JPH0217989B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor imaging device having an excellent so-called shutter function.

The image sensor previously invented by the inventor of the present application has (1) wide dynamic range, (2) high sensitivity, (3) low noise, and has a hook-structured light detection/storage area. (4) Achieves high resolution. One of the more significant features of this prior art image sensor is that non-destructive readout of optical information is possible due to the carrier accumulation effect unique to the hook structure.

In the photodetection/storage section of the example of the image sensor of the prior art described above, a first conductivity type, low resistance first region, a high resistance region, and a high resistance a second region, a third region of a second conductivity type opposite to the first conductivity type and low resistance; and a fourth region of the first conductivity type and low resistance.
A hook structure consisting of regions, e.g. n ⁺ region, p ^-
A hook structure consisting of a p + region, a p ⁺ region and an n ⁺ region is formed, of which the p ⁺ region and n ⁺ region on the inside side of the substrate are formed.
Only the regions are laterally separated by isolation regions to form cells with pn junctions.
That is, the n ⁺ region and p ^- region on the front side are common to each cell. Furthermore, a transparent electrode is formed on the outer surface of the n ⁺ region on the front side, while on the inner side
A read transistor is connected to the n ⁺ region. With a positive voltage applied to the transparent electrode, light is irradiated onto the substrate through the transparent electrode.

Electrons generated near the substrate surface by light irradiation
One of the electrons in the hole pair is immediately absorbed by the transparent electrode to which a positive voltage is applied, but the other hole is accelerated by the electric field and travels within the p ^- region and reaches the p ⁺ region inside it. flows into. A pn junction with a predetermined barrier voltage is formed at the boundary between this p ⁺ region and the n ⁺ region further inside it, and this barrier allows holes that have flowed into the p ⁺ region to further move inside the n ⁺ region. Prevent it from entering the area. In other words, the hole is
It is stored in the potential well of the hook structure formed in the p ⁺ region. As the holes accumulate, the barrier voltage of the pn junction decreases, and electrons are extracted from the internal n ⁺ region across the pn junction to the p ⁺ region. The extracted electrons are accelerated in the p ^- region and travel to the surface side, pass through the n ⁺ region on the surface side, and are absorbed by the transparent electrode to which a positive voltage is applied. As a result, the internal n ⁺ region, which is in a floating state and from which electrons are extracted, is positively charged and its potential increases. this
The rising potential of the n ⁺ region is read out using the readout transistor described above.

In the prior art image sensor described above, holes are not accumulated in the potential wells of the hook structure only when light is irradiated, and only when light irradiation and voltage application are performed at the same time, holes accumulate, that is, light does not accumulate. It has the characteristic that input is stored. This voltage application corresponds to the shutter function in a conventional purely optical camera.

However, the shutter function in the prior art described above has the following drawbacks. In other words, the holes in the electron-hole pairs generated by light irradiation before voltage application and remaining near the surface are attached to the potential wells of the hook structure together with the holes in the electron-hole pairs generated after voltage application. be included. This means that the optical information before the shutter is opened is mixed into the optical information after the shutter is opened, reducing the shutter function. Of course, the electron-hole pairs generated by light irradiation recombine and disappear at a constant speed unique to the material, but this alone is sufficient to prevent the optical information immediately before the shutter release from being mixed into the desired optical information immediately after the release. cannot be avoided.

The present invention has been made in view of the above-mentioned drawbacks of the conventional art, and its object is to provide an imaging device having an improved shutter function.

According to the present invention that achieves the above object, in a semiconductor imaging device including a photoelectric conversion cell including a photodetection section and a readout section coupled to the photodetection section,
The photodetector includes a first conductive type and low resistance first conductive type, which is formed sequentially in the vertical direction from the surface of the semiconductor substrate.
a second region having a high resistance, a third region having a second conductivity type opposite to the first conductivity type and having a low resistance, and extending from above the first region into the second region. and a fourth region of the second conductivity type and low resistance formed at appropriate intervals in the lateral direction, and an external voltage is applied during the period of accumulating the irradiation light so that the irradiation light is not accumulated. The first region is provided with an external voltage applying means that stops applying the external voltage during the period, and one of the electron-hole pairs generated near the first region by light irradiation is applied to the external voltage. By applying a voltage, the second region is caused to travel to the third region, and a signal charge corresponding to the light irradiation is transferred into a fifth region of the first conductivity type and low resistance adjacent to the third region. The readout section is composed of either a readout transistor that detects the signal charge or a transfer type scanning circuit, and by stopping the application of an external voltage, the signal charge generated by light irradiation before shutter operation is detected. A semiconductor imaging device has been proposed which is equipped with a shutter function that recombines and annihilates one of the electron-hole pairs through a fourth region. A semiconductor imaging device that recombines and annihilates electron-hole pairs and has an excellent shutter function is provided.
The details of the present invention will be explained below with reference to the drawings.

