Chipworks

Contributed by: St.J. Dixon-Warren, Rajesh Krishnamurthy, Tim White, and James Mihaychuk

The coupling of an electrical signal, while providing high voltage galvanic isolation between two parts of a circuit, has always been a technical challenge. The main solution, since at least the early 1960s, has been to use optical coupling. The electrical signal is converted to light using a light emitting diode (LED) or even a small incandescent lamp. The light crosses the gap that provides the electrical isolation, and then the signal is converted back to the electrical domain with a photodiode or a photoresistor. Typically, opto-isolators are unidirectional devices; however, it is possible to build a bidirectional device by using a pair of LEDs placed face-to-face.

An example of a unidirectional opto-isolator is the Renesas PS9402, which was recently analyzed by Chipworks. The PS9402 is an optically coupled isolator containing a GaAlAs LED on the isolated input side, and a photodiode, signal processing circuit, and power output transistor on one chip on the output side. It also includes an isolated fault output. The PS9402 provides 5000 V breakdown isolation between the input and output pins.

Figure 1 shows a cross section through the PS9402. A signal from the input pins is converted to the optical domain by LED1. The light propagates across a gel filled cavity and is absorbed by the photodiode integrated into the BCDMOS die. The high voltage isolation gap is between the upper and lower lead frames. The LED2 is used to couple the fault signal back across the high voltage gap.

Figure 1. Renesas PS9402 Package Cross-Section

Optical isolators are a well established technology; however, due to the nature of the devices, they tend to have high package integration costs, hence a number of vendors have recently developed so-called “digital isolators” which keep the signal entirely within the electrical domain. A variety of different methods are used by the various suppliers of this new technology. The suppliers include Analog Devices, Infineon, NVE Corporation, Silicon Labs, and Texas Instruments. Chipworks has completed a suite of reports on these technologies. Some highlights of these analyses will be summarized here.

Analog Devices iCoupler digital isolators use monolithic planar isolation transformer structures to couple a signal across the high voltage isolation. The Infineon devices are based on a similar transformer technology. Figure 2 shows an X-ray photograph of the Analog Devices ADUM1200 digital isolator. The package contains two die. The transmitter die features three transformer coils, seen in the X-ray, that couple the signal across a layer of polyimide dielectric material.

Figure 2. Analog Devices ADUM1200 Package X-Ray

A cross-sectional view of one of the transformer coil windings in the ADUM1200 is shown in Figure 3. The cross section shows the top and bottom transformer windings separated by the polyimide insulating layer. According to the device datasheet, this polyimide can support up to 2500 V RMS.

Figure 3. Analog Devices ADUM1200 Isolation Transformer Windings

Texas Instruments and Silicon Labs use capacitive coupling to bridge the high voltage gap, rather than inductive coupling. The Texas Instruments ISO7220A is a dual-channel digital isolator. This device has a logic input and output buffer separated by TI’s silicon dioxide (SiO₂) isolation barrier, providing galvanic isolation of up to 4000 V. Figure 5 shows a cross-sectional SEM picture of the edge of the pads. The bottom capacitor plate is formed with an N⁺ substrate diffusion, with the top and bottom plates being separated by the full dielectric stack on the die. The Silicon Labs technology, shown in Figure 6, is quite similar, except that the top plate was formed with metal 6 and the bottom plate with metal 1, in a six metal CMOS process.

Figure 4. Texas Instruments ISO7220A Isolation Capacitor Edge

Figure 5. Silicon Labs Si8422BD Isolation Capacitor Edge

NVE Corporation digital isolators are based on a novel technology called GMR, or giant magnetoresistance. The GMR effect is observed as a significant change in the electrical resistance, depending on whether the magnetization of an adjacent ferromagnetic layer is in a parallel or an anti-parallel alignment. The overall resistance is relatively low for parallel alignment and relatively high for anti-parallel alignment. Figure 7 is a schematic diagram illustrating the operation of the NVE digital isolators. The magnetic field from a winding coil induces a change in the resistance of the GMR layer, which is sensed using a Wheatstone bridge structure. The benzocyclobutene (BCB) provides the high voltage electrical isolation. The GMR film is comprised of a thin, less than 100 nm, stack of permalloy (FeNi), copper, and antiferromagnetic CrPtMn. These films are deposited in the presence of a magnetic field, as described in the NVE US patent 7,557,562 B2, 2009.

Figure 6. Illustration of GMR Isolator

The NVE IL715-3E is a four channel unidirectional high-speed digital isolator. It is a CMOS device manufactured with NVE’s patented IsoLoop® spintronic GMR technology. The device apparently will provide 2500 V RMS isolation. Figure 8 shows the planar magnetic winding coils found on the top surface of the IL715-3E. The magnetic field from this coil results in a change in the GMR film resistance in the underlying Wheatstone bridge, shown in Figure 9.

Figure 7. NVE IL715-3E GMR Isolator Winding Coil and Bond Pads

Figure 8. NVE IL715-3E GMR Sensor Wheatstone Bridge

This brief survey illustrates the broad variety of technologies that have been used to bridge a low voltage signal across a high voltage gap. As discussed, optical coupling has historically been the method of choice; however, this is now being replaced by electronic methods based on inductive capacitive or magnetic coupling.

Chipworks Report References

• Analog Devices ADUM1200WTRZ Digital Isolator Process Review
• Analog Devices ADUM2200SRWZ Digital Isolator Process Review
• Analog Devices AduM1100AR-RL7 Digital Isolator Digital Isolator Receiver Partial Circuit Analysis
• Infineon EICEDRIVER® 1ED020I12FA IGBT Driver with Paired ICs and Planar Coreless Transformers 0.8 μm CMOS Process Process Review
• NVE IL715-3E GMR Type Digital Isolator (30457 J Die Markings) 0.50 μm CMOS Process Process Analysis
• Renesas PS9402 IGBT Driver with Photocouplers and 0.5 μm BCDMOS Signal Processor Process Review
• Silicon Laboratories Si8422BD Low-Power, Dual-Channel Digital Isolator TEM Analysis of Isolator
• Silicon Laboratories Si8422BD Low Power Dual Digital Isolator Circuit Analysis
• Texas Instruments ISO7220A Capacitor Type Digital Isolator Process Review
• Texas Instruments ISO1050DUB Isolated CAN Transceiver Circuit Analysis of Logic I/O Buffer

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