Process window

Notes

A layer ships on the worst structure in it, so one pinch point can shrink the whole window.

Overview

A process window, in lithography, is the range of exposure dose and focus over which printed features stay within their specifications for size, shape and remaining resist thickness. It is drawn as a region on a focus and exposure plot, and the larger it is, the more tolerant the process is to the normal drift of real tools and wafers. The term is also used loosely for the acceptable operating range of any process step, but lithography is where it is most precisely defined.

How it works

Engineers measure the window by printing a focus-exposure matrix: a wafer on which each field is exposed at a different combination of focus and dose. The critical dimension (CD) of the printed features is measured and plotted against focus for each dose, giving the family of curves known as Bossung curves. Contours where the CD reaches its upper and lower specification limits bound the CD process window. Windows can be drawn the same way for sidewall angle and resist loss, and the usable window is the region where they all overlap.

Two numbers summarise a window. Exposure latitude is the range of dose, usually expressed as a percentage of the nominal, that keeps linewidth within its limits; depth of focus is the range of focus that keeps the feature within all its specifications. Different features on the same layer, such as dense and isolated lines, have different windows, and the layer can only run where they overlap. Even the resist thickness matters: Chris Mack has shown how thin-film interference in the resist can change the size of the window.

At advanced nodes the window shrinks, and finding its edges matters. KLA's Andrew Cross told Semiconductor Engineering that process window discovery identifies the structures that are weakest in focus and dose, and that in EUV lithography stochastic failures can occur even inside the window defined by systematic effects alone. Lithography and OPC engineers therefore work to enlarge the common window and keep the running process centred in it.

Examples

• A focus-exposure matrix wafer printed for a new contact layer, with CD measured in every field to map the window.
• A layer where dense and isolated lines print at different sizes, so their overlapping window is smaller than either window alone.
• Choosing a resist thickness at a swing curve maximum because it gives more exposure latitude, as in one of Chris Mack's published examples.
• Process window discovery on an EUV layer to find the patterns most likely to fail at the edges of focus and dose.

Related terms

• Dose
• DOF
• CD

Where it appears on the Hub

Used on 16 pages of the Hub. In each group, the pages that mention it most come first.