Abstract:
A method of fabricating a microlens includes forming layer of photoresist on a substrate, patterning the layer of photoresist, and then reflowing the photoresist pattern. The layer of photoresist is formed by coating the substrate with liquid photoresist whose viscosity is 150 to 250 cp. A depth sensor includes a substrate and photoelectric conversion elements at an upper portion of the substrate, a metal wiring section disposed on the substrate, an array of the microlenses for focusing incident light as beams onto the photoelectric conversion elements and which beams avoid the wirings of the metal wiring section. The depths sensor also includes a layer presenting a flat upper surface on which the microlenses are formed. The layer may be a dedicated planarization layer or an IR filter, interposed between the microlenses and the metal wiring section.

Description:
PRIORITY STATEMENT 
       [0001]    This application claims priority under 35 U.S.C. §119(a) from Korean Patent Application No. 10-2011-0019001 filed on Mar. 3, 2011, the disclosure of which is hereby incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    The inventive concept relates to range finding and/or range imaging and more particularly, to depth sensors of range finders and/or range imaging cameras such as time-of-flight (TOF) cameras. The inventive concept also relates to microlenses and to a method of fabricating the same. 
         [0003]    Range finding is a technique of measuring the distance to a target without using physical means to do so, and range imaging is a technique used to produce an image of a scene. Typical range imaging cameras, for example, include an illumination system for illuminating a subject, such as a person, and depth pixels for collecting light reflected from the subject and producing data representative of an image of the subject. A time-of-flight camera (TOF camera) is a range imaging system that has the ability to produce a 3-D image of a scene. To this end, the depth sensor of a TOF camera may include a light source for illuminating the subject (the scene), and depth pixels which receive light reflected from the subject and generate photons in response to incident light. The photons are sensed to provide information on the distance between the camera and the subject. That is, the generation of the photons allows the time of flight of the light to be measured, and the distance between the subject and the camera can be calculated based on the time of flight and the known speed of light. Thus, the performance of a depth sensor is crucial to the precise calculation of the distance between the camera and the subject and/or is crucial to the quality of the image of the subject that can be produced. 
       SUMMARY 
       [0004]    According to an aspect of the inventive concept, there is provided a method of fabricating a microlens, the method comprising: forming a layer of photoresist on a substrate including by coating the substrate with liquid photoresist whose viscosity is 150 cp to 250 cp, patterning the layer of photoresist to form a photoresist pattern, and reflowing the photoresist pattern. 
         [0005]    According to a similar aspect of the inventive concept, there is provided a microlens formed by forming a layer of photoresist on a substrate including by coating the substrate with liquid photoresist whose viscosity is 150 cp to 250 cp, patterning the layer of photoresist to form a photoresist pattern, and reflowing the photoresist pattern. 
         [0006]    According to another aspect of the inventive concept, there is provided a depth pixel comprising: a microlens of photoresist that focuses rays of light incident thereon, and a photoelectric element that converts light received thereby to electric charges. The photoelectric element is positioned relative to the microlens so as to receive rays of light incident on and focused by the microlens. 
         [0007]    According to still another aspect of the inventive concept, there is provided a depth sensor comprising: an array of depth pixels, and a processor operatively connected to the array of depth pixels. Each of the depth pixels of the array includes a microlens of photoresist that focuses rays of light incident thereon, and a photoelectric element that converts light received thereby to electric charges. Furthermore, each photoelectric element of the array is positioned relative to a respective microlens in the array so as to receive rays of light incident on and focused by the respective microlens. The processor is configured to calculate a distance between a target and the depth sensor based on signals output by the array of depth pixels. The distance may be used to produce a 3D image of the target. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The above and other features and advantages of the inventive concept will become more apparent from the detailed description of the preferred embodiments thereof that follows as made with reference to the attached drawings in which: 
           [0009]      FIGS. 1A through 1C  are perspective views of stages, respectively, in a method of fabricating a microlens according to the inventive concept; 
           [0010]      FIG. 2  is a block diagram of a depth sensor including a microlens according to the inventive concept; 
           [0011]      FIG. 3  is a schematic cross-sectional view of a depth pixel of the depth sensor of  FIG. 2 ; 
           [0012]      FIG. 4  is a flowchart of the method of fabricating a microlens according to the inventive concept; 
           [0013]      FIG. 5  is a graph of result of simulations conducted using a conventional depth sensor and a depth sensor according to the inventive concept; 
           [0014]      FIG. 6  is a block diagram of an image processing system including a color image sensor and a depth sensor according to inventive concept; and 
           [0015]      FIG. 7  is a block diagram of a signal processing system including a depth sensor according to the inventive concept. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Various embodiments and examples of embodiments of the inventive concept will be described more fully hereinafter with reference to the accompanying drawings. Like numerals are used to designate like elements throughout the drawings. 
