Using a HEBS Glass Calibration Plate
Since each user of HEBS-glass masks employs a particular
combination of a optical lithographic exposure tool, photoresist, and resist
processing, an experimental relationship of photoresist thickness vs. mask
density must be established by the user using a HEBS-glass calibration
plate. Gray level masks of your designed pattern having predetermined gray
levels can be then ordered from Canyon Materials, Inc.
Most binary photoresists (eg. positive and non-chemically
amplified Novalac based photoresist) maybe used to produce analog resist
profile using an analog processing scheme. An analog resist process makes
the contrast curve more linear and reduces the threshold effect. An analog
resist process can be derived from the standard binary process with the
following modification in the resist processing steps:
Since the dynamic range of photoresists in an analog
resist process is in general limited to about unity in log scale, an optical
density range of 1.2 or less is in general sufficient for a gray scale
A lower temperature and/or shorter duration in the pre-bake
The concentration of the developer is another variable
that can be used to achieve a desirable analog resist property.
It may be necessary to establish a new calibration
of resist thickness vs. mask density, whenever the parameters of the optical
exposure tool or photoresist or of photoresist development process are
altered. The calibration plate should be kept for future uses.
A standard HEBS-glass e-beam mask is a clear field mask.
The background areas of the plate are clear and transparent. Nevertheless,
a dark field HEBS-glass gray scale mask that is black outside the patterned
mask area is also available from Canyon Materials, Inc.. HEBS-glass mask
having a dark background outside the pattern area is fabricated by darkening
the area surrounding the mask pattern with a flood electron gun.