WO2000073821A1

WO2000073821A1 - System and method of multi-mode cantilever and multi-mode torsional micro-oscillators for force microscopy

Info

Publication number: WO2000073821A1
Application number: PCT/US2000/014774
Authority: WO
Inventors: John T. Markert
Original assignee: Xidex Corporation
Priority date: 1999-05-28
Filing date: 2000-05-26
Publication date: 2000-12-07
Also published as: WO2000073821A9; AU5447900A

Abstract

The present invention provides a multi-mode torsional oscillator for use in a force sensing microscopy tool, In one embodiment of the present invention a sample is mounted on a multi-mode torsional oscillator. A magnetic field generator polarizes the sample and couples a moment of the sample to the multi-mode torsional oscillator. A sensing system detects the motion of the multi-mode torsional oscillator in response to forces exerted on the sample and provides an output signal. A processor receives the output signal and processes the output signal to reproduce a data representative of the moments contained within the sample.

Description

SYSTEM AND METHOD OF MULTI-MODE CANTILEVER

AND MULTI-MODE TORSIONAL MICRO-OSCILLATORS

FOR FORCE MICROSCOPY

RELATED APPLICATIONS

This application claims priority to United States provisional patent application Serial No. 60/136,423 filed May 28, 1999, entitled "Multi-mode Torsional Oscillator Detection for Magnetic Resonance Force Microscopy," which is incorporated herein by reference,

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to the field of force sensing microscopy and, more particularly, to a force sensing microcopy tool that achieves enhanced sensitivity through the use of a multi-mode torsional micro-oscillator.

BACKGROUND OF THE INVENTION

Experiments have been previously conducted that report the force detection of nuclear magnetism. Such an experiment is shown in FIGURE 1, wherein the nuclear spins 10 in a sample 12 mounted on a mechanical oscillator 14 are polarized in a large, static magnetic field 16. The Z component of the nuclear magnetism is cyclically inverted at the mechanical oscillator resonant frequency by a modulated radio frequency field 18 from a nearby coil 20. This time-dependent nuclear moment is coupled to the field gradient 19 of a nearby permanent magnet 21 producing a resonant force on mechanical cantilever 14. Cantilever 14' s motion can be detected with a fiber optic interferometer 22. However, prior art systems, such as the one described in FIGURE 1 lack the sensitivity required to measure a single nuclear spin. Therefore, a force sensing system is needed that is capable of detecting and measuring individual nuclear moments. Such a system may utilize a multi-mode torsional oscillator to achieve this increased sensitivity.

SUMMARY OF THE INVENTION

The present invention provides a multi-mode torsional oscillator for use in a force sensing microscopy tool. In one embodiment of the present invention a sample is mounted on a multi-mode torsional oscillator. A magnetic field generator polarizes the sample and couples a nuclear moment of the sample to the multi-mode torsional oscillator. A position sensing system detects the motion of the multi-mode torsional oscillator in response to forces exerted on the sample and provides an output signal. A processor receives the output signal and processes the output signal to produce a data representative of the moments contained within the sample.

Micro-oscillators of various designs have become useful tools in many applications which requiring increased sensitivity. In particular, applications that measure small forces benefit from both low damping factors and low spring constants. These two requirements have been achieved by creating single- crystal silicon multi-mode torsional oscillators as illustrated in FIGURE 2 using semiconductor fabrication techniques . The present invention increases the sensitivity of a new type of force microscopy tool. Specifically, double-torsional oscillators have been used to enhance the signal-to-noise ratio in magnetic resonance force microscopy. Typically, force microscopy tools (atomic force microscopy tools, magnetic force microscopy tools, and others of the like) employ a cantilever oscillator (a small vibrating rod) as their sensing mechanism.

One application of such a microscopy tool is to detect nuclear or electronic spin magnetisms resonantly. The present invention uses multi-mode torsional oscillators for such applications requiring high sensitivity, and integrated them into a magnetic resonance force microscopy tool. The system and method of the present invention is scaled down to achieve unprecedented sensitivity in order to detect single nuclear spins or moments.

The present invention improves the sensitivity of force measures and provides a facile way of performing magnetic resonance force microscopy. The main limitation to existing prior art devices is that prior art devices have force sensitivities which are too poor for desired application. For example, no prior art known to the inventors has achieved the goal of detecting individual electronic or nuclear magnetic moments. This is because of the combined limits on three physical parameters of the cantilever oscillators used in conventional force microscopy: (1) the spring constant, (2) the resonant frequency, and (3) the quality factor (or "Q") of the oscillator. These parameters and the operating temperature determine an oscillator's sensitivity. A double or multi-mode torsional oscillator can be designed with sufficiently high frequency, low spring constant, and high quality factor that its force sensitivity far exceeds that of a simple cantilever as used in prior art applications.

