JP2000223502A - Manufacture of semiconductor substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a semiconductor substrate, which solves a problem in conventional technology, caused by oxidation of an AlAs layer from lateral directions. SOLUTION: The method of manufacturing a semiconductor substrate is provided by forming a buffer layer of compound semiconductors on a substrate 4, wherein an AlAs layer 2 of 50 nm-1,000 nm thickness and a cap layer 1 are formed in the buffer layer. Slit-shaped openings having a width L1=0.5-1 μm are formed in the cap layer 1 with an interval L2=0.5 μm or larger, and the AlAs layer 2 is oxidized from these openings.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a semiconductor substrate, and more particularly to a method for manufacturing a semiconductor substrate used for compound semiconductor devices such as MESFETs and HEMTs.

[0002]

2. Description of the Related Art In the manufacture of a compound semiconductor device, a single crystal substrate serving as a base material is limited by a target epitaxial layer. For example, a GaAs / AlGaAs-based MESFET or HEMT-based electronic device can use a GaAs single crystal substrate that can be epitaxially formed by matching a lattice. Instead of this GaAs single crystal substrate, S
If an i-substrate can be used, manufacturing costs can be reduced, and device performance can be improved due to the high thermal conductivity of Si.

However, an electronic device formed on a Si substrate does not have the same characteristics as an electronic device manufactured by homoepitaxial formation. This is because when heterogeneous semiconductors are heteroepitaxially grown, dislocation defects due to lattice mismatch are generated from the heterointerface and propagate to the upper part of the epitaxial layer. The dislocation defect prevents electrical separation between the substrate and the device operation layer or the formed electrode, and generates a leak current and a parasitic capacitance, thereby deteriorating device characteristics.

[0004] As a heteroepitaxial formation technique for forming a high-resistance layer on a Si substrate, a MOCVD apparatus or MB
2 seconds in forming a GaAs buffer layer using the E device
Although various techniques for reducing dislocation defect density, such as step growth, thermal cycling, and strained superlattice (SLS) structure, have been proposed, device characteristics are still higher than electronic devices formed on semi-insulating GaAs substrates. Is inferior. The reason that the electronic device formed on the Si substrate has inferior device characteristics as compared with the electronic device formed on the semi-insulating GaAs substrate is that the defect density of the buffer layer is insufficiently reduced.

As a related technique for improving the electrical isolation performance, there is a technique for forming a high-quality insulating film on a GaAs substrate. The periphery of the element is removed by etching except for the element portion to form a mesa structure.
The oxidation insulating layer is formed by laterally steam-oxidizing the lAs layer (P. Parikh, P. Chavarkar, YFWu, PP
insukanjana and UKMishra: IEDM 96,929 (1996)).

In this prior art, although an insulating layer is formed by an AlAs oxide layer, the leakage current is reduced, but there is a problem that the high frequency characteristics are extremely poor. This is caused by deterioration of the crystallinity of the epitaxial layer which occurs when the AlAs layer is oxidized.

In the oxidation of a large-area AlGaAs layer, it has been reported that at the AlGaAs interface near the lateral surface where oxidation starts, the crystal changes into a porous state and the crystallinity deteriorates. Oxidation of a large area can be realized by diffusion of water vapor necessary for the oxidation reaction in the porous interface (S. Guh
a, F.Agahi, B.Pezeshki, JAKash, DWKisker and
NABojarczuk: Appl. Phys. Lett., 68,906 (1996)).
It is considered that the same phenomenon occurs in the oxidation of the AlAs layer, and that the crystallinity is deteriorated. In order to prevent the crystallinity from deteriorating, the oxidation must be completed in a short time in the initial stage of oxidation in which the water vapor contributing to the oxidation reaction is uniformly diffused in the AlAs layer. In other words, in oxidizing the AlAs layer from the lateral direction, the limit is to form an oxide insulating layer of a device having an area of about 100 μm square, and a device such as a power FET that requires a large-area oxide insulating layer. Not suitable for

SUMMARY OF THE INVENTION The present invention has been made in view of such problems of the prior art, and provides a method of manufacturing a semiconductor substrate which solves the problems of the prior art caused by oxidizing the AlAs layer from the lateral direction. The purpose is to provide.

[0009]

In order to achieve the above object, a method of manufacturing a semiconductor substrate according to the present invention comprises the steps of: forming a buffer layer made of a compound semiconductor and an active layer on the substrate; An AlAs layer having a thickness of 50 nm to 1000 nm and a cap layer are formed on the buffer layer, and a plurality of slit-shaped openings having a width L1 of 0.5 to 1 μm are formed in the cap layer at an interval L2 of 0.5 μm or more. The AlAs layer is oxidized from the opening.

