CS215816B1 - Circuitry for corrccting measured values of ellipsometric measurements - Google Patents

Description

The invention relates to a circuit for correcting the measured values of an ellipsometric measurement obtained on a rotating analyzer device by means of the measured values of the actual angular speed of the motor. The invention further relates to a device for measuring the actual angular velocity and a circuit for determining the correction to the measured ellpsometric values.

Previously known solutions use precise stabilization of engine speed. Engine speed fluctuations are small, but due to the required accuracy of the measurement of the angular displacement of the optical frequency, which is twice the engine speed, it can be said that at 0.1% and at best 0.01% the ellipsometric accuracy is at most 2 , 5 arc minutes.

When processing data from ellipsometric measurements, errors due to the optical path and errors in the electronic part have a major influence on the accuracy of the measured data. Using engine speed stabilization to rotate the analyzer solves this problem, but at the cost of high precision and stability.

When controlling angular velocity with high precision, a motor, sensor and control system with top parameters are needed. An incremental encoder is suitable for high accuracy measurements of the actual angular velocity of the motor, since the resulting quality of the ellipsometric measurement correction depends primarily on the encoder quality. For accurate reading of the angular velocity, a sufficiently accurate time standard and path stability processing the magnitude of the actual angular velocity of the motor are then required.

The disadvantages of the high-speed engine stabilization solution eliminate the circuitry for correcting the measured values of the ellipsometric measurement of a rotating analyzer ellipsometer according to the invention, which is based on the radiation source being coupled via an electromagnetic coupling through a sample with the rotating analyzer simultaneously coupled to the motor. The motor is further coupled to an incremental encoder whose first output is connected to the deviation evaluation block and the second output is to the second sampling circuit input and the second correction block input is where the deviation evaluation block is connected to the first correction block input whose output is connected to the second sum sum input. The first input of the rotating analyzer is coupled to the sampling circuit through series connected sensor blocks and a low frequency filter. The output of the sampling circuit is connected to an analog to digital converter, the output of which is connected to the first input of the sump. The summator output is connected to a block of measured values. .

Using the measured values of the actual angular speed of the motor, the actual measured values are transformed into new values, ie more accurate. The measured waveform of the sensor input signal, after passing through a low-pass filter that removes noise from the signal, is sampled at points corresponding to the individual points of the incremental encoder, independent of the actual angular velocity of the motor, and quantized in the analog-digital converter. The angular velocity evaluation loop provides the necessary correction error magnitude, which, after multiplying by a constant, defines by correcting the measured values of the ellipsometric measurement after the summation of the measured and correction values, the exact value of the signal size at a given point is calculated.

This ensures that the accuracy of the ellipsometric measurement data processing, even in the case of long-term variations in angular velocity, is only due to the error with which we measure the actual angular velocity of the motor.

The advantage of this circuit for correcting the measured values of the ellipsometric measurement is that there is no need for highly accurate and stable motor rotation control, and yet the accuracy of the measurement when not considering the optical path is due to the error with which we measure the actual angular speed of the motor.

The drawing shows a block diagram for correcting the measured values of an ellipsometric measurement according to the invention.

The radiation source 1 is connected via a sample 13 to a rotating analyzer 2, which is simultaneously connected to the motor 7. The motor 7 is further connected to an incremental encoder 8, the first output of which is connected to the deviation evaluation block 9 and the second output to the second input. the sampling circuit 5 and the second input of the correction block 10, wherein the deviation evaluation block 9 is connected to the first input of the correction block 10, the output of which is connected to the second input of the summator 11. The first input of the rotating analyzer 2 is connected to the sampling circuit 5 3 and a low-frequency filter 4. The output of the sampling circuit 5 is connected to an analog-to-digital converter 8, the output of which is connected to the first input of the sump 11, where the output of the sump 11 is connected to the measured value block 12.

