PROCESS FOR FABRICATING A SELF-ALIGNED BIPOLAR TRANSISTOR

Abstract: Using a single mask pattern (36,37) on a semiconductor substracte (31), a doped base contact region (39) adjacent the surface of the substrate, a buried insulating region (38) below the base contact region, and an insulating layer (40) on the base contact region are formed. Optionally, a metal or metal silicide base-electrode-taking-out is formed on the base contact region. Doped emitter (44) and intrinsic base (41) regions are formed below the mask pattern. A collector region (33) is defined by the base contact region and the buried insulating layer to be therebetween i.e. below the mask pattern. Hence, the bipolar transistor formed thereby has a size which is no larger than necessary, thereby reducing the collector-base capacitance, the base resistance, and the size of the device.

Description: This invention relates to a process for fabricating a bipolar transistor. More specifically, it relates to a process for fabricating a self-aligned bipolar transistor with a reduced collector-base junction capacitance and base resistance. A typical bipolar transistor comprises a semiconductor substrate having an n-type epitaxial semiconductor layer at the surface side thereof, in which an n-type region is isolated by a pn-junction or an insulating region to constitute a collector region. A base region is a p-type doped region formed in the collector region at the surface of the epitaxial layer, and an emitter region of an n⁺-type doped region is formed in the base region. An emitter electrode is formed on the surface of the n⁺-type emitter region and a base electrode is formed on the surface of the p-type base region. In this typical bipolar transistor, the base region must have a considerably larger area than that of the emitter, in order to provide insulation between the emitter and base electrodes based on a mask alignment. However, the area of the base region should preferably be as small as possible, since an essential active portion of the base region necessary for transistor operation is only the portion underlying the emitter region. The other portion of the base region, which is used only for taking out a base electrode, increases the base resistance and the collector-base junction capacitance, thereby decreasing the operating speed of the transistor, and prevents an increase of the degree of integration of the device. In order to alleviate the above problems, a bipolar transistor has been proposed in which an intrinsic base region is formed to have an area essentially necessary for transistor operation and a base-electrode-taking-out portion is made of polycrystalline silicon (T.Nakamura et al, "Self-Aligned Transistor with Sidewall Base Electrode", IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. DE-29, No. 4, April 1982, 596-600). In this transistor, the area of the collector-base junction is small, so that the collector-base junction capacitance is reduced and the operating speed of the transistor is increased. However, this device is fabricated by a process which is undesirably complex. EP-A-0036499 discloses a process for fabricating a bipolar transistor with an extrinsic base connection region. A number of deposition steps are followed by providing a mask, which is etched through to allow ion implantion of the exposed regions, followed by a single heat cycle. An object of the invention is to alleviate the above problems and to provide a simple process for fabricating a self-aligned bipolar transistor with a reduced base resistance and collector-base capacitance. According to the invention, there is provided a process for fabricating a self aligned bipolar transistor, comprising the steps of providing a semiconductor substrate of a second conductivity type; forming a first doped region of a first conductivity type adjacent to the surface of the semiconductor substrate; forming a mask layer on the first doped region, the mask layer having a pattern masking an active area of the transistor to be formed; locally implanting ions into the first doped region, using the mask layer as a mask, to form a buried insulating layer; locally introducing dopants in the first doped region at the surface of the substrate and above the buried insulating layer, using the mask layer as a mask, to form a base contact region of a second conductivity type opposite to the first conductivity type; forming a second insulating layer using the mask layer as a mask