Understanding electron charging, beam deflection and pattern distortion in Electron Beam Lithography
When we work with Electron Beam Lithography (EBL), we spend considerable time making sure the electron beam is properly focused and aligned.
We adjust the beam position, check the focus, correct astigmatism and try to obtain a stable writing condition.
But I came across an interesting question:
What if the electron beam is correctly adjusted, but the written pattern still appears shifted or distorted?
My first thought would be to check the electron optics, stage movement or alignment.
However, while learning more about EBL, I found another possible reason.
The sample itself can affect the electron beam.
This problem is called electron charging, and it becomes particularly important when writing on insulating materials.
Let’s understand why it happens.
1. What Happens When an Electron Beam Hits a Sample?
In EBL, a focused electron beam travels through the electron column and reaches the resist-coated sample.
The primary electrons interact with the resist and the underlying substrate.
During these interactions, several things can happen:
- Some primary electrons penetrate deeper into the material.
- Secondary electrons are generated and may escape from the surface.
- Some electrons are backscattered.
- Electrical charge can remain within the resist or substrate.
This last point is particularly important.
When electrons enter a material, the total charge entering and leaving the irradiated region may not be equal.
If the material cannot remove or redistribute the resulting charge quickly enough, electrical charge can accumulate.
That is the beginning of the charging problem.
2. Why Does the Substrate Material Matter?
Consider two samples.
Sample A: Conductive substrate
For example, a resist-coated silicon wafer with a suitable conductive path, or a metal substrate.
In a conductive material, electrical charge can move relatively easily.
If the sample has an appropriate connection to ground, accumulated charge may be dissipated.
However, even a conductive substrate can experience surface charging when covered by an insulating resist layer.
Sample B: Insulating substrate
For example:
- Glass
- Quartz or fused silica
- Sapphire
- Other electrically insulating materials
These materials do not allow electrical charge to flow away as easily.
When the electron beam exposes the resist, charge may accumulate within the resist or near the substrate surface.
The amount and sign of charging depend on factors such as incident electron energy, electron emission yields, material properties, exposure conditions and electrical grounding.
This means that two samples exposed using similar beam conditions may behave differently.
3. How Does Charging Affect the Electron Beam?
This was the part I found most interesting.
An electron beam consists of negatively charged particles.
Because electrons have electrical charge, their trajectories can be affected by electric fields.
Now imagine that charge has accumulated on the sample surface.
That charge creates a local electric field.
As the incoming electron beam approaches the sample, it can experience an additional electrostatic force.
The force on a charged particle is:
F = qE
Where:
- F = electric force
- q = particle charge
- E = electric field
For an electron, the charge is negative.
Therefore, the direction of the force is opposite to the local electric-field direction.
If the charging field has a component perpendicular to the intended beam path, the incoming electrons can be deflected.
The result?
The beam may land at a slightly different position from the intended writing coordinate.
The EBL system may command one position, but the electrons can arrive at another.
That difference can produce placement errors or distorted patterns.
4. Does This Mean the Electron Beam Is Actually Drifting?
We need to distinguish two different situations.
Case 1: Electron-optical or mechanical drift
The beam position changes because of problems involving the electron optics, electrical stability, mechanical movement or thermal effects.
Case 2: Charging-induced beam deflection
The electron optics may be operating correctly, but electric fields produced by accumulated sample charge change the trajectory of incoming electrons.
Both situations can appear as pattern-placement errors.
However, their causes are different.
This is why I think it is important not to immediately assume that every pattern shift is caused by the electron column or stage.
5. What Happens to the Written Pattern?
Charging can affect the writing result in several ways.
Pattern displacement
The entire exposed feature may appear shifted from its intended location.
Pattern distortion
Different parts of a structure may experience different deflection conditions.
As a result, straight lines may become curved or distorted.
Placement errors between features
If the charging condition changes during exposure, features written at different times may not have the same positional accuracy.
