JP2003059937A - Semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heterojunction type bipolar transistor with which high- speed operation is enabled and a high current amplification factor can be stably provided. SOLUTION: SiGe-HBT is provided with an SiGe film 18 and an Si film 19 by sequential epitaxial growth. The SiGe film 18 is composed of an SiGe buffer layer 18x, an SiGe gradient composition layer 18a and an upper SiGe layer 18b, with which a Ge content is almost fixed or change in the Ge content is smaller than that of the SiGe gradient composition layer 18a. Even when the position of an EB bonding part 33 is fluctuated, since the EB bonding part 33 is located in one part of the upper SiGe layer 18b, the fluctuation of the Ge content in the EB bonding part 33 can be suppressed so that the high current amplification factor can be stably provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device functioning as a heterojunction bipolar transistor, and more particularly to a measure for suppressing variations in characteristics such as current amplification factor.

[0002]

2. Description of the Related Art In recent years, a bipolar transistor formed using a silicon substrate has been miniaturized and speeded up by the progress of fine processing technology and self-alignment technology. A general bipolar transistor is a so-called homojunction bipolar transistor that uses a silicon substrate and a silicon single crystal layer epitaxially grown on the silicon substrate.

On the other hand, in order to achieve higher speed operation, a heterojunction bipolar transistor (hereinafter referred to as "HB
Research and development of "T") is actively carried out. Particularly recently, a SiGe layer, which is a mixed crystal of silicon and germanium, is epitaxially grown on a silicon substrate, and the HBT (hereinafter, referred to as
"SiGe-HBT") is being actively developed.

FIG. 8 is a sectional view of a conventional SiGe-HBT. As shown in the figure, the conventional SiGe-HBT
Are formed using a Si substrate 101 and a Si epitaxial layer 102 epitaxially grown on the Si substrate 101. Further, an N ⁺ type buried layer 110 provided over each part of the Si substrate 101 and the Si epitaxial layer 102 and a part of the Si epitaxial layer 102 by introducing a relatively high concentration of N type impurities are provided. The N
⁺ Collector extraction layer 111, and a portion of the Si epitaxial layer 102 excluding the N ⁺ collector extraction layer 111 is an N ⁻ collector diffusion layer 112 containing a low concentration N-type impurity. In addition, the Si epitaxial layer 10
A LOCOS isolation 116 for partitioning 2 into each bipolar transistor formation region and a deep trench isolation 117 extending below the LOCOS isolation 116 and reaching the Si substrate 101 are provided. However, in the bipolar transistor formation region, the N ⁺ collector extraction layer 1
11 and the N ^- collector diffusion layer 112 are divided into LOCO
The deep trench isolation 117 is not provided below the S isolation 116.

Further, the N of the Si epitaxial layer 102 is
^- S containing P-type impurities is formed on the collector diffusion layer 112.
The SiGe film 108 made of the iGe mixed crystal semiconductor layer and the capacitor
And the Si film 109 to be a top layer are epitaxial
It is formed by growth. In addition, the SiGe film 108 and
And the area from the side surface of the Si film 109 to the upper surface of the Si film 109.
P formed on the region and containing a high concentration of P-type impurities⁺ Basepo
Li silicon film 114, P⁺ Base polysilicon film 11
High-concentration N-type impurity provided on the opening formed in FIG.
N including⁺ And an emitter polysilicon film 113
It However, P ⁺ Base polysilicon film 114 and N⁺ D
The miter polysilicon film 113 and the
It is electrically separated.

Here, the SiGe film 108 and the Si film 1
09 is MBE method, UHV-CVD method or LP-CV
It is epitaxially grown using the D method. And
In the region of the Si film 109 immediately below the N ⁺ polysilicon film 113, the N ⁺ emitter polysilicon film 113 to the R
N-type impurities (phosphorus, arsenic, etc.) diffused by TA are doped. That is, N ⁺ of the Si film 109
The type region functions as the emitter region of the NPN bipolar transistor, and the P ⁺ type region of the SiGe film 108 is the NPN.
It functions as the base region of the bipolar transistor, and N
^- collector diffusion layer 112, N ⁺ -type buried layer 110 and N ⁺ collector lead layer 111 functions as a collector region of the NPN bipolar transistor.

In the semiconductor device manufacturing process, Si
After the SiGe film 108 is epitaxially grown on the epitaxial layer 102, Si is formed on the SiGe film 108.
The film 109 is continuously epitaxially grown. The Si film 109 is also required to prevent Ge contamination of the manufacturing line, mainly in the post-process after the SiGe epitaxial growth. However, the film thickness of the Si film 109 and the N type in the emitter polysilicon film 113 are required. Depending on the concentration of impurities,
By appropriately selecting the heat treatment conditions for diffusing the N-type impurities, the emitter / base junction (hereinafter referred to as “EB junction”) can be formed at a desired depth position in the Si film 109.

The conventional SiGe-H thus formed
BT has the advantage of higher emitter injection efficiency without doping the emitter region with a high concentration of impurities, as compared with a homojunction bipolar transistor composed of only a Si layer, and is expected to have a high current amplification factor (h _FE ). To be done.

FIG. 9 shows Si / SiGe having a graded composition.
Heterojunction bipolar transistor (SiGe-HB
T) and Si homojunction bipolar transistor (Si-
It is an energy band diagram for comparing the band structure with (BT). In the SiGe-HBT, the height of the barrier for holes injected from the base region to the emitter region can be made higher than the height of the barrier for electrons injected from the emitter region to the base region. Therefore, even if the impurity concentration in the emitter region is lowered and the impurity concentration in the base region is raised, the emitter injection efficiency does not decrease.

