High-throughput inspection of semiconductor wafers, surface coatings, and other materials requires imaging systems that deliver exceptional contrast without sacrificing speed or flexibility. Circular Differential Interference Contrast (C-DIC) microscopy enhances the visibility of subtle surface features while eliminating the sample alignment requirements of traditional DIC, making it particularly well suited for automated inspection. By leveraging the modularity of Zaber's Nucleus microscope platform, our Custom Engineering Team delivered an automated C-DIC microscope for material inspection which combines high-contrast imaging with rapid switching between imaging modes and ultra-repeatable sample positioning. Our unique motorized C-DIC slider simplifies automation reducing setup time and increases imaging throughput.
Understanding DIC and C-DIC Contrast Enhancement
A DIC microscope’s optical path enhances sample contrast by splitting input light into two closely spaced orthogonally polarized beams using a bifringent Nomarski prism. As the two beams pass through the sample, they take slightly different paths, encountering regions with different thicknesses and indices of refraction. These differences shift the phase of the two beams relative to each other. An analyzer prism recombines the two light beams into a single beam before it reaches the detector. As it does so, the phase difference between the two beams results in constructive and destructive interference, which creates the increased contrast (Fig. 1).
C-DIC Enables High-Contrast Imaging Without Sample Alignment
While traditional DIC relies on plane polarized light, circular DIC (C-DIC) uses circularly polarized light. While the basic operating principle is the same, the use of circularly polarized light does not require the sample to be aligned with the angle of the plane polarized light. By eliminating the angular dependence of the sample on the resulting contrast enhancement, C-DIC-based systems are ideal for high-throughput automation (Table 1). The sample can be placed at any orientation and imaged immediately without requiring any alignment. The elimination of angular dependence also supports high-contrast imaging of complex staples with features in many different orientations. Such samples are common in material science and semiconductor applications.
| Category | Reflected Brightfield | Reflected DIC | Reflected C-DIC |
|---|---|---|---|
| Ease of Automation | +++ | + | +++ |
| Throughput | +++ | + | +++ |
| Contrast when all features are in the same orientation | + | +++ | +++ |
| Contrast when features are in multiple orientations | + | + | +++ |
| Total System Cost* | $ | $$$ | $$ |
Table 1. Relative advantages and disadvantages of different reflected imaging modes. *Based on 10× and 20× objectives supporting both C-DIC and standard brightfield.
Automated C-DIC for Higher Throughput and Lower System Costs
Zaber’s automated C-DIC material inspection microscope solution uses Zeiss optical elements, guaranteeing exceptional optical performance when used in conjunction with Zeiss EC Epiplan objectives. This unique design enables high-throughput automation and cost savings compared to DIC microscope configurations.
By mounting the C-DIC slider (Zeiss PN: 000000-1105-192) on a motorized actuator in the infinity space just below the objective, up to four objectives can share a single C-DIC slider. By eliminating the costly requirement of one slider mounted below each objective (Fig. 2).This greatly reduces the overall system cost.
Figure 2. Zaber’s Custom Engineering Team mounted the C-DIC slider (red) behind the objective on the filter cube turret body. This enables all four objectives carried by the X-MOR4 objective turret to share a single C-DIC slider. The slider can be automatically moved in and out of the optical path to switch between reflected brightfield and C-DIC imaging with any mounted objective.
The motorized slider mount also enables a reduction in total system cost compared to configurations with a slider mounted below each objective. In those systems, switching between C-DIC and standard reflected imaging is accomplished by installing two copies of each objective. One with a DIC slider installed, and the other without. The shared, motorized slider on the Zaber system eliminates this requirement. This configuration also provides greater flexibility by supporting the installation of up to four unique objectives.
The analyzer module is housed in a standard Zeiss “Push & Click” filter cube (Zeiss PN: 424941-9000-000) which is mounted in the Zaber X-FCR06 filter cube turret. With the prism mounted in a motorized slider and the analyzer in a motorized turret, it is possible to quickly and automatically switch between standard brightfield or fluorescence imaging and DIC imaging.
For maximum C-DIC imaging throughput, Zaber’s high-speed direct drive linear motor XY stages can be synchronized with high speed line scan and TDI cameras. For a deep dive on how Zaber’s high-performance direct drive linear motor stages can work together with the latest TDI cameras, check out our whitepaper here.
Conclusion
By combining the contrast advantages of C-DIC with automated imaging mode switching and high-speed motion control, Zaber’s unique solution enables efficient, high-throughput inspection of semiconductor wafers and other materials. Its shared C-DIC slider design further simplifies system configuration and reduces component costs, providing a flexible, scalable solution for both research and industrial inspection applications.