JPS62223609A - Method for measuring film thickness and refractive index of semiconductor thin film - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Example technical field r /l”t W 111 days /-) + road r
Hc j 仁f/7't μ 11 Worker W 'r
4q f-m path This invention relates to a method for measuring the thickness and refractive index of a thin film.

B) Prior Art The thickness of a semiconductor thin film laminated on a semiconductor substrate has conventionally been measured by the following method.

(1) The substrate is folded and its cross section is etched.

Etching creates steps due to different materials, so
Observe this using a microscope. This is a method suitable for relatively thick films.

(2) Rather than exposing the interfold surfaces, angle polishing is performed in a direction that makes a small angle to the surface, etching is performed to expose the boundary between the thin film and the substrate, and the polished width of the thin film is measured using a microscope. The film thickness can be found by multiplying this value by the sine sin O of the inclination angle θ. Suitable for measuring thin films.

(3) Polish the thin film and substrate into a cylindrical shape to expose the boundary. Etch the border to make it easier to see, and measure the width of the border between the top and bottom surfaces using a microscope.

Since the radius of curvature of the cylinder is known, the thickness of the thin film is A% (approximately 10'Wr7+A) (4) Calculate the film thickness by irradiating the semiconductor substrate with monochromatic light and measuring the reflected light. An ellipsometer uses this.

(1) Problems with the Prior Art Of the methods described above, (1) to (3) are destructive inspections. It is not possible to fabricate a device in the area required for inspection.

For this reason, it is not possible to obtain information on the part where the device is actually manufactured. Also, it takes time to etch the lag. Zarani (
1) had the disadvantage that there was a large thickness error when measuring the film thickness using a microscope.

Method (4) is a non-destructive inspection. Therefore, it is possible to know the film thickness of the part where the element is actually manufactured.

However, method (4) has a drawback in that it takes time and effort to find the conditions under which the reflected light becomes linearly polarized. Another drawback is that it is difficult to adjust the position and angle of the sample, making measurements time-consuming. Also, the light source had to be coherent, so a laser was essential.

A first object of the present invention is to provide a method capable of non-destructively measuring the thickness and refractive index of a thin film formed on a semiconductor substrate.
This is the purpose of

A second object of the present invention is to provide a method for measuring film thickness and refractive index that allows easy adjustment of the position and angle of the sample and does not significantly impair measurement accuracy even if there is a deviation in position or angle. be.

A third object of the present invention is to provide a film thickness measuring method that not only determines the film thickness but also the refractive index at the same time as in the method described above.

Troubled invention method The present invention uses light for non-destructive testing.

In addition, transmitted light is used for measurements to facilitate sample handling.

Monochromatic light is incident on the sample and the intensity of the transmitted light is measured. This determines the transmittance of the sample at that wavelength. The light source emits wavelength-tunable monochromatic light.

Transmittance is measured while continuously changing the wavelength of monochromatic light.

Let T(λ) be the transmittance, which varies depending on the wavelength. The film thickness and refractive index of the semiconductor thin film laminated on the semiconductor substrate can be calculated from the change in transmittance depending on the wavelength.

The principle of this method will be explained below.

As shown in FIG. 3, it is assumed that a medium P with a refractive index of no and a medium Q with a refractive index of old are in contact at a boundary S of a plane.

Assume that monochromatic light is incident from P to Q. This is divided into those that are incident on Q and those that are reflected and return to P. The electric field of the incident wave is assumed to be perpendicular to the boundary S, the electric wave of the refracted wave entering the EOlQ is assumed to be El, and the electric field of the light reflected at the boundary S is assumed to be Fo.

From the boundary conditions of the electromagnetic field near the boundary surface, Elha et al. will discuss reflected waves and refracted waves in this book.

Next, consider the case where there are three media P, Q, and λ, as shown in FIG. The refractive index of each is no.

Assume that the old name is n2.

Of the light EO entering the boundary S from P, as already mentioned,
Some of it is reflected and lost. The remaining El is incident on Q.

A part of this is emitted to K. The strength of the electric field of this light is 0
Set to 1.

However, the remainder of El is reflected at the boundary T.

A part of this light Hx is emitted to the medium P at the boundary S, and the rest is reflected. Let the intensity of this light be El.

El hits the boundary T, and a part of it is emitted. Let the electric field of this light be 02. The remainder of El is reflected at T.

This is called H2.

FIG. 5 shows the relationship among these reflected, emitted, refracted lights, etc. In order to show the optical axes separately, they are drawn diagonally, but in reality all the optical axes are perpendicular to the planes S and T.

The reflectance on one surface is n, and the reflectance on five surfaces is m. Let the thickness of the medium Q be d. Let k be the wave number in medium d. This is the predetermined wave number of the wave number in vacuum.

It is.

El, Ez, Ej, . . . are the electric fields of light traveling in the medium Q in the direction of P→Q.

