JP3420801B2 - Thin film evaluation method and evaluation apparatus - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film evaluation method and evaluation apparatus, and more particularly to a method and apparatus for evaluating a thin film state by interference of light reflected from the thin film.

[0002]

2. Description of the Related Art A functional material having a thin film structure is widely used. For example, in the field of a bipolar transistor or a bipolar IC, a silicon epitaxial growth technique is used, and the formation state of the thin film has a great influence on the quality. By the way, the film thickness of the thin film is not strictly constant not only macroscopically but also microscopically, and has variations. In the past, it was admired that the variation in quality was related only to macroscopic variation, and the in-plane distribution was measured and evaluated by mapping measurement and the like.

Conventionally, as a general method for measuring the thickness of a thin film, there is a method of breaking the thin film and observing a cross section with a scanning microscope, or a method of measuring the electric resistance value and estimating the film thickness. It was These methods can measure the film thickness relatively directly, but in any case, they involve destruction of the thin film and are difficult to use for actual product management, whether they are for research purposes only. It was On the other hand, using a Fourier transform spectrophotometer, the interference light of the light reflected from the front and back surfaces of the thin film is obtained, and the film thickness is evaluated from the side burst information generated depending on the film thickness on the interferogram. Is also being developed.

This is excellent in that it does not cause destruction of the thin film, but on the other hand, the measurement accuracy of the thin film is low and the correlation with the SR value that is generally used for measuring the film thickness of the thin film is low. was there. Furthermore, even microscopic variations are rarely concerned, and were not targeted for measurement in any of the above methods. However, due to the need for more strict control of the quality of thin film materials, in addition to the average film thickness, microscopic variations (distribution) have recently become apparent.
Are also interested in, and the measurement method is being sought.

In the field of semiconductor devices such as ICs, the function is created by doping the substrate by thermal diffusion, but since it has a film structure, its film thickness can be measured. Has become essential. However, when the film structure is formed by doping,
A strict boundary surface is not formed, but a boundary layer having a continuous concentration gradient with doped species is formed. For such a structure, the “film thickness” required to be measured is the thickness of the boundary layer, the thickness up to the boundary layer defined in such a manner that the impurity concentration is 1/1000 level, The measurement is substantially the same as the view of the distribution of the average film thickness.

Under the circumstances as described above, it has become necessary to measure not only the film thickness that has been measured until now, but also the distribution thereof. In the conventional film thickness measuring method, an average value can be obtained. However, it was impossible to obtain the distribution. Further, when there is a local variation in the film thickness in the minute area range of the thin film, the importance of the measurement is increasing.

Therefore, the transition region of the epitaxial wafer has been conventionally evaluated (Japanese Patent Laid-Open No. 2-143).
543). That is, when light is reflected and transmitted through the thin film sample, the reflected light on the front surface and the back surface of the film interfere with each other, and an interference waveform is observed. Therefore, we obtained a scanning interferogram,
The transition region of the epitaxial wafer is evaluated by measuring the peak-to-peak or peak-to-bottom distance of one of the side burst waveforms.

[0008]

However, the above-mentioned conventional film thickness evaluation method has not always obtained satisfactory results, and a more accurate thin film evaluation method has been demanded.
The present invention has been made in view of the above-mentioned problems of the prior art, and an object thereof is to provide a film thickness evaluation method and an evaluation device capable of performing an appropriate evaluation of a film thickness doped by epitaxial growth or thermal diffusion. .

[0009]

In order to achieve the above-mentioned object, a thin film evaluation method according to the present invention comprises irradiating a thin film sample with light and reflecting or transmitting light reflected or transmitted on the front and back surfaces of the thin film. The peak envelope of the interference light spectrum of
From the characteristics due to the distribution of the depth of the reflecting surface of light at the tangent line
It is characterized in that the film thickness of the thin film is evaluated. Here, it is preferable to obtain the distribution of the film thickness from the attenuation term of the envelope. Further, it is preferable to calculate the thin film thickness proportional to the reciprocal of the wave number from the wave number at which the amplitude becomes zero due to the envelope characteristic.

