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Piezoelectric and ferroelectric films for microelectronic applications
Materials Science and Engineering B99 (2003) 159 /163 www.elsevier.com/locate/mseb
Piezoelectric and ferroelectric films for microelectronic applications Tian-Ling Ren *, Hong-Jin Zhao, Li-Tian Liu, Zhi-Jian Li Institute of Microelectronics, Tsinghua University, Beijing 100084, China Received 14 June 2002; received in revised form 11 September 2002; accepted 21 October 2002
Abstract Piezoelectric and ferroelectric films are very promising materials for microelectronic applications. In this paper, some important issues for these materials and applications are reviewed, and recent progresses on integrated ferroelectrics have been given. The physical and chemical preparation methods of the silicon-based piezoelectric and ferroelectric films, such as Sol /Gel, sputtering, metal organic chemical vapor deposition, have been described and compared. To realize a microelectronic device, the integrated circuits compatible ferroelectric/piezoelectric etching method is very important. The wet-chemical etching methods and dry-etching methods, such as reactive ion etching, have been introduced. There are many important applications for the silicon-based piezoelectric and ferroelectric films. One is the micro-sensors or micro-actuators or micro-electro-mechanical-system. Another is the memory devices. Some typical devices using piezoelectric and ferroelectric films have been introduced. # 2002 Published by Elsevier Science B.V. Keywords: Ferroelectric; Piezoelectric; Thin film; Silicon; Integrated circuits compatible; Microelectronic device
1. Introduction Applications of the piezoelectric effect have expanded into many fields since the Curie brothers discovered this effect in 1880/1881. Since then, based on the type of piezoelectric materials, four stages of historical development may be identified. The representative materials for the four stages were respectively single crystal quartz, single-crystal Rochelle salt, barium titanate (BT) ceramics, and lead /zirconate /titanate (PZT) ceramics. Today, we have much more than several thousand piezoelectric and ferroelectric materials, including inorganic and organic materials. Piezoelectric and ferroelectric materials have been widely used as electronic materials from the very beginning. Quartz crystals were first used for underwater transducers during World War I, and then for quartz crystal oscillators. Rochelle salt was used for underwater transducers and phonograph pickups. BT ceramics were discovered at the end of World War II and were first used for underwater transducers, com-
* Corresponding author. Tel.: /86-10-6278-2712; fax: 86-10-62771130 E-mail address: [email protected] (T.-L. Ren).
munication devices, and dielectric components such as capacitors. PZT ceramics were discovered in 1954 and replaced BT ceramics in all fields of piezoelectric applications. PZT materials are the most widely used because of their high electromechanical coupling factor, good frequency /temperature characteristics, and suitable quality factor. During the last 30 years, a tremendous increase in the investigation of piezoelectric and ferroelectric materials took place [1 /23]. One of the most important reason for this is that piezoelectric and ferroelectric films are very promising materials for microelectronic applications. The cross-fertilization between research in piezoelectric/ferroelectric films and semiconductor materials and technology proved to be the key for the takeoff of integrated ferroelectrics. During the last 10 years advances in this field were made at an ever increasing pace. There are two key points for the microelectronic applications of the piezoelectric and ferroelectric films. First, the films should be silicon-based and with high quality. Second, the process technique including the preparation and etching method should be integrated circuits (IC) compatible. Many studies on these subjects have been done from several decades ago world-widely.
0921-5107/02/$ - see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 4 6 6 - X
160
T.-L. Ren et al. / Materials Science and Engineering B99 (2003) 159 /163
Although there are many problems and they are far from satisfactory until now, many kinds of microelectronic devices have been realized successfully based on piezoelectric and ferroelectric films. In this paper, some important issues of piezoelectric and ferroelectric films for microelectronic applications are reviewed.
