Measuring and controlling deposition on a piezoelectric monitor crystal

A method of measuring and or controlling the rate of material deposition onto a piezoelectric crystal that has a fundamental resonance frequency f.sub.1 and at least one additional resonance frequency f.sub.2 next above said fundamental resonance frequency, said fundamental and additional resonance frequencies having predetermined initial frequencies f.sub.1u and f.sub.2u prior to any material being deposited on the crystal, and shifting to lower frequencies f.sub.1c and f.sub.2c under mass loading of the crystals as the material is deposited thereon, the method comprising:

• generating successive sweeps of frequencies in a frequency generator and applying said sweeps of frequencies to said crystals as said material is being deposited thereon;
• analyzing a response characteristic of said crystal to said sweeps of frequencies to identify the shifted fundamental resonance frequency f.sub.1c and the shifted additional resonance frequency f.sub.2c for the crystal as it is being coated;
• computing an areal mass density m of the coating of material on the crystal based on the initial resonance frequencies f.sub.1u and f.sub.2u for the crystal being coated;
• computing the rate of material deposit based on changes in said areal mass density m for successive sweeps of frequencies;
• and
• controlling the rate of material deposition based on the computed rate of material deposit;
• wherein said computing the areal mass density m is carried out by calculating parameters
• R.sub.uncoated and R.sub.coated, where ##EQU12## applying these parameters to the following approximation to determine a value (mz) which is the product of the mass area density m times the acoustic impedance ratio z of the coating of the deposited material relative to the piezoelectric crystal:
• ##EQU13## applying the value mz to the Lu-Lewis relation to obtain the value z:
• ##EQU14## calculating the areal mass density m from the value z and frequency shifts.

A method of measuring and or controlling the rate of material deposition onto a piezoelectric crystal that has a fundamental resonance frequency f.sub.1 and at least one additional resonance frequency f.sub.2 next above said fundamental resonance frequency, said fundamental and additional resonance frequencies having predetermined initial frequencies f.sub.1u and f.sub.2u prior to any material being deposited on the crystal, and shifting to lower frequencies f.sub.1c and f.sub.2c under mass loading of the crystals as the material is deposited thereon, wherein said resonances have mode assignments [1,0,0] and [1,0,2] which have indices to indicate the number of phase reversals along three crystal axes, and said fundamental resonances f.sub.1 corresponds to a fundamental [1,0,0] mode of vibration for the crystal and the additional resonance frequency f.sub.2 corresponds to an anharmonic [1,0,2] mode, the method comprising:

• generating successive sweeps of frequencies in a frequency generator and applying said sweeps of frequencies to said crystal as said material is being deposited thereon;
• analyzing a response characteristic of said crystal to said sweeps of frequencies to identify the shifted fundamental resonance frequency f.sub.1c and the shifted additional resonance frequency f.sub.2c for the crystal as it is being coated;
• computing an area mass density m of the coating of material on the crystal based on the initial resonance frequencies f.sub.1u and f.sub.2u for the uncoated crystal and the shifted resonance frequencies f.sub.1c and f.sub.2c for the crystal being coated;
• computing the rate of material deposit based on changes in said areal mass density m for successive sweeps of frequencies;
• and
• controlling the rate of material deposition based on the computed rate of material deposit.