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Lithography

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EUV Lithography - A new Manufacturing Paradigm
Meeting industry needs, Dr. Samir Ellwi, Vice President of Strategic Innovations Powerlase discusses EUV lithography

Extreme Ultraviolet Lithography – A new Manufacturing Paradigm

EUV Lithography technology is proving to be a most promising new technology for the fabrication of semiconductor chips. Dr. Samir Ellwi, Vice President of Strategic Innovations UK-based laser innovator, Powerlase, discusses advances in Extreme Ultraviolet Lithography (EUVL) sources.

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EUV Lithography technology in semiconductor chip fabrication has already demonstrated its potential to meet standards set by the International Technology Roadmap for Semiconductors for 2012, meaning semiconductors can be produced at 32nm and below. The improved EUVL technique exploits both Laser Produced Plasma (LPP) and Discharge Produced Plasma (DPP) as a source. The EUVL requires an efficient, highly scalable and flexible, cost effective and environmentally friendly EUV source in order to achieve high wafer throughput. Hence making the EUVL cost effective and flexible when it is introduced for 32nm node production and beyond.

Meeting the industry need for 2012
The current established method of microlithography for the production of semiconductor chips uses an excimer laser with a wavelength of 193 nm as a light source. This source is used in conjunction with a machine called a ‘stepper’ and involves a process of focusing light from the laser source through a prepared mask onto a wafer (the thin slice of semiconductor material that microelectronics are built upon), which sits inside the stepper machine. The light source passes through the mask and onto the wafer through a series of lenses, imprinting the desired image on the wafer.

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In the past this method has been perfectly adequate for semiconductor chip fabrication. However, the 193 nm standard will soon become out-dated as industry requirements have advanced to a point where manufacturers require better, more accurate printing for semiconductor advancements. The use of 193 nm has advanced as far as it can, following attempts to extend its usefulness by adapting it to Immersion Lithography. This involves replacing the air between the optics and the wafer with a liquid, such as water. The adaptation enables the creation of semiconductors at 45nm, an improvement over the previous 65nm standard. However, the creation of 45nm could be the limit for the Immersion Lithography technique as it cannot be improved upon any further without replacing the water with another liquid with a higher refractive index or, alternatively, lowering the wavelength of the light source used in the lithography process. It is clear the semiconductor industry demands a higher resolution standard of printing, which enables the creation of smaller nodes. This increase in higher resolution requires a much lower wavelength than that of 193 nm being used at present.

As such, Extreme Ultraviolet Lithography has been identified as the most promising next-generation lithography (NGL) technology and has set a new wavelength standard for the light source at 13.5nm. Meeting the new standard will greatly improve the accuracy of printing semiconductors, meaning they can now be produced at an accuracy of 32nm. In the future the EUVL technique will allow this to be refined further still, with accuracy reaching 22nm and even 16nm. This is exactly what the industry is demanding; smaller, more accurate printing, resulting in better performing microprocessors and semiconductors.

By 2012 the industry will have reached a point where a machine capable of producing semiconductor chips at 32nm will be a standard requirement. This requirement was set by the International Technology Roadmap of Semiconductors, which has been reviewed every two years since the early 1980s. This is not an uncommon practice. Computer processors, for example, have long had their development governed by Moore’s law. This was an observation made by Gordon Moore, one of the founders of Intel, in the mid-1960s. He believed that the power of computer processors would double every 18 months to 2 years. This is similar to the SEMATECH roadmap, which also follows a 2-year cycle.

The 32nm is scheduled to be in use by 2012 for high volume manufacturing of electronics goods such as Plasma Display Panels (PDPs) and computers.

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What Is The Solution?
The EUV Lithography stepper machine consists of three compartments; the source collector compartment, the illumination compartment and projection optics and the wave pattern photo resist compartment. Powerlase is currently contributing to the source collector compartment by providing lasers to create EUV radiation as a light source. It’s main objective is to provide lasers for both EUV sources; laser produced plasma (LPP) and discharged produced plasma (DPP). Recently, it provided the Extreme Ultraviolet Lithography System Development Association (EUVA), a world leading Governmental research association based in Japan, with one of its Starlase lasers in order for initial Research and Development work on a DPP source. Additionally, it has formed a collaborative research programme with the University of Central Florida (UCF) in the US to demonstrate an LPP EUV source.

The process uses the Starlase product design specifically to drive the EUV target and hence generate high EUV conversion efficiency.

Powerlase have already provided UCF with a kilo-class Starlase laser to irradiate the UCF’s tindoped micro-droplet laser plasma source. This source has demonstrated the highest conversion efficiency with a minimum amount of contamination. The combination of a high EUV conversion efficiency and the elimination of neutral and charged particles was the aim of this collaborative work. A second laser has also been delivered to UCF in order to increase the EUV output power. Professor Martin Richardson, Trustee Chair and Northrop-Grumman Professor of X-ray Photonics at UCF, says, “Powerlase’s investment in this collaboration demonstrates the potential advantages of solid-state laser driven laser plasma sources. In the short time of this collaboration we have already demonstrated EUV powers approaching 10 Watts, and expect further significant gains in the near future. This advance provides a viable technical pathway towards satisfying the power and cost requirements for EUV lithography.”

The combination of the Starlase lasers and UCF’s tin doped mass limited target has produced a high EUV radiation (13.5nm) with controllable charged particles that can be diverted or collected away from the EUV collection optics. The collection optics, EUV’s first collector, is a very sensitive and expensive optic that needs to last for a long period of time without any degradation.

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Greater Scalability
The LPP approach has some advantages in comparison to the DPP EUV source. It creates a much brighter source and has a much larger angle to collect the light, meaning more light is produced and collected, and therefore, better results are achieved without increasing the power to the source. The same result could be achieved in the discharge source, but this would require an increase in electrical power. This creates a heat management issue, which would result in damage to the electrodes.

Increased efficiency
There is also an environmental benefit. Efficient use of power is increasingly important to manufacturers, not only because of the cost issue, but also due to pressures from governments and environmental bodies to minimise the effects industry has on the environment. The LPP approach, particularly when driven by a diode pumped solid state laser (DPSSL) - that are very environmentally friendly, means advanced semiconductors can be produced at vastly increased throughput levels without an associated increased negative impact upon the environment.

Greater Throughput
The cost of producing semiconductor chips is measured initially by the cost of the manufacturing infrastructure required (such as the stepper machine) and in the longer-term by the throughput potential. In this instant, throughput is measured in terms of wafers produced per hour. Previously, the lack of a sufficiently bright source at a lower wavelength than 193nm has meant EUVL methods have not been cost effective, with throughput as low as 5-10 wafers per hour.

At present, the 193nm method limits the throughput of any manufacturing to between 60 and 80 per hour wafers per hour and with the new LPP approach the throughput can be increased to 100 wafers per hour – an increase of 25% and more. This increase also represents a potential reduction in the product pricing, as the increase in throughput will reduce the cost of producing wafers. Ultimately, the cost of semiconductors will be driven down, but more will be available.

Flexibility
The stepper manufacture currently requires the photo resist to have a sensitivity of 5 mill-Joules per cm squared. This determines how much power the source generates, as the higher the sensitively, the more power is required. If this sensitivity requirement were to be increased, say to 10 mill-Joules per cm squared, more power would be required by the source. With the LPP approach the power can be increased without increasing the number of lasers used.

Conclusion
The most significant impact of using the LPP approach is it provides manufacturers a greater level of scalability and flexibly in production. The fact the accuracy of printing can be improved without significant power increases combined with the ability to increase power depending on the level of sensitivity required by the resist, means manufacturers can tailor this technology to their specific needs. All of this can be done with minimal and in many cases, less negative impact on the environment than current processes.

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