2. Fabrication techniques of piezoelectric and ferroelectric films Preparation of the silicon-based piezoelectric and ferroelectric films is the first step to form microelectronic devices. Generally, the film fabrication involves three stages: (1) production of the appropriate atomic or molecular species from source materials; (2) their transport to the substrate through a medium by vacuum deposition, spraying, or dipping, and (3) condensation, diffusion and crystallization on the substrate via various solid state reactions. Depending upon the initial state, and the origin of the atomic and molecular specie forming the films, the techniques can be broadly classified into the categories of: (a) chemical (such as metal organic chemical vapor deposition (MOCVD) and Sol /Gel) and (b) physical methods (such as sputtering and pulsed laser deposition (PLD)). The starting materials used in the preparation of the ‘sol’ are usually inorganic metal salts or metal organic compounds such as metal alkoxides. In a typical sol /gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, or a ‘sol’. Thin films can be produced on a piece of substrate by spin-coating or dip-coating. When the ‘sol’ is cast into a mold, a wet ‘gel’ will form. With further drying and heat-treatment, the ‘gel’ is converted into dense ceramic or glass articles, and the silicon-based films can be formed. Sol/Gel fabrication of piezoelectric and ferroelectric thin films gains much interest and is widely adopted because of its simplicity, low processing temperature, well chemical homogeneity, easily stoichiometry control and its ability to produce uniform film over a large area that can provide integration with other circuit elements [15,16]. Gurkovitch et al. were the first to synthesize lead titanate, PZT and lead lanthanum zirconium titanate films using this method [8]. The low capital cost and simplicity of Sol /Gel method make it an excellent technique for formulating new ferroelectric compositions, and there is continuing progress towards improving the properties of thin films in the thickness range from 0.1 to several micrometers. MOCVD is currently paid more attention for piezoelectric and ferroelectric film preparations [8,13]. The advantages of MOCVD are (1) the opportunity to
deposit epitaxial thin film relatively easily because of the molecular reaction, lay-down, and incorporation into the crystal lattice at the surface, (2) homogeneous deposition over large areas for some of the reactor designs, (3) compatibility with the semiconductor processing, and (4) opportunity to achieve a high-quality step coverage even for 3D structures of high aspect ratio. This capability is unique to MOCVD compared to all other thin film deposition techniques. MOCVD method is now thought to be most promising way for high density ferroelectric memory applications. As for the physical preparation techniques, sputtering method has the advantages of homogeneous and large area deposition, and the disadvantage of low-quality of step coverage [11,12]. The PLD method is very convenient to prepare various piezoelectric and ferroelectric films for its very simple process, but it is very hard to form large area films [12,16]. To prepare thick piezoelectric and ferroelectric films with the thickness ranging from 1 to 100 micrometers on the silicon substrate, tape casting or thick film printing method can be used. The thick films have some applications on micro-actuators and other devices [24,25].
3. Etching techniques of piezoelectric and ferroelectric films To realize a microelectronic device, the IC compatible ferroelectric/piezoelectric etching method is very important. The wet-chemical etching can be used in some of the piezoelectric/ferroelectric sensor application, but the roughness is not so satisfactory indeed. Various dryetching processes including ion beam etching, plasma etching and reactive ion etching (RIE) have been studied to define patterns on the silicon-based piezoelectric and ferroelectric films. Great progresses have been made on the dry-etching techniques. As a typical example, Desu et al. has proposed a RIE characteristics of PZT using monochlortetrafluoroethane, HC2ClF4. This etching gas, a hydro chlorofluorocarbon (HCFC-124), is environmentally attractive due to its lower ozone depletion potential and lower global warming potential. The RIE of PZT is conducted in a self-made reactor chamber, as shown in Fig. 1. It is a standard parallel plate reactor system in which etchant gases (HCFC-124) are introduced into the bottom of the chamber and pumped out from the other side of the bottom. The substrate is positioned on a stainless steel cathode and is surrounded by a plastic annular ring to enhance etch uniformity. Positive photoresist is used as a mask to
T.-L. Ren et al. / Materials Science and Engineering B99 (2003) 159 /163
161
Fig. 1. Silicon-based PZT film prepared by Sol /Gel method.
