We have optimized and automated the experimental in-situ reflectivity measurement system for the laser diode (LD) facet coating. We have also developed a reflectivitysimulator
program that gives the reflectivity data as a function of the thickness of the film (single or multi-layer) for a given wavelength, which aids in optimizing the above parameters while monitoring the coating of the films in-situ. We report the
results for the in-situ reflectivity of a single layer MgF2 and a quarter-wave optical thick three bi-layer pairs of MgF2 and silicon on GaAs as a substrate for both the cases. We have achieved up to 83 % experimental reflectivity for the latter case.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2207

Bhatt, G.G.

Desai, M.S.

Kheraj, V.

Panchal, C.J.

Electronic Sumy State University Institutional Repository

   J. Nano- Electron. Phys. 3 (2011) No1, P. 721-727 Ó 2011 SumDU (Sumy State University)   721 PACS numbers: 68.65.Ac, 78.20.Ci  THE OPTIMIZATION OF OPTICAL THIN FILMS DEPOSITION USING IN-SITU REFLECTIVITY MEASUREMENTS AND SIMULATION   G.G. Bhatt1, Vipul Kheraj2, M.S. Desai1, C.J. Panchal1  1 Applied Physics Department, Faculty of Tech. & Engg.,  The M. S. University of Baroda, Vadodara (Gujarat), India  Email: cjpanchal_msu@yahoo.com 2 Department of Applied Physics,  Sardar Vallabhbhai National Institute of Technology, Surat, India   We have optimized and automated the experimental in-situ reflectivity measurement system for the laser diode (LD) facet coating.  We have also developed a reflectivity-simulator program that gives the reflectivity data as a function of the thickness of the film (single or multi-layer) for a given wavelength, which aids in optimizing the above parameters while monitoring the coating of the films in-situ.  We report the results for the in-situ reflectivity of a single layer MgF2 and a quarter-wave optical  thick three bi-layer pairs of MgF2 and  silicon  on  GaAs  as  a  substrate  for  both  the  cases. We have achieved up to 83 % experimental reflectivity for the latter case.   Keywords: THIN FILM, MULTILAYERS, IN-SITU, REFLECTIVITY, SIMULATION.  (Received 04 February 2011, in final form 13 October 2011)  1. INTRODUCTION  Thin film optical coating is used for modulating the reflectivity in different kinds of optical components viz. beam splitters, optical filters, polarizers, lenses  of  cameras  and  telescopes,  including  anti-reflection  (AR)  and  high-reflection (HR) coating on the laser diode facets [1]. Owing to its importance, it is essential to monitor the thin film deposition parameters like the deposition rate, substrate temperature, and thickness of the thin film during the deposition.  Although various optical in-situ measurement systems are available, most of these systems employ a very sophisticated instrumentation and are quite com-plex. We have developed an in-situ reflectivity measurement system for optimi-zation of facets-coating process for laser diodes. We have performed in-situ ref-lectivity measurements on single layer MgF2 and a quarter-wave optical thick (QWOT) three bi-layer pairs of MgF2 and  silicon  on  GaAs  as  a  substrate  for  both the cases. The measurements were optimized using a simulation program that gives the reflectance of non-absorbing dielectric single or multilayer, or QWOT bi-layer optical facet coatings for the Laser Diode (LD) using LabVIEW (laboratory virtual instrument engineering workbench) (version 8.2). Since the set-up is custom-built for our specific application, it is quite simple, cost-effec-tive and efficient tool for quick optimization and automation of the process.  The mathematical equations used in the simulation to calculate reflectivity of the optical thin-films followed by program execution are discussed in the next section. The experimental detail for the in-situ measurement of reflecti-vity has been described in section 3. Results of the simulated reflectivity spectra for various optical thin-films are presented in section 4.    722 G.G. BHATT, VIPUL KHERAJ, M.S. DESAI, C.J. PANCHAL  2. THEORETICAL BACKGROUND  2.1 Mathematical treatment    Fig. 1 – The light reflected from the air-film and the film-substrate boundary produces interference.  Fig. 1 shows light incident on an air-film interface. The light reflected from the air-film and the film-substrate interface give rise to the interference phenomenon, which results in the change in the reflected beam intensity as the film grows on to the substrate. The reflectance co-efficient, r, of a non-absorbing multilayer dielectric thin-film is given as [2]:   + - -=+ + +0 11 0 12 21 220 11 0 12 21 22            s ss sY m Y Ym m YmrY m Y Y m m Y m (1)  where, e m= '0 0 0 0Y n  and e m= '0 0S SY n with n0¢ and ns¢ being the effective refractive indices of the incident medium and the substrate, respectively, and are given as n¢0 = n0cosq0 and n¢s = nscosqs for s or perpendicular polarization. In case of p or parallel polarized light, n¢0 and n¢s are given as n¢0 = n0/cosq0 and n¢s = ns/cosqs.  