cmos fabrication

ENSC-E131 NanoFabrication and NanoAnalysis

Thin Film deposition

Jiangdong (JD) Deng, Ph.D

Metrology

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Thin Films

(Photo)-lithography

Cleaning

Front-EndProcesses

EtchIon

Implantation

Planarization

Test & Back End

DesignWafer

Preparation Design Wafer Preparation

- Material Growth, cutting, polishing Front-end Processes

- LPCVD, MOCVD, MBE, ALD, wafer-bonding Lithography (photo-, E-Beam, FIB) Etch

- RIE, wet etching, Cleaning

- Wet, plasma, O-zone Thin Films

- PVD, PECVD, ALD, LPCVD Ion Implantation Planarization

- Spin coating, CVD, Test and Back-end

- Dicing/cleaver, Wire bonding, assembly Metrology

Micro/Nano Fabrication Processes

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Outline of Thin Film Deposition

1. General characteristic and consideration of thin film deposition

2. Physical Vapor Deposition (PVD) Evaporation (Thermal & E-beam evaporation) Sputtering (DC & RF sputtering)

3. Chemical Vapor Deposition (CVD) Low-Pressure CVD (LPCVD) Plasma-Enhanced CVD (PECVD) Atomic Layer Deposition (ALD)

4. Other deposition Methods: Oxidization ,

General Characteristics of Thin Film Deposition

5Conformality and Arrival Direction

Thicker deposition at outside corners plugs gap at top before bottom is completely full.

Directional arrival of depositingspecies

6Conformality and Surface Mobility

Low surface migration rate may result in non-conformal films and poor step coverage

High surface migration rate results in conformal films.

Conformality is improved if surface mobility of the deposition species is high. Surface mobility is enhanced by high temperature and ionicbombardment, as in plasma processes, and is influenced by the molecular species.

7Film Design for Device fabrication

Choosing Right Film Deposition Methods Suitable film properties/qualities for device and

process (design) Step coverage, conformality, trench filling ability Temperature required (process, device) Deposition rate, throughput, cost, automation, etc.

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Thin Film Deposition

Physical Vapor Deposition (PVD)-Film is formed by atoms directly transported from source to the substrate through gas phase

Evaporation (Thermal & E-beam evaporation) Sputtering (DC & RF sputtering) Reactive PVDChemical Vapor Deposition (CVD)

-Film is formed by chemical reaction on the surface of substrate Low-Pressure CVD (LPCVD) Plasma-Enhanced CVD (PECVD) Atomic Layer Deposition (ALD) Atmosphere-Pressure CVD (APCVD) Metal-Organic CVD (MOCVD)Other deposition Methods: Oxidization , Spin coating, Plating

Evaporation

Crucible Load the source material-to-be-deposited (evaporant) into the container (crucible)

Heat the source to high temperature

Source material evaporates Evaporantvapor transports to

and Impinges on the surface of the substrate

Evaporant condenses on and is adsorbed by the surface

Langmuire-Knudsen Relation

Uniformity on a Flat Surface

Uniform Coating

Thickness Deposition Rate vs. Source Vapor Pressure

Deposition Rate vs. Source Temperature

Thermal Evaporation

Thermal Evaporator

Typical Boat/Crucible Material

E-beam Evaporation

E-Beam Evaporator

Comparison

Stoichiometrical Problem of EvaporationCompound material breaks down at high temperatureEach component has different vapor pressure, therefore different deposition rate, resulting in a film with different stoichiometry compared to the source

DC Diode Sputtering

Self-Sustained Discharge

Sheath Sheath zonezone

Requirement for Self-Sustained Discharge

Deposition Rate vs. Chamber Pressure

DC Magnetron Sputtering

Impact of Magnetic Field on Ions


As a result


RF (Radio Frequency) Sputtering

Comparison between Evaporation and Sputtering

Chemical Vapor Deposition (CVD)

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Film Formation Steps

1. Diffusion across stagnant layer2. Adsorption on surface3. Surface reaction, migration, film formation

4. Desorption of by-products5. Diffusion of products back into

gas stream

Types of CVD Reactions

Low Pressure CVD (LPCVD)

Plasma Enhanced CVD (PECVD)

What is ALD?

