Fabrication of high aspect ratio AFM probes with different materials inspired by TEM lift-out method

Supplemental material forFabrication of high aspect ratio AFM probes with different materials inspired by TEM "lift-out" method Swee Ching Tan a * Department of Materials Science and Eng

Supplemental material for

Fabrication of high aspect ratio AFM probes with different materials inspired by TEM "lift-out" method

Swee Ching Tan a) *

Department of Materials Science and Engineering, National University of Singapore, 5 Engineering Drive 2, Singapore 117579

Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.

Hangbo Zhao *

Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.

Carl V Thompson Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.

a)Electronic mail: msetansc@nus.edu.sg

* These authors contributed to the work equally.

Si NW Array Fabrication

The Si NW array was fabricated using a combination of interference lithography and metal assisted chemical etching The interference lithography was employed for

photoresist PFI-88 (Sumitomo Chemical Co.) The trilayer stack on the Si wafer was then exposed with a HeCd laser (λ = 325 nm) using a Lloyd’s mirror setup Two perpendicular exposures were performed to form diperiodic post structures By choosing the half angle

of the two incident beams,  , in the Lloyd’s mirror and the laser exposure time, post arrays of specific period and diameter can be produced For post arrays of 900 nm diameter and 1800 nm period, a half angle  = 5.18° and exposure time of 4 min 18s for a laser power of 0.165 µW was used After exposure, the sample was immersed in CD-26 developer (Shipley Micropost®) for 60 seconds to remove the unexposed PFI-88 photoresist In the subsequent reactive ion etching (RIE) process, a CF4 plasma (of power

150 W, pressure 10 mTorr) and He + O2 plasma (of power 300 W, pressure 10 mTorr) were used to transfer the photoresist post array pattern into the underlying SiO2 and ARC layers A 15 nm-thick gold layer was deposited onto the sample using electron beam evaporation A lift-off process was performed by soaking and sonicating the samples in N-Methyl-2-pyrrolidone (NMP) at 85°C for 5 min and 2 min, respectively The samples were then etched in a solution of HF, H2O2 and H2O (6:1:36) at room temperature For a nanowire height of 2.2 µm shown in Figure 2(b), the etch time was about 15 min

FIG S1 Photograph of the SEM stub with a sloped (22.1°) sample mounting surface

The actual height of the nanopillars was calculated based on the SEM software

measurement (2.06 µm, see Fig 2(b)) and the inclination angle of the slope of the SEM sample mounting stub (22.1 °, see Fig S1) Therefore the actual nanopillars height is

2.06m/ cos 22.1 2.22m