Solid-state image pickup device

Abstract: A liquid crystal layer (105) having upper and lower transparent electrodes (103, 109) is disposed above a photosite (113) for photo-electric converting and storing signal charges, and the liquid crystal layer (105) has an incident light transmission which is modulated by voltage, and upper (light incident side) transparent electrode (103) is supplied with a bias voltage, and the other transparent electrode (109) is in a floating condition (open circuit condition).

Description: The present invention relates to a solid-state image pickup device according to the preamble of claim 1. Such a device is known from EP-A-0 326 467. Heretofore there has been a problem that video cameras have used a lens aperture or electronic shutter to control the luminous energy of incident light, whereby it is impossible to control the luminous energy of the incident light in one picture element unit, and when an intense light is present locally in an image pickup plane, brume occurs at the point by the excessive luminous energy or an entire image plane becomes remarkably dark because of excessive backlight correction, that is, excessive luminous energy reduction in backlight condition by excessive shutting down of the iris (cf. U.S.P. 5,047,847). Thus, the luminous energy of incident light must be controlled in each pixel in order to solve the problem. And a technique of controlling the energy in each pixel is known (cf. Japanese Laid-open Patent No. 1988/35081). However, the prior art has a problem that in a system in which a feedback loop is formed over two or more frames/fields, an external circuit for feedback loop is necessary, and in addition, a time shift of at least one frame occurs unavoidably between the frame/field of detecting the luminous energy of incident light and the frame/field of feedback destination. The prior art cannot deal with a high speed moving luminous body, and needs the external circuit, thereby preventing the miniaturization of video cameras. Thus, a solid-state image pickup element becomes necessary in which the luminous energy of incident light is corrected in one pixel for the miniaturization, and a feedback loop is formed in the same frame/field for a high speed feedback. EP-A-0 326 467 discloses a selective dampening device, preferably applied to an image detector. Above the image detector, a liquid crystal layer is disposed. This liquid crystal layer is confined between control electrodes. One of the control electrodes is a light dependent resistor having its characteristic adapted such that when incident light quantity increases the voltage applied to the liquid crystal layer increases as well. It is the object of the invention to provide a solid-state image pickup device having an incident light control function of simple structure. This object is solved in accordance with the features of independent claim 1. Dependent claims are directed on preferred embodiments of the invention. The response speed of the incident light transmission control is fast. It takes place for a pixel during one frame/field even if an intense light occurs locally on any place of an image plane. By restricting an area of the incident light transmission reduction to a part of the photo-electric transfer area, it is possible to improve the modulation transfer function (MTF), thereby making the open area of the photo-electric transfer area narrow. A solid-state image pickup device comprises a photosite ( an photo-electric transfer part) disposed in matrix on a substrate ,where incident light is converted into signal charge and stored , for converting incident light to electron and for storing a photo-electrically converted signal charge, and an incident light control layer disposed at the light incident side of the photosite , having such a characteristics that an incident light transmission of the incident light control layer is modulated corresponding to the accumulated signal charge quantity. Fig. 1 is a perspective view of a floating electrode type pixel according to an embodiment of the present invention. Fig. 2 is an equivalent circuit of the floating electrode type pixel. Fig. 3 (A) is a graph showing an applied voltage-transmissions characteristics of liquid crystal and Fig.3(B) is a graph showing photo-electric conversion characteristics in the embodiment. Fig. 4 is a perspective view of a diffusion electrode-type pixel according to an embodiment of the present invention. Fig. 5 is an equivalent circuit of the diffusion electrode-type pixel. Fig. 6(A) is a perspective view of the structure of a pixel having an aperture formed by using transparent resin, Fig. 6(B) is a perspective view of the structure of a pixel having a lower electrode aperture, Fig. 6(C) is a perspective view of the structure of the pixel having an upper electrode aperture of the floating