DFB and DBR Lasers

Distributed Feedback (DFB) and Distributed Bragg Reflector (DBR) Lasers are used in applications such as optical communication and sensing where a narrow spectral linewidth is required. Bragg reflection from diffraction gratings patterned on these lasers ensures that they operate in single longitudinal mode. The fabrication of the grating is one of the most critical steps in the production of these lasers. Depending on the laser material and emission wavelength the period (pitch) of the gratings is usually deep in the sub-micron range, e.g. around 200nm or below.

Conventionally, e-beam lithography and interference lithography are the techniques most often used to print such high-resolution gratings. But both techniques suffer from serious drawbacks. E-beam lithography has low throughput and high-cost. Interference lithography systems are known to have reproducibility problems and have generally low yield. In addition, pitch uniformity, reproducibility and accuracy, as well as very large footprint on factory floor are serious issues faced with interference or holographic lithography systems.

PHABLE technology offers an ideal solution for production of DFB and DBR gratings bringing together many advantages in one tool. The DUV versions of the PhableR 100 exposure tool can print gratings with periods down to 120nm including around 200nm, which is the important region for fiber optic communication lasers. Pitch accuracy and reproducibility are inherent properties of the technique. Different periods can be printed simultaneously on the same wafer for fabrication of multi-wavelength lasers. Wafers can be patterned completely or in selected areas with grating patterns. Printed gratings can be accurately aligned to wafer flats, cleaved edges or alignment marks present on the wafer.

Nanowire Devices

Semiconductor nanowires are the subject of much development effort for use in new high-performance electronic and optoelectronic devices. For example, nanowire based LEDs are being developed for new display devices to provide higher brightness with lower energy consumption. Semiconductor nanowires in such devices are grown on patterned templates. For example, nanometer scale holes etched in a dielectric film deposited on a growth substrate (e.g. GaN) maybe used as seed sites for growth of the nanowires in the vertical direction away from the substrate surface. The holes opened in the dielectric film are typically around 100nm in diameter.

A high resolution lithographic technique is required for printing of the required hole arrays uniformly and reproducibly on growth substrates. Nanoimprint and e-beam lithography can be used for this purpose but ebeam is slow and expensive and nanoimprint suffers from process difficulty and reliability issues. PHABLE technology can produce the required periodic structures with excellent uniformity and reproducibility. Holes measuring around 100nm in diameter can be produced independent of the pitch, i.e. both dense and sparse hole arrays can be produced. Eulitha serves this application with its deep UV (DUV) PHABLE systems that have the required resolution.

Patterned Sapphire Substrates (PSS)

Eulitha's PHABLE photolithography technology is the ideal tool for printing patterns on sapphire substrates widely used in LED manufacturing. Patterned sapphire substrates (PSS) facilitate growth of high quality semiconductor layers resulting in higher performance LEDs. The structures required are typically arrays of columns or holes arranged on a hexagonal grid with a period in the range 1-3 μm for PSS, down to a few hundred nanometers in the case of Nano-Patterned Sapphire Substrates (NPSS), whose finer structures permit a further gain in light extraction efficiency. The formation of such high-resolution structures with good uniformity and reproducibility on sapphire substrates, which generally have poor surface flatness, is difficult to achieve at acceptable cost and yield using conventional lithography technologies. PHABLE technology makes it possible to print seamless patterns through full-field exposures at high-yield and throughput. The resulting structures in thick photoresist can be easily etched into the sapphire substrates.

Photonic Crystals for Light Extraction from LEDs

Much of the light generated inside a GaN based LED is lost because of total internal reflection at the interface between the high refractive index semiconductor material and air. This problem can be effectively addressed by creating a photonic crystal pattern on the LED surface. For example, 100nm-scale holes drilled into the device surface in the pattern of a photonic crystal drastically increase the so-called light extraction efficiency. Some of the highest performance LEDs demonstrated to date have used this technology. Eulitha's PHABLE technology is ideal for creation of such photonic patterns on LED surfaces. Eulitha provides wafer processing service to LED companies and researchers for this purpose. In addition, photonic nanoimprint stamps covering large areas are available which can be used to imprint such patterns on LED surfaces. Printing of similar patterns on sapphire substrates on which LED epitaxial layers are often formed, also leads to high extraction efficiency. Moreover, recent studies have shown much higher crystalline quality of layers deposited on such Nano patterned sapphire substrates (NPSS). Higher crystalline quality is important in improving the internal quantum efficiency.

