Name: Stanford Nanofabrication Facility: Dry Etching - Dry Etching Mechanisms (Part 3 of 4)
Uploaded: 2025-06-07T15:31:42.000Z
Duration: 18 min 38 s
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Hi, I'm Derek Bitty of the Stanford Nanofabrication Facility here at Stanford University.

This is part three of a four-part lecture series on dry etching.

In this presentation, I'm going to cover dry etching mechanisms.

Previously, the first lecture was on introduction to dry etching.

The second was on the basics of plasmids and the types of dry etch tools.

And the next lecture after this one will be on choosing a dry etching processing tool.

In this presentation, I'm going to cover the following areas.

Followed by ion-enhanced chemical etching.

I'm then going to talk about x-configuration and x-mechanism comparisons.

And finally, non-ideal pattern transfer issues in dry etching.

I'm going to start with physical spotting.

Which is, I think, well, an ion knocking atoms off a surface, or can be thought of as atomic sandblasting.

The diagram here shows an ion coming in and material coming off the surface.

Now, the sputtering tools, I'm going to say, typically use argon, but any ion can cause sputtering.

We like argon because it has no chemistry and all that, and we tend to get it for the same power condition, we get higher ion fluxes with argon and other things.

Physical sputtering tends to be used for anisotropic etching, but it has issues of damage, low selectivity, redeposition.

And we typically use physical sputtering when there's no volatile byproducts.

So we prefer using chemistry because chemistry, I'll go along and show you, gives you a lot of advantages in terms of rate and profile control.

But sometimes there's no byproduct that comes off the surface, so it has to be all physical.

I want to show you some typical rates for sputtering.

So here's a case of rates dependent on ion current, energy, and the material.

So I'm going to show you a case of 500 eV argon at 1 milliamp per square centimeter. 1 milliamp per square centimeter corresponds to 10 to the 15th ions per centimeter squared per second.

So here I have a list of materials and x-rays of sputter rates.

The first thing we see is that the numbers are relatively low in terms of rates.

The second thing I want to show you is that when you talk about the comparison of a sputter rate of metal versus metal oxides, you tend to have big differences.

In the case of aluminum, the ratio between the oxide and aluminum, all are 10 to 1.

So aluminum sputters 10 times faster than the oxide.

The other point to look at here is the low selectivity.

For the most part, they're not that different.

Silicon and silicon oxide sputter rates are not that much different from photoresist.

Let's go on here and talk about the next case of pure chemical etching.

Here we have radicals reacting spontaneously with terrific atoms.

The typical case we use a lot is fluorine, CF4 from fluorine, basically etching silicon.

Fluorine will spontaneously etch with silicon.

So you don't need any ion bombardment with this.

In this process, we're basically getting a neutral coming down and forming a byproduct.

We typically use it with free fluorine for silicon etching and free oxygen for polymer etching.

They tend to show strong temperature and loading dependence.

By this, I mean as the rate increases significantly with temperature, typically an activation energy of about 1 eV, and the rate decrease which increases both areas.

So basically, as your larger area, your radical density has to be used, your source of radicals have to be used over a larger area, so the rate goes down.

It can be a lot faster in the etching than sputtering, but a lot of times we need something more than isotropic.

So that's where some of the next mechanisms will come into play.

And the characteristics of the pure etching is it's stopped by any surface deposition or any formation of non-volatile products.

Say you have a contaminate in the material you're etching, and that contaminate can build up to the surface and finally stop the etching.

Here's an example of near-pure chemical etching.

This was done on our LAM ICP etcher using SFC and fluorine, but it was set up to be a very isotropic etching, so we see here that the lateral etch rate is almost equal to the vertical etch rate.

Now I'm going to talk about the next two mechanisms I'm calling mixed mechanisms.

The first one is going to be the ion-enhanced chemical etching, and then I'm going to talk about inhibitor or deposition-controlled etching.

