Analysis on vibrations and infrared absorption of uncooled microbolometer

Chen, Chao; Zhang, Long; Zhou, Yun; Zheng, Xing; Dong, Jianghui

Journal of Vibroengineering

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Published: 15 August 2015

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Analysis on vibrations and infrared absorption of uncooled microbolometer

Chao Chen¹

Long Zhang²

Yun Zhou³

Xing Zheng⁴

Jianghui Dong⁵

^{1, 2, 3, 4}School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu, 610054, China

⁵School of Natural and Built Environments, University of South Australia, Adelaide, 5095, Australia

Corresponding Authors:

Chao Chen, Jianghui Dong

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Abstract

The characteristics of vibrations in microbolometer had significant impact on the performances of its infrared absorption. Due to the complex architectures, leading to the unfavorable connection between the analysis of infrared absorption and vibrations. To solve this issue, a finite element analysis (FEA) method was designed to make better compatible with infrared absorption and vibrations, as well as the resonant frequency analysis was completed. A vanadium oxide (VO₂) based microbolometer was designed, and the corresponding three-dimensional (3D) modeling was also built. By vibrations and resonant frequency FEA, mechanics and frequency characteristic were studied. 200 G, 500 G and 1000 G acceleration vibrations were loaded on the 3D model at $Z$ axis, which perpendicular to the bridge-like structure. It shows that under 500 G acceleration vibration, the deformation of the model was small enough to ensure the resonant cavity maintained $λ / 4$ which means a high IR absorption for the microbolometer. The first order modal frequency, the second order modal frequency and the third order modal frequency of the 3D model were also analyzed. Purpose of resonant frequency analyzing of microbolometer was to avoid devices work on this frequency result of failure. Finally, an uncooled infrared focal plane was fabricated, and the experimental data matched the simulation fitting results. Perfect performance in mechanical properties, IR absorption and imaging effect of experimental device indicating a shorter design cycle and low cost potential. The fast, efficient FEA design method enables simulating infrared absorption and vibrations together.

1. Introduction

Uncooled infrared focal plane arrays (IRFPAs) were typical Micro-Electro-Mechanical Systems (MEMS) monolithic integrated device which operated at room temperature [1]. Compared to cooled detectors, it provided the advantages of small size, low power dissipation, less weight and low cost [2]. At the detect pixel of IRFPAs, microbolometer had a bridge-like structure insulated with substrate [4]. It absorbed IR radiation and caused temperature increasing which resulting in a change in the resistance of the bridge-like structure. ROIC was monolithically integrated with the bridge-like structure provided microbolometer with a bias voltage or current. Then resistance change can be detected as a change in the electrical signal.

In case of mixed domain of microbolometer, bridge-like structure design starting with mechanical CAD tools and thermoelectric behavior of the device was studied via theoretical calculation [5, 6], the finite element method (FEM) [7, 8], PSPICE model [9] or sub-circuit model analysis [10]. Mechanical properties had an important influence on the thermo-electric properties of microbolometer [11, 12]. Especially under a certain vibration, structure of microbolometer should keep the tiny deformation to ensure the devices work properly [13, 14]. Furthermore, it suggested that the device must avoid working under resonant frequency which may be cause serious mechanical damage [15, 16]. There was no direct way between simulations of IR absorption and vibration behavior of microbolometer.

In this study, IR absorption and vibration analysis of microbolometer were discussed by optical admittance matrix methods [17] and FEA respectively. According to the designing, a IRFPAs were fabricated to verify the results.

2. Method and basic theory

2.1. Simulation flowchart

Firstly, three-dimensional (3D) bridge-like structure of a VO₂ microbolometer was modeling, as well as an IR absorption equivalent model was established based on the 3D model. IR absorption properties were studied by optical admittance matrix methods and an optimization resonant cavity was given. Then vibration and resonance frequency analysis of microbolometer were finished by the FEA method which should be ensured a good mechanical properties to maintain the resonant cavity of microbolometer as $λ /$ 4 [4].

Fig. 1Simulation flowchart

2.2. Modelling of microbolometer

A 3D model of a VO₂ microbolometer shown in Fig. 2 was established. Vibrations and resonant frequency FEA were used to obtain the mechanics and frequency characteristic of the model.

Bridge-like structure, support layer, VO₂ resistance layer, passivation layer, absorption layer, electrode, and resonant cavity were established in details by using Intellisuite MEMS software (Fig. 2(a)). 3D mesh diagrams were shown in Fig. 2(b) which the node number of meshing diagrams was 83848. Such an accurate 3D model provides guarantee for an accurate result of FEA.

The types of elements were SOLID95. During FEA, the material properties of 3D mesh Model were defined according to the actual process parameters of their materials. The material properties were listed in Table 1. And then the IR absorption equivalent model was established based on the 3D model was shown in Fig. 3.

