WO2021125953A1

WO2021125953A1 - Novel atomic force microscopy probes with phononic crystals

Info

Publication number: WO2021125953A1
Application number: PCT/NL2020/050797
Authority: WO
Inventors: Simon GROEBLACHER; Milan P ALLAN; Richard Alexander NORTE; Maarten LEEUWENHOEK
Original assignee: Technische Universiteit Delft; Universiteit Leiden
Priority date: 2019-12-18
Filing date: 2020-12-16
Publication date: 2021-06-24
Also published as: NL2024495B1

Abstract

The present invention is in the field of an atomic force microscopy probe, which probes generally consist of four cooperating elements, namely a sharp tip to sense the surface, an optical resonator usually being a cavity, a drive usually being a piezoelectric, and a read out usually being a laser beam and a photodiode to measure deflection, a sensor comprising said probe, and a method of fabricating said probe.

Description

Novel atomic force microscopy probes with phononic crystals

FIELD OF THE INVENTION

The present invention is in the field of an atomic force microscopy probe, which prior art probes often consist of four cooperating elements, namely a sharp tip to sense the surface, a mechanical resonator usually being a cantilever, a drive usually being a piezoelectric, and a read out usually being a laser beam and a photodiode to measure deflection, a sensor comprising said probe, and a method of fabricating said probe.

BACKGROUND OF THE INVENTION

The present invention is in the field of an atomic force microscopy probe. An atomic force microscopy (AEM) provides a very high spatial resolution down to the order of Angstroms or better, e.g. nm scale. The microscope may be used for force measurement, such as for measurement of mechanical properties, for imaging, such as for imaging of topographic, electronic, mechanical, and other material properties, and for manipulation of e.g. individual atoms on a surface. So, an AEM can be used to measure the forces between the AEM probe and a sample, typically as a function of separation. For imaging a response of the probe to forces experienced by said probe due to a sample under observation is calculated into a three-dimensional image of the sample surface. For manipulation the tip is used to change the sample or atoms on a sample in a controlled way.

An AEM typically consists of a cantilever, a support for the cantilever, a piezoelectric element for oscillating the cantilever, a tip fixed to the cantilever, which tip in fact acts as the actual probe), and a detector for establishing deflection and motion of the cantilever. Further a typical microscope xyz drive, and a sample stage may be present. The interaction between tip and sample is typically on an atomic scale, whereas the relative motion of the cantilever is on a macro scale. The detector measures the deflection of the cantilever and typically converts said measurement into an electrical signal. The intensity thereof is proportional to the displacement of the cantilever. Various methods of detection can be used.

AEM probes may use photonic crystals or a piezo-effect based so-called qPlus sensor.

However, prior art probes typically have too much acoustic noise and/or vibrational noise, and is not sensitive enough in the readout, in particular for dedicated and sophisticated applications .

RU2610351 (C2) recites a method of measuring energy spectra of quasi-particles in a condensed medium comprises exciting quasi-particles with the required properties, propagation, reflection, re-propagation of the reflected quasi-particles, detection of the reflected quasi-particles, processing of the obtained information and reconstruction of the quasi-particle spectrum in the investigated sample. Measurement can be carried out once or twice. Therewith simplified adjustment, improved stability of operation and reduced distortion is obtained.

WO 2013/019719 A1 recites an ultra-compact nanocavity- enhanced scanning probe microscope and a method. Techniques for measuring the topography of a surface using a device including a semiconductor slab having a distal end and a base region, and an air slot therein are recited. A sensor tip can be coupled to the slab below the air-slot. A photonic crystal including a lattice pattern with a cavity region defined by a local perturbation in the lattice pattern can be integrated into the semiconductor slab above and below the air slot, thereby providing a split- cavity photonic crystal resonator integrated into the semiconductor slab.

DE 19651 636 A recites an apparatus, such as a raster tunnel microscope, which is vibration-free suspended by a carrier plate, suspended from a frame via sprung elements. The frame comprises a base plate, to which are secured vertical support posts with horizontally coupled yokes. The carrier plate is set between the support posts, and the sprung elements are fastened to the yokes. The rectangular base plate and two U-shaped carriers may form the frame, with the carriers containing two support posts interconnected by the yoke on opposite sides of the base plate.

