Isotropic and Anisotropic Silicon Wet Etching Processes

The silicon wet etching of monocrystalline wafers produces microscopic structures that are used in mechanical devices and semiconductor parts. When the silicon wafer is immersed in a chemical bath, only the exposed parts of the silicon wafer are subject to etching. Its other parts, that are not to be etched, are protected by a mask.

Find additional details about isotropic and anisotropic silicon wet etching in these documents:

•  University of Maryland - Silicon Wet Isotropic and Anisotropic Etching

•  University of California at Irvine - Anisotropic Silicon Etch Using KOH

In isotropic etching, the isotropic etchant etches the silicon wafer equally in all directions. This means that the wafer is etched downward and sideways under the mask, resulting in a cavity which usually consists of rounded corners and edges which are bigger than the opening in the mask.

In anisotropic etching, the anisotropic etchant such as potassium hydroxide (KOH), etches at varying speeds in different directions. This means that the etch rate in the downward direction is faster than the etch rate in the sideways direction. This leaves to cavities with straight sides and less undercutting of the mask.

  

The silicon wafer mask defines where etching can take place, however, the depth of the etched cavity and its shape can be made by selecting the right etchant and controlling the etching rate. Isotropic etching is often used to achieve larger features in the initial stages of the silicon wafer processing, while anisotropic etching is used to produce straight-edged microstructures in the final stages.

The etch rate is also a significant factor. It depends on the concentration of the etchant and the etching solution temperature. Once the etch rate is determined at a certain temperature, the masked silicon wafer is immersed into the etchant. The etching should be long enough to produce the required cavity size. The subsequent wafers will undergo the same process for reproducible results.

Silicon wet etching equipment that includes Modutek’s Teflon tanks can be used to support both isotropic and anisotropic processes. For processes that are temperature-dependant, the tank temperature controller provides rapid and accurate heating control.

The complete article “Isotropic and Anisotropic Silicon Wet Etching Processes”, explains the processes in further detail. If you have questions about selecting the right equipment for your silicon wet etching process, contact Modutek at 866-803-1533 or email Modutek@modutek.com.