By: Henry Medina Nanoelectronics Laboratory National Tsing Hua University.

By: Henry Medina Nanoelectronics Laboratory

National Tsing Hua University

Riikka L. Puurunen. “Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process”. JOURNAL OF APPLIED PHYSICS 97, 121301 (2005)

Cambridge Nanotech website for ALD◦ http://www.cambridgenanotech.com/ald/

1. First Part1. What is ALD process2. Applications3. Basic Characteristics of ALD

2. Second Part1. Chemical adsorption Mechanism2. Growth per cycle (GPC) Effects with T on GPC3. Number of cycles vs. GPC

3. Third Part1. Requirements for Precursors

1. Classes2. Patterning of ALD growing layers (Techniques)

1. First Part1. What is ALD process

ALD is a CVD technique suitable for inorganic material layer as oxides (high-k dielectrics), nitrides and some metals.◦ The success of ALD is to divide CVD process in half

Perfect for deposition of very thin layers of the size of a monolayer* coating complex shapes with high quality (good step coverage).

Definition: “Film deposition technique based on sequential use of self terminating* gas-solid reactions” **

• *Definition of Monolayer and Self terminating reaction will be explained later

• ** Definition taken from the main reference

1. First Part1. What is ALD process2. Applications

Semi & Nanoelectronics Optical Humidity Barriers MEMS Nanostructures Chemical

Semi & Nanoelectronics◦ Gate Dielectrics Intel has recently announced that their 45 nm generation

processors will include a high-k HfO2 gate dielectric made by ALD. There are various reasons that ALD has become the method of choice

Unlike ALD, conventional evaporated films suffer from porosity and sputtering creates defects in the sensitive silicon surface layer.

In addition, ALD guarantees extremely uniform and reproducible thickness, low stress, growth on amorphous structure and low defect density.

Besides the industrial silicon platforms, ALD has proven essential to create gate dielectrics on device substrates without native oxides

Semi & Nanoelectronics◦ Gate Dielectrics and Gate Electrodes

TEM micrograph of 45nm Intel high-k and metal gate pMOS transistor. (Source: IEDM2007 10.2)

Gate Electrode (TiN by ALD)

High K dielectric (HfO2 by ALD)

• Prevents Cu diffusion into silicon • Refractory nature • Amorphous • Acts as an adhesion promotor for Cu and Co

33 nm

32 nm

Top

Bottom

ALD Tungsten nitride (WN)

Cambridge NanoTech co-authored publication

Zaitsu et al. Applied Phyics Letters, 80, 2442, 2002

ALD good for AR coatings: large area precision thickness control and batch coating.

=> Graded index coatings posssible by varying the number of Al2O3/TiO2 low n/high n layers inside a nanolaminate stack

ALD-ZnO transparent conductors advantages:

No costly indium as in ITO Good optical transmission Low resistivity (1 mOhmcm) Large area uniformity Very smooth films in contrast to ITO

Thin film transistors:

ALD of ZnO active matrix thin filmtransistors possible as well.

Water vapor transmission rate of 25 nm ALD Al2O3 better than 1 mm polymer encapsulation!

WVTR <10−5 g/m2 day demonstrated

Applied Physics Letters, 89, 031915 2006

SiO2

Dielectric

Top electrode

poly Si

Hausmann et al. Thin Solid films 2003.

3D DRAM needs conformal coatingof high-k dielectric and metal electrode

C=kA/d: Al2O3, ZrO2, Ta2O5

DRAM crown DRAM trench

Samsung uses ALD for DRAM manufacture!

High aspect ratio ALDof Ta2O5 in vias of 170 nm dia, 7 micronsdeep

100 nm

100 nm

(a) Schematic of finger gated devices. Mo gates (150nm wide 10 nm thick) were defined lithographically on a Si/SiO2 substrate and subsequently coated with 25 nm of HfO2 grown by low-temperature ALD. Nanotubes were grown across these local gates by CVD and contacted with Ti/Au electrodes. Not to scale.

(b) Atomic force micrograph of nanotubes grown across Mo finger gates and contacted (far left and far right) by Ti/Au leads. Note that one finger gate passes directly underneath the nanotube-metal contact. Arrows indicate the location of the nanotube. Finger gates are labeled as in the text.

Local gating of carbon nanotubes, Biercuk, Nano Letters 2003

Client: Prof. C.M. Marcus, Harvard University.

Client: Nobel laureate Prof. Tsui, Princeton University

Client: Prof. Ohno, Tohoku University, Japan.

Client: C.M. Marcus, Harvard University.

Cambridge NanoTech co-authored publication, Applied Physics Letters 2003.

