Masks and Reticles: Tools for Pattern Formation on Semiconductor Wafers

Masks and Reticles

Semiconductor manufacturing entails the formation of various patterns on wafers. These patterns define the structure of and interconnection between the different components and features of the integrated circuit. The patterns are formed on wafers using patterning tools known as masks and reticles. Below are some key points about masks and reticles.

- A mask is defined as a tool that contains patterns which can be transferred to an entire wafer or another mask in just a single exposure.

- A reticle is defined as a tool that contains a pattern image that needs to be stepped and repeated in order to expose the entire wafer or mask.

- Reticles have two major applications: 1) printing of images directly onto wafers in equipment known as step-and-repeat aligners; and 2) printing of images onto masks which, in turn, transfer the images onto wafers.

- The patterns on a reticle are usually 2X to 20X the size of the patterns on the substrate. However, some reticle patterns are 1X the substrate pattern.

- The equipment used for printing patterns on substrates that are smaller than the patterns on the reticles is also referred to as a 'reduction stepper', while one that's used for printing equal-size patterns is known as a 1X stepper.

- The 'polarity' of a mask or reticle can either be positive or negative. A positive mask or reticle has background areas (or fields) that are clear or transparent, which is why a positive mask or reticle is also known a 'clear-field' tool. A negative mask or reticle has fields that are opaque, which is why a negative mask or reticle is also known a 'dark-field' tool.

- There are many ways by which a pattern may be transferred to a wafer using a mask, a reticle, or a combination of both. Regardless of the pattern transfer process, everything starts with a set of pattern data that are converted into an actual pattern by a 'pattern generator.' Commonly-used pattern generators include: 1) plotters; 2) optical pattern generators; and 3) electron beam pattern generators.

- The patterns generated by the pattern generators are formed on either a mask or reticle. For example, plotter-generated patterns can be photo-reduced and formed on 10X emulsion reticle, while optically generated patterns can be formed on 5-20X hard-surface reticles. E-beam generated patterns can be formed on a 5-10X reticle, a 1X reticle, a 1X hard surface mask, or even directly to the wafer.

- The patterns formed on a reticle can be transferred directly onto the wafer, or they may first go to a mask which is the one that transfers the patterns to the wafer. Patterns on masks generally get transferred to the wafer directly.

Primary Reference: Silicon Processing for the Vlsi Era: Process Technology

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