Silicon with superdeep nanopores excludes light
Researchers from the MESA+ Institute for Nanotechnology at the University of Twente, the FOM Institute for Atomic and Molecular Physics in Amsterdam and ASML have developed a new method for producing nanopores in silicon. Thanks to this approach they have succeeded in making nanopores with a record depth. These new structures can, for example, be used in chemical sensors and in capacitors in high-frequency electronics. As the method developed is compatible with the technique already used by industry, integration with new structures in silicon chips is possible. The researchers published their results on 9 April 2008 in the leading British journal Nanotechnology.
The success of the ICT industry depends on the ability to produce large quantities of chips. Machines usually use silicon wafers as large as LPs on which a quantity of the same micro- and nanostructures are fabricated. This is realised using Complementary Metal Oxide Semiconductor (CMOS) technology. The smaller the structures, the greater the quantity of information that can be handled by a chip. A new challenge is combining electronic chips with optical communication so that considerable information densities can be conveyed. This can only be realised if the chips are made suitable for optical structures, especially for periodic arrays of nanostructures in silicon.
New method
Researchers at the MESA+ Institute for Nanotechnology at the University of Twente, the FOM Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam and ASML, global leader in lithographic systems for the semiconductor industry, have developed a new method for the manufacture of nanopores in silicon. The researchers applied the desired structures in a photosensitive mask using so-called deep-ultraviolet 'step-and-scan' lithography, which was developed by ASML. The mask changes under the influence of UV light and served as a mask: the covered silicon remains and the exposed silicon is removed by etching. Then the researchers substantially modified the plasma-etching process routinely used by industry so that it could produce very deep nanopores.
Using this method the researchers managed to realise an aspect ratio of more than 16, a world record. The aspect ratio is the ratio between the depth of the pore and its diameter, an important measurement for this type of nanopores. The nanopores have a diameter of up to 8 micrometres (1 micrometre is a millionth of a metre) and a diameter of 310 to 515 nanometres (1 nanometre is a thousandth of a millionth of a metre). Distances between the pores varied from 440 to 750 nanometres. The researchers discovered that limiting unwanted etching of the sidewalls during the etching process is the crucial factor that enabled them to achieve such large depths.
Applications
The nanostructures make many interesting applications possible. Optical reflection measurements demonstrated that the new structures act like very good photonic crystals. A photonic crystal is a highly ordered nanostructure that functions as a hall of mirrors for photons. Interference effects make the propagation of light with certain colours impossible in many directions. The forbidden colour spectrum is termed the ‘photonic gap’. The availability of these deep nanopores with a wide range of diameters brings the emergence of a three-dimensional photonic crystal a step closer. Such a crystal can be used to direct photons with extreme precision. Furthermore, it is important to make the nanostructures as deep as possible so that a photonic crystal with a large volume can be obtained. The silicon structures produced have an intensive reflectivity at wavelengths of 1330 and 1550 nanometres and are therefore interesting for the telecom industry. Other applications are chemical sensors and capacitors in high-frequency electronics for, amongst other things, mobile phones.
Being able to use such structures in existing silicon chips opens up a wealth of possibilities. The compatibility of the manufacturing process with existing CMOS technology means that it is possible to incorporate optical structures in silicon chips together with electronics. These types of structures can also be used for rapid optical switches.
The research team
Léon Woldering is a PhD student in the group Complex Photonic Systems (COPS) at University of Twente. Willem Tjerkstra and Henri Jansen are researchers at the MESA+ Institute at the University of Twente. Irwan Setija is senior researcher at ASML in Veldhoven. Willem Vos is group leader at FOM Institute for Atomic and Molecular Physics in Amsterdam and professor at the University of Twente. This research was funded by NanoNed/STW, FOM and NWO.
Reference
L.A. Woldering, R.W. Tjerkstra, H.V. Jansen, I.D. Setija and, W.L. Vos, Periodic arrays of deep nanopores made in silicon with reactive ion etching and deep UV lithography , Nanotechnology 19 (2008) 145304.
You can find the article here:
http://www.iop.org/EJ/article/0957-4484/19/14/145304/nano8_14_145304.pdf or on
http://cops.tnw.utwente.nl/pdf/08/woldering_online-version_nano8_14_145304.pdf.
Information about optical switches can be found here:
/live/nieuws/archief_persberichten/2007/artikel.pag?objectnumber=66552 and
http://cops.tnw.utwente.nl/research/newsfiles/Euser_switching.pdf.
More information: Léon Woldering, Universiteit Twente, telefoon: 00 31 (0) 53 489 53 90.