Higher performance, greater functionality, and dramatically reduced die size enable delivery of superior bipolar products in smaller packages such as the popular SOT-23.
By Craig D. Wilson, PhD, Key Technical Contributor, ADSiV Process Development, Analog Devices, Inc.
The introduction of the Analog Devices iPolar™ manufacturing process represents the industry's first major re-engineering of 36-volt bipolar processing in nearly two decades. Designed for industrial applications that require bipolar analog components with high voltage tolerances, the new iPolar process yields devices that consume 75 percent less silicon area than earlier bipolar processes while offering unusually high performance and functionality. This, in turn, is fostering the introduction of families of off-the-shelf industrial devices that offer industrial systems designers a new array of lower noise, lower cost options in much smaller SOT-23 packages.
How Analog Devices came to develop iPolar
As semiconductor process technology continues to mature, much of the spotlight has focused on CMOS and the needs of mobile, battery-powered consumer and communications applications for components with ever-lower supply voltages. Unlike the digital CMOS domain, however, where designers work with sub-micron geometries and hundreds of thousands to many millions of devices per die, the industrial marketplace typically uses process geometries as large as 2.5 microns that yield perhaps a hundred active bipolar devices per die.
While many semiconductor manufacturers were driving CMOS-based chip voltages down and performance up, the needs of industrial users for higher speeds, more features, lower noise, lower power, and smaller packages did not keep pace. Indeed, until the advent of Analog Devices' iPolar process, most 36-V bipolar process advances were modest and depended on huge diffusion areas to adequately separate the transistors, which in turn required the use of larger die and wafers.
Figure 2. A smaller footprint - An 8-lead SOIC (small outline IC) package (left) is shown next to a 5-lead SOT-23, which at 8.3 mm2 is nearly 75 percent smaller. The more efficient use of board real estate dramatically improves performance and functionality.
Why iPolar is so revolutionary
With the iPolar 36-V process, Analog Devices tapped years of bipolar process design experience by employing a proprietary deep trench isolation scheme to minimize transistor size without compromising performance. This was achieved by maintaining a junction isolation scheme between the devices and the substrate, rather than using a full dielectrically-isolated (DI) scheme that requires buried-oxide wafers (SOI substrates). While integrating the deep trench isolation scheme with a bulk substrate presents an additional process development challenge, when compared to a DI process the circuit benefits are clear. In addition to dramatically lowering costs relative to SOI (silicon-on-insulator) wafers, the iPolar process uses bulk substrates to provide much improved thermal conductivity for the active transistors, promoting better transistor characteristics over a wider temperature and voltage range.
Additionally, the high-performance precision bipolar devices that iPolar enables were constructed without poly emitters to create the best possible 1/f noise performance, while the emitter/base junction formation was carefully engineered to avoid the burst (or popcorn) noise often associated with older bipolar processes.
Though revolutionary, iPolar is flexible enough to have allowed Analog Devices to migrate popular design techniques to the new process, including ADI's patented XFET technology as well as trimmable thin-film and high-performance capacitor technologies. Rather than simply port older techniques to a new process, however, ADI re-engineered each to provide the highest performance and process uniformity while minimizing die consumption. At the same time, the iPolar process uses more a advanced wafer fabrication facility than other 36-volt bipolar processes, which allows ADI to take advantage of cleaner, more advanced process techniques to maximize yields by minimizing defects and lot-to-lot wafer and die variations.
Analog Devices also has improved the 'analog' performance of various transistor combinations, using device matching and a new emitter-base-collector architecture to better control important circuit parameters such as offset voltage and noise.
Figure 1 shows the layouts of two similar transistors. The iPolar-based transistor at the bottom features a 4× area reduction, making the resulting circuit small enough to fit into a SOT-23 package. In the case of an amplifier IC, the added space enables designers to add features like bias current cancellation and shut-down capability.
Figure 1. Before and after iPolar - At the top is a layout of a traditional 36-volt device. At the bottom is the same function tooled in the new iPolar 36-volt technology which brings about a far superior product in a much smaller die.
In addition to more efficient use of available die area, iPolar enables transistors with bandwidth of at least 750 MHz or more for both the NPN and PNP transistors, a significant improvement over standard 36-volt bipolar performance.
An overview of iPolar Products
Let's look next at some of the products that are enabled by the Analog Devices iPolar process. iPolar products being introduced now are listed in Table 1. iPolar products to be introduced soon are listed in Table 2.
The iPolar AD8675 brings significant improvements to the OP27 - Holding the bias current Ib as low as possible is essential in an operational amplifier. That is why Ib cancellation circuitry is essential, for otherwise the Ib rises with increases in voltage and temperature. Device designers create circuits that compensate, so that the b stays relatively constant over variations in voltage and temperature. This has played a major role in the design of the AD8675 iPolar amplifier. It exhibits improved precision and noise performance - vis-à-vis the low-noise, precision OP27 operational amplifier plus a 50 percent reduction in power and a 20 percent improvement in bandwidth.
1 Also available in dual and quad versions.
iPolar AD8677 outperforms the popular OP07 – Analog Devices' iPolar AD8677 is a redesign of the popular ultra-low offset voltage OP07 operational amplifier, bringing improved phase margin and stability. It fits into the SOT-23 package illustrated in Figure 2 and is far smaller than the SOIC. The SOT-23 is a surface mount package with overall dimensions, including leads of 0.112" x 0.115" (2.84 mm x 2.92 mm). Some of the salient features of the AD8677 are listed in Table 1.
Table 2. iPolar Products to be introduced soon
|ADR 12x, ADR13X - Voltage Reference||
|ADR82x - Voltage Reference + Amplifier||
|High Speed Amplifier at 26 V|
iPolar is the first significant effort to re-engineer 36-volt bipolar devices in a generation. It has made possible the introduction of a family of analog devices for the industrial marketplace that deliver dramatically improved performance and far greater functionality in just 25 percent of the board space. Together with iCMOS, the high-voltage industrial manufacturing CMOS process introduced in November 2004, Analog Devices reaffirms its continuing commitment to the industrial marketplace.