KR20070011433A - Metallic thin film piezoresistive transduction in micromechanical and nanomechanical devices and its application in self-sensing spm probes - Google Patents

Abstract

Thin metallic films are used as the piezoresistive self-sensing element in microelectromechanical and nanoelectromechanical systems. The specific application to AFM probes is demonstrated. ® KIPO & WIPO 2007

Description

METALLIC THIN FILM PIEZORESISTIVE TRANSDUCTION IN MICROMECHANICAL AND NANOMECHANICAL DEVICES AND ITS APPLICATION IN SELF-SENSING SPM PROBES }

Cross Reference of Related Application

This application claims the priority on US Provisional Patent Application No. 60 / 468,452, filed May 7, 2003, with partial pending US Patent Application No. 10 / 826,007, filed on April 16, 2004. Part of the call is filed. This application also claims priority to US Provisional Patent Application No. 60 / 562,652, filed April 15, 2004. The entire contents of all these applications are incorporated herein by reference.

The present invention relates to piezo-resistive sensors for micro electro mechanical systems (MEMS) and nano electro mechanical systems (NEMS).

Since electromechanical systems (MEMS) and nano electromechanical systems (NEMS) are fully integrated and easy to use, piezo-resistive displacement detection technology is used in both electromechanical systems (MEMS) and nano electromechanical systems (NEMS). It is getting attention. The applications include inertial sensors such as scanning probe microscopes, force and pressure sensors, flow sensors, chemical and biometric sensors, accelerometers and motion transducers. These applications mostly include p-type doped silicon layers as sensing elements. Doped silicon has a very high gauge factor 20-100, but also has a high sheet resistance (10 mA / square), and therefore has a relatively large thermal noise floor. Also, the 1 / f noise is expected to be higher because of the small carrier density. In addition, the manufacturing process of semiconductor piezo resistors such as ion implantation or molecular beam epitaxy is expensive and very complicated. Finally, semiconductor materials are susceptible to damage during processing. Thus, these materials are not suitable for use in nanoscale sizes.

In order to solve such incompatibility, piezo-resistive sensing elements for electro-mechanical systems and nano-electro-mechanical systems with higher sensitivity and lower manufacturing costs are required.

One embodiment of the present invention provides a micromechanical device and a nanomechanical device including a movable element and a metal thin film used to sense movement of the movable element in a piezoelectric resistance manner.

1A and 1B show SEM images of exemplary devices in accordance with embodiments of the present invention.

2 and 3 show the resonance response curves of the device shown in FIG. 1A.

4 shows the thermodynamic noise spectral density of the device shown in FIG. 1B. Two modes are shown. The data is tailored to the Lorentz function.

5A shows a three-dimensional schematic diagram of a device in accordance with embodiments of the present invention.

5B shows an SEM image of an AFM probe in accordance with embodiments of the present invention.

7, 14 and 15 show schematic diagrams for testing setups used to test a device of embodiments of the present invention.

6 and 8 show plots of piezoresistive responses of devices in accordance with embodiments of the present invention.

9 shows a noise spectral plot from a metal thin film piezo resistor.

FIG. 10 is a plot showing force indentation of a piezoresistive probe. FIG.

FIG. 11A shows a 3D topography image obtained from a direct tapping mode AFM.

11B shows a 3D topography image obtained from a lock-in measurement of a metal thin film piezo resistor.

12A-12H are side cross-sectional views illustrating steps in a manufacturing process flow for a self-sensing non-contact / tapping mode piezoresistive SPM probe.

13A-13H are side cross-sectional views illustrating steps in the manufacturing process flow for a self-sensing contact mode piezoresistive SPM probe.

In an embodiment of the present invention, a metal thin film used as a piezo-resistive self-sensing element in a micromechanical system and a nanomechanical system is disclosed. These systems preferably include micro electro systems and nano electro systems.

Micro-electro and nano-electro systems include devices comprising features having sizes in the range 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. Preferably, these features include movable features or elements such as cantilevers, diaphragms, clamped beams, wires and the like. Micro electro mechanical systems and nano electric mechanical systems include, but are not limited to, scanning probe microscopes ("SPM"), force and pressure sensors, flow sensors, chemical and biological sensors, and accelerometers, such as atomic force microscopy ("AFM"). And inertial sensors such as motion transducers. For example, chemical and biological sensors may include one or more cantilevers with surfaces coated with materials that selectively bind to chemical or biological assays (ie, gas or liquid assays that include or consist of the species or biological species of interest). It may include.

The term "thin film" includes a relatively thin metal film having a thickness of about 100 nm to about 10 microns, as described below, a metal film having a thickness of about 10 nm to about 100 nm, and a thickness of less than about 10 nm. It may include an ultra-thin film, such as a metal film of a non-retardant or island form having a. The term "metal" includes pure or essentially pure metals or metal alloys.

