High Sensitivity Surface Defect Inspection of SiC and SmartSiCTM Substrates Using a DUV Laser-Based System

SmartSiCTM technology enables the supply of cost-effective and high-quality substrates to support the manufacturing of Silicon Carbide (SiC) Power Devices and the transition to High Volume Manufacturing (HVM) [1]. As detailed in [2] SmartSiCTM is prepared using a poly-crystalline handle wafer, it combines the benefit from both an optimized high quality epi-ready 4H-SiC layer and an ultra high conductivity handle material. Smart CutTM technology can be extended to SiC 200mm substrates and first SmartSiCTM 200mm sample has been prepared [2].SmartSiCTM substrates crystal quality is inherited by donor wafers [1, 2] and do not require a systematic control, enabling a new defects monitoring strategy, focusing on surface defects.This paper describes how a commercially available DUV inspection system was utilized for high sensitivity, high-throughput inspections of 150 and 200 mm 4H-SiC and SmartSiCTM substrates, for the HVM environment. The KLA Surfscan® SP A2 unpatterned wafer inspection system offers the opportunity to complement other inspection technologies to optimize SiC substrate defect control, with low threshold detection, below 150 nm.


Introduction: SmartSiC TM Defects Inspection Challenges
4H-SiC defect characterization is crucial to guarantee the epitaxial layers' yield. The link between epitaxial layers' extended defects and devices performances has been investigated since many years [4]. Both crystallographic defects inherited from the substrate (such as Basal Plane Dislocations, Stacking Faults and Micro-Pipes) and surface defects were studied as impacting the epitaxial layers' growth. Surface defects of 4H-SiC substrates, such as particles and scratches, can act as nucleation centers for epitaxial layer crystallographic defects, and propagate to product yieldlimiting defects, as triangular stacking faults [3,5].
Production-grade SiC wafers quality control must include defect inspection, as an early, costeffective means to control quality and final device yield.
As illustrated in Fig. 1 and Fig. 2, SmartSiC TM is a bi-layer SiC engineered substrate composed of a thin layer of 4H-SiC material, ranging from 700 nm to 350 nm, transferred to a specific handle wafer. Typical inspection tools for SiC substrates include surface microscopy, laser light scattering and photoluminescence data collection, to characterize both surface and crystal defects. A lower system throughput and complex inspection setup, however, can limit the process control sampling volume of SiC wafers in a production environment. On the other hand, specific SmartSiC TM design creates the opportunity of using laser scattering-based tools designed for HVM, offering high throughput and high sensitivity.

Influence of SiC optical properties on inspection systems performance
SiC material exhibits a very low absorption coefficient, resulting in high penetration depth and transparency on a wide portion of the light spectrum. Values reported in the literature for optical penetration depth are extremely high in the UV range (48 µm penetration at room temperature for a 355 nm laser, 8.4 µm at 325 nm [6]).
These material properties can cause surface defects sensitivity limitation, due to sub-surface signal, and local reflectance variation on thin layers stacks. In order to limit the influence of high penetration depth, we considered the optical behavior of SmartSiC TM substrates at 266 nm DUV wavelength.
Most of the reference values for 4H-SiC optical constants are measured in the ultraviolet range, 300 to 360 nm wavelength, commonly used for epitaxial layers characterization. In addition, as shown in [7], dopants concentration strongly influences optical properties. Therefore for our study k extinction coefficient was measured in the DUV range, below 300 nm, on a production grade 4H-SiC wafer used as donor for SmartSiC TM manufacturing (n-doped 4H-SiC with dopants concentration ~5.10 18 at/cm 3 ). We calculated the penetration depth based on the fit of n and k optical constants using a variable angle spectroscopy ellipsometer (Woollam, RC2).

