The challenges of growing the world’s largest sapphire slab

Can optical quality sapphire be grown in a different way to create a large, thick window to protect equipment on the belly of an aircraft?

This is the question that members of the Rubicon Technology design team were asked when the United States Air Force Research Lab (AFRL) called upon them about three years ago to develop a new growth platform that could produce very large polished sapphire infrared (IR) windows.

In the past, these dimensions were met by piecing together several smaller sapphire windows, with less-than-desired performance.

Despite high interest from the defense and aerospace industries, no method for producing sapphire for optical-grade windows of such great size and thickness existed at the time. Sapphire had been limited to smaller sizes and shapes using traditional growth methods. Rubicon studied the problem and agreed that it could be done. In 2012, the Air Force Research Laboratory granted Rubicon a three-year research contract to develop this product.

With several “firsts” already under its belt, such as having been the first to commercialize 6-inch and 8-inch wafers for the LED industry and the first to produce large-diameter patterned sapphire substrates, Rubicon has now successfully become the first to grow a 36 x 18 x 2-inch sapphire slab.

Drawing upon its strong history of scaling up sapphire growth processes, Rubicon successfully produced large sapphire blanks by highly modifying the Horizontal Directional Solidification (HDS) process and developing LANCE. Unlike the ES2 method, which produces high-quality sapphire crystals in the shape of roughly cylindrical boules, the HDS method produces sapphire plates that are advantageous for shaping into large windows.

Scaling up to larger crystal sizes while maintaining crystal quality was, of course, the real test and involved development of a new type of furnace.

Devising how to maintain crystal quality at such a large size proved to be experimental and time-intensive for the designers. By combining multiple platforms, materials and heater types, Rubicon ultimately demonstrated the ability to pull a horizontal boat through a heater surrounded by thermal insulation to achieve the desired thick slab while maintaining its high quality.

Over the past two years, the design team has continued to tweak and perfect the LANCE process, keeping the ultimate goal in mind of producing a sapphire slab with the cut measurements of 36 x 18 x 2 inches. The project is on track to produce optical quality sapphire at this immense size.

Looking to the future, this monolithic sheet is a hugely innovative advance that will provide a large, seamless optical field for military sensing applications and beyond.

For more information on the LANCE project, visit http://www.rubicontechnology.com/research-development/project-lance-very-large-sapphire-windows.

Case Number 88ABW-2015-1571

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