Real-time particle counter for liquids with nebulizer and dryer

A real-time particle counter apparatus for monitoring contaminant particles in liquids includes a nebulizer, a drying chamber and a counter. The nebulizer provides an aerosol of droplets containing particles to the drying chamber, and the droplets are evaporated leaving solid particles to be counted and sized by the counter. Liquid is recirculated in a closed loop by pumping means that moves the liquid between the liquid source and the nebulizer. A constant air flow containing particles is provided to the counter and the apparatus is self-regulating so that no operator is required to maintain the level of liquid in the nebulizer.

CROSS-REFERENCE TO COPENDING APPLICATION 
A particle detector for flowing liquids with the ability to distinguish 
bubbles is disclosed in allowed copending U.S. patent application Ser. No. 
07,/012,976, filed Feb. 10, 1987 , now U.S. Pat. No. 4,783,599, on behalf 
of Peter G. Borden, and assigned to the same assignee. 
Field of the Invention 
This invention relates to a particle counter for sensing particle 
contamination in liquids. 
BACKGROUND OF THE INVENTION 
Description of the Prior Art 
During the manufacture of semiconductor devices, such as VLSI devices, 
semiconductor wafers that are being processed are exposed to liquids in a 
liquid bath or spray, for example. Typical liquids include deionized (DI) 
water, hydrogen peroxide, hydrochloric acid, hydrofluoric acid and 
sulphuric acid. Contamination of a liquid occurs when spurious particles 
enter the processing environment and become mixed in the liquid being 
used. These contaminants can contact and be deposited on the wafers, which 
would result in defective wafers thereby significantly lowering production 
yield. 
In prior art systems, light scattering is used to detect particles by means 
of a very bright light beam that is directed through the liquid being 
sampled. If particles are present in the liquid, the light is scattered to 
a photodetecting means. Similar types of methods have been used to count 
airborne particles, such as described in the textbook by William C. Hinds, 
entitled "Aerosol Technology", published by John Wiley & Sons, Inc., New 
York, 1987. 
It is known that sensitivity in liquids is much less than air, because the 
amount of light scattered scales as the ratio of the index of refraction 
of the particle to the index of refraction of the liquid medium. Although 
it is possible to detect particles in the 0.1 to 0.2 micron diameter size 
range in air, the typical detection limit in liquids is in the 0.3 to 0.5 
micron size range. Therefore, it has been difficult to detect small 
contaminant particles that appear in liquids used for processing wafers. 
An additional problem that exists in prior art particle detection systems 
for liquids is the presence of bubbles in the liquid. A bubble appears as 
a particle to the detector and thus it is generally difficult to 
distinguish bubbles from particle contaminants by simple light scattering 
technology. The conventional method to eliminate bubbles is to pressurize 
the liquid. This approach requires testing of the liquid sample, rather 
than monitoring the liquid used in the process continuously. Also, a 
pressurization apparatus is complex, expensive and may be dangerous to 
use. An improved system for distinguishing bubbles from particles in a 
liquid is disclosed in the aforementioned patent application. 
In some particle detectors used for monitoring liquids, a nebulizer is used 
to aspirate the liquid and form an aerosol of small droplets. Solid 
particles are obtained that can be detected. In known systems that employ 
nebulizers, the user needs to add liquid manually to the nebulizer, by 
means of an eye dropper, for example. This method does not afford the 
monitoring of the liquid on a real-time continuous basis. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a particle counter useful for 
continuously monitoring liquids in which contaminant particles appear. 
Another object of this invention is to provide a particle counter that 
counts very small particles in liquids without being affected by the 
presence of bubbles. 
Another object is to provide a particle counter for detecting small 
contaminant particles in liquids quickly in real-time in a semiconductor 
wafer production line prior to the number of contaminant particles 
reaching high abnormal levels, thereby precluding a significant loss in 
production yield of semiconductor devices. 