The optical admittance matrix model of the IR absorption equivalent model was shown in Fig. 4.

Fig. 2Model of microbolometer

a) 3D model details

b) 3D mesh diagrams

Fig. 3. IR absorption equivalent models ( $d 1$ , $d 2$ , $d 3$ , $d 4$ , $d 5$ , $d 6$ were the thickness of Si₃N₄, VO₂, resonant cavity, reflective aluminum and substrate of microbolometer)

Fig. 4The optical admittance matrix model (add and subtract in Ej where the direction of propagation, the dj was thickest, Nj was the plural refractive index of each membrane in microbolometer and given by Nj=nj-ikj, where, nj was the refractive index, kj was extinction coefficient

$The optical admittance matrix model (add and subtract in Ej where the direction of propagation, the dj was thickest, Nj was the plural refractive index of each membrane in microbolometer and given by Nj=nj-ikj, where, nj was the refractive index, kj was extinction coefficient$

Table 1Microbolometer material properties

Material	Thermal conductivity ( $W$ / cm·K)	Thermal capacitance ( $J$ / kg·K)	Density ( $g$ / cm³₎	Resistivity (Ω·Cm)	Young modulus (GPa)	Poisson’s ratio
Si₃N₄	0.02	3.33	2.3	1×10¹⁵	300	0.26
NiCr	0.4	3.92	8.5	1×10^-4	200	0.312
VO₂	0.05	5.00	4.34	1	80	0.24

2.3. IR absorption

The transmittance $T (λ)$ and the reflectivity $R (λ)$ could be obtained by calculating the gain matrix of membrane in the infrared wavelength ranged from 8-14 μm and then IR absorption $A (λ)$ could given by:

1

A (λ) = 1 - R (λ) - T (λ) .

Assuming that each membrane were well-proportioned and isotropic, input IR was S-polarized and vertical incidence, the gain matrix of thin film layer $j$ was given by:

2

M_{i} = [\begin{matrix} c o s δ_{i} & j s i n δ_{i} / β_{i} \\ j β_{i} s i n δ_{i} & c o s δ_{i} \end{matrix}],

where, $δ_{j}$ was aphasic thicker and $η_{j}$ was revised admittance of each membrane respectively. $δ_{j}$ was given by:

3

δ_{1} = \frac{2 π}{λ} N_{1} d_{1} c o s θ_{1},

where $θ_{j}$ was refractive angle. On the condition of input IR was vertical incidence, the value of $θ_{j}$ was zero. Then, the gain matrix of the entire film series can be expressed by Eq. (4):

4

[\begin{array}{l} B \\ C \end{array}] = \prod_{j = 1}^{k} ‍ [\begin{matrix} c o s δ_{j} & \frac{i}{η_{j}} s i n δ_{j} \\ i η_{j} s i n δ_{j} & c o s δ_{j} \end{matrix}] \cdot [\begin{matrix} 1 \\ β_{k + 1} \end{matrix}],

where, $β_{k + 1}$ was the modified admittance. Hence, the combined admittance of multilayer film is $Y = C / B$ . Then the $T (λ)$ and $R (λ)$ were expressed by Eq. (5) and Eq. (6) respectively:

5

T (λ) = \frac{4 β_{0} β_{k + 1}}{(β_{0} B + C) (β_{0} B + C)^{*}},

6

R (λ) = (\frac{β_{0} B - C}{β_{0} B + C}) {(\frac{β_{0} B - C}{β_{0} B + C})}^{*},

where, $β_{0}$ was the optical admittance of incidence medium. Finally, IR absorption could be obtained by the Eq. (1) via the matlab program. Optical parameters of the IR absorption equivalent model were listed in Table 2.

Table 2Optical parameters of materials

Material	$n$	$k$	Thickness (μm)
Si₃N₄ (infrared absorber layer)	2.1	0.01	$d 1 =$ 0.03
VO₂	2	3	$d 1 =$ 0.07
Si₃N₄ (support layer)	2.1	0.01	$d 1 =$ 0.3
Resonant cavity	1	0	$d 1 =$ 2.5
Al reflector	25.4	67.3	$d 1 =$ 0.5
Si substrate	3.5	0	$d 1 =$ 5

Fig. 5 was the result of IR absorption analysis. It is shown that when the height of resonant cavity was 2.5 μm, the average IR absorption rate had a maximum value as 85.8 % in 8-14 μm.

2.4. Vibrations finite element analysis

Mechanical properties also had an impact effect on the performance of the device. A micro bolometer pixel with a $λ$ /4 resonant cavity had a maximal infrared absorption rate. The larger or smaller $λ$ /4 was, the lower infrared absorption rate was. Especially under certain vibrations, the structure of microbolometer should keep small deformation to ensure the resonant cavity was $λ$ /4. For an IR detector response in 8-14 μm, then $λ$ was 10 μm [21]. Accelerations vibration shown in Table 3 were loaded on the 3D model at $Z$ axis, which perpendicular to the bridge-like structure. Mechanical properties of the model were shown in Table 1.