FIG. 1 is a sectional view of an embodiment of the present invention. a
The figure is a cross-sectional view parallel to the normal direction (hereinafter referred to as "vertical direction") of the semiconductor substrate 10, and figure b is a cross-sectional view of the substrate 10.
2 is a cross-sectional view parallel to the tangential direction (hereinafter referred to as "lateral direction") of this substrate on the surface of the substrate. The device of this embodiment has a structure of a so-called image sensor in which a large number of photoelectric conversion cells are arranged in a matrix, separated by insulating separation regions 6, in a single semiconductor substrate 10 such as Si. Each cell consists of a light sensing region and a readout transistor, which are vertically stacked to form a very compact configuration.

The light sensing area of each cell is n ⁺ area 1, p ^- area 2,
n ^{+ ・}p ^-・p ⁺・n ⁺ consisting of p + area 3 and n ⁺ area 4
It consists of a hook structure and a p ⁺ region 5 extending from above the n ⁺ region 1 into the p ^- region 2, and an electrode 7 formed on the surface of the n ⁺ region 1 has a positive electrode of an appropriate width. A pulse voltage V _s (write pulse) is supplied.
The read transistor has n ⁺ source region 9, p
It is composed of a channel region 8, an n ⁺ drain region 4, a source electrode 9' and a gate electrode 8', and a bit line 11 is connected to the source electrode 9' and gate electrode 8', respectively.
and word line 12 are connected. Bit line 1
1 is made of a metal such as Al or a semiconductor such as doped polysilicon, and the word line 12 is made of silicide of a high-melting point metal such as Mo, W, or Ti, that is, MoSi ₂ ,
It is made of semiconductors such as WSi ₂ , TiSi ₂ and doped polysilicon. The electrode 7 is made of polysilicon,
A transparent electrode made of a transparent material such as SnO ₂ or In ₂ O ₃ or a thin metal film. Insulating layer 13 forms a gate insulating layer, and insulating layer 14 insulates bit line 11 and word line 12.

As described above, the equivalent circuit of a unit cell composed of a light sensing region and a readout transistor is expressed as shown in FIG. 2a. Regarding the light sensing region, a diode D1 similar to a pin diode (more precisely, a p ⁺ p ^- n ⁺ diode) is formed by the n ⁺ region 1, the high resistance p ^- region 2, and the p ⁺ region 3, and the p ⁺
Region 3 and n ⁺ region 4 form a diode D2. The p ^- region 2 may be an i-region or an n ^- region as long as it is a high-resistance region. Since the vertical length of the high-resistance p ^- region is sufficiently large, the junction capacitance of diode D1 is sufficiently small compared to the junction capacitance C _f of diode D2 and the reverse bias resistance of diode D1, so it can be omitted in the equivalent circuit. can. On the other hand, an n ⁺ source region 9, a p channel region and an n ⁺ drain region 4
constitutes an electrostatic induction transistor Q1 for reading. The connections for eight of these cells are illustrated in FIG. 2b.

Now, in the apparatus shown in FIG. 1, light (indicated by an arrow) is irradiated through the transparent electrode 7, and in this state the electrode 7
When a positive voltage (write pulse) is applied to , the shutter release operation is performed, but first, the operation before the shutter release, that is, the operation with the electrode 7 maintained at the ground potential under light irradiation will be explained.

By light irradiation, electron-hole pairs are generated in the p ^- region 2 near the surface of the substrate 10 . The generated electrons enter the nearby n ⁺ region 1. On the other hand, holes enter the p ⁺ region 5 side. In this way, the electrical balance between the n ⁺ region 1 and the p ⁺ region 5 is maintained by the electrons that have entered the n ⁺ region 1 and the holes that have entered the p ⁺ region 5 . As a result, the bias voltage
The electron-hole pair generated by light irradiation before V _s (+) is applied, that is, before the shutter opens, is
Absorbed in n ⁺ region 1 and p ⁺ region 5, p ^-
It does not exist in area 2 at all. The p ⁺ region 5 functions as a suction port for holes generated by polarization.

