Chemical Vapor Deposition - University of Tennesseeweb.utk.edu/~prack/Thin films/cvd2.pdfChemical...

Chemical Vapor Deposition


Dr. Philip D. RackAssistant Professor

Department of Materials Science and Engineering

The University of TennesseeTel (865) 974-5344

Fax (8654) [email protected]


Step Coverage of CVD Films

High Surface Migration rateConformal coating

Low Surface Migration Rate

Arrival Angleθ = 180°

θ = 270°

θ = 90°

Flux is a function of arrival angle (θ)


Flow of Phospho-Silicate Glass (PSG),Boro-Silicate (BSG) or BPSG

Flow - Elevated temperature step to close pin-holes and smooth out topographyMore flow (lower glass viscosity @ high temperatures) for higher phosphorus content (% by weight)A re-entrant profile can cause opens or stringers after etching

Re-entrantprofile

gap

If you do fill it in, you may get anEtch Stringer

Sem cross-sections (10,000X) of samples annealed in steam at 1100°C for 20 minfor the following weight of phosphorus: (a) 0.0 wt. % P; (b) 2.2 wt. % P; (c) 4.6 wt.% P; (d) 7.2 wt. % P.

Reference: Silicon Processing for the VLSI Era Vol. 1 by Wolf and Tauber, page 187


Re-flow of PSG and BSG to flatten the contour around contact openings

Reflow - Allows the surface tension of the glass to draw back the edges of the contact openingsSurface 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) ReflowedBPSG film with 4 wt.% P and 4 wt.% B. Reflow was 930°C in N2 for25 minutes

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


Epitaxial Growth

Literally means “arranged upon”A continuation of the single crystal substrate which acts as the seedVapor Phase Epitaxy will be discussed here, as opposed to liquid or solid phase epitaxy

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

Thickness Range

Thin Thick> 20 microns< 1 micron

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

Doping Range

Light Heavy> 1017 cm-3< 1015 cm-3

Processing becomes difficult at the extremesof thickness and doping


Epitaxial CVD Chemistry

Reactant gas in H2 ambientSiCl4 silicon tetrachloride, deposition rate of 0.4-1.5 microns/minute @ temperatures of 1150-1250 °CSiHCl3 trichlorosilane, deposition rate of 0.4-2.0 microns/minute @ temperatures of 1100-1200 °CSiH2Cl2 dichlorosilane, deposition rate of 0.4-3.0 microns/minute @ temperatures of 1050-1150 °CSiH4 - silane deposition rate of 0.2-0.3 microns/minute @ temperatures of 950-1050 °CSi2H6 - DiSilane, has an even lower deposition temperature

Relative Merits

SiH4SiCl4Low TempHigh Growth Rate

SiHCl3 SiH2Cl2

Relative Drawbacks

BOOM !

High temp, wafer warpageOut Diffusion

Low Growth Rate

Gas Phase Nucleation


Current RIT CVD Processes

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 uniformitySilicon Nitride - Deposition from ammonia and dichlorosilaneat ~800°C. Good uniformityOxide - Deposition from silane and oxygen at ~400°C. No insitudoping of the oxide films, i.e. no PSG. Poor Uniformity, extremely large gas depletion effect.


Types of CVD

Epitaxy - “Arranged Upon”APCVD - Atmospheric Pressure CVD

Mass Transfer LimitedGas phase nucleation

LPCVD - Low Pressure CVDReaction Rate Limited

PECVD - Plasma Enhanced CVDReaction Rate Limited

MOCVD - Metal Organic CVD

Process Advantages Disadvantages

APCVD Simple Reactor Poor Step CoverageFast Deposition Particle ContaminationLow Temperature

LPCVD Excellent Purity High TemperaturesExcellent Uniformity Low Deposition RateConformal Step Complex Reactor

CoverageLarge Wafer Capacity

PECVD Low Temperature Chemical and ParticulateFast Deposition ContamitationGood Step Coverage Complex Reactor


CVD Equipment Components

Reactor ZoneBell Jar Quartz Tube

Gas Distribution SystemMass Flow ControllersFlow Meters

Wafer Heating SystemInfraredrf inductionResistive

Exhaust System


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 changesand greater gas loading effect but lower particulate count

Barrel type

wafers

rf coils

graphitesusceptor

Bell Jar

Quartz Lamps

Gas Input

Wafers

Cut-away view

Gas Input

Exhaust

Exhaust Reactant Gases


Low Pressure CVD

oooooooooooooooooooo

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Microcontroller

Thermocouples

Mass flow controllers (MFCs)VacuumPump

Burn Box

~ 200 mTorr

Hot wall Heater Element

gasmonitor

“sniffer”

Gas DistributionSiH4

NH3

Building Exhaust

SiH2Cl2O2 N2


Plasma Enhanced CVD

Why?

LPCVD Silicon Nitride, SiH4 + NH3 @ 700-900°C

PECVD Silicon Nitride, SiH4 + NH3 @ 200-350°C

Using an rf source to create a plasma SIGNIFICANTLY reduces thedeposition temperature and reduces the thermal budget

~rf supply 13.56MHz

Graphite Electrode (wafer holder)

Insulating Spacers

rf must be fed throughthe furnace door

Near Electrode is shown transparent

Chemical Vapor Deposition - University of Tennesseeweb.utk.edu/~prack/Thin films/cvd2.pdfChemical...

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