SELF-ALIGNED POWER FIELD EFFECT TRANSISTOR IN SILICON CARBIDE

Abstract: The present invention is directed to a silicon carbide field effect transistor. The FET is formed on a silicon carbide monocrystalline substrate. An insulative material gate having a pair of spaced apart sidewalls is patterned on the substrate. The insulative material comprises a first insulation material overlayed by an electrically conductive layer. The substrate is lightly doped base regions located partially under the sidewalls of the gate and extending into the exposed substrate. Associated with the lightly doped base regions are heavily doped source regions aligned with the exposed substrate. On the underside of the substrate is a drain region to form the FET. A method to fabricate a field effect transistor is disclosed. The transistor is formed in a monocrystalline substrate of silicon carbide. Forming a transistor on the silicon carbide substrate entails depositing a first electrically insulative layer over the substrate. Next, an electrically conductive layer is deposited over the first insulative layer and then a second electrically insulative layer is deposited over the conductive layer. The following step includes partially removing the second insulative layer and exposing a portion of the first conductive layer to obtain two spaced apart regions of the silicon carbide substrate for forming lightly doped base regions. Then the substrate aligned with the exposed portion of the first conductive layer is lightly implanted with a dopant to form lightly doped base regions. Next, a layer of a third insulative material is formed on the sidewalls of the second insulative material. The following step entails removing the exposed conductive layer not aligned with the second insulative material and third insulative material.

Description: SELF-ALIGNED POWER FIELD EFFECT TRANSISTOR IN SILICON CARBIDE The present invention relates to a field effect transistor ("FET") device and, in particular, to an improved FET and process for forming same. Silicon carbide as a substrate is an excellent material for power devices due to its wideband gap and high blocking voltage. However, dopants do not readily diffuse in silicon carbide substrate. Typically, dopants have been implanted into a silicon carbide substrate to form lightly doped base regions and heavily doped source regions by using two separate masks. An example of this typical technique is disclosed in the specification of U.S. Patent No. 5,338,945 which discloses "the base region of the silicon carbide field effect transistor is formed by amorphizing a first portion of a silicon carbide substrate, providing an implant of the base region dopant ions into the first portion and then recrystallizing the first portion to thereby create a substantially monocrystalline base region." It also discloses "Next, the source region is formed by amorphizing a second portion of the substantially monocrystalline base region, providing an implant of the source region dopant ions and then recrystallizing the second portion to thereby create a substantially monoαystalline source region in the base region." In other words, implanting dopants into a silicon carbide substrate has required at least two separate masks. The two mask technique is costly because two masks are used instead of one. More importantly, the two mask technique is difficult to use because the two masks must be properly aligned. The present invention includes a silicon carbide field effect transistor comprising a silicon carbide monocrystalline substrate, an insulative material gate patterned on said substrate and having a pair of spaced apart sidewalls to form a gate insulator, an electrically conductive layer on said insulative material to form a gate, a lightly doped base regions in said substrate located partially under the sidewalls of said gate and extending into said exposed substrate, a heavily doped source regions located in said exposed substrate within its associated lightly doped base regions and self-aligned with the sidewalls of the gate insulator; and a drain region located on the underside of said substrate. Advantageously, the silicon carbide field effect transistor, is formed on a silicon carbide monocrystalline substrate. An insulative material gate having a pair of spaced apart sidewalls is patterned on the substrate. The insulative material comprises a first insulation material overlayed by an electrically conductive layer. Within the substrate is lightly doped base regions located partially under the sidewalls of the gate and extending into the exposed substrate. Associated with the lightly doped base regions are heavily doped source regions aligned with the exposed substrate. On the underside of the substrate is a drain region to form the FET. The invention also includes a method to fabricate a field effect transistor in a monoαystalline substrate of silicon carbide, comprising the steps of forming a first electrically insulative layer over the surface of the silicon carbide substrate depositing an electrically conductive layer over the surface of said first insulative layer depositing a second electrically insulative layer over the surface of said conductive layer removing partially said second insulative layer and exposing a portion of said first conductive layer to obtain two spaced apart regions of the silicon carbide substrate for forming lightly doped base regions implanting lightly the substrate through the exposed conductive layer with a dopant to form lightly doped base regions in the substrate forming a third insulative material on the sidewalls of said second insulative material and removing said exposed conductive layer not layered with said second or third insulative material to expose portions of the first insulating layer and to define a second set of spaced regions of the silicon carbide substrate implanting heavily the substrate through the exposed portions of the first insulating layer to form heavily doped base regions in the substrate, and removing said second and third insulative material to form a