Deposition Process

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The Deposition Process


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Learning Objectives

You Will Be Able to Explain The need for Deposition Processes The methods used to perform physical and

chemical deposition processes The advantages of different deposition

processes

The use of plasma for enhancing deposition


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Purpose of Deposition

Deposition places conductive or insulatinglayers on a substrate

Deposition processes create locallyconductive paths that can be used tointerconnect devices

Deposition can be used to build up morecomplex structures one layer at a time


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Deposition Types

Silicon Dioxide SiO 2 Insulating layers Protective coatings Gate oxides

Silicon Nitrides Protective layers Isolation


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Deposition Types

Polysilicon Heavily doped silicon Conductive Used for interconnects and gate

Metals Aluminum/Aluminum-Copper Tungsten Titanium Alloys


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CVD

Silicon Nitride

Silicon dioxide Polycrystalline Silicon

Epitaxial Layers Customized Surfaces

Insulator Conductors

Barriers

Chemical Vapor Deposition CVD Applications


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Deposition Processes

Physical or Chemical (or both?) Physical Processes Deposit the material

without chemical reactions

Chemical processes utilize liquid or vaporforms of precursors that react with the surfaceto form the desired deposition

It is possible to combine the processes andgain the benefits of each

Many processes are carried out in reducedpressure (partial vacuum) environments


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Requirements of Deposition

This process use many layers to form aproduct, any deposited layer must becompatible in many ways with what is

below it Film Stress Conformality Uniformity Step Coverage Thermal compatibility


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Result of Non-Uniform Deposition


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Conformal Coverage

Good ConformalCoverage

Poor ConformalCoverage


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Step Coverage


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Physical Vapor Deposition: PVDAdvantages:Versatile deposits almost anymaterial

Very few chemical reactions

Little wafer damageLimitations:

Line-of-sight

Shadowing

Thickness uniformity

Difficult to evaporate materials withlow vapor pressures

2 types : evaporation andsputtering


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Physical Deposition Processes

Sputtering Plasma is created by RF or HV DC source Inert gas such as Ar is used in a low pressure

environment Free electrons strike Ar atoms, causing

positive ions to be formed

Negatively charged target material attractsions

Ions dislodge particles that are deposited


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Practice Questions

1. What are the two main types of depositionprocesses?

Click once for each question.

Physical and Chemical Deposition2. What are commonly used metals for deposition?

Aluminum, tungsten, and copper

3. What does conformality of a deposition refer to?

The ability of the deposition to follow surface contoursevenly


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Sputtering


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Evaporative Deposition

Advantages Uniformly covers substrate Simple process without chemicals or gases

Disadvantages Alloys are difficult to deposit

Different metals have different vapor pressures High aspect ratio features are difficult to cover

Trajectory of evaporated particles tends to bevertical, which may not pattern sidewalls evenly


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Practice Questions

1. Which physical deposition process uses plasma?

Click once for each question.

Sputtering

2. What is an advantage of sputtering?

Low temperature process, good conformal coating

3. What is a disadvantage of evaporativedeposition?Difficult to deposit alloys, difficult to get good high aspect

ratio feature deposition


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Chemical Deposition Processes

Wet or Dry? Wet processes use liquids and immersion

Electroplating Electroless deposition Wet growth of SiO2 insulating layer (water vapor)

Dry processes use chemical vapors Atmospheric Pressure Chemical Vapor Deposition

Low Pressure Chemical Vapor Deposition Plasma Enhanced Chemical Vapor Deposition


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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, siteincorporation, etc.

5. Desorption from surface

6. Transport byproducts through boundary layer

7. Transport byproducts away from deposition region


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Chemical Deposition Processes Atmospheric

Chemical VaporDeposition (CVD) Wafers are heated Chemical gases are

introduced A temperature

dependent deposition

rate Mass transport limited

at higher temperatures


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Chemical Deposition Processes

Low Pressure (CVD) Surface reaction

limited at low pressure

Chamber may also beheated or unheated Low pressure

environment increasesmean free path

Better Step Coverageand conformality than

APCVD


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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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Chemical Deposition Processes Plasma Enhanced Low

Pressure (CVD) Lower Temperature

Process due to Plasma

Enhancement Disassociation of precursor

gas molecules(Homogeneous reactions)

Ions bombard surface

making it more reactive Higher rates of deposition

are possible than withLPCVD

From MATEC Module 54


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Plasma Enhanced CVD(PECVD)

Good when temperature isrestricted

Provides reasonable depositionrates

Good film qualityConformal

May leave unwantedbyproducts on film


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New Methods forNanomanufacturing

Thinner layers are necessary for higherspeed transistors in IC design Gate oxide thickness < 50 A Approaches atomic layer dimensions

Atomic Layer Deposition A 2 step process of deposition and re-layering SiOH* + SiCl4 Si O-SiCl3 + HCl SiCl* + H2O SiOH* + HCl


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New Methods forNanomanufacturing

Molecular vapor deposition Anti-stiction layers in MEMS are needed to

avoid structures fusing to substrates Vapor deposition of compounds avoids

contamination found in liquid processes Oxygen plasma clean operation precedes

deposition process


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LIGA Process

LIGA includes X-Ray lithography, electroforming,and plating operations that construct high aspectratio features on substrates

Precision patterning of a deposited PMMA resist layerusing X-Ray lithography Areas remaining after development are plated with

metal

Photo resist and excess metal removed Remaining features are high aspect ratio metal

Documents

Deposition Process