Durable infrared optical coatings based on pulsed DC-sputtering of hydrogenated amorphous carbon (a-C:H)

Des Gibson; Shigeng Song; Lewis Fleming; Sam Ahmadzadeh; Hin On Chu; Stephen Sproules; Ryan Swindell; Xiaoling Zhang; Parnia Navabpour; Caspar Clark; Mark Bailey

doi:10.1364/AO.378266

Durable infrared optical coatings based on pulsed DC-sputtering of hydrogenated amorphous carbon (a-C:H)

Des Gibson, Shigeng Song, Lewis Fleming, Sam Ahmadzadeh, Hin On Chu, Stephen Sproules, Ryan Swindell, Xiaoling Zhang, Parnia Navabpour, Caspar Clark, and Mark Bailey

Applied Optics
Vol. 59,
Issue 9,
pp. 2731-2738
(2020)
•https://doi.org/10.1364/AO.378266

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Des Gibson, Shigeng Song, Lewis Fleming, Sam Ahmadzadeh, Hin On Chu, Stephen Sproules, Ryan Swindell, Xiaoling Zhang, Parnia Navabpour, Caspar Clark, and Mark Bailey, "Durable infrared optical coatings based on pulsed DC-sputtering of hydrogenated amorphous carbon (a-C:H)," Appl. Opt. 59, 2731-2738 (2020)

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Abstract

Optical properties of low-temperature pulsed DC-sputter deposited ($ {\le} {70° {\rm C}}$) hydrogenated carbon are presented. Increasing hydrogen incorporation into the sputter deposited carbon significantly decreases infrared optical absorption due to a decrease in deep absorptive states associated with dangling bonds. Hydrogen flow is optimized (hydrogen flow 3 sccm), achieving the best compromise between increased infrared transmittance and hardness for durable coating performance. Optical, environmental, and durability performance of pulsed DC-sputtered carbon incorporated in multilayer (a-C:H/Ge) infrared antireflective coatings indicates suitability as a durable infrared optical coating for commonly used infrared substrates, including temperature sensitive chalcogenide glass.

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Parameter	Amorphous Carbon	Polycrystalline Germanium
Ar flow (sccm)	100	190
$H_{2}$ flow (sccm)	0–18	3
Gas purities (%)	99.99	99.99
Process pressure (mBar)	$5 \times 10^{- 3}$	$4.6 \times 10^{- 3}$
Base pressure (mBar)	$5 \times 10^{- 7}$
Power (kW)	4	2.5
Current (A)	10	5.92
Voltage (V)	400	423
Pulse Conditions
Pulsed DC frequency (kHz)	46	75
Pulse duration (µs)	4	13
Reverse pulse (as percentage of forward pulse voltage) (%)	20	20
Deposition rate (Å/s)	0.3	1.1
Target purity (%)	99.99	99.99
Preclean microwave plasma power (kW)	3 ^a
Chamber preconditioning and substrate preclean time (mins)	30
Thickness (nm)	200–1200	200–700

Test	Specification	Description
Adhesion	MIL-C-48497A Section 4.5.3.1	Press adhesive cellophane tape firmly against the coated surface and quickly removed at an angle normal to the coated surface
Severe abrasion	MIL-C-675C Section 4.5.10	40 strokes with rubber eraser loaded at 1 kg
Humidity	MIL-C-48497A Section 4.5.3.2	24 h at $49 ° C$ and humidity greater than 95%
Temperature	N/A	16 h at $+ 95 ° C$ , 16 h at $- 54 ° C$
Salt water solubility	MIL-C-48497A Section 3.4.3.2	24 h immersion in salt water solution

	Pulsed DC-Sputtered a-C:H (Hydrogen Flow 3 sccm)	PECVD a-C:H [2,3,11,20]
$n$ at 10 µm	2.03	2.05
$k$ at 10 µm	0.01	0.02
Vickers hardness $k g / {m m}^{2}$	1800	1800
Young’s modulus GPa	130	150
Stress MPa (compressive)	−700	−1100

Layer Number (from Substrate)	Material	Physical Thickness (nm)
1	Germanium	262
2	Carbon	229
3	Germanium	649
4	Carbon	1155

	Wavelength Range (µm)
	8–12		8–14
	Design	Measured	Design	Measured
Transmittance	92.8%	91%	87%	85%
Reflectance	1.1%	1.3%	2.8%	3.5%

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