JP6913345B2 - Nitride and oxide film forming methods and film forming equipment - Google Patents

Description

The present invention relates to a film forming method and a film forming apparatus for nitrides and oxides, particularly a film forming method and a film forming apparatus for forming nitrides and oxides on a group III nitride semiconductor substrate.

Group III nitrides, for example, wide bandgap semiconductors typified by GaN, are being put to practical use as power transistors.
Conventionally, the MBE (MBE: Molecular Beam Epitaxy) method has been used to form a film of GaN or the like. However, the MBE method requires an ultra-high vacuum to obtain good quality crystals, the material must be extremely hot to obtain the required vapor pressure, and a cooling mechanism for the entire device is required.

In recent years, film formation by the PLD (PLD: Pulsed Laser Deposition) method has attracted attention instead of the MBE method. The PLD method is a method of irradiating the surface of a target made of a film-forming material with a short-pulse laser beam (pulse-shaped laser beam) to evaporate the material and form a film on a substrate. In particular, by using a short pulse laser beam of about picoseconds or less, the material can be evaporated before the thermal energy diffuses deep into the target. That is, the short pulse laser beam can evaporate the material without heating the target.

Regarding the method for forming a group III nitride by the PLD method, for example, Patent Document 1 discloses a method for forming a group III nitride by targeting a Ga-based group III metal in an ammonia or nitrogen radical atmosphere. ing.
Further, Patent Document 2 discloses a method of forming a Group III nitride on a nitrided sapphire substrate by a PLD method.

Japanese Unexamined Patent Publication No. 2005-22940 Japanese Unexamined Patent Publication No. 2012-188294

As disclosed in Patent Document 1, the conventional research is exclusively related to the improvement of the crystallinity of the group III nitride semiconductor to be formed. No studies have been conducted on the effect on the underlying group III nitride semiconductor when the group III nitride semiconductor is regrown or when an insulating film or contact electrode is formed on the group III nitride semiconductor.

For example, a FET (Field Effect Transistor), which is one of semiconductor devices, controls electrical conduction in a channel region on a semiconductor surface by an electric field generated by a gate electrode. In the FET, the interface state between the GaN surface on which the channel is formed and the gate insulating film has a great influence on the characteristics of the FET, and the drive current and the response speed of the FET are deteriorated.
Further, the contact resistance (contact resistance) between the contact electrode and the GaN surface is affected by the crystallinity of the GaN surface in contact with the contact electrode, which may cause an increase in the contact resistance.

Further, in the PLD method, the laser is irradiated to the target surface to evaporate the material, so that the material evaporates from the local region (spot) irradiated with the laser. Therefore, there is a problem that it is difficult to form a film having a uniform thickness on a large-diameter substrate.
In order to improve the uniformity of the film thickness, a mechanism for revolving the substrate to be filmed, scanning the laser beam on the target surface, and a control system for that purpose are required, which causes a problem that the equipment becomes expensive. There is.

Further, a film deposited due to the influence of an oxynitride formed on the target surface due to oxygen, nitrogen, etc. remaining in the film forming chamber of the film forming apparatus by the PLD method (hereinafter, may be referred to as PLD apparatus). There is also a problem that the purity of the film is lowered.

In view of the above problems, the present invention presents a method for regrowth on a group III nitride semiconductor and a film forming method for forming a nitride, oxide or oxynitride, which can obtain good electrical characteristics, and a film forming method thereof. It is an object of the present invention to provide a film forming apparatus.

The film forming method according to the present invention is
The first step of heating a substrate having a group III nitride on its surface while irradiating the surface of the substrate with nitrogen radicals, and
It is characterized by including a second step of forming a film on the surface of the substrate after the first step.

By adopting such a film forming method, defects due to nitrogen desorption (nitrogen loss) that may occur in the pre-process of the semiconductor device manufacturing process on the surface of the group III nitride can be repaired, and nitrogen desorption before film formation can be performed. A film can be formed on a group III nitride with reduced separation and reduced crystal defects near the surface. Therefore, in an electronic device such as a FET to which this film forming method is applied, it is possible to prevent deterioration of electrical characteristics due to unnecessary interface states and the like.

Further, the film forming method according to the present invention is
In the second step, the surface of the target made of the film-forming material is irradiated with laser light in a pulsed manner.
It is characterized in that the film forming material is vapor-deposited on the substrate.

Further, the film forming method according to the present invention is
In the second step, the surface of the substrate is irradiated with nitrogen radicals, oxygen radicals, or a mixture of nitrogen radicals and oxygen radicals to form a nitride, oxide, or oxynitride of the film-forming material on the substrate. It is characterized by that.

