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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 12, Iss. 9 — May. 3, 2004
  • pp: 1803–1809
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Quasi-direct writing of diffractive structures with a focused ion beam

Yongqi Fu, Ngoi Kok Ann Bryan, and Wei Zhou  »View Author Affiliations


Optics Express, Vol. 12, Issue 9, pp. 1803-1809 (2004)
http://dx.doi.org/10.1364/OPEX.12.001803


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Abstract

A new method for fabrication of diffractive structures, which we call quasi-direct writing, is illustrated. The diffractive structures can be generated by changing the pixel spacing along the direction of the cross scan (with zero overlap) and keeping the pixel spacing constant along the other scan direction, with a normal overlap of 50%–60%, while the substrate surface is scanned with a focused ion beam (FIB). Quasi-direct writing is a method for achieving special customer designs when the milling machine has no computer programming function. Diffractive structures with various periods and depths can be derived by controlling the parameters of pixel spacing, beam current, ion incidence angle, and the scan time or ion dose. The method is not restricted to any one material and can be used for metals, insulators, and semiconductors.

© 2004 Optical Society of America

1. Introduction

Currently there are three main direct writing methods for fabrication of micro-optical elements: laser direct writing [1

1. Andrei Y. Smuk and Nabil M. Lawandy, “Direct laser writing diffractive optics in glass,” Opt. Lett. 22, 1030 (1997). [CrossRef] [PubMed]

,2

2. Michael T. Gale, Markus Rossi, Jorn Pedersen, and Helmut Schutz, “Fabrication of continuous-relief micro-optical elements by direct laser writing in photoresists,” Opt. Eng. 33, 3556 (1994). [CrossRef]

] (LDW), electron-beam lithography [3

3. T. Fujita, H. Nishihara, and J. Koyama, “Fabrication of microlenses using electron-beam lithography,” Opt. Lett. 6, 613 (1981). [CrossRef] [PubMed]

] (EBL), and focused ion-beam direct milling [4

4. Fu Yong-Qi and Ngoi Kok Ann Bryan, “Diffractive optical elements with continuous relief fabricated by focused ion beam for monomode fiber coupling,” Opt. Express 7, 141–147 (2000). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-7-3-141. [CrossRef] [PubMed]

,5

5. Yongqi Fu and Ngoi Kok Ann Bryan, “Hybrid micro-diffractive-refractive optical element with continuous relief fabricated by focused ion beam for single-mode coupling,” Appl. Opt. 40, 5872 (2001). [CrossRef]

] (FIBM). FIBM direct writing requires a programming function (called MDDL in the operation software for our machine, model Micrion 9500EX) that enables the users to write customer-designed programs; the programming function has special commands provided by the machine manufacturer to control the whole milling process to yield the desired structures. In this way the designed gratings can be directly milled point by point and line by line according to the computer program. (This process transfers the designed patterns by direct impingement of the ion beam on the substrate. Different points correspond to different milling depths of the designed continuous relief. The depth is derived from discrete data, which have been converted from the designed continuous-relief profile.) All the controlled design parameters (depth and lateral dimensions) are accounted for in this special program. However, not every model of the FIB machines has the MDDL programming function; e.g., the FEI Quanta series and Seiko series lack this function. It is difficult to fabricate diffractive structures by FIBM with FIB machines that lack the programming function. In this paper we introduce a method for which the programming function is not required. In other words, it is programming-function free. We call our method quasi-direct writing. Its principle is changing the pixel spacing of the direction along the cross scan (Y, represented by psy ), as shown in Fig. 1 and keeping the pixel spacing of the other scan direction (the X direction, represented by psx ) constant, and vice versa. (The normal overlap for the X direction is 50%–60%). Overlap in the cross-scan direction is zero. Gratings can be directly generated with different periods that correspond to the different Y pixel spacings. The method reported here also differs from our previously reported method of purely self-organized formation [6

6. Yongqi Fu and Ngoi Kok Ann Bryan, “Self-organized formation of a blaze grating like structure on Si(100) induced by focused ion beam scanning,” Opt. Express 12, 227 (2004). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-2-227 [CrossRef] [PubMed]

