KR20120001337A - A unit picture element including a photodiode including a light anti-absorption layer formed thereon, back-side illumination cmos image sensor including the unit picture element - Google Patents

Abstract

PURPOSE: A unit pixel and a backside illumination complementary metal oxide semiconductor image sensor including the same are provided to an improve trade off relation of sensitivity about white spot and light. CONSTITUTION: A photo diode(120) is formed on the upper side of a wiring layer(110). An optical absorption blocking layer(130) is formed on the upper side of the photo diode. An anti-reflection layer(140) is formed on the upper side of the optical absorption blocking layer. A color filter(150) is formed on the upper side of the anti-reflection layer. A micro lens(160) is formed on the upper side of the color filter. The anti-reflection layer is formed into a compound semiconductor in which energy band gap is bigger than a semiconductor which constitutes the photo diode. A visible ray passes through the compound semiconductor.

Description

A unit picture element including a photodiode including a light anti-absorption layer formed thereon, back-side illumination CMOS image sensor including a unit pixel including a light absorption prevention layer at a photodiode interface, and a backside illumination CMOS image sensor including the unit pixel including the unit picture element}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to unit pixels, and more particularly, to a unit pixel in which a portion which does not absorb light at an interface between an anti-reflection layer (ARL) and a photodiode is formed.

The CMOS image sensor includes a plurality of unit picture elements, and converts an image signal detected by each of the plurality of unit pixels into an electrical signal. Each unit pixel includes a photodiode for sensing an incident image signal, and a plurality of MOS transistors for converting the image signal detected from the photodiode into an electrical signal. Conventionally, an image signal, that is, light, is received from an upper portion of a chip on which a photodiode and a MOS transistor are formed. Since not only the photodiode but also the MOS transistors are formed in the unit pixel, the area of the photodiode receiving the light when viewed from the top of the chip is inevitably allocated to a part of the unit pixel.

The newly proposed back-side illumination CMOS image sensor is a method of receiving light from the bottom of the chip, i.e. from the substrate, rather than from the top of the chip. That is, after forming both the photodiode and the MOS transistors constituting the image sensor, the lower part of the chip (grinding) to a thickness enough to receive the light optimally, and the color filter and the lower part of the polished portion To form more microlenses.

The technical problem to be solved by the present invention is to provide a unit pixel in which a material that does not absorb light at the interface of the photodiode-reflective layer.

Another technical problem to be solved by the present invention is to provide a backside illumination CMOS image sensor having a pixel array composed of unit pixels in which a material that does not absorb light at the photodiode-reflective layer interface is formed.

According to an aspect of the present invention, a unit pixel includes a wiring layer, a photodiode formed on the wiring layer, a light absorption prevention layer formed on the photodiode, and an antireflection layer formed on the light absorption prevention layer (ARL). Anti-Reflection layer). The light absorption prevention layer is formed of a compound semiconductor having a larger energy band gap (Eg) than a semiconductor constituting the photodiode.

Preferably, the compound semiconductor may transmit visible light.

Preferably, the semiconductor may include silicon (Si).

Preferably, the compound semiconductor may be silicon carbide (SiC).

Preferably, the compound semiconductor may be formed on the photodiode by a deposition process.

Preferably, the light absorption layer may be doped with a P-type high impurity.

Preferably, the P-type high impurity may include boron (B).

Preferably, a color filter layer may be further formed on the anti-reflection layer.

Preferably, a micro lens may be further formed on the color filter layer.

Preferably, the wiring layer may include a plurality of metal layers and an insulating layer for insulating the plurality of metal layers.

According to another aspect of the present invention for achieving the above technical problem, a backside illumination CMOS image sensor includes a pixel array, a row decoder, and a column decoder. In the pixel array, a plurality of unit pixels are arranged in two dimensions. The row decoder controls the operation of the unit pixels arranged in the pixel array in units of horizontal lines. The column decoder controls the operation of the unit pixels arranged in the pixel array in a vertical line unit. Each of the unit pixels includes a wiring layer, a photodiode formed on the wiring layer, a light absorption prevention layer formed on the photodiode, and an antireflection layer formed on the light absorption prevention layer. The light absorption prevention layer is formed of a compound semiconductor having a larger energy band gap (Eg) than the semiconductor constituting the photodiode.

According to the present invention, a material that does not absorb light is formed at an anti-reflection layer (ARL) -silicon interface, thereby improving a trade-off relationship between white spots and sensitivity to light. There is an advantage.

