U.S. patent application number 13/608878 was filed with the patent office on 2013-01-03 for arrangement and method to perform scanning readout of ferroelectric bit charges.
This patent application is currently assigned to INTEL CORPORATION. Invention is credited to Donald E. Adams, Yevgeny V. Anoikin, Nathan R. Franklin, Byong M. Kim, Qing Ma, Valluri Rao, Robert N. Stark, Quan Anh Tran, Li-Peng Wang.
Application Number | 20130003521 13/608878 |
Document ID | / |
Family ID | 40798275 |
Filed Date | 2013-01-03 |
United States Patent
Application |
20130003521 |
Kind Code |
A1 |
Tran; Quan Anh ; et
al. |
January 3, 2013 |
ARRANGEMENT AND METHOD TO PERFORM SCANNING READOUT OF FERROELECTRIC
BIT CHARGES
Abstract
An arrangement, a method and a system to read information stored
in a layer of ferroelectric media. The arrangement includes a layer
including a ferroelectric media having one or more ferroelectric
domains holding bit charges, a domain corresponding to information;
a probe having a tip, wherein the media and the tip are adapted to
move relative to one another such that the tip scans the
ferroelectric domains of the media while applying a contact force
to the domains to generate a direct piezoelectric effect within the
domains; and circuitry coupled to the tip and adapted to generate a
signal in response to an electrical coupling between the tip and
the domains while scanning the tip in contact with the domains, the
signal corresponding to a readout signal for ferroelectric bit
charges stored in the media
Inventors: |
Tran; Quan Anh; (Fremont,
CA) ; Kim; Byong M.; (Fremont, CA) ; Stark;
Robert N.; (Saratoga, CA) ; Franklin; Nathan R.;
(San Mateo, CA) ; Ma; Qing; (San Jose, CA)
; Rao; Valluri; (Saratoga, CA) ; Adams; Donald
E.; (Pleasanton, CA) ; Wang; Li-Peng; (San
Jose, CA) ; Anoikin; Yevgeny V.; (Fremont,
CA) |
Assignee: |
INTEL CORPORATION
Santa Clara
CA
|
Family ID: |
40798275 |
Appl. No.: |
13/608878 |
Filed: |
September 10, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11964580 |
Dec 26, 2007 |
8264941 |
|
|
13608878 |
|
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Current U.S.
Class: |
369/126 ;
G9B/9.012 |
Current CPC
Class: |
G11B 9/02 20130101; B82Y
10/00 20130101; G11B 9/1409 20130101 |
Class at
Publication: |
369/126 ;
G9B/9.012 |
International
Class: |
G11B 9/02 20060101
G11B009/02 |
Claims
1. An arrangement to store information, comprising: a layer
including a ferroelectric media having one or more ferroelectric
domains holding bit charges, a domain corresponding to information;
a probe having a tip, wherein the media and the tip are adapted to
move relative to one another such that the tip scans the
ferroelectric domains of the media while applying a contact force
to the domains to generate a direct piezoelectric effect within the
domains; circuitry coupled to the tip and adapted to generate a
signal in response to an electrical coupling between the tip and
the domains while scanning the tip in contact with the domains, the
signal corresponding to a readout signal for ferroelectric bit
charges stored in the media.
2. The arrangement of claim 1, wherein the circuitry includes a
charge amplifier device coupled to the tip, and wherein the signal
is a voltage signal.
3. The arrangement of claim 1, wherein the circuitry includes an
electrical current detector coupled to the tip, and wherein the
signal is a current signal.
4. The arrangement of claim 1, wherein the tip includes a plurality
of tips.
5. The arrangement of claim 1, wherein the tip is adapted to apply
a force to domains between about 1 nN and about 3 .mu.N.
6. The arrangement of claim 1, wherein the tip is adapted to apply
contact forces of differing magnitudes.
7. The arrangement of claim 6, wherein the tip is adapted to apply
a modulated contact force to each of the domains.
8. The arrangement of claim 7, further including a mechanical
vibrator coupled to the tip and adapted to modulate the contact
force on said each of the domains.
9. The arrangement of claim 7, further including a lock-in
amplifier coupled to the tip and adapted to pass signals at a force
modulation frequency only.
10. The arrangement of claim 1, wherein the tip is adapted to scan
the ferroelectric domains at a plurality of scanning speeds.
11. The arrangement of claim 1, wherein the tip is one of
electrically conductive and semiconductive.
12. The arrangement of claim 1, wherein the media and the tip are
adapted to move relative to one another such that the tip scans the
ferroelectric domains of the media while applying a first contact
force to the domains, and scans intermediate regions of the media
in between the domains while applying a second contact force to the
intermediate regions, the second contact force being smaller than
the first contact force.
13. The arrangement of claim 1, wherein the media and the tip are
adapted to move relative to one another such that a scanning speed
of the tip on the domains is about 2.5 cm/s.
14. A method of reading information stored in a layer of a
ferroelectric media including ferroelectric domains having bit
charges using a tip, the method comprising: scanning the tip over
the domains while applying a contact force to the domains with the
tip to generate a direct piezoelectric effect within the domains;
using circuitry coupled to the tip to generate a signal in response
to an electrical coupling between the tip and the domains while
scanning the tip in contact with the domains, the signal
corresponding to a readout signal for ferroelectric bit charges
stored in the domains.