The term "self-sensing" does not require a piezoelectric piezoelectric sensor, such as a photodetector used to detect reflected radiation that determines deflection of the cantilever or diaframe and an external motion sensing device such as a laser used to irradiate the cantilever or diaframe with radiation. It means that a metal thin film is used as a resistive element. However, if desired, an external motion sensing device may be used in combination with a self sensing metal thin film. In addition, the metal thin film may be provided with a current source providing a current across the metal thin film and a voltage across the metal thin film to determine the amount of movement (ie, wavelength, frequency, direction and / or any other characteristic) of the metal thin film and the movable element. And a detector for detecting.

Strain gauge factor (“gauge factor”) is often used to represent the electrical resistance and strain relationship of piezo-resistive materials, which is defined as dR / Rdε = (1 + 2ν) + dρ / ρdε, where ν is Poisson's ratio, ρ is resistivity, ε is strain and R is resistance. The first term is simply derived from geometric deformation, and the second term is the term representing the physical change in resistivity due to strain, which is simply generated by the strain propagated in the average free path of the material.

Even when the metal thin films have gauge factors 1 to 4 which are much lower than semiconductor piezo resistors, they can have a low resistivity and produce higher current densities, so that significant signal strength can be obtained. Low resistivity results in low thermal noise. Since metal thin films have a higher carrier density on the order of several orders of magnitude than semiconductor piezo resistors, their 1 / f noise is considerably lower. The lower the noise floor, the more similar the resolution of the silicon device.

Compared with semiconductor piezo resistors, metal thin film piezo resistors can be manufactured at a significantly reduced cost. Metal films of 10 nm to 10 microns thick can be simply deposited or sputtered on any substrate, such as Si, SiC, SiN, Si02, glass and even plastic materials. The damage to a metal thin film can also be minimized. Metal thin films can also be patterned down to nanoscale size and fabricated on a device in large arrays.

In a preferred embodiment of the present invention, a gold thin film is used for the piezo-resistive sensing element. Other pure or essentially pure metals including, but not limited to, Ag, Ni, Pt, Al, Cr, Pd, W, and Constantan, Karma, Isoelastic, Nichrome Similar results can be obtained with metal alloys such as (Nichrome) V, Pt-W, Pt-Cr and the like.

In another preferred embodiment, metal thin films are used to coat movable elements, such as micron (eg, 1-100 micron) or nanometer (eg, 1 micron or less) size devices. The movable element can be any element in a movable MEMS or NEMS. Preferably, the movable element may comprise a flexible resilient element (eg, a "flexture"), such as a resonator.

For example, the resonator may preferably include micron or nanometer sized cantilevers. However, the present invention is not limited to these, but may be used in other resonators including double clamped beam resonators, torsional resonators and diaphragm resonators. Examples of double clamped beam resonators, torsional resonators and diaphragm resonators are described in US Patent Application No. 10 / 826,007, US Patent Publication No. 6,593,731 and PCT Application PCT / US03 / 14566 (WO / 2004/041998). Publication) and their corresponding US patent application Ser. No. 10 / 502,641, all of which are incorporated herein by reference in their entirety. For example, a double clamped beam resonator is fixed at both ends but includes a beam that is freely suspended so that it can move or bend vertically with respect to its length. In one example, as described and disclosed in US Pat. No. 6,593,731, a torsional resonator is disposed at two vertices and is of flexible diamond or polygonal shape that can be twisted or rotated about an axis between the vertices. It can contain a structure. The diaphragm resonator may include any plate-shaped resonator that is fixed to one or more edges and whose middle portion is freely suspended so that it can move or bend in one or more directions. One example of a diaphragm resonator is a trampoline resonator.

In another preferred embodiment of the present invention, the use of a metal thin film as a sensor in an AFM probe has been described. Processing is provided to fabricate both self-contact sensing / tapping mode piezoresistive SPM probes and contact sensing mode piezoresistive SPM probes. As described above, metal thin film piezo-resistive sensing elements can also be used in force and pressure sensors, flow sensors, chemical and biochemical sensors, inertial sensors such as accelerometers and motion transducers, and other MEMS or NEMS devices. . The example device has a displacement sensitivity estimated at 1.6 × 10 ⁻⁶ / nm or more, a force resolution of at least 3.8 fN / √Hz equivalent to a doped silicon counterpart and a noise level of less than 1 nV / √Hz.

Finally, a highly sensitive electronic downmix readout scheme is employed to extract the piezoelectric resistance response from the metal thin film. In a preferred embodiment, this detection scheme and appropriate circuit (s) are used for self-sensing metallic piezo resistor probes for contact mode and non-contact mode AFM operation.

Metal thin film

The metal thin film has a slightly lower gauge factor (2-4). Since they have a very low resistance and can produce a higher current density than semiconductor piezo resistors, they can produce signal strength equivalent to semiconductor piezo resistors. The lower the resistance, the less thermal noise. Since metal thin films have carrier densities that are orders of magnitude higher than semiconductor piezo resistors, their 1 / f noise is significantly lower. In devices that operate at the resonant frequency and perform ac measurements, the metal thin film has high sensitivity.