Fig. 3. Top view and cross section of a piezoelectric film based accelerometer.
can be as high as 420 mV g 1, and the size of the device can be smaller than 1 mm2. Fig. 2. Schematic view of the RIE reactor chamber.
selectively protect the PZT regions to be preserved Fig. 2.
4. Applications of piezoelectric and ferroelectric films There are many important applications for the siliconbased piezoelectric and ferroelectric films. Generally, there are two kinds of applications. One is the microsensors or micro-actuators or micro-electro-mechanicalsystem (MEMS). For this application, the piezoelectric, pyroelectric and the high dielectric (high K) properties are often utilized. The second application is the memory devices. The high dielectric (high K) and remanent polarization properties are utilized to fabricate the ferroelectric based DRAM, FeFET, or FeRAM memory devices. Fig. 3 shows the top view and cross section of a piezoelectric ceramic film based accelerometer [26]. Principle of the device is as following: . The seismic mass exerts a force on the piezoelectric layer. . Charges generated due to the piezoelectric effect. . Generated charges are sensed by a preamplifier. The Sensitivity of the micro-accelerometer
Fig. 4 is a PZT ceramic film actuated micro-pump [27]. The device is fabricated using silicon micromachining hybrid technology. The pump rate can be higher than 900 ml min 1, and the maximum backpressure is 70 kPa at a driving voltage of 40 V. The micro-pump has very promising uses in medical, printer, and many other applications. With the worldwide booming interests in wireless communication, especially the personal communication system and Bluetooth, front-end RF filters that have low insert loss and wide bandwidth are of great demand. Thanks to the development of micro-machining and thin film deposition techniques, film bulk acoustic resonator (FBAR) is becoming a new approach towards RF filters of high quality [28 /31]. This resonator has the advantage of small size, especially in height, and the feasibility of system on a chip when made on a silicon substrate. PZT and PT are recently reported as the thin films in FBARs because they have much better piezoelectric property than ZnO and AlN. Especially their larger electromechanical coupling coefficient (K2t ) makes bandwidth more than 100 MHz possible. Fig. 5 is a radio frequency (RF) bulk acoustic wave (BAW) filter composed of several FBARs based on PZT films. The central frequency of the filter is about 1.8 GHz, and the bandwidth is about 120 MHz. As it is known that memory is the most important microelectronic device, and it has the biggest market of
162
T.-L. Ren et al. / Materials Science and Engineering B99 (2003) 159 /163
Fig. 4. PZT film actuated micro-pump.
Currently, and in the foreseeable future, NVFeRAM have the potential of importance three major markets in the $150B range [7]. First, as stand-alone memories, NVFeRAM have the potential to replace the exited FLASH, EEPROMs, DRAMs and SRAMs. In the second market, NVFeRAM is already enjoying a great position in cost and functionality. In this case, the fast write-speed and low power allows the use of smart cards or tags in a variety of applications such as ticketing, fare collections and inventory control. Fig. 6 is an 8 K bit contactless FeRAM IC card designed and fabricated by Tsinghua University and Panasonic Corporation. Finally, where the logic units is a microcontroller or digital signal processor , NVFeRAM already has shown interesting ‘system-on-a-chip’ capabilities without the
Fig. 5. RF BAW filter using PZT film.
all the microelectronic products. Ferroelectric thin film based memory, such as the non-volatile ferroelectric memory (NVFeRAM), is thought to be one of the most promising candidate for future memory applications [7,32/35]. Use of ferroelectric thin film with thickness of 100 nm yields NVFeRAM write of about 1 V. Writing speeds higher than 6 ns can be achieved at the capacitor level, even for capacitors with thick ferroelectric films (thicker than 180 nm) and large areas. For these devices, the speed is limited by the CMOSs, not by the intrinsic characteristics of the ferroelectric layer; NVFeRAM is the only fast write non-volatile memories existing today. This characteristic and the nearly fatigue free behavior with low power make NVFeRAM the mature evolution of CMOS in the non-volatile memory area.
