Performance optimization of lateral-mode thin-film piezoelectric-on-substrate resonant systems

At a Glance

Metadata	Details
Publication Date	2015-01-01
Journal	STARS (University of Central Florida)
Authors	Hedy Fatemi
Citations	1
Analysis	Full AI Review Included

Thin-Film Piezoelectric-on-Substrate (TPoS) Resonators utilizing Ultra Nano Crystalline Diamond (UNCD)

Executive Summary

This research demonstrates the performance optimization of lateral-mode Thin-film Piezoelectric-on-Substrate (TPoS) resonators and filters by utilizing Ultra Nano Crystalline Diamond (UNCD) substrates manufactured via Chemical Vapor Deposition (CVD).

Frequency Scaling: Diamond (TPoD) substrates enable resonance frequencies up to two times higher than equivalent silicon (TPoS) devices due to diamond’s superior Young’s modulus (up to 933 GPa).
Thermal Stability: UNCD resonators exhibit a significantly improved Temperature Coefficient of Frequency (TCF), measured as low as -11 ppm/°C, substantially better than Si-based TPoS devices (-20 to -30 ppm/°C).
Power Handling & Robustness: Diamond substrates provide superior mechanical robustness, with fracture energy densities measured at 3.1 x 10⁶ J/m³, over 2x higher than silicon, making them ideal for high-power oscillator applications.
Surface Quality Requirement: Achieving high-performance devices requires an ultra-smooth UNCD surface roughness (Ra < 1nm) achieved via Chemical Mechanical Planarization (CMP) to ensure highly oriented Aluminum Nitride (AlN) piezoelectric film growth.
Filter Performance: A monolithic lateral-mode filter achieved a record low Insertion Loss (IL) of 3.7 dB at ~900 MHz with 50Ω termination, confirming the TPoD platform’s potential for integrated RF filtering.
Advanced Applications: TPoD resonators enabled state-of-the-art applications, including very low-noise oscillators (lowest reported Phase Noise for MEMS) and high-resolution passive wireless temperature sensors.

Technical Specifications

The table below summarizes key performance characteristics comparing TPoS resonators built on standard Silicon (Si) and Ultra Nano Crystalline Diamond (UNCD).

Parameter	Value (Si)	Value (UNCD, E=933GPa)	Unit	Context
Substrate Material	Silicon (SOI, 5µm layer)	UNCD (~3µm)	-	Thin-film Piezoelectric-on-Substrate (TPoS)
Young’s Modulus (E)	~160	491, 650, 933	GPa	Directly influences resonance frequency scaling.
Highest Resonance Frequency (f_c)	543.6	1076	MHz	21^st order resonator design.
*Maximum fQ Product**	1.49 x 10¹²	2.22 x 10¹²	Hz	Figure of Merit for resonators.
Min. Insertion Loss (IL)	4.3	3.7	dB	37^th harmonic monolithic filter at ~900 MHz.
Max. Fractional Bandwidth (FBW)	-	1.0	%	Achieved using novel hourglass filter design (HG2).
Critical Energy Density (Bifurcation)	0.47 x 10⁶	0.28 x 10⁶	J/m³	Energy stored before hysteresis observed.
Fracture Energy Density	1.2 x 10⁶	3.1 x 10⁶	J/m³	Energy stored before mechanical failure (2x greater for Diamond).
Temperature Coefficient of Frequency (TCF)	-20 to -30	-11	ppm/°C	Low TCF crucial for stable oscillators/sensors.
Metal Layer Thickness	100	100	nm	Molybdenum (Mo) electrodes.
Piezoelectric Film Thickness	0.5 - 1	0.5 - 1	µm	Aluminum Nitride (AlN).
UNCD Surface Roughness (post-CMP)	-	< 1	nm (rms)	Critical for high-quality AlN orientation (FWHM ~3°).
Impedance Transformation Ratio	-	19:1	-	Demonstrated in a 10^th order transformer design.

Key Methodologies

The following ordered list summarizes the crucial material preparation and deposition steps required to realize high-performance TPoD devices:

UNCD Film Deposition: Ultra Nano Crystalline Diamond (UNCD) films (thickness ~3µm) were deposited on polished silicon wafers using the Hot Filament Chemical Vapor Deposition (HFCVD) technique.
Young’s Modulus Control: Diamond deposition temperature was increased (660 °C to 810 °C) to control the Young’s modulus (up to 933 GPa) and shift residual stress from compressive (300-400 MPa) to tensile (~100 MPa).
Two-Step Chemical Mechanical Planarization (CMP): An aggressive slurry and high force polishing step reduced the initial rms roughness (60nm) down to ~15nm. This was followed by conventional CMP to achieve the required ultra-smooth surface roughness of less than 1nm (Figure 10-b).
Piezoelectric and Metal Stack Deposition: The Mo/AlN/Mo stack was sputtered directly onto the ultra-smooth UNCD surface. Molybdenum (Mo, 100nm) was chosen for low acoustic loss and low thermal expansion mismatch with AlN.
AlN Orientation Verification: X-ray Diffraction (XRD) and rocking curve analysis confirmed the resulting AlN film quality was highly textured with a Full-Width Half Maximum (FWHM) of approximately 3°, similar to films grown on polished single-crystal silicon.
MEMS Fabrication and Release: Standard microfabrication techniques were employed, including dry etching (Cl₂ for AlN, SF₆ for Si/UNCD), wet etching (TMAH for AlN), and sacrificial layer release (BOE for oxide layer on SOI, or backside dry etch for UNCD-on-Si).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced diamond materials and processing services necessary to replicate, optimize, and extend the high-frequency TPoS research detailed in this analysis.

Research Requirement	6CCVD Capability & Solution	Value Proposition for the Customer
Substrate Material (UNCD)	Polycrystalline Diamond (PCD) Wafers. 6CCVD supplies high-quality MPCVD polycrystalline diamond substrates, the ideal platform for replicating TPoD (Thin-film Piezoelectric-on-Diamond) performance gains.	Enables immediate access to substrates proven to achieve 2x frequency scaling and lower TCF (-11 ppm/°C) compared to silicon.
Ultra-smooth Surface Quality	Precision Polishing Service (Ra < 1nm). The demonstrated need for roughness < 1nm is a standard offering for 6CCVD’s PCD material (up to inch size).	Guarantees the necessary surface finish for depositing highly oriented piezoelectric films (e.g., AlN, ZnO) crucial for high electromechanical coupling and low insertion loss (IL).
Custom Substrate Thickness	Custom PCD Substrates (0.1µm - 10mm). The paper utilized ~3µm thick UNCD films; 6CCVD offers custom thicknesses for device layers and bulk substrates.	Allows engineers to precisely tune material stack stiffness for optimized resonance frequency and acoustic velocity based on specific device design needs.
Electrode and Buffer Layers	Custom Metalization (Mo, Au, Ti, Pt, etc.). 6CCVD offers in-house metal deposition (sputtering/evaporation) for stacks like Mo/AlN/Mo, or Cr/Au adhesion layers.	Provides turnkey integration of the full thin-film stack, minimizing material interface defects and ensuring compatibility with Mo and AlN processing.
High Power Handling Applications	High Mechanical Robustness PCD. Diamond’s 2x superior fracture energy density (3.1 x 10⁶ J/m³) is critical for high-power oscillators and impedance transformers.	Reduces device failure rates in non-linear operation, enabling maximum carrier power and optimal phase noise performance in [Low-Noise Oscillator] projects.
Global Manufacturing & Logistics	Global Shipping (DDU/DDP available). 6CCVD manages international logistics for sensitive wafer materials.	Ensures reliable, timely, and secure delivery of custom diamond substrates and processed wafers worldwide.

Engineering Support

6CCVD’s in-house PhD material science team can assist design engineers in selecting the optimal polycrystalline diamond grade and processing recipe (including bespoke polishing and metalization specifications) required for high-performance Thin-Film Piezoelectric-on-Substrate Resonator and Filter projects. We specialize in tailoring diamond properties—such as Young’s Modulus, grain size, and surface chemistry—to maximize device performance stability (low TCF) and power handling capacity.

Call to Action: For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

The main focus of this dissertation is to characterize and improve the performance of thin-film piezoelectric-on-substrate (TPoS) lateral-mode resonators and filters. TPoS is a class of piezoelectric MEMS devices which benefits from the high coupling coefficient of the piezoelectric transduction mechanism while taking advantage of superior acoustic properties of a substrate. The use of lateral-mode TPoS designs allows for fabrication of dispersed-frequency filters on a single substrate, thus significantly reducing the size and manufacturing cost of devices. TPoS filters also offer a lower temperature coefficient of frequency, and better power handling capability compared to rival technologies all in a very small footprint. Design and fabrication process of the TPoS devices is discussed. Both silicon and diamond substrates are utilized for fabrication of TPoS devices and results are compared. Specifically, the superior acoustic properties of nanocrystalline diamond in scaling the frequency and energy density of the resonators is highlighted in comparison with silicon. The performance of TPoS devices in a variety of applications is reported. These applications include lateral-mode TPoS filters with record low IL values (as low as 2dB) and fractional bandwidth up to 1%, impedance transformers, very low phase noise oscillators, and passive wireless temperature sensors.

Tech Support

Original Source

DOI: None