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

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

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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.


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


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


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.


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.


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


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


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


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


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