Table 3Acceleration shock FEA parameters

Test equipment	Shock direction	Acceleration shock	Shock pulse width
Electromagnetic vibration system (D3000-36)	$Z$ axis	200 G	1 ms
	$Z$ axis	500 G	1 ms
	$Z$ axis	1000 G	1 ms

Fig. 5Result of IR absorption analysis

a) IR absorption vs wavelength

b) Average IR absorption rate vs height of resonant cavity

Fig. 6Vibrations FEA results

a) Deformation under 200 G

b) Deformation under 500 G

c) Deformation under 1000 G

d) Stress distribution under 200 G

e) Stress distribution under 500 G

f) Stress distribution under 1000 G

Fig. 6 were the deformation result under acceleration vibration. It can be observed from Fig. 6 that under 200 G and 500 G acceleration, shock at $z$ axis, deformation of the model were very small. While the stress of structure under 1000 G acceleration, shock was bigger. It means that under 1000 G acceleration, shock, the structure would be fractured and caused the device failure [18].

Fig. 7 shown the first order modal frequency, the second order modal frequency and, the third order modal frequency was 151215 Hz, 213236 Hz and 318524 Hz respectively. Such a high resonant frequency shown that the device had an excellent mechanical properties under different environment [19].

Fig. 7Results of resonance frequency analysis

a) Resonant frequency

b) First order modal frequency

c) The second order modal frequency

d) The third order modal frequency

3. Results and discussion

A 320×240 microbolometer with pixels measuring 37×37 μm² was fabricated according to the design in this study to verify the simulation results. In order to validate properties of the microbolometer in this study, random vibration test was finished according to the test parameters shown in Table 4.

Table 4Random vibration test parameters

Test equipment	Random vibration direction	The intensity of vibration
Vibration system (M18500 Acceleration METER)	$Z$ axis	2 (m/s²) ²/Hz

From the SEM of random vibration test shown in Fig. 8, there was little change of internal bridge-like structure and the devices worked properly before and after the test.

Fig. 8SEM of random vibration test on Z axis

a) Before the vibration

b) After the vibration

Fig. 9SEM of acceleration, shock test on Z axis

a) 200 G

b) 500 G

c) 1000 G

d) FIB cross section (1000 G)

Fig. 9(a) and (b) were SEM of microbolometer under 200 G and 500 G acceleration shock. It is shown that the deformation of the model were very small. The resonant cavity was $λ /$ 4 which means high IR absorption for the microbolometer. While under 1000 G acceleration vibration, it can be observed from Fig. 9(c) and (d) that serious deformation and fracture were occurring.

For the excellent mechanical properties, the IR absorption rate of the device was 84.8 % (Fig. 10(a)). High IR absorption result in good responsivity, low noise voltage and NETD which were shown in Figs. 10(b), (c) and (d). Experimental results were listed in Table 5.

A scanning electron micrograph of the product was shown in Fig. 11(a), (b) was the image captured by the device under 60 Hz.

It is shown the deviation between the measured IR absorption rate was 1.2 %. Experiments versus simulations were as expected. Acceleration shock test results were also in accordance with the vibrations FEA. It suggested that the FEA method was very effective and provided advantages in accuracy.

Fig. 10Noise voltage, Responsivity, NETD distributions and IR absorption histograms

a) 3-D Distribution of noise

b) Responsivity histogram

c) NETD histogram

d) IR absorption histogram

Fig. 11Microbolometer as manufactured, and as used

a) Scanning electron microscope (SEM)

b) Image captured (60 Hz)

Table 5Performance of microbolometer (experimental)

Experimental result	$V_{n}$	Responsivity	NETD	IR absorption rate (8-14 μm)
Experimental result	8.61×10^-5 V	7.42×10⁶ V/W	61.5 mK	84.8 %

4. Conclusions

A novel method for resonant frequency and vibration analysis based on the FEA method was introduced which provide the structure of microbolomer with a good mechanical properties result in a high IR absorption. Finally, a 320×240 IRFPA was fabricated based on this design which had a good mechanical properties leading to excellent performance of the device. Random vibration test and acceleration, shock test was finished and it is showing the experimental results matched simulation results well. This FEA method simulation method offered an efficient ways of microbolometer design and fabrication.

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About this article

Received

18 January 2015

Accepted

05 May 2015

Published

15 August 2015

SUBJECTS

Oscillations in electrical engineering

Keywords

microbolometer

finite element analysis

vibration analysis

infrared absorption

Acknowledgements

This research was supported by the National Science Foundation of China (Grant No. 61235006, 61421002).

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.