JP2001035838A - Manufacture of silicon carbide semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate influence such as abnormal deposits, composition change and change of surface configuration in an SiC surface after anneal by carrying out surface etching by means of plasma after annealing impurities of ion implantation. SOLUTION: An epitaxial wafer wherein an epitaxial layer 2 is formed on an n-type 4H-SiC substrate 1 is used as a wafer. In a surface layer of the epitaxial layer 2, an ion implantation layer 3 of aluminum of an average impurity concentration of 1×1018/cm3 is formed up to the depth of 0.5 μm, for example. Then, aluminum ion implantation and following high temperature anneal at 1500 deg.C for 30 minutes, for example, are carried out and thereafter RCA cleaning is carried out. Then, dilute HF treatment is carried out again for removing a natural oxide film which is newly generated in the process, and rinse by means of pure water is carried out between the treatments. After RCA cleaning is finished, plasma etching by plasma of mixed gas of H2 and O2 is carried out.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to silicon carbide (hereinafter referred to as Si)
C) as a material.

[0002]

2. Description of the Related Art In recent years, attention has been paid to SiC as one of semiconductor materials replacing silicon (hereinafter referred to as Si). Si
C has a band gap of 4.25 eV in 4H-SiC,
Since it is nearly three times as large as that of Si (1.12 eV), the operating upper limit temperature can be increased. Further, the breakdown electric field strength of 3.0 MV / cm for 4H-SiC is higher than that of Si (0.
25 MV / cm), which is about one order of magnitude larger, so that the on-resistance that is effective by the reciprocal of the cube of the breakdown electric field strength is reduced, and power loss in a steady state can be reduced. Furthermore, the thermal conductivity of 4.9 W / cmK for 4H-SiC and that of Si (1.5 W / cmK)
Since it is at least three times higher than that of K), there is an advantage that the cooling effect is high and the size of the cooling device can be reduced.

[0003] For these reasons, SiC is expected to be applied to power devices, high-frequency devices, high-temperature operating devices, and the like. At present, MOSFETs, pn diodes, Schottky diodes, and the like have been prototyped, and devices that exceed the characteristics of Si in terms of withstand voltage and on-resistance continue to appear.

[0004] In order to produce these devices, a technique for controlling the conductivity type and carrier concentration in a selected region is required. The method includes a thermal diffusion method and an ion implantation method. Since the diffusion coefficient of impurities is very small in SiC,
The thermal diffusion method widely used for semiconductor chips is difficult to apply to SiC. Therefore, the ion implantation method is usually used in SiC.

As ion species to be implanted, nitrogen (hereinafter referred to as N) and phosphorus (hereinafter referred to as P) are used for the n-type, and aluminum (hereinafter referred to as Al) for the p-type.
Alternatively, boron (hereinafter referred to as B) is often used.

[0006] The ion implantation and the subsequent process are as follows:
It is performed as follows. First, for selective ion implantation, a film such as a resist or an oxide film is formed and patterned as a mask, and then ion implantation is performed. In this case, ion implantation is sometimes performed in an atmosphere at a temperature of several 100 ° C. to 1000 ° C. in order to minimize crystal damage due to the ion implantation.

After the implantation, all the resist, oxide film and the like used as a mask are removed to leave the SiC surface exposed. This is because if a thermal oxide film or the like is deposited on the SiC during the subsequent high-temperature annealing, a reaction with the SiC occurs to prevent the etching from occurring. In particular, attention must be paid to the fact that the ion-implanted region has crystal damage and is weaker in bonding strength between atoms, so that it is more easily etched than other regions.

Thereafter, high-temperature annealing for electrically activating the implanted impurities is performed. In order to completely activate the impurities, a high temperature of 1500 ° C. is required for N and Al, and 1700 ° C. for B. During high temperature annealing,
The ion-implanted SiC sample is placed in a polycrystalline SiC container. This is to prevent sublimation of atoms near the surface at high temperatures.

After the high-temperature annealing, the surface is cleaned before the subsequent process. As a pretreatment method for surface cleaning, for example, the following so-called RCA cleaning method using a hydrogen peroxide solution is known.

First, in order to remove organic substances and precious metals, sulfuric acid and hydrogen peroxide (H ₂ SO ₄ : H ₂ O ₂ = 4: 1, 120 to 150)
C.) for 10 minutes, and then a dilute HF (0.5%, RT) treatment is performed to remove the natural oxide film. Thereafter, ammonium hydroxide (NH ₄ OH: H ₂ O ₂ : H ₂ O = 0.0) is used to remove particles existing in the natural oxide film.
05: 1: 5, 80-90 ° C.). Thereafter, Nature hydrochloric acid to remove metals that were present in the oxide film peroxide _{(HCl: H 2 O 2:} H 2 O = 1: 1: 6,80~9
(0 ° C.). Finally, a dilute HF treatment is performed again to remove a natural oxide film newly generated in these processes. Rinse with pure water for about 5 minutes between these treatments. Thereafter, for example, MOS having an insulated gate structure
In the case of an element, a thermal oxide film is formed. In the case of a Schottky diode, a Schottky electrode is formed.

