Semiconductor device

A semiconductor device, including a semiconductor substrate of a first conductivity type, a first semiconductor layer of the first conductivity type, provided on the semiconductor substrate and having an impurity concentration lower than that of the semiconductor substrate, a second semiconductor layer of a second conductivity type, selectively provided on the first semiconductor layer, a plurality of first semiconductor regions of the first conductivity type, selectively provided in the second semiconductor layer at a surface thereof, a plurality of gate insulating films in contact with the second semiconductor layer, a plurality of gate electrodes respectively provided on the gate insulating films, a plurality of first electrodes provided on the second semiconductor layer and the first semiconductor regions, and a second electrode provided on a back surface of the semiconductor substrate. The semiconductor substrate contains boron, a concentration of the boron therein being in a range from 5×1015/cm3 to 5×1016/cm3.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2020-036303, filed on Mar. 3, 2020, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to a semiconductor device.

2. Description of the Related Art

Silicon carbide (SiC) is expected to replace silicon (Si) as a next generation semiconductor material. Compared to a conventional semiconductor device in which silicon is used as a semiconductor material, a semiconductor device in which silicon carbide is used as a semiconductor material (hereinafter, silicon carbide semiconductor device) has various advantages as such as enabling use under higher temperature environments (at least 200 degrees C.) and reducing device resistance in an ON state to one of a few hundredths of that of the conventional semiconductor device. These advantages are due to characteristics of the material itself such as the bandgap of silicon carbide being about three times that of silicon and dielectric breakdown electric field strength being nearly ten times greater than that of silicon.

As silicon carbide semiconductor devices, Schottky barrier diodes (SBDs) and vertical metal oxide semiconductor field effect transistors (MOSFETs) having a planar gate structure or a trench gate structure have been made into products.

A trench gate structure is a MOS gate structure in which a MOS gate is embedded in a trench formed in a semiconductor substrate (semiconductor chip) at a front surface of the semiconductor substrate and in which a channel (inversion layer) is formed along sidewalls of the trench, in a direction orthogonal to the front surface of the semiconductor substrate. Therefore, compared to a planar gate structure in which a channel is formed along the front surface of the semiconductor substrate, unit cell (constituent unit of a device element) density per unit area may be increased and current density per unit area may be increased, which is advantageous in terms of cost. A planar gate structure is a MOS gate structure in which a MOS gate is provided in a flat plate-like shape on the front surface of a semiconductor substrate.

A structure of a conventional silicon carbide semiconductor device is described taking a trench-type MOSFET as an example.FIG. 10is a cross-sectional view of the structure of the conventional silicon carbide semiconductor device. As depicted inFIG. 10, in a trench-type MOSFET150, an n+-type buffer layer116and an n-type silicon carbide epitaxial layer102are deposited on a front surface of an n+-type silicon carbide substrate101. On a surface of the n-type silicon carbide epitaxial layer102, opposite a surface thereof facing the n+-type silicon carbide substrate101, an n-type high-concentration region106is provided. Further, in the n-type high-concentration region106at a surface thereof opposite that facing the n+-type silicon carbide substrate101, first p+-type base regions104are selectively provided. In the n-type high-concentration region106, second p+-type base regions105are selectively provided so as to underlie an entire area of a bottom of each of the trenches118.

Further, in the conventional trench-type MOSFET150, a p-type silicon carbide epitaxial layer103, n+-type base regions107, p++-type contact regions108, gate insulating films109, gate electrodes110, an insulating film111, source electrodes113, a back electrode114, the trenches118, a source electrode pad115, and a drain electrode pad (not depicted) are further provided. The source electrodes113are provided on the n+-type base regions107and the p++-type contact regions108, and the source electrode pad115is provided on the source electrodes113.

In the trench-type MOSFET150, a parasitic pn diode formed by the p-type silicon carbide epitaxial layer103and the n-type silicon carbide epitaxial layer102is built-in as a body diode between a source and drain. Thus, a freewheeling diode used in an inverter is formed, thereby reducing cost, and contributing to reductions in size.

