Periodic Reporting for period 1 - PRISME (PRogram for ISolation Manufacturing in Europe (PRISME))
Reporting period: 2019-09-01 to 2021-08-31
The aim of the PRISME project is to enable the next ‘Electrical’ revolution by developing isolation technology that protects people and equipment from high voltages (HV). This will be delivered by combining innovations in isolation materials and device design to meet higher voltage requirements. Novel test structures and protocols will be designed to enable rapid characterisation to ensure reliable lifetime operation of these materials and devices in HV applications.
Electric Vehicles (EV’s) have gained increased popularity in recent years as consumers look for alternative fuel vehicles to conserve energy and reduce CO2 emissions and Governments work to meet UN Sustainable Development Goal 13 to combat climate change and its impacts. The Roadmap to a Resource Efficient Europe (COM (2011) 571) outlines how we can transform Europe's economy into a sustainable one by 2050 and one of the key aspects is sustainable transport. Electric motors are more energy efficient than conventional combustion engines, and they can dramatically reduce emissions. As the voltage of EV battery packs increases, downstream system and component suppliers must meet the safety and performance challenges in these high power conversion applications. While batteries are at the heart of EVs, they are also the source of many barriers to adopting EVs because of reliability, safety, weight and cost.
To increase the battery efficiency high voltages are needed and this poses two major challenges:
1. Signal isolation – transmitting high voltage signals to low power electronics to monitor safe operation,
2. Power isolation – protecting people and hardware from HV.
PRISME will focus on isolation technology achieved by physical separation of two metal coils by high performance insulating polymer: polyimide (PI). PI layer is an electrical insulator widely used in microelectronics due to its excellent electrical properties and ease of processing. Its high breakdown electric field (>400V/um) makes it particularly suitable as an insulating material for digital isolators. This work aims to show how the PI electrical isolation performance is significantly enhanced by introducing thin PECVD silicon nitride (SiN) layers at the PI-electrode interface. Insight into the physical origins of these improvements is obtained by careful wafer-level characterizations and product-level aging tests.
The two critical developments required to deliver the next generation isolation technology are:
1. New aging methods to accelerate isolation technology reliability validation
2. New advanced materials that meet the breakdown requirements for higher voltage isolation.
Electric Vehicles (EV’s) have gained increased popularity in recent years as consumers look for alternative fuel vehicles to conserve energy and reduce CO2 emissions and Governments work to meet UN Sustainable Development Goal 13 to combat climate change and its impacts. The Roadmap to a Resource Efficient Europe (COM (2011) 571) outlines how we can transform Europe's economy into a sustainable one by 2050 and one of the key aspects is sustainable transport. Electric motors are more energy efficient than conventional combustion engines, and they can dramatically reduce emissions. As the voltage of EV battery packs increases, downstream system and component suppliers must meet the safety and performance challenges in these high power conversion applications. While batteries are at the heart of EVs, they are also the source of many barriers to adopting EVs because of reliability, safety, weight and cost.
To increase the battery efficiency high voltages are needed and this poses two major challenges:
1. Signal isolation – transmitting high voltage signals to low power electronics to monitor safe operation,
2. Power isolation – protecting people and hardware from HV.
PRISME will focus on isolation technology achieved by physical separation of two metal coils by high performance insulating polymer: polyimide (PI). PI layer is an electrical insulator widely used in microelectronics due to its excellent electrical properties and ease of processing. Its high breakdown electric field (>400V/um) makes it particularly suitable as an insulating material for digital isolators. This work aims to show how the PI electrical isolation performance is significantly enhanced by introducing thin PECVD silicon nitride (SiN) layers at the PI-electrode interface. Insight into the physical origins of these improvements is obtained by careful wafer-level characterizations and product-level aging tests.
The two critical developments required to deliver the next generation isolation technology are:
1. New aging methods to accelerate isolation technology reliability validation
2. New advanced materials that meet the breakdown requirements for higher voltage isolation.
1. New aging methods to accelerate isolation technology reliability validation:
Standard accelerated lifetime tests on industrial parts are usually performed at 60 Hz, by varying the applied voltage over and above the specification working conditions. An extrapolation fit (e.g. inverse power or exponential law) is then used to establish the working voltage, that is, the voltage at which the lifetime is practically ‘infinite’ (~30 years). However, new applications such as isolated power converters in EVs with fast power devices control (i.e. SiC, GaN) by pulse-width modulation (PWM), involve higher working frequencies potentially ranging up to 100 kHz with very high slew rate >10 kV/μs. It is therefore critical to evaluate the intrinsic behavior of the isolation barrier in high frequency.
A comprehensive investigation assessing both the intrinsic and extrinsic properties of polyimide films at high voltage (up to 10 kHz) and the HVE performance of fully built units also at higher frequency in square wave (>10 kHz) has been performed. Investigations was performed at 2 scales using (i) wafer-level testing and (ii) package-level HVE testing.
It was clearly demonstrated that the nonlinear DC and AC conductivities converge indicating partly common conduction mechanisms close to breakdown. The frequency independent nature of the polyimide AC breakdown was measured at wafer-level up to 10 kHz. A confirmation result of recent square wave high frequency HVE tests at package unit level was carried out at 2kVrms up to 50kHz.
2. New advanced materials that meet the breakdown requirements for higher voltage isolation
This part of the work aims to show how the PI electrical isolation performance is significantly enhanced by introducing thin PECVD silicon nitride (SiN) layers at the PI-electrode interface.
To evaluate the effects of tailoring the interfaces in a production device, 4 different wafer-level test structures have been adapted using the same process flow used in production. Spin-coated PI interfaces with Au and TiW/Au metals were processed with SiN adjacent to either the cathode, anode, neither or both. The test structures are labelled PI, PI-SiN, SiN-PI and SiN-PI-SiN.
