JP3537732B2 - Ellipsometer using voltage controlled liquid crystal retarder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ellipsometer using a voltage-controlled liquid crystal retarder.

[0002]

2. Description of the Related Art An ellipsometer is a device for obliquely incident light of a known polarization state on a sample surface and measuring a change in the polarization state of reflected light. A method of analyzing this and calculating the complex refractive index and film thickness of the sample is called ellipsometry, and as a means for measuring the structure of ultra-thin films and solid surfaces with high sensitivity,
Has a history of over 100 years. The device has a relatively simple configuration and can be incorporated into vacuum equipment, etc.
It is also useful for measurement, and its use is extremely wide.

[0003] are actually determined by ellipsometer, the real part of the ratio of the complex amplitude reflectance r _p and r _s for p and s components at the surface of the sample (absolute value: tan) and imaginary part (position
Phase: Δ), which is usually defined as follows :

[0004] Ψ and Δ uniquely give the polarization state of the reflected light. However, the calculation of the thickness and the complex refractive index of the thin film from these is complicated, and the analysis has been laborious until now.

However, since the analysis time has been greatly shortened due to the recent development of computers, attention has been paid again in recent years.

[0006]

The structure of the error descriptor Someta INVENTION SUMMARY is], there are two main types of quenching methods and photometry separately, any structure is as shown in FIG.

[0007] FIG. 4 is a diagram showing a configuration of a conventional e descriptor Someta according.

In this figure, L is a light source, P is a polarizer,
S is a sample, C is a compensator (1/4 wavelength plate), A is an analyzer,
D is a photodetector.

In the extinction method, polarizers P, which are optical elements,
A point at which the intensity of the detected light becomes zero by alternately rotating the compensating plate (1/4 wavelength plate) C and the analyzer A (called an extinction position).
And complex reflectance is determined from the azimuth of each element at this time. In this case, it takes time to find the extinction position, but the accuracy is extremely high.

In the photometric method, on the other hand, the light intensity is measured by continuously rotating a polarizer P as an optical element, a compensator (a quarter-wave plate) C, and an analyzer A (whichever may be used) with a motor or the like. The complex reflectance is obtained by performing a Fourier analysis on this.

In this case, a compensating plate (1/4 wavelength plate) C is not always necessary, and therefore, a configuration in which only the polarizer P and the analyzer A rotate the analyzer A is often used. The photometric method is less accurate than the quenching method, but the time is shortened.

In any case, at present, the optical element must be mechanically rotated using a hand or a motor. In particular, in the quenching method, when performing a highly accurate experiment, it is necessary to perform a troublesome work of rotating at least two elements alternately and searching for the quenching position. Reading errors are also likely to occur.

The present invention has been made in view of the above circumstances, and has as its object to provide an ellipsometer using a voltage-controlled liquid crystal retarder that can easily and quickly measure a sample with high accuracy.

[0014]

According to the present invention, there is provided an ellipsometer using a voltage-controlled liquid crystal retarder, comprising: a first liquid crystal retarder for receiving reflected light from a sample; A second liquid crystal retarder R2 for receiving the output light from the first liquid crystal retarder R1 is disposed, and only a voltage applied to the first liquid crystal retarder R1 is applied.
To obtain a value at which the final light intensity is minimized, and then
With the voltage of the first liquid crystal retarder R1 fixed at this value
Changes the voltage applied to the second liquid crystal retarder R2 in the state
Finally light intensity Me seek a value that is a minimum in a complex reflectance ratio from the voltage of the first liquid crystal retarder R1 r _p / r
_s = the imaginary part of tanΨexp (iΔ) Δ, the real part ta of voltage from the complex reflectance ratio of the second liquid crystal retarder R2
nΨ is independently obtained .

[2] In an ellipsometer using a voltage-controlled liquid crystal retarder, a light source, a polarizer that transmits light from the light source, a sample to be irradiated with light from the polarizer, and reflected light from the sample are incident on the ellipsometer. A first liquid crystal retarder, a second liquid crystal retarder for receiving output light from the first liquid crystal retarder, an analyzer for receiving light from the second liquid crystal retarder, and a light from the analyzer. as well as and a light detector to incident, the
Change only the voltage applied to the first liquid crystal retarder to final
A value that minimizes the light intensity is obtained, and then the first liquid crystal
The voltage of the second liquid crystal is fixed with the voltage of the
Change the voltage applied to the retarder to minimize the final light intensity
From the voltage of the first liquid crystal retarder.
The elementary reflectance ratio r _p / r _s = tanΨexp (iΔ)
Imaginary part delta, the double the voltage of the second liquid crystal retarder over the
It is characterized in that the real part tan の of the elementary reflectance ratio is obtained independently .

