Fully isolated high-voltage MOS device

Abstract

A semiconductor structure includes a semiconductor substrate; an n-type tub extending from a top surface of the semiconductor substrate into the semiconductor substrate, wherein the n-type tub comprises a bottom buried in the semiconductor substrate; a p-type buried layer (PBL) on a bottom of the tub, wherein the p-type buried layer is buried in the semiconductor substrate; and a high-voltage n-type metal-oxide-semiconductor (HVNMOS) device over the PBL and within a region encircled by sides of the n-type tub.

Claims

1. A method comprising: implanting a p-type semiconductor substrate to form an n-type buried layer (NBL); implanting a surface portion of the p-type semiconductor substrate directly over the n-type buried layer (NBL) to form a p-type buried layer (PBL); implanting the p-type semiconductor substrate to form a first n-type ring encircling and contacting the NBL, wherein a top surface of the first n-type ring is higher than a top surface of the NBL; epitaxially growing a semiconductor layer over the p-type semiconductor substrate; and implanting the semiconductor layer to form a second n-type ring directly over and vertically overlapping the first n-type ring. 2. The method of claim 1 further comprising forming a high-voltage n-type metal-oxide-semiconductor (HVNMOS) device directly over the NBL and in a region encircled by the second n-type ring. 3. The method of claim 1 , wherein the second n-type ring extends from a top surface to a bottom surface of the semiconductor layer, and wherein the second n-type ring contacts the first n-type ring. 4. The method of claim 1 further comprising implanting the semiconductor layer to form a high-voltage n-well (HVNW) region within a region encircled by the second n-type ring, wherein a bottom surface of the HVNW region is vertically spaced apart from the NBL by the PBL. 5. The method of claim 4 , wherein the HVNW region and the second n-type ring are formed by a same implantation step. 6. The method of claim 1 , wherein after the step of epitaxially growing the semiconductor layer, the PBL diffuses into the semiconductor layer, with no portion of the PBL remaining in the p-type semiconductor substrate. 7. The method of claim 1 further comprising: forming a high-voltage p-well (HVPW) region directly over the PBL; forming a HVNW region over the PBL; forming a gate dielectric extending from directly over the HVPW region to directly over the HVNW region; forming a gate electrode over the gate dielectric; forming a source region in the HVPW region and adjacent the gate electrode; and forming a drain region in the HVNW region and adjacent the gate electrode. 8. A method comprising: forming an n-type tub comprising a bottom buried in a semiconductor substrate, and an n-type ring extending from a top surface of the semiconductor substrate to the bottom of the n-type tub, wherein the n-type ring encircles and contacts the bottom of the n-type tub; forming a p-type buried layer (PBL) directly over the bottom of the n-type tub, wherein the PBL is buried in the semiconductor substrate; and forming a high-voltage n-type metal-oxide-semiconductor (HVNMOS) device directly over the PBL and within a region encircled by the n-type ring. 9. The method of claim 8 , wherein the step of forming the n-type tub comprises: implanting a lower portion of the semiconductor substrate to form the bottom of the n-type tub and a lower portion of the n-type ring; epitaxially growing an upper portion of the semiconductor substrate over the lower portion of the semiconductor substrate; and implanting the upper portion of the semiconductor substrate to form an upper portion of the n-type ring. 10. The method of claim 9 , wherein the upper portion of the n-type ring has a lower n-type impurity concentration than the bottom of the n-type tub and the lower portion of the n-type ring. 11. The method of claim 9 further comprising forming a high-voltage n-well (HVNW) region in the upper portion of the semiconductor substrate, wherein the step of forming the HVNW region and the step of forming the upper portion of the n-type ring are performed by a same implantation step. 12. The method of claim 9 , wherein the step of forming the PBL is performed by implanting into the lower portion of the semiconductor substrate before the step of epitaxially growing the upper portion of the semiconductor substrate. 