PbTiO3 thin films for pyroelectric application

Thin Solid Films 371 Ž2000. 254᎐258

Preparation and properties of multilayer PbŽ Zr,Ti. O3rPbTiO3 thin films for pyroelectric application Weiguo LiuU , Jong Soo Ko, Weiguang Zhu Microelectronics Center, School of Electrical and Electronic Engineering, Nanyang Technological Uni¨ ersity, Singapore 639798 Singapore Received 4 January 2000; received in revised form 10 April 2000; accepted 25 April 2000

Abstract To develop a high performance pyroelectric infrared ŽIR. detector, Pb1.1ŽZr0.3Ti0.7 .O3rPbTiO3 ŽPZTrPT. multilayer thin films were deposited onto the top of a PtrTirSi3 N4rSiO2 membrane by a modified sol-gel process. For the comparison purpose, Pb1.1ŽZr0.3Ti0.7 .O3 ŽPZT. thin films were also prepared with the identical method under same conditions. X-Ray diffraction measurement revealed that the diffraction pattern of the multilayer film was the superimposition of the PZT and PT patterns. At 1 kHz, dielectric constant of 389 and 558, dielectric loss of 1.2 and 1.1% were obtained, respectively, for the PZTrPT and PZT thin films. The PZTrPT film showed a lower dielectric constant as expected and a similar dielectric loss compared with those of the PZT film, which is beneficial to use the multilayer thin films as the pyroelectric IR detecting element. Pyroelectric coefficients for the PZTrPT film and the PZT film were correspondingly 380 and 400 ␮Crm2 K. Calculated detectivity figures of merit for the PZTrPT and PZT thin films were 20.3= 10y6 Pay1r2 , and 18.7= 10y6 Pay1r2 , and values of the voltage response figures of merit were 0.038 m2rC and 0.028 m2rC, respectively. At 20 Hz, the dynamic pyroelectric voltage responsivity of 132 VrW Žin rms. was obtained for the PZTrPT film and 98 VrW Žin rms. for PZT film with the same element size of 240 = 360 ␮m2. High response of the multilayer thin film was ascribed to its relatively lower dielectric constant when compared to the PZT thin films. Experimental results showed the PZTrPT multilayer thin film is a good candidate material for developing high performance IR detectors. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Multilayers; Pyroelectricity; Dielectric properties; Sensors

1. Introduction Lead zirconate titanate ŽPZT. thin films have been extensively studied for their interesting ferroelectric, piezoelectric and pyroelectric properties w1,2x. To develop high performance pyroelectric infrared detectors, efforts have been made to improve the desired proper-

U

Corresponding author. Sensors and Actuators Lab., S1, School of EEE, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore. Tel.: q65-790-6923; fax: q65-791-2687. E-mail address: [email protected] ŽW. Liu..

ties of PZT thin films. The sol-gel technique is one of the most important methods in preparing high quality PZT thin films w3x. The properties of the pyroelectric IR detectors made of pyroelectric PZT thin films can be described by three major ‘figure of merit ŽFOM.’, as current response FOM Fi , voltage response FOM Fv , and detectivity FOM FD w4x. For high performance pyroelectric detectors, high FOMs are required, and they are related to the material parameters of the PZT thin films. Among the material parameters, dielectric constant and dielectric loss are of the most importance. An effective way to improve the FOMs is to reduce the dielectric loss by controlling the microstructure of the