[0011]

Prior to the above-described high-temperature annealing, the inside of the heat treatment container is sufficiently evacuated and replaced with gas. However, some residual components such as moisture and residual air are present. If the trace amounts of residual moisture, oxygen, and the like react with the SiC surface during high-temperature annealing, a surface oxide film is formed.

[0012] The surface adsorbed substances on the heater such as moisture may be desorbed during heating and deposited on the SiC surface to form a natural oxide film, or the heater material itself may evaporate and deposit on the SiC surface. In addition, by high-temperature annealing, Si having a high vapor pressure in SiC may be preferentially evaporated, and the surface may become C (carbon) rich.

Further, in the case of an epitaxial wafer, the surface is usually <11-20> or <11-20> from the (0001) plane.
An off-substrate polished so as to be inclined several degrees in the <1-100> direction is used. Since the epitaxial layer grows laterally in the off direction, the sectional shape of the final surface is a step shape in which each atomic layer has a width of several nm.

Due to the high-temperature annealing at 1500 ° C. or more, there is a case where a phenomenon called step bunching, in which the steps on the surface are integrated and the surface unevenness becomes severe, occurs.
As described above, undesirable deposits are present on the SiC surface after annealing, the composition is changed, and the surface shape is changed.

In order to avoid such a problem, a cleaning process is performed. However, if the process is continued with insufficient cleaning to produce a device, for example, in a Schottky barrier diode, the barrier height is reduced. However, problems such as an increase in leakage current at the time of reverse bias and a decrease in breakdown voltage occur. In MOSFET,
There is a problem that carriers are accumulated at the interface between the oxide film and the semiconductor, and that the moving carriers are scattered due to surface irregularities, thereby lowering the mobility. Therefore, it is recommended to form an oxide film once after the high-temperature annealing and to remove the oxide film.

FIGS. 2A to 2C are sectional views in the order of steps for explaining the steps. The above-mentioned RCA cleaning using a hydrogen peroxide solution is performed on a wafer having an epitaxial layer 2 grown on a substrate 1 and an ion-implanted layer 3 formed on the surface layer of the epitaxial layer 2 [FIG. 2 (a)]. RCA
By cleaning, deposits of the heater material metal from the container wall and natural oxide films formed during annealing can be removed to some extent.

Thereafter, pyrogenic oxidation is performed at 1100 ° C. for 5 hours with a mixed gas of hydrogen (hereinafter referred to as H ₂ ) and oxygen (hereinafter referred to as O ₂ ) to form a thermal oxide film 4 having a thickness of about 30 nm. [FIG. 2 (b)].

Etching is performed for 10 minutes using a BHF solution to remove the thermal oxide film 4 to make a clean surface appear (FIG. 3C). By forming an oxide film and removing the oxide film with a BHF solution, if a C-rich layer is formed, it is removed as CO or CO ₂ during thermal oxidation. For this reason, after this process, a SiC surface close to the stoichiometric composition can be obtained.

However, there is a problem that it takes a long time to form and remove the oxide film. In view of such problems, an object of the present invention is to provide an abnormal deposit on the SiC surface after annealing, a change in composition,
An object of the present invention is to provide a method for manufacturing a device having good characteristics by removing the influence of a change in surface shape or the like.

[0020]

According to the present invention, in order to solve the above-mentioned problems, plasma etching is performed after annealing for activating ion-implanted impurities. Any of plasma using a mixed gas of H ₂ and O ₂ , plasma using a gas containing fluorine atoms, and surface etching using a high-temperature molten salt such as a molten alkali may be used.

When a mixed gas of H ₂ and O ₂ is used, not only is the surface etched away, but also a C-rich layer and surface contaminants containing C atoms are removed from the surface as CO or CO _2. The effect is further enhanced.

Since the gas containing fluorine atoms has a higher activity than plasma of a mixed gas of H ₂ and O ₂ , the etching rate is high, the etching can be performed in a short time, and the effect on deep and large defects can be obtained. large. Surface etching with a high-temperature molten salt such as a molten alkali also has a high etching rate and can be etched in a short time. Etching by these is effective in removing step unevenness and surface irregularities generated during substrate growth.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below based on examples. [Embodiment 1] FIGS. 1 (a) and 1 (b) are sectional views in the order of steps for explaining a method of the present invention.

As the wafer, an epitaxial wafer in which an epitaxial layer 2 was grown on an n-type 4H-SiC substrate 1 at a plane 8 ° off from the (0001) Si plane was used. The carrier concentration of the substrate 1 is 1 × 10 ¹⁸ / cm ³ , the carrier concentration of the epitaxial layer 2 is 1 × 10 ¹⁶ / cm ³ ,
The thickness is 10 μm. The surface layer of the epitaxial layer 2 has an average impurity concentration of 1 × 10 ¹⁸ /0.5 μm.
An ion implanted layer ³ of aluminum (hereinafter, referred to as Al) of cm ³ is formed.

