JPH02268465A - Solid infrared image sensor - Google Patents

Abstract

PURPOSE:To enable high photosensitivity and high charge transfer capacity by making a conductivity type of a photoelectric transducing region and a carrier injection region of an infrared ray sensor an n-type, by making a conductivity type of a potential barrier region between the regions p-type, and by reading an electric charge from the carrier injection region of n-well structure. CONSTITUTION:An infrared sensor 15 is formed to a p-type single crystalline silicon substrate 4 having a reflection preventing film 14 together with a transfer gate 16 and a vertical CCD 17 to constitute a solid infrared image sensor 15. The sensor 15 is provided with a photoelectric transducing region 1 consisting of a first conductivity type semiconductor, a carrier injection region 3 of a first conductivity type whose impurity concentration is lower than that of the region 1, a potential barrier region 2 of a second conductivity type which is completely depleted when operated between the regions 1, 3, and so forth. When first and second conductivity types are made n-type and p-type, respectively, the sensor 15 becomes highly photosensitive compared with a case that they are made p-type and n-type, respectively. Thereby, optical signal charge which does not become a hole is read from the region 3 of n-well structure and charge transfer capacity in the CCD can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a solid-state infrared image sensor that converts infrared image information into time-series electrical signals.

(Prior Art) A conventional solid-state infrared image sensor of this type has been disclosed in the patent application No. 63-237583 (Japanese Patent Application No. 62-73
No. 240), this was presented as a second embodiment.

First, the operating principle of the infrared sensor that constitutes the light receiving section of the image sensor will be described.

The energy band structure of the infrared sensor is shown in Fig. 2(a).
It is as shown in (b). In this figure, (a) shows the case where the first conductivity type is p type and the second conductivity type is n type, and (b) shows the case where the first conductivity type is n type and the second conductivity type is p type. Although infrared rays are incident from the carrier injection region side, they may be incident from the photoelectric conversion region side. Photoelectric conversion fJT region 19 or 25 and carrier injection region 21 or 2
A potential barrier that becomes an obstacle to the majority carriers in 7 is formed at the interface between the photoelectric conversion region and the potential barrier region and at the interface between the carrier injection region and the potential barrier region. In particular, the potential barriers for majority carriers in photoelectric conversion regions 19 and 25 are respectively Φ!
1 and Φ. It is. The height of this potential barrier varies from zero to zero depending on the conductivity type of the potential barrier region, the thickness of the potential barrier region, the balance of impurity concentrations in the three regions, and the bias voltage applied between the photoelectric conversion region and the carrier injection region. It can be arbitrarily set up to an energy level corresponding to the diffusion potential formed in a normal pn junction. If the dark current is large at room temperature due to the small potential barrier, cooling is required.

Infrared light 3 whose energy is smaller than the forbidden band width Eg of semiconductors
5 (or 36) is the carrier injection region 21 (or 2
7) and around the potential barrier region 2 or 26), the light is transmitted without being absorbed, and the photoelectric conversion region 19 (
or 25).

When the first conductivity type is p type and the second conductivity type is n type in (a), in the photoelectric conversion region 19, electrons below the Fermi level Ef in the valence band 22 are exposed to the incident infrared light. 35 energy h, and transitions from below the Fermi level E1 to an empty level between the Fermi level Er and the valence band edge Ev,
A child 31 and a hot hole 32 are formed using hot'. These hot electrons 31 and hot holes 32 move within the photoelectric conversion region 19 until they are recombined. When a moving hot hole 32 reaches the potential barrier region 20 and has energy greater than the potential barrier ΦI8, the probability that it passes through the potential barrier region 20 and is injected into the carrier injection region 21 is be. When the real holes 32 are injected into the carrier injection region 21, the hot electrons 31 left behind in the photoelectric conversion region 19 and the hot holes 32 injected into the carrier injection region 21 become signal charges.

