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>.
Micro-joining and hermetic sealing of dissimilar and biocompatible materials is a critical issue for a broad spectrum of products such as micro-electronics, micro-optical and biomedical products and devices. Today, biocompatible titanium is widely applied as a material for orthopedic implants as well as for the encapsulation of implantable devices such as pacemakers, defibrillators, and neural stimulator devices. Laser joining is the process of choice to hermetically seal such devices.
Laser joining is a contact-free process, therefore minimizing mechanical load on the parts to be joined and the controlled heat input decreases the potential for thermal damage to the highly sensitive components. Laser joining also offers flexibility, shorter processing time and higher quality. However, novel biomedical products, in particular implantable microsystems currently under development, pose new challenges to the assembly and packaging process based on the higher level of integration, the small size of the device's features, and the type of materials and material combinations. In addition to metals, devices will also include glass, ceramic and polymers as biocompatible building materials that must be reliably joined in similar and dissimilar combinations. Since adhesives often lack long-term stability or do not meet biocompatibility requirements, new joining techniques are needed to address these joining challenges. Localized laser joining provides promising developments in this area. This paper describes the latest achievements in micro-joining of metallic and non-metallic materials with laser radiation. The focus is on material combinations of metal-polymer, polymer-glass, metal-glass and metal-ceramic using CO<sub>2</sub>, Nd:YAG and diode laser radiation. The potential for applications in the biomedical sector will be demonstrated.
Implantable microsystems currently under development have the potential to significantly impact the future treatment of disease. Functions of such implants will include localized sensing of temperature and pressure, electrical stimulation of neural tissue and the delivery of drugs. The devices are designed to be long-term implants that are remotely powered and controlled for many applications. The development of new, biocompatible materials and manufacturing processes that ensure long-lasting functionality and reliability are critical challenges. Important factors in the assembly of such systems are the small size of the features, the heat sensitivity of integrated electronics and media, the precision alignment required to hold small tolerances, and the type of materials and material combinations to be hermetically sealed. Laser micromachining has emerged as a compelling solution to address these manufacturing challenges. This paper will describe the latest achievements in microjoining of metallic and non-metallic materials. The focus is on glass, metal and polymers that have been joined using CO<sub>2</sub>, Nd:YAG and diode lasers. Results in joining similar and dissimilar materials in different joint configurations are presented, as well as requirements for sample preparation and fixturing. The potential for applications in the biomedical sector will be demonstrated.
Advanced microsystems for optoelectronic and biomedical applications incorporate a variety of non-metallic materials such as glass, silicon, sapphire and polymers. Examples include switches and multiplexers for fiber-optical data transmission in telecommunications, and innovative implantable microsystems currently being developed to monitor, stimulate and deliver drugs. Laser micromachining has proven to be an effective tool to address specific manufacturing challenges for these devices. Investigations have been conducted on laser ablation for precise localized material removal, laser cutting, and drilling; and application data for a range of relevant materials already exists. In contrast, applications of laser joining are currently limited to microwelding and soldering of metals. The assembly of SMD’s and the sealing of pacemakers are typical examples.
This paper will describe the latest achievements in laser microjoining of dissimilar materials. The focus will be on glass, metal and polymer that have been joined using CO<sub>2</sub>, Nd:YAG and diode lasers. Results in joining similar and dissimilar materials in different joint configurations will be presented, as well as requirements for sample preparation and fixturing. The potential for applications in the optoelectronic and biomedical sector will be demonstrated.
New joining techniques are required for the variety of materials used in the manufacture of microsystems. Lasers are emerging as a useful tool for joining miniaturized devices. The beam can be focused to less than .001 inch allowing localized joining of very small geometries. There is minimal heat input into the part so distortion and change in material properties is minimal. The high quality of the laser welds and the precise process control enable hermetic sealing.
Commercially available high power diode lasers (HPDLs) with output powers of up to 6 kW have been recognized as an interesting tool for industrial applications. In certain fields of application they offer many advantages over Nd:YAG and CO<SUB>2</SUB> lasers because of their low maintenance, compact design and low capital costs. Examples of successful industrial implementation of HPDLs include plastic welding, surface hardening and heat conduction welding of stainless steel and aluminum. The joining of plastics with an HPDL offers the advantages of producing a weld seam with high strength, high consistency and superior appearance. One example is the keyless entry system introduced with the Mercedes E-class where the microelectronic circuits are embedded in a plastic housing. Other applications include instrument panels, cell phones, headlights and tail lights. Applications in the field of surface treatment of metals profit from the HPDL's inherent line-shaped focus and the homogeneous intensity distribution across this focus. An HPDL system is used within the industry to harden rails for coordinate measurement machines. This system contains a customized zoom optic to focus the laser light onto the rails. With the addition of a temperature control, even complex shapes can be hardened with a constant depth and minimum distortion.