WO2005081929A2 - Detection d'un deplacement de resonateur faisant appel a un melangeur abaisseur de signal piezoresistant - Google Patents

Detection d'un deplacement de resonateur faisant appel a un melangeur abaisseur de signal piezoresistant Download PDF

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Publication number
WO2005081929A2
WO2005081929A2 PCT/US2005/005597 US2005005597W WO2005081929A2 WO 2005081929 A2 WO2005081929 A2 WO 2005081929A2 US 2005005597 W US2005005597 W US 2005005597W WO 2005081929 A2 WO2005081929 A2 WO 2005081929A2
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Prior art keywords
resonator
frequency
cantilever
mechanical
piezoresistive element
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PCT/US2005/005597
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English (en)
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WO2005081929A3 (fr
Inventor
Igor Bargatin
Edward B. Myers
Mo Li
Jessica Arlett
Benjami Gudlewski
Michael L. Roukes
Darron K. Young
Hong X. Tang
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California Institute Of Technology
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Priority claimed from US11/010,578 external-priority patent/US7434476B2/en
Application filed by California Institute Of Technology filed Critical California Institute Of Technology
Priority to US10/590,485 priority Critical patent/US7552645B2/en
Publication of WO2005081929A2 publication Critical patent/WO2005081929A2/fr
Publication of WO2005081929A3 publication Critical patent/WO2005081929A3/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
    • G01Q20/00—Monitoring the movement or position of the probe
    • G01Q20/04—Self-detecting probes, i.e. wherein the probe itself generates a signal representative of its position, e.g. piezoelectric gauge

Definitions

  • This invention is related to micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS).
  • MEMS micro-electro-mechanical systems
  • NEMS nano-electro-mechanical systems
  • MEMS microelectromechanical systems
  • the piezoresistor containing devices achieve high strain sensitivity by using semiconductor-based piezoresistors, primarily dop ed Si or AlGaAs.
  • piezoresistive systems htave the advantages of being fully integrated sensors that operate from room temperat ⁇ ure down to at least 4 K and do not require a magnetic field.
  • Resonance detection of cantilevers up to 9 MHz has been achieved previously using doped Si piezoresistors.
  • direct current (DC) biasing is used in the piezoresistive resonance detection of cantilevers.
  • An embodiment of the invention provides a system containing a micro- mechanical or nano-mechanical device and a method of operating the same.
  • the device includes a resonator and a piezoresistive element connected to the resonator.
  • the method includes AC biasing the piezoresistive element at a first frequency, driving the resonator at a second frequency different from the first frequency, and detecting a mechanical response of the resonator at one or both of a difference frequency and a sum frequency of the first and second frequencies.
  • Figure 1A is a schematic of a circuit with a DC readout scheme.
  • Figure IB is a schematic of a circuit of the first embodiment of the present invention.
  • Figure 2A is a scanning electron micrograph of a cantilever according to an example of the first embodiment of the invention.
  • Figure 2B is an optical micrograph of a cantilever and dummy resistor according to an example of the first embodiment of the invention.
  • Three gold leads at the bottom of the image connect the components to the rest of the circuit shown in Figure IB.
  • Figures 3 A and 3B are plots of signal amplitude versus frequency.
  • the plot in Figure 3 A was measured using the down conversion scheme of the first embodiment of the invention and the plot in Figure 3B was measured directly with a network analyzer.
  • Figure 4 is a plot of voltage noise versus frequency which illustrates thermomechanical noise peaks from the fundamental vibration mode; of a 2.6 micron long cantilever. The inset shows one of the noise peaks from the second vibration mode of the same cantilever.
  • Figure 5A is a three dimensional schematic view of a device according to an embodiment of the invention.
  • Figure 5B is an SEM image of an AFM probe according to an embodiment of the invention.
  • Figures 7A and 7B are schematics of a testing set up used to test the devices of one example of the present invention.
  • Figures 6 and 8 are plots of piezoresistive response o devices of an embodiment of the invention.
  • Figure 9 is a plot of a noise spectrum from a metallic thin film piezoresistor.
  • Figure 10A is a 3D topographical image obtained from direct optical tapping mode AFM.
  • Figure 10B is a 3D topographical image obtained from lock-in measurement of the metallic thin film piezoresistor.
  • Figures 11 and 12A-12F show steps in the manufacture of a device of the second embodiment of the invention.
  • Figure 13 is a top view of an in-process device of the second embodiment of the invention.
  • Figure 14 is a micrograph of an SPM probe of the second embodiment of the invention.
  • Figures 15 and 16 are plots of power and S v versus frequency, respectively, for an SPM probe of the second embodiment of the invention.
  • the present inventors have developed a system and method for measuring RF-range resonance properties of nano- or micro- mechanical devices which comprise entire or parts of micro- or nano-electro- mechanical systems (MEMS and NEMS, respectively), with integrated piezoresistive strain detectors serving as signal downmixers.
  • the piezoresistive elements comprise thin film elements and the technique takes advantage of the high strain sensitivity of thin film piezoresistors, while overcoming the problem of RF signal attenuation due to a high source impedance.
  • the technique also reduces the effect of the cross-talk between the detector and actuator circuits.
