The processing of gate-all-around (GAA) Si transistors requires several isolated and vertically stacked nanometer-thick Si sheets or wires. For this purpose, the sacrificial SiGe layers of a SiGe/Si superlattice are etched selectively and laterally. Controlling the quantity of etched SiGe material, i.e., the so-called SiGe cavity depth, is critical for optimal device performance. Unfortunately, this critical dimension can only be measured by time-consuming cross-sectional transmission electron microscopy (TEM), which results in limited statistics and hence insufficient control of the cavity depth across wafers and batches. This paper evaluates the capabilities of micro hard x-ray fluorescence (μHXRF) for the determination of cavity depth as a fast and non-destructive alternative to TEM. As we show, μHXRF provides cavity depth values in excellent agreement with TEM. In addition, two critical advantages of the technique demonstrated here are that, thanks to the very high energy of the incoming and emitted X-rays, the SiGe volume is extracted without requiring any complex model and without any correlation to other geometrical parameters of the complex GAA device.
GeSbTe (GST) compounds have been implemented in Phase Change Random Access Memory (PCRAM) devices. 2D scaling or stacking of PCRAM is limited by cost and therefore the development of 3D architectures is expected. A key requirement for the fabrication of this 3D architecture is the etch-back of the GST. The main objective of this study is to partially recess GST in a controllable way, leaving the surface of the GST smooth after recess. Wet isotropic recess of amorphous and crystalline blanket films as well as patterned samples was explored using commodity chemistries like APM and HPM. Due to some shortcomings of these peroxide-containing solutions, the oxidizing agent was changed from H2O2 to O3. In the ozone-containing solutions the roughness of the GST after etch-back could be controlled as well as the selectivity towards Al2O3, SiO2 and TiN ensured.
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 1st generation X-point memory [4] and might be adopted in a 2nd 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.
The use of SiGe substrate as a semiconductor material is increasing because of its unique properties. In order to manufacture high-performance devices, it is necessary to develop SiGe selective etching technology. In this study, SiGe epi and oxide substrates with varying germanium percentages (15, 25, and 40 %) were used for the investigation of the selective etching process. As the etchant, APM (1:4:20) solutions were used, and added HF and HCl to confirm the pH effect. The evaluation was conducted while adjusting the pH level. In the case of the SiGe epi substrate, the etching rate was very low at high pH, but the etching rate rapidly increased at a specific pH. And then, the etch rate gradually decreased. On the other hand, the etch rates of the oxide substrate rapidly increased as the pH decreased. To explain the etch rate behavior due to the difference in Ge content and type of substrates, the surface chemistry was measured, and the speciation of the solution was analyzed.
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