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An infrared pyroelectric detector improved by cool isostatic pressing with cup-shaped PZT thick film on silicon substrate
Infrared Physics & Technology 61 (2013) 313–318
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Regular article
An infrared pyroelectric detector improved by cool isostatic pressing with cup-shaped PZT thick film on silicon substrate Q.X. Peng, C.G. Wu, W.B. Luo ⇑, C. Chen, G.Q. Cai, X.Y. Sun, D.P. Qian State Key Lab of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China
h i g h l i g h t s The surface of the thick film was at the same height with the substrate. Metal thin film was used to connect the top electrode and the bonding pad. The pyroelectric properties was improved by cool isostatic pressing.
a r t i c l e
i n f o
Article history: Received 6 May 2013 Available online 29 September 2013 Keywords: PZT Infrared Pyroelectric Detectivity
a b s t r a c t In this paper, we presented a new pyroelectric detector with back to back silicon cups and micro-bridge structure. The PZT thick film shaped in the front cup was directly deposited with designed pattern by electrophoresis deposition (EPD). Pt/Ti Metal film, which was fabricated by standard photolithography and lift-off technology, was sputtered to connect the top electrode and the bonding pad. The cold isostatic press (CIP) treatment could be applied to improve the pyroelectric properties of PZT thick film. The infrared (IR) properties the CIP-optimized detector were measured. The voltage responsivity (RV) was 4.5 102 V/W at 5.3 Hz, the specific detectivity (D*) was greater than 6.34 108 cm Hz1/2 W1 (frequency > 110 Hz), and the thermal time constant was 51 ms, respectively. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Pyroelectric materials can generate electrically measurable signal, which results from the spontaneous polarization change related to temperature variation [1]. Pb(ZrxTi1x)O3 (PZT) is a typical pyroelectric material, which attracted great attention in the IR radiation detection application in the wavelength ranges of 3–5 lm and 8–14 lm [2,3]. Accompanied by the development of MEMS technology, pyroelectric device based on PZT thick film with silicon substrate became a hot topic in the past decades [4,5]. Various pyroelectric detectors with small size, light weight, robust structure could be realized by this method [6,7]. The conventional structure of a thick film detector was shown in Fig. 1a schematically. There height difference between the top electrode and the bonding pad was about 10 lm. A metal wire was generally bonded on the surface of top electrode and the bonding pad to conduct electric signal. This wire bonding connection would bring about two shortcomings. On one side, the wire would increase the heat capacity leads to heat loss because of its good heat conduction ability. On the other side, it was not benefit for ⇑ Corresponding author. Tel.: +86 83202140. E-mail addresses: [email protected] (Q.X. Peng), [email protected] (C.G. Wu), [email protected] (W.B. Luo). 1350-4495/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2013.09.002
mass-produce for fussy repeated bonding action. By considering of the above aspects, connections using hundreds thick metal film is a better candidate than the wire bonding. Unfortunately, when the pattern of the top electrode and the connection was transferred by photolithography, the height difference would distort, or even fracture the pattern at the edge of the thick film. Obviously, reducing the height difference value of the top electrode and the bonding pad was the simplest and most feasible way to solve this technical problem. To obtain a high performance pyroelectric sensor, methods such as sintering aid [8], doping [9], changing material component [10], were used to enhance the pyroelectric properties. These methods improved pyroelectric properties from the point of phase or micro-structure variation by another chemical addition. Moreover, CIP was used as a physical method to optimize material properties. Hindrichsen et al. [11] found that the piezoelectric properties of the PZT thick film was enhanced when the thick film was pressed by CIP. Zhang et al. [12] also improved the micro-structure and dielectric properties of Ba0.6Sr0.4TO3 film by CIP treatment. However, it was found that the thick films were liable to peel-off from the substrate because the shape of the thick films should be changed during CIP treatment. To solve these two problems, a new sensor structure shown in Fig. 1b was designed. In this structure, the thick film was shaped
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PZT thick film on silicon substrate was described in detail. The CIP treatment was used to improve the pyroelectric properties of thick film. The IR properties of the CIP-optimized sensor were measured to show the CIP treatment effects on the performance of the detector. 2. Experiment details 2.1. MEMS fabrication processes
Fig. 1. Two structure designed for thick film sensor. (a) Conventional structure, (b) thick film shaped in silicon cup.
