JP2855964B2 - Method for measuring thickness of single crystal thin film on SOI substrate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention measures a single crystal thin film having a highly doped layer on an SOI substrate formed by bonding the single crystal thin film on a dielectric substrate using a Fourier transform infrared spectrophotometer. About the method.

[0002]

2. Description of the Related Art Conventionally, as a method of forming a single crystal semiconductor thin film having a thickness of 1 μm or more on a dielectric substrate, a technique of epitaxially growing a single crystal silicon film or the like on a single crystal sapphire substrate is well known. However, in this technique, since there is a mismatch in lattice constant between the dielectric substrate and the silicon single crystal to be vapor-grown, a large number of crystal defects occur in the silicon vapor-phase growth. Poor practicality.

Therefore, in recent years, SOI (Si On Insulator)
Particularly, a bonded substrate having a structure (hereinafter, referred to as an SOI substrate) has attracted attention. In this SOI substrate, for example, at least one of two single crystal silicon wafers is oxidized to form an oxide film on at least one surface of the wafer.
After laminating these two wafers so that the oxide film becomes an intermediate layer, they are heated and bonded to a predetermined temperature, and one of the wafers is surface ground, and the surface is further polished. It is obtained by making this into a thin film to form a single crystal silicon thin film (hereinafter referred to as an SOI film).

By the way, the SOI on such an SOI substrate
Conventionally, the thickness of the I film has been measured by a spectral interference method using visible light.

[0005] However, the above-mentioned film thickness measurement by the spectral interference method is effective for an SOI film having a film thickness of about 10 μm or less. The measurement becomes impossible due to the influence of the absorption.

On the other hand, as another method for measuring the film thickness,
Fourier transform infrared spectrophotometer (hereinafter referred to as FTIR)
(Hereinafter, referred to as the FTIR method) is known. However, this method does not cover the thickness of a silicon epitaxial layer formed on a silicon support substrate layer having a doping concentration of about 10 ¹⁸ atoms / cm ³ or more. It has been used exclusively for measurement.

Here, the principle of measuring the thickness of the silicon epitaxial layer by the FTIR method will be described with reference to FIGS. FIG. 6 is a diagram showing the basic configuration of a film thickness meter by the FTIR method, FIG. 7 is a diagram showing the state of reflection of infrared light on a silicon epitaxial layer, and FIG. 8 is a graph showing an optical path difference-reflected infrared light intensity curve. is there.

[0008] As shown in FIG.
Is generated as interference light using a Michelson interferometer composed of a fixed mirror 2, a movable mirror 3, and a beam splitter 4, and the interference light is formed on a silicon substrate 5. Is irradiated on the epitaxial layer 6.

By the way, as shown in FIG. 7, the reflected light R of the incident infrared light (Michelson interference light) L on the silicon substrate 5 is composed of various components. Among them, the main ones are those which are reflected at the surface c of the epitaxial layer 6 to become the reflected light R1, and those which pass through the epitaxial layer 6 and are reflected again at the interface g between the epitaxial layer 6 and the support substrate layer 7. The silicon substrate 5 through the surface h of the epitaxial layer 6
Two of them are emitted to the outside and become reflected light R2.

Since the two reflected lights R1 and R2 interfere with each other, the thickness of the epitaxial layer 6 can be obtained by analyzing the interference light. That is, although a detailed description is omitted here, when the fixed mirror 2 and the movable mirror 3 (see FIG. 6) that generate the incident interference light have a certain optical path difference, the combined reflected light (two reflected lights) Light R
1, R2) show a unique behavior. That is, a portion called a side burst (peak aggregation portion) where the reflected light intensity has a peak value occurs on the optical path difference-reflected infrared light intensity curve shown in FIG. Since there is a correlation with the thickness of the epitaxial layer 6, the thickness of the epitaxial layer 6 can be obtained from FIG.

The present inventors have found a condition for applying the FTIR method to the measurement of the thickness of the SOI film,
A method for non-destructively measuring the thickness of a single crystal having a thickness of μm or more with high accuracy has been previously proposed. That is, the method comprises irradiating the SOI substrate with interference light obtained by continuously changing the optical path difference between a fixed mirror and a movable mirror constituting a Michelson interferometer, and forming an optical path difference-reflected infrared light intensity curve. And selecting the one having the smallest absolute value of the optical path difference from the minimum peaks present in each of the plurality of side bursts in this curve, and obtaining the thickness of the single crystal thin film from the optical path difference of the minimum peak. .

