Electret atomic force microscope bending beam

Abstract

The invention relates to an electret atomic force microscope bending beam for measuring surface potentials and / or surface properties by means of an atomic force microscope, comprising a bending beam having a bending beam tip, characterized in that at least the bending beam tip is coated with a thin electret layer.

Description

[0001] ELECTRICAL ATOMICROSCOPE BEND BEAM

[0002] The invention relates to an electret atomic force microscope cantilever for measuring surface potentials and / or surface properties using an atomic force microscope with a cantilever with a cantilever tip.

[0003] Electrets are dielectric layers that have a (quasi-)permanent surface potential and thus provide a permanent electric field, similar to a permanent magnet. Due to this special property, electrets are used in a wide variety of applications today and have become indispensable in our daily lives.

[0004] Current state-of-the-art electrical surface characterization techniques include atomic force microscopy (AFM), electrostatic force microscopy (EFM), and Kelvin probe force microscopy (KPFM). Electrical surface characterization techniques are used for electrical fault analysis, trapped charge detection, electrical polarization imaging, and electrical reading / writing, among other applications. Both technologies are based on non-contact atomic force microscopy and utilize a bias voltage to oscillate the cantilever. The amplitude depends on the electrostatic force between the tip of the cantilever and the sample surface. In electrostatic force microscopy, the electrostatic force is measured directly, whereas in Kelvin probe force microscopy, quantitative measurements of the local surface potential are performed using a compensation technique.A DC / AC voltage is often applied between the cantilever and the surface to modulate the electrostatic interaction, making small electrostatic forces detectable. Various modes of amplitude and frequency modulation are available. For conductive samples, for example, the contact potential difference, which is related to the work function, is measured. This also provides information about local charges or dipole distributions.

[0005] US Pat. No. 9,869,694 B2 describes an apparatus and method for collecting topography, mechanical property data, and electrical property data using an atomic force microscope (AFM) either in a single pass or in a double-pass operation. Preferably, the atomic force microscope is used in PFT (peak force tapping) mode or using Kelvin probe force microscopy (KPFM), which allows the use of a wide range of probes, increasing, among other things, the sensitivity of the electrical property measurement.

[0006] US Pat. No. 8,726,411 B1 discloses a charged probe and a method for measuring the electric field. A nanoparticle attached to the tip of a probe can be electrically charged with a polarity. This probe can be used to measure the electric fields of objects in the nanometer range.

[0007] In addition, the document US 4,397,702 A shows a sensor probe unit comprising an electrically non-conductive polymer layer charged with a permanent electrical charge to form an electret and which is closely connected to a compensating metal electrode.

[0008] The document EP 2 359 148 B1 discloses a method for operating a scanning probe microscope comprising the steps of: generating a relative movement between a probe having a tip and a sample, determining a background signal by sufficiently increasing a separation between tip and sample to a controlled distance such that only parasitic forces contribute to a detected deflection of the probe without an interaction between tip and sample, and detecting a movement of the probe by measuring a total probe deflection.

[0009] The publication KR 102007 0 009 789 A discloses a probe for an atom force microscope cantilever which uses a ferroelectric and which uses the electrical polarization of the ferroelectric for precise measurement by using the ferroelectric in the probe of the cantilever.

[0010] JP H09 - 21 829 A discloses, for evaluating an insulating film, the possibility of capacitance measurement through an insulating film on a metal, wherein in a scanning capacitance microscope, an object in which a dielectric layer is applied to a conductive needle-like element is used as a probe, and the probe is brought into contact with an insulating film formed on a conductive base material at a predetermined contact pressure, and wherein at least one alternating bias voltage is applied between the conductive needle-like element of the probe and an insulating base material.The dielectric constant of a ferrodielectric substance layer is changed in proportion to the bias voltage, and accordingly, a capacitance between the conductive needle-like element of the probe and the conductive base material is modulated in the state of sandwiching between the ferrodielectric substance layer and the insulating film of a sample.

[0011] US 2014 / 0203707 A1 discloses electron emission devices and device components for optical, electronic and optoelectronic devices, including MEMS and NEMS devices based on cantilever-based instrumentation, wherein devices according to certain aspects of the invention comprise a dielectric, pyroelectric, piezoelectric or ferroelectric layer on the receiving surface of a substrate with an integrated actuator, such as a temperature controller or mechanical actuator, optionally in the form of a cantilevered device with an integrated heating thermometer.

[0012] The problems in the state of the art are essentially that both electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) require careful control of all atomic force microscope parameters for precise measurement.

[0013] When using the KPFM frequency-modulated non-contact mode, for example, there are four independent control loops operating simultaneously. This requires very precise process control to prevent overlap between the individual loops.

