JP4895189B2 - Oxide formation method - Google Patents

Description

  The present invention relates to a method for forming an oxide, and more particularly to a method for oxidizing a hardly oxidizable semiconductor material.

  In a semiconductor element manufacturing process, a technique for oxidizing a semiconductor material to form an oxide film on the surface thereof has an important significance as an elemental technique for forming a gate oxide film of MOS transistors, element isolation, and the like.

  However, in an element manufacturing process using a compound semiconductor, it is generally difficult to form an oxide film by thermal oxidation like a silicon semiconductor process. In particular, gallium nitride semiconductors are known to have high chemical stability and cannot be easily oxidized.

  Under these circumstances, compound semiconductors such as gallium nitride semiconductors are currently subject to various restrictions on the manufacturing process and device structure as compared to silicon processes. For example, in the process of forming a transistor with a MIS structure, a technique for forming a gate insulating film by oxidizing a base GaN film has not been established, and a method of forming a gate oxide film by a CVD method or the like is employed. ing.

  As a conventional method for oxidizing such a hardly oxidizable semiconductor, there is one described in Patent Document 1. This document describes a method of forming a thermal oxide film on the GaN surface by heat treatment at a high temperature of 800 to 1000 ° C.

  However, such a high-temperature heat treatment has problems that the crystal quality of the underlying GaN film is deteriorated or that the composition fluctuates due to nitrogen desorption.

In addition, since the formation of the thermal oxide film requires a long time, the yield is poor, and it is difficult to control the film thickness by the heat treatment time, which is far from practical use.
JP 2005-183733 A

  As described above, in the prior art, it has been difficult to stably oxidize refractory semiconductor films such as GaN at a low temperature that does not impair the crystal quality of the underlying semiconductor film.

According to the present invention, there is provided a method for forming an oxide comprising a step of forming an oxide of the semiconductor material by bringing the semiconductor material into contact with water at a temperature of 200 ° C. or higher and a pressure of 1.5 MPa or higher ,
The semiconductor material contains M as a constituent element;
Formula M _p O _q (however, p and q are greater than zero.) When the standard enthalpy at normal temperature of the oxide represented by △ and H _{0
△ H 0 / q> -450 (} kJ / mol)
A method for forming an oxide is provided.

  As described above, an oxide can be formed by reaction with water whose temperature and pressure are in a specific range. In the present invention, such water is brought into contact with the semiconductor material and oxidized, so that the oxide can be stably formed at a lower temperature than in the thermal oxidation method. For this reason, it is possible to obtain a good quality oxide without deteriorating the underlying semiconductor material as in the case of using the thermal oxidation method.

  As the “water” in the present invention, water in a subcritical state or a supercritical state is preferably used. From the viewpoint of further improving the controllability of the oxide formation amount by allowing the oxide formation to proceed under milder conditions, subcritical water, specifically, a temperature of 260 ° C. or higher and lower than 374 ° C. and a pressure of 15 MPa or higher and 50 MPa. It is preferable to use water. On the other hand, it is preferable to use supercritical water, specifically, water having a temperature of 374 ° C. or higher and 650 ° C. or lower and a pressure of 22 MPa or higher and 50 MPa from the viewpoint of allowing the oxide formation reaction to proceed more rapidly.

  The “water” in the present invention may contain various solutes such as an oxidizing agent and a reducing substance.

  In the present invention, the oxide may contain nitrogen or the like as another constituent element. That is, the oxide in the present invention includes oxynitrides and the like.

  Since the method according to the present invention includes a step of bringing a semiconductor material into contact with water whose temperature and pressure are in a specific range, an oxide can be stably formed at a lower temperature than a thermal oxidation method. For this reason, it is possible to obtain a good quality oxide while suppressing deterioration of the semiconductor material.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings, taking as an example a case where a GaN film is formed on a base substrate such as sapphire and the surface is oxidized using supercritical water.

(First embodiment)
FIG. 1 is a diagram showing a schematic configuration of a semiconductor material oxidation treatment apparatus according to the present embodiment.

  The oxidation treatment apparatus 100 includes a reactor 113, a supply pipe 133, and a discharge pipe 135. A substrate 117 is disposed inside the reactor 113, and an oxide film is formed on the surface of the substrate 117.

  As the reactor 113, one having a structure capable of accommodating supercritical water is used. The reactor 113 includes a heater (not shown) that heats the inside of the reactor 113 to a predetermined temperature.

