Aligners orient wafers by detecting the notch or flat and rotating to a standard position for consistent processing. Purpose: Tools require wafers in known orientation for pattern placement, alignment marks, and consistent processing. Detection methods: Optical sensors detect notch (300mm) or flat (200mm and earlier) as wafer spins. Edge grip: Gripper or chuck holds wafer by edge while rotating. No contact with active surface. Rotation: Precision rotation stage positions wafer to specified angle. Sub-degree accuracy. Integration: Usually built into EFEM. Wafer aligned before entering process chamber. Wafer mapping: May also perform wafer mapping - detect which slots have wafers, detect double-slotted or cross-slotted wafers. OCR: Some aligners read wafer ID (OCR or RFID) simultaneously with alignment. Throughput consideration: Alignment adds cycle time. Optimized for speed while maintaining accuracy. Notch location: Standard specifies notch at specific position (e.g., 3 oclock or 6 oclock) to match tool requirements. Pre-aligning: Some tools have pre-aligners and fine aligners for multi-stage alignment.