An uncooled optically readable infrared imaging detector - CiteSeerX

Sensors and Actuators A 133 (2007) 236–242

An uncooled optically readable infrared imaging detector Dong Fengliang a , Zhang Qingchuan a,∗ , Chen Dapeng b , Pan Liang a , Guo Zheying a , Wang Weibing b , Duan Zhihui a , Wu Xiaoping a a CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei, Anhui 230027, China b Institute of Microelectronics, Chinese Academy of Science, Beijing 100029, China

Received 2 December 2005; received in revised form 28 March 2006; accepted 6 April 2006 Available online 6 June 2006

Abstract This paper presents a design and fabrication of bi-material micro-cantilever array (focal plane array, FPA) made of silicon nitride (SiNx ) and gold (Au) for uncooled optical readout infrared (IR) imaging system, in which silicon (Si) substrate is removed. Compared with the conventional thermal imaging detectors where the FPA must be put in high vacuum, IR thermal images can be obtained even though the cantilever array is placed in the atmosphere. The reason is the elimination of air gap (∼2 ␮m) between the cantilever beam and substrate, which introduces the air conduction of high temperature gradient. The preliminary experimental results with the micro-cantilever array of 140 × 98 elements and a 12-bit charge-coupled device (CCD) indicate that objects at temperature of higher than 120 ◦ C can be detected and the noise-equivalent temperature difference (NETD) is ∼7 K. Also, the experimental results are well accordant with the thermomechanical analysis of designed micro-cantilever array. © 2006 Elsevier B.V. All rights reserved. Keywords: Uncooled; Infrared imaging; Optical readout; Bi-material micro-cantilever array

1. Introduction In recent years, much attention has been paid to uncooled infrared (IR) imaging [1–3] because of their wide applications in military and civil fields, such as remote sensing, night vision, environmental monitoring, etc. These applications not only demand low NETD, but also require low cost and economical power consumption. It is important for an IR camera to be sensitive in the 8–14 ␮m spectral range because both the atmospheric transmission window and the peak of the blackbody spectrum for objects near room temperature are in this range. IR detectors can be broadly divided into cooled detectors (photonic detectors) and uncooled ones (thermal detectors). Although photonic detectors, based on semiconductors such as HgCdTe, have been commercialized because of their high detectivity (NETD ∼5–10 mK [1]), the application is limited by their high cost and excessive power consumption because they require auxiliary cooling devices, which cool the detectors to about −200 ◦ C (∼77 K) for eliminating the thermal noise of electrons. For a

∗

Corresponding author. E-mail address: [email protected] (Q. Zhang).

0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.04.031

conventional thermal IR imaging device (NETD ∼20–50 mK [1]), the temperature rise in each pixel is measured electrically by changes either resistance or capacitance. The need for electrical interconnect to each pixel leads to fabrication complexity as well as scanning electronics and a display system, all of which have kept the cost prohibitively high for many commercial applications. Additionally, because electrical interconnects are used for each pixel, it is difficult to increase the thermal isolation to the radiation limit. From the 1990s, the interest in optically readable bi-material micro-cantilever array has become more and more unquenchable. The optical readout system eliminates the need for the highly sensitive readout integrated circuits and scanning electronics, thus reducing fabrication complexity and costs. The references [4–9] have demonstrated varying degrees of success, showing that the NETD of optomechanical IR imaging system is ∼2 K [4–7]. However, the interferometry optical readout system used in [4–7] is highly susceptible to oscillation noise, while the sensibility and space resolution of the pinhole-filtering optical readout method introduced by [8,9] is not gratifying. Likewise, the FPA architecture in [4–9] are fabricated employing sacrificial layer technique, which makes the fabrication of FPA complicated due to the difficulty of release of sacrificial layer and

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causes the structure disabled easily because of the adherence of cantilevers to substrate. In addition, the thermal conduction of great temperature gradient of the air gap (∼2 ␮m) makes FPA workable only in vacuum of about 0.01 Pa [6]. From 2003, the author’s group has developed bi-material FPA for infrared imaging using diffractive optical readout and succeeded in obtaining the IR images of thermal objects of 200 ◦ C [10,11]. Based on the previous work, this paper presents a design and fabrication of FPA architecture made of bi-material, in which Si substrate is removed, and gives the analysis of design parameters. In the thermal imaging experiment, FPA array was laid not in vacuum, but in atmospheric environment. The thermal image of objects of higher than 120 ◦ C could be obtained and the measured thermomechanical response of FPA’s elements is about 0.013 deg/K. 2. Concept of FPA architecture and optical readout system Compared with the reported FPA [4–9] of bi-material cantilever arrays supported on the Si substrate, a design of Si substrate-removed architecture, in which no air gap exists, is introduced. Fig. 1 gives the schematic view of bi-material microcantilever element of FPA, made of two different materials (SiNx and Au) having evidently different coefficients of thermal expansion (CTE). It comprises three portions: reflector, bi-material cantilever beam and frame. The reflector, 90 ␮m long, 60 ␮m wide, is made of layers of SiNx (1.8 ␮m thick) and Au (0.4 ␮m thick). The SiNx portion of reflector serves as the incident IR absorber, while the part of gold layer acts as a reflector of readout light. The folded region in Fig. 1 is bi-material cantilever beam 3 ␮m wide; the total length Lleg except last fold is about 550 ␮m. The cantilever element is supported on the frame though the leg of last fold. The FPA architecture has mainly the characteristics: (1) the optical readout does not require metal interconnection on each pixel for measuring output as electrical readout system and thus

Fig. 2. Schematic diagram of the optical readout system.

enables the thermal isolation to improve; (2) the length of bimaterial cantilever is fabricated as possible as long to reduce thermal conduction of cantilever, which maximizes the temperature rise of cantilever element, while bi-material having evidently different coefficients of thermal expansion makes the deformation of cantilever maximized; (3) the architecture without sacrificial layer and Si substrate eliminates adhesion, which is a vital defect of micro-fabrication, thus simplifying greatly the fabrication process; (4) silicon substrate is removed that blocks about 40% IR radiation of the thermal object, the transmissivity of radiation is improved; especially (5) without air gap, reducing air conduction to 1/104 of traditional thermal detectors [6], thus it is unnecessary to put the array in vacuum; (6) the manufacture of FPA is compatible with conventional IC fabrication, which enhances the reliability and yield. Fig. 2 shows the schematic principle diagram of the optical readout system. When the bi-material cantilever absorbs IR

Fig. 1. Schematic sectional view (a) and top view (b) of the micro-cantilever.

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flux bringing a temperature rise, it bends, inducing a change of inclination angle of reflector (in Fig. 1(a)), which is proportional to the temperature rise. The readout system converts the angle change of micro-cantilever array to a visible image onto the CCD. The visible readout light from LED through a pinhole, which is located on the focus of the collimating lens L1, becomes a parallel beam before illuminating on the FPA. The reflected diffracting flux by BS synthesizes the spectra of FPA on the rear focal plane of transforming lens (also L1). A knifeedge placed on the focal plane is a spatial filter, defining two statuses for reflected light flux, transmission or block. When the direction of reflected readout light changes, the spectra of FPA shifts, therefore, the light flux passing through the knife-edge filter increases or reduces, so does the image intensity of cantilever elements. A visible grey image is formed on the CCD by subtracting of images of the deformed and previously undeformed FPA elements. Note that, in order to optimize the image grey, the knife-edge should be placed at the 0th order of the spectra of FPA and the readout illumination should approach the maximum of measuring range of the CCD. 3. FPA design For a given incident flux of IR, under the constraints of predetermined element size, the design of FPA should (1) maximize the pixel’s temperature rise, (2) optimize the thermomechanical response and (3) maximize the length of each reflector along the direction of cantilever beams to make the spectra of readout light the narrowest, thus improving optical sensitivity. In addition, the thermal time constant of the element should be compatible with the frame rate of CCD system to achieve real-time detection.

