Solid state image pick-up device

Abstract: A photoelectric conversion device comprises a plurality of first pixels for outputting a first color signal and second pixels for outputting a second color signal different from said first color signal arranged therein, characterized in that said first and second pixels have semiconductor junctions of different capacitances; and said device further comprises a first light shielding cell having a semiconductor junction of substantially the same capacitance as that of said first pixel; and a second light shielding cell having a semiconductor junction of substantially the same capacitance as that of said second pixel.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a solid state image pick-up device for use in an information processing apparatus such as a copying machine or a facsimile terminal equipment, and more particularly to a solid state image pick-up device with multiple pixels arranged and having a photoelectric conversion unit in which a signal corresponding to a electric charge stored in the control electrode area by light illumination is output from one of the second main electrode areas. Related Background Art In recent years, solid state image pick-up devices of the CCD type in which a CCD is used in the scanning circuit have been typically well known. Also, since the output of a photoelectrically converted signal may decrease with higher definition of photoelectric conversion elements, photoelectric conversion elements of the amplification type in which a photoelectrically converted signal can be amplified for the output have gained popularity. Such amplification type photoelectric conversion elements include a bipolar transistor type, a field effect transistor and an electrostatic induction transistor type. Among them, photoelectric conversion elements of the bipolar transistor type include a normal photo-transistor, as well as a photo-transistor in which the emitter is connected to a capacitive load as disclosed in USP No. 4,791,469 issued to the inventors Omi and Tanaka. Such a photoelectric conversion element of the amplification type with a bipolar transistor will be exemplified in the following. Fig. 15 is a schematic plan view showing a linear solid state image pick-up device using the above-mentioned bipolar sensor. In Fig. 15, 1 is a part of a semiconductor substrate which is a collector region of the bipolar type sensor. 2 and 3 are a base region and an emitter region of the bipolar type sensor, respectively, and 4 is a read-out circuit system which outputs a signal from each pixel sequentially. U (as indicated in the broken line) shows a light sensitive pixel (photoelectric conversion element) to provide a bipolar type sensor. Such light sensitive pixels of the same structure are arranged in a linear form. It is to be noted herein that each pixel is shielded against light except for a light receiving face of open area (x x y), wherein if these light receiving faces are equal in size (x x y), each pixel theoretically will have the same light sensitivity. Fig. 16 is a schematic plan view showing a two-dimensional solid state image pick-up device using the above-mentioned bipolar type sensors. In Fig. 16, 11 is a part of a semiconductor substrate which is a collector region of the bipolar type sensor. 12 and 13 are a base region and an emitter region of the bipolar type sensor for each pixel, respectively, and 4 is a read-out circuit system which outputs a signal from each pixel sequentially. U' (as indicated in the broken line) shows a light sensitive pixel (photoelectric conversion element) to provide a bipolar type sensor. Such light sensitive pixels of the same structure are arranged in a two-dimensional form of m x n. It is to be noted herein that if these light receiving faces are equal in size of light receiving area or open area, each pixel theoretically will have the same light sensitivity. For the purpose of reading a monochromatic image, the constitutions of Figs. 5 and 6 are most suitable. However, since the light signals from solar rays or lamp rays have spectral characteristics over wide wavelength regions, if a plurality of light signals which lie in mutually different wavelength regions are photoelectrically converted by different pixels, there may occur differences in the sensitivity. To facilitate the understanding of such a technical problem, the reading of a color image will be described below. When reading a color image with a solid state image pick-up device having red light signal reading pixels (R pixels), green light signal reading pixels (G pixels) and blue light signal reading pixels (B pixels), it is common to dispose one of the color filters on each pixel in accordance with a certain rule. In this case, because the spectral transmission characteristics of color filters are different with the color, the sensitivity of the pixel having a color filter arranged is a value determined by the characteristic of the color filter, which may be different for each color. For example, when using red R, green G and blue B primary color filters, the sensitivity