Electron Beam Lithography

Technical Staff

Principle

We use, on our electron-beam writer, a convergent beam to expose a resist, electrosensitive, coated on a sample to be structured.
The beam is driven according to a drawing designed by a computer assisted design software. Each drawing is made point per point. After development, the resist can be used as a mask and the sample can follow a typical micro/nano fabrication flowchart: dry or wet etching, lift-off, …

Substrates

We work on all kind of substrates which size can vary from 4mm to 150mm.  Their thickness mustn’t exceed 5mm.

The use of charged particles lead to work on conductive samples or made conductive (contact us).

Laser interferometric measurement role

An e-beam writer uses electron optics which does not allow to use big fields without distortions.  In order to realize a structure bigger than a field size, stitching between fields is needed.

To do so, our machine is fitted with a laser interferometric measurement which allows to know the sample position with a precision of 2nm. This precision gives us the possibility to calibrate the field size and its position as well as the distortion measurement. According to these measurements, a software calculates the offsets to be applied and allow: fields stitching, drift compensation and  overlay.

The global precision obtained for these operations is at best 10nm and at worst 60nm.

The resolution is field dependent. We use the following field-size:

- nanostructures :100x100µm2 
- microstructures 1x1mm2 

Facilities


Electron beam writer :

RAITH150 : up to 30kV



Other equipements


Coating System:
GATAN PECS

Precision Etching Coating System for: Cr, Ni, Au, SiO2, AuPd, Co, … for thin films deposition (<100nm) on samples size lower than 1"

Know How

The size of the realized structures depend on the combination of : e-beam parameters (current, energy, working distance field size), substrate, resist and developer.


We can achieve resolution of:

  • 20nm on positive organic resist (positive means that the exposed area is removed - Figure 1)
  • 50nm on negative organic (negative means that the exposed area is kept - Figure 2)
  • 6nm on negative inorganic resist (Figure 3)

The resist used depend on the purpose : contact us.


Positive organic resists


  • PMMA : high resolution, low reactive etching resistance
  • ZEP520 : better reactive ion etching resistance


Fig.1 - 22nm linewidth grating with 80nm period in PMMA (150nm thick)


Negative organic resists


SU8 : extremely sensitive (1 µC/cm2 @ 30kV) – better reactive ion etching resistance than PMMA.

maN2403: better reactive ion etching resistance than PMMA.

Fig.2 - 45° tilt view of 50nm width insolated line in 300nm thick maN2403



Inorganic negative resist

Backscattered electrons are mainly involved in inorganic resist exposure leading to resolution improvement (Figure 3) and period improvement (Figure 4).
Due to that, the main drawback is the increase of dose exposure and consequently the proportional increase of exposure time.

  • HSQ : XR1541 – 40nm thick



Fig.3 - HSQ XR-1541 : isolated line of 6-10 nm width – roughness is mainly due to e-beam writer mechanical perturbations





Fig.4 - HSQ XR-1541 : 20nm linewidth grating with 50nm pitch without residues - roughness is mainly due to e-beam writer mechanical perturbations


  • HSQ : diluted Fox-15 – 150nm thick exhibiting vertical walls without residues between pillars




Fig.5 – diluted HSQ Fox-15 - 22nm width pillars grating with 150nm spacing without residues