Jim J. Chang

Laser‐plasma interaction during visible‐laser ablation of methods

We have investigated the dynamics of high‐radiance visible‐laser interaction with a metal vapor plasma during laser ablation of aluminum and carbon steel. The experiment with a copper vapor laser reveals strong plasma‐absorption induced ignition at laser intensities above 1–2 GW/cm2. Approximation based on hydrodynamic relations indicates that the vapor density at the end of the 40‐ns laser pulse is 3×1020–1×1021 cm −3 with a pressure of a few thousand atmosphere at the target surface. This high‐density vapor with a temperature exceeding 10 000 K leads to pronounced plasma absorption via photoionization. Plasma absorption via inverse bremsstrahlung is determined to be negligible because of a relatively low electron density, measured to be peaked at ∼5×1018 cm−3.

Precision micromachining with pulsed green lasers

We have developed high-precision machining based on high-power pulsed green lasers (30–40 ns pulse width) at multi-kHz repetition rate. Dynamics of material removal has been investigated using a copper vapor laser. We found that noticeable surface evaporation starts to appear as laser intensity exceeds 107 W/cm2. Material removal is then dominated by ablation at higher laser intensities. However, strong plasma absorption starts to appear as laser intensity exceeds 2 GW/cm2. This prolongs material heating by hot plasma via electron conduction, resulting in noticeable melt formation and expulsion. Maintaining laser radiance below the plasma-ignition threshold minimizes this melt formation. The optimum rate of ablation on metals was found to be ∼1 μm/pulse with a laser fluence of 50 J/cm2. Higher material removal rate can be achieved at higher fluence, but is mostly accompanied with unwanted melt formation and ejection. By keeping laser intensity within a few GW/cm2, we have demonstrated high-aspect-ratio machining with micron-scale accuracy and negligible heat affected zone. High-quality percussion drilling, trepanning, grooving, and slotting were demonstrated on metals and ceramics with a negligible heat affected zone. Straight holes with sizes varying from 500 to less than 25 μm were consistently drilled with a height-to-diameter ratio up to 40. The high quality machining with scalable machining speed promises expanded use of pulsed green lasers in micromachining.We have developed high-precision machining based on high-power pulsed green lasers (30–40 ns pulse width) at multi-kHz repetition rate. Dynamics of material removal has been investigated using a copper vapor laser. We found that noticeable surface evaporation starts to appear as laser intensity exceeds 107 W/cm2. Material removal is then dominated by ablation at higher laser intensities. However, strong plasma absorption starts to appear as laser intensity exceeds 2 GW/cm2. This prolongs material heating by hot plasma via electron conduction, resulting in noticeable melt formation and expulsion. Maintaining laser radiance below the plasma-ignition threshold minimizes this melt formation. The optimum rate of ablation on metals was found to be ∼1 μm/pulse with a laser fluence of 50 J/cm2. Higher material removal rate can be achieved at higher fluence, but is mostly accompanied with unwanted melt formation and ejection. By keeping laser intensity within a few GW/cm2, we have demonstrated high-aspect-ratio ma...

Time-resolved beam-quality characterization of copper-vapor lasers with unstable resonators

Beam quality (BQ) of a 4-cm copper-vapor laser (CVL) with unstable resonators of different magnifications was characterized based on time-resolved far-field measurement. It was found that the BQ improvement after each round trip of the cavity cannot be predicted correctly from resonator theory. With a cavity Fresnel number of ~ 300, the achievable CVL BQ at the later part of the pulse was limited to approximately 4 times diffraction limited (×DL), even with a cavity magnification of 130. A pronounced temporal BQ oscillation, which is synchronized with the temporal pulse modulation, was also observed throughout the entire pulse. Examination of the temporal evolution of the far-field spot with use of a gated camera revealed that the strong presence of amplified spontaneous emission (ASE) in the cavity during the entire laser pulse severely limited the achievable BQ because of consecutive cavity feedback that included this highly divergent ASE. BQ deterioration caused by intense ASE throughout the pulse was reduced when a cavity with a smaller Fresnel number was used.

