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The core components of this probe storage system are a two-dimensional
array of silicon probes (cantilevers) and a micro-mechanical scanner which
moves the storage medium relative to the array. A sophisticated design
levels the probes above the storage medium with high precision and ensures
that external vibrations and impacts are absorbed. For the device to perform
its reading, writing and erasing functions, the cantilever tips are brought
into contact with the storage medium — a thin film of a custom designed
cross-linked polymer coated on a silicon substrate, which is moved in
the x- and y-directions. The storage medium is positioned with nanometer-scale
accuracy relative to the cantilever array.
Our most recent array design consists of an array of 64 × 64 cantilevers
(4096) on a 100 µm pitch. The 6.4 × 6.4 mm² array is fabricated
on a 10 × 10 mm² silicon chip using a newly developed "transfer
and join" technology that allows the direct interconnection of the
cantilevers with CMOS electronics used to control the operation of the
cantilevers. With this technology the cantilevers and CMOS electronics
are fabricated on two separate wafers, allowing the processes used in
the fabrication to be independently optimized. This is a critical feature,
as many of the processes used to fabricate mechanical structures such
as cantilevers are not compatible with the fabrication of CMOS electronics.
Using a few additional processes steps, the cantilevers are transferred
onto the CMOS wafer, using a soldering process that provides a mechanical
and electrical interconnect to the CMOS wafer. This process is done at
the wafer level and is therefore compatible with low-cost batch fabrication.
The cantilevers used in the array are of a three-terminal design, with
separate heaters for reading and writing, and a capacitive platform for
electrostatic actuation of the cantilevers in the z-direction. The cantilevers
are approximately 70 µm long, with a 500-700 nm long tip integrated
directly above the write heater. The apex of each tip has a radius on
the scale of a few nanometers allowing data to be written at extremely
high densities (greater than 1 Tb/in²). In addition to the cantilevers,
the array chip also carries eight thermal sensors which are used to provide
x/y positioning information for closed-loop operation of the microscanner.
Movement of the storage medium relative to the cantilever array is achieved
using a silicon-based x/y microscanner. The scanner consists of a 6.8
× 6.8 mm² scan table, which carries the polymer medium, and
a pair of electromagnetic actuators. Both the scan table and the actuators
are supported by silicon springs that are 10–12 µm wide and
approximately 400 µm thick. The scan table, spring system, and actuator
frames are fabricated on a silicon wafer using a deep trench etching process.
The scanner chip is mounted on a silicon base plate, which acts as the
mechanical ground of the system and provides a clearance of approximately
20 µm between its top surface and the bottom surface of the moving
parts of the scanner. The scan table can be displaced approximately 120
µm in two orthogonal directions (x and y) using the two electromagnetic
actuators. Each actuator consists of a pair of permanent magnets mounted
in a silicon frame, with a miniature coil mounted between them on the
base plate. The actuator motion is coupled to the scan table using a pivot
and a mass-balancing scheme, which makes the system robust against external
vibrations and shock.
Positioning information for the closed-loop operation of the scanner
is provided by two pairs of thermal position sensors. These sensors are
fabricated on the cantilever-array chip and positioned directly above
the scan table. The sensors consist of thermally isolated, resistive strip
heaters made of moderately doped silicon. Each sensor is positioned above
an edge of the scan table and heated by applying a current. A fraction
of this heat is conducted through the ambient air into the scan table,
which acts as a heat sink. Displacement of the scan table gives rise to
a change in the efficiency of this cooling mechanism, resulting in a change
in the temperature of the heater and thus a change in its electrical resistance.
These sensors provide an effectively linear position signal over the entire
120 µm range of the scanner, with a resolution of less than 2 nm
in a 10 kHz bandwidth.
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