Optical particle counter and method

Abstract

This specification relates to an optical particle counter, a method for using an optical particle counter, and a method for reducing particle counting errors during use of an optical particle counter. The method includes placing an optical isolator between a laser and a flow cell to allow laser light to pass through the flow cell and to reduce the intensity of light returned to the laser as optical feedback that could cause mode hopping.

Description

Technical Field

[0001] This specification relates to an optical particle counter, a method of using an optical particle counter, and a method of reducing particle counting errors during use of an optical particle counter.

[0002] Cross - Reference to Related Applications This application claims the benefit under 35 U.S.C. § 119 of U.S. Provisional Patent Application No. 63 / 471,410, filed Jun. 6, 2023, the disclosure of which is incorporated herein by reference in its entirety.

Background Art

[0003] Optical particle counters have various commercial and industrial uses for identifying the presence or amount of specific types of particles in a fluid. The fluid may be a liquid (e.g., an aqueous or organic liquid) or a gas (e.g., the clean air in a “clean room,” or an exhaust gas).

[0004] In certain applications, the fluid is a “slurry” (also referred to as a “particle dispersion” or “particle suspension”) that contains small particles dispersed or suspended in a liquid medium. Specifically, the slurry may be of the type used with a CMP process (“chemical - mechanical planarization” or “chemical - mechanical polishing”) to treat the surface of a semiconductor wafer or a micro - electronic device (“CMP substrate”). In these applications, special optical particle counters are used to detect relatively large particles (e.g., agglomerated particles) present in the slurry.

[0005] CMP slurries contain small abrasive particles dispersed in an aqueous medium. The slurry is used to planarize the surface of a semiconductor wafer by applying the slurry to the surface and using a moving pad (“CMP pad”) to bring the pad and the slurry into pressure - contact with the surface. The slurry and the pad are pressed against the surface to abrasively wear away a small amount of material from the semiconductor wafer surface, thereby producing a very flat, i.e., “planarized,” surface.

[0006] The particle size in the slurry can be selected to provide a desired polishing effect. The slurry can be prepared to have particles within a desired size range, often with a particle size distribution that has the morphology of a typical (i.e., bell-shaped or Gaussian) curve. However, over time, after the slurry is prepared and before it is used, the individual particles in the slurry tend to coalesce, forming larger "aggregated" particles. These aggregated particles can be large enough to generate scratches on the semiconductor wafer surface during the planarization step.

[0007] An optical particle counter can be used to detect undesirable particles in a slurry, including aggregates formed from multiple individual polishing particles. When used to detect aggregates in a slurry, the optical particle counter directs focused laser light through the slurry sample as the sample moves through a small transparent conduit called a "flow cell." The laser light passing through the moving slurry may be reflected or interrupted as aggregates pass through the light. The particle counter includes one or more photodetectors positioned relative to the moving slurry sample, which receive the light from the laser passing through or reflected by the slurry. Changes in the intensity of the laser light received by the detector may indicate undesirable large particles in the slurry, such as aggregates. [Overview of the Initiative]

[0008] An optical particle counter is a highly sensitive measuring device that relies on a precisely tuned combination of highly refined optical components such as lasers, transmissive optical devices such as flow cells, optional filters, photodetectors and electronic control systems, and mechanical components. The combined components are controlled and monitored by computer software, which must be programmed and applied to the specific measurement application to accurately interpret the electronic signals generated by the photodetector, which receives and measures the intensity of laser light passing through or reflected by the slurry. The system can be highly sensitive to very small differences in operating conditions, variability in the performance of system components (e.g., consistency of laser wavelength and output, which can be affected by laser temperature and current input), and the properties of the slurry and its components (liquid medium and particles). One or more changes or variations in these can cause erroneous readings, i.e., "particle counting errors," in the optical particle count.

[0009] When an optical particle counter produces a particle counting error, the cause of the error may be obvious or it may be latent and extremely difficult to identify from among the numerous variations, inputs, and operating conditions that combine to produce the measurement result. When an optical particle counter produces a reading that is suspected to be erroneous, there may be many potential causes of the error. Identifying the specific cause of the error can be an extremely difficult task requiring a thorough examination of all the different optical, computer, and electronic components of the instrument.

