Silicon carbide and method of manufacturing the same Number:7,166,523 from the United States Patent and Trademark Office (PTO) owispatent

Title: Silicon carbide and method of manufacturing the same

Abstract: In a method of manufacturing a silicon carbide substance, such as a film, a layer, a semiconductor, which is doped with an impurity, a carbonization process is executed after formation of a doped silicon substance which is obtained by carrying out a silicon deposition process and by a doping process of the impurity. Both the silicon deposition and the doping processes may be simultaneously or separately carried out prior to the carbonization process or may be continued during the carbonization process also. At any rate, the carbonization process is intermittently carried out. A unit process of composed of a combination of the silicon deposition process, the doping process, and the carbonization process may be repeated a plurality times, for example, 2000 times.

Patent Number: 7,166,523 Issued on 01/23/2007 to Nagasawa

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Inventors:	Nagasawa; Hiroyuki (Hachiouji, JP)
Assignee:	Hoya Corporation (Tokyo, JP)
Appl. No.:	10/890,155
Filed:	July 14, 2004

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Foreign Application Priority Data

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Aug 10, 2000 [JP]			2000/242171

Current U.S. Class:	438/478 ; 438/479; 438/483
Current International Class:	H01L 21/36 (20060101)
Field of Search:	438/478,479,483,505,105

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Primary Examiner: Schillinger; Laura M.
Attorney, Agent or Firm: Sughrue Mion, PLLC

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Parent Case Text

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This is a divisional of application Ser. No. 09/924,872 filed Aug. 9, 2001 now abandoned. The entire content of the disclosure of which is incorporated herein by reference.

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Claims

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What is claimed is:

1. A method of depositing a doped silicon carbide on a substrate from a vapor phase or a liquid phase, comprising, as a unit process, the following steps of: depositing a silicon layer on the substrate; doping the silicon layer with an impurity composed of at least one element selected from a group consisting of N, B, Al, Ga, In, P, As, Sb, Se, Zn, O, Au, V, Er, Ge, and Fe, to form a doped silicon layer; and carbonizing the doped silicon layer into a silicon carbide layer of the silicon carbide doped with the impurity, after the depositing and the doping steps are executed, wherein the unit process is repeated a plurality of times to deposit the silicon carbide layer to a desired thickness.

2. A method as claimed in claim 1, wherein the silicon layer depositing step, the doping step, and the carbonizing step are carried out during epitaxially growing a thin film on the substrate by the use of a chemical vapor deposition technique; the silicon layer deposition step being carried out by using a gas of a silane group or a dichlorosilane group as a silicon raw material while the carbonizing step is carried out by the use of an unsaturated carbonhydrate gas.

3. A method as claimed in claim 1, wherein the silicon layer depositing step is followed by the doping step and the carbonizing step is carried out after the doping step.

4. A method as claimed in claim 1, wherein the silicon layer depositing step and the doping step are simultaneously carried out and are followed by the carbonizing step.

5. A method as claimed in claim 1, wherein the silicon layer depositing step and the doping step are simultaneously carried out while the carbonizing step is carried out when a predetermined time lapses after the start of both the silicon depositing and the doping steps.

6. A method as claimed in claim 1, wherein an amount of impurity is varied during each doping step of the unit processes to provide a plurality of silicon carbide layers which have different impurity concentrations in a thickness direction, respectively.

7. A method as claimed in claim 1, wherein the doping step controls an amount of impurity so that impurity concentrations in the silicon carbide fall within a range between 1.times.10.sup.13/cm.sup.3 to 1.times.10.sup.21/cm.sup.3.

8. A method as claimed in claim 1, wherein the doping step controls an amount of impurity so that an impurity concentration gradient falls within a range between 10.times.10.sup.18/cm.sup.4 and 4.times.10.sup.24/cm.sup.4 in a thickness direction of the silicon carbide layer.

9. A method as claimed in claim 1, wherein the substrate has a surface which is structured by either one of a single crystal silicon, a silicon carbide of a cubic system, and a silicon carbide of a hexagonal system while the silicon carbide layer deposited on the surface of the substrate is structured by silicon carbide of a cubic system or a hexagonal system.

10. A method of depositing a doped silicon carbide on a substrate from a vapor phase or a liquid phase, comprising, as a unit process, the following steps of: depositing a silicon layer on the substrate; doping the silicon layer with an impurity composed of at least one element selected from a group consisting of N, B, Al, Ga, In, P, As, Sb, Se, Zn, O, Au, V, Er, Ge, and Fe, to form a doped silicon layer; and carbonizing the doped silicon layer into a silicon carbide layer of the silicon carbide doped with the impurity, further comprising the step of: removing the substrate from the silicon carbide layer after the formation of the doped silicon carbide, to leave a silicon carbide wafer.

