Bipolar transistor with low parasitic CCB heterojunction

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for fabricating a Heterojunction Bipolar Transistor (HBT), using Aspect Ratio Trapping (ART) technology in semiconductor ridge structures, like nanoridges. The HBT manufactured by the method according to the invention has a very low parasitic base-collector capacitance.BACKGROUND OF THE INVENTION

[0002] Two of the primary performance metrics for a HBT for Radio Frequency (RF) applications, are the cut-off frequencies for maximum current gain, Ft, and power gain, F max , respectively. As the consumer requirements continue to push higher, parasitic capacitances and resistances are becoming the most critical factors in limiting the HBT performance. In particular, the base-collector (depletion) capacitance, C cb , is one of the most critical parasitic capacitances, because it affects both F t and F max .

[0003] FIG. 6 illustrates a conventional HBT 60, which comprises a sub-collector region 61, a collector region 62 arranged on the sub-collector region 61, a base region 63 arranged on the collector region 62, and an emitter region 65 arranged on the base region 63. The emitter region 65 is narrower than the base region 63, to provide space to form base contacts 64 contacting the base region 63 next to the emitter region 65. The emitter region 65, and the parts of the base region 63 and collector region 62 located beneath the emitter region 65, together form the active (transistor) area of the HBT 60.

[0004] The parasitic C cb is indicated in FIG. 6 beneath the base contacts 64. It could be reduced by eliminating the part of the collector region 62, which is located underneath the base contacts 64, since this part of the collector region 62 is not part of the active transistor area. However, the challenge in controlling, and in particular reducing, the value of the parasitic C cb for an optimized performance of the HBT 60 is twofold.

[0005] Firstly, scaling the width of the collector region 62 would limit the space for the base contacts 65, thereby severely limiting the base resistance, R bb , which would adversely affect F max . Secondly, selectively etching out the unwanted parts of the collector region 62 is not only difficult, but also limits the stability of the HBT and thus its yield, especially, in case of a Schottky HBT (SHBT), where this is not a suitable solution in a traditional process flow.

[0006] One conventional approach to reduce the parasitic C cb in a HBT is to deliberately over-etch the collector region 62 in a process flow based on blanket wafers. However, this still leads to significant stability and yield issues.

[0007] Another conventional approach to reduce the parasitic C cb in a HBT is the transferred substrate method. In this method, the HBT is fabricated on a blanket substrate up to the formation of the base contacts 65. Afterwards, the HBT is transferred to a host wafer in an upside-down orientation, and is then bonded to the host wafer. Following the removal of a carrier wafer from the top, the collector region can then be patterned to reduce the parasitic C cb . However, this is a very complex process, and thus also not the ideal solution in terms of yield.

[0008] US 2014 / 084420 A1 discloses a method for forming an HBT, wherein the method comprises bridging an extrinsic base and an intrinsic base by a selective epitaxial growth process.

[0009] US 2015 / 137185 A1 discloses an HBT and a fabrication method for the HBT. In particular, the HBT is provided with an airgap located vertically between an extrinsic base and a collector of the HBT.SUMMARY OF THE INVENTION

[0010] In view of the above-mentioned challenges and disadvantages of the conventional approaches, embodiments of the invention aim to provide an improved method to fabricate a HBT and to provide an improved HBT. An objective is, in particular, to provide a HBT having enhanced cut-off frequencies for both maximum current gain and power gain. To this end, the aim is to fabricate a HBT with reduced parasitic base-collector capacitance. However, the stability of the HBT should thereby not be compromised. In addition, a high fabrication yield is of course another important goal.

[0011] The objective is achieved by the embodiment of the invention provided in the enclosed independent claim. Advantageous implementations of this embodiment are defined in the dependent claims.

[0012] The embodiments of the invention base on fabricating a HBT using the ART technology in ridge structures, in particular using nanoridges.

[0013] A first aspect of the invention provides a method for fabricating a HBT, the method comprising: providing a semiconductor support layer; forming an even number of at least four elongated wall structures on the support layer, wherein the wall structures are arranged side-by-side at a regular interval; forming an odd number of at least three semiconductor collector-material ridge structures on the support layer, wherein each ridge structure is formed between two adjacent wall structures; forming a semiconductor base-material layer on a determined ridge structure of the at least three ridge structures, wherein the determined ridge structure is surrounded by the other ridge structures of the at least three ridge structures; forming a semiconductor emitter-material layer on the base-material layer; extending the base-material layer epitaxially so that it coherently covers all the wall structures and all the ridge structures; and selectively removing all the other ridge structures surrounding the determined ridge structure and not removing the determined ridge structure; and filling spaces formed by removing the other ridge structures with an insulator material or air.