Fig. 3. Real (η) and imaginary (κ) parts of the refractive index of a LPCVD SiNx .

materials used for bi-material cantilever array. By comparison, it can be seen that low-stress SiNx and Au show the desirable large mismatch in CTE. Fig. 3 [4,6] provides the real (η) and imaginary (κ) parts of the refractive index of SiNx , indicating the absorption peak in the 8–14 ␮m spectral range, which is suitable for the requirement of detection. In addition, low-stress SiNx also has low thermal conductivity, which is significant for thermal isolation. 3.2. Thermal design of FPA Consider a cantilever element in the FPA initially at thermally equilibrium with the surroundings. The temperature rise TC in the cantilever can be expressed as: qApixel FF , G

3.1. Selection of materials

TC =

The two materials selected for FPA should meet following basic features: (1) one of the materials must efficiently absorb IR in the range of 8–14 ␮m, and the other a good reflector in the visible spectrum for optical readout; (2) the two materials must have a large mismatch in CTE; (3) at least one material’s thermal conductivity must be low; (4) the films of the materials must have low residual stress. Additionally, the materials must be compatible with micro-fabrication processes and should possess good chemical inertness. The two materials of FPA used in this paper are low-pressure chemical vapor deposited (LPCVD) low-stress SiNx and Au. Table 1 [4,12] lists the physical properties of some optional

where q is absorbed IR flux, Apixel the element area, FF the fillfactor and G is the thermal conduction between the cantilever element and the environment:

(1)

G = Gleg + Grad + Gair .

(2)

Here the leg’s conductance Gleg can be expressed as: −1 Lleg Llast + , Gleg = 2 i ki A i i ki A i

(3)

where k is the thermal conductivity of the selected material, A the cross-section area of the beam, Lleg the total length of leg (except

Table 1 Properties of optional materials for bi-material cantilever array

SiNx Au Al Si

Density ρ × 10−3 (kg m−3 )

Young’s modulus E (GN m−2 )

Thermal cond. k (W m−1 K−1 )

Expansion coeff. α (×10−6 K−1 )

Heat Capacity (J kg−1 K−1 )

2.40 19.3 2.7 2.33

180 77 80 100

5.5 ± 0.5 296 237 135

0.8 14.4 23.6 2.6

691 129 908 700

F. Dong et al. / Sensors and Actuators A 133 (2007) 236–242 Table 2 Design parameters of Si substrate-removed cantilever array

the fraction of the radiative energy emitted by the object at temperature Tt (∼300 K). Within the 8–14 ␮m spectral band, dP/dTt = 0.63 W m−2 K−1 sr−1 .

150 × 100 550 8.2 × 10−7 7.4 × 10−8 2.0 × 10−3 1.7 × 10−4 2 × 10−8 24

Element size (␮m2 ) Lleg (␮m) Gleg (W K−1 ) Grad (W K−1 ) H ST (rad K−1 ) Gair (W K−1 ) Thermal time constant τ (ms)

3.3. Thermomechanical design of FPA

last fold) and Llast is the length of last fold leg (∼90 ␮m). The subscripts i = 1, 2 denote the Au and SiNx layers in a bi-material cantilever, respectively. Grad is the radiative conductance of the pixel, which follows: Grad = 4σApixel (εAu + εSiNx )T , 3

(4)

where σ is the Boltzmann constant, the emissivities of Au and SiNx are 0.01 and 0.8, respectively, T the element temperature (∼300 K). Gair is the thermal conductance of air: Gair =

kair Apixel , D

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(5)

To determine the inclination angle of the cantilever as a function of temperature rise, one must solve the thermomechanical governing equation for a bi-material cantilever beam, which is given as [6]: dθ tAu + tSiNx d2 z = 2 = 6(αAu − αSiNx ) T (x), 2 dx dx tSiN K x z = 0; θ =

dz = 0 at x = 0, dx

(8)

where α is the thermal expansion coefficient, t the cantilever thickness, z the cantilever deflection at a distance, T(x) the temperature rise of the cantilever and K is a structure parameter given as: K = 4 + 6n + 4n2 + Φn3 +