], in which the diffractive structure is spontaneously generated during sputtering erosion under ion bombardment, a self-organized formation caused purely by the competition between smoothing, driven by surface energy, and roughening, induced by the sputtering removal of material. The process of radiation-enhanced surface transport, resulting in surface smoothing in which relaxation by viscous flow (for amorphous materials) and surface diffusion (for crystal materials), plays a dominant role. But the purely self-organized formation strongly depends on the substrate material (it has strong material selectivity). We have only found two materials that exhibit this effect, TiNi thin film and Si (100) [6

6. Yongqi Fu and Ngoi Kok Ann Bryan, “Self-organized formation of a blaze grating like structure on Si(100) induced by focused ion beam scanning,” Opt. Express 12, 227 (2004). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-2-227 [CrossRef] [PubMed]

]. Quasi-direct writing is a guided or pixel-spacing-controlled self-formation induced by changing the pixel spacing of the cross-scan direction.

2. Experimental setup

The milling experiments were carried out by our FIB machine (a Micrion 9500EX dual-beam system), which is equipped with a liquid-gallium ion source and is integrated with a scanning electron microscope (SEM), an energy-dispersion x-ray spectrometer (EDX) facility, and gas-assisted etching (GAE) functions. This machine uses a focused Ga+ beam with an energy ranging from 5 to 50 keV, a probe current from 4 pA to 19.7 nA, and a beam-limiting aperture size from 25 to 350 μm. For the smallest beam currents the beam can be focused to as small as 7 nm in diameter at full width at half-maximum (FWHM). The substrate material in our experiments is Si (111). The diffractive structures were characterized by an atomic force microscope (AFM), model Nanoscope (R) IIIa from Digital Instruments.

Fig. 1. Schematic of FIB digital scanning steps and sizes in the defined scan area of L × W. Overlap (h) in both the X and the Y direction can be controlled by varying the X- and Y-direction pixel spacing. (a) Overlap of 50%–60% in both the X and the Y scan direction for FIB direct writing; (b) overlap of 50%–60% in the X direction and zero in the Y direction for quasi-direct writing.

3. Results and discussion

Fig. 2. Ion beam current versus lateral dimension and depth of the diffractive structures measured with an AFM in a 15 μm × 15 μm area. Scanning was carried out with an ion energy of 40 keV, an ion incidence angle of 0°, ion dose of 1.5 nC/μm2, and beam current of 569 pA. X and Y pixel spaces are 0.02 and 0.6 μm, respectively. The inset FIB images are for beam currents of 99 pA and 2 nA. The substrate material is Si (111).
Fig. 3. Ion energy versus lateral dimension and depth of the diffractive structures measured with an AFM in a 15 μm × 15 μm area. Scanning was carried out with an ion incidence angle of 0° and beam current of 569 pA. X and Y pixel spaces are 0.02 and 0.8 μm, respectively. The ion dose is 1.5 nC/μm2. The substrate material is Si (111). The dotted line indicates that no ripples are observed for ion energies below 25 keV.
Fig. 4. Ion incidence angles versus lateral dimension and depth of the diffractive structures measured with an AFM in a 15 μm × 15 μm area. The scanning was carried out with ion energy of 40 keV, a scan time of 64 min, and a beam current of 569 pA. X and Y pixel spaces are 0.02 and 0.6 μm, respectively. The inset AFM images are for ion incidence angles of 0° and 70°. The substrate material is Si (111).
Fig. 5. Y pixel spaces versus lateral dimension and depth of the diffractive structures measured with an AFM in a 15 μm × 15 μm area. X pixel spaces were fixed at 0.02 μm. The scanning was carried out with an ion energy of 40 keV, an ion dose of 1.5 nC/μm2, ion incidence angles of 0° 45°, and 80°, and a beam current of 569 pA. The inset FIB images correspond to Y pixel spaces of 0.1 and 2 μm, indicated by the arrows. The white arrows in the FIB images show the projection direction of the ion beam and scan path. The substrate material is Si (111).
Fig. 6 Ion-beam scanning time versus lateral dimension and depth of the diffractive structures measured with an AFM in the scanned 15 μm × 15 μm area. The scanning was carried out with ion energy of 40 keV and beam current of 569 pA. X and Y pixel spaces are 0.02 and 0.6 μm, respectively. The inset AFM images correspond to scanning times of 4 and 64 min, indicated by the arrows. The diffractive structures changed from sinusoidal to blazelike topographies. The substrate material is Si (111).
Fig. 7. Diffractive structure change for various process parameters with quasi-direct FIB writing on quartz, measured with an AFM. (a) Three-dimensional (3D) image of the grating with a line density of 787 lines/mm, milled with scan time 8 min, beam current 569 pA, and incidence angle 15°. (b) 2D profile with depth 24.4 nm and width 1270 nm. (c) 3D image of the grating with a line density of 900 lines/mm, milled with scan time 32 min, , beam current 569 pA, and incidence angle 60°. (d) 2D profile with depth 439 nm and width 1110 nm.

