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is frequently expressed as noise counts per volume of fluid
measured. Particles counters for these applications must be
designed to have an extremely low background pulse noise level.
A POU high purity water particle counter
Given the future trends for the use of high purity water in
semiconductor manufacturing and an understanding of optical
particle counters, the requirements for a point of use high purity
liquid particle counter are summarised as follows:
ring650 nanometer sensitivity
ring6Adequate sample rate
ring6Low background false count level
Fig 2. Layout of an optical system in a light scattering ring6Small size
liquid particle counter ring6Optimum flow rate for directly monitoring processes
ring6Fast clean up time for mobile applications
ring6Low CoO including maintenance and calibration costs
reduce clean up times and eliminate lag times in response to
changes in the process being monitored depending on location. To achieve 50 nanometer sensitivity requires a number of
technologies be employed simultaneously. The first technology is
Performance considerations a high brightness diffraction limited single mode laser diode. In
In high purity water applications the particle levels are addition, the use of this lower cost type of light source minimises
extremely low and therefore one must carefully consider other the cost of ownership of the instrument. This light source is then
factors that affect the instrument’s measurement. Optical coupled with a low aberration optical system to produce a very
particle counters have a measurement uncertainty that is a bright laser spot in the centre of the flow cell.
function of the particle’s time of arrival which is governed by a Another improvement factor comes from using a high gain
Poisson statistical process. Therefore there is a fluctuation in the photodetection system which offers a substantial gain in the
actual number of counts reported in a sample about the long- signal to noise of the system. A final critical element of the
term average particle level. The amount of fluctuation has a design is a very high numerical aperture light collection optical
direct relation to the number of observed events or particles in a system that has minimal aberration and a low magnification.
sample. The standard deviation of this fluctuation in reported The high numerical aperture increases the signal to noise of the
counts is equal the square root of the number of counts. system. The low aberration and magnification permits the use of
Therefore, to reduce the fluctuation in a particle counter’s a small photodetector to reduce the system noise including the
measurement one must improve the number of particles counted background pulse noise or false count level of the instrument.
in a sample.
To understand this performance aspect of a specific liquid Summary
particle counter in a high purity water application one must High purity water is playing an increasing role in semiconductor
consider the sample rate of the counter which is defined as the manufacturing. Current trends in semiconductor processing are
view volume times the flow rate. In addition, the sensitivity of resulting in an increased need for liquid particle counters
the particle counter as well as the distribution of particles being capable of monitoring high purity water in point of use
monitored must be considered. For example, if the distribution of applications. These applications require particle counters that
particles follows the inverse third power law then there are eight have a small footprint and low cost of ownership. To meet these
times as many particles present at 50 nanometers than at 100 requirements a number of technologies have been employed in
nanometers. Therefore a more sensitive instrument may provide the design of a liquid particle counter resulting in an instrument
a more statistically significant measurement. In addition, the well suited for point of use monitoring.
number of particles counted is directly proportional to the
sample rate of the sensor. Thus higher sample rate sensors with
equivalent sensitivity produce a more statistically significant REFERENCES
measurement. [1] V. Libman, A. Neuber, ‘Water Conservation Challenges in
Another point that must be considered in high purity water the Electronics Industry,’ Ultrapure Water, Micro 2007
particle counters is the fact that at low particle levels these Conference, 2007.
instruments are also subject to cosmic ray noise. Cosmic rays in [2] V. Libman, A. Neuber, ‘Water Reuse Trends in the
the atmosphere bombard the detector and if they have enough Electronics Industry’, Presented at the Ultrapure Water, Micro
energy they create a voltage pulse output from the detector with 2007 Conference, 2007.
sufficient amplitude that it is falsely counted as a particle. The [3] B. Parekh, ‘Emerging Applications of UPW: Meeting
cosmic ray pulse noise limits the particle levels that the Challenges of Advanced Semiconductor Manufacturing,’
instrument can measure accurately. This background count level Ultrapure Water, Micro 2007 Conference, 2007.
July 2008
www.micronanosystems.info
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