Measuremeni Of Sidewall Roughness Of InP Etched By RIBE

Abstract

The dry etching is going to be used as an effective microfabrication technique to form micro-optical devices such as microcavity surface emitting (SE) lasers’, ” and etched facet lasers.’-’’ In these applications, the etched surfaces should be so smooth and vertical as to avoid light scattering and maintain high reflectivity, that is sensitive to the facet roughness.” Also, the smooth etched facet is needed to reduce the etching induced damage@ at sidewalls. Many authors have reported on InP reactive ion beam etching (RIBE) with Cl, electron cyclotron resonance (ECR) p la~ma.~ .~’ In these works, the etched bottom roughness reported is, for example, less than lOOnm at high ion extraction voltage of 1450V”. So far, the quality of the etched mirror reflectivity has been estimated mainly by scanning electron microscope (SEM) observation and by measuring the external differential quantum efficiency and the threshold of etched-facet semiconductor However, the quantitative evaluation of the sidewall roughness has never been reported. In this paper, we have measured the sidewall roughness using an electron probe surface roughness analyzer for the first time. The roughness of InP sidewalls etched by MBE is discussed under different etch conditions. In this experiment, we used a high-vacuum RJBE system with an electron cyclotron resonance (ECR)-type ion source. The etching gas is pure chlorine or chlorine including 33% argon. We measured the sidewall roughness using a field emission electron probe surface roughness analyzer. The roughness resolution of this system is about lnm. Since the roughness measurement by this system can be performed by a noncontact manner, the sample is not damaged by this measurement. The samples prepared here were (100) oriented InP substrates with an electron beam resist mask which was patterned by an electron beam lithography system to suppress mask edge fluctuations. In this experiment, the etching temperature was fixed to be 140°C. Figure 1 shows the 3-D plot of the sidewall roughness and the SEM image of the sample which was etched at an ion extraction voltage of 300V and a gas (Ar : Cl,= 1 : 2 ) pressure of 4.5 X 10-4Torr. The roughness of the etched sidewall is 2.5nm in center line average and lOnm in peak to peak roughness. Figure 2 summarizes the roughness measurement of the sidewall etched under various etching conditions. Fig. 2 (a) and (b) show the roughness of the sidewall and the bottom surface of the sample shown in Fig. 1, respectively. These results indicate that the sidewall and the surface of InP substrate etched by RIBE have almost equivalent roughness. In Fig. 2, it is realized that the minimum value of the average roughness of the etched sidewall is about lnm and the value of the peak to peak roughness is 5.8 nm at an ion extraction voltage of 4OOV and a gas pressure of 1.2 X 10-3Torr (h). This etched sidewall is as smooth as the cleaved facet of an InP substrate (i). It is found that the etched sidewall roughness is decreased by a lower ion extraction voltage etching in a constant gas pressure((c), (d)), as well as by a higher gas pressure etch in a constant ion extraction voltage ((0, (g), (h)). Also, the sidewall becomes smoother as the etching time decreases. For a smooth InP etch, we should reduce icn extraction voltage and increase gas pressures as far as we could, while maintaining anisotropic etch profiles. The minimum roughness is much smaller than the wavelength of semiconductor laser light. If etching mask edge fluctuation or undulation can be decreased, RIBE is an effective dry etching technique for the fabrication of microsized SE lasers and optoelectronics devices.