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

First, the voltage control liquid crystal retarder will be described.

Acquisition of R and 1 / V Linearity by Current Control Circuit In order to completely control the phase difference of the liquid crystal retarder by voltage, it is necessary to form a current control circuit in order to take the influence of the electric capacity of the cell. is there. It uses passive or active circuits such as:

(1) Passive Circuit FIG. 2 is a circuit diagram (part 1) of a liquid crystal retarder showing an embodiment of the present invention.

In this figure, reference numeral 11 denotes an AC voltage source V _IN ,
Reference numeral 12 denotes an impedance Z, which is connected in series to the nematic liquid crystal cell 5 which is homogeneously aligned.

[0021] In the circuit shown in FIG. 2, the impedance additional resistance of the liquid crystal cell 5 of the impedance Z 12 1
When the current is sufficiently larger than / iωC, the current flowing through the circuit becomes Thus, it is not affected by the capacitance of the liquid crystal cell 5. At this time, the CV applied to the liquid crystal cell 5 is The CV, that is, the phase difference R can be accurately controlled only by the reciprocal of the voltage regardless of other physical properties.

(2) Active Circuit FIG. 3 is a circuit diagram (part 2) of a liquid crystal retarder showing an embodiment of the present invention.

In this figure, a voltage control gain variable amplifier 22 is connected in series to one end of an AC voltage source 21, and the output of the voltage control gain variable amplifier 22 is connected to an AC ammeter 2.
3, the liquid crystal cell 5 is connected to the AC ammeter 23, the other end of the liquid crystal cell 5 is connected to the other end of the AC voltage source 21, and a current I is supplied to the liquid crystal cell 5.

On the other hand, the current | I |
(Absolute value) is added to the divider 24 and the subtractor 25. The control voltage V is set as a target value in the subtracter 25, and the output of the subtracter 25 is configured to be fed back to the variable voltage gain amplifier 22 via the PID circuit 26.

Then, a constant current I flows through the liquid crystal cell 5. That is, a voltage V (proportional to the phase difference R) with the control voltage V as a target value is applied.

Therefore, if the constant current circuit shown in FIG. 3 is used, the CV can be more accurately determined with a constant current, and the phase difference of the liquid crystal retarder can be controlled.

Hereinafter, embodiments of the present invention will be described.

FIG. 1 is a block diagram of an ellipsometer using a voltage controlled liquid crystal retarder according to an embodiment of the present invention.

In this figure, L is a light source, P is a polarizer,
S is a sample, R1 is a first liquid crystal retarder, R2 is a second liquid crystal retarder, A is an analyzer, and D is a photodetector.

As shown in this figure, in the present invention, instead of passing through one of the compensating plates (1/4 wavelength plates) C for giving a fixed phase difference, the conventional ellipsometer shown in FIG.
Two liquid crystal retarders R1 and R2 capable of giving an arbitrary phase difference by the voltage control of the present invention are used.

Here, the axis of the first polarizer P is at 45 degrees from the plane of incidence, and the axis of the analyzer A is the fast axis of the first liquid crystal retarder R1 (the retardation size δ ₁ ) perpendicular to the plane of incidence. Is parallel to the plane of incidence, and the axis of the second liquid crystal retarder R2 (retardation magnitude δ ₂ ) is fixed at 45 degrees. At this time, when the light intensity I that ultimately reaches the photodetector D is differentiated by [delta] _1, Get. Here, ρ = | r _p | / | r _s | = tanΨ. When an appropriate value of δ ₂ > 0 is given and the point where the light intensity is minimized by changing the voltage applied to the first liquid crystal retarder R1 is obtained. From the equation (4), δ ₁ at this time is δ ₁ + Δ
= Π / 2.

When the voltage-controlled liquid crystal retarder of the present invention is used, the phase difference of the sample S is determined only by the voltage V _1IN applied to the first liquid crystal retarder R1, and is given by the following equation.

[0033] Here, A is a known constant unique to the liquid crystal retarder.
When the voltage applied to the first liquid crystal retarder R1 that minimizes the final light intensity is obtained from the equations (4) and (5), the position of the sample S is determined from the value regardless of the second liquid crystal retarder R2. The phase difference Δ can be uniquely obtained.