It is also preferable to handle the distribution of the thin film as a Gaussian characteristic. On the other hand, the thin film evaluation apparatus according to the present invention includes a light beam irradiation means, a detection means, an envelope calculation means, and an evaluation means. And, the luminous flux irradiating means,
A thin film sample is irradiated with a light beam for obtaining a spectrum.

Further, the detecting means detects the interference light due to the transmitted or reflected light from the thin film sample. The envelope calculating means obtains the envelope characteristic of the peak of the spectrum of the interference light from the spectrum information obtained from the detecting means. The evaluation means is for the envelope light output from the envelope calculation means .
From the characteristics resulting from the distribution of the reflection surface depth, the film thickness of the thin film
Carry out the evaluation.

[0012]

The present inventors have paid attention to the attenuation of the interference spectrum when proceeding with the examination of the film thickness evaluation method by the interferometry method. That is, the amplitude of the interference waveform used for film thickness evaluation generally becomes smaller as the wavelength becomes shorter. This is because many substances tend to absorb light in the short wavelength region, and when light absorption occurs, the reflected light on the back surface is attenuated, which may cause attenuation of the amplitude of the interference waveform. In fact, in an absorbing film, an interference waveform attenuated by the absorption is observed.

However, even in a film which seems to have no absorption, the amplitude is attenuated as the wavelength becomes shorter. Then, the present inventors have found that this attenuation characteristic, particularly the envelope characteristic of the interference spectrum waveform expressed by the wave number and the amplitude suggests the thin film characteristic, and the film thickness or the film thickness in a minute region is more than the envelope characteristic. We decided to find the distribution.

[0014]

1 is a schematic view of a thin film evaluation apparatus according to an embodiment of the present invention. A film thickness evaluation apparatus 10 shown in the figure includes a spectroscope 12 as a luminous flux irradiating means, a reflecting mirror 16 for guiding the luminous flux emitted from the spectroscopic means 12 onto a measured object (wafer) 14, and a measured object. And a reflecting mirror 20 for guiding the reflected light from the object 14 to the detecting means 18.

The spectroscope 12 is a Fourier transform spectroscope in this embodiment, and includes a light source 21, a beam splitter 22, a fixed mirror 24, and a movable mirror 26. The light beam from the light source 20 is split into two by the beam splitter 22, one of which is reflected by the fixed mirror 24 and returned to the beam splitter 22, and the other of which is reflected by the moving mirror 26 and similarly returned to the beam splitter 22. . Then, the beam splitter 22 combines the reflected lights from the two reflecting mirrors 24 and 26 and emits the interference light beam toward the reflecting mirror 16.

The interferogram signal (FIG. 2) output from the detecting means 18 is Fourier transformed by the Fourier transforming means 30 to obtain a spectrum signal (FIG. 3). Then, the envelope calculating means 32 obtains the envelopes 33a and 33b of the interference spectrum waveform expressed on this spectrum. Then, in the present embodiment, a wafer obtained by doping by epitaxial growth or thermal diffusion is used as an object to be measured, and the evaluation means 34 performs thin film evaluation from envelope information. The evaluation result is displayed on the display 36, and the suitability of the wafer manufacturing state is shown.

Next, an example of measuring the film thickness distribution of a wafer doped by epitaxial growth or thermal diffusion using the film thickness evaluation apparatus according to the present invention will be described in more detail. Film Thickness Distribution First, the evaluation of the film thickness distribution by the apparatus shown in FIG. 1 will be described. Further, as shown in FIG. 4, even when the film component 52 is formed on the specific substrate 50 by epitaxial growth and a relatively clear boundary surface 54 is present and exists, microscopically, the unevenness of the boundary surface exists. However, the unevenness causes variations in the depth of the reflecting surface.

Therefore, also in the case of FIG. 4, it is necessary to grasp the unevenness of the boundary surface as the film thickness distribution. In this case, the inventors have found that the envelope is very closely related to this film thickness distribution. That is, in evaluating the film thickness distribution, the evaluation means 24 performs the following calculation. For simplification, consider a case where a light beam is incident vertically as shown in FIG.

