« PreviousContinue »
ACTUATING AND SENSING DEVICE FOR
SCANNING PROBE MICROSCOPES
FIELD OF THE INVENTION
 The invention relates to the field of scanning probe microscopes. It specially relates to an actuating and sensing device for scanning probe microscopes as disclosed in the independent claim.
BACKGROUND OF THE INVENTION
 Microfabricated cantilever beams including a probing tip attached thereto are one of the main components of scanning probe microscopes (SPM), e.g. atomic force microscopes (AFM). A measuring mode of an SPM is the so called "dynamic" mode. In this mode a tip is brought very close to a sample surface and the cantilever is vibrated with a frequency, which is close to its resonance frequency. In different measuring modes, e.g. so called "tapping" or intermittent contact modes, the tip is allowed to touch the surface as the cantilever vibrates. While a sample is scanned, the distance between the tip and features of the sample surface vary. This variation causes changes in the gradient of interaction forces, e.g. van der Waals forces, between tip and surface. The resulting changes in the mechanical characteristics of the cantilever, e.g. resonance frequency, phase, vibration amplitude and Q-factor, are detected with external systems, e.g. optical deflection detection systems. Usually, the distance between cantilever and sample surface is controlled by a feedback system to maintain the characteristics property at a constant value.
 Cantilevers are typically vibrated by using a piezo slab attached to a cantilever chip. Spring constants k of conventional cantilever beams used in the tapping mode are usually k=l-100 N/m with resonance frequencies of 5-300 kHz. Low spring constant cantilevers are preferred because in that way the tip is less damaged or worn during operation. High resonance frequency cantilevers are preferred for high throughput or speed SPM measurements. As an example, it is difficult to use very soft cantilevers for dynamic mode measurements in air, e.g. with a spring constant with a value less than 0.1 N/m, if there is no sufficient vibrating amplitude provided, e.g. up to 1 fim. Water on a sample surface e.g. traps the tip to the surface without releasing it again. A piezo slab attached to the cantilever cannot effectively provide the tip with a sufficient excitation if the cantilever is vibrated at a frequency different from the first resonance frequency of the cantilever. The frequency of such a system—piezo slab and cantilever chip—is also not effective at higher frequencies than the first resonance frequency of the system.
 In other implementations of SPMs quartz tuning forks are used instead of microfabricated cantilevers. Tuning forks are electrical components mainly developed for electronic circuits. They are small mechanical resonators of a few mm in size and have a very high Q-factor, i.e. they are very sensitive to applied forces. The relatively easy accessibility to their resonance characteristics, e.g. by measuring the electrical conductance, make tuning forks attractive candidates for SPM applications. In SPM applications using a tuning fork, an SPM tip is attached to one prong of the tuning fork. The tip is attached either sideways or on top of the prong as disclosed e.g. in the documents U.S. Pat. No.
6,094,971 and EP 0 864 846. The disadvantages of such cantilever systems arise from the fact that the tips are fixed directly to one prong of the tuning fork. The symmetry of the tuning fork is broken. This reduces the mechanical Q-factor and makes it less sensitive to applied forces. Further, the vibration amplitude of the tip is always the same as that of the tuning fork itself. In addition, these probes are very stiff compared to conventional microfabricated cantilevers, i.e. tips are easily damaged during operation. Typical spring constants of prongs of tuning fork resonators are 1.8 kN/m with a resonance frequency of about 30 kHz.
 It is an object of the invention to provide an actuating and sensing device for scanning probe microscopes overcoming drawbacks of existing scanning probe microscopes, especially of scanning probe microscopes using tuning fork resonators.
 In particular, it is an object of the invention to provide an actuating and sensing device, which includes a tuning fork and a microfabricated cantilever.
 It is a further object of the invention to provide a symmetric actuating and sensing device that enhances Q-factor and/or resonance frequency of state of the art tuning fork resonators.
 These objects are achieved by the actuating and sensing device as defined in the claims.
SUMMARY OF THE INVENTION
 The actuating and sensing device of the present invention includes a fork-like device including two prongs, e.g. a tuning fork, and connection means with a probing tip, wherein the tip is connected to both of said two prongs of the fork-like device via said connection means. The fork-like device is used as a mechanical resonator to vibrate. The movements of the prongs are transformed via said connection means into movements of said probing tip, wherein said tip movements can be in different planes than in a movement plane of the prongs. The actuating and sensing device is preferably used in scanning probe microscopes (SPM), e.g. atomic force microscopes.
 One advantage of a tip being connected to two prongs of a tuning fork is that the symmetry of the tuning fork can be maintained. In this way the Q-factor of the tuning fork can be much higher than that of a state of the art tuning fork SPMs, e.g. the tuning fork is much more sensitive to applied forces.
 The tip can be connected to the prongs of the tuning fork using flexible and elastic connection means, e.g. spring means like plate or leaf springs, resilient elastic pieces of sheet materials, foils or wires. The small mass of the connection means added to the tuning fork tends to decrease the resonance frequency of the tuning fork. This also occurs in the case of state of the art tuning fork SPMs. However, in the present invention, the connection means gives an additional stiffness to the tuning fork and enhances the resonance frequency of the tuning fork. This effect is much more pronounced compared to that of the additional mass. Therefore, the resonance frequency of the tuning fork becomes higher than the original value, which is preferred for example in high speed SPMs. The system properties can be noticeably improved by using a tuning fork with attached connection means.
