WO2012018869A1 - Particle counter with vertical-cavity surface-emitting laser - Google Patents

Abstract

An improvement to optical particle counter design using the vertical-cavity surface- emitting laser (VCSEL), giving significant advantages over the existing edge-emitting laser diodes commonly used in optical particle counters. VCSELs have relatively low threshold currents, thus consuming less power and generating less heat than standard semi-conductor laser diodes of the same power output. In addition to lower power consumption, the VCSELs have better sensor resolution, improved reliability, lower temperature sensitivity and higher immunity to mode hopping than the edge-emitting laser diodes. In addition, because VCSEL can be tested at the wafer level, lower cost potentials can be achieved. This overall allows the creation of a more sensitive, higher reliability, lower cost, and smaller package size particle counter.

Description

PARTICLE COUNTER WITH VERTICAL-CAVITY SURFACE-EMITTING LASER

CONTINUITY DATA

This is a non-provisional patent application claiming priority to U.S. provisional patent application no. 61/369,941, filed on August 2, 2010.

FIELD OF THE PRESENT INVENTION

The present invention is an optical particle counter that utilizes a vertical-cavity surface- emitting laser (VCSEL). The present invention gives distinct advantages over the existing particle counters that use edge-emitting laser diodes. Laser diode based particle counters have a number of limitations, including sensitivity to optical feedback that causes a reduction in the signal to noise ratio, especially in light obscuration particle counters; limited resolution with high power multimode strip laser diodes; and a limited operating range. In contrast, VCSELs have immunity to optical feedback, relatively low threshold currents, and consume less power and generate less heat than standard semiconductor laser diodes of the same power output. The higher power efficiency of VCSELs allows for the creation of smaller sensor designs and increased battery life in portable applications. In addition, the VCSELs have lower temperature sensitivity and higher immunity to mode hopping (mode hopping creates optical noise in the system) than the edge-emitting laser diodes. Furthermore, because VCSELs can be tested at the wafer level, there is a lower cost of production. These overall factors allow the creation of a more sensitive, more reliable, lower cost, and smaller sized particle counter.

BACKGROUND OF THE INVENTION

The present invention in general relates to systems which utilize light scattering principles to detect and count single particles in fluids, and more particular to such a particle counter that utilizes a VCSEL light source.

The principles of light scattering are widely used for detecting and analyzing particles in or of a fluid. The present invention relates to the science of utilizing the principles of light scattering to detect and measure the size of individual particles suspended in a fluid. Each particle that is detected is counted, and an indication of the number of particle counts within a channel, with each channel corresponding to a particular size range, is provided. For particle counters to operate effectively, the density of particles in the fluid must be very small— indeed, the particles are generally considered to be contaminants. It is important to distinguish the science of particle counting from other scientific fields, such as photometry and cytometry, which also utilize scattered light, but in which the density of the particles in the fluid is relatively large; often it is the particles of the fluid itself that are detected and analyzed. These latter systems rely on collecting scattered light from thousands, millions, and even billions of particles; therefore, their principles of operation are very different from the principles used in particle counters. Particle counters are generally used to detect contaminants in extremely pure fluids, such as those used in high tech electronics and the pharmaceutical industry. Generally, small samples of the fluids used in the manufacturing processes are diverted to the particle counters, which sound an alarm if the number and/or size of the particles detected is above a predetermined threshold. Since a small sample of the manufacturing fluid is generally not completely representative of the entire volume of the manufacturing fluid, statistics is used to extrapolate the state of the manufacturing fluid from the sample. The larger the sample, the more representative it is, and the more quickly an accurate determination of the number and size of particles in the manufacturing fluid can be made. Thus, it is desirable for a particle counter to detect particles as small as possible, as fast as possible, in as large a sample as possible.

