KR100426323B1 - Ultra High Speed Ellipsometer using Division-of-Amplitude Photopolarimeter - Google Patents

Abstract

The present invention relates to an ultrafast ellipsometer having a measurement time of nanoseconds, comprising: a He-Ne laser light source for emitting linearly polarized light, a first light splitter for dividing the He-Ne laser light source light into reflected light and transmitted light, and each separated Two wool prisms that subdivide light into two orthogonal polarizations, four photometers each measuring the intensity of light having four different polarization states divided through the wool prisms, and In an ultra-high speed ellipsometer using an amplitude-division polarization meter including an oscilloscope that converts an analog signal detected by a measuring device into a digital signal and outputs it as a constant waveform, the linearly polarized state of the light emitted from the He-Ne laser light source is circularly polarized. A λ phase plate to be converted into; A linear polarizer for transmitting the light passing through the 1/4? Phase plate at a desired azimuth angle and transmitting the light; The transmission depth of light on the c-Si substrate, which is a single crystal silicon, is arranged to be refracted by the light emitted from the He-Ne laser light source and passed through the 1/4 λ phase plate and the linear polarizer to be incident to the first light splitter. Thicker coated crystallized Ge-Sb-Te alloy sample; Variable wavelength high power Nd-YAG / which emits a range of pulses incident on the sample to check whether the measured values output from the photometer and oscilloscope match the phase change of the sample and to adjust the phase change of the sample OPO laser, shutter that emits light from Nd-YAG / OPO laser from 1/1000 second to several seconds so that only a single pulse of the laser pulse is incident on the sample, and emits from Nd-YAG / OPO laser Light generating means having a red filter for filtering only pulses of a predetermined range among the pulses; A second optical splitter disposed on a movement path of the laser pulse between the Nd-YAG / OPO laser and the sample to divide a portion of the laser pulse and transmit the divided portion to a photometer associated with the oscilloscope; Is more included.

Description

Ultra High Speed Ellipsometer using Division-of-Amplitude Photopolarimeter

The present invention relates to an ultrafast ellipsometer having a measurement time of nanoseconds. In particular, the present invention relates to an information recording or erasing of a phase change type optical recording medium (Ge ₂ Sb ₂ Te ₅ ) used in a DVD. The present invention relates to an ultrafast ellipsometer using an amplitude-division polarimetry for simultaneous measurement of optical amplitude and phase in real time.

In general, this measuring method, called ellipsometry, allows to acquire not only information about the light intensity but also information about the light amplitude by measuring the change in the polarization state of the light.

Among the advantages of the simultaneous acquisition of amplitude and phase, the light-absorbing material can simultaneously determine the complex refractive index of the material, resulting in a phase-change type such as Ge ₂ Sb ₂ Te ₅ (Ge-Sb-Te). It is also usefully used to measure the refractive index and extinction coefficient of an optical recording medium.

In addition, the method using ellipsometer distinguishes two consecutive phase change processes from the reflectance measurement method, which is a general optical method for measuring the phase change process of Ge-Sb-Te alloy. Allows you to determine the Avrami constants, the fundamental constants that define the change process.

Therefore, when looking at the elliptic system commonly used until recently, the rotation analyzer (rotation polarizer) method, the rotation compensator method or the phase modulation method is the main.

In this case, the ellipsometer of the rotation analyzer as shown in FIG. 1 or the phase modulation method as shown in FIG. 2 may be used to rotate an analyzer (polarizer) or a compensator using a motor. Since the phase is modulated by applying a modulation voltage to the piezoelectric element, it takes several tens of milliseconds to several tens of ms to obtain one elliptic constant pair (Δ, Ψ).

That is, when looking at the detailed problem, it has been presented as a problem to have a mechanical speed limit according to the rotation of the motor because the rotating probe method shown in Figure 1 attached to rotate the polarizer by using a motor. In the case of the phase modulation type shown in FIG. 2, the piezoelectric element is used to change the polarization, and it has been proposed to have a limitation of speed due to the voltage response time of the piezoelectric element.

