Alexander Garifal, marketing engineer, and Joyce Kilmer, director of marketing, Photonics Industries
Photonics Industries’ portfolio of products includes diode-pumped solid-state (DPSS) ultraviolet (UV) nanosecond lasers for microprocessing applications such as those in the defence, industrial, medical and scientific sectors. These lasers, known as the DX series, combine the benefits of end pumping diodes and proficient intracavity harmonic generation. This paper provides a brief explanation of intracavity harmonic generation, the benefits thereof and the wide variety of industrial applications for an ultraviolet (UV) nanosecond laser in the context of specific application of DX series lasers. The industrial microprocessing applications covered herein are flexible circuit board (FPCB) cutting, via hole drilling, silicon (Si) and glass wafer singulation (dicing), indium tin oxide (ITO) film patterning and marking, and metal cutting.
Introduction to intracavity harmonics
Dr. Yusong Yin developed intracavity harmonic generation and established Photonics Industries (PI) in 1993 to create DPSS lasers that utilised this technology in an industry dominated by conventional extra-cavity harmonic generation methods.
Optical second-harmonic generation was first theorised in 1970. It was shown that optical second-harmonic generation equalled that of fundamental generation in utilisation of internal cavity harmonics1. Power conversions for subsequent harmonics were thus significantly optimised, having reduced the deterioration in power found in extra-cavity harmonic generation. However, the theory had yet to be effectively applied on a wide scale, as there were some observed chaotic phenomena that kept the industry in the realm of extra-cavity harmonic conversions2.
Photonics Industries’ intracavity harmonic generation technology disproved the notion that intracavity generation yielded unpredictable results and chaotic behaviour3. Over the years, the company has produced many laser product lines for the microprocessing market. A number of patents have been granted over the years for innovations in intracavity harmonic generation as well as optimisation of power efficiency, pulse-to-pulse stability and overall efficiency4.
Overview of ultraviolet laser microprocessing
For industrial microprocessing applications, precision and longevity of the laser system are paramount considerations. In the case of high-precision ablative methods in very minute applications, the UV wavelength is almost always utilised because it can be focused on a smaller spot size. In addition, the ablation threshold is lower for UV than other higher wavelengths such as infrared (IR). For the UV range, ablation is more a photochemical process than a photothermal process, as it is for IR. This means the heat affected zone (HAZ) at the IR (1,064 nm) wavelength has a much larger thermal effect on the material the laser is being applied to. Shorter wavelengths, going down the harmonics of green (532 nm) and UV (355 nm), have a much smaller thermal affect (lower HAZ), resulting in better microprocessing precision and quality. This general rule applies over a variety of materials, but it is important to note that different material categories will have slight variance in wavelength dependency. For instance, it is beneficial to use shorter wavelengths for insulators than those used for conductors/metals, which do not have such a strong wavelength dependency.
DX series lasers for microprocessing
Photonics Industries’ DX series lasers offer foundational assemblage and additive features that make them suitable for a wide range of industrial microprocessing applications.
One of the key foundational assemblage features of DX series lasers is their excellent TEM00 transverse mode beam profile, with M2 values ranging as low as 1.05 to 1.1. For microprocessing, low M2 values lead to better focus and longer depth of focus (DOF), which is especially useful for micromachining and drilling.
Another significant foundational assemblage feature of DX series lasers is their diode end pumping design. In DPSS lasers, diode end pumping from the diode into the main laser cavity along the beam axis, where resonance occurs, allows for a more efficient amplification schema compared with conventional diode side pumping. Diode end pumping contributes toward high accuracy circular beam profiles, which is 90 percent and above for DX series lasers. For industrial microprocessing, laser scanning heads need highly accurate circular beam profiles to avoid beam ellipticity problems such as differing line widths when scanning along varying axial directions. Diode end pumping configurations also allow for highly cost-effective field replaceability because the interfacing is fibre coupled and engineered more simply than side pumped laser systems.
The additive features of DX series lasers allow for high adaptability and subsequent high efficiencies. They include dynamic pulse control (DPC) by duty control or pulse energy control (PEC), burst mode functionality and gating with first pulse kill (FPK) and/or last pulse kill (LPK). DX series lasers can therefore be considered as having an encompassing functionality for total pulse control.
Industrial microprocessing application examples
The foundational assemblage and additive features of DX series lasers are examined here in the industrial microprocessing applications of FPCB cutting, via hole drilling, Si and glass wafer singulation (dicing), ITO film patterning and marking, and metal cutting.
Flexible printed circuit board cutting
For FPCB cutting, short pulse width together with low repetition rate in the laser is ideal. This article focuses on the range of nanosecond pulse widths, so short will not be referring to the picosecond range or any number of magnitudes shorter; but rather, for this application, in the realm of <20 ns. Repetition rates are ideally in the range of 20–30 kHz.
DX series lasers operate within these parameters and even to single-shot repetition rates. The FPCB cross-section sample shown in figure 1 was cut using the DX-355 15W laser, which incorporates a typical galvanometer setup, 2x beam expander and 100 mm LINOS F-Theta lens.
A flexible printed circuit board (FPCB) cross-section that was cut using the DX-355 15W laser (front-left image, back-right image).
