Optics and Photonics Journal, 2013, 3, 281-283
doi:10.4236/opj.2013.32B066 Published Online June 2013 (http://www.scirp.org/journal/opj)
Copyright © 2013 S ciRes. OPJ
Hardening and Optimizing of the Black Gold Thin Film
as the Absorption Layer for I n f rared Detector
Dong-pei Qi an, Chuan-gui Wu, Yao Shuai, Wen-bo Luo, Qiang-xiang Peng,
Xiao-yong Chen, Wan-li Zhang
State Key Laboratory of Electronic Thin Films and Integrated Devi ces ,
Universit y of Electronic Science and Technology of China, Chengdu, China
Received 2013
ABSTRACT
This paper reports on the study of the black gold thin film as the absorption layer in the near infrared spectrum. The
fabrication of the black gold thin film was achieved by a thermal evaporation technique in N2 atmosphere. Different
evaporation conditions were attempted to optimize the absorbance of the black gold coating, especially the atmosphere
pressure and the mass of evaporation source. The long-standing problem of black gold’s adhesion with the substrate
was solved by fuming 502 superglues into the black gold layer, which had almost no impact on the absorbance perfor-
mance. Layers produced at N2 atmosphere of 3×103 Pa show an absorbance exceeding 0.9 in the near infrared.
Keywords: Black Gold; 502 Fuming; Absorption Layer
1. Introduction
An absorption layer is an indispensable part of a high-
performance infrared detector for transforming infrared
radiation into heat [1,2]. For high frequency application,
the radiation absorption layer requires small heat capaci-
ty and high absorbance to achieve maximal sensitivity
and minimal response time. Consequently, infrared ab-
sorbing lacquers or printing ink [3] may not be the best
option as for their large masses and correspondingly high
heat capacitance while black gold coating is preferred
due to its thin thickness and therefore, low heat capacity.
In this paper, the absorbance of black gold will be
mainly investigated. Black gold has been criticized for its
rigidity and its weak adhesion with the substrate. In this
paper, 502 superglues will be fumed into the black gold
layer to harden the black gold coating [4]. And the ab-
sorbance will be tested to see if the absorbance is af-
fected by this method. Further measurements will aim at
the detailed relations between the evaporation conditions
(like the pressures of N2 atmosphere, masses of gold) and
the absorbance. Considering th e heat capacitance, the high
absorbance in the near-infrared region should be achieved
under the condition of keeping the mass of evaporation
source as low as po ssi ble.
2. Experimental
2.1. Fabrication of Black Gold and Study of Its
Influencing Factors
We used the thermal resistance coating machine (MN-
300) as the thermal evaporator. Firstly, pure gold gra-
nules were laid in a dip in the middle of a molybdenum
foil with width of 10 mm and thickness of 0.3 mm, and
one sample was attached to a copper block mounted on
the c ylinder whic h is 5 cm above the evaporation so urce .
Then, the vacuum chamber was evacuated just by a me-
chanical pump to a pressure of 1 Pa for around 1h before
nitrogen gas was introduced up to a pressure of 104 Pa.
This pressure lasted for 5 min to reduce the content of
oxygen which is known to lower the absorbance [2]. Af-
ter the rinsing, the pressure was adjusted to evaporation
pressure (1.5 × 102 Pa, 1.8 × 102 Pa, 2 × 102 P a , 2 .5 × 102
Pa, 3 × 102 Pa, 5 × 102 Pa , 1 × 103 Pa , 2 × 103 Pa, 3 × 103
Pa) to start the film coati ng. 150 A current was applied to
meet the requirement of 0.5 to 3.3 mg/s evaporation rate.
And Evaporation time depends on the mass of gold (0.1 g,
0.2 g, 0.3 g).
Different pressures of nitrogen atmosphere and differ-
ent masses of gold granules were attempted as variables
to explore their influences on the absorbance of the black
gold coatings, which were tested by Fourier transform
infrared spectroscopy.
