FOBIC Process

Consider making a contact hole through the CVD oxide that was deposited over the finished transistors.

Since the source/drain area are as small as possible, you have to make not only a small hole but you must align it precisely relative to the gate stack of the transistors.

Small misalignements would produce a short circuit between the gate electrode and the Al that will be put in the contact hole eventually.

Now cover the whole gate stack with Si₃N₄ that will resist whatever etching procedure you use to remove the oxide in the future contact hole. You gain a lot in your "process window" for the contact hole - misalignements don't matter as much any more as shown below.

That looks pretty good - but there is a prize to pay:

First of all - how do we make the nitride encapsulation? And don't forget; Si₃N₄ must never come in contact with Si - you always need a thin layer of buffer oxide underneath (not shown in the picture). There is quite a bit of added process complexity!

The topography gets worse. The aspect ratio of the contact hole - the relation between depth and diameter - increases and with it the problems of filling it with Al.

Still, starting around 1987, FOBIC was used and helped to get the next generation onto the market.

The picture below is a cross section through a 16 Mbit DRAM memory cell shown before. It shows one of the contacts to a transistor (which is connected on the other side to a trench capacitor). The FOBIC structure has been outlined; it is clearly visible.

With frame

6.2.2 Oxide CVD