Abstract:
A microlens has a surface with an effective index of refraction close to the index of air to reduce reflection caused by change in indices of refraction from microlens to air. The microlens having an index of refraction approximately the same as that of air is obtained by providing a rough or bumpy lens-air surface on the microlens. Features protrude from the surface of a microlens to create the rough surface and preferably have a length of greater or equal to a wavelength of light and a width of less than a sub-wavelength of light, from about 1/10 to ¼ of the wavelength of light. The features may be of any suitable shape, including but not limited to triangular, cylindrical, rectangular, trapezoidal, or spherical and may be formed by a variety of suitable processes, including but not limited to mask and etching, lithography, spray-on beads, sputtering, and growing.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to a microlens for use in a CMOS image sensor and in particular to a microlens having reduced microlens surface reflection, and a method for producing the same. 
   BACKGROUND OF THE INVENTION 
   Microlenses are used to funnel light of a larger area into a photodiode of an imager pixel, for example. Microlenses also can be used to trap light into a solar cell, as well as to project light from a light-producing component of a display. Advanced products and systems that utilize microlenses in these and other similar ways include, without limitation, digital cameras, flat-panel visual displays, and solar panels. Such products and systems are used in a wide variety of applications ranging from mobile phone displays and flat-screen televisions to mapping the solar system, and beyond. 
   The direction that light is propagated through two media, such as air and a lens, is based on the relationship between the refractive indices of the media. Snell&#39;s Law (Eq. 1) relates the indices of refraction n of the two media to the directions of propagation in terms of angles to the normal:
 
n 1  sin θ 1 =n 2  sin θ 2   (1)
 
   The index of refraction (n) is defined as the speed of light in vacuum (c) divided by the speed of light in the medium (v), as represented by Eq. 2:
 
 n=c/v   (2)
 
   The refractive index of a vacuum is 1.000. The refractive index of air is 1.000277. Representative materials used in microlens and semiconductor device fabrication include oxide, with a refractive index of 1.45, and nitride, with a refractive index of 2.0.  FIG. 1  illustrates the relationship between the indices of refraction at the air-microlens interface. The graph on the right side of  FIG. 1   a  shows a constant index of refraction in the air and a different constant index of refraction at all depths of the microlens, and therefore a sharp increase in the index of refraction at the air-microlens interface. 
   When light travels from a medium with a low refractive index, such as air, to a medium with a high refractive index (the incident medium), such as nitride, the angle of light with respect to the normal will increase. In addition, some light will be reflected. This will reduce the efficiency of the imaging system, since not all of the light hitting the lens will travel through the lens to the photodiode, for example. 
   Reflection at the interface of two different media can be quantified by the following formula (Eq. 3):
 
 R =( n   1   −n   2 ) 2 /( n   1   +n   2 ) 2   (3)
 