[0014] The resolution and the imprecisely determined work functions for small structures are limited by an averaging effect. This results from the long-range nature of the electrostatic forces.

[0015] A probe with a tip containing electret nanoparticles, as described in US Pat. No. 8,726,411 B1, enables precise measurement of surface potentials. However, such a probe is only partially reproducible and complex to manufacture, making it unsuitable for large-scale production. The electret nanoparticles cannot be reproducibly attached to the probe tip, nor can they be reproducibly loaded.

[0016] The present invention is based on several objects.

[0017] One task is to provide an electret atomic force microscope cantilever that can be produced reproducibly and in an industrially viable manner.

[0018] A further task is to provide an electret-containing atomic force microscope cantilever, wherein the electrets have a homogeneous and reproducibly distributed surface potential.

[0019] These tasks are solved with an electret atomic force microscope cantilever according to the main claim.

[0020] The electret atomic force microscope cantilever for measuring surface potentials and / or surface properties by means of an atomic force microscope with a cantilever with a cantilever tip is characterized in that at least the cantilever tip is coated with an electret thin film.

[0021] The entire cantilever beam or a limited portion can be coated with a thin electret layer. In one variant, the entire cantilever beam can also be coated. This can be achieved, for example, by:

[0022] Make more sense from a production perspective.

[0023] In addition, the electret thin film can be negatively or positively charged.

[0024] The electret thin film can in particular be evenly distributed over the bending beam tip.

[0025] Furthermore, the electret thin film or insulator or dielectric thin film used as a charged electret can be produced by means of

[0026] - chemical vapor deposition of polymers and / or

[0027] - wet chemical polymer thin film production process with self-charging during the production process and / or subsequent charging preferably by corona discharge and / or electrowetting and / or electron beam and / or the triboelectric effect.

[0028] In a preferred embodiment, the electret thin film can be produced by means of initiated chemical vapor deposition with self-charging during the manufacturing process.

[0029] The electret atomic force microscope cantilever can be used to measure surface potentials using an atomic force microscope in DC / AC mode, contact mode, or non-contact mode.

[0030] The electret atomic force microscope cantilever according to the invention has a novel cantilever tip due to its coating with an electret thin film. The positive or negative charge of the electret thin film enables interactions with charges on the surface of a sample, making measurements of small surface charges possible or more precise. As already described, the electret atomic force microscope cantilever can be used in various operating modes, including DC / AC mode, contact mode, and non-contact mode.

[0031] If the electret thin film is produced using chemical vapor deposition, the options include parylene deposition, plasma-enhanced chemical vapor deposition, initiated vapor deposition, and / or molecular layer deposition. After the polymer thin film has been produced using either chemical vapor deposition or wet-chemical polymer thin film production processes, charging is performed, preferably using corona discharge and / or electrowetting, if no self-charging has occurred during the thin film production process. For electret thin films, which must be charged in separate process steps, the possibilities for applying charges in a truly homogeneous and reproducible manner are very limited. Precise potential control is not feasible.For example, the corona discharge method achieves very high electric field strengths, which can lead to dielectric breakdown in the thin films and produce inhomogeneous surface charges or surface potentials.

[0032] Electrets typically use polymers that are initially uncharged and then subjected to a subsequent polarization process or charge trapping. Polymers used for this purpose are hydrophobic, exhibit high electrical resistance, and are particularly suitable for permanently storing applied charge. Examples include polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), polypropylene (PP), polyvinylidene fluoride (PVDF), polytetrafluoroethylene propylene (PTFEP), and their copolymers.

[0033] The electrets used here are characterized, in a particularly preferred embodiment, by the fact that they have no crystalline components or are formed without crystalline components. This property can be verified, for example, using XRD. Nevertheless, these electrets exhibit a potential, which distinguishes them from ferroelectrics. Thus, one property in a particularly preferred embodiment is the absence of ferroelectric behavior in the electrets to be used here.

[0034] Self-charging electret thin films are preferably used for coating, as this eliminates the need for a separate charging process. A preferred embodiment is initiated chemical vapor deposition with self-charging during the manufacturing process for producing an electret thin film on the cantilever tip of an atomic force microscope.

[0035] By eliminating a separate downstream charging step, the manufacturing process can be simplified and made more cost-effective.

[0036] This is made possible by a process for producing a charged electret formed from monomers with an initiator (not required if the monomer is used self-initiating). The process comprises the following steps:

[0037] Depositing a polymer thin film via an initiated chemical vapor deposition, wherein at least some of the monomers have at least two terminal acrylate and / or methacrylate groups, the initiator is a radical initiator and / or the monomer is self-initiating and a spontaneous polarization to an electret thin film takes place in the formed polymer thin film without application of an electric field.

