Feature: Semiconductors
Te production of semiconductors can be divided into three
stages: design, front-end and back-end. Commonly, defects occur during the front-end stage. Here, processes take place within enclosed chambers that are supplied with ESGs, to create the necessary surface features on the wafer by processes such as deposition, etching and doping. Te danger here is that undetected trace impurities in the gas supply can be deposited directly onto the surface of the wafer, permeating into the bulk layers, affecting devices’ properties. For example, moisture absorption in low dielectrics is known to cause device reliability issues, and even tiny defects can lead to poor device performance, rendering chips unusable. Tis can result in wastage and production line suspension, in turn costing the fab millions of dollars in losses. Additionally, if defective products reach the market undetected, a semiconductor company’s reputation may be damaged. Hence, there is a crucial need to ensure that UHP gases and chemicals are continuously monitored, making sure impurities don’t enter the manufacturing processes.
Gas analysers with accurate detection limits Historically, semiconductor plants have used either gas chromatography (GC), thermal desorption or inductively-coupled plasma analyses to verify the purity of ESGs in quality control and assurance operations. However, these techniques can only detect impurities to the level of part per billion (ppb), an insufficient detection limit to remain competitive in an industry where gas purity is critical. While no single analyser can monitor every impurity in every ESG, API-MS analysers offer fast, accurate detection limits down to ppt levels for a wider range of impurities – moisture, oxygen, carbon dioxide, carbon monoxide and methane – compared to chromatography applications. As a result, API-MS are fast becoming the most important instruments for purity analysis of UHP gases. A strategically installed API-MS can greatly benefit operations when paired with GCs for continuous quality control (CQC). API-MS performs analysis of the sample gas directly, with no
requirement for sample pre-conditioning. It performs ionisation at atmospheric pressure in a two-stage process. A primary ionisation of the bulk gas molecules occurs due to the high electric field from a corona discharge needle. Tese ions readily transfer the charge to trace contaminant molecules, with lower ionisation energies in a secondary stage. Te overall result is a very efficient ionisation of the trace contaminant molecules, which pass with a portion of the sample gas into a quadruple mass spectrometer, where the complete contaminant spectrum is measured at the multiplier detector. API-MS offers increased sensitivity over conventional electron
impact MS, meaning that an increased portion of the trace component can be ionised by charge transfer. As the method relies on the relative ionisation potential of gas molecules, the ionisation is highly selective for trace contaminant molecules when detecting impurities in helium, neon, argon, nitrogen and hydrogen as matrix gases, but is less effective with oxygen, which has a lower ionisation potential compared to common contaminants. API-MS therefore offers detection levels down to 10-50ppt, far exceeding the established gas phase concentration limits of 100ppt per impurity outlined by the International Technology Roadmap (ITR) for semiconductors.
Installing an API- MS capable of online measurements and ppt
detection will provide more control over semiconductor manufacturing processes
Comprehensive measurements At the semiconductor fabrication plant, single or multiple UHP gas lines are linked to an API-MS for immediate and continuous analysis. When a measurement is performed, the instrument’s reaction chamber is flooded with the sample gas, maximising the sample volume for ionisation and reducing background interference, to achieve the best possible performance. Tis atmospheric- pressure ionisation also removes any issues associated with vacuum background. Te Termo Fisher Scientific APIX δQ and APIX Quattro systems
are the first API-MS solutions on the market with online calibration, offering detection limits to ppt per impurity, to protect the integrity of UHP gas supplies for competitive and reliable semiconductor manufacturing. Tese process-orientated analysers combine advanced electronics with powerful soſtware to offer a cost-effective alternative to conventional quality control techniques. Te APIX δQ is a single API-MS analyser, whereas the APIX
Quattro is four analysers in one instrument. Each instrument offers continuous analysis of four UHP gas types, and is supplied with manifolds for switching between different gas streams, allowing the systems to analyse up to eight and 16 sample streams, respectively. Every instrument is also equipped with a gas blender for automatic generation of ppb calibration gases from part per million (ppm) standards for straightforward self-calibration. Te hood assembly in both configurations incorporates a hydrogen safety system that makes the equipment safe. Just as importantly, the systems meet industry-standard communications protocols for plant control and integration, offering seamless incorporation of CQC analysis into existing workflows.
It’s an investment In semiconductor manufacturing, it is crucial to monitor the impurity levels in UHP gases to ensure maximum yields by eliminating even the smallest wafer defects. Investing in fast, sensitive gas analysers can help maximise time, profit and end-user satisfaction by avoiding costly defects. Installing an API-MS capable of on-line measurements and ppt
detection will provide more control over manufacturing processes, by allowing the operator to proactively stop impurities from even entering the chamber, increasing the overall yield and reducing operating and downtime costs.
www.electronicsworld.co.uk June 2023 39
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