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techniques-Physical Vapor Deposition and Chemical Vapor Deposition Dr. Marc Madou, Winter 2011 UCI Class 6

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Pattern Transfer: Additive techniques-Physical Vapor Deposition and Chemical Vapor Deposition Dr. Marc Madou, Winter 2011 UCI Class 6

Contentx

Physical vapor deposition (PVD) Thermal evaporation Sputtering Evaporation and sputtering compared MBE Laser sputtering Ion Plating Cluster-Beam

x

x x

Chemical vapor deposition (CVD) Reaction mechanisms Step coverage CVD overview Epitaxy Electrochemical Deposition

Physical vapor deposition (PVD)x

The physical vapor deposition technique is based on the formation of vapor of the material to be deposited as a thin film. The material in solid form is either heated until evaporation (thermal evaporation) or sputtered by ions (sputtering). In the last case, ions are generated by a plasma discharge usually within an inert gas (argon). It is also possible to bombard the sample with an ion beam from an external ion source. This allows to vary the energy and intensity of ions reaching the target surface.

Physical vapor deposition (PVD): thermal evaporationN = No expHeat S ources Resistance e-beam RF Laser

e kT

The number of molecules leaving a unit area of evaporant per secondAdvantages No radiation Low contamination No radiation No radiation, low contamination Dis advantages Contamination Radiation Contamination Expensive

6

Physical vapor deposition (PVD): thermal evaporationN (molecules/unit area/unit time) = 3. 513. 10 22Pv(T)/ (MT) 1/2SiA r bitr a r y sur fa c e e le m e nt

This is the relation between vapor pressure of the evaporant and the evaporation rate. If a high vacuum is established, most molecules/atoms will reach the substrate without intervening collisions. Atoms and molecules flow through the orifice in a single straight track,or we have free molecular flow :Kn = /D > 1

R e sist

d

E v a po r a nt c o nta ine r D with o r ific e dia m e te r D

The fraction of particles scattered by collisions with atoms of residual gas is proportional to:

The source-to-wafer distance must be smaler than the mean free path (e.g, 25 to 70 cm)

The cosine lawA ~ cos cos /d2

Physical vapor deposition (PVD): thermal evaporationFrom kinetic theory the mean free path relates to the total pressure as: = (RT/2M) 1/2 /PTt2

Substrate

Surface featuret1

Since the thickness of the deposited film, t, is proportional to the cos , the ratio of the film thickness shown in the figure on the right with = 0 is given as:

t1 cos 1 = 3 t2 cos 2

1 = 00

0 2 = 70

Source

t1/t2=cos1/cos2Shadow

Source

Physical vapor deposition (PVD): sputteringW= kV i PTd

Momentum transfer

-V working voltage - i discharge current - d, anode-cathode distance - PT, gas pressure - k proportionality constant

Evaporation and sputtering: comparison

Ev apo ratio n Rate Cho ice o f materials P urity S ubs trate heating S urf ace damag e In- s itu cleaning Allo y co mpo s itio ns , s to chio metry X -ray damag e Chang es in s o urce material Deco mpo s itio n o f material S caling - up U nif o rmity Capital Equipment N umber o f depo s itio ns Thicknes s co ntro l Adhes io n S hado w ing ef f ect F ilm pro perties ( e. g . g rain s ize and s tep co v erag e)Thousand atomic layers per second (e.g. 0.5 m/min for Al) Limited

S putteringOne atomic layer per second Almost unlimited

Better (no gas inclusions, very high Possibility of incorporating vacuum) impurities (low-medium vacuum range) Very low Unless magnetron is used substrate heating can be substantial Very low, with e-beam x-ray Ionic bombardment damage damage is possible Not an option Easily done with a sputter etch Little or no control Alloy composition can be tightly controlled Only with e-beam evaporation Radiation and particle damage is possible Easy Expensive High Difficult Difficult Low cost Only one deposition per charge Not easy to control Often poor Large Difficult to control Low Good Easy over large areas More expensive Many depositions can be carried out per target Several controls possible Excellent Small Control by bias, pressure, substrate heat

Physical vapor deposition (PVD): MBE, Laser Ablationx

x

-

MBE Epitaxy: homo-epitaxy hetero-epitaxy Very slow: 1m/hr Very low pressure: 10-11 Torr Laser sputter deposition Complex compounds (e.g. HTSC, biocompatible ceramics)

