One of the major challenges for x-ray lithography as a contender for next generation lithography Lies in patterning small features-below 100 nm-with a mask-to-wafer gap greater than 20 mum. X-ray phase masks (XPM) provide an opportunity to alleviate this constraint, if the XPM can be constructed with good radiation and thermal stability for practical applications. In this article, we examine a robust silicon nitride based XPM with improved thermal and radiation stability for sub-70 nm imaging. The impact of various factors, such as exposure dose, phase-shifter thickness, sidewall slope, and mask-to-wafer gap, were studied. Phase-shifter slope and mask-to-wafer gap appeared to have a substantial impact on the printability of the XPMs. However, patterning sub-70 nm features could be achieved at a gap of 20 mum. Phase-shifter thickness variation within 10% appeared to have negligible impact on the linewidth with a synchrotron source. Extensive simulations and exposures were conducted to study the dose and gap dependency of different photoresists. Experimental results for Shipley UV-6 and UVN-30 confirmed the simulation results.