The quality factor of an oscillator is limited for practical purposes by losses. These losses are due to the motion of dislocations in the solid and by coupling losses in the base or mount of an oscillator. A single-crystal oscillator minimizes dislocations and their associated motions and in so doing the losses are primarily determined by coupling. Driving the multi-mode torsional oscillator in an anti-symmetric mode can minimize coupling losses. In an antisymmetric mode a head of the oscillator moves much more than the base. Therefore, most of the energy of the oscillator is stored within head and is effectively isolated from coupling losses. For example, a typical cantilever oscillator may have a Q of 100 to 1,000, where a double-torsional oscillator may typically have a Q of at least 100,000 and 1,000,000. Such a double- torsional oscillator has been incorporated into a magnetic resonance force microscopy tool and demonstrated its practical use for the force detection of nuclear magnetism.

One key advantage of the increased sensitivity for magnetic resonance force microscopy is subsurface sensitivity. Most prior art conventional scanning probe imaging systems detect only the surface layer of the material of interest via scanning probe techniques.

The current uses of the double-torsional mode oscillator with its greatly enhanced sensitivity are in magnetic resonance force microscopy (MRFM) . This includes the three-dimensional imaging of various materials and biological samples. Since protons have one of the strongest nuclear spin signals, any sample containing hydrogen is especially suited for MRFM. However, other nuclei can be similarly detected. Thus, imaging of processes in individual biological cells has been made possible. Also, the imaging of solid-state materials such as polymers is possible. For example, composite materials utilizing polymers may be quickly and efficiently inspected using existing scanning techniques coupled with magnetic resonance force microscopy.

Multi-mode torsional oscillators allow single-spin sensitivity to which other applications include the imaging of single molecules and the individual detection of quantum spin states. The former would have immense value for the development of tailored drugs and vaccines, while the later may help realize quantum computing. A disadvantage associated with the present invention is the extremely high sensitivity. For example, a high-Q system has a slow response to a changing signal. This slows down data acquisition rates. However, there are standard techniques to overcome such problems, including anharmonic modulation and non-linear feedback methods, which overcome the high Q slow response without sacrificing signal-to- noise . More importantly, a methodology can be developed as part of the present invention for developing multi- mode torsional oscillators having geometry' s in moments of the oscillator which achieve a desired ratio of the moments of inertia of the wing and the head of the double-torsional oscillator. In this manner, a multi- mode torsional oscillator may be formed wherein the moment of a first portion such as the head is decoupled in the moment of inertia of at least one second portion of the multi-mode torsional oscillator such as the wing.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and the advantages thereof may be acquired by referring to the following description, taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein:

FIGURE 1 illustrates prior art setup for force sensing microscopy tool; FIGURE 2 provides an example of a double torsional oscillator of the present invention;

FIGURE 3 illustrates the use of a multi-mode torsional oscillator within a force sensing microscopy tool;

FIGURE 4 shows a resonance curve of a double- torsional oscillator;

FIGURE 5 is a photograph from a scanning electron micrograph of double-torsional oscillators; FIGURE 6 depicts an experimental probe used by the present invention; and

FIGURE 7 provides an illustration of a double torsional oscillator;

FIGURE 8 provides a plot of the lock-in output as a function of time;

FIGURE 9 is a flow chart depicting a method for fabricating multi-mode torsional oscillators of the present invention;

FIGURES 10 depict one possible fabrication process flow;

FIGURES 11 present a 2-D multi-mode torsional oscillator having a triple torsional design;

FIGURE 12 provides a template of a photo-mask outlining various multi-mode torsional oscillators; FIGURE 13 depicts a schematic of a fiber optic sensing system with a feedback array;

FIGURE 14 shows a small magnetic particle coupled to an oscillator of the present invention;

FIGURE 15 is a resonant sweep of an oscillator; FIGURE 16 shows the test results from a typical frequency sweep of a multi-mode torsional oscillator;

FIGURES 17A-17F illustrate the first six natural modes of a simulated multi-mode torsional oscillator; FIGURE 18 compares a prior art cantilever's Q to that of the present invention;

FIGURE 19 illustrates a realistically simulated oscillator;

FIGURE 20 depicts values used to model the oscillator of one embodiment of the present invention; and

FIGURE 21 demonstrates the validity of such modeling techniques as applied to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention are illustrated in the FIGURES, like numerals being used to refer to like and corresponding parts of various drawings.

Micro-oscillators of various designs have become useful tools in many applications that require high sensitivity. In particular, applications that measure small forces benefit from high-quality factor and low spring constant. These two requirements have been achieved by creating single-crystal silicon multi-mode torsional oscillators using micro fabrication techniques .

The present invention increases the sensitivity of a force sensing microscopy tools. Specifically, double-torsional oscillators have been used to enhance the signal-to-noise ratio in magnetic resonance force microscopy (MRFM) . Typically, force microscopy tools (atomic force microscopy tools, magnetic force microscopy tools, and others of the like) employ a cantilever oscillator (a small vibrating rod) as their force sensing mechanism. One such cantilever is discussed extensively in U.S. patent application Serial No. 09/499,101 filed on February 4, 2000, entitled "System and Method of Multi-dimensional Force Sensing for Atomic Force Microscopy, " which is incorporated herein by reference.

