A review of ion projection lithography

Abstract

Although optical lithography has been extended to far smaller dimensions than was predicted 15 years ago, there are definite physical barriers to extending it to the minimum dimensions of 70 nm that are projected to be required 15 years from now. Both focused, point electron beams and ion beams have been used to write dimensions in resist well below 20 nm, albeit at speeds far too slow for production lithography. Projection systems, which employ a mask and, in effect, produce a large array of beams, can provide both small minimum dimensions and high throughput. 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 is relatively independent to resist thickness or ion energy. IMS in Vienna, Austria has built two generations of ion projection lithography systems which have demonstrated many of the features needed for high throughput lithography. In these systems a stencil mask is irradiated with a uniform beam of light ions, H+, H2+, or He+, and the transmitted pattern is demagnified (by 10× to 3×) and focused on a resist covered wafer at energies in the 70–150 keV range. So far, minimum dimensions down to 70 nm line-space pairs have been demonstrated, drift has been eliminated with a “pattern lock” servo system, and field distortion of less than 0.15 μm over 8×8 mm has been measured in agreement with calculations. Based on these achievements a new generation ion lithography machine has been designed which uses 3× demagnification and will expose a 20×20 mm field at 0.12 μm minimum dimensions with less than 10 nm of distortion introduced by the ion optics. Global and stochastic space charge effects have been modeled and, in some cases, measured 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 μA. At this total current the time to expose one chip 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 (∼2 eV) have been developed and will provide uniform illumination of the mask. Stencil mask fabrication on membranes of 2.5-μm-thick silicon has been developed. Distortion of the pattern cut in the membrane 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 calculations and measurements mask distortion due to ion beam heating can be reduced 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. The next step in the development of ion projection lithography is being conducted by the MEDEA program in Europe, which will develop a full field processing tool.