[0010] As described herein, certain types of particle counting errors can be caused by laser "mode hopping." In addition, in addition to many other potential causes, mode hopping can be caused by "optical feedback," i.e., light from a laser used to perform particle size measurements of a slurry is scattered and returned to the laser from the particles in the slurry. When certain types of optical particle counters are used to measure the presence of undesirably large particles in a slurry, such as a CMP slurry, the light guided into the slurry may hit small particles (micron or nanoscale particles) in the slurry, be scattered, and returned in large quantities to the laser. The scattered and re-induced light re-incident to the laser can induce an effect known as "mode hopping" of the light emitted by the laser. Laser mode hopping has the effect of causing variability in the light emitted from the laser, such as undesirable fluctuations in the intensity or frequency of the emitted laser light. When fluctuating (irregular) laser light is generated by the laser of an optical particle detector, the optical sensor of the optical particle counter receiving that light from the laser may produce erroneous signals, such as signals indicating a much higher concentration of relatively large particles detected in the slurry. Potentially, the error could cause the optical particle counter to report results (particle counts) that are more than 100 times higher than the expected or known particle count of the slurry.

[0011] In one embodiment, the present invention relates to an optical particle counter. The particle counter comprises a flow cell adapted to accommodate a flow of liquid dispersion; a laser that generates a beam of laser light directed towards the flow cell; an optical isolator positioned between the laser and the flow cell and adapted to allow light to pass from the laser towards the flow cell and to prevent laser light reflected from the flow cell from entering the laser as optical feedback; and an optical detection system for detecting the laser light leaving the flow cell.

[0012] In another aspect, the present invention relates to a method for detecting particles in a liquid dispersion using an optical particle counter. The method provides a flow of a liquid dispersion through a flow cell, wherein the liquid dispersion comprises a liquid medium having particles dispersed in a liquid medium; a laser is used to generate a beam of laser light; the laser light is passed through a flow of liquid dispersion in the flow cell; the laser light is passed through an optical isolator placed between the laser and the flow cell before passing through a flow of liquid in the flow cell, so that the laser light reaches the flow cell and prevents the laser light scattered by the liquid dispersion in the flow cell from being incident on the laser as optical feedback; and the laser light is detected exiting the flow cell.

[0013] In another aspect, the present invention relates to a method for reducing laser mode hopping in an optical particle counter comprising a flow cell containing a liquid dispersion containing particles dispersed in a liquid. The laser directs laser light into the flow cell and the liquid dispersion. A photodetector detects the laser light leaving the flow cell. The method includes placing an optical isolator between the laser and the flow cell to allow the laser light to pass through the flow cell and to reduce the intensity of the light returned to the laser as optical feedback that could cause mode hopping. [Brief explanation of the drawing]

[0014] [Figure 1] This is a diagram of a conventional optical particle counter. [Figure 2] This is a diagram showing the optical particle counter described. [Modes for carrying out the invention]

[0015] The following describes optical particle counters that are useful for detecting and measuring the presence of specific types of particles in liquid slurries.

[0016] An optical particle counter (or “counter”) may be useful for detecting relatively large particles (e.g., relatively large “aggregates”) in a slurry containing concentrations of these larger particles along with smaller particles suspended in a liquid medium. The slurry may contain any type of liquid medium and any type of solid particles. The following description relates to one specific example of the application of the optical particle counter and method of the present invention for detecting certain relatively large particles that may be present in a slurry used for chemimechanical planarization of semiconductor substrates.

[0017] Chemical mechanical planarization (CMP) is a process that uses chemical and mechanical forces together to polish a surface. It is used by semiconductor device manufacturers in various specific processes to prepare semiconductor wafers for the manufacturing process. Before the deposition or fabrication of complex micro and nanoscale semiconductor components, the working surface of a substrate (e.g., a 300mm semiconductor wafer containing microelectronic devices) must meet tight tolerances for flatness and smoothness to maximize quality and repeatability. The process ("planarization") is carried out using a CMP slurry, which is a liquid dispersion containing a low concentration (e.g., nanoscale) of abrasive particles suspended ("dispersed") in a liquid medium.

