Name: Stanford Nanofabrication Facility: Dry Etching - Introduction (Part 1 of 4)
Uploaded: 2025-06-07T15:27:36.000Z
Duration: 13 min 11 s
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Hi, I'm Jim McVitie of Stanford's Nanofabrication Facility at Stanford University.

In this series of four short presentations, I'm going to discuss the basics of dry or plasma etching and give some information to help you choose the right type of plasma etched tool and process for fabricating your nanostructures.

So in this first lecture, I'm going to have the introduction of dry etching.

Following lecture will be on the basics of plasma and the type of dry etching tools, dry etching mechanisms, and choosing a dry etching process and tool.

In the presentation, I'm going to cover first, what is dry etching, second, where dry etching is used in fabrication, third, etch profile control and selectivity definitions, and finally, why use plasma for etching.

In nanofabrication, etching is the process of removing regions of deposited films or subgrades.

Two of the other major processes used in nanofabrication, deposition, the process of laying down films, and lithography, the process of patterning the matte, usually films of fold resist, will be covered in other units of this course.

As I mentioned, here we are covering the important process of dry etching.

Basically dry etching is vapor or gas-based etching.

Etching come in as a gas and byproduct leave as a gas.

So having a volatile byproduct is essential for dry etching.

Down here I show a diagram of the basics of dry etching.

We have a reaction, in this case fluorine, coming down the surface, etching, reacting with the surface, and a byproduct, silicon tetrafluoride, coming off.

In this lecture, I'm going to limit our discussion to plasma dry etching.

In this case, the plasma offers an extra control not available by wet etching where we use liquids.

So down in the diagram here we have an extension.

Rather than going out with fluorine, we start out with CF4, which is a very stable gas.

We use a plasma to break up the gas and generate ions, electrons, but also react with free fluorine that then does the etching that I showed you before.

Where is dry etching used in the fabrication?

First, it's used for removing blanket layers, such as, well, we can use it for removing resist and their process or other things.

Secondly, and most importantly, for patterning.

In patterning, we use a mask to block the reaction in ions so that we can duplicate a copy of the mask in the material on the surface of our wafer.

And we usually, the mask is often a full resist.

So let's show a diagram of how this works.

So here, let's say I start out with a substrate with a uniform film, and I want to pattern that film on the top.

And the way I get there, first I put out a photoresist layer and pattern it typically with optical beams or electron beams.

And then I expose it to my plasma etching process that selectively removes the film and then opens the area.

And then I remove the photoresist, so now I have my patterned film on my substrate.

Okay, here I want to do some definitions for X-profile control.

So typically, X-profile is critical to the structure we want.

So for nanostructures, I want to have a high-fidelity reproduction of the structure or pattern in my mask layer in the underlying structure, whereas in other cases, I want to have some slope and maybe a little bit undercut, typically because of step coverage issues.

So here I show the case I'm going to define, making anisotropic etching with completely vertical etching, and it completely replicates the mask in the film.

The second case is isotropic etching, and this is where we get the pure chemical etching, where you have undercut equal to the X-depth.

So if I go down a nanometer thick, I go sideways a nanometer.

And again, this can be very useful for some structures where we really have a difficult time getting step coverage.

And lastly, I want to talk about the neo-anisotropic or directional etching.

This is a little probably more typical of a lot of things we're going to actually see.

We have a little bit of undercut, and we have some slope.

And a lot of times we want to do this intentionally because, like I say, step coverage, when we have a film going over a sharp step, having some slope improves our ability, well, reduces the chance of having a breakage of the film at the corners.

Okay, let's have a definition of X-selectivity.

Selectivity is the ratio of the X-rate of the material to be removed, divided by the material to remain.

And so let's say here is my diagram of the substrate, the film, and the photoresist pattern.

Now I've shown some etching, so I've removed some of my photoresist, and I've etched in the material.

And the reason I want selectivity is I don't want to lose the matte material before I get all the way through the film I'm etching.

Secondly, a second selectivity is when I go all the way through the material and I hit the substrate material.

And there the selectivity can also be very important, such as if I'm etching a gate over a gate oxide, a thin gate oxide, I have to stop immediately before going through that thin oxide.

So therefore I want a very high selectivity to the material removed to my underlying gate oxide.

So normally we want a high selectivity, we want the rate of removal of my film to be much greater than the removal rate of my mask layer.

So here's an example of the etching of polysilicon over a silicon dioxide layer.

Here I have a top layer right here is the photoresist layer.

Below that I have the polysilicon layer that I'm patterning.

And finally I have the silicon dioxide layer down below.

And for this process we have a polysilicon to photoresist selectivity of 6 and a polysilicon oxide selectivity ratio of 13.

These numbers, these selectivity ratios can be from 1 to over 100, even up to 1000 depending on the process and the details.

First we can use it for generating local sources.

So like I say before I showed etching of silicon with free fluorine.

Free fluorine is very reactive and it's much easier and safer to generate the fluorine locally using a PLAPA.

Second, PLAPA etching is often much easier to do than wet.

Case in point is the case of polysilicon etching.

Wet polysilicon etching is very difficult to do with a mask layer.

Next we can get very high aspect ratios using PLAPA etching.

So an aspect ratio is a ratio of the depth divided by the width.

So here I'm showing an etch into silicon of an aspect ratio of 40 and 1.

In PLAPA etching we have ions and the ion can be used to bombard the surface and can do a number of useful things.

One, the ion can clean the surface, remove oxide, polymers left over from lithography before your etching process.

In addition they can activate chemical reactions.

So I'll show you how sometimes the chemical, the radicals will just sit on the surface and do nothing and the ion can come in, break bonds and allow the reaction to proceed.

And ions are nice in that we can control the ion energy and the directionality by using the geometry of the system and by having, by judicial use of RF power.

Ion energy and directionality are key to anisotropic etching.

Higher energy means more directional ions and that leads to more anisotropic etch problems.

In this diagram I show symbolically the etching process and role of ions.

So here on the top part here I'm saying here I have an electron colliding with a source molecule generating a free radical.

This free radical then diffuses to the surface that I want to etch, it absorbs in the surface and if it's like chlorine it'll just sit there.

But we have an ion come in, the ion can actually create site by breaking bonds, let's say silicon bonds, and when the bonds are broken the absorbed etching can then react with the materials, let's say silicon, and then form a volatile product or lead to a chain reaction of spontaneous reactions that form a volatile product.

And so the product then comes off and we're then removing, let's say silicon, we remove silicon from the surface.

Here I show an example reaction here of chlorine.

So we have chlorine gas going into the plasma, electrons in the plasma breaking the chlorine gas into two chlorine atoms, then on the surface silicon plus an ion plus the chlorine atoms yield silicon dichloride, that comes off, we can either have dichloride or trichloride come off, and if you have the dichloride case then it can react in the gas phase to form the more stable form of silicon tetrachloride.

Etching, a dry etching is a gas phase etching, byproduct of volatility is essential.

Plasmas offer lots of advantages over wet etching.

And one of the big advantages is the flexibility of the etch profile, we have isocropic, anisocropic, near anisocropic, and we have local generation of reaction for radicals.

And we have controlled ion bombardment, which can be used to activate surfaces in addition to cleaning, and the ions have directionality.

In the next video, I'll talk about the basics of plasmas and the types of dry etching tools.