US 2019/267965 A1 recites a micromechanical vibrasolator isolating vibration of a micromechanical resonator and includes: phononic bandgap mirrors, monophones connected serially; phonophore arms in an alternating sequence of phonophore arm- monophone-phonophore arm; abutments in acoustic communication with the phononic bandgap mirrors; wherein the micromechanical resonator is interposed between the phononic bandgap mirrors with phononic bandgap mirror arranged in parallel on opposing sides of the micromechanical resonator arranged perpendicular to a direction of vibration of an in-plane vibrational mode of the micromechanical resonator.

The present invention therefore relates to an improved atomic force microscope probe, which solves one or more of the above problems and drawbacks of the prior art, providing reliable results, without jeopardizing functionality and advantages .

SUMMARY OF THE INVENTION

It is an object of the invention to overcome one or more limitations of the prior art probes and provide an improved probe. In a first aspect the present invention relates to an atomic force probe assembly, comprising (i) a tip for sensing surface characteristics, (ii) an mechanical resonator in contact with the tip or incorporated in the tip, wherein the resonator comprises at least one phononic crystal, and (iii) at least one read-out, wherein the resonator provides output to at least one read-out, preferably an optical read-out, a capacitive read-out, or a combination thereof. The tip may relate to the tip per se, or also include a cantilever. The at least one phononic crystal may be provided on the tip or be incorporated therein. For instance, the cantilever and the phononic crystal may be the same. A photonic crystal relates to a periodic optical nanostructure that affects the motion of photons. A phononic crystal relates to a periodic optical nanostructure that affects the motion of phonons. They need not be crystalline or semi crystalline, but could be. The present probe comprises three of the typical AEM elements integrated on a chip, using a suspended high-tension membrane to build some or all of the parts. The tip can be an overhanging (i.e. suspended) membrane with other layers on top. The membrane can be SiN, such as a high-tension SiN membrane. The resonator can be a resonator based on phononic crystals, or shielded by phononic crystals. The read out can be made from optical waveguides, or from combined phononic and photonic crystals. It is noted for clarity that photonic crystals and phononic crystals are different from one and another. Both may comprise periodic nanopatterns. Photonic crystals give a material special optical ("photon") properties, whereas phononic crystals influence vibrational modes ("phonons"). Phononic crystals can also be used to isolate the probe from unwanted vibrations. The fabrication process is considered simple, and includes dry etch and dry release.

It is found that the present phononic resonators provide a higher Q and hence a better sensitivity, and a highest quality photonic/phononic coupling. Also, an integrated construction allows for simpler design, and a better sensitivity. The present probe can be used for existing AEM's and industry, in in particular for speciality AEM, such as to characterize wafers for nanofabrication.

In a second aspect the present invention relates to an atomic force probe assembly, such as the present one, comprising at least one shield for acoustic noise and/or vibrational noise, wherein the shield preferably is at least one phononic crystal.

In a third aspect the present invention relates to a sensor comprising an atomic force probe assembly according to the invention, such as a surface profile sensor.

Advantages of the present description are detailed throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

In an exemplary embodiment of the present atomic force probe assembly the resonator may be selected from photonic crystals.

In an exemplary embodiment of the present atomic force probe assembly the at least one shield for acoustic noise and/or vibrational noise may be at least one phononic crystal .

In an exemplary embodiment of the present atomic force probe assembly the optical read-out may be selected from an optical guide, and a combined phononic and photonic crystal.

In an exemplary embodiment of the present atomic force probe assembly the combined phononic and photonic crystal may comprise at least two periodic nanopatterns, one periodic nanopattern providing optical (photon) properties, and one periodic nanopattern providing vibrational (phonon) properties.

In an exemplary embodiment of the present atomic force probe assembly the combined phononic and photonic crystal may comprise one periodic nanopattern, simultaneously providing optical (photon) properties and vibrational (phonon) properties.

In an exemplary embodiment of the present atomic force probe assembly the phononic and photonic crystal may each be independently provided as a layer, or may be a combined layer, such as a layer with 10-500nm thickness.

