JP3779214B2 - Semiconductor device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device that functions as a heterojunction bipolar transistor, and more particularly, to a countermeasure for suppressing variation in characteristics such as current amplification factor.
[0002]
[Prior art]
In recent years, bipolar transistors formed using a silicon substrate have been miniaturized and speeded up by progress in microfabrication technology and self-alignment technology. A general bipolar transistor is a so-called homojunction bipolar transistor using a silicon substrate and a silicon single crystal layer epitaxially grown on the silicon substrate.
[0003]
On the other hand, in order to achieve higher speed operation, research and development of heterojunction bipolar transistors (hereinafter referred to as “HBT”) are being actively conducted. Particularly recently, the development of an HBT (hereinafter referred to as “SiGe-HBT”) using a SiGe layer as a base layer by epitaxially growing a SiGe layer, which is a mixed crystal of silicon and germanium, on a silicon substrate has been actively conducted. Has been made.
[0004]
FIG. 8 is a cross-sectional view of a conventional SiGe-HBT. As shown in the figure, the conventional SiGe-HBT is formed using a Si substrate 101 and a Si epitaxial layer 102 epitaxially grown on the Si substrate 101. Further, N provided over each part of the Si substrate 101 and the Si epitaxial layer 102. ⁺ N-type impurity introduced into the buried type layer 110 and part of the Si epitaxial layer 102 by introducing a relatively high concentration of N-type impurities ⁺ A collector lead layer 111, and N of the Si epitaxial layer 102 ⁺ The portion excluding the collector extraction layer 111 is N containing low-concentration N-type impurities. ^- A collector diffusion layer 112 is formed. Further, a LOCOS isolation 116 that partitions the Si epitaxial layer 102 for each bipolar transistor formation region and a deep trench isolation 117 that extends below the LOCOS isolation 116 and reaches the Si substrate 101 are provided. However, in the bipolar transistor formation region, N ⁺ Collector extraction layer 111 and N ^- The deep trench isolation 117 is not provided below the LOCOS isolation 116 that partitions the collector diffusion layer 112.
[0005]
Further, N of the Si epitaxial layer 102 ^- On the collector diffusion layer 112, an SiGe film 108 made of a SiGe mixed crystal semiconductor layer containing a P-type impurity and an Si film 109 serving as a cap layer are formed by epitaxial growth. In addition, a P containing a high-concentration P-type impurity formed on a region extending from the side surface of the SiGe film 108 and the Si film 109 to the upper surface of the Si film 109. ⁺ Base polysilicon film 114 and P ⁺ N provided on the opening formed in the base polysilicon film 114 and containing high-concentration N-type impurities ⁺ And an emitter polysilicon film 113. However, P ⁺ Base polysilicon film 114 and N ⁺ The emitter polysilicon film 113 is electrically isolated from each other by an insulating film.
[0006]
Here, the SiGe film 108 and the Si film 109 are epitaxially grown using the MBE method, the UHV-CVD method, or the LP-CVD method. In the Si film 109, N ⁺ In the region immediately below the polysilicon film 113, N ⁺ N-type impurities (phosphorus, arsenic, etc.) diffused by RTA from the emitter polysilicon film 113 are doped. That is, N in the Si film 109 ⁺ The mold region functions as the emitter region of the NPN bipolar transistor, and the P region of the SiGe film 108 ⁺ The mold region functions as the base region of the NPN bipolar transistor, and N ^- Collector diffusion layer 112, N ⁺ Mold buried layer 110 and N ⁺ The collector lead layer 111 functions as a collector region of the NPN bipolar transistor.
[0007]
In the semiconductor device manufacturing process, after the SiGe film 108 is epitaxially grown on the Si epitaxial layer 102, the Si film 109 is continuously epitaxially grown on the SiGe film 108. The Si film 109 is mainly required in the subsequent process after the SiGe epitaxial growth in order to prevent Ge contamination on the production line. The film thickness of the Si film 109 and the N-type in the emitter polysilicon film 113 are used. An emitter-base junction (hereinafter referred to as “EB junction”) is formed at a desired depth position of the Si film 109 by appropriately selecting a heat treatment condition for diffusing the N-type impurity according to the impurity concentration. can do.
[0008]
The conventional SiGe-HBT formed in this way has an advantage that the emitter injection efficiency is high even if the emitter region is not doped with a high concentration, compared with a homojunction bipolar transistor consisting only of the Si layer. Current gain (h _FE ) Is expected.
[0009]
FIG. 9 is an energy band diagram for comparing the band structures of a Si / SiGe heterojunction bipolar transistor (SiGe-HBT) having a graded composition and a Si homojunction bipolar transistor (Si-BT). In SiGe-HBT, the height of the barrier against holes injected from the base region into the emitter region can be made larger than the height of the barrier against electrons injected from the emitter region into the base region. Therefore, even if the impurity concentration in the emitter region is lowered and the impurity concentration in the base region is increased, the emitter injection efficiency does not decrease.
[0010]
In other words, SiGe-HBT can realize a higher current amplification factor without doping the emitter region at a higher concentration than Si-BT by utilizing the narrow band gap property of SiGe.