In other words, in SiGe-HBT, S
Utilizing the narrow band gap property of iGe, Si-B
Compared with T, a high current amplification factor can be realized without doping the emitter region with a high concentration.

The band gaps of Si and Ge are about 1.1 eV and about 0.7 eV, respectively. In the case of a SiGe film having a Ge content of 10 to 15%, the band gap is 1. It will be about 0 eV. For this reason,
By monotonically increasing the Ge content in the SiGe film 108 from the emitter side to the collector side (gradient composition), the energy band gap Eg goes from the emitter side to the collector side as shown by the solid line in FIG. Becomes a continuously decreasing slope structure. Therefore, a built-in electric field E represented by the following formula (1) E (eV) = (1.1-1.0) / qW (1) (q: charge amount, W: base width) is generated in the base layer. However, the minority carriers injected from the emitter to the base can be accelerated by the electric field E. Therefore, as compared with the conventional Si-BT in which the minority carriers travel in the base region only by diffusion, it is possible to easily realize high-speed operation.

[0012]

However, the conventional SiGe-HBT has the following drawbacks.

FIG. 10 is a diagram showing the impurity concentration distribution in the depth direction and the Ge content change in the XX line cross section shown in FIG. As shown in the figure, the SiGe film 108 is partitioned into a SiGe buffer layer 108x, which is an undoped layer, and a SiGe gradient composition layer 108y into which a high concentration P-type impurity is introduced and the bandgap continuously changes. .
The P-type impurity diffusion region 132 serving as the base region is formed of SiG.
Although formed on the e film 108, the N-type impurity diffusion region 131 serving as an emitter region is formed from the Si film 109 to the S film.
It is formed over a part of the iGe film 108. That is, the P-type impurity diffusion region 132 and the N-type impurity diffusion region 1
31 and overlap. The reason is that during the heat treatment for forming the emitter region, the N-type impurities reach not only the emitter polysilicon film 113 but also the Si film 109 and a part of the SiGe film 108 thereunder.

FIG. 11 is a diagram for explaining variations in the diffusion depth of N-type impurities in the SiGe gradient composition layer 108y. As shown in the figure, depending on the heat treatment conditions, the EB
Since the diffusion depth of the N-type impurity that defines the junction 133 also greatly varies, it is very difficult to precisely control the position of the EB junction 133 and eliminate the variation in the position.

As described above, in the conventional SiGe-HBT, since the diffusion depth of the emitter region changes, the Ge content in the EB junction 133 substantially changes. As a result, for example, G in the EB joint 133
When the e content increases, the bandgap at the EB junction 133 becomes smaller, so that the collector current increases and, as a result, the current amplification factor increases. That is, since a slight change in the Ge content in the EB junction 133 causes a large change in the current amplification factor, it is difficult to obtain a transistor with a constant current amplification factor and less variation.

In particular, when the base layer is thinned in order to increase the speed of the transistor, the gradient of the change in the Ge content increases, so that the variation in the current amplification factor due to the subtle change in the position of the EB junction 133 is caused. The effect becomes noticeable.

On the other hand, the EB junction portion 133 has the SiGe film 10.
If it is formed in the Si film 109 instead of 8
Since the junction portion 133 deviates from the heterojunction portion and the effect of the narrow band gap is lost, a high current amplification factor cannot be obtained, and the original effect of the heterojunction cannot be obtained.

Further, in the conventional SiGe HBT,
There is also a problem that it is difficult to control the concentration of boron (B) in the base region.

For example, when B ₂ H ₆ is used as the source gas of B, even if the flow rate of B ₂ H ₆ is constant during the deposition of the SiGe film, the content of Ge in the film decreases as the Ge content decreases. It is known that the B concentration of B decreases (the B incorporation rate decreases). That is, in the case of having a gradient composition of SiGe, a gradient of B concentration occurs in the same direction as the gradient of Ge.

FIG. 12 is a diagram showing the impurity concentration distribution in the depth direction and the Ge content change in the conventional SiGe-HBT when the flow rate of B ₂ H ₆ is constant. As shown in the figure, the B concentration of the P-type impurity diffusion region 132 decreases as the Ge content of the SiGe gradient composition layer 108y decreases. When the position of the EB junction 133 is varied, the B concentration of the EB junction 133 is also greatly varied, and as a result, the EB breakdown voltage characteristic is also greatly varied.

By the way, in SiGe-HBT,
The base resistance is reduced by increasing the B concentration in the base region (in some cases, higher than the N-type impurity concentration in the emitter region) as compared with normal Si-BJT. On the other hand, if the B concentration in the base region is too high, the EB breakdown voltage is lowered. Therefore, the B concentration needs to be adjusted so that the base resistance and the EB breakdown voltage fall within a desired range. However, in the SiGe-HBT, if the variation in the EB breakdown voltage is large, the design margin is narrowed, and it may be difficult to adjust the B concentration in order to keep the base resistance and the EB breakdown voltage within a desired range.

On the other hand, the Ge content in the SiGe film is
Is controlled by the flow rate of the source gas (for example, GeH ₄ ) for use, so the Ge content rate is reduced (the GeH ₄ flow rate is reduced).
To compensate for the reduction of a captured amount of B due to, B ₂
By increasing the flow rate of H ₆ , B in the SiGe film is increased.
It is possible in principle to keep the concentration constant. However, in reality, the GeH ₄ flow rate and the Ge content in the SiGe film are not proportional to each other, and the B ₂ H ₆ flow rate and the Si content are not proportional to each other.
Since it is not proportional to the B concentration in the Ge film, such control of the gas flow rate causes the process to be complicated.

