EE3013: Deposition and Physical Vapour Deposition (PVD)

• This lesson is about Deposition and Physical Vapour Deposition (PVD), within the context of EE3013/Semiconductor Devices and Processing at the School of Electrical and Electronic Engineering.
• At the end of it, one should be able to:
• Describe properties of a high quality thin film
• Understand fundamental concepts in Physical Vapour Deposition (PVD) and Chemical Vapor Deposition (CVD)
• Understand advantages/limitations of PVD and CVD techniques and ways to improve them
• Three main categories in the semiconductor fabrication process:
• Lithography: Patterning of substrate (silicon wafer)
• Etching: Removal of materials from substrate
• Deposition: Deposit materials (metal/non-metal) on the substrate

Thin Films

• Thickness is typically less than 1000nm
• Thin films have special properties unlike bulk materials
• Not fully dense
• Under stress
• Different defect structures from bulk
• Strongly influenced by the surface and interface effects

Mechanism in Thin Film Deposition

• Typical steps to deposit thin films:
• Emission of particles from source via heating or high voltage
• Transport of particles to substrate
• Condensation of particles on substrate

Conformality

• Conformality in filling a hole/trench defines the step coverage of a deposition
• A good step coverage film is needed for electrical connection
• Conformal step coverage = constant thickness on horizontal and vertical surfaces
• Non-conformal step coverage = thinner on vertical surfaces

Aspect Ratio

• A measure of height to width, given by AR = height/width
• It is generally more difficult to fill higher aspect ratio trenches, as the filling may have poor step coverage, due to poor conformality
• It is important to understand the thin film deposition techniques to produce high quality thin films

Thin Film Deposition Methods

• Two main deposition methods are used today:
• Physical Vapor Deposition (PVD): Does not involve chemical reaction. Creates vapor of thin film materials inside the chamber. Condensation occurs on the wafer surface leading to a solid thin film deposition. Evaporation and sputter deposition are examples, and are most commonly used for metals.
• Chemical Vapor Deposition (CVD): Involves chemical reaction. Reactant gases are introduced into the chamber, where chemical reactions occur on the wafer surface leading to the deposition of a solid thin film. APCVD, LPCVD, and PECVD are examples, and are most commonly used for dielectrics and Si.

General Characteristics of Thin Film Deposition

• Properties that define quality of a film:
• Physical and chemical properties
• Electrical property, eg. breakdown voltage
• Mechanical properties, eg. film stress and substrate adhesion
• Optical properties, eg. transparency and refractive index
• Composition
• Film density, defect (pinhole...) density
• Texture
• Impurity level, doping
• Conformality (step coverage)
• Trench/Hole filling

Physical Vapor Deposition (Evaporation)

• Chemical reactions are not involved
• The material source is heated to a high temperature in a vacuum, either by:
• Thermal methods
• E-beam methods
• Material is vapor transported to a target in vacuum
• Film quality (density) is often not as good as sputtered film
• Film thickness can be precisely monitored using a quartz balance
• In thermal evaporation, the source material is heated in a high vacuum chamber (P < 10-5 Torr)
• High vacuum is required to minimize collisions of source atoms with air molecules
• Heating is done by resistive or e-beam sources
• Surface interactions are physical; can be very fast (> 1 µm/min possible, but the film quality may suffer; R&D is typically 0.1 ~ 1nm/sec)
• Has poor conformal coverage.

Types of Evaporation Methods

• Two types of evaporators
• Thermal evaporator:
• Resistive heating
• Relatively old deposition technique
• Electron beam evaporator:
• Heated by electron beam
• The most popular technique
• More expensive than thermal evaporator

Thermal Evaporation

• Widespread use for materials whose vapor pressure can be reasonable at 1600°C or below
• Common evaporant materials: Au, Ag, Al, Sn, Cr, Sb, Ge, In, Mg, Ga
• It can be heated by:
• Heating source rod using a heated spiral
• Heating the source material using a dimpled boat

Electron Beam Evaporation

• Use a focused electron beam to heat and evaporate metals; electrons are accelerated by DC 10kV, and current 10s-100s of mA
• The target material temperature can be very high
• Suitable for high melting point metals like W, Ta, etc
• Evaporation occurs at a highly localized point near the beam bombardment spot on the source surface, so little contamination occurs from the crucible
• Evaporation is initiated by heating the target with e-beam collision, but heating target cannot be terminated instantly
• A mechanical shutter is needed to terminate the deposition instantly.

