Basic considerations about SEM to be used
in connection with NanoMaker for e-beam lithography
Most important thing is that microscope must be equipped with interface
for external XY scan control (most modern SEMs have). If the interface is
already used by an microanalysis or digital imaging system, a manual switch
or relay box will be required. The external scan interface must have XY scan
inputs within a ±10 Volts range with input impedance greater than 2 kOhms.
The bandwidth of external XY scan control circuit must be better then 100
kHz.
Secondary electrons image signal output in external scanning mode is required.
Maximum signal altitude in range -2... +10 Volts on 15 kOhms impedance (typically
in range of 0...+5 Volts ).
Long-term stability of probe current (less then 1%/hour drift and noise)
at typical writing accelerated voltage 25 - 30 keV. The best choice is a SEM
with Schottky field emission (SFE) source.
SEM resolution close to value declared by vendor. Good resolutions and
linear deflection along scan field of 500 x 500 um at least, if system intended
to draw large structures.
Beam current measurement is required. Measurement scheme can be different
depending on the microscope type. In some SEMs a Faraday cup located in block
of apertures can be used for measuring. You can also measure beam current
by placing a Faraday cup on a sample holder. Picoammeter is required for e-beam
lithography with the range from 1 pA to microamperes at least. It can be integrated
with a SEM or standalone.
An electrostatic beam blanker is highly recommended. A magnetic beam blanker is
suitable in most cases also. NanoMaker allows to exposure without beam blanker
at all, but this constricts the range of tasks to be accomplished. Blanker
should have electronics that will accept the TTL (or comparable) level of
signal.
An universal specimen holder with clamping springs should have sufficiently large
area to carry simultaneously a Faraday cup, calibration grid and substrate
to be exposed.
Computer controlled motorized stage is optional. Cable connection between
NanoMaker's computer port and stage controller has to be provided.
Load-lock system for sample loading is preferable because significantly
improves productivity.
