Quartz Crystal MicroBalance (QCM)

Abstract

This project aims to measure the thickness of thin films using a quartz crystal microbalance(QCM). The thin films of interest are grown using magnetron sputtering at the eMagic Lab. In particular, Au will be deposited on a 10mm x 10mm x 0.5mm MgO(001) substrate. Thickness measurements are important in characterizing the sample, and it is helpful to get an estimate of the thickness which serves as a reference when fitting data from spectroscopic ellipsometry(SE). We can corroborate the measurements with atomic force microscopy(AFM) and SE data.

Team members

Tan Jhoon Yong

Idea

Measurements of the thickness of thin films are essential to get a preliminary idea of the thin-film interference effects. Getting a more accurate measurement of the thickness allows us to probe the optical properties of thin films better with SE data fitting.

Sample Preparation

7 samples of varying thicknesses of Au are deposited on a 10mm x 10mm x 0.5mm MgO(001) substrate using magnetron sputtering in an Ar-filled chamber. Insitu thickness measurement results are as follows: 20.6nm, 12.4nm, 8.3nm, 4.2nm, 2.1nm, 1.0nm.

Working Principle

The working principle behind the QCM is based on the piezoelectric effect. The piezoelectric effect is the generation of electric charges by a crystalline material upon subjecting it to stress.[1] An oscillating electric field applied across the device induces an acoustic wave that propagates through the crystal and meets minimum impedance when the device's thickness is a half-integer multiple of the wavelength of the acoustic wave.[2] It is a shear mode device in which the acoustic wave propagates in a direction perpendicular to the crystal surface.[3] Therefore, crystals cut in specific crystal axes have to be used to utilize the shear mode.

Orientation of AT- and BT- cut crystals

A resonant oscillation in the crystal is achieved by putting the crystal in an oscillation circuit where the electrical and mechanical oscillations are close to the fundamental frequency of the crystal. Changes to the mass of the crystal would result in a proportional change in its natural frequency according to the Sauerbrey equation[4]

Δf = -CΔm/A

where Δf is the change in resonance frequency, C is the specific sensitivity of the crystal, Δm is the change in mass, and A is the active area of the crystal. C describes how much material per unit area is required to change the resonant frequency of the crystal by 1Hz. A smaller C value gives a higher mass sensitivity. This value depends on the fundamental resonant frequency of the crystal, and it is defined as

C = v₀ρ₀/2f₀²

Therefore, we can determine the mass of the deposited material. For the scope of this project, energy dissipation is assumed to be negligible,

Setup

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Measurements

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References

1. Meissner A (1927) € Uber piezoelectrische Krystalle bei Hochfrequenz. Z Tech Phys 8:74
2. O’Sullivan, C. K., & Guilbault, G. G. (1999). Commercial quartz crystal microbalances – theory and applications. Biosensors and Bioelectronics, 14(8), 663–670.
3. Ebersole, R., Miller, J., Moran, J., & Ward, M. (1990). PZ quartz sensors for use in clinical analysis. J. Am. Chem. Soc, 112, 3239.
4. Sauerbrey, G. (1959). Verwendung von Schwingquarzen zur Wägung dünner Schichten und zur Mikrowägung. Zeitschrift für Physik, 155(2), 206–222.