Metal thin film piezo resistors can be manufactured at significantly reduced cost. The metal thin film can be simply deposited or sputtered on almost any substrate. The processing damage to the metal thin film is also minimized. Metal thin films can be scaled down to nanoscale sizes and easily patterned and fabricated on devices in large arrays.

In a preferred embodiment, a thin film of gold is used as the piezoelectric resistance sensing element. For bulk gold, v is 0.42 and typical gauge factors are 1-4. The gold thin film can be divided into three different regions depending on the thin film thickness. Films with a thickness of more than 100 nm are close to bulk. Films with a thickness of 10 nm to 100 nm have a continuous thin film section. At thicknesses below 10 nm, thin films often have discontinuous sections. These discrete films have very large strain gauge factors due to the nature of the metal island gaps. In the above-described embodiment, a thick film of 30 nm-50 nm thick gold, which proceeds into the continuous thin film region, is used as the piezo resistor layer. However, the size of the metal thin film can be changed considerably. For example, in all successive gold thin films from 30 nm gold thin films up to 10 micron thin films, the piezo-resistance response is about the same magnitude based on the measurement results. The metal thin film can have any suitable length and width. For example, the metal thin film may be a narrow wire (a wire having a cross-sectional area of about 100 nm or less), may cover some or all of the surface of the movable element of the device, and may be about 100 nm, such as 200 nm to 2 microns. Can have a width of up to 10 microns.

In addition to gold, a broad group of pure or essentially pure metals includes, but is not limited to, nickel, platinum, palladium, tungsten, aluminum, and the like, and can be used for piezoelectric resistance sensing elements. Metal alloys including, but not limited to, constantan, karma, isoelastic nichrome V, Pt-W, and Pd-Cr can also be used for piezo-resistive sensing elements. The table below lists examples of some metals and gauge factors for some of these metals.

TABLE 1

Resonator

As mentioned above, preferred resonator structures include cantilevers such as cantilevered NEMS or MEMS structures. SEM images of two exemplary cantilevers are shown in FIGS. La and 1b. La is a SEM image of a cantilever 10 μm long and 2 μm wide at f ₀ = 1.5 MHz. 1B is an SEM image of a 33 μm long and 4 μm wide cantilever at f ₀ = 52 KHz. The device has a final resistance of 150 ohms. As shown in FIGS. La and 1b, the cantilever 1 preferably comprises an opening or notch 3 near the cantilever base 5 comprising a contact pad. A portion of the cantilever 1 surrounding the notch 3 is referred to as a "leg" 7. The gold thin film 9 is preferably formed at least on the leg 7 portion of the cantilever. If necessary, the notch 3 may be omitted.

Preferably, the cantilevered NEMS structure shown in FIGS. La and 1b is described in Appl. Phys. Lett. 78,162 (2001), manufactured using the method disclosed herein, the contents of which are incorporated by reference. An exemplary method is described below. The starting material is silicon carbide epitaxially grown to 80 nm thickness on the silicon wafer. Silicon carbide is chosen by convenience rather than by necessity. For example, silicon and other components such as silicon nitride and silicon oxide may be used instead. First, a gold contact pad is formed by photolithography. Secondly, a strain concentrating leg such as leg 7 shown in Fig. 1A is formed by a metal wiring pattern and then drawn by electron beam lithography. A 1 nm chromium adhesive layer is followed by a heat deposition of a 30 nm gold layer on the photoresist pattern located on the silicon carbide, followed by lift off to form the desired pattern. In subsequent electron beam lithography, a 50 nm chromium layer is patterned as an etch mask over the entire cantilever formation region using a 1: 1 Ar: NF ₃ mixed gas. Thereafter, the sample is etched with an electron cyclone reaction (ECR) etch. A 250V bias voltage is used to anisotropically etch the SiC layer. Thereafter, the bias voltage is reduced to 100V, which enables isotropic etching. After the silicon layer under the device cantilever is etched, the silicon carbide structure is revealed. The etching ends when the cantilever is undercut to the extent that it is exposed. The chrome mask is removed by wet etching to reduce the stress on the cantilever, which causes the cantilever to roll up. Finally, a very short ECR dry etch is used to reveal the entire cantilever. Thereafter, the sample is adhered on the piezoelectric ceramic (PZT) actuator, the entire device is mounted on the chip carrier, and electrical connection is made by wire bonding.

Thereafter, the sample is loaded into a vacuum tank and the measurement is performed at room temperature. The measurement system includes a battery set such as a 1/2 dc bridge, a dc bias source and an ac-dc bias tee. The PZT actuator is driven by the output of the network analyzer. After a 67 dB gain, 50 Hz input and output two stage impedance preamplifier, the ac portion of the signal is fed back by the network analyzer.

