Deposition & Growth

8/13/2019 Deposition & Growth

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Deposition

Different kinds of films are needed to bedeposited Thermal oxides Dielectric layers Epitaxial layers Polycrystalline silicon Metal films


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Issues related to film deposition

Quality of deposited films Composition Contamination levels Defect density Mechanical & Electrical properties

Uniform thickness Filling issues


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Step coverage

Film thickness decreases as it crosses a step(non planar

topography) leading to change in electrical resistance andcause mechanical cracking and failure


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Step coverage

Coverage on the side of a step in topographyis called step coverage.

Conformal step coverage refers to uniformfilm thickness on both horizontal and verticalsurfaces

surfacehorizontaltoptheondepositedthicknessstepaof sidetheondepositedthicknessMinimum

CoverageStep


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Filling issues

Void in filling cause change in resistance (metals) and cracks (dielectrics). Aspect ratio is another important parameter.

Aspect ratio AR hw


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Deposition

Requirements for deposition1. Desired composition, low contaminates,

good electrical and mechanical properties.2. Uniform thickness across wafer, and

wafer-to-wafer.3. Good step coverage (conformal coverage). 4. Good filling of spaces.5. Planarized films.


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Deposition

Two main deposition methods are usedtoday: Chemical Vapor Deposition (CVD)

APCVD, LPCVD, PECVD, HDPCVD

Physical Vapor Deposition (PVD) evaporation, sputter deposition


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APCVD

H 2Ar

H 2+PH 3

H 2+B2H 6

HClSiCl 4 H 2

Silicon wafe rsGraphite susceptor

Quartz reaction chamberRF induction (heat ing) coils

vent

SiCl 4 + 2H 2 Si + 4HCl


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v F

Nk S h G

k S h G

C GN

k S h Gk S h G

C TN

Y

1. If k S


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Key points: k S limited deposition is VERY temp

sensitive.

h G limited deposition is VERY geometry(boundary layer) sensitive.

Si epi deposition often done at high T toget high quality single crystal growth.

h G controlled. horizontal reactor

configuration. h G corresponds to diffusion through a

boundary layer of thickness . S

But typically is not constantas the gas flows along a surface.

\ special geometry is required foruniform deposition.

S


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Low Pressure Chemical Vapor Deposition(LPCVD)

Atmospheric pressure systems havemajor drawbacks:

At high T, a horizontal configuration must be used (few

wafers at a time). At low T, the deposition rate goes down and throughput

is again low.


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The solution is to operate at low pressure. In the mass transferlimited regime,

h G D GS

But D G 1

Ptotal


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D G will go up 760 times at 1 torr,while increases by about 7 times.Thus h

G will increase.

Transport of reactants from gas phaseto surface through boundary layer isno longer rate limiting.

Process is more T sensitive, but canuse resistance heated, hot-walled

system for good control of temperatureand can stack wafers.



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Plasma Enhanced CVD (PECVD)RF power input

Electrode

ElectrodeWafers

Plasma

Gas outle t, pump

Heater

Gas inlet( SiH 4, O 2)

Non -thermal energy to enhance processes at lower temperatures. Plasma consists of electrons, ionized molecules, neutral molecules, neutral and

ionized fragments of broken-up molecules, excited molecules and free radicals. Free radicals are electrically neutral species that have incomplete bonding and

are extremely reactive. (e.g. SiO, SiH 3, F) The net result from the fragmentation, the free radicals, and the ion bombardment

is that the surface processes and deposition occur at much lower temperaturesthan in non-plasma systems.


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High Density Plasma (HDP) CVD

Remote high density plasma with independent RF substrate bias. Allows simultaneous deposition and sputtering for better planarization and

void-free films (later).

Mostly used for SiO 2 deposition in backend processes.


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Physical Vapor Deposition (PVD)

Wafer holder

Wafers

Vacuum Source material

Vacuum system Exhaust

Heater (resistancor E-beam)

Atomicflux

PVD uses mainly physical processes to produce reactant species in the gas phaseand to deposit films .

In evaporation, source material is heated in high vacuum chamber. (P < 10 -5 torr). Mostly line -of-sight deposition since pressure is low. Deposition rate is determined by emitted flux and by geometry of the target and

wafer holder.


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Molecular Beam Epitaxy (MBE)

Thin film growth under ultra high vacuum. Reactants introduced by molecular beams.

Create beams by heating source of material in an

effusion (or Knudsen) cell. Several sources, several beams of different

materials aimed at substrate

Can deposit 1 atomic layer or less! Very precisely defined mixture of atoms to giveexactly the desired material composition!


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MBE can achieve precise control of both chemicalcompositions and doping profiles.

Single crystal multilayer structures withdimensions on the order of atomic layers can begrown using MBE.

MBE can use wide variety of dopants whencompared to other methods and the dopingprofile can be exactly controlled.


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Substrate temperature for MBE ranges from400C to 900C and growth rate ranges from0.001 to 0.3m/min.

Many unique profiles and alloy compositionsare possible with MBE which are not possible

by conventional CVD methods.

Many novel structures are possible with MBE.



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Structures and Defects in EpitaxialLayers

2 types of Epitaxial layers Lattice-Matched Strained-Layer


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Epitaxial Layers

Lattice Matched Strained Relaxed


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Thin film deposition is a key technology in modern ICfabrication.

Topography coverage issues and filling issues are very

important, especially as geometries continue to decrease. CVD and PVD are the two principal deposition techniques.

CVD systems generally operate at elevated temperatures

and depend on chemical reactions.

In general either mass transport of reactants to thesurface or surface reactions can limit the deposition ratein CVD systems.

Summary


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Summary

In low pressure CVD systems, mass transport isusually not rate limiting.

However even in low pressure systems, shadowing by

surface topography can be important.

In PVD systems arrival angle distribution is veryimportant in determining surface coverage. Shadowingcan be very important.

A wide variety of systems are used in manufacturingfor depositing specific thin films.

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Deposition & Growth

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