A method of manufacturing a vertical-channel MOSFET

Abstract: The following steps are performed on a wafer of semiconductor material having a layer (1) with n conductivity: a) implanting n impurity ions and p impurity ions in an area of the layer and subjecting the wafer to a high-temperature treatment; the impurities, the implantation doses and energies, and the high-temperature treatment time and temperature being such as to form a first, p region (49) and a second, n region (50) which forms a pn junction with the first region (49); b) hollowing out a trench (43) which intersects the first region (49) and the second region (50), c) forming a dielectric coating (44) on the lateral surface of the trench (43), d) depositing electrically-conductive material (51) in the trench (43) in contact with the dielectric (44), and e) forming elements (60, 61, 62) for electrical contact with the layer (1), with the second region (50), and with the electrically-conductive material (51) inside the trench (43), in order to produce drain (D), source (S) and gate (G) electrodes of the MOSFET, respectively. A submicrometric vertical-channel MOSFET of optimal quality and reproducibility is thus produced by a method compatible with DPSA technology.

Description: The present invention relates to a method of manufacturing a semiconductor device comprising a vertical-channel MOSFET transistor. As is known, a vertical-channel MOSFET transistor has its source and drain regions in layers which have the same type of conductivity and which are parallel to one another and to the front surface of the wafer of semiconductor material in which the transistor is formed, these layers being separated by a body region having the second type of conductivity. The gate dielectric and the gate electrode are formed on the lateral walls of a trench hollowed out in the wafer so as to intersect the source and drain regions. The channel of the MOSFET is included in the portion of the body region delimited by the side walls of the trench. EP-A-0617 468 discloses a vertical - channel MOS-transistor. A technique for the manufacture of electronic devices with high dynamic performance, that is, with a high frequency or fast switching speed, which can achieve the very small dimensions and the extremely thin junctions necessary for such performance, is known. Isolation between active areas of the device is achieved by the hollowing-out of narrow trenches which are filled at least partially with dielectric material (trench isolation). This technique also permits the formation of self-aligned structures with the use of two polycrystalline silicon layers and is therefore known as the "double polysilicon self-aligned" or DPSA technique. With this technique, it is possible to produce vertical npn bipolar transistors capable of operating at transition frequencies greater than 50 GHz, but, up to now, this technique has not been suitable for the production of other components of equally good quality, except at the cost of process complications which involve unacceptable increased expense. For example, lateral pnp transistors can be produced without modification of the standard process, but with rather limited performance. There is therefore a need, within the scope of this technology, to produce good-quality circuit components for use in combination with the npn bipolar transistors in order to extend the application of this technology to the design of more complex integrated circuits. The main object of the present invention is to propose a method of manufacturing a semiconductor device comprising a vertical-channel MOSFET transistor which is compatible with the DPSA technique. This object is achieved, according to the invention, by the method defined and characterized in the first claim. The invention will be understood better from the following detailed description of an embodiment thereof given with reference to the appended drawings, in which: Figure 1 shows, in section, a vertical npn transistor formed by the DPSA technique on a wafer of semiconductor material, and Figures 2a to 2m show a portion of the wafer, in section, during various successive steps of the method according to the invention for the manufacture of a vertical n-channel MOSFET transistor (to simplify the drawings and in view of the symmetry of the structure, Figures 2g to 2m show only a portion of the structure). Figure 1 shows, in section, a portion of a wafer of semiconductor material, for example, monocrystalline silicon, comprising a p- substrate 2 and an n layer 1 comprising a buried n+ layer 3 and an epitaxial n- layer 4. A trench isolation structure, generally indicated 6, formed from silicon dioxide 7 and polycrystalline silicon 8, extends from the front surface 35 of the wafer as far as the substrate 2, forming a frame which encloses the structure of the vertical npn transistor, isolating it from the rest of the wafer. Plaques 10 and 11 of silicon dioxide (field oxide) extend on the surface 35 and are partially embedded in the epitaxial layer 4. The plaque 10 completes the isolation of the structure of the vertical npn transistor on the surface and the plaque 11 delimits and isolates from one another two areas indicated 12 and 13. A diffused n+ region 14 (a sinker) extends from the area 12, through the epitaxial layer 4 as far as the buried layer 3 and has the function of a deep collector contact. A layer 15 of n+ doped polycrystalline silicon 15 is formed on the area 12 in contact with the sinker region 14 and a metal strip 16 is in contact with the layer 15 to form the collector electrode C of the transistor. Two diffused p+ regions 18 and 19 extend in the epitaxial layer 14 from the area 13 and together form the so-called extrinsic base of the transistor. They are joined by a diffused p region 20 which forms the so-called