned with the imager dies  108   a - 108   d  to form a plurality of imager modules  150   a ,  150   b  (other imager modules are formed, but not shown in  FIG. 4A ). As shown in  FIG. 4B , the imager modules  150   a ,  150   b  may then be separated by dicing into individual imager modules  150   a ,  150   b.    
     The process of forming multiple lens elements  111  detailed above, however, suffers from several problems. First, it is difficult to maintain thickness uniformity of the lens elements  111  because bonding is done polymer-to-glass. The cured polymer that comprises lens elements  111   a - 111   d  is co-extensive with the edges of the lens wafer and so any stacking elements must be bonded to the polymer. A uniform thickness among the lens elements  111  would lower adhesive bond line thickness and make the adhesion more reliable. Second, chipping or delamination of the polymer can occur during a dicing stage of production, which can lead to decreased image quality. 
     Other known methods of forming multiple lens elements  111 , such as using a jet dispense process suffer from problems as well. First, a jet dispense process is comparably low-throughput because lenses must be formed individually. Second, jet dispense processes commonly produce residual polymer volume (e.g., sputter) outside the lens area, which can cause problems with formation of other lenses on the lens wafer. Third, controlling polymer dispense volume is much more difficult and must be precisely maintained for each lens. Fourth, lenses produced by jet dispense processes can have voiding problems as a result of trapped air bubbles. Last, accuracy of individual lens alignment on the lens wafer varies directly with the accuracy of the dispense process. Accordingly, there is a need for a method of fabricating lens elements that yields discrete lens wafers which mitigates against such drawbacks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an imager module. 
         FIGS. 2A-2B  illustrate an imager wafer assembly process. 
         FIGS. 3A-3B  illustrate a lens wafer assembly process. 
         FIGS. 4A and 4B  illustrate an imager module assembly process. 
         FIGS. 5A-5B  illustrate top and cross-sectional views, respectively, of steps of a method of making a stamp according to an embodiment described herein. 
         FIGS. 6A-6D  illustrate steps in a method of making a stamp, according to an embodiment described herein. 
         FIGS. 7A and 7B  illustrate steps in a method of making lens elements, according to an embodiment described herein. 
         FIGS. 8A and 8B  illustrate steps in a method of making lens elements, according to an embodiment described herein. 
         FIGS. 9A and 9B  illustrate top and cross-sectional views, respectively, of assembled imager modules constructed in accordance with an embodiment described herein. 
         FIG. 10  illustrates a lens stack structure containing lenses constructed in accordance with an embodiment described herein. 
         FIG. 11  illustrates a block diagram of a CMOS imaging device constructed in accordance with an embodiment described herein. 
         FIG. 12  depicts a system constructed in accordance with an embodiment described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustrations specific embodiments that may be practiced. It should be understood that like reference numerals represent like elements throughout the drawings. These example embodiments are described in sufficient detail to enable those skilled in the art to practice them. It is to be understood that other embodiments may be utilized, and that structural, material and electrical changes may be made, only some of which are discussed in detail below. 
     Embodiments described herein relate to a method of making a stamp having a mask pattern and methods of making discrete lenses on a wafer by using an ultraviolet replication process and the stamp. A method of forming a stamp is now described. Referring to  FIGS. 5A and 5B , to form stamp  300 , a mask  320  is formed on a glass substrate  310  and patterned to form a plurality of aperture openings  330   a - 330   f . Optional alignment marks  340   a ,  340   b  can also be formed on the mask  320 , depending on the alignment method chosen. The aperture openings  330   a - 330   f , although illustrated as circular in  FIG. 5A , may be rectangular or other shapes as necessary to correspond to a desired lens shape. 
     In one embodiment, the glass substrate  310  may comprise a float glass. One example of a float glass that may be used is a boro-float glass with a coefficient of thermal expansion between 2 and 5, such as Borofloat® 33 from Schott North America, Inc. The mask  320  can be deposited on the surface of the glass substrate  110  by any suitable method. The mask  320  can be formed of a metal, such as black chromium, or another appropriate light absorbing material, such as dark silicon or black matrix polymer, such as PSK™ 2000, manufactured by Brewer Science Specialty Materials, or JSR 812, manufactured by JSR Corporation. The aperture openings  330   a - 330   f  can be formed by photo patterning the mask  320  so that deposition of the light absorbing material does not occur on certain portions of the glass substrate  310 , or by removing light absorbing material from mask  320  using other suitable methods. The optional alignment marks  340   a ,  340   b  can be formed by the same methods used to form aperture openings  330   a - 330   f.    
