JPH0715987B2 - Schottky barrier infrared image sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an infrared image sensor for converting information of an infrared image into a time-series electric signal, and more particularly to a Schottky barrier infrared image sensor.

[Conventional technology]

Conventionally, Schottky barrier infrared image sensors are classified into a front side illumination type and a back side illumination type. Comparing the two, the latter has a higher sensitivity because the reflection of incident light on the Schottky electrode can be reduced. Further, in the case of the backside illumination type, because of high sensitivity, an optical resonance mechanism is usually provided. With this mechanism, it is possible to effectively use infrared rays having a specific wavelength band that have been transmitted without being absorbed by the Schottky electrode. The optical resonance mechanism is, as shown in FIG. 2 (a), a silicon (Si) substrate 15 / Schottky electrode 16 /
It is composed of a multi-layer structure of the dielectric film 8 / the metal reflection film 17.

[Problems to be solved by the invention]

As shown in FIG. 2 (a), the above-mentioned optical resonance mechanism is provided between the metal reflection film 17, the Schottky electrode 16 and the Schottky electrode 1.
The interference phenomenon caused by multiple reflections of infrared rays 14 between the front and back surfaces of 6 is used. Due to this interference phenomenon, a standing wave of infrared rays 14 is generated in the optical resonance mechanism, and the infrared ray intensity incident on the Schottky electrode 16 is large at a wavelength at which the "antinode" portion of the standing wave hits the position of the Schottky electrode 16, The "node" portion becomes smaller at the wavelength hit. Therefore,
The infrared absorption rate of the Schottky electrode of a Schottky barrier type infrared sensor having an optical resonance mechanism shows wavelength dependence.

This situation is clear from FIG. 2 (b) cited from the literature (WF Kosonocky, FVShall-cross, TSVillani).
and JVGroppe, “160 × 244 Element PtSi Schottky−
Barrier IR-CCD Image Sensor, “IEEE Transactions o
n Electron Devices, vol.ED−32, no.8, pp.1564-1573,1
985.). This figure shows the dielectric film thickness dependence of the infrared absorption rate at the Schottky electrode obtained by simulation. When "WITH AL-MIRROR" has an optical resonance mechanism, "WITHOUT AL-MIRROR" has If not. It deals with the case where the incident infrared wavelength is 4 μm and the thickness of the Schottky electrode made of platinum monosilicide (PtSi) is 20Å which is said to be the optimum value. Dielectric No. 1 is silicon monooxide (SiO), and dielectric No. 2 is silicon dioxide (SiO ₂ ). The difference in the dielectric film thickness dependency depending on the type of the dielectric is due to the difference in the refractive index. It can be seen that the infrared absorptivity may be rather lowered due to the provision of the optical resonance mechanism depending on the film thickness of the dielectric.

In general, the thickness of the Schottky electrode in the Schottky barrier infrared sensor is negligibly thin compared to the wavelength of infrared rays, so that when the Schottky electrode is used alone, the optical characteristics with respect to infrared rays have almost no wavelength dependence. Therefore,
The wavelength dependence of the infrared absorption rate in the case of having an optical resonance mechanism is almost governed by the film thickness of the dielectric. In Fig. 2 (b), in the case of dielectric No. 1, the fact that the absorption coefficient shows a peak at a film thickness of about 6000Å at a wavelength of 4 µm means that
It means that the absorptance peaks at a film thickness of about 1 / 6.7, and that the absorptivity shows a dip at a film thickness of about 9500Å at a wavelength of 4 μm means that the film absorbs at a film thickness of about 1 / 4.2 of the wavelength. The rate is a dip. In other words, the absorptance peaks at a wavelength of about 6.7 times the film thickness of the dielectric No. 1, and the absorptance shows a dip at a wavelength of about 4.2 times the film thickness.

