On semiconductor wafers there is a straight line, which is called Orientation Flat.
What exactly are these and how they are formed?
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3 Answers
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As stevenh says one of the purposes is to ensure correct orientation during handling. The other reason is to indicate the crystal orientation of the lattice and the wafer doping. From BYU's cleanroom website:
Wafer Flats
Purpose and Function
Orientation for automatic equipment
Indicate type and orientation of crystal.
Primary flat – The flat of longest length located in the circumference of the wafer. The primary flat has a specific crystal orientation relative to the wafer surface; major flat.
Secondary flat – Indicates the crystal orientation and doping of the wafer. The location of this flat varies.
For large crystals no flats are ground. Instead a notch is machined for positioning and orientation purposes.
The crystal lattice is defined by how the wafer grew from the initial seeding.
It is important to cut the wafers along the crystal lattice if you want a clean break.
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2\$\begingroup\$ the #2 reason is the primary reason, crystal orientation. Since 200 mm wafers only a few companies use flats and everyone else uses notches (typcially Japan and Korea use the old flat style). In earlier days different ctrytal orientaions and wafer polarity were used and the flats told you what the wafer was. from 200 mm and up, there are all <100> n-type, so they only need the orientation in one location. Also notches and flats are made in the boule BEFORE slicing into wafers. \$\endgroup\$ Commented Oct 4, 2012 at 16:52
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As their name suggests they're to ensure correct orientation during handling. If they were completely round there would be no way to know if the wafer wouldn't be slightly rotated, and then for instance in the process called dicing the saw would cut through all the dies. Which would be, er, pretty bad :-(.
The ingot the wafers are sliced from is cylindrical, and the wafers are originally round. The flats are created by cutting off a part of it.
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2\$\begingroup\$ @stevenh that is not the reason at all. The saw operators can easily see the scribe lines in the saw microscope and the wafer flats are NEVER used during sawing because the wafer is mounted on sticky tape during the process. \$\endgroup\$ Commented Oct 4, 2012 at 16:49
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1\$\begingroup\$ @rawbrawb - What's that? Do they have a carpenter with a laser-guided Black & Decker circular saw, following scribe lines to dice the wafer? \$\endgroup\$– stevenvhCommented Oct 4, 2012 at 16:58
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Orientation Flat is usually formed by grinding one side at the boule level. XRD measurments are used to check crystal orientation.
Certain semiconductor devices (edge emitting lasers in particular, but also SOA, SLED, PICs) are formed by cleaving wafers to dies after processing = breaking them along crystal planes.
There are also substrates / wafers with Precision Flats = cleaved flats (not grinded), which are made to ensure precise alignment of devices along crystal planes, which then help to obtain clean cleave, which is later coated with reflective or antireflective dielectric layer stacks, to make mirrors for lasing = edge emitters (lasers, SOA, DFB, DBR).
