Niels Kramer

Fabrication of metallic nanowires with a scanning tunneling microscope

A procedure to pattern thin metal films on a nanometer scale with a scanning tunneling microscope (STM) operating in air is reported. A 30 nm film of hydrogenated amorphous silicon (a‐Si:H) is deposited on a 10 nm film of TaIr. Applying a negative voltage between the STM tip and the a‐Si:H film causes the local oxidation of a‐Si:H. The oxide which is formed is used as a mask to wet etch the not‐oxidized a‐Si:H and subsequently, the remaining pattern is transferred into the metal film by Ar ion milling. Metal wires as narrow as 40 nm have been fabricated. Since a‐Si:H can be deposited in very thin layers on almost any substrate, the presented procedure can be applied to structure all kind of thin films on a nanometer scale.

Nanometer lithography on silicon and hydrogenated amorphous silicon with low energy electrons

The oxidation of a hydrogen terminated Si surface can locally be induced with a scanning tunnelling microscope (STM) operating in air or with a beam of free electrons in a controlled oxygen environment. The oxidation mechanism of both processes was studied and compared. The oxidation with the STM in air depends strongly on the applied tip‐substrate voltage and writing speed, but is not proportional to the tunnelling current. This is in contrast to the process with a beam of free electrons. The thickness of the electron beam induced oxide is studied as a function of electron energy, electron dose, and oxygen pressure. Oxide thicknesses of 0.5–3 nm are measured using Auger spectroscopy. The initial step of the oxidation process is the electron beam induced removal of hydrogen from the surface. The electron dose requirement for this step was determined as a function of electron energy. The dose is found to be minimal for 100 eV electrons, and is ≊4 mC/cm2. Oxide lines made with the STM on Si(110) were used as a mask to wet etch the pattern into the Si(110). With tetramethyl ammonium hydroxide, a selective anisotropic etch liquid, trenches with a width of 35 nm and a depth of 300 nm were made. We show that it is also possible to locally oxidize hydrogenated amorphous silicon (a‐Si:H) and use the oxide as an etching mask. Hydrogenated amorphous silicon has the advantage that it can be deposited in very thin layers on almost any substrate and therefore has great potential as STM and electron‐beam resist.

Resistless high resolution optical lithography on silicon

In this letter, we report on the high resolution patterning of a silicon surface without using a resist layer. A hydrogen passivated silicon surface is chemically modified by illumination with ultraviolet light (UV, λ=350.7 nm) in air. Auger electron spectroscopy (AES) revealed that silicon oxide was formed at the illuminated areas. A light interference pattern was made on the silicon surface by two UV laser beams, oxidation occurred only at the maximum intensity, but not at the minimum. In this way oxide lines were fabricated with a width below 200 nm on a 500 nm period. The oxide lines were used as a wet etch mask to etch more than 25 nm into Si(110) without affecting the oxide. The advantage of this technique is that it is a very simple process which allows the high resolution patterning over large areas of silicon without using a resist.

Nanolithography on hydrogen-terminated silicon by scanning-probe microscopy

Scanning-probe microscopes (SPM), i.e. the scanning-tunneling and force microscopes, can be used to locally oxidize hydrogen-terminated silicon and hydrogenated amorphous silicon. Because of its reliability and potential for pattern transfer, this lithography process has found great attention and has become a prototype process for SPM nanolithography. The local oxidization can be performed in ambient or ultra-high vacuum (UHV), and it is initiated by strong electric fields, electron impact, or by short-wavelength light. In this article, the progress of this subfield of nanolithography is reviewed. Emphasis will be on the process conducted in humid environments were a fairly solid understanding is emerging. For completeness, important experiments performed in UHV will be discussed briefly. Finally, recent applications of this process technique to the fabrication of electronic devices will be presented.