in silicon cup to avoid the thick film peeling off the substrate. Moreover, its surface and the substrate were in the same plane, which could flattened the photoresist and make the Cr mask contacted closely with both the thick film and the substrate in light exposure process. Thus, the photolithography could be conducted almost the same as on a plane substrate. In this paper, the sensor fabrication process of the infrared detector improved by cool isostatic pressing with cup-shaped
The fabrication process of the pyroelectric sensor was shown step by step in Fig. 2. The starting material was cleaned double polished (1 0 0) silicon-on-insulator (SOI) substrate with 30 lm silicon on 1 lm insulator SiO2 layer and 265 lm carrier substrate (Fig. 2a). Firstly, the 3 3 mm front silicon cup (the depth decided by the depth of the buried SiO2 layer) was developed by lithograph technology and TMAH wet etching (Fig. 2b). The heat isolation channel was also formed at this step. Then, 500 nm SiO2 was grown by dry-oxygen oxidation which would not only sever as mask layer but also passive layer for dry etch-stop technology (Fig. 2c). 300 nm TiO2 diffusion barrier layer was oxidized at 700 °C for 1 h from melt Ti layer which was sputtered by direct current (DC) magnetron sputtering (Fig. 2d). After that, the substrate was carefully cleaned by ultrasonic washing for 30 min with acetone and deionized water. Finally, 200 nm patterned Pt bottom electrode was deposited by DC magnetron sputtering method (Fig. 2d).
Fig. 2. A cross sectional schematic of the MEMS route for the PZT thick film pyroelectric sensor (a) SOI substrate. (b) Si cup and isolation channel etching. (c) Thermal oxidation of SiO2. (d) TiOx diffusion barrier layer and bottom electrode. (e) Thick film deposition and sintering. (f) Top electrode. (g) Absorbing layer evaporation and (h) micro-bridge release.
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2.2. Structure and pyroelectric measurements The phase identification of the sintered samples was performed by XRD (TongDa TD3000) and their interface structures were characterized by Scan Electron Microscope (SEM, FEI INSPECT F). Pyroelectric coefficient (Pc), voltage responsivity (RV) and unit bandwidth noise voltage (VN) were tested using dynamic pyroelectric measuring system [14]. The measurements were carried out with the chopper blackbody radiation source at a temperature of 500 K. The incident power (PI) calculated from the black body parameters was 2.29 106 W. The voltage response (VS) and VN was traced by oscilloscope and lock-in amplifier. 3. Result and discussion 3.1. Pyroelectric properties of the thick film
Fig. 3. Micro-scope image of the thick film surface.
The detail of the PZT thick film preparation process was described in our previous work [13]. To densify the green film, the samples were treated by CIP under the pressure 260 MPa prior to sintering. The rising rate of pressure was 2.5 MPa/s and the decline rate was 0.5 MPa/s for all the samples. After CIP pretreatment, the PZT thick film was sintered at 800 °C for 1 h with rising rate 4 °C and cooling rate 2.5 °C/min, respectively. After sintering process, AZ6112 photoresist was used to transfer the top electrode pattern. 100 nm Pt film with 30 nm Ti adhesive layer was prepared as top electrode by DC sputtering and lift-off method (Fig. 2f). At the same time, the connection of the top electrode and the bonding pad was fabricated in this process. The micro-scope image of the thick film surface was shown in Fig. 3. 400 nm gold black layer was evaporated as absorbing layer (Fig. 2g). To pole the films, 5 V/lm DC voltage was applied at 150 °C for 15 min at ramp rates of 3 V/min. The poling voltage was not shut off until the temperature cooled down to room temperature. In order to reduce the cost, the back silicon was firstly etched to tens of micrometer by using 44% KOH solution at 70 °C. The thick film was hermetically protected from contamination by Apizon wax until the wet etching finished. The residual wax was cleaned by tetrachloroethylene. And then, the micro-bridge structure was released through real time monitor etching system in which SF6 was used as etchant (Fig. 2h). The upward view micro-scope image of the released micro-bridge was shown in Fig. 4.