  • the method also allows a way to circumvent the problem of impedance mismatch and extends the usability of low-frequency, high-input-impedance preamplifiers and other equipment.
  • the method includes using AC biasing and the intrinsic properties of the piezoresistor to perform heterodyne downmixing of the signal to a much lower frequency, which can then be detected by standard circuitry without significant signal loss. Not only does this increase the detected signal, it also reduces unwanted background from the cross-talk between the detector and actuator circuits.
  • this downmixing scheme is applied to the detection of mechanical response of cantilever NEMS with fundamental mode frequencies of 5-25 MHz to achieve thermal noise-limited detection of mechanical resonances in these devices at room temperature, demonstrating the ultimate sensitivity of downmixed piezoresistive signal detection in resona it high- frequency NEMS applications.
  • this downmixing scheme is applied to the detection of mechanical response of a cantilever probe resonator in an atomic force microscope (AFM) (i.e., in a MEMS application).
  • AFM atomic force microscope
  • mechanical response is not limited to mechanical displacement (i.e., movement), and includes other mechanical responses, such as stress.
  • the examples illustrate the detection of mechanical displacement of a resonator, the amount of stress in the resonator can also be detected instead of or in addition to the detection of the mechanical displacement.
  • the system containing the resonator comprises a chemical or biological sensor
  • th ⁇ e stress in the resonator arising from binding of chemical or biological species to the resonator may be detected.
  • FIG. 1A illustrates the difficulties in a prior art method of applying low- frequency techniques to high-frequency piezoresistive NEMS.
  • the piezoresistor R c is placed in a bridge or half-bridge configuration with a fixed dummy resistor R d .
  • the ends of the resistors are oppositely DC biased at +V ⁇ 2 and - V b /2., so that the voltage at the bridge point is zero when the NEMS is not moving.
  • the bridge output must be connected in some way to a circuit, e.g., the input of a high-input-impedance preamplifier.
  • a circuit e.g., the input of a high-input-impedance preamplifier.
  • this effectively introduces capacitances C par to ground in parallel with the cantilever and dummy resistor, effectively foraiing a low-pass filter with a cut-off frequency of ( ⁇ tRC par ) ⁇ .
  • typical amplifier-input and caMe capacitances C par >10 pF, and R ⁇ IO k ⁇ the AC output is strongly attenuated, at wjl ⁇ > 2 MHz.
  • the measurements are further complica_ted by cable resonances.
  • FIG. IB illustrates a circuit schematic of a system 101 according to one example of the first embodiment of the invention.
  • the system includes a micro- mechanical or nano-mechanical device 103.
  • the device 103 may comprises ibr example, a scanning probe microscope (SPM) probe, such as an AFM probe, a mass sensor, a charge sensor, a force sensor, a pressure sensor, a flow sensor, a chemical sensor, a biological sensor, an inertial sensor, a biological imaging device or any ottier suitable device which comprises an entire MEMS or NEMS or a portion thereof.
  • SPM scanning probe microscope
  • the device 103 includes a resonator and a piezoresistive element connected to the resonator (not shown in Figure IB for clarity).
  • the resonator may comprise a cantilever, a torsional resonator, a doubly clamped beam, a diaphragm resonator, which are described in more detail below, or any other type of resonating element.
  • the piezoresistive element may comprise a thin film which is coated on a surface of the resonator or which is encapsulated in whole or in part within the resonator.
  • the piezoresistive element may comprise a metal or semiconductor fiLm coated on a surface of the resonator.
  • the resonator may comprise a doped region in a portion of a semiconductor cantilever, such as a p+ doped region in a silicon cantilever.
  • the system 101 also includes an AC bias source 105.
  • the AC bias source is electrically connected to the piezoresistive element of the device 103.
  • the source 105 is either directly or indirectly connected to the piezoresistive element to provide an AC bias (i.e., an alternating voltage) to the piezoresistive element at a
  • the system 101 also includes an AC drive source 107.
  • the AC drive source 107 is operatively connected to the resonator of the device 103.
  • the source 107 is either directly or indirectly comiected to the device 103 to drive the
  • the AC drive source 107 may be electrically connected to a piezoactuator (not shown in Figure 1) which is adapted to oscillate the resonator of the device 103 at the second frequency.
  • the second frequency comprises the resonator's resonance frequency and which differs from the first frequency by 100 kHz or less.
  • the system 101 also includes a phase sensitive detector 109.
  • the detector is electrically connected to the piezoresistive element of the device and is adapted to detect a mechanical response of the resonator.
  • the detector is adapted to receive an electrical signal from the piezoresistive element to detect a mechanical response of the resonator, such as the oscillation or changes in oscillation of the resonator.
  • the detector 109 comprises a lock-in amplifier, but may comprise other suitable phase sensitive detectors.
  • the detector 109 is adapted to detect the mechanical response of the resonator at one or both of a difference frequency and a sum frequency of the first and second frequencies.
  • the detection is conducted at the difference frequency, as will be described in more detail
  • the system 101 also contains a dummy resistor 111 located in a bridge configuration (i.e., in series) with the device 103.
  • the AC bias source 105 provides a first voltage (V b ) having the first frequency to both the piezoresistive element of the device 103 and to the resistor 111.
