JPH05306910A - Method for measuring film thickness of single-crystal thin film on soi substrate - Google Patents

Abstract

PURPOSE:To highly accurately measure an SOI film having a high-concentration doped layer (buried layer) by using the resultant curve of a curve obtained by performing cosine Fourier transformation and inverse transformation on a curve representing the relation between optical path differences and intensities of reflected infrared rays and another curve obtained by performing sine Fourier transformation and inverse transformation on the same curve. CONSTITUTION:The film thickness of an SOI film (single-crystal film) 12 is found from the peak optical path difference at which the absolute value of the optical path difference DELTAbetween a fixed mirror and mobile mirror constituting a Michelson's interferometer among a plurality of peaks existing in the optical path difference area corresponding to the film thickness of the film 12 by using the resultant curve Y(DELTA) of a curve Cf(DELTA) obtained by performing cosine Fourier transformation and inverse transformation on a curve f(DELTA) representing the relation between the optical path differences DELTA and intensities of reflected infrared rays and obtained by irradiating an SOI substrate 11 with interference light obtained by continuously changing the optical path difference DELTA and another curve Sf(DELTA) obtained by performing sine Fourier transformation and inverse transformation on the curve f(DELTA). Therefore, the film thickness of the film 12 which has a high-concentration doped layer (buried layer) having a dopant concentration of >=1X10<17>atms/cm<3> and is formed adjacent to a dielectric layer 13 can be measured with high accuracy in a nondestructive state.

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 a dielectric substrate by measuring the single crystal thin film on an SOI substrate by using a Fourier transform infrared spectrophotometer. Regarding 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, a large number of crystal defects occur in the silicon vapor phase growth due to the mismatch of the lattice constants between the dielectric substrate and the vapor-grown silicon single crystal. Practicality is poor.

Therefore, in recent years, SOI (Si On Insulator)
A bonded substrate having a structure (hereinafter referred to as an SOI substrate) has come to be particularly noted. In this SOI substrate, for example, at least one of the two single crystal silicon wafers is oxidized to form an oxide film on the surface of at least one of the wafers,
After laminating these two wafers so that the oxide film serves as an intermediate layer, they are heated to a predetermined temperature to bond them, and one of the wafers is surface-ground and then the surface thereof is further polished. This can be obtained by thinning it into a single crystal silicon thin film (hereinafter referred to as an SOI film).

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

However, the film thickness measurement by the above-mentioned spectral interferometry is effective for the SOI film having a film thickness of about 10 μm or less, but when the film thickness of the SOI film exceeds the above-mentioned thickness, it becomes visible light silicon. Measurement becomes impossible due to the absorption of.

On the other hand, as another method for measuring the film thickness,
Fourier transform infrared spectrophotometer (hereinafter referred to as FTIR)
There is known a method (hereinafter, referred to as FTIR method) using the above method. In this method, the thickness of a silicon epitaxial layer formed on a supporting substrate layer of silicon having a doping concentration of about 10 ¹⁸ atoms / cm ³ or more is used. 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. 6 to 8. 6 is a basic configuration diagram of a film thickness meter by the FTIR method, FIG. 7 is a diagram showing a state of reflection of infrared light on a silicon epitaxial layer, and FIG. 8 is an optical path difference-reflected infrared light intensity curve diagram. is there.

As shown in FIG. 6, an infrared ray generating lamp 1
The continuous infrared light having a wavelength of 2.5 to 25 μm generated by is converted into interference light by using a Michelson interferometer composed of the fixed mirror 2, the movable mirror 3 and the beam splitter 4, and this interference light is placed on the silicon substrate 5. The epitaxial layer 6 is irradiated.

Incidentally, as shown in FIG. 7, the reflected light R of incident infrared light (Michelson interference light) L on the silicon substrate 5 is composed of various components. Among them, the main ones are those reflected by the surface c of the epitaxial layer 6 to be reflected light R1 and those transmitted through the epitaxial layer 6 and reflected again at the interface g between the epitaxial layer 6 and the supporting substrate layer 7. , The silicon substrate 5 through the surface h of the epitaxial layer 6
They are two that 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 moving mirror 3 (see FIG. 6) that generate incident interference light have a certain optical path difference, the combined reflected light (two reflections Light R
1, which is a composite of R2) shows a peculiar behavior. That is, a portion called a side burst (peak aggregation portion) where the reflected light intensity shows a peak value is generated 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.