intrinsic base of the transistor. Two regions 22 and 23 of p+ doped polycrystalline silicon extend on the regions 18 and 19, respectively. The regions 18 and 19 are formed by the diffusion into the epitaxial layer 4 of p-type impurities contained in the polycrystalline silicon of the regions 22 and 23. The region 23 also extends partly on the field-oxide plaque 11, a metal element 24 which forms the base electrode B of the transistor being disposed above the region 23. A layer 25 of dielectric material, for example, vapour-phase deposited silicon dioxide (VAPOX) covers the polycrystalline silicon regions 15, 22 and 23, except for the contact areas of the metal collector and base electrodes 16 and 24. The opposite side edges of the regions 22 and 23 and of the overlying portions of the silicon dioxide layer 25, and the regions of the area 13 adjacent these edges are covered by structures with L-shaped cross-sections, constituted by silicon dioxide 26 and silicon nitride 27. An n+ doped polycrystalline silicon insert 28 fills the space delimited by the L-shaped structures 26 and 27 and is in contact, on the area 13, with an extremely thin n+ diffused region 29. This region is formed by diffusing n-type impurities contained in the polycrystalline silicon of the insert 28 into the intrinsic base region 20. A metal strip 30 extends on the insert 28 and forms the emitter electrode E of the transistor. According to the invention, vertical-channel MOSFET transistors are formed by a process fully compatible with the DPSA technique. With reference to Figures 2a to 2m, in which references the same as those of Figure 1 indicate the same elements, the method provides for the following steps: a wafer of semiconductor material ready to undergo DPSA treatment such as that for producing the npn transistor structure of Figure 1 described above, is covered with a layer 40 of material impermeable to the ionic implantations to be performed, for example, a photoresist layer; a window 41 is opened in the layer 40; the wafer is subjected to implantation with ions of an n-type doping impurity with low diffusivity, for example, arsenic, with a low energy, for example, 10-40 KeV and a high dose, for example, 1-5·10 15 atoms/cm 2 , and to implantation with ions of a p-type doping impurity with a high energy, for example, 80-250 KeV and a medium dose, for example, 1-7·10 13 atoms/cm 2 (Figure 2a); after the removal of the photoresist layer (Figure 2b), the wafer is subjected to selective dry etching of the silicon in accordance with the standard DPSA method in order to hollow out an isolation trench 42 and a gate trench 43, for example 1µm wide, extending from the front surface into the substrate 2, for example, to a depth of 6µm, as shown (not to scale) in Figure 2c; the isolation trench 42 extends as a rectangular frame in plan and delimits within it a portion of silicon which is to contain the vertical-channel MOSFET transistor; the gate trench 43 extends, in plan, along the centre-line of the rectangular portion of silicon, naturally without touching the isolation trench 42; the wafer is subjected to a high-temperature treatment (for example, at 920°C for 60-120 min.) in an oxidizing atmosphere so as to form a thin (for example, 10nm) layer 44 of silicon dioxide to constitute the gate dielectric; a layer 45 of polycrystalline silicon (for example 0.25µm thick) is then deposited and, on top of this, a layer 46, for example, of the same thickness, of silicon nitride (Figure 2d) ; a photoresist mask 47 is formed by a normal photolithographic process (Figure 2d); the portions of the nitride which are not covered by the mask are removed by chemical etching and the mask 47 is removed so that only the portions 46' of the layer 46 which were protected by the mask remain (Figure 2e); the wafer is subjected to a high-temperature treatment (for example, at 950°C for 60-120 min.) in an oxidizing atmosphere, which causes the growth of a relatively thick layer (for example 0.3µm) of silicon dioxide on the portions of the wafer which are not covered by the nitride 46', including the walls of the isolation trench 42 (Figure 2f); in this step, all of the uncovered polycrystalline silicon is transformed into silicon dioxide and the doping impurities implanted in the monocrystalline silicon diffuse to form a p+ region, indicated 49, which forms a pn junction with the buried n+ layer 3, and an n+ region, indicated 50, which forms a pn junction with the region 49 and is delimited by the front surface of the wafer; the regions 49 and 50 constitute the body region and the source region of the MOSFET transistor, respectively, and are intersected by the side walls of the gate trench 43; the nitride layer 46' is removed by chemical etching (Figure 2g); n-doped polycrystalline silicon, indicated 51, (doped, for example, with arsenic) is deposited until the trenches are completely filled and the wafer is then subjected to controlled dry etching to remove the polycrystalline silicon until the field oxide 48 is uncovered (Figure 2h); a further layer 52 of silicon nitride is deposited and is removed by photolithography over the field oxide 48 and over the isolation trench 42 (Figure 2i); the wafer is subjected to a high-temperature treatment (for example, at 1000°C for 60-100 min.) in an oxidizing atmosphere so as to cause the growth of a "cap" 53 of silicon dioxide on the polycrystalline silicon of the isolation trench 42 and to increase the thickness of the field oxide, indicated 48' in Figure 2j and in the following figures; the residual nitride 52 is removed by chemical etching and the underlying polycrystalline silicon 51 is removed in controlled manner by further chemical etching until the portion of the silicon dioxide layer 44 which is on the front surface of the wafer is uncovered; after this