Therefore, in order to measure the phase change process of the Ge-Sb-Te alloy sample which is an optical recording medium with this time resolution, the experiment was carried out at around 135 ° C. at which the phase change occurs relatively slowly (see FIGS. 3A and 3B attached). Should be done.

Referring to the measurement graphs shown in FIGS. 3A and 3B, in the change of the elliptic constant (tanΨ, cosΔ) reflecting the change of crystal structure with temperature of Ge ₂ Sb ₂ Te ₅ , a phase change type optical recording medium, Amplitude information Can only observe changes similar to conventional reflectivity, In, we can clearly observe the change caused by the two phase change. It can be seen that the phase change occurs rapidly as the phase change temperature increases.

Therefore, at a high temperature at which information is recorded or erased on an actual optical recording medium, a phase change process occurs within several tens of nanoseconds. Therefore, a need for an ultra-fast ellipsometer is needed to analyze such a process in real time.

Recently, a ZLE (Zeeman Le-Poole ellipsometer) high speed ellipsometer has been reported as a technique that has been proposed as an ellipsometer with a fast data acquisition rate. However, the ZLE ellipsometer has a time resolution of 1-2 ㎲. This method does not satisfy the requirements, and furthermore, this method uses a Zeeman energy level difference, and thus has a technical limitation that the possibility of developing an ultra-fast ellipsometer with a nanosecond time resolution is rare.

An object of the present invention for solving the above-mentioned problems relates to an ultra-fast ellipsometer having a measurement time of nanoseconds, in particular, information recording of a phase change type optical recording medium (Ge ₂ Sb ₂ Te ₅ ) used in DVD Or to provide an ultra-fast ellipsometer using an amplitude-division polarization meter to simultaneously measure the optical amplitude and phase in real time during the process, such as information erasing within a few nanoseconds.

1 is a diagram illustrating a basic structure of a rotational analyzer type ellipsometer

2 is a diagram illustrating a basic structure of a phase modulated ellipsometer

3A and 3B are graphs for measuring a phase change process of a Ge-Sb-Te alloy sample as an optical recording medium

4 is an exemplary diagram for explaining a typical method of measuring Stokes parameters using a phase retarder and a polarizer having a phase delay angle of 90 degrees.

5 is a schematic diagram showing a basic configuration of DOAP

6 is a graph of the pulse width measurement results of a high power Nd-YAG / OPO laser having a pulse width of 7 ns.

7 is a basic structural example of an ultra-fast elliptic system according to the present invention

8A to 8D are graphs calculated by computing a change in intensity of light measured by each photometer as the amorphous Ge-Sb-Te changes to crystalline Ge-Sb-Te.

9A and 9B are intensity graphs of light measured by photometric devices D1 and D2 while high power Nd-YAG / OPO laser pulses are subjected to crystallized Ge-Sb-Te coated on a c-Si substrate.

The present invention for achieving the above object, the He-Ne laser light source for emitting linearly polarized light, the first light splitter for dividing the He-Ne laser light source light into reflected light and transmitted light, and each of the separated light orthogonal to each other Two uraston prisms that are subdivided into two linearly polarized light, four quantometers each measuring the intensity of light with four different polarization states divided through the ulaston prism, and analogues detected from the photometer In the ultra-high speed ellipsometer using an amplitude division polarization meter including an oscilloscope that converts a signal into a digital signal and outputs it as a constant waveform, 1 / of converting the linearly polarized state of the light emitted from the He-Ne laser light source into circularly polarized light. 4 λ phase plate; and linear polarizer for transmitting the light passing through the 1/4 λ phase plate to a desired azimuth angle; When the light emitted from the Ne laser light source passes through the 1/4 λ phase plate and the linear polarizer is irradiated, it is refracted and disposed so as to be incident to the first light splitter. A crystallized Ge-Sb-Te alloy sample; and a range of incident incident to the sample to check whether the measured values output from the photometer and the oscilloscope match the phase change of the sample and to control the phase change of the sample. The irradiation time of the variable wavelength high power Nd-YAG / OPO laser emitting the pulse and the light emitted from the Nd-YAG / OPO laser is controlled from 1/1000 to several seconds so that only a single pulse of the laser pulse train is incident on the sample. A light generating means having a shutter and a red filter for filtering only a pulse of a predetermined range among pulses emitted from the Nd-YAG / OPO laser; and the Nd-YAG / OPO laser; A second optical splitter disposed on the movement path of the laser pulses between the specimens and dividing a portion of the laser pulses and transmitting the divided optical beams to the photometer connected to the oscilloscope; and the reading means is emitted from the He-Ne laser light source. Changes in the Mueller matrix of the sample appearing as the amorphous Ge-Sb-Te turns into crystalline Ge-Sb-Te when the light is reflected off the surface of the Ge-Sb-Te sample after passing through a linear polarizer with + 45 ° azimuth. According to dyadM` (m_ij), (However, it is calculated by defining as j j == 1, 2, 3, 4), wherein the first light splitter is a sample of 78 nm thick MgF ₂ coated on a ZnS substrate. The above object and various advantages of the present invention are more clearly understood by those skilled in the art from the preferred embodiments of the present invention described below with reference to the accompanying drawings. Will be.

In detail, the objective of the present invention is that a conventional ellipsometer based on a modulation scheme requires a response time of several nanoseconds compared to a measurement time of several tens of microseconds to several tens of ms to obtain one elliptic constant pair (Δ, Ψ). In related elliptic applications they show limitations. For example, in the high temperature region where data is actually recorded and erased on a phase-change optical recording medium, Ge-Sb-Te, the phase change process occurs within several tens of nanoseconds, and thus, in the nanosecond time zone, such as a phase change process that changes to an amorphous or crystalline phase. In order to observe the reaction in real time, an ultra-fast ellipsometer with nanosecond time resolution is required. The problem to be solved by the present invention is to overcome the conventional concept of measuring the polarization state based on the mechanical modulation method, the electrical modulation method or the interference method (Division-of-amplitude photo-polarimeter; Hereinafter, the purpose is to develop an ultra-fast ellipsoidal system using DOAP).

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

First, the background of the technical concept applied to the present invention should be defined for Stokes variables.

The Stokes parameter, proposed by Stokes in 1852, is a powerful way of describing the polarization state of light. Stokes variables are the average values of two orthogonal electric field components of light traveling in the z-axis, E _x and E _y , respectively.

That is, when the electric field is given as a variable of time, the electric field may be defined as in Equation 1 below.

In this case, the polarization state may be represented by an equation of an ellipse that changes with time as shown in Equation 2 below.

In Equation 2 Represents a phase difference. The above elliptic equation is usually expressed in the form of an average over a period of time, as shown in Equation 3 below.

In Equation 3, four Stokes parameters representing polarization from two electric field components orthogonal to the x and y directions are defined as shown in Equation 4 below.

The four Stokes parameters make it possible to clearly describe not only the complete polarization state but also general polarization states such as no polarization and partial polarization.

Thus, by determining the Stokes parameters, one can completely describe any polarization state, and one typical method of measuring Stokes parameters is to use a phase delay and a linear polarizer with a 90 ° delay angle.

Therefore, in the arrangement as shown in FIG. 4, the fast axis of the phase delay forms an angle between the x axis and phi` (the azimuth angle of the phase delay is phi`), and the quadrature wavelength phase delay (phase delay between the fast axis and the slow axis). An angle of 90 °), the azimuth of the linear polarizer's transmission axis with the x axis is theta, and the electric field of the light entering the phase retarder is The intensity may be defined as in Equation 5 below.

In this case, using the four Stokes parameters of the light intensity can be expressed as shown in Equation 6 below.

Therefore, the Stokes variable can be determined by measuring the intensity of light by changing the azimuth angle of the phase delay and the linear polarizer, and one method of determining the Stokes variable using a combination of several simple azimuth angles is presented in Equation 7 below.

In addition to the method of using the phase delay and the linear polarizer described above, there is a method using a division-of-amplitude photo-polarimeter (DOAP) as a method for determining the Stokes parameters.

The core polarizers constituting the DOAP include two first beam splitters (BSs) for dividing incident light into reflected light and transmitted light, and two uraston prisms for dividing each of the separated light into two linearly polarized lights that are orthogonal to each other. Wollaston prism (WP).

That is, as shown in FIG. 5, the light splitting through the first splitter passes through the ulaston prism and is divided into four different polarized lights. The basic structure of is completed.

The DOAP can be used to obtain Stokes variables as follows. Stokes vector representing the polarization state of the light incident on the first splitter , Stock vectors of reflected light among light split by the first splitter Stock vectors of light and transmitted light Can be written as Equation 8 below using dyadR `=` (r_ij), which is the reflection Mueller matrix of the first optical splitter, and dyadT `=` (t_ij), which is the transmission Mueller matrix.

On the other hand, a vector created as the first row in the Mueller matrix representing the polarization of an ideal linear polarizer with an azimuth angle of θ. Is defined as in Equation 9 below.

Therefore, when the light reflected from the sample surface passes through the first light splitter and passes through the ulastone prism having the azimuth angles of θ ₁ and θ ₂ , respectively, the intensity of light detected by each photometer is expressed by Equation 10 below. .

Equation 10 is a 4x4 characteristic matrix dyadM` (m_ij) representing the polarization of the first light splitter and two wool prisms and a stock state vector indicating the polarization state of the incident light as shown in Equation 11 below. Can be redefined as a product of

In Equation 11, the component of the characteristic matrix dyadM` (m_ij) is represented by Equation 12 below.

However, j `=` 1, `2,` 3, `4.

Lastly, by inversely converting Equation 11 to obtain an expression regarding the Stokes vector of the incident light, Equation 13 may be defined as follows.

At this time, the dyadM` matrix can be calculated directly from the Mueller matrix of the first light splitter and the ulaston prism, that is, the polarizing elements constituting the dyadM`_BS` and dyadM`, respectively. _ {WP} ` Becomes

On the other hand, as shown in Figure 4 attached by using a phase retarder and a linear polarizer to adjust their azimuth angle in the optical system to create four different polarization states and each of them incident on the DOAP to measure the intensity of light measured by the four light meter After the calculation, inverse calculation of Equation 11 may be performed to determine the dyadM` matrix.

On the basis of the above-described theory will be described in detail with respect to the ultra-fast ellipsometer according to the present invention.

DOAP is a passive method of obtaining the polarization state from the intensity of light passing through a static polarizer without relying on active methods such as mechanical rotation or electrical phase modulation. It is determined by the response time of the photometric device and the signal processing speed for A / D conversion of the measured light intensity.

Therefore, in order to verify the ultra-high speed ellipsometer using the DOAP to achieve in the present invention, the ultra-high speed light quantity measuring system was constructed as follows.

Using a photometric device (ThorLabs, DET210) with a response time of 1 바꾸, the light intensity is converted into an electrical signal, and a 4-channel digital oscilloscope with a frequency band of 500 M㎐ per channel is used to convert the electrical signal into a digital signal. (Yokogawa, DL7100) was also used to have a time resolution of 1 ns.

First, in order to verify the response time of this light quantity measuring device, the second harmonic of the high-power Nd-YAG laser (BM Industries: ND 6000) is passed through an optical parametric oscillator (OP) (OPO) so that the pulse width is 7 ns. The pulses were detected and viewed.

In FIG. 6, it can be seen that the present photometric device, which captures the intensity change of a laser having a pulse width of 7 ns and is a combination of a photometric device and a digital oscilloscope, can obtain data with a time resolution of nanoseconds.

