JPH07106260A - Compound semiconductor epitaxial wafer and measuring method for its film thickness - Google Patents

Abstract

PURPOSE:To enable measuring the absolute value of film thickness with high reliability, improve the quality ensurance of a product epitaxial wafer, and facilitate failure analysis. CONSTITUTION:An epitaxial wafer for an MESFET has a buffer layer 20 on a semiinsulative GaAs substrate 10 in which layer at least one or more very thin hetero epitaxial layer is inserted, and has, on the layer 20, an N-type GaAs layer 30 as an active layer, and an N<+> type GaAs layer 40 as a contact layer for forming an ohmic electrode. The buffer layer 20 consists of the following in order from below; an un type GaAs layer 23 of 200nm in thickness, an un type Al0.3Ga0.7As layer 24 of 2nm in thickness, an un type GaAs layer 25 of 3nm in thickness, an un type Al0.3Ga0.7As layer 26 of 2nm in thickness, and an un type GaAs layer 27 of 200nm in thickness. Since the hetero epitaxial layer is inserted in the buffer layer having little influence upon device characteristics, film thickness absolute value measurement using a TEM (transmission electronic microscope) is enabled without deteriorating characteristics.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compound semiconductor epitaxial wafer having an active layer on a substrate via a buffer layer, and a film thickness measuring method thereof.

[0002]

2. Description of the Related Art As a wafer used for a compound semiconductor device, there is a compound semiconductor epitaxial wafer in which a buffer layer is formed on a compound semiconductor substrate by a vapor phase growth method and an active layer of a compound semiconductor crystal is formed thereon. An epitaxial wafer for GaAs MESFET manufactured by using the metal organic chemical vapor deposition (MOVPE) method will be described below.

FIGS. 4 and 5 show the MESF used conventionally.
It is a typical example of an epitaxial wafer for ET. Figure 4 is G
aAs system, FIG. 5 shows GaAs / AlGaAs system. Both epitaxial wafers are semi-insulating GaAs substrate 1
0 has a buffer layer, an active layer, and a contact layer on the surface. Undoped (un type) that is a buffer layer
GaAs layer 20 (thickness 500 nm) or un-type GaA
s layer 21 (thickness 300 nm) and un-type Al _0.3 Ga _0.7
The As layer 22 (thickness: 200 nm) is inserted between the substrate 10 and the n-type GaAs 30 which is the active layer in order to buffer the influence from the substrate 10. Surface n ⁺ type GaAs layer 40
(Thickness 100 nm) is a contact layer for forming an ohmic electrode. These wafers are manufactured as follows.

First, the pretreated GaAs substrate is MOVP.
It is placed in the reactor of the E unit. An organic metal of Ga is used as a Ga raw material, an organic metal of Al is used as an Al raw material, and AsH ₃ gas is used as an As raw material. These raw material gases are caused to flow in the reaction furnace to be thermally decomposed at a high temperature, and a GaAs layer as shown in FIGS. 4 and 5 or a GaAs layer and AlGaAs are formed on the substrate.
A buffer layer 20 consisting of layers is grown.

Next, the valve for introducing the doping source gas into the reaction furnace is switched to perform n-type doping, for example SiH.
₄ , Si source gas such as Si ₂ H ₆ is introduced to grow the n-type GaAs layer 30 which is an active layer. After the active layer is formed, the n-type doping amount is increased to grow the n ⁺ -type GaAs layer 40 which is the ohmic electrode forming contact layer. The film thickness of these epitaxial layers 20, 30 and 40 is controlled by the flow rate of the raw material gas and the time during which the gas is flowing in the reaction furnace.

Now, the GaAsME manufactured in this way
Most of the electrical characteristics of the SFET epitaxial wafer are determined by the film thickness d and the carrier concentration n of the n-type GaAs layer 30, which is the active layer. Since the threshold voltage V _th, which is a typical characteristic example of the MESFET, is proportional to n · d ² , the dependence of the film thickness d that can be squared is larger than the carrier concentration n. Therefore, the n-type G that is the active layer for the wafer
How to control the film thickness d of the aAs layer 30 is the most important technique.

[0007]

In order to control the film thickness of the epitaxial layer, it is important to measure not the relative value of the film thickness but the absolute value thereof accurately. G
Conventional film thickness measurement methods such as aAs and AlGaAs are
(1) Cross-sectional SEM (scanning electron microscopy) measurement to detect the intensity of secondary electrons and backscattered electrons due to electron beam collisions,
(2) FTIR (Fourier Transform Infrared Spectroscopy) measurement using infrared interference, (3) Ellipsometry measurement to detect the polarization state at the time of reflection, (4) Touch to read by changing the vertical movement of the needle into an electrical signal (5) Cross-section TE to obtain a magnified image of diffraction or transmission by primary thermoelectrons or field electrons
Examples include lattice image observation by M (transmission electron microscopy). Of these measuring methods, the most reliable method for measuring the absolute value of the film thickness is only the film thickness measurement by observing the lattice image with the TEM in (5). Other measurements are unsuitable as absolute value measurements for various reasons. In the absolute film thickness measurement by observing the lattice image of the cross-section TEM, the electron beam is transmitted through the heterostructure laminated thin film (heteroepitaxial layer) formed on the substrate, and each layer is displayed as an enlarged dark field image of the lattice image pattern. This is done by observing this dark field image.

However, in order to measure the film thickness with high resolution by observing the lattice image by TEM, it is necessary to measure at a high magnification of about 400,000 times, but when the film thickness becomes 60 nm or more,
It becomes impossible to measure in one view. In this respect, in the conventional epitaxial wafer for MESFETs shown in FIGS. 4 and 5,
The absolute value of the film thickness cannot be accurately measured because there is no heterogeneous epitaxial layer having a thickness of 60 nm or less. Therefore,
Regarding the film thickness of the epitaxial wafer as a product, sufficient quality assurance cannot be achieved.

Further, in manufacturing a device, the above-mentioned heterogeneous epitaxial layer for film thickness measurement is not necessary, but when some defect occurs in the product, the heterogeneous epitaxial layer is very necessary for structural analysis or defect analysis. is important.
However, since the epitaxial wafer for MESFET of the product does not have the heterogeneous epitaxial layer for measuring the film thickness, it is difficult to perform the analysis when performing the structural analysis or the defect analysis.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a compound semiconductor epitaxial wafer capable of measuring the absolute value of the film thickness of an epitaxial layer and a film thickness measuring method thereof.

[0011]

A first invention is a compound semiconductor epitaxial wafer having an active layer on a substrate via a buffer layer, and an epitaxial layer for forming a heterojunction in the buffer layer for film thickness measurement. At least one layer is inserted, and the film thickness of the epitaxial layer for film thickness measurement is in the range of 1 to 60 nm.

A second aspect of the present invention has a buffer layer on a semi-insulating GaAs substrate, and the buffer layer has 1 to 60 in the middle of an undoped GaAs layer which serves as an original buffer layer above and below.
Undoped A used as an epitaxial layer for measuring the thickness of nm
lGaAs layer, undoped GaAs layer, undoped Al
It has a laminated structure in which a GaAs layer is inserted, has an n-type GaAs layer serving as an active layer on a buffer layer, and an n ⁺ -type GaAs serving as a contact layer for forming an ohmic electrode thereon.
It has a layer.

A third aspect of the present invention has a buffer layer on a semi-insulating GaAs substrate, and the buffer layer has 1 to 60 in the middle of an undoped GaAs layer which serves as an original buffer layer above and below.
Undoped A used as an epitaxial layer for measuring the thickness of nm
lGaAs layer, undoped GaAs layer, undoped Al
It has a laminated structure in which a GaAs layer is inserted, has an n-type AlGaAs layer serving as an electron supply layer on the buffer layer, and has an n ⁺ -type GaAs layer serving as a contact layer for ohmic electrode formation thereon. .

