BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a magnetic scanning or positioning system with at least two degrees of freedom.
2. Discussion of the Related Art
Magnetic actuators are ubiquitous. They are economical, reliable, easy to power and provide good power to weight and power to volume ratios. They can be found in a large variety of applications ranging from a large train to the smallest timepiece. However, most magnetic actuators—even the smallest ones—are still made using wound coils rather than a batch fabrication process.
Hard disk drives contain magnetic actuators for positioning the read/write heads. The actuators are usually compact and flat, but they are only able to generate a one-dimensional rotational movement. This limits their use to specific applications. Another design is needed for storage systems where a surface of a storage medium is to be scanned in x and y direction.
In the PCT patent application WO 96/07074, as published on Mar. 7, 1996, and currently owned by the present applicant, a fine positioning apparatus with atomic resolution is described. The fine positioning apparatus basically comprises magnetic actuators and—in the preferred embodiment—mechanical means for damping or decreasing the motion of said magnetic actuators. The driving system of said magnetic actuators is similar to a voice coil. The fine positioning apparatus according to WO 96/07074 is therefore referred to as ‘voice coil actuator’. It can be used in the field of Scanning Probe Microscopy such as Scanning Tunneling Microscopy (STM) or Atomic Force Microscopy (AFM) and/or in the field of data storage, where precise positioning of magnetic, optical, electrical or mechanical writing and sensing devices is crucial.
In the paper ‘Microfabrication and parallel operation of 5×5 2D AFM cantilever arrays for data storage and imaging’ by M. Lutwyche et al., Proc. IEEE Int'l Workshop on MICRO ELECTROMECHANICAL SYSTEMS (MEMS' 98), Heidelberg, Germany, Jan. 25-29, 1998, a fine positioning system with 5 degrees of freedom is presented. A 2D AFM cantilever array is scanned in x and y direction—i.e. parallel to a surface of the array—using voice coil actuators with ranges of 30 μm and 15 μm, respectively. Three additional voice coil actuators, also with a 30 μm-range, are used in a triangular arrangement to move and level the sample in z direction—i.e. perpendicular to a surface of the array. The main disadvantage of said fine positioning system is its volume and weight.
For data storage applications as well as in other applications such as optical beam scanners or optical focusing and alignment systems, a scanning or positioning system is needed, which is small, flat, lightweight, and shock resistant and which features fast response, low power consumption, and a large range of motion.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the drawbacks of known scanning or positioning systems.
It is still another object of the present invention to provide a scanning or positioning system with at least two degrees of freedom, which is small, flat and lightweight, and which features fast response, low power consumption and a potential range of motion between 1 μm and 10 mm.
It is still another object of the present invention to provide a scanning or positioning system which can be fabricated using common batch fabrication techniques, and to provide a method for making such scanning or positioning systems.
This is accomplished by the scanning or positioning system and fabrication process described in the present application. The scanning or positioning system comprises a supporting base equipped with at least one magnet, a movable platform equipped with at least two electrical coils, and suspension elements providing an elastic connection between the movable platform and the supporting base. The magnet and the electrical coils are arranged in such a way that translational and/or rotational relative movements of the movable platform and supporting base are generated when a current is passed through the electrical coils. The electrical coils are positioned flat on or in the movable platform, thereby forming an essentially flat arrangement with the movable platform.
The working principle and the basic arrangement of the coil windings and permanent magnets are shown in FIGS. 2 and 3. In FIG. 2 the windings are placed in the vertical field of permanent magnets, whereby one half of the electrical coil is located over a N-pole and the other half over a S-pole. When a current is passed through the electrical coil, a force is generated moving the electrical coil to the left (direction of current flow and magnetic field as indicated in the drawing).
In FIG. 3 the windings are placed in the horizontal or fringe field of permanent magnets, whereby one half of the electrical coil is located between the poles and the other half beside one of the poles. When a current is passed through the electrical coil, a force is generated moving the electrical coil upwards (in z direction; direction of current flow and magnetic field as indicated in the drawing). Using two electrical coils located on opposite sides of the movable platform allows to generate a tilt about a horizontal axis (x or y tilt). The up and down movement as well as the x and y tilt have only a limited range of motion. But nevertheless they are very important for many applications.
The flat arrangement of the movable platform and the electrical coils opens new possibilities for the construction and fabrication of the movable parts. Lightweight construction is enhanced in particular. A feature which is desired for fast response and low power consumption. The sensitivity to shocks and vibrations is reduced too. The resonant frequency is in the order of 100 Hz to 1 kHz for a range of motion between 100 μm and 1 mm.
Another advantageous feature of the scanning or positioning system with at least two degrees of freedom is its relatively large range of motion in horizontal direction.
