Bipolar power transistor with high collector breakdown voltage and related manufacturing process

Abstract: There is described a bipolar power transistor with high breakdown voltage, obtained in a heavily doped semiconductor substrate (1) of the N type over which a lightly doped N type layer (2), constituting a collector region of the transistor, is superimposed; the transistor has a base region comprising a heavily doped P type diffusion (4) which extends into the lightly doped N type layer (2) from a top surface, and an emitter region constituted by a heavily doped N type diffusion (11) extending from said top surface within said heavily doped P type diffusion (4); the heavily doped P type diffusion (4) is obtained within a deep lightly doped P type diffusion (3), extending from said top surface into the lightly doped N type layer (2) and formed with acceptor impurities represented by atoms of aluminium.

Description: The present invention relates with a bipolar power transistor with high collector breakdown voltage and a related manufacturing process. It is known that in bipolar power transistors fabricated by planar technology, the collector region sustains almost the entire reverse voltage applied to the base-collector junction. This is a consequence of the electric field distribution across the junction, depending on the doping concentration profile of the transistor's base region, which, in the case of an NPN transistor, is obtained by diffusion of a high concentration of boron into a lightly doped N- epitaxial layer constituting the transistor's collector region. Because of such limitation, the structure used for today high voltage bipolar transistors is not the planar one, but the so-called "mesa": this name stems from the fact that each transistor resembles a little mesa whose vertical walls terminate the base-collector junction. One of the basic technologies by which mesa transistors are fabricated is known as MultiEpitaxial Mesa (MEM); in the MEM technology, described for example in pages 29-32, Solid State Technology, March 1976, the base region comprises a lightly doped P- epitaxial layer, grown over an N- epitaxial layer (from which the name of the technology) and in which a P+ region is diffused; the corresponding base doping concentration profile causes a significant percentage of the total reverse voltage applied to the base-collector junction to be sustained by the P- epitaxial portion of the base region, and not only by the N- epitaxial collector. The electric field distribution in both the base and the collector regions determines some of the electrical characteristics of the transistor such as the gain dependance on the collector current and the ruggedness in secondary breakdown and in direct breakdown. Even though this technology allows the fabrication of transistors with collector breakdown voltages up to 2000V, it involves the passivation of the base-collector junction with glass, which is deposited, by means of a complex and expensive process, in an annular moat obtained by etching the silicon down to the N- epitaxial layer; such moat, circumscribing each transistor to isolate its base from the other transistors' bases, gives rise to the mesa structure of the transistor. As a consequence, mesa transistors show low reliability with respect to transistors obtained by planar technologies, wherein the base-collector junction is passivated by means of thermal oxide. US-A-4,074,293 discloses a bipolar power transistor according to the preamble of claim 1. In view of the state of the art described, the object of the present invention is to provide a bipolar power transistor obtained with a planar manufacturing process, which however shows a high base-collector breakdown voltage, and an electric field distribution in the base region like that of mesa technology. According to the present invention, such object is attained by means of a bipolar power transistor as set forth in claim 1. In a preferred embodiment of the invention, around said deep lightly doped P type diffused region there is provided a plurality of merged concentric rings, each of which is constituted by an annular P type region extending from said top surface into the lightly doped N type layer and formed with acceptor impurities represented by atoms of aluminium, each ring having a dopant concentration which is lower than that of the adjacent internal ring and higher than that of the adjacent external ring. According to the invention, there is also accomplished a process for the manufacturing of said bipolar power transistor, characterized in that it comprises the following succession of steps: a) growth of a lightly doped N type epitaxial layer over a heavily doped semiconductor substrate of the N type; b) oxide growth over a top surface of said epitaxial layer; c) masked implant of aluminium atoms into said epitaxial layer; d) drive-in diffusion of said aluminium atoms into the epitaxial layer to form a deep lightly doped P type base region; e) masked implant of a high concentration of acceptor dopants into said deep lightly doped P type base region; f) drive-in diffusion of said acceptor dopants into the deep lightly doped P type base region to obtain a heavily doped P type base region within said deep lightly doped P type base region; g) masked implant of a high concentration of donor dopants into said heavily doped P type base region; h) drive-in diffusion of said donor dopants into the heavily doped P type base region to obtain a diffused heavily doped emitter region. Thanks to the present invention, it is possible to produce bipolar power transistors with planar structure, which however, having a base doping profile similar to that of mesa transistors obtained in MEM technology, show high breakdown voltages. The features of the present invention shall be made more evident