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Electron-beam lithography provides better resolution then photolithography. This is possible because of small wavelength of the 10-50 KeV electrons. The resolution of electron-beam lithography system is not limited by diffraction, but by electron scattering in the resist and by the various aberrations of the electron optics. The electron-beam exposure system (EBES) machine has proved to be the best photomask pattern generator. However, the pattern writing is in serial form. Therefore, the throughput is much less than for optical systems. In the earlier years of development, electron-beam lithography was employed in the production of low-volume integrated circuits.
There is a formation of bonds or cross-links between polymer chains when negative resist is exposed to electron beam. However, bond breaking occurs in positive resist when it is exposed. The electron-beam induced cross-links between molecules of negative resist make the polymer less soluble in the developer solution. Resist sensitivity increases with increasing molecular weight. In positive resist the bond breaking process predominates. Thus exposure leads to lower molecular weight and greater solubility.
The polymer molecules in the unexposed resist will have a distribution of length or molecular weight and thus a distribution of sensitivities to radiation.
The narrower the distribution, the higher will be the contrast. High molecular weight and narrow distribution are advantageous.
The resist resolution is limited by swelling of the resist in the developer and electron scattering. Swelling is of more concern for the negative resist and this occurs in all types of lithography, that is, optical, electron, or X-ray. Swelling leads to poor adhesion of resist to the substrate. This problem becomes less severe as resist thickness is reduced.
There is also a fundamental process limitation on resolution. When electrons are incident on a resist or other material, they inter the material and lose energy by scattering, thus producing secondary electrons and X-rays. This limits the resolution to an extent that depends on resist thickness, beam energy, and substrate composition.
For thinner resist layers the resolution is better. Minimum thickness, however, is set by the need to keep defect density low and by resistance to etching as used while device processing. For photomasks where the surface is fiat and only a thin layer of chrome must be etched with a liquid etchant, resist thickness in the range of 0.2 to 0.4 micro meters are used. In case of more severe dry gas plasma etching process employed, thickness of 0.5 micro meters to 2 micro meters are required. One way to overcome this problem is to use a multilayer resist structure in which the thick bottom layer consists of the process-resistant polymer. A three-layer resist structure may be used in which the uppermost layer is used to pattern a thin intermediate layer, such as SiO2 which serves as a mask for etching the thick polymer below. For electron lithography a conducting layer can be substituted for the SiO2 layer to prevent charge build-up that can lead to beam placement errors.
Multilayer resist structure also alleviates the problem of proximity effect encountered during electron-beam exposure. In this, an exposed pattern element adjacent to another element receives exposure not only from the incident electron beam but also from scattered electrons from the adjacent elements. A two- layer resist structure is also used. In such structure, both the thin upper and the thick lower layer are positive electron resist, but they are developed in different solvents. The thick layer can be overdeveloped to provide the undercut profile that is ideal for lift-off process.
The first widespread use of electron-beam pattern generators has been in photomask making as discussed in previous section. The EBES machine, as stated earlier, has proved to be the best photomask pattern generator. Scanning electron-beam pattern generators are similar to scanning electron microscopes, from which they are derived. A basic probe-forming electron optical system may consist of two or more magnetic lenses and provisions for scanning the image and blanking the beam on the wafer image plane. Typical image spot sizes are in the range from 0.1 to 2 micro meters. This is for from the diffraction limits. Hence diffraction can be ignored. However, abberations of the final lens and of the deflection system will increase the size of the spot and can change its shape as well.
Electron Projection Printing
Electron projection system provides high resolution over a large field with high throughput. Rather than a small beam writing the pattern in serial fashion, a large beam provides parallel exposure of large area pattern. In a 1:1 projection system parallel electric and magnetic fields image electrons onto the wafer. The mask is of quartz and is patterned with chrome. It is covered with CsI on the side facing the wafer. Photoelectrons are generated on the mask/cathode by backside UV illumination.
The advantages of the projection system are stable mask, good resolution, fast step-repeat exposure with low sensitivity electron resists, large field, and fast alignment. The limitations of the system include proximity effects of electrons and shorter life of cathode.
Electron Proximity Printing
This is a step-repeat system in which a silicon membrane stencil mask containing one chip pattern is shadow printed onto the wafer. The mask cannot accommodate re-entrant geometries. Registration is accomplished by reference to alignment mask on each chip. An advantage of electron proximity printing is its ability to measure and compensate for mask distortions. Proximity effects must be treated by changing the size of pattern elements. The main limitation of the system is the need for two masks for each pattern.