Cambridge NanoTech Inc. Confidential

Nickel nanotubes grown in porous alumina, then alumina etched away

K. Nielsch, Max Planck Institute, 2006

Client: K. Nielsch, Max Planck Germany

TiO2-Al2O3-TiO2 coaxial nanotubes grown with ALD inside porous alumina.

K. Nielsch, Max Planck Institute, 2006


Nature Materials 2007 Published online: 2 July 2006; doi:10.1038/nmat1673



Deposition of Al2O3 inside and around tubular shaped tobacco mozaic virus length 300 nm, OD 18 nm, ID 4 nm. Grown < 80C



Steps:◦ Self-terminating reaction of the first reactant

(Reactant A)◦ Purge or evacuation to remove non-reacted

reactant and by products◦ Self-terminating reaction of the second reactant

(Reactant B)◦ Purge

This is considered as one reaction cycle


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, Trimethyl Aluminum (TMA) is pulsed into the reaction chamber.

Tri-methylaluminumAl(CH3)3(g)

CH

HH

H

Al

O

Hydroxyl (OH)from surfaceadsorbed H2O

Methyl group(CH3)

Substrate surface (e.g. Si)

2005 © All rights reservedCambridge NanoTech Inc.


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

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

C

H

H

H

H

Al

O

Reaction ofTMA with OH

Methane reactionproduct CH4

H

HH

HH C

C




C

HH

Al

O

Excess TMAMethane reactionproduct CH4

HH C

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.




C

HH

Al

O

H2O

HH C

OHH

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



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

H

Al

O

O

H2O reacts with the dangling methyl groups on the new surface forming aluminum-oxygen (Al-O) bridges and hydroxyl surface groups, waiting for a new TMA pulse.

Again metane is the reaction product.

O

Al Al

New hydroxyl group

Oxygen bridges

Methane reaction product

Methane reaction product



H

Al

O

O

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.

O 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 OO OO

Al Al AlO O

O OO

Al Al AlO O

O OO

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:


The surface must be in a controlled state, e.g. heated

Parameters to be adjusted: ◦ Reactants (precursors)◦ Substrate◦ Temperature


2. Second Part1. Chemical adsorption Mechanism

Both have been seen in ALD applications

Self terminating Reaction

Both have been seen in ALD applications but Chemisorption by ligand exchange is preferred because is associated with (a)

For ALD process ligand exchange is preferred

◦ For ligand exchange saturation in the process is due to 2 factors: (a) Steric Hidrance (b) Number of Reactive

Surface Sites


2. Second Part1. Chemical adsorption Mechanism2. Growth per cycle (GPC) Effects with T on GPC

Unit used in ALD system to describe different process.

Ideal would be growth one monolayer per cycle but this is not true due to steric hindrance

3 Models to describe the effect of steric hindrance on GPC

Model I: Just few processes can be assumed as model I ( GPC is around 25% of a monolayer)

Model II: Often used to model many ALD processes. (Gives growths of GPC under 20% of a monolayer)

Model III: Is applied only for some ideal ALD processes as AlMe3/H2O. Most optimistic (GPC around 30% of a monolayer)

Basically depends on the surface and the reactant, an there are 4 possibilities

Most of the papers said AlMe3/H2O for Al2O3 over SiO2 is rather insensitive to T, there are documentation from 80oC to 300oC so it’s assumed as model (b)

This is not completely true because T affect the adsorption time on surface so the cycle time per precursor should be adjusted to wait for the self terminating reaction*.

*On FAQ of ALD Savannah is possible to find some information related of how to fix this problem



Due to the substrate there are 4 types:




1. Classes

Ligand precursor◦ To prepare the surface for next layer, and define

the kind of material to growth i.e. H2O for oxides, N2 or NH3 for nitrides, etc.

Main Precursor (metallic precursor)◦ Highly reactive (usually this means volatile

precursors), thermally stable, and full-fill the requirement for self terminating reaction

◦ For Oxides H2O this is the material preferred because of its physical properties

Easily decompose at low temperature by ligand exchange on H2 (Gas) and O attached to the substrate

O2 O3 ROH (Alcohols with organic Chains)

◦ Nitrides NH3 N2

◦ To growth pure materials the ligand precursor should be selected depending on the main precursor


Recent Cambridge NanoTech experiment: replacement of H2O with

beer

+ =


Al2O3 grown with beer/TMA

Al2O3 grown with H2O/TMA

Both same thickness. Beer LD is similar because the water vapor is distilled from the cylinder with beer, and thus pure H2O.This demonstrates how the intrinsic distillation of the vapor draw process in the system reduces the need for high purity precursors.