2.2. 502 Fuming and Its Influence on Absorbance
A 502 fingerprint fuming cabinet (BTSP-II) is available in
our laboratory. To begin with, a cup of water was placed
on one heater, 4.5 gram 502 superglues were loaded in an
aluminum foil dish on another heater and the sample was
suspended 30 cm above the heater with the black gold

D.-P. QIAN ET AL.
Copyright © 2013 S ciRes. OPJ
282
coating facing the aluminum foil dish. After that, the
whole fuming procedure was automatically follo win g the
timer order which was set before. Specifically, the humi-
dification lasted for 25 min to reach the humidity of 80%
RH in t he ca bine t, a nd the n t he evap ora tion o f glue s co n-
tinued for 30 min. At last, another 20 min was taken to
make sure that 502 glues filled sufficiently into black
gold coating.
The absorbance of the black gold film after 502 fum-
ing was tested to make a comparison with the raw black
gold film.
3. Results and Discussion
The near-infrared reflectivity (R) measurements of the
black gold are performed using a Fourier transform
infrared spectroscopy. As the black gold thin film is de-
posited on t he no n -transparent substrate, absorbance A of
the film can be calculated by A = 1-R [2].
3.1. Absorbance Properties of Black Gold Layers
Prepared at Various N2 Pressures
As seen in Figure 1, the reflectivity of the black gold
layer is reduced with increasing pressure. Layers pro-
duced at N2 atmosphere of 3 × 103 Pa show an absor-
bance higher than 0.9 at wavelength below 8 μm and
exceeding 0.8 at wavelength below 15 μm. A higher N2
atmosphere pressure requires a closer distance between
the sample and the evaporation source, where most of the
materials cannot sustain under such a high temperature.
Hence, N2 atmosphere pressure of 3 × 103 Pa is an op-
timal choice.
3.2. Absorbance Properties of Black Gold Layers
Prepared Consuming Different Masses of
Evaporation Sources
Three samples were prepared consuming evaporation
sources of 0.3 g, 0.2 g, and 0.1 g at N2 atmosphere pres-
sure
Figure 1. Reflectivity of black gold in various N2
atmosphe res with same amount of source.
of 3 × 103 Pa, cor resp ond ing to the thi ckne ss o f the black
gold layer of 900 nm, 400 nm, and 200 nm respectively.
As we can see from the Figu re 2, the sample with eva-
poration source of 0.3 g gold granules exhibits a higher
absorbance than others at wavelength above 7μm, but
lower at wavelength belo w 6 μm. As a whole, the black
gold coating consuming evaporation source of 0.3 g ex-
hibits better absorbance performance in wider range of
the near-infrared spectrum. On top of that, the less gold
used, the thinner film is, correspondingly a lower heat
capacity obtained, which is directly related to heat re-
sponse t ime.
3.3. Absorbance Property after 502 Fum i ng
Figure 3 illustrates two curves of reflectivity in almost
similar tracks. We can see that the reflectivity of black
Figure 2. Reflectivity of black gold with different amount of
source at same N2 pressure of 300 0 pa.
Figure 3. Reflectivity of black gold both before and after
502 fuming at N2 pressure of 3000 pa.

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Copyright © 2013 S ciRes. OPJ
283
gold after 502 infusion increases just marginally. It i ndi-
cates that the 502 superglues fuming method does not
obviously influence the absorbance performance of the
black gold coating. Additionally, the black gold coating
after the fuming is extraordinarily strong and can barely
be scraped off.
4. Conclusions
Under the condition of N2 atmosphere pressure of 3×103
Pa and evaporation source of 0.3 g, the absorbance o f t he
black gold coating deposited meets the requirement for
the appli catio n in the near-infrared spectrum. More evapora-
tion sources can be consumed to achieve a higher absor-
bance though the black gold coating will be thicker, re-
sulti ng in a highe r response time of the infrared detector.
The method of fuming 502 superglues to reinforce the
adhesion between the black gold layer and the substrate
is proved to be a viable method.
5. Acknowledgements
This work has been supported by the National Natural
Science Foundation of China (NSFC. 51102037) and the
Fund ame ntal Rese ar c h F u nd s fo r t he Ce nt ra l Uni ve rsi ties
from UESTC (No. ZYGX2010J030).
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