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EUV: Extreme Ultraviolet Lithography

EUV lithography is a soft X-ray technology.
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Description

Extreme ultraviolet (EUV) lithography is a soft X-ray technology, which has a wavelength of 13.5nm. Today’s EUV scanners enable resolutions down to 22nm half-pitch. In a system, an EUV light source makes use of a high power laser to create a plasma. This, in turn, helps emit a short wavelength light inside a vacuum chamber. In the chamber, the system uses several multilayer mirrors, which act to reflect light via interlayer interference.

EUV was originally slated to be introduced at the 45/40nm process node, but problems with the power source to achieve sufficient throughput have forced multiple delays. It was considered a critical component at 16/14nm as a way of avoiding double patterning using 193nm immersion. EUV systems began shipping for 7nm processes.

At 7nm, single patterning is possible with EUV, but is a relatively slow operation with today’s resists and can cause unwanted random or stochastic defects in patterns, which affect yield.

At 5nm, double patterning will be required on the critical layers even with EUV. Even though it requires more expensive steps, double patterning means the pitches of features can be relaxed and then processed, which can reduce the number of defects.

How EUV Works

Key to an EUV system is the source. Based on a laser-produced-plasma (LPP) technology, the EUV source consists of several parts, including a carbon dioxide (CO²) laser. The laser, which provides power for the source, is located under the fab floor in the sub-fab.

The laser consists of two parts—a seed laser (pre-pulse and main pulse) and a power amplifier. Today’s EUV sources use a 20-kilowatt laser.

The actual EUV source is situated on the fab floor. Attached to the EUV scanner, the source consists of a droplet generator, collector and a vacuum chamber. In EUV, the process takes place in a vacuum environment, because nearly everything absorbs EUV light.

The droplet generator is a small vessel. In operation, tin is loaded into the droplet generator and then heated. At that point, a train of tiny tin droplets flow out from the droplet generator, through a filter and into the vacuum chamber in the source. The droplets are 25 microns in diameter and are falling at a rate of 50,000 times a second.

In the vessel, there is a camera. A droplet passes a certain position in the chamber. Then, the camera tells the seed laser in the sub-fab to fire a laser pulse into the main vacuum chamber. This is called the pre-pulse.

Then comes the really hard part. The pre-pulse laser hits the spherical tin droplet and turns it into a pancake-like shape. Then the laser unit fires again, representing the main pulse. The main pulse hits the pancake-like tin droplet and vaporizes it.

At that point, the tin vapor becomes plasma. The plasma, in turn, emits EUV light at 13.5nm wavelengths.

The goal is to hit a droplet with precision. This determines how much of the laser power gets turned into EUV light, which is referred to as conversion efficiency (CE).

Meanwhile, once the EUV light is generated, the photons hit a multi-layer mirror called the collector. The light bounces off the collector and travels through an intermediate focus unit into the scanner.

In the scanner, the light bounces off a complex scheme of 10 surfaces or multi-layer mirrors. First, the light goes through a programmable illuminator. This forms a pupil shape to illuminate the right amount of light for the EUV mask.

Then, EUV light hits the mask, which is also reflective. It bounces off six multi-layer mirrors in the projection optics. Finally, the light hits the wafer at an angle of 6%. Each multi-layer mirror reflects about 70% of the light. Based on various calculations, the EUV scanner itself has a transmission rate of only 4%.

The light then hits the photoresist on the wafer.

Books
Semiconductor Devices: Physics and Technology

Extreme Ultraviolet (EUV) Lithography

EUV Lithography

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