Patent Publication Number: US-2007121477-A1

Title: Cantilever with control of vertical and lateral position of contact probe tip

Description:
CLAIM OF PRIORITY  
      This application claims benefit to the following U.S. Provisional Patent Application:  
      U.S. Provisional Patent Application No. 60/813,959 entitled CANTILEVER WITH CONTROL OF VERTICAL AND LATERAL POSITION OF A CONTACT PROBE TIP, by Nickolai Belov et al., filed Jun. 15, 2006, Attorney Docket No. NANO-01044US0.  
     CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
      This application incorporates by reference all of the following co-pending applications and the following issued patents:  
      U.S. patent application Ser. No. 11/177,550, entitled “Media for Writing Highly Resolved Domains” by Yevgeny Vasilievich Anoikin et al., Attorney Docket No. NANO-01032US1, filed Jul. 8, 2005;  
      U.S. patent application Ser. No. 11/177,639, entitled “Patterned Media for a High Density Data Storage Device” by Zhaohui Fan et al., Attorney Docket No. NANO-01033US0, filed Jul. 8, 2005;  
      U.S. patent application Ser. No. 11/177,062, entitled “Method for Forming Patterned Media for a High Density Data Storage Device,” by Zhaohui Fan et al., attorney Docket No. NANO-01033US1, filed Jul. 8, 2005;  
      U.S. patent application Ser. No. 11/177,599, entitled “High Density Data Storage Devices with Read/Write Probes with Hollow or Reinforced Tips,” by Nickolai Belov, Attorney Docket No. NANO-01034US0, filed Jul. 8, 2005;  
      U.S. patent application Ser. No. 11/177,731, entitled “Methods for Forming High Density Data Storage Devices with Read/Write Probes with Hollow or Reinforced Tips,” by Nickolai Belov, Attorney Docket No. NANO-01034US1, filed Jul. 8, 2005;  
      U.S. patent application Ser. No. 11/177,642, entitled “High Density Data Storage Devices with Polarity-Dependent Memory Switching Media,” by Donald E. Adams et al., Attorney Docket No. NANO-01035US0, filed Jul. 8, 2005;  
      U.S. patent application Ser. No. 11/178,060, entitled “Methods for Writing and Reading in a Polarity-Dependent Memory Switching Media,” by Donald E. Adams, Attorney Docket No. NANO-01035US1, filed Jul. 8, 2005;  
      U.S. patent application Ser. No. 11/178,061, entitled “High Density Data Storage Devices with a Lubricant Layer Comprised of a Field of Polymer Chains,” by Yevgeny Vasilievich Anoikin et al., Attorney Docket No. NANO-01036US0 filed Jul. 8, 2005;  
      U.S. patent application Ser. No. 11/004,153, entitled “Methods for Writing and Reading Highly Resolved Domains for High Density Data Storage,” by Thomas F. Rust et al., Attorney Docket No. NANO-01024US1, filed Dec. 3, 2004;  
      U.S. patent application Ser. No. 11/003,953, entitled “Systems for Writing and Reading Highly Resolved Domains for High Density Data Storage,” by Thomas F. Rust, et al., Attorney Docket No. NANO-01024US2, filed Dec. 3, 2004;  
      U.S. patent application Ser. No. 11/004,709, entitled “Methods for Erasing Bit Cells in a High Density Data Storage Device,” by Thomas F. Rust et al., Attorney Docket No. NANO-01031US0, filed Dec. 3, 2004;  
      U.S. patent application Ser. No. 11/003,541 entitled “High Density Data Storage Device Having Erasable Bit Cells,” by Thomas F. Rust et al., Attorney Docket No. NANO-01031US1, filed Dec. 3, 2004;  
      U.S. patent application Ser. No. 11/003,955, entitled “Methods for Erasing Bit Cells in a High Density Data Storage Device,” by Thomas F. Rust et al., Attorney Docket No. NANO-01031US2, filed Dec. 3, 2004;  
      U.S. patent application Ser. No. 10/684,661, entitled “Atomic Probes and Media for high Density Data Storage,” by Thomas F. Rust et al., Attorney Docket No. NANO-01014US1, filed Oct. 14, 2003;  
      U.S. patent application Ser. No. 11/321,136, entitled “Atomic Probes and Media for high Density Data Storage,” by Thomas F. Rust et al., Attorney Docket No. NANO-01014US2, filed Dec. 29, 2005;  
      U.S. patent application Ser. No. 10/684,760, entitled “Fault Tolerant Micro-Electro Mechanical Actuators,” by Thomas F. Rust, Attorney Docket No. NANO-01015US1, filed Oct. 14, 2003;  
      U.S. patent application Ser. No. 09/465,592, entitled “Molecular Memory Medium and Molecular memory Integrated Circuit,” by Joanne P. Culver et al., Attorney Docket No. NANO-01000US0, filed Dec. 17, 1999;  
      U.S. Pat. No. 6,985,377, entitled “Phase Change media for High Density Data Storage,” Attorney Docket No. NANO-01019US1, issued Jan. 10, 2006, to Thomas F. Rust, et al.;  
      U.S. Pat. No. 6,982,898, entitled “Molecular Memory Integrated Circuit Utilizing Non-Vibrating Cantilevers,” Attorney Docket No. NANO-01011US1, issued Jan. 3,2006, to Thomas F. Rust, et al.;  
      U.S. Pat. No. 5,453,970, entitled “Molecular Memory Medium and Molecular Memory Disk Drive for Storing Information Using a Tunnelling Probe,” issued Sep. 26, 1995 to Rust, et al. 