Fig. 6. An 8 K bit contactless FeRAM IC card.
T.-L. Ren et al. / Materials Science and Engineering B99 (2003) 159 /163
added complications of power transistors such as in EEPROMs and FLASH.
5. Summary Silicon-based piezoelectric and ferroelectric films can be widely used in microelectronic devices. The IC process compatible preparation and etching methods are the key integration techniques for the applications of the piezoelectric/ferroelectric films. Sol/Gel, MOCVD, and sputtering are the usual preparation methods presently used, and MOCVD is thought to be the most promising one. Compared with wet etching, dryetching of the piezoelectric/ferroelectric films should be more suitable for microelectronic integration. Based on their prominent properties of piezoelectric/ferroelectric films, such as ferroelectric, dielectric, piezoelectric, pyroelectric, etc, various microelectronic devices can be realized. These novel MEMS or memory devices can be widely used in future electronic products.
Acknowledgements Thanks for the financial support by the National 973 Project (G1999033105) and the National ‘985’ Project of China.
References [1] K.R. Udayakumar, S.F. Bart, A.M. Flynn, J. Chen, L.S. Tavrow, L.E. Cross, R.A. Brooks, D.J. Ehrlich, Proc. IEEE-MEMS 14 (1991) 109 /113. [2] R.M. Moroney, R.M. White, R.T. Howe, Proc. IEEE-MEMS (1990) 182 /187. [3] P.R. Scheeper, A.G.H. Van der Donk, W. Olthuis, P. Berveld, Sens. Actuators A44 (1994) 1. [4] P.R. Scheeper, A.G.H. Van der Donk, W. Olthuis, P. Berveld, J. MEMS 1 (1992) 147. [5] S.S. Lee, R.P. Ried, R.M. White, J. MEMS 5 (1996) 238. [6] R.P. Ried, E.S. Kim, D.M. Hong, R.S. Muller, Proc. Micromech. Syst. (1992) 23 /32. [7] R. Ramesh, S. Aggarwal, O. Auciello, Mater. Sci. Eng. 32 (2001) 191.
163
[8] R. Waser, T. Schneller, S. Hoffmann-Eifert, P. Ehrhart, Integrated Ferroelectrics 36 (2001) 3. [9] C.A. Paz de Araujo, J.D. Cuchiaro, L.D. McMillan, M.C. Scott, J.F. Scott, Nature (London) 374 (1995) 627. [10] J.C. Crawford, F.L. English, IEEE Trans. Electron Devices ED16 (1969) 525. [11] H. Adachi, T. Mitsuyu, O. Yamazaki, K. Wasa, J. Appl. Phys. 60 (1986) 736. [12] (a) O. Auciello, K.D. Gifford, A.I. Kingon, Appl. Phys. Lett. 64 (1994) 2873; (b) O. Auciello, C.M. Foster, R. Ramesh, Annu. Rev. Mater. Sci. 28 (1998) 501. [13] K. Tominaga, M. Miyajima, Y. Sakashita, H. Segawa, M. Okada, Jpn J. Appl. Phys. Part 1 (29) (1990) L1874. [14] J.S. Horowitz, K.S. Grabowski, K.B. Chrisey, R.E. Leuchmer, Appl. Phys. Lett. 59 (1991) 1565. [15] R.W. Schwartz, T.J. Boyle, S.J. Lockwood, M.B. Sinclair, D. Dimos, C.D. Buchheit, Integr. Ferroelectr. 7 (1995) 259. [16] S.K. Dey, D.A. Payne, K.D. Budd, IEEE Trans. Ultrason., Ferroelectr., Freq. Contr. 35 (1988) 80. [17] G.F. Derbenwick, A.F. Isaacson, IEEE Circuits Devices 17 (2001) 20. [18] I.K. Yoo, S.B. Desu, Mater. Sci. Eng. B13 (1992) 319. [19] S.B. Majumder, S. Bhaskar, P.S. Dobal, R.S. Katiyar, Integr. Ferroelectr. 29 (2000) 87. [20] G. Velu, T. Haccart, D. Remiens, Integr. Ferroelectr. 23 (1999) 1. [21] J. Im, S.K. Streiffer, O. Auciello, A.R. Krauss, Appl. Phys. Lett. 77 (2000) 2593. [22] A.R. Krauss, O. Auciello, A.M. Dhote, et al., Integr. Ferroelectr. 32 (2001) 813. [23] J.F. Scott, C.A. Paz De Araudjo, Science 246 (1989) 1400. [24] Z.J. Li, T.L. Ren, L.T. Liu, Chin. J. Semiconductors 20 (1999) 177. [25] R. Maas, M. Koch, N.R. Harris, Mater. Lett. 31 (1997) 109. [26] Y. Nemirovsky, A. Nemirovsky, P. Muralt, Sens. Actuators A56 (1996) 239. [27] M. Koch, N. Harris, R. Maas, Meas. Sci. Technol. 