KR20030041043A - Image sensor for measuring the dark signal - Google Patents

Abstract

PURPOSE: An image sensor for measuring dark current is provided to be capable of reducing dark current by using a test pattern. CONSTITUTION: An image sensor for measuring dark current comprises a pixel array for driving the image sensor and a test pattern for measuring the dark current generated at interface between an active and field region. The test pattern further includes the first test pattern(A,B,C) having the first photodiode and the second test pattern(A',B',C') having the second photodiode. At this time, the area and the size of depletion region of the second photodiode is substantially same to the first photodiode.

Description

Image sensor for measuring the dark signal

The present invention relates to a semiconductor device, and more particularly, to an image sensor, and more particularly, to an image sensor having a test pattern for measuring the dark current of the image sensor.

In general, an image sensor is a semiconductor device that converts an optical image into an electrical signal. In a double charge coupled device (CCD), individual metal-oxide-silicon (MOS) capacitors are very different from each other. A device in which charge carriers are stored and transported in a capacitor while being located in close proximity, and CMOS (Complementary MOS) image sensor is a CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits. Is a device that employs a switching method that creates MOS transistors by the number of pixels and sequentially detects the output using them.

In manufacturing such various image sensors, efforts are being made to increase the photo sensitivity of the image sensor, and one of them is a light condensing technology. For example, a CMOS image sensor is composed of a photodiode for detecting light and a portion of a CMOS logic circuit for processing the detected light into an electrical signal to make data. To increase light sensitivity, the ratio of the photodiode to the total image sensor area is increased. Efforts have been made to increase (usually referred to as Fill Factor).

FIG. 1 is a unit pixel circuit diagram of a conventional CMOS image sensor, and a submicron CMOS Epi process is applied to increase sensitivity and reduce cross talk effects between unit pixels.

FIG. 2 is a plan view showing the layout of an image sensor having a general 4 Tr structure. The current generated from the photodiode PD is transferred to the FD via the Tx Tr, and the changing FD voltage is used as the gate input of the Dx Tr. To achieve the structure.

The unit pixel is composed of one low voltage buried photodiode and four NMOS transistors. Unlike the conventional photo gate structure, the low voltage buried photodiode has a polysilicon with a light sensing region. Not only is it covered, it has excellent light sensitivity for short wavelength blue light as well as increase the depth of depletion in the light sensing area, so the light sensitivity for long wavelength red or infrared light is also excellent. On the other hand, the low-voltage buried photodiode structure allows photogenerated charges in the photosensitive area to be completely transported to the Floating Sensing Node, which significantly increases the charge transfer efficiency. There are advantages to it.

In addition, the transfer gate (Tx), that is, the gate electrode and the reset gate (Rx), which transfer photocharges among the four transistors, is caused by a voltage drop due to a positive threshold voltage. In order to prevent the loss of charge transport efficiency, the NMOS transistor has a negative threshold voltage. In this case, the N-LDD ion implantation is omitted so that the overlap capacitance between the gate electrode and the reset gate and the floating sensing node is reduced. (Overlap Capacitance) can be lowered to amplify the change in potential of the floating sensing node according to the amount of charge carried. (△ V-ΔQ / C)

Meanwhile, the drive gate (Sx) serving as a source follower is composed of a general submicron NMOS transistor. This structure was constructed with minimal changes to the submicron CMOS Epi process, and the thermal cycle was designed to be completely unchanged. On the other hand, a color filter array composed of red, green, blue or yellow, magenta, cyan, and the like on a unit pixel array for color image realization. (Color Filter Array) The process of forming.

The operation principle of obtaining an output from such a unit pixel is as follows.

end. Turn off Tx, Rx, Sx. The low voltage buried photodiode is then fully depletion.

I. Photogenerated charge is collected in a low voltage buried photo diode.

All. After a proper integration time, the Rx is turned on to reset the floating sensing node first.

la. The unit pixel is turned on by turning on Sx.

hemp. Measure the output voltage (V1) of the source follower buffer. This value simply means the CD level shift of the Floating Sensing Node.

bar. Turn on Tx.

four. All photogenerated charges are transported to Floating Sensing Nodes.

Ah. Turn off Tx.

character. Measure the output voltage (V2) of the source follower buffer.

car. The output signals V1-V2 are the result of the photocharge transport resulting from the difference between V1 and V2 and are pure signal values without noise. This method is called CDS (Corelated Double Sampling).