[0018] Abrasive particles can be any type of abrasive particle useful in chemical machining compositions. Examples include various forms of zirconia, alumina, ceria, silica, and other ceramic materials. None of these may be doped or undoped, and they may be prepared by any of the various known methods for different types of particles. The abrasive particles are dispersed or suspended throughout a liquid medium. Various types of abrasive particles (e.g., charged, aggregated, uncharged, non-aggregated) are known and commercially available. Individual particles may be spherical or nearly spherical in shape, but they can also have other shapes such as elliptical, square, or rectangular cross-sections. Individual particles are called primary particles. Generally, individual (primary) particles cluster or combine to form larger particles ("aggregated particles") from multiple individual particles.

[0019] Aggregated particles, consisting of several to several individual particles (e.g., 2, 3, 5, or 10 particles), are common and useful in slurries. However, aggregated particles can grow to a size large enough to cause damage to the substrate during processing. In CMP slurries, abrasive particles can coalesce to form larger aggregated particles due to pH shifts, shear stress, or temperature effects. Some particles may become excessively large, taking the form of large "aggregated" or "gel" particles. In slurries, the size of individual useful abrasive particles and acceptable aggregated particles can be submicron, e.g., on the nanometer scale. Useful abrasive particles (individual or acceptable aggregated particles) may have an average particle size ranging from about 0.1, 0.5, 1, 5, 10, or 20 microns to about 100 microns. Larger, undesirable aggregated particles are larger than the size of useful abrasive particles, e.g., larger than D90, D95, or D99 of useful particles, and may be large enough to damage the substrate during processing.

[0020] The slurry analyzed by the described system may contain any appropriate amount of abrasive particles, e.g., about 0.01, 0.05, 1, 2, or 5% by weight to about 10, 20, or 30% by weight based on the total weight of the slurry. The particles have a particle size distribution that includes a high proportion of all particles having particle sizes in the micron range, for example, at least 90 or 95% of the particles in the slurry have a particle size of less than 100, 50, 10, 1, 0.5, or 0.1 microns, for example, at least 90 or 95% of the particles have a particle size of less than 5, 1, 0.5, or 0.1 microns, i.e., measured D90 or D95 of less than 100 microns, or measured D90, D95, or D99 of less than 5, 1, 0.5, or 0.1 microns.

[0021] The slurry typically comprises abrasive particles dispersed in a liquid medium containing a high concentration of water and any small amount of organic additive such as a solvent, the solvent of which may, among other things, be a lower alcohol (e.g., methanol, ethanol, etc.) or an ether (e.g., dioxane, tetrahydrofuran, etc.). An exemplary liquid carrier may contain, or essentially consist of, water, more preferably deionized water. A carrier consisting essentially of water may contain up to (or less) 3, 2, 1, 0.5, 0.1, or 0.05 wt% of another liquid, such as a non-aqueous (organic) solvent such as a lower alcohol (e.g., methanol, ethanol, etc.) or an ether (e.g., dioxane, tetrahydrofuran, etc.).

[0022] To achieve the high-quality and high-process-yield requirements necessary for current microelectronic device manufacturing, all variables present in the manufacturing steps, including the composition of the CMP slurry used during the planarization step, must be monitored and controlled as strictly as possible. Potential variations in the CMP slurry include the concentration and distribution of abrasive particles within the slurry, particularly the presence and concentration of larger aggregated particles. Large aggregated particles in the CMP slurry (e.g., particles larger than 0.5 microns) can cause scratches and dig defects on the wafer surface during the CMP process. Consequently, it is crucial to monitor the size distribution and concentration of fine particles in the CMP slurry, especially the presence of undesirably large aggregated particles.

[0023] Generally, an optical particle counter uses a focused laser beam to detect particles contained in a liquid slurry. The laser beam is directed at a moving sample of the slurry and passes at least partially through the slurry. The light of the laser beam may pass through the entire slurry, or may be blocked, scattered, or reflected by certain particles in the slurry as the sample passes through the laser beam. Certain small particles, such as useful micron and nanoscale particles in the slurry, may not block or reflect the light of the laser, but larger sized particles, such as larger sized aggregates having a particle size larger than the useful slurry particles, may block the light or may reflect, deflect, or scatter the light.

