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Micro-robotics, observation and actuation
The driving forces behind micro-scale experimentation hang on appropriate observation, actuation and control tools which are the centrepiece of micro-robotics. Micro-robotics as a field has been developed since the 1990s, and consists in the application of micro- or macro-scale robotic tools in order to affect micro-scale objects – where micro-scale originally refers to sizes between a micrometre and a millimetre. This extends further into actuation and manipulation down to the nanometre scale, and although the transition is not trivial, the physics here will pertain to both the micro- and nano-world. This section will address the main aspects of physics and robotics which are specific to these scales: physical phenomena, observation tools, actuation systems, and, as pertains to the experiments conducted in this work, their applications under scanning electron microscopes.
Physics of the micro-world
The physical phenomena which form the basis of interaction and sensing at the micro-scale are not those we are normally familiar with. Although the laws of classical (as opposed to quantum) physics still apply at this scale, different forces prevail in the micro-world. This difference is referred to as the scale effect [?].
Scale effect
The prevailing forces at our scale (the human or macro-scale) are « volumetric forces », so called because they scale with the volume of the objects involved: gravity and inertial forces. However, at the micro-scale, the power balance is reversed and « surface forces » take over.
This phenomenon can be illustrated through the « Square-Cube Law »: a cube with edges 10 cm has 100 cm2 sides and a volume of 1000 cm3. A cube with ten times smaller edges would have a side surface of 1 cm2 (e.g. a hundred times smaller) and a volume of 1 cm3 (e.g. a thousand times smaller). Therefore, through this evolution in scale, the effects of forces that apply on surfaces progressively catch up with those of volumetric forces (Fig. ??).
Figure 1.1 – Evolution of the balance of forces when going down in scale [?] : tension forces (Ftens), van der Waals forces (Fvdw), and electrostatic forces (Felec) progressively get to overcome gravity (Fgrav).
Beyond surface and volume forces, the scale effect also affects behaviour as they relate to time (time dilatation) and all physical phenomena of the micro-world, from solid or fluid mechanics to heat transfers. The immediate consequence is the apparition of significant interaction forces between objects: attractive forces if they are very close, adhesion forces if they are in contact, but also attractive or repulsive electrostatic forces which can be completely unpredictable. These behaviours result in both challenges and opportunities for experimentation at the micro-scale.
Surface forces
Surface forces can be attractive or repulsive. Attractive forces notably contribute to adhesion effects, which are omnipresent in micro-robotics. The three main categories of adhesion forces at the micro-scale are : [?].
Van der Waals forces: interaction forces between two permanent or induced dipoles, which cover most intermolecular forces aside from covalent or hydrogen bonds. They are dependent upon the materials of the objects, and become significant at distances around a dozen nanometres.
electrostatic forces, or classical Coulomb forces, which act between all charged objects. They are dependent upon the accumulation of electrostatic charges by the objects. When in the presence of even weak charges, electrostatic interaction of modest magnitude can be detected from over a hundred nanometres, but compared to Van der Waals forces, it only becomes significant in closest proximity [?].
capillary forces, which govern surface tensions and liquid menisci, according to the hu-midity of the medium. They appear at air-liquid interfaces, or on immersed hydrophobic surfaces. They are dependent upon the nature of the liquids, materials and geometry of the system.
In the context of this work, quartz probes will mainly seek to sense attractive or repulsive van der Waals forces, which are reliably short-ranged. Electrostatic forces apply at a greater and potentially more variable range, and may for this reason be an obstacle, especially in the experiments combined with electron microscopy in Chapter 4.
Observing the micro- and nano-world
Various means of observation are available when it comes to obtaining global vision feed-back into the micro-world. Depending on the scale aimed for (Fig. ??), they rely on distinct physical principles of operation, and may interact directly or indirectly with objects to build an image. Alternatives to optical vision therefore come with their advantages as well as additional hindrances; in this regard, the microscopes relevant to this work are the optical, electron and local probe microscopes, the characteristics of which are as follows.