As described above, the basic structure of the ultrafast elliptic system manufactured by using the DOAP method according to the present invention is as shown in FIG. 7 and it was confirmed that the ultrafast elliptic system had a nanosecond time resolution.

Using a shutter that can adjust the time from 1/1000 second to several seconds, only a single pulse of the laser pulse train emitted at a repetition rate of 10 Hz from the variable wavelength high-power Nd-YAG / OPO laser is incident on the sample.

Part of the incident light was sent to the photometric device connected to the external trigger signal terminal of the oscilloscope using a second optical splitter. In order to control the elliptic state, the light incident on DOAP was passed through a 1 / 4lambda phase plate into a linearly polarized light emitted from 632.8 nm He-Ne laser, and then passed through a linear polarizer set to a desired azimuth. As a sample, a crystallized Ge-Sb-Te alloy sample coated on the c-Si base layer of single crystal silicon thicker than the transmission depth of light was used.

When the signal is input to the external trigger terminal, the photometric system is initially set so that the A / D conversion of the signal input to the four channels of the digital oscilloscope starts, that is, the synchronized A / D conversion. The change in refractive index that occurs in the process of changing the amorphous Ge-Sb-Te into the crystalline Ge-Sb-Te is identified as the change in the intensity of light measured by each photometer.

The computational process of this procedure shows that the amorphous Ge-Sb-Te is crystalline Ge-Sb- when the light emitted from the He-Ne laser light source reflects off the surface of the Ge-Sb-Te sample after passing through a linear polarizer with + 45 ° azimuth. An approximate method was used to obtain the change in the Mueller matrix of the sample appearing by changing to Te in equation (11). When Ge-Sb-Te is crystallized, using the elliptic constants (tanΨ, cosΔ) observed in FIGS. 3A and 3B, the change of the Mueller matrix may be defined as in Equation 14 below.

The first light splitter was used by arranging the sample coated with 78 nm thick MgF ₂ on the ZnS substrate so that the incident angle was 78.33 °.

The azimuth angle of the optical axis of the wool prisms to the incidence plane is set to 0 °, 30 °, 45 °, 60 °, and 90 °, respectively. In each case, the graph shows the change in the intensity of light measured by the photometer. As shown in Figs. 8A to 8D.

The light reflected from the first splitter during the crystallization process showed the largest change when the azimuth angle of the uraston prism was 90 °, and the light transmitted through the first splitter was one of the two orthogonal components that passed through the uraston prism. One shows the maximum change at an azimuth of 90 ° and the other at 0 °. On average, they show relatively large changes in common at 30 ° azimuth.

Ge-Sb-Te observed with a single pulse of high power Nd-YAG / OPO laser with a wavelength of 540 nm and a pulse width of 7 ns on a sample with a crystallized Ge-Sb-Te thin film with a thickness of 2000 μs on the c-Si substrate. Changes in the sample are the same as in FIGS. 3A and 3B.

The laser energy density at the sample surface is approximately 35 mW / cm ² per pulse. Since the depth of light penetration into the Ge-Sb-Te of a laser with a wavelength of about 540 nm is about several hundreds of microns, most of the absorbed laser energy is converted into thermal energy in the Ge-Sb-Te thin film layer, and this amount of heat gradually goes toward the c-Si substrate Delivered. The temperature of Ge-Sb-Te and c-Si is changed by the thermal action of the high power laser pulse, and the polarization state of the reflected light is changed by the complex refractive index change caused by this temperature change.

When measured without using the red filter (see attached FIG. 9A), the peak of the same shape as the laser output of FIG. 6 was observed simultaneously with the laser pulse, and then a relatively small change was observed and about 30 ns was observed. After elapse, the light intensity decreases to the size before the laser pulse.

In the case of using a red filter on the front of the photometric device (see FIG. 9B attached), the peak portion caused by the laser pulse observed in the accompanying FIG. 9A is greatly reduced, while the subsequent relatively small change shows that the magnitude thereof remains the same. can see.

