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New application of polyimide in uncooled a-Si TFT infrared sensors
ARTICLE IN PRESS
Microelectronics Journal 38 (2007) 278–281 www.elsevier.com/locate/mejo
New application of polyimide in uncooled a-Si TFT infrared sensors Xing-Ming Liu, Lin Han, Li-Tian Liu Institute of Microelectronics, Tsinghua University, Beijing 100084, China Received 23 November 2006; accepted 26 November 2006
Abstract New application of polyimide (PI) is introduced in this paper. PI film of 27 mm is achieved, and the excellent thermal-isolation performance of the film is simulated by ANSYS. The surface of film is flat, which is suitable for the fabrication of other materials such as aluminum and silicon nitride. The PI film is used as thermal-isolation layer of uncooled a-Si thin film transistor infrared sensors in this research, and the fabrication process is greatly simplified. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polyimide; Thermal-isolation layer; Uncooled infrared sensors
1. Introduction Recently, uncooled infrared (IR) images have shown considerable progress in performance, size, and cost, both for military and commercial applications, because they do not require cryogenic equipments, which significantly reduce their price and increase their portability. For uncooled IR sensors, thermal-isolation structure, sensitive device and high IR absorption coefficient are the most important parts. In order to attain high-performance uncooled IR sensors, substantive research on these three parts have to be done. In order to fabricate the thermal-isolation structure, traditional surface micromachining techniques are often used, where several materials such as polyimide (PI) [1], polysilicon [2], PSG [3], and TEOS [4] act as sacrifice layers. But the thermal isolation is limited by the thickness of the sacrifice layer. Another optional approach is bulk micromachining techniques using anisotropic silicon etching (ASE) [5] or DRIE [6]. However, it has drawbacks: double-side lithography is required, the membranes may break, and space on the chip is lost due to the slope of the /1 1 1S planes used for etch stopping in ASE. Recently, porous silicon is used as sacrificial layer, which can provide Corresponding author. Tel.: +86 10 62789151 x 320;
fax: +86 10 62771130. E-mail address: [email protected] (X.-M. Liu). 0026-2692/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2006.11.019
a considerable deep air gap about 7.5 mm [7]. However, this method is complicated because of the slow growth of porous silicon on high-resist silicon substrate and special protective films are required (shown as Fig. 1) to prevent the substrate from erosion of HF. In this paper, PI is gelatinized on silicon substrate and film thickness of 27 mm can be easily attained, then thin film transistor (TFT) is fabricated on PI. The thermal conductance of PI is 0.4 W m 1 K 1. PI film has an excellent adherence to the silicon substrate, and its stress is very low. The decomposed temperature of PI is 475 1C, which is enough for the fabrication of uncooled a-Si TFT IR sensors as the fabrication temperature is no higher than 400 1C in the total fabrication process. By using PI as thermal-isolation layer, the fabrication process of uncooled IR sensors is greatly simplified. After the fabrication of TFT, SiON passivation layer and IR absorption layer are deposited on it, which greatly increase IR absorption of uncooled IR sensor.
SiO2/SiN x/SiO 2/SiNx protective layer
Air gap
Fig. 1. Schematic structure of air thermal-isolation layer.
