Extending Reticle Life Through Better Cleaning Budgets
Michael A. Peters and Bob Puharic
Production fabs are constantly balancing the cost of manufacturing against yield. As device sizes shrink, there is an additional factor with which lithography engineers and fab manager must now content: the relationship between reticle cleaning and printed image quality.
It is well known that reticles require maintenance in order to provide high-quality images necessary for device yield. Historically, reticles made of chrome and quartz protected with a pellicle were relatively simple to clean and maintain. Unless there was handling damage, reticles had a long service life. When cleaning was required, traditional chemical cleaning processes, complimented by soft brush scrubbing were quite effective. Even though the cleaning changed the reticle characteristics, the allowance was large enough to enable a few cleaning cycles prior to reticle “retirement.”
Due to reduced feature sizes and changing lithography technology, this allowance is no longer as forgiving. Advanced product designs, tighter process tolerances and yield goals require today’s advanced reticles to print a near-perfect image quality onto the wafer. Furthermore, with the increasing cost of reticle purchases, extending the usable life is a key element of every fab manager’s objectives.
These trends and requirements are driving the leading edge of production reticle management. Like the industry facing the challenges of new materials, larger wafer sizes and smaller linewidths, lithography faces a similar challenge of smaller linewidths with optical proximity correction (OPC) features, new reticle materials such as MoSi, and new image projection techniques using EPSM and EUV. With all of these changes taking place while managers take aim on the goal of printing a perfect images on each wafer produced, managing the reticles in production is becoming more complex with each technological advancement.
Image Degradation and Reticle Life
When a reticle is used in production, handling, exposures and particles all contribute to degeneration in the printed image quality.
The basic goal of cleaning a reticle is to remove organics, ionics, and particles. Organics, which attenuate the exposure, and particle adders, which affect the image, both decrease the resulting image quality and directly affect the device yield.
The International Technology Roadmap for Semiconductors (ITRS) has an aggressive schedule continuing through the 45 nm node that identifies defect reduction technology as one of the top 10 goals to be met.
Organic Contaminant Removal
Reticle maintenance technology is also changing. Contact cleaning with brushes is now unacceptable. It is both inefficient for small geometries, and its contact causes damage to the delicate structures.
For maximum flexibility to address the cleans and the available budget, a reticle manager may choose to utilize a mix of sulfuric peroxide and ozonated DI for a process best suited to the individual needs of the production fab.
The reticle manager now has enormous cleaning flexibility and a set of guidelines in which to match reticle maintenance programs to image quality objectives. Manufacturers with different production models can now tailor reticle maintenance programs based on their specific needs. High-product-mix producers can clean more often, stay within budget and reap better yields. High-volume producers can keep the same cleaning tolerances and, with less budget expended per clean, significantly extend the reticles’ useful production life. Regardless of the manufacturing model, users will benefit either their yields or manufacturing costs with a reticle management program and newer cleaning technologies.