[0014] The determined ridge structure may be the central ridge structure, i.e. the ridge structure arranged centrally in the odd number of ridge structures arranged next to each other. The formation of the different layers and ridge structures may be implemented by growth or deposition, particularly by epitaxial growth. For example, Metalorganic Vapor Phase Epitaxy (MOVPE), Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) may be employed.

[0015] The method of the first aspect is able to fabricate an improved HBT. In particular, since the other collector-material ridge structures, other than the determined ridge structure, are removed by the method, the parasitic base-collector (depletion) capacitance, C cb , is significantly reduced in the final HBT. The HBT has accordingly extremely low, or possibly even zero, C cb . Accordingly, the cut-off frequencies for both the maximum current gain, Ft, and the power gain, F max , are improved for the HBT.

[0016] Suitable insulator materials may improve further the stability of the HBT, without adding parasitic Ccb.

[0017] In an implementation of the method, the semiconductor collector-material and the semiconductor emitter-material comprise a first-conductivity-type III-V semiconductor material; and the base-material comprises a second-conductivity-type III-V semiconductor material.

[0018] The III-V semiconductor material allows a well-controlled formation, in particular growth, of the ridge structures. The ridge structures may be grown using the ART technology, wherein the ridge structures may be nanoridges.

[0019] In an implementation of the method, the wall structures comprise a silicon oxide; and / or the semiconductor support layer comprises a silicon-based layer.

[0020] In an implementation of the method, the forming of each ridge structure is confined in and guided by a trench formed in the support layer.

[0021] In an implementation, the method further comprises forming each ridge structure in a V-groove, wherein the V-groove is disposed in the support layer.

[0022] The support layer trench and / or V-groove are characteristic for the ART technology. Defects can be trapped in a lower part of the ridge structures, and the parts of the ridge structures above can be made more or less defect-free.

[0023] In an implementation of the method, extending the base-material layer comprises: epitaxially growing base-material on the other ridge structures than the determined ridge structure, until the base-material merges together over the wall structures and with the base-material layer formed on the determined ridge structure.

[0024] This provides a simple but efficient way to extend the base-material layer.

[0025] In an implementation of the method, removing the other ridge structures comprises: selectively etching the semiconductor collector-material using wet chemistry.

[0026] Thus, the material responsible for the parasitic C cb can be easily but efficiently removed.

[0027] In an implementation, the method further comprises, before extending epitaxially the base-material layer: forming an etch-stop layer, in particular a not-intentionally-doped III-V semiconductor etch-stop layer, on each other ridge structure than the determined ridge structure.

[0028] In an implementation, the method further comprises, after removing the other ridge structures: removing the etch-stop layers by selectively etching using wet chemistry.

[0029] For instance, for forming the etch-stop layer, a thin InGaP re-growth under the extended base-material layer can be performed. The etch-stop layer protects the extended base-material layer from getting etched during the above-described collector-material ridge structure removal. In this way, the base-collector junction can be completely removed under the extended base-material layer, and therefore, C cb can be reduced to zero.

[0030] In an implementation, the method further comprising, before removing the other ridge structures: forming a protective layer on the base-material layer and on the emitter-material layer of the ridge structure, respectively.

[0031] This protects the top surface of these layers from the processing that removes the ridge structures, and therefore leads to higher stability and yield.

[0032] In an implementation of the method, forming the ridge structures comprises, for each ridge structure: forming a narrower ridge portion to confine all defects on the support layer, and forming a wider defect-free ridge portion on top of the narrower ridge portion, wherein the forming of the wider ridge portion is guided by the corresponding two adjacent wall structures.

[0033] This improves the performance of the HBT, since the collector-material ridge structure that remains to from the active transistor area is of high quality. The wall structures act as a template to grow the ridge structures, and benefit the stability of the final HBT.

[0034] In an implementation of the method, epitaxially forming the ridge structures comprises, for each ridge structure: forming a sub-collector region on the support layer and forming a collector region on the sub-collector region.

[0035] A HBT fabricated according to the above-described method is clearly distinguishable from a HBT formed by another method. For instance, remainders of the collector-material ridge structures may remain in the support layer (e.g. in trenches). Further, the wall structure template that provides stability to the HBT is not present in other HBTs.

