Method for manufacturing semiconductor device

A method for manufacturing a semiconductor device according to an embodiment includes: forming an insulating layer having a first plane in contact with a nitride semiconductor layer and a second plane opposite to the first plane and containing at least one of an oxide and an oxynitride; and performing first heat treatment at 600° C. or more and 1100° C. or less in a state where a voltage making a first plane side positive relative to a second plane side is applied to the insulating layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2018-029318, filed on Feb. 22, 2018, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a method for manufacturing a semiconductor device.

BACKGROUND

Semiconductor devices such as transistors and diodes are used in circuits such as switching power supply circuits and inverter circuits. These semiconductor devices are required to have a high breakdown voltage and a low on-resistance. There is a trade-off relationship determined by the element material in the relationship between the breakdown voltage and the on-resistance.

Due to advances in technology development, the low on-resistance has been implemented for semiconductor devices up to near the limit of silicon being the major semiconductor material. In order to further improve the breakdown voltage or further reduce the on-resistance, it is necessary to change the semiconductor material. By using a nitride semiconductor such as gallium nitride or aluminum gallium nitride as a semiconductor material of a semiconductor device, the trade-off relationship determined by the semiconductor material can be improved. Therefore, it is possible to drastically increase the breakdown voltage and reduce the on-resistance of the semiconductor devices.

However, in a transistor having a metal insulator semiconductor (MIS) structure using a nitride semiconductor, there is a problem that the threshold voltage fluctuates due to charges contained in a gate insulating layer.

DETAILED DESCRIPTION

A method for manufacturing a semiconductor device according to an embodiment includes: forming an insulating layer having a first plane in contact with a nitride semiconductor layer and a second plane opposite to the first plane, and the insulating layer containing at least one of an oxide and an oxynitride; and performing first heat treatment at 600° C. or more and 1100° C. or less in a state where a voltage making a first plane side positive relative to a second plane side being applied to the insulating layer.

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the following description, the same members and the like are denoted by the same reference numerals, and the description of the members once described is omitted as appropriate.

In the present specification, a “nitride semiconductor layer” includes a “GaN-based semiconductor”. A “GaN-based semiconductor” is a generic name for gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN) and semiconductors having intermediate compositions thereof.

In the present specification, “undoped” means that the impurity concentration is 1×1015cm−3or less.

In the present specification, in order to indicate the positional relationship of components and the like, the upward direction of the drawing is described as “above” and the downward direction of the drawing as “below”. In this specification, the concepts of “above” and “below” are not necessarily terms indicating the relationship with the direction of gravity.

First Embodiment

A method for manufacturing a semiconductor device according to a first embodiment includes: forming an insulating layer having a first plane in contact with a nitride semiconductor layer and a second plane opposite to the first plane and containing at least one of an oxide and an oxynitride; and performing first heat treatment at 600° C. or more and 1100° C. or less in a state where a voltage making a first plane side positive relative to a second plane side is applied to the insulating layer. Further, before the first heat treatment is performed, a conductive layer in contact with the second plane is formed. Also, after the first heat treatment is performed, second heat treatment is performed in a state where a voltage making the first plane side negative relative to the second plane side is applied to the insulating layer.

FIG. 1is a schematic sectional view of a semiconductor device fabricated by a method for manufacturing a semiconductor device according to the first embodiment. The semiconductor device is a high electron mobility transistor (HEMT)100having a MIS structure using a GaN-based semiconductor. The HEMT100has a gate recess structure in which a gate electrode is provided in a trench (recess).

The HEMT100includes a substrate10, a buffer layer12, a channel layer14(nitride semiconductor layer), a barrier layer15(nitride semiconductor layer), a gate insulating layer16(insulating layer), a gate electrode18, a source electrode20, a drain electrode22, an interlayer insulating layer30, and a trench40.

The bottom of the trench40is located inside the channel layer14. The gate insulating layer16and the gate electrode18are formed inside the trench40. By locating the bottom of the trench40inside the channel layer14, a two dimensional electron gas under the gate electrode18disappears. Therefore, a normally-off operation can be implemented.

The substrate10is formed of, for example, silicon (Si). Apart from silicon, for example sapphire (Al2O3) or silicon carbide (SiC) can also be applied.