Unstable imaging and alignment
Charging can also influence electron-based observation and alignment measurements.
For example, an alignment mark may appear to move or distort during repeated scanning.
However, these symptoms are not unique to charging. Similar problems can result from stage errors, calibration problems or electron-optical instability.
So charging should be considered as one possible cause, not automatically treated as the confirmed diagnosis.
6. Why Can the Problem Become Worse During Writing?
Imagine that we start exposing a pattern on an insulating sample.
At the beginning, the sample may have relatively little accumulated charge.
As exposure continues, additional electrons interact with the material.
Depending on the balance between incoming electrons, emitted electrons and charge dissipation, the surface potential can change.
This means that the electrical condition near the sample may not remain constant throughout the writing process.
The charging effect can therefore become dependent on exposure history.
For example, a region exposed earlier may have a different charging condition from a region exposed later.
That is one reason charging-related placement errors can be difficult to understand.
The problem may depend not only on the material but also on the sequence in which the pattern is written.
7. Can Charging Be Positive as Well as Negative?
Yes.
At first, I assumed that because the electron beam brings negative electrons to the sample, the surface must always become negatively charged.
But that is not necessarily correct.
When primary electrons strike the material, secondary and backscattered electrons can leave it.
If more electron charge leaves a region than enters it, positive charging can occur.
If more negative charge remains than escapes or dissipates, negative charging can occur.
The charging behavior depends on the material and electron-beam conditions.
This is important because the direction of the resulting electric field determines how the incoming beam is affected.
8. How Can Charging Be Reduced?
Several approaches are used in electron-beam processes.
A. Conductive coating
A thin conductive layer can sometimes be added to provide a path for accumulated charge.
For example, certain processes use thin conductive coatings or conductive polymers with electron-beam resists.
The coating must be compatible with the resist, exposure process and subsequent processing steps.
B. Proper grounding
When an appropriate conductive path is available, grounding helps charge dissipate.
However, grounding the sample holder alone does not necessarily eliminate charging in an insulating resist or substrate.
C. Adjusting exposure conditions
Beam current, electron energy, writing sequence and exposure strategy can influence charging behavior.
Changing these conditions requires careful process evaluation because they also affect other aspects of lithography.
D. Charge-dissipation layers
Specialized conductive layers may be used to reduce charging during electron-beam exposure.
These are particularly useful for some insulating-substrate processes.
There is no single solution that works for every sample.
The correct approach depends on the substrate, resist and required pattern accuracy.
9. How Would I Think About This During EBL Troubleshooting?
Suppose an EBL system is producing patterns that appear shifted or distorted.
Instead of immediately changing beam alignment, I would first try to understand the nature of the error.
Some useful questions are:
- Is the problem occurring on every sample or only on insulating substrates?
- Does the pattern shift change during repeated exposure or observation?
- Is the stage position stable?
- Are beam focus, astigmatism and calibration behaving normally?
- Is the sample properly mounted and electrically connected where required?
- Does the process use a suitable charge-dissipation method?
These questions do not prove that charging is responsible.
But they help separate possible sample-related effects from equipment-related problems.
In actual equipment work, any changes to beam conditions or process parameters should follow the approved machine and customer procedures.
10. What I Understood
Before studying this topic, I mainly thought about beam placement in terms of the electron column.
The beam is focused, deflected and directed toward a particular coordinate.
So I assumed that if the electron optics were properly adjusted, the beam would always arrive at the intended position.
But now I understand that the situation can be more complicated.
The electron beam does not travel through an environment completely unaffected by the sample.
If charge accumulates near the surface, the resulting electric field can influence the incoming electrons.
Therefore, accurate EBL writing depends not only on the machine’s beam-control system but also on the electrical behavior of the sample.
This is especially important for insulating substrates.
Gurwinder Singh
GSingh-EB Semiconductor Insights
www.gsingh-eb.com
