Lowering defect densities and increasing yields are the key challenges for chipmakers and chip designers who use hundreds of methods for both tasks. This is because semiconductor fabrication technologies involve thousands of steps, and each can affect defect rates and yields. A recent discovery by researchers at Chinese universities has revealed how resists behave during development and how the post-exposure bake (PEB) step can reduce defect density by up to 99% in some cases, according to a paper published in Nature. However, despite these bold claims, the study has dubious practical use.
Researchers from Peking University and Tsinghua University have managed to visualize how photoresist molecules dissolve, migrate, and entangle within developer liquid during the pattern-forming (development) step. To do so, the team used cryogenic electron tomography (cryo-ET) to reconstruct the true 3D structure of photoresist polymers in their hydrated state at sub-5nm resolution.
The study revealed that most photoresist molecules accumulate in clusters at the gas–liquid interface rather than being evenly distributed in solution, which generates defects. The scientists claim that a slight increase in post-exposure bake (PEB) temperature — from 95°C to 105°C in their case — and maintaining a continuous developer layer prevented these clusters, cutting defect density on 300mm wafers by over 99% using existing resists and DUV equipment.
However, it is worth noting that chipmakers carefully select PEB temperatures for each process technology to achieve the best possible results, which limits the practical implications of the research.
To make it easier to understand what was discovered, here's a sequence of steps within a lithography step.
Coating: The wafer is spin-coated with photoresist. Exposure: Ultraviolet (UV) or Extreme Ultraviolet (EUV) light passes through a mask to selectively expose regions of the resist. Post-Exposure Bake (PEB): The exposed resist is gently heated to activate the acid-catalyzed chemical reactions that change solubility. Development: The wafer is rinsed with a developer solution (often TMAH in water for DUV), which dissolves the exposed or unexposed parts of the resist, depending on the resist type, to create a thin liquid film of developer and form the patterns. This step was the focus of the research. Rinse and Dry: The remaining pattern is cleaned and dried for subsequent processing.
The study in the development phase discovered that these photoresist molecules form weak, reversible entanglements that lead to microscopic clusters, which turn out to be the hidden source of pattern defects seen on processed semiconductor wafers.
A hidden process
In immersion and EUV lithography, the developer's liquid film dissolves light-exposed regions of the resist, transferring the pattern to the wafer. While the process is well known across the industry, until now there was no clear understanding of the microscopic behavior of chemically amplified resists (CARs) during pattern development, as existing methods such as scanning electron microscopy (SEM) could only observe dried residues or indirect effects. As a result, process engineers usually rely on trial-and-error tuning of resist chemistry and developer composition, since nobody has observed the real-time behavior of photoresists during development.
Instead of using SEM or atomic-force microscopy, the researchers used a cryo-electron tomography tool — usually used in structural biology to study cells, protein complexes, or viruses in frozen states — to visualize the behavior of photoresists inside a developer liquid at nanometer resolution. To do so, they had to go to great lengths in sample preparation, vitrification speed (the cooling rate at which a liquid changes state without crystallizing), and electron-beam control.
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