While it is always possible to use a measured etch rate and a specified time to determine the depth of etched features, it is highly desirable to be able to use a well-defined structural feature to stop an etch. There is a rich array of possible etch-stop structures in use.

Electrochemical Etch Stop

• When a silicon wafer is biased with a sufficiently large anodic potential relative to the etchant, it tends to oxidize.
• This process is called electrochemical passivation, because if there is oxide on the surface, the oxide masks the etch.

• This passivation step is a redox reaction and requires current. If the supply of that current is blocked with a reverse biased pn junction, the passivation cannot occur and the etching can proceed.

  

• The use of this electrochemical property to control the thickness of an etched diaphragm is illustrated in Figure above.

• A p-type wafer has a diffused n-region at its upper surface. The depth of the junction can be accurately controlled by the combination of ion implantation and drive in.
• The backside of the wafer is covered with a masking layer that contains an opening.
• When this wafer is placed into the anisotropic etchant with an anodic bias applied as shown, the reverse biased p-n junction prevents the passivation current from flowing.
• As a result, the p-silicon remains unpassivated and hence etches.
• However, the n-silicon becomes passivated and is not etched.
• Furthermore, as anisotropic etching proceeds, when the junction is reached there is no longer anything blocking the flow of passivation current.
• The n-silicon quickly passivates, terminating the etch. The resulting diaphragm thickness is equal to the depth of the original junction, at least ideally.
• In practice, it is necessary to control the junction leakage and current supply paths to be certain, first, that there is no pathway that can provide enough current to passivate the p-region, and, second, that once the junction is reached, all features have enough current supply so that they passivate at the same point.

p+ Etch Stop

• If instead of using an n-type diffusion into a p-wafer, a heavily boron doped p+ layer is formed by ion implantation and diffusion, the anisotropic etch will terminate without requiring application of anodic bias.
• This greatly simplifies the tooling required to perform the etch.
• However, the etch-rate selectivity is not quite as high as for passivating oxides.
• Furthermore, the p+ diaphragm will have residual tensile stress that might affect the performance of any device, such as a pressure sensor, that depends on the mechanical properties of structure.
• Finally, it is not possible to diffuse piezoresistors into already-heavily-doped p+ silicon.
• So while the p+ etch stop offers some advantages and can be used with great effect in some processes, it also has some important limitations.

Dielectric Etch Stop

• If the n-layer of Figure above were replaced by a material that is not etched, for example, silicon nitride, then the result of the anisotropic etch is a silicon nitride diaphragm suspended over a hole through the silicon.
• LPCVD silicon nitride is very brittle, and has a very high residual tensile stress, making it prone to fracture.
• However, if the ratio of dichlorosilane to ammonia is increased during silicon nitride deposition, a silicon-rich nitride layer is deposited which has lower stress.
• Membranes of silicon rich nitride can be used as the basis for mechanical devices.
• Masks for X-ray lithography can also be made in this fashion, depositing gold on the membrane to create the masking regions.