Pressure dependence of copper laser output characteristics

Output characteristics of a large-bore (8-cm-diameter) copper laser were analyzed at different neon pressures. Radial delay decreased from ∼40 ns at 30 Torr to ∼10 ns at 110 Torr because of increased plasma impedance. When the laser was running as an amplifier, the best power was achieved at approximately 80 Torr (small-bore copper lasers normally optimize at 30-40 Torr). This occurs because a more pronounced reduction of radial delay at higher pressures can be achieved in large-bore devices. As a result, a more uniform beam profile was obtained at higher pressures because of more efficient axial pumping. The improved coupling between the laser head and the pulse modulator at higher pressures also translates to a substantial improvement in laser efficiency (>50%) as the pressure rises from 30 to 100 Torr. The same laser optimizes at a much lower pressure (30-40 Torr) when its energy is extracted with a flat-flat resonator, because resonator loss increases with increasing gas pressure because of faster gain rise time at higher pressures.

Copper-laser oscillator with adjoint-coupled self-filtering injection

A new injection-controlled laser resonator developed to achieve diffraction-limited beam quality for high-gain short-pulse lasers is reported. The resonator is seeded with a short-pulse laser signal by adjoint-coupled injection. The two-times diffraction-limited injection beam is self-filtered through a prepulse cavity propagation to improve its beam quality. The use of a self-imaging unstable resonator diminishes the edge-diffraction-induced beam deterioration. A beam quality of 1.1-1.3 times diffraction limited is achieved throughout the entire 70-ns laser pulse of a 30-W copper-vapor laser.

Beam characteristics of a large-bore copper laser with radiatively cooled plasma

In a large-bore copper vapor laser (CVL), excessive gas heating at the axial region of the discharge lowers its efficiency by thermally populating the metastable lower laser levels. The associated lower gas density also lengthens the discharge field- diffusion time, leading to weaker axial pumping and undesired beam characteristics. A novel approach to circumvent this obstacle has been developed by cooling the plasma radiatively via a series of segmented metal plates (septa) placed vertically along the length of the tube. This improved tube design significantly lowers the average gas temperature and shortens the radial delay. A 27% increase in laser power was observed with the addition of septa. We have characterized the beam intensity profile, spatial and temporal pulse variation, and beam polarization through extensive laboratory measurements. A detailed computational model of the laser has been used to characterize and interpret the laboratory results.

High-power copper vapour lasers and applications

Expanded applications of copper vapor lasers has prompted increased demand for higher power and better beam quality. This paper reports recent progress in laser power scaling, MOPA operation, beam quality improvement, and applications in precision laser machining. Issues such as gas heating, radial delay, discharge instability, and window heating will also be discussed.

Development of an electric capillary discharge source

We report on the development of an electric capillary discharge source that radiates with comparable efficiency at both 13.5 nm and 11.4 nm, two wavelengths of interest for EUV lithography. The discharge source is comprised of a low- pressure, xenon-filled, small diameter capillary tube with electrodes attached to both ends. A high-voltage electric pulse applied across the capillary tube generates an intense plasma that radiates in the EUV. This source is capable of producing 7 mJ/steradian per pulse in a 0.3 nm bandwidth centered at 13.4 nm. In this paper we will address three significant issues related to the successful development of this source: minimization of debris generation, thermal management, and imaging quality.

Improvement of high-current large-volume discharge with profiled hollow-cathode electrodes

Filamentary discharge at the electrode of high-power copper vapor lasers prevents stable operation when the buffer gas (Ne) pressures exceeds 30 torr. Experimental evidence indicates that the discharge constriction starts in the cathode fall region. The volumetric power deposition at the cathode fall is estimated to be /spl sim/130 MW/cm/sup 3/ at 40 torr during a peak discharge current of 2.5 KA. This highly pressure-dependent (/spl alpha/ P/sup 7/3/) large thermal loading at cathode fall is likely to initiate thermal instability as pressure increases. This discharge instability can be mitigated by taking advantage of the hollow-cathode effect even for pressures exceeding 100 torr. We have designed a large-area electrode with many hollow-cathode grooves spreading over the uniform-field-profile area of the electrode for enhanced electron emission. This unique design lowers the cathode-fall voltage, and as a consequence, reduces the thermal loading at the cathode fall. With this profiled, hollow-cathode electrode, we have successfully extended stable discharge from /spl sim/30 to /spl sim/100 torr. We believe this profiled electrode design with multiple hollow-cathode grooves will expand hollow-cathode electrodes to many applications that require high-current large-volume discharge at elevated pressures.

Industrial applications of high-power copper vapor lasers

A growing appreciation has developed in the last several years for the copper vapor laser because of its utility in ablating difficult materials at high rates. Laser ablation at high rates shows promise for numerous industrial applications such as thin film deposition, precision hole drilling, and machining of ceramics and other refractories.