Metal Patterning Based on Femtosecond Laser Direct Writing(Invited)
Objective Femtosecond laser direct writing is a new nanofabrication technology.However,achieving high-quality pattern transfer directly on metal surfaces is difficult owing to the photothermal effect between the femtosecond laser and metal,which can result in laser ablation.To address this,we develop a new metal pattern transfer technology that combines femtosecond laser direct writing technology and the lift-off process.This technology enables precise metal pattern transfer at a sub-micron level(0.89 μm).We explore the effects of femtosecond laser direct writing parameters and development methods and parameters on the performance of metal transfer.We observe that the accuracy of metal transfer increases with higher femtosecond laser exposure doses.Meanwhile,the dry development method outperforms the wet development method.Increasing the development time of the sacrificial layer results in a gradual increase in the transfer line width,accompanied by improved roughness of the line.By incorporating a specific undercut angle in the metal transfer process,we successfully address the edge warpage and high roughness of the metal lines.In this study,a chromium-based transfer grating with 2 inch(1 inch=2.54 cm)diameter is fabricated using the new approach.Additionally,the feasibility of using the prepared chromium pattern as a mask plate is verified using projection exposure on SU-8 photoresist,thus demonstrating the potential of the strategy to locally replace the e-beam lithography for mask plate processing.In addition,Au and Pt patterns are successfully transferred using this approach,demonstrating its universality and wide range of potential applications.Methods The lift-off process based on femtosecond laser processing comprises sample preparation,femtosecond laser exposure,development,sputtering of metal,and stripping.1)Sample preparation:The quartz substrate is cleaned using an ultrasonic machine and vacuum plasma machine to remove impurities.Thereafter,a sacrificial layer is spin-coated onto the quartz substrate and baked on a hot plate.Next,photoresist is added dropwise onto the sacrificial layer.2)Femtosecond laser direct writing:A femtosecond laser is used to expose the prepared samples.3)Development:The exposed samples are developed by immersing them successively in propylene glycol methyl ether acetate(PGMEA)and isopropyl alcohol(IPA)to wash away the unexposed photoresist.The sacrificial layer is then removed by immersion in NMD-3 solution.Dry development is performed on a microwave plasma debonder.4)Sputtering of metals:Chromium and gold plating is performed using magnetron sputtering equipment,and platinum plating is performed using a magnetron ion sputtering apparatus.5)Stripping:The residual photoresist and sacrificial layer are removed using the degumming solution,N-methyl pyrrolidone(NMP),through immersion and ultrasonication,followed by rinsing with deionized water.The samples are left to air dry to complete the transfer of the pattern of the metal.Results and Discussions This study explores the effects of sacrificial layer development methods and parameters on chrome transfer.The results in Figs.3(a)-(c)indicate that the wet developing method has many defects and is ineffective,whereas the dry development method achieves high-quality chrome transfer within an optimal development time of 20 min[Figs.3(d)-(f)].Next,we investigate the effect of femtosecond laser direct writing process parameters on chrome transfer properties.The results show that increasing the exposure dose decreases the width of the chrome lines(Fig.4).At the direct writing speed of 20 mm/s and direct writing power of 47.5 mW,the line width is as small as 890 nm,albeit with warpage at the line edges.To solve these problems,we design and introduce the undercut angle in the photoresist to protect the chrome lines from the negative effects of the lift-off process.Figure 5(c)shows that the transferred chrome line warps severely on both sides without the undercut angle.When the undercut angle is 10°,the edges of the chrome lines are smoother and the chrome lines tightly adhere to the substrate without any warpage[Fig.5(d)].Thereafter,the edge roughness of the chrome lines is reduced by optimizing the sacrificial layer dry-developing process(Fig.6).Finally,we explore the large-area pattern transfer and multi-metal pattern transfer capabilities of this strategy.Combining the femtosecond laser direct-writing technique and lift-off process,we successfully achieve a 2-inch chromium-based transfer grating and verify the feasibility of using the chromium pattern as a mask plate for projected exposure on SU-8 photoresist.Meanwhile,Au and Pt patterns are successfully transferred using this strategy,demonstrating its universality.Conclusions A novel metal pattern transfer technique is developed by combining femtosecond laser direct writing technology and the lift-off process.In the lift-off process,a sacrificial layer is introduced and a dry development strategy is used to achieve metal pattern transfer with submicron(0.89 μm)precision.The effects of femtosecond laser direct writing parameters and development methods and parameters on metal transfer are explored.We observe that the accuracy of metal transfer increases with an increase in the femtosecond laser exposure dose.Increasing the development time of the sacrificial layer increases the transfer line width and significantly improves the roughness of the line.Moreover,the problems of edge warpage and high roughness of metal lines are successfully addressed by incorporating a specific undercut angle in the metal transfer process.In addition,a 2-inch chrome-based transfer grating is achieved using the proposed approach.The feasibility of using the prepared chrome pattern as a mask plate for projected exposure on SU-8 photoresist is confirmed.Finally,the successful transfer of Au and Pt patterns demonstrates the universality of this approach.
laser techniquefemtosecond laser direct writinglift-off processmetal patterningmask plate processingphotoresist
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维普
