In the process of fabricating 3D NAND devices, the complex deposition and etch have been proven to be challenging. Two etch processes: silicon nitride sacrificial removal and W etch-back in the 3D NAND word-line formation have been identified as the two critical steps that significantly impact the 3D NAND product yields. In this paper, we present the results of an automated on-line chemical management system that were specifically developed to enable real-time monitoring and control of both the sacrificial silicon nitride removal and W etch-back processes.
Keywords—Silicon Nitride Etch, Tungsten Etch, 3D NAND, Phosphoric-Acetic-Nitric Acid, PAN
One of the key challenges of 3D NAND is scaling stack height for higher bit density. Unlike 2D planar NAND that is constrained by lithography, the bit density of 3D NAND is limited by the complex deposition and etch process steps while stacking the NAND structures in the vertical direction. The process of fabricating 3D NAND begins with multilayered silicon nitride and oxide deposition, followed by high aspect-ratio hole etch for the channel and word-line. The silicon nitride in the word-line is a sacrificial layer that is removed by immersion wet-etch, followed by dielectric (ONO) and tungsten metal gate, deposition and etch-back . In this process flow, the silicon nitride sacrificial removal and W etch-back have been identified as the two critical steps that require accurate real-time metrology and process control.
Critical Wet Etch Processes
A. Sacrificial Silicon Nitride Etch using Hot Phosphoric Acid
The method of using hot phosphoric (Hot Phos) acid to etch silicon nitride is well understood and has been used in semiconductor manufacturing for many years. The control of temperatures and water content in H3PO4 was found critical in controlling the nitride and oxide etch rates. It was also found that seasoning the Hot Phos etching bath with silicate can further reduce the etching rate of SiO2 and improve the etch selectivity. Theoretically, a critically high etch selectivity can be achieved by seasoning the H3PO4 with high concentration of silica. Nevertheless, maintaining a stable etch process with such a high etch selectivity over time has been proven difficult to achieve without real-time monitoring and control, due to the dynamic bath loading behavior and etch by-products. A reliable real-time monitoring and control of Si is also important to prevent process induced defects due to Si precipitation.
At ECI, we have developed a suite of methods [2,3] designed to accurately analyze the components of the Hot Phos etch bath for stable and reliable monitoring and control of the etch process. These methods not only enable a reliable and stable etch process in the life time of the etching solution, but also the feed and bleed and cost savings that extend the lifetime of the etching baths. We have demonstrated that real-time results can be obtained using the methods implemented in our automated on-line system QualiSurf QSF-500 (see figure 1). To ensure the real-time results are accurate, we measured and compared the results with off-line Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) (see Figure 2).
In the experiments of seasoning and etch, we demonstrated the capability of accurate monitoring and control of the Hot Phos silicon nitride etch process. The results are shown in Figure 2 and Figure 3.
B. PAN Tungsten (W) Etch
For a well-controlled selective etch of aluminum over Si or SiO2, PAN (Phosphoric-Acetic-Nitric acid) is commonly used. PAN is also considered for the W etch-back in the 3D NAND process. Similar to Aluminum etch, W oxidizes in the nitric acid forming a by-product W(NO3)x, which dissolves in the phosphoric acid. Acting as a wetting agent, the acetic acid in PAN facilitates the etch process by removing the H2 by-product.
Over the lifetime of the PAN solution, the concentration of H3PO4 increases due to the evaporation loss of the Nitric/Acetic/H20. To maintain a stable and consistent etch rate of W, the H3PO4 concentration must be controlled. Figure 5 illustrates the consequences of inconsistent etch of W where 3D NAND devices will short when W under-etches. At ECI, we have developed an on-line automated chemical management system that accurately monitors and controls the components of PAN. In a spiking experiment, different concentrations of H3PO4 were added into the bath. The system accurately measured the H3PO4 component concentrations as shown in Figure 6.
The demand for a higher bit density in 3D NAND will continue to push the limits of the fabrication process and stack height. The monitoring and control of the process becomes critically important as the number of stacking layers increases. In this paper, we presented the results of real-time on-line automated solutions in accurately monitoring and control Si3N4 and W etch.
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 ECI Technology, Inc. Press Release, “Quali-Surf Qualifies in Japan and Taiwan Fabs”, Totowa, NJ, Feb 2, 2012.
 C. N. Bai, G. Liang, E. Shalyt, “Metrology for High Selective Silicon Nitride Etch”, Solid State Phenomena, Vol. 255, pp. 81-85, 2016.