by the following detailed description of its preferred embodiment, illustrated as a non-limiting example in the annexed drawings, wherein: Figure 1 is a cross-sectional view of a bipolar power transistor according to the preferred embodiment of the invention; Figure 2 is a doping concentration profile diagram (Concentration versus Depth) of said bipolar power transistor; Figure 3 is a diagram (Electric field versus Distance from the base-collector junction) showing the electric field distribution across a reverse-biased base-collector junction; Figures 4 to 7 are cross-sectional views of the bipolar power transistor taken at intermediate steps of the manufacturing process according to the invention. With reference to Figure 1, a bipolar power transistor according to the invention is obtained into a semiconductor substrate 1 of the N+ type, generally with (111) crystal orientation; over the substrate 1 there is superimposed a lightly doped N- type epitaxial layer 2, having dopant concentration ranging form 10 13 to 10 14 atoms per cm 3 and thickness of 100-150µm. The epitaxial layer 2, together with the semiconductor substrate 1, forms a collector region of the bipolar power transistor. A deep, aluminium-doped P type region 3 extends from a top surface into the epitaxial layer 2 to a depth of approximately 30-40µm, while a heavily doped P+ type region 4, doped for example with boron, extends from said top surface within the deep P type region 3 to a depth of about 10µm; the P+ type region 4, together with the P type deep region 3, forms a base region of the bipolar power transistor. A heavily doped N+ type region 11 extends from said top surface within the P+ type region 4, and forms an emitter region of the bipolar power transistor. Three concentrical rings 5, 6 and 7, constituted by lightly doped P- type aluminium-doped regions extending from the top surface into the epitaxial layer 2, are provided around the deep P type region 3; the dopant concentration of the ring 5 is higher than that of the ring 6, which in turn has a dopant concentration higher than that of the ring 7. An insulating oxide layer 8 is superimposed over the top surface, and contact areas are provided to allow strips 9 and 10 of a conductive layer to contact the P+ type region 4 and the N+ type region 11, respectively. The strip 9 is connected to a base terminal B, while the strip 10 is connected to an emitter terminal E. The substrate 1 is also connected to a collector terminal C. Thanks to the fact that aluminium in silicon behaves as an acceptor dopant and has a high diffusivity (approximately ten times higher than the diffusivity of boron), it is possible to obtain a deep base-collector junction having the dopant concentration profile shown in Figure 2; it appears that the base region has a high dopant concentration near the emitter-base junction (region 4), suitable to achieve low base resistance, while near the base-collector junction (region 3) the base dopant concentration is much lower, and comparable with that of the epitaxial layer 2; this allows the base region to sustain a significant part of the total reverse voltage applied to the base-collector junction, and consequently leads to a higher breakdown voltage, since the peak value of the electric field at the metallurgical junction diminishes. This is shown in Figure 3, wherein Vp and Vn are the fractions of the total reverse voltage applied to the base-collector junction sustained by the base and collector regions respectively, and are given by the areas subtended by the electric file distribution in said two regions. A distinguishing characteristic of aluminium with respect to other high-diffusivity species is that aluminium does not introduce deep states responsible of an increase in leakage currents and consequently of a decrease in the transistor's current gain. The presence of the lightly doped rings 5, 6 and 7 leads to a further increase in the base-collector breakdown voltage, since said rings, smoothing the corners of the P type region 3, reduce the edge effects on the electric field. The manufacturing process according to the present invention starts with the growth of the N- epitaxial layer 2 over the N+ semiconductor substrate 1; after the oxidation of the entire top surface of the epitaxial layer 2 (Fig. 4), the oxide layer 12 thus formed is selectively etched, and atoms of aluminium are implanted into selected areas of the epitaxial layer 2; with a subsequent drive-in diffusion step the deep P type region 3 and the rings 5, 6 and 7 are simultaneously formed; the higher dopant concentration of ring 5 with respect to ring 6, and of ring 6 with respect to ring 7, is achieved by etching the oxide layer 12 to form annular windows 13, 14 and 15 with decreasing area, so to decrease the aluminium dose implanted into silicon (Fig. 5). From now on, the process is similar to a traditional planar process. After the oxidation of the top surface, there is performed a selective implant of a high dose of boron into the deep P type region 3; a subsequent drive-in diffusion allows the formation of the heavily doped P+ type region 4, and the simultaneous further diffusion of the aluminium atoms of region 3 makes it possible to achieve a base-collector junction depth of about 30-40µm (Fig. 6). A further oxidation of the top surface is then performed, and donor impurities are selectively implanted into the P+ type region 4 to form, after a drive-in diffusion, the heavily doped N+ emitter region 11 (Fig. 7). After the definition of contact areas, a conductive layer is deposited over the entire surface and selectively removed to allow the formation of the strips 9 and 10; an electrical contact to the bottom surface of the substrate 1 is also provided. The surface of the chip is finally passivated, for example by means of silicon nitride.

