Crystal Growth

Sand to Silicon

Silicon Purification

Crystal Growth Techniques

• Polycrystalline silicon has randomly oriented crystallites – electrical characteristics are not ready for device fabrication

• Convert EGS grade polysilicon into single crystal silicon– High Temperature– Single crystal silicon seed as template

• Commonly used Methods– Bridgman Method– Czochralski Method– Float zone Method

Bridgman Method

• Evacuated and sealed silica ampoule• Quartz boat carrying seed crystal and charge (powdered

high purity EGS polysilicon• Charge alone is heated to melting point• Heating mechanism

Bridgman Method

• Requires seed crystal.• Directional solidification.• Precise temperature gradient – only the charge should

melt not the seed crystal.

Bridgman Method

• Drawbacks– Si expands on cooling. As it starts to cool, the

confining boundary of quartz boat excretes a shearing stress leading to crystal dislocations.

– Contamination from boat– Difficult to make round wafers in horizontal

furnace

Czochralski Method

• EGS melted in slowly rotating quartz crucible at 1415°C

• It consist of:– Furnace– Crystal pulling mechanism– Ambient control– Control systems

Czochralski Method

Seed

Single Crystal Silicon

Quartz Crucible

Water Cooled Chamber

Heat Shield

Carbon Heater

Graphite Crucible

Crucible Support

Spill Tray

Electrode

Czochralski Method

• Single crystal silicon seed is mounted on a slowly rotating chuck and lowered into the molten silicon.

• Portion of the seed crystal which is in contact with the molten Si begins to melt.

• However the seed crystal temperature is precisely controlled to be just below that of molten Si.

Czochralski Method

Czochralski Method