Large Area irradiation

Nuclear microprobes typically form a demagnified image of an object aperture using quadrupole lenses, with the beam focused in the chamber. Since their maximum scan size is only a few square millimeters, larger wafer areas cannot easily be patterned, which is most direct beam writing applications are restricted to millimeter-size areas. Also, since the beam current within the focused spot is only picoamperes, the time required to pattern large areas is prohibitive. A further limitation is that any fluctuations in beam current results in variations in fluence at different positions, resulting in rough surfaces after anodization.

At CIBA we have made simple modifications were made to a megavolt accelerator and nuclear microprobe to give a facility capable of large area irradiation at MeV ion energies. This provides a uniform patterning of exposed areas, though more complex patterning with different fluences may be achieved using a multilevel photoresist mask. The large-area irradiation facility shown in Fig. 1 uses the standard microprobe lens focusing system but with the collimator and object apertures opened wide to give a focused beam current of several hundred nanoamperes within the microprobe chamber. For a beam divergence angle into the quadrupole lenses of, say, ±0.4 mrad the beam convergence at the focal plane is ±32 mrad (about ±2º) for a microprobe demagnification of 80. The wafer is positioned about 50 cm downstream of the focal plane, where the highly divergent beam is uniformly distributed over more than 30 x 30 mm2. A longer drift length would provide an even larger irradiated area but 50 cm provides the capability to irradiate areas of 1 square inch. A fluorescent screen is first placed at the wafer location to view the beam uniformity, since the distribution passing through the large collimator aperture may not be uniform.

Fig 1. Schematic of microprobe ion optical system used to project a large area, uniform intensity of beam over wafer surface which is located downstream of the chamber.

Large area patterning through multilevel photoresist mask

The fabrication and use of a multilevel photoresist (PR) mask structure gives the capability of creating more complex large-area patterned regions in conjunction with the irradiation system described above. Such a multilevel mask is necessary because different amounts of damage to the underlying silicon are required to produce different amounts of damage to the underlying silicon. Ions of two or three different energies are used to irradiate the same area, and only those above a certain energy determined by the mask thickness penetrate the wafer, see Figure 2.

Fig. 2. SRIM plots of 400 and 700 keV H2 + ions incident on different thicknesses of PR on top of a 7-μm thick Si layer (shown in gray). From left to right the PR thicknesses are 7μm, 3μm and 0 (a) trajectory plots showing how far each ion penetrates through the layer structure. (b) defect density in each case. Both beam energies are shown, with the higher energy to the right

Figure 2 shows SRIM simulations which summarize the principles underlying the use of a multilevel PR structure. The wafer and PR are irradiated with two molecular hydrogen (H2+) ion energies, 400 keV and 700 keV, equivalent to twice the fluence of 200 keV and 350 keV protons. Molecular hydrogen is used rather than protons since with a megavolt accelerator higher energies are more easily attained with better beam brightness. A PR layer, which is thick enough to provide good energy differentiation between these two energies, is required. The thickest PR mask regions (left-most case) stop both beam energies leaving those wafer portions unirradiated. Both beam energies are transmitted with their full energy through the exposed wafer surface (right-most case) so both contribute to wafer damage. For the intermediate PR thickness (centre case), the layer is thick enough to stop the lower energy ions, and only the higher energy ions contribute to damaging the wafer.