Bi-level resist systems are very promising with deep-UV lithography for obtaining high-aspect patterns on high-step substrates. Three-dimensional polysilphnylenesiloxane (TSPS), a negative deep-UV bi-level resist we developed, has high resolution and high sensitivity. The TSPS molecule is structured as a rigid three-dimensional mesh consisting of a silphenylenesiloxane core surrounded by functional groups. The rigid silphenylene core provides low swelling, high thermal stability, high O2-RIE resistance, and high deep-UV absorption for the initiation of crosslinkage. We found that adding the photoinitiator 2, 2-dimethoxy-2-phenylacetophenon (DMPA) to TSPS raises sensitivity to about 20 times that of TSPS alone. TSPS sensitivity to deep-UV light is 20mJ/cm2 at Dg0.5. A 0.25μm pattern can be delineated with TSPS/MP-1300 bi-level resist systems using a KrF-excimer laser stepper.