1 illustrates a cross-sectional structure of a unit pixel including a light absorption prevention layer at an interface portion of a photodiode and an antireflection layer according to the present invention.
2 shows an example of a cross-sectional structure of a unit pixel not including a light absorption prevention layer at an interface portion of a photodiode and an antireflection layer.
3 is a circuit diagram of a unit pixel including a light absorption prevention layer at an interface portion of a photodiode and an antireflection layer according to the present invention.
4A to 4D show the relationship between the energy band gap, the silicon ratio, the visible light absorption rate, and the defect concentration according to the z-axis depth of the unit pixel of FIG. 3.
5 is a cross-sectional view of a unit pixel of a CMOS image sensor.
6 is a cross-sectional view of unit pixels generated using a CMOS process.
7A is a cross-sectional view of a unit pixel when a photodiode is formed.
7B is a cross-sectional view of the unit pixel when the light absorption prevention layer is formed on the photodiode.
7C is a cross-sectional view of the unit pixel when the light absorption prevention layer is doped with P-type high impurity.
7D is a cross-sectional view of the unit pixel when the antireflection layer and the color filter layer are formed on the light absorption prevention layer.
7E is a sectional view of a unit pixel when an island of a certain form is formed.
7F is a cross-sectional view of a unit pixel when a photon refracting microlens is formed by applying heat to an island formed on an upper portion of the photodiode.
8 shows the configuration of the CMOS image sensor.

In order to fully understand the present invention and the operational advantages of the present invention and the objects achieved by the practice of the present invention, reference should be made to the accompanying drawings, which are provided for explaining exemplary embodiments of the present invention, and the contents of the accompanying drawings.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Like reference numerals in the drawings denote like elements.

The core idea of the present invention is to form a material at the interface between the photodiode and the antireflective layer (ARL) that does not generate interface defects and does not absorb light by a method such as doping or compound semiconductor formation. By doing so, the absorption of light and the generation of electrons occur at a certain depth away from the backside surface, thereby being less susceptible to dark defects on the backside surface. As a result, the formation of a photodiode with a high electric field does not increase the white spot.

1 illustrates a cross-sectional structure of a unit pixel including a light absorption prevention layer between a photodiode and an antireflection layer ARL according to the present invention.

The cross-sectional structure 100 shown in FIG. 1 represents two of the plurality of unit pixels constituting the CMOS image sensor. The wiring layer 110 and the photodiode layer 120 are stacked in order from the bottom. In the photodiode layer 120, two photodiodes PD1 and PD2 are formed on a semiconductor substrate. Hereinafter, the photodiode and the photodiode layer are used interchangeably. Each unit pixel may be distinguished by a trench (not shown) filled with an insulator. The photodiode layer 120 may be formed of a semiconductor. The light absorption prevention layer 130 is stacked on the photodiode layer 120. The light absorption prevention layer 130 may be formed of a compound semiconductor. However, the compound semiconductor should have a larger energy band gap than the semiconductor constituting the photodiode (~ 1.3 eV or more when Eg (Si) = 1.1 eV). The antireflection layer (ARL) 140, the color filter 150, and the microlens 160 are sequentially stacked on the light absorption prevention layer 130.

The unit pixel 100 according to the present invention is used for a backside illumination CMOS image sensor. Referring to FIG. 1, light primarily reaches a photodiode. In the case of the general CMOS image sensor, the light incident on the image sensor is a method of finally entering the photodiode after passing through the wiring layer 110. The difference between these two methods will be described later.

In summary, in order to achieve the problem to be solved in the present invention, preferably, the material constituting the light absorption prevention layer 110 is formed of a silicon doping layer or a similar material without generating an interface defect. It must be a material that does not absorb light. To this end, the energy band gap Eg of the compound semiconductor constituting the light absorption prevention layer 110 should be higher than that of the semiconductor constituting the photodiode layer 120. For example, assuming that the semiconductor constituting the photodiode layer 120 is silicon (Si), the compound semiconductor constituting the light absorption prevention layer 130 may be any material having a larger energy gap than silicon. Preferably, the compound semiconductor constituting the light absorption prevention layer 130 may use silicon carbide (SiC).

Hereinafter, an optical principle for generating the effect of the present invention will be described. For convenience of explanation, one unit pixel will be described.

2 is a cross-sectional view 200 of a unit pixel without a light absorption prevention layer in the unit pixel 100 according to the present invention.

