KR20030084491A - Unit Pixel with improved fill factor and dark signal property in cmos image sensor - Google Patents

Abstract

PURPOSE: A unit pixel of a CMOS image sensor is provided to improve dark current property by directly connecting a reset transistor to a photodiode and to improve fill factor by using an improved drive transistor. CONSTITUTION: A photodiode(201) senses a light and generates optical carriers. A reset transistor(204) is directly connected to the photodiode(201) so as to improve dark current. A floating diffusion region(203) stores the carrier generated in the photodiode. A transfer transistor(202) is connected between the photodiode and the floating diffusion region. A drive transistor(206) detects an electrical signal from the floating diffusion region. A select transistor(208) is connected to the drive transistor. At this time, a gate and a drain of the drive transistor(206) are connected each other by using butting contact, thereby improving fill factor.

Description

Unit pixel with improved fill factor and dark signal property in cmos image sensor

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CMOS image sensor, and in particular, by changing a reset transistor and a drive transistor, to increase the removal efficiency of electrons present in the photodiode and at the same time to fill factor. It is an improved invention.

In general, an image sensor is a semiconductor device that converts an optical image into an electrical signal. Among them, a charge coupled device (CCD) includes individual metal-oxide-silicon (MOS) capacitors. A device in which charge carriers are stored and transported in a capacitor while being in close proximity to each other. Complementary MOS image sensors use CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits. A device employing a switching scheme that creates MOS transistors as many as pixels and sequentially detects outputs using the MOS transistors.

CCD (charge coupled device) has many disadvantages such as complicated driving method, high power consumption, high number of mask process steps, complicated process, and difficult to implement one processing chip because signal processing circuit cannot be implemented in CCD chip. In order to overcome such drawbacks, the development of a CMOS image sensor using a sub-micron CMOS manufacturing technology has been studied in recent years. The CMOS image sensor forms an image by forming a photodiode and a MOS transistor in a unit pixel (Pixel) and sequentially detects signals through a switching method.As a CMOS manufacturing technique, the power consumption is low and the number of masks is 20 to 30 to 40. Compared to CCD process that requires two masks, the process is very simple, and it is possible to make various signal processing circuits and one chip, which is attracting attention as next generation image sensor.

FIG. 1A is a circuit diagram showing a unit pixel composed of one photodiode PD and four NMOS transistors in a conventional CMOS image sensor, and includes a photodiode for generating photocharges by receiving light and a photodiode ( A transfer transistor Tx for transporting the photocharges collected from the PD) to the floating diffusion region FD, and a reset for setting the potential of the floating diffusion region FD to a desired value and discharging electric charges to reset the floating diffusion region. A transistor Rx, a drive transistor Dx serving as a source follower buffer amplifier, and a select transistor Sx for addressing can be configured as a switching role. Outside the unit pixel, a load transistor is formed to read an output signal.

Operation of the image sensor unit pixel configured as described above is performed as follows. Initially, the reset transistor Rx, the transfer transistor Tx, and the select transistor Sx are turned on to reset the unit pixels. At this time, the photodiode PD starts to be depleted to generate charge charging on the photodiode, and the floating diffusion region FD is charged and stored in proportion to the supply voltage VDD.

Thereafter, the transfer transistor Tx is turned off, the select transistor Sx is turned on, and the reset transistor Rx is turned off. In such an operating state, the first output voltage V ₁ is read from the unit pixel output terminal Out, stored in a buffer (not shown), and then the transfer transistor Tx is turned on to change the intensity of the photodiode. After the charges are moved to the floating diffusion region, the second output voltage V ₂ is read again from the output terminal Out to change the analog data for the two voltage differences 'V ₁ -V ₂ ' into digital data. One operation cycle is completed.

In the unit pixel of the CMOS image sensor having such a configuration, before performing the operation of reading the photocharges stored in the photodiode, the operation of resetting the photodiode by using the reset transistor is performed.

That is, the reset transistor and the transfer transistor are turned on in order to perform a reset operation to remove free electrons existing in the photodiode, in order to remove noise components and obtain more accurate image values.

In the unit pixel of the conventional structure shown in Fig. 1A, the force for attracting the free electrons present in the photodiode in this reset operation depends on the power supply voltage V _DD . It is used to remove electrons applied to the photodiode.