 In the present invention the movements of the prongs of a tuning fork cause movements of the connection means. While the tuning fork is vibrated at a frequency the connection means is vibrated at the same frequency. Large vibration amplitudes of the connection means, e.g. spring means, are obtained also when the connection means are vibrated at higher frequencies than their first resonance frequency. As an example, if the first resonance frequency of a tuning fork is 50 kHz and that of the spring means, e.g. a plate spring, is 8 kHz, the plate spring can be vibrated at 50 kHz with a vibration amplitude of e.g. 500 nm at its free end. The resonance frequency of the connection means needs not be the same as that of the tuning fork and can be chosen according to the needs of the user. This is not possible in state of the art SPMs. There the cantilever or tuning fork is vibrated with a frequency, which is close to its resonance frequency. Reasonably large vibration amplitudes of the cantilever or prong can only be obtained around the first resonance frequency of the cantilever or tuning fork, respectively.
 In the present invention, the connection means preferably have a shape with at least one axis or plane that is essentially a symmetry axis or plane of the connection means. Preferably the connection means has one or more symmetry axis and the tip lies on at least one of these symmetry axis. It is especially preferable if a symmetry axis of the connection means lies in or is parallel to a symmetry axis or plane of the tuning fork. With a symmetric arrangement of connection means, tip and tuning fork, the complete symmetry of the cantilever beam is maintained. This not only enhances the Q-factor of the tuning fork but also simplifies the movements of the tip, i.e. one degree of freedom out of three is limited so that the tip moves in a plane.
 In an embodiment of the present invention, the tip is connected to the tuning fork in at least three points: first connection points on or at the two prongs of the tuning fork and a further connection point. This further connection point is preferably located at the base of the tuning fork, e.g. on a symmetry axis or plane of the fork. A further connection of the tip with the tuning fork can e.g. be used as coupling means for the tip to an external source or unit. A coupling means can e.g. be an optical or electronical coupling location, e.g. a contact point for a voltage to be applied to the tip by an otherwise electrically isolated tip and preferably independent of any driving signal of the tuning fork. It can also be used e.g. for an optical coupling of light into or out of an actuating and sensing device as can e.g. be used in a scanning near field optical microscope (SNOM). A connection means having at least a further connection point is designed accordingly, preferably in a way to keep the symmetry of the tuning fork.
 Tuning fork, connection means and tip can be one single or separate parts of a scanning probe device. According to an embodiment of the present invention a tip and the connection means are one part, e.g. fabricated out of one block of material in one or several processing steps as is well known in microtechnology. However, a tip can also be part of a connection means in a way that the connection means itself serves as probing tip. The connection means are then provided with e.g. a sharp edge or corner. This can for example be achieved by shaping the connection means as a triangle. However, a probing tip can also be glued or
attached to a connection means by any suitable fixing technique. Conventionally available tuning forks like quartz or piezoelectric tuning forks, are usually covered by a metallic layer, e.g. a gold layer, that serves as electrical contact. Therefore, the connection means can also be attached to the tuning fork by other fixing techniques e.g. by welding or bonding. Preferably piezoelectric tuning forks are used, e.g. quartz tuning forks as used in watch applications. Depending on the material used to fabricate the actuating and sensing device, the tuning fork is not piezoelectric. This can e.g. be the case when tuning fork, connection means and tip are made of the same material, e.g. fabricated from one block of material, as e.g. silicon. In that case, the tuning fork can at least partially be covered with e.g. a piezoelectric layer. For tuning forks which contain semiconductor or conductor materials, it is possible to generate a movement of the prongs by applying electrostatic forces to the prongs. This can e.g. be realized by placing counter electrodes near the prongs to form a capacitor and apply a potential between the prongs and the electrodes.
 In SPM measurements, the tuning fork is resonated. Due to interaction forces between tip and sample surface changes in the mechanical characteristics of the tuning fork, e.g. resonance frequency, phase, vibration amplitude and Q-factor, or changes of the connection means, e.g. mechanical or positional changes, are occurring and are detected. As power source for a piezoelectric tuning fork resonator a current or voltage signal is applied to the tuning fork and a movement of the prongs is generated. For different kinds of tuning forks, e.g. electro-magnetically resonated tuning forks or conventional metal tuning forks, suitable power sources have to be chosen. Direct and indirect ways are possible for the detection of changes in mechanical characteristics of the tuning fork or for changes of the connection means. In a direct way the internal signals from the tuning fork are measured, e.g. a change in the electrical conductance of the tuning fork is sensed. In an indirect way external detection systems are used, e.g. optical deflection detection systems that detect e.g. changes of the position of a connection means due to the interaction of a tip with a sample surface.
 Connection means according to the present invention are preferably designed as spring means, e.g. plate springs, resilient stripes of solid material. These spring means can have spring constants with values lower than 0.1 N/m. Typical values of spring constants in experimental set-ups lie in a range of 0.03-80 N/m and preferably in a range of 0.04-30 N/m, e.g. 0.07 N/m. This is much lower than spring constants of state of the art tuning fork resonators that have values around 2 kN/m. It is even lower than typical spring constant values of 1-100 N/m of conventional silicon cantilevers. The cantilever system of the present invention is therefore much softer than state of the art cantilevers, i.e. tips are e.g. less often damaged during operation.
BRIEF DESCRIPTION OF THE DRAWINGS
 In the following, preferred embodiments of the invention are described with reference to drawings.
 FIG. 1 schematically shows state of the art tuning forks including a probing tip.