15. The method of claim 14, wherein using includes using a charge
amplifier device coupled to the tip, wherein the signal is a
voltage signal.
16. The method of claim 14, wherein using includes using an
electrical current detector coupled to the tip, wherein the signal
is a current signal.
17. The method of claim 14, wherein the ferroelectric media has a
continuous surface including surfaces of the domains, and wherein
scanning includes scanning across the continuous surface while
applying a contact force continuously during scanning.
18. The method of claim 14, wherein the tip includes a plurality of
tips.
19. The method of claim 14, wherein applying a contact force
includes applying a contact force to the domains between about 1 nN
and 3 .mu.N.
20. The method of claim 14, wherein applying a contact force
includes applying contact forces of differing magnitudes to the
domains.
21. The method of claim 20, wherein applying includes using to the
tip to apply a modulated contact force to each of the domains.
22. The method of claim 21, wherein applying includes using a
mechanical vibrator coupled to the tip to modulate the contact
force on said each of the domains.
23. The method of claim 21, wherein the circuitry includes a
lock-in amplifier coupled to the tip and adapted to pass signals at
a force modulation frequency only.
24. The method of claim 14, wherein scanning includes scanning the
tip at a plurality of scanning speeds.
25. The method of claim 14, wherein the tip is one of electrically
conductive and semiconductive.
26. The method of claim 14, wherein scanning comprises scanning a
surface of the media while applying a first contact force to the
domains when scanning the domains and while applying a second
contact force to intermediate regions of the media in between the
domains when scanning the intermediate regions, the second contact
force being smaller than the first contact force.
27. A system comprising: an electronic assembly comprising: an
arrangement to store information, comprising: a layer including a
ferroelectric media having one or more ferroelectric domains
holding bit charges, a domain corresponding to information; a probe
having a tip, wherein the media and the tip are adapted to move
relative to one another such that the tip scans the ferroelectric
domains of the media while applying a contact force to the domains
to generate a direct piezoelectric effect within the domains;
circuitry coupled to the tip and adapted to generate a signal in
response to an electrical coupling between the tip and the domains
while scanning the tip in contact with the domains, the
polarization signal corresponding to a readout signal for
ferroelectric bit charges stored in the media; and a graphics
controller coupled to the electronic assembly.
28. The system of claim 27, wherein the circuitry includes a charge
amplifier device coupled to the tip, and wherein the signal is a
voltage signal.
29. The system of claim 27, wherein the circuitry includes an
electrical current detector coupled to the tip, and wherein the
signal is a current signal.
30. The system of claim 27, wherein the tip is adapted to apply a
modulated contact force to each of the domains.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/964,580, filed 26 Dec. 2007, and claims
priority therefrom under 35 U.S.C. .sctn.120. The priority
application is currently pending.
FIELD
[0002] Embodiments relate to high density data signal readout
arrangements and methods, and specifically to arrangements and
methods involving the use of a probe tip sensor to perform a
piezoresponse signal readout from memory media which include
ferroelectric bit charges.
BACKGROUND
[0003] Conventional piezoresponse signal readout techniques, such
as piezoresponse force microscopy (PFM) rely on the detection of
bit signals from a ferroelectric memory medium by virtue of a
cantilever probe tip sensor's mechanical motion on the surface of
the medium. Such probe storage devices typically use two parallel
plates. A first plate includes the cantilevers with contact probe
tips extending therefrom for use as read-write heads and a second,
complementary plate includes memory media for storing data. The
plates can be moved relative to one another in an X-Y plane while
controlling the Z-spacing between the plates. Motion of the plates
relative to one another allows scanning of the memory media by the
contact probe tip and data transfer between the two.
[0004] Disadvantageously, PFM relies on complex laser and optical
setup for alignment and detection of the cantilever probe tip
deflection, which deflection is typically less than about 1 nm. In
PFM, the laser beam is focused onto the cantilever probe tip by
using the optical setup. The reflected laser beam from the
cantilever probe tip is then aligned to a center of a photodiode
detector. In PFM, an AC voltage is applied between the cantilever
probe tip and the ferroelectric sample. The AC voltage results in
an expansion and contraction of the ferroelectric at the same
frequency as a frequency of the AC voltage. Consequently, the
cantilever probe tip deflects in unison with the expansion and
contraction of the ferroelectric sample, in this way causing the
reflected laser beam to oscillate about the center of the
photodiode detector. The changing position of the reflected laser
beam relative to the center of the photodiode detector in turn
generates current which PFM uses to calculate the cantilever tip
deflection.
[0005] Alternatively, a conventional scanning nonlinear dielectric
microscopy (SNDM) technique may be used to read bits. SNDM,
however, disadvantageously requires complicated resonance circuitry
operating at the GHz range to detect atto-farad ranges in
capacitance. SNDM aims to detect changes in capacitance as the tip
goes from an UP domain to a DOWN domain. However, this change in
capacitance has proven to be extremely difficult to detect,
requiring complicated circuitry operating at high frequencies. SNDM
further requires that the tip be made coaxial in order to provide a
constant impedance environment for the system, in this way adding
to the complexity of the same.
