ICP silicon etching
Deep reactive ion etching or DRIE technology enables very high aspect ratio etches to be performed into silicon substrates. The sidewalls of the etched holes are nearly vertical and the depth of the etch can be hundreds or even thousands of microns into the silicon substrate.
Aligned Silicon Bonding
Aligned bonding is performed in an EV240 system. Bonding can be performed in a number of environments typically nitrogen or vacuum. Alignment tolerances of 4µm.
Alignment to the back side of the substrate is again carried out on the EV240 system and is typically used in conjuntion with deep silicon etchin to free MEMS structures define in SOI substrates.
Precision Grinding and Polishing
As well as having a number of polishing systems on site, the QAMEC centre has access to state of the art silicon grinding and polishing systems. QAMEC engineers have developed expertise in the polishing of Si, LT oxide and copper. There is also significant knowledge and experience in the polishing of germanium substrates.
Silicon Wet Etching
QAMEC engineers have developed wet etches such as KOH, Si/SiGe selective etches for the fabriction of BESOI substrates. The fexibilty of the laboratory allowed the development of masking layer used to confine the etch.
Wafer bonding is the joining of two (or more) wafers together to create a multi-wafer stack. There are three basic types of wafer bonding 1) direct or fusion bonding; 2) field-assisted or anodic bonding; and 3) bonding using an intermediate layer. In general, all bonding methods require substrates that are very flat, smooth, and clean, in order for the wafer bonding to be successful and free of voids.
1) Direct or fusion bonding is typically used to mate two silicon wafers together and is used in the fabrication of Silicon on Insulator (SOI) substrates. Direct wafer bonding can be performed on other combinations, such as bare silicon to a silicon wafer to produce an abrupt doping profile.
Wafer bonding enables a thick layer of single crystal silicon to be attached to another wafer. This can be extremely useful when fabricating MEMS stuctures with multiple layers/cavities or a thick layer of material for applications requiring appreciable mass or in applications where the material properties of single crystal silicon are advantageous over those of thin-film LPCVD materials.
Silicon to silicon fusion bonding is considered one of the best hermetic seals available, ensuring long lifetime of sealed devices.
- Silicon to silicon
- Silicon Dioxide to silicon
2) Anodic bonding. A silicon wafer is bonded to a Pyrex 7740 wafer using an electric field and elevated temperature. The two wafers can be pre-processed prior to bonding and can be aligned during the bonding procedure. The mechanism by which anodic bonding works is based on the fact that Pyrex 7740 has a high concentration of Na+ ions; a positive voltage applied to the silicon wafer drives the Na+ ions from the Pyrex glass surface, thereby creating a negative charge at glass surface. The elevated temperature during the bonding process allows the Na+ ions to migrate in the glass with relative ease. When the Na+ ions reach the interface, a high field results between silicon and glass, and this combined with the elevated temperatures fuses the two wafers together. As with direct wafer bonding, it is imperative that the wafers are flat, smooth, and clean and that the anodic bonding process is performed in a very clean environment. An advantage of this process is that Pyrex 7740 has a thermal expansion coefficient nearly equal to silicon and therefore there is a low value of residual stress in the layers. Anodic bonding is a widely used technique for MEMS packaging.
- Silicon to Glass
- Silicon Dioxide to Glass
- Silicon Nitride to Glass.
3) Intermediate layer bonding methods include:
Eutectic bonding; which involves the bonding of a silicon substrate to another silicon substrate at an elevated temperature through the formation of an alloy.
Polymers; epoxy resins, photoresists, polyimides, silicones, BCB etc. A polymer is spun onto the substrate surface, the two surfaces are brought together typically uner pressure and at elevated temperature to produce permanent bond.
- Au/Sn bonding
- BCB bonding