CHARACTERIZATION AND PREPARATION OF ANTI- REFLECTION COATINGS IN THE RANGE OF 3-5 µm FOR Si OPTICAL WINDOW
FC= 0.39
1.05
K. Iqbal*, A. Maqsood, M. Mujahid and M. H. Asghar School of Chemical and Materials Engineering (SCME), National University of Sciences and Technology (NUST), Sector H-12, Islamabad, Pakistan E-mail Address: khurramiqbal.nust@gmail.com
DESIGN OF Si/SiO2 THIN FILMS
ABSTRACT
■ The hybrid antireflective coating model was designed using thin film design TFCALC software. ■ SiO2 was used as the low index material, while high index material was Si ■
Refractive indices were calculated by Sellmeiers dispersion equation. n2 - 1 = A1λ2 / (λ2-C12) + A2λ2/ (λ2-C22) + A3λ2/ (λ2-C32),
[C1, C2, C3 and λ] = [µm]
Material
A1
A2
A3
C1
C2
C3
1 2
SiO2 Si
0.696166 10.668429
0.407943 0.00304347
0.897480 1.5413341
(µm) 0.068404 0.30151648
(µm) 0.116241 1.1347511
(µm) 9.89616 1104.0
Parametric values of Sellmeier equation
Antireflection (AR) coating has significant role on optical and electro-optical applications[1]. This work was designed at 4.2µm wavelength, prepared and characterized for non-quarter-wave thick multilayer AR coatings based on low-high refractive indexes in the 3–5µm bands and total thickness was determined by Fresnel equations. At 4.2µm wavelength, mainly carbon dioxide takes part in reducing the %transmittance [2].
1µm
Refractive indices of SiO2 & Si in the 3-5µm wave band
Transmittance (%) vs Wavelength (µm) profile of Si/SiO2 layers
RF magnetron Sputtering
Target Materials
Si / SiO2
Target Size
4 inch
Forward Power
2.74 kW
Reflected Power
0.16 kW
Chamber Pressure
S.No Material 1 2
SiO2 Si
5.0 ×
10-4
Deposition rate Argon flow rate (Å/sec) (sccm) 1.1 190 1.5 130
Pa
Oxygen flow rate (sccm) 21 -
1000
1000
Sample deposited
SiKa
900
■ Atomic compositions were computed with the help of ZAF method. ■ The crystal structure was determined and lattice parameters a = 5.44 Å of the samples were calculated by using Bragg’s equation. ■ Satellite peaks (or doublet or rocking curves) 0 near at 69 degree shows unstrained from top layer to Si substrate. 2 2 2 2 2 2 (4) Sin θ / (h + k + l ) = λ / 4a
800 700
: As-
800
Acc. Voltage : 10.0 kV
Acc. Voltage kV
700
Probe Current : 1.0 nA
600
: 10.0
Probe Current : 1.0 nA
600
500
: 650 0C
Sample
900 OKa
1. Structural identification Counts
Sputtering System
Before Annealing
RESULTS
MATERIALS AND METHODS Sputtering system
Before Annealing
Before Annealing
200X
(1) (2)
OKa
n2AR = ns ×nair d = λ0 / (4 nAR )
(3)
S.No.
SiKa
Thin film multilayer anti-reflection coatings (SiO2/Si/SiO2) having thicknesses 286/571/143nm were deposited by RF magnetron sputtering deposition technique on 0.5mm thick Si(100)-substrates. Post-deposition annealing is also carried out in the temperature range 150-6500C for 4hr at the rate of 100C/min. Si Optical window was designed at 4.2µm wavelengths and correlated with modeling software TFCAL. The films are transparent in the 3‐5µm band of the electromagnetic spectrum, firmly adhered to the substrate. The prepared multilayer thin films are characterized optically and structurally using UV/VIS/IR spectrophotometer, Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Microscopy (EDS). . INTRODUCTION
500
400
200X 300
400 300
200
200
100
100
0
0 0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
0.00
1.00
2.00
3.00
4.00
keV
5.00
6.00
7.00
8.00
9.00
10.00
keV
EDS analysis of As-deposited and 650 0C sample
1 µm
Si (100)
As- deposited
6500C
Deposition rate, Argon flow rate and Oxygen flow rate for the individual layers 5500C
Layer
Material
Physical thickness (nm) 4500C
1
SiO2
143
2
Si
571
3
SiO2
286
3500C
Si/O ratio of the surface of Multi-layer Thin Films 2500C
Physical thickness SiO2/Si/SiO2
1 µm
0.5µm
1500C
Annealing at 650 0C
SiO2
Characterization Techniques
Si As-deposited
SiO2
Structural and Optical Analysis by following techniques: Si substrate
Technique
Model
Scanning Electron Microscope (SEM) / Energy Dispersive Microscopy (EDS)
JSM-6490A, Joel
X- Ray Diffraction (XRD)
Siemens / Bruker D 8
XRD analysis of As-deposited and annealed samples between intensity and angle 2θ
2. Film surfaces and roughness ■ Films have columnar structure and the surface of the film is smooth and featureless.
3. Spectral distribution of Si/SiO2 coatings transmittance
Conditions 40 kV
Tube Current
40 mA
Wavelength CuKα
0.154178 nm
Step
0.04
Scan Speed
1°/ min
AFM
JSPM5200, Jeol
RMS Roughness (nm)
25
Tube Voltage
Typical SEM image of cross-sectional morphologies of the as-deposited sample and surface of the as-deposited and 650 0C sample
As-deposited 20
T1 = 150 C
■An Average transmission of Si/SiO2 coatings is achieved 75% in the 3-5µm wave bands.
T2 = 250 C
15
T3 = 350 C 10 T4 = 450 C 5
T5 = 550 C T6 = 650 C
0
Sample
Conditions Cantilever tip
Si3N4
Operation Mode
AC
Cantilever Frequency
174.161 kHz
Force Constant
1.00 N / m
Scan Speed
1°/ min
UV/VIS/IR Spectrometer
U-3501, Hitachi
RMS roughness of the as-deposited and annealed samples
CO2
Annealing of Samples Temperatures
1 50 - 650 0C
Time
4 hr
Rate
10 0C Three-dimensional AFM image of the as-deposited and 650 0C sample
ACKNOWLEDGEMENTS This project was funded by NUST and the assistance offered by greatly acknowledged: ■ A. A. Khan ■ M. Islam NANOPAPRIKA POSTER 2011
Measured transmission spectra for the as-deposited and annealed samples
CONCLUSIONS
REFERENCES
Multilayer thin‐films of Si and SiO2 are successfully prepared by RF magnetron sputtering. Annealed at 6500C generates smooth films as well as enhanced optical properties. The resulting models were helpful for determining the errors in deposition processes of each of the utilized deposition techniques, and this was the main goal.
[1] M. H. Asghar, M. Shoaib, F. Placido and S. Naseem, Cent. Eur. J. Phys., Vol. 6, No. 4, 2008, pp. 853 - 863. [2] M. H. Asghar, M. B. Khan, and S. Naseem, Semiconductor Phys: Quan. Elect. & Optoelect., Vol. 6, No. 4, 2003, pp. 508- 513.