The Story of Silicon
what is silicon?
Silicon (Si) is the element with atomic number 14, positioned just below carbon in the periodic table of elements. Although it has some characteristics in common with carbon, silicon possesses some truly unique properties that are key to the vast field of semiconductor technology. Most hi-tech electronics, from computers to clock-radios to auto fuel-injection systems, depend on silicon-based devices. Despite its importance, however, silicon is often confused with the plastic substance silicone, and few are aware of the complex processes needed for its purification and use for semiconductor devices.
Fortunately, silicon is extremely easy to find on earth; more than 25% by weight of the earth's crust is composed of silicon dioxide (silica and silicate), occurring as clay, feldspar, granite, quartz and sand. The grains of sand on a beach that sparkle in the sun are usually made of silicon dioxide.
purification
Silicon used for electronic devices must be extremely pure; the presence of just a few atoms of other elements is enough to compromise the operation of a semiconductor device. The required purity is achieved using a complex multistep process.
First, pure sand is carefully selected and mined, then melted in an electric arc furnace. At temperatures over 1900°C, carbon reduces the silica to silicon. Liquid silicon collects in the bottom of the furnace, and is then drained and cooled. The result is >97% pure metallurgical-grade silicon (MG-Si).
Second, the MG-Si is further purified by converting it to a silicon compound that can be more easily purified than silicon itself, and then converting that silicon compound back into pure silicon. Trichlorosilane (TCS) is most commonly used as the intermediate, and is formed by reacting MG-Si with HCl in a fluidized bed reactor. Next, the TCS is fractionally distilled to attain a high level of purity. The hyper-pure TCS is then vaporized and diluted with high-purity hydrogen in preparation for the next step.
Finally, the hyper-pure TCS, now with impurity levels of less than 1 part per billion, must be converted back to silicon. This is achieved using the Siemens process, in which high-purity silicon rods are exposed to TCS vapor at 1150°C. The TCS decomposes and deposits additional silicon onto the rods, which grow in diameter to about 20 centimeters. Silicon produced from this and similar processes is called polycrystalline silicon (polysilicon).
crystal growth
In order to use silicon in a semiconductor device, it must be in the form of a single crystal, with specific orientation. This can be achieved by recrystallizing the polysilicon using the Czochralski process. High-purity polysilicon is melted down in a crucible. Impurity atoms such as boron or phosphorus can be added to the molten silicon in precise amounts (a technique known as doping) in order to control the electrical conductivity of the silicon.
Next, a seed crystal (a small, high-purity piece of single-crystal silicon) mounted on a rod is dipped into the molten silicon. The silicon atoms in the melt line themselves up with the atoms in the seed crystal, growing the crystal one layer at a time. The seed crystal's rod is pulled upwards and rotated at the same time. By precisely controlling the temperature gradients, rate of pulling, and speed of rotation, a large, single-crystal, cylindrical ingot is extracted from the melt. This process is normally performed in an inert atmosphere, such as argon, and in an inert chamber, such as quartz.
The silicon ingot is then shaped into a perfectly round cylinder of the desired diameter, and a length-wise notch is ground at the correct point to indicate crystallographic orientation.
slicing
From this ingot of pure, single-crystal silicon, individual wafers are sliced, each one from 2 inches to 12 inches in diameter (depending on the original diameter of the ingot), and less than a millimeter thick. The edges of the wafers are ground to the correct shape. Next, each wafer is lapped to achieve a high degree of flatness, cleaned, and polished in preparation for patterning.
patterning
To build integrated circuits on silicon, a sequence of steps are carried out on each wafer to create conducting areas (analogous to wires) and non-conducting areas (analogous to insulation). To create insulating areas, the silicon surface can be oxidized to create silicon dioxide. Additional insulating layers can be introduced by depositing polymer films on the wafer. Metals are deposited to create conducting lines and contacts. In order to control the structure of the insulating and conducting areas, photolithography is used. In this process, a light-sensitive layer is coated on the wafer, and masks are used to expose selected areas to light. The exposed and unexposed areas can be treated differently, creating patterns in the underlying layers. After many processing steps, the end result is a wafer with multiple devices on it, which is then cut into chips.
packaging
Next, precise electrical connections must be made to specific locations on each chip. In addition, the fragile chip must be protected from excessive heat, moisture, and general handling. To achieve both these ends, chips are packaged in carefully designed materials that protect them, allow them to dissipate heat efficiently, and have connectors of a standard size and shape that can be used on a standard motherboard.
And so silicon is transformed from grains of sand, to single-crystal wafers, to integrated circuit chips.
