METHOD OF MAKING A BIPOLAR JUNCTION TRANSISTOR

Abstract: A method of making a bipolar junction transistor includes providing a substrate (10) having an n-type collector region (13) therein. A thin layer of polycrystalline or amorphous silicon (19) is formed by LPCVD while in-situ doping the silicon with boron as it is deposited. The deposited layer is thermally annealed to recrystallize the deposited silicon, thereby forming an epitaxial base region (11), having an abrupt junction with the collector region (13). A portion of the deposited silicon is then implanted with boron ions to form a base contact region (37). Next, a layer of LPCVD silicon doped with arsenic or phosphorus is provided and formed to provide an emitter region (51) and a collector contact region (52), a further annealing step increasing the concentration of active donors in these regions (51, 52) while simultaneously annealing the base contact region (37). Controlled annealing can result in the retention of a polycrystalline emitter structure.

Description: METHOD OF MAKING A BIPOLAR JUNCTION TRANSISTOR Technical Field This invention relates to a method of the kind for making an NPN bipolar junction transistor, including the step of providing a semiconductor substrate having a single crystal silicon n-type collector region therein, depositing so as to overlie the collector region a layer of polycrystalline or amorphous silicon, and annealing the deposited sili- con. Background Art Bipolar junction transistors are well known semiconductor devices which are widely used, e.g., as high-speed switches in integrated logic circuits and for processing microwave signals. The term "bipolar" is utilized because charge carriers of both positive and negative polarities participate in the operation of the transistors. Typically, a bipolar transistor includes a collector region, a base region and an emitter region. In accordance with the type of bipo¬ lar transistor under consideration herein, the collec¬ tor and emitter regions are made of n-type semi¬ conductor material, while the base region is made of p-type semiconductor material. Such a transistor is generally referred to as an NPN transistor. By applying a positive potential to the base with respect to the emitter, the collector current can be switched on. By controlling the dimensions and other parame¬ ters of the regions, the collector current can exceed the base current by an order of magnitude or two which increase is known as "gain". In order to achieve high switching speeds and gain, it is important to have a "narrow" or thin base so as to reduce the transit time for an electron injected by the emitter to pass through the base and OMPI so that virtually all the electrons injected by the emitter are transported across the base to the collec¬ tor. In order to minimize the base width, a great deal of effort has been expended to control doping concentrations and diffusing processes. In recent years, this control has been effected by various ion- implantation processes and by computer-controlled diffusion furnaces. As a general practice, the boron-doped base region of an NPN bipolar transistor is obtained by diffusion and must be annealed to relieve the implant damage and to activate the acceptors. This results in diffusion profile characteristics which make it diffi- cult to maintain an abrupt base-collector junction and a well-defined, and controlled, base thickness. Thermal annealing is still necessary when the boron atoms have been implanted by ion implantation. In accordance with one approach for producing very narrow base width, ultra-high-speed bipolar transistors, epitaxial deposition is used to form a single crystal doped base region. Such an approach is disclosed in the article "An Uncompensated Silicon Bipolar Junction Transistor Fabricated Using Molecular Beam Epitaxy" by Swartz et al., IEEE Electron Device Letters, Vol. EDL-2, No. 11, November 1981, pages 293- 295. However this process is done at 850°C which can cause diffusion at junctions, thus reducing switching speed and gain. Moreover, this appears to be a time- consuming operation, and the equipment for performing molecular beam epitaxy is very expensive. A method of the kind specified is known from an article in IBM Technical Disclosure Bulletin, Vol. 22, No. 11, April 1980, pages 5053-5054. According to the known method a substrate in the form of a thin single-crystal silicon membrane has provided thereon a polysilicon base region into which is evaporated or OMPI shallowly implanted a p-dopant. A CVD layer of sili¬ con dioxide having an emitter hold therein is next formed. Then an n+ polysilicon region for the emitter is formed in the hole. The base and the emitter poly- silicon regions are annealed with a laser or an elec¬ tron beam to crystallize them with the same orien¬ tation as the collector. This known method has the advantage of providing a narrow width base. Disclosure of the Invention According to the present invention, there is provided a method of the kind specified, characterized by the steps of: depositing said polycrystalline or amorphous silicon while in-situ doping with boron the silicon being deposited; thermal pulse annealing the deposited silicon to recrystallize the deposited silicon overlying the collector region into a single epitaxial crystalline base region; providing n-type doped polycrystalline silicon to form an emitter region overlying said base region; and effecting a thermal pulse operation to increase the concentration of active donors in said emitter region. It will be appreciated that