Embedded high‐precision capacitor measurement system based on ring‐oscillator

Abstract

A direct way to measure the electrical value of capacitors embedded in a circuit using a ring-oscillator is presented. A calibration system ensures the robustness of the measurement process against temperature, power supply and process variations. Silicon results show the ability of the system to measure robustly a large range of small capacitors. The system also provides a noise immunity as the system provides a digital signature of the capacitor value.Introduction: The determination of capacitances formed by metal-oxide semiconductor transistor gate oxide or inter-metal dielectric (IMD) is essential for monitoring insulating layer thicknesses. Capacitor measurement methods can be divided into two categories: external and embedded methods, each having advantages and drawbacks.The external method consists in measuring capacitors directly on silicon, with an external capacitance metre. With this technique, only large capacitors (several picofarads) can be measured because of the kT/C thermal noise introduced by the cable lengths used for the measurement [1]. Embedded measurement methods consist in using on silicon a system that converts the electrical capacitance value into another physical quantity that is easier to measure.A widely used technique is the charge-based capacitance measurement (CBCM) technique [2] where the measured capacitor is alternatively charged and discharged via a controlled pseudo-inverter. The capacitance value is obtained by measuring and averaging the total current flowing through the pseudo-inverter. The technique has been improved so that the charge injection phenomenon induced by the pseudo-inverter can be compensated. The method is called CBCM charge injection-induced errors free (CIEF) [3]. The working principle is based on the measurement of the total current of the pseudo-inverter without the capacitor (i.e. the capacitor is 'cancelled'). This 'cancellation' is performed by applying the same voltage across the capacitor.

Self‐sensing estimator for micromachined electrostatic actuators and parameters’ optimisation

“…This capacitive measurement approach has certain advantages: (i) its interface circuits can be packaged with the micro‐electro‐mechanical systems (MEMS) unit and (ii) it is minimally sensitive to changes in temperature. However, this practical method often requires high‐frequency excitation signal sources or additional sensing structures [6]. The excitation power source not only affects the steady‐state performance of the actuator, but it may also generate harmonics that distort the measurement signals [7].…”