Combined design and realization of Al-SiN bi-material optic readout infrared focal plane array with 320 × 240 pixels

Sensors and Actuators A 273 (2018) 256–265

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Combined design and realization of Al-SiN bi-material optic readout infrared focal plane array with 320 × 240 pixels Xia Zhang a , Dacheng Zhang b,∗ a b

Department of Photo-Electronics, Communication University of China, Beijing 100024, China Institute of Micro-Nano Electronics, Peking University, Beijing 100871, China

a r t i c l e

i n f o

Article history: Received 19 August 2017 Received in revised form 13 January 2018 Accepted 12 February 2018 Available online 26 February 2018 Keywords: Combined design Al-SiN bi-material UOR IR FPA Microelectronic process Imaging

a b s t r a c t A combined design of an aluminum (Al)-nitride silicon (SiN) bi-material uncooled optic readout (UOR) infrared (IR) focal plane array (FPA) with 320 × 240 pixels, based on microelectronic technology, is presented. The combined design includes that the materials of the pixel are selected by analyzing the established bi-material beam model on mechanics of materials, the IR absorption and visible light reflectivity of materials, as well as the etching selectivity and material compatibility in fabricating; the film thicknesses of the bi-material beam and reflector of the pixel are determined according to the mechanics model and the equations derived from optoelectronics principle; the self align approach to be able to eliminate the misalignment between two patterns is proposed taking consideration of the etching selectivity and material compatibility. By means of the combined design, the Al-SiN bi-material UOR IR FPA with higher thermal sensitivity and temperature resolution as well as faster response time is successfully fabricated. The measured value of the thermo-mechanical response of the pixel achieves 3.4 × 10−3 rad/K. The IR image video clips are obtained at room temperature. © 2018 Elsevier B.V. All rights reserved.

1. Introduction IR imaging systems are widely applied in numerous fields such as remote sensing, medical diagnosis, fire fighting, composition identifications, and so on. There has been a growing interest in the imaging systems with uncooled IR FPAs having the capability to absorb incident IR radiation in the wavelength range from 8 to14 ␮m which is not only the atmospheric transmission window but also contains the peak of the blackbody spectrum for objects at around room temperature and fabricated by using microelectronic process, because of their small size, low power consumption and cost. The FPAs depending on different working principles have their own advantages and drawbacks. In comparison with the FPAs by electrically measuring the change in either resistance or capacitance of pixels with temperature, bi-material UOR IR FPAs have the advantages such as lower thermal noise and simpler signal processing. Various bi-material UOR IR FPAs have been reported since 1997 [1,2]. Many groups develop bi-material UOR IR FPAs and the imaging systems based on them. Some of the groups reported obtaining IR images, as follows. Perazzo et al. presented that the objects at

∗ Corresponding author. E-mail address: [email protected] (D. Zhang). https://doi.org/10.1016/j.sna.2018.02.018 0924-4247/© 2018 Elsevier B.V. All rights reserved.

temperatures of 100 ◦ C was imaged by the Au-SiN bi-material UOR IR system with the noise equivalent temperature difference (NETD) in the range of 10 K, the pixel pitch is 140 ␮m [3]. Ishizuya et al. developed the Al-SiN bi-material UOR IR FPA with 266 × 194 pixels, one pixel size of 55 × 55 ␮m2 , and obtained an infrared image of a person by adding an offset correction to the raw image taken through CCD camera, as well as, an infrared image of an electric heater without correction [4]. Zhao et al. reported that the thermal images of human hands were obtained by means of using the Au-SiN bi-material FPA containing 50 × 70 pixels, one pixel size of 100 × 100 ␮m2 , the NETD in the range of 2–5 K [5], and the Al-SiN bi-material FPA with 300 × 300 pixels, one pixel size of 65 × 65 ␮m2 , the NETD of 200 mK [6]. Grbovic et al. reported the AuSiN bi-material FPA consisted of a 75 ␮m pitch 256 × 256 pixels. The best pixel NETD is below 500 mK, with average NETD of 1.5 K, and a 6 ms response time. Thermal images were obtained at room temperature [7]. Li et al. proposed the freestanding Au-SiN bi-material FPA and the optical out IR system. The images of hand and person were obtained by using FPAs containing 100 × 100 pixels, with 200 ␮m pixel pitch, the NETD of 200 mK [8]. Yu et al. reported Au-SiN bimaterial FPA with 160 × 120 pixels, one pixel size of 146 × 110 ␮m2 [9]. Thermal images of a person were captured. Steffanson et al. reported the real-time, full-frame thermal imagery of a hand is generated by an UOR IR system based on the Polymer and