The buffer layer12is provided on the substrate10. The buffer layer12has a function of mitigating a lattice mismatch between the substrate10and the channel layer14. The buffer layer12is formed as a multilayer structure of, for example, aluminum gallium nitride (AlWGa1-WN (0≤W≤1)).

The channel layer14is provided on the buffer layer12. The channel layer14is also called an electron transit layer. The channel layer14contains gallium (Ga). The channel layer14is, for example, undoped AlXGa1-XN (0≤X<1). More specifically, the channel layer14is, for example, undoped gallium nitride (GaN). The film thickness of the channel layer14is, for example, 0.1 μm or more and 10 μm or less.

The barrier layer15is provided on the channel layer14. The barrier layer15is also called an electron supply layer. The band gap of the barrier layer15is larger than that of the channel layer14. The barrier layer15contains gallium (Ga). The barrier layer15is, for example, undoped aluminum gallium nitride (AlYGa1-YN (0<Y≤1, X<Y)). More specifically, the barrier layer15is, for example, undoped Al0.25Ga0.75N. The film thickness of the barrier layer15is, for example, 10 nm or more and 100 nm or less.

Between the channel layer14and the barrier layer15is a heterojunction interface. A two dimensional electron gas (2 DEG) is formed at the heterojunction interface and becomes a carrier of the HEMT100.

The source electrode20is provided on the channel layer14and the barrier layer15. The source electrode20is electrically connected to the channel layer14and the barrier layer15.

The source electrode20is, for example, a metal electrode. The source electrode20has, for example, a stacked structure of titanium (Ti) and aluminum (Al). The source electrode20and the barrier layer15are desirably in ohmic contact.

The drain electrode22is provided on the channel layer14and the barrier layer15. The drain electrode22is electrically connected to the channel layer14and the barrier layer15.

The drain electrode22is, for example, a metal electrode. The drain electrode22has, for example, a stacked structure of titanium (Ti) and aluminum (Al). The drain electrode22and the barrier layer15are desirably in ohmic contact.

The distance between the source electrode20and the drain electrode22is, for example, 5 μm or more and 30 μm or less.

Note that a structure in which the source electrode20and the drain electrode22are in contact with the channel layer14may also be adopted.

At least a portion of the gate electrode18is formed inside the trench40. The gate electrode18is provided on the barrier layer15. The gate electrode18is provided between the source electrode20and the drain electrode22.

The gate electrode18is, for example, polycrystalline silicon containing a conductive impurity. Also, the gate electrode18is, for example, a metal. The gate electrode18is made of, for example, titanium nitride (TiN).

At least a portion of the gate insulating layer16is formed inside the trench40. The gate insulating layer16is located between the channel layer14and the gate electrode18.

The gate insulating layer16has a first plane in contact with the channel layer14and the barrier layer15and a second plane opposite to the first plane. The gate electrode18is in contact with the second plane.

The gate insulating layer16is also formed on the barrier layer15between the gate electrode18and the drain electrode22. The gate insulating layer16is also formed on the barrier layer15between the gate electrode18and the source electrode20.

The gate insulating layer16contains at least one of an oxide and an oxynitride. The oxide is, for example, silicon oxide or aluminum oxide. The oxynitride is silicon oxynitride or aluminum oxynitride.

The thickness of the gate insulating layer16is, for example, 20 nm or more and 100 nm or less. The equivalent oxide thickness (EOT) of the gate insulating layer16is, for example, 20 nm or more and 40 nm or less.

Next, a method for manufacturing a semiconductor device according to the first embodiment will be described.FIGS. 2, 3, 4, 5, 6, 7, and 8are schematic sectional views showing the method for manufacturing a semiconductor device according to the first embodiment.

First, the substrate10, for example, a silicon substrate is prepared. Next, for example, a multilayer structure of aluminum gallium nitride to be the buffer layer12is formed by epitaxial growth on the silicon substrate. For example, the buffer layer12is grown by a metal organic chemical vapor deposition (MOCVD) method.

Next, undoped gallium nitride to be the channel layer14(nitride semiconductor layer) and undoped aluminum gallium nitride to be the barrier layer15(nitride semiconductor layer) are formed on the buffer layer12by epitaxial growth (FIG. 2). The channel layer14and the barrier layer15are grown by, for example, the MOCVD method. The channel layer14and the barrier layer15contain gallium (Ga).