Here,  n0 and ns are the refractive indices of the incident medium and the substrate whereas q0 and qs are the angles of incidence in the incident medium and the substrate, respectively. In equation (1), m11, m12, m21, and m22 are the elements of characteristic matrix, M, of the entire system with p layers in a multilayer stack, which is the resultant of the product (in proper sequence) of the individual 2 ´ 2 matrices for each layer in the multilayer stack, that is:    é ù= = ê úë û11 121 2 3 p21 22    M  M M M ... ... M     m mm m  (2)  However, the layer matrix is a complex matrix [2]. Hence, elements m12 and m21 in above equations are purely complex whereas the other two elements, i.e. m11 and m22, are real. Thus, the reflectance coefficient r is an imaginary quantity. So, the reflectance, R = r×r* gives,   ( ) ( )( ) ( )- + -=+ + +2 2' ' ' '0 11 22 0 12 212 2' ' ' '0 11 22 0 12 21s ss sn m n m n n m mRn m n m n n m m (3)  Equation (3) determines the reflectance of the non-absorbing multilayer dielectric thin-film. This equation is used in the computer simulation program.  Substrate Thin Film Air  d n0nf ns Incident light Interference occurs betweenreflected beams     THE OPTIMIZATION OF OPTICAL THIN FILMS DEPOSITION... 723   Fig. 2 – The front-panel of the LabVIEW program for reflectivity simulation of the optical thin film  2.2 Program execution  In the case of optical films, two parameters are necessary to determine the reflectivity of the thin film viz. the physical thickness and the refractive index of the material [3]. The other input parameters are the refractive index of the substrate material, wavelength of interest, and thickness (step size). All these parameters are provided on the front panel of the program, shown in Fig. 2. Fig. 3 shows the other control parameters to be fed in the input in program. The program executes as per the data flow arranged using the graphical programming code in the back panel of the program viz. block diagram.    Fig. 3 –  The  input  parameters  for  single  layer  (a)  and  multilayer  (b)  in-situ  reflectivity simulation  3. EXPERIMENTAL  3.1 Sample preparation   The deposition of single layer MgF2 and a QWOT three bi-layer pairs of MgF2 and silicon on GaAs as a substrate for both the cases was carried out under the high vacuum (10 – 5 mbar) using e-beam evaporation system (EBG-6K, HINDHIVAC). (b)(a)   724 G.G. BHATT, VIPUL KHERAJ, M.S. DESAI, C.J. PANCHAL  The GaAs test substrate was used to optimize the deposition conditions as well as the reflectivity of the coated material. The quartz crystal monitor was used for measuring the thickness of the sample. For the better control of the deposition parameters viz. thickness and deposition rate, e-beam evaporation system is interfaced with the thin film deposition controller (SQC-112C, Sigma Instruments). The substrate was rotated with 120 rpm inside the vacuum chamber during the deposition with the help of a dc motor in order to get uniform coating. The substrate was maintained at 100 °C substrate temperature using radiant heater during the deposition.  The deposition rate was 4 Å/s.  3.2 In-situ reflectivity measurement  LDPDSubstrateBeamsplitterChamber Ambientreflectingmirror   Fig. 4 – A schematic diagram of the experimental in-situ reflectivity measurement setup  Fig. 4 shows a schematic diagram of the experimental setup for measuring in-situ reflectivity. The laser beam enters in to the vacuum chamber through a glass view port of the chamber and reflects back on the photo-detector (BPW-34) from the sample at normal incident angle. We have used a     Fig. 5 – The LabVIEW front panel showing real time data acquisition as the light reflects from the sample and is detected by the photodetector     THE OPTIMIZATION OF OPTICAL THIN FILMS DEPOSITION... 725 commercially purchased red laser diode (l = 650 nm) as a probe light source for the experiment. To check the stability of the optical power of the laser diode, we operated the laser diode in CW mode continuously for a few minutes before starting the deposition and monitored its optical power as a function of time. We observed negligible variation in optical power. Thus, we  can  consider  the  optical  power  output  of  the  laser  diode  to  be  stable  during the in-situ reflectivity measurements.  