Electron-beam lithography facility

The electron-beam lithography (EBL) facility enables writing patterns of arbitrary geometries with minimum features sizes as fine as 5 nm. The facility is based on an Elionix FS-125 electron-beam lithography tool operating at 125 keV with a field emission source and a laser interferometer controlled stage capable of holding wafers op to 200 mm diameter. Other features of the tool include laser height sensing of the sample, with automatic Z stage compensation, minimum beam diameter of 1.8 nm, maximum beam current of 100 nA and Proximity Effect Correction (PEC) with Genisys Beamer software.

Facility procedures

The Elionix FS-125 operates at 125 keV acceleration with a field emission source capable of 25 pA to 100 nA of beam current. The pattern generator maximum speed is 16 MHz with field sizes of 75, 150, 300 and 600 microns at 125 keV. Pixel step sizes from 0.3nm to 25 nm are available. The stage accommodates wafers from 50 mm up to 200 mm and masks to 150 mm. Up to 9 small pieces may also be mounted at one time.

The Elionix FS-125 is a user-run tool. First time e-beam lithography users should contact Mark K Mondol with a proposed process. After meeting to discuss the process the first training session can be arranged. To become a qualified user of the FS-125 requires 4-6 hours of training, after 2-3 initial training sessions users can book and use the tool on their own. However, to fully use all the functionality the tool offers requires many hours of training and experimentation.

What you need to know

There are many and varied parameters that need to be considered before writing a pattern with e-beam lithography. Here is a short list:

  • Substrate- thickness , diameter, conductivity, flatness. 
  • Resist type needed - positive (PMMA) or negative (HSQ)*. 
  • Pattern transfer- how will the pattern be transferred into/onto the substrate, does it need to be transferred? How thick does the resist need to be? 
  • What are the smallest and largest features (or spaces) in your pattern? 
  • E-beam lithography produces big patterns by stitching together smaller fields, what sort of stitching errors (between fields) can you tolerate? 
  • What are the tolerances of your pattern; i.e. if you want 100 nm wide lines will you accept 110 nm lines or even 150 nm lines? 
  • Do your patterns conform to “Manhattan geometries” (i.e. orthogonal patterns) or will you require acute/obtuse angles or curves? 
  • Do you have precise placement objectives, either in relation to other features or for specific periodicities in your pattern, what are your tolerances? 
  • What is the total written area of your pattern? A grating consisting of 2500 lines that are 500 microns long about 100 nm wide and spaced 200 nm apart (i.e. 50% coverage of an area 500 x 500 microns), in PMMA, takes about 3.5 minutes to write, Using an aggressive process with 10 nA of beam current. A more conservative approach might take 14 minutes, with a 2.5 nA beam current. In other words writing 1 cm square, of 50% duty cycle or coverage, with moderate resolution takes something like 15- 60 minutes to write in PMMA; depending on your requirements for line edge roughness. Times for HSQ exposures will be significantly longer, times for ZEP will be shorter. 
  • What format is the pattern in, or do you still need to generate the pattern data? 
  • Do you want to align the pattern to some pre-existing pattern? Will you want to align subsequent e-beam patterns to previous ones? Will you want to do an optical alignment to the e-beam patterns? Do you need to align to a previously existing optically produced pattern? 
  • How many times might you want to write the pattern? How often will you want to change the pattern you write? * Positive resists get developed away in exposed areas, negative resists leave resist behind in exposed areas.

Sponsors

The MIT Electron-Beam Lithography facility was made possible in part by generous support from the Gordon and Betty Moore Foundation, the Department of Electrical Engineering & Computer Science, Microsystems Technology Laboratories, Office of the Provost, Office of the Vice President for Research, Research Laboratory of Electronics, School of Engineering, School of Science, and faculty from across the Institute.

Contact the EBL facilities manager at sebl@rle.mit.edu with questions.