CN102856337B

CN102856337B - The making method of pixel cell, cmos image sensor and pixel cell

Info

Publication number: CN102856337B
Application number: CN201210304225.7A
Authority: CN
Inventors: 令海阳
Original assignee: Shanghai Huahong Grace Semiconductor Manufacturing Corp
Current assignee: Shanghai Huahong Grace Semiconductor Manufacturing Corp
Priority date: 2012-08-23
Filing date: 2012-08-23
Publication date: 2016-06-08
Anticipated expiration: 2032-08-23
Also published as: CN102856337A

Abstract

A kind of pixel cell, comprising: have the substrate of photosensitive region, the silicon oxide layer being positioned on described photosensitive region from bottom to top and silicon nitride layer. In addition, present invention also offers the making method of this pixel cell, comprise the cmos image sensor of this pixel cell. Adopt the technical scheme of the present invention, it is possible to improve the problem that the dark current of pixel cell is excessive preferably, the problem that the sensitivity of the cmos image sensor comprising this pixel cell is poor can be improved simultaneously.

Description

The making method of pixel cell, cmos image sensor and pixel cell

Technical field

The present invention relates to semiconductor applications, in particular to a kind of pixel cell and making method thereof, the cmos image sensor that comprises this pixel cell.

Background technology

The effect of image sensor is that optical imagery is converted into corresponding electrical signal. Image sensor is divided into CMOS (CMOS) image sensor and charge-coupled device (CCD) image sensor. The advantage of ccd image sensor is that image sensitivity is higher, and noise is little, but ccd image sensor and other devices integrated compares difficult, and the power consumption of ccd image sensor is higher. By contrast, to have technique simple, easily and other devices are integrated, volume is little, weight is light, power consumption is little, low cost and other advantages for cmos image sensor. Current cmos image sensor has been widely used in static digital camera, camera cell phone, Digital Video, medical shooting device (such as gastroscope), automobile-used shooting device etc.

The parts that optical imagery is converted in cmos image sensor corresponding electrical signal are pixel cell. With reference to, shown in Fig. 1, pixel cell comprises photosensitive unit 11 and pixel circuit region 12. Fig. 2 is the sectional structure schematic diagram along the A-A straight line in Fig. 1. It may be seen that photosensitive unit 11 is formed with photosensitive region 111, pixel cell is formed with the transistor 121 gone out by the electric signal transmission that photosensitive unit 11 converts to. In addition, for preventing photosensitive region 111 by the pollution of the steam etc. in successive process or use procedure, the semiconducter substrate that this comprises photosensitive region 111 is formed with one layer of good silicon nitride layer 13 of compactness. As required, follow-up transistor 121 is formed with metal interconnect structure, photosensitive region 111 is formed optical filtering element and microlens etc.

But, above-mentioned pixel cell in use has the excessive problem of dark current, correspondingly, and the problem that the cmos image sensor comprising this pixel cell has sensitivity poor.

For the problems referred to above Producing reason, it is considered that be cause by the defect of photosensitive region 111. In order to improve above-mentioned defect, prior art also has some schemes to be suggested. Bottom pixel circuit region 12 is formed, such as it is positioned at the through hole at the interface of semiconducter substrate and silicon nitride layer 13 when forming transistor 121 and metal interconnect structure simultaneously, and lead to into hydrogen in this through hole, to utilize the hydrogen atom in hydrogen that the bound not strong feature of its unbound electron is improved the defect of photosensitive region 111.

But the improvement effect of such scheme is very limited.

Summary of the invention

The problem that the present invention solves is the cmos image sensor proposing a kind of new pixel cell and making method thereof, comprising this pixel cell, improve the problem that the dark current of pixel cell is excessive preferably, and the problem that the sensitivity comprising the cmos image sensor of this pixel cell is poor.

For solving the problem, the present invention provides a kind of pixel cell, comprising:

There is the substrate of photosensitive region;

The silicon oxide layer being positioned on described photosensitive region;

The silicon nitride layer being positioned on described silicon oxide layer.

Can selection of land, the thickness range of described silicon oxide layer is: 50 microns-500 microns.

Can selection of land, the thickness range of described silicon nitride layer is: 100 microns-500 microns.

Can selection of land, described substrate comprises semiconducter substrate, is positioned at the epitaxial film of described semiconductor substrate surface.

