Wafer Aligner

A wafer aligner (prealigner) is the station where a robot-held wafer gets spun under an optical or capacitive sensor so the tool can find its notch or flat and correct its center offset before anything else happens to it. It is a small, unglamorous step that sits between every FOUP pick and every process chamber placement — and it is the reason a 300 mm wafer with roughly a millimeter of random position error coming out of its pod can still be placed into a chamber with micron-level repeatability.

It is solving two different errors at once: where the center is, and which way the wafer is rotated. A FOUP slot holds a wafer loosely enough that it can shift by close to a millimeter and rotate by several degrees between loads. A process chamber, by contrast, needs the wafer centered on its chuck or susceptor to within microns and rotated to a known reference angle, because downstream lithography overlay, etch uniformity, and even simple edge-exclusion zones all assume the wafer sits exactly where the recipe expects. The prealigner is what converts "wherever the FOUP happened to leave it" into "exactly where the chamber needs it."

Two sensing approaches dominate, and they trade speed against wafer type. Optical prealigners shine a beam across the wafer edge and use a linear or line-scan sensor to find the notch (or the older flat) as the wafer spins — fast and standard for patterned production wafers. Capacitive or mechanical-contact aligners instead sense the physical edge profile without light, which matters for wafers with highly reflective or transparent films where an optical edge signal gets unreliable; some fabs run both types depending on which layer a given lot is at.

Reference angle accuracy compounds across the whole tool, which is why aligners are specified almost as tightly as lithography stages. A notch-finding error of even a fraction of a degree, multiplied across repeated loads into a chamber with a fixed alignment key, shows up downstream as rotational misregistration between layers — so prealigner accuracy specs (sub-0.1° rotational, single-digit-micron centering) sit in the same conversation as overlay budgets, even though the aligner itself never touches a mask or a beam.

The wafer aligner rarely gets mentioned next to lithography scanners or etch chambers in a fab tour, but it runs more cycles per day than almost any other single station — every wafer, every tool, every load — making its speed and repeatability a quiet multiplier on the whole fab's throughput and yield.