PREDICTABILITY IN LOW TEMPERATURE PLASMAS: FROM LABORATORY ...

PREDICTABILITY IN LOWTEMPERATURE PLASMAS:

FROM LABORATORY TO TECHNOLOGY*

Mark J. KushnerUniversity of Michigan

Dept. of Electrical Engineering and Computer ScienceAnn Arbor, MI 48109 USA

[email protected] http://uigelz.eecs.umich.edu

November 2008

* Work supported by NSF, SRC, Applied Materials, TEL Inc., 3M Inc., AFOSR

AGENDA

• Low Temperature Plasma Science – Providing Societal Benefit

• LTPS – Its role in the IT Infrastructure

• Two Convergent Science Based Technologies

• Electric Discharge Excimer Lasers

• Plasma Materials Processing

• Where to from here? Low Temperature Plasma Science Workshop

University of MichiganInstitute for Plasma Science and Engineering

• Low temperature plasmas are a power transfer medium.

• Electrons transfer power from the "wall plug" to internal modes of atoms / molecules to "make a product”, very much like combustion.

• The electrons are “hot” (several eV to 10 eV) while the gas and ions are cool, creating “non-equilibrium” plasmas.

WALL PLUG

POWER CONDITIONING

ELECTRIC FIELDS

ENERGETIC ELECTRONS

COLLISIONS WITHATOMS/MOLECULES

EXCITATION, IONIZATION, DISSOCIAITON (CHEMISTRY)

LAMPS LASERS ETCHING DEPOSITIONE

eA

PHOTONS RADICALS

IONS

COLLISIONAL LOW TEMPERATURE PLASMAS


• Displays• Microelectronics Processing

SOCIETAL BENEFIT – SCIENCE BASED TECHNOLOGIES

• Lighting

• Healthcare

• Thrusters• Jet Engine Spray

Coatings

IMPACT OF LTPS ON DAILY LIFE…INCREDIBLE


• Ref: Plasma 2010 Decadal Study

PLASMA LIGHTING AND THE ENERGY ECONOMY

• Annual US electrical power consumption: 3.5 x 1012 kW-Hr

• Electrical power expended in lighting: 22% (7.6 x 1011 kW-Hr)

• Expended in fluorescent lamps: 9% (3.1 x 1011 kW-Hr)

• 35 1-GWe power plants are used to excite a single multiplet of Hg states in fluorescent lamps.http://www.eia.doe.gov/cneaf/electricity/epa/epates.htmlhttp://antwrp.gsfc.nasa.gov/apod/ap970830.html http://www.eere.energy.gov/buildings/info/documents/

pdfs/lmc_vol1_final.pdfUniversity of Michigan

Institute for Plasma Science and Engineering

• Conversion of incandescent to plasma lighting would save 2.2 x 1011 kW-Hr a year…the equivalent of 24 1-GWe power plants.

http://www.gelighting.com/na/

• GE-A19 Incandescent 17 Lumens/W

• GE-T3 Plasma Fluorescent 52 Lumens/W

• GE BD-17 High Intensity Discharge87 Lumens/W

PLASMA LIGHTING AND THE ENERGY ECONOMY

• Optimizing the electron f(ε) in plasma lighting by 0.1 eV translates into three 1-GWe plants….and this has been done.

• This is an incredible accomplishment and mastery of discharge physics.


ON THE GROUNDDISCHARGE PHYSICScollisions

vx tff

mEqfv

tf

⎟⎠⎞

⎜⎝⎛∂∂

−∇⋅−∇⋅−=∂∂

rr

• Reaction mechanism

• Boltzmann’s Equation

• Cross Sections

• Electron Energy Distributions

• Power Flow to Excited States


• Optimizing Ar/Hg fluorescent lamps.

LTPS – A VERY DIVERSE FIELD

• The diversity of field makes leveraging advance in science to produce society benefiting technologies challenging.


• Applied Materials PECVD for LCD panels and solar cells.

• Microplasma arrays (Ref: J. G. Eden)

• Example - Size: Large, stable plasmas (5 m2 plasmas) for LCD television panels to tiny (100 µm2) plasmas so intense that the plasma electrons merge with solid electrodes.

