The measured conductivity of resist layers is strongly influenced not only by the temperature but also directly dependent on the air humidity. After a softbake, the now almost anhydrous layer gradually takes up water at room temperature from the ambient air due to the slightly hygroscopic properties of the polymer. The measured resistance increases significantly. In order to assess the conductivity properties of Electra 92 under the conditions of e-beam lithography, Mr. Riebe (University of Potsdam) conducted corresponding measurements in a high vacuum. In these tests, Electra 92 was coated on quartz plates (film thickness about 190 nm), the polymer film contacted with clamps, and the measurement arrangement transferred into a vacuum chamber.
Measurement arrangement to determine the conductivity under vacuum conditions
Under atmospheric conditions, a resistance of > 21 MΩ was measured. The resistance decreased rapidly to 16.6 MΩ directly after applying the vacuum (2*10-3 Torr) and finally decreased further to about 12.7 MΩ (at 2*10-4 Torr). After ventilation, the measured resistance quickly increased again to 20 MΩ.
Interestingly, almost a doubling of the measured conductivity is observed in a vacuum (the resistance is accordingly halved). Under these conditions, the conductive layer is virtually anhydrous and thus shows a higher conductivity.
In e-beam applications with a much stronger vacuum consequently a conductivity of up to about 1 S/m can be assumed, which is completely sufficient for most applications.