0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 0 1 2 - 9

W. Liu et al. r Thin Solid Films 371 (2000) 254᎐258

PZT thin films. Contemporarily, the sol-gel technique has left little room for the improvement of the crystalline property of the thin films, thus the reduction of the dielectric loss. Other measures have been taken to improve the FOMs. By using low dielectric constant material, for example, the Fv and FD of the pyroelectric detectors can be increased accordingly. But unfortunately, low dielectric constant materials are generally not suitable for the developing of IR array detectors. In the array, the size of the detector element is very small and the capacitance of each element is small too, thus it becomes very difficult for matching it with the input capacitance of the signal processing circuit. This conflict between the high figure of merit for high performance and a high dielectric constant for high output capacitance seems inevitable, it has to be compromised, therefore, thin film with moderate dielectric constant seems to be desirable. In the PZT family thin films, the film with a ZrrTi ratio of 30:70 shows higher pyroelectric properties w5x, and its relative dielectric constant is approximately 550, which is relatively high for getting higher FOM. On the other hand, the relative dielectric constant for a PT thin film is approximately 260 w6x, which is beneficial to get a high FOM, but the output capacitance of small detector element made of the PT thin film is too small as the size of the element is scaled down to several tens of micrometers. Recent studies revealed that multilayer PZT and PT structure showed improved dielectric and ferroelectric properties, where, very thin PT layers were used as seeding interlayers. Low dielectric loss was found in the stacking PZTrPT structure w7x, which is an important feature for the films to be used as a pyroelectric IR detector. The insertion of a thin PT layer also improved the crystalline property of the PZT thin films w8x. Improvement of the electric fatigue property of PZTrPbZrO3 multilayer thin films w9x implies that the multilayer structure is a good choice in controlling a specific property of ferroelectric thin films. By combining the PZT and PT thin films ‘in serial’, the effective dielectric constant of the stacked PZT and PT thin film can be adjusted and controllable. This leads to the idea in this paper to develop pyroelectric IR detectors using the multilayered PZT and PT thin films. The objectives of the present study are to investigate the preparation and the dielectric, ferroelectric, and especially the pyroelectric properties of the PZTrPT multilayer thin films, and to search the possible application of this multilayer thin film in developing high performance IR detectors. 2. Experimental The PZT thin film with the mole ratio of ZrrTis 30:70 was chosen in this study. The PZTrPT multilayer thin films and PZT thin films were prepared using a

255

modified sol-gel process, in which, solid precursors of PZT and PT were prepared first, and then the precursor was dissolved in 2-methoxyethanol Ž2-MOE. to form the sols for the spin-coating deposition. Chemicals used in the experiments were lead acetate trihydrate ŽRiedel-de Haen ¨ ., zirconium acetylacetonate ŽFluka Scientific., titanium isopropoxide ŽSigmaAldrich., and acetylacetone ŽFluka Scientific.. Solid precursor preparing processes were: Ž1. chelating the titanium isopropoxide with acetylacetone in a mole ratio of 1:1 by heating and stirring the mixture at 80⬚C for 10 min; Ž2. dissolving lead acetate trihydrate into the stabilized titanium isopropoxide at 120⬚C with continuous stirring for 10 min, excess lead acetate trihydrate Ž10%. was added to compensate the possible lead loss during the heat treatment of the thin films; Ž3. dissolving zirconium acetylacetonate into the mixture of Ti and Pb compounds at 110⬚C with continuous stirring for 30 min. In preparing the PT precursor, this step was omitted. A light yellow clear sticky solution resulted in this step; and Ž4. removing volatile products under reduced pressure at 80⬚C for more than 4 h. Light yellow porous Pb1.1ŽZr0.3Ti0.7 .O3 ŽPZT. precursor powder and Pb1.1TiO3 ŽPT. precursor powder were finally obtained. The powder precursors were insensitive to moisture, offering the sol-gel process to be carried-out in a less critical environment, and ensuring the repeatability in preparing PZT and PT thin films. The PZT and PT powders were dissolved in 2-MOE in weight percentage of 20% at 124⬚C by stirring for 30 min. The sols were filtered with a 0.1-␮m micro-pore filter before spin-coating. Substrates used in the experiment were PtrTirSi3 N4rSiO2rSiŽ100.. The thickness of each layer in the substrate was 100 nm, 50 nm, 200 nm, 500 nm, and 450 ␮m, respectively. The spincoating deposition was carried-out at 3000 rev.rmin for 30 s for each layer. Multilayer PZTrPT thin films were prepared by alternatively depositing PZT and PT layers for five times, and finally a PZT layer was deposited with the PZT as the first and the final layer in the multilayer thin film. In addition, PZT thin films alone were also prepared by repeating the spin-coating steps for 11 times. The fired thickness of each layer of PZT or PT thin film in one spin-coating step was approximately 50 nm, thus, total thickness for both the PZTrPT and the PZT thin film were approximately the same as 550 nm. After each spin-coating step, the wet PZT Žor PT. thin film was baked at 400⬚C on a hot plate for 2 min. The resultant films were annealed at 600⬚C for 30 min in a furnace to crystallize the films. The standard lithography process was adopted to pattern the PZTrPT and PZT thin films. HCl Ž50%. aqueous solution with 1% HF as the additive was used as the PZTrPT etchant to pattern the sensing elements. The size of each sensing element was 240 = 360 ␮m2. A 10-nm thick nickel layer was deposited on the

W. Liu et al. r Thin Solid Films 371 (2000) 254᎐258

256

Fig. 1. Cross-section of the PZTrPT or PZT thin film element.

