Related pages: Vacuum Oven at BRK2031

What's suitable for use in vacuum?

The NASA Outgassing Database (potentially more modern link here) may be used as a guide for materials suitable for use in vacuum. This report includes data on "materials meeting two criteria: a maximum total mass loss (TML) of 1.0 percent and maximum collected volatile condensable material (CVCM) of 0.10 percent." Similarly, materials with TML < 1% are CVCM <0.1% are generally judged as suitable for use the flexible compatible high vacuum systems. The "clean" systems may generally be even more restrictive.

Polyimide

Spin on films

Spin on polyimide may come as photoresist, or non-photosensitive forms.

These spin on formulations rely on NMP (or NMP + hydrocarbon) carrier solvent. The mixture is a combination of polyimide precursors, which only form polyimide when properly thermally cured (a thermoset polymer formulation). This requires significantly elevated temperatures (~300 C is typical) compared to normal photoresist. Uncured polyimide precursor consists of low molecular weight components that will readily outgass in vacuum. Curing proceeds via step-growth polymerization, meaning that low molecular weight component will predominate until very close to the curing temperature.

From the PI-2500 Series Product Bulletin:

"The cure heating cycle imidizes the polyimide precursor converting it to a polyimide and driving out remaining solvent. This process requires elevated temperatures and controlled environments to achieve the best results. There is sufficient energy at 180°C to complete the imidization of the polyimide, but higher temperatures are required to completely drive off solvents, thus achieving the ultimate electrical and mechanical properties. A programmable high temperature oven with typical nitrogen flow rate of 10 litres per minute is recommended for best results. To activate the adhesion promoter, it is recommended that the cure be carried out up until 200°C in air (min 50% RH). Above this temperature, a nitrogen atmosphere should be used. The ramp rates (up and down) should be low to avoid high stress in the polyimide. The maximum cure temperature may be higher than 200°C when the coating is to be subjected to a high temperature process after curing. In such cases, temperatures up to 400°C have been used to ensure that there is no outgassing during subsequent processes."

Solvent components remain present until the crosslinking process is completed (Kotera 2000 and Chen 2017).

Tape

Very small amounts of Kapton tape may be used in the organic systems, as long as it is based on acrylic adhesive. The acrylic adhesive will be the main outgassing component, and will outgass acrylic monomers and some other low molecular weight products. This has been directly reported [Laikhtman 2009, color here], and it was noted (as would be expected) that outgassing greatly increases with temperature.

Vendors are Caplinq, Polyonics, and AccuGlass.

For thin tapes, the thinnest available is Polyonics XT-701.  

Substrates

Commercial Kapton film may be used as a substrate in the organic systems after properly cleaned via the TAI standard clean.