JP2013149914A - Photoelectrochemical device and manufacturing method for semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoelectrochemical device and a manufacturing method for a semiconductor device, which can precisely control a threshold of a nitride semiconductor by using photoelectrochemical reaction.SOLUTION: The manufacturing method includes: an oxidation step in which, while controlling a power supply 8 connected between a counter cathode 7 and an acting anode 5, light 4 having energy higher than a bandgap in a GaN layer of a sample 3 is irradiated upon the sample 3 to generate a pair of electrons and holes and the holes having moved to the surface of the sample 3 are subjected to oxidation reaction in the interface between the surface of the sample 3 and an oxidizing solution 2 to oxidize an AlGaN layer on the surface of the sample 3 to turn it into a thin film; and a threshold monitor step in which, by sweeping voltage of the power supply 8, measurement is taken of a threshold level Vth at which electrons are present in the GaN layer causing current to stop flowing any more. The oxidation step is carried out until the threshold level Vth measured in the threshold monitor step reaches a target threshold.

Description

  The present invention relates to a photoelectrochemical device used for manufacturing a semiconductor device such as a nitride semiconductor high electron mobility transistor (hereinafter referred to as HEMT) typified by GaN, and a method for manufacturing the semiconductor device.

Nitride semiconductors typified by GaN have a higher dielectric breakdown electric field than conventional GaAs semiconductors, and have properties not found in GaAs semiconductors such as spontaneous polarization and piezo polarization, and generate high-concentration two-dimensional electron gas. it can. By using these properties, a high-frequency and high-output power amplifier that cannot be realized with a GaAs semiconductor, and a switch device with high power resistance and low on-resistance can be realized.
There is HEMT as a device used for these, and one of the important characteristics is a threshold value. The threshold value indicates a voltage at which the transistor is turned on, and is an important parameter that determines the characteristics of the amplifier and the switch. For this reason, the manufacturing method of HEMT excellent in the controllability of a threshold value is desired.

In a HEMT using a heterojunction of an AlGaN layer and a GaN layer, as will be described later with reference to FIG. 1, the threshold value (gate voltage necessary for interrupting current) depends on the film thickness of AlGaN, and the film thickness decreases. The threshold increases. That is, the threshold voltage can be increased by reducing the thickness of the AlGaN barrier layer and reducing the distance between the gate electrode and the GaN channel layer.
Therefore, in manufacturing the HEMT, it is important to control the threshold value by reducing the thickness of AlGaN.
Conventionally, dry etching described in Non-Patent Document 1 has been a common method for thinning AlGaN. Non-Patent Document 2 discloses a method of thinning AlGaN by a photoelectrochemical reaction.

Takeshi Nakata et al., “Normally Off AlGaN / GaN HEMT Using Recessed Gate”, IEICE Technical Report, ED2005-129, p. 51 N. Harada et al. “Formation of recessed-Oxide gate for normally-off AlGaN / GaN high electron mobility transistors using selective electrochemical oxidation”, Applied Physics Express 4 (2011) 021002.

In the conventional AlGaN thinning by dry etching, the HEMT threshold cannot be monitored during the process, and the film thickness is controlled by the etching rate.
Here, the HEMT threshold Vth will be described.
First, the threshold value Vth is expressed by the following formula (1). Where φ _b is the work function, ΔE _c is the conduction band difference at the heterojunction, q is the elementary charge, N is the impurity concentration, d _d is the film thickness of the doped region, ε is the dielectric constant, and σ is the fraction The maximum strength, d _i, is the thickness of the region where no impurities are doped.
Vth = φ _b −ΔE _c −qNd _d ² / 2ε−qσ (d _d + d _i ) / ε (1)

In the HEMT of a nitride semiconductor, the contribution of the third term on the right side of the above formula (1) is considered to be small. The values of the conduction band difference ΔE _c and the polarization strength σ are determined by the material, and it is difficult to change them.
Therefore, to control the HEMT threshold, it is conceivable to change the work function φ _b or the film thickness d _i . Here, φ _b can be changed by changing the metal, but the value cannot be changed greatly. Therefore, it is efficient to change the film thickness d _i for controlling the threshold.

