【Introduction】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 are used in many markets including automotive, industrial automation, medical and energy. The main reason for the high voltage performance of these digital isolators is the use of a polyimide film between the top and bottom helical windings of stacked winding transformers.
This article will describe the structure of a digital isolator, in which a polyimide film is used as the isolation layer. In order to meet various safety standards, such as UL and VDE, digital isolators need to withstand various high voltages such as short-time withstand voltage, surge voltage, operating voltage, etc. The aging behavior of polyimide under various high-voltage waveforms such as AC or DC was studied, and the working voltage of the isolator was calculated through the polyimide life model. In addition, improving the high-voltage life of polyimide by improving the structure will also be discussed.
The isolation between circuit elements is generally to ensure high voltage safety or data integrity. For example, isolation protects sensitive circuit components and human-machine interfaces on the system side, preventing damage or injury from dangerous voltages on the field side, which has more robust components such as sensors and actuators. Isolation also eliminates common-mode noise or ground loops that can affect data acquisition accuracy. While optocouplers have provided isolation for decades, they have significant limitations, including low speed, high power consumption, and limited reliability. Their low bandwidth and long transmission delay times make 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 consumption, which greatly limits the total system power budget of 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 decreases, making it no longer possible to meet the reliability requirements of demanding applications such as automotive.
Digital isolators eliminate the pitfalls 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 microtransformers support the integration of multiple transformers and other necessary circuit functions. The stacked helices used in digital isolators provide tight magnetic coupling between the top and bottom coils and very low magnetic coupling between adjacent helices. In this way, multiple channels can be integrated with little interference with each other. The magnetic coupling between the top and bottom helices depends only on the size and separation distance. Unlike optocouplers, the current transfer ratio 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 and can achieve frequencies from 150 Mbps to 600 Mbps for digital isolators. The high quality factors of these transformers are well above 10, making these digital isolators orders of magnitude lower power dissipation than 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 primarily determined by a 20 μm to 40 μm thick polyimide layer between the top and bottom coils of the chip-scale microtransformer. We will describe 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 digital isolators.
Figure 1. (a) Schematic diagram of the optocoupler, (b) cross-sectional view of the optocoupler package.
Figure 2. (a) Digital isolator in plastic package, (b) transformer cross-section.
Digital isolators use polyimide film
Polyimides are polymers composed of imine monomers. Polyimide is used as an insulating material in many digital isolators for a number of reasons, including excellent breakdown strength, thermal and mechanical stability, chemical resistance, ESD performance, and relatively low dielectric constant. In addition to its good high voltage performance, polyimide also has excellent ESD performance, able to withstand EOS and ESD events in excess of 15 kV. During an energy-limited ESD event, the polyimide polymer absorbs some of the charge, forming stable free radicals that interrupt the avalanche process and dislodge some of the charge. Other dielectric materials, such as oxides, typically do not have this ESD tolerance, and once the ESD level exceeds the dielectric strength, avalanches can occur even at low ESD energy. Polyimide also has high thermal stability, with a weight loss temperature of over 500°C and a glass transition temperature of about 260°C; and high mechanical stability, with a tensile strength of over 120 MPa and an elastic elongation of over 30 %. Although polyimide has high elongation, its Young’s modulus is about 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 films can be coated on semiconductor wafer substrates, and their 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, a thick polyimide film with a dielectric constant of 3.3 works well with small diameter Au transformer coils to minimize the capacitance of the isolation barrier. Most iCoupler products have less than 2.5 pF capacitance between input and output. Because of these properties, polyimide is increasingly used in microelectronics applications and is an excellent insulating material for iCoupler’s high-voltage digital isolators.
Construction and fabrication of digital isolators
The digital isolator is mainly composed of three parts: isolation barrier coupling element, insulating material and signal transmission modulation and demodulation circuit. The insulating material is used to allow the isolation barrier to achieve a certain isolation level, and the isolation level mainly depends on the dielectric 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. Both oxides and nitrides have excellent dielectric strengths ranging from 700 V/µm to 1000 V/µm. However, their inherent high stress also prevents reliable formation of 15 μm to 20 μm thick films on large-scale modern IC wafers. Another disadvantage of organic films is their susceptibility to ESD; small voltage overstresses can lead to catastrophic avalanche breakdown. Organic films such as polyimide are composed of very long CH chains, and a small energy-limited ESD event may destroy some local CH chains without destroying the structural integrity of the material, showing higher resistance to ESD. Tolerance. In terms of dielectric strength, polyimide is inferior to oxide or nitride – 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 30 µm polyimide film has a withstand voltage range of 18 kV to 24 kV, which is better than that of the 20 µm oxide (14 kV to 20 kV). Polyimide-based isolators are a good choice for applications with robust ESD performance and high withstand voltages against impulse voltages such as those found in lightning strikes.
