Step Coverage of CVD Films - Thin Film Materials Processing - Lecture Slides, Slides of Material Engineering

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Chemical Vapor Deposition

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Step Coverage of CVD Films

High Surface Migration rate

Conformal coating

Low Surface Migration Rate

Arrival Angle θ = 180° θ = 270°

θ = 90°

Flux is a function of arrival angle (θ)

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Re-flow of PSG and BSG to flatten the

contour around contact openings

ƒ Reflow - Allows the surface tension of the glass to draw back the edges of the contact openings

ƒ Surface tension of the film at high temperatures causes the edges to draw back, lowering the side wall angle.

Θ - reflow angle

a b

(a) SEM of dry-etched contact window before reflow, (b) Reflowed BPSG film with 4 wt.% P and 4 wt.% B. Reflow was 930°C in N 2 for 25 minutes

Reference: Silicon Processing for the VLSI Era Vol. 1 by Wolf and Tauber, page 191

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Epitaxial Growth

ƒ Literally means “arranged upon”

ƒ A continuation of the single crystal substrate which acts as the seed

ƒ Vapor Phase Epitaxy will be discussed here, as opposed to liquid or solid phase epitaxy

ƒ Allows lightly doped layers to be grown on top of heavily doped material, which is impossible through diffusion.

Thickness Range

Thin Thick

< 1 micron > 20 microns

Fast digital devices (3.3V) Power Analog, 100’s of Volts

Doping Range

Light Heavy

< 10 15 cm -3 > 10 17 cm -

Processing becomes difficult at the extremes

of thickness and doping

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Current RIT CVD Processes

ƒ Polycrystalline silicon - Deposition from Silane at 610°C, amorphous silicon occurs if the temperature is reduced to ~550°C. The use of disilane would allow the deposition temperature to be reduced even further. No insitu doping of the polysilicon. Good uniformity

ƒ Silicon Nitride - Deposition from ammonia and dichlorosilane at ~800°C. Good uniformity

ƒ Oxide - Deposition from silane and oxygen at ~400°C. No insitu doping of the oxide films, i.e. no PSG. Poor Uniformity, extremely large gas depletion effect.

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Types of CVD

ƒ Epitaxy - “Arranged Upon”

ƒ APCVD - Atmospheric Pressure CVD

 Mass Transfer Limited

 Gas phase nucleation

ƒ LPCVD - Low Pressure CVD

 Reaction Rate Limited

ƒ PECVD - Plasma Enhanced CVD

 Reaction Rate Limited

ƒ MOCVD - Metal Organic CVD

Process Advantages Disadvantages

APCVD Simple Reactor Poor Step Coverage Fast Deposition Particle Contamination Low Temperature

LPCVD Excellent Purity High Temperatures Excellent Uniformity Low Deposition Rate Conformal Step Complex Reactor Coverage Large Wafer Capacity

PECVD Low Temperature Chemical and Particulate

Fast Deposition Contamitation Good Step Coverage Complex Reactor

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Epitaxial Reactor Designs,

Barrel vs. Pancake type

Horizontal Cold Wall (rf heating), lower gas loading, higher particulate count, high power rf supply required

Pancake type

Vertical Cold wall (radiant heating), frequent lamp changes and greater gas loading effect but lower particulate count

Barrel type

wafers

rf coils

graphite susceptor

Bell Jar

Quartz Lamps

Gas Input

Wafers

Cut-away view

Gas Input

Exhaust

Exhaust (^) Reactant Gases

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Low Pressure CVD

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Microcontroller

Thermocouples

Mass flow controllers (MFCs)

Vacuum Pump

Burn Box

~ 200 mTorr

Hot wall Heater Element

gas monitor “sniffer”

Gas Distribution

SiH 4

NH 3

Building Exhaust

O 2 N 2 SiH^2 Cl 2

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