A Comprehensive Explanation of Primary Flat and Secondary Flat
As power electronics and wide-bandgap semiconductors continue to advance, silicon carbide (SiC) substrates have become essential materials for electric vehicles, photovoltaic inverters, high-voltage power supplies, and RF devices. Among the many parameters of a SiC substrate, the flats—directional edges on the wafer—play a critical role in semiconductor manufacturing and automated processing.
For 2–4 inch SiC substrates, the industry widely adopts two flats, namely:
Primary Flat
Secondary Flat
This article provides a systematic explanation of why two flats are necessary and how they function in actual production processes.

1. Definition of Primary Flat and Secondary Flat

Primary Flat

The primary flat is used to identify the crystal orientation of the SiC wafer.
In 4H-SiC and 6H-SiC, the primary flat corresponds to a specific crystallographic direction (e.g., <11-20>), serving as the reference for all orientation-dependent fabrication steps.

Secondary Flat

The secondary flat is used to distinguish:
Conductivity type (N-type / P-type)
Polytype (4H / 6H)
Its position relative to the primary flat is defined by SEMI standards, enabling automated equipment to quickly recognize wafer properties.
 

2. Why Do 2–4 Inch SiC Substrates Require Two Flats?

1. Precise Crystal Orientation Recognition

Hexagonal SiC (4H-SiC, 6H-SiC) exhibits high crystallographic symmetry. A single flat may not be sufficient to prevent orientation misalignment.
However, crystal orientation is critical for SiC device fabrication, such as:
Trench orientation in MOSFETs
Ion implantation direction
Identification of Si-face or C-face
Wafer dicing direction
The primary flat provides a precise orientation reference to avoid directional errors during processing.
 

2. Differentiation of Conductivity Type or Polytype

The conductivity type (N-type or P-type) directly impacts:
Epitaxial growth parameters
Doping conditions
Device threshold and structural design
According to SEMI standards, the location of the secondary flat varies depending on conductivity type or polytype, allowing automated systems to correctly identify the material.
This is crucial for epitaxy suppliers, wafer fabs, IDMs, and packaging facilities.

3. Compatibility with Small-Diameter Wafer Processing Equipment

Compared with larger wafers, small-diameter equipment (legacy CVD epitaxy tools, ion implanters, CMP tools, etc.) relies more on flats rather than notches for alignment.
Thus, two flats provide more reliable orientation recognition, reducing alignment errors and ensuring stable processing.
As the industry moves toward 6-inch and 8-inch SiC wafers, single-notch designs are becoming the norm.
However, for 2–4 inch SiC wafers, dual-flat configurations remain the mainstream standard.
 

4. Preventing 180° Misalignment During Processing

With only one flat, the wafer could easily be placed in the reverse direction on automated tools, causing complete inversion of processing orientation.
Dual flats ensure correct placement and alignment throughout the manufacturing line.
 

3. SEMI Standards Define Flat Positions

The SEMI (Semiconductor Equipment and Materials International) standards specify:
The crystallographic direction associated with the primary flat
The angular position of the secondary flat for different material types
Flat lengths and identification conventions
These standards allow wafer manufacturers and equipment suppliers to maintain consistent, industry-wide wafer recognition systems.
 

Conclusion: Dual Flats Are Essential for Small-Diameter SiC Wafers

In summary, the two-flat structure on 2–4 inch SiC substrates exists because:
It provides an accurate crystal orientation reference
It differentiates conductivity type or polytype
It meets the alignment requirements of legacy processing equipment
It prevents directional misalignment during fabrication
It complies with SEMI international standards
Dual flats enhance process accuracy and stability while ensuring consistency in SiC device performance.

About JXT Technology Co., Ltd.

JXT Technology Co., Ltd. supplies:
Silicon carbide (SiC) substrates
Sapphire wafers
Quartz wafers
Silicon wafers
We are committed to providing high-performance, high-reliability crystalline materials for power electronics, optoelectronics, and semiconductor manufacturing, supporting customers in achieving superior process stability and yield.
For more information or custom specifications, please feel free to contact us.