[0011]
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 about 1.0 eV between Si and Ge. Become. Therefore, by increasing the Ge content in the SiGe film 108 monotonously from the emitter side toward the collector side (gradient composition), the energy band gap Eg is increased from the emitter side to the collector as shown by the solid line portion in FIG. The inclined structure is continuously reduced toward the side. Therefore, the following formula (1)
E (eV) = (1.1−1.0) / qW (1)
A built-in electric field E represented by (q: charge amount, W: base width) is generated in the base layer, and minority carriers injected from the emitter to the base can be accelerated by the electric field E. Therefore, the operation speed can be easily increased as compared with the conventional Si-BT in which minority carriers travel in the base region only by diffusion.
[0012]
[Problems to be solved by the invention]
However, the conventional SiGe-HBT has the following problems.
[0013]
FIG. 10 is a diagram showing the impurity concentration distribution in the depth direction and the Ge content rate 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 that is an undoped layer and a SiGe graded composition layer 108y in which a high-concentration P-type impurity is introduced and the band gap changes continuously. . The P-type impurity diffusion region 132 serving as the base region is formed above the SiGe film 108, while the N-type impurity diffusion region 131 serving as the emitter region is formed from the Si film 109 to a part of the SiGe film 108. Has been. That is, the P-type impurity diffusion region 132 and the N-type impurity diffusion region 131 overlap. The reason is that during the heat treatment for forming the emitter region, the N-type impurities reach not only the Si film 109 but also a part of the SiGe film 108 below the emitter polysilicon film 113.
[0014]
FIG. 11 is a diagram for explaining the variation in the diffusion depth of the N-type impurity in the SiGe graded composition layer 108y. As shown in the figure, the diffusion depth of the N-type impurity that defines the EB junction 133 varies greatly depending on the heat treatment conditions, so that the position of the EB junction 133 is precisely controlled and the variation in the position is eliminated. It is very difficult.
[0015]
As described above, in the conventional SiGe-HBT, since the diffusion depth of the emitter region varies, the Ge content in the EB junction 133 varies substantially. As a result, for example, when the Ge content in the EB junction 133 is increased, the band gap in the EB junction 133 is reduced, so that the collector current is increased, resulting in a phenomenon that the current amplification factor is increased. That is, a slight variation in the Ge content at the EB junction 133 causes a large variation in the current amplification factor, making it difficult to obtain a transistor with a constant current amplification factor and little variation.
[0016]
In particular, if the base layer is thinned in order to increase the speed of the transistor, the slope of the change in the Ge content increases, so the effect of variations in the current amplification factor due to subtle variations in the position of the EB junction 133 is significant. become.
[0017]
On the other hand, if the EB junction 133 is formed not in the SiGe film 108 but in the Si film 109, the EB junction 133 is separated from the hetero junction and the effect of the narrow band gap is lost. It cannot be obtained, and the original effect of the heterojunction cannot be obtained.
[0018]
Further, the conventional SiGeHBT has a problem that it is difficult to control the concentration of boron (B) in the base region.
[0019]
For example, B as a source gas of B ₂ H ₆ When B is used, B during the deposition of the SiGe film ₂ H ₆ It is known that the B concentration in the film decreases (the B incorporation rate decreases) as the Ge content decreases, even if the flow rate of is constant. That is, in the case of having a SiGe gradient composition, a B concentration gradient occurs in the same direction as the Ge gradient.
[0020]
FIG. ₂ H ₆ It is a figure which shows the impurity concentration distribution of the depth direction in conventional SiGe-HBT, and Ge content rate change when supposing that the flow volume of 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 graded composition layer 108y decreases. When the EB junction part 133 has a variation in position, the EB junction part 133 has a large variation in B concentration. As a result, the EB breakdown voltage characteristic varies greatly.
[0021]
By the way, in SiGe-HBT, the base resistance is increased 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. We are trying to reduce it. On the other hand, if the B concentration in the base region is too high, the EB breakdown voltage is lowered. Therefore, it is necessary to adjust the B concentration so that the base resistance and the EB breakdown voltage are within a desired range. However, in the SiGe-HBT, when the variation in the EB breakdown voltage increases, the design margin may be narrowed, and it may be difficult to adjust the B concentration to keep the base resistance and the EB breakdown voltage within a desired range.
[0022]
On the other hand, the Ge content in the SiGe film depends on the source gas for Ge (for example, GeH _Four ), The Ge content is reduced (GeH _Four In order to compensate for the decrease in the amount of B taken in with the decrease in flow rate) ₂ H ₆ In principle, it is possible to keep the B concentration in the SiGe film constant by increasing the flow rate of. However, in reality, GeH _Four The flow rate is not proportional to the Ge content in the SiGe film. ₂ H ₆ Since the flow rate of B is not proportional to the B concentration in the SiGe film, such control of the gas flow rate leads to a complicated process.
[0023]
The main object of the present invention is to provide a heterojunction bipolar transistor having a structure in which the band gap is inclined for carrier acceleration, even if the position of the EB junction varies during manufacturing, and the variation in the band gap at the EB junction is reduced. By taking measures to suppress, it is to stably exhibit a high current gain while maintaining high-speed operation. Furthermore, in the present invention, variations in the EB breakdown voltage are suppressed while avoiding complication of the process.