Can selection of land, described substrate has multiple photosensitive region, between neighboring photosensitive region by shallow trench isolation from.

Can selection of land, the substrate closing on described photosensitive region has transistor.

Can selection of land, described transistor is transfering transistor.

Can selection of land, the dopant ion type of described photosensitive region is N-type or P type.

Based on above-mentioned pixel cell, present invention also offers a kind of cmos image sensor comprising above-mentioned pixel cell.

In addition, present invention also offers the making method of above-mentioned pixel cell, comprising:

Substrate is formed photosensitive region;

Described photosensitive region forms silicon oxide layer;

Described silicon oxide layer forms silicon nitride layer;

Can selection of land, substrate is formed photosensitive region by dopant ion inject formed.

Can selection of land, the technique forming photosensitive region in substrate is selective epitaxial growth process and in-situ doped.

Can selection of land, described photosensitive region is formed silicon oxide layer be formed by chemical vapour deposition or selective oxidation method formed.

Can selection of land, also form the part of the grid oxic horizon of the transistor closing on region when described photosensitive region forms silicon oxide layer.

Compared with prior art, the present invention has the following advantages: 1) interface of semiconducter substrate and silicon nitride layer is processed, by forming through hole in pixel circuit region, the defect improving photosensitive region by existing scheme, but this photosensitive region is covered by the silicon nitride layer of densification, thus, improve limited efficiency. Being different from such scheme, the present invention proposes to arrange silicon oxide layer between photosensitive region and silicon nitride layer, and during to avoid silicon nitride layer directly to contact with photosensitive region, defect occurs in the surface causing excessively by force photosensitive region to contact with it due to silicon nitride layer bonding force.

2) in optional scheme, the forming method of silicon oxide layer is chemical Vapor deposition process or selective oxidation method, relative to selective oxidation method, chemical Vapor deposition process does not use the thickness of the photosensitive region of former substrate, thus, this photosensitive region still can make according to the specifications parameter of former pixel cell so that this scheme is good with existing processing compatibility.

3) in optional scheme, the thickness range of described silicon oxide layer is: 50 microns-500 microns, and the thickness range of silicon nitride layer is: 100 microns-500 microns, and the defect namely avoiding photosensitive region produces, and achieves again the pollutions such as photosensitive region carries out isolated steam.

4) in optional scheme, the substrate closing on described photosensitive region has transistor, and namely pixel circuit region has the transistor realizing transfer, selection etc., forms the part of the grid oxic horizon of this transistor when forming zone of oxidation on photosensitive region simultaneously.

Accompanying drawing explanation

Fig. 1 is the vertical view of existing pixel cell;

Fig. 2 is the pixel cell local cross section structure schematic diagram of A-A straight line along Fig. 1;

Fig. 3 is the schematic circuit of the 4T transistor pixel cell in embodiment one;

Fig. 4 is the making method schema of the pixel cell in embodiment one;

Fig. 5 is the cross section structure schematic diagram of the pixel cell in embodiment one;

Fig. 6 is the cross section structure schematic diagram of the pixel cell in embodiment two.

Embodiment

For enabling above-mentioned purpose, the feature and advantage of the present invention more become apparent, below in conjunction with accompanying drawing, the specific embodiment of the present invention is described in detail.

The present inventor is in the dark current process of research pixel cell, finding that this dark current Producing reason is not only that the defect of the existing photosensitive region it is generally acknowledged produces, prior reason is relevant with the silicon nitride layer formed on this photosensitive region surface to realize other pollutents such as isolated steam. Owing to nitrogen-atoms is stronger to the out-shell electron adsorptivity of other atom in this silicon nitride layer, Siliciumatom or other doped element of photosensitive region is easily caused to lose electronics, this loses the silicon of electronics or other doped element easily forms unsettled unsaturated link(age), i.e. defect, this defect trapped electron after light is converted into electronics, affect the number of electrons being detected, cause photoelectric transformation efficiency low. The problems referred to above unglazed shine photosensitive region time, the problem that dark current can be caused bigger.

For the problems referred to above, the present invention proposes to arrange silicon oxide layer between the silicon nitride layer on photosensitive region and its, Sauerstoffatom in this silicon oxide layer is more weak to the out-shell electron adsorptivity of other atom, thus, Siliciumatom or other doped element of photosensitive region can not be caused to lose electronics, thus avoid defect and produce.

