JP2002134702A - Semiconductor device and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor device or the like wherein the noise from a digital circuit is restrained from propagating in a substrate to an analog circuit, when the digital circuit and the analog circuit are formed mixed on the same substrate. SOLUTION: This semiconductor device consists of a P-type silicon substrate 11 and an epitaxial layer 12, which is formed on the silicon substrate 11 and composed of N-type silicon. A well 13 for a noise barrier is formed in the silicon substrate 11. In the inside of the epitaxial layer 12, a well 15 for a digital circuit on which a digital circuit 14 is to be formed and a well 17 for an analog circuit on which an analog circuit 16 is to be formed, are formed on both upper sides of the well 13. The depths of the well 15 and the well 13 are set as values, which can restrain the propagation of noise generated in the digital circuit 14 to the analog circuit 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device in which digital circuits and analog circuits are mixedly formed on the same semiconductor substrate, and a method for manufacturing the semiconductor device.

[0002]

2. Description of the Related Art In recent years, system LSIs have been required to be miniaturized and operated at high speed. In the course of this trend, digital circuits and analog circuits have been mixedly formed on the same silicon substrate. . However, in this type of system LSI, crosstalk, in which clock noise generated in a digital circuit propagates to an analog circuit and adversely affects the analog circuit, is becoming a serious problem.

In addition to the type of crosstalk caused by capacitive coupling between metal wirings, there is a crosstalk of noise propagating in a substrate by forming a digital circuit and an analog circuit on the same silicon substrate. As a method of suppressing the propagation of noise in a substrate, a method as shown in FIG. 3 is known. According to this method, as shown in FIG. 3, a high-resistance epitaxial layer 2 is formed on a low-resistance silicon substrate 1, and wells 3 and 4 are formed in the epitaxial layer 2, respectively. In addition to forming the digital circuit 5, an analog circuit 6 is formed in the well 4 so as to suppress the noise from the digital circuit 5 from being propagated in the horizontal direction toward the analog circuit 6.

[0004]

However, in this method, as shown in FIG. 3, the noise 7 from the digital circuit 5 reaching the low-resistance silicon substrate 1 propagates in the silicon substrate 1 in the lateral direction. Then, after propagation to below the analog circuit 6, there is an inconvenience that it easily propagates through the epitaxial layer 2 and reaches the analog circuit 6.

In order to eliminate such inconvenience, a deep well is provided in the substrate, and a normal shallow well is provided in the deep well to form a double structure, and a digital circuit is formed in the normal well. There is a method of confining noise from a digital circuit in a deep well. However, in this method, the noise leaked from the deep well is easily propagated in the horizontal direction and the vertical direction, and is not necessarily perfect as a noise prevention technique.

On the other hand, SOI (Silicon On I)
Although there is a technique for completely suppressing the noise in the downward direction by using an nsulator, there is a disadvantage that the cost of the SOI substrate is almost one digit higher than that of a normal silicon substrate. Further, the SOI has an inconvenience that the difficulty from the design side remains due to a problem (floating body) inherent to the SOI substrate.

In view of the above, it is an object of the present invention to provide a case in which a digital circuit and an analog circuit are mixedly formed on the same substrate and noise from the digital circuit propagates through the substrate. It is an object of the present invention to provide a semiconductor device which is prevented from reaching an analog circuit, and a method for manufacturing the semiconductor device.

[0008]

Means for Solving the Problems In order to solve the above-mentioned problems and achieve the object of the present invention, the inventions according to claims 1 to 7 are configured as follows. That is, claim 1
The invention described in (1) includes a silicon substrate and an epitaxial layer made of silicon formed on the silicon substrate, and a noise barrier well is formed in the silicon substrate, and the noise barrier is formed in the epitaxial layer. On both sides above the barrier well, a digital circuit well for forming a digital circuit and an analog circuit well for forming an analog circuit are formed, respectively.
The depth of each of the digital circuit well and the noise barrier well is such that noise generated in the digital circuit can be suppressed from propagating to the analog circuit.

According to a second aspect of the present invention, in the semiconductor device according to the first aspect, the noise barrier well is located between the digital circuit well and the analog circuit well when viewed from above. Is characterized by being interposed. According to a third aspect of the present invention, in the semiconductor device according to the first or second aspect, the epitaxial layer is made of a high resistance, and the silicon substrate is made of a different type from the epitaxial layer; The digital circuit well and the analog circuit well are of a different type from the epitaxial layer, and the noise barrier well is of a different type from the silicon substrate.

According to a fourth aspect of the present invention, in the semiconductor device according to the third aspect, between the digital circuit well and the epitaxial layer, between the analog circuit well and the epitaxial layer, and between the digital circuit well and the epitaxial layer. A reverse bias voltage is applied between the layer and the silicon substrate and between the noise barrier well and the silicon substrate.