After organics removing process, the green films were treated by CIP under 260 MPa pressure. After CIP treatment, no peel-off morphology were observed at the surface of the films deposited on substrate with silicon cup. In order to demonstrate the effects of CIP treatment, green films treated with and without CIP were sintered at 800 °C for 1 h. The SEM cross sectional images of the PZT thick films before and after sintering were shown in Fig. 5. PZT film pressed by CIP showed more dense structure than that without CIP treatment before sintering. The porous structure of the green film without CIP treatment was caused by two reasons. Firstly, the PZT powders were driven toward to the substrate and finally deposited on it by the applied electric field. The increased viscosity of the suspension caused by organic binder blocked the fine particles moving into the pores formed by large particles. Secondly, the removal of residual organics left a volume of pores in the film. However, the density of the PZT film was increased by CIP treatment because the fine particles could be pressed into the pores under proper pressure. The particles were more close to each other in CIP treated PZT films and this could make the mass transition more easily in the sintering process. As a result, the CIP treated PZT films were more compact after sintering as it could be seen by comparing Fig. 5b and d. According to the XRD patterns of the PZT films shown in Fig. 6, all the reflection peaks were corresponding to PZT perovskite structure (JCPDF-500346), and the (1 1 1) Pt orientation was retained. The Pc of the sintered PZT thick films with and without CIP treatment were measured by dynamic method and the dielectric properties were also tested by impedance analyzer (Agilent4284A). It is very clear that high Pc of PZT thick film should be obtained when we fabricate a PZT thick film detector. Parameters other than Pc, such as FV and FD, should be taken into consideration to evaluate the pyroelectric properties of the thick film. FV and FD can be calculated by the following equations:
FV ¼
FD ¼
Fig. 4. Upward view micro-scope image of the micro-bridge released by dry etching.
Pc C e0 er
ð1Þ Pc
Cðe0 er tan dÞ1=2
ð2Þ
Where C is the volume specific heat, e0 is the permittivity of free space, er is the relative permittivity, and tan d is the dielectric loss. Depending on detector operation mode, FV and FD should be as high as possible [15]. So, how to enhance Pc and lower dielectric loss is an important issue in PZT thick film detector fabrication process. Table 1 listed the main property parameters of the two films treated by CIP or not. As it could be seen from the table, Pc was increased and tan d was decreased by CIP treatment. These results
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Fig. 5. The cross sectional SEM images of the thick films before and after sintering at 800 °C for 1 h. (a) PZT without CIP before sintering, (b) sintered PZT without CIP press, (c) PZT treated with 260 MPa before sintering and (d) sintered PZT treated with 260 MPa.
RV ¼
Fig. 6. XRD patterns of the PZT thick film with CIP treatment sintered at 800 °C for 1 h.
Table 1 Comparison of PZT films with and without CIP. Pc (C/cm2 K) No CIP CIP
8
0.83 10 1.7 108
er
tan d
FV (m2/C)
FD (Pa1/2)
245 330
0.02 0.015
0.015 0.023
0.5 105 1.0 105
VS PI
According to Fig. 7, it could be seen from the results that the maximum RV up to 4.5 102 V/W occurred at 5.3 Hz. The RV decreased gradually when the frequency (f) was lower than 100 Hz. It became flat and stability when the f increased sequentially. The variation of RV could be explained by the relation of the heat transmission time (tr) and the 1/f. When tr was much smaller than 1/f, the temperature fluctuation stimulated by one incident radiation cycle can reach the largest amplitude before the next one attended. Therefore, the temperature variation dependent of RV showed a larger value. In contrast, the temperature of the sensing area could not decline to the lowest value before the next incident flow attended when tr was comparable to 1/f. The real temperature fluctuation was smaller than that at a lower frequency, resulting in the decline of Pc. So, the RV which was related with Pc decreased. Noise sources of a pyroelectric detector can be mainly classified into thermal noise, dielectric noise and amplifier noise [16,17]. The electric relevant noise sources are normally due to resistance in equivalent circuit, voltage or current noise in amplifier. The noise of the detector is not determined by one single noise source but
demonstrate that FV and FD were increased obviously by CIP treatment. These improvement would benefit for the performance of the infrared detector, which will be disused in the following part. 3.2. Pyroelectric response of the sensor The pyroelectric response of the sensor fabricated by this MEMS technology was measured by dynamic pyroelectric measuring system. All these parameters were tested based on the testing platform described in the experiment part. When an incident radiation made the temperature of the pyroelectric sensor changed, a voltage VS was produced between the top and bottom electrodes. RV reflected the transformation capability of radiation to voltage signal which was expressed as:
ð3Þ
Fig. 7. The voltage response of the detector at different frequency.
