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Rev. November 2012

PRODUCT BULLETIN

P I 2525, P I 2555 & P I 2574

PI 2525, PI 2555 and PI 2574 polyimide precursors

are suitable for applications where the high

temperatures typically used for polyimide curing

(350°C) cause problems. Typical applications for

these materials are as stress buffer or interlayer

dielectric layers over low temperature substrates.

These materials imidize faster and at lower

temperatures than for standard polyimide precursors.

The materials are supplied as solutions suitable for

spin or roller coating application. A separate

adhesion promoter is recommended with PI 2525

and PI 2555 to improve adhesion to oxides and to

metals. PI 2574 is self priming and does not require

an adhesion promoter. Processing by wet or plasma

etch is possible. Cured film thicknesses from 2 µm to

9 µm can be obtained.

Process Details

Coating

These products can be coated onto a variety of

metals, alloys, semiconductor and ceramic

substrates. HD MicroSystems adhesion promoters

VM651 or VM-652 are recommended to provide good

processability and adhesion with PI 2525 and PI

2555. Bonding of the polyimide precursor to the

substrate is achieved during the softbake cycle as the

priming chemistry is activated by temperature.

These solutions are highly viscous. There are some

guiding principles for dispensing materials of this

type. Always coat substrates which are at room

temperature. Never trap air into the solution. This

can occur for example when the solution is moved

during dispense. All bubbles take time to dissipate

out of solution. If left in, coating "comets“ will result.

Dispensing should be in the centre and as close to

the substrate as possible. A clean cut-off at dispense

is neccessary before the spin process starts. It may

be neccessary in the case of highly viscous solutions

to have a short delay prior to spin to allow the

polyimide to flow as far as possible and relax.

Both static and dynamic dispense may be used.

Static dispense is the easiest, but requires more

material to be dispensed for each substrate.

Dynamic dispense uses less material, but requires

greater control during dispense to ensure that the

polyimide strikes the exact centre of the substrate.

Any deviation will result in poor coating quality. The

acceleration to final speed should be as low as

possible to allow gradual flow of the polyimide across

the substrate. Often one or more intermediate spin

speeds are used to allow the polyimide to gradually

cover more than 80% of the substrate before

continuing on to the final speed. To reduce the

backside contamination potential it is often beneficial

to prolong the spread cycle until the bulk of the

excess polyimide has been removed from the

substrate.

The final spin speed and time is determined by the

film thickness required (see spin speed curves on

following page. Longer spin times will improve

coating uniformity, but will also reduce the film

thickness. In semiconductor applications, an Edge

Bead Removal (EBR) and Backside Rinse process

may be added to the coating cycle to remove

polyimide precursor from the edge and back of the

wafer prior to baking. NMP (N-methyl-2-pyrrolidone)

or NMP/IPA (isopropanol) can be used for this

purpose.

Soft Bake

After application of the polyimide, a bake process is

required. Both convection oven and hotplate bake

methods may be used. The purpose of this stage is

to partially cure the polyimide precursor prior to

patterning. This bake stage leaves the coating dry,

yet soluble in the etchant solution.