Linear image sensor

Abstract: In a linear image sensor, a charge transfer part is disposed between a one-dimensional array of photodetectors and an overflow drain and comprises a CCD including four or more transfer gates for each photodetector and transferring signal charges from the photodetector in a direction of the photodetector array, transfer gates controlling charge transfer from the photodetectors to the CCD, and shutter gate controlling charge transfer from the charge transfer part to the overflow drain. Each transfer gate is disposed between each photodetector and a prescribed one of the four or more CCD transfer gates, and each shutter gate is disposed between the prescribed CCD transfer gate and the overflow drain. The four or more CCD transfer gates are controlled by four or more phase driving clocks. Therefore, during the charge transfer operation, there is a state where at least adjacent three CCD transfer gates are all closed, so that signal charges are drained from the photodetector into the overflow drain through the CCD transfer gate in the middle of the three CCD transfer gates without mixing the drained signal charges with the signal charges transferred by the CCD. That is, the charge drain operation is carried out during the charge transfer operation in the charge transfer part, whereby a timing for opening the shutter gate can be set at an arbitrary time in the charge read cycle.

Description: The present invention relates to a linear image sensor with an electron shutter mechanism and a method for driving the linear image sensor. With the progress in silicon LSI technology in recent years, a linear image sensor, in which a one-dimensional array of photodetectors and a charge coupled device (hereinafter referred to as CCD) transferring signal charges generated in the photodetectors are combined on a semiconductor substrate, has been put to practical use. Photodiodes utilizing p-n junction or Schottky barrier diodes are used as the photodetectors. The linear image sensor is called as a visible image sensor or an infrared image sensor, depending on a wavelength detected by the image sensor. Some of these image sensors are provided with an electron shutter mechanism for preventing saturation of signal charges in the photodetectors. Figures 15(a) and 15(b) illustrate a prior art infrared linear image sensor with such an electron shutter mechanism, in which figure 15(a) is a plan view schematically showing a layout of elements constituting the sensor and figure 15(b) is a sectional view taken along a line XVb-XVb of figure 15(a). In figure 15(a), an infrared linear image sensor 200 includes a p type silicon substrate 9, a one-dimensional array of infrared detectors 1, a CCD 12, and an overflow drain 13. The CCD 12 and the over-flow drain 13 run parallel with each other with the array of the infrared detectors 1 between. Each infrared detector 1 is connected to the CCD 12 through a transfer gate 4 which controls transfer of charges from the infrared detector 1 to the CCD 12. In addition, each infrared detector 1 is connected to the overflow drain 13 through a shutter gate 15 which controls transfer of charges from the infrared detector 1 to the overflow drain 13. Gates of transistors constituting the transfer gates 4 and the shutter gates 15 are connected to input terminals It and Is, respectively, and controlled by clock signals from the input terminals It and Is. In this structure, the CCD 12 constitutes a charge transfer circuit which transfers signal charges generated in the infrared detectors 1 in a direction of the array of infrared detectors, and an output amplifier 8 is connected to an output end of the charge transfer circuit. The overflow drain 13 is biased with positive voltage from the outside and signal charges generated in the detectors 1 are drawn into the overflow drain 13. A description is given of a sectional structure of the linear image sensor 200 with reference to figure 15(b). In figure 15(b), reference numeral 10 designates a PtSi/Si Schottky barrier diode as the above-described infrared detector 1, which comprises a platinum silicide layer 1a selectively disposed on the p type silicon substrate 9 and a p type silicon region 9a contacting with the platinum silicide layer 1a to produce a Schottky junction. In addition, a guard ring 11 is disposed around the p type silicon region 9a, so that leakage current from the infrared detector 1 is reduced. The guard ring 11 and the platinum silicide layer 1a serve as a charge storage region of the infrared detector 1. An n type impurity diffused region 12d as a CCD channel and an n type impurity diffused region as the overflow drain 13 are disposed at opposite sides of the p type silicon region 9a of the infrared detector 1 with prescribed intervals from the p type silicon region 9a. In addition, a CCD transfer gate 12a comprising polysilicon is disposed on the n type impurity region 12d through a gate insulating film 10a. The CCD transfer gate 12a is connected to an input terminal Ic that gives a transfer clock to the CCD 12. A p type impurity region 4b having a prescribed concentration is disposed between the guard ring 11 and the CCD channel 12d and a p type impurity region 15b having a prescribed concentration is disposed between the guard ring 11 and the n type impurity region of the overflow drain 13. Gate electrodes 4a and 15a comprising polysilicon are disposed on the p type impurity region 4b and the p type impurity region 15b, respectively, through gate insulating films 10a. Thus, transistors as the transfer gate 4 and the shutter gate 15 are constituted. Characteristics of the transistors, i.e., potential levels in their ON and OFF states, are varied by variation in concentrations of the p type impurity regions 4b and 15b. Therefore, the concentrations are set so that the potential levels of the transistors as the transfer gate and the shutter gate may be equal to each other. Figure 16 is a schematic diagram for explaining operations of the transfer gate and the shutter gate in a charge read cycle Xr of the image sensor of figures 15(a) and 15(b). Figures 17(a)-17(f) are schematic diagrams illustrating potential profiles during the charge read operation in a cross-section taken along a line XVb-XVb of figure 15(a). The charge read operation will be described using figures 16 and 17(a)-17(f). When a driving clock CLt of the transfer gate 4 rises at time Ta0, the transfer gate 4 opens, that is, potential level of the transfer gate 4 falls down to an ON level Et on from an OFF level Et off as shown in figure 17(f), and signal charges Cs 2 stored in the photodetector 10 till then are transferred to the CCD channel 12d through the p type impurity region 4b. When the driving clock CLt falls down at time Ta1, the potential level of the transfer gate 4 returns to the level Et off , and the transfer gate 4 closes. At this time, the potential level Eid of the infrared detector 1 is reset to the ON level Et on (energy level t) of the transfer gate 4. More specifically, when the transfer gate 4 opens, the platinum silicide layer 1a in its floating state is connected to the CCD channel 12d whose potential level is lower than the potential level of the platinum