By combining the original LIGA-process with a sacrificial layer technique, movable microstructures are fabricated to realize microsensor and microactuator elements. As examples of movable microstructures a capacitive acceleration sensor, a microturbine, and a linear comb-drive have been fabricated. Their characteristics and results concerning their behavior are presented.<>

Electrostatic stepping micromotors have been designed with regard to low rotor friction on the axle. The design rules are determined using two-dimensional FEM calculations and taking into consideration the particular rotor position during motion. The motors are fabricated by a fully integrated LIGA process. The minimum driving voltages needed are measured to be about 60 V. Determination of the coefficients of friction in operation completes the rules for future designs.

Combination of the LIGA process with a sacrificial layer technique offers the possibility of fabricating in one process sequence metal capacitive acceleration sensors with a movable seismic mass and stationary electrodes on top of the same substrate. The process allows micromechanical devices with a free geometry to be designed. Use has been made of this advantage to fabricate extremely temperature-resistant acceleration sensors. Temperature resistance is achieved by designing the sensor partly with a positive and partly with a negative temperature coefficient. The design presented here is for a 1 g sensor with a measured temperature coefficient of offset (TCO) of 1.02×10−4g/K in the temperature range −10-100 °C.

AbstractDie Röntgentiefenlithographie mit Synchrotronstrahlung, kombiniert mit galvanischer Metallabscheidung und verschiedenen Abformprozessen (LIGA‐Verfahren), erlaubt den Aufbau von Strukturen beliebiger lateraler Gestalt mit kritischen Abmessungen im Mikro‐ und Submikrometerbereich sowie Strukturhöhen von bis zu einem Millimeter: Mikroturbinen von der Größe eines Staubkornes rotieren mit 150 000 U/min; Beschleunigungssensoren messen „unfallrelevante”︁ Verzögerungen an einem Kraftfahrzeug, um im richtigen Moment den Airbag auszulösen; Spektrometer, die nicht größer als ein Fingernagel sind, bilden zusammen mit chemischen Mikrosensoren kompakte und preiswerte Analysegeräte im Umweltschutz und in der Medizintechnik. Den heutigen Entwicklungsstand und die Perspektiven des LIGA‐Verfahrens beschreibt der folgende Beitrag.

In the first step of the LIGA process a resist layer is patterned by deep etch X-ray lithography. The exposed parts are dissolved by an organic developer. To achieve perfect microstructures the development must be free of residue, and the unexposed parts should not be attacked.Our studies showed that the solid PMMA material in the resin, as well as the unexposed resist contains at the applied developer temperature soluble parts. Therefore the molecular weight distribution of the resist and the solid PMMA material was investigated as a function of the hardener system respectively the initiator and the regulator concentration. Formation of low molecular weight molecules in the resist layer can only be suppressed if the hardener content is small and if the resin contains no low weight molecules. On the basis of these findings an optimised resist was produced and the quality of the microstructures considerably improved.

Three-dimensional microscale structures can be fabricated with the Liga process, which uses deep etch X-ray lithography, electroforming and plastic moulding. The basic process has been extended in order to increase the number of possible applications. Some of the extensions include: movable microscale structures produced using a sacrificial layer technique; structures with different shapes in the third dimension obtained by patterning plastic substrates as well as by a moulding technique and by X-ray lithography; and micropumps developed by combining Liga with a membrane technology. The improved process together with the great variety of materials (plastics, metals, alloys, ceramics) that can be used opens up many potential applications of Liga in microsystem technologies. A few prototype applications are briefly described: microturbines with diameters of the order of 100 mu m, rotating at 150000 rev min(-1) for 10(8) revolutions; microsensors which can measure acceleration with high accuracy, e.g. of crashing cars; micromotors with diameters of 0.25 mm driven by electrical forces; and micropumps which can deliver very small amounts of liquids and even gases. In the future all these microcomponents will be integrated with microelectronic devices to create 'intelligent' microsystems which are expected to change our life in the way that the microelectronic revolution did in the past.

Summary form only given. A prototype micropump made of a gold structure 100 μm in height, covered by a titanium membrane on one side and a glass plate on the other, is reported. The titanium membrane is 2.7-μm thick and deflects to near the bottom of the pump chamber (100 μm) under a pressure of 500 hPa. Pump chambers with diameters of 2 to 5 mm have been fabricated with volumes from 0.3 μl to 2 μl. To fabricate the pump, a low-adhesive carbon layer was deposited onto a silicon wafer and covered by a titanium layer. Openings in the titanium were patterned by photolithography and etching. On top of these openings perforated polyimide membranes were built up, forming inlet valves on the titanium layer. Then, the gold structure, aligned relative to the microvalves, was fabricated with the LIGA process. The microstructure was closed with a glass plate

Using the LIGA process combined with a sacrificial layer technique, microstructures with great structural heights and lateral dimensions in the micrometer range have been fabricated. These structures are used as microactuators. By using electrostatic linear actuators large displacements are achieved. A large torque is realized by a micromotor with a toothed rotor, friction being diminished by a rolling movement. Bimaterial elements based on PMMA and metal are designed to achieve a movement parallel to the substrate, to fabricate, for example, a gripper. Electromagnetic activation has been used to measure the fatigue behavior of LIGA micromechanical elements.

