dept. physics & astronomy space research group nick nelms, john dowson space research group...
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Dept. Physics & AstronomySpace Research Group
Nick Nelms, John Dowson
Space Research Group
Dept. Physics & Astronomy
University of Leicester
UK
GERB-4 Detector SystemGoldblack Coating Status Report
3rd March 2004
Dept. Physics & AstronomySpace Research Group
Goldblack Coating
What is goldblack coating?It is a very low density deposit of amorphousgold which appears black at both visible and IRwavelengths.
Why is goldblack coating required?Applying a goldblack coating to thermal IRdetectors improves spectral response andflatness and extends absorption range.
Dept. Physics & AstronomySpace Research Group
Goldblack Facility
• Stainless steel vacuum chamber (<10-6 mbar)
• Precision gas pressure control (0 – 100 mbar)
• High current evaporation (0 – 150 A)
• LN2 cooled substrate plate
• PC control and data logging
• Large viewing port
Dept. Physics & AstronomySpace Research Group
Goldblack Deposition
Typical goldblacking procedure• Install substrate and gold metal• Evacuate chamber overnight• Cool substrate (typ. -55 degC)• Set chamber pressure (e.g. 10 mbar N2)• Melt gold metal, hold for 60 sec• Increase current slowly to evaporation level• Allow gold to fully evaporate (10’s to 100’s sec)• Warm up substrate• Vent chamber to dry N2
• Remove substrate
Dept. Physics & AstronomySpace Research Group
Goldblack Evaluation
Variables under investigation
• Gold quantity• Chamber pressure• Evaporation rate• Substrate temperature• Internal geometry
Dept. Physics & AstronomySpace Research Group
Goldblack Evaluation
Properties under
investigation
• Coating thickness• Coating appearance• Area density• Fill factor• Deposition rate• Spectral response
Gold coated microscope cover-glass used as substrate
Dept. Physics & AstronomySpace Research Group
Goldblack Evaluation
Parameter evaluation methods
Thickness – Calibrated optical microscope
Appearance – Optical microscope, SEM
Area density - Gold coated substrate weighed before and after deposition
Fill factor – Combination of thickness and area density
Evaporation rate – Gold metal quantity and evaporation time measured
Dept. Physics & AstronomySpace Research Group
Goldblack Evaluation
Deposition rate – Deposition time (= evaporation time) measured and combined with sample weight measurements
Spectral response –
IR response measured with P-E Spectrum One (1.2 – 27 um). Gold coated substrate gives 100% reflectance, absorption calculated as 1 - %R of goldblack measurement.
Visible response measured using P-E Lambda 35 (0.2 – 1.1 um). Aluminium coated substrates used.
Dept. Physics & AstronomySpace Research Group
Goldblack Status
Coating thickness
Typical thickness from previous work 10 – 100 um
Coating thickness for GERBs 1-3 ~20 um
LU trial thickness results 5 – 70 um
Dept. Physics & AstronomySpace Research Group
Goldblack Status
Coating appearance
Photograph of blacked substrate
SEM micrograph of goldblack sample
Dept. Physics & AstronomySpace Research Group
Goldblack Status
Area density
Area densities from previous work 95 – 338 ug/cm^2
LU trial area densities 140 – 1380 ug/cm^2
Fill factor
Fill factors from previous work 0.0015 – 0.02
LU trial fill factors 0.006 – 0.04
Evaporation rate
Evaporation rate from previous work 0.3 – 3.3 mg/s
LU trial evaporation rates 0.1 – 1.7 mg/s
Dept. Physics & AstronomySpace Research Group
Goldblack Results
From previous work, good spectral response is given by low area density and low fill factor (fill factor is ratio of goldblack density to bulk gold density).
Using LU goldblack facility, this indicates a requirement for cooled substrate, evaporation pressures of at least 10 mbar and low deposition rate.
Dept. Physics & AstronomySpace Research Group
Goldblack Reflectivity Measurements
Goldblack spectral response
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30
Wavelength (um)
%R
103 - 5mbar, 3cm
105 - 5mbar, 2cm
106 - 5mbar, 1cm
107 - 10mbar, 3cm
110 - 10mbar, 2cm
111 - 10mbar, 1cm
112 - 10mbar, 1cm
113 - 10mbar, 2cm
114 - 10mbar, 2cm
116 - 10mbar, 3cm
Dept. Physics & AstronomySpace Research Group
Goldblack Results
Sample 110
Area density = 1020 ug/cm^2
Fill factor = 0.019
Deposition rate = 25 ug/s
Sample 112
Area density = 141 ug/cm^2
Fill factor = 0.002
Deposition rate = 0.5 ug/s
Sample 114
Area density = 167 ug/cm^2
Fill factor = 0.004
Deposition rate = 0.43 ug/s
Dept. Physics & AstronomySpace Research Group
Goldblack Reflectivity Measurements
Spectral Response
0
5
10
15
20
25
30
35
40
45
50
0 5 10 15 20 25 30
Wavelength (um)
%R
110 - 10mbar, 2cm
112 - 10mbar, 1cm
114 - 10mbar, 2cm
GERB Specification
GERB4 Spec
Dept. Physics & AstronomySpace Research Group
Goldblack Issues
Aging effects
Most previously reported goldblacks undergo some aging resulting in a degradation of spectral performance. This can be countered by introducing 1-2% Cu into the gold melt as in the case of GERB 1-3.
The aging may be dependent upon water content during deposition. Initial investigations show no degradation due to aging but further work is required.
Dept. Physics & AstronomySpace Research Group
Goldblack Issues
Reticulated blacking
The aim of this work is to produce reticulated blacking – a goldblack deposit on each individual pixel only.
The method uses a custom bi-metal mask and alignment tool.
The alignment tool is in house and has been tested successfully – it allows sub-micron alignment of detector and mask.
The photo-masks are also in-house.
An initial blacking trial using the photo-mask and a silicon substrate has had partial success with goldblack being deposited through the 50 x 45 um holes.