Online proceedings - EDA Publishing Association

The fabrication of optical Cap-Wafers, i.e. the fabrication of
cavity windows with high optical performance (Fig. 1b) on
wafer-level processing, poses an extra challenge for
manufacturing.
Conventional methods forming a cavity are commonly using
subtractive processes such as plasma etching, wet etching or
even more coarse processes like sandblasting and ultrasound
milling. These processes remove material from a wafer
substrate (e.g. a polished glass wafer) in unmasked areas thus
forming a cavity; whereas the material which is protected by a
masking technology remains and forms bond frames or
similar structures. It should be noted, that processing takes
place in the bottom of the cavity,
In case of an optical cavity this surface represents the optical
active surface and is, especially with shallow cavities, very
close to the focal plane of an optical sensor. Furthermore any
optical surface finish in the cavity area such as highly
polished surfaces, antireflective / filter coatings or optical
elements like gratings and apertures are damaged by the
cavity formation using subtractive processes.
These drawbacks can be avoided by using additive techniques.
In this case bond frames or similar structures are formed
on top of a high quality optical wafer by depositing material
whereas the optical area in the bottom of the cavity shall
remain intact.
Most commonly photoactive polymers are used to form the
frames like BCB or SU-8. These materials are typically
applied by spin- or spray-on, structured using lithography
where the photosensitive material is selectively exposed and
removed in a development step thus forming the bond frames
very precisely. This method is the mainstream solution for
low-end product, where the disadvantages of polymers like
limited reliability, moisture uptake, oxygen or moisture
transmission or degradation at elevated temperatures play a
minor role.
In the effort to meet reliability requirements for advanced
products stencil printing of frit-glass or solder glasses is used.
However these glass-powder based materials generate
particles during processing and the resolution of the lateral
dimensions as well as the height of the frames structures is
limited by the minimal grain size of the glass-powder, the
stencil printing process and the need for a reflow process for
post-processing the glass-slurries.
In order to overcome the limitations of todays mainstream
solutions, we propose a novel method to fabricate precise and
hermetic Cap-Wafers with high optical quality:
Shallow Cavities using Additive Microstructuring of Glass
– Lithoglas process
An advanced, additive microstructuring process of glass
(Lithoglas process) is used to fabricate a “glass-only” cap
wafer providing utmost optical performance at a very low
level of defects. The novel deposition and microstructuring of
glass allows the formation of thin films of dense borosilicate
glass on a broad range of substrates at substrate temperatures
below 100 °C [3, 4].
The deposition of the glass is done by a plasma-assisted e-
beam evaporation process. It is a high rate deposition process
11-13 May 2011, Aix-en-Provence, France
with deposition rates of several hundred nm/min and at the
same time low substrates temperature. With the high
deposition rate typical film thicknesses of 3 – 20 µm can be
achieved easily and production can be run as a cost effective
batch process. Thicker layers as thick as 100 µm have been
done in R&D – this is only possible due to the outstanding
control of stress in the deposited layers.
The glass layer can be microstructured by lift-off (Fig. 2a).
Since the deposition process is working at low temperatures
standard photo resists can be used for masking.
Process Step 1 – Lithography: As a first step photo resists
is deposited by spin-on. It is exposed by mask aligner or
stepper and developed. The photo resist carries the negative
image of the target structures in the glass layer.
Process Step 2 – Glass Deposition: The borosilicate glass
layer is deposited by plasma-assisted e- beam deposition on
the full wafer. The substrate temperature stays below 100 °C
during this process.
Process Step 3 – Lift-Off: The glass microstructures are
developed by dissolving the photo resist mask and with this
removing the glass on top of it. A structured glass layer
remains at the locations which were not covered by the resist;
areas covered by photo resist were protected trough out the
process and reveal their original surface finish after lift-off.
Depending on the layer thickness an aspect ratio of 1.8 can
be achieved yielding 1.3 µm fine structures in a 3 µm glass
layer by using a mask aligner for lithography (Fig 2b).
Glass, especially borosilicate glass, is well known for its
chemical inert behavior and stability. It is temperature stable
and hardly dissolves in most acids, bases and solvents. It is
the close-to-perfect material for semiconductor packaging in
respect to its electrical, chemical and physical properties. The
use of Lithoglas microstructured thin-film glass as passivation
and functional layer and its unique advantages is described
elsewhere [5]. In this paper we focus on use of Lithoglas as
bond frame.
Fig. 2.a) Lithoglas Process Flow – Microstructuring of
Glass by Lift-Off Borosilicate glass (blue) can be deposited at
low temperatures of below 100°C on a variety of substrates
(grey) by using plasma-assisted e-beam deposition. This
permits the microstructuring of the glass thin-film by lift-off
using standard photo resists (red)
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