8 (1997) 49. [28] N. Hanajima, S. Tsursumi, T. Yonezawa, K. Hashmoto, R. Nanjo, M. Yamaguchi, Jpn J. Appl. Phys. 36 (1997) 6069. [29] A. Yamada, C. Maeda, T. Umemura, F. Uchikawa, K. Misu, S. Wadaka, T. Ishikawa, Jpn J. Appl. Phys. 36 (1997) 6073. [30] J. Joseph Lutsky, S. Rajan Naik, Rafael Reif, Sodini, G. Charles, Proceedings of IEDM, 441 (1996). [31] T.L. Ren, Y.X. Liu, L.T. Liu, Z.J. Li, Integr. Ferroelectr. 41 (2001) 1725. [32] K. Kim, Integr. Ferroelectr. 36 (2001) 21. [33] J.T. Evans, R. Womack, IEEEJ. Solid-State Circuit 23 (1988) 1171. [34] Semiconductor World. Process Focus, vol. 17, No. 7, July 1998, pp. 85 /86. [35] Y.B. Chung, B.G. Jeon, K.D. Suh, IEEE J. Solid-State Circuit 35 (2000) 697.
Piezoelectric and ferroelectric films for microelectronic applications Tian-Ling Ren *, Hong-Jin Zhao, Li-Tian Liu, Zhi-Jian Li Institute of Microelectronics, Tsinghua University, Beijing 100084, China Received 14 June 2002; received in revised form 11 September 2002; accepted 21 October 2002
Abstract Piezoelectric and ferroelectric films are very promising materials for microelectronic applications. In this paper, some important issues for these materials and applications are reviewed, and recent progresses on integrated ferroelectrics have been given. The physical and chemical preparation methods of the silicon-based piezoelectric and ferroelectric films, such as Sol /Gel, sputtering, metal organic chemical vapor deposition, have been described and compared. To realize a microelectronic device, the integrated circuits compatible ferroelectric/piezoelectric etching method is very important. The wet-chemical etching methods and dry-etching methods, such as reactive ion etching, have been introduced. There are many important applications for the silicon-based piezoelectric and ferroelectric films. One is the micro-sensors or micro-actuators or micro-electro-mechanical-system. Another is the memory devices. Some typical devices using piezoelectric and ferroelectric films have been introduced. # 2002 Published by Elsevier Science B.V. Keywords: Ferroelectric; Piezoelectric; Thin film; Silicon; Integrated circuits compatible; Microelectronic device
1. Introduction Applications of the piezoelectric effect have expanded into many fields since the Curie brothers discovered this effect in 1880/1881. Since then, based on the type of piezoelectric materials, four stages of historical development may be identified. The representative materials for the four stages were respectively single crystal quartz, single-crystal Rochelle salt, barium titanate (BT) ceramics, and lead /zirconate /titanate (PZT) ceramics. Today, we have much more than several thousand piezoelectric and ferroelectric materials, including inorganic and organic materials. Piezoelectric and ferroelectric materials have been widely used as electronic materials from the very beginning. Quartz crystals were first used for underwater transducers during World War I, and then for quartz crystal oscillators. Rochelle salt was used for underwater transducers and phonograph pickups. BT ceramics were discovered at the end of World War II and were first used for underwater transducers, com-
* Corresponding author. Tel.: /86-10-6278-2712; fax: 86-10-62771130 E-mail address: [email protected] (T.-L. Ren).