    
    
     COPYRIGHT NOTICE  
      A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to he facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.  
     TECHNICAL FIELD  
      This invention relates to high density data storage using molecular memory integrated circuits.  
     BACKGROUND  
      Software developers continue to develop steadily more data intensive products, such as evermore sophisticated, and graphic intensive applications and operating systems (OS). Each generation of application or OS always seems to earn the derisive label in computing circles of being “a memory hog.” Higher capacity data storage, both volatile and non-volatile, has been in persistent demand for storing code for such applications. Add to this need for capacity, the confluence of personal computing and consumer electronics in the form of personal MP3 players, such as the iPod, personal digital assistants (PDAs), sophisticated mobile phones, and laptop computers, which has placed a premium on compactness and reliability.  
      Nearly every personal computer and server in use today contains one or more hard disk drives for permanently storing frequently accessed data. Every mainframe and supercomputer is connected to hundreds of hard disk drives. Consumer electronic goods ranging from camcorders to TiVo® use hard disk drives. While hard disk drives store large amounts of data, they consume a great deal of power, require long access times, and require “spin-up” time on power-up. FLASH memory is a more readily accessible form of data storage and a solid-state solution to the lag time and high power consumption problems inherent in hard disk drives. Like hard disk drives, FLASH memory can store data in a non-volatile fashion, but the cost per megabyte is dramatically higher than the cost per megabyte of an equivalent amount of space on a hard disk drive, and is therefore sparingly used.  
      Phase change media are used in the data storage industry as an alternative to traditional recording devices such as magnetic recorders (tape recorders and hard disk drives) and solid state transistors (EEPROM and FLASH) CD-RW data storage discs and recording drives use phase change technology to enable write-erase capability on a compact disc-style media format. CD-RWs take advantage of changes in optical properties (e.g., reflectivity) when phase change material is heated to induce a phase change from a crystalline state to an amorphous state. A “bit” is read when the phase change material subsequently passes under a laser, the reflection of which is dependent on the optical properties of the material. Unfortunately, current technology is limited by the wavelength of the laser, and does not enable the very high densities required for use in today&#39;s high capacity portable electronics and tomorrow&#39;s next generation technology such as systems-on-a-chip and micro-electric mechanical systems (MEMS). consequently, there is a need for solutions which permit higher density data storage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Further details of the present invention are explained with the help of the attached drawings in which:  
       FIGS. 1A and 1B  illustrate displacement of a contact probe tip due to friction force at the interface with the media.  
       FIGS. 1C and 1D  illustrate displacement of contact probe tip having a smaller height relative to the contact probe tip of  FIGS. 1A and 1B , the displacement occurring due to friction force at the interface with the media.  
       FIGS. 2A-2C  illustrate an effect of thermal oxidation on a sharpness of the contact probe tip.  
       FIGS. 3A and 3B  are plan views of a straight bar shaped contact probe cantilever and a chevron shaped contact probe cantilever.  
       FIGS. 4A and 4B  are plan and cross-sectional views, respectively, of an embodiment of an electrostatic actuator with one stop for use with a cantilever having a contact probe tip in accordance with the present invention.  
       FIG. 4C  is a cross-sectional view of the cantilever of  FIGS. 4A and 4B  deflected by electrostatic actuation.  
       FIG. 5A  is plan view of an embodiment of an electrostatic actuator with two stops for use with a cantilever having a contact probe tip in accordance with the present invention.  
       FIGS. 5B and 5C  are cross-sectional views of the electrostatic actuator of  FIG. 5A .  
       FIG. 6A  is a plan view of a straight bar shaped contact probe cantilever.  
       FIG. 6B  is a cross-sectional view of the same cantilever in a cross-section along its longitudinal axis.  
       FIGS. 6C, 6D  and  6 E are cross-sectional views of a straight bar shaped contact probe cantilever in a cross-section perpendicular to its longitudinal axis.  
       FIGS. 7A, 7B  and  7 C are cross-sectional views of contact probe cantilever with vertical electrostatic actuator and stops before etching of sacrificial layers.  
       FIGS. 8A and 8B  are plan views of embodiments of cantilevers in accordance with the present invention.  
       FIGS. 9A and 9B  are plan and cross-sectional views, respectively, of an embodiment of an electrostatic actuator for controlling lateral position of a cantilever having a contact probe tip in accordance with the present invention.  
       FIG. 9C  is a cross-sectional view of a cantilever with AFM tip deflected horizontally in the longitudinal direction of the beam.  
       FIG. 9D  is a plan view of an electrostatic actuator utilizing comb-structure for controlling lateral position of a cantilever having a contact probe tip in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION  
      Probe storage devices enabling higher density data storage relative to current technology can include cantilevers with contact probe tips as components. 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. At least one of the plates can be moved with respect to the other plate in a lateral X-Y plane while maintaining satisfactory control of the Z-spacing between the plates. Motion of the plates with respect to each other allows scanning of the memory media by the contact probe tips and data transfer between the contact probe tips and the memory media.  
