A REVIEW OF ION PROJECTION LITHOGRAPHY

Although optical lithography has been extended to far smaller dimensio ns than was predicted 15 years ago, there are definite physical barrie rs to extending it to the minimum dimensions of 70 nm that are project ed to be required 15 years from now. Both focused, point electron beam s and ion beams have been used to write dimensions in resist well belo w 20 nm, albeit at speeds far too slow for production lithography. Pro jection systems, which employ a mask and, in effect, produce a large a rray of beams, can provide both small minimum dimensions and high thro ughput. Ions are particularly well suited for this because they suffer little or no scattering in the resist, the linewidth is not a strong function of dose (good process latitude), and the resist sensitivity i s relatively independent to resist thickness or ion energy. IMS in Vie nna, Austria has built two generations of ion projection lithography s ystems which have demonstrated many of the features needed for high th roughput lithography. In these systems a stencil mask is irradiated wi th a uniform beam of light ions, H+, H-2(+), Or He+, and the transmitt ed pattern is demagnified (by 10x to 3x) and focused on a resist cover ed wafer at energies in the 70-150 keV range. So far, minimum dimensio ns down to 70 nm line-space pairs have been demonstrated, drift has be en eliminated with;a ''pattern lock'' servo system, and field distorti on of less than 0.15 mu m over 8x8 mm has been measured in agreement w ith calculations. Based on these achievements a new generation ion lit hography machine has been designed which uses 3x demagnification and w ill expose a 20x20 mm field at 0.12 mu m minimum dimensions with less than 10 nm of distortion introduced by the ion optics. Global and stoc hastic space charge effects have been modeled and, in some cases, meas ured with the existing machines. Stochastic space charge effects will not cause unacceptable blur in the new design if the total ion current is kept below 3 mu A. At this total current the time to expose one ch ip is still of order 0.5 s so that the calculated throughput is about 70 wafers (200 mm) per hour. Ion sources with low energy spread (simil ar to 2 eV) have been developed and will provide uniform illumination of the mask. Stencil mask fabrication on membranes of 2.5-mu m-thick s ilicon has been developed. Distortion of the pattern cut in the membra ne due to stress relief has been modeled, and with proper mask design can be kept below 20 nm (6.7 nm on the wafer). According to calculatio ns and measurements mask distortion due to ion beam heating can be red uced to a negligible level if a radiation cooling cylinder is used. As a result of the building and evaluation of the-existing machines and the design of the next generation, significant progress has been made in ion projection lithography which we will review in this article. Th e next step in the development of ion projection Lithography is being conducted by the MEDEA program in Europe, which will develop a full fi eld processing tool. (C) 1998 American Vacuum Society.