Boat Crucible Material

• Refractory Materials
• Tungsten (W): 3380°C
• Tantalum (Ta): 3000°C
• Molybdenum (Mo): 2620°C
• Refractory Ceramics
• Graphite (C): 3799°C
• Alumina (Al2O3): 2030°C
• Boron Nitride (BN): 2500°C
• Considerations: Thermal conductivity, thermal expansion, electrical conductivity, reactivity
• Graphite is the most popular, but avoid cracking the crucible due to stress/temperature gradients.
• Tungsten dissolves in aluminum, so aluminum and tungsten are not compatible
• Quartz Crystal Micro-balance (QCM), monitors the thickness deposited on the wafer by measuring the shift of resonance frequency on its surface with sub-Å accuracy

Advantages of electron beam evaportation

• Films can be deposited at high rates (up to ~100 Å/s)
• Low energy atoms (~0.1 eV)
• Leave little surface damage
• Little residual gas and impurity incorporation due to high vacuum conditions.
• Very little substrate heating

Limitations of electron beam evaportation

• Accurately controlled alloy compounds are difficult to achieve
• Poor step coverage
• X-ray damage

Comparison of Thermal and E-Beam Evaporation

• Thermal:
• Metal of low melting point materials
• Typical evaporating materials: Au, Ag, Al, Cr, Sn, Sb, Ge, In, Mg, Ga
• Impurity: High
• Deposition rate: 1 ~ 20 Å/s
• Temperature range: ~ 1800°C
• Cost: Low
• E-Beam
• Both metal and dielectrics
• Typical evaporating materials: same 'Thermal' plus Ni, Pt, Ir, Rh, Ti, V, Zc, W, Ta, Mo, Al2O3, SiO, SiO2, SnO2, TiO2, ZrO2
• Impurity: Low
• Deposition rate: 10 ~ 100 Å/s
• Temperature range: ~ 3000°C
• Cost: High

Sputter Deposition

• Material exists as solid target
• Material is removed from target by momentum transfer
• Gas particles (often Argon) are ionized by plasma; these ions strike the target and remove/sputter away the atoms in the target
• Sputtered atoms condense on the substrate
• Involves energetic bombardment of ions, which makes the film denser

Sputtering System

• The sputtering target material is placed in the cathode of an electrical circuit and supplied with high negative voltage
• A substrate is placed on an electrically grounded anode
• Electric field accelerates electrons and turns the gas into plasma
• High energy electrons from the plasma collide with argon atoms to form Ar+ ions and secondary electrons
• Ar+ ions are accelerated towards the sputtering target through negative bias, which transfers momentum of the argon to the target material to dislodge one or more atoms
• The ejected (sputtered) atoms move through the plasma, land on the substrate on the anode, condense there, and form a thin film
• Cue ball (Ar+) striking the billiard balls (target atoms)

General Properties of Sputtering Deposition

• Energy of each bombarding ion: 500-1000eV. Energy of sputtered atoms: 3-10eV
• Sputtering process is very inefficient from the energy point of view; 95% of incoming energy goes to target heating & secondary electron
• High rate sputter processes need efficient cooling techniques to avoid target damage from overheating
• The sputtered species, in general, are predominantly neutral or not charged particles
• Energies of atoms or molecules sputtered at a given rate are about one order of magnitude higher than those thermally evaporated at the same rate, which often lead to better film quality
• Sputter-deposition rates are invariably one to two orders of magnitude lower compared to thermal evaporation rates under normal conditions. Sputter yield is dependent on various factors