ARTICLE IN PRESS X.-M. Liu et al. / Microelectronics Journal 38 (2007) 278–281
279
2. Thermal-isolation characteristic simulation of PI film
3. Fabrication of PI thermal-isolation membrane
It is obvious that substituting cavity thermal isolation structure with thermal-isolation film can greatly simplify the process of fabrication. However, it is hard to attain good thermal isolation by using usual IC fabrication materials. Table 1 shows properties of these materials, which are needed in the finite element analysis (FEA) in this paper. Silicon oxide and silicon nitride both have high thermal conductivity, l that is much higher than that of air. Furthermore, their thickness is limited. The oxidation speed of silicon is slow, which is about 20 nm/min under the thickness of 300 nm, and oxidation speed becomes slower with the increase of the thickness of SiO2, so it is difficult to attain SiO2 film of several microns. As to silicon nitride, because the stress of film is great, SiNx is easy to break when the thickness exceeds 500 nm. Compared with silicon oxide and silicon nitride, the thermal conductivity of PI is much lower. PI is able to endure high temperature of 400 1C, which is enough for the fabrication of uncooled TFT IR sensors. And it has excellent chemical stability under 400 1C. It is very easy to attain excellent PI membrane of about 30 mm through several times of gelatinizing. To compare thermal isolation of this several materials, their stable thermal properties are simulated by FEA software ANSYS8.0, and results are shown in Fig. 2 and Table 2. The thickness of air cavity is assumed to 7.5 mm, and the thickness of silicon oxide, silicon nitride, and PI is set as 30 mm. Their material properties are fixed as shown in Table 1. When the same heat flux of 0.05 W m 2 is loaded on these thermal-isolation layers, the surface temperature of PI increases by 3.75 K which is about 30 times of that of SiO2 and 80 times of that of SiNx with the same thickness. It is obviously that the thermal isolation of PI is much better than silicon oxide and silicon nitride. Though the thermal isolation of PI membrane is not as good as that of the air, and the surface temperature increase of it is about 1/4 of that of air cavity, it can be compensated by increasing the aspect ratio of TFT to maintain or even improve the whole performance of uncooled a-Si TFT IR sensors, as the detectivity of uncooled a-Si TFT sensors increases with the aspect ratio of TFT. In addition, the convection is another important thermal transfer mode of air cavity structure, and the thermal-isolation performance of the structure becomes worse if thermal convection is considered at the same time.
In this paper, PI membrane is made from ZKPI-type PI which is viscid prepolymer liquor. The glutinosity of PI is 40,000–50,000 centipoises at room temperature. Its reaction temperature of heat treatment dehydration is 300 1C, whose chemical principle is shown as
Fig. 2. Temperature distribution of PI thermal-isolation structure at the heat flux of 0.05 W/m2.
Table 1 Properties of materials Material
Thermal conductivity, l (W m
Air [8] Silicon oxide [8] Silicon nitride [9] Polyimide
0.025 12 32 0.4
1
K 1)
Heat capacity, Cp (J kg 773 800 691 1130
1
K 1)
Density, r (kg m 3) 1.29 2360 2400 1460
ARTICLE IN PRESS X.-M. Liu et al. / Microelectronics Journal 38 (2007) 278–281
280
Table 2 Surface temperature increase with different materials used as thermalisolation layer Polyimide
SiO2
SiNx
Surface temperature increase, DT (K)
15
3.75
0.125
0.047
PI of ZKPI type has good conglutination with Al, Au, Si, SiNx, and SiO2. It is easy to attain PI membrane of 30 mm through gelatinizing. The fabrication of PI film can be presented by three steps: gelatinizing, solidification, and cross-linking.
30min
Temperature
Air
300°C 250°C 40 min
120°C 1hour
160°C 1hour
200°C 1hour
70°C
Thermal isolation layer
350°C 30min
Time Fig. 3. Process of cross-linking process of PI.
3.1. Gelatinizing Before gelatinizing, PI is taken out of the cold storage of 5–10 1C and is placed at room temperature for about an hour to thaw. Silicon wafers are put in the oven of 110 1C or bombarded by O2 plasma so that the surface of silicon wafer is activated, which can enhance the conglutination between PI and silicon wafer. Gelatinizing of PI is processed on spin-coater. Rotary speed of spin-coater is 1400–4000 rpm. The thickness of PI film is mainly determined by glutinosity of prepolymer and the rotary speed of gelatinizing. The thickness of PI film becomes thinner with the increase of rotary speed and the decrease of glutinosity of prepolymer. The rotary speed used in this paper is 3000 rpm. The uniformity of PI film is greatly influenced by the quality of the surface of silicon wafer and the rotary speed of gelatinizing. If the surface is not sufficiently activated, PI perhaps cannot adhere to it. And pinholes appear if the surface of wafer is contaminated. Too slow rotate speed can result in strips of PI membrane. After gelatinizing, wafers must be placed horizontally in order to keep the flatness of PI film surface.
