Design, Fabrication and Characterization of a MEMS Thermal Displacement Sensor

Design, Fabrication and Characterization of a MEMS Thermal Displacement Sensor

Hogervorst, R.P.

Khiat, A. (mentor)

Mechanical, Maritime and Materials Engineering

Precision and Microsystems Engineering

2010-03-16

This thesis is a part of the CLEMPS (Closed Loop Embedded MEMS-based Precision Stage) project. Within this project a MEMS-stage should be positioned using a feedback (displacement) signal with 10-nm accuracy over a 100 ?m range. This thesis investigates if thermal sensing can be used as a reliable sensing mechanism within the CLEMPS project. Lantz et al demonstrated a promising thermal displacement sensor that achieves nanometre resolution on a 100 ?m range [1]. Using this sensor as a basis, five conceptual designs for thermal displacement sensors are introduced, of which one concept operates in vacuum and four concepts operate in a normal environment. Basic analysis showed that each conceptual design is feasible, except for thermal sensing in vacuum. The designs have large variety in fabrication complexity and estimated performance. The behaviour of the sensors is strongly nonlinear due to dependencies of the material properties on temperature. Different thermo electrical models are introduced that quantify the performance of the designs. Those models are used to gain extensive insight in the behaviour of the sensors and to provide comparison between the performances of the different sensor designs. The models are also applied as design tool for investigating the influence of geometric variations on the sensing performance. Only two of the designs are compatible with the available SOI-wafer based processing. These sensor designs were actually fabricated and integrated within an actuated MEMS stage. For experimental evaluation of the general feasibility of thermal displacement sensing, the thesis focuses on the simplest design. An electronic measurement circuit is designed and fabricated that provides the actuator voltages and performs the differential current-to-voltage amplification for the measurement signal. The mechanical behaviour of the stage is experimentally characterized to determine the relation between the actuator voltage and stage displacement. Now, the (quasi-) static measurement performance of the fabricated sensor can be experimentally characterized by actuating the stage at a low frequency, while recording the applied actuator voltage, the resulting individual heater currents and the differential measurement current in xPC. This data is analysed to obtain the sensor performance in terms of offset, sensitivity, nonlinearity, noise and power consumption. The static measurements are repeated on many sensors to investigate the repeatability and influence of geometric variations. The influence of varying conditions in terms of heater supply voltage, environmental temperature and humidity is determined by performing many characterizations on one device using controlled measurement conditions. Also, the dynamic performance is evaluated in terms of sensor drift, frequency behaviour and hysteresis of the device by using supplementary measurements The feasibility of thermal displacement sensing within the CLEMPS is evaluated using the results on the sensor performance for design IV. Using acquired insight, the performance limiting effects are identified and recommendations for improvements are presented.

MEMS
sensor
thermal displacement

http://resolver.tudelft.nl/uuid:08358c91-bb35-44c8-83e2-9de17b078f05

2010-03-25

Student theses

master thesis

(c) 2010 Hogervorst, R.P.