[0006] The prior art fails to provide an arrangement and method
that avoid the need for complex optical setups and/or for complex
resonance circuitry operating in the GHz range as noted above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a schematic representation of an arrangement
according to a first embodiment;
[0008] FIG. 2 is a graph of an exemplary voltage signal obtained
from using the arrangement of FIG. 1;
[0009] FIG. 3 is a schematic representation of an arrangement
according to a second embodiment;
[0010] FIG. 4 is a graph plotting voltage signal versus contact
force while using the arrangements of FIG. 1 within specific
exemplary parameters;
[0011] FIG. 5 is a flowchart showing stages of a method embodiment;
and
[0012] FIG. 6 is a schematic view of an embodiment of a system
incorporating an arrangement as shown in FIG. 1 or FIG. 3.
[0013] For simplicity and clarity of illustration, elements in the
drawings have not necessarily been drawn to scale. For example, the
dimensions of some of the elements may be exaggerated relative to
other elements for clarity. Where considered appropriate, reference
numerals have been repeated among the drawings to indicate
corresponding or analogous elements.
DETAILED DESCRIPTION
[0014] In the following detailed description, an arrangement and
method to perform scanning readout of ferroelectric bit charges are
disclosed. Reference is made to the accompanying drawings within
which are shown, by way of illustration, specific embodiments by
which the present invention may be practiced. It is to be
understood that other embodiments may exist and that other
structural changes may be made without departing from the scope and
spirit of the present invention.
[0015] The terms on, above, below, and adjacent as used herein
refer to the position of one element relative to other elements. As
such, a first element disposed on, above, or below a second element
may be directly in contact with the second element or it may
include one or more intervening elements. In addition, a first
element disposed next to or adjacent a second element may be
directly in contact with the second element or it may include one
or more intervening elements. In addition, in the instant
description, figures and/or elements may be referred to in the
alternative. In such a case, for example where the description
refers to Figs. X/Y showing an element A/B, what is meant is that
Fig. X shows element A and Fig. Y shows element B. In addition, a
"layer" as used herein may refer to a layer made of a single
material, a layer made of a mixture of different components, a
layer made of various sub-layers, each sub-layer also having the
same definition of layer as set forth above.
[0016] Aspects of this and other embodiments will be discussed
herein with respect to FIGS. 1-6 below. The figures, however,
should not be taken to be limiting, as they are intended for the
purpose of explanation and understanding.
[0017] FIGS. 1 and 3 pertain to respective embodiments where a
force is either constant or actually modulated to create a direct
piezoelectric effect in the memory media, in this way generating a
charge that may be measured to allow a reading of the media. A
direct piezoelectric effect may be demonstrated by the following
equation (1):
P.sub.i=d.sub.ij.sigma..sub.j (1)
where P is the polarization charge per unit area, d is the
piezoelectric coefficient, and .sigma. is the force per unit area.
Equation (1) suggests that when a force is applied onto a sample of
material that exhibits piezoelectric characteristics, charge is
built up in that sample. Ferroelectric materials exhibit
piezoelectric characteristics and obey equation (1). The polarity
of the built up charge reverses when the ferroelectric domain upon
which force is applied reverses it direction from UP to DOWN and
vice versa. Thus, the domain orientation can be determined by
measuring the built up charge. Alternatively, if the built up
charge changes over time, current is generated and can be used to
determine the domain orientation. The direct piezoelectric effect
current is brought about in the embodiment of FIG. 1 by the fact
that when a cantilever tip is scanned across piezoelectric domains
of alternating polarization while applying a constant force to the
domains, the built up charge resulting from the applied force
changes its sign during scanning. The direct piezoelectric effect
current is brought about in the embodiment of FIG. 3, on the other
hand, by the fact that, when a cantilever tip is used to apply an
actually modulated force to the top of a domain, as per equation
(1) above, the charge changes in magnitude over time. As compared
with readout arrangements of the prior art, the readout techniques
as disclosed herein advantageously dispense with the need for
complicated optical setups and alignments as required with PFM or
with the need for complicated resonance circuitry at GHz range as
required with SNDM, for example, and offer a simple,
well-established circuitry to detect current. The embodiments of
FIGS. 1 and 3 will be respectively described below.
[0018] Referring first to FIG. 1, an arrangement 100 is shown
according to a first embodiment. Arrangement 100 comprises a layer
102 including a ferroelectric media 104 having one or more
ferroelectric domains 106 including bit charges, each of the
domains corresponding to information stored in the arrangement 100.
The ferroelectric media 104 may include, for example, a PZT layer
(lead zirconate titinate, or PbZr.sub.xTi.sub.1-xO3, where x ranges
from 0.10 to 0.55) disposed over a substrate 105. Alternative
materials for the ferroelectric media may similarly be employed,
such as, for example, Bi.sub.3Ti.sub.4O.sub.12 and
Pb1.sub.-xLa.sub.x(Ti.sub.1-yZr.sub.y)O.sub.3 (i.e., PLZT), where
x=0 to 0.2; y=0.1 to 0.55. The substrate 105 may include, in one
embodiment, a layer of strontium titanate oxide (STO) (although
alternative materials may be used, as readily recognizable by those
skilled in the art), and a layer of strontium ruthenate oxide
(SRO). A wavelength .lamda. of recorded information in the form of
the domains 106 associated with alternating polarization. The A in
FIG. 1 represents the pitch, that is, the periodic spacing between
two domains of the same polarization. Accordingly, .lamda./2, which
representing a half pitch, refers to a spacing between domains
having alternating polarizations with respect to one another. The
pitch may be leveraged using a probe tip or tip 110 with scanning
speed v to modulate a signal frequency for the readout signal
corresponding to the scanned information. According to one
embodiment, the speed v would be chosen to modulate the signal
frequency into the RF range, such as, for example, into the low RF
range.
[0019] Referring still to FIG. 1, the arrangement further includes
a probe 108 including tip 110. The probe 108, including tip 110,
may include, in one embodiment, a straight cantilever wherein the
tip 110 extends from near a free end of the cantilever. According
to a preferred embodiment, the tip may be electrically conductive.
For example, the tip may comprise a metal. In one embodiment, the
tip may be made entirely of metal. Alternatively, the tip may
include a metallic coating, and may comprise, for example, a Si tip
coated with a metallic layer, such as, for example, a Pt layer.
Optionally, an adhesion layer, such as one including Cr or Ti, may
be provided between the Si material of the tip and the metallic
layer. According to another embodiment, the tip may be
semiconductive. In such a case, the tip may include Si. Although
the embodiment of FIG. 1 shows a single probe tip, it is noted that
embodiments are not so limited, and include within their scope the
use of an arrangement including multiple tips such as tips 110, for
example, which tips may be controlled either independently with
respect to one another, or in unison, or in interdependent
manner.
[0020] According to embodiments, the media 104 and the tip 110 may
be adapted to more relative to one another such that tip 110 scans
the domains 106 while applying a contact force F to the domains to
generate a direct piezoelectric effect within the domains. A direct
piezoelectric effect may occur as noted above with respect to
equation (1) when a force F is exerted onto a piezoelectric and/or
ferroelectric thin film. In such a case, a charge Q may be
generated, which is quantified by the following equation:
Q=d.sub.33F (2)
where d.sub.33 is a piezoelectric coefficient of the media 104 and
F is the magnitude of the force applied. It is clear to see that
equation (2) above is a derivation of equation (1) in an "out of
plane" direction of the media top surface 112. For example, for an
epitaxial ferroelectric PZT film, d.sub.33 may be about 100 pC/N.
According to an embodiment, the tip 110 may be adapted to scan each
of the domains on a top surface 112 thereof (top surfaces 112 being
part of top surface 122 of media 104), and, while scanning each of
the domains, the tip 110 may apply a contact force F to said each
of the domains, in this way bringing about a direct piezoelectric
effect within any given one of the domains. Merely scanning the tip
110 over each of the domains 106 would lead to capacitive coupling
across the tip-domain junction in any event, in this way generating
a bit signal within the tip 110, so that the tip 110 would be
acting as an antenna for detecting a bit signal from the domains,
the signal having a frequency that is a function of the scanning
speed v of the tip 110. According to embodiments, applying a
contact force F to each of the domains using the tip 110 while
scanning each of the domains 106 with the tip allows a direct
piezoelectric effect generated by the contact force F to amplify
the bit signal, such that the tip 110 may more readily detect, by
way of electrical capacitive coupling with each domain 112 across
the tip-domain junction, bit signals corresponding to bit charges
or information stored in the media 104.
[0021] Referring now still to FIG. 1, as the tip pushes down on the
media top surface 112, charge builds as a result of the direct
piezoelectric effect described in relation to equations (1) and (2)
above. As the tip moves across a domain boundary 113, the built-up
charge changes its sign. Assuming that the tip 110 is moving with a
velocity v and that dx is the width of the boundary region 113, the
time required for the tip to cross the boundary region would be
given by the following equation (3):
dt=dx/v (3)
Assuming that, based on equation (2) above, the charge at location
xo is given by Q.sub.xo=+d.sub.33F and the charge at location x1 is
given by Q.sub.x1=-d.sub.33F, then, because built-up charge changes
over the time dt when the tip crosses the domain boundary 113 from
xo to x1 in FIG. 1, a current thus generated may be given by the
following equation (4):
i = ( Q x 1 - Q xo ) / t = - ( d 33 F + d 33 F ) / ( x / v ) = - 2
( v / x ) d 33 F ( 4 ) ##EQU00001##
In general, the tip radius r would represent the limitation in
resolving the domain boundary. Thus r can be used in the place of
dx in equation (4) above. Therefore, a more practical rendition of
current would be given by equation (5):
i=-2(v/r)d.sub.33F (5)
Equations (4) and (5) suggest that the generated current i can be
increased by scanning at a higher speed v, applying a higher force
F, or using a smaller tip radius r. Equations (4) and (5) also
suggest that the sign of the current depends on whether the tip
moves from an UP domain to a DOWN domain or vice versa. Assuming
for example that the following values apply: v=1 cm/s; d33=100
pC/N; F=1 .mu.N; and r=20 nm, then i as given by equation (5) above
would equal about 100 pA.
