The silicon wet etching of monocrystalline wafers produces microscopic structures that are used in micromechanical devices and semiconductor components. Areas of the silicon wafer not to be etched are protected by masks made of materials such as silicon dioxide or silicon nitride. The exposed areas of the silicon are etched when the wafer is immersed in a chemical bath.
In isotropic etching, an isotropic etchant such as hydrofluoric acid etches the silicon equally in all directions. This means that the wafer is etched directly downwards and also sideways under the mask. The resulting cavity has rounded corners and edges and is larger than the opening in the mask.
For anisotropic etching, the anisotropic etchant such as potassium hydroxide (KOH) etches with different speeds in different directions. This means that the etch rate in the downward direction can be faster than in the sideways directions under the mask. When properly designed, the anisotropic etch can produce cavities with straight sides and less undercutting of the mask.
More details on isotropic and anisotropic silicon wet etching can be found in the following documentation:
- Silicon Wet Isotropic and Anisotropic Etching, University of Maryland
- Anisotropic Silicon Etch Using KOH, University of California at Irvine
How Anisotropic Etching Works
In monocrystalline silicon wafers, the silicon atoms are arranged in a crystal lattice. The crystal has planes which have different atom densities. For example, the 111 plane in silicon is a diagonal plane that has a different atom density than the vertical 100 or 110 planes. As a result, the etching speed for certain etchants differs according to the plane of the silicon crystal through which etching takes place.
To design a successful anisotropic etching process, the etching speed of the etchant and the orientation of the crystal planes have to be coordinated. For example, if the aim is to etch a deep cavity with straight walls, the crystal planes with a slow etching speed have to be oriented along the cavity walls while the plane with a high etching speed should form the bottom. Rather than producing etched cavities with rounded walls and edges, anisotropic etching can create clearly defined straight-walled cavities along the silicon crystal planes.
Controlling the Etching Speed to Obtain Specific Structures
The silicon wafer masks define where etching can take place but the depth of the etched cavity and its shape can be determined by choosing the right etchant and controlling the etching rate. Isotropic etching is often used to create larger features in the initial stages of silicon wafer processing while anisotropic etching can produce straight-edged microstructures in the finishing stages.
Beyond the choice of the type of etching used, the etch rate depends on the concentration of the etchant and the etching solution temperature. Once the etch rate is determined for an etchant concentration at a specific temperature, the masked wafer is immersed in the etchant just long enough to produce the size of cavity required. In each case the precise concentration has to be mixed and the temperature has to be maintained at the target level. Subsequent wafers can be processed exactly the same way for reproducible results.
Differences between Isotropic and Anisotropic Silicon Wet Etching
While isotropic etching is harder to control precisely and hydrofluoric acid is difficult to handle, isotropic etching is faster than anisotropic etching. As a result it is often used for large geometries for which etching speed is an important factor. Anisotropic KOH etching is more precise but it depends on the orientation of the silicon crystal planes. If the wafer crystal orientation doesn’t match the requirements of the process, the resulting cavity will not have the desired characteristics.
Silicon wet etching equipment such as Modutek’s Teflon tanks support both types of processes and they come in temperature controlled and ambient versions. For processes dependent on temperature for controlling etch rate, the tank temperature controllers provide rapid and accurate heating. Concentration can be maintained through supplementation of de-ionized water and custom tank sizes are available if needed. For a free consultation or quote on selecting the right silicon wet etching equipment for your etching process contact Modutek at 866-803-1533.