Further, in a known semiconductor device, by depositing a buffer layer having an impurity concentration about equal to that of the substrate, a thickness of the buffer layer may be suppressed and even when bipolar operation is performed by large current, an occurrence of triangular and bar-shaped stacking faults in the substrate may be effectively suppressed and the thickness of the buffer layer may be measured by a conventional FT-IR method (for example, refer to Japanese Laid-Open Patent Publication No. 2019-012835).

Further, a known silicon carbide semiconductor device enables enhanced reliability of a product at a low cost by forming an n+-type buffer layer by an n-type impurity and additionally adding vanadium that forms recombination centers (for example, refer to Japanese Laid-Open Patent Publication No. 2019-134046).

SUMMARY OF THE INVENTION

According to an embodiment of the invention, a semiconductor device includes a semiconductor substrate of a first conductivity type, and having a front surface and a back surface opposite to each other, the semiconductor substrate containing boron, a concentration of the boron therein being in a range from 5×1015/cm3to 5×1016/cm3; a first semiconductor layer of the first conductivity type, provided on the front surface of the semiconductor substrate and having an impurity concentration lower than an impurity concentration of the semiconductor substrate, the first semiconductor layer having a first surface and a second surface opposite to each other, the second surface facing the semiconductor substrate; a second semiconductor layer of a second conductivity type, selectively provided on the first surface of the first semiconductor layer, the second semiconductor layer having a first surface and a second surface opposite to each other, the second surface facing the semiconductor substrate; a plurality of first semiconductor regions of the first conductivity type, selectively provided in the second semiconductor layer at the first surface thereof; a plurality of gate insulating films in contact with the second semiconductor layer, each having a first surface and a second surface opposite to each other, the second surface being in contact with the second semiconductor layer; a plurality of gate electrodes provided on the first surfaces of the gate insulating films, respectively; a plurality of first electrodes provided on the first surface of the second semiconductor layer and surfaces of the first semiconductor regions; and a second electrode provided on the back surface of the semiconductor substrate.

DETAILED DESCRIPTION OF THE INVENTION

First, problems associated with the conventional techniques are discussed. In an instance in which the body diode of the trench-type MOSFET150is used, a lifetime of the n-type silicon carbide epitaxial layer102that forms the drift layer is increased, whereby an effect of conductivity modulation is utilized to reduce a resistance of the n-type silicon carbide epitaxial layer102and operate the body diode under a lower resistance.

Nonetheless, when the lifetime of the drift layer is increased, carriers occur in the drift layer. In this state, when reverse recovery is performed, the carriers in the drift layer flow as reverse recovery current. As a result, a problem arises in that, during switching operation of the trench-type MOSFET150, the reverse recovery current increases, loss during switching increases, increases in surge voltage occur, and reliability of the semiconductor device is affected.

Embodiments of a semiconductor device according to the present invention will be described in detail with reference to the accompanying drawings. In the present description and accompanying drawings, layers and regions prefixed with n or p mean that majority carriers are electrons or holes. Additionally, + or − appended to n or p means that the impurity concentration is higher or lower, respectively, than layers and regions without + or −. Instances where symbols such as n's and p's that include + or − are the same indicate that concentrations are close and therefore, the concentrations are not necessarily equal. In the description of the embodiments below and the accompanying drawings, main portions that are identical will be given the same reference numerals and will not be repeatedly described. Further, in the present description, when Miller indices are described, “−” means a bar added to an index immediately after the “−”, and a negative index is expressed by prefixing “−” to the index. Further, throughout the entire specification, “equal” impurity concentrations means the impurity concentrations are within a range of one another, the range being specified with consideration of variation, the range being ±20%, or the range may be preferably ±10%, or more preferably may be ±5%.

A semiconductor device according to an embodiment contains a semiconductor having a bandgap that is wider than that of silicon (Si) (hereinafter, wide bandgap semiconductor). A structure of the semiconductor device according to the embodiment is described taking, as an example, an instance in which, for example, silicon carbide (SiC) is used as the wide bandgap semiconductor.FIG. 1is a cross-sectional view of a silicon carbide semiconductor device according to the embodiment.

