U.S. patent number RE36,603 [Application Number 08/700,090] was granted by the patent office on 2000-03-07 for distance-controlled tunneling transducer and direct access storage unit employing the transducer.
This patent grant is currently assigned to International Business Machines Corp.. Invention is credited to Wolfgang D. Pohl, Conrad W. Schneiker.
United States Patent |
RE36,603 |
Pohl , et al. |
March 7, 2000 |
Distance-controlled tunneling transducer and direct access storage
unit employing the transducer
Abstract
A distance-controlled tunneling transducer comprises a plurality
of tunnel tips arranged in an array at a tunneling distance from an
electrically conductive surface of a storage medium. Each tip is
attached to a respective cantilever beam permitting the distance
between each tip and the surface to be individually pre-adjusted
electrostatically. Arranged in juxtaposition with each cantilever
beam is an active control circuit for adjusting the tip-to-surface
distance during operation of the storage unit, thus preventing
crashes of the associated tip into possible asperities on the
surface of the recording medium. Each control circuit is designed
such that its operating voltage concurrently serves to pre-adjust
its associated cantilever beam and to maintain the tip-to-surface
distance essentially constant.
Inventors: |
Pohl; Wolfgang D. (Adliswil,
CH), Schneiker; Conrad W. (Tucson, AZ) |
Assignee: |
International Business Machines
Corp. (Armonk, NY)
|
Family
ID: |
27411304 |
Appl.
No.: |
08/700,090 |
Filed: |
August 20, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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440597 |
May 11, 1995 |
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421207 |
Oct 13, 1989 |
5043577 |
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Reissue of: |
674264 |
Mar 25, 1991 |
05210714 |
May 11, 1993 |
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Current U.S.
Class: |
365/151; 250/306;
365/118; 365/153; 365/217; 369/126 |
Current CPC
Class: |
G11B
9/14 (20130101); G11B 9/1445 (20130101); G11B
9/1454 (20130101); B82Y 10/00 (20130101); G11B
5/00 (20130101); G11B 5/012 (20130101); G11B
9/08 (20130101) |
Current International
Class: |
G11B
9/00 (20060101); G11B 9/08 (20060101); G11B
5/00 (20060101); G11B 5/012 (20060101); G11C
011/00 (); G01N 023/00 (); G11B 009/00 () |
Field of
Search: |
;365/217,118,151,153
;250/306,307 ;369/126 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2-260668 |
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Oct 1990 |
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JP |
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1536441 |
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Jan 1990 |
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SU |
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91/00592 |
|
Jan 1991 |
|
WO |
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Primary Examiner: Tran; Andrew Q.
Attorney, Agent or Firm: Gunster, Yoakley, Valdes-Fauli
& Stewart, P.A.
Parent Case Text
This application .Iadd.is a continuation of application Ser. No.
08/440,597, filed May 11, 1995, now abandoned, which was filed as a
reissue of U.S. Pat. No. 5,210,714, which issued May 11, 1993, from
application Ser. No. 07/674,264, filed Mar. 25, 1991, which
.Iaddend.is a division of application Ser. No. 07/421,207, filed
Oct. 13, 1989, now U.S. Pat. No. 5,043,577.
Claims
What is claimed is:
1. A direct access storage unit comprising a recording medium; a
plurality of tunnel tips arranged on a transducer in proximity to
said recording medium, and means for generating a .[.mutual.].
