Introduction

Various theoretical models have been proposed to describe material removal from a solid surface heated by laser irradiation. The thermal models of Afanas'ev and Krokhin (1967), Anisimov (1968), and Olstad and Olander (1975) represent early theoretical contributions to this problem. Chan and Mazumder (1987) developed a one-dimensional steady-state model describing the damage caused by vaporization and liquid expulsion due to laser–material interaction. Much of the above work was driven by laser applications such as cutting and drilling, and was thus focused primarily on modification of the target's morphology, with no particular interest in the detailed description of the properties and dynamics of the evaporated and ablated species. Moreover, these models dealt with continuous-wave (CW) laser sources, or relatively long (millisecond) time scales.

During the first stage of interaction between the laser pulse and the solid material, part of the laser energy is reflected at the surface and part of the energy is absorbed within a short penetration depth in the material. The energy absorbed is subsequently transferred deeper into the interior of the target by heat conduction. At a later stage, if the amount of laser energy is large enough (depending upon the pulse length, intensity profile, wavelength, and thermal and radiative properties of the target material), melting occurs and vaporization follows. The vapor generated can be ionized, creating high-density plasma that further absorbs the incident laser light. Effects of this laser-plasma shielding have been shown via the simplified one-dimensional model of Lunney and Jordan (1998).

Introduction

Lasers that can produce coherent photon pulses with durations in the femtosecond regime have opened up new frontiers in materials research with extremely short temporal resolution and high photon intensity. The ultrafast nature of femtosecond lasers has been used to observe, in real time, phenomena including chemical reactions in gases (Zewail, 1994) and electron–lattice energy transfer in solids (Shah, 1996). On the other hand, ultra-short laser pulses impart extremely high intensities and provide precise laser-ablation thresholds at substantially reduced laser energy densities. The increasing availability of intense femtosecond lasers has sparked a growing interest in high-precision materials processing. In contrast to material modification using nanosecond or longer laser pulses, for which standard modes of thermal processes dominate, there is no heat exchange between the pulse and the material during femtosecond-laser–material interactions. As a consequence, femtosecond laser pulses can induce nonthermal structural changes driven directly by electronic excitation and associated nonlinear processes, before the material lattice has equilibrated with the excited carriers. This fast mode of material modification can result in vanishing thermal stress and minimal collateral damage for processing practically any solid-state material. Additionally, damage produced by femtosecond laser pulses is far more regular from shot to shot. These breakdown characteristics make femtosecond lasers ideal tools for precision material processing.

Thorough knowledge of the short-pulse-laser interaction with the target material is essential for controlling the resulting modification of the target's topography.

Introduction

Effective contamination control and development of an efficient cleaning technology are critical in the semiconductor-device manufacturing and data-storage industry (Mittal, 1988). Especially, sub-micrometer-contaminant removal is becoming more and more important as tighter microscale integration of devices is constantly being pursued in the industry. The most effective way to solve the contamination problem is to avoid contamination by adequate design of a manufacturing process based on careful analysis of the contamination sources. However, in many cases, the process itself is a source of contamination and the development of a cleaning tool may often be unavoidable. In fact, a large percentage of the fabrication cost is attributed to several elaborate cleaning steps. Several conventional cleaning techniques are currently in wide industrial use. Nevertheless, laser cleaning (LC) is attractive because of the following advantages over the conventional cleaning techniques.

• It is effective for sub-micrometer- to macroscopic-sized contaminants.

• The cleaning process is environmentally sound, not involving bulk usage of toxic solvents.

• The chance of causing mechanical damage to delicate parts is relatively small.

• Selective cleaning of a part is possible through search-and-clean procedures.

Much research work has been done on LC schemes for a variety of substrates since the 1980s. A few notable examples of earlier work should be mentioned. Zapka et al. (1989) demonstrated LC applied to the cleaning of electron-beam-lithography masks (delicate Si membranes of thickness just 3 μm with transmission apertures of dimensions about 1 μm); such structures are too vulnerable to damage or contamination if cleaned by conventional megasonic techniques, scrubbing/wiping, high-pressure jets or other means but were found to be effectively cleanable simply by irradiation with a few ultraviolet (UV) laser pulses at a wavelength of 248 nm and energy fluence typically lower than 0.3 J/cm2.

