Needle shaped probes with a dual electrode system in submicron size have been developed for electrochemical analyses of living single cells. The probe system is designed for local probing of the cytosolic cell environment and cell organelles using amperometric, potentiometric and impedance spectroscopic methods. Silicon nitride cantilevers with an electrode metal layer system are fabricated on four-inch wafers using conventional micro fabrication techniques. The probe needle structures with a tip in sub micron scale are patterned using Focus Ion Beam (FIB) technology. A focused ion beam is utilized to write the probe needle shape into the pre-shaped cantilever and, for a dual electrode system, the probe is divided into two parts to create two separate electrodes. Subsequently, the needle structures are released from the supporting bulk silicon during a wet etching step, and a silicon nitride layer is deposited to isolate and embed the electrode metal layer. Finally, FIB milling is used for a precise exposure of the buried metal layer by cutting the top of the tip.
Electrochemical characterization of nano-probes showed full functionality of Ag/AgCl as well as of platinum transducer systems. The sharpness of the probe tip with a radius of smaller than 50nm and the mechanical robustness of the needle structure allow for a reliable penetration of cell membranes. Initial measurements of cell membrane potentials and cell membrane impedances of rat fibroblast cells using Ag/AgCl transducer probes demonstrate the analytical capability of these probes in biological environments.
A multi-layer technology for electrical high-density connections between the two opposing sides of a wafer has been developed. Openings in a double-side polished wafer were created by applying a deep reactive ion etching technique. Hole structures with a diameter of 20 μm were formed through a 350-μm thick wafer. A multi-layer system of up to eight layers consisting of alternating conducting layers (N-type doped poly-silicon) and isolating layers (silicon-oxide) were grown until the vias were filled. Subsequently, all layers on the wafer surface were then removed in a CMP process. In this way, a multi-connection structure embedded in the silicon wafer can be fabricated.
The applied low-pressure chemical vapor deposition techniques guarantee a sufficient homogenous coating outside and inside of the entire structure to a minimum layer thickness of one µm. The connection quality has been examined combining impedance spectroscopy and Focused Ion Beam technology. Depending on the geometry and the doping profile of the poly-silicon layers, a connection resistance of less than 80 Ohms can be achieved with sufficient DC isolation.
In this way, a multi-connection of up to four isolated signal lines per opening was manufactured. This corresponds to a local connection density higher than 30.000/cm2. The achievable connection density and the full CMOS compatibility of the applied processes make this multi-layer connection technology particularly well suited for combined MEMS and CMOS applications