The silicon carbide semiconductor device according to the embodiment is a trench-type MOSFET50that includes MOS gates having a trench gate structure in a semiconductor base, at a front surface thereof (surface of a p-type silicon carbide epitaxial layer3described hereinafter). A silicon carbide semiconductor base is formed by epitaxially growing an n-type silicon carbide epitaxial layer (first semiconductor layer of a first conductivity type)2and the p-type silicon carbide epitaxial layer (second semiconductor layer of the second conductivity type)3sequentially on an n+-type silicon carbide substrate (semiconductor substrate of the first conductivity type)1containing silicon carbide. An n+-type buffer layer (third semiconductor layer of the first conductivity type)16may be epitaxially grown on the n+-type silicon carbide substrate1. Further, n-type high-concentration regions6may be epitaxially grown on the n-type silicon carbide epitaxial layer2.

Here, the n+-type silicon carbide substrate1contains boron (B) and a concentration of the boron is in a range from 5×1015/cm3to 5×1016/cm3. More preferably, the concentration of boron may be in a range from 1×1016/cm3to 5×1016/cm3. Further, the n+-type silicon carbide substrate1contains nitrogen (N) and a concentration of the nitrogen is in a range from 1×1018/cm3to 2×1019/cm3. In other words, in the n+-type silicon carbide substrate1, the concentration of boron is in a range from 1/100 to 1/10 of the concentration of nitrogen.

Configuration may be such that the concentration of boron described above is only in a region of the n+-type silicon carbide substrate1, near a surface of the n+-type silicon carbide substrate1facing the n-type silicon carbide epitaxial layer2. Nonetheless, preferably, in an entire area of the n+-type silicon carbide substrate1, the concentration of boron may be in the range described above.

Further, the n-type silicon carbide epitaxial layer2may contain boron, the concentration of boron in the n-type silicon carbide epitaxial layer2being lower than the concentration of boron in the n+-type silicon carbide substrate1and, for example, being less than 1×1013/cm3. In an instance in which the n+-type buffer layer16is provided, the n+-type buffer layer16contains boron. A concentration of the boron in the n+-type buffer layer16is lower than the concentration of boron in the n+-type silicon carbide substrate1and higher than of the concentration of boron in the n-type silicon carbide epitaxial layer2. The n+-type buffer layer16may have a multilayered structure. In this instance, at least a layer thereof facing the n-type silicon carbide epitaxial layer2contains boron and preferably, all of the layers may contain boron.

In the embodiment, the concentration of boron in the n+-type silicon carbide substrate1is higher than that in the n-type silicon carbide epitaxial layer2, whereby the lifetime of carriers (electrons) in the n+-type silicon carbide substrate1becomes shorter than the lifetime of the carriers in the n-type silicon carbide epitaxial layer2. For example, the lifetime of the carriers in the n-type silicon carbide epitaxial layer2is at least 0.5 μs and the lifetime of the n+-type silicon carbide substrate1is shorter than this.

The lifetime of the carriers in the n-type silicon carbide epitaxial layer2is at least 0.5 μs and therefore, when the n-type silicon carbide epitaxial layer2of the embodiment is measured by a deep level transient spectroscopy (DLTS) method, majority carrier traps (electron traps) of the n-type silicon carbide epitaxial layer2are at most 5×1013/cm3.

In this manner, the lifetime of the carriers of the n+-type silicon carbide substrate1is reduced, whereby during reverse recovery, carriers remaining in the n-type silicon carbide epitaxial layer2may be recombined near the n+-type silicon carbide substrate1and reverse current may be reduced.

