Optics: measuring and testing – For size of particles – By particle light scattering
Reexamination Certificate
2002-07-05
2004-03-23
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For size of particles
By particle light scattering
C356S246000
Reexamination Certificate
active
06710874
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to the field of detection and characterization of particles in concentrated liquid systems, such as slurries and suspensions.
2. Background Information
Liquid systems with high particulate concentrations are widely used in industry. Examples of such systems are slurries used in Chemical Mechanical Planarization (CMP) processes for the semiconductor industry and emulsions used in the pharmaceutical industry.
Slurry systems used for CMP can have a complicated chemical and colloidal composition. Regarding the chemical composition, a slurry can be either basic (relatively high pH) or acidic (relatively low pH). Keeping the pH value in a predetermined range is often desired for slurries because the pH value often determines the stability of the colloidal composition of the slurry. The colloidal composition of the slurry can have a wide distribution of particle sizes, with diameters ranging from, for example, tens of nanometers (nm) to tens of micrometers (um). The concentration of particles may vary from, for example, ~10
12
particles per cubic centimeter (#/cc) for submicron particles to ~1-100 #/cc for larger (>1 &mgr;m) particles. The size distribution of particles in slurries typically can not be described by a single function but rather, often includes several independent modes. Monitoring particle size parameters is often desired for CMP applications because the size distribution of the small particles can determine the CMP performance. For example, large particles with diameters larger than a few micrometers can cause wafer damage.
Optical methods of detection and characterization have been used for non-intrusive, on-line monitoring of particle parameters in gas and liquid media. See, for example, U.S. Pat. No. 6,159,739, the disclosure of which is hereby incorporated by reference in its entirety. Optical methods include irradiation of the sample with light of known parameters (e.g. wavelength and intensity) and analysis of the scattered and/or transmitted light using different optical detectors.
Integral optical methods, also known as “ensemble” optical methods, allow in principle for the determination of the particle parameters of the scattering system as a whole. With these methods, particles of different sizes and compositions contribute simultaneously to the detected “signal ”. Deconvolution of this composite “signal”, using an appropriate mathematical algorithm, is used to arrive at an estimate of the underlying particle size distribution. These integral optical methods usually involve several assumptions, such as the type (e.g., shape) of the size distribution, refractive index, and so forth.
Differential optical methods allow for the determination of the parameters of single particles. Accumulation of the information relating to the plurality of individual particles allows for the determination of the parameters of the system at whole. Because of the statistical nature of this process, the sensing volume is an important parameter for differential optical methods. The sensing volume is defined as that part of the sample suspension or dispersion, which is irradiated and from which the optical signal is collected. The larger the sensing volume, the greater the number of particles can typically be detected per given sampling period. At the same time, a larger sensing volume can generate larger scattering noise, which can ultimately determine the limit for particle detection. The optimal sensing volume can be determined by taking into account the degree of sample transparency, the concentration of smaller particles, the concentration of bigger particles, and so forth.
Application of optical methods for particle characterization of slurries and suspensions is often limited because of the high optical density of the sample, caused primarily by the high concentration of smaller particles. The high optical density of slurries and suspensions can cause multiple light scattering and a large blockage of light propagation through the sample. At these conditions it can be hard to obtain reliable and accurate information about the size distribution of the particles that scatter and/or block light.
The frequently wide variation in particle parameters of a liquid system can also create conflicting requirements. The sensing volume should be large enough to provide a statistically representative signal for the largest particles, having the lowest number concentration. However, the larger sensing volume can cause a higher optical density (turbidity) of the sample and more extensive multiple light scattering. In addition, a high concentration of abrasives in a slurry can damage optical cell surfaces that are in contact with the sample flow.
It would be desired to have the largest possible sensing volume, consistent with limitations imposed by the optical density and multiple light scattering of the sample. In addition, it would be desired to employ a differential method of optical detection to retain high resolution and sensitivity to the small number of relatively large particles that populate the particle size distribution and which largely define the quality of the slurry. Integral, or “ensemble”, methods of optical detection are significantly limited in their resolution and sensitivity in this respect.
U.S. Pat. No. 5,710,069 (Farkas et al.) discloses a method for measuring slurry particle size during substrate polishing, the disclosure of which is hereby incorporated by reference in its entirety. This method includes shining a light into a portion of the moving liquid-particle mixture having small numbers of relatively large particles and detecting and measuring the reflected light to determine the sizes of the particles. The signal includes a background due to scattered light from a relatively large, indeterminate number of particles. This scattered light is caused in part by the plurality of smaller particles that are present in a slurry at high concentrations.
Another known optical method which does not require slurry dilution, is described by Cerni and Sehler (Particle Optical Sensing for CMP Slurry, MICRO, 2000), and based on measuring the intensity of transmitted and scattered light of different wavelengths. This method involves data processing to reconstruct the underlying particle size distribution. It is an indirect method, which does not allow for the detection of single particles of larger size existing on a background of smaller particles at much higher concentration.
Another optical detection method is described in U.S. Pat. No. 5,818,583 by Sevick-Muraca et al. A system and method are disclosed for the self-calibrating, on-line determination of size distribution and volume fraction of a number of particles dispersed in a medium by detecting multiple scattered light from the particles. The multiple scattered light is re-emitted in response to exposure to a light source configured to provide light of time varying intensity at selected wavelengths. An estimation approach based on an expected shape of the size distribution and the mass of the particles is also disclosed.
U.S. Pat. No. 5,835,211 (Wells et al.) discloses an optical sensor for counting and sizing particles, the disclosure of which is hereby incorporated by reference in its entirety. This method includes measuring a light extinction (LE) and a light scattering (LS) signal representative of the particles. The light scattering and light extinction signals are combined to form a single composite signal, which increases in magnitude monotonically with an increase in the size of the particle passing through a beam of light. The combination of LS and LE signals allows an accurate measurement of particle size in both an upper range and lower range of particle sizes. The slurry is diluted for this monitoring technique to work properly, because only one particle should pass through the beam of light at a time. Additionally, a small sample of the slurry is first extracted and then substantially diluted before being analyze
Burns Doane , Swecker, Mathis LLP
Font Frank G.
Lauchman Layla
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