MSE-630

Thin Film Deposition

Topics:

•Chemical Vapor Deposition

•Physical Vapor Deposition

•Evaporation

•Sputtering

•Strengths and Weaknesses

•Basic Calculations

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Issues related to thin film deposition

• Quality:– Composition– Defect density (e.g. pinholes)– Contamination– Mechanical and electrical properties– Good adhesion– Minimum stress

• Topography– Uniform thickness on non-planar surfaces– Step coverage– Conformal coverage: uniform– Space filling in holes, channels– Voids

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Thin film filling issues: (a) shows good metal filling of a via or contact hole in a dielectric layer

(b) silicon dioxide dielectric filling the space between metal lines, with poor filling leading to void formation

(c) poor filling of the bottom of a via hole with barrier or metal

SEM photo showing typical coverage and filling problems

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Two types of thin film deposition: CVD and PVD

• CVD– Reactive gases interact with substrate– Used to deposit Si and dielectrics– Good film quality– Good step coverage

• PVD– Used to deposit metals– High purity– Line of sight

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CVDGases react with substrate

Various types of CVD:

Atmospheric pressure – APCVD

Low pressure – LPCVD

Plasma enhanced – PECVD

High density plasma - HDPCVDCVD systems

(a)APCVD w/cold wall for deposition of epitaxial silicon

(b) LPCVD w/hot wall for depositing polycrystalline and amorphous silicon

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Steps in CVD

1. Transport reactants via forced convection to reaction region

2. Transport reactants via diffusion to wafer surface

3. Adsorb reactants on surface

4. Surface processes: chemical decomposition, surface migration, site incorporation, etc.

5. Desorption from surface

6. Transport byproducts through boundary layer

7. Transport byproducts away from deposition region

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Flux through boundary layer:

F1 = hG(CG-Cs) (molecules/cm2/s)

Flux of reactants consumed at surface:

F2 = ksCs (molecules/cm2/s)

Process is limited by slowest step, thus F = F1 = F2

G

s

Gs

hk

CC1

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Film Growth

NC

hkhkvC

hkhkCkF

scm

NFv

G

Gs

GsG

Gs

Gsss

Define Y = CG/CT = PG/Ptotal YNC

hkhkv T

Gs

Gs

If ks << hG, then v ≈ CT/N ksY

If hG<< ks , then v ≈ CT/N hGY

ks = hGexp(-Ea/kT) Ea ≈ 1.6 eV

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Boundary layer velocities along susceptor. s is the thickness of the boundary layer. The boundary layer increases with distance in the direction of gas flow

The susceptor in a horizontal epitaxial reactor is tilted so that the cross-sectional area of the chamber is decreased, increasing the gas velocity along the susceptor. This compensates for both the boundary layer and depletion effects.

hG = DG/s

The position of the boundary layer changes wrt x:

Uxxs

)(

= viscosity = density of gasU = gas velocity

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AutodopingGases may be doped, e.g.,

AsH3, PH3, B2H6

Autodoping occurs when dopant atoms adsorbed on (1) wafer frontside (2)

wafer backside and edges (3) other wafers and (4)

susceptor are reemitted.

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LPCVD

Growth velocity vs 1/T for APCVD (760 torr) and LPCVD (1 torr) systems. The lower total pressure (with PG and CG fixed) shifts the hG curve upward, extending the surface reaction regime to higher temperatures.

Recall that:

totalG

s

GG

T

Gs

Gs

PDandDh

YNC

hkhkv

1

Decreasing Ptotal increases DG, hG and v

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• Advantages of LPCVD– Faster growth– Less autodoping– Little diluent gas needed– Lower gas consumption– Fewer byproducts (particles)

• Disadvantages:– Line of sight– Poorer step coverage– shadowing

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Plasma Enhanced CVD (PECVD)

Good when temperature is restricted

Provides reasonable deposition rates

Good film quality

Conformal

May leave unwanted byproducts on film

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Physical Vapor Deposition: PVDAdvantages:

Versatile – deposits almost any material

Very few chemical reactions

Little wafer damage

Limitations:Line-of-sight

Shadowing

Thickness uniformity

Difficult to evaporate materials with low vapor pressures

2 types: evaporation and sputtering

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kevap

k rR

F cos2

Geometries of flux and deposition of small areas on a flat wafer holder for (a) a point source and (b) a

small planar surface source

ikevap

NrR

v coscos2

Flux from a point source

Deposition rate from a surface source:

esevap PTmAR

2/121083.5

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Deposition rate of evaporated film as function of position on substrate for point and surface sources. i = k in this configuration for both point and surface sources.

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Mean free path :ePd

kT22

k = 1.36 x 10-2 erg/at-K

d≈.4 x 10-8 cm

Pe = partial pressure (torr)

Vapor pressure as a function of temperature of commonly evaporated metals

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The depositing species have a high sticking coefficient (close to 1) in (a), so that they are deposited where they

first strike. In (b) the depositing species have a low sticking coefficient (<<1) so that man are reemitted and deposit

elsewhere on the topography, such as the sidewalls.

Sticking coefficient: Sc = Freacted/Fincident

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Sputtering

Schematic diagram of DC-powered sputter deposition equipment Plasma structure and voltage

distribution in DC sputter system

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Processes in sputter deposition

Distribution of arrival fluxes for (a) uniform or isotropic arrival distribution and (b) directed

or anisotropic arrival distribution. Arrival angle distribution (cosn) is defined by arrival flux

relative to unit surface area. This flux is equal to the normal component of incoming

flux, relative to the vertical direction for a horizontal surface.

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Effect of arrival angle distribution of depositing species on filling trenches

or holes. In (a) a relatively wide arrival angle distribution leads to poor bottom filling or coverage, while (b) a narrower arrival angle distribution leads to better bottom filling. The higher the aspect ratio of the feature, the narrower the arrival angle distribution must be for

adequate coverage.

Schematic diagram of ionized sputter deposition system (ionized PVD)

showing atomic flux lines

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