Polyimide Films for Digital Isolators

Polyimide Film Uses for Digital Isolators

Polyimide Films for Digital Isolators

summary

Compared with traditional optocouplers, digital isolators have advantages in high speed, low power consumption, high reliability, small size, high integration, and ease of use. Billions of digital isolators using microtransformers have been widely used in many markets, including automotive, industrial automation, medical, and energy. The main reason why these digital isolators have high voltage performance is that a polyimide film is used between the top spiral winding and the bottom spiral winding of the stacked winding transformer. This article will introduce the structure of a digital isolator in which a polyimide film is used as an isolation layer. In order to meet various safety standards such as UL and VDE, digital isolators need to have various high voltage performances such as short-time withstand voltage, surge voltage, and working voltage. The aging behavior of polyimide under various high voltage waveforms such as AC or DC is studied, and the working voltage of the isolator is deduced through the polyimide life model. In addition, improving the high voltage service life of polyimide by improving the structure will be discussed.

Introduction

Isolation between circuit elements is generally used to ensure high voltage safety or data integrity. For example, isolation protects sensitive circuit elements and human-machine interfaces on the system side from damage or injury caused by dangerous voltages on the field side, where more robust components such as sensors and actuators are located. Isolation also eliminates common-mode noise or ground loops that can affect data acquisition accuracy. Although optocouplers have been used to provide isolation for decades, they have significant limitations, including low speed, high power consumption, and limited reliability. They use low bandwidth and have long transmission delays, which makes it difficult for them to meet the increasing speed requirements of many isolated fieldbus communications, such as RS-485 in industrial automation systems.

Their LEDs have high power dissipation, which significantly limits the total system power budget for power-constrained industrial systems, such as 4 mA to 20 mA process control systems. Over time, especially at high temperatures, the current transfer ratio of optocouplers degrades, making them no longer able to meet the reliability requirements of demanding applications such as automotive.

Digital isolators eliminate the deficiencies of traditional isolation and offer advantages over optocouplers in terms of high speed, low power consumption, high reliability, small size, high integration, and ease of use. Digital isolators using microtransformers1,2 allow integration of multiple transformers and other necessary circuit functions. The stacked spirals used in digital isolators provide tight magnetic coupling between the top and bottom coils and very low magnetic coupling between adjacent spirals. This allows multiple channels to be integrated together with little interference between the channels. The magnetic coupling between the top and bottom spirals depends only on the size and separation distance. Unlike the current transfer ratio of optocouplers, it does not degrade over time, so these transformer-based digital isolators are highly reliable. The self-resonant frequencies of these transformers range from a few hundred MHz to several GHz, enabling frequencies of 150 Mbps to 600 Mbps for digital isolators. The high quality factor of these transformers is well above 10, making the power consumption of these digital isolators orders of magnitude lower than that of optocouplers.

The optocoupler shown in Figure 1 achieves isolation by filling a few millimeters of molding compound between the LED die and the photodiode die. For the transformer-based digital isolator shown in Figure 2, the isolation performance is mainly determined by the 20 μm to 40 μm thick polyimide layer between the top and bottom coils of the chip-scale microtransformer. We will introduce the detailed structure of these isolators, the deposition method of these polyimide films, the characteristics of the polyimide films, the high-voltage performance, and the aging behavior of the digital isolators.

Figure 1. (a) Schematic diagram of an optocoupler, (b) cross-sectional view of an optocoupler package

Figure 2. (a) Digital isolator in plastic package, (b) transformer cross-section

Digital isolators use polyimide film

Polyimide is a polymer composed of imide monomers. Polyimide is used as an insulating material in many digital isolators for many reasons, including excellent breakdown strength, thermal and mechanical stability, chemical resistance, ESD performance, and relatively low dielectric constant. In addition to good high-voltage performance, polyimide also has excellent ESD performance, capable of withstanding EOS and ESD events exceeding 15 kV. 3 In an energy-limited ESD event, the polyimide polymer absorbs some of the charge, forming stable free radicals that interrupt the avalanche process and discharge some of the charge. Other dielectric materials, such as oxides, generally do not have this ESD tolerance, and once the ESD level exceeds the dielectric strength, avalanches may occur even if the ESD energy is very low. Polyimide also has high thermal stability, with a weight loss temperature of more than 500°C and a glass transition temperature of approximately 260°C; as well as high mechanical stability, with a tensile strength of more than 120 MPa and an elastic elongation of more than 30%. Although polyimide has a high elongation, its Young's modulus is approximately 3.3 GPa, so it is not easily deformed.

