James Bovatsek

Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate

High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for rapid writing of low-loss optical waveguides in transparent glasses. A novel femtosecond fiber laser system (IMRA America, FCPA muJewel) providing variable repetition rate between 0.1 and 5 MHz was used to study the relationship between heat accumulation and resulting waveguide properties in fused silica and various borosilicate glasses. Increasing repetition rate was seen to increase the waveguide diameter and decrease the waveguide loss, with waveguides written with 1-MHz repetition rate yielding ~0.2-dB/cm propagation loss in Schott AF45 glass. A finite-difference thermal diffusion model accurately tracks the waveguide diameter as cumulative heating expands the modification zone above 200-kHz repetition rate.

“Quill” writing with ultrashort light pulses in transparent materials

A remarkable phenomenon in ultrafast laser processing of transparent materials, in particular, silica glass, manifested as a change in material modification by reversing the writing direction is observed. The effect resembles writing with a quill pen and is interpreted in terms of anisotropic trapping of electron plasma by a tilted front of the ultrashort laser pulse along the writing direction.

Self-assembled periodic sub-wavelength structures by femtosecond laser direct writing

Self-assembled, sub-wavelength periodic structures are induced in fused silica by a tightly focused, linearly polarized, femtosecond laser beam. Two different types of periodic structures, the main one with period (Lambda(E)) in the direction of the laser beam polarization and the second with period (Lambda(k)) in the direction of the light propagation, are identified from the cross-sectional images of the modified regions using scanning electron microscopy. We demonstrate the spatial coherence of these nanogratings in the plane perpendicular to the beam propagation direction. The range of effective pulse energy which could produce nanogratings narrows when the pulse repetition rate of writing laser increases. The period Lambda(E) is proportional to the wavelength of the writing laser and period Lambda(k) in the head of the modified region remains approximately the wavelength of light in fused silica.

Highest-speed dicing of thin silicon wafers with nanosecond-pulse 355nm q-switched laser source using line-focus fluence optimization technique

Due to current and future anticipated widespread use of thin silicon wafers in the microelectronics industry, there is a large and growing interest in laser-based wafer dicing solutions. As the wafers become thinner, the laser advantage over saw dicing increases in terms of both the speed and yield of the process. Furthermore, managing the laser heat input during the dicing process becomes more important with increasingly thin wafers and with increasingly narrow saw streets. In this work, shaped-beam laser-cutting of thin (100 μm and below) silicon is explored with Newport / Spectra- Physics Pulseo 20-W nanosecond-pulse 355-nm DPSS q-switched laser system. Optimal process conditions for cutting various depths in silicon are determined, with particular emphasis on fluence optimization for a narrow-kerf cutting process. By shaping the laser beam into a line focus, the optimal fluence for machining the silicon can be achieved while at the same time utilizing the full output power of the laser source. In addition, by adjusting the length of the laser line focus, the absolute fastest speed for various cutting depths is realized. Compared to a circular beam, a dramatic improvement in process efficiency is observed.

Ultrashort pulse micromachining with the 10-μJ FCPA fiber laser

IMRAs ultrashort pulse fiber laser products continue to evolve to expand the application scope. The latest prototype FCPA produces pulses with less than 500-fs pulse duration at a 50-kHz repetition rate. At the fundamental wavelength of 1045 nm, the pulse energy is greater than 10 mJ. The increase in pulse energy over the standard FCPA μJewel permits greater flexibility in the focusing conditions applicable for micromachining, enabling a wider variety of laser-machined structures and profiles. This paper describes the latest micromachining application areas being studied with this new laser.

Laser ablation threshold and etch rate comparison between the ultrafast Yb fiber-based FCPA laser and a Ti:sapphire laser for various materials

Ti:Sapphire lasers remain the most widely used utlrafast laser. However, precise optical alignment and environmental control are necessary for continuous, long-term stable operatoin of the laser. IMRAs FCPA laser is an air-cooled, Yb fiber-based ultrafast laser designed to operate in an industrial environment and provide a stable, high-quality laser beam. In this work, the micromachining performance of the FCPA laser is directly compared with a conventional Ti:Sapphire regenerative amplifier laser. An experimental study was conducted to determine the ablation threshold and etch rate for a variety of materials (including metals, semiconductors, and dielectrics). The materials chosen for the experiments cover a wide range of optical, mechanical and physical properties. Similar focusing conditions were used for both lasers in order to ensure that any differences in the results are primarily due to the different characteristics of each laser. For materials with a relatively low ablation threshold, the full energy of the Ti:Sapphire laser is not needed. Furthermore, it is near the ablation threshold where ultrafast laser processing provides the benefit of minimal thermal damage to the surrounding material. Although the relatively low pulse energy of the FCPA limits its ability to ablate some harder materials, its high repetition rate increases the material processing speed and its good beam quality and stability facilitates tight, efficient focusing for precise machining of small features.