periodic displacement between said recording medium and said
transducer, wherein each tip is operated as a detector for the
current flowing across a gap (Z.sub.G) between said tip and a
surface of said recording medium, and wherein each tip is attached
to a respective cantilever beam with the .[.mutual.]. distance
between said tip and the surface of the recording medium being
adjustable by electrostatic means, characterized in that each tip
is associated with an active control circuit integrated on said
transducer in proximity to the respective cantilever beam
associated with a particular tip and comprising a transistor, a
load resistance, and a capacitance, and that the circuit is
connected such that the supply voltage (U.sub.0) creating the
current across the gap between said tip and the surface of said
recording medium also serves to adjust, during operation, the gap
distance (Z.sub.G) between said tip and the surface of said
recording medium. .Iadd.2. A direct access storage unit comprising
a recording medium, one or more tips arranged on a transducer in
proximity to said recording medium, and means for generating a
displacement beetween said recording medium and said transducer,
wherein each tip is operated as a detector for a current flowing
across a gap distance (Z.sub.g) between each said tip and a surface
of said recording medium, and wherein each tip is attached to a
respective cantilever beam with the gap distance (Z.sub.g) between
each said tip and the surface of the recording medium being
adjustable, characterized in that each tip is associated with an
active control circuit integrated on said transducer in proximity
to the respective cantilever beam associated with the particular
tip and comprising a transistor, a load resistance, and a
capacitance, and that the circuit is connected such that the supply
voltage (U.sub.0) creating the current across the gap distance
(Z.sub.g) between said tip and the surface of said recording medium
also serves to adjust, during operation, the gap distance (Z.sub.g)
between said tip and the surface of said
recording medium..Iaddend..Iadd.3. The direct access storage unit
of claim 2, further comprising an electrostatic means for adjusting
the gap distance (Z.sub.g) between the recording medium and the
tip..Iaddend..Iadd.4. The direct access storage unit of claim 2,
further comprising a piezo means for adjusting the gap distance
(Z.sub.g) between the recording medium and the
tip..Iaddend..Iadd.5. The direct access storage unit of claim 2,
wherein the gap distance (Z.sub.g) between the recording medium and
the tip is less than 20 nm..Iaddend..Iadd.6. The direct access
storage unit of claim 2, wherein the gap distance (Z.sub.g) between
the recording medium and the tip is less than 2
nm..Iaddend..Iadd.7. The direct access storage unit of claim 2,
wherein the active control circuit comprises an open loop
circuit..Iaddend..Iadd.8. The direct access storage unit of claim
2, wherein the means for generating a displacement allows each tip
to scan a plurality of concentric circles of tracks on the surface
of the recording medium..Iaddend.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a distance-controlled tunneling
transducer for use in a direct storage unit, having a plurality of
tunnel tips arranged facing a recording medium. In particular the
invention teaches improved gap control means for implementation in
micromechanical techniques. The invention is also applicable to
low-voltage field-emission environments where the gap dimension is
somewhat larger in the tunneling regime. Therefore, where in the
following description reference is made to tunneling phenomena,
those skilled in the art will be able to easily apply what is said
to field-emission phenomena as well.
In the tunneling regime, the tip/surface distance typically is less
than 2 nm, and in the field-emission environment, that distance is
considered to be on the order of 20 nm. Small local deviations from
planarity of the surface of the recording medium, say on the order
of tenths of a nanometer, may result in relatively large changes of
the tip current, inparticular in the tunnel regime, where the
dependence of the tunneling current on the tip-to-surface distance
is exponential. Because of the fact that the operating distances in
tunneling as well in in field-emission environments are so small,
it may even happen that the tip crashes into a surface it and thus
suffers damage, unless some measure is taken to maintain the
tip-to-surface distance essentially constant. In conventional
tunneling microscopes, this problem is solved by means of a
feedback loop operating from the tip distance above the surface,
with the aim of keeping the tip current constant. It will be
obvious to those skilled in the art that in view of the relatively
large velocity with which surface asperities may be encountered as
the tip scans across the surface of the recording medium, and
because of the possibly abrupt change in tip-to-surface distance, a
very fast response of the feedback loop is required.
Recently, direct access storage devices have been proposed which
operate on the principle of the scanning tunneling microscope. The
basic reference in this area is EP A-0 247 219. The reference
teaches a storage unit comprising an array of tunnel tips arranged
at a tunnel distance from the surface of a recording medium which
is capable of permitting digital information to be written or read
the variations of the tunneling current. The tunnel is maintained
by means of a feedback loop, and the idea of integrating the
control circuitry of that feedback loop on the tunnel tip array is
mentioned. No details of the circuitry nor of the way the
integration can be achieved are, however, given.