Sputter Yield

• It is the number of sputtered atoms per bombarding (impinging) ion. Higher yield gives higher sputter deposition rate
• The yield is rather insensitive to the target temperature up to very high temperatures where it show rapid increase due to the accompanying thermal evaporation
• It depends on:
• Ion energy
• Ion incident angle
• Ion mass
• The yield increases with ion energy
• For higher ion energies, yield approaches saturation, which occurs at higher energies for heavier bombarding particles
• Sometimes, at very high energies the yield decreases as Argon ions are penetrated into sputter target, due to atoms beneath target no longer reach the surface to escape
• The yield increases with increasing obliqueness of the incident ions
• At large angles of incidence the surface penetration effect decrease the yield drastically
• An optimal angle is needed to achieve high sputtering yield
• 60° – 70°
• An optimal angle is needed to achieve high sputtering yield
• Sputter yield increases with ion mass.
• Sputter yield is maximum for ions with full valence shells: noble gasses such as Ar, Kr, Xe have large yields

Radio Frequency Sputter Deposition

• DC sputtering is unable to sputter insulating/ dielectric materials
• Radio Frequency (RF) potential is applied to the cathode (target) of the sputtering system
• Without RF, positive charge (Ar+) builds up on the cathode (target) in DC sputtering systems
• Alternating potential can avoid charge build-up
• Sputtering of insulator become possible because of the RF on the target

Magnetron Sputter Deposition

• Low ionization efficiency in electron-Argon gas collision: In DC & RF sputtering, the efficiency of ionization from energetic collisions between the electrons and gas atoms is low; most electrons lose energy in non-ionizing collisions or are collected by the electrodes
• Solution, magnets used to increase Argon ionization, which increases deposition rates
• A magnetic field is applied perpendicularly to the electric field, to trap electrons near the target surface, causing them to move in a spiral motion until they collide with an Ar atom
• The ionization and sputtering efficiencies are increased significantly; deposition rates increase by 10-100×, to 1 µm per minute
• In magnetron sputtering, magnets are employed to capture and restrict the electrons in front of the target
• Increase ion bombardment rate on the target, produce more secondary electrons, increase ionization rate in the plasma
• More ions cause more sputtering of the target, increase deposition rate without increasing the chamber pressure
• Higher deposition rate

Advantages of Sputter Deposition

• Able to deposit a wide variety of metals, insulators, alloys and composites
• Able to deposit compound/alloy thin film; the film has the same composition as the sputter target
• Better film quality (densified) and step coverage as adatoms are more energetic
• More reproducible deposition control; same deposition rate for same process parameters, which makes film thickness control easier over time.
• Uniform thickness over large substrates
• Can use large area targets
• Sufficient target material for many depositions
• No x-ray damage

Disadvantages of Sputter Deposition

• Substrate damage to possible ion bombardment
• Higher pressures, more contaminations
• Low deposition rate for some materials
• energy incident on the target becomes and needs to be rejected

Comparison between Evaporation and Sputtering

• Evaporation
• Rate: 1000 atomic layer/s
• Thickness control: Possible
• Materials: Limited
• Contaminants: Low
• Surface roughness: Little
• Adhesion: Medium
• Film properties: Difficult to control
• Step coverage: Poor
• Equipment cost: Medium
• Sputtering
• Rate: 1 atomic layer/s
• Thickness control: Easy
• Materials: Almost unlimited
• Contaminants: High
• Surface roughness: High
• Adhesion: Good
• Film properties: Can be controlled
• Step coverage: Good
• Equipment cost: Expensive

PVD summary

• Step coverage and trench filling are important figures of merit in a thin film deposition; trench filling is highly dependent on the aspect ratio of the trench
• PVD can be carried out using evaporation technique or sputtering technique
• Evaporation technique can either be carried out using thermal evaporation or electron beam evaporation
• Sputtering process uses high energetic argon ions to dislodge atoms from target materials for deposition.
• Sputter yield depends on ion energy, ion mass, and the incident angle of the ion.
• RF sputtering can prevent charge accumulation on the substrate hence makes dielectric deposition possible; whereas magnetron sputtering induce spiral motion on ions, which increases the sputter yield