MEMS based voltage references
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Doctoral thesis (article-based)
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Date
2006-11-03
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Mcode
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Language
en
Pages
109, [42]
Series
VTT publications, 613
Abstract
Voltage references are fundamental building blocks in many instruments like data logging systems, digital multimeters, and calibrators. State-of-the-art DC voltage references are large and expensive Josephson voltage standards, operated at cryogenic temperatures. On the other hand, small and affordable Zener diodes are noisy and require temperature compensation, so there is a gap to be filled between these devices. The situation regarding AC voltage references is even worse. There are no references fundamentally based on AC, beside the AC Josephson voltage standard. Usually AC references are based on generating an AC voltage from DC, and respectively, an AC voltage is measured by converting it to DC. Hence, a small and affordable MEMS based AC voltage reference would be a very unique device. The excellent mechanical properties of silicon microelectromechanical systems (MEMS) have been demonstrated in many commercial applications. Currently the performance of the components is limited by electrostatic instability phenomena and mechanical stress effects arising from component mounting and packaging. However, when these problems are solved, new application areas open up for micromechanical components, for example, in voltage metrology. The stability of a MEMS based voltage reference is ultimately based on mechanical properties of one of the most stable materials: single crystal silicon. This dissertation reports a DC voltage reference and an AC voltage reference based on the pull-in voltage, a characteristic property of an electrostatic MEMS component. First a brief introduction to voltage metrology and MEMS is given, then methods available for making MEMS based voltage references are discussed, and finally results are presented. The results are divided into three Sections: design and manufacturing of the components, readout electronics, and measurement results of the reference long-term stability. The stability of the reference voltage is of major importance in metrological applications and it is studied both theoretically and experimentally in this work. A detailed analysis of the electromechanical coupling of MEMS components is presented. Due to the lack of an appropriate text book, a majority of the formulas are derived from the basic equations by the author, including also those presented in the Methods Section. Component manufacturing, design and materials choices are also discussed focusing on the stability issue. In the experimental part of this work a DC voltage reference and an AC voltage reference were designed, manufactured and characterized. Also two MEMS moving plate capacitors were designed: one optimised for use as a DC reference and the other optimised for use as an AC reference. The capacitor electrodes required metallizing which could not be manufactured using the existing processes. Hence a new silicon-on-insulator (SOI) manufacturing process utilising low temperature fusion bonding was developed. The stability of the AC voltage reference presented in this dissertation is at ppm-level (10−6). This level of performance is sufficient for several applications and outstanding compared to results published earlier for MEMS based voltage references. In the beginning of this work, a slow electrostatic charging effect of the MEMS component was the major factor limiting the device stability. The charging was significantly reduced by using AC voltage (instead of DC) to actuate the component to the pull-in point. However, even in the absence of an external DC voltage, there is an internal DC voltage due to the component built-in voltage. AC voltage actuation together with built-in voltage compensation removed the charging effect in the first order and