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46 The PCB Design Magazine • June 2017 Inner sheet uniformity = A resistance instrument is applied to mea- sure resistance and each piece of base sheet has to sacrifice 60 points for measurement. Table 2 shows the measurement result. Table 2 indicates the resistance distribution of TaN thin film on a base sheet whose size is four inches. Accordingly, it can be summarized that a base sheet with No. 2 position features the best uniformity of inner sheet, while a base sheet close to the plate edge or target material edge features relatively bad square resistance variation, and the inner sheet uniformity of tar- get material close to target material edge is the worst. TaN thin films with bad uniformity can have a huge effect on high-accuracy network re- sistor manufacturing. To defeat non-uniformity of thin film close to the target material edge, a uniform plate can be installed to adjust deposited thin film be- cause it is capable of selectively covering depo- sition area to control film uniformity. Deposition Scanning Speed Analysis With the acceleration of scanning, square re- sistance of TaN thin film displays a trend of en- largement with linear improvement. The higher scanning speed is, the shorter the deposition time will be and so will the number of atoms on thin film. Film will be thinner as well. Three thin-film structures are available for processing: island-shaped structures, net-shaped structures, and continuous structures. The properties of thin film are closely related to its structure and elements. When the film is relatively thin, the film comes in a structure of island. With the film becomes thick, the island structure transforms into net structure and continuous structure. When it comes to resistor thin film, how- ever, three types of phase structures are avail- able: conductive, semiconductor, and insula- tion. In an island structure, conductive phase particles are scattered in thin film like shutter islands that are surrounded by insulation phase. Therefore, the film square resistance is relatively high. The net-shaped structure, however, is ac- tually a conductive network composed by the interconnection between conductive particles. Insulation phases are scattered inside the net- work with a low square resistance. Continuous structure is a type of continuously thin film composed of accumulating conductive particles densely, containing few insulation elements. As a result, thin-film square resistance is shrinking. Nitrogen Flow Analysis With the improvement of nitrogen flow ra- tio, square resistance of TaN thin film gradually goes up. This law works dramatically especially when nitrogen flow rate increases from 15% to 20%. That's because the increasing of nitrogen partial pressure leads to the increasing of Ta cav- ities and conductive type of thin film will con- vert from electronic conduction to cavity con- duction. Thus, square resistance will finally rise. The increasing of nitrogen flow rate leads to gradual reduction of TaN thin film thickness, which is opposite to the change trend of square resistance. Film thickness is closely associated with mean free path of sputtered particles and sputtering rate of target material. In summary, thin-film embedded resistors feature agreeable uniformity, leading to its suc- cessful applications in a wide range of industries. A great number of tests and experiments have been performed to demonstrate the reliability of thin-film embedded resistors. Therefore, it can be expected that thin-film embedded resistors can be highly relied in numerous electronic ap- plications. PCBDESIGN Dora Yang is a technical engineer from PCBCart, a China-based PCB production service provider. Contact the author through Twitter: @dorayang0227. Table 2: Sheet resistance of TaN thin film. PERFORMANCE EVALUATION OF THIN-FILM EMBEDDED RESISTORS