A scanning tunneling microscope realized in micromechanical
techniques is disclosed in U.S. Pat. No. 4,520,570. A semiconductor
chip has slots etched into it in a pattern resulting in a plurality
of tunnel tips to be formed that are hinged by stripes of
semiconductor material to the main body of the chip. Again in this
reference, an area is provided on the semiconductor chip to contain
control circuitry associated with the tunnel tip, in casu a
transistor acing as an impedance transformer.
While it is acknowledged that the idea has occurred to integrate
the tunnel tip feedback loop into the semiconductor chip on which
the tunnel tip is formed, it has turned out that conventional
control circuitry for scanning tunneling microscope and field
emission microscope applications is by far too complex and, hence,
too bulky to be installed on the semiconductor chip if the chip is
to carry a plurality of tips arranged in a small array.
As a possible alternative, one may requires that both the array of
tips and the use of the recording medium be sufficiently flat, for
instance within 0.1 nm over the area to be scanned, to allow for
global gap width control averaging over the currents of all tips.
Such a requirement would, however, impose undesirable constraints
with regard to precision of manufacturing and alignment, as well as
to the choice of materials for the recording medium.
SUMMARY OF THE INVENTION
A primary object of the present invention is to solve this problem
by foregoing the idea of perfect feedback regulation, i.e. with
zero error. Instead, an open loop circuit is provided which
compensates for distance variations though less completely, but
which is sufficiently simple to be easily integrated into a
multiple-tip scanner head. The achievable reduction in distance
variation as result of variation of the tip-to-surface distance
typically is a factor of from 30 to greater 100. The result is
relaxed manufacturing tolerance requirements with regard to
flatness of the recording medium to values that can easily be
maintained.
Details of an embodiment of the invention will hereafter be
described by way of example having reference to the attached
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top plan view onto a section of an array of tunnel
tips;
FIG. 2 is an enlarged cross-section of the tunnel region along line
A--A of FIG. 1;
FIG. 3 is a schematic representation of the tunnel tip, showing the
different positions of the cantilever beam during operation;
FIG. 4 is a circuit diagram of the circuitry associated with the
tunnel tip;
FIG. 5 is a graph used in the determination of the operating point
of the circuitry of FIG. 4;
FIG. 6 is a section of the graph of FIG. 5 having a larger
scale;
FIG. 7 shows a characteristic of the control circuitry in
accordance with the parameters chosen; and
FIG. 8 shows another characteristic of the control circuitry in
accordance with the parameters chosen.
DETAILED DESCRIPTION
While the invention will be described in connection with the
electrostatic deflection of a cantilever beam as conventionally
used in micromechanical arrays, it will be obvious to those skilled
in the art that the invention is also applicable to piezoceramic
scanners.
FIGS. 1 and 2 show the contemplated arrangement of the elements of
the transducer in accordance with the invention in a
semi-schematical way. Referring to FIG. 1, there is shown a section
of the transducer 1 with three tunnel tips 2', 2", 2"' out of a
plurality of tunnel tips (or field-emission tips) arranged in an
array. The tunnel tips are attached to cantilever beams 3',3",3"'
respectively which are formed, e.g. by etching from body 4, FIG. 2,
of transducer 1 as an integral part thereof. Transducer 1 is
mounted to a conventional xyz-drive 5 which provides lateral
deflection as well as coarse approach and adjustment of the average
distance between tunnel tip 2 and the oppositely disposed surface
6, by keeping the total tunneling current essentially constant.
Surface 6 may actually be the surface of a sample to be inspected
by means of a scanning tunneling. However, since the present
invention is intended particularly for use in with information
storage devices, for the purposes of this description surface 6
will be assumed to be the surface of a recording medium 7. The
medium 7 may comprise, for example, a magnetizable material. Since
the funnel effect requires the surface opposite by the tunnel tip
to be electrically conductive, any nonconductive materialized as
the recording medium may be provided with a very thin conductive
coating.
