Abstract:
A topography sensor and method include a probe configured to traverse a surface to determine a topography. A stray magnetic field is disposed in proximity to the probe. A magneto-resistive sensor is configured so that the stray magnetic field passing through it changes in accordance with positional changes of the probe as the probe tip traverses the surface.

Description:
RELATED APPLICATION INFORMATION 
       [0001]    The present application is related to U.S. application Ser. No. (TBD), Attorney Docket Number CH920080085US1 (163-273), entitled “MAGNETO-RESISTANCE BASED NANO-SCALE POSITION SENSOR”, filed currently herewith and incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to topography mapping of surface forces, and more particularly to a system, device and method for mapping a surface using magneto-resistance. 
         [0004]    2. Description of the Related Art 
         [0005]    Typically, the optical detection technique is used for topography sensing at the nanometer scale, because of its high bandwidth and sub-nanometer resolution capability. However, such setups are expensive and bulky. The optical detection concept is not suitable for batch fabrication of large arrays of integrated sensors, which are required for parallel operation to provide high throughput in nano-scale probing systems, with applications in lithography, storage, and nano-fabrication. 
         [0006]    Other sensing concepts, e.g., thermo-electric and piezo-resistive, provide a solution to this problem, as they can be fabricated in integrated form. However, all such known concepts suffer from either low bandwidth (thermo-electric) or low resolution (piezo-resistive) or both of the problems. Therefore, a new solution is needed that combines the advantages of optical sensing with the miniaturization and integration capability of the latter techniques. 
       SUMMARY 
       [0007]    A topography sensor and method include a probe configured to traverse a surface to determine a topography. Nanometer scale spatial and temporal resolution is achieved by utilizing a nanometer scale sharp probe tip and a probe deflection detection sensor having nanometer scale resolution. A magnetic field is disposed in proximity to the probe by using a permanent magnet, magnetic layer or a current-loop, etc. A magneto-resistive sensor is configured so that the stray magnetic field passing through the sensor changes in accordance with positional changes of the probe as the probe tip traverses the surface. 
         [0008]    A method for topography sensing includes providing a stray magnetic field disposed in proximity to a probe; traversing a surface with the probe; and sensing changes in the stray magnetic field in accordance with positional changes of the probe as the probe traverses the surface using a magneto-resistive sensor. 
         [0009]    These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]    The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein: 
           [0011]      FIGS. 1A-B  are cross-sectional views showing a magnet and a sensor sensing topographical changes on a medium in accordance with the present principles; 
           [0012]      FIGS. 2A-B  are cross-sectional views showing a magnet and a sensor on opposite sides of a medium; 
           [0013]      FIGS. 3A-B  are cross-sectional views showing a magnet and a sensor spaced apart on a same side of a medium; and 
           [0014]      FIGS. 4A-B  are cross-sectional views showing a frame structure having a magnet and a sensor integrated therein. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]    The present principles provide new techniques for topography sensing (including surface forces such as van der Waals, electric and magnetic forces, etc.) with high bandwidth and high resolution. The present principles provide the capability to include integrated sensors. The sensors can be fabricated, e.g., using existing micro-electro-mechanical-systems (MEMS)-based fabrication processes. Particularly useful embodiments offer the combined potential for high bandwidth, well in excess of 1 MHz, and high resolution at the sub-nanometer level. 
         [0016]    One application of high bandwidth and high resolution integrated sensors lies in probe-based high density data storage, especially in applications where high data-rate is paramount, as, for example, in archival storage. In particular, the field of archival storage is currently dominated by magnetic tape, which is expected to extend, in current form, for another 5-10 years. Around that time, tape heads will take the form of dense arrays of miniature heads, bundled together to read magnetically sub-micrometer sized information carriers (domains) in a storage medium of a particular structure. As such, ways to sense some medium property (magnetic flux, topography, etc.) with magnetic means, such as the one in accordance with the present principles, gain significant importance for the scaling of established technologies in the area of archival storage. 
