Christof Asbach

Nanoparticle exposure at nanotechnology workplaces: A review

Risk, associated with nanomaterial use, is determined by exposure and hazard potential of these materials. Both topics cannot be evaluated absolutely independently. Realistic dose concentrations should be tested based on stringent exposure assessments for the corresponding nanomaterial taking into account also the environmental and product matrix. This review focuses on current available information from peer reviewed publications related to airborne nanomaterial exposure. Two approaches to derive realistic exposure values are differentiated and independently presented; those based on workplace measurements and the others based on simulations in laboratories. An assessment of the current available workplace measurement data using a matrix, which is related to nanomaterials and work processes, shows, that data are available on the likelihood of release and possible exposure. Laboratory studies are seen as an important complementary source of information on particle release processes and hence for possible exposure. In both cases, whether workplace measurements or laboratories studies, the issue of background particles is a major problem. From this review, major areas for future activities and focal points are identified.

Comparability of Portable Nanoparticle Exposure Monitors

Five different portable instrument types to monitor exposure to nanoparticles were subject to an intensive intercomparison measurement campaign. Four of them were based on electrical diffusion charging to determine the number concentration or lung deposited surface area (LDSA) concentration of airborne particles. Three out of these four also determined the mean particle size. The fifth instrument type was a handheld condensation particle counter (CPC). The instruments were challenged with three different log-normally distributed test aerosols with modal diameters between 30 and 180 nm, varying in particle concentration and morphology. The CPCs showed the highest comparability with deviations on the order of only ±5%, independent of the particle sizes, but with a strictly limited upper number concentration. The diffusion charger-based instruments showed comparability on the order of ±30% for number concentration, LDSA concentration, and mean particle size, when the specified particle size range of the instruments matched the size range of the aerosol particles, whereas significant deviations were found when a large amount of particles exceeded the upper or lower detection limit. In one case the reported number concentration was even increased by a factor of 6.9 when the modal diameter of the test aerosol exceeded the specified upper limit of the instrument. A general dependence of the measurement accuracy of all devices on particle morphology was not detected.

Comparison of different characterization methods for nanoparticle dispersions before and after aerosolization

A well-known and accepted aerosol measurement technique, the scanning mobility particle sizer (SMPS), is applied to characterize colloidally dispersed nanoparticles. To achieve a transfer from dispersed particles to aerosolized particles, a newly developed nebulizer (N) is used that, unlike commonly used atomizers, produces significantly smaller droplets and therefore reduces the problem of the formation of residual particles. The capabilities of this new instrument combination (N + SMPS) for the analysis of dispersions were investigated, using three different dispersions, i.e. gold–PVP nanoparticles (∼20 nm), silver–PVP nanoparticles (∼70 nm) and their 1 : 1 (m : m) mixture. The results are compared to scanning electron microscopy (SEM) measurements and two frequently applied techniques for characterizing colloidal systems: Dynamic light scattering (DLS) and analytical disc centrifugation (ADC). The differences, advantages and disadvantages of each method are discussed, especially with respect to the size resolution of the techniques and their ability to distinguish the particle sizes of the mixed dispersion. While DLS is, as expected, unable to resolve the binary dispersion, SEM, ADC and SMPS are able to give quantitative information on the two particle sizes. However, while the high-resolving ADC is limited due to the dependency on a predefined density of the investigated system, the transfer of dispersed particles into an aerosol and subsequent analysis with SMPS are an adequate way to characterize binary systems, independent of the density of concerned particles, but matching the high resolution of the ADC. We show that it is possible to use the well-established aerosol measurement technique (N + SMPS) in colloid science with all its advantages concerning size resolution and accuracy.

Nanoparticle release from dental composites

Dental composites typically contain high amounts (up to 60 vol.%) of nanosized filler particles. There is a current concern that dental personnel (and patients) may inhale nanosized dust particles (<100 nm) during abrasive procedures to shape, finish or remove restorations but, so far, whether airborne nanoparticles are released has never been investigated. In this study, composite dust was analyzed in real work conditions. Exposure measurements of dust in a dental clinic revealed high peak concentrations of nanoparticles in the breathing zone of both dentist and patient, especially during aesthetic treatments or treatments of worn teeth with composite build-ups. Further laboratory assessment confirmed that all tested composites released very high concentrations of airborne particles in the nanorange (>10(6)cm(-3)). The median diameter of airborne composite dust varied between 38 and 70 nm. Electron microscopic and energy dispersive X-ray analysis confirmed that the airborne particles originated from the composite, and revealed that the dust particles consisted of filler particles or resin or both. Though composite dust exhibited no significant oxidative reactivity, more toxicological research is needed. To conclude, on manipulation with the bur, dental composites release high concentrations of nanoparticles that may enter deeply into the lungs.

Experimental Investigations on Particle Contamination of Masks Without Protective Pellicles During Vibration or Shipping of Mask Carriers

Extreme ultraviolet lithography (EUVL) is considered the next generation lithography to produce 32-nm feature-size or smaller. The challenge is that conventional pellicles are unavailable for protecting the EUVL masks against contaminant particles, because the EUV beam is easily absorbed by most solid materials. The masks are usually transported or stored in mask carriers. Without the protective pellicles, particles generated inside the mask carrier may deposit on the critical surface of the mask. It is therefore important to identify where the particles are generated inside the mask carrier during shipping. In this paper, two shipping carrier models of different mask holder designs were used. The mask carriers with quartz mask blanks inside were shaken manually, vibrated with a computer-controlled vibration table, or shipped via air freight. In order to simulate the EUVL mask shipping, no pellicles were used. Several online and offline particle detection techniques were employed to investigate particle generation inside the mask carrier during vibration or shipping. It was shown that particles were mostly generated at the contact points between the mask surface and the carrier element. The design of the mask-holding element in the mask carrier played an important role in reducing particle generation.

