Pyrex Glass Wafer - Defects Observed on the Etched Glass Surface

Pyrex Glass Wafer - Defects Observed on the Etched Glass Surface

Material Name: Pyrex glass wafer
Record No.: 9
Primary Chemical Element in Material: Si
Sample Type: Wfer
Uses: Etching
Etchant Name: None
Etching Method: Wet etching
Etchant (Electrolyte) Composition: No data
Procedure (Condition): No data
Note: This paper addresses the main issues related to wet micromachining of one of the mostly used BioMEMS materials - glass - and proposes two optimized solutions for deep wet etching. As a result, 500 µm-thick Pyrex glass wafer was etched using an etching mask consisting of low stress amorphous silicon (a:Si) and photoresist. Moreover we report the successful through etching of 1mm Pyrex glass wafer using a combination of low stress a:Si/SiC/photoresist mask.

There are two types of defects in the glass etching process: pinholes and notching defects on the edges. These can be observed after certain etch time, and are the result of the interaction between the etchant and mask. Figure 1 illustrates these defects. The main reasons of defect generation are:
- residual stress in the masking layer,
- stress type (tensile or compressive),
- stress gradient existing in multilayer mask,
- hydrophilicity of the surface.

The residual stress, as we mentioned in ref., has a strong influence on the defect generation. The small defects on the surface are generated during masking layer depositions. If the stress is tensile, the residual stress generates small microchannels in the masking layer. If the masking layer surface is hydrophilic the etching solution will be “sucked” in these microchannels. As a consequence, pinholes will start to generate in a short time. For this reason, a masking layer with compressive stress, which commonly exists in amorphous silicon or polysilicon, is sometimes preferred.

The existence of notching defects on the edges is a typical characteristic of metal masks such as Cr/Au and Cr/Cu that present tensile stress as high as 1 GPa. These notching defects are generated due to the stress gradient or the breakage of the edge of the mask. During isotropic etching process, due to underetching, the edge of masking layer becomes a freestanding structure. Due to the stress and/or stress gradient, the mask can break and leave some areas uncovered.

Hydrophilic or hydrophobic property of the masking layer surface, as we previously mentioned, also play an important role. A small defect, for example micro-creeps on a hydrophobic surface, is very difficult to be filled with the etching solution, resulting in a better etching protection. For this reason, amorphous silicon or polysilicon, as same as silicon, presents a hydrophobic behavior and is preferred to be used as etching mask layer compared to Cr/Au mask.
Reference: Ciprian Iliescu, Bangtao Chen and Jianmin Miao, DEEP WET ETCHING-THROUGH 1 mm PYREX GLASS WAFER FOR MICROFLUIDIC APPLICATIONS, February 2007, Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), DOI: 10.1109/MEMSYS.2007.4433150.

Figure 1: Defects observed on the etched glass surface.