CVD, Oxidation, and Diffusion Thin-Film Deposition

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Fundamentals of Micromachining. Dr. Bruce K. Gale. BIOEN 6421. EL EN 5221 and 6221 ME EN 5960 and 6960. Thin-Film. Deposition. •. S pin-on F ilm s. –. P.

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Dept. of ECE, Univ. of Texas at Austin

Evaporation

Condensed Phase (solid or liquid)

Gas Phase

Transport

R. B. Darling / EE-527

Condensed Phase (usually solid)

Condensation

Gas Phase

Physical Vapor Deposition

Dean P. Neikirk © 2001, last update February 2, 2001

• PVD: physical vapor deposition • CVD: chemical vapor deposition

– vapor phase deposition

• not standard IC process

– liquid phase deposition

• not standard IC process

– crystalline, poly crystalline, amorphous – electro-deposition

• “deposited” films

• example: SiO2 formed by oxidation of Si substrate

– typically “converted” from original substrate material

• “grown” films

Thin film processes

– Oxidation – LPCVD – PECVD

• Chemical Vapor Deposition (CVD)

– Evaporation – Sputtering

• Physical Vapor Deposition (PVD)

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

• Spin-on Films

Thin-Film Deposition

Fundamentals of Micromachining Dr. Bruce K. Gale BIOEN 6421 EL EN 5221 and 6221 ME EN 5960 and 6960

CVD, Oxidation, and Diffusion

diffusion masks surface passivation gate insulator (MOSFET) isolation, insulation

Dept. of ECE, Univ. of Texas at Austin

hydroxyl group

network former

network modifier

silicon

non-bridging oxygen

bridging oxygen

1 atm , 1000 C

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

• Si (s) + 2H2O --->SiO2 + 2H2

– Wet Oxidation

• Si(s) + O2 --> SiO2

– Dry Oxidation

• Formation of the oxide of silicon on the silicon surface is known as oxidation . • Thermal Oxidation is characterized by high temperatures (900 - 1200 C) . • Two main processes :

Thermal Oxidation of Silicon

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– BUT devitrification rate (i.e. crystallization) below 1000Û&QHJOLJLEOH

m.p. 1732Û&JODVVLV³XQVWDEOH´EHORZ 1710Û&

– can also find “crystal quartz” in nature

vitreous silica: material is a GLASS under “normal” circumstances

• C V D, evaporate, sputter

– deposited:

• thermal: “highest” quality • anodization

– grown / “native”

Formation:

– – – –

Uses:

Dean P. Neikirk © 2001, last update February 2, 2001









Silicon Oxides: SiO2 • • • •

R. B. Darling / EE-527

J. N. Stranski and L. Krastanov, Ber. Akad. Wiss. Wien 146, p. 797 (1938).

(3) Stranski-Krastanov: (layers + islands):

R. B. Darling / EE-527

F. C. Frank and J. H. Van der Merwe, Proc. R. Soc. London, Ser. A 198, p. 205 (1949).

(2) Frank-Van der Merwe: (layer growth; ideal epitaxy):

M. Volmer and A. Weber, Z. Phys. Chem. 119, p. 277 (1926).

(1) Volmer-Weber: (island growth):

Modes of Thin Film Growth

Island Stage Coalescence Stage Channel Stage Continuous Film Stage

Stages of Thin Film Growth

j’

gas oxide x

N0

N1

moving growth interface silicon

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Dept. of ECE, Univ. of Texas at Austin

j = −D ⋅

∂ N oxidizer  N −N  ≈ −D ⋅  − 0 1  ∂x x  

j’ = k ⋅ N1

=

steady state

j



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‡

j =

D ⋅ N0 x + Dk Dept. of ECE, Univ. of Texas at Austin

solve for N1, sub back into flux eq

 N − N1  k ⋅ N1 = − D ⋅ − 0  x 

– k is the chemical reaction rate constant in steady state, flux in must equal flux consumed

Dean P. Neikirk © 2001, last update February 2, 2001





silicon

N − N1 ∂ N = − 0 ∂ x x

N1

N0 is limited by the solid solubility limit of the oxidizer in the oxide! – N0O2 ~ 5 x 1016 cm-3 @ 1000Û& – N0H2O ~ 3 x 1019 cm-3 @ 1000Û& flux of oxidizer j’ at SiO2 / Si interface consumed to form new oxide

concentration

Oxidizer concentration gradient and flux

Dean P. Neikirk © 2001, last update February 2, 2001



x

oxide

• simplest approximation:

gas

N0

moving growth interface

∂x

– supply of oxidizer is limited by diffusion through oxide to growth interface ∂N • Fick’s First Law: flux j = − D oxidizer

