Lecture 6:Recent Advances in Surface

Cleaning and Preparation Techniques

2

Outline• Introduction• Why Surface Cleaning is so Important?• Basic Concepts• External Cleaning

– Abrasive Cleaning– Aqueous Etching– Ultra-sonic and Mega-sonic Cleaning – Vapor-phase cleaning , etc.

• In-situ / Fine Cleaning– Plasma cleaning– Ion Scrubbing– Reactive Plasma etching / Cleaning (RIE)

• Clean Rooms and Their Needs• Surface Modifications• References

3

It controls so many things that relate to interface and coating performance

Coated systemJ.Musil, J.Vyskocil, S. Kadlec. Hard coatings prepared

by arc evaporation and sputtering. Physics of Thin Films 14, 79-144, 1993

Substrate

Interface

Surface

Coating

Does interface influences performance

significantly?Tool life

Corrosion resistance

C. SchönjahnSheffield Hallam University, UK

4

Chief among them is adhesion

Fundamental adhesion:∑ nsinteractiolecular

-intermo linterfacia Maximum number of atoms in minima of corrugation potential

Enhance Adhesion

Local Epitaxy

Strong chemical bonding

increased surface area

K.L. Mittal: Adhesion measurement of films and coatings, p1-13 VSP, Utrecht, The Netherlands 1995

C. SchönjahnSheffield Hallam University, UK

5

Interface Properties & AdhesionArc-PVD Deposited TiAlN Coating

0

10

20

30

40

50

60

70

L cin

N o

n M

2

Ar 1200V Cr 600V Cr 1200VC. SchönjahnSheffield Hallam University, UK

6

Interface & corrosion resistance

Potentiodynamicpolarisation curves of Nb coated stainless steel in 3% NaClsolution

Hilke Paritong, Thesis submitted February 2000 at Sheffield Hallam University, p.75

7

FundamentalsSurface Cleaning involves removal of dirt and

contaminants. It is an integral part of any type of surface modification and coating processes.

Contaminant can be defined as any foreign material on the surface that

– Interferes with film formation– Affect film properties– Influences film stability

8

Classification of Contaminants

-Impair film adhesion-Nucleate defects in films-Form deleterious decomposition products

Films, discrete particulates, micro-droplets

Molecular

-Little practical consequence (degasified) Adsorbed gases and vapors

Gaseous

-Nucleate crystal defects-Deteriorate device performance-Can change surface composition (localized)

Elemental metal films and particles

Atomic

-Cause crystal defects-Diffuse on surface changing film properties e.g. resistivity

Physisorbed / chemisorbed cations and anions

Ionic

EffectsSources of particlesBasic Type

9

Forces Holding Contaminants• Chemical Adsorption

– Involves formation of chemical bond between surface and contaminant/s– Chemical bonds can be ionic, covalent or metallic– Thermodynamically irreversible, not possible to rinse off contaminant with

solvent

• Physical Adsorption– Either Van der Walls or Inter-molecular– Bonds are dipole-dipole, dipole induced dipole, dispersion force– Thermodynamically reversible and can be rinsed Care – water can form strong physisorbed bond with surface contaminant

• Other physical forces– Gravity – it can hold particle on the surface– Electrostatic forces– Environmental surroundings e.g. humidity– Substrate surface texture

10

Fast – since it is a non-activated process

Very variable –often an activated process

Kinetics of Adsorption

Multilayer uptake possible

Limited to one monolayer

Saturation Uptake

Non-dissociative, reversible

Often dissociative, mostly irreversible

Nature of Adsorption

Virtually independent of surface atomic geometry

Marked variation between crystal Planes

Crystallographic Specificity

Related to factors like molecular mass and polarity

Wide range (related to chemical bond strength)

Adsorption Enthalpy

PhysisorptionChemisorptionCharacteristics

General Characteristics of Physisorbed and Chemisorbed Contaminants

11

< 12< 5< 0.5< 10

Intermolecular (Van der Waals) Hydrogen bondsDipole-dipoleDipole induced dipoleDispersion

140 – 25015 – 17027 - 83

Chemical BondsIonicCovalentMetallic

Energy (kcal/mole)Types of force

Relative Strength of Forces Between Surfaces and Contaminant Layers

12

Selection Of Cleaning Technique / s• Which forces are prevalent on contaminated surface ?• Contaminant, an atom, a particle, molecule or ion ?• Organic or inorganic nature (surface and contaminant) ?• How clean is CLEAN!!

External Cleaning Techniques

Physical ChemicalAbrasion, Ultrasonic, etc Aqueous cleaning, specific solvent removal etc.

