Optical Silicon Wafers 

Prime Grade vs. Optical Grade Silicon Wafers

Prime grade silicon wafers and optical grade silicon wafers may share characteristics such as high-purity monocrystalline material and polished surfaces, but the terms describe different sets of requirements.

Prime-grade wafers are generally specified for semiconductor processing, where parameters such as crystal orientation, resistivity, TTV, bow, warp, particles, surface condition, and crystallographic quality can be important. For optical applications, additional requirements may include wavelength-dependent transmission, absorption, surface roughness, flatness, scratch/dig, parallelism, and optical coatings.

Therefore, a wafer described as prime grade should not automatically be assumed to satisfy a particular infrared optical specification. Likewise, the term optical grade should be accompanied by measurable specifications appropriate to the intended wavelength range and application.

How Optical Silicon Wafers Are Manufactured

Optical silicon substrates begin with monocrystalline silicon produced by crystal-growth methods such as the Czochralski (CZ) or float-zone (FZ) process. The grown silicon crystal is sliced into wafers, mechanically processed, cleaned, and polished to obtain the required thickness, geometry, and surface finish.

Double-side polishing (DSP) is particularly useful for transmission optics because both major surfaces of the substrate are polished. Depending on the optical application, additional specifications may include surface roughness, flatness, parallelism, TTV, bow, warp, scratch/dig, and edge geometry.

Optical performance ultimately depends on both the quality of the polished surfaces and the properties of the silicon bulk material.

Optical Properties of Crystalline Silicon

Crystalline silicon is an indirect-band-gap semiconductor with a room-temperature band gap of approximately 1.12 eV. Strong interband absorption occurs at photon energies above the band-gap region, while silicon becomes substantially more transparent at longer infrared wavelengths.

This behavior makes silicon useful for selected near-infrared and mid-infrared optical applications. However, there is no single transmission range that applies equally to every silicon wafer.

Practical infrared transmission depends on:

• Wavelength
• Wafer thickness
• Doping concentration
• Free-carrier concentration
• Resistivity
• Crystal growth method
• Oxygen, carbon, and other impurities
• Temperature
• Surface polish and roughness
• Surface reflection
• Anti-reflection coatings, when present

Why Silicon Reflects Infrared Light

Silicon has a relatively high refractive index in the infrared. As a result, a significant portion of incident radiation can be reflected at each uncoated air/silicon interface even when absorption within the silicon itself is relatively low.

This distinction is important: low absorption does not necessarily mean high measured transmission. Reflection losses at the front and rear surfaces can substantially reduce external transmission through an uncoated silicon window.

For optical systems requiring higher transmission, an anti-reflection (AR) coating can be designed for a particular wavelength or wavelength band. AR performance depends on the coating materials, thicknesses, wavelength, polarization, and angle of incidence.

High-Resistivity Silicon for Infrared Transmission

High-resistivity silicon is frequently selected for infrared applications because its lower free-carrier concentration can reduce free-carrier absorption compared with more heavily doped silicon.

Free-carrier absorption generally becomes increasingly important at longer wavelengths and at higher carrier concentrations. Consequently, electrical resistivity can be an important optical specification even when the wafer is not being used as an electronic device substrate.

However, resistivity alone cannot predict the complete transmission spectrum. Wafer thickness, carrier type and concentration, wavelength, impurity absorption, surface reflection, and temperature must also be considered.

Float-Zone vs. Czochralski Silicon for Optical Applications

Both float-zone (FZ) and Czochralski (CZ) silicon can be used for optical and photonic research, but their material characteristics differ.

FZ silicon generally contains less oxygen than conventional CZ silicon because the float-zone process does not use a silica crucible during crystal growth. FZ material is also commonly available at very high resistivity.

These properties can make FZ silicon attractive when low free-carrier absorption or low oxygen content is important. However, CZ silicon can be entirely suitable for many infrared applications and may offer advantages in availability, diameter, specification options, and cost.

The correct material should therefore be selected from the required wavelength range and optical performance rather than assuming FZ silicon is universally superior.

Infrared Absorption Mechanisms in Silicon

Several physical mechanisms can limit infrared transmission through crystalline silicon. Understanding these mechanisms helps researchers select an appropriate substrate for a particular wavelength range.

Interband Absorption

At sufficiently high photon energies, photons can promote electrons across the silicon band gap. This produces strong absorption toward the short-wavelength side of silicon's infrared transmission region.

Free-Carrier Absorption

Free electrons and holes introduced by doping can absorb infrared radiation. This is one reason high-resistivity silicon is often preferred when low optical absorption is required.

Multiphonon Absorption

At longer infrared wavelengths, interactions involving multiple lattice vibrations can contribute to absorption and eventually limit the useful long-wavelength transmission of silicon.

