1、飞秒激光器的应用Industrial femtosecond lasers and material processing01/22/2019NORMAN HODGSON, MICHAEL LAHA, TONY S. LEE, ALBRECHT STEINKOPFF,andSEBASTIAN HEMINGOver the last five years, material processing with femtosecond pulses in the range of 300 to 900 fs has gained in popularity due to the small heat-
2、affected zone (HAZ) and increased energy penetration depth resulting from the high laser pulse intensity. Industrial ultrashort-pulse (USP) diode-pumped solid-state and fiber lasers are now being used to cut foils for flat panel displays, to cut stents, and to drill fuel injector nozzles, as well as
3、 for wafer scribing and surface microstructuring.The first industrial use of femtosecond laser pulses for microprocessing dates back to the late 1990s, where titanium sapphire (Ti:sapphire) amplifiers were used to repair lithography masks in integrated circuit fabrication. At that time, the only cho
4、ice in commercial laser sources wereQ-switched, neodymium (Nd)-doped solid-state lasers delivering pulse duration of tens of nanoseconds, and ultrafast Ti:sapphire amplifiers that exhibited pulse durations of 100 fs and provided output power at the 1 W level at 1 kHz (FIGURE 1). The small feature si
5、ze of the chromium layer on top of the fused silica substrate prohibited the use of nanosecond pulses due to thermal damage of the chromium, and even the substrate. The necessity for a small HAZ outweighed the high cost of a Ti:sapphire amplifier system, which was about $300,000.FIGURE 1.Lithography
6、 mask repair using a 100 fs Ti:sapphire laser is shown, where a chromium layer on a fused silica substrate is ablated; width of the ablated lines is 750 nm.1Today, many different femtosecond lasers are available, providing output pulse energies of up to 200 J, with average output powers in the kilow
7、att range. Especially over the last decade, a large variety of femtosecond solid-state laser and fiber laser architectures have entered the material processing field, all of them based on ytterbium (Yb)-doped gain materials. Average output powers of up to 100 W are currently employed in industrial a
8、pplications, with pulse durations between 300 and 900 fs and repetition rates of up to 2 MHz. Compared to the original Ti:sapphire amplifier systems, the output powers have increased by two orders of magnitude, while system cost has been considerably decreased at the same time.A majority of the low-
9、power lasers have been deployed in ophthalmic applications. In the 2000s, Nd:glass regenerative amplifiers were used to cut the corneal flap in LASIK surgery by generating a bubble plane inside the cornea. These Nd:glass systems were later replaced by Yb:fiber master-oscillator power-amplifiers (MOP
10、As), which allowed higher repetition rates at a lower cost (FIGURE 2). Typical pulse energies required for flap cutting are 24 J at repetition rates of 50 to 200 kHz and pulse durations of around 300 fs. A second, more recently emerging ophthalmic application is lens dissection as part of cataract s
11、urgery. In this case, the pulse energies employer are in the range of 20 to 40 J at repetition rates of 50 to 100 kHz, and pulse durations are preferably below 800 fs.FIGURE 2.The evolution of the deployment of femtosecond lasers in microprocessing applications is shown; Ti:sapphire regenerative amp
12、lifiers for mask repair have been replaced by picosecond Nd:YVO4lasers and Nd:glass regenerative amplifiers by femtosecond Yb fiber MOPAs.According to a forecast by Strategies Unlimited in 2016, the total market in 2019 for femtosecond and picosecond lasers used for material processing (including op
13、hthalmic applications) is projected to be $460 million (TABLE 1).2Half of this revenue is generated by picosecond lasers, which have been widely used in microelectronic manufacturing. The other half is split between ophthalmic femtosecond lasers ($136 million) and femtosecond lasers for non-biologic
14、al material processing ($98 million).TABLE 1.The 2016 Revenue Forecast of femtosecond and picosecond lasers in material processing is shown, where values for 2014 and 2015 are actual revenues; femtosecond lasers include ophthalmic lasers.2Mechanism and benefits of femtosecond laser processingThe int
15、eraction of femtosecond and picosecond pulses with matter is governed by the absorption of the light by the electrons and subsequent energy transfer to the lattice. In the case of metals, the photons are absorbed by the electron gas, which increases its temperatures to values of several 10,000C. The
16、 electrons will transfer their energy to the lattice within the electron-phonon relaxation time, which for most materials is in the range of 100 fs to 1 ps at room temperature. The lattice has about 100X higher heat capacity compared to the electrons. This leads to a substantial delay between the in
