METHOD OF FABRICATING A LATERAL PNP TRANSISTOR

Abstract: A method for making a lateral PNP transistor (simul­ taneously with an NPN transistor) and the resultant device wherein a first mask (76) defines a base-width by the resistor implant (78, 80) for a P-type resistor and a second mask (86) is overlaid asymmetricially on said first mask to partially cover the collector (80). At the same time that the NPN extrinsic base contact is made, P-type dopants are intro­ duced in the areas (88, 89) exposed by the first and second masks to provide an emitter (92) and a collector contact (94) for the PNP transistor, said transistor thus featuring a graded collector (96, 94) with a high punch-through voltage.

Description: Background of the Invention Technical Field This invention relates to a method of fabricating an improved lateral PNP transistor, and in particular relates to a method of fabricating a lateral PNP transistor with a graded collector simultaneously with fabricating NPN transistors and P-type resistors. Prior Art One type of transistor which has found wide application in integrated circuits is the lateral PNP transistor. In this type of transistor, two regions near the surface of a semiconducting material are doped P-type and form respectively the emitter and collector. The emitter and collector are separated along the surface by an N-type region, which forms the base of the transistor. In order to obtain high operating speeds, the base is made as narrow as possible. However, two problems ensue. If the base-collector junction is reverse-biased, the depletion region extends into the narrow base region and at a punch-through voltage will extend through the base to the emitter. Once the depletion region has punched through, current control of the transistor is lost. If however, the depletion width is maintained relatively narrow for a given voltage by increasing the doping level of the base region, the resultant high electric field in the depletion region may exceed the threshold electric field for avalanche breakdown. The current multiplication associated with avalanche also destroys current control. The fabrication of PNP transistors becomes particularly difficult when they are to be included in an integrated circuit fabricated with an NPN technology. That is, the integrated circuit is to include both NPN and PNP transistors. As with any integrated circuit method, the number of processing steps and particularly of mask levels should be minimized. The necessity for simultaneously fabricating both NPN and PNP transistors has caused design trade-offs which sacrifice the performance of the PNP transistors built. A typical processing sequence for fabricating a conventional vertical NPN bipolar transistor will now be described. As shown in FIG. 1, a P-type silicon substrate 20 is overlaid with an oxide layer 22 which is etched to provide a collector window 24 and an isolation window 26 surrounding the collector window 24. Then with a series of photo-resist masking steps and diffusion steps, an N⁺ sub-collector 28 and a P⁺ isolation region 30 are formed on the surface of the substrate 20. Thereafter the oxide layer 22 is stripped and an N⁻ epitaxial layer 32 is grown, as shown in FIG. 2. Thereafter, as shown in FIG. 3, additional masking and diffusion steps are used to diffuse N-type and P-type dopants from the surface to form an N⁺ sub-collector reach-through 34 that joins the N⁺ sub-collector 28 and to form a larger P⁺ isolation region 36. A field oxide 38 is grown on top of this structure that has a thickness greater over the P⁺ isolation region 36. Then, as shown in FIG. 4, ion implantation is used to form a P-type base P(B) 40 at the surface of a part of the N⁺ sub-collector 28 within the isolation region 36. The ion implantation is done through the field oxide 38 and the resultant base 40 further defines an N⁻ collector 42. The field oxide 38 serves to protect the surface of ion-implanted regions. A blocking mask 44 of photo-resist is used as shown in FIG. 5 to cover the transistor region within the P⁺ isolation region 36 and thereafter another ion implantation of P-type dopants is performed in the outer region 74 of the N⁻ epitaxial layer through the field oxide 38 to form a P-type resistor P(R) 46. It should be noted that the use of two ion implantation steps for forming the P(B) 40 and P(R) 46 regions, respectively, is predicated upon the ion implant conditions for those regions being specified differently. Prior art processes typically require resistor region sheet rho (i.e. sheet