Pollution Monitoring Sensor for a Micro-Factory

Miroslav Král and Reymond Clavel Ecole Polytechnique Federale de Lausanne (EPFL) Laboratory of Robotics Systems (LSRO)

Lausanne, Switzerland Miroslav.Kral@epfl.ch, Reymond.Clavel@epfl.ch

Abstract— On the market there are many systems for particle sizing and counting, but unfortunately no one meets the parameters to be integrated into the Micro-Factory (FM) concept. For this reason pollution-monitoring sensor which meets the conditions of MF applications is under development. A MF is a new approach to producing and assembling products having a size of millimetres or microns. A MF is based on a modular concept of several operation units, each having a size of 1dm3, customer-defined conditions (temperature, humidity), and a clean room environment. The detection system is designed to detect each single particle passing through the chamber. It is a hybrid system combining two different detection principles. Both principles are dedicated to detecting particles of a different size. The detection of particles ranging from a size of 100 microns to 10 microns can be achieved using the first method, while the second method can succeed the detection of pollutants having a size of 0.5 micron.

Keywords-miro-factory, clean room, polution monitoring, light scattering

I. INTRODUCTION The developing of a new type of pollution sensor is

motivated by an application for a clean room control in a Micro-Factory (MF) [1] [2], Fig.1. A MF is a new approach to producing and assembling products having a size of millimeters or microns. A MF concept has been under study for more than 10 years by different research groups [3]. In our laboratory we are dealing with the development of a MF which will integrate a clean room environment. A MF is based on a modular concept of several operation units, each having a size of 1dm3, customer-defined conditions (temperature, humidity), and a clean room environment. Each unit is dedicated to a certain operation and the product passes by each of them using a conveyer. The idea behind using a MF is led by the fact that by reducing the size of the production volume we decrease the cost of production and the principal source of pollution which is, in a standard clean room, the presence of a human being. The aim of the research project is to have a production module able to manufacture a product for diverse high-tech applications. The concept of a MF comprises environment quality regulation and control, different methods of fabrication, manipulation or micromanipulation with components, their inspection and assembly, and finally packaging. All these operations can be performed in a clean room environment, without being touched by humans during the entire process. The main possible impact is expected on high-tech applications oriented towards MEMS, with an impact on medical

applications, material research - material testing, defect detection, validation of FEM, measurement of material properties, micro-crack evaluation, microscopic inspection by observation, pollution of samples, determination of residual stresses in micro-components. The well-regulated environment allows the testing of components subjected to different conditions – high pressure, low pressure up to vacuum, humidity, acceleration, jerk, shock, radiation. The micro factory concept is feasible for both – mass production and single product production or prototype fabrication, due to the implemented modularity concept and variability of instrumentation and technology present inside the MF. In order to satisfy the demand for production of medical devices, a clean environment respecting clean room standards ISO 14644-1 class ISO 1 is needed [4]. Clean rooms are classified according to the number and size of particles permitted per volume of air. The number 1 specifies the decimal logarithm of the number of particles of a size of 0.1 μm or larger permitted per cubic metre of air.

The operating volume of such a factory is around a few litres. This also puts heavy constraints on the system for pollution monitoring. The amount of particles present is expected to be no more than 5 per 1 m3 of a size of 0.1 micron. The conditions in a micro-factory are different to those of a typical clean room. The volume of a micro-factory is to the order of 50 dm3. Commercially-available instruments [5],[6] used to detect the contamination of a standard volume clean room, which is typically tenths of a m3, are not feasible for this kind of application. Dust varies from a few nanometers to a fraction of a millimeter in size. Dust also offers essentially unlimited choice in material composition and structure.

Figure 1. The micro-factory in the laboratory of robotics systems in EPFL [1]

Figure 2. An optical layout for detection of particles by diffraction method

II. METHODS A new kind of pollution-detecting single particle sensor is under development and being tested in our laboratory. Pollution detection means a determination of size and number of particles. The detection system is designed to detect each single particle passing through the chamber. It is a hybrid system combining two different detection principles. Each principle is dedicated to detecting particles of a different size. The first is the diffraction of light on single particles and analyses the image captured by CCD and the second based on the scattering of light by a single particle. From an angle of scattering and intensity of light under this direction detected by a photodiode, we deduced the size of the particle by using the Rayleigh theory of scattering. By illuminating the flow of particles with monochromatic highly coherent light, the diffraction pattern captured by CCD can be analysed and the scattered light can be detected.