Ka. Repeat the process of 'a' to 'tea'. However, the low voltage buried photodiode is fully depleted during the 'dead' process.

On the other hand, electron-hole pairs generated by defects contained in the substrate itself, dislocations at the surface of dangling bonds and the active / field region interface, etc. Pair) produces a certain level of output signal even when the input is low, that is, when the light is weak or absent. This is called dark signal (Dark signal) and is one of the main factors to determine the performance of the device.

However, in the test pattern for identifying the characteristics of the device in the current image sensor as described above, it is possible to check the total amount of the dark current described above, but it is difficult to determine the main cause.

Therefore, improvement of dark current becomes difficult.

The present invention proposed to solve the problems of the prior art as described above, it is an object of the present invention to provide an image sensor that can improve the dark current characteristics by being able to determine the main cause of the dark current by using an appropriate test pattern.

1 is a unit pixel circuit diagram of a conventional CMOS image sensor;

2 is a plan view showing a layout of an image sensor having a general 4 Tr structure;

3 is a plan view illustrating a test pattern layout of a general image sensor;

4 is a cross-sectional view of the test pattern of FIG.

5 is a schematic circuit diagram of the test pattern of FIG.

6 (a) is a schematic plan view showing a test pattern for measuring a first dark current;

6 (b) is a schematic plan view showing a test pattern for measuring a second dark current;

Figure 6 (c) is a schematic cross-sectional view of the image sensor for the third dark current measurement.

Explanation of symbols on the main parts of the drawings

A, B, C: First test pattern

A ', B', C ': Second test pattern

In order to achieve the above object, the present invention is a pixel array for driving a substantial element; And a test pattern integrated on the same substrate as the pixel array and measuring a dark current generated at an interface between an active region and a field region, wherein the test pattern comprises: a first test pattern including a first photodiode; And a second photodiode having an area substantially the same as that of the first photodiode, and having a charge depletion region from the substrate surface to a bottom thereof substantially the same as the first photodiode. An image sensor for measuring dark current occurring at an interface between an active region and a field region, the second test pattern having a more interface between the field region and the active region than the pattern.

In addition, the present invention for achieving the above object is a pixel array for driving a substantial element; And a test pattern integrated on the same substrate as the pixel array and configured to measure a dark current generated by a crystal defect on a photodiode surface, the test pattern comprising: a first test pattern including a first photodiode; And a second photodiode having a charge depletion region from the substrate surface to a bottom thereof substantially the same as the first photodiode, but having a different area from the first photodiode. The present invention provides an image sensor for measuring dark current caused by crystal defects on a surface of a photodiode, comprising a second test pattern having the same peripheral area.

In addition, the present invention for achieving the above object is a pixel array for driving a substantial element; And a test pattern integrated on the same substrate as the pixel array and configured to measure a dark current generated by a defect in a photodiode bulk, wherein the test pattern includes a first photodiode. Test pattern; And a second photodiode having substantially the same area as that of the first photodiode, but having a different size of the charge depletion region from the surface of the substrate to the bottom thereof, the second photodiode being different from the first photodiode. And a second test pattern having substantially the same periphery area as and an image sensor for measuring a dark current caused by a defect in a photodiode bulk.

Preferably, the first and second test pattern of the present invention further comprises a reset transistor and a transfer transistor electrically connected to the first and second photodiodes, respectively.

The first test pattern further includes first light blocking means formed on the first photodiode, and the second test pattern further includes second light blocking means formed on the second photodiode. do.

In order to identify the main causes of dark current, the present invention separates candidates which are the main causes of dark current separately and analyzes only the process variables for the factors with the same process variables such as the area of the device, which is another factor to analyze the components. By using different pairs of test patterns, it is possible to improve the dark current characteristics by measuring the dark current in the test pattern integrated under the same conditions as the pixel array and identifying the main factors.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the technical idea of the present invention. do.

3 is a plan view illustrating a test pattern layout of a general image sensor.

Referring to FIG. 3, the types of pixels constituting the test pattern are designed to be integrated on the same substrate as the pixel array in order to reduce potential variables such as potential change, and a light blocking layer on the photodiode to secure a dark state. Shielding using (M2).

However, since pad 3 is a voltage output terminal, the pad 3 is designed in a floating form, and by simplifying the test pattern, a transistor structure composed of PD, FD, and Tx is used.

Here, the test pattern includes a photodiode, and unlike the pixel array, for example, in the case of a 4 Tr structure, the test pattern further includes a transfer transistor Tx and a reset transistor Rx electrically connected to the photodiode. .