In the embodiment, the n-type silicon carbide epitaxial layer2having a long lifetime is used and during body diode operation, conductivity modulation is used, thereby reducing the resistance. On the other hand, the lifetime of the carriers in the n+-type silicon carbide substrate1, closer to a back electrode14than to the n-type silicon carbide epitaxial layer2, is reduced, whereby the effect of conductivity modulation, as is, may reduce the reverse recovery current. Therefore, with the resistance of the body diode of the MOSFET being reduced, the reliability of the semiconductor device may be enhanced by a reduction of switching loss and a reduction of surge current.

The MOS gates having the trench gate structure are configured by the p-type silicon carbide epitaxial layer3, n+-type source regions (first semiconductor regions of the first conductivity type)7, p++-type contact regions8, trenches18, gate insulating films9, and gate electrodes10.

In particular, the trenches18penetrate the p-type silicon carbide epitaxial layer3in a depth direction z from a front surface of the semiconductor base and reach the n-type high-concentration regions6(in an instance in which the n-type high-concentration regions6are not provided, the n-type silicon carbide epitaxial layer2, hereinafter, simply (2)). The depth direction z is a direction from the front surface toward a back surface thereof. The trenches18, for example, are disposed in a stripe pattern.

In the trenches18, the gate insulating films9are provided along inner walls of the trenches18, and the gate electrodes10are provided so as to be embedded in the trenches18on the gate insulating films9. One unit cell of a main semiconductor device element is configured by one of the gate electrodes10in one of the trenches18, and one of the gate electrodes10between adjacent mesa regions (region between adjacent trenches of the trenches18). InFIG. 1, while only two trench MOS structures are depicted, further MOS gate (insulated gate including a metal, an oxide film, and a semiconductor) structures having a trench structure may be disposed.

On a front surface of the n+-type silicon carbide substrate1, the n+-type buffer layer16may be provided. The n+-type buffer layer16has an impurity concentration that is equal to that of the n+-type silicon carbide substrate1and, for example, is a buffer layer doped with nitrogen. Recombination of electron-holes progresses in the n+-type buffer layer16, suppressing hole density injected into the n+-type silicon carbide substrate1, whereby the occurrence of triangular and bar-shaped stacking faults may be effectively suppressed.

In the n-type silicon carbide epitaxial layer2, in a surface layer thereof facing source electrodes13described hereinafter, n-type regions (hereinafter, n-type high-concentration regions)6may be provided so as to be in contact with the p-type silicon carbide epitaxial layer3. The n-type high-concentration regions6are a so-called current spreading layer (CSL) that reduces carrier spreading resistance. The n-type high-concentration regions6, for example, are provided uniformly in a direction parallel to a substrate front surface (the front surface of the semiconductor substrate) so as to be exposed at the inner walls of the trenches18.

The n-type high-concentration regions6, from respective interfaces thereof with the p-type silicon carbide epitaxial layer3, reach positions deeper on a drain side (deep positions closer to the back electrode14) than are bottoms of the trenches. In the n-type high-concentration regions6, first and second p+-type base regions4,5may each be selectively provided. The first p+-type base regions4are provided between adjacent trenches of the trenches18(mesa regions) to be separate from second p+-type base regions5and the trenches18, and to be in contact with the p-type silicon carbide epitaxial layer3. Of the bottoms and bottom corner portions of the trenches18, the second p+-type base regions5underlie at least the bottoms of the trenches18. The bottom corner portions of the trenches18are borders between the bottoms and sidewalls of the trenches18.

Pn junctions between the first and the second p+-type base regions4,5and the n-type silicon carbide epitaxial layer2are formed at deep positions closer to the back electrode14than are the bottoms of the trenches18. The first and the second p+-type base regions4,5may be provided in the n-type silicon carbide epitaxial layer2without providing the n-type high-concentration regions6. Each of the first and the second p+-type base regions4,5has an end facing the back electrode14, at a depth position so that the pn junctions between the first and the second p+-type base regions4,5and the n-type silicon carbide epitaxial layer2are closer to the back electrode14than are the bottoms of the trenches18, the depth position being changeable according to design conditions. Application of high electric field to portions of the gate insulating films9along the bottoms of the trenches18may be prevented by the first and the second p+-type base regions4,5.

