Enabling Technologies for Nano-scale Measurements Using Near-field Optics
The objective of this project is to develop a nano-scale optical measurement technology based on the near-field imaging principle. The development is aimed to improve the performance of the commercial SNOM systems for specific industrial applications in the failure analysis of nano-electronics and this would involve developing the nano-scale probe-sample sensing and control, the near-field modelling, the near-field signal detection, and the SNOM image processing. The technologies will be integrated to deliver a SNOM system capable of 50nm spatial resolution.
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To achieve high quality nano-scale probe regulation, adaptive control schemes are developed to overcome hysteresis and creep. This method is able to identify the nonlinearities and compensate them to a satisfactory level.
Near-field modelling will be investigated by a systematic approach. The functional model of the near-field system is used to explore the theoretical aspects of characterising the near-field. The FDTD approach will be tailored to numerical analysis of near-field modelling.
In near-field detection, homodyne and heterodyne approaches are explored to extract the near-field signals. To remove the low frequency noises such as drift resulting from the thermal effect on fibres, active homodyne and heterodyne schemes are developed.
To remove the noise effects in the SNOM images, adaptive 2D filtering technologies are to be developed to deblur the images from the distortions resulting from scanning motion. The near-field model developed earlier will also be used to remove the artefacts in the SNOM images.
There are four major application areas which use SNOM measurements extensively: Semiconductor, data storage, material science and life science. SNOM is used for failure analysis and would lead to tremendous savings. For example, in a typical semiconductor front-end fabrication unit, this can save about US$1.5 million per day in yield gain.
- SNOM in Semiconductor
In the semiconductor sector, older generation technologies were based on 200 mm wafers with typically 2 to 3 layers of about 0.25 microns feature sizes, aluminum interconnects etc. The next generation technologies involve 300 mm wafers, with 5 to 7 layers of feature sizes down to 0.045 um. Additionally, copper interconnects and low-k dielectric films posed greater requirements on the accuracy of failure analysis. This technological migration requires the demand from conventional 2D metrology to 3D metrology to increase. According to VLSI, the overall process diagnostic equipment growth rate till 2008 is around 15% annually, with a total market size of US$1.5 billion. The profilometry market, a relatively small segment, with an estimated market size will not exceed US$300 million by 2008.
- SNOM in Data Storage
The data storage sector is not as vibrant as the semiconductor industry in term of the technological transition. However, global consumers continue to drive strong demand for smaller and faster data storage. The growth rate in this sector is 24% till 2008 (VLSI). This strong demand is due primarily to the expanded application space of hard disk drives in MP3 players, digital video and storage networks.
- SNOM in Nanotechnology
World research funding in nanotechnology will increase at a double digit rate. It was estimated by the Nano Business Alliance that world-wide research funding in nanotech reached US$ 4 billion in 2004. Among the big spenders are Japan, US, Europe and China.
- SNOM in Life Sciences
SNOM applications in life sciences are yet to be popularised. Conventional measurement tools used are scanning electron microscopes, and transmission electron microscope. The unique capability of SNOM to reveal transparent top layers will see wider applications of SNOM.
The objective of the project is to establish a core competence in nano-scale measurements by using a SNOM system. To this end, the following enabling technologies have been identified and are to be developed in the project.
- Nano-scale probe-sample separation sensing: this is the first step to ensure proper probing of the near-field of optical signals.
- Nano-scale probe regulation using PZT: accurate regulation requires a good automatic compensation control of the PZT nonlinearities including hysteresis and creep which have been identified as the key factors limiting the SNOM imaging quality.
- Near-field modelling: as the near-field is formed by small aperture and small separation, the near-field optical field distribution depends on many parameters. Near-field modelling is required to quantify the relationship between the parameters.
- Near-field signal detection: as the near field optical signals decay very fast and are buried within significant background light signals, they are too weak to be detected. This technology is to detect the near-field signals from the noisy background.
- Scanning near-field image formation and processing: as the near-field is detected using a small aperture, the optical properties over a large area are obtained by scanning the probe over the area to form an image. The scanning motion causes distortion to the SNOM image. Moreover, probe wear also brings about artefacts in SNOM images. The technology is developed to remove the distortion and artefacts.