11. A method as claimed in claim 1, wherein the doping step of each process unit is carried out by varying a species of the impurities from one to another at each process unit to provide a pn junction in the doped silicon carbide layer.

12. A method as claimed in claim 1, further comprising the step of: using, as a seed crystal, the doped silicon carbide obtained in claim 1; and further growing a silicon carbide on the seed crystal by a vapor deposition method, a sublimation re-crystallization method, or a liquid deposition method.

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Description

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BACKGROUND OF THE INVENTION

This invention relates to silicon carbide and in particular, to a method of manufacturing a film of silicon carbide, a device including the silicon carbide, an ingot of silicon carbide, and the like. In this event, it is to be noted that the silicon carbide is used for a substrate material of a semiconductor device, a sensor, a dummy wafer in a semiconductor manufacturing process, an X-ray mask, a solar cell, and so on.

It is known in the art that silicon carbide itself is a semiconductor which has a forbidden band as wide as 2.2 eV or more and which is formed by thermally, chemically, and mechanically stable crystals. In addition, consideration has been made about applications of the silicon carbide to a semiconductor substance which is used on conditions of a high frequency and a high electric power because the silicon carbide has a high thermal conductivity. As a method of manufacturing the silicon carbide, have been known in the art the Acheson method and the sublimation and recrystallization method (will be also called an improved Lely method). Specifically, the Acheson method is for reacting silicon on heated coke to deposit the silicon carbide on the surface of the coke while the sublimation and recrystallization method is for heating the silicon carbide obtained by the Acheson method to sublimate and thereafter recrystallize it. In addition, is also known a liquid deposition method which melts silicon within a carbon crucible to pulling the silicon carbide with reacting floating carbon in the crucible with the silicon.

Moreover, any other methods have also been proposed so as to obtain a silicon carbide film which has a high purity and reduced crystal defects. Specifically, as such methods, have been known a chemical vapor deposition (CVD) method and an atomic layer epitaxy (ALE) method. In the CVD method, the silicon carbide is deposited on a surface of a substrate by thermally reacting a carbon source gas with another silicon source gas in a normal or a reduce pressure atmosphere. On the other hand, silicon source molecules and carbon source molecules are alternately adsorbed on a substrate surface and epitaxial growth of the silicon carbide proceeds with crystallinity of the substrate kept unchanged in the silicon carbide.

Herein, it is to be noted that, when the silicon carbide is used as a material of a semiconductor device, controlling an impurity is extremely important. For example, let the silicon carbide be used as a substrate for a power semiconductor device of a discrete type, such as a Schottky-barrier diode. In this event, the device has a series resistance or an on-resistance when the device is put in an on-state and the on-resistance is preferably small because of a reduction of a power loss within the device. In order to decrease the on-resistance, the substrate must be doped with an impurity of an amount as large as 10.sup.21/cm.sup.3 at maximum.

On the other hand, consideration should be made about a breakdown voltage of a semiconductor device. Such a breakdown voltage of the semiconductor is generally proportional to -0.5 power (namely, minus square root) of the impurity concentration. Taking this into account, the impurity concentration should be reduced to 1.times.10.sup.14/cm.sup.3 at a portion of the device at which an electric field is concentrated.

In the meanwhile, a thermal diffusion method is used to dope the impurity into the substrate on manufacturing the semiconductor device which uses silicon as a base material. The thermal diffusion method is for adding the impurity into the substrate by coating an impurity on a substrate surface or by exposing the substrate in an impurity atmosphere and by thereafter heating the substrate. However, such a thermal diffusion method can not be applied to a silicon carbide substrate. This is because a diffusion coefficient within the silicon carbide is extremely slow as compared with that within the silicon. This make it very difficult to diffuse an impurity to a depth (deeper than 1 .mu.m with a concentration range between 1.times.10.sup.14 and 1.times.10.sup.21/cm.sup.3) which is available for manufacturing the semiconductor device.

Under the circumstances, an ion injection method is usually used to add an impurity to silicon carbide and is useful to widely control an impurity concentration. However, restriction is inevitably imposed in the ion injection method on a distribution of impurity along a depth direction due to a range of injected ions. In other words, the distribution of impurity depends on the range of the injected ions. Taking this into consideration, Japanese Unexamined Patent Publication No. Hei.11-503571, namely, 503571/1999 discloses a method of introducing a dopant into a semiconductor layer of silicon carbide. More specifically, the method should have a step of ion injecting a dopant into a semiconductor layer at a low temperature and a step of annealing the semiconductor layer at a high temperature. In this event, the ion injecting step is performed at the low temperature so that an amorphous layer is formed near to a surface of the semiconductor while the annealing step is performed at the high temperature so that the dopant is diffused into an un-injected layer laid under the amorphous layer. Even when this method is used, it is difficult to diffuse the impurity with a high concentration over a whole of the substrate.