  • V b first voltage
  • the AC bias source provides
  • the system 101 also contains a first low pass filter 117 whose output is electrically connected to an input of the lock-in amplifier 109 and whose input is electrically connected to a bridge point between the resistor 111 and the device 103.
  • the downmixed output from the bridge point is provided through the filter 117, then amplified in amplifier 119, and then provided into the lock-in amplifier 109.
  • the system 101 also contains a mixer 121 whose first input is electrically connected to an output of the AC bias source 105, whose second input is electrically connected to an output of the AC drive source 107, and whose output is electrically connected via a second low pass filter 123 to a reference input of the lock-in amplifier 109.
  • the AC bias source 105 is connected to the mixer 121 via the first power splitter 113 and the AC drive source 107 is connected to the mixer 121 via a second power splitter 125.
  • the mixer 121 provides a downmixed reference signal to the lock-in amplifier 109.
  • the piezoresistive element i.e., piezoresistor
  • the voltage is applied oppositely to the ends of the device 103 and resistor 111, to null the bias voltage at the bridge point.
  • the output signal at the bridge point contains two frequency components, at the sum and difference of the drive and bias frequencies.
  • Aw sufficiently small, preferably 200 kHz or less, such as 100 kHz or less
  • the downmixed frequency component is attenuated minimally by the parallel capacitances.
  • the output is then sent through the low pass filter 117 to remove any residual carrier and the upper sideband, amplified in amplifier 119, and finalLy fed into the lock-in amplifier 109 for detection.
  • the lock-in reference is generated by splitting off the bias and drive voltages with power splitters 113 and 125 and sexiding the voltages into a mixer 121, such as any commercial mixer, which generates a downmixed signal in parallel with the device 103.
  • This electromechanical downmixing effect is a fully linear property of the piezoresistor.
  • the piezoresistor down-converts any signal to an amplitude haLf that of the signal that would be generated with a DC bias of the same magnitude, hi contrast, any parasitic cross-talk signal from the actuation circuit would need to pass through an element with a nonlinear I-N response in order to mix with the; bias voltage down to the frequency Aw.
  • the I-N nonlinearity of the piezoresistors of the examples of the present invention is quite small, so that even extremely weak displacement signals can be downmixed and extracted from the device with minimal attenuation and background.
  • the downmixing scheme is tested using high-frequency piezoresistive cantilevers as the NEMS device 103.
  • the cantilevers are fabricated from silicon-on-insulator (SOI) wafers, where the top Si layer consists of 80 nm Si plus a 30 nm layer of boron-doped p-Si to act as the piezoresistive strain sensor (i.e., a semiconductor piezoresistive film).
  • SOI silicon-on-insulator
  • the cantilevers were fabricated in a manner similar to that of described in J. A. Harley and T. W. Kenny, Appl. Phys. Lett.
  • a backside KOH etch suspends the top Si layer as a membrane, and a combination of electron beam lithography, liftoff, and fluorine/chlorine-based plasma etching steps forms the cantilever from the membrane.
  • a typical cantilever is shown in Fig. 2A. Cantilever lengths ranged between about 2-3 ⁇ m and widths were approximately 700 nm. For example, the exemplary cantilever shown in Figure 2 A is 3.2 microns long, 700 nm wide and 110 nm thick.
  • the cantilever includes a notch and leg portions surrounding the notch.
  • the piezoresistive film is located at least on the leg portions of the cantilever.
  • the dummy resistor 111 is fabricated on-chip using the same p-Si material that provides the cantilever strain sensor, in order to minimize parallel capacitances.
  • the cantilever and dummy resistances varied from device to device, and are in the range of 50-150 k ⁇ .
  • the cantilever chip is mounted onto a piezoelectric ceramic actuator disk with a thickness of approximately 80 ⁇ m, corresponding to a thickness-mode resonance frequency of about 25 MHz. This assembly was in turn mounted onto a circuit board and placed into a vacuum chamber for measurements at room temperature.
  • Figure 3 A shows a resonance curve for a 1.7- ⁇ m long cantilever using the above described downmixing method.
  • N b o 3 N
  • a peak-to-peak voltage of 1.9 N is applied to the piezoactuator.
  • the amplitude is normalized to drive and bias levels.
  • the square of the voltage signal is proportional to the energy in the cantilever. Fitting this quantity to a Lorentzian yields a width of 18.2 kHz, implying a quality factor of Q «1300.
  • Fig. 3B shows the same cantilever resonance measured in the standard DC half-bridge configuration (see M. Tortonese, R. C. Barrett, and C. F. Quate, Appl. Phys. Lett. 62, 834 (1993)) using a DC bias of 5 N across R c and Ra and a 50-Ohm high-frequency network analyzer for drive (approximately 1 V pp ) and detection. In both measurements no impedance matching had been performed, and the signal amplitude is normalized to the drive and bias levels. The resonant signal in the downmixing scheme is approximately 1000 times larger than in the direct measurement scheme.
  • the relative magnitude of the background, caused by parasitic coupling of the drive signal to the detection circuit, is nearly three orders of magnitude smaller in the downmixing case.
  • the downmixed resonance is essentially Lorentzian, while the network analyzer resonance is highly distorted.
