Thermal Oxide Silicon Wafers 

What Are Thermal Oxide Silicon Wafers?

Thermal oxide silicon wafers are silicon substrates with a layer of silicon dioxide (SiO₂) grown by oxidizing the silicon surface at elevated temperature. Unlike a deposited oxide, thermal SiO₂ is formed by consuming a portion of the underlying silicon as oxidizing species react at the Si/SiO₂ interface.

Thermal oxidation is valued for producing a uniform, electrically insulating oxide with a high-quality interface to crystalline silicon. These properties make thermally oxidized wafers useful for semiconductor processing, microfabrication, MEMS, surface passivation, electrical isolation and research applications.

UniversityWafer supplies silicon wafers with thermal oxide in a variety of substrate specifications and oxide thicknesses.

How Is Thermal SiO₂ Grown?

Thermal oxide is commonly grown by exposing a silicon wafer to an oxidizing ambient at elevated temperature. Two important processes are dry oxidation and wet oxidation.

During oxidation, molecular oxygen (O₂) or water-derived oxidizing species are transported through the existing oxide and react with silicon at the Si/SiO₂ interface. As the oxide grows, part of the silicon substrate is consumed and incorporated into the SiO₂ layer.

Dry Thermal Oxidation

Dry oxidation typically uses molecular oxygen (O₂). Compared with wet oxidation at a similar temperature, dry oxidation has a slower growth rate and is commonly selected when relatively thin, dense oxide layers and good electrical interface properties are important.

Dry thermal oxide has historically been important for MOS structures and other applications requiring high-quality dielectric interfaces.

Wet Thermal Oxidation

Wet oxidation uses water vapor or steam as the oxidizing species. It generally produces SiO₂ considerably faster than dry oxidation and is therefore useful when thicker oxide layers are required.

Wet-grown oxide is commonly considered for field isolation, masking, sacrificial oxide and other processes where greater oxide thickness or faster growth is advantageous.

Thermal Oxide Wafer Applications

Thermally oxidized silicon is used throughout semiconductor research and microfabrication. The appropriate oxide thickness and substrate specification depend on the intended process.

Electrical Insulation

Silicon dioxide is an electrical insulator and can electrically separate conductive or semiconducting regions from the silicon substrate. Thermal SiO₂ is therefore useful in MOS structures, test devices and numerous microfabrication processes.

MOS Devices and Capacitors

The high-quality Si/SiO₂ interface made thermal oxide historically fundamental to metal-oxide-semiconductor (MOS) technology. Thermal oxide remains useful for MOS capacitors, research structures and many microelectronic processes.

In advanced commercial CMOS technology, however, the primary gate dielectric is not necessarily conventional SiO₂. High-k dielectric materials are used in many modern transistor technologies to achieve greater effective gate capacitance while limiting leakage current.

Surface Passivation

A properly grown oxide can passivate the silicon surface by reducing the density of electrically active interface states associated with unsatisfied silicon bonds. Surface and interface quality are important in many electronic, sensor and research applications.

Masking During Semiconductor Processing

SiO₂ can be used as a masking material during selected diffusion, etching and ion-implantation processes. The effectiveness of an oxide mask depends on oxide thickness, dopant species, implantation energy, temperature and the specific fabrication process.

Because the required stopping or diffusion-barrier capability varies with the process, oxide thickness should be selected according to the actual fabrication conditions rather than assuming that any SiO₂ layer will provide sufficient masking.

MEMS and Microfabrication

In MEMS fabrication , silicon dioxide can function as an electrical insulator, etch mask, sacrificial material or part of a multilayer device structure.

Thermal oxide is particularly useful when a well-controlled SiO₂ layer is required directly on a silicon substrate before subsequent lithography, etching, deposition or bonding processes.

Device Isolation

Thermal oxidation has also been used for electrical isolation between semiconductor devices. One historical example is Local Oxidation of Silicon (LOCOS), in which relatively thick field oxide regions were selectively grown to isolate neighboring devices.

Modern integrated circuits commonly use other isolation technologies, such as shallow trench isolation (STI), but LOCOS remains important in the history and study of semiconductor processing.

Wafer Bonding and Research Structures

Oxidized silicon wafers are also useful in wafer bonding experiments, dielectric-stack development, thin-film research and the fabrication of specialized structures where a controlled insulating layer is required between silicon and another material.

Silicon dioxide is also an important component of silicon-on-insulator (SOI) structures, where a buried oxide (BOX) electrically isolates a thin silicon device layer from the underlying silicon handle wafer.

Thermal Oxide Thickness

Oxide thickness is one of the most important specifications when selecting a thermal oxide wafer. The required thickness depends on whether the oxide will be used for electrical isolation, masking, optical experiments, microfabrication, surface passivation or another purpose.

UniversityWafer can supply thermally oxidized silicon substrates across a range of oxide thicknesses. Availability depends on the requested wafer diameter, silicon specification, oxide thickness and quantity.

When requesting a quote, specify the desired SiO₂ thickness in nanometers (nm), micrometers (µm), or angstroms (Å), together with the acceptable thickness tolerance.

Thermal Oxide on One Side or Both Sides

Depending on the oxidation process and masking configuration, oxide may be required on one or both wafer surfaces. For many furnace oxidation processes, exposed silicon surfaces oxidize wherever they contact the oxidizing ambient.

If your experiment specifically requires oxide on only one side, or requires different frontside and backside conditions, include that requirement when requesting a quotation.

Thermal Oxide vs. Deposited SiO₂

Thermal oxidation and oxide deposition both produce SiO₂-containing films, but they are fundamentally different processes.

Thermal oxide is grown by chemically converting the surface of a silicon substrate into SiO₂. This produces an oxide with an excellent interface to the underlying crystalline silicon.

Deposited oxide is added to the surface rather than grown by consuming silicon. Deposition techniques can include chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) and processes using tetraethyl orthosilicate (TEOS) as a precursor.

Deposited oxide can be advantageous when oxidation temperatures are incompatible with the process, when SiO₂ must be formed over materials other than silicon, or when the required film structure cannot be produced conveniently by thermal oxidation.

The best choice therefore depends on the required interface quality, thickness, thermal budget, substrate material and downstream processing.

Choosing a Thermal Oxide Silicon Wafer

Oxide thickness alone is not enough to fully specify an oxidized wafer. The properties of the underlying silicon can be just as important to the experiment or fabrication process.

Useful specifications to provide include:

• Wafer diameter
• Thermal oxide thickness and tolerance
• Dry or wet oxide, when required
• Oxide on one side or both sides
• Crystal orientation, such as <100> or <111>
• P-type or N-type conductivity
• Dopant species
• Resistivity range
• Silicon wafer thickness
• Single-side-polished (SSP) or double-side-polished (DSP)
• Prime, test or other required wafer grade
• Quantity

Providing complete specifications makes it easier to identify an available substrate or determine whether custom oxidation is appropriate for the application.

Why Use Thermally Oxidized Silicon?

Thermally grown SiO₂ remains one of the most useful materials in silicon processing because it combines electrical insulation, chemical stability, compatibility with lithographic processing and a high-quality interface with crystalline silicon.

These characteristics make oxidized silicon wafers useful for university research, semiconductor process development, MEMS, MOS structures, dielectric studies, wafer bonding and many other microfabrication applications.

Related Thermal Oxide Resources

• Silicon Wafers
• Silicon Dioxide (SiO₂)
• Prime Grade Silicon Wafers
• Silicon-on-Insulator (SOI) Wafers
• MEMS Substrates