gate. Conveniently, a method to fabricate a field effect transistor is formed in a monocrystalline substrate of silicon carbide. Forming a transistor on the silicon carbide substrate entails depositing a first electrically insulative layer over the substrate. Next, an electrically conductive layer is deposited over the first insulative layer and then a second electrically insulative layer is deposited over the conductive layer. The following step includes partially removing the second insulative layer and exposing a portion of the first conductive layer to obtain two spaced apart regions of the silicon carbide substrate for forming lightly doped base regions. Then the substrate aligned with the exposed portion of the first conductive layer is lightly implanted with a dopant to form lightly doped base regions. Next, a layer of a third insulative material is formed on the sidewalls of the second insulative material. The following step entails removing the exposed conductive layer not aligned with the second insulatative material and third insulative material. Thereby, exposing portions of the first insulating layer and defining a second set of spaced regions of the substrate. The second set of spaced regions are then heavily implanted to form heavily doped source regions. The second insulative material and third insulative material are then removed to form a gate. The invention will now be described by way of example with reference to the accompanying dreawings in which; Figure 1 is a schematic representation of a transistor; and Figures 2a-g are a schematic depicting a process of the present invention. In this invention, a self-aligned implanted power FET, preferably a MOSFET, in silicon carbide is fabricated with a single mask. The cost for masks are decreased and alignment problems are diminished in relation to the two step technique. Figure 1 illustrates a power FET device 10 formed in a monocrystalline silicon carbide substrate 12. An insulative material gate 20 having a pair of spaced apart sidewalls 34, 36 is patterned on substrate 12. Gate 20 comprises a first insulative material 14 overlayed with an electrically conductive layer 22. Within substrate 12 is lightly doped base regions 16 located partially under sidewalls 34, 36 and extending into exposed substrate region 30. Associated with the lightly doped base regions 16 are heavily doped source regions 18 aligned with the exposed substrate 30. On the underside of substrate 12 is a drain contact 32 to form the FET 10. The present invention illustrated in figure 1 is attained through a method illustrated in figure 2. Turning to figure 2a, a layer of first insulative material 14 is applied over a monocrystalline substrate 12. In particular, substrate 12 is a starting wafer composed for example of monocrystalline silicon carbide. First insulation material 14, preferably an oxide layer, is formed on the upper surface 38 of wafer 12. Oxide layer 14 is formed by low temperature chemical vapor deposition techniques, epitaxiaUy, or rapid thermal oxidation processes that form an insulative material ranging from 250 to 350 angstroms. These processes ensure first insulative material 14 is adequately attached on substrate 12 for further deposition of elements. After the initial step of applying insulative material 14 over substrate 12, a layer of electrically conductive material 22, as illustrated in figure 2b, is applied on insulative material 14. Conductive material 22 ranges from 4,500 to 5,500 angstroms thick and can be applied by a low-pressure chemical vapor deposition (LPCVD) process. In this embodiment of the present invention conductive material 22 is polysilicon. The next step, shown in figure 2c, entails depositing a layer of a second insulative material 24 on conductive material 22. Second insulative material 24, preferably an oxide layer, is deposited by the same techniques as first insulation material 14. Moreover, second insulation material 24, preferably, is thinner than the desired thickness of substrate 12. Turning to figure 2d, an etch resistant mask 40 is patterned on second insulative material 24. Portions of second insulative material 24 not aligned with mask 40 are removed by conventional etching techniques, such as reactive ion etching, to expose first portion 26 of conductive material 22. As shown in figure 2e, a first dopant of one polarity is implanted in substrate 12 5 aligned with first portion 26. The first dopant is a high implant energy source used to penetrate conductive material 22, first insulative material 14 and substrate 12. Conventionally, the implant is a dose of 2el2, energy of 360KeV of boron and forms a lightly doped base regions 16, commonly a p-well. Second insulative material 24 and mask 40 block the boron ions from penetrating into substrate 12 aligned with material 24 and mask 40. 0 After the implantation substrate 12, mask 40 is removed exposing the remaining second insulation material 24. As illustrated in figure 2f, a third insulative material 28 is formed on the sidewalls of second insulative material 24. Portions of conductive material 22 not aligned with second insulative material 24 and third insulative material 28, preferably a nitride material, is removed. Conductive material 22 is removed by conventional etching techniques 15 disclosed above. Thereby, exposing area 30 of the first insulative material 14 that is smaller than first portion 26. Turning to figure 2g, a second dopant of opposite polarity to the first dopant is then implanted in substrate 12 aligned with area 30. The second dopant normally forms a heavily doped source regions 18, commonly a n-well, with arsenic ions. Heavily doped source regions 20 18 are located within associated lightly doped base regions 16. Second insulative material 24 and third insulative material 28 blocks the arsenic ions from entering the underlying substrate 12. Turning to figure 1, remaining second insulative material 24 and third insulative material 28 are removed. This removal process, preferably, occurs by known stripping 25 procedures known in the semiconductor industry. Thereby, remaining