Microsystem Technologies 2 (1996) 178 —181  Springer-Verlag 1996 Electrochemical post-processing of LIGA structures S. Akkaraju, Y.M. Desta, B.Q. Li, M.C. Murphy Abstract The Louisiana State University (LSU) Microsystems Engineering Team ( SET) is developing techniques for producing microstructures with predictable 3-D geometry. One approach is based on controlled anodic dissolution of electroplated structures to obtain desired shapes. A finite element model was developed to predict the resulting shapes. As a demonstration of the shaping process, development of LIGA-based scanning probe microscope tips was investigated. An array of micro-posts 1.8 micrometers in diameter and 12 micrometers high was produced using x-ray lithography and subsequent electrodeposition. The posts were sharpened electrochemically. Other geometries are being investigated. 1 Introduction The LIGA technique has been demonstrated as a viable method for mass production of high aspect ratio microstructures (Becker et al. 1986; Ehrfeld et al. 1988; Guckel et al. 1990). A wide range of microdevices and structures ranging from micropumps, micromotors, optical components and accelerometers have been fabricated using the process. One advantage, and limitation, of the process is the restriction to projected 2-D patterns along the line of the incident x-ray beam. A new technique of electrochemical post-processing to obtain cross-sectional shape control of LIGA structures is proposed. This would be extremely useful in making devices with true three-dimensional geometries. The LSU Microsystems Engineering Team (SET), is investigating the possibility of shaping the LIGA structures using anodic dissolution techniques. Electrochemical shaping, Received: 30 October 1995/Accepted: 18 December 1995 S. Akkaraju, Y.M. Desta, B.Q. Li, M.C. Murphy Microsystems Engineering Team, 2508 CEBA, Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803, USA Correspondence to: M.C. Murphy We would like to thank all the faculty members of the Microsystems Engineering Team, Dr. Marc Madou of Microfabrication Applications, and Dr. R.L. McCarley of the Department of Chemistry, LSU, for the discussions and valuable input. Thanks also to Mr. Z. Ling and Vikas Galhotra for technical assistance, Yuli and Olga Vladimirsky at CAMD for the exposures, and David Ortte for helping out in the electrochemical sharpening experiments. developed circa 1950, has been used in wide range of applications. These include sharpening surgical tools, electrochemical machining (ECM) of machine parts, and electrochemical micromachining of (EMM) thin films (Datta 1992, 1995). As a demonstration of the technique, the fabrication of LIGA-based probes for scanning probe microscopy (SPM) was investigated. Extremely fine, high aspect ratio tips are essential in atomic force microscopy (AFM) and scanning probe metrology. The state-of-the-art for fabricating SPM tips involves silicon micromachining techniques [Albrecht (1989)]. These techniques place limits on both the choice of materials and aspect ratio. Better tips are essential for improved resolution and imaging. The range of potential applications includes direct DNA sequencing (Hansma et al. 1993), investigation of living cells (Ho ¨ rber et al. 1992), and metrology applications in microelectronics (Vasile et al. 1991). Each of these applications imposes special needs on the design of the probes. For example, low aspect ratio tips with magnetic properties may be beneficial for metrology applications, multiplexing tips may be needed for gene sequencing, and flared tips would help in profiling vertical side walls. A ‘tool-set’ of tips made specifically to meet the needs of the different applications would help improve on imaging capability. 2 Proposed process outline Electrochemical shaping of the structures is based upon the principle of anodic dissolution. The LIGA structures are electrochemically machined to obtain the desired shape. The counter electrode geometry determines the final shape of the structures. The electrochemical etching of the structures can be carried out using a platinum mesh electrode positioned 2—3 centimeters away from the microstructures. The anodic dissolution is not isotropic due to the uneven current distribution and the microstructures can be shaped accordingly. If specific shaping is desired, the counter electrode may be lithographically patterned and metallized around the LIGA structures. Figure 1 shows a possible shaping process. In the first step, synchrotron x-ray lithography is used to pattern a photoresist. This is followed by development of the resist, leaving a primary plastic template corresponding to the mask. The plastic template is then filled by electrodeposition to obtain the initial LIGA structures. 178