CN102800686B - Back-illuminated type CMOS - Google Patents

Abstract

The invention provides a back-illuminated CMOS image sensor, comprising: a device wafer, the device wafer has a front side and a back side, a photodiode is formed in the device wafer, and the photodiode is close to the device wafer and a positively charged film layer covering the back side of the device wafer. The positively charged film layer increases the energy band barrier close to the back of the device wafer, which increases the difficulty for electrons to escape from the photodiode, thereby reducing the probability of electrons escaping from the photodiode, thereby improving the photon conversion efficiency and improving The imaging quality of the back-illuminated CMOS image sensor is improved.

Description

Back-illuminated CMOS image sensor

technical field

The invention relates to the technical field of image sensors, in particular to a back-illuminated CMOS image sensor.

Background technique

A digital camera is an electronic product widely used today, and an image sensor is included in the digital camera, which is used to convert light into electric charge. Image sensors can be divided into charge-coupled device (Charge-Coupled Device) image sensors (commonly known as CCD image sensors) and CMOS (Complementary MetalOxide Semiconductor) image sensors according to the principles they use, in which CMOS image sensors are based on complementary metal oxide Manufactured using CMOS technology. Since the CMOS image sensor is manufactured using a traditional CMOS circuit process, the image sensor and its required peripheral circuits can be integrated.

Traditional CMOS image sensors use Front Side Illumination (FSI) technology to manufacture pixels on the pixel array, and the incident light needs to pass through the front side of the pixel to reach the photo-sensing area. That is to say, the structure of the traditional front-illuminated CMOS image sensor makes the incident light need to pass through the dielectric layer (dielectric layer) and the metal layer (metal layer) before reaching the photo-sensing area, which leads to the traditional CMOS image sensor It needs to face problems such as low photon conversion efficiency (quantum efficiency), serious cross talk between pixels, and dark current (dark current).

For this reason, another CMOS image sensor is proposed in the prior art, which is a Back Side Illumination (BSI) CMOS image sensor, also called a back side illuminated CMOS image sensor. Different from the front lighting technology, the back-illuminated CMOS image sensor is constructed by the front end of the silicon crystal (silicon) image sensor, which places the color filter (color filter) and microlens (microlens) on the back side of the pixel (back side), The incident light enters the image sensor from the back of the image sensor. Compared with front-illuminated CMOS image sensors, this back-illuminated CMOS image sensor has less light loss and higher photon conversion efficiency.

Among them, the photon conversion efficiency is an important parameter of the CMOS image sensor, generally, the higher the photon conversion efficiency is, the higher the imaging quality of the CMOS image sensor is. Although the photon conversion efficiency of the existing back-illuminated CMOS image sensors is better than that of the front-illuminated CMOS image sensors, it is still desired to improve the photon conversion efficiency of the back-illuminated CMOS image sensors in order to obtain higher imaging quality.

Contents of the invention

The object of the present invention is to provide a back-illuminated CMOS image sensor to improve the photon conversion efficiency of the back-illuminated CMOS image sensor.

In order to solve the above technical problems, the present invention provides a back-illuminated CMOS image sensor, comprising:

a device wafer, the device wafer has a front side and a back side, a photodiode is formed in the device wafer, and the photodiode is close to the back side of the device wafer; and

A positively charged film layer, the positively charged film layer covers the back surface of the device wafer.

Optionally, in the back-illuminated CMOS image sensor, the material of the positively charged film layer is oxide.

Optionally, in the back-illuminated CMOS image sensor, the material of the positively charged film layer is one or more of borosilicate glass, indium glass and titanium glass.

Optionally, in the back-illuminated CMOS image sensor, the positively charged film layer has a thickness of 20 angstroms to 300 angstroms.

Optionally, in the back-illuminated CMOS image sensor, the positively charged film layer is formed by a chemical vapor deposition process.

Optionally, in the back-illuminated CMOS image sensor, a metal shielding layer is further included, and the metal shielding layer covers the positively charged film layer.

Optionally, in the back-illuminated CMOS image sensor, the material of the metal shielding layer is aluminum or tungsten.

