The energy influx from an rf plasma to a substrate during plasma processing

1. 13 n The energy influx from an rf plasma to a substrate during plasma processing W.W. Stoffels, E. Stoffels, H. Kersten*, M. Otte*, C. Csambal* and H. Deutsch Department of Physics, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven * Institute for Physics, University of Greifswald, Domstr. 10a, D-17487 Greifswald, Germany. Acknowledgment The work has been supported by the Royal Dutch Academy of Sciences (KNAW) and the Deutsche Forschungsgemeinschaft (DFG) under SFB198/A14.

2. Abstract • Aim: determine the energy flux to a substrate in an low pressure rf plasma • Method: calorimetric probe • Results: • Argon: • heat flux is few times 10-3 W/cm2 • heating mainly due to ions and electrons • Oxygen • 50% higher heat flux than argon • molecular surface processes are important as well

3. Substrate heating: csdT/dt = Fin-Fout • Fin = Heat flux Jx times probe surface: • ions: kinetic recombination • electrons:kinetic • neutrals: kinetic, internal, association, chemical • photons: blackbody, plasma • Fout: • thermal conduction of gas and substrate • radiation The ion and electron heating depends on surface potential: =>Separation of neutral component possible by using a bias voltage Cs: heat capacity substrate; ji,je ion/electron flux; Vpl -Vfl acceleration voltage of ions in sheath Note

4. Thermal probe • Principle: heat flux determines the heating time of the probe The probe is a Cu plate, diameter 3.4 cm, height 0.002 cm. Mounted to a thermocouple and shielded from below (see picture). It can be moved (x,y,z) and rotated. Photograph of the thermal probe placed in the glow at substrate position.

5. Thermal probe: raw data TS(t)-curves as measured during the argon plasma process (p=1Pa, P=15W) for three substrate voltages (0, -46, -95V). Rising edge plasma on, decreasing edge plasma off. The plasma heat flux is determined from the derivative signal Current-voltage characteristic of the thermal probe for argon and oxygen. The measured electron and ion flux is used to separate ion and electron heating from neutral heating.

6. Experimental setup • Capacitively coupled 13.56 MHz plasma. • Al electrode D=130mm • Spherical reactor D=400mm. • Diagnostics: • Thermal probe • Langmuir probe • CCD camera • Typical conditions: • 1Pa, 15W Ar or O2 • Argon: Te = 3.5 eV ne = 2 1015 m-3

7. Results: Argon Calculated contributions by ions (Ji, Jrec) and electrons (Je) to the thermal balance of the substrate. The calculations are based on ne measured by the Langmuir probe and a Bohm flux. For the electron current (right branch) the measured substrate current is used. Measured data fitted by the model results. Left Right

8. Results: Oxygen • Similar trends for oxygen and argon • Overall higher heat flux in oxygen due to neutral heating • ne(oxygen) < ne(argon) so electron branch is smaller Measured integral energy influx (Qin) for argon and oxygen, respectively, for the same macroscopic discharge conditions. Comparison with argon

9. Conclusions • Thermal heat flux to a substrate can be measured by probe • Electron, ion and neutral heating can be separated • Argon 15W, 1Pa: • heat flux few times 10-3 W/cm2. • Increases with bias voltage • mainly ion (and electron) heating • Oxygen 15 W, 1Pa: • same trends • significant influence of neutral heating • These data are also valid for heat flux in dusty plasmas