A fourth invention is the method for measuring the film thickness of the compound semiconductor epitaxial wafer according to the first invention to the third invention, wherein the film thickness of the epitaxial layer for film thickness measurement inserted in the buffer layer. Is measured by observing a lattice image by the TEM method, and the film thickness of the active layer or the electron supply layer is calculated based on the measured film thickness.

[0015]

In the first aspect of the invention, the epitaxial layer for measuring the film thickness is inserted in the buffer layer because it has a small influence on the device characteristics of the buffer layer. Also,
The reason why the heterojunction is formed in the buffer layer is to enable a lattice observation image of the film due to a difference in lattice constant. The reason for limiting the thickness to a very thin range of 1 to 60 nm is that if the thickness is 60 nm or more, the lattice image observation by TEM cannot be measured in one visual field, and if it is 1 nm or less, the measurement becomes difficult. Because.

In the second invention in which the active layer is a GaAs layer, the epitaxial layer for measuring the film thickness inserted between the upper and lower buffer layers is not a single undoped GaAs layer but an undoped AlGaAs layer or undoped GaAs.
The three-layer structure of the layer and the undoped AlGaAs layer is
This is because a heterojunction cannot be formed in the buffer layer made of the same material as the undoped GaAs layer alone.

Further, in the third invention in which the electron supply layer is an AlGaAs layer, an epitaxial layer for film thickness measurement inserted between the upper and lower buffer layers is an undoped AlG layer.
aAs layer, undoped GaAs layer, undoped AlGa
The reason why the As layer has a three-layer structure is that the wafer structure up to the buffer layer can be shared with the second invention in this way.

In the fourth invention, when the epitaxial layer for measuring the film thickness inserted in the buffer layer is observed by the TEM method, each layer appears as an enlarged dark field image of a lattice image pattern. By observing this enlarged dark field image, the film thickness of each layer can be measured in one field. A proportional relationship is established between the measured film thickness and the film thickness of the active layer or electron supply layer formed of the same material as this film. Therefore, the film thickness of the active layer or the electron supply layer can be calculated based on the measured film thickness.

[0019]

EXAMPLES Hereinafter, the compound semiconductor epitaxial wafer of the present invention was used as a GaAs MESFET and a GaAs HEM.
An example applied to an epitaxial wafer for T will be described.

The structure of the epitaxial wafer for GaAs MESFETs shown in FIG. 1 has a buffer layer 20 in which at least one very thin heterogeneous epitaxial layer is inserted on a semi-insulating GaAs substrate 10, and an active layer is formed thereon. It has an n-type GaAs layer 30 and an n ⁺ -type GaAs layer 40 which is a contact layer for forming an ohmic electrode. The buffer layer 20 is an un-type GaA having a thickness of 200 nm in order from the bottom.
s layer 23, un-type Al _0.3 Ga _0.7 As layer 2 having a film thickness of 2 nm
4, un-type GaAs layer 25 with a thickness of 3 nm, un with a thickness of 2 nm
It is composed of a type Al _0.3 Ga _0.7 As layer 26 and an untype GaAs layer 27 having a film thickness of 200 nm.

The structure of the epitaxial wafer for GaAs HEMT shown in FIG. 2 is the same as that of FIG. 1 with respect to the substrate, the buffer layer, and the contact layer for forming the ohmic electrode, except that the active layer, that is, the electron supply layer. The point is that a certain n-type Al _0.3 Ga _0.7 As layer 31 is formed with a thickness of 45 nm.

When these epitaxial layers are grown using the MOVPE apparatus, trimethylgallium (Ga raw material), trimethylaluminum (Al raw material), AsH ₃ gas (As raw material), Si ₂ H ₆ gas (dopant Si raw material) are used as raw materials. ) Was used. The growth pressure was atmospheric pressure. The Al mixed crystal ratio of AlGaAs is 30%, that is, Al
_{It was set to 0.3} Ga _0.7 As.