In the back-illuminated CMOS image sensor provided by the present invention, the energy band potential barrier close to the back of the device wafer is improved through the positively charged film layer, which increases the difficulty for electrons to escape from the photodiode, thereby reducing the electron flow rate from the photodiode. The probability of escaping in the medium increases, thereby improving the photon conversion efficiency and improving the imaging quality of the back-illuminated CMOS image sensor.

Description of drawings

FIG. 1 is a schematic cross-sectional view of an existing back-illuminated CMOS image sensor;

2 is a schematic cross-sectional view of a back-illuminated CMOS image sensor according to an embodiment of the present invention;

FIG. 3 is a comparison diagram of energy band potential wells of photodiodes in the back-illuminated CMOS image sensor of this embodiment and the conventional back-illuminated CMOS image sensor.

detailed description

The back-illuminated CMOS image sensor provided by the present invention will be further described in detail below with reference to the drawings and specific embodiments. Advantages and features of the present invention will be apparent from the following description and claims. It should be noted that all the drawings are in a very simplified form, and are only used for the purpose of conveniently and clearly assisting in describing the embodiments of the present invention.

As mentioned in the background technology, the technology of back-illuminated CMOS image sensor is developed on the technology of front-illuminated CMOS image sensor. Some improvements have been made to low problems. As shown in FIG. 1 , it is a schematic cross-sectional view of a conventional back-illuminated CMOS image sensor. The existing back-illuminated CMOS image sensor 1 includes:

A device wafer 11, the device wafer 11 has a front side 111 and a back side 112, a photodiode 12 is formed in the device wafer 11, and the photodiode 12 is close to the back side 112 of the device wafer 11; and

A high-K dielectric layer (ie, a high-k dielectric layer) 13 , the high-K dielectric layer 13 covers the back surface 112 of the device wafer 11 .

Although the photon conversion efficiency of the existing back-illuminated CMOS image sensor 1 is improved compared with the front-illuminated CMOS image sensor, based on the continuous pursuit of the image quality of the CMOS image sensor, people hope to further improve the back-illuminated CMOS image sensor. photon conversion efficiency, but this point has always had little effect.

For this problem, the inventor has carried out continuous in-depth research and finally found that:

In the existing back-illuminated CMOS image sensor 1, when the light 14 is incident on the photodiode 12 and a large number of electrons 15 are generated in the photodiode 12 to realize the photon conversion process of the CMOS image sensor, there will be a certain amount of The electrons 15 escape from the constraints of the photodiode 12 , and it is precisely because of the escape of these electrons 15 that the photon conversion efficiency of the CMOS image sensor is low.

After discovering the problem that hinders the photon conversion efficiency of the CMOS image sensor, the inventors made further research, and based on this, proposed a solution to improve the photon conversion efficiency of the back-illuminated CMOS image sensor.

Specifically, please refer to FIG. 2 , which is a schematic cross-sectional view of a back-illuminated CMOS image sensor according to an embodiment of the present invention. As shown in Figure 2, the back-illuminated CMOS image sensor 2 includes:

A device wafer 21, the device wafer 21 has a front side 211 and a back side 212, a photodiode 22 is formed in the device wafer 21, and the photodiode 22 is close to the back side 212 of the device wafer 21; and

A positively charged film layer 23 , the positively charged film layer 23 covers the back surface 212 of the device wafer 21 .

In this embodiment, the material of the positively charged film layer 23 is oxide, specifically, it may be one or more of borosilicate glass, indium glass and titanic glass. Preferably, the thickness of the positively charged film layer 23 is 20 angstroms to 300 angstroms, for example, the thickness of the positively charged film layer 23 can be 40 angstroms, 60 angstroms, 80 angstroms, 100 angstroms, 120 angstroms, 150 Angstroms, 170 Angstroms, 200 Angstroms, 230 Angstroms, 250 Angstroms, or 270 Angstroms.

In this embodiment, the energy band barrier close to the back 212 of the device wafer 21 can be improved through the above-mentioned positively charged film layer 23; When the back side 212 of the wafer 21 is incident on the photodiode 22, the electrons 25 formed in the photodiode 22 are difficult to escape from the photodiode 22, thereby reducing the probability of electrons 25 escaping from the photodiode 22, thereby improving the The photon conversion efficiency of the back-illuminated CMOS image sensor is improved, and the imaging quality of the back-illuminated CMOS image sensor is improved.