The main object of the present invention is to provide a heterojunction bipolar transistor having a structure in which the bandgap is inclined for carrier acceleration, even if the position of the EB junction changes during manufacturing, even if the position of the EB junction changes. By taking measures to suppress the variation of
It is to stably exhibit a high current amplification factor while maintaining high-speed operation. Further, in the present invention, the variation of the EB breakdown voltage is suppressed while avoiding the complication of the process.

[0024]

A first semiconductor device of the present invention comprises a substrate having a first semiconductor layer and a band which is provided on the first semiconductor layer and has a band higher than that of the first semiconductor layer. A second semiconductor layer having a small gap and made of a mixed crystal semiconductor; and a third semiconductor layer provided on the second semiconductor layer and having a band gap larger than that of the second semiconductor layer. At least a part of the first semiconductor layer is a collector region containing an impurity of the first conductivity type,
, Which is a base region containing impurities of the second conductivity type, and at least a part of the third semiconductor layer is an emitter region containing impurities of the first conductivity type, which functions as a heterojunction bipolar transistor. In the device, the second semiconductor layer has a graded composition layer having a composition in which a band gap increases in a direction from the collector region to the emitter region, and a rate of change of the band gap is a band gap of the graded composition layer. An upper layer having a composition smaller than the rate of change, and an emitter-base junction is formed in the upper layer of the second semiconductor layer.

Since the emitter / base junction is formed in the upper layer of the second semiconductor layer made of a mixed crystal semiconductor, a high current amplification factor due to the narrow band gap can be exhibited. In addition, since the upper layer of the second semiconductor layer has a smaller bandgap change rate than the graded composition layer, even if the introduction range of the first conductivity type impurity for forming the emitter region varies, the emitter-base junction portion may be changed. The variation of the bandgap in the area is reduced, and the variation range of the characteristics of the bipolar transistor such as the variation of the current amplification factor can be suppressed.

Since the composition of the mixed crystal semiconductor in the upper layer of the second semiconductor layer is substantially constant and the band gap in the upper layer is substantially constant, impurities of the first conductivity type for forming the emitter region are formed. Even if the introduction range of the bipolar transistor fluctuates, the fluctuation of the characteristics of the bipolar transistor can be suppressed to be smaller.

The composition of the mixed crystal semiconductor in the upper layer of the second semiconductor layer changes substantially continuously so that the band gap of the upper layer increases in the direction from the collector region to the emitter region. Due to the change, the carrier acceleration function by the built-in electric field can be more effectively exhibited in the entire base layer.

On the upper layer, the second semiconductor layer has a larger bandgap in the direction from the collector region to the emitter region, and its change rate is higher than the bandgap change rate of the upper layer. Of the second semiconductor layer and the third semiconductor layer
Since the lattice strain due to the difference in lattice constant at the boundary with the semiconductor layer becomes smaller, the generation of crystal defects due to the lattice strain in the second and third semiconductor layers can be suppressed.

The second semiconductor layer is a SiGe layer, the third semiconductor layer is a Si layer, and the second semiconductor layer is a SiGe layer.
When the Ge content in the upper layer of the semiconductor layer is 2% or more, it is 8
Within the range of not more than%, it becomes easy to realize both the high current amplification factor due to the narrow band gap of the second semiconductor layer made of a mixed crystal semiconductor and the high speed base running due to the gradient composition.

The second semiconductor layer is a SiGe layer,
Since the third semiconductor layer is a Si layer and the change rate of the Ge content in the upper layer of the second semiconductor layer is within 4%, the mixed crystal semiconductor is suppressed while suppressing the generation of crystal defects. It is easy to realize a high current amplification factor due to the narrow band gap of the second semiconductor layer made of.

The second semiconductor layer is made of Si, Ge and C.
May be a ternary mixed crystal semiconductor layer, and the third semiconductor layer may be a Si layer.

The emitter-base junction is the second
It is preferable that the semiconductor layer is located substantially in the center of the upper layer.

The impurity concentration in the graded composition layer of the second semiconductor layer decreases as the band gap increases in the direction from the collector region to the emitter region.
Since the impurity concentration in the upper layer of the semiconductor layer is substantially constant, it is possible to suppress variations in the EB breakdown voltage.

Since the second semiconductor layer is a SiGe layer and the impurity in the second semiconductor layer is boron (B), the remarkable effects of the present invention can be exhibited.

[0035]

(First Embodiment) FIG. 1 is a sectional view of a SiGe-HBT according to a first embodiment of the present invention. As shown in the figure, the SiGe-H of this embodiment is
The BT is formed using the Si substrate 11 and the Si epitaxial layer 12 epitaxially grown on the Si substrate 11. In addition, the N ⁺ type buried layer 20 provided over each part of the Si substrate 11 and the Si epitaxial layer 12 and the part provided with a relatively high concentration of N type impurities are introduced into the part of the Si epitaxial layer 12. and a N ⁺ collector lead layer 21 that is, the portion except the N ⁺ collector lead layer 21 of the Si epitaxial layer 12 is N includes a low concentration N-type impurity ^- has a collector diffusion layer 22. In addition, the LOCOS isolation 26 that partitions the Si epitaxial layer 12 into each bipolar transistor formation region is provided.
And a deep trench isolation 27 extending below the LOCOS isolation 26 and reaching the Si substrate 11.
However, in the bipolar transistor formation region,
N ⁺ collector extraction layer 21 and N ⁻ collector diffusion layer 22
The deep trench isolation 27 is not provided below the LOCOS isolation 26 that partitions the and.