As described above, according to the semiconductor device of the present invention,
A digital circuit well and an analog circuit well are formed in the epitaxial layer, and the depth of the digital circuit well is set so that noise generated in the digital circuit can be suppressed from propagating to the analog circuit. Therefore, it is possible to suppress noise from the digital circuit from propagating in the epitaxial layer and reaching the analog circuit.
When the resistance of the epitaxial layer is increased, the effect of suppressing the resistance is increased.

According to the semiconductor device of the present invention, the noise barrier well is formed in the silicon substrate. Therefore, even if noise from the digital circuit reaches the silicon substrate, the noise barrier well forms the noise. Noise can be suppressed from reaching the analog circuit. In particular, when the noise barrier well is formed at an intermediate position between the digital circuit well and the analog circuit well, the effect of suppressing the noise is increased.

Further, according to the semiconductor device of the present invention, the well for the digital circuit and the epitaxial layer are formed by PN junctions, respectively, and a reverse bias voltage is applied to each PN junction. For this reason, when a reverse bias voltage is applied, a depletion layer can be formed at each PN junction, and the depletion layer can be used as a noise propagation barrier due to the high resistance of the depletion layer.

According to a fifth aspect of the present invention, there is provided a first step of forming a noise barrier well having a predetermined depth in a silicon substrate, and forming a silicon epitaxial layer on the silicon substrate on which the noise barrier well is formed. A second step,
A digital circuit well having a predetermined depth for forming a digital circuit and an analog circuit having a predetermined depth for forming an analog circuit, on both sides of the noise barrier well in the epitaxial layer. And a third step of forming each of the use wells.

According to a sixth aspect of the present invention, in the method of manufacturing a semiconductor device according to the fifth aspect, the first step includes:
Forming a photoresist pattern on the silicon substrate with the first mask for forming the noise barrier well, forming the noise barrier well in the silicon substrate after the formation, and in the third step, Forming a photoresist pattern on the epitaxial layer with a second mask for forming the digital circuit well and the analog circuit well based on the mask; After the formation, the digital circuit well and the analog circuit well are respectively formed in the epitaxial layer.

According to a seventh aspect of the present invention, in the method of manufacturing a semiconductor device according to the fifth or sixth aspect, the epitaxial layer is formed of a high resistance and the silicon substrate has a different shape from the epitaxial layer. And wherein the digital circuit well and the analog circuit well are of a different type from the epitaxial layer, and the noise barrier well is of a different type from the silicon substrate. is there.

As described above, according to the method of manufacturing a semiconductor device of the present invention, it is possible to realize a semiconductor device capable of suppressing noise from a digital circuit from propagating through an epitaxial layer or a silicon substrate and reaching an analog circuit.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of a semiconductor device according to an embodiment of the present invention will be described below with reference to FIG. As shown in FIG. 1, the semiconductor device according to this embodiment includes a P-type silicon substrate 11 and an epitaxial layer 12 made of N-type silicon formed on the silicon substrate 11. Digital circuit 1 inside
4 and an analog circuit 16 are formed. Here, the epitaxial layer 12 preferably has a high resistance.

More specifically, the silicon substrate 1
1, a noise barrier well 1 having a predetermined depth
3 are formed, and the well 13 is formed of an N-type well.
In the epitaxial layer 12, a digital circuit well 15 for forming a digital circuit 14 and an analog circuit 16 are formed in the epitaxial layer 12 on both the left and right sides above the noise barrier well 13. Circuit wells 17 are formed.

The digital circuit well 15 comprises a P-type well, and the analog circuit well 17 comprises an N-type well. The depth of the digital circuit well 15 is such that the noise generated in the digital circuit 14 can be suppressed from propagating to the analog circuit 16. The depth of the analog circuit well 17 may be the same as the depth of the digital circuit well 15, but may be shallower. The depth of the noise barrier well 13 is set to a depth at which when noise generated in the digital circuit 14 leaks to the silicon substrate 11, the noise is suppressed from propagating in the silicon substrate 11 in the lateral direction. preferable.

As shown in FIG. 1, the well 13 for the digital circuit and the well 17 for the analog circuit, and the well 13 for the noise barrier It is preferable to be at an intermediate position between the well 15 for analog and the well 17 for analog circuit. As shown in FIG. 1, a P-type well 18 and an N-type well 19 are formed in the digital circuit well 15.
4 are formed (not shown). As shown in FIG. 1, a P-type well 20 and an N-type well 21 are formed in the analog circuit well 17.
6 are formed (not shown).

An N-type well 22 is formed in the epitaxial layer 12 between the digital circuit well 15 and the analog circuit well 17. The N-type well 22 is connected to a DC power supply (VDD = 5 V). When an AC current (noise) in the epitaxial layer 12 enters the N-type well 22, the AC noise is passed through the DC power supply. It works outside.