By the way, in order to reduce the collector resistance when fabricating a bipolar transistor having an SOI structure, a dopant concentration of 1 × 10 ¹⁷ atoms / cm ³ or more is formed at the bottom adjacent to the dielectric layer 13 as shown in FIG. Having a highly doped layer (hereinafter referred to as a buried layer) 12A of
An SOI structure for forming the OI film 12 is generally used. In FIG. 1, reference numeral 11 denotes an SOI substrate, and 12B denotes an SOI substrate.
The low doping concentration layer 14 in the I film 12 is a supporting substrate layer.

[0013]

As an example, the buried layer (sheet resistance: 10Ω / □, thickness: 3 μm)
m), the thickness of the SOI film having a thickness of 4 to 35 μm measured by a scanning electron microscope (hereinafter, referred to as SEM), and the result measured by the method (FTIR method) previously proposed by the present inventors. FIG. 9 shows the deviation from (δ2). The figure shows that the larger the SOI film thickness, the larger the deviation between them, and the method proposed by the present inventors cannot measure the thickness of the SOI film having a buried layer with high accuracy. .

The present invention has been made in view of the above-mentioned problems, and an object thereof is to provide a highly doped layer (buried layer) having a dopant concentration of 1 × 10 ¹⁷ atoms / cm ³ or more in a portion adjacent to a dielectric layer. It is an object of the present invention to provide a method for measuring the thickness of a single crystal thin film on an SOI substrate, which can measure the thickness of an SOI film having the above characteristics nondestructively and with high accuracy.

[0015]

In order to achieve the above object, according to the present invention, a dopant concentration of 1 × 10 ¹⁷ atoms / cm is provided on the dielectric side.
^The present invention proposes a method for measuring the thickness of a single crystal thin film on an SOI substrate formed by bonding a single crystal thin film having ^three or more heavily doped layers on a dielectric substrate using a Fourier transform infrared spectrophotometer. And an optical path difference-reflected infrared light intensity curve obtained by irradiating the SOI substrate with interference light obtained by continuously changing the optical path difference Δ between the fixed mirror and the movable mirror constituting the Michelson interferometer. a curve Cf (Δ) obtained by performing cosine Fourier transform and inverse transform of f (Δ), and a curve f (Δ)
Sf obtained by performing sine Fourier transform and inverse transform on
And a curve Y (Δ) obtained by combining (Δ): Y (Δ) = √ (Cf (Δ) ² + Sf (Δ) ² ) where a plurality of curves exist in the optical path difference region corresponding to the thickness of the single crystal thin film. Out of the peaks, the thickness of the single crystal thin film is determined from the optical path difference of the peak having the smallest absolute value of the optical path difference.

[0016]

In general, infrared light is easily absorbed in a silicon layer having a high impurity concentration. Therefore, when measuring the SOI film thickness of the SOI substrate by the FTIR method, if a high impurity concentration layer exists at the boundary between the SOI film and the dielectric layer, infrared light is somewhat absorbed in the high impurity concentration layer. Absorbed. for that reason,
The reflection of infrared light at the interface between the SOI film and the dielectric layer is different from the case where there is no light absorption band (that is, no high impurity layer).

In the case of FTIR, a relation curve (interferogram) f (Δ) between the optical path difference Δ between the fixed mirror and the movable mirror of the Michelson interferometer and the reflected light intensity is subjected to a sine Fourier transform. Is again transformed into a curve Sf
(Δ) is called the imaginary part, which is considered to represent the effect when there is light absorption on the optical path.

By the way, the method proposed by the present inventors as a method for measuring the thickness of an SOI film having no buried layer is based on the optical path difference Δ between the fixed mirror and the movable mirror of the Michelson interferometer in FTIR and the reflected light. An intensity relationship curve (interferogram) f (Δ) is obtained, and an SOI film thickness is obtained from a peak position (optical path difference Δ) having the smallest absolute value of the optical path difference Δ among a plurality of peaks in the side burst portion. I did it. In practice, in order to improve the shape of the peak, the relation curve (interferogram) f (Δ) is once subjected to cosine Fourier exchange, and the inverse exchange is performed to obtain a curve (cepstrum) Cf (Δ). The SOI film thickness was determined only from this curve Cf (Δ).

When the method of the present invention is applied to the measurement of the SOI film thickness of an SOI substrate having no buried layer, that is, a relation curve (interferogram) f (Δ ) Is cosine-Fourier-exchanged and further inverse-exchanged, and a curve (cepstrum) Cf (Δ) and a relational curve f (Δ) are similarly sine-Fourier-transformed, and a curve Sf obtained by inversely transforming the same.
(Δ) and the result obtained by measuring the SOI film thickness using a composite curve Y (Δ) obtained by synthesizing the SOI film with the method proposed earlier, that is, the SOI using only the curve (cepstrum) Cf (Δ). The results obtained when measuring the film thickness were substantially the same. This is based on the fact that the absorption of the infrared light in the SOI substrate having no buried layer is small, so that the contribution of the curve Sf (Δ) can be almost neglected. It goes without saying that the same value can be obtained by using either Sf (Δ) or the composite curve Y (Δ).

