A brief study of LIGA Process
By —
KSHITIJ DWIVEDI
ADITYA KULKARNI
ROHAN KULKARNI
GANESH LOHIYA
NIKHIL MAHALE
(Students Of Vishwakarma Institute Of Technology)
LIGA is a German acronym for, Lithography — Galvanoformung — Electroplating Abformung — Molding Introduction. LIGA, the MEMS micromachining process, employs the three fabrication process steps listed below. LIGA fabrication is used to create high-aspect-ratio structures using x-rays generated by a synchrotron or relatively low-aspect-ratio structures using UV (ultraviolet) light.
Today, several microfabrication technologies are available for the fabrication of micro components and systems. Silicon-based processing is one of the most successful micromachining technologies that has been developed as extensions of standard IC and microelectronics planar processing. Others rely on high-precision engineering and laser structuring. Individual technologies, such as Si-micromachining or laser structuring, are, however, far from sufficient to meet the needs of a wide range of problems. The wide range of functions of most devices to be manufactured, the specificity of the environment in which they will operate, and the best cost/performance ratio for the targeted application. The following deficiencies of IC-based machining techniques have sparked interest in several non-Si based machining methods: The requirement for the use of application-specific materials in order to optimize the functions and performance of various devices , The desire to save money by using low-cost materials, The difficulty of creating truly 3D objects with planar-based processing remains a challenge.
Precision and ultra-precision mechanical, electro-discharge, LIGA-based, and laser-based micromachining techniques, to name a few, are examples of such alternatives, each with their own application domains and relative merits. LIGA-based processing, a series of microfabrication steps combining a step of deep X-ray lithography [(DXRL), also known as deep etch X-ray lithography], and subsequent additive plating-through-mask and molding, has progressed from emerging to well-established non-silicon alternative microfabrication technology for MEMS. In the fabrication of microstructures, LIGA technology offers distinct advantages over other manufacturing methods. Several R&D institutes around the world use and develop LIGA-based technologies. The LIGA process is being used for commercial purposes. LIGA technology has been developed over the course of two decades. During that time, other high aspect-ratio technologies, such as UV photolithography in thick resist like SU8, also known as ‘UV–LIGA,’ and Deep Reactive Ion Etching (DRIE) of silicon, have evolved and successfully challenged LIGA in some specific application areas. The basic LIGA process and some aspects of it are recalled here to highlight its strengths and discuss challenges, not in terms of material properties but of applications. The goal of this blog is to contribute to the discussion of the LIGA potential by summarizing proposals and ideas for LIGA applications that have been discovered.
The LIGA process involves the following steps:
l. A very thick (up to hundreds of microns) resist layer of polymethylmethacrylate (PMMA) is deposited onto a primary substrate.
2. The PMMA is developed after being exposed to columnated X-rays.
3. Metal is electrodeposited onto the primary substrate.
4. The PMMA is removed or stripped, resulting in a freestanding metal structure.
5. Plastic injection molding takes place.
The LIGA-fabrication process is composed of:
1. Exposure, 2. Development 3. Electroforming 4. Stripping
Step-1: Coating: Coat a layer with an electrically conductive surface with dense photoresist (300 m to > 500 m).
Step-2: X-Ray Radiation — X-ray lithography with prolonged penetration from heavily collimated X-rays to penetrate dense materials with well-defined sidewalls, can be resisted. Irradiation means exposing a dense coating of resistance to a synchrotron’s high-energy x-ray beam. The mask membrane is usually made of a low atomic number material like diamond or beryllium, or a thin membrane of a higher atomic number material like silicon or silicon carbide.
Step-3: Development: The design is engraved into the resist substrate using x-rays in this process, and the desired shape is formed.
Step-4: Electroforming: On the exposed conductive substrate base, metal is electroplated. Electroforming is synonymous with electroplating. The term “electroforming” refers to the use of plating to produce an individual metal object.
Step-5 A metal structure shaped after the photoresist is removed may be used as a mold.
To build partially freed, flexure-suspended objects, or fully freed instruments, sacrificial techniques are combined with the simple LIGA method.
Components for LIGA process
● MASKS
A translucent, low-Atomic number carrier, a patterned high-Atomic number absorber, and a metallic ring for orientation and heat removal include masks. Carriers are made of high thermal conductivity materials to minimize thermal gradients. Vitreous carbon and graphite are examples, as are silicon, silicon nitride, titanium, and diamond. Gold, nickel, copper, tin, lead, and other X-ray-absorbing metals are absorbers.
Masks come in a variety of styles. Electron beam lithography is used to produce the mask. (4 µm thick) This is a plated photomask that provides. Direct photomask, which is available. (80 µm thick).
● SUBSTRATE
A flat substrate, such as a silicon wafer or a polished disc of beryllium, copper, titanium, or another element, is used as the starting material. Electrical current may flow through the substrate as it is electrically conductive.
● PHOTORESIST
A photoresist is needed for the fabrication of high-aspect-ratio structures. As applied in thick layers, it shapes a mould of vertical sidewalls which must be stress-free. The usage of photoresist is determined by the type of Liga Process employed. PMMA (Polymethyl methacrylate) is used for x-rays . For UV-Ray, SU-8, a glue-down technique is used to apply a photoresist with a high molecular weight to the substrate.
· TYPES OF LIGA
There are two major LIGA-fabrication technologies: X-Ray LIGA, which utilizes synchrotron X-rays to make high-aspect-ratio structures, and UV LIGA, which is a more affordable approach that uses ultraviolet light to create structures with very low aspect ratios.
