Integrated edge structure for high voltage semiconductor devices and related manufacturing process

Abstract: An integrated edge structure for a high voltage semiconductor device comprising a PN junction represented by a diffused region (3,7) of a first conductivity type extending from a semiconductor device top surface is described. The edge structure comprises a first lightly doped ring (4) of the first conductivity type obtained in a first lightly doped epitaxial layer (2) of a second conductivity type and surrounding said diffused region (3,7), and a second lightly doped ring (8) of the first conductivity type, superimposed on and merged with said first ring (4), obtained in a second lightly doped epitaxial layer (6) of the second conductivity type grown over the first epitaxial layer (2).

Description: The present invention relates to a process for manufacturing a high voltage semiconductor device. High voltage semiconductor devices include PN junctions which must withstand high reverse voltages; an example is represented by the base-collector junction of a bipolar NPN transistor. Most of PN junctions fabricated by planar technology essentially consist of a first semiconductor region of a given conductivity type diffused into a second semiconductor region of opposite conductivity type. An insulating oxide Layer is superimposed to the two regions and metal contacts are provided for electrical connection of the two semiconductor regions. A depletion region is associated with the PN junction, which can be considered as made up of two regions, a first one along the plane portion of the junction, a second one at the edges of said plane portion. The electric field has a different behaviour in the two regions. In the plane portion the equipotential lines are parallel to the junction, the maximum electric field is located at the junction and the breakdown occurs when it reaches the critical value Ecrit. At the junction edges, because of the finite junction depth, the equipotential lines are curved, and less spaced than in the plane portion. As a consequence, the electric field increases, higher electric fields are associated with smaller curvature radii, i.e. shallower junctions. The breakdown voltage of a PN diffused junction is usually lower than that of the corresponding plane junction, since the electric field in the edge region is much higher. The ratio between the breakdown voltage of the edge and the plane portion is thus below the unity. Several techniques have been developed to increase said ratio, essentially by changing the size of the depletion layer so to avoid local increases in the electric field which can lead to early breakdown. In one of said techniques, the metal contact of the diffused region of the junction is extended over the insulating oxide layer; in this way, a metal field-plate is formed which, acting as a shield, forces the equipotential lines to extend over a wider region, thus reducing the electric field. The high density of the equipotential lines in the oxide Layer does not represent a problem, because of the higher dielectric strength of the oxide layer with respect to the silicon. At the metal field-plate edge, however, the shielding action ceases, with an increase of the electric field in the surface region. To reduce the surface electric field, the metal field-plate can be extended over a thicker region of the oxide layer. Surface states, fixed interface charges and mobile charges can however lead to surface breakdown. In another technique, described in GB-A-2163597, one or more high resistivity rings are provided around the lateral edges of the junction; in this way the depletion layer spreads over wider regions, so that the spatial charge distribution is widened and the electric field is consequently reduced. The rings are formed by implantation and diffusion of dopants; by controlling the implanted dose and the diffusion process, it is possible to achieve the desired resistivity. Two or more concentrical rings, with increasing resistivity from the inner to the outer one, are necessary when the device must withstand high reverse voltages. However, peaks in the electric field value are observed at the interface between two rings, and at the edge of the outer ring. An increase in the number of rings leads to larger spreading of the depletion Layer, and the peaks in the electric field value are lowered. The effectiveness of the high resistivity rings could be improved by increasing their depth; this reduces the electric field, especially near the surface of the semiconductor device, where the existence of surface charges causes local variations of the electric field value. To obtain a deep junction by means of implantation and diffusion, it is necessary to use dopants having high diffusivity, and the diffusion process shall be extended in time and/or performed at higher temperatures; both of these solutions are disadvantageous for an industrial application. EP-A-322040 discloses a monolithic semiconductor device including at least one transistor of an integrated control circuit and a power transistor. The device comprises a semiconductor body comprised of two superimposed epitaxial layers. In the lower epitaxial layer buried portions of insulation doped regions are formed, and in the upper epitaxial layer annular portions of the insulation regions are formed, which extend to the buried portions. In view of the state