Optical microscopes
Optical microscopy relies on mirrors and lenses to redirect photons and provide the user with an enlarged image. Photons can be reflected, as is the case for human vision, or transmitted (going through a transparent sample). Although it is very commonly used and easy to set up, optical microscopy is limited by: field of view, which decreases in the same manner.
Electron microscopes
Electron microscopy relies on electrons instead of photons. Electrons are emitted by a field electron gun (electromagnetically induced) or a thermoionic field gun (tungsten filament, LaB6 cathode). As with photonic microscopy, these electrons can then be reflected (SEM: Scanning Electron Microscopy) or transmitted (TEM: Transmission Electron Microscopy). The electron beam interacts with matter as it hits its surface, which results in emissions e.g. secondary, Auger, back-scattered electrons, or X-rays. These emissions are intercepted by specific sensors, and treated so as to reconstruct an image of the object’s surface, topography, atomic composition…
A SEM can reach sub-nanometre resolutions, and a TEM ten times finer resolutions yet on thin samples. However, the electron beam also interacts with the environment along the distance that separates it from the samples, including air molecules – an electron microscope therefore requires a vacuum chamber (or at least, in the case of the more recent « environmental » models2,
1The Rayleigh criterion or Rayleigh limit: half the illumination wavelength.
2 These models, based on technology enabling airtight transmission between the gun and sample chambers, are meant to extend their application range to the observation of e.g. hydrated objects around the triple point of water, or objects that would be damaged by low pressure.
1. Micro-robotics, observation and actuation
an environment with controlled pressure and composition). Further, the samples and sample holders must be conductive, lest electric charges accumulate on them and interfere with the behaviour of the microscope. Despite these drawbacks, electron microscopy is the most popular when real-time vision feedback is required at resolutions not reached by optical microscopy.
Figure 1.3 – Images of a 20 30 m membrane by optical (left) and electron (right) microscopy.
Local probe microscopes
These microscopes use end-tools of various natures, sizes and operating principles, which have in common that they probe the observed surfaces with a stiff tip. In each case, the apex of this tip is of a nano- or micrometre scale and is brought very close or even in contact with the zones of interest. Imaging conducted with quartz probes falls into this category.
Atomic Force Microscopes (AFM) [?] : probes are either soft cantilevers deflecting and reflecting a laser, or more recently so-called « self-sensing » probes, such as the quartz resonators used in this work. Measured data are the Z (vertical) position of the probe and the interaction forces applied on the apex of its tip, be they attractive or repulsive. This combined information allows the reconstitution of a topographic profile of the sample’s surface, and/or the measurement of forces at the nano- or pico-Newton scales. The three main AFM modes are contact mode, intermittent contact (or « tapping »), and « non-contact » (or « near-contact ») modes. These modes of operation, as well as the state of the art in AFM, will be further elaborated on in the next section of this chapter. Aside from (or in conjunction with) microscopy, AFM probes can also be used as self-sensing manipulators [?].
Scanning Tunnelling Microscopes (STM) [?] can be used for conductive or semi-conductive samples. As the tip remains within 0.1 to 1 nanometre of the object’s surface, informa-tion is obtained through the electric current flowing between the two. Tunnelling can also be combined with atomic force microscopy on a same probe [?], with the drawback that any deformation caused by one mode of observation is then itself observed by the other with a phase shift [?]. Just as AFM can be used to push objects along with their imaging capabilities, so have STM been used for electrodeposition – for instance to build nanometre-scale batteries [?].
