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Line Edge Roughness (LER)

Deviation of a feature edge from ideal shape.
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Description

Line edge roughness, or LER, is defined as a deviation of a feature edge from an ideal shape.

Semiconductor features are not perfectly smooth. LER describes the amount of variation on the edges of features, according to Fractilia, a startup that develops measuring techniques for LER. At 10nm and below, LER can become as large as the size of the features on the chip. This in turn impacts transistor performance and yield.

Another issue is that the features sizes become smaller at each node, but LER doesn’t scale. As line edge roughness becomes large relative to feature width, it limits the effective resolution that can be achieved.

LER has been an issue for years, but it becomes more problematic as chipmakers began to utilize extreme ultraviolet (EUV) lithography in IC production. EUV, a next-generation lithography technology, involves the use of a 13.5nm wavelength scanner to print fine features on a wafer.

In an EUV scanner, photons hit a resist and cause a reaction. But with EUV, there might be a new and different reaction during each event or multiple events. And so EUV is prone to events involving stochastics or random variations in chips.

LER is just one of those variations. EUV stochastics can cause unwanted LER in patterns.

The EUV resists themselves involves a complex process. Photons absorbed by the resist simply trigger a chain of events leading to deprotection. In EUV resists in particular, the absorbed photon actually ionizes a monomer, setting off a chain of secondary electron emissions that, in turn, trigger photoacid generation. The diffusion distance of the photoacid molecules defines the “deprotection blur.”

A number of other effects can lead to line edge roughness, with shot noise an important contributor. Shot noise is caused by the random distribution of photons from a light source. For example, let’s say EUV light hits the resists in three consecutive and separate events. In one simple example here, the goal for a resist is to absorb an equal number of photons at each event.

In the first event, the resist absorbs 10 photons. Then it absorbs 9 the next time, and 11 after that. The variability from one event to the next is a phenomenon called photon shot noise.

Critical-dimension scanning electron microscopes (CD-SEM) are one of the most common tools used to measure LER. However, signal-to-noise ratio in a CD-SEM can become an issue.

Other types of pattern roughness include linewidth roughness (LWR) and pattern placement roughness (PPR).