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 , 30µg/Hz1/2 , and 20µg/Hz1/2
.
For more technical
developments see www.eurekamagazine.co.uk