2 and 3, the resonance signal of the device of FIG. 1A is shown at different drive levels with a constant dc bias of 50 mV. The cantilever includes two legs 5 μm long and 500 nm wide and pads 5 μm long and 2 μm wide. This cantilever has a fundamental resonant frequency at 1.5 MHz and a second resonant mode at 14.8 MHz. 2 shows a resonance curve in the basic resonance mode, and FIG. 3 shows a resonance curve in the second resonance mode. 2 and 3 show peak amplitudes as a function of operating level or operating amplitude. In the linear response region, the amplitude at the resonant frequency is proportional to the amplitude of the ac signal applied to the piezoelectric actuator. In the basic resonant mode, the cantilever has a Q factor (quality factor) that is 1000 in vacuum. The cantilever also operates at a low Q factor of 90 in air, as shown by the dashed line in FIG. 2. The second resonant mode has a Q factor of approximately 700.

These piezoresistive cantilevers have sufficient sensitivity to detect their thermoelectric noise. 4 shows the noise spectrum of the device shown in FIG. 1B, which has a 52 KHz fundamental frequency and a second frequency mode at 638 KHz. From the noise spectral density and using the calculated spring constant, the sensitivity of the cantilever can be corrected. By applying the energy equalization law, K <z ² > = k _b T to the cantilever potential energy, we can define the sensitivity in the static state as C _s = ΔR / RΔz, where K is the spring constant and k _b Is a Boltzmann constant, T is an absolute temperature, <z ² > is the average square displacement change of the cantilever, ΔR / R is the resistance change ratio of the piezo resistor, and Δz is the cantilever static displacement. By integrating the noise spectral density curve, the expected value can be obtained to obtain <ν ² > = C _s ² <z ² >, where <ν ² > is the mean square voltage variation. From the device geometry and the elastic properties of SiC and gold, the spring constant K ≒ 0.0024 N / m can be calculated. The sensitivity is determined to be C _s = 1.6 × 10 ⁻⁷ / kHz. This measurement has a noise floor of 3 nV / √Hz at 52 KHz and 1.4 nV / √Hz at 638 KHz and is limited by Johnson and amplifier noise. The cantilever has a force resolution of 3.8 fN / √Hz at 500 KHz. The sensitivity of the device is relatively low, but due to the low noise floor, sufficient resolution can be obtained.

The geometry of the device shown in FIG. 1B can be modified to further enhance the force resolution or displacement resolution. In the device of FIG. 1B, only the strain sensing element 9 occupies one tenth of the length of the cantilever. Given a spring constant, the force sensitivity is proportional to l / t ² , so the thinner and shorter the cantilever, the better the force sensitivity can be obtained. Doped silicon devices can also receive large 1 / f noise. Hooge's law indicates that 1 / f noise is inversely proportional to the total number of carriers, so that the smaller the cantilever, the worse the noise performance. In the case of a metal thin film, the number of carriers (˜10 ²² / cm ³ ) is about 4 orders larger than a typical doped semiconductor (˜10 ¹⁸ / cm ³ ). 1 / f noise is not a limited problem.

In addition to cantilevers, metal thin films can also be used as another geometry. Accordingly, metal thin film piezo-resistive sensors may be used with, but not limited to, other resonators such as double clamped beams, torsional resonators, and diaphragm resonators.

Device application example

5A and 5B show examples of the use of metal thin films in self-sensing cantilever probers for atomic force microscopy. FIG. 5A is a schematic diagram and FIG. 5B shows an SEM image of a micromachining cantilever (ie, probe) 11 having a microscale size of 150 μm length × 30 μm width × 4 μm thickness. The probe 11 includes a cantilever 1, notches 3, a base 5, a leg 7, a metal thin film 9 and an AFM tip 13. Preferably, but not necessarily, the metal thin film 9 is formed on the same side of the cantilever as the tip 13. The particular probe 11 shown in FIG. 5 is designed for tapping mode AFM. As will be described later, the probe for contact mode AFM can also be designed preferably, although the spring constant is very small.

12 and 13 show respective manufacturing processes for both the non-contact mode probe probe 11 and the contact mode probe 21. As shown in Figs. 12A to 12H, the method of manufacturing the non-contact / tapping mode piezo-resistive SPM probe 11 into the metal thin film 9 includes the following steps.

First, as shown in FIG. 12A, the starting substrate 101 is prepared. This substrate can be any suitable silicon on insulator (SOI) substrate, such as, for example, a SIMOX (ie oxygen injected) SOI substrate or a UNIBOND (ie bonded) SOI substrate. The substrate may have a thickness of 400 to 900 microns, such as 550 microns thick with an oxide layer 103 of 0.5 to 5 microns thick between the two silicon portions 105, 107.

Then, as shown in FIG. 12B, masking layers 109 and 111, which may be made of any material available as a mask for silicon etching, are deposited on both sides of the substrate 101. As shown in FIG. For example, masking layers 109 and 111 may comprise a silicon nitride layer deposited by LPCVD at a thickness of 400 to 2000 angstroms, such as 550 angstroms thick. Another material may be used, such as silicon oxynitride or aluminum oxide.