         [0017]    Other terminology used herein for the purpose of describing particular examples or embodiments of the inventive concept is to be taken in context. For example, the terms “comprises” or “comprising” when used in this specification specifies the presence of stated features or processes but does not preclude the presence or additional features or processes. 
         [0018]    A method of fabricating a microlens according to the inventive concept will now be described with reference to  FIGS. 1A through 1C , and  4 . In general, a microlens is any small lens whose diameter is less than a millimeter (mm) 
         [0019]    Referring to  FIGS. 1A and 4 , a layer of photoresist  1  is formed on a substrate  3  (S 10 ). The layer of photoresist  1  has a thickness of 0.1 μm to 9.9 μm. To this end, the substrate  3  is coated (e.g., by spin coating) with liquid photoresist. The viscosity of the photoresist is 150 cp to 250 cp at the time the photoresist is caused to reflow (as described below). 
         [0020]    Referring to  FIGS. 1B and 4 , the layer of photoresist  1  is then exposed and developed to form a photoresist pattern  5  (S 20 ). In this embodiment, the photoresist pattern  5  comprises one or more cylindrical members each having a diameter of 10 μm to 99 μm. For example, the photoresist pattern  5  is a 2-dimensional array (rows and columns) of cylindrical members each having substantially the same diameter within a range of 10 μm to 99 μm, and the same height. Referring to  FIGS. 1C and 4 , the photoresist pattern  5  is then subjected to a thermal reflow process (e.g., by curing) which melts the photoresist pattern  5  (S 30 ). As a result, each cylindrical member is transformed into a microlens  7 . For example, after the reflow process, the height of each microlens ranges from 0.1 μm±0.01 μm to 9.9 μm±0.01 μm. The thickness of each microlens ranges from 10 μm±1 μm to 99±1 μm. 
         [0021]    One embodiment of a depth sensor  10 , having microlenses, according to the inventive concept will now be described with reference to  FIG. 2   
         [0022]    The depth sensor  10  comprises a semiconductor chip  20 , a light source  32 , and a lens module  34 . The semiconductor chip  20  of this example includes an array  22  of depth pixels  50 , a row decoder  24 , a timing controller  26 , a photo gate controller  28 , a light source driver  30 , and a logic circuit  36 . 
         [0023]    The row decoder  24  selects one row from among the plurality of rows of the depth pixels  50  in response to a row address output from the timing controller  26 . 
         [0024]    The photo gate controller  28  generates a plurality of photo gate control signals and provides them to the array  22  under the control of the timing controller  26 . 
         [0025]    The light source driver  30  generates a clock signal MLS for driving the light source  32  under the control of the timing controller  26 . The light source driver  30  also provides the clock signal MLS or information about the clock signal MLS to the photo gate controller  28 . 
         [0026]    The light source  32  emits a modulated optical signal toward a target  40  in response to the clock signal MLS. The light source  32  may comprise a light emitting diode (LED), an organic light emitting diode (OLED), or a laser diode. The form of the modulated optical signal may be that of a sine wave or a square wave. In  FIG. 2  the light source  32  is illustrated for the sake of simplicity as having only one light emitting element, but the light source  32  includes a plurality of light emitting elements arranged in circle around the lens module  34 . 
         [0027]    The logic circuit  36  processes signals output by the plurality of depth pixels  50  in the array  22  and outputs the processed signals to a processor under the control of the timing controller  26 . The processor may be a chip separate from that of the semiconductor chip  10 . Furthermore, the depth sensor  10  and the processor may be an integral unit regardless of whether the processor is part of the chip  20 , i.e., the depth sensor  10  and processor may be a range imaging camera. In any case, using the processed signals, the processor may calculate the distance between the depth sensor  10  and the target  40  based on the time-of-flight principle. Thus, the processor and the depth sensor  10  may constitute a TOF camera. 
