Abstract:
A storage device includes a storage medium and a probe having plural tips. The storage medium has a surface in which storage cells are to be formed. The tips of the probe form plural perturbations in the surface in at least one of the storage cells for representing a data bit.

Description:
BACKGROUND  
       [0001]     In computing systems, such as desktop computers, portable computers, personal digital assistants (PDAs), servers, and others, storage devices are used to store data and program instructions. One type of storage device is a disk-based device, such as magnetic disk drives (e.g., floppy disk drives or hard disk drives) and optical disk drives (e.g., CD or DVD drives). Such disk-based storage devices have a rotating storage medium with a relatively large storage capacity. However, disk-based storage devices offer relatively slow read-write speeds when compared to operating speeds of other components of a computing system, such as microprocessors and other semiconductor devices.  
         [0002]     Another type of storage device is a solid state memory device, such as a dynamic random access memory (DRAM), static random access memory (SRAM), flash memory, and electrically erasable and programmable read-only memory (EEPROM). Although solid state memory devices offer relatively high read-write speeds, usually on the order of nanoseconds, they have relatively limited storage capacities.  
         [0003]     With improvements in nanotechnology (technology involving microscopic moving parts), other types of storage devices are being developed. One such storage device is based on atomic force microscopy (AFM), in which one or more microscopic scanning probes are used to read and write to a storage medium. Typically, a scanning probe has a tip that is contacted to a surface of the storage medium. Storage of data in the storage medium is based on perturbations created by the tip of the probe in the surface of the storage medium. In one implementation, a perturbation is a dent in the storage medium surface, with a dent representing a logical “1,” and the lack of a dent representing a logical “0.” Other types of perturbations that can be created in the surface of the storage medium include creating or altering the topographic features or composition of the storage medium, altering the crystalline phase of the medium, filling or emptying existing electronic states of the medium, creating or altering domain structures or polarization states in the medium, creating or altering chemical bonds in the medium, employing the tunneling effects to move and remove atoms or charge to or from the medium, or storing/removing charge from a particular region.  
         [0004]     One of the issues associated with a probe-based storage device is the reliability of each storage bit. Because the perturbations created to store data bits is based on some alteration of a characteristic in the surface of the storage medium, reliability of successfully generating the perturbations or detecting such perturbations can pose a challenge. For example, when a tip scans over a portion of the storage medium in which a perturbation has been created in the storage medium, the tip may miss the presence of the perturbation. As a result, a read error may occur, which reduces reliability of storage device operation. The push to create these types of devices on the nano-scale and to increase their density makes noise and reliability issues even more challenging. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  illustrates a portion of a probe-based storage device that includes a storage substrate defining a storage medium, along with a probe having multiple tips to create redundant perturbations in the storage medium.  
         [0006]      FIG. 2  is a schematic diagram of a probe substrate containing an array of probes and peripheral circuitry to interact with such probes.  
         [0007]      FIG. 3  illustrates the probe substrate positioned to face the storage substrate in the probe-based storage device of  FIG. 1 .  
         [0008]      FIG. 4  illustrates the tips of a probe in contact with a surface of the storage medium.  
         [0009]      FIG. 5  illustrates creation of redundant perturbations in the surface of the storage medium with the tips of the probe.  
         [0010]      FIG. 6  illustrates the reading of the perturbations created by the probe.  
         [0011]      FIG. 7  illustrates several probes formed in the probe substrate.  
         [0012]      FIG. 8  is a block diagram of a system that includes a computing device having a port to connect to a probe-based storage device.  
     
    
     DETAILED DESCRIPTION  
       [0013]      FIG. 1  shows an example probe-based storage device that includes a storage substrate  10  that provides a storage medium. The storage medium has a storage surface  20  on which perturbations can be formed by multiple tips  14  and  16  of a probe  12 . According to some embodiments, the probe  12  is a very small probe (on the order of micrometers, nanometers, or even smaller) that is built using nanotechnology techniques.  
         [0014]     The tips  14  and  16  of the probe  12  are attached to and extend outwardly from a cantilever  13  of the probe  12 . Note that in some embodiments, multiple probes (such as an array of probes), each with multiple tips, are provided in the probe-based storage device. In other embodiments, a single probe with multiple tips can be provided in the probe-based storage device. As discussed further below, the probe  12  is formed from a probe substrate that is positioned in a plane that is generally parallel to the storage substrate  10 . The probe tips protrude from the main surface of the probe substrate to enable the tips to contact the storage surface  20 .  
         [0015]     As shown in  FIG. 1 , perturbations  18  are formed in the surface  20  of the storage medium on the storage substrate  10 . In one embodiment, the perturbations  18  are dents, pits, indentations, or markings formed in the surface  20  of the storage medium. In this embodiment, the material providing the surface  20  of the storage medium is formed of a relatively soft material, such as polymer (e.g., PMMA or polymethylmethacrylate). In other embodiments, the material providing the storage surface  20  of the storage medium can be a liquid crystal, a phase change material, or any other suitable material. In one implementation, the layer made of the soft material can be formed over the rest of the storage substrate  10 , with the top layer defining the storage surface  20 . Alternatively, the entire substrate  10  can be formed of the soft material.  