We did not check for the variation of wavelength of the laser diode with time, but as we operate the laser diode at a very low power with controlled operating temperature, we assume that the wavelength of the laser diode is stable over the duration of the experiment. The signal from the photo-detector is fed to a computer through data acquisition card (NI-USB-6251). Fig. 5 shows the acquired data in the front panel of the program built in LabVIEW.  During the deposition, the film grows vertically on the rotating substrate and the light undergoes interference. The change in the intensity of the reflected laser light will produce a sinusoidal curve as the film grows. Fig. 6 shows the real time intensity data acquisition of the reflected laser beam from the sample during the deposition as a function of time.  4. RESULTS AND DISCUSSIONS  The single layer MgF2 film was deposited at room temperature on the GaAs substrate at the deposition rate of 4 Å/s. Fig. 6 shows the acquired data of the reflected laser light intensity versus time. It is well known that there is direct relation between the film thickness and the film deposition rate. So, one can easily estimate the thickness of the deposited film.    Fig. 6 – Intensity measurement curve as a function of time for the MgF2 thin film deposited on GaAs substrate.  Moreover, the natural reflectivity of the substrate GaAs is ~ 32 % and will be modified as the film grows on it. Hence, we can directly get the film reflectivity from the reflected intensity. Fig. 7 shows the experimental as well as the simulated data of the reflectivity of single layer MgF2. Likewise Fig. 8 shows the reflectivity plot for multilayer stack of material. The three QWOT bi-layer pair of MgF2 and Si where the former has a low refractive index and the latter has a high refractive index. Starting with the low refractive  index  material  coating  on  the  GaAs  substrate,  the  stack  of  the     726 G.G. BHATT, VIPUL KHERAJ, M.S. DESAI, C.J. PANCHAL  layers were deposited in alternation. We have achieved up to 83 % experimental reflectivity. The simulated data, which we have fitted, was fed refractive indices 1.38 and 3.5 for MgF2 and Si, respectively. In order to confirm the obtained results, we measured the reflectivity spectrum  of  the  coated  samples  ex-situ  with  the  standard  setup  using  a  broad-band light source, a monochromator, photo-detectors and a lock-in amplifier. Figure 9 shows the reflectivity spectrum for a three QWOT bi-layer pairs of MgF2-Si measured ex-situ. As shown in the figure, the reflectivity value obtained ex-situ at 650 nm is in good agreement with the value obtained in-situ at the end of the deposition.    Fig. 7 – In-situ reflectivity and simulated reflectivity plots for a single layer MgF2 (n = 1.43) deposited on GaAs substrate    Fig. 8 – In-situ reflectivity plot for three QWOT bi-layer pairs of MgF2-Si  There is a noticeable difference between experimental and simulated value of the reflectivity, especially at the higher thickness as shown in fig. 8. The measured  value  is  lower  as  compared  to  the  simulated  value  of  the  reflectivity. This is mainly due to the usage of bulk values of the material refractive indices during reflectivity simulations. However, it is known that oxides film grown using e-beam evaporation exhibit a variation in refractive     THE OPTIMIZATION OF OPTICAL THIN FILMS DEPOSITION... 727 index with film thickness [4]. We can easily modify the refractive index for a particular layer and estimate the value of its refractive index by matching the best-fit curve.  Wavelength (nm)550 600 650 700 750 800 850 900 950 1000Reflectivity (%)020406080100R=83.33 %  Fig. 9 – Reflectivity spectrum for three QWOT bi-layer pairs of MgF2-Si measured ex-situ  5. CONCLUSION  The in-situ reflectivity measurement of the single and multilayer optical thin films has been demonstrated. The change in intensity of the laser light after reflection has been measured to obtain the reflectivity as a function of thickness. Also, we have developed an in-situ reflectivity simulation for the optimization of the in-situ film deposition. The reflectivity simulator provides a very good tool for designing the optical thin films with desired reflectivity response. Also, one can use the simulator for determining the material and the deposition conditions viz., film thickness, and deposition rate for single or multilayer films.  REFERENCES 1. R. Overend and D.R. Gibson, Vacuum 43, 71 (1992). 2. V.A. Kheraj, C.J. Panchal, M.S. Desai, V. Potbhare, Pramana – Journal of Physics 72, 1011 (2009). 3. E. Hecht, Thin Film Optical Filters, 2nd ed. (Bristol: Adam Hilger Ltd: 1986). 4. S. Pellicori, Coating Mater. News 13, 1 (2003). 

