Nanoelectronics MEMS structures for advanced characterization techniques


The silicon micromachining capability of INL for processing substrates with diameters up to 8 inch will allow obtaining cost-effective MEMS structures used in advanced characterization techniques. Atomic force microscopy (AFM) tips, tip arrays for current in-plane tunnelling (CIPT) magnetoresistance measurements and advanced windows for transmission electron microscopy (TEM) will be produced and applied in the context of biochemical/medical analysis and related sensors development using those advanced methods.

Conventional AFM tips are usually obtained from silicon substrates that are bulk micromachined using KOH/TMAH solutions. This etching method is anisotropic leading to devices, both AFM cantilever and its base, with sidewalls that have an angle of 54.74º, i.e. the angle between (100) and (111) planes. Since the fabrication of such platforms involves machining the entire thickness of the silicon substrate, a large part of the wafer is lost and density of devices obtainable is low. A substrate width of ca. 1 mm is lost for perforating a 700-µm-thick wafer in this case. To overcome this issue, the fabrication of AFM tips at INL relies solely on DRIE and the resulting chips have 90º sidewalls, wafer device density being limited in this case by the characteristic 1:20-1:30 aspect ratios of the Bosch process. Only 20-30 µm of substrate width are lost for the etch through step of one device.

A similar approach is being employed to process CIPT tip arrays, which allow the current-perpendicular-to-plane (CPP) transport properties of a magnetic tunnel junction to be extracted from current-in-plane (CIP) 4-point resistance measurements with variable mean pitch.  This type of characterization is very important specially in an industrial context since it allows wafer properties to be extracted prior to the expensive and time consuming microfabrication method of a full wafer.

Environmental TEM imaging is a powerful technique to observe physical transformations, chemical reactions and even biological reactions. Special windows are necessary in this case to keep the species being measured at atmospheric pressures, while the overall holder is inside an ultra high vacuum ambient. This is typically achieved by front-to-front bonding two wafers with silicon nitride (Si3N4) membranes, machined using KOH anisotropic etching, and spacers. In addition to the inefficient usage of wafer area because of the etching, the bonding procedure is critical and often unreliable, leading to undesired leaks that contaminate the TEM chamber. The solution being developed at INL produces such windows in which Si3N4 pockets with fluidic feedthroughs are machined in a single substrate by combined DRIE/XeF2/HF vapor etching and PECVD passivations.