Optimizing the processing of sapphire with ultrashort laser pulses Geoff Lott 1, Nicolas Falletto 1, Pierre-Jean Devilder, and Rainer Kling 3 1 Electro Scientific Industries, Eolite Systems, 3 Alphanov October, 15 ICALEO
Motivation Chemically inert Biocompatible Scratch resistant Intrinsic properties of sapphire Optical transparency Hardness
Laser processing of sapphire QCW lasers for cutting and dicing Internal features with helical cutting Zibner, F. et al. (1), Ultra-high precision helical laser cutting of sapphire, ICALEO, San Diego, USA, M31. Mendes, M. et al. (15), Fiber laser micromachining in highvolume manufacturing, www.industrial-lasers.com.
Motivation Broader utilization of sapphire for many applications has been slowed by the difficulty of laser machining fine features onto it with industrially acceptable throughput and quality What is the optimized industrially-viable process for micromachining of sapphire with current state-ofthe-art laser systems and standard beam delivery components? What are the limitations of this process, and expectations going forward?
Average Power (w) Pulse Energy (µj) Laser and experimental apparatus Chinook IR Wavelength: 3nm Pulse duration:.ps M <1. 7 5 3 λ/ Max pulse energy: 5uJ spec. Max average power : W spec. Repetition rate: up to 3MHz 3 Seeder Repetition Rate (khz) x beam expander Data analysis with Keyence laser profilometer Scanlabs hurryscan galvo mm f(θ) Aerotech ALS13H-15 (Z) ABL15 (X-Y) Sapphire wafers: c-plane sapphire, double side polish Thickness: 3µm (effective sample thickness of 5µm) TTV: µm Micro-roughness:.3nm
Bottom-up ablation process Scanlabs hurryscan galvo mm f-theta lens Beam waist starts below bottom surface; translated upwards at constant speed while pattern is repeated continuously µm diameter pattern consisting of: Inward spiral + Outward spiral + Outer circle
Methodology and process parameters Generate holes with µm diameter (aspect ratio ~1) Learn general rules that can be modified to suit smaller or larger features Consider realistic throughput goals: are taper (if any) and throughput related? Process Parameters Pulse energy:.µj on sample Waist diameter: 1µm Polarization: circular Spiral pitch: 9µm Repetition rate varied: 1kHz, khz, khz, 51kHz khz 9 1kHz not shown poor hole quality Overlap varied: 7%, %, 9%, 95%, 9% Dynamic z-speed: varied from µm/s to >µm/s 9 lower speed determined by customer cycle time requirements (equivalent to 5s/hole) 9 cycle time inversely proportional to z-speed for complete bottom-up ablation process
Bottom-up hole quality comparison for sapphire High quality Low quality Top surface x x Good process low taper, no cracks or chips x x Bad process Significant taper, cracking, damage rings Bottom surface Low taper, not zero taper due to molten sapphire redeposition
Taper vs. z-speed for varied overlap and repetition rate Available overlap conditions limited by max galvo speed In general, taper is smaller at lower z-speed values Taper decreases at higher repetition rates for identical overlap Not a cold ablation process thermal accumulation plays a critical role Taper (degrees) Taper (degrees) khz 51kHz Z-axis Translation Speed (µm/s) khz 1 1 1 1 1 1 1 1 95% 95% 9% % 7% 9% s s 1s 5s khz 9% 95% 9% 9% 1 1 1 1 1 1 1 1 Z-axis Translation Speed (µm/s)
Accumulation and how it affects the bottom-up ablation process Full bottom-up ablation large accumulation effects Threshold position for top surface machining Initiation of ablation Process window below top surface threshold sapphire top sapphire bottom khz, 9% overlap µm/s khz, 9% overlap 5µm/s Bottom-up Hybrid Transition from bottom-up to top-down (hybrid) lower accumulation Process window overlaps top surface threshold khz, 9% overlap 15µm/s Top-down Observation of switch from bottom-up to top-down process was easily observed by eye, but curvature of wall taper can also be used to identify process type(s).
Taper vs. z-speed for varied overlap and repetition rate Two regions: High speed = top-down Low speed = bottom-up Inflection between regions signifies transition from bottom-up to hybrid process At this point, cycle time is no longer inversely proportional to z-axis translation speed Taper (degrees) Taper (degrees) khz 51kHz Z-axis Translation Speed (µm/s) khz 1 1 1 1 1 1 1 1 95% 95% 9% % 7% 9% s s 1s 5s khz 9% 95% 9% 9% 1 1 1 1 1 1 1 1 Z-axis Translation Speed (µm/s)
Damage rings on back-side of sample onset of damage ring Bottom-up/topdown hybrid onset Damage rings observed for top-down process Entrance edge acts as focusing lens Top-down process Origin previously observed experimentally and modeled by Wolfgang Schulz et al. Sun, M. et al. (13), Numerical analysis of laser ablation and damage in glass with multiple picosecond laser pulses. Optics Express 1(7), 75-77.
µm/s: 5s/hole 3µm/s: s/hole 5µm/s: 5s/holes khz, 9% overlap These processes are too cold (low thermal accumulation, transition to top-down process more likely) 15um max 15um max 3um min taper 35um min.3 taper khz, 9% overlap 51kHz, 95% overlap These processes are just right khz, 9% overlap These processes are too hot (melt on surface, HAZ, filamentation)
Cracking/damage vs. average taper No cracks/damage observed1 < 5 taper: No cracks observed for % of holes > 5 taper: No cracks observed for % of holes Cracks/damage observed Average Taper (degrees) One data point for each individual set of examined process parameters (not a yield measurement) Low taper high chance of excellent hole quality Small, tightly spaced arrays of holes demonstrate repeatable, robust process High confidence that many sets of process parameters result in very high yield
Best process windows Taper (degrees) Taper (degrees) khz 51kHz Z-axis Translation Speed (µm/s) khz 1 1 1 1 1 1 1 1 95% 95% 9% % 7% 9% khz 9% 95% 9% 9% 1 1 1 1 1 1 1 1 Z-axis Translation Speed (µm/s) We are able to drill holes with less than degrees taper in under 1 seconds, or less than 5 degrees taper in under 5 seconds
Conclusions We have demonstrated the ability to drill small holes in sapphire wafers with throughput that meets known customer demands µm diameter holes with <5 taper in less than 5 seconds µm diameter holes with < taper in less than 1 seconds Process speed and quality both benefit from a bottom-up process Avoid damage rings and cracks, minimize taper Aggregates of melted sapphire particulates re-adhere to the hole sidewalls, leading to non-zero taper for all examined parameter combinations Post-processing did not eliminate aggregates
Thank you for your attention!
Can a post-process KOH bath decrease taper? Before KOH: One hour KOH etching bath, agitated with stir bar Aggregated material decreases, but is not eliminated After KOH:
Bottom-up Debris Accumulation At the start of processing, the trench is very clean, and mostly free of debris. This trench is < µm in depth (the Step value on the left) Note that for diagnostic purposes, the outer diameter of these features is large (~1.35 mm).
Bottom-up Debris Accumulation As the trench is made deeper, debris starts to accumulate along the side walls. A maximum depth of ~1 µm is reached before debris accumulation clogs the trench.
Bottom-up Debris Accumulation With further movement along the z-axis, the trench has become completely clogged with sapphire particulates.