Fabrication of novel AFM probe with high-aspect-ratio ultra-sharp three-face silicon nitride tips

Abstract

In this paper we present the wafer-scale fabrication of molded AFM probes with high aspect ratio ultra-sharp three-plane silicon nitride tips. Using (111) silicon wafers a dedicated process is developed to fabricate molds in the silicon wafer that have a flat triangular bottom surface enclosed by three {111} side planes. By conformally coating the mold with a sufficient thick layer, the mold is sharpened, removing the flat bottom surface in the silicon mold, leaving a mold ending in three {111} side planes, which always intersect in a point. This ultimately results in AFM probes with tetrahedral tips consisting of three planes, thus the tips have ultra sharp apexes. We used silicon nitride to mold the probes in order to obtain more wear resistant probes compared with commonly used silicon. Inspection of the fabricated tips shows a tip radius of less than 4 nm.

Ultrasharp High-aspect-ratio Tetrahedral Molded Tips

“…(23,24) We previously presented the results of conformally filling the mold with a layer of TEOS and subsequently depositing the SiRN device layer. (17) In addition to the work presented there, it is important to note that for this process to result in a sharp tip, the conformal deposition of the fill-up TEOS layer plays a crucial role. If for some reason the thickness of the fill-up layer on the inclined surface, t incl , is a factor X less, which is practically common, (25) the ratio of the thickness on the inclined surface to that on the flat surfaces is given by Eq.…”

“…We previously presented a general process to develop tips using the molds. (17) Starting with a silicon wafer with tetrahedral pits (a), the process comprises filling the mold with a fill-up layer, conformally deposited by the low-pressure chemical vapour deposition (LPCVD) of tetraethyl orthosilicate (TEOS). Owing to the geometry of {111} side planes in combination with the bottom (111) surface, the size of the bottom surface decreases with increasing layer thickness.…”

“…For the fabrication of cantilever probes, as previously published, (17) the cantilever width, thickness, and length of 30 µm, 600 nm, and 100 µm, respectively, have been chosen, such that the cantilever will have the theoretical stiffness and resonance frequency of 0.5 N/m and 110 kHz, respectively. (29) This is in the range of typical AFM tapping mode probes and therefore the dimensions have been chosen as such.…”

“…(16) To circumvent the potential imperfections of a pyramidal mold, or of a mold made of multiple deposited and lithographically defined layers, we previously presented a method in which a tetrahedral mold is made in (111) silicon. (17) In this way, the tetrahedral silicon crystal structure is used to obtain a fully consistent, symmetric, and predictable mold with three side planes that generally intersect in a single point. With that approach, it has been demonstrated that tetrahedral molded tips are successfully fabricated.…”

“…In Sect. 2, the method of mold fabrication described by Vermeer et al (17) is reiterated. Subsequently, the methods of fabricating tips out of this are described, followed by the results of the fabrication of such tips in Sect.…”