As shown in FIG. 12C, the masking layer 111 is patterned using photolithography (ie, applying / spin coating a photoresist on the masking layer, then baking the photoresist and optionally After exposure, the photoresist is patterned and the mask layer is selectively etched). In more detail, crystal axis exposure pit 113 and cantilever region opening 115 are formed in layer 111 and then extended to silicon portion 107 of substrate 101. The pit 113 may be formed by KOH pit etching and the opening 115 may be formed by reactive ion etching the layer 111 using a photoresist mask.

Thereafter, as shown in Fig. 12D, a tip mask is formed. Preferably, tip mask 117 is formed by photomasking patterning masking layer 109 to leave tip mask 117. For example, portions of the layer 109 not covered by the patterned photoresist layer may be subjected to reactive ion etching to form the mask 117. Preferably, but not necessarily, the photoresist used in this step may be removed before the tip etch step.

Thereafter, as shown in Fig. 12E, a tip mask 117 is used for the tip etching step. The tip 13 is formed by etching the silicon portion 105 using the tip mask 117. For example, silicon 105 may be subjected to anisotropic etching with KOH and an oxidation / HF etching cycle to form tip tip 13. In this step, the silicon portion 105 of the substrate 101 is processed thinner so that its thickness is approximately equal to the desired cantilever 1 thickness.

Thereafter, as shown in Fig. 12F, a metal pad and a metal piezoelectric resistive film 9 are formed. Preferably, a metal film such as 30 to 70 nm, for example a 50 nm thick Au layer, is formed on the silicon portion 105 adjacent to the tip tip 13 (ie, the metal film is formed in front of the substrate or on the substrate). It is preferably formed in the shortest side of the layer). The metal pads and film 9 can be patterned into the desired shape using a photolithography process. Alternatively, the metal pad and film 9 deposit a metal on the photoresist pattern and then lift off the photoresist pattern to leave the patterned metal on the silicon portion 105 of the substrate 101. It may be patterned by a liftoff method.

Then, as shown in FIG. 12G, the cantilever 1 is patterned using photolithography. For example, the silicon portion 105 of the substrate 101 may be patterned using RIE or wet etching.

Thereafter, as shown in FIG. 12H, the backside of the substrate 101 is etched to expose the cantilever 1. This is, for example, KOH etching the backside silicon portion 107 of the substrate 101 through the opening 115 in the masking layer 111 and then HF etching to remove the oxide layer 103 under the cantilever 1. Is performed.

Of course, other materials and etching methods / mediators may be used. In addition, the photoresist layer may be removed immediately after the etching step or after time. For example, the photoresist used to form the opening 115 may be removed as soon as the opening 115 is formed and may be removed after the step shown in FIG. 12H.

13A to 13H show a method of forming the contact mode piezo-resistive SPM probe 21. In FIGS. 13A-H, the front face of the probe is shown somewhat lower than the top face of the probe. Of course, "top" and "bottom" are terms of relative concept that depend on how the probe is positioned and are used only to describe the components in the figures.

First, as shown in FIG. 13A, the starting substrate 101 is prepared. This substrate may be any suitable semiconductor or insulator substrate, for example a silicon wafer. Thus, SOI substrates do not necessarily need to be used in this method. The wafer has the same thickness as the SOI substrate in FIG. 12A.

Thereafter, as shown in Fig. 13B, masking layers 109 and 111, which can be formed of any material that can be used as a mask for silicon etching, are deposited on both sides of the substrate 101. For example, masking layers 109 and 111 may be silicon nitride layers deposited by LPCVD at low stresses of 800 to 1500 angstroms thick, such as 1000 angstroms thick. It is also possible to use another material such as silicon oxynitride or aluminum oxide.

Thereafter, as shown in FIG. 13C, the backside masking layer 111 is patterned using photolithography to form an alignment hole 114 extending to the backside of the substrate 101. This hole 114 reactively etches the patterning layer 111 using a photoresist mask, followed by a patterning layer 111 and optionally a photoresist (if the photoresist has not yet been removed). Can be formed by KOH etching the substrate 101. KOH etching may include, for example, etching using a 30% KOH solution at 60 ° C.

Thereafter, as shown in FIG. 13D, a membrane mask is formed in the layer 111. In more detail, a membrane opening 116 extending to the substrate is formed in layer 111 using a photolithographic method. Opening 116 is reactive ion etched layer 111 using a photoresist mask, followed by patterning layer 111 and optionally photoresist (if photoresist has not yet been removed) as mask. It can be formed by KOH etching the substrate 101. KOH etching of the substrate deepens the hole 114 until it extends to the front masking layer 109.