The forming method of silicon oxide layer has two kinds, and a kind of is vapour deposition, adopts the extraneous method introducing this layer, and another kind is selective oxidation method (LOCOS), and the latter adopts the silicon in existing substrate to form silicon oxide layer.

Below for the first method, introduce embodiments of the invention one.

Embodiment one

The structure that pixel cell in CMOS image sensor is conventional has 3T transistor, 4T transistor. Below for 4T transistor, introduce the pixel cell adopting the technical scheme of the present invention to make.

With reference to the circuit diagram of the 4T transistor shown in Fig. 3, its pixel cell 2 comprises: optical sensor 21, for generation of the electric charge also collecting the light generation being radiated on photosensitive region; Transfering transistor 22, for transferring to sense node (being generally floating diffusion region 23) by photoelectricity electric charge from optical sensor 21. Floating diffusion region 23 is electrically connected to the grid that transistor 24 is followed in source. In addition, pixel cell 2 also comprises: reset transistor 25, for floating diffusion region 23 is reset to predetermined voltage V_DD; Row selecting transistor 26, for response address signal by signal V_outFollow transistor 24 from source and output to output terminal.

With reference to the cross section structure schematic diagram of the schema shown in Fig. 4 and the pixel cell shown in Fig. 5, first perform step S11, form photosensitive region 211 on the substrate 20.

In the present embodiment, optical sensor 21 can be photorectifier, such as PN knot, PIN junction etc. For forming photorectifier, this substrate 20 is P type or N-type silicon substrate, correspondingly, defines trap district on the substrate 20, and implanting p-type dopant ion district or N-type dopant ion district are to form photosensitive region 211. The formation process of above-mentioned photorectifier is common process, and concrete parameter, with reference to existing parameter, does not repeat them here.

In addition, in concrete making processes, this step forms multiple photosensitive region 211 in substrate 20, is isolated by shallow trench (STI) 27 between neighboring photosensitive region 211.

It should be noted that, in the substrate 20 closing on region of photosensitive region 211, namely pixel circuit region can form various transistor by other technique, transfering transistor 22 as shown in Figure 5 and reset transistor 25. Thus, this step also forms the source and drain district (sign) of each transistor in the pixel circuit region of substrate 20.

Then perform step S12, still with reference to shown in Fig. 5, described photosensitive region 211 forms silicon oxide layer 28.

In the present embodiment, forming silicon oxide layer 28 on described photosensitive region 211 is formed by selective oxidation method (LOCOS). This selective oxidation method is common process, and the concrete parameter of formation can with reference to existing technique.

As previously mentioned, it is utilize Sauerstoffatom wherein that the out-shell electron adsorptive power of the silicon in substrate 20 and other dopant ion is not strong that this step forms the object of silicon oxide layer 28, not easily causing substrate 20, the unsaturated link(age) that especially the surface appearance of photosensitive region 211 is unsettled, namely avoids the generation of defect. Meanwhile, based on the characteristic of this silicon oxide layer 28, it is good not as silicon nitride to the isolated effect of the pollutents such as steam. Thus, follow-up on it silicon nitride layer also can be formed. The present inventor finds, the thickness range of choice of silicon oxide layer 28 is: can better realize above-mentioned purpose when 50 microns-500 microns.

It should be noted that, the various transistors of pixel circuit region, transfering transistor 22 as shown in Figure 5 and the grid oxic horizon of reset transistor 25 can also be three layers of ONO structure of oxidenitride oxide. Thus, this step, when forming silicon oxide layer 28, defines the first layer oxide compound of the grid oxic horizon of the transistor in the substrate 20 closing on region of photosensitive region 211.

Then perform step S13, described silicon oxide layer 28 forms silicon nitride layer 29.

The technique forming silicon nitride layer 29 in this step can be the common process such as chemical vapour deposition, physical vapor deposition. The present inventor finds, the thickness range of choice of silicon nitride layer 29 is: the object that can better realize the pollutents such as isolated steam when 100 microns-500 microns.

In addition, this step, when forming silicon nitride layer 29, defines the second layer nitride of the grid oxic horizon of the transistor in the substrate 20 closing on region of photosensitive region 211.

Afterwards, prospective region on the surface of silicon nitride layer 29 forms third layer oxide compound (sign), the grid (sign) of other transistor of image element circuit from bottom to top successively. Other transistor etc. of pixel cell 2 completes by existing making method.