The optimization of optical thin films deposition using in-situ reflectivity measurements and simulation

SocioEconomic Challenges Journal (Sumy State University)

 
 
 
J. Nano- Electron. Phys. 
3 (2011) No1, P. 721-727 
Ó 2011 SumDU 
(Sumy State University) 
 
 
721 
PACS numbers: 68.65.Ac, 78.20.Ci 
 
THE OPTIMIZATION OF OPTICAL THIN FILMS DEPOSITION USING 
IN-SITU REFLECTIVITY MEASUREMENTS AND SIMULATION  
 
G.G. Bhatt1, Vipul Kheraj2, M.S. Desai1, C.J. Panchal1 
 
1 Applied Physics Department, Faculty of Tech. & Engg., 
 The M. S. University of Baroda, Vadodara (Gujarat), India 
 Email: cjpanchal_msu@yahoo.com 
2 Department of Applied Physics, 
 Sardar Vallabhbhai National Institute of Technology, Surat, India 
  
We have optimized and automated the experimental in-situ reflectivity measurement 
system for the laser diode (LD) facet coating.  We have also developed a reflectivity-
simulator program that gives the reflectivity data as a function of the thickness of 
the film (single or multi-layer) for a given wavelength, which aids in optimizing the 
above parameters while monitoring the coating of the films in-situ.  We report the 
results for the in-situ reflectivity of a single layer MgF2 and a quarter-wave optical  
thick three bi-layer pairs of MgF2 and  silicon  on  GaAs  as  a  substrate  for  both  the  
cases. We have achieved up to 83 % experimental reflectivity for the latter case.  
 
Keywords: THIN FILM, MULTILAYERS, IN-SITU, REFLECTIVITY, SIMULATION. 
 
(Received 04 February 2011, in final form 13 October 2011) 
 
