The Physics of Laser Ablation in Microchemical Analysis

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A N A LY T I C A L C H E M I S T R Y / F E B R U A R Y 1 , 2 0 0 2

The Physics of Laser Ablation in Microchemical Richard E. Russo •Xianglei Mao •Samuel S. Mao

Lawrence Berkeley National Laboratory

I

f you were lucky enough to find some old coins, maybe dating back about 2000 years to the Roman Empire, you would need to validate your discovery by determining their elemental composition. Spectroscopic analysis with laser ablation (LA) as a sampling technique could provide a swift “nondestructive” answer. You would be surprised to learn from such an analysis that the coins had very different compositions. In

In laser ablation, there are no sample-size requirements, sample preparation is simple, and solid samples can be spatially characterized.

fact, brass, a mixture of zinc and copper, was first widely used at the time of Augustus’s coin reforms (23 BC); but, as analysis reveals, its use gradually declined until the third century, when coins contained virtually no zinc (1). This example is only one of the many steadily growing number of LA microchemical analysis applications. LA is the process in which an intense burst of energy, delivered by a short duration

F E B R U A R Y 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y

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Onset of ablation FIGURE 1.Laser ablation. (a) Photo shows an LA plasma at a copper sample surface. The glowing blue plume is due to recombination of copper ions expanding out of the sample. The sample was ablated using a 10-J/cm2, 30-ns UV excimer laser pulse in 0.1-torr atmosphere. (b) Hyperlinear dependence of single-pulse ablation characteristics on laser intensity.

laser pulse, is used to remove a tiny amount of material from a sample (Figure 1a). LA holds the promise of becoming the standard technique for direct solid sampling. Analytical applications of LA now cover a great range of academic and industrial fields, including geology, environmental science, forensics, semiconductor manufacturing, and archaeology (2–9). In principle, any type of solid sample can be ablated and there are no sample-size requirements. LA does not require complicated sample preparation procedures, so the risk of contamination or sample loss can be avoided. Chemical analysis using LA requires much smaller amounts of material (typically less than a microgram) than those required for solution nebulization (about a milligram). Depending on the analytical detection system, subpicogram quantities may be sufficient for analysis, rendering the technology essentially nondestructive. In addition, a focused laser beam permits spatial characterization of heterogeneity in solid samples (e.g., inclusions), typically with micrometer resolution in terms of lateral and depth dimensions.

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A basic LA microanalysis system consists of a laser, an ablation chamber, and a detector. Short-pulse solid-state or excimer lasers have been most often used to produce light pulses for ablation. Samples are usually housed in a pressurevariable chamber mounted with an adjustable sample stage. One widely used detection technique is laser-induced breakdown spectroscopy (LIBS), in which an optical spectrometer is used to collect light emission from the laser-produced mass plasma at the sample surface (10–13). Typical LIBS spectra include atomic and ionic emission lines superimposed on a broadband continuum resulting primarily from electron–ion recombination. Identification of the spectral lines of different constituents of the sample provides qualitative analytical characterization. As an alternative to LIBS, LA microanalysis can work with a mass or optical spectrometer with inductively coupled plasma (ICP) as the analytical excitation source (14–19). In this experiment, inert gases such as argon and helium are typically used to carry laser-ablated mass to the ICP excitation source. The resulting spectra from ICPMS or ICP-atomic emission spectroscopy (AES) will show the elemental or molecular species in the sample. LA-ICPMS and LA-ICP-AES are promising analytical approaches for essentially nondestructive determination of elements with very low detection limits. Variations in analytical instrumentation, as well as different chemical aspects of LA, have been discussed in many recent reviews (14–19). In this article, we will focus on some physical aspects of the LA process that are important for microchemical analysis.

Physics The primary goal of LA for microchemical analysis is to transform a desired amount of sample into the vapor phase with a stoichiometric representation of the sample. It is therefore necessary to know how much mass is ablated by each laser pulse, and whether the ablated mass has the same composition as the sample. However, it has not been easy answering these seemingly simple questions. For example, if 1 ng of mass can be ablated using 1 mJ of laser energy, doubling the energy may ablate more than 10 ng of mass with the same sample under certain laser intensity conditions, and may ablate less than 2 ng under other conditions. A similar situation can occur for the composition of the ablated mass. For example, if a sample is a 50/50 mixture of copper and zinc, under different LA conditions, the ablated mass may contain 30% copper and 70% zinc, or vice versa. Indeed LA remains a perplexing process, governed by a variety of distinct nonlinear mechanisms. Ablation is inherently a multidimensional event because the mass leaves the surface in the form of electrons, ions, atoms, molecules, clusters, and particles separated in time and space. Understanding fundamental LA mechanisms is critical for efficiently coupling the laser beam into the sample and producing stoichiometric vapor for microchemical analysis. Because LA processes occur over several orders of magnitude in time (20), starting with the electronic absorption of laser energy (10–15 s) and continuing until particle ejection (10–6 s) after the laser pulse is completed, an effective way to elucidate the underlying physics of LA is to look at ablation events at various times.