   Therefore, reflection from the interface between the two media can be reduced by matching their indices of refraction as closely as possible. As noted above, the refractive index of oxide is significantly closer to 1.0 than that of nitride. By providing an outer layer on a lens having an index of refraction closer to that of the surrounding medium, such as that of air, reflection is reduced and the efficiency and accuracy of the lens is improved. 
   Thus, it would be useful to have a microlens having a graded refractive index profile to reduce light reflection. 
   BRIEF SUMMARY OF THE INVENTION 
   Exemplary embodiments of the invention provide a microlens having a surface with an index of refraction close to the index of air to reduce reflection caused by the sharp reflective index change from microlens to air. A gradual index change is obtained at the surface by providing a microlens having a rough or bumpy lens-air surface. Features protrude from the surface of a microlens to create the rough surface and preferably have a length of greater or equal to a wavelength of light and a width of less than a sub-wavelength of light, from about 1/10 to ¼ of the wavelength of light. The wavelength is in the range of 400 nm to 700 nm, however the performance of the invention is not necessarily related to the visible wavelength of light. The features may be of any suitable shape, including but not limited to triangular, cylindrical, rectangular, trapezoidal, or spherical and may be formed by a variety of suitable processes, including but not limited to mask and etching, lithography, spray-on beads, sputtering, and growing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other advantages and features of the invention will be more clearly understood from the following detailed description which is provided in connection with the accompanying drawings. 
       FIG. 1   a  is a graph of depth vs. indices of refraction at the air-microlens interface of a microlens of prior art; 
       FIG. 1   b  is a graph of depth vs. indices of refraction at the air-microlens interface of a microlens according to an exemplary embodiment of the invention; 
       FIG. 2   a  is a cross-sectional view of a microlens according to an exemplary embodiment of the invention; 
       FIG. 2   b  is a cross-sectional view of a plurality of microlenses shown in  FIG. 2   a;    
       FIG. 2   c  is a three-dimensional close-up view of the microlens of  FIG. 2   a;    
       FIG. 3  is a cross-sectional view of a microlens according to another exemplary embodiment of the invention; 
       FIG. 4  is a cross-sectional view of a microlens according to another exemplary embodiment of the invention; 
       FIG. 5   a  is a cross-sectional view of a microlens according to another exemplary embodiment of the invention; 
       FIG. 5   b  is a cross-sectional view of a microlens according to another exemplary embodiment of the invention; 
       FIG. 6  is a schematic of an imaging device using a pixel having a microlens constructed in accordance with an embodiment of the invention; and 
       FIG. 7  illustrates a schematic of a processing system including the imaging device of  FIG. 6 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In the following detailed description, reference is made to the accompanying drawings which form a part hereof and illustrate specific exemplary embodiments by which the invention may be practiced. It should be understood that like reference-numerals represent like-elements throughout the drawings. 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 term “substrate” is to be understood as including 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 “substrate” in the following description, previous process steps may have been utilized to form regions or junctions in the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, germanium, or gallium arsenide, for example. 
   The term “light” refers to electromagnetic radiation that can produce a visual sensation (visible light) as well as electromagnetic radiation outside of the visible spectrum. In general, light as used herein is not limited to visible radiation, but refers more broadly to the entire electromagnetic spectrum, particularly electromagnetic radiation that can be converted by a solid state photosensor into a useful signal. 
   Referring now to the drawings, where like elements are designated by like reference numerals,  FIG. 2   a  illustrates a cross-section of a microlens  10  in an exemplary embodiment of the invention.  FIG. 2   b  illustrates a cross-section of a plurality of such microlenses  10 . The microlens  10  is formed above a pixel cell  4 , which is formed in a substrate  5 . There may be one or more intermediate layers between the substrate  5  and microlens  10 , including, but not limited to, interlayer dielectric layers, metal layers, passivation and/or insulation layers. These intermediate layers are represented in  FIG. 2   a  as layer  6 . 
   Bumps  20 , or grating, are formed on the surface of microlens  10 . The bumps  20  have a trapezoidal shape. The bumps  20  preferably have a height h 1  equal to or greater than a wavelength of light. The wavelength of light is in the range of approximately 400 nm to 700 approximately nm. The bumps  20  preferably have a width w 1  along the x- and z-axes, as shown in the close-up three-dimensional illustration of  FIG. 2   c . Width w 1  is much less than a subwavelength of light, preferably in the range of approximately 10 nm to approximately 100 nm. The bumps  20  may be spaced apart by s 1 , wherein s 1  is preferably in the range of approximately 10 nm to approximately 100 nm. For ease of explanation, the bumps  20  that are located behind the section plane of  FIG. 2   a  are not shown. In the preferred embodiment, however, the bumps are located across the entire curved surface of the microlens  10 . Thus, the spacing s 1  separates the bumps  20  that are shown in  FIG. 2   a  from those located (but not shown) in front of and behind the illustrated bumps. 
   The bumps  20  may comprise any suitable material such as oxides, nitrides, and metals. Light transmissive materials are more preferable. 
   The bumps  20  may be formed by methods such as depositing the material and ion-etching the trapezoidal shape over the microlens  10 . Forming the bumps  20  by etching provides accurate control in shaping the bumps, however other methods are also possible. For example, the bumps  20  may also be formed by depositing a photoresist over the material and etching by photolithography. If formed by photolithography, the bumps  20  would have a parallel orientation relative to each other, as represented by bumps  40  in  FIG. 4 . The bumps  20  may also be formed by preparing the microlens  10  surface with a plasma vapor deposition process, spin developing, or spin wetting to deposit the material and growing the bumps  20 . The bumps  20  may also be formed by forming studs in the microlens  10  and etching away the surface of the microlens to expose the bumps  20 . 
