Spin Coating PEDOT:PSS

[prix27]

I have got a Si wafer 4" in diameter. I am cleaning it using Piranha solution. I am spin coating water soluble PEDOTSS on top of it with static dispensing ( I am dispensing the material around 5ml before the spin starts). The material I deposit covers the substarte completely. I am spinning at the speed of 4000 rpm for 1min.

The issue is I am always getting comet tail defect or streak through out the wafer after spinning.

Before spin coating I am using syringe filter to filter out the particles from the solution. I am using a laurell 400 lte spin coater without a fragment adapter. I am just lying the wafer on chuck as the wafer is too big.

Please help. Your suggestions are highly appreciated. Please let me know IF I am doing anything wrong.

Best Regards
Prix

 

[IRstuff]

Can you actually see particulates in the "streaks"

Is it possible that the film top layer is crusting and you're seeing a wrinkling?

Have you talked to your spin coater's manufacturer, or talked to whoever has acquired Shipley or MacDermid? Their apps engineers might have some insight.

TTFN
faq731-376

 

[prix27]

No I cant see the particulates. I an seeing just streaks from center. I dont know what to call this defects. The chances of comet tail defects are very rare as I am filtering the solution before spin coating.

Manufacturer has suggestion of playing with speed and time. I played with it. Its not helping.

 

[IRstuff]

Is there any way to confirm or deny some sort of crusting? What about mixing? Viscosity? Surface prep?

I did find this:

http://www.ossila.com/support/Fab_pedot_pss.php

also:

http://www.ossila.com/oled_opv_ofet_catalogue3/M121-PEDOT-PSS(AI 4083).php

5000 rpm at 30s for that one.

TTFN
faq731-376

 

[prix27]

I am using same PEDOTSS which you mentioned..I am using it as it is. the viscosity is same as of water....I dint try dynamic dispensing as it never worked for me.....I have tried 5000 rpm for 1 min...and it dint work either...Thanks for you suggestions though...

 

[TomDOT]

Is your surface at a truly pristine level of cleanliness before dispensing? Is your Piranha fresh, with good peroxide? 3:1 ratio? Have you tried increasing your dwell time in the Piranha?

Alternately:

Are the streaks thinner or thicker than the main coating film?

Is your dispersion flocculating? ie - is it really homogeneous before dispensing? Loose flocs can reform fairly quickly.

 

[prix27]

I am keeping my wafer in piranha for 45 mins. .then I rinse the wafer three times in different beaker of DI water...Then dry it with N2....my H2SO4:H2O2 ratio is 3:1.....Th streaks become thinner as it hoes away from the center.. ...

AT first I am using syringe to filter out the solutions...and then couple of minutes of stirring....and then finally spin coating it using static dispensing method

Thank You.

 

[TomDOT]

Peroxide's fresh, and you're making up fresh batches of Piranha?

 

[IRstuff]

Can you tell whether the streaks are on the surface of the film, in the bulk of the film, or on the surface of the wafer? I'm thinking maybe you're trapping some air during the dispense?

TTFN
faq731-376

 

[prix27]

First of all thank you for all the inputs that you are giving...yes H2o2 is fresh and piranha is from new batch made just 10 mins before cleaning...

@IRstuff- That might be the case...I am dispensing it manually using a 5 ml pipette..how to avoid this air during dispense...while dispensing I am keeping the tip at 1 cm above the substrate and dispensing it slowly..may be a pause in between just to verify there is no excess liquid...any tips on how to dispense it manually by avoiding air...

Automatic dispenser might solve this issue but I dont have one...

 

[IRstuff]

It's been a LONG while since I've seen a spin coater in operation, but my recollection was that it was continuous and relatively quick. The height was probably 0.5cm, and there might have been a slow spin followed by the high speed spin.

TTFN
faq731-376

 

[TomDOT]

When drawing liquid into a pipette, ensure the tip is deep enough in the liquid to avoid drawing in air. The more viscous the liquid and more rapid the draw, the deeper you need to have the tip.

 

[rsw11]

Might try a step wise spin coat program, 10 seconds @ 500 rpm (accel 100 rpm/sec) + 10 sec @ 1500 (accel 300 rpm/sec) rpm then to the needed rpm to achieve the coating thickness needed. Comet like defects are often from too high a speed and/or acceleration rate.