Uncooled IR imaging using optomechanical detectors - CiteSeerX

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Ultramicroscopy 107 (2007) 610–616 www.elsevier.com/locate/ultramic

Uncooled IR imaging using optomechanical detectors Zhengyu Miaoa, Qingchuan Zhanga,, Dapeng Chenb, Zheying Guoa, Fengliang Donga, Zhiming Xionga, Xiaoping Wua, Chaobo Lib, Binbin Jiaob a

CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei 230027, China b Institute of Microelectronics, Chinese Academy of Science, Beijing 100029, China Received 27 July 2006; received in revised form 25 November 2006; accepted 6 December 2006

Abstract In this study, we present an uncooled infrared imaging detector using knife-edge filter optical readout method. The tilt angle change of each cantilever in a focal plane array (FPA) can be simultaneously detected with a resolution of 1051. A deformation magnifying substrate-free microcantilever unit is specially designed. The multi-fold legs of microcantilever are interval metal coated to form a thermal deformation magnifying structure. Thermal and thermomechanical performance of this microcantilever unit are modeled and analyzed. An FPA with 100 100 pixels is fabricated and thermal images of human body are obtained by this detector. r 2007 Elsevier B.V. All rights reserved. Keywords: Uncooled IR imaging; Microcantilevers; Optical readout

1. Introduction Uncooled infrared (IR) imaging technique has attracted much attention in recent years due to its low cost and high performance [1–3], in which FPA that consisted of bimaterial microcantilevers is often used. Absorption of the incident IR radiation raises the cantilever temperature, resulting in proportional deflection due to mismatch in thermal expansion of the two materials. The deflections of microcantilever array can be detected by capacitive, piezoresistive, or optical method [4–9]. Among these methods, the optical readout method does not require any micro readout circuit interconnect on each pixel, and better performance can be obtained through it than electrical readout due to better cantilever thermal isolation, less electronic noise, and fabrication complexity. Zhao et al. [9] analyzed that such optomechanical detectors have the potential of reaching NETD below 3 mK. Two key issues of the optomechanical detectors are the optical readout method and the FPA design. For optical readout method, although the deflection of a single cantilever has been measured with sub-angstrom resolution Corresponding author. Tel.: +86 551 3607613; fax: +86 551 3606459.

E-mail address: [email protected] (Q. Zhang). 0304-3991/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2006.12.003

using optical beam steering [10–13], such far-field technique is susceptible to crosstalk when applied to cantilever array. To solve the problem, sequential position readout from a microcantilever array by switch on/off prearranged micromode fibers guided onto a sensor solves the crosstalk of optical readout [14,15]; however, it is not suitable for large imaging array. Researchers in Berkeley use interferometry methods with coherent light to detect the vertical displacement of diffraction grating or the reflector in microcantilever to get thermal images [8,9]. Their optical detecting resolution is typically l/10l/100 (l is the wavelength of readout light) and is sensitive to environment vibration. Researchers in Oak Ridge National Laboratory use quad-cell method for single microcantilever and CCD readout method for microcantilever array [16]. Their CCD readout method converts the tilt angle change of reflective area in the cantilever to the displacement of the reflected light spots on CCD plane, which easily causes crosstalk when applied to imaging array and results in poor image quality, and the resolution of this method is low due to the limitation of CCD pixel size. They designed an interval metal-coated leg structure that can increase the thermal isolation of the detector and thereby increase the temperature gradient across the bimaterial portion of the leg. However, only single pixel detector with

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interval metal-coated leg structure is fabricated by focus ion beam milling, and detector array of this structure was not realized. Moreover, in these reported FPA structures, the cantilever array [Fig. 2(a)] is standing on the silicon substrate using sacrificial technique [4,8,9,16]. The incident IR radiation energy will be attenuated by the substrate before reaching the cantilever, and stiction will easily occur due to thermal or mechanical shock in such structure. Here, we present an optical readout method using knifeedge filter that can simultaneously detect the tilt angle changes of microcantilever array with a resolution of 1051. This optical readout method converts the tilt angle change to the intensity change of fixed CCD pixel, which eliminates the crosstalk between CCD pixels. To solve the problems in a traditional FPA with substrate and meet the requirement of the knife-edge filter optical readout method that demands a large tilt angle change of the microcantilever for a small temperature change, a deformation magnifying substrate-free (DMSF) microcantilever array is designed and fabricated. Thermal and thermomechanical performance of a microcantilever unit are modeled and analyzed. An FPA with 100 100 pixels is fabricated. By using this detector, thermal images of human body are obtained. 2. Principle 2.1. Presented optical readout method and IR imaging system The schematic diagram of the optical readout IR imaging system is shown in Fig. 1. A thermal object is imaged on the bimaterial microcantilever array by an IR lens. The visible readout light of a cold light source (LED) emitting from a pin hole becomes parallel through a collimating lens. The light arrives at the cantilever array and is then reflected by the reflector of the cantilever unit. These reflected rays are synthesized into the spectra of the reflectors on the focal plane of the collimating lens (also acting as a Fourier lens), and a knife-edge filter is placed on the focal plane to filter the spectra. The readout light passing through the knife-edge filter forms an image of the