         [0016]     To create the dents  18 , the tips  14  and  16  are locally heated to a predetermined temperature (e.g., up to about 400° C.) for some amount of time. The heat from the tips melts the storage surface  20  at the contact points of the tips  14  and  16 . When a downward force is applied onto the probe  12 , tips  14  and  16  imprint the dents  18 . The applied downward force can be an incremental, applied downward force, or alternatively, a constant downward force due to the elastic nature of each cantilever. For example, the device is assembled such that the cantilevers are bent back a little and are always applying some pressure on the storage substrate.  
         [0017]     The presence of a dent represents a logical “1,” while the absence of a dent represents logical “0.” During write operations, use of the multiple tips  14  and  16  causes two redundant dents  18  to be created for each given storage cell  19 . In the example of  FIG. 1 , four storage cells  19  are illustrated, with each storage cell  19  including a pair of redundant dents  18 . Note that if dents are not formed in a given storage cell  19 , then that represents a logical “0.” Alternatively, if two dents are formed in a given storage cell, then the cell represents a logical  
         [0018]     In other embodiments, to provide even greater redundancy, a probe with more than two tips can be used for generating more than two perturbations in each storage cell. The redundant dents  18  (or other type of perturbation) in each storage cell  19  are spaced apart by a spacing defined by the distance between the probe tips  14  and  16 .  
         [0019]     Once dents are formed, they can be erased by also using the tips  14  and  16  of the probe  12 . During erase, the tips  14  and  16  engage the dents  18 , with the tips being heated locally to melt the material surrounding the dents  18  such that the material flows into the dents to remove the dents.  
         [0020]     Heating of the tips  14  and  16  can be achieved in one of several ways. For example, an electrical pulse can be sent along a conductor through the cantilever  13  to the tips  14  and  16 , which causes the tips  14  and  16  to be heated to the desired temperature. The heating can be achieved by local heating elements such as resistors (which heat up in response to current passing through the resistors). Alternatively, laser beams or other heat sources can be used to perform heating.  
         [0021]     Instead of redundant dents formed in a storage cell  19  by the tips  14  and  16  of the probe  12 , other types of redundant perturbations can be created in each storage cell  19 . Perturbations can include, but are not limited to, the following: creating or altering the composition of the storage medium; altering the crystalline phase of the medium; filling or emptying existing electronic states of the medium; creating or altering domain structures or polarization states in the medium; creating or altering chemical bonds in the medium; employing tunneling effects to move and remove atoms or charge to or from the medium; or storing/removing charge from a particular region.  
         [0022]      FIG. 2  illustrates the probe substrate  50 , which includes an array of probes  12  formed in the substrate  50 . Peripheral circuitry  52  and  54  are provided on the peripheral sides of the probe substrate  50 . For example, peripheral circuitry  52  and  54  can drive X and Y select lines to select bits of the storage array to read from or write to. A row of probes  12  may be activated by the select lines to read from or write to storage cells that the probes are in contact with. This structure enables concurrent access of multiple cells in one operation, which improves access speeds. Alternatively, one of the probes may be activated to read from or write to a storage cell. The peripheral circuitry  52  and  54  also include sensing devices and decoders to detect analog signals from the probes and to convert the analog signals to a digital representation of a logical “0” or a logical “1.” 
         [0023]     As shown in  FIGS. 1 and 3 , the probe substrate  50  is placed with the surface containing the probes  12  facing the storage surface  20  of the storage substrate  10 , on which the storage cells are formed. The probe substrate  50  is positioned over the storage substrate  10  so that the probe tips  14  and  16  ( FIG. 1 ) of each probe point downwardly to engage the storage surface  20  of the storage substrate  10 . In an alternative arrangement, the storage substrate  10  is positioned over the probe substrate  50  so that the probe tips  14  and  16  point upwardly to face the storage surface  20 . In other arrangements, the probe substrate  50  and the storage substrate  10  can be positioned laterally or diagonally.  
         [0024]     The storage substrate  10 , in the example of  FIG. 3 , is coupled to an actuator  100  that is designed to move the storage substrate  10  in both X and Y directions such that probes  12  ( FIG. 1 ) can be placed over desired storage cells on the storage substrate  10 . Data sensed by the probes  12  is provided to buffers  102 , which store output data for retrieval by an external device. The buffers  102  may also store write data to be written to storage cells  19  ( FIG. 1 ) in the storage substrate  10 .  