  • the downmixing technique is sensitive enough to detect the thermomechanical fluctuations of the cantilever when it is not driven externally. This is done by applying only a bias voltage, and, while sweeping the bias, detecting narrowband noise at the offset frequency Aw.
  • thermomechanical noise and Johnson noise the preamplifier input noise is negligible in the present case.
  • the precise origin of this excess noise is unclear. However, the noise floor can be reduced to the expected Johnson noise level
  • the network analyzer measurement has input-referred
  • thermomechanical noise amplitude of about 7
  • thermomechanical noise peak remains visible in the downmixed scheme at even higher frequencies, as demonstrated by detection of the thermal noise peak from the second vibration mode of the 11.9-MHz cantilever (inset to Fig. 4), whose driven resonance is detected at approximately 71 MHz. This demonstrates that the technique as presented here is viable for even smaller NEMS whose fundamental modes lie in the VHF range. In addition, while the measurements were performed at room temperature, both Johnson and thermomechanical voltage noise vary with
  • FIG. 5A is a schematic and figure 5B is an SEM image of an exemplary micromachined cantilever (i.e., probe) 103 with the following micro-scale
  • the probe 103 contains a
  • the metal film 9 acts as the piezoresistive element.
  • a semiconductor film or doped region may be used instead.
  • the metal film 9 is formed on the same side of the cantilever as the tip 13.
  • the specific probe 103 shown in Figure 5B is designed for tapping mode AFM. Similar probes may be used for non-contact mode AFM.
  • Figure 6 illustrates a piezoresistance response of a metal film on the cantilever similar to that in Figure 5B according to another aspect of the second example of the present invention.
  • the cantilever is 125 microns long, 40 microns wide and 4 microns thick with a conventional tip.
  • the cantilever is suitable for a self-sensing probe 103 designed for tapping mode AFM applications.
  • a gold thin film covers the two legs of the cantilever and forms a current loop.
  • a very strong piezoresistance response is observed, as shown in Fig. 6.
  • Non-resonant background signal is subtracted from the raw data.
  • the quality factor for this specific cantilever is about 220 in air. Under vacuum conditions, the piezoresistance response is stronger, with quality factor rising above 360. The data for vacuum is shifted to the right for better visual inspection.
  • Figure 7 A illustrates the measurement set up of the AFM probe 103 and Figure 7B schematically illustrates a circuit scheme to measure the piezoresistive response of the AFM probe 103 when the probe is used in tapping/non-contact/ AC mode AFM.
  • the bias current through piezoresistive element 9 is modulated at one frequency, while the cantilever 1 is driven at another, different frequency.
  • the mechanical response of the cantilever is detected at their difference frequency and/or their sum frequency.
  • the AC drive source 107 is used to drive the cantilever 1 through a piezo drive source in the base 5.
  • the drive source 107 is synchronized with the AC bias source 105 and the outputs of the AC drive and bias sources are provided into different inputs of the mixer 121.
  • the output of the mixer 121 is provided through a low pass filter 123 into the lock-in amplifier 109 as a reference signal.
  • the AC bias source 105 is used to bias the metal film 9, whose output is also provided into the lock-in amplifier 109 through another LPF 117 and amplifier 119.
  • the AC drive source 107 can be used to drive the cantilever 1 at resonant frequency, such as 240 kHz for example.
  • Direct lock-in measurement can be employed to detect the amplitude of the oscillation.
  • the second example also employs the down-mix detection scheme.
  • the sample bias current may be applied at a frequency that is 200 kHz or less higher, such as 10-50 kHz higher, for example 20 kHz higher than the drive frequency (e.g. 260 kHz for a 240 kHz drive frequency).
  • Lock in measurement is performed at 20 kHz or 500 kHz, for example.
  • the probe 103 is then tested with a commercial AFM system (Dl dimension 3100 system) equipped with a signal access module for external signal access and control.
  • a commercial AFM system Dl dimension 3100 system
  • the standard Dl probe holder is modified to facilitate the testing of the metallic piezoresistive probes.
  • the electrical connections from the chip holder to the AFM headstage are disconnected.
  • four wires 31, 33, 35, 37 are soldered to the chip holder to enable an electrical connection to the piezo actuator 5 under the probe 103 and connections to two electrical contacts pads 39, 41 on the self-sensing probe.
  • self-sensing means that the probe does not require external optics, such as a laser and photodetector to measure its mechanical response.
  • the drive signal is applied to piezo actuator 5 through the AC drive source, such as an external function generator, 107 (Stanford Research System DS345).
  • a bias voltage is supplied across the two legs 7 of the cantilever 1.
  • a resistor 111 with a resistance value similar to that of the cantilever is used as a balance resistor (20.3 Ohms) for extraction of resonant AC signal.
  • the voltage change across the probe 103 is further amplified through a low-noise voltage amplifier 119 (Stanford Research System SR560). This oscillating AC voltage is then fed into a lock-in amplifier 109 (Stanford Research System SR830).
  • the measurement is locked into the drive signal provided by the function generator, x-output of the lock-in amplifier is supplied to one input channel of the nanoscope controller through signal access module after the phase extender box.