On the other hand, in the case where the first conductivity type is n type and the second conductivity type is p type in (b), in the photoelectric conversion region 25, there is a gap between the Fermi level Ef in the conduction band 30 and the conduction band edge Ec. The electrons absorb the energy hv of the incident infrared light 36 and transition from below the Fermi level to an empty level above the Fermi level, forming hot electrons 33 and hot holes 34.

These hot electrons 33 and hot holes 34 move in the photoelectric conversion region 25 until recombined, as in the case described above. When the hot electron 33 in motion reaches the potential barrier region 26 and has energy greater than the potential barrier Φ8, the probability that it passes through the potential barrier region 26 and is injected into the carrier injection region 27 is There is. When the hot electrons 33 are injected into the carrier injection region 27, the potholes 34 left behind in the photoelectric conversion region 25 and the hot electrons 33 injected into the carrier injection region 27 become signal charges.

Note that in Figure 2 (a) and (b), the photoelectric conversion region is depicted as being in a degenerate state, but even if it is in a non-degenerate state, if the impurity concentration is significantly higher than that in the carrier injection region, it may not be used as a sensor. Operate.

Next, a solid-state infrared image sensor presented as a second embodiment in the application specification of this infrared sensor will be described.

The structure of the solid-state infrared image sensor is shown in FIGS. 3 and 4. These are examples of the two-dimensional interline transfer COD method, and Figure 3 is a cross-sectional view of a unit pixel.
FIG. 4 is an overall configuration diagram.

Unit pixels are an infrared sensor 52 and a transfer gate 53
, and a vertical CCD 54. The area in the p-type nested crystal 8i substrate 40 that is in the same state as when the substrate was fabricated in the infrared sensor portion is the carrier injection region 39, and the carrier injection region 39, that is, the p-type photoconductor containing impurities at a higher concentration than the substrate, is on the surface. A conversion area 37 is provided.

A potential barrier region 38 is formed between the photoelectric conversion region 37 and the carrier injection region 39. The potential barrier region 38 is either p-type, which has a lower impurity concentration than carrier injection region 39, or is intrinsic, or is n-type, which is fully depleted under at least operating conditions. An n-type guard ring 41 is provided around the light W, the conversion region 37, and the potential barrier region 38 in order to alleviate electric field concentration. On the side of the transfer gate 53 of the n-type guard ring 41, an n-type region 42 containing a high concentration of donor impurity and serving as a source region when reading optical signal charges is formed.

In order to short-circuit the photoelectric conversion region 37 and the n-type region 42, a silicide film 43 is formed in ohmic contact with both. Silicide is used because it has the advantage of having excellent stability of properties.
You don't absolutely have to use this. It can also be replaced by a simple metal film or metal wiring. Further, if the contact portion of the photoelectric conversion region 37 with the n-type region 42 and the n-type region 42 both contain impurities at a high concentration so that a tunnel current can easily flow, the silicide film 43 may not be provided. Can be shorted.

The photoelectric conversion region 37 and the potential barrier region 38 are finally electrically connected by the silicide film 43, the n-type region 42, and the n-type guard ring 41, and transition between energy bands occurs to cause the conduction band of the potential barrier region 38. Even if electrons accumulate inside, they are quickly removed.

The structure of the vertical CCD 54 is a buried channel type.

A poly-Si electrode 47 serving as a transfer electrode is provided from above the Si substrate at the transfer gate 53 portion to above the n-type channel region 45 with a thermal oxide film 46 interposed therebetween. To limit the channel width in the lateral direction with respect to the charge transfer direction, the vertical C
The thermal oxide film 46 is thickened at both end portions of the CD 54 to provide a p-type channel blocking region 44. Since FIG. 3 shows a cross section of a portion including the transfer gate 53, there is no thick oxide film or p-type channel blocking region at the end of the vertical CCD 54 on the infrared sensor 52 side, but the transfer gate 53 occupies it. There is only a small area, and where there is no area, there is a thick oxide film and a p-type channel blocking region at both ends of the vertical C0D 54.