      In some probe storage devices, for example utilizing phase change materials in a stack of the memory media, both mechanical and electrical contact between the contact probe tips and the memory media enables data transfer. In order to write data to the memory media, it is necessary to pass current through the contact probe tips and the phase change material to generate heat sufficient to cause a phase-change in some portion of the phase change material (said portion also referred to herein as a memory cell). Electrical resistance of the memory media can vary depending on the parameters of the write pulse, and therefore can represent data. Reading data from the memory media requires a circuit with an output sensitive to the resistance of the memory cell. An example of one such circuit is a resistive divider. Both mechanical and electrical contact between the contact probe tip and the memory media may also enable data transfer where some other memory media is used, for example memory media employing polarity-dependent memory.  
      A data transfer rate of a contact probe tip is determined in part by the scanning speed of the contact probe tip, a distance between memory cells, and a number of bits stored in a memory cell. For example, if a scanning speed of a contact probe tip is 3.2 cm/s, the distance between neighboring memory cells is 32 nm, and each cell contains 2 bits, then a raw data rate per contact probe tip is 2 megabits per second. However, the effective data transfer rate can be lower because of two factors: (a) some percentage of the memory cells may be used for error correction, and to store navigation and/or other information that is not transferred to the user, and (b) although the movable plates move (relative to one another) with approximately constant speed through a central portion of the scan area of the memory media, motion may slow down, stop, and reverse in direction when reading data at the ends of the scan area (such portions of the scan area can be referred to as turnaround areas). If a contact probe tip performs read-write operations in the turnaround areas the data transfer rate in these areas is expected to be lower than the data transfer rate in the central portion of the scan area where contact probe tip moves with a relatively constant speed.  
      Data intensive applications (e.g., recording and/or playing video) can require data transfer rates as high as 10-20 megabytes per second. In order to achieve this range of data transfer rates, multiple contact probe tips can be employed to transfer data to and from the memory media. For example, if the effective data transfer rate per contact probe tip is 1.25 megabit per second and the required data transfer rate is 160 megabits per second (20 megabytes at 8 bits per byte), then at least 128 contact probe tips can be used simultaneously for data transfer.  
      The contact probe tips should be positioned over the same tracks during writing of data and reading of the written data to read data without errors. Factors such as temperature can cause shifting of a contact probe tip with respect to the data tracks on the memory media and with respect to other contact probe tips. Fine position control of the contract probe tips can compensate for shifting by enabling adjustment of the lateral position of the contact probe tips at least in cross-track direction. Position adjustment in the down-track direction is less applicable because drift can be effectively handled by data processing means as timing error.  
      Fabrication of Low-Height Contact Probe Tips  
      Random movement of a contact probe tip with respect to the data track due to friction force at the contact probe tip and memory media interface is a factor that may not be easily compensated for by fine position control. Several parameters can affect the random movement of the contact probe tip due to friction force, including the coefficient of friction between the tip and the memory media, the natural frequency of the cantilever, and the height of the contact probe tip.  FIGS. 1A-1D  illustrate the affect of the height of a contact probe tip  12 , 22  on random movement due to friction force. A contact probe tip  22  having a smaller height (as shown in  FIGS. 1C and 1D ) exhibits less positional displacement for a similar value of friction force as a contact probe tip  12  having a larger height.  FIG. 1A  shows a cantilever  11  with a “tall” contact probe tip  12  not loaded with a friction force.  FIG. 1B  shows the same contact probe tip  12  loaded with a friction force F f1  . The friction force creates a torque T proportional to the product of the contact probe tip height h dp1  (T=F fP h tip1 ). The torque T torque causes some twisting of the cantilever  11 . The angle of twisting α is proportional to the applied torque T. The resulting displacement δ tip1  of the contact probe tip  12  is proportional to the product of the angle of twisting α and the tip height h tip1  (δ tip1 ≈h tip1  α). The lateral displacement of the contact probe tip  12  is therefore proportional to a square of the contact probe tip height h tip1 (δ tip1 ≈F fr h 2   tip1 ).  
       FIG. 1C  shows a cantilever  21  with a “short” contact probe tip  22  not loaded by a friction force.  FIG. 1D  shows the same contact probe tip  22  loaded with the friction force F fr . The height h tip2  of the contact probe tip  22  is smaller than that of the contact probe tip  22  shown in  FIG. 1A , and the torque T created by the friction force F fr  and the twisting angle α of the cantilever  21  is smaller. The lateral displacement δ tip2  of the “short” contact probe tip  22  is smaller than the lateral displacement δ tip  of the “tall” contact probe tip  12 . The difference in lateral displacement is roughly proportional to the squared decrease of the contact probe tip height. Thus, decreasing the tip height can be desirable and can decrease random movement by decreasing lateral displacement of the contact probe tip due to friction force at a contact probe tip and memory media.  
      Short contact probe tips can be desirable in probe storage devices due to the smaller torque that the cantilever  21  is subjected to when scanning the surface of the memory media. Reducing the lateral movement of the contact probe tips  22  can improve control tip position by reducing tip displacement, thereby increasing the tracking precision of the device. Short contact probe tips can be fabricated through a series of standard semiconductor processes.  
