Page 310 - A Practical Guide from Design Planning to Manufacturing

280 Chapter Nine

The wafer must then be properly aligned before exposure. Each mask
has special alignment marks (called fiducials) that will be patterned into
each layer on the die. A laser is shone through the alignment marks on
the mask and reflected off the surface of the wafer. By analyzing the
reflection, the mask and wafer are precisely aligned. This is a process
similar to how the laser in an audio CD player stays aligned with the
proper track on the CD by monitoring its own reflected light.
Once aligned, shining light through the mask and focusing the image
onto the surface of the wafer expose the photoresist. Most processing steps
affect the entire wafer simultaneously, but it is not possible to create a
focused image across the entire wafer. Instead, each exposure projects
onto an area of only a few square centimeters. This could be enough to
expose only a single die at a time or several, depending on the die area of
the design. After each exposure, the wafer must be aligned again to expose
the next region of the wafer. Because of this serial stepping from one area
to the next to complete exposure of the entire wafer, the photolithography
machines that perform this operation are commonly called steppers.
After the photoresist has been exposed, it is developed much like a
photograph in a chemical bath. There are two basic classes of resists,
positive resist and negative resist. Positive resists become more soluble
when exposed to light, negative resists become less soluble. The exam-
ple in Fig. 9-12 shows the use of positive resist, which is much more
common. After development, the wafer is heated, in a step called hard
bake, to cure the remaining resist to a hard lacquer-like surface.
The developed wafer is then etched. The photoresist prevents the
etching of material it covers while material in exposed areas is removed.
After etching, the remaining photoresist is stripped off and the original
deposited layer is left behind, now patterned into the same shapes
drawn on the mask. A modern fabrication process might employ as
many as 20 to 30 separate masks for patterning different layers of mate-
rial. The creation of these masks has become one of the most important
steps in the manufacturing process.

Masks
Semiconductor manufacturing may start with making wafers, but blank
wafers can be used to make any integrated circuit. The first step in
making a particular design is making photolithography masks. Each
layer of layout is made into a separate mask (also called a reticle) to be
used to pattern that particular layer on the wafers. Creating masks
uses the same manufacturing steps of deposition, patterning, and etch-
ing used to process the wafers. See Fig. 9-13.
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The starting point for a mask is typically a 5-in piece of quartz (crys-
talline SiO ) a few millimeters thick. Sputtering is used to deposit a thin
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