thin film processing

8/7/2019 Thin Film Processing

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Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Thin Film Processing

Gary Mankey

MINT Center and Department of Physics

http://bama.ua.edu/~gmankey/[email protected]




Vacuum

A vacuum is defined as lessthan 1 Atmosphere of pressure.

1 Atm = 105 Pa = 103 mbar =760 Torr

Below 10-3 Torr, there are moregas molecules on the surface of the vessel then in the volume of the vessel.

High Vacuum < 10-3 Torr Very High Vacuum < 10-6 Torr

Ultra High Vacuum < 10-8 Torr

760 mm Hg

Vacuum

ATM




Why do we need a vacuum?

Keep surfaces free of contaminants.

Process films with low densityof impurities.

Maintain plasma discharge for sputtering sources.

Large mean free path for electrons and molecules(P = 1 m @ 7 x 10-5 mbar).

P

Mean free path for air at

20 ºC:P = 7 x 10-3 cm / P(mbar)




Vacuum Systems

A vacuum system consists of chamber, pumps and gauges.

Chambers are typically made of glass or stainless steel and

sealed with elastomer or metalgaskets.

Pumps include mechanical,turbomolecular, diffusion, ion,sublimation and cryogenic.

Gauges include thermocouplefor 1 to 10-3 mbar and Bayard-Alpert for 10-3 to 10-11 mbar.




Alabama Deposition of Advanced Materials

All materials are either glass, ceramics, stainlesssteel, copper and puremetals.

A turbomolecular pumpand a cryo pump createthe vacuum.

Sputtering sources areused for deposition.

Characterizationmethods includeRHEED, and Auger electron spectroscopy.




1 cm

e-

e-

e-

Bayard-Alpert or Ionization Gauge

Electrons, e-, produced by thehot filament are acceleratedthrough the grid acquiringsufficient energy to ionizeneutral gas atoms, n.

The ionized gas atoms, I+, arethen attracted to the negatively,

biased collector and their current is measured with an

electrometer. Typical ion gauges have a

sensitivity of 1-10 Amp / mbar and range of 10-3-10-11 mbar.

Electrometer +150V-45V

6VAC

e-

n n

nn

n

n

n

n

I+

I+I+

Filament Collector

Grid




Residual Gas Analysis

A quarupole mass spectrometer analyzes the composition of gasin the vacuum system.

The system must be ³baked´ at

150 - 200 ºC for 24 hours toremove excess water vapor fromthe stainless steel walls.

The presence of an O2 peak atM/Q = 32 indicates an air leak.

At UHV the gas composition isH2, CH4, H2O, CO and CO2.




Monolayer Time

W

e define the monolayer time asthe time for one atomic layer of gasto adsorb on the surface:X = 1 / (SZA).

At 3 x 10-5 Torr, it takes about one

second for a monolayer of gas toadsorb on a surface assuming asticking coefficient, S = 1.

At 10-9 Torr, it takes 1 hour to form

a monolayer for S = 1. For most gases at room

temperature S<<1, so themonolayer time is much longer.

Impingement rate for air:

Z = 3 x 1020 P(Torr) cm-2 s-1

Sticking CoefficientS = # adsorbed / # incident

Area of an adsorption site:A } 1 Å2 = 10-16 cm2




Vapor Pressure Curves

The vapor pressures of mostmaterials follow an Arrheniusequation behavior:PVAP = P0 exp(-EA/kT).

Most metals must be heated totemperatures well above 1000 K to achieve an appreciable vapor

pressure.

For PVAP = 10-4 mbar, thedeposition rate is approximately10 Å / sec.




Physical Evaporation

A current, I, is passed throughthe boat to heat it.

The heating power is I2R, whereR is the electrical resistance of

the boat (typically a few ohms). For boats made of refractory

metals (W, Mo, or Ta)temperatures exceeding 2000º Ccan be achieved.

Materials which alloy with the boat material cannot beevaporated using this method.

High CurrentSource

Substrate

Flux

Boat Evaporant




Limitation of Physical Evaporation

Most transition metals, TM,form eutectics with refractory

materials. The vapor pressure curvesshow that they must be heatedto near their melting points.

Once a eutectic is formed, the boat melts and the heatingcurrent is interrupted.




Electron Beam Evaporator

B

Substrate

Flux

Crucible

e-beam

e-gun

The e-gun produces a beam of electrons with 15 keV kineticenergy and at a variable current of up to 100 mA.

The electron beam is deflected

270º by a magnetic field, B. The heating power delivered to a

small (~5mm) spot in the evaporantis 15 k V x 100 mA = 1.5 k W.

