While learning Electron Beam Lithography, I kept seeing different beam-current values such as 25 pA, 1 nA and 12 nA.
At first, I thought the choice should be simple.
If a higher beam current means more electrons are reaching the sample, why not always use the highest current available?
For example, if I can use 12 nA, why would I ever choose 25 pA and make the exposure much slower?
Then I noticed that beam current is also connected with another part of the electron-optical system: the aperture. And when we talk about writing very small patterns, we also start talking about the beam spot or probe size.
That made me realize that choosing beam current is not simply about getting more electrons.
There is a balance between how quickly we can deliver charge and what kind of focused electron beam we need for our pattern.
So in this article, I want to understand that relationship in a simple way.
1. What Does Beam Current Actually Mean?
An electron beam is a flow of electrons.
Because electrons carry electrical charge, beam current tells us how much charge is flowing through the beam per unit time.
The basic relationship is:
where:
- = beam current
- = electrical charge
- = time
In EBL, the currents are very small, so we often see units such as pA (picoampere) and nA (nanoampere).
For example:
while:
So a 1 nA beam delivers charge much faster than a 25 pA beam.
This is why higher current can be useful when writing a large amount of pattern.
But this still doesn’t answer my original question.
Why don’t we always use the highest current?
To understand that, I first needed to understand the aperture.
2. What Does the Aperture Do?
Inside an EBL electron column, electrons generated by the source travel through an electron-optical system before reaching the sample.
An aperture is basically a plate containing a very small opening.
A simple way I imagine it is:
Electron Source
↓
↓ ↓ ↓ ↓ ↓
─────────────
○ ← Aperture
─────────────
↓
↓
Focused Beam
↓
Resist
The aperture helps select which part of the electron beam continues through the column.
As a general idea, a larger aperture can allow more beam current, while a smaller aperture restricts the beam more and usually gives lower current.
So I can think about it like this:
Larger aperture → potentially more current
Smaller aperture → generally less current
But the aperture is not only related to how many electrons pass through.
It is part of the electron-optical system that determines the beam reaching the sample.
And this brings us to spot size.
3. What Is Spot Size?
When I first heard the term spot size, I imagined the electron beam as a perfectly straight cylinder with a fixed diameter.
But a real electron beam is not quite like that.
The focused beam has a spatial distribution of electrons. The probe or spot size describes how tightly the electron beam is focused at the sample.
For very fine patterns, we generally want a small and well-controlled probe.
This is where the trade-off starts to appear.
To obtain more beam current, the system generally has to accept more electrons and a wider range of electron trajectories. Depending on the electron-optical design and settings, this can make it harder to maintain the smallest possible focused probe because lens aberrations and other effects become more important.
So my simple mental model became:
More current → faster exposure
but
Fine-feature writing → may require beam conditions optimized for a smaller probe
This doesn’t mean that high current always gives bad resolution.
That would be too simple.
The actual probe size depends on the electron source, aperture, acceleration voltage, lenses, aberrations and the design of the EBL system.
So beam current is only one part of the complete picture.
4. Why Would I Use 25 pA Instead of 1 nA?
Now I can return to the question that originally confused me.
Suppose my EBL system gives me different beam-current conditions:
25 pA
1 nA
12 nA
If I am trying to write an extremely fine nanoscale structure, I may choose a lower-current condition because that condition may provide a smaller or more suitable probe for high-resolution writing on that particular system.
The disadvantage is that it takes longer to deliver the required charge.
On the other hand, if my pattern does not require the smallest possible probe, a higher-current condition can deliver the required charge much faster.
So neither one is automatically better.
It depends on what I am trying to write.
5. A Simple Example That Made This Clear to Me
In my previous article about electron dose, I calculated a pattern that required:
Now suppose I want to deliver exactly the same amount of charge using different beam currents.
The relationship is:
Let’s compare 25 pA, 1 nA and 12 nA.
| Beam Current | Ideal Time to Deliver 1.2 nC |
|---|---|
| 25 pA | 48 s |
| 1 nA | 1.2 s |
| 12 nA | 0.1 s |
This example made the importance of beam current very clear to me.
With 25 pA, theoretically it takes 48 seconds to deliver the charge.
With 1 nA, it takes only 1.2 seconds.
And with 12 nA, it takes only 0.1 seconds.
So why not choose 12 nA every time?
Because exposure speed is not our only requirement.
We also need beam conditions suitable for the dimensions and quality of the pattern we want to fabricate.
The fastest beam condition is therefore not automatically the best beam condition.
Also, these values represent the ideal charge-delivery time. The actual EBL writing time can be longer because a real system also has beam movement, blanking, settling and other operations.
6. Does Changing Beam Current Change the Dose?
This was another thing I wanted to understand.
Suppose my process requires the same nominal dose, but I change the beam current.
Do I automatically change the dose?
Not necessarily.
Dose tells us how much charge we want to deliver per exposed area.
Beam current tells us how quickly charge is being delivered.
So, in a simplified example, I can deliver the same charge using:
Lower current + longer exposure time
or
Higher current + shorter exposure time
For example:
If the required charge remains the same, increasing current means the required ideal delivery time becomes shorter.
This is why I now try not to mix up beam current and electron dose.
They are connected, but they are not the same parameter.
7. Does Spot Size Equal the Final Linewidth?
One final point was important for me.
Suppose the electron probe has a certain spot size.
Does that mean the final line written in the resist will have exactly the same width?
No.
The final developed linewidth also depends on the resist, dose, electron scattering, substrate and development process.
So:
Beam spot size ≠ automatically final pattern linewidth
A small probe is important for fine-feature writing, but it is only one part of the complete lithography process.
This also means I should be careful about saying something like:
“Use 25 pA for a 20 nm line.”
There is no universal rule like that.
Different EBL systems can have different probe characteristics even at the same nominal beam current.
The correct beam condition has to be selected based on the actual system and process.
What I Understood
Before learning about this, I looked at beam-current values such as 25 pA, 1 nA and 12 nA and thought:
If 12 nA gives me more electrons, why would I ever use 25 pA?
Now I understand that this is not the right way to look at it.
Beam current tells me how quickly charge can be delivered.
The aperture helps control the portion of the electron beam used by the electron-optical system and therefore influences the available current and probe conditions.
And the spot size tells me something about how tightly the beam is focused at the sample.
So I now think about it like this:
Aperture + Electron Optics
↓
Beam Current & Probe Characteristics
↓
Exposure Speed & Fine-Feature Capability
↓
Resist Exposure
↓
Final Pattern
The goal is not simply to choose the highest current.
It is to choose a beam condition that gives enough current for reasonable writing time while still providing the beam performance required for the pattern.
Conclusion
Beam current, aperture and spot size initially looked like separate EBL parameters to me.
But they make much more sense when I think about them together.
A higher beam current can deliver the required charge much faster, which can reduce exposure time.
But EBL is not only about speed.
When we want to fabricate very small features, the characteristics of the focused electron probe also become important.
So instead of asking:
“What is the highest beam current I can use?”
I think the better question is:
“What beam condition is suitable for the feature I actually want to write?”
That small change in thinking helped me understand why an EBL system needs different apertures and different beam-current conditions instead of simply operating at maximum current all the time.
Next topic
Field Size in EBL: Why Can’t the Electron Beam Write an Unlimited Area at Once?
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