Wafer Defect Inspection: Solving the "Auto‑Focus Tracking" Challenge to Improve Detection Accuracy and Efficiency

Image source: "Research on Multi‑Scan Patterned Wafer Inspection Technology Based on Bright‑Field and Dark‑Field Imaging" by Chen Shiwei (removed upon request)

In our previous application introductions, we discussed two common methods for wafer defect inspection in the semiconductor industry: Automated Optical Inspection (AOI) and Scanning Electron Microscope (SEM) systems. Today, we focus on a key challenge in AOI systems: auto‑focus tracking.

Schematic diagram of a typical AOI inspection system

Relationship between DOF and resolution (Image source: Edmund Optics, removed upon request)

03 The Effect of Wafer Warpage

Wafers are subjected to mechanical stress during various process steps. In most cases, unbalanced stress on the front and back surfaces leads to warpage. This is particularly evident in grinding processes, where strong residual stresses can cause convex warpage (positive warp) or concave warpage (negative warp). Additionally, due to processing tolerances, wafer thickness can vary within a certain range.

Wafer thickness variation and warpage

In conventional inspection systems, for wafers with warpage exceeding a certain level, focus is typically set at the wafer edge. This is done so that whenever the wafer moves into the camera's field of view, the camera can be triggered for sampling immediately, improving inspection efficiency. However, the consequent issue is that due to warpage, while the wafer edge remains within DOF, a large portion of the wafer centre falls outside DOF. This results in poor imaging quality due to defocus, leading to missed defects.

Poor imaging quality in the wafer centre due to defocus

"To maintain overall inspection system performance, the XY scanning stage must have excellent accuracy and flatness. Otherwise, poor stage flatness itself introduces significant focus errors. During high‑speed Y‑axis scanning, the dynamic following error must be sufficiently small; otherwise, Y‑axis‑based trigger signals will be inaccurate.

The Z‑axis requires high accuracy and dynamic performance, including minimal following error and high no‑load cutoff frequency, enabling continuous real‑time focusing within extremely short time frames. Furthermore, every time the wafer enters and exits the optical field of view, the system turns auto‑focus on and off, so the Z‑axis must have sufficient rigidity to minimise overshoot and settle quickly after overshoot."

Achieving rapid auto‑focus tracking during high‑speed wafer defect inspection is a considerable challenge for the motion system.

Auto‑focus principle

One of Coretech's classic solutions employs the ONEXY‑350‑350 open‑frame XY integrated stage, paired with the SurfaceZ Z‑lift stage. For a 6‑inch or 8‑inch wafer, the wafer is vacuum‑chucked onto the stage surface. The X‑axis performs step motions, while the Y‑axis executes raster‑type scanning. Using position‑based triggering (PEG/PSO) from the controller, image sensors can be triggered at intervals as small as 1 μm or even less, enabling easy scanning inspection. The Z‑lift stage compensates for wafer height variations in real time, keeping the wafer within the camera's focal range.

ONEXY‑350‑350 with SurfaceZ auto‑focus solution

The XY axes utilise Coretech's ONEXY series high‑precision integrated XY motion stage. The ONEXY series employs Coretech's self‑developed direct‑drive motors, with an open‑frame structure and integrated XY design. The lower axis features dual‑motor drive for excellent dynamic performance, superior geometric performance, and higher load capacity. With low lateral height and reduced Abbe error, travel ranges up to 500 mm × 500 mm are available, along with multiple feedback options (including zero‑expansion grating scales). Its outstanding mechanical performance makes it ideal for wafer defect inspection applications.

ONEXY‑350‑350 integrated XY motion stage

SurfaceZ high‑precision direct‑drive Z‑lift stage

Z‑axis real‑time focus tracking

Wafer inspection results with auto‑focus enabled

Comparison of wafer inspection results before and after enabling auto‑focus