         [0017]    Embodiments of the present invention can take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment including both hardware and software elements. In a preferred embodiment, the present invention is implemented in hardware with software support. The software may include but is not limited to firmware, resident software, microcode, etc. 
         [0018]    Furthermore, aspects of the invention can take the form of a computer program product accessible from a computer-usable or computer-readable medium providing program code for use by or in connection with a computer or any instruction execution system. For the purposes of this description, a computer-usable or computer readable medium can be any apparatus that may include, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device) or a propagation medium. Examples of a computer-readable medium include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Current examples of optical disks include compact disk-read only memory (CD-ROM), compact disk-read/write (CD-R/W) and DVD. 
         [0019]    A data processing system suitable for storing and/or executing program code may include at least one processor coupled directly or indirectly to memory elements through a system bus. The memory elements can include local memory employed during actual execution of the program code, bulk storage, and cache memories which provide temporary storage of at least some program code to reduce the number of times code is retrieved from bulk storage during execution. Input/output or I/O devices (including but not limited to keyboards, displays, pointing devices, etc.) may be coupled to the system either directly or through intervening I/O controllers. 
         [0020]    Network adapters may also be coupled to the system to enable the data processing system to become coupled to other data processing systems or remote printers or storage devices through intervening private or public networks. Modems, cable modem and Ethernet cards are just a few of the currently available types of network adapters. 
         [0021]    The sensors as described herein may include of be part of the design for an integrated circuit chip. The chip design may be created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer transmits the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., Graphic Data System II (GDSII)) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
         [0022]    The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
         [0023]    Referring now to the drawings in which like numerals represent the same or similar elements and initially to  FIGS. 1A and 1B , a sensing device  100  is illustratively shown. Sensing device  100  is employed to measure non-magnetic surfaces although magnetic surfaces may be measured as well with an appropriate configuration of the magneto resistive sensor, probe and the micro-magnet. Sensing device  100  is based on magneto-resistive (MR) sensing which is used for measuring stray magnetic fields. MR sensor  100  provides both high bandwidth (e.g., greater than 1 MHz) and high resolution (e.g., less than 1 nm). 
         [0024]    Device  100  may include a plurality of different configurations where a magnetic component  102  and magneto resistive (MR) sensor  104  are employed, but may have their locations transposed or their locations may be altered or integrated into different components of the designs. For illustrative purposes, one preferred embodiment is depicted in  FIGS. 1A and 1B . 
         [0025]    It should be understood that magnetic component  102  may include a magnetic field or any device that generates a magnetic field. For example, magnetic component (hereinafter magnet for simplicity)  102  may include a permanent magnet, a magnetic layer, a current loop or coil, an inductor, etc. 
         [0026]    Magneto-resisitive (MR) sensing may employ any sensor  104  belonging to the MR sensing family, e.g., Anisotropic Magneto-Resistive (AMR), Giant Magneto-Resistive (GMR), Tunnel Magneto-Resistive (TMR), etc.). These MR sensors may be employed to probe a stray field  112  of a magnet in different architectures for topography sensing. When a tip  106  of a microcantilever  108  traverses over nanometer-scale features of a sample surface or medium  110 , there is a change in a magnetic field B (indicated by stray-field vector, V x ) passing through the MR sensor  104  due the deflection of the microcantilever  108  with respect to the stray magnetic field  112  from the magnet  102 . The magnetic field V x  changes between  FIG. 1A  and  FIG. 1B  as the microcantilever  108  is deflected. The MR sensor  104  senses resistance changes (ΔR) versus the magnetic field (B and/or V x ). The sensitivity ΔR/R&gt;10 −2 /nm. 
         [0027]    Topography variations induce a modulation of the magnetic field V x , which is sensed by the MR sensor  104 . Either the magnet  102  or the MR sensor  104  may be placed at the end of the microcantilever  108 . The other of the magnet  102  or the MR sensor  104  is fixed on the cantilever or to a nearby structure. To optimize the arrangement, the magnetic field strength (V x ) and/or the orientation of the MR sensor  104  are selected so as to provide a highest sensitivity. As the tip  106  traverses the topography of medium  110 , the magnetic field strength through the MR sensor changes, and the topography signal is generated. 
