JPH01500471A - Electronic shutter for image sensor using photodiode - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Electronic shutter for image sensor using photodiode This invention is based on an image of an electronic shutter that empties the accumulated charge in the photodiode. It's related to the sensor.

Background technology Image sensors are made up of different types of sensors that collect electrical charge in response to incident light from the scene. It is equipped with a photodetector element of the formula.

A photodiode is often advantageously used as this detection element. Photodiode It has a high quantum efficiency and is of low limitation.

In many applications, the integration period (exposure time) is too long, so the die Things with aether are full of electrical charge. Excess charge spills out and becomes ``bloomin''. This causes a problem known as 'gagging'. To shorten the integration period (for example, for example, by a mechanical shutter or by resetting the photodiode to empty its charge. The light must be blocked by methods such as

The photodiode is prepared, as is well known, at the junction of p-type and n-type materials. . A potential well is generated in the depletion region at the interface between these two amounts of material. incident illumination An electron-hole pair is formed by The electron (the charge in question) moves into the potential well I was caught by this. Two methods have been devised to eliminate blooming. Ru. This is known as a “lateral overflow drain” and a “vertical overflow drain.” It is.

Figure 1 shows a cross-sectional view of a typical prior art structure illustrating a lateral drain. Ru. The sensor 10 includes an a-shaped substrate 12 and three spaced n-shaded areas. one The five-shaped area is marked with the reference numeral 14. This n shadow area and the substrate 12 are assembled Together they form a photodiode. Reference numeral 14 also indicates a photodiode. It can also be considered as pointing. Similarly, area 16 is an interline CCD (see Figure 3). Region 18 is part of the buried channel structure and is used to remove excess charge. is part of the lateral drain. The upper surface of the substrate 12 is provided with a transparent material for incident illumination. There is a thin oxide layer 20. Two polysilicon electrodes 22 and 24 are prepared. Ru. Electrode 22 is configured to be connected to a source of potential φ1. The electrode 22 rotates It is said to be used as a sending electrode. It is also the reset electrode. The electrode 24 has a potential of - is configured to be connected to a source of 7 and is known as an overflow drain electrode. There is. Above these electrodes 22 and 24 another oxide layer is provided. child The upper surface of the oxide layer prevents incident light from entering the substrate 12 except in the area of the photodiode 14. There is an aluminum layer 26 that prevents this from occurring. Separate adjacent photodiodes A channel blocking section 28 is provided for this purpose. only one such channel A block is shown. Channel blocking portion 28 includes a relatively thick oxide portion and p + It consists of a piloseneous entry.

In operation, electrode-hole pairs are formed after a photon of light illuminates the diode. Ru. Electrons are accumulated in potential wells located in the depletion regions of the three regions 14. bury the charge A positive potential is applied to fl for transfer to the input channel.

Electrons flow along the surface channel and then in the buried channel of the COD. flows into the potential well. At this point, the potential of φ is removed. This time an unwanted call The load will continue to accumulate on the photodiode. To prevent this from happening A positive potential is applied to φ. For this reason, the charge accumulated in the photodiode is It is transferred to the area 18 of the screen where it is removed in a known manner.

All of the '1tN24's for each photodiode are connected together. similar , all electrodes 22 are connected together. Unfortunately, the electrical characteristics of the device The changes are reset φ, and read φ, the diode of the potential well given by the voltage. This will inevitably result in changes between the two modes.

This results in sensor spatial pattern noise that can be explained by the following equation: Become.

here, Noise is calculated by the number of electrons, C is the capacitance of photodiode 14, and V81 and V#! are each is the RMS value of the surface potential under the electrode.

Such spatial pattern noise can severely limit sensor sensitivity in certain applications. It may be reduced significantly.

A prior art vertical drain configuration is shown in FIG. The components are shown in Figure 1. Where the same reference numerals are used, the same reference numerals are given. This element (device) An n-type substrate 3o is prepared. The upper surface of the external substrate 30 has p There is a shaped well 32. The type 3 substrate 3o is connected to a source of potential Vh, and the p-well 32 is Grounded. Immediately below the center portion of the photodiode 14 is a contour substrate region 30. extends upwardly so as to be relatively close to the photodiode 14. In this configuration , when Vb is a positive potential and the potential well of the photodiode begins to overflow, Electrons flow into the n-type substrate 30 through the narrow portion of the p-well 32. This configuration is vertical Shown as a directional overflow drain. It is the lateral direction of excess charge to COD. Prevent blur. Lateral and vertical overflow drains minimize blooming problems Although it is effective for reducing the does not preclude storage of potential in the potential well of the board.

