Process Overview Polyimide Bake.
Koyo Thermo Systems

Koyo Thermo Systems Co., Ltd. is represented in Europe by
Crystec Technology Trading GmbH

Crystec
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Polyimide Bake Processvertical furnace

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.

buffer layer

There are three types of polyimide available:

chemistry of poylimide cure process

CLH oven, kiln

Non-photosensitive 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 Polyimide

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.

photo sensitive polyimide

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 necessary.

exhaust in PI process

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):

oxygen level in ppm

Please 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 laboratory.

Crystec Technology Trading GmbH will be pleased to further discuss details with you.
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