[0022] Referring still to FIG. 1, according to one embodiment, the
media and the tip are adapted to move relative to one another by
virtue of a movement of the tip, the media being stationary with
respect to the same, or, in the alternative, by virtue of a
movement of the media, the tip being stationary with respect to the
same. Actuation control systems to move a probe tip or a media in a
scanning or lateral motion in the scanning direction as shown by
arrow v in FIG. 1, are well known in the art. Such actuation
control systems may include, for example, at least one of
electrostatic actuators, piezoelectric actuators, electromagnetic
actuators and thermal actuators, as would be recognized by the
skilled person. Where the media is being moved with respect to the
tip in a scanning or lateral motion, such media may be disposed on
a movable media platform 114 (shown in broken lines in FIG. 1) in a
well known manner. In the shown embodiment of FIG. 1, a scanning
actuation control system 116 is shown in broken lines as being
coupled to the tip 110 to actuate a lateral motion of the same with
respect to the media 104 as described above. In the alternative, a
scanning actuation control system 116' is shown in broken lines as
being coupled to the platform 114 to actuate a lateral motion of
the same with respect to the tip 110 as described above. The
actuation control systems 116 and 116' are shown in broken lines in
order to suggest that they may represent alternatives with respect
to one another for scanning tip 110 onto the domains 106. According
to one embodiment, the actual control system 116/116' may be
adapted to allow a scanning of the domains 106 at a plurality of
scanning speeds v. Preferably, as noted above, the scanning speed v
is set such that the frequency of the bit signal corresponds to a
RF. For example, the scanning speed may be about 2.5 cm/s according
to one embodiment.
[0023] With respect to the application of the contact force F
between tip 110 and domains 106, actuation control systems may be
used to move a probe tip toward a media in a "Z direction," that
is, in a direction toward or away from one another as shown by
arrow Z in FIG. 1, in order to urge the tip 110 toward the domains
106 to effect a contact force F. Such actuation control systems may
include, for example, at least one of electrostatic actuators,
piezoelectric actuators, electromagnetic actuators and thermal
actuators, as would be recognized by the skilled person. In the
shown embodiment of FIG. 1, a Z actuation control system 118 is
shown as being coupled to the tip 110 to actuate a Z direction
motion of the same with respect to the media 104 as described
above. It is preferable to arrange the tip and the media as level
to each other as possible to insure a minimal force variation while
scanning. According to a preferred embodiment, the contact force
may range anywhere between about 1 nN and about 3 .mu.N, and may
preferably range between about 1 nN and about 200 nN. According to
one embodiment, the Z direction actuation control system 118 may be
adapted to generate a contact force F having differing magnitudes.
Thus, the Z direction actuation control system 118 may be adapted
to allow a first contact force F.sub.1 of a first magnitude be
applied to all of the domains in one scanning operation, and a
second contact force F.sub.2 of a second magnitude different from
the first magnitude be applied to all of the domains in a
subsequent scanning operation. In the alternative, the Z direction
actuation control system 118 may be adapted to allow differing
contact forces to be applied as between differing domains in a
single scanning operation.
[0024] Referring still to FIG. 1, arrangement 100 may include
circuitry 118 which includes a charge amplifier device 120
ohmically coupled to the tip 110. The charge amplifier device may
amplify the bit signal to a voltage signal V.sub.pp in response to
the charge Q from the bits of pitch written in the domains of the
media that the tip 110 detects by the electrical capacitive
coupling across the tip-domain junction. The voltage signal
V.sub.pp may be given by the following equation:
V.sub.pp=Q/C.sub.f (6)
where Cf represents the capacitance of a feedback capacitor
installed on the charge amplifier circuit assembly to convert the
charge input signal Q to the voltage output signal Vpp. According
to an embodiment, the arrangement may be configured such that
C.sub.f is equal to about 0.5 pF. According to embodiments, the
direct piezoelectric effect in the embodiment of FIG. 1 amplifies
the bit signal to V.sub.pp while the tip 110 is scanned on a domain
106 while applying a contact force F thereto at a scan speed v
necessary to retrieve the bit signal with a desirably high data
transfer rate. A high data transfer rate would correspond, for
example, to about a 500 kbps (1 Mbps) readout of 50 nm pitch (half
pitch) bits with the tip scanning at about 2.5 cm/s. A desirably
high data transfer rate would correspond, for example, to a 5 Mbps
(10 Mbps) readout of bits 10 nm pitch (half pitch) bits with the
tip scanning at about 5 cm/s. Referring to FIG. 2, an exemplary
schematic graph is shown plotting V.sub.pp versus time as a
function of F, or of the direct piezoelectric effect in the
embodiment of FIG. 1, for an arrangement similar to arrangement 100
of FIG. 1. As suggested in FIG. 2, a larger contact force generates
larger piezoelectric charge, which in turn results into a more
amplified V.sub.pp signal as plotted versus time.