In addition, injected ions are insufficient with electrical activation and the ion injection brings about the crystal defects within the silicon carbide. Under the circumstances, proposal has been made in Japanese Unexamined Patent Publication No. Hei 12-068225, namely, 068225/2000 about a method of additionally ion injecting carbon atoms (C) to improve electrical activation of acceptors injected into the silicon carbide. This method is also effective to suppress diffusion resulting from heat treatment. Furthermore, Japanese Unexamined Patent Publication No. Hei 11-121393, namely, 121393/1999 discloses a method of forming a mask of SiO.sub.2 on a silicon surface of a silicon carbide substrate and thereafter carrying out ion injection of nitrogen as impurity element. After injection of the impurity, this method should further carry out ion injection (channeling injection) from a direction perpendicular to the silicon surface and another ion injection (random injection) from another direction oblique from the perpendicular direction by 7 degrees. As pointed in Japanese Unexamined Patent Publication No. Hei 11-121393, when phosphorus atoms are ion injected into the semiconductor of silicon carbide, the temperature on the ion injection should be kept at a high temperature, such as 1200.degree. C. or more.

Herein, let an impurity be added all over a substrate. In this case, use is made of a method which forms silicon carbide simultaneously with doping an impurity and which may be called in-situ doping. In such in-situ doping, restrictions are inevitably imposed on an impurity source and a concentration to be added. For example, disclosure is made in Japanese Unexamined Patent Publication No. Hei 09-063968 about a method which causes a boron inclusion gas to flow simultaneously with feeding a mix gas of carbon and silicon and which serves to grow a semiconductor layer of p-silicon carbide in a vapor phase. In this event, when a supply quantity of carbon and a supply quantity of silicon, both of which contribute to crystal growth, stand for Q.sub.C and Q.sub.Si, respectively, the following relationship should hold: 1<Q.sub.C/Q.sub.Si<5.

As regards the semiconductor layer of the p-silicon carbide deposited in the above-mentioned manner, the following relationship between atomic density d.sub.C of the carbon and atomic density d.sub.Si of the silicon should be satisfied: 1<d.sub.C<d.sub.Si<32/31.

As mentioned in Japanese Unexamined Patent Publication No. Hei 10-507734, namely, 507734/1998, trialkylboron should be used as an organic boron compound in a CVD process or a sublimation process. Specifically, let use be made of the organic boron compound which has, in a molecule, at least one boron atom chemically bonded to at least one carbon atom, when doping is carried out in a single crystal of silicon carbide by each of the CVD and the sublimation process. The above-mentioned Publication points out that trialkylboron effectively acts as such an organic boron compound.

In order to vary a concentration of nitrogen as an impurity over a wide range by using in-situ doping technique, Applied Physics letters 65(13), 26 (1994) reports about varying a concentration of carbon which competes with nitrogen in an occupancy ratio of crystal lattices in silicon carbide. In this case, since the concentration of nitrogen arranged in positions of the crystal lattice in place is sensible against the concentration of carbon, a composition ratio of a silicon source and a carbon source should be strictly controlled on growing the silicon carbide. This makes mass-production of the silicon carbide difficult.

Alternatively, let an impurity be doped with silicon carbide by using the sublimation and recrystallization method. In this event, silicon carbide powder and an impurity source (such as Al, B) which act as raw materials should be mixed at a predetermined ratio and sublimated to be recrystallized on a seed crystal. Herein, it is noted that a vapor pressure of the impurity source is very higher than that of the silicon carbide at a sublimation temperature. In consequence, an impurity concentration in the silicon carbide inevitably becomes high at a beginning of silicon carbide growth and becomes low at an end of the growth because the impurity source is wasted and extinct.

Such a variation of the impurity concentration gives rise to a variation of resisitivity among silicon carbide substrates when the silicon carbide formed by the sublimation and recrystallization method is sliced to obtain the silicon carbide substrates. This makes it difficult to realize a stable characteristic of a device. In addition, the silicon carbide grown by the sublimation and recrystallization method does not always have a flat surface and a sharp pn junction or a flat pn junction can not be attained by the use of such silicon carbide.

In the conventional in-situ doping which dopes an impurity during growing the silicon carbide by a vapor growth method, capturing the impurity proceeds simultaneously with growth of silicon carbide. Taking this into consideration, let a pn junction be formed by the use of the above-mentioned in-situ doping. In this case, impurity materials should be switched from one to another during the doping. On switching the impurity materials, a previous impurity gas is inescapably left in a reaction system at the beginning of doping another impurity. In consequence, it is difficult to obtain a sharp pn junction which has a clear junction boundary between p and n regions. In addition, donor impurities and acceptor impurities coexist in a portion adjacent to the junction boundary and such coexistence brings about a high compensation degree and which makes it difficult to enhance mobility in the pn junction.

