Development of Special Hardware for RF-GD-OES

Dr. V. Hoffmann, Dr. L. Wilken

Our research in GD-OES in the last years focused on further development of radio frequency (RF) powered glow discharges in order to analyze non-conductive materials or non-conductive layers. Within a BMBF project in cooperation with LECO INSTRUMENTE GMBH a free running rf generator was developed [1]. The free running generator is easy to use. After the ignition of the discharge the RF voltage is stabilized in less than 10 ms, which is necessary to analyze thin layers in the nanometer range. The RF generator supplies voltages up to 2.5 kV (root-mean-square) that are required for the measurement of thick (up to 3 mm) non-conductive materials like glass.

The glow discharge analyzer measures the intensities dependent on time. To calculate a concentration depth profile it is necessary to sputter all analytical samples with the same discharge that is in DC characterized by voltage and current. For RF discharges similar signals should be available. Within a EU-Project and in cooperation with the Spectruma Analytik Mr. Wilken developed in his PhD a glow discharge source with integrated current and voltage sensors [2] (Figure 1).

Fig. 1 left: schema of the glow discharge source with integrated current and voltage
Fig. 1 right: GD source that was developed for the Spectruma Analytik.
Fig. 1 left: schema of the glow discharge source with integrated current and voltage
Fig. 1 right: GD source that was developed for the Spectruma Analytik.

The current signal is not disturbed by the leak current of the water cooling and it is only slightly disturbed by a displacement current. In Figure 2 the electron current and the ion current are visible. An electrical equivalent circuit is used to evaluate time averaged parameters of the signals [3,5]. 

We found that the ion current describes best the sputter and excitation process. Several certified reference materials are sputtered for constant ion current. The measured intensities Ii and sputtering rates SR are shown in Figure 3. Linear calibration curves results that are similar to that of DC-GD-OES measurements. We showed that the electrical signals are useable for the quantitative measurement of insulating materials [5]. The signals could also be used to evaluate characteristic discharge parameters [5]. In plasma reactors for etching or deposition the electrical probes could also deliver valuable information about the discharge.

Figure 2: Voltage and current signal sputtering a copper sample ( 770 Pa, f = 3.3 MHz, Ppl = 9,8 W).
Figure 2: Voltage and current signal sputtering a copper sample ( 770 Pa, f = 3.3 MHz, Ppl = 9,8 W).
Figure 3: Measurements at different certified reference materials (CRM) that are sputtered for constant voltage and current (Al-line, 396.15 nm, Uk =700V, Ik =20mA).
Figure 3: Measurements at different certified reference materials (CRM) that are sputtered for constant voltage and current (Al-line, 396.15 nm, Uk =700V, Ik =20mA).

Literature including links:

[1] Hoffmann V., Uhlemann H.-J., Präßler F., Wetzig K., Birus D., New Hardware for Radiofrequency Powered Glow Discharge Spectroscopies and its Capabilities for Analytical Applications, Fresenius’ Journ. Anal. Chem. 355, 1996, 826-830
[2] Wilken L. , Hoffmann V.,  Uhlemann H.-J., Siegel H., Wetzig K., Development of a Radio-Frequency Glow Discharge Source with Integrated Voltage and Current Probes, J. Anal. At. Spectrom. 18, 646 (2003).
[3] Wilken L.  Hoffmann V., Wetzig K., Analysis of new electrical signals in respect to quantification of radio frequency glow discharge emission spectrometry, Appl. Surf. Sci. 252, 261-265 (2005).
[4] Wilken L.  Hoffmann V., Wetzig K., Radio frequency glow discharge source with integrated voltage and current probes used for sputtering rate and emission yield measurements at insulating samples, Anal. Bioanal. Chem. 383 [3], 424-433 (2005)
[5] Wilken L., Theoretische und experimentelle Untersuchungen zur Optimierung einer Glimmentladungsquelle für spektroskopische Messungen von elektrisch leitenden und nichtleitenden Materialien, Shaker-Verlag, 2004, ISBN: 1610-4773/3-8322-3446-2.