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317
Fig. 8. Unit bandwidth noise voltage and noise equivalent power at different frequency.
the quadratic sum of the various contribution of every noise source. The voltage noise at different chopper incident frequency f was showed in Fig. 8. At the frequency lower than 100 Hz, the noise decreased exponentially, indicating the dominating noise of 1/f noise source. The constant noise voltage revealed the Johnson noise at frequency larger than 100 Hz. The noise equivalent power (NEP) of a pyroelectric detector is given as a ratio of total noise voltage and voltage responsivity, meaning the incident radiation power needed to produce a VS equal to VN:
NEP ¼
PI V S =V N
ð4Þ
The calculated NEP was about 2.5 109 W between 5–28 Hz and 5 1010 W at f >120 Hz, respectively. The performance of a pyroelectric detector was assessed by specific detectivity D* which was defined as:
D ¼
pffiffiffiffiffiffiffiffiffiffiffi A D Df V S PI VN
ð5Þ
Where AD is the sensing area and Df is the system bandwidth (which is 1 Hz in our measurement). The specific detectivity D* was plotted at Fig. 9. The D* were 6.34 108 cm Hz1/2 W1 at 100 Hz and 8.03 107 cm Hz1/2 W1 at 5.3 Hz, which is one order higher than the reported pyroelectric detector [18]. Pyroelectric response for step-function input of infrared radiation was measured to obtain the thermal time constant (sT). Using MIRL17-900 as infrared source, the measured RV at different frequency was plotted in Fig. 10. The source power could be treated as a constant value at the testing frequency range. sT was calculated from the RV change where it varied only 3 dB. The sT of this fabricated detector was 51 ms.
Fig. 9. The specific detectivity of the detector at different frequency.
Fig. 10. Voltage response at different modulation frequency.
4. Conclusion A 3 3 mm PZT thick film pyroelectric detector with back to back silicon cups structure was developed in which the thick film was shaped in the front silicon cup. The metal film connection of the top electrode and bonding pad has been realized completely by standard photolithography and lift-off technology. The Pc, FV and FD of the thick film have been improved by CIP pretreatment. The tested results of the pyroelectric detectors show that this fabricated detectors have great potential in smart weapon and rapid response infrared gas concentration sensor.
References [1] Kang Min Ok, Eun Ok Chi, P. Shiv Halasyamani, Bulk characterization methods for non-centrosymmetric materials: second-harmonic generation, piezoelectricity, pyroelectricity, and ferroelectricity, Chem. Soc. Rev. 35 (2006) 710–717. [2] R. Bruchhaus, D. Pitzer, M. Schreiter, W. Wersing, Optimized PZT thin films for pyroelectric IR detector arrays, J. Electroceram. 3 (1999) 151–162. [3] M.P. Thompson, J.R. Troxell, M.E. Murray, C.M. Thrush, J.V. Mantese, Infrared absorber for pyroelectric detectors, J. Vac. Sci. Technol. A: Vacuum Surfaces Films 25 (2007) 437–440. [4] N.M. Shorrocks, A. Patel, M.J. Walker, A.D. Parsons, Integrated thin film pyroelectric detector arrays, Microelectron. Eng. 29 (1995) 59–66. [5] L. Kozielski, M. Adamczyk, J. Erhart, M. Pawelczyk, Application testing of Sr doping effect of PZT ceramics on the piezoelectric transformer gain and efficiency proposed for MEMS actuators driving, J. Electroceram. 29 (2012) 133–138. [6] C.C. Hindrichsen, T. Pedersen, E.V. Thomsen, K. Hansen, R.L. Mller, Investigation of top/bottom electrode and diffusion barrier layer for PZT thick film MEMS sensor, Ferroelectrics 367 (2008) 201–213. [7] R. Xu, A. Lei, C. Dahl-Petersen, K. Hansen, M. Guizzetti, K. Birkelund, E.V. Thomsen, O. Hansen, Screen printed PZT/PZT thick film bimorph MEMS cantilever device for vibration energy harvesting, Sens. Actuat. A – Phys. 188 (2012) 383–388. [8] H.B. Zhang, S.L. Jiang, J.Z. Xiao, K. Kajiyoshi, Low temperature preparation and electrical properties of sodium–potassium bismuth titanate lead-free piezoelectric thick films by screen printing, J. Euro. Ceram. Soc. 30 (2010) 3157–3165. [9] H. Nakaki, H. Uchida, S. Koda, S. Okamoto, H. Funakubo, Enhancement of spontaneous polarization in lead zirconate titanate thin film by Dy3+ substitution, Appl. Phys. Lett. 87 (2005) 182906–182913. [10] R.A. Dorey, R.W. Whatmore, Pyroelectric PZT/PMNZTU composite thick films, J. Euro. Ceram. Soc. 25 (2005) 2379–2382. [11] C.G. Hindrichsen, R. Lou-MØller, K. Hansen, E.V. Thomsen, Advantage of PZT thick film for MEMS sensors, Sens. Actuat. A – Phys. 163 (2010) 9–14. [12] M.H. Zhang, H. Wang, H.B. Yang, W.H. Liu, H.F. Zhou, X. Yao, Enhanced dielectric properties of low temperature sintered Ba0.6Sr0.4TiO3 thick film, J. Electroceram. 