Abstract:
A probe-based storage device comprises a storage surface for storing data represented by deformations in the surface. A probe faces the surface and includes a resonant circuit having a reactance dependent on deflection of the probe relative to the surface. A scanner is provided for scanning the probe across the surface such that the probe follows said deformations. A detector reads data stored on the surface by detecting variation of the resonant frequency of said circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a method and apparatus for reading data in probe-based data storage devices. 
     2. Discussion of Related Art 
     Techniques employing nanometer-sharp tips for imaging and investigating the structure of materials down to the atomic scale, such as the atomic force microscope (AFM) and the scanning tunneling microscope (STM), also find application in ultra high density storage devices. See, for example, U.S. Pat. No. 4,575,822 and P. Vettiger et al., “The Millipede—More than one thousand tips for future AFM data storage,” IBM Journal of Research and Development, vol. 44 No. 3 May 2000, pp. 323–340. In the device described by Vettiger, information is stored as sequences of “pits” and “no pits” written on a polymer storage surface via an array of cantilevers each carrying a tip. The cantilevers, and the tips thereon, are selectively heated to write data onto the surface. The heating of each tip to a sufficient level produces a corresponding deformation of the surface adjacent the tip. The stored information is read back by treating each cantilever as a thermo-mechanical sensor in a circuit which is electrically equivalent, to a first degree of approximation, to a current source or a voltage source in cascade with a variable resistor. The sensor transforms the physical value carrying the read information into an electrical signal. The value of the variable resistor depends on the temperature at the tip of the cantilever. During the read process, the cantilever reaches different temperatures whether it moves over a “pit” (bit “1”) or a “no pit” (bit “0”). A detection circuit senses a voltage which is dependent on the value of the cantilever resistance to make a decision on whether a “1” or a “0” is detected. 
     Conventionally, to read the recorded information, the cantilever employed for writing is provided with the additional function of a thermal read back sensor by exploiting its temperature dependent resistance. In general, the resistance increases non-linearly with heating power/temperature from room temperature to a peak value of 500–700 degrees C. The peak temperature is determined by doping concentration in the variable resistance of the cantilever, which ranges from 1×10 17  to 2×10 18  cm −3 . Above the peak temperature, the resistance drops as the number of intrinsic carriers increases through thermal excitation. For sensing, the resistor is operated at about 350° C. This temperature is not high enough to deform the surface as in the case of writing. 
     The principle of thermal sensing is based on the thermal conductance between the heater platform and the surface changing according to the distance between them. The medium between the heater platform and the surface, such as air, transports heat from the cantilever to the surface. When the distance between cantilever and surface is reduced as the tip moves into a pit, the heat transport through the air becomes more efficient. As a result, the evolution of the heater temperature in response to a pulse applied to the cantilever is different and, in particular, the maximum value achieved by the temperature is smaller than in the case in which no pit is present. As the value of the variable resistance depends on the temperature of the cantilever, the maximum value achieved by the resistance will be smaller as the cantilever moves over a pit. Therefore, during the read process, the cantilever resistance reaches different values whether it moves over a pit (bit “1”) or no pit (bit “0”). 
     The thermo-mechanical cantilever sensor, which transforms temperature into an electrical signal that carries information, is electrically equivalent, to a first degree of approximation, to a variable resistance. A detection circuit should therefore sense a voltage that depends on the value of the cantilever resistance to make a decision on whether a “1” or a “0” is written. The relative variation of thermal resistance is typically around 10 −5 /nm. Hence, a written bit “1” typically produces a relative change of the cantilever thermal resistance ΔR Θ /R Θ  of about 10 −4 ˜5×10 −4 . The relative change of the cantilever electrical resistance is of the same order of magnitude. As a consequence, an important issue in detecting the presence or absence of a pit is a sufficiently high resolution to permit extraction of the signal that contains the information about the bit being “1” or “0”. The signal carrying the information can be viewed as a small signal superimposed to a very large offset signal, which can be three to four orders of magnitude larger. 
     Parallel operation of large two-dimensional arrays can be achieved by a row/column time-multiplexed addressing scheme similar to that implemented in DRAMs. In the device described in [5], such a multiplexing scheme is employed to address the array column by column for parallel write/read operation within one column. In particular, read back signal samples are obtained by applying a read pulse to the cantilevers in a column of the array, low-pass filtering the cantilever response signals, and sampling the filter output signals. This process is repeated sequentially until all columns of the array are addressed, and then restarted from the first column. The time between two pulses corresponds to the time needed for a cantilever to move from one bit position to the next. Another problem encountered with time-multiplexed read operations based on thermo-mechanical sensing stems from an inherent limitation to achievable data rate which is determined by the cantilever thermal time constant. Specifically, a read pulse needs a duration at least equal to the time taken for the cantilever to achieve a temperature of about 350 degrees C., at which reading can take place. 
     SUMMARY OF INVENTION 
     In accordance with the present invention, there is now provided a probe storage device comprising: a storage surface for storing data represented by deformations in the surface; a probe facing the surface and including a resonant circuit having a reactance dependent on deflection of the probe relative to the surface; a scanner for scanning the probe across the surface such that the probe follows said deformations; and, a detector for reading data stored on the surface by detecting variation of the resonant frequency of said circuit. This advantageously permits detection of data from the storage surface without involving heating the probe. The device can thus operate with lower power dissipation. In addition, increased reading speeds can be achieved because consideration need not be given to the thermal time constant of the probe. 
     The reactance may comprise a variable inductance. Preferably, the variable inductance comprises a ferromagnetic element and a coil defining current path moveable relative to the ferromagnetic element in response to deflection of the probe. Alternatively, the reactance may comprise a variable capacitance. 
     In a preferred embodiment of the present invention to be described shortly, the detector comprises a first signal generator connected to the resonant circuit for generating a first signal in the resonant circuit and, a mixer for multiplying the output from the resonant circuit by a second signal synchronized to and phase shifted from the first signal and having a similar wave form to that of the first signal. The detector may comprise a second signal generator for generating the second signal, the first and second signal generators being synchronized by a synchronization signal. Alternatively, in the interests of simplicity, the detector may comprise a phase shifter having an input connected to the output of the first signal generator and an output connected to the mixer for generating the second signal by phase shifting the first signal. In a particularly preferred embodiment of the present invention, the detector comprises: a low pass filter for filtering the output of the mixer; and sample and hold circuit for sampling the output of the low pass filter; and a detection circuit for converting samples from the sample and hold circuit into binary values. The first and second signals preferably vary at substantially the resonant frequency of the resonant circuit when the probe is not deflected. 
