IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 10, OCTOBER 2009 1045 Fabrication of Large-Area Suspended MEMS Structures Using GaN-on-Si Platform Jianan Lv, Zhenchuan Yang, Member, IEEE, Guizhen Yan, Member, IEEE, Wenkui Lin, Yong Cai, Baoshun Zhang, and Kevin J. Chen, Senior Member, IEEE Abstract—In this letter, piezosensitive elements featuring large- size suspended gallium nitride (GaN) microstructures are fabri- cated with a two-step dry-release technique using the GaN-on-Si platform. The suspended microstructures are integrated with highly piezosensitive AlGaN/GaN heterostructures as sensing units to realize the GaN-based integrated microsensors. To characterize the residual-stress distribution of the fabricated mi- crostructures, micro-Raman spectroscopy is employed. A microac- celerometer structure with a 250 × 250-μm 2 proof-mass area is fabricated with the proposed fabrication technique, and the piezoresponse properties of the integrated sensing elements are characterized through bending experiment. Index Terms—Accelerometer, gallium nitride (GaN), micro- electromechanical system (MEMS), piezosensitivity, sensor. I. I NTRODUCTION G ALLIUM nitride (GaN) and related compound semicon- ductors have been successfully applied in optoelectronic devices such as high-brightness light-emitting diodes and high- frequency power transistors [1] owing to the materials’ intrinsic properties, including wide bandgap, high electron-saturation velocity, and high-electric-breakdown field. GaN also exhibits promising mechanical and piezosensitive properties [2]–[11] along with excellent thermal and chemical stability, making them strong candidates for integrated micro/nanosensors used in harsh environments such as automotive and aviation systems. To develop microsensors based on microelectromechanical- system (MEMS) structures, effective means to pattern and release GaN microstructures are essential, particularly in some MEMS devices that require large vertical operating space, e.g., accelerometers or other inertial sensors with movable proof mass. However, due to the difficulties of fabrication and fracture-related problems caused by large built-in residual Manuscript received May 12, 2009; revised July 20, 2009. First pub- lished September 11, 2009; current version published September 29, 2009. This work was supported in part by National Natural Science Foundation of China under Grant 60706028. The review of this letter was arranged by Editor J. A. del Alamo. J. Lv, Z. Yang, and G. Yan are with the National Key Laboratory of Science and Technology on Micro/Nano Fabrication and Institute of Microelectronics, Peking University, Beijing 100871, China (e-mail: z.yang@ime.pku.edu.cn). W. Lin, Y. Cai, and B. Zhang are with Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215125, China (e-mail: wklin2007@sinano.ac.cn; ycai2008@sinano.ac.cn; bszhang2006@ sinano.ac.cn). K. J. Chen is with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong (e-mail: eekjchen@ust.hk). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2009.2028905 stress in GaN thin films, previously reported works mainly fo- cused on small structures, e.g., cantilevers and microbeams. Davies et al. [3] demonstrated free-standing GaN cantilevers on Si (111) by inductively coupled plasma reactive-ion etching (ICP-RIE) and wet KOH etching. GaN-on-patterned-silicon techniques were developed in our previous work to release sus- pended GaN microstructures using GaN selective area growth and sacrificial wet etching of the silicon [4], [5]. One draw- back of the wet-etching technique is the stiction between the wetted suspended microstructures and the nearby substrates. To overcome the stiction problem, Zimmermann et al. [6] used anisotropic silicon dry etching through the wafer from the back side to achieve deeply released GaN structures. Vicknesh et al. [7] and Brueckner et al. [8] demonstrated front-side dry-release techniques to realize suspended GaN structures on Si. However, undesirable features such as severe undercut and nonvertical sidewalls of the supporting silicon are observed and impose obstacles to the achievement of high fabrication accuracy and precision. In this letter, we report a fabrication process that was devel- oped for making large-area GaN MEMS structures by a dry- etch-only fabrication technique. The importance for fabricating large-area structure is to increase the inertia of the proof-mass structure for improved sensitivity. To obtain deeply released suspended GaN microstructures, a two-step silicon-releasing process is developed by combining both anisotropic and isotropic dry etching of the silicon substrate. The anisotropic etching produces high-aspect-ratio silicon trenches. This step enables the following conditions: 1) vertical profiles in the silicon sidewalls and 2) independent control of the suspension distance of the GaN MEMS structures. The isotropic etching is used to completely release the MEMS structure laterally. This combined dry-etching technique is fully compatible with not only the common silicon micromachining process but also the fabrication of conventional AlGaN/GaN high-electron-mobility transistors (HEMTs). II. EXPERIMENTAL PROCEDURE The AlGaN/GaN heterostructure sample used in this let- ter was grown by MOCVD (metal–organic–chemical–vapor- deposition) on (111) Si substrate. The epilayers consist of an ∼1-μm GaN buffer layer followed by a 20-nm Al 0.26 Ga 0.74 N barrier layer. The fabrication process flow is shown in Fig. 1. At first, the active-sensing regions with piezosensitive 2DEG channels were defined by 200-nm-deep mesa etching using Cl 2 -based ICP-RIE at 5 mTorr. Low coil power (200 W) and 0741-3106/$26.00 © 2009 IEEE