Introduction

Fundamental understanding of microscale phenomena has been greatly facilitated in recent years, largely due to the development of high-resolution mechanical, electrical, optical, and thermal sensors. Consequently, new directions have been created for the development of new materials that can be engineered to exhibit unusual properties at sub-micrometer scales. Surface engineering is a critical sub-field of nanotechnology because of the paramount importance of surface-interaction phenomena at the micro/nano-machine level. Nanofabrication of complex three-dimensional patterns cannot be accomplished with conventional thermo-chemo-mechanical processes. While laser-assisted processes have been effective in component microfabrication with basic dimensions in the few-micrometer range, there is an increasing need to advance the science and technology of laser processing to the nanoscale. Breakthroughs in various nanotechnologies, such as information storage, optoelectronics, and bio-fluidics, can be achieved only through basic research on nanoscale modification and characterization of surfaces designed to exhibit special topographical and compositional features at high densities.

Since their invention in the 1980s, scanning-microscopy techniques such as scanning tunneling microscopy (STM), atomic-force microscopy (AFM), scanning near-field optical microscopy, and further variants thereof, have not only become indispensable tools for ultrahigh-resolution imaging of surfaces and measurement of surface properties but also offered hitherto unexplored possibilities to locally modify materials at levels comparable to those of large atoms, single molecules, and atomic clusters. These nanometric investigation tools have been used extensively in numerous high-resolution nanostructuring studies, to manipulate single atoms, and also as effective all-inclusive nanofabrication tools.

Lasers are effective material-processing tools that offer distinct advantages, including choice of wavelength and pulse width to match the target material properties as well as one-step direct and locally confined structural modification. Understanding the evolution of the energy coupling with the target and the induced phase-change transformations is critical for improving the quality of micromachining and microprocessing. As current technology is pushed to ever smaller dimensions, lasers become a truly enabling solution, reducing thermomechanical damage and facilitating heterogeneous integration of components into functional devices. This is especially important in cases where conventional thermo-chemo-mechanical treatment processes are ineffective. Component microfabrication with basic dimensions in the few-microns range via laser irradiation has been implemented successfully in the industrial environment. Beyond this, there is an increasing need to advance the science and technology of laser processing to the nanoscale regime.

The book focuses on examining the transport mechanisms involved in the laser–material interactions in the context of microfabrication. The material was developed in the graduate course on Laser Processing and Diagnostics I introduced and taught in Berkeley over the years. The text aims at providing scientists, engineers, and graduate students with a comprehensive review of progress and the state of the art in the field by linking fundamental phenomena with modern applications.

Samuel S. Mao of the Lawrence Berkeley National Laboratory and the Mechanical Engineering Department of UC Berkeley contributed major parts of Chapters 5, 6, and 9. I wish to acknowledge the contributions of all my former and current students throughout this text.

Rapid vaporization of liquids on a pulsed-laser-heated surface

Background

The laser-beam interaction with materials in liquid environments exhibits unique characteristics in a variety of technical applications. The explosive vaporization of liquids induced by short-pulsed laser irradiation is utilized in laser cleaning to remove micro-contaminants (Park et al., 1994) and in medical laser surgery. Physical understanding of superheated liquids and liquid-to-vapor phase transitions has been sought in order to achieve better control of such applications. The transient development of the bubble-nucleation process and the onset of phase change were monitored by simultaneous application of optical-reflectance and -scattering probes (Yavas et al., 1993). The numerical heat-conduction calculation also shows that the solid surface achieves temperatures of tens of degrees of superheat (Yavas et al., 1994). Real-time measurement of the surface temperature transient in the course of the laser-induced vaporization process is needed, since the surface temperature is an important parameter in heterogeneous nucleation. The kinetics of heterogeneous bubble nucleation and the growth dynamics have long been a subject of intense research interest (Skripov, 1974; Stralen and Cole, 1979; Carey, 1992).

Enhanced pressure is produced in the interaction of short-pulsed laser light with liquids (Sigrist and Kneubühl, 1978). The efficient coupling of laser light into pressure is of interest in many technical and medical areas, such as laser cleaning to remove surface contaminants, laser tissue ablation, corneal sculpturing (Vogel et al., 1990), and gall-stone fragmentation (Teng et al., 1987).