The capabilities of local probe microscopes vary widely across all types – the technology is however the most precise currently in use, and can reach the atomic resolution [?]. The main performance downside of local probe microscopy is the length of time it takes to scan an image: several minutes for micrometre-sized samples, and up to half an hour or more when aiming for higher-resolution images. This is one of the motivations behind seeking higher-frequency quartz resonators in the next two chapters, and the literature on the subject will be examined in Sect. ??. Although quartz local probes will be used as imaging tools in Chapter 3, the sample characterisation experiments that follow in Chapter 4 will require real-time vision: electron microscopy (SEM) will therefore be the observation method of choice. Whether it be for imaging or its applications in force sensing or characterisation, local probe microscopy will equally rely on micro-actuation systems; we now turn towards these sytems and their combined use with SEM.
Micro-robotic systems and SEM-controlled characterisation
Micro-robotic setups involve accurate movements at the micro-scale, carried through actu-ating systems at the end of which effectors are displaced by micrometric or nanometric steps. These systems can be composed of one or more actuators, and assembled into micro-robotic plat-forms with one or several effectors adapted to the environment, sample objects and observation method.
Micro-robotic actuators
Micro-robotic actuators exploit displacement phenomena which are not significant at the macro-scale but offer great accuracy in micro-manipulation. The main actuating principles used in micro-robotics are piezoelectric; electrostatic; thermal; SMA (shape memory alloys) or EAP (electroactive polymers) [?].
To this day, actuators used in micro-robotics have for the most part been based on the piezoelectric effect: deformation occurs when current is applied through piezoelectric ceramics in a quantified and repeatable manner, which is easily controlled in closed loop. This deforma-tion, when exploited directly, offers the ability to apply continuous displacement at extremely high resolutions proportional to the actuator’s size. Hence, the best resolutions and speeds [?] cannot be directly applied on a large range. A common way to remedy this problem is to have a small-range, accurate positioner placed at the end of a rougher, larger-range positioner chain. Another way to exploit the piezoelectric effect for larger-range positioning exists through the stick-slip principle [?] : during the « stick » phase, a progressive displacement is applied by a piezoelectric actuator, during which the effector is carried by its guide through friction; then the electric signal driving this displacement is suddenly inverted, and the subsequent, comparatively much quicker deformation or relaxation of the piezoelectric actuator withdraws the guide but lets the end effector « slip » (Fig. ??). Whereas the actuators used in Chapter 3 are piezo-stacks with a range limited to 50 micrometres, Chapter 4 will make use of stick-slip actuators with a range over a centimetre, which represents an advantage over a combination of distinct coarse-and fine actuators in that the position sensors remain the same throughout both large-range and close-range motion – meaning that the reference position between effector and sample is not lost after large-range relocation, in turn enabling enough flexibility for operations such as the local characterisation of samples on a larger-scale batch to go on uninterrupted.
Other types of positioners are found in the literature, especially in order to satisfy specific requirements with regard to performance or environmental conditions. General-purpose micro-robotic manipulation platforms (such as the SmarPod system used in this work), however, all rely on the piezoelectric effect.
SEM-integrated platforms
In order to use a manipulation system at the nanometric scale, the positions of the tools and effectors relative to the objects with which they interact need to be observed in a fairly precise manner. In the case of Atomic Force Microscopy (AFM), the manipulator itself can be used as a probe to obtain images before or during the manipulation, but more complex or exploratory operations call for more direct visual feedback. Using scanning electron microscopes (SEMs)
Figure 1.5 – Micro-robotic platforms and positioners designed for SEM integration; from left to right and top to bottom from the Universities of Toronto [?], Shizuoka [?], Oldenburg [?] and Nagoya [?].
Micro-robotics, observation and actuation
is advantageous in this regard. This choice is mainly motivated by performances that classical optical microscopy cannot offer: the nanometric imaging resolution, and the depth of field which is instrumental to simultaneously observing tools and samples. Indeed, in a typical setup, tools and samples end up being superimposed vertically during operations, and the field of view needs to be tilted at an angle in order to discern the precise interactions and contact points between the two (Fig. ??). In these conditions, it is often impossible to focus on both using optical microscopy. Without a depth of field such as that offered by a SEM, it is possible for AFM and derived technologies to rely on force-feedback to control the position of the tools relative to substrates and samples; one can also tilt the effectors rather than the whole system, or use angled tool tips [ ?] as is often the case for cantilevers. However, in some cases tools must stay perpendicular to the surface of the sample (see Chapter 2, Sect. ??) and, short of designing specific interface tools, a tilted field of view with a high depth of field may indeed be required for human-operated experimentation.
