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SMT007-Aug2019

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AUGUST 2019 I SMT007 MAGAZINE 31 easier to determine consistency with thick- ness. In general, the brighter the coating, the thicker the application. Dewetting of coating is common when residues are present, and many times, on sharp edges of leads and component bodies. This may leave some areas more vul- nerable to the operating environment. Determining the level of adhesion can be done using IPC TM-650 2.4.1.6. This method uses a 10 x 10 grid of 1 mm x 1 mm squares etched into the coating with subsequent appli - cation of tape and removal of the tape with a steady motion at a 180° angle. Inspection after the tape pull is based on how many of the grid pieces were removed with the tape. The remaining grid area is judged on a scale from zero to five with zero being more than 65% removed and five having none of the coating removed. This is primarily done on test cou - pons during process evaluation and not on actual product. We recommend this test on the actual product if possible because that will give you a much better idea of what to expect when the coating is combined with the chosen material set. Now, it's time to circle back to the title of this month's installation: "Sealing Your Fate." After you apply your coating of choice, that is nor- mally the end of the process with no easy way to make repairs if necessary. You can certainly remove coating with a wide range of chemis- tries or dry ablation processes, but these are time-consuming and have their own inherent risk of introducing failure opportunities. The important lesson to take away from this col- umn is that coating does not always prevent failures; it is just as important to look at your cleanliness levels just as you would with an assembly that is not bound for coating. If you have a dirty assembly, you might be buying a little time, but ultimately, you've sealed your own fate. SMT007 Eric Camden is a lead investigator at Foresite Inc. To read past columns or contact Camden, click here. Direct After-fabrication Tailoring of MoS2-FET Transistors Transition metal dichalcogenides are discrete, 2D, atomically thin layers bound together by Van der Waals forces. They exhibit thickness-dependent variations in their physical properties that can be exploited in distinct optoelectronic applications. For example, the band structure of molybdenum disulfide (MoS2) has a direct bandgap of 1.8 eV in a single layer that narrows down with thickness, being 1.2 eV indi- rect bandgap in bulk. The atomically thin layers of MoS2 can be sep- arated by micromechanical exfoliation; nonethe- less, the fabrication of optoelectronic devices from mechanically exfoliated MoS2 is an intricate process. The geometry of the device is limited in all cases by the shape of the exfoliated flake, even when a deter- ministic stamping method is employed. Thus, developing techniques to tailor the device geometry after the fabrication steps are completed is of great interest. The group led by Professor Dan- iel Granados at IMDEA Nanociencia has come to a smart solution to modify the geometry of several field-effect transistors (FET) fabricated out of exfoli- ated MoS2. The proposed method uses a variation of focused electron beam-induced etching (FEBIE) with a pulsed electron beam. The beam scans the surface into a designed geometry, employing a pattern gen- erator, modifying the conduction channel between the source and the drain of the transistor, and allow- ing a tailor-made device performance. This method presents several advantages. It com- bines patterning and etching into a single step, allows electronic and optical characterization before and after the tailoring step, and the pulsed-FEBIE method has an electron beam energy lower than other stud- ies, reducing the sample damage and prevents the distortion of the MoS2 lattice. (Source: IMDEA Nanociencia)

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