The LIGA process has become an accepted technology for fabrication of three-dimensional microstructures. The main process step of this technology is the deep X-ray lithography with synchrotron radiation (SR). In order to get a good aspect ratio the critical wavelength of the SR has to be 0.2 nm. Creating the radiation in a normal bending magnet with a flux density of 1.65 T the electron beam must have an energy of 2.36 GeV. For the generation of this characteristic SR a design of storage ring with a fourfold symmetry is proposed. Each quadrant has a DRA structure with 2 quadrupoles for matching the betatron functions to the desired values within the straight sections. Both the bending magnets and the quadrupoles of the DBA arc are split in order to insert the sextupoles. The ring has a circumference of 70.0 m with an emittance of 7110-7 mar-ad. The design allows the insert.ion of two wigglers at a later stage each with a length of 3 m.

It is known that the extension of the LIGA process by a sacrificial layer technique allows the fabrication of movable microstructures with great structural height without the need to assemble individual components. With this technique an electrostatic stepping micro motor has been fabricated and tested. The design of the motor has been optimized with regard to large torque by finite-element-method calculations. The radii of the rotor and the axle have been adjusted to the step width of the motor in order to realize a rolling movement of the rotor on the axle, which minimizes the friction.<>

The authors report that the use of a resist layer, which can be structurized either by plastic molding or by deep-etch X-ray lithography, in addition to the LIGA process allows the fabrication of stepped microstructures. This technique is based on the combination of a high-molecular base layer made of polymethyl methacrylate (PMMA) with an internal adhesion promoter and a low-molecular PMMA, which is suitable for the molding of microstructures due to the addition of an internal mold release agent. This resist system is applied to produce stepped microstructures on the basis of which mold inserts are manufactured by means of electroforming. Using these mold inserts, the molding of stepped LIGA structures of PMMA and of LIGA structures of polyvinylidene fluoride (PVDF) was carried out

The authors have demonstrated that it is possible to detect the number of revolutions of the LIGA microturbines easily and with high accuracy using a fiber optical system. The main advantage is that the fibre is integrated in the microstructure level. The microturbines have been driven for nearly 100 million revolutions. So far flow rates from 10 sccm min-1 to 50 sccm min-1 have been detected. As there is also a defined correlation between the number of revolutions and the flux of the gas it seems possible to use these microturbines as volumetric flow sensors.

By combination of the LIGA (German Acronym for Lithografie, Galvanoformung, Abformung) process with a sacrificial layer technique, movable microstructures with great structural heights can be fabricated. As a first example, a movable mass suspended on a cantilever which can be used as a capacitive acceleration sensor is presented. Calculation of the sensor characteristics shows that a capacitor gap width of 3-mu-m is needed. This capacitor has been fabricated up to a height of more than 100-mu-m. The movability of these microstructures is demonstrated by applying electrical and magnetic fields.

Moveable and flexible microstructures can be fabricated by a combination of the LIGA process and sacrificial layer techniques. A promising application of such flexible structures can be seen in the field of integrated optics, where the problem of precisely coupling fibers to integrated optical chips still demands a satisfactory solution. In the LIGA structure the fibers are guided by precisely positioned stop faces and then precisely located and prefixed by integrated spring elements. The fabrication of these spring-element arrays by the LIGA process has several advantages: the thermal expansion coefficient of the substrate can be matched to the optical chip material; the use of spring elements for prefixing simplifies the handling; and adhesives and related problems can be avoided. Additionally, a small center spacing of the fibers can be reached with a curved parabolic profile of the spring elements using the LIGA advantage of unrestricted design in the cross-sectional shape. First measurements of load deformation on the spring elements show that the values for the modulus of elasticity and for tensile strength found in literature for bulk nickel can be used for electrodeposited nickel too.

X-ray masks with up to 10-mu-m high absorbers made from gold and mask membranes made from titanium and beryllium fulfil the special requirements of the LIGA process. For titanium mask blanks a special technique has been developed to transfer the PVD-membrane form the substrate to a rigid frame. To adjust the X-ray masks optically, the metallic membrane is selectively etched under a self-supporting alignment mark.

The LIGA process is based on deep-etch lithography with synchrotron radiation, electroforming and plastic molding. This process allows three-dimensional microstructures to be fabricated from a variety of materials. By means of a sacrificial layer technique freely movable oscillating and rotating microstructures have been fabricated. The potential of the LIGA technology is demonstrated by the presentation of components for integrated optics, bandpass and highpass filters for the far infrared, acceleration sensors, and microgears.

By the LIGA process microstructures with structural heights of several hundred micrometers and dimensions down to one micrometer can be produced in submicron accuracy. Using a special sacrifical layer technique it is possible to fabricate movable as well as fixed microstructures in one process step. These microstructures are used as microsensors or microactuators. As first examples capacitive acceleration sensors as well as microgears with a shaft bearing used as microturbines have been produced by this extended LIGA technique.