Figure 1 is a graph showing the relationship between the thickness _{d i} of the threshold and the AlGaN HEMT utilizing heterojunction between AlGaN and GaN. In the HEMT of FIG. 1, the film thickness d _d of the impurity-doped region is 0 nm, and the film thickness d _i is the thickness of AlGaN. For this reason, the horizontal axis is expressed as the AlGaN film thickness.
In the graph shown in FIG. 1, φ _b is 1.1 eV, ΔE _c is 0.2 eV, N is 1e ¹⁵ cm ⁻³ , _dd is 0 nm, and σ is 1e ¹³ cm ⁻² . Since AlGaN / GaN has a strong polarization effect and often does not use an impurity region, d _d is calculated at 0 nm as described above.

  As shown in FIG. 1, the threshold value increases as the AlGaN film thickness decreases. The slope of the threshold with respect to the AlGaN film thickness is 0.2 V / nm, and when the AlGaN film thickness is changed by 1 nm, the threshold value also changes by 0.2 V. For this reason, highly accurate film thickness control is desired. In particular, an enhancement type (normally off type) having a threshold value of 0 or more is desired for a switch or a single power supply compatible transistor. In this case, the film thickness of AlGaN needs to be about 5 nm or less, and more accurate threshold control is required.

As a general method for thinning AlGaN, there is dry etching described in Non-Patent Document 1. In Non-Patent Document 1, the AlGaN layer under the gate electrode is thinned by inductively coupled reactive ion etching (ICP-RIE) using chlorine gas (see FIG. 5 of Non-Patent Document 1). The etching rate is optimized by controlling to 6 nm / min, and the etching time is changed between 0 and 150 seconds.
Further, as the etching time increases, that is, the barrier film thickness decreases, the HEMT threshold increases, and the enhancement type is 0.3 V in 150 seconds (see FIG. 6 of Non-Patent Document 1). Since the original barrier layer thickness is 25 nm, the thickness is reduced to 10 nm by dry etching.

In the conventional technique represented by Non-Patent Document 1, the film thickness of AlGaN is controlled by the etching time as described above. Therefore, the etching rate is an important parameter that determines the accuracy of film thickness control. The etching rate is determined by the reaction between the gas and the semiconductor, and this reaction varies depending on various factors such as the aperture ratio of the mask (percentage of the portion to be etched in the wafer), how dirty the etching chamber is, and the reproducibility of the etching conditions. It depends on various factors.
That is, the etching rate is likely to vary depending on the ratio of the openings to be etched and the process environment, and the film thickness of AlGaN varies accordingly, which causes the threshold to vary.
As described above, in the conventional dry etching, the HEMT threshold value cannot be measured in situ during the AlGaN thinning process, and thus the variation in the threshold value is large.
For example, when the thickness is reduced to 15 nm and the process variation is 10%, the threshold value changes by 0.3 V as shown in FIG.

  Further, as shown in Non-Patent Document 2, a method of thinning AlGaN by a photoelectrochemical reaction has been proposed. In Non-Patent Document 2, AlGaN is thinned by replacing the upper part of AlGaN having an Al composition x of 0.25 and a film thickness of 25 nm with a gate oxide film (see FIG. 1 of Non-Patent Document 2). . An enhancement type HEMT with a threshold value of 1.2 V is realized by forming a gate oxide film of 20 nm and thinning AlGaN to about 5 nm.

However, Non-Patent Document 2 describes that the voltage is simply set so that a current for proceeding the photochemical reaction flows, but it is considered that the HEMT threshold is measured by changing the voltage during the process. It has not been.
That is, it is not assumed that the threshold is controlled by measuring the current-voltage characteristics in situ using the principle that the current changes according to the amount of electrons in the channel that changes with voltage.

  The present invention has been made to solve the above-described problems, and provides a photoelectrochemical device and a semiconductor device manufacturing method capable of precisely controlling a threshold value of a nitride semiconductor using a photoelectrochemical reaction. The purpose is to obtain.

  The photoelectrochemical device according to the present invention includes a container that holds an oxidizing solution, a cathode that is in contact with the oxidizing solution in the container, and a semiconductor sample that is electrically connected to the oxidizing solution in the container together with the semiconductor sample. In a photoelectrochemical apparatus comprising an anode, a semiconductor sample is irradiated with light having energy higher than the band gap of the channel layer of the semiconductor sample while controlling a power source connected between the cathode and the anode. The holes that have moved to the surface of the semiconductor sample are subjected to an oxidation reaction at the interface between the surface of the semiconductor sample and the oxidizing solution, thereby oxidizing the barrier layer on the surface of the semiconductor sample and making it thin A controller that performs an oxidation step and a threshold monitoring step of measuring a threshold at which electrons are present in the channel layer and no current flows by inserting a voltage of a power source; Threshold measured in value monitor process executes an oxidation step to reach the target threshold.

