Etching behaviour of sputter-deposited aluminium nitride thin films in H3PO4 and KOH solutions

Aluminium nitride (AlN) reactively sputter deposited from an aluminium target is an interesting compound material due to its CMOS compatible fabrication process and its piezoelectric properties. For the implementation in micromachined sensors and actuators an appropriate patterning technique is needed to form elements made of AlN. Therefore, the influence of different sputtering conditions on the vertical etch rate of AlN thin films with a typical thickness of 600 nm is investigated in an etch mixture based on phosphoric acid. Under comparable conditions, such as temperature and concentration of the etchant, thin films with a high c-axis orientation are etched substantially slower compared to films with a low degree of (002) orientation. When a high c-axis orientation is present detailed analyses of the etched topographies reveal surface characteristics with a low porosity and hence, low roughness values. From temperature dependant etching experiments an activation energy of 800(± 30) meV is determined showing a reaction-controlled etching regime independent of sputter deposition conditions. For comparison, AlN films synthesized under the same conditions were etched in potassium hydroxide (KOH) at room temperature revealing comparable etching characteristics as a function of deposition parameters. Depending on the degree of (002) orientation the topography of the etched samples show a strong increase in surface roughness with time due to a selective etching behaviour between (002) and residual crystallographic planes.