X. Zhang, D. Zhang / Sensors and Actuators A 273 (2018) 256–265

SiN bi-material FPA, with a Polymer-SiN 40 × 30 arch type pixels and the NETD of 600 mK [10,11]. Various bi-material UOR IR FPAs may be divided into two types. One has a substrate. The other has no substrate whose structure and fabricating process are simpler. Except for the arch type pixel, no matter which type an UOR IR FPA belongs to, its pixel typically consists of a bi-material reflector and bi-material beams composed of a layer of silicon based material with higher IR absorptivity, lower coefficient of thermal expansion (CTE) and residue stress, and a layer of metal with higher CTE and reflectance of visible light. When the bi-material beams with thermal expansion mismatch effect bend due to their temperature change attributed to the incident IR flux from targets, driving the reflectors to rotate, the change in direction of the visible light reflected by the reflectors into a visible-spectrum charge-coupled device (CCD) camera results. Thus, a thermal image is yielded by the CCD camera. When designing IR detectors, some aspects should be considered such as efficient absorption of IR radiation, sensitive thermometer, thermal isolation for good temperature rise with absorbed IR power, and low thermal mass for fast response time. For a pixel in UOR IR FPAs, considering its working principle, these aspects are associated with the IR absorption material used for absorbing incident radiation in wavelength range from 8 to 14 ␮m and the metal with higher CTE and reflectance of visible light, the sensitive element size related to IR absorption power, the film thicknesses of bi-material beam determining the thermal sensitivity of the pixel for given materials, the film thicknesses of bimaterial reflector influencing the optical beam quality captured by the CCD camera, the pixel structure and size related to its thermal isolation and response time, the FPA patterns and fabrication process determining the size of the misalignment between patterns, all of which have influence on the sensitivity and resolution of the FPA, which are two key parameters used to describe FPA performance. FPAs are kernel components in IR imaging systems, directly influencing the quality of thermal image. Several researches have been endeavoring to improve the performance of UOR IR FPAs for practical applications. But, some key factors directly affecting the performance of the FPAs are not given enough attention. For example, the metal thickness of the bi-material reflector is designed to be equal to that of the bi-material beam in the pixel [12], resulting that the higher the thermal sensitivity of bi-material beams, the benter the reflector is. Besides, the larger the stress between two selected materials combined together, which is caused by material property, the benter the reflector is also. The bending reflector leads to the dispersion of diffraction spectrum of the sample light reflected by the reflector into CCD camera, resulting in a decrease in sensitivity [13]. Another example is related to FPA patterns and fabrication process designing. The absorber film pattern is first fab-

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ricated; then, metal film pattern is fabricated on it [6,12,14]. As a result, there must be the misalignment between patterns, causing a drop in the thermal sensitivity of the bi-material beam, leading to the non-uniformity of the FPA sensitivity, as well as hindering the resolution of the FPA enhanced. At the same time, the noise increases. The misalignment is one of the main reasons, resulting that the measurement value is far from the design value. And the smaller the structure, the larger the affection caused by the misalignment, i.e., the larger the difference between design and the measurement values. In order to obtain a substrateless UOR IR FPA with higher performance, a combined design of the Al-SiN bi-material UOR IR FPA with 320 × 240 pixels, one pixel size of 60 × 60 ␮m2 , based on microelectronic process, is presented in the paper, which includes the material selection of the FPA, the film thickness optimization of the bi-material beams and reflector of the pixel and the self align approach to be able to eliminate the misalignment between patterns. The UOR IR FPA by means of the combined design is successfully fabricated. Not only the problems affecting the FPA performance mentioned above are effectively solved, but also the IR video images are obtained.

2. Thermo-mechanical response and material selection 2.1. Mechanics model of a bi-material micro beam Assume a bi-material beam is composed of materials 1 and 2 as illustrated in Fig. 1(a). L and b represent the length and wide of the beam respectively. h1 and h2 are the thicknesses of materials 1 and 2. E1 and E2 are the elastic modulus of materials 1 and 2. ␣1 and ␣2 are the CTEs of materials 1 and 2, and ␣1 < ␣2 . When the temperature of the beam is raised by T due to it absorbing incident IR flux, two materials expand. At the same time, material 1 is extended by material 2 and material 2 is compressed by material 1, because the CTE of material 1 is smaller than that of material 2. The CTE difference of two materials causes the beam bending, as shown in Fig. 1(b). Assume FN1 and FN2 represent the axial forces in beam direction, M1 and M2 are the bending moments of the beam, as illustrated in Fig. 1(c). 1 and 2 are the stresses at the surface of materials 1 and 2 respectively, as shown in Fig. 1(d). According to the theory of mechanics of materials [15], when the beam is in equilibrium, there are

FN1 + FN2 = 0

Fig. 1. (a) Bi-material beam; (b) beam bending; (c) axial forces and the bending moments; (d) stresses.

