Abstract
Ultra-thin and large-area silicon wafers with a thickness in the range of 20–70 μm, were produced by spalling using a nickel stressor layer. A new equation for predicting the thickness of the spalled silicon was derived from the Suo–Hutchinson mechanical model and the kinking mechanism. To confirm the reliability of the new equation, the proportional factor of stress induced by the nickel on the silicon wafer, was calculated. The calculated proportional factor of λ = 0.99 indicates that the thickness of the spalled silicon wafer is proportional to that of the nickel layer. A similar relationship was observed in the experimental data obtained in this study. In addition, the thickness of the stressor layer was converted to a value of stress as a guide when using other deposition conditions and materials. A silicon wafer with a predicted thickness of 50 μm was exfoliated for further analysis. In order to spall a large-area (150 × 150 mm2 or 6 × 6 in2) silicon wafer without kerf loss, initial cracks were formed by a laser pretreatment at a proper depth (50 μm) inside the exfoliated silicon wafer, which reduced the area of edge slope (kerf loss) from 33 to 3 mm2. The variations in thickness of the spalled wafer remained under 4%. Moreover, we checked the probability of degradation of the spalled wafers by using them to fabricate solar cells; the efficiency and ideality factor of the spalled silicon wafers were found to be 14.23%and 1.35, respectively.
Highlights
Since the trajectory of spalling propagation was parallel to the nickel surface, the control of uniform deposition of the nickel stressor layer was a critical factor of large-area spalling (Suo and Hutchinson, 1990)
The nickel film deposited with a shield had a uniform thickness of about 7 μm, which demonstrates that the shield assisted uniform growth of the nickel stressor layer, when it was deposited by controlling the electric field in the bath
A kerf-less thin silicon wafer with a large area was successfully fabricated by spalling, and its thickness was calculated from the steady-state crack depth, using the proposed equation based on the Suo–Hutchinson model and the kinking mechanism
Summary
Silicon solar cells are the focus of considerable research efforts because of their high energy-conversion efficiency (∼25%) (Green et al, 2015), stability, and so on (Bruel, 1995; Dross, 2008; Shahrjerdi et al, 2012; Radhakrishnan et al, 2014; Kobayashi et al, 2015; Lee et al, 2016; Green et al, 2017; Wang et al, 2017). Because the induced stress in the silicon wafer caused crack propagation inside the silicon wafer at a steady-state crack depth, the thickness of the spalled silicon wafer was higher when the internal stress of the nickel stressor layer was lower.
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