EUREKA SEPTEMBER 2004 COVER FEATURE STORY

Gyro compasses become devices on chip

Tom Shelley reports on the latest breakthrough in silicon technology, bringing missile guidance technology down to the man in his car

A 4mm diameter electrostatically levitated and capacitance driven ring motor has been developed for use as a gyroscope sensor for control systems for cars.

The first version of the device rotates at up to 20,000 rpm and is suspended with 2.5 micron wide axial and 5 micron wide radial gaps between itself and its surrounds. A second version is 1mm in diameter and rotates at up to 100,000 rpm.

First test samples have already been released and full scale production is expected to commence in the Spring of 2005.

The devices were revealed by Professor Masayoshi Esashi of the New Industry Creation Hatchery Centre in Tohoku, Japan, at a seminar in Cambridge on Nano Technology. The Centre is part of Tohoku University which has a linking arrangement with Cambridge.


Professor Masayoshi Esashi

The gyroscope has been developed at and in conjunction with Tokimec Inc, formerly Tokyo Keiki, a company that has been making somewhat larger sized compasses for the marine market since 1901 and gyroscope compasses since 1918. The present development is aimed at improved motion control and navigation in cars.

Previous low cost angular rate sensors have been vibratory gyroscopes, which respond to Coriolis forces. A rotating gyroscope is potentially much better, and miniature gyroscope systems, costing typically 100,000s have for some time formed the basis of missile warhead navigation systems. The new device is not only much smaller than these, but avoids all friction by doing away with bearings and running in a vacuum, and costs a very small fraction of the price.


Rotor and stator

In it, the ring shaped spinning rotor is made of silicon, with capacitor electrode plates which are charged to induce rotation and other plates that can be energised to maintain the rotor in its null position.


Rotor

Rotation occurs because a voltage applied to the rotational electrodes induces a charge on the silicon rotor so that it rotates to minimise the field energy. The rotor is driven by three phases, with 18 stator poles and 20 rotor poles. Step angle is 6 degrees.

When an angular rate movement is applied at right angles to the axis of spin, the precession torque is counteracted by charge applied to the electrode plates to return the rotor to its null position. The device acts as a dual axis gyroscope in the plane of the rotor, and because of the additional levitation control to keep the rotor at its correct height, acts as a three axis accelerometer. The rotor speed is detected by differential capacitance between the two stators and the rotor. The output periodically changes as the rotor rotates and the frequency of the output indicates the speed of the rotor. The rotational speed is compared with a reference speed. When the rotational speed is lower than the reference speed, a feedback voltage is applied through an inverter.

The manufacturing process begins with wafers of 'Pyrex' glass, which are patterned and etched with hydrofluoric acid to form capacitance gaps for the axial control and also stoppers to prevent sticking of the rotor to the glass substrates during the anodic bonding steps.

Subsequently, metal layers are deposited and patterned to form the electrodes and electrode pads. The glass is anodically bonded to a silicon wafer, 150 microns thick. The silicon wafer is then subjected to deep reactive ion etching to release the rotor. At the same time, islands are formed to serve as feed throughs as is a capacitance gap for radial control. The stacked wafer is then bonded to the bottom glass, which forms a cavity to encapsulate the rotor. Finally, the wafer is diced and the diced chips are mounted in 44 pin metal packages and bonded with gold wire. The package is sealed in a vacuum environment by laser welding.

As an accelerometer, sensitivity to feedback voltage is 0.76V/g for axial acceleration and 1.92V/g for radial acceleration. As a gyroscope, sensitivity is 6.5mV/(deg s) and resolution 0.05 deg/s, whereas the noise floor is 0.15deg/h1/2 . Rotation drive voltage is 9V.


Drivers of the more upmarket 2005 model Japanese manufactured cars can expect to be benefiting from these extraordinary devices working to improve both road handling and navigation.


Latest 1mm diameter rotor version

If this was not enough, Professor Esashi also revealed an even more extraordinary accelerometer device based on an electrostatically levitated 1mm diameter silicon sphere weighing 1.2mg. The sphere is free to move within its exactly spherical enclosure, from which it is separated by a constant narrow gap.

The process starts with the silicon ball, upon which patterned electrodes are deposited, followed by various other layers. The balls are made by a company called Ball Semiconductor Inc, which is headquartered in Texas, but is led by its two Japanese born founders, Akira Ishikawa and Hideshi Nakano.

The Ball Semiconductor company web site gives no clue as to exactly how the balls are patterned with electrodes and integrated circuits apart from mentioning its possession of a maskless exposure system based on use of Texas Instruments Digital Mirror Devices.

The gap in the accelerometer device comes from a sacrificial layer of polysilicon, beyond which is deposited a thick layer of gas permeable ceramic forming the outer shell. The final process step is to implement xenon tetrafluoride etching through the gas permeable shell, to remove the sacrificial layer of polysilicon. The etching method is remarkable because it produces a precise, narrow, gap enclosure without requiring accurately aligned assembly. The team believes it may also offer a route to manufacturing other micro and nano electromechanical constructions of multiple parts, which may be of any shape, not necessarily spherical.

Greater gaps and accelerations require working at higher voltages. The device described to the audience at Cambridge had a 4 micron thick sacrificial layer and required a + 30VDC peripheral circuit to make measurements at up to + 2g. The device produces a linear output with minimal cross axis sensitivity.

Professor Esashi says it is yet to be commercialised and that research into its development continues.

http://www.tokimec.co.jp/english Tokimec Inc
http://www.ballsemi.com Ball Semiconductor Inc
mailto:esashi@cc.mech.tohoku.ac.jp Professor Masayoshi Esashi at the New Industry Creation Hatchery Centre

Pointers

* The ring shaped rotor is 4mm across, 150 microns thick and 300 microns wide. The capacitance gaps is 2.5 microns axial and 5 microns vertical. Normal rotation speed is 12,300 rpm.

* Dynamic ranges are 200 deg/s (gyroscope) and 4g (accelerometer)

* Sensitivity is 6.5mV/(deg s) as a gyroscope and 0.76v/g axial and 1.92v/g radial as an accelerometer

* Noise floors are 0.15 deg/h1/2 , 30g/Hz1/2 , and 20g/Hz1/2 .

For more technical developments see www.eurekamagazine.co.uk

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