Moreover, gas flows and the like give rise to an uneven distribution of impurity concentrations in a plane and a uniform impurity concentration can not be obtained over a wide range. Hence, the impurity concentrations can not be strictly controlled and the silicon carbide which has desired impurity concentration distributions can not be attained with a high yield.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, it is an object of this invention to provide a method of manufacturing silicon carbide, which can dope an impurity over an area wider than four inches in diameter with high controllability kept and which can achieve high productivity.

It is another object of this invention to provide a method of the type described, which can dispense with or substantially eliminate any restrictions related to impurity sources, impurity concentrations, and/or depths of impurity doped regions.

It is still another object of this invention to provide silicon carbide and a semiconductor device manufactured by the above-mentioned method.

A method to which this invention is applicable is for use in depositing a silicon carbide on a substrate from a vapor phase or a liquid phase. According to a first aspect of this invention, the method comprises the steps of depositing a silicon layer on the substrate, doping the silicon layer with an impurity composed of at least one element selected from a group consisting of N, B, Al, Ga, In, P, As, Sb, Se, Zn, O, Au, V, Er, Ge, and Fe, to form a doped silicon layer, and carbonizing the doped silicon layer into a silicon carbide layer of the silicon carbide doped with the impurity.

According to a second aspect of this invention, the silicon layer depositing step, the doping step, and the carbonizing step are carried out during epitaxially growing a thin film on the substrate by the use of a chemical vapor deposition technique. The silicon layer deposition step being carried out by using a gas of a silane group or a dichlorosilane group as a silicon raw material while the carbonizing step is carried out by the use of an unsaturated carbonhydrate gas.

According to a third aspect of this invention, the silicon layer depositing step is followed by the doping step and the carbonizing step is carried out after the doping step.

According to a fourth aspect of this invention, the silicon layer depositing step and the doping step are simultaneously carried out and are followed by the carbonizing step.

According to a fifth aspect of this invention, the silicon layer depositing step and the doping step are simultaneously carried out while the carbonizing step is carried out when a predetermined time lapses after the start of both the silicon depositing and the doping steps.

According to a sixth aspect of this invention, the silicon carbide layer doped with the impurity is deposited to a desired thickness by repeating a process unit composed of the silicon depositing step, the doping step, and the carbonizing step a plurality of times.

According to a seventh aspect of this invention, an amount of impurity is varied during each doping step of the unit processes mentioned in the sixth aspect to provide a plurality of silicon carbide layers which have different impurity concentrations in a thickness direction, respectively.

According to an eighth aspect of this invention, the doping step controls an amount of impurity so that impurity concentrations in the silicon carbide fall within a range between 1.times.10.sup.13/cm.sup.3 to 1.times.10.sup.21/cm.sup.3.

According to a ninth aspect of this invention, the doping step controls an amount of impurity so that an impurity concentration gradient falls within a range between 10.times.10.sup.18/cm.sup.4 and 4.times.10.sup.24/cm.sup.4 in a thickness direction of the silicon carbide layer.

According to a tenth aspect of this invention, the substrate has a surface which is structured by either one of a single crystal silicon, a silicon carbide of a cubic system, and a silicon carbide of a hexagonal system while the silicon carbide layer deposited on the surface of the substrate is structured by silicon carbide of a cubic system or a hexagonal system.

According to an eleventh aspect of this invention, the method further comprises the step of removing the substrate from the silicon carbide layer after the formation of the doped silicon carbide, to leave a silicon carbide wafer.

According to a twelfth aspect of this invention, the doping step of each process unit is carried out by varying a species of the impurities from one to another at each process unit to provide a pn junction in the doped silicon carbide layer.

According to thirteenth aspect of this invention, the method further comprises the steps of using, as a seed crystal, the doped silicon carbide obtained in claim 1 and further growing a silicon carbide on the seed crystal by a vapor deposition method, a sublimation re-crystallization method, or a liquid deposition method.

According to a fourteenth aspect of this invention, a silicon carbide has a thickness and a region which has an impurity concentration gradient between 1.times.10.sup.22/cm.sup.4 and 4.times.10.sup.24/cm.sup.4 in the thickness direction.

According to a fifteenth aspect of this invention, a semiconductor device has the silicon carbide manufactured by the method according to the first aspect mentioned above.

According to a sixteenth aspect of this invention, a semiconductor device is structured by the silicon carbide according to the fourteenth aspect.

In the first through the fifth aspects of this invention, the impurity doped silicon is carbonized into the silicon carbide doped with the impurity. With this method, it is possible to easily and accurately dope a desired impurity into the silicon with a high concentration because the impurity can easily be doped into the silicon in comparison with the silicon carbide. More specifically, the impurity and the impurity concentration are limited which can be doped into the silicon carbide while the silicon is easily doped with various kinds of impurities in a high concentration. This shows that impurity doping can be done to a high concentration which exceeds limits determined for a diffusion coefficient and a soluability of the impurity in the silicon carbide.

