Dual-beam stealth laser dicing based on electrically tunable lens
Introduction
Conventionally, silicon wafers have been separated into dies by blade dicing and laser ablation [[1], [2], [3]]. Mechanical blade dicing has been the industry standard for a long time, which typically has problems of vibration, stress loads and contaminations associated with wet processing. Although the use of laser ablation improves the dicing quality, it still has issues of surfaces debris and excessive heat generation during the dicing process [4,5]. In recent years, stealth laser dicing (SLD), i.e., laser processes performed inside the wafer, as a dry and high-quality dicing technology has received much attentions. Its advantages over other dicing techniques include high precision (i.e., kerf width ∼ 2 μm vs. 100 μm), minimal thermal effect, and free of debris [6]. In a typical SLD system, a nanosecond laser with photon energy less than the bandgap of silicon and a wavelength (λ ∼ 1064 nm) translucent to the material penetrates and tightly focuses inside a silicon wafer. By scanning the laser focus along directions parallel to the sample surface, thermal micro-cracks and a high dislocation density layer, i.e., stealth dicing (SD) modified layer, are formed inside the wafer [7,8]. The wafer is then mounted on a tape for separation via a wafer expander. SLD was first developed for high-quality dicing of ultra-thin semiconductor wafers (thickness < 50 μm); due to its remarkable performance, SLD was soon adapted for dicing regular wafers, which accordingly requires multiple scans (e.g., 10–15) at different depths to induce sufficient microcracks for separation. The multi-depth serial scanning requirements lowers the throughput of SLD. To address the issue, the concept dual-beam processing, which was previously developed for laser welding [9] and laser imaging [10,11] applications, was introduced in the SLD systems. However, the current dual-beam design is less efficient and flexible due to the use two light sources [9,12], custom-built optical components [13,14], and fixed distance between the two laser foci [15]. Another issue that SLD technology encounters frequently is wafer warpage and misalignment, which renders the SD layers not parallel to the wafer surface, leading to suboptimal dicing results [16]. (Note that the required dicing power varies with depth due to material absorption.) Common strategies to address the warpage issue include dynamic focus control via a scanning optical element [17], piezoelectric objective scanner [18], or fast wafer scanning via a precision stage [19]. Angular wafer misalignments are typically compensated by the use of a precision tip-tilt stage. Overall, the speeds of these methods are too low to match the desired laser scanning speeds for SLD.
In this work, we present the design and characterization of a dual-beam stealth laser dicing (D-SLD) system based on electrically tunable lenses (ETLs) that realizes high-speed focus-adjustable multi-depth laser scanning. ETLs are shape-changing lenses that modulate the laser focus at high speed (i.e., ∼1 kHz) by controlling the fluid-polymer membrane interface via a voice coil actuator [[20], [21], [22], [23]]. By integrating ETLs in the SLD system, each laser focus can be rapidly and individually modulated to compensate errors induced by wafer warpage and misalignments without requiring other expensive components, e.g., precision tip-tilt stages.
Fig. 1 presents the optical configuration of the D-SLD system. The light source is a Q-switched nanosecond laser (MATRIX 1064-10-30, Coherent Inc.) that generates 1064 nm laser pulses with a tunable repetition rate up to 100 kHz and a pulse width of ∼50 ns. A shutter (Shutter 1) is placed at the laser output port for controlling the laser system. The laser beam is first expanded via a beam expander (BE-02-05-C, Thorlabs Inc.); the polarization beam splitter (PBS1) and half wave plate (HWP) split the laser into two beams, i.e., Beam 1 and Beam 2 (90°), with continuously adjustable power ratio. Next, Beam 1 passes through ETL1 (EL-10-42-OF-1064-4D, Optotune AG) and a 4-f system (i.e., L1 and L2) and combines with Beam 2 at PBS2, where HWP1 and HWP2 control the power of Beam 1 and Beam 2 respectively. Lastly, the combined beam enters ETL2 and is relayed to the objective lens (Olympus LMPLN-IR, 50×, NA 0.65) via L3 and L4. Note that ETL2 controls the positions of the two foci together for ±75 μm, while ETL1 axially adjusts the focus of Beam 1 from −75 to 65 μm in reference to the focal position of Beam 2. As shown in Fig. 1., an epi-illumination microscope is built in the system to monitor the SLD process in situ, where a lamp is coupled to the objective lens via an 800 nm short pass filter (SP); the processing field is imaged to a charge-coupled device (CCD) camera via the SP, beam splitter (BS), and tube lens. The wafer is mounted on a precision XYZ stage (Planar-300XY & WaferMax Z stages, Aerotech Inc.) via a custom-designed vacuum chuck for high-speed laser scanning operations. To monitor the wafer position errors and curvature, a height sensor (LK-H022K, KEYENCE) is placed next to the objective lens. As laser dicing begins, the height sensor measures the wafer surface profile along the dicing direction in real time and sends the calculated correction signals to ETL2 to compensate the position and geometric errors at different dicing speeds, enabling real-time dual-focus error correction and high-throughput SLD.
Section snippets
Optimization of dual-beam operation
The intensity profile of a Gaussian beam that propagates along the z-axis can be expressed as [24,25]:where is the central intensity at the beam waist; is the size of the beam waist (radius) at z = 0 and is the beam radius at z. For a collimated Gaussian beam focused by a lens with a given numerical aperture (NA), the beam radius at the focal point is:where n is the refractive index of the medium that the laser beam propagates and
Calibration of ETLs
To ensure precise focus position control, ETL1 and ETL2 are calibrated before use. Fig. 5 presents the measured relationships between the focus displacements and drive voltage in a silicon wafer, which shows a linear relationship.
Correction of tip-tilt errors
We first demonstrate real-time compensation for positioning errors, i.e., tip-tilt errors during SLD. As shown in Fig. 1, the height sensor measures the wafer surface profile as the laser beam scans through the dicing trajectories; and the ETL2 corrects the position
Conclusion
In this work, we present the design and characterization of a dual-beam stealth laser dicing system (D-SLD) based on ETLs. In the system, two ETLs are employed to achieve fast and independent focus control that realize real-time correction of wafer misalignments and geometric errors, e.g., warpage. Theoretical and experimental studies have been performed to minimize the shielding effect for the two-focus operation by introducing a lateral focus shift. Dual-beam SLD experiments have been
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is supported by the HKSAR Innovation and Technology Commission (ITC), Innovation and Technology Fund (ITF), ITS/179/16FP.
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