[0025] Referring next to FIG. 3, a second embodiment is shown for
an arrangement to store and read information. Arrangement 300 shown
in FIG. 3 is similar to arrangement 100, except for the following
differences: (1) arrangement 300 includes a tip 110 which may be
adapted to apply an actually modulated contact force F to each of
the domains 106; (2) according to a preferred embodiment,
arrangement 300 is adapted to retrieve a current signal for the
change in piezoelectric charges in the domains, as opposed to
voltage signals as described above in relation to FIG. 1.
Arrangement 300 may thus include an electric current detector 320
coupled to the tip, the current detector being adapted to detect a
current signal for current induced as a result of the media 102
being compressed by the tip 110 in a modulated fashion. As a result
of the above, like components as between arrangements 100 on the
one hand, and 300 on the other hand, have been denoted with like
reference numerals, and will not be described again with respect to
FIG. 3, as they have already been described in relation to FIG.
1.
[0026] With respect to the application of a modulated contact force
F between tip 110 and domains 106 as applicable to arrangement 300
of FIG. 3, the magnitude of the contact force may vary with time
with respect to each one of the domains to allow a detection or
characterization of ferroelectric domains or domains 106 in media
104 using tip 110. To detect the domains in the media, the tip may
be moved using scanning actuation control system 116 in a manner as
described above in relation to FIG. 1 to scan the media surface
122. At the same time, the tip or the media may be controlled to
modulate the contact force F between the tip and the surfaces 112
of the domains for each given one of the domains. Due to a direct
piezoelectric effect, a current generated when the contact force
between tip 110 and the surfaces 112 of domains 106 is modulated is
shown in equation (4) below:
i=(dQ/dt)=d.sub.33(dF/dt) (4)
where dF/dt is the changing rate of contact force, d.sub.33 is the
piezoelectric modulus of the media in the direction perpendicular
to the media surface 122, and dQ/dt is the changing rate of
polarization charge induced when the media 104 is compressed by the
tip 110. According to the above regime, while the amplitude of the
current in equation (4) above can be the same for both the UP
domain (indicated with an upwardly pointing broken-lined arrow in
FIG. 3) and for the DOWN domain (indicated with an downwardly
pointing broken-lined arrow in FIG. 3), the current i for
respective ones of the UP and DOWN domains will be 180 degrees out
of phase with respect to one another. In order to address the
above, electrical current detector 320 could be used to resolve or
detect the above phase difference by detecting the domain
orientations of the domains. The amplitude as obtained by equation
(4) and detected by the electrical current detector 320 would then
yield more detailed information about location polarization
changes. Optionally, the arrangement 300 may include a lock-in
amplifier 322 coupled to the tip, and adapted to pass signal only
at a force modulation frequency of the tip in order to improve
signal to noise ratio.
[0027] Referring still to FIG. 3, actuation control systems may be
used to move a probe tip and the media toward one another in the Z
direction in order to urge the tip 110 toward the domains 106 to
effect a contact force F in a modulated fashion. Such actuation
control systems may include, for example, at least one of
electrostatic actuators, piezoelectric actuators, electromagnetic
actuators and thermal actuators, as would be recognized by the
skilled person. In the shown embodiment of FIG. 3, a Z actuation
control system 318 is shown as being coupled to the tip 110 to
actuate a Z direction motion of the same with respect to the media
104 as described above. The actuation control system 318 may
include a mechanical vibrator to allow the tip to apply a modulated
contact force to the domains. Alternatively, a function generator
may be coupled to the PZT material of the media in order to drive
the same, thus moving the media surface toward and away from the
tip in a modulated fashion (not shown). According to a preferred
embodiment, the contact force may be modulated within an amplitude
range between about 1 nN and about 3 .mu.N. Preferably, the contact
force F is modulated in ranges below about 200 nN.
[0028] Referring still to FIG. 3, and referring also to equations
(1) and (2) above with respect to a direct piezoelectric effect, a
charge generated as a result of the application of a modulated
force may be given by equation (7) below:
Q=d.sub.33F=d.sub.33F.sub.o sin .omega.t (7)
and a current generated as a result of the above may be given by
equation (8) below:
i=dQ/dt=d.sub.33F.sub.o.omega. cos .omega.t (8)
where F.sub.o is the peak amplitude of the modulated force, and w
is given by the equation (9):
.omega.=2.pi./T=2.pi.f (9)
where T is the period, and f is the frequency, of the modulation
signal. In the case of the above, i may be given by its root mean
square value to the extent that the force may be assumed to be
sinusoidally modulated. In that case, current i may be given by
equation (10):
i.sub.rms=(d.sub.33F.sub.o.omega.)/ 2 (10)
Equation (9) above suggests that the generated current can be
increased in the case of the embodiment of FIG. 3 by increasing the
force modulation frequency or by increasing the modulated force
peak amplitude. The equation also suggests that the sign of the
generated current will reverse when the domain polarity changes.
Assuming for example that the following values apply: d.sub.33=100
pC/N; F=1 .mu.N; and f=310 kHz, then i as given by equation (10)
above would equal about 138 pA.