With such a configuration, the reactivity with the nitrogen radicals and oxygen radicals supplied to the substrate surface is controlled while adjusting the film formation rate by controlling the frequency of the laser beam in the PLD method, and the quality is good. A film can be formed.

Further, the film forming method according to the present invention is
The laser beam irradiating the target surface is characterized by being optically branched into a plurality of laser beams.

With such a configuration, the uniformity of the film thickness is improved, and it is possible to cope with an increase in the diameter of the substrate.

The film forming apparatus according to the present invention is
A substrate support device that supports a substrate to be formed, a substrate heating device that heats the substrate, a target container that houses a target made of a film-forming material, a target support device that supports the target container, and radical irradiation. Equipped with a device and a laser irradiation device,
The radical irradiation device generates nitrogen radicals and supplies the nitrogen radicals to the surface of the substrate.
The laser irradiation device is characterized by irradiating the target surface with a pulsed laser beam.

With such a configuration, it is possible to form a film such as a nitride while reducing crystal defects on the surface of the Group III nitride. As a result, the performance of electronic devices such as FETs formed by using this device is improved.

Further, the film forming apparatus according to the present invention is
It is characterized by having an optical element for branching a laser beam into a plurality of parts between the laser irradiation device and the target.

Since the laser beam is optically branched by the optical element in this way, it is not necessary to provide the PLD device with a complicated device such as a substrate rotation device or a laser scanning device.
Further, for example, by branching the laser beam so as to be arranged in a straight line and providing an irradiation spot arranged in a straight line on the target surface, the uniform film thickness of the film to be formed can be improved only by rotating the substrate. Can be done. Therefore, film thickness uniformity can be realized with a simple device configuration.

Further, the film forming apparatus according to the present invention is
The wall surface of the target container in contact with the target is provided with irregularities.

Further, the film forming apparatus according to the present invention is
The wall surface of the target container in contact with the target is made of a substance having poor wettability with the film-forming material constituting the target.

Further, the film forming apparatus according to the present invention is
The target support device is characterized in that the target container can be swung or rotated.

With such a configuration, unnecessary slag such as oxides formed on the target surface moves to the inner wall surface side of the target container (crucible), and the slag is prevented from being irradiated with the laser beam. , Deterioration of purity of the film formed by the PLD method can be suppressed.

The conceptual diagram of the film forming apparatus in the 1st Embodiment which concerns on this invention. The schematic diagram explaining the function of the optical element which branches a laser beam into a plurality of. FIG. 5 is a cross-sectional view schematically illustrating a film forming method using a branched laser beam. The conceptual diagram explaining the principle of improving the film thickness uniformity. The graph which shows the transistor characteristic of an insulated gate FET. Top view and cross-sectional view showing a modified example of the target container.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, none of the following embodiments give a limiting interpretation in finding the gist of the present invention. Further, the same or the same type of members may be designated by the same reference numerals and the description thereof may be omitted.

Further, for the sake of simplicity, in the following description, GaN may be described as a typical example of the group III nitride semiconductor, but the description is not limited to GaN. For example, a mixed crystal of InN, GaN, AlN, and BN is also included in the group III nitride semiconductor.

FIG. 1 is a conceptual diagram showing a main configuration of a PLD device.

The PLD device includes a substrate support device 2 and a target support device 3 inside a chamber 1 which is an airtight vacuum container. A substrate 4, which is an object for forming a film, is placed on the substrate support device 2.
The substrate 4 is exemplified by a circular wafer having a GaN layer on the surface, but the substrate 4 is not limited thereto.

The substrate support device 2 can heat the substrate 4 to 200 [° C.] or higher by a built-in heating device and maintain the substrate 4 at a constant temperature. The heating device may be a method of heating by heat conduction from a heater that generates Joule heat, or a method of heating by radiant heat such as a lamp.

The target container 5a is supported on the target support device 3. The target container 5a stores the target 6a, which is a film-forming material. Therefore, the target support device 3 supports the target 6a via the target container 5a.
Further, the target support device 3 includes a temperature control device. Therefore, the target support device 3 can maintain the temperature of the target 6a at a constant temperature via the target container 5a.

The temperature control device may be configured to include a heating device, heat the target 6a to a melting point or higher, and use the target 6a as a liquid target.
Further, the temperature control device may be provided with a cooling device such as a Pertier element or a heat exchanger so that the temperature rise of the target due to the laser beam can be suppressed. Further, both a heating device and a cooling device may be provided so as to enable a high degree of temperature control.