  • FIG. 8 shows three resonant curves from the same cantilever.
  • Trace 1 bottom curve
  • Trace 2 middle curve
  • Trace 3 upper curve
  • Comparable signal strengths are observed in all three curves. In optical data, side bands due to non-flexural resonance are apparent. They are absent in the electrical measurement curves. Apparently electrical measurements are immune to the shear motion displayed in the optical measurement data.
  • a comparison between trace 2 and trace 3 shows that the downmixing scheme can effectively eliminate the cross-talk signal that is usually inevitable in such a measurement.
  • the noise spectrum measurement is then performed on the metallic thin film containing probe, as shown in Figure 9. Very low noise spectrum is observed. For frequency > 1000 Hz, the noise level is below lnV/ Hz, smaller than the Johnson noise generated by a 50 Ohm resistor at room temperature. Generally speaking, at the same frequency range, the noise level in p+ silicon is about 30 ⁇ V/ ⁇ z. The noise performance of metallic piezoresistor is at least about 30 times better than semiconducting Si material.
  • a standard SPM calibration grating is employed to demonstrate the imaging capability of the exemplary metallic thin film containing probes.
  • the grating is a 1-D array of rectangular SiO 2 steps on a silicon wafer with 3 micron pitch.
  • the step height is 20 nm ⁇ 1 nm.
  • the topographic image shown in Figure 10B is acquired from monitoring the output of the lock-in amplifier when AFM is operated at "lift mode”.
  • Optical tapping mode AFM image is shown in Figure 10A for comparison. Even without signal conditioning, the metallic thin-film piezoresistor yields very high signal to noise ratio. The image quality is comparable to that of the optical measurement result.
  • the SPM such as the AFM is used to either determine and/or image characteristics of a surface being examined by the AFM probe 103 based on the piezoresistive response of the metal film 9.
  • the AFM probe may be used to image a surface of a material as shown in Figure 10B or to determine one or more characteristics of the surface of a material, as may be carried out with an AFM.
  • a data processing device such as a computer or a dedicated processor, is used to process the signal from the AFM probe and the associated equipment, such as the lock-in amplifier, to create, store and/or display the image and/or data corresponding to the surface characteristics.
  • a metal film has been described above as the preferred piezoresistive film.
  • microelectromechanical and nanoelectromechanical systems include devices with features having a size of 1 micron to 100 microns and 1 nanometer to less than 1 micron, respectively, in at least one dimension, and preferably in two or three dimensions.
  • these features comprise movable features or elements, such as cantilevers, diaphragms, clamped beams, wires, etc.
  • Microelectromechanical and nanoelectromechanical systems include, but are not limited to, scanning probe microscopes ("SPM”), such as atomic force microscopes (“AFM”), force and pressure sensors, flow sensors, chemical and biological sensors, and inertial sensors, such as accelerometers and motion transducers, mass sensors, charge sensors, and a biological imaging devices.
  • SPM scanning probe microscopes
  • AFM atomic force microscopes
  • chemical and biological sensors may comprise one or more cantilevers having a surface coated with a material which selectively binds to a chemical or biological analyte (i.e., gas or liquid analyte containing or consisting of the chemical or biological species of interest).
  • film includes relatively thin metal films, having a thickness of about 100 nm to about 10 microns, thin metal films, having a thickness of about 10 nm to about 100 nm, and ultra thin metal films, such as discontinuous or island type metal films having a thickness of less than about 10 nm, as will be discussed in more detail below.
  • metal includes pure or essentially pure metals and metal alloys.
  • the resonators may be made of any suitable material, such as inorganic materials, including semiconductor materials, such as Si, SiC, III-N and II-NI materials, insulating materials, such as metal or semiconductor oxides, nitrides or carbides, including SiN and SiO 2 , glass and even organic materials, such as polymeric / plastic materials.
  • semiconductor materials such as Si, SiC, III-N and II-NI materials
  • insulating materials such as metal or semiconductor oxides, nitrides or carbides, including SiN and SiO 2 , glass and even organic materials, such as polymeric / plastic materials.
  • Metal piezoresistive element may comprise pure or essentially metals including, but not limited to, Au, Ag, Ni, Pt, Al, Cr, Pd, W, and metal alloys such as Constantan, Karma, Isoelastic, Nichrome V, Pt-W, Pt-Cr, etc.
  • Semiconductor piezoresistive elements may comprise doped silicon, AlGaAs or other suitable semiconductor materials.
  • the resonator preferably comprises a micron or nanometer sized cantilever.
  • the invention can be used with other resonators, including, but not limited to, doubly clamped beams, torsional resonators, and diaphragm resonators.
  • doubly clamped beam resonators, torsional resonators and diaphragm resonators are disclosed in U.S. Patent Application Number 10/826,007, U.S. Patent No. 6,593,731 and PCT Application PCT/US03/14566 (published as WO/2004/041998) and its counterpart U.S. Patent Application Number 10/502,641, all incorporated herein by reference in their entirety.
  • a doubly clamped beam resonator comprises a beam that is fixed on both ends, but whose middle portion is free hanging so that it can flex or move perpendicular to its length.