conductive material 22 and first insulation material 14 aligned with conductive material 22 is gate 20. In an alternative embodiment, first insulation material 14 not aligned with conductive material 22 can be removed as well by conventional semiconductor processes. Underside of substrate 12 is subsequently planarized to a desired thickness. Once the 30 underside of substrate 12 is planarized, a drain contact 32 is applied to the underside. In one embodiment of drain contact 32, underside of substrate 12 is metalized. The present invention is directed to a silicon carbide field effect transistor. The FET is formed on a silicon carbide monocrystalline substrate. An insulative material gate having a pair of spaced apart sidewalls is patterned on the substrate. The insulative material comprises a first insulation material overlayed by an electrically conductive layer. The substrate is lightly doped base regions located partially under the sidewalls of the gate and extending into the exposed substrate. Associated with the lightly doped base regions are heavily doped source regions aligned with the exposed substrate. On the underside of the substrate is a drain region to form the FET. A method to fabricate a field effect transistor is disclosed. The transistor is formed in a monocrystalline substrate of silicon carbide. Forming a transistor on the silicon carbide substrate entails depositing a first electrically insulative layer over the substrate. Next, an electrically conductive layer is deposited over the first insulative layer and then a second electrically insulative layer is deposited over the conductive layer. The following step includes partially removing the second insulative layer and exposing a portion of the first conductive layer to obtain two spaced apart regions of the silicon carbide substrate for forming lightly doped base regions. Then the substrate aligned with the exposed portion of the first conductive layer is lightly implanted with a dopant to form lightly doped base regions. Next, a layer of a third insulative material is formed on the sidewalls of the second insulative material. The following step entails removing the exposed conductive layer not aligned with the second insulatative material and third insulative material.

Claims: CLAIMS: 1. A method to fabricate a field effect transistor in a monocrystalline substrate of silicon carbide, comprising the steps of forming a first electrically insulative layer over the surface of the silicon carbide substrate depositing an electrically conductive layer over the surface of said first insulative layer depositing a second electrically insulative layer over the surface of said conductive layer removing partially said second insulative layer and exposing a portion of said first conductive layer to obtain two spaced apart regions of the silicon carbide substrate for forming lightly doped base regions implanting lightly the substrate through the exposed conductive layer with a dopant to form lightly doped base regions in the substrate forming a third insulative material on the sidewalls of said second insulative material and removing said exposed conductive layer not layered with said second or third insulative material to expose portions of the first insulating layer and to define a second set of spaced regions of the silicon carbide substrate implanting heavily the substrate through the exposed portions of the first insulating layer to form heavily doped base regions in the substrate, and removing said second and third insulative material to form a gate. 2. A method as claimed in claim 1 including the step of planarizing the underside surface of the substrate to a predetermined thickness, and in which the depositing of said second insulative material is thinner than the predetermined substrate thickness. 3. A method as claimed in claim 1 wherein the depositing of said tfiird insulative material is a nitride material. 4. A method as claimed in any one of claims 1 to 3 wherein the depositing of said second insulative material is an oxide material, the growing of said first insulative material is also an oxide material. 5. A method as claimed in any one of claims 1 to 4 wherein the deposition of said conductive material is a polysilicon material. 6. A method as claimed in claim 5 including the steps of forming a drain region in the underside surface of the substrate, applying a drain contact to said underside, planarizing the underside of said silicon carbide layer to a predetermined thickness, metalizing said underside, and in which each heavily doped base is entirely within its associated lightly doped base. 7. A silicon carbide field effect transistor comprising a silicon carbide monocrystalline substrate, an insulative material gate patterned on said substrate and having a pair of spaced apart sidewalls to form a gate insulator, an electrically conductive layer on said insulative material to form a gate, a lightly doped base regions in said substrate located partially under the sidewalls of said gate and extending into said exposed substrate, a heavily doped source regions located in said exposed substrate within its associated lightly doped base regions and self-aligned with the sidewalls of the gate insulator; and a drain region located on the underside of said substrate. 8. A silicon carbide field effect transistor as claimed in claim 7 wherein said lightly doped base regions are a p well, and said heavily doped base regions are a n well. 9. A silicon carbide field effect transistor as claimed in claim 8 wherein said electrically conductive material is a polysilicon material. 10. A silicon carbide field effect transistor as claimed in any one of claims 7 to 9 wherein said electrically insulative material is an oxide material, and said electrical drain contact includes a metalized layer underlying said substrate and electrically connected to said metalized layer is a drain contact.

Inventors: FULLER ROBERT T, FULLER, ROBERT, T.
Applicants: HARRIS CORP, HARRIS CORPORATION
Application year: 1998
Main classification: H01L21
IPC codes: H01L21, H01L29