resistivity) to be different from base region sheet rho. Regions 40 and 46 typically have different depths, region 46 usually being shallower than region 40. However, both can have about the same depth, as shown here. As shown in FIG. 6, with yet another mask and another ion implantation through the field oxide 38, P-type dopants are used to form a P⁺ base contact 48 and a P⁺ resistor contact 50. Then, as shown in FIG. 7, an N⁺ dopant is diffused into the base 40 to form an N⁺ emitter 52. The result is a vertical NPN transistor between the emitter 52, the base 40 and the collector 42. The field oxide is preferentially etched above the base contact 48 part of the base 40, above the emitter 52, above the N⁺ sub-collector reach-through 34 and above the resistor contact 50. Finally, interconnection metallurgy is evaporated onto the surface in a predetermined pattern to form a base lead 54, an emitter lead 56, a collector lead 58 and a resistor lead 60. Although methods are known to fabricate PNP transistors and particularly lateral PNP transistors within this sequence of NPN fabrication steps, these transistors have suffered the previously described problems of low punch-through voltage, low avalanche breakdown voltage or poor performance as measured by gain and frequency response. Methods have been described which increase the performance of transistors by providing graded doping within the base region. Such a method has been described for a lateral PNP transistor by E.A. Valsamakis in an article entitled "Lateral PNP with Gain Bandwidth Product", in the IBM Technical Disclosure Bulletin , Vol. 13, 1970, pg. 1457. A completely different method has been described for a graded base PNP transistor in U.S. Patent 3,873,989 issued to Schinella et al. In U.S. Patent 4,283,236 a method for fabricating improved double diffused lateral PNP transistors with substantially reduced series base resistance has been described. However, it is not known how any of these methods can be easily incorporated within an NPN fabrication technology. Accordingly, it is an object of this invention to provide a method of fabricating a high performance lateral PNP transistor with a high punch-through voltage and a high avalanche breakdown voltage. It is yet a further object to provide a method of fabricating a PNP transistor with a minimal change from standard and conventional NPN fabrication technology. The invention, as claimed, can be summarized in one aspect thereof as a method of fabricating a lateral PNP transistor consistent with NPN technology and providing PNP transistors having high performance and high operating voltages. In this method the P-type resistor implant through a field oxide is elsewhere also used to define a base width between an emitter and a collector of a PNP transistor. A second level photo-resist is then applied over part of the base photo-resist and part of the collector. The field oxide is removed from the emitter and the area of the collector not covered by either level of photo-resist. The P⁺ contact formation in the NPN technology is used to form the emitter and collector of the PNP transistor. As a result, the PNP transistor thus fabricated has a graded collector. Brief Description of the Drawings FIGS. 1-7 describe a prior art method of fabricating a vertical NPN transistor, as well as a P-type resistor. FIGS. 8-11 describe a method of fabricating the PNP transistor of this invention. FIG. 12 is a graph of the doping profile of a PNP transistor built according to this invention. FIG. 13 is an alternative form of a PNP transistor built according to this invention. Detailed Description of the Preferred Embodiments The fabrication of a lateral PNP transistor, according to this invention, can be easily integrated into the NPN fabrication previously described with reference to FIGS. 1-7. In this respect, the PNP transistor can be considered a free lateral PNP transistor because it involves no major additional processing steps, although the steps are somewhat modified, from those required for the NPN technology. Referring now to FIG. 8, a structure of an N⁻ epitaxial layer 70 and an overlying field oxide 72 are fabricated according to the previously described NPN technology at the step shown in FIG. 3. The N⁻ epitaxial layer can, for instance, be that part 74 of the N⁻ epitaxial layer isolated from the NPN transistor by the isolation region 36, as shown in FIG. 3. Although not shown in FIG. 8, a buried N⁺ region provided with a reach-through N⁺ region similar to the regions 28 and 34 of FIG. 3 