Medusa 84 SiH, an alternative HSQ resist for realizing DFB lasers diodes

Medusa 84 SiH, an alternative HSQ resist for realizing DFB lasers diodes P. Sammeta a, O. Brox a, D. Rentner a, P. Della Casa a, R.-S. Unger a, H. Wenzel a, M. Schäfer a, A. Knigge a, H. Biller b, M. Schirmer b a FerdinandBraun-Institut (FBH), Berlin, 12489, Germany b Allresist GmbH, Strausberg, 15344, Germany e-mail: [email protected] In this paper we investigate the performance of the edge emitting distributed feedback (DFB) lasers fabricated using hydrogen silsesquioxane (HSQ) polymer-based electron beam resist Medusa 84 SiH by Allresist GmbH (Germany). DFB laser realized with hydrogen silsesquioxane resist were analyzed and presented in [1]. Limited shelf life of HSQ resist poses a concern while producing high-quality DFB lasers. In order to make the DFB fabrication process more reliable and cost-effective, an HSQ resist variant Medusa 84 SiH – SX ARN 8400.08 was tested. In addition, the solvent used for Medusa 84 SiH is butyl acetate thereby avoiding a health hazardous solvent methyl isobutyl ketone (MIBK) which is commonly used for HSQ resist [2]. The first epitaxial growth step by metalorganic vapour-phase epitaxy (MOVPE) on a n-GaAs substrate terminates with InGaP/GaAs/InGaP grating layers. The DFB lasers were designed for an emission wavelength of 1064 nm. Grating lines of period 330 nm and width of 90 nm were patterned into 100 nm thick Medusa 84 SiH resist using e-beam lithography (Raith EBPG 5200) operating at 100 keV. The writing time (~12.5 hrs for a 3-inch wafer) was similar to the writing time of the process with different HSQ resist (~12 hrs). A post development bake at 120 °C was carried out to for an improved plasma etch resistance. The gratings were then plasma etched and the resist was removed with diluted hydrofluoric (HF) acid, followed by 10 mins DI water rinse. Fig. 1 exhibits an SEM micrograph of gratings taken after the dry etching and cleaning step. The gratings were finally buried during the second regrowth step, thereby completing the vertical laser epitaxial stack. The SEM image displayed in Fig. 2 showcases the cross-section of the gratings realized with Medusa 84 SiH resist. From the SEM micrograph it can be seen that there are less morphological defects at the interface indicating a good regrowth interface. The DFB laser diodes of cavity length 1500 µm and 2.2 µm ridge width were facet coated with the front and rear facet reflectivity of 0% and 96%, respectively. Continuous wave measurements were then carried out on bar level. The output power and applied voltage versus injection current are shown in Fig. 3(a). The output spectrum for different injection current can be seen in Fig. 3(b). The output results were found to be similar to the results of the DFB laser fabricated with the other HSQ resist. In this work frame a DFB process fabricated with Medusa 84 SiH was developed and analyzed as an alternative source of HSQ resist. The results presented in this paper corroborate that the Medusa 84 SiH by Allresist GmbH is an equivalent replacement for similar HSQ variants resist without compromising in the quality of the DFB laser performance. [1] O. Brox “Buried grating fabrication process for 7xx nm DFB laser diodes,” MNE 2024 - 50th International Micro and Nano Engineering Conference (MNE2024), Montpellier, France. [2] O. R. Opaluch et al., “Medusa 84 SiHA novel high selectivity electron beam resist for diamond quantum technologies”, 2025, DOI 10.48550/arXiv.2503.04845. Figure 1. SEM micrograph at 30° tilt of the gratings post plasma etching and Medusa 84 SiH resist removal. Figure 2. SEM (BSE) image of the gratings cross-section. Figure 3. Measurement results, output PI and VI curve (a) and the optical spectra as a function of injection current (b).