and then removing the mask layer; forming an intrinsic base region of the second conductivity type in the first doped region adjacent to the base contact region using the second insulating layer as a mask; forming an emitter region of the first conductivity type in the intrinsic base region and adjacent to the surface of the substrate; and using the first doped region of the first conductivity type below the intrinsic base region as a collector. In this process, preferably a metal or metal silicide layer, or a refractory metal or silicide thereof, may be inserted between the base contact region and the second insulating layer to reduce the base resistance. Such a metal or metal silicide layer may be formed by locally depositing a metal or metal silicide on to the base contact region, optionally followed by siliciding the deposited metal. In this case, a second insulating layer is formed above the base contact region. The second insulating layer may be formed by oxidizing, e.g., thermal oxidizing or anodizing, the surface of the base contact region itself or the surface of the above-mentioned metal or metal silicide layer, or by depositing an insulating material such as silicon dioxide on the base contact region or on the above-mentioned metal or metal silicide layer, or a combination thereof. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Fig. 1 is a sectional side view of a typical bipolar transistor of the prior art; Fig. 2 is a sectional side view of another self-aligned bipolar transistor of the prior art; Figs. 3 to 6 are sectional side views showing the main steps in a process for fabrication of a bipolar transistor in accordance with the invention; and Figs. 7 to 9 are sectional side views showing the main steps in another process for fabrication of a bipolar transistor in accordance with the invention. Before describing the present invention in detail, the prior art will be briefly described with reference to Figures 1 and 2 of the drawings. Fig. 1 illustrates a typical known bipolar transistor comprising a p-type semiconductor substrate 1, an n⁺-type buried region 2, an isolation insulating region 3, an n-type epitaxial semiconductor layer or a collector region 4, an n⁺-type collector contact region 5, a p-type base region 6, an n⁺-type emitter region 7, a collector electrode 8, a base electrode 9 and an emitter electrode 10. In this transistor, the p-type base region 6 is quite large in order to provide proper insulation between the base and emitter electrodes 9 and 10. Fig. 2 illustrates a self-aligned bipolar transistor proposed by T. Nakamura et al in "Self-Aligned Transistor with Sidewall Base Electrode", mentioned above. In Fig. 2 the transistor comprises a p-type silicon substrate 11, an n⁺-type buried region 12, a silicon dioxide layer 13, an n-type collector region 14, a p-type intrinsic base region 15, a p⁺-type extrinsic base region 16, a n⁺-type emitter region 17, an n⁺-type collector contact region 18, a collector electrode 19, a base electrode 20 and an emitter electrode 21. This bipolar transistor is fabricated by (1) locally etching the n-type epitaxial layer except at the portion where the collector region 14 and the intrinsic base region 15 are to be formed, (2) oxidizing the exposed surface of the epitaxial layer to form the oxide layer 13, (3) depositing and patterning polycrystalline silicon to form the extrinsic base region 16, (4) carrying out doping in a portion of the n-type expitaxial layer to form the p-type intrinsic base region 15, (5) oxidizing the surface of the polycrystalline extrinsic base region 16, and (6) carrying out doping in a portion of the p-type intrinsic base region 15 to form the n⁺-type emitter region. This fabrication process is rather complex. Figs. 3 to 6 illustrate a process for fabricating a self-aligned bipolar transistor according to the present invention. Referring to Fig. 3, the transistor comprises a p-type silicon substrate 31; an n⁺-type buried region 32 formed by implanting arsenic ions at an energy of 60 keV and a dosage of 10¹⁶ cm⁻² into the silicon substrate 31; an n-type epitaxially grown silicon layer 33; a locally oxidized layer 34 for isolation; an n⁺-type collector contact region 33c formed by phosphorus ion implantation at 100 keV and 5 x 10¹⁵ cm⁻²; a silicon dioxide (SiO₂) layer 35 having a thickness of 50 nm; a silicon nitride (Si₃N₄) layer 36 having a thickness of 150 nm, as an anti-oxidation layer, formed by chemical vapour deposition (CVD); and a phospho-silicate glass (PSG) layer 37 having a thickness of 1 to 1.5µm, formed by CVD. The SiO₂ layer 35 is formed by thermal oxidization in dry oxygen at 1000°C. The Si₃N₄ layer 36 is deposited by thermal decomposition of a gaseous mixture of monosilane (SiH₄) and ammonia (NH₃) under a reduced pressure of 133.3 Pa (1 Torr) at 900°C. The PSG layer 37 is deposited by thermal decomposition of a gaseous mixture of SH₄ and phosphine (PH₃) under a reduced pressure of 4 x 10⁴ Pa (300 Torr) at 400°C. By conventional lithography, a resist pattern (not shown) is formed on the PSG layer 37 at a portion under which the active region of the bipolar transistor is to be formed. The PSG and Si₃N₄ layers are locally etched by reactive ion etching (RIE), using the resist pattern as a mask. The RIE is carried out by applying an electric power of 100 W at a radio frequency of 13.56 MHz in a reduced atmosphere of trifluoromethane (CHF₃) as an etching gas. The thus locally etched PSG and Si₃N₄ layers 37 and 36 are shown in Fig. 3. Referring to Fig. 4 oxygen ions are implanted into the n-type expitaxial layer 33 and the substrate 31 spaced below the surface of the epitaxial layer 33, using the PSG and Si₃N₄ layers 37 and 36 as a mask, to form a buried insulation region 38. The implantation of oxygen ions is conducted at 200 keV and 10¹⁸ cm⁻². After implantation, annealing at 1100 to 1200°C in a nitrogen atmosphere is carried out to stabilize the buried insulating region 38, i.e. an SiO₂ region. Nitrogen ions, for example, may be implanted in place of oxygen ions to form the buried insulating region 38, i.e. an Si₃N₄ region. Further, boron ions are implanted into the n-type epitaxial layer 33 in a region from the surface thereof to the buried insulating region 38, using the PSG and Si₃N₄ layers 37 and 36 as a mask, to form a p⁺-type base contact region (extrinsic base region) 39. The implantation of boron ions is conducted at 30 keV and 10¹⁵ cm⁻². This implantation of boron may be carried out without the PSG layer 37. The sequence of the steps of the implantation of oxygen and boron, i.e., formation of the buried insulating region 38 and the base contact region 39 may be reversed. Referring to Fig. 5, the entire PSG layer 37, and the Si₃N₄ layer 36 except a region for transistor operation, are removed and thermal oxidization is carried out, using the Si₃N₄ layer 36 as a mask, to form an SiO₂ layer 40 having a thickness of 300 nm as a field oxidized layer defining the region of operation of the transistor therein. The oxidization is conducted by thermal oxidization in dry oxygen at 900°C. The Si₃N₄ layer 36 is removed by etching with hot phosphoric acid. Referring to Fig. 6, using the field SiO₂ layer 40 as a mask, boron ions are implanted at 30 keV and 10¹⁴ cm⁻² to form a p-type intrinsic base region 41. The SiO₂ layer 35 is then removed by fluoric acid to expose the surface of the epitaxial layer, which now has a p-type conductivity adjacent to the surface thereof. A polycrystalline silicon layer doped with arsenic at 10²¹ cm⁻³ is deposited by CVD over the entire surface in a thickness of 300 nm, which is then patterned to leave the polycrystalline layer only in a region for operation of the transistor, forming an emitter electrode 43. Alternatively, a non-doped polycrystalline silicon layer is deposited over the entire surface, followed by implanting arsenic ions at 150 keV and 10¹⁶ cm⁻² and patterning to form the emitter electrode 43. The CVD of the polycrystalline silicon is conducted by thermally decomposing SiH₄ gas under a reduced pressure of 133.3 Pa (1 Torr) at 600°C. An annealing or emitter drive is carried out at 950°C for 20 minutes to form an n⁺-type emitter region 44. The emitter region 44 may be made by ion implantation, etc. Thus, the main parts of a bipolar transistor are fabricated. Then, a base electrode of, e.g., aluminum, is formed on the field SiO₂ layer 40 to make contact with the base contact region 39 through a window formed in the field SiO₂ layer 40. A collector electrode 46 of, e.g., aluminum, is made concurrently with the base electrode 45. Thus, a self-aligned bipolar transistor is simply fabricated in accordance with the present invention. In this process, the buried insulating region 38, the p⁺-type base contact region 39 and the field SiO₂ layer 40 are made in alignment with the pattern of the original Si₃N₄ and PSG layers 36 and 37. Further, the p-type intrinsic base region 41 and n⁺-type