APPARATUS AND METHOD FOR DETERMINING PHYSICAL PROPERTIES OF FERROELECTRIC SINGLE CRYSTAL USING SPECTROSCOPIC ELLIPSOMETRY FIELD OF THE INVENTION The present invention relates to a spectroscopic ellipsometry apparatus for determining physical properties of a ferroelectric single crystal comprising a series of an optical device, a temperature controlling device and a computer; and a method of determining the dielectric constant or phase transition temperature of the single crystal using same.

BACKGROUND OF THE INVENTION Spectroscopic ellipsometry is capable of providing optical, surface and structural properties of an object under a working condition instead of a high vacuum environment by way of measuring the phase and amplitude changes of a polarized light reflected by the object surface, and therefore, it is widely used in physics, chemistry, semiconductor and material applications.

The conventional method of determining the dielectric constant of a ferroelectric material is based on obtaining two ellipsometric angles with a conventional ellipsometry apparatus and calculating the dielectric constant based on the following equation: s s=ff2=sin20 (1+tall20 p/l+p) 2) wherein, s is dielectric constant; N, refractive index; 0, incident angle; and p, complex reflection coefficient rate.

However, the frequency of the light used in spectroscopic ellipsometry is much higher than that of the AC power generally used in measuring the dielectric constant of a single crystal material, and as the dielectric constant value varies widely with the frequency, spectroscopic ellipsometry has not, hitherto, been used in measuring the dielectric constant of a single crystal.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an

apparatus which can be used to rapidly determine the physical properties of a ferroelectric single crystal using spectroscopic ellipsometry.

It is another object of the present invention to provide a method of determining the dielectric constant or phase transition temperature of a single crystal using same.

In accordance with one aspect of the present invention, there is provided an apparatus for determining physical properties of a ferroelectric single crystal, which comprises; (A) an optical device comprising a visible light source, a single crystal holder, a polarized light generator located between the light source and the single crystal, a polarized light analyzer which is reflected by the single crystal, and a spectrum detector which determines spectral characteristics of the polarized light passed through the polarized light analyzer; (B) a temperature controlling device comprising a heater disposed in close contact with the single crystal to control the temperature thereof and a heater-controlling means; and (C) a computer connected with said optical device which converts spectrum values detected by the spectrum detector to the desired property values via programs.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and features of the present invention will become apparent from the following description of the invention, when taken in conjunction with the accompanying drawings, which respectively show: FIG. 1 : a schematic diagram of the apparatus of the present invention for determining physical properties of a ferroelectric single crystal; FIG. 2: the distribution of dielectric constant values on the surface of a ferroelectric single crystal wafer measured at regular intervals by spectroscopic ellipsometry; FIG. 3: a graph exhibiting the correlation between the dielectric constant values of a ferroelectric single crystal obtained by spectroscopic ellipsometry and those by AC application; and FIGs. 4A and 4B: graphs depicting phase transition temperatures of a ferroelectric single crystal obtained by the inventive method and the

conventional method, respectively.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 represents a schematic diagram, of the apparatus of the present invention for determining physical properties of a ferroelectric single crystal, and it comprises an optical device composed of a light source (10), a polarized light generator (20), a single crystal holder (30), a polarized light analyzer (40) and a spectrum detector (50); a temperature controlling device composed of a heater (31) and a heater-controlling means (70); and a computer (100). The inventive apparatus is manipulated by an electricity source feeder (80) and a driving unit (60) after a ferroelectric single crystal sample is inserted in the holder (30). For convenience, the optical device and other parts may be enclosed in a case (90).

A representative procedure for analyzing the properties of a ferroelectric single crystal using the inventive apparatus is as follows; the light emitted from the light source (10) passes through the polarized light generator (20) controlled by the driving unit (60) to be polarized to a set direction. The polarized light whose wavelength is varied in the range from 200 to 600 nm is guided to the surface of a single crystal with a specific incident angle. The incident light is reflected on the surface of the single crystal. The resulting reflected polarized light whose amplitude and phase depend on the surface characteristics of the crystal is analyzed with the polarized light analyzer (40), and its intensity and two ellipsometry angles, A and lIJ, are determined with the spectrum detector (50). The measured values are then sent to the computer (100) to be converted to the desired property values using specific programs.

The temperature-dependent properties of a single crystal may be measured by changing the temperature profile thereof with the heater (31).

As one embodiment in accordance with the present invention, there is provided a method of determining the dielectric constant of a ferroelectric single crystal using the inventive apparatus, which comprises the steps of : (a) determining low-frequency dielectric constant values measured by the alternative currency (AC) application method at preset sites of a ferroelectric single crystal sample; (b) determining high-frequency dielectric constant values measured by

spectroscopic ellipsometry at said preset sites of the single crystal sample ; (c) establishing a correlation between the low-frequency and high- frequency dielectric constant values obtained in steps (a) and (b), respectively, and inputting the correlation to the computer (100) of the inventive apparatus; and (d) measuring the high-frequency dielectric constant value of a ferroelectric single crystal target having the same composition as the sample, which is converted to the low-frequency dielectric constant value using the computer.