As a result, since the AlAs oxide region exists in the buffer layer portion, it is possible to form a second buffer layer having a high quality insulating layer and further having a low dislocation defect region in the opening of the cap layer. Become.

[0011]

The AlAs layer formed on the buffer layer is oxidized from the opening of the cap layer, that is, from above, so that the AlAs layer can be oxidized over a large area in a short time.

After the AlAs layer is oxidized, a second buffer layer is laterally grown in the opening of the cap layer, so that the second buffer layer has a region with few dislocation defects on the opening of the cap layer. It becomes possible to form a buffer layer. In particular, by forming a gate electrode of a field effect transistor in a region where dislocation defects are small, a compound semiconductor device having excellent characteristics can be formed.

[0013]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. An example of heteroepitaxial formation using a Si single crystal substrate, which is most effective for reducing the manufacturing cost of the entire compound semiconductor device, will be described for the manufacture of a GaAs MESFET substrate having a relatively simple structure. Further, the epitaxial formation apparatus will be described by taking an example of epitaxial formation by a MOCVD apparatus. Note that sapphire, GaAs, Ga
A single crystal substrate such as P can also be used.

[0014] GaAs heteroepitaxial formation on a Si single crystal substrate generally involves two-step growth of GaAs. First, a surface oxide layer is removed by heat-treating a (100) Si single crystal substrate having an off-angle of several degrees in the (110) direction at 900 ° C. or higher by an MOCVD apparatus.

Then, AsH ₃ and TMG which is an organic metal are used.
Epitaxial formation is performed by using (trimethylgallium) or TEG (triethylgallium) as a raw material gas and spraying the same on a single crystal substrate.

After forming a GaAs amorphous layer at a substrate temperature of 300 to 450 ° C. on a Si single crystal substrate 4 to a thickness of 10 to 100 nm,
The temperature is raised to 600 to 700 ° C. to form a GaAs layer 3 of 2 to 5 μm (FIG. 1).

GaAs formed on Si single crystal substrate 4
On the buffer layer 3 composed of
(Trimethylaluminum) is used to form an AlAs layer 2 having a thickness of 0.05 to 1 μm. Thereafter, a cap layer 1 made of GaAs having a thickness of 0.05 to 0.1 μm is formed on the AlAs layer 2. Thereafter, the epitaxial growth is temporarily suspended, and the substrate 4 is taken out of the reaction furnace. The cap layer 1 is formed to prevent the AlAs layer 2 from spontaneously oxidizing in a short time when the substrate 4 is taken out into the atmosphere (FIG. 1).

In order to oxidize the AlAs layer 2 from the upper surface,
An opening 5 is formed in the cap layer 1 by photolithography (FIG. 2-5). The width L1 of the opening 5 is 0.5 to 1 μm
m, and the interval L2 between the openings 5 is 0.5 μm or more. The etching solution is a mixture of ammonia water and hydrogen peroxide.
When a material having H = 7.04 ± 0.02 is used, AlA
Etching stops on the upper surface of the s layer 2. If the distance L2 between the openings 5 of the cap layer 1 is smaller than 0.5 μm, the AlA
When the s layer 2 is oxidized, the cap layer 1 may peel off (FIG. 2). The width L1 of the opening 5 is set to 1 μm or less in order to minimize the natural oxidized region of the Al arsenic layer generated for providing the opening. Further, in order to make the region of the opening 5 smaller than the remaining cap layer region for the same purpose, the interval L2 is set to 0.5 μm or more to reduce the size of the natural oxidation region.

After the opening 5 is formed in the cap layer 1, A
The lAs layer 2 is oxidized. In order to acidify AlAs, wet oxidation is performed at 400 to 500 ° C. by introducing pure water at 60 to 90 ° C. into the oxidation furnace by bubbling with nitrogen at 1 to 10 L / min. Depending on the width and spacing of the openings 5 and the oxidation conditions,
Assuming that the interval L1 between the openings 5 is 10 μm, A
The entire surface of the lAs layer 2 can be oxidized (FIG. 2).

After oxidizing the AlAs layer 2, a second buffer layer 7 made of GaAs is formed to a thickness of about 2 to 3 μm.
The second buffer layer 7 grows from the cap layer 1 remaining on the AlAs layer 2, but can be formed on the entire surface by lateral growth. Thereby, the cap layer 1
The defect density also decreases in the region 7a above the opening 5 (FIG. 3).