  According to the present invention, there is an effect that the threshold value of the nitride semiconductor can be precisely controlled using a photoelectrochemical reaction.

It is a graph showing the relationship between the threshold and the AlGaN with a thickness _{d i} of the HEMT utilizing heterojunction between AlGaN and GaN. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows schematically the photoelectrochemical apparatus which concerns on Embodiment 1 of this invention. It is a figure which shows the outline | summary of the photoelectrochemical reaction which concerns on this invention. It is a graph which shows the current-voltage characteristic at the time of drawing a power supply voltage from negative to positive in the photoelectrochemical apparatus of FIG. It is a band figure of an AlGaN thin film formation process. 2 is a diagram showing a process sequence for thinning using a photoelectrochemical reaction according to Embodiment 1. FIG. It is a figure which shows another structure of the photoelectrochemical apparatus which concerns on Embodiment 1. FIG. It is a graph which shows the current-voltage characteristic with the passage of time concerning Embodiment 3 of this invention. 6 is a diagram showing a process sequence for thinning using a photoelectrochemical reaction according to Embodiment 3. FIG. It is a figure which shows the manufacturing method of HEMT using the photoelectrochemical apparatus which concerns on Embodiment 4 of this invention.

Embodiment 1 FIG.
FIG. 2 is a diagram schematically showing a photoelectrochemical apparatus according to Embodiment 1 of the present invention. 2, in the photoelectrochemical device according to Embodiment 1, the working anode 5, the reference electrode 6 and the counter cathode 7 to which the sample 3 is attached are arranged in the oxidizing solution 2 of the container 1, and the working anode 5 and A power source 8 and an ammeter 9 are connected in series between the counter cathode 7 and a voltmeter 10 is connected between one end of the ammeter 9 connected to the reference electrode 6 and the working anode 5. The control unit 8A is composed of a microcomputer or the like, and controls the irradiation of the ultraviolet light 4 by the light source 4A and the DC voltage of the power supply 8 according to a preprogrammed sequence based on the measurement value of the ammeter 9 or the voltmeter 10. To perform the photoelectrochemical reaction.

The oxidation solution 2 is a solution containing hydroxide ions (OH ⁻ ) such as an aqueous glycol solution, and may be an aqueous potassium hydroxide solution or an aqueous hydrochloric acid solution.
Sample 3 is a semiconductor sample in which a barrier layer is formed on at least a channel layer, similar to HEMTs using this, and for example, a buffer layer, a GaN channel layer, and an AlGaN barrier layer are formed on a Si substrate. A crystal grown in this order is used. Further, the AlGaN layer side is a surface on which the light 4 is irradiated.

  The light 4 is light for generating a pair of electrons and holes in the GaN layer in the sample 3. The light 4 is desirably light having energy higher than the band gap of the GaN layer, that is, ultraviolet light having a wavelength shorter than the band gap energy of 3.4 eV (365 nm or less).

Further, the sample 3 is electrically connected to the working anode 5 as indicated by a symbol a in FIG. A saturated calomel electrode is used for the reference electrode 6 and a Pt electrode is used for the counter cathode 7. In this apparatus, the photoelectrochemical reaction proceeds according to the following formula (2).
However, the first term on the left side of the following formula (2) is GaN or AlN which is a semiconductor oxidized on the surface of the AlGaN layer. If it is AlGaN, it may be an intermediate (mixture) of GaN and AlN.
2GaN (or 2AlN) + 6h ⁺ (hole) + 6OH ⁻ → Ga ₂ O ₃ (Al ₂ O ₃ ) + 3H ₂ O + N ₂ ↑ (2)

FIG. 3 is a diagram showing an outline of the photoelectrochemical reaction according to the present invention. In the reaction process (A) of FIG. 3, the sample 3 is irradiated with light 4 (ultraviolet light) having energy higher than the band gap energy of GaN (channel layer), so that electrons 11 and h ⁺ ( 12 pairs (hereinafter referred to as holes) are generated. The holes 12 generated at this time move from the inside of the sample 3 to the surface as shown in the reaction process (B).

Next, as shown in the reaction process (C), hydroxide ions OH ^- 13, holes 12, GaN (or AlN) present in the oxidation solution 2 at the interface between the oxidation solution 2 and the surface of the sample 3 are shown. Causes an oxidation reaction to form an oxide film Ga ₂ O ₃ (Al ₂ O ₃ ), water, and nitrogen. In this way, the AlGaN layer is oxidized and thinned.