Thereafter, as shown in Fig. 13E, a tip mask is formed. Preferably, the tip mask 117 is formed by patterning the masking layer 109 in a photolithographic manner, leaving the tip mask 117. For example, portions of layer 109 not covered by the patterned photoresist layer may be subjected to reactive ion etching to form mask 117. Preferably, but not necessarily, the photoresist used in this step is removed before the tip etch step. E-beam lithographic alignment masks may also be formed during this step.

Thereafter, as shown in Fig. 13F, the tip mask 117 is used for the tip etching step. The tip 13 is formed by etching the front side of the substrate using the tip mask 117. For example, the silicon substrate 101 may be anisotropically etched using KOH. Thereafter, the remaining masking layer 111 is preferably removed.

Then, as shown in FIG. 13G, an optional low pressure silicon nitride layer 118 is formed on the front side of the substrate 101 and above the tip 13, whereby the tip surface is coated with silicon nitride. It is also possible to use another coating material. Thereafter, a metal pad and a metal piezoelectric resistive film 9 are preferably formed just in front of the layer 118 on the front surface of the substrate 101. The metal thin film 9 may be the same as the film 9 shown in FIG. 12F. The metal thin film 9 can be patterned using any suitable method, for example electron beam lithography.

Thereafter, as shown in FIG. 13H, the backside of the substrate 101 is etched to expose the cantilever 1. This can be done, for example, by KOH etching the back side of the substrate through the opening 116 in the masking layer 111.

In addition, other materials and etching methods / mediators may be used. In addition, the photoresist layer may be removed immediately after the etching step or after time.

Thus, a general method of manufacturing the probe 11 and the probe 21 includes preparing a substrate, forming a mask layer on the front and back surfaces of the substrate; Patterning the mask layer; Forming an SPM tip on the front side of the substrate using a mask layer patterned on the front side of the substrate; Forming a piezo-resistive sensor of the patterned metal thin film in front of the substrate; Etching the substrate from the back through the opening of the back masking layer to form a cantilever supporting the SPM tip and the metal thin film.

Piezoresistive Response Example

FIG. 6 illustrates the piezoelectric resistance response of the metal thin film on the cantilever in the same manner as in FIG. 5B according to another example of the present invention. The cantilever has a conventional tip and is 125 microns long, 40 microns wide and 4 microns thick. This cantilever is suitable for a self-sensing probe 11 designed for tapping mode AFM applications (ie, this cantilever is a MEMS device that is somewhat larger than the NEMS device shown in FIG. 1A). The gold thin film covers the two legs of the cantilever and forms a current loop. As shown in Fig. 6, a very strong piezoelectric resistance response is observed. The non-resonant background signal is extracted from raw data. The Q factor for this particular cantilever is about 220 in air. Under vacuum conditions, the piezoelectric resistance response becomes stronger and the Q factor rises to 360 or more. The data in the vacuum is shifted to the right for a more visible inspection.

Measurement circuit set-up for this experiment is shown in FIGS. 14 and 15. 14 illustrates a method of measuring the piezoelectric resistance response of an SPM probe when the probe is used in contact mode AFM. In one embodiment shown in FIG. 14, an ac bias current passes through the piezo resistor and the ac voltage is measured to improve measurement sensitivity. The scheme in FIG. 14 provides a simple bridge resistance measurement scheme. In addition, the DC measurement can be directly adopted to detect the warp of the cantilever. A lock-in at a fixed frequency (eg 20 KHz) may be employed to enhance measurement sensitivity.

FIG. 15 illustrates a method of measuring piezoelectric resistance response of an SPM probe when the probe is used for tapping / contactless / ac mode AFM. The bias current through the piezo resistor is modulated at one frequency while the cantilever is driven at another frequency. The mechanical response of the cantilever is detected at these and other frequencies or at the sum of them.

Therefore, as shown in FIG. 15, the cantilever 1 is driven by the piezoelectric drive source in the base 5 using an ac drive source. The drive source can be synchronized with the ac bias source and the outputs of the ac drive source and the bias source are provided as individual inputs of the mixer. The output of the mixer is provided as a reference signal to the lock-in amplifier through a low pass filter (LPF). The metal thin film 9 is biased using an ac bias source, the output of which is also provided to the lock-in amplifier via another LPF and amplifier. In the scheme shown in FIG. 15, the cantilever can be driven at a resonant frequency, for example 240 KHz, using an ac drive source. A lock-in direct measurement can be adopted to detect the amplitude of oscillation. To remove the electrical background signal due to crosstalk, the example shown above employs a down mixing detection scheme, which is described in detail below. The sample bias current can be applied at resonant frequencies above 10-50 KHz, such as 20 KHz higher than the drive frequency (eg 260 KHz at 240 KHz drive frequency). Lock in measurements can be performed, for example, at 20 KHz or 500 KHz (see US Provisional Patent Application No. 60 / 562,652, the contents of which are incorporated by reference in further detail).