      For example, in an embodiment, a contact probe tip having a desirably short height can be formed in a series of process steps. A thin silicon dioxide layer can be formed on a substrate. Preferably, thermal oxidation is used to form the layer. A thermal silicon dioxide (also referred to herein as a thermal oxide) layer can be as thin or as thick as needed (500 A to 1 um for example). A thin silicon nitride film can be deposited over the thermal oxide. The thermal oxide can serve as an adhesion layer for silicon nitride. For example, low pressure chemical vapor deposition (LPCVD) silicon nitride or plasma enhanced chemical vapor deposition (PECVD) silicon nitride can be preferred to withstand high process temperatures. The silicon nitride film is a masking layer for later processing steps. A thickness of the silicon nitride film is determined so as to act as a barrier during subsequent thermal oxidation step(s) and so as to protect the underlying silicon substrate from etching during the dry silicon etch. For example, typically LPCVD nitride film can be chosen in the range of 500 A to 3500 A. Both the silicon dioxide and silicon nitride layers are sacrificial in the tip forming process, but they can also be incorporated into the probe storage device.  
      Photolithography can define areas where contact probe tips will be formed. A tip area can consist of a small square, polygon or circle area protected by a dielectric stack of silicon nitride and silicon dioxide surrounded by an open area. Linear dimensions of the small tip area protected by many typical photolithographic processes can range from 0.2 μm to 5 μm. Silicon nitride and silicon dioxide are both selectively etched away in the open areas, leaving silicon exposed. Etching of silicon nitride and thermal oxide layers is followed by a dry silicon etching step. Dry anisotropic etching of both dielectric layers and silicon provides preferred control for etching small features. Etching of silicon undercuts the edges of tip areas. The resulting structure is mushroom-like, with a silicon leg  34  and a dielectric stack  33  as a cap as shown in  FIG. 2A . Thermal oxide  35 , 45  is then re-grown, as shown in  FIGS. 2B and 2C . During thermal oxidation, the silicon leg  34  of the mushroom structure is oxidized, forming a silicon tip  32 , 42  beneath the oxide. The thermal oxide  35 , 45  is preferably thick enough to pinch off the silicon near the dielectric stack  33  and disconnects the silicon leg  34  between the dielectric stack  33  and the silicon tip  32 , 42 . The dielectric stack  33  causes oxidation to occur from the sides, creating sharper tips  32 , 42 . A thickness of the thermal oxide affects tip shape. The thermal oxide  35 , 45  is then stripped using a wet etch process (e.g. buffered oxide etch (BOE)). The dielectric stack  33  is also removed during this step. The silicon nitride layer can be removed completely at this step using a wet process (e.g. etching in hot phosphoric acid). A final layer of thermal oxide can be grown if oxide tips are required. A metal coating can be deposited over the tip to make the tips conductive.  
      To achieve high resolution and lower random movements of a contact probe tip due to friction force (as described above), it can be desirable to form a silicon tip shape that is short and sharp. Embodiments of methods for forming a probe storage device in accordance with the present invention include controlling several factors during fabrication of contact probe tips. In an embodiment, tip height can be controlled by reducing the tip pattern size defined during photolithography. A pattern having smaller feature sizes will result in an smaller overall tip height, for a given etch process. Tip pattern size is constrained by the capability of the photolithographic tool and photolithographic process including pattern resolution and repeatability. Further, tip pattern shapes can affect tip height. At larger tip pattern sizes, for a given width dimension, tip height will be greatest with a shape having a larger area, such as a square pattern as compared with a polygon or circle, for example. As width dimension decreases the differences between, for example, a square, a polygon, and a circle become negligible due to decreased resolution at small feature sizes.  
      Tip height can also be affected by the thermal oxidation after the dry silicon etching step. As can be seen in  FIG. 2C , a thick oxide  45  can decrease tip height, but at the cost of increased tip radius or poor “sharpness.” Tips with large radius of curvature are considered “dull,” while tips with small radius of curvature are “sharp.” Thick oxides (typically thicker than 1 um) can be used to create short tips with large radius of curvature. Thin oxides (typically thinner than 1 um) can be used to create taller tips with small radius of curvature. After tips are formed, their height can be reduced using subsequent thin thermal oxidations (&lt;0.5 um) and oxide etching (wet). This is important because each set of oxidation and oxide etching steps reduces tip height while keeping the tip radius relatively constant. Final tip metallization can further influence tip sharpness. A thick metal coating can increase tip radius of curvature. It is better to form a sharp silicon tip during the process because subsequent processing (final oxidation and/or metallization) can be used to increase the tip radius to reach requirements for probe storage device. Tip height can be controlled by tip pattern size and subsequent oxidations.  
      Actuator for Control of Z-Position of Contact Probe Tips  
      In probe storage device architectures employing a large number of contact probe tips, it can be advantageous to use only a small portion of the contact probe tips for data transfer at any given moment of time. A reduced portion of “active” contact probe tips can significantly reduce a number of electrical interconnects needed for the probe storage device architecture. For example, a probe storage device with a target capacity of 16 gigabytes with 2 bits stored in each of the memory cells and a hypothetical 25% formatting overhead requires N=(16×1024×1024×1024×8)/2/(1−0.25)≈9.16·10 10  memory cells. If a cell size is 32 nm, the size of the area used to store this amount of data can be evaluated as approximately 93.2 mm 2 . If the plates have a ±75 μm range of motion relative to one another, approximately 4,170 read-write heads can access the surface of the memory media. However, only a smaller number of contact probe tips are actually used for data transfer (e.g. 128 contact probe tips for 20 megabytes per second data transfer rate).  