[0024] Photodetectors placed around the sample collect light that passes through the slurry or that is reflected, scattered, or deflected by the particles of the slurry. The particle counter uses information from the transmitted light or the scattered or deflected light to calculate the amount or size of the particles in the slurry sample.

[0025] An exemplary system includes a photodetector on the opposite side of the laser that detects light from the laser after the beam has passed through the slurry. In these exemplary systems, slurry particles of a sufficient size passing through the path of the laser beam block the light of the laser and temporarily reduce the light intensity. The particles are recognized by the temporary reduction in the intensity of the laser light detected by the photodetector. This type of system is called a "light extinction" type optical particle counting system.

[0026] Another type of exemplary system includes one or more photodetectors positioned relative to the slurry sample that enable the detector to receive light from the laser that is scattered or deflected by relatively large particles in the slurry sample. In these exemplary systems, relatively large slurry particles passing through the path of the laser beam scatter or deflect the light of the laser and are recognized as scattered or deflected light detected by a photodetector configured to sense this type of scattered or deflected light. This type of system is called a "scattering" type particle detection system.

[0027] Referring to FIG. 1, a prior art optical particle counter 2 for detecting relatively large (very large agglomerated) particles in a CMP slurry containing undesirable large agglomerated particles combined with smaller, useful slurry particles on the micron or nanoscale is shown. The flow cell 30 is made of an optically transparent material and includes vertical sidewalls that form the sides of a channel adapted to contain the flow of the sample CMP slurry 35. The flow cell may be made of a transparent material such as quartz or sapphire. An exemplary flow cell may define an internal channel having a rectangular cross-section with a width and thickness useful for allowing the flow of the CMP slurry through the channel during the process of counting particles using the optical particle counter. An exemplary thickness (the direction in which the laser is aligned when the laser enters the channel) may range from 200 to 400 um. An exemplary width (the direction across the laser when the laser enters the channel) can be within the range of 1.5 to 2.0 mm.

[0028] The laser 10 generates a laser light beam 15 directed to pass through the lens 18 and the flow cell 30. The laser light passes through the slurry and the useful micron or nanoscale particles but is interrupted or scattered by the larger agglomerated particles. The light transmitted through the flow cell 30, including the amount of scattered light 19, passes through the lenses 22 and 24 and is detected by a photodetector 20 capable of detecting the scattered light focused by the lenses 22 and 24. In response to the photodetector 20 detecting the amount of scattered light 19, the detector 20 generates an electronic digital or analog output signal that correlates to the intensity of the received scattered light 19.

[0029] During use, large or aggregated particles in the CMP slurry passing through the flow cell 30 are scattered as the beam passes through the flow cell 30 and the sample 35, forming scattered light 19. The scattered light 19 is received and detected by the photodetector 20 and converted into an electronic signal. By monitoring the amount of scattered light 19 received and detected by the photodetector 20, the operating software of the optical particle counter 2 can detect, monitor, and report the presence and concentration of large aggregated particles present in the sample CMP slurry 35 as the sample passes through the flow cell 30.

[0030] The laser 10 and detector 20 can be lasers and detectors useful for optical particle counters, respectively, as shown in the figure. An example of a useful laser may be a single-mode laser that emits light of a single frequency with a wavelength in the range of 600 to 700 nanometers. Examples are commercially available, such as a semiconductor laser diode type laser that emits light at a wavelength of 635 nanometers with an output in the range of 7 to 50 milliwatts. Examples of useful photodetectors 20, such as the Hamamatsu S2506-02 and Hamamatsu S6967, are also commercially available.

[0031] An optical particle counter, such as the one shown in Figure 1, can be highly sensitive to even very small differences in operating conditions, variability in the performance of system components (e.g., consistency of laser wavelength and output, which can be affected by laser temperature and current input), and the properties of the slurry and slurry components (liquid medium and particles). A change or variation in one or more of these characteristics of counter 2, as well as other conditions and characteristics of its operation, can cause counter 2 to produce an incorrect reading, i.e., a "particle counting error."

