Polycrystalline Silicon (Poly Si) has been commonly used for micro electromechanical systems (MEMS) applications, yet the main drawback of this material is that it requires a high processing temperature (higher than 800°C) in order to achieve the required mechanical properties. Such a temperature is too high when MEMS devices are fabricated after the electronics circuitry (especially if non refractory metals are used for CMOS back end technology). On the other hand, Poly-SiGe material has physical properties comparable to those of Poly-Si, yet it can be processed at low enough temperature to avoid damage on chips electronics. This makes Poly-SiGe very attractive for the integration of MEMS after standard electronics.
In order to achieve an optimal integration of MEMS devices with CMOS circuitry, the electrical connection between the MEMS and the electronics circuit needs to introduce the minimum possible interconnect parasitics. Therefore, direct deposition of SiGe onto metal is desirable. In addition, the specific contact resistivity between the two materials needs to be low enough, allowing it to be comparable to the state-of-the-art specific interconnect resistivity requirement (less than 1 ohm-cm2).
Using the well-known Kelvin structures, research of SiGe deposition on metal films is continuing with the prospect of determining the contact resistivity of p+ SiGe films deposited onto TiN of different contact area sizes. We are also interested to see how increasing or decreasing SiGe processing temperature (between 400°C and 450°C) affects the contact resistivity. The initial results obtained from the contact resistance measurements are very promising. Depending on the contact hole dimensions as well as the SiGe deposition temperature, it is possible to reduce the specific contact resistivity to less than 1 ohm-cm2. One of the main issues we've encountered is a proper cleaning process of the wafers prior to SiGe deposition. This is critical as it helps to achieve a good interface between SiGe and the metal film beneath, thus reducing the contact resistance. Using Argon Sputtering plasma rather than Helium plasma, we observed that the contact resistivity could also be reduced.