Further, N ^{− of the} Si epitaxial layer 12
SiG containing P-type impurities is formed on the collector diffusion layer 22.
A SiGe film 18 made of an e-mixed crystal semiconductor layer and a Si film 19 made of a cap layer are formed by epitaxial growth. In addition, the SiGe film 18 and the Si film 1
P ⁺ base polysilicon film 2 formed on the region extending from the side surface of Si 9 to the upper surface of Si film 19 and containing a high concentration of P-type impurities.
4 and an N ⁺ emitter polysilicon film 23 which is provided on the opening formed in the P ⁺ base polysilicon film 24 and contains a high concentration of N-type impurities. However, the P ⁺ base polysilicon film 24 and the N ⁺ emitter polysilicon film 2
3 is electrically isolated from each other by an insulating film.

Here, the SiGe film 18 and the Si film 19
Is epitaxially grown using the MBE method, the UHV-CVD method or the LP-CVD method. And Si
In the region immediately below the N ⁺ polysilicon film 23 of the film 19, N ⁺ emitter poly silicon film 23 is diffused by RTA N-type impurity (phosphorus, arsenic, etc.) is doped. That is, the N ⁺ -type region of the Si film 19 mainly functions as the emitter region of the NPN bipolar transistor, and the P ⁺ -type region of the SiGe film 18 mainly serves as the NP region.
Functions as the base region of the N bipolar transistor,
N ⁻ collector diffusion layer 22, N ⁺ type buried layer 20, and N
^{+ The} collector extraction layer 21 functions as the collector region of the NPN bipolar transistor.

In the manufacturing process of the semiconductor device, Si
After the SiGe film 18 is epitaxially grown on the epitaxial layer 12, the Si film 19 is formed on the SiGe film 18.
Are continuously epitaxially grown. The Si film 19 is
Mainly in the post-process after SiGe epitaxial growth,
It is necessary to prevent Ge contamination of the manufacturing line, but a heat treatment for diffusing N-type impurities according to the thickness of the Si film 19 and the concentration of N-type impurities in the emitter polysilicon film 23. By properly selecting the conditions, the emitter / base junction (hereinafter referred to as “EB junction”) can be formed at a desired depth position of the Si film 19.

FIG. 2 shows S according to the first embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (refer FIG. 1) of iGe-HBT. As shown in the figure, in the present embodiment, Si
The Ge film 18 is a SiGe buffer layer 18x which is an undoped layer, and SiG in which the Ge content changes monotonously and continuously.
e Gradient composition layer 18a and SiG having a substantially constant Ge content
e upper layer 18b. This point
This is a feature of this embodiment. That is, in the SiGe gradient composition layer 18a, the Ge content is the minimum value (for example, 2%).
Value of ˜8%), increases in the direction from the emitter region to the collector region, and reaches the maximum value of Ge content when reaching the SiGe buffer layer 18x (for example, 20% to 30%).
Value). That is, in the SiGe graded composition layer 18a, the band gap decreases in the direction from the emitter region to the collector region, thereby accelerating the carriers. And the SiGe upper layer 1
In 8b, the Ge content is almost constant.

On the other hand, the P-type impurity diffusion region 3 serving as the base
2 is the SiGe gradient composition layer 18a and the SiGe upper layer 18
It is formed over b. The N-type impurity diffusion region 31 is formed over the Si film 19 and a part of the SiGe upper layer 18b. That is, the EB junction 33, which is the boundary between the P-type impurity diffusion region 32 and the N-type impurity diffusion region 31, is configured to exist at one site in the SiGe upper layer 18b. In other words, during the heat treatment for forming the emitter region, the N-type impurities are not only from the emitter polysilicon film 23 to the Si film 19 but also to the Si below the Si film 19.
Although it reaches the Ge film 18, even if the position of the EB junction 33 changes due to fluctuations and variations in manufacturing process conditions,
The thicknesses of the SiGe gradient composition layer 18a, the SiGe upper layer 18b, and the Si film 19 are set so that the EB junction portion 33 is located at one site in the SiGe upper layer 18b.

That is, due to fluctuations and variations in manufacturing process conditions, between manufacturing lots, between wafers, and within a wafer surface,
The diffusion depth of the N-type impurities from the N ⁺ -type emitter polysilicon film 23 varies, but the variation range can be empirically known from the process conditions. If the thickness of the
It can be set so that the B junction 33 is almost certainly formed in the SiGe upper layer 18b.

In particular, the N-type impurity is removed from the emitter polysilicon film 23 at the center position in the thickness direction of the SiGe upper layer 18b.
It is preferable to set so that the center of the variation range of the depth of diffusion from is substantially coincident with. In addition, since the upper limit of the Ge content in the SiGe film is generally around 30%, S
In order to realize both the high current amplification factor due to the narrow band gap of iGe and the high-speed base traveling due to the gradient composition, the Ge content ratio of the SiGe upper layer 18b is
It is preferably in the range of 2 to 8%.

According to the present embodiment, even if the position of the EB bonding portion 33 changes due to fluctuations or variations in the manufacturing process conditions, the EB bonding portion 33 is formed in the SiGe upper layer 18b in which the Ge content is almost constant. Therefore, the Ge content in the EB joint portion 33 is kept substantially constant. Therefore, the SiGe-H of the present embodiment is
BT, by EB junction 33 is present in the SiGe upper layer 18b, while maintaining a high current amplification factor h _FE due to the narrow band gap, it is possible to exert a relatively stable current amplification factor.

