In-situ metrology for ‘lights-out’ ALD processes

Atomic Level Deposition (ALD) is a process that is used extensively and increasingly in advanced semiconductor manufacturing of memories (3D-NAND and emerging Stacked DRAM) and advanced logic processes (such as gate-all-around). These processes are characterized by the need for precisely controlled thin-film layers which are a few tens of molecules thick, often measuring only a few 10’s of angstroms (1Å=1x10-10m).

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A wide range of materials can be deposited using ALD, including oxides, nitrides and metals. The ALD process is widely used, as it provides ultra-thin highly controllable mono-layers of material which are by nature conformal and pinhole free. The ALD market is expected to grow by a CAGR of 16%-20% from 2020 to 2025 (source: ASM).

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For effective ALD process monitoring and control, a high speed chemically specific quantified metrology solution is needed that can work with harsh process gases such as hydrochloric or hydrofluoric acid by products, and can handle condensate particles which can form on chamber surfaces during processing. 

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The metrology solution must quantify the gases present, to enable accurate, rapid and efficient transitions between the multiple phases of operation: precursor gas injection, gas purge, reactant gas injection and by-product gas removal. Typically, each complete cycle takes just a few seconds, so the metrology solution must work in real-time with a high sample rate and sensitivity.

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However, most ALD processes have no plasma or use a weak, remote plasma source. This means that legacy in-situ metrology techniques such as Optical Emission Spectroscopy (OES) are left in the dark. With no strong plasma source to enable their operation, they are unable to provide the information required due to low signal-to-noise ratio or no signal at all.  

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Without in-situ metrology, these process step transitions are typically run for a fixed duration, which leads to processing inefficiency, because sufficient margin between steps must be left to ensure precursor and reactant gases do not inadvertently mix in the chamber. Running ALD processes without metrology also runs the risk of significant line yield loss or process excursions for example, if one of the reactant gas concentrations fluctuates high or low.

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Fast, robust chemically specific mass spectrometry, such as Aston from Atonarp, is a new solution to provide in-situ metrology and control in these non-plasma (‘lights-off’) processes. It enables rapid, chemically specific in-situ quantitative gas analysis down to the parts per billion level, delivering the real-time data needed for effective ALD process control.

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Learn more about Aston and download our Application Brief below for more details on how Aston supports atomic level deposition.