Thereafter, a positive voltage _Vs is applied to the electrode 7, and the shutter opening operation is performed. The energy diagram of the hook structure when a positive voltage _Vs is applied to the electrode 7 is as shown in FIG. A depletion layer is formed in the high-resistance p ^- region 2, which is a pin diode (more precisely
p ⁺ p ^- n ⁺ diode) corresponds to D1 being reverse biased. One preferred embodiment of the present invention is the p ^- region 2
The thickness of the region, the impurity concentration, and the applied voltage _Vs are selected so that the entire region is in a depleted state.
This condition applies to the aforementioned patent application (Japanese Patent Application No. 55-54001,
55-60316 and 55-69257). On the other hand, in Fig. 3, p ⁺ region 3
and has a constant barrier voltage at the boundary of n ⁺ region 4
A pn junction is formed, but since most of the positive voltage applied to the electrode 7 is applied to the high resistance p ^- region 2, the forward bias applied to the pn junction remains at an extremely small value.

After a positive voltage is applied to the electrode 7 and the energy diagram shown in FIG. 3 is achieved, the electrons generated by the light irradiation are immediately absorbed by the electrode 7, while the holes are accelerated by the electric field. Tep ^-
It travels in region 2 and is accumulated in p ⁺ region 3.

Now, if we assume that the quantum efficiency is 1 under a one-dimensional model, the unit charge is q, the speed of light is C, the photon density is S(t) (photons/cm ³ ), and the time after the start of irradiation is t.
Then, the positive charge due to holes per unit area accumulated in p ⁺ region 3 is △Q=C・q∫ ^t _p S(t)
given in dt. By accumulating holes, that is, positive charges, in the p ⁺ region 3 in this way, the barrier voltage of the pn junction between the p ⁺ region 3 and the n ⁺ region 4 increases as △V=△
It decreases by Q/Cf. This corresponds to diode D2 being forward biased by ΔV in the equivalent circuit of FIG. 2a.

Therefore, electrons in n ⁺ region 4 pass through p ⁺ region 3.
p ^- Flows into region 2. These electrons are accelerated and travel within the p ^- region 2, pass through the n ⁺ region 1, and are absorbed by the electrode 7 to which a positive voltage is applied. As a result, the n ⁺ region 4 in the floating state is positively charged by the charge of the extracted electrons, and the barrier potential of the pn junction formed between it and the p ⁺ region 3 gradually increases. This corresponds to the forward bias ΔV applied to the diode D2 in the equivalent circuit of FIG. 2a being gradually canceled as electrons are extracted. Such charging of n ⁺ region 4 is caused by n ⁺ region 4
It stops when the amount of positive charge due to the lack of electrons becomes equal to the amount of charge of holes accumulated in the p ⁺ region 3. At this time, the barrier potential becomes equal to the value in the thermal equilibrium state before hole accumulation, and the potential of the n ⁺ region 4 increases by ΔV compared to before hole accumulation.

The thus increased voltage of n ⁺ region 4 is read out to bit line 11 using read transistor Q1. That is, the word line 12 in FIG. 1a is opened and the read transistor Q1 is turned on. As a result, electrons flow from n ⁺ region 9 to n ⁺ region 4 via p region 8, and a positive voltage is read out to bit line 11. The amount of positive charge in the n ⁺ region 4 before reading is neutralized by the charge of electrons that flowed in during reading, and when the read transistor Q1 is turned off, the amount of positive charge in the n ⁺ region 4 decreases. is a somewhat small value. This value is determined by a combination of the amount of charge accumulated before reading, the amount of accumulated charge replenished during reading, and various read conditions. No matter what the value is, if the amount of positive charge accumulated in the n ⁺ region 4 is smaller than the amount of positive charge of the hole accumulated in the p ⁺ region 3, the charges of both The barrier voltage decreases by a value obtained by dividing the difference in the amount by the junction capacitance C _f , and electrons in the n ⁺ region 4 flow out through the p ⁺ region 3 to the electrode 7 to which a positive voltage is applied.
This outflow of electrons continues until the amount of positive charge in the n ⁺ region 4 becomes equal to the amount of positive charge of the holes accumulated in the p ⁺ region 3, as described above. In this way, in the device of the present invention, information (the increased voltage in the n ⁺ region 4) that is once destroyed by reading is automatically reproduced.