In the above-mentioned aspects, an amount of the impurity finally doped into the silicon carbide layer is coincident with an amount of the impurity previously doped into the silicon. Therefore, the amount of the impurity in the silicon carbide can be strictly controlled by impurity doping conditions, such as a doping amount of the impurity and a supply time, on forming the silicon layer. Moreover, it is to be noted that, since the diffusion coefficient of the impurity in the silicon carbide is by far lower than that in the silicon, it is possible to prevent an impurity distribution of the impurity in the silicon carbide from being disturbed due to inner diffusion. Accordingly, a doping region of the impurity can be precisely controlled by considering a thickness of a previously deposited silicon layer and may exceed a thickness not thinner than 1 .mu.m. In addition, an impurity concentration in the silicon carbide can be readily varied over a very wide range, as mentioned in the eighth aspect of this invention. This makes it possible to apply this invention to a wide variety of semiconductors.

Herein, the silicon layer deposition step and the impurity doping step must be executed in the absence of any carbon material. In the absence of any carbon material, the impurity doping step may be carried out simultaneously with or after the silicon layer deposition step. At any rate, the carbonization step should be carried out after both the silicon deposition step and the impurity doping step. The reasons will be described later in detail.

Let carbon materials and impurity materials be simultaneously introduced onto the substrate having the previously silicon layer. In this event, the silicon carbide is predominantly formed on the silicon layer as compared with impurity doping. This is because carbonization takes place on a surface of a silicon as a surface phenomenon while impurity diffusion is progressive in proportion to a square root of a time and, as a result, the carbonization is quickly finished within a short time in comparison with impurity doping. Therefore, it is difficult to sufficiently dope an impurity in the presence of carbon. Moreover, since the carbon material quickly sticks to the substrate surface, diffusion of the impurity materials can not be prevented into the substrate. From this fact, it is understood that the impurity materials should be doped into the silicon layer deposited on the substrate in the absence of the carbon materials in order to sufficiently dope impurities.

In this case, the silicon materials and the impurity materials may be simultaneously introduced. The carbon materials may be introduced onto the substrate after deposition of the silicon layer doped with the impurities. During the carbonization step, the impurity materials may be introduced together with the carbon materials. This is because carbonization is predominantly caused to occur even when the impurity and the carbon materials coexist and, as a result, this situation is similar to the case where the carbon materials alone are introduced.

In addition, the silicon materials may coexist with the carbon materials during the carbonization step as long as the carbon materials and the silicon materials are supplied within a predetermined flow rate range which is determined by a ratio of attachment coefficients. The reasons will be as follows. Within the predetermined flow rate range, the carbon materials act to form an adsorption layer on a substrate surface and the adsorption layer serves to prevent formation of the silicon carbide even when the carbon and the silicon materials are simultaneously given.

In order to deposit a thin film from a vapor phase or a liquid phase, may be used a chemical vapor deposition method (CVD), a molecular beam epitaxy (MBE) method, a liquid-phase epitaxy (LPE) method, or the like. The substrate or base may have a silicon, a silicon carbide, TiC, a sapphire, or the like on at least surface thereof.

As the impurities doped, may be exemplified N, B, Al, Ga, In, P, As, Sb, Se, Zn, O, Au, V, Er, Ge, and Fe. Among others, the third group of elements, such as B, Al, Ga, In, act as acceptors to form a p-type semiconductor while the fifth group of elements, such as N, P, As, Sb, act as donors to form an n-type semiconductor. In addition, the fourth group of elements, such as Se, acts to form an energy level in a forbidden band due to a difference of electron affinities and to vary an electric resistance. The other elements, such as Zn, O, Au, V, Ge, Fe, serve to form deep energy levels and, as a result, to vary a life time of minority carriers and resistivity.

The above-enumerated impurities may be used individually or combined. For example, even when the impurities are of the same type to act as donors, acceptors, such impurities of the same type often form different energy levels in the forbidden band and provide different temperature variations of resistivity from each other. In the case where the resistivity at a certain temperature is to be adjusted, a plurality of the impurities of the same type may be added simultaneously. Specifically, two elements, for example, N and P, may be selected from the fifth group of elements mentioned above and be added simultaneously.

Alternatively, a donor and an acceptor may be added at the same time to compensate for carriers and, as a result, to increase the resistivity. This technique makes it possible to form, for example, an i (intrinsic) layer in a PIN diode. The donor and the acceptor may be, for instance, N and B, respectively. In order to avoid an inevitable increase of resistivity resulting from an impurity inescapably invaded during crystal growth, another impurity that has an inverse characteristic to the impurity may be added. On simultaneously adding the donor and the acceptor, adjustment may be carried out such that cancellation is made about natures of the impurities, which is helpful to obtain a silicon carbide of a high resistivity.