Physical vapor deposition (PVD): Ion cluster platingx

x

Ionized cluster: it is possible to ionize atom clusters that are being evaporated leading to a higher energy and a film with better properties (adherence, density, etc.). From 100 mbar (heater cell) to 10-5 to 10-7 mbar (vacuum)-sudden cooling Deposits nanoparticles Combines evaporation with a plasma faster than sputtering complex compositions good adhesion

Physical vapor deposition (PVD):Ion cluster plating and ion platingx

x

Gas cluster ions consist of many atoms or molecules weakly bound to each other and sharing a common electrical charge. As in the case of monomer ions, beams of cluster ions can propagate under vacuum and the energies of the ions can be controlled using acceleration voltages. A cluster ion has much larger mass and momentum with lower energy per atom than a monomer ion carrying the same total energy. Upon impact on solid surfaces, cluster ions depart all their energy to an extremely shallow region of the surface. Cluster plating material is forced sideways and produces highly smooth surfaces. Also individual atoms can be ionized and lead to ion plating (see figure on the right, example coating : very hard TiN)

Chemical vapor deposition (CVD): reaction mechanismsx x

x x x

x x

x x

Mass transport of the reactant in the bulk Gas-phase reactions (homogeneous) Mass transport to the surface Adsorption on the surface Surface reactions (heterogeneous) Surface migration Incorporation of film constituents, island formation Desorption of by-products Mass transport of by-produccts in bulk

CVD: Diffusive-convective transport of depositing species to a substrate with many intermolecular collisions-driven by a concentration gradient

SiH4 SiH4

Si

Chemical vapor deposition (CVD): reaction mechanismsx

Energy sources for deposition: Thermal Plasma Laser Photons Deposition rate or film growth ratec Fl = D x

Laminar flow

(U)(x)

x

(Ficks first law)1 2

x (x) = U

L(Boundary layer thickness)

dx

(gas viscosity , gas density , gas stream velocity U)L 1 2 2 = (x)dX = L L0 3 UL 1

(Dimensionless Reynolds number)ReL = UL = 2L 3 ReL

Fl = D

c 3 Re L 2L

(by substitution in Ficks first law and x=)

Chemical vapor deposition (CVD) : reaction mechanismsx

x

Mass flow controlled regime (square root of gas velocity) (e.g. AP CVD~ 100-10 kPa) : c Fl = D 3 Re 2L FASTER Thermally activated regime: rate limiting step is surface reaction (e.g. LP CVD ~ 100 Pa----D is very large) : Ea SLOWER R= R eL

o

kT

Chemical vapor deposition (CVD): step coveragex

Step coverage, two factors are important Mean free path and surface migration i.e. P and T kT Mean free path: =2 PT a1 2

>

Ea R = Ro e - kT

2

Fl = D

c 3 Re L 2L

z

Fld = arctan

0 = 180

is angle of arrival0 = 90 0 = 270

w

w z

Chemical vapor deposition (CVD) : overviewx

x x x x

CVD (thermal) APCVD (atmospheric) LPCVD ( Ni Oxidation (anode reaction): H2PO 2- + H2O> H2PO3- +2H+ +2e-----------------------------------------Ni+2 + H2PO2- + H2O > Ni + H2PO3- + 2H+ e.g. electroless Cu: 40 mhr-1

Electrochemical deposition: electrolessx

x x x

x

Evans diagram: electroless deposition is the combined result of two independent electrode reactions (anodic and cathodic partial reactions) Mixed potential (EM): reactions belong to different systems ideposition = ia = ic and I=A x i deposition Total amount deposited: m max= I t M/Fz (t is deposition time, Molecular weight, F is the Faraday constant, z is the charge on the ion) CMOS compatible: no leads required

+ Evans diagram

-

F= 96,500 coulombs=1, 6 10 -19 (electron charge) x 6. 02 10 23 (Avogadros number)

Electrochemical deposition :electrodepositionthermodynamicsx

Electrolytic cell Au cathode (inert surface for Ni deposition) Graphite anode (not attacked by Cl2)

x

x

Two electrode cells (anode, cathode, working and reference or counter electrode) e.g. for potentiometric measurements (voltage measurements) Three electrode cells (working, reference and counter electrode) e.g. for amperometric measurements (current measurements)