Nanoimprint Lithography

Nanoimprint lithography (NIL) is a fast and efficient technology for replicating nanoscale patterns of a stamp on a substrate. Because of its versatility and potential in terms of resolution and throughput, NIL has been included on the roadmap to future chip fabrication (32 nm node and beyond). It is also considered as the prime candidate for mass production of patterned magnetic media. The quality of the resulting NIL patterns depends mainly on the quality of the used stamps. High-resolution stamps are mainly fabricated by e-beam lithography. However, e-beam has limited throughput and it suffers from stitching related uniformity problems among others.

NIL stamps fabricated by EULITHA possess a high degree of uniformity due to the holographic nature of our lithography technique. Our unique technology makes large area nanostructing affordable. EULITHA offers standard high-resolution NIL stamps and custom designs for thermal-NIL or UV-NIL.

INVITED poster at the MNE 2008 conference, Athens, Greece:
Fabrication of a stitching free 38 nm half-pitch NIL template with EUV interference lithography
H. H. Solak, M. Saidani, C. Spreu, K. Vogelsang, H. Schift, J. Gobrecht

Nanophotonics - sub-wavelength Optics

Optical components based on sub-wavelength periodic nano-structures (e.g. wire-grid polarizers, beam splitters, filters, etc) for the visible and UV regions offer new opportunities in optical system design and performance For instance, wire-grid polarizers manufactured with EUV-IL technology that exhibit high performance in the visible and UV regions were recently demonstrated [1]. Their compact and planar design makes them especially attractive for display technologies especially for the blue region. Further applications include improvement of emission efficiency for LED's and structured substrates for bio-sensing.

L. Wang et al; Proc. SPIE 8424, Nanophotonics IV, 842429.

Nanowire circuits

A number of devices in future nanoelectronic applications may be based on arrays of metal nanowires, such as the crossbar latch devices proposed by HP labs [1]. This new concept is based on two stacked layers of parallel wires which are aligned perpendicular to each other. A layer of a bistable molecules is sandwiched between the wire layers and can be electronically switched from one state to another. Data storage and Boolean logic functions were already successfully tested in this straightforward device concept.

Our PHABLE technique is ideally suited to produce large arrays of metal or semiconductor nanowires with the required periodicity and uniformity for research on these systems.

[1] P. Kuekes et al, J. of Appl. Phys. 97, 034301 (2005).

Templated Self Assembly

Templated self-assembly is the combination of top-down lithography with bottom-up self-assembly. Recently, templated self-assembly is gaining interest in various fields of research. Templates which are fabricated with our EUV-IL technique are already used in various self-assembly approaches, like guided self-assembly of block copolymers [1] or the fabrication of three-dimensional SiGe quantum dot crystals [2,3]. Usually, several parameters have to be adjusted to study and to understand the particular self-assembly process. Hence, an adequate amount of identical templates is required. EULITHA provides you with highly uniform templates, which can be individually tailored to your self-assembly problem.


[1] M. Stoykovich et al, Phys. Rev. Lett. 97, 147802 (2006).

[2] D. Grützmacher et al, Nano Lett. 7(10), 3150 (2007).

[3] C. Dais et al, Nanotechnology 26(25), 255302 (2015).

2" sapphire wafer with PSS pattern produced on the PhableR 100 system
high aspect-ratio photoresist pillars for PSS
Photoresist pillars exposed on PhableR 100 tool for PSS fabrication
PSS cones on sapphire
PSS pattern etched on a sapphire substrate
AFM image of conical PSS pattern etched in sapphire (image courtesy of Force Precision Instrument, Taiwan)