1. Definition of TTV

TTV (Total Thickness Variation) is defined as the difference between the maximum and minimum thickness of a wafer. It is a key parameter used to evaluate thickness uniformity across the wafer surface.

In semiconductor manufacturing, wafer thickness must be highly uniform over the entire surface to ensure process stability and device performance. TTV is typically determined by measuring wafer thickness at five representative locations and calculating the maximum difference among them. The resulting value serves as an important criterion for assessing wafer quality.

In practical applications, the TTV requirement is generally:

• 4-inch wafers: TTV < 2 μm

• 6-inch wafers: TTV < 3 μm

2. Measurement Methods

2.1 Double-Sided Alignment Method

In this approach, the surface topography of the front side and back side of the wafer is measured separately:

• Front surface profile: ${\mathrm{<span\; style="font-size:18px;">z</span>}}_{\mathrm{<span\; style="font-size:18px;">f</span>}}<span\; style="font-size:18px;">(</span>\mathrm{<span\; style="font-size:18px;">x</span>}<span\; style="font-size:18px;">,</span>\mathrm{<span\; style="font-size:18px;">y</span>}<span\; style="font-size:18px;">)</span>$z_f(x, y)zf​(x,y)

• Back surface profile: ${\mathrm{<span\; style="font-size:18px;">z</span>}}_{\mathrm{<span\; style="font-size:18px;">b</span>}}<span\; style="font-size:18px;">(</span>\mathrm{<span\; style="font-size:18px;">x</span>}<span\; style="font-size:18px;">,</span>\mathrm{<span\; style="font-size:18px;">y</span>}<span\; style="font-size:18px;">)</span>$z_b(x, y)zb​(x,y)

The local thickness distribution is obtained by differential calculation:

$$t(x,y)=z_f(x,y)-z_b(x,y)$$t(x,y)=zf​(x,y)−zb​(x,y)

Single-side surface measurements may be performed using techniques such as:

• Fizeau interferometry

• Scanning white-light interferometry (SWLI)

• Confocal microscopy

• Laser triangulation

Accurate alignment of the coordinate systems for the front and back surfaces is critical. In addition, measurement time intervals should be carefully controlled to minimize thermal drift effects.

2.2 Transmission / Reflection Coupled Methods

Dual-head opposing displacement sensor method:

Capacitive or eddy-current sensors are symmetrically positioned on both sides of the wafer to synchronously measure the distances $d_1$d1​ and $d_2$d2​ from each surface. If the baseline distance between the two probes $l$$l$ is known, the wafer thickness is calculated as:

$$\text{TTV}=l-d_1-d_2$$

$${\mathrm{}}_{}$$​

Ellipsometry or spectral reflectometry:

Wafer or film thickness is inferred by analyzing the interaction between light and the material. These methods are well suited for thin-film uniformity measurements but offer limited accuracy for measuring the TTV of the wafer substrate itself.

Ultrasonic method:

Thickness is determined based on the propagation time of ultrasonic waves through the material. This technique is applicable to opaque materials or specialized measurement scenarios.

All of the above methods require appropriate data-processing procedures—such as coordinate alignment and thermal drift correction—to ensure measurement accuracy.

In practical applications, the optimal measurement technique should be selected based on wafer material, wafer size, and required measurement precision.