RU2377692C1 - Cmos photodetector element with high fill factor - Google Patents

Abstract

FIELD: physics; conductors.

SUBSTANCE: invention can be used for making photosensitive digital and analogue devices. According to the invention, the CMOS photodetector element comprises a photodiode and transistors of the photodiode control circuit. According to the invention, the photodiode is formed by two layers: an embedded low-doped n-type layer (grown on a blank substrate of p-type conductivity, a p-type pocket forms on the surface of the given n-epitaxial layer), which serves as the body of the photodiode, and a surface n⁺-layer which is aligned with the embedded photodiode layer at a certain depth. The substrate and the p-type pocket are connected to an earth bus and the n-region of the photodiode is connected to the positive supply potential bus. The n⁺-region of the photodiode is the source of the control transistor of the photodetector element circuit. With the respective selection of doping levels of diffused layers and control voltages, the n-layer of the photodiode in accumulation mode appears completely depleted, which enables transfer of photogenerated charge carriers into the n⁺-layer of the photodiode through directed electric fields surrounding a considerable part of the body of the photodetector element.

EFFECT: increased sensitivity of the CMOS photodetector element and reduced noise.

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Description

The invention relates to the field of semiconductor ICs and can be used to create photosensitive devices.

CMOS photodetectors are known in which each photodetector contains a photodiode and MOS control transistors (see HSWong, "Technology and Device Scaling Considerations for CMOS Imagers", IEEE Trans. On Electron Devices, vol. 43, No. 12, pp. 2131-2142, 1996; A. Theuwissen, E. Roks, "Building a Better Mousetrap", Photonics spectra, No. 2, pp. 29-32, 2001).

The efficiency of receiving input radiation by a CMOS photodetector element is estimated using a parameter called the Fill Factor (FF). The filling factor is the ratio of the area of the photodiode to the total area of the photodetector element (PFE). With a decrease in design standards, FF can be increased by reducing the fraction of the area of the photodetector occupied by the control transistors. Other things being equal, a photodetector having a higher FF has a higher photosensitivity.

At the same time, the photosensitivity of the photodetector directly depends on the efficiency of the collection of charge carriers in the photodiode from the volume of the semiconductor in which they are generated by the input optical radiation. It is obvious that the photosensitivity of the PES will increase with an increase in the fraction of the volume of the element from which the charge carriers generated by the light enter the photodiode. When calculating the magnitude of the filling factor FF, these volumes are not taken into account.

CMOS photodetector elements are known that have a multilayer structure for collecting photogenerated charge carriers from the largest possible volume of a semiconductor substrate, which increases their photosensitivity compared to elements collecting carriers only from the body volumes of the photodiode and its depletion region [1, 2, 3]. It is believed that the photodiode area of such an element increases by the projection area of the volume from which the carriers are collected onto the surface of the photocell. With this approach, the element filling factor can reach 100% [1, 2].

The main prototype is a CMOS photodetector element, in which a structure with an internal potential barrier between strongly and lightly doped p-substrate layers is used to collect charge carriers from the depth of the substrate and transfer them to the region of the photodiode [1, 2]. However, the disadvantages of this photodetector element, leading to the loss of part of the photogenerated charge carriers, are:

- the use of a diffusion carrier transfer mechanism that does not provide a strict directivity of their flow in the region of the photodiode;

- a small thickness of the collection area of the media, which does not provide coverage of most of the volume of the element;

- location of the collection area at a depth from the surface of the crystal.