The above-mentioned silicon layer deposition step, impurity doping step, and carbonization step may be executed by controlling supplies of raw materials of a vapor phase or a liquid phase. For example, such adjustment may be made about a concentration ratio of impurity sources mingled in the raw material of the silicon carbide and a time of mixing the impurities.

As gases of a silane group and a dichlorosilane group, are exemplified dichlorosilane, tetrachlorosilane, trichlorosilane, hexachlorosilane, or the like. As unsaturated hydrocarbon gases, are exemplified acetylene, ethylene, propane, or the like. Using these raw materials can reduce a temperature of forming a silicon carbide, widen a tolerance range accepted for varying a gas flow rate, and improve crystallinity of the silicon carbide obtained.

In the case where manufacturing is made in accordance with the second aspect, a base sheet, such as a substrate during reaction, is kept at a temperature range between 900.degree. C. and 1400.degree. C., preferably, between 1000.degree. C. and 1400.degree. C. When the reaction temperature is too high, the silicon is fused when it is used as the base sheet while, when the reaction temperature is too low, the reaction speed becomes too slow.

According to the sixth aspect, it is possible to attain an impurity doped layer which has an optional or desirable concentration profile. The method according to this aspect dispenses with any limitations related to impurities when they are diffused into the silicon.

The seventh aspect can realize a desirable concentration gradient that may be, for example, a sharp concentration gradient which can not be obtained by a conventional method. Such a concentration gradient may be a steep concentration gradient defined by the ninth aspect mentioned above.

The tenth aspect according to this invention can realize an excellent silicon carbide. When the silicon carbide is deposited on a silicon carbide substrate, the silicon carbide substrate may be used as a silicon carbide substrate for a semiconductor as it is. Such a silicon carbide substrate may be formed by the Acheson method or the sublimation and recrystallization method or may be a carbonized silicon surface. In this connection, such a substrate and a surface may be collectively called a base body.

The base body may be a substrate which has a surface with a slightly inclined crystal normal axis (namely, the substrate with an off angle). Such a substrate may be exemplified by a silicon substrate of (100) which has a surface normal angle slightly inclined from [001] direction towards [110] one. In addition, the substrate may have a plurality of undulations extended in parallel with one another on a surface in a predetermined single direction. As such a substrate, are exemplified a silicon substrate having undulations extended in parallel in a direction of [011] on a substrate surface defined by (001) plane, a silicon carbide substrate of a cubic system, or the like. The above-enumerated substrate is helpful to reduce defects on a silicon carbide layer to be deposited thereon and therefore serves to obtain a silicon carbide of a high quality.

The twelfth aspect of this invention is helpful to obtain a silicon carbide semiconductor wafer that has a desirable and uniform impurity concentration or an impurity concentration distribution strictly controlled. At any rate, the base body may be removed. When the silicon substrate is used as the base body, removal of the base body can be accomplished after deposition of the silicon carbide layer on the silicon substrate by etching or cutting the base body. When a thick silicon carbide layer is deposited on the substrate, the deposited thick silicon carbide layer may be sliced by a wiring saw and so on to be left as a wafer. More particularly, when the wafer has a diameter of 6 inches, the thickness of the silicon carbide layer may be, for example, 0.65 mm. On the other hand, when the wafer has a diameter of 5 inches, the silicon carbide layer may be 0.5 mm thick. Alternatively, the silicon carbide layer may be 0.36 mm thick or so when the wafer has a diameter of 3 inches or 4 inches. From this fact, it is readily understood that the impurity concentration can be precisely controlled over a whole of the wafer even when the wafer is wide in area and the silicon carbide layer is comparatively thick. Specifically, it is possible to reduce a variation of the impurity concentration over a whole surface (wider than 4 inches in diameter) to less than 5% and to reduce a variation of the impurity concentration in a thickness direction to less than 5%. The variation of the impurity concentration represented by % is calculated by: (variation of the impurity concentration)=[(maximum concentration-minimum concentration)/(average concentration)].times.100 (%). The impurity concentration gradient may be precisely controlled in accordance with the ninth or the fourteenth aspect of this invention mentioned above.

The twelfth aspect of this invention serves to form a pn junction which is precisely controlled in impurity concentration and is helpful to manufacture a desirable semiconductor device with a good yield in bulk.

The thirteenth aspect of this invention can suitably select an impurity concentration of the silicon carbide used as a seed crystal. This enables to control electric resistivity to an appropriate value and, thereby, serves to avoid adhesion of particles appearing during the crystal growth. As a result, an excellent silicon carbide ingot can be obtained.