The purpose of this invention is to prevent charge from accumulating on the photodiode during the non-integration period. An object of the present invention is to provide an electronic shutter that allows

Disclosure of invention The purpose is to integrate multiple photodiodes that integrate light from the scene and store charge in potential wells. ode, associated with each photodiode and used to transfer stored charge from this diode. imager with electronic shutters for each photodiode. (α) a shallow diode placed above each photodiode; connected to a source of reference potential that clamps its top surface to a fixed voltage. and (b) removing the potential well from the photodiode. A device for selectively applying a potential to empty the charge of the photodiode. This is achieved in the above-mentioned image sensor.

Drawing description Figure 1 is an image of the conventional technology using a photodiode with a lateral drain. FIG. 2 is a cross-sectional view of the sensor, using a photodiode with a vertical tray 7. FIG. 3 is a cross-sectional view of a prior art image sensor used according to the present invention. FIG. 4 is a plan view of a portion of an interline image sensor that may be used; The figure shows a vertical drain image sensor according to the present invention taken along line 4--4 of FIG. Figures 5a and 5b are cross-sectional views taken from Figures 2 and 4, respectively. This example shows an electric potential diagram for a vertical drain image sensor. FIG. 6 is a cross-sectional view of a lateral drain image sensor according to the present invention.

How to carry out the invention Turning now to FIG. 3, a schematic configuration of the interline image sensor IO is shown. ing. The sensor IO comprises an array of photodiodes 14. Is it a scene? In response to incident light, each photodiode stores electrons in a potential well. The hole is not important. Electrons are transferred through the surface channel to the buried channel C0D16. It will be done. Vertical CCD embedded on either side of adjacent rows of photodiodes 14 There is a channel 16. This architecture is called "interline". ing. The entire surface of the sensor, except for the photodiode 14, is covered with an aluminum layer 26 (first (See Figure 4). The charge is transferred from the vertical CCD 16 to the horizontal CCD 37. It will be done. In a similar manner, the charge is generated from the horizontal CCD, producing an output voltage (Vost). Transferred to floating diffusion 938. The details of this particular configuration are It is well understood by those skilled in the art, so there is no need to discuss it further here. There's no need.

Turning now to Figure 4, the longitudinal drain diagram taken along line 4-4 of Figure 3 A portion of the image sensor is shown in cross-section. The configuration of this image sensor is Similar to that shown in Figure 2 and therefore identical where the constituent parts correspond. The same numbers are used. For example, the 3-type substrate 30, the p-well 32.3 region (photo diodes) 14 and a buried channel CCD 16. In addition to these, a potential source φ1 There is a polysilicon transfer electrode 22 connected to. Also, a normal channel blocking section 28 is being prepared.

In accordance with the present invention, the top surface of n-region 14 has a shallow p-decade region 40. area 40 gives a shallow diode in combination with region 14. It should be noted here Although the sensor is shown as having a contoured substrate, Reversed impurity polarity is possible (total substitution of p-type materials for 3-type materials) layer 40 is shallow to provide short wavelength quantum efficiency and Does not block incoming photons of light. As shown in the figure, the left channel blocking section 28 is The p+ fibrotic input is grounded. This p+ injection part is directly connected to the p+ layer 40. , thus grounding this layer. Additionally, p-well 32 is also electrically grounded. It is. 1140 to ground on the shallow photodiode layer 40. The surface position is clamped to the reference potential regardless of the voltage CVb applied to the substrate.

This is illustrated in Figures 5a and 5b.

Figure 5a shows the potential diagram (in schematic form) for the longitudinal drain shown in Figure 2. It is shown. In the best situation, assume that about 5 volts are being applied to Vb. Let's decide. A potential well is formed in the photodiode to collect electrons (a-). potential For metallurgical wells where the well is to be overloaded, electrons cross the p-well 32 to the substrate 30. Flow into.