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microcantilever array on a CCD camera (12 bit, 70 db) by an imaging lens. As the temperature changes, the reflector tilts due to thermal deformation of the bimaterial legs and its spectra shift; therefore, the light flux passing through the knife-edge filter increases or decreases, and so does the image intensity of each microcantilever unit on the CCD. To realize the highest sensitivity of the optical readout system, the knife edge is located at the center of the zerothorder diffractive spectra of the reflectors and the readout light intensity should approach the full measuring range of the CCD. To avoid CCD saturation, the knife-edge filter should be located on the side, through which the passing light flux decreases when the cantilever temperature rises. In order to get a high-quality thermal image, a background picture is taken as a reference before IR exposure, and then thermal images are obtained by a realtime subtraction of the reference from the IR exposure pictures. As a result, an invisible IR radiation of the thermal object can be represented by a visible image on CCD. Although an LED is used as readout light here, laser can be also used. The sensitivity of the knife-edge filter optical readout method can be derived from the following consideration. The half-width of the zeroth-order diffractive spectra of the reflector on the focal plane is given by Dd ¼ fl/L [17], where f is the focal length of Fourier lens, l the wavelength of readout light, and L the reflector length. Then the measurable minimum tilt angle change ymin of reflector is Dd/fN by supposing that the half-width is quantized to N gray levels of the CCD, i.e. ymin ¼ l/NL (rad) ¼ 1.2 106 rad or 7 105 deg for N ¼ 4096 (12 bit CCD), l ¼ 0.5 mm and L ¼ 100 mm. Thus, the measurable minimum displacement (Lymin ¼ l/N) at the reflector tip using the knife-edge filter optical readout method is l/4096, which is much smaller than that of the interference method (typically l/100). From the analysis it can be seen that the optical detecting sensitivity do not rely on the length of the optical level, which means that a compact system could be realized. 2.2. Multi-fold interval metal-coated leg DMSF structure A DMSF FPA is patterned on a substrate-free SiNx thin film [Fig. 2(a)], which has the following merits: increasing

CCD Imaging Lens

FPA in Vacuum Chamber

IR Object

IR Lens

Collimating Lens

Knife-edge Filter Pin Hole

Half Mirror

LED

Fig. 1. Schematic diagram of knife-edge filter optical readout IR imaging system.

Fig. 2. Side view of the traditional FPA structure with substrate (a) and the substrate-free structure (b).

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3. Thermomechanical analysis and design of microcantilever unit 3.1. Thermal analysis of microcantilever unit An important parameter of the performance of a thermal detector is the IR energy conversion efficiency, i.e. the temperature change in the microcantilever DTc due to the temperature change of the target IR source DTs (the IR power qs absorbed within the microcantilever is determined by DTs for a given IR optical system), which can be expressed as [3], H¼

Fig. 3. Schematic diagram of microcantilever.

DT c Aab SiNx t0 pðdP=dT t Þ ¼ . DT s 4F 2no G total

(1)

Here, Gtotal is the total thermal conductance, Aab the area of absorber, e the emissivity of the pixel (eAu ¼ 0.01, SiNx ¼ 0:8), t0 the transmissivity of the IR optical head ( ¼ 0.9), Fno the f/# of the IR imaging lens ( ¼ 0.8), and dP/dT the fraction of the radiative energy emitted by the target source at temperature Tt (300 K). Within the 8–14 mm spectral band, dP/dTt ¼ 0.63 W m2 K1 sr1. The total conductance Gtotal contains three parts, i.e. supporting legs, radiation, and air. The thermal conductance of air can be neglected after vacuum package. Thus, Gtotal ¼ Gleg þ Grad ðW=KÞ,

(2)

where Grad is the radiation conductance of the pixel, which satisfies Grad ¼ 4sAab ðAu þ SiNx ÞT 3 ðW=KÞ, Fig. 4. Schematic diagram of TDMS.