often has a relation of red sensitivity SR > green sensitivity SG > blue sensitivity SB. An image read by such a color solid state image pick-up device is converted into an electric signal for each color, and output to a signal processing circuit at a later stage. In this case, because the sensitivity is different for each color owing to the reason as previously mentioned, it is necessary to apply a different gain to each color output signal prior to the input into the signal processing circuit at the later stage, for example, so that the signal level of each color in reading with white uniform light may be equal. Such a processing is referred to as a white balance processing. This white balance processing designates the necessity of excess hardware or excess control from the point of the system level, leading to the increase in cost. As one means for resolving this problem, it is contemplated that the sensitivity of each color is adjusted by varying the open area of the pixel for each color filter, but there is a risk that a different aperture characteristic for each color may be caused. Document US-4 242 694 discloses a color imaging device having three semiconductive photoelectric elements for light of the respective primary colors red, green and blue (R, G, B). In order to adapt the photoelectric elements to the particular wavelength, there is provided a transparent conductive film for each of the elements, each film having a certain length so as to fully or partially cover a photodiode provided underneath each film. Another image sensor is known from EP-A-0 437 340. It is an object of the present invention to provide an image pickup device producing low noise output signals. According to the invention, this object is accomplished by an image pickup device as set out in claim 1. That is, the semiconductor junctions of two pixel types being provided for converting light of two different wavelengths have different capacitances. To provide a dark signal output for each of these pixel types in order to compensate dark noise, one of each pixel type is shielded against light. Further, the semiconductor junctions of these light-shielded pixels which are arranged on the same substrate have substantially the same capacitances as those of the pixels producing the regular (non dark signal) output. The present invention will be described hereinbelow by way of example and with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A to 1C are schematic top views for explaining the pixel constitution in a solid state image pick-up device. Fig. 2 is a graph showing examples of the spectral sensitivity characteristic in a semiconductor for use with a solid state image pick-up device. Fig. 3 is a graph showing an example of the spectral sensitivity characteristic of a color separation filter for use with the solid state image pick-up device. Fig. 4 is a schematic top view of a solid state image pick-up device. Fig. 5 is a schematic cross-sectional view showing a pixel in the solid state image pick-up device of Fig. 4 l in cross section taken along the line A-A'. Fig. 6 is a schematic top view of a solid state image pick-up device. Fig. 7 is a schematic cross-sectional view of a pixel in the solid state image pick-up device. Fig. 8 is a schematic top view of a solid state image pick-up device according to an embodiment of the present invention. Fig. 9 is a schematic cross-sectional view showing a pixel in a solid state image pick-up device according to the embodiment of Fig 8 in cross section taken along the line B-B' of Fig. 8. Fig. 10 is a schematic cross-sectional view showing a pixel in a solid state image pick-up device in cross section taken along the line C-C' of Fig. 8. Fig. 11 is a circuit configuration diagram of a solid state image pick-up device. Fig. 12 is a circuit diagram of a pixel in the solid state image pick-up device. Fig. 13 is timing diagrams for explaining the driving method of a solid state image pick-up device. Fig. 14 is a control block diagram of an image information processor using a solid state image pick-up device. Fig. 15 is a schematic top view showing a line sensor as a conventional solid state image pick-up device. Fig. 16 is a schematic top view showing an area sensor as the conventional solid state image pick-up device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. First, a solid state image pick-up device as shown in Figs. 1A to 1C will be described to facilitate the understanding of the present invention. Figs. 1A, 1B and 1C show pixel portions of a solid state image pick-up device in three examples. In Fig. 1A, the lengths y 1 , y 2 in semiconductor regions 102, 112 for a left pixel 101 and a right pixel 111 are varied to provide substantially the same sensitivity. The constitution in other portions is the same. Namely, when the lights in different wavelength regions are photoelectrically converted, the sensitivity of pixel 101 is higher and the sensitivity of pixel 111 is lower, if y l is equal to y 2 . For this reason, the lengths of the semiconductor regions 102, 112 are made different so that the