emitter region 44 also may be made in alignment with the thus formed field SiO₂ layer 40. Thus, all of these layers are made from only one mask pattern. In this bipolar transistor, the emitter region 44, the intrinsic base region 41, and the collector region 33 are made so that they involve only a region essentially necessary for transistor operation. Furthermore, there is the buried insulating region 38 between the base contact-region 39 and the collector region 33 and 32, thus reducing the collector-base junction capacitance and the base resistance. The reduced base resistance is also due to a reduced resistance of the p⁺-type base contact region 39 due to a high concentration of dopants in a single crystalline region, which is enabled by a process according to the present invention. Figures 7 to 9 illustrate another embodiment of a process for fabricating a bipolar transistor according to the present invention. Also in this process, the same procedures described before with reference to Figs. 3 and 4 may be carried out. Thus, after forming the buried insulating region 38 and the p⁺-type base contact region 39, referring to Fig. 7, the SiO₂ layer 35 on the p⁺-type base contact region 39 is locally removed to expose the surface of the base contact region 39. On to the exposed base contact region 39, tungsten is locally deposited to form a tungsten layer 51 having a thickness of 100 nm. This local deposition is conducted by CVD from a gaseous mixture of tungsten fluoride (WF₆) and argon or nitrogen at a reduced pressure of 13.3 to 666.5 Pa (0.1 to 5 Torr) at a low temperature of 250 to 500°C (see, E.K. Broadbent and C.L. Ramiller "Selective Low Pressure Chemical Vapor Deposition of Tungsten", J. Electrochem. Soc. SOLID-STATE SCIENCE AND TECHNOLOGY, June 1984, pp 1427-1433). This tungsten layer 51 may be converted to silicide WSi₂ by annealing at 600 to 1000°C. Alternatively, titanium silicide (TiSi₂) may be locally deposited onto the exposed base contact region 39 to form a TiSi₂ layer having a thickness of 100 nm. This local deposition is conducted by CVD from a gaseous mixture of tetrachlorotitanium (TiCl₄) and trichlorosilane (SiHCl₃) in a carrier gas of hydrogen under a pressure of less than 1.33 x 10⁴ Pa (100 Torr) at 600 to 900°C (see Japanese Patent Application No. 60-23480). Further, implantation of boron ions to form the p⁺-type base contact region may be carried out after the metal or metal silicide layer 51 is formed, not before. After the metal or metal silicide layer, for example, WSi₂ layer 51, as a base-taking-out electrode, is formed, the surface thereof is oxidized by anodizing to form an anodized film 52. The surface of the WSi₂ layer 51 may be oxidized by thermal oxidization. If thermal oxidization is effected, an anti-oxidization layer 36 is necessary where an emitter is to be formed. Annealing is then conducted at 600 to 1000°C to accelerate the alloying of the WSi₂ layer 51. The metal or metal silicide layer 51 may be formed by a process other than local deposition, for example by a lift-off technique. Referring to Fig. 8, the Si₃N₄ layer 36 is removed and an SiO₂ layer 53 having a thickness of 200 nm, shown by a broken line in Fig. 8, is formed by CVD, followed by RIE, to remove the SiO₂ layer 53 uniformly from the surface, leaving a side wall 54. The thickness of the side wall 54 may be controlled by the degree of RIE. In this RIE, the surface of the epitaxial region 33 is exposed, but the SiO₂ layer 53 may be left on the anodized film 52. The formation of the SiO₂ layer 53 by CVD is not always necessary. After anodizing the surface of the TiSi₂ layer 51, local etching may be conducted to open a window in the Si₃N₄ and SiO₂ layers 36 and 35 where an emitter region is to be formed. Referring to Fig. 9, a p-type intrinsic base region 55, a polycrystalline emitter electrode 56, an n⁺-type emitter region 57, a base electrode 58, and a collector electrode 59 may be formed by similar procedures to those described before with reference to Fig. 6. Thus, a self-aligned bipolar transistor is accomplished. This bipolar transistor not only has the same advantages as those of the former bipolar transistor in Fig. 6, but also has an advantage in that the base resistance is further reduced since the intrinsic base region 55 and the base electrode 58 are electrically connected through the metal or metal silicide layer 51, which has a lower electrical resistance than the p⁺-type doped semiconductor region 39.