Figure 1.6 – Illustration of the reduced depth of field in optical microscopy and its impact when the observed surface is tilted – a.: Cantilever tool viewed from above; b.: Focal plane in a tilted setup; the depth of field cannot fully include both the probe and sample.
Dissuasive factors, on the other hand, are the cost and upkeep of a SEM. Besides, the tech-nical challenges related to the use of a SEM are themselves the subject of much work, usually concerning the vacuum chamber which is part of a SEM. When it comes to effectors and tools, these challenges include the potentially limited space of the chamber, the restriction to specific materials (vacuum-compatible, and nonmagnetic so as not to interfere with the electron beam), and the absence of thermal dissipation through convection (which is a limitation to how much heat the sensors and actuators are allowed to generate). For samples, difficulties include the fixation, loading and unloading inside the chamber, and specific conditions required for the ob-servation of hydrated or biological samples which otherwise exsiccate. Further, observed objects have to be sufficiently conductive so as not to be electrically charged (and thus rendered un-observable or even damaged) under the effect of the electron beam – this is often, for normally non-conductive surfaces, done through metallisation.
The overview of SEM-integrated nanomanipulation platforms as described in the literature shows that electron microscopy is a viable and valued tool in micro-robotics. The main char-acteristics for the micro-robotic platforms found in recent papers are summarised in table ??: embedding into a SEM correlates with nanometre-range experiments, and with flexible degrees of freedom; both are exemplified in the specific advantages of self-sensing AFM probes.
The operation of quartz probes in force sensing or mechanical characterisation, just like topographical imaging, finds its roots in Atomic Force Microscopy (AFM). This particular branch of local probe microscopy was developed by Binnig, Quate and Gerber in 1986, following the invention of the Scanning Tunneling Microscope (STM). AFM and STM have since been pre-eminent methods of observation and measurement at the nanoscale and below. Although AFM can be used for manipulation, i.e. the pushing and positioning of small objects, this aspect will not addressed here, the focus being on force sensing and imaging. This section will first summarise the general principles and control modes of AFM, then the use of tuning forks as AFM probes, the medium specificities of ambient and liquid environments, and the evolution towards faster AFM imaging.
General principles of AFM
AFM as it was first conceived [?] consists in using a cantilever as a probe: a soft horizontal beam, clamped at one end and with a sharp vertical tip at the other; this initial setup also allowed some degree of observation under an optical microscope. Countless other applications based on AFM were then developed. AFM can be combined together with another observation technique, such as optical microscopy for coarse positioning, an electron microscope for more precise yet relatively large-scale observation, or complementary tools such as fluorescence [?] for biological applications.
Figure 1.7 – AFM setup with a photodiode measuring laser deflection [?].
AFM modes
The term AFM in and of itself only describes the use of nanoscale force sensing, but can refer to very different ways of using a probe. There are three main variations of AFM: contact mode, or static mode: the cantilever’s tip is and remains in contact with the imaged surface, and the beam’s deflection is measured – usually through laser reflection (Fig. ??). During the scan, the cantilever or the substrate are moved along the vertical axis so as to maintain a constant application of force. This mode obviously involves high forces and frictions, and can damage both the tool and the observed surfaces.
« non-contact » or « near-contact » mode: the cantilever is excited by an oscillator and vibrates close to its resonance frequency. When the tip is brought onto the surface of the sample, the interaction forces influence the frequency, amplitude and phase of the oscillation, whence data is extracted to reconstitute an image. The oscillation amplitude must remain small, at a sub-nanometre or even sub-Angström scale, to remain within the range of attractive forces (Fig. ??).
