A New Liftoff Method for Small and Dense Nanostructures
Authors: Duan Tian Li , Wang Yao , Zhang Rui , Ma Xu Hang , Qu Xue Xuan 
Published: February 2020

Introduction

Nanostructures have unique properties which attract much attention and have a wide application. For example, metalens could replace the traditional optical components to develop a simple and effective optical system[1]. The key elements of AR and VR smart glasses are based on grating patterns with small dimension[2,3]. Nanostructures are mainly fabricated by electron beam lithography (EBL) which provides excellent patterning solution and flexible pattern variation[4,5].
After the EBL, nanostructures are transferred from the resist to the substrate by etching or liftoff. Because the resist is not a good hard-mask for dry etch and the nanostructures could be distorted during etching, a liftoff process is adopted for the nanostructures transfer[6]. Moreover, small and dense patterns in irregular shape are frequently designed in the nanostructures, so it becomes difficult to fabricate the nanostructures same as the designed-mask by a liftoff process. Therefore, a simple and effective liftoff method should be developed.

Experiment

A p type silicon wafer is used as the substrate, after the standard cleaning procedure (SC1, SC2, BOE), the wafer is baked at 180℃ for 10 mins. Following by the polymethyl methacrylate (PMMA) coating, the resist thickness is 230 nm. After coating, the wafer is baked at 150℃for 3 mins. The EBL with 80 KV acceleration voltage and a beam current of 9 nA is used. After the exposure, the sample is developed in a developer (IPA:MIBK=3：1) for 3 minutes. Then the wafer is dried by N2 gas.

Results and Discussions

The pattern used in the EBL is shown in Fig. 1 to explain the role of dummy pattern. Fig. 1(a) is the original mask pattern and the blue region is for exposure. A line of small square patterns with the dimension of 20 nm is added in the no-exposure region, as shown in Fig. 1(b) and (c).
After exposure, a 70 nm Chromium (Cr) film is deposited on the PMMA by ebeam evaporator, as shown Fig. 2(a), following by a liftoff process in acetone for about 5 mins. Then the sample is cleaned in DI water and dried by N2 gas, as shown in Fig. 2(b).
With dummy small patterns added on the liftoff region, after exposure and development, the dummy patterns could form a deep and thin valley on the resist but not contact the substrate. Following the metal deposition, the metal film may not fully cover the valley or form a week connection in the valley region. Therefore, the metal film is very easy to break in the valley when the sample is dipped in the acetone, which could results in a successful liftoff.
However, the exposure dose for the dummy pattern should be tested to find the optimum value; otherwise, the dummy pattern could appear in the nanostructure, as shown in Fig. 2(c), and destroy the function of the nanostructure.

Conclusions

A new liftoff method is present. With dummy small patterns added to the original mask pattern, a deep and thin valley on the resist but not contact the substrate is formed. This method leads to a simple and effective liftoff process.

Acknowledgements

The research was founded by “Research and application on high dense nanopillar by electron beam lithography”(Grant no:JCYJ20170817105420497)

References

1 J. Park, S. Bae, and K. Jeong, presented at the 2018 International Conference on Optical MEMS and Nanophotonics (OMN), 2018.

2 H. Raad, C. White, H. Schmitzer, D. Tierney, A. Issac, and A. Hammoodi, presented at the 2017 Progress in Electromagnetics Research Symposium - Fall (PIERS - FALL), 2017.

3 L. Silin, T. Shiqi, and X. Jinsong, presented at the 2016 Progress in Electromagnetic Research Symposium (PIERS), 2016 (unpublished).

4 H. Duan, D. Winston, J.K.W. Yang, B.M. Cord, V.R. Manfrinato, and K.K. Berggren,Sub-10-nm half-pitch electron-beam lithography by using poly(methyl methacrylate) as a negative resist Journal of Vacuum Science & Technology B 28, (2010) C6C58.

5 C. Nien, L.-C. Chang, J.-H. Ye, V.-C. Su, C.-H. Wu, and C.-H. Kuan,Proximity effect correction in electron-beam lithography based on computation of critical-development time with swarm intelligence Journal of Vacuum Science & Technology B 35, (2017)

6 M. Hatzakis, B.J. Canavello, and J.M. Shaw,Single-Step Optical Lift-Off Process IBM Journal of Research and Development 24, (1980) 452.

Figure 1