Technology suitable for small quantity production
The LIGA technology comprises the processes of X-ray lithography, electroforming and moulding. The process combines photoetching and electrical plating. In the first step, a deep etching pattern is made in a photoresist after exposure to synchrotron radiation (SR) through a patterned mask. In the etched grooves, a metal is electrically plated. Finally, the plated part is demolded as a metal product.
LIGA enables the manufacture of micro-components made of non-silicon materials like plastics, metals and ceramics with almost any kind of lateral geometry and very high aspect ratios. This technique can basically produce patterns with straight walls. Since the horizontal section of the product does not change in relation to the vertical position, these kinds of shapes are often called 2.5dimensional shapes.
Owing to the straightness and high resolution of SR which is alight in the X-ray range, a high-aspectratio product with high horizontal resolution of submicron range is obtainable (The LIGA Technique, Catalogue of MicroParts Gesellschaft fur Mikrostrukturtechnik mbH).
For LIGA, in most cases, PMMA is used as resist material. In X-ray-lithography almost parallel high energy synchrotron rays enable the manufacture of very deep structures (up to 1000 µm depth, lateral dimensions down to 0.2 µm, surface quality Ra 0.3 nm) with almost vertical and very smooth side walls (aspect ratios from 50 to 500) [44]. If UV light or lasers are used instead of X-rays, less impressive resolutions and aspect ratios are obtained at relatively low cost. When these structures are produced in polymers, the exposed structured areas can be filled by electroplating with different metals like nickel, gold, copper or certain alloys. Once the PMMA is dissolved, metallic micro structures are left. The metal structures produced can be the final product; however it is common to produce a metal mould. This mould can then be used for injection moulding [45].
Although the materials that can be electroplated are limited to metals, the wide freedom in patterning leads to a wide range of applications.
This technology is also used for fabrication of tools for embossing or coining of polymers, although the development of new plateable alloys with improved thermal and mechanical properties are necessary if temperatures increase above 400 °C.
LIGA competes with WEDM because they target similar types of products. Compared to WEDM, the main advantages of LIGA are:
- Narrower space in the pattern is possible
- Surface of vertical Wall is smoother
- Batch process can cope better with mass production
The main disadvantages are:
- Formation of inclined walls or tapered holes is more difficult
- Choice of materials is narrow, while WEDM can machine most metals and alloys
- Lead time is longer because it requires the use of masks. Consequently, it is not suitable for small-series production.
[44] McGeough J.A. et al, 2001, Electroforming process and application to micro/macro manufacturing, Annals of the CIRP, 50/2: 499-514
[45] Kiaer E.M. et al, 2000, Micro injection moulding, Proc. of the 1st Euspen topical conference on fabrication and metrology in nanothecnology, Copenhagen, Denmark: 259-267