JPH10150180A - Solid-state image sensing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to prevent the effective region of photodiodes from being reduced due to the concentration in a p-type well, to make it possible to prevent carriers, which are generated at deep positions in a substrate by a photoelectric conversion, from flowing in the photodiodes and to contrive the enhancement of the sensitivity of a solid-state image sensing device and the reduction in a color mixture and a dark current in the device. SOLUTION: This device is constituted into a structure, wherein photodiodes 33, which consist of an n-type layer, are two-dimensionally arranged in the surface of a p-type silicon substrate 31 and each of the photodiodes 33 is provided with a readout transistor for reading out an obtained signal charge, an amplitude transistor, which amplifies and takes out the signal charge, and a reset transistor for eliminating the signal charge. In this case, a p<+> well 32 is buried-formed in the substrate 31 by an ion implantation of boron (B) and a boundary 35 on the surface side, where the impurity concentration in this well 32 is reduced to a concentration equal with the impurity concentration in the substrate, of the substrate is made to coincide with the lower end of a depletion layer 34 which is formed of the photodiodes 33.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifying solid-state imaging device for amplifying and extracting a signal charge obtained by a photoelectric conversion unit such as a photodiode, and more particularly to an improvement in a p-type well structure provided in a substrate. The present invention relates to a solid-state imaging device.

[0002]

2. Description of the Related Art In recent years, a solid-state imaging device using an amplification type MOS sensor has been proposed as one of the solid-state imaging devices. This solid-state imaging device amplifies a signal detected by a photoelectric conversion unit (photodiode) for each cell by a transistor, and has a feature of high sensitivity. In particular,
The signal charge generated by the photoelectric conversion modulates the potential of the signal charge storage section, and the potential modulates the amplification transistor inside the pixel to provide an amplification function inside the pixel, thereby increasing the number of pixels and reducing the image size. It is expected as a solid-state imaging device suitable for reduction in pixel size by reduction.

In forming a well for forming an element of such an amplification type solid-state imaging device, boron (B) is implanted into a p-type or n-type silicon substrate by an ion implantation method, and then heat diffusion (for example, at 1190 ° C.). Is performed for 2 hours. As a result, 1 μm from the substrate surface
m, there is a boron concentration peak, and boron is diffused to a depth of 7 to 8 μm, so that a p-type well having a boron concentration gradient (hereinafter, referred to as DIPWELL) can be formed.
Subsequently, a method of forming a photodiode serving as a photoelectric conversion unit by implanting phosphorous (P) into a desired position patterned by a resist is generally used.

However, this type of device has the following problems. In other words, the photodiode of the photoelectric conversion part has a thermal diffusion of boron to a depth of 7 to 8 μm.
The photodiode (n) is formed in IPWELL.
(Depletion type) (effective area of photodiode:
The area where carriers are collected by the expansion of the depletion layer of the photodiode) was dependent on the DIPWELL concentration. For this reason, when the concentration of DIPWELL is high, there is a problem that the effective area of the photodiode is narrowed and the sensitivity is lowered.

[0005] Further, since the DIPWELL is formed from the substrate surface to a deep position of the substrate (usually 7 μm or more), the DIPWEL is formed.
The concentration gradient of the boron concentration of L is relatively small. This is DI
This means that the gradient of the potential barrier formed by PWELL is small. For this reason, carriers generated by photoelectric conversion at a deep position (for example, 3 μm) on the substrate become DIPW
There was a problem that the color flow was caused by crossing over the ELL barrier and flowing into the adjacent photodiode. Further, there is a problem that carriers generated at a deep position in the substrate leak into the photodiode due to diffusion and increase dark current.

Here, the factors that increase the color mixture and the dark current are the carriers generated at a position deeper than the depth at which the depletion layer spreads by the photodiode, and the carriers generated at the deep position are used for imaging. The light has a long wavelength. Therefore, the wavelength of the irradiation light that is a problem is about 650 nm or more (red light). At this time, the reciprocal of the light absorption coefficient of the silicon substrate is about 3.2 μm.
(H. Melcior, "Demodulation
and Photodetection Techniques, "in FTArecchi and
EOSchulz-Dubois, Eds., Laser Handbook, Vol.1, North
-Holland, Amsterdam, 1972, pp725-835).

