JPH05280937A - Measuring method of film thickness - Google Patents

Abstract

PURPOSE:To prevent occurrence of an error in measurement of a film thickness and to enable execution of highly precise measurement by measuring unevenness or a curve of a wafer and by correcting nonuniformity of an incident angle caused by the unevenness or the curve. CONSTITUTION:When a height of the surface of a wafer 6 is measured, a polarizer 2, a 1/4 wave plate 3 and an analyzer 4 provided on the optical axis of an ellipsometer are made to escape from this optical axis, a wafer stage 7 is moved up and down so that the reflection intensity of an incident light from a light source 1 be maximum, and the height of the wafer at that point is measured. Then, the stage 7 is tilted by a corrective angle (d) determined by tan (d)=a/b by using a difference (a) between the heights of two points in the vicinity of the measuring point and a distance (b) between the planes of the two points, so that an angle of incidence on a sample from the light source 1 be calibrated. In a state wherein this angle of incidence is fixed at any measuring point, a film thickness at a prescribed measuring point is measured by the ellipsometer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a film thickness measuring method,
Particularly, it relates to a measuring method using an ellipsometer.

[0002]

2. Description of the Related Art When light is reflected on the surface of an object, the polarization state of light changes before and after reflection. The ellipsometry method makes use of this point. That is, by measuring the change in the polarization state, the optical constants of the object and the surface properties can be known.

An ellipsometer is a device for realizing this ellipsometry method. FIG. 5 is a block diagram of an apparatus used in the conventional example. A slit 58, on the optical axis of the light source 51,
Optical chopper 70, rotating polarizer 52, quarter wave plate 53,
A wafer stage 57 on which a wafer 61 whose film thickness is to be measured is placed is provided, and an aperture 59, a rotation analyzer 54, and a photomultiplier 55 are arranged on the optical axis of the reflected light reflected by the wafer 61. The amplifier 60 and the oscilloscope 50 are sequentially connected.

In the conventional ellipsometer having this structure, first, the light passing through the slit 58 of the light source 51 is modulated by the optical chopper 70, and the light passing through the rotating polarizer 52 and the quarter wavelength plate 53 is linearly polarized. And is incident on the wafer 61. The reflected light of the light incident on the wafer 61 is detected by the rotation analyzer 54. When this light is received by the photomultiplier 55, a photocurrent is generated, but this photocurrent is monitored by an electronic circuit to minimize the amplitude of the rotational vibration of the polarized light.
The angle between 2 and the rotation analyzer 54 is obtained. This photocurrent passes through the selective amplifier 60, becomes image information, and its waveform is displayed on the oscilloscope 50.

With the above-described structure and principle, the wafer 61
The thickness of the thin film can be obtained by applying polarized light to and reflecting the reflection coefficient ratio and the phase difference of the reflected light. That is,
The reflectances r _p and r _s of the vibration component (p component) of the electric field parallel to the incident surface of the light incident at the wavelength λ and the incident angle ψ and the vibration component (s component) of the perpendicular electric field are (1 ) And
It becomes as shown in equation (2).

[0006]

[Equation 1] Here, the phase difference δ is given by equation (3), where n is the refractive index. The reflection coefficient ratio (r _p / r _s ) is given by the equation (4).

[0007]

[Equation 2]

[0008]

[Equation 3] As described above, the p component and the s component have different amplitude ratios and a relative phase difference occurs, so that the linearly polarized light is reflected as elliptically polarized light. Therefore, if the setting angle of the polarizer and the analyzer as P _0, A _0, respectively, (4) relative shown by formula phase difference Δ and the incident angle ψ, respectively (5), shown in equation (6) Like

[0009]

[Equation 4]

[0010]

By the way, in the prior art, as a method for measuring a semiconductor insulating film, for example, a thin oxide film, the ellipsometry method as described above has the highest accuracy and is widely used. However, in an actual LSI wafer, unevenness or warpage occurs on the wafer due to the influence of heat treatment or the like, which causes an error during measurement.

This is because the measured value depends on the incident angle ψ of the wafer, so that the incident angle varies due to unevenness and warpage of the wafer. The present invention has been made in view of the above points, and an object of the present invention is to provide a method for measuring a film thickness that does not cause an error in measured values due to unevenness or warpage of a wafer.

[0012]

In order to achieve the above object, the film thickness measuring method of the present invention comprises a polarizer for converting the light of an ellipso light source into linearly polarized light along the optical axis, and the light thereof. A wafer mounted on a stage on which is incident, and an analyzer for analyzing reflected light reflected from the wafer are arranged in order,
In the method for measuring the film thickness of the sample using an ellipsometer having an analyzing means for determining the thickness of the thin film according to the photocurrent of the detected light, in the plurality of measurement points in the wafer surface, The stage is moved up and down so that the reflection intensity of the emitted light is maximized, the height from the reference plane at the plurality of measurement points is obtained from the moving distance, and the film thickness at a predetermined measurement point is measured in the vicinity of the predetermined measurement point. By using the height difference a between the two points and the distance b between the planes of the two points, the stage is tilted by the correction angle α obtained by the following equation (A), and the sample is incident from the ellipso light source. It is characterized by calibrating the incident angle to be measured and measuring the incident angle with the ellipsometer at a constant state at any of the measuring points.

Tan α = a / b ... (A)

[0014]

When the film thickness is measured by the ellipsometer, if the wafer to be measured has irregularities or warps, the incident angle varies, resulting in an error in the measured value.