Lasers for materials processing

Lasers (the acronym from light amplification by stimulated emission of radiation), with their unique coherent, monochromatic, and collimated beam characteristics, are used in ever-expanding fields of applications. Different applications require laser beams of different pulse duration and output power. Lasers employed for materials processing range from those with a high peak power and extremely short pulse duration to lasers with high-energy continuous-wave output.

Continuous-wave – millisecond – microsecond lasers

Continuous-wave (CW) and long-pulsed lasers are typically used to process materials either at a fixed spot (penetration material removal) or in a scanning mode whereby either the beam or the target is translated. Millisecond- and microsecond-duration pulses are produced by chopping the CW laser beam or by applying an external modulated control voltage. Fixed Q-switched solid-state lasers with pulse durations from tens of microseconds to several milliseconds are often used in industrial welding and drilling applications. Continuous-wave carbon dioxide lasers (wavelength λ = 10.6μm and power in the kilowatt range) are widely employed for the cutting of bulk and thick samples of ceramics such as SiN, SiC, and metal-matrix ceramics (e.g. Duley, 1983). Continuous-wave laser radiation allows definition of grooves and cuts. On the other hand, low-power CO2 lasers in the 10–150-W range are used for marking of wood, plastics, and glasses. Argon-ion lasers operating in the visible range (λ = 419–514 nm) are utilized for trimming of thick and thin resistors. In the biomedical field various CW lasers have been used.

Laser chemical vapor deposition

Consider a target material immersed in a reactive ambient medium. An incident laser beam may excite and dissociate the reactant molecules. Consequently, excited molecules or radicals diffuse to the solid surface and may interact with the target surface, resulting in etching or deposition. These processes are thoroughly discussed in Bäuerle (2000). Figure 13.1 gives a schematic illustration of the laser-induced chemical-processing systems utilizing either direct beam incidence onto the substrate or processing via a beam propagating in a direction parallel to the substrate. Thermal or pyrolytic chemical laser processing is characterized by the rapid dissipation of the excitation energy into heat. In this case, the particular excitation mechanisms are not significant and the processing rate is chiefly determined by the induced temperature distribution. However, the physicochemical processes involved may be drastically different from the conventional “thermal-processing” treatment. This is highlighted by the extremely confined laser-beam radiant energy and hence the temperature-distribution localization to high peak temperatures and steep temperature gradients. Furthermore, pulsed-laser-induced heating rates can be very fast, reaching 1012 K/s, even 1015 K/s, leading to a regime where the chemical reaction deviates greatly from equilibrium. Consequently, one may expect the formation of new phases, microstructures, and morphologies through novel chemical-reaction pathways. On the other hand, photochemical or photolytic laser chemical-processing conditions apply when the thermalization of the excitation energy is slow.

Introduction

Laser ablation of polymers and other molecular materials constitutes the basis for a range of well-established applications, such as matrix-assisted laser desorption–ionization (MALDI) (Hillenkamp and Karas, 2000), laser surgery (Niemz, 2003) including the widely used laser-assisted in situ keratomileusis (LASIK) technique, surface microfabrication and lithography (Lankard and Wolbold, 1992), and pulsed laser deposition (PLD) of organic coatings (Bäuerle, 2000; Chrisey and Hubler, 1994). Interaction of UV laser pulses with an organic substance typically results in photothermal and/or photochemical processes in the irradiated material. Generally, photothermal processes, which produce heat in the sample, dominate when the laser photon energy is small, whereas photochemical processes occur when the laser photon energy is larger than the chemical-bond energies of the molecules.

For lasers operating at near-IR wavelengths, photothermal processes usually play a major role. With deep-UV (wavelength shorter than ∼200 nm) laser irradiation, in which the photon energy is larger than the typical energy of the chemical bonds of molecules, photochemical processes are usually responsible for the onset of ablation. For laser ablation of organic materials with wavelengths between the near IR and deep UV, photothermal and photochemical processes are often interrelated (Ichimura et al., 1994). On the other hand, the characteristics of material ejection depend on the nature of the ablation process (Srinivasan, 1986; Georgiou et al., 1998). For instance, ablation of organic materials results in little lateral damage in the sample when the photochemical processes are dominant.