Purging Performance Research Articles

A comparative study of a front opening unified pod (FOUP) moisture removal efficiency with different purging gases

Airborne molecular contamination (AMC) is one of the main causes of chip products defects during manufacturing. The downward moist air flow originated from the fan filter unit (FFU) inside the mini-environment (ME) can penetrate into the front opening unified pod (FOUP) and cause wafer defects. Purging nitrogen or clean dry air (CDA) are two common techniques used to minimize the relative humidity (RH) level inside the FOUP. This computational fluid dynamics (CFD) study compares purge performance of CDA and nitrogen when the FOUP door is open. The RH contours and moist air stream line were simulated inside the FOUP during purge. The FFU velocity, purging devices flow rate and temperature are kept constant during the simulation. An innovative large eddy simulation (LES) model was performed with the assistance of a small mesh size to simulate accurately the flow behavior inside the FOUP and ME. The results show that purging the FOUP with either CDA or nitrogen can significantly decrease the RH level to below 5% (RH < 5%). However, the FOUP purge with nitrogen is preferred due to the higher moisture removal efficiency. The CFD simulation results are verified by RH measurements with SHT-35 sensor inside the FOUP under the same processing conditions.

The Effect of Purge Flow Rate and Wafer Arrangement on Humidity Invasion Into a Loaded Front Opening Unified Pod (FOUP)

In recent years, the semiconductor industry has continued to advance production techniques and reduce chip feature size. The presence of unwanted moisture during the construction of these features increases the risk of chip defects and reduced yield. During production, smaller integrated circuit (IC) products are more sensitive to the presence of moisture. Therefore, increasing the efficiency of moisture control techniques is very important to decrease the product defect rate. Front opening unified pods (FOUPs) are the plastic enclosure boxes that are designed to provide a cleaner environment for semiconductor wafers during manufacturing and storage. Clean dry air (CDA) or nitrogen (N₂) can purge moist air out from the FOUP. For the first time, in this study the effect of purge flow rate on the moisture removal efficiency of a loaded FOUP was examined accurately via smoke flow visualization, particle image velocimetry (PIV) and relative humidity (RH) measurement. Moreover, two different wafer arrangements of top-empty and bottom-empty FOUP were investigated. The findings show the bottom-empty FOUP arrangement results in higher purge performance compared to the top-empty FOUP arrangement. Four purge flow rates of 130, 200, 300 and 400 LPM were examined. The findings also show that when the purge flow rate was set to 200 LPM, the lowest level of relative humidity was measured inside the FOUP. Therefore, the purge flow rate of 200 LPM and the bottom- empty FOUP arrangement can minimize the humidity invasion into a loaded FOUP. The findings from this study can be beneficial to the industry to optimize the purge flow rate and to define the most efficient arrangement of the wafers inside the FOUP during manufacturing and storage. This can increase the product quality and reduce energy consumption by decreasing product defects and increasing yield.

Minimising exposure to droplet and aerosolised pathogens: a computational fluid dynamics study

BackgroundHazardous pathogens are spread in either droplets or aerosols produced during aerosol-generating procedures (AGP). Adjuncts minimising exposure of healthcare workers to hazardous pathogens released during AGP may be beneficial. We used state-of-the-art computational fluid dynamics (CFD) modelling to optimise the performance of a custom-designed shield. MethodsWe modelled airflow patterns and trajectories of particles (size range 1–500 μm) emitted during a typical cough using CFD (ANSYS Fluent software, Canonsburg, PA, USA), in the presence and absence of a protective shield enclosing the head of a patient. We modelled the effect of different shield designs, suction tube position, and suction flow rate on particle escape from the shield. ResultsUse of the shield prevented escape of 99.1–100% of particles, which were either trapped on the shield walls (16–21%) or extracted via suction (79–82%). At most, 0.9% particles remained floating inside the shield. Suction flow rates (40–160 L min−1) had no effect on the final location of particles in a closed system. Particle removal from within the shield was optimal when a suction catheter was placed vertically next to the head of the patient. Addition of multiple openings in the shield reduced the purging performance from 99% at 160 L min−1 to 67% at 40 L min−1. ConclusionCFD modelling provides information to guide optimisation of the efficient removal of hazardous pathogens released during AGP from a custom-designed shield. These data are essential to establish before clinical use, pragmatic clinical trials, or both.