The distribution state of the Ge content is SiG.
By controlling the partial pressure ratio of each source gas of Ge and Si (for example, GeH ₄ and SiH ₄ ) during the epitaxial growth of the e layer, it is possible to easily control to an arbitrary pattern.

Further, in place of the Si film 19 formed on the SiGe film 18, a small amount of Ge containing S is contained.
Even if an iGe film is provided, the same effect as this embodiment can be exhibited.

(Second Embodiment) In the first embodiment, the minority carriers (electrons) injected from the emitter region to the base region are the regions (SiGe gradient) of the base region where the Ge content changes. While traveling in the SiGe upper layer 18b until reaching the composition layer 18a), the SiGe upper layer 18b moves only by diffusion, and during that time, the effect of acceleration by the built-in electric field cannot be obtained, which is slightly disadvantageous from the viewpoint of high-speed operation. .

SiGe-HB of the second embodiment of the present invention
T is provided with means for eliminating this disadvantage in the first embodiment. That is, the present invention relates to a structure for suppressing the variation of the current amplification factor caused by the variation of the EB junction position while maintaining the minority carrier acceleration function by the built-in electric field in the entire base region.

FIG. 3 shows the S according to the second embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (refer FIG. 1) of iGe-HBT. The basic structure of the SiGe-HBT of this embodiment is as shown in FIG. 1 in the first embodiment.

As shown in FIG. 3, the Si according to the present embodiment is
In the Ge-HBT, the Ge content in the SiGe upper layer 18b is not constant, but gradually increases in the direction from the emitter region to the collector region. That is,
The band gap in the SiGe upper layer 18b gradually decreases in the direction from the emitter region to the collector region, which enhances the carrier acceleration function. The EB junction 33 is formed in the SiGe upper layer 18b, and the G of the SiGe gradient composition layer 18a is formed.
Similar to the first embodiment, the e content changes greatly in the direction from the collector region to the emitter region.

Also in the present embodiment, even if the position of the EB junction 33 is changed due to the variation or variation of the manufacturing process conditions, the EB junction 33 is located at one site in the SiGe upper layer 18b, and the SiGe gradient composition is obtained. Layer 18a, SiG
e The thicknesses of the upper layer 18b and the Si film 19 are set respectively.

Then, due to fluctuations and variations in manufacturing process conditions, between manufacturing lots, between wafers, and within a wafer surface,
The diffusion depth of the N-type impurities from the N ⁺ -type emitter polysilicon film 23 varies, but the variation range can be empirically known from the process conditions. Therefore, the variation range of the diffusion depth of the N-type impurities is larger than that of the SiGe upper layer 18b. If the thickness of the
It can be set so that the B junction 33 is almost certainly formed in the SiGe upper layer 18b.

In particular, the central position in the thickness direction of the SiGe upper layer 18b and the N-type impurities are the emitter polysilicon film 23.
It is preferable to set so that the center of the variation range of the depth of diffusion from is substantially coincident with. Since the upper limit of the Ge content in the SiGe film is generally around 30%, S
In order to realize both the high current amplification factor due to the narrow band gap of iGe and the high-speed base traveling due to the gradient composition, the Ge content ratio of the SiGe upper layer 18b is
It is preferable that the content changes in the range of 2 to 8%, and the change rate of the content rate is within 4%.

In this embodiment, as in the first embodiment described above, the EB junction 33 can be formed in the SiGe upper layer 18b to maintain the high current amplification factor h _FE due to the narrow band gap. Further, even if the diffusion depth of the N-type impurity from the emitter polysilicon film 23 changes,
Since the change in the Ge content in the SiGe upper layer 18b is relatively small, the EB junction 33 of the SiGe upper layer 18b is changed.
It is possible to suppress the variation in the Ge content rate due to the change in the position of. Moreover, when minority carriers (electrons) are injected into the SiGe upper layer 18b, SiG having a graded composition is obtained.
e When traveling in the upper layer 18b, the function of accelerating the minority carriers by the built-in electric field can be obtained.
The Ge-HBT can further improve the operation speed as compared with the first embodiment.

In the present embodiment, there is a bending point in the change line of the Ge content in the boundary between the SiGe gradient composition layer 18a and the SiGe upper layer 18b in the SiGe film 18, but at the boundary between the two, the Ge is changed. By continuously changing the content rate, it is also possible to prevent a bending point from occurring in the Ge content rate change line. With such a structure, the function of accelerating carriers by the built-in electric field is more effectively exhibited.

(Third Embodiment) In the SiGe-HBT of the third embodiment of the present invention, the variation of the current amplification factor caused by the variation of the position of the EB junction is the same as in the first and second embodiments. While further suppressing the above, a means for suppressing the generation of crystal defects between the SiGe film and the Si film is provided.

FIG. 4 shows the S according to the third embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (refer FIG. 1) of iGe-HBT. The basic structure of the SiGe-HBT of this embodiment is as shown in FIG. 1 in the first embodiment.