The reflected light A ₁ on the front surface of the thin film and the reflected light A ₂ on the back surface of the thin film have a phase difference of 4πnd / λ because of the optical path difference 2nd corresponding to the film thickness of the film 52 having a refractive index n. If nd = D,

## EQU1 ## A ₁ = a ₁ expiωt A ₂ = a ₂ expi (ωt + 4πD / λ) The intensity I (λ, D) of the interference light is

## EQU2 ## I (λ, D) = (A ₁ + A ₂ ) ・ (A ₁ + A ₂ ) ^* = a ₁ ² + a ₂ ² + 2a ₁ a ₂ cos 4πD / λ Meanwhile, D is Ρ _(D) d _D Given a distribution,

[Equation 3] It should be noted that the above-mentioned Ρ _(D) has a Gaussian distribution shown in Equation 4.

[0020]

[Equation 4] In this case, the actually observed interference light intensity is

[Equation 5] It is represented by. When there is no variation in film thickness, that is, σ _p
= 0, the interference light intensity is

[Equation 6] And the damping term is

Equation 7] becomes q (λ) = exp-8π 2 (σ D / λ) 2.

Next, when the film thickness distribution is obtained from the actually measured interference waveform, the attenuation term can be obtained as envelopes 33a and 33b of the interference waveform as shown in FIG. It is assumed that the value of this attenuation term is obtained at an appropriate wavelength λ _i , i = 1 to n, and this is q (λ _i ). In this case, λ _i , q (λ _i )
Is

Since q (λ _i ) = exp−8π ² (σ _D ² / λ _i ) ² is satisfied,

Lnq (λ _i ) =-8π ² σ _D ² (1 / λ _i ) ² and the measured values (λ _i , q (λ _i )) are plotted along the horizontal axis (1 /
When plotted as λ _i ) ² and the vertical axis lnq (λ _i ), FIG.
As shown in, they will be arranged on a straight line with a negative slope.

If this inclination is G,

## EQU10 ## G = -8π ² σ _D ² , and the standard deviation σ _D is

## EQU11 ## σ _D = {-G / 8π ² ) ^1/2 .

This σ _D is the standard deviation of the optical path difference D = nd. To obtain the standard deviation σ _d of the film thickness d, σ _D is defined as the refractive index n.
It is necessary to divide by.

According to the present embodiment in the Equation 12] σ _d = σ _D / n above, it is possible to determine the film thickness distribution from the envelope of the interference light spectrum.

In addition to the intensity change due to the interference, the interference waveform is overlapped with the change due to the absorption of the sample and the wavelength dependence of the reflectance. Since these depend on each sample, it is difficult to handle them in general, and it may be necessary to exclude them when estimating q (λ _i ). Therefore, q (λ _i ) and −q from the upper and lower envelopes at a certain wavelength λ _i .
Estimate (λ _i ) and take the difference

[Equation 13] It is easiest to obtain as q (λ _i ) − [− q (λ _i )] = 2q (λ _i ).

Further, when the slope of the plot of {(1 / λ _i ) ² , lnq (λ _i )} is obtained, it is general to apply a straight line by the method of least squares. In this case, the error [ln
It is formulated so that the sum of squares of q (λ _i ) − {k ₁ (1 / λ _i ) ² + k ₂ ] is minimized. However, in reality q
It is considered that the smaller (λ _i ) is, the lower the relative reliability is, and the application of the straight line as the envelope may not be appropriate.

In such a case, a non-linear envelope type application can be considered. That is, if σ _D ² = S,

[Mathematical formula-see original document] If q (λ _i ) = exp−8π ² / λ _i ² · S S is improved by its initial value S ₀ and the variation ΔS,

F _(s) = exp-8π ² / λ _i ² · S

F (S ₀ + ΔS) = f (S ₀ ) + (df / dS) ₀ · ΔS + Φ = {1- (8π ² / λ _i ² ) · ΔS) exp-8π ² / λ _i ² · S ₀ and the residual sum of squares R is

[Equation 17] From dR / dΔS = 0,

[Equation 18] Is obtained.

