Workplace Emissions and Exposures During Semiconductor Nanowire Production, Post-production, and Maintenance Work

Our plain-English summary of Workplace Emissions and Exposures During Semiconductor Nanowire Production, Post-production, and Maintenance Work, by C. Isaxon, K. Lovén, L. Ludvigsson, et al (Annals of Work Exposures and Health, 2020).

The study

• Authors: C. Isaxon, K. Lovén, L. Ludvigsson, S. Sivakumar, A. Gudmundsson, M.E. Messing, J. Pagels, M. Hedmer
• Journal: Annals of Work Exposures and Health
• Published: 2020;64(1):38-54
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Nanowires are a high-aspect-ratio material of increasing interest for a wide range of applications. A new and promising method to produce nanowires is by aerotaxy, where the wires are grown in a continuous stream of gas. The aerotaxy method can grow nanowires much faster than by more conventional methods. Nanowires have important properties in common with asbestos fibers, which indicate that there can be potential health effects if exposure occurs. No conclusive exposure (or emission) data from aerotaxy-production of nanowires has so far been published.

Methods: Different work tasks during semiconductor nanowire production, post-production, and maintenance were studied. A combination of direct-reading instruments for number concentration (0.007-20 µm) and filter sampling was used to assess the emissions (a couple of centimeter from the emission sources), the exposure in the personal breathing zone (max 30 cm from nose-mouth), and the concentrations in the background zone (at least 3 m from any emission source). The filters were analyzed for metal dust composition and number concentration of nanowires. Various surfaces were sampled for nanowire contamination.

Results: The particle concentrations in the emission zone (measured with direct-reading instruments) were elevated during cleaning of arc discharge, manual reactor cleaning, exchange of nanowire outflow filters, and sonication of substrates with nanowires. In the case of cleaning of the arc discharge and manual reactor cleaning, the emissions affected the concentrations in the personal breathing zone and were high enough to also affect the concentrations in the background. Filter analysis with electron microscopy could confirm the presence of nanowires in some of the air samples.

Conclusions: Our results show that a major part of the potential for exposure occurs not during the actual manufacturing, but during the cleaning and maintenance procedures. The exposures and emissions were evaluated pre- and post-upscaling the production and showed that some work tasks (e.g. exchange of nanowire outflow filters and sonication of substrates with nanowires) increased the emissions post-upscaling.

© The Author(s) 2019. Published by Oxford University Press on behalf of the British Occupational Hygiene Society.

In plain English

This study did not examine asbestos. It measured airborne and surface particles at a company making gallium arsenide (GaAs) semiconductor nanowires by aerotaxy. The authors discuss asbestos because nanowires share its fiber shape, and suggest a similar lung risk if GaAs nanowires do not dissolve in the lungs, which this study did not test. Measurements were taken in 2014, before production was scaled up, and in 2016, after. They covered production, post-production and maintenance, measuring near the source, in the breathing zone and in the background. Manual reactor cleaning in 2014 gave the highest emissions. A filter at the source found 98 nanowires per cm3. The breathing-zone filter, covering all production-laboratory work, found 0.025 per cm3, an 8-hour average of 0.013 per cm3. Emitted nanowires were up to almost 25 µm long, much longer than the roughly 1 µm wires produced. After scale-up, filter exchange and sonication of silicon nanowire samples gave more emissions near the source. Arsine gas was not detected in 2014 and was not sampled in 2016. The authors concluded that most potential exposure came from cleaning and maintenance rather than manufacturing.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Workplace exposure measurement study (described by the authors as an intervention study), with measurements before (2014) and after (2016) production was scaled up
• Who was studied: Work tasks performed by workers at one producer of GaAs nanowires made by aerotaxy, measured in 2014 (Study 1) and 2016 (Study 2), in production and post-production laboratories; the number of workers is not stated
• Exposure: Airborne particles, gold seed particles, gallium and arsenic, arsine gas, and nanowires during work with GaAs nanowires (target length 1 µm, diameter 120 nm); silicon nanowires were also handled as example nanowires
• Main results:
  • Study 1, manual reactor cleaning: particle concentrations were several orders of magnitude larger than during any other work task
  • Study 1: 98 nanowires per cm3 in the emission zone during manual reactor cleaning. The personal breathing zone sample, which covered all work tasks in the production laboratory, held 0.025 per cm3, corresponding to an 8-hour time-weighted average of 0.013 per cm3
  • Assuming the nanowires meet the WHO fiber criteria and are as toxic as Mitsui carbon nanotubes, the authors compared the result with a suggested exposure limit for fibrous nanomaterials of 0.01 per cm3 (8-hour average). They stated that a measured average of 0.013 per cm3 can pose a health risk
  • The emitted nanowires were much longer than the roughly 1 µm wires being produced: 100 nanowires were measured, with a mean length of 3.88 µm, a median of 3.25 µm, and a range of 0.52 to 24.76 µm
  • In their discussion, the authors stated that if GaAs nanowires are insoluble in the lungs, their stiff fiber shape poses potential for a lung risk similar to inhaling asbestos and carbon nanotubes
  • Study 2, cleaning of the arc discharge and DMA and aerosol filter exchange: gold particles were found on filters from both the emission zone and the personal breathing zone
  • Study 2, exchange of the nanowire outflow filter: emission-zone filters showed gallium at 0.51 µg/m3 and arsenic at 3.6 µg/m3
  • Study 2, sonication of substrates with silicon nanowires: short particle peaks occurred in the emission zone when the Para film was removed after sonication; breathing-zone instruments showed no significant increase
  • Other tasks (inspection of reactor, automatic reactor cleaning, leak test, work in glove box, production of GaAs nanowires, handling of substrates with silicon nanowires) gave no significant emissions above background
  • No arsine gas was detected in any Study 1 (2014) sample; for that reason, no arsine sampling was done in Study 2 (2016)
  • Surface contamination with nanowires was found on 1 of 32 surfaces sampled in Study 1 and 1 of 18 in Study 2, both times the floor inside the reactor enclosure
• The authors' conclusion: The authors concluded that most of the potential exposure occurred during cleaning and maintenance rather than during the actual manufacturing, and that some tasks gave more emissions after upscaling
• Limitations the authors state:
  • There are no standard workplace methods for collecting nanofibers, and fiber counting varies between laboratories, which adds uncertainty to the measured concentrations
  • Fibers not lying flat on the filter may have had their lengths underestimated, which would undercount fibers meeting the WHO criteria
  • The acid used to dissolve the filter samples may not have dissolved all the GaAs, so gallium and arsenic levels may be somewhat underestimated
  • It is not known whether GaAs nanowires dissolve in the lung, so any health risk remains uncertain
  • Surface contamination inside the reactor enclosure could not be quantified because the tape samples were overloaded
  • No nanomaterial was detected by electron microscopy during the Study 2 outflow filter exchange, which the authors attributed to the short sampling time
  • Inspection of reactor and automatic reactor cleaning were still performed during Study 2 but could not be measured because of scheduling

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