On the other hand, when the buried layer exists in the SOI film, that is, when the influence of infrared light absorption in the SOI substrate cannot be ignored, the contribution of the curve Sf (Δ) cannot be ignored. Therefore, if the composite curve Y (Δ) is not the exact S
The OI film thickness cannot be determined, and the present invention has been made by paying attention to this point.

[0021]

An embodiment of the present invention will be described below with reference to the accompanying drawings.

According to the present invention, the SOI substrate 11 shown in FIG.
The FTIR method is applied to the measurement of the thickness of the OI film 12, and the bottom of the SOI film 12 adjacent to the dielectric layer 13 has a buried layer having a dopant concentration of 1 × 10 ¹⁷ atoms / cm ³ or more (high impurity concentration). Layer 12A is formed.

By the way, the present inventors have found that the thickness of the oxide film constituting the dielectric layer is 0.5, 1.0, 1.5, 2.0,
An SOI substrate having a buried layer formed by diffusing Sb as an impurity to a sheet resistance of 10Ω / □ and a thickness of 3.0 μm in an SOI film having a thickness of 2.5 μm and a thickness of 30 μm; SO without layer
The I-substrate is irradiated with interference light obtained by continuously changing the optical path difference Δ between the fixed mirror and the moving mirror (see FIG. 6) of the Michelson interferometer in FTIR, and the optical path difference-reflected infrared light intensity curve (Interferogram) f (Δ) is obtained, and then the curve f (Δ) is cosine Fourier-exchanged, and the curve (cepstrum) Cf is obtained by performing the inverse exchange.
(Δ) and similarly the curve f (Δ) is subjected to a sine Fourier transform,
Further, a composite curve Y (Δ) obtained by synthesizing a curve Sf (Δ) obtained by performing the inverse conversion with the curve Sf (Δ):

[0024]

Y (Δ) = √ (Cf (Δ) ² + Sf (Δ) ² ) The results are shown in FIGS. 2 and 3, respectively (FIG.
Is a composite curve Y for an SOI substrate having a buried layer
(Δ), and FIG. 3 shows a composite curve Y (Δ) for an SOI substrate without an embedded device, respectively).

Next, each SOI substrate is cleaved and its cross section is observed with an SEM to measure the thickness of the SOI film for each SOI substrate. Which position of the curve Y (Δ) (optical path difference Δ)
I checked whether it corresponds. As a result, although the shape of the composite curve Y (Δ) differs depending on the presence or absence of the buried layer and the thickness of the oxide film, the SOI film thickness of both the SOI substrate having the buried layer and the SOI substrate having no buried layer is obtained. Corresponds to the peak position existing in the side burst portion on the composite curve Y (Δ). In particular, when there are a plurality of peak positions, it has been found that the peak position having the smallest absolute value of the optical path difference Δ corresponds to the SOI film thickness.

Accordingly, the present inventors have determined that the composite curve Y (Δ)
In the side-burst section of the above, the positions (optical path differences Δ) and the magnitudes of all peaks are stored in a computer, and among the peaks having a size of 30% or more of the maximum peak size, the optical path difference Δ is the largest. The SOI film thickness corresponding to the peak position having a small absolute value was displayed on a CRT of a computer.

Here, the reason why the above result is obtained will be considered.

Generally, infrared light is easily absorbed in a silicon layer having a high impurity concentration. Therefore, the SOI of the SOI substrate
When the film thickness is measured by the FTIR method, if a high impurity concentration layer is present at the boundary between the SOI film and the dielectric layer, some infrared light is absorbed in the high impurity concentration layer. Therefore, the reflection of infrared light at the interface between the SOI film and the dielectric layer is:
This is different from the case where there is no light absorption band (that is, there is no high impurity layer).

In the case of FTIR, a relation curve (interferogram) f (Δ) between the optical path difference Δ between the fixed mirror and the movable mirror of the Michelson interferometer and the reflected light intensity is subjected to a sine Fourier transform. Is again transformed into a curve Sf
(Δ) is called the imaginary part, which is considered to represent the effect when there is light absorption on the optical path.

