Keng H. Leong
Improvements in femtosecond laser technology have allowed the development of turnkey integrated amplified systems
The development of ultrafast or femtosecond laser technology and the subsequent research on material interactions have resulted in many new and unique applications in diverse fields.1-4 In addition to the unique capability to ablate virtually any solid without significant heat effects, femtosecond lasers are used in multiphoton absorption for microscopy and diagnostics, terahertz generation, waveguide production for telecom components and tabletop X-ray sources. The improvement of ultrafast lasers from initial finicky and alignment-sensitive instruments to turnkey tools has made industrial applications much more viable.
There are hundreds of femtosecond laser oscillators installed mostly for diagnostic applications. Rudolph Technologies (www.rudolphtech.com), for example, uses a femtosecond oscillator as part of its picosecond ultrasonic laser sonar system for opaque film thickness determination. More than 200 installations are in operation in semiconductor plants. There are more than 400 installations of integrated amplified femtosecond laser systems in the world today, mostly at research and development facilities.
Femtosecond laser ablation of materials initially has been carried out in vacuum.5 Recent efforts have shown that ablation in ambient air can be achieved with similar results,4 thereby removing another hurdle to the acceptance of the technology. The availability of a turnkey amplified system resulted in the first (published) industrial femtosecond micromachining application as a dielectric mask repair tool used in photolithography replacing the pulsed Nd:YAG laser.6 The ultrashort pulses enabled removal of selected chromium layers on the mask without affecting the optical transmission of the dielectric substrate.
Femtosecond lasers are available from a number of manufacturers in the U.S. and Europe. A list of manufacturers of turnkey femtosecond oscillators and amplified systems is provided in Table 1.
Table 1. Manufacturers of turnkey femtosecond oscillators and amplified systems
All oscillators (fiber oscillator or Ti:Sapphire) use diode pumping for stability. The amplified systems use chirped pulse amplification.1 The Clark-MXR amplifier uses a fiber oscillator and a regenerative amplifier with gratings for the stretcher and compressor. The Quantronix Vitesse Duo-RegA system is a two-box system using a Ti:Sapphire oscillator and amplifier. The Femtolasers oscillator and multipass amplifier use Ti:Sapphire and dispersive mirror technology for dispersion compensation enabling amplified pulses of <30fs. IMRA uses fiber oscillator and fiber chirped-pulse amplification (FCPA). A palm-size Femtolite laser head illustrates the compactness of fiber oscillators. The Quantronix Integra integrates the Vitesse, a green pump source and a sequential Regen and Multipass Ti:Sapphire amplifier stage for high energy operation. The Spectra-Physics Hurricane integrates the Mai Tai and a green diode-pumped source (Evolution) and a Ti:Sapphire regenerative amplifier from Positive Light (www.poslight.com).
The important parameters for the different amplified laser systems are listed in Table 2.
Table 2. Beam parameters for amplified femtosecond lasers
Available output powers are generally in the 1W range. Quantronix offers a system with pulse energies exceeding 3.5 mJ and >3.5W. The high repetition rate and stable (but low energy) output of the RegA and the FCPA (prototype) are suited for diagnostics, waveguide manufacturing and microablation. The high-energy Ti:Sapphire amplifiers operate in the kiloHertz range. Most of the systems allow user-settable repetition rates. Pulsewidths vary from 160fs to <30fs. For most materials, the ablation characteristics are not sensitive to femtosecond pulsewidth or wavelength for sufficient irradiance and fluence. However, shorter pulsewidths in the 10-fs range appear to ablate transparent materials better.7 Quantronix and Spectra-Physics offer a factory-tunable range from 780 to 820nm useful for diagnostics. All amplified systems offer access to the oscillator output.
Beam quality affects the minimum spot size achievable. With a near-diffraction-limited beam and short focal length focusing optics, spot sizes of approximately 1 micron are attainable. The roundness of the beam is important in getting symmetric ablation. For precision and repeatability, pulse energy and beam profile stability are required. Pointing stability determines the precision and repeatability of the desired ablated feature. A 25µrad pointing stability would imply <1 µm precision for a laser processing workstation. Manufacturers have only recently started to list pointing stability data. Beam mode stability data appear to be unavailable. As manufacturers become more cognizant of users' requirements, such data would be readily available.
Diode pumping is used by most systems and requires minimal maintenance but has finite life and substantial replacement costs for the diodes, which are guaranteed only for one year even though >10,000-hr lifetimes are claimed. The CPA-2010 uses a telecom-grade diode with 20,000-hr MTTF (five-year warranty) for the oscillator but a lamp-pumped green laser (frequency-doubled Nd:YAG) for pumping the TI:Sapphire amplifier. Quantronix offers both lamp-pumped and diode-pumped green sources. Lamp changes are required every 500-600 hrs. The cost of the lamp and deionizing and filter cartridges is approximately $400, compared to diode cost of >$10,000 for each pump source. The user should factor in the nominal downtime of <30 min for lamp replacement every 600 hrs, compared to a replacement of the diodes after 10,000 hrs, to determine the total cost and effectiveness. Newer versions of diode-pumped sources offer easy user replacement of the diode module. As diode cost and reliability improves, we expect that all systems would be diode-pumped.
All femtosecond laser systems are sensitive to changes in the ambient environment. The stability values listed in Table 2 are obtained in an ambient environment with ±1 or 2 °C control. For a cleanroom or semiconductor facility, such an ambient requirement usually does not present a problem. However, beam stability is sensitive to temperature changes and a two-degree change in temperature may double the pointing stability specification and may also affect the energy and mode stability. Clark-MXR and Spectra-Physics use water cooling controlled to 0.1 °C such that specifications are valid for ±3°C.
All the laser systems listed do not provide integrated diagnostics for beam monitoring necessary for process diagnostics. An external beam train prior to the workstation is necessary for diagnostics and also for pulse energy control. The laser is usually run at one pulse energy setting for maximum stability, and attenuating waveplates are used to decrease/control the pulse energy. A CCD camera can be used for beam profiles, a photodiode for pulse energy measurement or a frequency-resolved optical gating (FROG) scheme for ultrashort-pulse diagnostics. FROG hardware and software used to be complex and required tedious alignment to operate. A recent innovation (www.swampoptics.com) has resulted in a low-cost and easy-to-operate device for pulsewidth, intensity and phase of ultrashort pulses >50fs.8
The stability of the integrated system is sensitive to temperature variations because of the nature of ultrashort pulses compared to conventional laser systems. The potential development of an all-fiber system capable of high-energy pulses may improve the cost, stability and size of an amplified system. With good ambient environmental control, current systems are capable of precision laser manufacturing. As installations increase and manufacturers are able to decrease unit cost and improve stability and reliability, we expect to see a more rapid deployment of this unique manufacturing tool.
This work was supported by the Office of Naval Research through the Navy MANTECH Electro-Optics Center under Cooperative Agreement N00014-99-2-0005.
Keng Leong is a contributing editor for ILS. He can be reached via e-mail at email@example.com.
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