Thermal Anneals for MEMS

A thermal anneal is a controlled heat treatment that holds a wafer at a chosen temperature, for a chosen time, in a chosen gas, to bring about a beneficial change in a film or structure. Unlike deposition or etching, an anneal usually adds and removes nothing; it simply gives atoms the energy to move, react, and reorganize. In MEMS fabrication, anneals are used to densify deposited films, passivate interfaces, relieve or stabilize film stress, form and improve metal contacts, and activate implanted dopants such as the boron in a piezoresistive pressure sensor.

The result of an anneal is set by four variables, temperature, time, ambient gas, and the ramp and cool rate, balanced against the thermal budget that the rest of the device can tolerate. This guide explains what anneals do, the main ambients, the difference between furnace and rapid thermal annealing, the common MEMS uses, and how Rogue Valley Microdevices anneals fit, for MEMS and related microfabrication applications.

What a Thermal Anneal Does

Heat is energy, and at anneal temperatures that energy lets atoms diffuse, bonds rearrange, and films settle into a lower energy, more stable state. Depending on the temperature and ambient, the same basic step can densify a porous film, drive hydrogen out of a dielectric, let a metal react with silicon to form a low resistance contact, move and activate implanted dopants, or relax built in stress. Because nothing is patterned, an anneal acts on the whole wafer at once, which is why it is defined by process conditions rather than by a mask.

Why Anneal in MEMS

MEMS devices are built from stacks of deposited and grown films whose mechanical and electrical properties must be predictable and stable. Anneals are one of the main tools for making them so. A nitrogen anneal densifies a PECVD oxide or nitride so it does not shift during later high temperature steps. A forming gas anneal passivates the dangling bonds at a silicon to oxide interface and alloys metal contacts. An inert anneal relieves or stabilizes the residual stress that determines how a membrane deflects or how a released beam curls. A high temperature anneal activates implanted dopants and repairs the damage left by implantation. In every case the goal is the same: to turn an as deposited or as implanted film into a stable, well characterized part of the device.

Anneal Ambients

The gas in the furnace is as important as the temperature, because it decides what chemistry can and cannot happen at the surface during the anneal.

Furnace Anneal and Rapid Thermal Anneal

Anneals are carried out in two main ways. A furnace anneal heats a batch of wafers together in a resistively heated tube, ramping slowly and holding for minutes to hours, which gives excellent uniformity and high throughput and suits densification, stress control, interface passivation, and longer diffusions. A rapid thermal anneal heats a single wafer with lamps, reaching temperature in seconds and cooling quickly, which keeps the total thermal budget low. That short, hot cycle is used to activate implanted dopants while limiting how far they diffuse, to form silicides, and for other steps that need high temperature but very little time.

Thermal Anneals and Film Stress

Residual film stress is a central concern in MEMS, because it sets how a membrane bows, how a cantilever curls after release, and how a resonant structure behaves. As deposited films, especially PECVD dielectrics and evaporated or sputtered metals, often carry significant intrinsic stress and can contain trapped hydrogen or other species that make the stress drift when the wafer is heated later in the flow. Annealing addresses this in two ways. It relaxes some of the intrinsic stress as the film reorganizes, and, just as important, it stabilizes the film by driving out volatile species and densifying the structure so the stress no longer changes during subsequent thermal steps. For a released MEMS structure, a predictable, stable stress is often more valuable than simply a low one.

Thermal Budget and Material Compatibility

An anneal never happens in isolation, so its temperature and time must respect everything already on the wafer. Aluminum metal, for example, limits many back end anneals to roughly 450 °C, which is why forming gas alloy anneals are run in that range. Existing dopant profiles will diffuse further at high temperature, and some films degrade if overheated. The general rule is to use the lowest temperature and shortest time that achieve the goal, and to reach for a rapid thermal anneal when a step genuinely needs high temperature but cannot afford the diffusion of a long furnace cycle.

Common MEMS Anneal Applications

• Dopant activation for piezoresistors, activating and driving in implanted boron and repairing implant damage in pressure and force sensors.
• Metal contact alloy and sinter, a forming gas anneal that lowers contact resistance and forms reliable ohmic contacts.
• PECVD film densification, a nitrogen anneal that stabilizes deposited oxide and nitride against later thermal cycling.
• Interface passivation, a forming gas anneal that neutralizes dangling bonds at the silicon to oxide interface and improves dielectric quality.
• Film stress relief and stabilization for membranes, cantilevers, and other released structures.
• Polysilicon anneal, activating dopants and relieving stress in structural polysilicon layers.
• Bond strengthening for fusion bonded and silicon on insulator wafers.

Thermal Anneals at Rogue Valley Microdevices

• Nitrogen, nitrogen with oxygen, and forming gas anneals, run in a horizontal furnace.
• Nitrogen anneal to densify PECVD oxide and nitride films so they stay stable through later high temperature processing.
• Forming gas anneal, recommended after dry chlorinated oxidation, to passivate dangling bonds at the silicon to oxide interface and secure the full benefit of the oxide.
• Processed on both sides, on 50, 100, 125, 150, and 200mm wafers from about 100 to 2,000 µm thick, in silicon, silicon on insulator, and quartz.
• The published thermal anneal process runs at 450 °C, well suited to densification, interface passivation, and metal contact alloying, and compatible with aluminum metal.
• Annealing integrates with RVM thermal oxidation, LPCVD and PECVD films, and metal deposition; for higher temperature anneals, discuss your thermal budget with the team.

Purpose and Anneal Selection

Choosing an Anneal

Start from the change you need and work back to the conditions. Decide the goal, whether it is densification, passivation, stress stabilization, contact alloying, or dopant activation, then choose the ambient that enables it, then set the lowest temperature and shortest time that reach the goal within the thermal budget the wafer allows.

A Simple Selection Workflow

1. What is the goal: densify a film, passivate an interface, stabilize stress, alloy a contact, or activate dopants?
2. Which ambient does it need: inert nitrogen or argon, forming gas, or an oxidizing gas?
3. What temperature does the wafer allow? If aluminum is present, stay near 450 °C or below.
4. Does the step need high temperature with minimal diffusion? Consider a rapid thermal anneal rather than a long furnace cycle.
5. Can the anneal be combined with an existing thermal step, such as after oxidation or PECVD, to save a cycle?

Frequently Asked Questions