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Deposition of TCO films by Atomic Layer Deposition (ALD)

Gurram Sanjeev Kumar

Fraunhofer Institute for Surface Engineering and Thin Films (IST), Germany

TCM-2010, 3rd International Symposium on Transparent Conductive Materials

ORAMA Summer School ,15-17 Oct, 2010

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Outline

ALD Fundamentals

ALD of TCOs

ALD in ORAMA Project

Summary

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Atomic Layer Deposition Fundamentals

Introduction

Atomic Layer Deposition follows

- Sequential steps from [1] to [4] - Self limiting property- Reaction completion/cycle- No gas phase reactions

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Matti Putkonen and lauri Niinisto, 2005

Condensation Dec

ompo

sitio

n

Desorption

Low

rea

ctiv

ity

Low reactive sitesEa – activation energyEd - desorption energy

T. Suntola, M.S. Report 4, 1989

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Atomic Layer Deposition Fundamentals

Comparision with other techniques

Technology Backgrounder: Atomic Technology Backgrounder: Atomic Layer Deposition," IC Knowledge LLC, Layer Deposition," IC Knowledge LLC, 24 April 06. 24 April 06.

VTT Finland

Properties

PVD CVD ALD

Uniformity ~80 Å range

~10 Å range

Å range

Conformity < 50% < 70% 100%

Cleanliness Particles Particles No particles

Vacuum High High /Med. Medium

Temp. range

Low Low Wide

Technology ~100 nm ~90 – 65nm

No limit

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Atomic Layer Deposition Fundamentals

History

Picosun Oy Beneq Oy

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Atomic Layer Deposition Fundamentals

Thermal ALD

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-Thermal energy is the main source

- ALD without radicals or plasma

-Negative H (heat of reactions, sponteneous at any temperature)(eg. ZnO, Al2O3, CdS ..)

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Atomic Layer Deposition Fundamentals

Plasma Enhanced ALD (PEALD)

PEALD is a useful extenstion of thermal ALD adding additional capabilties:

- Low temperatures and higher deposition rates (eg. SiO2 )- Enhanced film quality and tunability- It has good film adhesion than thermal ALD (Creation of more active sites)- Enhances reaction chemistry for specific films- Cycle time can be lowered ( easy switch on and off of plasma)- It helps in the deposition of single elements ( eg. Noble metals like Pt, Cu..)

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Steven M George, Chem. Rev. 2010

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Atomic Layer Deposition Fundamentals

Advantages of Plasma over Thermal ALD

Processes requiring Plasma ALD

Example:

Problem : Desorption of precursor and no ALD processs

Solution : Use Plasma of second reactant and reduce deposition temperature

Solution with Plasma ALD

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Atomic Layer Deposition Fundamentals

Precursor Chemistry

The basic requirements for a good precursor for ALD processes are:

- Sufficient volatility at the deposition temperature

- No self-decomposition at the deposition temperature

- Preferably gases and liquids, solids as well (if no sintering problems)

- Precursors must adsorb or react with the surface sites

- Sufficiently reactivity towards the other precursor

- No etching of the substrate or the growing film

- Safe for handling and lower costs

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Atomic Layer Deposition Fundamentals

Precursor ChemistryInorganic

Metal Halides

Metallo organic

ß-diketone complexes and Alkoxides

Organo metallic

Adv: Thermal stability Reactivity

Disadv: By products Vapour pressure

Adv: Vapour pressure

Disadv: Thermal stability Reactivity Molecule size

Adv: Thermal stability Reactivity By products Vapour pressure

Disadv: Cost and availability

H2O or O3 are commonly used as one of the reactant for Oxide materials

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High-k dielectrics :Al2O3, HfO2, ZrO2, Ta2O5, La2O3

Conductive gate electrodes : Ir, Pt, Ru, TiN,

Metal interconnects and liners : Cu, WN, TaN, WNC, Ru, Ir

Catalytic materials : Pt, Ir, Co, TiO2, V2O5

Nanostructures : DRAM, MEMS

Biomedical coatings: TiN, ZrN, CrN, TiAlN, AlTiN

Metals :Ru, Pd, Ir, Pt, Rh, Co, Cu, Fe, Ni

Piezoelectric layers :ZnO, AlN, ZnS

Transparent Electrical Conductors : ZnO:Al, ITO)

UV blocking layers :ZnO, TiO2

OLED passivation : Al2O3

Photonic crystals : ZnO, ZnS:Mn, TiO2, Ta2N5,

Anti-reflection and optical filters : Al2O3, ZnS, SnO2, Ta2O5

Electroluminescent devices : SrS:Cu, ZnS:Mn, ZnS:Tb, SrS:Ce

Optical applications : SnO2, ZnO, MgF2

Sensors : SnO2, Ta2O5

Wear and corrosion inhibiting layers : Al2O3, ZrO2

Atomic Layer Deposition Fundamentals

Applications

Special structures requiring high conformity

Ali Javey et al 3D solar cells

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ALD of Transparent Conductive Oxides

TCOs Deposition and Types

There are different TCOs :

N-type TCOs P-type TCOs

ZnO based for eg. ZnO, ZnO:Al CuO based for eg. CuO, CuAlOx

ZnSn2O4 , ITO, SnO2 (delafossite structures), ZnO based amorphous Spinel structures, ZnO:N

