I. Introduction
The sensing of charge with high resolution plays an important role widening the use of electrometers or charge sensors in many diverse applications, such as surface charge analysis, mass spectrometry, tunneling microscopy, powder technology and aerosol science, bimolecular charge detection and ionization current measurement in ionization chambers [1]. The commercial transistor based electrometer, such as, the Keithley 6517A has a minimum charge resolution of 10 fC (~63k electrons) [2]. However, for bespoke scientific applications where charge noise of less than 1 fC is required, Keithley 6517A’s resolution is insufficient to meet the scientific needs. Recently, both resonator type and vibrating-reed (VR) type microelectromechanical systems (MEMS) electrometers exhibit significant improvement on charge sensing [1]. Nevertheless, the resonant MEMS sensors operated at room temperature suffer a poor charge resolution due to high phase noise. Thus, a vacuum environment is vital to achieve high quality factor (Q factor) for overcoming phase noise issue to improve the device performance [3]–[8]. On the other hand, compared to the existing commercial macro-scale and resonator type MEMS charge sensors, the vibrating-reed based MEMS charge sensors exhibit superior charge resolution at room temperature, and vacuum environment is not required [9]–[13].