4. Summary

In summary, the quasi-direct writing method has the advantages of requiring no programming function and not excluding any material. The substrate materials can be metals, insulators (e.g., BK7 glass or quartz), or semiconductors (e.g. Si, GaN, GaAs, or InP). It is essential for users with FIB machines that have no customer programming function. It also provides an additional flexible option for those people who want to use FIB machines to fabricate their diffractive structures. It is more suitable for locally writing the gratings used in distributed feedback laser diodes (DFB) and the Bragg gratings in distributed Bragg reflector (DBR) laser diodes. Any FIB machine can fabricate the gratings with this method. Diffractive structures with circular symmetry, such as Fresnel plates, also can be fabricated if the FIB machine scans with a circular path.

Acknowledgments

This research was supported in part by the Funding for Strategic Research Program on Ultraprecision Engineering of the Agency of Science, Technology and Research, Singapore, and by the Innovation in Manufacturing Systems and Technology Program in Singapore— Massachusetts Institute of Technology Alliance.

References

1.

Andrei Y. Smuk and Nabil M. Lawandy, “Direct laser writing diffractive optics in glass,” Opt. Lett. 22, 1030 (1997). [CrossRef] [PubMed]

2.

Michael T. Gale, Markus Rossi, Jorn Pedersen, and Helmut Schutz, “Fabrication of continuous-relief micro-optical elements by direct laser writing in photoresists,” Opt. Eng. 33, 3556 (1994). [CrossRef]

3.

T. Fujita, H. Nishihara, and J. Koyama, “Fabrication of microlenses using electron-beam lithography,” Opt. Lett. 6, 613 (1981). [CrossRef] [PubMed]

4.

Fu Yong-Qi and Ngoi Kok Ann Bryan, “Diffractive optical elements with continuous relief fabricated by focused ion beam for monomode fiber coupling,” Opt. Express 7, 141–147 (2000). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-7-3-141. [CrossRef] [PubMed]

5.

Yongqi Fu and Ngoi Kok Ann Bryan, “Hybrid micro-diffractive-refractive optical element with continuous relief fabricated by focused ion beam for single-mode coupling,” Appl. Opt. 40, 5872 (2001). [CrossRef]

6.

Yongqi Fu and Ngoi Kok Ann Bryan, “Self-organized formation of a blaze grating like structure on Si(100) induced by focused ion beam scanning,” Opt. Express 12, 227 (2004). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-2-227 [CrossRef] [PubMed]

OCIS Codes
(050.1950) Diffraction and gratings : Diffraction gratings
(220.4000) Optical design and fabrication : Microstructure fabrication

ToC Category:
Research Papers

History
Original Manuscript: March 22, 2004
Revised Manuscript: April 5, 2004
Published: May 3, 2004

Citation
Yongqi Fu, Ngoi Bryan, and Wei Zhou, "Quasi-direct writing of diffractive structures with a focused ion beam," Opt. Express 12, 1803-1809 (2004)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-9-1803


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