Contact-free handling using actively controlled electrostatic levitating fields

Abstract

In general electric field forces have the distinctive property of being able to mediate forces to virtually any material in a fully non-invasive and contact-free fashion. Based on this property, electrostatic levitation holds great promise for the semiconductor, solar panel, and flat-panel display industry since the handling of (semi)conducting and dielectric materials in a contact-free manner can bring many advantages and solve long-standing contamination and particulate control problems. These problems arise from the direct mechanical contact through which dielectric and semiconductor materials are commonly handled by process equipment in these industrial areas. Direct mechanical contact can cause chemical and particulate contamination of the handled material. Furthermore, it can result in electrostatic charging through tribo-electric effects, which raises the electrostatic potential of the handled material causing air-borne particles to be attracted to it. Ultimately, chemical and particulate contamination can critically affect performance, reliability, and product yield of the manufactured devices. Electrostatic levitation offers the inherent capability to avoid these contamination problems. In addition to enhancing contamination control, electrostatic levitation can provide uniformly distributed suspension forces. This is an advantage in the handling of very large and thin glass substrates used in the manufacture of flat panel displays as it leads to minimal mechanical deformation of the substrates. This thesis presents a comprehensive and in-depth study on the use of electrostatic fields for the contact-free suspension of (semi)conducting and dielectric materials. The electrostatic levitation devices that have been developed are specifically geared toward novel applications in the semiconductor and flat-panel display industry. Closed-loop feedback control is necessary to stabilize the position and attitude of the levitated object. In order to stably levitate an object, only three degrees of freedom are required to be actively controlled, i.e. the vertical motion and the angular motions represented by the pitch and roll angles. The lateral and longitudinal movements are passively stabilized by restrictive forces originating from the fringing fields existing between the outer edges of the stator electrodes and the suspended object. A generic voltage-controlled electrostatic levitator for the contact-free suspension of conducting disks or panels was developed first. Its main building blocks consist of a feedback controller, high-voltage dc amplifiers, displacement sensors, and a stator electrode structure. Simple guidelines based on the assumption of uniform electric fields are established for the design of suitable stator electrode patterns and applied voltage distributions, which guarantee electric potentials on disks/panels close to zero volts. Squeeze film air damping plays a major role since it may impact the dynamic behavior of the levitator significantly. The reason for this lies in the fact that the forces arising from squeeze film damping can approach values that are of the same order of magnitude as the electrostatic suspension forces. We also investigated both theoretically and experimentally the restrictive lateral forces produced by the fringing fields. These forces are weaker than the levitation forces. In line with the theoretical model, the measurements show that the lateral force can be increased by applying higher stator voltage magnitudes or by decreasing the air gap separation. Based on these observations, the stator design was improved by adding a ring of peripheral sector electrodes having the primary function of generating the fringing fields. Levitation experiments have been conducted in an atmospheric environment, demonstrating the successful suspension of a 4-inch silicon wafer, having a mass of 9.4 g, at a nominal gap separation of 300 µm utilizing centralized PID feedback control. At constant ring electrode voltages of ±1.2 kV, a lateral stiffness value of 0.84 N/m was produced by the improved stator produced, which corresponds to an increase of a factor of 5.5 relative to the conventional stator. A major and growing industrial area of potential application of electrostatic levitation constitutes liquid crystal display (LCD) manufacturing. Glass substrates are basic and essential components in LCDs. They belong to the class of lossy dielectrics. Their charge relaxation times may complicate the task of levitating them stably. A stator electrode suitable for the levitation of lossy dielectrics consists of a regular planar array of parallel bar electrodes to which voltages of differing polarities are alternatingly applied. We develop a general analytical model of the levitation field and force on a lossy dielectric plate produced by this stator electrode. This model takes into account the influence of the atmospheric humidity on the electrostatic charging dynamics. The levitation force dynamics are studied by evaluating the transient response of the field under a step in the applied voltages. In this context, the rate of electric charge build up on the plate is characterized by the suspension initiation time (TSI), which is defined as the time elapsed between applying step voltages to the stator electrodes and start of lift-off of the dielectric plate from its initial position. TSI is theoretically predicted for 0.7 mm thick soda-lime glass substrates, typically used in the manufacturing of liquid crystal displays (LCDs), as a function of electrode geometry, air gap separation, ambient humidity, and step voltage magnitudes. The predicted results are shown to be in good agreement with previously published experimental data. Position measurement based on capacitive sensing technology has been investigated as well since it can lead to an improved level of levitator cleanliness. This stems from the fact that the sensing electrodes can be integrated into the stator electrodes rendering compact, planar structures. A charge-discharge capacitive displacement sensor with improved stray capacitance immunization capabilities was developed. In addition a simple and cost-effective capacitive sensor using the oscillation principle was designed and realized. Both sensors have been calibrated for different target materials, i.e. silicon, aluminum, and soda-lime and quartz glass. Measurements demonstrate a good linear behavior for both sensors. The final part of this thesis deals with cost-effective and compact electrostatic levitator designs. These designs are characterized by small footprints to ensure cleanliness and scalable to many degrees of freedom multi-electrode levitators without incurring excessive and prohibitive economic costs. The first designed levitator is driven by a relay based switching controller. Its key properties are that it is devoid of high-voltage dc amplifiers and a maximum number of only two high-voltage power supplies, capable of delivering constant dc voltages, are required. This number is entirely independent of the number of individually controlled stator electrodes. The inherent switching nature of the system imposes limit cycle oscillations on the levitated object. It is due to the squeeze-film air damping that these oscillations can be significantly suppressed, in particular at small gap separations down to 100 µm or lower. Successful levitation has been achieved for 4-inch silicon wafers, 100×100 mm quartz glass substrates, and a highly flexible aluminum sheet measuring 280×280 mm, respectively. Experiments with the silicon wafer and glass substrates at reference air gap separations down to 100 µm or lower demonstrate limit cycle amplitudes below 1 µm. The second levitator is based on hysteresis control and represents a fundamental improvement on the first levitator in that it enables incorporating active damping in the system through derivative control. This fact opens up the possibility of operating the levitator in vacuum without degradation in performance as demonstrated by simulations. Overall, the proposed levitator retains largely the advantages of the relay control driven levitator. A 4-inch silicon wafer was levitated successfully at a reference gap separation of 200 µm exhibiting a good transient and steady-state suspension performance. The measured switching period was 1.5 ms and the amplitude of the voltage ripple was 10 V. The stability of the limit cycles in both levitators has been analyzed using the describing function method and Filippov’s theory. The first method follows an approximate approach while the second method offers a more precise avenue of tackling the stability analysis coupled with the key capability of analyzing multi-DOF systems.