Electron beam lithography

To obtain resolutions better than the few μm of photolithography it is necessary to use either X-ray lithography or electron beam lithography. Here we give a brief overview of the latter technique. After development of the resist one can choose to etch the exposed part of the wafer. Acid will typically not etch the polymer photoresist but only the substrate, so etching will carve out the design defined by the mask. The shape of the etching depends on the acid and the substrate. It can be isotropic and have the same etch rate in all spatial directions, or it can by anisotropic with a very large etch rate in some specific directions. One can choose the etching process that is most suitable for the design. Metal deposition followed by lift-off is another core technique. Here a thin layer of metal (less than 500 nm) is deposited by evaporation technique on the substrate after de veloping the resist. At the exposed places the metal is deposited directly on the substrate, and elsewhere the metal is residing on top of the remaining photoresist. After the metal deposition the substrate is rinsed in a chemical that dissolves the photoresist and there by lift-off the metal residing on it. As a result a thin layer of metal is left on the surface of the wafer in the pattern defined by the photography mask. The above mentioned process steps can be repeated many times with different masks

and very complicated devices may be fabricated that way.

Electron beam lithography is based on a electron beam microscope, in which a focused beam of fast electrons are directed towards a resist-covered substrate. No mask is involved since the position of the electron beam can be controlled directly from a computer through electromagnetic lenses and deflectors. The electrons are produced with an electron gun, either by thermal emission from hot tungsten filament or by cold field emission. The emitted electrons are then accelerated by electrodes with a potential U ≈ 10 kV and the beam is focused by magnetic lenses and steered by electromagnetic deflectors. the electron is both a particle and a wave. The wavelength λ of an electron is given in terms by the momentum p of the electron and Planck’s constant h by the de Broglie relation Eq. λ = h/p. In the electron beam microscope the electron acquires a kinetic energy given by the acceleration voltage U as 1/2mv² = eU, where m and −e is the mass and charge of the electron, respectively. Since p = mv the expression for the wavelength λ becomes

λ = h/ √2meU

which for a standard potential of 10 kV yields λ = 0.012 nm .However, the resolution of an electron beam microscope is not given by λ. First of all, one can not focus the electron beam on such a small length scale. A typical beam spot size is around 0.1 nm. But more importantly are the scattering processes of the electrons inside the resist and the substrate. As illustrated by the computer simulation the backscattering of the electrons implies that an area much broader area is exposed to electrons than the area of the incoming electrons. This results in an increase of the resolution. It turns out that in practice it is difficult to get below a minimum linewidth of 10 nm. Electron beam lithography is still the technique with the best resolution for lithography. A major drawback of the method is the long expose time required to cover an entire wafer with patterns. The exposure time texp is inversely proportional to the current I in the electron beam and proportional to the clearing dose D (required charge per area) and the exposed area A,

texp = DA I I

In photolithography the entire wafer is exposed in one flash, like parallel processing, whereas in electron beam lithography it is necessary to write one pattern after the other in serial processing. For mass production electron beam lithography is therefore mainly used to fabricate masks for photolithography .