The inner wall 115 of the reactor 113 is made of a material that is stable against supercritical water.
Examples of such a material include refractory metals such as Ti and Ta;
Examples thereof include noble metals such as Au, Ru, Rh, Pd, Os, Ir, and Pt; or alloys containing one or more of the above metals; or quartz.

  The supply pipe 133 communicates with the reactor 113 and supplies water into the reactor 113. The supply pipe 133 communicates with the first supply pipe 129 and the second supply pipe 131. The first supply pipe 129 and the second supply pipe 131 communicate with the first tank 101 and the second tank 103, respectively. Pure water 102 is accommodated in the first tank 101. The second tank 103 contains a hydrogen peroxide solution 104 as a component added to the pure water 102.

  The first supply pipe 129 and the second supply pipe 131 are provided with a valve 105 and a valve 107 that can be opened and closed, respectively. By adjusting the open / closed state of the valve 105 and the valve 107, it is possible to adjust whether or not the contents of the first tank 101 and the second tank 103 are supplied to the supply pipe 133 and the supply amount. The substances pressurized and supplied by the pump 109 from the first supply pipe 129 and the second supply pipe 131 are mixed in the supply pipe 133 and supplied into the reactor 113 in a supercritical state.

  The oxidation processing apparatus 100 also includes a heater 111 that heats the supply pipe 133. The heater 111 is disposed at a predetermined position between the pump 109 and the reactor 113.

  The discharge pipe 135 communicates with the reactor 113 and discharges the supercritical water in the reactor 113. The discharge pipe 135 passes through the cooler 119. The discharge pipe 135 is provided with a pressure control valve 121 that adjusts the pressure of the reactor 113.

  The discharge pipe 135 communicates with the gas-liquid separator 123. The exhaust gas and waste liquid separated by the gas-liquid separator 123 are discharged out of the apparatus through an exhaust pipe 137 and a waste liquid pipe 139 communicating with the gas-liquid separator 123, respectively. The exhaust pipe 137 and the waste liquid pipe 139 are provided with a pressure control valve 125 and a pressure control valve 127, respectively.

  The substrate 117 has a structure in which a single crystal GaN film is formed on the surface of a sapphire base substrate.

Hereinafter, a method for forming an oxide using the oxidation treatment apparatus 100 will be described.
This method includes a step of bringing a GaN film on the surface of the substrate 117 into contact with water in a high temperature and high pressure state to form an oxide film on the surface of the substrate 117.

  Specifically, first, the substrate 117 is placed in the reactor 113. Then, the temperature inside the reactor 113 is raised to a predetermined temperature.

  Further, the valve 105 is opened, and the pure water 102 in the first tank 101 is supplied to the supply pipe 133 via the first supply pipe 129. Then, the pressure control valve 121, the heater 111, and the heater 140 are controlled to adjust the inside of the supply pipe 133 to a predetermined temperature and pressure, and are supplied into the reactor 113. At this time, in order to adjust the pressure of water, the reactor 105 is controlled by the pressure control valve 121 and the pure water 102 is discharged while supplying the pure water 102 with the valve 105 opened.

  The water supplied to the reactor 113 is supercritical water. At this time, the temperature of the water supplied to the reactor 113 is set to 374 ° C. or higher, and the pressure is set to 22 MPa or higher.

In addition, although there is no restriction | limiting in particular in the upper limit of the temperature of water, From a viewpoint of forming an oxide at lower temperature than a thermal oxidation method, it is set to 650 degrees C or less, for example, 450 degrees C or less, for example.
Also, the lower limit of the water pressure is not particularly limited, but it is, for example, 50 MPa or less, preferably 30 MPa or less, from the viewpoint of further improving the controllability of the oxide formation reaction.

  In the oxidation processing apparatus 100, the presence / absence of addition of the hydrogen peroxide solution 104 and the ratio of the hydrogen peroxide solution 104 to the pure water 102 are adjusted by adjusting the open / close state of the valve 105 and the valve 107. In the case of supplying the hydrogen peroxide solution 104, for example, after the valve 105 is first opened and the valve 107 is closed, the pure water 102 is adjusted to a predetermined temperature and pressure and the reactor 113 reaches a predetermined condition. With the valve 105 closed and the valve 107 open, the hydrogen peroxide solution 104 is supplied.

  After performing the oxidation reaction for a predetermined time, the heater 111 and the heater of the reactor 113 and the pump 109 are stopped to lower the temperature of the oxidation treatment apparatus 100, and then the system is depressurized. After decompression, the reactor 113 is opened and the substrate 117 is taken out. By the above procedure, a gallium oxide film is formed on the surface of the substrate 117.

