Improvement of the crystallinity of ZnO thin films and frequency characteristics of a film bulk acoustic wave resonator by using an Ru buffer layer and annealing treatment

https://doi.org/10.1016/j.spmi.2005.08.066Get rights and content

Abstract

We deposited zinc oxide (ZnO) thin films on an Ru buffer layer in order to protect the amorphous layer at the ZnO and Al interface. Also, ZnO thin films grown by means of an annealing treatment were investigated as regards improvement of the c-axis orientation and morphology properties. In the X-ray diffraction (XRD) pattern, it was observed that there was an improvement of the (002) orientation achieved by the variation of the annealing treatment temperature. Also, the surface roughness and specific resistance were increased by the annealing treatment but the full width at half-maximum (FWHM) was decreased. For film bulk acoustic resonators (FBARs) fabricated using these results, we finally confirmed that a resonant frequency of 0.79 GHz, without a shift, was measured. In addition, the values of the return loss were improved by the annealing treatment.

Introduction

In recent years, zinc oxide (ZnO) thin films have been extensively studied with a view to a variety of applications, such as in varistors [1], short-wavelength laser diodes [2] and film bulk acoustic resonators (FBARs) [3], due to their unique combination of electrical, optical and piezoelectric properties [4]. Among these applications, FBARs have been used for providing band pass filters in the range from 400 MHz to 10 GHz [5], [6], [7]. Also FBARs, which are can be highly integrated in monolithic microwave integrated circuits (MMICs), have become one of the most promising components for device use due to their small size and high performance [8].

Most FBARs have a sandwich structure which is composed of metal–piezoelectric–metal layers. Among the various metals used, when aluminum (Al) is selected as the bottom electrode, one observes poor c-axis orientation with an amorphous layer (Al2O3) at the ZnO and Al interface [9]. In order to obtain better orientation and morphology of ZnO thin films, researchers have used buffer layers such as platinum (Pt) [10], gallium nitride (GaN) [11], and magnesium oxide (MgO) [12].

In this paper, we deposited ZnO thin films on an Ru buffer layer in order to protect the amorphous layer at the ZnO and Al interface. Also, ZnO thin films grown by means of an annealing treatment were investigated with a view to improvement of the c-axis orientation and morphology properties. From these results we finally identified improvement of the crystalline quality of the ZnO thin films grown by means of the annealing treatment on the Ru buffer layer and the return loss of the membrane-type FBARs.

Section snippets

ZnO thin film preparation

ZnO thin films with 1.4 μm thickness were prepared by asymmetrical bipolar pulsed dc magnetron sputtering on Ru(50 nm)/Al(400 nm)/ SiO2(500 nm)/Si substrate. The sputtering chamber was pumped down and the process of fabrication of the ZnO thin films was as indicated in Table 1. The microstructure and crystal orientation of the ZnO thin films were analyzed using X-ray diffraction (XRD) (Mac Science M18XHF-SRA), field emission scanning electron microscopy (FE-SEM) and atomic force microscopy

Results and discussion

Fig. 2 shows the XRD patterns measured from ZnO thin films deposited on Ru/Al/ SiO2/Si under the annealing treatment from 300 to 500 C. All samples showed only the (002)-oriented peak. As the annealing temperature was increased, the (002) peak was improved. The c-axis orientation of the ZnO thin films seems to be improved by the buffer layer and increase of the annealing treatment temperature because Ru metal applied as a buffer layer has the hexagonal-closed-packed (HCP) structure, which is

Conclusions

ZnO thin films were prepared on Ru/Al/ SiO2/Si substrate by asymmetrical bipolar pulse dc magnetron sputtering. In our study, ZnO thin films with an Ru buffer layer showed good c-axis orientation. Also, we investigated the material and electrical properties of ZnO thin films obtained by means of annealing treatments. The FWHM was decreased by increasing the annealing temperature. In contrast, the values of the rms and resistance were increased. Finally, the frequency characteristics of FBARs

Acknowledgements

This work was supported by EESRI (R-2003-B-070-02), which is funded by the Ministry of Commerce, Industry and Energy (MOCIE).

References (15)

  • J. Shi et al.

    Materials Science and Engineering

    (2003)
  • S.Y. Lee et al.

    Thin Solid Films

    (2005)
  • J.B. Lee et al.

    Thin Solid Films

    (2003)
  • D.J. Kang et al.

    Thin Solid Films

    (2005)
  • H. Yamada et al.

    Vacuum

    (2004)
  • N. Matsunami et al.

    Nuclear Instruments and Methods in Physics Research

    (2003)
  • W.T. Lim et al.

    Thin Solid Films

    (2001)
There are more references available in the full text version of this article.

Cited by (17)

  • Fabrication and RF characterization of zinc oxide based Film Bulk Acoustic Resonator

    2018, Superlattices and Microstructures
    Citation Excerpt :

    Due to advancement in technology, there has been an increased need of high frequency bandpass filters and duplexers for LTE advanced (4G) and LTE advance pro (4.5G pro/pre-5G) frequency bands [4]. FBARs have four basic device structures: a membrane over an air-gap, back air cavity membrane [5], polymer as a substrate [6], and solidly mounted resonator (SMR) [7]. FBARs employ a film of PZE material with a metal electrode on both sides, excited by radio frequency (RF) signal.

  • Advances in piezoelectric thin films for acoustic biosensors, acoustofluidics and lab-on-chip applications

    2017, Progress in Materials Science
    Citation Excerpt :

    The amorphous layer can result in ZnO films deposited on Ni, Cu, and Cr substrates with poor film textures. However, no such amorphous intermediate layer has been observed on Au, Ru, Pt, Al or sapphire substrates [360,361]. The ZnO films deposited on these substrates show a strong (0 0 0 2) orientation.

  • TiN/TiCN multilayer films modified by argon plasma treatment

    2013, Applied Surface Science
    Citation Excerpt :

    Many of these applications require high hardness, excellent chemical stability, low friction coefficient and good wear resistance [1–4]. Post-processing technique including annealing and plasma treatment is an effective method for further improving film structure and enhancing the mechanical and tribological properties of the film [5,6]. Currently, plasma treatment has drawn the attention of the researchers due to high cleanliness, low distortion, high hardness and good wear resistance [7].

  • The effect on the annealing temperature of Li doped ZnO thin film for a film bulk acoustic resonator

    2007, Superlattices and Microstructures
    Citation Excerpt :

    The sputter time was 1 h, the rf power was 125 W, and the ZLO film thickness was 1 μm, and then these samples were annealed from 300 to 600 ∘C in oxygen ambient atmosphere for 20 min. Finally, the FBAR with piezoelectric ZLO films was designed with the following fabrication process [7]. ( 1) Both-side-polished p-type Si(100)/back-polished SiO2 wafer (0.5 μm) was prepared as a substrate. (

View all citing articles on Scopus
View full text