For example, when epitaxially growing a group III nitride with n-type doping, a target material of Ga, Al, In alone or an alloy to which Ge is added in an amount of 0.5 to 5 [mol%] can be used. When the group III nitride of p-type doping is epitaxially grown, a simple substance or alloy of the target material to which 0.5 to 5 [mol%] of Mg is added can be used. Further, it is also possible to continuously form a film by arranging an oxide target such as Si and NiO or C12A7 on such an epitaxial growth layer and moving it to the position of the laser spot.

A plurality of target containers can be provided in the chamber 1. FIG. 1 shows an example in which different targets 6a and 6b are housed in two target containers 5a and 5b, respectively. The target container 5a or the target container 5b can be selected according to the material to be formed, supported by the target support device 3, and maintained at a constant temperature. Therefore, it is possible to continuously form a plurality of different types of films.
The number of target containers is not limited to two, and may be more than two, or may be one.

The chamber 1 includes a light transmitting window 7 for introducing a laser beam. The pulsed laser emitted from the laser irradiation device 8 installed outside the chamber 1 is focused by the lens 9, introduced into the chamber 1 through the light transmission window 7, and irradiated onto the target 6a.
The frequency of the laser pulse is set to be equal to or higher than the threshold value of the repetition frequency capable of maintaining continuous excitation with respect to the film-forming material on the surface of the target 6a as described later.
The center wavelength of the pulsed laser, the energy of one pulse, and the pulse width can be, for example, 1064 [nm], 50 [μJ], and picoseconds, respectively, but are not limited thereto. Further, the irradiation region (spot) diameter of the laser on the target 6a is focused to, for example, 100 [μm] or less, but is not limited thereto.

This PLD device includes a diffraction optical element 10 (Diffractive Optical Element: DOE) between the laser irradiation device 8 and the lens 9, and the laser beam is branched into a plurality of parts by the diffraction optical element 10. The branched laser light is focused by the lens 9.
FIG. 2 is a schematic diagram showing the branching of the laser light by the diffractive optical element 10 and the focusing of the laser light by the lens 9.

As shown in FIG. 2, the laser beam 101 emitted from the laser irradiation device 8 is branched into a plurality of laser beams 102 by the diffractive optical element 10. The branched laser beam 102 is focused on the surface of the target 6a by the lens 9. The focused laser beam 103 irradiates the surface of the target 6a and supplies light energy.
Further, by using the diffractive optical element 10, a desired branching pattern can be easily controlled.
As the diffractive optical element, a commercially available diffractive optical element can be used.

An F-θ lens (telecentric F-θ lens or non-telecentric F-θ lens) is used for the lens 9 that collects the branched laser beam.

When a telecentric F-θ lens is used for the lens 9, each laser beam is incident on the surface of the target 6a at the same angle regardless of the position on the surface of the target 6a. As a result, each branched laser beam can obtain the same spot shape on the surface of the target 6a, and can supply the same energy per unit area to a plurality of irradiated regions.
On the other hand, when the lens 9 is a non-telecentric Fθ lens, the laser beam spreads, so that the area larger than the light transmitting window 7 can be irradiated with the laser beam. Since the distance from the light transmitting window 7 to the target 6a is sufficiently longer than the diameter of the target 6a, it can be considered that the branched laser beam is incident on the surface of the target 6a at substantially the same angle even in this case.

As will be described later, the multi-branch (multi-split) irradiation in which the laser beam is branched into a plurality of laser beams can improve the film thickness distribution (film thickness uniformity) formed on the substrate 4.

In the above example, a diffractive optical element was used as an optical element for branching the laser beam, but a so-called microlens in which minute lenses were arranged in a specific pattern after the beam diameter of the laser beam was expanded by a beam expander. The laser beam may be branched using an array.
In the case of a microlens array, each lens can be used as a condenser lens together with the branching of the laser.

The film-forming material evaporates at the portion of the surface of the target 6a that is irradiated with the laser beam. The film-forming material released from the surface of the target 6a forms a plume 11, and a film is formed by the film-forming material reaching the opposing substrate 4.

The chamber 1 includes a gas introduction port 12 and an exhaust port 13, and can control pressure and atmosphere.
A vacuum pump is connected to the exhaust port 13, and the inside of the chamber 1 can be exhausted to maintain a vacuum state.

Atmosphere control gas can be introduced from the gas introduction port 12 as needed. The pressure inside the chamber 1 can be kept constant by controlling the opening degree of the APC (Auto Pressure Controller) valve installed between the exhaust port 13 and the vacuum pump and the rotation speed of the vacuum pump.

