Thermal Oxide vs. PECVD Oxide for MEMS
Two ways to form silicon dioxide, two very different films. This guide explains how each process works, compares their material properties, and gives practical guidance for choosing the right oxide for your device.
Thermal oxide and Plasma Enhanced Chemical Vapor Deposition (PECVD) oxide are the two primary methods used to form silicon dioxide (SiO₂) during MEMS fabrication, silicon photonics, and semiconductor wafer processing. In MEMS and silicon photonics, silicon dioxide serves as an etch mask, a sacrificial layer, an electrical insulator, and an optical cladding. Although both films are silicon dioxide, they are produced by fundamentally different mechanisms. Those differences determine interface quality, dielectric performance, film density, residual stress, thermal budget, substrate compatibility, and where each process belongs within a MEMS fabrication flow.
Choosing the right oxide is often one of the earliest process decisions an engineer makes, because it influences downstream lithography, etching, metallization, release steps, optical performance, and long-term device reliability.
Same Material, Two Different Processes
The shortest way to describe the difference is growth versus deposition. Thermal oxidation grows silicon dioxide by consuming silicon from the wafer itself. PECVD deposits silicon dioxide onto an existing surface from plasma-activated process gases. That single distinction drives nearly every practical difference that follows, from process temperature to which substrates each method can coat.
How Thermal Oxidation Works
Thermal oxidation is a growth process performed in a high-temperature diffusion furnace. Silicon wafers are loaded into a horizontal or vertical furnace where carefully controlled flows of dry oxygen (O₂) or water vapor (H₂O) react directly with exposed silicon at approximately 900 to 1200 °C. As the oxidizing species reach the surface, they chemically convert the substrate into silicon dioxide.
Unlike a deposition process, thermal oxidation consumes silicon from the wafer. Approximately 44 percent of the finished oxide thickness grows below the original silicon surface while 56 percent grows above it. Because the film is formed directly from the silicon crystal lattice, the resulting oxide is dense, stoichiometric, contains essentially no bonded hydrogen, and forms an exceptionally clean silicon-to-oxide interface with very low interface trap density, high dielectric strength, and excellent long-term reliability.
Two oxidation chemistries are used. Dry oxidation reacts silicon with oxygen, producing slower growth rates but the highest quality, thinner films with superior electrical properties. Wet oxidation uses water vapor to increase the growth rate significantly and is commonly selected for thick oxides used in electrical isolation, MEMS sacrificial layers, and DRIE masking.
Because oxidation occurs wherever silicon is exposed, both sides of the wafer and the exposed wafer edge oxidize simultaneously unless a masking or protection method is applied.
Oxidation Kinetics
Thermal oxidation follows predictable kinetics. Thin oxides initially grow rapidly because oxidizing species reach the silicon interface easily. As the oxide thickens, oxygen or water vapor must diffuse through the existing film to reach the interface, which steadily reduces the growth rate. Dry oxidation produces the highest quality thin oxides, while wet oxidation provides the faster growth needed for thick isolation and MEMS applications.
Dry Chlorinated Oxide
Dry chlorinated oxide is a refinement of dry thermal oxidation in which a small, carefully controlled amount of a chlorine source is added to the oxygen ambient inside the furnace. The chlorine is incorporated near the silicon-to-oxide interface as the film grows, where it improves the electrical quality, cleanliness, and long-term reliability of the oxide. Rogue Valley Microdevices uses Trans-LC as the chlorine source for this process.
The chlorine performs two related functions. It getters mobile ionic contamination, particularly sodium, that would otherwise drift through the oxide and shift device threshold voltages, and it traps heavy metals such as iron, chromium, and nickel before they can degrade the interface. The same chemistry continuously cleans the furnace tube by converting metallic residue into volatile chlorides that are carried away in the gas flow.
Compared with standard dry oxidation, a chlorinated oxide provides:
- Higher dielectric breakdown strength and lower leakage current
- Lower fixed oxide charge and interface trap density
- Reduced oxidation-induced stacking faults
- Improved minority carrier lifetime
- Effective gettering of mobile ions and heavy metal contamination
- A slightly higher oxidation rate than chlorine-free dry oxidation
Trans-LC is a liquid chlorine source based on trans-1,2-dichloroethylene (trans-DCE). It is metered into the furnace as a vapor carried in the oxygen flow during oxidation. Trans-LC replaced earlier chlorine sources such as TCA (1,1,1-trichloroethane) and TCE, which are restricted as ozone-depleting compounds under the Montreal Protocol, and it is preferred over corrosive anhydrous HCl gas. It carries a low toxicity hazard rating, is not regulated as an ozone-depleting chemical, and decomposes at a relatively low temperature, which gives consistent, repeatable results across the dry oxidation temperature range.
Dry chlorinated oxide is selected wherever oxide integrity and device reliability are critical and mobile ion control matters most, including gate dielectrics, high-reliability MEMS, and sensor devices.
How PECVD Forms Silicon Dioxide
Plasma Enhanced Chemical Vapor Deposition (PECVD) is a low-temperature deposition process performed inside a vacuum reaction chamber. Process gases, typically silane (SiH₄) together with nitrous oxide (N₂O) or oxygen (O₂), are introduced into the chamber. Radio-frequency power generates a low-pressure plasma that dissociates the precursor gases into highly reactive radicals and ions.
These activated species migrate to the wafer surface where they recombine to form a deposited silicon dioxide film. Because the plasma supplies the activation energy, deposition occurs at only 200 to 400 °C without consuming the substrate. This allows oxide to be deposited after temperature-sensitive materials, aluminum interconnects, and completed MEMS structures are already present on the wafer.
PECVD deposits oxide onto silicon, metals, dielectric films, quartz, sapphire, glass, and silicon-on-insulator (SOI) wafers. The deposited film contains small amounts of bonded hydrogen and is slightly less dense than thermal oxide, but it offers outstanding flexibility for late-stage integration. Because the wafer rests on a temperature-controlled chuck or electrode that shields the backside from the plasma, deposition is typically limited to the front side of the wafer.
How the Two Processes Differ
Electrical and Mechanical Properties
Thermal oxide remains the benchmark dielectric because of its dense stoichiometric structure, exceptionally clean silicon interface, extremely low interface trap density, high dielectric strength, and low leakage current. It is the preferred choice for gate oxide, field oxide, and any application requiring the lowest possible leakage.
PECVD oxide provides excellent electrical insulation, passivation, and environmental protection. It is generally selected where interface perfection is less critical and where the ability to tune film stress is valuable for managing the mechanical behavior of MEMS structures.
Optical Properties
Thermal oxide is commonly selected for silicon photonics because of its excellent optical quality, stable refractive index, and extremely low defect density, which make it well suited to optical waveguide undercladding. PECVD oxide is frequently used as an overcladding or protective dielectric because it can be deposited after waveguide fabrication without exposing the device to high temperature.
Step Coverage and Topography
Thermal oxidation grows uniformly on exposed silicon and faithfully follows the underlying topography. PECVD oxide provides good step coverage over moderate topography and is well suited to passivation and interlayer dielectric applications. For very high-aspect-ratio features that demand greater conformality, engineers may evaluate LPCVD or ALD processes instead.
Process Integration
Many production MEMS and photonics flows intentionally use both processes. Thermal oxide is typically grown early, before metal deposition, to provide electrical isolation, hard masking, sacrificial layers, or optical undercladding. Once high-temperature processing is complete, PECVD oxide is deposited to provide passivation, interlayer dielectric insulation, encapsulation, or optical overcladding. Combining the two processes lets engineers leverage the strengths of each while minimizing process compromises.