reduced the drift of the reference below 2 ppm during the three week measurement period. The next challenge is to improve the component mechanical stability including the reduction of the component temperature coefficient. Before commercialisation component mounting and hermetic packaging need further attention as well. Also, the DC voltage reference showed a significant improvement compared to results published earlier. In addition to the actions mentioned above, the DC voltage reference would still require improvement in the feedback electronics.Mikroelektromekaanisiin systeemeihin, eli MEMSiin, perustuvat jännitereferenssit ovat ainutlaatuisia sekä ominaisuuksiltaan että toimintaperiaatteeltaan. Ne ovat pieniä ja tarkkoja, ja niillä on hyvä hinta-laatusuhde. Referenssin toiminta perustuu mikromekaanisesti valmistettuun levykondensaattoriin, jonka toinen levy on ripustettu piijousien varaan. Levyjen väliin kytketty jännite pyrkii lähentämään levyjä toisiinsa, kun taas piijousi vastustaa tätä liikettä. Voimatasapainoa levyjen välillä voidaan kuvata massa-jousi-vaimennin-mallilla ja sillä on kaksi keskeistä ominaisuutta. Sähköstaattinen voima levyjen välillä on verrannollinen jännitteen neliöön, eli sitä voidaan käyttää todellisena tehollisarvomuuntimena. Lisäksi kondensaattorin ulostulojännitteellä on maksimi, jota voidaan käyttää stabiilina jännitereferenssinä, sillä sen arvo riippuu vain yksikiteisen piin materiaalivakiosta ja jousen geometriasta. Koska referenssijännite on samalla myös käännepiste, ohjausjännitteen pienet vaihtelut eivät vaikuta referenssijännitteen arvoon ensimmäisessä kertaluvussa. Näiden ominaisuuksien ansiosta MEMS-jännitereferenssit tulevat haastamaan olemassa olevat jännitereferenssit, esimerkiksi Zenerdiodit. Väitöskirjassa tutkitaan sekä teoreettisesti että kokeellisesti kahta jännitereferenssiä, joista toinen on suunniteltu tasajännitteelle ja toinen vaihtojännitteelle. Molemmille suunniteltiin omat MEMS-komponentit. Niiden suunnittelu, valmistus ja ominaisuudet on kuvattu yksityiskohtaisesti samoin kuin referenssien lukuelektroniikat ja valmiiden laitteiden ominaisuudet. Erityisesti huomiota on kiinnitetty referenssien sähköiseen stabilisuuteen, joka on myös nykyisin kaupallisessa tuotannossa olevien MEMS-komponenttien ongelma. Väitöstyön merkittävin saavutus on MEMS-komponenttien sähköisen stabiilisuuden parantaminen tasolle, joka mahdollistaa niiden hyödyntämisen vaativissa metrologisissa sovelluksissa. Näitä innovaatioita, samoin kuin työssä kehitettyä uutta valmistusmenetelmää, voidaan geneerisesti hyödyntää myös muihin MEMS-sovelluksiin.Description
Keywords
MEMS, micro electromechanical systems, DC voltage reference, AC voltage reference, electrostatic charging, pull-in voltage, long-term stability, micromachining
Other note
Parts
- Kärkkäinen, A., Oja, A., Kyynäräinen, J., Kuisma, H. and Seppä, H. Stability of Electrostatic Actuation of MEMS. Physica Scripta, Vol. T114, 2004, pp. 193-194. [article1.pdf] © 2004 The Royal Swedish Academy of Sciences. By permission.
- Kärkkäinen, A., Pekko, P., Dekker, J., Pesonen, N., Suhonen, M., Oja, A., Kyynäräinen, J. and Seppä, H. Stable SOI Micromachined Electrostatic AC Voltage Reference. Microsystem Technologies, Vol. 12, Dec. 2005, pp. 169-172. [article2.pdf] © 2005 Springer Science+Business Media. By permission.
- Kärkkäinen, A., Pesonen, N., Suhonen, M., Oja, A., Manninen, A., Tisnek, N. and Seppä, H. MEMS-Based AC Voltage Reference. IEEE Transactions on Instrumentation and Measurement, Vol. 54, Apr. 2005, pp. 595-599. [article3.pdf] © 2005 IEEE. By permission.
- Kärkkäinen, A., Awan, S. A., Kyynäräinen, J., Pekko, P., Oja, A. and Seppä, H. Optimized Design and Process for Making a DC Voltage Reference Based on MEMS. IEEE Transactions on Instrumentation and Measurement, Vol. 54, Apr. 2005, pp. 563-566. [article4.pdf] © 2005 IEEE. By permission.
- Kärkkäinen, A., Tisnek, N., Manninen, A., Pesonen, N., Oja, A. and Seppä, H. Electrical stability of a MEMS-based AC voltage reference. Sensors and Actuators A, submitted for publication.
