Abstract
Most accelerometers today are based on the capacitive principle. However, further miniaturization for micro integration of those sensors leads to a poorer signal-to-noise ratio due to a small total area of the capacitor plates. Thus, other transducer principles should be taken into account to develop smaller sensors. This paper presents the development and realization of a miniaturized accelerometer based on the tunneling effect, whereas its highly sensitive effect regarding the tunneling distance is used to detect small deflections in the range of sub-nm. The spring-mass-system is manufactured by a surface micro-machining foundry process. The area of the shown polysilicon (PolySi) sensor structures has a size smaller than 100 µm × 50 µm (L × W). The tunneling electrodes are placed and patterned by a focused ion beam (FIB) and gas injection system (GIS) with MeCpPtMe3 as a precursor. A dual-beam system enables maximum flexibility for post-processing of the spring-mass-system and patterning of sharp tips with radii in the range of a few nm and initial distances between the electrodes of about 30–300 nm. The use of metal–organic precursor material platinum carbon (PtC) limits the tunneling currents to about 150 pA due to the high inherent resistance. The measuring range is set to 20 g. The sensitivity of the sensor signal, which depends exponentially on the electrode distance due to the tunneling effect, ranges from 0.4 pA/g at 0 g in the sensor operational point up to 20.9 pA/g at 20 g. The acceleration-equivalent thermal noise amplitude is calculated to be 2.4–3.4 mg/. Electrostatic actuators are used to lead the electrodes in distances where direct quantum tunneling occurs.
Highlights
The basic principle of an accelerometer is the deflection of a proof mass
With respect to previous work, the results show that instead of increasing the sensor resolution of an accelerometer [25,26,33,42,43,44,45,47], the highly sensitive tunneling effect can be used to miniaturize the required sensor area
The further miniaturization of acceleration sensors reaches its limits using conventional transducer principles due to scaling laws when the sensor structures are isometrically reduced in size
Summary
The basic principle of an accelerometer is the deflection of a proof mass. Today’s commercial accelerometers are based on established transducer principles, such as capacitive, piezoresistive, piezoelectric, and thermal effects. In order to increase the performance and sensitivity, large proof masses and long beams are provided since noise mechanisms lead to limiting properties [1,2,3,4]. With respect to the scaling laws [5], an isometric reduction of the proof mass leads to a substantial reduction of the deflection at an applied acceleration. In the case of the piezoresistive or piezoelectric principle, the reduced deflection of the mass leads to a lower deformation and a significantly reduced measurement signal. Since the sensor properties are mainly dependent on the signal-to-noise ratio, the miniaturization of today’s conventional methods for measuring acceleration is limited. A further decrease of the adequate sensing area size leads to the need of new methods and transducer principles
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