In the SiGe-HBT according to this embodiment, the SiGe film 18 is the SiGe gradient composition layer 18a.
And a SiGe upper layer 18b, and a SiGe uppermost layer 18c. And the SiGe upper layer 18b
Has a substantially constant Ge content, and the SiGe uppermost layer 18
The Ge content of c rapidly decreases in the direction from the collector region to the emitter region and becomes almost 0 in the portion adjacent to the Si film 19. That is, the Ge content in the SiGe uppermost layer 18c sharply decreases in the direction from the emitter region to the collector region, and the upper end of the SiGe uppermost layer 18c has the same bandgap as Si. The EB junction 33 is formed in the SiGe upper layer 18b. Further, the P-type impurity (boron) is S
It is introduced into the iGe gradient composition layer 18a and the SiGe upper layer 18b, but is hardly introduced into the SiGe uppermost layer 18c. As in the first and second embodiments, the Ge content of the SiGe gradient composition layer 18a increases in the direction from the emitter region toward the collector region.

Also in the present embodiment, even if the position of the EB junction 33 is changed due to the variation or variation of the manufacturing process conditions, the EB junction 33 is located at one site in the SiGe upper layer 18b, and the SiGe gradient composition is obtained. Layer 18a, SiG
e upper layer 18b, SiGe uppermost layer 18c, and Si film 1
9 thicknesses are set respectively.

Then, due to fluctuations and variations in manufacturing process conditions, between manufacturing lots, between wafers, and within a wafer surface,
The diffusion depth of the N-type impurities from the N ⁺ -type emitter polysilicon film 23 varies, but the variation range can be empirically known from the process conditions. Therefore, the variation range of the diffusion depth of the N-type impurities is larger than that of the SiGe upper layer 18b. If the thickness of the
It can be set so that the B junction 33 is almost certainly formed in the SiGe upper layer 18b.

Also in this embodiment, the SiGe upper layer 1
It is preferable to set the center position of 8b in the thickness direction and the center of the variation range of the depth in which the N-type impurity diffuses from the emitter polysilicon film 23 substantially coincide with each other. In addition,
Generally, since the upper limit of the Ge content in the SiGe film is around 30%, in order to realize both the high current amplification factor due to the narrow band gap of SiGe and the speeding up of the base running due to the gradient composition, the SiGe upper part is required. The Ge content of the layer 18b is preferably in the range of 2% or more and 8% or less.

Similar to the first embodiment, in this embodiment, the Ge content in the EB bonding portion 33 is substantially constant, so that the same effect as in the first embodiment can be obtained. In addition, the Ge content in the SiGe uppermost layer 18c continuously decreases upward,
Since it is 0 in the portion adjacent to the film 19, the following effects can be exerted.

In the case of the first and second embodiments, SiG
G at the upper end of the e film 18 (SiGe upper layer 18b)
The e content has a constant value instead of 0. However, since the lattice constants of Si and Ge differ from each other by about 4%, there is a lattice mismatch between the SiGe upper layer 18b and the Si film 19, and as a result, the Si film 19 and the SiGe film 18 are formed.
There is a risk that crystal defects may occur due to lattice distortion.

On the other hand, in the present embodiment, Si
The upper end portion of the Ge film 18 (SiGe uppermost layer 18c) and Si
Since there is almost no lattice mismatch with the film 19,
It is possible to effectively suppress the occurrence of crystal defects due to lattice distortion in the SiGe film 18 and the Si film 19.

-Modification- FIG. 5 shows a SiG according to a modification of the third embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (refer FIG. 1) of e-HBT. The basic structure of the SiGe-HBT of this modification is as shown in FIG. 1 in the first embodiment.

In this modification, the Ge content in the SiGe upper layer 18b is not constant and gradually increases in the direction from the emitter region to the collector region, so that the function of accelerating carriers due to the decrease in band gap is enhanced. Is configured. Then, the EB joint 33 is made of Si.
It is formed in the Ge upper layer 18b, and the Ge content of the SiGe uppermost layer 18c sharply decreases in the direction from the collector region to the emitter region and becomes almost 0 in the portion adjacent to the Si film 19. This is the same as in the third embodiment.

In this modification, in addition to the same effect as the third embodiment described above, a small number of electrons injected from the emitter region to the base region by the built-in electric field in the SiGe upper layer 18b as in the second embodiment. The acceleration function for carriers (electrons) can be improved.

In order to realize both the high current amplification factor due to the narrow band gap of SiGe and the speeding up of base running due to the graded composition while suppressing the generation of crystal defects, the Ge content ratio of the SiGe upper layer 18b is set to It is preferable that the range is 2% or more and 8% or less, and the change width of the Ge content in the SiGe upper layer 18b is 4% or less.

The Ge content in the portion of the SiGe uppermost layer 18c in contact with the Si film 19 in the third embodiment or its modification does not necessarily have to be 0, and if it is a value close to 0, it will depend on the lattice strain. Although the effect of suppressing the generation of crystal defects can be obtained, in order to more effectively exert the effect of the present embodiment, the Ge content of the portion of the SiGe uppermost layer 18c adjacent to the Si film 19 is 0. It is preferable.

(Fourth Embodiment) As in the first to third embodiments, the SiGe-HBT of the fourth embodiment of the present invention has a variation in the current amplification factor caused by a change in the position of the EB junction. While further suppressing the above, a means for suppressing the variation in the EB breakdown voltage is further provided.

FIG. 6 shows an S according to the fourth embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (refer FIG. 1) of iGe-HBT. The basic structure of the SiGe-HBT of this embodiment is as shown in FIG. 1 in the first embodiment.

As shown in the figure, in the present embodiment, the SiGe film 18 is composed of the SiGe buffer layer 18x which is an undoped layer, the SiGe gradient composition layer 18a whose Ge content changes monotonously and continuously, and the Ge layer. It is composed of the SiGe upper layer 18b having a substantially constant content. Then, in the SiGe graded composition layer 18a, the Ge content becomes the minimum value (for example, a value of 2% to 8%) and continuously increases in the direction from the emitter region to the collector region to form the SiGe buffer layer 18x. When it reaches, the Ge content becomes the maximum value (for example, a value of 20% to 30%). Then, in the SiGe upper layer 18b, the Ge content is almost constant.