silicide layer 1a, whereby the signal charges Cs2 are transferred toward the CCD channel 12a through the platinum silicide layer 1a and the transfer gate 4. When the transfer gate 4 is closed, the platinum silicide layer 1a is separated from the CCD channel 12d and returns into the floating state, so that the platinum silicide layer 1a is charged in accordance with the amount of the transferred charges. In addition, the transfer of signal charges is carried out until the potential level of the infrared detector 1 becomes equal to the lowered potential level Et on of the transfer gate 4. Meanwhile, in the infrared detector 1, signal charges Cs1 are stored in accordance with the amount of incident infrared light during a period of Ta1 < t < ta2, until the shutter gate 15 opens, as shown in figure 17(b). When the driving clock Cls of the shutter gate 15 rises at time Ta2, the shutter gate 15 opens, that is, potential level of the shutter gate 15 lowers to an ON level Es on from an OFF level Es off as shown in figure 17(c), and the signal charges Cs1 stored in the photodetector 10 till then are drawn into the n type impurity region of the overflow drain 13 through the n type impurity region 15b. When the driving clock CLs falls down at time Ta3, the potential level of the shutter gate 15 returns to the level Es off and the shutter gate 15 closes as shown in figure 17(d). At this time, the potential level Eid of the infrared detector 1 is reset to the ON level Es on of the shutter gate 15 (energy level s) like the above-described opening and closing operations of the transfer gate 4. This reset level s of the shutter gate 15 is equal to the reset level t of the transfer gate 4. Thereafter, in the infrared detector 1, signal charges Cs2 are stored in accordance with the amount of incident infrared light as shown in figure 17(e) during a period of Ta3 < t < ta4 until the shutter gate 15 opens, i.e., during a shutter period Xs. The signal charges Cs2 stored during the shutter period Xs are transferred to the CCD channel 12 within a pulse period of the driving clock Clt of the transfer gate 4, i.e., a period of Ta4 ≦ t < Ta5 (figure 17(f)). Thereafter, the charge read operation from the infrared detector 1 to the CCD 12 is carried out in the charge read cycle Xr as described with respect to figures 17(a) to 17(f). In the charge read cycle Xr, the CCD 12 conducts the transfer operation of signal charges from the infrared detector 1. Hereinafter, the charge transfer operation will be described with reference to figures 18 to 20. Figure 18 is a plan view illustrating a structure of CCD transfer gates of the linear image sensor 200 and connections between the CCD transfer gates and driving clocks therefor. Figure 19 is a schematic diagram illustrating waveforms of three phase driving clocks 1 to 3 for the CCD 12 in addition to the driving clocks CLt and CLs of figure 16. Figure 20 is a schematic diagram for explaining a manner of transferring signal charges in an infrared detector to a position corresponding to the next infrared detector in the CCD 12. As shown in figure 18, the CCD 12 is a three-phase driving CCD which includes three transfer gates 12a to 12c for one infrared detector 1. In this structure, a transfer gate 4 is disposed between each infrared detector 1 and each transfer gate 12a, and signal charges generated in the infrared detector 1 are transferred to a region directly under the transfer gate 12a of the CCD channel 12d. In addition, a shutter gate 15 is disposed between each infrared detector 1 and the overflow drain 13. As shown in figure 20, when a pulse of the driving clock CLt is applied to the transfer gate input I t in the period of Ta0 ≦ t < Ta1 in which potential wells are formed directly under the transfer gates 12a, signal charges A to D are sent to the potential wells under the first transfer gates 12a from the respective infrared detectors 1. When the driving clock CLt falls down at time Ta1, the charge transfer operation begins. More specifically, phases of the CCD driving signals 1 to 3 are different by 120 degrees from each other. Therefore, the potential wells are formed beneath the first and second transfer gates 12a and 12b during the first transfer period of Ta1 ≦ t < t1, directly beneath the second transfer gates 12b during the second transfer period of t1 ≦ t < t2, and beneath the second and third transfer gates 12b and 12c during the third transfer period of t2 ≦ t <t3. When the fourth transfer period of t3 ≦ t < t4 and the fifth transfer period of t4 ≦ t < t5 have passed away, signal charges A to D are present in the potential wells beneath the next transfer gates 12a. That is, during the above-described period of Ta0 ≦ t < t6, the signal charges A to D move a distance corresponding to one infrared detector. The CCD 12 repeats the transfer operation until the next rise of the driving clock CLt of the transfer gate 4 to finish the transfer operation. In the linear image sensor 200 having the electron shutter mechanism, however, if the potential level t of the infrared detector 1 which is reset at the time of the opening and closing operations of the transfer gate 4 is different from the potential level s of the infrared detector 1 which is reset at the time of the opening and closing operations of the shutter gate 15 even in a small degree, an accurate amount of the infrared light cannot be detected. That is, the detector reset level t at the time of reading charges depends on characteristics of the transfer gate transistor and the detector reset level s at the time of sweeping charges depends on characteristics of the shutter gate transistor. Therefore, if there is a difference, even a slight difference, in characteristics between the transfer gate transistor and the shutter gate transistor due to variations in the wafer process, the reset level of the infrared detector at the time of transferring charges from the infrared detector to the CCD is unfavorably different from the reset level of the infrared detector at the time of sweeping charges into the overflow drain 13, whereby some signal charges remain in the infrared detector without being transferred to the CCD or excessive charges are transferred to the CCD. This problem will be described in more detail. Figures 21(a)-21(f) and figures 22(a)-22(f) illustrate potential profiles each corresponding to the potential profile of figures 17(a)-17(f). Figures 21(a)-21(f) show a case where the potential level s of the infrared detector 1 which is reset at the time of sweeping charges is lower than the potential level t of the infrared detector 1 which is reset at the time of transferring charges. In this case, when the shutter gate 15 operates, charges C0 are excessively drawn into the overflow drain in addition to the stored charges Cs1 as shown in figure 21(c). Thereafter, when the transfer gate 4 conducts the charge read operation, signal charges Cs3 at a storage level lower than a difference between the level Et on at the ON time of the transfer gate 4 and the level Es on at the ON time of the shutter gate 15, i.e., an energy difference of t - s, unfavorably remain in the infrared detector 1. On the other hand, figures 22(a)-22(f) show a case where the potential