In the semiconductor device according to the embodiment,
A P-type digital circuit well 15 and a P-type analog circuit well 17 with a N-type epitaxial layer 12 and a PN junction.
PN junction with the N-type epitaxial layer 12, the PN junction with the N-type epitaxial layer 12 and the P-type silicon substrate 11, and the PN junction with the N-type noise barrier well and the P-type silicon substrate. In addition, a reverse bias voltage is applied to each of the PN junctions so that the PN junction can be used.

As described above, according to the semiconductor device of this embodiment, the well 15 for digital circuit and the well 17 for analog circuit are formed in the epitaxial layer 12 and the depth of the well 15 for digital circuit is formed. To
The noise generated in the digital circuit 14 is
To a depth that can suppress propagation to For this reason, it is possible to suppress noise from the digital circuit 14 from propagating through the epitaxial layer 12 and reaching the analog circuit 16. In particular, since the epitaxial layer 12 has a high resistance, the effect of suppressing the noise is great.

According to the semiconductor device of this embodiment, the noise barrier well 13 is formed in the silicon substrate 11.
Is formed, even if the noise from the digital circuit 15 reaches the silicon substrate 11, the noise barrier well 13 can suppress the noise from reaching the analog circuit 16. In this example, the noise barrier well 1
3 is formed at an intermediate position between the digital circuit well 15 and the analog circuit well 17, so that the suppression effect is large.

Further, according to the semiconductor device of this embodiment, the P-type digital circuit well 15 and the N-type epitaxial layer 12 are each formed into a PN junction, and a reverse bias voltage is applied to each PN junction. It can be used by applying voltage. Therefore, when a reverse bias voltage is applied, a depletion layer can be formed on each PN junction surface, and the high resistance of the depletion layer can make the depletion layer a barrier for noise propagation.

Next, an embodiment of a method of manufacturing a semiconductor device according to the present invention will be described with reference to FIG. First, using a mask for the noise barrier well 13, a photoresist pattern (not shown) is formed on the P-type silicon substrate 11 shown in FIG. P) ion implantation is performed. This ion implantation is performed by applying an acceleration energy of about 100 keV to the ion, and the ion implantation amount is 10 ¹³ /
cm ² .

When the ion implantation is completed, heat of about 1200 ° C. is applied to the silicon substrate 11 for about 10 hours in order to thermally diffuse the implanted phosphorus. At this time, in order to prevent an oxide film from being formed on the surface of the silicon substrate 11, an inert gas (for example, nitrogen) which does not react with silicon is used. When the thermal diffusion is completed, as shown in FIG. 2A, a noise barrier well 13 composed of an N-type well is formed in a P-type silicon substrate 11.
Is formed. The noise barrier well 13 has a minimum length (or width) of about 20 μm, for example.
Its depth is about 10 μm.

Next, on a P-type silicon substrate 11, FIG.
As shown in FIG. 1B, an N-type and high-resistance epitaxial layer 12 is formed. This N-type epitaxial layer 1
2 has a thickness of 30 μm and a resistivity of 10 μm, for example.
It is about 00 [Ωcm]. Next, the black and white of the mask for the noise barrier well 13 is inverted, and the mask for forming the digital circuit well 15 and the analog circuit well 17 is used to form a mask on the epitaxial layer 12 shown in FIG.
A photoresist pattern (not shown) is formed. And
Using the photoresist pattern, boron (B) ions are implanted into the epitaxial layer 12 as impurities. The implantation of this ion is 100 ke
The acceleration energy of about V is applied, and the amount of implanted ions is about 10 ¹² / cm ² .

When the implantation of the ions is completed, heat of about 1200 ° C. is applied to the epitaxial layer 12 for about 10 hours in order to thermally diffuse the implanted boron. At this time, an inert gas (for example, nitrogen) is used to prevent an oxide film from being formed on the surface of the epitaxial layer 12. When the thermal diffusion is completed, as shown in FIG.
A well 15 for a digital circuit and a well 17 for an analog circuit composed of a mold well are respectively formed. The digital circuit well 15 and the analog circuit well 17
Has a depth of, for example, about 12 μm.

Thereafter, as shown in FIG. 2D, the P-type well 18 and the N-type well 1 are placed in the digital circuit well 15.
9, a P-type well 20 and an N-type well 21 are formed in the analog circuit well 17, and
An N-type well 22 is formed in the epitaxial layer 12 between the digital circuit well 15 and the analog circuit well 17.

