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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
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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
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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
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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
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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
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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
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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
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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
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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
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< 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
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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
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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
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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
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• 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
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Chemical Cleaning/ Etching-Used for removal of Chemisorbed contaminants
-Removes virtually any foreign material or contaminant
-Causes an etch effect on the surface
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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
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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
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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
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• Ion Scrubbing• Reactive Plasma Etching/ Cleaning (RIE)• Laser Cleaning / Ablation• And others..
Plasma-based cleaning techniques
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Some Plasma Etching Processes
Handbook of Deposition Technologies for films and coatings – Science, Technology and Applications (2nd Edition)
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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
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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
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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