[0032] In general, particle counting errors can arise from multiple causes, which may be obvious or latent, and are extremely difficult to identify among the numerous variations in input and operating conditions of the optical particle counter combined to produce the measurement results. To reduce or eliminate particle counting errors, numerous potential error sources must be identified and controlled. Identifying or controlling potential particle counting error sources can be an extremely difficult task, requiring a close examination of many different optical components, computer and software programming, as well as all the electronic components of the optical particle counter, including operating conditions (temperature) and all operating inputs (e.g., the current used to operate the laser).

[0033] Considering the potential causes of particle counting errors, the operation of the various optical components of the counter, individually or in combination, can be one of many potential causes. Laser operation is one possible cause, including how conditions (temperature) and input (current) can affect the optical output. Lasers are highly sensitive to variations in input and operating conditions, including operating temperature ("laser case temperature"), variations in input current, and "optical feedback," which is light entering the laser from outside the laser.

[0034] Optical feedback can affect the performance of a laser by altering the properties of the light emitted from the laser. Under certain conditions, light reflected back from a laser can cause the laser to experience "mode hopping," which means that the laser's optical output shifts from one longitudinal mode (wavelength) to a different mode. Mode hopping can result in irregular changes in the wavelength of the light emitted from the laser, along with intensity fluctuations (intensity "noise"). There are various potential causes of mode hopping, including the operating conditions of the laser (e.g., the laser's case temperature and current), the variability of the current used to operate the laser, and optical feedback.

[0035] The applicant has determined that when using an optical particle counter as shown in Figure 1 to detect large aggregated particles in a slurry containing mostly smaller particles (e.g., a CMP slurry), the light emitted from the laser and directed towards the flow cell and slurry can be scattered by the smaller particles in the slurry and returned to the laser in sufficient quantities to cause mode hopping. Mode hopping due to optical feedback is not an obvious cause of particle counting error in the system in Figure 1. Other possible causes of particle counting error include the focus shape of the laser beam, the laser temperature which may cause mode hopping, or the beam not being centered in the flow cell.

[0036] As described herein, in order to avoid particle counting errors caused by mode hopping due to light 17 scattered by smaller particles in the slurry 35, an optical isolator is placed between the laser 10 and the flow cell 30 of the counter 2, as shown in Figure 1.

[0037] More generally, when using an optical particle counter (e.g., the type shown in Figure 1) as specified by the applicant and described herein, to detect relatively large aggregated particles in a slurry containing mostly smaller (micron or nanoscale) particles that are useful, optical feedback can be generated when light emitted from a laser is scattered by the slurry containing the smaller particles that are useful and returned to the laser as optical feedback. Optical feedback can generate mode hopping, which can be the cause of particle counting errors generated by the optical particle counter.

[0038] According to the optical particle counter described herein, the counter may include an optical isolator positioned between the laser and the flow cell. The optical isolator acts to control the passage of light in the laser beam by directing light from the laser to the flow cell, while reducing the amount of light or substantially preventing light from passing in the reverse direction from the flow cell into the laser. The optical isolator substantially reduces or effectively prevents light scattered by smaller particles contained in the slurry flowing through the flow cell from being scattered and returned to the laser as optical feedback. The reduced optical feedback prevents mode hopping by the laser.

[0039] An exemplary optical isolator may be effective in allowing light to pass effectively in one direction, i.e., from the laser to the flow cell, while reducing the intensity of the light or essentially preventing the light from passing in the opposite direction. The optical isolator allows a substantial amount of laser light to pass from the laser to the flow cell, for example, up to 90, 95, or 99% or more of the light emitted from the laser to pass through the optical isolator in the direction from the laser to the flow cell. The optical isolator can reduce the intensity of light reflected by the slurry or substantially prevent it from being incident back into the laser, for example, by reducing the intensity of reflected light by at least 50, 80, 90, 95, or 99%.

