JPH04320372A - Image sensor - Google Patents

Abstract

PURPOSE:To provide an image sensor having best spectral distribution for each color to be used. CONSTITUTION:In an imaging system comprising a plurality of image sensors having gate conductors disposed at an opening, an optical color separating means 2 separates an incident light beam into a plurality of color beams and the thickness of gate conductor of each image sensor is set so that a peak value appears in the spectral distribution of the color beam at a wavelength same or close to that at which spectral sensitivity distribution of image sensors 3, 4, 5 has a peak value.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging device, and more particularly to an imaging device using a transistor image sensor element.

0002

2. Description of the Related Art Various systems have been proposed for image pickup devices. That is, the main types are CCD type and MOS type. However, today, when there is a need to obtain high-resolution video as typified by high-definition television, the following problems are expected when adopting the above method.

The first problem is a decrease in aperture ratio due to the multi-pixel arrangement.

For example, in the case of a CCD type image sensor with 1/2 inch size and 400,000 pixels, the aperture ratio is 30.
%, and the influence of optical shot noise is increasing. Number of pixels required for high-quality video (13
00,000 to 2 million pixels) in 1 inch to 2/3 inch size, the aperture ratio is 10 to 20%.
This is extremely inconvenient.

The second problem is the incompatibility with increasing the speed of read operations. For example, in the CCD type, there is a concern that the transfer efficiency will decrease.

Furthermore, problems with the conventional method include smear,
There is blooming. It is said that it is difficult to completely solve these problems using the conventional methods described above.

[0007] As an image sensor that solves the problems seen in the above-mentioned conventional methods, a new method is introduced in Japanese Patent Application Laid-Open No. 14959/1983 entitled "Element for Sensing the Threshold of a Substrate Charge Modulation Type Transistor and its Manufacturing Method". Image sensors have been proposed. A feature of this type of image sensor is that each photosensitive cell is constituted by a substrate charge modulation type transistor.

[0008] First, the general configuration of the above-mentioned image sensor will be explained using FIGS. 6 and 7. 6 is a plan view of the photosensitive cell, and FIG. 7 is a sectional view taken along the line A--A in FIG. 6.

As shown in FIG. 6, each photosensitive cell includes a drain region 18 and a gate region 2 surrounded by the drain region 18 and further arranged in a honeycomb shape.
4, and a source region 26 located in the center of the gate region 24. The source region 26 is connected to each adjacent cell by a signal readout line 30.

On the other hand, to explain the depth direction using FIG. 7, a P-type layer 38 and an N-type layer 36 are further formed on the buried channel 34 formed on the silicon substrate 11 to form a gate region. 24 is constructed and an oxide layer 40 is formed to separate gate conductor 28 from gate region 24, and an oxide layer 42 is formed to separate gate conductor 28 from source conductor 30.

Next, FIG. 8 is a diagram showing the potential of a photosensitive cell. Due to the layer structure of the gate region 24 described above,
Depending on the depth, respective potential curves 96, 98 of the conduction band and the valence band are generated. Distance Xd
1 and Xd2 indicate the respective thicknesses of the two depletion regions. The potential curve includes a hole potential well 100, a first well 102, and a second well 103. light (hν)9
When zero is incident, holes 94 generated in the Xd1 region gather in the well 100. This changes the potential distribution and causes a probing current to flow from the source to the drain.

Thereby, the number of electrons stored in the well 102 is kept constant, and as a result, it is sensed as a change in the threshold value of the MOS transistor. Holes stored in well 100 are flushed out to the substrate by pulsing the transistor gate with a positive pulse. Furthermore, since the well 100 stores holes away from the substrate surface, they can be completely removed by positive pulsing without being affected by traps at the interface with the oxide and causing a reset failure as in the conventional case. Can be done. This is reset noise (KTC noise)
It has the advantage of causing almost no damage.

Furthermore, FIG. 9 shows an equivalent circuit of the image sensor. A drive pulse is applied to the address line 28 selected by the decoder 110 to which the address data 112 is applied, that is, the gate conductor 28 . The photosensitive cell includes a drain 18, a gate portion 24, and a source 26. A bias transistor 118 is connected to the source 26 and connected to a bias power supply 117 . In addition, the source 26 is connected to the readout line 120, and the clamp circuit (clamp capacitor Co122, transistor 12
6 and a pulse source 128), the signal is inputted to the shift register circuit 46 via a sampling circuit (consisting of a transistor 132), and a signal output is outputted through an output amplifier (not shown).

