Antti Peltonen

The fabrication of silicon nanostructures by local gallium implantation and cryogenic deep reactive ion etching

We show that gallium-ion-implanted silicon serves as an etch mask for fabrication of high aspect ratio nanostructures by cryogenic plasma etching (deep reactive ion etching). The speed of focused ion beam (FIB) patterning is greatly enhanced by the fact that only a thin approx. 30 nm surface layer needs to be modified to create a mask for the etching step. Etch selectivity between gallium-doped and undoped material is at least 1000:1, greatly decreasing the mask erosion problems. The resolution of the combined FIB-DRIE process is 20 lines microm(-1) with the smallest masked feature size of 40 nm. The maximum achieved aspect ratio is 15:1 (e.g. 600 nm high pillars 40 nm in diameter).

Dry fabrication of microdevices by the combination of focused ion beam and cryogenic deep reactive ion etching

In this paper, we demonstrate silicon microdevice fabrication by a combination of focused ion beam (FIB) and cryogenic deep reactive ion etching (DRIE). Applying FIB treatment only to a thin surface layer enables very high writing speed compared with FIB milling. The use of DRIE then defines the micro- and nanodevices utilizing the FIB-modified silicon as a mask. We demonstrate the ability to create patterns on highly 3D structures, which is extremely challenging by other nanofabrication methods. The alignment of optically made and FIB-defined patterns is also demonstrated. We also show that complete microelectromechanical systems (MEMS) can be fabricated by this method by presenting a double-ended tuning fork resonator as an example. Extremely short process time is achieved as the full fabrication cycle from mask design to electrical measurements can be completed during one working day.

Fabrication of Dual-Type Nanowire Arrays on a Single Substrate

A novel method for fabricating dual-type nanowire (NW) arrays is presented. Two growth steps, selective-area epitaxy (SAE) in the first step and vapor-liquid-solid (VLS) in the second step, are used to grow two types of NWs on the same GaAs substrate. Different precursors can be used for the growth steps, resulting in sophisticated compositional control, as demonstrated for side-by-side grown GaAs and InP NWs. It was found that parasitic growth occurs on the NWs already present on the substrate during the second growth step and that the SAE NWs shadow the growth of the VLS NWs. Optical reflectance measurements revealed the dual-type array having improved light trapping properties compared to single-type arrays. Dual-type NW arrays could be practical for thermoelectric generation, photovoltaics and sensing where composition control of side-by-side NWs and complex configurations are beneficial.

Localized gallium doping and cryogenic deep reactive ion etching in fabrication of silicon nanostructures

We present a novel fabrication method to create controlled 3-dimensional silicon nanostructures with the lateral dimensions that are less than 50 nm as a result of a rapid clean room compatible process. We also demonstrate periodic and nonperiodic lattices of nanopillars in predetermined positions with the minimum pitch of 100 nm. One of the uses of this process is to fabricate suspended silicon nanowhiskers.

Effect of Surface Wear on Corrosion Protection of Steel by CrN Coatings Sealed with Atomic Layer Deposition

Corrosion protection of steel obtained with physical vapor deposition (PVD) coatings can be further improved by sealing the intrinsic pinholes with atomic layer deposition (ALD) coatings. In this work, the effect of surface wear on corrosion protection obtained by a hybrid PVD CrN/ALD Al2O3/TiO2 nanolaminate coating was studied. The samples were investigated by alternating surface wear steps and exposure to salt solution and consecutively the progression of corrosion after each wear and each corrosion step was evaluated. Optical microscopy, scanning electron microscopy (SEM), and energy-dispersive spectroscopy showed that the rust spots were almost exclusively located on positions at which the wear steps had removed the top surface of the PVD CrN coating. Nevertheless, even after complete removal of the ALD nanolaminate from the top of the CrN surface by sandpaper grinding, the corrosion current density was less than half compared to the PVD CrN coating alone without surface wear. Cross-sectional SEM images obtained with focused ion beam milling showed not only the presence of the ALD coating at the CrN defects but also the opening of new pathways for the corrosion to attack the substrate. A mechanism for the effect of wear on the structure and corrosion protection of hybrid PVD/ALD coatings is proposed on the basis of this investigation.

Milling a silicon nitride membrane by focused ion beam

An ultrathin amorphous membrane, such as that made of silicon nitride (SiN) suspended on silicon substrate, is a popular platform for various applications. However, its hardness causes many difficult technical problems for patterning, especially when combined with other fabrication processes. In nanofabrication, focused ion beam (FIB) is a popular milling technique. It would be a perfect tool for perforating the SiN membrane, but the ion beam charges the membrane, induces stress, and breaks them sporadically. The authors design a metallic structure near the cutting area to neutralize the charges. It reduces stress on the membrane and enables the perforation. Commercial SiN membranes are perforated with FIB and are suspended on thin legs on silicon chip. The authors study and discuss various designs and the applicability of this approach.

Galvanic corrosion of silicon-based thin films: A case study of a MEMS microphone

MEMS microphones are widely utilized in commercial applications such as mobile phones and laptops. Microphones cannot be sealed completely from the atmosphere since sensing requires propagation of sound waves to the sensing element. Therefore, the sensing element is vulnerable to airborne impurities and humidity. The prevalent material of choice for MEMS microphones is silicon, which is generally considered reliable and inert material in normal atmospheric conditions, although micron-scale silicon is known to be susceptible to delayed fracture failures under cyclic stresses. Especially humidity has been shown to affect its fatigue properties. Furthermore, the fabrication history of the device can affect the mechanical performance of the devices significantly. Hence, there exist large discrepancy for the fatigue limit and fracture strength of micron-scale silicon in the literature. In addition, there is little information on how harsh environments can affect the reliability of silicon-based thin films in MEMS applications. Therefore, harsh environmental tests were conducted on MEMS microphones. The silicon-based sensing elements of the microphones were observed to degrade during the testing. Failure analysis was conducted employing microscopy and chemical analysis techniques. The degree of degradation was evaluated qualitatively with nanoindentation, and finite element method was employed in explaining the influence of the observed degradation.