Each of the cantilever beams can be deflected electrostatically by
application of voltages U.sub.1 and U.sub.2, respectively, between
an electrode 8', 8", 8"' on cantilever beam 3', 3", 3"' and a
counter-electrode 9 at the bottom of the recess 10 etched into body
4 underneath each cantilever beam, and between the electrode 8 and
the surface 6 of the recording medium 7. This deflection is used to
control the width of the tip-to-surface gap, in particular during
operation in the tunneling mode.
Arranged on body 4 of transducer 1 are electric circuit elements 11
through 17 which eve to control the deflection of the cantilever
beams 3 in the array and to create the tunneling currents across
the gaps between the tuned tips and the surface 6 of the common
recording medium 7. FIG. 3 is an enlargement of a portion of FIG. 2
to better show the relevant z-coordinates, distances and voltages.
In its home position, i.e., in a voltageless state, cantilever beam
3 assumes a position designated in FIG. 3 as z.sub.CB a distance
Z.sub.1 from counter-electrode 9 the surface of the
counter-electrode being located at Z.sub.BOT, and at a distance
Z.sub.2 from the surface 6 of recording medium 7 which is located
at Z.sub.S. In the home position of tunnel tip 2, and considering
that the tip has a height dimension Z.sub.P, its apex is located at
Z.sub.TIP, i.e. a gap width Z.sub.G away from surface 6.
Application of voltages U.sub.1, U.sub.2 between electrode 8 on
cantilever beam 3 and surface 6 causes the cantilever beam 3 to
deflect by a distance .DELTA.Z to a new position Z.sub.CB0. The
voltages required for deflecting cantilever beam 3 are provided by
circuitry comprising a field-effect transistor 11 connected between
supply line 12 and electrode 8 on cantilever beam 3. Field-effect
transistor 11 acts as a constant current source which is set by
means of a the line 13. Counter electrode 9 is part of supply line
12 and is at a constant potential U.sub.0. Hence the deflection
voltages are U.sub.1 =U.sub.0 -U.sub.1, U.sub.2 =U.sub.t.
When tunnel tip 2 is far from surface 6, i.e. gap Z.sub.G is large,
then the resistance across the tunnel gap R.sub.1 .fwdarw..infin.,
U.sub.1 .fwdarw.0, U.sub.2 .fwdarw.U.sub.0. As cantilever beam 3
becomes most deflected towards surface 6 of recording medium 7:
.DELTA.Z.sub.max .tbd.B(U.sub.0 /Z.sub.P).sup.2, where it is
assumed that Z.sub.G <Z.sub.P. The term B will be explained
below.
When tip 2 gets closer to surface 6 so that tunneling current
I.sub.t becomes finite, i.e. larger than the avoidable leakage
current of field-effect transistor 11, which is typically <100
pA, then the voltage ratio U.sub.1 /U.sub.2 increases resulting in
a retraction of cantilever beam 3. Thus, any increment in z.sub.3,
that is, for example, some surface roughness, produces a much
smaller decrease of Z.sub.G.
