One approach to realize a back contact solar cell design is to ‘wrap’ the front contacts to the backside of the cell . This results in significantly reduced shadowing losses, possibility of simplified module assembly process and reduced resistance losses in the module; a combination of measures, which are ultimately expected to lower the cost per watt of PV modules. A large number of micro-vias must be drilled in a silicon wafer to connect the front and rear contacts. Laser drilling was investigated using a pulsed disk laser which provided independent adjustment of pulse width, repetition rate and laser power. To achieve very high drilling rates, synchronization of the laser pulses with the two-axis galvanometer scanner was established using a FPGA controller. A design of experiments (DOE) was developed and executed to understand the key process drivers that impact the average hole size, hole taper angle, drilling rate and hole quality. Laser drilling tests were performed on wafers with different thicknesses between 120 μm and 190 μm. The primary process parameters included the average laser power, pulse length and pulse repetition rate. The impact of different laser spot sizes (34 μm and 80 μm) on the drilling results was compared. The results show that average hole sizes between 30 – 100 μm can be varied by changing processing parameters such as laser power, pulse length, repetition rate and spot size. In addition, this study shows the effect of such parameters on the hole taper angle, hole quality and drilling rate. Using optimized settings, 15,000 holes per second are achieved for a 120 μm thick wafer with an average hole diameter of 40μm.
Today's complexity in packaging of MEMS and BioMEMS requires advanced joining techniques that take the specific
package integration for each device into account. Current focus on reducing investment and operating costs for device
packaging require a flexible and reliable joining approach for similar and dissimilar materials such as metals, polymers,
glass and silicon to manage increasing system complexity. Depending on the application, packaged devices must fulfill
tough requirements regarding strength, thermal stress, fatigue and hermeticity and long-term stability.
This research is focused on laser microjoining of polyimide and PEEK polymers to metals such as nitinol,
chromium and titanium using fiber laser. Our earlier investigations have demonstrated the potential of this unique
joining technique, which successfully addresses the existing microjoining challenges including high precision, localized
processing capability and biocompatibility. Our current study further defines the key processing parameters for joining
novel dissimilar material combinations based on the characterization of such laser joints by means of mechanical failure
tests and the bond area analysis using optical microscope, scanning electron microscopy (SEM) and X-ray photoelectron
The results compare operating windows for generating quality bonds for different material joining
configurations. They also provide an initial approach to characterize laser-fabricated microjoints that can be potentially
used for the optimization of the design process of devices utilizing these materials. Potential packaging applications
include microsystems used for chemical or biological assays (lab-on-a-chip), implantable devices used for pressure or
temperature sensing, neural stimulation and drug delivery.
Fiber lasers in MOPA configuration are a very flexible tool for micromachining applications since they allow to independently adjust the pulse parameters such as pulse duration, repetition rates and pulse energy while maintaining a constant beam quality. The developed fiber laser provides an average power of 11 W and maximum pulse energy of 0.5 mJ for a wide range of pulse parameters at diffraction limited beam quality. Its pulse duration and repetition rate are continuously adjustable from 10 ns to cw and from 10kHz to 1MHz respectively. Ablation experiments were carried out on stainless steel, nickel and silicon with the goal of optimizing removal rates or surface finish using nanosecond pulses of different parameters. Maximum removal rates are achieved on all three materials using relatively similar pulse parameters. For silicon, pulse duration of 320ns at 100kHz and 45mJ resulted in optimum removal. In single shot experiments on silicon a significant influence of the pulse duration was found with a distinct optimum for removal rate and surface finish. The optimum intensity at the work piece is in the range of 35MW/cm<sup>2</sup> to 70MW/cm<sup>2</sup>. Lower values are below the ablation threshold, while the plasma shielding effect limits considerable increases in removal rates for intensities exceeding 70MW/cm <sup>2</sup>.