FIG. 4 is a graph showing current-voltage characteristics when the power supply voltage is inserted from negative to positive in the photoelectrochemical apparatus of FIG. As shown in FIG. 4, when the voltage of the power supply 8 is pulled from a negative value to a positive value, a current flows in a state where there are no electrons in the channel, and when the voltage is increased so that electrons exist in the channel. Current stops flowing. The voltage at which this current does not flow is the HEMT threshold Vth.
In the present invention, the voltage at which no current flows when the voltage of the power supply 8 applied between the working anode 5 and the counter cathode 7 is inserted in the photoelectrochemical reaction represented by the above formula (2) corresponds to the threshold voltage of the HEMT. Focusing on this, the AlGaN layer is thinned while measuring the threshold value in situ during this photoelectrochemical process.

As described above, the present invention measures the threshold value Vth in the current-voltage characteristic on the spot using the principle that the current changes according to the amount of electrons of the channel that changes with voltage.
In a conventional apparatus using a photoelectrochemical reaction, it is common to perform control to simply advance the photoelectrochemical reaction, and control in consideration of the characteristics of the subsequent device using the product of the photoelectrochemical reaction. Did not do.
The present invention pays attention to the fact that the photoelectrochemical reaction of the above formula (2) can measure a voltage corresponding to the threshold Vth of HMET during the process, and by performing thinning (oxidation) while measuring the threshold Vth. Highly accurate threshold control is possible.

Next, the photoelectrochemical reaction in the present invention will be described in detail with reference to band diagrams.
FIG. 5 is a band diagram of the AlGaN thinning process. FIG. 5A shows a case where the voltage measured by the voltmeter 10 is equal to or lower than the threshold value Vth, and FIG. FIG. 5C shows the case where the voltage measured by the voltmeter 10 is equal to the threshold value Vth.

In the band diagram shown in FIG. 5A, since the gap between the working anode 5 and the counter cathode 7 is biased to a value lower than the threshold value Vth, no electrons exist in the channel. Here, irradiation with ultraviolet light 4 generates a pair of electrons 11 and holes 12 in the GaN layer 14. At this time, the hole 12 moves to the surface of the AlGaN layer 15 (left side in the figure) by the electric field in the GaN layer 14.
The holes 12 that have moved to the surface oxidize the AlGaN layer 15 together with the hydroxide ions OH ^- 13 at the interface between the oxidation solution 2 and the surface of the AlGaN layer 15 as described above with reference to FIG. It will become. What is important here is that the holes 12 need to move to the surface of the AlGaN layer 15 in order to proceed with oxidation.

  In the band diagram shown in FIG. 5B, since a voltage higher than the threshold value Vth is applied between the working anode 5 and the counter cathode 7, electrons 17 exist in the channel. In this case, the holes 12 generated by irradiating the ultraviolet light 4 recombine with the electrons 12 in the channel 16 and cannot move to the surface of the AlGaN layer 15. Therefore, the oxidation reaction does not proceed at the interface between the oxidation solution 2 and the surface of the AlGaN layer 15, and no current flows.

In the band diagram shown in FIG. 5C, a voltage equal to the threshold value Vth is applied between the working anode 5 and the counter cathode 7. In this case, there are no electrons in the channel, but there is no band bending. For this reason, the holes 12 do not move to the surface of the AlGaN layer 15 and no current flows.
5A to 5C, the voltage applied to the channel of the sample 3 by the power source 8 is changed, and the current (hydroxide ions OH ^- 13 accompanying oxidation are generated by the ammeter 9). The current-voltage characteristics as shown in FIG. 4 are obtained by monitoring the current generated by consumption. That is, since the current changes rapidly before and after the threshold value Vth, the threshold value Vth itself can be measured.

FIG. 6 is a diagram showing a process sequence (voltage setting method of the power supply 8) for thinning using the photoelectrochemical reaction according to the first embodiment. As shown in FIG. 6, the photoelectrochemical reaction of the above formula (2) is controlled so as to become a target threshold value as a HEMT threshold value.
Periods A, C, and E are periods in which the AlGaN layer is oxidized by this photoelectrochemical reaction. The broken line b is the threshold value Vth of each period, and the voltage applied to the sample 3 by the oxidation in the periods A, C, E (hereinafter referred to as the oxidation voltage) is as shown in FIG. The voltage is lower than the threshold value Vth at the time (hereinafter, the threshold value Vth until reaching the target threshold value is appropriately described as a monitor voltage). Furthermore, periods B, D, and F are periods during which the monitor voltage (threshold value) changed by the previous oxidation is measured.

