6.2.2 LOCOS Process

Basic Concept of Local Oxidation

The abbreviation "LOCOS" stands for "Local Oxidation of Silicon" and was almost a synonym for MOS devices, or more precisely, for the insulation between single transistors. LOCOS makes the isolation between MOS transistors considerably easier then between bipolar transistors, cf. the drawings discussed before:

For bipolar transistors, you have to separate the collectors. This involves an epitaxial layer and some deep diffusion around every transistor.

For MOS transistors, no isolation would be needed weren't it for the possible parasitic transistors. And this problem can be solved by making the "gate oxide" of the parasitic transistors - which then is called field oxide - sufficiently thick.

The thick field oxide has been made by the LOCOS process from the beginning of MOS technology until presently, when LOCOS was supplanted by the "box isolation technique", also known as "STI" for "Shallow trench isolation".

Since the LOCOS technique is still used, and gives a good example of how processes are first conceived, are optimized with every generation, become very complex, and are finally supplanted with something different, we will treat it here in some detail

As the name implies, the goal is to oxidize Si only locally, wherever a field oxide is needed. This is necessary for the following reason:

Local (thermal) oxide penetrates into the Si (oxidation is using up Si!), so the Si - SiO₂interface is lower than the source - drain regions to be made later. This could not be achieved with oxidizing all of the Si and then etching off unwanted oxide.

For device performance reasons, this is highly beneficial, if not absolutely necessary.

For a local oxidation, the areas of the Si that are not to be oxidized must be protected by some material that does not allow oxygen diffusion at the typical oxidation temperatures of (1000 - 1100) ⁰C. We are talking electronic materials again!

The only material that is "easily" usable is Silicon nitride, Si₃N₄. It can be deposited and structured without too much problems and it is compatible with Si.

However, Si₃N₄ introduces a major new problem of its own, which can only be solved by making the process more complicated by involving yet another materials. This gives a succinct example of the statement made before: That materials and processes have to be seen as a unit.

Lets see what would happen with just a Si₃N₄ layer protecting parts of the Si from thermal oxidation.

Oxygen diffusion through the oxide already formed would also oxidize the Si under the Si₃N₄; i.e. there would be some amount of lateral oxidation. Since a given volume of Si expands by almost a factor of 2 upon oxidation (in other words: Oxidizing 1cm³ of Si produces almost 2 cm³ of SiO₂), the nitride mask is pressed upwards at the edges as illustrated.

With increasing oxidation time and oxide thickness, pressure under the nitride mask increases, and at some point the critical yield strength of Si at the oxidation temperature is exceed. Plastic deformation will start and dislocations are generated and move into the Si. Below the edges of the local oxide is now a high density of dislocations which kill the device and render the Si useless - throw it out.

This is not "theory", but eminently practical as shown in the TEM picture from the early days of integrated circuit technology:

We are looking through a piece of Si. The dark lines are the projections of single dislocations, the "dislocations tangles" corresponds to oxide edges; "E" shows contact areas (emitters) to the Si. Another picture can be found in the link.

Actually, it doesn't even need the oxidation to produce dislocations. Si₃N₄ layers are always under large stresses at room temperature and would exert great shear stresses on the Si; something that can not be tolerated as soon as the nitride films are more than a few nm thick.

We arrive at a simple rule: You cannot use Si₃N₄ directly on Si - never ever. What are we to do now, to save the concept of local oxidation?

Buffer Oxide

We need something between the Si₃N₄ mask and the Si; a thin layer of a material that is compatible with the other two and that can relieve the stress building up during oxidation. Something like the oil in you motor, a kind of grease.

This "grease" material is SiO₂, as you might have guessed - it was already mentioned before under its proper name of "buffer oxide". The hard Si₃N₄ (which is a ceramic that is very hard not yielding at a "low" temperature of just about 1000 ^oC), is now pressing down on something "soft", and the stress felt by the Si will not reach the yield stress - if everything is done right.

The situation now looks like this

No more dislocations, but a comparatively large lateral oxidation instead, leading to a configuration known as "birds beak" for the obvious reason shown in the picture to the right (the inserts just are there to help you see the bird).

So we got rid of one problem, but now we have another one: The lateral extension of the field oxide via the birds beak is comparable to its thickness and limits the minimum feature size.

While this was not a serious problems in the early days of IC technology, it could not be tolerated anymore around the middle of the eighties.

One way out was the use of a poly-Si layer as a sacrificial layer. It was situated on top of the buffer oxide below the nitride mask and was structured with the mask. It provided some sacrificial Si for the "birds beak" and the total dimension of the field oxide could be reduced somewhat.

This process is shown in comparison with the standard process in the link.

But even this was not good enough anymore for feature sizes around and below 1 µm. The LOCOS process eventually became a very complicated process complex in its own right; for the Siemens 16 Mbit DRAM it consisted of more than 12 process steps including:

2 oxidations, 2 poly-Si deposition, 1 lithography, 4 etchings and 2 cleaning steps.

It was one of the decisive "secrets" for success, and we can learn a simple truth from this:

Before new materials and processes are introduced, the existing materials and processes are driven to extremes! And that is not only true for the LOCOS process, but for all other processes.

Still, with feature sizes shrinking ever more, LOCOS reached the end of its useful life-span in the nineties and had to be replaced by "Box isolations", a simple concept in theory, but hellishly difficult in reality.

The idea is clear: Etch a hole (with vertical sidewalls) in the Si wherever you want an oxide, and simple "fill" it with oxide next. More about this process can be found in the link above.