Fig. 4. Cross-sectional SEM images of polyimide film.
3.2. Solidification and cross-linking Gelatinized PI prepolymer is not suitable for preservation and using, and it has to be solidified. Gelatinized wafers are put in the oven of 80 1C for 30 min until solvent in PI is totally vaporized. Then solid PI prepolymer is attained. Such solid PI prepolymer is not stable and cannot endure high-temperature fabrication process, which exceeds 100 1C. So cross-linking must be progressed in time to achieve stable PI film. The detailed process of crosslinking is presented in Fig. 3. Scanning electronics microscope (SEM) image is pictured to examine the thickness of as-prepared PI film. The result of four times’ gelatinizing is shown in Fig. 4. Thickness of 27 mm has been achieved. Atom force microscope (AFM) pictures of silicon nitride film grown on PI and substrate are shown in Fig. 5 (SiNx is deposited by plasma-enhanced chemical vapor deposition).
Fig. 5. AFM picture of SiNx grown on (a) PI and (b) silicon substrate.
ARTICLE IN PRESS X.-M. Liu et al. / Microelectronics Journal 38 (2007) 278–281
SiNx grown on PI has the same quality as that grown on silicon substrate. These AFM pictures show that the surface of films is flat, and that PI film made in this method is suitable for the deposition of SiNx which is required in uncooled a-Si TFT IR sensors. 4. Conclusion PI of high glutinosity is compatible with the fabrication of a-Si process and is promising thermal-isolation material. According to the simulation result, thick PI membrane has excellent thermal isolation and flat surface, which is suitable for the fabrication of a-Si TFT. Applications of PI greatly simplify the fabrication process of uncooled a-Si TFT IR sensors, and good performance is expected. References [1] P. Eriksson, J.Y. Andersson, G. Stemme, Thermal characterization of surface micromachined silicon nitride membranes for thermal infrared detectors, J. Microelectro-mech. Syst. 6 (4) (1997) 55–61.
281
[2] M.V.S. Ramakrishna, G. Karunasiri, P. Neuzil, U. Sridhar, W.J. Zeng, Highly sensitive infrared temperature sensors using selfheating compensated microbolometers, Sensors Actuators A 79 (2000) 122–127. [3] L. Pham, et al., Surface-micromachined pyroelectric infrared imaging array with vertically integrated signal processing circuitry [J], IEEE Trans. Ultrason. Ferroelectr. Frequency 41 (4) (1994) 552–555. [4] S. Sedky, P. Fiorini, M. Caymax, et al., IR bolometers made of polycrystalline silicon germanium, Sensors Actuators A 66 (1998) 193–199. [5] A.D. Oliver, K.D. Wise, A 1024-element bulk-micromachined thermopile infrared imaging array, Sensors Actuators A 73 (1999) 222–231. [6] S.-J. Liu, X.-B. Zeng, J.-H. Chu, Thermal-sensitive BST thin film capacitors for dielectric bolometer prepared by RF magnetron sputtering, Microelectron. J. 35 (2004) 601–603. [7] L. Dong, R.F. Yue, L.T. Liu, 8 8 monolithic uncooled infrared imaging arrays using micromachined amorphous silicon thin film transistors, in: Proceedings of the 14th Micromechanics Europe Workshop (MME’03), 2003, 275pp. [8] E.C. Guyer, D.L. Brownell, Handbook of Applied Thermal Design, McGraw Hill, New York, 1989. [9] J. Lai, T. Perazzo, Z. Shi, et al., Optimization and performance of high-resolution micro-optomechanical thermal sensors 58 (1997) 113.
