2 19-Oct-2007
Product Description
The SC22 is a fourth generation Magnetic Field
Cancelling System, designed to improve the
performance of electronic instruments that are
sensitive to magnetic fields, such as electron
microscopes and electron beam metrology tools.
The SC22 is a replacement for the SC12 system,
which has an installed base of over 1000 units
world wide.
It is important to note that mechanical vibration,
acoustic noise and ground loops in the electron
beam tool installation can produce imaging
defects similar to magnetic fields. The SC22
cannot improve images which are affected by
these other interfering sources because they are
not magnetic fields.
A typical SC22 system installation on an SEM is
shown in Fig. 1. The control unit is not shown.
The cables make one turn and are shown in red,
green and blue. The actual cables are grey and
usually installed in white plastic conduits. Where
the room has a false ceiling, the Z cable is usually
installed above it. The magnetic field sensor is
located close to the bottom of the electron beam
column. An optional mount enables it to be
strapped to the column if required.
The amount by which the field is reduced is
determined by the loop gain of the system, which
is automatically set by the SC22 to 50 times. The
system does not cancel the earth’s DC magnetic
field, nor does it cancel the field everywhere in
Fig. 1 Typical SEM Room Installation
the room. It creates a region around the magnetic
field sensor where the AC field is much reduced.
The volume of this region depends mostly on the
gradient of the ambient field and the positioning
of the field cables.
The SC22 control unit displays the amplitudes of
the X, Y & Z field components and the total
vector field on its LCD panel. Tesla and Gauss
units, RMS & pk-pk can be selected. The
measuring system can resolve 1µG (100 pT) field
changes. The real-time measured fields are
available on front panel BNC’s as analog voltage
levels for oscilloscope display.
The magnetic field amplitude is continuously
monitored and compared with preset “trip levels”
to provide “GO/NOGO” indication of the field
quality. The LCD panel and a small green LED
on the sensor indicate that the field is “OK”.
The SC22 controls are much simpler than the
SC12 and are supervised by the embedded
microcomputer. There are just 3 control buttons
on the front panel. The “units” button enables
choice of the displayed field units. It has no effect
on field cancelling. The “setup” button starts a 4
second program that measures and sets the gain
and phase of the feedback loop. The “cancel/
standby” button turns cancelling on and off.