     Viewing the present invention from another aspect, there is now provided a method for detecting data in a probe storage device, the method comprising: storing data as deformations in a storage surface; positioning a probe facing the surface and including a resonant circuit having a reactance dependent on deflection of the probe relative to the surface; scanning the probe across the surface such that the probe follows said deformations; and, reading data stored on the surface by detecting variation of the resonant frequency of said circuit. 
     In a preferred embodiment of the present invention, there is provided a method for reading back information written in a probe storage device based on an AFM cantilever array, in which each cantilever circuit is electrically equivalent to an RLC circuit comprising a resistance R, an inductance L, and a capacitance C. Rather than relying on variation of a cantilever resistance in dependence on temperature at the tip of a heated cantilever, sensing whether a “1” or a “0” is recorded is achieved by applying to the “cold” cantilever a sinusoidal wave form having a frequency equal to the resonant frequency of the RLC circuit, and observing the wave form at the output of the RLC circuit to detect a variation in the circuit transfer characteristics. For example, a variation of the phase characteristic can be obtained from variation of one or both reactance values of the RLC circuit (L and/or C). Such a variation is induced by movement of the probe over a “pit” or a “no pit”. Many advantages over conventional thermo-mechanical reading techniques are achieved by this technique. As the read back process does not involve heating the cantilever to a relatively high temperature of about 350 degrees C., the rate at which reading can be performed is not limited by a thermal time constant. Thus, higher data rates can be achieved. Furthermore, a lower signal power is involved in detection because no signal energy is converted into thermal energy for heating the cantilever. Additionally, because the identity of a bit as a “1” or a “0” is encoded as a variation of phase and/or amplitude of a sinusoidal signal, detection of the useful signal does not take place in the presence of a large offset signal, as occurs in the case of thermo-mechanical reading. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a probe storage device; 
         FIG. 2  is simplified circuit diagram of a read channel for the probe storage device; 
         FIG. 3  is a frequency response in terms of amplitude characteristics corresponding to a cantilever in the probe storage device. 
         FIG. 4  is a frequency response in terms of phase characteristics corresponding to a cantilever in the probe storage device. 
         FIG. 5  is a graph showing Signal to Noise Ratios of the read channel as a function of the power of applied read pulse; 
         FIG. 6  is simplified circuit diagram of another read channel for the probe storage device; 
         FIG. 7  is a side view of a cantilever of the array; and, 
         FIG. 8  is a plan view of a cantilever of the array. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring first to  FIG. 1  an example of a probe storage device embodying the present invention comprises a substrate  10  having a two dimensional array  120  of probe cantilever sensors  20  facing a storage surface  30 . The cantilevers  20  are connected to row conductors  40  and column conductors  50 . Each cantilever  20  is addressed by a different combination of a row conductor  40  and a column conductor  50 . The row conductors  40  are selectively addressed via a row multiplexer  60 . Similarly, the column conductors  50  are selectively addressed via a column multiplexer  70 . The storage surface  30  is mounted on a scanner mechanism comprising an x position transducer  80 , a y position transducer  90  and a z position transducer  100 . In operation, the z transducer  100  moves the storage surface  30  towards or away from the array  120 . The x transducer  80  and the y transducer  90  move the storage surface  30  in orthogonal directions relative to and within a plane parallel to the array  120 . The transducers  80 – 100  may be piezoelectric, electromagnetic, or similar position control devices. The ranges of travel of the x transducer  80  and the y transducer  90  are such that, in use, each cantilever  20  is scanned across its own field of the storage surface  30  during both data reading operations and data writing operations. Such scanning may be performed in a raster like fashion. Each cantilever  20  carries at its distal end a tip or probe facing the storage surface and a resistive heater element adjacent the tip. During a write operation, each cantilever  20  from which data is to be written is engaged with the storage surface  30  via the z transducer  100  and connected to a write channel via the multiplexers  60  and  70 . The write channel applies a write signal indicative of data to be stored to the cantilever  20 . As herein before described, to write a binary “1”, the write signal is of a magnitude sufficient to heat the tip via the heater element to a level sufficient to produce a local deformation or pit  110  in the storage surface  30  in the region of the tip. To write a binary “0”, the write signal is maintained sufficiently low that no such local deformation of the storage surface  30  occurs. Multiple bits are thus written as the tips are scanned across the storage surface  30 . Each cantilever  20  comprises an RLC circuit having a resistance R, a capacitance C, and a variable inductance L. During a read operation, each cantilever  20  from which data is to be read is engaged with the storage surface  30  via the z transducer  100  and connected to a read channel via the multiplexers  60  and  70 . The tips are then scanned across the storage surface  30  and recorded data is read out via the read channel in a manner to be described shortly. The read channel described herein advantageously avoids the aforementioned problems associated with the prior art because it does not rely on a temperature dependent cantilever resistance to generate the read back signal. 
     Referring now to  FIG. 2 , in a preferred embodiment of the present invention, the read channel comprises: a first signal generator  200  having an internal resistance R 0 ; a buffer amplifier  210 ; a second signal generator  220 ; a mixer  230  having gain K m , a low pass filter  240 , a sample and hold circuit  250  and a threshold detector  260 . The output of the second signal generator  220  is synchronized to the output of the first signal generator  210  via a synchronization signal  270 . As shown in  FIG. 2 , in a preferred embodiment of the present invention, the synchronization signal  270  may be derived from the output of the first signal generator  210 . However, in other embodiments of the present invention, both the first and second signal generators  210  and  220  may be synchronized by a common, independently generated synchronization signal. In operation, as will be described herein, the first and second signal generator  200  and  220  both generate sinusoidal signals. However, in other embodiments of the present invention, different wave forms may be employed. In operation, a pulse of AC signal or burst V p (t) generated by the signal generator  200  is applied to the cantilever RLC circuit. The expression of the applied burst is given by 
                         V   p     ⁡     (   t   )       =     A   ⁢           ⁢     rect   ⁡     (     t   τ     )       ⁢           ⁢     sin   ⁡     (     2   ⁢           ⁢   π   ⁢           ⁢     f   0     ⁢   t     )           ,     
     ⁢   where           (   1   )                 rect   ⁡     (     t   τ     )       =     {         1           if   ⁢           ⁢   0     ≤   t   ≤   τ             0       otherwise                   (   2   )               
A denotes the burst amplitude, and f 0  is chosen approximately equal to the resonant frequency of the RLC circuit, i.e. f 0 ≈1/(2π√{square root over (LC)})
 
