Sputtering and CVD/PVD Technology

NANO54Foothill College

Overview• Overview of thin film stack• Fundamentals of sputtering

– Glow discharge, atoms in a gas state• Techniques of sputtering

– DC and RF sputtering– DC Magnetron and reactive sputtering

• Glow discharge• CVD and PVD processes

– MO-CVD, PE-CVD, plasma deposition

Thin Film Applications• Window coatings• Magnetic media• Semiconductors• Biomedical devices• Optics• Automotive• Aerospace

http://spectrum.ieee.org/energy/renewables/first-solar-quest-for-the-1-watt/0

Thin Film Stack• A thin film Bragg reflector consists of a multilayer-

stack of alternate high- and low-index films, all one quarter wavelength thick (see figure left). The geometrical thicknesses of the high- und low-index films are tH=l/(4nH) and tL=l/(4nL) respectively.nH and nL are the indices of refraction of the high- and low-index films, respectively and l is the center wavelength of the Bragg mirror.  On every interface in the stack a part of the incident beam is reflected. The reflected parts have a phase shift of 180° only if the incident light goes from low-index medium in a high-index medium. The relative phase difference of all reflected beams is zero or a multiple of 360° and therefore they interfere constructively. The intensity of the incident light beam decreases during his travel trough the quarter-wave stack and at the same time the reflected light increases, if the absorptance ‘A’ of the stack is negligible.

http://www.batop.de/information/r_Bragg.html

Thin film stack SEM image (FIB cross sectionSEM cross-section image of a thin film stack. Each layer is approximately 500 Angstroms thick. - http://www.jwdstaging6.com/gallery

XPS sputter depth profile of an architectural glass coating obtained using 2 kV argon ions and Zalar rotation to enhance depth resolution. [PHI AES]

Thin films and coatings are used or applied to an almost limitless range of applications to provide a specific performance characteristic for a broad range of industrial products. These include coatings that provide antistatic properties, corrosion resistance, reduce wear, and promote adhesion; as well as thin films for optical devices, mirrors, semiconductor devices, magnetic media, and food wrap. Architectural glass, lenses, mirrors, and many other products are coated to provide specific optical properties. These coatings are often a stack of thin layers that can be characterized using PHI's surface analysis instruments  to verify the composition of the layers, detect contaminants, and estimate layer thickness.

Thin film coatings on glass (XPS, AES, TOF-SIMS, D-SIMS)

http://www.phi.com/surface-analysis-applications/thin-films-and-coatings.html

Southwall XIR Film

AES depth profile of Southwall Technologies XIR Film – data from Nanolab Technologies

Deposition Processes• Evaporation (thermal)• Sputter deposition (DC, RF, PE)• CVD, MO-CVD, PE-CVD• Plasma polymerized films• Solution cast (spin coating) films• Self Assembled Monolayers

(SAMs)

Glow Discharge• Glow-discharge sputtering means the energetic particles

used to strike target is generated by glow-discharge. Creation of glow-discharge

• Tube filled with argon, neutral, no charged particles between cathode and anode

• Elastic collision, no energy exchange • Inelastic collision

– Energy not enough high, excite electrons, emitting photons – Energy high enough, ionize electrons, cause secondary free

electrons – Both free electrons are accelerated again, so cause

cascading or gas break down – Flow of electrons is collected by anode, current will quickly

decay to zero

Glow Discharge Processes

Atoms into Gas State

• At target:

– target atoms ejected– target ions ejected (1 - 2 %)

• Electrons emitted – helps keep plasma going

• Ar+ ions reflected as Ar neutrals• Ar buried in target• Photons emitted

Vacuum DepositionPhysical Vapor Deposition• Evaporation [ HV, UHV,

Inert Gas Reactive]• Sputtering [ HV, UHV,

Inert Gas, Reactive] • Pulsed Laser

Deposition [HV, UHV, Inert Gas, Reactive ]