Polyimide has excellent chemical resistance, which is one of the reasons why it is widely used as an insulating coating for high-voltage cables. Polyimide film can be coated on semiconductor wafer substrates, and its excellent chemical resistance also helps facilitate IC processing on top of the polyimide layer, such as the Au plating used to make iCoupler® transformer coils. Finally, thick polyimide film with a dielectric constant of 3.3 is well suited for use with small diameter Au transformer coils to minimize the capacitance of the isolation barrier. Most iCoupler products have a capacitance of less than 2.5 pF between input and output. Due to these characteristics, polyimide is increasingly used in microelectronic applications and is a very suitable insulation material for iCoupler high-voltage digital isolators.

Digital Isolator Structure and Fabrication

Digital isolators are mainly composed of three parts: isolation barrier coupling elements, insulating materials, and signal transmission modulation and demodulation circuits. Insulating materials are used to achieve a certain isolation level for the isolation barrier, and the isolation level is mainly determined by the insulation strength and its thickness. There are two main types of dielectric materials: organic materials (such as polyimide) and inorganic materials (such as silicon dioxide or silicon nitride). Oxides and nitrides both have excellent dielectric strengths of 700 V/μm to 1000 V/μm. However, their inherent high stress also prevents the reliable formation of 15 μm to 20 μm thick films on large-scale modern IC wafers. Another disadvantage of organic films is that they are easily affected by ESD; even a small voltage overstress can cause catastrophic avalanche breakdown. Organic films such as polyimide are composed of very long CH chains. A small ESD event with limited energy may destroy some local CH links, but will not destroy the structural integrity of the material, showing higher tolerance to ESD. In terms of dielectric strength, polyimide is not as good as oxides or nitrides - about 600 V/µm to 800 V/µm. However, due to the low stress of the film itself, thicker polyimide layers of 40 µm to 60 µm can be formed without excessive cost. The withstand voltage range of 30 µm polyimide film is 18 kV to 24 kV, which is better than the withstand voltage range of 20 µm oxide (14 kV to 20 kV). For applications with strong ESD performance and high withstand voltage capability to withstand surge voltages (such as those seen in a lightning strike), polyimide-based isolators are a good choice.

Commercial polyimide films are available in the form of photoresists that are deposited on wafers at a tightly controlled thickness and then patterned using standard photolithography. Figure 3 shows the process flow for an isolation transformer used in a digital isolator. The CMOS wafer with the top metal layer forming the bottom coil is spin-coated with a first layer of photosensitive polyimide, and then the polyimide layer is formed using photolithography. The polyimide is then thermally cured to achieve high structural quality. The top coil layer is electroplated, and then a second polyimide layer is applied, patterned, and hardened to form the top coil package. Since the deposited polyimide film has no voids (as shown in Figure 4), corona discharge does not occur, so the transformer device also has good aging characteristics and is well suited for operation under continuous AC or DC voltage.

Figure 3. Industrial flow chart of isolation transformer

Figure 4. Cross-sectional view of the fabricated isolation transformer

High voltage performance for digital isolators

The isolation rating is determined by the maximum withstand voltage for a duration of 1 minute, according to UL 1577. When the digital isolator is factory tested, it is tested for 1 second using 120% of its rated voltage. For a 2.5 kV rms 1 min rated digital isolator, the corresponding factory test setting is 3 kV rms for 1 second. In practical applications, there are two important high voltage performance parameters to note. One is the maximum working voltage, at which the insulation layer needs to remain intact throughout continuous AC or DC operation. For example, according to VDE 0884-11, the life of an isolator providing reinforced isolation needs to be greater than 37.5 years at a failure rate of 1 ppm at a voltage of 120% of the rated voltage. For example, if the reinforced digital isolator is rated for a working voltage of 1 kV rms, its life at 1.2 kV rms with a failure rate of 1 ppm needs to be greater than 37.5 years. Similarly, the life of an isolator providing basic insulation needs to be greater than 26 years at a failure rate of 1000 ppm at a voltage of 120% of the rated voltage. Another important application parameter is the maximum transient isolation voltage that the device can withstand. Transient test waveforms may vary, Figure 5 shows an example waveform according to EN 60747-5-5 or IEC 61010-1. The time taken to rise from 10% to 90% is about 1.2 μs, and the time taken to decrease from the peak to 50% is 50 μs. This is to simulate lightning strike conditions, so it is very important for the isolator to have strong surge performance that can meet field requirements. ESD tolerance is an important feature of semiconductor devices. Having high surge performance means that it also has excellent ESD tolerance.