Next, the trench40penetrating the barrier layer15and reaching the channel layer14is formed (FIG. 3). The trench40is formed by, for example, a reactive ion etching method. Though not shown, before forming the trench40, for example, a silicon nitride film is formed as a mask material. The process may proceed while leaving the mask material unremoved. In this case, a stacked film of the silicon nitride film and the gate insulating film16is formed on the barrier layer15. From the viewpoint of protecting the front surface of the barrier layer15, the silicon nitride film is preferably left.

Next, the gate insulating layer16(insulating layer) is formed on the channel layer14and the barrier layer15(FIG. 4). The gate insulating layer16has a first plane in contact with the channel layer14and the barrier layer15and a second plane opposite to the first plane.

The gate insulating layer16contains at least one of an oxide and an oxynitride. The oxide is, for example, silicon oxide or aluminum oxide. The oxynitride is, for example, silicon oxynitride or aluminum oxynitride.

The gate insulating layer16is formed by, for example, a chemical vapor deposition (CVD) method.

Next, a conductive layer118in contact with the second plane of the gate insulating layer16is formed (FIG. 5). The conductive layer118is, for example, polycrystalline silicon containing a conductive impurity.

The conductive layer118is, for example, a metal. The conductive layer118is, for example, titanium nitride (TiN).

Next, the first heat treatment is performed (FIG. 6). Densification of the gate insulating layer16is performed by the first heat treatment. Positive charges in the gate insulating layer16are removed by the first heat treatment. The positive charge is, for example, a hydrogen ion or a gallium ion.

The first heat treatment is performed at 600° C. or more and 1100° C. or less. The first heat treatment is performed, for example, in a non-oxidizing atmosphere. The first heat treatment is performed in an inert gas atmosphere, for example, a nitrogen atmosphere, an argon atmosphere, a helium atmosphere, or the like. The time period of the first heat treatment is, for example, 5 minutes or more and 60 minutes or less.

The first heat treatment is performed in a state where a voltage is applied to the gate insulating layer16. A voltage making the first plane side of the gate insulating layer16positive relative to the second plane side is applied. For example, a voltage is applied to the gate insulating layer16by bringing electrodes into contact with the substrate10and the conductive layer118. A voltage making the substrate10positive relative to the conductive layer118is applied.

The electric field strength in the gate insulating layer16during the first heat treatment is, for example, 2 MV/cm or more and 10 MV/cm or less.

Next, the second heat treatment is performed (FIG. 7). Negative charges in the gate insulating layer16are removed by the second heat treatment. The negative charge is, for example, a fluoride ion or a nitrogen ion.

The temperature of the second heat treatment is, for example, lower than that of the first heat treatment. The temperature of the second heat treatment is 400° C. or more and 1000° C. or less.

The second heat treatment is performed, for example, in a non-oxidizing atmosphere. The second heat treatment is performed in an inert gas atmosphere, for example, a nitrogen atmosphere, an argon atmosphere, a helium atmosphere, or the like. The time period of the second heat treatment is, for example, 5 minutes or more to 60 minutes or less.

The second heat treatment is performed in a state where a voltage is applied to the gate insulating layer16. A voltage making the first plane side of the gate insulating layer16negative relative to the second plane side is applied. For example, a voltage is applied to the gate insulating layer16by bringing electrodes into contact with the substrate10and the conductive layer118. A voltage making the substrate10negative relative to the conductive layer118is applied.

The electric field strength in the gate insulating layer16during the second heat treatment is, for example, 2 MV/cm or more and 10 MV/cm or less.

Next, the conductive layer118is patterned to form the gate electrode18(FIG. 8). The patterning of the conductive layer118is performed using, for example, a lithography method and a reactive ion etching method.

Next, the source electrode20, the drain electrode22, and the interlayer insulating layer30are formed by a known method.

By the above manufacturing method, the HEMT100shown inFIG. 1is formed.

Hereinafter, the function and effect of the method for manufacturing a semiconductor device according to the first embodiment will be described. Hereinafter, a case where the gate insulating layer16is silicon oxide is taken as an example.