Chemical Vapor Deposition• Thermal Growth

[Plasma Assisted ]• Polymerization

• Plasma assisted • Chemical growth

• Plasma enhanced

Sputter Deposition

Fundamentals of Sputter Deposition

Sputter Deposition

•momentum transfer process •involves top 10 Å•model as hard sphere collisions •good for energies < 50 keV

•95 % of incident energy goes into target => COOL the target

•5 % of incident energy is carried off by target atoms •typical energies of 5-100 eV

•target atoms come off with a non-uniform distribution •more atoms normal to the surface•cosine distribution (like surface source)

•characterize process by sputter yield (S)•S = number ejected / number incident

Sputtering Process

Sputter Yield

S depends on:• target material

– binding energy– mass of atoms

• sputtering gas – mass of atoms (S increases for heavier gasses)– incident energy (S increases for higher energies)

• geometry – most efficient 20-30 degrees from glancing

for normal incidence sputtering: – maximum around 10 kV

• sputtering threshold• S is about 1-10 typically

Calculating S• For calculating S we need:• number of atoms ejected

– depends on momentum and energy transferred • these depend on relative masses and collision angle• maximum energy transferred to target atom in hard sphere collision

» depends on binding energy of target atom

• number of layers involved in process – mean free path of ion in target

• typically about 2 layers• surface density of target atoms• collision cross section of ion with target atom

PVD Sputtering Tool

PVD by Sputtering– Sputtering: Sputtering is a process whereby

coating material is dislodged and ejected from the solid surface due to the momentum exchange associated with surface bombardment by energetic particles. Processes:

– Gas ions are accelerated by a high voltage, producing a glow discharge or plasma

– A source (the cathode, also called the target ) is bombarded in high vacuum by gas ions

– Atoms from the target are ejected by momentum transfer and move across the vacuum chamber

– Atoms are deposited on the substrate to be coated and form a thin film.

Sputtering Mechanism• Bounce back :

when very low energy (<5eV); when the collision is head-on or nearly so

• Embedded : when much higher energy (>10KeV), the impinging particles are most likely to be embedded in the target, which is the basis of Ion Implantation

• If energy level is between the two extremes:

– some fraction of the energy of the impinging ions is transferred to the solid in the form of heat, and lattice damage

– another fraction of such energy causes atoms from the surface to be dislodged and ejected into the gas phase----Sputtering

The kinetic energy of the impinging particles largely dictates what event will happen

Unique Characteristics Uniform thickness over large area. Simple thickness control. The alloy composition maintains stoichiometry

with the original target composition. Deposition rates do not differ a great deal from

one material to another. Sputtering-cleaning prior to initiating film

deposition. The surface is not again exposed to ambient after such cleaning.

The lifetime of a sputtering target may be as long as hundreds of runs and is seldom less than 20.

Coating Materials Metals: Al, Cu, Zn, Au, Ni, Cr, W, Mo, Ti Alloys Ag-Cu, Pb-Sn, Al-Zn, Ni-Cr Nonmetals: graphite, MoS2, WS2, PTFE

Refractory oxides: Al2O3, Cr2O3, Al2O3-Cr2O3, SiO2, ZrO2-Y2O3

Refractory carbides: TiC, ZrC, HfC, NbC, Tic-Ni, Tic-ZrC Refactory nitrides: TiN, Ti2N, ZrN, HfN, TiN-ZrN, TiN-AlN-ZrN Refractory borides: TiB2, ZrB2, HfB2, CrB2, MoB2Refactory silicides: MoSi2, WSi2, Cr3Si2