Thermal Oxide vs. PECVD Oxide at a Glance
| Property | Thermal Oxide (Grown) | PECVD Oxide (Deposited) |
|---|---|---|
| Formation | Silicon converted into SiO₂ | SiO₂ deposited from plasma-activated gases |
| Process equipment | Quartz diffusion furnace | RF PECVD reactor |
| Process chemistry | Dry O₂ or H₂O | SiH₄ + N₂O or O₂ plasma |
| Temperature | 900–1200 °C | 200–400 °C |
| Silicon consumed | Yes; about 44% grows below the original surface | No |
| Frontside / backside coverage | Both wafer sides and exposed edges oxidize unless protected | Typically front side only; backside shielded by the chuck |
| Compatible substrates | Exposed silicon | Silicon, metals, dielectrics, quartz, sapphire, glass, SOI |
| Film density | Highest | High |
| Hydrogen content | Essentially none | Low bonded hydrogen |
| Interface quality | Excellent | Good |
| Film stress | Typically compressive | Tunable (compressive or tensile) |
| Step coverage | Uniform growth on exposed silicon | Good over moderate topography |
| Growth / deposition rate | Slow | Fast |
| Thermal budget | High | Low |
| Thickness at RVM | 500Å to 10µm | Up to 5µm |
| Typical process sequence | Early fabrication | Mid-stage to late-stage fabrication |
| Typical applications | Gate oxide, field oxide, DRIE hard mask, sacrificial oxide, waveguide undercladding | Passivation, ILD, encapsulation, optical overcladding |
Typical Applications
Thermal Oxide
- Gate oxide
- Field oxide
- Electrical isolation
- DRIE hard mask
- MEMS sacrificial layers
- Optical waveguide undercladding
PECVD Oxide
- Passivation
- Interlayer dielectric (ILD)
- Device encapsulation
- Optical overcladding
- Low-temperature insulation
MEMS Application Examples
| Application | Typical Oxide Strategy |
|---|---|
| Pressure sensors | Thermal oxide for electrical isolation and masking; PECVD oxide for final passivation. |
| Accelerometers | Thermal oxide sacrificial layers combined with PECVD dielectric protection. |
| Microfluidic devices | Thermal oxide for channel insulation; PECVD oxide for sealing or encapsulation. |
| Silicon photonics | Thermal oxide undercladding followed by PECVD oxide overcladding. |
Choosing the Right Oxide
Choose thermal oxide when interface quality, dielectric strength, thick high-quality oxide, DRIE masking, sacrificial oxide, or optical undercladding are the primary requirements. Choose PECVD oxide when thermal budget is the limiting factor, when oxide must be deposited over metals or completed device structures, or when the substrate is something other than silicon.
- Is the wafer still bare silicon? If yes, evaluate thermal oxidation.
- Does the process already include aluminum or temperature-sensitive structures? If yes, evaluate PECVD.
- Is the silicon-to-oxide interface electrically critical? Choose thermal oxide.
- Is the objective passivation, encapsulation, or interlayer insulation? Choose PECVD.
- Remember that many successful fabrication flows intentionally use both processes at different stages.
Frequently Asked Questions
Is PECVD oxide the same as thermal oxide?
No. Both are silicon dioxide, but thermal oxide is grown by consuming silicon while PECVD oxide is deposited from plasma-activated process gases.
Why can’t thermal oxide be grown over metal?
Thermal oxidation requires exposed silicon and furnace temperatures well above the limits of common interconnect metals. PECVD oxide is used when a dielectric is needed over metal.
Which oxide has better electrical performance?
Thermal oxide. Because it is grown directly from the silicon crystal lattice, it has very low interface trap density, high dielectric strength, and low leakage current.
Can PECVD oxide replace thermal oxide?
Not always. Although PECVD oxide provides excellent insulation and passivation, it cannot match the silicon-to-oxide interface quality, dielectric integrity, or electrical performance of thermally grown oxide. The appropriate choice depends on process requirements, thermal budget, and device architecture.
Can both processes be used together?
Yes. Many MEMS and photonics flows grow thermal oxide early in the process, then deposit PECVD oxide later for passivation, encapsulation, or overcladding after metal and temperature-sensitive layers are in place.
Talk to a MEMS Foundry
Have a device in development or a process you want to outsource? Rogue Valley Microdevices is a pure play MEMS foundry offering wafer services, thin films, photolithography, metal deposition, and silicon etching on 100mm, 150mm, and 200mm substrates. Contact us to discuss your project and find the right process for your device.