In addition, this device has a load lock chamber for substrate replacement to prevent unnecessary oxidation of the target surface, and a turbo molecular pump is used to exhaust the chamber 1 for film formation, thereby applying base pressure. ^{Can be 10-5} [Pa] or less.
Further, the nitrogen gas used is a high-purity gas compatible with semiconductors, for example, a gas purifier is used, and a gas having an oxygen concentration of 1 [ppb] or less is preferably used.

Further, the chamber 1 is equipped with a radical irradiation device 14, for example, a radical gun.
Nitrogen gas whose flow rate is controlled by an MFC (mass flow controller), for example, 2 [sccm] of nitrogen (N ₂ ) gas is introduced into the radical irradiation device 14, and high frequency, for example, 13.56 [MHz] of electric power is supplied. .. The radical irradiation device 14 generates nitrogen radicals 18 from nitrogen gas by the supplied electric energy, and irradiates the surface of the substrate 4 with the generated nitrogen radicals 18 from the nozzle 15 which is the outlet. Since the nitrogen radicals are active, they have a nitriding action on the film-forming material flying from the surface of the substrate 4 and the target 6a to the substrate 4.
The frequency of the applied power is not limited to the above.

(Improvement of film thickness uniformity)
Hereinafter, the principle of improving the film thickness uniformity formed on the substrate 4 by this PLD apparatus will be described.

The amount of the film-forming material released from the surface of the target 6a has an angle distribution depending on the angle α formed by the direction perpendicular to the surface of the target 6a (normal direction). In general, the angular distribution of the amount of emission is maximum in the vertical direction, that is, in the direction of zero α, and becomes 0 (zero) in the horizontal direction, that is, in the direction of 90 °. It is empirically known that the angular distribution of the amount of emission can be approximated by the cosine law, that is, an equation proportional to the nth power of cosα. (Usually n is a value of 1 or more.)

A film-forming material having the above angular distribution is emitted from the spot irradiated with the laser on the surface of the target 6a. Therefore, the film formation rate is the highest at the intersection of the normal of the spot surface irradiated with the laser and the surface of the substrate 4, and the film formation rate decreases as the distance from the intersection increases.
For example, when a laser beam is applied to one central portion of the surface of the target 6a, the film formed on the substrate 4 has a thicker film thickness at the central portion and a thinner film thickness at the outer peripheral portion. Therefore, as the size of the substrate 4 increases, the difference in film thickness between the central portion and the outer peripheral portion increases.

FIG. 3 is a cross-sectional view schematically illustrating a situation in which a plurality of plumes are emitted from the surface of the target 6a by a plurality of branched laser beams. As shown in FIG. 3, the laser beams 103a, 103b, and 103c branched into a plurality of laser beams by the diffractive optical element 10 are irradiated to a plurality of spots on the surface of the target 6a. A plurality of plumes 11a, 11b, 11c of the film-forming material are emitted from the plurality of spots irradiated with the laser beam.
The location where the plume is generated is determined by the irradiation position of the laser beam, and the irradiation position of the laser beam can be determined by the diffractive optical element 10 and the lens 9.

FIG. 4 shows a conceptual diagram when a film is formed on the substrate 4 by the plume 11.
FIG. 4A shows an example in which one plume forms a film on the substrate 4, and FIG. 4B shows an example in which nine plumes form a film on the substrate 4. In the example shown in FIG. 4B, the plume generation locations formed on the target 6a are arranged concentrically so as to surround the central portion and the central portion.

In the example shown in FIG. 4A, since the film is formed around the region 16 in the central portion of the substrate 4 by one plume, the film thickness is the thickest in the region 16 and gradually approaches the outer peripheral portion. The film thickness decreases.
On the other hand, in the example shown in FIG. 4B, the nine plumes centered on the central region 16a of the substrate 4 and the outer peripheral regions 16b, 16c, 16d, 16e, 16f, 16g, 16h, and 16i. Since the film is formed, the film thickness is prevented from being reduced at the outer peripheral portion, and the uniformity of the film thickness is improved.
It should be noted that the regions 16, 16a and the like conceptually show the region where the film thickness is formed to be the thickest for the sake of explanation, and do not indicate that the film is formed only in this region.

In the above example, an example in which the laser beam is branched into nine is shown for understanding, but the number of branches is not limited to nine.
The number and arrangement of the lasers branched by the diffractive optical element 10 may be determined from the requirements for the film thickness distribution (film thickness uniformity) formed on the substrate 4.

