Innovation Foundry System Using ROHM’s Piezoelectric MEMS Technology

1. Introduction

Piezoelectric elements possess the unique property of generating a voltage when force is applied and, conversely, producing force when voltage is applied. They also require very little electricity during standby, providing greater energy savings. These attributes have proven useful in a variety of electronic applications, from industrial inkjet printheads and autofocus motors in cameras to the wearable and infrastructure markets.

Another technology, MEMS (Micro Electrical Machine Systems), combines mechanical elements with micron-level (1/1000 ^th of a millimeter) electronic circuits produced using semiconductor microfabrication technology and is typically used in high-speed sensors and gyroscopes. Being able to form a thin-film piezoelectric element would make it possible to create an extremely small controller for processing the output of a MEMS drive block, contributing to greater miniaturization, increased performance, and lower costs.

Piezoelectric MEMS is quickly becoming an essential technology for next-generation devices that require smaller form factors, higher performance and lower energy consumption. However, thin-film deposition possessing excellent piezoelectric properties as well as piezoelectric body micromachining and molding have proven difficult (Photo 1). In addition, MEMS drive block processing requires high precision, and extensive knowledge and expertise are needed to develop new technologies and apply them to a variety of applications, creating a high barrier to entry.

In response,  ROHM has made it possible to implement integrated product development and manufacture, from wafer injection to mounting, that meet market needs by leveraging ferroelectric technology cultivated for many years in memory development along with MEMS miniaturization and mounting technologies. Performing joint development with customers from the project development stage in particular has allowed ROHM to succeed in formulating optimized processes and establishing an innovation foundry system that supplies piezoelectric MEMS for various markets and applications.

Photo 1: Sample Piezoelectric MEMS Devices

2. Thin-Film Piezo

Voltage and ferroelectric characteristics are important factors in determining the performance of piezoelectric MEMS. ROHM is focusing on developing a thinner lead zirconate titanate (PZT) material featuring superior voltage and ferroelectric properties. However, the performance required for PZT thin-film membranes differs depending on the device, making it necessary to create a thin film that possesses the optimal characteristics for each need.

For example, in the field of actuators that convert electricity into mechanical deformations, PZT thin-film membranes with excellent piezoelectric characteristics are required (Figure 1-a). While in the sensor field there is a need for products that can sensitively respond to changes in temperature and pressure in the target object (Figure 1-b). To meet these demands, ROHM has succeeded in establishing thin-film deposition technology that achieves PZT characteristics optimized for various devices through proprietary technology development.

The amount of PZT thin film deformation required relative to the voltage suitable for the actuator field is high. Therefore, in theory the crystals should be lined up in the (001) orientation. However, in order to ensure consistency with semiconductor processes it is common practice to use platinum (Pt), which has an orientation of (111), as the lower electrode, making it difficult to orient the PZT thin-film membrane on top of this in a stable (001) manner. In response, ROHM established technology to insert a seed layer on top of the lower electrode that acts as the source for crystal control, enabling stable formation of a PZT thin-film membrane that orders the crystals in the (001) direction. This allows for a larger amount of deformation at the same voltage compared with conventional PZT (Figure 2).

On the other hand, PZT thin film for the sensor field requires large changes in the electrical charge in response to changes in temperature and pressure. And since PZT thin film is piezoelectric, they maintain charge even when the applied voltage is 0V. The larger this charge is at 0V (referred to as residual polarization), the larger the change in charge relative to variations in temperature and pressure, resulting in superior sensor sensitivity. By optimizing the composition ratio of Zr and Ti in PZT thin-film membranes, ROHM has succeeded in improving residual polarization and obtaining PZT thin-film membranes ideal for sensors (Figure 3).