an advantage of the invention is that the utilization of two separate and distinct thermal pulsing steps provides the capa- bility of making a transistor having a polysilicon emitter. This is advantageous as compare with tran¬ sistors having single crystal silicon emitters since it is found that a relatively high gain is exhibited for a polysilicon emitter, apparently due to a low hole recombination lifetime due to the presence of grain boundaries in the polysilicon. A further advan¬ tage is that abrupt base-collector and base-emitter junctions can be realized. Still another advantage is that relatively simple deposition techniques are utilized, obviating the complexities and expense in¬ herent in molecular beam epitaxy. In brief summary of a preferred embodiment of the invention, there is disclosed a method of treating polysilicon deposited on previously fabricated single crystal material wherein the polysilicon is in-situ doped with impurities and thereafter recrystallized by thermal-pulse annealing. The thermal-pulse annealing is performed in a vacuum at a temperature high enough to effect recrystallization and solid phase epitaxy but low enough to avoid migration of dopants from the polysilicon into the previously-fabricated material so as to maintain the abrupt junction with the previous¬ ly-fabricated material that existed subsequent to the deposition of polysilicon. Preferably, the temperature at which recrys- tallization and solid phase epitaxy is effected is within the range of approximately 600°C - 650°C. The afore-described steps are particularly suitable for converting a thin layer of polysilicon into a base region having an abrupt junction with the collector region. The preferred embodiment further contemplates depositing the polysilicon to a thickness of 0.02 micrometer (urn) to 0.5 urn and in-situ doping the poly¬ silicon with boron to a concentration of approximately 10 16 to 1019 boron atoms per cubic centimeter of polysilicon, while maintaining a deposition temper¬ ature in the range of approximately 600°C to 650°C, to thereby initially form a thin base layer with an abrupt base-collector junction. The preferred embodiment further contemplates performing the aforecited steps in combination with subsequently forming emitter and collector contract regions from a layer of n+ polysilicon which is ther¬ mal pulse annealed, to maintain an abrupt base-emitter junction and to anneal a previously implanted p+ base contact region. Brief Description of the Drawings One embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a diagrammatic, cross-sectional elevation of a bipolar junction transistor substruc¬ ture, prior to fabrication of the base region; Fig. 2 illustrates the transistor structure of Fig. 1 after a boron-doped polysilicon layer has been deposited thereon; Fig. 3 illustrates the structure of Fig. 2 after a layer of silicon dioxide insulation has been deposited over the structure and after the base con¬ tact region deposited in Fig. 2 has been formed; Fig. 4 illustrates a subsequent deposition of n+ polysilicon over the silicon dioxide; Fig. 5 illustrates the transistor structure after the subsequent layer of polysilicon has been etched to form an emitter and collector contact to prepare the transistor structure for a subsequent metallic deposit, which metallic deposit establishes lead connections with the base contact, the collector contact and the emitter, and Fig. 6 illustrates the completed structure with metal connections deposited on the structure of Fig. 5. Best Mode for Carrying Out the Invention Referring now to Fig. 1, there is shown a typical transistor substructure, designated. generally by the numeral 10, which is ready to receive deposi¬ tion of a base region 11 (see Figs. 2-6). The sub¬ structure 10 includes a heavily doped p ++ silicon substrate 12 upon which a lightly doped, n" epitaxial, single crystal collector 13 has been formed. The collector 13 is surrounded by an isolation oxide 14. Optionally, the collector 13 may have an n + subcollector 16 which is accessed from the surface 17 of the collector, via an n + contact which is formed by sinker diffusion 18. The isolation oxide 14 shown in Fig. 1 is preferably formed by a conventional recessed LOCOS process. Referring now to Fig. 2, the substructure 10 receives a layer of polysilicon 19 on the surface 17, which layer of polysilicon 19 overlaps the entire substrate. The polysilicon layer 19 is subsequently processed to form the base 11. The silicon layer 19 can either be polycrystalline or amorphous. The polysilicon layer 19 is deposited in a thin layer having a thickness in the range of 0.02 to 0.5 urn by using low pressure chemical vapor deposition (LPCVD) . While the polysilicon layer 19 is being deposited, it is in-situ doped with boron atoms to a concentration of approximately 1016 to 1019 boron atoms per cubic centimeter of polysilicon. As has been previously emphasized in this application, it is important to keep the polysilicon layer 19 thin be¬ cause the transit time for injected electrons through the subsequently-formed base 11 needs to be kept short in order to provide a fast switching speed for the transistor. By using a LPCVD process, layers as thin as 0.01 um may be deposited and uniformity of + 0.005 um may be maintained across large silicon wafers. In addition, LPCVD processes are ideal for forming very thin layers of p-type silicon on n-type substrates. Moreover, LPCVD deposition of poly- crystalline silicon can be done at temperatures of about 620°C to 700°C, while deposition of amorphous