Positive and Negative Resist in EBL — What Is Actually the Difference?

Published by Gurwinder Singh

1,107 words

5–7 minutes

Introduction

While learning Electron Beam Lithography, I came across two words again and again:

Positive resist and negative resist.

At first, I thought the names might have something to do with electrical charge. We are using electrons, which have a negative charge, so that was the first connection that came into my mind.

But that is not what positive and negative mean here.

The difference is actually about what happens to the resist after electron-beam exposure and development.

This also made me ask another question.

If the electron beam hits the resist, why does the exposed area disappear in one resist but remain in another?

I wanted to understand this without going too deep into chemistry, so in this article I will try to explain it in the same simple way that I understood it.

First, What Is the Resist Doing?

Before using EBL, as we coat the sample with a thin layer of resist.

Then the electron beam writes the pattern according to our design.

But the electron beam is not acting like a knife and physically cutting the pattern into the resist.

This was important for me to understand.

The electrons are changing the material chemically.

After exposure, we use a developer. This is when we can actually remove certain parts of the resist and get our pattern.

But which part gets removed depends on the type of resist.

And this is where positive and negative resist come in.

Positive Resist

Let’s take PMMA as an example because it is a very common material we hear about in EBL.

PMMA contains long polymer chains.

I find it easier to imagine these chains like long pieces of spaghetti.

When electrons transfer enough energy to PMMA, some of these long polymer chains are broken into shorter chains.

So, very simply:

Long chains → Electron exposure → Shorter chains

Why does that matter?

Because the exposed material can now be dissolved more easily by the developer.

After development, the area where the electron beam wrote the pattern is removed.

This gives me a very simple way to remember it:

Positive resist: the exposed area is removed.

Suppose I write a small rectangle with the electron beam.

After development, that rectangle becomes an opening in the resist.

The beam didn’t dig the opening itself. It changed the resist so that the developer could remove it later.

That distinction is important.

Then What Happens With Negative Resist?

Negative resist behaves almost in the opposite way.

Instead of making the exposed region easier to remove, electron exposure can make that region more resistant to the developer.

One example used in EBL is HSQ.

The chemistry of HSQ is different from PMMA.

The important idea is:

Electron exposure → exposed material becomes more strongly connected/networked → exposed region remains after development

So my simple rule becomes:

Positive = exposed part goes away.
Negative = exposed part stays.

Once I remembered those two sentences, positive and negative resist became much easier for me to understand.

But Why Do We Need Both?

This was my question TO MY PROFESSOR.

If PMMA works well, why not just use PMMA for everything?

ANSWER IS;

Because researchers are not always trying to make the same structure.

For example, one researcher may want to make metal electrodes using a lift-off process.

Another may want very small lines or dense nanoscale structures.

Another device may require the resist to survive an etching process.

So the choice of resist depends on the process.

Researchers may have to think about things such as feature size, resist thickness, exposure dose, development, lift-off, etching and the material underneath the resist.

This is something I am starting to notice again and again while learning nanofabrication:

There is usually not one material or one setting that is best for every device.

The process depends on what we are trying to make.

Let’s Take a Simple Example

Imagine I want to fabricate two small metal electrodes.

First, I coat the substrate with a positive resist such as PMMA.

Then I make my electrode pattern in CAD and send that pattern to the EBL system.

The beam exposes the areas where I want openings.

After development, the exposed PMMA is removed.

Now I have openings in the resist.

Next, metal can be deposited.

After lift-off, the unwanted metal sitting on top of the remaining resist is removed together with the resist.

The metal inside the openings stays on the substrate.

So the process looks something like:

Design → EBL → Development → Metal Deposition → Lift-Off → Metal Pattern

When I look at the process this way, it also becomes clear that EBL is not making the whole device by itself.

EBL is one step in the fabrication process.

One Thing That Confused Me

There is one mistake I think a beginner can make very easily.

Positive and negative resist have nothing to do with positive and negative electrical charge.

The names tell us how the resist behaves during processing.

For a simple beginner explanation:

Positive Resist	Negative Resist
Beam exposes an area	Beam exposes an area
Exposed region becomes easier to remove	Exposed region becomes harder to remove
Exposed region is removed during development	Exposed region remains after development
PMMA is a common example	HSQ is a common example

That’s the comparison I would remember first.

The deeper chemistry can come later.

Is Choosing the Correct Resist Enough?

Unfortunately, no.

What if the dose is too high?

Even if we choose a suitable resist, we still need the correct process conditions.

For example, what happens if I use PMMA but my electron dose is too low?

Maybe the resist does not receive enough exposure.

So now another question appears:

The exposed feature may not come out with the dimensions I expected.

Then we also have resist thickness, beam energy, development conditions and other parameters.

What I Understood From This

Before studying this topic, positive and negative resist sounded like two complicated chemical categories.

Now I think about them much more simply.

If the electron beam writes on a positive-tone resist, the exposed area is removed during development.

If it writes on a negative-tone resist, the exposed area remains.

Of course, the actual chemistry and process are more complicated than these two sentences. But as a starting point, this makes the concept much easier to understand.

It also helped me understand something bigger about EBL.

We often focus on the electron beam because that is the impressive part of the machine. But producing a good nanoscale pattern also depends heavily on the resist, dose and development process.

NEXT TOPIC;

Electron Dose in EBL: What Happens When the Dose Is Too Low or Too High?

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