The n+-type source regions7are selectively provided in the p-type silicon carbide epitaxial layer3. The p++-type contact regions8may be selectively provided so as to be in contact with the n+-type source regions7. The n+-type source regions7are in contact with the gate insulating films9at the sidewalls of the trenches18and face the gate electrodes10, across the gate insulating films9at the sidewalls of the trenches18.

An interlayer insulating film11is provided in an entire area of the front surface of the semiconductor substrate so as to cover the gate electrodes10. In the interlayer insulating film11, contact holes penetrating through the interlayer insulating film11in the depth direction z and reaching the substrate front surface are opened.

The source electrodes (first electrodes)13are in ohmic contact with the semiconductor substrate (the n+-type source regions7) in the contact holes and are electrically insulated from the gate electrodes10by the interlayer insulating film11. A source electrode pad15is provided on the source electrodes13. In an instance in which the p++-type contact regions8are provided, the source electrodes13are in contact with the p++-type contact regions8. In an instance in which the p++-type contact regions8are not provided, the source electrodes13are in ohmic contact with the p-type silicon carbide epitaxial layer3.

The back electrode (second electrode)14that is a drain electrode is provided on the back surface of the semiconductor substrate. A drain electrode pad (not depicted) is provided on the back electrode14.

Next, a method of manufacturing the silicon carbide semiconductor device according to the embodiment is described.FIGS. 2, 3, 4, 5, 6, and 7are cross-sectional views of states of the silicon carbide semiconductor device according to the embodiment during manufacture.

First, the n+-type silicon carbide substrate1that contains an n-type silicon carbide and having a concentration of boron that is in a range from 5×1016/cm3to 5×1016/cm3is prepared. In an instance in which the concentration of boron in the n+-type silicon carbide substrate1is less than 5×1016/cm3, for example, the concentration of boron may be set to be in the range described above by ion implantation of boron.

Here, the n+-type buffer layer16may be epitaxially grown on a first main surface of the n+-type silicon carbide substrate1while an n-type impurity, for example, nitrogen (N) atoms, is doped. The n+-type buffer layer16has an impurity concentration that is equal to the impurity concentration of the n+-type silicon carbide substrate1. Next, on the surface of the n+-type buffer layer16, a first n-type silicon carbide epitaxial layer2acontaining silicon carbide is epitaxially grown to have a thickness of, for example, about 30 μm while an n-type impurity, for example, nitrogen atoms, is doped. Here, the n+-type buffer layer16is formed so that the concentration of boron in the n+-type buffer layer16is lower than the concentration of boron in the n+-type silicon carbide substrate1. Similarly, the first n-type silicon carbide epitaxial layer2ais formed so that a concentration of boron therein is lower than the concentration of boron in the n+-type buffer layer16and in the n+-type silicon carbide substrate1. The state up to here is depicted inFIG. 2.

Next, on the surface of the first n-type silicon carbide epitaxial layer2a, an ion implantation mask having predetermined openings is formed by a photolithographic technique using, for example, an oxide film. Further, a p-type impurity such as aluminum is implanted in the openings of the oxide film, thereby forming lower first p+-type base regions4aand the second p+-type base regions5at a depth of about 0.5 μm.

Further, formation is such that a distance between one of the lower first p+-type base regions4aand an adjacent one of the second p+-type base regions5is about 1.5 μm. An impurity concentration of the lower first p+-type base regions4aand the second p+-type base regions5is set to be, for example, about 5×1018/cm3.

Next, portions of the ion implantation mask may be removed, an n-type impurity such as nitrogen may be ion implanted in the openings, whereby in surface regions of the first n-type silicon carbide epitaxial layer2a, lower n-type high-concentration regions6aat a depth of, for example, about 0.5 μm may be formed. An impurity concentration of the lower n-type high-concentration regions6ais set to be, for example, about 1×1017/cm3. The state up to here is depicted inFIG. 3.

