Fused silica (a-SiO 2) exposure to low-energy femtosecond laser pulses (below ablation threshold) introduces a local increase of the HF etching rate. This property has been used to fabricate a variety of structures ranging from simple fluidic channels to more complex optofluidics and optomechanical devices. In practice, the desire patterns are written by contiguously stacking laser exposed regions, which defined the volume to be removed.. In previous work, we showed that there was an optimum energy level for maximizing the efficiency of the etching process. Here, we focus on the interaction between adjacent laser affected zones and its effects on the overall etching process. Experimentally, we exposed fused silica specimens to patterns consisting of matrices of lines with varying density, under various laser exposure conditions. Surprisingly, we show that for certain laser affected zone densities and pulse energies, the exposed regions do not etch while their constitutive elements (i.e. the single laser affected zones) do. This paper describes our recent experimental observations and proposes a qualitative model to explain these findings. © 2012 SPIE.