Initially, the voltage of the power source 8 is negative and oxidation is performed in the period A, thereby thinning the AlGaN layer.
A predetermined period in which the oxidation does not proceed excessively is set as the period A, and when the thinning of the film has progressed to some extent (when the oxidation in the period A is completed), the voltage of the power source 8 shown in FIG. Then, the monitor voltage at this time is measured (period B). At this time, if the monitor voltage does not reach the target threshold value, oxidation is similarly performed in the period C, and the monitor voltage at that time is measured in the period D. This process is repeated until the monitor voltage reaches the target threshold value, and the process ends when the target threshold value is reached.

As described above, in the present invention, the AlGaN layer can be thinned while checking the HEMT threshold Vth itself, so that the HEMT threshold Vth can be controlled to a desired value with higher accuracy than the conventional process. Is possible.
In addition, during the periods A, C, and E during oxidation, the oxidation rate can be estimated by conducting preliminary experiments in advance. However, in the present invention, the threshold value Vth is measured even if there are some variations in the situation of the photoelectrochemical apparatus (intensity of the ultraviolet ray 4 and the concentration of the oxidizing solution 2) and the structure of the sample 3 (AlGaN composition and film thickness). However, since the process proceeds, the estimation of the oxidation rate may be a rough guide.

Further, the above process can be automatically executed by programming the sequence shown in FIG. 6 in the control unit 8A.
Furthermore, even if the AlGaN layer 15 is not a single composition but a plurality of layers (for example, a layer made of InAlGaN, GaN, InN, InAlN), the present invention can measure the threshold value on the spot, so Good threshold control is possible.

  The photoelectrochemical device according to the present invention may be various types of devices, the outline of which is the same as that shown in FIG. A simple structure can be realized by putting an oxidizing solution in a container such as a beaker and hanging an electrode. In this case, the sample is affixed to the working anode with wax or the like, and a wiring such as a wire is attached to the sample with solder or a conductive adhesive, and is electrically connected to the working anode. The ultraviolet light may be irradiated from the outside of the beaker.

  FIG. 7 is a diagram showing another configuration of the photoelectrochemical apparatus according to Embodiment 1, and is an apparatus suitable when the sample 3 has a fixed shape such as a wafer shape. As shown in FIG. 7, a window 18 a made of a material that transmits ultraviolet light 4 is provided at one end of the container 1 </ b> A. The side of the container 1A facing the window 18a is opened, and a ring groove for arranging the O-ring 19 is formed on the peripheral edge of the opening. As shown in FIG. 7, a reference electrode 6 and a counter cathode 7 are attached inside the container 1 </ b> A, and the sample 3 is disposed so as to contact the oxidizing solution 2.

  In the wafer-shaped sample 3, an electrode formed on the surface facing the window 18a when attached to the container 1A is electrically connected to the working anode 5. The sample 3 is attached to the opening of the container 1A via the O-ring 19 disposed in the ring groove. Then, the container 1A is sealed with the O-ring 19 by attaching the lid 18b from the back side of the sample 3 (one side which is not the window 18a). A power source 8 and an ammeter 9 are connected in series between the working anode 5 and the counter cathode 7, and a voltmeter 10 for measuring the voltage of the power source 8 applied to the channel of the sample 3 is connected.

In this state, the ultraviolet lamp 4a is turned on by the lamp power source 4b, and the sample 3 is irradiated with the ultraviolet light 4 through the window 18a, whereby the photoelectrochemical reaction shown in the above formula (2) proceeds, and the AlGaN layer is Can be thinned. At this time, by performing oxidation while measuring the monitor voltage with the voltmeter 10, a nitride semiconductor having a target threshold value can be obtained.
The control unit 8A controls the irradiation of the ultraviolet light 4 by the ultraviolet lamp 4a and the direct current voltage of the power supply 8 according to a sequence programmed in advance based on the measured value of the ammeter 9 or the voltmeter 10 to perform photoelectric operation. Perform a chemical reaction.

By configuring in this way, the same effect as that of the apparatus shown in FIG. 2 can be obtained.
Note that the reference electrode 6 is an electrode for monitoring the potential of the oxidation solution 2 and does not directly participate in the photoelectrochemical reaction, and thus may be omitted.