26 (2011) 99–104. [13] Q.X. Peng, C.G. Wu, W.B. Luo, L. Jin, W.L. Zhang, C. Chen, X.Y. Sun, The improvement of pyroelectric properties of PZT thick films on Si substrate by TiOx barrier layer, Infrared Phys. Technol. 58 (2013) 51–55. [14] Ch.G. Wu, W.L. Zhang, Y.R. Li, X.Zh. Liu, J. Zhu, Measurement of induced pyroelectric coefficient using dynamic method: theory and experiment, Jpn. J. Appl. Phys. 45 (2006) 2674–3677.
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[15] R. Köhler, N. Neumann, N. Heb, R. Bruchhaus, W. Wersing, M. Simon, Pyroelectric devices based on sputtered PZT thin film, Ferroelectrics 201 (1997) 83–92. [16] P. Muralt, Micromachined infrared detectors based on pyroelectric thin films, Rep. Prog. Phys 64 (2001) 1339–1388.
[17] R.W. Whatmore, Pyroelectric arrays: ceramics and thin films, J. Electroceram. 13 (2004) 139–147. [18] M.C. Kao, H.Z. Chen, S.L. Yang, P.T. Hsieh, C.C. Yu, Pyroelectric Ta-modified LiNbO3 thin films and devices for thermal infrared detection, Thin Solid Films 56 (2008) 5518–5522.
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Regular article
An infrared pyroelectric detector improved by cool isostatic pressing with cup-shaped PZT thick film on silicon substrate Q.X. Peng, C.G. Wu, W.B. Luo ⇑, C. Chen, G.Q. Cai, X.Y. Sun, D.P. Qian State Key Lab of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China
h i g h l i g h t s The surface of the thick film was at the same height with the substrate. Metal thin film was used to connect the top electrode and the bonding pad. The pyroelectric properties was improved by cool isostatic pressing.
a r t i c l e
i n f o
Article history: Received 6 May 2013 Available online 29 September 2013 Keywords: PZT Infrared Pyroelectric Detectivity
a b s t r a c t In this paper, we presented a new pyroelectric detector with back to back silicon cups and micro-bridge structure. The PZT thick film shaped in the front cup was directly deposited with designed pattern by electrophoresis deposition (EPD). Pt/Ti Metal film, which was fabricated by standard photolithography and lift-off technology, was sputtered to connect the top electrode and the bonding pad. The cold isostatic press (CIP) treatment could be applied to improve the pyroelectric properties of PZT thick film. The infrared (IR) properties the CIP-optimized detector were measured. The voltage responsivity (RV) was 4.5 102 V/W at 5.3 Hz, the specific detectivity (D*) was greater than 6.34 108 cm Hz1/2 W1 (frequency > 110 Hz), and the thermal time constant was 51 ms, respectively. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Pyroelectric materials can generate electrically measurable signal, which results from the spontaneous polarization change related to temperature variation [1]. Pb(ZrxTi1x)O3 (PZT) is a typical pyroelectric material, which attracted great attention in the IR radiation detection application in the wavelength ranges of 3–5 lm and 8–14 lm [2,3]. Accompanied by the development of MEMS technology, pyroelectric device based on PZT thick film with silicon substrate became a hot topic in the past decades [4,5]. Various pyroelectric detectors with small size, light weight, robust structure could be realized by this method [6,7]. The conventional structure of a thick film detector was shown in Fig. 1a schematically. There height difference between the top electrode and the bonding pad was about 10 lm. A metal wire was generally bonded on the surface of top electrode and the bonding pad to conduct electric signal. This wire bonding connection would bring about two shortcomings. On one side, the wire would increase the heat capacity leads to heat loss because of its good heat conduction ability. On the other side, it was not benefit for ⇑ Corresponding author. Tel.: +86 83202140. E-mail addresses: [email protected] (Q.X. Peng), [email protected] (C.G. Wu), [email protected] (W.B. Luo). 1350-4495/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2013.09.002