This can eliminate the influence of the scattered light of the high power laser pulse by using a red filter, and the change observed using the red filter is due to the change in temperature and complex refractive index of the sample by the laser pulse. The change due to temperature or complex refractive index does not show monotonic decrease, and the increase and decrease in the form of the heat transfer to each part of the sample during and after the laser pulse and the temperature change of each part Provides dynamic information at a nanosecond time resolution.

Moreover, when normalizing and comparing the intensity of light measured by four light quantity measuring devices, it is expected that information on the complex refractive index change of the sample will be obtained in addition to the dynamic heat transfer and temperature distribution information.

While the invention has been shown and described in connection with specific embodiments thereof, it is well known in the art that various modifications and changes can be made without departing from the spirit and scope of the invention as indicated by the claims. Anyone who owns it can easily find out.

Providing the ultra-fast ellipsometer according to the present invention as described above, has the advantage of providing additional phase information compared to the existing optical equipment, but has a nanosecond time resolution. As observed by the change in temperature or refractive index of the phase-change optical recording medium, the process of increasing or decreasing the temperature or complex refractive index during the nanosecond time interval changes the laser energy to heat and transfers the heat amount to each part of the sample. And dynamic information occurring in the sample, such as temperature changes in each section, and nanosecond time resolution.

In addition, the ultra-fast ellipsometer according to the present invention will be a useful tool suitable for measuring and analyzing many dynamic mechanisms with complex refractive index changes or surface or thin film structure changes with nanosecond time resolution as well as phase change mechanism analysis. .

Claims (6)

He-Ne laser light source that emits linearly polarized light, a first splitter that divides the He-Ne laser light source light into reflected and transmitted light, and two urastones that divide each separated light into two orthogonal linearly polarized light. Four photometers, each measuring the intensity of light having four different polarization states divided through the prism and the uraston prism, and converting analog signals detected from the photometers into digital signals and outputting them as constant waveforms. In the ultra-fast ellipsometer using an amplitude division polarization meter containing an oscilloscope A 1/4 λ phase plate for converting a linearly polarized state of light emitted from the He-Ne laser light source into circularly polarized light; A linear polarizer for transmitting the light passing through the 1/4? Phase plate at a desired azimuth angle and transmitting the light; The transmission depth of light on the c-Si substrate, which is a single crystal silicon, is arranged to be refracted by the light emitted from the He-Ne laser light source and passed through the 1/4 λ phase plate and the linear polarizer to be incident to the first light splitter. Thicker coated crystallized Ge-Sb-Te alloy sample; Variable wavelength high power Nd-YAG / which emits a range of pulses incident on the sample to check whether the measured values output from the photometer and oscilloscope match the phase change of the sample and to adjust the phase change of the sample OPO laser, shutter that emits light from Nd-YAG / OPO laser from 1/1000 second to several seconds so that only a single pulse of the laser pulse is incident on the sample, and emits from Nd-YAG / OPO laser Light generating means having a red filter for filtering only pulses of a predetermined range among the pulses; A second optical splitter disposed on a movement path of the laser pulse between the Nd-YAG / OPO laser and the sample to divide a portion of the laser pulse and transmit the divided portion to a photometer associated with the oscilloscope; Ultra-fast ellipsometer using an amplitude division polarization meter characterized in that it further comprises. delete delete delete The method of claim 1, The reading means indicates that the amorphous Ge-Sb-Te is crystalline Ge-Sb-Te when the light emitted from the He-Ne laser light source reflects off the surface of the Ge-Sb-Te sample after passing through a linear polarizer having an azimuth angle of + 45 °. Change the Mueller matrix of the sample According to dyadM` (m_ij), (Where j `=` 1, `2,` 3, `4) An ultra-fast ellipsometer using an amplitude division polarization meter, characterized in that it is calculated approximately by defining it as. The method of claim 1, The first optical splitter is an ultra-fast ellipsometer using an amplitude division polarization meter, characterized in that the sample coated with 78 nm thick MgF ₂ on the ZnS substrate at an angle of 78.33 °.