**2. Digital isolator construction and fabrication**

There are three major components for a digital isolator, isolation barrier coupling element, insulation material, and signaling schemes through the isolation barrier. Insulation material is used for the isolation barrier to achieve certain isolation rating, and the isolation rating mainly depends on the dielectric strength and its thickness. There are two main types of dielectric materials, organic such as polyimide and inorganic such as silicon dioxide or silicon nitride. Oxide or nitride has an excellent dielectric strength of 700–1000 V/μm; however, it has an inherent high stress to prevent film thicker than 15–20 μm to be formed reliably on a large-scale modern IC wafer. The other limitation to organic films is that they are susceptible to ESD, and a tiny energy of voltage overstress will lead to catastrophic avalanche breakdown. Organic films such as polyimides consist of long C-H chains, and a small ESD event with limited energy may break some local C-H links without compromising material structural integrity, and they tend to be much more ESD tolerant. Polyimide does not compare favorably to oxide or nitride in terms of dielectric strength, around 400–700 V/μm; however, with inherent low film stress, much thicker polyimide layers as much as 40–60 μm can be formed economically. Thus, 30 μm polyimide films provide withstand voltages of 12–21 kV, comparable to 20 μm oxide with withstand voltages of 14–20 kV. For applications with robust ESD performance and high voltage withstand capability against impulse voltages, such as those present during lightning strikes, polyimidebased isolators provide the most robust choice.

Commercial polyimide films are available in photoresist forms that are deposited on wafers with well-controlled thicknesses and then easily patterned with standard photolithography processes. Here is the process flow as shown in **Figure 3** for the isolation transformers used for the digital isolators. A CMOS wafer with its top metal layer forming the bottom coil is spin-coated with the first photosensitive polyimide, and the polyimide layer is patterned through photolithography. The polyimide is then thermally cured to achieve high structural quality. Top coil layer is plated after which a second polyimide layer is coated, patterned, and cured to form the encapsulation for the top coil. Because deposited polyimide films are free of voids as shown in **Figure 4** and do not suffer from corona discharge, the transformer devices also exhibit good aging behavior and work well under continuous AC voltages and DC voltages.

### **Figure 3.**

*Isolation transformer process flow. (a) CMOS substrate with top metal, (b) polyimide layer spin coated, (c) polyimide layer patterned & cured, (d) top coil plated, (e) 2nd polyimide layer spin coated and (f) 2nd polyimide layer patterned & cured.*

**Figure 4.** *Cross section for the fabricated isolation transformer.*
