ECE614: Device Modelling

and Circuit Simulation

Unit 2 PVD & CVD

By Dr. Ghanshyam Singh

Content

� Physical vapor deposition

(PVD)

– Thermal evaporation

– Sputtering

– Evaporation and sputtering

compared

– MBE

– Laser sputtering

– Ion Plating

– Cluster-Beam

� Chemical vapor deposition

(CVD)

– Reaction mechanisms

– Step coverage

– CVD overview

� Epitaxy

Physical vapor deposition (PVD)

� 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 evaporation

Heat Sources Advantages Disadvantages

Resistance No radiation Contamination

e-beam Low contamination Radiation

RF No radiation Contamination

Laser No radiation, lowcontamination

Expensive

N = No exp- Φe

kT

6

The number of molecules

leaving a unit area of evaporant

per second

No = slowly varying function of T

ϕe = activation energy required to

evaporate one molecule of

material

k = Boltzman’s constant

T = temperature

Physical vapor deposition (PVD): thermal

evaporation

Si

Resist

d

ββββ

θθθθEvaporant container with orifice diameter DD

Arbitrary surface element

Kn = λλλλ/D > 1

A ~ cosββββ cos θθθθ/d2

N (molecules/unit area/unit time) =3. 513. 1022Pv(T)/ (MT)1/2

The cosine law

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 :

The fraction of particles scattered by collisions

with atoms of residual gas is proportional to:

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

Physical vapor deposition (PVD): thermal

evaporation

ββββ2222 = 70 = 70 = 70 = 700000ββββ1111 = 0 = 0 = 0 = 00000

t2

t1

Substrate

t1

t2

= cos ββββ1

cos ββββ2

≈≈≈≈ 3

Surface feature

Source

Source

Shadow

t1/t2=cosββββ1111/cosββββ2222

λλλλ = (ππππRT/2M)1/2 ηηηη/PT

From kinetic theory the mean free path relates

to the total pressure as:

Since the thickness of the deposited film, t, is proportional

to the cos β, the ratio of the film thickness shown in the

figure on the right with θ = 0° is given as:

Physical vapor deposition (PVD): sputtering

W= kV iPTd

-V working voltage

- i discharge current

- d, anode-cathode distance

- PT, gas pressure

- k proportionality constant

Momentum transfer

Evaporation

and

sputtering:

comparison

Evaporation Sputtering

Rate Thousand atomic layers per second

(e.g. 0.5 µm/min for Al)

One atomic layer per second

Choice of materials Limited Almost unlimited

Purity Better (no gas inclusions, very high

vacuum)

Possibility of incorporating

impurities (low-medium vacuum

range)

Substrate heating Very low Unless magnetron is used substrate

heating can be substantial

Surface damage Very low, with e-beam x-ray

damage is possible

Ionic bombardment damage

In-situ cleaning Not an option Easily done with a sputter etch

Alloy compositions,stochiometry

Little or no control Alloy composition can be tightly

controlled

X-ray damage Only with e-beam evaporation Radiation and particle damage is

possible

Changes in sourcematerial

Easy Expensive

Decomposition ofmaterial

High Low

Scaling-up Difficult Good

Uniformity Difficult Easy over large areas

Capital Equipment Low cost More expensive

Number ofdepositions

Only one deposition per charge Many depositions can be carried

out per target

Thickness control Not easy to control Several controls possible

Adhesion Often poor Excellent

Shadowing effect Large Small

Film properties (e. g.grain size and stepcoverage)

Difficult to control Control by bias, pressure,

substrate heat

Physical vapor deposition (PVD): MBE,

Laser Ablation

-

� MBE

– Epitaxy: homo-epitaxy

hetero-epitaxy

– Very slow: 1µm/hr

– Very low pressure: 10-11

Torr

� Laser sputter deposition

– Complex compounds (e.g.

HTSC, biocompatible

ceramics)

Chemical vapor deposition (CVD): reaction

mechanisms

� 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

SiH4SiH4

Si

Chemical vapor deposition (CVD):

reaction mechanisms

Fl = D∆c

∆x

δ(x) =ηx

ρU

1

2

δ =1

Lδ(x)dX =

2

30

L

∫ Lη

ρUL

1

2

ReL =ρUL

ηδ = 2L

3 ReL

� Energy sources for deposition:

– Thermal

– Plasma

– Laser

– Photons

� Deposition rate or film growth rate

(Fick’s first law)

(gas viscosity η, gas density ρ, gas stream velocity U)

(Dimensionless Reynolds number)

Laminar flow

L

δ(x)

dx

(U)

(Boundary layer thickness)

Fl = D∆c

2L3 ReL (by substitution in Fick’s first law and ∆x=δ)

� Mass flow controlled regime

(square root of gas

velocity)(e.g. AP CVD~ 100-10

kPa) : FASTER

� Thermally activated regime:

rate limiting step is surface

reaction (e.g. LP CVD ~ 100

Pa----D is very large) :

SLOWER

Chemical vapor deposition (CVD)

: reaction mechanisms

Fl = D∆c

2L3 ReL

R = Ro e -

Ea

kT

Chemical vapor deposition (CVD):

step coverage

Fl = D∆c

2L3 ReL

R = Ro e -

Ea

kT

� Step coverage, two factors are

important

– Mean free path and surface

migration i.e. P and T

– Mean free path: λ =

αααα

w

zθ=180θ=180θ=180θ=1800000

θ=270θ=270θ=270θ=2700000θ=90θ=90θ=90θ=900000

θ is angle of arrival

kT

2

1

2 PTπa2

> α

Fldθ∫

θ = arctanw

z

Chemical vapor deposition (CVD) :

overview

� CVD (thermal)

– APCVD (atmospheric)

– LPCVD (<10 Pa)

– VLPCVD (<1.3 Pa)

� PE CVD (plasma enhanced)

� Photon-assisted CVD

� Laser-assisted CVD

� MOCVD

� The L-CVD method is able to fabricate continuous thin rods by pulling the substrate away from the stationary laser focus at the linear growth speed of the material while keeping the laser focus on the rod tip, as shown in the Figure . LCVD was first demonstrated for carbon and silicon rods. However, fibers were grown from other substrates including silicon, carbon, boron, oxides, nitrides, carbides, borides, and metals such as aluminium. The L-CVD process can operate at low and high chamber pressures. The growth rate is normally less than 100 µm/s at low chamber pressure (<<1 bar). At high chamber pressure (>1 bar), high growth rate (>1.1 mm/s) has been achieved for small-diameter (< 20 µm) amorphous boron.

Chemical vapor deposition (CVD) : L-CVD

Epitaxy

� VPE:

– MBE (PVD) (see above)

– MOCVD (CVD) i.e.organo-metallic

CVD(e.g. trimethyl aluminum to

deposit Al) (see above)

� Liquid phase epitaxy

� Solid epitaxy: recrystallization of

amorphous material (e.g. poly-Si)

Liquid phase epitaxy

Epitaxy

� Selective epitaxy

� Epi-layer thickness:

– IR

– Capacitance,Voltage

– Profilometry

– Tapered groove

– Angle-lap and stain

– Weighing

Selective epitaxy

Homework

� Homework: demonstrate equality of λ = (πRT/2M)1/2 η/PT and λ = kT/2 1/2 a 2 π 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.