Electrochemical deposition :electrodeposition-thermodynamics (E)1. Free energy change for ion in the solution to atom in the metal (cathodic reaction):

G = G m ( f r e e e n e -r gGe ( y f r pe eu r e e n me r eg t ya l o) or alson f io G = G 0 -R T ln a M z+ = G 0 -R T ln C M z+ z+ (1)

in

th e e le c tr o l

2. The electrical work, w, performed in electrodeposition at constant pressure and constant temperature: G = - w + PV and since V =0

G = - E zF (2)3. Substituting Equation (2) in (1) one gets E = E0 + R T lna Mz+ (Nernst equation) zF

4. Repeat (1) and (2) for anodic reaction:

G = G 2 - G 1

or G = -(E 2 -E 1 )zF = -E cell zFE2 > E1 : - battery E2 < E1 : + E ext > E cell to afford deposition

Electrochemical deposition :electrodeposition-thermodynamics ()x

x

x

A thermodynamic possible reaction may not occur if the kinetics are not favorable Kinetics express themselves through all types of overpotentials E -E o = ( + anodic and - is cathodic)

Electrochemical deposition :electrodeposition-kinetics-activation controlx _ G # kT RT k = e c h

(without field) G* = G#+

x

kT k = kc e h

FRT

Understanding of polarization curves: consider a positive ion transported from solution to the electrode Successful ion jump frequency is given by the Boltzmann distribution theory (h is Planck constant):

(with field)

F kT RT i = k z F = k cz F e h (1 )F kT RT i = k zF = kc z F e h

Electrochemical deposition :electrodeposition-kinetics-activation controlx

At equilibrium the exchange current density is given by: kT i = i = kc zF e e h

(1 )F e RT

= i = i c zF

F e kT e h RT

x

The reaction polarization is then given by:=e

x

The measurable current density is then given by:

i= i i

x

For large enough overpotential: (1 )F F

i = ie (e

RT

e

RT

)

(Butler-Volmer)

= a + blog(i)

(Tafel law)

Electrochemical deposition :electrodeposition-kinetics-diffusion controlx

From activation control to diffusion control:dC C x= 0 C x=0 = dX

C x=0 1- i = 0 C ili = ilnFc (1 e RT

we get :

)

x

Concentration difference leads to another overpotential i.e. concentration polarization:c = RT C x=0 ln nF C 0

x

Using Faradays law we may write also: C 0 Cx=0i = nFD 0

x

At a certain potential C x=0=0 and then:C 0 I l = nFAD 0

Electrochemical deposition :electrodeposition-non-linear diffusion effectsx

Nonlinear diffusion and the advantages of using microelectrodes: 0I l = nFAD 0 C

x

An electrode with a size comparable to the thickness of the diffusion layer

= (D0 t )x

1 2

The Cottrell equation is the current-vs.-time on an electrode after a potential step: t For micro-electrodes it needs correction : Il = n F A1 0 D0 2 C

x

Il = n F

1 D 2 A 0C 0

t

C0 + A n F0 D r

Electrochemical deposition :electrodeposition-non-linear diffusion effectsx

The diffusion limited currents for some different electrode shapes are given as (at longer times after bias application and for small electrodes):I l,m = rnFD0 C (disc)0

I l,m = 2rnFD0 C0 (hemisphere) I l,m = 4rnFD0 C0 (sphere) x

If the electrodes are recessed another correction term must be introduced:I l,m = AnFD 0 C0 r+L

Homeworkx x

x x

x

Homework: demonstrate equality of = (RT/2M)1/2 /PT and = kT/2 1/2 a 2 PT (where a is the molecular diameter) What is the mean free path (MFP)? How can you increase the MFP in a vacuum chamber? For metal deposition in an evaporation system, compare the distance between target and evaporation source with working MFP. Which one has the smaller dimension? 1 atmosphere pressure = ____ mm Hg =___ torr. What are the physical dimensions of impingement rate? Why is sputter deposition so much slower than evaporation deposition? Make a detailed comparison of the two deposition methods. Develop the principal equation for the material flux to a substrate in a CVD process, and indicate how one moves from a mass transport limited to reaction-rate limited regime. Explain why in one case wafers can be stacked close and vertically while in the other a horizontal stacking is preferred. Describe step coverage with CVD processes. Explain how gas pressure and surface temperature may influence these different profiles.