Experimental Investigations of Protection Schemes for Extreme Ultraviolet Lithography Masks in Carrier Systems Against Horizontal Aerosol Flow

In extreme ultraviolet lithography (EUVL), conventional pellicles are unavailable for protecting the EUVL masks, since they highly absorb the EUV radiation. One of the serious challenges is therefore to prevent particulate contamination of the EUVL masks. In this paper, EUVL mask protection schemes proposed by Asbach were experimentally challenged against horizontal aerosol flow simulating particle transport from the side during mask handling, shipping, and storage at atmospheric pressure. The protection schemes include mounting the critical surface facing down, using a cover plate with particle trap, and applying electrophoresis or thermophoresis. Both electrophoresis and thermophoresis showed very good protection capabilities. Electrophoresis, however, might be counterproductive due to the unknown particle charge polarity in real situations. A particle trap, on which contaminant particles can deposit before they reach the critical surface, could then be used to collect all particles irrespective of their polarity with a sufficiently high electric field but might not work against zero-charged particles. On the other hand, thermophoresis acts on all particles and transports them in the same direction. Therefore, the upside-down mounting and thermophoresis with the cover plate and particle trap are considered the promising protection schemes for the EUVL mask carrier systems

Investigation of thermophoretic protection with speed-controlled particles at 100, 50, and 25 mTorr

Thermophoresis is considered as a candidate for protection of extreme ultraviolet lithography masks from particle contamination during vacuum exposures. A thermophoretic force is exerted on a particle by surrounding gas molecules within a temperature gradient. Gas molecules on the “warm side” of the particle provide more momentum than on the “cool side,” so particles move from the warm to the cool region. In this study, thermophoretic protection of a critical surface from particles injected with known initial speeds into a quiescent gas has been investigated at 100, 50, and 25mTorr. Initial particle speed was varied from 10to31m∕s depending on the gap distances (1, 2, and 3cm), particle sizes (125 and 220nm), and system pressures. A pinhole plate is used to supply speed-controlled particles with almost no accompanying gas flow. The results demonstrate that the window of protection offered by thermophoresis is very narrow for inertial particles, and that thermophoresis offers the greatest protection for lo...

Numerical Evaluation of Protection Schemes for EUVL Masks in Carrier Systems Against Horizontal Aerosol Flow

A numerical model, based on the commercial code Fluent, was developed to study the effectiveness of protection schemes for extreme ultraviolet lithography (EUVL) photomasks in mask carriers against particle contamination under atmospheric pressure conditions. The model included the effect of gravity, diffusion, drag force, thermophoresis, and electrophoresis on the particles and was validated against experimental data. Due to good agreement, the model could be extended down to a particle size of 50 nm, which could not experimentally be detected. It was found that electrophoresis can offer very effective protection if the particle charge distribution is unipolar. Thermophoresis also showed very promising results, however, only a small fraction of the particles could be intentionally deposited within a particle trap, surrounding the mask. Maintaining the mask facing down mainly protects the mask against large gravity-driven particles, whereas the protection against small particles requires the use of phoretic contamination control.

Protection schemes for critical surface in vacuum environments

This article presents a study of particle formation during vacuum pump down and methods that can be incorporated to protect critical surfaces (like semiconductor wafers or masks) from those particles. Particle formation during pump down was reexamined with temperature measurements. Particles were intentionally produced with hard pump down for studying protection schemes. For the first step, a face-down approach for a critical surface was used to investigate the effect of protection from particle contamination. It was very effective to hold a critical surface face down for protection with the use of high gravitational settling velocity in vacuum environment. However, the face down approach did not sufficiently protect the critical surface. For the second step, a bottom protective plate was introduced below the critical surface to improve the protection efficiency. The bottom plate played a great role in protection of the critical surface with preventing particle formation near the critical surface by keeping the surrounding gas temperature high enough to avoid particle formation as well as with potential blocking of incoming particles toward the critical surface. Higher gas temperature intrinsically avoids formation of residue particles by the condensation process during pump down.

Mathematical Description of Experimentally Determined Charge Distributions of a Unipolar Diffusion Charger

The charge distributions of an improved opposed flow unipolar diffusion charger were measured using a tandem differential mobility analyzer (DMA) set up in a size range of approximately 20–400 nm. The charger is intended to be used in a portable aerosol sizer to measure particle size distributions. The determined charge distributions were represented by lognormal distributions, and a set of equations and coefficients was developed to calculate the charge distributions. These equations can be easily implemented in software for size distribution measurements. The agreement between the mathematically derived and measured charge distributions is very good, with regression coefficients R 2 > 0.96. The investigations showed that approximately 55% of 20-nm particles remain uncharged, while up to 25 elementary charges need to be considered for multiple charge correction of 400-nm particles. Comparison with the Fuchs theory delivered satisfying agreement with the measured average charge levels, but charge distributions cannot be described by the Fuchs theory, likely caused by the charger geometry. Copyright 2012 American Association for Aerosol Research