      Further, contact probe tips can wear due to friction at the interface between the contact probe tips and the memory media, and due to material transfer processes associated with electrical current flow. Wearing of the contact probe tips can be decreased by disengaging non-active contact probe tips from the surface of the memory media. Disengagement can also decrease the overall friction force between the contact probe tips and the memory media, and consequently can decrease positional errors associated with random movement caused by friction forces acting on the movable parts of the probe storage device. Control of z-positioning of the contact probe tips with respect to the memory media can enable both engaging and disengaging contact probe tips with the memory media.  
       FIG. 3A  illustrates a straight cantilever  101  for use in a probe storage device.  FIG. 3B  illustrates a chevron type, dual-leg cantilever  701  for use in a probe storage device. A contact probe tip  102  extends from near a free end of the cantilever  101 . The length, width, and thickness of a cantilever  101  can influence the bending stiffness of the cantilever  101  (i.e. the amount of normal-to-cantilever plane force applied at the free end of cantilever to cause a unit deflection). Where the contact probe tip  102  is located approximately near the end of the cantilever  101 , a normal force applied to the contact probe tip  102  will cause about substantially the same displacement as the normal force applied to the end of the cantilever  101 . Thus, the force applied to the end of cantilever  101  is referred to herein as a tip force. The stiffness of a cantilever  101  is proportional to its width and the cube of its thickness, as well as the Young&#39;s modulus of the material of which its composed. The stiffness of the cantilever  101  is further inversely proportional to the cube of its length.  
      A gap between the surface of a memory media and a platform from which a cantilever  101  extends can be closed due to bending of the cantilever  101  toward the memory media. Bending of the cantilever  101  is preferably large enough to urge the contact probe tip  102  against the memory media with a force sufficient for creating stable electrical contact. Sufficient force depends on multiple factors including physical properties (e.g. electrical conductivity, Young&#39;s modulus) of the materials used for forming the contact probe tip  102 , the radius of curvature of the contact probe tip  102 , surface properties (e.g., roughness, microstructure) of the contact probe tip, an overcoat material applied to the memory media surface and/or the surface of a structure having memory media, and physical properties of the materials forming the memory media stack. In some applications, the tip force at the interface of the contact probe tip  102  and memory media should be in the range of hundreds of nanoNewtons in order to establish a reliable electrical contact between the contact probe tip  102  and the memory media.  
      Z-actuators used for disengaging (or engaging) contact probe tips with the memory media should be capable of generating forces that exceed the force urging the contact probe tip against the memory media (or away from the memory media). Several actuation techniques can be applied for control of the z-position of the cantilevers. In an embodiment of a device in accordance with the present invention, a cantilever can include z-position control by thermal actuation. In such an embodiment, a cantilever can be formed of a stack of materials having different thermal expansion coefficients. One or more of the layers of the stack of materials is conductive or semi-conductive. If layers nearer the surface of the cantilever from which the contact probe tip extends have a higher thermal expansion coefficient than layers generally farther from the contact probe tip, then heating the multi-layer cantilever can cause bending of the cantilever so that the contact probe tip is disengaged from the media stack. This design of thermal actuator for control of vertical position of the cantilevers and contact probe tips can require that initially the cantilevers be bent toward the memory media and pressed against the surface of the memory media with a force for establishing electrical contact. In an alternative embodiment, the cantilevers can be disengaged from the media stack when not actuated. If layers nearer the surface of the cantilever from which the contact probe tip extends have a lower thermal expansion coefficient than layers generally farther from the contact probe tip then heating the multi-layer cantilever can cause bending of the cantilever so that the contact probe tip engages the memory media.  
      In still another embodiment of a device in accordance with the present invention, a cantilever can include z-position control by electrostatic actuation.  FIG. 4A  is a plan view and  FIGS. 4B and 4C  are cross-sectional views of an exemplary structure of a cantilever  101  having a contact probe tip  102  extending from the cantilever  101 , and an electrostatic actuator for z-position control. The cantilever  101  with contact probe tip  102  and the electrostatic actuator are formed on a silicon substrate  107  covered by a field dielectric layer  104 . The electrostatic actuator is formed by the conductive cantilever  101 , which serves as a first electrode, and a metal layer  103 , which serves as a second electrode (also referred to herein as an actuator electrode) of the electrostatic actuator. Electrostatic force is generated by applying voltage between the cantilever  101  and the actuator electrode  103 . Electrodes  101 , 103  of the electrostatic actuator are separated by an air-gap  109  and by a dielectric layer  105 . To ensure current flow at the interface of the contact probe tip  102  and the memory media, during actuation it is possible to change the electrical potential of the actuator electrode  103  with respect to the cantilever  101  without changing the electrical potential of the cantilever  101 . In order to prevent sticking between the cantilever  101  and the actuator electrode  103 , at least one stop  106  is formed beneath the cantilever  101 . A height of the stop  106  is, preferably, smaller than the depth of the air-gap  109  between the cantilever  101  and the actuator electrode  103  provided by the isolation dielectric  105 . The stop  106  can be formed using the same isolation dielectric deposited directly on the field dielectric layer  104 . The air gap  109  is formed by etching of a sacrificial layer. Different materials can be used to form a sacrificial layer. For example, metal, poly-silicon and dielectric layers as PECVD oxide and LPCVD nitride and combination of these materials can serve as a sacrificial layer.  
