By Roberta Kelly

Supervisor: Jan Kycia

Electron Beam Lithography

Overview

1. Scanning Electron Microscope Basic2. Electron beam lithography basics3. Introduction to the Nanometer Pattern Generation

program. 4. Instructions on how to do electron beam lithography5. Issues that need to taken into consideration while

doing lithography.6. Some samples that show these issues

Scanning Electron Microscope

• Electrons are accelerated through an aperture and down a column to interact with a sample.

• The beam is focused with both electric and magnetic lenses. In the LEO 1530, the electron beam is maintained through the length of the column and focusing is only done at the very end of the column.

Electron Beam Lithography Basics• A resist, a long chained polymer, is spun onto a substrate usually silicon.• Electrons from a scanning electron microscope are accelerated through the

resist and into the silicon where secondary electrons are produced. These electrons travel through the resist where they break the bonds of the polymer chain.

• When the sample is developed, the now short chained polymers are dissolved, leaving the written pattern behind.

• To create a device, a metal (we will be using aluminum) is evaporated onto the pattern.

Nanometer Pattern Generation System NPGS• NPGS is a program that controls the SEM electron beam.• There are two parts to simple electron beam lithography,

writing a pattern, and running the pattern.• Writing a pattern is done in a CAD program provided with

NPGS. It has been modified to add useful commands for electron beam lithography

• Running a pattern involves specifying parameters that NPGS uses to calculate the location of each dot in the pattern and for how long it is exposed.

Pattern file• Patterns are made up of lines and

areas. • A line is exposed one point at a

time and has a width of the minimum width allowed by how well the user focused.

• An area is also exposed one point at a time. Then one line at a time. Based on the dimensions of the area and other parameters, NPGS automatically calculates the locations of the points. The lines can be exposed in a serpentine manner, or they can be exposed from lines always written in the same direction

Run File•• The run file saves all parameters provided by the user. These The run file saves all parameters provided by the user. These

parameters determine how many point, the locations of the pointsparameters determine how many point, the locations of the points, and , and how long each point is exposed.how long each point is exposed.

•• The magnification at which the pattern is being written must be The magnification at which the pattern is being written must be inputted so NPGS can calculate how much voltage must be suppliedinputted so NPGS can calculate how much voltage must be supplied to to the SEM in order to place each point correctly.the SEM in order to place each point correctly.

•• The specimen current is a measure of how many electrons hit a poThe specimen current is a measure of how many electrons hit a point int each second. each second.

•• The dosage is a measure of how much charge a point, line or areaThe dosage is a measure of how much charge a point, line or areashould get. Each part of a pattern that is a different colour cashould get. Each part of a pattern that is a different colour can get a n get a different dosage.different dosage.

•• The centre to centre distance and the line spacing determine howThe centre to centre distance and the line spacing determine how far far apart each point is. apart each point is.

•• These last 3 points are used to calculate how long each point shThese last 3 points are used to calculate how long each point should be ould be exposed.exposed.

•• The only parameters that are changed during electron beam The only parameters that are changed during electron beam lithography are the specimen current and maybe the magnificationlithography are the specimen current and maybe the magnification..

Resist• Resist is a polymer dissolved in a solvent. • The resist is spin coated onto a substrate, usually Si or GaAr. This

ensures a uniform thickness of the resist on the substrate.• The thickness of the resist depends on the dilution of the polymer and

the spin speed.• After spinning the polymer onto the substrate, it is baked at 170°C for

30 minutes

Preparing for writing (on the SEM)1. Load a faraday cup, high resolution gold on carbon sample, and the

sample to be written, into the scanning electron microscope.2. Turn off any rotation or tilt. Make sure that the aperture that you wish to

write with (10µm for small features) is the one that is loaded. Turn the EHT to 20kV.

3. Roughly focus on the faraday cup sample. Measure the beam current with the faraday cup. Check again that the correct aperture is loaded.

4. Switch to the high resolution gold on carbon sample. Focus to 1,000,000x. Make sure that astigmatism is minimized. Get rid of all aperture alignment problems.

5. Switch to the sample with the substrate on it. Find the edge of the silicon, without exposing the resist.

6. Find the location of the four corners and focus at these corners. Take the co-ordinates and the WD values at these corner.

7. Average these values to get the centre of the silicon and the focus at that point.

Lithography1. Turn on the beam blanker.2. Switch the beam control and the imaging output box to NPGS3. Goto Control Panel – choose stage tab and input the coordinates of the

centre of the silicon. 4. Goto Panel and choose external scan control and turn in on5. Set the magnification on the SEM to the magnification that you will be

writing on.6. Set the WD to the value calculated in preparation.7. Open the run file in the editor and make sure that the magnification in the

file matches the magnification on the SEM.8. In the run file, input the beam current in the relevent box.9. Save and close the run file.10. Click on process run file.11. Check for any error messages.12. Press space to run the file13. Check to make sure the beam blanker is working.

Finishing

1. Turn the SEM to TV mode and rotate the stage away from the sample that has just been written.

2. Turn off the beam blanker, and flip the switches back to their normal positions.

3. Turn the EHT off.4. Remove the sample.5. Develop the sample by immersing it for 15-30 seconds in

developing solution.6. Rinse for 30-60s in IPA.7. Blow dry.8. Examine as desired.

Issues with e-beam Lithogrpahy

• Uneven distribution of resist on the silicon- this changes the WD along the silicon so some areas will be better focused than others.

• Improper or insufficient focusing – the pattern may be under dosed, it may not allow as fine features as desired.

• Astigmatism- the pattern will lack uniform ness that may be desired.• Under/Overexposing the resist – each substrate takes a different dosage

and different sized patterns will have variations in optimal dosage.• Under/Over developing the resist• Proximity effect- features close to other features need less dosage

because of the spreading of the secondary charges.

Dosage Calibration Pattern0.25 nC/cm 0.25 nC/cm

0.25 nC/cm

0.5 nC/cm

1nC/cm

2 nC/cm

4 nC/cm

6 nC/cm

8 nC/cm

1 nC/cm

10 nC/cm

Proximity Effect

Overexposure

Astigmatism

Thinnest Lines•This pattern shows the finest line made to date. •They have a width of 58 nm at their thinnest point.•They were actually written using areas.•The dark parts of the pattern indicate the areas that were fully exposed and developed.

•The bottom picture shows how this pattern looks when written by Joe Nabity. •His wires are 30 nm wide.•His pattern is much more even

This pattern was written at a time in which the beam current was not steady and the SEM was not working optimally. The effects of this (and possibly bad focusing or charging) can be seen by the uneven spacing of the wires.

Thank You

• Jan Kycia (my supervisor)• Tong Leung• Nina Heinig• Anyone else who helped for 20s at a time or

who answered questions for me about the SEM or the NPGS program.