         [0028]    With different designs of the microcantilever  108  and topological configurations of the MR sensor  104  and magnet  102 , a high bandwidth and high resolution topography imaging signal can be obtained. For parallel operation of arrays of microcantilevers  108 , one possible embodiment of the present sensing scheme would be to integrate micromagnets and MR sensors on the same microcantilever structure. 
         [0029]    A type of magneto resistive sensor, i.e., giant magneto resistive (GMR) sensor is a stack including anti-Ferro magnetically pinned layers and soft magnetic free layers having conductive nonmagnetic inter-layers. In a high resistance state, i.e., in the absence of an external magnetic field the magnetic moments in the two magnetic layers are opposite to each other due to ferromagnetic coupling. In the presence of an external magnetic field, the magnetic moment of the magnetically free layer aligns itself in the direction of the external magnetic field by overcoming the anti-Ferro magnetic coupling. Due to interfacial spin-polarized scattering between the ferromagnetic layers separated by conductive layers, the electrical resistance of the sensor changes. The resistance varies as a cosine function of the angle between the magnetic moments of the pinned layer and the free layer. The thinner the layers, the higher the resistance change is. The maximum resistance change of a GMR sensor is typically between 10% and 20% and can be as high as 110% at room temperature. 
         [0030]    When the probe tip traverses the surface, the stray magnetic field through the magneto resistive sensor changes. The magnetic moment of the soft layer aligns itself along the component of the stray magnetic field in its plane and the resistance of the sensor changes. Typically, a constant current is passed through the sensor and voltage output from the sensor is used as the imaging signal. 
         [0031]    The sensitivity and resolution of topography sensing scales proportionally to the sensitivity and resolution of the magneto resistive sensor. The sensitivity of the present methods is improved by tuning the spatial distribution of the stray magnetic field through the magneto resistive sensor to exploit the full range of operation of the MR sensor. 
         [0032]    The spatial distribution of the stray magnetic field is not linear for a large range of operation (in the micrometer range). However, for all practical purposes, the imaging signal can be assumed a linear function of the topography of surfaces. For large motion of the probe, the mapping between the imaging signal and the topography is a static nonlinear map which can be used to interpret the imaging signal. This method senses the movement of the probe tip similar to the optical deflection method. Various image processing techniques such as those employed on the optical deflection signal may be utilized to interpret the imaging signal from the magneto resistive sensor. 
         [0033]    The sensitivity/resolution may be optimized by trial and error, by computation, by design, by experience or combinations thereof. Magneto resistive sensors usually operate at low field strengths starting from zero Oersted to a few hundreds of an Oersted. At higher field strengths, the soft magnetic layers get saturated and the sensor loses sensitivity. The stray magnetic field at the MR sensor in its sensitive direction can be oriented by carefully choosing the shape, size and material of a permanent magnet, and the relative position of the magnet with respect to the MR sensor. The stray magnetic field at the MR sensor can also be oriented by using combinations of more than one permanent magnet, magnetic layer and current loop. Magnetism simulation tools can be utilized to simulate various configurations and compute the stray magnetic field. 
         [0034]    The medium and the topography thereof advantageously may include non-magnetic materials. Referring to  FIGS. 2A and 2B , cross-sections of an illustrative MR topography sensing device  200  are shown.  FIG. 2A  shows an MR sensor mounted on the cantilever  201  while  FIG. 2B  shows a magnet  202  mounted on the cantilever  201 . Either the MR sensor  204  or the magnet  202  may be mounted on the cantilever  201 . A tip  206  is coupled to the cantilever  201  and contacts a surface of a sample or movable medium  210  to be topographically measured or mapped. The sample  210  may include a printed circuit board (PCB), an integrated circuit, a biological material, or any other non-magnetic material. The magnet  202  ( FIG. 2A ) or the sensor  204  ( FIG. 2B ) is fixed on an opposite side of the medium  210 , opposite the tip  206 . The other of the sensor  204  or the magnet  202  is fixed to the cantilever  201 . During operations, the medium  210  is moved relative to the tip  206  and the vertical tip movement is measured using the sensor  204 . FIGS.  2 A- 2 B may be employed in archival storage applications although other applications are also contemplated. 
