BACKGROUND OF THE INVENTION
The present invention relates generally to a Contact Probe Storage
System (CPS) arrangement and more specifically to a sensor arrangement for use with
a CPS which exhibits improved response.
It has been hitherto been proposed to sense data which is written
onto a movable medium using a probe that is supported on a cantilever and used to
contact the medium. By using heat transfer characteristics between the movable medium
and the probe (or a portion of the cantilever), it is possible to determine minute
changes in distance between the movable medium and the cantilever on which the probe
is carried, and use this as a means for reading out the data stored on the movable
In the above type of arrangement, both imaging and reading are carried
out using a thermomechanical sensing concept. The heater in the cantilever that
is used only for writing is also used as a thermal readback sensor by exploiting
a temperature-dependent resistance function. That is to say, in this type of arrangement,
the resistance (R) increases nonlinearly with heating power/temperature from room
temperature to a peak value of 500 - 700°C. The peak temperature is determined by
the doping concentration in the heater platform, which ranges from 1 x 1017
to 2 x 1018. Above the peak temperature, the resistance drops as the
number of intrinsic carriers increases because of thermal excitation.
For sensing, the resistor is operated at about 200°C. This temperature
is not high enough to soften the polymer, as is necessary for writing, but allows
the molecular energy between the cantilever on which the probe is carried, and the
moving medium, to remove heat and thus provide a parameter which allows the distance
between the cantilever on which the probe is carried and the medium on which the
probe is running to be measured.
That is to say, this thermal sensing is based on the fact that the
thermal conductance between the heater platform and the storage substrate changes
according to the distance between them. The medium between a cantilever and the
storage substrate, in this case air, transports heat from the heater/cantilever
to the storage/substate. When the distance between heater and sample is reduced
as the probe moves into a bit indentation, heat is more efficiently transported
through the air and the heater's temperature and hence its resistance decreases.
Thus, changes in temperature of the continuously heated resistor are monitored while
the cantilever is scanned over data bits, providing a means of detecting the bits.
Under typical operating conditions, the sensitivity of the thermomechanical
sensing is even better than that of piezoresistive-strain sensing inasmuch as thermal
effects in semiconductors are stronger than strain effects. A ΔR/R
sensitivity of about 10-4/nm is demonstrated by the images of the 40-nm-size
bit indentations using the thermomechanical sensing. This is better than the results
are obtained using the piezoresistive-strain sensing technique.
Nevertheless, the thermal response has been found to be slower than
desired and is significantly slower than the cantilever's ability to mechanically
follow the data pattern written in the medium. This leads to the system's read performance
being slower than it would be if it were not limited to the thermal response of
the sensing system.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic perspective view showing a cantilever having
a sensor pod arrangement according to the embodiments of the invention.
Fig. 2 is a schematic sectional view taken along section line II -
II of Fig, 3 showing a FET sensor arrangement according to a first embodiment of
Fig. 3 is a schematic plan view as seen from the probe side of the
cantilever arrangement shown in Fig. 2.
Fig. 4 is a schematic sectional side view of a second embodiment of
the invention as taken along section line IV - IV of Fig 5.
Fig. 5 is a schematic plan view as seen from under the cantilever
arrangement shown in Fig. 3.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
Figs. 1-3 show a first embodiment of the invention. Fig. 2 shows a
suitable medium 101 such as a layer of polycarbonate or polymethylmethacrylate (PMMA)
for example, which is formed over the surface of a suitable support substrate 102.
The media 101, which in this case is non-conductive, has been heated (for example)
locally to write data by forming a change in medium topography which can be detected
by lowering a probe 104, which is formed at the end of the cantilever 106 in the
manner depicted in Fig. 1, onto the medium.
Although the topography is shown as comprising a series of data recesses
or pits 101A, these can be replaced with humps (not shown) or a combination of pits
and humps. However, it should be noted that these humps (if used) should be isolated
from the pits so as not to be not confused with the raised ring that tends to form
around the mouth of the pits like a lunar impact crater.
