Oxidation and CVD

Prof. Tianhong Cui, Mechanical Engineering

ME 8254

Thin-Film Deposition Spin-on Films

– Polyimide (PI), photoresist (PR)– Spin-on glass (SOG)

Physical Vapor Deposition (PVD)– Evaporation– Sputtering

Chemical Vapor Deposition (CVD)– Oxidation– LPCVD– PECVD

Stages of Thin Film Growth

• Island Stage

• Coalescence Stage

• Channel Stage

• Continuous Film Stage

Thermal Oxidation of Silicon Formation of the oxide of silicon on the silicon

surface is known as oxidation .

Thermal Oxidation is characterized by hightemperatures (900 - 1200 Degree C) .

Two main processes :– Dry Oxidation

Si(s) + O2 --> SiO2 1 atm , 1000 C

– Wet Oxidation Si (s) + 2H2O --->SiO2 + 2H2

– Dry oxidation produces a better (more dense) oxide ascompared to wet oxidation.

Types of CVD

• Atmospheric Pressure CVD (APCVD)

• Low Pressure CVD (LPCVD)

• Plasma Enhanced CVD (PECVD)

Chemical Vapor Deposition(CVD)

CVD = formation of non-volatile solid filmon substrate by reaction of vapor phasechemicals

Steps in CVD– Gases are introduced into a reaction chamber

– Gas species move to the substrate

– Reactants are adsorbed on the substrate

– Film-forming chemical reactions

– Desorption and removal of gaseous by-products

CVD reactions Heterogeneous = occur at wafer surface

– Desirable

– Produce good quality films

Homogeneous = occur in gas phase– Undesirable

– Form gas phase clusters of material

– Consume reactants

– Reduce deposition rate

CVD Reaction Rate (R) R = R0 exp(-Ea/kT)

– where Ea = activation energy (eV)

– k = Boltzmann constant

– T = temperature (K)

Surface reaction rate increases with increasingtemperature at very high temperature– Reaction rate > reactant arrival rate

– Mass-transport limited

At low temperatures– Reaction rate < reactant arrival rate

– Reaction rate limited

Low Temperature Oxidation ofSilicon

(LTO) SiO2 is formed using three types of CVD Processes.

APCVD, LPCVD and PECVD

SiH4 + O2 : ----->SiO2 + 2H2 (240 - 550 C)

(200 - 500 nm/min optimal) and (1400 nm/min possible).

Deposition rate increases slowly with increased T (310- 450 C)

Deposition rate can also be increased by increasing the O2 /SiH4

ratio

Deposition can occur in the APCVD as low as 130 C

For LPCVD Window (100 - 330 C ) 2-12 torr, and 14 nm/min at300 C

PECVD SiH4 + 2N2O:SiO2 + 2N2 + 2H2 (200- 400 C) , RF,

0.1 - 5 torr .

Low ratio of N2O /SiH4 will increase “N” leading toformation of silicon rich films .

Lower deposition temperatures and higher ratios ofN2O/SiO2 will lead to less dense films and faster etchrates

HF etch rate is a measure of the film’s density Densification of films

Low Temperature oxide formation byAPCVD /LPCVD/ PECVD vs. Thermal

Oxidation of SiliconADVANTAGES

Low temperatures

Fast Deposition rates especially APCVD .

Good Step Coverage especially PECVD.

DISADVANTAGES

Contamination especially PECVD.

Inferior electrical properties of PECVD films as comparedwith thermally grown ones.

Less dense films are obtained .

CVD Chemistries

Silicon Oxide– Dry Oxidation: Si + O2 SiO2

– Wet Oxidation: Si + 2H2O SiO2 + 2H2

– SiH4 + O2 SiO2 + 2H2

– SiH4 + N2O SiO2 + by-products

– SiCl2H2 + N2O SiO2 + by-products

– Si(OC2H5)4 SiO2 + byproducts

CVD Chemistries

Silicon Nitride– 3SiH4 + 4NH3 Si3N4 + 12H2

– SiCl2H2 + NH3 Si3N4+ by-products

– SiH4 + 4N2O Si3N4 + by products

– SiH4 + N2 Si3N4 + by products

CVD Chemistries

Polysilicon: SiH4 Si + 2H2

Silicon Carbide

Polycrystalline Diamond

Parylene (polymerized p-xylylene)

Refractory Metals:– 2WF6 + 3SiH4 2W + 3SiF4 +6H2

II-VI compounds (e.g., CdSe)