Use of a line focus of a quadrupole multiplet for irradiating millimeter length lines B.Q. Xiong ⇑ , M.B.H. Breese, S. Azimi, Y.S. Ow, E.J. Teo Physics Department, National University of Singapore, Lower Kent Ridge Road, 119260 Singapore, Singapore article info Article history: Received 25 November 2010 Received in revised form 2 February 2011 Available online 12 February 2011 Keywords: Proton beam irradiation Long line focus Waveguides Quadrupole multiplet abstract We have developed a new method of irradiation suitable for uniformly irradiating long, straight lines for waveguide and channel fabrication in semiconductors and polymers. This is achieved by exciting the quadrupole lenses of a nuclear microprobe in a manner such that the beam is focused in one plane but highly defocused in the orthogonal plane. This method requires no surface mask and since the beam is not scanned, it produces uniformly-irradiated patterns. This approach has been used for rapid exposure of lines 8 mm in length with widths of about 3 lm in silicon and 1.5 lm in polymethyl methacrylate. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Nuclear microprobes can be used to micromachin semiconduc- tor materials such as silicon [1–4], gallium arsenide [5,6] and insu- lator materials such as lithium niobate [7], diamond [8] and photoresists such as polymethyl methacrylate (PMMA) [9,10] using a scanned, focused MeV ion beam, for a range of applications in optics, photonics, micro-electromechanical systems and micro- fluidics. For these purposes, sophisticated scanning software is used to control the position of the beam and to accurately deliver a certain fluence to each irradiated point. While capable of produc- ing patterns with very high resolution and structures of about 30 nm in polymer resists [11], this approach suffers from a slow throughput and limited scan area, as for all direct-write processes. Furthermore, direct writing is affected by fluence variations due to beam current fluctuations which results in increased surface roughness for silicon micromachining after electrochemical anodi- sation. [12,13]. Other types of facilities have been developed for either ion irra- diation over large areas or at high energies which seldom combine the two factors. An ion projection lithography system [14] based on energies of 100 keV was designed to uniformly illuminate a stencil mask [15], with the transmitted beam projected and focused using electrostatic lenses on the sample surface over an area of about 1 cm 2 . However, electrostatic focusing is unsuitable for MeV ion energies owing to the high voltages required to produce sufficient field strength. A different type of projection lens system was built [16] to irradiate samples with MeV heavy ions, where the beam passes through portions of a stencil mask at the object aperture and forms a demagnified image on the surface at the microprobe focal plane. Fabricating free-standing masks capable of withstand- ing a high beam power is challenging and such a system is not suit- able for irradiating large wafer areas. Recently a different mode of irradiation was introduced based on a quadrupole multiplet of a microprobe to focus the beam [17]. Instead of placing the sample at the image plane in the irra- diation chamber, it was placed about 50 cm downstream so that the beam which is focused in the chamber diverges over a large area of the sample. This approach requires a mask, usually in the form of a patterned photoresist on the sample surface to shield the silicon from the incoming ions. This method allows for short irradiation time as it uses high beam currents of hundreds of nano- amperes. Furthermore, the irradiation is uniform since any beam current fluctuations similarly affect the whole irradiated area. We now routinely use this method to irradiate different wafers for silicon micromachining, in order to produce large area arrays of Bragg reflectors [17], concave micro-mirrors, and waveguides. 2. Beam optical simulations of focused line formation This paper describes a mode of irradiation which also gives very uniform fluence distribution because the beam is not scanned. This is achieved by exciting a triplet of quadrupole lenses of a nuclear microprobe in a particular manner so that the beam is focused in one plane but highly defocused in the orthogonal plane. Our work is based on the high excitation ‘Oxford’ quadrupole triplet lens con- figuration [18,19], but it is similarly applicable to other quadrupole configurations for focusing MeV ion beams in microprobes such as doublets [19,20]. In the high excitation triplet, lenses 1 and 2 (L 1 , L 2 ) are coupled together, with lens 1 assumed to be positive in 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.02.002 ⇑ Corresponding author. E-mail address: g0800800@nus.edu.sg (B.Q. Xiong). Nuclear Instruments and Methods in Physics Research B 269 (2011) 729–732 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb