Method of determining multilayer thin film deposition on a piezoelectric crystal
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Publication Number
US-8438924-B2
Publication Date
2013-05-14
Expiration Date
Abstract
A method for accurately calculating the thickness of deposited thin film layers onto a piezoelectric crystal blank in which dissimilar materials can be utilized, enabling determinations for various applications employing exotic materials. Additionally, the specific acoustic impedance (or equivalent z-ratio) of an unknown deposited material can be determined. The exact analytical solution nearly eliminates thickness errors when several layers of different materials are sequentially deposited on the same monitor quartz crystal.
Core Innovation
The invention provides an analytically exact multi-layer thin-film thickness determination method using a piezoelectric crystal blank with an applied electrode and one or more sequentially deposited dissimilar film layers. It determines the fundamental resonance frequency of the piezoelectric crystal blank and the fundamental resonance frequency of the composite blank with the applied electrode, then applies a first deposited layer and determines the resonance frequency of the composite resonator comprising the blank, the electrode, and the deposited layer.
The method computes acoustic phase lag across the crystal blank, the electrode, and the deposited layer at the resonance frequency, and computes the deposited-layer thickness from phase-lag information and the density of the deposited material. Acoustic transfer-matrix formalism is used to derive recursive phase-lag relations, and a phase-lag parameter K is computed from specific acoustic impedance Z and tangent-function terms.
In a further aspect, the invention determines the specific acoustic impedance of an unknown material by using thin film deposition on the piezoelectric crystal blank. It measures resonance-frequency information for the blank, the electrode composite, and the composite resonator including the deposited unknown layer, while also using mass/area/thickness determination for the deposited film layer. Based on the computed acoustic phase lag information and the thickness measurement, the specific acoustic impedance of the deposited film layer is determined using a constructed non-linear equation involving acoustic phase-lag tangent functions.
Claims Coverage
Two independent claims define the invention: one for measuring deposited film thickness on a piezoelectric crystal by computing acoustic phase lag at resonance, and one for determining the specific acoustic impedance of an unknown deposited film by combining phase-lag information with mass/area/thickness and solving a non-linear equation. Overall, the claims emphasize acoustic phase lag computation, impedance-based phase-lag parameter relationships, and thickness/impedance inference grounded in resonance-frequency measurements.
Thickness from acoustic phase lag at resonance
Determining the fundamental resonance frequency of a piezoelectric crystal blank; determining the fundamental resonance frequency of the composite blank and applied electrode; applying a first deposited layer and determining the resonance frequency of the composite resonator comprising said blank, said electrode and said deposited layer; determining acoustic phase lag across each of the crystal blank, electrode and deposited layer as computed at said resonance frequency; and computing the thickness of said deposited layer from the phase lag information and density of said material.
Specific acoustic impedance from phase lag plus thickness
Determining the specific acoustic impedance of an unknown material by thin film deposition on a piezoelectric crystal blank; computing acoustic phase lag information across said crystal blank, said applied electrode and said deposited layer at said measured resonant frequency; determining the thickness of said electrode and the thickness of said deposited layer based on mass/area/thickness measurements and densities; and determining the specific acoustic impedance of said deposited film layer based upon said phase lag information and said thickness measurement.
The claim set covers two main capabilities: deriving deposited film thickness from resonance-frequency-derived acoustic phase lag across the piezoelectric blank, electrode, and deposited layer using density, and determining a deposited film’s specific acoustic impedance from resonance-frequency acoustic phase lag together with thickness information obtained from mass/area measurements, including constructed non-linear equation relationships and impedance/tangent-based terms.
Stated Advantages
Thickness error reduction and improved accuracy compared with a traditional Zmatch approach, including for thin layers.
Documented Applications
Determining thickness of deposited film layers on a piezoelectric crystal using analytically exact multi-layer thin-film thickness determination.
Determining the specific acoustic impedance of an unknown material using thin film deposition on a piezoelectric crystal blank.
Specific example context includes an OLED material (AlQ3) in the described system comparison.
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