Elsevier

Thin Solid Films

Volume 340, Issues 1–2, 26 February 1999, Pages 210-217
Thin Solid Films

Micro/nanomechanical characterization of ceramic films for microdevices

https://doi.org/10.1016/S0040-6090(98)01153-5Get rights and content

Abstract

Microelectromechanical systems (MEMS) are currently fabricated using single-crystal silicon, various polysilicon films and other ceramic materials. Silicon carbide (SiC) film has recently been pursued as a material for use in MEMS devices owing to its excellent mechanical properties and high-temperature capabilities. Since physical and chemical properties, friction and wear are important issues in such small-scale devices, it is essential that the materials used in MEMS have good micro/nanomechanical and tribological properties. Micro/nanomechanical characterization of single-crystal 3C-SiC (cubic or β-SiC) films, undoped and doped (n+-type) polysilicon films have been carried out. For comparision, measurements on undoped single-crystal Si(100) have also been made. Hardness, elastic modulus and scratch resistance of these materials were measured by nanoindentation and microscratching using a nanoindenter. Fracture toughness was measured by microindentation using a microindenter. Friction and wear properties were measured using an accelerated ball-on-flat tribometer. It is found that the 3C-SiC film exhibits higher hardness, elastic modulus and scratch resistance as well as lower friction compared to other materials. These results show that the 3C-SiC film possesses desirable micro/nanomechanical properties that make it an ideal material for use in MEMS devices.

Introduction

The advances in silicon process technology over the last three decades have led to the development of microcomponents known as microelectromechanical systems or MEMS. Researchers have fabricated a wide variety of sensors, actuators, valves, gear trains, turbines, nozzles and pumps (for a collection of works see Ref. [1]) with dimensions in the range of a couple to a few hundred micrometers. In MEMS devices, various forces associated with the devices scale down with the size. When the length of the machine decreases from 1 mm to 1 μm, the area decreases by a factor of a million and the volume decreases by a factor of a billion. The resistive forces such as friction, viscous drag and surface tension that are proportional to the area increase a thousand times more than the forces proportional to the volume such as inertial and electromagnetic forces. These forces lead to tribological concerns, which become critical because friction/stiction (static friction), wear and surface contamination affect device performance and in some cases can even prevent devices from working.

Although silicon based MEMS devices find such wide uses today, they lack high temperature capabilities with respect to both mechanical and electrical properties. Recently, researchers have been pursuing SiC as material for high-temperature microsensor and microactuator applications [2]. The high-temperature capability of SiC combined with its excellent mechanical properties, thermal dissipative characteristics, chemical inertness and optical transparency makes SiC an ideal choice for complementing polysilicon (polysilicon melts at 1400°C) in MEMS devices. Since MEMS devices need to be of low cost to be viable in most applications, researchers have found low-cost techniques of producing single-crystal 3C-SiC (cubic or β-SiC) films via epitaxial growth on large-area silicon substrates [3]. This technique allows high-volume batch processing and has the advantage of having silicon as the substrate, an inexpensive material for which microfabrication and micromachining technologies are well established. It is believed that these films will be well suited for MEMS devices.

Micro/nanomechanical and tribological characterization of these SiC films and their comparison to the polysilicon materials and films currently used in such small-scale devices is of critical importance. This paper presents the results of these studies conducted for the first time on 3C-SiC films as well as on the most commonly used materials in MEMS technology today: undoped single-crystal silicon and polysilicon films (undoped and doped).

Section snippets

Mechanical and tribological characterization

Hardness and elastic modulus were calculated from the load-displacement data obtained by nanoindentation using a Berkovich indenter on each sample at six different indentation loads ranging from 0.2 to 15 mN. In microscratch studies, a conical indenter having a tip radius of 1 μm and an included angle of 60°, was drawn over the sample surface, and the load was ramped up, until substantial damage occurred. The coefficient of friction was monitored during scratching. In order to obtain scratch

Hardness and elastic modulus

Representative load–displacement plots of indentations made at 15 mN peak indentation load on the undoped Si(100), undoped polysilicon film, doped polysilicon film, and SiC film are shown in Fig. 1. The SiC film exhibits the lowest indentation depth and highest slope of unloading curve, as compared to other samples, and the indentation depths and slopes of unloading curve for the undoped Si(100) are comparable to those of the undoped polysilicon and doped polysilicon films. The undoped Si(100),

Conclusions

The SiC film shows higher hardness, elastic modulus and scratch resistance as well as lower friction compared to the other materials currently used in MEMS devices. The fracture toughness of the SiC film is comparable to that of the undoped Si(100). In addition, the availability of a low-cost technique of producing the SiC film makes it an exceptional choice as a material for high-temperature MEMS applications.

Acknowledgements

The authors would like to thank Dr C. A. Zorman and Professor M. Mehregany of Case Western Reserve University for providing polysilicon and SiC films and for technical assistance. Financial support for this study was provided by the Office of Naval Research, Department of the Navy (Contract No. N00014-96-1-10292). The information herein does not necessarily reflect the position or policy of the government and no official endorsement should be inferred.

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