The P-type impurity diffusion region 3 serving as the base
2 is the SiGe gradient composition layer 18a and the SiGe upper layer 18
It is formed over b. The N-type impurity diffusion region 31 is formed over the Si film 19 and a part of the SiGe upper layer 18b. That is, the EB junction 33, which is the boundary between the P-type impurity diffusion region 32 and the N-type impurity diffusion region 31, is configured to exist at one site in the SiGe upper layer 18b. In other words, during the heat treatment for forming the emitter region, the N-type impurities are not only from the emitter polysilicon film 23 to the Si film 19 but also to the Si below the Si film 19.
Although it reaches the Ge film 18, even if the position of the EB junction 33 changes due to fluctuations and variations in manufacturing process conditions,
The thicknesses of the SiGe gradient composition layer 18a, the SiGe upper layer 18b, and the Si film 19 are set so that the EB junction portion 33 is located at one site in the SiGe upper layer 18b.

That is, due to fluctuations and variations in manufacturing process conditions, between manufacturing lots, between wafers, and within a wafer surface,
The diffusion depth of the N-type impurities from the N ⁺ -type emitter polysilicon film 23 varies, but the variation range can be empirically known from the process conditions. Therefore, the variation range of the diffusion depth of the N-type impurities is larger than that of the SiGe upper layer 18b. If the thickness of the
It can be set so that the B junction 33 is almost certainly formed in the SiGe upper layer 18b.

However, in this embodiment, SiG is used.
As the Ge content in the SiGe gradient composition layer 18a of the e film 18 greatly decreases toward the emitter side, SiG
The B concentration content in the e-gradient composition layer 18a is also reduced.
However, since the Ge content is substantially constant in the SiGe upper layer 18b, the B concentration in the SiGe upper layer 18b is kept substantially constant.

Also in this embodiment, the SiGe upper layer 1 is formed.
It is preferable to set the center position of 8b in the thickness direction and the center of the variation range of the depth in which the N-type impurity diffuses from the emitter polysilicon film 23 substantially coincide with each other.

In the present embodiment, as in the first embodiment, the Ge content in the EB junction portion 33 is substantially constant, so that the diffusion depth of the N-type impurity is the same as in the first embodiment. In consideration of the variation of the EB junction 33 in the SiGe upper layer 18b in which the Ge content is almost constant.
It is easy to adjust the film thickness of each layer so that In addition, since the B concentration in the SiGe upper layer 18b is maintained substantially constant, it is possible to maintain the B concentration Ge content rate in the EB junction 33 substantially constant even if the manufacturing process conditions fluctuate or vary. it can. Therefore, it is possible to suppress variations in the EB breakdown voltage.

On the other hand, as the Ge content decreases (the GeH ₄ flow rate decreases), the amount of B taken in is compensated for by B ₂
By increasing the flow rate of H ₆ , B in the SiGe film is increased.
It is possible in principle to keep the concentration constant,
This method complicates the process.

On the other hand, in the case of this embodiment, Si
Since it suffices to keep the flow rate of the source gas for B (B ₂ H ₆ ) at the time of forming the Ge film substantially constant, it is possible to reliably suppress the variation in the EB breakdown voltage by a simple process.

-Modification- FIG. 7 shows a SiG according to a modification of the fourth embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (refer FIG. 1) of e-HBT. The basic structure of the SiGe-HBT of this modification is as shown in FIG. 1 in the first embodiment.

In this modification, the Ge content in the SiGe upper layer 18b is not constant and gradually increases in the direction from the emitter region to the collector region, so that the function of accelerating carriers due to the decrease in the band gap is enhanced. Is configured. Then, the EB joint 33 is made of Si.
The fact that it is formed in the Ge upper layer 18b is the same as in the fourth embodiment.

In the present modification, in addition to the same effect as in the above-described fourth embodiment, a small number of electrons injected from the emitter region to the base region due to the built-in electric field in the SiGe upper layer 18b are used as in the second embodiment. The acceleration function for carriers (electrons) can be improved.

On the other hand, in the SiGe upper layer 18b,
Although the B concentration also changes as the Ge content changes, SiGe
Since the change in the Ge content in the upper layer 18b is smaller than the change in the Ge content in the SiGe gradient composition layer 18a, S
The change in B concentration in the iGe upper layer 18b is also small. Therefore, the variation of the EB breakdown voltage can also be suppressed by this modification.

In the fourth embodiment or its modification, the SiGe uppermost layer 18c having the graded composition of Ge as in the third embodiment can be provided.

(Other Embodiments) In the above-mentioned first to fourth embodiments, the present invention is applied to a bipolar transistor (SiGe-HBT) having a Si / SiGe heterojunction.
However, the present invention can be applied to Si / S
Even when applied to a bipolar transistor having an iGeC or SiGe / SiGeC heterojunction, the above first to fourth
The same effect as that of the above embodiment can be exhibited. In that case, the base layer is a ternary mixed crystal semiconductor layer containing Si, Ge, and carbon (C).

Further, the present invention is not limited to the HBT having the mixed crystal semiconductor layer as in the first to fourth embodiments, but a compound semiconductor layer containing, for example, indium (In), gallium (Ga) and P. Even when applied to the HBT having, the same effects as those of the above-described first to fourth embodiments can be exhibited.

