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ELECRON BEAM LITHOGRAPHY
General points:
Electron Beam Lithography (EBL) uses, on our machine, a focused beam in order to expose electrosensible polymers spin coated on a substrate to be structured.
The trajectory of the electron beam is linked to a drawing made by CAD/DAD software. Every exposure is made point per point. The roughness of the pattern obtained is thus dependant on the distance between these points.
After development, the resist can be further used as a mask in classical steps of micro/nanofabrication: wet or dry etching, lift-off, electrochemical growth, ….
An EBL writer with a convergent beam uses electron lenses that don't allow the use of very big field size without distortion. The resolution is thus dependant on the size of the field. Field size of up to 2x2mm2 can be used for microstructures realization while 100x100µm2 fields are generally used for nanostructures fabrication.
Laser Interferometer Stage:
In order to realize structures larger than the field size, stitching of the fields must be controlled. This function is performed by a Laser Interferometer which allows to know the mechanical position of the stage with a precision of 2nm. This Laser
Interferometer is thus used for: field size calibration and stitching, measurement of drifts (beam & mechanics) and alignment between multiple levels.
Substrates used:
We work on all kind of substrates: semiconductors Si, GaAs, GaSb, InP, and their compounds), glass, … with a size ranging from 4x4mm
Due to the use of charged particles, samples must be conductive or made conductive.
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E-beam writer RAITH150
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| Know-How : |
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The resolution depends on the parameters of the exposure: voltage, current, working distance, field size, sample and resist.
On our machine 20nm resolutionin positive resist can be achieved
(Figure 1) and 50nm using a negative one (Figure 2).
The resist used depends on its own properties and its use.
Positive resists:
PMMA (High resolution, "low" ICP-RIE resistance) and ZEP520 (better resistance to ICP-RIE)
Figure 1 : line grating in 150nm thick PMMA- 22nm linewidth and a period of 80nm
Negative resists used:
maN2403 and SU8 (extremely sensible - 1µC/cm2), both having a good resistance to ICP-RIE.
Figure 2: isolated line of 50nm width (at the bottom) viewed at 45° in 300nm thick maN2403.
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