1. INTRODUCTION 
 
Thin film optical coating is used for modulating the reflectivity in different 
kinds of optical components viz. beam splitters, optical filters, polarizers, 
lenses  of  cameras  and  telescopes,  including  anti-reflection  (AR)  and  high-
reflection (HR) coating on the laser diode facets [1]. Owing to its importance, 
it is essential to monitor the thin film deposition parameters like the 
deposition rate, substrate temperature, and thickness of the thin film during 
the deposition. 
 Although various optical in-situ measurement systems are available, most of 
these systems employ a very sophisticated instrumentation and are quite com-
plex. We have developed an in-situ reflectivity measurement system for optimi-
zation of facets-coating process for laser diodes. We have performed in-situ ref-
lectivity measurements on single layer MgF2 and a quarter-wave optical thick 
(QWOT) three bi-layer pairs of MgF2 and  silicon  on  GaAs  as  a  substrate  for  
both the cases. The measurements were optimized using a simulation program 
that gives the reflectance of non-absorbing dielectric single or multilayer, or 
QWOT bi-layer optical facet coatings for the Laser Diode (LD) using LabVIEW 
(laboratory virtual instrument engineering workbench) (version 8.2). Since the 
set-up is custom-built for our specific application, it is quite simple, cost-effec-
tive and efficient tool for quick optimization and automation of the process. 
 The mathematical equations used in the simulation to calculate reflectivity 
of the optical thin-films followed by program execution are discussed in the 
next section. The experimental detail for the in-situ measurement of reflecti-
vity has been described in section 3. Results of the simulated reflectivity 
spectra for various optical thin-films are presented in section 4. 
 
 
 
722 G.G. BHATT, VIPUL KHERAJ, M.S. DESAI, C.J. PANCHAL  
2. THEORETICAL BACKGROUND 
 
2.1 Mathematical treatment 
 
 
 
Fig. 1 – The light reflected from the air-film and the film-substrate boundary 
produces interference. 
 
Fig. 1 shows light incident on an air-film interface. The light reflected from 
the air-film and the film-substrate interface give rise to the interference 
phenomenon, which results in the change in the reflected beam intensity as 
the film grows on to the substrate. The reflectance co-efficient, r, of a non-
absorbing multilayer dielectric thin-film is given as [2]: 
 
 
+ - -=
+ + +
0 11 0 12 21 22
0 11 0 12 21 22
     
  
     
s s
s s
Y m Y Ym m Ym
r
Y m Y Y m m Y m
 (1) 
 
where, e m= '0 0 0 0Y n  and e m= '0 0S SY n with n0¢ and ns¢ being the effective 
refractive indices of the incident medium and the substrate, respectively, 
and are given as n¢0 = n0cosq0 and n¢s = nscosqs for s or perpendicular 
polarization. In case of p or parallel polarized light, n¢0 and n¢s are given as 
n¢0 = n0/cosq0 and n¢s = ns/cosqs.  Here,  n0 and ns are the refractive indices 
of the incident medium and the substrate whereas q0 and qs are the angles of 
incidence in the incident medium and the substrate, respectively. In 
equation (1), m11, m12, m21, and m22 are the elements of characteristic 
matrix, M, of the entire system with p layers in a multilayer stack, which is 
the resultant of the product (in proper sequence) of the individual 2 ´ 2 
matrices for each layer in the multilayer stack, that is:  
 
 
é ù= = ê úë û
11 12
1 2 3 p
21 22
    
M  M M M ... ... M  
   
m m
m m
  (2) 
 
However, the layer matrix is a complex matrix [2]. Hence, elements m12 and 
m21 in above equations are purely complex whereas the other two elements, 
i.e. m11 and m22, are real. Thus, the reflectance coefficient r is an imaginary 
quantity. So, the reflectance, R = r×r* gives, 
 
 ( ) ( )
( ) ( )
- + -
=
+ + +
2 2' ' ' '
0 11 22 0 12 21
2 2' ' ' '
0 11 22 0 12 21
s s
s s
n m n m n n m m
R
n m n m n n m m
 (3) 
 
Equation (3) determines the reflectance of the non-absorbing multilayer 
dielectric thin-film. This equation is used in the computer simulation program. 
 