electrode type, and Fig. 6(D) is a perspective view of the structure of a pixel having an upper electrode aperture of the embodiment of the diffusion electrode type. Fig. 7 is a perspective view of the structure of the solid-state image pickup device in another embodiment. With reference to drawings, embodiments of the present invention will be described hereinafter. Fig. 1 shows a perspective view of a floating electrode-type pixel. Incident light is irradiated through a polarizing plate 101, a cover glass 102, a transparent electrode 103, an insulator 104, a liquid crystal 105, an insulator 106, a polarizing plate 107, an insulator 108, and a transparent electrode 109 to a photosite 113 constituted by N layer 114 and p-well 115 , and is converted to electron ,and then the signal charge is stored in a depletion layer capacitor formed at a junction part of the N layer 114 and the p-well 115. The photosite is separated from VCCD 117 by a channel stopper 116. A bias 118 is applied to a substrate and a bias 119 is applied to the liquid crystal 105. Poly-Si 110,111 are employed as electrodes of the VCCD 117. The operation of the floating electrode-type pixel of Fig. 1 is described as follows referring to an equivalent circuit of Fig.2 . In the equivalent circuit , a depletion layer formed at a junction part of the N layer 114 and the p-well 115 corresponds to storing capacitor 204. A signal charge ΔQ sig generated by the incident light is stored in the storing capacitor of the depletion layer 204 and the capacitor of the insulator 202 as a signal charge quantity 203. As a result , the voltage V liq applied to the liquid crystal 105 by the bias 119 changes by a variation ΔV liq . Using a liquid capacitance C liq 201, an insulator capacitance C i 202, and a capacitance of the depletion layer C PD 204, the variation ΔV liq of the voltage applied to the liquid crystal 105 can be expressed with the following equation (1): ΔV liq = ΔQ sig C liq C i +C i C PD +C PD C liq C i As described above , the voltage V liq applied to the liquid crystal 105 is reduced to (V liq - ΔV liq )205 by the photo generated signal charge . Fig.3 (A) shows the applied voltage - transmission characteristics of the liquid crystal 105. First, the transmission of the liquid crystal is the maximum value LC OFF 302 by the voltage V liq1 301 applied to the liquid crystal 105 . As described above, the voltage applied to the liquid crystal 105 varies to (V liq1 - ΔV liq )304 by the signal charge Q sig generated by incident light. As a result , the transmission of the liquid crystal 105 is reduced. And the higher the incident light quantity is, the more the voltage applied to the liquid crystal 105 varies and the more the transmission of the liquid crystal 105 is reduced. The state corresponds to LC ON 306. Fig.3(B) shows the photo-electric conversion characteristics of the invented pixel having the transmission characteristics of Fig.3(A). The line 308 shows the invented one and the broken line 307 shows the conventional. Both photo-electric characteristics have the same saturation level. However in the present invention , the more the incident light quantity increases , and the more the signal charge ΔQ sig increases, the more the voltage applied to the liquid crystal 105 is reduced as shown in the equation (1), and the light transmission is reduced as shown in Fig.3(A). As a result , in the solid line 308 of the photo-electric conversion characteristics , the saturation level 311 of the present invention pixel is shifted to a point of higher illumination side, thus revealing an expanded incident light range 312 as compared to the saturation level of the prior pixel. Shifting a knee point(bent point)313 of the photo-electric conversion characteristics line 308 to a point of lower illumination is realized by making the liquid crystal capacitance C liq 201 and the charging capacitance C PD 204 small, and/or making the insulator capacitance C i 202 large, because the shifting is realized by making the voltage ΔV liq modulated by the stored signal charge large in the equation (1). Fig. 4 show a perspective view of a diffusion electrode-type pixel element. Incident light is irradiated through a polarizing plate 401, a cover glass 402, a transparent electrode 403, an insulator 404, a liquid crystal 405, an insulator 406, a polarizing plate 407, an insulators 408, and a transparent electrode 409 to a photosite constituted by N layer 414 and p-well 415 , and is converted to electrons, and then the signal electrons are stored in a capacitor of a depletion layer formed in a junction part of the N layer 414 and the p-well 415. A bias 418 is applied to a substrate and a bias 419 is applied to the liquid crystal 405. Poly-Si 410,411 are employed as conversion electrode forming the VCCD 117. The transparent electrode 409 is connected to N layer 414. Fig. 