level s of the infrared detector 1 which is reset at the time of sweeping charges is higher than the potential level t of the infrared detector 1 which is reset at the time of transferring charges. In this case, when the shutter gate 15 operates, a part of the stored charges Cs1, i.e., charges Cs5 are only drawn into the overflow drain leaving charges Cs4 as shown in figure 22(c). Thereafter, when the transfer gate 4 conducts the charge read operation, charges Cs4 whose amout is equivalent to a difference between the level Et on at the ON time of the transfer gate 4 and the level Es on at the ON time of the shutter gate 15, i.e., an energy difference of t - s, are excessively sent to the CCD in addition to the signal charges Cs2. As the result, contrast of signal is lowered. A monolithic integrated circuit for line scanning which is similar to the structure shown in Fig. 18 is disclosed in EP-A-0 026 380. This circuit also comprises a semiconductor substrate 1, a one-dimensional array of photodetectors, a charge transfer part and a charge drain part. Said charge transfer part having transfer gates and shutter gates. Instead of using three phase driving clocks which control said transfer gates as described with respect to Fig. 18 this circuit includes four transfer gates and therefore uses four phase driving clocks. Said array of photodetectors is disposed between said charge drain part and said charge transfer part. Since said charge transfer part and said charge drain part are arranged at opposite sides of said array of photodetectors a charge transfer operation and a charge drain operation can easily be performed without mutual influence but the same problems arise as they have been described before with respect to Fig. 18. In order to solve these problems, an improved linear image sensor has been developed by the inventor of the present invention. Figures 23(a) and 23(b) are diagrams illustrating the improved linear image sensor, in which figure 23(a) is a plan view schematically showing a layout of elements constituting the sensor and figure 23(b) is a sectional view taken along a line XXIIIb-XXIIIb of figure 23(a). In these figures, reference numeral 300 designates the improved linear image sensor. In the sensor 300, an overflow drain 3 and a CCD 12 are disposed on one side of an array of infrared detectors 1 and the CCD 12 is disposed between the overflow drain 3 and the infrared detector array. Shutter gates 5 are disposed between the CCD 12 and the overflow drain 3, so that signal charges generated in the infrared detector array are drawn into the overflow drain 3 through the CCD 12. In addition, a p type impurity region 5b is disposed between a transfer channel 12b of the CCD 12 and an n type impurity region of the overflow drain 3, and a polysilicon gate 5a is disposed on the p type impurity region 5b through a gate insulating film 10a. The p type region 5b and the polysilicon gate 5a constitute a shutter gate transistor. Other structures are the same as those of the linear image sensor shown in figure 15. Figure 24 is a plan view illustrating a structure of CCD transfer gates of the linear image sensor 300 and connections between the CCD transfer gates and driving clocks therefor. Figures 25(a)-25(g) are schematic diagrams illustrating potential profiles during a charge read operation of the linear image sensor 300 in a cross-section taken along the line XXIIIb-XXIIIb of figure 23(a). Figure 26 is a schematic diagram illustrating waveforms of signals applied to the respective gates of the linear image sensor 300. The charge read operation will be described using these figures. First of all, when the driving clock CLt of the transfer gate 4 rises at time Ta0, the transfer gate 4 opens, that is, the potential level of the transfer gate 4 falls to an ON level Et on from an OFF level Et off , and signal charges Cs stored in the infrared detector 1 till then are transferred to the CCD channel 12b through the n type impurity region 4b (figure 25(f)). When the driving clock CLt falls down at time Ta1, the potential level of the gate 4 returns to the level Et off and the gate 4 closes. At this time, the potential level Eid of the infrared detector 1 is reset to the ON level Et on of the transfer gate 4 (an energy level t) for the same reason as in the case of the sensor 200 shown in figure 15. At the same time, charge transfer operation begins in the CCD 12. Thereafter, during the period of Ta1 < t < Tb2 of figure 25(a), i.e., the transfer period Xt of figure 26, signal charges are successively stored in the infrared detector 1 as shown in figure 25(a) while the charge transfer operation is carried on in the CCD 12. Thereafter, during the period of Tb2 ≦ t < Tb3, i.e., the period until the shutter gate 5 opens, signal charges Ce is stored in the infrared detector 1 in accordance with the amount of incident infrared light as shown in figure 25(b). When the driving clock CLs of the shutter gate 5 and the driving clock CLt of the transfer gate 4 rise at time Tb3, these gates 5 and 4 open, that is, the potential levels of the gates 5 and 4 fall to the ON levels Es on and Et on from the OFF levels Es off and Et off , respectively, whereby signal charges Ce stored in the infrared detector 1 are drawn into the n type impurity region of the overflow drain 3 through the channel 12b of the CCD 12. When the driving clocks CLs and CLt rise at time Tb4, the potential levels of the gates 5 and 4 return to the levels of Es off and Et off , respectively, and the gates 5 and 4 close (figure 25(d)). At this time, the potential level Eid of the infrared detector 1 is reset to the ON level Et on (t) of the transfer gate 4, regardless of the ON level Es on of the shutter gate 5, like in the case of the opening and closing operations of the transfer gate 4, so that the potential level of the infrared detector 1 in the charge read operation is not different from that in the charge drain operation. Thereafter, in the infrared detector 1, signal charges Cs are stored in accordance with the amount of incident infrared light until the transfer gate 4 opens, during the shutter period Xs, i.e., the period of Tb4 < t < Tb5 (figure 25(e)). Charges Cs stored during this period are transferred to the CCD channel 12 within the pulse period of the driving clock CLt of the transfer gate 4, i.e., the period of Tb5 ≦ t < Tb6 (figure 25(f)). When the driving clock CLt rises at time Tb6, the CCD 12 begins the charge transfer operation. Thereafter, charges are successively transferred from the infrared detector 1 to the CCD 12 in the charge read cycle Xr as shown in figures 25(a) to 25(g). In order to achieve the above-described operation, it is necessary to set the potential levels so as to be deeper in the order of the transfer gate (Et on ), the CCD channel (E cc ), the shutter gate (Es on ) and the overflow drain (Eod). This is easily achieved by controlling impurity concentrations of the p type and n type impurity regions of the respective elements. In the above-described linear image sensor 300, unnecessary charges generated in the infrared detector are drawn into the charge exhaust part like the overflow drain through the charge transfer part. This idea is also