2

Choice of Photoresist

Files of this type can be patterned using common

photolithography techniques centered around positive

photoresist. The underlying polyimide is an effective

anti-reflective layer as there is significant absorption

between 350nm and 450nm. This absorption can

significantly reduce substrate reflection effects on the

photoresist, usually seen as “notching“ after

development.

The photoresist should be selected with the correct

wavelength to suit the exposure tool in use. As a

general guide, formulations with good adhesion in

“wet etch“ semiconductor applications perform well.

Other attributes include:

•

Compatability to standard alkaline positive

photoresist developers

•

Low contrast performance so that a soft sidewall

profile is always produced

•

Capability to produce cleanly developed via holes

in thick resist coatings

•

Good development latitude, especially when

over-developed

•

The ease of producing a minimum dried 2.5 µm

film thickness

The coating after softbake has minimum solubility in

typical photoresist solvents. Photoresist can therefore

be coated directly onto the polyimide coating without

layer inter-mixing occuring.

Photoresist Application

Substrates should be coated directly with the resist

selected. No dehydration bake should be given as

this would make the polyimide totally insoluble in the

developer. Instead an HMDS vapor prime is

permissable if installed on-line but is not really

neccessary for good resist adhesion to the polyimide.

Once coated, the resist should be given a softbake at

90°C either in a convection oven for 30 minutes or on

a vacuum hotplate for 60 seconds. Once coated and

baked, coatings may be held up to 24 hours before

exposure.

Photoresist Exposure

Typical exposure: 50 mJ to 150 mJ

Once exposed, development should take place within

8 hours.

Spin Speed Curves for Cured Polyimide

Pyralin PI 2525/PI2574

(30 s spin; 120 s at 100°C

Cure: 30 min at 200°C + 30 min at 350°C)

5

7

9

11

13

2000

3000

4000

5000

Spin speed (rpm)

Spin Speed Curve Pyralin PI 2555

(30 s spin; 120 s at 100°C

Cure: 30 min at 200°C + 30 min at 350°C)

1.5

2.5

3.5

4.5

2000

3000

4000

5000

Spin speed (rpm)

Photoresist Development

Polyimide Precursor Etch

A single step is used to develop the photoresist and

etch the polyimide precursor.

Most alkaline positive resist developers will dissolve

both exposed photoresist and polyimide precursor at

varying rates. The choice of developer affects the

quality of the image after development. Best results

have been obtained using a NaOH based developer.

Thin layers up to around 5 µm can usually be

developed quickly and cleanly with developer which is

at ambient temperature. After development using a

spray or puddle technique, a water rinse should be

used to remove the developer. The substrate should

subsequently be spun until dry.