      Fabrication of the contact probe tip  102  located at the end of the cantilever  101  can be accomplished using process steps described in the above section incorporated into a process flow suitable for fabrication of a structure as shown in  FIGS. 3A-3C  or a structure as shown in  FIGS. 4A-4C . When formed, the contact probe tip  112  is typically connected to the silicon substrate  107 . At least one etching step is used in order to release the contact probe tip  102 . A cavity  108  is formed under the tip  102  as a result of the at least one etching step. Contact probe tip release can be controlled by design of the etch mask, a type of etching agent, a recipe, etching time, and number of etching steps. A silicon structure  110  reinforcing the contact probe tip  102  can be retained at the end of an etching process. A size and shape of the reinforcing structure  110  can be controlled by the pattern used for etching (i.e., the etch mask), type of etching agent, recipe, etching time, and number of etching steps. For example, a contact probe tip  102  with a reinforcing structure  110  can be formed by a reactive ion etching (RIE) step followed by either anisotropic etching or isotropic etching. The RIE step enables profiles having substantially vertical sidewalls. A further etching step allows undercutting of the contact probe tip  102  and forms a reinforcing structure  100  under the contact probe tip  102 .  
       FIG. 5A  is plan view of another embodiment of an electrostatic actuator with two stops  306  for use with cantilever  301  having a contact probe tip  102  in accordance with the present invention.  FIG. 5B  is a cross-sectional view of the same structure parallel to the longitudinal axis of cantilever  301 .  FIG. 5C  is a cross-sectional view of the same structure perpendicular to the longitudinal axis of the cantilever  301  and to the stops  306 . As shown in  FIGS. 5A-5C , the actuator structure has two features: (a) the contact area between the cantilever  301  and the stops  306  is much smaller than surface area of the cantilever  301  and (b) the depth of the gap  319  between the cantilever  301  and the stops  306  is smaller than depth of the gap  309  between the cantilever  301  and the actuator electrode  303  located under the cantilever  301 . These features allow; (a) protection of the cantilever  301  from mechanical and electrical contact with the actuator electrode  303  and (b) protection of the structure from stiction. Mechanical and electrical contact between the cantilever  301  and the actuator electrode  303  is undesirable because it can cause both short electrical connection between electrodes  301 , 303  in the electrostatic actuator and sticking of the cantilever  301  to the actuator electrode  303 . Where a contact area between the cantilever  301  and the stops  306  is small, restoring force due to built-in stress in the cantilever  301  can be enough to overcome attraction forces acting at the interface between the cantilever  301  and the stops  306  when they are in a mechanical contact.  
      If a metal cantilever  301  is deposited on top of a sacrificial layer, which has the same thickness over the stops  306  as over the actuator electrode  303 , then after release the cantilever  301  will have travel distance to stops  306  approximately the same travel distance to the actuator electrode  303 . As a result, stops  306  will not prevent undesirable contact between the cantilever  301  and the actuator electrode  303 . Therefore, it is desirable to increase the thickness of the sacrificial layer between the cantilever  301  and the actuator electrode  303  bigger than thickness of a sacrificial layer between the cantilever  301  and the stops  306 .  
      The stops  306  are shown in  FIG. 5A-5C  as structures having a top surface above the actuator electrode  303 . Alternatively, the stops  306  can have a top surface at the same level, above or below the plane of actuator electrode  303 . The thickness of a sacrificial layer between the cantilever  301  and the stops  306  should be smaller than the thickness of a sacrificial layer between the cantilever  301  and the actuator electrode  303 .  
      Several options can be used in order to make thickness of sacrificial layer on top of the stops  306  smaller than thickness of sacrificial layer on top of the actuator electrode  303 . The first option is related to using two different stacks of sacrificial materials.  FIG. 7A  illustrates a stack of materials formed in the process of fabrication of cantilevers  301  with contact probe tips (not shown). One stack of sacrificial materials  321  is formed between the cantilever  301  and the stops  306  and the stops  306  and another stack of sacrificial materials  322  is formed between the cantilever  301  and the actuator electrode  303 . Thickness of stack of sacrificial materials  321  between the cantilever  301  and stops  306  is smaller than thickness of stack of sacrificial materials  322  between the cantilever  301  and actuator electrode  303 . After cantilever release, when a voltage drop is applied between the cantilever  301  and bottom actuator electrode  303 , the cantilever  301  is attracted to the actuator electrode  303  and deflects toward it. Distance between the cantilever and the stops  306  is smaller than the distance between the cantilever  301  and the actuator electrode  303 . Therefore, cantilever  301  will be stopped by stops  306  in its motion toward the actuator electrode  303  and will not contact the actuator electrode  303 . For example, sacrificial layer on top of stops  306  can be formed using a thin thermal oxide protected by a layer of LPCVD nitride while sacrificial layer between the cantilever  301  and the actuator electrode  303  can be formed using PECVD oxide. Thickness of the PECVD oxide layer can be bigger than at least thickness of the thermal oxide layer grown on top of stops  306 . Preferably, thickness of the PECVD oxide layer is bigger than combined thickness of the LPCVD nitride layer and the thermal oxide layer deposited on top of stops  306 . This method requires removing PECVD oxide from the top surface of the stops  306  before cantilever material deposition.  