Thus, when the medium 101 or the cantilever 106 has been moved relative
to the other to assume an exact coordinate relationship, the cantilever 106 can
be moved toward the medium 101. In the event that a pit 101A (or isolated hump)
is located under the probe 104, the entry of the probe into the pit (or engagement
with the top of the hump) indicates the presence of a data bit. Should the probe
104 not find a pit or a hump and seat on the flat surface of the medium then an
absence of a data bit is indicated.
Additionally, the bits may be coded such that a particular change
from one state (pit, hump or flat surface) to another state would indicate a bit
and other states or changes would indicate the lack of a bit. The invention can
use other coding techniques employed in contact storage device detection patterns
or other responses that are predominant in the response of the sensor-media systems.
The topography of the medium 101 is thus such that the distance or
air gap between the medium 101 and a cantilever 106 on which the probe 104. is formed,
varies. This distance variation allows an FET (field effect transistor) 108, which
is formed in the end of the cantilever 106 proximate the probe 104, to respond to
changes in an electric field which is generated between the substrate 102 and the
cantilever 106 and thus modulate a signal in the form of a current which passes
through the FET 108 in accordance with the amount of clearance between the medium
101 and the cantilever 106.
A sensor circuit 110 is arranged to be responsive to the change in
current passing through the FET 108 and thus detect the change in distance between
the cantilever 106 and the medium 101.
A sensor support extension or "pod" 114 is formed in a manner which,
in this embodiment, surrounds the probe 104. As shown in Fig. 3, the pod 114 has
a face oriented toward the medium 101 and has at least portions of the source 108A
and drain 108B of the FET formed therein. This brings the FET closer to the substrate
102 which interacts with the FET to generate the electric field. The FET, in the
illustrated embodiment, is a depletion mode FET which includes a channel 120 that
is formed in the face of the pod 114 juxtaposed the medium 101.
In this first embodiment, the source, drain and substrate 102 are
circuited as schematically illustrated in Fig. 2, so as to develop a bias voltage
between the source and drain of the FET 108 and the medium 101. This induces the
situation where the proximity of the substrate 102 effectively gates the FET 108
and modulates the amount of current which is permitted to flow from the source to
the drain through the channel which is interconnects the two.
Since the pod 114 brings the elements of the FET 108 closer to the
surface of the media 101 and reduces the distance from the substrate 102, the sensor's
response characteristics are improved. That is to say, with the provision of the
pod 114, not only is the FET exposed to a more intense electric field, but the relative
change in distance "h" between the FET and the media 101 (Δh/h) which occurs
in the event that the probe 104 enters a recesses formed in the medium 101, is increased.
In the embodiment illustrated in Figs. 1- 3, the medium 101 and the
cantilever 106 are operatively (mechanically) connected so that medium 101 is selectively
movable with respect to the cantilever 106 by way of a drive mechanism denoted by
numeral 119 (schematically depicted in Figs. 2 and 4). This mechanism 119 is arranged
to move the two elements (viz., the cantilever 106 and the medium 101) with respect
to one another to as to assume a selected coordinate relationship and position the
probe 104 so that it can detect if a data indicative change in topography (e.g.
a pit 101A) is present or absent at that set of coordinates.
A variant of the above embodiment uses an induced-channel type FET.
Unlike the depletion mode, this induced-channel or enhancement mode FET is such
that there is no intrinsic channel and the drain to source conductance is very low
until the gate voltage is applied. When the gate voltage exceeds a given threshold,
enough carriers are pulled into the channel region that the device starts to conduct.
In an N-channel enhancement type FET, the channel is p-type material that forms
a conduction band when sufficiently positive gate voltage is applied. When conducting,
the channel behaves like n-type material.