         [0028]    In one example of this embodiment, the logic circuit  36  includes an analog-to-digital converter which converts signals output from the array  22  into digital signals. The logic circuit  36  may also include a correlated doubling sampling (CDS) block (not shown) which performs CDS on the digital signals output from the analog-to-digital converter. Alternatively, the logic circuit  36  may include a CDS block (not shown) which performs CDS on the signals output from the array  22  and an analog-to-digital converter which converts CDS signals output from the CDS block into digital signals. The logic circuit  36  may also include a column decoder which transmits signals output by the analog-to-digital conversion block or the CDS block to the processor under the control of the timing controller  26 . 
         [0029]    The modulated optical signal emitted by the light source  32  is reflected from the target  40 . Reflected optical signals are input to the array  22  through the lens module  34 . 
         [0030]    When the target is three-dimensional, there are different distances Z 1 , Z 2 , and Z 3  between the depth sensor  10  and the target  40 . In this case, a distance Z (in particular, the distance between the light source  32  or the array  22  and the target  40 ) may be calculated as follows according to a TOF principle when the modulated optical signal has a waveform of cos ωt, and using a selected one  23  of the depth pixels  50 . 
         [0031]    The reflected optical signal is received by the depth pixel  23 , and the depth pixel  23  as a result outputs an optical signal having a waveform of cos(ωt+Φ), wherein (Φ) is the phase difference between the optical signals output by the light source  32  and the depth pixel  23 . This phase difference (Φ)=2*ω*Z/C=2*(2πf)*Z/C, wherein C is the speed of light, π it is the wavelength of the light emitted by the light source  32  and f is the frequency of the light. Accordingly, the distance Z=Φ*C/(2*ω)=Φ*C/(2*(2πf)). 
         [0032]    In this way, the optical signals input to the array  22  through the lens module  34  may be demodulated by the plurality of depth pixels  50 . In other words, the optical signals input to the array  22  through the lens module  34  are used to form an image of the target  40 . 
         [0033]    An individual depth pixel, namely depth pixel  23 , will now be described in more detail with reference to  FIG. 3 . It should be noted, however, that the various layers shown in  FIG. 3  are used in common in the array  22 . 
         [0034]    The depth pixel  23  includes a microlens  100 , an optional planarization layer  110 , an infrared (IR) filter  120 , a wiring section  130 , an interlayer insulating layer  140 , a substrate  150 , and a photoelectric conversion element  160 . 
         [0035]    In the illustrated example, the microlens  100  is formed on the planarization layer  110  using photoresist in the manner described above with reference to FIGS.  1 A- 1 C and  4 . In the array  22 , an array of the microlenses  100  are provided on the planarization layer  110 . The planarization layer  110  thus corresponds to substrate  3  and provides a smooth and flat surface (its upper surface) on which the microlens  100  is formed. The planarization layer  110  may be of an acrylic or epoxy material. Alternatively, the planarization layer  110  may be omitted, and the microlens may be formed on the upper surface of the IR filter  120  in the case in which that surface is smooth or polished. 
         [0036]    As mentioned above, when the viscosity of the photoresist is 150 cp to 250 cp, a relatively thick layer of photoresist (0.1 μm to 9.9 μm thick) can be formed on the planarization layer  110  or IR filter  120 . The microlens  100  can be relatively large when it is formed from such a layer of photoresist having a thickness of 0.1 μm to 9.9 μm. The significance of this resides in the fact that a depth pixel of a depth sensor is generally much larger than that of a typical pixel of an image sensor. Accordingly, the inventive concept can provide a microlens for use in the depth sensor. 
         [0037]    Light passing through and focused by the microlens  100  is received by the IR filter  120 . The IR filter  120  transmits only that part of the light having wavelengths in the IR spectrum. 
         [0038]    The wiring section  130  includes a plurality of dielectric layers  132 ,  134 ,  136 , and  138  and a plurality of metal wirings  131 ,  133 ,  135 , and  137 . Each of the dielectric layers  132 ,  134 ,  136 , and  138  may be an oxide layer or a composite layer of an oxide film and a nitride film. 
         [0039]    The metal wirings  131 ,  133 ,  135 , and  137  are connected with power or signal lines. Also, the metal wirings  131 ,  133 ,  135 , and  137  may be connected to each other by vias extending through the dielectric layers  134 ,  136 , and  138 . The microlens  100  serves to focus the light away from the metal wirings  131 ,  133 ,  135 , and  137  so that the majority of the light is incident on the substrate (as shown by the chained lines in  FIG. 3 ). 