The breakdown field of PI film is around 450 Vrms/um. It is increased up to 475 and 480 Vrms/um for PI-SiN and SiN-PI, respectively. Finally, the highest improvement is for the symmetrical SiN-PI-SiN structure with a dielectric strength around 510 Vrms/um. The SiN barrier efficiently enhances the dielectric behavior of the full stack in terms in HV. Tailoring both sides of PI film with a higher bandgap material at its interfaces with metal is a valuable strategy to limit injection in bulk. The electric field threshold for the initiation of charge injection into PI is significantly increased when a SiN layer is introduced at its interfaces with metallization and correlates with breakdown field enhancement, proving a superior injection barrier efficiency.
The dissemination of the project results has taken place in two ways: internally at ADI and externally to different audiences. Internally, the dissemination of the results occurred in ADI during the two main internal conferences and the internal workshop: i) ADLEC - Analog Devices Limerick Engineering Conference (attended by ADI engineers mostly based in Ireland) and ii) GTC - General Technical Conference (global ADI event) and the Isolation TechDay workshop. On top of that, appropriate intellectual property has been protected throughout the course of the project and 2 patents have been filed based on the project results before to communicate externally. The dissemination of the results occurred through the open access publications of 1 guest edited book, 2 book chapters, 3 journal papers (IEEE TDEI, JAP, MDPI Polymers), 1 White paper available on the ADI website, and 5 IEEE conference papers.
Standard accelerated lifetime tests on industrial parts are usually performed at 60 Hz, by varying the applied voltage over and above the specification working conditions. An extrapolation fit (e.g. inverse power or exponential law) is then used to establish the working voltage, that is, the voltage at which the lifetime is practically ‘infinite’ (~30 years). However, new applications such as isolated power converters in EVs with fast power devices control (i.e. SiC, GaN) by pulse-width modulation (PWM), involve higher working frequencies potentially ranging up to 100 kHz with very high slew rate >10 kV/μs. It is therefore critical to evaluate the intrinsic behavior of the isolation barrier in high frequency.
A comprehensive investigation assessing both the intrinsic and extrinsic properties of polyimide films at high voltage (up to 10 kHz) and the HVE performance of fully built units also at higher frequency in square wave (>10 kHz) has been performed. Investigations was performed at 2 scales using (i) wafer-level testing and (ii) package-level HVE testing.
It was clearly demonstrated that the nonlinear DC and AC conductivities converge indicating partly common conduction mechanisms close to breakdown. The frequency independent nature of the polyimide AC breakdown was measured at wafer-level up to 10 kHz. A confirmation result of recent square wave high frequency HVE tests at package unit level was carried out at 2kVrms up to 50kHz.
2. New advanced materials that meet the breakdown requirements for higher voltage isolation
This part of the work aims to show how the PI electrical isolation performance is significantly enhanced by introducing thin PECVD silicon nitride (SiN) layers at the PI-electrode interface.
To evaluate the effects of tailoring the interfaces in a production device, 4 different wafer-level test structures have been adapted using the same process flow used in production. Spin-coated PI interfaces with Au and TiW/Au metals were processed with SiN adjacent to either the cathode, anode, neither or both. The test structures are labelled PI, PI-SiN, SiN-PI and SiN-PI-SiN.
The breakdown field of PI film is around 450 Vrms/um. It is increased up to 475 and 480 Vrms/um for PI-SiN and SiN-PI, respectively. Finally, the highest improvement is for the symmetrical SiN-PI-SiN structure with a dielectric strength around 510 Vrms/um. The SiN barrier efficiently enhances the dielectric behavior of the full stack in terms in HV. Tailoring both sides of PI film with a higher bandgap material at its interfaces with metal is a valuable strategy to limit injection in bulk. The electric field threshold for the initiation of charge injection into PI is significantly increased when a SiN layer is introduced at its interfaces with metallization and correlates with breakdown field enhancement, proving a superior injection barrier efficiency.
The dissemination of the project results has taken place in two ways: internally at ADI and externally to different audiences. Internally, the dissemination of the results occurred in ADI during the two main internal conferences and the internal workshop: i) ADLEC - Analog Devices Limerick Engineering Conference (attended by ADI engineers mostly based in Ireland) and ii) GTC - General Technical Conference (global ADI event) and the Isolation TechDay workshop. On top of that, appropriate intellectual property has been protected throughout the course of the project and 2 patents have been filed based on the project results before to communicate externally. The dissemination of the results occurred through the open access publications of 1 guest edited book, 2 book chapters, 3 journal papers (IEEE TDEI, JAP, MDPI Polymers), 1 White paper available on the ADI website, and 5 IEEE conference papers.
Higher working voltage of digital isolators with PI was successfully achieved by tailoring PI interfaces with SiN layers. The reduced charge injection enabled by SiN delivers higher working voltage 900Vrms (i.e. 750Vrms after derating) for a single die or 1500Vrms for 2 dies with high reliability (>30 years-lifetime). More recently, a full encapsulation has demonstrated the achievements of a >1000Vrms working voltage. This has led to the release of new commercial parts for ADI in 2020.
It was clearly demonstrated that the frequency independent nature of the PI breakdown field revealing that under a PD-free regime, the frequency does not accelerate the underlying short-term mechanism of failure. This is proving the capabilities of using the parts in the high frequency range for the control of power devices without derating their working voltage.
It was clearly demonstrated that the frequency independent nature of the PI breakdown field revealing that under a PD-free regime, the frequency does not accelerate the underlying short-term mechanism of failure. This is proving the capabilities of using the parts in the high frequency range for the control of power devices without derating their working voltage.