Fiber chirped pulse amplification system for micromachining

Chirped Pulse Amplification (CPA) is widely used for generating high-energy femtosecond pulses. This is most commonly done with a solid-state Ti:Sapphire crystal through a free-space optical path. The high energy density in the crystal and the precise optical path required with the use of bulk optics make it difficult to design a simple system with good stability and beam quality over the environmental conditions typically encountered in the manufacturing environment. A CPA system using fiber architecture reduces the need for precise beam guiding since the light follows the fiber. The pump energy is more evenly distributed along the length of the amplifier fiber, reducing the thermal dissipation that is required (no water chiller is required) and improving the overall efficiency. The fiber architecture also produces a superior quality beam that does not require great care to maintain. IMRAs latest FCPA μJewel uses the inherent advantages of the FCPA architecture, along with extensive engineering, to produce a compact and stable femtosecond fiber laser system. Its high repetition rate and stable performance enables applications that were difficult to achieve previously. This paper will review the general design architecture of the FCPA μJewel and discuss several applications.

High speed micromachining with high power UV laser

Increasing demand for creating fine features with high accuracy in manufacturing of electronic mobile devices has fueled growth for lasers in manufacturing. High power, high repetition rate ultraviolet (UV) lasers provide an opportunity to implement a cost effective high quality, high throughput micromachining process in a 24/7 manufacturing environment. The energy available per pulse and the pulse repetition frequency (PRF) of diode pumped solid state (DPSS) nanosecond UV lasers have increased steadily over the years. Efficient use of the available energy from a laser is important to generate accurate fine features at a high speed with high quality. To achieve maximum material removal and minimal thermal damage for any laser micromachining application, use of the optimal process parameters including energy density or fluence (J/cm2), pulse width, and repetition rate is important. In this study we present a new high power, high PRF QuasarR 355-40 laser from Spectra-Physics with TimeShiftTM technology for unique software adjustable pulse width, pulse splitting, and pulse shaping capabilities. The benefits of these features for micromachining include improved throughput and quality. Specific example and results of silicon scribing are described to demonstrate the processing benefits of the Quasar’s available power, PRF, and TimeShift technology.

Optimized precision micromachining using commercially-available, high-repetition rate, microJoule femtosecond fiber lasers

Fiber lasers offer an excellent technology base for production of an industrial-quality tool for precision microfabrication, answering the need to expand the capabilities of laser material processing beyond traditional welding, cutting, and other industrial processes. IMRAs FCPA μJewelTM femtosecond fiber laser has been developed to address the particular need for direct-write lasers for creation of clean and high-quality micron and sub-micron features in materials of commercial interest. This flexible Yb:fiber chirped-pulse amplification architecture, capable of operating at rep-rates between 100 kHz and 5 MHz, balances the need for higher-repetition rate with that of sufficient pulse energy to work at or near ablation threshold, while meeting industrial standards for temperature, shock and vibration. Demonstration of the need for higher-repetition rates for direct write processes will be provided in this paper. Further, results of laser-processing of materials typically used in flat panel displays, photomasks, and waveguide production using the FCPA μJewelTM laser will be presented.

Advantages of using DPSS nanosecond laser with a Gaussian beam shape for scribing thin film photovoltaic panels

Laser scribing of various thin film materials is a key process in manufacturing of thin film photovoltaic (PV) panels. In recent years, PV industry has adopted the use of high-power nanosecond-pulse diode pumped solid state (DPSS) Q-switch lasers to increase precision and throughput of scribe processes. A major push for the use of lasers is made in order to increase the quality of scribes and hence the efficiency of a solar cell while reducing fabrication costs. This paper focuses on identifying advantages of using a Gaussian shaped laser beam from a DPSS Q-switch laser for thin film scribe processes. In particular, scribing with a Gaussian laser beam and a flattop shaped laser beam has been evaluated and compared. From a laser scribing system design perspective, the effect of beam intensity distribution on the process depth of focus has been characterized. In addition, scribing with a high quality low M2 Gaussian beam from a DPSS q-switch laser and a beam from a high M2 fiber laser has been compared. Again from a laser scribing design perspective, the effect of each laser on process depth of focus has been characterized.Laser scribing of various thin film materials is a key process in manufacturing of thin film photovoltaic (PV) panels. In recent years, PV industry has adopted the use of high-power nanosecond-pulse diode pumped solid state (DPSS) Q-switch lasers to increase precision and throughput of scribe processes. A major push for the use of lasers is made in order to increase the quality of scribes and hence the efficiency of a solar cell while reducing fabrication costs. This paper focuses on identifying advantages of using a Gaussian shaped laser beam from a DPSS Q-switch laser for thin film scribe processes. In particular, scribing with a Gaussian laser beam and a flattop shaped laser beam has been evaluated and compared. From a laser scribing system design perspective, the effect of beam intensity distribution on the process depth of focus has been characterized. In addition, scribing with a high quality low M2 Gaussian beam from a DPSS q-switch laser and a beam from a high M2 fiber laser has been compared. Aga...