Crystallization of PZT Film by Laser Annealing Technology
The Position of Newly Developed Technology
A laser annealing technology (*1) heat-treats required points only for the required time, using laser beam properties. Ricoh succeeded in crystallizing PZT (lead zirconate titanate) (*2), which was produced by CSD (Chemical Solution Deposition) method (*3), with laser annealing technology and proved that the film obtained has displacement characteristics proper to PZT. Ricoh is the first to have obtained the PZT film, which has displacement characteristics, with its original irradiation technology using a semiconductor laser among laser annealing technologies (as of May, 2013).
This technology, when combined with the formation technology of piezoelectric element (*4) by IJP method (an ink-jet printing technique) that Ricoh announced in May, 2012, makes it possible to plot an actuator (*5) of a minute size (several - several hundred microns) as a 3D modeling pattern on a silicon substrate. The technology uses piezo material and crystallizes it with laser annealing technology. In addition to the field of MEMS for imaging equipment, hard disk drives, and displays requiring a displacement function in minute area, these technologies can evolve for use in the field of sensors, capable of measuring minute pressure and acceleration.
The Advantage of the Heat Treatment by Laser Annealing Technology
Although PZT material is used for many piezo elements, such as sensors and actuators, what Ricoh developed this time applies laser annealing technology to produce PZT film using the CSD method in the crystallization process. Until now, this crystallization process was made by heating to about 750 ℃ in an electric furnace, where almost all heat capacity was used to raise and lower the temperature of the substrate and furnace itself, resulting in significant waste of heat capacity and run time. With the laser annealing technology, because only the section irradiated by the laser beam is heated locally, there is no wasted energy, as only the PZT film on the substrate is heated.
To produce a PZT film by the CSD method, heat for drying and pyrolyzing, other than for crystallization, is also required. Although laser annealing was applied to the most important crystallization process to determine the film properties here, it is also applicable to the drying and pyrolyzing processes. With the laser annealing technology adopted for all heating processes, our tentative estimation shows that the CO2 exhaust can be reduced to about one-tenth, and the processing time can be halved that of the process using an electric furnace.
Conquest of Crack and Film Peel Off
The problem with crystallization annealed by laser lays in how to uniformly improve crystallinity without causing cracking and peeling. To solve this problem, there was a need to make the temperature at the time of irradiation uniform, even with the laser beam. Ricoh achieved the solution by developing a unique optical system capable of (1) making the geometry of the laser beam spot rectangular (circular in usual) to conform with the device form(See Figure 1), and (2) making the intensity distribution of laser beam (beam profile) oblong (Gaussian distribution in usual). Beyond that, the cracking and peeling problem were solved by rationalizing laser power and irradiation time(See Figure 2).
Characteristics of the Obtained PZT Film
Figure 3 and 4 show the X-ray diffraction pattern, electrical properties, and displacement characteristics of PZT film crystallized by laser annealing technology. Also, as shown in Figure 3, the PZT film is mainly crystallized in a (111) orientation compared with that before treatment, confirming proper crystallization. Also, as shown in Figure 4, a) the polarization voltage characteristic draws a comparatively beautiful hysteresis curve and shows that it has good ferroelectric properties. Further, b) the displacement-voltage characteristic (butterfly curve) proves that the film can be displaced mechanically, with a coefficient of displacement of about 100 pm/V.
Globally, there are only a few cases where PZT film has been obtained with a displacement characteristic using laser annealing technology. The other cases use excimer laser irradiation, making the systems large and expensive; they also require long processing time because of pulsed laser radiation. Ricoh's process, on the other hand, uses a semiconductor laser, keeping the system small and inexpensive and yielding the further advantage that it can be processed in a short time because of continuous light.
This technology was developed through joint research with the Fraunhofer Institute for Laser Technology of Germany, (also known as: ILT) with RWTH Aachen University (Rheinisch-Westfaelische Technische Hochschule Aachen).
- *1Laser annealing technology: A technology to cause transformation in metal or semiconductor’s crystal lattice by thermal action of laser irradiation. This technology has the features that local annealing treatment (heat treatment to obviate internal distortion by heat) of the required section only is possible and that it can be processed in a short time.
- *2PZT (lead zirconate titanate): A solid solution ceramic of lead zirconate and lead titanate; it is used for the piezoelectric ignition element in household gas appliances, ultrasonic transducers, piezoelectric buzzers, etc. This piezoelectric material presently is the most versatile.
- *3CSD method: The abbreviation of Chemical Solution Deposition method, also called the sol-gel method, is to synthesize a material by starting with gelification of the sol solution. It is a low-temperature synthesis method and can be applied to synthesize materials of various microstructures, topologies and functions.
- *4Piezoelectric element: An element expanded and contracted by applying a voltage. Further, the phenomena of deformation when a voltage is applied or of being charged when external force is applied are also called the piezoelectric effect.
- *5Actuator: A mechanism to convert input energy into physical movement. An actuator, generally called, is a device to convert electrical energy into movement and is widely used for household appliances, in the aeronautics industry and in the research of artificial muscles.