Optics: measuring and testing – With sample preparation – Condensation nuclei detector
Reexamination Certificate
1998-12-21
2002-10-22
Font, Frank G. (Department: 2877)
Optics: measuring and testing
With sample preparation
Condensation nuclei detector
C356S335000, C356S336000
Reexamination Certificate
active
06469780
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH FOR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
The present invention relates to the field of particle counting in gases, particularly reactive, corrosive gases for the electronics fabrication industry.
Minute amounts of contamination can adversely affect the microchip fabrication process in the electronics industry. Contamination in the form of particles causes short circuits, open circuits, and other defects. These defects can cause finished micro-electronic circuits to fail. Such failures are responsible for significant yield reductions in the micro-electronics industry. Yield reductions caused by micro-contamination substantially increase processing costs.
Micro-electronic circuits require many processing steps. Processing is performed using extremely clean gases. However, the amount of contamination needed to produce fatal defects in micro-circuits is extremely small. For example, an individual particle as small as 0.02 micrometer in diameter can result in a fatal defect in a modern micro-circuit. Micro-contamination may occur at any time during the many steps needed to complete a circuit. Therefore, tight control of cleanliness in the processing gas is required.
Modern filters are able to remove particulate contaminants in process gases with an extremely high efficiency. However, the complete assurance of contamination control also requires verification of gas cleanliness. An accurate technique for detecting microscopic particles in filtered gases must be available. This technique must be capable of detecting microscopic contaminant particles as small as 0.02 micrometer in reactive or toxic gases used in microchip processing, including but not limited to the following gases: SiCl
4
, PH
3
, B
2
H
6
, AsH
3
, SiF
4
, Si
2
H
6
, NH
3
, BCl
3
, BF
3
, Cl
2
, H
2
, HBr, HCl, HF, NF
3
, N
2
O, O
2
, SiH
4
and WF
6
, as well as inert gases, including, but not limited to such gases as N
2
, Ar, He, CF
4
, CHF
3
, C
2
F
6
and SF
6
.
Ideally, the technique should perform the particle measurement at a pressure equal to the process gas system line pressure (about 60 psig). By performing the measurement at the gas line pressure, the sample gas would not need to be reduced in pressure before entering the measuring instrument. That is, the instrument could be connected directly to the gas line without intermediate pressure reducing devices. Measurement at gas line pressure would provide an advantage in particle measurement, since the process of pressure reduction in some gases can produce adverse effects such as particle shear-off or nucleation of impurities in the pressure reduction device. Such objects produced by shear-off or nucleation would be falsely interpreted by the downstream particle counting instrument as gas line contaminant particles.
Also, many pressure reduction devices require venting a portion of hazardous and expensive gases before the remaining sample can enter the instrument. Such venting is costly, environmentally damaging, and may require increased flow capacities through vent system emission control devices.
Finally, condensation of the process gas into liquid droplets may occur during the process of pressure reduction, especially when sampling high boiling point and easily condensed gases such as HF, WF
6
and BCl
3
. Such condensation droplets also would be falsely interpreted by the downstream particle counting instrument as gas line contaminant particles.
It is therefore advantageous to develop a sampling technique that can measure contaminant particles as small as 0.02 micrometer in toxic or reactive microchip processing gases without the need for intermediate gas pressure reduction.
Previous attempts to obtain continuous counting of contaminant particles in reactive or toxic process gases, as well as inert gases, have included laser particle spectrometers or laser particle monitors. These instruments determine the equivalent optical diameters of contaminant particles through a process of light scattering from individual particles. The number of light pulses scattered is equal to the number of particles passing through the optical sensing volume of the instrument. Such instruments have been developed for use with reactive or toxic gases, and for pressurized sample gases. Modern laser particle counters typically function with low background noise for particles larger than 0.1 micrometer, but are noise limited in lower size detection capability because of light scattering from sub-range particles and gas molecules. Consequently, such instruments cannot detect contaminant particles smaller than 0.1 micrometer.
Previous attempts to obtain low noise particle detection below 0.1 micrometer have included condensation nucleus counters (CNCs). These instruments use continuous conductive cooling, continuous cooling through dilution, or cooling through expansion to create a supersaturated aerosol mixture. Various substances have been used as a saturating medium, including water, alcohol (e.g., butanol) and perfluorinated organic compounds, such as perfluorodimethyldecalin. The fine particles act as nucleation sites for vapor condensation and subsequent droplet growth. Droplets grow to sufficient size to permit detection by conventional light scattering or light absorption techniques with negligible accompanying noise.
Such a CNC has been described in U.S. Pat. No. 4,790,650 wherein a device admits a gaseous flow into a saturator zone and then takes a portion of the flow through a chilled region to condense a working fluid on entrained particles to enlarge the diameter of the particle to facilitate counting by downstream means, such as an optical particle detection device.
Additional descriptions of CNCs are found in the dissertation by M. R. Stolzenburg, particularly Chapter 5, titled “An Ultrafine Aerosol Condensation Nucleus Counter”, and in an article titled “A Condensation Nucleus Counter Design for Ultrafine Particle Detection Above 3 nm Diameter” by P. B. Keady, V. L. Denier, G. J. Sero, M. R. Stolzenburg and P. H. McMurry.
U.S. Pat. No. 4,293,217 discloses a continuous flow CNC and process for detecting small contaminants in gas streams. Additional patents pertaining to CNC's include U.S. Pat. Nos. 3,806,248 and 3,632,210.
The theory and operation of one type of CNC is set forth in an article by M. R. Stolzenburg and P. H. McMurry, entitled “Counting Efficiency of an Ultrafine Aerosol Condensation Nucleus Counter: Theory and Experiment”.
The above CNCs were developed for use only with inert sample gases. A CNC designed for use in H
2
and O
2
, as well as inert gases such as N
2
and He, was described in an article by A. E. Holmer, M. L. Malczewski, J. Blesener and G. Schurmann entitled “Design and Calibration of a Condensation Nucleus Counter Suitable for Use in Hydrogen Service”.
A CNC designed for use in H
2
and O
2
, as well as inert gases such as N
2
, was described in an article by H. T. Sommer, J. R. C. Futrell, L. R. Dominguez-Sommer and D. D. Christman entitled “Condensation Nucleus Counter Evaluation for Hazardous Semiconductor Process Gases”.
An alternative method for measuring particles in reactive gases is disclosed in U.S. Pat. No. 5,231,865. This patent discloses a diffusion gas diluter device and a method wherein a particle-containing reactive gas, such as H
2
or O
2
, is diluted with an inert diluent gas to diminish the reactive characteristics of the particle-containing gas without disturbing the particle concentration of the gas, thus allowing it to be accurately and safely measured for its particle content using a conventional inert gas CNC.
The measurement techniques in the above references are capable in some cases of detecting particles in H
2
and O
2
, and can in some cases detect particles as small as 0.003 micrometer. However, those techniques do not detect particles as gas pressures above 0 psig in toxic or other reactive gases, such as SiCl
4
, PH
3
, B
2
H
6
, AsH
3
, SiF
4
, Si
2
H
6
, NH
3
, BCl
3
, BF
3
McDermott Wayne Thomas
Ockovic Richard Carl
Air Products and Chemicals Inc.
Chase Geoffrey L.
Font Frank G.
Punnoose Ray M.
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