[0038] The monomers can be formed as either homo- or comonomers. In particular, at least some of the monomers can be formed as dimethacrylates and / or trimethacrylates.

[0039] In addition, the surface potential of the electret thin film resulting from polarization increases linearly with the layer thickness.

[0040] The surface potential of the electret thin film can be adjusted with the initiator, whereby the surface potential is positive or negative.

[0041] The initiator di-tert-butyl peroxide (TBPO) can be used to set a positive surface potential of the electret thin film and the initiator perfluorobutanesulfonyl fluoride (PFBSF) can be used to set a negative surface potential of the electret thin film.

[0042] The monomers used may preferably have a dipole moment of at least 1 Debye and a vapor pressure of at least 16 Pa at 25 °C (at ambient pressure 1 atm, 12766.95 Pa).

[0043] In particular, the surface potential of the formed electret thin films is stable under standard conditions and the electret thin film can be easily used to coat the cantilever tip of an atomic force microscope.

[0044] In particular, initiators that form radicals with an electronegativity of more than 3 on the Pauling scale after their activation can lead to negative surface potentials in the formed electret thin films, and initiators that form radicals with an electronegativity of less than 3 on the Pauling scale after their activation can lead to positive surface potentials in the formed electret thin films.

[0045] Because the monomers can also be formed as comonomers, it is possible to combine monomers with at least two terminal acrylate and / or methacrylate groups with monomers without these end groups. Monomers with a vinyl group are also possible as comonomers if at least one comonomer has at least two acrylate or methacrylate groups. The resulting polymer layer is formed as an electret layer, making it possible to combine different functionalities of monomers or to adjust a desired charge / surface potential at a desired film thickness.

[0046] This process allows the surface potential to be adjusted extremely precisely in the millivolt range, something that is not possible with conventional electrets, which currently require charging during production. The effect is substrate-independent for various substrates, such as gold, silicon (conductive, heavily n-doped), copper, titanium oxide, and silicon.

[0047] Compared to conventional electrets, the charging process is eliminated. Furthermore, since the charges scale linearly with layer thickness (as previously observed), they can be applied with extreme precision, enabling entirely new applications for electret thin films, such as the electret atomic force microscope cantilever according to the invention.

[0048] As already explained, the disadvantage of currently used electret materials is that a charging process or polarization process is always necessary to generate a surface potential.

[0049] Polyacrylates and polymethacrylates are not suitable for a downstream charging process because they have difficulty storing separately applied charge and have therefore not typically been used as electret materials to date.

[0050] In polymer thin films deposited via the gas phase, particularly with dimethacrylates or trimethacrylates as homomonomers or comonomers, spontaneous polarization can be observed immediately after deposition, without the application of an electric field. This observation forms the basis for the manufacturing process. Deposition takes place via initiated chemical vapor deposition (iCVD). The surface potential of the thin films resulting from polarization increases linearly with film thickness.

[0051] Additionally, a gas barrier layer can be applied over the electret layer. This type of barrier layer enables the AFM electret cantilever to operate in various gas atmospheres, preventing the electret charge from being lost. For example, an organic-inorganic multilayer of organic and inorganic thin films can be used. This can protect the electret from water vapor penetration and thus potential / charge loss. For example, an ALD / iCVD or hot wire CVD / iCVD multilayer can be used.

[0052] This represents a significant advantage for use in industrial applications, as an entire process step (the charging step) is eliminated. This reduces costs, particularly in industrial applications. Another advantage is the highly precise control over the resulting surface potential. Furthermore, the layers can be applied homogeneously to large-area substrates using the typical CVD growth method.

[0053] The potential precision, reproducibility, and homogeneity of the electret thin films produced using this method on cantilever tips in atomic force microscopes are unparalleled. The substrate independence opens up the possibility of electret use on materials that make traditional charging impossible. Since iCVD is already industrialized, the self-charged electret thin films can be mass-produced. The electret atomic force microscope cantilever exhibits increased precision compared to the cantilever with a single nanoparticle on the tip according to US Pat. No. 8,726,411 B1. By using a single layer, an effective electric field comparable to that of a nanoparticle with the size of the cantilever tip can be used.The tip end of the electret-coated cantilever tip can theoretically be simplified as a nanoparticle, since only the part of the layer that is directly adjacent to the sample interacts with the sample.

[0054] In addition, the electret atomic force microscope cantilever makes it possible to measure electric fields without conductive substrates.

[0055] Since traditional AFM is based on charge interactions, improvements in measurements of uncharged surfaces are also possible.

