[Semiconductor Metrology] Measuring the Thickness of Invisible Films: Ellipsometry and Alpha Step

When manufacturing semiconductor chips, we say, "We deposited 5nm of HfO₂." But how do we know that's truly 5nm? In semiconductor manufacturing, accurately measuring the thickness and properties of thin films (metrol

In this article, we will summarize the principles of well-known optical methods (Ellipsometry) and physical methods (Alpha Step) for measuring the thickness of thin films, as well as the know-how for interpreting data in practice.

1. Using the polarization of light: Ellipsometry

Ellipsometry is the most powerful and precise instrument for simultaneously measuring the thickness, refractive index (n), and extinction coefficient (k) of thin films using light. Capable of measuring ultra-thin films in the sub-nm range, it is used as standard equipment in semiconductor mass production lines.

A caveat is that ellipsometry is difficult to use on non-transparent metals, as it compares the difference between the light reflected from the thin film surface and the light transmitted through the thin film and reflected from the substrate.

1.1. How it works: Changes in polarization

Ellipsometry does not directly measure thickness. It measures the "change in polarization state of light when it reflects from a thin film surface."

1. Incident: Light with a specific polarization state is shined onto the sample.
2. Reflection: As light passes through a thin film and is reflected from the substrate, the phase and amplitude of the light change depending on the thickness and properties of the film.
3. Detection: Analyzes the degree to which reflected light changes into an ellipse.

There are two key parameters measured here:

• Psi (Ψ): The ratio of the amplitudes of reflected light (p-wave and s-wave).
• Delta (Δ): The Phase Difference.

1.2. The Wall of Practice: Modeling and Fitting

This is the most confusing part for researchers new to ellipsometry. They press the "Start" button, but no thickness is displayed. Instead, only Ψ and Δ graphs appear. Because ellipsometry is an indirect measurement method, the measured data must be compared to a mathematical model to inversely estimate the thickness. This process is called fitting.

• MSE (Mean Squared Error): This error value indicates how well your model matches the actual measurements. Typically, an MSE < 5~10 is considered reliable.
• Model Selection Guide:
  • Transparent thin films (SiO₂, SiN): Cauchy Model(most basic)
  • Light-absorbing thin films (Poly-Si, Metal): Tauc-Lorentz Modelor Drude Modelmust be used.

💡 Pro Tip: Don't blindly believe that a fitting yields an MSE of 1.5. If the refractive index (n) value is physically implausible (e.g., n = 3.0 for SiO₂), it's simply a mathematical coincidence and not an incorrect result. Always compare the results to the reference n value.

2. Physically scraping to measure thickness: Alpha Step

Officially called a Surface Profiler, it's better known by its KLA-Tencor model name, the Alpha Step. Like a needle on an LP record, the tip scrapes across the surface, measuring the elevation.

2.1. How it works: Stylus contact

1. A fine diamond tip (stylus) is lowered onto the sample surface.
2. Moves horizontally for a specified distance and scratches the surface.
3. It converts the vertical movement of the tip into an electrical signal and draws a cross-section (profile).

2.2. When to use?

Ellipsometry can only measure the thickness of flat thin films, but Alpha Step is specialized in measuring step height.

• Etch Depth: After the etching process, when measuring the height difference between the area where the PR (photoresist) is and the area where it was cut.
• Metal thickness after lift-off: When you want to check the thickness of the metal electrode.

2.3. Pros and Cons

• Pros: The principle is intuitive, and you don't need to know optical constants like refractive index. Even opaque films like metals can be measured.
• Cons: The tip scratches the surface, causing damage to the sample. (Soft films, such as PR, can be damaged, resulting in errors.) Ultra-thin films, less than a few nanometers in thickness, are difficult to measure.

3. Summary Comparison: Ellipsometry vs. Alpha Step

Please refer to this when you are wondering what equipment to use.

Comparison items	Ellipsometry	Alpha Step (알파 스텝)
Measurement method	Optical (non-contact)	physical (contact)
Measurement target	Thin film thickness, refractive index, extinction coefficient	Step Height
Applicable materials	transparent/translucent thin film	All substances (including metals and PR)
Precision	very high	~ several nm
Difficulty	Difficult (requires modeling)	Easy (intuitive)
Disadvantage	Metal thickness cannot be measured	Sample damage, ultra-thin film not possible

4. Conclusion: “Is this data reliable?”

Metrology is the eye of the process.

• If you want to precisely measure transparent dielectrics (SiO₂, HfO₂) at the atomic level, you need to master Ellipsometry. In particular, rather than obsessing over reducing the MSE value, you should first develop the habit of questioning whether the refractive index is physically valid.
• If you need to check metal electrodes (Au, Pt) or etching depth, don't hesitate and use Alpha Step.

Only when you accurately understand the characteristics of both devices and cross-check them appropriately can your process data be trusted.

References: J.A.Woollam