CN112447775A

CN112447775A - CMOS image sensor pixel manufacturing method for improving quantum efficiency

Info

Publication number: CN112447775A
Application number: CN201910801765.8A
Authority: CN
Inventors: 徐江涛; 王瑞硕; 夏梦真; 史兴萍; 李凤
Original assignee: Tianjin University Marine Technology Research Institute
Current assignee: Tianjin University Marine Technology Research Institute
Priority date: 2019-08-28
Filing date: 2019-08-28
Publication date: 2021-03-05

Abstract

A method for fabricating a CMOS image sensor pixel with improved quantum efficiency, using high-concentration P-type ion doping implanted on the back of the pixel, and a method of depleting thicker sensitive semiconductor regions by applying a reverse bias on the substrate, thereby eliminating the field region and rapidly collect photogenerated charges. The magnitude of the reverse bias depends on the resistivity and thickness of the semiconductor substrate, and can be far in excess of any other voltage in the system. Ultimately, the quantum efficiency of the image sensor is further improved, and the imaging quality is greatly improved.

Description

CMOS image sensor pixel manufacturing method for improving quantum efficiency

Technical Field

The invention belongs to the field of CMOS image sensors, and particularly relates to a CMOS image sensor pixel manufacturing method for improving quantum efficiency, wherein incident light is in a near-infrared or soft X-ray waveband.

Background

A clamped photodiode (PPD) was originally used in a CCD image sensor, and was used in a CMOS image sensor over twenty years later. Fig. 1 is a PPD-based 4T pixel structure. The 4T pixel is composed of a PPD, a transmission tube, a reset tube, a row gate tube and a floating diffusion node. When light is incident on the semiconductor surface, a portion of the incident light is reflected and the remainder is absorbed by the semiconductor. When the photon energy entering the semiconductor is not lower than the forbidden bandwidth of the semiconductor material, the semiconductor material absorbs the energy with a certain probability, so that electron-hole pairs, namely photon-generated carriers, are generated. After illumination integration is completed, the transmission tube is conducted, photo-generated charges are transferred to the floating diffusion node from the photodiode region under the action of an electric field, namely, the charge-voltage information conversion process is completed, and finally, optical signals stored in the floating diffusion node are read out line by line through the line gate tube and the column-level reading circuit.

For incident light in the near infrared and X-ray bands of longer wavelengths, the absorption length can reach tens or even hundreds of microns. The PPD type 4T pixel using the conventional CMOS image sensor cannot reach such a deep depletion region depth, thereby causing a low quantum efficiency and reducing an imaging performance of the image sensor. In the pixel epitaxial layer, the parts except the depletion layer are neutral regions, and if incident light is absorbed in the neutral regions and photo-generated electron-hole pairs are generated, signal charges cannot be transferred to the floating diffusion nodes and finally cannot be successfully read out.

Disclosure of Invention

Aiming at the problems in the prior art, the invention discloses a CMOS image sensor pixel manufacturing method capable of improving quantum efficiency. The magnitude of the reverse bias depends on the resistivity and thickness of the semiconductor substrate and can far exceed any other voltage in the system. Finally, the quantum efficiency of the image sensor is further improved, and the imaging quality is greatly improved.

A CMOS image sensor pixel manufacturing method for improving quantum efficiency is disclosed, as shown in FIG. 2, firstly, a high-concentration P-type ion implantation is performed at the bottom of a substrate before a gate is formed, a P + + region is formed, namely, a depletion region 2 is added in a neutral region below an original PPD region. After the transmission gate is formed, N-type ions are implanted by adopting a self-alignment technology to form an N-photosensitive area and an FD area of PPD. Compared with the traditional 4T active pixel, the pixel structure provided by the patent needs to perform deep P-type ion implantation once before the gate is formed, and the substrate applies negative pressure, so that the two formed depletion regions are connected. When light with longer wavelength irradiates a PPD region, electrons in a valence band in a semiconductor material absorb the energy of photons, and the energy passes through a forbidden band from the valence band to reach a conduction band, so that a photogenerated electron-hole pair, namely a photogenerated carrier, is formed, and the photogenerated electrons are collected in the PPD region and the photogenerated holes are absorbed by a substrate under the action of an electric field in a depletion region; the photo-generated charge generated by photons absorbed in the neutral region at the bottom of the depletion region cannot be collected by the PPD region.

