High- Aspect-Ratio Processes

1. High- Aspect-Ratio Processes • High-aspect-ratio processes provide a means for fabricating N/MEMS components with small lateral dimensions in comparison with their thicknesses. • Relatively thick and narrow structures offer high rigidity in the direction perpendicular to the substrate. • For actuator applications, thick, high-aspect-ratio devices offer the possibility of compact production of high torque/force because of the larger interaction areas. • Several methods are being investigated for the fabrication of high-aspect-ratio structures. At the present time the LIGA process is the best known method.

2. The LIGA Process • Besides the silicon micromachining a further important technology for the realization and production of microstructural components was recently developed. • This new technology is LIGA, which is a German acronym for LIthografie, Galvanik, and Abformung: in English it is lithography, electroforming and injection molding. • The LIGA process was developed at the Karlsruhe Nuclear Research center in Germany in ~ 1986.

3. LIGA uses high-intensity, low-divergence x-rays as the exposure source for the lithography. These x-rays are usually produced by a synchrotron radiation source. • PMMA (polymethylmethacrylate) is used as the x-ray resist. • A characteristic x-ray wavelength of 0.2 nm allows the transfer of a pattern from a high contrast x-ray mask into layer of thickness of up to 1000 mm so that a resist may be generated with an extremely high depth to width ratio. • Thicknesses of several hundreds of microns and aspect-ratios of more than 100 can be achieved. • However, a synchrotron is necessary for LIGA and because of this only very few research groups are currently using this process.

4. The LIGA Process • A resist is spin cast to a thickness of up to a millimeter, then exposed through an x-ray mask. • The exposed resist is developed away, and a metal is plated through the openings. • Finally, the remaining resist is dissolved, leaving the plated metal structure.

5. The LIGA Process (cntd.) • A thick layer of x-ray resist most likely (PMMA) is exposed to synchrotron radiation using an x-ray mask. • By developing the exposed areas, a plastic template of the desired structure is generated.

6. The LIGA Process (cntd.) • In the next step, metal is deposited into the resist structure by electroforming. • After stripping the resist either a final metal structure or metal mold insert for subsequent replication processes is achieved.

7. The LIGA Process (cntd.) • In view of a cost-effective mass fabrication process, plastic microstructures fabricated by injection molding of polymer materials can be used as final products or as templates for a second electroplating process.

8. Synchrotron Radiation Lithography • At the present, synchrotron radiation lithography is used in IC with characteristic dimensions in submicron range; for this application resist layers of a few microns are sufficient. Hence relatively soft radiation (l = 2 nm) is required. • In LIGA, one is concerned with the production of a plastic templates where the structural heights are several hundred microns. The x-ray process in this case is sometimes called x-ray depth lithography. So when using x-rays in LIGA advantage is taken of its high intensity. • At a characteristic l ~ 0.7 nm, a structural height of 400 mm is obtained by four times irradiating and developing PMMA sheet. At l ~ 0.2 – 0.3 nm a height of 500 mm can be obtained by a single radiation and development step. • However the effect of diffraction and the effect of secondary electrons both increase with decreasing wavelength.

9. X-Ray Masks and Resists • Because of the relatively short wavelength used in synchrotron radiation lithography in LIGA processes and the high densities involved greater demands are put on the masks in terms of transparency of the membrane, its resistance against high doses, and its contrast than are usually needed in IC synchrotron radiation lithography. • Within the relevant wavelength range only beryllium has sufficient transparency for being used as a mask membrane at a thickness permitting easy processing and handling. • The necessary absorption of the absorber structures is obtained by application of 10 to 15 mm thick gold layer on the membrane. • For fully utilizing the accuracy potential of synchrotron radiation lithography it is essential to use a resist and developer system with a ratio of the dissolution rates in the exposed and unexposed areas of ~ 1000. A suitable system is PMMA with a mixture of glycolic, an azine, a primary amine, and water as a developer.

10. Sacrificial LIGA (SLIGA) • An integrated fabrication technology combining sacrificial layer and LIGA (SLIGA) technologies has been developed for generating partially or totally movable structures. • First a sacrificial layer is deposited onto a substrate and patterned by photolithographic and wet etching processes. This is followed by x-ray lithography.

11. SLIGA (cntd.) • Electroforming steps follow with the additional requirement of exact adjustment of the x-ray mask and pre-structured substrate. • Finally the sacrificial layer is selectively etched leading to movable structures.

12. Electroplating • An important part of LIGA process is electro-plating to form metallic micromechanical parts in the mold. • Electroplating is metal deposition from ions in solution following the shape of the plating mold. • The advantages of electroplating are: • It is additive. • The thickness of the plated metal can be large since the plating rate is high. • A variety of metals can be deposited and co-deposited. • Plating results in smooth reflective metal surfaces for optical applications

13. Different Shapes in the Third Dimension: 1. Stepped Structures • This is fabrication of structures with different level heights. • For this purpose a two-layer resist system has been developed; the bottom is a high-molecular and adhesion promoting PMMA layer, and the top is lower molecular weight. • First the top layer is patterned. The bottom layer is then patterned by a deep x-ray lithography where the mask is adjusted exactly to the top layer. • Using this template, a metallic model insert for the mass fabrication of secondary plastic microstructures with different level heights can be realized.

14. Different Shapes in the Third Dimension: 2. Inclined Side Walls • Structures with inclined side walls can be fabricated by varying the angle between the synchrotron radiation and the resist, which is usually 90 °C.

15. Materials for LIGA Polymers • PMMA, with excellent structural accuracy, is for deep x-ray lithography. • However, the low radiation sensitivity and the problem of stress cracking susceptibility require the development of new x-ray resist materials. • Three classes of materials with higher radiation sensitivity than PMMA are being examined. These materials are based on polymethacrylimide (PMI), polyoxymethylene (POM), and polyalkensulfone (PASO). • However there are still other problems like the side-wall roughness or the compatibility with the electrolyte. • For molding processes PMMA and POM are used in commercially available forms. • New investigations mainly involve materials for higher temperature applications; e. g., fluorinated polymers.

16. Metals and Ceramics • The first LIGA structures were made of nickel, copper, or gold deposited from suitable electrolytes. • Nickel-cobalt electrolyte for the deposition of corresponding alloys has been developed for the generation of microstructures with increased hardness and elastic limit. • Other nickel- and palladium-based alloys have been used; it all depends on the application. • Ceramics such as zirconium oxide and aluminum oxide are being used in fuel cells, nozzles, valves or filter membranes. • Piezoceramic materials are also used in ultrasonic transducer applications.

17. Substrates • Up to now LIGA microstructures are generated on the surface of metals, ceramics, plastic, or glass substrates as well as Si wafers. • Depending on the substrate material, adhesion, and metallization layers must be deposited to enable resist coating and electroplating. • A special development target is the fabrication of LIGA structures on the surface of a processed Si wafer with ICs: this would allow the combination of LIGA structures with microelectronics in an integrated manner.

18. Example 20: LIGA Structures • Fabrication starts with a pn junction diode and metallization in a Si substrate. • This is followed by an x-ray lithography and electroplated nickel. • A separate sacrificial layer x-ray lithography and plating (SLIGA) is performed to create the stator and rotor. This allows the thickness and material properties of the stator, rotor, and conductors to be specified independently. • By making the rotor thinner than the stator, it is possible to use magnetic field to levitate the rotor, thereby reducing friction.

19. Fully nickel structure with high aspect ratio. Stator with bearing supports for external gear boxes.

20. Loaded magnetic micromotor. SLIGA version of electrostatic actuator.