Electron Beam Lithography
Electron Beam Lithography is a technique vital to semiconductor manufacturing, where by a Specialized type of Scanning Electron Microscope is used to selectively expose a Electron / photosensitive chemical layer on wafers, or glass plates (or other suitable transparent substrate) in order to create the microstructure of semiconductor intigrated circuits.
The thus gained patterned photoresist is then used, after developing, to either directly etch or ion implant the semiconductor material (among other techniques) or used to create exposure masks for Photolithography machines used in large scale semiconductor manufacturing.
Electron beam lithography has the downside, that its write speed is exceptionally slow in comparison to photolithography. This can be easily understood by the means by which the writing is taking place. With a Photolithography machine, the entire area of the die is exposed at the same time (this usually being a small square that is duplicated over the area of a circular wafer by means of a wafer stepper), much like a photographic enlarger exposes the photopaper with the image of the negative, all at once. On the other hand, the electron beam lithography machine must expose everything by writing the areas with a single moving Pixel, of usually round shape. Thus in order to create a square area, such as is common in semiconductor circuits, the beam must Raster scan this area, thus taking a significant amount of time. There also exists a limit of write speed imposed by physics, this being the space charge limited spot size, in essence, the smaller the writing spot is to be, the less current density it must have, the higher the accelerating voltage must be, or the shorter the lens focal length has to be. The amount of charge needed to expose the photoresist is fixed, thus if a small spot is to be used, the current is usually reduced in order to obtain it, thus increasing dwell time, which in turn slows down the total write speed.
A notable interesting development to this was IBM's electron beam lithography systems, notably the ELS-1, EBS, FELS-1 which all notably featured the ability to write with a square beam profile, which could also be compressed in one axis to directly write the entire area in one shot. This significantly reduced the writing time needed to expose a wafer, to the point where IBM employed the ELS-1 in volume manufacturing of processors for a time. This technology however, as all things do, comes with a cost, this being the complexity of the apparatus, its difficulty of use and its immense cost to produce. These remarkable IBM machines have never been (at least to the knowledge of the author) made more then one or twice pre model. The ELS-1 being the only one to have been made twice. They where mostly used to pattern Photolithography masks for production, as well as prototype development, not direct production line use.
It can be seen, that in theory any Scanning Electron Microscope can in theory be used as a Electron Beam Lithography machine, this is indeed correct, but it requires the microscope to have the ability to switch off its writing beam. This is known as beam blanking. As well as, if circuits larger then the field of view of the microscope are to be made, be equipped with a closed loop stage of excellent prallelity. The latter is accomplished in commercial and dedicated machines, via the use of Laser Interferometers as feedback mechanisms to servomotor and or Piezo controlled stages. Additionally, the XY axis of writing must match to within acceptable tolerances those of the stage mechanism, or be compensated electrically, so as to align the FOV segments with one another.
Also of note, is that the optics must be of such quality, that at the maximum write extent from the electron optical column, which is to be used, the beam focus can not change more then the maximum allowable size error of the circuit, in addition to not be distorted away from the intended position by optical Aberration. Most of these problems however, only occur at very small feature sizes, and thus in laboratory enviroments, the dedicated electron beam lithography machine has been supplanted by modified scanning electron microscopes.