Reactive ion etching of features down to 100 nm in linewidth in tungsten has been studied using an SF6 based chemistry. The studies were carried out in a PlasmaTherm 500 etcher operated at low pressure (2 mTorr) and power (100 mWatts/cm2). Key processing parameters have been identified to achieve the resolution and aspect ratio required for high contrast x-ray masks. The critical parameters include sample temperature, gas dilution and end point detection. However, even with all of these parameters optimized, additional sidewall passivation is required to obtain the necessary 6.5:1 aspect ratio. A novel method of achieving such passivation based on an intermittent etching process is described.