High-resolution depth monitoring of reactive ion etching of InP/InGaAs(P) MQWs using reflectance measurements

Small-dimension photonic devices for telecommunications applications have recently attracted increasing interest along with the development of dry etching techniques. Extremely accurate control of etching depth is often needed for the fabrication of these devices containing multiple quantum well (MQW) structures. The excellent anisotropy, reasonable etching rates and excellent compatibility with resist masks of reactive ion etching (RIE) permit this technique to be used for fabrication of such devices. A drawback of the technique is that it is difficult to control etched depth with an accuracy of the order of the QW dimension. Laser interferometry (reflectometry) (LIR) is a powerful method for in situ monitoring of MQW structure etching. End-point detection (EPD) with very high depth resolution is obtained by appropriately combining the operating parameters of RIE and of LIR acquisition. In this paper we discuss the use of LIR at 670 nm to monitor the etching of inP/InGaAs(P) MQW structures by CH4/H-2/(Ar) RIE and we report the real-time monitoring for etching of a 0.5 nm thick InGaAsP layer embedded between InP layers which is, to the best of our knowledge, the highest resolution ever reported for this technique. In the MQW etching process, the use of LIR combined with RIE successfully detects all stack layers with a resolution better than the well/barrier thicknesses. This capability permits etching to be monitored with well thickness as small as 3 nm in the case of InP/InGaAs layers. For structures having well/barrier layers with similar composition and hence a small refractive index difference, such as two In1-xGaxAsxP1-y layers with y = 0.57 and y = 0.84 respectively, monitoring of the etching of MQW with well thicknesses as small as 5 nm is demonstrated. The technique illustrated in the paper also proved to be useful as a simple method of analysing structure layers: the sequence and the uniformity of the QWs can be characterized with very high resolution.