         [0025]     Alternatively, the actuator  100  is operatively coupled to move the probe substrate  50 , or to move both the probe substrate  50  and the storage substrate  10 . The actuator  100  is also able to move the probe substrate  50  and/or the storage substrate  10  in the Z direction, which is generally perpendicular to the X and Y directions.  
         [0026]      FIG. 4  is a side view of the tips  14  and  16  of the probe  12  in contact with the storage surface  20  of the storage substrate  10 . This position enables the probe  12  to write to a storage cell  19 . As shown in  FIG. 5 , heating of the tips  14  and  16  and downward pressure applied onto the cantilever  13  of the probe  12  causes dents  18  to be formed in the storage surface  20 .  
         [0027]     To read from the storage cell  19 , the cantilever  13  of the probe  12  is actuated to a slanted position (shown in  FIG. 6 ), such that the cantilever  13  is at a slanted angle (not parallel) with respect to the storage surface  20 . At the slanted angle, one tip ( 14 ) is in contact with the storage surface  20  of the storage substrate  10 . In another implementation, during read operations, both tips  14  and  16  can be in contact with the storage surface  20 . The probe  12  is scanned such that the probe  14  is dragged across the storage surface  20 . The tip  14  is dragged across both the redundant dents  18  that are part of one storage cell  19 . The redundant dents increase the likelihood that the tip  14  will accurately detect presence of at least one of the dents  18  in the storage cell. Therefore, reliability is enhanced and the number of read errors resulting from mis-detection of a dent  18  is reduced.  
         [0028]     As the probe tip is dragged across the storage surface  20 , the probe tip will deflect into the dent as the tip crosses a dent. Detection of either of the dents in the storage cell  19  is an indication of a logical “1.” In one implementation, during a read operation, the probe tip is heated to a temperature that is lower than the write temperature. When the heated probe tip encounters a dent, the probe tip transfers heat to the material of the storage surface  20  and electrical resistance falls. This reduction in electrical resistance is detected by peripheral circuitry  52  and  54  ( FIG. 2 ).  
         [0029]     In an alternative implementation, detection of the engagement of the probe tip with the dent is based on measurement of the deflection of the cantilever  13  in response to the probe tip engaging the dent. The detection of the cantilever deflection is performed by a piezoresistive resistor that has a resistance that varies with deflection of the cantilever  13 . The piezoresistive resistor can be provided at the fixed base of the cantilever  13 . Other methods to detect deflection of the cantilever  13  can be used as well.  
         [0030]      FIG. 7  shows several probes  12  formed in respective cavities  56  of the probe substrate  50 . This embodiment is merely one example of how probes  12  can be formed in the probe substrate  50 . Note that, in other embodiments, other techniques for forming probes in the substrate  50  can be employed. Each probe  12  is coupled to a local pivoting actuator  58  so that the probe  12  is pivotably coupled to the probe substrate  50 . Alternatively, instead of a pivoting attachment, some type of bending mechanism can be used, such as by use of a piezoresistive element that takes advantage of the inherent flexibility of the cantilever. The pivoting actuator  58  is adapted to pivot the probe  12  with respect to the storage medium surface  20 . In one position, the pivoting actuator  58  maintains the cantilever  13  of the probe  12  substantially parallel to the storage medium surface  20  (for performing write or erase operations as shown in  FIGS. 4 and 5 ). In a second position, the pivoting actuator pivots the cantilever  13  such that the cantilever  13  is at a slanted angled with respect to the storage medium surface  20  (as shown in  FIG. 6 ).  
         [0031]     In another embodiment, instead of using a local pivoting actuator  58  for each probe, one pivoting actuator can be used for an entire row of probes  12 . The pivoting actuator is a microelectromechanical system (MEMS) actuator, which is based on nanotechnology. Very small structures, on the order of nanometers, are formed in the probe substrate  50  to provide the moving parts that make up the pivoting actuator  58 . The pivoting actuator  58  can be responsive to input electrical signals. For example, if the input electrical signal is at a first state, the actuator provides a first position of the cantilever  13 ; on the other hand, if the input signal is at a second state, the actuator  58  provides a second, different position of the cantilever  13 .  
         [0032]     The probe-based storage device can be packaged for use in a computing system. For example, as shown in  FIG. 8 , a probe-based storage device  200  that incorporates the multi-tip probe(s)  12  as discussed above is attached or connected to an I/O (input/output) port  202  of a computing device  204 . The I/O port  202  can be a USB port, a parallel port, or any other type of I/O port. Inside the computing device  204 , the I/O port  202  is connected to an I/O interface  206 , which in turn is coupled to a bus  208 . The bus  208  is coupled to a processor  210  and memory  212 , as well as to mass storage  214 . Other components may be included in the computing device  204 . The arrangement of the computing device  204  is provided as an example, and is not intended to limit the scope of the invention. In alternative embodiments, instead of being coupled to an I/O port of the computing system, the probe-based storage device can be mounted onto the main circuit board of the computing system.  
         [0033]     In the foregoing description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details. While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.