Etch-stop layer

Electrochemical etching is one way of controlling the etch rate and determine a clear etch stop layer when bulk micromachining Silicon. In this case, the wafer is used as anode in an HF-Electrolyte. Sufficiently high currents lead to oxidation of the silicon. The resulting oxide which is dissolved by the HF-solution. Since lowly doped silicon material is not exhibiting a notable etch rate, it can be used as an etch stop. 

But even in a homogeneously doped material an etch stop layer can be generated by an inhomogeneous charge carrier distribution. If a positive bias is applied to the metal electrode of an MOS structure, an inversion layer is formed in the p-type semiconductor. The inversion layer passivates in alkaline solutions if it is kept at the PP using a second bias [Sm5], as shown in Fig. 4.16b. This method is used to reduce the thickness variations of SOI wafers [Og2]. Illuminated regions... 

Channel structures are etched on two plates which are later positioned face-to-face to give the overall fluid structure [140], In the region where the channels overlap, they are separated by the separation plate defined by an etch stop layer. The channel covered by this structured plate was generated by underetching in the <100> direction through slits in the plate. The micro mixer is assembled from a silicon and a glass wafer connected by anodic bonding. 

The cantilevers were defined by photolithography and dry etching. The silicon dioxide buffer layer was kept to be employed as an etch stop layer for DRIE. 

If silicon is highly doped, particularly with boron, then silicon will be etched very slowly by the previously mentioned wet chemical etchants such as KOH, EDP, and TMAH. One can use ion implanting or diffusing boron into silicon to form a thin etch-stop layer. Since non-highly doped silicon will be etched very fast, this thin etch-stop layer will stay as a free-standing... 

When ULK CMP is completed at a given metal level, the wafers then proceed to the next step where they are capped with an SiCN layer. This layer acts as a diffusion barrier for Cu, as well as acting as an etch-stop layer to permit the controlled exposure of copper at the bottom of vias at the next metallization level. The deposition of the SiCN layer is typically preceded by an ammonia-based plasma pretreatment process, which serves to reduce Cu oxides at the surface of the metal lines, and improve the adhesion of the capping layer to Cu. 

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