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A simple inexpensive method for thickness measurement of thin films E Sader, Physics Department, Faculty of Science, Birzeit University, Occupied West Bank

received 7 March 1987

A simple and inexpensive method for monitoring the thickness of thermally evaporated thin films is reported. The technique relies on a simple oscillating quartz crystal circuit using TTL. The operation of the monitor is illustrated for thin aluminium and copper films. Their thickness is determined, by this method, to an accuracy of a few percent, as the deviations of the mass densities between the bulk material and the films are very small.

Introduction 5V POWER SUPPLY

An inexpensive, simple and more reliable’ method for thickness measurement of thin films evaporated in vacuum is described

below. The technique used is the well-known oscillating quartz crystal

method’. The crystal oscillates in one of its resonant frequencies via the piezoelectric effect. The change in the resonant frequency, due to loading the optically polished surface of the crystal by the evaporant atoms, is a measure of the thickness of the thermally evaporated thin tilm3.4.

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Description and analysis of the monitor OSCILLATOR

The monitor consists of a 4.431 MHz optically polished AT-cut quartz crystal, an electronic oscillator to drive the crystal and a

frequency meter.

XTAL

Figure 1 is a circuit diagram of the electronic oscillator. On the left is an IC regulated 5 V dc power supply. To the right is the oscillator. The inverters A 1, A, and A, used in the oscillator circuit are three from the six available in the TTL 7404 integrated circuit.

Inverters 1 and 2 are used as amplifiers while inverter 3 serves as a buffer. Resistances R, and R, are used to set the gain. C, is a coupling capacitor. A frequency meter is connected to the BNC

socket.

Figure 1. Circuit diagram of the electronic oscillator

The crystal is installed in the vacuum chamber 25 cm above the evaporation crucible with a 5 mm diameter circular aperture

mounted just below it.

Conclusion

Calibration of the monitor was done, in situ, by Tolansky’s4 interferometric method. This calibration is illustrated in Figure 2 for thin metal films of aluminium and copper.

Referring to Figure 2, it is clear that differences between the film and bulk densities for aluminium and copper layers produced are small. It can therefore be shown that the thickness d of the film in angstroms can be written as

As long as the temperature of the crystal is not allowed to rise excessively (i.e. 40-50°C max typically) the resonant frequency will change very little with temperature variations (for an AT-cut

crystal).

d= KXKP 1 P As

Mailing address: Birzeit University, PO Box 14, Birzeit. West Bank, via Israel.

where p is the bulk density of the evaporant material in g cmm3 and AT is the change in period of oscillation of the crystal in seconds. The constant K, is dependent upon the acoustic impedance of the crystal at the resonant frequency. The constant

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ESader: Thickness measurement of thin films

Figure 2. Calibration of the monitor

K, is determined by the system geometry, i.e. crystal-source distance and orifice size and position. The combined constant was

found to be

bs 0.310+0.012 3

cm ’

Acknowledgement

The author wishes to thank Mr J Zagal for his assistance in contructing the oscillator circuit.

References

’ G K Mohan, V Toudon and P B Gael, Am J Ph~s, 50, 95C (1982). ’ G Sauerbrey, Z Phys, 155, 206 (1959). 3 W Steckelmacher, 7’llin Film Microelectronics. Chapman & Hall, London (1965). 4 L Eckertova, Physics of Thin Films. Plenum Press, New York (1977). 5 A R Nyaiesh and M A Baker, Vacuum, 32, 305 (1982).

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