Microelectronics Journal 38 (2007) 278–281 www.elsevier.com/locate/mejo
New application of polyimide in uncooled a-Si TFT infrared sensors Xing-Ming Liu, Lin Han, Li-Tian Liu Institute of Microelectronics, Tsinghua University, Beijing 100084, China Received 23 November 2006; accepted 26 November 2006
Abstract New application of polyimide (PI) is introduced in this paper. PI film of 27 mm is achieved, and the excellent thermal-isolation performance of the film is simulated by ANSYS. The surface of film is flat, which is suitable for the fabrication of other materials such as aluminum and silicon nitride. The PI film is used as thermal-isolation layer of uncooled a-Si thin film transistor infrared sensors in this research, and the fabrication process is greatly simplified. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polyimide; Thermal-isolation layer; Uncooled infrared sensors
1. Introduction Recently, uncooled infrared (IR) images have shown considerable progress in performance, size, and cost, both for military and commercial applications, because they do not require cryogenic equipments, which significantly reduce their price and increase their portability. For uncooled IR sensors, thermal-isolation structure, sensitive device and high IR absorption coefficient are the most important parts. In order to attain high-performance uncooled IR sensors, substantive research on these three parts have to be done. In order to fabricate the thermal-isolation structure, traditional surface micromachining techniques are often used, where several materials such as polyimide (PI) [1], polysilicon [2], PSG [3], and TEOS [4] act as sacrifice layers. But the thermal isolation is limited by the thickness of the sacrifice layer. Another optional approach is bulk micromachining techniques using anisotropic silicon etching (ASE) [5] or DRIE [6]. However, it has drawbacks: double-side lithography is required, the membranes may break, and space on the chip is lost due to the slope of the /1 1 1S planes used for etch stopping in ASE. Recently, porous silicon is used as sacrificial layer, which can provide Corresponding author. Tel.: +86 10 62789151 x 320;
fax: +86 10 62771130. E-mail address: [email protected] (X.-M. Liu). 0026-2692/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2006.11.019
a considerable deep air gap about 7.5 mm [7]. However, this method is complicated because of the slow growth of porous silicon on high-resist silicon substrate and special protective films are required (shown as Fig. 1) to prevent the substrate from erosion of HF. In this paper, PI is gelatinized on silicon substrate and film thickness of 27 mm can be easily attained, then thin film transistor (TFT) is fabricated on PI. The thermal conductance of PI is 0.4 W m 1 K 1. PI film has an excellent adherence to the silicon substrate, and its stress is very low. The decomposed temperature of PI is 475 1C, which is enough for the fabrication of uncooled a-Si TFT IR sensors as the fabrication temperature is no higher than 400 1C in the total fabrication process. By using PI as thermal-isolation layer, the fabrication process of uncooled IR sensors is greatly simplified. After the fabrication of TFT, SiON passivation layer and IR absorption layer are deposited on it, which greatly increase IR absorption of uncooled IR sensor.
SiO2/SiN x/SiO 2/SiNx protective layer
Air gap
Fig. 1. Schematic structure of air thermal-isolation layer.