The probe 11 can then be tested using an AFM system (DI Dimension 3100 system) equipped with a signal access module for external signal access and control. Measurement set-up is outlined in FIG. 7. The standard DI probe holder can be modified to facilitate the testing of metal piezoresistive probes. First, the electrical connection from the chip holder to the AFM head stage is disconnected. Secondly, the four wires 31, 33, 35, 37 are soldered to the chip holder to enable electrical connection to the piezoelectric actuator 5 under the probe 11 and two electrical contact pads on the self sensing probe ( 39, 41 to the electrical connection. A drive signal is applied to the piezoelectric actuator 5 through an external function generator 43 (Stanford Research System DS345). The dc bias voltage is applied between the two legs 7 of the cantilever 1. Resonance ac signal is extracted using a resistor 43 having a resistance value similar to that of the cantilever as the balance resistor (20.3 Ohms). The voltage variation across the probe 11 is further amplified by the low noise voltage amplifier 45 (Stanford Research System SR560). This oscillated ac voltage is then supplied to a lock-in amplifier 47 (Stanford Research System SR830). The measurement is locked with the drive signal provided by the function generator. The x-output of the lock-in amplifier is fed to one input channel of the nanoscope controller through a signal access module after the phase extender box.

Resonance curves from electrical measurements are first obtained. 8 shows three resonance curves obtained from the same cantilever. Trace 1 (bottom curve) is from the AFM embedded laser deflection measurement unit. Trace 2 (middle curve) shows the direct electrical lock-in measurement of the cantilever using dc bias current. Trace 3 (top curve) shows an improved measurement result with the ac bias current (downmixing method described above). Significant signal strength is observed in all three curves. In the optical data, the sidebands due to unstrained resonances are clearly shown. They do not appear in the electrical measurement curves. Electrical measurements are not affected by the shear strain movements shown in the optical measurement data. The comparison between trace 2 and trace 3 shows that the downmixing scheme can effectively eliminate crosstalk signals that are typically unavoidable in measurement.

Thereafter, as shown in FIG. 9, noise spectrum measurement is performed on the metal thin film probe. The noise spectrum is observed to be very low. At frequencies below 1000 Hz, the noise level is below l nV / √Hz, which is less than the Johnson noise generated by a 50 Ohm resistor at room temperature. Typically, in the same frequency range, the noise level in p + silicon is about 30 nV / √Hz. The noise performance of metal piezo resistors is about 30 times better than piezo resistors made of semiconductor Si material.

In contact mode AFM, the cantilever does not operate at the resonant frequency, but depends on the geometry of the sample surface. The cantilever's DC or quasi-DC response is of utmost concern for contact mode operation. AFM force indentation is employed to change the piezoelectric probe in the Z direction in a range after the cantilever is in contact with the sample surface. The cantilever is therefore bent at the modulation frequency. The modulated piezo resistor signal is picked up by a wideband dc amplifier and measured by an oscilloscope. Data for force indentation of the probe is shown in FIG. 10 (right axis). The voltage applied in the z direction of the piezoelectric tube is also shown in FIG. 10 (left axis) for comparison. This voltage modulation corresponds to 300 nm bending amplification. 55 V signal amplification is observed across the piezo resistor probe. This corresponds to 0.88 mV / nm after 2000 times voltage amplification. In shared standard optical AFM detection, the response is ˜20 mV / nm. If the exemplary piezo resistor is provided with exemplary low noise, a 30 times higher gain amplifier can be obtained and can operate on a significant noise floor with a signal response of 26.4 mV / nm, matching the performance of the optical cantilever.

Standard SPM calibration gratings can be employed to illustrate the imaging capabilities of the exemplary metal thin film probe. The grating is a one dimensional array of rectangular SiO ₂ steps on a silicon wafer with a 3 micron pitch. The height of the step is 20 nm ± 1 nm. The image of the geometry shown in FIG. 11B is obtained by monitoring the output of the lock-in amplifier when the AFM is operating in “lift mode”. For comparison, an optical tapping mode AFM image is shown in FIG. 11A. Even when there is no need to condition the signal, metal thin film piezo resistors produce a very low signal to noise ratio. Image quality corresponds to image quality as a result of optical measurement. Thus, an SPM such as AFM may be used to determine and / or image the properties of the surface irradiated by the AFM probes 11, 21 based on the piezoelectric resistance response of the metal thin film 9. That is, the AFM probe may be used to image the surface of the material shown in FIG. 11B or to determine one or more characteristics of the material surface when performed with FM. In addition, although not shown in the figure, signals are processed from an AFM probe and related devices such as a lock-in amplifier using a data processing device such as a computer or a dedicated processor to generate and store images and / or data corresponding to surface characteristics. And / or display. The metal thin film has been described above as a preferred piezo-resistive film. However, other piezo-resistive material films may be used instead. For example, a piezo-resistive semiconductor thin film, such as a doped silicon thin film, for example a p-type doped silicon thin film, may be formed on the resonator surface and used to detect movement of the resonator.