The entire surface of this image sensor is covered with an insulating film 48 made of silicon oxide (SiO, 5iO2), silicon nitride (SiN*813N4), etc., formed by CVD or the like. A metal reflective film 49 made of aluminum or the like is provided on a portion of the insulating film 48 facing the photoelectric conversion region 37.
It is equipped with This is to reuse the infrared light transmitted through the photoelectric conversion region 37, and the single crystal Si substrate 40
Parts of carrier injection region 39 and potential barrier region 38 of photoelectric conversion region 37/insulating film 48/metal reflective film 49
Since an optical resonance state occurs in the multilayer structure of the insulating film 4 , the insulating film 4 is adjusted so that the antinode of the standing wave generated by infrared light having a center wavelength in the wavelength band used by the sensor is located near the charge conversion region 37 .
Setting the thickness to 8 provides high performance.

Further, the entire surface thereof is covered with a protective film 50, and the entire back surface is covered with an antireflection film 51.

The entire groove bottom of the image sensor is as shown in FIG.
This is an interline transfer method, in which unit pixels are arranged two-dimensionally, and vertical and horizontal scanning is performed by a vertical CCD 54 and a horizontal CCD 56, respectively. Vertical CCD5
4 is a 4-phase drive, the horizontal CCD 56 is a 2-phase drive, and the horizontal COD outputs a signal to the outside via an output section 57.
It is made of tie.

The operation of this two-dimensional infrared COD image sensor is as follows.

Photoelectric conversion is performed with the infrared sensor in storage mode. In the accumulation mode, the transfer gate 53 is in an OFF state, and the infrared sensor 52 is in a biased state as shown in FIG. 2(a). When the infrared light 55 incident from the back surface of the infrared sensor 52 reaches the photoelectric conversion region 37 and is photoelectrically converted, electrons, which are generated signal charges, are transferred to the photoelectric conversion region 37, the n-type guard link 41, the n-type region 42,
and is accumulated in the silicide film 43. Photoelectric conversion area 37
During the period when signal charges are accumulated in the other three portions, the vertical CCD 54 is reading signals. After accumulating the signal charge for a certain accumulation time, the transfer gate 53
is turned on, and the signal charges accumulated in the charge conversion region and other three portions are read out to the vertical CCD 54. Thereafter, the transfer gate 53 is turned off, and the three portions of the photoelectric conversion region 3 start accumulating signal charges again.

The signal charge read out to the vertical CCD 54 is transferred to the horizontal CCD 56 for one horizontal line during the -horizontal period.
The signals are sequentially read out from the horizontal CD 56 via the output section 57. The output section 57 is composed of a capacitor, a source follower, and an amplifier, and performs charge-potential conversion and impedance conversion on the signal charge and outputs it in the form of a voltage change. This readout of signals for -horizontal lines is repeated every horizontal period, and the signals of all pixels are read out during the signal charge accumulation period of the photoelectric conversion area 37 and the other three portions.

In the above example, the first conductivity type has been described as a p-type and the second conductivity type as an n-type, but the reverse is also true, and in that case, pn in FIG. 3 are all replaced. Although this example is a back-illuminated image sensor, a front-illuminated image sensor is also possible, and in that case, the metal reflective film 49 and antireflection film 51 are unnecessary.

In addition, although only the two-dimensional interline transfer CCD method has been described in detail here, the prior art includes M
There are also signal readout systems such as the O8O8 system and one-dimensional array systems. Furthermore, even if semiconductor materials other than Si are used as the material,
This type of solid-state infrared image sensor is viable.

(Problem to be solved by the invention) In the photoelectric conversion region of the infrared sensor that constitutes the light receiving section, a high concentration and thin impurity distribution is required in consideration of infrared absorption and hot carrier release efficiency to the carrier injection region. It will be done.