are supposed to be present in FIG. 8; the said reach-through region would be located outside the cross-section shown in FIG. 8. A level of photo-resist 76 is deposited on top of the thin portion of the field oxide 72 and its width W B defines the width of the base of the PNP transistor. This photo-resist layer 76 is deposited preferably at the same time as the blocking mask 44 shown in FIG. 5 which is used to mask the implant of the P-type resistor 46. The photo-resist level 76 is then plasma-hardened or alternatively hardened by ultraviolet radiation and then high temperature post-baked. (This hardening of photo-resist 76 is necessary so that it is not dissolved during the development processing of the overlying photo-resist layer 86, to be described.) Thereafter the resistor implant passing through the field oxide 72 forms a nascent emitter region 78 and a nascent collector region 80. Then a second photo-resist layer 84 and 86 is simultaneously deposited and then patterned to define an emitter contact window 88 and a collector contact window 89 as shown in FIG. 9, and base contact and resistor contact windows (not shown) corresponding to base contact 48 and resistor contact 50 of FIG. 6. The second photo-resist layer 86 partially overlays the first photo-resist layer 76 and also overlays part of the nascent collector region 80, as shown in FIG. 9. As a result, the first photo-resist layer 76 continues to define the emitter-base junction of the PNP transistor. This junction is self-aligned and no additional level-to-level tolerances need to be assigned to the emitter-base junction. Then, as shown in FIG. 10, the emitter window 88 and collector window 89 are opened by etching off the silicon dioxide 72 by reactive ion etching. All of the nascent emitter region 78 and part of the nascent collector contact region 80 are exposed by this opening. Emitter and collector contacts are then formed by a high dose ion implantation of boron to make the two regions degenerate or nearly degenerate. Then, as illustrated in FIG. 11, both levels of photo-resist 76, 84 and 86 are stripped and an interconnection metallurgy 90 and 91 is deposited. The pattern covers all of the exposed emitter and collector regions 78 and 80. The interconnection metallurgy 90 and 91 can be either an aluminum-silicon-copper alloy or heavily doped P-type polysilicon. In the latter case, the emitter/collector second ion-implantation step can be avoided. Instead, a heat treatment step is carried out to drive the P-type dopants from the polysilicon 90 and 91 in to form a P⁺ emitter 92 and a P⁺ collector contact 94. The part of the nascent collector 80 which is not further doped in this drive-in forms a main collector 96. The combination of the main collector 96 and the P⁺ collector contact 94 forms a graded collector. The doping profile that results from this drive-in is shown by profile 98 in FIG. 12. Profile 82 is the doping profile for the epitaxial layer 70 which has a depth of about 0.7 µm on top of the substrate 20. Profile 83 represents the acceptor concentration introduced by the resistor ion implantation which has a depth of about 0.5 µm. The profiles shown in FIG. 12 are the final profiles which may have been broadened by heat treatment subsequent to implantation. Of course, when the acceptor concentration N A as represented by profile 83 falls below the donor concentration N D as represented by profile 82, then a reversal of conductivity type occurs, because the effective carrier type and concentration is represented by the larger value. As is evident by the two P-type profiles 83 and 98, the collector contact 94 is not simply a surface contact but extends approximately to the depth of the main collector 96. As a result, the conduction path is substantially horizontal into the collector 96 and 94. The absence of additional conduction paths vertically into the area of the emitter contact 92 eliminates slow reponse on the emitter side of the PNP transistor caused by vertical injection. The N-type doping profile 82 is the profile for the base, the P-type profile 83 is the profile for the main collector 96, and the P-type profile 98 is the profile for the emitter 92 and the collector 94. Also included in the structure for the lateral PNP transistor, but not shown in FIG. 11, is an N⁺ reach-through to the N⁻ base region 70. This reach-through is similar to the reach-through 34 to the N⁺ sub-collector 28 shown for the NPN transistor of FIG. 7. Both reach-throughs perform the same purpose of providing a surface contact to a buried