Referring to FIG. 2, in the back-side illumincation (BI) structure, defects that cause dark current and white spots on the silicon and anti-reflective layer (ARL) interfaces of the backside are caused by dark current and white spots. ) Is formed a lot. Defect means, for example, the likeage of a surface, ie electrons are produced on the surface.

The incident light has the most photoelectric conversion at the backside surface, and a photodiode with a strong electric field is used to reduce the crosstalk in which electrons from the surface move to neighboring pixels. ) Formation is required.

However, if a strong electric field (e-field) is formed in the photodiode, electrons generated in the defect are also attracted to the photodiode, so white spots and crosstalk are tradeoffs. In a relationship.

Therefore, there is a need for a structure that does not increase electrical crosstalk and also reduces the effects of defects.

3 illustrates a unit pixel having a light absorption prevention layer 130 at an interface where the photodiode layer 120 and the antireflective layer (ARL) 140 meet, according to an embodiment of the present invention. The stacked structure of the unit pixels 300 of FIG. 3 is the same as the stacked structure of the unit pixels 100 of FIG. 1. However, to clarify the difference from FIG. 2, the interface position of the silicon and the antireflection layer of FIG. 2 is indicated. Hereinafter, for convenience of description, the photodiode layer 120 is a silicon layer, and the light absorption prevention layer 130 is referred to as a silicon carbide (SiC) layer. However, as described above, the compound semiconductor constituting the light absorption prevention layer 130 may be any material having a larger energy gap than silicon. Referring to FIG. 3, when the compound semiconductor SixC (1-x) is formed to a thickness of several um or less at the interface where the silicon layer and the anti-reflection layer 140 meet, the band gap of SiC is wide to transmit visible light, thereby absorbing light. The light absorption prevention layer 130 may not be made. x is the silicon ratio.

In addition, the formation of a silicon carbide (SiC) layer to change the concentration of carbon in the silicon can minimize the occurrence of defects.

As a result, a certain distance can be minimized from the silicon layer and the antireflective layer (ARL) interface. Since the electrons do not occur in the silicon-carbide (SiC) layer, doping of the backside P + layer is within a certain range. Reducing the concentration or depth may not increase crosstalk.

4A through 4D illustrate a silicon ratio (x) 410, an energy band gap 420, a visible light absorbance 430, and a defect concentration varying according to the z-axis depth of the unit pixel 300 of FIG. 3. 440).

4A to 4D, z = 0 to z = a are spaced apart from the interface of silicon and the antireflective layer (z = 0) (z = a) to form a section in which the light absorption prevention layer 130 is formed. Indicates. In addition, z = a to z = b indicate a section in which the photodiode layer 120 is formed. In this case, a may be 50 nm or more and 3 um or less, and b may be 1 um or more and 20 um or less. a is less than b

Referring to FIG. 4A, in the period (z = 0 to z = a) in which the light absorption prevention layer 130 is formed, the ratio (x) of silicon is about 0.5 and in the period in which the photodiode layer 120 is formed (z = a z = b), the ratio of silicon is one.

Next, referring to FIG. 4B, the energy band gap of silicon carbide (SiC) is larger than that of silicon in the period (z = 0 to z = a) where the light absorption prevention layer 130 is formed.

Therefore, referring to FIG. 4C, silicon carbide (SiC) has a wide energy band gap, and thus is not excited by light in the visible region.

In order to absorb light in a semiconductor, that is, to excite electrons by photon energy, photons having a photon energy larger than a band gap are required. On the other hand, the relationship between the wavelength in micrometers and the photon energy in eV units is shown in Equation 1.

In this case, the visible light region is generally considered to be 400 nm or more and 700 nm or less, and becomes 3.1 eV at 400 nm having the highest energy. Since the band gap of silicon carbide can be made higher than 3.1 eV, electrons are not excited by the light in the visible region, and the incident photons pass through the silicon carbide as it is. On the other hand, when the photodiode is made to have a band gap of about 1.1 eV, the bandgap is smaller than the lowest photon energy of 1.77 eV (700 nm), so that electrons can be excited by light in the visible region.

As a result, referring to FIGS. 4C and 4D, since the electrons generated by the light in the visible region are generated at a location where the excitation is a predetermined depth away from the backside surface (z = a), the influence of the dark defect on the backside surface is affected. You will receive less.

5 is a circuit diagram of a unit pixel constituting the CMOS image sensor.