The reason is as follows.

Since the infrared absorption coefficient due to free carriers increases in proportion to the carrier concentration in the semiconductor, increasing the impurity concentration can increase the absorption of infrared rays. On the other hand, regarding the efficiency of releasing hot carriers into the carrier injection region, by reducing the thickness of the photoelectric conversion region, multiple reflections of hot carriers occur between the insulator interface and the potential barrier region interface, and the lifetime ends. If the number of times that hot carriers are incident on the potential barrier region interface is increased, it will increase. Therefore, light? l! If the conversion region has a high concentration and thin impurity distribution, a sensor with high sensitivity can be obtained. (However, if the impurity concentration is extremely high, it will impair crystallinity and shorten the life of hot carriers, so there is an optimal concentration even if it is called a high concentration. Also, if the thickness is made thin, the amount of infrared absorption (There is an optimal thickness that balances infrared absorption and hot carrier emission efficiency into the carrier injection region.) As a characteristic inherent to semiconductor materials, impurities that can easily form such an impurity distribution Or the conductivity type is determined. Typical examples of semiconductors with n-type conductivity that tend to form impurity regions with high concentration and shallow junction depth are Si and InP.
(Indium Phosphorus). Impurities suitable for this purpose are As (arsenic) for Si and Se (arsenic) for InP.
selenium). When forming the above-mentioned infrared image sensor using such a semiconductor material as a substrate, high performance can be achieved by forming the infrared sensor of the light receiving part so that the first conductivity type is n type and the second conductivity type is p type. However, in the case of an infrared sensor with this structure, the transferred optical signal charge becomes a hole, and compared to the case where the transferred charge is electrons, the transfer efficiency deteriorates in the COD method, and the response speed decreases in the MO8 method. This results in a decrease in In other words, when a conventional solid-state infrared image sensor is made of a semiconductor with an n-type conductivity type that tends to form a highly concentrated impurity region with a shallow junction depth, the light-receiving part has high performance and charge transfer is not possible. The disadvantage is that it is difficult to form something with excellent capabilities.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the solid-state infrared image sensor of the present invention includes a charge conversion region made of a first conductivity type semiconductor, and a first charge conversion region having a lower impurity concentration than the photoelectric conversion region. A carrier injection region made of a conductive type semiconductor, and a first conductive type semiconductor or an intrinsic semiconductor that exists between the photoelectric conversion region and the carrier injection region and whose impurity concentration is lower than that of the carrier injection region, or at least operate. An infrared sensor has a potential barrier region made of a second conductivity type semiconductor that becomes fully depleted under certain conditions, and the photoelectric conversion region, the potential barrier region, and the carrier injection region form a homojunction structure, In a solid-state infrared image sensor comprising an infrared sensor array arranged one-dimensionally or two-dimensionally, and an electronic scanning circuit that reads out electric charges generated by photoelectric conversion in the infrared sensor array to the outside as a time-series signal, a first conductivity type is n-type, the second conductivity type is p-type, the carrier injection region has an n-well structure, and optical signal charges in each infrared sensor are read out from the carrier injection region.

(Function) In the solid-state infrared image sensor of the present invention, since the first conductivity type is n type and the second conductivity type is p type, the conductivity type that tends to form an impurity region with high concentration and shallow junction depth is n type. The infrared sensor in the light receiving section has high performance when it is made of semiconductor material. However, since the carrier injection region has an n-well structure and the optical signal charges in each infrared sensor are read out from the carrier injection region, the transferred charges are electrons. Therefore, since the charge transfer ability is excellent, the above-mentioned problems can be solved.

(Example) Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a vertical cross section of a unit pixel according to an embodiment of the present invention.

