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

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

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 c s dt dt f in f out

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

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

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

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

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

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

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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