TTS Launches Comprehensive Upgrade of SiC Wafer Minority Carrier Lifetime Imaging System – TAU-9000

Minority carrier lifetime is a fundamental parameter that defines the physical properties of semiconductor materials. It directly determines carrier diffusion length and device switching speed/frequency, while also serving as a sensitive indicator of lattice quality—reflecting point defects, impurity incorporation, and doping concentration—thus establishing a critical benchmark for material qualification and performance assessment.

Point defects (e.g., carbon vacancies and impurities) and doping concentration (e.g., nitrogen doping) within SiC wafers critically influence minority carrier lifetime by introducing non-radiative recombination centers. Specifically, an elevated point defect density shortens the carrier lifetime, signaling degraded lattice quality, which in turn elevates on-resistance and compromises key electrical performance metrics, including leakage current and on-state voltage drop. Furthermore, the carrier lifetime exhibits a strong dependence on doping concentration. Hence, rapid, accurate, and high-resolution spatial imaging of minority carrier lifetime across SiC wafers is imperative for material quality assessment, doping uniformity monitoring, process optimization, and yield enhancement.

Fig.1 Schematic illustration of minority carrier lifetime measurement principles.

Currently, microwave photoconductive decay (μ-PCD) remains the industry-standard method for SiC minority carrier lifetime characterization. However, this technique was originally developed for conventional silicon materials, where lifetimes typically exceed 10 µs. In contrast, SiC differs substantially from Si in growth methodology, process parameters, and elemental composition. Moreover, variations in doping concentration and epitaxial layer thickness give rise to a broad SiC lifetime range—spanning from below 100 ns to several microseconds—imposing far more stringent demands on measurement accuracy that conventional μ-PCD inherently fails to meet:

• Limited temporal resolution (~50 ns for μ-PCD), precluding accurate characterization of thin epitaxial layers and substrates with short lifetimes;

• Poor spatial resolution, limited by the microwave probe to approximately millimeter scale (~1 mm), incapable of high-precision wafer-level distribution mapping;

• Slow point-scan acquisition, requiring tens of minutes to over one hour for a 6-inch epitaxial wafer, failing to meet future high-volume manufacturing inspection demands.

To directly address these critical industry pain points, Time-Tech Spectra has developed the TAU-9000 Wafer Minority Carrier Lifetime Imaging System, built upon time-resolved spectroscopic imaging technology. With the recent comprehensive functional upgrade (TAU-9000HR), this system represents a significant breakthrough in SiC wafer inspection capabilities.

The system enables high-precision (100-μm resolution) full-wafer minority carrier lifetime imaging and uniformity mapping, while also supporting ultra-high-resolution micro-zone imaging (sub-micron resolution) for localized areas. The excitation source offers switchable wavelengths at 355 nm and 266 nm, allowing for separate characterization of bulk and surface recombination velocities. Furthermore, the upgraded system can be optionally configured with a vacuum or nitrogen-purged chamber, supports temperature-controlled measurements from room temperature up to 200°C, and features fully automated operation—making it equally suitable for both in-line production monitoring and advanced R&D applications.

Fig.2 Overview of the TAU-9000HR High-Resolution Wafer Minority Carrier Lifetime Imaging System.

Core Highlights of the Upgraded TAU-9000 System

01 | Full-Wafer High-Precision Minority Carrier Lifetime Mapping

In full-wafer mode, the system delivers a spatial resolution of 100 μm, clearly resolving detailed lifetime distribution features across the entire wafer. The system architecture is designed with production scalability in mind, supporting 6-inch, 8-inch, and 12-inch wafer sizes to ensure inspection readiness for next-generation large-diameter SiC volume production. Leveraging a high-speed data acquisition design, the measurement time for a single 6-inch wafer is maintained within 10 minutes—achieving a several-fold improvement in throughput over conventional μ-PCD methods.