[0024]
[Means for Solving the Problems]
A first semiconductor device of the present invention includes a substrate having a first semiconductor layer, a band gap smaller than that of the first semiconductor layer, and a mixed crystal semiconductor provided on the first semiconductor layer. And a third semiconductor layer provided on the second semiconductor layer and having a band gap larger than that of the second semiconductor layer, wherein at least one of the first semiconductor layers is provided. A part is a collector region containing a first conductivity type impurity, at least a part of the second semiconductor layer is a base region containing a second conductivity type impurity, and at least a part of the third semiconductor layer is a first region. A semiconductor device that functions as a heterojunction bipolar transistor that is an emitter region containing one conductivity type impurity, wherein the second semiconductor layer has a band gap in a direction from the collector region toward the emitter region. And an upper layer having a composition in which the change rate of the band gap is smaller than the change rate of the band gap of the gradient composition layer, and the emitter-base junction is the second layer It is formed in the upper layer of the semiconductor layer.
[0025]
Thus, 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 band gap 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 The variation of the band gap in the transistor is reduced, and the fluctuation range of the characteristics of the bipolar transistor such as the fluctuation of the current amplification factor can be suppressed.
[0026]
The composition range 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, so that the first conductivity type impurity introduction range for forming the emitter region is obtained. Even if fluctuates, fluctuations in characteristics of the bipolar transistor can be further suppressed.
[0027]
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 changes so as to increase in the direction from the collector region to the emitter region. Therefore, the carrier acceleration function by the built-in electric field can be more effectively exhibited in the entire base layer.
[0028]
The second semiconductor layer has a band gap that is larger on the upper layer in a direction from the collector region to the emitter region, and the rate of change is larger than the rate of change of the band gap of the upper layer. By further including the upper layer, the lattice distortion caused by the difference in lattice constant at the boundary between the second semiconductor layer and the third semiconductor layer is reduced, so that the second and third semiconductor layers Generation of crystal defects due to lattice distortion can be suppressed.
[0029]
The second semiconductor layer is a SiGe layer, the third semiconductor layer is a Si layer, and the Ge content in the upper layer of the second semiconductor layer is 2% or more and 8% or less. Therefore, it is easy to realize both a high current amplification factor due to the narrow band gap of the second semiconductor layer made of a mixed crystal semiconductor and a high-speed base running due to the gradient composition.
[0030]
The second semiconductor layer is a SiGe layer, the third semiconductor layer is a Si layer, and the change width of the Ge content in the upper layer of the second semiconductor layer is within 4%, It is easy to realize a high current amplification factor due to the narrow band gap of the second semiconductor layer made of a mixed crystal semiconductor while suppressing the generation of crystal defects.
[0031]
The second semiconductor layer may be a ternary mixed crystal semiconductor layer containing Si, Ge, and C, and the third semiconductor layer may be a Si layer.
[0032]
It is preferable that the emitter-base junction is located approximately at the center of the upper layer of the second semiconductor layer.
[0033]
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, and the impurity concentration in the upper layer of the second semiconductor layer is approximately By being constant, variations in the EB breakdown voltage can be suppressed.
[0034]
The second semiconductor layer is a SiGe layer, and the impurity of the second semiconductor layer is boron (B), so that the remarkable effect of the present invention can be exhibited.
[0035]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
FIG. 1 is a cross-sectional view of a SiGe-HBT according to the first embodiment of the present invention. As shown in the figure, the SiGe-HBT of this embodiment is formed using a Si substrate 11 and a Si epitaxial layer 12 epitaxially grown on the Si substrate 11. Further, N provided over each part of the Si substrate 11 and the Si epitaxial layer 12. ⁺ N buried by introducing a relatively high concentration N-type impurity into part of the buried layer 20 and the Si epitaxial layer 12 ⁺ A collector extraction layer 21, and N of the Si epitaxial layer 12 ⁺ The portion excluding the collector extraction layer 21 is N containing low-concentration N-type impurities. ^- A collector diffusion layer 22 is formed. Further, a LOCOS isolation 26 that partitions the Si epitaxial layer 12 into each bipolar transistor formation region and a deep trench isolation 27 that extends below the LOCOS isolation 26 and reaches the Si substrate 11 are provided. However, in the bipolar transistor formation region, N ⁺ Collector extraction layer 21 and N ^- A deep trench isolation 27 is not provided below the LOCOS isolation 26 that partitions the collector diffusion layer 22.
[0036]
Further, N of the Si epitaxial layer 12 ^- On the collector diffusion layer 22, an SiGe film 18 made of a SiGe mixed crystal semiconductor layer containing a P-type impurity and an Si film 19 serving as a cap layer are formed by epitaxial growth. Further, P formed on the region extending from the side surface of the SiGe film 18 and the Si film 19 to the upper surface of the Si film 19 and containing a high concentration of P-type impurities ⁺ Base polysilicon film 24 and P ⁺ N provided on the opening formed in the base polysilicon film 24 and containing high-concentration N-type impurities ⁺ And an emitter polysilicon film 23. However, P ⁺ Base polysilicon film 24 and N ⁺ The emitter polysilicon film 23 is electrically isolated from each other by an insulating film.