Figures: Handbook of Semiconductor Wafer Cleaning Technology – Science technology and Applications

13

External Cleaning Mechanisms• Abrasive Cleaning• Aqueous / Chemical Cleaning• Ultra-sonic and Mega-sonic Cleaning• Specific solvent cleaning• Reactive Cleaning• Degreasing

and lots of others depending on application needs

Discussed here

14

Abrasive Cleaning-Removal of Gross contaminants from Surface

-Can be used wet and dry with various particle sizes

-Has high surface cleaning rate and no surface size limitations

Known Processes

Grit blasting, Shot peening, high pressure water jet blasting

Avg. particle size of Abrasive cleaning Materials

15

• To remove light / heavy oils and residue left by other cleaning processes

• Wide range of solvent availability and low cost • High contaminant removal selectivity

Drawbacks• Generation of large amounts of waste• Drying is difficult due to low vapor pressure of

water and cause recontamination (oxidation/corrosion)

• Difficult to couple with vacuum systems

Aqueous CleaningModern in-line cleaning system

16

Chemical Cleaning/ Etching-Used for removal of Chemisorbed contaminants

-Removes virtually any foreign material or contaminant

-Causes an etch effect on the surface

17

Ultra-Sonic Cleaning• Thoroughly removes adhering

physisorbed particles from surface (e.g. removal of abrasive after abrasive cleaning)

• Cleans holes cracks and recesses by cavitatingpressure waves

• Not suitable for submicron particle removal

Working Concept

Typical operation frequency 18-120KHzMega Sonic Cleaning-Very high frequency of operation (400 KHz and greater)

- non cavitating pressure wave and suitable for cleaning smooth surface

- widely used in Silicon wafer cleaning

18

Modern In-line Cleaning Systems

• The modern cleaning systems are designed to offer optimum cleaning and a very high level of productivity and reliability.

• Cleaning involves the uses of environmentally friendly aqueous solutions and is based on a combination of spray and immersion in heated baths with ultrasonic vibration.

• It provides residue and corrosion free surfaces after thorough rinsing and hot air drying.

• Ideal for components made of steel, carbide, nonferrous metals and alloys

• Closed loop circulation of rinsing media allows wastewater free operation

• System can automatically select a specific cleaning process out of four

De-Ionisedwater unit

Tap watter

Tank 1

Ultrasonics

Filtration

Heating (60 oC)Time: 5 mins

Solutions:4%HT0151%HT1169

Tank 2

Rinse

Tap water

Top up during operation

Time: 1 min

Tank 3

Ultrasonics

Heating(60 oC)

Time: 5 mins

Solutions:4% VP1233A

Tank 4

Rinse

Tap water

Time: 1 min

Top up during operation

Tank 5

Ultrasonics

Heating(40 oC)

Rinse

Time: 1 min

De-Ionisedwater

Tank 6

Heating(40 oC)

Rinse

Time: 1 min

De-Ionisedwater

Dryer

Vacuum

95 oC

Time: 15 mins

Schematic of the NOVATEC (Italy) Cleaning Line

Oil outTop up automatically

Top up automatically

20

1 2 3 4 5 6 7 8 9 10 Dryer

Tank Sulutions emperature (oC) Time (sec)

1 De-Ionised water+6L DECOSPRAY N 65 180

2 De-Ionised water+3kg GALVEX SU 93 65 180

3 Tap water+0.65L GALVEX 17.30 40 10

4 De-Ionised water+2.5kg RODASTEL 10 65 Skip

5 Tap water+0.65L GALVEX 17.30 40 Skip

6 De-Ionised+4.2LGALVEX 17.30 65 120

7 Tap water+1.30LGALVEX 17.30 35 10

8 De-Ionised water 50 10

9 De-Ionised water 50 10

10 De-Ionised water 50 5

Dryer 250 300

Schematic of the UCM (Switzerland) Cleaning Line

21

• Ion Scrubbing• Reactive Plasma Etching/ Cleaning (RIE)• Laser Cleaning / Ablation• And others..

Plasma-based cleaning techniques

22

Some Plasma Etching Processes

Handbook of Deposition Technologies for films and coatings – Science, Technology and Applications (2nd Edition)

23

Plasma Cleaning•Use for removal of small or monolayer of contaminants developed after external cleaning and before final deposition to clean surface to required standards for film deposition

For uniform surface cleaning-Surface potential be uniform over the surface-Plasma density be uniform over the surface-Plasma gas chemistry can be reactive or non-reactive with surface (as per requirements)-Metal ions can also be used to clean or etch substrate surfaces

Some industrial Plasma Cleaning systems

Argon Plasma Cleaning Process

24

Reactive Plasma/Ion Etching/Cleaning

• Energetic reactive ions bombard the surface using DC potential (for conductive surface) or RF/pulsed DC (for non-conductive surface)

• Less surface damage in semiconductor electronic properties than sputter etching

• Dry alternative to cleaning solvents• No hazardous waste production• Plasma chemistry can be tailored for specific surface cleaning

E.g. Cleaning of surface hydrocarbons by Oxygen plasma

25

Need for Ultra Clean Environments• Device size decreasing with

newer Chips, DRAMS, and MEMS devices

• Market driven by higher complexity and performance with lower cost

• Leads to a tight tolerance of the contaminant particle size, its proportion and distribution on the surface

Source: http://www.intel.com/research/silicon

26

Class1 Clean Rooms (IBM)

Touchless fabrication facility http://research.ibm.com for more info

27

28

Contaminants / Defect Analysis