FIG. 4 is a circuit diagram a the unit cell, viz. for the
electronic circuitry associated with one cantilever beam 3. The
tunneling current I.sub.t flowing across the tip-to-surface gap,
i.e. through the tunnel resistance R.sub.t, is monitored by means
of a load resistance 14 (R.sub.L) which is chosen such that R.sub.L
<R.sub.t under operating conditions. The signal U.sub.SIG
occurring across load resistance 14 is provided via lines 15 and
16. A load capacitance 17 (C.sub.L) introduces some inertia into
the compensation process characterized by a time constant
.tau..sub.L =R.sub.t C.sub.L. Hence, information-carrying
variations passing tip 2 within a time shorter than the time
constant .tau..sub.L are not compensated, giving rise to a large
variation in U.sub.SIG. The performance of the scheme is described
by the Z.sub.G (Z.sub.S) characteristic quasi-static conditions
(with reference to .tau..sub.L). Therefore, R.sub.L and C.sub.L are
ignored in this part of the discussion. Further ignored are the
stray capacitance C.sub.1 between cantilever beam 3 and supply line
12, and the stray capacitance C.sub.2 between cantilever beam 3 and
counterelectrode 9, because they are negligible compared to load
capacitance C.sub.L. Without loss of generality, it can be that
Z.sub.CB0 =0, hence, Z.sub.S <0. Under these conditions Z.sub.G
(Z.sub.S) can be derived from the following set of relations:
with .epsilon..epsilon..sub.0 =dielectric constant (.congruent.0,8
pF/m), E=elastic modulus, l=length and t=thickness of the
cantilever beam 3, respectively, and R.sub.t0 .congruent.40
k.omega., .kappa..congruent.10.sup.10 m.sup.-1.
Equations [1] through [7] cannot be solved analytically for Z.sub.G
but the derivative can be determined easily: ##EQU1##
With the parameters chosen below, and at realistic operating
conditions in the tunneling mode, Z.sub.G
<.DELTA.-Z<Z.sub.1,2 such that Z.sub.1 .congruent.z.sub.BOT
and Z.sub.2 .congruent.Z.sub.p ; further U.sub.1
-.congruent.U.sub.0 <U.sub.2 =U.sub.t, the quantity A
becomes:
For a numerical calculation, the following values have proven
appropriate: .epsilon.=1; .epsilon..sub.0 =8.times.10.sup.-12 F/m;
1=200 .mu.m; t=2 .mu.m; E=10.sup.11 N/m.sup.2 (silicon, quartz);
z.sub.BOT =Z.sub.P =3 .mu.m. With these values and a projected
width w=200 .mu.m of the cantilever beam, a spring constant of
C*=4,5 N/m results which is sufficient to prevent mechanical
instabilities due to interfacial forces. The first elastic
resonance of the cantilever beam occurs at >100 kHz which is
much better than in present-day scanning tunneling microscopes. The
deflection parameter becomes:
Assuming operation at U.sub.t =0.5 V, then A.congruent.100. Since
A>1, one may ignore the 1 in the denominator of equation [10],
hence ##EQU2##
Equation [13] means that a 10 nm variation in surface height
results in no more than a 0.1 nm on change in tunnel gap width.
For very small and very large tunnel gap widths, the above-given
set of equations is easily solved for z.sub.S (Z.sub.G). There is
no reduction effect to be expected: ##EQU3##
FIG. 5 depicts the linear and almost linear relations calculated in
accordance with Equation [14]. The dashed lines designated a, b,
and c, having a slope of -1, and being mutually parallel-displaced
by the amounts .DELTA.Z.sub.max, and 2 .DELTA.Z.sub.max,
respectively, arc the calculated asymptotes to the curve z.sub.S
(Z.sub.G) represented in a semi-quantitative way by the solid curve
d. The operating point is to be chosen on the horizontal plateau
(of the width 2 .DELTA.Z.sub.max) between dashed lines a and b.
Since the voltage U.sub.1 is generally kept small, the operating
point will preferably be chosen near the right-hand end of the
plateau.
For this purpose, the tunneling current from a few selected cells
can be fed into a regular feedback control circuit. This measure
assure that the overall system settles approximately at the
operating points of the individual elements. The quantitative
behavior of the response curve may be obtained by numerical
integration of Equation [13].The initial setting of the parameters
may be chosen as follows:
The resulting characteristics are shown in FIGS. 6 through 8. FIG.
