PROCESS YIELDS HIGH PRECISION
WITHOUT MATERIAL STRESSES
MARTIN HERMANS, JENS GOTTMANN,
and JÜRGEN ORTMANN
Selective laser-induced etching (SLE) is a new laser technology for rap- idly manufacturing true 3D high-precision devices made from transparent material such as fused silica, ultralow expansion glass (ULE), or sapphire. With SLE, parts can be produced that consist of cavities, tunnels, arbitrary undercuts,
and even mounted moving parts (FIGURE 1).
SLE: a two-step process
In a first step, ultrafast laser radiation is focused to microm-
eter-sized spots into a material that is transparent to the
wavelength of the laser radiation used. The laser radiation is
absorbed, only at the focal spot, by the material because of
nonlinear absorption processes occurring at the high intensi-ties that are applied (>1012W/cm2). The absorbed energy leads
to internal heating and subsequent quenching of the material in
a very confined volume, resulting in a permanent modification
of the transparent material. This laser modification does not
consist of cracks and can be applied with extreme precision.
This should not be confused with laser-produced 3D pictures
in glass that are widely available, although there are some simi-larities. Line by line and layer by layer, a complete 3D connected
volume is exposed inside the glass by 3D scanning of the focus.
Then, in the second step, the workpiece is taken off the laser
machine and placed into an etching bath, where only the modified material is dissolved in the fluid etching chemical. The
etching starts at the surface and works its way into the workpiece, washing out all the material that was previously modified with the laser radiation.
The high precision of SLE technology comes from extraordi-
narily high selectivity, as the acronym suggests—selectivity is
the ratio of the etching rates of modified material vs. untreated
material. For example, the selectivity found in fused silica is
larger than 1000: 1, resulting in long, fine channels with small
conicity coming from one single line of laser irradiation with
subsequent etching. Other transparent materials showing ele-
vated selectivities during the SLE process are, for example,
Borofloat33, sapphire, ULE, or soda lime glasses. High selec-
tivity is the basis for more complex 3D structures, as they are
produced by stacking lines of laser-induced modification next
to and on top of each other.
Advantages of SLE are the high precision that is possible
(±1µm), no remaining stresses in the material after etching,
and true 3D capability. Because of the several analogies to 3D
printing, SLE technology can be considered as 3D printing for
transparent materials, with the difference being that it works
subtractively. Contrary to additive 3D printing, there is no need
for supporting structures that must be removed afterwards.
In the SLE process, the remaining glass or crystal is the
original one from the datasheets with all the known specifica-
tions, as the changed glass material is removed by wet-chem-
ical etching. The advantage arising from this is that there is no
need for new certification of materials that are already certified
for their specific application. Although SLE surfaces feature an
initial roughness on the order of Ra ~200nm, the surfaces do
FIGURE 1. Selective
laser etching produces
parts that consist of
cavities, tunnels, arbitrary
undercuts, and even
mounted moving parts.