That is, the voltage V _FD finally applied to the photodiode is

V _PD = V _DD- (V _TX + V _FD + V _RX ) = 3.3-(i × R _TX + i × R _FD + i × R _RX ).

Here, R _TX is a resistance of a well in which a transfer transistor is formed, R _FD is a resistance of a floating diffusion region, and R _RX is a resistance of a well in which a reset transistor is formed.

As such, the voltage used to remove the electrons present in the photodiode is used to remove the electrons present in the photodiode with the voltage reduced by (V _TX + V _FD + V _RX ) from the power supply voltage. do.

That is, in the conventional unit circuit structure, as the voltage used to remove the residual electrons remaining in the photodiode and the floating diffusion region is lower than the power supply voltage, there is a problem that a dark signal due to the residual electrons is increased.

FIG. 1B is a graph showing a change in output voltage Vout according to a gate input voltage Vg of a drive transistor in a unit pixel of a conventional CMOS image sensor. It shows that it is turned on and outputs a voltage having a linear slope.

When the photocharge generated in the photodiode is transferred to the floating diffusion region via the transfer transistor, and this charge is input to the gate of the drive transistor, the output of the drive transistor changes accordingly. That is, a signal having information about an image is output by the photocharge generated in the photodiode.

FIG. 1C illustrates a circuit connection state of a conventional drive transistor and a cross-sectional structure corresponding thereto, and includes a spacer formed on a sidewall of a gate polysilicon 11 and a gate polysilicon of a drive transistor formed on a substrate 10. Referring to FIG. 12 and a tungsten plug 16 and a barrier metal 15 for contacting an upper conductor with an interlayer insulating film 14 formed on a substrate comprising a source / drain region 13 and a gate polysilicon. have.

Referring to FIG. 1C, a tungsten plug 16 connected to a gate of a drive transistor is conventionally applied with a general contact structure insulated from a drain / source region 13 of the drive transistor. It occupies a considerable area in unit pixels.

Also. FIG. 1D illustrates a layout of a unit pixel having a conventional structure, in which an isolation defining an active region in which a photodiode and a diffusion region are to be formed and polysilicon constituting a gate of each transistor are illustrated. Referring to this, the photodiode 101 has a square shape, and the gate polysilicon 102 of the transfer transistor is formed in contact with one side of the photodiode 101.

The floating diffusion region 103 is laid out in contact with the other side of the gate polysilicon 102 of the transfer transistor by being turned 90 ° in the X-axis direction in the Y-axis direction, and in contact with one side of the gate polysilicon 104 of the reset transistor.

The other side of the gate polysilicon 104 of the reset transistor is formed in contact with the drain region 105, and the drain region 105 is formed at an angle of 90 ° from the X-axis direction to the Y-axis direction, and then the gate polysilicon 106 of the drive transistor. It comes in contact with.

Subsequently, the gate polysilicon 108 of the select transistor is formed in the same direction, between the other side of the gate polysilicon 106 of the drive transistor and the gate polysilicon 108 of the select transistor, and the other side of the gate polysilicon 108 of the select transistor. Source / drain regions 107 and 109 are formed in the substrate.

In the layout of a conventional unit pixel configured as described above, the floating diffusion region 103 is formed in an active region between the transfer transistor 102 and the reset transistor 104 and the gate polysilicon of the floating diffusion region 103 and the drive transistor. 106 is electrically connected via a contact.

The fill factor represents the ratio of the area occupied by the photodiode to the total area of the unit pixel, which is one of the important factors related to the performance of the image sensor.

When the fill factor is calculated from the unit pixel of the conventional structure, the unit pixel size is 7.85 × 8 = 62.8 μm ² , and the photodiode size is 4.2 × 4.2 = 17.64 μm ² , and the fill factor is 17.64 ÷ 62.8 = 0.281 (28.1 It can be seen that the fill factor is not so large as%).

The larger fill factor means that the ability to receive light and convert it into an electrical signal means that the larger the fill factor, the larger the change in the output voltage of the unit pixel. In other words, the dynamic range of the CMOS image sensor increases.

The unit pixel of the CMOS image sensor having the same configuration as the related art has a disadvantage that the fill factor is not suitable for more accurate image reproduction.

Disclosure of Invention The present invention has been made in view of the above-described problems, and an object thereof is to provide a CMOS image sensor and a method of manufacturing the same, which improve the fill factor and improve the electron removal efficiency of the photodiode.

