CVD

MOS Transistor

CVD Thin Film Growth

1

2

34

5 6

Atmospheric Pressure CVD

Atmospheric Pressure CVD

Atmospheric Pressure CVD

Horizontal type Barrel type

Atmospheric Pressure CVD

Low Pressure CVD

Low Pressure CVD

Laser Enhanced CVD

Plasma Enhanced CVD

Plasma Enhanced CVD

Plasma Enhanced CVD

Thin FilmDeposition

• Quality – composition, defect density, mechanical and electrical properties

• Uniformity – affect performance (mechanical , electrical)

Thinning leadsto ↑ R

Voids: Trap chemicals lead tocracks (dielectric) large contactresistance and sheet resistance(metallization)

AR (aspect ratio) = h/w ↑ with↓ feature size in ICs.

Examples

Poor step coverage with increasing AR

Thinning causesmetal resistance toincrease,generates heatand lead to failure

Chemical Vapor Deposition

Flat on thesusceptor

Cold wallreactor

Methods of Deposition:

Chemical Vapor Deposition(CVD):APCD, LPCVD, HDPCVDPhysical Vapor Deposition (PVD:evaporation, sputtering)

Atmospheric Pressure : APCVD

Cold wall reactors (walls not heated -only the susceptor)

Low pressure: LPCVD – batchprocessing.

Hot wallreactor

Atmospheric Pressure Chemical Vapor Deposition

Transport by forcedconvection

By diffusionthroughboundary layer Diffusion through the B. L

Desorption of by products

Transport of byproducts byforced convection

@ the surface (4): decomposition,reaction, surface migration attachmentetc.

(3) May be desorption which dependson a sticking coefficient (4)

Growth rate for Si depositionN=5•1022cm-3

Mole fraction of theincorporating species in the gasphase.

Partial pressure

Total concentration in the gas phase

CT = 1 * 1019 cm-3 5 * 1022

V = 0.14 µm/min

PG @ 1 torr

Ptotal = 1 atm = 760 torr

adsorption

transport

reaction

Steady state

Steps in deposition

As in Deal-Grovemodel for oxidation

Growth Kinetics

Determined by the Smaller of ks or hG Two limiting cases

1) Surface reaction ks << hG

Control: fast transport

slow reactionYk

N

Cv

S

T=

2) Mass transfer or gasphase diffusion hG << ks

YhN

Cv

G

T=

Fast reaction and slow transport.

Temperature uniformitymore important than thegas flow wafersvertically poly-Si

Put wafersflat toensure flowuniformity@ the Sisurface.

Epitaxy

APCVD

SiO2

(111) Si shows slower v – fewerattachment sites than in (100) Si

Ea ≈ 1.6 eV for all Si sources H desorption from the Si surface.

With H2 as a gas carrier

Light mass

heavy

Limited by transport

Both are linear with time (t)

SiH4 the fastest growth

kS limited deposition is VERY temp sensitive.hG limited deposition is VERY geometry (boundary layer) sensitive

Boundary Layer – Diffusion to the Surface

Gas moves with the constant velocity U.

Boundary layer (caused by friction ) increasesalong the susceptor, mass transfer coefficient hGdecreases, gas depletion caused by consumptionof the reacting species (concentrations decrease)

Growth rate decreases along the chamber

• Use tilted susceptor

• Use T gradient 5-25°C

• Gas injectors along the tube

• Use moving belt

Deposition of alloys DIFFICULT – variousreactions, kinetics (species, precursors)

Use PVD rather than CVD

B.L.

viscosity

gas density