Manufacturing Bits: Dec. 8

Progress report on EUV resists; baby carbon nanotubes.


Progress report on EUV resists
The development of resists is a key part of extreme ultraviolet (EUV) lithography.

“EUV resists are production ready,” said Kevin Cummings, Sematech’s director of lithography. “However, through Sematech’s work with the resist suppliers, we have observed a deceleration in the rate of improvements. As a result, Sematech is working not only with the resist community to continue improvements, but also to help create new classes of resists and will allow improvements to accelerate EUV.”

One of the big challenges is resist outgassing. In outgassing, the mirrors used for EUV can become contaminated during exposure in the presence of some hydrocarbons. Resists are known to outgas during exposure to EUV radiation in a vacuum environment.

The reduction of resist outgassing to minimize or prevent possible contamination of EUV exposure tools is critical for the development of EUV resists. So, it is vital that resist chemistries meet stringent outgassing specifications before being used in an EUV scanner. But the resist learning cycle is long and inefficient. This is due in part to the lengthy outgas tests that resist material formulations must undergo before being subjected to exposure in a production EUV scanner.

In a joint collaboration with JSR, Sematech has experimentally demonstrated a new and improved evaluation method that reduces the amount of testing of commercial EUV resists from every formulation to just three samples per family. The results confirm that the concentration of the major components in a resist can be varied without the need for requalification. This, in turn, can result in a potential savings of a few hundred outgas tests for each resist family.

This achievement has the potential to realize substantial cost savings by significantly improving the resist learning cycle. “Sematech has been working with JSR over the last several months to make sure all test procedures meet industry guidelines for outgas testing. Now we are able to reduce the sample tests per each resist family for all resist suppliers, further enabling the infrastructure that will afford cost-effective EUV manufacturing,” Cummings said.

“As for outgas testing for EUV resists, there is still plenty of work to be done,” Cummings said. “For example, we still need to determine if novel (i.e., those that are not chemically amplified) resists can also be ‘family tested.’ What we have accomplished to date allows us to reduce of the number of outgas samples that need to be tested. We are estimating a 90% reduction. Our goal is to test all current EUV resists, and we believe this will still take a few years.”

Baby carbon nanotubes
Single-walled carbon nanotubes have the ability to conduct electricity at high rates of speed, making them attractive for use in next-generation transistors. But carbon nanotubes are challenging to make in production. The properties of carbon nanotubes are dependent on their structure. And the structure is determined when the nanotube is just beginning to form.

In a step towards understanding how nanotubes are formed, the National Institute of Standards and Technology (NIST), the University of Maryland, and Texas A&M have filmed these structures when they are only a few atoms old.

The research team was able to capture these images of carbon atoms forming into nanotubes by lowering the pressure to slow their growth rate. (Source: NIST)

Researchers were able to capture these images of carbon atoms forming into nanotubes by lowering the pressure to slow their growth rate. (Source: NIST)

These nanotube “baby pictures” give an insight into how they germinate and grow. The beginning of the growth process is called nucleation. To understand the process, researchers need to image the nucleation process as it happens. However, this is challenging. It involves a small number of fast-moving atoms. Researchers can image the atoms using fast, high-resolution cameras, but they are expensive.

To solve the problem, NIST slowed the growth rate by lowering the pressure inside a scanning transmission electron microscope. With the tool, researchers watched as carbon atoms generated from acetylene rained down onto 1.2nm bits of cobalt carbide, where they attached, formed into graphene, encircled the nanoparticle, and began to grow into nanotubes.

“Our observations showed that the carbon atoms attached only to the pure metal facets of the cobalt carbide nanoparticle, and not those facets interlaced with carbon atoms,” said NIST chemist Renu Sharma, on the agency’s Web site. “The burgeoning tube then grew above the cobalt-carbon facets until it found another pure metal surface to attach to, forming a closed cap. Carbon atoms continued to attach at the cobalt facets, pushing the previously formed graphene along toward the cap in a kind of carbon assembly line and lengthening the tube. This whole process took only a few seconds.”

According to Sharma, the carbon atoms seek out the most energetically favorable configurations as they form graphene on the cobalt carbide nanoparticle’s surface. The group’s next step will be to measure the chirality of the nanotubes as they grow. They plan to use metal nanoparticles with different facets to study their adhesive properties to see how they affect the tubes’ chirality and diameter.

  • How many times have we heard the following “EUV ________ is/are production ready, however ______________”. Readers should fill in the blanks (pardon the pun). For example, how about “masks” and “they cannot be inspected”. Or “throughput” and “we’ve had to change our productivity metric from wafers/hour to hours/metric”. OK, you try it.