(1)

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and M1 + M2 − FN1 (

h2 h1 + ) = 0. 2 2

(2)

The strain ε1 of material 1 is equal to the strain ε2 of material 2 at the interface of materials 1 and 2. Thus, ε1 = ε2 .

(3)

For a thin film beam, the materials 1 and 2 have same radiuses. Thus, 1 ≈ 2 ≈ .

(4)

And their radiuses meet 1 12M1 1 12M2 = and = . 1 2 E1 bh31 E2 bh32

(5)

The stress 1 of material 1 and the stress 2 of material 2 are 1 =

FN1 6M1 FN2 6M2 + and 2 = − . bh1 bh2 bh21 bh22

(6)

According to Hooke’s Law, and considering the temperature raised by T , the strains of the materials 1 and 2 are ε1 =

1 2 + ˛1 T and ε2 = + ˛2 T E1 E2

(7)

Using the radius  of the bending beam and its length L to express the bending angle  0 corresponding to the free end of the beam shown in Fig. 1(b), there is 20 =

L 

(8)

Using Eqs. (1)–(8), the thermo-mechanical response (TMR, rad/K) of the bi-material beam (the bending angle  0 of the beam while the temperature changes by 1 K,) is given by 0 = T

3L(˛2 − ˛1 )

h2 E2 h (1 + h2 ) h2 E1 1 1

h E2 1 E1

1 + 4 h2

+6

h2 E 2 2 h2 E1 1

+4

h3 E 2 2 h3 E1 1

+

h4 E 2 2 h4 1

(9)

2 E2 1

Eq. (9) indicates that the TMR of the beam is proportional to the beam length for two given materials and their thicknesses, and depends on the CTE difference between two materials. Eq. (9) may be used to guide the design for material selection and thickness optimization of bi- material beams and a bi-material reflector in an UOR IR pixel. 2.2. Selection of IR absorber and metal materials In view of the reported bi-material beam UOR IR FPAs, in general, SiN or oxide silicon (SiO) is used as an IR absorber, and gold (Au) or Al is used as a metal on it. The material parameters of SiN, SiO, Al and Au are tabulated in Table 1. As shown tabulated in Table 1, Both SiN and SiO have the capability to absorb incident IR radiation in the wavelength range from 8 to 14 ␮m. If using SiN or SiO to correspond to material 1 and Al or Au to correspond to material 2 in Fig. 1, using Eq. (9) and material parameters in Table 1, the graphs of the TMR of bi-material beams in length of 100 ␮m against the thickness ratio of a metal layer to an IR absorber layer whose thickness is 0.4 ␮m are shown in Fig. 2. The graphs demonstrate that the TMR has a maximum for each bi-material beam and the maximum TMR corresponds to a certain thickness ratio of a metal layer to an IR absorber layer, which means that the maximum TMR can be obtained by matching metal thickness after determining the thickness of IR absorber layer. Higher TMR is desired for bi-material beams of the pixel. The graphs also reveal that the TMR approaches zero, while the thickness ratio approaches zero, which means that the thinner the thickness of

Fig. 2. Graphs of the thermo-mechanical response of the beam against the thickness ratio.

the metal on the IR absorber layer, the lower the TMR is. Lower TMR is desired for bi-material reflectors of the pixel, ensuring the reflectors flatter when their temperature changes. For Al-SiO beam as shown in Fig. 2, its TMR has the largest value compared with that of others. However, while the thickness ratio approaches zero, its TMR is also much higher than that of others, which means that the reflector composed of Al and SiO films is benter, resulting from material property. Besides, after the Al-SiO material reflector is released, there is larger residual stress caused by fabricating process, resulting in the reflector bending [19]. The bent reflector caused by both material property and fabricating process gives rise to the dispersion of diffraction spectrum of the light reflected by the reflector into CCD camera and a loss of intensity in regular direction of propagation, resulting in a decrease in sensitivity of the pixel. For Au-SiO beam as shown in Fig. 2, its maximum TMR is lower than that of the Al-SiN beam, whereas, when thickness ratio is smaller, its TMR is higher than that of Al-SiN beam. Thus, Au-SiO bi-material is not suitable for fabricating the bi-material beam and reflector. In addition, the IR absorber material of the pixel is generally used as the FPA frame material. As the Young’s modulus of SiO is much smaller than that of SiN as shown in Table 1, SiO film is softer than SiN film for a given size. Thus, the SiO frame is not strong enough, bringing higher vibration noise to the FPA. For Au-SiN beam as shown in Fig. 2, while the thickness ratio approaches zero, its TMR value is smaller than others which is desirable for the bi-material reflector. But, its maximum TMR is also lower compared with others, which is undesirable for the bimaterial beam. For Al-SiN beam as exhibited in Fig. 2, even though its TMR is little higher than that of Au-SiN beam while the thickness ratio approaches zero, the maximum TMR of Al-SiN beam is much higher than that of Au-SiN beam. In addition, Al-SiN bi-material compared with Au-SiN bi-material has some advantages: (1) In visible wavelength range, the reflectivity of Al is 90.4%, but the reflectivity of Au is 47.7%, as tabulated in Table 1. If Al is selected as the surface material of the reflector, the light intensity reflected by the reflector increases compared with Au, which can compensate a loss of light intensity in regular direction of propagation caused by the dispersion of the light reflected by the bent reflector. Thus, the drop in sensitivity of the FPA caused by the dispersion can be reduced. In addi-