The radiation source 1 emits electromagnetic radiation, which after reflection from the sample 13 passes through a rotating analyzer 2, whose movement is determined by the motor 7 to the sensor 3, which transforms into an electrical signal and after processing in the low frequency filter 4 performs memorizing instantaneous values up to. until the next sampling pulse arrives. Downstream of the sampling circuit is an analogue to digital converter 6, which ensures the conversion of measured values into digital form. The actual angular speed of the motor 7 is sensed by the Incremental encoder 8, which controls the sampling circuit 5 and the correction block 10. The correction block 10 receives the order of the samples from the incremental encoder 8 and the deviation value from the reference speed from the deviation evaluation block 9. correction according to formula (1) in the correction block 10, the summation 11 is summed with the digital data from the analog-to-digital converter 6 and at the input of the measured value block 12 the results are correct.

The above-described solution can increase the achievable measurement accuracy by up to two orders, so that the resulting measurement accuracy depends only on the quality of the optical Information Path, now on the parameters of the radiation source, the quality of the polarizer, analyzer, mechanical parameters of the optical path. The following are formulas valid for correction calculation.

And. sin Ω t, = = A. sin _ω t, · A. cos ω t,.

where τ = the time constant of the low-frequency filter

T = reference speed dT = deviation from reference speed

Amplitude of the signal being processed ^{ti =} w ^{and i} = ° - ²⁵⁰ sampling point designation. sin Ω ti - corrected value, where ω = Ω =

2π ⁼ Τ

And. sin ω t, - measured value 2πτΑ ——-. cos ω ie. dT - correction

The phase angle with respect to the reference point is measured in an ellipsis measurement. If a frequency variation occurs during the measurement, this error due to the phase shift in the low pass filter is added to the measured values. The size of the error is given by:

(2)

If we want to move the measured waveform by this magnitude dg?, We use the following formula: sin (cot + g?) = Sin ω tcos φ + coscotsinp (3) Assuming sin φ = φ and cosgj = 1 (4) ).

For example: When using an uncontrolled motor with a flywheel that ensures a maximum variation in the angular speed of the motor of less than 1% during measurement, then all validity conditions (1) can be achieved and thus the possibility of correcting the measured values of the ellipsometric measurement. We choose t, A.

dT

Thus: _τ = 0.5 ms A = 10 V - - = 1θ - ² φ = 1 ° = 0.0175 rad, 2πτΑ dT 2π. 0.5 ΙΟ- ³ . 10 - φ 'Τ “δχΓΐ 75 = 17.9 ms, ie 56 revolutions per second.

And after reaching the result of the previous relation T> 17.9 ms we get n <3600 rpm.

Under the above assumptions, we conclude that for a value less than 3600 rpm

The circuit may be used to correct the measured values of the ellipsometric measurement according to the invention.

The described circuit for correction of measured values of ellipsometric measurement is suitable for ellpsometers with rotating analyzer, which are to achieve high accuracy but do not have highly stable motor speed. The method of determining the correction and the correction processing device to achieve higher measurement accuracy can advantageously be applied to all measurements where periodic signal processing is performed.

Claims (1)

SUBJECT Circuit for correcting the measured values of the ipsipsometric measurement, characterized in that the radiation source (1) is connected by electromagnetic coupling via a sample (13) to a rotating analyzer (2), which is simultaneously connected to the motor (7), connected to an incremental encoder (8), the first output of which is connected to the deviation evaluation block (9) and the second output to the second sampling circuit input (5) and the second correction block input (10), where the dT evaluation block (9) is connected to the first input of the reaction coefficient block (10j), the output of which is connected to the second input of the sump (11), the first input of the rotating analyzer (2) being coupled to the sampling circuit (5) via the serial connected sensor blocks (3) and low frequency filter ( 4j, and wherein the output of the sampling circuit (5) is coupled to an analog-to-digital converter (6), the output of which is connected to the first input of the sump (11), wherein the output of the sump (11) is coupled to bl. the measured values (12).