1 , Φn

tAu

n=

tSiNx

EAu , ESiNx (9)

;Φ =

where kair = 26.3 × 10−3 W m−1 K−1 , is the thermal conductivity of air at room temperature, D the typical value of the experimental setting (∼20 mm). Notice that in the previous FPA [4–9], D is the thickness of sacrificial layer (∼␮m), inducing Gair ∼10−4 W K−1 , 104 times of current value, thus heat absorbed by cantilever element can not be accumulated in atmospheric environment. Table 2 provides the thermal and mechanical parameters of this design. Notice that the value of Gair is only 2% of the total G, it can be neglected, thus Eq. (2) can be rewritten as:

in which E is the elastic modulus. Based on the fact that the cross-section area of IR absorber is much larger than that of beam, inducing that the thermal conductance of the former is much higher, it is reasonable to suppose that the temperature distribution of the IR absorber is uniform, while the distribution of beam is linear. Thus the temperature rise T(x) of the cantilever beam can be given as:

G = Gleg + Grad .

ΔT (x) =

(6)

It is clear from Eq. (1) that, to optimize the temperature rise of FPA element, the element area should be maximized and G must be reduced. The radiative conductance is the intrinsic conductance of the cantilever since the radiative emission is related directly to the absorption through Kirchoff’s law, and it is the limit for the total conductance. The leg’s conductance must be lowered. According to Eq. (3), to reach a small enough thermal conductance, it is necessary to choose a material of low thermal conductivity and to design a long supporting leg of the cantilever with small cross-sectional area. Here, the leg’s conductance is not low enough because Au is thermally conductive, thus a thermal isolation structure would be necessary in future improvement. The temperature rise TC in the cantilever due to the temperature rise TS of the IR object, educed from Eq. (1), can be expressed as [5]: H=

TC Aab ετ0 π(dP/dTt ) = , 2 (G Ts 4Fno leg + Grad )

(10)

where L is the equivalent length of cantilever beam. The last fold leg’s thermal conductance is equivalent to that of the leg whose length and cross-section area are both double. Here the equivalent leg is used for the simpleness of calculation. Eq. (8) can be solved and the thermomechanical sensitivity of the cantilever ST can be found to be L θmax tAu + tSiNx x ST = = 6(αAu − αSiNx ) dx 2 TC L tSiN K 0 x L n+1 = 6(αAu − αSiNx ) KtSiNx Lleg L−(1/6)Lleg L−(1/6)Lleg L−(1/3)Lleg L−(1/2)Lleg − + − L−(1/3)Lleg

(7)

in which Aab is the element absorption area, ε the emissivity of element, τ 0 (=0.4) and Fno (=0.7) are the transmissivity and f/# of the IR lens, respectively, and (dP/dTt ) is

x x TC , TC = Lleg + 2Llast L

+ =

L−(2/3)Lleg

L−(5/6)Lleg

1 (αAu − αSiNx ) 2

L−(1/2)Lleg

−

L−(5/6)Lleg

L−Lleg

n+1 K

L−(2/3)Lleg

+

2Llast

xdx

0

L2leg + 24L2last tSiNx (Lleg + 2Llast )

,

(11)

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NETD of system is expressed as: NETD =

Inoise TS . IN

(14)

Here, Inoise is the grey level of systemic noise, IN the grey level of IR object and TS is the temperature rise of object compared with ambient temperature. The thermal time constant τ of the element is determined by the element’s heat capacitance Cth , and thermal conductance G: τ=

Fig. 4. The influence of bi-material thickness ratio (n) on the thermomechanical sensitivity ST .