• basic model is the Grove and Deal Model

Oxide growth kinetics

concentration

d

0.44d

SiO2

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Dept. of ECE, Univ. of Texas at Austin

+ Si - Si → Si - O - Si

Si - O - Si

+ H2

Dean P. Neikirk © 2001, last update February 2, 2001

– ρwet §JPFP3

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Dept. of ECE, Univ. of Texas at Austin

• this results in a more open oxide, with lower density, weaker structure, than dry oxide

Si - O H

Si - OH

– H2O + Si-O-Si → Si-OH + Si-OH – diffusion of hydroxyl complex to SiO2 -Si interface

• proposed process

– Si + 2 H2O → SiO2 + H2

• overall reaction is

“Wet” oxidation of Si

Dean P. Neikirk © 2001, last update February 2, 2001

– it is possible to prepare a hydrogen terminated Si surface to retard this “native” oxide formation

• “bare” silicon in air is “always” covered with about 15-20 Å of oxide, upper limit of ~ 40 Å

original silicon surface

– ρSiO2 = 2.25 gm/cm3 , GMW = 60 – ρSi = 2.3 gm/cm3 , GMW = 28 – oxide d thick consumes a layer 0.44d thick of Si

• density / formula differences

• does the oxygen go “in” or the silicon go “out”?

– Si + O2 → SiO2 – once an oxide is formed, how does this chemical reaction continue?

• in dry ( 1, slow SiO2 diffuser

Oxidation thicknesses

Dean P. Neikirk © 2001, last update February 2, 2001

• little effect on parabolic rate constant B • increases linear rate constant B/A – again, really only significant for Nphosphorus > ~1020 cm-3

– k = Cox / CSi ~ 0.1 – dopants “pile-up” at silicon surface

• phosphorus

• little effect on linear rate constant B/A ( = Nok / n) • can increase parabolic rate constant B ( = 2DNo / n ) – really only significant for Nboron > ~1020 cm-3

– k = Cox / CSi ~ 3 – dopants accumulate in oxide

• boron

Effect of Si doping on oxidation kinetics

Oxide thickness (microns)

Pressure Effects on Oxidation

Oxide thickness (microns)

concentration concentration

CVD reactions

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

– – – –

Undesirable Form gas phase clusters of material Consume reactants Reduce deposition rate

• Homogeneous = occur in gas phase

– Desirable – Produce good quality films

• Heterogeneous = occur at wafer surface

– – – – –

• CVD = formation of non-volatile solid film on substrate by reaction of vapor phase chemicals • Steps in CVD

Chemical Vapor Deposition (CVD)

λ ranges from 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 APCVD : 325 C ratio 3:1 , 475 C ratio 23:1 , 550 C ratio 60: 1 LPCVD : 360 C ratio 1:1 , 450 C ratio 1.45 : 1 Deposition can occur in the APCVD as low as 130 C For LPCVD Window (100 - 330 C ) 2-12 torr and 14 nm/min at 300 C

Dean P. Neikirk © 2001, last update February 2, 2001

• • • •

• • • • • •

Low Temperature Oxidation of Silicon

load lock

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wafers

Dept. of ECE, Univ. of Texas at Austin

rf in

pump

chemical scrubbers filters vacuum pumps

graphite rf electrode

furnace heater

mass flow controller gas supply

furnace heater

wafers

gas flow

mass flow controller gas supply

gas flow

furnace heater

chamber wall (tube)

gas supply

load lock

mass flow controller gas supply

plasma assisted CVD: PECVD

– atmospheric: high deposition rates – low pressure (LPCVD): lower rates, good uniformity

• avoid unwanted contamination, escape of hazardous materials (the reactants)

thermally driven reactions – requires leak-tight, sealed system

heat entire system:

CVD system design: hot wall reactors

Dean P. Neikirk © 2001, last update February 2, 2001



ADVANTAGES Low temperatures Fast Deposition rates especially APCVD . Good Step Coverage especially PECVD.

• DISADVANTAGES • Contamination especially PECVD. • Inferior electrical properties of PECVD films as compared with thermally grown ones. • Less dense films are obtained .