Please refer to FIG. 3 , which is a comparison diagram of energy band potential wells of photodiodes in the back-illuminated CMOS image sensor of this embodiment and the conventional back-illuminated CMOS image sensor. As shown in Figure 3, the abscissa represents the depth in the photodiode, the unit is micron, the greater the depth is near the back of the device wafer; the ordinate represents potential energy, the unit is volts; the light is incident on the photodiode from the back of the device wafer Inside, as shown in Figure 3, is incident from the right side of the coordinate system.

It can be seen from FIG. 3 that in the part of the photodiodes far away from the backside of the device wafer, the energy bands of the photodiodes in the back-illuminated CMOS image sensor of this embodiment are basically the same as those in the existing back-illuminated CMOS image sensors, and the potential energy is uniform. Therefore, the electrons in this part of the photodiode are not easy to escape from the shackles of the photodiode; and in the part of the photodiode near the device wafer backside, the potential energy of the part of the photodiode of the back-illuminated CMOS image sensor provided by this embodiment (marked by L1 in the dotted line box in FIG. 3 ) which is significantly higher than the potential energy of some photodiodes of the prior art back-illuminated CMOS image sensor, marked by L2 in the dotted line box in FIG. 3 ).

Since the back-illuminated CMOS image sensor provided in this embodiment greatly improves the energy band barrier near the back of the device wafer, when light is incident on the photodiode from the back of the device wafer, compared with the back of the prior art, In the illuminated CMOS image sensor, the electrons formed in the photodiode of the back-illuminated CMOS image sensor in this embodiment are difficult to escape from the photodiode, thereby reducing the probability of electrons escaping from the photodiode, thereby improving the efficiency of the back-illuminated CMOS image sensor. The photon conversion efficiency of the illuminated CMOS image sensor is improved, and the imaging quality of the back-illuminated CMOS image sensor is improved.

Further, the positively charged film layer 23 can be formed by a chemical vapor deposition process. Specifically, the device wafer 21 can be placed in a chemical vapor deposition chamber, preferably, the temperature of the chemical vapor deposition chamber is 300°C to 800°C, and then the reaction gas (according to the selected material is different) , correspondingly injecting different reaction gases, which is a prior art, and will not be described in detail in this application), and the positively charged film layer 23 is formed by a chemical vapor deposition process.

In this embodiment, the back-illuminated CMOS image sensor 2 further includes a metal shielding layer (not shown in FIG. 2 ), the metal shielding layer covers the positively charged film layer 23, preferably, the metal The shielding layer is made of aluminum or tungsten. The problem of color crosstalk of the back-illuminated CMOS image sensor can be avoided through the metal shielding layer, and the quality of the back-illuminated CMOS image sensor can be further improved.

The above description is only a description of the preferred embodiments of the present invention, and does not limit the scope of the present invention. Any changes and modifications made by those of ordinary skill in the field of the present invention based on the above disclosures shall fall within the protection scope of the claims.

Claims (7)

1. a back-illuminated type CMOS, it is characterised in that including:

Device wafers, described device wafers has front and the back side, is formed with photodiode, described light in described device wafers Electric diode is near the back side of described device wafers；And

Positively charged film layer, described positively charged film layer covers the back side of described device wafers.

2. back-illuminated type CMOS as claimed in claim 1, it is characterised in that the material of described positively charged film layer For oxide.

3. back-illuminated type CMOS as claimed in claim 2, it is characterised in that the material of described positively charged film layer For one or more in Pyrex, indium glass and titanizing glass.

4. the back-illuminated type CMOS as described in any one in claims 1 to 3, it is characterised in that described band is just The thickness of electric charge film layer is 20 angstroms ~ 300 angstroms.

5. the back-illuminated type CMOS as described in any one in claims 1 to 3, it is characterised in that described band is just Electric charge film layer is formed by chemical vapor deposition method.

6. the back-illuminated type CMOS as described in any one in claims 1 to 3, it is characterised in that also include gold Belonging to shielding layer, described metal shielding layer covers described positively charged film layer.

7. back-illuminated type CMOS as claimed in claim 6, it is characterised in that the material of described metal shielding layer is Aluminum or tungsten.