E-beam Lithography Using NPGS System

Introduction  

The NPGS card connected at the back of the computer provides the voltage to X and Y scan coil of of the electron beam. It also provides TTL signal to the beam blanker. The beam blanker accepts 0V and 5V as trigger signal to blank the beam.

The NPGS software also communicate with XL30 ESEM software through serial port. The XL30 software accepts some command also known as "opcode" which can be used to remotely control the SEM microscope. The NPGS software can send command to XL30 software to read working distance (WD), move the stage, change magnification etc.  

Check List

1) Faraday Cup
2) Gold standard sample or 100nm colloidal gold nanoparticles (by from here . (Get 50nm colloidal gold nanoparticles as 100nm is too big). See SEM images at end

You can either use different stubs for Faraday cup, gold standard and your sample or buy e-beam lithography stub which as everything in one stub
   

You can buy this stub from here
 

2) 3% PMMA e-beam resist.
3) e-beam resist developer

4) Iso-propanol


1) Cleaning substrate 

1) Put Si substrate in spin coater vacuum chuck. Check if vacuum is holding substrate. Set the rpm at 500-600rpm, set timer to some large value say (10min) and start spin coater.

2) While sample is rotating, squirt acetone on substrate thoroughly from center to circumference. While continuing to squirt acetone, start also squirting IPA, this will make sure that you acetone doesn't dry before squirting IPA. Stop squirting acetone and continue to thoroughly was substrate with IPA. After that start squirting DI water and stop squirting IPA. After thoroughly washing with DI water, either increase the RPM to 4000 or use nitrogen gun to dry sample.

Step – 1: Sample Preparation

1) Spin coat 3% PMMA solution on silicon wafer at 4000rpm for 45sec. This will coat about ~300nm of PMMA on silicon substrate. It is necessary to have clean environment during spin coating otherwise one can observe strain due to dust particles on Si wafer. One can use disposable droppers to avoid dust particles during spin coating. Use the entire wafer as compare to small chips while spin coating. (Note that, PMMA coated wafers can be used for many months as long as kept in dark).
2) Bake the wafer on hotplate at 180C for 60s.

3) Scribe number 1, 2, 3 and 4 on the four corner of the sample just enough to only scratch the PMMA layer not the Si substrate itself. These scribes will be very useful later to focus the electron beam. You can also put 100-300nL of 50nm gold nano-particles at four corner, this will help you focus and condition electron beam.

Step - 2: Conditioning Electron Beam using Gold Standard

1) Change the setting of connector inside SEM so that beam current can be measured with external pico-ammeter. Since this disable automatic "sample touch alarm" be careful while moving samples.
2) Mount the sample on e-beam sample holder near the gold standard sample.  Put the sample inside the SEM chamber with working distance (WD) of ~5mm. WD determines largest patterning area. For small features use 30kv or more and spot size 1.

3)  To condition electron beam, navigate to gold standard sample and focus the beam. Adjust the astigmatism to get the best image at higher magnification (~1000,000x). With my system, the best image I was able to get is shown below


image here

Once the beam is conditioned, don’t change the focus setting from now onwards until patterning.

If you have an image of your sample holder then use it to navigate to Au standard sample. In my case Au sample is on the right side of the Faraday cup.

Step - 3:  Beam Current Measurement using Faraday Cup

1) Navigate to Faraday cup avoiding looking into the e-beam resist coated sample. 
2) To Focus on the Faraday cup don't use the normal control but change the z-height of the stage. 
3) Once the Faraday cup is in focus by changing sample stage height zoom into the black hole until the entire screen is black. Wait for 10 minutes to stabilize the beam in the Faraday cup and measure the current of ammeter.

Below is the Voltage/Spotsize and Current (as of 05/05/2014) for my XL30 system

kv  spotsize  current
10  
10  3 110.5pA

4) Enter this value into the NPGS software.

Step - 4: Writing Pattern

1) Navigate to one of the corner of the sample and focus by changing the z - value of the sample stage. Use the scribes you made on the surface of PMMA to focus.

2) In NPGS software go to Commands -> Direct Stage Control
The software will ask you to enter "rotation value". Hit Enter to skip that. The software will than ask you to focus on corner and hit Enter. This will collect the working distance. Repeat this step for all four corners. The software will that determine flatness of the sample surface. (you can skip this step if you don't have stage controller)
3) Turn on beam blanker and set the beam to "ON". 
4) Quickly navigate to the location you want to write and the set the beam in beam blanker to "EXT". This will give beam blanker control to NPGS software which by default blanks the beam. 

Note: Its good idea to set high contrast (~50) and low Brightness (~30) so that one can see the affect of e-beam exposure on PMMA during navigation.
5) Open the .rf6 file, enter the value of the beam current and select the desired magnification as indicated by maxmag or run file
6) In XL30 software, set Scan is as External XY.

7) Run the .rf6 file.The NPGS software will itself open the beam blanker and start writing pattern. This can be seen in ammeter as the current value increases from zero to finite value.

 

Step - 5: PMMA Development

1) Immerse sample in developer solution ( 3:1 IPA:MIBK) and put ultrasonic bath for 70 seconds.
2) Rinse sample with IPA and dry with nitrogen.
3) Post bake at 95 degree for 30 minutes in conventional oven.

Step - 6: SEM Imaging of Written Pattern 

1) Deposit 5nm of Au using Physical Vapor Deposition or Sputtering
2) For Imaging use 5kV and spot size - 3.
3) Ideally the detector should be TLD but I don't have that in my ESEM system.
Note - At higher magnification of about ~ 10,000 I could see the resist getting exposed and modified due to scanning (can be seen as a box in all the images below)

Recommendation to get fine Feature Size


- In general higher accelerating voltage will give least amount of back-scattering at the surface and consequently the smallest line-width

Advance Topics

Note – When writing the dual layer patter, the software communicate with XL30 software via serial port and changes the magnification by itself.


Appendix

1) SEM image of 100nm Au colloidal nanoparticles