13. The method of claim 8 , wherein after the step of forming the HVNMOS device, all n-well regions encircled by the n-type ring are spaced apart from the bottom of the n-type tub by the PBL. 14. A method comprising: implanting a p-type semiconductor substrate to form an n-type buried layer (NBL); implanting a surface layer of the p-type semiconductor substrate directly over the n-type buried layer (NBL) to form a p-type buried layer (PBL); implanting the p-type semiconductor substrate to form a first n-type ring encircling and contacting the NBL, wherein a top surface of the first n-type ring is higher than a top surface of the NBL; epitaxially growing a semiconductor layer directly over the p-type semiconductor substrate; implanting the semiconductor layer to form a second n-type ring directly over and overlapping the first n-type ring; implanting an upper portion of the semiconductor layer to form a high-voltage n-well (HVNW) region, wherein a bottom surface of the HVNW region is higher than a bottom of the semiconductor layer; and forming a high-voltage n-type metal-oxide-semiconductor (HVNMOS) device directly over the PBL and within a region encircled by the second n-type ring, wherein a gate dielectric of the HVNMOS device comprises a portion directly over the HVNW region. 15. The method of claim 14 further comprising implanting the upper portion of the semiconductor layer to form a high-voltage p-well (HVPW) region, wherein a bottom surface of the HVPW region is higher than a bottom of the semiconductor layer, and wherein the HVPW region comprises a portion directly under the gate dielectric. 16. The method of claim 14 , wherein the HVNW region and the second n-type ring are formed by a same implantation step. 17. The method of claim 14 , wherein the second n-type ring has a lower n-type impurity concentration than the first n-type ring. 18. The method of claim 14 , wherein during the step of epitaxially growing the semiconductor layer, the NBL and the PBL diffuse into the semiconductor layer, and no portion of the PBL exists in the p-type semiconductor substrate after the step of epitaxially growing the semiconductor layer. 19. The method of claim 14 , wherein after the step of forming the HVNMOS device, all n-well regions encircled by the second n-type ring are vertically spaced apart from the NBL by the PBL. 20. The method of claim 14 , wherein after the step of implanting the p-type semiconductor substrate to form the NBL and before the step of epitaxially growing the semiconductor layer, the NBL is buried in the p-type semiconductor substrate.
This application is a continuation of U.S. patent application Ser. No. 11/773,365, filed Jul. 3, 2007, and entitled “Fully Isolated High-Voltage MOS Device,” which application is hereby incorporated herein by reference. TECHNICAL FIELD This invention relates generally to high-voltage metal-oxide-semiconductor (HVMOS) devices, and more particularly to the isolation of HVMOS devices. BACKGROUND High-voltage metal-oxide-semiconductor (HVMOS) devices are widely used in many electrical applications, such as CPU power supplies, power management systems, AC/DC converters, etc. FIG. 1 illustrates a conventional n-type HVMOS (HVNMOS) device 2 formed over substrate 16 . HVNMOS device 2 includes gate electrode 4 , drain region 6 in high-voltage n-well (HVNW) 8 , and source region 10 in high-voltage p-well (HVPW) 12 . Shallow trench isolation (STI) region 5 spaces drain region 6 and gate electrode 4 apart, so that high drain-gate voltages can be applied. HVNMOS device 2 is formed on an n-type buried layer (NBL) 14 , which is further formed on p-type substrate 16 . The conventional HVNMOS device 2 suffers drawbacks when used in certain applications. For example, power IC or driver IC may be involved with negative voltages. Therefore, when used in power IC or driver IC, HVNMOS device 2 may have negative source-to-substrate voltages and/or negative drain-to-substrate voltages. In such a case, a forward bias is applied on diode 18 , which is formed of p-type substrate 16 and n-type NBL 14 . The n-type NBL 14 is further connected to n-type HVNW region 8 and N+ drain region 6 . Therefore, diode 18 will be turned on by negative drain-to-substrate voltages. This adversely causes the increase in leakage current, and possibly the reduction in breakdown voltage of HVNMOS device 2 . What is needed in the art, therefore, is a HVMOS device with good performance when positive source-to-substrate and drain-to-substrate voltages are applied, and is also capable of performing well for negative source-to-substrate and drain-to-substrate voltages. SUMMARY OF THE INVENTION In accordance with one aspect of the present invention, a semiconductor structure includes a semiconductor substrate; an n-type tub extending from a top surface of the semiconductor substrate into the semiconductor substrate, wherein the n-type tub comprises a bottom buried in the semiconductor substrate; a p-type buried layer (PBL) on a bottom of the tub, wherein the p-type buried layer is buried in the semiconductor substrate; and a high-voltage n-type metal-oxide-semiconductor (HVNMOS) device over the PBL and within a region