1A is a circuit diagram showing the configuration of a unit pixel in a CMOS image sensor according to the prior art;

1B is a graph illustrating input and output voltage relationships in a drive transistor according to the prior art;

Figure 1c is a view showing the configuration and contact of the drive transistor according to the prior art;

1D is a view showing a layout of a CMOS image sensor unit pixel according to the prior art;

FIG. 2A is a circuit diagram showing the configuration of unit pixels in a CMOS image sensor according to an embodiment of the present invention; FIG.

2B is a graph illustrating input / output voltage relationships of a drive transistor according to an embodiment of the present invention and input / output voltage relationships of a drive transistor according to the prior art;

2C illustrates a circuit and butting contact of a drive transistor according to an embodiment of the present invention;

2D is a diagram illustrating a layout of a CMOS image sensor unit pixel according to an embodiment of the present invention.

* Description of the symbols for the main parts of the drawings *

20: substrate

21: gate electrode

22: spacer

23 source / drain regions

24: interlayer insulating film

25: barrier metal

26: tungsten plug

201: photodiode

202: transfer transistor

203: floating diffusion area

204: reset transistor

206: Drive Transistor

208: Select Transistor

The present invention for achieving the above object, a photodiode for generating a photocharge by sensing light from the outside; A reset transistor connected between a power supply voltage terminal and the photodiode to reset the photodiode; A floating diffusion region configured to receive and store charge generated from the photodiode; A transfer transistor connected between the photodiode and the floating diffusion region to transfer charges generated from the photodiode to the floating diffusion region; A drive transistor for detecting an electrical signal from the floating diffusion region; And a select transistor having one side connected to the other side of the drive transistor.

In addition, the present invention provides a photodiode comprising: a first active region constituting a photodiode; A second active region formed in contact with one side of the first active region; A gate of a transfer transistor overlapping the first active region and the second active region; A gate of the drive transistor and the select transistor formed on the second active region and gradually spaced apart from the transfer transistor; And a gate of the reset transistor formed on the first active region and surrounding a portion of the first active region.

The present invention improves the fill factor by changing the structure of the drive transistor while simultaneously improving the electron removal efficiency by preventing the voltage applied to the photodiode from being reduced by directly connecting the reset transistor to the photodiode.

More specifically, the reset transistor is directly connected to the photodiode, but a power supply voltage contact connected to the reset transistor is formed in the photodiode, thereby greatly improving the fill factor, and using a butting contact to gate the drive transistor. By connecting the drain and drain stages, the layout area is reduced, contributing to the improvement of the fill factor.

When forming a drive transistor, when the butting contact connecting the gate and the drain to each other is used as described above, a power supply voltage terminal connected to the drain of the drive transistor is not required, and thus an active region for contacting the power supply voltage terminal is conventionally required. The layout area can be reduced.

Hereinafter, the most preferred embodiments of the present invention will be described with reference to the accompanying drawings so that those skilled in the art can easily implement the technical idea of the present invention.

FIG. 2A is a circuit diagram illustrating the configuration of unit pixels in a CMOS image sensor according to an exemplary embodiment of the present invention. Referring to FIG. 2A, the arrangement of the reset transistor and the structure of the drive transistor are different from those of the related art, and other parts are related to the related art. Similar to

In other words, the unit pixel of the CMOS image sensor according to an embodiment of the present invention composed of one photodiode and four transistors is a photodiode (PD) for generating and accumulating photocharges for use in image reproduction, and a photo A transfer transistor (Tx) for transporting the photocharges generated by the diode to the floating diffusion region, a reset transistor (Rx) connected to the photodiode to perform a reset operation for discharging electrons in the photodiode and the floating diffusion region, and a floating diffusion The display device is configured to output image information according to the voltage of the region, but includes a drive transistor Dx having a gate terminal and a drain terminal connected thereto, and a select transistor Sx for addressing by switching. Outside the unit pixel, a load transistor (load resistor) is formed to read an output signal.

Referring to the reset operation in the unit pixel of the CMOS image sensor having such a structure, when only the reset transistor is turned on, the electrons in the photodiode are removed by the power supply voltage (). Finally, the voltage VPD applied to the photodiode is

V _PD = V _DD- (V _RX ) = 3.3-(i × R _RX ).

That is, in the prior art, it can be seen that a voltage V _DD- (V _RX ) greater than the voltage V _DD- (V _TX + V _FD + V _RX ) applied to the photodiode is applied to the photodiode.

