The efficiency of luminescent solar concentrators could be enhanced by use of wavelength-selective filters, reducing the amount of luminescent light lost. To accomplish this, polarization-independent filters with reflectivity >97% were made by combining layers of cholesteric liquid crystals, either a right- with a left-handed layer, or two right-handed layers with a half-lambda waveplate. Normal cholesteric filters have a reflection bandwidth which is narrower than the spectral and angular range of the luminescent emission. The reflection band is broadened from 80 to 200 nm by employing a pitch gradient in the cholesteric layer. The measured transmission bands compare well with calculations.
Laser induced forward transfer (LIFT) is a freeform, additive patterning technique capable of depositing high resolution metal structures. A laser pulse is used to generate small droplets from the donor material, defined by the spot size and energy of the pulse. Metallic as well as non-metallic materials can be patterned using this method. Being a contactless, additive and high resolution patterning technique, this method enables fabrication of multi-layer circuits, enabling bridge printing, thereby decreasing component spacing. Here we demonstrate copper droplet formation from a thin film donor. The investigation of the LIFT process is done via shadowgraphy and provides detailed insight on the droplet formation. Of particular importance is the interplay of the droplet jetting mechanism and the spacing between donor and receiving substrate on a stable printing process. Parameters such as the influence of laser fluence and donor thickness on the formation of droplets are discussed. An angle deviation analysis of the copper droplets during flight is carried out to estimate the pointing accuracy of the transfer. The possibility of understanding the droplet formation, could allow for stable droplets transferred with large gaps, simplifying the process for patterning continuous high-resolution conductive lines.
Direct-write technologies can form a low-cost, alternative approach to create electrical interconnects by eliminating mask and etch costs. Also, direct-write is more efficient in creating complex structures as well as for producing small series. However, existing, industrially-mature direct-write technologies typically lack the resolution required for advanced IC packaging applications [1][2][3][4]. Laser Induced Forward Transfer (LIFT) is a direct write process which has been proven to be capable of patterning resolutions in the 1-5 µm range [5][6][7][8]. Thus far, a lack of deposition control resulting in contamination of the substrate has been a problem. The current paper shows an approach to come to a robust, contamination-free process window for LIFT of pure copper. Thus, we tackled a major roadblock towards the industrial feasibility of LIFT as a full metal direct-write technology that meets the current demands for IC packaging and integration.
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