NOTE: A piezoelectric actuator has a range of motion of about 10 μm or less.
Combining the flat arrangement of the movable platform and the electrical coils with a flat supporting base and flat (permanent) magnets yields a scanning or positioning system, which is potentially compact, lightweight, and flat and which has a good power to volume and power to weight ratio. The scanning or positioning system can be used in a large variety of applications including present and future data storage and imaging systems. The outer dimensions of such a storage system could be about 20 mm×20 mm×4 mm for the complete system.
NOTE: The smallest version of the known voice coil scanner has outer dimensions of about 30 mm×30 mm×30 mm.
Various modifications and improvements of the scanning or positioning system are as follows:
Power performance of the scanning or positioning system can be improved by placing a component part comprising a ferro-magnetic material on the side opposite to the permanent magnets e.g. by covering the system with a magnetic steel sheet which closes the magnetic circuit on top. This decreases the reluctance and makes the magnetic field more uniform. The cover sheet may have an opening, where the movable platform can be accessed.
The movable platform may be equipped with discrete flat coils which are attached e.g. by gluing or soldering.
Good flatness is achieved when the coils are spiral in shape, i.e. when the coil windings lay all in one single plane.
The fabrication is simplified considerably when the movable platform and the suspension elements are fabricated as one part. In this case it is possible to use the same substrate and/or process sequence for the fabrication. In the same way the movable platform and suspension elements can be combined with a supporting frame, to which said suspension elements are connected. Using the same substrate allows to apply batch or other mass production techniques.
Batch processing can also be applied when the electrical coils are an integral part of the movable platform. If the coils are located on the movable platform, the processes used are similar as in printed circuit board fabrication, whereby additive or subtractive processing may be used. Thick film processing may also be applied. If the coils are located in the movable platform, similar process steps as in the fabrication of integrated circuits are used. In both cases, the movable platform, the electrical coils and the electrical conductors needed to connect the coils can be fabricated in the same process sequence.
Of course it is also possible to combine the fabrication of the movable platform and electrical coils with the fabrication of the suspension elements and the supporting frame. Beside economical benefits batch processing has also the advantage that the resulting components exhibit maximum flatness.
Suitable substrates are e.g. oxidized silicon wafers or flat sheets consisting of SiNx or a ceramic material or a metal.
Optimum power to weight and power to volume ratios can be achieved when the electrical coils constitute the major part of the movable platform. Such a movable platform can be fabricated using the process described in the claims and the Detailed Description section.
Up to six degrees of freedom are possible, when the movable platform is equipped with additional coils and the supporting base with additional permanent magnets.
The suspension elements may have the form of long narrow beams. This has several advantages especially when the beams are fabricated together with the movable platform using the same substrate and/or the same process sequence. Long narrow beams will help to decrease stiffness of the platform suspension and to increase fatigue-life of the beams.
In a modification said long narrow beams are divided into at least two portions, whereby adjacent portions form a right angle. This allows an extended length by carrying the beams around the movable platform and free deformation in more than one direction.
System performance might be improved by using a ferro-fluid to close part of the air gap between the permanent magnets and the electrical coils. This allows better cooling of the electrical coils.
In a modification of the scanning or positioning system, the (permanent) magnets are located on or in the movable platform and the electrical coils on or in the supporting base. In this case the movable platform and the suspension elements can be made of a thin magnetic steel sheet. The supporting base is preferably also made of magnetic steel. This allows to achieve a thin air gap and a high magnetic induction giving the system a high power performance.
The scanning or positioning system can be used to advantage in data storage systems. Such a system may comprise a storage medium with nm-sized magnetic storage elements, one or several magnetic read/write heads located e.g. on an Atomic Force Microscope (AFM) cantilever, and said scanning or positioning system, which is used to approach, to align and to scan the storage medium with the magnetic read/write heads.
The invention can also be used to advantage in approach systems. Such a system comprises a planar device, e.g. a flat substrate which needs to be patterned, and a second device as e.g. an AFM cantilever or an array of AFM cantilevers each with one or several apertures. During patterning the cantilevers act as shadow masks as addressed in copending patent application 98118283.5 filed on Sep. 28, 1998, currently assigned to the present applicant. The approach system further comprises said scanning or positioning system, at least three controllers for the z movement and x and y rotation, and at least three sensors for detecting the bending of the cantilevers at three different locations. The output signal of the sensors is fed to the controllers. The approach system can be used to approach the substrate with the cantilever array so that the surfaces of the substrate and the cantilever array are essentially parallel, and to maintain the cantilever array at constant height with respect to the substrate.
The scanning or positioning system can also be used to advantage in scanning probe systems such as AFM or STM systems and applications. Due to the large range of motion it allows coarse as well as fine positioning.