« tapping » mode: this hybrid dynamic mode somewhat combines contact and dynamic modes. The cantilever is also excited, and the tip only briefly touches the surface, near the desired maximum amplitude when the oscillating movement reaches its end. This mode usually uses greater oscillation amplitudes than pure non-contact modes, as it needs to be able to pull free from adhesion forces.
Table of contents :
1 Probing into the micro-world: imaging, tools and state of the art
1 Micro-robotics, observation and actuation
1.1 Physics of the micro-world
1.1.1 Scale effect
1.1.2 Surface forces
1.2 Observing the micro- and nano-world
1.2.1 Optical microscopes
1.2.2 Electron microscopes
1.2.3 Local probe microscopes
1.3 Micro-robotic systems and SEM-controlled characterisation
1.3.1 Micro-robotic actuators
1.3.2 SEM-integrated platforms
2 Sensing and imaging through Atomic Force Microscopy
2.1 General principles of AFM
2.1.1 AFM modes
2.1.2 Dynamic AFM control modes
2.2 Tuning fork AFM
2.2.1 Operating principle
2.2.2 Force measurement
2.2.3 Conservative and dissipative forces
2.2.4 Stiffness measurement
2.3 AFM environments
2.4 High-frequency AFM
3 Objectives
2 Micro-robotic probes designed from standard quartz resonators
1 Mechanical properties of tuning forks
1.1 Quality factor
1.2 Stiffness
1.3 Sensitivity
2 Probe fabrication
2.1 Probe tip
2.2 Adhesive and balancing
2.3 Tip positioning
2.4 Probe holder and fixation
3 Towards higher frequencies
3.1 Tuning fork overtones
3.2 Other tuning fork frequencies
3.3 Thickness shear quartz resonators
3.3.1 Horizontally mounted resonators
3.3.2 Disc resonators
3.3.3 Contoured beam resonators
3.4 Comparison
4 MHz-range contoured beam quartz analysis
4.1 Hypotheses
4.2 Results
5 Conclusion
3 Ambient imaging with quartz probes
1 Experimental setup
1.1 Quartz control, electronics and software
1.2 Actuators
1.3 Control parameters
2 Experimental protocol and testing
2.1 Attractive and repulsive modes
2.2 Samples
2.3 Imaging criteria
2.4 Artefacts and causes
2.5 Preliminary internal characterisation
2.5.1 Actuator overshoot
2.5.2 Overall Z-axis drift
2.5.3 Adhesion and meniscus effect
2.6 Adopted conventions
3 Attractive mode FM-AFM
3.1 32.768 kHz probe on calibration grating
3.2 32.768 kHz and 196 kHz overtone probes on paraffin wax
3.3 3.58 MHz probe on silicon surface
3.4 Overview of the attractive mode
4 Repulsive mode FM-AFM
4.1 32.768 kHz probe on paraffin wax
4.2 100 kHz probe on paraffin wax
4.3 100 kHz probe on calibration grating
4.4 10 MHz probe on calibration grating
4.5 3.58 MHz probe on calibration grating
4.6 3.58 MHz probe on paraffin wax
5 Conclusion and perspectives
4 SEM-embedded micro-robotic sensing with quartz tuning forks
1 Quartz probe sensing and in air, vacuum and SEM environments
1.1 Setup and pressure
1.2 Effect on the piezo-actuating system
1.3 Quartz frequency, quality factor and dissipation
1.4 Influences on settling time
1.5 Influences on sensitivity
1.6 Influence of the electron beam in a Scanning Electron Microscope
1.7 Conclusion and perspectives
2 Membrane stiffness measurement in a SEM
2.1 Samples and setup
2.2 Membrane stiffness measurements
2.3 Results
3 Conclusion
Conclusion and Perspectives
