IMPROVEMENT OF DEVICES FOR AEROSOL PARTICLE MONITORING IN CLEAN ROOMS

V. I. Kalechits and O.Yu. Maslakov

 Laboratory of Optical Aerosol Devices,

Russian Research Center "Kurchatov Institute", Moscow, Russia

INTRODUCTION

Any high technology manufacture, as well as any technology using clean rooms or high efficiency filtration, require devices for the control of airborn particles concentration. The problem of development of aerosol particle monitoring system for clean rooms arised in Russia in the beginning of 90th simultaneously with the state program for microelectronics progress. Some models of optical (including laser) aerosol particle counters and spectrometers were designed in the former USSR before that moment /ref.1/, but the computer control systems were absent.

The first experimental specimen of control and monitoring system named LADA was elaborated in Russian Research Center "Kurchatov Institute" on the basis of experience in the field of aerosol control in the atomic energy industry /ref.2,3/.

AEROSOL CONTROL AND MONITORING SYSTEM LADA

Control and monitoring system LADA provides the continuous and periodical control of airborn particle concentration in the clean rooms, boxes and technological clean gases - under the normal condition or high pressure.

Figure 1

LADA System includes (see figure 1) the personal computer (IBM PC), connected to the multiplexers (up to 64 pieces). Each of them can be connected to 16 laser aerosol sensors LAD and (or) separate temperature, humidity and pressure (pressure difference) sensors.

The multiplexer is intended for the collection, processing and transfer to PC of the information from sensors. It uses microprocessor, ensuring simultaneous reception of information (counting pulses) from 16 sensors, computing of concentration, storage of information up to the moment of its transfer to PC. If the concentration measured in any channel will exceed beforehand given amount, the controller transmits the information to computer display outside of the rules installed by program polling.

LASER DIODE AEROSOL SENSOR

The compact aerosol sensor LAD-1 (see fig.2) with the solid state laser diode as light source was designed specially for LADA system. The sensor operation is based on analysing and counting of the light impulses scattered by the individual particles when passing through the laser beam. The light impulses are converting by photoreceiver to electrical ones with the amplitudes proportional to the geometric dimension of particles. The sensor provides the registration of the total number of impulses with amplitudes exceeding the definite level (i.e. the amount of particles with diameters up to the known value).

The optical scheme of a sensor provides the collection of forward-scattered laser light and its registration by photoreceiver.

The optical circuit contains also reference photodiode for stabilisation of laser diode power.

Figure 2

The signal from the photoreceiver passes to an amplifier, further consistently to discriminator, generator     of TTL-pulses and then to multiplexer. The discriminator has four amplitude thresholds, which are set before measurement.   LAD-1 sensor has the special electronic unit which cuts off the  pulses with the length more than

   3pDr²  / 2Q ,

where D is the laser beam diameter, r is the radius of aerosol stream, Q is the volume flow rate.

When the pulses with a duration, exceeding specified size, come to the input of a discriminator, an indicator "limit" flashes on a panel of sensor. Thus the coincidences ( i.e. simultaneous crossing of measuring volume by several particles ) because of high aerosol concentration or sensor inner pollution are indicated.

The sensitivity of LAD-1 sensor is up to 0.3 microns, the flow rate - up to 3 litres/min., dimensions 255 * 120 * 90 mm.

OPEN CAVITY LASER AEROSOL SENSOR

The laser aerosol sensor LAD-2 (fig. 3) was designed specially for high class clean room, where the small concentration of particles stipulates the necessity of high sample volume. The flow rate of LAD-2 sensor is up to 15 litres/min.

The optical unit of LAD-2 is based on the 2 mW He-Ne laser with a low voltage power supply. Aerosol particles are injected to the open laser cavity through the split aerosol jet orientated along the laser beam.

Figure 3

Unlike LAD-1 sensor the optical scheme of LAD-2 collects the light scattered to the right angular direction. Other LAD-2 features (reference photodiode for stabilisation of laser power, four amplitude thresholds, electronic selection of "long" pulses, indication of high aerosol concentration and sensor inner pollution) are the same as for LAD-1.

The sensitivity of LAD-2 varies from 0.2 to 0.4 microns in dependence of given flow rate, dimensions of LAD-2 are 520 * 170 *100 mm.

Either LAD-1 or LAD-2 sensors are operating with the external vacuum pump (one or more for the hole LADA system).

Also LADA System can include environmental quartz resonant sensors for temperature, pressure and pressure difference measurement. These sensors can be connected to any of multiplexer input.

SIMPLE SENSORS USAGE

Any described sensor (LAD-1, LAD-2 or environmental sensors) can be used without multiplexer. Special SIN (single interface) processor was designed to provide one channel of data processing capability. This board can be plugged into a card slot of any IBM compatible computer.

The software of LADA System is comfortable for a work and an information reading and is count upon the personnel with a minimal qualification. Constant menu, auxiliary windows, HELP screens are used in LADA Software.

HIGH PRESSURE APPLICATION

LADA System can be supplied by the reducer of special design - an apparatus for connection of any LAD type sensors to the high pressure volumes or pipelines (from 1.2 to 8.0 bar).

Operation principle of reducer bases on the calculations of gas flow through the calibrated pinhole.

The reducer has three coupling for connection to LAD sensor inlet, to high-pressure volume and for dumping of a redundant flow, as a rule, through the aerosol filter.

METROLOGICAL SUPPLY OF SYSTEM

Also LADA System contains a set of instruments for the metrological supply of a system (i.e. for sensors calibration and periodic check-up), including the generator of standard latex particles, set of the latex suspensions with different particle's radii and auxiliary instruments.

The dispersity of a generated aerosol is determined by the dispersity of initial suspension and does not exceed 0.05 for particles sizes less than 1.0 micron and 0.10 for particles sizes more than 1.0 micron. The flow rate of produced latex aerosol varies from 0.5 up to 3.0 litres/mines.

The complex of metrological equipment includes also methodical documentation which allows to user to have their own regularly practice of laser sensors calibration.

LADA SYSTEM IMPROVEMENT

During the work we have carried out the careful study of customer's requirements and the possibilities of the further system improvement. In 1993 the group of LADA system authors offered some new ways for the development of a new control and monitoring system (named LADA-M) on the base of the advanced laser aerosol sensors (LAD-10 and LAD-20) with the following extended possibilities:

a/ While developing of the system we have come to the conclusion that the present situation with the microprocessor technique allows to refuse a multiplexer and connect all sensors (of advanced types) directly to the computer.

The advanced particle sensor (either LAD-10 or LAD-20) includes an optic system for measuring of aerosol particles as well an internal multichannel microprocessor block for data collecting and processing. Small sensors of temperature, humidity and pressure can be additionally mount inside LAD case and connect to such an electronic block.

Nevertheless the sensitivity and flow-rate parameters are the same as in LAD-1 and 2, the over-all dimensions of a sensor are not essentially changing and parameters of information exchanging with IBM PC are the same that in the previous type of LADA monitoring system which uses a multiplexer.

b/ For the enlargement of possibilities of the LADA-M monitoring system the block of multichannel analysis (MCA) of pulse amplitude was developed. This block is a part of new microprocessor unit of LAD-10 or 20 sensors. MCA block can be produced in two variants. The first one operates in counting mode, and aerosol sensor works as a simple particle counter. The second variant of block operation is the MCA mode. In case of the connection of such aerosol sensor to the input of IBM PC a simple system of particle counting is turned to the aerosol laser spectrometer.

The integration of these (seeming so small) changes has resulted to absolutely new principle of LAD sensors construction. Pursuant to these "modular" principle any type of LAD sensor includes a certain set of "necessary" units or blocks (and each of them can be produced in several interchangeable modification). Sensor will not operate without these units. Some "additional" units can be added to sensors if it follows from the technical requirements or customer's demand. These blocks are not obligatory for sensor function.

MODIFIED AEROSOL CONTROL AND MONITORING SYSTEM LADA-M

The modified control and monitoring system LADA-M (see figure 4) includes the personal computer connected directly to LAD sensors (up to 64 pieces per 1 port). Each of LAD sensor has two obligatory blocks (optical and microprocessor ones) and optional units: inner temperature, humidity and pressure (pressure difference) quartz resonant sensors, reducer and some other additional blocks.

OPTICAL UNITS

The first variant of optical unit (for LAD-10 sensor) includes the compact solid state laser diode as light source. Special laser module was developed for these new sensors. It uses a solid state laser diode on the basis of quantumsized heterostructure AlGaAs (wavelength 0.82 mcm). The use of this type of laser gives the opportunity to lower an operating

Figure 4

current (less than 220 mA), to increase an efficiency (up to 15-20%) and to reach high output power (up to 100 mW).

For decrease of heat loads the laser diode is enclosed at TO-3 package including Peltier microcooler and thermosensor, connected to temperature control electronic circuit to ensure stability of laser characteristics. The feedback photodiode is also placed inside TO-3 package. The divergence or laser beam is decreased (about 10 times) by cylindrical microlens, placed directly at the radiating area of laser diode junction. The laser module is equipped with heat sink, as well as collimating lens focusing the laser radiation in measuring volume. The sensitivity of LAD-10 optical unit is the same as in LAD-10.

In the second variant of a optical unit ( for LAD-20 sensor) bases on 2 mW He-Ne laser, where aerosol particles are injected to the open laser cavity. All characteristics and features of LAD-20 optical unit are the same as for LAD-2.

The design of aerosol chamber both in LAD-10 and LAD-20 sensors foresees the possibility of use of three variants of aerosol jet unit. The first one is count on the flow rate from 0.1 to 1.0 l/min. In this variant the aerosol stream is blowing by the coaxial stream of clean air (air shield). The second construction is intended for measurement with the flow rates from 1 to 15 l/min but without clean air blowing (usual for measurements in clean rooms). The third variant of aerosol injection unit ensures the flow rate to 28 l/min; in this case the aerosol jet is formed by the split orientated along the laser beam.

MICROPROCESSOR UNITS

The microprocessor unit can be produced in two modifications too. Both variants can be used either in LAD-10 or LAD-20 sensors. The main part of microprocessor unit is the block of multichannel analysis (MCA) of a pulse amplitude (see above).

The first variant of MCA block operates in counting mode, and LAD device works as a simple particle counter provides the registration of the total number of impulses with amplitudes exceeding the definite level - as a rule, coinciding with sensor sensitivity).

The second variant of block operates in MCA mode. It includes a multichannel peak analyser of micro- and millisecond pulses, that permits to measure not only concentration, but also function of particle size distribution. Thus the use of this microprocessor unit turns a simple system of particle counting to the aerosol laser spectrometer with appropriate increase of information, accuracy and resolution.

Both modification of a microprocessor unit are supplied by a controller for primary data processing and its transfer directly to computer (without a multiplexer). Power supply block of microprocessor unit can be manufactured in two variants too: for CW operation mode or pulsed one. The use of pulses power supply mode for laser diode provides growth of laser output power, increase of laser lifetime and improvement of the heat characteristics.

The microprocessor chip used in both variants of units can realise parallel processing of the information in eight channels. It creates the opportunity of connection to any microprocessor unit of “additional” blocks - quartz resonant sensors of temperature, humidity and pressure ( difference of pressure ) mentioned above.

LAD-10 AND LAD-20 VARIANTS

Thus the both base models of laser aerosol sensors (LAD-10 and LAD-20) can be manufactured in the various modifications (more than 20 possible variants - without additional blocks) depending on the used microprocessor unit, aerosol jet unit and the mode of laser power supply. Such a variety of sensors' modifications is not a curious fact but, on the contrary, permits to achieve sharp specialisation of various variants of sensors.

For example, LAD-10C (laser diode sensor with counting mode) can be used as the main element of monitoring systems - but such a device with the second type of aerosol jet unit is the most expedient for the clean room control and with the first type of jet unit (with air shield) - for the atmosphere aerosol monitoring and air pollution control. LAD-10S (laser diode sensor with multichannel analysis) is necessary for the calibration and periodic check-up of LAD-10C for measurement of sizing accuracy and size resolution.

The use of laser aerosol spectrometer LAD-20S (laser open cavity sensor with MCA block) is expedient for applications where the particle size distribution is useful or necessary - the attestation of clean rooms, the testing of high efficiency filters, for scientific investigations. The choice of the aerosol jet unit depends from the aerosol concentration: LAD-20S device with the second or third types of aerosol jet unit is preferable for the measurements "after" tested filter and the same device with the first type of jet unit - "before" filter.

Just flexibility and variety of possible modifications of laser aerosol sensors are, in our opinion, main distinctive feature of the modified control and monitoring system LADA-M.

PERSPECTIVES OF DEVELOPMENT

At the same time the elaboration of principally new laser aerosol sensors is continued. Now the special sensor LAD-V for aerosol particle detection in vacuum pipelines and volumes is under development. It is distinguished by plane sensing area formed by laser beam and system of mirrors. In spite of difference in optical and electronic circuits, new sensor named LAD-V is compatible with a control and monitoring system LADA.

The development of  LADA compatible laser sensor for detection of particles in liquids is planning  in future.

An additional unit for the LAD type particle sensors with a simple display and keyboard have to be designed in future. After connection of this compact unit the particle sensor can be transformed to the separate device ready for usage without PC. One can notice that the connection of a portable pump can transform such a sensor to the fully autonomous device. These devices are now under development.

ACKNOWLEDGEMENT

The work was supported by RRC "Kurchatov Institute " grants NN 38-1994, 43-1995, 52-1996.

REFERENCES

1. Nakhutin I.E., Rubeszny Ju.G., Kalechits V.I., Sypailo I.P. Universal Laser Aerosol Spectrometer ULSA-1. Sov. Instruments and Experimental Techniques, 1988, N 6, 145-149 (in Russian).

2. Kalechits V.I., Kirsch A.A., Kulibaba V.I., Maslakov O.Yu., Pavlov Yu.V. Control and measurement system LADA for monitoring of clean rooms parameters. Technologiya Chistoty (Cleanness Technology), 1994, N 1, 19-24 (in Russian).

3. Kalechits V.I., Kirsch A.A., Kulibaba V.I., Maslakov O.Yu., Pavlov Yu.V. Aerosol control and monitoring system LADA. J. of Aerosol Sci., 1994, V.25, Suppl. 1, Abstract of the 1994 European Aerosol Conf., pp. S207 - S208.

ABSTRACT

Control and monitoring system for continuous and periodical detection of airborn particle concentration in the clean rooms, boxes and technological clean gases is described. The system includes the personal computer connected directly to LAD devices (up to 64 pieces per 1 port). Each of LAD has two obligatory blocks (optical and microprocessor units) and optional blocks: temperature, humidity and pressure (pressure difference) quartz resonant sensors. The first variant of optical unit (LAD-10) includes the compact solid state laser diode as light source. The second variant (LAD-20) bases on the 2 mW He-Ne laser, where aerosol particles are injected to the open laser cavity. The sensitivity of LAD-10 is up to 0.3 microns, the flow rate - up to 3 litres/min. The flow rate of LAD-20 is up to 15 litres/min., the sensitivity varies from 0.2 to 0.4 microns in dependence of given flow rate.

The microprocessor unit of LAD can be produced in two modifications too. The first one operates in counting mode, and LAD device works as a simple particle counter provides the registration of the total number of impulses with amplitudes exceeding the definite level. The second variant includes a board for multichannel analysis (MCA) of pulse amplitude. LAD device with MCA unit operates as laser aerosol spectrometer.

The diversity of sensor constructions is defined by the variety and the peculiarities of possible applications.