Then, the present inventors found out the conditions for applying the FTIR method to the thickness measurement of the SOI film, and determined the thickness 1
We have previously proposed a method that enables nondestructive and highly accurate measurement of the thickness of single crystals with a thickness of μm or more. That is, the method is that the optical path difference-reflected infrared light intensity curve is obtained by irradiating the SOI substrate with interference light obtained by continuously changing the optical path difference between the fixed mirror and the moving mirror that configure the Michelson interferometer. This is a method of obtaining the film thickness of the single crystal thin film from the optical path difference of the minimum peak by 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. ..

By the way, for the purpose of lowering collector resistance when a bipolar transistor having an SOI structure is manufactured, as shown in FIG. 1, a dopant concentration of 1 × 10 ¹⁷ atoms / cm ³ or more is provided at the bottom adjacent to the dielectric layer 13. Having a high-concentration doped layer (hereinafter referred to as a buried layer) 12A of S
An SOI structure forming the OI film 12 is generally used. In FIG. 1, 11 is an SOI substrate and 12B is an SO substrate.
In the I film 12, a lightly doped layer, 14 is a supporting substrate layer.

[0013]

Thus, as an example, the embedded layer (sheet resistance: 10 Ω / □, thickness: 3 μm) is used.
m), the film thickness of the SOI film is measured by a scanning electron microscope (hereinafter referred to as SEM) to be 4 to 35 μm, and the result measured by the method (FTIR method) previously proposed by the present inventors. The deviation from (δ2) is shown in FIG. From the figure, it is understood that the deviation between the two becomes larger as the SOI film thickness becomes larger, and the method proposed by the present inventors cannot measure the film thickness of the SOI film having the buried layer with high accuracy. ..

The present invention has been made in view of the above problems, and an object thereof is to provide a high-concentration 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. The object of the present invention is to provide a method for measuring the thickness of a single crystal thin film on an SOI substrate, which is capable of nondestructively and highly accurately measuring the thickness of an SOI film having

[0015]

In order to achieve the above object, the present invention has a dopant concentration of 1 × 10 ¹⁷ atoms / cm 3 on the dielectric side.
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 highly doped layers on a dielectric substrate using a Fourier transform infrared spectrophotometer In addition, the 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 that configure the Michelson interferometer. A curve Cf (Δ) obtained by cosine Fourier transform and inverse transform of f (Δ) and the same curve f (Δ)
Sf obtained by performing sine Fourier transform and inverse transform of
A curve Y (Δ) obtained by synthesizing (Δ): Y (Δ) = √ (Cf (Δ) ² + Sf (Δ) ² ) in which a plurality of optical path difference regions corresponding to the single crystal thin film thickness exist Among these peaks, the film thickness of the single crystal thin film is obtained 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 a high impurity concentration layer is present at the boundary between the SOI film and the dielectric layer when the SOI film thickness of the SOI substrate is measured by the FTIR method, some infrared light may be present in this high impurity concentration layer. Be 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, there is no high impurity layer).

In the case of FTIR, the relational 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 sine Fourier transformed, and The curve Sf obtained by inverse transformation of
(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, in the method previously proposed by the present inventors as a method for measuring the film thickness of an SOI film having no buried layer, the optical path difference Δ between the fixed mirror and the movable mirror of the Michelson interferometer in FTIR and the reflected light. The relationship curve (interferogram) f (Δ) with the intensity is obtained, and the SOI film thickness is obtained from the 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. Actually, in order to improve the shape of the peak, the relational curve (interferogram) f (Δ) is once subjected to cosine Fourier exchange, and then the inverse exchange thereof is performed to obtain the curve (cepstral) Cf (Δ), The SOI film thickness is determined only by this curve Cf (Δ).

When the method of the present invention is applied to the measurement of the SOI film thickness of the SOI substrate having no buried layer, that is, the relation curve (interferogram) f (Δ) for the substrate having no buried layer. ) Is subjected to cosine Fourier exchange, and then the inverse exchange thereof is performed, and a curve (cepstrum) Cf (Δ) and a relational curve f (Δ) are also subjected to sine Fourier transform, and a curve Sf obtained by further performing the inverse transform thereof.
(Δ) and the resultant curve when the SOI film thickness is measured using the synthetic curve Y (Δ) and the previously proposed method, that is, only the curve (cepstrum) Cf (Δ) is used. When the film thickness was measured, the results were almost the same. This is because the contribution of the curve Sf (Δ) is almost negligible since the absorption of infrared light in the SOI substrate having no buried layer is small, and the curve of the SOI film thickness is the curve. It is natural that the same value can be obtained by using either Sf (Δ) or the composite curve Y (Δ).