So far, the pixel cell 2 of the present embodiment one has made complete.

To sum up, this pixel cell 2 comprises:

There is the substrate 20 of photosensitive region 211;

The silicon oxide layer 28 being positioned on described photosensitive region 211;

The silicon nitride layer 29 being positioned on described silicon oxide layer 28.

Based on above-mentioned pixel cell 2, the present embodiment one additionally provides the cmos image sensor comprising above-mentioned pixel cell 2.

Embodiment two

The present embodiment two provide pixel cell and making method and embodiment one roughly the same. Difference is: as shown in Figure 6, and this substrate 20 ' comprises the semiconducter substrate (silicon substrate) 20 of the injection N in embodiment one or P type element, also comprises the epitaxial film 30 of its Epitaxial growth. The source and drain district of the transistor of the pixel circuit region that photosensitive region 211, photosensitive region 211 close on all adopts selective epitaxial growth method to be formed in this epitaxial film 30. In addition, the source and drain district of the transistor of the pixel circuit region that photosensitive region 211, photosensitive region 211 close on is limit growth, while carry out the process of ion doping, i.e. and in-situ doped technique. Above-mentioned in-situ doped technique can reduce the defect in photosensitive region 211.

Embodiment three

The present embodiment three provide pixel cell and making method and embodiment one roughly the same. Difference is: the silicon oxide layer 28 on photosensitive region 211 is not formed by LOCOS method, but is formed by chemical Vapor deposition process. This chemical Vapor deposition process such as adopts silane and oxygen reaction to generate.

It is understandable that, the benefit of the method is: the thickness that need not take the photosensitive region 211 formed, like this, the source and drain district of the transistor of the pixel circuit region that photosensitive region 211, photosensitive region 211 close on, the sti structure 27 of isolated each photosensitive region 211 still make according to existing specifications parameter, thus, present method and existing processing compatibility are good.

Although the present invention is with better embodiment openly as above; but it is not for limiting the present invention; any those skilled in the art are without departing from the spirit and scope of the present invention; can utilize the Method and Technology content of above-mentioned announcement that technical solution of the present invention is made possible variation and amendment; therefore; every content not departing from technical solution of the present invention; any simple modification, equivalent variations and the modification above embodiment done according to the technical spirit of the present invention, all belongs to the protection domain of technical solution of the present invention.

Claims

1. a pixel cell, it is characterised in that, comprising:

There is the substrate of photosensitive region;

The silicon oxide layer being positioned on described photosensitive region;

The silicon nitride layer being positioned on described silicon oxide layer;

Wherein, the thickness range of described silicon oxide layer is: 50 microns-500 microns, for avoiding the bonding force of silicon nitride layer to cause photosensitive region surface defect occur.

2. pixel cell according to claim 1, it is characterised in that, the thickness range of described silicon nitride layer is: 100 microns-500 microns.

3. pixel cell according to claim 1, it is characterised in that, described substrate comprises semiconducter substrate, is positioned at the epitaxial film of described semiconductor substrate surface.

4. pixel cell according to claim 1, it is characterised in that, described substrate has multiple photosensitive region, between neighboring photosensitive region by shallow trench isolation from.

5. pixel cell according to claim 1, it is characterised in that, the substrate closing on described photosensitive region has transistor.

6. pixel cell according to claim 5, it is characterised in that, described transistor is transfering transistor.

7. pixel cell according to claim 1, it is characterised in that, the dopant ion type of described photosensitive region is N-type or P type.

8. a cmos image sensor, it is characterised in that, comprise the pixel cell described in claim 1 to 7 any one.

9. the making method of a pixel cell, it is characterised in that, comprising:

Substrate is formed photosensitive region;

Described photosensitive region forms silicon oxide layer;

Described silicon oxide layer forms silicon nitride layer;

10. making method according to claim 9, it is characterised in that, substrate is formed photosensitive region and injects formation by dopant ion.

11. making methods according to claim 9, it is characterised in that, the technique forming photosensitive region in substrate is selective epitaxial growth process and in-situ doped.

12. making methods according to claim 9, it is characterised in that, forming silicon oxide layer on described photosensitive region is formed by chemical vapour deposition or the formation of selective oxidation method.

13. making methods according to claim 9, it is characterised in that, also form the part of the grid oxic horizon of the transistor closing on region when described photosensitive region forms silicon oxide layer.