Gerd Esser

Laser droplet weld: an innovative joining technology opens new application possibilities

Laser Droplet Welding is an innovative joining technology. The welding is realised by a laser generated liquid metal droplet which is deposited onto the parts to be joined. The raw material is a metal wire. In conventional laser welding a gap between the parts worsens the quality of the laser welded joint substantially. Contrarily a droplet offers sufficient material to bridge gaps. Even different gap sizes can be bridged by a suitable selection of the droplet size. A further advantage is the controllable heat transfer, only given by the heat content of a single drop that is sufficient to produce a high-temperature weld. The droplet heating provides the opportunity to weld small devices, thin coatings and even heat sensitive components without negative influence on their mechanical and electrical function. It is also possible to interconnect different materials by the addition of material supplied in form of drops. With the Laser Droplet Weld it is furthermore possible to join high reflective materials. This article describes the process and the system technology as well as achieved results. It will mainly focus on the droplet detachment which influences the complete process, e.g. the heat quantity or weld splashes.

Laser adjustable actuators for high-accuracy positioning of micro components

High-precision adjustment of smallest optical and electronic components is increasingly recognized as one of the key issues facing micromachining technology. As even narrow production tolerances for all individual parts are often not sufficient to match the tightly specified positioning accuracies of the complete assembly, in situ adjustment techniques are gaining more and more attention. Together with research partners from industry and science, the BLZ is developing a contact-free, laser-based adjustment method which allows high-accuracy adjustment of components mounted on specifically designed actuators. The underlying mechanisms do not depend on thermal effects but on selective laser ablation of prestressed layers of actuator substrate. This way, slightest deformations or modifications of particular mechanical properties can be initiated. The method promises to be more accurate and less time consuming than thermally induced laser bending.

Laser-based microalignment for fabrication of highly precise 2D fiber collimator arrays

In optical communication there are systems requiring a parallel transmission of several high bit rate data channels based on singlemode fibers. Especially for a free space transmission within such a system a precise collimation of the parallel channels is necessary to guarantee a low loss system. For this purpose compact two dimensional fiber collimator arrays can be used. A main quality characteristic for this arrays is the pointing accuracy, the angular deviation of the collimated beams to a theoretical optical axis. The angular deviation is caused by a lateral offset of the fiber axes to the axes of the corresponding micro lens. To minimize this offset we developed and analyzed an actor geometry for a fiber array which allows a laser based micro alignment of the fibers to the micro lenses. An alignment accuracy within submicrons can be realized which guarantees a pointing accuracy below 0.01 degrees. In this paper we demonstrate some important results of the FEM based analyzes to show the influence of different laser parameters for optimizing the alignment procedure and to minimize alignment time. Experimental results confirm the actor behavior calculated in the FE analyzes and demonstrate the qualification of laser based micro alignment of fibers for the assembly of highly precise two dimensional fiber collimator arrays.

Laser micro welding of copper and aluminium using filler materials

The most evident trend in electronics production is towards miniaturization. Regarding the materials involved, another trend can be observed: intelligent combinations of different materials. One example is the combination of copper and aluminium. Copper is the material of choice for electronic packaging applications due to its superior electrical and thermal conductivity. On the other hand, aluminium offers technical and economical advantages with respect to cost and component weight -- still providing thermal and electrical properties acceptable for numerous applications. Especially for high volume products, the best solution often seems to be a combination of both materials. This fact raises the question of joining copper and aluminium. With respect to miniaturization laser micro welding is a very promising joining technique. Unfortunately, the metallurgical incompatibility of copper and aluminium easily results in the formation of brittle intermetallic phases and segregations during laser welding, thus generating an unacceptable quality of the joints. This paper presents investigations on enhancing the quality during laser micro welding of copper and aluminium for applications in electronics production. In order to eliminate the formation of brittle intermetallic phases, the addition of a filter material in form of a foil has been investigated. It can be shown that the addition of pure metals such as nickel and especially silver significantly reduces the occurrence of brittle phases in the joining area and therefore leads to an increase in welding quality. The proper control of the volume fractions of copper, aluminium and filler material in the melting zone helps to avoid materials segregation and reduces residual stress, consequently leading to a reduction of crack affinity and a stabilization of the mechanical and electrical properties.

