Development Of Single-step And High-resolution ICP Dry Etching For A Wide Range Of InP-based Materials
The Optoelectronics Research Group in the Department of Electronics and Electrical Engineering at the University of Glasgow, UK has been engaged in research to develop techniques that integrate the three desirable properties of semiconductor lasers in a single mode-locked laser diode: high output power, high repetition rate and ultrashort pulses. All these aims of the project may be achieved with the implementation of properly designed photonic-band gap mirrors (intra-cavity reflectors) into laser gain sections  and/or periodic structures into ridge waveguide lasers [2, 3] in order to compress the pulses. Both intra- and extra-cavity structures in 1.55-μm emitting MLLs based upon InGaAs/InGaAsP/InP and InGaAs/AlGaInAs/InP quantum well systems are of the special interest. Appropriately designed and fabricated air-slots and grating structures will provide controlled degrees of reflectivity, optical dispersion and compression of the laser pulse.
The key to fabricate these fine-pitch patterns is the development of precise and reliable dry etching process to provide highly anisotropic transfer of both micron- and nanometer-scale features with minimized scattering loss. Furthermore, to ensure strong overlap between waveguided mode and periodic structure deep fabrication process is required to etch the structures sufficiently below laser active region, preferably into lower cladding layer. Therefore high-resolution, deep (> 3.0 mm) dry etching is a fundamental demand for the fabrication of submicron-sized gratings, photonic crystals and low-loss ridge waveguides in InP-based materials. For the purposes of the project, optimization of the dry etching process was carried out in an STS Multiplex ICP etch tool [4, 5], courtesy of the Institute of Photonics (IoP), Glasgow, UK. All experiments were performed on three different types of wafer: bare p-type InP substrates, MOCVD grown InP/InGaAsP passive waveguides and a commercial AlGaInAs-based 1550 nm laser diode (LD) wafer, supplied by the IQE company. An optimized ICP dry etching process based on a Cl2/Ar/N2 gas mixture was used in the experiment, with hydrogen silsesquioxane (HSQ) as the electron-beam lithography (EBL) resist/etch-mask. The highly anisotropic process resulted in near-vertical sidewalls on deeply etched structures, as presented in Fig. 1. Properly balanced process conditions were set to produce a strong passivation effect on the sidewalls while still maintaining a useful InP etch rate (560 ¸ 730 nm/min).
The low roughness of the etched sidewalls and surfaces was further confirmed using both optical waveguide propagation with Fabry-Pérot resonances (fringe contrast) and the roundtrip attenuation method. The scattering loss was measured for ridge waveguides with widths varying from 10 mm down to 3 mm and for different etch depths (between 1.5 mm and 3.2 mm). In all these cases, for both TE and TM polarized optical signals at l = 1.55 mm, low waveguide propagation loss was observed, estimated to be less than < 0.3 dB/mm.
For the process conditions given, transfer of small-feature-size patterns was accomplished at a very high aspect ratio, as shown in Fig. 2. A deeply etched first-order grating with a period of 236 nm was fabricated in a 3 mm-wide InP/InGaAsP ridge waveguide (Fig. 2a). The grating is etched directly into the sidewall of the ridge since, with this geometry, it is relatively easy to control the grating strength/reflectivity. Fabrication of such distributed Bragg reflectors (DBR) is less critical than for surface gratings etched deeply into the central part of the laser ridge. Fig 2b presents an angled-view SEM micrograph of deeply etched 86 nm-wide features in InP/InGaAsP, where aspect ratios as high as 30 have been obtained.
An additional outcome of the balanced process was equal-rate etching of all the materials investigated. Smooth transitions between the layers in InP/InGaAsP/InGaAs/AlGaInAs structure with different compositions for constant etching conditions are shown both in Fig. 1 and Fig. 2.
High-aspect ratio ICP dry etching is a crucial technological step to properly fabricate intra-cavity reflectors in Al-quaternary based material . Before the fabrication of photonic band-gap mirrors the theoretical response of device in terms of reflection (R), transmission (T) and loss (L) was calculated using two-dimensional (2-D) simulations. Cross-section approach was employed with CAvity Modelling Framework (CAMFR) simulation tool. Numerical results for 363 nm-wide air-slot for a fundamental TE0 optical mode at the constant wavelength of l = 1.55 µm are presented in Fig. 3. The single air-slot efficiency is highly dependent on total structure depth, as presented in Fig. 3c. For non-optimum etch depth conditions (1.96 mm-deep air-slot) both low reflectivity R of ~17% and high feature loss L (~78%) are observed. The reason for such a behavior is that a vast amount of power carried by TE0 optical mode, propagating from the left to the right where the mirror is located, is coupled into the substrate. The substrate coupling loss effect is indicated by a logarithmic plot of electric field distribution within the structure. In contrary, high reflectivity and low loss case may be observed for a deeper 2.95-mm-deep mirror, which provides reflectivity of R ~ 67% and negligible structure loss (~7%). Therefore, fabrication process for the experimental purposes involves deep anisotropic etching of 363-nm-wide features with use of optimized Cl2/Ar/N2 inductively coupled plasma etching.