4 is a cross-sectional view of the test pattern of FIG. 3.

Referring to FIG. 4, a device isolation film Fox is locally disposed on a substrate P-Sub, and ion implantation is performed from the surface of the substrate P-Bub to the bottom thereof so that one side of the device isolation film Fox comes into contact with the device isolation film Fox. The photodiode PD formed through the buried form is formed in a structure in which P0 and n- overlap.

The transfer transistor Tx is formed to contact the other side of the photodiode PD and one side thereof, and the gate of the transfer transistor Tx is connected to the control signal Test1 for controlling the on / off of the transfer transistor during the test. The gate is connected, and its source / drain (n +) shows a floating diffusion (FD) for a general unit pixel, but an output voltage (Vout) for a test pattern. On the other side of the transfer transistor Tx, a reset transistor Rx is formed and controlled by the reset control signal Test2, and the source / drain thereof is connected to the power supply voltage VDD, and the test pattern having the above-described configuration A schematic circuit configuration is shown in FIG.

Hereinafter, referring to FIG. 5, the operation of the device in the dark current measurement by the test pattern will be described. Since the control of Tx and Rx is performed through the control signals Test1 and Test2, Test1 and Test2 according to the above description are simplified. Omit the mention of.

First, when Tx and Rx are turned off, the photodiode PD is completely depleted. Then, Rx is turned on to reset the FD, that is, the output terminal Vout, and then the first output voltage of the output terminal Vout is measured, and then, Tx is turned on so that the second output voltage at Vout is caused by the dark current from the PD. Measure

Here, the difference between the second output voltage and the first output voltage at the output terminal indicates the output by the dark current.

On the other hand, the PD is in the light blocking state.

The dark current measurement using the test pattern of the present invention is performed through the series of measurement processes, and the above operation process will be omitted.

Candidates that are the main cause of dark current are as follows.

1. Dark current generated at the active area / field area interface (hereinafter referred to as first dark current)

2. Dark current due to crystal defects on the surface of the photodiode (hereinafter referred to as second dark current)

3. Dark current generated in photodiode bulk (hereinafter referred to as third dark current)

Therefore, by excluding the different dark current components to find the main dark current among the three dark currents as described above, that is, by making the process variables that provide the cause of causing the different dark currents the same between the two test patterns to be compared, By finding and comparing the dark current for each of the one cause, the main dark current can be found, and the main cause can be analyzed to improve its characteristics.

As a result, since a pair of test patterns for each dark current is needed, each test pattern pair can be used separately or simultaneously, so that up to six test patterns can be used.

FIG. 6A is a schematic plan view illustrating a test pattern for measuring a first dark current.

Referring to FIG. 6 (a), the main cause of generating the first dark current is an output signal generated by electron-hole pairs generated by dislocations in the field edge region, that is, the interface between the field and the active region. The second test pattern A 'which has more boundary surfaces between the field region and the active region than the first test pattern A and the first test pattern A having a shape, that is, the length of the peripheral region is longer. By comparing the dark current between the two test patterns by using the first dark current value can be obtained.

Here, the first test pattern A and the second test pattern A 'differ only in the length of the peripheral region, and the size of the photodiode and the charge depletion region of the photodiode from the substrate surface to the bottom thereof are equal to each other. Since the factors caused by the second dark current and the third dark current can be minimized, the value of the first dark current generated by the difference in the length of the peripheral region can be measured.

FIG. 6B is a schematic plan view illustrating a test pattern for measuring a second dark current. FIG.

Referring to FIG. 6B, since the main factors for generating the second dark current are due to crystal defects on the photodiode surface, for example, dangling bonds, the first test pattern B and the first test pattern having a basic pattern shape. The second dark current value can be obtained by comparing the dark current between the two test patterns by using the second test pattern B ′ having the different area (B) and the photodiode.

In this case, the first test pattern B and the second test pattern B 'have the same length of the peripheral region, and the first and second test patterns B' have the same size as the charge depletion region of the photodiode from the substrate surface to the bottom thereof. Since the factors caused by the dark current and the third dark current can be minimized, the value of the second dark current generated by the dangling bond on the surface of the photodiode can be measured.

That is, the larger the photodiode area of one test pattern, the greater the surface area, and therefore the greater the number of dangling bonds. Therefore, since a dark current difference occurs between the two test patterns, the dark current difference caused by other factors can be minimized and measured.