munication devices, and dielectric components such as capacitors. PZT ceramics were discovered in 1954 and replaced BT ceramics in all fields of piezoelectric applications. PZT materials are the most widely used because of their high electromechanical coupling factor, good frequency /temperature characteristics, and suitable quality factor. During the last 30 years, a tremendous increase in the investigation of piezoelectric and ferroelectric materials took place [1 /23]. One of the most important reason for this is that piezoelectric and ferroelectric films are very promising materials for microelectronic applications. The cross-fertilization between research in piezoelectric/ferroelectric films and semiconductor materials and technology proved to be the key for the takeoff of integrated ferroelectrics. During the last 10 years advances in this field were made at an ever increasing pace. There are two key points for the microelectronic applications of the piezoelectric and ferroelectric films. First, the films should be silicon-based and with high quality. Second, the process technique including the preparation and etching method should be integrated circuits (IC) compatible. Many studies on these subjects have been done from several decades ago world-widely.
0921-5107/02/$ - see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 4 6 6 - X
160
T.-L. Ren et al. / Materials Science and Engineering B99 (2003) 159 /163
Although there are many problems and they are far from satisfactory until now, many kinds of microelectronic devices have been realized successfully based on piezoelectric and ferroelectric films. In this paper, some important issues of piezoelectric and ferroelectric films for microelectronic applications are reviewed.
2. Fabrication techniques of piezoelectric and ferroelectric films Preparation of the silicon-based piezoelectric and ferroelectric films is the first step to form microelectronic devices. Generally, the film fabrication involves three stages: (1) production of the appropriate atomic or molecular species from source materials; (2) their transport to the substrate through a medium by vacuum deposition, spraying, or dipping, and (3) condensation, diffusion and crystallization on the substrate via various solid state reactions. Depending upon the initial state, and the origin of the atomic and molecular specie forming the films, the techniques can be broadly classified into the categories of: (a) chemical (such as metal organic chemical vapor deposition (MOCVD) and Sol /Gel) and (b) physical methods (such as sputtering and pulsed laser deposition (PLD)). The starting materials used in the preparation of the ‘sol’ are usually inorganic metal salts or metal organic compounds such as metal alkoxides. In a typical sol /gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, or a ‘sol’. Thin films can be produced on a piece of substrate by spin-coating or dip-coating. When the ‘sol’ is cast into a mold, a wet ‘gel’ will form. With further drying and heat-treatment, the ‘gel’ is converted into dense ceramic or glass articles, and the silicon-based films can be formed. Sol/Gel fabrication of piezoelectric and ferroelectric thin films gains much interest and is widely adopted because of its simplicity, low processing temperature, well chemical homogeneity, easily stoichiometry control and its ability to produce uniform film over a large area that can provide integration with other circuit elements [15,16]. Gurkovitch et al. were the first to synthesize lead titanate, PZT and lead lanthanum zirconium titanate films using this method [8]. The low capital cost and simplicity of Sol /Gel method make it an excellent technique for formulating new ferroelectric compositions, and there is continuing progress towards improving the properties of thin films in the thickness range from 0.1 to several micrometers. MOCVD is currently paid more attention for piezoelectric and ferroelectric film preparations [8,13]. The advantages of MOCVD are (1) the opportunity to
deposit epitaxial thin film relatively easily because of the molecular reaction, lay-down, and incorporation into the crystal lattice at the surface, (2) homogeneous deposition over large areas for some of the reactor designs, (3) compatibility with the semiconductor processing, and (4) opportunity to achieve a high-quality step coverage even for 3D structures of high aspect ratio. This capability is unique to MOCVD compared to all other thin film deposition techniques. MOCVD method is now thought to be most promising way for high density ferroelectric memory applications. As for the physical preparation techniques, sputtering method has the advantages of homogeneous and large area deposition, and the disadvantage of low-quality of step coverage [11,12]. The PLD method is very convenient to prepare various piezoelectric and ferroelectric films for its very simple process, but it is very hard to form large area films [12,16]. To prepare thick piezoelectric and ferroelectric films with the thickness ranging from 1 to 100 micrometers on the silicon substrate, tape casting or thick film printing method can be used. The thick films have some applications on micro-actuators and other devices [24,25].
