Substrates Used in Attenuated Total Reflectance (ATR)

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HR-Silicon Wafers Used For Mid-Infrared Spectroscopy

A PhD student requested a quote for the following.

I am looking for a high-resistivity silicon wafer to use as an Attenuated Total Reflectance (ATR) crystal for mid-infrared spectroscopy. My target spectral range includes 764 cm⁻¹ and 820 cm⁻¹, which correspond to approximately 13.09 µm and 12.20 µm, respectively. Therefore, the wafer must remain transparent in the 12–13 µm wavelength region.

I want to ask if you have any wafer recommendations that may be even more suitable for ATR use in the 12–13 µm wavelength range.

Reference # for specs and pricing.

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What Is Attenuated Total Reflectance (ATR) in FTIR Spectroscopy?

Attenuated Total Reflectance (ATR) is an infrared sampling technique used with FTIR spectrometers to analyze the near-surface region of solids, liquids, films, powders, and wafers. ATR relies on total internal reflection inside a high-index crystal and an evanescent wave that penetrates only about 0.5–2 µm into the sample, making it ideal for surface-sensitive measurements on silicon and specialty substrates.

How ATR Works

In an ATR setup, an infrared beam is directed into a crystal with a high refractive index (typically diamond, germanium, zinc selenide, or silicon). The beam reflects internally at the crystal–sample interface at an angle greater than the critical angle, so total internal reflection occurs.

At each reflection point, a non-propagating evanescent wave extends a short distance beyond the crystal surface into the sample. Molecules in this region absorb energy at specific infrared frequencies corresponding to their vibrational modes. The reflected beam is therefore attenuated, and the FTIR spectrometer converts this attenuation into a spectrum.

The effective penetration depth depends on the wavelength, the refractive indices of the crystal and sample, and the angle of incidence. Typical values for ATR penetration depth are in the range of 0.5–2 µm.

IR Source IR beam ATR crystal (diamond, ZnSe, Ge, Si) Attenuated beam to detector Sample (wafer, film, liquid, powder) Evanescent wave (~0.5–2 μm into sample) Detector
Schematic of an ATR-FTIR setup. The infrared beam reflects inside a high-index crystal and generates an evanescent wave that probes the sample surface.

Why ATR Is Useful for Wafer and Thin-Film Analysis

Compared to traditional transmission FTIR, ATR greatly simplifies sample preparation and excels at probing surfaces and coatings on thick or opaque substrates. This is particularly important in semiconductor and materials research where samples often include:

  • Polished or etched silicon wafers (100, 111, and custom orientations)
  • Silicon-on-insulator (SOI) wafers and bonded structures
  • Thermal and PECVD oxides, silicon nitride, and other dielectric films
  • Photoresist layers, polymer films, encapsulants, and adhesives
  • Sapphire, fused silica, and compound semiconductor substrates (GaAs, GaN, SiC)

Because the penetration depth is limited to the near-surface region, ATR-FTIR is well suited for:

  • Detecting organic residues from lithography, dicing, or packaging
  • Monitoring changes in surface termination (e.g., Si–H, Si–OH) after RCA, HF, or UV-ozone cleaning
  • Characterizing polymer and photoresist films on silicon or SOI
  • Assessing surface preparation quality before wafer bonding or coating steps

ATR vs. Transmission FTIR

The following table summarizes key differences between ATR-FTIR and traditional transmission FTIR measurements in the context of wafer and thin-film analysis:

Feature ATR-FTIR Transmission FTIR
Sample preparation Minimal; place wafer, chip, or drop of liquid directly on crystal. Requires thin films, pellets, or IR-transparent windows.
Sample thickness Works with thick, opaque wafers and substrates. Requires samples thin enough for IR light to pass through.
Information depth Surface-sensitive (0.5–2 µm evanescent wave). Bulk absorption over the entire path length.
Best suited for Wafers, surface films, coatings, polymers, and liquids. Thin films, gases, and well-defined optical path cells.
Accessory materials ATR crystals: diamond, ZnSe, Ge, Si. Windows: KBr, NaCl, CaF2, etc.

ATR Crystal Materials and Typical Use Cases

The choice of ATR crystal affects penetration depth, spectral quality, and chemical compatibility:

  • Diamond – Extremely robust; ideal for hard, abrasive wafers and rough surfaces.
  • Zinc Selenide (ZnSe) – General-purpose; good for many liquids and softer solids.
  • Germanium (Ge) – High refractive index; shallow penetration depth; suitable for strongly absorbing or high-index samples.
  • Silicon (Si) – Useful for certain non-aqueous systems and mid-IR applications.

Penetration Depth (Conceptual)

The ATR penetration depth, \( d_p \), depends on the wavelength \( \lambda \), crystal refractive index \( n_1 \), sample refractive index \( n_2 \), and angle of incidence \( \theta \). In qualitative terms, higher crystal index and incidence angle reduce penetration depth, while longer wavelengths increase it. In practice, most ATR accessories are configured to provide a penetration depth that works well for thin films and surface analysis.

Typical ATR Applications in Semiconductor and Materials Labs

  • Surface cleanliness checks: Detecting organics and processing residues on silicon wafers.
  • Oxide and nitride characterization: Monitoring SiO2 and Si3N4 bonding environments.
  • Polymer and photoresist monitoring: Thickness trends, crosslinking, and residual films.
  • Bonding and packaging: Evaluating adhesion layers, epoxies, and underfills.
  • Functionalized surfaces: Studying self-assembled monolayers (SAMs) or biochemical coatings on wafer substrates.
UniversityWafer, Inc. — Substrates Optimized for ATR and FTIR Measurements

UniversityWafer, Inc. supplies a wide range of silicon, SOI, sapphire, fused silica, and compound semiconductor wafers that are well suited for ATR-FTIR and other optical characterization techniques.

  • Prime and test-grade silicon wafers with DSP or SSP surfaces.
  • Custom thermal oxide and nitride thicknesses for optical and IR studies.
  • High-resistivity silicon for IR and THz transmission experiments.
  • Sapphire, fused silica, and specialty substrates for advanced spectroscopy.

Tell us your wavelength range, surface finish, and film stack, and we will suggest the most appropriate wafers for your ATR and FTIR experiments.

Request an ATR-Ready Wafer Quote

For related topics, you may also be interested in:
• Silicon Wafers  |  • Silicon-on-Insulator (SOI) Wafers  |  • Fused Silica and Quartz