Hong-Jin Zhao

Piezoelectric and ferroelectric films for microelectronic applications

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.

Preparation and properties of PLZT thick films on silicon

Abstract Lead-lanthanum-zirconate-titanate (PLZT) thick films were prepared on Pt/Ti/SiO 2 /Si substrates with PZT/PT seeding layer by the screen-printing method. Phase characterization and crystal orientation of the PLZT thick films were investigated by X-ray diffraction analysis (XRD). The ferroelectric hysteresis loop, high-frequency dielectric constant, dielectric loss and piezoelectric constant of the PLZT thick films were measured. The remnant polarization of the silicon-based PLZT thick films was about 32 μC cm −2 , the coercive field was about 20 kV cm −1 and the piezoelectric constant d 33 was about 630 pC N −1 . In the frequency range from 1 to 300 MHz, the dielectric constant was about 3000 and the dielectric loss was less than 0.03, respectively. The PLZT thick films with excellent ferroelectric, high frequency and force–electric coupling properties should be suitable for the ferroelectrics–silicon integrated system, and be a good candidate material for the third-generation (3G) mobile communication and the force–electric coupling microelectromechanical system (MEMS) device applications.

Characteristics of silicon-based BaxSr1-xTiO3 thin films prepared by a sol-gel method

Silicon-based BaxSr1-xTiO3 (BST) thin films have been prepared by a sol-gel method with rapid thermal annealing (RTA) processes. Phase structure of the films has been investigated by x-ray diffraction. Atomic force microscopy studies reveal a dense and smooth surface of the sol-gel prepared films. Microstructure and electrical properties of the BST films can be affected by the substrate and the annealing process. RTA method is found to be very efficient to improve the electrical properties of the films. Dielectric constant and dielectric loss of the BST films at 100 kHz are 230 and 0.02, respectively. Leakage current density of the BST capacitors is 1.6×10-7A cm-2 at 3 V.

High-frequency properties of PZT for RF-communication applications

Abstract Lead–zirconate–titanate (PbZr 0.53 Ti 0.47 O 3 ) nano powders and ceramics were prepared using a sol–gel method. Phase characterization of the lead–zirconate–titanate (PZT) material was identified by X-ray diffraction analysis (XRD). Micro-structure of the samples was examined by the scanning electron microscopy (SEM). Ferroelectric hysteresis loop, high-frequency dielectric response and piezoelectric constant d 33 were measured. The remnant polarization ( P r ) of the PZT ceramics pellet is about 20 μC cm −2 and the coercive field ( E C ) is about 30 kV cm −1 . The dielectric constant is about 900 and the dielectric loss is lower than 0.02 in the radio-frequency (RF) region. The piezoelectric constant d 33 is about 170 pC N −1 . The experimental results show that the PZT material can be suitable for the applications of miniature RF front-end devices and force-electric coupling devices.

Fabrication of high-quality PZT-based piezoelectric microphone

This paper proposes a novel lead-zirconate-titanate (PZT)-based structure used for the integrated microphone. The performance of the PZT thin films and the devices were improved due to adoption of the lead-titanate (PT) layer as the seeding layer. The PZT-based piezoelectric microphones with conductor-piezoelectric-conductor sandwich structure were fabricated. The size of the finished microphone ranged from 600/spl times/600 /spl mu/ m/sup 2/ to 1000/spl times/1000 /spl mu/ m/sup 2/. A high sensitivity of 38 mV/Pa can be obtained. This quality was prominent in the existed microphones. The frequency response of the microphone was very flat in the audio frequency range. The novel piezoelectric microphones should be very promising for acoustic applications.

High quality PZT thick films using silicon mold technique for MEMS applications

The present paper proposes a simple fabrication technique of high quality PZT thick films that call for silicon mold technique. The fabrication process adopts the silicon with back window obtained from silicon anisotropic etching as the silicon mold, and the improved PZT sol is dispensed in the silicon mold. The PZT films obtained using single spin coating with the thickness of l00 /spl mu/ m or higher, which is over the depth of back window of the silicon substrate, are crack-free and have good morphology. The PZT films with perfect perovskite structure have excellent piezoelectric property and the d/sub 33/ is about 170pC/N. Ferroelectric hysteresis loops are measured, and the remnant polarization (P/sub r/) of the PZT ceramics pellet is about 25 /spl mu/C/cm/sup 2/ and the coercive field (E/sub C/) is about 27kV/cm. In the radio-frequency (RF) region, the dielectric constant is about 350 and the dielectric loss is less than 0.01.

Microstructure and Electrical Properties of (Ba,Sr)TiO 3 Thin Films Prepared by a Sol-Gel Method

Silicon-based Ba x Sr 1 m x TiO 3 (BST) thin films have been prepared by a Sol-Gel method with rapid thermal annealing (RTA) processes. Phase structure of the films has been investigated by X-ray diffraction (XRD). Atomic force microscopy (AFM) studies reveal a smooth, dense and crack-free surface of the BST films. Microstructure and electrical properties of the BST films can be affected by the substrate and the annealing process. RTA method is found to be very efficient to improve the electrical properties of the films. Dielectric constant and dielectric loss of the BST films at 100 KHz are 230 and 0.02, respectively. Leakage current density of the BST capacitors is 1.6 2 10 m 7 A/cm 2 at 3V.

Modeling and Simulation of Thin-Film Bulk Acoustic Resonators

The PZT-based thin film bulk acoustic resonator (FBAR) structure had been simulated and optimized. The results revealed that the resonant frequency of the FBAR can be controlled by adjusting the mono-layer thickness of the sandwich structure. Dependence of resonant frequency upon the thickness of the piezoelectric film was obtained. The relation of the buffer layer thickness and the quality factor (Q value) had been investigated. The structure of a ladder-type FBAR filter with the center frequency of 2 GHz and the 3-dB bandwidth of 100MHz had been designed. The CPW for the FBAR with characteristic impedance of 50 z was optimized.

Preparation and Etching of Silicon-Based Piezoelectric Thin Films for Integrated Devices

Lead-zirconate-titanate (PZT) and ZnO thin films on silicon substrates were prepared by a sol-gel method. Phase characterization and crystal orientation of the films were investigated by X-ray diffraction analysis (XRD). It was shown that the PZT thin films had a perfect perovskite structure after annealed at a low temperature of 600 , and the ZnO thin films were well crystallized with (002) preferred orientation at a low annealing temperature of 400 . PZT thin films were chemically etched using HCl/HF solution through typical semiconductor lithographic process, and ZnO thin films were wet etched using acidic and alkaline etchants. The etching conditions were optimized. The etching precision can be higher than 1 w m.

Fabrication of (100) Orientated PZT Thin Films for MEMS Applications

High quality silicon-based PZT thin films were prepared using an improved sol-gel process. PT thin film was adopted as the seeding layer. The fabrication technique and the growth mechanism of the PZT thin films crystallized with (100) preferred orientation were studied through changing the pre-annealing temperatures, the annealing temperatures and the thickness of the PT seeding layer. It was shown that the PZT thin films with the suitable seeding layer had a more highly (100) preferred orientation. The dielectric response and dielectric loss of the PZT thin films were also measured. In the radio-frequency (RF) region, the dielectric constant was about 150 and the dielectric loss was less than 0.01, respectively.