Article for New Electronics
Published in October 2001 Issue
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Copyright 2001 New Electronics
Silicon Micromachining Technology
Increases Capacity of Hard Disk Drives
MEMS(Micro-Electro-Mechanical-System) sensors and microactuators compensate for
vibrations and allow finer head positioning, bringing more than tenfold improvement in
tracks-per-inch
Benedetto Vigna
STMicroelectronics
Demand for disk drive capacity increases steadily, driven by electronic photography,
media files and bloated operating systems. At the same time the amount of space
available for storing data is decreasing: today s business customers require more
compact servers to squeeze more into each rack, small form factor desktops to leave more
room for working, slimmer laptops that are easier to carry and even palmtop computers
with micro drives. This extra capacity can be provided by adding more platters, larger
disks or new media, but these are all expensive options. Silicon micromachining
technology is now bringing to the disk drive industry new solutions that allow drive
capacity and speed to be increased without any changes in the recording method, media
or heads.
Capacity of hard disk drives can be boosted by increasing the number of tracks on each
platter. Typically the data density of a drive along the recorded tracks  measured in
tracks per inch -- is as much as ten times that of the track density - expressed in bits-per-
inch. The reason for the relatively wide spacing between tracks is the limitation on the
precision of the head tracking servo and the need for drives to be vibration resistant, since
even in benign environments they are subject to vibrations from other drives.
One way to squeeze the tracks closer together is to compensate for vibrations with a
closed loop system that senses the vibration then drives the head actuator to ensure that it
stays in place over the recorded track. In addition to increasing capacity this method can
also improve the overall speed performance of the drive because there is less risk that
strong vibration could cause a temporary loss of tracking, forcing the drive to waste time
restoring the head to the right position  an operation which requires a minimum of one
disk rotation.
The concept of vibration compensation is not new, but what makes it so appealing today
is the emergence of MEMS sensors which combine high performance with the economic
benefits of mass production on silicon wafers, which both reduces cost and guarantees
uniformity.
For this application STMicroelectronics has developed angular accelerometer sensors that
measure the acceleration about the vertical axis of the drive. Typical disk drives use a
voice coil actuator to position the read/write head. This is called a  voice-coil actuator
because conceptually it is like a loudspeaker in the sense that it consists of a coil moving
in a magnetic field. Physically, however, it resembles more the tracking arm of an old-
time vinyl record turntable. Since the actuator rotates around an axis parallel with the
disk axis it is sensitive to the rotational component of vibrations, rather than lateral or
longitudinal vibration.
These vibrations can be sensed by using two linear accelerometers, but the positioning is
critical and repeatability less certain. To overcome these limitations ST has chosen a
rotational sensor where a wheel-like structure rotates inside a fixed array of spokes; this
structure is machined from a solid wafer of silicon using photo-chemical operations.
Movements of the drive and thus the sensor devices causes the rotor to move relative to
the stator cause a change in capacitance that is sensed, filtered and converted by an
integrated circuit housed in the same SO-24 package.
Called L6671, this combination of MEMS sensor and interface chip provides a digital
output through a 5V tolerant 3.3V three-wire serial bus and achieves a signal bandwidth
of 800Hz, a sensitivity of 2.5rads/s2, a full-scale sensitivity of 200rads/s2 and a signal-to-
noise ratio of 37dB over 30-800Hz. Capacitance changes as sma ll as 0.05fF (0.05
femtoFarad = 0.05x10-15F) can be measured by the interface chip in the L6671.
MEMS-based sensor devices like the L6671 will lead to a much greater use of
acceleration sensors in a wide range of end products because they are simple to use,
accurate, reliable and relatively inexpensive to manufacture. In the automotive field, for
example, rotational accelerometers can be used in stability control devices to sense yaw
movements of the car; in computer games sensor devices enable hand held gamepads that
are sensitive to movement and in computer pointing devices they can eliminate the need
to have a clean, flat work surface.
Microactuator for Dual Servo
Using just vibration sensors with a conventional electromagnetic head positioning
actuator already brings a significant improvement to disk drive performance by allowing
tracks to be spaced closer together. The number of tracks per inch can be further
increased by adding a second, fine-positioning actuator at the end of the head suspension.
This dual-servo approach can be implemented using a tiny electromagnetic actuator, but
this is costly to manufacture and also brings magnetic fields close to the head surface
where they can disturb the read and write operations. Using silicon micromachining
technology, however, today it is possible to build a MEMS microactuator based on
electrostatic principles that is ideal for this role.
STMicroelectronics is developing a commercial microactuator for this application that
has a basic wheel like structure similar to the rotational accelerometer and is made using
the same basic manufacturing process. This device has a wheel-like rotor, with many
spokes, that is free to move within two arrays of fixed spokes that form the stator
assembly. By applying a voltage of +/-10V to the stator elements the rotor can be moved
very precisely about the vertical axis over a limited angular range. A read/write head
attached to the rotor can thus be moved about a micron in each direction.
Rotational accelerometers like the L6671 are hermetically sealed before being mounted
on the lead frame to protect the moving structures from contamination, and also to assure
that the gas inside the device always has the same composition and pressure, otherwise
the characteristics of the structure would vary. In fact the effect of gas pressure on
resonant frequency provides a non-destructive method for measuring the pressure inside a
closed device.
In the case of the head positioning actuator the assembly is more complex. Since the
head must be free to move the rotor/stator assembly is covered by a layer that is etched to
create a very small circular gap. This allows the rotor and head to move freely, but
prevents contaminants entering the assembly. This protection is sufficient because the
microactuator will always be used inside a hard disk drive, which is a clean, dry, dust -
free environment.
Silicon micromachining technology provides engineers in many disciplines with new
solutions and new opportunities that will not only improve the performance of existing
products but also lead to the development of new end products. The key advantage of
MEMS is that it brings the economy of scale of silicon wafer technology to a broad range
of electro-mechanical, fluidic and optical components. The basic silicon micromachining
steps used in MEMS fabrication are identical or similar to those used in silicon chip
manufacture, so existing production equipment and knowhow can be applied to these
new components.
In addition to the electromechanical sensors and actuators, MEMS technology can be
used in all optical networks to make light switching devices and in radio frequency
circuits to make filters, switches and resonators. Applying fluidic MEMS technology we
can also make laboratory-on-chip solutions for sensing chemicals or even DNA, enabling
new portable analysis products that will have a major impact on the medical world and
could also be used by consumers.