advanced analytical chemistry chapter1

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Advanced Analytical Chemsitry- Surface and Interface Analysis

Low Operating Voltage Single ZnONanowire Field-Effect TransistorsEnabled by Self-Assembled OrganicGate Nanodielectrics

S. Ju et. al, Nanoletter, 5, 2281 (2005)

Energy-transfer pumping of semiconductor nanocrystalsusing an epitaxial quantumwell

M. Achermann et. al, Nature 429, 642 (2004)

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A first organic thin film transistor with a SAM insulator

Halik et al., Nature 431, 963 (2004)

Introduction

Surface and interface of solids is of importance in daily life.

It is one of the most cross-disciplinary area of science and technology.

Including physics, chemistry, electronic engineering, biology, etc.

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Introduction

What is a surface and a surface layer ?

1. A layer of zero thickness forming the boundary of an object.

2. The interface between a matter and vacuum.

3. The definition of a layer thickness is governed by the particular application.

Introduction

Elementary surface composition• nature surface elements• concentration of surface atoms• vertical distribution→ on the surface (adsorbates)→ in the surface near regime

Molecular state of surface compounds/Adsorbates

Oxidation state of surface species• Me0 ↔ Men+

Depth profiles

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Introduction

In thermodymanics point of view, the chemical composition, microstructure and atom behaviors of a solid surface is different from its bulk media.

Introduction

Since it requires energy to terminate the bonding, the surface is energetically less stable than the bulk.

This energy is known as the surface free energy. In the case of liquid interfaces, this energy is called surface tension.

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Problem definition

1. Is an identification of the chemical compound needed or is the composition sufficient?

2. Are quantitative results needed, or qualitative identification sufficient?

3. What is the detection limit needed for qualitative identification?

Characterization of surface layer properties : concentrations, maps, profiles and sections.

Sample

Probe Analyzer

Event

Techniques – General Principles

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Relevant are surface properties,not: bulk propertiesnot: gas phase properties

Sensitivity:• surface: 1015 atoms(molec.)/cm2• bulk: 1022-1023 atoms(molec.)/cm3• gas phase 1022 atoms(molec.)/l

(1020 per cm2 and 10 cm)

Surface Sensitivity:• absorption (strong interaction with

material)• symmetry requirements• specific spectroscopic surface

features

Techniques – Analytical techniques

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Techniques – Analytical techniques

Techniques- Characterization Techniques I

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Techniques- Characterization Techniques II

Techniques- Characterization Techniques III

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Techniques - In situ

Techniques– Advantages and Disadvantages

BadGoodYesBadYesGoodTheoretical compatibility

NoNoNoYesYesYesNon-destructiveGoodBadBadGoodGoodBadSpacial sensitivy

ML- MLs　MLMLsMLsMLsSurface sensitivity GoodBadBadGoodGoodGoodResolution

BadBadBadYesYesGoodChemical states analysis

BadGoodBadBadYesGoodQuantitative analysis

10-4-10-510-2-10-

310-2-10-

310-910-2-10-

310-2-10-

3Sensitivity

BadGoodGoodBadGoodGoodHomogeneity

YesNoNoNoNoNoHydrogen detectable

SIMS RBS ISS IELS AES XPS

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Techniques

Ultra-high Vacuum(UHV)

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Most surface analysis are carried out in HV or UHV, in order to get “atomic clean”surface.

Most thin film process are carried out in HV or UHV, in order to preventing from contaminations.

UHV- Introduction

Definition of Vacuum

“Vacuum” is a word from Latin, meaning “Empty”

A closed system (chamber) which the pressure inside is lower then the atmospheric pressure.

UHV- Introduction

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UHV- Introduction

Composition of atmosphere

Composition Pressure Gas Vol% ppm (Pa) N2 78.084 79.117 O2 20.946 21.223 CO2 0.033 33.437 Ar 0.934 946.357 Ne 18.18 1.842 He 5.24 0.51 Kr 1.14 0.116 Xe 0.087 0.009 H2 0.5 0.051 CH4 2 0.203 N2O 0.5 0.051

UHV- Introduction

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Classification

UHV- Introduction

Gas composition vs Pressure

When in low vacuum, the composition of gases inside a chamber is similar to its composition in atmosphere.

When in high vacuum, the partial pressure of water is increased with respect to the total pressure decreased.

When in UHV, the partial pressure of water is reached to 70%-90% of total pressure.

The quantity of CO2 is increased if we bake the chamber to remove water.

UHV- Introduction

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UHV- Introduction

1045 x 1053 x 101210-10UltraHigh

1503 x 101610-6High

10-35 x 10-23 x 101910-3Medium

10-65 x 10-53 x 10221Low

10-97 x 10-82 x 1025760Atmospheric

Time / ML, tML(s)

Mean Free Path(m)

Gas Density, ρ(molecules m-3 )

Pressure (Torr)

Degree of Vacuum

P ~ 10-10 TorrMaintain a Clean Surface:P ~ 10-6 TorrCollision Free Conditions:

• Ultimate Pressure– Low Vacuum (Rough) Pumps

• Rotary Vane Pumps• Sorption Pumps

– High Vacuum Pumps• Diffusion Pumps• Turbo Molecular Pumps

– Ultra-High Vacuum• Turbo Molecular Pumps• Ion Pumps• Titanium Sublimation Pumps

