ee c245 – me c218 introduction to mems design fall 2007ee245/fa07/lectures/lec4.filmdepii.pdf ·...

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C 245: Introduction to MEMS Design Lecture 4 C. Nguyen 9/6/07 1

EE C245 – ME C218Introduction to MEMS Design

Fall 2007

Prof. Clark T.-C. Nguyen

Dept. of Electrical Engineering & Computer SciencesUniversity of California at Berkeley

Berkeley, CA 94720

Lecture 4: Film Deposition II


Lecture Outline

• Reading: Senturia, Chpt. 3; Jaeger, Chpt. 2, 3, 6• Lecture Topics:

Example MEMS fabrication processesOxidationFilm Deposition

EvaporationSputter depositionChemical vapor deposition (CVD)Plasma enhanced chemical vapor deposition (PECVD)EpitaxyAtomic layer deposition (ALD)Electroplating

Lithography

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Chemical Vapor Deposition (CVD)

• Even better conformity than sputtering• Form thin films on the surface of the substrate by thermal decomposition and/or reaction of gaseous compounds

Desired material is deposited directly from the gas phase onto the surface of the substrateCan be performed at pressures for which λ (i.e., the mean free path) for gas molecules is smallThis, combined with relatively high temperature leads to

Types of films: polysilicon, SiO2, silicon nitride, SiGe, Tungsten (W), Molybdenum (M), Tantalum (Ta), Titanium (Ti), …

Excellent Conformal Step Coverage!


The CVD Process

Wafer

Gas Flow – Gas Stream(a)

Energy required to drive reactions supplied by several methods: Thermal (i.e., heat), photons, electrons (i.e., plasma)

(b)

(c) (d)

(e) (e)

(d)

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The CVD Process (cont.)

Step-by-Step CVD Sequence:a) Reactant gases (+ inert diluting gases) are introduced

into reaction chamberb) Gas species move to the substratec) Reactants adsorbed onto the substrated) Atoms migrate and react chemically to form films

This determines to a large extent whether or not a film is conformal (i.e. better step coverage)

e) Reaction by-products desorbed and removed from reaction chamber

Not Conformal Conformallow Tnot enough adatom migration

High TPlenty of adatom migration

Surfaceprocesses

Gas phaseprocesses

EE C245: Introduction to MEMS Design Lecture 4 C. Nguyen 9/6/07 6

CVD Modeling

gN

Simplified Schematic:

gJsJ

sN

δ

surface

Ng = conc. of reactant molecules in the gas stream

Ns = conc. of reactant molecules at the surface

Js = flux of gas molecules at the surface

Jg = flux of molecules diffusing in from the gas stream·

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CVD Modeling (cont.)


CVD Modeling (cont.)

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Temperature Dependence of CVD

log (growth rate) Mass

Transport Limited Regime

Reaction Rate Limited Regime

Arrhenius behavior

Slope = aE−

T1

temperatureDep. Rate less dependent on T, herefor better control, better to operate here (@ higher T)

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Atmospheric Pressure Reactor (APCVD)

•Once used for silicon dioxide passivation in integrated circuits

• Substrates fed continuously

• Large diameter wafers•Need high gas flow rates•Mass transport-limited regime (high pressure, so tougher for gas to get to the wafer surface)

• Problems/Issues:Wafers lay flat, and thus, incorporate foreign particlesPoor step coverage


Low Pressure Reactor (LPCVD)

•Many films available: polysilicon, SiGe, Si3N4, SiO2, phosphosilicate glass (PSG), BPSG, W

• Temp.: 300 → 1150°C• Press.: 30 → 250 Pa (200mTorr → 2Torr)

• Reaction rate limited; reduced pressure gives gas molecular high diffusivity; can supply reactants very fast!

• Can handle several hundred wafers at a time

• Excellent uniformity

• Problems:Low dep. rate (compared to atm.)Higher T (than atmospheric)In hot wall reactors, get deposition on tube walls (must clean)

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Plasma-Enhanced CVD Reactor (PECVD)

• RF-induced glow discharge + thermal energy to drive reactions → allows lower temperature deposition with decent conformability

• Still low pressure

• Problems:Pin-holesNon-stoichiometric filmsIncorporation of H2, N2, O2 contaminants in film; can lead to outgassing or bubbling in later steps


Polysilicon CVD

• In situ doping of polysilicon: n-type: add PH3 (phosphine) or Arsine gases (but greatly reduces dep. rate)p-type: add diborane gas (greatly increases dep. Rate)

Polysilicon Deposition:

600°CSiH4 Si + 2H2 (thermal decomposition of silane)

(conformal high T)

LPCVD (25 to 150 Pa) → 100-200Å/min

Fairly high temperature → conformal

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Silicon Oxide CVD

Silicon Dioxide Deposition:• After metallization (e.g., over aluminum)

Temperature cannot exceed the Si-Al eutectic pt.: 577oCActually, need lower than this (<500oC) to prevent hillocks from growing on Al surfacesSimilar issues for copper (Cu) metallization

• Low temperature reactions:

• Above reactions: not very conformal step coverage → need higher T for this

SiH4 + O2 SiO2 + 2H2(silane) 300-500°C

4PH3 + 5O2 2P2O5 + 6H2(phosphine) 300-500°C

LPCVDLTO

ReactionsPhosphosilicate

glass (PSG)


Silicon Oxide CVD (cont.)

• Phosphosilicate glass can be reflown6-8 wt. % allows reflow @ 1000-1100oCVery useful to achieve smoother topographyLower concentration → won’t reflowHigher concentration → corrodes Al if moisture is present5-15% P can be used as a diffusion source to dope Si

• Before metallization:Can use higher temperature → better uniformity and step coverage

(dichlorosilane) (Nitrous oxide)

(nice conformal step coverage)

HTOSiCl2H2 + 2N2O SiO2 + 2N2 + 2HCl

~900°C

or …

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Silicon Oxide CVD (cont.)