The effects of the oxide film forming method according to the present embodiment will be described below.
According to the method of this embodiment, since oxidation is performed using supercritical water, an oxide film can be formed on the surface of the GaN film in the substrate 117 at a lower temperature than the thermal oxidation method. Further, compared with the thermal oxidation method, the in-plane uniformity of the oxide film can be improved, and the flatness of the surface of the oxide film can be improved. Therefore, according to the present embodiment, a gallium oxide film having a good film quality can be formed.

In addition, since supercritical water is used, the surface of the substrate 117 can be completely oxidized to form a Ga ₂ O ₃ film having a stoichiometric composition.

  In this embodiment, the case where the hydrogen peroxide solution 104 is accommodated in the second tank 103 is described as an example. However, as will be described later, a reducing substance may be used instead of the hydrogen peroxide solution 104. Good.

(Second embodiment)
In the first embodiment, the method for forming an oxide film using supercritical water is exemplified, but high-temperature high-pressure water having a temperature of 200 ° C. or higher and a pressure of 1.5 MPa or higher may be used instead of supercritical water. . In this embodiment, subcritical water is used instead of supercritical water.

  Specific examples of the subcritical water include water having a temperature of 200 ° C. or more and less than 374 ° C. and a pressure of 1.5 MPa or more. Among them, those having a temperature of 260 ° C. or more and less than 374 ° C. and a pressure of 15 MPa or more and 30 MPa are preferable. By using such subcritical water, the oxide film formation reaction proceeds under mild conditions, and the controllability of the oxide film thickness can be improved.

  As mentioned above, although embodiment of this invention was described with reference to drawings, these are the illustrations of this invention, Various structures other than the above are also employable.

  For example, in the above-described embodiment, the configuration in which the first tank 101 and the second tank 103 communicate with the supply pipe 133 of the oxidation treatment apparatus 100 has been described as an example. However, the oxidation treatment apparatus 100 includes at least pure water. What is necessary is just to provide the 1st tank 101 in which 102 is accommodated, and the 2nd tank 103 may be abbreviate | omitted.

  Moreover, in the said embodiment, although the oxidation processing apparatus 100 demonstrated and demonstrated the structure which has the gas-liquid separator 123 as an example, the gas-liquid separator 123 can be abbreviate | omitted suitably.

  Further, the oxidation treatment apparatus 100 may be configured to send air together with water to the reactor 113 and to separate the discharge from the reactor 113 into exhaust gas and waste liquid by the gas-liquid separator 123.

  In the above-described embodiment, the case where the oxide film is formed on the surface of the substrate 117 by bringing the GaN film on the surface of the substrate 117 into contact with high-temperature and high-pressure water has been described as an example. However, the material of the semiconductor film is not limited to GaN. However, it can be applied to various materials. In particular, the present invention has a particularly remarkable effect when applied to a semiconductor film whose film quality is impaired when exposed to a high temperature or a hardly oxidizable semiconductor film.

  This is due to the following reason. That is, in the case of a silicon single crystal, since the crystal structure is relatively stable even when exposed to high temperatures, an oxide film can be easily obtained by a thermal oxidation method. On the other hand, in the case of the above films, it is difficult to form an oxide film by a thermal oxidation method.

As such a material, for example, In _x Ga _1-x As _y P _1-y (where x and y are each independently 0 or more and 1 or less; the same shall apply hereinafter), Ga _x Al _1-x. Examples include III-V group compound semiconductors such as As _y P _1-y and Al _1-xy Ga _x In _y N, compound semiconductors such as SiC, and single semiconductors such as Ge. Among these, Al _{1 -xy} Ga _x In _y N which is a nitride semiconductor, more specifically, Al _x Ga _1-x N with y = 0 (where x is 0 or more and 1 or less), or the like. Is mentioned.

In addition, as one measure of the poor oxidation property of the semiconductor material, for example, standard enthalpy of oxide generation can be used.
Specifically, when the standard enthalpy of formation of an oxide represented by the chemical formula M _p O _q (where p and q are greater than zero) at room temperature is ΔH ₀ , the value of ΔH ₀ / q It is considered that the smaller the absolute value of is, the less easily an oxide is formed.

As such a material ,
_{△ H 0 / q> -450 (} kJ / mol)
The material which is is mentioned. Among these, GaN corresponds to the case where the element M is Ga, and ΔH ₀ / q = −363 of Ga ₂ O ₃ (Edited by Chemical Society of Japan, “Chemical Handbook Basic Edition” Revised Edition, 2004, p.II-294). In addition, ΔH ₀ / q of SiO ₂ is −455 (The Chemical Society of Japan, “Chemical Handbook Basic Edition”, 5th revised edition, 2004, p . II-299) . According to the above method Symbol embodiment, these flame oxidation of the semiconductor film is reliably oxidized, it is possible to form an oxide film on the surface.