         [0035]    Referring to  FIGS. 3A and 3B , cross-sections of an illustrative MR topography sensing device  300  are shown.  FIG. 3A  shows an MR sensor  204  mounted on a cantilever  201  while  FIG. 3B  shows a magnet  202  mounted on the cantilever  201 . The MR sensor  204  or the magnet  202  may be mounted on the cantilever  201 . A tip  206  is coupled to the cantilever  201  and contacts a surface of a sample or movable medium  210  to be topographically measured or mapped. The sample  210  may include a printed circuit board (PCB), an integrated circuit, a biological material, or any other non-magnetic material. The magnet  202  is spaced apart from the sensor  204  ( FIG. 3A ) on a same side of the medium  210 . In  FIG. 3B , the sensor  204  is spaced apart from the magnet  202 . During operations, the medium  210  is moved relative to the tip  206  and the vertical tip movement is measured using the sensor  204 .  FIGS. 3A-3B  may be employed in mobile storage applications although other applications are also contemplated. 
         [0036]    Referring to  FIGS. 4A and 4B , cross-sections of another illustrative MR topography sensing device  400  are shown. Device  400  includes a rigid beam  407  and a cantilever which has two legs  401  connected by a beam  405 . The device  400  includes a gap or free space  403  between the magnet  402  and the sensor  404 . The cantilever including its two legs  401  and connecting beam  405  function as a spring element (similar to cantilever  201 ). A tip  406  is coupled to the beam  405  and contacts a surface of a sample or movable medium  210  to be topographically measured or mapped. Deflections of the tip  406 , in response to the movement of the medium  210 , cause fluctuations in the magnetic field as before. 
         [0037]    In  FIG. 4A , the magnet  402  is attached to the support arms  405  and held across the gap  403  from the sensor  404  within a frame formed by legs  401  and beam  407 . The magnet  402  and the magneto resistive sensor element  404  are integrated on the same cantilever structure to make the topography sensor. During operations, the medium  210  is moved relative to the tip  406  and the vertical tip movement is measured using the sensor  404  because it moves with respect to the magnet  402  along with the tip  406 . 
         [0038]    In  FIG. 4B , the sensor  404  is attached to the support arms  405  and the magnet  402  is held across the gap  403  from the sensor  404  within the frame formed by legs  401  and beam  407 . The magnet  402  and the sensor  404  within the frame move together on the same side of the medium  210 . During operations, the medium  210  is moved relative to the tip  406  and the vertical tip movement is measured using the sensor  404  because the magnet  402  moves with respect to the sensor  404  along with the tip  406 . 
         [0039]    The structures and methods of the present invention offer at least the following substantial advantages over the known solutions in topography sensing. 1) Very high bandwidth: At low-cost and miniature form-factors, in contrast to the bulky and expensive optical setups. 2) Very high resolution: The resolution of the magnetic sensing scheme can theoretically match the sub-nanometer resolution of optical sensing, by appropriate placement of sensor and magnet and miniaturization of both. 3) Potential for MEMS fabrication at low-cost: as opposed to optical sensing which requires bulky and expensive components 
         [0040]    The present magneto-resistive topography sensing techniques combine all the above-mentioned desirable features, e.g., a small form-factor and integrated fabrication of thermo-electric or piezo-resistive sensing, as well as the superb bandwidth/resolution performance of optical sensing. Advantageously, MR sensors are employed herein for measuring topography of a non-magnetic surface. 
         [0041]    Having described preferred embodiments of a system and method for magneto-resistance based topography sensing (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.