· X-Ray LIGA
The X-Ray LIGA is a microtechnology fabrication process developed in the early 1980s by a team led by Erwin Willy Becker and Wolfgang Ehrfeld at the Institute for Nuclear Process Engineering (Institut für Kernverfahrenstechnik, IKVT) at the Karlsruhe Nuclear Research Center, which has since been renamed the Institute for Microstructure Technology (Institut für Mikrostrukturtechnik, IMT) at the Karlsruhe Nuclear Research Center (KIT). LIGA was one of the first major techniques to allow the on-demand fabrication of high-aspect-ratio structures (structures that are much taller than wide) with lateral precision of less than one micrometer.
An X-ray-sensitive polymer photoresist, usually PMMA, bonded to an electrically conductive substrate is subjected to parallel beams of high-energy X-rays from a synchrotron radiation source through a mask partially filled with a solid X-ray absorption material in the process. Chemical removal of exposed (or unexposed) photoresist creates a three-dimensional matrix that can be filled with metal electrodeposition. To create a metallic mold, insert the resist is chemically stripped away. The mold insert may be used to make injection molded pieces out of polymers or ceramics.
The precision achieved by using deep X-ray lithography (DXRL) is the LIGA technique’s distinguishing feature. The technique allows the fabrication of microstructures with high aspect ratios and accuracy in a variety of materials (metals, plastics, and ceramics). Many of the practitioners and users are affiliated with or near synchrotron facilities.
· UV LIGA (X-ray-absorbing)
UV LIGA exposes a polymer photoresist, usually SU-8, to an affordable ultraviolet light source, such as a mercury lamp. Since heating and transmittance are not issues in optical masks, a plain chromium mask may be used in place of the technically advanced X-ray mask. Because of these reductions in complexity, UV LIGA is both cheaper and more available than its X-ray equivalent. UV LIGA, on the other hand, is less efficient at producing precision molds and is thus used where costs must be held low and very high aspect ratios are not needed.
· Micro-punching Head Fabrication.
Synchrotron Radiation Taiwan is the LIGA process’s lithography light source. X-ray has a small wavelength, less diffraction, high voltage, and energy. As a lithography light source, its composition allows for sub-micron accuracy, resolution, and a high aspect ratio. Figure 2 depicts the approximate finding. An electroforming method using Ni/Co alloy as an electroforming bath is attempted in this study to achieve extra-high hardness and strength. The micro-punch head produced by the Ni/Co alloy electroforming process is wear- and impact-resistant. The micro-punch method is based on the idea of conventional metallic material punching for obtaining well-finished products of high aspect ratio and multi-selective sheets.
· Deep X-ray lithography process.
To continue deep X-ray lithography, the normal LIGA method requires Synchrotron Radiation with a wavelength of 0.2–0.6 nm. Because of the light source’s outstanding collimation, a perpendicular sidewall may be achieved. Light’s high energy will drastically reduce exposure time. SRRC has increased the electron energy of the Taiwan Light Source to 1.5 GeV. To treat 4-inch standard wafers, a deep X-ray scanner with machine control was mounted. Helium at 100 bars is used to cool the uncovered samples. In this step, Figure 3 depicts the mask pattern. During a single exposure treatment, several pumping and ventilation steps are performed. Exposure time is 2 hours. After the absorber pattern has been moved to a PMMA resist sheet, the uncovered areas are dissolved with a developer. The production method determines the precision and aspect ratio possible for microstructures.
· Extra-high hardness electroforming process.
Phase of electroforming with extra-high hardness. After the resist pattern has been successfully transferred on a silicon wafer using deep X-ray lithography, the resist pattern is transferred into a Ni/Co mold using the electroforming technique. The metallization process consists of the following stages. First, a Ni thin film is sputtered on the resist template board, which serves as the seed sheet. Then the Ni/Co electroforming process is used to shape the Ni/Co mold with a thickness of 1 mm, followed by the chemical mechanic polishing (CMP) process to reduce the overall thickness difference of the Ni/Co mold after the electroforming process. The silicon wafer is then separated from the Ni/Co mold using KOH wet etching. and the resist is stripped off. Finally, with the metal mold, the nanoimprinting process is used to replicate the pattern. Figure shows the finished product of the Ni/Co electroforming process.
Micro punching process.
Following electroforming, a micro-punch head with the desired characteristics and extra-high stiffness is obtained. Polymer, paper, and metal can all be used in the imprinting process. Figure on the right depicts the micro-punching machine schematic, while figure on the left depicts real photographs of the equipment.
LIGA technology is particularly well suited for producing polymer components with high aspect ratios, smooth surfaces, and submicron accuracies. Mechanical systems and micro optics, for sensing, optical telecom, and datacom networks, are the major areas of modern applications. Large-area high-precision patterning and, more broadly, the fabrication of molding equipment for a variety of uses, including polymer fluidics, may be future high-impact areas. As opposed to more conventional fabrication methods such as milling or micro electro-discharge machining, the latter example’s incredibly smooth surfaces are the most appealing characteristic of LIGA.A second path, directly producing components with Deep-X-Ray-Lithography, is currently being investigated. Due to the long exposure times of the PMMA resist, the cost has been prohibitively high in this case.
References:
[1] Chantal Khan Malek, Volker Saile, “Applications of LIGA technology to precision manufacturing of high-aspect-ratio micro-components and -systems: a review”, Microelectronics Journal Elsevier Publication pp 131–143 (2004)
[2] Y. Chenga, B.-Y. Shew, M.K. Chyu, P.H. Chen, “Ultra-deep LIGA process and its applications” Nuclear Instruments and Methods in Physics Research A 467–468 (2001)
[3] C.T. Pan, P. J Cheng, S.C. Shen, M.F. Chen, R Y. Wang, M.C Chou, T.C Wu , “ Applications of LIGA on Micro-punching processes for Metallic Materials”, Materials Science Forum pp. 55–60 (2016)