For example, if an optical resonance mechanism using dielectric No. 1 with a thickness of about 6000 Å is provided to increase the absorption rate of infrared rays near the wavelength of 4 μm, the absorption rate of infrared rays near the middle of the wavelength of 2 to 3 μm will be obtained. Will lower it.

On the contrary, even if an optical resonance mechanism using a dielectric No. 1 with a thickness of about 3000 Å is provided to increase the absorption rate of infrared rays near the wavelength of 2 μm, the absorption rate of infrared rays near the wavelength of 4 μm is Is better than the case without. However, this time, the absorptance of infrared rays in the middle of the wavelength range of 1 to 1.5 μm is reduced.

As described above, the optical resonance mechanism used in the conventional back-illuminated Schottky barrier infrared image sensor only has the effect of improving the sensitivity in a very limited wavelength band, There is a drawback in that the wavelength band on the shorter wavelength side than the band has a wavelength band that lowers the sensitivity as compared with the case where no optical resonance mechanism is provided. Therefore, there is a problem that the optical resonance mechanism cannot be used when sensitivity is required in a wide wavelength band.

[Means for solving problems]

The Schottky barrier infrared image sensor provided by the present invention in order to solve the problems of the above-described conventional Schottky barrier infrared image sensor is a back-illuminated type of each sensor of the Schottky barrier infrared sensor array, and
It has an optical resonance mechanism composed of a laminated structure of Si substrate / Schottky electrode / dielectric film / metal mesh filter.

[Action]

As an example of the metal mesh filter, the one quoted from the literature is shown in FIG. 3 (K.Sakai, T.Fukui, Y.Tsunawaki an
d H. Yoshinaga, “Metallic Mesh Bandpass Filters and
Fabry-Perot Interferometer for the Far Infrare
d, "Jap.J.Appl.Phys.vol.8, no.8, pp.1046-1055,196
9). The material is nickel. FIG. 3 (a) shows the structure,
(B) is a reflectance characteristic, (c) is a transmittance characteristic. Third
As can be seen from FIG. (A), this metal mesh filter has a structure as if it was hollowed out from a single metal sheet. The reflectance and transmittance characteristics are the lattice constant g20 and the line width a.
The standardized value of 19 is a / g, and the horizontal axis is λ / g, which is the standardized wavelength λ with the lattice constant g20. Fig. 3 (b)
According to, the reflectance of the metal mesh filter is a / g = 0.2
From around λ / g = 3 for 5 and λ / g = for a / g = 0.3.
2 and from a near λ / g = 1.8 when a / g = 0.4, on the short wavelength side, in each case λ / g = 1 to 1.2
It is extremely low in the degree. According to FIG. 3 (c),
When λ / g = 1 to 1.2, the low reflectance is 100% transmittance.
This is because it will increase to a close level. For short wavelengths of λ / g <1, many components are diffracted, so the reflectance increases a little, but stays at a few tens of percent, and the transmittance is also a few tens of percent.
To a lesser extent.

Since the metal mesh filter has the above optical characteristics, the optical resonance mechanism of the Schottky barrier infrared image sensor of the present invention has a structural parameter a / g of the metal mesh filter, a / g = Λ / g = 3 when 0.25
From around, when a / g = 0.3, from around λ / g = 2, a / g =
In the case of 0.4, from around λ / g = 1.8, the long-wavelength side acts similarly to the conventional optical resonance mechanism, and λ / g
At wavelengths of about 1 to 1.2, the operation is almost the same as when the optical resonance mechanism is not provided. Then, at other wavelengths, the interference effect becomes intermediate. Therefore, in the case of the conventional structure, the reflectance is high in the wavelength band where the "antinode" of the standing wave hits the position of the Schottky electrode, and in the case of the conventional structure in the shorter wavelength side than that wavelength. By designing the metal mesh filter so that the transmittance or the ratio of diffraction is high in the wavelength band where the "node" of the standing wave hits the position of the Schottky electrode,
It is possible to increase the sensitivity in the former wavelength band and to secure the same sensitivity in the latter wavelength band as in the case of not having an optical resonance mechanism.

〔Example〕

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a vertical sectional structure of a unit pixel in a Schottky barrier infrared image sensor according to an embodiment of the present invention.
The electronic scanning circuit of this embodiment is composed of a CCD. In addition, as the electronic scanning circuit, there is a circuit including a combination of MOS switches, and the Schottky barrier type infrared image sensor of the present invention can be configured by using the combination. The unit pixel described here is common to one-dimensional and two-dimensional image sensors.

The structure of this embodiment will be described. This embodiment is constructed on a p-type Si substrate 1. A Schottky electrode 2 made of PtSi is provided on the surface of a p-type Si substrate 1. An n-type guard ring 3 is formed at the edge portion of the Schottky electrode 2 for the purpose of relaxing the electric field concentration and preventing the breakdown voltage from decreasing and the dark current from increasing. An n-type high-concentration impurity-added layer 4 that is in ohmic contact with the Schottky electrode 2 is provided on a part of the edge portion of the Schottky electrode 2.
The n-type layer 5 is formed to face this. The Si substrate around the Schottky electrode 2 is covered with a thermal oxide film (SiO ₂ ) 6. A poly-Si electrode 7 is formed at a position facing the portion extending from the edge portion of the n-type high-concentration impurity added layer 4 to the n-type layer 5 with the thermal oxide film 6 interposed therebetween, and the poly-Si electrode 7 is also thermally oxidized. It is covered with a film. The surface of the element formed up to this point is covered with a dielectric film 8 made of SiO, SiO ₂ or the like by a chemical vapor deposition method (CVD method). A metal mesh filter 9 is provided on the surface at a position facing the Schottky electrode 2. An antireflection film 10 is provided on the back surface of the P-type Si substrate 1.

The unit pixel includes an antireflection film 10 (part), p-type Si substrate 1 (part), Schottky electrode 2, n-type card ring 3, n-type high-concentration impurity added layer 4 (part), dielectric film 8 (part) , A light receiving part 11 composed of a metal mesh filter 9, a p-type Si substrate 1 (part), an n-type high-concentration impurity added layer 4 (part), an n-type layer 5
(Part), thermal oxide film 6 (part), poly-Si electrode 7 (part)
Surface-channel type transfer gate composed of
12, n-type layer 5 (part), thermal oxide film 6 (part), poly-Si
Bulk channel CCD composed of electrode 7 (part)
It consists of the register unit 13. The infrared rays 14 incident from the back surface of the substrate are photoelectrically converted in the Schottky electrode 2.
As a result, the generated signal charge is transferred to the transfer gate.
It is transferred to the CCD register unit 13 via 12, further transferred between CCD registers of other pixels, and read out to the outside as a time series signal.

Here, the optical resonance mechanism in the present embodiment will be described. The optical resonance mechanism of this embodiment is based on the p-type Si substrate 1 / Schottky electrode (PtSi) 2 / dielectric film 8 / metal mesh filter 9
It consists of Take the case where the film thickness of PtSi is 20Å as an example. At this time, the thickness of the dielectric film 8 is 6000Å in case of SiO.
Approximately 8000Å in the case of SiO ₂ . With this, when the reflectance from the surface side of the dielectric film 8 is high, the absorption of infrared rays in PtSi is large near the wavelength of 4 μm, and the absorption of the wavelength 2 to
As described above, it becomes smaller near the middle of 3 μm. Here, the line width a of the metal mesh filter 9 is 0.8 μ
m and the lattice constant g are 2 μm. Therefore, the parameter a /
g is 0.4. If the thickness of the metal film such as aluminum is 1 μm or more, the reflectance for infrared rays in the above-mentioned wavelength band becomes sufficiently high. Therefore, the thickness of the metal mesh filter 9 may be 1 μm or more.