of art described, object of the present invention is to provide a process for manufacturing a high voltage semiconductor device with integrated edge structure which does require neither the use of high-diffusivity dopant species nor long, high-temperature diffusion processes. According to the present invention, such object is attained by means of the process set forth in claim 1. Thanks to the present invention, it is possible to obtain deep edge structures which force the depletion layer to spread over regions sufficiently wide to decrease the electric field below the critical value and, being obtained in superimposed epitaxial layers, do not require high-diffusivity dopants or long, high-temperature diffusion processes. The features of the present invention shall be made more evident by the following detailed description of some embodiments, illustrated as non-limiting examples in the annexed drawings, wherein: Figure 1 is a cross-sectional view of a first semiconductor device for explaining the present invention; Figure 2 is a cross-sectional view of a second semiconductor device for explaining the invention; Figure 3-5 are cross-sectional views taken at different steps of a fabrication process in accordance with an embodiment of the present invention. Figure 6 and 7 are cross-sectional views of further semiconductor devices for explaining the invention. Figure 1 is a cross-sectional view of a discrete bipolar NPN power transistor; it is obtained in an N+ semiconductor substrate 1 over which a first N- epitaxial layer 2 is grown. A P-type diffused region 3 is obtained in said first epitaxial layer 2, and is surrounded by a first buried ring constituted by an annular P- region 4, also obtained by diffusion into the first epitaxial layer 2. An N+ region 5 is obtained by diffusion into the P-type diffused region 3. A second N- epitaxial layer 6 is grown over the first epitaxial layer 2, and an annular P+ region 7 is diffused into the second epitaxial layer 6 to contact the P-type diffused region 3. A second ring is constituted by an annular P- region 8, obtained by diffusion into the second epitaxial layer 6, and surrounds the annular P+ region 7. An N+ region 9 is diffused into the second epitaxial layer 6 to contact the N+ region 5. The entire surface is then covered with an insulating oxide layer 10, in which contact areas are provided to allow metal strips 11 and 12 to respectively contact the annular P+ region 7 and the N+ region 9. The bottom surface of the substrate 1 is contacted by a metal Layer 13. The N+ region 9 and the underlying N+ region 5 form an emitter region of the discrete bipolar NPN power transistor, which is connected by means of the metal strip 12 to an external emitter terminal E. The P-type diffused region 3 forms a base region of the discrete bipolar transistor, which is connected, by means of the annular P+ region 7 and the metal strip 11 to an external base terminal B. The first and second epitaxial Layers 2 and 6 and the semiconductor substrate 1 form a collector region of the discrete bipolar power transistor, which is connected by means of the metal layer 13 to an external collector terminal C. The two rings constituted by the annular P- regions 4 and 8 represent a high voltage edge structure for the bipolar power transistor, and prevent the base-collector junction from early breakdowns, induced by the finite curvature radius, under reverse bias. Figure 2 is a cross-sectional view of a bipolar low voltage transistor which can be integrated with other components on the same chip. In this case, the N+ region 9 has an annular shape, and delimits a portion 14 of the second epitaxial Layer 6; inside said portion 14 a P-type region 15 is diffused, and an N+ region 16 is then defined within said region 15. Contact areas are provided in the insulation oxide Layer 10 to allow metal strips 17, 18 and 19 to respectively contact the N+ region 16, the P-type region 15 and, through the N+ annular region 9, the N+ region 5. The N+ region 16 forms an emitter region of the bipolar low voltage transistor, while the P-type region 15 and the N+ region 5 form a base region and a collector region, respectively. The metal strips 17, 18 and 19 are respectively connected to external terminals E, B and C. The P-type region 3 and the P+ annular region 7 which surround the bipolar low voltage transistor constitute a region able to isolate the transistor from the substrate 1 of the chip: to ensure the electrical isolation, these P-type regions shall be negatively biased with respect to the substrate 1; again, the annular P- regions 4 and 8 form a high voltage edge structure which prevents the junction between the P-type region 3 and the epitaxial Layer 2 from early breakdowns. An embodiment of the manufacturing process according to the present invention, particularly suitable for obtaining the device shown in Figure 2, starts with the growth of the first lightly doped N- epitaxial layer 2 over the N+ semiconductor substrate 1. By means of a selective implant and a successive diffusion of dopants into the first epitaxial layer 2 the P-type region 3 is formed; by means of a further implant and diffusion, the N+ region 5, enclosed within said region 3, is then defined. A third masked implant, and a successive diffusion, allow the formation of the P- annular region 4, constituting the first buried ring, around the P-type region 3; it is to be