**1. Introduction**

Isolation between circuit components is typically required for safety and/or data integrity considerations. For example, isolation protects sensitive circuit components and human interface on the system side from dangerous voltage levels present on the field side, where more robust components such as sensors and actuators reside. Isolation can also eliminate common-mode noises or ground loops that affect data acquisition accuracy. While opto-couplers are choices of isolation for many decades, they present significant limitations in terms of low speed, high power consumption, and limited reliability. Its low bandwidth and long propagation delay presented significant challenges in meeting the ever-increasing speed requirements for many isolated field bus communications such as RS485 in industry automation systems. Its high power consumption due to the need to lighting up LED puts significant constraint on overall system power budget in power limited industry systems such as process control 4–20 mA systems. As current transfer ratio for the opto-couplers degrades over time, especially at high temperatures, they fail to meet reliability for demanding applications such as automotive.

Digital isolators remove penalties associated with isolation, provide compelling advantages over opto-couplers in terms of high speed, low power consumption, high reliability, small size, high integration, and ease of use. Digital isolators using

micro-transformers [1, 2] allow the integration of multiple transformers and with other necessary circuit functions. These stacked spirals used in digital isolators provide tight magnetic coupling between the top coil and bottom coil and very little coupling between spirals side by side. This enables multiple channel integration with little interference between the channels. The magnetic coupling between the top spiral and bottom spiral depends only on the size and separation, unlike the current transfer ratio for the opto-couplers, and does not degrade over time, which leads to the high reliability for these digital isolators based on transformers. These transformers have self-resonant frequency from a few hundred MHz to a few GHz and can be easily used to realize digital isolators from 150 to 600 Mbps. With highquality factor well over 10 for these transformers, the power consumption for these digital isolators is orders of magnitude lower than those of opto-couplers.

Opto-couplers as shown in **Figure 1** rely on a few mm thick molding compound between the LED die and photodiode die to achieve isolation. For transformerbased digital isolators as shown in **Figure 2**, isolation performance is mainly limited by 20 to 40 μm thick polyimide layers sandwiched between the top and bottom coils of the chip-scale micro-transformers. In this chapter, we will review detailed construction of these isolators, the deposition methods for these polyimide films, characterization of the polyimide films, the high voltage performance, and the aging behavior for the digital isolators.

Polyimide was chosen as the insulating material for many reasons, including excellent breakdown strength, thermal and mechanical stability, chemical resistance, ESD performance, and relatively low permittivity. Besides good high voltage performance, polyimide has an excellent ESD performance and is capable of handling EOS and ESD events exceeding 15 kV [3]. During energy-limited ESD events, the polyimide polymer absorbs some of the charges to form stable radicals that interrupt the avalanche process and bleeds away some of the charge. Other dielectric materials such as oxides typically do not have this ESD tolerant characteristic and may go into avalanche once the ESD level exceeds the dielectric strength, even if the ESD energy is low. The polyimide also has high thermal stability, with a weight loss temperature over 500°C and a glass transition temperature above 260°C. The polyimide also has high mechanical stability with a tensile strength over 120 MPa and a high elastic elongation over 30%. In spite of its high elongation, polyimide does not deform easily, because the Young's Modulus is about 3.3 Gpa.

The polyimide has excellent chemical resistance, which is one reason it has been widely used for insulation coatings for high voltage cables. High chemical resistance also helps to facilitate IC processing on top of polyimide layers, such as the Au plating used to create *i*Coupler transformer coils. Lastly, the thick polyimide layers, with a dielectric constant of 3.3, work well with the small diameter Au transformer coils to minimize capacitance across the isolation barrier. Most Coupler products exhibit

**Figure 1.** *(a) Opto-coupler schematic. (b) Opto-coupler package cross section.*

**Figure 2.** *(a) Digital isolator in a plastic package. (b) Transformer cross section.*

less than 2.5 pF capacitance between the input and output. Because of these characteristics, polyimide is increasingly used in microelectronics applications, and it is an excellent choice as insulating material for the *i*Coupler high voltage digital isolators.