On the other hand, when there is a buried layer in the SOI film, that is, when the influence of absorption of infrared light in the SOI substrate cannot be ignored, the contribution of the curve Sf (Δ) cannot be ignored. Therefore, if it is not the composite curve Y (Δ), the accurate S
Since the OI film thickness cannot be obtained, the present invention has been made paying attention to this point.

[0021]

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

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

By the way, the present inventors have found that the thickness of the oxide film forming the dielectric layer is 0.5, 1.0, 1.5, 2.0,
An SOI substrate having a thickness of 2.5 μm and a thickness of 30 μm, in which Sb as an impurity is diffused to have a sheet resistance of 10 Ω / □ and a thickness of 3.0 μm, and an SOI substrate having a thickness of 30 μm. SO without layers
The optical path difference-reflected infrared light intensity curve is obtained by irradiating the I substrate with interference light obtained by continuously changing the optical path difference Δ of the fixed mirror and the moving mirror (see FIG. 6) of the Michelson interferometer in FTIR. (Interferogram) f (Δ) is obtained, then the curve f (Δ) is subjected to cosine Fourier exchange, and the inverse exchange thereof is performed to obtain a curve (cepstral) Cf.
(Δ) and the curve f (Δ) are also subjected to sine Fourier transform,
Further, a synthetic curve Y (Δ) obtained by synthesizing the curve Sf (Δ) obtained by performing the inverse transformation thereof:

[0024]

## EQU1 ## Y (Δ) = √ (Cf (Δ) ² + Sf (Δ) ² ) was obtained. The results are shown in FIGS. 2 and 3, respectively (see FIG. 2).
Is a composite curve Y for an SOI substrate having a buried layer
(Δ), and FIG. 3 shows a synthetic curve Y (Δ) for an SOI substrate having no embedded device).

Next, each SOI substrate is cleaved, and the cross section thereof is observed by SEM to measure the thickness of the SOI film for each SOI substrate, and the film thickness is synthesized as shown in FIG. 2 or FIG. Which position on the curve Y (Δ) (optical path difference Δ)
I checked whether it corresponds to. As a result, the shape of the composite curve Y (Δ) varies depending on the presence or absence of the buried layer and the thickness of the oxide film, but the SOI film thickness is obtained for both the SOI substrate having the buried layer and the SOI substrate having no buried layer. Was found to correspond to the peak position existing in the side burst portion on the synthetic curve Y (Δ). In particular, it has been found that when there are a plurality of peak positions, the peak position where the absolute value of the optical path difference Δ is the smallest corresponds to the SOI film thickness.

Therefore, the present inventors have found that the composite curve Y (Δ)
In the side burst part in, the position (optical path difference Δ) and the size of all peaks are stored in a computer, and the optical path difference Δ of the peaks having a size of 30% or more of the maximum peak size is recorded. The SOI film thickness corresponding to the peak position with a small absolute value was displayed on the CRT of the computer.

Now, let us consider the reason why the above results are obtained.

In general, infrared light is easily absorbed in a silicon layer having a high impurity concentration. Therefore, the SOI of the SOI substrate
When a high impurity concentration layer exists at the boundary between the SOI film and the dielectric layer when the film thickness is measured by the FTIR method, some infrared light is absorbed in this 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, the relational 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 sine Fourier transformed, The curve Sf obtained by inverse transformation of
(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, in the method previously proposed by the present inventors as a method for measuring the film thickness of an SOI film having no buried layer, the optical path difference Δ between the fixed mirror and the movable mirror of the Michelson interferometer in FTIR and the reflected light. The relationship curve (interferogram) f (Δ) with the intensity is obtained, and the SOI film thickness is obtained from the 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. Actually, in order to improve the shape of the peak, the relational curve (interferogram) f (Δ) is once subjected to cosine Fourier exchange, and then the inverse exchange thereof is performed to obtain the curve (cepstral) Cf (Δ), The SOI film thickness is determined only by this curve Cf (Δ).

Thus, when the method of the present invention is applied to the measurement of the SOI film layer of the SOI substrate having no buried layer, that is, the relation curve (interferogram) f (Δ) for the substrate having no buried layer. ) Is subjected to cosine Fourier exchange, and then the inverse exchange thereof is performed, and a curve (cepstrum) Cf (Δ) and a relational curve f (Δ) are also subjected to sine Fourier transform, and a curve Sf obtained by further performing the inverse transform thereof.
(Δ) and the resultant curve when the SOI film thickness is measured using the synthetic curve Y (Δ) and the previously proposed method, that is, only the curve (cepstrum) Cf (Δ) is used. FIG. 4 shows a comparison with the result 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) is almost the same. This is because the contribution of the curve Sf (Δ) is almost negligible since the absorption of infrared light in the SOI substrate having no buried layer is small, and the curve of the SOI film thickness is the curve. Sf
It is natural that the same value can be obtained by using either (Δ) or the composite curve Y (Δ).