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Table 1 Material parameters [16–18]. material

SiN SiO Au Al

c(J/(kg·K))

IR absorptivity (8–14 ␮m) a (%)

Reflectance of IR (10 ␮m) R (%)

Reflectance of visible light R’ (%)

690 100 130 942

92 90 2 3

– – 98.4 98.1

– – 47.7 90.4

Young’s modulus E (109 Pa)

Expansion coefficient ˛ (10−6 /◦ C)

Specific heat

180 70 73 70

0.8 0.35 14.2 25.0

tion, if the light intensity reflected by Al is the same as Au, the light intensity incident to Al is almost half of that to Au, which reduces the noise brought by the incident light to the pixel. (2) As Fluoride gas mainly used for SiN etchant has very high etching selectivity to Al, Al can be directly used as mask material of SiN dry etching. If both Al and photoresist on SiN film together form the combined mask for SiN etching, the misalignment between two patterns can be eliminated, which can effectively improve the sensitivity of the UOR IR FPA. According to the above analysis, it is superior that SiN film is selected as IR absorber material of the pixel and the frame material of the FPA and Al is selected as the surface metal on it. 2.3. Thicknesses of IR absorber and metal films Using Eq. (9), material parameters from Table 1 and Matlab software, the dependence of the Al-SiN bi-material beam TMR on thicknesses of two materials is illustrated in Fig. 3. Each curve corresponds to a certain TMR, as rad per K. The curves demonstrate that the thinner the SiN and Al films, the higher the TMR of the bi-material beam is, which means that Al and SiN films should be as thin as possible in order to obtain higher TMR. When IR radiation propagates toward a bi-material beam and reaches its SiN film surface, if using I0 to denote the incident intensity and R to denote the reflectance that is the ratio of the IR intensity reflected by the SiN film surface to the incident intensity, in accordance with P. Bouguer’s law [20], the refracted intensity at a depth x below the SiN film surface is given by I = I0 (1 − R)exp(−˛x)

(10)

in which ␣ is the absorption coefficient and equal to ˛=

4 n

. 

(11)

Density  (10 kg/m )

Poisson’s ratios 

Thermal conductivity k (W/(m·K)) K)

ε (8–14 ␮m)

2.40 2.27 19.30 2.70

0.42 0.20 0.27 0.35

19 – – 236

0.8 – – 0.03

3

3

Emissivity

where n is the refractive index of SiN material, is the extinction coefficient,  is the vacuum wavelength. ˛−1 is the penetration depth corresponding to that the refracted intensity is reduced to e−1 times of the transmitted intensity (1-R) I0 . If the depth is expressed by dpenetration [21], thus, there is dpenetration =

 . 4 n

(12)

For IR radiation in the wavelength range from 8 to14 ␮m, the extinction coefficient and refractive index of SiN film with higher IR absorption coefficient, lower CTE and stress fabricated by using low pressure chemical vapour deposition(CVD) are approximately equal to 1 and 1.95, respectively. The penetration depth is calculated as 0.3–0.4 ␮m by using Eq. (12), which means that if the thickness of SiN film is set to be 0.4␮m, when the transmitted IR propagates in it, most of the IR energy can be absorbed, the rest reaching the SiN/Al surface. Since the reflectance of Al to IR is 98.1% (Table 1), most energy reaching the SiN/Al surface is reflected back [22,23] to SiN film and will be absorbed again, i.e., the SiN film thickness of 0.4 ␮m can ensure the pixel absorbing sufficient energy in the wavelength range from 8 to 14 ␮m. As analyzed above, to obtain higher TMR, Al and SiN films of bimaterial beam should be as thin as possible. And thinner SiN and Al films can meet the requirements of low thermal mass for fast response time. However, too thin SiN film is not beneficial to IR energy absorption, and causes an increase in mechanical noise of the pixel and a decrease in mechanical strength of the FPA. By comprehensively consideration, the SiN film thickness is determined as 0.4 ␮m. The analysis in Section 2.2 indicates the maximum TMR can be obtained by matching metal thickness for a given IR absorber thickness. Thus, under the condition of SiN film thickness of 0.4␮m, if the thickness of Al film is determined as 0.3 ␮m by using Eq. (9), the TMR of the Al-SiN bi-material beam approximately has a maximum. The analysis in Section 2.2 also indicates that the thinner the Al film on the absorber, the flatter the reflector is, when its temperature changes. However, too thin Al film is transparent to the incident sample visible light [24]. Thus, the metal thickness of the reflector should be larger than 40 nm, ensuring the intensity of the reflected visible light. In brief, SiN film thickness of the pixel is determined as 0.4 ␮m, Al film thicknesses of the bi-material beam and the bi-material reflector are determined as 0.3 ␮m and 40 nm. 2.4. Pixel size, TMR and thermal time constant 2.4.1. Pixel size In accordance with IR detector design principle, working principle of the UOR IR FPA and the results obtained by analyzing above, the optimized pixel structure and its dimensions are shown in Fig. 4(a). Violet and green areas in the figure correspond to Al-SiN bi-material and SiN material films, respectively.