top of the sensing elements to serve as the top electrode as well as the radiant absorption layer. After all the above processes, the PZTrPT and the PZT thin films were protected and a KOH etching process was used to etch off the backside silicon substrate under the sensing area to form a PtrTirSi3 N4rSiO2 membrane. The final structure of the element for the experiment is depicted in Fig. 1. Hereafter, the Pb1.1ŽZr0.3Ti0.7 .O3rPbTiO3 thin film is denoted as PZTrPT, and the Pb1.1ŽZr0.3Ti0.7 .O3 thin film is denoted as PZT. A Rigaku Ultima q X-ray diffractometer and a HP 4284A precision LCR meter were used to analyze the structure development and the dielectric properties of the PZTrPT and PZT thin films. Radiant RT-66A standardized ferroelectric tester was used to characterize the D᎐E hysteresis loops and the polarization property of the PZTrPT and PZT thin films. The Byer᎐Roundy method was used to measure the pyroelectric coefficient of the films w10x, and the dynamic pyroelectric responses of the fabricated thin films were recorded with a Chynoweth system w11x. 3. Results and discussion 3.1. XRD results Fig. 2 shows the XRD diffraction patterns of the PZTrPT and PZT thin films. It can be seen that both

Fig. 2. XRD patterns of the PZTrPT, PZT, and PT thin films.

Fig. 3. Dielectric constant and dielectric loss of: Ža. PZTrPT thin film; and Žb. PZT thin film.

films have their pure perovskite structure. The tetragonal structure is shown in the PZTrPT multilayer films, and it is the result of the superimposition of the diffraction peaks from PZT and PT in the films. For comparison, the XRD pattern of the PT film prepared under identical conditions is also shown in the figure. All the films show random crystalline orientation under present preparation conditions. 3.2. Dielectric properties The dielectric constants and the dielectric losses of the PZTrPT and PZT thin films before and after poling are shown in Fig. 3. For both PZTrPT and PZT thin films, lower dielectric constant and dielectric loss are found in the poled thin films. The PZTrPT thin film shows a lower relative dielectric constant than that of the PZT thin film. At 1 kHz, the relative dielectric constants and dielectric losses are 389, 558 and 1.2%, 1.1% for the PZTrPT and PZT thin film, respectively. The low dielectric constant of PZTrPT thin films can be well described by considering the films to be composed of PZT and PT layers in serial. It was reported that the dielectric constant of the PT film prepared with the sol-gel method is approximately 260 w6x. The calculated dielectric constant of the 11 layered PZTrPT film is approximately 360, which is in good agreement with our experiment result. It is obvious that the dielectric constant of the multilayer thin films can be easily controlled by adjusting the relative thickness of PZT

W. Liu et al. r Thin Solid Films 371 (2000) 254᎐258

257

Fig. 5. Pyroelectric coefficient of the PZTrPT and PZT thin films.

3.4. Pyroelectric properties

Fig. 4. Hysteresis loops of: Ža. PZTrPT thin film; and Žb. PZT thin film.

and PT layers in this multilayer structure. Dielectric losses of the PZTrPT and PZT thin films are almost the same, and it is a very important feature in improving the FOMs of the multilayer thin films. 3.3. Ferroelectric properties Fig. 4a,b shows the hysteresis loops of the PZTrPT and PZT thin films. The difficulty in poling the PZTrPT film due to the low dielectric constant of PT decreases the remnant polarization of the PZTrPT film when the film is poled at a low electric field. In this case, the applied external field is mainly dropped at the PT layers, which decreases the effective applied field at the PZT layers. It can be clearly seen from Fig. 4a that when the PZTrPT thin film is poled at a low electric field, its hysteresis loop shows a ‘narrow waist’ shape. A higher electric field is therefore required to pole the PZTrPT film to reach its saturated polarization when compared to the PZT thin film case.

The pyroelectric coefficient of these films was tested using a Byer᎐Roundy method. The Linkam hotstage with its temperature controller was utilized to ramp the temperature of the films at 8⬚Crmin. The pyroelectric current was recorded with a HP4155B semiconductor parameter analyzer. Results of the tested pyroelectric coefficients within a temperature range from 40 to 100⬚C of the PZTrPT and PZT thin films poled at 400 KVrcm are shown in Fig. 5. It can be seen that the pyroelectric coefficients of the PZTrPT and PZT thin films near room temperature are approximately 380 and 400 ␮Crm2 K, respectively. In Fig. 5, it can be seen that when the temperature is higher than 60⬚C, a higher pyroelectric coefficient is observed in the PZTrPT thin film compared with the PZT one. The reason is not clear at the present time, but the interfaces at the multilayered thin films may contribute to this phenomenon. The FOMs of the PZTrPT thin films and PZT thin films can be calculated from the dielectric properties and pyroelectric coefficients of the films. Parameters used in the calculation and the calculated results are shown in Table 1. Although the pyroelectric coefficient of the PZTrPT thin film is lower than that of the PZT thin film, for both poled at the same electric field, the voltage response FOM of the PZTrPT thin film is higher than that of the PZT thin film because of its lower dielectric constant. The detectivity FOM of the multilayer PZTrPT thin film is also higher than that of the PZT thin film.

