Micromachined probes for high-speed and high -sensitivity photoconductive sampling of mesoscopic devices.

Abstract

This dissertation investigates the fabrication and use of micromachined probes to measure picosecond signals on devices, especially on mesoscopic structures. Photoconductive sampling technique is chosen because of its high sensitivity and its ability to make direct voltage measurements. A miniature photoconductive probe is fabricated from low-temperature GaAs (LT-GaAs) using standard semiconductor processing techniques including etching, metallizing, and backside processing. From this micromachining process a probe is created with a 1-mum thickness of LT-GaAs and a tip having a 7-mum spatial resolution. The probe is mounted onto a single-mode optical fiber to couple the light onto the photoconductive switch and to allow the probe to be freely positionable. Because of the fast carrier lifetime of LT-GaAs, the probe has a 3.8-ps response time. The probe's capacitance affects its response as well as the responsivity. The sensitivity of the probe is limited to about 1-muV/(Hz)<super> 1/2</super>. However, when the probe is integrated with a JFET source follower, the sensitivity can be improved to tens of nV/(Hz)<super>1/2</super>. This higher sensitivity occurs since a higher frequency modulation can be used to suppress the effect of laser noise fluctuations. In addition to the higher sensitivity, absolute voltage measurements are also possible. An improved theoretical model explains the influence of the modulation frequency on the sensitivity and absolute voltage measurements. Other types of devices can be fabricated using micromachining techniques. A small cone tip is placed on the probe electrode to create an atomic force microscope probe with micron to sub-micron resolution to obtain topographical information of a device. Another micromachined device that can be fabricated is a miniature terahertz emitter. The device's emitted radiation is due to the magnetic moment since the electrode has closed loop geometry. A split-gate device is fabricated in order to measure the propagating signal using the probe. The current versus gate voltage shows quantized conductance under a low temperature environment. Theoretical calculations reveal that the temporal response of the split-gate transistor requires a probe with sub-microvolt sensitivity and picosecond resolution. The probe's capability to launch and detect signals in a cryogenic environment makes it an appropriate method for measuring the response of the split-gate device.