     The value of the variable inductance L depends on the extent of deflection of the cantilever  20  as the tip moves over a pit  110 . The relative variation of inductance is indicated by the parameter λ x =ΔL x /L. The subscript x indicates the x-distance in the direction of scanning from the initial point. Therefore, the parameter λ x  will take the largest absolute value when the tip of the cantilever  20  is located at the center of a pit  110 . However, as the time taken for the cantilever  20  to move from the center of a pit  110  to the next is much larger than the duration of a read pulse, the current through the inductance does not vary significantly as a function of x during the period for which a read burst is applied. The signal V b (t,x) at the output of the buffer amplifier  210  is then given by 
                       V   b     ⁡     (     t   ,   x     )       =         R   b       R   0       ⁢     (         V   p     ⁡     (   t   )       -       L   ⁡     (     1   +     λ   x       )       ⁢       ⅆ     i   L         ⅆ   t         -     Ri   L       )               (   3   )               
where i L  denotes the current through the inductance, which obeys the differential equation
 
                         L   ⁡     (     1   +     λ   x       )       ⁢     R   0     ⁢   C   ⁢           ⁢         ⅆ   2     ⁢     i   L         ⅆ     t   2           +       (         R   0     ⁢   RC     +     L   ⁡     (     1   +     λ   x       )         )     ⁢       ⅆ     i   L         ⅆ   t         +       (       R   0     +   R     )     ⁢     i   L         =       V   p     ⁡     (   t   )               (   4   )               
with initial conditions
 