     Referring now to  FIG. 6A , transparent material  410  is formed on the masked glass substrate  310  over the mask  320 . The transparent material  410  may be optionally bonded to the glass substrate by an adhesive agent, such as Hexamethyldisilazane (HMDS). The transparent material  410  can be any suitable material, such as a polymer, and need not have a high transparency ratio. In one embodiment, the transparent material  410  may be a material that is dissolvable in a weak solvent, for example, polyvinyl alcohol (PVA). In another embodiment, the transparent material  410  can be polydimethylsiloxane (PDMS). 
     As shown in  FIG. 6B , the stamp  300  is reoriented and lens features  430   a ,  430   b ,  430   c  of a lens master wafer  420  are aligned to the aperture openings  330   a ,  330   b , and  330   c  of the mask  320  ( FIG. 5A ). Lens master wafer  420  can be aligned to aperture openings  330   a ,  330   b , and  330   c  using the optional alignment marks  340   a ,  340   b , or, other suitable methods of alignment such as laser guiding can be used. As shown in  FIGS. 6C and 6D , the lens features  430   a ,  430   b ,  430   c  are pressed into the transparent material  410  to create lens cavities  440   a ,  440   b  and  440   c . Next, the lens master wafer  420  is removed and the transparent material  410  is cured. In another embodiment, the transparent material  410  may be a material that requires heating to soften it before lens master  420  can be pressed into it to create lens cavities  440   a ,  440   b  and  440   c.    
     While the embodiment described in  FIGS. 5A-6D  show cross-sectional views of a stamp  300  having six lens cavities (only three lens cavities  440   a - 440   c  are shown), it should be understood that, in practice, a stamp  300  may have tens, hundreds, or even thousands of lens cavities. It should also be understood that while the embodiments described in  FIGS. 5A-6B  detail the production of a single stamp  300 , in practice, many such stamps  300  could be produced at the same time. 
     A method of making a plurality of lens elements using the stamp  300  ( FIG. 6D ) is now described. As shown in  FIG. 7A , curable material  520  is applied to a lens wafer  510  and the lens wafer  510  is positioned under stamp  300  and optionally aligned with alignment marks  340   a ,  340   b . In one embodiment, the curable material  520  may be a low dispersion (Abbe number&gt;50) ultraviolet-curable resist or other hybrid polymer that requires curing, and may be optionally bonded to the wafer lens  510  by an adhesive agent, such as Hexamethyldisilazane (HMDS). One example of such an ultraviolet-curable hybrid polymer is Ormocomp® from Micro Resist Technology. 
     As shown in  FIG. 7B , stamp  300  is used to imprint curable material  520  into lenses  540   a ,  540   b ,  540   c  (as discussed above, stamp  300  has six lens cavities, thus, six lenses are formed but not all are shown). An ultraviolet source directs ultraviolet radiation  530  towards the glass substrate  310  of stamp  300 . The aperture openings  330   a ,  330   b , and  330   c  in mask  320  allow the ultraviolet radiation  530  to cure the lenses  540   a ,  540   b , and  540   c , while leaving a portion of uncured material between each lens  540   a - 540   c.    
     Referring now to  FIG. 8A , the lens wafer  510  is separated from the stamp  300 . In one embodiment, the stamp  300  and lens wafer  510  can be placed in a weak solvent bath to dissolve the transparent polymer material  410 , but not the cured lenses  540   a ,  540   b , and  540   c  or the uncured material  520  between them. In this embodiment, the glass substrate  310  and mask  320  used in the stamp  300  can be reused multiple times by forming a new layer of transparent material  410  on the mask  320  and imprinting lens cavities  440   a ,  440   b  and  440   c  with a lens master  420 . In another embodiment, transparent material  410  is not dissolved and stamp  300  can be mechanically separated from the lens wafer  510 . 
     In one embodiment, the uncured material  520  located between the lenses  540   a ,  540   b , and  540   c  is removed by a developer chemical  601 . One example of such a developer chemical is isopropyl alcohol. As shown in  FIG. 8B , the discrete lenses  540   a - 540   c  remain on the lens wafer  510 . 
     This particular method has several advantages over previous ultraviolet replication technology, due to the absence of polymer film present across the glass wafer: Bonding is done glass-to-glass (an exemplary illustration is shown in  FIG. 10 ), improving thickness uniformity and the dicing process for lens singulation is much easier since it can be done through plain glass. 
     The method proposed herein also has several key advantages over other known methods as well, such as a jet dispense process. First, the proposed process is high-throughput and multiple lenses are made in a single ultraviolet imprint. Second, the proposed process offers better control of residual polymer volume which resides outside the lens area because residual polymer (e.g. uncured material  520  shown on  FIG. 8A ) is removable. The residual polymer can be controlled by changing aperture size or stamp thickness. In a dispense approach the control of polymer volume is much more difficult and squeeze-out needs to be carefully maintained for each lens. Third, the present process does not have voiding problems due to trapped air bubbles. Last, lateral alignment accuracy is maintained very well. In a dispense approach lens alignment accuracy will depend on dispense alignment accuracy. 