In an exemplary embodiment of the present atomic force probe assembly the atomic force probe is adapted to operate at a first read-out frequency or first band of read out frequencies, and wherein the at least one phononic crystal damps at least one second frequency or band of second frequencies, wherein first and second frequency or first band and second band of frequencies do not overlap, preferably at least two second frequencies or two bands of second frequencies. By identifying a first read-out frequency or first band of frequencies the present at least one phononic crystal can be adapted, such as by design, to allow the first read-out frequency or first band of frequencies to pass, and to filter out, or to damp, other frequencies, typically frequencies providing noise to the read out. A read-out frequency of an AEM assembly may for instance be 10 kHz, or 15 kHz, and these frequencies may be filtered out. A first band may be selected around this first frequency, typically a band of a central frequency ±30% thereof suffices, preferably ±50% thereof. E.g. 10 ±3 kHz or 15 ±4.5 kHz. The band may be chosen asymmetrically.

In an exemplary embodiment of the present atomic force probe assembly the tip may comprise an overhanging, for example a suspended, membrane, such as a high-tension membrane, such as SiN, and optionally one or more layers on said membrane.

In an exemplary embodiment the present atomic force probe assembly may comprise (iv) a drive, such as a piezo drive.

In an exemplary embodiment of the present atomic force probe assembly the assembly may be integrated on a chip.

In an exemplary embodiment of the present atomic force probe assembly the dielectric structure (1) may be a large aspect ratio layer, such as with an aspect ratio of >10⁴, preferably >10⁶, more preferably >10⁷, such as >10⁸.

In an exemplary embodiment of the present atomic force probe assembly the dielectric layer may be selected from S1₃N₄, S1O₂, SiC, InGaP, Si, diamond, graphene, and combinations thereof.

In an exemplary embodiment of the present atomic force probe assembly the dielectric layer 12 may have a thickness of 3-300 nm, preferably 5-200 nm, more preferably 10-100 nm, such as 20- 50 nm.

In an exemplary embodiment of the present atomic force probe assembly the dielectric layer may be provided with a tensile strength > 0.05 GPa.

In an exemplary embodiment of the present atomic force probe assembly the dielectric layer may have a hardness of > 8.5 Mohs.

In an exemplary embodiment of the present atomic force probe assembly the dielectric layer may be ultra clean with < 10 ppm impurities in the layer.

In an exemplary embodiment of the present atomic force probe assembly the dielectric layer may have a mechanical quality factor of > 10⁶.

In an exemplary embodiment of the present atomic force probe assembly the dielectric layer may have a specific mass of < 1 gr/m², preferably < 0.1 gr/m².

In an exemplary embodiment of the present atomic force probe assembly the dielectric layer may be deposited by at least one of sputtering, evaporating, plasma-enhanced chemical vapor deposition (PECVD), and low-pressure chemical vapor deposition (LPCVD).

In an exemplary embodiment of the present atomic force probe assembly the dielectric layer may be provided with an etched patterned photonic crystal array having at least one n*m array with holes, wherein a surface area of the holes and array of holes are adapted for reflecting light.

In an exemplary embodiment of the present atomic force probe assembly for a given wavelength or range of wavelengths the holes may have a cross-sectional length which is 0.25-0.50 * wavelength or 0.25-0.50 * weighted mean of said range of wavelengths, respectively.

In an exemplary embodiment of the present atomic force probe assembly a space area between the holes may be in a range of 35- 97% of a surface area of the mirror, the remainder of the surface area of the mirror being formed by top surfaces of the holes.

In an exemplary embodiment the present atomic force probe assembly may comprise ie [2,2¹⁰] arrays with holes, wherein ni and mi of each array i are chosen independently.

The invention will hereafter be further elucidated through the following examples which are exemplary and explanatory of nature and are not intended to be considered limiting of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims.

SUMMARY OF THE FIGURES

Figs, la-g and 2a-c show details of the present invention. DETAILED DESCRIPTION OF FIGURES The figures are detailed throughout the description, and specifically in the experimental section below.

