34-36 ICP v2 10/9/09 13:31 Page 34
industry dry etching
rhombus4
Development of single-step and
high-resolution ICP dry etching for
a wide range of InP-based materials
Mode-locked lasers (MLLs) are effective sources of periodic trans of coherent optical pulses and
are fundamental components in a range of optical communications and spectroscopy applications.
Especially, integrated semiconductor lasers have advantages over other forms of laser since their
waveguide structure concentrates the optical intensity into the active medium, and the short cavity
lengths typically lead to repetition rates in the range 40 GHz to 2 THz. By Rafal Dylewicz,
Lianping Hou, Gábor MezŒsi and Catrina Bryce, Optoelectronics Research Group,
Department of Electronics & Electrical Engineering, University of Glasgow.
T
he Optoelectronics Research anisotropic transfer of both micron- was used in the experiment, with
Group in the Department of and nanometer-scale features with hydrogen silsesquioxane (HSQ) as
Electronics and Electrical minimized scattering loss. the electron-beam lithography (EBL)
Engineering at the University of Furthermore, to ensure strong resist/etch-mask. The highly
Glasgow, UK has been engaged in overlap between waveguided mode anisotropic process resulted in near-
research to develop techniques that and periodic structure deep vertical sidewalls on deeply etched
integrate the three desirable fabrication process is required to structures, as presented in Fig. 1.
properties of semiconductor lasers in etch the structures sufficiently below Properly balanced process
a single mode-locked laser diode: laser active region, preferably into conditions were set to produce a
high output power, high repetition lower cladding layer. strong passivation effect on the
rate and ultrashort pulses. All these sidewalls while still maintaining a
aims of the project may be achieved Therefore high-resolution, deep (> useful InP etch rate (560-730
with the implementation of properly 3.0 mm) dry etching is a fundamental nm/min).
designed photonic-band gap mirrors demand for the fabrication of
(intra-cavity reflectors) into laser gain submicron-sized gratings, photonic The low roughness of the etched
sections [1] and/or periodic crystals and low-loss ridge sidewalls and surfaces was further
structures into ridge waveguide waveguides in InP-based materials. confirmed using both optical
lasers [2, 3] in order to compress the For the purposes of the project, waveguide propagation with Fabry-
pulses. Both intra- and extra-cavity optimization of the dry etching Pérot resonances (fringe contrast)
structures in 1.55-µm emitting MLLs process was carried out in an STS and the roundtrip attenuation
based upon InGaAs/InGaAsP/InP Multiplex ICP etch tool [4, 5], method. The scattering loss was
and InGaAs/AlGaInAs/InP quantum courtesy of the Institute of Photonics measured for ridge waveguides with
well systems are of the special (IoP), Glasgow, UK. All experiments widths varying from 10 mm down to
interest. Appropriately designed and were performed on three different 3 mm and for different etch depths
fabricated air-slots and grating types of wafer: bare p-type InP (between 1.5 mm and 3.2 mm). In all
structures will provide controlled substrates, MOCVD grown these cases, for both TE and TM
degrees of reflectivity, optical InP/InGaAsP passive waveguides polarized optical signals at l = 1.55
dispersion and compression of the and a commercial AlGaInAs-based mm, low waveguide propagation loss
laser pulse. The key to fabricate 1550 nm laser diode (LD) wafer, was observed, estimated to be less
these fine-pitch patterns is the supplied by the IQE company. An than < 0.3 dB/mm. For the process
development of precise and reliable optimized ICP dry etching process conditions given, transfer of small-
dry etching process to provide highly based on a Cl2/Ar/N2 gas mixture feature-size patterns was
34
www.compoundsemiconductor.net September 2009
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