US20110180695A1 - Layered lens structures and methods of production - Google Patents

Abstract

A microlens structure includes lower lens layers on a substrate. A sputtered layer of glass, such as silicon oxide, is applied over the lower lens layers at an angle away from normal to form upper lens layers that increase the effective focal length of the microlens structure. The upper lens layers can be deposited in an aspherical shape with radii of curvature longer than the lower lens layers. As a result, small microlenses can be provided with longer focal length. The microlenses are arranged in arrays for use in imaging devices.

Description

CROSS REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of U.S. application Ser. No. 11/513,262, filed Aug. 31, 2006, which application is a divisional of U.S. application Ser. No. 11/244,101, filed Oct. 6, 2005, which issued on Apr. 3, 2007 as U.S. Pat. No. 7,199,347, which application is a divisional application of U.S. application Ser. No. 10/740,597, filed Dec. 22, 2003, which issued on Apr. 17, 2007 as U.S. Pat. No. 7,205,526, the disclosures of which are incorporated by reference herein.
FIELD OF THE INVENTION
• The present invention relates generally to improved lens structures, and in particular to a microlens system for an imager or display array.
BACKGROUND OF THE INVENTION
• The semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), complementary metal-oxide semiconductor (CMOS) active pixel sensors (APS), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others, in which an array of microlenses causes incident light to converge toward each of an array of pixel elements. Semiconductor displays using microlenses have also been developed.
• Microlenses are manufactured using subtractive processes and additive processes. In an additive process, lens material is formed on a substrate, patterned and subsequently formed into microlens shapes.
• In conventional additive microlens fabrication, an intermediate material is patterned on a substrate to form a microlens array using a reflow process. Each microlens is separated by a minimum distance from adjacent microlenses, typically no less than 0.3 micrometers. Distances less than 0.3 micrometers may cause unwanted bridging of neighboring microlenses during reflow. In the known process, each microlens is patterned as a single shape, typically square, with gaps around it. Heat is applied during the subsequent step of reflowing, which causes the patterned microlens material to form a gel drop 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, they may touch and merge, or bridge, into one larger 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 also allow unfocused photons through the empty spaces in the microlens array, leading to increased cross-talk between respective photosensors of adjacent pixel cells.
• In addition, 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 a photosensitive region. This problem is due in part to the increased difficulty in constructing a smaller microlens that has the optimal focal length 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 the distortion created by multiple layered regions above the photosensitive area, for example, color filter regions, which results in increased crosstalk between adjacent pixels. Consequently, smaller imagers with untuned or nonoptimized microlenses do not achieve optimal color fidelity and signal/noise ratios.
• It would be advantageous to have improved microlens structures and techniques for producing them.
BRIEF SUMMARY OF THE INVENTION
• Exemplary embodiments of the invention provide a microlens structure having at least two differing layers which together produce a desired microlens characteristic. In a two-layer exemplary embodiment, for example, the top layer can have a different shape than the bottom layer, thus obtaining a desired focal property. The top layer can be formed by off-angle deposition, e.g., sputtering, of a transparent glassy material, such as a silicon oxide, over a pre-formed lower layer.
• The invention also provides methods of producing microlenses. An exemplary method embodiment includes forming a bottom layer with precursor microlens material such as by photoresist reflow. A top layer is deposited over the precursor microlens material using a glass-forming oxide, for example. Deposition takes place by sputtering the oxide at an angle off normal by about 45°-60°. As a result of depositing the glass at an angle off normal, glass is deposited in greater amounts around the peripheral edges of the precursor material, thereby changing the shape and increasing the effective focal length of the lenses. According to one exemplary two-layer embodiment the resulting shape is aspherical.
• These and other features and advantages of various embodiments of the invention will be better understood from the following detailed description, which is provided in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 illustrates a cross sectional view of a portion of a microlens structure in accordance with an exemplary embodiment of the invention, and represents a cross-section taken along line I-I inFIG. 6;
• FIG. 2 illustrates a top view of a portion of theFIG. 1 embodiment;
• FIG. 3 illustrates the focal lengths of lower and upper lens regions of the microlens structure ofFIG. 1;
• FIG. 4 is a schematic illustration of an apparatus for manufacturing a microlens structure according to an exemplary embodiment of the present invention;
• FIG. 5 is a cross-section illustrating a method of manufacturing a microlens structure with the apparatus ofFIG. 4;
• FIG. 6 shows a block diagram of an imager integrated circuit (IC) in accordance with an exemplary embodiment of the invention
• FIG. 7 is a schematic diagram of a processor system with an imager IC as inFIG. 6.
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, with the exception of steps necessarily occurring in a certain order.
• The term “wafer” or “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 or insulating structures in, on, or at a surface of which circuitry or optical or electrical devices can be formed. Furthermore, when reference is made to a “wafer” 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, a semiconductor wafer or substrate need not be silicon-based, but could be based on silicon-germanium, germanium, gallium arsenide or other semiconductors.
