A differential mobility analyzer and a Faraday cup electrometer for operation at 200–930 Pa pressure
Section snippets
INTRODUCTION
A differential mobility analyzer (DMA) has been used as a size spectrometer for an aerosol in the atmosphere. Recently, Seto et al. (1997) succeeded in reducing the DMA operating pressure to several kPa. This study suggests the potential of a DMA for a particle monitor in the new field of the semiconductor fabrication industry where nanometer-sized particles formed at low pressures are a serious problem (Ziemann et al., 1995). In this paper, we report the successful development of a DMA and a
EXPERIMENT
Figure 1a and b show the schematics of the DMA (hereafter referred to as a very low-pressure DMA, VLPDMA) and the total system, respectively. The VLPDMA is a modified version of the low-pressure DMA reported by Seto et al. (1997). The column length of the DMA is 18 mm, and the radii of its external and internal electrodes are 33 and 25 mm, respectively. To reduce the pressure drop between the DMA and vacuum pumps, which is crucial for low-pressure operation, small radii and excessive lengths of
RESULTS
Figure 3a–d show the normalized mobility distributions as a function of obtained for various particle sizes by changing the voltage applied to DMA 2 operating at 200, 400, 665, and 930 Pa, respectively. As both the electrical mobility Z related to the particle diameter in the free molecular regime and the centroid mobility classified by the DMA are inversely proportional to the pressure P, i.e., Z=ZatmPatm/P and where the subscript atm indicates the value at
SUMMARY
We have succeeded in lowering the operating pressure of a DMA to a few hundred Pa by devising a system that features a low-pressure drop and a large evacuation capacity. It was demonstrated that the performance of the present DMA might be approximately described by a triangular transfer function. The system maintains sufficient resolution at a few hundred Pa for accurate mobility classification in this pressure regime. To our knowledge, the present DMA system presents the lowest operating
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