Test structure for measuring the selectivity in vapour etch processes

Etch selectivity between layers is critical in the fabrication of microelectronics and microsystems. This is particularly true in the case of isotropic gas/vapour etching methods used to release free standing structures through the selective etching of sacrificial layers. Commonly used structural materials have been reported to be largely inert when exposed to a given vapour etchant, indicating high selectivity when measured against typical sacrificial layers. However, there is growing evidence that these structural layers are actually etched at an enhanced rate if they are located in the proximity of the sacrificial layer being removed. Hence, removal rates given in the literature that have resulted from measurements of layers that have been etched in isolation can no longer be trusted to characterize critical etch processes in device fabrication. In this paper, a test structure is reported that enables a far more accurate determination of the etch selectivity between sacrificial and structural materials. The method is demonstrated by a XeF2 vapour etch of a polysilicon sacrificial layer located above a silicon nitride structural layer. A dataset is presented for the polysilicon and silicon nitride layers, which shows a selectivity of 5:4. The raw dataset for Fig. 6 is provided.