On the other hand, when there is a buried layer in the SOI film, that is, when the influence of absorption of infrared light in the SOI substrate cannot be ignored, the contribution of the curve Sf (Δ) cannot be ignored. Therefore, if it is not the composite curve Y (Δ), the accurate S
Since the OI film thickness cannot be obtained, the present invention has been made paying attention to this point.

Here, a specific example of measuring the SOI film thickness 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 supporting substrate layer 14 in FIG. 1) was doped with Sb as an impurity and had a sheet resistance
0Ω / □, thickness after diffusion to 3.0μm, thickness 1, 2,
Another single crystal silicon wafer on which three types of 3 μm oxide films are formed is bonded, and after bonding, the oxide film on the exposed surface of another single crystal silicon wafer is removed to expose the single crystal surface,
It was obtained by grinding the single crystal surface to form a thin SOI film having a thickness of 4 to 35 μm.

The SOI film thicknesses of the plurality of SOI substrates having different SOI film thicknesses and oxide film thicknesses obtained as described above were measured by the method of the present invention. After that, each SOI substrate was cleaved and the cross section thereof was observed with an SEM to measure the SOI film thickness.

FIG. 5 shows the 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 in overall without stratification by the oxide film thickness, the phase of the data of both is shown. The relation number is 0.999, and it can be seen that the two have a very high correlation. This proves the correctness of the method of the present invention, and according to the method of the present invention, the SOI film thickness having a buried layer can be measured nondestructively and with high accuracy regardless of the oxide film thickness. You can

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 the film is formed, the high-concentration doped layer side of the wafer B is transferred to the wafer A.
After the wafer B is bonded to the wafer B, the oxide film on the side opposite to the bonding part is removed to expose the single crystal surface, and the surface is further thinned by grinding, polishing or the like to form an SOI film. Is taken.

[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 which compose the Michelson interferometer can be obtained 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 film thickness of the single crystal thin film is obtained from the optical path difference of the peak having the smallest absolute value of the optical path difference among the plurality of peaks existing in the optical path difference region corresponding to the single crystal thin film thickness. , Non-destructively and highly accurately measure 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 region adjacent to a dielectric layer The effect that can be obtained is obtained.

[Brief description of drawings]

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

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

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

FIG. 4 is an SOI film thickness δ2 measured by the previously proposed method and an SOI film thickness δ0 measured by the method of the present invention.
It is a figure which shows the difference ((delta) 2- (delta) 0) with respect to a dielectric film thickness.

FIG. 5: SOI film thickness (δ measured by the method of the present invention
It is a figure which shows the deviation with respect to the SOI film thickness measured using the SEM of 0).

FIG. 6 is a basic configuration diagram of a silicon epitaxial layer thickness measuring system by an 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 optical path difference and 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 with respect to the SOI film thickness measured using the SEM.

[Explanation of symbols]

 1 Light Source 2 Fixed Mirror 3 Moving Mirror 11 SOI Substrate 12 SOI Film 12A Buried Layer (High Impurity Concentration Layer) 13 Dielectric Layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masatake Katayama 2-13-1 Isobe, Annaka-shi, Gunma Shin-Etsu Semiconductor Co., Ltd.

Claims (3)

[Claims] 1. A dopant concentration of 1 × 10 ¹⁷ at on the dielectric side.
A method for 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. Therefore, the optical path difference-reflected infrared light intensity curve f obtained by irradiating the SOI substrate with the 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 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): In Y (Δ) = √ (Cf (Δ) ² + Sf (Δ) ² ), the 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 Is characterized in that the film thickness of the single crystal thin film is obtained 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 is 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 atom.
Another single crystal silicon wafer B having a heavily doped layer of s / cm ³ or more is bonded on the heavily doped layer side to form a wafer B.
The method for measuring the film thickness of a single crystal thin film on an SOI substrate according to claim 1, wherein the exposed surface side is obtained by thinning. 3. The SOI substrate uses a single crystal or polycrystalline silicon wafer A as a dielectric substrate, and has a high concentration 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 bonded to the wafer A, and the oxide film on the exposed surface of the wafer B is removed to expose the single crystal surface, It is a thing obtained by layering, Claim 1 characterized by the above-mentioned.
A method for measuring the film thickness of a single crystal thin film on an SOI substrate as described above.