1. Reproducibility Analysis of E-beam Deposited Multilayer Dielectric Reflective Filter
Nancy Sharmaa , P Sunitaa, b, VSRS Praveen Kumara,b , Gaurav Dwivedia, Mukesh Kumara,b, P. Krishna Raoa, Neelam Kumaria,b, Vinod Karara,b, Amit L Sharmaa,b*
aCSIR-Central Scientific Instruments Organisation, Chandigarh, India,
bAcademy of Scientific & Innovative Research (AcSIR), CSIR-CSIO, Chandigarh
* Corresponding Author: Dr. Amit L. Sharma,
ABSTRACT
RESULTS
A Reflective filter consisting of 15 layers is designed and fabricated for a design wavelength of 660 nm.
Three deposition runs of the same design (with same thicknesses) are performed in different batches on BK-7 substrates using e-beam deposition technique.
Reflectance spectra is obtained for all three reflective filters at 45° angle of incidence and reproducibility analysis is performed by comparing the spectra.
Peak Reflectance are obtained at wavelengths 610 nm, 625 nm and 660 nm in the three deposition runs with reflectance amplitudes of 60.59%, 59.49% and 54.11% respectively.
Keywords: Optical reflective coatings, Interference, Multilayers, Reproducibility
Table 1: Observed parameters during analysis
Sample
Material deposited/layer
(gms)
Peak Obtained (nm) %R
Al2O3
SiO2 1
0.35
0.203
610
60.59 2
0.31
0.279
625
59.49 3
0.407
0.295
660
54.11
INTRODUCTION
Dielectric interference filters alter the transmission or reflection properties of a substrate to suit the application requirement.
These filters are used in the fields like display systems, avionics, protective coatings, etc.
For applications such as in avionics display systems which require very high precision filter performance, any process parameter instability can result in improper performance in terms of peak reflectance wavelength shift or change in filter bandwidth.
In the present work, to check the reproducibility, three films have been deposited in different batches with equal number of layers and same thicknesses (Al2O nm and SiO nm) using e-beam vacuum deposition technique.
Substrate
L- SiO2
H- Al2O3
Design Equation:
Glass/(HL)7H/Air
Figure 5: 15 layers with quarter wave stack of alternating high (Al2O3) and low (SiO2) indices
Figure 6: Layer structure for reflective filter
MATERIALS & METHODS
BK-7 Substrates (procured from SCHOTT, Malaysia) were used for deposition of films.
Al2O3 and SiO2 with refractive indices 1.63 and 1.46 respectively (procured from M/s Umicore, Thin Film Products with 99.99% purity) were used as coating materials .
FilmstarTM software is used for designing the optical filter.
Thin film deposition was done using Electron beam evaporation plant (PFEIFFER PLS570) and in-situ quartz crystal monitor was used for monitoring thickness.
Reflectance profile of the coated films was obtained using UV-Vis-NIR double beam spectrophotometer (Perkin-Elmer, Lambda 9) in the wavelength range of nm at oblique angles of incidence.
Weight of coating materials was measured before and after coating to check the reproducibility of coating.
Figure 7: Designed spectrum of Reflective filter
Figure 8: Experimental results for all three optical reflective filters
CONCLUSIONS
Analysis of results shows that experimental results deviates from the designed results for two samples and third sample was found to be consistent with design data.
A slight change in the reflectance peak amplitude as well as peak wavelength is observed in the three coated films.
Reason for this deviation is due to variations in initial deposition rate when the shutter opens resulting in unequal thicknesses and refractive indices of the individual thin films layers.
It was observed that for samples 1 and 2, the deposition rate of Al2O3 was stabilized after ~12 nm deposition and ~30 nm deposition in case of SiO2 after the shutter opens.
Filter Design using FilmstarTM
Weight measurement of coating materials
Film deposition using E-beam evaporation
Weight measurement after coating
Characterization using UV-Vis-NIR spectrophotometer
Comparison and analysis of results
Figure 1: A typical avionics display system(1)
ACKNOWLEDGEMENT
Authors are thankful to The Director, CSIR-CSIO, Chandigarh for his support. Funding from CSIR through 12th FYP Supra Institutional Project (SIP) grant OMEGA/PSC0202 is gratefully acknowledged.
Figure2: See-through display fitted in aircraft(2)
REFERENCES
Ronald R. Willey, ”Reproducibility in Optical Thin Film Processing” ,SVC Bulletin, 2014
Macleod, H. Angus. Thin-film optical filters. CRC Press, 2010.
Parinam Sunita, Vemuri SRS Praveen Kumar, Mukesh Kumar, Parinam Krishna Rao, Neelam Kumari , Vinod Karar and Amit L. Sharma, “Estimation of optical constants and thicknesses of E-beam deposited Metal Oxide films by envelope method“ in International Conference on Optics and Photonics (ICOP), February 20-22, 2015 held in Calcutta University, Kolkata.
ImageCourtesy: (1) (2)
Figure 3: Coated sample with reflectance peak at 660 nm wavelength
Figure 4: Flow chart for reproducibility analysis of reflective filters