[0037]
Here, the SiGe film 18 and the Si film 19 are epitaxially grown using the MBE method, the UHV-CVD method, or the LP-CVD method. In the Si film 19, N ⁺ In the region immediately below the polysilicon film 23, N ⁺ N-type impurities (phosphorus, arsenic, etc.) diffused from the emitter polysilicon film 23 by RTA are doped. That is, N in the Si film 19 ⁺ The mold region mainly functions as the emitter region of the NPN bipolar transistor, and the P region of the SiGe film 18 ⁺ The mold region mainly functions as the base region of the NPN bipolar transistor, and N ^- Collector diffusion layer 22, N ⁺ Mold buried layer 20 and N ⁺ The collector lead layer 21 functions as a collector region of the NPN bipolar transistor.
[0038]
In the semiconductor device manufacturing process, after the SiGe film 18 is epitaxially grown on the Si epitaxial layer 12, the Si film 19 is continuously epitaxially grown on the SiGe film 18. The Si film 19 is mainly required in order to prevent Ge contamination on the production line in the post-process after the SiGe epitaxial growth. The film thickness of the Si film 19 and the N-type in the emitter polysilicon film 23 are used. An emitter-base junction (hereinafter referred to as “EB junction”) is formed at a desired depth position of the Si film 19 by appropriately selecting a heat treatment condition for diffusing the N-type impurity in accordance with the impurity concentration. can do.
[0039]
FIG. 2 is a diagram showing an impurity concentration distribution in the depth direction and a change in Ge content in a cross section taken along the line Ia-Ia of the SiGe-HBT according to the first embodiment of the present invention (see FIG. 1). As shown in the figure, in the present embodiment, the SiGe film 18 includes an undoped SiGe buffer layer 18x, a SiGe graded composition layer 18a in which the Ge content changes monotonously and continuously, and the Ge content is It is constituted by a substantially constant SiGe upper layer 18b. This is a feature of this embodiment. That is, in the SiGe graded composition layer 18a, when the Ge content becomes the minimum value (for example, a value of 2% to 8%) and increases in the direction from the emitter region to the collector region, and reaches the SiGe buffer layer 18x. In addition, the Ge content becomes a maximum value (for example, a value of 20% to 30%). 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 carriers. In the SiGe upper layer 18b, the Ge content is substantially constant.
[0040]
On the other hand, the base P-type impurity diffusion region 32 is formed across the SiGe gradient composition layer 18a and the SiGe upper layer 18b. The N-type impurity diffusion region 31 is formed from the Si film 19 to a part of the SiGe upper layer 18b. That is, the EB junction 33 that 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. That is, during the heat treatment for forming the emitter region, the N-type impurity reaches not only from the emitter polysilicon film 23 to the Si film 19 but also to the SiGe film 18 therebelow. However, due to variations in manufacturing process conditions and variations, Even if the position of the EB junction 33 varies, 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 33 is in one part of the SiGe upper layer 18b. ing.
[0041]
In other words, due to fluctuations and variations in manufacturing process conditions, N ⁺ The diffusion depth of the N-type impurity from the n-type emitter polysilicon film 23 varies, but the variation range is empirically known from the process conditions, so the thickness of the SiGe upper layer 18b is larger than the variation width of the N-type impurity diffusion depth. Can be set so that the EB junction 33 is formed in the SiGe upper layer 18b almost certainly.
[0042]
In particular, it is preferable to set so that the center position in the thickness direction of the SiGe upper layer 18b and the center of the variation range of the depth at which the N-type impurity diffuses from the emitter polysilicon film 23 substantially coincide. In addition, since the upper limit of the Ge content in the SiGe film is generally around 30%, in order to realize both a high current amplification factor due to the narrow band gap of SiGe and a high-speed base running due to the gradient composition, The Ge content of the SiGe upper layer 18b is preferably in the range of 2 to 8%.
[0043]
According to the present embodiment, even if the position of the EB junction 33 varies due to variations or variations in manufacturing process conditions, the EB junction 33 is formed in the SiGe upper layer 18b having a substantially constant Ge content. Thus, the Ge content in the EB junction 33 is kept substantially constant. Therefore, the SiGe-HBT of the present embodiment has a high current gain h due to the narrow band gap because the EB junction 33 is present in the SiGe upper layer 18b. _FE While maintaining a relatively stable current amplification factor.
[0044]
In addition, the distribution state of the Ge content is determined when each of Ge and Si source gases (for example, GeH) is used when epitaxially growing the SiGe layer. _Four And SiH _Four ) Can be easily controlled to an arbitrary pattern.
[0045]
Even if a SiGe film containing a small amount of Ge instead of the Si film 19 formed on the SiGe film 18 is provided, the same effect as in the present embodiment can be exhibited.
[0046]
(Second Embodiment)
In the first embodiment described above, the minority carriers (electrons) injected from the emitter region into the base region reach the region (SiGe graded composition layer 18a) in which the Ge content changes in the base region. While traveling through the layer 18b, it 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.
[0047]
The SiGe-HBT of the second embodiment of the present invention is provided with means for eliminating this disadvantage in the first embodiment. That is, the present invention relates to a structure for suppressing variations in current gain caused by fluctuations in the EB junction position while maintaining the minority carrier acceleration function by the built-in electric field in the entire base region.
[0048]
FIG. 3 is a diagram showing an impurity concentration distribution in the depth direction and a change in Ge content in a cross section taken along the line Ia-Ia of the SiGe-HBT according to the second embodiment of the present invention (see FIG. 1). The basic structure of the SiGe-HBT of this embodiment is as shown in FIG. 1 in the first embodiment.
