The Photoresist Analog Transfer Scheme for HEBS-Glass Photomasks

Photoresists designed for IC fabrication are binary photoresists. Due to the existence of a threshhold effect, binary microstructures are produced with conventional photolithographic processes using all types of photomasks including chrome masks. To create binary structures in photoresist, it is necessary that the optical density (OD) of chrome is more than 1.2. When the OD of the mask is much below 1.2, even with the binary photoresist, there exists a dynamic range where the developed resist thickness is a function of the optical density of the photomask. This dynamic range can be extended by the analog transfer scheme described below. The dynamic range of photoresist in the analog transfer scheme, in general, cooresponds to OD values in the range of 0.1 to 0.85. The dynamic range of a positive non-chemically amplified, Novalac-based photoresist can, in general, be extended to about 1.0 in OD value.

• Clean substrate (e.g. quartz plates) with heated resist stripper.

• Rinse thoroughly with Di water.

• Dry out bake at about 120^oC for 1 hour.

• Allow to cool down and spin a layer of OeBR-514 (photoresist from Olin Ciba Geigy) at 2500 rpm for 30 seconds. This should give a layer slightly above 1 micron thickness.

• Softbake photoresist at a low temperature and for a shortened time from that which is normally used for binary lithography. For OeBr-514 use 90^oC in a convection oven for 15 minutes instead of the recommended 30 minutes. This increases the sensitivity and will also make the contrast curve more linear and increase the dynamic range of the analog transfer scheme.

• After exposure for a batch development with low agitation (move plate once every 5 seconds e.g. lift half an inch, slowly move down), use a standard Metal-Ion-Free developer which is rather weak and has lower contrast compared to the usual developers, e.g. Shipley MF-319.

• Immediately follow with a Di water rinse. Be very careful not to spray or force any features away.

• Postbake for several hours in a 90^oC oven. Higher temperatures will melt the features in the resist.

(Note: the above procedure was developed for Olin Ciba Geigy’s OeBR-514 resist which is no longer available for purchase. Please see below for available photoresists.)

• In principle, other resists can be used in a similar way. Just adjust the processing temperature or duration to reduce the heat schedules for softbake. Specifically, use a bake duration that is a factor of 0.5 to 0.67 of the manufacturer's recommendations for binary lithography (e.g. 15 to 20 minutes instead of 30 minutes). The photoresist should be a positive and non-chemically amplified, Novolac-based photoresist.

• Other photoresists that have also been used successfully are: Shipley STR1045, STR 1075 and S1650, and Clariant AZ P4620, AZ P4330-RS and AZ 4562.

• Resist features having thickness range from submicron up to more than 20 micron were produced with HEBS glass masks having gray levels in the OD range of 0.1 to 1.2.