Patent Publication Number: US-2005128927-A1

Title: Electrostatic actuator for contact probe storage device

Description:
BACKGROUND OF THE INVENTION  
      Contact probe storage technology provides a method for ultrahigh density storage at a high speed. Most contact probe storage devices utilize arrays of cantilever beams with heated tips. The tips of the probes are kept in contact with a polymer media with a load determined by the bending of a cantilever beam on which the probe is supported. When heated sufficiently the probe can write data by locally fusing and forming pits in the media. Reading is carried out electrically by sensing a change in the impedance between the probe tip and a conducting layer below the media, or thermally by the change in the heat transfer characteristics when the probe tip is in a pit. The media is placed on a platform that can be moved in the x and y directions with respect to the tip or probe by a precision micromover.  
      One problem with cantilever devices of this type is the force needed to overcome the frictional forces resulting from all the cantilevers in continuous contact with the media. In addition, keeping the probe tips in continuous contact with the media results in tip wear. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The various aspects and features of the exemplary embodiment of the invention will become more clearly appreciated as a description of thereof is given with reference to the following drawings.  
       FIG. 1  is a schematic plan view of an embodiment of the invention.  
       FIG. 2  is a side sectional view as taken along section line II-II of  FIG. 1 .  
       FIG. 3  is a side sectional view showing the sensing tip of the probe which is mounted on the actuator, resting against a portion of a medium in which there is no data indicative topographical feature (e.g. no recess, debit, or mound) at the coordinates at which the tip is located.  
       FIG. 4  is a side sectional view showing the sensing probe tip resting against a portion of a medium in which there is a data indicative topographical feature (e.g. a recess or debit) at the coordinates at which the probe tip is located and which reduces the amount by which the probe is deflected away from the medium.  
       FIG. 5  is a side sectional view showing the sensing arrangement withdrawn from the medium in response to the application of a voltage across the electrodes which form part of a capacitance sensing arrangement that detects the change in distance therebetween in response to the tip encountering a data indicative topographical feature.  
       FIG. 6  is a plan view of showing an example of the configuration in which the upper electrode and flexures/traces which are associated therewith, can be formed.  
       FIG. 7  is plan view of a lower electrode which is shaped so as to conform to the shadow of the upper electrode shown in  FIG. 6 .  
       FIGS. 8-23  are views depicting the steps which are carried out in connection with the fabrication of the exemplary embodiment. 
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS  
      The embodiments of the invention relate to an actuator for contact probe storage device which is configured to undergo electrostatic actuation, so the probe tip can be drawn away from a medium and allowed to contact the media only when needed. The ability to selectively disengage the probe tip from the media when not reading or writing data substantially reduces the force required from the mover. If the probe tips are disengaged from the media when not in use, wear is significantly reduced.  
      In addition to actuation, the parallel plate electrodes which enable the electrostatic attraction also allow capacitive sensing of data.  
      Although only one probe actuator is shown in the drawings it will be understood that a plurality of these arrangements can be placed in an array with a 40 μm (for example) pitch, with multiplexing used to address specific probes.  
       FIGS. 1 and 2  show an exemplary embodiment. In this arrangement an upper electrode  100  is supported in a spaced relationship over a fixed lower electrode  102 , by way of resilient flexures  104 - 110 . A heater generally denoted by the numeral  112  is supported on an upper side of the upper electrode  100  and electrically separated therefrom by a layer of non-conductive thermally insulative material  114 . The heater  112  comprises in part, a layer of metal  116  which in the embodiment takes the form of TaAl by way of example. The metal layer  116 , which is formed over the insulative layer  114 , is electrically connected with the traces  106  and  110  via portions of the layer  116  which extend through vias  117 . This establishes an electrical connection between the heater  112  and the flexures  106 ,  110 . Because all of the flexures are, in this embodiment, also formed of the same metal (TaAl) they can also function as traces via which electrical signals/current can be transmitted.  
      An aluminum layer  118  is formed over the TaAl layer  116  to complete the heater  112 . A recess is formed in the Al layer  118  to expose a portion of the TaAl layer  116 . A probe  120  is formed on the TaAl layer in the illustrated manner.  