The fourteenth aspect of this invention makes it possible to easily obtain a slice of a desired concentration by forming a silicon carbide having a concentration gradient in a thickness direction and by cutting the silicon carbide into a plurality of slices. This is effective to widen a control width of an inner electric field within a semiconductor and to design an operating characteristic of a semiconductor device.

The fifteenth aspect of this invention serves to obtain a power semiconductor or the like which has a high speed, a high efficiency, and a high breakdown voltage.

The sixteenth aspect of this invention serves to manufacture such a semiconductor.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a diagrammatic view for use in describing a CVD device available for a silicon carbide manufacturing method according to this invention;

FIG. 2 shows a timing diagram for use in describing a silicon carbide manufacturing method according to a first embodiment of this invention;

FIG. 3 shows a timing diagram for use in describing a silicon carbide manufacturing method according to a second embodiment of this invention;

FIG. 4 shows a timing diagram for use in describing a silicon carbide manufacturing method according to a third embodiment of this invention;

FIG. 5 shows a timing diagram for use in describing a silicon carbide manufacturing method according to a fourth embodiment of this invention;

FIG. 6 shows a timing diagram for use in describing a silicon carbide manufacturing method according to a fifth embodiment of this invention;

FIG. 7 is a graph which represents concentrations of electrons in silicon carbide obtained by varying start time instants (ts) and flow rates (fn);

FIG. 8 shows a timing diagram for use in describing a silicon carbide manufacturing method according to a first comparative example;

FIG. 9 shows a timing diagram for use in describing a silicon carbide manufacturing method according to a second comparative example;

FIG. 10 shows a timing diagram for use in describing a silicon carbide manufacturing method according to a third comparative example;

FIG. 11 is a graph which represents concentrations of electrons included in silicon carbide obtained by varying the flow rates; and

FIG. 12 shows a timing diagram for use in describing a silicon carbide manufacturing method according to a fourth comparative example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

Referring to FIGS. 1 and 2, description will be made about a silicon carbide manufacturing method according to a first embodiment of this invention. Herein, it is to be noted that the method according to the embodiment of this invention is executed by the use of a CVD device or apparatus illustrated in FIG. 1 in accordance with a timing chart shown in FIG. 2. In addition, description will be also made about silicon carbide according to this invention.

Briefly, the method according to the first embodiment of this invention is featured by carrying out an impurity doping process after deposition of a silicon layer and by thereafter carrying out a carbonizing process.

The CVD device illustrated in FIG. 1 has a deposition chamber 1 and a gas inlet pipe or conduit 2 which has a nozzle 2a positioned at a center portion of the deposition chamber 1 and standing substantially perpendicularly on the center portion. A plate 3 is fixed to a tip portion of the gas inlet pipe adjacent to the nozzle 2a. A heating member 5 is attached to a lower side of the plate 3 so as to heat substrates 4 or the like placed on an upper side of the plate 3. The substrates 4 serve as base members according to this invention.

Outside of the deposition chamber 1, a gas supply facility is placed so as to supply gases to the gas inlet pipe 2 and has gas supply sources (not shown) of H.sub.2 gas, N.sub.2 gas, SiH.sub.2Cl.sub.2 gas, and C.sub.2H.sub.2 gas and valves 2b, 2c, 2d, and 2e connected between the gas inlet pipe 2 and the above-mentioned gas supply sources. In addition, an exhaust pipe 6 is coupled to the deposition chamber 1 and is also coupled to an exhaust pump (not shown) or the like through a pressure control valve 7. Thus, the deposition chamber 1 can be exhausted by the exhaust pump through the pressure control valve 7.

The CVD device illustrated in FIG. 1 is so-called a known reduced CVD device of a cold-wall type and serves to grow a thin film on each substrate 4 in a known manner. Specifically, such growth of the thin film can be accomplished by heating the substrates 4 on the plate 3 by the heating member 5, by exhausting the deposition chamber 1 through the exhaust pipe 6, by supplying a selected one of the gases within the deposition chamber 1 through the nozzle 2a of the gas inlet pipe 2, and by forming a vapor phase within the deposition chamber 1.

The above-mentioned reduced CVD device can manufacture silicon carbide in the following manner. At first, a silicon substrate of a single crystal is prepared as each substrate 4 and has a diameter of six inches and a surface layer on which a {001} plane of the single crystal appears. The silicon substrate is subjected to pre-processing to carbonize the surface layer of the silicon substrate into a thin silicon carbide film. Such a thin silicon carbide film underlies a silicon carbide layer which is grown on the substrate and therefore serves as a buffer layer or an underlying layer of the silicon carbide layer so as to grow the silicon carbide layer of an excellent crystallinity.