[0040] One example of an optical isolator is a type called a "Faraday optical isolator." A Faraday optical isolator is a passive, unidirectional, non-reciprocal device that uses the phenomenon of magneto-optical rotation to isolate a laser beam source and protect the laser oscillator from reflections within the optical system. A Faraday isolator functions as a photodiode that propagates light in only one direction.

[0041] Referring to Figure 2, an optical particle counter 4 is shown for detecting very large aggregated particles in a CMP slurry containing aggregated particles in combination with smaller nanoscale particles. The counter 2 can be adapted to detect aggregated particles in a CMP slurry that are larger than 80, 90, 95, or 99% of the useful slurry particles, for example, to detect aggregated particles having a size larger than D80, D90, D95, or D99 of the useful slurry particles. For slurries containing at least 90, 95, or 99% of particles smaller than 0.7 microns, the counter can be adapted to detect aggregated particles larger than 0.7 microns, for example. For slurries containing smaller useful particles, the counter may be adapted to detect particles larger than 0.5 microns, or larger than 0.2, 0.15, 0.1, 0.5, 0.7, or 0.8 microns, up to approximately 20 microns, without detecting smaller useful slurry particles, such as those smaller than 0.8, 0.7, 0.5, 0.1, 0.15, or 0.2 microns.

[0042] The illustrated particle counter 4 includes a similar arrangement of components to the components of counter 2 in Figure 1, except for the presence of an optical isolator 50 added between the laser 10 and the flow cell 30. As with counter 2 in Figure 1, the laser 10 generates a laser light beam 15 that passes through the flow cell 30. The laser light passes through the slurry and useful slurry particles of the micron or nanoscale, but is interrupted or scattered by larger aggregated particles. The light (19) scattered while passing through the flow cell 30 is detected by the photodetector 20. The scattered light 17 is the light scattered by smaller particles in the slurry and returned to the laser 10.

[0043] During use, as the laser beam 15 passes through the flow cell 30 and the sample 35, large or aggregated particles in the CMP slurry passing through the flow cell 30 generate scattered light 19. The scattered light 19 is received and detected by the photodetector 20. By monitoring the amount (e.g., intensity) of the scattered light 19 (produced by large aggregated particles) received by the photodetector 20, the operating software of the optical particle counter 4 can detect, monitor, and report the presence and concentration of large aggregated particles in the sample CMP slurry 35 as the sample passes through the flow cell 30.

[0044] The illustrated optical particle counter 4 includes an optical isolator 50 positioned between the laser 10 and the flow cell 30 to prevent optical feedback from being received by the laser 10 in a manner that allows scattered and re-induced laser light 17 to return to the laser 10. The optical isolator 50 allows light from the laser 10 to pass through the flow cell 30 and the slurry 35 in a first direction and finally be received by the photodetector 20 as scattered light 19. The optical isolator 50 also has the effect of reducing or preventing scattered and re-induced light 17 from returning from the cell 30 to the laser 10 as optical feedback.

Claims

1. An optical particle counter, A flow cell adapted to accommodate the flow of a liquid dispersion, A laser that generates a beam of laser light directed towards the flow cell, An optical isolator positioned between the laser and the flow cell, configured to allow light to pass from the laser to the flow cell and to prevent laser light reflected from the flow cell from entering the laser as optical feedback, An optical particle counter comprising an optical detection system for detecting laser light emitted from the flow cell.

2. The counter according to claim 1, wherein the optical detection system comprises a photodetector that receives laser light scattered by particles in a liquid dispersion that has passed through the flow cell and through the laser light.

3. The counter according to claim 2, wherein the photodetector is adapted to detect particles larger than 0.15 microns in size passing through the laser beam.

4. The counter according to any one of claims 1 to 3, wherein the optical isolator is effective in reducing mode hopping by the laser compared to an equivalent counter that does not include the optical isolator located between the laser and the flow cell.

5. The counter according to any one of claims 1 to 4, wherein the flow cell is a microflow cell including a channel having a depth and width of 0.4 mm × 2 mm.