[0014]

[Problems to be Solved by the Invention] However, in the case of this method, since the gate conductor 28 is provided above the opening, it is preferable to form it with a conductive transparent electrode. Although it is possible to form it with tin oxide, there are many technical problems, and it is generally preferable to use polysilicon (polycrystalline silicon).

An object of the present invention is to provide an imaging device that can use a material such as polysilicon as a gate conductor.

[0016]

[Means for Solving the Problems] The imaging device of the present invention is an imaging device that uses a plurality of imaging elements each having a gate conductor in an aperture, and separates incident light into a plurality of colored lights by optical color separation means. , and the gate conductor thickness of each image sensor is set so that the peak of the spectral distribution of the colored light and the peak of the spectral sensitivity distribution of the corresponding image sensor are at the same or close wavelength.

[0017]

[Function] Polysilicon (polysilicon is taken up here, but is not necessarily limited to such materials) is used as a gate conductor with a thickness of 3000 Å to 500 Å.
If it is deposited at a thickness of 0 Å, it is expected to have a large effect on the spectral transmittance, and in particular to cause a decrease in so-called blue sensitivity around 400 nm to 500 nm. Therefore, it is desirable to deposit it as thinly as possible. However, it is also true that the thinner the material, the more difficult it becomes to control it by low-pressure CVD, etc., and it is extremely difficult to achieve both a good sensitivity distribution and ease of processing. For example, the appropriate gate conductor thickness can be predicted using an electronic computer based on the physical properties of polysilicon and multilayer film interference theory. Accordingly, it is possible to obtain an imaging device that has both high spectral sensitivity and ease of processing.

[0018]

Embodiments Hereinafter, embodiments of the present invention will be explained in detail with reference to the drawings.

In this embodiment, an imaging device will be described in which an appropriate gate conductor thickness is obtained by predicting the physical properties of polysilicon and multilayer film interference theory using an electronic calculator.

Note that the configuration and operation of the image sensor are the same as those of the image sensor described using FIGS. 6 to 9, so the explanation will be omitted here. The configuration, a method for obtaining an appropriate gate conductor thickness through calculation, and the results thereof will be explained.

FIG. 1 is a block diagram of an embodiment of an imaging apparatus according to the present invention.

As shown in the figure, the optical image captured by the optical system 1 is separated into R (red) by a prism 2 which is an optical color separation means.
, G (green), and B (blue), and enter the R, G, and B color sensors 3, 4, and 5. There, the signal is converted into an electric signal by photoelectric conversion, and is output as a video signal by the encoder 7 via the processing circuit 6. The R, G, and B color sensors 3, 4, and 5 have the same configuration as the image pickup device described using FIGS. 6 to 9. However, R in this example
, G, B The gate conductor thickness (polysilicon here) of each color sensor 3, 4, 5 is the same as that of each color (R, G, B).
) is set to the appropriate thickness.

FIG. 2 is a characteristic diagram showing the spectral sensitivity characteristics of the image sensor when the conductor thickness is set to suit R, G, and B according to the present invention.

As shown in the figure, the gate conductor thicknesses are 500 Å, 650 Å, and 800 Å, which greatly improve process ease compared to the extremely thin polysilicon thickness of 100 Å. ,green,
Since it is applied to a red element, the sensitivity of each color can be improved.

A method of calculating the spectral distribution of the image sensor will be explained below. By determining such a spectral distribution, an appropriate gate conductor thickness can be determined.

First, the refractive indices of polysilicon and bulk silicon are shown in FIGS. 3, 4, and 5. The refractive index is expressed by the complex refractive index n-iK, and the absorption coefficient α is α=(2π/
λ)·K. About polysilicon 10
The reason why the refractive index changes between about 0 Å (FIG. 4) and about 3000 Å (FIG. 3) is because the crystal state of polysilicon changes depending on the thickness. Note that the value of the complex refractive index n-iK changes depending on the polysilicon thickness in this way, but in the following calculations, data for the case where the polysilicon thickness is about 100 Å and data when the polysilicon thickness is about 3000 Å are used to supplement the data. and the complex refractive index n−iK at a given polysilicon thickness
I'm looking for.

