A high stability low cost film thickness monitor

Vacuum/volume 35/number Printed in Great Britain

Workshop

a/pages 325 to 32611985

0042-207X/8553.00+ .OO Pergamon Press Ltd

note

received 15 April 1985

Dual voltage-controlled

A high stability low cost film thickness monitor* In the search for improved film thickness monitoring circuits based on the oscillating quartz crystal principle, a simple but high stability circuit design has been developed by employing the voltage control oscillator (VCO) principle. The particular advantages of this design are: (i) An improvement in oscillator frequency and measurement stability over more conventional measuring methods as described in ref 1. (ii) The ready availability of ‘off the shelf VCOs at low cost suitable for this application, thus avoiding the usual circuit construction necessary to provide for crystal oscillation. (iii) The VCO provides a square wave and pulse circuit processing output which is well suited for counting and is far less susceptible to degradation and counting errors that occur with the use of sine wave outputs used with more conventional oscillators. (iv) The VCO is a self-contained integrated circuit chip of very small size. The VCO can thus be placed near monitoring crystals in the vacuum system. The ‘chip’ also is small enough that it can be shielded in a small Faraday cage for use in rf plasma applications (but can also be used with long leads), thus avoiding the wellknown problem of lead capacitance between crystal and oscillator. The voltage control oscillator. The inexpensive Texas Type SN54LS124 VCO used in the construction described here, is a dual voltage control oscillator. The chip in fact has two separate oscillator circuits but only one of these is needed in this application. The wiring arrangement is shown in the block diagram, Figure 1, with the monitoring crystal connected as shown. It should be noted that the VCO chip is wired to operate at the fundamental frequency of the quartz crystal (high stability running) without employing the voltage control facility. The waveform for counting is a square wave output (Figs. 2a, 2b). This square wave counting output is directly compatible with TTL counting circuitry and unlike sine wave outputs is quite immune to noise and distortion counting errors. It is also possible, * This work was carried out at the Unit of Plasma Processing, Sussex University, Brighton, UK.

oscillators

I I I I I I +5v

r-

-

-1

I L 1

auortz crystal

Figure 1. Block diagram

Freauency counter

of the VCO and the wiring arrangements.

although not vital, to include noise reduction by adjusting external C and R values for the crystal circuit. As the LS124 has two separate isolated oscillator circuits (identical) on chip, a second crystal can be operated on the second oscillator circuit for additional monitoring purposes. It is also possible to use the second oscillator with a non-monitoring crystal to ‘beat’ against the first so that a ‘frequency’ difference method of counting can be used as an alternative. It has been found in use that the crystal frequency is stable tof3Hz (6 MHz crystal) over a period of 8 h, and responds to mass loading in the normal manner. The monitor is directly compatible with frequency counting such as the Sinclair PFM200 and the Marconi TF2430. References 1C J Robinson

and M A Baker, .I Phys E: Sci Instrum, 11, 625 (1978).

A R Nyaiesh, The Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94305, USA

325

Workshop note

Figure 2. The wave form output for counting: (a) output wave form of a 2MHz quartz crystal; wave form of a 6MHz quartz crystal which has stop oscillating in sheer thickness mode.

326

(b) output

wave form of a 1OMHz quartz crystal;

(c)output