Method of fabricating a heterojunction bipolar transistor

Abstract: A heterojunction bipolar transistor having a planar surface topology, reduced lateral dimensions and a base electrode (40) aligned to both the emitter (38) and collector (26,36) electrodes is fabricated by forming sub-­ collector (12), collector (14), base (16) and one or more emitter (18,20) layers on a substrate (10). An opening (24) extending to the sub-collector layer (12) is then formed and a first portion (26) of the collector electrode (26,36) is formed therein so that the sidewalls of the opening (24) are not contacted by the first portion (26). Dielectric material (30) is then formed between the sidewalls of the opening (24) and the first portion (26) of the collector electrode (26,36). A second portion (36) of the collector electrode (26,36) then formed on the first portion (26) of the collector electrode (26,36) along with an emitter electrode (38) so that the second portion (36) of the collector electrode (26,36) and the emitter electrode (38) are substantially planar. After then exposing the base layer (16), the self-aligned base electrode (40) is formed between the second portion (36) of the collector electrode (26,36) and the emitter electrode (38).

Description: Background of the Invention This invention relates, in general, to semiconductor device fabrication and more particularly to a method of fabricating a heterojunction bipolar transistor that has relatively planar surface topology and reduced lateral dimensions. Conventional methods of fabricating heterojunction bipolar transistors include forming collector, base and emitter layers on a substrate. Typically, the processes then form an emitter electrode followed by etching down to the base layer and forming a base electrode. Finally, an opening is etched to the collector layer so that a collector electrode may be formed. These conventional methods generally result in a heterojunction bipolar transistor having non-planar surface topology due to the large height difference between emitter electrode and the collector electrode. Conventional single self-aligned heterojunction bipolar transistor fabrication processes generally align the base electrode only to the emitter electrode. This self-alignment results in a device configuration wherein the collector electrode is laterally distant from the base electrode. As a result, overall transistor size is sacrificed and the lateral dimensions of the transistor are larger than that obtainable using a process wherein the base electrode is automatically aligned to both the emitter and the collector electrodes. An article by Tully et al. entitled "A Fully Planar Heterojunction Bipolar Transistor", IEEE Electron Device Letters, Vol. EDL-7, No. 11, November 1986, page 615, discloses a planar, self-aligned heterojunction bipolar transistor fabrication process. This process utilizes a two-step epitaxial deposition with an intervening selective ion implant of bases, This epitaxial regrowth and selective ion implantation cause the disclosed process to be relatively complicated and difficult to employ. Summary of the Invention According to a first aspect of the invention there is provided A method of fabricating a heterojunction bipolar transistor comprising the steps of: forming a photoresist mask on an emitter layer; using said photoresist mask to form an opening wherein said opening has positively sloping, tapered sidewalls and extends through the emitter layer to a collector layer, and wherein said opening, undercuts said photoresist mask; forming a first portion of a collector electrode in said opening so that said first portion is not in contact with the sidewalls of said opening; isolating said first portion with a dielectric wherein said first portion, said dielectric and said emitter layer form a substantially planar structure; forming a second portion of a collector electrode on said first portion; forming an emitter electrode adjacent to and substantially planar with said second portion; exposing a portion of a base layer of the heterojunction bipolar transistor between said second portion of a collector electrode and said emitter electrode; and forming a base metal on the exposed portion of said base layer. According to a second aspect of the invention there is provided a heterojunction bipolar transistor as claimed in claim 9. Brief Description of the Drawings FIGS. 1-10 illustrate highly enlarged cross-sectional views of a portion of a heterojunction bipolar transistor during fabrication embodying the present invention. Detailed Description of the Invention FIG. 1 shows a substrate 10 having a series of active layers disposed thereon. In this embodiment, substrate 10 comprises semi-insulating gallium arsenide although substrates of other well known materials may be employed. Also in this embodiment, the active layers are sub-collector 12 comprised of gallium arsenide and having an N+ conductivity type, collector layer 14 comprised of gallium arsenide and having an N conductivity type, base layer 16 comprised of gallium arsenide