In a transistor having a MIS structure using a nitride semiconductor, there is a problem that a threshold voltage fluctuates due to charges contained in the gate insulating layer. According to the first embodiment, by performing heat treatment by applying a voltage to the gate insulating layer, the amount of charge contained in the gate insulating layer is reduced. Therefore, a transistor having the MIS structure capable of suppressing fluctuations of the threshold voltage is implemented. Details will be described below.

The threshold voltage of a transistor is considered to fluctuate because charges contained in the gate insulating layer move due to a gate voltage applied to the gate electrode during operation of the transistor. Alternatively, fluctuations of the threshold voltage of a transistor is considered to be caused by movement of charges contained in the gate insulating layer due to thermal diffusion. Therefore, in order to suppress fluctuations of the threshold voltage, the amount of charges contained in the gate insulating layer is desired to be reduced.

Charges contained in the gate insulating layer are, for example, ionized gallium (Ga), nitrogen (N), hydrogen (H), or fluorine (F). Gallium (Ga) and nitrogen (N) originate, for example, from the nitride semiconductor layer below the gate insulating layer. Hydrogen (H) originates, for example, from the source gas at the time of forming the gate insulating layer. Fluorine (F) originates, for example, from members of the reaction chamber of the process equipment.

Gallium (Ga) and nitrogen (N) are considered to be contained in the gate insulating layer by being diffused from the nitride semiconductor layer during the heat treatment after the formation of the gate insulating layer. From the viewpoint of improving the film quality of the gate insulating layer, it is preferable to perform densification at a relatively high temperature. However, when densification is performed at a high temperature, gallium (Ga) and nitrogen (N) are separated from the nitride semiconductor layer and diffuse into the gate insulating layer, so that fluctuations of the threshold voltage are more likely to occur.

FIGS. 9A, 9B, 9C, and 9Dare explanatory diagrams of functions and effects of the method for manufacturing a semiconductor device according to the first embodiment.FIG. 9Ais a diagram showing the energy level of gallium (Ga) in the band gap of silicon oxide.FIG. 9Bis a diagram showing the energy level of hydrogen (H) in the band gap of silicon oxide.FIG. 9Cis a diagram showing the energy level of fluorine (F) in the band gap of silicon oxide.FIG. 9Dis a diagram showing the energy level of nitrogen (N) in the band gap of silicon oxide.FIGS. 9A, 9B, 9C, and 9Dare based on results of a first principle calculation by the inventors.

As shown inFIG. 9A, gallium (Ga) in silicon oxide forms an energy level near the lower end of the conduction band of silicon oxide. Gallium (Ga) emits electrons to become a positive ion. In addition, gallium (Ga) becomes energy-stable by being positioned between lattices of silicon oxide. Therefore, diffusion of gallium (Ga) in silicon oxide is relatively fast.

As shown inFIG. 9B, hydrogen (H) in silicon oxide forms an energy level near the lower end of the conduction band of silicon oxide. Hydrogen (H) emits an electron to become a positive ion. In addition, hydrogen (H) becomes energy-stable by being positioned between lattices of silicon oxide. Therefore, diffusion of hydrogen (H) in silicon oxide is relatively fast.

As shown inFIG. 9C, fluorine (F) in silicon oxide forms an energy level near the upper end of the valence band of silicon oxide. Fluorine (F) becomes a negative ion by receiving an electron. In addition, fluorine (F) becomes energy-stable by being positioned between lattices of silicon oxide. Compared with, for example, hydrogen (H) positioned between lattices to become a positive ion, the diffusion of fluorine (F) in silicon oxide is slow. This is because fluorine (F) in silicon oxide is a negative ion and so has a large electron cloud.

As shown inFIG. 9D, nitrogen (N) in silicon oxide forms an energy level near the upper end of the valence band of silicon oxide. Nitrogen (N) becomes a negative ion by receiving an electron. In addition, nitrogen (N) becomes energy-stable by being positioned at the lattice point of silicon oxide. For this reason, the diffusion of nitrogen (N) in silicon oxide is relatively slow.

As described above, gallium (Ga) and nitrogen (N) originate from the nitride semiconductor layer. As a result of the first principle calculation by the inventors, it turned out that the energy of formation of gallium holes in gallium nitride is much higher than that of nitrogen holes in gallium nitride. The energy of formation of a gallium hole in gallium nitride is 8.4 eV and that of a nitrogen hole in gallium nitride is 3.2 eV.