6 depicts the relevant section of curve d in FIG. 5, namely the
relation of tunnel gap width versus sample position, for three
values of the tunneling current, viz. 0.1 nA, 1 nA, and 10 nA, at a
larger scale. FIG. 7 shows the corresponding relation between
dZ.sub.G /dz.sub.S versus U.sub.t which is independent I.sub.t. It
can be seen that dZ.sub.G /dz.sub.S =0.01 in the operating range
around U.sub.t =0.5 V. FIG. 8 relates the gap width to the
tunneling voltage for the three values 0.1 nA, 1 nA and 10 nA of
the tunneling current I.sub.t.
It will be apparent from the foregoing description that variations
in tunneling current occurring fast as compared to the time
constant .tau..sub.L of the R.sub.L -C.sub.L circuit are not
compensated for by the current stabilizer comprising field-effect
transistor 11. Small asperities on the surface 6 of the recording
medium 7 (topography) as well a local changes in workfunction of
the recording material, therefor, will show up in full strength in
the tunneling current I.sub.t. They create a voltage signal
U.sub.SIG across load resistance R.sub.L 14 can be wed for further
processing of the stored information
While the present invention is not directed to the storage medium
per se, it seems in order to briefly introduce a storage medium
capable of recording variations of the tunneling current. A
suitable storage medium may, for example, comprise a thin layer of
silicon dioxide on top of a silicon substrate. The oxide is
prepared to have a plurality of trapping sites of about 2.4 eV
depth. Electrons emitted from a tunnel tip can be stably trapped at
such sites at room temperature (cf. D. J. Dimaria in Pantelides
(Ed.) "The Physics of SiO2 and is Interfaces", Pergamon 1978, p.
160 et seq.). This mechanism qualifies for write/read
applications.
On conventional storage disks, the storage locations are arranged
in concentric circles or tracks about a single common center. By
contrast, on the storage medium proposed for we in connection with
the present invention, the storage locations are arranged in a
plurality of identical groups, with all storage positions in any
one group being arranged in concentric circles or tracks about the
group's own center, and with all centers being arranged in an
ordered array on the recording surface of the stage medium. The
concentric circles or tracks of storage locations of each group
form a "microdisk" with a diameter of less than 0.1 mm. Even with
several hundred "microdisks" per recording surface, the area
required for a given storage capacity is much smaller than required
in any other known storage devices.
The storage medium just described may be attached to the free end
of an elongated pizoceramic bendable tube, such as the one known
from EP-A-0 247 219, the other end of which is rigidly anchored.
Closely adjacent the recording surface at the free end of the tube
is mounted the transducer of the present invention with each tunnel
tip of its array of tunnel tip being aligned with one of the
microdisks. Each tunnel tip faces, and is spaced closely adjacent
to, the recording surface on the storage medium. The distance (in
the nanometer range) between each tunnel tip ad the recording
surface is at the beginning individually adjusted so that each tip
normally is at the same preselected tunneling distance from the
recording surface 6.
By successively energizing electrode pairs attached to the tube,
the free end of the tube and, hence, the recording surface is
forced to move in a circular orbit. The diameter of this orbit will
vary according to the potential differential that is selected.
Thus, the recording surface at the free end of the piezoceramic
tube can be caused to move in any one of a plurality of concentric
orbits relative to the tunnel tips of the transducer array. As a
result, each tip will scan that one of the plurality of concentric
circles of tracks of its microdisk corresponding to the selected
orbit diameter of the tube. Digital information is written into
and/or read out of the storage medium by varying the tunneling
current flowing across the gap between the tunnel tip and recording
surface.
Recording, therefore, involves addressing and moving a selectable
one of the tips 2 in the array of transducer 1 and concurrently
energizing the electrode pairs surrounding the tube to a potential
corrsponding to an orbit diameter that permits access to a desired
one of the concentric tracks on the associated microdisk.
It will be clear to those skilled in the art that several other
recording media, as well as other known schemes for the mutual
displacement of transducer and/or tunnel tips and recording medium
may be employed to achieve the desired result. The important point
is that with all those schemes the control circuitry necessary to
compensate the variations in the distance between each individual
the tunnel tip and the surface of the recording medium is
integrated on the transducer.
* * * * *