On the surface of the second buffer layer 7 in which the GaAs region 7a having a low dislocation density is formed, 1 × 10 ¹⁶ to 10 × 10 ^16
An active layer 9 of n-type GaAs doped with Si of about ¹⁷ cm ^{-3 is} epitaxially formed to a thickness of about 0.05 to 0.2 μm. If necessary, 1 ×
N ⁺ -type GaAs doped with about 10 ¹⁷ to ¹⁸ cm ^-3
Is epitaxially formed to a thickness of about 0.1 to 0.2 μm (FIG. 4).

A mesa for defining a MESFET operation region is formed. Usually photolithography, approximately several hundred μm
An etching mask is formed in the corner region, and etching is performed. For the etching, a mixed solution of hydrogen peroxide and an acid or an alkali is used. Generally, sulfuric acid: hydrogen peroxide: water = 1:
An 8: 8 mixture is used. The depth at which the mesa is formed depends on the device to be manufactured because the device operation layer 9 and the first buffer layer 3 are etched, but the depth is 1 μm to several μm (FIG. 4).

Thereafter, a MESFET structure is formed on the surface of the mesa region. At this time, the gate electrode 12 is positioned in the second buffer layer 7 in a region having a low defect density corresponding to the region 7 a on the opening 5 of the cap layer 1 of the first buffer layer 3. Source electrode 11 and drain electrode 12
Are located in the region 7b with a high defect density, but a part on the gate electrode side is present in the region 7b with a low defect density (FIG. 4).

[0024]

As described above, in the method of manufacturing a semiconductor substrate according to the present invention, a thickness of 50 to 100 nm is formed in the buffer layer.
An AlAs layer having a thickness of 0 nm is provided, a cap layer made of GaAs having a thickness of 50 nm to 100 nm is formed thereon, and a slit-like opening having a width L1 of 0.5 to 1 μm is formed on the cap layer at a distance L2 of 0.5 μm or more. Since the AlAs layer is oxidized from the opening, the MESFET fabricated on the heteroepitaxial substrate is electrically separated by the presence of the oxidized AlAs layer in the buffer layer. Therefore, there is no leakage current to the substrate side, and the parasitic capacitance of the electrode formed on the mesa region can be greatly reduced.

Also, since the AlAs layer is oxidized from the upper surface, a large-area MESFET such as a power amplifier can be formed in a short time.

Further, lateral growth is performed after AlAs oxidation, and a second buffer layer having few dislocation defects can be formed on the opening region of the GaAs cap layer. A gate electrode of the MESFET structure is formed on this region. When formed, device characteristics can be significantly improved.

[Brief description of the drawings]

FIG. 1 is a schematic view showing one embodiment of a method for manufacturing a semiconductor substrate according to claim 1;

FIG. 2 is a schematic view showing another embodiment of the method for manufacturing a semiconductor substrate according to claim 1;

FIG. 3 is a schematic view showing one embodiment of a method for manufacturing a semiconductor substrate according to claim 3;

FIG. 4 is a schematic view showing another embodiment of the method for manufacturing a semiconductor substrate according to claim 3;

FIG. 5 is a view showing a conventional method of manufacturing a semiconductor substrate.

FIG. 6 is a diagram showing another conventional method for manufacturing a semiconductor substrate.

[Explanation of symbols]

1: cap layer, 2: AlAs layer, 2a: AlAs oxide layer, 3: buffer layer, 4: substrate, 5: opening, 7: second
Buffer layer, 9: active layer, 10: contact layer, 1
1: Source electrode, 12: Gate electrode, 13: Drain electrode, 14: Barrier layer, 15: Channel layer, 16: Ga
As substrate, L1: width of opening, L2: interval between openings

Claims (3)

[Claims] 1. A method for manufacturing a semiconductor substrate, comprising forming a buffer layer made of a compound semiconductor and an active layer on a substrate,
A having a thickness of 50 nm to 1000 nm is formed on the buffer layer.
An As layer and a cap layer are formed.
1 = Slit-like opening of 0.5 to 1 μm is spaced L2 =
A plurality of layers having a thickness of 0.5 μm or more are formed.
A method for manufacturing a semiconductor substrate, comprising oxidizing an As layer. 2. The method according to claim 1, wherein said cap layer has a thickness of 50 nm to 100 nm.
2. The method according to claim 1, comprising GaAs of nm. 3. The method according to claim 1, wherein a second buffer layer is formed on the opening and the cap layer.