dept. physics & astronomy space research group nick nelms, john dowson space research group...

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


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.


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


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


Goldblack Evaluation

Variables under investigation

• Gold quantity• Chamber pressure• Evaporation rate• Substrate temperature• Internal geometry



Properties under

investigation

• Coating thickness• Coating appearance• Area density• Fill factor• Deposition rate• Spectral response

Gold coated microscope cover-glass used as substrate



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



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.


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


Goldblack Status

Coating appearance

Photograph of blacked substrate

SEM micrograph of goldblack sample


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


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.


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


Goldblack Results

Sample 110

Area density = 1020 ug/cm^2

Fill factor = 0.019

Deposition rate = 25 ug/s

Sample 112


Fill factor = 0.002

Deposition rate = 0.5 ug/s

Sample 114


Fill factor = 0.004

Deposition rate = 0.43 ug/s


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


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.


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.

dept. physics & astronomy space research group nick nelms, john dowson space research group...

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