As the voltage applied to the photodiode PD increases during the reset operation, the efficiency of removing electrons present in the photodiode is increased, which effectively removes dark current and enables more accurate image data processing. .

The dark current is generated by electrons moving from the photodiode to the floating diffusion region even in the absence of light, and the dark current decreases as the concentration of residual electrons is reduced by removing a lot of electrons in the photodiode during the reset operation.

2B is a graph showing a change in output voltage Vout according to a gate input voltage Vg in a drive transistor connected with a gate terminal and a drain terminal together with a conventional input / output graph. The graph showing the voltage and the graph ② is a graph showing the output of the drive transistor according to an embodiment of the present invention.

In the exemplary embodiment of the present invention, since the gate and the drain of the drive transistor are connected to each other, the output Vout according to the gate input Vg is as shown in graph ②. In more detail, in the section where the input voltage is small, the change of the output voltage ΔV 'according to the change of the input voltage ΔVg is smaller than that of the conventional No. 1 graph ΔV.

The drive transistor according to an embodiment of the present invention has a disadvantage in that the output voltage is less varied with respect to the input voltage input to the gate. Such a disadvantage is that if an external circuit is provided with an amplifier, it is expected that there will be no big problem in image reproduction.

That is, while the fill factor can be increased by using the butting contact, such a disadvantage may be solved by providing an amplifier externally.

FIG. 2C illustrates a circuit connection state of a drive transistor and a cross-sectional structure corresponding thereto according to an embodiment of the present invention. The gate polysilicon 21 and the gate polysilicon of the drive transistor formed on the substrate 20 are illustrated in FIG. Tungsten plug 26 and barrier metal contacting the upper conductor with the interlayer insulating film 24 formed on the substrate including the spacer 22 and the source / drain region 23 formed on the sidewall of the gate and the gate polysilicon 25 is shown.

Referring to FIG. 2C, since the tungsten plug 26 connected to the gate of the drive transistor is electrically connected to the drain region 23 of the drive transistor, the active region for connecting the power supply voltage terminal and the drain terminal is not necessary. Clothing area can be reduced, which can contribute to better fill factor in increasingly finer unit pixels.

Also. FIG. 2D illustrates a layout of unit pixels according to an embodiment of the present invention, in which an isolation defining an active region in which a photodiode and a diffusion region are to be formed, and polysilicon constituting a gate of each transistor are illustrated. Referring to this, the photodiode 201 has a square shape and is formed at a lower portion of the unit pixel, and the gate polysilicon 202 of the transfer transistor is in contact with one side of the photodiode 201.

On the other side of the gate polysilicon 202 of the transfer transistor, the drain region 203 of the drive transistor is laid out by 90 ° in the X-axis direction from the Y-axis direction, and the butting contact 300 is a gate polysilicon ( 206 and drain region 203 are electrically connected to each other. In the exemplary embodiment of the present invention, since the drain region 203 of the drive transistor is not connected to the power supply voltage terminal, the active region for forming the power supply voltage contact is not necessary.

Subsequently, the gate polysilicon 208 of the select transistor is formed in the same direction, and the source is formed between the gate polysilicon 206 of the drive transistor and the gate polysilicon 208 of the select transistor and on the other side of the gate polysilicon 108 of the select transistor. / Drain regions 207 and 209 are formed.

The gate polysilicon 204 of the reset transistor is formed in contact with the lower portion of the photodiode 201, and is formed by surrounding the power supply voltage contact 205 with the gate polysilicon 204 of the reset transistor.

That is, in one embodiment of the present invention, a power supply voltage contact connected to the drain of the reset transistor is formed in the photodiode without forming a separate active region and electrically insulated the power supply voltage contact with polysilicon.

Referring to FIG. 2D, it can be seen that the photodiode 201 occupies more than half of the lower portion of the unit pixel compared to the conventional layout. The size of the photodiode can be increased because the power supply voltage contact connected to one end of the reset transistor is formed in the photodiode. In other words, such a surplus space contributes to an increase in the photodiode area since a separate active region for forming a power supply voltage contact is not needed.

In the present invention, although the power supply voltage contact 205 and the photodiode 201 are electrically insulated by using the gate polysilicon 204 of the reset transistor, when the power supply voltage is applied to the power supply voltage contact, the photodiode is inserted into the photodiode. Electrons can penetrate.