      Another example of different sacrificial layers deposited on top of stops  306  and on top of actuator electrode  303  is illustrated in  FIG. 7B . A stack of sacrificial layers  421  is deposited both on top of stops  306  and on top of actuator electrode  303 . Stack of sacrificial layers  421  contains at least one sacrificial layer. At least one more sacrificial layer  422  is deposited on top of the actuator electrode  303 . Etching of sacrificial layers  421  and  422  creates a structure, which has a gap between cantilever  301  and stops  306  smaller than the gap between the cantilever  301  and the actuator electrode  303 . For example, structure shown in  FIG. 7B  can be formed by using a layer  421  of PECVD oxide both on top of stops  306  and on top of actuator electrode  303  and, in addition, a sacrificial metal layer  422  can be deposited on top of actuator electrode. Aluminum, titanium, tungsten and other metals can be used as a sacrificial metal. Thickness of the sacrificial metal determines the difference in the depth of the air gap between the cantilever  301  and stops  306  and depth of the air gap between cantilever  301  and actuator electrode  303 . Thickness of the PECVD oxide layer can be, preferably, in the range of 200 nm to 2000 nm. Thickness of the sacrificial metal layer can be, preferably, in the range of 10 nm to 1000 nm.  
      An alternative embodiment of stops to prevent stiction between cantilever and actuation electrode is shown in  FIG. 7C .  FIG. 7C  is a cross-sectional view of a cantilever  501 , actuation electrode  303  and stops  506  prior to removal of sacrificial layers  521  and  522 . Each of sacrificial layers  521  and  522  can be represented by only one layer or multiple layers. The sacrificial layer  521  is deposited on top of actuator electrode  303 . The stack of sacrificial layers  521  contains at least one sacrificial layer. At least one more sacrificial layer  522  is deposited on top of the actuator electrode  303  and on top of the stops  506 . The stops  506  can be on the same level as the actuation electrode  303 , below the actuation electrode  303 , or above.  
      The difference between  FIG. 7A ,  FIG. 7B , and  FIG. 7C  is that the part of the cantilever  501  that comes into contact with the stops  506  is underneath the cantilever  501 . During processing, for  FIG. 7C , the sacrificial layers  521  (for example, PECVD oxide) between the cantilever and actuation electrode is etched in such a way as to create “holes” in the area where stops  506  are located, which will be filled in by the cantilever metal  501  creating “bumps”. Another sacrificial layer  522  is deposited before the cantilever metal  501 , as a release layer to isolate cantilever  501  from both actuation electrode  303  and stops  506 . The thickness of sacrificial layer  521  determines the air gap between cantilever  501  and actuation electrode  303 . In all examples stiction can be further reduced by electrically isolating the stops  506  from the actuation electrode  303 .  
      Another process option, which allows providing different gaps and between cantilever and stops and between cantilever and actuator electrode, is related to using a combination of geometrical shape of the stops and deposition processes that results in different thickness of sacrificial layer deposited on top of the stops and on top of actuator electrode. For example, if stops have a shape of narrow ridges (as it is shown in  FIG. 5A-5C ), a spin-on material can be used as a sacrificial layer and this layer can be deposited on wafers by spinning. In that case thickness of the spin-on material on top of stops  306  is expected to be smaller than its thickness on top of actuator electrode  303 . Cantilever material can be deposited on top of this sacrificial layer. After etching off the sacrificial layer, depth of the air gap  319  between cantilever  301  and stops  306  is expected to be smaller than depth of the air gap  309  between cantilever  301  and actuator electrode  303 .  
      After release, cantilevers are bent out of the surface of the wafer due to a built-in stress gradient as it is illustrated in  FIGS. 6A and 6B  for a rectangular cantilever  101  with a probe contact tip  102 . Besides that, cantilever may have bending in the plane perpendicular to its longitudinal axis. Depending on process parameters, shape of the released cantilever  101  in cross-sections perpendicular to its longitudinal axis can be different. Some possible shapes are shown in  FIGS. 6C,6D  and  6 E. In order to prevent contact between cantilever  101  and actuator electrode (not shown in  FIG. 6 ) stops  106  can be positioned under the area of the cantilever, (e.g. central part or periphery) that is closer to the actuator electrode due to bending of the cantilever  101  in cross-sections perpendicular to its longitudinal axis. If bending of cantilevers  101  in the cross-sections perpendicular to its longitudinal axis is relatively small then contact between cantilever and the actuator electrode may occur in different areas. Some cantilevers will be contacting the actuator electrode in the central area of the cross-section while some other cantilevers will make this contact in the peripheral areas. Designs using stops  106  located both under the central part and under periphery of cantilevers  101 , as shown in  FIG. 6E , can be preferable, because these designs protect the cantilever bean from the direct contact with the actuator electrode regardless of the curvature of the cantilever beam in cross-sections perpendicular to its longitudinal axis.  