encircled by sides of the n-type tub. In accordance with another aspect of the present invention, a semiconductor structure includes a semiconductor substrate; a first high-voltage n-well (HVNW) region in the semiconductor substrate; a second HVNW region in the semiconductor substrate, wherein the first and the second HVNW regions are parallel; a third HVNW region between the first and the second HVNW regions; a first high-voltage p-well (HVPW) region between the first and the second HVNW regions; a first and a second n-type isolation (N-ISO) region underlying and adjoining the first and the second HVNW regions, respectively; an n-type buried layer (NBL) connecting the first and the second N-ISO regions; and a PBL on the NBL. In accordance with yet another aspect of the present invention, a semiconductor structure includes a semiconductor substrate of a p-type conductivity; an epitaxial layer on the semiconductor substrate; an n-type buried layer (NBL) extending from the semiconductor substrate into the epitaxial layer; a PBL on the NBL; an N-ISO ring extending from the semiconductor substrate into the epitaxial layer, wherein the N-ISO ring encircles and adjoins the NBL and at least a portion of the PBL; a HVNW ring on and adjoining the N-ISO ring; and a HVNMOS device over the PBL, wherein a portion of the HVNMOS device in the epitaxial layer is encircled by the HVNW ring. In accordance with yet another aspect of the present invention, a method of forming a semiconductor structure includes providing a semiconductor substrate having a p-type conductivity; forming an epitaxial layer on the semiconductor substrate; forming an NBL extending from the semiconductor substrate into the epitaxial layer; forming a PBL on the NBL; forming an N-ISO ring extending from the semiconductor substrate into the epitaxial layer, wherein the N-ISO ring encircles and adjoins the NBL; forming a HVNW ring on and substantially overlapping the N-ISO ring; and forming a HVNMOS device over the PBL, wherein a portion of the HVNMOS in the epitaxial layer is encircled by the HVNW ring. In accordance with yet another aspect of the present invention, a method of forming a semiconductor structure includes providing a semiconductor substrate having a p-type conductivity; forming an NBL in the semiconductor substrate; forming a PBL in the semiconductor substrate, wherein the NBL and the PBL substantially overlap; forming an N-ISO ring in the semiconductor substrate, wherein the N-ISO ring encircles and substantially adjoins the NBL and the PBL; epitaxially growing an epitaxial layer on the semiconductor substrate, wherein the PBL, the NBL and the N-ISO ring diffuse into the epitaxial layer, and wherein the PBL is on the NBL after the step of epitaxially growing; forming a HVNW ring on and adjoining all sides of the N-ISO ring; and forming a HVNMOS device over the PBL, wherein a portion of the HVNMOS in the epitaxial layer is encircled by the HVNW ring. The embodiments of the present invention can sustain both positive and negative source/drain-to-substrate voltages. BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: FIG. 1 illustrates a conventional high-voltage n-type MOS device; FIGS. 2A through 9 are cross-sectional views of intermediate stages in the manufacturing of embodiments of the present invention; and FIG. 10 illustrates a top view of an embodiment of the present invention. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. The embodiments of the present invention are described with reference to FIGS. 2A through 10 . Throughout the various views and illustrative embodiments of the present invention, like reference numbers are used to designate like elements. Referring to FIGS. 2A and 2B , substrate 20 is provided. Substrate 20 preferably includes a semiconductor material such as bulk silicon, although other semiconductor materials and structures such as silicon germanium (SiGe) and silicon-on-insulator (SOI) may also be used. Substrate 20 is preferably lightly doped with a p-type impurity. Implantations are performed to substrate 20 , forming doped layers. A photo resist (not shown) is formed and patterned, and an n-type buried layer (NBL) 22 is formed by implanting an n-type impurity, for example, phosphorous, antimony and/or arsenic, into substrate 20 . In an exemplary embodiment, the implantation has a dosage of between about 10 14 /cm 2 to about 10 16 /cm 2 . The photo resist is then removed. N-type isolation regions 24 , also referred to as N-ISO regions 24 , are also formed by an implantation, wherein a photo resist may be formed to define the boundaries of N-ISO regions 24 . Although N-ISO regions 24 are shown as two separated regions, in a top view, N-ISO regions 24 form a ring (refer to FIG. 10 ). Preferably, N-ISO regions 24 at least substantially