and having a P conductivity type, emitter layer 18 comprised of aluminum gallium arsenide and having an N conductivity type and emitter cap layer 20 comprised of gallium arsenide and having an N+ conductivity type. These active layers are formed by methods well known in the art. Although an NPN transistor is disclosed herein, it should be understood that the present invention may also be employed to fabricate a PNP transistor. A first photoresist mask 22 is formed on emitter cap layer 20 and is used to form an opening 24 that extends through emitter cap layer 20, emitter layer 18, base layer 16, collector layer 14 and stops on sub-collector layer 12 as shown in FIG. 2. Opening 24 may be formed by employing a wet isotropic etch of dilute phosphoric acid or one of many other wet, dry or combination of wet and dry etches well known in the art. It should be understood that these etches should etch laterally in opening 24 as well as vertically. This enables opening 24 to undercut first photoresist mask 22. The importance of this undercut will be explained presently. First portion 26 of the collector electrode is formed in opening 24 employing first photoresist mask 22. Standard deposition techniques are used. It is essential that first portion 26 does not touch the sidewalls of opening 24. Therefore, in order to employ first photoresist mask 22 in the deposition of first portion 26, it is important that the etching of opening 24 undercut first photoresist mask 22 as described above. First portion 26 comprises a layer of gold-germanium alloy disposed directly on sub-collector layer 12, a layer of nickel disposed on the alloy layer and a layer of gold disposed on the nickel layer. it should be understood that first portion 26 may also comprise other metal schemes. Further, first portion 26 should be substantially planar with the top surface of emitter cap layer 20. During the formation of first portion 26, excess metal 28 will deposit on first photoresist mask 22. First photoresist mask 22 and excess metal 28 are removed following the formation of first portion 26 by a lift-off process and the resultant structure is depicted in FIG. 3. Now referring to FIG. 4, a dielectric layer 30 is formed on the structure following the lift-off procedure. It should be understood that dielectric layer 30, silicon dioxide in this embodiment, also fills opening 24 around first portion 26. As shown in FIG. 5, a photoresist layer 32 is then formed on dielectric layer 30. Photoresist layer 32 may be spun-on or applied by other methods well known in the art. Photoresist layer 32 and dielectric layer 30 are then etched away, preferably using reactive ion etching with fluorine chemistry. This etch results in a planar structure as shown in FIG. 6. The portions of dielectric layer 30 disposed in opening 24 are not etched away and remain to insulate first portion 26 from active layers 14, 16, 18 and 20. As shown in FIG. 7, a second photoresist mask 34 is formed on the planarized structure. Mask 34 is used to form second portion 36 of the collector electrode above first portion 26 and also to form emitter electrode 38 as shown in FIG. 8. In this embodiment, both second portion 36 and electrode metal 38 are formed of a layer of gold-germanium alloy, a layer of nickel disposed on the alloy layer, and a layer of gold disposed on the nickel layer. Following the formation of second portion 36 and emitter electrode 38, emitter cap layer 20 and emitter layer 18 are etched away excepting those portions disposed beneath emitter electrode 38 as shown in FIG. 9. It should be understood that the removal of layers 20 and 18 expose base layer 16. The etch of layers 20 and 18 is preferably done with a well known wet etch so that undercutting of emitter electrode 38 occurs. However, one of skill in the art will understand that it is also possible to employ a dry etch or a combination of wet and dry etches to achieve the undercutting. Following the removal of layers 20 and 18, base electrode 40 is formed on base layer 16 between second portion 36 and emitter electrode 38. In this embodiment, base electrode 40 comprises a layer of titanium upon which is formed a layer of platinum and then a layer of gold although other metal schemes may be employed. It should be understood that the process disclosed herein allows for base electrode 40 to be self-aligned with both second portion 36 of the collector electrode and emitter electrode 38. This alignment allows base electrode 40 to be fabricated relatively closely to both emitter electrode 38 and the collector electrode which is comprised of first portion 26 and second portion 36. The space between second portion 36 and emitter electrode 38 defines the lateral dimension of base electrode 40 and therefore is extremely important in defining the lateral dimensions of the entire device. It should be understood that during the formation of base electrode 40, metal 42 will form on second portion 36 and also on emitter electrode 38. This metal may be left or may be removed so that an even more planar structure is achieved.

