Microlithography with Soft X Rays

The use of soft x rays as a practical means of replicating patterns in the fabrication of electronic and optical microdevices was suggested by Spears and Smith(1, 2) in 1972. The approach is similar to contact x-ray microscopy, which has been used for several decades (see Chapter 8). Typically, x-ray wavelengths from 0.4 to 8.0 nm are used to proximity print Au mask patterns supported by thin transparent substrates with 0.1-µm resolution. X-ray lithography is an important alternative to optical lithography because it overcomes the fundamental limitations of diffraction and of shallow depth of field. Although x-ray replication is itself dependent on electron beam lithography for generating masks, it has inherently higher resolution for making device features. More importantly, it is a parallel rather than a serial exposure process, which tends to make it much more cost effective effective than direct electron beam wafer writing. In fact from an economic point of view, x-ray lithography is potentially competitive with optical lithography for fabricating electronic devices with 1.0-µm features. At 0.5-µm feature sizes x-ray lithography may be the only viable approach that has a throughput on the order of one wafer per minute. X-ray lithography systems intended to meet these micron and submicron volume production goals are being developed commercially.