Silicon and the Czochralski process

Most people have no idea what the Czochralski process actually is and yet it is the first step in a chain of events and processes that have changed the world. The reason you can read this now is because of the Czochralski. This post is about his fantastic process.

The Czochralski process is a method of crystal growth that is named after Jan Czochralski who discovered the method in 1916 while investigating the crystallization rate of metals. This process is now used to produce single crystals of semiconductors such as silicon, germanium and gallium arsenide. It can also be used to create crystals of palladium, platinum, silver, gold and synthetic gemstones.

The most important use of this process is the growth of large cylindrical single crystals of silicon. It is these that are turned into the silicon chips used in computers, phones, DVD payers and many other devices.

The process:

Silicon is the most abundant solid element on earth, being second only to oxygen and it makes up more than 25% of the earth’s crust. It is almost always found in compounds rather than pure silicon. For silicon chips we need pure silicon. It is not possible to get the high quality, high purity silicon required for computer chips in a single step. In fact it takes a number of steps. The Czochralski process is the final step which results in high purity silicon.

Silicon typically starts out as an iron-silicon alloy and accounts for about 80% of the worlds production of  elemental silicon. 1- 2% of this is purified to be used in the electronics industry.

The use of silicon in semiconductor devices requires very pure silicon (>99.9%). This can be extracted directly from solid silica or other silicon compounds by molten salt electrolysis a process known since the 1850s.

The next step is to produce a higher purity silicon for use in the crucible. The silicon is purified further using a chemical process known as the Siemens process. The Siemens process itself starts with a high purity seed rod, silicon is then deposited onto this via a chemical reaction. The rod basically gets fatter and fatter and is very pure.

The rod from the Siemens process can then be broken up and placed in the Crucible ready for the next stage.

The silicon is melted in a crucible (typically quartz, which is silicon!) and a small Czochralski silicon crystal is lowered into the melt. At this point impurities such as boron of phosphorus can be introduced to alter the final silicon. This is done in a very controlled way using very specific amounts. So instead of pure silicon we get "doped" silicon known as either n-type or p-type silicon. Or the impurities can be left out an a pure silicon crystal is grown.

The temperature of the silicon in the crucible is very tightly controlled and is just above the melting temperature of silicon. The seed crystal is attached to a rod that is rotated and pulled upwards at the same time. The speed of rotation and the speed the crystal is pulled upwards determines the width of the final crystal. This is typically done in an argon atmosphere.

The finished crystals can be very large, crystals with a 400 mm diameter (that is a circumference of over a meter!) and 1 to 2 meters in length are standard these days. These large crystals are then cut up into wafers thick enough to be handled during the chip making process. This thickness is usually about 3/4 of a mm thick, so a 1 meter crystal will give about 1200 wafers.

The crystal is cut up using a special wire saw that can cut hundreds of slices simultaneously. This can take several hours. The shape edges are then smoothed down to prevent damage later in the process.  Then the wafers are mounted and polished using a slurry, this is a liquid with very small particles in it that act as an abrasive. The result is very flat wafers.

The wafers are then etched with nitric, hydrofluoric and acetic acids producing an even smoother and cleaner surface. This is then ready for chip production.

The silicon wafers used at the beginning of the integrated circuit making process must first be refined to "nine nines" purity (99.9999999%), a process which as we have seen requires repeated applications of refining technology.

That is pretty much it. Though by all accounts much of this is still an art form as much as it is process. There are some real artists out there it seems.