Ablation of metal surfaces by pulsed ultraviolet lasers underultrahigh vacuum
R. Viswanathan
Department of Chemistry, Beloit College, Beloit, Wisconsin 53511
Ingo Hussla*
IBM Almaden Research Center, K33/801, 650 Harry Road, San Jose, California 95120-6099
Received November 12, 1985; accepted January 27, 1986
A review of recent studies of ablation of metal surfaces by pulsed ultraviolet lasers under ultrahigh vacuumconditions is presented. Techniques employed include optical emission spectroscopy, time-resolved mass spec-trometry, and reflectance measurements. The results are discussed in terms of optical and thermal mechanismsthat could account for the observed ablation phenomena.
INTRODUCTION AND REVIEW OF PREVIOUSWORK
Recently, mechanistic studies of laser-solid interactionsl, 2
in the processes of laser-induced annealing, crystallization,vaporization, ion implantation, etc. have become importantbecause of potential applications in the very-large-scale in-tegrated-circuit fabrication and nuclear-fusion technol-ogies.3'4 Experimental and theoretical knowledge of theoptical and thermal processes occurring during the interac-tion of pulsed high-powered lasers with materials is also offundamental interest.5
Much of the early developmental work involving pulsed-laser interactions with solids has utilized high-poweredruby, YAG, and CO2 lasers.6 In particular, a vast body ofliterature exists on the interaction of such pulsed lasers withsemiconductor surfaces7' ; interactions of pulsed-laserbeams with silicon surfaces have been extensively studiedbecause of the obvious importance of silicon as a raw materi-al in the semiconductor industry. Recently, however, theinteraction of pulsed lasers with transition metals has alsobecome important because of the crucial need to fabricatedenser very-large-scale integrated circuits incorporatingmetallic substrates and interconnections with especiallygood thermal- and electrical-conductivity properties.8
Pulsed lasers operating in the ultraviolet region of the elec-tromagnetic spectrum are most suitable for use in material-processing applications such as ablation, etching, and photo-deposition of metals because of a number of advantages thatthey have when compared with other laser sources. Theseadvantages are summarized as follows:
(1) These lasers have shorter wavelengths, resulting inlarger photon energies and smaller diffraction-limited beamsizes.
(2) Most of the transition metals have much smallerreflectivities, i.e., absorption of incident laser power is moreefficient at ultraviolet wavelengths:
(3) Stable beams with uniform intensity profiles aremuch easier to generate by using ultraviolet lasers, especiallyexcimer lasers. In addition, the short temporal pulse width(approximately 15 nsec) and high fluence (up to 2 J/pulse forcommercially available systems) of an excimer laser allowsfor very high heating rates (of the order of 1010 K/sec) to beeasily achieved.
(4) Photolysis of molecules, of importance in chemicaletching and photodeposition processes, takes place efficient-ly in the ultraviolet.
This review will focus on recent work connected with oneaspect of ultraviolet-laser interactions with metals, namely,pulsed-laser-induced ablation of transition-metal surfacesby excimer lasers under ultrahigh vacuum. In particular,the laser ablation technique will be discussed in terms of arepresentative set of experiments on the ablation of cleanand carbon monoxide covered copper surfaces. In this con-text, it may be noted that only a limited number of experi-ments involving ultraviolet-laser-induced ablation of metalsurfaces under well-characterized conditions have been re-ported in the literature. Rothenberg and Koren 9 have pre-sented results for ablation and plasma formation of alumi-num and sapphire (a-A1203) substrates by 248-nm excimer-laser pulses of 15-nsec duration. The laser power densityfor plasma formation was estimated from the onset of the3961.5-A resonance optical emission of aluminum atoms inthe vapor ablated from the surfaces. It was found that thethreshold fluence for strong laser-produced plasma emissionin sapphire was significantly lower than the thresholdfluence needed in the case of clean aluminum. Marks andPollak' 0 have studied the surface oxidation of clean niobiummetal in an oxygen atmosphere using a pulsed excimer laser(308 nm, 40 nsec FWHM). X-ray photoelectron spectrosco-py (XPS), conducted after irradiation of the surface by thelaser, revealed a range of niobium oxidation states includingNb+5, Nb+4 , Nb+2 , and Nb metal ongoing from the surface tothe bulk. The XPS data also indicated enhanced oxidation
Table 1. Laser Power Density for Various Effects Observed during KrF-Excimer-Laser Pulse Interaction withPolycrystalline Copper Surfaces (Refs. 13 and 14)
Krypton Fluoride LaserAbsorbed Power Density Observed Effect Remarks
(a) Clean copper substrate at Ts = 90 K
300 MW/cm 2 Vaporization of neutral Thresholdcopper monomers
360 MW/cm 2 Neutral copper dimers Threshold-500 MW/cm 2 Uncharged copper trimers Ratio Cu:Cu 2:Cu3 = 10:12:1>500 MW/cm 2 Positively charged copper ions, Threshold for visual observation of
green plume green plume1 GW/cm 2 Plasma, bright plume Ratio Cu:Cu2:Cu3 = 10:3:0
(b) Substrate covered with submonolayer of CO at Ts = 90 K
6 MW/cm 2 LITD of carbon monoxide Threshold for LITD of carbonmonoxide from copper
40 MW/cm 2 LITD of carbon monoxide Complete desorption of carbonmonoxide
60 MW/cm 2 LITD of carbon monoxide no Alterationlonger reproducible of copper substrate
400 MW/cm 2 Vaporization of neutral Thresholdcopper monomers
>600 MW/cm2 Positively charged copper ions, Threshold for visual observationgreen plume of green plume
that was activated by laser heating of the surface and con-trolled by diffusion. Data on pulsed-laser annealing ofCu(001) surfaces using a 308-nm excimer laser with a 25-nsec pulse width have been presented by Kevan.11 Themaximum laser power density utilized in this work was ap-proximately 50 MW/cm 2 . High-resolution angle-resolvedphotoemission was utilized to examine the quality of thelaser-annealed surface after exposure to multiple pulsesfrom the excimer laser, and a significant degradation of theelectron coherence length was observed for laser-pulse pow-er densities above a certain threshold. A theoretical studyof surface melting of copper was also published recently byJayanthi et al.12
As part of a project investigating laser-induced thermaldesorption (LITD), Viswanathan and Hussla (see Refs.13-16) studied the effects of 248-nm excimer-laser pulses onclean and carbon monoxide covered copper surfaces underultrahigh vacuum conditions. Ablation of copper was de-tected by time-resolved mass spectrometry and optical emis-sion spectroscopy, both carried out simultaneously on apulse-to-pulse basis. The nature of the ablated species wascharacterized as a function of the absorbed laser power den-sity by time-resolved mass spectrometry. A significant dif-ference in the laser power-density thresholds for vaporiza-tion of copper from pure and carbon monoxide covered cop-per surfaces was found.' 3 At high laser power densities,neutral copper dimer and trimer species as well as ionizedcopper species were detected in the vapor phase. Pertinentresults are summarized in Table 1. In addition, translation-al temperatures of the ablated copper vapor were deter-mined by fitting the time-resolved mass spectral intensitysignals to a Maxwell-Boltzmann distribution.'4 Typicalspectra and fits are shown in Fig. 1. The following sectionscontain a description of the apparatus utilized in such ex-periments, new results on the ablation of copper surfaces by
.0
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D
0
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(a)
(b)
35 185 335
TIME (microseconds)Fig. 1. Time-resolved mass-spectrometric signal of copper species(amu 64) ablated from a polycrystal copper surface by a kryptonfluoride laser pulse. Points are the actual data, while the solid lineis the least-squares-fit signal shape obtained from a fit of the data toa Maxwell-Boltzmann distribution. (a) Neutral copper emission at300-MW/cm 2 laser power density; (b) Cu(+1) ions, detected at 550-MW/cm 2 absorbed laser power with the electron impact ionizerswitched off. (From Ref. 14.)
R. Viswanathan and I. Hussla
798 J. Opt. Soc. Am. B/Vol. 3, No. 5/May 1986
Fig. 2. Apparatus for optical emission studies during pulsed-ultra-violet-laser-induced ablation.
an excimer laser, and a discussion of the significance of thefindings.
EXPERIMENTAL CONSIDERATIONS
An ultrahigh vacuum (UHV) apparatus is necessary for astudy of the ablation of clean metal surfaces. As an illustra-tive example, a brief description of the apparatus utilized inexperiments on copper surfaces17 is given below. A focusedpulsed krypton fluoride excimer-laser beam (15 nsecFWHM; 248 nm; maximum pulse energy, 250 mJ; LambdaPhysik EMG101) was utilized to pulse-heat a polycrystallinesample of copper under UHV conditions. The intensityprofile of the laser was mapped using a photodiode array andwas found to be uniform within 5% over the entire cross-sectional area.17 A laser-in laser-out arrangement was uti-lized to avoid spurious effects due to laser light scatter with-in the chamber. The chamber had provisions to cool themetal sample to 90 K. Vaporized copper species and surfaceadsorbates desorbed by the laser pulse were detected in realtime by a UTI-1OOC quadrupole mass-spectrometer probe.The ion current from the probe was amplified by a custom-built fast electrometer amplifier, and the resulting time-of-flight (TOF) signal was digitized and stored by a high-speeddigitizer (Biomation 8100) interfaced to a signal averager(Nicolet 1170). The TOF distance was 218 nm, the detec-tion angle being coincident with the surface normal. Cop-per species ablated from the surface were also detected bymonitoring the characteristic optical emission from excitedelectronic states using a grating monochromator (JobinYvon) and a photomultiplier (RCA 1P28). The laser was op-erated at a 1-Hz repetition rate in order to maintain pulse-to-pulse power stability of its output. However, effectiverepetition rates down to 0.01 Hz for the laser beam hittingthe sample could be achieved by using a novel electrome-chanical shutter mechanism,' 6 which was synchronized tothe laser pulse. A custom-built dual-probe energy meterutilizing special photodiodes sensitive to ultraviolet light(Hammamatsu S1337-1O1OBQ) was used to carry out pulse-to-pulse reflectance measurements. The experimental ar-rangement is shown in Fig. 2. Since the energy-meterprobes were susceptible to damage by the high-energy laserpulse, an alternative arrangement in which the average pulseenergy was measured after reflection off the surface (posi-tion B in Fig. 2) using a disk calorimeter (Scientech, Inc.,
Model 380103) was also used periodically in order to checkthe pulse-to-pulse reflectance measurements. The ablationexperiments were carried out at base pressures of about 3 X10-10 mbar. The polycrystalline copper sample was pre-pared from an oxygen-free high-conductivity copper gasket,polished (0.2-Atm mesh) and cleaned by oxidation and argon-ion sputter cycles, followed by annealing of the surface.Carbon monoxide gas (99.995%, Matheson) was dosed bycontrolled leakage to build up a constant partial pressure of5 X 10-9 mbar during the ablation experiments with carbonmonoxide covered copper.
NEW RESULTS OF STUDIES ON EXCIMER-LASER-INDUCED ABLATION OF COPPERThe following results were obtained by using the apparatusdescribed above. Pulse-to-pulse reflectance measurementsyielded a reflectance of 15% (±2%) for the clean polycrystal-line copper surface at 90 K, for 248-nm radiation. Thisresult is not in agreement with the value (35%) reported byEhrenreich and Philipps,18 which was obtained under condi-tions in which the sample could conceivably have been sub-ject to atmospheric contamination. The optical emissionspectra for neutral copper species in the vapor had three
C
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C
0.w
CE
I0a
C
a)-J
500 510 520 530 540
Wavelength (nm)
Fig. 3. Optical emission spectra during ablation of polycrystallinecopper by a pulsed krypton fluoride excimer laser. Absorbed laserpower density was 450 MW/cm 2; (a) clean surface at 90 K, (b)carbon monoxide covered surface at 90 K.
I I I
522
(a)
515
510
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I I I
R. Viswanathan and I. Hussla
Vol. 3, No. 5/May 1986/J. Qpt. Soc. Am. B 799
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20. 160T
-7 180't..\...X.. \Copper'' .,X10
Fig. 4. Solid curve, angular distribution of the krypton fluoridelaser-induced optical emission during ablation of clean polycrystal-line copper at absorbed laser power density of 450 MW/cm2. Theoriginal photograph was taken through window W4. Dashed curve,evaporant distribution from a tube basket.24 Dotted curve, cosinedistribution for comparison.
peaks at the wavelengths corresponding to the strongestlines in the 400-600-nm region of the emission spectrum ofcopper reported in the literature. 19 -22 The peaks occurredat 510, 515, and 522 nm, respectively. A similar result hasbeen reported by Rothenberg et al.
2 3 in their study of theemission lines of neutral copper species during laser ablationof solid copper chloride. A striking finding was the fact thatthere was a general decrease in the intensity of all the peakson going from a clean copper surface to a carbon monoxidecovered copper surface, at constant laser power densitiesabove 450 MW/cm 2 . In particular, the integrated emissiondecreased by a factor of 2 on going from a clean [Fig. 3(a)] toa carbon monoxide covered [Fig. 3(b)] copper surface at alaser power density of 450 MW/cm 2 . A densitometric analy-sis indicated that the spatial distribution of the vapor plumecould be fitted to a cos8 0 distribution with a distinct peakingin the forward direction. The fit is shown in Fig. 4 alongwith a cos 0 evaporant distribution and a typical evaporantdistribution from a tube basket.24
DISCUSSION
It is possible to obtain an order-of-magnitude estimate of thelaser heating and subsequent cooling rates of clean coppersurfaces by using the formulation of Bloembergen. 25 Theassumption made is that all the thermal parameters areindependent of temperature and that there is no latent heatinvolved in phase transitions. For copper surfaces, the opti-cal absorption depth a'- at 248 nm is small compared withthe thermal diffusion length, which is directly proportionalto the square root of the pulse duration. Using the relevantthermal parameters for copper,25 an average surface tem-perature of approximately 6000 K is estimated for a 15-nseckrypton fluoride laser pulse with an absorbed power densityof 300 MW/cm 2. This may be compared with the observedtranslational temperature Td of 22 000 K for copper vaporreported previously. 4 In addition, the time taken for va-porization can be roughly estimated by following the methodof Ready6 and is found to be approximately 1 nsec. It can beseen from this result that the surface is vaporized in a timeduration (1 nsec) that is a fraction of the laser-pulse duration(15 nsec). According to Ready6 26 and others,2 7 slightly ther-mally ionized material is vaporized early on in the laserpulse, followed by an opaque high-temperature plasma.6 Itis possible that the laser light is effectively cut off from thesurface by the formation of the plasma, in which case several
new physical processes become important: interaction ofcopper vapor with the incident laser beam leading to absorp-tion of multiple photons of the incident -5-eV radiatiqp,followed by an expansion of the plasma to become transpar-ent again. It can be seen (Fig. 1) that fast copper (+1) ionsare detected by the mass spectrometer at laser power densi-ties above 500 MW/cm 2 . These ions have translational tem-peratures that are approximately twice the temperatures ofneutral species at the same laser power density. This phe-nomenon could be caused by formation of the plasma phasediscussed above. Also, it may be noted that no thermalizedcopper species were detected, indicating that evaporationfollowed by heating and ionization of vaporized copper ma-terial is a distinct possibility. The observed translationalenergy for the ionized copper species (7 eV) is roughly thesame as the ionization potential for neutral copper and onceagain supports the contention that multiphoton processesmay become important during some stage of the vaporiza-tion. Alternatively, it is possible that a mechanism wherebythe incident 5-eV laser radiation itself is involved in photo-ionization of the neutral copper species may also be opera-tive.2 8 In this context it is interesting to note that no ionizedcarbon monoxide species (ionization potential, 14 eV) wereobserved during ablation of carbon monoxide covered sur-faces. Photoionization of helium and nitrogen by ultravio-let radiation from a laser-induced copper plasma has beenobserved at higher gas densities and higher laser intensi-ties.2 9
A general decrease and a distinct change in the intensitypattern [Figs. 3(a) and (b)] of the three prominent neutralcopper lines in the optical emission spectrum is observed ongoing from a clean copper surface to a carbon monoxidecovered surface at laser power densities above 450 MW/cm 2 .In addition, the vapor ablated from copper surface has ahighly directed and narrow spatial distribution. It may benoted that similar effects were observed in a pulsed dye-laser (580-nm) microprobe experiment on a copper-alu-minum alloy sample,3 0 with the copper atomic fluorescencesignal decreasing by a factor of 2 when the exposure of thesample was changed from a inert argon atmosphere to anoxygen atmosphere because of energy transfer and subse-quent quenching of the copper emission. Presumably, ener-gy transfer between carbon monoxide and excited copperspecies in the vapor ablated from carbon monoxide coveredcopper surfaces could result in a similar quenching of thecopper optical emission. Finally, we would like to point outthat both the change in the nature of the'species in thecopper vapor and the change in laser vaporization thresholddensities on going from a pure to a carbon monoxide coveredcopper surface may be of some importance and utility inmaterial-processing applications such as laser writing, laservapor deposition, laser wire bonding, and laser circuit repair.
ACKNOWLEDGMENTS
We are grateful to Eric Weitz and Peter Stair, Departmentof Chemistry, Northwestern University, Evanston, Illinois,for providing laser and UHV facilities (U.S. Office ofNaval Research contract no. N00014-79-C-0794). IngoHussla would like to thank the Deutsche Forschungs-gemeinschaft for a fellowship.
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R. Viswanathan Ingo Hussla
R. Viswanathan is an assistant professorof chemistry and computer education atBeloit College, Wisconsin. He receivedthe bachelor's degree in chemistry(1973) from Bombay University and the
- master's degree in chemistry (1975) from@Vthe Indian Institute of Technology, Kan-
pur, India. In 1980, he received thePh.D. degree in physical chemistry fromthe University of Oregon, Eugene. Hisdissertation research involved the deter-mination of the structures of some novelweakly bound complexes involving hy-
drogen sulfide, produced in seeded supersonic nozzle beams, byradio-frequency and microwave molecular-beam electric resonancespectroscopy. He worked as a postdoctoral fellow with ProfessorEric Weitz in the Department of Chemistry at Northwestern Uni-versity (1980-1983). His research work there involved the develop-ment of pulsed-laser-induced desorption as a versatile tool to studythe dynamics of desorption and surface diffusion processes. Hiscurrent research interests include laser-surface interactions underUHV, studies of the dynamics of relaxation of excited electronicstates of molecules in the liquid and matrix phase, and applicationsof microprocessors in signal conditioning and high-speed data ac-quisition. He is a member of the American Chemical Society.
Ingo Hussla received the M.S. degrees inchemical engineering (1970) and inchemistry (1978). He earned the Dr.rer. nat. degree from Friedrich-Alexan-der-Universitiit Erlangen-Ndrnberg,Federal Republic of Germany, in 1980.The thesis concerned infrared spectros-copy of adsorbates at low temperaturesand under UHV. In 1982, he spent ayear as a visiting scientist from the Uni-versity of Hannover at NorthwesternUniversity, Illinois. Since 1983, he hasbeen working at the IBM Almaden Re-
search Laboratory, San Jose, California, in the Physical ScienceDepartment. His main fields of research are resonant and nonreso-nant laser-induced surface phenomena in adsorbates and conden-sates under UHV, such as photodesorption, reaction, migration, andluminescence. He is currently working in the area of laser- andplasma-induced surface processes of importance to microelectronicfabrication and tribology. He is a member of the American Chemi-cal Society, Gesellschaft Deutscher Chemiker, and Bunsen-Gesell-schaft ffr Physikalische Chemie. He recently joined Leybold-Her-aeus GmbH, Hanau, Federal Republic of Germany, as manager ofthe microprocessor wafer technology division.