The power is sufficient to heatmost materials to over 1000 ºC.

Heating power is adjusted bycontrolling the electron current.

Evaporant




The Sputtering Process

Electrons (e-) are localized in the plasma by a magnetic field.The e- collide with argon gasatoms to produce argon ions.The Ar+ are accelerated in an

electric field such that they strikethe target with sufficient energy toeject target atoms.The target atoms, being electricallyneutral, pass through the plasmaand condense on the substrate.N SS

SN N

-

+

Magnets

Target

PlasmaDischarge

Substrate

1 mTorr Ar




Measuring and Calibrating Flux

Many fundamental physical properties aresensitive to film thickness.

In situ probes which are implemented inthe vacuum system include a quartz crystalmicrobalance, BA gauge, Auger / XPS,

and RHEED. Ex situ probes which measure film

thickness outside the vacuum systeminclude the stylus profilometer,

spectroscopic ellipsometer, and x-raydiffractometer.

Measuring film thickness with sub-angstrom precision is possible.

?




Quartz Crystal Microbalance

The microbalance measures a shiftin resonant frequency of avibrating quartz crystal with a

precision of 1 part in 106.

f r = 1/2T sqrt(k/m) }f 0(1-(m/2m). For a 6 MHz crystal disk, 1 cm in

diameter this corresponds to achange in mass of several

nanograms. d = m / ( VA), so for a typical metal

d } 10 ng / (10 g/cm3*1 cm2) =0.1 Angstroms.

Quartz

Crystal

Substrate

FrequencyMeasurement

Conversion toThickness

Display

Flux




Auger / XPS

An x-ray source produces photoelectrons or a electron gun produces Auger electrons.

The electrons have kinetic

energies which are characteristicof the material.

The attenuation of substrateelectrons by the film is described

by Beer¶s law:I = I0 exp(-dcos5/0).

Since, 0 } 10 Å, this techniquehas a high sensitivity.

Photoelectrons &

Auger Electrons

Excitation:X-rays or keV Electrons

Electron EnergyAnalyzer




Auger Electron Spectroscopy

The excitation knocks a coreelectron out producing a core hole.

To lower the energy of the ion, anelectron from an upper shell decays

nonradiatively into the core hole. The Auger electron from the upper

shell acquires an energy equal to theenergy difference of the core hole

and upper shell. The kinetic energy of the electrons

are measured to identify thechemical species of the atoms.

EVAC

Excitation

CoreHole

Upper Shell

KineticEnergy




Secondary Electron Energy Distribution

The energy distribution ischaracterized by ± An elastic peak at the

incident electron energy.

± A low energy peak whichincreases as 1/E2 and dropsoff rapidly below 10 eV.

± Auger electrons, which can be measured to determinechemical composition.




Cu Auger Scan in Pulse Counting Mode

These transitionscorrespond to Auger electrons ejected from thevalence band by aneighboring electronfilling the L shell or 2plevels.




Cu Auger Spectrum for Analog Mode

In analog mode, theanalyzer energy ismodulated and a lock-inamplifier detects thederivative of the number of electrons or dN(e)/dEvs. E.

The high energy peakscorrespond to those on the

previous slide.




The Universal Curve for 0

The Universal Curve describes thedependence of electron mean free

path, 0, on energy for most

materials. In most cases, it is accurate to

within a factor of two.

0 = (35 / E)2

+ 0.5 Ewith 0 in Å and E in eV.




Reflection High-Energy Electron Diffraction

15 keV

electrons reflect from thesurface and are displayed as a spoton a phosphor screen.

The angle is adjusted such thatelectrons reflecting from adjacent

layers interfere destructively. When only one layer is exposed, the

spot is bright.

When the top layer covers half of

the surface, the spot is extinguished. The time between two maxima in

the intensity plot is the monolayer time.

d = atomicspacing

15 keVElectron Gun Screen

U

Path difference = 2dsin5 = (n+1/2) P

P = [150 / E(eV)]1/2




Epitaxial Growth

Epi-Taxi ( greek)epi meaning ³on´taxi meaning ³arrangementin relation to a source of stimulation´ The crystal structure of thefilm has a direct relationship

to that of the substrate

Film

Substrate




Growth Modes for Ultrathin Films

The growing film surface canexhibit different behaviorsdepending on substratetemperature, interfacial

strain, and alloy miscibility. The growth modes must becharacterized using acombination of chemicaltools such as Auger electron

spectroscopy and structuraltools such as RHEED andatomic force microscopy.

Stranski-Kastranov

Diffusion Limited

Surface AlloySurface Segregation

Volmer-Weber

Layer by Layer

thin film processing

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