Pure HBr based inductively coupled plasma vertical, anisotropic etching provides high aspect ratio (20-40) nanoscale trenches in InP at 165 degrees C processing temperatures. Since these temperatures are comparatively lower than chlorine based chemistries, HBr should yield improved device reliability. In addition to temperature dependence, other important considerations for integrated photonic applications are discussed. The phenomenon of aspect ratio dependent etching, or reactive ion etching lag, begins to manifest itself when the etch aspect ratio of InP approaches 30:1. No microloading effect is observed in the 100 nm scale trench etching. Physical etch dominates the etching mechanism in this regime, and acceptably smooth, 20 nm rms surface roughness is observed.