Using variable-delay optical shadowgraphy or interferometry to study the dynamics of time-resolved experiments in which a pump laser beam initiates the ablation of the sample have proven to be powerful approaches (20). Varying the time delay between two laser beams can provide transient dynamics of the ablation processes. Time-resolved shadowgraphs reveal how the laser-ablated mass evolves above the sample surface. Gas dynamic behavior in one, two, or three dimensions dominates the development of the mass plume or plasma on different timescales. Interference patterns can provide spatial–temporal information on the intrinsic properties of the ablated mass or plasma during the LA process. The shift of the interference fringes during the generation and propagation of a laser-produced plasma is related to the number density of the species (electrons and ions) inside the plasma. Instead of an interferometer, an optical spectrometer synchronized with the ablation laser pulse can be incorporated in the experiment to monitor the time-resolved atomic or ionic emission (or absorption) of the laser-ablated mass. This type of instrumentation is essentially a time-resolved LIBS system. Electron density and excitation temperature inside the laserproduced mass plume or plasma can be inferred from measuring the width and intensity ratio of characteristic spectral lines (21). By using a combination of time-resolved, pump–probe experiments, the physics underlying the phenomena associated with LA microchemical analysis can be unraveled. Data from microanalysis (mass removal per pulse, crater volume, ICP intensity, LIBS characteristics, etc.) versus laser intensity using a suite of available pulsed lasers indicates five distinct regimes with four transition points or thresholds. These transitions characterize the changes in the dominant ablation mechanism for different ranges of laser intensities (Figure 1b). The first transition point at the lower laser intensity represents the onset of ablation or irreversible microscale sample surface damage (usually invisible to the naked eye).

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Threshold 1: The onset of ablation For laser intensities below the ablation threshold, bulk massremoval suitable for microchemical analysis does not occur, although individual atoms or ions may be removed or desorbed from the sample surface. The study of laser–material interactions in this low-intensity regime will help researchers understand the microscopic pathway of mass removal induced by above-threshold laser intensities (22). When a pulsed laser beam impinges on a sample surface, laser-induced electronic excitation can be significant although melting and vaporization do not occur. A high density of laser-excited electrons can be inferred by examining time-resolved optical absorption inside a laser-irradiated transparent sample on the femtosecond timescale. Figure 2a shows an absorption image of a slab of SiO2 irradiated by an ultrashort (100 fs, 800 nm) pump laser pulse. Electronically induced optical absorption is evident along the path of the pump pulse, which arrives at the sample surface 200 fs earlier than the probe pulse (100 fs, 400 nm). Initially, light-induced hot electrons are generated that are not in equilibrium with the sample material or even with the

Eads

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Ef

FIGURE 2.Electronic excitation on a femtosecond timescale. (a) Absorption of a 100-fs, 400-nm laser beam by photoexcited electrons inside an SiO2 sample. Electrons were excited by a 100-fs, 800-nm laser pulse. The color bar indicates the transmittance scale for a 400-nm probe laser beam. (b) Schematic of energy exchange between a photoexcited electron and an adsorbate. Ef, Evac, and Eads are the Fermi energy, the vacuum energy, and the energy of the adsorbate, respectively.


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FIGURE 3.Development of an ionized plume of sample mass. (a) Spatial–temporal plume images recorded at three different times (35-ps, 1064-nm, single-pulse LA of copper with laser intensity ~1012 W/cm2). (b) Shielding of ablation laser pulse by the mass plume (30-ns, 248-nm LA of brass).

electron gas itself. Electron emission can take place at the sample surface through a combination of thermionic and photoelectric effects, when surface electrons acquire sufficient energy to surmount a surface energy barrier (work function) (23). For electrons not leaving the sample surface, there is competition between different relaxation processes (localized or delocalized) that generally include collisions with phonons and plasmons, defects and impurities, and electrons and holes (22). Desorption of isolated atoms or ions may take place if localized relaxation processes dominate. For instance, the absorbed laser energy of the electronic system may be transferred to a single site of atomic scale, such as an adsorbate or a defect. If this energy resides at the site long enough, typically a few vibrational periods, chemical bonds may be broken and the atom or ion will