Fig. 3. Dependence of the thermo-mechanical response  0 /T (rad/K) on material thicknesses.

2.4.2. TMR of the pixel According to the material parameters from Table 1 and the pixel dimensions shown in Fig. 4(a), the TMR simulation result of the Al-

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Fig. 4. (a) Al-SiN pixel dimensions; (b) TMR simulation of the pixel. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)

SiN bi-material pixel is shown in Fig. 4(b) by using ANSYS software. The red to blue areas in the pixel correspond to the rotation angles from large to small around the X-axis in the vertical direction. In the Al-SiN pixel, a SiN beam employed as a bridge to connect two Al-SiN bi-material beams not only plays a role in thermal isolation for good temperature rise, but also makes two bi-material beams rotating in same direction, as shown in Fig. 5(a). Thus, in Fig. 4(a), the effective length of the bi-material beam is equal to the sum of two bi-material beam lengths. Besides, the SiN single material beam removes the rotation axis X in the vertical direction from O to O’ point and the rotation angle  around the X-axis at O’ point is 2 times as large as the beam bending angle  0 around the X-axis at O point as shown Fig. 5(b).  = 2 0 .

(13)

Therefore, the TMR of the pixel is the ratio of the rotation angle  around the X-axis at the O’ point to the change in temperature T. According to Eqs. (9) and (13), the TMR of the pixel is expressed as  = T

6Leff (˛2 − ˛1 ) h E2 1 E1

1 + 4 h2

+6

h2 E2 h (1 + h2 ) h2 E1 1

h2 E 2 2 h2 E1 1

1

+4

h3 E 2 2 h3 E1 1

+

h4 E 2 2 h4 1

(14)

2 E2 1

Where Leff is the effective length of the bi-material beam in the pixel. For the pixel in Fig. 4(a), the effective length Leff is approximately 100 ␮m. Using Eq. (14) and the material parameters from Table 1, the maximum TMR of the pixel is calculated as 4.428 × 10−3 rad/K. By applying the model of multi-layer shell finite element of ANSYS software, the simulation value of the maximum TMR of the pixel is 4.431 × 10-3 rad/K. There is an excellent agree-

ment between the calculation and the simulation values, which proves that the mechanics model of the pixel TMR is effective. 2.4.3. Time constant of the pixel Based on heat transfer theory [25] and according to the pixel structure, the heat capacity of the pixel is given by C = cSiN SiN VSiN + cAl Al VAl , where c,  and V represent the specific heat, the density and the volume of materials, respectively. The heat capacity of 2.02 × 10−9 J/K is obtained by using material parameters from Table 1 and the dimensions provided in Fig. 4(a). The conductance related to heat conduction between the reflector and the frame includes bi-material beams and single material beams. According the pixel structure, this conductance is exposed as Gcond = 2 · [(kSiN ASiN /ıSiN ) · (kSiN ASiN /ıSiN + kAl AAl /ıAl )]/[kSiN ASiN /ıSiN + (kSiN ASiN /ıSiN + kAl AAl /ıAl )]where k, A and ı donate the thermal conductivity of materials, the crosssectional areas of material layers and the beam length, respectively. The conductance is calculated as 9.57 × 10−8 W/K by using material parameters and the dimensions of the pixel. The conductance related to thermal radiation of the pixel is given by Grad = 4 · (εSiN ASiN + εAl AAl )T 3 where ε and A are the emissivity of the pixel surface and the surface area of the pixel;  is the Stefan-Boltzmann constant ( = 5.76 × 10−8 W/(m2 ·K4 )); T is the absolute temperature of the pixel. This conductance at 300K is calculated as 1.35 × 10−8 W/K by using material parameters and the dimensions of the pixel. If the FPA is packaged in a vacuum chamber, heat convection can be ignored. Total thermal conductance is Gth = Gcond + Grad that is equal to1.092 × 10−7 W/K. The thermal time constant of the pixel is exposed as = C/Gth , which is equal to19 ms.