the absorption of IR flux, and eliminating stiction between the cantilever and the substrate. The microcantilever unit on FPA consists of reflector/IR absorber, deformation structure and frame (Fig. 3). One side of the reflector/IR absorber is Au film that plays a role as a reflector; the opposite side is SiNx film that plays a role as an IR absorber. Thermal deformation structure is supported by the frame and is symmetrically located along the reflector. The multi-fold legs are interval metal coated to form a thermal deformation magnifying structure (TDMS). The bimaterial legs will bend while singlematerial legs keep unbending as the temperature changes. In this study, the double-fold leg structure is used. Fig. 4 is the schematic diagram of TDMS. Supposing there is a temperature change on the bimaterial leg and y1 is the tilt angle change of the reflector for single-fold leg, then for double-fold interval metal-coated leg TDMS, y2 is the tilt angle change of the added bimaterial leg. The total tilt angle change of the reflector will be y1+y2. Based on this design, deformation-magnifying effect can be obtained.

(3)

where s is the Stefan–Boltzmann constant ( ¼ 5.67 108 W m2 K4) and T the pixel temperature (300 K). For the calculation of thermal conductance of supporting legs, the process is as following. The thermal resistance of bimaterial and single-material leg are Rb ¼ Lleg = ðkSiNx ASiNx þ kAu AAu Þ, Rs ¼ Lleg =kSiNx ASiNx , respectively. Then the thermal resistance of one-fold leg will be R ¼ Lleg =ðkSiNx ASiNx þ kAu AAu Þ þ Lleg =kSiNx ASiNx . For nfold structure, thermal resistance will be Rleg ¼ nLleg = ðkSiNx ASiNx þ kAu AAu Þ þ nLleg =kSiNx ASiNx . Thermal conductance is the reciprocal of thermal resistance, which can be expressed as G leg ¼ 1=Rleg ¼ ðnLleg =ðkSiNx ASiNx þ kAu AAu Þþ nLleg =kSiNx ASiNx Þ1 . For the presented symmetrical structure, the total thermal conductance of n-fold supporting legs can be expressed as 1 nLleg nLleg Gleg ¼ 2 þ ðW=KÞ, kSiNx ASiNx þ kAu AAu kSiNx ASiNx (4) where n is the multi-fold number, A the cross-sectional area of the leg, k the thermal conductivity of the leg material, and Lleg the leg length that equals to the length of reflector. For double-fold leg TDMS, n equals to 2.


3.2. Thermomechanical design In a microcantilever unit with single-fold deformation leg, the tilt angle change Dy of the reflector due to temperature change DTc on the microcanitlever is defined as thermomechanical sensitivity ST [9], Lleg Dy n1 þ 1 ST ¼ ¼ 6ðaAu aSiNx Þ ðrad=KÞ, (5) DT c K tSiNx

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pattern the wet-etch window on the backside of the wafer. Finally, silicon substrate is etched by KOH, and the silicon substrate under the main frame outside FPA area is held to maintain strength. SEM pictures of microcantilever array and one pixel structure that is 200 mm 200 mm in size are given in Fig. 6. Bimaterial leg and single-material leg is 180 mm in length and 2 mm in width. Microcantilever array is supported by frames of 10 mm in width. The total pixel number of current fabricated FPA is 100 100.

where a is the coefficient of thermal expansion (CTE) of the material, and K is given as K ¼ 4 þ 6n1 þ 4n21 þ Fn31 þ n1 ¼

5. Imaging results and the calculation of NETD

1 , Fn1

5.1. Imaging results

tAu E Au ; F¼ , tSiNx E SiNx

ð6Þ

where E is the elastic modulus, t the thickness, and n1 the thickness ratio of Au and SiNx. Considering that the thermal conductivity of Au is much greater than that of SiNx, the temperature gradient should be mainly located on the single-material legs. For the case of double-fold interval metal-coated leg TDMS, if the temperature on the frame is T and the temperature on the reflector/IR absorber is T+DTc, we can assume that the temperature on the bimaterial leg adjacent to the absorber is T+DTc, and the temperature on the other bimaterial leg adjacent to the frame is T+DTc/2. Thus, the total deformation is 32 times of that in single-fold leg structure, which can be expressed as the following: Lleg 3 Dy n1 þ 1 ðrad=KÞ. ST ¼ ¼ 6ðaAu aSiNx Þ DT c K tSiNx 2 (7) The design parameters of DMSF FPA, the thermal and mechanical properties of SiNx and Au, and the performance of DMSF FPA of current design with double-fold leg structure are listed in Tables 1–3, respectively.