Physical constraints require tradeoffs between the above goals. For example, sample volume and speed usually must be sacrificed to detect smaller particles. This is a direct result of the fact that, for particles to be detected in a particular fluid, the fluid must be constrained to flow through the monitoring region of a particle counter. Physical objects, such as nozzles and flow tubes, must be used to direct the fluid flow to the particle counter monitoring region. If it is desired to detect the particles in the entire sample flow, then scattered light from the entire sample flow must be collected. This generally results in light scattered from the physical constraining objects, such as a nozzle or flow tube, also being collected, which light creates noise in the output. The noise prevents detection of extremely small particles. This noise can be avoided by detecting particles in only a small portion of the sample flow. Particle counters that attempt to count all the particles in a fluid sample are generally referred to as volumetric particle counters, and particle counters that detect particles in only a small portion of the fluid flow are generally referred to as in-situ particle counters.

The word in-situ in Latin literally means in the natural state. That is, ideally, it refers to measurements unaffected by the measurement instrumentation. In an in-situ system, to be unaffected from the constraining elements, the detected particles must be far from the constraining elements, and only particles in a small fraction of the sample fluid flow are detected. In-situ systems commonly process 5% or less of the sampled fluid. An in-situ single pass particle counter is disclosed in U.S. Pat. No. 5,459,569, issued Oct. 17, 1995 to Knollenberg et al., which patent is hereby incorporated by reference. As a result of measuring only a selected fraction of fluid flow, however, in-situ systems take more time to achieve a statistically significant determination of the fluid cleanliness level or fluid quality. When measuring particle contamination levels in a clean room environment, this extended measurement time generally incurs the risk that an unacceptably high level of airborne or liquid particle concentration could go undetected for substantial time periods, thereby allowing a large number of manufactured parts to be produced under

unacceptably "dirty" conditions. This situation can lead to substantial economic loss owing to the waste of time and production materials in the affected facility.

Since it is practically impossible to actually measure 100% of the particles carried by flowing fluid, herein the term "volumetric" generally corresponds to systems which measure 90% or more of the particles flowing through a measurement device. Volumetric particle measurement systems generally provide the advantage of measuring a greater volume of fluid, whether liquid or gas, within a fixed time period, thereby enabling a more rapid determination of a statistically significant measure of fluid quality. In the case where the particle concentration exceeds a predetermined permissible limit, this more rapid fluid processing generally enables a defective manufacturing process to be halted more quickly and more economically than would be possible employing in-situ measurement systems. However, as indicated above, volumetric measurement systems generally experience more noise than do in-situ systems because the efforts expended to control the location and flow characteristics of the fluid being analyzed generally perturbs the characteristics being measured to a greater extent than does in-situ measurement. One example of a trade -off between measurement completeness and interference with measurement data is that which arises when establishing the proximity of placement of a fluid inlet nozzle to a laser beam. Generally, both the completeness of the measurement, i.e., the percentage of sample flow measured and the interference with this measurement, increase with increasing proximity of the nozzle to the laser beam.

In various circumstances, there may be measurement processes having characteristics which are intermediate between in-situ and volumetric processes. Thus, where in-situ measurement generally corresponds to particle measurement within 5% or less of fluid transported through a measurement device, and volumetric measurement generally corresponds to analysis of 90%> or more of such fluid, it will be recognized that measurement processes may be configured to process 10%>, 30%>, 50%>, or other percentages in between the levels associated with in-situ and volumetric operation. Accordingly, herein, the term "non-in-situ" measurement generally corresponds to measurement of a proportion of fluid equal to more than 5% of total fluid flow.

In the field of particle counting, the use of high power illumination generally enhances particle detection. Specifically, higher power levels generally enable the detection of smaller particles than lower power systems. Higher power levels also generally permit particles of a given size to be detected more quickly. Thus, lasers are generally used as the light source in particle counters. Laser particle counters are of two types: intracavity particle counters in which the sample volume passes through the laser cavity, and extracavity particle counters, usually referred to as "single pass" particle counters, in which the sample volume is located outside the laser cavity. Locating particle-containing fluid flow within the cavity of the laser illuminating the particles provides for higher illumination power levels than are available in single pass laser systems, because, to maintain the lasing action, only a limited amount of optical energy is allowed to pass out of the cavity. A state-of-the-art in-cavity laser particle measurement system is disclosed in U.S. Pat. No. 5,889,589, issued March 30, 1999 to Jon C. Sandberg, which patent is hereby incorporated by reference herein. However, in such particle counters, significant fluid flow through the laser cavity tends to modulate the characteristics of the laser cavity, thereby introducing undesired noise due to the medium, e.g., the air molecules. For this reason, fluid flow rates are commonly reduced when employing in-cavity systems to minimize the introduction of the cavity modulation-related noise.

Conventional laser pumping cavities also have cavity power fluctuations greater than 30% short term and 50% long term caused by such things as thermal effects, air density changes, and particulate contamination of the laser cavity. Further, it is difficult to maintain calibration with such power level changes, because not all noise levels track linearly with power. This results in calibration errors occurring in most systems as the power level decreases. For this reason, cavity systems need to be purged regularly, and the systems need to be disassembled regularly to mechanically clean them. Locating a fluid flow containing particles for counting and measurement outside a laser cavity in a "single pass" laser system utilizing a solid-state laser diode generally avoids all of these problems and permits larger fluid samples to be monitored, but at the expense of much lower available laser power. Finally, U.S. Patent No. 6,859,277 issued February 22, 2005 to Wagner et al. is for a particle counter that employs strip laser diodes, unlike the present invention which employs VCSELs.

The power available in particle counters that utilize VCSELs to detect particles in fluid is challenged by multi-element effects which necessarily arise in large laser diodes. The presence of multiple elements creating a laser beam makes it difficult to shape the beam. Accordingly, there is a need in the art for a particle counter system and method which provides high power illumination in a low noise environment and which produces a scattered light energy spectrum which is readily convertible into particle measurement data. Further, to accomplish this in a non-in-situ system would be highly advantageous. SUMMARY OF THE INVENTION

The present invention advances the art and helps to overcome the aforementioned problems by providing a particle counter utilizing a VCSEL that provides a high power beam in a single pass, low noise system for rapid detection and measurement of small particles.

Particle counters have for years used laser diodes as light sources. A laser diode is a quasi point source, so it must be combined with some form of beam shaping (at least a simple lens) to be used in a particle counter. When it is combined with beam shaping to illuminate the sample, it can easily be combined with an inlet and outlet nozzle and detector to create and optical particle counter.

However, laser diodes have characteristics that limit their performance in particle counters. First, their wavelength is sensitive to temperature, which gives them a limited operating temperature range. Second, they are sensitive to optical feedback, which also limits the signal to noise ratio of the system. Third, they are relatively inefficient light sources, and emit beam profiles that are not ideal for use in a particle counter. VCSELs, on the other hand, can be utilized in several ways to overcome the limitations of laser diode based particle counters.

In one embodiment, a VCSEL can be used to improve the signal to noise ratio of a light blocking optical particle counter due to the lack of sensitivity to optical feedback. In another embodiment, a one-dimensional VCSEL array can be used to make a solid state particle counter as an alternative to a strip laser diode based instrument. In another embodiment, the use of a VCSEL provides a particle counter that can be used for high temperature operation since the operating temperatures can be as high as 80 degrees centigrade. In yet another embodiment, a VCSEL array can be used to create a high resolution particle counter, since the beam profile is a quasi top hat instead of the common Gaussian profile of a single mode laser diode, or the erratic, almost Fresnel diffraction-like profile of a multimode laser diode.

The present invention provides a particle counter that utilizes a VCSEL. Several breakthroughs in employing VCSELs have been made. The ability to build two- dimensional array configurations allow for high power VCSELs needed to provide sufficient illumination for the particles of interest.

The present invention provides a device for optically detecting an unconstrained particle suspended in a flowing fluid, the device comprising: a sample chamber having a fluid inlet and a fluid outlet; a VCSEL producing a laser beam; a beam shaping system directing the laser beam at the flowing fluid in the sample chamber; a light collector located to collect light scattered by the particle in the sample chamber, the collector producing an electric signal characteristic of the scattered light; and an output device communicating with the collector to provide an output characteristic of the particle detected in the fluid in the sample chamber. Preferably, the fluid is a gas. Preferably, 10% or more of the particles suspended in the fluid passing through the sample chamber are detected; more preferably, 30% or more of the particles suspended in the fluid passing through the sample chamber are detected. More preferably, 50% or more of the particles suspended in the fluid passing through the sample chamber are detected; and most preferably, 80%> or more of the particles suspended in the fluid passing through the sample chamber are detected. In the preferred embodiment, the device is a volumetric particle counter. Preferably, the beam shaping system comprises a lens. Preferably, the lens comprises an aspheric collimating lens, an achromatic spherical lens, and a cylinder lens. In another embodiment, the lens comprises an aspheric collimating lens and two cylinder lenses. Preferably, the fluid flows in a first direction, the laser beam is single mode in a dimension substantially along the first direction, and the laser beam includes multiple modes in a dimension substantially in a direction perpendicular to the first direction. Preferably, the fluid flows along a first axis, and the energy distribution of the laser beam substantially along the first axis, in terms of distance from the center of the laser beam versus relative intensity as compared to the intensity at the center, is Gaussian. Preferably, the fluid flows along a first axis, and the energy distribution of the laser beam substantially along a second axis perpendicular to the beam and the first axis, in terms of distance from the center of the laser beam versus relative intensity as compared to the intensity at the center, is more uniform than a Gaussian distribution. Preferably, the output is substantially free of noise greater than the noise created by light scattered from molecules of the fluid.

In another aspect, the present invention provides a device for optically detecting an unconstrained particle suspended in a flowing fluid, the device comprising: a fluid inlet for producing a fluid flow; a VCSEL producing a laser beam; a beam shaping system directing the laser beam at the fluid flow; a light collector located to collect light scattered by the particle in fluid flow, the collector producing an electric signal characteristic of the scattered light; and an output device communicating with the collector to provide an output characteristic of the particle detected in the fluid.

The present invention also provides a method for optically detecting an unconstrained particle suspended in a fluid, the method comprising: flowing the fluid containing an unconstrained particle; providing a VCSEL producing a laser beam having one or more elements; directing the laser beam at the fluid flow; collecting light scattered by the particle in the fluid; and providing an output based on the collected light scattered by the particle detected in the flowing fluid. Preferably, the fluid flows substantially along a first direction, and providing and directing comprises controlling the laser beam in a dimension along a second direction perpendicular to the first direction and the direction of the laser beam. Preferably, the directing comprises focusing the laser beam with at least two lenses selected from the group consisting of an aspheric collimating lens, an achromatic spherical lens, and a cylinder lens. Preferably, the directing, collecting and providing an output are performed such that the output is substantially free of noise greater than the noise created by light scattered from molecules of the fluid.

The present invention enables much larger, and therefore more powerful, VCSELs to be used effectively in a fluid particle counter. As will be seen in more detail below, the present invention teaches how to control noise from spontaneous emission while examining a large portion of the fluid flow. While the system permits high-powered, low noise volumetric systems that were not previously possible, it should be understood that the present invention is not limited to volumetric systems. The present invention can be used to advantage in any particle counter, including non-in-situ and in- situ systems. The above and other advantages of the present invention may be better understood from a reading of the following description of the preferred exemplary embodiments of the present invention taken in conjunction with the figure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a particle counter according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In this disclosure, the term light is not limited to visible radiation but is used in a broad sense meaning any electromagnetic radiation. The terms in-situ and volumetric are used as described in the "Background of The Invention" above. It is also noted that this disclosure is limited to fluid particle counters, which is a term of art. There are particle counters that detect particle counters in a vacuum. Because there is no fluid present, or rather any fluid present is rarified as compared to normal fluids, problems associated with fluid flow, light scattering from the fluid and the apparatus used to control the fluid flow are absent and the physics of such particle counters is significantly different than that of fluid particle counters. Further, it should be noted that particle counters as disclosed herein are designed to be able to detect single particles which are unconstrained in a flowing fluid as distinguished from other systems that detect and analyze the particles of the fluid itself, clouds of particles suspended in a fluid, or particles which are constrained in the fluid, such as constrained to flow in a single line past a light beam. Those skilled in the art recognize that it is a much more difficult task to detect and size single particles flowing unconstrained in a fluid; therefore, the art of particle counting involves different technology than these other particle detection and analysis systems.

A basic design for the present invention is illustrated in FIG. 1. The components are shown suspended in space to facilitate understanding their relationship with one another. The three basic components are a VSCEL (10), a light detection device (20), and particle flow source (30). A first axis (1), second axis (2), and third axis (3) are illustrated that are normal to each of the three basic components. These axes can be normal to one another as in Euclidian geometry, but they need not be. The first arrow (31) indicates the direction of particle flow. The second arrow (11) indicates the direction of the light rays (15). In most relevant art, the light detection device (20) is a photodiode, and the particle flow source (30) is achieved with a conventional pump (not shown) pushing or pulling the particles along the third axis (3).

In FIG. 1, the light rays (15) being emitted by the VCSEL (10) are represented as lines, as is the custom with ray tracing. And the particles (35) are represented by small circles. The size of the particles (35) is exaggerated for illustrative purposes. The particles (35) come out of the particle flow source (30) with some velocity and intersect with the light rays (15). The view volume (40) corresponds to the three dimensional intersection of the particles (35) and the light rays (15). The view volume (40) is represented by a sphere, as is the custom in discussing particle detectors.

An example of how particles (35) are detected will now be explained. A simple model of light rays (15) will be used so that ray tracing can be used in order to illustrate the basic method of operation. The particles (35) go through the view volume (40), and light rays (15) strike the particles (35). The first light ray (101), third light ray (103), fourth light ray (104), and fifth light ray (105) have collided with particles (35) in FIG. 1 and are scattered. For purposes of discussion, these four light rays (101, 103, 104, 105) will be called scattered light rays (101, 103, 104, 105). The second light ray (102) does not collide with a particle (35). Most light rays (15) would not collide with a particle (35) and would merely pass through the view volume (40) and proceed down the first axis (1). In most relevant art, the light rays (15) will enter a conventional light trap (not shown). The fourth light ray (104) will collide with the light detection device (20); however, the first light ray (101), the third light ray (103), and fifth light ray (105) will not collide with the light detection device (20).

The fourth light ray (104) striking the light detection device (20) is used to define the existence of a struck particle (35) and, based on the signal strength, the size of the particle (35). The ability to accurately count and size particles is based on the signal strength above the background noise of the system. The greater the signal to noise ratio, the smaller the particle (35) that can be detected and sized. The noise of the system is caused by stray light striking the light detection device (20).

The more of the light rays (101, 103, 104, 105) scattered in the viewing sphere that are collected by striking the light detection device (20), the more sensitive the particle detector will be. Relevant art is concerned with increasing the particle detector's ability to record the scattered light rays (101, 103, 104, 105) by redirecting the scattered light rays (101, 103, 105) that would miss the light detection device (20). This redirection is accomplished with mirrors.

The basic principle is that the more of the scattered light rays (101, 103, 104, 105) that can be detected by the light detection device (20), then the more sensitive the light detection device (20) will be, and the less power the light detection device (20) will need to consume for a given sensitivity by the VCSEL (10).

Claims

1. A device for optically detecting an unconstrained particle suspended in a flowing fluid, the device comprising:

a sample chamber having a fluid inlet and a fluid outlet;

a vertical-cavity surface-emitting laser producing a laser beam;

a beam shaping system directing the laser beam at the flowing fluid in the sample chamber;

a light collector located to collect light scattered by the unconstrained particle in the sample chamber, the light collector producing an electric signal characteristic of the light scattered by the unconstrained particle; and

an output device communicating with the light collector to provide an output characteristic of the unconstrained particle detected in the flowing fluid in the sample chamber.