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leave its equilibrium position. Figure 2b illustrates schematic energy transfer to an adsorbate on a metal substrate (24). The photoexcited electrons of the metal substrate can tunnel through a surface barrier and become attached to the adsorbed atom to form a negative ion. After a short time, the electron is scattered back into an unoccupied electronic state of the substrate, leaving the adsorption system excited with an energy that may be large enough to free the adsorbate. The lifetime (~ femtoseconds) of the excited electrons inside the adsorbate can be determined from ultrafast pump–probe measurements, which have only been possible recently, thanks to advancements in femtosecond lasers (25, 26). As laser intensity increases, at the onset of ablation, a substantial fraction of the laser-irradiated region is removed. The simplest mechanism of ablation onset is laser-induced melting and vaporization, along with energy dissipation into the bulk of the sample through heat conduction. For laser wavelengths in the UV to near-IR range, the initial stage of ablation is the absorption of photon energy by free or bound electrons inside the sample. These energetic electrons (and holes for nonmetallic samples) collide with lattice phonons and subsequently pass energy to the lattice through delocalized relaxation mechanisms. When the lattice temperature exceeds the melting point of the sample material, irreversible surface damage will occur at the laser-irradiated spot. If additional laser energy is delivered to the melted sample, it will significantly vaporize. Because the pressure of the rapidly vaporized sample material is usually much larger than the ambient, a plume of mass vapor will form and expand away from the sample surface. Because thermal properties such as melting and vaporization temperatures vary by orders of magnitude, LA of multielement alloys may result in fractionation; that is, the ablated mass has a different composition from that of the sample. For example, in a classic LA-ICP experiment using a nanosecond laser to ablate a brass sample composed of 35% zinc (Tboiling = 907 K) and 65% copper (Tboiling = 2567 K), the vapor phase contained a significantly higher percentage of zinc for laser intensities just above the ablation threshold (27 ). Fractionation influences accurate chemical analysis, because the ablated mass composition differs from the actual sample composition. Although it is still a challenge, substantial research efforts have been devoted to understanding and eliminating elemental fractionation in LA microchemical analysis (14, 28–32). Laser wavelength, pulse duration, and transport of the ablated mass may contribute to fractionation for LA.

Threshold 2: Shielding by ionized mass As the laser intensity increases, the expanding mass plume of ablated sample material can partially ionize (33). This ionized plume contributes to the frequently observed reduction of ablation efficiency and other measurable effects at moderately high laser intensities. The associated behavior at threshold 2 is due to the shielding or absorption of the trailing part of the pulsed laser beam by the ionized mass plume from the sample. Because electron emission starts during the early stage of the LA process, the emitted electrons gain energy from the incom-

Analytical applications of LA cover a range of fields, including geology, environmental science, forensics, semiconductor manufacturing, and archaeology. A reduction of fractionation when laser intensity is increased has been observed in LA-ICP experiments (27 ).

Threshold 3: Explosive ablation Recent experimental and theoretical work implies that at higher laser intensities (>20 GW/cm2 for 3-ns, 266-nm LA of silicon), the sample surface could approach a state characterizing the material’s thermodynamic critical point. Near the critical point, the sample will experience a rapid transition from superheated liquid to a mixture of vapor and liquid droplets, which eject explosively from the sample surface. Although not yet confirmed by experiments, it is possible that fractionation may be reduced in this laser intensity regime because sample mass is removed in the form of micrometer-size droplets or particles. From the point of view of energy balance, the phase transition is essentially from solid to liquid to vapor before the critical temperature is reached. The energy necessary to transfer mass from solid to vapor includes a large fraction of absorbed laser energy to satisfy the latent heat of melting and vaporization. After the sample surface passes the critical temperature, the densities of the liquid and vapor allow them to merge smoothly, and hence, there is

(a)