• The term “pixel,” as used herein, refers to a picture element unit cell containing a photosensor and other components for converting electromagnetic radiation to an electrical signal. For purposes of illustration, a representative CMOS imager pixel cell is illustrated in the figures and description herein. However, this is just one example of the types of imagers and pixel cells with which the invention may be used. The invention may also be used to create microlens arrays for display devices.
• The term “microlens” refers herein to one of an array of optical components over an array of photosensors or photoemitters. In an imager array each microlens tends to focus incident light toward a respective photosensor. A microlens array may be part of a layered structure formed over a substrate using photolithographic techniques. Various processes have been developed for producing microlenses, including fluid self-assembly, droplet deposition, selective curing in photopolymer by laser beam energy distribution, photoresist reflow, direct writing in photoresist, grayscale photolithography, and modified milling. These processes are described in more detail in U.S. Pat. No. 6,473,238 to Daniell, the disclosure of which is incorporated herein by reference.
• While the invention is described with particular 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 includes a microlens, especially one that requires high quality microlenses for optimized performance. Other exemplary micro-optical devices that can include microlenses include CCDs and display devices, as well as others.
• Referring initially toFIGS. 1 and 2, an exemplary embodiment of an imager array2 is shown illustratively in cross sectional and top views, respectively. A plurality of microlens structures is provided, each having a lower lens region4 and an upper lens region6. The microlens structures are provided over passivation layer8, intervening layer10 (e.g., color filter array, metallization, etc.), and an array ofimaging pixels12, with one microlens structure over eachpixel12. Eachpixel12 includes a photosensor for converting photons to free electrical charges, and the array2 also includes structures that obtain electrical signals based on charge levels. Each pixel's microlens is structured in at least two layers, e.g., layers4 and6 shown inFIGS. 1 and 2, to increase the pixel's light collection efficiency.
• In the illustrated embodiment ofFIGS. 1 and 2, the two lens layers4,6 in each microlens structure cause light from a larger arc to converge onto a light sensitive photosensor of arespective pixel12 and to lengthen the effective focal length of each microlens structure. Lower lens layer4 covers a smaller area ofsubstrate14 than upper lens layer6 as shown inFIG. 2, so that alight ray16 is deflected onto the photosensor ofpixel12 from outside the area ofpixel12, increasing the percentage of incident light that reaches a corresponding photosensor of apixel12. In addition, upper layer6 has a shape that results in different focal properties than lower layer4.
• In the illustrated embodiment, the upper surface of upper layer6 has radii of curvature longer than the substantially uniform radius of curvature at the upper surface of lower layer4. As a result, the effective (or average) focal length of each microlens structure is longer than if both layers had the same shape.FIG. 3 illustrates schematically a focal length A of lower layer4 alone, as compared to a longer focal length B of combined layers4 and6. More generally, the upper surface of layer6 can have a shape that improves efficiency by distributing light onto the photosensor of a pixel in a way that improves conversion of photons to free charge carriers.
• The lens layers4,6 can be formed into various symmetrical geometric shapes, such as circles, squares, etc., and asymmetrical shapes to provide a path for incident light rays to reach the photo sensors of thepixels12.FIG. 2 shows the lower lens layer4 as having a substantially circular cross-section (FIG. 1). Lens layer6 has a rounded, rectangular perimeter. It should be understood, however, that a variety of shapes for each of layers4 and6 may be used in embodiments of the invention, as discussed below.
• Referring again toFIG. 2, upper lens layer6 has an aspherical shape with radial dimensions larger than that of spherical lens layer4. In the illustrated embodiment, lens layer6 extends horizontally to a boundary between adjacent microlens structures. Because lens layer6 is aspherical in shape, the radius of curvature of its upper surface varies with orientation, being near its minimum in the cross section ofFIG. 1 and near its maximum along a diagonal cross section (not shown). At all orientations, the radius of curvature is significantly longer than that of lower lens layer4. As a result, an effective focal length of the lens structure, made up of lens layers4 and6, is longer than an effective focal length of lens layer4 alone.
• Lens layers4 illustratively are substantially spherical and can be formed using a photo resist reflow technique, as is known to those of skill in the art for forming microlenses. The lens layers4 illustratively are formed from a layer of microlens material, such as photo resist, referred to herein as a “precursor microlens material.” Other inorganic, as well as organic and organic-inorganic hybrid materials, also could be used. The precursor microlens material is illustratively coated and patterned upon the passivation layer8. After patterning, a portion of the material over each pixel has a substantially rectangular or circular configuration and each portion is substantially equal in size with the others. Upon reflow, the precursor microlens material hardens and preferably is impervious to subsequent reflow processes. As a result of the reflow process, the patterned precursor microlens material is transformed into lens layers4. The lens layers4 each have a substantially circular perimeter configuration with a spherically curved profile.
• The layer8 upon which the lens layers4 are formed can be any suitable material that is transparent to electromagnetic radiation in the relevant wavelength range. The lens layers4, which are also transparent to electromagnetic radiation in the relevant wavelength range, will retain their shape even if a subsequent reflow process is performed. As shown inFIG. 2, there are spaces between the lens layers4 of adjacent microlenses.
• After patterning and reflowing the precursor microlens material to form lower lens layers4, upper lens layers6 are formed. Lens layers6 are deposited over lens layers4 by an off-angle deposition process, illustrated inFIGS. 4 and 5. In an exemplary embodiment, an SiO₂beam20 is supplied from a sputtering source22 through acollimator24. The SiO₂beam is directed toward a rotatingplatform26.Platform26 supports thesubstrate14, on which only two of lens layers4 are shown in cross-section inFIG. 5, formed on layer8