mass-produce for fussy repeated bonding action. By considering of the above aspects, connections using hundreds thick metal film is a better candidate than the wire bonding. Unfortunately, when the pattern of the top electrode and the connection was transferred by photolithography, the height difference would distort, or even fracture the pattern at the edge of the thick film. Obviously, reducing the height difference value of the top electrode and the bonding pad was the simplest and most feasible way to solve this technical problem. To obtain a high performance pyroelectric sensor, methods such as sintering aid [8], doping [9], changing material component [10], were used to enhance the pyroelectric properties. These methods improved pyroelectric properties from the point of phase or micro-structure variation by another chemical addition. Moreover, CIP was used as a physical method to optimize material properties. Hindrichsen et al. [11] found that the piezoelectric properties of the PZT thick film was enhanced when the thick film was pressed by CIP. Zhang et al. [12] also improved the micro-structure and dielectric properties of Ba0.6Sr0.4TO3 film by CIP treatment. However, it was found that the thick films were liable to peel-off from the substrate because the shape of the thick films should be changed during CIP treatment. To solve these two problems, a new sensor structure shown in Fig. 1b was designed. In this structure, the thick film was shaped
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PZT thick film on silicon substrate was described in detail. The CIP treatment was used to improve the pyroelectric properties of thick film. The IR properties of the CIP-optimized sensor were measured to show the CIP treatment effects on the performance of the detector. 2. Experiment details 2.1. MEMS fabrication processes
Fig. 1. Two structure designed for thick film sensor. (a) Conventional structure, (b) thick film shaped in silicon cup.
in silicon cup to avoid the thick film peeling off the substrate. Moreover, its surface and the substrate were in the same plane, which could flattened the photoresist and make the Cr mask contacted closely with both the thick film and the substrate in light exposure process. Thus, the photolithography could be conducted almost the same as on a plane substrate. In this paper, the sensor fabrication process of the infrared detector improved by cool isostatic pressing with cup-shaped
The fabrication process of the pyroelectric sensor was shown step by step in Fig. 2. The starting material was cleaned double polished (1 0 0) silicon-on-insulator (SOI) substrate with 30 lm silicon on 1 lm insulator SiO2 layer and 265 lm carrier substrate (Fig. 2a). Firstly, the 3 3 mm front silicon cup (the depth decided by the depth of the buried SiO2 layer) was developed by lithograph technology and TMAH wet etching (Fig. 2b). The heat isolation channel was also formed at this step. Then, 500 nm SiO2 was grown by dry-oxygen oxidation which would not only sever as mask layer but also passive layer for dry etch-stop technology (Fig. 2c). 300 nm TiO2 diffusion barrier layer was oxidized at 700 °C for 1 h from melt Ti layer which was sputtered by direct current (DC) magnetron sputtering (Fig. 2d). After that, the substrate was carefully cleaned by ultrasonic washing for 30 min with acetone and deionized water. Finally, 200 nm patterned Pt bottom electrode was deposited by DC magnetron sputtering method (Fig. 2d).
Fig. 2. A cross sectional schematic of the MEMS route for the PZT thick film pyroelectric sensor (a) SOI substrate. (b) Si cup and isolation channel etching. (c) Thermal oxidation of SiO2. (d) TiOx diffusion barrier layer and bottom electrode. (e) Thick film deposition and sintering. (f) Top electrode. (g) Absorbing layer evaporation and (h) micro-bridge release.
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2.2. Structure and pyroelectric measurements The phase identification of the sintered samples was performed by XRD (TongDa TD3000) and their interface structures were characterized by Scan Electron Microscope (SEM, FEI INSPECT F). Pyroelectric coefficient (Pc), voltage responsivity (RV) and unit bandwidth noise voltage (VN) were tested using dynamic pyroelectric measuring system [14]. The measurements were carried out with the chopper blackbody radiation source at a temperature of 500 K. The incident power (PI) calculated from the black body parameters was 2.29 106 W. The voltage response (VS) and VN was traced by oscilloscope and lock-in amplifier. 3. Result and discussion 3.1. Pyroelectric properties of the thick film
Fig. 3. Micro-scope image of the thick film surface.