As described above, according to the first embodiment, the power supply 8 connected between the counter cathode 7 and the working anode 5 is controlled (voltage is controlled), and is higher than the band gap of the GaN layer 14 of the sample 3. The sample 3 is irradiated with energy light 4 to generate a pair of electrons 11 and holes 12, and the holes 12 moved to the surface of the sample 3 are subjected to an oxidation reaction at the interface between the surface of the sample 3 and the oxidizing solution 2. Thus, the oxidation step of oxidizing the AlGaN layer 15 on the surface of the sample 3 to make it thin, and the threshold value Vth for measuring the threshold value Vth at which no current flows due to the presence of electrons in the GaN layer 14 by inserting the voltage of the power source 8. The control unit 8A executes the monitoring step, and the control unit 8A executes the oxidation step until the threshold value Vth measured in the threshold value monitoring step reaches the target threshold value.
In this way, the HEMT threshold Vth can be precisely controlled in thinning the AlGaN layer 15 using the photoelectrochemical reaction of the above formula (2).

Embodiment 2. FIG.
In the first embodiment, the case where the oxidation is performed while changing the voltage of the power supply 8 is shown. However, the photoelectrochemical reaction according to the present invention proceeds by moving the holes to the surface of the AlGaN layer, and the oxidation is performed during that time. A current is generated by the consumption of the hydroxide ions OH ⁻ of the solution 2. That is, a current flows at a voltage lower than the threshold value Vth, and no current flows at a voltage higher than that.

  Therefore, in the second embodiment, the voltage is monitored while changing the current of the power supply 8. Since the photoelectrochemical reaction proceeds according to the above formula (2), the value of the current corresponds to the number of Al or Ga atoms (a factor that determines the oxide film thickness of AlGaN). Thus, by monitoring the integrated amount of the current of the power source 8, the approximate oxide film thickness of AlGaN can be estimated, and the oxidation rate can be controlled.

  Note that the power supply 8 according to the second embodiment can be programmed to insert current and voltage by the control unit 8A as in the first embodiment. That is, by programming the sequence in the control unit 8A, it is possible to automate the thinning process by the photoelectrochemical reaction.

  As described above, according to the second embodiment, the AlGaN layer 15 of the sample 3 is thinned by the oxidation reaction by controlling the current of the power supply 8 energized between the counter cathode 7 and the working anode 5 in the oxidation process. Since the progress is controlled, the same effect as in the first embodiment can be obtained.

Embodiment 3 FIG.
In the second embodiment, the sequence in which the current for oxidation becomes constant is shown.
On the other hand, in the third embodiment, the number of times of measurement of the monitor voltage (threshold value Vth) is reduced by controlling the current while monitoring the voltage of the power supply 8.

  FIG. 8 is a graph showing current-voltage characteristics over time according to Embodiment 3 of the present invention. In FIG. 8, time elapses from time t1 to time t2, during which oxidation is performed. When the oxidation progresses (thinning progresses), the threshold voltage Vth increases as shown in FIG. 1, so that the monitor voltage increases in the positive direction of the horizontal axis. Here, if the current is a constant value Ib, the monitor voltage increases from Vm1 to Vm2 and approaches the target threshold value.

FIG. 9 is a diagram showing a process sequence of thinning using a photoelectrochemical reaction according to Embodiment 3, and shows a sequence using the current-voltage characteristics shown in FIG.
In FIG. 9, a broken line d indicates a value of a current that is passed during oxidation (hereinafter referred to as an oxidation current), and a broken line e is a value of the monitor voltage up to a target threshold value. As indicated by the broken lines d and e, the oxidation is continued with a constant oxidation current until the monitor voltage approaches the target threshold value.

When the difference ΔV between the monitor voltage and the target threshold value becomes smaller than a predetermined value, it is determined that the monitor voltage has approached the target threshold value, and the oxidation current is gradually lowered as indicated by the broken line d.
In this way, the monitor voltage can be brought close to the target threshold with high accuracy. Accurate monitoring up to the target threshold is performed by the method described with reference to FIG. 4 in the first embodiment, and the thinning process is completed.

  Note that the power supply 8 according to the third embodiment can be programmed to insert and remove current and voltage by the control unit 8A, as in the first embodiment. In other words, the above sequence can be automated by programming the control unit 8A.

  As described above, according to the third embodiment, by controlling the current while monitoring the voltage of the power supply 8, the same effect as in the first embodiment can be obtained, and the monitor voltage can be obtained during the process. The number of times (threshold) is measured can be reduced.