• Excimer Laser Lithography


• Roadrunner Supercomputer

TWO LTP TECHNOLOGIES HAVE ENABLED THE WORLDWIDE IT INFRASTRUCTURE

• Electric discharge excited excimer lasers…• RF discharge plasma etching, deposition and sputtering systems…• Two low temperature plasma systems singlehandedly responsible for

the worldwide information technology infrastructure.


• www.intel.com

• Moore’s law would not exist in the absence of LTPs.

• ….And it can all be traced to our mastering the application of Boltzmann’s equation to technological plasmas.

• Understanding the coupling of electron and ion velocity distributions to photon-generation and radical production have created our high technology infrastructure.


TWO LTP TECHNOLOGIES HAVE ENABLED THE WORLDWIDE IT INFRASTRUCTURE

CHAPTER 1 – ELECTRIC DISCHARGE EXCIMER LASERS FOR PHOTOLITHOGRAPHY

ELECTRIC DISCHARGE EXCIMER LASERS FOR PHOTOLITHOGRAPHY

• Microelectronics features are defined by photolithography –transferring a pattern from a mask to the silicon wafer.

• Feature sizes are limited by the wavelength - excimer UV lasers have enabled sub 0.1 µm features

• www.nature.com

University of MichiganInstitute for Plasma Science

and Engineering

EXCIMER LASERS FOR PHOTOLITHOGRAPHY

• …A triumph of applying incredibly complex plasma chemistry to development of 24/7 dependable technology.


TIGHT COUPLING OF PRODUCTS AND f(ε)

( ) ( )

( ) ( )collisions

v

x

ttrvftrvfa

trvft

trvf

⎟⎠⎞

⎜⎝⎛

∂∂

+∇⋅−

∇⋅−=∂

∂

,,,,

,,,,

rrrrr

rrrrr

ν


• Tight coupling between atomic-molecular properties, ionization fraction and excitation rates provides the means to customize plasmas to produce desire end products…this was known early on.

( )

( ) ( ) vdNNvtrvft

trN

ijijji

3

,,,

,

σν∑∫−

=∂

∂

rrr

r

( ) ( ) ( )( )

( ) ( ) ( )θ

θ

cost,r,vft,r,vf

cosPt,r,vft,r,vf

i

ii

i

rr

rrr

+≈

=∑0

• Early approximate methods for solving Boltzmann’s equation involved 2-term spherical harmonic expansions.

• First introduced by Morse, Allis and Lamar (1935) and popularized by Holstein (1946) in context of low pressure plasmas.

EARLY WORK ON BOLTZMANN’S EQUATION


• Deviation from Maxwellian is in part the anisotropy of f(v).

• Capture anisotropy in spherical harmonic expansion (SHE).

• In absence of statistical methods (e.g., Monte Carlo simulations), SHE of Boltzmann’s equation used for non-isotropic transport.

• Higher order terms enabled subtleties to be revealed and application to highly nonequilibrium situations.

IMPROVEMENTS IN EXPANSION THEORY


• Leveraging connection between f(ε) and power flow to specific excited states enabled a revolution in technology development.

• …Fostered by numerical solutions of Boltzmann’s equation.

LEVERAGING f(ε) TO POWER FLOW

• EEDs in CO2 vs E/N Nighan, PRA 2, 1989 (1970)


• These works provided fundamental guidance for developing highly efficient discharge excited lasers by optimizing f(ε) as the excited state manifold evolved.

OPTIMIZING f(ε) FOR LASER TECHNOLOGIES

• EEDs in CO2 vs T(vib)


• Became clear that optimizing rates of excitation of CO2(v) was incompatible with self sustained discharges.

• Motivated development e-beam sustained discharges.

• Fractional energy in CO2vs E/N.

• Nighan, PRA 2, 1989 (1970)

• E-beam sustained discharges optimized f(ε) to excite CO(v) while providing background ionization. Multi-kJ pulses were realized –but instabilities terminated the pulses.