One application of such a force sensing microscopy tool is to detect nuclear or electron spin magnetism resonantly. One embodiment of the present invention uses multi-mode torsional oscillators for applications requiring such high sensitivity. This can be accomplished by integrating those oscillators into a magnetic resonance force microscopy tool. Nuclear spin paramagnetism may be detected using the present invention with the improved force sensitivity as observed. The system and method of the present invention is scaled down to achieve unprecedented sensitivity in order to detect single nuclear spins. The present invention improves the sensitivity of force measures and provides a facile way of performing magnetic resonance force microscopy. The main limitation to available devices is that these prior art devices have force sensitivities, which are too poor for desired applications. For example, no prior art known to the inventors has achieved the goal of detecting individual electronic or nuclear magnetic moments. This is because of the combined limits on three physical parameters of the cantilever oscillators used in conventional force microscopy: (1) the spring constant, (2) the resonant frequency, and (3) the quality factor (or "Q") of the oscillator. These parameters and the operating temperature determines the minimum detectable force of an oscillator (i.e., the oscillator' s sensitivity) . A double or multi-mode torsional oscillator can be designed with sufficiently high frequency, low spring constant, and high quality factor that its force sensitivity far exceeds that of a simple cantilever as used in prior art applications. FIGURE 2 illustrates a simple example of the present invention taking the embodiment of a single- crystal, double-torsional oscillator 24 with a small moment of inertia on top (the head 26) , a greater moment of inertia in the center (the wings 28), and a base 30 for mounting.

The quality factor of an oscillator is limited for practical purposes by losses. These losses are usually due to motion of dislocations in the solid and by coupling losses in the base or mount of an oscillator. For oscillator 24 described previously, the single- crystal nature minimizes dislocation motion therefore losses are determined by coupling of the sensing portion of the oscillator 24 to base 30 through wing 28. Driving double-torsional oscillator 24 in an anti- symmetric mode minimizes the losses associated with the base coupling. In an anti-symmetric mode, head 26 moves much more than wing 28. Therefore, most of the energy of the oscillator is stored within head 26, thus effectively isolating head 26 from losses. For example, a typical prior art cantilever oscillator 14 of FIGURE 1, may have a Q between 100 to 1,000, where as a double-torsional oscillator 24 may have a Q between of at least 1,000, and more typically in the range of 100,000 and 1,000,000. Such a double- torsional oscillator has been incorporated into a magnetic resonance force microscopy tool and demonstrated its practical use for the force detection of nuclear magnetism.

One key advantage of the increased sensitivity for magnetic force microscopy is subsurface sensitivity. Most prior art conventional scanning probe imaging systems detect only the surface layer of the material of interest via scanning probe techniques.

Some practical applications of the double- torsional mode oscillator with its greatly enhanced sensitivity are in magnetic resonance force microscopy (MRFM) . This includes the three-dimensional imaging of various materials and biological samples. Since protons have one of the strongest nuclear spin signals, any sample containing hydrogen is especially suited for MRFM. However, other nuclei can be similarly detected. Thus, imaging of processes within individual biological cells has been made possible. The imaging of materials such as polymers is also possible. Additionally, composite materials utilizing polymers may be quickly and efficiently inspected using existing scanning techniques coupled with magnetic resonance force microscopy techniques.

Like conventional magnetic imaging, the force sensing microscopy tools of the present invention are sensitive not only to spin density, but also to dynamic processes. The motions and interactions of the nuclear spins affect their associated relaxation times, which are also detected. Multi-mode torsional oscillators allow single-spin sensitivity; applications of this, include the imaging of single molecules and the individual detection of quantum spin states. The former would have immense value for the development of tailored drugs and vaccines, while the later may help further realize quantum computing.

A disadvantage associated with the present invention is the extremely high-Q as related to a data acquisition rates. For example, a high-Q system has a slow response to a changing signal. This response slows data acquisition rates. However, standard techniques overcome such problems, including anharmonic modulation and non-linear feedback methods, which overcome the high Q slow response without sacrificing signal-to-noise . One embodiment of the multi-mode torsional oscillator detection system of the present invention is shown in FIGURE 3. Frequency modulated signal 32, carried on RF coil 34, induces cyclic inversions of nuclear moments or spins at the mechanical oscillator frequency. A permanent magnet 36 produces a magnetic field gradient dB/dz, which couples nuclear moment M to oscillator 38 through the force defined by Equation 1.

F = M dB/dz (EQN. i;

The force motion of oscillator 38 may be detected by conventional fiber optic interferometry system 40. FIGURE 4 shows a resonance curve of a double- torsional oscillator with a high Q of approximately 1,000,000.

FIGURE 5 is a photograph from a scanning electron micrograph of a double-torsional oscillator. Such double-torsional oscillators are on the order of 150 micrometers in length.

FIGURE 6 depicts an experimental probe used by the present invention. In FIGURE 6, experimental probe 70 is contained within a vacuum can (not shown) , and in the embodiment depicted in FIGURE 6 fits into a 60mm diameter variable temperature cryostat which itself has been inserted into the bore of an 8.2 T superconducting magnet. Rotary motion from outside the probe drives three mechanical translators 74, 76 and 78, one each for optical fiber course approach positioning 74, oscillator lateral movement 76, and permanent magnet positioning 78. A Piezo electric element provides finer adjustments. Piezo electric element swings fiber optics in and out of the plane of FIGURE 6. Stack piezo provides fiber optic 84 fine positioning and tube piezo 88 provides fine positioning for permanent magnet. Fiber optic interferometers known to those skilled in the art may be utilized in association with the present invention. In this embodiment, a 675-nanometer laser diode is coupled into one arm of a Gould directional coupler. Reflections from cleve and fiber and from nearby oscillator interfere with and are sensed by a PAR photo diode. A locked-in amplifier 100 detects an output of PAR photo diode. For the embodiment shown, the output current at photo diode is approximately 20 pA/A, where the photo diode noise level is less than 0.2 pA/VHz, thus providing a motion sensitivity of better than 0.01 A per VHz.

The present invention need not be limited to the interferometers setup and parameters ranges described in this embodiment. Rather, parameters and setups selected by those having skill in the art may be chosen. The described parameter ranges are for illustrative purposes only. The static field in this embodiment was varied as permanent magnet 90 approached a paraffin sample at the edge of oscillator 96 from a range of 8.5 to 8.55 T (8.2 T from the super-conducting magnet and 0.30-0.35 T from an iron wire permanent magnet 90) . Approximately a 20 G RF field was tuned at a 400 kHz off resonance swept to the NMR frequency of 363.5 mHz over a 0.5 millisecond period and then frequency modulated at the oscillator 10.3 kHz frequency with an amplitude of 50 kHz. The static field gradient at this sample was approximately 300 T/m.

For the above-discussed embodiment, a single- crystal, double-torsional oscillator as depicted in FIGURE 7 was used. Residual losses in single-crystal oscillators occur through coupling to base 101. Typical cantilever modes have a Q of approximately 1,000. For the double-torsional oscillator shown in FIGURE 7, and the anti-symmetric mode with a small moment of inertia at the top or head 102 most of the kinetic energy is stored within head 102 effectively isolating its Q. Thus, one is able to routinely obtain Qs of approximately 10⁵ at room temperature and 10⁶ at 77°C, and 10⁸ at 4.2°C. This is on the order of 2-5 orders of magnitude greater than typical prior art cantilevers having Qs of 10³.

For any mechanical oscillator, the theoretical sensitivity from thermal noise is described in Equation 2. F_mm = l4k_BTΔv/ Qω (Eqn. 2) where ω and k are the frequency and effective spring constants of the oscillator. Measured parameters indicate a F_mιn on the order of 3xl0^~13 N. This produces a factor of five smaller than the photo diode detection limit of 0.1 A -JJiz ■ Thus, the latter sets the noise level. For small oscillators, a predicted F_mιn = 2xl0^"18 N at 4.2°C. This will result in motional amplitude of 0.2 A, well above the photo diode limit. Thus, for the small oscillators, the noise becomes thermally limited. In the one-dimensional case, using a large oscillator, no signal is observed at the lock-in output when the spins in the sample are far from resonance. As the magnet approaches the sample, the sample reaches a point where a slice of the spins are resonant. Where thickness Z = B/(dbz/dz), B is approximately 20 Gauss as determined by the frequency modulation and the amplitude of the RF field. For the case where B/(dbz/dz) is approximately 300 T/M, Z of approximately 7 NM. Thus, for a 2 mm diameter sample, a volume of 0.02 mm³ was resonant, corresponding to a 6xl0¹⁷ spins for a moment via Currie's law of 6xl0^~14 J/T for tesla and thus a force of 2xl0^-11 N, well above the photo diode noise floor.

FIGURE 8 provides a plot of the lock-in output as a function of time for a 1.3 second RF pulse where the oscillators ring up to an amplitude of approximately 0.25 A before decaying back to the noise level of approximately 0.01 A, indicating a force approximately 3xl0^~ N. In the case of the small oscillator, the moment sensitivity is greatly enhanced due to the increased force sensitivity and the much larger field gradients available. For example, the maximum field gradient scales inversely with the diameter of the wire. For example, a 100 N diameter wire will produce a field gradient of about 10⁴ T/M, implying a resolvable moment of 10^~21 J/T, or about 10⁹ proton moments at 4.2°C. Such NMR sensitivity is entirely unprecedented.

Oscillator Fabrication

Large oscillators are typically fabricated using standard photo lithography and micro-machining techniques. These fabrication techniques are known to those skilled in the art of MEMS fabrication and semiconductor fabrication. Small multi-mode torsional oscillators of the present invention require extra sensitivity to defects. For example, the micro- oscillator depicted in FIGURE 5 was fabricated in the method depicted by the flow chart of FIGURE 9. In step 150, a substrate is masked. Typically, this mask patterns various size micro-oscillators typically on the order of 150 micrometers high.