  • a torsional resonator may comprise, in a non-limiting example, a flexible diamond or polygonal shaped structure mounted at two anchor points and which can move by twisting or turning about an axis between the anchor points, as described and illustrated in U.S. Patent No. 6,593,731.
  • a diaphragm resonator may comprise any plate shaped resonator which is anchored at one or more edges and whose middle portion is free hanging so that it can move or flex in one or more directions.
  • An example of a diaphragm resonator is a trampoline resonator.
  • a method of making nanoscale scanning probes is described. It should be noted that the method of the second embodiment may be used to make the resonator of the first embodiment. However, the resonator of the first embodiment may also be made by any other suitable method. Furthermore, the method of the second embodiment may be used to make a probe that is not used in the system and method of the first embodiment. Still further, while a cantilever shaped scanning probe is described, other resonators and sensor/imaging devices described above with respect to the first embodiment can be made instead.
  • the probes of the second embodiment are used in NEMS adapted for scanning probe microscopy of biological samples. These probes preferably, but not necessarily fall into nanoscale size regime described above.
  • the thickness of the cantilever can be as thin as 30 nm, such as 30 nm to 150 nm.
  • the width of the cantilevers is as narrow as 400 nm, such as 400 to 800 nm. These dimensions match the size of the scanned cells themselves and offer high resolution in scanning probe microscopes (SPM). Meanwhile, the small dimensions of the cantilevers allow them to be operated at much higher frequencies and yield faster temporal responses, namely as fast as 1 ⁇ s.
  • the mass fabrication process of the second embodiment employs a wafer scale nanofabrication technique that is capable of producing tens to hundreds of cantilevers from a single silicon wafer.
  • the silicon chips that house the cantilevers may be chosen to match the size of the chips used in the SPM market so that conventional installation techniques are used.
  • the semiconductor chips, such as silicon chips, also have electronic circuits integrated into them so that they can interface directly into the commercial scanning probe microscopes and provide a force-sensitive electrical
  • piezoresistive detection Two types of mechanical motion detection schemes are disclosed below for nanoscale cantilevers: piezoresistive detection and optical detection.
  • the former relies on the high piezoresistivity of a piezoresistive film, such as an epitaxially grown doped semiconductor layer, for example a p+ silicon layer, or a metal layer formed on or in the cantilever.
  • Optical transduction is realized by focusing a radiation beam, such as a laser beam onto the end of the cantilever with a micron-scale reflecting pad which is disposed near the tip of cantilever, whereby the cantilever deflection is amplified by the optical path traveled from the tip of the cantilever to a split photo detector.
  • the starting material used for wafer scale fabrication can be divided into three sections as diagrammatically depicted in Figure 11.
  • the first section is the substrate 28, such as a handle wafer, which may be a thick (300-500 ⁇ m) Si wafer or another semiconductor substrate which will be used to mechanically support the cantilever.
  • Handle wafer 28 in one embodiment is oriented in the [100] direction. This maximizes the piezoresistive constant along the [110] direction and eases the difficulty of fabrication and alignment of scanning probes fabricated from the structure.
  • the second section is the etch stop layer 24.
  • An etch stop layer may comprise any material which exhibits a higher resistance to a given liquid or gas etching medium than the substrate 28, such that when the substrate 28 is etched, the etch will terminate on the etch stop layer 24.
  • the etch stop layer 24 may be a thin (400 nm to l ⁇ m) layer of silicon oxide (i.e. SiO 2 ), silicon nitride or another insulating material.
  • the third section is the device layer section 22.
  • the device layer section 22 comprises the cantilever material 20 and the piezoresistive film 18.
  • the cantilever material 20 may be a layer of a semiconductor material, such as undoped silicon, a compound semiconductor material or metal or semiconductor oxide, nitride or carbide, such as SiO 2 , Si 3 N 4 , SiC, etc.
  • layer 20 may comprise a 60-100 nm, such as 80 nm undoped silicon layer.
  • the piezoresistive film 18 may comprise a metal or doped semiconductor film, such as a 30-50 nm, for example a 30 nm p-doped (boron doped) Si layer (4 x 10 19 cm “2 ).
  • Layer 18 may be grown epitaxially on layer 20, by MBE, CVD, etc.
  • a polysilicon bilayer 18/20 may be used instead.
  • the bilayer can be made by plasma enhanced chemical vapor deposition (PECVD) with in-situ or subsequent doping.
  • PECVD plasma enhanced chemical vapor deposition
  • the device section 22 may comprise any suitable layer which can be formed into a functional cantilever.
  • Materials such as Si, SiC, SiN, or metal (i.e. Au, Al, Pt, etc.) can be used. If the material is transparent (SiC or SiN) to the laser's emission used for the optical transduction, a small (l ⁇ m x 1 ⁇ m to lO ⁇ m x lO ⁇ m wide, 30 nm thick) Au pad shown in Figure 14 can be fabricated at the end of the cantilever to maximize reflectivity of the cantilever.
  • FIG. 12A is a side cross- sectional view of the starting material shown in Figure 11.
  • the electrodes 30 for the cantilevers are formed on the device layer section 22, as shown in Figure 12B.
  • the electrodes 30 may be formed by depositing a metal or polysilicon layer and then photohthographically patterning it into a desired shape.