[0007]

As described above, in the conventional amplifying type solid-state imaging device, the photodiode of the photoelectric conversion unit has a D-type in which boron is thermally diffused to a depth of 7 to 8 μm.
The photodiode (n) is formed in IPWELL.
Since the extent of the depletion layer depends on the concentration of DIPWELL, there is a problem that if the concentration of DIPWELL is high, the effective area of the photodiode becomes narrow and the sensitivity decreases. In addition, since the gradient of the potential barrier formed by the DIPWELL is small, carriers generated by photoelectric conversion at a deep position in the substrate flow over the DIPWELL barrier and flow into the adjacent photodiode, causing color mixing and increasing dark current. There was a problem.

The present invention has been made in consideration of the above circumstances, and has as its object to prevent a reduction in the effective area of a photodiode caused by the concentration of a p-type well and to prevent the substrate from being deepened. An object of the present invention is to provide a solid-state imaging device capable of preventing carriers generated by photoelectric conversion at a position from flowing into a photodiode, improving sensitivity, reducing color mixing, and reducing dark current.

[0009]

[Means for Solving the Problems]

(Structure) The present invention for solving the above problems employs the following structure. (1) The photoelectric conversion units are two-dimensionally arranged on the surface of the semiconductor substrate,
In a solid-state imaging device provided with an amplifying transistor for amplifying and extracting a signal charge obtained by the photoelectric conversion unit and a reset transistor for discharging the signal charge for each photoelectric conversion unit, A p-type well is buried by introducing impurities to a predetermined depth, and a depth from the substrate surface on the substrate surface side where the impurity concentration of the p-type well is reduced to the same as the impurity concentration in the substrate is formed by the photoelectric conversion unit. Characterized in that the depth substantially coincides with the depth of the end of the depletion layer. (2) a p-type well formed on the surface of the semiconductor substrate, a photoelectric conversion unit two-dimensionally arranged on the surface of the p-type well,
An amplifying transistor provided for each photoelectric conversion unit for amplifying and reading out the signal charge obtained by the photoelectric conversion unit; and a signal charge provided for each photoelectric conversion unit and obtained by the photoelectric conversion unit. And a reset transistor for discharging the light, the depth from the substrate surface where the impurity concentration of the p-type well is reduced to the same as the impurity concentration in the substrate, the light of the longest wavelength used for imaging. It is characterized by being set smaller than the reciprocal of the absorption coefficient. (3) In the above (1), the depth from the substrate surface on the substrate surface side where the impurity concentration of the p-type well decreases to the same level as the impurity concentration in the substrate, and the depth of the end of the depletion layer formed by the photoelectric conversion portion are A mechanism is provided for adjusting the potential of the photoelectric conversion unit so that they substantially match each other. (4) In the above (1) and (2), the substrate is p-type silicon,
The p-type well is formed by boron ion implantation. (5) In the above (2), the light having the longest wavelength to be used for imaging is red light, and the depth from the substrate surface at which the impurity concentration of the p-type well decreases to the same as the impurity concentration in the substrate is set to around 3 μm. That you set. (6) A read transistor for reading signal charges and a capacitor for storing signal charges are provided together with the amplification transistor and the reset transistor corresponding to the photoelectric conversion unit. (Operation) In the conventional p-type well structure, the impurity concentration gradient is gentle in the depth direction of the substrate. However, in the present invention, the well structure in which the boron concentration locally increases in the depth direction of the substrate is provided. (Hereinafter referred to as FPWELL).
Alternatively, the structure is such that the boron concentration gradient of the DIPWELL is steep. As a result, in the FPWELL structure or the DIPWELL steep well structure, the conventional DIPWELL structure (well structure of 7 to 8) is used.
The potential gradient becomes steeper than that of the potential gradient. FIG. 6A shows the potential distribution of the FPWELL structure, and FIG. 6B shows the potential distribution of the DIPWELL steep well structure.

[0010] For these reasons, the height of the potential barrier viewed from electrons generated at the same substrate depth depends on the well structure having a steep potential gradient (DIPWELL steepened well structure).
Alternatively, the FPWELL structure is higher than the well structure formed by the conventional method. For this reason, it is possible to prevent carriers generated by photoelectric conversion at a deep position in the substrate from flowing into the photodiode of the photoelectric conversion unit, and it is possible to reduce color mixture and dark current.

Further, since the formation position of the FPWELL can be formed irrespective of the formation position and the concentration of the photodiode, the FPWELL is formed.
It is also possible to make the impurity concentration of WELL extremely high.
As a result, the potential barrier can be changed freely,
Diffusion of carriers from a deep position in the substrate can be effectively suppressed, and the dark current can be significantly reduced.