The detail of the PZT thick film preparation process was described in our previous work [13]. To densify the green film, the samples were treated by CIP under the pressure 260 MPa prior to sintering. The rising rate of pressure was 2.5 MPa/s and the decline rate was 0.5 MPa/s for all the samples. After CIP pretreatment, the PZT thick film was sintered at 800 °C for 1 h with rising rate 4 °C and cooling rate 2.5 °C/min, respectively. After sintering process, AZ6112 photoresist was used to transfer the top electrode pattern. 100 nm Pt film with 30 nm Ti adhesive layer was prepared as top electrode by DC sputtering and lift-off method (Fig. 2f). At the same time, the connection of the top electrode and the bonding pad was fabricated in this process. The micro-scope image of the thick film surface was shown in Fig. 3. 400 nm gold black layer was evaporated as absorbing layer (Fig. 2g). To pole the films, 5 V/lm DC voltage was applied at 150 °C for 15 min at ramp rates of 3 V/min. The poling voltage was not shut off until the temperature cooled down to room temperature. In order to reduce the cost, the back silicon was firstly etched to tens of micrometer by using 44% KOH solution at 70 °C. The thick film was hermetically protected from contamination by Apizon wax until the wet etching finished. The residual wax was cleaned by tetrachloroethylene. And then, the micro-bridge structure was released through real time monitor etching system in which SF6 was used as etchant (Fig. 2h). The upward view micro-scope image of the released micro-bridge was shown in Fig. 4.
After organics removing process, the green films were treated by CIP under 260 MPa pressure. After CIP treatment, no peel-off morphology were observed at the surface of the films deposited on substrate with silicon cup. In order to demonstrate the effects of CIP treatment, green films treated with and without CIP were sintered at 800 °C for 1 h. The SEM cross sectional images of the PZT thick films before and after sintering were shown in Fig. 5. PZT film pressed by CIP showed more dense structure than that without CIP treatment before sintering. The porous structure of the green film without CIP treatment was caused by two reasons. Firstly, the PZT powders were driven toward to the substrate and finally deposited on it by the applied electric field. The increased viscosity of the suspension caused by organic binder blocked the fine particles moving into the pores formed by large particles. Secondly, the removal of residual organics left a volume of pores in the film. However, the density of the PZT film was increased by CIP treatment because the fine particles could be pressed into the pores under proper pressure. The particles were more close to each other in CIP treated PZT films and this could make the mass transition more easily in the sintering process. As a result, the CIP treated PZT films were more compact after sintering as it could be seen by comparing Fig. 5b and d. According to the XRD patterns of the PZT films shown in Fig. 6, all the reflection peaks were corresponding to PZT perovskite structure (JCPDF-500346), and the (1 1 1) Pt orientation was retained. The Pc of the sintered PZT thick films with and without CIP treatment were measured by dynamic method and the dielectric properties were also tested by impedance analyzer (Agilent4284A). It is very clear that high Pc of PZT thick film should be obtained when we fabricate a PZT thick film detector. Parameters other than Pc, such as FV and FD, should be taken into consideration to evaluate the pyroelectric properties of the thick film. FV and FD can be calculated by the following equations:
FV ¼
FD ¼
Fig. 4. Upward view micro-scope image of the micro-bridge released by dry etching.
Pc C e0 er
ð1Þ Pc
Cðe0 er tan dÞ1=2
ð2Þ
Where C is the volume specific heat, e0 is the permittivity of free space, er is the relative permittivity, and tan d is the dielectric loss. Depending on detector operation mode, FV and FD should be as high as possible [15]. So, how to enhance Pc and lower dielectric loss is an important issue in PZT thick film detector fabrication process. Table 1 listed the main property parameters of the two films treated by CIP or not. As it could be seen from the table, Pc was increased and tan d was decreased by CIP treatment. These results
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Fig. 5. The cross sectional SEM images of the thick films before and after sintering at 800 °C for 1 h. (a) PZT without CIP before sintering, (b) sintered PZT without CIP press, (c) PZT treated with 260 MPa before sintering and (d) sintered PZT treated with 260 MPa.
RV ¼
Fig. 6. XRD patterns of the PZT thick film with CIP treatment sintered at 800 °C for 1 h.
Table 1 Comparison of PZT films with and without CIP. Pc (C/cm2 K) No CIP CIP
8
0.83 10 1.7 108
er
tan d
FV (m2/C)
FD (Pa1/2)
245 330
0.02 0.015
0.015 0.023
0.5 105 1.0 105
VS PI
According to Fig. 7, it could be seen from the results that the maximum RV up to 4.5 102 V/W occurred at 5.3 Hz. The RV decreased gradually when the frequency (f) was lower than 100 Hz. It became flat and stability when the f increased sequentially. The variation of RV could be explained by the relation of the heat transmission time (tr) and the 1/f. When tr was much smaller than 1/f, the temperature fluctuation stimulated by one incident radiation cycle can reach the largest amplitude before the next one attended. Therefore, the temperature variation dependent of RV showed a larger value. In contrast, the temperature of the sensing area could not decline to the lowest value before the next incident flow attended when tr was comparable to 1/f. The real temperature fluctuation was smaller than that at a lower frequency, resulting in the decline of Pc. So, the RV which was related with Pc decreased. Noise sources of a pyroelectric detector can be mainly classified into thermal noise, dielectric noise and amplifier noise [16,17]. The electric relevant noise sources are normally due to resistance in equivalent circuit, voltage or current noise in amplifier. The noise of the detector is not determined by one single noise source but
demonstrate that FV and FD were increased obviously by CIP treatment. These improvement would benefit for the performance of the infrared detector, which will be disused in the following part. 3.2. Pyroelectric response of the sensor The pyroelectric response of the sensor fabricated by this MEMS technology was measured by dynamic pyroelectric measuring system. All these parameters were tested based on the testing platform described in the experiment part. When an incident radiation made the temperature of the pyroelectric sensor changed, a voltage VS was produced between the top and bottom electrodes. RV reflected the transformation capability of radiation to voltage signal which was expressed as:
ð3Þ
Fig. 7. The voltage response of the detector at different frequency.
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Fig. 8. Unit bandwidth noise voltage and noise equivalent power at different frequency.
the quadratic sum of the various contribution of every noise source. The voltage noise at different chopper incident frequency f was showed in Fig. 8. At the frequency lower than 100 Hz, the noise decreased exponentially, indicating the dominating noise of 1/f noise source. The constant noise voltage revealed the Johnson noise at frequency larger than 100 Hz. The noise equivalent power (NEP) of a pyroelectric detector is given as a ratio of total noise voltage and voltage responsivity, meaning the incident radiation power needed to produce a VS equal to VN:
NEP ¼
PI V S =V N
ð4Þ
The calculated NEP was about 2.5 109 W between 5–28 Hz and 5 1010 W at f >120 Hz, respectively. The performance of a pyroelectric detector was assessed by specific detectivity D* which was defined as:
D ¼
pffiffiffiffiffiffiffiffiffiffiffi A D Df V S PI VN
ð5Þ
Where AD is the sensing area and Df is the system bandwidth (which is 1 Hz in our measurement). The specific detectivity D* was plotted at Fig. 9. The D* were 6.34 108 cm Hz1/2 W1 at 100 Hz and 8.03 107 cm Hz1/2 W1 at 5.3 Hz, which is one order higher than the reported pyroelectric detector [18]. Pyroelectric response for step-function input of infrared radiation was measured to obtain the thermal time constant (sT). Using MIRL17-900 as infrared source, the measured RV at different frequency was plotted in Fig. 10. The source power could be treated as a constant value at the testing frequency range. sT was calculated from the RV change where it varied only 3 dB. The sT of this fabricated detector was 51 ms.
Fig. 9. The specific detectivity of the detector at different frequency.
Fig. 10. Voltage response at different modulation frequency.
4. Conclusion A 3 3 mm PZT thick film pyroelectric detector with back to back silicon cups structure was developed in which the thick film was shaped in the front silicon cup. The metal film connection of the top electrode and bonding pad has been realized completely by standard photolithography and lift-off technology. The Pc, FV and FD of the thick film have been improved by CIP pretreatment. The tested results of the pyroelectric detectors show that this fabricated detectors have great potential in smart weapon and rapid response infrared gas concentration sensor.
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