where θ max is the inclination angle of reflector. Notice that due to the counteraction of thermal deformation of neighboring bimaterial legs, the total deformation is weakened, which would be eliminated by the realization of interval Au-evaporation of multifold leg (thermal isolation legs connecting with bi-material legs alternately) in further work. It can be seen from Eq. (11) that the mismatch in thermal expansion must be large and the length must be maximized. It is shown in Fig. 4 and Eq. (11) that the cantilever thickness must be reduced in order to optimize ST . For chosen materials, ST is proportional to (n + 1)/K. The optimum value of n depends on EAu /ESiNx [6], which is theoretically predicted to be nop = 0.75, as shown in Fig. 4. However, the value of n is not optimized in this work (n = 0.22), nor is the value of ST (see Table 2). Compared with the reported result (4.5 × 10−3 rad/K) [6], the value still has considerable room for improvement. The optical readout section converts the inclination angle θ of cantilever array to the grey change IN of image on the CCD. Their linear relationship has been confirmed in previous work [13]: θ 1 π ≈ (deg) = (rad), IN 2.4N 432N

(12)

Cth , G

(15)

in which Cth = (ρAab tc)i , ρ is the material density, t the thickness, c the material heat capacity, and i indicates each material used in the cantilever element. τ must be compatible with realtime visible imagers (∼30 frames/s), in this work, which is 24 ms (see Table 2). 4. Fabrication of FPA The designed FPA, containing 140 × 90 elements, was fabricated using surface micromachining technology. The process consisted typically of four steps: deposition of the SiNx film, patterning the SiNx layer, evaporation of the Au layer and release of the structure, as shown in Fig. 5. The simplified process started with a silicon wafer washed using de-ionized water. The first step was deposition of a lowstress LPCVD SiNx film 1.8 ␮m thick. The initial bending of the cantilever, which originates from residual stress in or between the films, severely influences the performance of the optical readout system, hence, its reduction is a major issue. It is well known that the residual tensile stress in LPCVD SiNx film depends strongly on the deposition conditions such as temperature, pressure and the ratio of the flow rate of source gases during deposition. The second step was to lithograph photoresist mask and it is followed by the remove of unwanted SiNx layer employing reactive ion etching (RIE) technique. Third, a layer of gold 400 nm thick was evaporated, and then the cantilever patterns were defined. Finally, the Si substrate beneath the FPA elements was etched in KOH solution, while the substrate joint to the frame of FPA was partly reserved to enhance the bending modulus.

in which N is the quantization level of CCD (for the 12-bit CCD, N = 4096). From Eqs. (7), (11) and (12), the grey change of image as a function of the temperature rise of IR object, can be expressed as: IN IN × ST × H. = Ts θ

(13)

NETD is typically used to define the sensitivity performance of an infrared imaging system. It is the equivalent temperature rise in an IR object that can be detected with a signal-to-noise ratio of unit. In the optical readout system, the signal and the noise of system are presented in form of grey levels, hence, the

Fig. 5. Micro-fabrication process sequence of bi-material cantilever array: (a) deposition of SiNx film; (b) patterning SiNx film; (c) evaporation of Au layer; (d) release of the structure.


241

Fig. 6. SEM picture of micro-cantilever array.

Fig. 6 demonstrates a scanning electron micrograph (SEM) of FPA. The element extends from frame 3 whose width is 10 ␮m (in the inset). The width of frames 1 and 2, where there is still some Si substrate reserved, is 100 and 50 ␮m, respectively. There are 10 × 7 cantilevers in the region surrounded by frames 1 and 2. 5. Experimental results and discussion The optical readout system shown in Fig. 2 was employed in the experiment of thermal imaging with the background temperature around 25 ◦ C. Notice that the FPA was not placed in the vacuum chamber, but in atmosphere. Shown in Fig. 7 is the thermal image of a 250 ◦ C T-shaped copper plate (the inset of Fig. 7). To assess the validity of the modeling above, a temperatureadjustable electrical iron (see the inset of Fig. 8(a)) was used as the IR object. A series of thermal images were collected at different temperatures. Fig. 8(b) demonstrates the curve of the iron’s image grey levels at the position of five selected FPA

Fig. 7. T-shaped copper plate and its thermal image of 250 ◦ C.

