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 Greifswald, Germany. The work has been supported by the Royal Dutch Academy of Sciences (KNAW) and the Deutsche Forschungsgemeinschaft (DFG) under SFB198/A14. Acknowledgment 13 n

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 W/cm 2 heating mainly due to ions and electrons –Oxygen 50% higher heat flux than argon molecular surface processes are important as well

Substrate heating: c s dT/dt =  in -  out  in = Heat flux J x times probe surface: –ions: kinetic recombination –electrons:kinetic –neutrals: kinetic, internal, association, chemical –photons: blackbody, plasma  out: –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 C s : heat capacity substrate; j i,j e ion/electron flux; V pl -V fl acceleration voltage of ions in sheath Note

Thermal probe Principle: heat flux determines the heating time of the probe The probe is a Cu plate, diameter 3.4 cm, height 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.

Thermal probe: raw data T S (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.

Experimental setup Capacitively coupled MHz plasma. –Al electrode D=130mm –Spherical reactor D=400mm. Diagnostics: –Thermal probe –Langmuir probe –CCD camera Typical conditions: –1Pa, 15W Ar or O 2 –Argon: T e = 3.5 eV n e = m -3

Results: Argon Calculated contributions by ions (J i, J rec ) and electrons (J e ) to the thermal balance of the substrate. The calculations are based on n e 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

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

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 W/cm 2. –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