Commercially available polyimide films are available as photoresists, which are deposited on wafers at tightly controlled thicknesses and then patterned using standard photolithographic processes. Figure 3 shows the process flow of the isolation transformer used in digital isolators. A first layer of photosensitive polyimide is spin-coated on the CMOS wafer with the top metal layer forming the bottom coil, and then the polyimide layer is formed by photolithography. The polyimide is then thermally cured to achieve high structural quality. The top coil layer is electroplated, then a second polyimide layer is applied, shaped and hardened to form the top coil package. Since the deposited polyimide film has no voids (as shown in Figure 4), no corona discharge occurs, so the transformer equipment also has good aging characteristics and is very suitable for operation under continuous AC or DC voltage.
Figure 3. Industrial flow diagram of an isolation transformer.
Figure 4. Cross-sectional view of the fabricated isolation transformer.
High Voltage Performance for Digital Isolators
Isolation rating according to UL 1577 determined by the maximum withstand voltage for a duration of 1 minute. During factory testing, the digital isolator 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 1 second at 3 kV rms. In practical applications, there are two important high-pressure performance parameters to be aware of. One is the maximum operating voltage at which the insulation 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 120% of the rated voltage. For example, if a reinforced digital isolator is rated to operate at 1kv rms, its lifetime at 1.2 kV rms needs to be greater than 37.5 years at a failure rate of 1 ppm. Likewise, at 120% of the rated voltage, at a failure rate of 1000 ppm, the life of an isolator providing basic insulation needs to be longer than 26 years. Another important application parameter is the maximum transient isolation voltage that the device can withstand. Transient test waveforms may vary, Figure 5 shows example waveforms according to EN 60747-5-5 or IEC 61010-1. The time taken to go from 10% to 90% was about 1.2 μs, and the time to go from peak to 50% was 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 characteristic of semiconductor devices, and its high surge performance means that it also has excellent ESD tolerance.
Figure 5. IEC 61010-1 Surge Test Waveform
Properties of Polyimide Films
Figure 6 shows the main electrical properties of the spin-coated polyimide film itself measured at the wafer level: on the one hand, the DC bulk conductivity of polyimide is very low in the electric field range of 40 V/µm, about 10 -16, remains very low at least up to 150 V/µm. On the other hand, at 60 Hz, the AC breakdown electric field value of the polyimide film reaches the minimum value of 450 V rms/µm. All of these factors make spin-coated polyimide films an excellent insulating material for reliable digital isolator applications.
Figure 6. The main electrical properties of the spin-coated polyimide film itself measured at the wafer level: (A) DC conductivity versus electric field, (b) AC breakdown electric field distribution.
Figure 7 shows the surge performance of an isolator using a 30 μm thick polyimide film. These isolators will pass surge tests up to 18 kV with a first failure of 19 kV for negative pulses and 20 kV for positive pulses.
Figure 7. Surge performance of an isolator with 30 μm thick polyimide film.
Aging of polyimide film
We study the service life of polyimide through high pressure endurance tests. Any insulator can break down given enough time and voltage. Figure 8 shows an example setup. Multiple devices are connected in parallel, with different high voltages applied to groups of devices from a high voltage power supply, and switching/measuring devices (such as an Agilent 34980 and a computer) are used to monitor when these devices are broken down. This process can be lengthy, and it can take days to months to break down these devices.
Figure 8. Experimental setup for high voltage endurance testing.
The time distribution of device failures can be analyzed by a Weibull chart, as shown in Figure 9. Applying 6 different voltages to groups of 16 devices resulted in a nice Weibull plot for each group. From the Weibull plot, it is possible to estimate the mean time to failure (MTTF), or the time to failure at some failure rate (eg 1 ppm). It is clear that the time to failure is much shorter at high pressure than at low pressure. According to VDE 0884-11, the minimum to maximum MTTF needs to span at least two orders of magnitude, and at the lowest test voltage, 63% of the failure times will exceed 1E7 seconds or about 116 days. Figure 9 shows that the datasets generated at these 6 voltages meet these requirements.
Figure 9. Weibull profile of an isolator with a 20 μm thick polyimide layer.
In order to infer the operating voltage, the time to failure is plotted based on the stress voltage. For basic insulation, the working voltage is determined by derating the voltage by 20%, in which case the time to failure or service life at 1000 ppm is greater than 24 years. Similarly, for reinforced insulation, the operating voltage is determined by derating the voltage by 20%, at which time the service life at 1 ppm is greater than 30 years.
The main breakdown mechanism is through charge injection, which results from the direct impact of electrons from the electrodes into the polyimide surface area. Under HVac conditions, the breakdown process begins when charges are injected into the polyimide surface. Charges can accumulate at certain points of accumulation on the surface. After accumulation, the electrical energy is released, and the stored electrostatic energy can cause tension in the local mechanism. Through the quantum activation process, this tension eventually induces local free volumes (voids or microcracks), which form more local accumulation points. If the HVac persists long enough, this process will result in a continuous decrease in the insulating capacity and eventual breakdown by electrical breakdown.