Now, in the optical resonance mechanism configured as described above, λ / g of the metal mesh filter 9 is 2 with respect to infrared rays having a wavelength of 4 μm, which is sufficiently high according to the reflectance characteristic of FIG. 3 (b). Since the reflectance is shown, as can be seen from FIG. 2 (b), the absorptance in PtSi is improved by the interference effect as compared with the case without the optical resonance mechanism. Further, when the reflectance from the surface side of the dielectric film 8 is high, the absorption of infrared rays in PtSi is small, and λ / g of the metal mesh filter 9 is about 1.2 to 1.3 for infrared rays in the middle of the wavelength of 2 to 3 μm. Therefore, according to the reflectance characteristic of FIG. 3 (b), since it has only a low reflectance, the absorptance in PtSi is secured to the extent that there is no optical resonance mechanism.

Therefore, the Schottky barrier infrared image sensor of the present invention has improved sensitivity in a specific wavelength band,
Moreover, there is no wavelength band that lowers the sensitivity as compared with the case where the optical resonance mechanism is not provided on the shorter wavelength side than the wavelength band.

〔The invention's effect〕

As described above, the present invention can enhance the sensitivity in a specific wavelength band similarly to the conventional back-illuminated type Schottky barrier infrared image sensor having an optical resonance mechanism,
In addition, the disadvantage that the wavelength band shorter than the wavelength band, which is found in the conventional one, has a wavelength band lower in sensitivity than that in the case where the optical resonance mechanism is not provided is eliminated.

Therefore, according to the present invention, there is an effect that a weaker infrared image can be captured over a wider wavelength band than the conventional one.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional structural view of a unit pixel in a Schottky barrier infrared image sensor according to an embodiment of the present invention. FIG. 2 is a diagram for explaining an optical resonance mechanism provided in a conventional back-illuminated Schottky barrier infrared image sensor. FIG. 3A is a diagram showing the outline of the structure and the path of light inside. FIG. 6B shows the dielectric film thickness dependence of the infrared absorptance of the Schottky electrode of the back-illuminated Schottky barrier infrared sensor obtained by simulation. In this figure, "WITH AL-MIRROR" has an optical resonance mechanism and "WITHOUT AL-MIRROR" does not. It deals with the case where the wavelength of the incident infrared ray is 4 μm and the thickness of the Schottky electrode made of platinum monosilicide (PtSi) is 20 Å. Dielectric No. 1 is silicon monooxide (Si
O), dielectric No. 2 is silicon dioxide (SiO ₂ ). FIG. 3 is a diagram for explaining the metal mesh filter. An example of a metal mesh filter is shown, in which FIG. 7A shows the structure, FIG. 7B shows the reflectance characteristic, and FIG. 7C shows the transmittance characteristic. 1 ... p-type Si substrate, 2 ... Schottky electrode (PtSi), 3
...... n-type guard ring, 4 ...... n-type high-concentration impurity-added layer, 5 ...... n-type layer, 6 ...... thermal oxide film (SiO _2), 7 ...... poly Si electrode, 8 ...... dielectric film, 9 ... Metal mesh filter, 10 ... Anti-reflection film, 11 ... Light receiving part, 12 ... Transfer gate part, 13 ... CCD register part, 14 ... Infrared, 1
5 …… Si substrate, 16 …… Schottky electrode, 17 …… Metal reflective film, 19 …… Line width of metal mesh filter, 20 …… Lattice constant of metal mesh filter.

Claims (1)

[Claims] 1. A Schottky barrier type infrared sensor array arranged one-dimensionally or two-dimensionally on a silicon substrate, and electronic scanning for externally reading charges generated by photoelectric conversion in the Schottky barrier type infrared sensor array as a time series signal. A Schottky barrier type infrared image sensor including a circuit, wherein each sensor of the Schottky barrier type infrared sensor array is a back-illuminated type and has an optical structure including a laminated structure of silicon substrate / Schottky electrode / dielectric film / metal mesh filter. Schottky barrier infrared image sensor having a dynamic resonance mechanism.