Fig. 7 shows thermal images of human body and human hand, where the images were taken by IR lens (f/# ¼ 0.8), and the FPA was kept in the vacuum cavity (1 Pa). Figs. 7(a) and (b) show the thermal images of a man wearing and removing a coat at a room temperature of 25.5 1C, respectively. The imaging distance is 5 m. The gray change can be obviously observed since the radiation temperature is changed. Fig. 7(c) shows a thermal image of the human hand wearing a watch. The imaging distance is 1 m. As shown in Fig. 7(c), the contour of the hand and watch can be clearly distinguished. These thermal images were not done with any digital image processing. The bright spot array composing the thermal image represents the corresponding reflectors of microcantilever units on FPA. And in these images, there appears to be some blind spots. These spots are invalid cantilever units due to the failure in fabrication process.

Table 2 Mechanical and thermal properties of SiNx and Au

4. Microfabrication A silicon bulk micromachining technique is used to fabricate the uncooled IR FPA. The microfabrication process typically consists of five steps as shown in Fig. 5. It starts with 500 mm thick double-side polished P/1 0 0S silicon wafer. First, a low stress of SiNx film is deposited on both side of silicon substrate by LPCVD. RIE is then used to pattern SiNx film on the front side to define the mirror, the legs and the frame. Lift-off technique is followed to form interval metal-coated structure. The next step is to

SiNx Au

Density r 103 (kg/ m3)

Young’s modulus E (GN/m2)

Thermal condition K [W/(m K)]

Expansion coefficient a [ 106 K1]

2.40 19.3

180 73

5.570.5 296

0.8 14.2

Table 3 Performance of DMSF FPA with double-fold leg structure Gleg (W/K)

Grad (W/K)

H

ST (deg/K)

1.06e7

1.16e7

5.95%

0.023

Table 1 DMSF FPA design parameters Unit size (mm)

Reflector size (mm)

Bimaterial leg length (mm)

Bimaterial leg width (mm)

Single-material leg length (mm)

Single-material leg width (mm)

Au film thickness (mm)

SiNx film thickness (mm)

200 200

180 130

180

2

180

2

0.2

2


614

5.2. Estimation of NETD

change DI in imaging system due to the temperature change DTs of the target IR source.

According to the definition, NETD is equivalent temperature change in an IR source that can be detected with a signal-to-noise ratio of unity, that is NETD ¼

I noise ðmKÞ. DI=DT s

(8)

Here Inoise is the gray levels of system noise, DI/DTs the thermal response sensitivity of system, defined by the gray

a b c d e One pixel structure

f 100 × 100 pixels Si

SiNx

Au

Fig. 5. Microfabrication process sequence of DMSF FPA: (a) SiNx film was deposited on silicon substrate, (b) SiNx film was patterned to define the reflector and legs, (c) Cr–Au layer was deposited on SiNx and patterned by lift-off, (d) Wet-etch window was formed, (e) silicon substrate was removed, (f) full view of the FPA.

5.2.1. From experiment To evaluate the thermal response sensitivity, a black body source with temperature control precision of 0.01 K was used to provide a uniform calibrated IR flux onto FPA. By measuring the intensity change on the CCD pixels corresponding to fixed reflectors of the microcantilever units with temperature change, the thermal response sensitivity can be obtained. The experimental relationship between the gray level on the CCD pixels and the temperature of the black body source is shown in Fig. 8. The temperature of the black body source was increased by the step of 1 1C interval from 25 to 38 1C. The gray levels of the CCD pixels (which are bigger than 3000) are selected by taking an average gray level value of 3 3 neighborhood reflector images in the central imaging area. The experimental data in Fig. 8 are fitted to a line by least-squares fitting and the slope of the line is 15.32 gray level/K; that is, the experimental thermal response sensitivity DI/DTs is 15.32 gray level/K. To measure the system noises, the black body source was controlled at the temperature of 25 1C, and 300 thermal images were serially captured and the gray fluctuation of the CCD pixels whose gray levels (brightness) are bigger than 3000 is statistically calculated. The measured system noise distribution of the DMSF FPA is shown in Fig. 9. The average fluctuation is about 10 gray levels, which can be regarded as the system noise reasonably. Thus, NETD is 650 mK in the present experiment calculated by Eq. (8). 5.2.2. From modeling analysis Theoretically, optical detecting sensitivity of the knifeedge filter optical readout method is defined as the gray level change DI corresponding to the measurable minimum tilt angle change ymin of reflector DI NL p ¼ ¼ 6:3 N ðgray level=degÞ. Dy l 180

Fig. 6. SEM pictures of FPA: (a) microcantilever array, (b) one pixel structure.