Atomic Layer Depositiona.k.a. Molecular Layering (Rus.)Molecular Layer

Epitaxy, ALE (Atomic Layer Epitaxy)ALG (Atomic Layer Growth), etc

Invented by Russian researchers in 1960s, independently by Finns in 1970s. Roy Gordon group in Harvard, Micro- processing (IBM, Intel )Solar Cell, and other applications

In essence, a pulsed form of thermal CVDGrowth is controlled by total available precursor on surface, rather than precursor flux and reactivity, as in regular CVD.

What about ALD?

In ALD, reactants/precursors are introduced in alternating pulses, rather than concurrently.

First precursor is pulsed into reactor, lands on surfaces, and forms a "monolayer", which remains as excess precursor is purged from reactor and pumped away.

Second precursor is pulsed into reactor, lands on surfaces, and reacts with first monolayer, to completion, so that first monolayer is consumed.

Growth cycle is complete, leaving ~1 angstrom of material. Cycle is repeatedseveral times.

Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups, producing methane as the reaction product.

Tri-methylaluminumAl(CH3)3(g)

CH

H

H

H

Al

O

Hydroxyl (OH)from surfaceadsorbed H2O

Methyl group(CH3)

Substrate surface (e.g. Si)

Reaction of TMA with OH

CH

H

H

H

Al

O

Methane reactionproduct (CH4)


HCH

H

HC

Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups, producing methane as the reaction product.

In air H2O vapor is adsorbed on most surfaces, forming a hydroxyl group. With silicon this forms: Si-O-H (s). After placing the substrate in the reactor, TrimethylAluminum (TMA) is pulsed into the reaction chamber.

Al(CH3)3 (g) + : Si-O-H (s) :Si-O-Al(CH3)2 (s) + CH4

ALD Cycle for Al2O3 Deposition

Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups, until the surface is passivated. TMA does not react with itself, terminating the reaction to one layer. This causes the perfect uniformity of ALD. The excess TMA is pumped away with the methane reaction product.

CHH

Al

O

Excess TMA Methane reactionproduct CH4

HH C



After the TMA and methane reaction product is pumped away, watervapor (H2O) is pulsed into the reaction chamber.

CHH

Al

O

H2O

HH C

OHH


H2O reacts with the dangling methyl groups on the new surface forming aluminum-oxygen (AI-O) bridges and hydroxyl surface groups, waiting for a new TMA pulse. Again, methane is the reaction product.

2 H2O (g) + :Si-O-Al(CH3)2 (s) :Si-O-Al(OH)2 (s) + 2 CH4

New hydroxyl group

Oxygen bridges

Methane reaction product

Methane reaction product H

Al

O

OO

Al Al


The reaction product methane is pumped away. Excess H2O vapor does not react with the hydroxyl surface groups, again causing perfect passivation to one atomic layer.

H

Al

O

OO O

Al Al


One TMA and one H2O vapor pulse form one cycle. Here three cycles are shown, with approximately 1 Angstrom per cycle. Each cycle including pulsing and pumping takes, e.g. 3 sec.

O

H

Al Al Al

HH

OO

O OOOO

Al Al AlO O

O OOAl Al Al

O OOOO

Al(CH3)3 (g) + :Al-O-H (s) :Al-O-Al(CH3)2 (s) + CH4

2 H2O (g) + :O-Al(CH3)2 (s) :Al-O-Al(OH)2 (s) + 2 CH4

Two reaction steps

in each cycle:


ALD film control

Self-saturation process: Film growth during a cycle is controlled by the total

reactant present rather than a flux of reactant, depends on sufficient precursor to cover area of interest, temperature sufficient to allow reaction to proceed, time sufficient for each layer to react to completion. No process MFCs, no tuning networks, no pressure

controllers, no sophisticated power supplies, no process cooling, etc.

Exceedingly high conformality, aspect ratios from 100s to 100,000 demonstrated.

Excellent uniformity over 8" sample, 1% Very Slow. (~6 /min)

Lining and Filling Holes by ALD

AspectRatio ~50:1

ALD films

Oxides Al2O3, HfO2, SiO2, LaO2, TiO2

Metals and pure elements Pt, Cu, Si, Ge, Sb.

Nitrides. Rare-Earth Oxides.

Nano-Coaxial Cable or Transistor

Conducting tungsten nitride (WN) concentrically around insulating aluminum oxide (Al2O3) concentrically around a conducting carbon nanotube.