Referring to FIG. 5, the unit pixel 500 transmits charges generated by photodiodes PD for detecting light and photons focused on the photodiodes PD to the floating diffusion region F / D. Transistor M1, a reset transistor M2 for resetting the floating diffusion region F / D, a conversion transistor M3 for generating an electrical signal corresponding to the charges transferred to the floating diffusion region F / D, and a unit A selection transistor M4 for transmitting the electric signal converted in the pixel to the outside is provided.

The transmission transistor M1 is controlled by the transmission control signal Tx, the reset transistor M2 is the reset control signal RE, and the selection transistor M4 is controlled by the selection control signal Sx. According to the direction of light reception, the general CMOS image sensor and the backside illumination CMOS image sensor can be distinguished. That is, the general CMOS image sensor receives the light LIGHT1 incident on the N-type electrode of the photodiode, and the backside illumination CMOS image sensor receives the light LIGHT2 incident on the P-type electrode.

In the case of a general CMOS image sensor, part of the light LIGHT1 incident on the unit pixel is blocked from being incident on the photodiode by a MOS transistor or the like. On the other hand, in the case of the backside illumination CMOS image sensor, since the light LIGHT2 can be received by the entire unit pixel, the light receiving efficiency is relatively better than that of the general CMOS image sensor.

6 is a sectional view of a unit pixel generated using a CMOS process.

Referring to FIG. 6, the unit pixel 600 formed by the CMOS process is implemented by implementing a photodiode and a MOS transistor on a P-type substrate. One electrode of a photodiode having two electrodes is a substrate and the other One electrode is an N + type diffusion region. The MOS transistor is formed by a gate formed between one diffusion region N + and two diffusion regions N + forming a photodiode, and is operated by a signal applied to the gates of the MOS transistors. It is implemented by a silicon oxide film (SiO ₂ ) and a polycrystalline silicon (Poly-Silicon) formed on the silicon oxide film. In order to control the threshold voltage of the transistor, the silicon oxide film is preferably thermally grown.

Light LIGHT2 when light LIGHT2 is applied to the P- substrate of the photodiode compared to the area capable of receiving light LIGHT1 when light LIGHT1 is applied to the N + diffusion region N + of the photodiode. It can be easily seen that the area that can be received is relatively large. The present invention relates to a backside illumination CMOS image sensor corresponding to when light LIGHT2 is applied to a P- substrate of a photodiode.

Hereinafter, a method of forming the unit pixel 1000 according to the present invention shown in FIG. 1 will be described.

7A is a cross-sectional view of the unit pixel 710 when the wiring layer and the photodiode layer are formed.

Through a series of processes, photodiodes and MOS transistors are formed on one side of the wafer, and metal is completely formed for the electrical connection of the photodiodes and MOS transistors. Assume that the portion where the photodiode and morph transistors are formed is the top of the wafer. Of the plurality of layers constituting the backside illumination CMOS image sensor 100 shown in FIG. 1, the layers formed by the above processes are photodiodes PD1 and PD2 and wiring layers.

After the above process is completed, the lower part of the wafer is polished so that the wafer has a constant thickness. When the polished wafer is flipped over, the bottommost substrate is faced up. The unit pixel 710 illustrated in FIG. 7A illustrates a case in which a wafer including both a photodiode and a MOS transistor is inverted. Therefore, during the process, the photodiodes PD1 and PD2, which will be positioned at the bottom, are shown upward, and the wiring layer 110, which will be positioned at the top, is shown at the bottom. The substrate on which one electrode of the photodiode is formed is polished after both the photodiode and the transistor are formed to adjust the width of one terminal of the photodiode.

In the subsequent process, the light absorption prevention layer 130, the antireflection layer 140, the color filter 150, and the microlens 160 are formed in the upper direction of the inverted wafer. The subsequent steps will be described below with reference to Figs. 7B to 7F.

FIG. 7B is a cross-sectional view of the unit pixel 720 when the light absorption prevention layer 130 is formed on the photodiode. The light absorption prevention layer 130 may be formed by doping or depositing at an interface between the silicon and the antireflection layer ARL.

7C is a cross-sectional view of the unit pixel 730 when the P-type high impurity is injected into the light absorption prevention layer 130. Referring to FIG. 7C, the P-type high impurity is formed in the light absorption prevention layer 130 by performing an ion implantation process, but is not limited thereto. For example, the compound semiconductor constituting the light absorption prevention layer 130 may be formed to have a P-type high impurity. Furthermore, in one embodiment of the present invention, the P-type high fluoride implantation process of FIG. 7C may be omitted, and the light absorption prevention layer of FIG. 7B may be formed and the process may be directly skipped to the step of FIG. 7D.