Laser-assisted fabrication of electronic circuits using the ADDIMID process

A novel laser-assisted technology for the additive fabrication of microelectronic circuits on three-dimensional polymer substrates (Molded Interconnect Devices, 3-D MID) has been developed. Advantages of the ADDIMID-approach are: a very short process chain, no etchants, no coatings (important on 3D substrates), industry-proven laser technology (diode-pumped Nd:YAG) and high writing velocity (greater than 600 mm/s). An essential component of the process is a special composite substrate material. The material consists of a polymer matrix containing finely dispersed microcapsules. The microcapsules are fabricated by coating micron-scaled copper powder with nano-scaled SiO2. The SiO2 coating provides electrical insulation of the copper particles and promotes adhesion to the polymer matrix. The microcapsules are mixed with a thermoplastic base material to form a granulate. Polymer substrates are produced by injection-molding. A laser direct-write process with galvanometric beam deflection is used to generate the circuit pattern. The laser uncovers the microcapsules and removes the SiO2 coating. Metallic copper is exposed in the processed surface regions. The exposed copper acts as catalytic nucleation site. The circuitry is then formed by chemical copper-plating. This paper presents experimental investigations on direct writing with a CO2- and a diode-pumped Nd:YAG-laser. Effects of variations in focus position, writing velocity, and pulse frequency are described and specified with regard to their impact on the quality of the circuit patterns. A phenomenological model of the laser direct-write process is outlined.

Laser-assisted generation of electronic circuits on tailored thermoplastics

This paper describes the tailoring of thermoplastics and the laser assisted generation of active regions that catalyze a chemical metal deposition in a liquid plating solution on these thermoplastics. The plated regions may serve as an electronic circuit on the insulating thermoplastic substrate. Tailoring of the thermoplastic materials is achieved by addition of a special activator powder. This powder consists of micro-encapsulated catalytic core particles with a non-catalytic barrier layer on their surface. Different encapsulation methods are compared for a copper core material. Laser activation is examined using a cw CO2 laser and an Nd:YAG laser. No activation is achieved with the CO2 laser. With the Nd:YAG laser and suitable process parameters, activation is possible even at writing speeds of up to 650 mm/s. A qualitative model is presented that explains the basic mechanisms of the laser activation process.

New developments in laser processing of silicon devices

Silicon is the standard material for the production of integrated circuits and one of the most important substrates for micro systems technology. It can be produced with an extraordinarily high purity, homogeneity and crystal perfection. Today, laser processing of silicon is becoming increasingly more interesting. This can be partly attributed to the evolution of frequency-converted solid state lasers which emit visible or ultraviolet radiation that is readily absorbed by silicon. Another reason for the growing interest in laser processing of silicon devices is that conventional technologies are approaching their limits. Especially laser cutting of thin silicon wafers as an alternative to mechanical sawing represents a very promising option for industrial applications. This paper shows current research results on laser processing of silicon. Besides laser cutting and ablation with frequency-tripled Nd:YAG lasers and Ti:Sapphire femtosecond lasers, laser welding of silicon with millisecond pulses is a focus of the presented work. When welding Si, the brittle behavior of the material usually leads to thermally induced cracks. These cracks do typically not occur when cutting with short and ultrashort pulsed lasers. A controlled heating of the work piece can prevent cracks during welding with millisecond pulses as well. Together with laser cutting and welding, laser adjustment of silicon components by ultrashort pulse ablation of pre-stressed layer systems, which is also described in this paper, is another promising approach for high precision manufacturing of silicon micro devices.

(001)-Textured Laser-Crystallized Silicon thin Films on Glass Substrates

We investigate the microstructure of polycrystalline silicon films (grain size, texture and grain boundary population) on glass substrates. These films are produced from amorphous silicon precursor layers by scanning the raw beam of a continuous wave Ar + - ion laser operated at a wavelength of 514 nm over the amorphous silicon thereby crystallizing it. The materials applicability for devices in large area electronics strongly depends on the orientation of the surface normal, the average grain size and the defect density and population. Transmission electron microscopy together with electron back-scattering diffraction analysis of the crystallized layers reveal grain widths of about 10μm and grain lengths of several 10 μm. Under certain procesing conditions a preferred (001)-surface normal orientation (texture) forms. The grain boundary population is dominated in the textured films by coincidence boundaries, essentially twin boundaries of first and second order as well as Σ=5 boundaries.

Contact mask technique for laser structuring of 3-D MID

Molded interconnect devices (MID) are promising polymer components for many industrial applications to make complex functional components containing circuits possible, and to avoid material and production costs. Current examples in the automotive field are a car dash board and a rear stop light. For suitable large scale production technologies for complex 3D MIDs, development of new manufacturing processes is essential. Different technologies are used to generate circuit patterns on the substrate by photoimaging. The geometrical information can be transformed by using different mask technologies. Besides the common mask projection technique, a contact mask which contains the circuit geometry is also suitable. The circuits are therefore generated in original scale by IR laser radiation on a polyvinyl chloride (PVC) sheet. The blackened polymer is nontransparent for the UV radiation of common UV lamps and excimer lasers in particular, used for imaging of photosensitive resists. For three-dimensional masks, the sheet is formed by a deep drawing process.