Figure 1. Sample Actuator and Sensor

Figure 2. Improved Displacement Through Orientation Control of PZT Thin-Film

Figure 3. Improved Polarization Through Composition Control of PZT Thin-Film

3. Thin-Film Piezo Formation

In addition to the PZT characteristics described above, technology for mass production is also an important key for establishing a piezoelectric MEMS foundry. MEMS devices generally use mechanical movement, necessitating uniform mechanical strength of the structures created. And although the design rule for MEMS is only a few microns or more compared to semiconductor integrated circuits, a high degree of precision is required regarding thickness and dimensions. There are times when film thickness must be 5% or less and dimensions 1% (an accuracy of ±0.2 microns for a structure of 20 microns), making statistical control of film thickness and processing dimensions essential.

PZT film formation is particularly difficult due to the fact that j PZT film is very thick - on the micron-order scale, and k PZT characteristics have a strong effect on film formation conditions.

ROHM uses the sol-gel method of PZT film formation, in which sol-gel solution is spin coated onto wafers, then dried on a hot plate and fired in an infrared heating furnace to form a thin film. Since, as stated above, the required PZT film thickness for piezoelectric MEMS is on the micron-order scale, film formation is performed by repeatedly overlaying dozens of layers. The advantage of the sol-gel method is that a high breakdown voltage, high reliability piezoelectric PZT film can be formed with high reproducibility and in-plane uniformity.

One disadvantage, however, is the effect of particles on film deposition. Because of this, it is important to control the number of particles in each layer, which can significantly affect the final quality of the thin film. Since sol-gel solution is unique and differs from general insulating coating, more caution is required compared to normal spin coaters. For example, sol-gel solution attached to the nozzle tip and coater cup solidifies through normal drying, making it a factor in the creation of particles.

Also, in many cases the cleaning method of the wafer edge (side to periphery, where particles are easily generated) can become problematic, making airflow control within the device along with mist control generated in the coater cup – both of which are important for general insulating coating – important factors. As a countermeasure, it was possible to adopt existing equipment development and semiconductor process technologies to significantly reduce the number of particles by optimizing nozzle material and shape as well as introducing and improving cleaning mechanisms for devices.

In addition, regarding piezoelectric characteristics measurement, since the measurement of in-plane distribution itself is difficult due to the processing required for the cantilever and membrane structures (etching removal of rear Si), PZT characteristics variations between wafers and in-plane variations have proved to be challenging. In answer, ROHM investigated and introduced a device that can measure piezoelectric characteristics non-destructively in wafer form. It was discovered that the temperature distribution during the heating process, such as drying and baking, caused variations in the PZT piezoelectric characteristics. As a result, ROHM was able to keep PZT characteristics variations in check through precise temperature control and by suppressing in-plane distribution on the hot plate and in the infrared heating furnace. This makes it possible to establish technology that can stably create a thin film with superior PZT characteristics.

Photo 2: Thin-Film Piezo Formation Equipment

4. In Conclusion

While development in the semiconductor industry has focused mainly on silicon, a number of other alternatives, such as compound semiconductors comprised primarily of SiC or GaN, as well as organic materials, have surfaced as viable solutions. Therefore, in contrast to processes devised to exclude different elements, diffusion prevention technology is considered essential as a process technology to control the introduction of multiple materials.

In the MEMS field in particular there is a demand for distinctive properties such as piezoelectric, pyroelectric, and magnetic characteristics, making it increasingly important to develop unique materials in the future. This, along with methods to match these materials with CMOS lines, is seen as the key to future development.

In addition, piezoelectric materials are generally susceptible to reduction actions and experience progressive degradation as they pass through the normal CMOS process. The result is significantly decreased piezoelectric characteristics at the final stage, which often makes the products unusable.

Because of this it is important to devise solutions such as creating a barrier layer or making it difficult for reduction to occur, as well as preventing diffusion and providing a structure within the piezoelectric material along with establishing process conditions that prevent the introduction of foreign particles.

ROHM, by utilizing expertise in LED and LD optical devices that use compound materials based on LSI technology, along with discrete ceramic-based technology for resistors and capacitors, seeks to increase control knowledge of different materials and strengthen the development of new devices in the MEMS field.