Impurity-Related Absorption

Oxygen, carbon, dopants, and other impurities can introduce additional absorption features. Their importance depends on concentration, temperature, and wavelength.

Phonon Absorption in Optical Silicon

The silicon crystal lattice supports quantized vibrational modes known as phonons. Because silicon is an indirect-band-gap material, phonons also participate in some optical transitions near the absorption edge.

At longer infrared wavelengths, multiphonon processes contribute to intrinsic lattice absorption. This behavior is a fundamental material property and cannot simply be eliminated by using higher-purity silicon.

Researchers characterizing optical silicon may therefore measure:

• Spectral transmission
• Absorption coefficient
• Reflectance
• Temperature-dependent transmission
• Refractive index
• Free-carrier absorption
• Impurity-related absorption features

Optical Silicon for Windows and Infrared Components

Polished silicon can be fabricated into infrared optical windows and other components for systems operating within appropriate infrared wavelength ranges.

Applications can include:

• Infrared spectroscopy
• Thermal imaging and sensing
• Laser instrumentation
• IR detectors and sensor assemblies
• Optical filtering systems
• Beam delivery systems
• Semiconductor process instrumentation

Optical components may require specifications beyond those normally used for semiconductor wafers, including optical flatness, parallelism, scratch/dig, edge finishing, custom thickness, and AR coatings.

Silicon Wafers for Photonics

Silicon is also a major material platform for silicon photonics. Its relatively high refractive index allows strong optical confinement when silicon is combined with lower-index surrounding materials.

Photonic structures fabricated on silicon can include waveguides, resonators, interferometers, couplers, modulators, and other integrated optical structures.

Importantly, bulk optical silicon wafers and silicon photonics wafers are not always interchangeable. Many integrated silicon photonic devices use silicon-on-insulator (SOI) wafers because the buried oxide and silicon device layer enable strong vertical optical confinement.

Optical Silicon for MEMS and Sensors

Silicon's compatibility with lithography, etching, thin-film deposition, bonding, and micromachining also makes optical silicon useful in MEMS and sensor research.

Depending on the device, polished silicon substrates may be incorporated into optical MEMS, infrared sensor assemblies, micro-optical structures, spectroscopy platforms, and semiconductor instrumentation.

For these applications, optical requirements may need to be considered together with mechanical and fabrication parameters such as orientation, thickness, stress, surface finish, etch behavior, and wafer flatness.

DSP vs. SSP Silicon for Optical Applications

Silicon wafers can be supplied as single-side polished (SSP) or double-side polished (DSP).

SSP wafers have one polished major surface and may be suitable when only one surface is optically or lithographically important.

DSP silicon wafers have polished front and back surfaces and are often preferred for transmission measurements, optical characterization, wafer bonding, through-wafer optical systems, and processes requiring high-quality surfaces on both sides.

Silicon vs. Fused Silica for Optical Applications

Silicon and fused silica are both used as optical substrate materials, but their optical properties are very different.

Silicon is a crystalline semiconductor with a high refractive index and useful infrared transmission. Fused silica is an amorphous form of SiO₂ that is transparent through much of the ultraviolet, visible, and near-infrared spectrum.

Therefore, fused silica is not a processing method used to manufacture optical silicon. It is a separate optical material with a different transmission spectrum, refractive index, thermal behavior, and fabrication role.

Optical Silicon Wafer Specifications to Consider

Selecting an optical silicon substrate requires matching both the semiconductor and optical specifications to the intended application. Researchers should consider:

• Diameter or custom dimensions
• Thickness and thickness tolerance
• Crystal orientation
• FZ or CZ crystal growth
• N-type, P-type, or nominally undoped material
• Dopant and resistivity
• SSP or DSP surface finish
• Surface roughness
• Flatness and parallelism
• Total Thickness Variation (TTV)
• Bow and warp
• Scratch/dig specification
• Operating wavelength range
• Required spectral transmission
• Anti-reflection coating requirements

Choosing Silicon for an Infrared Application

When transmission is a critical design parameter, specify the actual wavelength range and minimum acceptable transmission rather than requesting only "optical grade silicon."

For example, two silicon wafers with similar surface polish may exhibit different infrared performance because they have different thicknesses, resistivities, dopant concentrations, impurity levels, or crystal-growth histories.

Providing the complete optical specification allows the substrate to be evaluated against the requirements of the experiment or optical system.

Related Optical Silicon & Photonics Resources

• Silicon Wafers
• Prime Grade Silicon Wafers
• Double-Side Polished Silicon Wafers
• Silicon-on-Insulator (SOI) Wafers
• Silicon Photonics
• Infrared Optical Windows
• Fused Silica Wafers
• Germanium Substrates