Diagnostics and Experiments on LAPPS D Leonhardt D

Diagnostics and Experiments on LAPPS* D. Leonhardt, D. P. Murphy, S. G. Walton, R. A. Meger, R. F. Fernsler, R. E. Pechacek Plasma Physics Division, U. S. Naval Research Laboratory, Washington, DC 20375 -5346 presented at ICOPS 99, Monterey, CA

ABSTRACT NRL is developing a new plasma processing reactor called the ‘Large Area Plasma Processing System’ with applications to semiconductor processing and other forms of surface modification. The system consists of a planar plasma distribution generated by a magnetically collimated sheet of 2 -5 k. V, 10 m. A/cm 2 electrons injected into a neutral gas background. This beam ionization process is both efficient at plasma production and readily scalable to large (square meters) area. The use of a beam ionization source largely decouples the plasma production from the reactor chamber. Ion densities (oxygen, nitrogen, argon, helium) of up to 5 x 1012 cm-3 in a volume of 2 cm x 60 cm have been produced in the laboratory. Typical operating pressures range from 20– 200 mtorr with beam collimating magnetic fields strengths of 10– 300 Gauss. Thus far the system has been operated with a pulsed (10 -2000 s pulse length, <10 k. Hz pulse repetition frequency) hollow cathode. Temporally resolved measurements of the plasma sheet using Langmuir probes, spectrally resolved optical emission, microwave interferometry, and cyclotron harmonic microwave emission will be presented. Results of initial processing tests using an oxygen plasma showing isotropic ashing of a photoresist will be shown. Progress in the development of a dc hot filament cathode will be presented along with the status of the 1 m 2 UHV chamber for future processing tests. An overview of the LAPPS process along with theoretical treatments and issues will also be presented by co-authors 1. 1 Presented in 5 A 01 -02 by R. F. Fernsler. Work supported by the Office of Naval Research

Plasma Production HOLLOW CATHODE BEAM SOURCE THIS PRODUCTION PROCESS THUS SCALES WITH THE ELECTRON BEAM SOURCE emits MAGNETIC FIELD KILOVOLT ELECTRON BEAM which efficiently IONIZES THE BACKGROUND GAS resulting in A COLD PLASMA DISTRIBUTION ANODE 1 -2 CM

Lapps Diagnostics A variety of diagnostics are necessary to determine the critical parameters in the plasma environment and surface interactions: • ELECTRON BEAM – Current and voltage monitors – Electron energy analyzer - beam energy loss, distribution • PLASMA – – – Langmuir probes - time resolved determination of floating potential, Te, ne Microwave transmission - highly accurate but global measurement of ne Charge collectors/photodetachment experiments - to study negative ion production Optical spectroscopy - non-intrusive determination of ionic species, temperature Laser induced fluorescence - non-intrusive determination of ion/neutral species with high spatial resolution • SURFACE – Quadrupole mass spectrometer - fluxes of charged and neutral particles to surfaces being studied as well as ion/neutral energy distributions – Topological diagnostics post processing - SEM/AFM

Acrylic Test Chamber Excellent for diagnostics • Linear hollow cathode beam source • 500 s, 2. 4 k. V pulse • base pressure ~ 10 m. Torr OPTICAL EMISSION SPECTROMETER low resolution, 350 -1100 nm, minimum integration time of 2 ms. Quickly gives entire emission spectrum of plasma 30 cm wide plasma layer Coils SIDE VIEW in operation TOP VIEW MICROWAVE TRANSMISSION AND NOISE MEASUREMENTS X band system operating 8. 5 -12. 5 GHz. Attenuation of microwaves can be directly related to ne LANGMUIR PROBE Th-W probe to temporally resolve plasma’s Te, ne, floating potential, saturation currents. . . PHOTOMULTIPLIER TUBE to determine temporal response of light emission. Can be coupled to 1/4 m monochrometer to temporally resolve specific lines when applicable

Oxygen Discharge: Temporal data 1 75 m. Torr/225 Gauss 75 m. Torr/210 Gauss

Oxygen Discharge: Temporal data 2 100 m. Torr/90 Gauss Basically, the O 2 discharge shows two preferred operating modes: (1) a short lived (~150 s) high density mode at lower pressures and high magnetic fields (2) a long lived high density /low impedance mode at higher pressures THE LANGMUIR PROBE QUICKLY BECOMES CONTAMINATED, so only Iesat, Iisat and Vfloat are shown. Presently we are looking at heated and emissive probes to circumvent this problem. USING MICROWAVE ( W) TRANSMISSION TO DETERMINE PLASMA DENSITY: Microwaves penetrate a finite distance into plasma even when below the critical frequency. Assuming a uniform plasma profile with thickness < W wavelength, attenuation of microwaves is (to first order) given by ne(cm-3) 1. 2 x 1012[f(GHz)/10]2. Thus for complete attenuation of 8. 5 GHz Ws, ne 9 x 1011 cm-3. For 12 GHz, ne 1. 9 x 1012 cm-3.

