Fachgebiet 3D-Nanostrukturierung, Institut für Physik

Contact: yong.lei@tu-ilmenau.de

yang.xu@tu-ilmenau.de

Office: Heisenbergbau (Gebäude V) 202, Unterpörlitzer Straße 38 (tel: 3748)

www.tu-ilmenau.de/nanostruk

Vorlesung: Mittwochs (G), 9 – 10:30, C 108 Übung: Mittwochs (U), 9 – 10:30, C 108

Prof. Yong Lei & Dr. Yang Xu

Techniken der Oberflächenphysik (Techniques of Surface Physics)

mailto:yang.xu@tu-ilmenau.demailto:yang.xu@tu-ilmenau.demailto:yang.xu@tu-ilmenau.de

How to fabricate: nano-fabrication

• Anodic aluminum oxide (AAO) template

• Ultrathin alumina membrane (UTAM)

• Polystyrene (PS) shpere template

• Chemical vapor deposition (CVD)

• Physical vapor deposition (PVD)

• Atomic layer deposition (ALD)

• Lithography, soft lithography, nano-imprinting

Types of Chemical Vapor Deposition (CVD)

• (thermal) Chemical vapor deposition • Plasma enhanced CVD (PECVD) • Metal organic CVD (MOCVD) • Atmospheric pressure CVD (APCVD) • Low-pressure CVD (LPCVD) • Ultrahigh vacuum CVD (UHVCVD) • Aerosol assisted CVD (AACVD) • Direct liquid injection CVD (DLICVD) • Microwave plasma-assisted CVD (MPCVD) • Remote plasma-enhanced CVD (RPECVD)

Thermal CVD

When a conventional heat source (furnace or oven) is used, the technique is called thermal CVD. It consists of a quartz tube inserted into a furnace and has a gas inlet on one side and a gas outlet on the other side. The sample is placed onto a quartz boat inside the tube.

Thermal CVD

Example Carbon Nanotubes: Hydrocarbons as precursor. A typical growth process: 1st: purge reactor with inert gas; 2nd: gas flow is switched for growth period; 3rd: gas flow is switched back to inert gas while the reactor cools down. For growth on substrates, catalysts are applied on substrate before loading it inside reactor. Typical T for catalytic CVD in CNT growth are 800–1500 K.

Reaction Process in CVD

a) Epitaxial Growth: ordered crystalline growth on a

monocrystalline substrate (acts as a seed crystal), the deposited

film has a lattice structure and orientation same as the substrate

Homoepitaxy: a crystalline film grown on a substrate of same material. grow more purified films than substrate, with different doping levels.

Heteroepitaxy: a crystalline film grown on a substrate of different materials, to grow e.g. GaN on Sapphire or AlGaInP on GaAs

Homoepitaxial growth of Si on a Si substrate

SiCl4(g)+2H2(g) = Si(s)+4HCl(g) at approx. 1000-1200 °C

b) Vapor-Liquid-Solid (VLS) growth • Catalytic nanodots on substrate • Catalyst must be inert

CVD growth of large-scale ordered carbon nanotube (CNT)

arrays initiated from highly ordered catalyst arrays on

silicon substrates

Importance of ordered aligned CNTs: field-emission devices

(flat-panel display).

Two methods to fabricate ordered aligned arrays of CNTs:

EBL method: graphitized CNTs; adjustable diameters and

spacing of the CNTs.

(limited pattern area; high capital cost)

Template method: large pattern areas; low equipment costs.

(difficult to obtain CNT arrays on substrate; poor crystallinity)

Combine the advantages of the template and the EBL methods.

Use the UTAM-prepared metallic nanodots as the catalysts.

Fabrication Process of highly ordered CNT arrays on Si substrate

Ultra-thin alumina mask

Si wafer

Ultra-thin alumina mask

Si wafer

Ultra-thin alumina mask Catalyst array

Si wafer

Catalyst array

Si wafer

Carbon nanotube array

Y Lei, Chem. Mater., 16, 2757, 2004

(a)

Ni particle area

Ni film area

(a3)

(a2)

50nm

(b)

Ni film area

Ni particle area

(b3)

(b2)

50nm

(a) Images of as-prepared Ni nanoparticle arrays; (b) heat-treated Ni nanoparticle array

(in ammonia at 700 oC for 5 min).

PE-CVD growth of CNTs: 100 sccm ammonia + 10 sccm acetylene, 700oC, 7mBar.

(c) Ordered CNT array grown from

the Ni nanoparticle array.

(d) CNTs grown from Ni film for the

comparison with CNTs.

(c)

(c5) (c4)

(c3)

Ni film area

Ordered CNT array on Ni

particle area

(c2)

(d)

(d2)

200nm (d3)

a b

1. Ordered arrays of CNTs with monodisperse diameters have been successfully

fabricated on Si substrates with quite large areas.