The measurement of the low-frequency dielectric constant by AC application in step (a) is performed by depositing electrode materials on the upper and lower sides of the single crystal and then applying an AC field thereto. The frequency of the applied AC is low in the range of 1 to 100 lcHz, and the frequency of the light used in spectroscopic ellipsometry, high in the range of 1.2 x 10'5 to 6.0 x 10'S Hz.

The inventive method can provide the dielectric constant value of a strong dielectric (the low-frequency dielectric constant value) more conveniently and rapidly than the conventional AC application method, and it can further be used to establish a dielectric constant map of the whole surface of a target.

For instance, a high-frequency dielectric constant map of a ferroelectric single crystal (PMN-PT: 0.65 [Pb (Mgli3Nb2/3) 03] 0. 33 [PbTiO3] 0. 02 [LiTaO3]- l [Pt] l [NiO]) wafer prepared by the method disclosed in Korean Publication Patent No. 2001-96505 can be determined by spectroscopic ellipsometry as shown in FIG. 2.

The measuring points are arranged at intervals of 5 mm width x 5 mm length, and the frequency of the incident light was 1015 Hz. The area (200) shown in FIG. 2 represents an area having an even distribution of large dielectric constant values.

FIG. 3 depicts a graph exhibiting the correlation between the above dielectric constant values and those of the single crystal wafer mentioned in FIG.

2 which are obtained by the AC application method. For the AC application, the single crystal wafer was coated with electrode materials prior to voltage application at a frequency of 104 Hz. The correlation established in FIG. 3 can be expressed by the following equation:

ioih, =-7278. 15 + 1382. 18 e lip,, Subsequently, the s ellipso value of a ferroelectric single crystal target having same composition as said wafer is measured and the c value thus determined is converted to the dielectric constant value (e lo/, H,) using the above-mentioned equation, wherein e io/c is the low-frequency dielectric constant value obtained by AC application at a frequency of 1 01çHz ; and 8 eZlipso is the high-frequency dielectric constant value obtained by spectroscopic ellipsometry.

As another embodiment in accordance with the present invention, there is provided a method of determining the phase transition temperature of a ferroelectric single crystal by way of raising the temperature of the single crystal mounted in the inventive apparatus and observing the temperature at which the ellipsometric angle obtained in the spectrum detecter suddenly changes.

A ferroelectric material exhibits a ferroelectric property around room temperature, but tends to exhibit a reduced ferroelectric property or completely lose such a property due to the alteration of its crystal structure at above a certain temperature. Such a temperature is referred as to Curie temperature (Tc). The phase transition temperature has been conventionally determined by observing the change in the dielectric constant with temperature while heating in a liquid medium a single crystal sample having both sides coated with electrode materials.

In accordance with the present invention, as the dielectric and optical properties of a material are of the same origin, the phase transition temperature can be more conveniently and rapidly determined by detecting the temperature- <BR> <BR> dependent change in the optical property, i. e. , ellipsometric angle, using the inventive apparatus as compared with the conventional method, and furthermore, it is possible to judge the uniformity of the composition of the whole target surface by determining a phase transition temperature map thereof.

For instance, the surface distribution of the phase transition temperature of a ferroelectric single crystal (PMN-PT: 0.65 [Pb (Mgi/3Nb2/3) 03] 0. 33 [PbTiO3] 0. 02 [LiTaO3]-1 [Pt] 1 [NiO]) wafer prepared by the method disclosed in Korean Publication Patent No. 2001-96505 can be determined by the inventive method.

FIG. 4A shows the temperature-dependent change in ellipsometric

angle,, as determined in accordance with the inventive method, and FIG. 4B, the temperature-dependent change in the dielectric constant, in accordance with the conventional method.

First, it can be seen in FIG. 4B that there are sudden changes in the dielectric constant at about 105°C and about 140°C, wherein 140°C corresponds to the above-mentioned Curie temperature, and 105°C, the temperature (Trt) at which the rhombohedral crystal structure of the wafer shifts to a tetragonal crystal structure.

FIG. 4A reveals that relatively sudden changes in the ellipsometric angle occur at two temperature points, 105°C and 136°C, which respectively match with the phase transition temperatures observed in FIG. 4B.

As described above, the apparatus and method of the present invention provide a simple and economical means for determining the dielectric constant and phase transition temperature of a ferroelectric single crystal of any type under any condition, and, it is particularly useful in establishing the uniformity of the dielectric constant and the composition of the entire surface of a single crystal.

While the invention has been described with respect to the above specific embodiments, it should be recognized that various modifications and changes may be made to the invention by those slcilled in the art which also fall within the scope of the invention as defined by the appended claims.