A. Light difraction technique In this approach the pollutants particles are illuminated by a monochromatic plane wave which after diffracted and impinging to the detector surface. Particle is making a shadow which is represented as the diffraction pattern. These diffraction patterns are analyzed by using the pattern reorganization algorithm. Matlab script was developed and standard algorithms were used to analysis the image of different shape. For the particles of more or less spherical shape the Hough transformation were used. Measuring information is the first diffraction maximum diameter. For particles of size bigger that 50 microns there is no diffraction pattern and the size of shadow is analyzed just from geometrical conditions. Diffraction patterns are analyzed in the framework of scalar diffraction theory [12]. Detection apparatus is adapted to satisfy the condition form Fraunhofer diffraction theory. From analysis one can deduce the size and amount of particles present in volume. The image of diffracted particles impinges on the CCD and is stored. The optical layout can be seen in Fig.2.

The open laboratory set up is made from optical and mechanical parts from commercial suppliers and they are fixed

on the optical breadboard Fig 3. The light source is a Ne-Ne laser having the power of P = 5 mW and operating in TEM00 (more than 99% energy is contained in 00 mode) mode at a wavelength of =623,32 nm. The laser beam is partially linearly polarized 1:500, the output radius is 1.01 mm and divergence is 1 mrad. A filter made from two polarizers is placed in front of the laser in order to trim the laser power. The fixed polarizer P1 is used to establish and maintain horizontal polarization in the transmitted beam while the second variable polarizer is set up between 55-80 degrees used to prevent the saturation of the CCD camera. The polarizers are slightly inclined with respect to the optical axis to avoid parasitic reflections. This element is followed by a beam expander with integrated spatial filter. The beam expander is a Keplerian one, where the objective is a plan convex lens made from BK7, with a focal length of 60 mm and diameter of 2 inches and oriented with planar side towards the eyepiece in order to reduce spherical aberration. The eyepiece lens is an aspheric lens with a focal distance of 15.64 mm and a numerical aperture of NA = 0.23. The pinhole placed inside the telescope in the focal plane of the objective is fixed on the mount placed on the home-made barrel and adjusted by two screws to the optical axis.

B. Light scatering technique

The arrangement of the detection apparatus is done in order to detect the Rayleigh scattering (RS). RS is a low-energy elastic scattering process. The application of RS scattering is widely used in experiments in chemistry, biology and physics and principles are extendly describe in literature [7] - [11]. This theory can be used only when certain conditions are satisfied. The size of a particle must be much smaller than the wavelength of the incident light (a << /2 ). In our case this means that we can detect a particle with a size of 0.1 μm. For the scattered intensity of polarised light of the impinging intensity I0 in the direction :

2cos1

2116,

2

2

2

24

64

0 mm

raIrI (1)

where a is the radius of a spherical particle, r is the distance between the centre of the particle and the photodiode, is the wavelength of light and m is the complex refractive index. In our case we assume that a scattering medium are small dielectric spheres with a refracting index only slightly different from 1. With our instrument the scattered power corresponds to 280 fW. This corresponds to the amount of 20x103 photons per second. This is rather hard to achieve, but not completely impossible. The experimental layout for the second detection method is illustrated in Fig. 4. The detection of pollution particles of a size of less than 1micron is based on the scattering of light. Particles drift by the stream of air. In the chamber they are illuminated by the laser beam. Two photodiodes are used for light intensity detection. The beam is polarized by a Glan-Thomson polarizer and then passes through a mechanical modulator – chopper, which provides an intensity modulation of light in the range of 1 kHz to 10 kHz. The best results were achieved with a modulation frequency of 5.05 kHZ.

Figure 3. Sensor set-up, partially disassembled

With this frequency the chopper is at its most stable and the variation of the rotation of the disc is at a minimum (+/-4 Hz). Scattered light is detected at certain angles and its intensity is measured on the photodiode. With our set-up most measurements were made under 10 degree of light scattering. This is limited by our mechanical configuration and size of optical elements. This is limited by our mechanical configuration and size of optical elements. The second photodiode is in the direction of the beam. Heterodyne detection is used in order to measure the signal on each photodiode. Two independent lock-in amplifiers are used to measure the voltage on the photodiode. The analogy signal from both lock-ins is triggered and processed in a DAQ card. The correlation of both signals is calculated and only significant values are considered for analysis. This allows the singling out of the signal which does not originally come from

scattering. The optical output is calculated by using the Jones’s matrix formalism which considers all optical elements in the detection set-up having an influence on the polarization state of light. The detection of scattered light always leads to the detection of very weak signals perturbed by many different noises. In the case of a micro-factory clean room this problem is even more significant. For this purpose, the problem is overcome by applying several different techniques. Since the event of passing a particle through a laser beam is rather rare and happens on average every 1 minute for class ISO 1, we have the time reserve for the integration constant. This means that for testing particles of the smallest size, 0.1 micron, we can have time integration of the order of 100ms, limited by other related noise sources. This offers the possibility to adapt the instrument for different particle size detection. The particles are delivered to the place where they interact with the laser beam by a stream of air which is ventilated through a glass pipe and the interaction chamber is made of cuvette made from optical glass.

The beam-dust interaction region is 10 mm in length. The speed of ventilated air is 0.1 m/s. The total volume of the MF changes in 300 s, which means that it changes by 1l/min. The flow is laminar with Reynolds number Re=855. The laminarity was checked by visual a optical method. The duration of the illumination of the particle is 0.2s. This gives the limit for the time integration constant. We expected that the particles, if present, are distributed uniformly in the whole of the scanned volume, so that there is no passing of several particles simultaneously in the chamber.

Figure 4. Optical layout for the scattering method

III. RESULTS The first approach was successful for detecting particles

ranging from 10 to 100 microns. The error of size measurement was of the order of a few microns. Several different materials were tested. The examples of results are listed in Tab.1. For example, a particle having a diameter of 8 microns according to CCD reading had a real size of 10 microns. The comparison measurement was done using different methods based on a microscope technique. This imperfection mostly came from the non-symmetrical form of particles and their different orientation during the measurement. The analysis is based on the scalar diffraction theory. Example of an image detected by sensor is in Fig. 5. Polarization effect and conductivity of material was not considered.

When using the scattering method the lowest scattered intensity of light of 280 fW was systematically detected. This corresponds to a particle size of 0.5 microns. The event of such a detection appears on average 40-50 times per minute, depending on the general conditions of operation, while nearly 90 percent of signals from the first photodiode is invalidated because of no correlation with the second photodiode. These results were achieved by using as a detector a Si photodiode with an extremely high gain of around 1012 V/A. A photodiode is less sensitive compared to a photomultiplier, but the signal is less noisy. With this detection technique and multiple passes of light through a modulator, we reduce the signal-to-noise ratio (SNR). This can lead in the future not only to the detection of the presence of a particle, but also to the discovery of the accuracy of its size. This is an important milestone in developing such an instrument.

Figure 5. Diffraction spots of polution particles

TABLE I.

Sample Diffraction method results

Particle Sample Origin CCD

sensor [μm]

Microscope [μm]

1 Dust sediment collection 23 27

2 Metalic powder from polishing 50 51

3 Dust sediment 10 11

4 Wood powder 80 88

5 Optical polishing powder 8 10

IV. CONCLUSION In this paper the concept of a pollution monitoring system

was presented with the preliminary tests. This set-up has been successfully tested. The detection of particles proves to be possible even in the particular conditions of a MF. However, some problems appear during the detection, mostly related to the instability of the laser intensity and fluctuation of the modulation frequency. This may be overcome by using a more stable laser source, such as a laser diode, and replacing the mechanical chopper by an electro-optic modulator with even higher modulation frequency and stability. Such an attempt will be investigated with further tests in the real conditions of a MF during and after the fabrication process, in order to prove the reliability of this technique.

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