Atomic Layer Deposition

 July 11, 2012, FlexTech Alliance, San Francisco—Ritwik Bhatia from Cambridge NanoTech described the details of processing engineered materials in atomic layer granularity. This process allows for the conformal coating of materials on to a substrate a layer of material at a time.

Atomic layer deposition is a process that uses a number of precursor materials, separated by purge cycles, to create a self limiting atomic layer. As a result, the thickness of the layer depends on the number of cycles, and not on any other variables, and all reactions are at the surface. Layers are deposited about one angstrom at a time.

Applications for this type of process include optical coatings, semiconductors, nano materials, MEMS, and chemicals. The process works well with most oxides, between 15 and 20 metals, between 10 and 15 nitrites, and various quantities of sulfites carbides and fluorides. It’s possible to dope the films, either as they are being deposited or after deposition.

The technology enables nano-laminate materials. The only real requirements are that materials be at a common deposition temperature. More than two materials can be used for a deposition layer, and compositions can be changed during deposition to create graded films. For example, Al3O2 is good as a moisture barrier, but is subject to corrosion. ZnO2 is not easily corroded but is a poor moisture barrier. Putting the zinc over the aluminum creates a non-corroding moisture barrier.

Adding in a metal to a film can change the film characteristics. Doping a zinc oxide with aluminum or zinc can reduce resistivity by a factor of 10. Similarly, tin doped indium oxide is much more conductive than the base oxide. The resistivity is tunable by the amount of dopant added to the film. It’s also possible to deposit partial layers of compound materials like GIZO, by limiting the amount of material available for the chemical reactions.

These processes are changing deposition technologies and techniques by allowing the simplify deposition of one metal and then another one. The resulting compound may require annealing or other processing steps before the materials are usable, but the annealing process is just another step in production. The theoretical underpinnings and demonstration materials both exist, but production equipment for atomic layer deposition still needs extensive development.

In addition to the base materials, they also produced ALD gates and films. The electrical properties for active devices are affected by deposition temperatures and anneal time and temperature. The zinc oxides are intriguing due to their higher connectivity at lower deposition temperatures. Transistor characteristics using ALD are comparable to those using other deposition methods.

Normally, ALD is done on a pulse of precursor material in, vacuum to remove all excess precursor traces, pulse in second precursor material, vacuum, and continue this loop. The challenges with this flow include the need for full vacuum seal between the equipment and the substrate, and wasted precursor materials. Obviously, this flow would not work in a continuous roll to roll process. Instead, they are proposing a zone separated flow where the precursor zones are separated by inert gas zones. The inert gases act as barriers between the precursors and hold the substrate at minimal distances from the deposition heads.