Compared with the traditional 4T active pixel, the pixel manufacturing method provided by the invention has the advantages that the heavily doped P-type ion implantation is added at the bottom of the substrate, the negative voltage is applied, so that depletion regions formed by the PPD and the substrate as well as the substrate and the heavily doped P-type region are connected, the depth of the depletion region is increased equivalently finally, when light with longer wavelength irradiates the PPD region, photo-generated charges are absorbed by the formed deep depletion region and are transferred to the floating diffusion node end at the conduction stage of the transmission tube and are read out finally, the collection rate and the quantum efficiency of the photo-generated charges are effectively improved, and the imaging quality of the sensor is improved.

Drawings

FIG. 1 is a basic structure diagram of a 4T pixel;

fig. 2 shows a pixel structure with the addition of P + + ion implantation at the bottom of the substrate.

Detailed Description

The present invention will be described in detail below with reference to the drawings and examples.

According to the invention, a P + + region is formed by carrying out high-concentration P-type ion implantation at the bottom of a substrate once, a depletion region 2 is added in a neutral region below an original PPD region, the doping concentrations of an epitaxial layer and the P + + region and negative voltage applied by the substrate are well controlled, so that the depletion region 1 and the depletion region 2 can be connected together to form a complete depletion region, photogenerated charges generated by near infrared rays or soft X rays at a deep position are collected in the depletion region and are transmitted to a floating diffusion node through a transmission tube, and finally read out. The structure can effectively improve the quantum efficiency and further improve the imaging quality of the image sensor.

The CMOS image sensor pixel structure for improving the quantum efficiency is suitable for pixels of which incident light is near infrared rays or soft X rays in a longer wave band, and the PPD depletion region 1 is connected with the depletion region 2 formed by the substrate and the epitaxial layer. For example, for a pixel with near infrared light as incident light, a P-type epitaxial layer is formed by doping B ions with 2e13/cm2, a heavily doped P + + region is formed at the bottom of the substrate by implanting B ions with a concentration of 1e15/cm2, and a negative voltage of-4.0V is applied to the substrate. The depth of a depletion region 1 formed by PPD and a P-type epitaxial layer can reach 3 um; the P + + layer forms a depletion region with the epitaxial layer having a depth in the range of about 10um to about 15 um. For the pixel with the epitaxial thickness of 12um, the depletion region 1 and the depletion region 2 formed according to the process conditions are overlapped in space, so that the photosensitive area is effectively increased, and further the quantum efficiency and the imaging quality of the image sensor are improved.

By adopting the pixel structure, structural optimization based on a PPD structure can be realized, and the pixel unit design of low neutral region diffused dark current can be realized on the basis of ensuring a large photosensitive area.

Claims

1. A CMOS image sensor pixel manufacturing method for improving quantum efficiency is characterized by comprising the following steps: firstly, carrying out high-concentration P-type ion implantation at the bottom of a substrate before forming a gate to form a P + + region, namely adding a depletion region in a neutral region below an original PPD region, and applying negative pressure to the substrate to connect the two depletion regions; after the transmission gate is formed, injecting N-type ions by adopting a self-alignment technology to form an N-photosensitive area and an FD area of PPD; when light with longer wavelength irradiates a PPD region, electrons in a valence band in a semiconductor material absorb the energy of photons, and the energy passes through a forbidden band from the valence band to reach a conduction band, so that a photogenerated electron-hole pair, namely a photogenerated carrier, is formed, and the photogenerated electrons are collected in the PPD region and the photogenerated holes are absorbed by a substrate under the action of an electric field in a depletion region; the photo-generated charge generated by photons absorbed in the neutral region at the bottom of the depletion region cannot be collected by the PPD region.