E-BEAM SUSTAINEDCO(v) LASER - INSTABILITIES


• High power laser development was put on science basis by experimentally tracking power flow through excited states.

BOLTZMANN KINETICS AND DIAGNOSTICS LEADTO SCIENCE BASED DESIGN


• Complex, validated, design capable computer models were developed based on Boltzmann kinetics.


BOLTZMANN KINETICS AND DIAGNOSTICS LEADTO SCIENCE BASED DESIGN


• Streamers originating from cathode fall perturbations imprinted by low pre-ionization densities were diagnosed…..

GLOW-TO-ARC TRANSITIONS TERMINATE LASER

• Kinetics were understood but discharge instabilities prematurelyterminated laser pulses…Realization of importance of pre-ionization revolutionized the field.

REMEDYING DISCHARGE INSTABILITIES IN EXCIMER LASERS



• Models using Boltzmann analysis verified need for critically large pre-ionization density to prevent micro-arcs.

MODEL BASED SCALING LAWS


TECHNOLOGY’S RESPONSE• An entire new generation of laser technology was developed built

upon the fundamental understanding of micro-arc generation – and how to prevent it – using x-ray preionization.


INDUSTRY QUALIFIED ArF EXCIMER LASER PHOTOLITHOGRAPHY – NEXT GENERATION EUV

• ArF discharge lasers enable 32 nm photolithography.

• Discharge laser produced plasmas are basis for next generation EUV sources.

• Cymer, Inc.

• Cymer 50 W EUV demonstration –Semi. Intl., Nov. 2008

CHAPTER 2 – PLASMA MATERIALS PROCESSING

PLASMAS FOR NANOSCALE FABRICATION

• Plasmas are and will continue to be indispensable for etching, deposition and cleaning in microelectronics fabrication.

• Control of dimensions at 22 and 35 nm nodes requires resolution of a few nm or less.

• http://www.intel.com http://realworldtech.com


and Engineering

• Required: Unprecedented control of reactant fluxes from the plasma onto the wafer: Uniformity, Composition, Energies

ACTIVATION ENERGY: SUB-eV, SUB-DEGREE CONTROL

• Activation energy is largely delivered through ion bombardment.

• Distinguishing between materials will be determined by sub-eV and sub-degree control of ion energies.

• Intel Fin-FET University of Michigan

Institute for Plasma Scienceand Engineering

• …and with excimer laser photolithography

MULTI-FREQUENCY CAPACITIVELY COUPLED PLASMA ETCHER:APPLIED MATERIALS CENTURA ENABLER

• Plasma etching of dielectric materials for logic contacts and interconnect – 300 mm wafers at the 45 nm node.

• Ref: S. Rauf, AMAT University of MichiganInstitute for Plasma Science and Engineering

TYPICAL PLASMA ETCHING REACTOR

• Hitachi XT ECR

http://www.hitachi-hta.com University of MichiganInstitute for Plasma Science and Engineering

ION NEUTRAL SYNERGISM• The basis of plasma etching is the synergism between neutral

surface chemistry and ion activation.

• Separately controlling composition and energies of radicals and ions will enable precise feature evolution.


SELECTIVITY IN PLASMA ETCHING

• Fabricating microelectronics devices requires preferential etching a material – selectivity.

• Selective etching occurs by controlling radical fluxes and the ion energy and angular distribution (IEAD) to wafer.

• Sheath physics will dominate

mainetch_ied


EARLY PROGRESS IN SHEATH DYNAMICS

• With the realization that sheath physics would dominate the ability to selectively etch material, early work addressed their dynamics.


• Analytic approaches provided keen insights into scaling.

• Analytic models were later incorporated into large scale computer models (which did not resolve sheaths) as “jump” boundary conditions.

DIAGNOSTICS: SOPHISTICATED PROBES• The rf plasma environment probe measurements difficult. Mastery

of the technique enabled in depth analysis of rf discharge kinetics.

LASER ELECTRIC FIELD MEASUREMENTS• Stark shift laser-induced-fluorescence spectroscopy of increasing

greater sophistication enabled measurements of sheath properties.