◦ Inorganic Elements

React with nonmetal compounds and hidrogen compoundsAs elements don’t carry extra ligands*Selective reactivity (just few elements)

* Free ligands are associated with impurities grown in the film

◦ Inorganic Halides

Variety of materials grown ReactiveStable in TemperatureNo extra ligands

Drawbacks: gaseous byproducts containing hydrogen nonmetal reactants e.g. HCl Corrosive etching the film and can be readsorb at the surface

Metal Organic Non direct metal carbon bounds Organometallic

Metal Organic Non direct metal carbon bounds

◦ Alkoxides◦ ß-diketanones◦ Aminidates


◦ Alkoxides Decompose at low temperature, usually lower than

200oC The decomposition produces the oxide already

If deposited on the surface the process lose the conformality

Alcohol as by-product, promote readsorption The chains have high content of carbon and

hydrogen


◦ Alkoxides


◦ ß-diketanones Before Cyclopentadienyls were widely used to grow

alkaline-earth metals Bulky chains, causing a marked steric hindrance so

GPC very low As alkoxides the M-O ligand is difficult to remove or

change for Nitrogen so is not suitable for nitrides


◦ ß-diketanones Before Cyclopentadienyls were widely used to grow

alkaline-earth metals Bulky chains, causing a marked steric hindrance so

GPC very low As alkoxides the M-O ligand is difficult to remove or

change for Nitrogen so is not suitable for nitrides


◦ ß-diketanones


◦ Amidinates New… from 2003 not widely studied Decomposition at around 300oC Seems to be self terminating process but GPC larger

than a monolayer has been reported Maybe for decomposition or readsorption

Metal Organic Non direct metal carbon bounds Organometallic

Alkyls Cyclopentadienyls

Metal Organic Organometallic

◦ Alkyls: M-Cn-H2n+1

The reactivity of Alkyls reactant based is considered of the highest in ALD process e.g. AlMe3/H2O has a GPC of 30% of a monolayer at 300 oC

◦ Alkyls: M-Cn-H2n+1

The reactivity of Alkyls reactant based is considered of the highest in ALD process e.g. AlMe3/H2O has a GPC of 30% of a monolayer at 300 oC

Metal Organic Organometallic

◦ Cyclopentadienyls : M-5 Carbon ringFor deposition of pure metals as Ru (RuCp 2/O2). These

reactants have been used since long time but gain popularity after 2000 however there are few information about the process.

Metal Organic◦ Cyclopentadienyls : M-5 Carbon ring

For deposition of pure metals as Ru (RuCp 2/O2). These reactants have been used since long time but gain popularity after 2000 however there are few information about the process.




1. Classes2. Patterning of ALD growing layers (Techniques)

Photoresist PMMA Surface modification chemicals to become

hydrophobic Etching

On the other hand surface modification is also used to promote adhesion (hydrophilic)

Photoresist*◦ Rhodium growth Rh(acac)3/O2.◦ Temperature too high for photoresist ??(300oC)◦ Additional the use of HMDS inhibit the film

growth.◦ Film grown over Oxide

* K. J. Parka and G. N. Parsons. “Selective area atomic layer deposition of rhodium and effective work function characterization in capacitor structures”. Applied Physics Letters 89, 043111 2006

PMMA*◦ Titanium Dioxide TiCl4/H2O and Ti(OCH(CH3)2)/H2O.◦ At 160oC◦ For TiCl4.

Cl React with PMMA, PMMA difficult to remove when more than 150 cycles are applied.

◦ For Ti(OCH(CH3)2). No reaction with PMMA, PMMA easy to remove by normal

solvents. ◦ There’s no specification about the substrate

* Ashwini Sinha, Dennis W. Hess, and Clifford L. Henderson. “Area selective atomic layer deposition of titanium dioxide: Effect of precursor chemistry”. J. Vac. Sci. Technol. B 24(6) Nov/Dec 2006

Surface Modification*◦ APS ◦ HMDS

APS treatment at 150oC

APS treatment at 150oC

Surface Modification*◦ APS

Not good results but I never try to modify parameter 150oC too high for APS??

◦ HMDS

Surface Modification*◦ ODTS and Octadecene. Liquid and are baked for

thermal reaction◦ For positive and negative patterning.◦ Surface treated before growth

* Rong Chen, Stacey F. Bent. “Chemistry for Positive Pattern Transfer Using Area-Selective Atomic Layer Deposition”. Adv. Mater. 2006, 18, p.1086–1090

Surface Modification◦ Surface treated before growth to increase

adhesion

◦ For Si. Piranha treatment (H2SO4/H2O2 7:3) HF 2%Note: Sample are immediately transfer to next process

Generate Hydride termination (Si-H).

Etching

After coating the surface with ALD film, the excess can be removed by wet etching or dry etxhing

Wet etching: NaOH for Al2O3 has given good results

By: Henry Medina Nanoelectronics Laboratory National Tsing Hua University.

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