6. The counter according to any one of claims 1 to 5, wherein the laser is a semiconductor laser.

7. The counter according to any one of claims 1 to 6, wherein the laser light includes electromagnetic radiation having a wavelength in the range of 600 to 700 nanometers.

8. The counter according to any one of claims 1 to 7, wherein the laser has an output in the range of 7 to 50 milliwatts.

9. The counter according to any one of claims 1 to 8, comprising a liquid dispersion in the flow cell, wherein the liquid dispersion comprises a liquid medium and particles dispersed in the liquid medium.

10. The aforementioned liquid dispersion Based on the total weight of the liquid dispersion, At least 90 weight percent of a liquid medium, The counter according to claim 9, comprising less than 10 weight percent of dispersed particles.

11. The counter according to claim 9 or 10, wherein the particles are ceramic particles.

12. The counter according to any one of claims 9 to 11, wherein the particles include silicon dioxide (silica), cerium oxide (ceria), zirconia, or alumina.

13. The counter according to any one of claims 9 to 12, wherein the particles have an average particle size (D50) of less than 0.15 microns.

14. A method for detecting particles in a liquid dispersion using the counter described in any one of claims 1 to 13.

15. A method for detecting particles in a liquid dispersion using an optical particle counter, To provide a flow of a liquid dispersion through a flow cell, wherein the liquid dispersion includes a liquid medium having particles dispersed in the liquid medium, Using a laser to generate a beam of laser light, Passing the laser light through the flow of the liquid dispersion in the flow cell, Before the laser light passes through the liquid flow in the flow cell, the laser light is passed through an optical isolator placed between the laser and the flow cell to allow the light to reach the flow cell, thereby preventing the laser light scattered by the liquid dispersion in the flow cell from being incident on the laser as optical feedback. A method comprising detecting the laser light emitted from the flow cell.

16. The method according to claim 15, wherein a photodetector receives laser light passing through the flow cell and detects a decrease in the intensity of the laser light caused by particles contained in a liquid dispersion passing through the laser light.

17. The method according to claim 16, wherein the photodetector detects a decrease in the intensity of the laser light caused by particles having a size greater than 0.15 microns.

18. The method according to any one of claims 15 to 17, wherein the optical isolator is effective in reducing mode hopping by the laser compared to an equivalent counter that does not include the optical isolator located between the laser and the flow cell.

19. The method according to any one of claims 15 to 18, wherein the flow cell is a microflow cell comprising a channel having a depth and width of 0.4 mm × 2 mm.

20. The method according to any one of claims 15 to 19, wherein the laser is a semiconductor laser.

21. The method according to any one of claims 15 to 20, wherein the laser light includes electromagnetic radiation having a wavelength in the range of 600 to 700 nanometers.

22. The method according to any one of claims 15 to 21, wherein the laser has an output in the range of 7 to 50 milliwatts.

23. The aforementioned liquid dispersion Based on the total weight of the liquid dispersion, At least 90 weight percent of a liquid medium, The method according to any one of claims 15 to 22, comprising less than 10 weight percent of dispersed particles.

24. The method according to any one of claims 15 to 23, wherein the particles are ceramic particles.

25. The method according to any one of claims 15 to 24, wherein the particles include silicon dioxide (silica), cerium oxide (ceria), zirconia, or alumina.

26. The method according to any one of claims 15 to 25, wherein the particles have an average particle size (D50) of less than 0.15 microns.

27. A method for reducing laser mode hopping of an optical particle counter comprising a flow cell comprising a liquid dispersion comprising particles dispersed in a liquid, wherein the laser directs laser light into the flow cell and the liquid dispersion, a photodetector detects the laser light leaving the flow cell, and the method comprises placing an optical isolator between the laser and the flow cell to allow the laser light to pass through the flow cell and to reduce the intensity of the laser light returned to the laser as optical feedback which may cause mode hopping.

28. The method according to claim 27, wherein the photodetector measures the decrease in the intensity of the laser light when particles contained in the liquid dispersion pass through the laser light.

29. The method according to claim 27 or 28, wherein the laser experiences reduced mode hopping compared to an equivalent optical particle counter that does not include the optical isolator between the laser and the flow cell.

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