     
       
         
           
             
               
                 
                   
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     Assuming, for the purpose of explanation, that the time of application of a read burst corresponds either to the cantilever  20  being located at the center of a pit  110  for detecting a bit “1”, or away from a pit  110  for detecting a bit “0”. Two possible responses are obtained at the output of the buffer amplifier as solutions of (3–4), denoted by V b (t,x|a x =1) and V b (t,x|a x =0), respectively. Assuming, also for the purpose of explanation, that the duration of the transients is small compared to the duration of the burst τ, in the time interval[0,τ] the two responses are approximately given by:
 
 V   b ( t,x|a   x =0)≈| H (f 0 )| A  sin(2πf 0   t +φ),  t ε[0,τ]  (5)
 
and,
 
 V   b ( t,x|a   x =1)≈| H (f 0 )| A  sin(2πf 0   t +φ+Δφ),  t ε[0, τ]  (6)
 
where |H(f 0 )| and φ denote the amplitude and phase of the frequency response of the internal resistance R 0  of the pulse generator  210  and the RLC circuit in series, respectively. The term Δφ stems from the variation in phase characteristic of the frequency response. In turn, this stems from the variation of the inductance as the tip moves over a pit  110 .
 
       FIG. 3  illustrates the amplitude characteristics of the frequency response obtained for R 0 =100Ω, R=50Ω, L=1 μH, and C=145 pF.  FIG. 4  illustrates the corresponding phase characteristics based on the same parameters. 
     The buffer output signal is multiplied by the mixer  230  with a sinusoidal wave form cos(2πf 0 t+φ) from the second signal generator  220 . For the two responses (5) and (6), the expressions of the resulting signals are given by: 
                           V   b     ⁡     (     t   ,       x   |     a   x       =   0       )       ⁢           ⁢     cos   ⁡     (       2   ⁢           ⁢   π   ⁢           ⁢     f   0     ⁢   t     +   φ     )         ≈           K   m     ⁢          H   ⁡     (     f   0     )            ⁢   A     2     ⁢   sin   ⁢     (     2   ⁢     (       2   ⁢           ⁢   π   ⁢           ⁢     f   0     ⁢   t     +   φ     )       )         ,     t   ∈     [     0   ,   τ     ]               (   7   )               and   ,                                   V   b     ⁡     (     t   ,       x   |     a   x       =   1       )       ⁢           ⁢     cos   ⁡     (       2   ⁢           ⁢   π   ⁢           ⁢     f   0     ⁢   t     +   φ     )         ≈           K   m     ⁢          H   ⁡     (     f   0     )            ⁢   A     2     ⁡     [       sin   ⁡     (     2   ⁢     (       2   ⁢           ⁢   π   ⁢           ⁢     f   0     ⁢   t     +   φ     )       )       +     sin   ⁡     (     Δ   ⁢           ⁢   φ     )         ]         ,     t   ∈     [     0   ,   τ     ]       ,           (   8   )               
respectively.
 