[0056] The invention is described below with reference to the accompanying figures in the description of the figures, which are intended to illustrate the invention and are not to be considered limiting. They show:

[0057] Figure 1 is an exemplary schematic representation of the structure of an electret atomic force microscope cantilever according to the invention;

[0058] Figure 2 shows an exemplary schematic representation of the processes in the iCVD process and

[0059] Figure 3 shows an exemplary schematic representation of the reaction processes in the iCVD process according to the manufacturing method according to the invention.

[0060] Fig. 1 shows an exemplary schematic representation of the structure of an electret atomic force microscope cantilever according to the invention. The electret atomic force microscope cantilever comprises a cantilever 4 with a cantilever tip 1. The cantilever tip is coated with a thin electret film 2. The coated cantilever tip 1 is oriented toward a sample surface 3.

[0061] Depending on the polarity, the coated cantilever tip 1 is attracted or repelled by the sample surface 3 depending on the surface potential or charge.

[0062] This method can also be applied to measurements in AC mode. In particular, the repulsive regime of charges can be specifically targeted to measure the greatest possible effects.

[0063] Fig. 2 shows an exemplary schematic representation of the processes in iCVD process 5. In the exemplary iCVD process 5 shown in Fig. 2, a thin polymer layer is created by free radical polymerization. Process 5 takes place under vacuum conditions (process pressure approximately 40-50 Pa). Typically, one or more monomers 51 and an initiator 52 are fed into the reactor in the form of vapors. To prevent condensation on the reactor walls, the reactor and all inlet and outlet lines are heated to approximately 90-130°C (depending on the monomer 51 used). To start process 5, for example, a filament / filament arrangement located above the substrate holder is heated (approximately 250-400°C), causing the initiator molecules 52 to decompose into free radicals 53. This can also occur in other ways, for example, by UV, plasma, thermally, or chemically. The monomer molecules 51 do not decompose and adsorb on the substrate holder.For the monomer molecules 51 to adsorb, the substrate holder must be cooled to approximately 20-35 °C. The free radicals 53 then collide with the adsorbed monomer molecules 51 and initiate a free radical polymerization 5 on the cooled substrate holder. The process is typically carried out in a flow-through process. New (co-)monomer 51 and initiator molecules are continuously added to the reactor, while byproducts and unreacted molecules are simultaneously pumped out. A batch process is also possible.

[0064] Figure 3 shows an exemplary schematic representation of the reaction processes in iCVD process 5 according to Figure 2. Using a combination of the monomer PEGDMA and the initiator TBPO results in positive surface potentials. Combining PEGDMA with the initiator PFBSF instead of TBPO results in negative surface potentials.

[0065] To achieve a long-term stable electret thin film, a thin hydrophobic layer can be applied to the polymer.

[0066] In addition, electret thin films produced using this manufacturing process have proven to be stable with regard to their charge against mechanical influences.

[0067] List of reference symbols

[0068] 1 bending beam tip

[0069] 2 electret thin film

[0070] 3 Sample surface

[0071] 4 bending beams

[0072] 5 initiated chemical vapor deposition process (iCVD process)

[0073] 51 Monomer

[0074] 52 Initiator

[0075] 53 free radical

Claims

CLAIMS 1. Electret atomic force microscope bending beam for measuring surface potentials and / or surface properties by means of an atomic force microscope with a bending beam (4) with a bending beam tip (1), characterized in that at least the bending beam tip (1) is coated with an electret thin film (2).

2. Electret atomic force microscope cantilever according to claim 1, characterized in that the electret thin film (2) is negatively or positively charged.

3. Electret atomic force microscope cantilever according to claim 1 or 2, characterized in that the electret thin film (2) is uniformly distributed at least over the cantilever tip (1).

4. Electret atomic force microscope cantilever according to one of the preceding claims, characterized in that the electret thin film (2) can be produced by means of - chemical vapor deposition of polymers and / or - wet chemical polymer thin film manufacturing process with self-charging during the manufacturing process and / or subsequent charging by corona discharge and / or electrowetting and / or electron beam and / or the triboelectric effect.

5. Electret atomic force microscope cantilever according to one of the preceding claims, characterized in that the electret thin film (2) can be produced by means of initiated chemical vapor deposition (5) with self-charging during the production process.

6. Electret atomic force microscope cantilever according to one of the preceding claims, characterized in that a barrier layer against gases is applied over the electret thin film (2).

7. Use of the electret atomic force microscope cantilever according to any one of the preceding claims for measuring surface potentials by means of an atomic force microscope in DC / AC mode or contact mode or non-contact mode.

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