5.1.4 Integrated MOS Transistors

MOS transistors are quite different from bipolar transistors - not only in their basic function, but also in the way they are integrated into a Si substrate. Lets first look at the basic structure

We have a source and drain region in the Si (doped differently with respect to the substrate) with some connections to the outside world symbolically shown with the blue rectangles. Between source and drain is a thin gate dielectric - often called gate oxide - on top of which we have the gate electrode made from some conducting material that is also connected to the outside world.

To give you some idea of the real numbers: The thickness of the gate dielectric is below 10 nm, the lateral dimensioon of the source, gate and drain region is well below 1 µm.

You know, of course, what a MOS transistor is and how it works - at least in principle. If not: Use the link Basic MOS transistor.

If we integrate MOS transistors now, it first appears (wrongly!) that we can put them into the same Si substrate as shown below:

There seems to be no problem. The transistors are insulated from each other because one of the pn-junctions between them will always be blocking. However: We must also consider "parasitic transistors" not intentionally included in our design!

If in the space between transistors a wire is crossing on top of the insulating layer as shown in the illustration, it will, on occasion be at high potential. The drain of the left transistor together with the source of the right transistor will now form a parasitic transistor with the insulating layer as the gate dielectric, and the overhead wire as the gate electrode.

Everything being small, the threshold voltage may be reached and we have a current path where there should be none.

This is not an academic problem, but a typical effect in integrated circuit technology, which is not found in discrete circuits: Besides the element you want to make, you may produce all kinds of unwanted elements, too: parasitic transistors, capacitors, diodes, and even thyristors.

The solution is to make the threshold voltage larger than any voltage that may occur in the system. The way to do this is to increase the local thickness of the insulating dielectric.

This gives us the structure in the next illustration

How we produce the additional insulator called field oxide between the transistors will concern us later; here it serves to illustrate two points:

Insulation is not just tricky for bipolar transistors, it is a complicated business with MOS technology, too

There is now some "topology" - the interfaces and surfaces are no longer flat. Looks trivial, but constitutes one of the major problems of large-scale integration!

Note that the gate - substrate part of a MOS transistor is, in principle, a capacitor. So we can now make capacitors, too.

However, if we need a large capacitance - say some 50 fF (femto Farad) - we need a large area (several µm²) because we cannot make the dielectric arbitrarily thin - we would encounter tunneling effects, early breakdown, or other problems. So we have to have at least a thickness of around 5 nm of SiO₂ . If the capacitor area than gets too large, the escape road is the third dimension: You fold the capacitor! Either into the substrate, or up into the layers on top of the Si.

The "simple" way of folding integrated capacitors into the substrate is shown in the right hand side of the next illustration


Planar capacitor	"Trench" capacitor

The planar capacitor (on the left) and the "trench" capacitor (on the right) have a doped region in the Si for the second electrode, which must have its own connection - in the drawing it is only shown for the trench capacitor. We learn two things from that:

1. Large scale integration has long since become three-dimensional - it is no longer a "planar technology" as it was called for some time. This is not only true for truly three-dimensional elements like the trench capacitor, but also because the processes tend to make the interfaces rough as we have seen already in the case of the field oxide.

2. The names for certain features generally accepted in the field, are on occasion simply wrong! The capacitor shown above is not folded into a trench (which is something deep and long in one lateral direction, and small in the other direction), but into a hole (deep and small in both lateral directions). Still, everybody calls it a trench capacitor.

The key processes for ICs more complex than, say, a 64 Mbit memory, are indeed the processes that make the surface of a chip halfway flat again after some process has been carried out.

Again, there is a special message in this subchapter: Integrating MOS transistors, although supposedly simpler than bipolar transistors (you don't need all those pn-junctions), is far from being simple or obvious. It is again intricately linked to specific combinations of materials and processes and needs lots of ingenuity, too.

But we are still not done in trying to just get a very coarse overview of what integration means. If you take an arbitrary chip of a recent electronic product, changes are that you are looking at a CMOS chip, a chip made with the "Complementary Metal Oxide Semiconductor " technology.

So lets see what that implies.

Questionaire
Multiple Choice questions to 5.1.4