Scanning tunneling microscope lithography: A solution to electron scattering

The scanning tunneling microscope (STM) operated in the field emission mode is shown to have important lithographic applications. The technological potential of the technique is demonstrated by patterning films up to 80 nm thick of SAL-601-ER7, a negative resist from Shipley. With the STM, 22 nm lines of developed resist have been written on Si and 35 nm lines on GaAs. For comparison, exposures were made with a 50 kV, 17 nm 1/e diameter electron beam in identically prepared and processed resist films on a variety of substrates. The 50 kV probe produced minimum linewidths of: 60 nm on a 200 nm Si3N4 membrane; 70 nm on a 200 nm Si3N4 film on a bulk Si substrate; 95 nm on a bulk Si substrate; and 186 nm on a bulk GaAs substrate. The strong substrate dependence indicates that the resolution, at 50 kV, is determined by electron scattering rather than the post exposure processing of the resist. Low voltage lithography with an STM offers a technique which greatly reduces the effects of electron scattering with a consequent improvement in resolution. In addition, the results of the Si3N4 film suggest a novel way to reduce the effects of backscattered electrons in 50 kV lithography.