disclosed in Japanese Published Patent Applications 62-200761 and 60-120555. More specifically, Japanese Published Patent Application 62-200761 discloses a solid-state imaging element in which a charge transfer part is disposed between a one-dimensional array of infrared detectors and an overflow drain, and charges flown into the charge transfer part due to excess light incident on the infrared detector are drawn into the overflow drain. However, this device has no electron shutter mechanism. In addition, Japanese Published Patent Application 60-120555 discloses a solid-state imaging device in which two CCD registers are disposed on opposite sides of a one-dimensional array of infrared detectors and two dummy shift registers are disposed on the outer sides of the CCD registers and charges remaining in the infrared detectors are drained into the dummy shift registers through the CCD shift registers. In the above-described linear image sensor 300, since the signal charges Ce stored in the infrared detector 1 are drawn into the overflow drain 3 through the CCD 12, when the charge drain operation is conducted during the charge transfer operation of the CCD 12, transferred signal charges are sometimes mixed with the drawn charges. In this case, the shutter gate 5 cannot be turned ON during the transfer operation of the CCD 12. In order to avoid this, the CCD transfer period and the charge drain period are set in different periods of time. However, the shutter period Xs is limited to a short period after the charge transfer period Xt in the charge read cycle Xr, so that signal charges are not sufficiently stored especially when the amount of incident infrared light is small. It is an object of the present invention to provide a linear image sensor in which a perfect electron shutter operation is conducted without a difference in reset levels of a detector between a charge read operation and a charge drain operation, a shutter period is set in an arbitrary period in a charge read cycle, and sufficient amount of signal charges are stored even if the amount of incident infrared light is small. This object is solved by the subject matter of claim 1. A method of driving such a linear image sensor is disclosed in claim 7. Further advantageous modifications are subject matter of the subclaims. According to the subject matter of claim 1, there is therefore, during the charge transfer operation, a state where at least adjacent three CCD transfer gates are all closed, so that signal charges are drained from the photodetector into the overflow drain through the CCD transfer gate in the middle of the three CCD transfer gates without mixing the drained signal charges with signal charges transferred by the CCD. That is, the charge drain operation is carried out during the charge transfer operation in the charge transfer part, whereby a timing for opening the shutter gate can be set at an arbitrary time in the charge read cycle. According to the subject matter of claim 2, even when adjacent three CCD transfer gates among the five CCD transfer gates are used for the charge drain operation during the charge transfer operation, remaining two CCD transfer gates are used for the charge transfer operation, so that signal charges from the photodetector is received by a potential well equivalent to the two CCD transfer gates and the amount of transferred signal charges can be doubled. According to the subject matter of claim 5, as compared with the five-phase driving CCD according to claim 2, one CCD transfer gate and a driving clock therefor are dispensed with, whereby the structure of the CCD is simplified. In the drawings: Figure 1 is a plan view illustrating a structure of an infrared linear image sensor according to a first embodiment of the present invention; Figure 2 is a diagram illustrating waveforms of signals for explaining a charge read operation of a five-phase driving CCD in the linear image sensor of figure 1; Figures 3(a)-3(g) are diagrams illustrating potential profiles in a cross-section taken along a line III-III of figure 1 during the charge read operation; Figure 4 is a diagram illustrating potential profiles in a charge transfer direction during a charge transfer operation of the five-phase CCD of figure 2; Figure 5 is a diagram illustrating potential profiles of the CCD for explaining stored charge drain operation during the charge transfer operation of the five-phase CCD; Figure 6 is a plan view illustrating a structure of an infrared linear image sensor with a four-phase driving CCD in accordance with a second embodiment of the present invention; Figure 7 is a diagram illustrating waveforms of signals for explaining a charge read operation of the four-phase driving CCD of the image sensor of figure 6; Figures 8(a)-8(g) are diagrams illustrating potential profiles in a cross section taken along a line VIII-VIII of figure 6 during the charge read operation; Figure 9 is a diagram illustrating potential profiles in a charge transfer direction during the charge transfer operation of the four-phase CCD; Figure 10 is a diagram illustrating potential profiles for explaining stored charge drain operation during the charge transfer operation of the four-phase CCD; Figure 11 is a plan view illustrating a structure of an infrared linear image sensor in which the CCD of the second embodiment is driven by eight-phase driving clocks, in accordance with a third embodiment of the present invention; Figure 12 is a diagram illustrating waveforms of signals for explaining a charge read operation of the eight-phase driving CCD of the image sensor of figure 11; Figure 13 is a diagram illustrating potential profiles in a charge transfer direction during the charge transfer operation of the eight-phase driving CCD; Figure 14 is a diagram illustrating potential profiles for explaining stored charge drain operation during the charge transfer operation of the eight-phase driving CCD; Figures 15(a) and 15(b) are diagrams illustrating an infrared linear image sensor with an electron shutter mechanism according to the prior art, in which figure 15(a) is a schematic plan view and figure 15(b) is a sectional view taken along a line XVb-XVb of figure 15(a); Figure 16 is a diagram illustrating a relation between the charge read cycle and the shutter period of the prior art infrared linear image sensor; Figures 17(a)-17(f) are diagrams illustrating potential profiles a cross-section taken along a line XVb-XVb of figure 15(a) during the charge read operation of the prior art infrared linear image sensor; Figure 18 is a plan view illustrating a layout of gate electrodes of respective elements in the prior art infrared linear image sensor; Figure 19 is a diagram illustrating waveforms of signals for the charge transfer operation of the prior art infrared linear image sensor; Figure 20 is a diagram illustrating potential profiles in the CCD during the charge transfer operation; Figures 21(a)-21(f) are diagrams illustrating potential profiles for explaining problems in the case where the potential of the transfer gate at ON time is higher than that of the shutter gate; Figures 22(a)-22(f) are diagrams illustrating potential profiles for explaining problems in the case where the