Comparing Ez and El, Ez is reflected once each on the T and S planes, and also travels back and forth in the medium Q. The phase difference becomes 2 kd, and the amplitude becomes mn times. However, since the medium Q is sufficiently thin, absorption here is ignored.

Ez = El m n e ''kd (5) Similarly, E j + x has the relationship of equation (5) with respect to E j.

Now, on the plane T, the light of Ej is transmitted to the medium and becomes the light of Gj. Let the transmittance be (, which is given by. Between Gj and Ej, Gj = t Ej
(There is a power relationship.

Assume that the medium K is a substrate, and the intensity of light emitted to the back surface of the substrate is measured. After entering the medium,
It is assumed that the light emitted from the back surface is five times as large as Gj, including all losses due to absorption and reflection from the point where the light passes through K and is emitted from K. This is constant regardless of the magnitude of the electric field.

In the end, the total sum of light exiting the substrate (medium R) is:

This is a function of the wavelength λ.

Since the monochromatic light is made incident while changing its wavelength continuously, the transmitted light intensity L(λ) is determined as a function of the wavelength (S).

The refractive indices no, nl, and n2 also have wavelength dependence, but this is slight. El also varies as a function of the wavelength λ depending on the spectrum of the light source. However, what mainly changes depending on the wavelength is the 1kd e (11 part. Since Imn+<1, the denominator is (1-1mn+
), (9) becomes maximum, and the denominator becomes (1+1
mn1), (9) becomes the minimum.

Transmittance T is defined by IL/Eol.

From equations (2) and (9), this ratio is calculated as s, t,
Contains values such as no and old. That is, if the coefficient of equation (2) is set to W, the transmittance T is. T is a measurable quantity. It is assumed that nO and n2 are known. This is because the refractive index of air and the refractive index of the substrate. Since n2 is measured in advance by other means, it is assumed that it is known. The unknowns are the refraction weight and thickness d of the thin film.

The maximum transmittance Tmax is the minimum transmittance Tmin. These are measurable quantities. Subtract T min from T max and convert this to (Tmax −1
-Tmin), this ratio U becomes. From the measured values of T max and T min,
We can find the ratio U of ('9).

In equation (3), if it is known that nt-no)Q, m is positive, and in equation (4), if the sign of (nl-n2) is known in advance, (3), (4) From αe, the refractive index nl of Q can be calculated.

Since the thickness d of the medium Q is determined, the thickness d of the medium Q can also be determined.

From the formula for α, there are many wavelengths that have maximum transmittance,
This is expressed as λ0, λ1. ...'Aj,...
.....

The corresponding wave number is 1. k2.・・・・・・,k
If j,..., it should be (j=o, 1.2...). The wavelength interval Δλj is Δλj=λj+
1−λj (defined by 1υ), the thickness d of the thin film can be found by:

FIG. 1 is a block diagram of a defining device for carrying out the method of the present invention.

A white light source 1 emits white light. This is white light with a continuous spectrum. In the present invention, since it is necessary to continuously measure the transmittance T (or transmitted light intensity L) as a function of wavelength, a white light source is used.

A monochromator 2 extracts monochromatic light of wavelengths. The monochromatic light passes through an attenuator 3 and is attenuated to a suitable intensity. This is to make the incident light intensity EO a constant value independent of the wavelength λ.

The monochromatic light having a constant intensity enters the demultiplexer 4 and is demultiplexed to the light intensity meter 5 and the sample 7 at a constant ratio.

It is desirable that this ratio be constant regardless of the wavelength.

Even if it is not constant, it is sufficient if the relationship between wavelength and ratio is known.

Since the ratio is constant, the electric field EO of the light irradiated onto the sample 7 as incident light is determined by the light intensity measured by the light intensity meter 5.
can be calculated.

The light intensity meter 5 measures the electric field E obtained by multiplying EO by a certain ratio.

should be received. However, due to the spectrum of the white light source 1 and the characteristics of the monochromator 2, the intensity of monochromatic light exiting the monochromator 2 is not constant regardless of the wavelength. Therefore, if the electric field E of the light entering the light intensity meter 5 is larger than Eo, the attenuation of the attenuator 3 is increased, and if E is smaller than EO, the attenuation of the attenuator 3 is decreased. In this way, the attenuation of the attenuator 3 is set so that the electric field of the light entering the light intensity meter 5 becomes EO.

The incident light EO enters the upper surface of the sample 7 in the vertical direction.

Since the sample 7 has a thin film formed on a substrate, the incident light EO enters the thin film, but undergoes multiple reflections at the boundary between the thin film and the substrate and at the boundary between the thin film and air.

While undergoing multiple reflections, a portion of the light passes through the substrate and is emitted to the outside.

The intensity of the transmitted light is measured by a light intensity meter 8.