      As shown in  FIG. 6 , the traces/flexures  106 ,  110  are separated from the upper electrode  100  by gaps  106 A and  110 A. With this arrangement the heater  112  can be supplied current via flexures/traces  106  and  110 , while the upper electrode can separately have a voltage applied via flexures/traces  104 ,  108 . The overall configuration of the upper electrode  100  and the flexures  104 - 110  can be likened to a “pinwheel” wherein opposed pairs of flexures extend at right angles to each of a pair of axes which pass through an imaginary center of rotation of the pinwheel and which axes intersect each other essentially at right angles.  
      The flexures  104  and  108  also provide part of a circuit whereby the upper electrode  100 , the lower electrode  102  and an air gap  122  therebetween, forms a capacitor which enables the change in distance between the upper and lower electrodes  100  and  102 , which is induced by the probe  120  engaging a data indicative topographical feature, to be sensed.  
      The areas of the upper and lower electrodes  100 ,  102  are selected to minimize the mass, while providing adequate area for the parallel plate capacitor arrangement just mentioned. The gap between the fixed lower electrode  102  and the actuating upper electrode  100  is small to maximize the capacitance, but large enough to provide adequate displacement without pulling down when a voltage is applied across the upper and lower electrodes  100 ,  102 .  
      As will be appreciated, although the upper and lower electrodes  100 ,  102  constitute an essential part of an actuator arrangement, they also constitute a part of a sensing arrangement which enables the change in distance to be detected during the periods the actuating voltage is not applied.  
      A spacer arrangement comprising spacers  124  are provided on the mounting portions  126  which support the free ends of the flexures  104 - 110 . These spacers  124  have dimensions which are selected allow the probe  120  to protrude, when the flexures  104 - 110  are fully relaxed, above the upper level of the spacers  124  to a degree that disposition of a medium  128  (supported on a substrate or die  129 ) on the spacers  124  (in the manner shown in  FIG. 3 ) deflects the probe platform (viz., the upper electrode  100 /heater  112 ) downwardly toward the lower fixed electrode  102  and causes the probe  120  to apply a predetermined load to the surface of medium  128 .  
      This embodiment provides a high resonant frequency for fast operation. The stiffness of the flexures  104 - 110  are low enough to allow adequate z-axis displacement of the probe platform  100 / 112  at a reasonable voltage (e.g. 16 volt), while the load on the media from the suspension restoring force provided by the flexures  104 - 110  is within allowable limits. A suitable load is, merely by way of example, is 100 nN. This is based on current CPS devices is merely an example and in no way limiting as to load which can be selectively exerted.  
     EXAMPLE  
      An optimized device exhibits parameters which are summarized in the table below. The load on the probe tip when writing to media is near the desired target of 100 nN if the gap is set so that the probe tip is deflected 200 nm from the relaxed position. The voltage needed to pull down the probe tip ⅓ of the 900 nm gap is 16 volts.  
      The nominal capacitance is 1.3 fF. The difference in capacitance 0.2 fF is produced assuming a media film of 150 nm. Although small, this capacitance is of a detectable magnitude.  
      The fundamental frequency of the device is 300 kHz. However, this design could be optimized for a higher frequency by increasing the stiffness of the flexures and enable a target of 1 Mhz for example, to be achieved. Nevertheless, this would involve a tradeoff wherein increased load on the media and/or increased voltage needed to withdraw the probe  120  from contact with the medium  128 .  
      Lowering the mass of the device could also increase the frequency. This, however, tends to lower capacitance and raise actuation voltage.  
                           TABLE                                   Parameter   Value                          Write Load   90 nN           Actuation Voltage   16 V           Capacitance   1.3 fF           Capacitance Delta   0.2 fF           Effective Mass   1.4 e-13 kg           Frequency (1 st  Mode)   300 kHz           Spring Constant   0.45 N/m                      
 
       FIGS. 8-23  show steps which can be used to fabricate the above described embodiment. It will be appreciated that these steps which are set forth below are merely exemplary and that variations/modifications are possible. The materials which are mentioned in connection with each of the layers can be varied as deemed appropriate.  
      In  FIG. 8 a  silicon wafer  130  is treated to produce a thermal oxide layer  132  having a thickness of about 500 nm. Next, as shown in  FIG. 9 , an aluminum (Al) layer is deposited on the oxide layer  132  and etched to form the lower fixed electrode  102 . In this embodiment, this electrode  102  has a shape similar to that shown in  FIG. 7 . Following this, a 1 μm of PECVD oxide is deposited and etched to set an electrode gap (viz., a gap between the upper and lower electrodes  100 ,  102 ) via the formation of mounting portions  126  in the manner illustrated in  FIG. 10 .  