6 (c) is a schematic plan view of the image sensor for measuring the third dark current.

Referring to FIG. 6 (c), since the main factor for generating the third dark current is due to defects in the photodiode bulk, that is, in the substrate itself, the first test pattern C and the second test pattern C ′. By varying the size of the charge depletion region of the photodiode from the substrate surface of the photodiode to the bottom thereof, the third dark current value can be obtained by comparing the dark current between the two test patterns.

In this case, the first test pattern C and the second test pattern C 'may have the same length as the peripheral area and the planar area of the photodiode, thereby minimizing the factors caused by the first dark current and the second dark current. Therefore, the value of the third dark current generated by the bulk of the photodiode can be measured.

Here, the depth of the charge depletion region of the photodiode may vary depending on the concentration of each impurity region constituting the photodiode and its ion implantation depth according to the ion implantation energy. In the present invention, the photodiode is formed between two test patterns. It is desirable to vary the ion implantation energy for each other.

In addition, when the dark current component in the bulk according to the third dark current is detected, ion implantation of P-type impurities may block the effect at the edge of the field region.

Meanwhile, in the present invention as described above, each test pattern is simultaneously integrated with the pixel array in the substrate.

The present invention made as described above, by making it possible to more accurately measure the main cause of the dark current of the image sensor, it is possible to improve the operating characteristics of the image sensor by improving the characteristics according to the cause of the dark current, as well as device development period It was found through the examples that it can be shortened.

Although the technical idea of the present invention has been described in detail according to the above preferred embodiment, it should be noted that the above-described embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

The present invention described above, by allowing the main cause of the dark current of the image sensor can be measured, it can be expected to have an excellent effect that can ultimately shorten the device development period of the image sensor.

Claims (9)

A pixel array for substantially driving the device; And It is integrated on the same substrate as the pixel array, and includes a test pattern for measuring the dark current generated at the interface between the active region and the field region, The test pattern is, A first test pattern comprising a first photodiode; And And a second photodiode having a substantially same area as that of the first photodiode, and having a charge depletion region from the surface of the substrate to the bottom thereof, the second photodiode having substantially the same size as the first photodiode. Second test pattern having more interface between field area and active area than Image sensor for measuring the dark current generated at the interface between the active region and the field region comprising a. The method of claim 1, The first and second test patterns may further include reset transistors and transfer transistors electrically connected to the first and second photodiodes, respectively. Image sensor. The method of claim 1, The first test pattern further includes first light blocking means formed on the first photodiode, and the second test pattern further includes second light blocking means formed on the second photodiode. An image sensor for measuring dark current generated at an interface between an active region and a field region. A pixel array for substantially driving the device; And It is integrated on the same substrate as the pixel array, and includes a test pattern for measuring the dark current caused by the crystal defects on the photodiode surface, The test pattern is, A first test pattern comprising a first photodiode; And A charge depletion region from the surface of the substrate to the bottom thereof includes a second photodiode having substantially the same size as the first photodiode but having a different area from the first photodiode, and substantially having the first test pattern. Second test pattern having the same peripheral area area Image sensor for measuring a dark current caused by a crystal defect on the surface of the photodiode, comprising a. The method of claim 4, wherein The first and second test patterns may further include reset transistors and transfer transistors electrically connected to the first and second photodiodes, respectively. Image sensor. The method of claim 4, wherein The first test pattern further includes first light blocking means formed on the first photodiode, and the second test pattern further includes second light blocking means formed on the second photodiode. An image sensor for measuring dark current generated at an interface between an active region and a field region. A pixel array for substantially driving the device; And It is integrated on the same substrate as the pixel array, and includes a test pattern for measuring the dark current caused by a defect in the photodiode bulk, The test pattern is, A first test pattern comprising a first photodiode; And The area of the first photodiode is substantially the same, but includes a second photodiode having a different charge depletion region from the surface of the substrate to the bottom thereof, the second photodiode being different from the first photodiode. Second test pattern having substantially the same peripheral area area Image sensor for measuring the dark current caused by a defect in the photodiode bulk, comprising a. The method of claim 7, wherein The first and second test patterns may further include reset transistors and transfer transistors electrically connected to the first and second photodiodes, respectively. Image sensor. The method of claim 7, wherein The first test pattern further includes first light blocking means formed on the first photodiode, and the second test pattern further includes second light blocking means formed on the second photodiode. An image sensor for measuring dark current generated at an interface between an active region and a field region.