1. ION ENERGY DISTRIBUTIONS IN INDUCTIVELY COUPLED PLASMAS HAVING A BIASED BOUNDARY ELECTRODE* Michael D. Logue and Mark J. Kushner Dept. of Electrical Engineering and Computer Science University of Michigan, Ann Arbor, MI 48109, USA mdlogue@umich.edu, mjkush@umich.edu Hyungjoo Shin, Weiye Zhu, Lin Xu, Vincent M. Donnelly, and Demetre J. Economou University of Houston, Department of Chemical & Biomolecular Engineering, Houston, TX 77204 November 2011 GECNov2011 * Work supported by SRC and US Dept. of Energy Office of Fusion Energy Sciences

2. AGENDA University of Michigan Institute for Plasma Science & Engr. GECNov2011  Control Of Ion Distribution Functions  Description of the Model and Geometry  Ion Energy Distributions (IEDs) and Plasma Parameters for Pulsed ICP  Pulsed ICP With Constant DC Boundary Electrode (BE) Bias  Pulsed ICP With Pulsed DC Boundary Electrode (BE) Bias  Concluding Remarks

3. CONTROL OF ION DISTRIBUTION FUNCTIONS University of Michigan Institute for Plasma Science & Engr. GECNov2011  In plasma materials processing there is a need to control the ion energy distributions (IEDs) to surfaces with increasing precision.  A recent development controlling of IEDs in inductively coupled plasmas (ICPs) is use of a boundary electrode.  A dc or pulsed dc bias to the boundary electrode shifts the quasi-dc plasma potential and in turn the peak in the IED incident onto grounded surfaces.  Using results from a computational investigation, the effect of pressure and BE bias (both dc and pulsed dc) on plasma parameters and IEDs will be discussed.  Example: Simulated IEDs and etch profiles for Ar/Cl 2 =80/20, 10 mTorr, 300 W peak ICP power, 100 W peak bias power, and 5 kHz pulse frequency, Duty Cycle=50%  Agarwal, J. Vac. Sci. Technol. A, Vol 29 (2011)

4. DESCRIPTION OF HPEM  Modular simulator that combines fluid and kinetic approaches.  Resolves cycle-dependent phenomena while using time-slicing techniques to advance to the steady state. GECNov2011 University of Michigan Institute for Plasma Science & Engr.

5. HPEM-EQUATIONS SOLVED -  Maxwell’s Equations – Frequency Domain Wave Equation GECNov2011  Azimuthal antenna currents – retain only E , B rz  Plasma currents  Collisional ion currents  Kinetically derived non-local electron currents capture nonlocal effects. University of Michigan Institute for Plasma Science & Engr.

6.  Electron Energy Distributions – Electron Monte Carlo Simulation GECNov2011  Cycle dependent electrostatic fields  Phase dependent electromagnetic fields  Electron-electron collisions using particle-mesh algorithm  Phase resolved electron currents computed for wave equation solution.  Captures long-mean-free path and anomalous behavior. University of Michigan Institute for Plasma Science & Engr. HPEM-EQUATIONS SOLVED -

7. GECNov2011 University of Michigan Institute for Plasma Science & Engr. HPEM-EQUATIONS SOLVED -

8. GECNov2011 University of Michigan Institute for Plasma Science & Engr. PLASMA CHEMISTRY MONTE CARLO MODULE  Pseudo-Particles are launched at random times during the rf cycle based on ion source functions vs position.  Trajectories are integrated based on stored electric fields interpolated as a function of time during pulse period.  Null collision techniques are used to account for elastic and inelastic collisions.  Since transit time of ions may exceed pulse period, the stored electric fields during pulse are treated as being periodic. For species K For (r,z) mesh cell I,J Get Source Function for Species K Determine # of particles To launch Launch particle at random time during rf cycle Follow particles until a surface Is hit. Include acceleration from Fields and particle collisions Collect statistics on particles That hit specified surface For particle 1:Total Particles Go to top of particle loop Go to top of mesh cell loop Go to top of species loop

9. BOUNDARY ELECTRODE ICP (BE-ICP): EXPERIMENT  BE-ICP is a cylindrical plasma driven by a solenoidal coil.  The biased electrode is at the top boundary consisting of annular rings.  Ion energies measured by retarding field energy analyzer (RFEA).  Electron, ion densities, temperatures: Langmuir probe  Argon, 10s mTorr, 40 sccm University of Michigan Institute for Plasma Science & Engr. GECNov2011

