Name: Stanford Nanofabrication Facility: Dry Etching - Choosing a Dry Etching Process & Tool (Part 4 of 4)
Uploaded: 2025-06-07T15:44:21.000Z
Duration: 18 min 58 s
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Hi, I'm Jim McVitie, and this is part four of a four-part series of lectures on dry etching.

In this lecture, I'm going to talk about choosing a dry etching process and tool.

Previously, I talked about, I gave an introduction to dry etching.

I've been followed by basic applied methods and types of dry etching tools, followed by dry etching mechanisms.

In this lecture, I'm going to cover what are the issues that control etch process choice, etch chemistry choices, and I'll talk about the role of the use of boiling points to help determine useful etch products, and polymer crayoffs in oxide etching.

I'll then cover etch crayons in different materials, and end with a comparison between different etch configurations.

Here we show a diagram we showed previously of our photoresist over a film that was being etched over a substrate in the polysilicon etch case.

What etch tools and process should we use to get desired etch characteristics for our structure?

So here's going through some of the issues.

First is, are there volatile etch products?

The etch depth controls selectivity means, what is the matte material, stopping material, allow losses.

Deeper depths require higher rates and higher selectivities.

If we're talking about really deep, and we're talking about silicon, then we can use what we call the deep silicon etch process, with fluorine based, with a switching between etch and deposition.

We have a control, we can set the temperature in the reactor, and then we also have wafer heating depending on the process conditions, how much power we have, and by power.

Temperature affects the etch and depth rates.

It affects profile, selectivity, and it can affect resistive degradation.

If the resistive gets too hard, it can degrade and be very difficult to remove, and actually affect the profile as we're going deeper.

And finally, in terms of etch temperature, many of our tools have backside helium used to improve the heat transfer between the wafer and the electrodes.

And finally, we have the issue of initial breakthroughs.

Some of the steps we will begin with a step that will just remove any oxide or polymer before we begin the process.

This is very important when you have a very high selectivity process.

You have to start with your material and oxide can slow you down.

So sometimes you go in and just remove that oxide and then switch to the high selectivity process.

For etching we have, for the case of silicon and germanium, we use halogens, fluorine, chlorine, bromine, to form halides, volatile halides.

And the fluorine tends to be faster, but more isocropic than the fluorine and bromine.

For metals, we use the halogens also to form metal halides, which may or may not be volatile.

We may need boron trifluoride and a sputtering to remove an initial oxide.

Boron trifluoride is a very interesting chemical in that it not only removes a lot of oxides, it also, when in the gas chemistry, it removes any water vapor or oxygen that may be coming from the surfaces or other places.

So it really, it generates an environmental void of any oxidizers.

Metal oxides and nitrites, we use halogens again, plus ion bombardment and maybe boron trifluoride.

For silicon oxide, we tend to use carbon fluorides, mixtures, plus maybe CHF3.

The CF4 compound may have CHF3 added or not, or maybe hydrogen added.

Plus we'll add in additives that will enhance or reduce deposition.

So oxygen and fluorine that tend to remove the polymer and therefore give it better control.

For nitrite, we can either use an oxide etch or we use a fluorine-based etch where we add oxygen or nitrogen.

For polymer, we typically use oxygen, but we can add either argon or fluorine to help the process.

Fluorine added, when you have a really tough polymer, sometimes a little bit of fluorine added to oxygen will break through a tough polymer that has a lot of cross-linking.

And argon is used when we want to add more ion bombardment.

Inhibitory deposition, we use the fluorocarbons, CHF3, hydrogen, and sometimes both nitrogen or oxygen can help with inhibitory deposition.

And this is for controlling sidewalls and anisotropic etching.

When we have marginal byproduct volatility, we often increase the temperature and also increase ion bombardment.

The downside is that the increased temperature and ion bombardment can damage the photoresist and lower selectivity.

This slide, I'm going to talk about the use of boiling points to evaluate possible byproducts.

So what I have here is a table of different elements, and I have chloride products, fluoride products, and their boiling points.

And the reason we look at boiling points is boiling points are often easily available, whereas from possible products, we often do not have the data for vapor pressures at the temperature we're doing the etching is not available.

But we can extrapolate roughly what the vapor pressure is at the operating temperature from boiling points.

What I have here is a plot of temperature versus vapor pressure for the case of gallium tichochloride.

And we see here at the top here, here's the atmospheric pressure.

We have the boiling point at about 200 degrees C.

Now down here at 50 degrees C, what might be a typical etching temperature, we see the vapor pressure is on the order of one millitor.

And millitor, one to ten millitor is what we like to have for the byproduct so that they come off the surface.

So we see here there's a relationship between boiling point and vapor pressure at the operating temperature.

So we can use boiling point to help us determine possible byproducts.

And so typically, we're interested in boiling point less than 200 degrees for vapor that we're going to be operating near room temperature or less than 300 degrees for vapor that might be heated.

So aluminum chloride has a boiling point that actually sublimes at 180 degrees C.

Whereas aluminum fluoride has a boiling point of 1,200 degrees or 1,300 degrees.

So we see obviously if we're going to etch aluminum, we want to use chlorine.

When we take a look at copper, we see copper chloride has a boiling point of 1,500 degrees C.

So basically, we're not going to etch very well copper, either chlorine or fluorine.

Whereas silicon tetrafluoride has a boiling point of minus 86 degrees.

So it means that fluorine is going to come off easier.

So we're probably going to have a higher equity than we know.

But we can easily etch this temperature, a boiling point of 60 degrees C for silicon tetrafluoride is well within the range that we can easily do etching.

And as we've shown throughout the lectures, that chlorine does a very good job at etching silicon.

So here I show a polymer payoff for oxide etching.

What I have here, I'm showing basically pressure on one side.

I'm showing basically very low pressure to a high pressure one torr.

And typically, we're going to get much at the high pressure.

We can have low ion energy because of all the collisions.

At low pressure, we're going to have high ion energy.

In the x-axis here, I'm showing basically the fluorocarbon composition.

So that as one side, I'm adding oxygen and fluorine.

And so in this case, I'm going to have a lot more free fluorine going on.

But at high pressures, with the high fluorine case, we can actually get isocropic etching of oxide.

At low pressures, rather than adding oxygen, we typically go for maybe hydrogen or adding carbon.

We go more toward the polymer deposition range.

Before we get to actual deposition of polymer, we get the most anisotropic etching.

If we go more towards the deposition threshold, we actually start getting wall slope and the etch rate drops down.

And we see that at high pressures, we get polymer deposition that occurs much earlier than at low pressures.

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Stanford Nanofabrication Facility: Dry Etching - Choosing a Dry Etching Process & Tool (Part 4 of 4)

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