potential of the transfer gate at ON time is lower than that of the shutter gate; Figures 23(a) and 23(b) are diagrams illustrating an infrared linear image sensor according to the prior art wherein the problems of the image sensor of figure 15 are solved, in which figure 23(a) is a schematic plan view and figure 23(b) is a sectional view taken along a line XXIIIb-XXIIIb of figure 23(a); Figure 24 is a plan view illustrating a layout of gate electrodes of respective elements in the prior art image sensor; Figures 25(a)-25(g) are diagrams illustrating potential profiles in a cross-sectional taken along the line XXIIIb-XXIIIb during the charge read operation of the prior art image sensor; and Figure 26 is a diagram illustrating waveforms of signals for conducting the charge read operation of the prior art image sensor. Figure 1 is a plan view showing a structure of an infrared linear image sensor according to a first embodiment of the present invention, in which the same reference numerals as in figure 24 designate the same or corresponding parts. In figure 1, a CCD 2 is driven by first to fifth phase clock signals 1 to 5 and includes first to fifth transfer gates 2a to 2e for one infrared detector 1. The clock signals 1 to 5 are applied to the transfer gates 2a to 2e, respectively. Other structures are the same as those of the conventional linear image sensor shown in figure 24, that is, the transfer gate 4 is disposed between the first transfer gate 2a and the infrared detector 1 and the shutter gate 5 is disposed between the first transfer gate 2a and the overflow drain 3. In addition, reference numeral 2f designates an n type channel region of the CCD 2. A description is given of the operation. Figure 2 illustrates clock signals applied to the respective gates of the linear image sensor of figure 1 during the charge read operation. Figures 3(a)-3(g) illustrate potential profiles in a cross section taken along a line III-III of figure 1 during the charge read operation. Figure 4 illustrates a manner of transferring signal charges in the CCD. Figure 5 illustrates a timing for draining signal charges during the charge transfer operation. First of all, an ordinary charge transfer operation of the CCD 2 will be described. During a period of Ta0 ≦ t < Ta1 in which a potential well is created directly under the first and second transfer gates 2a and 2b of the CCD 2, the transfer gate control clock CLt rises at time Ta0 and the transfer gate 4 opens as shown in figure 3(f), whereby signal charges Cs stored in the infrared detector 1 flow into the potential well. When the control clock CLt falls down at time Ta1, the transfer gate 4 closes as shown in figure 3(g) and transfer of the signal charges Cs begins as shown in figure 4. In figure 4, reference characters A to D designate charges transferred from the infrared detectors to the potential wells beneath the first and second transfer gates 2a and 2b corresponding to the respective infrared detectors. This transfer operation will be described with reference to figure 4. In figure 4, since the CCD driving clocks have five phases, i.e., the phases of clocks 1 to 5 are delayed by 72 degrees (2π/5) from each other, a period for transferring signal charges from a position corresponding to an infrared detector to a position corresponding to the next infrared detector is divided into first to tenth transfer periods. First of all, during the first transfer period of Ta1 ≦ t < t1, the potential wells created beneath the first and second transfer gates 2a and 2b extend beneath the third transfer gates 2c, and the potential wells beneath the first transfer gates 2a disappear during the second transfer period of t1 ≦ t < t2, whereby the signal charges A to D transferred from the infrared detectors 1 to the CCD 2 move a distance equivalent to one transfer gate. Subsequently, during the third transfer period of t2 ≦ t < t3 and the fourth transfer period of t3 ≦ t < t4, the signal charges A to D move a distance equivalent to one transfer gate. Thereafter, the signal charges move in this way during the fifth and sixth transfer periods, the seventh and eighth transfer periods, and the ninth and tenth transfer periods, whereby the signal charges A to D move a distance equivalent to one infrared detector 1. Repeating the transfer operation, the CCD 2 successively outputs the signal charges to the outside through the output amplifier 8. A description is given of a shutter operation during the charge transfer operation, i.e., an operation for draining signal charges out of the infrared detectors 1. Figures 3(a)-3(g) show potential profiles in a cross-section taken along a line III-III of figure 1 during the transfer period of tn0 ≦ t ≦ tn7 in which the shutter operation is carried out. Especially figure 3(a) shows a period of tn0 < t < tn1 in which a potential well is created beneath the first transfer gate 2a, and figure 3(b) shows the third transfer period of tn3 ≦ t < Tc2 prior to the fourth transfer period of Tc2 ≦ t < Tc3 in which the shutter operation is carried out. In the fourth transfer period, no potential well is created beneath the first transfer gate 2a and the fifth and second transfer gates 2e and 2b disposed at opposite sides of the first transfer gate 2a during the normal charge transfer operation. In the transfer cycle for conducting the shutter operation during the charge transfer operation, the driving clock 1 for the first transfer gate 2a and the transfer gate control clock CLt rise together with a shutter gate control clock CLs at time Tc2 in the fourth transfer period, whereby, as shown in figure 3(c), the potential levels of the transfer gate 4 and the shutter gate 5 fall down to the levels Et on and Es on , respectively, and at the same time, the potential level of the first transfer gate 2a falls down to the level Ecc at which the transfer well is created. Thereby, signal charges Ce stored in the infrared detectors 1 are drawn into the overflow drain 3 through the potential wells beneath the first transfer gates 2a as shown in figure 5. When the control clocks CLt and CLs fall down as shown in figure 3(d), the infrared detector 1, the CCD 2, and the overflow drain 3 are separated from each other. Thereafter, storage of signal charges is carried on in the infrared detectors 1 and signal charges are successively transferred through the first transfer gates 2a in the CCD 2. Figure 3(e) shows a potential profile at time tn7 during such an operation. When the transfer gate control clock CLt rises at time Ta4, the transfer gate 4 opens as shown in figure 3(f) and signal charges Cs stored in the infrared detector 1 flow into the potential well beneath the first and second transfer gates 2a and 2b. When the control clock CLt falls down at time Ta5, the transfer gate 4 closes as shown in figure 3(g) and transfer of the signal charges Cs starts in the CCD 2. According to the first embodiment of the present invention, the CCD 2 disposed between the one-dimensional array of infrared detectors 1 and the overflow drain includes five transfer gates 2a to 2e for each infrared detector 1, and five-phase driving clocks 1 to 5 are applied to the transfer gates 2a to 2e, respectively. Therefore, during the charge