IAN LANG ELECTRONICS

 

Shown left is a boule made by Intel Corporation in preparation for computer chips. A boule like this can be intrinsic, in which case it is pure silicon, or more commonly the silicon has been doped with either phosphorous or boron before being made into a boule in which case the resulting material will display N or P properties respectively. There is no way to tell just by looking at it and they have to be marked.

 It is very large, having a diameter of about 12 inches, and clearly we are not going to be able to fit it into the confines of the average laptop. The "wobbly" end will be cut off and recycled, and the boule itself will be cut into thin wafers.

The trend in commercial production has been to increase the size of the wafer since the bigger the wafer the more chips can be etched on to it at one go and  there is less peripheral waste. Currently the largest size is 12 inches but Intel are experimenting with the 18 inch wafer and expect to have it in production by 2017.

Wafers are cut with a diamond saw and have certain standard thicknesses; the 12 inch, which is now a standard, has a thickness of  0.775 mm. The 2 inch had 0.275mm, it is not made any more as the cost is too great for the return, and the 8 inch (which some manufacturers still use since the cost of tooling for a 12 inch outweighs the potential benefit in returns) has a thickness of  0.725mm.

 

Integrated Circuits

First Diffusion >

Making Wafers.

This picture comes from an Intel basic education programme aimed at schoolchildren ("From Sand to Silicon") and is an excellent illustration of the process showing how the boule is sawn up into wafers.

 

However the story does not end here. Once the wafers have been created the ongoing problem is that they are not smooth enough to be used in production. Now we must grind and polish them.

 

This is done by a mild abrasive grinder which removes irregularities on each side of the wafer until it is perfectly flat. The wafer is then cleaned with weak acids.