      A force F el  provided by the electrostatic actuator formed by the electrodes  101 , 103  is directly proportional to the overlapping area A of the electrodes  101 , 103  and the squared actuation voltage V applied between the electrodes  101 , 103 , and inversely proportional to the squared gap d between the electrodes  101 , 103  (i.e. F el˜A·U   2 /d 2 ). The maximum voltage that can be used for actuation can be determined either by a voltage supplied to the probe storage device or by an output voltage of special circuits used to increase the maximum voltage available for actuation (e.g. voltage multiplication circuits). Voltage multiplication circuits are often used in devices utilizing low-voltage supply (e.g. handheld devices, batter-operated devices) in order to generate internally voltages, which are higher than the voltage supply. Operating electrostatic actuators at low voltages allows voltage multiplication circuits to be eliminated. The electrostatic force F el  is increased by decreasing the gap d between the cantilever  101  and the actuator electrode  103  and increasing the overlapping area A of the electrodes  101 , 103 . Referring to  FIGS. 8A and 8B , the overlap area A can be increased by increasing the width of the straight bar cantilever  801  of  FIG. 3A  or filling the hole between legs of the chevron cantilever  901  of  FIG. 3B . An increase in overlapping area A also makes the cantilevers  801 , 901  mechanically stronger. Increased tip force can cause faster wear of one or both of the contact probe tips and the memory media. It can therefore be desirable to compensate tip force increase by one or both of decreasing thickness of the cantilever and increasing cantilever length. Cantilever stiffness is proportional to a cube of its thickness and inversely proportional to a cube of its length. However, cantilever stiffness is a linear function of its width for the straight bar geometry. Therefore, an increase in the overlapping area A can be compensated by relatively small adjustments of cantilever length and thickness. This allows increasing the electrostatic force F el  without changing the bending stiffness of the cantilever and without changing the tip force, which electrostatic force F el  should overcome.  
      Actuator for Control of Lateral Position of Contact Probe Tips  
      An embodiment of an actuator for fine control of the lateral positions of contact probe tips in accordance with the present invention is shown in  FIGS. 9A-9C . Preferably, such an actuator can be used to adjust position of the contact probe tips, for example within 1 to 2 tracks. Assuming a pitch between tracks in the range of 30 nm to 50 nm, contact probe tip displacement provided by such an actuator could be in the range of 60 nm to 100 nm. In an embodiment, fine control of the lateral position of a contact probe tip can be used to compensate for shifts between contact probe tips, for example as caused by thermal drift, variation of the gap between plates of the probe storage device, and variation of cross-track deflection of the tips due to variations in cantilever stiffness and friction force at tip-media stack interface. In such embodiments, a control loop for adjusting the lateral position can be independent of servo control and can provide alignment of a group of tips by both initial alignment (i.e. calibration) and tracking environmental conditions. Alternatively, fine control of the lateral position of a contact probe tip can compensate for some other shift between contact probe tips, for example variation in distances between contact probe tips created during manufacturing. This shift also can be compensated for a group of tips during an initial alignment step.  
      Referring to FlGS.  9 A- 9 D), the actuator includes a flexible structure  205 , for example a beam suspended over a cavity  212  and connected to a substrate  207  in one or more areas. A cantilever  201  having a contact probe tip  202  extending from the distal end of the cantilever  201 , is connected with the flexible structure  205  at a proximal end of the cantilever  201 . The actuator applies lateral force to the flexible structure  205 , causing bending of the flexible structure  205  in the plane of the substrate  207  and corresponding lateral displacement of the tip  202 . Electrostatic actuation can be used to deflect the flexible structure  205  from a neutral position. In such an embodiment, an electrode  213  comprising a metal is formed on the flexible structure  205 . A second electrode  211  is disposed over the substrate  207 . Both electrodes  211 , 213  can extend along the length of the flexible structure  205 . When voltage is applied between the electrodes  211 , 213 , an electrostatic force attracts the electrodes  211 , 213  to each other to cause lateral bending of the flexible structure  205  and corresponding deflection of the contact probe tip  202 . Alternatively, electrostatic actuator with comb-shaped electrodes  611 , 613  shown in  FIG. 9D  can be used in order to increase electrostatic force and allow actuation at low voltage.  
      The cavity  212  under the flexible structure  205  can be formed by etching trenches  206  adjacent to the flexible structure  205  at first and then undercutting the flexible structure  205 . Openings  216  in the cantilever  201  can be implemented in order to simplify undercutting of the flexible structure under the proximal end of the cantilever  201 . Initial etching of the trenches can be done, for example, using reactive ion etching (RIE) process, which allows making profiles with almost vertical side walls. Undercutting of the flexible structure  205  and forming cavity  212  can be done using either anisotropic or isotropic etching. These process steps can be integrated with the discussed above micromachining steps for forming contact probe tips  202  with reinforcing structures (not shown in  FIGS. 9A-9D ).  
      In still other embodiments, different actuation methods can be employed for lateral actuation of the flexible structure  205 , including piezoelectric, electromagnetic, thermal, and electrostatic. For example, in an embodiment, where a piezoelectric actuator is used a piezoelectric material can be deposited on a side wall of the flexible structure  205 . Applying a voltage to the piezoelectric material can cause the flexible structure  205  to bend and the contact probe tip  202  to move laterally. Alternatively, where an electromagnetic actuator is used a magnetic field can be applied perpendicular to the substrate  207  while current flows along the flexible structure  205 . A Lorentz force acts on the flexible structure  205  in the plane of the substrate  207  in a direction perpendicular to the flexible structure  205 , causing the flexible structure  205  to bend resulting in lateral displacement of the contact probe tip  202 . Direction of the tip deflection can be changed by changing the direction of the current.  
      In still another embodiment, thermal actuation of the flexible structure  205  can result where current is passed through a conductor or semi-conductor disposed along the flexible structure  205  so that heating occurs, causing the flexible structure  205  to deflect and the contact probe tip  202  to be displaced laterally. In order to define the preferable direction of the flexible structure  205  deflection, the flexible structure  205  can be shaped as an arc. Thermal actuator can consume low power because very small overheating of the arc-shaped flexible structure  205  is enough for 100 nm deflection of the contact probe tip  202 . Thermal actuator provides unidirectional motion of the contact probe tip  202 .  
      The foregoing description of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.