According to this invention, a particle counter for liquids incorporates, 
inter alia, a novel nebulizer that produces from a liquid being monitored 
an aerosol of small particle size, a drying chamber for evaporating the 
aerosol droplets to leave small particles suspended in the air of the 
drying chamber, and a counter for counting airborne particles drawn from 
the drying chamber. The air flow of contaminant particles is continuously 
monitored. In the event that the number of contaminant particles exceeds a 
predetermined level, the process is temporarily halted and the 
contaminated liquid is exhausted as waste and replenished with a new 
liquid.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the drawing, a liquid used in a semiconductor wafer 
process flows through a pipe 13, shown partially broken away. Samples of 
the liquid are drawn through an inlet or orifice 12 by the intake of a 
pump 4 and flows through a pipe 7 through an inlet port 2 into a nebulizer 
1. At the same time, air is directed through an inlet 8 and is exited at 
an orifice 9 within the nebulizer. The orifice 9 through which the air is 
expelled into the nebulizer is higher than the level of liquid 11 that is 
pumped into the nebulizer by means of the pump 4 through the port 2. The 
air that is directed into the nebulizer creates a vertical jet that 
aspirates liquid through an orifice 10. The aspirating action creates an 
aerosol within the nebulizer. 
The aerosol of small droplets is drawn from the nebulizer with a drying air 
flow that enters tube 17 and passes through an inlet 18 of a drying 
chamber 20, which may be a large plastic bottle, for example. The very 
small droplets quickly evaporate leaving small particles suspended in the 
air in the drying chamber. The small particles are drawn through an exit 
aperture 19 and flow through a pipe 22 to an airborne particle counter 23. 
The counter 23 includes a pump (not shown) that draws the air exiting from 
the drying chamber through the pipe 22. The air through pipe 22 carries 
the airborne particles, which are counted and sized in the airborne 
counter. The counter is a standard instrument commercially available for 
such purposes. The air through the counter exhausts through a port 24. 
The sum of the volumes of air provided to the nebulizer through inlet 8 and 
the drying air flow provided to the drying chamber through inlet 18 is 
greater than the volume of air expelled at the exhaust 24 by the pump of 
counter 23. The difference is exhausted as excess air at exhaust 21 that 
is coupled to the drying chamber exit port 19, so that the air leaving the 
drying chamber is effectively divided into the constant flow to the 
counter and excess air portion. 
The aerosol that is drawn from the nebulizer with the drying air flow 17 
consists of water droplets having particles contained therein. Because the 
droplets are small in volume, the probability is very high that the 
droplets will each contain at most one particle. The airborne counter 23 
counts and sizes these particles and provides an effective monitor to 
determine whether the liquid being used in the process has been unduly 
contaminated. 
During operation of the particle counter system, if the liquid 11 in the 
nebulizer rises above the level of outlet port 3, liquid is drawn from the 
outlet port by means of a pump 5 via tube 6. The speed of pump 5 is 
greater than the speed of pump 4 so as to enable drawing off liquid that 
is higher than orifice 9 and the outlet port 3. The outlet port 3 is at 
lower level than the orifice 9 so that liquid does not flow down the air 
tube 8. A continuing flow of water is maintained through a loop including 
an outlet pipe 26 that is coupled to the pump 5, a return pipe 15 that is 
connected between the outlet pipe 26 and the main pipe 13 in which the 
water flows as indicated by arrow 14, the pump 4 and nebulizer 1. The 
liquid level in the nebulizer is thus self-regulating and does not require 
manual replenishment or control by the user. 
In some cases, it is not desired to recirculate the liquid from the 
nebulizer 1 back into the main fluid flow 14. In this case, a two-way 
valve 25 and waste line 16 is provided, so that liquid from the nebulizer 
can be expelled from the system. 
It should be understood that the scope of the invention is not limited to 
the specific configuration disclosed with reference to the drawing. In an 
alternate embodiment, for example, the sample orifice 12 may be disposed 
in a liquid bath or vat instead of in a pipe through which the liquid 
flows. Various types of counters may be used for counting and sizing the 
particles. 
By means of the novel arrangement disclosed herein, a continuous flow 
through a nebulizer is realized to allow continuous sampling of a liquid 
flow and real-time counting and sizing of particles in a liquid. The novel 
design of the nebulizer used in the system affords self-regulation of the 
level of the liquid, and thereby does not require any additional operator 
control. Also, the evaporation of the droplets in the drying chamber 
virtually eliminates bubbles that may appear in liquids, but are no longer 
present with the remaining solid particles, so that the counter senses 
actual contaminants and not spurious bubbles.