         [0040]    The interlayer insulating layer  140  may be of silicon nitride, silicon oxide, or an acrylic resin. The interlayer insulating layer  140  may cover transistors (e.g., transfer transistors) on the substrate  150 . 
         [0041]    The substrate  150  may be of silicon (Si), silicon germanium (SiGe), or germanium (Ge). The photoelectric conversion element  160  is integrated with the substrate  150  and is generally formed at the upper surface of the substrate. For instance, the photoelectric conversion element  160  may be a photodiode or a photogate which generates photocharges in response to IR rays incident thereon. In the array  22 , a plurality of the photoelectric conversion elements  160  are provided across the substrate  150 , and each microlens  100  focuses the light onto a respective photoelectric conversion element  160  while avoiding the metal wirings  131 ,  133 ,  135 , and  137  that run across the dielectric layers  134 ,  136 , and  138 . 
         [0042]      FIG. 5  shows the results of simulations conducted using a conventional depth sensor and the depth sensor  10  illustrated in  FIG. 2 . The conventional depth sensor is similar to that of  FIG. 2  but does not include a microlens. That is, in the conventional depth sensor, the upper surface of the planarization layer is exposed to receive the incident light. 
         [0043]    As the results of  FIG. 5  show, the conventional depth sensor has a fill factor of 0.18% irrespective of the thickness of the planarization layer. On the other hand, the depth sensor  10  has a fill factor that increases as the thickness of the planarization layer increases. 
         [0044]    Furthermore, when the thicknesses of the planarization layers are 1 μm, the depth sensor  10  has a fill factor two times greater than that of the conventional depth sensor. When the thicknesses of the planarization layers are 4 μm, the depth sensor  10  has a fill factor three times greater than that of the conventional depth sensor. Thus, it can be seen that the microlens  100  provides the depth sensor  10  with increased performance. 
         [0045]    An example of an image processing system  700  according to the inventive concept will now be described with reference to  FIG. 6 . The image processing system  700  may be employed by a 3D range finder, a game controller, a camera, or an image (gesture) recognition device. 
         [0046]    The image processing system  700  includes a color image sensor  310 , a depth sensor  10  ( FIG. 2 ), a processor  210 , and a memory  220  connected through a bus  301 . Note, although the depth sensor  10  and the color image sensor  310  are illustrated in  FIG. 6  as physically separate components, this is just for the sake of clarity, and the depth sensor  10  and the color image sensor  310  may be embodied as an integrated component having common signal processing circuitry. 
         [0047]    In this example of the image processing system  700 , the color image sensor  310  includes a pixel array which includes a red pixel, a green pixel, and a blue pixel but does not include a depth pixel. Accordingly, the processor  210  may generate three-dimensional (3D) image information based on depth (range) information output by the depth sensor  10  and color information (e.g., at least one of red, green, blue, magenta, cyan, and yellow color information) output by the color image sensor  310 . The 3D image information generated by the processor  210  can be stored in the memory device  220 . The image processing system  700  may also include a display (not shown) on which the 3D image information is displayed. 
         [0048]    An example of a signal processing system  800  according to the inventive concept will now be described with reference to  FIG. 7 . 
         [0049]    The signal processing system  800 , which simply functions as a range finder, includes a depth sensor  10  ( FIG. 2 ), a processor  210  controlling the operations of the depth sensor  10 , a memory device  220  and an interface (I/F)  410  connected to one another through a bus  401 . 
         [0050]    The processor  210  calculates the distance between the signal processing system  800  and an object (target) based on depth information output by the depth sensor  10 . The distance calculated by the processor  210  can be stored in the memory device  220 . The interface (I/F)  410  may input user information and output the distance information. The I/F  410  may be a wireless interface. 
         [0051]    Finally, embodiments of the inventive concept and examples thereof have been described above in detail. The inventive concept may, however, be embodied in many different forms and should not be construed as being limited to the embodiments described above. Rather, these embodiments were described so that this disclosure is thorough and complete, and fully conveys the inventive concept to those skilled in the art. Thus, the true spirit and scope of the inventive concept is not limited by the embodiment and examples described above but by the following claims.