[0049]
As shown in FIG. 3, in the SiGe-HBT according to the present embodiment, 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. That is, the band gap in the SiGe upper layer 18b gradually decreases in the direction from the emitter region to the collector region, thereby enhancing the carrier acceleration function. The fact that the EB junction portion 33 is formed in the SiGe upper layer 18b and that the Ge content of the SiGe gradient composition layer 18a is greatly changed in the direction from the collector region to the emitter region is the first embodiment. It is the same as the form.
[0050]
Also in the present embodiment, even if the position of the EB junction 33 varies due to variations or variations in manufacturing process conditions, the SiGe graded composition layer 18a, The thicknesses of the SiGe upper layer 18b and the Si film 19 are set.
[0051]
Then, due to fluctuations and variations in manufacturing process conditions, N ⁺ The diffusion depth of the N-type impurity from the n-type emitter polysilicon film 23 varies, but the variation range is empirically known from the process conditions, so the thickness of the SiGe upper layer 18b is larger than the variation width of the N-type impurity diffusion depth Can be set so that the EB junction 33 is formed in the SiGe upper layer 18b almost certainly.
[0052]
In particular, it is preferable to set so that the center position in the thickness direction of the SiGe upper layer 18b and the center of the variation range of the depth at which the N-type impurity diffuses from the emitter polysilicon film 23 substantially coincide. In general, since the upper limit of the Ge content in the SiGe film is around 30%, in order to realize both a high current amplification factor due to the narrow band gap of SiGe and a high-speed base running due to the gradient composition, The Ge content of the SiGe upper layer 18b varies in the range of 2 to 8%, and the variation width of the content is preferably within 4%.
[0053]
In the present embodiment, as in the first embodiment described above, the EB junction 33 is formed in the SiGe upper layer 18b, and a high current gain h due to the narrow band gap is obtained. _FE Can be maintained. Even if the diffusion depth of the N-type impurity from the emitter polysilicon film 23 varies, the change in the Ge content in the SiGe upper layer 18b is relatively small, so that the position of the EB junction 33 in the SiGe upper layer 18b is Variations in the Ge content due to fluctuations can be suppressed. In addition, when minority carriers (electrons) are injected into the SiGe upper layer 18b, the minority carrier acceleration function by the built-in electric field can be obtained even when traveling through the SiGe upper layer 18b having a gradient composition. The operation speed of SiGe-HBT can be further improved compared to the first embodiment.
[0054]
In the present embodiment, there is a bending point in the change line of the Ge content rate at the boundary between the SiGe graded composition layer 18a and the SiGe upper layer 18b in the SiGe film 18, but the Ge content is reduced at the boundary between the two. By continuously changing, it is possible to prevent a bent point from being generated in the change line of the Ge content. By having such a structure, the carrier acceleration function by the built-in electric field is more effectively exhibited.
[0055]
(Third embodiment)
Similar to the first and second embodiments, the SiGe-HBT of the third embodiment of the present invention further suppresses variations in the current amplification factor caused by the variation in the position of the EB junction, and further, the SiGe film and the SiGe Means are provided for suppressing the occurrence of crystal defects between the film.
[0056]
FIG. 4 is a diagram showing an impurity concentration distribution in the depth direction and a change in Ge content in a cross section taken along the line Ia-Ia (see FIG. 1) of SiGe-HBT according to the third embodiment of the present invention. The basic structure of the SiGe-HBT of this embodiment is as shown in FIG. 1 in the first embodiment.
[0057]
In the SiGe-HBT according to the present embodiment, the SiGe film 18 includes the SiGe uppermost layer 18c in addition to the SiGe gradient composition layer 18a and the SiGe upper layer 18b. The Ge content of the SiGe upper layer 18b is substantially constant, and the Ge content of the SiGe uppermost layer 18c decreases rapidly in the direction from the collector region to the emitter region, and is substantially 0 in the portion adjacent to the Si film 19. It has become. That is, the Ge content in the SiGe uppermost layer 18c rapidly decreases in the direction from the emitter region toward the collector region, and the upper end of the SiGe uppermost layer 18c has the same band gap as Si. The EB junction 33 is formed in the SiGe upper layer 18b. Further, the P-type impurity (boron) is introduced into the SiGe graded composition layer 18a and the SiGe upper layer 18b, and is hardly introduced into the SiGe uppermost layer 18c. Similar to the first and second embodiments, the Ge content of the SiGe graded composition layer 18a increases in the direction from the emitter region to the collector region.
[0058]
Also in the present embodiment, even if the position of the EB junction 33 varies due to variations or variations in manufacturing process conditions, the SiGe graded composition layer 18a, The thicknesses of the SiGe upper layer 18b, the SiGe uppermost layer 18c, and the Si film 19 are set.
[0059]
Then, due to fluctuations and variations in manufacturing process conditions, N ⁺ The diffusion depth of the N-type impurity from the n-type emitter polysilicon film 23 varies, but the variation range is empirically known from the process conditions, so the thickness of the SiGe upper layer 18b is larger than the variation width of the N-type impurity diffusion depth Can be set so that the EB junction 33 is formed in the SiGe upper layer 18b almost certainly.