DC Sputtering

DC Sputter Deposition

Sputter deposition is a physical vapor deposition (PVD) method of depositing thin films by sputtering, that is ejecting, material from a "target," source, which then deposits onto a "substrate," such as a silicon wafer. Sputtered atoms ejected from the target have a wide energy distribution, typically up to tens of eV (100,000 K). The sputtered ions (typically only a small fraction (~1% of the ejected particles are ionized) can ballistically fly from the target in straight lines and impact energetically on the substrates or vacuum chamber (causing resputtering).  Energetic ions sputter material off the target which diffuse through the plasma towards the substrate where it is deposited. There is no strong plasma glow around the cathode since it takes a certain distance for the plasma to be generated by electron avalanches started by a few secondary electrons from the sputtering process.

http://en.wikibooks.org/wiki/Microtechnology/Additive_Processes

Sputtering System

Thin Film Sputtering Lines

6 Chamber Thin Film Sputtering Line

RF Sputter Deposition

RF Sputter Deposition

Cathode Sputter Arrangement

Cathode PlanarSputtering System

Reactive Sputtering

In Line Deposition Tools

Roll to Roll Coater

TEM Characterization

TEM Characterization

• Sputtering alloy targets• composition of alloy in film is approximately the same as alloy in target

(unlike evaporation)

• rapid mixing in liquids (evaporation)• slow diffusion mixing in solids (sputtering)

– target reaches steady state– surface composition balances sputter yield

• Process:• Initial alloy of A and B .......................... ABABABABABABAB• If SA > SB , remove more A

– enriches surface in B .................... . BAB. B. BA. B . AB• More B on surface => more B sputtered ....... ABABBABABBAB• surface composition reaches steady state

– surface enriched in B– bulk composition sputtered– fASA / CA = fBSB / CB

• where f = surface fraction and C = bulk composition• alloy targets need to be conditioned by sputtering a few hundred Å

before depositing

• 2. Transport to substrate• Target atoms pass through Ar gas and plasma

environment – one Ar+ ion for every 10,000 Ar neutrals– electrons in plasma collide with Ar neutrals to form ions and

more electrons• Target atoms collide with Ar atoms, Ar+ ions and

electrons – treat as random walk "diffusion" through gas– target atoms lose energy (down to 1-10 eV)– chemical reactions may occur in gas– not a line of sight process (unless pressure reduced)

• can coat around corners

• 3. Deposit on substrate• target atoms and ions impinge• electrons impinge• Ar atoms impinge

– Ar pressure about 0.1 torr– Ar may be incorporated into film

• energetic particles may modify growth• substrates heat up

– 100 - 200 C is common

– for a thermally isolated sample (no heat conduction) = substrate density (g/cm3)

– c = substrate heat capacity (J/gûC)– d = substrate thickness (cm)– W = film atomic volume (cm3/atom)– D-DOT = deposition rate (Å/min)– t = time (secs)– energies are in eV/atom– note: the constant does have units

Parameters:Argon Pressure

– optimum deposition rate around 100 mTorr– compromise between

• increasing number of Ar ions• increasing scattering of Ar ions with neutral Ar atoms

– if you can increase the number of ions without increasing the number of neutrals, you can operate at lower pressures

• Sputter voltage – maximize sputter yield (S)– typically -2 to -5 kV

• Substrate Bias Voltage – substrate is being bombarded by electrons and ions from target and plasma

• sputtering film while you deposit– neutral atoms deposit independently– put negative bias on the substrate to control this– can significantly change film properties

• Substrate temperature – control with substrate heater– heating from deposited material

• increases with increasing sputter voltage• decreases with increasing substrate bias

• Deposition rate – changes with Ar pressure– increases with sputter yield

• usually increases with high voltage• Particle Energy

– increases with increasing sputter voltage– decreases with increasing substrate bias– decreases with increasing Ar pressure

Good for insulating materialsin DC systems, positive charge builds up on the cathode (target) need 1012 volts to sputter insulators !!avoid charge build up by alternating potential