silicon can be done at temperatures below 600°C. Accordingly, a very abrupt pn base-collector junction is formed with minimal inter-diffusion of impurities when depositing either amorphous or polycrystalline silicon since diffusion at these temperatures is negligible. The polysilicon 19 is then photomasked (not shown) and etched to remove the dotted line portion 19' so as to expose the n+ contact 18. The photo¬ resist is then stripped to reexpose the polysilicon layer 19. In order to be suitable for the base region 11 of a transistor, the polysilicon layer 19 must be "regrown" as a single crystal by epitaxial recrystallization. This regrowth of the polysilicon layer 19 utilizes the single crystal, epitaxial n- region 13 as a "seed". It is performed in a vacuum or inert ambi¬ ent by thermal pulse annealing. By limiting the thermal pulse heating step to less than 1200°C for less than 10 seconds, interdiffusion of dopants across the surface 17 from either the collector material or base material is negligible and the abrupt junction between the collector 13 and base 11 is maintained. By using thermal pulse annealing, the energy suffi¬ cient to recrystallize the polysilicon layer 19 may be imparted to the polysilicon layer in less than one second of exposure time, although 5 to 10 seconds is typical. Thermal-pulse recrystallization can be performed using laser annealing, infrared radiation, arc lamps, or strip heaters. Preferably, recrystalli- zation is effected by solid phase epitaxy using a temperature in the range of approximately 600°C - 650°C. After the base region 11 is formed, the sub¬ sequent regions of the vertical NPN transistor are deposited. Referring now to Fig. 3, a layer 26 of silicon dioxide is deposited by chemical vapor deposi¬ tion over the entire structure shown in Fig. 2. The silicon dioxide layer 26 is then photomasked by a layer of photoresist (not shown) over the portions of silicon dioxide shown remaining in Fig. 3 and etched to expose a base contact surface 27 and an emitter contact surface 28 on the base 11, and to expose a ' ' OM?X V'. ive collector contact surface 29 on the collector sinker 18 of the subcollector 16. The photoresist (not shown) is then stripped, and a layer of photoresist 36, shown in Fig. 3, is deposited over the entire structure with the exception of the base contact surface 27 and portions of the silicon dioxide 26 adjacent the base contact surface. The base contact 27 is then implanted with additional boron atoms to form a p + region 37. The photoresist 36 (shown in Fig. 3) is then stripped so that the fabrication is ready to receive a subsequent layer of polysilicon, as is shown in Fig. 4. Referring now to Fig. 4, a subsequent, thin layer of polysilicon 40 is deposited by LPCVD at a temperature in the range of 600°C to 700°C over the structure of Fig. 3 after the photoresist 36 has been stripped. The polysilicon layer 40 has a thickness in the range of 0.1 to 0.4 um and is in-situ doped with phosphorus or arsenic atoms during the deposition. The density of the phosphorus or arsenic atoms in the polysilicon 40 is approximately 10^0 to 5 x 1021 atoms per cubic centimeter. Preferably, phosphorus is used to dope the polysilicon layer 40 because phosphorus has a higher active donor concentration than arsenic and arsenic tends to cluster. Arsenic is more widely used because it tends to form more abrupt junctions due to a lower diffusion coefficient. However, by keeping the deposition temperature low, i.e. 600°C to 700°C, and utilizing in-situ doping during deposition, phosphorus-doped polysilicon can be deposited with an abrupt junction with underlying surfaces and with no need for further diffusion. Consequently, the disad¬ vantages of phosphorus doping are minimized so that the advantages can be utilized. Referring now to Fig. 5, the n ++ polysilicon 40 of Fig. 4 is photomasked over all areas with the exception of an emitter area 51 and a collector con- tact area 52. Upon etching the unmasked polysilicon away, only the emitter 51 and collector 52 remain. The surface 27 of the base contact 37 is exposed by the etching. After the photoresist (not shown) used to protect the emitter 51 and collector contact 52 is removed, the base contact, emitter, and collector contact are all exposed for subsequent connections to the circuit in which the transistor is used. Preferably, the etch to define areas 51 and 52 is performed with potassium-hydroxide, or an EPW (ethylene-diamine, pyrocatechol, water) solution which is a selective etchant for n-type silicon in that it etches p-type silicon about thirty times slower than n-type silicon. Consequently, the etch does not erode the surface 27 of the p + base contact- 37. Areas 51 and 52 of n-type polysilicon are then thermal-pulsed annealed to increase the concen¬ tration of active donors, while simultaneously anneal¬ ing the p + implanted base contact region 37. The thermal-pulse anneal preferably takes place in a very short length of time, i.e. less than 10 seconds, and at a temperature less than 1200°C. It will be appre¬ ciated that by suitably choosing the thermal-pulse annealing parameters, the emitter 51 can obtain a polycrystalline structure. As is seen in Fig. 6, the procedure for establishing contact with the base contact 37, emitter 51 and collector contact 52 can be the standard proce¬ dure wherein metal 53, such as aluminum or aluminum alloy, is deposited on the transistor structure, photomasked and etched to delineate the connections 54B, E, and C, respectively, to the base 37, emitter 51 and collector 52 contacts.