[0060]
Also in this embodiment, it is preferable to set the center position in the thickness direction of the SiGe upper layer 18b so that the center of the variation range of the depth at which the N-type impurity diffuses from the emitter polysilicon film 23 substantially coincides. . In general, since the upper limit of the Ge content in the SiGe film is around 30%, in order to realize both a high current amplification factor due to the narrow band gap of SiGe and a high-speed base running due to the gradient composition, The Ge content of the SiGe upper layer 18b is preferably in the range of 2% to 8%.
[0061]
In the present embodiment, as in the first embodiment, the Ge content in the EB junction portion 33 is substantially constant, so that the same effect as in the first embodiment can be obtained. In addition, since the Ge content in the SiGe uppermost layer 18c is continuously reduced upward and becomes 0 in the portion adjacent to the Si film 19, the following effects can also be exhibited. .
[0062]
In the case of the first and second embodiments, the Ge content in the upper end portion of the SiGe film 18 (SiGe upper layer 18b) is not 0 but has a constant value. However, since the lattice constants of Si and Ge are different by about 4%, there is a lattice mismatch between the SiGe upper layer 18b and the Si film 19, and as a result, lattice distortion occurs in the Si film 19 and the SiGe film 18. May cause crystal defects.
[0063]
On the other hand, in the present embodiment, since there is almost no lattice mismatch between the upper end of the SiGe film 18 (SiGe uppermost layer 18c) and the Si film 19, lattice distortion occurs in the SiGe film 18 and the Si film 19. It is possible to effectively suppress the occurrence of crystal defects due to the above.
[0064]
-Modification-
FIG. 5 is a diagram showing an impurity concentration distribution in the depth direction and a change in Ge content in an Ia-Ia cross section (see FIG. 1) of SiGe-HBT according to a modification of the third embodiment of the present invention. . The basic structure of the SiGe-HBT of this modification is as shown in FIG. 1 in the first embodiment.
[0065]
In this modification, 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, and is configured to enhance the carrier acceleration function by reducing the band gap. ing. Then, the EB junction portion 33 is formed in the SiGe upper layer 18b, and the Ge content of the SiGe uppermost layer 18c rapidly decreases in the direction from the collector region to the emitter region and is adjacent to the Si film 19. As in the third embodiment, the value is almost zero.
[0066]
In this modification, in addition to the same effects as those of the third embodiment described above, minority carriers (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. ) Acceleration function can be improved.
[0067]
In order to achieve both a high current amplification factor due to the narrow band gap of SiGe and a high-speed base running due to the gradient composition while suppressing the occurrence of crystal defects, the Ge content of the SiGe upper layer 18b is 2% The above range is preferably 8% or less, and the change width of the Ge content in the SiGe upper layer 18b is preferably within 4%.
[0068]
Note that the Ge content of 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. If the value is close to 0, crystal defects due to lattice distortion may occur. Although the effect of suppressing the generation can be obtained, in order to exhibit the effect of this embodiment more effectively, the Ge content of the portion adjacent to the Si film 19 of the SiGe uppermost layer 18c is preferably 0. .
[0069]
(Fourth embodiment)
Similar to the first to third embodiments, the SiGe-HBT according to the fourth embodiment of the present invention suppresses the variation in current amplification factor caused by the variation in the position of the EB junction, and further varies the EB breakdown voltage. Means for suppressing this are provided.
[0070]
FIG. 6 is a diagram showing an impurity concentration distribution in the depth direction and a change in Ge content in a cross section taken along the line Ia-Ia (see FIG. 1) of SiGe-HBT according to the fourth embodiment of the present invention. The basic structure of the SiGe-HBT of this embodiment is as shown in FIG. 1 in the first embodiment.
[0071]
As shown in the figure, in the present embodiment, the SiGe film 18 includes an undoped SiGe buffer layer 18x, a SiGe graded composition layer 18a in which the Ge content changes monotonously and continuously, and the Ge content is It is constituted by a substantially constant SiGe upper layer 18b. In the SiGe graded composition layer 18a, the Ge content becomes a minimum value (for example, a value of 2% to 8%), and continuously increases in the direction from the emitter region to the collector region. When reached, the Ge content becomes the maximum value (for example, a value of 20% to 30%). In the SiGe upper layer 18b, the Ge content is substantially constant.
[0072]
The P-type impurity diffusion region 32 serving as a base is formed across the SiGe gradient composition layer 18a and the SiGe upper layer 18b. The N-type impurity diffusion region 31 is formed from the Si film 19 to a part of the SiGe upper layer 18b. That is, the EB junction 33 that 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. That is, during the heat treatment for forming the emitter region, the N-type impurity reaches not only from the emitter polysilicon film 23 to the Si film 19 but also to the SiGe film 18 therebelow. However, due to variations in manufacturing process conditions and variations, Even if the position of the EB junction 33 varies, 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 33 is in one part of the SiGe upper layer 18b. ing.
[0073]
In other words, due to fluctuations and variations in manufacturing process conditions, N ⁺ The diffusion depth of the N-type impurity from the n-type emitter polysilicon film 23 varies, but the variation range is empirically known from the process conditions, so the thickness of the SiGe upper layer 18b is larger than the variation width of the N-type impurity diffusion depth Can be set so that the EB junction 33 is formed in the SiGe upper layer 18b almost certainly.
[0074]
However, in the present embodiment, as the Ge content in the SiGe graded composition layer 18a of the SiGe film 18 greatly decreases toward the emitter side, the B concentration content in the SiGe graded composition layer 18a also decreases. Yes. 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.