A CMOS photodetector element comprising a photodiode and control transistors is proposed, characterized in that

- on the substrate of the first type of conductivity, the first layer of the second type of conductivity is grown, which is the buried region of the photodiode;

- in the first layer of the second type of conductivity, a second layer of the first type of conductivity is formed over the entire area, which is a pocket in which the surface region of the photodiode and the region of the transistors of the photodetector are then formed;

- in the second layer of the first type of conductivity, a second layer of the second type of conductivity is formed (the surface region of the photodiode) so that it closes with the first layer of the second type of conductivity and occupies only part of the area of the second layer of the first conductivity type;

- in the second layer of the first conductivity type, outside the second layer of the second conductivity type, the third and subsequent regions of the second conductivity type are formed, which are regions of the element transistors;

- the first layer of the first type of conductivity at the periphery has an ohmic contact connected to the ground bus;

- the first layer of the second type of conductivity at the periphery has an ohmic contact connected to the "power" bus;

- a system of dielectric layers and gates is formed on the surface of the element, which implements the electrical circuit of the photodetector.

In this element, the second region of the second conductivity type plays the role of the “accumulation region” of photogenerated charges, the first region of the second conductivity type plays the role of the “collection region” of photogenerated charge carriers, from which the carriers are directed to the “accumulation region”. Moreover, with an appropriate selection of the doping levels of the diffusion regions of the element and the values of the control voltages, the transfer of photogenerated charge carriers from the “collection region” to the “accumulation region” will be carried out using directed electric fields.

The technical result of this invention is to increase the photosensitivity of the photodetector element and improve its noise characteristics.

The invention is illustrated by the drawings.

Figure 1 presents a schematic section of a CMOS photodetector element according to the present invention. The area of the photodetector element is limited by a rectangle indicated by a dashed line. In the second, smaller, indicated by the dotted line region C1, transistors of the photodetector element V ₁ , V ₂ , V _{3 are} shown, their number may be different for different types of circuits.

The photodetector contains:

- a substrate of the first type of conductivity 1;

- the first layer of the second type of conductivity 2 grown using epitaxy operations on the substrate 1, this layer is a buried region of the photodiode;

- the second layer of the first type of conductivity 3, which is a pocket for the formation of the transistor regions of the photodetector circuit;

- the second layer of the second type of conductivity 4 and 4.1, which merges with the first layer of the second type of conductivity;

- the third region of the second type of conductivity 5 and the subsequent regions of the second type of conductivity 5.1, 5.2, etc., which are the areas of the transistors of the element;

- gates 6, 6.1, 6.2 of transistors of the photodetector circuit;

- a system of dielectric layers 7 on the surface of the photodetector element.

Ohmic contacts to the regions of the photodetector element are located on the periphery of the crystal (the diagram shows their connection diagram): 8 to the first layer of the first type of conductivity and 9 to the second layer of the first type of conductivity (3), which are connected to the ground bus; 10 to the first layer of the second type of conductivity, which is connected to the bus "power".

Figure 2 shows a schematic section of a CMOS photodetector element according to the present invention during its manufacturing after the operation of forming a buried region of the photodiode 2 by epitaxial growth of an n-type film on the substrate 1.

Figure 3 shows a schematic section of a CMOS photodetector element according to the present invention in the manufacturing process after the operation of forming the region of the pocket 3 by implantation of p-type impurities.

Figure 4 shows a schematic section of a CMOS photodetector element according to the present invention during its manufacturing after the operation of forming the surface n ^{+ region of} photodiode 4 by implantation of an n-type impurity, while the doping energy level and subsequent thermal operations should ensure the closure of this region with buried area of the photodiode.

Figure 5 shows a schematic section of a CMOS photodetector element according to the present invention during its manufacturing after operations of forming source-drain regions of transistors of an element circuit by implanting an n-type impurity impurity.

In the mode of accumulation of photogenerated charge carriers, the photodetector operates as follows.

By applying the corresponding control voltages to the ohmic contacts of the buried n-region of the photodiode, the n-region of the photodiode, the substrate, the p-pocket, the areas of transistors and the gates of the photodetector circuit, the mode of complete depletion of the buried n-region of the photodiode is established. In this case, the electric fields in the pn junctions formed by the recessed n-region of the photodiode and the p-substrate and the recessed n-region of the photodiode and the p-pocket are directed so that the electrons photogenerated in the regions covered by the electric fields of the data pn- transitions will drift to the n ⁺ -region of the photodiode. For this, it is necessary that the potential of the n ^{+ region of the} photodiode be higher than the depletion potential of the buried n-region of the photodiode.

