# 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

© H. Föll (Semiconductor Technology - Script)