• • • •

/LPCVD/ PECVD vs. Thermal Oxidation of Silicon

Low Temperature oxide formation by APCVD

2

4

2

2

2

2

4

2

2

• SiH + 2N O:→SiO + 2N + 2H (200- 400 C) , RF, 0.1 - 5 torr . • Low ratio of N O /SiH will increase “n” leading to formation of silicon rich films . • Lower deposition temperatures and higher ratios of N O/SiO will lead to less dense films and faster etch rates • HF etch rate is a measure of the film’s density • Densification of films

PECVD

– Reaction rate < reactant arrival rate – Reaction rate limited

• At low temperatures

– Reaction rate > reactant arrival rate – Mass-transport limited

• Surface reaction rate increases with increasing temperature at very high temperature

– where Ea = activation energy (eV) – k = Boltzmann constant – T = temperature (K)

• R = R0 exp(-Ea/kT)

CVD Reaction Rate (R)

graphite susceptor

pump

gas inlet

Dept. of ECE, Univ. of Texas at Austin

Dry Oxidation: Si + O2 Æ SiO2 Wet Oxidation: Si + 2H2O Æ SiO2 + 2H2 SiH4 + O2 Æ SiO2 + 2H2 SiH4 + N2O Æ SiO2 + by-products SiCl2H2 + N2O Æ SiO2 + by-products Si(OC2H5)4 Æ SiO2 + byproducts

• Silicon Oxide

– – – – – –

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pump

gas flow

rf electrodes

– “pancake” configuration is similar

gas inlet wafers

gas flow

plasma

• parallel plate plasma reactor

CVD Chemistries

Dean P. Neikirk © 2001, last update February 2, 2001

from: http://www.appliedmaterials.com/prod ucts/pdd.html

• barrel reactor • single wafer systems

wafers

gas flow

rf excitation coil

• horizontal tube reactor

Basic configurations

Dept. of ECE, Univ. of Texas at Austin

substrate

x

d

d =

µ ⋅x ρ ⋅v

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substrate

lines gas flow Dept. of ECE, Univ. of Texas at Austin

– v: velocity; ρ: density; µ: viscosity • reactant supply limited by diffusion across boundary layer • geometry of wafers relative to gas flow critical for film thickness uniformity – to improve boundary layer uniformity can tilt wafer wrt gas flow

gas flow lines

• causes formation of stagnant boundary layer

– away from surfaces, flow is primarily laminar – friction forces velocity to zero at surfaces

interaction of gas flow with surfaces

• typical of LPCVD tube furnace design

– reactants are well mixed, no “geometric” limitations on supply of reactants to wafer surface

purely “turbulent” flow

Dean P. Neikirk © 2001, last update February 2, 2001





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Gas flow in CVD systems

Dean P. Neikirk © 2001, last update February 2, 2001

• inherently a non-isothermal system

– more complex to achieve temperature uniformity – hard to measure temperature

• disadvantages

– reduces contamination from hot furnace walls – reduces deposition on chamber walls

• advantages

– resistive heating (pass current through “susceptor”) – inductive heating (external rf fields create eddy currents in conductive susceptor) – optical heating(lamps generate IR, absorbed by susceptor)

• heat substrate “only” using

Cold wall reactors

• TiN often used to improve adhesion • causes long “initiation” time before W deposition begins

WF6 + 3H2 D W + 6HF cold wall systems ~300Û& can be selective adherence to SiO2 problematic

Dept. of ECE, Univ. of Texas at Austin

– Cu β-diketones, ~100Û-200Û&

copper

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– tri-isobutyl-aluminum (TIBA) – LPCVD – ~200Û-300Û&WHQVQPPLQGHSRVLWLRQUDWH

aluminum

Dept. of ECE, Univ. of Texas at Austin

– frequently used to fill deep (“high aspect ratio”) contact vias

– – – – –

tungsten

Dean P. Neikirk © 2001, last update February 2, 2001







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Metal CVD

Dean P. Neikirk © 2001, last update February 2, 2001

• dope after deposition (implant, diffusion)

• can cause substantial decrease in deposition rate

– n-type: arsine AsH3, phosphine PH3 : ρ ~ 0.02 Ω-cm

• can cause substantial increase in deposition rate

– p-type: diborane B2H6: ρ ~ 0.005 Ω-cm (B/Si ~ 2.5x10-3)

• in-situ doping

• 950Û&SKRVSKRUXVGLIIXVLRQPLQa—PJUDLQVL]H • 1050Û&R[LGDWLRQa-3 µm grain size

– grain size dependent on growth temperature, subsequent processing

• atmospheric, cold wall, 5% silane in hydrogen, ~1/2 µm/min • LPCVD (~1 Torr), hot wall, 20-100% silane, ~hundreds nm/min