  In the above embodiment, from the viewpoint of promoting oxidation, it is preferable to supply oxygen into the reactor 113 and dissolve it in water. Note that as a method for supplying oxygen, the above-described method for supplying the hydrogen peroxide solution 104 can be used, but another oxidizing agent can be used, or oxygen gas can be supplied to the supply pipe 133.

  There is no restriction | limiting in the oxygen content of water, According to the material of the board | substrate 117, an oxidation reaction rate, etc., it can set suitably. Further, from the viewpoint of stably forming the oxide with high controllability and reproducibility, the oxygen content of water is, for example, 20 ppm or less, preferably 15 ppm or less, and more preferably 10 ppm or less.

  However, even when oxygen is not contained in water, the oxide formation reaction proceeds.

  From the viewpoint of improving the controllability of the oxidation rate, it is preferable not to supply oxygen but to use the amount of oxygen originally dissolved in the raw material water or to deaerate.

  Further, from the viewpoint of controlling the oxidation rate and improving the film thickness controllability of the oxide film to be formed, the reactor 113 contains an organic carbon compound such as hydrogen, alcohol, organic acid and urea, and an inorganic substance such as carbon monoxide. It is preferable to supply a reducing substance such as a carbon compound and ammonia and dissolve it in water. By adding a reducing substance to water, the progress of excessive oxidation can be suppressed.

  For example, in the oxidation treatment apparatus 100 shown in FIG. 1, when a reducing substance is added to water, a predetermined reducing substance is contained in the second tank 103 instead of the hydrogen peroxide solution, What is necessary is just to supply in the reactor 113 via the supply pipe | tube 133 from the supply pipe | tube 131. In addition to the supply lines for the pure water 102 and the hydrogen peroxide solution 104, the oxidation treatment apparatus 100 may further include a supply line for a reducing substance. The supply line of the reducing substance is connected to the supply pipe 133, for example.

  Moreover, in the said embodiment, although the oxidation processing apparatus which supplies water continuously was mentioned as an example, the batch-type apparatus which puts water in a container, seals and heats can also be used. At this time, when the supercritical state is 374 ° C. or higher, the pressure is adjusted by the filling amount of water, and when the subcritical water is lower than 374 ° C., the saturated vapor pressure is obtained.

  In the following examples, the semiconductor material was oxidized using the oxidation processing apparatus 100 shown in FIG.

Example 1
FIG. 2 is a cross-sectional view showing the configuration of the sample used in this example.

  A sample 150 shown in FIG. 2 is obtained by forming a low-temperature buffer layer 153 and a GaN epitaxial layer 155 in this order on a sapphire substrate 151.

  Sample 150 was produced by the following procedure. That is, after forming a GaN layer as the low-temperature buffer layer 153 on the 2-inch sapphire substrate 151, an n-type GaN layer was formed as the GaN epitaxial layer 155 by MOCVD (Metal Organic Chemical Vapor Deposition) method.

  The surface of the GaN layer of the obtained sample 150 was ultrasonically washed with methanol and then washed with pure water. This was immersed in 28 wt% aqueous ammonia, washed with pure water, and stored in pure water until immediately before the experiment.

After performing the above pretreatment, the sample 150 was placed in the reactor 113 of the oxidation treatment apparatus 100 (FIG. 1) and subjected to oxidation treatment. Pure water 102 was supplied in a subcritical state at a temperature of 260 ° C. and a pressure of 25 MPa, and 30% by weight of hydrogen peroxide solution 104 was accommodated in the second tank 103 and supplied to water. The H ₂ O ₂ concentration in water was 15% by weight. When this is converted into O ₂ partial pressure, it is 1.0 MPa. In this example, the oxidation treatment time was 10 minutes.

  The surface of the sample 150 after the oxidation treatment was analyzed by XPS (X-ray Photoelectron Spectroscopy) while etching. FIG. 3 is a diagram showing the analysis results. In FIG. 3, the horizontal axis corresponds to the depth from the surface of the sample 150, and the etching time of 100 seconds corresponds to the depth from the surface of 100 nm. In addition, the vertical axis in FIG. 3 indicates the intensity ratio (%) of each element of Ga, N, and O.

  FIG. 3 shows that a gallium oxynitride film is formed on the surface of the sample 150.