[0029] It is to be noted that, notwithstanding FIGS. 1 and 3,
embodiments are not limited to the use of an arrangement where a
single tip is used. Thus, an arrangement according to embodiments
encompasses within its scope a plurality of probes including probe
tips (not shown), such as, for example, tips extending from
respective probes in the shape of cantilevers toward the surfaces
112 of the domains 106. Such probes may, for example, be arranged
in a row with respect to top surfaces 112 of the domains 106. In
addition, in the instant description with respect to the "applying
a contact force F" is not meant to connote that the tip necessarily
applies the same contact force F (i.e. a contact force F having the
same magnitude) as between various ones of the domains. Rather,
"applying a contact force F" as used herein connotes a force
application between the tip and each of the domains during a
scanning of said each of the domains by the tip, the contact force
F possibly having: (1) different magnitudes (possible in the
embodiment of FIG. 1) or different average magnitudes (possible in
the embodiments of FIG. 3, for example) as between various ones of
the domains; (2) having a modulated magnitude on each individual
domain (possible in the embodiment of FIG. 3, for example); or (3)
having the same magnitude (possible in the embodiment of FIG. 1,
for example) or the same average magnitude (possible in the
embodiment of FIG. 3, for example) throughout each scanning
operation, based on application needs.
[0030] Referring now to FIG. 4, a graph is shown of experimental
data regarding readout signal V.sub.pp in Volts plotted versus
contact force in .mu.N for an exemplary arrangement similar to
arrangement 100 according to an embodiment. Here, C.sub.f=0.5 pf,
and the voltage signal V.sub.pp=Q/0.5 pf. The solid line in FIG. 4
predicts the linear amplification of the readout signal output with
increasing contact force F according to the direct piezoelectric
effect Q=(100 pC/N).times.F. The solid squares in FIG. 4 represent
a set of readout signal outputs from experimentation, where a 50 nm
thick PZT epitaxially grown film was used on a SRO/STO substrate. A
conductive probe tip was used to write a ferroelectric bit line
array having a 60 nm width and a pitch of 160 nm. The tip was then
scanned in contact with the media surface across the bit array at
the speed 0.814 mm/s under a wide range of contact forces while
recording the readout signal traces averaged over four times on the
oscilloscope. The experimental data scatters around the dashed
linear fit line in FIG. 4. The offset between the prediction and
the experiment may be largely attributed to the presence of
contamination layers on the tip and the media surfaces, which
layers tend to make the tip-media junction electrical coupling less
than ideal. The readout signal output may amplify when the
contamination layers on the PZT are removed using oxygen plasma
followed by nitrogen aided surface passivation for read/write
stability. A tip surface treatment as noted above allows the
tip-media contact to establish a better electrical coupling, and
aids the process of signal boosting by a direct piezoelectric
effect.
[0031] Referring next to FIG. 5, a flowchart 600 is shown of a
method of reading information stored in a layer of ferroelectric
media according to embodiments. In the description below, reference
to components X/Y/Z is meant to signify a refers to component X in
FIG. 1, or component Y in FIG. 3. The method depicted schematically
in flowchart 600 may be performed for example using either of
arrangements 100 and 300 shown in FIGS. 1 and 3, respectively. At
block 602, method embodiments may include scanning the tip, such as
tip 110 of FIG. 1/3, over the domains 106/106, while applying a
contact force F to the domains with the tip to generate a direct
piezoelectric effect within the domains. At block 604, method
embodiments may further include using circuitry, such as, for
example, the circuitry including the charge amplifier device 120 of
FIG. 1, or, in the alternative, the electrical current detector 320
of FIG. 3, coupled to the tip 110, to generate a signal in response
to a electrical coupling between the tip 110 and the domains 106
while scanning the tip in contact with the domains, the signal
corresponding to a readout signal for ferroelectric bit charges
stored in the domains. According to one embodiment, the tip 110 may
be scanned over the domains at a velocity to result in the signal
having a frequency in the RF range. In the embodiments of FIGS. 1
and 3, for example, the tip 110 may be scanned across the surface
while applying a contact force continuously during scanning. On the
other hand, for the embodiments of FIGS. 1 and 3, for example, the
tip 110 may be scanned across the surface of the media while
applying a contact force during scanning only on the top surfaces
112 of the domains 106. The latter method embodiment may be
performed using the arrangement 100 of FIG. 1 or the arrangement
300 of FIG. 3, for example, where tip 110 may apply a contact force
F during scanning only to the top surfaces 112 of the domains 106,
and not to intermediate boundary regions 113 disposed in between
the domains 106. Additionally, in the embodiment of FIG. 1,
according to another method embodiment, the tip 110 may be scanned
across the surface of the media 104 while applying a first contact
force during scanning on the top surfaces 112 of the domains 106,
and while applying a second contact force during scanning on top
surfaces of intermediate regions 113 disposed in between the
domains 106, the second contact force being less than the first
contact force. In the case of the latter embodiment, a desirable
readout amplification by piezoelectricity may be achieved with a
lower average contact force as compared with applying a constant
contact force across the top surface 122 of media 104, in this way
advantageously reducing wear on the tip.