stressed out that this step could as well be performed before the formation of the N+ region 5, or before the definition of the P-type region 3 (Fig. 3). The second lightly doped N- epitaxial Layer 6 is then grown over the entire top surface of the chip (Fig. 4). The annular P+ region 7 is then diffused into the second epitaxial layer 6 to contact the P-type region 3, followed by the masked implant and diffusion of the P- annular region 8, constituting the second ring, around the annular P+ region 7, in order to completely define the edge structure. Each of said P- annular regions 4 and 8 could be made up by two or more merged concentric regions with dopant concentration decreasing from the inner to the outer one, in order to further reduce the surface electric field, with techniques known to anyone skilled in the art. Up to now, the described succession of steps is suitable to obtain both the structures of Figures 1 and 2. To proceed with the fabrication of the structure of Figure 2, the N+ region 9 is formed by means of selective implant and diffusion (Fig. 5); the region 9 extends into the second epitaxial layer 6 to contact the underlying N+ region 5. In this case it has an annular shape and delimits a portion 14 of the second epitaxial Layer 6 into which the P-type region 15 forming the base region of the bipolar low voltage transistor is diffused; to form the emitter region, the N+ region 16 is diffused within the P-type region 15. The following process steps are similar to known processes, including the oxidation of the entire surface, the definition of contact areas and the deposition of a metal conductive layer which, after being selectively removed, forms the metal strips 17, 18 and 19. The surface of the chip is eventually passivated, for example with glass, and the bottom surface of the substrate 1 is covered with a metal conductive layer 13. In a possible variation to the previous embodiments, always within the scope of the present invention as defined by the appended claims, the definition of regions 3, 5 and 9 is omitted, the region 7 does not have an annular shape, and the metal strip 12 is not defined, so that a PN high voltage diode is formed (Figs. 6 and 7). This only requires not to perform some process steps, such as the implantation and diffusion of dopants into the first epitaxial layer 2 to form the diffused region 3, the implantation and diffusion of region 5 and of the annular region 9. In this case the anode region 7 is shallower than the edge structure formed by regions 4 and 8, and may as well extend into the first epitaxial layer 2 or be confined within the second epitaxial layer 6. It is also possible to use the teachings of the present invention to manufacture semiconductor devices such as those described in EP-459578 and EP-453026, as it would be if a Power MOSFET or an IGBT (Insulated Gate Bipolar Transistor) should be manufactured. In case of processes in which three or more epitaxial layer are grown, the edge structure manufactured in accordance with the present invention is defined in the upper two of such layers.

Claims: Process for manufacturing a high voltage semiconductor device comprising a lightly doped semiconductor body (2,6) of a first conductivity type having a top surface, a doped region (3,7) of a second conductivity type extending from the top surface of the semiconductor body thereinto and forming a PN junction with said semiconductor body, and a lightly doped ring (4,8) of the second conductivity type formed in the semiconductor body and laterally surrounding said doped region, characterized by comprising the following succession of steps: a) growth of a first lightly doped epitaxial layer (2) of the first conductivity type over a heavily doped semiconductor substrate (1); b) forming a first lightly doped ring (4) of the second conductivity type by masked implant and subsequent diffusion of dopants into said first epitaxial layer (2); c) growth of a second lightly doped epitaxial layer (6) of the first conductivity type over said first epitaxial layer (2); d) forming a diffused region (7) of the second conductivity type of said doped region (3,7) by masked implant and subsequent diffusion of dopants into said second epitaxial layer (6); e) forming a second lightly doped ring (8) of the second conductivity type disposed on and merged with said first ring (4) and laterally surrounding said diffused region (7) by masked implant and subsequent diffusion of dopants into said second epitaxial layer (6). Process according to claim 1, characterized in that , between steps a) and b), a buried region (3) of the second conductivity type is formed by masked implant and subsequent diffusion of dopants into said first epitaxial layer (2), said first ring (4) laterally surrounding the buried region (3), and said diffused region (7) being disposed on and merged with said buried region (3). Process according to claim 1, characterized in that the semiconductor substrate (1) is of the first conductivity type. Process according to claim 1, characterized in that the semiconductor substrate (1) is of the second conductivity type.

Inventors: CACCIOLA GIOVANNA, LEONARDI SALVATORE, ZAMBRANO RAFFAELE, LEONARDI, SALVATORE, ZAMBRANO, RAFFAELE
Applicants: CONS RIC MICROELETTRONICA, CONSORZIO PER LA RICERCA SULLA<BR>MICROELETTRONICA NEL MEZZOGIORNO
Application year: 1993
Main classification: H01L29
IPC codes: H01L29, H01L21