Abstract:
An image sensor test pattern provides time efficient optical testing of CMOS image senors at a single luminous intensity. These test patterns include at least first and second arrays of pixels having different light-accumulating characteristics. The different light-accumulating characteristics may be achieved multiple different ways. In some cases, the photodiodes in the first array of pixels are larger than photodiodes in the second array of pixels. In other cases, the photodiodes in the first array of pixels have open holes of a first size and the photodiodes in the second array of pixels have open holes of a second size less than the first size. In still other cases, the photodiodes in the first array of pixels have colors filters of a first density (or first thickness) and the photodiodes in the second array of pixels have color filters of a second density (or second thickness) less than the first density (or first thickness).

Description:
REFERENCE TO PRIORITY APPLICATION  
       [0001]     This application claims priority to Korean Patent Application No. 2004-115046, filed Dec. 29, 2004, the disclosure of which is hereby incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to image sensors and methods of testing image sensors.  
       BACKGROUND OF THE INVENTION  
       [0003]     An image sensor is a semiconductor device for converting optical images into electric signals. Charge coupled devices (CCDs) and CMOS image sensors are examples of image sensors. The CCD is a device in which MOS capacitors are located adjacent to each other, and charge carriers are stored on each capacitor. The CMOS image sensor is a switch type device which uses a control circuit and a signal processing circuit as a peripheral circuit, with as many MOS transistors as the number of pixels, and sequentially detects outputs from the capacitors using the MOS transistors. The CCD has the disadvantages of a relatively complex driving scheme, high power consumption, and a complex manufacturing process requiring a number of mask steps. Further, since the CCD cannot have a built-in signal processing circuit, it is difficult to realize the signal processing circuit in one chip.  
         [0004]     The CMOS image sensor has recently been developed by using sub-micron CMOS manufacturing technology to overcome such disadvantages. The CMOS image sensor realizes images by forming a photo diode and a MOS transistor within a unit pixel and sequentially detecting signals with a switch scheme. The CMOS image sensor has low power consumption, due to the CMOS manufacturing technology, which is simple compared to the CCD process. Further, the CMOS image sensor can combine a plurality of signal processing circuits in one chip, placing it in the spotlight as a next-generation image sensor. The realization of images in the CMOS image sensor is dependent on the number of electrons generated in a photodiode, according to the number of photons of incident light. Therefore, it is important to test the characteristics of the image sensor according to the number of photons of incident light, before image sensor products are brought to the market. The characteristics of the image sensor according to the number of photons is represented by sensitivity, quantum efficiency, conversion gain, signal to noise ratio, photon shot noise, etc. To observe such characteristics, the luminous intensity of a light source is varied to adjust the number of incident photons, while monitoring the characteristics of the image sensor. However, this process is time consuming, which increases the production cost.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides a test pattern of optical characteristics of a CMOS image sensor capable of reducing the time required for testing the CMOS image sensor.  
         [0006]     The present invention also provides a test method of optical characteristics of a CMOS image sensor-capable of reducing the time required for testing the CMOS image sensor.  
         [0007]     According to an aspect of the present invention, there is provided a test pattern of optical characteristics of a CMOS image sensor, the test pattern including a pixel array region having optically blocked photodiodes, a pixel array region including photodiodes of a normal size, and a plurality of pixel array regions including photodiodes of sizes proportional to the normal size.  
         [0008]     According to another aspect of the present invention, there is provided a test pattern of optical characteristics of a CMOS image sensor, the test pattern including a pixel array region having optically blocked photodiodes, a pixel array region including photodiodes having an open hole of a normal size, and a plurality of pixel array regions including photodiodes having open holes of a size proportional to the normal size.  
         [0009]     According to another aspect of the present invention, there is provided a test pattern of optical characteristics of a CMOS image sensor, the test pattern including a pixel array region having optically blocked photodiodes, a pixel array region including photodiodes having a color filter of a normal thickness or density, and a plurality of pixel array regions including photodiodes having color filters of a thickness or density proportional to the normal thickness or density.  
         [0010]     According to another aspect of the present invention, there is provided a test method of optical characteristics of a CMOS image sensor, the test method including setting a test pattern region for monitoring optical characteristics, in a CMOS image sensor chip, locating a pixel array including photodiodes of a normal size, in the test pattern region, locating a plurality of pixel array regions including photodiodes of sizes proportional to the normal size, in the test pattern region, and testing the optical characteristics of the CMOS image sensor at a single luminous intensity using the pixel arrays within the test pattern region.  
         [0011]     According to another aspect of the present invention, there is provided a test method of optical characteristics of a CMOS image sensor, the test method including setting a test pattern region for monitoring the optical characteristics, in a CMOS image sensor chip, locating a pixel array including photodiodes having an open hole of a normal size, in the test pattern region, locating a plurality of pixel array regions including photodiodes having open holes of sizes proportional to the normal size, in the test pattern region, and testing the optical characteristics of the CMOS image sensor at a single luminous intensity using the pixel arrays within the test pattern region.  
