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 HNO₃ 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 H₂O₂ 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 H₂O₂ concentration was monitored by ICPMS and showed a well-controlled etch rate. However, some shortcomings of these H₂O₂-containing solutions, like roughness and selectivity, lead to a change of oxidizing agent from H₂O₂ to O₃. In the O₃-containing solutions, the selectivity towards Al₂O₃, SiO₂, and TiN could be secured. The impact of the dissolved O₃ concentration on surface roughness and etch rate as well as the uniformity of this wet etching process were assessed on a single wafer tool. Finally, the bulk and surface GST composition and oxidation post-recess were verified through XPS and ERD.

REFERENCES:

[1] D. Loke et al., “Breaking the Speed Limits of Phase-Change Memory.” Science, 2012, 336, 6088, 1566.

[2] K. Ding et al., “Recipe for ultrafast and persistent phase-change memory materials.” NPG Asia Mater 12, 63, 2020.

[3] F. Rao et al., “Reducing the stochasticity of crystal nucleation to enable sub-nanosecond memory writing.” Science, 2017, 358, 6369, 1423.

[4] [internet] https://www.techinsights.com/blog/intel-3d-xpoint-memory-die-removed-intel-optanetm-pcm-phase-change-memory

[5] [internet] https://www.techinsights.com/blog/memory/intels-2nd-generation-xpoint-memory

[6] [internet] https://www.imec-int.com/en/imec-magazine/imec-magazine-october-2017/in-pursuit-of-high-density-storage-class-memory

[7] H.Y. Cheng et al., “Wet-Etching Characteristics of Ge₂Sb₂Te₅ Thin Films for Phase-Change Memory.” IEEE Trans. Magn., 41, 2, 2005.

[8] L. Wang et al., “Basic Wet-Etching Solutions for Ge₂Sb₂Te₅ Phase Change Material.” J. Electrochem. Soc., 157, H470, 2010.

[9] C. Deng et al., “XPS study on the selective wet etching mechanism of GeSbTe phase change thin films with TMAH.” Proc. of SPIE, 8782, 87820N, 2012.