Through thermodynamic analysis, the service life L can be expressed as Equation 1:
where Et is the threshold field at which no charge injection occurs, and m and n are proportionality constants.
We analyzed the HVac withstand data for iCoupler devices following the procedure specified in ANSI/IEEE Standard 930-1987 (“IEEE Guide for Statistical Analysis of Electrical Insulation Voltage Endurance Data”), resulting in:
As shown in Equation 2, this phenomenological fit is used to calculate the shortest lifetime because it assumes that the thermodynamic model does not specify a threshold field. If we try to measure the threshold field, the duration of the HV test becomes very long. We use Equation 2 to simulate the failure time in Figure 10. As you can see, the model matches the data very well.
Figure 10. Failure time diagram for an isolator with a 20 μm thick polyimide layer.
We also found that the lifetime of iCoupler devices at DC or unipolar AC voltages is much longer than at bipolar AC voltages; at least two orders of magnitude higher. For unipolar waveforms, the accumulated charge creates an internal field barrier region around the electrode, further preventing charge injection into the polyimide layer, as shown in Figure 11. In bipolar AC waveforms, the reversal of the electric field prevents the formation of this stable field barrier, and the accumulation area continues to invade the polyimide layer, eventually leading to electrical breakdown. On the other hand, at DC or unipolar AC voltage, SiO2 has a shorter lifespan.
Figure 11. Field barrier region with accumulated charge resulting in a zero net electric field.
The lifetimes shown in Figure 10 are based on worst-case bipolar AC waveforms. For unipolar AC or DC waveforms, the HV lifetime is even longer. The model used in this paper is related to polyimide insulation, not to insulators that use SiO2 insulators as the primary means of isolation. Likewise, the models used to predict the HV lifetime of SiO-based digital isolators are independent of polyimide-based isolation systems.
Figure 12 shows a comparison of the lifetime of polyimide films at unipolar and bipolar voltages. It can be seen that under the same fault time, the peak stress voltage of the unipolar voltage is twice that of the AC bipolar voltage. Fundamentally, the lifetime is determined by the peak-to-peak voltage of the polyimide film, not its peak stress voltage.
Figure 12. Time-to-failure comparison between AC bipolar voltage and unipolar voltage.
Structure Improvement of Polyimide Film
In order to improve the high voltage resistance of polyimide, the charge injection barrier shown in FIG. 13 can be used. The charge injection barrier is best to use oxides or nitrides with large band gaps and high dielectric constants. A high dielectric constant helps reduce the electric field near the electrodes, while a large band gap can increase the electrical barrier to charge injection.
Figure 13. (a) Transformer isolation without and (b) with nitride charge injection barrier.
To analyze charge injection for a given isolated system, a band diagram can be drawn, as shown in Figure 14. Figure 13 shows the 4 important materials used in the isolation system, namely: Au for the top coil, polyimide for the isolation between the top and bottom coils, oxide as a barrier for charge injection, and the seed crystal below the Au layer TiW. From the band diagram, the amount of charge injected by Au or TiW into polyimide, oxide (electrons) or holes can be calculated.
Figure 14. Band diagram of charge injection.
Figure 15 shows the measured charge current versus time for polyimide and polyimide with SiN implant barrier at 1000 V. Steady-state current was reduced by more than 5 times when using polyimide with SiN barrier compared to using polyimide alone. This highlights a significant reduction in the charge injection process, which is known to be a major contributor to electrical aging at high electric fields.
Figure 15. Comparison of charging current (at 1 kV) for polyimide and polyimide with SiN implanted barrier.
Figure 16 shows the time to failure (HVE test) versus applied AC voltage at 60 Hz, 1 kV rms up to 3.5 kV rms for single-die configurations with polyimide and with polyimide/SiN barrier. The graph shows the lifetime at 50%, and the extrapolated values for the dataset at 1 ppm. In addition, for both cases, the operating voltage (presumably) at a lifetime of 30 years is also reported. Digital isolators with polyimide insulation operate at 400 V rms, while improved digital isolators with SiN injection barriers operate at >900 V rms (750 V at 1 ppm with 20% derating ). Based on the wafer-level analysis and comparison results, it can be concluded that the SiN implant barrier between the polyimide and the metal coil has improved lifetime and operating voltage. These thin SiN layers reduce bipolar charge injection at the onset of space charge formation, thereby reducing current flow, reducing associated thermal effects, and (most likely) extending lifetime at a given voltage.
Figure 16. Comparison of failure time of polyimide isolators with and without SiN charge injection barrier.
From surge voltage to high voltage withstand, polyimide films have excellent high voltage performance. We have identified the properties of these films that their anti-aging behavior can be further enhanced by using charge injection barriers with large dielectric constants and large band gaps. This article describes the use of polyimide films in digital isolators, which are an excellent choice for building isolation barriers for digital isolators.