transfer operation, there is a state where adjacent three transfer gates are all closed. When the adjacent three transfer gates 2e, 2a, and 2b are all closed, signal charges are drained from the infrared detector 1 into the overflow drain 3 through the first transfer gate 2a in the center of the three transfer gates without mixing the drained signal charges into the signal charges transferred by the CCD 2. That is, the signal drain operation is conducted in the charge transfer part during the charge transfer operation, whereby a timing for opening the shutter gate can be set at an arbitrary time in the charge read cycle. In addition, the CCD 2 includes the five transfer gates 2a to 2e for one infrared detector and these transfer gates 2a to 2e are controlled by the five-phase driving clocks 1 to 5, respectively. Therefore, a margin equivalent to one transfer gate is obtained in the charge transfer operation and this can be used for the transfer operation. As the result, signal charges from the infrared detector are transferred to the potential well equivalent to two transfer gates, so that the amount of the transferred charges can be doubled. Furthermore, a difference in detector reset levels between the charge read operation and the charge drain operation can be avoided. More specifically, in the charge storage period, the potential level Eid of the infrared detector 1 is reset to the potential level Et on of the transfer gate 4. In the charge drain period, a positive voltage is applied to the transfer gate 4 and the shutter gate 5, and the potential levels of the transfer gate 4 and the shutter gate 5 fall down to the levels Et on and Es on , respectively, as shown in figure 3(c), whereby signal charges Cs are transferred to the overflow drain 3 through the CCD channel 12. Also in this case, the potential level Eid of the detector is reset to the potential level Et on of the transfer gate 4. Therefore, the reset level of the detector only depends on the characteristics of the transistor of the transfer gate 4, and an energy level T is always obtained in the charge read period and the charge drain period, so that the both levels do not shift due to variation in wafer process and a perfect electron shutter operation is performed. While in the above-described first embodiment the five-phase driving CCD is employed, a four-phase driving CCD may be employed when a smaller amount of charges are transferred. A description is give of a linear image sensor with a four-phase driving CCD in accordance with a second embodiment of the present invention. Figures 6, 7, 8(a)-8(g), 9, and 10 are diagrams for explaining the linear image sensor according to the second embodiment of the present invention and correspond to figures 1, 2, 3(a)-3(g), 4, and 5 used for explaining the first embodiment, respectively. Figure 6 is a plan view illustrating a layout of each element of the infrared linear image sensor. Figure 7 illustrates clock signals applied to the respective gates of the linear image sensor during the charge read operation. Figures 8(a)-8(g) illustrate potential profiles in a cross section taken along a line VIII-VIII of figure 6 during the charge read operation. Figure 9 illustrates a manner of transferring signal charges in the CCD. Figure 10 illustrates a timing for draining signal charges during the charge transfer operation. In this second embodiment, a four-phase CCD 20 includes first to fourth transfer gates 20a to 20d for one infrared detector 1, and four-phase driving clocks 1 to 4 are applied to the transfer gates 20a to 20d, respectively. Other structures are the same as those of the device according to the first embodiment. Reference numeral 20e designates an n type charge transfer channel of the CCD 20. A description is given of the operation. First of all, an ordinary charge transfer operation of the CCD 20 will be described. During a period of Ta0 ≦ t < Ta1 in which a potential well is created directly under the first transfer gate 20a of the CCD 20, the transfer gate control clock CLt rises at time Ta0 and the transfer gate 4 opens as shown in figure 8(f), whereby signal charges Cs stored in the infrared detector 1 flow into the potential well. When the control clock CLt falls down at time Ta1, the transfer gate 4 closes as shown in figure 8(g) and transfer of signal charges Cs begins as shown in figure 9. In figure 9, reference characters A to E designate charges transferred from the infrared detectors to the potential wells beneath the first transfer gates 2a corresponding to the respective infrared detectors. The transfer operation will be described with reference to figure 9. In figure 9, since the CCD driving clocks have four phases, i.e., the phases of clocks 1 to 4 are delayed by 90 degrees from each other, a period for transferring signal charges from a position corresponding to an infrared detector to a position corresponding to the next infrared detector is divided into first to eighth transfer periods. First of all, in the first transfer period of Ta1 ≦ t < t1, the potential wells created beneath the first transfer gates 20a extend beneath the second transfer gates 20b, and the potential wells beneath the first transfer gates 2a disappear in the second transfer period of t1 ≦ t < t2, whereby the signal charges A to E transferred from the infrared detectors 1 to the CCD 20 move a distance equivalent to one transfer gate. Subsequently, during the third transfer period of t2 ≦ t < t3 and the fourth transfer period of t3 ≦ t < t4, the signal charges A to E more a distance equivalent to one transfer gate. Thereafter, the signal charges move in this way in the fifth and sixth transfer periods and the seventh and eighth transfer periods, whereby the signal charges A to E move a distance equivalent to one infrared detector 1. Repeating the transfer operation, the CCD 20 successively outputs the signal charges to the outside through the output amplifier 8. A description is given of a shutter operation during the charge transfer operation, i.e., an operation for draining signal charges out of the infrared detector 1. Figures 8(a)-8(g) show potential profiles in a cross-section taken along a line VIII-VIII of figure 6 during the charge transfer period of tnO ≦ t ≦ tn7 in which the shutter operation is carried out. Especially figure 8(a) shows a period tn0 < t < tn1 in which a potential well is created beneath the first transfer gate 20a, and figure 8(b) shows the third transfer period of tn3 ≦ t < Td2 prior to the fourth transfer period of Td2 ≦ t < Td3 in which the shutter operation is carried out. In the fourth transfer period, no potential well is created beneath the first transfer gate 20a and the fourth and second transfer gates 20d and 20b disposed at opposite sides of the first transfer gate 20a during the ordinary charge transfer operation. In the transfer cycle for conducting the shutter operation during the charge transfer operation, the driving clock 1 for the first transfer gate 20a and the transfer gate control clock CLt rise together with the shutter gate control clock CLs at time Td 2 in the fourth transfer period, whereby, as shown in figure 8(c), the potential levels of the transfer gate 4 and the shutter gate 5 