Wet Chemical Recess Etching of Ge 2 Sb 2 Te 5 for 3D PCRAM Memory Applications

Memory cells comprising a Phase Change Material (PCM) are the building blocks of fast and non-volatile memory devices called Phase Change Random Access Memory (PCRAM) [1-3]. The working principle of this memory involves data retention in the form of a phase (amorphous or crystalline) and the set and reset can be done by Joule heating to induce an amorphous-to-crystalline or crystalline-to-amorphous transition respectively. Some chalcogenide materials experience this thermally driven phase change, GeSbTe (GST) being one of those alloys extensively studied. GST has also been adopted for the fabrication of the 1 st generation X-point memory [4] and might be adopted in a 2 nd generation X-point memory of a four-layer PCM structure [5]. However, this 2D scaling or stacking of PCRAM is limited by cost and therefore the development of 3D architectures is envisaged for decreasing the cost/bit [6]. A key requirement for the fabrication of this 3D architecture is the conformal deposition and etch-back of GST. Dry plasma etching might be limited to anisotropic recess while isotropic lateral recess is needed. Therefore, wet isotropic etching might be the process of choice. A few chemical solutions have been proposed in previous studies. Cheng et al. showed that GST could be etched in HNO 3 but with a very high etch rate and with an unwanted surface composition change due to different oxidation and dissolution rates of the metalloids [7]. Wang et al. demonstrated that basic wet etching solutions led to a slower etch rate and a much smoother surface compared to acidic wet etching solutions [8]. Deng et al. showed a switch in the etch rate order between crystalline and amorphous GST depending on the H 2 O 2 concentration in TMAH [9]. In this work, we present a controllable partial recess solution that leaves the GST surface smooth after recess. Wet recess of amorphous and crystalline blanket films, as well as patterned samples, was initially explored using the commodity chemistries Ammonium Peroxide Mixture (APM) and (Hydrochloric Peroxide Mixture) HPM. The etching of GST in HPM as a function of the H 2 O 2 concentration was monitored by ICPMS and showed a well-controlled etch rate. However, some shortcomings of these H 2 O 2 -containing solutions, like roughness and selectivity, lead to a change of oxidizing agent from H 2 O 2 to O 3 . In the O 3 -containing solutions, the selectivity towards Al 2 O 3 , SiO 2 , and TiN could be secured. The impact of the dissolved O 3 c