As already mentioned, the GaAs MESF of FIG.
In ET, the thickness of the n-type GaAs layer 30 of the active layer is important in terms of characteristics. If the absolute value of the film thickness of the un-type GaAs layer 25 in the heterogeneous epitaxial layer in the buffer layer 20 is measured by observing the lattice image with a cross-sectional TEM, the film thickness of the n-type GaAs layer 30 which is an active layer of the same kind of material as this Can be measured accurately.

Further, in the GaAs HEMT shown in FIG. 2, the film thickness of the n-type Al _0.3 Ga _0.7 As layer, which is the electron supply layer, is important in the characteristics. When the absolute film thickness of the un-type Al _0.3 Ga _0.7 As layer 24 or 25 in the buffer layer 20 is measured in the same manner as in the previous example, the n-type Al _0.3 G of the same material as this is measured.
The film thickness of the a _0.7 As layer 31 can be accurately measured.

Next, a specific example of measurement by the above-mentioned cross-sectional TEM method will be described. FIG. 3 is a photograph result of observing a lattice image of the ultrathin heterogeneous epitaxial layer inserted in the buffer layer of the embodiment of FIGS. 1 and 2 by a cross-sectional TEM method.

The design values are as follows: the central un-type GaAs layer 25 has a thickness of 3.0 nm, and the un-type Al _0.3 Ga _0.7 As layer 2 above and below it.
Both 6 and 24 are 2.0 nm. However, the TEM measurement result shows that the GaAs layer 25 is 2.8 as shown in the figure.
3 nm, Al _0.3 Ga _0.7 As layers 24 to 26 are 2.2
It was 6 nm.

Based on these measurement results, the thicknesses of the n-type GaAs layer 30 which is the active layer in FIG. 1 and the n-type Al _0.3 Ga _0.7 As layer 31 which is the electron supply layer in FIG. 2 are calculated proportionally. , A deviation of −5.7% at 188.7 nm from the designed film thickness of the n-type GaAs layer 30 of 50 nm, and a deviation of 50.50 from the designed film thickness of the n-type Al _0.3 Ga _0.7 A layer s31 of 45 nm. It was found that the deviation was + 11.6% at 9 nm.

The deviation from the designed film thickness is a numerical value which is problematic in terms of quality assurance. Therefore, if the product is shipped as it is without noticing it, the quality cannot be sufficiently guaranteed. In this respect, according to the present embodiment, since the accurate film thickness measurement result can be fed back to the manufacturing process, it is possible to obtain a product wafer as close as possible to the designed film thickness, and to ensure sufficient quality assurance. become able to.

As described above, according to the above-mentioned embodiment, since the heterogeneous epitaxial layer having a thickness of 60 nm or less is inserted into the buffer layer of the product wafer, 4
Even if the measurement is performed at a high magnification of about 100,000 times, the magnified lattice image fits in one visual field, and as a result, the absolute value of the film thickness can be measured.

In the above-mentioned embodiment, the case where the GaAs layer and the Al _0.3 Ga _0.7 As layer are used as the heterogeneous epitaxial layers has been described, but the present invention does not limit the epitaxial species as long as the TEM lattice image can be taken. Absent. For example, InGaAs or InAlAs may be used.

[0031]

【The invention's effect】

(1) According to the invention described in claim 1, since the epitaxial layer for measuring the film thickness is inserted in the buffer layer, it is possible to measure the film thickness of the product without deteriorating the characteristics of the device. Moreover, since the film thickness is set to 1 to 60 nm, TE
The lattice image observation of the epitaxial film thickness by M can be contained in one visual field, and the absolute value of the film thickness of the epitaxial wafer can be measured. Furthermore, since the epitaxial layer for film thickness measurement is formed of the same material as the active layer, even if the active layer cannot be directly measured in absolute value due to its thick film thickness, it is based on the film thickness of the epitaxial layer for film thickness measurement. The product quality can be improved. In addition, when the failure analysis is performed, the film thickness can be accurately measured, which facilitates the analysis.