– silane pyrolysis: 600Û-700Û&6L+4 ê Si + 2H2

• deposition

– gates, high value resistors, “local” interconnects

• uses

Material examples: polysilicon

– 2WF6 + 3SiH4 Æ 2W + 3SiF4 +6H2

Polysilicon: SiH4 Æ Si + 2H2 Silicon Carbide Polycrystalline Diamond Parylene (polymerized p-xylylene) Refractory Metals:

CVD Chemistries

3SiH4 + 4NH3 Æ Si3N4 + 12H2 SiCl2H2 + NH3 Æ Si3N4+ by-products SiH4 + 4N2O Æ Si3N4 + by products SiH4 + N2 Æ Si3N4 + by products

• II-VI compounds (e.g., CdSe)

• • • • •

– – – –

• Silicon Nitride

CVD Chemistries

Dept. of ECE, Univ. of Texas at Austin

– – – – –

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Dept. of ECE, Univ. of Texas at Austin

• with both layer “below” and “above” • at room temperature and under deposition conditions

good electrical characteristics free from pin-holes, cracks low stress good adhesion chemical compatibility

general requirements

– chemical vapor deposition (CVD)

• thermal evaporation • sputtering

– physical vapor deposition (PVD)

deposition methods

– both conductors and insulators

need to be able to add materials “on top” of silicon

Dean P. Neikirk © 2001, last update February 2, 2001







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Deposited thin films

Dean P. Neikirk © 2001, last update February 2, 2001

– double walled tubing, all welded distribution networks

• helps with dispersal problem associated with gases

– monitor! – limit maximum flow rate from gas sources

• how to deal with this?

• toxic, corrosive

– ammonia

• very toxic, flammable

– phosphine

• toxic, burns on contact with air

– silane, SiH4

• most gases used are toxic, pyrophoric, flammable, explosive, or some combination of these

Safety issues in CVD

• • • • •

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aSiH4 + bNH3 ê SixNyHz + cH2 aSiH4 + bN2 ê SixNyHz + cH2 Si/N ratio 0.8-1.2, ~20% H ρ ~ 2.4-2.8 g/cm3 ; n ~ 1.8-2.5; k ~ 6-9 stress: ~2C - 5T Gdyne/cm2

– PECVD: ~250Û&- 350Û&

Dept. of ECE, Univ. of Texas at Austin

• 3SiH4 + 4NH3 ê Si3N4 + 12H2 ; 3Si2Cl2H2 + 4NH3 ê Si3N4 + 6HCl + 6H2 • Si/N ratio 0.75, 4-8% H • ρ ~ 3 g/cm3 ; n ~ 2.0; k ~ 6-7 • stress: ~10 Gdyne/cm2, tensile

– LPCVD: ~700Û&- 900Û&

• in practice Si/N ratio varies from 0.7 (N rich) to 1.1 (Si rich)

– stoichiometric formulation is Si3N4

deposition

• protect against mobile ion contamination

– diffusivity of Na also very low

• mask against oxidation, protect against water/corrosion

– diffusivity of O2, H2O is very low in nitride

uses

Dean P. Neikirk © 2001, last update February 2, 2001





Dept. of ECE, Univ. of Texas at Austin

Silicon nitride Si3N4

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• gate insulators, de-coupling caps

– high k dielectrics: k > ~25-100’s

• interlevel insulation with lower dielectric constants (k < ~3) – fluorinated oxides, spin-on glasses, organics

– low “k” dielectrics

new materials

• Si(OC2H5)4 ê SiO2 + by-products

– tetraethyl orthosilicate (TEOS)

high temperature: ~700Û&

– low temperature: ~250Û&

plasma-enhanced reaction (PECVD)

– SiH4 + O2 ê SiO2 + H2 – cold-wall, atmospheric, ~0.1 µm/min – hot-wall, LPCVD, ~0.01 µm/min

• “LTO” (low-temp. oxide) T < ~500Û&

– mid-temperature: ~ 500Û&

thermally driven reaction

Dean P. Neikirk © 2001, last update February 2, 2001









CVD silicon dioxide

3.5 x 1013

3.5 x 1010

3.5 x 107

3.5 x 104

10-3

10-6

10-9

10-12

rough vacuum

high vacuum 100 %

98 %

λ system, λ >> step

Dean P. Neikirk © 2001, last update February 2, 2001







180ÛLQFLGHQFH

randomizing collisions

180ÛLQFLGHQFH

assumes material does NOT migrate after arrival!!

substrate

no randomizing collisions

BUT no scattering inside “hole”!!

– small compared to system, large compared to wafer features – isotropic arrival at “flat” surface

Case II: 10-1 Torr « λ = 0.5 mm

Dean P. Neikirk © 2001, last update February 2, 2001







“low” pressure: λ step

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