[0036] The HBT is not claimed but it is useful for understanding the present invention.

[0037] In summary, by using e.g. the ART approach to form the ridge structures, the base-material layer can be separately formed around, but outside, the active transistor area. Thus, space is provided to form base contacts to the base-material layer. The template of the wall structures provides the necessary stability to the HBT. Further, the extended base-material can be made independent of the parasitic base-collector junction, which leads to easier scaling control over the base parasitics.BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The above described aspects and implementations are explained in the following description of embodiments with respect to the enclosed drawings: FIG. 1shows a method according to an embodiment of the invention, and the fabricated HBT . The HBT is not claimed but it is useful for understanding the invention. FIG. 2shows steps of an exemplary method according to an embodiment of the invention. FIG. 3shows a further step of the exemplary method according to the embodiment of the invention. FIG. 4shows a further step of the exemplary method according to the embodiment of the invention. FIG. 5shows a further step of the exemplary method according to the embodiment of the invention. FIG. 6shows a conventional HBT. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0039] Figure (FIG.) 1 shows a general method 100 according to an embodiment of the invention. The method 100 is suitable to fabricate a HBT 10, in particular to fabricate a HBT 10 with very low or even zero parasitic base-collector capacitance C cb .

[0040] The method 100 comprises the following steps. In a first step, a semiconductor support layer 11 or substrate or wafer is provided. In a second step, an even number of at least four elongated wall structures 12 are formed on the support layer 11. That is, for example, four, six, eight, ten, or more, wall structures 12 are formed. The wall structures 12 are arranged side-by-side at a regular interval, i.e., with the same distance between each two adjacent wall structures 12. The wall structures 12 may all extend into the same direction and may thus be arranged in parallel. In a third step, an odd number of at least three semiconductor collector-material ridge structures 13 is formed on the support layer 11. That is, for example, three, five, seven, nine, or more ridge structures 13 are formed. Each ridge structure 13 is formed between two adjacent wall structures 12, wherein these adjacent wall structures 12 may guide a growth of the ridge structure 13.

[0041] In a fourth step, a semiconductor base-material layer 14 is formed on a determined ridge structure 13c of the at least three ridge structures 13, or between two determined adjacent wall structures 12, wherein the determined ridge structure 13c is surrounded by the other ridge structures 13 of the at least three ridge structures 13. The determined ridge structure 13c may be a central ridge structure (e.g. in case of three ridge structure 13 it may be the middle ridge structure), and / or may be formed between the central adjacent pair of the wall structures 12 (e.g. in case of four wall structures 12 it may be formed between the two inner wall structures 12. The forming of ridge structures 13 may be confined in and guided by a trench or groove, e.g. V-groove, formed in the support layer 11, thus, the ridge structures 13 can be formed using the ART technology.

[0042] In a fifth step, a semiconductor emitter-material layer 15 is formed on the base-material layer 14. The first to fifths steps lead to the intermediate structure shown in (a) of FIG. 1.

[0043] In a sixth step, the base-material layer 14 is epitaxially extended, so that it coherently covers all the wall structures 12 and all the ridge structures 13. This sixth step leads to the intermediate structure shown in (b) of FIG. 1. The sixth step may be performed by epitaxially growing base-material on the ridge structures 13, other than the determined ridge structure 13c, until the base-material merges together over the wall structures 12 and with the base-material layer 14 formed on the determined ridge structure 13c.

[0044] In a seventh step, the other ridge structures 13 surrounding the determined ridge structure 13c are selectively removed and the determined ridge structure 13c is not removed. Further, spaces 16 that are formed by removing the other ridge structures 13 are filled with an insulator material or air. This seventh step leads to the finally fabricated HBT 10 that is shown in (c) of FIG. 1. The seventh step may be performed by selectively etching the semiconductor collector-material using wet chemistry.

[0045] The fabricated HBT 10 shown in (c) of FIG. 1 accordingly comprises the semiconductor support layer 11, and the at least four elongated wall structures 12 arranged on the support layer 11, wherein the wall structures 12 are arranged side-by-side at a regular interval. Further, the HBT 10 comprises the determined semiconductor collector-material ridge structure 13c arranged on the support layer 11, wherein the determined ridge structure 13c is arranged between the two determined adjacent wall structures 12 of the at least four wall structures 12. Further, the HBT 10 comprises the (extended) semiconductor base-material layer 14 arranged on the determined ridge structure 13c and over the wall structures 12, wherein the base-material layer is supported by the wall structures 12, and the semiconductor emitter-material layer 15 arranged above the ridge structure 13c on the base-material layer. In the spaces 16 formed between the wall structures 12 other than the two determined adjacent wall structures 12 (formed by removing the collector material ridge structures 13 other then the determined ridge structure 13c) are filled with air and / or with an insulator material.