Unit pixel is infrared sensor 15, transfer gate 16
, and a vertical CCD 17, as in the conventional case. The region of the well structure formed in the p-type nested crystal Si substrate 4 of the infrared sensor 15 portion is the carrier injection region 3. An n-type photoelectric conversion region 1 containing impurities at a higher concentration than the carrier injection region 3 is provided on the surface. A potential barrier region 2 is formed between the photoelectric conversion region 1 and the carrier injection region 3. The potential barrier region 2 is either n-type, which has a lower impurity concentration than carrier injection region 3, or intrinsic, or at least p-type, which is fully depleted under operating conditions. Around the photoelectric conversion region 1 and potential barrier region 2, p
This p-type guard ring 5 is located in the well of the carrier injection region 3 on the side where the vertical CCD 17 is located, and on the side where the vertical CCD 17 is not located.
It reaches outside the carrier injection region 3 and overlaps with the p-type channel blocking region 7 provided to limit the channel width of the n-type channel region.

The transfer gate 16 side of the carrier injection region 3 becomes a source region when reading optical signal charges. In order to short-circuit the photoelectric conversion region 1 and the p-type guard ring 5, a silicide film 6 is formed in ohmic contact with both. The reason why silicide is used is the same as in the past, and it can also be replaced by a simple metal film or metal wiring. Further, if the contact portion of the photoelectric conversion region 1 with the p-type guard ring 5 and the p-type guard ring 5 both contain impurities at a high concentration so that a tunnel current can flow easily, the silicide film 6 may not be provided. It can also be short-circuited. Photoelectric conversion region 1 and potential barrier region 2 are connected to p-type Si substrate 4 by silicide film 6, p-type guard ring 5, and p-type channel blocking region 7.

Structure of vertical CCD 17, insulating film 11, metal reflective film 12,
The protective film 13 and the anti-reflection film 14 are the same as those of the conventional ones, so their explanation will be omitted.

The overall configuration of the image sensor is also the same as that shown in FIG. 4, if illustrated. In other words, the overall configuration is an interline transfer method, in which unit pixels are arranged two-dimensionally,
Vertical and horizontal scanning are performed by vertical CCD 17 and horizontal CCD, respectively.
56. Vertical CCD 17 is 4
Phase drive, horizontal CCD56 is two-phase drive, horizontal CCD
The structure is such that a signal is output from 56 to the outside via an output section 57.

The operational differences between this two-dimensional infrared COD image sensor and the conventional one are shown below.

The infrared sensor 15 is operated in the accumulation mode as in the conventional case, but at that time the infrared sensor 15 is operated as shown in FIG.
), it is in a biased state. When infrared light 18 incident from the back surface of the infrared sensor 15 reaches the photoelectric conversion region 1 and is photoelectrically converted, hot electrons are emitted to the carrier injection region 3 and holes remain in the photoelectric conversion region 1. However, in the present invention, since the photoelectric conversion region 1 is connected to the p-type Si substrate 4 by the silicide film 6, the p-type guard nongap 5, and the p-type channel blocking region 7, holes escape to the p-type Si substrate 4. On the other hand, the hot electrons emitted into the carrier injection region 3 are transferred to the transfer gate 16.
is in an OFF state and the carrier injection region 3 is in a floating state, so that the carrier injection region 3 is accumulated there. After accumulating the signal charge for a certain accumulation time, the transfer gate 16 is turned on.
Signal charges (electrons) accumulated in the carrier injection region 3 are read out to the vertical CCD 17. The read signal charge is transferred in the same manner as a conventional infrared image sensor,
The output unit 57 outputs the signal as a time series signal.

As described above, in the infrared image sensor of the present invention, although it is an n-type photoelectric conversion region, the transferred charge can be converted into electrons, so it is possible to use an infrared image sensor with a high performance light receiving part and excellent charge transfer ability. Can be provided.

In this embodiment, the photoelectric conversion region 1 and the potential barrier region 2 are short-circuited by the silicide film 6 and the p-type guard ring 5, and the photoelectric conversion region 1 and the potential barrier region 2 are connected to the p-type Si substrate. 4 (ground) is connected to the P-type guard ring 5 and the p-type channel blocking region 7.
This is achieved through a structure in which the two overlap.
Other structures are also possible.