The film thickness distribution can be predicted, for example, by actually measuring the film thickness distribution data emitted from one central location on the surface of the target 6a and superimposing the measured data. That is, first, the film thickness distribution data emitted from one irradiation spot on the target surface is actually measured by the laser beam that does not branch. Next, the actually measured film thickness distribution data is applied to each irradiation spot of the laser beam branched into a plurality of branches, and these film thickness distributions are added together to obtain the entire film thickness distribution on the substrate 4.
Alternatively, assuming that the distribution function emitted from the laser spot is proportional to the nth power of cosα, the scale factor n and the proportional coefficient are calculated from the measured data of the film thickness distribution emitted from one irradiation spot. Next, the film thickness distribution can be obtained by applying the obtained distribution function to the angular distribution emitted from each irradiation spot of the laser beam branched into a plurality of branches and integrating at the surface position of the substrate 4.

Since the film thickness distribution on the substrate 4 can be predicted as described above, the laser by the diffractive optical element 10 meets the requirements of the size of the substrate 4, the film thickness uniformity, the growth rate, and the processing capacity (throughput) of the apparatus. The branching pattern of light can be determined. Further, various demands can be met by exchanging the diffractive optical element 10.

In addition, the laser beam is branched so as to be arranged in a straight line, the film-forming material is ablated from the irradiation spots arranged in a straight line on the target surface, and the substrate 4 is formed by simply rotating (not rotating). It is possible to improve the uniform film thickness of the film. That is, film thickness uniformity can be realized with a simple device configuration in which a mechanism for rotating the substrate 4, for example, an electric motor, is provided on the substrate support device 2.

(Film formation method)
Hereinafter, a film forming method using the PLD apparatus will be described.

(Pretreatment process)
First, the substrate 4 supported by the substrate support device 2 is heated to a predetermined temperature of, for example, 200 [° C.] or more by the heating device built in the substrate support device 2.
The method for heating the substrate 4 is, for example, a method in which the substrate 4 is placed on the substrate support device 2 held at a constant temperature and heated by heat conduction from the substrate support device 2, and after the substrate 4 is placed on the substrate support device 2, the substrate 4 is heated. There is a method of raising the temperature of the substrate support device 2 and heating by heat conduction, or a method of irradiating the back surface of the substrate 4 with light from the substrate support device 2 side with a lamp and heating the substrate 4 by radiant heat. Any of the above methods may be used as the heating method, and the heating method is not limited to the above.

Then, the surface of the substrate 4 is irradiated with nitrogen radicals from the radical irradiation device 14. At this time, the nitrogen radical has the effect of nitriding the surface of the substrate 4.
When there is a group III nitride, for example, a GaN layer on the surface of the substrate 4, the nitrogen vacancies can be reduced by supplying active nitrogen to the nitrogen vacancies desorbed from the surface.

By heating the substrate 4, it is possible to promote heat diffusion between the nitrogen radicals and the GaN layer near the surface, and to enhance the reactivity of the nitrogen radicals with the nitrogen vacancies.

In addition, crystal defects may occur in the GaN layer in the manufacturing process of electronic devices using group III nitride semiconductors, such as plasma treatment such as dry etching and ion implantation. Such crystal defects existing in the GaN layer can be recovered by performing heat treatment, for example, 200 [° C.] or higher while irradiating nitrogen radicals. In this case, by supplying nitrogen radicals, it is possible to recover defects while preventing nitrogen from being desorbed from the GaN layer by heat treatment.
Although nitrogen radicals may be irradiated after reaching a predetermined temperature, nitrogen desorption at the time of temperature rise can be effectively prevented by raising the temperature while irradiating the nitrogen radicals and maintaining the temperature at the predetermined temperature. ..

For example, the substrate 4 may be supported by the substrate support device 2, and the substrate 4 may be heat-treated until the substrate 4 reaches a predetermined temperature while irradiating the surface of the substrate 4 with nitrogen radicals from the radical irradiation device 14.

(Film formation process)
Next, following the pretreatment step, a film formation treatment is continuously performed on the surface of the substrate 4 by using the target 6a by the PLD method without opening to the atmosphere. Therefore, it is possible to prevent the formation of a natural oxide film on the surface of the substrate 4 and the adhesion of contaminants in the atmosphere, and it is possible to eliminate the presence of unnecessary products and the like at the interface between the formed film and the substrate 4.

