Thickness determination of ultrathin metal films using the X-ray fluorescence technique

Thin Solid Films, 67 (1980) 353-356 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands

353

THICKNESS DETERMINATION OF ULTRATHIN METAL FILMS USING THE X-RAY FLUORESCENCE TECHNIQUE D. K. KAUSHIK, S. PRIYOKUMAR SINGH, CHANDER BHAN, S. K. CHATTOPADHYAYA AND N. NATH Physics Department, Kurukshetra University, Kurukshetra 132119 (India) (Received September 25, 1979; accepted October 29, 1979)

The X-ray fluorescence technique was used to determine the thickness of singlelayered, double-layered and triple-layered films of copper, bismuth and gold on mylar substrates. An annular 1°9Cd X-ray source of 5 mCi was used to excite the characteristic X-rays. The background was much lower as well as fiat in the present study in comparison with our earlier results using an 241Am exciter source. This resulted in a downward extension of the lower limit of thickness measurement of thin films coupled with an improved accuracy.

1. INTRODUCTION In a previous communication~we used the X-ray fluorescence (XRF) technique to determine the thickness of single and composite metal films of copper and silver. The spectrum due to the 24~Am exciter source as well as that due to the mica substrate used in that work caused some complications in analysis of the data, mainly on account of the higher background level. In the present study an annular lO9Cd X-ray photon exciter source of 5 mCi was used. This is a photon exciter source emitting Ag Kot and Ag K[3 X-rays which contrast with the large number of source peaks in the case of the 241Am source. The use of a mylar substrate instead of mica resulted in the elimination of the impurity peaks that were present in the earlier study. With these improvements, we extended the earlier study to multilayer thickness measurements of films of copper, bismuth and gold. 2. EXPERIMENTAL

To estimate the thickness, characteristic Kct X-rays from copper films and characteristic L~ X-rays from bismuth and gold films were employed. Single and composite thin films of ultrapure copper (99.995~), bismuth (99.995~) and gold (99.998~) were deposited by vacuum evaporation on mylar substrates which had been precleaned by ionic bombardment. The thickness of each film was monitored during deposition using a calibrated quartz crystal monitor of frequency 8.5 MHz. The sensitivity of the crystal monitor used was 14.6 Hz A- 1 for copper, 16.09 Hz A- 1 for bismuth and 31.6 Hz A- ~ for gold. The other experimental details were as reported earlier 1.

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D.K. KAUSHIK et al.

RESULTS

AND

DISCUSSION

Figure 1 shows typical XRF spectra obtained with triple-layered thin film samples of copper, bismuth and gold (a) on a mylar substrate using a l°9Cd source and (b) on a mica substrate using an 241Am source• The thicknesses of copper, bismuth and gold were approximately 265/~, 120 ~ and 65 A respectively. It is clear that the background contribution due to the source radiation is lower in curve (a) than in curve (b). A linear relationship between the intensity of characteristic X-rays and the corresponding mass concentration of the target films has already been established 1. Figure 2 is a plot of the count rates observed for the relevant characteristic X-ray peak and the thicknesses of the copper, bismuth and gold films deposited separately on mylar as measured using a calibrated quartz crystal monitor. Similarly, Figs. 3 and 4 give similar plots for double-layered films of copper and bismuth and triple-layered films on copper, bismuth and gold, respectively. All these plots are linear. The errors shown here in the count rates are purely statistical.

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The errors in the thickness measured by the quartz crystal monitor are not shown in the figures but they have been estimated to be not more than 10~ for film thicknesses greater than 30 A and 20% for thicknesses below this limit. These errors mainly arise from effects of temperature and pressure in the evaporation unit at the time of deposition 1. The estimated minimum detection limits for thickness measurement on the basis of XRF counts were found to be 15 A, 11 A and 7 A respectively for the copper, bismuth and gold films using the 241Am source and mica substrates. These values decreased to 7 A, 7 A and 4 A for copper, bismuth and gold respectively when the 1 ° 9 C d s o u r c e and mylar substrates were used. ACKNOWLEDGMENTS

D.K.K. and C.B. are thankful to the Council of Scientific and Industrial Research, New Delhi (India), and the Department of Atomic Energy, Bombay (India), respectively, for financial assistance in the form of Research Fellowships. The authors thank Dr. S. N. Chaturvedi for valuable suggestions in preparing the manuscript. REFERENCE 1 S. Priyokumar Singh, D. K. Kaushik, S. K. Chattopadhyaya and N. Nath, Thin Solid Films, 59 (1) (1979) 51.