fall down to the levels Et on and Es on , respectively, and at the same time, the potential level of the first transfer gate 20a falls down to the level Ecc at which the transfer well is created. In this way, the signal charges Ce stored in the infrared detectors 1 are drawn into the overflow drain 3 through the potential wells beneath the first transfer gates 20a as shown in figure 10. When the control clocks CLt and CLs fall down at time Td3, the infrared detector 1, the CCD 20, and the overflow drain 3 are separated from each other as shown in figure 8(d). Thereafter, storage of signal charges Cs is carried out in the infrared detector 1 while signal charges are successively transferred through the first transfer gate 20a in the CCD 20. Figure 8(e) shows a potential profile at time t7 in such an operation. When the transfer gate control clock CLt rises at time Ta4, the transfer gate 4 opens as shown in figure 8(f) and signal charges Cs stored in the infrared detector 1 flow into the potential well beneath the first transfer gate 20a. When the control clock CLt falls down at time Ta5, the transfer gate 4 closes as shown in figure 8(g) and the transfer of the signal charges Cs begins in the CCD 2. According to the second embodiment of the present invention, the CCD 20 of the charge transfer part disposed between the one-dimensional array of infrared detectors 1 and the overflow drain includes the four transfer gates 20a to 20d for each infrared detector 1 and the four-phase driving clocks 1 to 4 are applied to the transfer gates 20a to 20d, respectively. Therefore, the signal charge drain operation is carried out during the signal charge transfer operation of the CCD 20 without mixing the drained charges with the transferred charges in the CCD. Therefore, a timing for opening the shutter gate can be set at an arbitrary time in the charge read cycle. In addition, a transfer gate of the CCD and a driving clock therefor are decreased as compared with the five-phase driving CCD, whereby the structure of the linear image sensor is simplified. However, there is a disadvantage that the amount of transferred charges is reduced by half. Therefore, five or more phase CCD is favorable in some cases. In the above-described first and second embodiments of the present invention, the number of the transfer gates for each infrared detector is equal to the number of the phases of the transfer gate driving clocks. However, the transfer gates may be driven by driving clocks whose number of phases is larger than the number of the transfer gates. A description is given of a linear image sensor in which four transfer gates are prepared for each infrared detector and the transfer gates are driven by eight-phase driving clocks, in accordance with a third embodiment of the present invention. Figures 11, 12, 13, and 14 are diagrams for explaining the linear image sensor according to the third embodiment of the present invention, which correspond to figures 6, 7, 9, and 10 of the second embodiment, respectively. Figure 11 is a plan view illustrating a layout of each element of the infrared linear image sensor. Figure 12 illustrates clock signals applied to the respective gates of the linear image sensor of figure 11 during the charge read operation. Figure 13 illustrates a manner of transferring signal charges in the CCD. Figure 14 illustrates a timing for draining signal charges during the charge transfer operation. In addition, potential profiles in a cross section taken along a line X-X of figure 11 during the charge read operation are identical to the potential profiles shown in figures 8(a)-8(g). In this third embodiment, the eight-phase driving clocks 1' to 8' are used as driving clocks for the charge transfer element, and the first to fourth driving clocks 1' to 4' are applied to the transfer gates 20a to 20d of the infrared detectors 1 in odd numbers among the infrared detectors arranged in a one-dimensional array while the fifth to eighth driving clocks 5' to 8' are applied to the transfer gates 20a' to 20d' of the infrared detectors 1' in even numbers. Other structures are the same as those in the second embodiment of the present invention. Each of the first to eighth CCD driving clocks 1' to 8' has a half cycle of CCD driving pulse as compared with the CCD driving clock according to the second embodiment. In addition, phases of the CCD driving pulses are successively delayed by 45 degrees from each other. In other words, clocks obtained by overlapping the clocks 1' and 5', 2' and 6', 3' and 7', and 4' and 8' are equivalent to the driving clocks 1, 2, 3, and 4 used in the second embodiment, respectively. However, the first driving clock 1' and the fifth driving clock 5' generate pulses for draining the charges stored in the infrared detectors 1 at the same time (t = Td2) at which the shutter gate control clock CLs is generated (figure 12). According to the third embodiment of the present invention, the ordinary charge transfer operation of the CCD 20 is carried out as shown in figure 13 while the shutter operation during the charge transfer operation is carried out as shown in figure 14, in the same manner as the linear image sensor of the second embodiment, so that the same effects as described with respect to the second embodiment are achieved. As is evident from the foregoing description, according to the present invention, a CCD of a charge transfer part disposed between a one-dimensional array of photodetectors and an overflow drain includes four or more transfer gates for each photodetector, and four or more phase driving clocks are applied to the respective transfer gates. Therefore, during the charge transfer operation, there is a state where adjacent three transfer gates are all closed, so that signal charges are drained from the photodetector into the overflow drain through the transfer gate in the middle of the three transfer gates without mixing the drained signal charges with signal charges transferred by the CCD. That is, the charge drain operation is carried out during the charge transfer operation in the charge transfer part, whereby a timing for opening the shutter gate can be set at an arbitrary time in the charge read cycle. In addition, according to the present invention, the above-described CCD may include five transfer gates for each photodetector and the five transfer gates are controlled by five-phase driving clocks to conduct charge transfer. Therefore, even when adjacent three transfer gates among the five transfer gates are used for the charge drain operation during the charge transfer operation, remaining two transfer gates are used for the charge transfer operation, so that signal charges from the photodetector is received by a potential well equivalent to the two transfer gates and the amount of transferred signal charges can be doubled. In addition, according to the present invention, the above-described CCD may include four transfer gates for each photodetector and these transfer gates are controlled by four-phase driving clocks to conduct charge transfer. Therefore, as compared with the five-phase driving CCD, a transfer gate and a driving clock therefor are dispensed with, whereby the structure of the CCD is simplified.

