Hybrid FIB milling strategy for the fabrication of plasmonic nanostructures on semiconductor substrates
© Einsle et al; licensee Springer. 2011
Received: 16 August 2011
Accepted: 31 October 2011
Published: 31 October 2011
The optical properties of plasmonic semiconductor devices fabricated by focused ion beam (FIB) milling deteriorate because of the amorphisation of the semiconductor substrate. This study explores the effects of combining traditional 30 kV FIB milling with 5 kV FIB patterning to minimise the semiconductor damage and at the same time maintain high spatial resolution. The use of reduced acceleration voltages is shown to reduce the damage from higher energy ions on the example of fabrication of plasmonic crystals on semiconductor substrates leading to 7-fold increase in transmission. This effect is important for focused-ion beam fabrication of plasmonic structures integrated with photodetectors, light-emitting diodes and semiconductor lasers.
Plasmonic nanostructures are finding ever increasing number of applications in various areas of photonics and optoelectronics [1–3]. While initial investigations into the optical properties of plasmonic systems have been almost exclusively done with metallic nanostructures on 'passive' dielectric substrates, such as silica or quartz, the real-world applications in many cases require the use of semiconductor substrates. Recently, there has been a demand on incorporating plasmonic nanostructures in active photonic devices, such as light-emitting diodes (LEDs), semiconductor lasers and photodetectors, to improve their performance [4–7].
For applications in visible and near-infrared spectral ranges, the plasmonic structures need to be fabricated with a precision on the order of tens of nanometers. Conventional microelectronics fabrication methods, such as visible and UV lithography and broad-beam ion etch, do not allow controlling feature sizes on such length scales. The two main methods for the fabrication of plasmonic nanostructures relies on using charged particle beams to structure the material. For example, electron beam lithography can be combined with either lift-off or an etch step to produce nanoscale structures. Electron beam lithography though is not the most efficient process and requires further processing before the final device is created. While robust, this process does not offer sufficient flexibility for quick and rapid prototyping. On the other hand, focused ion beam (FIB) milling is widely accepted as a method of choice for rapid prototyping of electronic and photonic components requiring critical parameters at the subwavelength scale. FIB can sputter away bulk material with nanoscale spatial localisation. The FIB approach offers a simple method to structuring bulk materials, by providing a maskless process that circumvents the pitfalls of resist-based lithography processes. A large variety of photonic and plasmonic devices with structurally controlled optical properties can be created using FIB milling [1–3, 8–10]. While excellent for fabrication of plasmonic structures on dielectric substrates, FIB patterning results in the deterioration of optical properties of semiconductors because of ion-beam-induced amorphisation and Ga+ implantation [11–13]. From FIB applications for milling semiconductor materials for transmission electron microscope (TEM) investigations, it is known that the 30 kv FIB damages approximately 50 nm of the GaP crystal through amorphisation (see, e.g., [10, 12] and Peterson and Blackwood (2010, personal communication)).
GaP substrates were used in these experiments as a representative for the InGaAs family of semiconductors. The fabricated plasmonic nanostructures are plasmonic crystals consisting of arrays of periodically arranged cylindrical apertures in a 100-nm Au film deposited on GaP substrates. The Au films were magnetron sputtered on the GaP substrates. Then, an FEI Nova 600 Dual Beam equipped with a Sidewinder FIB column was used to etch away selected regions of the Au film. To precisely control FIB mills, stream patterning files were used. Using FEI's PS Convert, pattern files are generated by inputting FIB-milling parameters such as horizontal field width, spot size, beam overlap (space between points in the pattern) and dwell time. The software then generates a file specifying pixel location and dwell for each point in the pattern. The choice of the input parameters allows controlling the overall depth of the aperture arrays created. Stream files provide fast and easy control over the various patterns required to mill arrays with various accelerating voltage conditions.
Hybrid milling conditions used for SPPC fabrication
30-kV mill time (s)
5-kV mill time (s)
Structure B was fabricated to investigate the effect of imaging a 30-kV fabricated array with the 5-kV beam. Owing to the reduced signal in the low kV image, perfect overlay of the 5- and -30 kV patterns required in the hybrid FIB approach is challenging to achieve. As a result, the overlay alignment could not be achieved without taking a sequence of high resolution images with the 5 kV ion beam. These were used to bring the 30 kV structure into the field of view such that the 5 kV mill pattern could be accurately overlaid with the 30 kV milled features. The alignment images have the net effect of removing material from the entire region imaged. The amount of gold sputtered was measured via cross-sectional images to be around 10 nm. It would be ideal to be able to eliminate the two images, however the damage induced by taking these two images seems to be minimal as demonstrated in the results shown below.
Structures C and D represent removing 50 nm of Au using the 30 kV beam. The remaining 50 nm of Au in the aperture holes are removed via a combination of imaging and patterning. Finally, for structure E, 70 nm of Au was removed via 30 kV patterning leaving the remainder to be removed with the 5 kV beam. The fabricated structures show good quality of fabricated SPPCs (Figure 2b,c). The cross sections of the structures fabricated by different milling approaches are very similar.
Results and discussion
All five structures A-E exhibit similar transmission spectra with the same position of the plasmonic resonances, it can be concluded that all single high-energy and double high/low-energy mills have produced aperture arrays with similar parameters and not significantly altered the geometry of the apertures, since the transmission spectra are very sensitive to the shape of the apertures. The most prominent difference in the transmission of the structures is the significantly increased transmission for the structures milled with the low kV approach. The main transmission peak around 660 nm shows a greater than sevenfold increase for the structures made with the hybrid milling when compared to the 30 kV patterning. The standard 30-kV milled structure shows lowest transmission. As seen in device B, simply imaging the structure with low kV ions improves the transmission because of the partial removal of the semiconductor damaged layer. The three structures milled with hybrid approach exhibit highest transmission.
We have described a hybrid milling approach for fabricating plasmonic crystals on semiconductor substrates. Combining two different accelerating voltages to etch the plasmonic crystal, we can achieve minimal overall damage to the semiconductor substrate keeping high-resolution capabilities of the ion-beam-based techniques. Reducing the amorphisation of the substrate results in over sevenfold increase of the optical transmission of semiconductor/metal nanostructures because of the reduction of the semiconductor surface damage. This FIB-milling process extends the ability of the technology to fabricate plasmonic and other nanostructures on substrates which are usually damaged through traditional FIB patterning approach by maintaining high spatial resolution of high-energy milling and lower damage introducing by low-energy ion beams.
focused ion beam
light emitting diode
scanning electron microscope
surface plasmon polarition
surface plasmon polaritonic crystal
transmission electron microscope.
This study was supported in part by the EC FP7 project PLAISIR, EC FP6 project PLEAS and EPSRC (UK). The authors thank Juergen Moosburger (OSRAM Opto-Semiconductors) for providing LED samples and Brendan Peterson, J.J. Blackwood, Ross Stanley and Andrea Dunbar for the discussion of GaP amorphisation during FIB processing. JE would like to thank FEI for tuition assistance.
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