In the figures:

1 tip

2 cantilever

3 mechanical resonator

4 phononic crystal

4a phononic crystal element

5 photonic crystal

6 light source

7 detector

11 substrate

12 dielectric layer

13 photoresist 18 cavity

Figure 1 shows a method of preparing an AEM probe tip. In fig. la SiN is deposited on a Si wafer. In fig. lb a lithographic resist is spun on the SiN, and in fig. lc the resist is exposed and developed to provide a tip, optics and phononics. So a photonic structure and a phononic structure may be provided adjacent to one and another, in the same process, using different "parts" of a mask for instance. In fig. Id the resist is used as mask to transfer patterns from the mask to the SiN. In fig. le the chip is diced into smaller portions, providing a pointed tip for AEM use. Then in fig. If the resist is removed and the chip is cleaned. The tip, optical readout, and phononics shield are undercut providing a cavity. The process may be repeated to provide a photonic structure on top of a phononic structure, is in fig. 2b. In view of processing this is less preferred. Typically the AEM probe is cut from the Si wafer using standard technology, such as die slicing, such as in between figs. If and lg. A sacrificial layer could be provided to prevent debris from jeopardizing functionality of the probe.

Fig. 2a shows a prior art AEM tip, with a light source, and a detector for detecting reflected light.

Fig. 2b shows an AFM tip according to the invention. Therein two crystals, a top photonic crystal and a bottom phononic crystal, are capacitively coupled. The holes in the two crystals have different dimensions in terms of height and width. An optical beam is used to measure deflection in the photonic crystal, representing movement of the tip, which tip is attached to the phononic crystal.

Figs. 2c-e show further embodiments, wherein an phononic shield is used, such as at either side of the AFM tip. In fig.

2e also a central part with a combined phonon Crystal (acting as resonator) and photonic crystal (to read out) is shown.

Methods of preparation

The method of preparation has been subject of a scientific paper by the present inventors and the paper and its contents are hereby incorporated by reference (Nanotechnology 30 (2019) 335702 https://doi.org/10.1088/1361-6528/ablc7f). Therein, using SiN as a base for the present suspended STM tips has a number of key advantages. First, it has a large selectivity to the Si etch to suspend the tip, allowing for clear protrusions. On the contrary, making the tips solely out of metal without underlying SiN would limit the choice of metal to those compatible with the etch described below. Second, it has a high mechanical stiffness, yielding robust tips that are resistant to tip treatment, as detailed below. Also, it allows for a process where the metallic layer is added in a last step. This allows to avoid any contamination by chemicals such as etchants and resists.

The present process starts with a 500pm thick Si(100) chip covered on both sides with a 200 nm thick layer of high stress low-pressure chemical vapor deposition silicon nitride. The initial pattern, consisting of the tip shape and two shields, is provided using electron beam lithography and then transferred into the SiN using a reactive-ion CHF₃ etch (see figure 1(a) of the paper). The shields are slabs of SiN on both sides of the tip, which were included to minimize the undercut of the Si once the overhang is created: it is found that the 50 nm lines around the shields reduce the etch rate of the Si significantly compared to a large exposed region without shields. Then the chip was cleaned in a piranha solution to remove all traces of resist and protect it in a new layer of photoresist that can be easily removed later by acetone. In order to bring the tip close to the edge inventors proceeded with dicing the chip along the lines depicted in figure 1(a) of the paper resulting in figure 1(b) of the paper. This step is typically precise to within a few micrometers, leaving a straight sidewall (roughness <3 pm). The residue created by the dicing process is washed away with the removal of the protective photoresist layer. After this cleaning step, inventors isotropically removed part of the Si substrate using a dry reactive-ion etch (SF6). For improved selectivity of the SiN over the Si the chip is cooled to -50 °C. During a typical etch the thickness of the SiN reduces from 200 to ~120 nm. The exposed Si sidewalls are removed at a rate of around 4 pm min^-1 until both the tip and the two shields protrude by about 10-12 pm, causing the shields to fall off (figure 1(c) of the paper). The straight sides next to the tip can be made very small or even rounded to avoid accidental touches when aligning to a sample, this would not have been possible using an anisotropic KOH wet etch. The final step in fabricating the STM tip involves depositing a metal on the chip through sputtering to ensure proper coverage of both the top and the side of the SiN tip (figure 1(d) of the paper). In this study, inventors deposited 20 nm of gold using a Leica ACE200 as the tip material; it is found relatively straightforward to use other interesting materials for the tip.

