1. Large Area Plasma Processing System (LAPPS) R. F. Fernsler, W. M. Manheimer, R. A. Meger, D. P. Murphy, D. Leonhardt, R. E. Pechacek, S. G. Walton and M. Lampe Naval Research Laboratory Presented at ICOPS, June 1999 *Work supported by the Office of Naval Research

2. Outline General description of LAPPS Experiments u See Poster 6P01 for details. Physics Comparison with other sources Summary

3. LAPPS Overview A magnetically confined, sheet electron beam is used to ionize a gas. Purpose: Create a cold, weakly ionized, planar plasma for material processing. Key distinguishing features: u Beam ionization replaces plasma heating. u Large processing area (~ 1 m 2 ). Limited by the size of the vacuum chamber and power.

4. Experimental Program Status:  n e & 5x10 12 cm -3 over 60 x 60 x 2 cm. u Wide range of gases, 20-300 mtorr.  Etch rate in oxygen > 6  m/min. Grounded stage (no bias). Immediate Plans: u Extend area to 1 m 2. u Etch with rf bias. u Investigate other beam sources. Present source is a hollow-cathode discharge. Diagnostics: Langmuir probe,  -wave and optical, electron energy analyzer, mass spectrometer, SEM.

5. Processing Objectives Large area. u But thin to minimize electrical power. Independent control of ion and free-radical fluxes. u Wide operating range in power, gas and bias. High uniformity. Efficient. Low T e. u To maximize ion anisotropy.

6. Design Constraints Beam current and energy. Gas pressure and composition. Magnetic field. Particle fluxes.

7. Beam Energy Energy loss rate:  N = gas density, Z = atomic number,  o ~ 100 eV. Beam range : Increase R by raising  b or reducing N.  R > 1 m in 100 mtorr   b * 3 keV.  N * 20 mtorr to avoid two-stream instability.

8. Beam Current Gas ionization rate:  j b = beam current density, w i  30 eV per e-i pair. Loss rate, worst case:  Where  ~ 5x10 -8 cm 3 /s (for e+AB +  A+B).  n e = 10 12 cm -3 requires j b  10 mA/cm 2. n b /n e < 10 -4. Dissociation rate: u Metastable rate: S m << S i.

9. Beam Confinement Elastic collisions cause the beam to spread. Apply B z to keep the gyroradius r c <  x b.   x b = beam thickness. r c < 1 cm at ~ 3 keV  B z * 200 G.

10. Effect of B z on the Plasma Outward plasma flow produces a diamagnetic current circulating around the plasma: B z  u Only the electrons are strongly magnetized. Outward flow is impeded if  m = mass, = collision frequency of ions & electrons.   N  Flow is impeded when B z /N * 2 kG/torr.

11. Electron Temperature T e Initial mean energy of secondary electrons: u I i = ionization energy. Effective “heating” rate: Collisional cooling: T e = T e (Q). u Predict T e < 1 eV in molecular gases. Consistent with Langmuir probe. Predict T e > 1 eV in noble gases. u Can raise T e with external heating.

12. Particle Fluxes Fluxes: B z and recombination limit the ion flux to  Increase B z or  x to decrease F i.  Add an atomic gas (   0) to increase F i. Free-radical flux:  Independent of B z,  x, or atomic additives. xx beam/plasma substrate F i,F r

13. Uniformity j b  constant but  b falls with z. u S i and n e thus rise (smoothly) with z. One solution is to increase  b (0). u Decreases efficiency, unless the energy is recovered prior to the beam dump. Other solutions:  Vary B z (z) or  x(z) to make F i more uniform. u Vary N(z) to make F i and F r more uniform. u Move the stage.

14. “Conventional” Sources Heat the plasma to produce ionization. u T e > 2 eV. Inefficient: w i >> I 1 ( in processing gases). u Energy goes mainly to low-lying excitation. Restricted operating range. u In gas type, pressure and power. Limited control. u Species with low I i are ionized preferentially. u Heating zone is determined by E-to-n e coupling.

15. LAPPS Issues Requires: u Beam source. u Isolation of the source from the processing region. u B z. Beam-plasma instabilities. u Suppressed by collisions and beam velocity spread. RF bias: u Small ground electrode(s). u Effect of B z on the sheath.

16. LAPPS Summary Plasma density n e  5x10 12 cm -3 (above 20 mtorr). Area ~ 1 m 2. u Limited by chamber size and power. Works in all gases over a wide pressure range. u Ionization rate depends mainly on concentration. Independent control of ion and radical fluxes. Smooth (no hot spots). u But some variation along B z. Efficient. u Thin.  w i  30 eV, depending on tradeoff for uniformity along B z. T e < 1 eV.