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 ICOPS99, 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 2-5kV, 10 mA/cm2 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 5x1012 cm-3 in a volume of 2 cm x 60 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 kHz 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 m2 UHV chamber for future processing tests. An overview of the LAPPS process along with theoretical treatments and issues will also be presented by co-authors1.

1 Presented in 5A01-02 by R. F. Fernsler.

Work supported by the Office of Naval Research

Plasma Production

MA

GN

ET

IC F

IEL

D

HOLLOW CATHODEBEAM SOURCE

emits

KILOVOLT ELECTRON BEAM

which efficiently

IONIZES THE BACKGROUND GAS

resulting in

A COLD PLASMA DISTRIBUTION

ANODE

1-2 CM

THIS PRODUCTION PROCESS THUS SCALES WITH THE

ELECTRON BEAM SOURCE

Lapps DiagnosticsA 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

Coils

30 cm wide plasma layer

Excellent for diagnostics

• Linear hollow cathode beam source

• 500 s, 2.4 kV pulse

• base pressure ~ 10 mTorr

TOP VIEWSIDE VIEW in operationOPTICAL EMISSION

SPECTROMETERlow resolution, 350-1100nm, minimum integration time of 2ms. Quickly gives entire

emission spectrum of plasma

MICROWAVE TRANSMISSION AND

NOISE MEASUREMENTS X band system operating 8.5-12.5 GHz. Attenuation

of microwaves can be directly related to ne

LANGMUIR PROBETh-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 175mTorr/225Gauss 75mTorr/210Gauss

100mTorr/90Gauss

Oxygen Discharge: Temporal data 2

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) 1.2x1012[f(GHz)/10]2. Thus for complete attenuation of 8.5 GHz Ws,ne9x1011cm-3. For 12 GHz, ne1.9x1012cm-3.

Basically, the O2 discharge shows twopreferred operating modes: (1) a short lived (~150s) 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.

0

5

10

15

20

845/777nm O(3S/

5S)

O(3P/

5P)1

st negative system

O(3P) + O(

3P)

O(3P) + O

+(

4S

0)

O(1D) + O

+(

4S

0)

X 2

g

a 4

u

X 3

g

-

b 4

g

-

O2

+

O2

O2 & O

2

+ MOLECULAR CURVES

ENER

GY

(eV)

INTRANUCLEAR SEPARATION

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 195 mTorr/300Gauss 95 mTorr/270Gauss

Neon Plasma Emission

3p manifold (9 states)

3s manifold (4 states)

Ne ground state

~17 eV

85 mTorr/300Gauss

All observed emissions are from neutral atoms, specifically from the 3p manifold of states to the 3s manifold. The 3s manifold is the lowest in energy, ~ 17eV above the ground state and consists of two metastable and two resonant states.

Differences in O2 and Ne discharges• O2 plasma destruction is recombination dominated (~n2), specifically by e + O2

+ 2O (or O + O*) while the Ne discharge is diffusion limited (~Dd2n/dx2), since there are no strong neutralization reactions in the 100mTorr regime. Gas mixtures can be very interesting...

• Neon discharges readily form high density (~1012cm-3) plasmas with or without large electron beam currents. O2 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 (500s) the electron beam has been turned off. In sharp contrast to the O2 plasma whose charged particles densities rapidly diminish after the pulse (40-60s). 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

BEAM DUMP CATHODE

KV ELECTRONS

PLASMA

T ~ cm

STAGE RF & TEMP CONTROL

MATERI ALTO BE

PROCESSED

FREE RADICALS

IONS

PLASMA ELECTRONS

L (~ meters)

BEAM ELECTRONS

BACKGROUND GAS

MAGNETIC FIELD

Initial Material Processing Test: Setup

A B

METAL PLATE

BIASCONNECTOR

Ti FOIL

PLASMADISTRIBUTION

ANODE SURFACEWAFER

Ti FOILLIMITER

MAGNETICFIELD

BEAMELECTRONS

A

B

Discharge Current10 A/div

CollectorCurrent1.9 mA/cm2-div

10 mm

6 mm

Collector current from 40 cm2 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 Photoresist0.1% duty, 20 sec total50 mTorr Oxygen gas

20 micron

Aluminum

Photo-resist

Silicon Wafer

2 cm

LAPPS Prototype Processing Chamber

RF Bias, Diagnostics

Beam EnergyAnalyzer

Shielded Cathode

B FieldCoils

Plasma Layer

Ground Plane, Diagnostics

Anode

• Aluminum body construction• Base pressure ~10-7 torr• fine control over gas flow

– residence time– gas mixture

TOP VIEW

SIDE VIEW of empty chamber in lab

LAPPS Parameters to be Investigated

Neutral Gas 10-1000 mTorr process gas

Plasma Density N+, N-, Ne up to ~ 1012 cm-3

Plasma Potential Vp low during pulse, Vp 0 in afterglowPlasma Temp Tion < Te < 1 eV

Electron Temp Control Auxiliary heating could raise Te to several eV

Free Radical Production Direct beam and dissociative recombination

Free Radical Species Species control via Te and pulse length (and gas)

Plasma Duty Cycle DC or arbitrary pulse with 10 microsecond on/off timeBias and Temp Control Independent stage for RF or DC bias and temp control

Plasma Processing Area Square meters, arbitrary location relative to surface

Uniformity Better than 1% desired, adjustable in one dimension

LAPPS UHV Compatible Chamber

End View Side View

Beam ProductionChamber

Processing ChamberField Coils

Pumps

1 m

• Scheduled for delivery 8/99• stainless steel construction• can accommodate 1m2 stage• separable cathode and processing chamber for cathode development

LAPPS UHV Compatible Chamber: Internal Arrangement

ElectronEmittingFilament

BeamOptics

Aperture andThin Foil

StageAdjustment RF Bias

ThermalControl

Aperture

Beam DumpAuxilary GroundingPlane Electron Beam

Processing StageMaterial

Beam Sources: Hollow Cathode

Anode

Insulator

GroundedShield

Hollow Cathode

Plasma

HV ElectronAnodeGrid

ReflexingElectrons

Magnetic Field

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 mA/cm2 beams produced

– 1-5 kV, 10-5000 s pulse, 10 kHz prf

• Significant plasma current

Beam Sources: Hot Filament Cathode• LAPPS beam requirements

– CW or modulated pulse

– <50 mA/cm2

– 15-20 keV 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 mA beam

– space charged limited beam

• LaB6 cathode in preparation– Pierce design extraction cathode

– post accelerate beam to 15-20 kV

Focusing Element:-45V wrt the Filament

First Acceleration Stage: +300V wrt the Filament

Second Acceleration Stage: + 2-5kV wrt the Filament (grounded)

Heated Filament

Beam Collector (not in photo):grounded througha 5.4 Ohm resistor

1st Stage

Filament Heater Contacts

Focusing Element

WORKING PROTOTYPE ASSEMBLY

2nd Stage10 cm

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