Micromachining and Lithography

Lithography is the process of making mechanical parts and structures by photographically exposing a light sensitive material, or photo-resist, to create patterns that may either be used directly, or which may act as shields to allow selective etching of lower layers. Alternatively, it can produce moulds to fill with metals, ceramics, polymers, glasses or even bio- and nano-engineered materials. Lithography is the cornerstone of the semiconductor industry where the world's top ten manufacturers have combined annual sales in excess of US$400 billion.

The semiconductor microchip is a planar device and the lithography techniques used are essentially two-dimensional. The emerging field of microtechnology, or MEMS, where additional functions such as actuation, sensing or microfluidics are integrated with the intelligent microchip has brought the need to manufacture devices with three-dimensional structure. Techniques used for this to date include excimer laser micromachining, UV lithography, electrodischarge machining, and electrochemical machining. However none of these are capable of manufacturing structures with high depth to width ratios.

In 1991 researchers at the German atomic research centre (Forschungzentrum Karlsruhe) announced the development of a deep x-ray lithography technique using synchrotron light. They used this to manufacture arrays of fine gas nozzles used for the separation and enrichment of uranium.

They called the technique LIGA, a German acronym that stands for lithography, electroplating, and replication by injection moulding or embossing.

LIGA is a major advance; it produces structures with:

• micron-sized features
• aspect ratios (depth:width) of more than 100, which is essential for power transfer in micro devices
• optically smooth side walls, which is vital for telecommunications applications
• almost perpendicular walls, which is made possible by the highly collimated synchrotron x-ray beam.

By way of comparison, excimer laser lithography achieves aspect ratios in the region 4-16, has side-wall roughness of around 75 nm R.A. or more, and produces a 7-12 degree wall angle.

LIGA was developed using polymethylmethacrylate (PMMA) as the x-ray sensitive material. Precast sheets of PMMA are glued to a previously plated holder and exposed to x-rays. PMMA requires around 4500 J/cm³ to expose, leading to exposure times of around 2-3 hours on a synchrotron. More recently, an epoxy-based resist borrowed from the microelectronics industry called SU-8 has been used. Since this requires only 35 J/cm³ to expose, exposure times are reduced to the order of 60 seconds. This completely transforms the cost analysis of LIGA and has brought the process into prominence as a serious competitor in the market for mass-production processes.