R.A. Lawes

Future trends in high-resolution lithography

A perennial question is “what is the future of high-resolution lithography, a key technology that drives the semiconductor industry”? The dominant technology over the last 30 years has been optical lithography, which by lowering wavelengths to 193 nm (ArF) and 157 nm (F2) and by using optical “tricks” such as phase shift masks, off-axis illumination and phase filters, should be capable of 100 nm CMOS technology. So where does this leave the competition? The 100-nm lithography used to be the domain of electron beam lithography but only in research laboratories. Significant efforts are being made to increase throughput by electron projection (scattering with angular limitation projection electron beam lithography or SCALPEL). X-ray lithography remains a demonstrated R&D tool waiting to be commercially exploited but the initial expenditure to do so is very high. Ion beam lithography and extreme ultraviolet (EUV) (λ<12 nm) have also received attention in recent years. This paper will concentrate on some of the key issues and speculate on how and when an alternative to optical lithography will be embraced by industry.

The effect of excimer laser etching on thin film diamond

Excimer laser projection patterning with an ArF (193 nm) source has been employed in the irradiation of thin diamond films. The effect of a number of process parameters including laser fluence and processing ambient on the quality of the etch product has been investigated; scanning electron microscopy shows that good control of etch quality may be achieved with excellent lateral reproduction of images down to 2 μm. Raman scattering and Auger electron spectroscopy of irradiated films have been correlated, and modifications in the diamond surface have been quantified according to processing parameters. Electrical tests on laser modified surfaces show that the reactivities of metals have a major role in the performance of contact metallizations on such a material. The viability of excimer laser etching of diamond as a manufacturing technique is considered.

Comparison of focused ion beam and laser techniques for optimal mask repair

Abstract As the critical dimensions of masks and reticles approach 1 μm, the limitations of the laser repair technique are becoming increasingly apparent. The use of focused ion beams for mask repair offers a process with significantly improved resolution over that achievable using lasers. A comparison of the two techniques for opaque defect repair reveals a significant degree of mask damage associated with laser repair. In contrast, the FIB process shows much better edge acuity with little damage to the mask. Some staining of the glass substrate does occur but this is found not to be a problem during subsequent printing using standard processes.

Laminated dry film resist for microengineering applications

Abstract An innovative technology with a dry film resist (Riston) for use in microsystem engineering applications is presented. In view of the simplicity of forming a controllable resist thickness (20 – 100 μm/level), inherent planarization for multilevel processes in 3D microsystems and high, stable over-wafer thickness uniformity, we have investigated the use of dry film resists to realise high aspect ratio microstructures > 10:1 using both reactive ion etching (RIE) and excimer laser ablation.

Deep UV optics for excimer laser systems

Abstract The high irradiance at short wavelengths provided by Excimer Lasers has applications in conventional lithography, micromachining and laser chemical processing. In each case a new generation and type of projection optics will be required. In this paper, the problems of lens design in the deep uv for each of the above areas are reviewed. Emphasis is given to the practical designs actually constructed and results achieved at the Rutherford Appleton Laboratory. For some designs, diffraction limited imagery over large fields is shown to be possible. The theoretical and practical work reported here shows that certain catadioptric systems are sufficiently broadband to obviate the need to line-narrow the laser.

LIGA: a fabrication technology for industry?

LIGA is a technology that offers significant advantages where high accuracy, high aspect ratio microstructures are required. The application of LIGA to the manufacture of real products has been delayed by technical problems that exist with the individual process steps and the limited availability of integrated facilities, enabling users to subcontract the complete manufacturing process. These problems have been dominated by the limited availability of high quality masks, long and expensive exposure at synchrotron radiation sources and the electrodeposition of thick stress-free layers. This paper describes the practical solutions developed at the Central Microstructure Facility, RAL, for the key process steps of manufacturing high precision gold-on- beryllium masks, exposure of SU-8 resist using a 2 GeV synchrotron, electrodeposition of deep (

Sub-micron lithography techniques

GTR 500 mm), stress-free metal layers and resist stripping procedures fro 3 micrometers minimum features up to 500 mm deep on 4-6 inch wafers. A cost model shows that the reduction in the exposure time using SU-8 instead of PMMA resist may enable x-ray LIGA to be cost competitive with other techniques such as uv LIGA, DRIE or direct laser ablation.

LIGA for Boomerang

Abstract Improvements to the performance of semiconductor devices, such as increased speed and complexity, are achieved in part through the reduction of critical dimensions. Dimensions of approximately 1 μm are already being achieved on an industrial scale for silicon integrated circuits, while research and development into sub-micron fabrication techniques has been under way for some years. Techniques involving electrons, ions, deep UV light and X-rays are now being used for research purposes to produce devices with 0.1–0.05 μm dimensions.

193-nm imaging using a small-field high-resolution imaging resist exposure tool

Boomerang is a 3GeV synchrotron radiation accelerator, currently being constructed in the State of Victoria, Australia. The outline design of two beamlines, suitable for the fabrication of MEMS devices using the LIGA process, is presented, along with an estimate of the exposure doses throughout the resist. The most commonly used resist is PMMA, which requires a minimum dose of about 4500 J/cm3 for accurate microstructure definition. Exposure with such a dose, in resist thicknesses of several hundred microns, can take hours. Fortunately, SU-8 resist is becoming more widely used as the minimum dosage required is about 35 J/cm3, leading to exposure times of only a few minutes. Although Boomerang will shorten exposure times due to its higher irradiance at the substrate, the full benefits may not be realizable due to excessive resist heating. Heating effects have been simulated and suggest that helium cooling will be essential if the glass transition temperature of the resist (100°C for PMMA, 50°C for SU-8) and thermal distortion of the mask are to be avoided. The parameters chosen in this study of the future performance of Boomerang have been inserted into a cost model. The model shows that Boomerang exposure can become competitive with other exposure methods, particularly where large quantities of devices with deep structures are required.

Fabrication of 200nm field effect transistors by X-ray lithography using a laser-plasma X-ray source

A 193nm excimer laser microstepper has been developed for deep UV photolithography research at this wavelength. The system incorporates a x10, 0.5NA, 4mm field diameter, high-resolution imaging lens of either all-refractive or catadioptric design. An all-fused silica refractive lens has been used in the results reported here to carry out exposures in polymethylmethacrylate and polyvinylphenol photoresists. Well-resolved images of 0.2micrometers dense lines and spaces and 0.35micrometers diameter contact holes have been produced in PMMA and polyvinylphenol resists.