       FIGS. 9A and 9B  are top down and cross-sectional views, respectively, of assembled imager modules, constructed in accordance with an embodiment described herein. As shown in  FIGS. 9A and 9B , the lens wafer  510  may be optically aligned with imager dies on an imager wafer  710  to form a plurality of imager modules  750   a - 750   f , which may then be separated into individual imager modules. Alternatively, the lens wafer  510  may be separated prior to being joined with imager dies. 
       FIG. 10  shows a cross-sectional view of an embodiment of an exemplary imaging module  1000  with a lens stack  1001  comprising lenses  1010  and  1020  produced by the methods described herein. Outer positive lens  1010  is mounted on lens wafer  1008  and separated from inner positive lens  1020  mounted on lens wafer  1028  by a transparent substrate  1015  and spacers  1039 . In another embodiment, transparent substrate  1015  can be a photographic filter, such as an ultraviolet, polarizing, or fluorescent filter. A spacer  1033  separates the lens stack  1001  from an imager wafer  1038  having an image sensor  106  in the image plane. Spacer  1033 , which can be formed of the same material as imager wafer  1038 , can be bonded directly to both imager wafer  1038  and lens wafer  1028  with a glass-to-glass bond. 
       FIG. 11  shows a block diagram of an imaging device  1100 , (e.g. a CMOS imager), that may be used in conjunction with a lens  540  according to embodiments described herein. A timing and control circuit  1132  provides timing and control signals for enabling the reading out of signals from pixels of the pixel array  106  in a manner commonly known to those skilled in the art. The pixel array  106  has dimensions of M rows by N columns of pixels, with the size of the pixel array  106  depending on a particular application. 
     Signals from the imaging device  1100  are typically read out a row at a time using a column parallel readout architecture. The timing and control circuit  1032  selects a particular row of pixels in the pixel array  106  by controlling the operation of a row addressing circuit  1034  and row drivers  1140 . Signals stored in the selected row of pixels are provided to a readout circuit  1042 . The signals are read from each of the columns of the array sequentially or in parallel using a column addressing circuit  1144 . The pixel signals, which include a pixel reset signal Vrst and image pixel signal Vsig, are provided as outputs of the readout circuit  1042 , and are typically subtracted in a differential amplifier  1160  and the result digitized by an analog to digital converter  1164  to provide a digital pixel signal. The digital pixel signals represent an image captured by an exemplary pixel array  106  and are processed in an image processing circuit  1168  to provide an output image. 
       FIG. 12  shows a system  1200  that includes an imaging device  1200  and a lens  540  constructed and operated in accordance with the various embodiments described above. The system  1200  is a system having digital circuits that include imaging device  1100 . Without being limiting, such a system could include a computer system, camera system, scanner, machine vision, vehicle navigation, video telephone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, or other image acquisition system. 
     System  1200 , e.g., a digital still or video camera system, generally comprises a central processing unit (CPU)  1202 , such as a control circuit or microprocessor for conducting camera functions that communicates with one or more input/output (I/O) devices  1206  over a bus  1204 . Imaging device  1000  also communicates with the CPU  1202  over the bus  1104 . The processor system  1200  also includes random access memory (RAM)  1210 , and can include removable memory  1215 , such as flash memory, which also communicates with the CPU  1202  over the bus  1204 . The imaging device  1100  may be combined with the CPU processor with or without memory storage on a single integrated circuit or on a different chip than the CPU processor. In a camera system, a lens  540  according to various embodiments described herein may be used to focus image light onto the pixel array  106  of the imaging device  1100  and an image is captured when a shutter release button  1222  is pressed. 
     While embodiments have been described in detail in connection with the embodiments known at the time, it should be readily understood that the claimed invention is not limited to the disclosed embodiments. Rather, the embodiments can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described. For example, while some embodiments are described in connection with a CMOS pixel imaging device, they can be practiced with any other type of imaging device (e.g., CCD, etc.) employing a pixel array or a camera using film instead of a pixel array. 
     Although certain advantages have been described above, those skilled in the art will recognize that there may be many others. For example, the steps in the methods described herein may be performed in different orders, or may include some variations, such as alternative materials having similar functions. Furthermore, while the substrate and stamps are described above in various embodiments as being transparent, alternate embodiments are possible in which the substrate and stamps are opaque and an alternate form of radiation to ultraviolet is used to cure the lenses. Accordingly, the claimed invention is not limited by the embodiments described herein but is only limited by the scope of the appended claims.