Claims: A process for fabricating a self-aligned bipolar transistor, comprising the steps of providing a semiconductor substrate of a second conductivity type (31); forming a first doped region (32,33) of a first conductivity type adjacent to the surface of the semiconductor substrate; forming a mask layer (36,37) on the first doped region, the mask layer having a pattern masking an active area of the transistor to be formed; locally implanting ions into the first doped region, using the mask layer as a mask, to form a buried insulating layer (38); locally introducing dopants in the first doped region at the surface of the substrate and above the buried insulating layer, using the mask layer as a mask, to form a base contact region (39) of a second conductivity type opposite to the first conductivity type; forming a second insulating layer (40) using the mask layer (36) as a mask and then removing the mask layer (36); forming an intrinsic base region (41) of the second conductivity type in the first doped region adjacent to the base contact region using the second insulating layer (40) as a mask; forming an emitter region (44) of the first conductivity type in the intrinsic base region and adjacent to the surface of the substrate; and using the first doped region of the first conductivity type below the intrinsic base region as a collector (33). A process according to claim 1, characterised in that the step of forming the buried insulating region (38) is prior to the step of forming the base contact region (39). A process according to claim 1, characterised in that the step of forming the base contact region (39) is prior to the step of forming the buried insulating layer (38). A process according to claim 1, characterised in that the step of forming the intrinsic base region (41) is prior to the step of forming the emitter region (44). A process according to claim 1, characterised in that the step of forming the emitter region (44) is prior to the step of forming the intrinsic base region (41). A process according to claim 1, characterised by the step of opening a window in the second insulating layer (40) to expose the surface of the base contact region (39) in the window, followed by forming a base electrode (45) in contact with the base contact region (39) through the window. A process according to any one of claims 1 to 6, characterised by the step of locally forming a metal or metal silicide layer (51) on the base contact region (39) before the step of forming the second insulating layer (52). A process according to claim 7, characterised in that the metal or metal silicide layer (51) is formed by local deposition of a metal or metal silicide on the base contact region (39), or by local deposition of a metal on the base contact region followed by silicidation of the deposited metal. A process according to any preceding claim, characterised in that the second insulating layer (52) is formed by making the surface of the metal or metal silicide layer insulating. A process according to any preceding claim, characterised in that the second insulating layer (52) is formed by anodizing the surface of the metal or metal silicide layer (51). A process according to claim 7, characterised in that at least a part of the second insulating layer (52) is formed by depositing an insulating material above the metal or metal silicide layer (51). A process according to claim 11, characterised in that after depositing the insulating material above the metal or metal silicide layer (51) and above the first doped region where at least a part of the mask layer (36) is removed, anisotropic etching is conducted to remove the insulating material except a side wall portion (54) of the metal or metal silicide layer, whereby an insulating wall having a desired lateral thickness of the insulating wall is formed at the side wall portion of the metal or metal silicide layer. A process according to any preceding claim, characterised in that the semiconductor substrate (31) is formed of silicon. A process according to any preceding claim, characterised in that the buried insulating region (38) is formed by implanting oxygen or nitrogen ions. A process according to any one of claims 7 to 12, characterised in that the locally deposited metal or metal silicide (51) is a refractory metal or a silicide thereof.

Inventors: GOTO, HIROSHI, HIDESHIMA, OSAMU
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: H01L21
IPC codes: H01L29, H01L21