[0075]
Also in this embodiment, it is preferable to set the center position in the thickness direction of the SiGe upper layer 18b so that the center of the variation range of the depth at which the N-type impurity diffuses from the emitter polysilicon film 23 substantially coincides. .
[0076]
In the present embodiment, as in the first embodiment, the Ge content in the EB junction portion 33 is substantially constant. Therefore, as in the first embodiment, the variation in the diffusion depth of the N-type impurity is reduced. Even if it considers, it is easy to adjust the film thickness of each layer so that the EB junction part 33 exists in the SiGe upper layer 18b whose Ge content rate is substantially constant. In addition, since the B concentration in the SiGe upper layer 18b is maintained substantially constant, the B concentration Ge content in the EB junction 33 can be maintained substantially constant even if there are fluctuations and variations in manufacturing process conditions. it can. Therefore, variation in EB breakdown voltage can be suppressed.
[0077]
On the other hand, the decrease in Ge content (GeH _Four B to compensate for the amount of B taken in as the flow rate decreases) ₂ H ₆ In principle, it is possible to keep the B concentration in the SiGe film constant by increasing the flow rate of the SiGe film. However, in this method, the process becomes complicated.
[0078]
On the other hand, in the case of this embodiment, the source gas for B (B ₂ H ₆ ), The variation in the EB breakdown voltage can be reliably suppressed by a simple process.
[0079]
-Modification-
FIG. 7 is a diagram showing an impurity concentration distribution in the depth direction and a change in Ge content in a cross section taken along line Ia-Ia (see FIG. 1) of SiGe-HBT according to a modification of the fourth embodiment of the present invention. . The basic structure of the SiGe-HBT of this modification is as shown in FIG. 1 in the first embodiment.
[0080]
In this modification, 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, and is configured to enhance the carrier acceleration function by reducing the band gap. ing. The EB junction portion 33 is formed in the SiGe upper layer 18b as in the fourth embodiment.
[0081]
In this modification, in addition to the same effects as those of the fourth embodiment described above, minority carriers (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. ) Acceleration function can be improved.
[0082]
On the other hand, in the SiGe upper layer 18b, the B concentration also changes with the change in the Ge content. However, the change in the Ge content in the SiGe upper layer 18b is larger than the change in the Ge content in the SiGe gradient composition layer 18a. Since it is small, the change in the B concentration in the SiGe upper layer 18b is also small. Therefore, the variation in the EB breakdown voltage can also be suppressed by this modification.
[0083]
In the fourth embodiment or its modification, the SiGe uppermost layer 18c having the Ge gradient composition as in the third embodiment can be provided.
[0084]
(Other embodiments)
In the first to fourth embodiments, the example in which the present invention is applied to the bipolar transistor (SiGe-HBT) having the Si / SiGe heterojunction has been described. However, the present invention is not limited to the Si / SiGeC or SiGe / SiGeC heterojunction. Even when applied to a bipolar transistor having a junction, the same effects as those of the first to fourth embodiments can be exhibited. In that case, the base layer is a ternary mixed crystal semiconductor layer containing Si, Ge, and carbon (C).
[0085]
Furthermore, the present invention is not an HBT having a mixed crystal semiconductor layer as in the first to fourth embodiments, but an HBT having a compound semiconductor layer containing, for example, indium (In), gallium (Ga), and P. Even if it applies to, the effect similar to the said 1st-4th embodiment can be exhibited.
[0086]
【The invention's effect】
According to the semiconductor device of the present invention, in a heterojunction bipolar transistor having a structure in which the band gap of the base region is inclined for carrier acceleration, even if the position of the EB junction varies, the band gap of the EB junction can be reduced. Variations can be suppressed, and thus a high current gain can be stably exhibited while maintaining high-speed operation.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a SiGe-HBT in a first embodiment of the present invention.
FIG. 2 is a diagram showing an impurity concentration distribution in a depth direction and a Ge content change in a cross section (FIG. 1) of the SiGe-HBT according to the first embodiment of the present invention.
FIG. 3 is a diagram showing an impurity concentration distribution in a depth direction and a Ge content change in an Ia-Ia line cross section (FIG. 1) of SiGe-HBT according to a second embodiment of the present invention.
FIG. 4 is a diagram showing an impurity concentration distribution in a depth direction and a change in Ge content in an Ia-Ia line cross section (FIG. 1) of SiGe-HBT according to a third embodiment of the present invention.
FIG. 5 is a diagram showing an impurity concentration distribution in a depth direction and a change in Ge content in a cross section (FIG. 1) of a SiGe-HBT according to a modification of the third embodiment of the present invention.
FIG. 6 is a diagram showing an impurity concentration distribution in the depth direction and a change in Ge content in a cross section (FIG. 1) of a SiGe-HBT according to a fourth embodiment of the present invention.
FIG. 7 is a diagram showing an impurity concentration distribution in a depth direction and a change in Ge content in a cross section (FIG. 1) of a SiGe-HBT according to a modification of the fourth embodiment of the present invention.
FIG. 8 is a cross-sectional view of a conventional SiGe-HBT.
FIG. 9 is an energy band diagram for comparing band structures of SiGe-HBT and Si-BT having a gradient composition.