Table 1 Calculated figures of merit and the parameters used in the calculation Material

Pyroelectric coefficient p⬘ Ž␮Crm2 K.

Relative dielectric constant ␧ r

Dielectric loss tg ␦

Volume heat capacity c⬘ Ž10 6 Jrm3 K.

Fv s p⬘rc⬘␧ Žm2rC.

FD s p⬘rc⬘Ž ␧ tg ␦ .1r2 Ž10y6 Pay1r2 .

PZT PZTrPT

400 380

558 389

0.011 0.012

2.9 2.9

0.028 0.038

18.7 20.3

258

W. Liu et al. r Thin Solid Films 371 (2000) 254᎐258

determined by a spectrometer. At 5 Hz, the dynamic pyroelectric voltage responsivity of 132 VrW Žin rms. is obtained for the PZTrPT film with element size of 240 = 360 ␮m2 , and 98 VrW Žin rms. for the PZT film with the same element area. From the waveforms of the voltage responses, the dynamic pyroelectric voltage responsivity of 480 VrW from peak to peak is obtained for the PZTrPT films at 5 Hz, and 265 VrW from peak to peak for the PZT films. Although the PZTrPT thin films have lower pyroelectric, they yield a higher voltage response than that of PZT thin films due to their lower dielectric constant. 4. Conclusions

Fig. 6. Ža. Waveforms of the voltage response of the PZTrPT and PZT thin films; and Žb. voltage responsivity of the PZTrPT and PZT thin films.

One of the most interesting properties of the pyroelectric detectors is the dynamic pyroelectric response of the thin films under radiation. To demonstrate the potential applications of these multilayer thin films for IR detectors, both the PZTrPT and PZT thin film elements were fabricated on top of the Si3 N4rSiO2 membrane. The dynamic pyroelectric voltage response of the PZTrPT and PZT thin films to 632.8-nm laser radiation was characterized using a Chynoweth system. The system was composed of a 10-mW He᎐Ne laser, a SR540 mechanical chopper, a SR560 low-noise preamplifier, a HP35670A dynamic signal analyzer, and a SR850 lock-in amplifier. Waveforms of the voltage responses and the voltage responsivity of the PZTrPT and PZT thin films were recorded by the system. Results are shown in Fig. 6a,b. The effective absorption of the Ni on PZTrPT or on the PZT thin film to the 632.8-nm laser radiation is approximately 0.7, which is

PZTrPT multilayer thin films were prepared and characterized for their possible application as pyroelectric IR detectors. High voltage response and detectivity figures of merit were obtained from the multilayer PZTrPT thin films, and high dynamic pyroelectric response to 632.8 nm laser radiation was obtained as well. Combined with the micromachining process, multilayer PZTrPT thin films were shown to be good candidate materials for developing high performance pyroelectric IR detectors. References w1x H.D. Chen, K.R. Udayakumar, C.J. Gaskey, L.E. Cross, Appl. Phys. Lett. 67 Ž1995. 3411. w2x M. Kohli, C. Wuethrich, K. Brooks, B. Willing, M. Forster, P. Muralt, N. Setter, P. Ryser, Sensors Actuators A60 Ž1997. 147. w3x B.A. Tuttle, R.W. Schwartz, MRS Bull. ŽJune 1996. 49. w4x R.W. Whatmore, Rep. Prog. Phys. 49 Ž1986. 1335. w5x A. Patel, J.S. Obhi, GECJ. Res. 12 Ž1995. 141. w6x M. Kohli, Y. Huang, T. Maeder, C. Wuethrich, A. Bell, P. Muralt, N. Setter, P. Ryser, M. Forster, Microelectron. Eng. 29 Ž1995. 93. w7x K.H. Yoon, J.H. Shin, J.H. Park, D.H. Kang, J. Appl. Phys. 83 Ž1998. 3626. w8x K. Ishikawa, K. Sakura, D. Fu, S. Yamada, H. Suzuki, T. Hayashi, Jpn. J. Appl. Phys. 37 Ž1998. 5128. w9x J. Hyun Jang, K. Hyun Yoon, Appl. Phys. Lett. 75 Ž1999. 130. w10x R.L. Byer, C.B. Roundy, Ferroelectrics 3 Ž1972. 333. w11x W.G. Liu, Q. Kang, L.Y. Zhang, X. Yao, Ferroelectrics 154 Ž1994. 313.