Carbon Al2O3 WNAl2O3WN

SiO2 film growth is a key process step in manufacturing all Si devices Thick (- 1m) oxides are used for field oxides (isolate devices from

one another ) Thin gate oxides (-100 ) control MOS devices Sacrificial layers are grown and removed to clean up surfaces

SiO2 film can be deposited through PVD or CVD methods. However, Deposited oxides tend to possess low densities and large numbers of

defect sites. Not suitable for use as gate dielectrics for MOS transistors, (but still acceptable for use as insulating layers between multiple

conductor layers, or as protective overcoats).

Oxidation of Silicon

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Silicon Dioxide Structure

(a) Basic structural unit of silicon dioxide (tetrahedron); (b) two dimensional representation of a quartz crystal lattice; (c) two dimensional representation of the amorphous structure of silicon dioxide

( c )

Quartz Density: 2.65 g/cm3

AmorphousDensity: 2.21 g/cm3

The simplest method of producing an oxide layer consists of heating a silicon wafer in an oxidizing atmosphere.

Dry oxide - Pure dry oxygen is employedDisadvantage

- Dry oxide grows very slowly. Advantage

- Oxide layers are very uniform.- Relatively few defects exist at the oxide-silicon interface (These defects interfere with the proper operation of semiconductor devices)

- It has especially low surface state charges and thus make ideal dielectrics for MOS transistors.

Wet oxide - In the same way as dry oxides, but steam is injectedDisadvantage

- Hydrogen atoms liberated by the decomposition of the water molecules produce imperfections that may degrade the oxide quality.

Advantage- Wet oxide grows fast.- Useful to grow a thick layer of field oxide

Comparison of Typical Thin Film Deposition Technology

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Properties of PECVD Silicon Nitride and LPCVD Nitride

Property LPCVD PECVDSi/N ration 0.75 0.8 1.0Density 2.8 3.1 g/cm3 2.5 2.8 g/cm3Refractive index 2.0 2.1 2.0 2.1Dielectric strength 1x107 V/cm 6x106 V/cmBulk resistivity > 1015 - 1017 ohms/cm 1015 ohms/cmSurface resistivity >1013 ohms/cm ~ 1013 ohms/cmStep coverage Fair ConformalThermal stability Excellent Variable > 400CH2O permeability Zero Low-none49% HF etch rate 8 nm/min 150 300 nm/min

References

Stephen Campbell, The Science and Engineering of Microelectronic Fabrication, Oxford Univ. Press, 1996

Silicon Processing for the VLSI Era, Vol. 1: Process Technology by Stanley Wolf and Richard N. Tauber

Erli Chen, lecture notes for Applied Physics 298r, 2004

Outline of Thin Film Deposition General Characteristics of Thin Film DepositionConformality and Arrival DirectionConformality and Surface MobilityFilm Design for Device fabricationThin Film Deposition EvaporationLangmuire-Knudsen RelationUniformity on a Flat SurfaceUniform CoatingThickness Deposition Rate vs. Source Vapor PressureDeposition Rate vs. Source TemperatureThermal EvaporationThermal Evaporator Typical Boat/Crucible MaterialE-beam Evaporation E-Beam EvaporatorComparison DC Diode SputteringSelf-Sustained DischargeRequirement for Self-Sustained DischargeDeposition Rate vs. Chamber PressureDC Magnetron SputteringDC Magnetron SputteringDC Magnetron SputteringRF (Radio Frequency) SputteringRF (Radio Frequency) SputteringComparison between Evaporation and SputteringChemical Vapor Deposition (CVD)Film Formation StepsTypes of CVD ReactionsTypes of CVD ReactionsTypes of CVD ReactionsLow Pressure CVD (LPCVD)Plasma Enhanced CVD (PECVD)What is ALD?What about ALD?ALD Cycle for Al2O3 DepositionALD Cycle for Al2O3 DepositionALD Cycle for Al2O3 DepositionALD Cycle for Al2O3 DepositionALD Cycle for Al2O3 DepositionALD Cycle for Al2O3 DepositionALD film controlLining and Filling Holes by ALDALD films Nano-Coaxial Cable or TransistorOxidation of SiliconSilicon Dioxide StructureThe simplest method of producing an oxide layer consists of heating a silicon wafer in an oxidizing atmosphere.Comparison of Typical Thin Film Deposition Technology Properties of PECVD Silicon Nitride and LPCVD NitrideReferences

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