The light intensity output of the light intensity meter 8 and the wavelength λ of the monochromator 2
The values are input to the X and Y terminals of the XY recorder 3. For example, the wavelength λ is input to the X terminal, and the light intensity is input to the Y terminal. In the XY coordinate system, the light intensity is plotted at a position corresponding to the wavelength λ.

The wavelength λ of the monochromator 2 is continuously changed. Then, standardized monochromatic light corresponding to that wavelength λ is emitted from sample 7.
The transmitted light intensity is recorded on the XY recorder. A light intensity meter 5, a feedback 6, and an attenuator 3 keep the intensity of monochromatic light constant. Monochromatic light whose intensity is constant is called standardized monochromatic light.

Since the light intensity is measured according to the wavelength λ, a wavelength/transmitted light graph is obtained on the XY coordinates of the XY recorder 9, with the transmitted light wavelength on the horizontal axis and the transmitted light intensity L on the vertical axis.

Since the intensity EO of the incident light is constant, the vertical axis can be said to be the transmitted light intensity L or the transmittance T.

This graph consists of a part where the transmitted light is 0 up to a certain wavelength, and the transmittance increases beyond that wavelength, and a part where the intensity of the transmitted light oscillates as the wavelength changes.

The vibrating part has a maximum value T max and a minimum value T mi
It oscillates between n.

By measuring the intensity of transmitted light when there is no sample, T=
Since the height of l is known, the values of Tmax and Tmin can be found.

A value U is obtained by dividing the difference between the two by the sum of the two according to the α9 formula. From U, find the reflectance product m, n.

The thin film refractive index n1 can be determined by assuming that the other refractive indices no and n2 are known.

There are many wavelengths that take the maximum value T max, but one of these wavelengths λj is measured, and the adjacent maximum value wavelength λj
+x is also measured. The thickness d of the thin film can be calculated from the values of the differences Δj and λj.

F) Effects (1) The method of the present invention is a non-destructive inspection using transmitted light. Since it is not destroyed, information on the parts that are actually used as elements can be obtained.

(2) Since transmitted light is used, the sample only needs to be on the optical path, and its position can be easily adjusted.

(3) If the angle between the sample surface and the optical path deviates from the right angle, errors will occur in the measurement of film thickness. However, even if the deviation is 5 degrees, the error is 1/
It's only about 1000.

(4) By installing the apparatus of the present invention in the middle of a semiconductor substrate transport line, it becomes possible to measure the film thickness and refractive index simultaneously with the transport. Line design becomes easier.

(+) Applications The present invention can be widely used for measuring the thickness and refractive index of a transparent thin film formed on a substrate transparent to light. For example, when expressed using the notation (thin film)/(substrate), (1) InGaAs / InP (2) InGaAsSb / Ga5b (3)
GaAl! InSb/A/5b(4) GaAl
! It can be used to measure the film thickness and refractive index of thin films with thin film/substrate structures such as AsSb/AlAs.

(ri) Example InGaAs laminated on InP substrate (refractive index 3.2)
The film thickness and refractive index of the layer were measured by the method of the present invention.

As a result, n = 3.5 and d = 6.9/'m were obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a measuring device for carrying out the method of the invention. FIG. 2 is a graph showing measured values of thin film transmitted light intensity and transmitted light wavelength obtained by the method of the present invention. The horizontal axis is the wavelength (μm) of transmitted light, and the vertical axis is the transmitted light intensity (relative value). FIG. 3 is a diagram showing the directions of incident light and reflected light at the boundary between a medium P with a refractive index no and a medium Q with heavy refraction. FIG. 4 is a diagram showing multiple reflections in a thin film. S and T are boundaries between the thin film and air, and between the thin film and the substrate. FIG. 5 is an explanatory diagram showing the optical axis obliquely to show multiple reflection in more detail. (Actual optical axis is on the plane) 1... White light source 2... Monochromator 3... Attenuator 4... Minutes Wave device 5...Light intensity meter 6...Feedback 7...Sample 8...Light intensity meter 9...XY recorder invention person inside - inside
1) Kan 1st figure 2nd figure! 4-1 Figure 4

Claims (1)

[Claims] A semiconductor substrate on which a semiconductor thin film is laminated is irradiated with monochromatic light whose wavelength can be continuously changed from the direction of the surface where the thin film is present at right angles to the surface. The intensity is measured while continuously changing the wavelength of monochromatic light, and the maximum value Tmax and minimum value Tmi of transmittance are determined.
Find the refractive index n1 of the thin film from n and the refractive index n2 of the substrate,
Wavelength λj of monochromatic light giving maximum value Tmax or minimum value Tmin and its adjacent maximum value Tmax or minimum value Tmin
A method for measuring the film thickness and refractive index of a semiconductor thin film, characterized in that the film thickness of the thin film is calculated based on the difference Δλ between the wavelength of monochromatic light giving the wavelength λj and the wavelength λj.