Appl. Phys. B 67, 185–191 (1998) Applied Physics BLasersand Optics Springer-Verlag 1998

Influence of cathode roughness on discharge homogeneityof a high-pulse repetition frequency long-pulseXeCl laserO. Uteza, Ph. Delaporte, B. Fontaine, B. Forestier, M. Sentis, I. Tassy

Institut de Recherche sur les Phenomènes Hors Equilibre, U.M.R. 6594 C.N.R.S., Aix-Marseille Universities I & II, Case 918, 163, Av. de Luminy,13288 Marseille Cedex 9, France(Fax: +33-491/829-289, E-mail: olivier.uteza@irphe-lp3.univ-mrs.fr)

Received: 17 October 1997/Revised version: 3 March 1998

Abstract. This paper investigates the dependence of dis-charge uniformity of a high pulse repetition frequency (PRF)long-pulseXeCl laser on cathode roughness in single-shotregime and in burst mode (10th shot) as a function of PRF( f ≤ 500 Hz). The cathode roughness governing local distur-bances of the electric field acts on the stability of dischargeprocesses in excimer lasers. Space and time discharge sta-bility evolution is experimentally analysed from dischargephotographs obtained with a CCD video camera and pres-sure perturbation measurements achieved with a piezoelectricpressure probe set close to the discharge volume. The resultsclearly show that the cathode roughness plays an importantrole in discharge stability. The spatial and dynamic develop-ment of microarcs in the discharge is discussed in relation tothe cathode roughness. Finally we show that certain values ofcathode roughness may efficiently prevent an arc from form-ing during the laser emission. These cathode surface states arethen suitable to achieve high and reproducible laser perfor-mances at high PRF.

PACS: 42.55.Lt; 52.80.-s

Elaboration of reliable high-average-power excimer devicesis of great interest for many industrial applications rangingfrom microelectronics to medicine and materials processing.But the increase of the excimer laser average power throughPRF increase is often limited by the development of nonuni-form energy deposition processes which have deleterious ef-fects on the laser output energy [1, 2] and laser beam qual-ity [3, 4].

Previous detailed studies [5, 6] showed the importance ofthe processes taking place at the cathode on the formation ofinstabilities of excimer laser electric discharges. In particular,it is underlined that filamentation is mainly initiated by localdisturbances in the cathode emissivity. So, cathode properties,such as its surface state or roughness [7, 8], its material [9–11], its profile [12–14], and parallelism [8], are keystonesto produce uniform and stable discharges. Imperfections of

cathode profile and parallelism induce large distortions inthe electric field distribution resulting in large spatial-scalenonuniformities in the electronic distribution and, afterwards,to volumetric arcing [8, 15]. However, these problems can beovercome by carefully designing and positioning the elec-trodes in the laser cavity. The cathode material controls partsof its heating and of its electronic emission. The choice ofthe cathode material is then of fundamental importance forthe discharge stability resulting in limited diffusion of the in-formation for competing reasons. However, previous studiesdemonstrated that the use of a material having a high meltingpoint, latent heat of evaporation, and thermal conductivity [9],and a large secondary electron emission [16], is beneficial todischarge stability and uniformity. The influence of the cath-ode surface defects on discharge stability has been brieflydescribed elsewhere [7, 8]. In this paper, we present an experi-mental study of the effects of cathode roughness on dischargestability in order to point out the advantages of controlling thedensity and dimensions of the cathode surface defects.

After a brief description of the experimental setup andthe diagnostics involved in the experiments, the uniformityand stability of the discharge of a high-PRF long-pulseXeCllaser is investigated versus cathode roughness in single-shotand PRF-mode operations (PRF≤ 500 Hz, 10 shots) for anenergy deposition of∼ 20 J/l bar. Then, the results are dis-cussed to show the importance of the cathode roughness onthe creation and development of streamers during the dis-charge and furthermore to determine the most suitable rough-ness to achieve reliable high-PRF laser mode operation.

1 The laser system and diagnostics

1.1 Laser apparatus

All the experiments have been performed on a UV preionizedspiker-sustainer long-pulseXeCl laser [17] shown in Figs. 1and 2. The laser system is composed of a fast-flow subsonic

186

Magnetic switch

UV corona preionization

Ferrites H20Sustainercapacitors

9 kW engine

Centrifugalcompressor

water thermal exchanger 30 kW

NeXe

turbomolecularvacuum pump

Aerodynamicloop

(approx. 250 l)

Hcl

peakingcapacitors laser cavity

observationwindow

DICAM2lens

CCD videocamera

Fig. 1. LUX test-bed scheme and diag-nostics

closed-cycle wind tunnel(vflow = 40 m s−1), a laser chamber,and a high-average-power electrical system.

The electrical discharge circuit consists of a corona-typeUV preionizer system [18] and a magnetic-spiker-sustainercircuit using the overshoot mode [19]. The spiker drivercapacitor(Csp= 1.7 nF) is continuously charged at20 kV.The sustainer capacitors(Cs= 100 nF) are pulse chargedto 11 kV in ≈ 1.1µs from a 200-nF capacitor using a CX1725 thyratron switch. The peaking capacitor bank consistsof ten 100-pF capacitors(Cpk = 1 nF). The optimisation ofthis electrical circuit leads to no significant change in po-larity of the applied voltage before and during the laseremission.

The electrodes consist of an Ernst profile [14], mas-sive copper (CuAl Electrolytic:Cu 99% Al 1%) cathode anda perforated plate anode, both32 cm long. The dischargegap, width, and length are, respectively,3 cm, ≈ 1.5 cm, and≈ 25 cm. Careful attention has been paid to the realisationof the massive electrode to minimise profile design imperfec-tions and to its insertion in the laser cavity to reduce misalign-ment errors.

The study of the influence of the cathode roughness ondischarge quality and laser characteristics requires us to openthe laser and to take the cathode down and back many times.Therefore a strict procedure has been followed each time toensure the best reproducibility of the laser operating condi-tions. The laser was evacuated to10−2 Torr, and filled with1.3 bar of argon before its opening to extract the cathode.During the modification of the cathode roughness, the laserwas closed. Next, the electrode was taken back; its profile andassembly in the laser cavity was severely controlled, reveal-ing sufficiently low variations(< 0.2%) in the static electricfield distribution to prevent a significant influence of the pro-cesses related to these defects on the spatial development ofthe discharge [20]. Then the laser was again evacuated to10−2 Torr and filled with a gas mixture of99.44% Ne, 0.47%(∓0.01) Xe, 0.092% (∓0.001) HCl at a total pressure of3 bar. For each new electrode change, the laser optics (twoplane-plane mirrors of≈ 100% and≈ 40% reflectivity) werecleaned.

Though this procedure was different from the one used forindustrial laser openings, it allowed us to achieve stable andreproducible laser characteristics (i.e. energy, pulse duration,discharge stability, etc.) required for such study.

1.2 Diagnostics

An observation window set in one of the elbows of the sub-sonic flow loop allows the transverse visualisation of thevisible light emitted by the discharge plasma (see Fig. 1).A fast digitally controlled CCD video camera (PCO modelDicam 2) provides discharge-integrated photographs (expo-sure time≈ 400 ns).

The temporal evolution of the pressure perturbation am-plitude is measured by a fast piezoelectric transducer (PCB122A22) having the following characteristics: a10−3 to10+3 psi dynamic range, a2-µs signal risetime and a res-onance proper frequency of250 kHz. The pressure probe isinserted downstream the discharge volume in the output mir-ror inner wall at6.5 cm from the centre of the laser cavity asschemed in Fig. 2.

6,5 cm

Cathode

Piezoelectric pressure probeGrid anode

Current returns

50 cm

32 cm

25 cm6,5 cm

40 %Cathode Discharge area

Rmax

Piezoelectric pressure probe

Fig. 2a,b.

b

a

Front (a) and top (b) schematic views of the laser cavity

187

Notice that the correlated analysis of pressure perturba-tion signature in any point of the cavity and time-integrateddischarge photographs can detect the formation and locationof arcs in the active volume and therefore has been demon-strated as a powerful tool to assess the uniformity and stabil-ity of a discharge process [21].

For each measurement (single shot and versus PRF), anaverage of respectively, ten shots for the single-shot regimeand five shots for the PRF regime is performed defining thefollowing presented results. With regard to discharge pho-tographs, only one sample is kept due to their large memory-consuming size but it always corresponds to the average valuedefined for the measurement.

2 Characterisation of the cathode roughness

All the experiments have been performed with the same ini-tial massive copper cathode with different preparations of itssurface state according to the following operating proced-ure. The cathode surface is first sanded down with 3M Inc.600-grain sandpaper, then the cathode roughness is obtainedusing different grain sizes of sandpaper (3M Inc. 80, 320,or 600) to rub it down. Afterwards, the resultant roughnessof the cathode is controlled by profilometry measurements.The apparatus to characterise the cathode surface state is de-scribed in Fig. 3. The signal issued from the displacement ofa pickup along the electrode surface (6 ◦pickup head= 0.05µm,vpickup,translation= 0.5 mm s−1) is transmitted after amplifica-tion to a microcomputer to be analysed by an adapted soft-ware (Surfascan). Measurements have been performed on8 mm×8 mm cathode surface samples. As an example, re-sultant 2D and 3D roughness profiles are shown in Fig. 4a,b.As no significant roughness differences are observed betweenthe x (flow axis) andy axis (laser beam axis) defining thecathode surface, the roughness parameters commonly used inmetallurgy to characterise the surface state of a material [22],i.e. Ra and AR (see Fig. 5 for a short description), are givenwithout reference to any axis.

Table 1 lists the roughness parameters for the studiedcathode surface states obtained after successively sandingthe cathode with the sandpaper with a grain of 600 (lowroughness cathode named cathode A), 320 (medium rough-ness cathode named cathode B), and 80 (high roughnesscathode named cathode C). It is useful to scheme on Fig. 5the resultant surface states (2D views) of each cathode ac-cording to bothRa and AR parameters to better underlinetheir differences. Cathodes A and B have roughly the same“flaw” or “pin” periodicity ( ARA ≈ ARB) but the pin am-

Computer analysis(Surfascan)

Pickup

Cathode

Marble bench

ElectronicAmplifier

Electronicconverter

Fig. 3. Experimental apparatus used for the profilometry measurements

Fig. 4a,b.Example of 2D (a) and 3D (b) roughness profiles of the cathodeprepared with 3M Inc. sandpaper

Cathode C

Cathode B

Cathode A

AR

0

z(x)

x

L

Mean line

Fig. 5. Schematic representation of the resultant surface states of the cath-odes A, B, and C. The letters A, B, C refer, respectively, to the low,medium, and high roughnesses. The first scheme is added to defineAR(roughness mean wavelength) andRa [22] whose mathematical definitionis the following: Ra= 1

L

∫ L0 |z(x)| dx, with L, evaluation length andz(x),

analytical function corresponding to the signal amplitude with respect to themean line

plitude of cathode B is twice as high as that for cathodeA ( RaB ≈ 2RaA). On the other hand, cathode C presentshigher pins (RaC ≈ 5RaB ≈ 10RaA) but much more spaced(ARC≈ 2ARA ≈ 2ARB).

188

Table 1. Roughness parameters defining the studied cathode surface states.For convenience, the letters A, B, C refer to a cathode of, respectively, low,medium, and high roughness

Cathode A Cathode B Cathode C(low roughness) (medium roughness) (high roughness)

Ra 0.21 0.44 2.59AR (µm) 95 108 177

3 Single-shot regime

Table 2 shows the energetic laser characteristics versus cath-ode roughness obtained in single-shot regime. The cathode B(medium roughness) delivers the lowest energetic perfor-mances (≈ 10% less) with respect to the cathodes A (lowroughness) and C (high roughness) and presents the highestenergy fluctuation level (σB ≈ 3σA ≈ 3σC, whereσ , the sym-bol representing the root-mean-square parameter, defines theenergy fluctuations around its average value).

The discharge quality is conveniently evaluated by cor-related pressure-probe measurements and discharge pho-

Table 2. Energetic laser characteristics versus cathode roughness obtainedin single shot. The symbolσ refers to the root-mean-square deviation anddefines the energy fluctuations around its average value

Cathode A Cathode B Cathode C(low roughness) (medium roughness) (high roughness)

EnergyE±σ (mJ) 200±4 175±11 193±3.5Efficiency η(%) 3.3 2.9 3.2

Fig. 6. Temporal evolution of pressure fluctua-tions induced in the laser cavity by a dischargein the case of cathode A (left), correspondingtransverse photograph (400 nsexposure time) ofthe visible light emitted by the discharge plasma(right). The lower right scheme shows the elec-tric arc footprints on the cathode

Fig. 7. Temporal evolution of pressure fluctua-tions induced in the laser cavity by a dischargein the case of cathode B (left), correspondingtransverse photograph (400 nsexposure time) ofthe visible light emitted by the discharge plasma(right). The lower right scheme shows the elec-tric arc footprints on the cathode

tographs [21]. Experiments involving time-integrated dis-charge photographs and pressure-perturbation signatureshave been carried out and typical measurements are presentedin Figs. 6, 7, and 8 corresponding to the cathodes A, B, and C,respectively. The cathode B with a medium roughness showsa poor discharge homogeneity with four arcs clearly visibleon the photograph (Fig. 7) compared to the low-roughnesscathode (cathode A) where only one arc is visible (Fig. 6) andespecially to the high-roughness cathode (cathode C) whereno arc is identified (Fig. 8).

In order to explain such results and discharge behaviour,the dynamic development of microarcs had to be considered.Theoretical studies on this subject are very few, so our expla-nations are mainly based on a study presented by Kushner [8].The author numerically studies the formation and develop-ment of microarcs, resulting from an inhomogeneity in thepreionization electron density or in the electric field. In thatwork, this is conveniently described from nonuniformities inthe preionization electron density, which is mathematicallydefined by:

ne0(r)= ne0

(1+αe−(r/r0)

2).

Wherer refers to the radial coordinates,ne0 correspondsto the preionization density in the bulk plasma,α is themagnitude of the nonuniformity andr0 is its spatial extent(≈ 100µm). Kushner shows thatr0 and α are determiningfactors in the microarc formation, taking into account bothspatial and time aspects. In particular, he demonstrated thatthe ratior0/α

1/2 has to be as high as possible to prevent thearc development if considering no arc dynamic formation.Moreover the time aspect has to be taken into account and isillustrated by Fig. 9 which shows the time required to develop

189

Fig. 8. Temporal evolution of pressure fluctua-tions induced in the laser cavity by a dischargein the case of cathode C (left), correspondingtransverse photograph (400 nsexposure time) ofthe visible light emitted by the discharge plasma(right)

10 100 1000

r0 (µm)

Tim

e re

qu

ired

to

dev

elo

p a

mic

roar

c

αA

αB>αA

αC>αB

Fig. 9. Time required to develop a microarc versusr0 for different magni-tudesα of the nonuniformity.r0 is the spatial extent of the nonuniformity.From M.J. Kushner [8]

a microarc as a function ofr0 for different magnitudesα of thenonuniformity.

Now let us draw a parallel between Kushner’s model andour experimental study. Indeed, the cathode surface can beseen as a pin grating, their density and dimensions being re-lated to the roughness parametersAR and Ra as shown inSect. 2. Each pin locally distorts the distribution of the elec-tric field on the cathode surface and in this region close tothe cathode, the so-called cathode sheath, the electric fieldis very high [16, 23]. So, even very small surface irregular-ities can induce local electric field modifications and thenstrong local nonuniformities in the electronic distribution,which lead to a rapid degradation of the discharge homogene-ity [8, 20, 24, 25]. It is obvious that the importance of thesepin effects slightly depends on the pin grating characteristics(pin dimensions and shape, density, etc.) on the surface of thecathode.

In the present work, the spatial extent(r0) and magnitude(α) defining the nonuniformities issued from a pin are con-sidered to be directly dependent on the roughness parametersAR and Ra, respectively, characterising the cathode. If weassume equivalence between, respectively,r0 and AR, andαand Ra, the influence of these nonuniformities on dischargestability can be explained according to the above conclusionsof the model described in [8].

Calculations of ther0/α1/2 ratio for the three cathodes

gives the following results:(r0/α1/2)A = 207> (r0/α

1/2)B= 162> (r0/α

1/2)C = 110. As a consequence, cathode Ahaving the highestr0/α

1/2 ratio presents favourable condi-

tions for the non-development of strong arcs as confirmed bythe experimental results previously shown.

To explain the behaviour of cathode B, which gives thelowest laser energy and high-energy fluctuation shot to shot,the time formation of arcs as illustrated in Fig. 9 has also tobe considered. The nonuniformities produced with cathodesA and B have roughly the same spatial extent (ARA ≈ ARB)but their amplitude is weak enough for cathode A to suffi-ciently delay the microarc appearance. Indeed, even if theformation of a few numbers of arcs seems evident for cath-ode A (see, for instance, the pressure singularities on thepressure-perturbation signature on Fig. 6), they have no sig-nificant influence on the laser characteristics. In the case ofcathode C, the pin effects are presumed to be stronger dueto a higherRa value and a low value of

(r0/α

1/2)

C but thespatial extent of the nonuniformities may be large enough(ARC ≈ 2ARA ≈ 2ARB) to delay the microarc appearanceafter the end of the laser emission, which is not the casefor cathode B. Moreover, their magnitude is presumably lowdue to their establishment during the phase of termination ofthe discharge current. These explanations are in accordancewith Fig. 9, which shows that the time to develop anarc(Tarc)increases withr0 and that for certain values ofr0 we can ob-serveTarc,C (for αC and 2r0) > Tarc,B (for αB andr0).

This is experimentally confirmed by the good dischargehomogeneity shown in Fig. 8 (see the absence of significantpressure singularities on the pressure-perturbation signatureof cathode C) and by the FWHM pulse duration measure-ments (τFWHM(A, single shot)≈ 138 ns; τFWHM(B, singleshot)= 120 ns; τFWHM(C, single shot)≈ 135 ns). So the laserpulse duration is shortened for cathode B which is obviouslydue to the rapid disruption of the optical homogeneity in-duced by microarcs forming early (lowr0/α

1/2 value) and inlarge number (lowAR) with this medium roughness cathode.

4 High-PRF regime

Figure 10 presents the evolution of the 10th-shot laser en-ergy versus cathode roughness as a function of PRF (PRF≤ 500 Hz). The measurements have been averaged over fiveexperiments realised with the same conditions (20 J/l bar en-ergy deposition,vflow = 40 m s−1) and normalised with re-spect to the laser energy obtained at1 Hz. They have beencarried out on the 10th shot of each experiment for method-ological reasons. In fact, operating the laser on very shortbursts allows us to study precisely the PRF influence on laserperformances obtained with the three cathodes by minimis-

190

0 100 200 300 400 500 600

Pulse Repetition Frequency (Hz)

0.5

0.6

0.7

0.8

0.9

1

1.1N

orm

alis

ed la

ser

ener

gy E

/E1H

z (a

.u.)

Cathode ACathode BCathode C

Fig. 10. Normalised output laser energy versus PRF (1≤ f ≤ 500 Hz; 10thshot) for the cathodes A (low roughness), B (medium roughness), andC (high roughness)

ing efficiently the onset of thermal disturbances related tolonger laser-operation times (consider for instance the ther-mal boundary layers).

Figure 10 shows a30% energy decrease for cathodeB between1 Hz and 500 Hz, whereas the energy remainsroughly constant for cathodes A and C (∆EA,1–500 Hz≈∆EC,1–500 Hz≈ 10%).

As soon as the PRF is increased up to a few hun-dred Hz, it is reasonable to assume that previous dis-charge processes can affect the stability of the current dis-charge. Measurements of the residual pressure field am-plitude ((∆p/p)residual) in the laser cavity before the nextenergy deposition have then been carried out. A depen-dence of the laser energy shot-to-shot evolution on re-sidual pressure field amplitude is expected with PRF in-crease if the following condition,(∆p/p)residual≥ 5%, isreached [26, 27]. Results are presented in Fig. 11 and onecan see that(∆p/p)residualdoes not exceed3% whatever thePRF and cathode roughness. Therefore pressure perturba-tions induced by the previous discharge processes cannot beinvoked to explain the laser energy decreases observed athigh PRF.

Now consider the discharge visualisations presented inFigs. 6–8 and 12. Careful analysis of these time-integrateddischarge photographs performed in single shot and at 100and500 Hzwell illustrates the evolution of the laser energy

100 Hz Cathode A 500 Hz

100 Hz Cathode B 500 Hz

100 Hz Cathode C 500 Hz

Fig. 12. Time-integrated discharge pho-tographs at100 Hz (left) and 500 Hz(right) for 10th shots obtained with thecathodes A (low roughness), B (mediumroughness), and C (high roughness)

0 100 200 300 400 500 600

Pulse Repetition Frequency (Hz)

0

1

2

3

4

Res

idua

l pre

ssur

e fi

eld

ampl

itud

e(∆

p/p)

resi

dual

(%)

Cathode ACathode BCathode C

Fig. 11. Residual pressure-field amplitude versus PRF (1≤ f ≤ 500 Hz;10th shot) for the cathodes A (low roughness), B (medium roughness), andC (high roughness)

versus PRF and cathode roughness seen in Fig. 10. In par-ticular, it is obvious that the strong laser energy reduction forcathode B (medium roughness) with PRF increase is relatedto a significant degradation of the discharge homogeneity. Onthe other hand, only few hot spots are observed at high PRFwhen high (cathode C) and low (cathode A) roughness elec-trodes are used, which is in accordance with the evolutionof the laser energy versus PRF for both cathodes A and C.It seems evident that the facility for cathode B to form mi-croarcs pointed out in single-shot regime increases with PRF.This is probably due to a reduced thermal relaxation time ofthe hot spots at high PRF further providing more favourableinitial conditions to form streamers earlier and in larger num-bers during the following energy deposition processes. Com-parison of the temporal evolution of the laser power for themedium roughness cathode (cathode B) at 100 and500 Hz(see Fig. 13) confirms these assumptions. Indeed, the laserpulse terminates prior at500 Hz (τFWHM(B, single shot) =120 ns, τFWHM(B,500 Hz) = 105 ns) because of an increas-ing number (compare Figs. 7 and 12) and earlier onset ofarcs during the discharge process [8]. On the other hand, theFWHM pulse durations measured with the cathodes A andC are little reduced with respect to low PRF measurements(τFWHM(A,500 Hz)≈ τFWHM(C,500 Hz)≈ 128 ns) in accor-dance with the slight degradation of discharge homogeneityversus PRF (compare Figs. 6 and 12 with Figs. 8 and 12).

191

Instantaneous laser power (a.u.)

Time (0,2 µs/div)

100 Hz

500 Hz

Cathode B

Fig. 13. Laser power time evolution at 100 and500 Hz(10th shot) obtainedwith cathode B (medium roughness)

These results demonstrate the prime importance of controland optimisation of the cathode roughness to preserve the dis-charge homogeneity at high PRF and then to obtain high andreproducible laser performances.

5 Conclusion

The efficiency of the input energy deposition process is a cru-cial problem in excimer mixtures especially to achieve re-liable high-power high-PRF excimer laser systems. For in-stance, uniformly depositing power for a long time, per-manent mode operation at high PRF (for instance, billionsof shots at PRF≈ 1 kHz), and high-energy deposition, arenot yet realised due to the degradation of the dischargestability.

The discharge stability directly depends on the equilib-rium between the rate coefficients for ionisation and attach-ment in any point of the active volume. So to optimise thelaser performances one has to uniformly excite the gas mix-ture that results, in other words to ensure the most homoge-neous flow in time and space of the discharge current in theactive volume. In the first stage, this is largely determined bythe distribution of the preionization density and of the unifor-mity of the electric field provided by the electrode profiles.However when the conditions about the minimum preion-ization density are fulfilled [28] and a careful design andshape of the electrodes are realised, the homogeneity of thedischarge current is then ultimately limited by the electrodeprocesses establishing local disturbances in the distribution ofthe electric field and afterwards leading to the degradation ofthe discharge quality [5, 6].

The experiments described in this paper investigate therole of the cathode roughness on the onset of discharge insta-bilities in the active volume. Indeed, the cathode roughness,by defining the local homogeneity of the electric field distri-bution on the cathode surface, provides the initial conditionsto efficiently control the establishment and flow of the dis-charge current in the active volume. In our experiments, con-

trolled roughness cathodes having the following characteris-tics, Ra≈ 2.6 andAR≈ 200µm (high-roughness cathode) orRa≈ 0.2 and AR≈ 100µm (low-roughness cathode), havebeen the most efficient to prevent arc formation during thedischarge and therefore appeared to be the most promising todeliver high and reproducible laser performances at high PRF.Note that the results agree well with theoretical ones issuedfrom the modelisation developed in [8].

Now further experiments have to be carried out on perma-nent laser mode operation (billions of shots) to confirm theresults for industrial requirements.

Acknowledgements.The authors thank M. Fraisnais, director of CREPI(Centre de Recherche et d’Etudes des Procedes Industriels) from PSA Peu-geot Citröen for supplying profilometry measurements.

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