Further, in the FPWELL structure, since the FPWELL structure is formed at a position slightly away from the photodiode (deep position), the depletion layer formed by the photodiode is wider than that in the DIPWELL structure. As a result, the effective area of the photodiode is expanded, so that the sensitivity can be improved. In other words, FPWE
With the LL structure, it is possible to improve the sensitivity as well as to reduce the color mixture and the dark current.

In the DIPWELL steep well structure, the depth from the substrate surface to the lower end of the DIPWELL is set to be smaller than the reciprocal of the absorption coefficient of the light of the longest wavelength used for imaging. Here, since the depth at which the irradiation light on the substrate reaches the inside of the substrate is generally represented by the reciprocal of the absorption coefficient for the light, in the present invention, the irradiation light on the substrate is smaller than the depth at which the irradiation light reaches the inside of the substrate. The boundary of DIPWELL exists at a shallow position. Therefore, the potential barrier has a large gradient with respect to carriers generated at a deep position on the substrate due to light used for imaging, and it is possible to effectively suppress the occurrence of color mixing and dark current.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the illustrated embodiments. (First Embodiment) FIG. 1 is a circuit configuration diagram showing one pixel portion of an amplification type solid-state imaging device according to a first embodiment of the present invention, and FIG. 2 is a block diagram showing an overall configuration.

As shown in FIG. 1, one pixel portion includes a read transistor 12 for reading charges (electrons) generated by photoelectric conversion by a photodiode 11, and a capacitor 13 connected to a drain of the read transistor 12. , An amplification transistor 14 for signal amplification, and a reset transistor 15 for discharging signal charges. Furthermore, an address line 16 connected to the gate of the read transistor 12 for selecting a line, a signal line 17 connected to the drain of the amplifier transistor 14 for reading a signal, and a reset line 18 connected to the gate of the reset transistor 15 , A drain line 19 for discharging a signal is provided.

As shown in FIG. 2, the signal line 17 is connected to the output transistor 2 driven by the horizontal scanning circuit 21.
3 and the address line 16 and the reset line 18 are connected to the vertical scanning circuit 22. In addition,
FIG. 2 shows an example in which the one-pixel portions 20 having the configuration shown in FIG. 1 are arranged in 3 × 3, but a large number of pixels are actually arranged in a matrix.

Although the circuit configuration so far is basically the same as that of the conventional device, in this embodiment, the configuration of the p-well provided on the substrate for forming the elements is significantly different from that of the conventional device. FIG. 3 is a sectional view of an element structure near a photoelectric conversion unit (photodiode unit) of the amplification type solid-state imaging device according to the present embodiment.

A p-type well (FPWELL) 32 is buried at a depth of several μm from the surface of the p-type silicon substrate 31. The p-type well 32 is formed by implanting B (boron) into the p-type silicon substrate 31 at a high acceleration (for example, 2 MeV). At this time, if necessary, B can be thermally diffused. As a result, the FPWELL structure has a boron concentration peak at a position of about 2.5 (± 0.5) μm from the substrate surface.
An FPWELL structure having a concentration profile that becomes the substrate concentration at a position of 5 (± 1) μm can be formed.

On the surface of the substrate 31, a photodiode 33 (n layer) for performing photoelectric conversion is formed. In the formation of the photodiode 33, first, a resist is applied and patterned to leave a resist 39 in a region other than the region where the photodiode 33 is to be formed. Then, phosphorus is implanted by ion implantation at, for example, 90 keV to form an n layer 33 having an impurity concentration of 2 × 10 ¹⁷ cm ⁻³ .

At this time, the formation depth of the p-type well 32 of the FPWELL structure (the peak depth of the boron concentration is 2.5 μm, the depth at which the boron concentration is reduced to the substrate concentration is 1.5 μm,
5 μm) is an optimum formation depth.
For this reason, it is preferable that the expansion of the depletion layer 34 due to the photodiode 33 be in contact with the end 35 of the p-type well 32. This is because the depletion layer 34 can be sufficiently expanded. On the other hand, if the p-type well 32 is formed at an excessively deep position, the depletion layer 34 of the photodiode 33 does not reach the end 35 of the p-type well, so that a non-electric field region 36 is formed before the p-type well 32. This non-electric field region 36
Since the carriers generated in step (1) move only by diffusion, they easily flow into the adjacent photodiode 37, which causes color mixing.

For the above reasons, there is an optimum formation depth at the formation position of the p-type well 32 of the FPWELL structure. In the formation position of the p-type well 32, it is preferable that the extension of the depletion layer 34 by the photodiode 33 and the position of the end 35 (junction position) of the p-type well 32 on the photodiode side coincide.