100 µm

ing laser pulse through collisions with the ionized mass vapor leaving the sample surface. This is an example of the inverse Bremsstrahlung process—resistive damping of the light wave due to electron–ion collisions (34). Impact ionization of the mass vapor occurs when the kinetic energy of the energetic electrons is higher than the ionization potential of the evaporated atoms from the sample. As a result of vapor ionization, the ionized mass plume may strongly absorb the laser energy and shield the sample surface from further exposure to the incident laser light (the trailing part), thereby reducing the efficiency of mass removal. Figure 3a shows time-resolved shadowgraph images of the laser-ablated mass plume obtained from pump–probe experiments. The mass plume typically has a hemispherical shape that, after its emergence, will undergo gas dynamic expansion above the sample surface with a velocity typically on the order of 105–106 cm/s. The laser-ablated mass plume above the sample surface will significantly develop and ionize, usually in a matter of nanoseconds. Consequently, the absorption of laser energy by the ionized vapor plume of sample mass may be negligible, with a laser pulse duration much less than 1 ns. This shielding effect is present at the moderately high laser intensities used in commercial LA microchemical analysis systems, which typically use nanosecond lasers (~ 1–100 ns), several millijoules of energy, and spot sizes on the order of 10–200 µm. Time-resolved transmission measurements provide direct evidence of the shielding of the sample by the ionized mass plume. Transmitted laser temporal profiles were recorded as a function of laser intensity in an experiment using a 30-ns UV laser beam to ablate a glass sample (35). The transmitted laser profile was similar to the original laser pulse until the intensity of the ablation laser exceeded threshold 2. The weakly ionized mass plume absorbed ~50% of the incident laser energy at ~1-GW/cm2 intensity. A series of transmitted laser temporal profiles at various laser intensities is shown in Figure 3b. As intensity increased, shielding became more significant in reducing the efficiency of ablation (truncated transmission of pulse duration). For microanalysis of multicomponent alloys using moderately high laser intensities, fractionation may be less than that observed with ablation just above the onset at threshold 1. It is expected that when energy deposited into such a sample greatly exceeds the vaporization latent heat of all the constituents of the sample, the thermal properties of individual components will play a relatively small role in mass removal. In other words, all constituents can be vaporized and subsequently removed. In addition, with an increase in laser intensity, impact ionization as well as multiphoton absorption may take place before the formation of an expanding vapor plume, because a dense local electron excitation can be created at or near the sample surface. This nonthermal ablation mechanism may also contribute to the reduction of fractionation, because an increased percentage of sample mass can be removed independent of the thermal properties of the sample’s constituents.

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FIGURE 4.Droplet ejection on the microsecond timescale. (a) Shadowgraph images of droplet ejection (3-ns, 266-nm, single-pulse LA of silicon with laser intensity ~8.7 ⫻ 1010 W/cm2). (b) Crater profiles generated with laser intensities below and above the threshold for explosive ablation (3-ns, 266-nm, single-pulse LA of silicon).


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no latent heat contribution to the transition. As a consequence, when the sample surface temperature approaches the critical temperature, which is typically several times the vaporization temperature, the same amount of laser energy absorbed by the sample surface can convert significantly more mass into the vapor phase. With high laser intensity, the irradiated sample volume can be heated above its normal boiling temperature and become metastable (36, 37 ). Near the thermodynamic critical state, density fluctuations may generate vapor bubbles inside the superheated liquid sample volume. The rate of homogeneous vapor nucleation rises catastrophically near the critical temperature. Bubbles will grow if their radii are greater than certain critical values. Once large bubbles are generated in the superheated liquid layer, the sample will undergo a rapid transition

marily from the sample surface instead of from direct laser ionization of the air (39). At the early stage of laser irradiation, the emitted electrons (thermionic and photoelectric emission) from the sample surface collide with air molecules and absorb laser energy principally by inverse Bremsstrahlung processes. These energetic electrons subsequently ionize the air which expands rapidly during the laser pulse (the electrons contribute to the ionized air plasma expansion). The resulting early stage electronic plasma may absorb a significant amount of energy from the pulsed laser beam. As a result, mass-removal efficiency will drop due to a lower rate of energy hitting the sample. Approximately 50% of the incident laser energy may be absorbed by this early stage electronic plasma, as demonstrated in an experiment using a 35-ps LA of metallic material (40). Figure 5 shows three shadowgraph images of the electronic plasma recorded at different times after the pump laser beam irradiates the sample (38). The experiment was conducted using a 35-ps, 1064-nm laser pulse to ablate copper. The electronic plasma forms in picoseconds and immediately expands longitudinally. The cone-shaped geometry of the electronic plasma is substantially different from that of the mass plume, which has a hemispherical shape and forms in nanoseconds (Figure 3a). Because the electronic plasma forms in picoseconds, it will absorb the rising part of the laser energy for nanosecond LA, in contrast to the ionized mass vapor that absorbs the trailing part of the laser energy. In this high-intensity laser regime, the ablation efficiency will be reduced by plasma absorption, in particular for picosecond and short nanosecond LA. For microchemical analysis, mass or optical spectra of the expanding mass plume may yield stoichiometric representation of the sample, because the electron plasma and the sample mass plume develop on picosecond versus nanosecond timescales. The electronic plasma formed during the early stage of the laser pulse may help reduce elemental fractionation by interacting with the sample and enhancing surface ionization.