ARTICLE IN PRESS X.-M. Liu et al. / Microelectronics Journal 38 (2007) 278–281
279
2. Thermal-isolation characteristic simulation of PI film
3. Fabrication of PI thermal-isolation membrane
It is obvious that substituting cavity thermal isolation structure with thermal-isolation film can greatly simplify the process of fabrication. However, it is hard to attain good thermal isolation by using usual IC fabrication materials. Table 1 shows properties of these materials, which are needed in the finite element analysis (FEA) in this paper. Silicon oxide and silicon nitride both have high thermal conductivity, l that is much higher than that of air. Furthermore, their thickness is limited. The oxidation speed of silicon is slow, which is about 20 nm/min under the thickness of 300 nm, and oxidation speed becomes slower with the increase of the thickness of SiO2, so it is difficult to attain SiO2 film of several microns. As to silicon nitride, because the stress of film is great, SiNx is easy to break when the thickness exceeds 500 nm. Compared with silicon oxide and silicon nitride, the thermal conductivity of PI is much lower. PI is able to endure high temperature of 400 1C, which is enough for the fabrication of uncooled TFT IR sensors. And it has excellent chemical stability under 400 1C. It is very easy to attain excellent PI membrane of about 30 mm through several times of gelatinizing. To compare thermal isolation of this several materials, their stable thermal properties are simulated by FEA software ANSYS8.0, and results are shown in Fig. 2 and Table 2. The thickness of air cavity is assumed to 7.5 mm, and the thickness of silicon oxide, silicon nitride, and PI is set as 30 mm. Their material properties are fixed as shown in Table 1. When the same heat flux of 0.05 W m 2 is loaded on these thermal-isolation layers, the surface temperature of PI increases by 3.75 K which is about 30 times of that of SiO2 and 80 times of that of SiNx with the same thickness. It is obviously that the thermal isolation of PI is much better than silicon oxide and silicon nitride. Though the thermal isolation of PI membrane is not as good as that of the air, and the surface temperature increase of it is about 1/4 of that of air cavity, it can be compensated by increasing the aspect ratio of TFT to maintain or even improve the whole performance of uncooled a-Si TFT IR sensors, as the detectivity of uncooled a-Si TFT sensors increases with the aspect ratio of TFT. In addition, the convection is another important thermal transfer mode of air cavity structure, and the thermal-isolation performance of the structure becomes worse if thermal convection is considered at the same time.
In this paper, PI membrane is made from ZKPI-type PI which is viscid prepolymer liquor. The glutinosity of PI is 40,000–50,000 centipoises at room temperature. Its reaction temperature of heat treatment dehydration is 300 1C, whose chemical principle is shown as
Fig. 2. Temperature distribution of PI thermal-isolation structure at the heat flux of 0.05 W/m2.
Table 1 Properties of materials Material
Thermal conductivity, l (W m
Air [8] Silicon oxide [8] Silicon nitride [9] Polyimide
0.025 12 32 0.4
1
K 1)
Heat capacity, Cp (J kg 773 800 691 1130
1
K 1)
Density, r (kg m 3) 1.29 2360 2400 1460
ARTICLE IN PRESS X.-M. Liu et al. / Microelectronics Journal 38 (2007) 278–281
280
Table 2 Surface temperature increase with different materials used as thermalisolation layer Polyimide
SiO2
SiNx
Surface temperature increase, DT (K)
15
3.75
0.125
0.047
PI of ZKPI type has good conglutination with Al, Au, Si, SiNx, and SiO2. It is easy to attain PI membrane of 30 mm through gelatinizing. The fabrication of PI film can be presented by three steps: gelatinizing, solidification, and cross-linking.
30min
Temperature
Air
300°C 250°C 40 min
120°C 1hour
160°C 1hour
200°C 1hour
70°C
Thermal isolation layer
350°C 30min
Time Fig. 3. Process of cross-linking process of PI.
3.1. Gelatinizing Before gelatinizing, PI is taken out of the cold storage of 5–10 1C and is placed at room temperature for about an hour to thaw. Silicon wafers are put in the oven of 110 1C or bombarded by O2 plasma so that the surface of silicon wafer is activated, which can enhance the conglutination between PI and silicon wafer. Gelatinizing of PI is processed on spin-coater. Rotary speed of spin-coater is 1400–4000 rpm. The thickness of PI film is mainly determined by glutinosity of prepolymer and the rotary speed of gelatinizing. The thickness of PI film becomes thinner with the increase of rotary speed and the decrease of glutinosity of prepolymer. The rotary speed used in this paper is 3000 rpm. The uniformity of PI film is greatly influenced by the quality of the surface of silicon wafer and the rotary speed of gelatinizing. If the surface is not sufficiently activated, PI perhaps cannot adhere to it. And pinholes appear if the surface of wafer is contaminated. Too slow rotate speed can result in strips of PI membrane. After gelatinizing, wafers must be placed horizontally in order to keep the flatness of PI film surface.
