Wafer Manufacturing - Semiconductor Device Fabrication

Semiconductor device fabrication is a series of mechanical, thermal, photographic and chemical processing steps during which microelectronic circuits are created on a wafer made of pure semiconductor material.

In semiconductor device fabrication, contaminants are specified by the number of particles per cubic meter for a given particle size.

The various semiconductor device fabrication processes fall into five general categories:

1.    Wafer Manufacturing
2.    Oxidation
3.    Lithography
4.    Diffusion
5.    Packaging

A Silicon Wafer

The starting material for manufacturing microelectronic devices is pure mono crystalline silicon ingots but for IC fabrication Semiconductor Grade Silicon is used as the starting material.

In wafer manufacturing, trichlorosilane is dominantly used for the following reasons:

1.    It can be easily formed by the reaction of anhydrous hydrogen chloride at low temperature. (200°C - 400°C).
2.    It is liquid at room temperature. Therefore, distillation can be easily carried out.
3.    It can be deposited on heated silicon.
4.    It reacts at low temperature and at faster rates than SiCl₄.

The hydrogen reduction of trichlorosilane on heated silicon rod is called Siemens method.

Equation:   SiHCL₃ + H₂ => Si + 3HCL

Disadvantages of Siemens Method are:

1.    High Power Consumption
2.    Poor Silicon and Chlorine Efficiency

Granular Polysilicon Deposition uses the decomposition of monosilane in a fludized bed deposition reactor to produce free flowing granular polysilicon.

Equation:    SiH₄   =>   Si + 2H₂

Because of the large surface area, fluidized bed reactor are much more efficient than the siemens type rod reactor.

Merits of Using Fluidized Bed Reactor are:

1.    It is highly efficient.
2.    It is a closed loop and green process.
3.    It is easier to melt for the next process of growing monocrystals.

There are two techniques for the production of monocrystalline silicon which are:

1.    Float Zone Method
2.    Czochralski Method

The float zone method uses an induction heating radio frequency coil to melt the polysilicon rod. A seed crystal with the desired orientation is held by a rotating rod placed just below the polysilicon rod  and as the heater traverses the polysilicon rod, it moves with the molten zone, as the tip of the molten zone touches the rotating seed crystal it solidifies (freezes).
A monocrystalline rod grows an extension of the seed crystal as the seed crystal is being withdrawn. In the presence of any impurities or dislocation in the polysilicon they are drawn to the neck and cut off at the end of the process.

 Float Zone and Czochralski Method

The Czochralski method involves three steps which are:

1.    Melting Polysilicon
2.    Seeding
3.    Growing

In the Czochralski method, polysilicon chunks are filled in a quartz crucible and melted at temperatures higher than 1420°C in an inert gas or vacuum for a while to ensure complete melting and ejection of air bubbles.
Seeding then takes place where a monocrystalline seed crystal installed in a shaft is dipped into the melt until it begins to melt itself, the melt temperature is lowered until a small amount of crystalline material is solidified.
Growth then takes place, in this case the seed is gradually withdrawn from the melt and once the proper diameter is achieved the seed lift is increased.
Towards the end, the crystal diameter is gradually reduced to form an end cone. When the diameter becomes small enough, the crystal can be separated from the melt without the generation of dislocations
Once the growth process is complete, the crystal is cooled inside the furnace for up to 7 hours to stablize and ensure easier handling.

The float zone silicon crystal has a higher purity than the Czochralski silicon crystal but the Czochralski silicon crystal has a higher mechanical strength than the float zone seed crystal.
Also note that the Czochralski silicon crystals are cheaper and has larger diameters.

With monocrystalline silicon ingot as the starting material, the processes involved are:

1.    Trimming and Orientation
2.    Flattening
3.    Ingot Etching
4.    Slicing
5.    Edge Rounding and Lapping
6.    Slice Etching
7.    Polishing and Cleaning

Primary flat is used to correctly align the wafer during device fabrication while secondary flat is used to identify wafer plane orientation and conductive type.

Polishing has two process which are:

1.   Mechanical (to create flatness)
2.   Chemical (to create smoothness)

Contaminants of a wafer are categorized into three which are:

1.    Molecular Eg. resins, oils skin which are held to the wafer by weak electrostatic force.
2.    Ionic Eg: Na⁺, F^-, I^- which can be removed by chemical reaction
3.    Atomic Eg:  Au, Fe, transition metals which can be removed by chemical reagents.

Water - Hydrogen Peroxide - Ammonium Hydroxide is the compound for chemically cleaning organic contaminats

Water - Hydrogen Peroxide - Hydrogen Chloride is the compound for chemically cleaning alkali and transition metals.