Seung Soo Hong

Leak Rate Calibration System

The leak rate calibration system down to 10 -8 Pa m 3 /s (10 -7 mbar L/s, 10 -7 std. cc/s) is built at the Korea Research Institute of Standards and Science (KRISS) and its characteristics are elaborated in this paper. The calibration system consists of a constant-pressure type flowmeter, an orifice-flow-type ultra high vacuum system and a leak-mounting manifold. A known amount of gas flows measured by the flowmeter can be used to calibrate ‘standard leak’ by passing flows alternately from the leak and the flowmeter through a constant conductance. The flowmeter is designed to keep the pressure in a variable-volume chamber of the flowmeter constant by changing the volume with a moving piston. The piston is attached to a step motor by a micrometer screw and is fully automated by a computer. The flow rate is calculated from the pressure and the speed of the piston. The flowmeter and the ultra high vacuum system are evacuated by turbo molecular pumps to the base pressure below 10 -8 Pa. This system has been used in leak rate calibration by comparison with the reference leak and by the direct measurement method using the flowmeter. The calibration results of commercial standard leaks, which require calibration by users, show significant difference with nominal values.

Effects of higher‐order harmonics on phase determination in an ultrasonic interferometer

The effect of higher-order harmonic components on phase determination were analyzed to improve the accuracy of transient time measurement in an ultrasonic interferometer with a quadrature detector system. Under ideal conditions the locus of output signals of the phase sensitive detector forms a circle on the phase plane as the frequency changes and any nonideal factor would distort the circle, leading to erratic phase determination. This analysis has shown that the nth-order contribution to the phase is mostly determined by the product of the amplitudes of corresponding components of the reference and echo signals fed to the phase detector. The distortion parameters were obtained by least-squares fitting of the data to the distorted circle in the phase plane, and compared with those roughly estimated from the frequency components of the reference and echo signals. The exemplary numerical phase corrections for the third-order harmonics were carried out, so as to illustrate the improvement in the accuracy of the ultrasonic interferometer.

The Effect of the Operational Conditions on the Quantitative Leak Rate of Leak Detectors

The Korea Research Institute of Standards and Sciences has developed and maintained leak standards systems, and has accumulated much data on leak standards that have been disseminated during calibration and testing services. We have found that the indicative value of a leak detector and a halogen leak detector varies a lot, depending on the conditions encountered during sniffer mode operation. Therefore, to measure the reliability of the leak rate or when performing a leak test, proper test conditions are necessary. In addition, we found that the reliability increased when we used a guide tube that could influence the flow of air.

Measurement of Outgassing in a Vacuum Environment

Outgassing, the evolution of gas from the material in a vacuum, is not only a source of micro contamination in a semiconductor or the flat display panel production process, but it also a limitation factor in the ultra clean process of nano-technology. The outgassing from the materials of satellites and spacecrafts must be controlled for increased safety and function because space is also a vacuum environment. Several methods are used in outgassing measurement in general, but there is no one method suitable for obtaining all outgassing data. The most suitable method for a particular application must be chosen by the experimenter or user. Three types of outgassing measurement systems were fabricated and characterized, ‘Throughput method,’ ‘Rate of Rise method’ and ‘Mass Loss Measurement method’. The outgassing rates of many kinds of materials were measured and characterized using these systems.

Preliminary Study of the Apparatus for Electron Stimulated Desorption of Rare Gas Solids

An electron stimulated desorption (ESD) experimental system consisting of a ultrahigh vacuum chamber, micro channel plate, electron gun, quadrupole mass spectrometer, liquid helium and liquid nitrogen cryostat, and a gas supply tube etc., was set up and evaluated in order to investigate desorption particles at the surface of rare gas solids (RGS). Ar was deposited on the copper sample for 200 seconds at a pressure of 6.3 x 10–4 Pa. Ion currents from the RGS during bombardment at various electron gun energies were measured. These increased proportionally with filament currents between 1.5 A and 1.75 A. With the electron gun turned on, the dominant residual species in the vacuum chamber were H2O, H2, C, O, N2, Ar, and CO2, as measured using a quadrupole mass spectrometer (QMS). The H2O partial pressure was almost 50 % of the total pressure, and that of the other gas species were lower than 20 %. Before starting the experiments, the ultimate base pressure of the ultrahigh vacuum chamber reached 10–8 Pa.