Of course, for such regeneration to be complete, it is necessary that the holes accumulated in the p ⁺ region 3 do not disappear or flow out, especially when the barrier voltage between the p ⁺ region 3 and the n ⁺ region 4 is reduced. Also, it is desirable that the holes accumulated in the p ⁺ region 3 do not flow out into the n ⁺ region 4 as much as possible. That is, it is desirable that the current that crosses the junction during writing and during reproduction after reading is dominated by electronic current as much as possible. The general method for achieving this is the same as the method for increasing the emitter injection efficiency of bipolar transistors, and some of the specific methods are detailed in the patent application mentioned above (Japanese Patent Application No. 55-54001, etc.). is exemplified.

In this way, in the device of the present invention, once written information is not erased by reading it out, in order to erase it, the polarity of the voltage applied to the electrode 7 is reversed and the information inside the p ⁺ region 3 is The configuration is such that holes accumulated in the p ^- region 2 are sucked into the electrode 7 or electrons are flowed from the electrode 7 into the p ⁺ region 3 via the p ^- region 2 . As another configuration, it is also possible to have a configuration in which a short-circuiting transistor is built into the substrate 10.

Further, a fifth n-region is provided which extends tapered in the vertical direction from the surface facing the p ^- region 2 of the insulation isolation region 6 within the p ^- region 2, so that the applied voltage can be adjusted.
A lens effect may be performed to deflect holes to the p ⁺ region 3 when V _s (t) is applied.

FIG. 4 is a sectional view of another embodiment of the present invention,
Areas with the same reference numerals as in FIG. 1 are the same areas as in FIG. The device of this example is
Unlike the case of FIG. 1, a readout transistor is not provided within the same semiconductor substrate, and consists only of a light sensing region. Furthermore, instead of forming a transparent electrode over the entire back surface of the substrate 15, discrete small electrodes 17 are formed. Since the operation of the device of this embodiment is exactly the same as the operation of the light sensing region of the device of FIG. 1, there will be no need for redundant explanation. A positive voltage generated in the n ⁺ region 4 in proportion to the amount of light received is read out via the electrode 16. A readout circuit as a peripheral circuit is connected to the electrode 16, but is omitted here.

FIG. 5 is a sectional view showing an example of the manufacturing process of the device shown in FIG. 4. First, as shown in Figure a.
A part of the Si semiconductor substrate 15 of p ^- conductivity type and impurity density is oxidized to form an insulating isolation region 6 (b
figure). Thereafter, a large amount of p-type impurity is diffused into the Si substrate to form p ⁺ region 3 (see figure c). Subsequently, a large amount of p-type impurity is diffused from one side of the substrate 15 to form a p ⁺ region 5 (Fig. d), and finally the substrate 15 is
After a large amount of n-type impurity is diffused from both sides of the substrate 15 to form n ⁺ regions 1 and 4 (Fig. e), metal electrodes 16 and 17 are formed by vapor deposition or the like on both sides of the substrate 15, and the process is completed.

FIG. 6 is a sectional view of another embodiment of the present invention,
The photodetector section is composed of a pin photodiode, and the readout section is composed of a charge transfer type scanning circuit. p ⁺ area 21, i area 22 and n ⁺ area 23
The pin photodiode is configured by
Electrodes 24 and 25 are formed thereon. The transparent electrode 25 is connected to the positive terminal of a DC power source 34 via a switch 33. On the other hand, electrode 24 is connected to n ⁺ region 27 formed on the surface of Si substrate 20 and is separated from other parts of Si substrate 20 by insulating layer 31 . A negative terminal of the DC power supply 34 is connected to an electrode 32 formed on the back surface of the substrate 20. Incidentally, a p ⁺ region 26 extends from above the n ⁺ region 23 into the i region 22 . this
In this embodiment, the p ⁺ regions 26 are formed in a dispersed manner in the lateral direction at an appropriate density.