Fig. 5. Single material beam function: (a) Making two bi-material beams rotating in same direction; (b) removing the rotation axis X ( = 2 0 ).

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Fig. 6. Misalign between different patterns: (a) Reduced overlap area between metal and SiN patterns; (b) simulation result of the distorted reflector with the misalignment.

2.4.4. Noise equivalent power The noise equivalent power (NEP), the required input power to achieve a signal to noise ratio of 1, is an important property of an infrared detector.Within a bandwidth f of 1Hz, the NEP is expressed as NEP = 16kB TD5 AD /ε(W Hz−1/2 ) [26,27], where 

which is the Stefan-Boltzmann constant and kB = 1.38 × 10−23 J/K which is Boltzmann’s constant. Suppose that TD is the absolute temperature of 295K, AD is approximately 52 × 36 ␮m2 which is the sensitive area receiving the radiation and ε is the emissivity (absorption efficiency) of 0.8. Thus, the value of the NEP is calculated as 2.56 × 10−13 (W Hz-1/2 ). This value is small enough to meet the requirement for IR detection.

3. Self-align design of patterns and processes 3.1. Analysis of the misalignment in non-self-align processes Sensitivity and resolution are two important performance parameters for an IR FPA. The pixel with higher sensitivity is typically designed to have larger IR sensing area to collect IR energy. Higher resolution means more pixels. Thus, the size of the FPA with higher sensitivity and resolution is larger. If the UOR IR FPA has 320 × 240 to 480 × 320 pixels, one pixel area of 60 × 60 ␮m2 , the edge length of the FPA chip is as large as about 20–30 mm. For the chip in this size and double side patterning design, a contact mask aligner with a specification of misalignment about 0.5 to 1.0 ␮m is available. When the length and the width of the beam in the pixel are 50 ␮m and 2 ␮m or less, the misalignment caused by the contact mask aligner may be ignored in the length direction but not be negligible in the width direction. Thus, the misalignment should be given enough attention. In the non-self-align process of the FPA, the IR absorber film pattern is first fabricated; subsequently, the metal film pattern is fabricated on it. Thus, there is the misalignment between two patterns, which leads to an overlap area loss between metal and absorber patterns, as illustrated in Fig. 6(a), causing a decrease in the thermal stress between two layer films. As a result, the actual TMR of the pixel must be lower than the design value (misalign-

ment = 0). The misalignment also leads to the non-uniformity of the FPA sensitivity because the driving area loss for each pixel may be different. Besides, the misalignment causes that the mass center of the pixel is not at their geometric center, which distorts the reflector, as shown in Fig. 6(b). The distorted reflector leads to the change in direction of the light reflected by it into the CCD camera. In order to obtain the FPA with higher performance, the misalignment must be eliminated.

3.2. Self-align pattern and processes The self-align patterns combined with fabricating process, which can eliminate the misalignment, are designed as shown in Fig. 7. The patterns contain a reflector pattern (1# mask), a reflector and bi-material beam pattern (2# mask), a single material beam and frame pattern (3# mask), as well as a pattern of FPA released (4# mask). According to the dimensions of the pixel shown in Fig. 4(a), the self-align fabricating process flow corresponding to Fig. 7 is as follows: 0.4 ␮m low stress SiN film is firstly deposited on a silicon wafer by low pressure CVD. Next, Al film with 260 nm thickness is deposited on SiN by physical vapour deposition (PVD). This thickness is thinner than the designed Al film thickness (300 nm) of bi-material beams. Subsequently, the reflector area is patterned by mask 1# and dry etched for reducing the critical dimension loss of Al. Then, a layer of Al film with 40 nm thickness is deposited to form the metal film of the reflector and a part of metal of bi-material beams by PVD. Except for the reflector area, the thickness of Al film is 300 nm (260 nm plus 40 nm) which is equal to the designed Al film thickness of bi-material beams. The metal layer of reflector and bi-material beams is patterned by mask 2#, with subsequent, dry etching Al to form the metal layer of the pixel. SiN beams and pixel frame are patterned by mask 3#. Subsequently, by using a combined mask including both the photoresist on SiN beams and pixel frame, and the metal layer of reflector and bi-material beams, SiN is etched to form the pixel structure.

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Fig. 7. Self-align patterns, process flow, and cross-sectional views of the pixel and the FPA: (a) Al films deposited and the reflector patterned; (b) Al film patterned; (c) SiN beams and pixel frame patterned; (d) combined mask; (e) FPA released.