         [0012]     According to another aspect of the present invention, there is provided a test method of optical characteristics of a CMOS image sensor, the test method including setting a test pattern region for monitoring the optical characteristics, in a CMOS image sensor chip, locating a pixel array including photodiodes having a color filter of a normal thickness or a normal density, in the test pattern region, locating a plurality of pixel array regions including photodiodes having color filters of a thickness or a density proportional to the normal thickness or the normal density, in the test pattern region, and testing the optical characteristics of the CMOS image sensor at a single luminous intensity using the pixel arrays within the test pattern region. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a diagram illustrating a test pattern of optical characteristics of a CMOS image sensor according to a first embodiment of the present invention;  
         [0014]      FIG. 2  is a circuit diagram illustrating a circuit of a unit pixel in pixel array regions shown in  FIG. 1 ;  
         [0015]      FIG. 3  is a diagram illustrating the layout of the unit pixel;  
         [0016]      FIG. 4  is a diagram illustrating a test pattern of optical characteristics of a CMOS image sensor according to a second embodiment of the present invention;  
         [0017]      FIG. 5  is a sectional view taken along line  5 - 5 ′ of the layout of the unit pixel shown in  FIG. 3 ; and  
         [0018]      FIG. 6  is a diagram illustrating a test pattern of optical characteristics of a CMOS image sensor according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     The attached drawings illustrate exemplary embodiments of the present invention, and are referred to in order to gain a sufficient understanding of the present invention, the merits thereof, and the objectives accomplished by the implementation of the present invention. Like reference numerals denote like elements throughout the drawings.  
         [0020]      FIG. 1  is a diagram illustrating a test pattern of optical characteristics of a CMOS image sensor according to a first embodiment of the present invention. The test pattern  10  of the optical characteristics according to the first embodiment of the present invention includes a pixel array region  11  including optically blocked photo diodes, a pixel array region  12  including photodiodes of a normal size, and a plurality of pixel array regions  13  to  15  including photodiodes of a size proportional to the normal size. For example, the pixel array regions  13 ,  14 , and  15  may include photodiodes of a size proportional to 0.8 times, 0.6 times, and 0.4 times the normal size, respectively. Here, the normal size means the size of photodiodes used in a typical production scale image sensor chip.  
         [0021]      FIG. 2  is a circuit diagram illustrating a circuit of a unit pixel  20  in pixel array regions  11  to  15  shown in  FIG. 1 , and  FIG. 3  is a diagram illustrating the layout of the unit pixel  20 . A unit pixel  20  includes a photodiode (PD) which receives photons of light and generates electrons, a transfer transistor  21  which transfers the electrons generated at the photodiode (PD) to a floating diffusion region (FD), a reset transistor  22  which sets the electric potential of the floating diffusion region (FD) to a desired value and resets the floating diffusion region (FD) by emitting electric charge, a drive transistor  23  serving as a source follower buffer amplifier, and a select transistor  24  which performs the function of addressing. A voltage (Vout) is output through the select transistor  24 .  
         [0022]     As illustrated by  FIG. 2 , a transfer control signal (TG) is applied to the gate of the transfer transistor  21 , and a reset control signal (RG) is applied to the gate of the reset transistor  22 . The floating diffusion region (FD) is connected to the gate of the drive transistor  23 . A selection control signal (SEG) is applied to the gate of the select transistor  24 .  
         [0023]     A test method of the optical characteristics of the CMOS image sensor using the test pattern according to the first embodiment will be described in detail. Generally, in the CMOS image sensor, the number of electrons which are photoelectrically converted from photons of incident light is in proportion to the size of the photodiode (PD). Specifically, the number of electrons generated at a specific luminous intensity varies depending on the size of the photodiode (PD).  
         [0024]     In the test method using the test pattern of the optical characteristics according to the first embodiment, a test pattern region  10  for monitoring the optical characteristics is set in the CMOS image sensor chip, a pixel array  12  including photodiodes of a normal size, and a plurality of pixel array regions  13  to  15  including photodiodes of a size proportional to the normal size are located in the test pattern region  10 , and then the optical characteristics of the CMOS image sensor at a single luminous intensity are tested by using the pixel arrays within the test pattern region.  
         [0025]     In the test method using the test pattern of the optical characteristics according to the first embodiment, the optical characteristics of the image sensor are tested by using pixel arrays including photodiodes of different sizes, thus obtaining the same effect as varying the luminous intensity of photodiodes having uniform size. The optical characteristics are tested at a single luminous intensity, instead of a varying luminous intensity, thus reducing the time required for testing the optical characteristics of the image sensor. The optical characteristics of the CMOS image sensor are typically represented by sensitivity, conversion gain, signal to noise ratio, quantum efficiency, photon shot noise, etc.  