Claims: Linear image sensor, comprising: [a] a semiconductor substrate (9); [b] a one-dimensional array of photodetectors (1; 1') for producing signal charges in response to incident light, said signal charges being either first signal charges (Cs) to be transferred or second signal charges (Ce) to be drained to the outside; [c] a charge transfer part for transferring said first signal charges (Cs) stored in said array of photodetectors (1; 1'), disposed parallel with said array of photodetectors (1; 1'); [d] a charge drain part (3) for draining said second signal charges (Ce) to the outside through said charge transfer part, said charge drain part (3) being disposed on that side of said charge transfer part which is opposite to said array of photodetectors (1; 1'), said charge transfer part includes: [c-1] a CCD (2; 20) that includes four or more CCD transfer gates (2a to 2e; 20a to 20d; 20a' to 20d') for each photodetector (1; 1') and transfers said first signal charges (Cs) sent from each photodetector (1; 1') in a direction of said array of photodetectors (1; 1'); [c-2] transfer gates (4) for controlling a charge transfer from said array of photodetectors (1, 1') to said CCD (2; 20) during a charge transfer operation, each transfer gate (4) being disposed between each photodetector (1; 1') and a first one (2a; 20a; 20a') of said CCD transfer gates (2a to 2e; 20a to 20d; 20a' to 20d') corresponding to said photodetector (1; 1'); [c-3] shutter gates (5) for controlling said charge transfer from said charge transfer part to said charge drain part (3) during a drain operation, each shutter gate (5) being disposed between said first CCD transfer gate (2a; 20a; 20a') and said charge drain part (3); and [c-4] four or more phase driving clocks (1 to 5; 1' to 8') controlling said CCD transfer gates (2a to 2e; 20a to 20d; 20a' to 20d'), wherein said CCD transfer gates (2a to 2e; 20a to 20d; 20a' to 20d') are controlled by said phase driving clocks (1 to 5; 1' to 8') in such a manner that, during said charge transfer operation, there is a state where at the beginning and at the end of said state CCD transfer gates (2e, 2b; 20d, 20b; 20d', 20b') adjacent to and at opposite sides of said first CCD transfer gate (2a; 20a; 20a') and said first CCD transfer gate (2a; 20a; 20a') are all closed, and a shutter operation is performed within said charge transfer operation in such a manner that between said beginning and said end of said state said transfer gate (4) and said shutter gate (5) are turned on and said first CCD transfer gate is opened, while said two adjacent CCD transfer gates (2e, 2b; 20d, 20b; 20d', 20b') are kept closed, and potential levels are deeper in the order of said transfer gate (4), said CCD (2; 20), said shutter gate (5) and said charge drain part (3). Linear image sensor according to claim 1, characterized in that said CCD (2) includes five CCD transfer gates (2a to 2e) for each photodetector (1) and these CCD transfer gates (2a to 2e) are controlled by five-phase driving clocks (1 to 5). Linear image sensor according to claim 1, characterized in that said photodetector (1; 1') is a Schottky barrier diode for detecting infrared light, which comprises a platinum silicide layer disposed on a p type silicon substrate and a p type silicon region contacting with said platinum silicide layer to produce a Schottky junction. Linear image sensor according to claim 3, characterized in that said charge drain part (3) is disposed parallel with said array of photodetectors (1; 1') on said p type silicon substrate and comprises an n type diffusion region in which charge drain wells deeper than charge storage wells of said photodetectors (1; 1') and charge transfer wells of said CCD (2; 20) are formed when a positive voltage is applied from outside. Linear image sensor according to claim 1, characterized in that said CCD (20) includes four CCD transfer gates (20a to 20d) for each photodetector (1) and these CCD transfer gates (20a to 20d) are controlled by four-phase driving clocks (1 to 4). Linear image sensor according to claim 1, characterized in that these CCD transfer gates (20a to 20d, 20a' to 20d') are controlled by driving clocks (1' to 8') whose number of phases is larger than the number of the CCD transfer gates (20a to 20d, 20a' to 20d'). Method for driving a linear image sensor in which first signal charges (Cs) from a one-dimensional array of photodetectors (1; 1') are transferred by a charge transfer circuit and second signal charges (Ce) are transferred to a charge drain circuit (3) through said charge transfer circuit to be drained to the outside, comprising the following steps: [1] providing a CCD (2; 20) including four or more CCD transfer gates (2a to 2e; 20a to 20d; 20a' to 20d') for each photodetector (1; 1'); [2] applying four or more phase driving clocks (1 to 5; 1' to 8') to said CCD transfer gates (2a to 2e; 20a to 20d; 20a' to 20d') to conduct a transfer operation of said first signal charges (Cs) sent from said photodetector (1; 1'), wherein, during said transfer operation, there is a state where at the beginning and at the end of said state CCD transfer gates (2e, 2b; 20d, 20b; 20d', 20b') adjacent to and at opposite sides of a first one (2a; 20a; 20a') of said CCD transfer gates (2a to 2e; 20a to 20d; 20a' to 20d') and said first CCD transfer gate ((2a; 20a; 20a') are all closed; and [3] performing a shutter operation within said transfer operation in such manner that between said beginning and said end of said state a transfer gate (4) disposed between each photodetector (1; 1') and said first CCD transfer gate (2a; 20a; 20a') and a shutter gate (5) disposed between said first CCD transfer gate (2a; 20a; 20a') and said charge drain circuit (3) are turned on and said first CCD transfer gate is opened, while said two adjacent CCD transfer gates (2e, 2b; 20d, 20b; 20d', 20b') are kept closed, and potential levels are deeper in the order of said transfer gate (4), said CCD (2; 20), said shutter gate (5) and said charge drain part (3).

Inventors: NAKANISHI JUNJI, NAKANISHI, JUNJI, Nakanishi, Junji, c/o Mitsubishi Denki K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L27
IPC codes: H01L27, H04N1, H04N5, H04N3