. . . . . . . . . . . . . . . . TIME ------->sputter deposition occurs when target is negativesubstrate and chamber make a very large electrode - so not much sputtering of substratePhysical processfrequencies less than about 50 kHz

oelectrons and ions in plasma are mobile both follow the switching of the anode and cathode

obasically DC sputtering of both surfacesfrequencies above about 50 kHz

oions (heavy) can no longer follow the switchingoelectrons can neutralize positive charge build up

target - - - + - - - + - substrate + chamber

+ + + - + + + - +

Advantages:Easier to keep plasma going under these conditions Can operate at lower Ar pressures (1-15 mTorr)fewer gas collisions => more line of sight deposition

Magnetron Sputter Depositionuse with DC or RFgoal: increase ionization of ArWhy? Higher sputter rates at lower Ar pressures (down to 0.5 mTorr)

fewer gas collisions - more line of sightHow ? increase probability of electrons striking Ar

increase electron path lengthuse electric and magnetic fields

Most common configuration: crossed electric and magnetic fieldsPut magnets (200 Gauss) behind target:

traps electrons near cathode more ionization near cathodes (10x)fewer electrons reach substrate (less heating)

with evaporation or sputtering (or chemical vapor deposition)bombard surface with ions

not necessarily same type as in filmions typically NOT incorporated in film

relatively low voltages (50 - 300 eV)leads to

physical rearrangementlocal heating

can change film properties for better or worse

disruption of columnar growth requires about 20 eV of added energy per depositing atom

Ion Assisted Deposition

• Add reactive gas to chamber during deposition (evaporation or sputtering)

• oxygen, nitrogen• Chemical reaction takes place on substrate and target• Can poison target if chemical reactions are faster than sputter rate• Adjust reactive gas flow to get good stoichiometry without incorporating excess gas into film

Reactive Sputter Deposition

Gas Flow Sputtering

Comparison of Evaporation vs. Sputtering Process and Films

Comparison of evaporation and sputtering

EVAPORATION SPUTTERING low energy atoms higher energy atoms high vacuum path

few collisions line of sight deposition little gas in film

low vacuum, plasma path

many collisions less line of sight deposition gas in film

larger grain size smaller grain size fewer grain orientations many grain orientations poorer adhesion better adhesion

Growth Modes

Elements of Thin Film Growth

Chemical Vapor Deposition

• CVD => Chemical Vapor Deposition• PE-CVD => Plasma Enhanced CVD• MO-CVD => Metal Organic CVD• Plasma Polymerization

Chemical Vapor Deposition

Spin CoatSpin coating is a procedure used to apply uniform thin films to flat substrates. In short, an excess amount of a solution is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force. A machine used for spin coating is called a spin coater, or simply spinner. Rotation is continued while the fluid spins off the edges of the substrate, until the desired thickness of the film is achieved. The applied solvent is usually volatile, and simultaneously evaporates. So, the higher the angular speed of spinning, the thinner the film. The thickness of the film also depends on the concentration of the solution and the solvent. Spin coating is widely used in microfabrication, where it can be used to create thin films with thicknesses below 10 nm. It is used intensively inphotolithography, to deposit layers of photoresist about  micrometer thick.  http://en.wikipedia.org/wiki/Spin_coating

Al/Pd/GaN Thin Film Annealing

(cross section)

Al/Pd/GaN Profile Data

Al/Pd/GaN Atomic Concentration Data

Summary• Atoms in the gas phase• Glow discharge and evaporation• Sputtering process• Types of sputtering (DC, RF, PE)• Surface reactions and processes• Chemical Vapor Deposition (CVD)• Physical Vapor Deposition (PVD)

References• AVS American Vacuum Society)• Wikipedia Materials Science• Colorado Dept of Physics and Energy• EAG Labs – application notes• Nanolab Technologies (XPS/AES data)

Permissions• Permission to use this PPT presentation

explicitly granted by Tom Christianson• Physics of Thin Films• PES 449 / PHYS 549 Spring 2000• Department of Physics and Energy Science

University of Colorado at Colorado Springs P.O. Box 7150 Colorado Springs, CO 80933

• email:tchriste@uccs.edu• Please do not repost this PowerPoint lecture