      Next, as depicted in  FIG. 11 , a 2.0 μm 1st sacrificial polysilicon layer  140  is deposited and subsequently planarized using CMP (see  FIG. 12 ), to a level flush with the tops of the mounting portions  126 . Following this, a layer of TaAl is deposited over the surface of the polysilicon  140 ′ and the mounting portions  126  and etched ( FIG. 12 ) to form the flexures  104 - 110  and upper electrode  100  having configuration similar to that shown in  FIG. 6 . This being completed, the following sequence of operations, respectively depicted in  FIGS. 14-23 , is executed. 
           FIG. 14 —Depositing, masking and etching of a 200 nm of PECVD oxide to form the layer of non-conductive thermally insulative material  114  which insulates a resistor structure, that forms a part of the heater  112 , from the upper electrode  100 . This also forms pads of insulative material (designated by the same numeral  114 ) with the same thickness as that associated with the resistor structure, on top of the TaAl layer above the mounting portions  126 .      FIG. 15 —Etching of vias  117  to TaAl layer defining flexures/traces  106 ,  110  to form the resistor traces.      FIG. 16 —Depositing a metal stack  150  of 100 nm Al over 50 nm TaAl, masking and wet etching the Al layer to form a resistor which forms a functional part of the heater  112 .      FIG. 17 —Masking and etching the metal stack  150  to complete the resistor trace of heater  112 .      FIG. 18 —Depositing, masking and etching a 600 nm layer of Ta/Au to form spacers  124  and set the gap between media and probe tip.      FIG. 19 —Masking and etching PECVD oxide  114  to expose the upper surface of the upper electrode and traces  104 - 110 .      FIG. 20 —Depositing 1 um of a 2nd sacrificial polysilicon  160 .      FIG. 21 —Masking and etching opening  161  in the sacrificial polysilicon  160  to the level of the resistor of the heater  112 .      FIG. 22 —Depositing a 500 nm probe  120 .      FIG. 23 —Etching the sacrificial poly layers with SF6 or XeF2 to reveal the completed arrangement.        

      This results in the arrangement illustrated in  FIG. 2 . Accordingly, as shown in  FIG. 3  it is now possible to bond a 150 nm media  128  on a die or substrate  129 , and mount the same on a micromover (not shown) and move the die  129  into the illustrated position atop of the spacers  124 . As noted above, this induces an exemplary situation wherein the engagement between the medium  128  and the tip or probe  120  induces a deflection of about 200 nm from the relaxed position. This situation induces an exemplary load of about 90 N.  
      In the event that the surface of the medium  128  is not smooth or free of data indicative topographical features as in the case illustrated in  FIG. 3 , and the probe or tip  120  is able to engage a data indicative topographical feature such as shown in  FIG. 4 , the amount of probe deflection is reduced to about 50 μm with a reduced load of about 22 nN.  
      This change in deflection can be detected through the change in capacitance between the upper and lower electrodes  100 ,  102 . As shown in the above table, this change can be about 0.2 fF, which while being small is measurable and the change in deflection can be detected.  
      Although this invention has been described with reference to only a single embodiment, it will be understood that variants and modifications of the invention, which is limited only by the appended claims, will be readily envisaged by the person skilled in the art to which this invention pertains or most closely pertains, given the preceding disclosure. For example, as shown in  FIG. 6 , the length of the flexures are not fixed and can be elongated with respect to the area of the upper electrode for the purposes of modifying the resonance frequency etc. The shape of the electrodes is not fixed and can be circular or any other desired configuration.  
      In the above type of arrangement, it is additionally possible for the heater/probe arrangement to carry out both imaging and reading using a thermomechanical sensing concept. The heater  112  can be used for writing and thermal readback sensing by exploiting a temperature-dependent resistance function. For writing the heater can be elevated to a temperature of 500-700° C. (for example).  
      For sensing, the heater  112  can be operated at about 200° C. This temperature is not high enough to soften the polymer medium which can consist of one more polymer layers including an upper layer of polycarbonate or polymethylmethacrylate (PMMA), but allows the molecular energy transfer between the structure on which the probe is carried, and the medium, to remove heat and thus provide a parameter which allows the presence/absence of a data indicative topographical feature to be detected.