Therefore, it is considered difficult for gallium to be simply separated from gallium nitride to form a gallium hole. Three electrons are short in a gallium hole. Thus, when three nitrogen holes are formed and dangling bonds of gallium are formed, a gallium hole is considered to be stabilized. That is, gallium holes are more likely to be formed by nitrogen holes being formed.

Therefore, if the formation of nitrogen holes is suppressed, gallium holes are less likely to be formed. In other words, if separation of nitrogen (N) from the nitride semiconductor layer is suppressed, separation of gallium (Ga) from the nitride semiconductor layer is suppressed.

In the method for manufacturing a semiconductor device according to the first embodiment, after the formation of the gate insulating layer16, the first heat treatment is performed in a state where a voltage is applied to the gate insulating layer16. A voltage making the first plane side of the gate insulating layer16positive relative to the second plane side is applied. Thus, the movement of nitrogen (N) that becomes a negative ion in silicon oxide in the direction from the first plane of the gate insulating layer16toward the second plane is suppressed. Therefore, the separation of nitrogen (N) from the nitride semiconductor layer is suppressed. Consequently, the separation of gallium (Ga) from the nitride semiconductor layer is also suppressed.

Since the separation of nitrogen (N) and gallium (Ga) from the nitride semiconductor layer during the first heat treatment is suppressed, threshold voltage fluctuations of the HEMT100can be suppressed even if the first heat treatment is performed at a high temperature. Therefore, densification of the gate insulating layer16can be performed at a high temperature.

Also, in the first heat treatment, it becomes easier for an element that becomes a positive ion in silicon oxide, for example, hydrogen (H) to move in the direction from the first plane of the gate insulating layer16toward the second plane. Thus, positive ions inside the silicon oxide are removed from the inside of the gate insulating layer16. Therefore, threshold voltage fluctuations of the HEMT100can be suppressed.

The temperature of the first heat treatment is 600° C. or more and 1100° C. or less and preferably 800° C. or more and 1050° C. or less. Below the above range, there is a possibility that the densifying effect of the gate insulating layer16is not sufficiently obtained. In addition, if the above range is exceeded, there is a possibility that nitrogen (N) is separated from the nitride semiconductor layer.

The temperature of the first heat treatment is preferably determined in consideration of the heat resistance of the gate insulating layer16and the gate electrode18. For example, when the gate insulating layer16is aluminum oxide, from the viewpoint of suppressing the crystallization of aluminum oxide, the temperature of the first heat treatment is preferably less than 800° C. Further, when the gate electrode18is, for example, titanium nitride, the temperature of the first heat treatment is preferably 700° C. or less.

From the viewpoint of suppressing oxidation of the gate electrode18and the nitride semiconductor layer, the first heat treatment is preferably performed in a non-oxidizing atmosphere. That is, the first heat treatment is preferably performed in an atmosphere that does not allow to intentionally contain oxygen. For example, the first heat treatment is performed in an inert gas atmosphere such as a nitrogen atmosphere, an argon atmosphere, or a helium atmosphere.

The time period of the first heat treatment is preferably five minutes or more and 60 minutes or less. Below the above range, there is a possibility that the densifying effect of the gate insulating layer16is not sufficiently obtained. In addition, if the above range is exceeded, the fabrication time of the HEMT100may become longer and the fabrication cost may increase.

The electric field strength in the gate insulating layer16during the first heat treatment is preferably 2 MV/cm or more and 10 MV/cm or less, and more preferably 4 MV/cm or more and 8 MV/cm or less. Below the above range, there is a possibility that the effect of suppressing the separation of nitrogen (N) from the nitride semiconductor layer and the effect of removing positive ions from the inside of the gate insulating layer16are not sufficiently obtained. If the above range is exceeded, the reliability of the gate insulating layer16may decrease.

In the method for manufacturing a semiconductor device according to the first embodiment, the second heat treatment is further performed after the first heat treatment. The second heat treatment is performed in a state where a voltage is applied to the gate insulating layer16. A voltage making the first plane side of the gate insulating layer16negative relative to the second plane side is applied.