10. PULSED BE-ICP  In the BE-ICP system both the ICP power and the DC bias on the boundary electrode can be independently pulsed.  This can allow for greater tunability of the plasma parameters as well as the IED  Investigate effects of using pulsed ICP power with and without pulsed dc bias on the boundary electrode University of Michigan Institute for Plasma Science & Engr. GECNov2011 Time  = 1/  Duty Cycle Power(t) P max P min ICP Power Envelope Boundary Electrode DC Bias(t) Δt Bias

11. BOUNDARY ELECTRODE ICP (BE-ICP): MODEL  Model representation of ICP system.  RFEA is modeled as a flat, grounded metal. Gridded structure is not be resolved at this scale.  Cylindrically symmetric model resolves pump as annular port.  Integration in time to a pulse-periodic steady state. University of Michigan Institute for Plasma Science & Engr. GECNov2011

12. T e, Φ, n e DURING PULSE, Δt Bias = 42-60μs  Electron density peaks to 10 12 cm -3. Plasma potential controls the IED. University of Michigan Institute for Plasma Science & Engr. Animation Slide TeTe  Plasma Potential nene GECNov2011

13. T e, PLASMA POTENTIAL DURING PULSE PERIOD University of Michigan Institute for Plasma Science & Engr. GECNov2011  Experiment  T e and  both have “overshoot” at beginning of pulse period to re-establish plasma.  Modulation of  affords opportunity to shape IED.  Applying pulsed BE bias will further modulate , and so IED.  Duty cycle = 20%, PRF = 10 kHz, P(pulse average) = 120 W.  Model

14. University of Michigan Institute for Plasma Science & Engr. GECNov2011 IEDs – PULSED ICP WITH DC BOUNDARY  Experiment A B  IEDs consist of 2 peaks: High energy peak corresponding to large  during plasma on period.  Low energy peak due to smaller (and falling)  during plasma off period.  Vast majority of ionization occurs during plasma on part of cycle (20  s)  Ions “launched” during plasma on period require tens of  s to reach substrate.  High energy peak is broadened as ions sample dynamics of  during transport to substrate. Model A B Time Power(t) Boundary Electrode DC Bias (t) A B

15. University of Michigan Institute for Plasma Science & Engr. GECNov2011 IEDs – PULSED ICP WITH PULSED DC BOUNDARY  Experiment  Energy of peak is independent of pressure as  is determined by BE voltage.  Magnitude of peak decreases at high pressure due to collisionality, and populates intermediate energies.  Lower energy peak decreases as pressure increases due to increased collision frequency  Model predicts larger low energy component (very sensitive to acceptance angle of RFEA vs energy.) Model A A B B B Time Power(t) Boundary Electrode DC Bias (t) A B Δt Bias

16. University of Michigan Institute for Plasma Science & Engr. GECNov2011 IEDs – PULSED ICP, PULSED DC BOUNDARY IN EARLY AFTERGLOW  Experiment  The positions of the peaks in the IEADs are determined by the plasma potential  during the plasma on period, and the voltage of the BE.  The magnitude of the peak is determined by the length of time the BE is on.  Slight peak around 1.5 V in model due to non-zero minimum plasma potential. A B Time Power(t) Boundary Electrode DC Bias (t) Δt Bias Model B B B A A

17. University of Michigan Institute for Plasma Science & Engr. GECNov2011 IEDs – PULSED ICP WITH PULSED DC BOUNDARY IN LATE AFTERGLOW  Experiment  By moving pulse of BE to the late afterglow, the low and high energy peaks are more distinctively separate.  The magnitude of the high energy peak is controlled by the length of the BE pulse. A B Time Power(t) Boundary Electrode DC Bias (t) Model Δt Bias A A B B B

18. CONCLUDING REMARKS University of Michigan Institute for Plasma Science & Engr. GECNov2011  The use of a dc biased boundary electrode in ICP allows for control of the IED due to the shifting of the plasma potential by the dc bias.  Positive biases result in the increase of the IED peak energy by nearly the applied bias while negative biases resulted in a small, capped decrease in the IED peak energy.  Pulsing the dc bias on the boundary electrode in the ICP afterglow creates a narrower peak in the IED centered at approximately the applied bias potential. The height of the peak is determined by the length of the BE pulse.  This allows tuning of the energy and height of the peak energy of the IED.