Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Content or effect of a constituent of a liquid mixture
Reexamination Certificate
2001-03-02
2003-08-12
Lefkowitz, Edward (Department: 2856)
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Content or effect of a constituent of a liquid mixture
C073S579000
Reexamination Certificate
active
06604408
ABSTRACT:
FIELD OF THE INVENTION
The present invention related to methods and devices for use in determining the characteristics of particles dispersed in a medium, particularly to such devices that operate with reduced wear and maintenance issues, and which avoid changes in sample level during operation.
BACKGROUND OF THE INVENTION
It often is desirable to know the specific properties of particles dispersed in a liquid medium. For example, larger particles in chemical-mechanical planarization (CMP) often lead to scratches and variations in the median particle size and the size distribution sometimes lead to inconsistent polishing. Similarly, the properties of sol-gel derived bodies depend in large part on the characteristics of the particles in the dispersion. For these reasons, among others, various methods have been developed to determine the characteristics of particles in such dispersions.
One such characteristic is the particle size distribution (PSD). See, for example, U.S. Pat. Nos. 4,706,509, 5,121,629, and 5,569,844. U.S. Pat. No. 4,706,509 describes a method for ultrasonically measuring solids concentration and particle size distribution in a dispersion. Ultrasonic waves at a variety of frequencies are directed into the dispersion, and the attenuation at these frequencies is measured. A dimensional spectrum (across the range of particle dimensions) is divided into dimensional intervals, and a system of linear equations is developed to represent the concentration of particles in each dimension interval. The system of equations is then solved to determine the PSD.
The process described in U.S. Pat. No. 5,569,844 involves measuring the attenuation of both ultrasonic waves and electromagnetic radiation to determine particle size distribution. Specifically, ultrasonic velocity and ultrasonic attenuation are combined with the density, as determined from the electromagnetic radiation attenuation, to calculate the PSD. In addition to the problems inherent in generating x-rays or gamma rays, however, particle sizes of about 10 to 15 &mgr;m appear to be the lower limit for the process.
In U.S. Pat. No. 5,121,629, ultrasonic waves at a variety of selected frequencies are passed through a dispersion, and the attenuation at each frequency is measured to derive a measured attenuation spectrum over those frequencies. Separately, based on a theoretical model, a set of attenuation spectra are calculated for a variety of PSDs, and the calculated spectra are then compared to the measured spectrum to formulate a preliminary approximation of the PSD of the dispersion. Further calculations must be performed, starting from this approximation, to more accurately determine the PSD.
U.S. Pat.No. 6,119,510 describes an improved process for determining the characteristics of dispersed particles. The term particles is used to include solids, liquids, or gases dispersed in a continuous medium. Waves (acoustic or light) are directed into a dispersion, and the attenuation of the waves for particular frequencies is measured to provide an attenuation spectrum. The measured attenuation spectrum is then compared to a set of theory-based calculated attenuation spectra to determine the particle size distribution corresponding to the measured attenuation spectrum. Unlike previous processes, the particle size distribution is capable of being accurately determined by a single inversion algorithm. Inversion techniques involve taking a set of known particle size distributions, determining the attenuation spectrum that each PSD would theoretically produce, and comparing a set of such theory-based spectra to the actual, measured spectrum to find the actual PSD.
Acoustic attenuation techniques for characterizing particles in dispersions involve the interaction of applied sound waves with the dispersed particles. As a sound wave travels through a dispersion, the wave loses acoustic energy by various scattering mechanisms. Measurement of the attenuation at different frequencies of the sound wave leads to an acoustic attenuation spectrum. Models by Epstein and Carhart, and by Allegra and Hawley make it possible to predict the attenuation spectrum for particles of a given size distribution and concentration. The models require knowledge of several physical properties of the particles and the liquid medium, including density, thermal expansion coefficient, thermal conductivity, heat capacity, viscosity, and shear rigidity. It is then possible to construct a 3-D matrix that relates attenuation, frequency, and particle size.
Acoustic attenuation in a particle dispersion can be measured by use of an apparatus such as the Ultrasizer TM, made by Malvern Instruments, Ltd., Worcestershire, United Kingdom (the assignee of U.S. Pat. No. 5,121,629, discussed above). A schematic of the chamber
110
of such an apparatus is shown in FIG.
7
. Two pairs of broadband transducers
112
,
114
,
116
,
118
, are in contact with a sample located in a tank
120
, typically formed of stainless steel. The chamber
110
typically contains a temperature sensor, heater, and a propeller (not shown). It is possible to remove the sample by use of a drain valve at the base of the tank
120
. Typically, one pair of transducers
112
,
114
covers a lower frequency range, e.g., 1 to 20 MHz, and the other pair
116
,
118
a higher range, e.g., 15 to 200 MHz. Generally, the transmitting transducers
112
,
116
are capable of being moved to different positions in the chamber, but the receiving transducers
114
,
118
are fixed. The spacing between the transmitting transducers
112
,
116
and the receiving transducers
114
,
118
is controlled by a stepper motor. The transmitting transducers
112
,
116
direct a series of planar sound waves, generally at preselected frequencies, across the sample and the receiving transducers detect the attenuated waves. Computer and signal processing equipment is used to control the process of attaining the attenuation spectrum, monitor the detected sound waves, and manipulate the signals generated.
Problems with the prior art devices, such as the apparatus described above, include wear of the seal due to the translation movement of the receiving transducers. That wear causes misalignment of the receiving transducer with the emitting transducer causing major degradation of results due to minute levels of misalignment. The transducers move up to about four inches and, thus, considerable wobble can be experienced due to movement on the o-ring seal. The wear of the seals also can result in leaking of the sample, which can result in a change of the volume sample, i.e., the level of the sample in the sample cell changes. Further, the in/out movement of the variable position transducers in the sample cell causes significant changes in the sample level in the cell. The change in the volume/level of the sample can result in the introduction of sound waves reflected from the liquid surface or a change in the reflection pattern, thereby further degrading the results. Besides sample loss, leaks also can cause electrical damage to the device and sample change. For example, the percent of solids in the sample can change if the medium is leaked preferably to the particles. Also, the shear applied on samples by friction between a moving transducer and an o-ring can cause the particles to aggregate (to form clusters). This particle aggregation can shift the measured particle size to a larger value, thereby rendering the data inaccurate.
Moreover, the semiconductor polishing industry is keen to detect relatively few large particles in the presence of a bulk of small polishing particles. These larger undesirable particles can cause micro-scratches on silicon wafers that result in large financial losses. A particle size measuring instrument that promotes particle aggregation during the measurement can be detrimental to this industry.
Constant wear and tear of the o-ring seal and transducer causes progressive loss of alignment that leads to progressive degradation of data. Thus, frequent replacement of the o-ring seals is required along
Boulet Gary
Dosramos J. Gabriel
Oja Tonis
Reed Robert W.
Cygan Michael
Edwards & Angell LLP
Lefkowitz Edward
Matec Instrument Companies, Inc.
Neuner George W.
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