Study on Broad Spectral Properties of K 9 Glass Substrate by Ellipsometry

Tang Fanbin

(Public Education Department,Nanning College,Nanning,Guangxi 530200)

Abstract:In order to obtain the optical constant of K9 glass substrate,the SE850 broad spectral reflectance type ellipsometer produced by SENTECH in Germany was used to measure and analyze the K9 glass substrate,and the optical constant curve of K9 glass on the 380nm(2300nm)wide spectrum was obtained.Elliptical curves with incident angles of 60°,65°,and 70° were respectively measured,the simplest base/air model was adopted for fitting,chose the most appropriate description of amorphous semiconductor Tauc-lorentz dispersion formula model fitting for K9 glass,when the incident Angle was 70°,got minimum mean square error(MSE),at the same time,the refractive index curve of K9 glass in the spectral rangeof 380nm-2400nmwasobtained.

Keywords:K9 glass;ellipsometer;refractiveindex

K9 glass is a glass product made of K9 material,mainly used in optical coating and other fields.The chemical groups of K9 are:SiO2=69.13%，B2O3=10.75%，BaO=3.07%，Na2O=10.40%，K2O=6.29%，As2O3=0.36%.K9 glass has a high transmittance in the range of visible light to near infrared spectrum and is widely used as the substrate material for various optical instruments[1].Most of the studies on various properties of K9 glass focus on the anti-laser damage threshold of K9 thin film by laser,including the damage study of K9 glass by continuous laser[2][3]and the damage study of K9 glass by pulse laser[4].The ellipsometry method has many advantages,and there are few studies on the optical properties of K9 glass analyzed by ellipsometer.The study on the basic optical properties of the material has a good promotion effect on the subsequent studies on various properties.In this paper,the SE850 reflectance spectroscopic ellipsometer produced by SENTECH in Germany was used to obtain the ellipsometry curve of K9 glass sample in the wide spectral range of 380nm～2300nm,and the optical constant curve of K9 glass was obtained at the same time,providing a reference for the subsequent study of using K9 glass asthebasematerial.

I.Principle of Measurement

According to the propagation characteristics of ellipsoidal light,when the polarized light hits the film sample at a certain angle,the light reflects on the upper and lower surfaces of the film and meets a series of conditions of coherent light,which belongs to coherent light.According to the difference of phase difference,the polarization state of light will also change accordingly.Ellipsometry parameter(，)is introduced to express the change of polarization state before and after,and the parameter satisfiesthefollowing expression[5]:

In the form,rpand rsare Fresnel reflection coefficients of p component and s component of light respectively,is the difference of reflection phase of p component and s component,andis polarization angle.The phase differencevaries from 0 to 2,and the polarization Anglevaries from0 to/2.

The ellipsometry method is used to obtain the physical parameters of the ellipsometry,andthe physical parameters such as film thickness and optical constant(refractive index n and extinction coefficient k)are required for the measurement.The relation can beexpressed asfollows[6]:

Jellison and Modine creatively proposed tauclorentz model[7]in 1996,based on the relation between Tauc joint state density and Lorentz oscillator model in the study of applicable models of low-index materials.In the TL model,the real part e1and the imaginary part e2of the dielectric constant of thethin filmcan beexpressed as:

In the form,P represents the cauchy principal value of the integral.Among them,the parameters to be fitted for the TL model are:,E0(transition energy),Eg (bandgap energy),A (amplitude parameter),and C(broadening parameter).Whereis a constant and E is the photon energy.Elliptic partial reference number is obtained by TL model inversion(,),through type(2)(3)to get the parametersof themembranematerial.

The ellipsometry parameter curve is obtained by setting the incident angle of the instrument.Take the measured elliptical parameters sum as the target file,select an appropriate modelandset appropriate parameters in the model for fitting.When the mean square error(MSE)of fitting results is small enough,the set parameters and model are considered to be the real values of the tested material,and other parameters of the sample can be obtained[8].The evaluation of the fitting results is first to determine whether the fitting results conform to the characteristics of the material itself,and then to analyze fromthe size and value of MSE,where the mean squaredeviation is:

II.Measurement and Analysis

Place the cleaned K9 glass on the ellipsometer test bench,and then adjust the test bench and the ellipsometer light.In all of the measurement parameters (energy density, test points and measuring time,etc.)set up after the measured in the Angle of incidence is 60°,65°and 70°of the elliptic curve,the simplest base/air model was adopted for fitting,in which the K9 glass dispersion model was selected with Tauc-Lorentz dispersion formula suitable for describing amorphous semiconductor.The curves of the corresponding elliptical parameters(,)at three different incident angles obtained after final fitting are shown in FIG.1,FIG.2 and FIG.3 respectively.

Figure 1 The Ellipsometry Parameter Curveof the Incidence Angleof 60o

Figure2 The Ellipsometry Parameter Curve of the Incidence Angle of 65o

Figure3 The Ellipsometry Parameter Curve of the Incidence Angle of 70o

It can be seen from figure 1,2 and 3 that there are a few pulses in the elliptical curve of the elliptical parameter D from three angles,which is caused by the instability of the signal source,while the difference in the elliptical parameter Y is not significant.Finally,the mean square error(MSE)of the corresponding Angle and the refractive index and extinction coefficient of K9 glass calculated by inversion are listed in Table1.

Table1 Resultsof Ellipsometry Measurement of K9 Glass

It can be seen from table 1 that the MSE and extinction coefficient at the incident Angle of 70oare both smaller than the other two incident angles,and the refraction index at 550nm obtained at the incident Angle of 70ois 1.51368 and the extinction coefficient is 0.01115,which are consistent with the values in the optical manual[9].The refractive index curve of K9 glass at 380nm～2400nm at an incident Angleof 70oisalsoobtained,asshown in Figure 4.

Figure 4 The Refractive Index Curve of K9 Thin Filmt

III.Conclusion

In this paper, reflectance spectroscopic ellipsometry was used to measure the K9 glass sample from three different incident angles,and the measured data were fitted with the base/air model established by Tauc-Lorentz dispersion formula.After analyzing,we can get when the incident Angle is 70°mean square error(MSE)minimum,at the same time get the K9 glass from 380 nm to 2400 nm spectral range of refractive index curve,follow-up in K9 glass substrate for coating made the prophase research work.