Depending on the detection system, subpicogram quantities may be sufficient for analysis, rendering the technique essentially nondestructive. into a mixture of vapor and liquid droplets. Rapid expansion of the high-pressure bubbles in the melted sample will lead to violent ejection of molten droplets from the target surface. Experimentally, threshold 3 marks the transition at which the ablation mechanism changes to explosive ablation or phase explosion. Time-resolved shadowgraph images reveal that violent ejection of particles occurs on the microsecond timescale in this intensity range (20). Figure 4a shows images of particle ejection obtained at different times using pump–probe experiments to ablate a silicon surface with a 3-ns UV (266-nm) laser. Images recorded after ~1 µs clearly indicate the ejection of bulk mass from the sample surface, which appears to be in the form of micrometer-size particles or droplets. Evaporation of atomic, ionic, and molecular masses and shock wave propagation occur in nanoseconds. The ejection of droplets typically takes tens of microseconds. In this explosive ablation regime, craters in the sample are usually substantially deeper than those created in the lower intensity regime. Figure 4b shows two exemplary crater profiles for laser intensities (single laser pulse) below (18 GW/cm2) and above (21 GW/cm2) the threshold for explosive ablation. There is a substantial increase in the volume of the craters for a nominal increase in intensity.

Threshold 4: Shielding by electronic plasma The last transition point in Figure 1b, threshold 4, is related to a recently discovered laser plasma that forms in the early stages of the ablation process. When a focused, high-power, pulsed laser beam ablates a sample in an ambient gas such as air, a plasma may form on the picosecond timescale—long before the emergence of the mass plume of vaporized sample material. Pump–probe interferometry experiments performed at atmospheric pressure in air showed that the electron number density for this early stage plasma was ~1020 cm–3 (on the picosecond timescale) (38). This high density indicates that electrons within this plasma come pri-

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Plume of the future We need to understand the physics of LA in microchemical analysis because matrix matched standards are not available for many unknown samples. When matrix matched standards are available, and the ablation behavior of the standards is similar to that of the unknown sample, calibration curves and quantitative data can be obtained. We hope that, by understanding the physics of LA, analysts will be able to elucidate a set of conditions and specify a priori the quantity and composition of the ablated mass from any sample. The goal is to achieve matrix independence so that the quantity ablated is independent of the samples’ thermooptical properties and the vapor is stoichiometric (no fractionation). Fractionation in LA is a function of intensity and is reduced at higher laser intensities. It may be possible to use high-power, ultrashort (femtosecond) lasers to eliminate the fractionation (41, 42). When the intensity of an ultrashort laser pulse is large

This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division, and the Office of Nonproliferation and National Security at the Lawrence Berkeley National Laboratory under contract DE-AC03–76SF00,098.

Richard E. Russo is a senior scientist and Xianglei Mao and Samuel S. Mao are scientists in the advanced laser technologies group at Lawrence Berkeley National Laboratory. All three authors are interested in laser materials interactions. Russo’s research interests also include microchemical analysis and pulsed laser deposition. Xianglei Mao is also interested in non-linear optics. Samuel Mao is also interested in nanotechnology. Address correspondence about this article to Russo at Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., MS 70-108B, Berkeley, CA 94720 ([email protected]).

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enough (terawatt levels), very dense electronic excitation (number of electrons is 1022 cm–3) can be achieved in the sample, which may place individual ions of the irradiated volume layer into an anti-bonding state. The sample could then undergo a transition from a tightly bound solid state to a high-pressure gas of densely packed ions rapidly leaving the sample surface through, for example, Coulombic repulsion. Such an ablation process would be less dependent on the elemental thermal properties that influence the composition of the mass ablated by nanosecond pulsed lasers. High-power, femtosecond LA may be a promising scheme for stoichiometric ablation in microchemical analysis. Experiments using this time regime and terawatt intensities are just beginning. Through continuous research efforts in studying the underlying physics of LA, the best conditions will be identified for a rich variety of microchemical analysis applications.

50 ps

150 ps

500 ps

FIGURE 5.Development of an electronic plasma (35-ps, 1064-nm, single-pulse LA of copper with laser intensity ~4 ⫻ 1012 W/cm2).

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The Physics of Laser Ablation in Microchemical Analysis

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