(9)


615

Fig. 7. Thermal images of human using knife-edge filter optical readout method: (a) Man wearing a coat, (b) man removing the coat, (c) human hand wearing a watch.

Fig. 8. The experimental relationship between the gray level on the CCD pixels and the temperature of the black body source.

Fig. 10. The experimental relationship between gray level responses and inclination angle of microcantilever.

Fig. 9. Noise distribution of the system. The CCD pixels whose gray levels are greater than 3000 are statistically calculated. The average fluctuation is taken as the system noise. For the DMSF FPA at 25 1C, the system noise is about 10 gray levels.

Here, l ( ¼ 0.5 mm) is the wavelength of readout light, N ( ¼ 4096) the quantization level for a 12-bit CCD, and L ( ¼ 180 mm) the length of the reflector. However, an initial bending of the reflector (with curvature radius 5 mm measured) because of the residual stress in the microfabrication process will reduce the optical detecting sensitivity since the spectra of the reflector on the focalplane are dispersed by the bending. An FPA is fixed on a rotating stage to measure the optical detecting sensitivity. The FPA is rotated to make its spectrum shift from the passed area of filter to the blocked area by step of 0.0151. The gray level responses of microcantilever with different inclination angles are recorded, which is shown in Fig. 10. The solid line is the linear fit curve of the experimental data. The slope of this line, i.e. the optical detecting sensitivity, is 1.0 104 gray levels per degree. Consequently, the experimentally optical detecting sensitivity DI/Dy is about 1.0 104 gray level/deg. According to the modeling analysis results given in Table 3, the thermomechanical sensitivity ST of the microcantilever

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array is 0.023 deg/K, and the IR energy conversion efficiency H is 5.95%. Thus the theoretical thermal response sensitivity can be calculated as, DI DT c Dy DI DI ¼ H ST ¼ 13:69 ðgray level=KÞ. ¼ DT s DT s DT c Dy Dy (10) Thus, NETD from modeling analysis can be calculated to be 730 mK from Eq. (8) by taking Inoise as 10 gray levels and the theoretical thermal response sensitivity as 13.69 gray level/K. The theoretical analysis is well accordant with the experimental result, which confirms that the thermomechanical model is reasonable. Theoretical and experimental values of the detecting sensitivity of the knife-edge filter optical readout method also imply that the thermal response sensitivity DI/DTs can be improved by 2.6 times (6.3 N/10 000) if we can improve the technique to fabricate a flat reflector that has no initial bending; that is, the NETD will be reduced from 650 K to 250 mK. A potential improvement of this detector is to reduce the thickness of the SiNx film and optimize the thickness ratio of Au and SiNx to increase the thermomechanical sensitivity. The optical detection sensitivity can also be improved by using a more stable readout light source and a high quantized CCD. A limitation of this technique is that the stress-induced mirror curvature will degrade the detection sensitivity. Further work should be focused on controlling the residual stress.

microcantilever are interval metal coated to form a deformation magnifying structure. The thermomechanical sensitivity of TDMS is about 50% higher than that of a single-fold structure. Thermal and thermomechanical performance of the microcantilever unit are modeled and analyzed. Thermal images of a human body are obtained by this detector. The NETD is about 650 mK. The experimental results are well accordant with the thermomechanical model of microcantilever unit. Such optical readout system can be also used to detect deflections of microcantilever array in biochips or chemical sensors. Acknowledgments The authors would like to acknowledge support from the National Natural Science Foundation of China (Grant Nos. 10232030, 10627201, 10472111) and National Basic Research Program of China (2006CB300404). References [1] [2] [3] [4] [5] [6] [7]

6. Conclusion [8]

A knife-edge filter optical readout method that can simultaneously detect the tilt angle changes of microcantilever array with a resolution of 1051 is presented. It solves the crosstalk problem and it has higher sensitivity than presented optical readout methods used for imaging array. Furthermore, it can use either coherent or incoherent light and does not rely on the length of the optical lever, which means a compact system could be realized. We also designed and fabricated a deformation magnifying substrate-free (DMSF) FPA with 100 100 pixels. This kind of FPA does not require any micro readout circuit interconnect on each pixel, which reduces the thermal noise. And a large array (1000 1000 or more) can be easily fabricated without growing complexity and cost. The substrate-free structure can avoid IR flux loss compared with the structure that has a silicon substrate, and can eliminate stiction between the cantilever and the substrate, due to thermal or mechanical shock. The multi-fold legs of

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