Embodiment 4 FIG.
In the fourth embodiment, a method for manufacturing a HEMT using a nitride semiconductor by a thin film formation method by a photoelectrochemical reaction as described in the first to third embodiments will be described.
FIG. 10 is a view showing a method for manufacturing a HEMT using the photoelectrochemical apparatus according to Embodiment 4 of the present invention, and shows a case where the channel layer under the gate electrode is thinned by the photoelectrochemical reaction described above. ing. Note that the process proceeds from FIG. 10A to FIG. 10C.

In FIG. 10, a representative structure of the HEMT is described. However, the type of the substrate is not limited to the Si substrate 20, and may be a SiC, GaN substrate, and the layer structure is not limited to FIG. Absent. For example, instead of the GaN layer 22, an AlGaN / GaN layer may be used, and instead of the AlGaN layer 23, a plurality of layer configurations such as AlGaN / GaN may be used, and other structures such as InGaAlN are included. It may be a layer or the like.
Even in such a configuration, since the principle of measuring the HEMT threshold Vth is the same, the same effects as those described in the first to third embodiments can be obtained.

  In the step of FIG. 10A, the semiconductors constituting the buffer layer 21, the GaN layer 22, and the AlGaN layer 23 are sequentially grown on the Si substrate 20 and stacked. The GaN layer 22 is a HEMT channel, and the AlGaN layer 23 is a HEMT barrier. For this crystal growth, an existing method such as MOCVD or MBE can be used.

After the above-described semiconductor layer is crystal-grown, a source electrode 24 and a drain electrode 25 are formed on the AlGaN layer 23. At this time, a high-concentration impurity region can be formed under the source electrode 24 and the drain electrode 25 to reduce ohmic resistance.
Next, a region that is not oxidized (thinned) on the AlGaN layer 23 is covered with an antioxidant layer 26. This antioxidant layer 26 prevents reaction with the oxidizing solution 2 and may be, for example, an oxide film, a nitride film, a resist, or the like. The antioxidant layer 26 can be formed by an existing method such as thermal CVD, sputtering, cat-CVD, or plasma CVD.
Further, the pattern partially covered with the antioxidant layer 26 can be formed by performing normal photolithography and etching.
Although not shown in FIG. 10A, the source electrode or drain electrode of the sample is electrically connected to the working anode 5.

  Thereafter, the sample is placed in the photoelectrochemical apparatus shown in FIG. 2 or FIG. 7, and the sample is irradiated with light 4 (ultraviolet light) having energy higher than the band gap energy of GaN. As a result, an oxide layer 27 is formed in a partial region (region where the gate electrode 30 is formed) of the AlGaN layer 26 that is not covered with the antioxidant layer 26, and the AlGaN layer 26 in this region is thinned. At this time, the HEMT threshold value Vth is controlled by the method described in the first to third embodiments.

  In the step of FIG. 10B, the antioxidant layer 26 and the oxide layer 27 formed by the photoelectrochemical reaction are removed. As these removal methods, dry etching or wet etching is used. By removing the oxide layer 27, a groove 28 is formed in a partial region of the AlGaN layer 23 (region where the gate electrode 30 is formed).

Note that the gas or solution used for etching is different depending on the composition of the oxide film. For example, a GaN oxide film can be dissolved with hydrochloric acid, hydrofluoric acid, or the like. An AlN oxide film can be dissolved with hot phosphoric acid, hydrofluoric acid, ammonium fluoride, potassium hydroxide, or the like. If it is an oxide of AlGaN, it can be dissolved in a solution in which the acid is mixed. On the other hand, in the case of dry etching, plasma of gas such as Ar, CF ₄ , CHF ₃ can be used.

  In the step of FIG. 10C, an insulating film 29 is newly formed on the trench 28, and a gate electrode 30 is formed thereon. Thereafter, a HEMT having a desired threshold value Vth can be obtained by forming a protective film, wiring, via-hole wiring, capacitance, resistance, and the like as necessary.

In the above description, the oxide layer 27 formed by the photoelectrochemical reaction is removed. However, the oxide layer 27 may be used as the insulating film 29 under the gate electrode 30.
Alternatively, an oxide film etchant (hydrochloric acid, hydrofluoric acid, or the like) may be placed in the oxidizing solution 2 used in the photoelectrochemical reaction to remove the oxide layer 27 while performing oxidation.