The foregoing description of the invention has been presented for the purposes of illustration and description. The details disclosed are not provided to limit or limit the invention, and variations and modifications may be made or practiced in these teachings. The contents of the detailed description are chosen to explain the principles of the invention and its application. The scope of the invention is defined by the claims and their equivalents.

Claims (28)

A micromechanical or nanomechanical device comprising a movable element and a metal thin film used for sensing movement of the movable element in a piezoelectric resistance manner. The micromechanical or nanomechanical device of claim 1, wherein the movable element comprises a resonator. 3. The micromechanical or nanomechanical device of claim 2 wherein the resonator comprises a cantilever. 4. The micromechanical or nanomechanical device of claim 3, wherein the cantilever comprises a semiconductor material. The micromechanical or nanomechanical device of claim 3, wherein the metal thin film is coated on the surface of the cantilever. 6. The cantilever of claim 5, wherein the cantilever includes a notch and leg portions surrounding the notch, And the metal thin film is at least located on the leg portions of the cantilever. 7. The micromechanical or nanomechanical device of claim 6 wherein the notch comprises a through hole positioned adjacent the base of the cantilever and penetrating the cantilever. 4. The micromechanical or nanomechanical device of claim 3, wherein the device comprises an AFM probe comprising an AFM tip on the first surface of the cantilever. The micromechanical or nanomechanical device of claim 8, wherein the metal thin film is located on a first surface of the cantilever. The micromechanical or nanomechanical device of claim 8, wherein the metal thin film is located on a second surface of the cantilever opposite the first surface of the cantilever. The micromechanical or nanomechanical device of claim 8, wherein the AFM probe comprises a contact probe. The micromechanical or nanomechanical device of claim 8, wherein the AFM probe comprises a non-contact probe. The micromechanical or nanomechanical device of claim 8, further comprising a bias source configured to bias the metal thin film and a detector configured to detect a signal provided from the metal thin film. The micromechanical or nanomechanical device of claim 13, wherein the bias source comprises an ac bias source and the detector comprises a phase sensing detector. 15. The micromechanical or nanomechanical device of claim 14, wherein the detector comprises a lock in amplifier. 3. The micromechanical or nanomechanical device of claim 2 wherein the resonator is selected from the group comprising a torsional resonator, a double clamped beam resonator and a diaphragm resonator. The micromechanical or nanomechanical device of claim 1, wherein the metal thin film comprises a self sensing film having a thickness of between 10 nm and 10 microns, wherein the size of the movable element is greater than or equal to 100 microns in at least one dimension. The method of claim 1, wherein the metal thin film is Au, Pt, W, Al, Ni, Cu, Cr, Ag, Pd, Pt-Cr, nickel-copper, nickel-chromium, Pt-W, isoelastic, Micro mechanical or nano mechanical device selected from the group consisting of Karma, Ni-Ag and Armor D. 19. The micromechanical or nanomechanical device of claim 18, wherein the metal thin film comprises an Au film, a Pt film, a W film, or an Al film having a thickness of 10 to 100 nm. The micromechanical or nanomechanical device of claim 1, wherein the device is a MEMS or NEMS selected from the group comprising SPM probes, force sensors, pressure sensors, flow sensors, chemical sensors, biological sensors, and inertial sensors. As a method for producing an SPM probe, Preparing a substrate; Forming a masking layer on the front and back surfaces of the substrate; Patterning the masking layer; Forming an SPM tip on the front side of the substrate using a masking layer patterned on the front side of the substrate; Forming a patterned metal thin film piezoresistive sensor on a front surface of the substrate; Etching the substrate from the backside through the opening of the backside masking layer to form a cantilever that supports the SPM tip and the metal film Method for producing a SPM probe comprising a. A method of operating a micromechanical or nanomechanical device comprising a movable element and a piezo resistor, Biasing the piezo resistive film to detect the amount of movement of the movable element and detecting the piezo resistive response of the piezo resistive film. 23. The method of claim 22, wherein the piezoelectric resistive film comprises a metal thin film. 23. The method of claim 22, wherein the piezoelectric resistive film comprises a semiconductor thin film. 23. The method of claim 22, wherein the device comprises an AFM probe. 27. The method of claim 25, further comprising at least one of determining and imaging a property of a surface irradiated by an AFM probe based on the piezoelectric resistance response of the metal thin film. . The method of claim 25, wherein the AFM probe is used in a contact mode, Detecting the piezoelectric resistance response of the metal thin film comprises ac biasing the metal thin film to measure the ac voltage. The method of claim 25, wherein the AFM probe is used in a contactless mode, Detecting the piezoelectric resistance response of the metal thin film, Ac biasing the thin metal film at a first frequency; Driving the AFM probe at a second frequency different from the first frequency; Detecting a mechanical response of the AFM probe at another frequency different from the first and second frequencies or a sum of the first and second frequencies Method for operating a micro mechanical or nano mechanical device comprising a.

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