[0032] Advantageously, embodiments provide a scanning tip readout
arrangement and method for reading ferroelectric bit charges using
a tip sensor by applying a force to the domains using the tip. The
force may be constantly applied according to one embodiment on the
one hand (FIG. 1), or it may be modulated according to another
embodiment by virtue of modulating a force applied between the tip
and each given domain (FIG. 3). Conventional piezoresponse signal
readout arrangements and methods, such as PFM, disadvantageously
rely on the detection of a cantilever's mechanical motion, which
requires a complicated laser setup and optical alignment.
Embodiments as shown in FIGS. 1 and 3, however, obviate the need
for such complicated laser setup and optical alignment, as the
signal detection according to embodiments allows reliance on a
direct electrical coupling between the tip and the domain
surface.
[0033] Advantageously, the embodiment as depicted with respect to
FIG. 1 provides a faster scanning tip speed readout as compared
with arrangements of the prior art. While the prior art may allow
surface tracking at a 10 khz range at best, an embodiment such as
the one shown in FIG. 1 may advantageously allow a readout rate up
to a range within tens of Mhz. An embodiment according to FIG. 1
may thus increase tip speed and readout rate on the order 1000
times with respect to the prior art, thus being useful in next
generation data storage/memory devices and markets. A fast readout
rate is ensured according to embodiments by simply scanning the tip
at a desirably high speed while maintaining a contact force with
the surfaces of the domains. The direct piezoelectric effect
present in the embodiment of FIG. 1, as described above, aids in
the amplification of the fast readout signal, and thus improves the
signal to noise ratio. An upper readout speed of embodiments is set
electrically by the capacitance C.sub.j and the resistance R.sub.j
across the media-tip junction. Cj and Rj define the time constant
(RjCj) across the tip-media junction system. For example, bits can
routinely be written using a 50 ns voltage pulse across the
tip-media junction type. This indicates that one should be able to
operate with a data retrieval rate that is in the Mbps range (i.e.,
1/50 ns or 1/RjCj)] when one works with a similar type of tip-media
junction resistance/capacitance combination. A fast readout in the
Mhz range may be routinely achievable as the Cj (in the atto farad
range) and the Rj (in the Giga to tera ohm range) are typical for a
tip-media junction according to embodiments. A Mhz readout range is
believed to be compatible to or better than any of solid-state
non-volatile memory or data-storage devices available on the market
today. The fastest scanning speed determined experimentally
according to embodiments drove the tip in contact at about 2.5 cm/s
while retrieving the readout signal over a ferroelectric bit line
array (.about.120 nm width and pitch .about.330 nm), embodiments
not being limited to the above scanning speed. The above high speed
result translates to the readout data transfer rate of about 75
kbps pitch (or .about.150 kbps for a half pitch).
[0034] Advantageously, with respect to the embodiment of FIG. 3,
that embodiment advantageously provides a force modulation and
electrical current detection arrangement which is simple and which
can be easily integrated into a small form factor, thus obviating
the complicated optical setups (PFM) and high frequency electrical
setups (SNDM) of the prior art. In addition, for the embodiment of
FIG. 3, because the amplitude of the current signal depends on the
changing rate of polarization charger instead of on the
polarization charge itself when the domains are compressed by the
cantilever tip, the contact force can be reduced as compared with a
contact force required in the embodiment of FIG. 1, for example,
and thus, in turn, a lifetime of the electrical sensing cantilever
can be prolonged.
[0035] Referring to FIG. 6, there is illustrated one of many
possible systems 900 in which embodiments of the present invention
may be used. In one embodiment, the electronic assembly 1000 may
include an arrangement, such as arrangement 100 of FIG. 1, or
arrangement 300 of FIG. 3. Assembly 1000 may further include a
microprocessor. In an alternate embodiment, the electronic assembly
1000 may include an application specific IC (ASIC). Integrated
circuits found in chipsets (e.g., graphics, sound, and control
chipsets) may also be packaged in accordance with embodiments of
this invention.
[0036] For the embodiment depicted by FIG. 6, the system 900 may
also include a main memory 1002, a graphics processor 1004, a mass
storage device 1006, and/or an input/output module 1008 coupled to
each other by way of a bus 1010, as shown. Examples of the memory
1002 include but are not limited to static random access memory
(SRAM) and dynamic random access memory (DRAM). Examples of the
mass storage device 1006 include but are not limited to a hard disk
drive, a compact disk drive (CD), a digital versatile disk drive
(DVD), and so forth. Examples of the input/output module 1008
include but are not limited to a keyboard, cursor control
arrangements, a display, a network interface, and so forth.
Examples of the bus 1010 include but are not limited to a
peripheral control interface (PCI) bus, and Industry Standard
Architecture (ISA) bus, and so forth. In various embodiments, the
system 900 may be a wireless mobile phone, a personal digital
assistant, a pocket PC, a tablet PC, a notebook PC, a desktop
computer, a set-top box, a media-center PC, a DVD player, and a
server.
[0037] The various embodiments described above have been presented
by way of example and not by way of limitation. Having thus
described in detail embodiments of the present invention, it is
understood that the invention defined by the appended claims is not
to be limited by particular details set forth in the above
description, as many variations thereof are possible without
departing from the spirit or scope thereof.
* * * * *