As described above, according to the fourth embodiment, the counter cathode 7 and the working anode 5 electrically connected to the semiconductor sample shown in FIG. While controlling the power supply 8 connected between the semiconductor samples, the semiconductor sample is irradiated with light 4 having energy higher than the band gap of the GaN layer 22 of the semiconductor sample to generate electron-hole pairs and move to the surface of the semiconductor sample. The oxidized hole is subjected to an oxidation reaction at the interface between the surface of the semiconductor sample and the oxidizing solution 2 to oxidize and thin the AlGaN layer on the surface of the semiconductor sample, and the voltage of the power supply 8 is subtracted. And a threshold value monitoring step for measuring a threshold value Vth at which no current flows due to the presence of electrons in the GaN layer 14, and the oxidation step is performed until the threshold value measured in the threshold value monitoring step reaches a target threshold value.
By doing so, the HEMT threshold value Vth can be precisely controlled by thinning the AlGaN layer (barrier layer) using the photoelectrochemical reaction of the above formula (2), and the HEMT having the desired threshold value Vth can be controlled. Can be manufactured.

  In the present invention, within the scope of the invention, any combination of each embodiment, any component of each embodiment can be modified, or any component can be omitted in each embodiment. .

  1, 1A container, 2 oxidation solution, 3 sample, 4 light (ultraviolet light), 4a ultraviolet lamp, 4b lamp power supply, 4A light source, 5 working anode, 6 reference electrode, 7 counter cathode, 8 power supply, 8A control unit, 9 Ammeter, 10 Voltmeter, 11,17 electrons, 12 holes, 13 hydroxide ions, 14,22 GaN layer, 15,23 AlGaN layer, 16 recombination, 18a window, 18b lid, 19 O-ring, 20 Si Substrate, 21 buffer layer, 24 source electrode, 25 drain electrode, 26 antioxidant layer, 27 oxide layer, 28 groove, 29 insulating film, 30 gate electrode.

Claims (5)

A photoelectric device comprising a container for holding an oxidizing solution, a cathode brought into contact with the oxidizing solution in the container, and an anode electrically connected to the semiconductor sample and brought into contact with the oxidizing solution in the container together with the semiconductor sample. In chemical equipment,
While controlling the power source connected between the cathode and the anode, the semiconductor sample is irradiated with light having energy higher than the band gap of the channel layer of the semiconductor sample to generate a pair of electrons and holes. An oxidation step in which the hole moved to the surface of the semiconductor sample is subjected to an oxidation reaction at the interface between the surface of the semiconductor sample and the oxidizing solution, thereby oxidizing and thinning the barrier layer on the surface of the semiconductor sample;
A control unit that performs a threshold monitoring step of inserting a voltage of the power supply and measuring a threshold at which electrons are present in the channel layer and no current flows;
The said control part performs the said oxidation process until the said threshold value measured at the said threshold value monitoring process reaches a target threshold value, The photoelectrochemical apparatus characterized by the above-mentioned.   The photoelectrochemical apparatus according to claim 1, wherein the control unit controls the progress of thinning of the barrier layer of the semiconductor sample by the oxidation reaction by controlling the voltage of the power source.   The photoelectrochemical apparatus according to claim 1, wherein the control unit controls the progress of thinning of the barrier layer of the semiconductor sample by the oxidation reaction by controlling the current of the power source. In a method for manufacturing a semiconductor device including a gate electrode that controls a potential of the channel layer through the barrier layer, using a semiconductor sample in which a barrier layer is disposed on the channel layer.
Bringing the cathode and an anode electrically connected to the semiconductor sample into contact with an oxidizing solution;
While controlling the power source connected between the cathode and the anode, the semiconductor sample is irradiated with light having energy higher than the band gap of the channel layer of the semiconductor sample to generate a pair of electrons and holes. An oxidation step in which the hole moved to the surface of the semiconductor sample is subjected to an oxidation reaction at the interface between the surface of the semiconductor sample and the oxidizing solution, thereby oxidizing and thinning the barrier layer on the surface of the semiconductor sample;
A threshold monitoring step of measuring a threshold at which electrons are present in the channel layer and no current flows by inserting a voltage of the power source,
The method of manufacturing a semiconductor device, wherein the oxidation step is executed until the threshold value measured in the threshold value monitoring step reaches a target threshold value.   5. The method of manufacturing a semiconductor device according to claim 4, wherein the semiconductor device is a high electron mobility transistor in which the channel layer and the barrier layer are formed of a nitride semiconductor.

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