adjoin, and preferably have portions overlapping, NBL 22 . P-type buried layer (PBL) 26 is also implanted, and the implanted impurities preferably include boron. In the preferred embodiment, PBL 26 is blanket formed. The impurity dosage of PBL 26 is preferably lower than the impurity dosage of NBL 22 . In an exemplary embodiment, the dosage for implanting PBL 26 is between about 10 13 /cm 2 and about 10 15 /cm 2 . PBL 26 preferably has a lower impurity concentration than N-ISO region 24 . In an exemplary embodiment, the implantation dosage for forming N-ISO region 24 is greater than 2 times the dosage for implanting PBL 26 . A bottom surface 28 of NBL 22 is preferably lower than a bottom surface 30 of PBL 26 . This may be achieved by adjusting, and increasing if necessary, the implantation energy of NBL 22 . More preferably, NBL 22 is substantially buried into substrate 20 , as is shown in FIG. 2B . However, one skilled in the art will realize that it is inevitable that NBL will distribute all the way up to the surface of substrate 20 . However, the majority of the implanted impurity is concentrated in a sheet as shown in FIG. 2B . The implantations of NBL 22 , N-ISO regions 24 and PBL 26 may be performed in any order. FIG. 3 illustrates the epitaxial growth of epitaxial layer 32 on substrate 20 . Epitaxial layer 32 is preferably formed of a semiconductor material such as silicon, and can be the same or different from the material in substrate 20 . Dotted line 34 schematically illustrates an interface between substrate 20 and epitaxial layer 32 . Epitaxial layer 32 may be either un-doped, or lightly doped with a p-type impurity, which is preferably in-situ doped while the epitaxial growth proceeds. The thickness T of epitaxial layer 32 is substantially greater than the thickness of subsequently formed high voltage p-type and n-type well regions. In an exemplary embodiment, the thickness T of epitaxial layer 32 is greater than about 2 nm. Since the epitaxial growth is performed at an elevated temperature, NBL 22 , N-ISO regions 24 and PBL 26 out-diffuse into epitaxial layer 32 . Due to the relatively high impurity concentration in N-ISO regions 24 , the top surfaces of the diffused N-ISO regions 24 are substantially higher the top surface of the diffused NBL 22 . NBL 22 is likely to have a higher impurity concentration than in PBL 26 due to its smaller diffusion length (and hence ions are more concentrated), and further due to the higher implantation dosage of NBL 22 than PBL 26 . Furthermore, since boron has a greater diffusion distance than n-type impurities, PBL 26 extends further into epitaxial layer 32 than NBL 22 . As a result, PBL 26 resides on NBL 22 . PBL 26 may also extend below NBL 22 . Accordingly, between p-type substrate 20 and NBL 22 , there is a p-type region 26 1 having a higher impurity concentration than substrate 20 . The bottom surface of PBL 26 may be higher or lower then the bottom surface of N-ISO regions 24 . Referring to FIG. 4 , photo resist 36 is formed and patterned using lithography techniques. A p-type impurity implantation is then performed in order to form p-well regions 38 , also equally referred to as high-voltage p-well (HVPW) regions 38 . HVPW regions 38 may be implanted with boron and/or indium. After the implantation, HVPW regions 38 may have an impurity concentration of between about 10 15 /cm 3 and about 10 16 /cm 3 . The bottoms of HVPW regions 38 join PBL 26 . Photo resist 36 is then removed. Referring to FIG. 5 , photo resist 40 is formed and patterned. An n-type impurity implantation is then performed, forming n-well regions 42 and 44 , which are equally referred to as high-voltage n-well (HVNW) regions. HVNW regions 42 and 44 may be implanted with phosphorous, antimony and/or arsenic, which neutralizes the p-type impurities in PBL 26 (and epitaxial layer 32 , if it is in-situ doped) and converts the implanted region to n-type. HVNW regions 42 and 44 and N-ISO regions 24 may comprise the same, or different impurities. In an exemplary embodiment, after the implantation, HVNW regions 42 and 44 may have a net n-type impurity concentration of between about 10 15 /cm 3 and about 10 16 /cm 3 . The bottoms of the HVNW regions 42 join N-ISO regions 24 . Similar to N-ISO regions 24 , although HVNW regions 42 look like two separated regions in the illustrated plane, they also formed a ring (refer to FIG. 10 ). HVNW regions 42 preferably have same widths as, and substantially co-terminus with N-ISO regions 24 . Alternatively, N-ISO regions 24 and HVNW regions 42 have substantially different widths. Photo resist 40 is then removed. N-ISO regions 24 , HVNW regions 42 and NBL 22 form a tub. PBL 26 is located on the bottom of the tub. This structure advantageously isolates the subsequently formed HVMOS devices