The main important parameters for deposition of the TCO layers by ALD

- Precursors

- Growth temperature

- Type of ALD used (Thermal or Plasma)

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ALD of Transparent Conductive Oxides

ZnO based TCO

Zinc alkyl compounds

DEZ (Diethyl

Zinc)

DEZ (Diethyl

Zinc)

DMZ (Dimethyl

Zinc)

DMZ (Dimethyl

Zinc)+

H2O/O3

Precursor choice

The important precursors for Zinc Oxide deposition are

C2H5-Zn-C2H5 CH3-Zn-CH3

C2H5-Zn-C2H5 + H2O ZnO + 2C2H6

CH3-Zn-CH3 + H2O ZnO + 2CH4

C2H5-Zn-C2H5 + O3 ZnO + COx +H2O

CH3-Zn-CH3 + O3 ZnO + COx +H2O

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ZnO based TCO: Precursor effect with Thermal ALD

Dimethyl Zinc + H2O processDiethyl Zinc + H2O process

104 – 200 °C 100 – 150 °C

-Higher growth rate with DMZ because of Steric effect (molecular size)

-Decrease of growth rate at high temp.:- due to dehydroxylation

- Orientation in the ALD window is <100> direction and above 200°C is <200>

Grzegory Luka et al, Poland, Baltic ALD 2010 & GerALD2 in Hamburg, [1]

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Diethyl Zinc + H2O process Dimethyl Zinc + H2O process

ZnO based TCO: Precursor effect with Thermal ALD

Zn intestitialsOxygen vacancies

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ZnO based TCO: Precursor effect with Plasma Enhanced ALD

Dimethyl Zinc + O2 processDiethyl Zinc + O2 process

75 – 150 °C [*]

(104 – 200, thermal ALD)

85 – 125 °C

(100 – 150, thermal ALD)

-The growth rate of both precursors are higher than Thermal ALD

-The <002> orientation is obtained 50°C below 200°C (unlike Thermal ALD)

- Resistivities are relatively lower to the thermal ALD : Hydrogen incorporation

*Sang-Hee Ko Park et al, Elec. Solid-State Lett., 2006, Vol. 9

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ZnO based TCO: Precursor effect with Plasma Enhanced ALD

Better is DMZ than DEZ for PEALD-DEZ completely decomposes at low temperature (RT) in high vacuum.

-DMZ is stable with formation of monomethylzinc with oxygen and without oxygen flow (this avoid CVD unlike DEZ)

- A strong peak of <002> is obtained at 120°C with PEALD of DMZ and above 150°C for DEZ.Quadrupole Mass Spectroscopy

Pieter C. Rowlette et al, Chem. Vap. Deposition, 2009, 15

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ALD of Transparent Conductive Oxides

ZnO based TCO: ALD of ZnO with O3

C2H5-Zn-C2H5 + H2O ZnO + 2C2H6

C2H5-Zn-C2H5 + O3 ZnO + COx +H2O

Properties:

- ZnO with Ozone occurs at high temp.

- Hydroxyl desorption at high temp.

- ZnO ( O3 process) – SAW applications

- ZnO (H2O process) – TCO applications

- Increase in resistivity - aggresive O3 - Si diffusion in to the films

O3 H2O

Seong Keun Kim et al, Thin Solid Films 478, 2005

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ALD of Transparent Conductive Oxides

ZnO doped TCOs :ZnO:Al

For a film thickness of 200 nm

Grzegory Luka et al, Poland, Baltic ALD 2010 & GerALD2 in Hamburg

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ALD of Transparent Conductive Oxides

ZnO based P-TCOs : ZnO: N

Initial reaction mechanism by DFT(Lin Dong et al, Thin Solid Films 517, 2009)Chongmu Lee et al, Materials Letters 61, 2007

Doping of Zinc Oxide with Nitrogen

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ALD of Transparent Conductive Oxides

Copper Oxide based P - TCOs

Growth rate 0.038 nm/cycle

ALD window 140 – 230 °C

CuO deposition from Cu(acac)2 and O3

Mari Alnes , Norway, Baltic ALD 2010 & GerALD2 in Hamburg

With Solid state reactions: Delafossite, Spinel structures can be produced

CuOCuO

Al2 O3 (TMA + H2O)

CuAlOxCuAlOx

Annealing at

High temp.

N times

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ALD in ORAMA Project

To develop low defect density electronics with ALD

Deposition of Amorphous n-TCOs like Zn-Sn-O and Zn-Ga-Sn-O with ALD

Deposition of N-type ZnO and ZnMgO (counterpart to crystalline based p-n junction) having mobilities with high homogeneity and uniformity, using ALD technique.

ZnMgO to be deposited by ALD as new material for Ag seed layer

Novel ALD deposition of Poly-Crystalline (P type Active Semiconductor Oxides) e.g. Delafossite structures.

Single crystalline films to be grown on crystalline substrate

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Summary

A brief introduction to ALD process

Precursors effect on TCOs deposition

Different effects of Thermal and Plasma ALD on TCOs

ORAMA project goals with ALD

Thank you