The FPA frame is patterned on the backside by mask 4# (Fig. 7(e)). Subsequently, Si is etched, FPA is released. 3.3. Analysis of the self-align The idea of self-align pattern design is as follows: The first self-align is to uniform metal thickness of the reflector. As each side length of the reflector pattern in Fig. 7(a) is 2.0 ␮m longer than the actual side length in Fig. 7(b). If there is the largest misalignment of 1 ␮m between two reflector patterns, the actual reflector pattern in Fig. 7(b) is always located within the pattern in Fig. 7(a), as shown in Fig. 7(d). The second is to realize a self-aligned bi-material structure, ensuring the TMR of the pixel. The metal pattern of the reflector and bi-material beams shown in Fig. 7(b) is combined with the photoresist pattern shown in Fig. 7(c) together to form the combined mask for SiN etching. Metal can directly used as the mask, on which there is no photoresist. After SiN etching, the metal is exactly left on the top of SiN as a functional layer. The third self-align is to ensure SiN beams in contact with AlSiN reflector and bi-material beams. An overlap of 2 ␮m between the reflector and the single material beam next to the reflector and a cross of the ends of a single material and a bi-material beam is designed, as shown in Fig. 7(d). If there is the largest misalignment of 1 ␮m, the overlap and cross can ensure the contact. As the self-align approach eliminates the misalignment, the problems resulting from the misalignment are effectively solved. 3.4. Fabricated pixel and array Based on the combined design, an Al-SiN bi-material UOR IR FPA with 320 × 240 pixels, one pixel size of 60 × 60 ␮m2 , is fabricated. A chip photograph and scanning electron microscope (SEM) photographs of a portion of the array and a part of the pixel are shown in Fig. 8. The photograph of the part of the pixel clearly shows that the

self-align structures, including the aligned two reflector patterns, the overlap between Al and SiN patterns of bi-material beams and the cross of the single material and bi-material beams, are perfectly realized. 4. Pixel TMR and FPA imaging measurements 4.1. Pixel TMR measurement The fabricated Al-SiN bi-material FPA is put on a temperature control hot plate. The displacement of the reflector is measured by a Veeco profiler. By calculating, the rotation angles of the reflector are obtained. A graph of the rotation angle versus temperature is shown in Fig. 9, the pixel TMR of 3.4 × 10−3 rad/◦ C. The measured value is a reduction of 15.8% in comparison with the design value of 4.43 × 10−3 rad/◦ C (Section 2.4.2), but much close to the design value in comparison with the FPA fabricated by non-self-align process because the misalignment is effectively eliminated. The drop is mainly caused by the heat convection between the PFA and the air, because the FPA is not packaged in a vacuum chamber when measuring the displacement of the reflector. In addition, the drop is also caused by the thickness deviation of SiN and Al films in depositing as well as the oxygen or nitrogen atoms existing in Al film by PVD, the latter leading to Al film harder and its inherent stress larger. 4.2. FPA imaging measurement A simple optical readout experimental setup is illustrated in Fig. 10. The Al-SiN bi-material FPA is put in a vacuum chamber at pressure of 4.5 × 10−1 Pa and room temperature of 23 ◦ C. The incident IR flux from targets is focused to the back side of the FPA passing through an IR lens and the germanium window of the vacuum chamber. A beam of visible light coming from a light-emitting diode (LED, 550 nm) passing through a lens1 and a glass window of the vacuum chamber reaches the metal surface of the pixels.

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Fig. 8. FPA fabricated by using self-align approach (SEM, JSM7500F).

4.3. NETD estimation

Fig. 9. Graph of the rotation angle of the reflector versus temperature.

When the bi-material beams bending due to their temperature change attributed to the incident IR flux, the reflectors is driven by them to rotate, causing the change in direction of the visible light reflected by the reflectors to an 8-bit CCD camera. The real-time image is obtained by capturing the change in the light intensity. In IR imaging measurement, a uniform background image without IR stimulus is first captured, then, the background image is subtracted from the obtained each frame with IR stimulus during the real-time image acquisition; the result of this subtraction is displayed at a rate of 25 frames per second. The images captured from the IR video clips without the use of any advanced data processing are displayed in Fig.10. The one on computer screen is a person who is 5 m away from the FPA. Others are 1 m away from the FPA. The frame with a hand, a bottle with water at temperature of 45 ◦ C and a lace on the top of the bottle shows that the FPA has high enough temperature resolution. In addition, a group of images from a 25 fps IR video clip is shown in the figure. In each image, the hand located on the left represents the silhouette of cartoon rabbit, the thumb, middle finger and ring finger forming a circle which stands for its head, an upright forefinger and little finger standing for its long ears. The hands located on the right plays a counting game that the number from one to five in one second interval showed respectively. The images show that the FPA has shorter thermal response time, i.e., thermal time constant is smaller. In the FPA imaging, the FPA is not packaged in an independent vacuum chamber, but put in the vacuum chamber connected with a vacuum pump by a tube. Thus, there is substantial noise.