So here I show that I have a nucleus coming in, the ions coming down, activating.

I've talked about this in some of the earlier lectures.

So the ion activates or gets the process going, and then we form the volatile byproduct coming off.

Radicals, so we basically talk about radicals going down to the surface, but the reactant can be blocked.

Maybe it can't break, let's say, a silicon and chlorine.

The chlorine cannot break the silicon-silicon bond, so it gets stopped, or it can be a deposition going on.

So the ions break the reaction-blocking bonds and help the byproduct get off the surface.

So here I'm saying chlorine, silicon with chlorine, and the ion coming in, basically forming silicon dichloride, or silicon tetrachloride, coming off the surface.

The process tends to be less dependent on temperature, less loading-dependent, and less dependent on surfactant oxides than the pure chemical case.

You can get good selectivity, good anisotropic, but it is subject to some secondary effect, and I'll show that later on.

The second mechanism is ion-enhanced inhibitor etching.

And this is where we have simultaneous deposition and etching going on at the same time.

The diagram here I'm showing, here I have ions coming in, I have a neutral coming down, and I'm showing here a little inhibitor layer here, where the ions are coming in, breaking through the inhibitor layer, allowing the radical to come down and react with the material we want to etch, and then the radical product coming off.

So typically what we do, let's say chlorine chemistry or something, we add another gas element to promote deposition, and that can be hydrogen, it can be carbon fluoride, CHF3, O2, nitrogen, and boron trichloride can help us.

I'll tell you there are cases where boron trichloride is helpful in etching.

So we basically use local deposition to block or slow etching, and then we use ion bombardment to remove the deposition on the horizontal surfaces.

Temperature, the process tends to show temperature and loading dependencies, not as bad as a pure chemical case, but the typical is temperature dependence.

You can get good selectivity, good anisotropy, and the process is somewhat tunable by how much deposition we put in.

Here in the diagram I'm showing a case where we have a low amount of deposition, so we're not quite isocropic, but we still have a certain amount undercut.

As we increase the deposition, we can go from this case to a pure anisotropic, and if we keep on increasing deposition, then we get sloped walls.

Sometimes we want sloped walls, but a lot of times we want the anisotropic behavior.

And here's some SEMs of a case of silicon etching using HBR, so it's a bromine-NF3-fluorine-oxygen combination, and we had an oxide mask, and this was done in our P5000 etching tool.

And we show here ag-etched, you see the wall closer, so basically in the process we actually form the oxide, so we go in and with a 51-HF dip, it removes this oxide-like material on the side, and so here it's down to the mask material, and you see we get a fairly anisotropic, and this process, if we tune it right, we can get all the vertical sidewalls, very anisotropic behavior.

But it's a combination of fluorine and bromine chemistry with oxide formation.

And the oxide formation, we have oxygen reacting with the silicon coming off to form this silicon dioxide-like material on the sidewall.

So here I want to show mechanisms and configuration comparisons.

So here I'm talking about we have physical processes versus chemical processes.

At higher pressures, we tend to show much more chemical processes, higher energy, if not more physical processes.

The chemical processes give a better selectivity.

The physical processes, as we go more to ion-driven processes, we get the anisotropic behavior.

And so here, I mentioned sputtering on the more physical end, pure chemical, and then the mixed region where we have a lot to control.

This is where we like to operate with the chemistry and the ion bombardment to give a control of what we selectively, high selectivity, good profile control.

And here I show the different configurations.

So I'm saying downstream action is mainly for chemical processes.

And ICP, well, ICP allows us to give a very broad range.

We can go in and tune the amount of ion energy, and we can run...

ICP can actually run from low pressures down to a few millitor, all the way up to 100 millitor.

And then in between, we have the CCP modes, the plasma and the RIE, where the plasma mode tends to be lower ion energies, where the RIE mode tends to be the higher ion energies.

So I want to talk about some of the non-ideal pattern cramp for issues.