3. Etching techniques of piezoelectric and ferroelectric films To realize a microelectronic device, the IC compatible ferroelectric/piezoelectric etching method is very important. The wet-chemical etching can be used in some of the piezoelectric/ferroelectric sensor application, but the roughness is not so satisfactory indeed. Various dryetching processes including ion beam etching, plasma etching and reactive ion etching (RIE) have been studied to define patterns on the silicon-based piezoelectric and ferroelectric films. Great progresses have been made on the dry-etching techniques. As a typical example, Desu et al. has proposed a RIE characteristics of PZT using monochlortetrafluoroethane, HC2ClF4. This etching gas, a hydro chlorofluorocarbon (HCFC-124), is environmentally attractive due to its lower ozone depletion potential and lower global warming potential. The RIE of PZT is conducted in a self-made reactor chamber, as shown in Fig. 1. It is a standard parallel plate reactor system in which etchant gases (HCFC-124) are introduced into the bottom of the chamber and pumped out from the other side of the bottom. The substrate is positioned on a stainless steel cathode and is surrounded by a plastic annular ring to enhance etch uniformity. Positive photoresist is used as a mask to
T.-L. Ren et al. / Materials Science and Engineering B99 (2003) 159 /163
161
Fig. 1. Silicon-based PZT film prepared by Sol /Gel method.
Fig. 3. Top view and cross section of a piezoelectric film based accelerometer.
can be as high as 420 mV g 1, and the size of the device can be smaller than 1 mm2. Fig. 2. Schematic view of the RIE reactor chamber.
selectively protect the PZT regions to be preserved Fig. 2.
4. Applications of piezoelectric and ferroelectric films There are many important applications for the siliconbased piezoelectric and ferroelectric films. Generally, there are two kinds of applications. One is the microsensors or micro-actuators or micro-electro-mechanicalsystem (MEMS). For this application, the piezoelectric, pyroelectric and the high dielectric (high K) properties are often utilized. The second application is the memory devices. The high dielectric (high K) and remanent polarization properties are utilized to fabricate the ferroelectric based DRAM, FeFET, or FeRAM memory devices. Fig. 3 shows the top view and cross section of a piezoelectric ceramic film based accelerometer [26]. Principle of the device is as following: . The seismic mass exerts a force on the piezoelectric layer. . Charges generated due to the piezoelectric effect. . Generated charges are sensed by a preamplifier. The Sensitivity of the micro-accelerometer
Fig. 4 is a PZT ceramic film actuated micro-pump [27]. The device is fabricated using silicon micromachining hybrid technology. The pump rate can be higher than 900 ml min 1, and the maximum backpressure is 70 kPa at a driving voltage of 40 V. The micro-pump has very promising uses in medical, printer, and many other applications. With the worldwide booming interests in wireless communication, especially the personal communication system and Bluetooth, front-end RF filters that have low insert loss and wide bandwidth are of great demand. Thanks to the development of micro-machining and thin film deposition techniques, film bulk acoustic resonator (FBAR) is becoming a new approach towards RF filters of high quality [28 /31]. This resonator has the advantage of small size, especially in height, and the feasibility of system on a chip when made on a silicon substrate. PZT and PT are recently reported as the thin films in FBARs because they have much better piezoelectric property than ZnO and AlN. Especially their larger electromechanical coupling coefficient (K2t ) makes bandwidth more than 100 MHz possible. Fig. 5 is a radio frequency (RF) bulk acoustic wave (BAW) filter composed of several FBARs based on PZT films. The central frequency of the filter is about 1.8 GHz, and the bandwidth is about 120 MHz. As it is known that memory is the most important microelectronic device, and it has the biggest market of
162
T.-L. Ren et al. / Materials Science and Engineering B99 (2003) 159 /163
Fig. 4. PZT film actuated micro-pump.
