Measurements Should Help Control Photoresist Process

As lithography processes move toward light sources with ever shorter wavelengths, patterning chips with ever finer features, it's no surprise that business is getting a little tricky. A considerable amount of the grief is falling on the shoulders of photoresist developers. Not only must they create films that will remain stable in much thinner layers (because of the increasing optical absorption that comes with decreasing laser wavelengths), but tighter critical dimensions are calling for tighter control.

Recognizing the difficult state of resists these days, researchers from the National Institute of Standards and Technology(NIST, Gaithersburg, Md.), the IBM T.J. Watson Research Center (Yorktown Heights, N.Y.) and the University of Texas (Austin, Texas) also recognized that there was a profound need for better resist measurement techniques. The level of control needed has been limited by a lack of direct measurements of the reaction front in today's chemically amplified photoresists, they point out in a report in the July 19 issue of Science. In the paper, the researchers detail X-ray and neutron reflectometry techniques used to measure complex reaction-diffusion processes at the nanometer level, providing a means to better understand the mechanisms and resolution limitations of polymer-based resists.

As the researchers point out, sub-100 nm CDs must be controlled over 2-5 nm length scales — comparable to the size of the polymeric molecules in the resists. Also, the thinner resists that are required with 193 nm lithography, for example, can display properties quite different than those of bulk resists.

Although the chemically amplified resists used with 193 nm lithography enable fine resolution, they must walk a tightrope between good acid mobility and excessive diffusion. Resolution ultimately depends on a complex relationship of parameters and molecular interactions that take place during the resist process. However, the researchers contend, previous measurements have not provided adequate information about spatial resolution or the reaction-diffusion models.

This is where X-ray and neutron reflectometry come in. Using the complementary reflectivity techniques on model bilayer structures, the researchers directly measured the reaction front profile before and after development. With these, they were able to measure both compositional and density profiles with nanometer resolution, enabling a better understanding of a resist's applicability for sub-100 nm printing. Ultimately, the researchers propose, these techniques could be used to develop and validate advanced simulation software used to design resist processes.

For additional information on lithography, go to www.semiconductor.net/lithography.