     FIG. 3  illustrates a cross-section of microlens  10  in another exemplary embodiment of the invention. In this embodiment, the microlens  10  features bumps  30  having a triangular shape. Bumps  30  preferably have a height h 2  equal to or greater than a wavelength of light. The wavelength of light is in the range of approximately 400 nm to approximately 700 nm. The bumps  30  preferably have a width w 2  along the x- and z-axes, as with bumps  20  of  FIG. 2   c . Width w 2  is much less than a subwavelength of light, preferably in the range of approximately 10 nm to approximately 100 nm in the x- and z-axes of said features. The bumps  30  may be spaced apart by s 2 , wherein s 2  is preferably in the range of approximately 10 nm to approximately 100 nm. 
   Like the bumps  20  of  FIG. 2   a , the bumps  30  of  FIG. 3  may comprise any suitable material such as oxides, nitrides, and metals; light transmissive materials are more preferable and materials such as metals are less preferable. The bumps  30  may be formed by methods such as depositing the material and etching the triangular shape over the microlens  10 ′. Forming the bumps  30  by etching provides accurate control in shaping the bumps, however other methods are also possible. The bumps  30  may also be formed by depositing a photoresist over the material and etching by photolithography. If formed by photolithography, the bumps  30  would have a parallel orientation relative to each other, as represented by bumps  40  in  FIG. 4  (described below). The bumps  30  may also be formed by preparing the microlens  10 ′ surface with a plasma vapor deposition process, spin developing, or spin wetting to deposit the material and growing the bumps  30 . The bumps  30  may also be formed by forming studs in the microlens  10 ′ and etching away the surface of the microlens to expose the bumps  30 . 
     FIG. 4  illustrates a cross-section of microlens  10 ″ in another exemplary embodiment of the invention. In this embodiment, the microlens  10 ″ features bumps  40  having a rectangular shape and are arranged in an approximately parallel configuration. Bumps  40  preferably have a height h 3  equal to or greater than a wavelength of light. The wavelength of light is in the range of approximately 400 nm to approximately 700 nm. The bumps  40  preferably have a width w 3  along the x- and z-axes, as with bumps  20  of  FIG. 2   c . Width w 3  is much less than a subwavelength of light, preferably in the range of approximately 10 nm to approximately 100 nm. The bumps  40  may be spaced apart by s 3 , wherein s 3  is preferably in the range of approximately 10 nm to approximately 100 nm. 
   Like the bumps  20  and  30  of  FIGS. 2   a  and  3 , respectively, the bumps  40  of  FIG. 4  may comprise any suitable material such as oxides, nitrides, and metals; light transmissive materials are more preferable and materials such as metals are less preferable. The bumps  40  may be formed by depositing a photoresist over the material and etching by photolithography. Using this method, the bumps  40  may also be formed perpendicular to the surface of the microlens  10 ″, as represented by bumps  10  in  FIG. 2 . Forming the bumps  40  by photolithography requires few processing steps and accurate control in shaping rectangular bumps, however other methods are also possible. For example, the bumps  40  may be formed by methods such as depositing the material and etching the rectangular shape over the microlens  10 ″. The bumps  40  may also be formed by preparing the microlens  10 ″ surface with a plasma vapor deposition process, spin developing, or spin wetting to deposit the material and growing the bumps  40 . The bumps  40  may also be formed by forming studs in the microlens  10 ″ and etching away the surface of the microlens to expose the bumps  40 . 
     FIG. 5   a  illustrates a cross-section of microlens  10 ′″ in another exemplary embodiment of the invention. In this embodiment, the microlens  10 ′″ features bumps  50  having a spherical shape. Because the bumps  50  are spherical in shape, the height to wavelength and width to subwavelength ratios (and thus the height to width ratio) of the embodiments illustrated in  FIGS. 2-4  are not attainable and therefore cannot lower the index of refraction of the surface of the microlens  10 ′″ as much as the other embodiments. However, the embodiment of  FIG. 5   a  has the advantage of ease of processing. The bumps  50 , like the bumps  20 ,  30 , and  40  of  FIGS. 2-4 , may comprise any suitable material such as oxides, nitrides, and metals; light transmissive materials are more preferable and materials such as metals are less preferable. They may also be formed using the same methods of formation as described above with respect to bumps  20 ,  30  and  40 . However, the bumps  50  may also be sprayed and adhered to the surface of the microlens  10 ′″, which is a simpler processing method than any of the aforementioned processes for the formation of spherical bumps  50 . 
   The bumps  50  have a diameter d 1  in the range of approximately 100 nm to approximately 500 nm, preferably in the range of approximately 200 nm to 300 nm. The bumps  50  are preferably spaced apart by s 4 , wherein s 4  is preferably in the range of approximately 100 nm to approximately 500 nm. 
   Another embodiment employing bumps having a spherical shape is shown in  FIG. 5   b . The bumps  60 ,  70 ,  80  have diameters d 1 , d 2 , d 3  in the range of approximately 100 nm to approximately 500 nm, preferably in the range of approximately 200 nm to 300 nm. 
   The formation of bumps on the surface of a microlens creates a surface with an index of refraction close to the index of air to reduce reflection caused by the sharp reflective index change from microlens to air. A gradual index change is obtained at the surface by providing a microlens having a rough or bumpy lens-air surface. Therefore, reflection from the interface between the two media can be reduced by matching their indices of refraction as closely as possible. By providing an outer layer on a lens having an index of refraction closer to that of the surrounding medium, such as that of air, reflection is reduced and the efficiency and accuracy of the lens is improved. 
     FIG. 6  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  201  comprising a plurality of pixels  4  with microlenses  10  (or  10 ′,  10 ″,  10 ′″,  10 ″″) 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. 7  shows system  300 , a typical processor system modified to include the imaging device  200  ( FIG. 6 ) 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. 
   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. 
   Although the above discussion describes the bumps as being formed in trapezoidal, triangular, rectangular, and spherical, it should be noted that the bumps and their formation are not limited to such embodiments. Other materials and methods may be used to form the bumps that are used to lower the index of refraction of the surface of the microlens. 
   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.