operation, the level of the polycrystalline silicon which is in the gate trench 43 is substantially aligned with the front surface of the wafer (Figures 2k); a window 53 is opened in the oxide 48', above the epitaxial n- layer 4 by a photolithographic process and an n+ deep contact (sinker) region 54 which reaches the buried n+ layer 3 is formed by implantation of ions of an n-type doping impurity, for example, phosphorus; the silicon dioxide layer 44 is removed from the front surface of the wafer; the resulting structure is shown in Figure 21; a silicon dioxide layer 55 is formed by vapour-phase deposition (VAPOX) and then, with the use of the normal steps of the DPSA technique, elements are formed for electrical contact with the sinker region 54 to produce the drain electrode D of the MOSFET, with the n+ region 50 to produce the source electrode S of the MOSFET, and with the doped, and hence electrically-conductive, polycrystalline silicon 51 of the gate trench 43, to produce the gate electrode G of the MOSFET; in this embodiment, the source contact element is also in contact with the p+ body region 49; the resulting structure is shown in Figure 2m; the VAPOX layer 55 in which windows 56, 57, 58, 59 are formed for the drain, body, source and gate contacts, respectively, can be recognized therein; inside the drain, body and source windows 56, 57 and 58 there is a thin layer of doped polycrystalline silicon produced by the same process steps which are necessary to produce the layer 15 or the insert 28 of n+ doped polycrystalline for the collector or emitter contact, respectively, of the npn transistor such as that shown in Figure 1. Metal strips 60, 61, 62 are formed by the same process steps which are necessary to form the emitter, base and collector electrodes of the npn transistor of Figure 1. A vertical-channel MOSFET transistor is thus produced with only slight modification of the standard DPSA process. In fact the only steps to be added are the photolithography to define the area for the implantation of the body and source regions and the implantations themselves. By suitable selection of the implantation parameters, that is, the doping species to be implanted, and the implantation doses and energies, as well as the times and the temperatures of the heat treatments to which the wafer is subjected during the process as a whole, it is possible to produce channels of submicrometric length (for example 0.2µm) with optimal precision and repeatability. Naturally, precise control of the parameters for the formation of the epitaxial layer 4 is also necessary. Since all of these parameters can be controlled very accurately, the resulting vertical-channel MOSFET structures can be reproduced equally accurately. The method described above illustrates the invention used within the scope of DPSA technology. However, the invention may also advantageously be implemented independently of this technology when it is necessary to form vertical-channel MOSFET transistors with submicrometric channels. Although the method according to the invention has been described with reference to an example in which an n-type vertical-channel MOSFET transistor is produced, clearly the method can also be used to produce p-type vertical-channel MOSFET transistors, simply by forming drain, body and source regions with conductivities complementary to those of the example described. Naturally, pairs of complementary vertical-channel MOSFET transistors can also be formed together.

Claims: A method of manufacturing a semiconductor device comprising a vertical-channel MOSFET transistor on a wafer of semiconductor material having a layer (1) of a first type of conductivity (n) delimited by a front surface of the wafer and lying on a substrate (2) of a second conductivity type (p), the method comprising the following steps for the formation of the MOSFET: defining an area on the front surface, implanting impurity ions of a first type and impurity ions of a second type in the area thus defined and subjecting the wafer to a high-temperature treatment, the impurities of the first type and of the second type, the implantation doses and energies, and the high-temperature treatment time and temperature selected being such as to form a first region (49) of a second type of conductivity (p) which forms a pn junction with the rest of the layer (1), and a second region (50) of the first type of conductivity (n) which forms a pn junction with the first region (49) and is delimited by the front surface; hollowing out in the wafer a gate trench (43) which intersects the first region (49) and the second region (50) with at least one of its lateral surfaces and extends into the substrate (2), forming a coating (44) of dielectric material on the lateral surface of the gate trench (43) which intersects the first and second regions, depositing electrically-conductive material (51) in the gate trench (43) in contact with the coating (44) of dielectric material, and forming elements (60, 61, 62) for electrical contact, respectively, with the layer (1) outside the first region (49) and the second region (50), with the second region (50), and with the electrically-conductive material (51) inside the gate trench (43), in order to produce drain (D), source (S) and gate (G) electrodes of the MOSFET, respectively, in which the step of hollowing-out a gate trench (43) in the wafer takes place simultaneously with the hollowing-out of isolation trenches (42) between components of the semiconductor device. A method according to Claim 1, in which the element (61) for electrical contact with the second region (50) is also in electrical contact with the first region (49).

Inventors: PATTI DAVIDE, PINTO ANGELO, PATTI, DAVIDE, PINTO, ANGELO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: H01L29
IPC codes: H01L29, H01L21