Al ion implantation followed by 1500
After annealing at a high temperature of 30 ° C. for 30 minutes, RCA cleaning was performed in the following procedure. First, organic matter, SPM for the removal of precious metal _{(H 2 SO 4: H 2} O 2 = 4: 1,120~1
(50 ° C.) for 10 minutes, and then a dilute HF (0.5%, RT) treatment is performed to remove a natural oxide film. Thereafter, ammonium hydroxide (NHOH: H ₂ O ₂ : H ₂ O) is used to remove particles existing in the natural oxide film.
= 0.05: 1: 5, 80-90 ° C.).
Then, in order to remove the metal present in the native oxide film, hydrochloric acid / hydrogen peroxide mixture (HCl: H ₂ O ₂ : H ₂ O = 1: 1:
6, 80-90 ° C). Finally, a dilute HF treatment is performed again to remove a natural oxide film newly generated in these processes. Rinse with pure water for about 5 minutes between these treatments. [FIG. 1 (a)].

After the RCA cleaning, H_TwoAnd O_Two With
Performs plasma etching with mixed gas plasma
[FIG. (B)]. The condition of plasma etching is H_Two
/ O _Two Ratio = 1, 50 Pa, RF200W. Substrate temperature
Is 200 to 300 ° C. The etching time is 15 minutes
It was etched by 120 nm. Therefore, the etching rate is
It becomes 8 nm / min.

Auger electron spectroscopy (AES) of the surface subjected to the plasma etching described above showed that the C / Si ratio was 1.1 and the surface composition was close to the stoichiometric composition. That is, a C-rich layer having a C / Si ratio of 1.5 as in the related art was not detected.

When the surface was observed by AFM (atomic force microscope), the depth and width of the polishing flaw were reduced, and the surface roughness Ra was 1 nm or less. This value corresponds to about 1/5 of the conventional value.

Further, nickel (N
When a Schottky barrier diode using i) as a barrier metal was fabricated, the barrier height was increased from 1.3 eV to 1.
Increased to 6 eV and leakage current at reverse bias was -20
At 0 V, it decreased from 10 ⁻³ A / cm ² to 10 ⁻⁶ A / cm ² .

When a MOSFET was fabricated using this wafer and CV measurement was performed, the interface state was reduced from 10 ¹³ / cm ² when no treatment was performed to 10 ¹² / cm ² . Carrier mobility was also increased from 20 to 60 cm ² / Vs by the treatment of the present invention. That is, it can be seen that the surface of the method of the present invention was sufficiently cleaned despite the etching of only 15 minutes.

As the plasma etching gas,
H_Two, O_TwoOther than the mixed gas of carbon bromide (C)
BrF_Three ), Carbon tetrafluoride (CF_Four ), Sulfur hexafluoride
(SF ₆ ), Nitrogen trifluoride (NF_Three ) And other fluorine atoms
A gas containing (F) is used. Etching rate
NF_Three At 50-200 nm / min
Yes, H_TwoAnd O_Two 1 more than in the case of mixed gas plasma
Digit etching speed can be increased.

[Second Embodiment] After the RCA cleaning of the first embodiment is completed, etching may be performed using a high-temperature molten salt such as an alkali.

Potassium hydroxide (KOH) was placed in a Ni crucible, heated to 460 ° C. by an external ceramic heater and melted, and an epitaxial wafer was immersed in the melt and etched. The surrounding atmosphere was dry air.

When the etching time was set to 10 seconds, about 0.1 μm was etched. Therefore, the etching rate is 0.8 μm / min. The surface characteristics of the wafer etched with molten KOH were almost the same as those in Example 1. Since KOH is easily available and does not require a very high temperature, the method of this embodiment can be carried out extremely easily, and a sufficient cleaning effect can be obtained.

[0035]

As described above, according to the present invention, by performing surface etching with a plasma of a mixed gas of hydrogen and oxygen, a plasma of a gas containing fluorine atoms, etc., abnormal deposits and compositions on the SiC surface can be obtained. By removing the influence of the change, the change in the surface shape, and the like, a SiC semiconductor device having good characteristics can be manufactured. It has been shown that the same effect can be obtained by performing surface etching in a high-temperature molten salt such as a molten alkali. Therefore, the present invention makes a great contribution to the spread and development of silicon carbide semiconductor devices.

[Brief description of the drawings]

FIGS. 1A and 1B are cross-sectional views in the order of steps for explaining a pretreatment method of the present invention.

FIGS. 2A to 2C are cross-sectional views in the order of steps for explaining a conventional pretreatment method.

[Explanation of symbols]

 1… Substrate 2… Epitaxial layer 3… Ion implanted layer 4… Thermal oxide film

Claims (5)

[Claims] 1. A method of manufacturing a silicon carbide semiconductor device, comprising performing surface etching by plasma after annealing for activating an ion-implanted impurity. 2. The method according to claim 1, wherein the surface is etched by plasma using a mixed gas of hydrogen and oxygen. 3. The method according to claim 1, wherein the surface is etched by plasma using a gas containing a fluorine atom. 4. A method for manufacturing a silicon carbide semiconductor device, comprising performing surface etching in a high-temperature molten salt after annealing for activating an ion-implanted impurity. 5. The method of manufacturing a silicon carbide semiconductor device according to claim 4, wherein the surface is etched with a molten alkali.