in the tub. FIG. 6 illustrates the formation of shallow trench isolation (STI) regions 45 . In the preferred embodiment, STI regions 45 are formed by forming trenches in the epitaxial layer 32 ; filling the trench with a dielectric material, such as SiO 2 or other high-density plasma (HDP) oxides; and performing a chemical mechanical polish to level the surface. In other embodiments, local oxidation of silicon (LOCOS) may be used to form field oxides at the same locations as STI regions 45 . The process steps for forming field oxides may include forming a mask layer over HVPW regions 38 and HVNW regions 42 and 44 , patterning the mask layer to form openings, and performing an oxidation. Field oxides are thus formed through the openings of the mask layer. The mask layer is then removed. FIG. 7 illustrates the formation of gate dielectrics 50 , gate electrodes 52 and spacers 54 . The formation processes are well known in the art, and thus are not repeated herein. Lightly doped source regions (not shown) may also be formed. Preferably, side edges of gate electrodes 52 fall over STI regions 45 . Referring to FIG. 8 , photo resist 58 is applied and patterned. A p-type impurity implantation is performed, forming P+ regions 60 . Preferably, P+ regions 60 are heavily doped to a high impurity concentration, for example, greater than about 10 20 /cm 3 . In the described embodiments, heavily doped means an impurity concentration of above about 10 20 /cm 3 . One skilled in the art will recognize, however, that heavily doped is a term of art that depends upon the specific device type, technology generation, minimum feature size, and the like. It is intended, therefore, that the term be interpreted in light of the technology being evaluated and not be limited to the described embodiments. P+ regions 60 in HVPW regions 38 act as contact regions. Photo resist 58 is removed after the formation of P+ regions 60 . Photo resist 62 is then formed and patterned, as is shown in FIG. 9 . An n-type impurity implantation is performed, forming drain region 64 , source regions 66 and contact regions 68 . Preferably, the n-type impurity is heavily doped. After the implantation, photo resist 62 is removed. One skilled in the art will realize that the order of forming N+ regions, P+ regions and gates is a matter of mere design choice. Two HVNMOS devices 100 and 200 are thus formed. Preferably, source region 66 and gate 52 of HVNMOS device 100 are connected to the respective source region 66 and gate 52 of HVNMOS device 200 , so that HVNMOS devices 100 and 200 act as a single device. Typically, an HVNMOS device may include more than one set of HVNMOS devices 100 and 200 , wherein each of the sets is typically referred to as a leg. One skilled in the art will realize that a single leg or multiple legs of a HVNMOS device may be formed in the tub formed of NBL 22 , N-ISO regions 24 and HVNW regions 42 . FIG. 10 schematically illustrates a top view of the structure shown in FIG. 9 . For simplicity, only HVPW regions, HVNW regions and gate electrodes are illustrated. FIG. 10 shows that HVNW 42 and N-ISO regions 24 form a ring encircling HVNW regions 38 and HVPW region 44 . Therefore, the HVNMOS device formed in HVNW regions (ring) 42 is electrically isolated. Referring back to FIG. 9 , it is noted p-type substrate 20 , NBL 22 and PBL 26 form two diodes 70 and 72 , which are connected face-to-face. Therefore, one of the diodes 70 and 70 will be reverse biased, no matter the source-to-substrate, drain-to-substrate voltages are positive or negative. The embodiments of the present invention have several advantageous features. The HVNMOS devices formed using the embodiments of the present invention can be applied with both positive and negative source/drain-to-substrate voltages without causing significant leakage currents and/or early breakdown. Since NBL 22 and PBL 26 (refer to FIG. 9 ) are formed before the formation of HVPW and HVNW regions (as compared to be implanted after the formation of epitaxial layer 32 ), it is easy to increase the impurity concentrations of NBL 22 and PBL 26 . In an exemplary embodiment, NBL 22 may have an impurity concentration as high as about 10 18 /cm 3 or even higher. PBL 26 may have an impurity concentration of greater than about 10 16 /cm 3 . As a result, parasitic bipolar transistors having either NBL 22 or PBL 26 as bases may have high base impurity concentrations. An example of such parasitic bipolar transistors is formed of substrate 20 , NBL 22 and PBL 26 . With high impurity concentrations, the base resistances are reduced, and base voltages are reduced accordingly. Parasitic bipolar transistors thus become harder to be turned on. Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

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