3

When thicker polyimide layers need to be processed,

it is often beneficial to heat the developer to between

23°C and 25°C. This accelerates the dissolution of

the polyimide precursor while having minimal effect

on the solubility of the photoresist. In more extreme

situations, this may be coupled to a double puddle

process. The second puddle is used to develop only

the polyimide precursor layer. Once developed,

wafers may be held up to 8 hours before stripping the

resist.

Resist Strip

After developing, the photoresist needs to be stripped

off the polyimide precursor surface before curing.

This step is usually carried out on automated track

equipment to reduce surface contamination with

resist residue which may result if clean solvents are

not used. Resist solvent strippers are normally used.

Cure

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.

Storage/Shelf Life

Pyralin

® PI 2525 and PI 2555 are stable at

cleanroom temperatures (21°C) for about three

weeks with no significant change in properties.

When stored at -18°C, shelf-life is two years from

date of manufacture. PI 2574 should be used within

one week at room temperature and has a frozen

shelf life of one year. Moisture contamination is

detrimental to stability and must be avoided.

Containers should be brought to room temperature

before opening to avoid moisture condensation inside

the botttle.

Example of Typical

Process Conditions

Application of Adhesion Promoter

(not required for PI 2574)

(VM 652 or diluted VM 651)

Dispense on static substrate, 3 seconds

Hold for 20 seconds

Spin Dry for 30 Seconds

Apply Polyimide Precursor Coating

Dispense on static substrate

Spread at 500rpm for 5 seconds

Spin at final speed for 30 seconds

EBR / Backside rinse, 10 seconds

Spin Dry, 15 seconds

Hot plate bake at 120

oC for 30 seconds,

followed by 150

oC for 30 seconds.

Coat Photoresist

Dispense, 3 seconds

Spread at 500rpm for 5 seconds

Spin at final speed for 30 seconds

EBR / Backside rinse, 10 seconds

Spin Dry for 15 seconds.

Contact hot plate bake at 90

oC for 60

seconds

Expose Photoresist – 50mj to 150mj

Develop Photoresist / Polyimide Etch

Developer: TMAH, DE-1000, KOH or NaOH

Rinse: DI water

Double Puddle Development Process:

Spray (100rpm)

5 seconds

Puddle

20 seconds

Spray (100rpm)

5 seconds

Puddle

20 seconds

Rinse (1000rpm)

15 seconds

Spin Dry (5000rpm)

15 seconds

Resist Strip

Stripper: PGMEA, acetone, N-Butlyacetate

Spray (100rpm)

5 seconds

Puddle

20 seconds

Spray (100rpm)

5 seconds

Puddle

20 seconds

Spin Dry (5000rpm)

15 seconds

Polyimide Cure (in Nitrogen)

Heat from RT to 200

oC, ramp rate 4oC/min

Hold 200

oC, 30 minutes

Heat to 300

oC, ramp rate 2.5oC/min

Hold at 300

oC for 60 minutes

Gradual cooling to RT