Images of a typical device are shown in figures 2(a)-(c) of the paper. The diameter of the apex of the tip depends on the initial thickness of the SiN, the electron beam spot size and dose, the SF₆ etch time and temperature, and the metal film thickness. However, as can be seen from figure 2(c) of the paper, the tip diameter is found to be mostly determined by the grain size of the metal film. The present tips achieve radii of a few nanometers, which is comparable to specialized commercially available metal wire tips. The overall yield of the fabrication as described in this section is around 80%.

Claims

1. Atomic force probe assembly, comprising (i) a tip for sensing surface characteristics, (ii) a mechanical resonator in contact with the tip or incorporated in the tip, wherein the mechanical resonator comprises at least one phononic crystal, wherein the phononic crystal is a periodic optical nanostructure adapted to affect the motion of phonons, and (iii) at least one read-out, wherein the mechanical resonator provides output to at least one read-out, preferably an optical read-out.

2. Atomic force probe assembly according to claim 1, wherein the mechanical resonator is selected from phononic crystals, and comprising at least one shield for acoustic noise and/or vibrational noise selected from phononic crystals.

3. Atomic force probe assembly according to claim 1 or 2, wherein the optical read-out is selected from an optical guide, and a combined phononic and photonic crystal.

4. Atomic force probe assembly according to claim 3, wherein the combined phononic and photonic crystal comprises at least two periodic nanopatterns, one periodic nanopattern providing optical (photon) properties, and one periodic nanopattern providing vibrational properties, and/or wherein the combined phononic and photonic crystal comprises one periodic nanopattern, simultaneously providing optical (photon) properties and vibrational (phonon) properties, and/or wherein the phononic and photonic crystal each individually are provided as a layer, such as a layer with 5-500nm thickness, and/or wherein the atomic force probe is adapted to operate at a first read-out frequency or first band of read out frequencies, and wherein the at least one phononic crystal damps at least one second frequency or band of second frequencies, wherein first and second frequency or first band and second band of frequencies do not overlap, preferably at least two second frequencies or two bands of second frequencies.

5. Atomic force probe assembly according to any of claims 1-4, wherein the tip comprises an overhanging membrane, such as a high-tension membrane (12), such as SiN, and optionally one or more layers on said membrane.

6. Atomic force probe assembly according to any of claims 1-5, comprising (iv) a drive, such as a piezo drive.

7. Atomic force probe assembly according to any of claims 1-6, wherein the assembly is integrated on a chip, and/or comprising a membrane layer (12) with a large aspect ratio layer, such as with an aspect ratio of >10⁴, preferably >10⁶, more preferably >10⁷, such as >10⁸, and/or wherein the material of the membrane layer is selected from dielectric materials, such as S1₃N₄, S1O₂, SiC, InGaP, Si, from metals, from superconducting materials, from diamond, from graphene, and combinations thereof, and/or wherein the membrane layer (12) has a thickness of 3-10000 nm, preferably 10-3000 nm, more preferably 100-1000 nm, such as 500- 750 nm, and/or wherein the membrane layer has a mechanical quality factor of > 10⁶, and/or wherein the membrane layer is deposited by at least one of sputtering, evaporating, plasma-enhanced chemical vapor deposition (PECVD), and low-pressure chemical vapor deposition (LPCVD), and/or wherein the membrane layer is provided with an etched patterned photonic crystal array having at least one n*m array with holes, wherein a surface area of the holes and array of holes are adapted for reflecting light, and/or wherein for a given wavelength or range of wavelengths the holes have a cross-sectional length which is 0.25-0.50 * wavelength or 0.25-0.50 * weighted mean of said range of wavelengths, respectively, and/or wherein a space area between the holes is in a range of 35-97% of a surface area of the mirror, the remainder of the surface area of the mirror being formed by top surfaces of the holes, and/or comprising ie [2,2¹⁰] holes.

8. Atomic force probe assembly comprising at least one shield selected from phononic crystals, wherein the phononic crystal is adapted to shield the atomic force probe assembly from for acoustic noise and/or vibrational noise.

9. Sensor comprising an atomic force probe assembly according to any of claims 1-8, such as a surface profile sensor.