Research On The Method Of Hillock-Free Pure Aluminum Sputter Deposition

As a physical vapor deposition technology,pure aluminum sputtering is one of the main film-forming methods of TFT-LCD metal films,which has the advantages of low cost and stable process.However,because the thermal expansion coefficient of pure aluminum is much larger than that of glass,when heated,the expansion of aluminum film on the side of glass is limited.It will release compressive stress by atom diffusion and diffuse transversely along grain boundary to form hil ock.In TFT-LCD productions,hil ock will cause short circuit between gate and source,drain or transparent electrode（ITO）,and caused electrostatic damage,resulting in a large number of products scrapped,which is the major problem in the current pure aluminum magnetron sputtering process.At present,there are two solutions commonly used in the industry:one is to add capping layer with higher melting point（such as molybdenum）above the aluminum film,so that the compressive stress generated in the aluminum film is absorbed by the top metal film,so as to inhibit the growth of hills;the other one is to add a small amount of other elements（Nd,Ti,Zr,Ta,Si,SC,Cu）into the pure aluminum target to form solid solution through al oying,so as to increase the hardness of the metal film.The film formed at the grain boundary prevents the aluminum atoms from moving along the grain boundary,thus inhibiting the formation of hil ocks.But the first solution couldn’t restrain the occurrence of the hil ock at the edge of the line,while the second one would increase the resistivity and production cost.In this study,the formation mechanism of the hil ock was studied from the aspects of process parameters,residual gas,substrate carrier,product design,film thickness,glass thickness,CVD process,photoresist coating thickness of insulating protective layer and so on.It was found that sputtering temperature and annealing temperature have an important effect on the formation of the hil ock,and the formation of the hil ock can be inhibited by reducing the sputtering temperature and the preheating time of CVD.At the same time,this paper found that H₂O and N₂ in the residual gas have the greatest influence on the hil ock.By reducing the carrier stay time in the atmosphere side,prolonging the pretreatment time and optimizing the carrier design,the residual gas in the sputtering process can be effectively reduced,so as to inhibit the formation of the hil ock.The thickness and linewidth of the product film also affect the generation of hills.It was found that the thicker the metal film and the larger the linewidth,the easier the formation of hil ocks.It is worth mentioning that the self-cleaning cycle of CVD cavity and the photoresist thickness of insulation protection layer also have certain influe nce on the generation of hillocks.By adding process guard plate and increasing the photoresist thickness,the scrap ratio of the production can be further reduced.