The main objectives in cutting FPCBs are to produce a low HAZ without charring, especially near electrodes, and achieve uniform, smooth cutting, which are achieved by using DX series lasers’ DPC by duty control or PEC features.
Via hole drilling
For via hole drilling applications, such as drilling holes in FPCBs, the repetition rate must be high to prevent unwanted thermal effects. Repetition rates are ideally in the range of 80 kHz or higher. This requirement is necessary because usually holes must be drilled on a large-scale. However, as repetition rates are scaled up, pulse energies are scaled down. This is problematic because, in nanosecond processing, via hole depth is controlled by pulse energy5. A simultaneous requirement for high power is then necessary to ensure each pulse has the requisite energy for hole drilling.
DX series lasers operate within these parameters. The via holes in the metal and plastic chip sample shown in figure 2 were drilled using the DX-355 15 W laser, which incorporates a typical galvanometer setup, 5x beam expander and 100 mm LINOS F-Theta lens.
Via holes that were drilled in a metal and plastic chip using the DX-355 15W laser.
The main objectives in via hole drilling are to keep thermal effects to an absolute minimum and create holes that are perfectly circular. DX series lasers are able to meet these objectives because of their high beam circularity of at or above 90 percent.
Silicon and glass wafer singulation (dicing)
Common silicon or glass wafers, used in the semiconductor industry, undergo a fabrication process known as singulation, whereby they are separated into die (individual integrated circuits (ICs) or chips). The process involves creating a groove in the wafer that is then cut or diced along. Grooving is performed using a laser and cutting or dicing is usually a mechanical process conducted using a diamond saw.
For laser grooving, the favourable repetition rate is typically high, namely as high as 200 kHz. The required pulse width is also high, at around 350 ns. This typically results in lower peak power to ensure full cutting does not occur, while maintaining the goal of consistent, high groove quality. Silicon wafers typically have a thickness of around 500–700 µm. A groove depth of ~10 µm is sufficient for the subsequent guided sawing process.
DX-LP (long pulse) lasers are ideal for wafer singulation grooving requirements. The PEC feature adjusts pulse energy to corresponding groove depth. The glass sample shown in figure 3 was textured at different pulse energies using PEC on a DX-355-LP laser.
Glass that was textured at different pulse energies using the pulse energy control (PEC) on a DX-355-LP (long pulse) laser.
Indium tin oxide film patterning and marking
ITO is a transparent conductive oxide (TCO) widely used on account of its electrical conductivity and optical transparency for products such as flat-panel displays, photovoltaics and glass doors and windows. ITO is most commonly deposited on surfaces as thin films and therefore laser precision annealing is required to achieve quality patterning and marking of circuits.
For patterning and marking, high repetition rates of around 50–100 kHz are required, especially in glass substrate applications of ITO. However, the average power does not need to be high, namely around 5 W.
DX series lasers operate within these parameters. For selective patterning and marking of the ITO film, it is best to output a certain number of pulses in a sequence. The burst mode functionality feature on DX series lasers outputs a specified number of pulses in a specified sequence based on the command inputted by the user.
Metal cutting
Metal cutting is a micro rather than macro type of application for DPSS lasers because their power range is geared toward precision applications rather than, for example, bulk cutting of thick pieces of metal.
DX series lasers typically require average powers of greater than ~15 W and repetition rates of around 50–100 kHz for cutting metals such as steel, tungsten, copper, composites, etc. These requirements are specifically for the precision cutting of thin metals but are not stringent if taking into account the full range of metals and processing needs.
A feature that can be advantageous for metal-involved processes is FPK control. For q-switched lasers, the first pulse emitted is the largest. DX series lasers have the pulse control to kill, or switch off, this giant pulse on demand. When processing composite materials, this feature may be useful in modulating the pulse through different layers. For example, a copper plate with a polymer substrate would benefit from this modulation of the giant pulse to regular pulsing. The giant pulse can be switched on to cut through copper portions and switched off for regular pulsing on the polymer portions.
Conclusion
The combination of intracavity harmonic conversion and foundational assemblage and additive features afforded by DX series lasers allows manufacturers to address a myriad of existing and emerging material processing applications.
Photonics Industries International
References
1Smith, R. (1970). Theory of intracavity optical second-harmonic generation. IEEE (Institute of Electrical and Electronics Engineers) Journal of Quantum Electronics, volume 6, issue 5, pp.215–223.
2James, G.E., Harrell, E. and Rajarshi, R. (1990). Intermittency and chaos in intracavity doubled lasers. American Physical Society, volume 41, issue 5, pp. 2778–2790.3Kilmer, J. (2007). Photonics Industries: intracavity harmonic laser pioneer [article]. Photonics Media. October 23.Available at: https://bit.ly/3t97AKa
4Patents by inventor Yusong Yin [web page]. JUSTIA Patents.
Available at: https://bit.ly/3toQw2Z
5Kilmer, J., Terraciano, M. and Yin, Y. (2013). Sub-ns and ps laser performance and results. Society of Photo-Optical Instrumentation Engineers (SPIE) proc. volume 8607: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XVIII.