In the second heat treatment, it becomes easier for an element that becomes a negative ion in silicon oxide, for example, fluorine (F) and nitrogen (N) to move in the direction from the first plane of the gate insulating layer16toward the second plane. Thus, negative ions inside the silicon oxide are removed from the inside of the gate insulating layer16. Therefore, threshold voltage fluctuations of the HEMT100can be suppressed.

At the time of performing the second heat treatment, the gate insulating layer16has been densified by the first heat treatment. Thus, it is difficult for nitrogen (N) separated from the nitride semiconductor layer to enter the lattice point of oxygen in the gate insulating layer16. Therefore, even if nitrogen (N) is removed from the inside of the gate insulating layer16, the separation of nitrogen (N) from the nitride semiconductor layer hardly occurs.

The temperature of the second heat treatment is preferably lower than that of the first heat treatment from the viewpoint of suppressing the separation of nitrogen (N) from the nitride semiconductor layer. The temperature of the second heat treatment is preferably 400° C. or more and 1000° C. or less. Below the above range, there is a possibility that the effect of removing negative ions in the gate insulating layer16is not sufficiently obtained. In addition, if the above range is exceeded, there is a possibility that nitrogen (N) is separated from the nitride semiconductor layer.

From the viewpoint of suppressing oxidation of the gate electrode18and the nitride semiconductor layer, the second heat treatment is preferably performed in a non-oxidizing atmosphere. For example, the second heat treatment is preferably performed in an inert gas atmosphere such as a nitrogen atmosphere, an argon atmosphere, or a helium atmosphere.

The time period of the second heat treatment is preferably five minutes or more and 60 minutes or less. Below the above range, there is a possibility that the effect of removing negative ions in the gate insulating layer16is not sufficiently obtained. If the above range is exceeded, the fabrication time of the HEMT100may become longer and the fabrication cost may increase.

The electric field strength in the gate insulating layer16during the second heat treatment is preferably 2 MV/cm or more and 10 MV/cm or less, and more preferably 4 MV/cm or more and 8 MV/cm or less. Below the above range, there is a possibility that the effect of removing negative ions from the inside of the gate insulating layer16is not sufficiently obtained. If the above range is exceeded, the reliability of the gate insulating layer16may decrease.

According to the fabrication method in the first embodiment, as described above, the amount of charges contained in the gate insulating layer16can be reduced. Therefore, a transistor having the MIS structure capable of suppressing fluctuations of the threshold voltage is implemented.

Second Embodiment

A method for manufacturing a semiconductor device according to a second embodiment is different from that in the first embodiment in that the first heat treatment is performed before a conductive layer in contact with the second plane is formed. Hereinafter, the description of content overlapping with that in the first embodiment will be partially omitted.

FIGS. 10 and 11are schematic sectional views showing a method for manufacturing the semiconductor device according to the second embodiment.

As in the first embodiment, the gate insulating layer16(insulating layer) is formed on the channel layer14and the barrier layer15. The gate insulating layer16has the first plane in contact with the channel layer14(nitride semiconductor layer) and the barrier layer15(nitride semiconductor layer) and the second plane opposite to the first plane.

Next, the first heat treatment is performed (FIG. 10). Densification of the gate insulating layer16is performed by the first heat treatment. Positive charges in the gate insulating layer16are removed by the first heat treatment. The positive charge is, for example, a hydrogen ion or a gallium ion.

The first heat treatment is performed at 600° C. or more and 1100° C. or less. The first heat treatment is performed, for example, in a non-oxidizing atmosphere. The first heat treatment is performed in an inert gas atmosphere, for example, a nitrogen atmosphere, an argon atmosphere, a helium atmosphere, or the like. The time period of the first heat treatment is, for example, five minutes or more and 60 minutes or less.

The first heat treatment is performed in a state where a voltage is applied to the gate insulating layer16. A voltage making the first plane side of the gate insulating layer16positive relative to the second plane side is applied. For example, a voltage is applied to the gate insulating layer16by bringing electrodes into contact with the substrate10and the gate insulating layer16.

The electric field strength in the gate insulating layer16during the first heat treatment is, for example, 2 MV/cm or more and 10 MV/cm or less.