Substrate 
Thin Film 
Air  
d 
n0
nf 
ns 
Incident light 
Interference occurs between
reflected beams 
 
 
 
 THE OPTIMIZATION OF OPTICAL THIN FILMS DEPOSITION... 723 
 
 
Fig. 2 – The front-panel of the LabVIEW program for reflectivity simulation of the 
optical thin film 
 
2.2 Program execution 
 
In the case of optical films, two parameters are necessary to determine the 
reflectivity of the thin film viz. the physical thickness and the refractive index 
of the material [3]. The other input parameters are the refractive index of the 
substrate material, wavelength of interest, and thickness (step size). All these 
parameters are provided on the front panel of the program, shown in Fig. 2. 
Fig. 3 shows the other control parameters to be fed in the input in program. 
The program executes as per the data flow arranged using the graphical 
programming code in the back panel of the program viz. block diagram. 
 
 
 
Fig. 3 –  The  input  parameters  for  single  layer  (a)  and  multilayer  (b)  in-situ  
reflectivity simulation 
 
3. EXPERIMENTAL 
 
3.1 Sample preparation  
 
The deposition of single layer MgF2 and a QWOT three bi-layer pairs of 
MgF2 and silicon on GaAs as a substrate for both the cases was carried out 
under the high vacuum (10 – 5 mbar) using e-beam evaporation system (EBG-
6K, HINDHIVAC). 
(b)(a)
 
 
 
724 G.G. BHATT, VIPUL KHERAJ, M.S. DESAI, C.J. PANCHAL  
The GaAs test substrate was used to optimize the deposition conditions as 
well as the reflectivity of the coated material. The quartz crystal monitor 
was used for measuring the thickness of the sample. For the better control 
of the deposition parameters viz. thickness and deposition rate, e-beam 
evaporation system is interfaced with the thin film deposition controller 
(SQC-112C, Sigma Instruments). The substrate was rotated with 120 rpm 
inside the vacuum chamber during the deposition with the help of a dc 
motor in order to get uniform coating. The substrate was maintained at 
100 °C substrate temperature using radiant heater during the deposition.  
The deposition rate was 4 Å/s. 
 
3.2 In-situ reflectivity measurement 
 
LD
PD
Substrate
Beam
splitter
Chamber Ambient
reflecting
mirror  
 
Fig. 4 – A schematic diagram of the experimental in-situ reflectivity measurement setup 
 
Fig. 4 shows a schematic diagram of the experimental setup for measuring 
in-situ reflectivity. The laser beam enters in to the vacuum chamber through 
a glass view port of the chamber and reflects back on the photo-detector 
(BPW-34) from the sample at normal incident angle. We have used a  
 
 
 
Fig. 5 – The LabVIEW front panel showing real time data acquisition as the light 
reflects from the sample and is detected by the photodetector 
 
 
 
 THE OPTIMIZATION OF OPTICAL THIN FILMS DEPOSITION... 725 
commercially purchased red laser diode (l = 650 nm) as a probe light source 
for the experiment. To check the stability of the optical power of the laser 
diode, we operated the laser diode in CW mode continuously for a few 
minutes before starting the deposition and monitored its optical power as a 
function of time. We observed negligible variation in optical power. Thus, 
we  can  consider  the  optical  power  output  of  the  laser  diode  to  be  stable  
during the in-situ reflectivity measurements. 
 We did not check for the variation of wavelength of the laser diode with 
time, but as we operate the laser diode at a very low power with controlled 
operating temperature, we assume that the wavelength of the laser diode is 
stable over the duration of the experiment. The signal from the photo-detector 
is fed to a computer through data acquisition card (NI-USB-6251). Fig. 5 
shows the acquired data in the front panel of the program built in LabVIEW. 
 During the deposition, the film grows vertically on the rotating substrate 
and the light undergoes interference. The change in the intensity of the 
reflected laser light will produce a sinusoidal curve as the film grows. Fig. 6 
shows the real time intensity data acquisition of the reflected laser beam 
from the sample during the deposition as a function of time. 
 
4. RESULTS AND DISCUSSIONS 
 
The single layer MgF2 film was deposited at room temperature on the GaAs 
substrate at the deposition rate of 4 Å/s. Fig. 6 shows the acquired data of 
the reflected laser light intensity versus time. It is well known that there is 
direct relation between the film thickness and the film deposition rate. So, 
one can easily estimate the thickness of the deposited film. 
 