sensitivity to the light to be photoelectrically converted may be substantially equal. In this case, each of the pixels 101, 111 may be a photo-diode or photo-transistor using a P-type semiconductor in the semiconductor regions 102, 112 and an N-type semiconductor in the semiconductor regions 103, 113. On the other hand, in Fig. 1B, the lengths y R , y G and y B of the semiconductor regions 122R, 122G and 122B for three pixels 121R, 121G and 121B are only different, while the widths of the semiconductor regions 123R, 123G and 123B are the same, i.e., X R = X G = X B , with the length y being equal, and the widths of the semiconductor regions 122R, 122G and 122B are the same. Further, Fig. 1C is different from Fig. 1B as above described only in that the length of the semiconductor regions 122R, 122G is made equal to y R . Even if three pixels 121R, 121G, 121B are of the same structure, the sensitivity of each of three pixels may be possibly different depending on the spectral characteristic of light to be detected and the light transmission characteristic of color separation filter. In such a case, it is desirable to take a constitution of Fig. 1C. In order to make all of uniform sensitivity in the invention, various methods can be adopted. For example, it is preferable that each pixel is constituted of a photo-diode, as shown in Figs. 1A to 1C, with the area of semiconductor junction (PN junction) changed, to adjust the sensitivity. Specifically, if the length or width of PN junction is changed with the depth from the surface of semiconductor substrate being substantially constant, the PN junctions of all the pixels can be formed in a process using the same mask pattern. Of course, such pixels can be also formed by using an FET or SIT as a bipolar transistor in which the above PN junction is replaced with the base-collector junction, or a unipolar transistor in which the PN junction is replaced with the gate-drain junction. And when the light signals over wide wavelength regions from solar rays or halogen lamp rays are processed through the color separation and photoelectric conversion, it is desirable that a color separation filter is provided for each pixel of R, G, B to selectively transmit a respective light of R, G, B therethrough to make photoelectric conversion on a pixel made of a semiconductor such as a single crystal silicone. In this case, the transmissivity of the filter may be as shown in Fig. 2, while the sensitivity of single crystal silicon may be as shown in Fig. 3. Accordingly, since the blue sensitivity only is extremely lower in the characteristic, it is desirable to have a pixel structure as indicated in Fig. 1C. Of course, when red and blue lights are only dealt with, or when red and green lights are only dealt with, or when green and blue lights are only dealt with, for example, a structure in which two pixels are made one pair as indicated in Fig. 1A, should be adopted. By providing the junction capacities with differences sufficient to compensate for the spectral characteristics such as a transmissivity of filter or a sensitivity of semiconductor, the excellent color separated signal can be obtained. Also, if such pixels shielded against the light are disposed on the same substrate to provide a pixel for dark signal output, the excellent removal of dark noise can be achieved. In this case, it is desirable to operate on the signal from the shielded dark pixel and the light sensitive pixel having the same capacity. The embodiments of the present invention will be described below with reference to the drawings, whereas the present invention is not limited to such embodiments, but various variations in design are possible within a range where the objects of the present invention can be accomplished. Fig. 4 is a plan view showing the constitution of a solid state image pick-up and Fig. 5 is a longitudinal cross-sectional view of one pixel (U R ) of Fig. 4. In Figs. 15 and 4, like numerals indicate like components, the explanation of which is omitted. This example is a linear color solid state image pick-up device, with three color filters (not shown) of primary colors R, G, B. Note that the color filter is not shown in Figs. 4 and 5, but is provided on the opening portion of each pixel. In Fig. 5, 5 is a light shielding membrane, 6 is an insulation layer, and 7 is an epitaxial layer on a semiconductor substrate 1, which constitute a part of the collector region. Note that one pixel with R filter disposed, one pixel with G filter disposed, and one pixel with B filter disposed constitute one triad, and such triads are arranged as an array. As shown in Fig. 4, in order to make equivalent the sensitivity of each color pixel for R, G, B, the shape of base region 2 for the pixel U R , U G , U B having color filter of R, G, B disposed is changed in accordance with the magnitude relation (R > G > B) in the spectral transmission characteristic of color filter, such that the length of base region 2 is greater in the order of R, G, B. In this way, the base-collector capacity C bc is varied such as C bcR > C bcG > C bcB (C bcR : base-collector capacity for pixel U R , C bcG : base-collector capacity for pixel U G , C bcR : base-collector capacity for pixel U B ), corresponding to the magnitude relation in the spectral transmission characteristic of color filter. This is based on a theory that the light-voltage conversion gain of pixel is inversely proportional to the base-collector capacity C bc . When reading a white uniform light, the output signal level (sensitivity) of each color is substantially equal, whereby it is possible to perform directly the signal processing without needs of adjusting the gain for each color in a subsequent circuit. The depth of base region 2 is equal for all the pixels U R , U G , U B . Of course, since the thickness of substrate t 1 , the thickness of n - epitaxial layer 7 t 2 , and the thickness of emitter 3 t 3 are the same, the junction capacity (storage capacity) can be easily changed by changing only the length of base region 2 in an opening portion 20 as indicated by y R , y G , y B in Fig. 4. Such a constitution allows the sensitivity to be adjusted without greatly changing the current amplification factor (h fe ) of transistor or the withstand voltage. Fig. 6 is a plan view showing the second example of a solid state image pick-up device, and Fig. 7 is a longitudinal cross-sectional view of one pixel as shown in Fig. 1. In Figs. 16 and 6, like numerals indicate like components, the explanation of which is omitted. This example is an application example of two-dimensional solid state image pick-up device, in which m x n pixels are arranged in two dimensions, with a first region consisting of a x b (m > a, n > b) pixels and a second region consisting of pixels except for those in the first region, as shown in Fig. 6. It is to be noted that m x n pixels are arranged at an equal pitch in both the row and column directions, with the opening area sensitive to the light being equal in size. In Figs. 6 and 7, 12 is a base region (base area A bc2 ) of pixel in the second region, 13 is an emitter region of pixel in the second region, while 12' is a base region (base area A bc1 ) of pixel in the first region, and 13' is an emitter region of pixel in the first region. Also, in Fig. 7, 17 is an epitaxial layer (constituting a part of collector region) on a semiconductor substrate 11, and 18 is a selective oxide region for the isolation of each pixel. y' is a width of the opening portion. The base area A bc1 of pixel in the first region is smaller than the base area A bc2 of pixel in the second region (A bc1 < A bc2 ). Accordingly, there is a relation of S 1 > S 2 between the sensitivity S 1 of pixel in the first region and the sensitivity S 2 of pixel in the second region, because if the opening area is equal, the sensitivity is inversely proportional to the base-collector base-collector capacity C bc (S ∝ 1/C bc ), and further the capacity C bc is substantially proportional to the base area A bc (C bc ∝ A bc ). This means that the two-dimensional area of m x n pixels is divided into two areas (i.e., a x b area and the other area) which have different weightings of sensitivity. For example, for a certain image, when weighting information in the central area of the image, in the conventional two-dimensional solid state image pick-up devices, a method in which the whole image is first read and converted into electric signal, and only signals in the weighted region are sampled from output signals to apply gain to those signals, but in the two-dimensional solid state image pick-up device the base-collector capacity C bc of pixel in the weighted region is only changed, whereby the weighting can be easily made. In this case, the output signal is directly a weighted signal, and therefore, there will be no needs of performing the excess processing at the later stage, so that the simplification and lower costs for the later stage circuit can be realized. As above described, the sensitivity of pixel can be varied without needs of changing the opening area of pixel and providing any excess signal processing circuit at the later stage, so that a color solid state image pick-up device at lower costs and with higher functions can be provided. This embodiment of Fig 8 involves a line sensor as a color solid state image pick-up device having a pixel structure as indicated by Fig. 1C. Fig. 8 is a schematic plan view showing a line sensor 300 in this embodiment, in which a pixel array 330 and a read-out circuit 320 are juxtaposed on the surface of a semiconductor chip. Fig. 9 is a longitudinal cross-sectional view of a pixel near a light receiving portion thereof, taken along the line B-B' of Fig. 8, and Fig. 10 is a transverse cross-sectional view of a pixel near the light receiving portion, taken along the line C-C' of Fig. 8. Fig. 11 is a circuit configuration diagram of the line sensor 300, and Fig. 12 is a circuit diagram of one pixel. Fig. 13 is timing charts for explaining a driving method of the line sensor 300. In the line sensor 300 of this embodiment, light is permitted to be incident only upon the base and collector portions of a bipolar