In the figure, reference numeral 38 denotes a photodiode 33
Is the gate of the read transistor for reading the signal charge obtained in step (1). Although not shown, a reset transistor, an amplification transistor, and the like are formed on the substrate 31 in addition to the read transistor. Then, by performing electrode wiring, an amplification type solid-state imaging device is completed.

As described above, according to the present embodiment, the p-type well 32 of the FPWELL structure having a thickness of about 1 μm is formed in the p-type silicon substrate 31 at a position of about 2.5 μm from the substrate surface, whereby As shown in FIG. 6A, the gradient of the potential barrier by the p-type well 32 can be increased, and carriers generated by photoelectric conversion at a deep position (for example, 3 μm) of the substrate are transferred to the photodiode 33 and the adjacent photodiode 37. Leakage can be prevented beforehand. For this reason, it is possible to suppress the occurrence of color mixture and an increase in dark current.

Also, by making the extension of the depletion layer 34 by the photodiode 33 coincide with the position of the end 35 of the p-type well 32 on the photodiode side, an electric field-free region is not generated under the photodiode, and the The effective area can be widened, and the sensitivity can be improved. (Second Embodiment) FIG. 4 is a cross-sectional view of an element structure near a photoelectric conversion unit of an amplification type solid-state imaging device according to a second embodiment of the present invention.

DIPWELL on the surface of the p-type silicon substrate 41
A p-type well 42 having a steep well structure is formed.
The p-type well 42 is formed, for example, to a depth of about 3 μm from the substrate surface by implanting boron ions into the p-type silicon substrate 41 at, for example, 200 keV.

On the surface of the p-type well 42, a photodiode 43 (n layer) for performing photoelectric conversion is formed. In forming the photodiode 43, first, a resist is applied and patterned to leave a resist in a region other than a region where the photodiode 43 is to be formed. Then, phosphorus is implanted at, for example, 90 keV by an ion implantation method to form an n layer having an impurity concentration of 2 × 10 ¹⁷ cm ⁻³ .

In the figure, reference numeral 48 denotes a photodiode 43
Is the gate of the read transistor for reading the signal charge obtained in step (1). Although not shown in the figure, the p-type well 4
2, a reset transistor, an amplification transistor, and the like are formed in addition to the read transistor. Then, by performing electrode wiring, an amplification type solid-state imaging device is completed.

By the way, the formation of the p-type well 42 in the present embodiment is specifically performed as follows. That is,
After boron ions are implanted into the p-type silicon substrate 41 at, for example, 200 keV, the substrate 41 is
Heat treatment for a minute to effect thermal diffusion of boron. According to this method, the depth (bottom of the well) 45 at which the boron concentration of the p-type well 42 is reduced to the same level as the boron concentration in the substrate can be set to about 3 μm from the substrate surface, and the conventional (7 to 8 μm)
A well having a steeper boron concentration gradient can be formed. Thereby, the distance from the substrate surface to the position where the boron concentration of the p-type well decreases to the boron concentration in the substrate can be made smaller than about 3.2 μm, which is the reciprocal of the red light absorption coefficient in the silicon substrate.

Here, the reason why the depth at which the boron concentration of the p-type well 42 is reduced to the substrate concentration (Xj of the p-type well) is smaller than the reciprocal of the light absorption coefficient of the substrate will be described.

In general, the amount (G) of carriers generated by light irradiation is calculated by using the absorption coefficient (α) and the substrate depth (x).
And the following proportional relationship: G〜exp (−αx) (1) The barrier height (V) of the well viewed from electrons generated at the substrate depth x is approximately VＶV0 [1-exp {-a (x-x0)] }] (2) Here, x0 is the peak position of the potential barrier of the well. A indicates the gradient of the potential barrier.

For this reason, the ratio (R) of electrons generated at a certain position x flowing over the well barrier and flowing into the photodiode is expressed by the following equation. R = G · exp (−qV / kT) (3) where q is the elementary charge, k is the Boltzmann constant, and T is the absolute temperature. By integrating the equation (3) from the peak position of the well to infinity (entire in the depth direction of the substrate), the amount of electrons flowing into the photodiode can be obtained. (3)
When the expression is integrated over the entire depth direction of the substrate, the result is as shown in FIG.

FIG. 5 shows that the amount of carriers generated in the substrate leaking into the photodiode tends to be substantially constant when Xj of the well is about 3.2 μm (reciprocal of the absorption coefficient). . Therefore, by making Xj of the well smaller than the reciprocal of the absorption coefficient of the irradiation light (in particular, the light having the longest wavelength), carriers flowing into the photodiode can be reduced.