     After multiplication by the sinusoidal wave form, the signal is filtered by the low pass filter  240  and sampled at the instant t s =τby the sample and hold circuit  250 . 
     Assuming, for the purpose of explanation that the high-frequency signal components are completely suppressed by the low pass filter  240  with time constant τ lpf =1/(R lpf C lpf ), the values taken by the signal samples are
 
 V   out ( a   x =0)≈0  (9)
 
and
 
                         V   out     ⁡     (       a   x     =   1     )       ≈           K   m     ⁢          H   ⁡     (     f   0     )            ⁢   A       2   ⁢     C   lpf         ⁢     (     1   -     ⅇ       -   τ     /     τ   lpf           )     ⁢           ⁢     sin   ⁡     (     Δ   ⁢           ⁢   φ     )           ,           (   10   )               
for the two cases of bit “0” and bit “1”, respectively. The threshold detector  260  then detects a written bit, where the value of the threshold is given by
 
                     V   Th     =       1   2     ⁢       V   out     ⁡     (       a   x     =   1     )                 (   11   )               
To determine performance of the channel, the signal-to-noise ratio (SNR) may evaluated at the detection point, according to
 
                   SNR   =     10   ⁢           ⁢       log   10     ⁡     (       V   Th   2       σ   w   2       )                 (   12   )               
where the variance of the noise is dependent on the thermal noise introduced by the resistors and the equivalent input voltage noise power spectral density of the buffer amplifier.  FIG. 5  shows the SNR as a function of the power of the applied pulse for various values of the parameter λ x , assuming, for the purpose of explanation, a cantilever  20  having the characteristics illustrated in  FIG. 3 , a sinusoidal wave form frequency f 0 =14 MHz, a low pass filter time constant τ lpf =12 μs, noise sources given by the resistors R 0  and R at room temperature, and a buffer amplifier  210  with one-sided noise power spectral density equal to 2×10 −17  V 2 /Hz.
 
     Referring now to  FIG. 6 , in a modification of the read channel herein before described with reference to  FIG. 2 , the second signal generator  220  is replaced by a phase shifter  280  having an input connected to the output of the first signal generator  200  and an output connected to the mixer  230 . In operation, the phase shifter produces the second signal cos(2πf 0 t+φ) based on the output from the first signal generator  200 . Thus phase shifter  280  thus insures that the second signal is synchronized to the first signal. 
     Referring to  FIGS. 7 and 8  in combination, in a particularly preferred embodiment of the present invention, each cantilever  20  comprises first, second, and third electrically conductive limbs  300 ,  310  and  320 . The first limb  300  is connected to a temperature dependent resistor  340 . The second, centrally disposed limb  310  provides a common return current path from the first limb  300  and the third limb  320 . The third limb  320  acts as a coil. The reference numeral  320  will hereafter be interchangeably used in connection with both the third limb and the coil. A ferromagnetic element  330  is disposed in proximity to the third limb  320  such that the third limb  320  is deflected relative to the ferromagnetic element  330  as the cantilever  20  is deflected to provide the variable inductance L. The three-limb cantilever  20  can be used for the aforementioned thermo-mechanical writing method by applying a pulse  350  to the first limb  300 , as well as for the disclosed reading method by applying a burst  360  from the signal generator  210  to the third limb  320 . 
     The variable inductance is given by the ratio of the flux of the magnetic field through the plane defined by the coil  320  and the current through the coil  320 , i.e. L=Φ L /i L . The cantilever  20  is subject to a deflection as the tip moves over a pit  110 . The flux of the magnetic field through the plane defined by the coil  320  thus varies as the tip moves over an pit  110 . Thus, the value of the inductance of the cantilever  20  varies. Assuming, for the purpose of explanation, that the cantilever  20  acts as a coil  320  with a diameter of 50 μm, and the ferromagnetic element  330  is formed from a material having a relative permeability of 100000, values of inductance of the order of 1 μH can be obtained. 
     In alternate embodiments of the present invention, the aforementioned variation in phase characteristic of the cantilever  20  may be achieved by arranging for the capacitance of the cantilever  20  to be variable, with the inductance remaining fixed. Note however, that the values of capacitance obtained by considering, for example, a limb of the cantilever  20  and the substrate  10  as two plates of a capacitor with air as dielectric material are much smaller than 1 pF. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the present invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.