Next, on the surface of the first n-type silicon carbide epitaxial layer2a, a second n-type silicon carbide epitaxial layer2bdoped with an n-type impurity such as nitrogen is formed to have a thickness of about 0.5 μm. An impurity concentration of the second n-type silicon carbide epitaxial layer2bis set to be about 3×1015/cm3. The second n-type silicon carbide epitaxial layer2bis formed so that a concentration of boron therein is equal to the concentration of boron in the first n-type silicon carbide epitaxial layer2aand lower than the concentration of boron in the n+-type silicon carbide substrate1. Hereinafter, the first n-type silicon carbide epitaxial layer2aand the second n-type silicon carbide epitaxial layer2bcombined form the n-type silicon carbide epitaxial layer2.

Next, on the surface of the second n-type silicon carbide epitaxial layer2b, an ion implantation mask having predetermined openings is formed by photolithography using, for example, an oxide film. Further, a p-type impurity such as aluminum is implanted in the openings of the oxide film, thereby forming upper first p+-type base regions4bat a depth of about 0.5 μm, so as to overlap the lower first p+-type base regions4a. The lower first p+-type base regions4aand the upper first p+-type base regions4bform connected regions that are the first p+-type base regions4. An impurity concentration of the upper first p+-type base regions4bis set to be, for example, about 5×1018/cm3.

Next, portions of the ion implantation mask may be removed, an n-type impurity such as nitrogen may be ion implanted in the openings, whereby upper n-type high-concentration regions6bmay be formed at a depth of, for example, about 0.5 μm in surface regions of the second n-type silicon carbide epitaxial layer2b. An impurity concentration of the upper n-type high-concentration regions6bis set to be, for example, about 1×1017/cm3. The upper n-type high-concentration regions6band the lower n-type high-concentration regions6aare formed to at least partially contact one another, thereby forming the n-type high-concentration regions6. Nonetheless, the n-type high-concentration regions6may be formed in an entire area of a substrate surface or may be omitted. The state up to here is depicted inFIG. 4.

Next, on the surface of the n-type silicon carbide epitaxial layer2, the p-type silicon carbide epitaxial layer3is formed by epitaxial growth to have a thickness of about 1.1 μm. An impurity concentration of the p-type silicon carbide epitaxial layer3is set to be about 4×1017/cm3. After formation of the p-type silicon carbide epitaxial layer3by epitaxial growth, in the p-type silicon carbide epitaxial layer3, a p-type impurity such as aluminum may be further ion implanted in channel regions of the p-type silicon carbide epitaxial layer3.

Next, on the surface of the p-type silicon carbide epitaxial layer3, an ion implantation mask having predetermined openings is formed by photolithography using, for example, an oxide film. In the openings. an n-type impurity such as nitrogen (N) or phosphorus (P) is ion implanted, thereby forming the n+-type source regions7in the p-type silicon carbide epitaxial layer3, at the surface thereof. Next, the ion implantation mask used in forming the n+-type source regions7is removed and an ion implantation mask having predetermined openings may be formed by a similar method, a p-type impurity such as phosphorus may be ion implanted in the p-type silicon carbide epitaxial layer3, at the surface thereof, whereby the p++-type contact regions8may be formed. An impurity concentration of the p++-type contact regions8is set to be higher than the impurity concentration of the p-type silicon carbide epitaxial layer3. The state up to here is depicted inFIG. 5.

Next, a heat treatment (annealing) is performed in an inert gas atmosphere of a temperature of about 1700 degrees C., whereby an activation treatment for the first p+-type base regions4, the second p+-type base regions5, the n+-type source regions7, and the p++-type contact regions8is implemented. As described above, ion implanted regions may be collectively activated by a single session of the heat treatment or the heat treatment may be performed each time the ion implantation is performed.

Next, on the surface of the p-type silicon carbide epitaxial layer3, a trench forming mask having predetermined openings is formed by photolithography using, for example, an oxide film. Next, the trenches18that penetrate the p-type silicon carbide epitaxial layer3and reach the n-type high-concentration regions6(2) are formed by dry etching. The bottoms of the trenches18may reach the second p+-type base regions5formed in the n-type high-concentration regions6(2). Next, the trench forming mask is removed. The state up to here is depicted inFIG. 6.