 
 
Fig. 6 – Intensity measurement curve as a function of time for the MgF2 thin film 
deposited on GaAs substrate. 
 
Moreover, the natural reflectivity of the substrate GaAs is ~ 32 % and 
will be modified as the film grows on it. Hence, we can directly get the film 
reflectivity from the reflected intensity. Fig. 7 shows the experimental as 
well as the simulated data of the reflectivity of single layer MgF2. Likewise 
Fig. 8 shows the reflectivity plot for multilayer stack of material. The three 
QWOT bi-layer pair of MgF2 and Si where the former has a low refractive 
index and the latter has a high refractive index. Starting with the low 
refractive  index  material  coating  on  the  GaAs  substrate,  the  stack  of  the  
 
 
 
726 G.G. BHATT, VIPUL KHERAJ, M.S. DESAI, C.J. PANCHAL  
layers were deposited in alternation. We have achieved up to 83 % 
experimental reflectivity. The simulated data, which we have fitted, was fed 
refractive indices 1.38 and 3.5 for MgF2 and Si, respectively. 
In order to confirm the obtained results, we measured the reflectivity 
spectrum  of  the  coated  samples  ex-situ  with  the  standard  setup  using  a  
broad-band light source, a monochromator, photo-detectors and a lock-in 
amplifier. Figure 9 shows the reflectivity spectrum for a three QWOT bi-
layer pairs of MgF2-Si measured ex-situ. As shown in the figure, the 
reflectivity value obtained ex-situ at 650 nm is in good agreement with the 
value obtained in-situ at the end of the deposition. 
 
 
 
Fig. 7 – In-situ reflectivity and simulated reflectivity plots for a single layer MgF2 
(n = 1.43) deposited on GaAs substrate 
 
 
 
Fig. 8 – In-situ reflectivity plot for three QWOT bi-layer pairs of MgF2-Si 
 
There is a noticeable difference between experimental and simulated value 
of the reflectivity, especially at the higher thickness as shown in fig. 8. The 
measured  value  is  lower  as  compared  to  the  simulated  value  of  the  
reflectivity. This is mainly due to the usage of bulk values of the material 
refractive indices during reflectivity simulations. However, it is known that 
oxides film grown using e-beam evaporation exhibit a variation in refractive 
 
 
 
 THE OPTIMIZATION OF OPTICAL THIN FILMS DEPOSITION... 727 
index with film thickness [4]. We can easily modify the refractive index for 
a particular layer and estimate the value of its refractive index by matching 
the best-fit curve. 
 
Wavelength (nm)
550 600 650 700 750 800 850 900 950 1000
R
ef
le
ct
iv
ity
 (%
)
0
20
40
60
80
100
R=83.33 %
 
 
Fig. 9 – Reflectivity spectrum for three QWOT bi-layer pairs of MgF2-Si measured ex-situ 
 
5. CONCLUSION 
 
The in-situ reflectivity measurement of the single and multilayer optical 
thin films has been demonstrated. The change in intensity of the laser light 
after reflection has been measured to obtain the reflectivity as a function of 
thickness. Also, we have developed an in-situ reflectivity simulation for the 
optimization of the in-situ film deposition. The reflectivity simulator 
provides a very good tool for designing the optical thin films with desired 
reflectivity response. Also, one can use the simulator for determining the 
material and the deposition conditions viz., film thickness, and deposition 
rate for single or multilayer films. 
 
REFERENCES 
1. R. Overend and D.R. Gibson, Vacuum 43, 71 (1992). 
2. V.A. Kheraj, C.J. Panchal, M.S. Desai, V. Potbhare, Pramana – Journal of 
Physics 72, 1011 (2009). 
3. E. Hecht, Thin Film Optical Filters, 2nd ed. (Bristol: Adam Hilger Ltd: 1986). 
4. S. Pellicori, Coating Mater. News 13, 1 (2003). 


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The optimization of optical thin films deposition using in-situ reflectivity measurements and simulation

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