Oxygen Plasma Emission • High-lying excited states are seen in visible regime with atomic emissions apparently dominant. • Excited atomic states have possible channels from molecular parentage or purely atomic precursors after dissociation of ground state molecule. Time resolved line emissions should assist in this determination (in progress. . . )

Neon Discharge: Temporal Data 1 95 m. Torr/300 Gauss 95 m. Torr/270 Gauss

Neon Plasma Emission 85 m. Torr/300 Gauss 3 p manifold (9 states) 3 s manifold (4 states) ~17 e. V Ne ground state All observed emissions are from neutral atoms, specifically from the 3 p manifold of states to the 3 s manifold. The 3 s manifold is the lowest in energy, ~ 17 e. V above the ground state and consists of two metastable and two resonant states.

Differences in O 2 and Ne discharges • O 2 plasma destruction is recombination dominated (~ n 2), specifically by e + O 2+ 2 O (or O + O*) while the Ne discharge is diffusion limited (~Dd 2 n/dx 2), since there are no strong neutralization reactions in the 100 m. Torr regime. Gas mixtures can be very interesting. . . • Neon discharges readily form high density (~1012 cm-3) plasmas with or without large electron beam currents. O 2 discharges were less forgiving. For materials processing applications, all possibilities should be explored; fluxes to the surface are to be measured via in situ mass spectrometry as well basic materials’ test exposures. • Ne plasma shows significant charged particle densities well after (500 s) the electron beam has been turned off. In sharp contrast to the O 2 plasma whose charged particles densities rapidly diminish after the pulse (40 -60 s). Conclusive measurements of specific species (charged and neutral) along with their time dependencies will also be studied via mass spectrometry. • Argon shows very similar behavior as Neon, but Ar+ emission lines are also seen in the visible spectrum. The analogous behavior is reassuring; Ne+ emission may merely be out of the spectral region we have access to. • Hollow cathode operation also varies, although this dependence is difficult to pinpoint at the present time. Hence, we are intending to measure the electron beam energy/distribution at the anode with a hemispherical energy analyzer. Additional work with different cathode shapes show a variety of plasma operating conditions. • Langmuir probe data closely mirrors the dependencies of the optical emission and electron beam current (somewhat) although the probe has a much smaller dynamic range (changes in factors or 2 -4) vs. the non-intrusive techniques (10 -100’s). It is unclear at this time whether this phenomena is a technical issue of probe applications.

LAPPS for Materials Processing PLASMA MAGNETIC FIELD BEAM DUMP CATHODE T ~ cm KV ELECTRONS L (~ meters) MATERI AL TO BE PROCESSED STAGE RF & TEMP CONTROL BEAM ELECTRONS PLASMA ELECTRONS IONS BACKGROUND GAS FREE RADICALS

Initial Material Processing Test: Setup A 10 mm B 6 mm Collector Current 1. 9 m. A/cm 2 -div Discharge Current 10 A/div Collector current from 40 cm 2 plates located 10 mm and 6 mm from oxygen plasma edge for -20 V bias and total discharge current

Actual Material Modification: Aluminum Mask on Photoresist Etched Photoresist 0. 1% duty, 20 sec total 50 m. Torr Oxygen gas

LAPPS Prototype Processing Chamber • Aluminum body construction • Base pressure ~10 -7 torr • fine control over gas flow – residence time – gas mixture Ground Plane, Diagnostics Plasma Layer Anode Shielded Cathode Beam Energy Analyzer SIDE VIEW of empty chamber in lab B Field Coils RF Bias, Diagnostics TOP VIEW

LAPPS Parameters to be Investigated

LAPPS UHV Compatible Chamber • • Scheduled for delivery 8/99 stainless steel construction can accommodate 1 m 2 stage separable cathode and processing chamber for cathode development Field Coils Processing Chamber Beam Production Chamber Pumps End View Side View 1 m

LAPPS UHV Compatible Chamber: Internal Arrangement Auxilary Grounding Plane Beam Dump Electron Beam Aperture Stage Adjustment Thermal Control RF Bias Processing Stage Aperture and Thin Foil Electron Emitting Filament Material Beam Optics

Beam Sources: Hollow Cathode Pulsed linear hollow cathode used extensively to date • Beam electrons produced by secondary emission from ion bombardment – eff < 0. 2 – cathode mat. , ion species, energy dependent – resonance with B • 60 cm long, 50 m. A/cm 2 beams produced – 1 -5 k. V, 10 -5000 s pulse, 10 k. Hz prf • Significant plasma current

Beam Sources: Hot Filament Cathode Beam Collector (not in photo): grounded through a 5. 4 Ohm resistor • LAPPS beam requirements – – CW or modulated pulse <50 m. A/cm 2 15 -20 ke. V beam energy linear cathode with 1 cm x 10 -100 cm width – ~1% uniformity • Initial experiments with thoriated tungsten filament – – 1 cm x 10 cm beam aperture 20 Gauss, 240 V extraction 3 cm FWHM, 50 m. A beam space charged limited beam • La. B 6 cathode in preparation Second Acceleration Stage: + 2 -5 k. V wrt the Filament (grounded) First Acceleration Stage: +300 V wrt the Filament Focusing Element: -45 V wrt the Filament Heated Filament WORKING PROTOTYPE ASSEMBLY 1 st Stage 10 cm – Pierce design extraction cathode – post accelerate beam to 15 -20 k. V Filament Heater Contacts Focusing Element 2 nd Stage

Acknowledgements We greatly appreciate the assistance of Dr. W. E. Amatucci with the Langmuir probe measurements. SGW is a National Research Council Postdoctoral Research Associate and REP is a member of SFA, Inc. , (Landover, MD). This work is supported by the Office of Naval Research