This is indicated by electron-1 with an arrow.

Let us assume that the voltage at Wb is increased to about 15 volts. This is shown in Figure 5a. The situation is shown in dotted lines. The top part (Vl) of the shallow photodiode is now The voltage will be approximately 7.5 volts. The term Va actually refers to the surface potential of a shallow photodiode. represents. A potential well is again formed.

In both cases (Vb=5V and F6=l5V), the excess charge is Potential can be accumulated in the well. This excess charge causes noise. This issue The Ming Dynasty recognized this problem and applied a sufficiently high voltage to Vb, as illustrated in Figure 5b. Eliminate potential wells by selectively adding.

As shown in the solid line section of Figure 5b, the potential applied to Vb is about 5 volts. Let's assume that. The potential (V, ) of the shallow diode layer 40 near the silicon surface is It is maintained at a fixed reference potential. Since the charge is collected in the potential well, the accumulated surplus charge Charge can flow into the substrate 30 through the p-well 32. This time around 15 volts Let us assume that a high potential of is applied to Vb. In this case, as illustrated, Since the potential well is virtually eliminated, all generated electrons flow "downhill". will flow through region 32 to substrate 30 and be dissipated. 5th G Without this top diode as illustrated in the figure, the voltage V# would float. be able to. In practice, this means that adjacent electric field regions (a pair of lateral capacitors) erodible splitting effects due to capacitance and direct transfer gate. It depends. These capacitances are similar to those of photodiodes. The surface potential of the photodiode is generally much smaller and effective for controlling the surface potential of the photodiode. Not.

Turning now to Figure 6, a cross-sectional view of a portion of the lateral drain image sensor is shown. has been done. This diagram is similar to that shown in FIG. Here, Figure 4 As shown in FIG. is formed. This shallow region 40 is also connected to a source of reference potential through the p-type substrate. has been done. Each component of this device is identical to that shown in FIG. Same reference numerals are given. Positive potential is φ.

When applied to n, and in a manner similar to that shown in the dotted section of Figure 5b, the potential well The door is completely removed and the charge flows into the drain 18. Of course, in a similar way , charge is to be transferred to the buried channel 16 and a positive potential is l11. join When the potential well of the photodiode is also completely removed, the charge is transferred to the surface. through the channel into the buried channel 16.

Industrial applicability and advantages The image sensor of this invention is effective for electronic cameras.

A feature of this invention is that the photodiode can be completely emptied of charge and that The pattern noise described by equation l can be removed.

The advantage of this invention is that it uses an electronic shutter and eliminates the need for a mechanical shutter. It is something that can be done.

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Claims (7)

[Claims] 1. used to store charge and in conjunction with overflow drain and readout element , the unwanted charge is overflowed and transferred to the drain by the first level potential. This is a method of removing the well of a photodiode that is The potential is used to drain the photodiode and force the photodiode charge to flow to the drain. The above method is characterized by: 2. 2. The method according to claim 1, characterized in that the reference potential is grounded. How to put it on. 3. Multiple photodiodes each integrating light from the scene and storing charge in a potential well photodiode, each photodiode is associated with the potential of this photodiode. A device capable of transferring accumulated charge representing scene brightness from a well, and a potential well An image sensor comprising a drain for removing excess charge from the image sensor, (a) Placed above the photodiode and clamping the top surface to a fixed potential. a shallow diode connected to a source of reference potential for (b) removing the potential well from the photodiode and draining the photodiode charge; a device for selectively applying a potential to flow into the The image sensor described above is characterized by: 4. Claim characterized in that the shallow diode is grounded. The image sensor described in 3. 5. characterized by the drain being a vertical overflow drain; The image sensor according to claim 3 or 4. 6. selectively applying a first potential level to the substrate to act as a vertical drain; and also apply a second potential level to remove the potential well of the photodiode and close this well. characterized by having a device for emptying the charge from the The image sensor according to claim 5. 7. (a) transferring the accumulated charge for further processing in response to the first voltage ψ1; (b) removing the potential well in response to the second voltage ψ2; It is equipped with a reset device that can empty the charge of the diode. Image sensor according to claim 3 or 4, characterized in that: