Fast and wide-band response infrared detector using porous PZT pyroelectric thick film

Infrared Physics & Technology 63 (2014) 69–73

Contents lists available at ScienceDirect

Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared

Fast and wide-band response infrared detector using porous PZT pyroelectric thick film C.G. Wu, X.Y. Sun, J. Meng, W.B. Luo ⇑, P. Li, Q.X. Peng, Y.S. Luo, Y. Shuai State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China

h i g h l i g h t s  We fabricated pyroelectric infrared detector based on porous lead zirconate titanate (PZT) thick film.  Porous PZT thick film performs good pyroelectric property at high frequency range.  Properties of detector demonstrate the porous structure of PZT thick film has great effects on high frequency detectivity.

a r t i c l e

i n f o

Article history: Received 22 September 2013 Available online 12 December 2013 Keywords: Porous PZT Infrared detector Fast respond Wide band

a b s t r a c t Porous lead zirconate titanate (PbZr0.3Ti0.7O3, PZT30/70) thick films and detectors for pyroelectric applications have been fabricated on alumina substrates by screen-printing technology. Low temperature sintering of PZT thick films have been achieved at 850 °C by using Li2CO3 and Bi2O3 sintering aids. The microstructure of PZT thick film has been investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The dielectric properties were measured using HP 4284 at 1 kHz under 25 °C. The permittivity and loss tangent of the thick films were 94 and 0.017, respectively. Curie temperature of PZT thick film was 425 °C as revealed by dielectric constant temperature measurement. The pyroelectric coefficient was determined to be 0.9  108 Ccm2 K1 by dynamic current measurement. Infrared detector sensitive element of dual capacitance was fabricated by laser directly write technology. Detectivity of the detectors were measured using mechanically chopped blackbody radiation. Detectivity ranging from 1.23  108 to 1.75  108 (cm Hz1/2 W1) was derived at frequency range from 175.5 Hz to 1367 Hz, and D*’s 3 dB cut-off frequency bandwidth was 1.2 kHz. The results indicate that the infrared detectors based on porous thick films have great potential applications in fast and wide-band frequency response conditions. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Lead zirconate titanate (PZT) thick films have been considered as one of the most promising candidates for integrated piezoelectric devices and pyroelectric infrared sensors [1–4]. There were two significant advantages comparing with other competitive materials. Firstly, PZT has excellent piezoelectric and pyroelectric properties, and were widely used in various sensors and transducers, for examples, pyroelectric infrared sensors, ultrasonic sensor and actuator, micro-generator [5–10]. Secondly, thick film technology is preferred in industry because it can provide benefits of costeffective, mass production and better material properties than that of thin films. In recent years, research efforts have been made on porous pyroelectric PZT films. The porosity of PZT films benefited low ⇑ Corresponding author. Fax: +86 28 8320 2140. E-mail address: [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.2013.11.013

dielectric constant, low heat capacity and low thermal conductivity, and resulted in figures of merit better than the corresponding values either for ceramic PZT or other pyroelectric materials (including TGS and LiTaO3) [11–13]. Moreover, research and applications of pyroelectric infrared sensor using porous PZT thick films were performed at low chopping frequency (<100 Hz). However, there is no report on infrared photoelectric response measurement at high chopping frequency (>1000 Hz). In this paper, the pyroelectric infrared sensors using porous PZT thick film fabricated by low temperature chemical liquid sintering technology have been studied. PZT with a composition of 30/70 is selected because this proportion exhibits good pyroelectric property and can effectively avoid piezoelectric effect, which is essential for pyroelectric applications. The microstructure, dielectric properties, pyroelectric properties and infrared photoelectric response property were investigated.

70

C.G. Wu et al. / Infrared Physics & Technology 63 (2014) 69–73

2. Experiment Lead zirconate titanate powder with a Zr/Ti mole ratio of 30/70 was obtained commercially from Hayashi chemical industry company (Japan). 3 mol% Li2CO3 and 3 mol% Bi2O3 were mixed with PZT powders as the sintering aids by dry-milling. Thick film paste was prepared by ball-milling powders with 50 wt.% organic vehicle (a-terpineol and ethyl cellulose) for 24 h to get homogeneity. The paste was then screen-printed onto 96% alumina substrate with 500 nm sputtered platinum bottom electrode. After that, all films were dried at 80 °C for 15 min and sintered at 850 °C for 30 min. The thickness of the films after a single print and sintering process was about 15–20 lm. The crystalline phase of powders and thick films were studied using X-ray diffraction (XRD, Dandongfangyuan DX-2600). The microstructure of thick films was examined by scanning electron microscopy (SEM, JEOL JSM-6490lv). To enable the electrical measurements, 200 nm platinum top electrodes (5  5 mm2) were sputtered to form a capacitor structure. All the capacitors were poled by a 5 MV/m electric field at 150 °C for 15 min before electric measurements. The dielectric properties of thick films were measured by precision LCR meter (Agilent 4284A). The pyroelectric coefficients of thick films were measured by test system based on dynamic current method made by Kunming Institute of Physics. For pyroelectric infrared detector, procedures are the same as above at the stage of thick film preparation except two differences. First, instead of platinum, NiCr metal layer was used as both top electrodes and infrared absorption layer. Secondly, the bottom electrode was divided equally to form two capacitors. The top view and cross section of the device structure are shown in Fig. 1a and b, respectively. Pyroelectric current is detected from both half of the bottom electrode. This structure makes the two capacitors back-toback connected (Fig. 1c). The equivalent circuit of this structure is shown in Fig. 1d. According to the circuit, net pyroelectric current is given by:

Ip ¼ I1  I2  Iloss

ð1Þ

where Iloss is the current dissipated by the sensor itself and I1, I2 are pyroelectric current excited in each unit which can be obtained with equation:

I1 ; I2 ¼ Ap

dT dt

ð2Þ

That means the net current is decided by (dT1  dT2)/dt which is different from single capacitor devices. Benefiting from the differential configuration, this structure could effectively reduce the

Fig. 2. Configuration of devices after laser etching.

induction charge excited by piezoelectric effect. When thick films were made, thermal insulation structures in Fig. 2 was made by laser etching. The infrared photoelectric response property of pyroelectric detector was investigated by mechanically chopped (Signal recovery, light chopper model 651) blackbody radiation (Isotech, R970 blackbody source). 3. Results and discussion 3.1. Microstructures of the porous PZT thick films The XRD patterns of the commercial powders and the thick films were shown in Fig. 3. Both of the powders and the thick films show perfect perovskite phase because all the peaks were correspond to perovskite PZT(30/70) [14]. The peaks of the thick films are a little bit sharp and narrow compared to that of PZT powders, indicating grain growth occurred during sintering. Fig. 4 and Fig. 5 are the cross-section SEM photographes of PZT thick film sample on Pt coated alumina substrate. The sintered PZT films thickness was about 17 lm. No obvious holes and cracks were observed in the bulk film. The sample gave an interconnected well-densified 3–3 ceramic/pore structure. The chain of grains sintered to form bigger grains that are connected almost grain to grain, and formed a continuous skeleton. The PZT grain exhibited diameter ranging from 1 to 2 lm, and the pore size of thick film ranged from 0.1 to 1 lm. The porous microstructure in our PZT thick film samples can be attributed to interparticle porosity, or interaggregate porosity [15]. As shown in Fig. 4, no vacancy and delamination were observed at Pt/PZT interface, indicating PZT porous films strongly adhere to the Pt bottom electrodes. The good adhesion can be attributed to the bonding among the PZT grains and between the grains and the substrate, which was essentially important technology factor to prevent PZT thick film to peel off from the substrate during micromachining process of infrared sensor. 3.2. Effects of the porous structure on PZT thick films

Fig. 1. Double-capacitor structure: top view (a), cross section (b), back-to-back configuration (c), and equivalent circuit (d).

The effects of the porous structure on the properties of the PZT thick film have been studied. Firstly, the dielectric properties of the PZT thick film were measured. Fig. 6 shows the relative dielectric constant and dissipation factor at the frequency of 1 kHz as a function of temperature. It indicates that the dielectric loss increase dramatically with temperature, the variation of dielectric constant is consistent with Curie–Weiss law. The Curie point of 425 °C is consistent with PbZrO3–PbTiO3 phase diagram. Loss tangent measured at room temperature was 0.017 and dielectric constant was only 94. The dielectric constant is much lower than the value of 30% porosity PZT, indicating a higher porosity of our PZT film [16]. According to relevant research [17], pores have a great influence on permittivity. The effective permittivity, e*, of porous thick films can be calculated by equation [18,19]:

C.G. Wu et al. / Infrared Physics & Technology 63 (2014) 69–73

71

Fig. 3. XRD patterns for PZT powders and thick films sintered at 850 °C. Fig. 6. Relative dielectric constant and dielectric loss of PZT thick films as a function of temperature at the frequency of 1 kHz.

the porosity volume fraction. Assuming that interspaces are welldistributed in PZT thick film, CV could be calculated by:

C 0V ¼ C V  ð1  xÞ

ð4Þ

where CV refers to volume specific heat of PZT ceramic, C 0V indicates volume specific heat of PZT thick film, x is the porosity volume fraction. If we estimate x as the value of p in Eq. (3) of 46%, C 0V of 2.3  106 J m3 K1 can be obtained. Finally, the Pyroelectric coefficient of the thick film was measured by dynamic current method to establish the effects of porous structure. A baseline temperature modulated by a small sinusoidal variable was given to thick films. Induced charge was converted to voltage signal by a load resistor R. When the voltage signal was measured from load resistor, pyroelectric coefficient can be calculated by Eq. (5).

Fig. 4. Interfacial SEM photograph of PZT thick film and Pt electrode.

V ¼ Rip ¼ RApxDTcosðxtÞ

ð5Þ

Pyroelectric signal measured in our experiment was shown in Fig. 7, a cosine-type current was obtained under sinusoidal thermal variations with period of 1 min and peak amplitude of about 1 °C. With a 5  5 mm2 sensor area, the calculated value of pyroelectric coefficient is 0.9  108 Ccm2 K1. The pyroelectric coefficient is comparable with Bi0.5(Na0.82K0.18)0.5TiO3 [21] and PZT porous film [22]. In summary, the porous PZT thick film show good pyroelectric properties and great reduced dielectric constant and volume specific heat. These improvements will benefit the performance of the pyroelectric detector because it can increase both the merit figure Fd of materials and detectivity of the detectors. Fig. 5. Cross-sectional SEM photograph of PZT thick films sintered at 850 °C.

2



e ¼e 1

P3 2

K3

! ð3Þ

where e is the relative permittivity of ceramic component, p stands for the porosity volume fraction of thick films and K is a coefficient decided by the shape of pores. This equation indicates that the presence of pores reduces the measured permittivity. According to our experiment, we choose 0.5 as the value of K [20]. Porosity volume fraction thus can be obtained for 46%. Secondly, the porous structure of the PZT thick film can impact the volume specific heat of porous PZT thick films. The volume specific heat is calculated from

Fig. 7. Pyroelectric current of thick films under sinusoidal temperature variations with period of 1 min and peak amplitude of about 1 °C.

72

C.G. Wu et al. / Infrared Physics & Technology 63 (2014) 69–73

3.3. Photoelectric responses of the pyroelectric detector As we know, voltage response of pyroelectric detector is one of the most important parameter of detector. It is measured by Eq. (6):

Rv ¼



 gxpA  G

1 þ x2 s2T

12 

1 þ x2 s2e

12

ð6Þ

In this equation, g is absorption coefficient of sensitive surface, p is pyroelectric coefficient, A is sensitive surface area, x is angular frequency of incident radiation, G is thermal conductivity, sT and se are thermal time constant and electrical time constant, which are proportional to thermal capacity and permittivity respectively. According to Eq. (6), it can be deduced that se and sT can influence the dependence of Rv on x (Rv–x curve). When both of them decrease, the Rv–x curve would be flat, indicating response frequency band width increase, and the frequency corresponding to the maximum value of Rv would increase. Thus, to increase high frequency response properties, porous PZT material of low thermal capacity and low permittivity was preferred, as sT and se are proportional to thermal capacity and permittivity respectively. Clearly, the porous PZT thick film fabricated in our experiments can exactly meet the requirements. The IR response of the infrared sensor was measured using the dynamic method [23]. A 500 K blackbody with a chopping frequency from 5.3 Hz to 187.3 Hz was used as radiation source. The IR response voltages caused by the temperature oscillation in the specimen was amplified by a preamplifier (model 8153, EG&G, USA) with an amplification of 1000. Then, the output voltage signal was measured by a lock-in amplifier (model 7265, EG&G, USA) and was monitored by a digital HP54615B oscillation. The noise of detector was measured by Signal recovery Lock in amplifier (model 7265). The parameters of the test system were shown in Table 1. Voltage responses between 8.5 Hz and 2217 Hz could be measured when adjusting chopper frequency. Responses at different chopper frequencies at 500 K are shown in Fig. 8(a). The noise of detector was measured by Signal recovery lock-in amplifier model 7265, which is shown in Fig. 8(b). The radiation power P can be calculated by Eq. (7) [24]:

P ¼ C RMS

erðT 4BB  T 4C Þ AS AD pd2

ð7Þ

According to Table 1 we can calculate the radiation power P is 2.29  106 W. The most important parameter of pyroelectric sensor is the detectivity (D*). Sensor detectivity is denoted as D*(T, Df, 1), where T is the temperature in Kelvin degrees, Df is the frequency and 1 stands for bandwidth of 1 Hz. D* can be calculated by expression below [25]:

D ¼

pffiffiffiffiffiffiffiffiffiffiffi A D Df V S  P VN

ð8Þ

Table 1 The test parameters of infrared detector. Physical symbol

Meaning

Value

CRMS

Energy RMS conversion coefficient Blackbody emission rate Stepan–Boltzmann constant Blackbody temperature Surface temperature of chopper Distance between device and diaphragm Area of diaphragm

0.447 0.98 5.67  1012 500 298 8.5 0.096

e r (W cm2 K4) TBB (K) TC (K) d (cm) AS (cm2)

Fig. 8. Voltage response as a inverse proportion to frequency (a) and noise response versus frequency (b).

Fig. 9. Directivity as a function of chopping frequency.

It can be seen from Fig. 9 that the value of D* was ranging from 0.68  108 to 1.75  108 (cm Hz1/2 W1) at frequency ranging from 55.3 Hz to 1367 Hz. When the modulation frequency was between 55.3 Hz and 137.3 Hz, the D* increased with the frequency increased. The highest value of D* was 1.75  108 cm Hz1/2 W1 at 137.3 Hz modulation frequency. After that, this trend started to reverse. The D* value of our sensor is comparable as PM622 infrared detector made by Nicera Sensor Company (9.3  107 cm Hz1/ 2 W1) using PZT ceramic slice, which is large enough for commercial applications [26]. The lower frequency (fL) and the upper frequency (fH) at which the D* is at 3 db below the maximum value was 175.5 Hz and 1367 Hz, respectively. It can be concluded that the 3 db cutoff frequency bandwidth of D* was about 1.2 kHz. The value was about tens to hundreds times larger than that of pyroelectric infrared sensors using LTO single crystalline slice, modified PbTiO3 ceramic slice, PZT thin film and PVDF/TrFE thin

C.G. Wu et al. / Infrared Physics & Technology 63 (2014) 69–73

film [27–30]. The results indicate that the infrared detectors based on porous thick films have great potential applications in quick and wide-band frequency response conditions, for example, smart munition and high frequency laser energy measuring system. 4. Conclusion Pyroelectric detectors using PZT(30/70) porous thick films have been prepared by screen-printing and laser etching. The permittivity and loss tangent at 1 kHz under 25 °C were 94 and 0.017. Curie point obtained from e-Temperature curve is 425 °C. pyroelectric coefficient is 0.9  108 Ccm2 K1. Detectivity D* of pyroelectric detector was measured at 500 K under different chopper frequency. A remarkable high frequency performance was shown in this test, D* ranging from 1.09  107 to 1.75  108 (cm Hz1/2 W1) and 1.2 kHz bandwidth of 3 db D* cutoff frequency were derived. Sensors fabricated by this technology are qualified for infrared applications. Acknowledgments This work has been supported by the National Natural Science Foundation of China (NSFC. 51102037) and the Fundamental Research Funds for the Central Universities from UESTC (Nos. ZYGX2010J030 and ZYGX2011J023). References [1] L. Capineri, L. Masotti, V. Ferrari, D. Marioli, A. Taroni, M. Mazzoni, Comparisons between PZT and PVDF thick films technologies in the design of low-cost pyroelectric sensors, Rev. Sci. Instrum. 75 (2004) 4906–4910. [2] M. Koch, N. Harris, A. Evans, N. White, A. Brunnschweiler, A novel micromachined pump based on thick-film piezoelectric actuation, Sens. Actuators, A 70 (1998) 98–103. [3] R. Kohler, N. Neumann, N. Hess, R. Bruchhaus, W. Wersing, M. Simon, Pyroelectric devices based on sputtered PZT thin films, Ferroelectrics 201 (1997) 83–92. [4] C.C. Chang, C.S. Tang, Integrated pyroelectric infrared sensor with a PZT thin film, Sens. Actuators, A 65 (1998) 171–174. [5] V. Ferrari, D. Marioli, A. Taroni, E. Ranucci, Multisensor array of mass microbalances for chemical detection based on resonant piezolayers of screen-printed PZT, Sens. Actuators, B 68 (2000) 81–87. [6] B. Morten, G. DeCicco, M. Prudenziati, Resonant pressure sensor based on piezoelectric properties of ferroelectric thick films, Sens. Actuators, A 31 (1992) 153–158. [7] S.P. Beeby, N. Ross, N.M. White, Thick film PZT micromachined silicon accelerometer, Electron. Lett. 35 (1999) 2060–2061.

73

[8] L. Capineri, V. Ferrari, F. Naldoni, L. Masotti, D. Marioli, A. Taroni, 3  3 matrix of thick-film pyroelectric transducer, Electron. Lett. 34 (1998) 1486–1487. [9] N.M. White, P.G. Jones, S.P. Beeby, A novel thick-film piezoelectric microgenerator, Smart Mater. Struct. 10 (2001) 850–852. [10] M. Koch, A.G.R. Evans, A. Brunnschweiler, The dynamic micropump driven with a screen printed PZT actuator, J. Micromech. Microeng. 8 (1998) 119–122. [11] C.P. Shaw, R.W. Whatmore, J.R. Alcock, Porous, functionally-gradient pyroelectric materials, J. Amer. Ceram. Soc. 90 (1) (2007) 137–142. [12] A. Seifert, P. Muralt, N. Setter, High figure-of-merit porous Pb1xCaxTiO3 thin films for pyroelectric applications, Appl. Phys. Lett. 72 (1998) 2409–2412. [13] S.P. Beeby, N. Ross, N.M. White, Thick film PZT/micromachined silicon accelerometer, Electron. Lett. 35 (1999) 2060–2061. [14] B. Malic, J. Cilensek, M. Mandeljc, M. Kosec, The adhesion phenomena in polypropylene/wollastonite composites, Acta Chim. Slov. 52 (2005) 259–263. [15] W.F.M. Groot Zevert, A.J.A. Winnubst, G.S.A. Theunissen, A.J. Burgraaf, Powder preparation and compaction behaviour of fine-grained Y–TZP, J. Mater. Sci. 25 (1990) 3449–3455. [16] Sidney B. Lang, Erling Ringgaard, Measurements of the thermal, dielectric, piezoelectric, pyroelectric and elastic properties of porous PZT samples, Appl. Phys. A 107 (2012) 631–638. [17] J.F. Fernandez, E. Nieto, C. Moure, P. Duran, R.E. Newnham, Processing and microstructure of porous and dense PZT thick films on Al2O3, J. Mater. Sci. 30 (1995) 5399–5404. [18] W. Wersing, K. Lubitz, J. Mohaupt, Dielectric, elastic and piezoelectric properties of porous PZT ceramics, Fer. 68 (1986) 77–97. [19] Z. Tao, D.X. Lin, M.C. Liang, L.R. Hong, Y. Hong, Research on effects of porosity and grain size on porous PZT ceramic dielectric and piezoelectric properties and its mechanism, Act. Phys. Sin. 55 (2006) 3074–3079. [20] H. Banno, Effects of shape and volume fraction of closed pores on dielectric, elastic, and electromechanical properties of dielectric and piezoelectric ceramics-a theoretical approach, Am. Ceram. Soc. Bull. 66 (1987) 1332–1337. [21] Haibo Zhang, Shenglin Jiang, Koji Kajiyoshi, Enhanced pyroelectric and piezoelectric figure of merit of porous Bi0.5(Na0.82K0.18)0.5TiO3 lead-free ferroelectric thick films, J. Am. Ceram. Soc. 93 (7) (2010) 1957–1964. [22] Y. Kuroda, Y. Koshiba, M. Misaki, K. Ishida, Y. Ueda, Pyroelectric response of submicron free-standing poly(vinylidene fluoride/trifluoroethylene) copolymer thin films, Appl. Phys. Exp. 6 (2013). 021601-021601-3. [23] R. Ciupa, A. Rogalski, Performance limitations of photon and thermal infrared detectors, Opto-Electr. Rev. 5 (4) (1997) 257–266. [24] S.T. Liu, D. Long, Pyroelectric detectors and materials, Proc. IEEE 66 (1) (1978) 14–26. [25] Z. Dong, D.G. Huang, D.Y. Zhang, Research of an Integrated LiTaO3 pyroelectric infrared fire detector, in: 2nd International Symposium on Advanced Optical Manufacturing and Testing Technologies, Adv. Opt. Manuf. Tech. 6149 (2006). [26] Pyreos Co., Ltd., Pyroelectric Infrared Sensor Datasheet PY-ITV-SINGLE– T046(3+1), 2012, p. 1. [27] R.A. Zakaria, NDIR Instrumentation design for CO2 gas sensing, PhD. Thesis, 2010, pp. 83–88. [28] PerkinElmer Co., Ltd., Infrared Sensing Technologies Catalog, 2009, p. 26. [29] Infratec Co., Ltd., Pyroelectric Infrared Sensor Datasheet LIM212, 2008, p. 1. [30] Norbert. Neumann, Reinhard. Köhler, Günter. Hofmann, Infrared sensor based on the monolithic structure Si–P(VDF/TrFE), Ferroelectrics 171 (1995) 225– 238.