Next, a transistor (not shown) for the digital circuit 14 is utilized by using the wells 18 and 19 described above.
And a transistor (not shown) for the analog circuit 16 is formed using the wells 20 and 21 described above. Thus, a desired semiconductor device is completed. As described above, according to the embodiment of the method of manufacturing a semiconductor device of the present invention, it is possible to suppress noise from the digital circuit 14 from propagating in the epitaxial layer 12 or the silicon substrate 11 and reaching the analog circuit 16. Semiconductor device that can be realized.

[0033]

As described above, according to the semiconductor device of the present invention, a digital circuit well and an analog circuit well are formed in an epitaxial layer, and the depth of the digital circuit well is reduced by the digital circuit. The depth is set so that the generated noise can be suppressed from propagating to the analog circuit. For this reason, it is possible to suppress the noise from the digital circuit from propagating through the epitaxial layer and reaching the analog circuit. In particular, when the resistance of the epitaxial layer is increased, the effect of suppressing the noise is increased.

Further, according to the semiconductor device of the present invention, since the noise barrier well is formed in the silicon substrate, even if noise from the digital circuit reaches the silicon substrate, the noise barrier well forms the noise. Noise can be suppressed from reaching the analog circuit. In particular, when the noise barrier well is formed at an intermediate position between the digital circuit well and the analog circuit well, the effect of suppressing the noise is increased.

Further, according to the semiconductor device of the present invention, the digital circuit well and the epitaxial layer are formed by PN junctions, respectively, and a reverse bias voltage is applied to each PN junction. For this reason, when a reverse bias voltage is applied, a depletion layer can be formed at each PN junction, and the depletion layer can be used as a noise propagation barrier due to the high resistance of the depletion layer.

On the other hand, according to the method of manufacturing a semiconductor device of the present invention, a semiconductor device capable of suppressing noise from a digital circuit from propagating in an epitaxial layer or a silicon substrate and reaching an analog circuit can be realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a configuration of an embodiment of a semiconductor device of the present invention.

FIG. 2 is a process diagram illustrating each process in an embodiment of the method of manufacturing a semiconductor device according to the present invention.

FIG. 3 is a cross-sectional view illustrating a configuration of a conventional semiconductor device.

[Explanation of symbols]

 Reference Signs List 11 P-type semiconductor substrate 12 N-type epitaxial layer 13 Well for noise barrier 14 Digital circuit 15 Well for digital circuit 16 Analog circuit 17 Well for analog circuit

Claims (7)

[Claims] 1. A semiconductor device comprising: a silicon substrate; and an epitaxial layer made of silicon formed on the silicon substrate. A noise barrier well is formed in the silicon substrate, and the noise barrier is formed in the epitaxial layer. A digital circuit well for forming a digital circuit and an analog circuit well for forming an analog circuit are formed on both sides of the upper side of the well for the digital circuit and the well for the noise barrier, respectively. A semiconductor device, wherein each depth is such that noise generated in the digital circuit can be suppressed from propagating to the analog circuit. 2. The noise barrier well according to claim 1, wherein the noise barrier well is interposed between the digital circuit well and the analog circuit well when viewed from above. Semiconductor device. 3. The epitaxial layer has a high resistance, the silicon substrate has a different type from the epitaxial layer, and the digital circuit well and the analog circuit well have a different type from the epitaxial layer. 3. The semiconductor device according to claim 1, wherein the noise barrier well has a different type from the silicon substrate. 4. 4. The method according to claim 1, further comprising: a step between the digital circuit well and the epitaxial layer; a step between the analog circuit well and the epitaxial layer; a step between the epitaxial layer and the silicon substrate; 4. The semiconductor device according to claim 3, wherein a reverse bias voltage is applied between the semiconductor device and the silicon substrate. 5. A first step of forming a noise barrier well having a predetermined depth in a silicon substrate; a second step of forming an epitaxial layer of silicon on the silicon substrate on which the noise barrier well is formed; Inside and on both sides above the noise barrier well, a digital circuit well of a predetermined depth to form a digital circuit, and an analog circuit well of a predetermined depth to form an analog circuit. And c. Forming a semiconductor device, respectively. 6. In the first step, a photoresist pattern is formed on the silicon substrate using the first mask for forming the noise barrier well, and after the formation, the noise barrier well is formed in the silicon substrate. In the third step, the first mask is a mask obtained by inverting black and white of the first mask, and the second mask for forming the digital circuit well and the analog circuit well based on the mask is used. 6. The semiconductor device according to claim 5, wherein a photoresist pattern is formed on the epitaxial layer, and after forming the photoresist pattern, the digital circuit well and the analog circuit well are respectively formed in the epitaxial layer. Production method. 7. The epitaxial layer has a high resistance, the silicon substrate has a different type from the epitaxial layer, and the digital circuit well and the analog circuit well have a different type from the epitaxial layer. 7. The method according to claim 5, wherein the noise barrier well is of a different type from the silicon substrate.