Dear Glen, I've worked pretty extensively, and have noticed this phenomenon as well. In my opinion, the following items helped to resolve this issue: 1 - Use of high acceleration to ramp up to spin off speed. I know that this is contrary to what some others on the group have said, but the principle of the high spin off makes sense. The reason for the "starfish" pattern has more to do with the surface tension of the resist than of its viscosity. Basically, once a "tendril" of resist has made it to the edge of the wafer, the path of least resistance for the flow of resist is within that "tendril", since the surface tension of the walls of the "tendril" is so high. By using high acceleration (>10,000 rpm/sec, ideally), there is a greater amount of force present to draw the resist outwards on the wafer. Additionally, the rapid acceleration motion breaks up the surface, allowing smoother flow. 2 - As one reader mentioned, the use of a cover for the coater bowl will also help more even spreading of the resist. Again, this is an attempt to reduce surface tension, this time by preventing drying of the outer surface of the resist. Because of the very high solids content (~70%), evaporation of solvent causes a very rapid increase of viscosity at the surface of the resist, and with an increase of viscosity comes an increase surface tension. The use of a cover traps evaporating solvent, and greatly reduces drying. In addition to allowing a better spread of resist, the use of a cover will improve resist uniformity as well. Best Regards, Chad Brubaker EV Group-Technology, Tel: (602) 437 9492 x 119, Fax: (602) 437 9435 E-mail: C.Brubaker@evgroup.com, Web: www.EVGroup.com, 03/15/02 -----Original Message----- From: Glen Landry [mailto:landry@anvil.nrl.navy.mil] Sent: Friday, March 15, 2002 6:24 AM To: mems-talk@memsnet.org Subject: [mems-talk] SU-8 spinning problems I have a problem spinning SU-8 2050 photoresist (Microchem corp). When I place the required amount on my 2" silicon wafer and spin, it does not spread uniformly. Instead it all goes to one side (if it is not exactly centered) or it spreads out in a starfish pattern, leaving large portions of the wafer uncovered. I have tried manually spreading the SU-8 across the wafer, with my pipette, but this introduces bubbles. It was also suggested to tilt the wafer to get the SU-8 to spread, or to sit it on a table. Neither of these suggestions worked, since the SU-8 2050 is so viscous and it takes so long to spread with gravity, a skin forms on the SU-8 before it spreads. Does anyone have suggestions? Glen Landry _______________________________________________ mems-talk@memsnet.org mailing list: to unsubscribe or change your list options, visit http://mail.mems-exchange.org/mailman/listinfo/mems-talk Hosted by the MEMS Exchange, providers of MEMS processing services. Visit us at http://www.mems-exchange.org/