Next, along the surfaces of the n+-type source regions7and the bottoms and sidewalls of the trenches18, the gate insulating films9are formed. The gate insulating films9may be formed by thermal oxidation of a temperature of about 1000 degrees C. in an oxygen atmosphere. Further, the gate insulating films9may be formed by a deposition method such as that for a high temperature oxide (HTO).

Next, on the gate insulating films9, for example, a polycrystalline silicon layer doped with phosphorus atoms is provided. The polycrystalline silicon layer may be formed so as to be embedded in the trenches18. The polycrystalline silicon layer is patterned by photolithography to be left in the trenches18and thereby form the gate electrodes10.

Next, for example, phosphate glass is deposited so as to cover the gate insulating films9and the gate electrodes10and have a thickness of about 1 μm to thereby form the interlayer insulating film11. The interlayer insulating film11and the gate insulating films9are patterned by photolithography, thereby forming contact holes in which the n+-type source regions7and the p++-type contact regions8are exposed. In an instance in which the p++-type contact regions8are not formed, the n+-type source regions7and the p-type silicon carbide epitaxial layer3are exposed in the contact holes. Thereafter, a heat treatment (reflow) is performed, thereby planarizing the interlayer insulating film11. The state up to here is depicted inFIG. 7. Further, after formation of the contact holes in the interlayer insulating film11, a barrier metal formed by titanium (Ti) or titanium nitride (TiN) or stacked layers of titanium and titanium nitride may be formed. In this instance, the contact holes exposing the n+-type source regions7and the p++-type contact regions8are further formed in the barrier metal.

Next, in the contact holes provided in the interlayer insulating film11and on the interlayer insulating film11, a conductive film that forms the source electrodes13is formed. The conductive film, for example, a nickel (Ni) film. Further, on a second main surface of the n+-type silicon carbide substrate1, a nickel (Ni) film is similarly formed. Thereafter, for example, a heat treatment of a temperature of about 970 degrees C. is performed, whereby the nickel film in the contact holes is converted into a silicide, thereby forming the source electrodes13. Concurrently, the nickel film formed on the second main surface becomes the back electrode14that forms an ohmic contact with the n+-type silicon carbide substrate1. Thereafter, unreacted portions of the nickel film are removed, thereby leaving the nickel film in, for example, only the contact holes as the source electrodes13.

Next, the source electrode pad15is formed so as to be embedded in the contact holes. A portion of a metal layer deposited to form the source electrode pad15may be used as a gate pad. On the back surface of the n+-type silicon carbide substrate1, a metal film such as a nickel (Ni), a titanium (Ti) film, etc. is formed in a contact portion of the back electrode14, using sputtering deposition. The metal film may be formed by a combination of stacked Ni films and Ti films. Thereafter, annealing such as rapid thermal annealing (RTA) is implemented so as to convert the metal film into a silicide and form an ohmic contact. Thereafter, for example, a thick film in which a Ti film, a Ni film, and a gold (Au) film are sequentially stacked is formed by electron beam (EB) deposition, whereby the back electrode14is formed.

In the epitaxial growth and ion implantation described above, for example, nitrogen (N) or phosphorus (P) that are an n-type with respect to silicon carbide, arsenic (As), antimony (Sb), etc. may be used as an n-type impurity (n-type dopant). As a p-type impurity (p-type dopant), for example, boron (B) or aluminum (Al) that are a p-type with respect to silicon carbide, gallium (Ga), indium (In), thallium (TI), etc. may be used. In this manner, the trench-type MOSFET50depicted inFIG. 1is completed.