P-type high impurity may be boron (P). The reason for injecting the P-type high impurity is to suppress dark current by reducing the minority carrier by forming the P-type high impurity layer. In general, if only the boron doped layer is formed without a material that does not absorb light between the photodiode and ARL, electron-hole pairs may be generated, thereby reducing sensitivity to light. There was also a drawback to increasing torque. On the contrary, the present invention has the advantage that it is possible to reduce the influence of surface package or defect without loss of sensitivity to light by doping a P-type high impurity in a compound semiconductor that does not absorb light.

FIG. 7D is a cross-sectional view of the unit pixel 740 in which the anti-reflection layer 140 and the color filter 150 are sequentially stacked on the light absorption prevention layer 130.

7E is a cross-sectional view of the unit pixel 750 when an island of a certain shape is formed on the photodiode. An island ISLAND having a smaller size than the area defined by the photodiodes PD1 and PD2 shown in FIG. 7E is formed on the photodiodes PD1 and PD2. It is preferable to form one island for each unit pixel, and the size of the island may vary depending on a subsequent process selected. The shape of the island is preferably reduced in proportion to the shape defined by the photodiode. For example, if the shape of the photodiode is a square, the shape of the island is also a square, and if the shape of the photodiode is hexagonal or octagonal more than a square, the shape of the island is preferably implemented as a hexagon or an octagon. In some cases, however, it is also possible to round the island.

The process of forming an island varies, but only one example is given for clarity.

-First, the material implementing the photonic refraction micro lens is applied.

Create a mask that defines an island.

-Use an mask to define the island in the photo-resistor,

-After removing other parts of the photoresist except the part defined as Ireland

The islands are formed using an etchant used to etch the materials used to implement the islands.

7F is a cross-sectional view of a unit pixel when the microlens 160 is formed by applying heat to an island formed on an upper portion of the photodiode.

8 shows the configuration of the CMOS image sensor.

Referring to FIG. 8, the CMOS image sensor 800 includes a row decoder 810, a column decoder 820, a pixel array 830, a selector 840, and a buffer 850.

In the pixel array 830, a plurality of unit pixels are arranged in two dimensions. The row decoder 810 controls the operation of the unit pixels arranged in the pixel array 830 in units of horizontal lines. The column decoder 820 controls the selector 840 to control the operations of the unit pixels distributed in the pixel array 830 in the vertical line unit. The electrical signal converted from the pixel array 830 is output through the buffer 850.

When the unit pixel constituting the pixel array 830 has the form shown in FIG. 1, the present invention will be implemented.

While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. In addition, it is obvious that any person having ordinary knowledge in the technical field to which the present invention belongs can make various modifications and imitations without departing from the scope of the technical idea of the present invention.

Claims (10)

Wiring layer;
A photodiode formed on the wiring layer;
A light absorption prevention layer formed on the photodiode; And
An anti-reflection layer formed on the light absorption prevention layer;
The light absorption prevention layer is a unit pixel, characterized in that formed of a compound semiconductor having a larger energy band gap (Eg) than the semiconductor constituting the photodiode. The method of claim 1,
Wherein said compound semiconductor transmits visible light. The method of claim 1,
The unit pixel of claim 1, wherein the semiconductor comprises silicon (Si). The method of claim 1,
And the compound semiconductor is silicon carbide (SiC). The method of claim 1,
And the compound semiconductor is formed on the photodiode by a deposition process. The method of claim 1,
Unit pixel, characterized in that the dopant P-type high impurity to the light absorption prevention layer. The method of claim 6,
The unit type pixel, characterized in that the P-type high impurity comprises boron (B). The method of claim 1,
A color filter layer is further formed on the antireflection layer.
The unit pixel, characterized in that a micro lens is further formed on the color filter layer. The method of claim 1,
And the wiring layer comprises a plurality of metal layers and an insulating layer for insulating the plurality of metal layers. A pixel array in which a plurality of unit pixels are arranged in two dimensions;
A row decoder for controlling the operations of the unit pixels arranged in the pixel array in units of horizontal lines; And
A column decoder configured to control the operation of the unit pixels arranged in the pixel array in a vertical line unit;
Each of the unit pixels,
Wiring layer;
A photodiode formed on the wiring layer;
A light absorption prevention layer formed on the photodiode; And
It is provided with an antireflection layer formed on the light absorption prevention layer,
The light absorption preventing layer is formed of a compound semiconductor having a larger energy band gap (Eg) than a semiconductor constituting the photodiode.

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