  • the electrodes 30 comprise ohmic contacts for the cantilever.
  • membranes 32 are formed, as shown in Figure 12C.
  • the membranes 32 are formed by masking the bottom of the substrate 28, etching the substrate until the etch stop layer 24 is reached, and then etching the etch stop layer with a different etching medium to stop on the device layer 22.
  • the substrate 28 may be etched using deep reactive ion etching (DRIE) or other dry etching methods and the etch stop layer 24 may be etched using wet etching, such as a buffered oxide wet etch.
  • An opening 20 is located under the membrane 32.
  • DRIE deep reactive ion etching
  • the support beams 46A, 46B and 48 shown in Figure 13 are fabricated in the same step depicted in Figure 12C by etching the substrate 28 and etch stop layer 24 to create the support beams at the same time as the membranes 32.
  • the support beams 46A, 46B and 48 temporarily connect the probe chip 42 to the remainder of the wafer 28 as shown in Figure 13.
  • Support beams 46 A, 46B and 48 bridge the thin membrane 32 from the wafer 28 and provide a full thickness element to provide mechanical connection strong enough to withstand normal wafer process handling.
  • supports 46A, 46B and 48 are not so large that they cannot be easily broken when desired by manual or robotic tweezer manipulation.
  • a cantilever etch mask 34 is photohthographically defined over each membrane 32.
  • the mask extends in part over wafer 28/electrodes 30 partially and over the membranes 32.
  • the cantilevers 10 are then defined using the etch mask 34 as shown in Figure 12E.
  • the exposed portions of the membranes may be etched away using any suitable etching method, such as electron beam patterning, for example.
  • Cantilever fabrication is completed with the removal of etch mask 34 as depicted in Figure 12F.
  • Figure 14 is a top plan view of a micrograph of the SPM probe.
  • the piezoresistive film is preferably positioned on legs 36 of the cantilever, as shown in Figure 14 and in Figures 5 A and 5B of the first embodiment.
  • Laying out a scanning probe using cantilever 10 with electrode legs forms a stress concentration region where the current passes through the most compliant area of the probe and serves to maximize the piezoresistive response with actuation.
  • the scanning probe is designed to include at least approximately a 1 ⁇ m reflective pad 38 near the tip of cantilever 10 as illustrated in Figure 14, in which case legs 36 do not need to contain the piezoresistive film.
  • a metal pad such as a 20 nm thick pad 38 of Au, is evaporated or sputtered onto the selected area near the probe tip. This reflective area 38 allows diffraction limited optical feedback to be utilized in commercial scanning probe microscope.
  • FIG. 13 shows electrodes 30 deployed on device layer section 22 and cantilevers 10 in the chips 42.
  • the device in Figure 13 is adapted for piezoresistive detection and pairs of electrodes 30 provide electrical paths for leads.
  • one chip 42 may contain two cantilevers, with one cantilever being the sensing cantilever and the other cantilever being a dummy cantilever. Movement of the cantilevers may be detected using the circuits of Figures 1 A or IB, for example. Other suitable circuits may also be used.
  • Figure 13 shows two release areas 44 adjacent to the chips 42.
  • the release areas enable removal of each chip or chips 42 from the wafer by any suitable means, such as manual or robotic tweezers or other removal tools.
  • the chips 42 may be separated from the substrate 28 without dicing the substrate 28.
  • Figure 13 also shows three support beams 46 A, 46B and 48 which support each chip 42 on the substrate 28 prior to trie removal of the chips from the substrate. However, two or more than three support beams may also be used. Figure 13 shows two support beams near the tip of the chip and one beam in the back of the chip. However, the beams may be arranged at other suitable locations as well. Thus, the number and location of the support beams shown in Figure 13 is exemplary and should not be considered limiting.
  • two support beams 46A and 46B are located near the tip of chip 42. These beams support and protect membrane 32 from tearing prior to defining the scanning probe cantilever 1O. Membrane 32 is the portion of device layer section 22 which is left unsupported when the underlying portions of the etch stop layer 24 and substrate 28 are removed.
  • a third support beam 48 is also connected to the substrate 28 and is used to support rear end of chip 42. Additional support beams 50A and 50B may be used to connect and support the release areas 44 to the substrate 28.
  • the support beams are strong enough to withstand the stresses and handling of subsequent wafer-scale processing of the chips while remaining compliant enough to be broken to allow chip removal from substrate 28. hi the illustrated embodiment, the support beams have the dimensions of 25 x 70 x 0.1 microns, but other dimensions could be employed as long as the functional requirements of support are satisfied.
  • each chip 42 and release area 44 is surrounded by a thin peripheral membrane 52.
  • the peripheral membrane may be formed during the same step as the device membrane 32, as shown in Figure 12C.
  • the peripheral membrane 52 may comprise the same material as the device membrane 32 (i.e., the device layer 22) and may have the same thickness as the device layer 22, such as 500 nm or less, for example 80 to 200 nm.
  • the thin peripheral membrane 52 also connects the chips 42 and release areas 44 to the substrate 28, but does not provide a sufficient amount of support to support the chips 42 on the substrate without the thicker support beams.
