What key specifications should be considered when selecting silicon wafers?

To manufacture the desired devices from silicon wafers, the first step is to choose the right wafer. But what are the key specifications to focus on?

Wafer Thickness (THK):
The thickness of the silicon wafer is a critical parameter. During wafer fabrication, precise control over thickness is essential, as both the accuracy and uniformity of wafer thickness directly impact device performance and the stability of the manufacturing process.

Total Thickness Variation (TTV):

TTV refers to the maximum difference in thickness between the thickest and thinnest points across the wafer surface. It is an important parameter used to assess the uniformity of wafer thickness. Maintaining low TTV ensures a consistent thickness distribution during processing, which helps prevent issues in subsequent manufacturing steps and ensures optimal device performance.

Total Indicator Reading (TIR):
TIR represents the flatness of the wafer surface. It is defined as the vertical distance between the highest and lowest points on the wafer surface. TIR is used to evaluate whether the wafer has any deformation or warpage during the manufacturing process, ensuring that the flatness of the wafer meets the required process specifications.

Bow:
Bow refers to the vertical displacement of the wafer’s center point relative to the plane of its edges, primarily used to evaluate the local bending of the wafer. It is measured by placing the wafer on a flat reference surface and determining the vertical distance between the wafer center and the reference plane. The Bow value typically focuses only on the central area of the wafer and indicates whether the wafer exhibits a convex (domed) or concave (dished) overall shape.

Warp:
Warp describes the deviation of the wafer’s overall shape from its ideal reference plane. Specifically, Warp is defined as the maximum deviation between any point on the wafer surface and the best-fit reference plane (typically calculated using a least-squares method). It is determined by scanning the entire wafer surface, measuring the height of all points, and calculating the maximum deviation from the best-fit plane. Warp provides an overall indicator of the wafer’s flatness, capturing both bending and twisting across the entire wafer.

Difference Between Bow and Warp:
The key difference between Bow and Warp lies in the area they evaluate and the type of deformation they describe. Bow only considers the vertical displacement at the wafer center, providing information about local bending around the center area—ideal for assessing localized curvature. In contrast, Warp measures deviations across the entire wafer surface relative to the best-fit plane, offering a comprehensive view of overall flatness and twisting—making it more suitable for evaluating the wafer’s global shape and distortion.

Conductivity Type / Dopant:
This parameter identifies the conductivity type of the wafer—that is, whether electrons or holes are the primary charge carriers. In N-type wafers, electrons are the majority carriers, typically achieved by doping with pentavalent elements such as phosphorus (P), arsenic (As), or antimony (Sb). In P-type wafers, holes are the majority carriers, created by doping with trivalent elements such as boron (B), aluminum (Al), or gallium (Ga). The choice of dopant and conductivity type directly influences the electrical behavior of the final devices.

Resistivity (RES):
Resistivity, often abbreviated as RES, refers to the electrical resistivity of the silicon wafer. Controlling the resistivity during wafer fabrication is critical, as it directly impacts the performance of the resulting devices. Manufacturers typically adjust the wafer’s resistivity by introducing specific dopants during processing. Typical target resistivity values are provided in specification tables for reference.

Surface Particle Count (Particles):
Particles refer to the contamination of the silicon wafer surface by small particles. These particles can originate from residual materials, process gases, dust, or environmental sources during manufacturing. Surface particle contamination can negatively impact device fabrication and performance, so strict control and cleaning of wafer surfaces are essential during production. Manufacturers typically employ specialized cleaning processes to reduce and eliminate surface particles to maintain high wafer quality.

How to Select the Appropriate Silicon Wafer?
Selection of the proper silicon wafer can be guided by the inspection standards and typical parameters shown in the table below for 6-inch wafers. Key considerations include:

• Thickness Variation: Variations in thickness often cause deviations in etching and corrosion processes, requiring compensation during manufacturing.

• Diameter Variation: Diameter deviations can lead to lithography misalignment, but the impact is generally considered minor.

• Conductivity Type and Dopants: These have a significant effect on device performance. Choosing the correct doping type is especially important.

• Resistivity: The uniformity of resistivity across the wafer surface must be carefully considered, as non-uniformity can seriously reduce device yield.

• Crystal Orientation: This greatly influences wet etching processes. If wet etching is involved, orientation deviations must be taken into account.

• Bow and Warp: Wafer bending and warpage strongly affect lithography accuracy, particularly when dealing with small critical dimensions (CD) in patterning.