Claims: A method of fabricating a heterojunction bipolar transistor comprising the steps of: forming a photoresist mask (22) on an emitter layer (18,20); using said photoresist mask (22) to form an opening (24) wherein said opening has positively sloping, tapered sidewalls and extends through the emitter layer (18,20) to a collector layer (12), and wherein said opening (24) undercuts said photoresist mask (22); forming a first portion (26) of a collector electrode (26,36) in said opening (24) so that said first portion (26) is not in contact with the sidewalls of said opening (24); isolating said first portion (26) with a dielectric (30) wherein said first portion (26), said dielectric (30) and said emitter layer (18,20) form a substantially planar structure; forming a second portion (36) of a collector electrode (26,36) on said first portion (26); forming an emitter electrode (38) adjacent to and substantially planar with said second portion (36); exposing a portion of a base layer (16) of the heterojunction bipolar transistor between said second portion (36) of a collector electrode (26,36) and said emitter electrode (38); and forming a base metal (40) on the exposed portion of said base layer (16). The method of claim 1 further comprising the steps of: prior to forming the photoresist mask (22); forming one or more collector layers (12,14) on a substrate (10); forming a base layer (16) on said one or more collector layers (12,14); and forming one or more emitter layers (18,20) on said base layer (16). The method of claim 2 wherein said substrate (10) is comprised of semi-insulating gallium arsenide, said one or more collector layers (12,14) and said base layer (16) are comprised of gallium arsenide, and said one or more emitter layers (18,20) are comprised of aluminum gallium arsenide. The method of claim 2 wherein the one or more collector layers (12,14) comprise a sub-collector layer (12) and a collector layer (14) both being comprised of gallium arsenide, the base layer (16) comprises gallium arsenide and the one or more emitter layers (18,20) comprise an aluminum gallium arsenide emitter layer (18) disposed on said base layer (16) and a gallium arsenide emitter layer (20) disposed on said aluminum gallium arsenide emitter layer (18). The method of claim 1 wherein the photoresist mask (22) is lifted-off following the forming a first portion (26) step. The method of claim 1 wherein the isolating said first portion (26) step further comprises the steps of: forming a dielectric layer (30) in the opening and on the first portion (26); forming a photoresist layer (32) on the dielectric layer (30); and etching said photoresist layer (32) and said dielectric layer (30) to the emitter layer (18,20). The method of claim 1 wherein the exposing said base layer (16) step includes removing the one or more emitter layers (18,20) from the structure excepting that disposed beneath the emitter metal (38). The method of claim 1 wherein the forming base metal (40) step includes forming base metal (40) between the emitter metal (38) and the second collector metal (36) so that said base metal (40) is self-aligned to said emitter metal (38) and said second collector metal (36). A heterojunction bipolar transistor comprising: a semi-insulating gallium arsenide substrate (10); a gallium arsenide sub-collector layer (12) on said semi-insulating gallium arsenide substrate (10); a gallium arsenide collector layer (14) on said gallium arsenide sub-collector layer (12); a gallium arsenide base layer (16) on said gallium arsenide collector layer (14); an aluminum gallium arsenide emitter layer (18) on said gallium arsenide base layer (16); a gallium arsenide emitter cap layer (20) on said aluminum gallium arsenide emitter layer (18); the invention being characterized by: an opening (24) having positively sloping, tapered sidewalls, wherein said opening (24) extends to said gallium arsenide sub-collector layer (12); a first collector metal (26) in said opening (24) so that said first collector metal (26) is not in contact with sidewalls of said opening (24), the surface of said first collector metal (26) being substantially planar with the top surface of the emitter cap layer (20); a dielectric layer (30) in said opening (24) between the sidewalls of said opening (24) and said first collector metal (26), wherein said dielectric layer (30) and said gallium arsenide emitter cap layer (20) are substantially planar; an emitter metal (38) on a second area of said gallium arsenide emitter cap layer (20); a second collector metal (36) patterned on said first collector metal (26); and a self-aligned base metal (40) on said gallium arsenide base layer (16) between said emitter metal (38) and said second collector metal (36).

Inventors: HUANG JENN-HWA, MANG LUKE, HUANG, JENN-HWA, MANG, LUKE
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1990
Main classification: H01L21
IPC codes: H01L21, H01L29