FIG. 8is a graph depicting reverse recovery current of the silicon carbide semiconductor device according to the embodiment and reverse recovery current of a conventional silicon carbide semiconductor device.FIG. 9is a graph depicting surge voltage of the silicon carbide semiconductor device according to the embodiment and the conventional silicon carbide semiconductor device. InFIGS. 8 and 9, the silicon carbide semiconductor device according to the embodiment is the trench-type MOSFET50formed on the n+-type silicon carbide substrate1in which the concentration of boron is 5×1015/cm3and inFIGS. 8 and 9, the conventional silicon carbide semiconductor device is the trench-type MOSFET150formed on the n+-type silicon carbide substrate101in which the concentration of boron is less than 1×1013/cm3. The trench-type MOSFETs50,150were formed as 1200V and 30A elements, operated at 600V, which is a half of the rated voltage, and the reverse recovery current and the surge voltage were measured. Further, the trench-type MOSFETs50,150have equivalent upper structures higher than the n+-type silicon carbide substrate1. In other words, the lifetime of the n-type silicon carbide epitaxial layer2that is a drift layer is long and the resistance of the body diode is low.

InFIG. 8, a horizontal axis indicates time from the time of recovery in units of “s” and a vertical axis indicates current between the drain and source in units of “A”. Here, a reverse recovery charge amount Qrr is an area of the graph for a portion greater than the rated current of 30A. As depicted inFIG. 8, the silicon carbide semiconductor device according to the embodiment has a reverse recovery charge amount Qrr of 0.50 μC and the conventional silicon carbide semiconductor device has a reverse recovery charge amount Qrr of 0.71 μC. In this manner, in the silicon carbide semiconductor device according to the embodiment, the reverse recovery charge amount Qrr is reduced by at least 30% relative to the conventional silicon carbide semiconductor device.

InFIG. 9, a horizontal axis indicates time from the time of recovery in units of “s” and the vertical axis indicates the voltage between the drain and source in units of “V”. Here, the surge voltage is a peak voltage until the reverse recovery current disappears during reverse recovery operation of the body diode. As depicted inFIG. 9, the silicon carbide semiconductor device according to the embodiment has a surge voltage Vr that is 859V and the conventional silicon carbide semiconductor device has a surge voltage Vr that is 1008V. In this manner, in the silicon carbide semiconductor device according to the embodiment, the surge voltage Vr is reduced at least 150V relative to that of the conventional silicon carbide semiconductor device.

From the results depicted inFIGS. 8 and 9, the silicon carbide semiconductor device according to the embodiment, with the resistance of the body diode reduced as is, suppresses the reverse recovery current to a greater extent than does the conventional silicon carbide semiconductor device, and the surge voltage decreases and reverse recovery characteristics are improved.

As described above, according to the semiconductor device according to the embodiment, the n+-type silicon carbide substrate contains boron, and the concentration of the boron is in a range from 5×1015/cm3to 5×1016/cm3. As a result, the lifetime of carriers (electrons) in the n+-type silicon carbide substrate is reduced, thereby enabling carriers remaining in the n-type silicon carbide epitaxial layer during reverse recovery operation to be recombined at the n+-type silicon carbide substrate side and the reverse recovery current to be reduced. Therefore, with the resistance of the body diode reduced as is, switching loss is reduced, enabling surge current to be reduced and the reliability of the semiconductor device may be enhanced.

In the foregoing, various modifications within a range not departing from the spirit of the invention are possible; for example, in the embodiments, for example, dimensions, impurity concentrations of regions, etc. are variously set according to necessary specifications. Further, in the embodiments described above, while an instance in which silicon carbide is used as a semiconductor is described as an example, other than silicon carbide, for example, silicon (Si), gallium nitride (GaN), etc. may be applied as a semiconductor. Further, in the embodiments, while the first conductivity type is assumed to be an n-type and the second conductivity type is assumed to be a p-type, the present invention is similarly implemented when the first conductivity type is a p-type and the second conductivity type is an n-type.

A semiconductor device according to an embodiment of the present invention achieves an effect in that with the resistance reduced during body diode operation, the reverse recovery current and surge voltage may be reduced.

As described above, the semiconductor device according to an embodiment of the present invention is useful for power semiconductor devices used in power converting equipment such as for inverters, power supply devices such as for various types of industrial machines, and igniters of automobiles.