Then, based on the calculation flow chart shown in FIG. 8, the nonlinear envelope attenuation characteristic can be obtained by performing the calculation until ΔS becomes substantially zero. Next, an example in which the film thickness distribution evaluation method according to the present embodiment is actually used will be described. As shown in FIG. 9, when the transmission spectrum of the thin film sample is measured, a spectrum as shown in FIG. 10 is obtained. In the figure, 1000 to 250
The waveform seen in the region of 0 nm is the interference waveform, and the amplitude becomes smaller as the wavelength becomes shorter.

This interference waveform represents the intensity distribution of the light which the directly transmitted transmitted light A ₁ ′ shown in FIG. 9 interferes with the transmitted light A ₂ ′ which has been reflected once on the front surface and once on the rear surface. Here, assuming that the film thickness D = nd has the Gaussian distribution, this intensity is shown in the above equation 5, and the attenuation term is as shown in the above equation 7. Attenuation term q (λ) at each wavelength
In order to estimate, the wavelength and intensity of peak and valley positions of the interference wavelength were first obtained (see FIG. 11). Note that the wavelength uses the position of the valley (λ _i : i = 1 to 1
n).

The value of the lower envelope at this wavelength is I
(Λ _i ). On the other hand, since it is not possible to directly obtain the value X of the upper envelope, as shown in FIG.
It is obtained by proportional distribution from the peak values on both sides of the wavelength. If each value is as shown in Fig. 11,

[Formula 19] Therefore, the attenuation term q (λ _i ) at the wavelength λ _i is

[Equation 20] 2q (λ _i ) = X−I (λ _i ).

From here, ln (2q (λ _i )), (1000
When / λ _i ) ² is calculated and the envelope is obtained, the result is as shown in FIG. 12, and the inclination thereof is -2.7291. −8π ² σ _D ² = −2.7291 Therefore, σ _D = 0.186 (μm) σ _D = 0.186 / n (μm). As described above, according to the film thickness distribution evaluation method according to the present embodiment, it is possible to obtain the variation in the film thickness in a minute region as, for example, the standard deviation.

Film Thickness Measurement The diffusion wafer 14 to be measured in this example is
As shown in FIG. 5, an I layer (non-diffusion layer) 50 made of a silicon single crystal substrate and a diffusion layer 52 formed by thermal diffusion
Consists of. Then, due to the nature of the diffusion process, the interface between the diffusion layer 52 and the non-diffusion layer 50 reflects the concentration profile of the diffusion impurities and has a spread in the depth direction. When this diffused wafer is irradiated with light, multiple reflection occurs with a certain impurity concentration (about 5 × 10 ¹⁷ atom / cm ³ ) as an optical interface as described above. Therefore, the film thickness can be evaluated by measuring the depth to this optical boundary surface.

However, this optical interface 54 is
Since it is located deeper than the diffusion layer thickness X _i defined by the R method, the diffusion layer thickness X _i ′ by the interferometry becomes thick. FIG. 13 shows the diffusion layer thickness X defined by the SR method.
_i and shows the relationship of the measured diffusion layer thickness X _i 'by interferometry.

From the figure, the diffusion layer thickness X _i 'by the interferometry is not only measured thicker than the diffusion layer thickness X _i defined by the SR method, but there is a problem in the correlation between the two.
Here, the total thickness X _{0 of the} wafer is unknown. Therefore, the inventors of the present invention focused on the envelope, and calculated the reciprocal of the wave number at the intersection of the envelopes 33a and 33b in FIG.
It was found that the correlation with the layer thickness X _i by the R method is extremely high (see FIG. 14).

On the other hand, the present inventors have found that the impurity concentration distribution of the diffusion wafer generally exhibits a Gaussian distribution. Then, as shown in FIG. 5, the maximum impurity concentration of the wafer: N ₀ = 10 ²⁰ atoms / cm ³ The impurity concentration at the optical interface 54 by the interferometry: N ₁
= 5 × 10 ¹⁷ atoms / cm ^{3 Since} the impurity concentration in the diffusion layer is N ₂ = 10 ¹⁴ atoms / cm ³ , the P ₁ point of the Gaussian curve is X = 0, N ₀ = 10 ²⁰
First, it is specified as atoms / cm ³ , and then the point P ₂ is specified by obtaining X _j from the wavelength σ of the envelope intersection point from the relational expression shown in FIG. As a result, the Gaussian curve L is specified, and the X coordinate position X _j ′ of the optical interface 54 can be calculated.

