A microfabricated silicon device can locally measure the intensity of an electric field without any significant field distortion.
The measurement of electric fields is important in a variety of applications, including the identification of electrostatic hazards during process control in industry1, the study and prediction of weather phenomena such as lightning2, and the monitoring of power utilities or high-voltage power lines to ensure the safety of humans and equipment3. For such applications, methods and devices are required to measure low-frequency (a.c.) and static (d.c.) electric fields. Most electric field sensors, which are also called field meters, are based on induction probes, optical sensors or field mills4. A fundamental limitation of these field meters is that the devices themselves often perturb the field to be measured, leading to field distortions and inaccurate measurements. This is due to the fact that dielectric parts in the instruments can have surface charges that produce moderate distortions, and electrical conducting parts in the instruments can generate significant field distortions, which are made worse if the instruments have to be grounded or connected to large conductors to set a reference potential. These traditional electric field sensors can also be bulky. Therefore, in an attempt to reduce their size, as well as their power consumption, MEMS (microelectrochemical system) technology has recently been explored5,6,7. However, despite the compact size of MEMS field meters, the devices can still be problematic as they include active electrical parts and components that can cause non-negligible field perturbations at this low size scale. Writing in Nature Electronics, Andreas Kainz and colleagues now report purely passive MEMS devices that can be used to measure electric field strength without introducing severe field distortion8.
The researchers — who are based at TU Wien and Danube University Krems — fabricated their MEMS devices using wafer-scale silicon-on-insulator technology, which relies on a silicon/insulator/silicon substrate. The devices measure the electric field strength using a spring-suspended millimetre-sized seismic mass that is microfabricated in silicon. When placed in an electric field, the mass can experience a force (F es) due to electrostatic induction6, which results in a mechanical displacement (δx) that is optically detected (Fig. 1a). The relative displacement between arrays of holes in the movable seismic mass and opaque patterned areas in a fixed glass layer on top modulates the light path between an LED and a photodiode and produces the output signal (Fig. 1b,c). Using finite element, multi-domain simulations, the researchers also explore possible layouts for their electric field sensor. In particular, they show that a fixed conductive section, situated to the side of the seismic mass at a gap smaller than 200 μm, can act as a field concentrator and can increase the electrostatic force.
A key feature of the MEMS device design is that it does not contain electrical parts or circuitry, and so reduces field distortion to a minimum. The LED and photodiode that are used for detection could also potentially operate from a remote location via fibre-optic links, which would mean the sensor head could become totally passive. The sensor has an overall size of a few square millimetres (Fig. 1d), and therefore it offers a highly localized probe of the electric field: that is, it can provide a good spatial resolution.
The electrostatic force and the resulting displacement (δx) are proportional to the square of the electric field (E 2) along x, and therefore the device is not able to discriminate the oriented direction of the electric field, that is, the sign, which could be a limitation in applications where it is necessary to detect voltage or charge polarity. Indeed, due to this quadratic transduction relationship, the device is, strictly speaking, a field intensity (or magnitude) sensor rather than a field sensor.
The nonlinearity of the sensor response means that an electric field that is sinusoidal at the frequency f will produce a sinusoidal sensor output at the double frequency, 2f. Kainz and colleagues exploit this characteristic of their devices to test them in a set up where an a.c. input electric field at frequency f is applied and the output signal is detected at frequency 2f using a lock-in technique. Applying this approach out of the lab would be equivalent to performing a frequency analysis of the unknown field magnitude at half the reference frequency used in the lock-in. The researchers fabricated a series of sensors with slightly different dimensions, resulting in each having different values for the input frequency bandwidth, the maximum measurable field and the equivalent field fluctuation spectral density, which defines the resolution of the device. For one set of dimensions, values of around 75 Hz, 100 kV m–1, and 200 V m–1 Hz–1/2, respectively, were obtained.
The sensors were also tested under a d.c. electric field and the expected quadratic response observed. As often happens with d.c. field meters, the d.c. measurements proved problematic in terms of stability. In particular, due to the finite conductivity of air, charge leakage occurred, which prevented true d.c. measurements and limited the lower frequency to about 0.5 Hz. The effect could be mitigated by operating the sensor in a vacuum. The researchers also analysed the influence of temperature on the device. They found that between –40 and 50 °C, the elastic stiffness of the MEMS sensors exhibits a temperature dependency of around 10%, which could possibly be reduced in a new design. While work remains to be done to optimize and refine the performance of these devices, Kainz and colleagues have introduced a novel approach to the field of microscale electric field sensors. Their compact MEMS devices, which can provide localized measurements of electric field strength with limited field distortion, could provide a valuable method for diverse applications.