Hardening and Optimizing of the Black Gold Thin Film as the Absorption Layer for Infrared Detector

doi:10.4236/opj.2013.32B066

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Optics and Photonics Journal, 2013, 3, 281-283

doi:10.4236/opj.2013.32B066 Published Online June 2013 (http://www.scirp.org/journal/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

Email: [email protected]

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.

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.

D.-P. QIAN ET AL.

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).

REFERENCES

[1] T. W. Lang, K. Kühl and H. S andmaier, “Absorbing Lay-

ers for Thermal Infrared Detectors,” Sensors And Actua-

tors A:Physical , Vol. 34, No. 3, 1992, pp.

243-248. doi:10.1016/0924-4247(92)85007-O

[2] W. Becker, et al., “Black Gold Deposits As Absorbers for

Far Infrared Radiation,” Physica stat us solidi(b), Vol. 194,

No. 1, 1996, pp. 241-255. doi:10.1002/pssb.2221940123

[3] P. Fiorentin, “Response of Printing Coloured Ink to Light

Exposure Measur ement and Analysis,” I2MTC, 2011, pp.

10-12.

[4] J. W. Clear y, R. E. P eale, M. Ishigami, et al., “Effects of

Polymer Infusion and Characteristic Length Scale on

Gold-Black Long-Wave and Far-infrared Absorbance,”

Materials Science and Engineering, Vol. 5, 2011, pp.

171-176.