region. Aside from the compatibility of this process with NPN technology, the lateral PNP transistor of FIG. 11 has the following advantages. The base width W B is defined by a single level of photo-resist 76, although additional side diffusion occurs in a controlled way in subsequent heat treating steps. The emitter-base junction is self-aligned so that the emitter 92 can be designed with minimal surface area. A small emitter 92 reduces vertical injection and provides higher lateral injection efficiency, has a low capacitance and thus provides a higher speed transistor and one with higher current gain. The main collector 96 has a low doping and a high resistance (same sheet rho as resistor region 46 of FIG. 5) and thus helps to reduce the field concentration at the base-collector junction, resulting in an increased punch-through voltage and reduced avalanche multiplication at the collector-base junction. The highly doped and relatively large P⁺ collector contact 94 reduces the collector resistance. An alternative form of the resultant PNP transistor is shown in FIG. 13. Device definition is provided by thick oxide regions 100 and 102 (or called shallow SiO₂ trenches) that reach down to the buried N⁺ layer 71. Device isolation is provided by an even deeper oxide region 104 (or called deep SiO₂ trenches) that reaches down to the P⁻ substrate 20 and replaces the P⁺ isolation region 36. An N⁺ reach-through 106 extending down to the N⁺ layer 71 provides a contact to the base epitaxial N⁻ region 70. In other respects, this device corresponds to the PNP transistor shown in FIG. 11. A numerical simulation of the PNP transistor built according to this invention illustrated in FIG. 13 shows that if the base width W B equals 1.5 µm and if the area of the emitter 92 equals 8.2 µm², then the current gain is equal to 43 at V EB = 0.6 V (I C = 82 nA) and is 2.2 at V EB = 0.9 V (I C = 0.32 mA). In comparison with a conventional lateral PNP transistor with an emitter area of 12.7 µm² and with resistor ion implants in both the emitter and collector, this device at V EB = 0.9 V has twice the current gain and 27% less capacitance across the emitter-base junction.

Claims: A method for simultaneously making an asymmetrical lateral PNP transistor having a graded collector junction and NPN transistors and P-type resistors, including the steps of: providing a semiconducting region (20) into which said PNP and NPN transistors and said P-type resistors (46) are to be formed, said semiconducting region (20) containing isolated buried N⁺ regions (71) and overlying epitaxial N⁻ regions (70,74); simultaneously forming said P-type resistors (46) and a P⁻ region 80, adjacent a portion of one of said N⁻ regions (70) to define the collector-base junction of said PNP transistor by the steps of: forming a first mask layer (76, 44) over portions of said N⁻ regions (70, 74), said first mask layer (76,44) having portions defining a base width for said PNP transistor and an area for said P-type resistors (46), and introducing a P-type dopant into the areas (80, 78, 46) of said N⁻ regions (70) not covered by said first mask layer (76,44); simultaneously forming the base contact regions (48) of said NPN transistors, a P⁺ emitter region (92) adjacent another portion of said N⁻ region (70) to define the emitter-base junction of said PNP transistor, and a P⁺ collector contact region (94) of said PNP transistor adjacent to said P⁻ region (96) by the steps of: removing said first mask layer (44) in areas of said NPN transistors and said P-type resistors; then forming a second mask layer (86, 84) over said portion of said first mask layer (76) defining the PNP base width and over an adjacent portion of said P⁻ region (96), said second mask layer (86,84) further defining base contacts (48) for said NPN transistors and contacts (50) for said P-type resistors (46); and then introducing further P-type dopants into the unmasked portions of P⁻ regions (78, part of 80); wherein said P⁺ collector contact region (94) is formed substantially to the depth of said P⁻ region (80,96); and forming an N+ region (106), simultaneously with forming the collector reach-through (34) of said NPN transistors, extending from the surface of said semiconducting region (20) to said isolated N⁺ region (71) for establishing the base contact (106) of said PNP transistor.

Inventors: ANANTHA, NARASIPUR G., GAUR, SANTOSH P., HUANG, YI-SHIOU, TSANG, PAUL J.
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1984
Main classification: H01L21
IPC codes: H01L29, H01L21