Silicon MEMS for Detection of Liquid and Solid Fronts

High-precision manipulation of small-size objects is an attractive and challenging topic for both industrial production and fundamental scientific research. The capability of monitoring micro-samples during handling is essential to the accuracy and efficiency of a handling system for both liquid and solid samples. When handling liquid samples with small volume, tools (such as drop-on-demand inkjet dispensers and on-chip micro-fluidic channels) are often used to generate droplets or liquid segments. The typical volume ranges from nano-liters to several pico-liters, and an accurate control of the sample volume is critical in many applications. The liquid volume can be derived from the detection of the position of the liquid surface in a fluidic channel. On the other hand, when manipulating micro-scale solid objects, such as micromechanical components or living cells, with mechanical tools, a reliable and efficient handling is highly desired. In this case, the in-situ detection on the solid-solid contact between the samples and the mechanical tools, including the contact force, position and orientation, are essential for successful carrying out the high-precision manipulation. With MEMS technology, the position detection for both liquid surface and solid contact can be achieved in a similar fabrication platform. Current existing inspection tools for micro-manipulation have limited capabilities, in terms of resolution, complexity in assembly, and often rely on external equipments, such as microscopes. Their application in high-performance in-situ monitoring is thus limited. Recently, MEMS technology has been widely used to fabricate different kinds of handling tools to miniaturize the micro-manipulation system, for its small-scale, low cost and better reliability. This gives great opportunities to integrate sensing elements directly into the handling tools during the fabrication. With properly-selected sensing principles and the-state-of-the-arts fabrication technologies, compact manipulation system with integrated sensing capability can be implemented. Apart from the design and fabrication of the MEMS sensors, dedicated readout circuitry is required to achieve a good inspection. With the trend of miniaturization of MEMS devices, the level of the output signal is becoming smaller and smaller, and can often be buried in electrical noise as well as undesired signals introduced by parasitic components. In this thesis, we address the detection principles for liquid surface position and solid contact position, as well as their integration and readout issues, by presenting two MEMS approaches using the capacitive and piezoresistive principle, respectively. The first approach is a capacitive sensor integrated into an inkjet nozzle to monitor the position of the ink meniscus inside the nozzle orifice. The second one is a piezoresistive sensor, which monitors contact positions and forces between solid samples and contact surfaces of micro-manipulation tools. The two types of devices share a similar fabrication platform and readout strategy. The fabrication platform is introduced in chapter 2. A bulk micromachining process, employing both the anisotropic wet etching of silicon and the DRIE process, is chosen as a general platform (semi-SOI) to implement the proposed devices. Chapter 3 develops a readout strategy based on the lock-in principle. By modulating the signal of interest to a high-frequency band, the influence of the flicker noise in the readout circuit, which dominates the noise in the low-frequency band, is eliminated. The strategy also suppresses the influence of parasitic capacitances, which otherwise can be fatal to capacitive detection. A femto-Farad capacitive test structure is designed and fabricated. It has a capacitance of two to three orders lower than the conventional MEMS capacitor sensors (typically at pico-Farad level), allowing an experimental study of the proposed readout method for femto-Farad capacitive sensors. A minimum detectable capacitance variation as low as 1 aF, with a signal bandwidth of 100 Hz, is achieved. A capacitive sensor to monitor the position of the ink meniscus in an inkjet nozzle is presented in chapter 4, as a demonstration of liquid surface position detection. Capacitive sensors are very suitable for liquid level detections with a sub-micron resolution thanks to their low intrinsic noise, flexibility in the arrangement of electrodes and possibility to be embedded on the sidewalls of fluidic channels without influencing the liquid flow. However, to integrate vertical electrodes on the sidewall of liquid channel is a challenging task. An IC-compatible process is developed for this purpose, allowing the integration of multiple electrodes with micron-level accuracy. The capacitance of the fabricated sensor varies from 1.5 fF (empty channel) to 13.1 fF (channel filled with 63 pL of water), and a 60 dB resolution is achieved with a 33 kHz measurement bandwidth. The system has a capacitive sensing resolution of 0.057 aF/Hz1/2, which corresponds to a volumetric resolution of 0.22 fL/Hz1/2 and a liquid surface position resolution of 0.17 nm/Hz1/2 in a vertical channel with a diameter of 40 ?m. To investigate the principle and technique for detecting the contact position between a micro-object and the contact surface of a handling tool (such as MEMS micro-grippers), miniaturized piezoresistive MEMS sensors are integrated (chapter 5). Piezoresistive sensors are chosen for their small dimensions and straightforward readout method, which allows multiple sensing elements being integrated in a single device and being monitored at a same moment. The developed sensing principle allows an in-situ monitoring of contact conditions, including contact forces and contact positions. Based on this principle, devices have been implemented to realize different functionalities: 1) an array of five sensing plates detecting the orientation of an object and its force distribution on the plates, and 2) a 2D sensing plate detecting a contact force and its 2D contact position on the plate. The devices are fabricated with the “semi-SOI” platform introduced in chapter 2, and are compatible with existing micro-gripper processes developed in the DIMES lab. A basic sensing cantilever is capable of detecting a vertical contact position on its contact surface. The resolution of position detection is 3 ?m under an applied contact force of 1 mN. The resolution of the force detection is 3.6 ?N. A sensitivity of 2.8 V/N is obtained with an effective supply voltage of 177 mV across the Wheatstone bridge. Combining five sensing cantilevers into a sensing array, a distribution of contact forces between the object and the five adjacent contact surfaces can be monitored, thus allowing the detection of the shape of the object contact front. At the same time, the five measured vertical contact positions allow an estimation of the object position and orientation. The fabricated 2D sensing plate detects the contact position of an applied contact force in 2D. Taking into account the electrical noise, the theoretical spatial resolution of the plate is 5 ?m under a contact force of 1 mN, and the force resolution is 10-20 ?N. Although the developed devices are designed to be integrated with silicon MEMS based micro-grippers, the concept behind the devices is suitable for many other applications. The devices presented in this thesis successfully demonstrate two different approaches (capacitive and piezoresistive) for in-situ monitoring liquid and solid micro-samples during handling, based on integrated MEMS sensors. The developed principles and techniques can potentially improve the accuracy and efficiency of micro-handling systems. Further research needs to be carried out to investigate the performance of the fabricated devices in existing handling tools, such as inkjet printheads and micro-grippers. This raises several new challenges for future research, including assembly of the sensors on the handling tools, analysis of the detected information and implementation of closed-loop control systems.