Current lithography systems are mostly all optical, deep UV systems. These systems use light in the deep UV region, i.e. 193 nm. Due to the fact that these systems are all optical, the resolution is limited.
One way of realising smaller resolutions is by using particle beams, especially electron beams. A known system uses masks just like all optical systems. The masks are situated between a substrate and an electron source in order to blind off parts of the electron beam. In that way, patterns are transferred to a resist. The system, however, has its drawbacks. First, the details on the mask have to be very small, about 100 nm, making these masks very difficult to produce. Furthermore, as electron beams are more energetic than light beams, the mask heats up.
Another way of increasing the resolution is disclosed in WO 98/54620. In this system, a conventional optical system using a mask is combined with an electron beam system. A light source produces a light beam, preferably in deep UV. The light beam impinges on a micro lens array having a plurality of lenses. The micro lens array divides the light beam into light beamlets. In practice, there may be as many as 107-108 light beamlets. The lenses of the micro lens array focus the light beamlets on a mask. The light leaving the mask passes a de-magnifier. The demagnifier focuses the light beamlets on a converter plate having a plurality of converter elements, Each converter element arranged for converting impinging light into an electron beam. The spot size of each electron beamlet is 100 nm or smaller, making the lithography system capable of writing details smaller than 100 nm. This system uses a mask and a complex optical system. The distance between two adjacent converter elements is in general larger than the width of an electron beam resulting from a converter element. A method of transferring a pattern onto a wafer, is scanning the mask with the light beamlets and simultaneously scanning the wafer with the electron beamlets. The mask is moved in one direction and at the same time, the wafer is moved in the opposite direction. The lithography system disclosed in WO 98/54620 uses a system of demagnifying optics, micro lens array, UV beam and mask to activate converter elements.
An alternative to these systems is a mask-less lithography system or so called ‘direct write’ system. Many direct write systems, in particular direct write systems using electron beams, are known in the art.
A very simple embodiment uses one cathode producing an electron beam with a very small diameter, less than 100 nm. By scanning this beam over a substrate and switching it on and off, a pattern can be transferred to the substrate. This is called raster scanning. Such a system using raster scanning method is very slow.
Alternatively, a system using line of cathodes is known. Using a line of electron beams, an entire strip of a pattern can be transferred at the same time. Still this system is not fast enough for transferring an entire pattern onto a wafer fast enough for mass production purposes.
Another system, for instance disclosed in WO 98/48443, comprises an array of cathodes. By switching individual cathodes on and off, all at the same time, a first part of a pattern is created. Using electron lenses, this part of the pattern is reduced in size in its entirety as if the electron beams were one single beam, and the part of the pattern is transferred to the substrate. After this step, a second part of the pattern is created by switching other cathodes on and off. This second part is subsequently transferred to the substrate, and so on, until a complete pattern is transferred to the substrate. One disadvantage of this method is that the electron beams are very close together. Due to aberrations of the electron lenses, a lot of distortion occurs. Furthermore, as the beams have to be focussed, use is made of lenses causing the beams to converge at one point along the beam path, causing even more problems due to coulomb interactions. Furthermore, processing time is a problem, because the writing field is small, which necessitates many movements of the wafer stage.
Yet another known system is described in U.S. Pat. No. 5,969,362. This system requires a multitude of cathodes, very closely spaced: 600 nm or less. The cathodes are electrically activated using a grid of wires. The system thus requires complex electrical systems for controlling a large number of cathodes. It is difficult to prevent crosstalk between the electrical systems as they are very close together. An entire pattern is transferred by moving the wafer in the X-Y plane using the wafer stage, putting a heavy burden on the mechanical system.
Another known system is described in U.S. Pat. No. 6,014,203. In this system, a field emission array comprising as many as 107 cathodes per square cm is used as an electron beam source. The field emission array is provided with photodiodes. These photodiodes are optically activated and on their turn electrically activate the cathodes. A pattern is transferred by projecting a multitude of LCD displays subsequently onto the photodiodes of the field emission array, requiring a complex optical and mechanical system. The system further comprises a focussing magnet and a steering magnet. Using the steering magnet, each electron beam is scanned in the X- and Y direction. All the electron beams are scanned simultaneously. In order to realise a high data rate, a multitude of LCD screens are one by one projected on the field emission array, requiring a complex optical and mechanical system. And even using very many LCD screens, it is still not possible to realise the data rate needed for the economical feasible production of chips.
Still another approach concerns a system which splits one electron beam up into a plurality (for instance 64×64) of small electron beams. Each small beam has its own electrostatic lens system reducing the size of each small beam. Furthermore, the lens system scans each beam over an area of, e.g., 4×4 microns. Furthermore, a blanking aperture array is provided and a deflector for each small beam. The deflector is capable of deflecting a small beam out of the aperture area, thus blanking the small beam when needed. The system uses wafer stage scanning to transfer a complete pattern. With this system, however, it is not possible to obtain high productivity, because the writing field is small, which also necessitates many movements of the wafer stages.