  • peripheral membrane 52 is omitted, and each chip 42 and release area 44 are surrounded by a gap instead of the peripheral membrane 52.
  • each chip 42 and release area 44 are connected to the rest of the substrate 28 only by the support beams, and each chip 42 has an island-like structure which is structurally connected to the rest of the substrate 28 only by the support beams.
  • each chip 42 and release area 44 are surrounded either by a thin peripheral membrane 52 (which provides little if any mechanical support to chips 42) or by a gap.
  • Release areas 44 can be manually or mechanically twisted or removed by tweezers or other instruments applied to release areas 44 to break support beams 50A and 50B.
  • the peripheral membrane 52 surrounding release areas 44 is also torn, but almost all of the mechanical coupling of the release areas 44 to substrate 28 is provided by the support beams 50A and 50B.
  • the tweezers or other instruments may be inserted into the gaps where the release areas 44 where previously located to grasp the chips 42.
  • the chip 42 removal from the substrate 28 may be achieved by manually or mechanically twisting or stressing supports 46A, 46B and 48 to break them.
  • the peripheral membrane 52 surrounding each chip 42 is also torn apart, but again with little mechanical resistance due to its low thickness.
  • Cantilever 10 which was etched from the device membrane 32 is sufficiently separated from the peripheral membrane 52 so that tearing or stress applied to the peripheral membrane 52 is well isolated and discomiected from cantilever 10.
  • the completed device is now ready to be installed in a scanning probe microscope or other suitable sensor device described in the first embodiment in the conventional manner.
  • Figure 15 shows a plot of power versus frequency for cantilevers 10 fabricated according to the second embodiment for optically detected signals at room temperature in a vacuum.
  • Figure 16 shows a plot of S v (i.e., power/frequency) versus frequency for cantilevers 10 fabricated according to the second embodiment for piezoresistively detected signals at a cryogenic temperature of 5°K.
  • Figure 15 shows a cantilever with a Q factor of 218 and a resonance peak at 27.3 kHz while Figure 16 shows a cantilever with a Q factor of 13,231 and a resonance peak at about 620.22 kHz.

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  • Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
  • Radiology & Medical Imaging (AREA)
  • Micromachines (AREA)
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Abstract

L'invention concerne un système contenant un dispositif micromécanique ou nanomécanique et une méthode de fonctionnement de ce système. Le dispositif de l'invention comprend un résonateur et un élément piézorésistant relié à ce résonateur. La méthode de l'invention consiste à orienter en CA l'élément piézorésistant à une première fréquence, piloter le résonateur à une seconde fréquence, différente de la première fréquence, et détecter une réponse mécanique du résonateur à une fréquence différente ou aux deux fréquences différentes, et additionner cette fréquence à la première fréquence et à la seconde fréquence.
PCT/US2005/005597 2003-05-07 2005-02-24 Detection d'un deplacement de resonateur faisant appel a un melangeur abaisseur de signal piezoresistant WO2005081929A2 (fr)

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US54716804P 2004-02-25 2004-02-25
US60/547,168 2004-02-25
US56265204P 2004-04-15 2004-04-15
US60/562,652 2004-04-15
US11/010,578 2004-12-14
US11/010,578 US7434476B2 (en) 2003-05-07 2004-12-14 Metallic thin film piezoresistive transduction in micromechanical and nanomechanical devices and its application in self-sensing SPM probes

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2793399A1 (fr) 2013-04-17 2014-10-22 Commissariat A L'energie Atomique Et Aux Energies Alternatives Circuit de mésure de frequence de resonance de nano-résonateurs
FR3050820A1 (fr) * 2016-04-29 2017-11-03 Commissariat Energie Atomique Systeme de mesure resonant a resolution amelioree

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US5804709A (en) * 1995-02-07 1998-09-08 International Business Machines Corporation Cantilever deflection sensor and use thereof
US5839062A (en) * 1994-03-18 1998-11-17 The Regents Of The University Of California Mixing, modulation and demodulation via electromechanical resonators

Patent Citations (2)

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Publication number Priority date Publication date Assignee Title
US5839062A (en) * 1994-03-18 1998-11-17 The Regents Of The University Of California Mixing, modulation and demodulation via electromechanical resonators
US5804709A (en) * 1995-02-07 1998-09-08 International Business Machines Corporation Cantilever deflection sensor and use thereof

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2793399A1 (fr) 2013-04-17 2014-10-22 Commissariat A L'energie Atomique Et Aux Energies Alternatives Circuit de mésure de frequence de resonance de nano-résonateurs
US9100025B2 (en) 2013-04-17 2015-08-04 Commissariat A L'energie Atomique Et Aux Energies Alternatives Circuit for measuring the resonant frequency of nanoresonators
FR3050820A1 (fr) * 2016-04-29 2017-11-03 Commissariat Energie Atomique Systeme de mesure resonant a resolution amelioree
EP3244169A1 (fr) * 2016-04-29 2017-11-15 Commissariat à l'énergie atomique et aux énergies alternatives Systeme de mesure resonant a resolution amelioree
US10416003B2 (en) 2016-04-29 2019-09-17 Commissariat A L'energie Atomique Et Aux Energies Alternatives Resonating measurement system using improved resolution

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