Ref: E. Aydil

80 mTorr SF6, 200 W

Ref: R. Dorai

Ref: W. Holber

DATA BASES OF ATOMIC AND MOLECULAR PROPERTIES

• The allied science areas [AMO (Atomic, Molecular, Optical), Surface Science] were and continue to be critical in developing knowledge bases of fundamental parameters.

• Many of the diverse molecular gases relevant to the field had no prior database.

• At first stifled industrially relevant modeling – later remedied by improved databases.


MODELING AND SIMULATION – FIRST PROVIDED INSIGHTS

• The computationally harsh environment of rf discharges in complex gas mixtures at first greatly challenged the community.

• Early efforts addressed fundamental transport properties, and transition between stochastic and resistive heating.


• Collisional heating:

• Anomalous skin effect:

• Regions of time averaged positive and negative power deposition; and non-monotonic E-field

ANOMALOUS SKIN EFFECTAND POWER DEPOSITION IN ICP

( ) ( )∫∫=

×=>

'dt'rd't,'rE't,t,'r,r)t,r(J

BvF,

e

skinmfp

rrrrrrr

rvr

σ

δλ

( ) ( )trEtrtrJeskinmfp ,,),(, rrrrrσδλ =<


ADVANCES IN MODELING CAPTURE ANOMALOUS BEHAVIOR

• 2-d models with non-local, kinetic solutions of Boltzmann’s equation coupled to electromagnetics capture experimentally observed anomalous sheath behavior – and show importance axial magnetic forces.

• Ar, 10 mTorr, 7 MHz, 100 WANIMATION SLIDE

• E-field • Power• Ionization


and Engineering

FINITE WAVELENGTH EFFECTS

• With increasing wafer size and rf frequency, and plasma shortened wavelengths CCPs become inductive with finite wavelengths.

Applied Materials External

Design Capable Models: Effect of B Field on [e] – 13.5, 162 MHzElectrostatic edge effects dominate at low frequency while at high frequency plasma is center peaked due to the standing electromagnetic wave.

HF w/B-Field: E×B drift shears plasma in opposite directions (top, bottom).LF w/B-Field: Asymmetry produced dc bias produces drift in one direction.

S. Rauf, J. Kenney, and K. Collins, Applied MaterialsAVS Symposium, Nov. 2008


• Science based design of plasma processing tools:

• Diagnostics

• Modeling

• Experience….

Refs: AMAT, AIST, Japan; Intel

• Excimer Discharge Laser Lithography


A CONVERGENCE OF SCIENCE BASED TECHNOLOGIES

( ) ( )

( ) ( )cols

v

x

tt,r,vft,r,vfa

t,r,vft

t,r,vf

⎟⎠⎞

⎜⎝⎛

∂∂

+∇⋅−

∇⋅−=∂

∂

rrrrr

rrrrr

ν

• Could Boltzmann have predicted that conservation of fluxes in phase space would one day produce super-computers?

LOW TEMPERATURE PLASMA SCIENCE WORKSHOP

• Convened by DOE-Office of Fusion Energy Science.• Summarize status of research in LTPS.• Identify outstanding major scientific

questions.• Articulate their importance – science

and relation to technology.• Describe basic research to address

questions.• Develop a prioritized roadmap.

• Report: Low Temperature Plasma Science: Not only the Fourth State of Matter but All of Them

• Published: September 2008


LTPS – PRIORITIES• 1 – Predictive Control of Plasma Kinetics

• Plasma kinetics underlie the means of utilizing LTPs and the generation of chemically reactive species.

• Crafting and controlling the velocity distributions electrons and ions are key to optimizing the end product.

• 2 – Collective Behavior and Non-linear Transport• LTPs produce unique collective behavior & nonlinear transport.• With a broad array of positive and negative ions there is a rich

possibility of waves and instabilities.

• 3 - Interfaces and Multiple Phases in Plasmas • LTPs uniquely interact with all phases: solid, liquid and gas.• Plasmas in liquids are now surgical instruments while low

pressure plasmas create nano-crystals of unique composition.


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PREDICTABILITY IN LOW TEMPERATURE PLASMAS: FROM LABORATORY ...

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