• Platform26 rotates relative to sputtering source22 as indicated by the arrows inFIG. 4. Collimated SiO₂is directed toward the rotatingplatform26, and condenses as a glass to form lens layers6 deposited directly on lens layers4. The speed of rotation ofplatform26 can be varied to allow for sufficient deposition of material without disturbing the integrity of the deposited glass.
• SiO₂beam20 is directed at an angle ✓ away from normal such that most of the glass deposition takes place around the perimeters or peripheral edges of the lens layers4 and little is deposited at the tops or central surfaces. The angle ✓ can range between about 0° and about 90°, and preferably is between about 45° and about 60°. Accordingly, the layer of deposited glass on lens layers6 is thicker toward the bottoms of lens layers4, near layer8, than it is toward the tops of lens layers4. To obtain a rectangular shape as inFIG. 2, the speed of rotation ofplatform26 or the rate of emission from source can be varied as a function of orientation ofplatform26. Consequently, the shape of the lens structures is changed from that of spherical lens layers4 such that horizontal aspherical radial dimensions of the lens layers6 are larger than a horizontal radius of the lens layers4. As a result of the off-angle deposition, the radius of curvature of the upper surfaces is increased, so that the effective focal length of the lens structures is increased from that of lens layers4.
• Various materials can be used for both the lens layers4 and6. Exemplary materials for lens layers6 are those that provide a substantially transparent layer and are amenable to physical vapor deposition. In addition to SiO₂, exemplary materials include nitrides such as Si₃N₄, borophosphosilicate glass (BPSG), phosphosilicate glass (PSG), and zinc selenide. Advantageously, a refractive index of the deposited lens layers and the lens layers4 will be substantially identical to minimize loss of incident light that otherwise would occur as the result of reflections from the interface between layers6 and4.
• Layers6 also provide a protective layer for later processes, and can have excellent optical properties. In particular layers6 can have lower absorption than lower layers4 formed of an organic microlens material. Further, the layers can protect organic microlenses4 to prevent cracking, oxidation, aging during high temperature baking processes, and physical or chemical attack in subsequent processes, for example.
• Advantageously, deposition continues until gaps between the lens layers4 are substantially filled with glass, thereby increasing the area of coverage of each lens. Consequently, a greater portion of light incident upon the lens structure array is captured and focused towardpixels12. The deposition process may take several minutes, for example, depending on the rate of deposition, desired thickness, subsequent processing requirements, etc. Typically, deposition takes place at least until gaps between individual lens in the lower layers4 are filled. Exemplary, non-limiting thicknesses of the resulting lens layers6 can be in the range of 0.1-2.0 micrometers, most preferably 0.4-0.8 micrometers, for example.
• FIG. 6 illustrates a block diagram of an imager integrated circuit (IC)308 having apixel array200 containing a plurality of pixels arranged in rows and columns. A cross-section taken along line I-I inarray200 would be the same as the cross-section illustrated inFIG. 1. In other words,pixel array200 includes at least one microlens structure, illustratively with components as inFIGS. 1 and 2, formed over an associated pixel cell. The pixels of each row inarray200 are all turned on at the same time by a row select line, and the pixels of each column are selectively output by respective column select lines. The row lines are selectively activated by arow driver210 in response torow address decoder220. The column select lines are selectively activated by acolumn selector260 in response tocolumn address decoder270.
• Thepixel array200 is operated by the timing and control circuit250, which controlsaddress decoders220,270 for selecting the appropriate row and column lines for pixel signal readout. The pixel column signals, which illustratively include a pixel reset signal (Vrst) and a pixel image signal (Vsig), are read by a sample and hold circuit261 associated with thecolumn selector260. A differential signal (Vrst−Vsig) is produced bydifferential amplifier262 for each pixel, and the differential signal is amplified and digitized by analog to digital converter (ADC)275. ADC275 supplies the digitized pixel signals to an image processor280 which can perform image processing before providing image output signals.
• Imager IC308 can be a CMOS imager or CCD imager, or can be any other type of imager that includes a microlens structure.
• FIG. 7 showssystem300, a typical processor based system modified to include animager IC308 as inFIG. 6. Processor based systems exemplify systems of digital circuits that could include animager IC308. Examples of processor based systems include, without limitation, computer systems, camera systems, scanners, machine vision systems, vehicle navigation systems, video telephones, surveillance systems, auto focus systems, star tracker systems, motion detection systems, image stabilization systems, and data compression systems for high-definition television, any of which could utilize the invention.
• System300 includes animager IC308 having the overall configuration depicted inFIG. 6 witharray200 including a microlens structure in accordance with any of the various embodiments of the invention.System300 includes aprocessor302 having a central processing unit (CPU) that communicates with various devices over abus304. Some of the devices connected to thebus304 provide communication into and out of thesystem300; an input/output (I/O)device306 andimager IC308 are examples of such communication devices. Other devices connected to thebus304 provide memory, illustratively including a random access memory (RAM)310,hard drive312, and one or more peripheral memory devices such as afloppy disk drive314 and compact disk (CD)drive316. Theimager IC308 may receive control or other data fromCPU302 or other components ofsystem300. Theimager IC308 may, in turn, provide signals defining images toprocessor302 for image processing, or other image handling operations.
• While exemplary embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.