The absorption rate of light by a substance is expressed using an absorption coefficient α.

I(x)=Io exp(-αx) where Io represents the intensity of light incident on the surface. Light absorbed inside the Si substrate generates carriers.

The generation rate of photoexcited carriers at the X position from the substrate surface is:

[0030]

[Equation 1] Here, Io is the intensity of the incident light, λ is the wavelength, C is the speed of light, and h is Planck's constant.

Consider an image sensor having the structure described above. It is thought that carriers generated within the space charge region (Xd1<X<Xd2) contribute to the photocurrent. The photocurrent J generated there is

[0032]

[Math 2] It is expressed as

By the way, the surface of a semiconductor device is covered with a silicon oxide film or a thin electrode film due to its structure. Therefore, light is reflected or absorbed at the interface between each layer or the semiconductor substrate, resulting in a loss in the amount of incident light.

[0034] When the j-th plane is added to the multilayer film up to the (j-1) plane, the reflectance is:

[0035]

[Equation 3] (assuming the incident angle of light is 0°) Energy reflectance R
is expressed as follows.

R=|rj,o|2 however,

[0037]

##EQU00004## Here, N is a refractive index, which is usually expressed as N=n-iK.

Therefore, the energy transmittance T of a multilayer structure containing a light absorbing material is expressed by the following equation.

T=(1-R)・A Here, A is the amount of attenuation of light by the absorbing substance.

Therefore, the amount of light I entering the silicon substrate
o is Io, where I is the amount of light incident on the sensor surface.
=I・T.

[0041] In this calculation, the following numerical values were used. That is, Xd1 = 0.05 (μm) Xd2 = 1.0 (μm) Silicon oxide film 1 (40 in Figure 7) = 1000 Å silicon oxide film 2 (42 in Figure 7) = Refractive index of 3000 Å silicon oxide film = 1 .46 These values, excluding the refractive index, are based on JP-A-14959-1989.
This is the data presented in the publication.

[0042] In this example, we will mainly focus on
Although the description has been made regarding the "element that senses the threshold value of a substrate charge modulated transistor" disclosed in the above publication, the present invention can also be applied to other elements that use a gate conductor in the opening.

Although this embodiment has been explained using a three-panel type camera as an example, it goes without saying that a two-panel type camera may also be used.

[0044]

According to the present invention, since it is possible to provide an image sensor having the best spectral distribution for each color used, the effect on sensitivity is extremely large and the effect greatly contributes to image quality.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a block of a three-panel camera according to the present invention.

FIG. 2 is a diagram showing the spectral sensitivity of a sensor for each of R, G, and B colors according to the present invention.

FIG. 3 is a diagram showing the refractive index of polysilicon (100 Å).

FIG. 4 is a diagram showing the refractive index of polysilicon (3000 Å).

FIG. 5 is a diagram showing the refractive index of bulk silicon.

FIG. 6 is a diagram (plan view of a photosensitive cell) illustrating a substrate charge modulation type element.

FIG. 7 is a diagram (cross-sectional view of a photosensitive cell) for explaining a substrate charge modulation type element.

FIG. 8 is a diagram (potential diagram of a photosensitive cell) for explaining a substrate charge modulation type element.

FIG. 9 is a diagram (equivalent circuit of the element) for explaining a substrate charge modulation type element.

[Explanation of symbols]

1 optical system, 2 prism, 3 red sensor, 4 green sensor, 5 blue sensor, 6 processing circuit, 7 encoder, 11
silicon substrate, 18 drain region, 24
Gate region, 26 Source region, 28 Gate conductor, 30 Signal readout line (source conductor), 34 Embedded channel, 36
N-type layer, 38 P-type layer, 40 oxide layer,
42 Oxide layer.

Claims (1)

[Claims] 1. An imaging device using a plurality of imaging elements each having a gate conductor in an aperture, wherein incident light is separated into a plurality of colored lights by an optical color separation means, and the color light corresponds to the peak of the spectral distribution of the colored lights. The gate conductor thickness of each image sensor is set so that the peak of the spectral sensitivity distribution of the image sensor has the same or close wavelength.