System for detection of extremely small particles in a low pressure environment

#This invention relates to a system for detecting extremely small particles in a low pressure environment or a vacuum, particularly during semiconductor wafer fabrication.

#Semiconductor wafer fabrication processes are especially susceptible to yield loss due to particle contamination. Free particles landing on a wafer surface during processing can produce defects in patterns and deposited layers that result in device failure. A primary source of these free particles is the vacuum processing equipment that is used for process steps such as the deposition of dielectric and conduction layers, the plasma etching of patterns, the electron beam writing of fine patterns, ion implantation of dopants into the wafer surface, and the dry stripping of photoresist.

#Such contaminant particles present in the processing equipment range in diameter from less than 100 angstroms to several microns. Typical sources include debris broken from wafers during handling, chemical reactions in the equipment, and the flaking of material from the walls of the equipment. Particles greater in size that one tenth of the smallest pattern geometry are generally of concern as they may produce defects in the wafers that are being fabricated. For example, a 1 megabit dynamic random access memory (DRAM) will use 1 micron lines, and 0.1 micron particles will be of concern, and a 16 megabit DRAM will use 0.5 micron lines, and 0.05 micron particles will be of concern.

#The invention will be described in greater detail with reference to the drawing in which:

#FIG. 1 is a schematic sectional diagram of a particle detector system, made in accordance with this invention; and

#FIG. 2 is a series of waveforms A-D representing the gravitational settling of particles for different size particles over a defined pressure range.

#With reference to FIG. 1, a system for detecting extremely small particles comprises a vapor column 3 having a reservoir 2 with an oil disposed at the bottom of the column 3. The oil is preferably Fomblin 425/9 diffusion oil, well known in the technology.

#During operation, a heater 1 heats the oil to a temperature that will cause vaporization. The oil vapors rise in the vapor column 3 and are directed down through a nozzle 4 into a saturation column 5. The saturation column is made of a heat conductive material, such as copper or brass, and has water cooling tubes or a coil 6 soldered to its exterior. In this manner, the temperature of the saturation column or tube 5 can be made lower than the temperature of the vapor column 3, causing the oil vapors to supersaturate in the saturation column. The cool walls of the saturation column also allow the vapors to condense. The condensed oil returns to the oil reservoir 2 through an oil return conduit 7. The downward spray of the oil vapors out of nozzle 4 result in a pumping action in the saturation column 5.

#A sample probe 12 fits into a vacuum chamber 13, for sampling a very low pressure gas. The low pressure gas is drawn with suspended particles by the pumping action into the saturation column 5. The cooling coil 6 extends up to the sample probe 12 to effectuate condensation of the oil vapors, so that they do not backflow into and contaminate the vacuum chamber 13.