(2) According to the invention described in claim 2, Ga
The quality of the epitaxial wafer for AsMESFET can be improved.

(3) According to the invention of claim 3, Ga
The quality of the AsHEMT epitaxial wafer can be improved.

(4) According to the invention of claim 4, the film thickness of the epitaxial layer for film thickness measurement is absolutely measured, and the film thickness of the active layer or the electron supply layer is calculated from this. Therefore, accurate film thickness measurement can be performed even with a thick active layer or electron supply layer.

[Brief description of drawings]

FIG. 1 is a sectional view of a GaAs MESFET epitaxial wafer which is an example of a compound semiconductor epitaxial wafer of the present invention.

FIG. 2 is a sectional view of a GaAs HEMT epitaxial wafer which is another embodiment of the compound semiconductor epitaxial wafer of the present invention.

FIG. 3 is a cross section T of the ultrathin heterogeneous epitaxial layer of the present embodiment.
Explanatory drawing which shows the lattice image observation result by EM.

FIG. 4 is a sectional view of a conventional GaAs-based MESFET epitaxial wafer.

FIG. 5 Conventional GaAs / AlGaAs MESFET
Sectional drawing of an epitaxial wafer.

[Explanation of symbols]

10 semi-insulating GaAs substrate 20 buffer layer 23 un-type GaAs layer (buffer layer) 24 un-type Al _0.3 Ga _0.7 As layer (buffer layer) 25 un-type GaAs layer (buffer layer) 26 un-type Al _0.3 Ga _0.7 As layer ( Buffer layer) 27 un-type GaAs layer (buffer layer) 30 n-type GaAs layer (active layer) 31 n-type GaAs layer (electron supply layer) 40 n ⁺ -type GaAs layer (contact layer for ohmic electrode formation)

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H01L 29/812

Claims (4)

[Claims] 1. A compound semiconductor epitaxial wafer having an active layer on a substrate via a buffer layer, wherein at least one epitaxial layer for film thickness measurement for forming a heterojunction is inserted in the buffer layer, and A compound semiconductor epitaxial wafer, wherein the thickness of the epitaxial layer for measuring the film thickness is 1 to 60 nm. 2. A buffer layer is provided on a semi-insulating GaAs substrate, and the buffer layer is provided above and below an undoped GaAs layer, which is an original buffer layer, in the middle.
Layer, an undoped GaAs layer, an undoped AlGaAs layer having a laminated structure of 1 to 60 nm for measuring the thickness of the epitaxial layer is inserted, and an n-type GaAs layer serving as an active layer is provided on the buffer layer. A compound semiconductor epitaxial wafer for GaAs MESFET, which has an n ⁺ -type GaAs layer serving as a contact layer for ohmic electrode formation thereon. 3. A buffer layer is provided on a semi-insulating GaAs substrate, and the buffer layer is vertically provided with undoped AlGaAs in the middle of an undoped GaAs layer serving as an original buffer layer.
Layer, an undoped GaAs layer, an undoped AlGaAs layer having a laminated structure of 1 to 60 nm for measuring the thickness of the epitaxial layer is inserted, and an n-type AlGaAs layer serving as an electron supply layer is provided on the buffer layer. , N ⁺ -type GaAs, which will be a contact layer for ohmic electrode formation thereon
A compound semiconductor epitaxial wafer for a GaAs HEMT, which has a layer. 4. The method for measuring the film thickness of a compound semiconductor epitaxial wafer according to claim 1, wherein the film thickness of the epitaxial layer for film thickness measurement inserted in the buffer layer is transmitted. Electron microscope (TEM)
A method for measuring the film thickness of a compound semiconductor epitaxial wafer, which comprises measuring the film thickness of the active layer or the electron supply layer based on the measured film thickness by observing a lattice image by the method.