Thermal vs plasma ALD: what a research lab needs

Atomic layer deposition has moved from a specialist technique to a standard tool in many research cleanrooms. Groups use it for gate dielectrics, passivation, diffusion barriers, coatings on nanostructures and thin seed layers. When a lab buys its first ALD system, used or new, the main choice is between a thermal ALD tool and one that adds a plasma source.

Thermal systems are simpler and coat deep structures very evenly. Plasma systems reach lower temperatures and a wider range of materials, but cost more to buy and maintain. This article explains the difference in practical terms and what to check on a used ALD system.

How ALD works, briefly

ALD builds a film one atomic layer at a time by alternating two self-limiting surface reactions. A precursor pulse, for example trimethylaluminium (TMA), reacts with the surface until every available site is used. A purge removes the excess. A second reactant, such as water, then reacts with the adsorbed layer, and a second purge follows. Each cycle adds a fixed thickness, often in the order of 0.1 nm for common oxides, so thickness is set by counting cycles.

Because each step saturates, the film grows evenly over every exposed surface, including the sidewalls and bottoms of deep trenches, as long as the precursor has time to reach them. That conformality is the main reason labs buy ALD.

Thermal ALD: simple and conformal

In thermal ALD, the energy for both reactions comes from the heated substrate. The co-reactant is usually water, ozone or ammonia. Typical research materials include:

• Al2O3 from TMA and water, the standard test and teaching process
• HfO2 and ZrO2 from metal-organic precursors and water
• TiO2 and ZnO for optical and sensor work
• Some metals, such as platinum, with oxygen at elevated temperature

Thermal tools are mechanically simple: a heated chamber, fast valves, heated precursor sources, a pump and a controller. That simplicity makes them reliable and easy to service. They also give the best conformality, because molecular precursors and water diffuse into high-aspect-ratio features without losing reactivity.

The Veeco Savannah, originally from Cambridge NanoTech, is one of the most common thermal ALD systems in university labs. Substrate capacity depends on the model, with the widely installed S200 taking substrates up to 200 mm. Besides its standard continuous-flow mode, it offers an exposure mode that holds precursor in the chamber for longer to coat very deep or porous structures, at the cost of longer cycles.

The limits of thermal ALD show up with materials that need more reactive co-reactants, such as many nitrides and some metals, and with temperature-sensitive substrates. Water also purges slowly at low temperatures, which lengthens cycle times.

Throughput is the other practical limit. At roughly a tenth of a nanometre per cycle and cycle times measured in seconds, a 50 nm film can take an hour or more on a single-wafer research tool. For shared tools, that makes scheduling and recipe discipline as important as the hardware.

Plasma ALD: lower temperature, more materials

Plasma-enhanced ALD replaces or supplements the thermal co-reactant step with a plasma of oxygen, nitrogen, hydrogen, ammonia or mixtures. The plasma provides reactive radicals, so reactions that would need high temperature thermally can run at much lower temperatures.

This opens up:

• Nitrides such as TiN, TaN, AlN and SiNx
• Some pure metals that are hard to grow thermally
• Oxides at low temperature, for deposition on polymers, organic layers or finished devices
• Denser films with lower impurity levels for some processes

Most research plasma ALD tools use a remote plasma source, often inductively coupled, above the substrate. Keeping the plasma away from the wafer reduces ion damage while delivering radicals.

The trade-offs:

• Conformality. Radicals recombine on surfaces, so in deep or narrow features they may not reach the bottom. Coverage in high-aspect-ratio structures is generally worse than with thermal ALD.
• Substrate effects. Oxygen plasma can oxidise sensitive surfaces before the first layer forms. Some substrates and interfaces need a thermal first cycle or a different co-reactant.
• Complexity. An RF generator and match, a plasma source with ceramic parts, extra gases and more interlocks add cost and service work.

Many plasma ALD tools can also run thermal processes, so a lab gets both modes in one chamber.

Choosing for a shared research lab

Ask your users three questions: which materials, on which substrates, and over what structures.

• If most requests are Al2O3, HfO2, TiO2 and ZnO on silicon, glass or nanostructures, a thermal tool covers them and will be the easiest to keep running.
• If users need nitrides, low-temperature films on polymers, or metals, a plasma-capable tool becomes necessary.
• If users coat very high aspect ratio structures, such as porous membranes, nanowire arrays or deep trenches, thermal ALD will usually give better results, regardless of material.

In a shared facility, contamination also matters. Some labs keep a dedicated tool for gate dielectrics and a second for general and metal processes, so that precursors used by one group do not affect the electrical results of another.

What to check on a used ALD system

ALD systems have specific wear points:

• ALD valves. The fast pneumatic valves cycle millions of times. Ask for cycle counts or service records and listen for valves that stick or leak.
• Heated lines and precursor manifolds. Cold spots cause condensation and particles. Check heater zones and controllers.
• Precursor cylinders. Cylinders that held pyrophoric precursors such as TMA need proper handling and are normally removed before shipping. Expect to buy fresh precursors and check that your supplier offers cylinders that fit the tool's source manifold.
• Pump and trap. Film deposits in the foreline and pump. Check the trap and pump condition, and ask what pump the tool used.
• Chamber and lid seals. O-rings and chamber surfaces collect film and need cleaning or replacement.
• Plasma source, on plasma tools: the ceramic tube or window, the RF generator and match, and the plasma gas lines.

A test deposition of Al2O3 with thickness and uniformity measured by ellipsometry is the standard acceptance check. Our guide to ellipsometers and reflectometers explains why ellipsometry suits thin ALD films.

Facilities and safety

Both types need a well-ventilated precursor enclosure, exhaust, nitrogen for purge, a vacuum pump and electrical supply suited to the heaters. TMA and some other common precursors are pyrophoric, so the cabinet, interlocks and handling procedures need to meet your site's safety rules before the first cylinder arrives. Plasma tools add process gases such as oxygen, hydrogen or ammonia, with their own handling requirements.

What we can do

We inspect and refurbish thermal and plasma ALD systems, replacing valves, heaters and seals, and can run a test deposition before shipping where facilities allow, or after installation at your site. Our preventive maintenance plans cover valve service and trap cleaning, and our spare parts service can source wear parts for common platforms.