5 shows a circuit equivalent to the diffusion electrode-type pixel. The capacitor in the depletion layer formed at the junction part of the N layer 414 and the p-well 415 for accumulating the signal charge as described above corresponds to a photosite capacitor 503. The signal charge Q sig generated by the incident light is stored in the storing capacitor 503 and the liquid crystal capacitor 501 as signal charge quantity 502. As a result, the voltage Viiq applied to the liquid crystal 405 by the bias 419 varies by ΔV liq . The variation ΔV liq of the voltage applied to the liquid crystal 405 can be expressed in the following equation (2) by using a liquid capacitance C liq 501 and an storing capacitance C PD 503. ΔV liq = ΔQ sig C PD +C liq In the embodiment , the upper limit of the incident light range is shifted to a point of higher illumination as compared to the prior CCD, as described in the photo-electric conversion characteristics line 308. Since the transparent electrode 409 is connected to N layer 414, more dynamic voltage modulation can be obtained. In the embodiment, shifting a knee point(bent point)313 of the photo-electric conversion characteristics line 308 to a point of lower illumination is realized by making the liquid crystal capacitance C liq 501 and the photodiode capacitance C PD 503 small, because the shifting is realized by making the voltage ΔV liq modulated by the stored signal charge large in the equation (2). Fig. 6(A) shows a perspective view of the structure of a floating gate type improving MTF. Since the outside appearance of the structure is the same as that of Fig.1, Fig.6(A) shows such structure that the polarizing plate 101 and the cover glass 102 are removed from the structure of Fig.1. Now 603 is a transparent electrode , 604 is an insulator , 605 is a liquid crystal , 606 is an insulator , 607 is a polarizing plate , 608 is an insulator , 609 is a transparent electrode , 614 is N layer, 615 is a p-well , 617 is VCCD , and 616 is a channel stopper. 618 is a bias and 619 is a bias and 610 and 611 are Poly-Si. A SiO 2 or a transparent resin 620 is formed boring through the liquid crystal 605 . The width of the transparent resin 620 is X-on 601 and the width of the photo-electric conversion area is X-off 602 which is larger than X-on 601. Fig.6(A) is a view in which the transparent electrode 603, the insulator 604 and the liquid crystal around the transparent resin 620 are removed for illustration convenience. When the incident light quantity is large , the light transmission decreases and the aperture for the N layer 614 varies from X-off 602 to X-on 601. Using a picture element pitch p and one side X of the aperture , the MTF of the element can be expressed in the following equation(3): MTF(X) = sin (2·π·X/p) 2·π·X/p Accordingly, in the above case, (MTF(X=X ON )) > (MTF(X=X OFF )) is obtained, which means that where there is a large amount of the luminous energy of incident light, the MTF can be improved. Fig. 6(B) shows a perspective view of the structure of a diffusion electrode type with improved MTF. Since the outside appearance of the structure is the same as that of Fig.4, in Fig.6(B) the polarizing plate 401 ,the cover glass 402, the transparent electrode 403 , the insulator 404 , the liquid crystal 405, the insulator 406, the polarizing plate 407 and the insulator 408 are not shown 623 is a lower transparent electrode , and 624 is an aperture of the lower transparent electrode 623 , and X- off 621 is the width of the aperture 624 and the X-on 622 is the width of the photo-electric conversion area ( the lower transparent electrode 623) . The structure of Fig.6(B) can improve MTF without using the transparent resin 620 of Fig.6(A), by using the lower transparent electrode 623 having the aperture 624. In the structure of Fig. 6(B), the part of the liquid crystal above the aperture 624 is not effected by the modulation voltage of the signal charge ΔQ sig , and the liquid crystal part becomes a state of LC OFF 302 and thereby the light transmission above the aperture 624 becomes constant irrespectively of the incident light quantity . The liquid crystal part between the transparent electrode 623 and the transparent electrode 103 is supplied with the modulation voltage of the signal charge ΔQ sig as shown by the equation (1). As a result, as indicated with solid line 308 of the photo-electric conversion characteristics , the saturation level 311 of the present invention pixel is shifted to a point of higher illumination, thus reveiling an expanded incident light range 312 as compared to the saturation level of the prior pixel. Fig.7 is an embodiment of the solid-state image pickup device applying the structure of the element unit of Fig.6(A), Fig6(B). 701 indicates p-layer, 702 indicates n-layer, 703 indicates 1st-Poly Silicon and 704 indicates 2nd-Poly Silicon. 