[0086]

According to the semiconductor device of the present invention, in the heterojunction bipolar transistor having a structure in which the bandgap of the base region is inclined for carrier acceleration, even if the position of the EB junction changes, It is possible to suppress variations in band gap between the parts, and thus it is possible to stably exhibit a high current amplification factor while maintaining high-speed operation.

[Brief description of drawings]

FIG. 1 is a SiGe-H according to a first embodiment of the present invention.
It is sectional drawing of BT.

FIG. 2 is a SiGe-HB according to the first embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (FIG. 1) of T.

FIG. 3 is a SiGe-HB according to a second embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (FIG. 1) of T.

FIG. 4 is a SiGe-HB according to a third embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (FIG. 1) of T.

FIG. 5 is a SiG according to a modification of the third embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (FIG. 1) of e-HBT.

FIG. 6 is a SiGe-HB according to a fourth embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (FIG. 1) of T.

FIG. 7 is a SiG according to a modification of the fourth embodiment of the present invention.
It is a figure which shows the impurity concentration distribution and Ge content change of the depth direction in the Ia-Ia line cross section (FIG. 1) of e-HBT.

FIG. 8 is a cross-sectional view of a conventional SiGe-HBT.

FIG. 9: SiGe-HBT and Si-B having a graded composition
It is an energy band diagram for comparing the band structure with T.

10 is a diagram showing an impurity concentration distribution in the depth direction and a Ge content change in a cross section taken along line XX shown in FIG.

FIG. 11 is a diagram for explaining variations in diffusion depth of N-type impurities in the SiGe graded composition layer.

FIG. 12 is a diagram showing the impurity concentration distribution in the depth direction and the Ge content change in the conventional SiGe-HBT when the flow rate of B ₂ H ₆ is constant.

[Explanation of symbols]

11 Si substrate 12 Si epitaxial layer 18 SiGe film 18a SiGe gradient composition layer 18b SiGe upper layer 18c SiGe uppermost layer 18x SiGe buffer layer 19 Si film 20 N ⁺ type buried layer 21 N ⁺ type collector extraction layer 22 N ⁻ type collector diffusion Layer 23 N ⁺ type emitter polysilicon film 24 P ⁺ type base polysilicon film 26 LOCOS isolation 27 deep trench isolation 31 N type impurity diffusion region 32 P type impurity diffusion region 33 EB junction

Claims (10)

[Claims] 1. A substrate having a first semiconductor layer, and a second semiconductor provided on the first semiconductor layer, having a bandgap smaller than that of the first semiconductor layer, and made of a mixed crystal semiconductor. A semiconductor layer; and a third semiconductor layer provided on the second semiconductor layer and having a bandgap larger than that of the second semiconductor layer, wherein at least a part of the first semiconductor layer is the first semiconductor layer. A collector region containing a conductivity type impurity, at least a part of the second semiconductor layer being a base region containing a second conductivity type impurity, and a part of the third semiconductor layer containing a first conductivity type impurity. A semiconductor device that functions as a heterojunction bipolar transistor that is an emitter region including the second semiconductor layer, the bandgap of which increases in a direction from the collector region to the emitter region. And a top layer having a composition in which the rate of change of the bandgap is smaller than the rate of change of the bandgap of the graded composition layer, and the emitter-base junction has the second semiconductor layer of A semiconductor device characterized by being formed in an upper layer. 2. The semiconductor device according to claim 1, wherein the composition of the mixed crystal semiconductor in the upper layer of the second semiconductor layer is substantially constant, and the band gap in the upper layer is substantially constant. Characteristic semiconductor device. 3. The semiconductor device according to claim 1, wherein the composition of the mixed crystal semiconductor in the upper layer of the second semiconductor layer changes substantially continuously, and the band gap of the upper layer is the collector. The semiconductor device is characterized in that it is changed so as to increase in the direction from the region to the emitter region. 4. The semiconductor device according to claim 1, wherein the second semiconductor layer has a band gap that increases in a direction from the collector region to the emitter region on the upper layer. A semiconductor device further comprising an uppermost layer having a rate of change larger than a rate of change of the band gap of the upper layer. 5. The semiconductor device according to claim 4, wherein the second semiconductor layer is a SiGe layer, the third semiconductor layer is a Si layer, and the second semiconductor layer is an upper layer of the second semiconductor layer. Ge content is 2%
The semiconductor device is in the range of 8% or less. 6. The semiconductor device according to claim 1, wherein the second semiconductor layer is a SiGe layer, and the third semiconductor layer is a Si layer. 2. The semiconductor device characterized in that the variation rate of the Ge content in the upper layer of the second semiconductor layer is within 4%. 7. The semiconductor device according to claim 1, wherein the second semiconductor layer is a ternary mixed crystal semiconductor layer containing Si, Ge and C. The semiconductor device, wherein the third semiconductor layer is a Si layer. 8. The semiconductor device according to claim 1, wherein the emitter-base junction is located substantially in the center of the upper layer of the second semiconductor layer. A semiconductor device characterized in that 9. The semiconductor device according to claim 1, wherein an impurity concentration in the graded composition layer of the second semiconductor layer is in a direction from the collector region to the emitter region. The semiconductor device is characterized in that the impurity concentration in the upper layer of the second semiconductor layer is substantially constant and decreases as the band gap increases. 10. The semiconductor device according to claim 9, wherein the second semiconductor layer is a SiGe layer, and the impurity in the second semiconductor layer is boron (B). Semiconductor device.