A blog from Intermolecular
What is a ferroelectric material and how is it used?
A ferroelectric material develops and retains an apparent stored charge after the application of an electric field. As a result, it can be “switched” between two polarization states, and its polarization-voltage characteristic shows hysteresis.
There are many potential uses of ferroelectric materials in the semiconductor industry. Because ferroelectrics have two stable polarization states, they can be used to store information in computer memory. For example, if a thin ferroelectric material is sandwiched between two metals, it acts as a barrier to the flow of current. Flipping the polarization direction of the ferroelectric material changes the height of the barrier that electrons see as they try to move from one metal electrode to the other, modulating current across the device.
Another important development is a negative capacitance field effect transistor (nc-FET) for use in computer processors. In this type of device, a ferroelectric and traditional dielectric are connected in series under the gate of a transistor. Under the proper conditions, this stack exhibits a greatly increased sub-threshold slope, which reduces power consumption and opens the door for further miniaturization of logic devices and more energy-efficient electronics.
Hafnium oxide and crystal phases
Before 2011, traditional ferroelectric materials were hard to integrate into semiconductor processing technology. All of that changed once researchers discovered that hafnium oxide can exhibit ferroelectric properties. Hafnium oxide is a commonly used material in the semiconductor industry. It is compatible with traditional microfabrication processes and can be deposited conformally and in very thin layers using atomic layer deposition (ALD). Under normal processing conditions, the arrangement of atoms in hafnium oxide crystals results in a typical dielectric material. However, by carefully tuning processing conditions, hafnium oxide can be coaxed into a different crystal phase that exhibits ferroelectricity.
For example, Intermolecular found that replacing a TiN bottom electrode with an Ir bottom electrode improves the remanent polarization of a blended hafnium-zirconium oxide ferroelectric material. We also found that the remanent polarization of pure hafnium oxide ferroelectrics can be enhanced by reduing the ozone dose time during ALD growth.
In fact, experiments at Intermolecular and elsewhere have shown that subtle changes in the interfaces, oxygen vacancy concentration, dopants, and thermal treatment of hafnium oxide can result in drastically different electric and ferroelectric properties. Because of this large parameter space, optimizing ferroelectric performance can be difficult and time consuming.
Intermolecular’s ferroelectric tools
Intermolecular’s high-throughput experimentation platform has allowed us to strategically and quickly explore a large parameter space for improving the performance of ferroelectric hafnium oxide. For example, Intermolecular can use its P-30 PVD system to quickly explore new electrode materials and interlayers. The P-30 has four independently controlled sputtering targets. By changing the power, position, and deposition time of each target, we can control the composition and thickness of the film deposited during each experiment. Using shutters and apertures, each deposition can be isolated to a small portion of a 300 mm wafer, allowing for up to 24 different experiments on a single 300 mm substrate.
For changing the properties of the ferroelectric films themselves, Intermolecular’s A-30 ALD chamber enables us to explore both full wafer processing and quadrant-isolated processing for experimentation. Using our proprietary lid design, we can deposit up to four different thicknesses or compositions of ALD material on a single 300mm wafer, greatly accelerating experimental throughput.