Effects of high relative humidity and dry purging on VOCs obtained during breath sampling on common sorbent tubes

Offline breath analysis by thermal desorption gas chromatography mass spectrometry (TD-GC-MS) requires the use of sorbent traps to concentrate and store volatile compounds. The selection of which sorbent to use and best practices for managing high relative humidity are important considerations to allow for reproducible, untargeted, biomarker discovery in water saturated breath samples. This work aims to assess three commonly used sorbent materials for their use in breath volatile sampling and determine how the high relative humidity inherent in such samples effects the capture of volatile compounds of interest. TenaxGR, TenaxTA/Carbograph1TD and TenaxTA/Carbograph5TD tubes were selected as they are the most commonly used sorbents in the breath sampling literature. The recovery of 29 compounds in a standard mix loaded using high humidity gas was tested for each sorbent and compared to loading in dry gas. Water retention and dry purge rates were determined for each sorbent for 500 ml and 1000 ml breath collections. Finally, breath samples were collected simultaneously on to each sorbent type using the ReCIVA and analysed by TD-GC-MS. All three sorbents exhibited acceptable reproducibility when loaded with the standard mix in dry gas (RSD < 10%). Loading the standard mix in humid gas led to reduced recovery of compounds based on their chemical properties. Dry purging performance for each sorbent material was assessed and was shown to be 1.14, 1.13 and 0.89 mg H2O min−1 for TenaxGR, TenaxTA/Carbograph1TD and TenaxTA/Carbograph5TD respectively when flushed with 50 ml min−1 of N2. A comparison of breath profiles on different sorbents showed differences in background artefacts (sulfur dioxide, cyclopenten-1-one and 3-nonene) and endogenous breath compounds (2-methyl-furan and furfural). This work demonstrates that high relative humidity during sampling reduces the ability of sorbent tubes to capture volatile compounds and could impact method detection limits during breath sampling. Sufficient water to impair accurate analysis was retained on all tubes. Minimal differences were observed between sorbent materials when used to sample breath, however, suggestions are provided for sorbent selection for future studies.

Modeling of the operation conditions on the gas purging performance of polymer electrolyte membrane fuel cells

Gas purging has been a common and favorable strategy to improve the cold-start possibility of a polymer electrolyte membrane fuel cell (PEMFC). In the present study, water removal processes in a PEMFC during the gas purging prior to its shutdown are numerically investigated with a transient two-fluid model. The dryness in the sub-regions of the PEMFC is analyzed. Effects of the operation conditions (such as the gas flow rate, relative humidity and temperature) are also explored. The results indicate that water removal processes are not sequential and the main drying mechanism in the porous medium is the surface evaporation under the pendular stage. The overall dryness highly depends on the inlet water flux other than the convective water flux under the same gas flow rate. The operation parameters have a significant impact on the overall drying performance in terms of the purging effectiveness and the energy consumption.

Numerical study of gas purge in polymer electrolyte membrane fuel cell

Gas purge for eliminating water from gas diffusion layer (GDL) and membrane is of great importance for polymer electrolyte membrane fuel cell (PEMFC) start-up at subfreezing temperature. A gas purge model was developed to investigate the water transport phenomenon in GDL and membrane. In simulations, hydrogen and air were used as purge streams in anode and cathode, respectively. Effects of purge conditions on gas purge were numerically studied. In addition to relative humidity, flow rate and temperature, the flow configuration of purge gases was studied for the first time. In order to improve purge performance, several purge protocols were proposed and discussed in detail with respect to purge effectiveness and energy saving.

Performance of Seven Tree Breeding Strategies Under Conditions of Inbreeding Depression.

