1. A method for manufacturing a silicon carbide semiconductor device comprising steps of:
preparing a silicon carbide substrate including a first semiconductor layer, a second semiconductor layer, and a third semiconductor layer, said first semiconductor layer having a first conductivity type, said second semiconductor layer being provided on said first semiconductor layer, said second semiconductor layer having a second conductivity type, said third semiconductor layer being provided on said second semiconductor layer, said third semiconductor layer being separated from said first semiconductor layer by said second semiconductor layer, said third semiconductor layer having said first conductivity type; and
forming a trench having an opening in said silicon carbide substrate, said trench including a bottom surface and a side wall surface, said bottom surface being constituted of said first semiconductor layer, said side wall surface having first to third side surfaces respectively constituted of said first to third semiconductor layers, said trench having a corner portion formed by said first side surface and said bottom surface meeting each other; and
forming a corner insulating film to cover said corner portion and expose a region extending from said opening onto said second side surface;
forming a gate insulating film to cover a region extending from said opening to said corner portion, the step of forming said gate insulating film including a step of thermally oxidizing said trench provided with said corner insulating film, the step of thermally oxidizing said trench including a step of heating said silicon carbide substrate at not less than 1300\xb0 C.; and
forming a gate electrode on said trench with said gate insulating film being interposed therebetween.
2. The method for manufacturing the silicon carbide semiconductor device according to claim 1, wherein
the step of thermally oxidizing said trench includes steps of:
increasing a temperature of said silicon carbide substrate from a temperature of less than 1300\xb0 C. to a temperature of not less than 1300\xb0 C. in a non-oxidizing atmosphere; and
changing an atmosphere surrounding said silicon carbide substrate from the non-oxidizing atmosphere to an oxidizing atmosphere while maintaining the temperature of said silicon carbide substrate at not less than 1300\xb0 C.
3. The method for manufacturing the silicon carbide semiconductor device according to claim 2, wherein
the step of thermally oxidizing said trench includes steps of:
changing the atmosphere surrounding said silicon carbide substrate from the oxidizing atmosphere to the non-oxidizing atmosphere while maintaining the temperature of said silicon carbide substrate at not less than 1300\xb0 C.; and
cooling said silicon carbide substrate from the temperature of not less than 1300\xb0 C. to a temperature of not more than 1000\xb0 C. while maintaining, as the non-oxidizing atmosphere, the atmosphere surrounding said silicon carbide substrate.
4. The method for manufacturing the silicon carbide semiconductor device according to claim 2, wherein the step of changing the atmosphere surrounding said silicon carbide substrate from the non-oxidizing atmosphere to the oxidizing atmosphere includes a step of introducing oxygen gas into the atmosphere surrounding said silicon carbide substrate.
5. The method for manufacturing the silicon carbide semiconductor device according to claim 1, wherein
the step of forming said corner insulating film includes steps of:
forming a covering insulating film to cover the region extending from said opening to said corner portion; and
patterning said covering insulating film.
6. The method for manufacturing the silicon carbide semiconductor device according to claim 5, wherein
the step of forming said covering insulating film includes steps of:
forming a silicon film on said trench; and
thermally oxidizing said silicon film.
7. The method for manufacturing the silicon carbide semiconductor device according to claim 6, wherein the step of forming said covering insulating film includes a step of forming a thermal oxidation film on said trench before the step of forming said silicon film.
8. The method for manufacturing the silicon carbide semiconductor device according to claim 7, wherein the step of thermally oxidizing said silicon film is performed at less than 1300\xb0 C.
9. The method for manufacturing the silicon carbide semiconductor device according to claim 5, wherein
the step of patterning said covering insulating film includes steps of:
forming a resist layer by applying a resist liquid to fill said trench with said covering insulating film being interposed therebetween;
patterning said resist layer by etching back said resist layer such that said resist layer partially remains on said trench; and
etching said covering insulating film using said resist layer as a mask after the step of patterning said resist layer.