As another example of the structure, (i) the p-type guard ring 5 and the p-type channel blocking region 7 are separated, and a metal wiring is provided from the p-type guard ring 5 to the ground electrode, or the p-type guard ring 5 and p-type Chao Lu blocking region 7
(ii) The guard ring is a p-type without a high impurity concentration, and there is an n-type region with a high impurity concentration inside the guard ring or in a position in contact with the guard ring. There is a +14 structure in which the n-type region and the photoelectric conversion region are connected by metal wiring, and the metal wiring is led to the ground electrode or connected to the p-type channel blocking region.

Also, in the infrared image sensor of the present invention, a front-illuminated image sensor can be realized, and
Similar to the prior art, it includes a signal readout method such as the MO8O8 method and a one-dimensional array.

(Effects of the Invention) As explained above, according to the present invention, when the substrate material is a semiconductor with an n-type conductivity type, such as Si or InP, in which it is easy to form an impurity region with a high concentration and a shallow junction depth. To,
This has the effect of making it possible to form an infrared image sensor with a high-performance light-receiving section and excellent charge transfer ability.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional structural diagram of a unit pixel according to an embodiment of the present invention. FIGS. 2(a) and 2(b) are explanatory diagrams of the energy band structure and photoelectric conversion mechanism of the light receiving part in an infrared image sensor of the conventional or the present invention, in which the first conductivity type is p type and the second conductivity type is When the conductivity type is n type, (b) is the case where the first conductivity type is n type and the second conductivity type is p type. FIG. 3 is a vertical cross-sectional structural diagram of a unit pixel of a conventional infrared image sensor of this type. FIG. 4 is an overall configuration diagram of a conventional infrared image sensor or an infrared image sensor of the present invention. 1.250.・Photoelectric conversion region (n type), 2, 20, 26
．． 38... Potential barrier region, 3,27... Carrier injection region (n type), 4.40... P type wheel crystal S
i-substrate, 5... p-type guard ring, 6.43... silicide film, 7, 44... p-type channel blocking region, 8
, 45... n-type channel region, 9, 46... thermal oxide film, 10.47... poly-Si electrode, 11, 48...
Insulating film, 12.49...Metal reflective film, 13.50...
・Protective film, 14.51...Antireflection film, 15.52・
...Infrared sensor, 16.53...Transfer gate, 17.54...Vertical CCD, 18,35,36.
55... Infrared light, 19.37... Charging conversion area (p
type), 21.39...Carrier injection region (p type), 2
2.28...Valence band, 23.29...Forbidden band, 2
4.30... Conduction band, 31, 33... Hot electron,
32.34...hot hole, 41. n-type guard ring, 42... n-type region, 56.・Horizontal CCD, 57
...Output section.

Claims (1)

[Claims] A photoelectric conversion region made of a first conductivity type semiconductor, a carrier injection region made of a first conductivity type semiconductor whose impurity concentration is lower than that of the photoelectric conversion region, and a carrier injection region which exists between these photoelectric conversion regions and the carrier injection region and has an impurity concentration. is a first conductivity type semiconductor with a lower conductivity than this carrier injection region, an intrinsic semiconductor, or a second conductivity type semiconductor that is fully depleted under at least operating conditions.
and a potential barrier region made of a conductive semiconductor,
The infrared sensor consists of a homojunction structure of the photoelectric conversion region, the potential barrier region, and the carrier injection region. In a solid-state infrared image sensor equipped with an electronic scanning circuit that reads out electric charge as a time-series signal to the outside, the first conductivity type is an n-type, the second conductivity type is a p-type,
A solid-state infrared image sensor, wherein the carrier injection region has an n-well structure, and optical signal charges in each infrared sensor are read out from the carrier injection region.