Next, the second heat treatment is performed (FIG. 11). Negative charges in the gate insulating layer16are removed by the second heat treatment. The negative charge is, for example, a fluoride ion or a nitrogen ion.

The temperature of the second heat treatment is, for example, lower than that of the first heat treatment. The temperature of the second heat treatment is 400° C. or more and 1000° C. or less.

The second heat treatment is performed, for example, in a non-oxidizing atmosphere. The second heat treatment is performed in an inert gas atmosphere, for example, a nitrogen atmosphere, an argon atmosphere, a helium atmosphere, or the like. The time period of the second heat treatment is, for example, five minutes or more to 60 minutes or less.

The second heat treatment is performed in a state where a voltage is applied to the gate insulating layer16. A voltage making the first plane side of the gate insulating layer16negative relative to the second plane side is applied. For example, a voltage is applied to the gate insulating layer16by bringing electrodes into contact with the substrate10and the gate insulating layer16.

The electric field strength in the gate insulating layer16during the second heat treatment is, for example, 2 MV/cm or more and 10 MV/cm or less.

Next, the conductive layer118in contact with the second plane of the gate insulating layer16is formed. The conductive layer118is, for example, polycrystalline silicon containing a conductive impurity. Also, the conductive layer118, for example, a metal. The conductive layer118is, for example, titanium nitride (TiN).

Next, the conductive layer118is patterned to form the gate electrode18. The patterning of the conductive layer118is performed using, for example, a lithography method and a dry etching method.

Next, the source electrode20, the drain electrode22, and the interlayer insulating layer30are formed by a known method.

According to the fabrication method in the second embodiment, as described above, as in the first embodiment, the amount of charges contained in the gate insulating layer16can be reduced. Therefore, a transistor having the MIS structure capable of suppressing fluctuations of the threshold voltage is implemented. Further, before a conductive layer in contact with the second plane is formed, the first heat treatment is performed. Therefore, process conditions, particularly the temperature, of the first heat treatment and the second heat treatment are not limited by the material of the conductive layer.

In the first and second embodiments, gallium nitride or aluminum gallium nitride containing gallium (Ga) is taken as an example of the nitride semiconductor, but, for example, indium gallium nitride or indium aluminum gallium nitride containing indium (In) can also be applied. It is also possible to apply aluminum nitride, indium nitride, or indium aluminum nitride not containing Ga. Stacked structures of these nitride semiconductors can also be applied.

Also in the first and second embodiments, undoped aluminum gallium nitride is taken as an example of the barrier layer15, but n-type aluminum gallium nitride can also be applied.

Also in the first and second embodiments, the case where the bottom of the trench40is located in the channel layer14is taken as an example, but a structure in which the bottom of the trench40is located in the barrier layer15can also be adopted. Further, a structure in which aluminum gallium nitride, aluminum nitride, or the like is re-grown on the bottom of the trench can be adopted.

Also in the first and second embodiments, the HEMT having the gate recess structure is taken as an example, but the present disclosure can also be applied to a HEMT having a planar gate structure without the gate recess structure.

Also in the first and second embodiments, the HEMT using a two dimensional electron gas as a carrier is taken as an example, but the present disclosure can also be applied to an ordinary metal oxide semiconductor field effect transistor (MOSFET) not using a two dimensional electron gas.

Also in the first and second embodiments, the case where the gate insulating layer16is made of silicon oxide has been mainly described, but the present disclosure is not limited to the case where the gate insulating layer16is made of silicon oxide and the present disclosure can also be applied when at least one of oxides and oxynitrides such as aluminum oxide, silicon oxynitride, and aluminum oxynitride is contained. The gate insulating film16may contain, for example, a nitride. For example, the nitride is silicon nitride or aluminum nitride. In addition, different stacked films may be applied to a portion of the gate insulating film16in contact with the channel layer14and a portion of the gate insulating film16in contact with the barrier layer15.

Also in the first and second embodiments, as an example of the method of applying a voltage to the insulating layer, the case of bringing electrodes into contact has been described, but the method of applying a voltage to the insulating layer is not limited to the above method. For example, in the second embodiment, it is also possible to apply a voltage to the gate insulating layer16by generating plasma in the space above the gate insulating layer16and inducing a positive charge or a negative charge.