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When the chips are down

The effects of the crash in the semiconductor market are being felt throughout the photonics industry. While the timescale of the slump is unknown, once growth does recover lasers will be at the forefront of silicon chip production. Greg Blackman looks at some of the applications

The semiconductor industry is suffering one of the worst crashes in its history as the global recession drives demand for electronic devices down. The market research firm, Gartner, recorded a drop in semiconductor wafer demand of 36.3 per cent in the fourth quarter of 2008. And this is by no means a trend describing the state of the market solely in the west: China’s semiconductor industry is predicted to fall by 5.8 per cent to $72bn in 2009, compared to 2008’s $76.5bn, according to electronics market analysis provider iSuppli. The company also predicts the global market will decline by 9.4 per cent in 2009, but there will be renewed growth in 2010.

The slump has had a knock-on effect on equipment providers for semiconductor manufacture. However, lasers play a key role in improving semiconductor manufacture and, once growth in the industry does return, laser systems will be used increasingly to produce the next generation of silicon chips.

Semiconductor fabrication is the process whereby an integrated circuit (IC) is formed on a wafer of semiconductor material. The process involves numerous photographic and chemical stages to gradually build up electrical circuitry on a semiconductor wafer. The pattern of the circuit is marked out by photolithography, etched, and then doped with metal ions to define circuit elements.

Typical lithography techniques used in microfabrication of silicon chips utilise deep ultraviolet (DUV) light at around 248nm. The wafer is coated with a light-sensitive photoresist and irradiated with DUV light, shone through a mask, to create the pattern of the circuitry. A chemical etching process then follows to wear away the marked areas.

Technological advances in semiconductor manufacture are geared towards squeezing more and more transistors onto a chip, as processing speed and storage capacity are both improved with higher transistor numbers. The lithographic printing process depends, in part, on the wavelength of light used: the shorter the wavelength, the smaller the printing process – and, therefore, the more transistors that can be fitted onto a wafer. The often-quoted Moore’s Law, stating that the number of transistors that can be placed on an integrated circuit will double every two years, will be maintained by various advances in semiconductor manufacture, but developments in the lithography process must be among those.

Next-generation lithographic processes include using extreme ultraviolet (EUV) light at 13.5nm to provide increased resolution. Powerlase, based in Crawley, UK, manufactures high peak-power, high average-power lasers for EUV generation that can be used in lithography or mask inspection. ‘EUV wavelengths of 13.5nm are difficult to produce and require a plasma environment of tens of thousands of Kelvin,’ explains Samir Ellwi, VP of strategic technology at Powerlase. The plasma environment is generated by ionising specific material using a laser beam focused to a very small spot size (approximately 30-100μm). The technology is also easily scalable: adding more laser power will result in a more powerful EUV emission.

Lithography is used during front-end processing, or the formation of the transistor directly onto the silicon, but laser systems are also used in processes such as dicing. Wafer dicing separates individual silicon chips from the wafer or ceramic tile and can be carried out by mechanical sawing, laser-scribing and breaking, or laser-cutting.

‘Wafer singulation is one application where laser systems are proving an efficient alternative to mechanical sawing,’ notes Ralf Schmidt, international sales manager for the semiconductor industry at Rofin, a global provider of lasers for industrial materials processing. ‘Using a laser to make the cuts allows various geometries to be processed. Mechanical methods are limited to cutting in only the x-y direction.’ The rounded edges of micro SD cards in mobile phones, for instance, are produced using laser singulation.

Laser systems are used to produce high quality cuts in wafer singulation applications. Image courtesy of Rofin.

A further advantage of lasers is the production of high-quality cuts that place minimal stress on the chip. Mechanical sawing is subject to smearing of the copper plating if the blade is overused and therefore, to avoid this, the blade must be changed regularly. Using an ultrafast, pulsed laser exerts less stress on the chip than mechanical methods and feed rates are also higher.

Lasers are cost-effective

The type of laser used depends very much on the application. For scribing, a picosecond fibre laser has the advantage over CW lasers of reducing the amount of heat the device is subjected to, and also produces high-quality edges. The heat-affected zone on the material is reduced by using an ultrafast, pulsed laser system such as a picosecond laser.

Regardless of the downturn in chip demand, the applications within semiconductor fabrication remain the same and the industry is always looking for cost-effective methods to increase production. Ellwi of Powerlase comments: ‘While the slump in semiconductor chip manufacture has certainly had a negative impact on laser system sales in some areas, in other areas the downturn has actually worked in the laser provider’s favour.’ He notes that the cost of ownership of certain machinery used in semiconductor fabrication is high and, in response to economic constraints, companies are re-evaluating manufacturing processes. Laser-based methods are often more cost-effective than alternative mechanical methods and, in an effort to lower production costs, laser systems will be implemented to replace the existing equipment.

For example, as part of thin film solar cell production, each panel is electrically isolated to ensure that no current can pass between the active area and the mounting frame. One way of doing this is through sand blasting, a costly procedure requiring a specific type and grade of sand that can’t be reused. The alternative is to use a laser system with nanosecond pulse duration, which is much more cost-effective.

Schmidt of Rofin says that most of the semiconductor industry is currently running reduced production rates and this has impacted on laser providers. There are, however, still applications that are relatively healthy in production turnover. Renewable energy is an area that continues to grow and production levels of solar cells and LEDs remain high.

Ellwi is optimistic that new semiconductor applications will emerge in which lasers will play a role. Over the last decade, the automotive industry has seen much of its manufacturing processes replaced with more efficient technology with the laser at the forefront. Ellwi feels that a similar occurrence is likely to happen in the semiconductor industry. The use of laser systems in wet lithography in the production of plasma screens is a further example of how photonics is improving the efficiency of production. Lasers have replaced the vast majority of processes in the production of plasma TVs and have cut the overall price by 30 to 40 per cent.

‘No one is sure when the semiconductor market will recover, but there will always be a demand for the latest version of mobile phones or computers,’ Schmidt says. The market has seen rapid growth in the past and once it begins to recover Schmidt expects further growth in the sector.