Surface micromachining | EPFL Graph Search

Surface micromachining builds microstructures by deposition and etching structural layers over a substrate. This is different from Bulk micromachining, in which a silicon substrate wafer is selectively etched to produce structures. Layers Generally, polysilicon is used as one of the substrate layers while silicon dioxide is used as a sacrificial layer. The sacrificial layer is removed or etched out to create any necessary void in the thickness direction. Added layers tend to vary in size from 2-5 micrometres. The main advantage of this machining process is the ability to build electronic and mechanical components (functions) on the same substrate. Surface micro-machined components are smaller compared to their bulk micro-machined counterparts. As the structures are built on top of the substrate and not inside it, the substrate's properties are not as important as in bulk micro-machining. Expensive silicon wafers can be replaced by cheaper substrates, such as glass or plast

Micro-Four-Point probe based on SU-8 cantilevers

Stephan Keller

The four-point probe is used for the measurement of the resistivity of thin metal or semiconductor films. There is an interest in miniaturization of the probes to obtain higher surface sensitivity, an increased spatial resolution and less damage to the sample. In this project the design, fabrication and first characterisation of a microscopic four-point device using SU-8-cantilevers were realized. A 200-µm-thick SU-8 body chip allows the handling of the device and the electrical connection to the measurement set-up. A SU-8 cantilever with 10 µm thickness is split into four equally spaced micro-cantilevers at its freestanding extremity, each of them supporting a probe tip. This design and the high flexibility of SU-8 ensure a stable electrical point-contact between samples and probe tips. The microscopic four-point probe is fabricated by surface micromachining using silicon as substrate and polysilicon as sacrificial layer. The mould for the probe tips is etched by KOH. 50 nm-Pt, 20 nm-Ti and 300 nm-Al are deposited by sputtering. The metal layer is structured using a chlorine-based plasma dry etch. Two steps of SU-8 photolithography result in the structure of the thin cantilevers and the thick body chip. The entire device is finally released by dry etch of the sacrificial layer with SF6. The smallest probe spacing achieved is 10 µm. The fabricated micro-four-point probes were used to measure the sheet resistance of a 140–nm-thick evaporated gold film. A first series of four-point measurements with a fabricated microscopic SU-8-device having a probe-to-probe spacing of 20 µm resulted in a mean value of sheet resistance Rs(µ4pp)=1.57E-6 Ω/square. This value is comparable to the one of Rs(omnimap)=1.52E-6 Ω/square measured with the commercialised Omnimap RS75 resistivity meter from KLA Tencor.

2005

Monolithic integration of Si nanowires with metallic electrodes: NEMS resonator and switch applications

Yusuf Leblebici, Davide Sacchetto, Izzet Yildiz

The challenge of wafer-scale integration of silicon nanowires into microsystems is addressed by developing a fabrication approach that utilizes a combination of Bosch-process-based nanowire fabrication with surface micromachining and chemical-mechanical-polishing-based metal electrode/contact formation. Nanowires up to a length of 50 mu m are achieved while retaining submicron nanowire-to-electrode gaps. The scalability of the technique is demonstrated through using no patterning method other than optical lithography on conventional SOI substrates. Structural integrity of double-clamped nanowires is evaluated through a three-point bending test, where good clamping quality and fracture strengths approaching the theoretical strength of the material are observed. Resulting devices are characterized in resonator and switch applications-two areas of interest for CMOS-compatible solutions-with all-electrical actuation and readout schemes. Improvements and tuning of obtained performance parameters such as resonance frequency, quality factor and pull-in voltage are simply a question of conventional design and process adjustments. Implications of the proposed technique are far-reaching including system-level integration of either single-nanowire devices within thick Si layers or nanowire arrays perpendicular to the plane of the substrate.

Institute of Physics2011

Surface Microstructures on Planar Substrates and Textile Fibers Guide Neurite Outgrowth: A Scaffold Solution to Push Limits of Critical Nerve Defect Regeneration?

Thomas Lehnert, Li Yao

The treatment of critical size peripheral nerve defects represents one of the most serious problems in neurosurgery. If the gap size exceeds a certain limit, healing can't be achieved. Connection mismatching may further reduce the clinical success. The present study investigates how far specific surface structures support neurite outgrowth and by that may represent one possibility to push distance limits that can be bridged. For this purpose, growth cone displacement of fluorescent embryonic chicken spinal cord neurons was monitored using time-lapse video. In a first series of experiments, parallel patterns of polyimide ridges of different geometry were created on planar silicon oxide surfaces. These channel-like structures were evaluated with and without amorphous hydrogenated carbon (a-C:H) coating. In a next step, structured and unstructured textile fibers were investigated. All planar surface materials (polyimide, silicon oxide and a-C:H) proved to be biocompatible, i.e. had no adverse effect on nerve cultures and supported neurite outgrowth. Mean growth cone migration velocity measured on 5 minute base was marginally affected by surface structuring. However, surface structure variability, i.e. ridge height, width and inter-ridge spacing, significantly enhanced the resulting net velocity by guiding the growth cone movement. Ridge height and inter-ridge distance affected the frequency of neurites crossing over ridges. Of the evaluated dimensions ridge height, width, and inter-ridge distance of respectively 3, 10, and 10 mu m maximally supported net axon growth. Comparable artificial grooves, fabricated onto the surface of PET fibers by using an excimer laser, showed similar positive effects. Our data may help to further optimize surface characteristics of artificial nerve conduits and bioelectronic interfaces.

Public Library Science2012

Surface Microstructures on Planar Substrates and Textile Fibers Guide Neurite Outgrowth: A Scaffold Solution to Push Limits of Critical Nerve Defect Regeneration?

Thomas Lehnert, Li Yao

Public Library Science2012