705(real line hatching ) shows such light shut state LC ON of the liquid crystal ON by strong incident light and 706(chain line ) shows such light transparent state LC OFF of the liquid crystal OFF by non-strong incident light. Thus the liquid crystal ON realizes reduction of light transmission, and thereby the intensity level of the incident light can be fed back in same frame/field period and the element unit can be controlled individually. And as shown in Fig. 6(C), the upper transparent electrode 103 can be partly removed ,thereby forming an aperture and improving MTF. Meanwhile as a material for controlling the light transmission, electrochromism and so on which have voltage-light transmission characteristics as shown in Fig. 3(A) can be used. And further as shown in Fig. 6(D), the upper transparent electrode 103 can be partly removed in the embodiment of the diffusion electrode type ,thereby forming an aperture and improving MTF. Further MTF can be improved by partly restricting the liquid crystal 105 by using other method than the above mentioned using of the transparent resin (SiO 2 ) 620. For example, glass material can be used. The aperture or transparent resin can be disposed anywhere within the area of the photo-electric conversion area to improve MTF. As described above, by using a material for controlling light transmission, the luminous energy of the incident light can be corrected on a chip during the same frame/field in element unit , whereby the upper limit of the range of the luminous energy of the incident light may be shifted to a point of higher illumination. Then, a wide dynamic range can be obtained. Also, an aperture window narrower in area than that of the photo-electric conversion part is provided , thereby allowing an element in which the MTF is improved when there is a large amount of the luminous energy of incident light.

Claims: A solid-state image pickup device comprising: photosites (114, 115; 413, 414, 415) disposed in matrix form on a substrate, for converting incident light to photoelectrons and for storing a photo-electrically converted signal charge, and an incident light control layer (105; 405) disposed at a light incident side of said photosites, characterized by means (109,409) for modulating the light transmission of said incident light control layer corresponding to the stored signal charge quantity. A device in accordance with claim 1, characterized in that said incident light control layer (105; 405) has a transparent electrode (109; 409); said incident light transmission is modulated by voltage, and said stored signal charge effects said transparent electrode (109; 409), thereby modulating said incident light transmission. A device in accordance with claim 1, characterized in that said incident light control layer (105; 405) has a first transparent electrode (103) disposed at the light incident side of said incident light control layer and a second transparent electrode (109) disposed between said incident light control layer and said photosite (113, 114, 115), said incident light transmission is modulated by voltage, said first transparent electrode (103) is supplied with a bias voltage, and said second transparent electrode (109) is in a floating condition. A device in accordance with claim 1, characterized in that said incident light control layer (405) has a first transparent electrode (403) disposed at light incident side of said incident light control layer and a second transparent electrode (409) disposed at said photosite (413, 414, 415), said incident light transmission is modulated by voltage, said first transparent electrode (403) is supplied with a bias voltage, and said second transparent electrode (409) is connected to a depletion impurity diffusion part of said photosite (413, 414, 415). A device in accordance with claim 1, characterized in that the area of a part where said incident light transmission is modulated, is narrower than an area of an aperture section of said photosite. A device in accordance with claim 3 or 4, characterized in that said first transparent electrode (103; 403) has an aperture section narrower than an aperture section of said photosite (113; 114). A device in accordance with claim 3 or 4, characterized in that said second transparent electrode (109; 409) has an aperture section narrower than an aperture section of said photosite. A device in accordance with claim 1, characterized in that said incident light control layer is disposed at a partial section above an aperture section of said photosite. A device in accordance with claim 8, characterized in that at other section above said aperture section of said photosite, a transparent member is disposed.

Inventors: KOMOBUCHI HIROYOSHI, KOMOBUCHI, HIROYOSHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1992
Main classification: G02F1
IPC codes: G02F1, H04N5