Never Too Thin

Exactly how thin is a thin film? Think in terms of molecules. Thin films may only be a few molecules thick—a thousand times thinner than a human hair.

Sandwip Dey knows all about thin films. Dey is an ASU materials scientist and electrical engineer. He and his colleagues create these films on a regular basis in laboratories located in basement of the Goldwater Engineering Research Center.

To begin, Dey must decide which materials to use to create the thin film. A variety of different chemicals or combinations of chemicals can be used depending on the properties desired in the final thin film. Many of the chemicals Dey uses are not commercially available. He uses chemistry to transform other material into the compounds he desires.

With the chemicals in hand, Dey uses two main methods to create thin films. The first is called chemical vapor deposition, or CVD. Chemical vapor is released in a chamber that contains a silicon wafer. The vapor reacts with the wafer’s surface. A thin film of material is deposited along that surface.

The second method is called sol-gel. The sol is a solution of both metal and organic compounds. This solution becomes a gel when applied to a spinning wafer of silicon.

The gel has no coherent structure. Using a rapid thermal annealing furnace, Dey removes all the organic compounds. Only inorganic molecules are left on the wafer. These molecules form the thin film, which does have a definite structure.

“We do all our processing through chemistry,” Dey says. “Without chemistry we wouldn’t get anywhere.”

Dey monitors the thin film as it is forming. He tests the film’s chemical and electrical properties afterward.

For example, he uses a mass spectrometer to measure the masses of individual molecules during the vapor phase. This device allows a researcher to learn the number of different compounds, their identities, and their relative amounts. This helps Dey understand how the chemical vapor reacts on the silicon wafer surface.

After creating the thin films, Dey’s research group takes them to ASU Nanofab’s clean room facilities to perform lithography. Lithography is the process of creating metallic patterns in the wafer surface so that it can connect to an external surface, as it would in an electrical device.

The researchers then test the patterned structures for their electrical properties and response speed. With this data in hand, they can model the electrical response and create a simulation of a device contained on the wafer. This helps Dey understand how a specific fabrication process controls the structure and properties of the finished product.

Dey’s research group makes many films that are processed identically through control of a number of variables. The temperature, amount and type of chemicals used, length of deposition, and other processing variables can affect the resulting film’s structure and its properties.

By making repeated, controlled experiments, Dey can zero in on the combination of factors that will create electro-ceramic thin films to solve the problem at hand.—Linley Erin Hall