Abstract
We have developed an additive process fabricating copper gate of amorphous thin-film transistor (α-Si-TFT) by selective electrodeposition through mask. A tapered edge shape of Cu deposit is a key requirement for gate electrode of α-Si-TFT, which can be achieved by the addition of organic additives (e.g. polyethylene glycol (PEG), bis-3-sodiumsulfopropyl disulfide (SPS)) in basic Cu bath. The influence of individual organic additive on edge shape of Cu deposit on Cu seed was also clarified. It was found that Cu electrodeposition without organic additive results in rough deposit and a pattern shape defined by the mask shape. PEG additive can refine the deposit morphology but the pattern shape is still defined by the mask shape. It is interesting that the deposit with SPS alone grows in an oblique direction away from the mask sidewall on the edge of deposited pattern and produces a tapered pattern independent of the mask sidewall. When the Cu bath contains both PEG and SPS, the combined effect results in a tapered pattern with fine morphology, smooth surface and low sheet-resistance, which is suitable for TFT array. And, SPS plays a key role in creating a tapered Cu deposit when the bath contains PEG and SPS. How SPS causes the tapered pattern during electrodeposition could be explained in terms of its self-assembled monolayers (SAMs) behavior. A tapered shape of Cu deposit could also form on copper seed pre-adsorbed with SPS although the copper bath itself did not contain SPS. The influence of SPS and Cl- on the sidewall shape of Cu pattern was identified. The influence of self-assembled disulfide and related thiolates with various end-groups on the edge shape of Cu pattern was also explored. A tapered pattern disappears during electrodeposition through mask when a Cu bath contains SPS in the absence of Cl-. Sulfonate end-group of thiolate also plays a key role in forming sloped sidewall of Cu pattern. Therefore, mechanism illustrating sloped sidewall of Cu deposit adjacent to photoresist mask during electrodeposition in a Cu bath containing SPS and Cl- was proposed in terms of SAMs and electrochemical behavior of SPS and Cl-. Besides, the influence of PEG with various molecular weights on the electrochemical behavior, surface morphology and edge shape of Cu deposit was studied. Low polarization, deposit with big grain and tapered edge shape of Cu pattern were typical for a Cu bath containing PEG with MW 200. In contrast, high polarization, deposit with fine grain and shape of Cu deposit defined by the shape of P/R mask represent for a Cu bath containing PEG with MW 2000, 4000 and 10000. Furthermore, after suitable acid pretreatment, the tapered Cu deposit could also be achieved on Ni seed layer as well as on Cu seed. The multilayer copper gate was fabricated after selectively etching of exposed nickel layer. The new method provides an alternative wet process for fabricating Cu gate of α-Si-TFT.