Load cell with positive, negative and zero stiffness operating modes for milli- to nanonewton contact force monitoring in electrical probing

Monolithic integrated circuits (ICs) have been miniaturized over the past five decades, and today their components range in size from hundreds of microns to several nanometers. Making point contact with electrical samples under a microscope is referred to as microprobing or nanoprobing and is carried out using micromanipulators equipped with sharply-ended probe needles. Probe landing on the sample is a critical step in this process since physical contact with the sample can damage the probe tip or the sample itself. Hence, this research was motivated primarily by the desire to find a novel method of monitoring and controlling physical contact between probe tip and sample, increasing its reliability in micro- and nanoprobing. Micro- and nanoprobing often involve qualified operators using microscope visual feedback to land the probe. This method, however, has several limitations, including a constrained field of view and no qualitative contact assessment. A more versatile solution is to measure the contact force between the probe tip and the sample since it does not rely on microscope images and provides an additional feedback channel that complements visual-servoing and facilitates micromanipulation. This work reviews a number of known methods of measuring force on a small scale, but none fully meets the needs of micro- and nanoprobing. The main challenges of these applications include, among others, measuring forces in the nanonewton to millinewton range, using different probe tips based on the type of sample, controlling contact force to obtain stable reliable conditions for electrical measurement, and protecting the probing system against excessive pressure exerted by the probe tip. The core objective of the thesis is to find a method for measuring and controlling the contact force between a probe tip and a sample that addresses the micro- and nanoprobing requirements. The proposed concept is based on a compliant mechanism that transduces the contact force applied to the probe tip into an output displacement. Through stiffness adjustment, the mechanism is capable of handling a wide range of forces that occur during micro- and nanoprobing. As a result of stiffness adjustment, the load cell can be designed at mescoscale, which provides high stiffness in transverse directions, and facilitates the interchangeability of probe tips. Using different probe tips necessitates analyzing gravity effects on the operation of the load cell and compensating for zero offset. Additionally, the study examines active force control implemented through stiffness and offset force adjustment, which provides a constant-force characteristic, bi-stable mode with overload protection, and allows fine probe tip positioning. Several polymer demonstrators and metal prototypes were fabricated as part of the work, allowing the experimental validation of subsequent stages. The performance of these mechanisms was compared with that of the analytical models and the finite element