Claims (9)

1-19. (canceled)

20. A processor based system comprising:

an imager integrated circuit substrate with a microlens array thereover, the microlens array comprising a lower lens layer, said lower lens having a first focal length; and an upper lens layer over the lower lens layer, said upper and lower lenses having a combined second focal length different from said first focal length, and wherein the upper lens layer has a curvature having a first radius and the lower lens layer has a curvature having a second radius, wherein the first and second radii are different from one another and the first and second curvatures extend in the same direction, the microlens array supported by the imager IC substrate;

a central processing unit in communication with the imager IC, said communication occurring over a system bus;

a system memory in communication with the central processing unit and imager IC over the system bus; and

an input/output device connected to and in communication with the system bus.

21. The processor based system ofclaim 20 wherein there is an intervening layer between the imager integrated circuit substrate and the microlens array.

22. The processor based system ofclaim 20 wherein the lower lens layer of the microlens array is supported by a microlens array substrate interposed between the lower lens layer and the imager integrated circuit substrate.

23. The processor based system ofclaim 20, wherein the processor based system is selected from the group consisting of computer systems, camera systems, scanners, machine vision systems, vehicle navigation systems, video telephones, surveillance systems, auto focus systems, star tracker systems, motion detection systems, image stabilization systems, and data compression systems for high-definition television.

24. The processor based system ofclaim 20 wherein the imager integrated circuit substrate has a pixel array wherein the pixel array is arranged in rows and columns.

25. The processor based system ofclaim 24 wherein the pixel array is operated by a control circuit.

26. The processor based system ofclaim 20 wherein the imager integrated circuit substrate is selected from the group consisting of a CMOS imager and a CCD imager.

27. The process based system ofclaim 20 wherein the imager integrated circuit further comprises an image processor that processes an image before output from the imager integrated circuit to the system bus.

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