1. Thin Films of Deposition

2. Thin film Thin film: thickness typically <1000nm.
Special properties of thin films: different from bulk materials, it may be –
Not fully dense
Under stress
Different defect structures from bulk
Quasi ‐ two dimensional (very thin films)
Strongly influenced by surface and interface effects
Typical steps in making thin films:
Emission of particles from source (heat, high voltage . . .)
Transport of particles to substrate
Condensation of particles on substrate
Lithography, thin film deposition and its etching are the three most important processes for micro-nano fabrication.

3. General characteristics of thin film deposition
Deposition rate
Film uniformity:
Across wafer uniformity.
Run-to-run uniformity.
Materials that can be deposited: metal, dielectric, polymer.
Quality of film:
Physical and chemical properties
Electrical property, breakdown voltage
Mechanical properties, stress and adhesion to substrate
Optical properties, transparency, refractive index
Composition, stoichiometry
Film density, defect (pinhole…) density
Texture, grain size, boundary property, and orientation
Impurity level, doping
Deposition directionality:
Directional - good for lift-off, trench filling
Non-directional - good for step coverage
Cost of ownership and operation.

4. Four equilibrium growth/deposition modes

5. Types growth modes
Layer by layer growth (Frank ‐ van der Merwe): film atoms more strongly bound to substrate than to each other and/or fast diffusion
Island growth (Volmer ‐ Weber): film atoms more strongly bound to each other than to substrate and/or slow diffusion.
Mixed growth (Stranski ‐ Krastanov): initially layer by layer then forms three dimensional islands.

6. Thin Film Deposition Techniques
PVD (Physical Vapor Deposition)
Thermal Evaporation
Electro-deposition
Magnetron sputtering (DC, RF)
Pulsed Laser Deposition
MBE (molecular beam epitaxy)
CVD (Chemical Vapor Deposition)
APCVD (atmospheric CVD)
LPCVD (low-pressure CVD)
PECVD (Plasma-enhanced CVD)
LECVD (Laser-enhanced CVD)
MOCVD (Metal-organic CVD)
Solution based deposition techniques
Sol-gel
PAD (Polymer assisted deposition)

7. Chemical Vapor Deposition (CVD)
CVD steps:
Introduce reactive gases to the chamber.
Activate gases (decomposition) by heat or plasma.
Gas absorption by substrate surface .
Reaction take place on substrate surface, film firmed.
Transport of volatile byproducts away form substrate.
Exhaust waste
CVD : deposit film through chemical reaction and surface absorption.

8. CVD advantages and disadvantages (as compared to physical vapor deposition)
Can be used for a wide range of metals and ceramics
Can be used for coatings or freestanding structures
Fabricates net or near-net complex shapes
Is self-cleaning—extremely high purity deposits (>99.995% purity)
Conforms homogeneously to contours of substrate surface
Has near-theoretical as-deposited density
Has controllable thickness and morphology
Forms alloys
Infiltrates fiber preforms and foam structures
Coats internal passages with high length-to-diameter ratios
Can simultaneously coat multiple components
Coats powders
Disadvantages:
High process temperatures.
Complex processes, toxic and corrosive gasses.
Film may not be pure (hydrogen incorporation…).

9. Types of CVD reactions Thermal decomposition Reduction (using H2)
AB(g) ---> A(s) + B(g)
Si deposition from Silane at 650oC: SiH4(g) → Si(s) + 2H2(g)
Ni(CO)4(g)  Ni(s) + 4CO(g) (180oC)
Reduction (using H2)
AX(g) + H2(g)  A(s) + HX(g)
W deposition at 300oC: WF6(g) + 3H2(g)  W(s) + 6HF(g)
SiCl4(g) + 2H2(g)  Si(s) + 4HCl (1200oC)
Oxidation (using O2)
AX(g) + O2(g)  AO(s) + [O]X(g)
SiO2 deposition from silane and oxygen at 450oC (lower temp than thermal oxidation): SiH4(g) + O2(g) --- > SiO2(s) + 2H2(g)
2AlCl3(g) + 3H2(g) + 3CO2(g)  Al2O3 + 3CO + 6HCl (1000oC)
(O is more electronegative than Cl)
Compound formation (using NH3 or H2O)
AX(g) +NH3(g)  AN(s) + HX(g) or AX(g) + H2O(g )  AO(s) + HX(g)
Deposit wear resistant film (BN) at 1100oC: BF3(g) + NH3(g)  BN(s) + 3HF(g)
(CH3)3Ga(g) + AsH3(g)  GaAs(s) + 3CH (650 – 750oC)

10. Types of CVD
APCVD (Atmospheric Pressure CVD), mass transport limited growth rate, leading to non-uniform film thickness.
LPCVD (Low Pressure CVD)
Low deposition rate limited by surface reaction, so uniform film thickness (many wafers stacked vertically facing each other; in APCVD, wafers have to be laid horizontally side by side.
Gas pressures around mTorr (lower P => higher diffusivity of gas to substrate)
Better film uniformity & step coverage and fewer defects
Process temperature 500°C
PECVD (Plasma Enhanced CVD)
Plasma helps to break up gas molecules: high reactivity, able to process at lower temperature and lower pressure (good for electronics on plastics).
Pressure higher than in sputter deposition: more collision in gas phase, less ion bombardment on substrate
Can run in RF plasma mode: avoid charge buildup for insulators
Film quality is poorer than LPCVD.
Process temperature around °C.
MOCVD (Metal-organic CVD, also called OMVPE - organo metallic VPE), epitaxial growth for many optoelectronic devices with III-V compounds for solar cells, lasers, LEDs, photo-cathodes and quantum wells.