The pulsed laser light emitted from the laser irradiation device 8 is branched into a plurality of pieces by the diffractive optical element 10 which is an optical branching device of the laser light, then is condensed by the lens 9, and passes through the light transmission window 7. The surface of the target 6a is irradiated.
As the pulsed laser beam, for example, a nanosecond pulse, a picosecond pulse, or a femtosecond pulse laser beam can be used.

The material of the target 6a evaporates from the spot irradiated with the laser beam on the surface of the target 6a and is emitted toward the opposite substrate 4.
The growth rate of the film formed on the substrate 4 depends on the repetition frequency of emitting the laser beam, and when the repetition frequency becomes equal to or higher than the threshold value, the film formation rate starts to increase.
For example, when the target is Ge-doped Ga, the center wavelength of the laser is 1064 [nm], the pulse width is 24 [ps] (picoseconds), and the fluence is 1.1 [J / cm ² ], the film formation rate. Allows stable film formation at a repetition frequency of 70 [kHz] or higher.
The short pulse laser preferably has a pulse width of picoseconds. If the energy of one laser pulse is too high, the target may be uneven.

The film formation rate can be controlled by the energy (fluence) of the laser, but the film formation rate can be adjusted with high accuracy by adjusting the repetition frequency. Thereby, the reaction with the nitrogen radical and the oxygen radical supplied from the radical irradiation device 14 can be adjusted.

The target 6a may be a solid, but by heating it above the melting point and liquefying it, Ge-doped Ga having a stable concentration can be evaporated.

The film-forming material is evaporated from the surface of the target 6a by the laser beam to the surface of the substrate 4, and for example, when forming a nitride, the surface of the substrate 4 is irradiated with nitrogen radicals from the radical irradiation device 14.

On the surface of the substrate 4, the film-forming material evaporated from the surface of the target 6a reacts with nitrogen radicals to form a nitride, for example, a Ge-doped GaN film.
At this time, the nitriding reaction can be promoted by heating the substrate 4 to, for example, 200 [° C.] or higher.

Further, as described above, due to the requirements for the size and film thickness of the substrate 4, the laser beam is branched into a plurality by the diffraction optical element 10 and irradiated to the target 6a.
Since the energy of the laser beam is dispersed according to the number of branches, the repetition frequency depends on the number of branches.

By preparing targets made of multiple different materials in the device, different types of films can be formed, and multiple films can be continuously formed in a controlled atmosphere without exposure to the atmosphere. You can also do it.

For example, it is also possible to adopt Si (silicon) as the target 6b, evaporate Si with a laser beam, and react it with nitrogen radicals on the surface of the substrate 4 to form a silicon nitride film.

Further, for example, after forming a GaN film using the target 6a made of GaN housed in the target container 5a, the target container 5b containing the target 6b made of Si and the target container 5a are exchanged, and the target support device 3 The target container 5a may be supported by the target container 5a, and the target 6b may be irradiated with a laser beam to form a silicon nitride film on the GaN film.

It is also possible to form a silicon oxide film by reacting with oxygen radicals on the surface of the substrate 4. Oxygen gas is supplied to the radical irradiation device 14 to irradiate the oxygen radical. Oxygen radicals supplied to the surface of the substrate 4 can react with the film-forming material evaporated from the target 6a to form an oxide.

It should be noted that a substance such as a metal other than silicon can be reacted with nitrogen or oxygen to form a nitride or oxide such as a metal.

Further, it is also possible to supply the evaporated film-forming material to the surface of the substrate 4 and simultaneously mix and supply nitrogen radicals and oxygen radicals to form an oxynitride such as SiON.

As described below, the film forming method of the present embodiment can be advantageously applied to, for example, the gate insulating film of the FET and the contact electrode to the source / drain region.

When a gate insulating film is formed on the channel region of a group III nitride semiconductor such as GaN, the gate insulating film can be formed while preventing the occurrence of defects due to nitrogen desorption and recovering the defects. Therefore, it is possible to prevent the generation of unnecessary interface states, improve the performance of the FET, and suppress the variation.

Further, when the contact electrode is formed on the source / drain, the crystal defect at the interface can be recovered while preventing nitrogen desorption, so that the contact resistance between the contact electrode and the source / drain region should be prevented from increasing. Can be done.

In particular, when impurities are doped into the source / drain region by ion implantation, activation annealing treatment is performed while irradiating nitrogen radicals to prevent the occurrence of defects due to nitrogen desorption from the surface of group III nitride semiconductors. However, the dopant can be activated. Therefore, it is possible to improve the performance of the transistor and suppress the variation.