[0046] The primary difficulty in controlling the parasitic C cb of the base-collector region in a HBT is separating the formation (e.g. growth) and the processing of the base-material with respect to the collector-material. The parasitic collector-material needs to be removed, with good control, e.g. control over the extent of etching, without etching or otherwise negatively affecting the stability of the base-material. In, for example, a conventional HBT based on blanket epitaxial growth and processing, this is not only difficult to achieve, but, in case of an SHBT, not a viable solution at all.

[0047] However, in the embodiments of the invention, by forming the ridges structures 13, for example using ART based growth of collector-material, the above-mentioned difficulty can be inherently overcome, due to the separation of each ridge structure 13 by a wall structure 12 (acting as a template barrier). As a result, a single determined ridge structure 13c can be used for the active transistor area, while the nearby ridge structures 13 can still be separately processed (here removed). This separation, can lead to zero parasitic C cb , namely by removing all the collector-material from the ridge structures 13 surrounding the determined ridge structure 13c.

[0048] The base-material layer 14 around the active transistor area can be created by overgrowing the ridge structures 13 and wall structures 12 to enable a merging of the base-material across different ridge structures 13. The separation between the base-material and collector-material, to ensure selective removal of only the collector-material, may be achieved by growing a (very thin) etch-stop layer 31, particularly from a material known to have a high selectivity towards a solution used to etch.

[0049] In this respect, the FIGs. 2-5 show more details of the method 100 according to an embodiment of the invention, which builds on the embodiment of the method 100 shown in FIG. 1. That is, the method 100 is exemplarily shown in the FIGs. 2-5 with more specific and / or additional steps and features. Same elements in FIG. 1 and in the FIGs. 2-5 are labelled with the same reference signs and function likewise. In particular, a HBT 10 obtainable by a method according to an embodiment of the invention, as shown in FIG. 5, can finally be obtained by using the illustrated process flow.

[0050] In FIG. 2, the ridge structures 13 are formed on the support layer 11 and between the wall structures 12 provided also on the support layer 11. In particular, they may be grown epitaxially.

[0051] Further, an active transistor area is then formed from the determined collector-material ridge structure 13c, the base-material layer 14 arranged on the ridge structure 13c, and the emitter-material layer 15 arranged on the base-material layer 14. A protective cap 22 may further be provided on the base material layer 14. The semiconductor emitter-material may comprise a first-conductivity-type III-V semiconductor material (e.g., including an n-doped InGaP emitter 15a and an n ++< -doped GaAs emitter cap 15b). The semiconductor base-material may comprise a second-conductivity-type III-V semiconductor material (e.g., p-doped GaAs). The semiconductor collector-material may comprise a first-conductivity-type III-V semiconductor material (e.g., including an n-doped GaAs collector region 23b and an n+-doped GaAs sub-collector region 23a). The active transistor area may be defined by etching, for example, by using a resist with / without SiO 2 as a hard mask. In this case, the emitter-material, the base-material and the SiO 2 mask may be provided on top of and over all ridge structures 13 and all wall structures 12, respectively, and may then be etched until the semiconductor collector-material of the ridge structures 13 is reached. Thereby, the pillar-like structure may be formed above the determined ridge structure 13c, as shown in FIG. 2. A spacer layer may further be provided to protect this area from further processing.

[0052] FIG. 3 shows further that an etch-stop layer 31 (e.g., a thin InGaP layer), may be formed, particularly grown, on the exposed collector-material surface, i.e., on the ridge structures 13 and wall structures 12, respectively. The etch-stop layer 31 enables a selective etching of the collector-material.

[0053] FIG. 4 shows further that the base-material layer 14 can be epitaxially extended, namely by epitaxially growing base-material (e.g., p-doped GaAs) onto the etch-stop layer 31. The base-material may specifically be grown until the base-material merges together over the ridge structures 13 other than the determined ridge structure 13c, and merges together with the base-material layer 14 that is formed on the determined ridge structure 13c. This enables good contacting to the base-material layer 14 arranged on the determined ridge structure 13c. The extended base-material layer 14 finally covers coherently all the wall structures 12 and all the ridge structures 13. The extended base-material layer 14 provides space for and enables good base contacts.