Currently, and in the foreseeable future, NVFeRAM have the potential of importance three major markets in the $150B range [7]. First, as stand-alone memories, NVFeRAM have the potential to replace the exited FLASH, EEPROMs, DRAMs and SRAMs. In the second market, NVFeRAM is already enjoying a great position in cost and functionality. In this case, the fast write-speed and low power allows the use of smart cards or tags in a variety of applications such as ticketing, fare collections and inventory control. Fig. 6 is an 8 K bit contactless FeRAM IC card designed and fabricated by Tsinghua University and Panasonic Corporation. Finally, where the logic units is a microcontroller or digital signal processor , NVFeRAM already has shown interesting ‘system-on-a-chip’ capabilities without the
Fig. 5. RF BAW filter using PZT film.
all the microelectronic products. Ferroelectric thin film based memory, such as the non-volatile ferroelectric memory (NVFeRAM), is thought to be one of the most promising candidate for future memory applications [7,32/35]. Use of ferroelectric thin film with thickness of 100 nm yields NVFeRAM write of about 1 V. Writing speeds higher than 6 ns can be achieved at the capacitor level, even for capacitors with thick ferroelectric films (thicker than 180 nm) and large areas. For these devices, the speed is limited by the CMOSs, not by the intrinsic characteristics of the ferroelectric layer; NVFeRAM is the only fast write non-volatile memories existing today. This characteristic and the nearly fatigue free behavior with low power make NVFeRAM the mature evolution of CMOS in the non-volatile memory area.
Fig. 6. An 8 K bit contactless FeRAM IC card.
T.-L. Ren et al. / Materials Science and Engineering B99 (2003) 159 /163
added complications of power transistors such as in EEPROMs and FLASH.
5. Summary Silicon-based piezoelectric and ferroelectric films can be widely used in microelectronic devices. The IC process compatible preparation and etching methods are the key integration techniques for the applications of the piezoelectric/ferroelectric films. Sol/Gel, MOCVD, and sputtering are the usual preparation methods presently used, and MOCVD is thought to be the most promising one. Compared with wet etching, dryetching of the piezoelectric/ferroelectric films should be more suitable for microelectronic integration. Based on their prominent properties of piezoelectric/ferroelectric films, such as ferroelectric, dielectric, piezoelectric, pyroelectric, etc, various microelectronic devices can be realized. These novel MEMS or memory devices can be widely used in future electronic products.
Acknowledgements Thanks for the financial support by the National 973 Project (G1999033105) and the National ‘985’ Project of China.
References [1] K.R. Udayakumar, S.F. Bart, A.M. Flynn, J. Chen, L.S. Tavrow, L.E. Cross, R.A. Brooks, D.J. Ehrlich, Proc. IEEE-MEMS 14 (1991) 109 /113. [2] R.M. Moroney, R.M. White, R.T. Howe, Proc. IEEE-MEMS (1990) 182 /187. [3] P.R. Scheeper, A.G.H. Van der Donk, W. Olthuis, P. Berveld, Sens. Actuators A44 (1994) 1. [4] P.R. Scheeper, A.G.H. Van der Donk, W. Olthuis, P. Berveld, J. MEMS 1 (1992) 147. [5] S.S. Lee, R.P. Ried, R.M. White, J. MEMS 5 (1996) 238. [6] R.P. Ried, E.S. Kim, D.M. Hong, R.S. Muller, Proc. Micromech. Syst. (1992) 23 /32. [7] R. Ramesh, S. Aggarwal, O. Auciello, Mater. Sci. Eng. 32 (2001) 191.
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