As a result, the film thickness X _i to be obtained is

It can be calculated by X _i = X _i ′ − (X _j −X _j ′). As a result, the film thickness X _i of the diffusion layer obtained by the SR method is calculated from X _i ′ obtained by the interference method,
It can be calculated.

As described above, the diffusion layer thickness X _i defined by the SR method can be accurately calculated from the wave number σ at the intersection of the envelopes of the interference light spectrum obtained from the thin film and the correction equation of the above equation 21. it can. Incidentally, X _i shown in FIG.
The correlation between X _i 'and X _i ' is corrected as shown in FIG. 16, and it is understood that the two agree very well. In the above embodiments, an example using a Fourier transform type spectroscopic means has been described, but the present invention is not limited to this.
For example, the spectrum can be directly obtained by using a dispersive spectroscopic means.

[0037]

As described above, according to the film thickness evaluation method and the evaluation device of the present invention, the thin film is evaluated from the envelope characteristic of the interference light spectrum. It is possible to accurately and accurately obtain the thin film information of.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a schematic configuration of a thin film evaluation apparatus according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of an interferogram obtained by the device shown in FIG.

FIG. 3 is an explanatory diagram of an interference light spectrum and its envelope obtained from the interferogram shown in FIG.

FIG. 4 is an explanatory diagram of a reflection state in a thin film having a clear boundary surface.

FIG. 5 is an explanatory diagram of a reflection state in a thin film having a boundary layer.

FIG. 6 is an explanatory diagram of a measurement state of a film thickness distribution.

FIG. 7 is a correlation diagram between a wavelength and an envelope.

FIG. 8 is a flow chart diagram in the case of calculating a non-linear envelope.

FIG. 9 is an explanatory diagram of a thin film evaluation state according to an example of the present invention.

FIG. 10 is an interference spectrum diagram of the thin film shown in FIG.

11 is an explanatory diagram of how to obtain an envelope curve in the interference spectrum diagram shown in FIG. 10. FIG.

12 is a correlation diagram of a wavelength envelope based on the interference spectrum diagram shown in FIG.

FIG. 13 is a comparison diagram of a film thickness and an SR value based on an interference method according to a conventional method.

FIG. 14 is a correlation diagram between the reciprocal of the wave number at the intersection of the envelopes and the SR value.

FIG. 15 is an explanatory diagram of the impurity concentration in the thin film.

FIG. 16 is a correlation diagram between a film thickness correction value and an SR value according to the present invention.

[Explanation of symbols]

10 ... Thin film evaluation means 12 ... Spectrometer (light flux irradiation means) 18 ... Detection means 32 ... Envelope computing means 34 ... Evaluation means

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01B 11/00-11/30 G01N 21/00-21/88

Claims (5)

(57) [Claims] 1. A thin film evaluation method for irradiating a thin film sample with light to evaluate a thin film from characteristics of interference light of each reflected or transmitted light reflected or transmitted on the front surface and the back surface of the thin film, Of the light at the envelope of the spectral peak
A thin film evaluation method, characterized in that the film thickness of a thin film is evaluated from the characteristics resulting from the distribution of the depth of the reflecting surface . 2. The thin film evaluation method according to claim 1, wherein a film thickness distribution is obtained from an attenuation term of the envelope. 3. The thin film evaluation method according to claim 1, wherein the thin film thickness proportional to the reciprocal of the wave number is calculated from the wave number at which the amplitude becomes zero due to the envelope characteristic. 4. The thin film evaluation method according to claim 1, wherein the distribution of the thin film is treated as a Gaussian characteristic. 5. A light beam irradiating means for irradiating a thin film sample with a light beam for obtaining a spectrum, a detecting means for detecting an interference light due to transmitted or reflected light from the thin film sample, and spectrum information obtained from the detecting means. And envelope reflection means for obtaining the envelope characteristic of the peak of the spectrum of the interference light, and the light reflection surface of the envelope output from the envelope calculation means
A thin film evaluation apparatus comprising: an evaluation unit that evaluates the film thickness of the thin film based on the characteristics caused by the depth distribution .