transistor BPT constituting a pixel with a light shielding layer 314. In Fig. 9, on a P-type silicon substrate 301 is disposed a pixel array 330 surrounded by an element isolation region 303 of P-type silicon, in which a transistor BPT constituting one pixel involves a collector region having n + type buried layer 302, n + type contact layer 304, and n - type epitaxial layer 305, a base region having P-type semiconductor layer 307, and an n + type emitter region 308. Also, on the surface, there are provided an insulation layer 309, a collector electrode 310, an emitter electrode 312, an insulation layer 313, a light shielding layer 314, a planerized layer 315, and a filter 316R. The light shielding layer 3l4, made of Al, Cr or a black organic resin, is provided on a P-type semiconductor layer 306 having a deep junction as the base, the emitter region 308 provided inside thereof, and the n + contact layer 304 of collector, thereby preventing light from directly entering these regions. Also, by enclosing the periphery and bottom face of the n - epitaxial layer 305 which mainly produces light carriers within the n + buried layer 302 and the n + contact layer 304, light carriers are prevented from escaping into adjacent pixels, while under the action of the light shielding layer 314. And the adjustment of stored capacity is conducted such that the length of P-type semiconductor layer 307, i.e., the length in a sub-scan direction, is made equal to almost 1/20 when the length of R pixel and G pixel is 1. Further, the line sensor 300 has a pair of light shielding pixels 201, 299 of the same transistor structure on both ends of the light sensitive pixels 202, 203, ····. These are of the same constitution as a pair of light sensitive pixels 202, 203 except that they are shielded against light by the light shielding layer 314. The operation of reading a signal based on carriers stored in the base region of each pixel in the line sensor 300 will be explained below. A basic operation principle is described in USP No. 4,810,896, issued to the inventors Tanaka et al. A triad of R, G, B signals are output to respective three output lines simultaneously by a scan transistor array STA and a shift transistor SR. This is sequentially scanned from a leftmost triad 201 of the light shielding pixel to a rightmost triad 299 of the light shielding pixel. Each pixel is constituted as shown in Fig. 12, with a PMOS transistor Q P connected to the base of a photo-transistor and the nMOS transistors Q N1 and Q N2 connected to the emitter thereof. A voltage source V CC is connected to the collector. At the timings as shown in Fig. 13, after the base is first connected to a voltage source V BB during the reset period, the emitter is connected to a voltage source V EE . At this time, the potentials of V BB and V EE are set so that the base-emitter junction of transistor BPT be sufficiently forward biased. Specifically, V BB is set to about +3.0V and V EE is set to about ±0V. Then, in a storage period T AC , the base and the emitter are floating, whereby holes of carriers arising upon incident light are stored in the base. Thereafter, if the nMOS transistor Q N2 is turned on with a pulse ρ T to connect the emitter to a capacitive load C T , the signal voltage is read out into the load C T owing to dividing capacity. Then, as previously described, the signal voltage is read out sequentially as R, G, B signal to three output lines by the scan transistor array STA and the shift register SR. With this line sensor, it is possible to remove the noise effectively by compensating color signal in such a way as to adjust the length of the base, with the same constitution of the light shielding pixel and the light sensitive pixel. Specifically, by calculating differences (R b -R d , G b -G d , B b - B d ) between the signals (R d , G d , B d ) of light shielding pixels R, G, B stored in memory, and the signals (R b , G b , C b ) of light sensitive pixels to be read later, the light signal with dark signal removed can be obtained. Another method is to calculate the difference between the first read signal and the later read signal which is delayed by delaying means. Such a differential calculation can be made easily by means of a differential amplifier. Fig. 14 is a control system block diagram of an image information processing apparatus using a solid state image pick-up device of the present invention. In an example of using the line sensor 300, 401 is a signal processing circuit, including a circuit for converting R, G, B signals into yellow (Y), cyan (C) and magenta (M) signals, respectively, in addition to the memory and the differential amplifier as previously described. Also, 402 is a head driving circuit, in which converted Y, C, M signals are supplied to a head 403. The head 403 is an ink jet head for discharging ink droplets by heat energy, having three color heads of Y, C, M, in which the recording or print is performed onto a recording medium using a yellow ink, a cyan ink and a magenta ink. The entire control thereof can be performed by a central processing unit CPU.