[0054] FIG. 5 shows further that a protective layer 51 may be provided, or may be extended from the protective cap 22, to the extended base-material layer 14. For example, by using a resist. Thereafter, the underlying collector-material of the ridge structures 13 other than the determined ridge structure 13c can be removed, for example, by wet etching. This is then followed by removal of the etch-stop layer 31, for example, again by wet etching.

[0055] The embodiments of the invention provided in this disclosure are applicable to both a SHBT and Double HBT (DHBT), which may be based particularly on III-V semiconductor materials. De-convoluting the base and collector designs in the HBT 10 opens the possibility of independently optimizing both. In addition, the embodiments of the present invention result in less issues with yield and HBT stability, as is the case with the conventional approaches. Also, the embodiments of the invention remove the requirement for a substrate transfer for fabricating the HBT 10, which further reduces the process complexity and yield issues. Hence, the embodiments of the invention are more suitable for 3D integration of III-V on silicon for RF applications.

Claims

1. Method (100) for fabricating a Heterojunction Bipolar Transistor (10), the method (100) comprising: providing a semiconductor support layer (11); forming an even number of at least four elongated wall structures (12) on the support layer (11), wherein the wall structures (12) are arranged side-by-side at a regular interval; forming an odd number of at least three semiconductor collector-material ridge structures (13) on the support layer (11), wherein each ridge structure (13) is formed between two adjacent wall structures (12); forming a semiconductor base-material layer (14) on a determined ridge structure (13c) of the at least three ridge structures (13), wherein the determined ridge structure (13c) is between two ridge structures of said at least three ridge structures (13); forming a semiconductor emitter-material layer (15) on the base-material layer (14); extending the base-material layer (14) epitaxially so that it coherently covers all the wall structures (12) and all the ridge structures (13); selectively removing all said ridge structures (13) except the determined ridge structure (13c); and filling spaces (16) formed by removing said ridge structures (13) with an insulator material or air.

2. Method (100) according to claim 1, wherein: the semiconductor collector-material and the semiconductor emitter-material comprise a first-conductivity-type III-V semiconductor material; and the base-material comprises a second-conductivity-type III-V semiconductor material.

3. Method (100) according to claim 1 or 2, wherein: the wall structures (12) comprise a silicon oxide; and / or the semiconductor support layer (11) comprises a silicon-based layer.

4. Method (100) according to one of the claims 1 to 3, wherein: the forming of each ridge structure (13) is confined in and guided by a trench (21) formed in the support layer (11).

5. Method (100) according to one of the claims 1 to 4 further comprising: forming each ridge structure (13) in a V-groove, wherein the V-groove is disposed in the support layer (11).

6. Method (100) according to one of the claims 1 to 5, wherein extending the base-material layer (14) comprises: epitaxially growing base-material on the other ridge structures (13) than the determined ridge structure (13c), until the base-material merges together over the wall structures (12) and with the base-material layer formed on the determined ridge structure (13c).

7. Method (100) according to one of the claims 1 to 6, wherein removing the other ridge structures (13) comprises: selectively etching the semiconductor collector-material using wet chemistry.

8. Method (100) according to one of the claims 1 to 7, further comprising, before extending epitaxially the base-material layer (14): forming an etch-stop layer (31), in particular a not-intentionally-doped III-V semiconductor etch-stop layer, on each other ridge structure (13) than the determined ridge structure (13c).

9. Method (100) according to claim 8, further comprising, after removing the other ridge structures (13): removing the etch-stop layers (31) by selectively etching using wet chemistry.

10. Method (100) according to one of the claims 1 to 9, further comprising, before removing the other ridge structures (13): forming a protective layer (22, 51) on the base-material layer (14) and on the emitter-material layer (14), respectively.

11. Method (100) according to one of the claims 1 to 10, wherein forming the ridge structures (13) comprises, for each ridge structure (13): forming a narrower ridge portion to confine all defects on the support layer (11), and forming a wider defect-free ridge portion on top of the narrower ridge portion, wherein the forming of the wider ridge portion is guided by the corresponding two adjacent wall structures (12).

12. Method (100) according to one of the claims 1 to 10, wherein epitaxially forming the ridge structures (13) comprises, for each ridge structure (13): forming a sub-collector region (13a) on the support layer (11) and forming a collector region (13b) on the sub-collector region (13a).