Thin Solid Films, 67 (1980) 347-351 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands 347

MEASUREMENT OF THIN FILM THICKNESS BY MEANS OF A SIMPLE NON-DESTRUCTIVE RADIOISOTOPIC TECHNIQUE

G. LUZZI, A. MAZZEI, A. NERI, M. SALMI AND G. SCHIRRIPA SPAGNOLO

Dipartimento di Fisica, Universith della Calabria, Cosenza 87100 (Italy)

(Received September 27, 1979; accepted October 26, 1979)

A simple quick and cheap technique for the measurement of thin metallic film thicknesses is described. The intrinsic resolution of the method is not as high as those of other techniques but its simplicity is very appreciable. Several samples, which were obtained by the vacuum deposition of nickel, chromium and zinc and which were accurately measured using a quartz microbalance system, were used to test the method. The films were successively examined with a radioisotopic gas detector X- ray fluorescence apparatus, producing several calibration curves. The results so far obtained show that with the aid of such a calibration the rough estimation of an unknown coating thickness is a very quick and easy task.

I. INTRODUCTION

The measurement of thicknesses of thin coatings is a fundamental requirement in industrial use (such as occurs in work on corrosion and catalysis) and in scientific investigations.

It is frequently very useful to have a method of measuring film thicknesses of up to 10 000 A, which uses a non-destructive technique.

From the industrial point of view, it is important to control deposition processes for economic reasons (process rapidity and optimal use of the depositing material); for scientific purposes it can be useful to have a method of verifying an unknown film thickness quickly, for calibration problems and when conventional methods are too long or complicated 1-3.

In this work an experimental study was performed to investigate the possibility of applying the radioisotopic X-ray fluorescence (XRF) technique to the measurement of thin metallic film thicknesses.

2. PROPOSED TECHNIQUE

The proposed method of analysis is by radioisotopic XRF. The sample to be measured is irradiated using a sealed-off radioisotopic source

that emits X-rays of the appropriate energy to excite the fluorescence X-rays characteristic of the element under examination (the coating element). The radiation emitted from the excited sample is detected by a proportional gas counter and its energy is electronically selected. Such a selection permits attention to be directed to

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the range of the emitted spectrum that is of interest for our proposal, i.e. the characteristic fluorescent peaks of the coating element.

The unknown thickness is obtained by means of a comparison of the experimental results (counts per second) with a calibration curve (which has a rectilinear shape for coatings of up to about 20 000 A) that has previously been obtained using standard samples.

The advantages of the proposed technique, if adopted with correct instrumentation, are fundamentally due to its intrinsic simplicity and rapidity and to the possibility of monitoring and controlling industrial processes on line.

3. EXPERIMENTAL RESULTS

Several experiments were performed to investigate the validity and limits of the proposed method.

The experimental apparatus consists of two units: a measuring head (a gas proportional counter coupled with a sealed-off radioisotopic source of 2~4Cm of activity 30 mCi) and an electronic apparatus for data handling and elaboration. The latter, coupled with an alternative specially designed measuring head, can operate automatically and on line.

In order to verify the potentiality of the technique, coating/substrate couples of industrial interest were chosen. The samples were prepared by vacuum evaporation. The vacuum was produced by the sequential use of a rotary oil pump (down to about 10 2 Torr), a zeolite pump cooled with liquid nitrogen (down to about 10-3 Torr) and ion pumps (down to 10-8-10- 9 Torr in about 10 h).

The background pressure sometimes increased during deposition, particularly for high deposition rates (due to crucible degassing, emission of gases from evaporating materials etc.); in these cases it was useful to aid the ion pumps with a titanium evaporation pumping system coupled with a cold finger or Meissner trap cooled with liquid nitrogen.

Nickel, chromium and zinc evaporations were performed using electron beam heating. The deposition rates were maintained at 0.5-2 A s -1. A quartz microbalance was used to measure the thicknesses and deposition rates.

All the coatings employed were obtained with a vacuum of better than 10 -6 Torr (to guarantee substantial purity of the deposited films).

Particular care was taken to ensure that the geometric conditions and operating procedures during the XRF measurements were constant. Figure 1 shows the adopted geometry.

Figure 2 shows three examples of calibration curves obtained so far with counting times of 200 s for three coating/substrate couples available to us; the plots of the obtainable precision refer to a confidence level of 95°/,.

4. CONCLUSIONS

Table I shows the results obtained in terms of minimum detectable limits (MDLs). Table II shows the smallest values of coating thicknesses that are normally of interest in industrial applications 4.

By comparing these two tables we observe that the MDLs obtained are

RADIOISOTOPIC MEASUREMENT OF THIN FILM THICKNESS 351

TABLE I MINIMUM DETECTABLE LIMITS a

Coating Substrate MDL (.~)

Cr Glass 160 Cr AI 145 Cr Steel 265 Ni Glass 61 Ni AI 59 Ni Steel 258 Zn Glass 38 Zn AI 55

a Detector: proportional gas counter. Source: 244Cm (30 mCi). Time: 200 s.

TABLE II MINIMUM THICKNESS OF COATINGS OF INDUSTRIAL INTEREST

Coating Minimum thickness (lam)

Sn 0.03 Zn 0.1 Pb 3 AI 4 Ni 2.5 Cu 2.5 Cr 10 Cd 2.5

substantially adequate, and it is clearly worthwhile employing the XRF technique, even using our simple instrumental version, in the industrial and scientific fields for the measurement of thin metallic film thicknesses.

REFERENCES

1 J.S. Watt, Int. J. Appl. Radiat. Isot., 16 (1965) 9-14. 2 F.V. Frazzoli and M. Salmi, Kerntechnik, 4 (1974) 163-164. 3 M. Salmi, A. Magrini, G. E. Gigante and O. A. Barra, lsotopenpraxis, 11 (1978) 382-383. 4 J .E. Reider, Iron Steel Eng., 39 (1972) 73.