In the domestication and breeding of tree species that suffer from inbreeding depression (ID), the long-term performance of different breeding strategies is poorly known. Therefore, seven tree breeding strategies including single population, subline, selfing, and nucleus breeding were simulated using a multi-locus model with additive, partial, and complete dominance allele effects, and with intermediate, U-shaped, and major allele distributions. The strategies were compared for genetic gain, inbreeding accumulation, capacity to show ID, the frequencies and fixations of unfavorable alleles, and genetic variances in breeding and production populations. Measured by genetic gain of production population, the nucleus breeding and the single breeding population with mass selection strategies were equal or superior to subline and single breeding population with within-family selection strategies in all simulated scenarios, in spite of their higher inbreeding coefficients. Inbreeding and cross-breeding effectively decreased ID and could in some scenarios produce genetic gains during the first few generations. However, in all scenarios, considerable fixation of unfavorable alleles rendered the purging performance of selfing and cross-breeding strategies ineffective, and resulted in substantial inferiority in comparison to the other strategies in the long-term.

Pre-particle filtration and moisture control by efficient purging in various inlet and outlet of a 450 mm wafer front-opening unified pod

The trend toward narrower line width in semiconductor manufacturing has made contamination control more and more important. The presence of moisture in wafer containers, such as front opening unified pods (FOUP), can lead to the native oxide residues growth, metal corrosion, and thin film cracking on wafer surfaces. Accordingly, decreasing contamination methods and improving factory efficiency are continuously researched. Single or multi-layer particulate shields on top of wafers in FOUPs may be used to prevent pollutant accumulation. In addition, point-of-use (POU) filtration may also been used to control particle contamination in FOUPs during wafer transformation and storage. The demand for stricter filtration led to the usage of 0.10 and 0.20 µm membranes to control the contamination. However, with the introduction of finer membranes end users may have concerns about deleterious remainders on wafers from undergoing filtration. There are a total of 25 pieces of wafers in the FOUP and the arrangement is from the bottom (wafer No. 1) to the top (wafer No. 25) with arising manner. Purging FOUPs to expel moisture vapors with Clean Dry Air (CDA) is one of the most popular methods.There was no previous research for investigating the purge performance on new-generation 450mm FOUPs. This research aims to study main factors influencing the performance of the purge system on 450mm FOUPs, including moisture concentration, CDA flow rate, and filter pressure.

Effective purge method with addition of hydrogen on the cathode side for cold start in PEM fuel cell

The purge process is essential for successful cold start of fuel cell vehicles during winter, and it plays an important role in the removal of the residual water inside the fuel cell in a short time. In this study, a new purge method is introduced by adding a small amount of hydrogen to the cathode gas flow in order to increase the purge performance. The experimental results demonstrate that the hydrogen addition purge method is very effective in removing the residual water near the catalyst layer. The water removal is verified by measuring the resistance of the fuel cell, dew point temperature of the outlet purge gas, and weights of the membrane electrolyte assembly (MEA) and gas diffusion layer (GDL). In addition, the image of the GDL after the purge process is captured to show the advantage of the hydrogen addition purge method. Cold start experiments are also conducted after the optimal purge process. It is also found that the degradation of the catalyst layer is not serious after the hydrogen addition purge process.

Water removal from a PEFC during gas purge

Abstract Gas purge intended to minimize residual water in a polymer electrolyte fuel cell (PEFC) during shutdown is critically important for cell performance and durability under freeze/thaw and cold-start cycling. This paper presents an experimental study of short-duration gas purge relevant to automotive application. A novel experimental procedure has been devised to achieve excellent reproducibility and consistency of purge data. Using the high-frequency resistance (HFR) of the cell as an indicator of membrane water content and hence of purge effectiveness, it is found that the purge performance can be uniquely described by two characteristic parameters, one representing the diffusive flux of water vapor across the catalyst layer and gas diffusion layer, and the other standing for the convective flux of water vapor down the channel with purge gas. A large set of purge data obtained over a range of purge cell temperature, purge gas flowrate and relative humidity and for two purge gases (N 2 and He) have been organized as function of these two parameters. Due to its threefold higher water diffusivity, Helium gas purge is seen to be superior over N 2 , particularly under purge conditions controlled by through-plane vapor diffusion such as lower purge temperatures and shorter purge durations. It is also shown that the HFR after purge typically decreases in a time scale of 1–2 h and that the extent of HFR relaxation generally increases with higher HFR or lower membrane water content after purge.