Taking the above into consideration, description will be made about the method according to the first embodiment of this invention. At first, after the deposition chamber 1 is exhausted through the exhaust pipe 6, the valves 2b and 2c are opened. As a result, the H.sub.2 gas and the C.sub.2H.sub.2 gas are introduced at flow rates of 200 sccm and 50 sccm to a pressure of 100 mTorr into the deposition chamber 1, respectively. Under the circumstances, the substrates 4 are heated by the heating member 5 to 1200.degree. C. within about 1 minute. Each resultant surface layer of the substrates 4 is previously carbonized into the silicon carbide film which may be called a previous silicon carbide film. After completion of the previous carbonization, the valve 2e is closed to temporarily stop supplying the C.sub.2H.sub.2 gas. Thereafter, the H.sub.2 gas is caused to continuously flow at a flow rate of 200 sccm with the substrate temperature kept at 1200.degree. C. In this event, the deposition chamber 1 is kept at the pressure of 60 mTorr by exhausting the deposition chamber 1. To this end, the pressure control valve 7 is controlled so as to adjust an exhaust rate of a gas exhausted through the exhaust pipe 6. Under the circumstances, the following deposition is executed.

As shown in FIG. 2, the H.sub.2 gas is caused to continuously flow over a whole processing duration, as mentioned before, while the valve 2c is opened for five (5) seconds to introduce the SiH.sub.2Cl.sub.2 gas at a flow rate of 30 sccm into the deposition chamber 1 and to deposit a silicon layer on each substrate 4 subjected to the preprocessing mentioned in the above-mentioned manner. Subsequently, the valve 2c is closed to stop supplying the SiH.sub.2Cl.sub.2 gas while the valve 2d is opened to introduce the N.sub.2 gas into the deposition chamber 1 at a flow rate of 50 sccm for five (5) seconds, as illustrated in FIG. 2. Such introducing the N.sub.2 gas is for adding donor impurities into the silicon layer to obtain a doped silicon layer doped with the nitrogen. Next, the valve 2d is closed to stop supply of the N.sub.2 gas while the valve 2e is opened to introduce the C.sub.2H.sub.2 gas into the deposition chamber 1 at a flow rate of 10 sccm for five (5) seconds. Such introducing C.sub.2H.sub.2 gas serves to carbonize the doped silicon layer into a doped silicon carbide.

Herein, it is assumed that a unit process is defined by a sequence of the silicon layer deposition process, the process of doping the nitrogen into the silicon layer, and the process of carbonizing the doped silicon layer. In this case, the unit process is repeated two thousands (2000) times in the illustrated example. Thus, it has been confirmed that the doped silicon carbide layer of a cubic system is epitaxially grown on the substrate 4 in the above-mentioned process.

Furthermore, it has also been confirmed that the silicon carbide layer is deposited on each substrate 4 to a thickness of 54 micron meters after 8.3 hours lapses and is composed of a single crystal. After deposition of the carbonized doped silicon carbide layer on the silicon substrate, a nitrogen (N) concentration in the doped silicon carbide has been measured by a SIMS (secondary ion mass spectrometer). As a result, it has been proved that the nitrogen concentration is equal to 7.4.times.10.sup.19/cm.sup.3 and the nitrogen is uniformly distributed in the doped silicon carbide. Furthermore, it has been found out that a concentration of electrons in the doped silicon carbide is as high as 7.2.times.10.sup.19/cm.sup.3 at a room temperature when it has been measured by the use of a Hall effect. In addition, the resistivity or specific resistance of the doped silicon carbide has been found to be 0.007 .OMEGA.cm. Moreover, a variation of the impurity concentration has been 3.7% within a plane and in a depth direction.

Second Embodiment

Referring to FIG. 3, description will be made about a silicon carbide manufacturing method and silicon carbide according to a second embodiment of this invention. In FIG. 3, the method will is specified by a time diagram for describing timing of supplying raw material gases and is similar to that illustrated in FIG. 2 except that a unit process shown in FIG. 3 is different from that illustrated in FIG. 2. Therefore, description will be mainly directed to the unit process of FIG. 3 and will be omitted about portions or processes common to the first and the second embodiments. Briefly, the method according to the second embodiment of this invention is featured by simultaneously executing both the silicon layer deposition process and the impurity doping process and by thereafter executing the carbonization process.

As shown in FIG. 3, the unit process according to the second embodiment of this invention has a doping process which is executed simultaneously with the depositing process of depositing a silicon layer on a previous silicon carbide layer. Specifically, the valve 2c is opened to supply a SiH.sub.2Cl.sub.2 gas to the deposition chamber 1 (FIG. 1) at a flow rate of 50 sccm for five seconds and, at the same time, the valve 2d is opened to supply a nitrogen gas (N.sub.2) at a flow rate of 50 sccm for fiv