Molecular Sensing for Semiconductor Etch Applications

As process technologies have advanced, the difficulties in performing etch operations have increased. New structures and chemistries have created challenges in monitoring these complex operations. For instance, 3D-NAND structures call for high aspect ratio (HAR) trench etching. Likewise, in addition to involving Al, W, Cu, Ti and TiN, etching is now used for high-k dielectrics, metal gates and rare earth metals used in memory stacks. In-situ quantitative gas metrology by using molecular sensing is proving to be an essential tool to qualify process chambers and monitor process chemistries to ensure high yields. Its use is not, however, without some challenges.

Atonarp, a leader in molecular sensing technology, has written a white paper that identifies the issues encountered with conventional gas analysis metrology and shows how their Aston molecular sensor product addresses them. Key among these issues is durability, chamber matching, system integration and ease of use.  Aston can monitor precursors, reactants and by-products during processing steps. Aston works in real time for baseline chamber and process fingerprinting, chamber clean, process monitoring (including in the presence of corrosive gases), particle deposition and gaseous contaminant condensation.

Aston offers two new features that enable it to work more reliably and longer before maintenance is needed. Their ReGen^TM mode uses energetic plasma ions to remove deposits on the sensor chamber walls. Also, the sensor offers two ionization sources; the μPlasma ionization feature preserves filament life from the effects of aggressive gases such as NF₃, CF₄ Cl₂ and there is also a separate electron impact (EI) filament ionizer for baselining and calibration. These features combined with the removal of particles such as (Tetraethyl Orthosilicate) TEOS and vapor contaminant deposits concurrently with regularly scheduled chamber clean cycles, dramatically extends the sensor lifetime between scheduled maintenance cycles by up to x100.

The Atonarp white paper reviews the metrology difficulties encountered when high aspect ratio masks are needed for 3D device structures. These are seen in 3D NAND flash and DRAM as well. Aspect ratios of greater than 100:1 are common. Etching through alternating layers of silicon nitrate and silicon oxide demand high speed sensing and quantitative end point detection, which is increasingly challenging for legacy metrology solutions. Variation from wafer to wafer must be minimized and held to tight margins or else yield will be compromised. New materials such as tungsten and copper call for increased contamination analysis to ensure protocol zones are managed within the FAB.

According to the Atonarp white paper, Aston provides comprehensive in-situ gas metrology. It can detect and quantify contamination, cross-contamination, gas impurities and process chemistry inside the process chamber. It can help assess performance of etch recipes. Aston also measures post-etch clean to help eliminate process drifts. Lastly, it uses plasma or gas monitoring to provide fast and accurate end point detection (EPD). For instance, it can do this by monitoring CO by-product decline during dielectric etching, or Cl reactant rise for polysilicon and metal at endpoint.

Low Open Area (OA) and HAR designs are making optical emission spectroscopy (OES) less desirable for EPD. OES already was facing challenges for OA due to background noise levels that inhibited detection of changes in emitting species. For tungsten etch there is a reduction in Cl reactants with smaller OA. With HAR the etch tends to slow down due to reduced material transport. The combination of both of these effects decreases the consumption of reactant gas, making it difficult to see observable changes in the OES signal due to depletion of reactants from the plasma. The white paper points out that Aston is capable of looking at both reactants and by-products for accurate EPD.

Aston is useful for atomic level etch (ALE) because it can report on chamber and process health. While ALE is basically self-limiting, it can still be useful to monitor process drifts for maximum throughput. Aston does not need plasma to make observations, so it is a good fit for ALE, which is chemical de-absorption, not plasma based. With ALE Aston can be used during the process to monitor all the chemical changes and ensure the process is properly going through all the steps.

The semiconductor etch process has become much more complex in recent years and calls for more advanced monitoring. Atonarp’s Aston in-situ metrology product looks like an excellent fit for these applications. In this article I was only able to skim the white paper, which offers much more detail on the uses of Aston. The full white paper is available in the Atonarp website by registering at this link  https://www.atonarp.com/contact/aston