(Example 2)
In Example 1, except that the water was supercritical water at a temperature of 400 ° C. and a pressure of 25 MPa, the sample 150 was oxidized according to Example 1 and subjected to surface analysis. FIG. 4 is a diagram showing the analysis results. The horizontal and vertical axes in FIG. 4 are the same as those in FIG.

FIG. 4 shows that an oxide film is formed on the surface of the sample 150. Further, the contents of O, Ga, and N in the oxide film are 60 atomic%, 39 atomic%, and 1 atomic%, respectively, and the surface of the sample 150 is almost completely oxidized, so that the substantially stoichiometric composition is obtained. It was confirmed that a Ga ₂ O ₃ film was formed.

(Example 3)
In Example 1, except that water was supercritical water at a temperature of 600 ° C. and a pressure of 25 MPa, the sample 150 was oxidized according to Example 1 and subjected to surface analysis. As a result, as in Example 2, it was confirmed that a Ga ₂ O ₃ film was formed on the top of the sample 150.

Example 4
In Example 2, the sample 150 was oxidized according to Example 1 without supplying the hydrogen peroxide solution 104, and surface analysis was performed. As a result, it was confirmed that a layer mainly composed of Ga and O was formed on the upper part of the sample 150 as in Example 2, and the composition of the layer was almost equal to that obtained in Example 2.

  In Example 1, it was confirmed that oxynitride was produced even when the water temperature was 200 ° C. and the pressure was 25 MPa.

  Moreover, it replaced with GaN used in Examples 1-4, SiC and AlN were used, and the oxide was formed according to each of Examples 1-4. As a result, in both cases of SiC and AlN, it was confirmed that oxides were formed for the cases according to each of Examples 1 to 4.

It is a figure which shows schematic structure of the oxidation processing apparatus in embodiment. It is sectional drawing which shows the structure of the sample in an Example. It is a figure which shows the XPS analysis result after the oxidation process of the sample in an Example. It is a figure which shows the XPS analysis result after the oxidation process of the sample in an Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Oxidation processing apparatus 101 First tank 102 Pure water 103 Second tank 104 Hydrogen peroxide solution 105 Valve 107 Valve 109 Pump 111 Heater 113 Reactor 115 Inner wall 117 Substrate 119 Cooler 121 Pressure control valve 123 Gas-liquid separator 125 Pressure control valve 127 Pressure control valve 129 First supply pipe 131 Second supply pipe 133 Supply pipe 135 Discharge pipe 137 Exhaust pipe 139 Waste liquid pipe 140 Heater 150 Sample 151 Sapphire substrate 153 Low temperature buffer layer 155 GaN epitaxial layer

Claims (11)

A method of forming an oxide, comprising a step of bringing a semiconductor material into contact with water at a temperature of 200 ° C. or more and a pressure of 1.5 MPa or more to form an oxide of the semiconductor material ,
The semiconductor material contains M as a constituent element;
Formula M _p O _q (however, p and q are greater than zero.) When the standard enthalpy at normal temperature of the oxide represented by △ and H _{0
△ H 0 / q> -450 (} kJ / mol)
A method for forming an oxide.   The method according to claim 1, wherein the semiconductor material is a compound semiconductor.   The method according to claim 2, wherein the semiconductor material is a III-V compound semiconductor. The method for forming an oxide according to claim 3, wherein the semiconductor material is Al _x Ga _1-x N (where x is 0 or more and 1 or less). The method according to any one of claims 1 to 4, contacting the semiconductor material below the water temperature 260 ° C. or higher 650 ° C. or less and a pressure 15MPa or 50 MPa, method of forming the oxide. The method according to any one of claims 1 to 4, contacting the semiconductor material to a temperature 374 ° C. or higher 650 ° C. or less and more pressure 22 MPa 50 MPa below the water, the method of forming the oxide. The method according to any one of claims 1 to 6, wherein the oxygen content of the water is 20 ppm or less. The method according to any one of claims 1 to 7,
The step of forming an oxide of the semiconductor material is performed in the reactor;
A method for forming an oxide, wherein a material of an inner surface of the reactor is Ti, Ta, Au, Ru, Rh, Pd, Os, Ir, Pt, an alloy thereof, or quartz. The method according to any one of claims 1 to 8, wherein the water contains a reducing substance, the method of forming the oxide.   10. The method for forming an oxide according to claim 9, wherein the reducing substance is at least one selected from the group consisting of hydrogen, a carbon compound, and ammonia. The method according to any one of claims 1 to 10, the water is brought into contact with the surface of the film made of the semiconductor material to form a film made of the oxide on the surface of the film, forming the oxide Method.

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