Performance Assessment of Probes Dedicated ... - Wiley Online Library

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Part. Part. Syst. Charact. 29 (2012) 156–166

Performance Assessment of Probes Dedicated to the Monitoring of Surface Particle Contamination Isabelle Tovena Pecault*, François Gensdarmes**, Guillaume Basso**, Francoise Sommer*** (Received: 13 February 2012; in revised form: 24 April 2012; accepted: 25 April 2012)

DOI: 10.1002/ppsc.201200005

Abstract Surface probes, based on airflow particle detachment coupled with optical particle counter, are actually available to measure the particle cleanliness of surfaces in cleanroom. No reliable data exits dealing with the performance assessment of these probes in unstuck and counting particles deposited on surfaces. This work presents a method for determining the efficiency of instruments dedicated to particle surface cleanliness measurements. The method is based on the realisation of standard particle deposits by sedimentation and analysis of whole of the contaminated surface by microscopy combined with a micrometric displacement bench. The method is used to assess a trial surface probe with 30 lm and 80 lm glass beads deposited on transparent (glass) or opaque (aluminium) surfaces at concentrations ran-

ging from 1 particle·cm–2 to 50 particles·cm–2. The results obtained show that the overall efficiency of the instrument tested is less than 5 %. The detailed analysis of results shows that this low value is mainly due to poor efficiency of sampling and detection of particles in the optical counter. When analysed in terms of particle detachment efficiency, the results agree qualitatively with a force balance analysis taking into account the friction by airflow and the distribution of adhesion forces of glass particles to glass substrates or rough aluminium substrates. Such result shows that airflow based surface probes for particle cleanliness measurements should be systematically qualified for representative conditions of operations whereas analytical microscopy measurements could be considered as reference.

Keywords: cleanroom, contamination, counting efficiency, optical particle counter, surface cleanliness measurement

1 Introduction The contamination control is crucial in more and more industries that are sensitive to particles such as microelectronics, optics and lasers [1, 2], pharmacy, engine, nanotechnologies. Efforts were made, at first, to control particulate contamination in the air or the in fluids involved in the industrial processes [3]. Surface particle contamination control is a more recent concern and

*

Dr. I. T. Pecault (corresponding author), CEA/DAM CESTA, DLP/SCAL/LPO, BP 2, 33114 Le Barp (France). E-mail: [email protected] ** F. Gensdarmes, G. Basso, IRSN, Aerosol Physics and Metrology Laboratory, BP 68, 91192 Gif-sur-Yvette cedex (France). *** F. Sommer, Biophy Research S.A., Actipôle St Charles, 131 Avenuede l’Etoile, 13710 Fuveau (France).

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gave rise last year to the ISO-FDIS 14644-9 standard [4] for the surface particle contamination classification. This standard suggests a classification of surface cleanliness according to the number of particles sized that are equal or larger than 0.05 lm and 500 lm per square metre of surface. This standard will supplement the IEST 1246D [5] which classification starts at 1 lm and ends at 1000 lm on 0.1 m2 surfaces. Level 100 according to IEST 1246D corresponds to a single particle larger than 100 lm, fewer than 11 particles larger than 50 lm, fewer than 78 particles larger than 25 lm. To comply with this standard, in the 1990s industrialists developed the first surface probes to monitor particulate contamination on surfaces. One notable example is the QIII, developed and patented by Dryden [6] in the USA. These instruments are based on the principle of optical aerosol counters with the addition, upstream, of an aeraulic probe that detaches the particles from the surface by airflow.

© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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The airflow is controlled by the sampling flowrate of the optical counter, which generally operates at 28.3 L/min. In the work presented here, we study the performance of this surface probe as a function of particle size (30 lm and 80 lm) for different types of surface (anodized aluminium, glass). This was done thanks to a specific bench test based on optical microscopy. The results are analysed on one hand in terms of particle resuspension efficiency and on the over hand in terms of sampling and detection efficiency by the optical particle counter. These results are then compared to the roughness measurements on glass and aluminium substrates and the adhesion forces between the glass particles and these substrates.

2 Material and Methods 2.1 Probe Efficiency Measurement Method 2.2.1 Description of the Surface Probe

Pump 28.3 L/min

Figure 1 shows a block diagram of the instrument prototype to be tested, which consists of a probe whose base is applied to the surface and an optical particle counter. As it can see on Figure 1, right side, the base of the probe consists of 4 blowing points and grooves and a central suction point. The particles are then brought through that point to the optical counter cell detection. The counter operates on the principle of measuring at 90° the light diffusion caused by a particle in the laser beam. To improve the particle sampling efficiency of the prototype tested, the detection cell has been incorporated in the probe as close as possible to the particle detachment area. The entire assembly is connected to a central unit containing the data acquisition system and the pump that controls the blower and suction flowrates on the surface. The counter has six channels to measure

Optical detection cell Suction point Blowing point

Base of the probe

4.5 cm

Fig. 1: Block diagram of the surface cleanliness measuring instrument.


cumulative number of particles size upper than 5 lm, 10 lm, 25 lm, 40 lm, 50 lm, and 100 lm respectively. To take a measurement, the base of the probe is applied to the surface, and then the sampling and acquisition are triggered simultaneously. The sampling time is adjustable from a few seconds to several minutes. The surface exposed to the 28.3 L/min flowrate is a disc of diameter 4.5 cm, i.e. a surface area of 15.9 cm2. One must notice that it is not obvious to determine the airflow velocity at the surface due to the particular shape with the four grooves. 2.1.2 Presentation of the Method for Efficiency Measurement The method used to determine the efficiency of the instrument is based on the realisation of a particle deposit by sedimentation and analysis of the particle number on the surface by an optical microscope combined with a micrometric displacement bench. For a given particle size, the overall efficiency of the instrument is defined as the ratio of number of particles detected by the laser optical particle counter to number of particles counted visually by microscopy. One must notice that the micrometric displacement bench allows to count the particles on the entire surface scanned by the base of the aeraulic probe. For the deposit to be realised, the surface sample is placed in a cylindrical vessel: 80 cm high and 30 cm diameter (see Figure 2), in which a small quantity of powder is dispersed during a short time (typically 1 s). Dispersion is performed by using a venturi-type system supplied with compressed air which is dried and HEPA H14 (High Efficiency Particulate Air [7]) filtered. After the dispersion, particles settle on the surface. The powder consists of 30 lm or 80 lm glass beads with certified diameters in reference to NIST. The amount of particles, pressure supply of the venturi and dispersion time are empirically fixed in order to obtain low particle surface concentrations. The deposits obtained by this method does not show particle aggregates or disparities on the surface as it can be seen on the mapping represented Figure 5. Two types of surface are used in the experiments: transparent glass surfaces (discs of 10 cm diameter, 3 mm thick) and opaque metallic surfaces made of smooth anodised aluminium (10 cm squares, 4 mm thick). To avoid particulate pollution of the sample, the experiments are performed in an ISO 8 dust-controlled cleanroom. The relative humidity of the air in the cleanroom is between 32 % and 38 % and the temperature is between 20 °C and 22 °C.


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Part. Part. Syst. Charact. 29 (2012) 156–166 Powder feeder

Venturi disperser

Sedimentation chamber

80 cm

Dry and filtered compressed air

HEPA filter Surface sample Ø 10 cm

The surface analysed by microscopy must correspond exactly to the surface diagnosed by the aeraulic probe (45-mm diameter disc). To ensure this, the area of the surface to be analysed is marked out using two fixed pins and a tag (see Figure 4). The two pins allow the base of the sampling probe to be positioned in a repeatable manner. When the surface is placed on the displacement table, a first disc is defined manually by the operator, who uses the table control software to perform three sightings on the pins and the tag. The software then automatically defines the surface to be analysed as a concentric disc of diameter specified by the operator, taken to be equal to the internal diameter of the probe. This ensures that the diameter of the surface analysed matches the surface examined by the aeraulic probe.

Exhaust 30 cm

Pins to mark out the position of the aeraulic probe

Fig. 2: Diagram of the sedimentation deposit apparatus

The number of particles deposited on a specific area of the surface sample is performed by a count from photographs taken using a long focal length optical microscope combined with a two axis XY displacement bench. This system allows to define an area to be analysed on the surface and automatically photograph the entire selected zone. Figure 3 shows various elements of the counting bench. The microscope is used at a focal distance equal to 15 cm, which means that reference measurements can be taken whilst remaining far from the surface. The measurement probe to be tested can therefore be positioned on the bench without displacing or touching the surface.

Long focal length optical microscope with CCD camera

Surface to be analysed Light source

3rd tag to mark out the area for microscope counting

Area analysed Circle marking out the area to be analysed

Fig. 4: Diagram showing identification of the area to be analysed and positioning of the probe.

Taking into account the magnification factor selected on the microscope, complete analysis of a 45-mm disc requires 768 photos to be taken. Figure 5 shows a sample mapping of a deposit of 80 lm particles on a tested surface, each point represents the position of a particle. To carry out the count, a sighting of the particles is performed on each photograph. Figure 6 shows examples of sightings performed on 30 lm and 80 lm particles deposited on glass and aluminium surfaces. One must notice that for each experiment the number of particles on the reference area is determined by counting manually the particles on each of the 768 photographs taken. This protocol allows reliable counting on a rough opaque surface with direct lightening. 2.1.3 Experimental Procedure

XY automatic displacement table

Fig. 3: Photographs of the counting bench.


The experiments are carried out to study the effect of particle diameter and the chemical nature of the surface on the response of the instrument. For this purpose, four types of experiment are performed using 30 lm and


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Part. Part. Syst. Charact. 29 (2012) 156–166 ●

●

●

●

● ●

●

●

Fig. 5: Mapping of a deposit of 80 lm particles on an aluminium surface.

Positioning the surface on the microscopy counting bench. Adjusting the microscope optics and the light by focusing on a particle. Defining the positioning circle and the surface area to be analysed. First acquisition of photographs on the defined surface area (768 photographs). Positioning the aeraulic probe on the surface. Carry out three consecutive 20 s measurements without moving the probe and remove the probe. Second acquisition of 768 photographs on the predefined surface area. Manual counting of particles by sighting on the two series of 768 photographs.

2.2 Measurement of Substrate Roughness by Topographical Analysis using Atomic Force Microscope in Tapping Mode

The atomic force microscope (AFM) used to measure the topography of the surfaces is a multimode Nanoscope V manufactured by VEECO, operating in tapping mode. In tapping mode, the cantilever oscillates and a sufficiently high oscillation amplitude is selected for the tip to pass through the contamination layer that is usually present on the surfaces analysed. The tip comes into contact with the surface only periodically. An analysis in tapping mode therefore allows a topographical surface study without disturbance of the sample by eliminating frictional forces in particular. In tapping mode, the cantilever used has Glass surface Aluminium surface a stiffness constant of approximately 50 N/m and an oscillation frequency in the region of 350 kHz. The piezoelectric ceramic element with the following maximum sweep characteristics was used: x = y = 12 lm, z = 3.9 lm. The image in height mode represents 30 µm 30 µm the actual topography of the surface. The roughness parameters are measured Glass surface Aluminium surface and the profiles are traced on this image. The analysis is performed at constant amplitude, which means that the variation of the oscillation amplitude of the tip is used as a control signal in order to correct the z-displacement to keep the amplitude constant and follow the topo80 µm 80 µm graphy of the surface. The image in amplitude mode represents the variation of the square root of the Fig. 6: Example of sighting of 30 lm and 80 m particles. 80 lm glass beads deposited by sedimentation on glass and anodized aluminium surfaces (mean roughness less than 1.6 lm). The experimental procedure is described below: ● Cleaning the surface using alcohol, a special tissue for use in cleanrooms and compressed air. ● Carrying out three blank measurements under a laminar-flow hood using the aeraulic probe. ● Carrying out particle deposition in the sedimentation chamber.



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2.3 Measurement of Adhesion Force by Atomic Force Microscopy

and then retracts, approaching and separating the sample and the probe tip. The force applied to measure the adhesion force is very low; we set a trigger of 4 nm. A force curve obtained by AFM has the typical shape represented on Figure 8. Cantilever deflection

amplitude. The z-scale is therefore difficult to interpret, but this image is included because it has the advantage that it generally provides a better view of the details. Phase contrast mode is used in tapping mode. The phase shift between the oscillation curve of the cantilever offload and when it is traversed by the sample is measured. This phase shift recording gives a contrast which is characteristic of the adhesion and/or elasticity according to the nature of the samples.

3

1 2

Over the past few decades, many techniques have been developed to characterize particle-surface adhesion: electric field detachment, centrifugal detachment or Atomic Force Microscopy (AFM) [8]. AFM can accurately and precisely measure the adhesion of single particles on a surface [9, 10, 11]. To measure the adhesion force between the glass beads and the glass or aluminium substrates, we stick a glass bead to the AFM cantilever. Cantilevers with stiffness constants in the region of 0.58 N/m are used as a mount for the particle. A microdroplet of cyanoacrylate glue is deposited on the tip of the lever arm using a capillary. A glass bead is then deposited on the microdroplet of glue using a fine needle. It required several attempts to secure the 30 lm particle as seen in Figure 7. It was not, however, possible to reliably bond an 80 lm diameter glass bead to the end of the cantilever. The overall dimensions and the greater weight (by a factor of 20) of an 80 lm glass bead compared to the 30 lm one make reliable quantitative adhesion measurements impossible using AFM. A force curve is obtained in contact mode by applying a saw tooth voltage (in z axis) to the piezoelectric ceramic element, which extends to contact between the tip and the surface or to a value set by the operator (trigger), µm 100

75

50

4

5

Tip – Surface distance

Deflection dz corresponding to pull-off force F

Fig. 8: Force curve obtained in contact mode.

The deflection is related to the applied force by Hooke’s law: F = k dz, where k is the stiffness constant of the cantilever equal to 0.58 N/m for the one used. The various parts of the curve represented in Figure 8 are: 1. The cantilever is far from the surface; the deflection is zero. The tip approaches without entering into contact with the surface: the cantilever senses the long-distance attractive interactions such as Van der Waals forces and electrostatic forces. The deflection is negative and decreases. 2. When the tip is very close to the surface, the attractive force is sufficient to bring the tip into abrupt contact with the sample. 3. As the tip approaches when in contact with the surface, the deflection of the cantilever increases. 4. The movement is inverted. As the tip moves away from the surface, the interaction forces make the tip adhere until a greater distance than the point where the contact occurred on the outward trip. The deflection becomes negative again until the adhesion is broken (5) and the cantilever is released from the surface.

3 Experimental Results

25

3.1 Characterisation of the Standard Particles used in the Experiments 0

25

50

75

100

µm

Fig. 7: 30 lm diameter glass bead bonded to the tip of the AFM cantilever.


The particles used to perform the experiments are calibrated glass beads whose average diameter is certified with reference to NIST methods. We nevertheless per-


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formed our own measurements of the particle diameters in order to qualify the purity of the samples on the size spectra that the optical particle counter detects. The size distributions (numbers) are measured using a Coulter Multisizer II. The Coulter determine the volume-equivalent diameter of the particles dispersed in an electrolytic solution. The principle is based on measuring the electrical resistance at the terminals of a circular orifice of about a hundred micrometres through which the solution is sampled (suspension of particles in the electrolyte). When a particle passes through the orifice, it displaces a quantity of electrolyte equal to its volume, which produces a resistance change proportional to the volume of the particle. Figure 9 represents the size distributions measured for both glass bead powders used in the tests. The measurements taken using the Coulter device reveal the presence of a significant quantity of particles finer than the diameter specified by the manufacturer for both powders. These fine particles could be generated over time during storage and transport, by abrasion of the calibrated particles with each other.

In order to analyse the size distributions, taking parasitic particles into account, we consider the 25 lm to 60 lm domain only for the 30 lm powder, and the 50 lm to 130 lm domain for the 80 lm powder. The count median diameters obtained are represented in Table 1. We find very good agreement between the median diameters and geometric standard deviations determined by the Coulter method and those specified by the manufacturer. These size ranges can therefore be used to define the measurement channels of the optical counter to be considered for the analysis of results.

3.2 Measurement of Overall Efficiency of the Surface Probe The overall efficiency of the instrument Eg (%) is defined by the following relationship: Eg

N 100; N1ref

(1)

Table 1: Comparison of the median diameters measured using the Coulter method to manufacturer specifications. Coulter data 5 lm < dp < 60 lm

Manufacturer data

Powder

Median diameter D50 (lm)

Coefficient of variation CV (%)

Geometric standard deviation rg

D50 (lm)

rg

30 lm standard glass beads

30.1 ± 2.1

7.6

1.08

30.7

1.08

80 lm standard glass beads

79.1 ± 4.0

3.5

1.03

Coulter data 50 lm < dp < 130 lm

D50 (lm)

rg

81.5

1.03

100 90

Cumulative number fraction (%)

80 70 60 50 40 30 µm glass beads

30 20

80 µm glass beads

10 0

1

10 Volume-equivalent diameter (µm)


100

Fig. 9: Number size distributions of 30 lm and 80 lm glass bead powders (Coulter measurements).


162


where N1ref corresponds to the number of particles counted manually on the 768 photographs taken by microscopy before the measurements with the aeraulic probe, and N represents for each acquisition the particle count on a measurement channel of the optical counter. The standard uncertainty rEg on Eg is expressed as: 2 Eg

r

100 N1ref

2

r 2 N

100 N N1ref 2

2 r2N1ref :

(2)

In our application, we assume that the uncertainty on the reference particle counting rN1ref is negligible compared to the uncertainty rN on the particle counting performed by the optical counter. The standard uncertainty rN on the counting of N particles by the optical particle p counter is given by Poisson’s law and is equal to N. Consequently the expanded uncertainty 2 × rEg (corresponding to a 95 % confidence interval) on the overall efficiency of the instrument is expressed by: p 200 N : (3) 2 rEg N1ref In each experiment, three consecutive 20-s measurements are performed without moving the probe on the surface. Three or four repetitions of each type of experiment are performed. For the tests performed with the 30 lm monodispersed glass beads, we use the cumulative counting of particles larger than 25 lm (N for dp > 25 lm) to calculate the ef-

ficiency of the instrument. For the tests carried out with 80 lm glass beads, we use the cumulative counting of particles larger than 50 lm (N for dp > 50 lm). As a general rule, the results obtained show that the overall efficiencies calculated on 1 or 2 successive acquisitions are not significantly different, considering the associated uncertainties. Consequently, we consider only the particle counting on the first 20-s measurements to perform the calculations and represent the results graphically. Figures 10 and 11 represent the overall efficiencies Eg obtained for 30 lm and 80 lm particles respectively on both types of surface. Data are represented with a 95 % confidence interval. The number of particles obtained for the reference counts (N1ref) is also mentioned. Note that the experiments are carried out with numbers of deposited particles from 20 to 856. The counts are performed on a 45-mm diameter disc, i.e. a surface of 15.9 cm2, consequently the corresponding surface concentrations are between 1 particle per cm2 and 54 particles per cm2. Results for 30 % lm particles shows large disparities for the glass surface and also for the aluminium surface, the overall detection efficiency of the aeraulic probe is ranging from 0 % to 22 %. As the disparities between the results are not explained by the data uncertainties, one could conclude that the measurements done by the aeraulic probe are not repeatable for the 30 lm glass bead particles. Some experiments show 0 % efficiency values which mean that no particle were detected by the optical particle counter during measurements.

Overall efficiency of the instrument Eg (%)

25,0

20,0

Exp. Exp. Exp. Exp. Exp. Exp. Exp.

1: N1ref 2: N1ref 3: N1ref 4: N1ref 5: N1ref 6: N1ref 7: N1ref

= = = = = = =

56 110 337 856 113 33 33

15,0

10,0

5,0

0,0 1

2

3

4

5

6

7

Experiment No.

Glass surface

Anodised aluminium surface

Fig. 10: Overall efficiency of the probe for deposits of 30 lm particles on glass and anodised aluminium surfaces.



163


Overall efficiency of the instrument Eg (%)

15,0

Exp. Exp. Exp. Exp. Exp. Exp. Exp.

10,0

1: N1ref 2: N1ref 3: N1ref 4: N1ref 5: N1ref 6: N1ref 7: N1ref

= = = = = = =

79 70 20 67 63 38 43

5,0

0,0 1

2

Glass surface

3

4

Experiment No.

5

6

7

Anodised aluminium surface

Fig. 11: Overall efficiency of the probe for deposits of 80 lm particles on glass and anodised aluminium surfaces.

Results obtained for the 80 lm particle shows good repetabilities the overall detection efficiencies are between 1.5 % and 5 %, no significant discrepancy is observed for the two kind of surfaces. One must notice that expended relative uncertainties represented Figure 11 are ranging from 100 % to 200 %. These high values come from the low number of particles measured by the instrument; this is due on one hand to the low overall efficiency of the probe, and on the other hand to the low surface concentrations of the deposits. Remind that low surface concentrations are needed in order to be representative of clean room surface contaminations and to avoid interactions between particles by saltation during airflow resuspension.

3.3 Measurement of Roughness of Glass and Aluminium Substrates After cleaning operations the roughness parameters are measured for glass and aluminium surfaces, mean value on three measurements are represented in Table 2. Rq is the standard deviation of the measurements of height of the surface, Ra is the mean roughness corresponding to the distance between planes defined by mean and median heigh values. Rdiff is the percentage of surface increase between the developed surface and the swept surface. The developed surface is calculated by triangulation. The principle of triangulation involves breaking the surface down into its smallest elements, i.e. triangles,


Table 2: Roughness of glass and aluminium surfaces, 5 lm × 5 lm area. Glass

Rq (nm)

Ra (nm)

Rdiff (%)

Glass surface Mean ± 1 r

0.81 ± 0.22

0.36 ± 0.04

0.14 ± 0.09

Aluminium surface Mean ± 1 r

19.74 ± 1.62

15.99 ± 1.80

3.12 ± 1.79

and then summing all the surface area elements defined in this way. The mean roughness and the developed area are 50 to 60 times lower for glass than for anodised aluminium surface, as shown in Figure 12.

3.4 Measurement of Adhesion Forces between the 30 lm Glass Bead and the Glass and Aluminium Substrates The same cantilever equipped with the same 30 lm particle was used to measure the glass/glass and glass/metal adhesion forces in air, so that the values measured on both substrates are directly comparable. Here we present only the statistical analysis of the results of the measurement of adhesion forces between the 30 lm glass bead and the glass substrate or the aluminium substrate (see Figure 13). For each case 50 force measurements were taken, the mean value for 30 lm particle on the glass surface was F = 147 ± 11 nN and the mean value for


164


µm

µm 8

8 6

6 4

4 2

2

Fig. 12: 3D images (10 lm × 10 lm area) obtained on glass surface (left) and anodised aluminium surfaces (right).

where N2ref is the number of particles counted manually on the 768 photographs taken after the measurement with the surface probe. It represent the fraction of particles detatched from the surface by the airflow generated during the measurement with the probe. The transmission and detection efficiency Etd (%) is defined by the following relationship: Etd

N N1ref

N2ref

100 :

(5)

30 lm on anodized aluminium was F = 120 ± 64 nN. As principally a consequence of surface roughness, the adhesion of glass bead on the aluminium surface show a lower mean value and a force spectrum broadenned thank to the one obtained for the smooth glass surface.

It represent the probability of sampling and detection of a particle in the instrument after its detatchment from the surface. Finally, the overall efficiency of the instrument define by Eq. 1 could be expressed simply as a function of Ed and Etd by:

4 Interpretation of Results

Eg Ed Etd :

(6)

The overall efficiency of the instrument can be expressed in terms of the efficiencies of the various elementary mechanisms acting on the probe response; such as particle detachment, transmission to the measurement cell, and detection. Thus, the particle detachment efficiency Ed (%) is defined by the following relationship:

Number of measurements

Figure 14 represents the mean values of detachment efficiency calculated by Eq. 4. These data are represented with uncertainties corresponding to the standard deviation calculated over the three or four repetitions of each type of experiment. Remind that uncertainties on the particles counting of photographs are neglected because they are done manually by sighting. On average, the detachment efficiency of glass beads is N1ref N2ref higher for the anodised aluminium surface than the glass 100 ; (4) Ed surface. This result can be explained by higher adhesion N1ref forces between the glass beads and the glass surface than between the glass beads and the 40 aluminium surface. This may be due to the inGlass surface fluence of roughness on the Van der Wall 35 forces. The mean roughness Ra of glass is Aluminium surface 30 0.36 ± 0.04 nm, compared to 15.99 ± 1.80 nm for the aluminium surface. Note that the principle 25 of detachment of particles by the probe is an airflow blowing on the particles and then an as20 piration to carry the particles into the cell of 15 the optical counter. On a rough substrate, the particles are deposited on peaks of the relief 10 and are in unstable equilibriums. Conse5 quently, the detachment of particles will be more efficient than in the case of a less rough 0 substrate with a larger surface area of interac0 50 100 150 200 250 300 350 tion between the base of the particle and the Adhesion force (nN) substrate. The micro wrinkles enable the particles to be farther away from the substrate, Fig. 13: Distribution of adhesion forces for 30 lm glass bead on glass surface or rought aluminium. reducing the Van der Waals interactions that



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controlled particles deposited on glass and anodised aluminium surfaces which 90 Glass surface are very common in cleanroom. The 80 method used to determine probe effiAnodised aluminium surface 70 ciency is based on the making of particle deposits by sedimentation and manual 60 counting of particles by using a micro50 scope combined with a micrometric dis40 placement bench. This systems allows to analyse the same area as the one mea30 sured by the probe under test. 20 The prototype of aeraulic probe tested 10 in this study reveals a low overall efficiency for 30 lm and 80 lm glass beads 0 20 30 40 50 60 70 80 90 100 deposited on glass and aluminium surfaces, the values are mostly lower than Particle diameter (µm) 5 %. The detailed analysis of results Fig. 14: Comparison of the mean detachment efficiencies obtained for the various shows that these low values are mainly surfaces as a function of particle diameter. due to a poor efficiency of transmission and detection of particles in the optical are at short range. This has already been demonstrated counter detector. When analysed in terms of detachfor different particles shapes (rough and smooth one) ment efficiency, the results agree qualitatively with theoonto flat surface [11]. To check that particle adhesion is retical values concerning the suspension of particles by reduce according to an increase of surface roughness, we airflow [12] and shows dependancy according to particle measured the adhesion forces between the 30 lm glass diameter and surface roughness. High roughness reparticle and the glass and aluminium substrates. The duces the contact surface area between the particle and adhesion forces measured between glass and glass are the substrate, and at least enhance the particle detatchhigher on average (147 nN) than between glass and me- ment efficiency of the probe. tal (120 nN), moreover the standard deviation of the As the ISO 14644-9 standard has been published, it measurements is six times higher on the glass/metal seems necessary to asess systematically in realistic conmeasurements, and the distributions are not identical dition of operation, the performance of probes based on for the two populations. The two populations are there- particles detachment by airflow in order to provide relifore difficult to compare, with very different roughness able surface contamination classification. values for glass and aluminium. The morphology of metal (with mean roughness 50 times higher than that of glass) leads to highly variable contact surface areas from 6 References one measurement point to another, which gives rise to a very high standard deviation for the glass/metal mea- [1] I. A. Fersman, L. D. Khazov, The effect of surface cleanliness of optical elements on their radiation resistance. surements (coefficient of variation 53 %). Translation of Soviet Journal of Optical Technology 1971, Moreover, for both types of surface, we note that the de37, 627. tachment efficiencies are higher for the 80 lm particles than the 30 lm particles. This result is explained by the [2] S. Palmier, I. Tovena, R. Courchinoux, J. L. Rullier, B. Bertussi, J. Y. Natoli, L. Servant, D. Talaga, Laser dafact that as the particle diameter increases, the aeraulic maged optics induced by chromium particle contaminaforce caused by the airflow friction increases more ration. Annual Symposium on optical materials for high pidly than the adhesion force. We generally consider power lasers: Boulder damage XXXVI. Colorado-USA – that the adhesion forces are proportional to the diaSeptember 2004. meter dp of the particles and that the aeraulic forces are n [3] ISO 14644-1, Cleanrooms and associated controlled enproportional to dp with 2 < n < 4 [12]. Detachment efficiency Ed (%)

100

5 Conclusion For the first time, an aeraulic surface probe used for particle cleanliness measurement have been assessed with


[4]

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[9]


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