The substrate is than implanted with boron ions in step 152 at 134 keV at a dose of at least 1.4xl0¹⁶ Cm² in order to provide an etch-stop for the final structures. This implant and dosage was chosen for a target depth of 4,000 A and an initial profile thickness of approximately 1,000 A. Other doses and energy may be determined through a DOE process or other technique as known by those skilled in the arts. In step 154, the implanted substrates are annealed. A typical anneal may be one at 1,000°C for 15 minutes to 4 hours. The anneal re-crystallizes lattice damage to the substrate and spreads the profile to any desired thickness up to 4,000 A.

Following the anneal, in step 156 the substrate is placed in anisotropic one word etch in order to fully undercut the oscillators from the substrate. A typical anisotropic silicon etch known to those skilled in the art may consist of a 48% H₂0, 32% ethanol, and 20% KOH for approximately two days. An impediment to removing the micro-oscillators from the substrate is in the drying process. Here surface tension actually tends to pull oscillators towards the remaining substrate. To overcome this impediment, a freeze drying technique was successfully developed and employed to eliminate this surface tension completely.

The most critical step in the processing is the etch. This process for the present invention had to be completely reversed from those commonly used in processing large oscillators. These prior art processes utilize a high-temperature, short duration etch. A design of experiments (DOE) to characterize the etch process examined the differential etch between pure silicon and boron dope silicon. This experiment found a slight dependence on the etch solution (KOH concentration and water ethanol mixture) . Also, the DOE showed that the temperature of the etch was a far more important parameter in determining an enhanced differential etching. Thus, the DOE yielded an optional process having low temperatures 0°C etch for a long period (several days) for a slow reduced defect etch process.

The process flow is depicted in FIGURE 10, where an oscillator pattern is placed onto a substrate. In this embodiment, silicon substrate 170 is coated with an oxide 172 and a resist layer 174. This resist layer is masked and exposed. Following resist 174 removal, boron implant 175 into the unmasked regions 176 of the substrate to produce a boron-implanted region within the substrate D. A HF etch, 178, follows to remove masking oxide layer 172. There are two etch processes (180 and 182) capable of removing the oscillator pattern from the substrate. Front etch 180 process depicted beginning in STEP 184 utilizes an isotropic etch. As the etch progresses from an initial to an intermediate stage 188, and a final stage 190, oscillator 192 is freed from substrate 170. Front view 190 provides an isometric view of the oscillator still attached to the substrate. Alternatively, back etch, process 182, may be incorporated to reduce detectivity (thus reducing dislocation losses) and the time associated with the etch. In this case, a back etch or lapping technique known to those skilled may be used to remove or reduce the substrate thickness without exposing the front masked pattern portion of the silicon to detectivity. Typically backside lapping is only used for packaging purposes. This backgrind is often used on larger diameter wafers. This is extremely important as silicon-manufacturing techniques migrate from small diameter substrates to larger diameter substrates. Larger diameter substrates require a thicker substrate to maintain the structural uniformity of the substrate during the fabrication process. Additionally, this back etch process may be used in conjunction with a front etch process to provide a more expeditious means of releasing the patterned oscillators 192 from substrate 170. The need for backs etch processes derives from the need for an extremely selective differential etch for a front etch process. When etching from the front, an etchant should only remove excess material surrounding and beneath patterned oscillator 192. Typically, to etch all the way around an oscillator approximately 100 microns of silicon are etched in order to preserve 0.5- micron thick oscillators. For many regions of parameter space the oscillators may be completely etched away during the etch process.

The multi-mode torsional oscillator of the present invention need not be limited to the embodiment disclosed in FIGURE 2. More importantly, the present invention includes a methodology for developing specific multi-mode torsional oscillators having geometries and moments that achieve a desired ratio of the moments of inertia of large portions or wing and the smaller portion or head of the double-torsional oscillator. In this manner, a multi-mode torsional oscillator may be formed wherein the moment of the head is decoupled from the moment of inertia of the wings of the multi-mode torsional oscillator. Additionally, other embodiments may include having a base of the multi-mode torsional oscillator anchored at both a top end and bottom end of said torsional oscillator, as shown in FIGURE 11. FIGURE 11 present a two-dimensional multi-mode torsional oscillator having a triple torsional design. Similarly, multi-mode torsional oscillator can be constructed having a triple-torsional design in the X, Y and Z axes. FIGURE 12 illustrates a template of a photolithography mask outlining various potential multi-mode torsional oscillators and arrays of multi- mode torsional oscillators.

Manufacturing the multi-mode torsional oscillators of the present invention highlighted the potential need for an alternate processing approach. This was due to the large defectivity associated with the front etch of a multi-mode torsional oscillator or any MEMs type device from an associated substrate. Since a large amount of silicon must be etched, this places a strict differential etch requirement on the process. Thus, as part of the multi-mode torsional oscillators and a method for manufacturing these multiple-oscillators, a two-sided process culminating in a back etch was developed. The advantage here is that the oscillators remain pristine and defect free as the etchant eats all the way through the wafers on the order of 300-400 microns and only attacks the oscillators for the final 0.5 microns. In this manner, smooth and defect free, or almost defect free, oscillators can be obtained.

This process may be completed with either positive or negative photolithography.