As described above, the step of heating while irradiating nitrogen radicals and the step of forming a film by the PLD method were carried out in the same chamber, but they were carried out in different chambers, and the substrate was transferred between different chambers by a vacuum transfer device. 4 may be transferred.
In this case, the target 6a, the target container 5a, and the target support device 3 are not required in the device used for the step of heating while irradiating the nitrogen radicals, and the laser irradiation device 8 that irradiates the target surface with the laser beam is unnecessary. be. However, a configuration in which a laser beam is used to heat the substrate 4 from the surface may be used.

(inspection)
By measuring the transistor characteristics of the gate-insulated FET, we verified the effect of reducing the interface state between the silicon nitride film formed by this device and the group III nitride semiconductor.

For verification, two types of FET samples in which a silicon nitride film was formed as a gate insulating film were prepared by this device, and the transistor characteristics were compared and investigated.

The outline of the FET sample preparation procedure used for the verification is as follows.

First, using the MOCVD method on sapphire or silicon, a GaN layer is _{continuously epitaxially grown at 1 [μm] and an Al 0.2} Ga _0.8 N layer at 20 [nm], and then about 800 [° C.]. A substrate that had been subjected to high temperature treatment was prepared.
Next, the substrate was irradiated with nitrogen radicals at room temperature, and sample A in which a silicon nitride film was formed at 20 [nm] by the PLD method using silicon as a target and the temperature was raised to 200 [° C.] while irradiating with nitrogen radicals. After that, a sample B in which a silicon nitride film was formed at 20 [nm] by the PLD method using silicon as a target was prepared.

A gate electrode was formed on the channel region by depositing Ni having a film thickness of 20 [nm] and Au having a film thickness of 200 [nm] on the above two types of samples A and B and patterning them by a lift-off method. ..
Then, by combining lithography and dry etching using, for example, CF ₄ , the silicon nitride film is removed so that the surface of the substrate to be the source and drain regions is exposed.
Next, Ti having a film thickness of 20 [nm] and Al having a film thickness of 200 [nm] are vapor-deposited and patterned by a lift-off method to form contact electrodes on regions serving as sources and drains.
Then, heat treatment was performed at about 500 [° C.] in a nitrogen atmosphere to obtain ohmic contact between the contact electrode and the source and drain regions of the substrate.

FIG. 5 shows the transistor characteristics of an FET in which a silicon nitride film is formed by this apparatus as a gate insulating film on a Ga nitride semiconductor substrate, and changes in drain current (Id-Vg) when the gate voltage is stepped up or down. Characteristics) are shown. 5 (a) and 5 (b) show the transistor characteristics of sample A and sample B, respectively.
In FIG. 5A, a clear difference in drain current is observed between when the gate voltage is stepped up and when the gate voltage is stepped down, and hysteresis is observed in the Id-Vg characteristic. However, in FIG. 5B, no hysteresis is observed in the Id-Vg characteristic.
Therefore, by heating the substrate at 200 [° C.] while irradiating it with nitrogen radicals, the interface state between the surface of the Ga nitride semiconductor substrate and the silicon nitride film which is the gate insulating film is reduced, which is good. It was verified that the transistor characteristics can be obtained.
Further, from the dependence of the substrate heating temperature, it can be understood that the nitrogen radicals promote the recovery of defects of the Group III nitride by heating the substrate to 200 [° C.] or higher.

(Modification example)
6A and 6B show a modified example of the target container 5a, FIGS. 6A and 6B are top views of the target container 5a accommodating the target 6a, and FIG. 6C is a cross-sectional view of the target container 5a.
The target 6a housed (held) in the target container 5a can be heated by the target support device 3. By heating and holding the target 6a at a temperature equal to or higher than the melting point, the target 6a can be used as a liquid target.
In this case, the target container 5a is used as a crucible.

When a liquid target is used in the PLD method, for example, if the surface of the target 6a is oxidized or nitrided, oxynitride or the like may precipitate (or aggregate) as slag on the surface of the target 6a due to surface tension.
When the slag (precipitate) is irradiated with laser light, it may be vapor-deposited on the substrate 4 and the purity of the formed film may be deteriorated.
Hereinafter, a method for preventing deterioration of the purity of the film due to this slag will be described.

This slag floats and moves on the surface of the target 6a by thermal energy. By binding this slag to the outside of the laser beam irradiation region, it is possible to prevent vapor deposition from the slag and prevent deterioration of the purity of the formed film.

As shown in FIG. 6A, a groove is formed in the inner wall of the target container 5a, and the contact area between the target 6a and the inner wall of the target container 5a can be increased by providing unevenness on the inner wall surface. can.

