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Photomask

A template of what will be printed on a wafer.
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Description

A photomask is basically a “master template” of an IC design. A mask comes in different sizes. A common size is 6- x 6-inch. A basic and simple mask consists of a quartz or glass substrate. The photomask is coated with an opaque film. More complex masks use other materials.

At one time, the term “photomask” was used to describe a “master template” used with a 1X stepper or lithography system. The term “reticle” was used to described a “master template” used in a 2X, 4X or 5X reduction stepper. Today, the terms “photomask” and “reticle” are used interchangeably. They are basically the same thing.

Where does the mask fit?
In the semiconductor process flow, a chipmaker first designs an IC, which is then translated into a file format. Then, in a photomask facility, a photomask is produced based on that format. The mask is a master template for an IC design. It replicates the original IC design.

In a fab, the mask as well as a wafer are inserted in a lithography scanner. A photoresist, a light-sensitive material, is applied on the wafer. In operation, the scanner generates light, which is transported through a set of projection optics and the mask in the system. This process patterns the desired features on the wafer.

Making masks
To mask a photomask, the first step is to create a substrate or mask blank. A basic blank consists of a quartz or glass substrate, which is coated with an opaque film.

At a photomask manufacturer, the materials on the blank are patterned using an e-beam mask writer. Then, the pattern is etched and cleaned, creating a photomask.

The mask is then inspected for defects. Finally, a pellicle, a thin membrane, is mounted on top of the mask, which protects the mask from falling particles or contamination. The mask with the pellicle on top is then shipped to the fab.

Mask set
Generally, a photomask consists of templates of several dies of a given IC design. The dies are aligned in rows and columns. This all depends on the device type.

For production purposes, you wouldn’t use a single mask. One device requires a “mask set.” In other words, a single device may require between 5 to 40 (or more) individual photomasks, called a “mask set,” according to Compugraphics. One mask is used for each step in the fabrication process, according to Compugraphics.

This depends on the complexity of a device. A complex device would require more masks. A 10nm optical mask may require 76 individual masks, compared with roughly 46 for a 28nm node mask. At each node, the mask is more expensive.

Mask types — Optical
Different photomask types are used for today’s optical-based lithography systems. An optical lithography system incorporates a light source with different wavelengths. Today’s most common lithography systems use a light source at 248nm and 193nm wavelengths.

In optical lithography, a mask consists of an opaque layer of chrome on a glass substrate. One simple photomask type is called a binary mask.

For this, a photomask maker etches the chrome in select places, which exposes the glass substrate. The chrome materials aren’t etched in other places. In operation, light hits the mask and goes through the areas with the glass, which exposes the wafer. Light doesn’t go through the areas with the chrome.

Another optical photomask type is called a phase-shift mask.  Developed in 1980s, phase-shift masks use different materials and structures, which improve the image quality in patterning.

There are two types of phase-shift masks, alternating and attenuated. Alternating phase-shift masks resemble a binary mask. The difference is that glass regions are made thinner or thicker.

“In an alternating aperture phase shifting mask, the light on one side of every dark line is 180 degrees out-of-phase with the light on the other side. That creates destructive interference between the apertures on either side, making the line dark even if it is out of focus a bit. This destructive interference effect also relaxes the usual wavelength-dependent Rayleigh limit on the width of a resolved feature,” explained Marc David Levenson, who invented the phase-shift mask while at IBM in the 1980s. (Levenson has since retired.)

Attenuated phase-shift masks also resemble a binary mask. The difference is that a molybdenum silicide (MoSi) material replaces the chrome. In operation, light hits the mask.

“Since the MoSi is not opaque like chrome, light is partially transmitted (typically 6%) and the phase is shifted, so it is roughly 180 degrees different from the light that goes through the glass only,” explained Bryan Kasprowicz, a distinguished member of the technical staff at Photronics.

 


Fig. 1: A schematic illustration of various types of masks: (a) a conventional (binary) mask; (b) an alternating phase-shift mask; (c) an attenuated phase-shift mask. Source: Wikipedia

EUV masks
Using 13.5nm wavelengths, extreme ultraviolet (EUV) lithography is a next-generation technology that patterns tiny features on wafers.

EUV masks are different than optical masks. Unlike optical masks, which transmit light, today’s binary EUV masks reflect light at 13.5nm wavelengths. An EUV mask consists of 40 to 50 alternating layers of silicon and molybdenum on a substrate, resulting in a multi-layer stack that is 250nm to 350nm thick. A ruthenium capping layer is deposited on the multi-layer stack, followed by a tantalum absorber.

The absorber is a 3D-like feature that juts out on top of the mask. In operation, EUV light hits the mask at a 6° angle. The reflections potentially cause a shadowing effect or photomask-induced imaging aberrations on the wafer. This issue, known as mask 3D effects, can result in unwanted pattern placement shifts.

 

Fig. 2: Cross-section of an EUV mask. Source: Luong, V., Philipsen, V., Hendrickx, E., Opsomer, K., Detavernier, C., Laubis, C., Scholze, F., Heyns, M., “Ni-Al alloys as alternative EUV mask absorber,” Appl. Sci. (8), 521 (2018). (Imec, KU Leuven, Ghent University, PTB)

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