17、cidence of the laser pulse and the time when the lattice has reached melting temperature (FIGURE 3). For high laser fluences, the ablation of the heated material occurs several tens of picoseconds after the laser pulse is absorbed.FIGURE 3.Interaction of ultrafast pulses with a metal is shown. The e
18、lectron gas absorbs the laser light, leading to a hot, thermalized electron distribution within 100 fs; the lattice will heat up with a delay of 4 to 30 ps.The light-matter interaction for ultrashort laser pulses can be mathematically described by the Two-Temperature Model, which provides the tempor
19、al and spatial evolution of the temperature of electron gas and lattice by incorporating the coupling between both systems via the electronphonon relaxation time.3This model has been used very successfully over the last decades to calculate damage threshold fluence, ablation rates, and HAZ for ultra
20、short pulse processing (FIGURE 4).4-8The main results are that for pulses that are shorter than 10 ps, the damage threshold fluence remains constant, while for larger pulse durations, the threshold fluence increases in proportion to the square-root of the pulse duration, independent of incident lase
21、r pulse wavelength.FIGURE 4.Calculated temporal temperature distribution of electron gas (left) and lattice for copper after irradiation with a 100 fs pulse at an average fluence of 0.14 J/cm2and a wavelength of 800 nm.7Similarly, the HAZ remains constant for pulse durations below 10 ps, again indep
22、endent of the wavelength of the laser light. The basic reason for this behavior is the delay in the temperature increase of the lattice and of heat conduction into the material. This regime of pulsed laser-matter interaction is therefore referred to as cold ablation, since the lattice stays cold dur
23、ing irradiation by the laser pulse. This name is a bit misleading, though, since the material will have to reach melting temperature to induce ablation.The most interesting effect of ultrashort pulse interaction, however, is the increase in the energy penetration depth and ablation depth with decrea
24、sing pulse duration. Decreasing the pulse duration at a given energy fluence leads to an increase in the temperature of the electron gas and a simultaneous increase of the electron-phonon relaxation time.In a more mechanical model, this can be easily understood: the velocity of the electrons traveli
25、ng through the lattice can become as high as 100,000 m/s for femtosecond pulses due to the very high intensity, and this high speed results in deeper penetration of the electron into the lattice without transferring energy to the lattice.8 The material processing efficiency and quality depends on th
26、e duration of the laser pulses. For pulses in the nanosecond regime, the absorption of the laser pulse is determined by the linear optical absorption depth of the laser light and the energy dissipation is a result of heat conduction into the material (FIGURE 5). For pulses that are shorter than 10 p
27、s, the initial energy penetration depends strongly on the light intensity and leads to deeper penetration depth for femtosecond pulses. In addition, the lack of thermal conduction during the pulse and the time of lattice heating results in a very low HAZ. For metals, HAZ of less than 5 m can be achi
28、eved, while for plastic materials, the HAZ is typically in the range of 30 to 50 m.FIGURE 5.Absorption of a laser pulse in a medium is shown, where energy penetration is represented in blue and volume heated via heat conduction in red.The increase of the penetration depth for shorter pulses leads to
29、 an increase in the maximum ablation rates when the pulses become shorter than about 20 ps (as shown in FIGURE 6 for aluminum). As will be discussed subsequently, at any pulse duration the maximum ablation rate is achieved at a pulse fluence of about 7.5X the ablation threshold fluence. Compared toQ
30、-switched laser pulses with pulse durations of tens of nanoseconds, the increased electron velocity in the material leads to ablation rates for sub-picosecond pulses that are only a factor of three lower.FIGURE 6.Maximum ablation rates of aluminum with Coherents measured values (red dots) and using
31、values taken from data published by Breitling et al. (blue dots).9For pulsed lasers, ablation becomes most efficient at a pulse energy fluence equal to e2times the threshold fluence. This is a result of the saturation of the ablated volume with increasing pulse fluence. At a fixed output power, more
32、 volume can therefore be ablated if the energy fluence is lowered, while simultaneously increasing pulse repetition rate and therefore throughput (FIGURE 7). The maximum value is achieved at the optimum fluence. The ablation rate, C (in mm3/W/min), is given by:whereFis the peak fluence in J/mm2,Fthis the peak threshold fluence, anddis the energy penetration depth per pulse in millimeters.10Typical values for the energy penetration depth per pulse are in the range of 20 to 100 nm for metals, semiconductors, and plastics, and 500 nm for glasses and transparent crystals.