As described above, in the present embodiment, the light having the longest wavelength among the light to be used for imaging is converted to red light (wavelength 650n).
m), the depth of the p-type well 42 (distance from the substrate surface to the lower end) of the DIPWELL steepened well structure is defined as the reciprocal of the absorption coefficient of the silicon substrate for red light (3.2 μm).
3 m, which is smaller than m). For this reason, the gradient of the potential barrier is large with respect to the carriers generated at a deep position in the substrate, and it is possible to sufficiently suppress the occurrence of color mixing and dark current.

The present invention is not limited to the above embodiments. Although a p-type Si substrate is used in the embodiment, an n-type Si substrate may be used. In addition, the read transistor, the reset transistor, the amplification transistor, and the like are formed after the photodiode is formed. However, each transistor can be formed before the photodiode is formed.

The configuration of one pixel portion is not limited to that shown in FIG. 1. Any configuration using an amplifying CMOS sensor can be appropriately changed according to specifications. Further, the present invention is not necessarily limited to the one using the amplification type MOS sensor, but can be applied to a CCD type.

In the first embodiment, the DIPWEL
Although only FPWELL was formed without forming L, DIPWELL
It is also possible to form an FPWELL after that. In addition, various modifications can be made without departing from the scope of the present invention.

[0037]

As described in detail above, according to the present invention, the improvement of the p-type well formed inside or on the surface of the substrate allows the diffusion of carriers (electrons) generated at a deep position in the substrate and the adjacent photo. Since leakage into the diode can be reduced, color mixing and dark current can be reduced. Further, since the effective area of the photodiode can be made wider than before, the sensitivity can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram showing one pixel portion of an amplification type solid-state imaging device according to a first embodiment.

FIG. 2 is a block diagram showing the overall configuration of the amplification type solid-state imaging device according to the first embodiment.

FIG. 3 is an element structure cross-sectional view showing the vicinity of a photoelectric conversion unit of the amplification type solid-state imaging device according to the first embodiment.

FIG. 4 is an element structure cross-sectional view showing the vicinity of a photoelectric conversion unit of an amplification type solid-state imaging device according to a second embodiment.

FIG. 5 is a diagram showing the relationship between the junction depth of a well and the amount of leaked carriers into a photodiode.

FIG. 6 is a diagram for explaining the operation of the present invention, showing a potential distribution of a p-type well in a substrate depth direction.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Photodiode 12 ... Read-out transistor 13 ... Capacitor 14 ... Amplification transistor 15 ... Reset transistor 16 ... Address line 17 ... Signal line 18 ... Reset line 19 ... Drain line 31,41 ... P-type silicon substrate 32,42 ... P-type well 33, 43 ... photodiodes 34, 44 ... depletion layers 35, 45 ... p-type well boundaries 36 ... field-free regions 37, 47 ... adjacent photodiodes 38, 48 ... gates 39 ... resists

Claims (3)

[Claims] 1. A photoelectric conversion unit is two-dimensionally arranged on a surface of a semiconductor substrate, an amplifying transistor for amplifying and extracting a signal charge obtained by the photoelectric conversion unit for each photoelectric conversion unit, and discharging a signal charge. In a solid-state imaging device provided with a reset transistor, a p-type well is formed by burying impurities into the substrate at a predetermined depth from the substrate surface, and the impurity concentration of the p-type well is reduced to the same level as the impurity concentration in the substrate. A solid-state imaging device, wherein a depth from a substrate surface on a substrate surface side to be formed substantially coincides with a depth of a depletion layer end formed by the photoelectric conversion unit. 2. A p-type well formed on a surface portion of a semiconductor substrate, a photoelectric conversion portion two-dimensionally arranged on the surface of the p-type well, and a photoelectric conversion portion provided for each photoelectric conversion portion. An amplification transistor for amplifying and reading out the signal charge obtained in the above, and a solid-state imaging device including a reset transistor provided for each photoelectric conversion unit and discharging the signal charge obtained in the photoelectric conversion unit, The depth from the substrate surface at which the impurity concentration of the p-type well decreases to the same level as the impurity concentration in the substrate is set to be smaller than the reciprocal of the absorption coefficient of the light of the longest wavelength used for imaging. Solid-state imaging device. 3. The solid-state imaging device according to claim 1, wherein said substrate is p-type silicon, and said p-type well is formed by ion implantation of boron.