Elsevier

Microelectronics Journal

Volume 35, Issue 2, February 2004, Pages 131-143
Microelectronics Journal

Applications of LIGA technology to precision manufacturing of high-aspect-ratio micro-components and -systems: a review

https://doi.org/10.1016/j.mejo.2003.10.003Get rights and content

Abstract

The by far leading technology for manufacturing MEMS devices is Si-micromachining with its various derivatives. However, many applications of microsystems have requirements on materials basis, geometry, aspect ratio, dimensions, shape, accuracy of microstructures, and number of parts that cannot be fulfilled easily by mainstream silicon-based micromachining technologies. LIGA, an alternative microfabrication process combining deep X-ray lithography, plating-through-mask and molding, enables the highly precise manufacture of high-aspect-ratio microstructures with large structural height ranging from hundreds to thousands of micrometers thick. These tall microstructures can be produced in a variety of materials with well-defined geometry and dimensions, very straight and smooth sidewalls, and tight tolerances. LIGA technology is also well suited for mass fabrication of parts, particularly in polymer.

Many microsystems benefit from unique characteristics and advantages of the LIGA process in terms of product performance. The LIGA technology is briefly reviewed. The strengths of the manufacturing method and its main fields of application are emphasized with examples taken from various groups worldwide, especially in micromechanics and microoptics.

Introduction

Several microfabrication technologies are available today and are used to fabricate microcomponents and systems. The most successful micromachining technologies have been developed as extensions of standard IC and microelectronics planar silicon-based processing. Others are based on advanced precision engineering and laser structuring. However, individual technologies including Si-micromachining or laser structuring are far from being sufficient to fulfill the needs of the variety of problems posed by:

  • •

    The great variety of functions of most devices to be made,

  • •

    The specificity of surroundings in which they will operate,

  • •

    The optimum cost/performance ratio for the targeted application.

Interest in a number of non-Si based machining methods stems from major deficiencies of IC-based machining techniques:

  • •

    The need for using application-specific materials to optimize the functions and performance of various devices,

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    The need to reduce cost by choosing low-cost materials,

  • •

    The difficulty to construct truly 3D objects with planar-based processing continues to be a challenge.

Precision and ultra-precision mechanical, electro-discharge, LIGA-based, and laser-based, micromachining techniques, to mention the most current ones, are such alternative techniques, each with their specific application domains and relative merits. LIGA-based processing, a sequence of microfabrication steps combining a step of deep X-ray lithography [1] [(DXRL), also called by some authors deep etch X-ray lithography], and subsequent additive processing of plating-through-mask and molding [2], has moved from emerging microfabrication technology to well-established non-silicon alternative microfabrication technology for MEMS. The LIGA technology provides unique advantages over other manufacturing methods in the fabrication of microstructures. LIGA-based technologies are used and further developed in a number of R&D institutes around the world. Spin-off companies and commercial companies have also evolved around large-scale synchrotron facilities. Commercial application of the LIGA process is occurring.

This short review will not go back to the physical and technological fundamentals of the technique [3], [4], [5]; it is rather intended to recall the essential steps of the process sequence and focus on a number of selected examples from recent work performed in various LIGA groups around the world and show the usefulness and advantages of this technology.

The LIGA technology has been developed over a rather long time span of two decades. During that time other high-aspect-ratio technologies such as UV photolithography in thick resist like SU8, often referred to as ‘UV–LIGA’ and Deep Reactive Ion Etching (DRIE) of silicon have evolved as well and challenge LIGA successfully in some specific application areas. For planning LIGA role in future manufacturing, a review of potential applications may serve as a basis.

The fabrication of LIGA-parts concerning in particular the lithographic aspect, materials base expansion through replication technologies such as electroplating and moulding and some associated challenges, as well as some materials issues was reviewed in a former article by one of the authors [6].1 The basic LIGA process and some aspects of the process are recalled here to illustrate its strengths and discuss challenges, not in terms of materials properties [7] but in terms of applications. The purpose of this article is thus providing input on the discussion of the LIGA potential by summarizing proposals and ideas for LIGA applications found in literature.

Section snippets

Basic process

The basic LIGA process is described in Fig. 1. In the first step of the LIGA process, an X-ray sensitive polymer (resist) layer up to several millimeters thick, typically polymethylmethacrylate (PMMA) is coated onto a conductive substrate. A pattern from a mask is therefore transferred into the thick resist layer via a 1:1 shadow proximity printing scheme using hard X-rays from a synchrotron radiation source. After exposure, selective dissolution of the chemically modified irradiated parts of

Applications of LIGA

Applications for deep microstructures exist in many sectors of R&D activity and as industrial products, worldwide [44], [45], [46], [47], [48], [49], [50]. The presence of thick, deep and highly precise microstructures with high aspect ratios is a requirement for a number of micromechanical, optical or packaging applications, as well as in other fields.

LIGA products

Though there is active R&D in LIGA in several groups around the world and prototyping with commercial companies is active, commercialization of LIGA products has been rather slow but is developing. Products manufactured by using LIGA and associated processes have emerged worldwide in companies [192], [193], [194], [195], [196]. In addition, LIGA foundry capabilities have been established, providing easy access to synchrotron radiation [197], [13]. Several groups are also offering process

Conclusion

LIGA is a flexible technology that offers several advantages over other microfabrication techniques. The fabrication of components, systems or molds using Deep-X-Ray-Lithography remains the most precise batch technique available for the manufacture of microobjects with large structural height and high aspect ratio. Most of the interest in LIGA is associated with the ability to provide these microstructures in a large selection of materials, in particular in metal or polymer, through replication

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