Substrate temperature and strain during sputter deposition of aluminum on cast borosilicate glass in a Gemini Observatory coating chamber

Jacques Sebag; John Andrew; Douglas Neill; Michael Warner

doi:10.1364/AO.49.004610

Applied Optics
Vol. 49,
Issue 24,
pp. 4610-4620
(2010)
•https://doi.org/10.1364/AO.49.004610

Substrate temperature and strain during sputter deposition of aluminum on cast borosilicate glass in a Gemini Observatory coating chamber

Jacques Sebag, John Andrew, Douglas Neill, and Michael Warner

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Jacques Sebag, John Andrew, Douglas Neill, and Michael Warner, "Substrate temperature and strain during sputter deposition of aluminum on cast borosilicate glass in a Gemini Observatory coating chamber," Appl. Opt. 49, 4610-4620 (2010)

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Abstract

Temperature and strain measurements obtained during coating of spin-cast borosilicate samples are presented here with an analysis of these results. These tests were performed for the Large Synoptic Survey Telescope (LSST) project to verify the possible use of sputtering deposition of optical coating on its large $8.4 m$ diameter primary–tertiary mirror. Made of spin-cast borosilicate glass, the working stress of the mirror’s nonpolished surfaces is $100 p s i$ ( $0.69 MPa$ ), resulting in a local temperature difference limit of $5 ° C$ . To ensure representative environmental conditions, the tests were performed in the Gemini Observatory coating chamber located in Hawaii, whose design was utilized to develop the LSST coating chamber design. In particular, this coating chamber is equipped with linear magnetrons built with cooled heat shields directly facing the mirror surface. These measurements have demonstrated that it will be safe for the LSST to use a magnetron sputtering process for coating its borosilicate primary–tertiary mirror.

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Thickness (mm)	Dimensions and Mass	Surface	Material	Plate (Quantity)
2	$152.4 \times 152.4 {mm}^{2}$ $112 g$	Polished	Float	A(1)
12.7	$50.8 \times 50.8 {mm}^{2}$ $75 g$	Polished	Pyrex	A(3), B(4), C(4)
28	$165 mm$ radius $1.3 kg$ for B and $0.7 kg$ for C	Polished (B) and Rough (C)	BSC	B(1), C(1)

	Plate A	Plate B	Plate C
Temperature Sensors	6	12	12
Strain Gauges	2	4	4

	$2 mm$ Thick	$12.7 mm$ Thick	$28 mm$ Thick
Measured temperature increase on top surface during coating	$7 ° C$ in $80 s$	$1.8 ° C$ in $50 s$	$2.3 ° C$ in $50 s$
Measured temperature decrease on top surface after coating	$0 ° C$	$1 ° C$ in $100 s$	$1.3 ° C$ in $250 s$
Measured temperature increase on back surface during coating	$4 ° C$ in $150 s$	$1.2 ° C$ in $150 s$	$0.8 ° C$ in $250 s$
Predicted mean temperature increase	$8.0 ° C$	$1.4 ° C$	$0.74 ° C$
Predicted front-to-back temperature difference during coating	$0.58 ° C$	$1.8 ° C$	$2.0 ° C$

Values Utilized In Calculations
		Thickness (mm)
Symbol	Variable	2	12.7	28	Units
Q	substrate heat load	16.26	1.81	14.97	W
A	radiative area	0.0465	0.0052	0.0428	$m^{2}$
σ	Boltzmann constant	$5.67 E - 08$	$5.67 E - 08$	$5.67 E - 08$	$W / m^{2} K^{4}$
ε	emissivity	0.048	0.048	0.048	na
m	mass	0.112	0.075	1.30	kg
$T_{2}$	ambient temperature	282.5	278.0	285.3	K
r	density	2230	2230	2180	$k g / m^{3}$
k	thermal conductivity	1.10	1.13	1.10	$W / m C$
c	specific heat of substrate	830	837	710	$J / k g C$
α	thermal diffusivity	$5.94 E - 07$	$6.05 E - 07$	$7.11 E - 07$	$m^{2} / s$
t	coating time	44	44	44	s

Power (kW)	Coating Duration (s)	Rotational Speed (rph)	Predicted Front-to-Back Temperature Difference for BSC Sample (C)
10	47	0.25	2
20	34	0.32	3.2
30	23	0.48	3.9
40	17	0.64	4.5

	Temperature Range ( $° C$ )	Thermal Output Correction ( $μ strain$ )
Plate B	$12.5 / 13.35$	4.7
Plate C	$14.6 / 16.6$	10.0

	Thermal Strain ( $μ strain$ )	Total Strain ( $μ strain$ )	Mechanical Strain ( $μ strain$ )	Stress (psi)
Plate B	2.24	1.3	$- 0.94$	$- 8$
Plate C	5.6	4	$- 1.6$	$- 14$

	Temperature Range ( $° C$ )	Thermal Output Correction ( $μ strain$ )
Plate B	$12.5 / 13.35$	4.7
Plate C	$14.6 / 16.6$	10.0

	Thermal Strain ( $μ strain$ )	Total Strain ( $μ strain$ )	Mechanical Strain ( $μ strain$ )	Stress (psi)
Plate B	2.24	1.3	$- 0.94$	$- 8$
Plate C	5.6	4	$- 1.6$	$- 14$

Abstract

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Applied Optics