Effects of Laser Beam Shape on Process Depth of Focus

The role of laser processes is crucial in the reduction of production cost and improvement of efficiency of solar cells. Laser scribing of different thin film materials is a critical step in thin film solar cell production. A powerful integrated laser scribing system is required to accommodate a depth range of 1–3 mm for large size glass solar panels without compromising the scribe quality.

Diode-pumped solid state (DPSS) q-switched lasers are most commonly employed for P1, P2, and P3 scribe processes. This article discusses the impact of laser beam shape on process depth of focus. In this scribe process, ~70µm wide scribes were micromachined on ~300-nm-thick molybdenum thin film grown on a glass substrate at different defocus planes.

Experimental Procedure and Results

This study used a Spectra-Physics Pulseo® 532-34 laser system operating at 100kHz to achieve ~68µm wide scribes at a scribe speed of 5.6m/s using a flat-top beam shape and ~70µm wide scribes at 6m/s using a Gaussian beam shape (Figures 1 and 2).

Figure 1. Schematic illustration of flat-top experimental optical setup and intensity distribution

Figure 2. Gaussian machining (top) and flat-top machining (bottom) microscope image and 3D depth profile trace

In this study, the process depth of focus was characterized based on the electrical resistance and quality of the scribes. The use of Gaussian beam machining resulted in high quality scribes within ±17mm defocus range, whereas the defocus range observed was only < ±1 mm in the case of flat-top beam machining. Electrical isolation of scribes was determined by measuring their electrical resistance at different defocus planes (Figures 3 and 4).

Figure 3. Microscope pictures of Gaussian beam scribes at different defocus positions (ΔZ) w.r.t focal plane showing quality of scribes

Figure 4. Microscope pictures of flat-top beam scribes at different defocus positions (ΔZ) w.r.t flat-top plane showing quality of scribes

All the scribes within the defocus range of ±17mm were electrically isolating for Gaussian beam machining, whereas scribes only within defocus range of < 0.5mm were shown good electrical isolation for flat-top machining (Figure 5).

Figure 5. Electrical resistance measurement of scribes processed at various defocus positions

Spectra-Physics’ DPSS Q-Switched Lasers

Spectra-Physics’ DPSS q-switched lasers have proven to deliver reliable operation in challenging around-the-clock industrial applications. With an extensive range of q-switched lasers (Table 1) and a deep applications know-how, Spectra-Physics can provide suitable laser solutions to customers for their industrial applications.

Table 1. Spectra-Physics’ broad portfolio of laser solutions

Power 1064 nm 532 nm 355 nm/349 nm 266 nm
0.5 W Explorer Explorer
1 W Tristar Navigator
2 W HIPPO
3 W Explorer XP Navigator
4 W
5 W Navigator HIPPO
6 W Navigator
7 W
8 W
9 W
10 W Navigator
11 W Mosaic, HIPPO
12 W
15 W HIPPO
17 W HIPPO
20 W Pulseo
27 W HIPPO
34 W Pulseo

Conclusion

Although high-quality electrically isolating scribes can be produced using both flat-top and Gaussian laser beam shapes, the flat-top beam scribing process has a very limited process depth of focus tolerance. This quality is detrimental from the perspective of system design due to the typical system requirement of accommodating up to 1–3 mm variations related to processing of larger glass panels with deviations in thickness and other system tolerances.

After defining acceptable process parameters, the use of Gaussian DPSS laser beams leads to a larger process window, thereby accommodating large process defocus variations for economical laser scribing with high yield.

About Spectra-Physics

Spectra-Physics, a Newport Company, is the world's premier supplier of innovative solutions for precision laser applications. Our broad portfolio of lasers are proven in 24/7 applications and are backed by our global support team. Our innovation driven culture strives to create breakthrough products that achieve new levels of cost-performance that power our customers' success.

This information has been sourced, reviewed and adapted from materials provided by Spectra-Physics.

For more information on this source, please visit Spectra-Physics.

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