The noise equivalent temperature difference (NETD) is an important property of an IR imaging system, which is used to describe the thermal sensitivity of it. The NETD is the smallest detectable temperature difference of the target source allowed by the system noise and defined as the ratio of the noise standard deviation ␴ to thermal response sensitivity RT , i.e., NETD = /RT [28]. For an imaging system based on an UOR IR FPA, the NETD of the system can be expressed as NETD = Inoise /(I/Ts ), where Inoise is the standard deviation of the system noise including the thermomechanical fluctuation noise of the pixel structure, the temperature fluctuation noise of the FPA, the background fluctuation noise, as well as the used visible light source noise and CCD shot noise; I/Ts is the thermal response sensitivity of the system, which is defined as the change in the gray level of the image of the IR target captured by using the CCD per Kelvin blackbody calibration source temperature change. To evaluate the system noise, with the absence of IR target, 50 frame thermal images of the background are serially captured by using the CCD, then, the root mean square of the fluctuation (the standard deviation) of the pixel gray obtained by using the CCD is calculated. The average standard deviation is about 1 Gy level determined as the system noise Inoise . In addition, the ratio of the difference between average grey levels of two points in the obtained IR image to the temperature difference between the IR sources of these two points is used to estimate the thermal response sensitivity of the system I/Ts , the value of which is 17.4 Gy/K. As a result, the NETD of the system is calculated as 57 mK at room temperature of 23 ◦ C. 5. Conclusion A combined design for the UOR IR FPA with 320 × 240 pixels, whose size is 60 × 60 ␮m2 , based on microelectronic processes, is presented. Applying the established bi-material beam model on mechanics of materials, by analyzing material IR absorption and visible light reflectivity, etching selectivity and process compatibility in fabricating, Al and SiN are selected as pixel materials. In order to obtain higher TMR of bi-material beams and ensure bimaterial reflector flatter, according to the analysis of the model on mechanics of materials and the equations derived from optoelectronics principle, SiN film thickness is optimized as 0.4 ␮m; Al film thicknesses of the bi-material beam and reflector are optimized as 0.3 ␮m and 40 nm. In order to ensure that the measured value

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Fig. 10. Schematic illustration of a simple optical readout experimental setup and IR images.

of the pixel TMR is closer to its design value, taking consideration of etching selectivity and material compatibility in fabricating, the self align approach to eliminate the misalignment between patterns is proposed. The design value of thermal time constant is 19 ms. By means of the combined design, the bi-material UOR IR FPA with higher thermal sensitivity and temperature resolution as well as faster response time is successfully fabricated. The measured value of the thermo-mechanical response of the pixel achieves 3.4 × 10−3 rad/◦ C. The real-time IR image video clips are captured by the an 8-bit CCD camera at room temperature. Of course, there are still some works to be done. As a certain amount of alignment distance is not required when designing the patterns, it is possible to improve the resolution of the FPA by means of increasing quantity of pixels for a given FPA size. In addition, the dimension of the pixel structure will be optimized to improve the dynamic range of the FPA. Acknowledgments The authors would like to thank National Key Laboratory of Science and Technology on Micro/Nano-Fabrication in Peking University for their help on the fabrication. They would also like to thank T. Wang in Beijing University of Technology for his help on the pixel TMR measurement and L.Q. Dong in Beijing Institute of Technology for his help on the IR imaging measurement. References [1] R.G. Buser, M.F. Tompsett, Uncooled infrared imaging arrays and systems: historical overview, Semicond. Semimet. 47 (1997) 1–14. [2] S.R. Manalis, S.C. Minne, C.F. Quate, G.G. Yaralioglu, A. Atalar, Two-dimensional micromechanical bimorph arrays for detection of thermal radiation, J. Appl. Phys. Lett. 70 (1997) 3311–3313. [3] T. Perazzo, M. Mao, O. Kwon, A. Majumdar, J.B. Varesi, P. Norton, Infrared vision using uncooled micro-optomechanical camera, J. Appl. Phys. Lett. 74 (1999) 3567–3569.

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Biographies Xia Zhang received her BS degree in semiconductor physics from Beijing University of Technology. She received her PhD degree in microelectronics and solid state electronics from Institute of Microelectronics, Chinese Academy of Sciences. She is currently a professor in the Communication University of China. Her main research interests include infrared thermal sensors and arrays. Dacheng Zhang received his MS and PhD degrees in microelectronics and solid state electronics from Peking University, Beijing, China. Currently, he is a professor in the Institute of Micro-Nano electronics, Peking University. His main research interests include micro-nano fabrication technology, mechanics property analysis of microstructure and MEMS devices.