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Optical, Electronic and Special Materials At 266 nm DUV wavelength, the extinction coefficient k was measured at 5.10 -2 , lowering the penetration depth into 4H-SiC to 400 nm, reducing the influence of embedded and interface defects for thin layers and limiting local reflectivity variations. The leverage of limiting penetration depth helps to improve surface inspection sensitivity at this wavelength.
To overcome the technical barriers related to high 4H-SiC penetration depth in visible and UV spectral range, a DUV wavelength laser-based inspection system (KLA's Surfscan ® SP A2) was used to inspect surface defects on various 4H-SiC and SmartSiC TM substrates.
The scattering of a 60 nm silicate sphere on the top and the bottom of the top SiC layer of a SmartSiC TM stack was simulated as a function of SiC layer thickness for the Surfscan ® SP A2 optical system (Fig.3). The graph is based on numerical simulation of light scattering of a sphere on top of film stack, performed based on Mie scattering theory. The signal from the bottom interface is 2 to 3 orders of magnitude less than the signal from the top surface. As a result, a DUV laser-based system can detect and segregate surface defects to achieve a sensitivity of a few tens of nanometers on a conventional 4H-SiC substrate or a SmartSiC TM stack.

DUV inspection sensitivity
Experimental data confirmed the simulation predictions: deposited 50 nm polystyrene latex (PSL) spheres were successfully detected on a production-grade 4H-SiC substrate and 126 nm spheres were likewise detected on various SmartSiC TM stacks (Fig.4). Moreover, these results were obtained at a throughput compatible with HVM requirements. For example 150 mm SmartSiC TM wafers were measured at 125 wafers per hour throughput.

Real Defects Detection and Classification
Production grade SmartSiC TM substrates, both 150 and 200 mm, were inspected in the same configuration as the wafers deposited with artificial defects, to confirm the sensitivity results on real defects. The defects coordinates issued from the DUV inspection system were reviewed using a scanning electron microscope (SEM, FEI Versa 3D). As illustrated in Fig. 5, surface particles below 300 nm were successfully detected on SmartSiC TM substrates.

Fig. 5. SEM review images gallery illustrating SmartSiC TM defect types
Not all SiC defects types are of equal significance from a manufacturing point of view. The ability to identify and separate "cleanable" versus "non-cleanable" defects is crucial to achieving "defect-free" substrates. Simultaneous brightfield and darkfield data collection channels were used on the Surfscan ® SP A2 system to classify defects based on the optical signature. "Non-cleanable" defects showed a stronger signal on the brightfield channel, whereas "cleanable" defects are mostly detected on the darkfield channel. Based on this observation, a classification rule was established to segregate defect categories with no impact on inspection sensitivity or throughput.
Detection and classification results are illustrated in Fig. 6 for a production grade 150 mm SmartSiC TM substrate: Fig. 6. SmartSiC TM defects detection and classification using DUV darkfield and visible brightfield inspections: maps of signal-based classification for "cleanable" (Left) and "non-cleanable" defects (Right) To demonstrate the classification accuracy on a statistic sample of wafers, SmartSiC TM substrates were inspected using a Nomarski confocal differential interference contrast microscope (Lasertec, SICA88) and images were manually reviewed. Classification accuracy achieved is above 85%, as summarized in Table 1.

Summary
In this work, a production-worthy, DUV laser-based inspection system, the Surfscan ® SP A2 was used to perform high-sensitivity defect inspection on various SiC substrates. Multiple SmartSiC TM stacks were inspected at a 126 nm detection threshold. Sub-micron particles detection was confirmed on real particles and defects classification was successfully demonstrated. All the results were obtained at HVM compatible requirements on both 150 and 200 mm substrates.
SmartSiC TM technology, replicating the crystal quality of a donor wafer, allows for a new approach for defect monitoring, focusing on surface defects, similar to Silicon. This creates the opportunity of an inspection combining high sensitivity and high throughput. Unique SmartSiC TM engineered substrates coupled with well-suited inspection and metrology systems will help to drive lower manufacturing costs to enable the evolving SiC power device market.