As described above, the first liquid crystal retarder R1
When the retardation is determined, the final light intensity I is given by the following equation. Clearly the equality holds Therefore, if the voltage applied to the second liquid crystal retarder R2 is determined so that the light intensity becomes zero, the value V
The real part of the complex reflectance ratio is found as follows from _{2I N.}

[0035]

[0036] As is apparent from the above, tan Ψ is uniquely determined only from V ₂ IN.

Summarizing the above, the voltage applied to the first liquid crystal retarder R1 and the second liquid crystal retarder R2 are independently changed to obtain values that minimize the final light intensity. the imaginary part of the complex reflectance ratio <br/> from the voltage, the real part of the complex reflectance ratio is determined independently from the second voltage of the liquid crystal retarder R2.

(1) Here, the second liquid crystal retarder R
The case where the fast axis 2 is set at +45 degrees from the P plane is considered, but may be 45 degrees or less.

(2) Here, although the analyzer A is installed in the S direction, it may be installed in the P direction. In this case, instead of equation (4) Δ ₁ that minimizes the light intensity for positive δ ₂
Then, the phase difference of the sample at this time is expressed by the following equation instead of the equation (5). Given by

When δ ₁ is determined in this way, the light intensity becomes Therefore, the light intensity becomes zero because δ ₂ is replaced by the equation (7). Only when is satisfied. Therefore, from the voltage applied to the second liquid crystal retarder R2 that gives δ ₂ , the real part of the complex reflectance ratio becomes , instead of the equation (8),

[0041] Given by

It should be noted that the present invention is not limited to the above-described embodiment, and various modifications are possible based on the gist of the present invention, and they are not excluded from the scope of the present invention.

[0044]

As described above in detail, according to the present invention, it is possible to obtain an ellipsometer using a voltage-controlled liquid crystal retarder that can easily and quickly measure a sample with high accuracy. .

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an ellipsometer using a voltage controlled liquid crystal retarder according to an embodiment of the present invention.

FIG. 2 is a circuit diagram (No. 1) of a liquid crystal retarder showing an embodiment of the present invention.

FIG. 3 is a circuit diagram (part 2) of a liquid crystal retarder showing an embodiment of the present invention.

4 is a diagram showing a configuration of a conventional e descriptor Someta.

[Explanation of symbols]

L light source P polarizer S sample R1 First liquid crystal retarder R2 Second liquid crystal retarder A Analyzer D Photodetector 5 Liquid crystal cell 11, 21 AC voltage source 12 Impedance Z 22 Variable voltage gain amplifier 23 AC ammeter 24 divider 25 Subtractor 26 PID circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G01J 4/00-4/04 G01M 11/00 G01B 11/00-11/30 102 G01N 21/21 G02B 5 / 30 G02F 1/13 505

Claims (2)

(57) [Claims] 1. An ellipsometer using a voltage-controlled liquid crystal retarder, comprising: (a) a first liquid crystal retarder for entering reflected light from a sample; and (b) a first liquid crystal retarder for entering output light from the first liquid crystal retarder. a second liquid crystal retarder is arranged, varying only the voltage applied to the (c) said first liquid crystal retarder over
To obtain a value that minimizes the final light intensity.
With the liquid crystal retarder voltage of this
Me values final light intensity by changing the voltage of the second liquid crystal retarder is minimum calculated, the complex reflectance ratio from the voltage of the first liquid crystal retarder r _p / r _s = tanΨe
imaginary part delta, the second ellipsometer using a voltage controlled crystal retarder and obtaining from the voltage of the liquid crystal retarder independently real part tanΨ of the complex reflectance ratio of xp (i?). 2. An ellipsometer using a voltage-controlled liquid crystal retarder, comprising: (a) a light source; (b) a polarizer for transmitting light from the light source; and (c) a sample for irradiating light from the polarizer. (D) a first liquid crystal retarder that causes reflected light from the sample to enter, (e) a second liquid crystal retarder that causes output light from the first liquid crystal retarder to enter, and (f) the second liquid crystal. (G) a photodetector for receiving light from the analyzer , and (h) changing only a voltage applied to the first liquid crystal retarder.
To obtain a value that minimizes the final light intensity.
With the liquid crystal retarder voltage of this
By changing the voltage applied to the second liquid crystal retarder, the final light
The value at which the intensity is minimized is determined, and the first liquid crystal retarder
From the complex reflectance ratio r _p / r _s = tan 比 e
xp (iΔ), the imaginary part Δ of the second liquid crystal retarder,
An ellipsometer using a voltage-controlled liquid crystal retarder, wherein a real part tan の of the complex reflectance ratio is independently determined from pressure .