Fig. 4. Cross-sectional SEM images of polyimide film.
3.2. Solidification and cross-linking Gelatinized PI prepolymer is not suitable for preservation and using, and it has to be solidified. Gelatinized wafers are put in the oven of 80 1C for 30 min until solvent in PI is totally vaporized. Then solid PI prepolymer is attained. Such solid PI prepolymer is not stable and cannot endure high-temperature fabrication process, which exceeds 100 1C. So cross-linking must be progressed in time to achieve stable PI film. The detailed process of crosslinking is presented in Fig. 3. Scanning electronics microscope (SEM) image is pictured to examine the thickness of as-prepared PI film. The result of four times’ gelatinizing is shown in Fig. 4. Thickness of 27 mm has been achieved. Atom force microscope (AFM) pictures of silicon nitride film grown on PI and substrate are shown in Fig. 5 (SiNx is deposited by plasma-enhanced chemical vapor deposition).
Fig. 5. AFM picture of SiNx grown on (a) PI and (b) silicon substrate.
ARTICLE IN PRESS X.-M. Liu et al. / Microelectronics Journal 38 (2007) 278–281
SiNx grown on PI has the same quality as that grown on silicon substrate. These AFM pictures show that the surface of films is flat, and that PI film made in this method is suitable for the deposition of SiNx which is required in uncooled a-Si TFT IR sensors. 4. Conclusion PI of high glutinosity is compatible with the fabrication of a-Si process and is promising thermal-isolation material. According to the simulation result, thick PI membrane has excellent thermal isolation and flat surface, which is suitable for the fabrication of a-Si TFT. Applications of PI greatly simplify the fabrication process of uncooled a-Si TFT IR sensors, and good performance is expected. References [1] P. Eriksson, J.Y. Andersson, G. Stemme, Thermal characterization of surface micromachined silicon nitride membranes for thermal infrared detectors, J. Microelectro-mech. Syst. 6 (4) (1997) 55–61.
281
[2] M.V.S. Ramakrishna, G. Karunasiri, P. Neuzil, U. Sridhar, W.J. Zeng, Highly sensitive infrared temperature sensors using selfheating compensated microbolometers, Sensors Actuators A 79 (2000) 122–127. [3] L. Pham, et al., Surface-micromachined pyroelectric infrared imaging array with vertically integrated signal processing circuitry [J], IEEE Trans. Ultrason. Ferroelectr. Frequency 41 (4) (1994) 552–555. [4] S. Sedky, P. Fiorini, M. Caymax, et al., IR bolometers made of polycrystalline silicon germanium, Sensors Actuators A 66 (1998) 193–199. [5] A.D. Oliver, K.D. Wise, A 1024-element bulk-micromachined thermopile infrared imaging array, Sensors Actuators A 73 (1999) 222–231. [6] S.-J. Liu, X.-B. Zeng, J.-H. Chu, Thermal-sensitive BST thin film capacitors for dielectric bolometer prepared by RF magnetron sputtering, Microelectron. J. 35 (2004) 601–603. [7] L. Dong, R.F. Yue, L.T. Liu, 8 8 monolithic uncooled infrared imaging arrays using micromachined amorphous silicon thin film transistors, in: Proceedings of the 14th Micromechanics Europe Workshop (MME’03), 2003, 275pp. [8] E.C. Guyer, D.L. Brownell, Handbook of Applied Thermal Design, McGraw Hill, New York, 1989. [9] J. Lai, T. Perazzo, Z. Shi, et al., Optimization and performance of high-resolution micro-optomechanical thermal sensors 58 (1997) 113.