Claims: WHAT IS CLAIMED IS: 1. A method of making an NPN bipolar junc¬ tion transistor, including the step of providing a semiconductor substrate (10) having a single crystal silicon n-type collector region (13) therein, deposit- ing so as to overlie the collector region (13) a layer of polycrystalline or amorphous silicon (19) , and annealing the deposited silicon, characterized by the steps of: depositing said polycrystalline or amor¬ phous silicon while in-situ doping with boron the silicon being deposited; thermally annealing the deposited silicon (19) to recrystallize the deposited silicon overlying the collector region (13) into a single epitaxial crystalline base region (11) ; provid¬ ing n-type doped polycrystalline silicon to form an emitter region (51) overlying said base region (11) ; and effecting a thermal pulsing operation to increase the concentration of active donors in said emitter region (51). 2. A method according to claim 1, char¬ acterized in that after said step of thermally annealing there is effected the step of implanting boron ions into a portion of the deposited silicon (19) to form a base contact region (37) for the base region (11) , and in that said thermal pulsing opera¬ tion is additionally effective to anneal the base contact region (37) . 3. A method according to claim 2, char¬ acterized in that said thermal annealing is performed by thermal pulsing at a temperature range of approxi¬ mately 700°C to 1200°C for less than ten seconds such that substantially no interdiffusion of impurities takes place between said collector region (13) and said base region (11) . 4. A method according to claim 1, char¬ acterized in that said step of depositing silicon is effected by low pressure chemical vapor deposition to a thickness of about 0.02 micrometers to 0.5 micro¬ meters, and in that the concentration of boron ions is approximately 10l6 to 1019 ions per cubic centimeter. 5. A method according to claim 4, char¬ acterized in that said step of depositing silicon is effected by depositing polycrystalline silicon at a temperature in the range of approximately 600°C to 700°C. 6. A method according to claim 1, char¬ acterized in that said step of depositing silicon is effected by depositing amorphous silicon at a temperature less than about 600°C. 7. A method according to claim 1, char¬ acterized in that said n-type doped polycrystalline silicon is formed by low pressure chemical vapor deposition at a temperature in the range of approxi¬ mately 600°C to 700°C, to provide a deposited thick¬ ness in the range of approximately 0.1 micrometers to 0.4 micrometers, while effecting in-situ doping with phosphorus or arsenic to a concentration of approxi¬ mately lθ20 to 5 x lθ21 atoms per cubic centimeter. 8. A method according to claim 7, char¬ acterized in that said n-type doped polycrystalline silicon is additionally formed to provide a collector contact (52) for the collector region (13), said thermal pulsing operation additionally serving to increase the concentration of active donors in the collector contact (52) . 10. A method according to claim 1, char¬ acterized in that said emitter region (51) retains a polycrystalline structure after said thermal pulsing operation. OMPI

Inventors: COLLINS GEORGE JOSEPH, SULLIVAN PAUL ANDREW, COLLINS, GEORGE, JOSEPH, SULLIVAN, PAUL, ANDREW
Applicants: NCR CO, NCR CORPORATION
Application year: 1984
Main classification: H01L21
IPC codes: H01L21, H01L29