Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2002-03-08
2003-08-26
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C250S306000
Reexamination Certificate
active
06611140
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to magnetic sensing of motion in a microfabricated device. More particularly the invention relates to a magnetic sensing unit for measuring displacements or flections on a nanometer scale.
BACKGROUND OF THE INVENTION
The miniaturization of mechanical components and devices provides new applications in various fields and allows insight in and the use of a new world, the so called nano-world. At the end of the 20th century the basis for the age of micro- and nanomechanics has been created. Batch fabrication based on today's chip manufacturing methods has been introduced which provides considerable potential for creating high-performance systems and devices at low cost. Applications in the field of mass data storage achieve much smaller storage devices and opens up the possibility of achieving storage densities in the range of hundreds of gigabits per square inch. In the field of microscopy, for example, the scanning tunneling microscope (STM) and the atomic force microscope (AFM) has been introduced successfully in recent years for atomic-scale surface analysis. Such microscopy techniques, in general called scanning probe microscopy (SPM), use a flexible cantilever of very small dimension whereby the cantilever is fabricated by micro machining techniques. The cantilever with a sharp tip is scanned across a surface of a sample and the displacement or the motion of the cantilever is detected in order to achieve an image having an atomic resolution. A variety of optical methods have been devised to detect the cantilever deflection. Typical forces between tip and sample range from 6 to 11 nN, and deflections as small as 0.001 nm can be detected. The three different ways of operation are contact mode, non-contact or dynamic force mode, and tapping mode which allow the detection of lateral, magnetic, electrostatic and Van der Waals forces. Also, such a cantilever can be used to write and read data.
Although the present invention is applicable in a variety of micro mechanical applications it will be described with the focus put on an application to cantilevers.
Today, several techniques are known to measure displacements or the motion of cantilevers used in scanning probe microscopy (SPM), for example, or other microfabricated devices.
The optical technique, also referred to as laser detection or optical beam deflection, uses either the reflection of a laser beam at the surface of a cantilever and therewith the change of the laser beam's angle during deflection or the interference effects between the incident and reflected beams. The deflection of the cantilever is monitored by reflecting the laser beam off the cantilever into a photodiode. During scanning, an image can be formed by mapping this laser-detected deflection.
With a piezoresistive technique the change of resistance of a piezoresistive path defined at the surface of a flexed arm of the cantilever can be measured. In the article “Atomic force microscopy using a piezoresitive cantilever”, by M. Tortonese et al., Proc. of the Int'l Conf. on Solid State Sensors and Actuators, San Francisco, Jun. 24-27, 1991, pp. 448-451, the fabrication of a silicon cantilever beam with an integrated piezoresistor for sensing its deflection is described. A silicon on insulator material was used for the fabrication.
From U.S. Pat. No. 5,345,815 a microminiature cantilever structure is known having a cantilever arm with a piezoresistive resistor embedded close to the fixed end of the cantilever arm. Deflection of the free end of the cantilever arm produces stress in the base of the cantilever. That stress changes the piezoresistive resistor's resistance at the base of the cantilever in proportion to the cantilever arm's deflection. A resistance measuring apparatus is coupled to the piezoresistive resistor to measure its resistance and to generate a signal corresponding to the cantilever arm's deflection.
U.S. Pat. No. 5,444,244 is related to a cantilever for a scanning probe microscope that includes a piezoresistor. A process of fabricating such a cantilever is further described, the process yielding a tip which has a high aspect ratio and a small radius of curvature at its apex. A combined atomic force/lateral force microscope including two or more piezoresistors responsive to both the bending and torsion of the cantilever is also disclosed.
However, piezoresistive cantilevers in spite of almost similar sensitivity as optical schemes, suffer from low frequency noise and temperature drift inherent to all semiconductor strain gauges. They require furthermore that the cantilevers be formed of single-crystal silicon.
IBM's U.S. Pat. No. 5,856,617 describes an atomic force microscope (AFM) that uses a spin valve magnetoresistive strain gauge integrated on the AFM cantilever to detect its deflection. The spin valve strain gauge operates in the absence of an applied magnetic field. The spin valve strain gauge on the AFM cantilever is made of a plurality of films, one of which is a free ferromagnetic layer that has nonzero magnetostriction and whose magnetic moment is free to rotate in the presence of an applied magnetic field. In the presence of an applied stress to the free ferromagnetic layer due to deflection of the cantilever, an angular displacement of the magnetic moment of the free ferromagnetic layer occurs, which results in a change in the electrical resistance of the spin valve strain gauge. An electrical resistance detection circuitry coupled to the spin valve strain gauge is used to determine cantilever deflection.
Document WO 96 03641 is related to a scanning probe microscope assembly that has an atomic force measurement (AFM) mode, a scanning tunneling measurement (STM) mode, a near-field spectronomy mode, a near-filed optical mode, and a hardness testing mode for examining an object.
The European patent application EP 0 397 416 A1 describes an apparatus for the high resolution imaging of macromolecules and interactions involving macromolecules. The apparatus comprises a surface on which the macromolecule under test is placed and a plurality of fine probes. Means are provided for scanning each of the probes across a small area of the surface in such a way that the total output from the probes covers the whole surface. Means such as a scanning tunneling and/or atomic force detector are used to monitor the movement of the individual probes in a direction transverse to the surface and display means are used to display the transverse movement of the probes, being illustrative of the topography of the surface.
An other European patent application EP 0 306 178 A2 is related to an acceleration sensor including a cantilever beam having a free end to which a permanent magnet is attached. A pair of magnetic sensors, each consisting of a barber-pole type magnetoresistive sensing element, are arranged opposite to and symmetrically with respect to the magnet. The cantilever is bent and the magnet is moved according to an acceleration, which is detected as outputs from the magnetoresistive sensing elements.
The German publication DE 41 03 589 A1 is related to a sensor device with a mechanical resonant oscillation element. The structure is similar to that of the acceleration sensor mentioned in the preceding paragraph with the little difference that only one sensor element is arranged in the prolongation of the beam.
U.S. Pat. No. 4,954,904 is related to an apparatus and method for controlling the flying height of a head over a rotating medium, such as used in a rigid disk drive employing magnetic, magneto-optic or optical recording techniques. The flying height is controlled via magnetic attraction or repulsion to maintain a selected and substantially uniform flying height of the head with respect to the rotating medium.
The optical technique and the piezoresistive technique are the most widely used techniques today. But other techniques, like capacitive, piezoelectric, or thermal techniques, can also be used instead of the optical or piezoresistive technique to
Bloechl Peter
Rossel Christophe
Willemin Michel
Gill William D.
Laveri Subhash
Lefkowitz Edward
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