10. The method for manufacturing the silicon carbide semiconductor device according to claim 9, wherein the step of etching said covering insulating film is performed by wet etching.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. A process for partial upgrading of properties of heavy andor extra-heavy crude oils, by catalytic hydrotreatment, which comprises the following steps:
1) desalting of the heavy or extra-heavy crude oil by an arrangement of two (2) desalters connected in series;
2) catalytic hydrotreating of the heavy andor extra-heavy desalted crude oil at maximum temperatures of 400\xb0 C. and 100 kgcm2 of pressure or less in a single reaction step to obtain a partially upgraded crude oil; and
3) separation of partially upgraded oil;
wherein said heavy andor extra heavy crude oils have an API gravity of 3-16 units, and said partially upgraded oil has a kinematic viscosity equal to or less than 250 cSt at 37.8\xb0 C., and API gravity increase of 4 to 8 degrees and where said upgraded oil has better quality for its transportation from platforms to maritime terminals or to refining centers.
2. The process according to claim 1, wherein the desalters employed in step 1) are of the dielectric type, for crude oil containing less than 200 pounds salt per 1,000 barrels.
3. The process according to claim 1, wherein step 1) is carried out under pressure of 7 to 14 kgcm2 and temperature of 125 to 150\xb0 C.
4. The process according to claim 1, wherein in step 1) the pressure value is preferably at least 2 kgcm2 above the vapor pressure of a crude oil-water mixture at the operating temperature.
5. The process according to claim 1, wherein the catalytic hydrotreatment of crude oil in step 2) is performed in a fixed bed reactor with a catalyst containing metals selected from the group consisting of Pt, Pd, Ni, Mo and Co.
6. The process according to claim 1, wherein the catalyst employed in step 2) has a low metal loading: molybdenum from 2 to 8 weight % and nickel or cobalt from 0.1 to 3 weight % in the catalyst, supported on gamma alumina, with surface area of 180 to 200 m2g, pore volume of 0.7 to 0.8 cm3g, and having a shape profile selected from the group consisting of cylindrical extrudates, lobular and spheres with a diameter of 1 to 3 mm.
7. The process of claim 1, wherein the catalytic hydrothermal treatment is carried out in a fixed bed reactor with a catalyst selected from the group consisting of Ni, Mo and Co, nickel-molybdenum (Ni\u2014Mo) mixtures and cobalt-molybdenum (Co\u2014Mo) mixtures supported on a support selected from aluminum oxide (alumina), silicon, titanium and mixtures thereof, wherein said aluminum oxide is in the gamma alumina phase.
8. The process according to claim 1, wherein the catalytic hydrotreatment of crude oil in step 2) in addition to the catalyst bed materials further includes pressure drop relaxers, with or without catalytic activity of hydrogenation, hydrocracking, or both, with shapes selected from the group consisting of spheres, tablets, and raschig rings.
9. The process according to claim 1, wherein step 2) is carried out at pressure of 40 to 100 kgcm2, hydrogen-to-hydrocarbon ratio of 2,000 to 5,000 ft3bbl, temperature of 360 to 400\xb0 C. and space velocity or volumetric flow relative to the volume of catalyst (LHSV: liquid hourly space velocity) of 0.25 to 3 h\u22121.
10. The process according to claim 1, wherein the separation of the partially upgraded oil in step 3) removes sour gases produced in the hydrotreatment from the partially upgraded oil.
11. The process according to claim 10, wherein up to 63% of sulfur is removed from the partially upgraded oil.
12. The process according to claim 1, wherein up to 66% of the metal (Ni+V) is removed with a global deposit rate equal to 0.0168 weight % per hour, equivalent to a catalyst life of 10 months.
13. The process according to claim 1, wherein the partially upgraded oil has a formed sediment content is less than 0.04 weight %.
14. A method of transporting heavy andor extra heavy crude oil including the steps of
hydrotreating heavy andor extra heavy crude oil at a temperature of not higher than 400\xb0 C. and pressure of 100 kgcm2 or less in a single reaction step to increase the API about 4 to 8 degrees,
recovering the partially upgraded crude oil, and
transporting the crude oil through a pipeline.
15. The method of claim 14, wherein said catalytic hydrotreatment is carried out under a pressure of 7 to 14 kgcm2 and a temperature of 125\xb0 C. to 150\xb0 C.