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