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Silicon Wafers

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Submitted By fellowgeek
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Silicon wafer is a thin slice of silicon crystal used in the fabrication of integrated circuits and other micro devices. Silicon is widely used in the semiconductor industry because it remains a semiconductor at higher temperatures than other semiconductor materials and because its native oxide is easily grown. Once grown its native oxide forms a better semiconductor/dielectric interface than other materials.

The purpose of this article is to give a brief and basic explanation of the many different silicon wafer options available. If you are interested in purchasing any quantity of silicon wafers then Mi-Net Technology Ltd will be able to help. Please contact us for further information.

Quality

There are different qualities of silicon wafer available. The main qualities are PRIME, MONITOR and TEST.

Prime wafers are wafers of top quality. They will generally be extremely flat and have a very low variation in thickness across the entire wafer. These wafers will be extremely clean and have a very low number of defects.

Monitor and test wafers are used less for final manufacturing and more for research and quality control. They are not as clean or as flat as a prime wafer and may have many more defects across the wafer. They will however be cheaper.

Growth Methods

There are two common growth methods for silicon wafers. These are Czochralski and Float Zone.

In the Czochralski method, high purity semiconductor grade silicon is melted down into a crucible. A quantity of an impurity (dopant) such as boron or phosphorous is added to the melt to precisely determine the type (N or P type) and conductivity of the silicon. Once the melt is ready, a silicon seed crystal is introduced into the melt and then gradually over time the seed is withdrawn upwards from the melt. By controlling the upwards speed and rotation of the seed crystal along with the temperature of the melt it is possible to extract a cylindrical silicon ingot from the melt. This ingot is then processed to create the final silicon wafers.

In the Float Zone method a polycrystalline rod of ultra-pure silicon is passed through an RF heating coil. This coil creates a molten zone from which the crystal ingot grows. There is a seed crystal at one end which is used to start the growth and specialised methods are then used to introduce required impurities. Again the final result is an ingot of silicon which is processed to create the final wafers.

Doping

Doping is the precise introduction of impurities into the silicon melt to control wafer properties such as type (N or P) and resistivity. Typical dopants are boron, phosphorus, arsenic or antimony.

Orientation

Once grown, the silicon ingot is sliced into wafers. The ingots are sliced at an angle that will be aligned to one of several crystal orientations. These orientations are defined by the Miller Index with common values being , or .

Flats

Flats are used to identify the orientation of a wafer and the dopant type.

The main flat is called the primary flat. This is the longest flat on the wafer and is also known as the major flat.

Next there is the secondary flat. This flat is shorter than the primary flat and its relative position to the primary flat is used to identify the type and orientation of the wafer. The secondary flat is also known as the minor flat.

Other options are to have a jeida flat which is a Japanese standard for major/minor flat length or a Notch which is an indent of specified shape and dimensions situated to allow identification of orientation,

Lapping

Once the silicon ingot has been sliced, we are left with rough cut silicon wafers. At this point the wafers can be left as they are but will have defects and saw marks. These wafers are called as-cut and will be used for processes where wafer quality isn’t important.

In most cases a more refined finish is required and so most wafers will be lapped. The lapping process removes the defects, saw marks, thins the wafer and relieves some of the stress accumulated in the wafer from the sawing process.

There is one more stage after lapping and this is called etching. This is a cleaning stage using acids to remove microscopic defects caused by the lapping process.

Polishing

Wafers come as single side polished (SSP) and double side polished (DSP). Double side polished wafers are generally used when an extremely high level of flatness is required or if there is a need for double side patterning and device manufacturing projects.

- Thickness and Diameter

Silicon wafer diameters can typically vary from as small as 1” up to as large as 18”. Thickness can also vary depending on the requirements for the wafer. In each case a tolerance will usually be specified to allow for slight variations in the manufacturing process.

Resistivity

This is the measure of how easy / difficult it is for charged carriers to flow through the wafer. Resistivity can be controlled when the wafer is fabricated by varying the impurities added into the silicon melt. Resistivity is specified as ohm-cm.…...

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