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Date: 21 May 2018
NIST Scientists measure nanoscale details of photolithography process  

Topic Name: NIST Scientists measure nanoscale details of photolithography process
Category: Electronics
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Research persons: NIST Research Team

Location: National Institute of Standards and Technology (NIST), United States


NIST Scientists measure nanoscale details of photolithography process

Scientists at the National
Institute of Standards and Technology
(NIST) have made the first direct
measurements of the infinitesimal expansion and collapse of thin polymer films
used in the manufacture of advanced semiconductor
devices. It’s a matter of only a couple of nanometers, but it can be enough to
affect the performance of next-generation chip manufacturing. The NIST
measurements, detailed in a new paper, offer a new insight into the complex
chemistry that enables the mass production of powerful new integrated circuits.
The smallest critical features in memory or processor chips include
transistor “gates.” In today’s most advanced chips, gate length is about
45 nanometers, and the industry is aiming for 32-nanometer gates. To build the
nearly one billion transistors in modern microprocessors,
manufacturers use photolithography,
the high-tech, nanoscale version of printing technology. The semiconductor wafer
is coated with a thin film of photoresist, a polymer-based formulation, and
exposed with a desired pattern using masks and short wavelength light (193 nm).
The light changes the solubility of the exposed portions of the resist, and a
developer fluid is used to wash the resist away, leaving the pattern which is
used for further processing.
Exactly what happens at the interface between the exposed and unexposed
photoresist has become an important issue for the design of 32-nanometer
processes. Most of the exposed areas of the photoresist swell slightly and
dissolve away when washed with the developer. However this swelling can induce
the polymer formulation to separate (like oil and water) and alter the unexposed
portions of the resist at the edges of the pattern, roughening the edge. For a
32-nanometer feature, manufacturers want to hold this roughness to at most about
two or three nanometers.
Industry models of the process have assumed a fairly simple relationship in
which edge roughness in the exposed “latent” image in the photoresist
transfers directly to the developed pattern, but the NIST measurements reveal a
much more complicated process. By substituting deuterium-based heavy water in
the chemistry, the NIST team was able to use neutrons to observe the entire
process at a nanometer scale. They found that at the edges of exposed areas the
photoresist components interact to allow the developer to penetrate several
nanometers into the unexposed resist. This interface region swells up and
remains swollen during the rinsing process, collapsing when the surface is
dried. The magnitude of the swelling is significantly larger than the molecules
in the resist, and the end effect can limit the ability of the photoresist to
achieve the needed edge resolution. On the plus side, say the researchers, their
measurements give new insight into how the resist chemistry could be modified to
control the swelling to optimal levels.
Note for Photoresist
Photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.
Photoresists are classified into two groups, positive resists and negative resists.
A positive resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer and the portion of the photoresist that is unexposed remains insoluble to the photoresist developer.
A negative resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes relatively insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer.
Photoresists can be exposed by electron beams, producing the same results as exposure by light. The main difference is that while photons are absorbed, depositing all their energy at once, electrons deposit their energy gradually, and scatter within the photoresist during this process. As with high-energy wavelengths, many transitions are excited by electron beams, and heating and outgassing are still a concern. The dissociation energy for a C-C bond is 3.6 eV. Secondary electrons generated by primary ionizing radiation have energies sufficient to dissociate this bond, causing scission. In addition, the low-energy electrons have a longer photoresist interaction time due to their lower speed. Scission breaks the original polymer into segments of lower molecular weight, which are more readily dissolved in a solvent.

The research, funded by SEMATECH, is part of a NIST-industry effort to better
understand the complex chemistry of photoresists in order to meet the needs of
next-generation photolithography.

In figure, Schematic of the photolithography process shows the formation of a gradient extending from the photoresist material to be removed (center) into the unexposed portions of the resist on the sides. NIST measurements document the residual swelling fraction caused by the developer that can contribute to roughness in the final developed image.

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