A variant of the distribution of electric potential in the volume of the photodetector that meets the specified conditions is shown in Fig.6. The distribution of the electric potential of Fig. 6 shows that the electric fields in the volume of the recessed region of the photodiode have directions that ensure the movement of electrons to the surface region of the photodiode (some lines of electron motion are shown by arrows).

Electrical isolation of adjacent photodetector elements is ensured by volume electric fields directed to the n ⁺ regions of the photodiodes of each element, which prevents carriers photogenerated in the region of the volume electric field of the photodiode of one element from moving to the region of the photodiode of the other element. This improves the noise characteristics of the photodetector elements.

Thus, in the proposed photodetector element, an increase in the filling factor is achieved by creating a low-doped n-epitaxial layer (2), which serves as a buried body of the photodiode, which, with appropriate selection of the doping levels of the diffusion regions of the element and the values of the control voltages, provides the occurrence of volume electric fields covering a significant part of the volume photodetector element and directed to the surface n ⁺ -region of the photodiode (4), which increases the photosensitivity of the element. In addition, these electric fields provide mutual electrical isolation of the photodetector elements.

The photodetector element according to the present invention may have the following impurity profile parameter values: the depth of the n-epitaxial layer (2) is 3.0 μm; the concentration of donors in the n-epitaxial layer (2) - 8.0 · 10 ¹³ cm ^-3 ; the depth of the p-pocket (3) is 0.78 microns; the average concentration of acceptors in the p-pocket (3) is 9.0 · 10 ¹⁶ cm ^-3 ; the average concentration of donors in the surface n ^{+ region of the} photodiode (4) is 4.9 · 10 ¹⁷ cm ^-3 . These values do not exhaust all the options for manufacturing a photodetector.

The photodetector element according to the present invention can be used to create array photodetector microcircuits for various purposes.

Literature

1. US patent No. 6.225.670.

2. B. Dierickx, G. Meynants, D.Scheffer, "Near 100% fill factor CMOS active pixels", IEEE CCD & AIS workshop, Brugge, Belgium, 5-7 June (1997); Proceedings p.PI.

3. B. Dierickx, J. Bogaerts, "NIR-enhanced image sensor using multiple epitaxial layers", Electronic Imaging, San Jose, 21 Jan 2004; SPIE Proceedings vol. 5301, p.204 (2004).

Claims (1)

CMOS photodetector containing a photodiode and transistors of a photodiode control circuit, characterized in that a first layer of a second type of conductivity is grown on a substrate of the first type of conductivity, which is a buried region of the photodiode, in the first layer of the second type of conductivity a second layer of the first conductivity type is formed over the entire area , which is a pocket in which the surface region of the photodiode and the transistor region of the photodetector element are then formed, in the second layer of the first type of conductivity the second layer of the second conductivity type (surface region of the photodiode) is mapped so that it closes with the first layer of the second conductivity type and occupies only part of the area of the second layer of the first conductivity type; in the second layer of the first conductivity type, the third and subsequent regions are formed outside the second layer of the second conductivity type the second type of conductivity, which are the areas of the transistors of the circuit element, the first layer of the first type of conductivity at the periphery has an ohmic contact connected to the ground bus, the first the second type of conductivity at the periphery has an ohmic contact connected to the "power" bus, a system of dielectric layers and gates is formed on the surface of the element, which implements the electric circuit of the photodetector element, the second region of the second type of conductivity acts as the "accumulation region" of photogenerated charges, the first region of the second of the type of conductivity plays the role of the “collection region” of photogenerated charge carriers, directing them to the “accumulation region”, while the transfer of photogenerated charge carriers Yes, from the "collection area" to the "area" of accumulation is carried out using directed electric fields.

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