Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 1551 – 1554
30th Eurosensors Conference, EUROSENSORS 2016
Low-voltage, high-tuning range MEMS variable capacitor using closed-loop control E.E.Moreiraa,b,*, J. Cabralb, J. Gasparc, L. A. Rochaa,b a
CMEMS-UM, Univeristy of Minho, 4800-058 Guimarães, Portugal ALGORITMI CENTER, University of Minho, Campus Azurém, 4800-058 Guimarães, Portugal c International Iberian Nanotechnology Laboratory (INL), 4715-330 Braga, Portugal
b
Abstract A low-voltage, high-tuning range variable MEMS capacitor using parallel-plates electrostatic microactuators operated in close-loop is introduced in this paper. The structures were fabricated using an in-house 2-masks process on SOI wafers and the use of a closed-loop controller increases the stable displacements of the structure beyond the pull-in limitation along the full available gap (the limitation are the mechanical stoppers). Additionally, the structure has two pairs of actuators, in opposing directions, enabling bi-directional displacement leading to extended tuning range. The fabricated capacitors have a designed tuning range close to 17. A lower actuation voltage is also a main benefit for future integration on low-power devices (measured pull-in voltage of approximately 3.25V). The devices were characterized and a capacitance variation of 2.3pF was experimentally verified. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Type your keywords here, separated by semicolons ;
1. Introduction Over the past years, MEMS variable capacitors were presented as a suitable alternative for radio frequency (RF) devices such as tunable filters, phase shifters and tunable antennas [1]. Variable capacitors require low actuation voltage, high tuning range, high-Q and high reliability [2] for proper use in RF applications. Control laws for parallelplate microactuators have already demonstrated their efficiency to improve the total stable displacement (up to 88.9%)
* Corresponding author. Tel.: +351 253 510 190; fax: +351 253 510 189. E-mail address:
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1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.458
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of the full-gap including bi-directionality, enabling extended travel range [3, 4]. Closed-loop control can be applied to parallel-plate actuated variable capacitors to create a low-voltage, high-tuning range variable capacitor. 2. Variable Capacitor Design The variable capacitor system introduced here is composed by two sets of actuators, differential reading electrodes for position sensing and a variable capacitor (Fig 1).
Fig 1. SEM image of the fabricated device.
The tunable capacitor includes two sets of electrodes, in series, with the central mass of the structure connecting the two capacitors (C1 and C2 in Fig 2). This configuration reduces the series resistance of the tunable capacitor contributing to an increase in the Q-factor of the capacitor. It also creates a path for the variable capacitor electrodes that is fully decoupled from the sensing and actuation control mechanism, as long as the signals have different ground references.
Fig 2. Configuration of the capacitors that result on the final variable capacitor.
The devices were fabricated using a SOI (silicon on insulator) wafer (active layer of 25 µm) and fabrication steps are shown in Fig 3. To fabricate the devices, the following steps were performed: a) 300nm of AlSiCu were deposited on top of a SOI wafer, followed by lithography and patterning. b) The active layer was patterned using DRIE (deep reactive ion etching) using a 2µm photoresist as mask. c) The structures were released using HF etching to remove the buried oxide (approximately 2µ m).
Fig 3. Fabrication steps used to fabricate the variable capacitor devices.
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The designed structure parameters are presented in Table 1. The main target design of the MEMS device was to allow a high tuning range with a low actuation voltage and therefore the devices were designed to have a low pull-in voltage around 3V in order to enable the application of the capacitors in low-power applications. Table 1. Designed structure parameters. Parameters
Value
Spring constant (k)
3.54 N/m
Pull-in voltage (Vpi)
3.04V
Mass (m)
67.45µg
Rest gap distance (d0)
2.25µm
Tunable capacitor
110fF – 1.77pF
3. Experimental Results Initially, the pull-in voltages were measure and measurements show that they are in close agreement with the expected values (Fig 4).
Fig 4. Measured pull-in voltages (both actuation sides).
Fig 5. Readout circuit characterization.
Next, the output of the readout circuit was calibrated in order to obtain the equivalent travel distance of the movable structure (see Fig 5). The behavior of the output is non-linear, as expected, and the curve obtained is used to convert voltage to distance, for the controller with feedback linearization. A PID controller with feedback linearization was implemented in a FPGA (Field-programmable gate array) [3, 4] and when applied to the fabricated devices, full-gap traveling (limited by mechanical stoppers) was successfully performed (see Fig 6). The theoretical tuning range factor is 17, corresponding to a capacitance variation from 110fF to a maximum of 1.66pF (this corresponds to a total displacement of 4 µm between the capacitor plates). The designed gap between the actuators plates is 2.25µm but the available gap (due to the mechanical stoppers) is just 2µm. Nonetheless, during characterization, and using a similar methodology as used in [4], a gap at rest around 3µm was estimated. This gap increase as compared to the designed one is due to the process over-etch. Measurements of the capacitance curve along the full-gap was performed by placing the variable capacitor in a high-pass filter while measuring the filter characteristics (a lock-in was used to do the required sweep measurements to retrieve the bode-plots while the MEMS capacitor value was changed). A capacitance variation of approximately 2.3pF was experimentally measured (see Fig 7). This value is slightly higher than the expected value, however, the models used do not take into account the fringe fields, that can explain the higher values measured. In Fig 7, only the capacitance variation is presented, and not the nominal values, since the parasitic capacitances of the setup make it extremely difficult to accurately measure the absolute capacitor values.
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Fig 6. Experimental results showing travel along the full available gap (3µm) and the respective actuation voltage required to move the structure to the desired reference.
Fig 7. Capacitance variation of the capacitor along the full gap. The voltage reference is the voltage given to the controller to get a given position.
4. Conclusions and Future Work Structures composed by parallel-plate electrostatic micro-actuators, differential reading electrodes and additional plates for a variable capacitor are proposed here. The devices were fabricated in a two-mask SOI process. A closedloop control was implemented on a FPGA and allows travel extension about 267%, corresponding to a displacement of 88.9% of the full gap. Two-sets of actuation and reading electrodes enable bi-directional travelling, doubling the available gap and consequently the capacitor tuning range. The control law was tested and stable displacements along the entire gap were validated. The pull-in voltage of the structure was specially designed to be in agreement with the specification for low-power applications and the experimentally measured values are close to the prerequisite. The capacitance variation, of the tunable capacitor, was measured and was found to be 2.3pF. In the future, research will focus on methods to better decouple the tunable-capacitor from the actuation and sensing capacitors.
Acknowledgements This work is supported by FCT with the reference project UID/EEA/04436/2013, by FEDER funds through the COMPETE 2020 – Programa Operacional Competitividade e Internacionalização (POCI) with the reference project POCI-01-0145-FEDER-006941.
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