Si(OC2H5)4 SiO2 + by-products650-750°C

(Tetraethylorthosilicate) (TEOS)

(excellent uniformity & conformal step coverage)


Silicon Nitride CVD

Silicon Nitride Deposition:• First, note that thermal growth is possible:

Si in NH3 @ 1000-1100oCBut very slow growth rate, thus, impractical

• LPCVD reactions:

Silane reaction: 3SiH4 + 4NH3 Si3N4 + 12H2

700-900°C

(Atm. Press.)Dichlorosilane reaction:

3SiCl2H2 + 4NH3 Si3N4 + 6HCL + 6H2

700-800°C

(LPCVD)Increase and T = 835°C Si rich nitride low stress

Problem: Clobbers your pumps! Expensive to maintain!

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Silicon Nitride CVD (cont.)

• Comments on LPCVD nitride films:Hydrogen rich: ~8% H2High internal tensile stresses: films >1000Å crack and peel due to excessive stressCan get 2μm films with Si-rich nitrideLPCVD gives high resistivity (1016 Ω-cm) and dielectric strength (10 MV/cm)

PECVD Nitride:

orSiH4 + N2 2SiNH + 3H2

Nitrogen discharge

SiH4 + NH3 SiNH + 3H3Ar plasma

PECVD films:Non-stoichiometricnitride20-25% H2 contentCan control stress

resistivity)10( 6 cm−Ω


Metal CVD

CVD Metal Deposition:

Tungston (W) – deposited by thermal, plasma or optically-assisted decomposition

WF6 W + 3F2

or via reaction with H2:WF6 + 3H2 W + 6HF

Other Metals – Molybdenum (Mo), Tantalum (Ta), and Titanium (Ti)

where M = Mo, Ta, or Ti2MCl5 + 5H2 2M + 10HCl,

(Even Al can be CVD’ed with tri-isobutyl Al … but other methods are better.)(Cu is normally electroplated)

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Epitaxy

• Use CVD to deposit Si on the surface of a Si waferSi wafer acts as a seed crystalCan grow a single-crystal Si film (as opposed to poly-Si)

Epitaxy:

Modeling –similar to CVD in fact, the model discussed so far for CVD is more relevant to epitaxy than CVD!

get similar curve:

Log (growth rate)

T1

Mass transport limited

Reaction rate limited

Reactions – can use SiCl4, SiH4, SiH2Cl2 for vapor phase epitaxy.

SiCl4: Silicon tetrachlorideSiH4: silaneSiH4Cl2: dichlorosilane


Epitaxy (cont.)

Most popular: SiCl4 (gas) + 2H2 (gas) Si (solid) + 4HCl (gas)

(Note that this is reversible!)

Reverse reaction (i.e., etching) if have excessive HCl sometimes used before deposition to clean the Si wafer surface.

SiCl4 (gas) + Si (solid) 2SiCl2 (gas)

Also get a competing reaction.

Important that the right conc. of SiCl4 is used!

See Figure 4.2

Too much SiCl4 etching rather than growth takes place!Growth rate too fast get polysiliconinstead of Si. (> 2μm/min.)

1200°C

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Epitaxial Growth Rate Dependencies

Figure 4.2


Epitaxy (cont.)Alternative reaction: pyrolytic decomposition of silane:

SiH4 Si + 2H2

650°C

not reversible, low T, no HCl formation

however, requires careful control of the reaction to prevent formation of poly-Sialso, the presence of an oxidizing species

causes silica formation

1. Just add impurities during growth: Arsine, diborane, Phosphine

Control resistivity by varying partial pressure of dopantspecies

i. Arsine, Phosphine → slow down the growth rateii. Diborane → enhances growth rate

Doping of Epitaxial Layers:

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Doping of Epitaxial Layers

2. Use “autodoping” → when growing own heavily-doped substrates

Impurity evaporates from wafer (or liberated by Cletching of surface during dep.)Incorporates into gas streamImpurities dope new layerExamples of autodoping:

Bipolar Processing:

n+

MOS:

n+P-Si

n n-n+

n+P-Siepitoxy

Buried collector to reduce

collection R

Dopantgradient helps to prevent latch up and punch through

epitoxy


Atomic Layer Deposition (ALD)

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• Fundamental Components:Self-limiting surface reactions of suitable precursor compounds A & BA & B then form the desired product S in a binary reaction cycle consisting of two sequential half-reactions

Precursor A

Precursor B



• Remarks:Both half-reactions must be complete and self-limiting at the monolayer levelThe total film thickness d(tot) can be “digitally”controlled by the number of applied deposition cycles N(A/B):

The reagents A & B in the half reactions are normally chemical reactions

But they don’t need to beThey can also represent a physical process, e.g., heating, irradiation, electrochemical conversion

d(tot) = d(mono) · N(A/B)

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Advantages of ALD

• Surface limited reactionexcellent step coverage and refilling

• Self-limiting mechanismMonolayer depositionComposition controlThickness control (∝ # of cycles)Less sensitive to flow rate & temperature

Note, though, that there’s still a temperature window:

growth cycle

temperature

ALD Window

Decomposition

Re-evaporationIncomplete Reaction

Condensation

monolayer


ALD Reactor

Usually mixed w/ an inert gas to achieve lower effective vapor

pressures → slows reaction, but needed to allow rapid pulsing & purging

200oC to 400oC needed Must purge completely before the next pulse

ee c245 – me c218 introduction to mems design fall 2007ee245/fa07/lectures/lec4.filmdepii.pdf ·...

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