         [0026]      FIG. 4  is a diagram illustrating a test pattern of optical characteristics of a CMOS image sensor according to a second embodiment of the present invention. The test pattern  40  of the optical characteristics according to the second embodiment of the present invention includes a pixel array region  41  including optically blocked photodiodes, a pixel array region  42  including photodiodes having an open hole of normal size, and a plurality of pixel array regions  43  to  45  including photodiodes having open holes of sizes proportional to the normal size. For example, the pixel array regions  43 ,  44 , and  45  may include photodiodes having open holes of a size proportional to 0.8 times, 0.6 times, and 0.4 times the normal size, respectively. Here, the normal size means the size of the open hole of the photodiodes used in a production-scale CMOS image sensor chip.  
         [0027]      FIG. 5  is a sectional view taken along line  5 - 5 ′ of the layout of the unit pixel shown in  FIG. 3 . Here, METAL 1  to METAL 3 , CF, ML, and PL 1  and PL 2  represent metal lines, a color filter, a microlens, and planarization layers, respectively. “L” represents the distance between the metal lines, and corresponds to the open hole of the photodiode through which light passes. The first metal line (METAL 1 ) is used for connecting the output voltage (Vout) and the drive transistor  23  to the FD shown  FIG. 3 . The second metal line (METAL 2 ) is used for routing signals RG, TG, and SEL shown in  FIG. 3 . The third metal line (METAL 3 ) is used for routing a power source voltage and for an optical shield.  
         [0028]     A test method of the optical characteristics of the CMOS image sensor using the test pattern according to the second embodiment will be described in detail. In the CMOS image sensor, the number of electrons which are photoelectrically converted from photons of incident light is in proportion to the size of the photodiode (PD) and the size of the open hole (L) of the photodiode (PD). Specifically, the number of electrons generated at a specific luminous intensity varies depending on the size of the open hole (L) of the photodiode (PD).  
         [0029]     In the test method using the test pattern of the optical characteristics according to the second embodiment, a test pattern region  40  for monitoring the optical characteristics is set in the CMOS image sensor chip, a pixel array  42  including photodiodes having an open hole of a normal size and a plurality of pixel array regions  43  to  45  including photodiodes having open holes of a size proportional to the normal size are located in the test pattern region  40 , and then the optical characteristics of the CMOS image sensor are tested at a single luminous intensity using the pixel arrays  42  to  45  within the test pattern region.  
         [0030]     In the test method using the test pattern of the optical characteristics according to the second embodiment, the optical characteristics of the image sensor are tested by using pixel arrays, each including photodiodes having open holes of different sizes, thus obtaining the same effect as varying the luminous intensity. The optical characteristics are tested at a single luminous intensity instead of a varying luminous intensity, thereby reducing the time required for testing the optical characteristics of the image sensor.  
         [0031]      FIG. 6  is a diagram illustrating a test pattern of optical characteristics of a CMOS image sensor according to a third embodiment of the present invention. The test pattern  60  of the optical characteristics according to the third embodiment of the present invention includes a pixel array region  61  including optically blocked photodiodes, a pixel array region  62  including photodiodes having a color filter (CF shown in  FIG. 5 ) of a normal thickness or density, and a plurality of pixel array regions  63  to  65  including photodiodes having color filters of a thickness or density proportional to the normal thickness or density.  
         [0032]     For example, the pixel array regions  63 ,  64 , and  65  may include photodiodes having a color filter of a thickness or density proportional to 0.8 times, 0.6 times, and 0.4 times the normal thickness or density, respectively. Here, the normal thickness and the normal density mean the thickness and the density of the color filter actually used in a CMOS image sensor chip.  
         [0033]     A test method of the optical characteristics of the CMOS image sensor by using the test pattern according to the third embodiment will be described in detail. In the CMOS image sensor, the number of electrons which are photoelectrically converted from photons of incident light may be controlled according to the thickness and density of the color filter (CF). Specifically, the number of electrons generated at a specific luminous intensity varies depending on the thickness and density of the color filter (CF).  
         [0034]     In the test method using the test pattern of the optical characteristics according to the third embodiment, a test pattern region  60  for monitoring the optical characteristics is set in the CMOS image sensor chip, a pixel array  62  including photodiodes having a color filter (CF) of a normal thickness and density and a plurality of pixel array regions  63  to  65  including photodiodes having color filters (CF) of a thickness or density proportional to the normal thickness or the normal density are located in the test pattern region  60 , and then the optical characteristics of the CMOS image sensor at a single luminous intensity are tested using the pixel arrays  62  to  65  within the test pattern region.  
         [0035]     In the test method using the test pattern of the optical characteristics according to the third embodiment, the optical characteristics of the image sensor are tested by using pixel arrays including photodiodes having color filters of different thickness or density, thus obtaining the same effect as varying the luminous intensity. The optical characteristics are tested at a single luminous intensity instead of a varying luminous intensity, thereby reducing the time required for testing the optical characteristics of the image sensor.  
         [0036]     As described above, in a test method using a test pattern of optical characteristics according to the present invention, the optical characteristics are tested at a single luminous intensity, without varying the luminous intensity. Therefore, the optical characteristics of the image sensor can be tested much more quickly.  
         [0037]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.