11. Physical Vapour Deposition(PVD) 1
Physical Vapour Deposition(PVD) 1.Physical vapour deposition (PVD) is fundamentally a vaporisation coating technique, involving transfer of material on an atomic level. It is an alternative process to electroplating. 2.The process is similar to chemical vapour deposition (CVD) except that the raw materials/precursors, i.e. the material that is going to be deposited starts out in solid form, whereas in CVD, the precursors are introduced to the reaction chamber in the gaseous state.
Working Concept PVD processes are carried out under vacuum conditions. The process involved four steps: 1.Evaporation 2.Transportation 3.Reaction 4.Deposition

12. Variants of PVD include ; Evaporative Deposition: In which the material to be deposited is heated to a high vapor pressure by electrically resistive heating in "high" vacuum. Electron Beam Physical Vapor Deposition: In which the material to be deposited is heated to a high vapor pressure by electron bombardment in "high" vacuum. Sputter Deposition: In which a glow plasma discharge (usually localized around the "target" by a magnet) bombards the material sputtering some away as a vapor. Cathodic Arc Deposition: In which a high power arc directed at the target material blasts away some into a vapor. Pulsed Laser Deposition: In which a high power laser ablates material from the target into a vapor.

13. Summary of Merits and Demerits of evaporation methods
E-Beam Evaporation
1. high temp materials 2. good for liftoff 3. highest purity
1.  some CMOS processes sensitive to radiation 2. alloys difficult 3. poor step coverage
Filament Evaporation
1. simple to implement 2. good for liftoff
1. limited source material (no high temp) 2. alloys difficult 3. poor step coverage
Sputter Deposition
1. better step coverage 2. alloys 3. high temp materials 4. less radiation \damage
1. possible grainy films 2. porous films 3. plasma damage/contamination

14. Importance of PVD Coatings
PVD coatings are deposited for numerous reasons. Some of the main ones are:
Improved hardness and wear resistance
Reduced friction
Improved oxidation resistance
The use of such coatings is aimed at improving efficiency through improved performance and longer component life.
They may also allow coated components to operate in environments that the uncoated component would not otherwise have been able to perform.

15. Advantages
Materials can be deposited with improved properties compared to the substrate material
Almost any type of inorganic material can be used as well as some kinds of organic materials
The process is more environmentally friendly than processes such as electroplating
Disadvantages
It is a line of sight technique meaning that it is extremely difficult to coat undercuts and similar surface features
High capital cost
Some processes operate at high vacuums and temperatures requiring skilled operators
Processes requiring large amounts of heat require appropriate cooling systems
The rate of coating deposition is usually quite slow

16. Deposition By Sputtering

17. Sputtering – General
Sputtering is a term used to describe the mechanism in which atoms are ejected from the surface of a material when that surface is stuck by sufficiency energetic particles.
Alternative to evaporation.
First discovered in 1852, and developed as a thin film deposition technique by Langmuir in 1920.
Metallic films: Al-alloys, Ti, TiW, TiN, Tantalum, Nickel, Cobalt, Gold, etc.

19. Reasons for sputtering
Use large-area-targets which gives uniform thickness over the wafer.
Control the thickness by Dep. time and other parameters.
Control film properties such as step coverage (negative bias), grain structure (wafer temp), etc.
Sputter-cleaned the surface in vacuum prior to deposition.

20. Sputtering steps Ions are generated and directed at a target.
The ions sputter targets atoms.
The ejected atoms are transported to the substrate.
Atoms condense and form a thin film.

21. Application of Sputtering
Thin film deposition:
Microelectronics
Decorative coating
Protective coating
Etching of targets:
Microelectronics patterning
Depth profiling microanalysis
Surface treatment:
Hardening
Corrosion treatment

22. Sputter sources Magnetron
Magnetic field traps freed electron near target
Move in helical pattern, causing large number of scattering events with Ar gas – creating high density of ionized Ar
Ion beam
Plasma of ions generated away from target and then accelerated toward start by electric field
Reactive sputtering
Gas used in plasma reacts with target material to form compond that is deposited on wafer
Ion-assisted deposition
Wafer is biased so that some Ar ion impact its surface, density the deposited film. May sputter material off of wafer prior to deposition for in-situ cleaning.

23. Process conditions
Type of sputtering gas. In purely physical sputtering (as opposed to reactive sputtering) this limits to noble gas, thus Argon is generally the choice.
Pressure range: usually 2-3 mTorr (by glow discharge).
Power
Heating of target material
Low temperature metals can melt from temperature rise caused by energy transfer from Ar ions
Electrical conditions: selected to give a max sputter yield (Deposition rate).

24. Sputter deposition film growth
Sputtered atoms have velocities of 3-6E5 cm/sec and energy of eV.
Desire: many of these atoms deposited upon the substrate.
Therefore, the spacing is 5-10 mm.
The mean free path is usually <5-10 mm.
Thus, sputtered atoms will suffer one or more collision with the sputter gas.

25. Sputtering Advantages Disadvantages
Large-size targets, simplifying the deposition of thins with uniform thickness over large wafers
Film thickness is easily controlled by fixing the operating parameters and simply adjusting the deposition time
Control of the alloy composition, step coverage, grain structure is easier obtained through evaporation
Sputter-cleaning of the substrate in vacuum prior to film deposition
Device damage from X-rays generated by electron beam evaporation is avoided.
Disadvantages
High capital expenses are required
Rates of deposition of some materials (such as SiO2) are relatively low
Some materials such as organic solids are easily degraded by ionic bombardment
Greater probability to introduce impurities in the substrate because the former operates under a higher pressure