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 powerful experimental tool, ultra-sharp nano-electrode array is designed, fabricated and characterized. The application on
a combination of Scanning Electrochemical Microscopy (SECM) and the Atomic Force Microcopy (AFM) is demonstrated. It can measure sample electrochemically initiated by SECM changes of topography while detecting topography using AFM. In order to realize this, a specialized probe system that is composed of a micro-mechanical bending structure necessary for the AFM mode and an electrochemical UME-tip required for a high performance SECM is crucial. The probe array is a row of silicon transducers embedded in silicon nitride cantilever array. The sharp high-aspect ratio (20:1) silicon tips are shaped and a thin layer of silicon nitride is deposited, which embeds the silicon tips in a silicon nitride layer so that they protrude
through the nitride. Thus, the embedded silicon tips with a diameter less than 600 nm, the top radius less than 20 nm, and the aspect ratio as high as 20 can be achieved. A metal layer and an insulator layer are deposited on these tip structures to make each probe selectively conductive. Finally, cantilever structures are shaped and released by etching the silicon substrate from the backside. Electrochemical and impedance spectroscopic characterization show electrochemical functionality of the transducer system.
An electrochemical transducer system embedded in silicon nitride cantilevers has been fabricated for simultaneous Scanning Electrochemical Microscopy (SECM) and Atomic Force Microscopy (AFM) analysis. Sharpened high-aspect ratio silicon tips are shaped combining isotropic and anisotropic deep-reactive etch processes and form the body of the transducer. Deposition of a silicon nitride followed by a back-etch step allows embedding these silicon tips in a silicon nitride layer so that they protrude through the nitride. This way, embedded silicon tips with a diameter smaller than 600 nm, a radius smaller than 50 nm, and an aspect ratio higher than 20 can be achieved. Subsequently, a platinum layer and an insulator layer are deposited on these tip structures. Introducing a metal masking technology utilizing Focused Ion Beam (FIB) technology, a precise exposure of the buried metal layer can be achieved to form ultra-micro electrodes on top of the tip. Finally, cantilever structures are shaped and released by etching the silicon substrate from the backside. Electrochemical and impedance spectroscopic characterization show electrochemical functionality of the transducer
system. Due to the high aspect ratio topography of the tip structure and low spring constant of silicon nitride cantilevers, these probes are particularly suited for high resolution SECM and AFM analysis. Furthermore, this technology allows a production of both linear probe-arrays and two-dimensional probe-arrays.
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/cm<sup>2</sup>. 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
A pencil-shaped electrochemical transducer system for analysis
or surface modification in nanometer dimension has been
developed. High aspect ratio tip structures are shaped
combining isotropic and anisotropic deep reactive etch
processes to form the body of the transducer. In this way, tips
with an aspect ratio higher than 20 and a tip radius of smaller
than 50 nm can be achieved. Subsequently, a three-layer
system (an isolation layer: silicon nitride, a metal layer:
platinum or gold and an isolation layer: silicon nitride) was
deposited on the tip structure. Planarization of this structure
in combination with a back etch process enables a precise
exposure of the buried metal layer down to an electrode
dimension of 200 nm on the tip.
Electrochemical and impedance spectroscopic characterization
showed full electrochemical functionality of the transducer
system. Due to the high aspect ratio topography, this probe is
particularly suited for Scanning Electrochemical Microscope
(SECM) - methodologies.
Furthermore this technology promises a feasible production
possibility for both probe-arrays and probes on cantilevers.