By the way, the method proposed by the present inventors as a method for measuring the thickness of an SOI film having no buried layer is based on the optical path difference Δ between the fixed mirror and the movable mirror of the Michelson interferometer in FTIR and the reflected light. An intensity relationship curve (interferogram) f (Δ) is obtained, and an SOI film thickness is obtained from a peak position (optical path difference Δ) having the smallest absolute value of the optical path difference Δ among a plurality of peaks in the side burst portion. I did it. In practice, in order to improve the shape of the peak, the relation curve (interferogram) f (Δ) is once subjected to cosine Fourier exchange, and the inverse exchange is performed to obtain a curve (cepstrum) Cf (Δ). The SOI film thickness was determined only from this curve Cf (Δ).

When the method of the present invention is applied to measurement of an SOI film layer of an SOI substrate having no buried layer, that is, a relation curve (interferogram) f (Δ ) Is cosine-Fourier-exchanged and further inverse-exchanged, and a curve (cepstrum) Cf (Δ) and a relational curve f (Δ) are similarly sine-Fourier-transformed, and a curve Sf obtained by inversely transforming the same.
(Δ) and a result obtained by measuring the SOI film thickness using a composite curve Y (Δ) obtained by synthesizing the SOI using only the method (Cepstrum) Cf (Δ) previously proposed. FIG. 4 shows a comparison with the result obtained when the film thickness was measured. FIG. 4 shows the SOI measured by the previously proposed method.
The difference (δ2−δ0) between the film thickness (δ2) and the SOI film thickness (δ0) measured by the method of the present invention is shown with respect to the dielectric film thickness. , (Δ
It can be seen that 2) substantially coincides. This is based on the fact that the absorption of the infrared light in the SOI substrate having no buried layer is small, and the contribution of the curve Sf (Δ) can be almost neglected. Sf
It goes without saying that the same value can be obtained using either (Δ) or the composite curve Y (Δ).

On the other hand, when the buried layer exists in the SOI film, that is, when the influence of infrared light absorption in the SOI substrate cannot be ignored, the contribution of the curve Sf (Δ) cannot be ignored. Therefore, if the composite curve Y (Δ) is not the exact S
The OI film thickness cannot be determined, and the present invention has been made by paying attention to this point.

Here, a specific example of SOI film thickness measurement by the method of the present invention will be described.

The SOI substrate to be measured has a diameter of 12
A 5 mm, N-type <100> single-crystal substrate (corresponding to the support substrate layer 14 in FIG. 1) was doped with Sb as a sheet resistance of 1
After spreading to 0 Ω / □ and thickness of 3.0 μm,
Bonding another single-crystal silicon wafer on which three types of oxide films of 3 μm are formed, removing the oxide film on the exposed surface of the another single-crystal silicon wafer after bonding to expose the single-crystal surface,
This single crystal plane was obtained by grinding to form an SOI film thinned to a thickness of 4 to 35 μm.

With respect to a plurality of SOI substrates having different SOI film thickness and oxide film thickness obtained as described above, the SOI film thickness was measured by the method of the present invention. Thereafter, each SOI substrate was cleaved, and the cross section was observed by SEM to measure the SOI film thickness.

FIG. 5 shows SO measured by the method of the present invention.
The deviation between the I film thickness (δ0) and the SOI film thickness measured using the SEM is shown. According to this figure, when viewed overall without layering by oxide film thickness, the phase difference between the two data The relationship number is 0.999, which indicates that there is a very high correlation between the two. This proves the correctness of the method of the present invention. According to the method of the present invention, the SOI film thickness having the buried layer is measured non-destructively and with high accuracy regardless of the oxide film thickness. Can be.

As a method of manufacturing an SOI substrate, a single crystal or polycrystalline silicon wafer A is used as a dielectric, and the wafer A is oxidized to another single crystal silicon wafer B having a buried layer on one of its surfaces. After forming the film, the side of the heavily doped layer of the wafer B is
After the wafer B is bonded to the wafer B, the oxide film on the side opposite to the bonded portion is removed to expose a single crystal surface, and the surface is further thinned by grinding, polishing, or the like to form an SOI film. Is adopted.

[0038]

As is apparent from the above description, according to the present invention, the interference light obtained by continuously changing the optical path difference Δ between the fixed mirror and the movable mirror constituting the Michelson interferometer is converted on the SOI substrate. Path difference-reflected infrared light intensity curve f obtained by irradiating
A curve Cf (Δ) obtained by cosine Fourier transform and inverse transform of (Δ) and a curve Sf (Δ) obtained by sine Fourier transform and inverse transform of the same curve f (Δ) are obtained. In the curve Y (Δ), the thickness of the single crystal thin film is determined from the optical path difference of the peak having the smallest absolute value of the optical path difference among a plurality of peaks existing in the optical path difference region corresponding to the thickness of the single crystal thin film. The thickness of a single crystal thin film (SOI film) having a high concentration doped layer (buried layer) with a dopant concentration of 1 × 10 ¹⁷ atoms / cm ³ or more in a portion adjacent to the dielectric layer is measured non-destructively and with high accuracy. The effect that it can be obtained is obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an SOI substrate having a buried layer.