Interferometery The fiber optic interferometer capabilities have been significantly improved within the present invention. There now exists sufficient stability for long-term tracking of oscillator motion with a sensitivity of better than 0.001 nm/ Afϊz . This has been enabled by two changes over prior art systems. First, a high-power laser diode 202 was incorporated and aligned. This increased the interferometer signal to a much higher level. Where in prior art systems one could only detect a very few interference fringes such as those with the fiber closest to the oscillator, the system depicted in FIGURE 13 is capable of detecting several hundred interference fringes over a distance of several microns.

Next, and more importantly for practical detection purposes, feedback system 204 as provided in FIGURE 13 is incorporated into the fiber optic interferometer capabilities. Feedback system 204 removes problems due to low frequency vibration, noise and the drift of the interferometer cavity. This feed-back system removes problems associated with low frequency vibration, noise, and drift of the interferometer cavity by monitoring the DC level of the interference fringe from the fiber stage any such drift is immediately corrected by the feed-back circuit and stack piezo which are coupled to the mechanical translator.

Feedback system 204 allows interferometer 200 to maintain a sensitivity to AC signals which is stable to about 1% even in the presence of large thermal drifts and mechanical vibrations which can be on the order or microns. In one embodiment, the crossover frequency is set to approximately 1 kHz. This is faster than most drift time scales but slower than most true oscillator responses, which are in the 5-100 kHz ranges or higher.

Magnetic Excitation of Multi-mode torsional Oscillators

Conventional NMR force microscopy requires micro- magnets. Similar micro-magnets have been used in the characterization DOE' s that study the excitation of multi-mode torsional oscillators for convenience. The torsional oscillator modes are not easily excited with linear (piezoelectric) excitation, whereas magnetic excitation can impart considerable torque to the oscillator. Other forms of excitation include PZT, magnetic, capacitive and the like as known to those skilled in the art. FIGURE 15 depicts a small magnetic particle 210 coupled to multi-mode torsional oscillator 208. Magnetic particle 210 typically has dimensions of approximately three microns. Magnetic particle 210 in the embodiment depicted can be glued to oscillator 208. This may be accomplished by bulk, or thin film deposition of cobalt nickel or iron. This may be accomplished through the use of a small micromanipulator for applying glue such as STYCAST 1266 epoxy. Although the use of fast-curing epoxy type glues is disclosed, other techniques known to those skilled in the art may be used. Micromanipulator can then be used to both apply the glue to the magnetic particle 202 and to locate magnetic particle 202 into place on oscillator 200. In another embodiment, a micro-magnet can be deposited by thin film e-beam deposition through the use of a mask. Other possibilities for depositing micro-magnets on the multi-mode torsional oscillator include the creation of a multi-layered magnetic structure comprising bulk magnetic materials, known to those skilled in the art. By building a multi-layered thin film magnetic structure, the directionality and magnitude of the magnetic field gradient can be controlled. FIGURE 14 depicts a resonant sweep obtained from a magnetic excitation of an oscillator mounted magnet whose polarization is perpendicular to the plane of the oscillator. A small AC magnetic field (on the order of 0.2 Gauss) was applied perpendicular to the moment and perpendicular to the axis of the torsional motion. Thus, a torque excited torsional motion. A test was conducted within room temperature in air. The data resulting from the experiment would be smooth had the experiment been performed after the interferometer stabilization took place. However, the experiment clearly demonstrates that the magnetic excitation is a viable way to excite torsional modes of an oscillator. However, as previously stated capacitive, and PZT and excitation can be used to excite torsional and bending modes of an oscillator.

Another means of exciting torsional modes would be for capacitive excitation. This technique has long been used for larger oscillators but has previously been considered too weak for the small oscillators of the present invention. In this instance, an optical fiber is coated and the ends of the optical fiber can be used to form one capacitor plate. The oscillator can be used to form the other capacitor plate. Since the fiber diameter is small, the electrostatic force can be applied locally to the oscillator head and thus excite torsional motion. Additionally, as used with larger oscillators, a high-voltage DC bias can be applied in conjunction with the AC excitation to increase the size of the excitation. The advantage there being that a very small capacitive gap can be achieved on the order of one micron.

Oscillators with High-Force Sensitivity Determination of the resonant frequencies and the quality factors of the oscillator as seen previously determine the force sensitivity of these oscillators. Additionally, observed noise levels can be compared with predicted thermal noise levels. FIGURE 16 depicts a typical frequency sweep of a micro-oscillator of the boron implanted multi-mode torsional design. Four resonances 212, 214, 216 and 218 are observed. These resonances are assigned in order of increasing frequency to the symmetric cantilever resonance 212, symmetric torsional resonance 214, anti-symmetric cantilever resonance 216, and anti-symmetric torsional mode resonance 218. The type of mode can be verified by moving the interferometer laser spot to various points on the oscillator while maintaining a constant excitation signal.

FIGURES 17A-17F illustrate the first six natural modes of a multi-mode torsional oscillator having a head and wings. In the symmetric cantilever mode, shown in FIGURE 17A, for resonance 212, both sides of the head and both sides of the wing move in phase. In the symmetric torsional mode, shown in FIGURE 17B, for resonance 214, the two sides of the head and the two sides of the wing move with opposite phases, while the same side of the head and wing move together. Conversely, for the anti-symmetric cantilever mode, shown in FIGURE 17C, for resonance 216, the head and wing move in opposite directions while each side of either the head or the wing move together. Finally, for the anti-symmetric torsional mode, shown in FIGURE 17D, for resonance 218, the two sides of the head and the two sides of the wing move with opposite phases. The same side for the head and the wing move also with opposite phases. These general behaviors have been verified and are illustrated in the simulations of the first six natural modes of such an oscillator provided in FIGURES 17A-17F.

FIGURE 18 provides a comparison of a commercially available AFM cantilever resonance 230 and one of the highest-Q resonances 232 attained at room temperature from a multi-mode torsional oscillator. Whereas the amorphous commercial prior art oscillator has a low Q of approximately 10, the single-crystal, double- torsional oscillator has a Q of approximately 10,000. Using a typical value for the effective spring constant of k is approximately equal to 5xl0^~3 N/m (for a 300 nm thick oscillator) . The force sensitivity given via Equation 1 for the 62 kHz mode is equal to 1.5x10^"

¹⁶ N/V/_z at room temperature. This exceeds target sensitivity, thus processing, geometry and fabrication for the multi-mode torsional oscillator have not been fully optimized but clearly demonstrates the current force sensitivity and advantage of using multiple oscillator modes. Higher sensitivities are expected for even thinner oscillators. Modeling Multi-mode torsional Oscillators

A process was developed and verified having a simulation model to be used to design future oscillators with fewer fabrication trials. These oscillators have been modeled using ANSYS finite- element analysis software. The models depicted in FIGURES 17A-17F are the simulations of the first six natural modes of the oscillators, where the oscillators are perfectly flat with squared corners. FIGURE 19 illustrates a more realistic oscillator 234 to be modeled, being slightly tapered with rounded corners that often result from the etch process. FIGURE 20 depicts the mesh and thickness values used for modeling this more realistic oscillator. SEM images were used to estimate thickness and variations contained without the oscillator.

FIGURE 21 provides a comparison of the four modes detected experimentally are represented by curve 238 with simulated Eigen frequencies 236. Deviation between experimental and analytically determined resonances was 0.2%, 4.4%, 3.8% and 4.7%, respectively. This agreement demonstrates confidence in this ANSYS modeling of the oscillators.

Although the present invention has been described in detail herein with reference to the illustrative embodiments, it should be understood that the description is by way of example only and is not to be construed in a limiting sense. It is to be further understood, therefore, that numerous changes in the details of the embodiments of this invention and additional embodiments of this invention will be apparent to, and may be made by, persons of ordinary skill in the art having reference to this description. It is contemplated that all such changes and additional embodiments are within the spirit and true scope of this invention as claimed below.

Claims

WHAT IS CLAIMED IS

1. A force sensing microscopy tool comprising: a mechanical oscillator, wherein said mechanical oscillator is a multi-mode torsional oscillator; an excitation mechanism to excite said oscillator at at least one resonance frequency; a uniform magnetic field that polarizes a sample; a magnetic field gradient that couples a nuclear or electron moment of a sample to said oscillator; a cyclical magnetic field that cyclically inverts said moment of said sample; a sensing system to detect said oscillator' s response to a coupling between said oscillator and said sample, and wherein said sensing system provides at least one output signal; and a processor operable to process said at least one output signal and produce at least one data signal representative of said sample.

2. The force sensing microscopy tool of Claim 1, wherein said sample and said oscillator are coupled by at least one force selected from the group consisting of nuclear force, electro-magnetic force, and gravitational force.

3. The force sensing microscopy tool of Claim 1, wherein said cyclical magnetic field is generated by an RF coil.

4. The force sensing microscopy tool of Claim 1, wherein said moment of said sample is inverted or excited at a resonant frequency of said oscillator.

5. The force sensing microscopy tool of Claim 1, wherein said uniform magnetic field is provided by an electromagnet .

6. The force sensing microscopy tool of Claim 1, wherein said magnetic field gradient is provided by a permanent magnet or an electromagnet.

7. The force sensing microscopy tool of Claim 6, wherein said permanent magnet or electromagnet is located on said oscillator, and wherein said sample is not physically affixed to said oscillator.

8. The force sensing microscopy tool of Claim 6, wherein said sample is located on said oscillator, and wherein said permanent magnet or electromagnetic is not physically affixed to said oscillator.

9. The force sensing microscopy tool of Claim 1, wherein said excitation mechanism is at least one selected from the group consisting of PZT, magnetic, capacitive and electro-static.

10. The force sensing microscopy tool of Claim 1, wherein said oscillator is a high-Q torsional oscillator comprising: at least one first component having a first inertial moment; at least one second component having a second inertial moment greater than said first moment; and at least one connecting segment to physically couple said at least one first inertial moment to said at least one second inertial moment.

11. The force sensing microscopy tool of Claim 10, wherein said high-Q torsional oscillator is fabricated from a single crystal substrate to reduce dislocation losses.