Thus, this alternative induced-channel FET conducts current from source
to drain in response to the electric field that is created when a voltage is applied
to the gate (viz., the substrate 102) and a channel being induced by the migration
of free electrons. As in the case of depletion mode FET, the gate signal determines
the amount of current flow through the channel as long as the source and drain voltages
remain constant. When the gate voltage is at zero, essentially no current flows
since a gate voltage is required to form a channel.
Since either one or both of the probe 104 and the medium 101 are electrically
non-conductive surface, the modulation of the current passing through the FET 108
is due solely to the changes in the electrical field which are produced between
the cantilever 106 and the substrate 102. The substrate 102, of course, must be
sufficiently conductive to allow for the required electrical field to be established.
The cantilever 106 is etched out of silicon and, as shown in Fig.
1, extends from a base block 106A and is provided with layers of cantilever activation
material 106B which are formed of intrinsically stressed material and which are
used to induce flexure in the cantilever 106 and move the probe 104 with respect
to the medium 101. The terminal ends of the doped traces 118A and 118B are located
on the base block 106A and a provided with electrical contacts 118E. The activation
material 106B, however, is not limited to the use of intrinsically stressed material
and can be alternatively formed of a piezoelectric material if so desired.
Inasmuch as doping and etching techniques are well within the purview
of the person skilled in the art of semiconductor fabrication, no description of
this aspect will be given for brevity.
Through the use of a FET as a sensor, a good sensor bandwidth can
be expected inasmuch as the electric field responds as fast as the cantilever can
move. The FET is thus able to respond quickly to the variations in field strength
and has the potential to make the mechanics of the cantilever the limiting factor
in the bandwidth. The signal to noise (SNR) for this arrangement can be expected
to be improved as compared to the above-mentioned thermal type sensor in that, with
the latter, much of the signal is filtered out by the thermal lowpass function.
The provision of the pod 114 improves these characteristics.
Since the FET is capable of producing gain, the sensor can be expected
to produces a relatively large output signal with respect to the various noise sources
and thus reduce signal degradation due to these noises. The SNR of the sensor will
be improved since the relative change in distance (Δh/h) will be larger as
A second embodiment of the invention is shown in Figs. 4 and 5. In
this embodiment the FET 108 which is shown in Figs. 1 and 2 is replaced with sensor
elements 116 which juxtapose the medium 101 and which are circuited with the sensor
circuit 110 so that a variable, which varies with the variation in the gap between
the sensor elements 116 and the media 101, is monitored.
In this second embodiment, the sensor elements 116 comprise heated
elements which are heated via the passage of current therethrough and thus responsive
to the change in distance between the media 101 and the cantilever 106. The juxtaposed
disposition of the sensing elements 116 at the end of the pod represents an improvement
over the arrangements discussed in the opening paragraphs of this disclosure, in
that, due to their reduced distance from the medium, the sensing elements 116 are
exposed to conditions which render them more responsive in that the amount of heat
which is removed from the heated elements is increased and the sensor arrangement
is able to exhibit better response characteristics.
It should be noted however, that there is a limit to how close heated
sensing elements can be brought to the upper surface of the medium in that the mean
free path of heat flow causes the heat flux to go to zero when the air gap becomes
The gaseous medium in the gap effects the minimum gap value. For example,
immersing the device in an atmosphere of nitrogen, argon or other gases such as
carbon dioxide or a hydrocarbon or fluorocarbon based gas, modifies the minimum
gap. However, these latter mentioned gases tend to have drawbacks associated with
their use which generally limit their application.
Although the invention has been disclosed with reference to a limited
number of embodiments, the various modifications and variations which can be made
without departing from the scope of the invention, which is limited only by the
appended claims, will be self-evident to those skilled in the art of Atomic Resolution
Storage (ARS) and Contact Probe Storage (CPS) technology. The provision of the pod
114 renders it possible to bring sensing elements provided in the cantilever 106
closer to the substrate and thus enable improvements in sensing sensitivity.