The present invention has solved this problem.
The operation will be described below with reference to FIG. (A)
When the wafer is flat as shown in the figure, the incident angle θ ₁ is the same at any measurement point on the wafer, and the reflection angle θ ₂ is the same. Assuming that the incident angle θ ₁ and the reflection angle θ ₂ are on the reference plane, when the wafer is convexly distorted as shown in FIG.
The incident angles at the points B, C and C have different values. Therefore, the incident angle at point A is θ ₁ + α, and in order to calibrate the incident angle to be equal to the incident angle θ ₁ when the wafer is flat, in this method, for example, when the incident angle at point A is obtained, it is already measured. The incident angle α to be calibrated can be obtained by the equation (B) using the film thicknesses h ₂ and h _{1 at the} points B and C and the distance between the planes of the points B and C using b.

Tan α = (h ₁ −h ₂ ) / b ... (B) Therefore, by tilting the incident surface of the wafer by α, it is possible to always maintain a constant incident angle with respect to the optical axis, As a result, the measurement error of the film thickness does not occur.

[0017]

1 is a block diagram of an ellipsometer used in an embodiment of the present invention. On the optical axis of the light source 1, the polarizers 2 and 1
A quarter wave plate 3, a wafer stage 7 on which a wafer 6 whose film thickness is to be measured is placed, an analyzer 4 for analyzing reflected light that reflects the wafer 6, and a photomultiplier 5 are arranged. Further, an amplifier 9 and an oscilloscope 10 are arranged in order. The wafer stage 7 is vertically and horizontally movable, and the wafer stage 7 is provided near the wafer stage 7.
A stage drive unit 11 is provided for driving the lens to a predetermined position. Further, in order to calibrate the incident angle, a data section 8 is provided which detects the difference between the height from the reference plane and a predetermined height at a plurality of measurement points within the plane of the wafer 6 and stores it as height data. ing. The process of creating this height data is configured to be performed automatically.

The embodiment of the present invention is carried out by using the ellipsometer having the above structure. The method will be described below. First, the height of the surface of the wafer 6 to be measured is measured. When performing this measurement, the polarizer 2, the quarter-wave plate 3, and the analyzer 4 provided on the optical axis of this ellipsometer are escaped from this optical axis, and the reflection intensity of the incident light from the light source 1 is maximized. Then, the wafer stage 7 is moved up and down so that the wafer height at that point is measured. The difference between this height and the predetermined height is stored in the data section 8 as height data as shown in FIG. From this height data, FIG. 3A shows the flatness contour lines showing the unevenness of the wafer 6 as a solid line with a predetermined height, with a broken line above a predetermined height, and with a dashed-dotted line below a predetermined height. It is a thing. Further, FIG. 3B is a three-dimensional representation of the state of the surface of the wafer by performing simulation from these data.

Next, as shown in FIG. 4, for example, when the incident angle at point A is calibrated, the incident angle at point A is made equal to the incident angle θ ₁ when the wafer is flat, as described above. Calibrate to For example, when obtaining the incident angle at the point A, the film thicknesses h ₂ , h _{1 at the} points B and C, and the distance between the planes of the points B and C, which have already been measured, are calculated by the equation (B) using b. , The incident angle α to be calibrated is determined.

Then, the stage driving unit 11 is driven to tilt the wafer stage 7 by α,
It is possible to always maintain a constant incident angle with respect to the optical axis, and it is possible to eliminate the error in the measured value of the film thickness.

The incident angle is set as described above,
The film thickness at points on the wafer 6 is sequentially measured, but as described in the conventional example, the principle of obtaining the thickness of the thin film is the same in this embodiment as well, and is obtained from the reflection coefficient ratio and the phase difference. ..

[0022]

As described above, according to the film thickness measuring method of the present invention, the unevenness or warp of the wafer is measured, and the variation of the incident angle caused by the unevenness or warp is corrected, so that the Since the measurement is performed by the ellipsometry method while keeping a constant incident angle with respect to the axis, it is possible to perform high-precision measurement without causing an error in film thickness measurement.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an ellipsometer used in an embodiment of the present invention.

FIG. 2 is a view showing height data of a wafer used in an embodiment of the present invention.

FIG. 3 is a diagram showing a surface of a wafer used in an example of the present invention.

FIG. 4 is a diagram illustrating an embodiment of the present invention.

FIG. 5 is a diagram illustrating a conventional example.

[Explanation of symbols]

 1 ... Light source 2 ... Polarizer 3 ... Quarter wave plate 4 ... Analyzer 5 ... Photomultiplier 6 ... Wafer 7 ... Wafer Stage 8 ... Data section 10 ... Oscilloscope

Claims (1)

[Claims] 1. A polarizer that converts light from an ellipso light source into linearly polarized light along an optical axis, a wafer mounted on a stage on which the light is incident, and reflected light reflected by the wafer is analyzed. In the method of measuring the film thickness of the sample by using an ellipsometer having an analyzing means for determining the thickness of the thin film according to the photocurrent of the analyzed light, which is sequentially arranged with an analyzer to For a plurality of measurement points, the stage is moved up and down so that the reflection intensity of the incident light is maximized, the height from the reference plane at the plurality of measurement points is obtained from the moving distance, and the film at a predetermined measurement point is obtained. Inclining the stage by a correction angle α obtained by the following equation (A) using the thickness difference a between two points in the vicinity of the predetermined measurement point and the distance b between the planes of these two points. By the above ellipso light Calibrating the incidence angle of incident on the sample from, in any of the measurement points the incident angle, the film thickness measuring method and measuring by the ellipsometer while constant. tan α = a / b ... (A)