elements (see Fig. 8(a)) versus the iron’s temperatures. It can be seen that the magnitudes of grey values experimentally measured are in good quantitative agreement with the results of modeling. It is also shown in Fig. 8(b) that the grey levels of different cantilever elements were not uniform at the same temperature due to element non-uniformity. Experimental data indicate that the thermomechanical sensitivity of FPA is about 0.013 deg/K, which matches well with modeling (1.7 × 10−4 rad/K). It can be seen from Fig. 8(b) that one grey level is corresponding to the temperature of about 2.5 K, while the measured grey levels of systemic noise is ∼3, thus NETD equals to ∼7 K. Considering the possible detective limit (∼␮K) [14] of an optomechanical bi-material micro-cantilever beam, with further development of micro-fabrication technique, it is potential that optically readable radiation-displacementconversion IR imaging will be applicable to uncooled IR imaging product.

Fig. 8. (a) An iron head and its thermal image. (b) The comparison of grey levels measured with calculated values. In the experiment, the grey levels of five elements at the different temperatures are measured.

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6. Conclusions This paper presents a design concept of core component (FPA) of an uncooled optically readable IR imaging system. The Si substrate-removed FPA has the attractive advantages: (1) without sacrificial layer, the fabrication process of FPA is greatly simplified; (2) without air gap, the architecture eliminates the air conduction of high temperature gradient, which makes it possible that the thermal image of an object can be obtained placing FPA in atmosphere etc. The optical readout system was established, and the thermal images of higher than 120 ◦ C objects were preliminarily exhibited. The systemic NETD was ∼7 K. Comparison between experimental data and theoretical calculation demonstrates the self-consistency. The FPA design presented in this paper has still improvement room. The reduction of film thickness, the optimization of bi-material thickness ratio and the realization of interval Auevaporation of multifold leg will improve the special resolution and reduce the systemic NETD of the proposal infrared imaging detector. Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 10232030, 50076040, 10472111) and National Basic Research Program of China (2006CB300404). References [1] R. Antoni, Infrared detectors: status and trends, Prog. Quant. Electron. 27 (2003) 59–210. [2] D. Jakonis, C. Svensson, C. Jansson, Readout architectures for uncooled IR detector arrays, Sens. Actuators A 84 (2000) 220–229. [3] L.R. Senesaca, J.L. Corbeila, B.S. Rajicab, N.V. Lavrik, P.G. Datkos, IR imaging using uncooled microcantilever detectors, Ultramicroscopy 97 (2003) 451–458. [4] M. Mao, T. Perazzo, O. Kwon, A. Majumdar, Direct-view uncooled microoptomechanical infrared camera, in: Proceedings of 12th IEEE International Conference, MEMS, 17–21 January, 1999, pp. 100–105. [5] Y. Zhao, M. Mao, R. Horowitz, A. Majumdar, J. Varesi, P. Norton, J. Kitching, Optomechanical uncooled infrared imaging system: design, microfabrication, and performance, J. MEMS 11 (2) (2002) 136–146.

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Biographies Dong Fengliang received his B.Tech. degree in design and fabrication of mould from Hefei University of Technology (HUT), Hefei, China, in 2002. He is currently working towards the Ph.D. degree in mechanical engineering at University of Science and Technology of China (USTC), Hefei, China. His research interests include design and analysis of micro-cantilever array for infrared imaging, optical detection technique. Zhang Qingchuan is a Professor in the CAS Key Laboratory of Mechanical Behavior and Design of Materials, USTC. His research interests in his group currently range from the Portevin-Le Chatelier effect in metal alloy, optical readout infrared imaging to micro-bio-sensor with optical measurement methods. Chen Dapeng is a Professor in the Institute of Microelectronics, Chinese Academy of Science, Beijing, China. He researches on MEMS design and fabrication. Wu Xiaoping is a Professor in the CAS Key Laboratory of Mechanical Behavior and Design of Materials, USTC. Her research interest is the application of optical measurement methods in the field of advanced science.