FIG. 2 is a diagram showing a composite curve Y (Δ) for an SOI substrate having no buried layer.

FIG. 3 is a diagram showing a composite curve Y (Δ) for an SOI substrate having a buried layer.

FIG. 4 shows the SOI film thickness δ2 measured by the previously proposed method and the SOI film thickness δ0 measured by the method of the present invention.
FIG. 6 is a diagram showing a difference (δ2−δ0) with respect to a dielectric film thickness.

FIG. 5 shows the SOI film thickness (δ) measured by the method of the present invention.
FIG. 11 is a diagram showing a deviation with respect to the SOI film thickness measured using the SEM of FIG.

FIG. 6 is a basic configuration diagram of a silicon epitaxial layer thickness measuring system by the FTIR method.

FIG. 7 is a diagram showing a state of reflection of infrared light on a silicon epitaxial layer.

FIG. 8 is a diagram showing a relationship between an optical path difference and a reflected infrared light intensity in a silicon epitaxial substrate.

FIG. 9 is a diagram showing a deviation of the SOI film thickness (δ2) measured by the previously proposed method from the SOI film thickness measured by using the SEM.

[Explanation of symbols]

 Reference Signs List 1 light source 2 fixed mirror 3 movable mirror 11 SOI substrate 12 SOI film 12A buried layer (high impurity concentration layer) 13 dielectric layer

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Masatake Katayama 2-13-1, Isobe, Annaka-shi, Gunma Shin-Etsu Semiconductor Co., Ltd. Inside Semiconductor Isobe Research Laboratory (56) References JP-A-55-78203 (JP, A) JP-A-61-140806 (JP, A) JP-A-2-152250 (JP, A) JP-A-3-110405 (JP, A) JP-A-4-120404 (JP, A) JP-A-4 −236306 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01B 11/00-11/30 102 H01L 21/64-21/66

Claims (3)

(57) [Claims] 1. A dielectric material having a dopant concentration of 1 × 10 ¹⁷ at
A method of measuring the thickness of the single crystal thin film on an SOI substrate formed by bonding a single crystal thin film having a highly doped layer of oms / cm ³ or more on a dielectric substrate using a Fourier transform infrared spectrophotometer. Then, an optical path difference-reflected infrared light intensity curve f obtained by irradiating the SOI substrate with interference light obtained by continuously changing the optical path difference Δ between the fixed mirror and the movable mirror constituting the Michelson interferometer f A curve Cf (Δ) obtained by cosine Fourier transform and inverse transform of (Δ) and a curve Sf (Δ) obtained by sine Fourier transform and inverse transform of the same curve f (Δ) are obtained. Curve Y (Δ): Y (Δ) = √ (Cf (Δ) ² + Sf (Δ) ² ) where, among a plurality of peaks existing in the optical path difference region corresponding to the thickness of the single crystal thin film, the absolute value of the optical path difference Is characterized in that the thickness of the single crystal thin film is determined from the optical path difference of the smallest peak. A method for measuring the thickness of a single crystal thin film on an SOI substrate. 2. The SOI substrate uses a single crystal silicon wafer A having an oxide film formed on its surface as a dielectric substrate, and the wafer A has a dopant concentration of 1 × 10 ¹⁷ atoms.
Another single crystal silicon wafer B having a heavily doped layer of s / cm ³ or more is joined on the heavily doped layer side to form a wafer B.
2. The method for measuring the thickness of a single crystal thin film on an SOI substrate according to claim 1, wherein the thickness is obtained by thinning the exposed surface side of the SOI substrate. 3. The SOI substrate uses a single crystal or polycrystalline silicon wafer A as a dielectric substrate, and has another heavily doped layer having a dopant concentration of 1 × 10 ¹⁷ atoms / cm ³ or more on one of its surfaces. Single crystal silicon wafer B
After an oxide film is formed on the wafer B, the high-concentration doped layer side of the wafer B is joined to the wafer A, and the wafer B is subjected to removal of the oxide film on the exposed surface to expose a single crystal face, and then to a thin film. 2. The method according to claim 1, wherein the layer is obtained by layering.
A method for measuring the thickness of a single-crystal thin film on an SOI substrate as described in the above.