2. Due to the high uniformity of such aligned arrays of graphitized CNTs, their

properties, especially the field-emission properties, are worthy of further study.

3. By changing the diameters of the pores of the UTAMs, the size of the catalyst and

thus that of the CNTs can be adjusted.

CNT array fabricated from Ni nanoparticle. The tilted view (a) and top view (b) were

obtained from the same CNT area; the lengths of nanotubes are about 100-200 nm.

CVD

Advantages:

• high growth rates possible

• can deposit materials which are hard to evaporate

• good reproducibility

• can grow epitaxial films

Disadvantages

• high temperatures

• complex processes

• toxic and corrosive gasses

Physical Vapor Deposition

• Thermal evaporation

• Electron beam evaporation

• Sputtering

Physical Vapor Deposition - PVD

Source Condensed Phase

(mostly solid e.g. Au)

Gas Phase

Resulting structures Condensed Phase

(usually solid)

Gas Phase

evaporation condensation

transport

Thermal evaporation holder

Resistance heated evaporation sources

Alumina crucible with wire basket

Thermal evaporation

• Simple and widely used

• Common evaporation materials: - Au, Ag, Al, Sn, Cr, Sb, Ge, In, Mg, Ga … - CdS, PbS, CdSe …

• Use W, Ta or Mo filaments to heat evaporation source

• Typical filament currents are 200-300 A

• Typical deposition rates are 1-20 Angstrom/second

• Can only achieve temperatures of about 1800°C

Electron beam evaporation Electron beam heated evaporation source

• More complex, but extremely versatile

• Achieves temperatures up to 3000 °C

• Typical emission voltage is 8 – 10 kV

• Typical deposition rates 0.2-100 Angstrom/second

• Common evaporation sources - all materials accommodated by the thermal evaporation - Ni, Pt, Ir, Rh, Ti, V, Zr, W, Ta, Mo - Al2O3, SiO, SiO2, SnO2, TiO2, ZrO2

Sputtering

Substrate placed in a vacuum chamber with source material (target), an inert gas (argon) is introduced at low pressure. A gas plasma is generated using an RF power, causing gas to become ionized. Ions are accelerated towards target surface, causing atoms of source to break off from target in vapor form and condense on surfaces on substrate.

PVD

Advantages

• Low substrate temperature

• Conformal film

• Relatively fast process

• Comparatively low cost

• Excellent thickness control

Disadvantages

• By-products incorporated • Cracking • Peeling • No high aspect ratio

materials • No stoichiometric films

Evaluation of film thickness – oscillating crystal

Film thickness by thermal or E-beam evaporation can be measured continuously during deposition by an oscillating crystal.

The measuring method is based on the frequency shift of oscillating crystal, which is caused by the material being evaporated onto crystal. Thereby the resonance frequency is decreased with increasing material being deposited.

5. The thickness of the deposited layer:

Under consideration of 4

Frequency shift for different materials Fl

im t

hic

knes

s

Frequency shift

Atomic Layer deposition Introduced with a name of Atomic Layer Epitaxy in 1974 by Dr. T. Suntola (Picosun Board Member)

Mr. Sven Lindfors (Picosun CTO) and the early

ALD reactor in 1978

Picosun ALD in Ilmenau

Principle of ALD ALD is a chemical gas phase thin film deposition method

based on alternate, saturative, surface reaction

The ALD process window

Factors affecting ALD surface reactions

• Growth rate in ALD is typically

1Å/cycle or less.

◦ Cycle time varies

◦ Higher growth rates indicate

in most cases the CVD growth

• ALD surface reactions can be affected by ◦ Reactivity of the precursor

Reaction mode (ligand exchange, dissociation, agglomeration)

◦ Reactivity of the ligand removal agent at the selected temperature

◦ Number of the reactive sites

Reaction mode (monofunctional, bifunctional)

◦ Size of the precursor, i.e. steric hindrance

Reviews about ALD mechanisms

‘Atomic layer deposition: an overview’, Chemical Reviews 110, 111 (2010)

‘Surface chemistry of atomic layer deposition: a case study for the TMA/water process’, Journal of Applied Physics 97, 121301 (2005)

‘Atomic layer deposition chemistry: recent developments and futrure challenges’, Angewandte Chemie, international edition 42, 5548 (2003)

‘Atomic layer deposition: from precursors to thin film structures’, Thin Solid Films 409, 138 (2002)

Advantages of ALD Surface controlled (self-limiting) thin film

• ~100% conformal

• Precise thickness control

• Excellent uniformity

• Pinhole-free films

• Repeatable process

• Low process temperature

• Graded or mixed

layers/nanolaminates

• High aspect ratio materials

Multiple Materials

‘Atomic layer deposition of transition metals’, Nature Materials 2, 749 (2003)

Thank you!