Claims: A bipolar power transistor with high breakdown voltage, obtained in a heavily doped semiconductor substrate (1) of the N type over which a lightly doped N type layer (2), constituting a collector region of the transistor, is superimposed, said transistor having a base region comprising a deep lightly doped P type diffused region (3), extending from a top surface of the lightly doped N type layer (2) thereinto and a heavily doped P type diffused region (4) extending from said top surface within the deep lightly doped diffused region (3), and an emitter region constituted by a heavily doped N type diffused region (11) extending from said top surface within said heavily doped P type diffused region (4), characterized in that said deep lightly doped P type diffused region (3), is formed with acceptor impurities represented by atoms of aluminium. A bipolar power transistor according to claim 1, characterized in that around said deep lightly doped P type diffused region there is provided a plurality of merged concentric rings (5,6,7), each of them constituted by an annular P type region extending from said top surface into the lightly doped N type layer (2) and formed with acceptor impurities represented by atoms of aluminium, each ring (5,6,7) having a dopant concentration which is lower than that of the adjacent internal ring and higher than that of the adjacent external ring. A bipolar power transistor according to claim 1, characterized in that said lightly doped N type layer (2) is constituted by an epitaxial layer. A bipolar power transistor according to claim 1, characterized in that said heavily doped P type diffused region (4) contains acceptor impurities which are represented by atoms of boron. A process for the manufacturing of a bipolar power transistor with high breakdown voltage, characterized in that it comprises the following succession of steps: a) growth of a lightly doped N type epitaxial layer (2) over a heavily doped semiconductor substrate (1) of the N type; b) oxide growth over a top surface of said epitaxial layer (2); c) masked implant of aluminium atoms into said epitaxial layer (2); d) drive-in diffusion of said aluminium atoms into the epitaxial layer (2) to form a deep lightly doped P type base region (3); e) masked implant of a high concentration of acceptor dopants into said deep lightly doped P type base region (3); f) drive-in diffusion of said acceptor dopants into the deep lightly doped P type base region (3) to obtain a heavily doped P type base region (4) within said deep lightly doped P type base region (3); g) masked implant of a high concentration of donor dopants into said heavily doped P type base region (4); h) drive-in diffusion of said donor dopants into the heavily doped P type base region (4) to obtain a diffused heavily doped emitter region (11). A process according to claim 5, characterized in that a plurality of P type aluminium-doped concentrical rings (5,6,7) are formed around and simultaneously with said deep lightly doped P type base region (3). A process according to claim 6, characterized in that said rings (5,6,7) have a dopant concentration which diminishes going from the most internal ring (5) to the most external ring (7). A process according to claim 5, characterized in that said acceptor dopants are ions of boron.

Inventors: COFFA SALVATORE, FRISINA FERRUCCIO, COFFA, SALVATORE, FRISINA, FERRUCCIO
Applicants: CONS RIC MICROELETTRONICA, CONSORZIO PER LA RICERCA SULLA MICROELETTRONICA NEL MEZZOGIORNO
Application year: 1993
Main classification: H01L29
IPC codes: H01L29, H01L21