As shown in FIG. 6B, when the target 6a is heated to a temperature equal to or higher than the melting point and liquefied, the surface tension of the target 6a acts to reduce the contact portion with the inner wall of the target container 5a, and further swings the target 6a. As a result, the outer edge of the target 6a becomes circular and is separated from the concave portion of the inner wall of the target container 5a. The slag 7 on the surface of the target 6a moves to the recess on the inner wall. As a result, the slag 7 is removed from the surface of the target 6a irradiated with the laser beam, and the surface of the target 6a becomes a smooth mirror surface. The phenomenon that the surface of the target 6a is mirrored can also be visually confirmed.
A target having a slag-free surface obtained by this swinging and rotating operation can be vapor-deposited by the PLD method after stopping the swinging or rotating.
For example, when a group III metal is targeted, one containing Ge, Mg, and Si as impurities in advance can be used. However, it goes without saying that the melting point of these alloys (mixtures) may rise, and it is necessary to change the heating temperature depending on the type and amount of impurities added.

As described above, the material of at least the inner wall surface of the target container 5a is a material that does not react with the target 6a and has poor wettability with the target 6a. By adopting a ceramic such as boron) or DLC (Diamond Like Carbon), it is possible to promote the movement of the slag 17 toward the inner wall surface side.
The target container 5a does not need to be made of the above material, and the inner wall of the target container 5a may be coated with the ceramic material, for example, by ion plating or the like.

FIG. 6C is a cross-sectional view of the target container 5a. As shown in FIG. 6C, for example, in order to facilitate the loading and unloading of the target material, the inner wall of the target container 5a may be provided with a taper, or the inner wall may have a vertical shape.

Further, the target support device 3 that supports the target may be provided with a function of swinging (or rotating) the target container 6a. By swinging, the movement of the slag to the inner wall surface of the target container 5a can be promoted.
The motor may be installed outside the chamber 1 and the oscillating motion may be transmitted directly to the target support device 3 by the shaft, indirectly by the magnetic coupling, or the motor may be transmitted to the chamber 1. It may be installed inside. In addition, it may be a swing motion in a predetermined angle range, or it may be a rotary motion.

1 Chamber 2 Board support device 3 Target support device
4 Substrate 5a, 5b Target container 6a, 6b Target 7 Light transmission window 8 Laser irradiation device 9 Lens 10 Diffractive optical element 11 Plume 12 Gas inlet 13 Exhaust port 14 Radical irradiation device 15 Nozzle 16, 16a, 16b, 16c, 16d, 16e, 16f, 16g, 16h, 16i area (spot)
17 Slag 18 Nitrogen radical 101 Laser light 102 Laser light 103, 103a, 103b, 103c Laser light

Claims (7)

The first step of heating a substrate having a group III nitride on its surface while irradiating the surface of the substrate with nitrogen radicals, and
After the first step, a second step of forming a film on the surface of the substrate is included.
In the second step, the surface of the substrate is irradiated with oxygen radicals or a mixture of nitrogen radicals and oxygen radicals, and the surface of the target made of the film-forming material is irradiated with laser light in a pulsed manner.
A film forming method comprising forming an oxide or an oxynitride of the film forming material on the substrate. The film forming method according to claim 1, wherein the laser beam irradiating the target surface is an optically branched laser beam. A substrate support device that supports a substrate to be formed, a substrate heating device that heats the substrate, a target container that houses a target made of a film-forming material, a target support device that supports the target container, and radical irradiation. Equipped with a device and a laser irradiation device,
The radical irradiation device generates nitrogen radicals and supplies the nitrogen radicals to the surface of the substrate.
The laser irradiating device is a film forming device characterized in that the target surface is irradiated with a pulsed laser beam and the inner wall surface of the target container is provided with irregularities. The film forming apparatus according to claim 3, further comprising an optical element for branching a laser beam into a plurality of layers between the laser irradiation apparatus and the target. The film forming apparatus according to claim 3 or 4, wherein at least the inner wall surface of the target container or the surface material thereof is made of a material that does not react with the target. The film forming apparatus according to any one of claims 3 to 5, wherein the target support device can swing or rotate the target container. The first step of heating a substrate having a group III nitride on its surface while irradiating the surface of the substrate with nitrogen radicals, and
After the first step, a second step of forming a film on the surface of the substrate
Including
In the second step, the surface of the substrate is irradiated with nitrogen radicals, and the surface of the target made of the film-forming material is irradiated with a pulse-like optically branched laser beam.
A film forming method characterized by forming a nitride of the film forming material on the substrate.

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