Process Overview Polyimide Bake
Koyo Thermo Systems is represented in Europe by
Crystec Technology Trading GmbH
Polyimide Bake Process
Koyo Thermo Systems furnaces are available for
many processes. Koyo Thermo Systems has an application laboratory in Tenri, Nara, Japan and
an application center in Uppsala, Europe in cooperation with the
University of Uppsala. The technology of new furnaces and new processes are
developed in close contact with the customers. Koyo Thermo Systems can also
assist the customer to fulfill special process demands. On these page, we
discuss the polyimide bake process, which is a speciality of Koyo Thermo
Systems. Koyo Thermo Systems is a real specialist for baking of polyimide.
Application and Chemistry
Polyimides are high temperature engineering polymers with excellent
mechanical, thermal, and electrical properties. Microelectronic applications
include stress buffer, passivation layer, chip bonding, and interlayer
dielectric. Polyimides are typically applied in liquid form, then thermally
cured into a film or layer with the desired properties. For most products,
the film can be patterned using photographic processes.
Most customers of this process use it as an adhesion promoter for the
molding compound and for buffer coating in order to reduce product losses
during IC packing as well as short life time problems created by temperature
variations. Polyimide layers also help to avoid problems, in case that the
assembly is done in a different factory than the IC-production.
The polyimide layer reduces the stress in the silicon that is created
by the package of the chip and avoids cracks at the edges. The polyimide
has to be cured in a furnace with a very good temperature uniformity in order
to avoid cracks in the polyimide layer and colour variations. Colour uniformity
is important for the pattern recognition systems, used in the assembly procedure.
Low oxygen values are necessary during this process in order to get bright
material and good adhesion.
There are three types of polyimide available:
- Non-photosensitive polyimide
- Photosensitive, ionic type polyimide
- Photosensitive, ester type polyimide
Non-photosensitive polyimide is rather cheap
and easy to handle. By-products that occur during thermal curing are liquid and do not form hard
coatings within short time. For this type of polyimide, you can use our
clean oven type CLH. A special version has been
developed for polyimide baking.
Photosensitive Material shows better process
performance e.g. higher resolution, has a wider margin in photolithography steps and a longer
shelf-life. By using photosensitive Polyimide, the number of process steps can be
reduced from 9 to 4. This is a big saving and the higher costs for a Koyo
Thermo Systems vertical bake furnace that is necessary for photosensitive
polyimide baking compared with a Koyo Thermo Systems conventional bake oven can
be earned back within short time. By introducing direct etching you can
save even 3 more process steps.
Ester bond types are more stable than ionic
types and have a longer lifetime in the shelf. The viscosity is not so much age
dependent and the contrast between exposed and unexposed areas is higher. The solubility
of unexposed areas is better.
Koyo Thermo Systems has big experience
with the baking of photosensitive polyimide. Together with Asahi, a new
vertical furnace concept has been
developed especially for this process and meanwhile >90 vertical
furnaces of Koyo Thermo Systems have been sold for this application.
However photosensitive polyimide,
especially the ester type with the best performance of all
polyimides, creates byproducts during the curing process that are very
difficult to remove after the deposition. Together with one of the
largest polyimide producers, the company of Asahi Chemicals, Japan,
Koyo Thermo Systems developed a special model of a vertical furnace.
Special LGO heating elements make sure that
you will get an excellent temperature control and temperature
uniformity at low temperatures (350°C - 400°C) that are used
normally for this cure process. In addition Koyo Thermo Systems
installs a special quartzware in this furnace for this process, that
avoids the deposition of curing by-products. An efficient exhaust is
The furnace is maintenance free and
we can prove the superior performance of this furnace type by our very
long reference list of satisfied customers. A vertical furnace that
combines hydrogen anneal and polyimide process is available now also.
For the performance of the polyimide it is very important, that the
oxygen content during the process is less than 20ppm. In the Koyo furnace,
values down to 8 ppm oxygen can be achieved. This low oxygen content does
not require evacuation! Koyo Thermo Systems developed an atmospheric furnace
(which is much cheaper), that can fulfil the needs for polyimide cure
and deliver excellent process results. The oxygen is measured by using a
zirconia oxygen sensor. In the following graph
you can see the decrease of the oxygen level over the time (from right to
left, using several scale changes for the oxygen concentration):
contact us in order to receive more information about polyimide
and about our equipment. Test runs can be done on our furnace installed in the Tenri application