On the other hand, the Si substrate 20, the n ⁺ regions 27 and 28, the gate insulating layer 29, and the gate electrode 30 constitute one horizontal unit of the BBD type scanning circuit.

In this embodiment, closing the switch 33 and applying a reverse bias to the pin photodiode described above corresponds to the shutter operation. Similar to the embodiment shown in FIG. 1, before the switch 33 is closed, electrons of the electron-hole pairs generated in the i region 22 near the n ⁺ region 23 enter the nearby n ⁺ region 23, while the positive The hole enters the p ⁺ region 26 side. In this way, the electrons that entered the n ⁺ region 23 and the p ⁺ region 2
The holes that entered into the n ⁺ region 23 and
Electrical balance between p ⁺ regions 26 is maintained. As a result, before the bias voltage V _s (+) is applied,
That is, electron-hole pairs generated by light irradiation before the shutter opening operation are absorbed in the n ⁺ region 23 and the p ⁺ region 26, respectively, and do not exist in the i region 22 at all.

Next, when the switch 33 is turned on, the electrons of the electron-hole pairs generated by the light irradiation are immediately absorbed by the transparent electrode 25, while the holes are accelerated and travel through the i region 22, and the holes are accelerated and travel through the n ⁺ region 27. An equal amount of electrons are extracted from the two via the electrode 24 and recombined and annihilated. As a result, the n ⁺ region 27 becomes depleted of electrons. Electrons in n ⁺ region 28 are transferred to n ⁺ region 27 via substrate 20 by applying a positive voltage to gate 30 . The resulting depletion state in the n ⁺ region 28 is further transferred in the vertical direction, ie, perpendicular to the plane of the drawing, by known transfer means (not shown), and read out. Although a BBD type scanning circuit is shown as an example, a CCD type scanning circuit may also be used.

The thickness of the i region 22 in FIG. 6 may be set to an appropriate value in terms of light absorption since most of the incident light is absorbed by the n ⁺ region ²³ , but holes generated before the shutter operation It is desirable to set the value to a certain degree so as not to reach the region 21, for example, a value larger than about 3 microns.

As described above in detail, the semiconductor imaging device of the present invention includes forming a low resistance region of a conductivity type opposite to the surface region in the high resistance region from above the low resistance region of the surface in the vertical direction from the surface of the semiconductor substrate. Therefore, electron-hole pairs generated by light irradiation before applying a voltage no longer exist in the high resistance region, so that a good shutter function can be exhibited.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of one embodiment of the present invention, a is a vertical cross-sectional view, b is a horizontal cross-sectional view, and FIG.
3 is an energy diagram of the hook structure in FIG. 1, FIG. 4 is a sectional view of another embodiment of the present invention, and FIG. 5 is a manufacturing process of the device shown in FIG. 4. FIG. 6 is a cross-sectional view of another embodiment of the present invention. 1, 23...n ⁺ area, 2, 22...p ^- or i area,
3,21...p ⁺ area, 4...n ⁺ area, 5,26...n ⁺
Region, 6... Insulation isolation region, 7, 24, 25, 32
...electrode, 8...p channel region, 9...n ⁺ drain region, 10, 15, 20...substrate, 11...bit line, 12...word line, 33...switch, 34...DC power supply.

Claims (1)

[Scope of Claims] 1. In a semiconductor imaging device including a photoelectric conversion cell consisting of a photodetection section and a readout section coupled to the photodetection section, the photodetection sections are formed sequentially in the vertical direction from the surface of the semiconductor substrate. a first region having a first conductivity type and a low resistance; a second region having a high resistance; a third region having a second conductivity type opposite to the first conductivity type and having a low resistance; a fourth region of the second conductivity type and low resistance extending from above the region into the second region and separated at an appropriate interval in the lateral direction; and accumulating irradiated light. During this period, an external voltage is applied,
The first region is provided with an external voltage applying means for stopping the application of the external voltage during a period in which the irradiation light is not accumulated, and the electron-hole pairs generated near the first region by the light irradiation are is made to travel within the second region to the third region by applying the external voltage, and a signal charge corresponding to the light irradiation is transferred to the first conductivity type, low resistance, adjacent to the third region. Fifth
The signal charges are accumulated in a region of A semiconductor imaging device equipped with a shutter function, characterized in that one of the electron-hole pairs generated by irradiation is recombined and annihilated via a fourth region.