The electrons penetrating into the photodiode may be a component that causes dark current or noise. Such a problem can be overcome by process control. That is, in one embodiment of the present invention, since the size of the photodiode is very large, if the n-type impurity region of the photodiode is formed apart from the gate polysilicon of the reset transistor, penetration of electrons can be prevented.

In general, in a unit pixel of a CMOS image sensor, a photodiode is formed by vertically bonding p-type impurity regions formed on an n-type impurity region to each other. In an embodiment of the present invention, an n-type impurity region is formed of a gate polysilicon of a reset transistor. The formation of electrons at a distance from 204 can be prevented. That is, while electrons are transferred from the power supply voltage terminal to the photodiode, recombination occurs to form a buffer zone for the electrons to disappear.

When the n-type impurity region is formed to be spaced apart from the gate polysilicon 204 of the reset transistor, the area of the photodiode is reduced. In an embodiment of the present invention, the photodiode is so large that the photodiode is large. The reduction in diode area does not have a significant effect on the increase in fill factor.

In one embodiment of the present invention, the unit pixel size is 9.15 × 8.65 = 79.1 μm ² and the photodiode size is 8.35 × 5.0 = 41.75 μm ² , and the fill factor is 41.75 ÷ 79.1 = 0.538 (53.8%).

In other words, applying the layout according to an embodiment of the present invention increases the fill factor by about 25% compared to the conventional one. (The increase from 28.1% to 53.8%.)

The present invention reduces the possibility of dark current by increasing the removal efficiency of free electrons during the reset operation by directly connecting the reset transistor to the photodiode, and by forming a power supply voltage contact in the photodiode in contact with the drain region of the reset transistor, Increased. In addition, the butting contact is applied to the structure of the drive transistor to contribute to the increase of the fill factor.

In the exemplary embodiment of the present invention, the drive transistor to which the butting contact is applied may be used, but a conventional drive transistor may be used.

As described above, the present invention is not limited to the above-described embodiments and the accompanying drawings, and the present invention may be variously substituted, modified, and changed without departing from the spirit of the present invention. It will be apparent to those of ordinary skill in the art.

When the present invention is applied to the CMOS image sensor, more accurate image reproduction is possible by increasing the removal efficiency of electrons in the reset operation and reducing the dark current, and the layout is changed by the circuit change of the unit pixel, and the fill factor is significantly increased than before. Therefore, the dynamic range of the CMOS image sensor is increased.

Claims (9)

In the unit pixel of the CMOS image sensor, A photodiode that senses light from the outside and generates photocharges; A reset transistor connected between a power supply voltage terminal and the photodiode to reset the photodiode; A floating diffusion region configured to receive and store charge generated from the photodiode; A transfer transistor connected between the photodiode and the floating diffusion region to transfer charges generated from the photodiode to the floating diffusion region; A drive transistor for detecting an electrical signal from the floating diffusion region; And A select transistor having one side connected to the other side of the drive transistor Unit pixel of the CMOS image sensor comprising a. The method of claim 1, The power supply voltage terminal connected to the reset transistor is formed in the photodiode unit pixel of the CMOS image sensor. The method of claim 2, And a power supply voltage terminal formed in the photodiode is electrically insulated by the gate polysilicon of the reset transistor. The method of claim 1, And the drive transistor is butted contact with the floating diffusion region. In the unit pixel of the CMOS image sensor, A first active region constituting the photodiode; A second active region formed in contact with one side of the first active region; A gate of a transfer transistor overlapping the first active region and the second active region; A gate of the drive transistor and the select transistor formed on the second active region and gradually spaced apart from the transfer transistor; And A gate of the reset transistor formed on the first active region and surrounding a portion of the first active region Unit pixel of the CMOS image sensor comprising a. The method of claim 5, And a first active region surrounded by a gate of the reset transistor is connected to a power supply voltage. The method of claim 5, The unit pixel of the CMOS image sensor, characterized in that the gate and the drain of the drive transistor is butted contact. The method of claim 5, The first active region constituting the photodiode, A unit pixel of a CMOS image sensor comprising a p-type impurity region formed on an n-type impurity region. The method of claim 8, In the first active region constituting the photodiode, And the n-type impurity region is formed to be spaced apart from a gate of a reset transistor formed to surround a portion of the first active region.