• Oil / Oil-Free– Oil

• Rotary Vane Pumps• Diffusion Pumps• Turbo Molecular Pumps

– Oil-Free• Turbo Molecular Pumps• Ion Pumps• Titanium Sublimation

Pumps

UHV Technique-How to Obtain Vacuum

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Rotatory PumpGas enters the inlet port and is trapped between the rotor vanes and the pump body. The eccentrically mounted rotor compresses the gas and sweeps it toward the discharge port. When gas pressure exceeds atmospheric pressure, the exhaust valve opens and gas is expelled.Atmosphere to 10-3 torrRobust, inexpensiveOil lubricated


Cyro-Pump

LN2 cooled molecular sieve with large surface areaAtm to 10-3 Torr (two units working alternately)Quickly becomes saturatedMust be baked at >200 °C to remove adsorbed gasesSimple, inexpensive, oil-free


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Diffusion PumpMomentum transfer to gas molecules through collision with directed jet of oil moleculesRequire cooling water, backing pump10-3 to 10-7 Torr (to 10-9 Torr with LN2 cooling)

Advantages• Robust• High pumping speed for relatively low cost. • No vibration or noise.

Disadvantages• Oil as a pumping medium, high risk of

back-streaming oil, cold traps required• Potential for serious vacuum problems


Turbo Molecular Pump

Molecules mechanically pumped by collision with angled high speed turbine blades (rotor).Several rotor arranged in a series spinning at 30,000-60,000 rpm.Rotor tangential velocity is on the order of the average thermal velocity of molecules. Atmosphere to 10-10 TorrOil/grease/electromagnetic bearingsMost common HV/UHV pump.


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Advantages• Correctly operated they do not back-

stream oil into the vacuum system at any time.

• They can be started and stopped in a few minutes time.

Disadvantage• Turbo pump can be noisy and they

induce vibration. • Turbo pumps are expensive.


Ion PumpHigh voltage between anode and cathode (~5 kV)Electrons are captured in anode and spiral due the to magnetic field (longer path-length).Gas molecules are ionized by collisions with electrons and are accelerated to cathode.Ions embedded in cathode material (titanium) and sputter titanium atoms from surface.Sputtered Ti atoms act as "getter" for reactive gases.10-4 Torr to 10-11 Torr


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AdvantagesClean, oil-free.No moving parts, no vibrations, quiet.Low power consumption and relatively long operating livesDisadvantageDo not pump noble gases well.Requires “regeneration” of Ti every 4-6 years.


Titanium sublimation pumpHeated Ti filament evaporates Ti film onto cooled surface.Ti getters reactive gases by reaction.Operate at 10-8-10-11 TorrInexpensive, reliablePeriodic operation - not primary pumping mechanism


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Materials Considerations

Outgassing rates

Producing “virtual” leaks

Mechanical stability

Temperature stability

Conductivity

Chemical inertness

Weldability


Oxygen free high-purity copper (OFHC) Be-Cu alloyTantalum, Molybdenum, Tungsten TeflonMACOR (machinable glass composite) Alumina Quartz, pyrex

"μ-metal" magnetic shielding (Co, Ni, Fe) Molybdenum disulfide (lubricant)

Compatible Materials


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Materials to be Avoided (high vapor pressures)Zn, Cd: especially be careful of fasteners, bolts Brass Certain soldersAny type of grease or oils

Common Vacuum ProblemsImproper cleaning or handling techniquesUsing incompatible materialsLeaksVirtual leaks



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Pump DownTypically follows a well-defined sequence according to the types of pumps on the vacuum systemFor UHV systems, typically requires a few hours to reach a medium vacuum after a vent to air

Bake OutHeat the chamber to temperatures between 100oC and 200oC for days.Rapidly remove adsorbed gases from the chamber walls at high temperatures in order to lower the outgassing rates at RTGenerally it takes another day to cool and recover base pressure.


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Heat filament with a constant current.Measure filament temperature with thermocouple.Gas molecules collide with and cool the filament.Voltage increases to keep filament at constant current.Atm to 10-4 TorrFast, simple, inexpensive.Thermocouple Gauge


Two identical heated filaments; one sealed at HV, one exposed to system.Current flows through Wheatstone bridge circuit.Pressure difference indicated by meter (non-linear).Atm to 10-4 torr.Simple, reliable, inexpensive.Pirani Gauge


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Heated filament produces electrons via thermionic emission.Electrons are accelerated towards anode grid.Many electrons pass through the grid and create positive ions from collisions with gas molecules.Ions are accelerated to collector wire.Measure the current between anode and collector.Operate at 10-4 to 10-11 TorrSensitive, high accuracy, widely used.Ion Gauge (Bayard-Alpert)


Quadrupole mass spectrometer - RGA (residual gas analyzer)10-4 to <10-14 torrTotal pressure mode integrates all ion intensitiesPartial pressure mode indicates residual vacuum compositionHighly accurate, preciseComplex, expensive.Mass Spectrometer


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Typical UHV mass spectrum of background gases after bakeout.

H2O CO2N2

H2

• Leak rates– Flanges– Virtual leaks

• Outgassing rates– Contaminants– Materials– Samples

• Pumping Rates– Type of pumps– Type of gases

being pumped


Typical UHV mass spectrum of residual gases before bakeout.


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END

advanced analytical chemistry chapter1

Documents

surface properties

surface adsorbates

solid surface

surface tension

surface free energy

layer thickness

chemical composition

bulk propertiesnot