Highly dependent on temperature (usually 1000C to 1200C).
2 steps
pre-deposition: continuous flow of impurities.
drive-in: move impurities to desired depth.
Cooled to freeze impurities.
Ion Implantation
Impurity ions accelerated by electric field to strike silicon.
Can control dose (in ) and energy (in ).
Impurity ions sorted by magnetic field.
Used when accurate control of dopant is required.
Impurity needs high temperature to reduce crystal lattice damage.
Silicon lattice leave large open space, ions may channel deep into substate through gaps. Tilting wafer helps with this.
Chemical Vapor Deposition (CVD)
Gases or vapors are reacted to form solid on a substrate.
Epitaxy
Saline gas creates silicon layer (epitaxial layer), where size of crystalline depends on temperature.
Low temperatures leads to a layer (polycrystalline) that has many mis-aligned silicon crystals.
Poly-silicon layers are heavily doped to form high conductivity interconnects.
Metallization
Vaporized Aluminum (way past 660C) deposits metal over entire silicon surface.
Layer is selectively etched.
When alloyed together, Si + Al have melting point of 577C. Metallization is performed at late stages to avoid these eutectic properties.
Some silicon can diffuse into aluminum during Al annealing (30 min 450C). This leads to spiking, where metal fills voids left in silicon.
Sputtering
Photolithography
Si surface is coated with photosensitive layer (photoresist).
Spin coated to achieve uniform thickness.
Photoresist can be softened with positive (exposed) or negative (not exposed) resist and be chemically removed. Effectively creating mask for below layers.
Wafer Exposure Systems
Contact can damage photoresist layer, but minimized diffraction
Gap has diffraction
Projection solve both damage and diffraction issues
Etching
uses liquid or gas to remove non-hardened material
wet etch uses liquid:
Buffered Hydrofluoric for Si
Potassium Hydroxide for Si
Phosphoric Acid for Al
Hot Phosphoric Acid for SiN
limited to 3m
requires rinse and dry steps and potential for contamination
Isotropic refers to non directional (wet)
Anisotropic refers to directional (dry)
dry etch is a generic term, usually involving gases
plasma uses corrosive gases and plasma energy
iron-milling uses ion-beam to blast wafer (inert gas)
some material is lost
poor selectivity
reactive ion etching combination of plasma and iron-milling
good selectivity
fine geometries
Wafer Probing
dice are checked using fine needles
limited to DC or simple testing. ink spots are used to indicate bad die
Die Attachment
epoxy cement is used to attach die to chip carrier since it is a poor conductor
alumina, gold, silver can be mixed to increase thermal conductivity
gold is deposited to back of wafer and heated to 390C to form eutectic bond
Wire Bonding
fine gold wires are sowed to bond aluminum bonding pads (100 to 125um)
Steps
gold ball is formed on bonding pad and pressed into nail head