 Fig.3 Comparative minority carrier lifetime imaging: TAU-9000HR vs. μ-PCD. (a) Lifetime mapping acquired on a 6-inch SiC wafer using the TAU-9000HR system, showing a spatial resolution of 0.1 mm. (b) Lifetime mapping of the same wafer acquired by μ-PCD. Note the spatial resolution (>1 mm) limited by the microwave probe.

02 | Switchable Excitation Wavelength (355/266 nm) with Bulk/Surface Dual-Mode Operation — No Substrate Interference

Equipped with dual excitation wavelengths at 355 nm and 266 nm, the system leverages the wavelength-dependent absorption depth in semiconductors to enable seamless switching between bulk and surface characterization modes. Under 266 nm excitation, the signal originates primarily from the near-surface region (~1 µm depth), whereas 355 nm excitation probes the bulk from tens of micrometers in depth—effectively eliminating substrate-related signal interference. By comparing lifetime mappings acquired at the two wavelengths, the system provides independently resolved imaging of bulk minority carrier lifetime and surface recombination velocity.

Fig.4 Comparative results on the same thick epitaxial wafer under 266 nm and 355 nm excitation. (a) Lifetime mapping under 355 nm excitation (bulk-dominated). (b) Lifetime mapping under 266 nm excitation (surface-dominated). (c) Decay kinetics at Plot1 for both excitation wavelengths. (d) Intrinsic bulk lifetime (τ_b) at Plot1, extracted via global fitting that accounts for carrier diffusion and surface recombination. (e) Mapping of surface recombination velocity S (cm/s). (f) Mapping of bulk lifetime τ_b (ns).

03 | High Temporal Resolution, Compatible with Both Epitaxial and Substrate Wafer Characterization

With a temporal resolution of <2 ns, the system substantially enhances its time-resolved detection capability. Beyond precise capture of rapidly decaying signals, the system accurately characterizes both short-lifetime substrates  (heavily doped, with lifetimes ranging from several to tens of nanoseconds) and relatively long-lifetime epitaxial layers (tens to hundreds of nanoseconds).

Fig.5 Minority carrier lifetime mapping on a SiC substrate. (a) Lifetime mapping acquired under 355 nm excitation. (b) Decay kinetics comparison at Plot1 (center) and Plot2 (edge). Compared to the relatively defect-free central region, the defect-rich edge regions exhibit shorter minority carrier lifetimes.

04 | Extended Ultra-High-Resolution Microscopic Imaging for Defect Visualization

The TAU-9000HR not only performs full-wafer macroscopic imaging but also delivers unique high-resolution capability for microscopic region characterization, achieving spatial resolution down to 0.9 μm. Imaging results confirm that this high-resolution mode clearly resolves crystalline defects in the epitaxial layer, including threading dislocations (TDs) and stacking faults (SFs)—providing direct, intuitive evidence for defect source identification and mechanism studies.

Fig.6 High-resolution minority carrier lifetime imaging. (a) and (b) High-resolution minority carrier lifetime mappings from distinct regions, clearly resolving stacking faults (SFs) and threading dislocations (TDs, appearing as point-like features). (c) and (d) Corresponding decay kinetics at selected points. Defect-affected regions consistently exhibit shorter minority carrier lifetimes than defect-free areas.

05 | Dual Sample Protection Modules for Sensitive Materials

To prevent environmental interference with measurement results, the system offers two independent module options: a nitrogen purge chamber and a vacuum sample chamber. Measurements conducted in inert gas or vacuum environments effectively preserve sample surface condition, ensuring high data fidelity and excellent repeatability.

06 | Wide-Temperature-Range Variable-Temperature Module for Mechanism Studies

An integrated variable-temperature module, spanning room temperature to 200°C, supports temperature-dependent minority carrier lifetime analysis, providing critical insights into recombination mechanisms and trap-state characteristics.

 Fig.7 Minority carrier lifetime measurements at different temperatures. (a) Room temperature measurement. (b) Measurement at 175°C. The epitaxial wafer exhibits a longer minority carrier lifetime at elevated temperatures (175°C) compared to room-temperature conditions.

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