Claims: An image pickup device comprising a plurality of first pixels (101; 121R) for outputting a first color signal and second pixels (111; 121G) for outputting a second color signal different from said first color signal arranged therein, characterized in that said first and second pixels (101; 121R, 111; 121G) have semiconductor junctions of different capacitances; and one of said first pixels (101; 121R) and one of said second pixels (111; 121G) are shielded against light, have a semiconductor junction of substantially the same capacitance as that of said first pixel (101; 121R) and said second pixel (111; 121G), respectively, and are disposed on the same substrate (1; 11; 301) to provide pixels for dark signal output for compensating the dark noise. A device according to claim 1, characterized in that said semiconductor junction is formed between a first semiconductor region (102, 112) of a first conductivity type and a second semiconductor region (103, 113) of a second conductivity type, and said second semiconductor regions (103, 113) of said first pixel (101; 121R) and said second pixel (111; 121G) have the same depth and a different area. A device according to claim 1, characterized in that said semiconductor junction is formed between a first semiconductor region (102, 112) of a first conductivity type and a second semiconductor region (103, 113) of a second conductivity type; and said second semiconductor regions (102, 112, 103, 113) of said first and second pixels (101; 121R, 111; 121G) have substantially the same depth and width, and different lengths. A device according to claim 1, characterized by color filters corresponding to said first and second pixels (101; 121R, 111; 121G). A device according to claim 1, characterized in that said first and second pixels have color filters of different spectrum transmittances, and capacitances compensating the difference between the spectrum transmittances of said filters. A device according to claim 1, characterized by further comprising a plurality of third pixels (121B) for outputting a third color signal, said third pixels having semiconductor junctions, and at least two of capacitances of said semiconductor junctions of said first, second and third pixels (101; 121R, 111; 121G, 121B) being different; and wherein one of said third pixels (121B) is shielded against light and has a semiconductor junction of substantially the same capacitance as that of said third pixel (121B). A device according to claim 6, characterized in that said semiconductor junctions of said first, second and third pixels (101; 121R, 111; 121G, 121B) have mutually different capacitances. A device according to claim 6, characterized in that said semiconductor junction is formed between a first semiconductor region (102, 112, 123B) of a first conductivity type and a second semiconductor region (103, 113, 122B) cf a second conductivity type; and said second semiconductor region (103, 113, 122B) of said first or second pixel ((101; 121R); (111; 121G)) and said second semiconductor region (122B) of said third pixel (121B) have the same depth and different areas. A device according to claim 6, characterized in that said semiconductor junction is formed between a first semiconductor region (102, 112, 123B) of a first conductivity type and a second semiconductor region (103, 113, 122B) of a second conductivity type; and said second semiconductor regions (103, 113, 122B) of said first, second and third pixels (101; 121R, 111; 121G, 121B) have the same depth and different areas. A device according to claim 1 or 6, characterized in that said semiconductor junction is a pn junction of a photodiode. A device according to claim 1 or 6, characterized in that said semiconductor junction is a base-collector junction of a phototransistor. A device according to claim 6, characterized by color filters corresponding to said third pixels (121G, 121B). A device according to claim 12, characterized in that said first, second and third pixels (101; 121R, 111; 121G, 121B) are red, green and blue pixels. An apparatus comprising: a device according to claim 1 or 6; and a converter (401) for converting said first, second and third color signals from said apparatus to other color signals. An apparatus comprising: a device according to claim 1 or 6; a converter (401) for converting said first and second color signals from said apparatus to other color signals; and a calculator for calculating said first, second and third color signals. An apparatus according to claim 14 or 15, characterized in that said apparatus has an ink jet recording head (403). An apparatus according to claim 14 or 15, characterized in that said apparatus has an ink jet recording head (403) for discharging ink by the use of heat energy. An apparatus according to claim 16 or 17, characterized in that said apparatus performs the recording by using inks including yellow, cyan, and magenta.

Inventors: OHZU HAYAO, OHZU, HAYAO, OHZU, HAYAO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: H01L27
IPC codes: H01L27