4H-Silicon Carbide as an Acoustic Material for MEMS

This article discusses the potential of 4H-silicon carbide (SiC) as a superior acoustic material for microelectromechanical systems (MEMS), particularly for high-performance resonator and extreme environments applications. Through a comparison of the crystalline structure along with the mechanical, acoustic, electrical, and thermal properties of 4H with respect to other SiC polytypes and silicon, it is shown that 4H-SiC possesses salient properties for MEMS applications, including its transverse isotropy and small phonon scattering dissipation. The utility and implementation of bonded SiC on insulator (4H-SiCOI) substrates as an emerging MEMS technology platform are presented. Additionally, this article reports on the temperature-dependent mechanical properties of 4H-SiC, including the temperature coefficient of frequency (TCF) and quality factor ( ${Q}$ -factor) for Lamé mode resonators. Finally, the 4H-SiC MEMS fabrication including its deep reactive ion etching is discussed. This article provides valuable insights into the potential of 4H-SiC as a mechanoacoustic material and provides a foundation for future research in the field.

I. Introduction

For decades, silicon has served as the backbone of the modern electronics industry due to its exceptional electrical, mechanical, and acoustic properties. However, as technological boundaries continue to be pushed, silicon has revealed limitations in high-performance and extreme-environment applications. As an alternative material, monocrystalline silicon carbide (SiC) has emerged as a compound semiconductor of interest with industrial substrates available in various sizes, including 6” and 8” wafers. SiC exhibits a unique combination of mechanical, acoustical, electrical, and chemical properties, including high elastic modulus, high fracture strength, chemical inertness, excellent thermal conductivity, high acoustic velocity, and a wide bandgap, as summarized in Table I [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. These attributes make SiC a pre-eminent contender in microelectromechanical systems (MEMS) applications, such as radio frequency communications [29], sensors and actuators [3], and nanophotonic structures [30]. Remarkably, the array of characteristics inherent to SiC endows it with high resistance to extreme temperatures, pressure, corrosive chemicals, high power, high radiation, large vibration, and high shock, all of which are typically encountered in harsh environments, such as aerospace, nuclear, oil and gas industries, and defense and automotive applications [13], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]. Owing to its exceptional characteristics, SiC has garnered burgeoning interest as a mechanoacoustic material for applications that demand top-tier performance even in the most challenging and rigorous conditions.TABLE I Mechanical and Electrical Properties Comparison

Property	Unit	(100) Si	(111) Si	(100) 3C-SiC	(0001) 6H-SiC	(0001) 4H-SiC
Density	Kg /m3	2230	2230	3166	3211	3210
Young’s Modulus	GPa	130	174	314	450	481
Fracture Strength	MPa	47.1	31.8	800	– *	200
Yield Strength	GPa	2.7	2.7	12.0	14.3	11.8
Poisson’s Ratio	1	0.28	0.26	0.237	0.207	0.205
Volumetric Heat Capacity	106 J/ (m3K)	1.58	1.58	2.24	2.22	1.92
Thermal Conductivity	W/cm/°C	1.3	1.48	3.6	3.5	3.7
Thermal Expansion	ppm°C−1	2.6	2.6	2.4	3.4	4.1
Melting Temperature	°C	1412	1412	2830	2830	2830
Acoustic velocity	103 m/s	9.1	9.1	11.9	11.9	11.9
Piezoelectric Coefficient e33	10−5C/ cm2	N/A	N/A	4.0 **	4.0	3.4
Bandgap	eV	1.1	1.7	2.4	3.0	3.3
f.	1013Hz	2 - 3	2 - 3	10 - 50	20 - 60	20 - 60

^*

No data available in bending test, only compressive facture strength at 24 GPa.

^**

The e₃₃ is under 3C-SiC trigonal structure symmetry with the z-axis along (111).