12. The force sensing microscopy tool of Claim 10, wherein said at least one first component is a single head and said at least one second component is a body.

13. The force sensing microscopy tool of Claim 10, wherein said head is sensitive to detect individual moments selected from the group consisting of magnetic, nuclear and gravitational moments.

14. The force sensing microscopy tool of Claim 10, wherein said oscillator may be represented as a system of coupled individual inertial moments.

15. The force sensing microscopy tool of Claim 10, wherein said oscillator has a high-Q between 1,000 and 1,000,000.

16. The force sensing microscopy tool of Claim 13, wherein said detected moment is an individual proton spin, neutron spin or electron spin.

17. The force sensing microscopy tool of Claim 16, wherein said proton is located within a hydrogen atom contained within a polymer, biological or organic structure.

18. The force sensing microscopy tool of Claim 1, wherein said oscillator mounted on a scanning system in order to inspect samples having a complex structure.

19. A high-Q torsional oscillator comprising: at least one first component having a first inertial moment; at least one second component having a second inertial moment greater than said first inertial moment ; and at least one connecting segment to physically couple said at least one first inertial moment to said at least one second inertial moment.

20. The oscillator of Claim 19, wherein said oscillator is fabricated from a single crystal substrate to reduce dislocation losses.

21. The oscillator of Claim 19, wherein said at least one first component is a single head and said at least one second component is a body.

22. The oscillator of Claim 21, wherein said head is sensitive to detect individual nuclear or electron spin moments selected from the group consisting of magnetic, nuclear and gravitational moments.

23. The oscillator of Claim 19, wherein said oscillator may be represented as a system of coupled individual inertial moments.

24. The oscillator of Claim 19, further comprising at least one base to mount said oscillator within a microscopy tool.

25. The oscillator of Claim 19, wherein said oscillator has a high-Q between 1,000 and 1,000,000,

26. The oscillator of Claim 19, wherein said oscillator may be incorporated into a magnetic resonance force microscopy tool.

27. The oscillator of Claim 22, wherein said nuclear moment is a individual proton spin.

28. The oscillator of Claim 27, wherein said proton spin is within a hydrogen atom forming a polymer, biological or organic structure.

29. A force sensing microscopy tool comprising: at least one mechanical oscillator, wherein said mechanical oscillator is a multi-mode torsional oscillator; an excitation mechanism to excite said oscillator at at least one resonance frequency; a coupling mechanism to couple said oscillator to said sample; a sensing system to detect motion of said oscillator coupled to said sample, and wherein said sensing system provides at least one output signal; and a processor operable to process said at least one output signal and produce at least one data signal representative of said sample.

30. The force sensing microscopy tool of Claim 29, wherein said coupling mechanism comprises: a uniform magnetic field that polarizes a sample; a magnetic field gradient that couples a nuclear moment of said sample to said oscillator; and a cyclical magnetic field that cyclically inverts said moment of said sample.

31. The force sensing microscopy tool of Claim 29, wherein said sensing system is a fiber optic interferometric system.

32. The force sensing microscopy tool of

Claim 31, wherein said sensing system further comprises a feedback system to minimize drift.

33. The force sensing microscopy tool of Claim 29, wherein said sensing system is an optical lever arm.

34. The force sensing microscopy tool of

Claim 29, wherein said magnetic field gradient is provided by a permanent magnet, wherein said permanent magnet is a multi-layered thin filmed magnetic structure formed on said oscillator to control a vector of said magnetic field gradient.

35. The force sensing microscopy tool of

Claim 29, wherein said coupling mechanism uses nuclear, electromagnetic, or gravitational forces.

36. The force sensing microscopy tool of Claim 29, wherein said at least one mechanical oscillator form an array of mechanical oscillators.

37. The force sensing microscopy tool of Claim

31, wherein a first coating on said oscillator and a second coating .on said fiber optic interferometric system together form a capacitor.

38. A method of force sensing via a force sensing microscopy tool comprising the steps of: at least one mechanical oscillator, wherein said mechanical oscillator is a multi-mode torsional oscillator; coupling said oscillator to a sample; sensing said oscillators response to said oscillator coupled to said sample outputting at least one output signal; and processing said at least one output signal to produce at least one data signal representative of said sample.

39. The method of Claim 38, wherein said coupling mechanism comprises: a uniform magnetic field that polarizes a sample; a magnetic field gradient that couples a nuclear moment of said sample to said oscillator; and a cyclical magnetic field that cyclically inverts said moment of said sample.

40. The method of Claim 38, wherein said step of sensing uses a fiber optic interferometric system.

41. The method of Claim 40, wherein said fiber optic interferometric system further comprises a feedback system to minimize drift.

42. The method of Claim 38, wherein said step of sensing using an optical lever arm.

43. The method of Claim 38, wherein said coupling is accomplished with a magnetic field gradient provided by a permanent magnet, wherein said permanent magnet is a multi-layered thin filmed magnetic structure formed on said oscillator to control a vector of said magnetic field gradient.

44. The method of Claim 38, wherein said coupling is achieved via nuclear, electromagnetic, or gravitational forces.