10 is a diagram showing an impurity concentration distribution in the depth direction and a change in Ge content in the XX line cross section shown in FIG. 8. FIG.
FIG. 11 is a diagram for explaining variation in diffusion depth of N-type impurities in a SiGe graded composition layer.
FIG. 12B ₂ H ₆ It is a figure which shows the impurity concentration distribution of the depth direction in conventional SiGe-HBT, and Ge content rate change when supposing that the flow volume of is constant.
[Explanation of symbols]
11 Si substrate
12 Si epitaxial layer
18 SiGe film
18a SiGe graded composition layer
18b SiGe top layer
18c SiGe top layer
18x SiGe buffer layer
19 Si film
20 N ⁺ Mold embedding layer
21 N ⁺ Type collector extraction layer
22 N ^- Type collector diffusion layer
23 N ⁺ Type emitter polysilicon film
24P ⁺ Type base polysilicon film
26 LOCOS separation
27 Deep trench isolation
31 N-type impurity diffusion region
32 P-type impurity diffusion region
33 EB joint

Claims (8)

A substrate having a first semiconductor layer;
A second semiconductor layer provided on the first semiconductor layer and having a band gap smaller than that of the first semiconductor layer and made of a mixed crystal semiconductor;
A third semiconductor layer provided on the second semiconductor layer and having a larger band gap than the second semiconductor layer;
At least a part of the first semiconductor layer is a collector region containing a first conductivity type impurity, at least a part of the second semiconductor layer is a base region containing a second conductivity type impurity, the third semiconductor layer A semiconductor device that functions as a heterojunction bipolar transistor in which at least a part of a semiconductor layer is an emitter region containing a first conductivity type impurity,
The second semiconductor layer is a layer made of a mixed crystal semiconductor containing Si and Ge, and has a band gap that continuously increases in a direction from the collector region to the emitter region in the base region, and the collector region A graded composition layer having a composition that increases the band gap in the direction from the emitter region to the emitter region; An upper layer having a composition in which the rate of change is smaller than the rate of change of the band gap of the gradient composition layer,
The impurity concentration in the gradient composition layer decreases as the band gap increases in the direction from the collector region to the emitter region,
In the upper layer, an emitter-base junction is formed,
The variation width of the Ge content in the upper layer is within 4%,
The impurity concentration of the upper layer is, Ge content in the upper layer, decreases with the above collector region to decrease toward the emitter region, and the change of the impurity concentration of the upper layer is the inclination A semiconductor device characterized by being smaller than a change in impurity concentration in a composition layer. A substrate having a first semiconductor layer;
A second semiconductor layer provided on the first semiconductor layer and having a band gap smaller than that of the first semiconductor layer and made of a mixed crystal semiconductor;
A third semiconductor layer provided on the second semiconductor layer and having a larger band gap than the second semiconductor layer;
At least a part of the first semiconductor layer is a collector region containing a first conductivity type impurity, at least a part of the second semiconductor layer is a base region containing a second conductivity type impurity, the third semiconductor layer A semiconductor device that functions as a heterojunction bipolar transistor in which at least a part of a semiconductor layer is an emitter region containing a first conductivity type impurity,
The second semiconductor layer is a layer made of a mixed crystal semiconductor containing Si and Ge, and has a band gap that continuously increases in a direction from the collector region to the emitter region in the base region, and the collector region A graded composition layer having a composition in which the band gap increases in the direction from the emitter region to the emitter region, and the band gap increases in the direction from the collector region to the emitter region formed on the graded composition layer. An upper layer having a composition in which the rate of change is smaller than the rate of change of the band gap of the gradient composition layer; and the band gap is increased in the direction from the collector region to the emitter region, A composition in which the change rate of the band gap is larger than the change rate of the band gap of the upper layer. And a top layer having,
In the upper layer, an emitter-base junction is formed,
The width of change of the Ge content in the upper layer is within 4%. The semiconductor device according to claim 1 ,
The second semiconductor layer has a band gap larger on the upper layer in a direction from the collector region to the emitter region, and the rate of change of the band gap is higher than the rate of change of the band gap of the upper layer. A semiconductor device comprising an uppermost layer having an increasing composition. The semiconductor device according to claim 1 or 2 ,
The third semiconductor layer is a Si layer;
A semiconductor device characterized in that the Ge content in the upper layer of the second semiconductor layer is in the range of 2% to 8%. The semiconductor device according to claim 1 or 2 ,
The second semiconductor layer, Ri mixed crystal semiconductor layer der ternary containing Si, Ge, and C,
A semiconductor device, wherein a Ge content in the graded composition layer decreases from the collector region toward the emitter region . The semiconductor device according to claim 1 or 2 ,
The semiconductor device according to claim 1, wherein the emitter-base junction is located at the center of the upper layer in the thickness direction . The semiconductor device according to claim 2 ,
The impurity concentration in the gradient composition layer decreases as the band gap increases in the direction from the collector region to the emitter region,
The impurity concentration in the upper layer decreases as the Ge content in the upper layer decreases from the collector region toward the emitter region ,
A change in impurity concentration in the upper layer is smaller than a change in impurity concentration in the gradient composition layer. A semiconductor device according to claim 1 or 7 ,
The semiconductor device, wherein the impurity in the second semiconductor layer is boron (B).

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