3 System Description
20 User Guide
3.1.3 OPTICS SYSTEM
DIGITAL MICROMIRROR DEVICE (DMD)
The DMD is an electrical input and optical output micro-electric-mechanical system
for spatial light modulation. It is composed of an array of micro light switches
containing individually controlled micro mirrors which correspond to the pixels in the
image to be displayed. Laser light is modulated by this device to project and transfer
a pattern into positive or negative resist.
LASER UNIT AND OPTICAL ELEMENTS FOR BEAM GUIDANCE
The laser beam is emitted by several laser diodes, coupled in an optical fiber and
projected on the DMD. The optical elements guide the modulated light through the
system. The light beam passes the write head and is finally projected on the
substrate.
CAMERA UNIT
The camera unit comprises one camera with low resolution, one with high resolution
and one overview camera. The cameras also feature a light source and make it
possible to inspect and measure the exposed substrates. Additionally, the cameras
are used for accurate alignment of a design with existing structures. Here, the High
Res camera is the appropriate choice due to its high resolution. In cases where the
alignment marks are larger than the field of view of the High Res camera, switching
to the macro camera is recommended. The overview camera is not appropriate for
alignment. Additionally, it does not show the complete surface of the substrate due
to mechanical limits. The camera image is transferred to the User PC via camera
software that opens in its own window. Systems with backside alignment have a
second camera unit for back side viewing mounted inside the stage and looking
pointed to the substrate.
PNEUMATIC AUTOFOCUS
The pneumatic autofocus serves to keep the focal point of the write beam stable on
the surface of the substrate. For probing the distance, compressed air passes
through the write head and leaves it through a nozzle. Once the write head is close
enough to the substrate, a pressure builds up between the nozzle and the substrate,
which is monitored by a pressure sensor. The pressure sensor transmits the
information to a controller which regulates the write head movement. The defoc
value shows the focus position of the write head with respect to the offered
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