High-frequency design engineers currently enjoy access to an unprecedented variety of semiconductor foundries and semiconductor processes. Most commercial foundries provide at least two wafer runs per year for their major processes and, in addition, often offer opportunities to experiment by sharing space on a multiple project wafer with other customers. The electrical functions offered by these many processes range from low-noise and high-power analog circuits to dense, high-speed digital circuits and wafers with combinations of analog and digital circuits. Some of the latter extend well beyond 100 GHz.
Many semiconductor suppliers, notably those without their own foundries, rely on foundry services to create their semiconductor-based products. Not having a foundry on premises has advantages, since it shifts the responsibilities for the care and maintenance of the foundry and its associated test equipment to the vendor. But this also sacrifices total control over the process and the vast opportunities for experimentation. Still, semiconductor foundries that sell their services generally pride themselves on their capabilities. They typically offer customers many levels of service, with as little or as much help as needed.
Most semiconductor foundry services start with a process design kit (PDK). This is a software-based tool built around the latest computer-aided-engineering (CAE) software, such as Microwave Office® from AWR Corp., the Advanced Design System (ADS) from Agilent Technologies, and Ansoft Designer with Nexxim from Ansoft. When contracting with a gallium-arsenide (GaAs) foundry such as United Monolithic Semiconductors, a customer receives a PDK that is designed for a suitable semiconductor process (such as a low-noise or power process), and compliant with the customer’s computer operating system and other simulation tools.
The PDK incorporates active and passive device models developed (and known to be effective) by the foundry, and essential to creating GaAs monolithic-microwave-integrated-circuit (MMIC) active and passive circuits with the foundry’s various semiconductor processes. Such software design tools are vital for creating a MMIC layout that the foundry can translate into a real wafer, using its own design-rule-check (DRC) process to verify the accuracy and validity of the layout.
Following a successful DRC process, the foundry will create a wafer mask that aids in fabricating the many physical levels of a GaAs semiconductor. Then, depending upon the foundry arrangement, the mask is used to produce two or more wafers filled with the customer’s design. In the case of the UMS foundry, production wafers are inspected visually, as well as via RF and direct-current (DC) process-control-monitoring (PCM) measurements that help determine final wafer quality and acceptance. UMS provides delivery of wafers by means of a GelPak® box or in diced form on UV film. Of special benefit to many customers, UMS offers two-day training courses that help to convey the foundry’s GaAs MMIC design methodology. Topics covered include process, modeling, CAE tools, reliability, electrical measurement, picking, packaging, and industrialization.
Semiconductor foundries share special relationships with CAE software developers because of the strict requirements for PDKs. In many cases, a foundry will support multiple technology processes—each with its own set of PDKs, and each designed for use with a specific commercial electronic-design-automation (EDA) software tool. For example, one of the largest silicon semiconductor foundries, IBM Microelectronics, offers RF CMOS, RF silicon-on-insulator (SOI), and silicon-germanium (SiGe) BiCMOS technologies at different locations, using 8-in. wafers for most of the semiconductor processes and 12-in. wafers for its standard silicon CMOS process in its Fishkill, NY facility.
As different as the capabilities of each process are, they all depend on their own PDKs developed for use with the ADS software from Agilent Technologies. In order for the combination to be effective, the performance capabilities of each semiconductor process must be accurately reflected by the simulation capabilities of the EDA software. Any update in the process technology must trigger an update with the EDA tools for this combination of process and software to provide optimum results.
For many years, the choice of foundry for an RF/microwave customer was between high-frequency silicon or GaAs process. But even one of the most successful GaAs foundries, TriQuint Semiconductor, now offers a number of different process technologies. TriQuint, which is one of the world’s largest commercial GaAs foundries, offers GaAs pseudomorphic high-electron-mobility-transistor (pHEMT) and metal-epitaxial-semiconductor field-effect-transistor (MESFET) semiconductor technologies. Using 100- and 150-mm wafers, the foundry can deliver both low-noise and high-power GaAs MMIC circuits at frequencies beyond 100 GHz with a 0.6-μm MESFET process and with pHEMT processes supporting device features as small as 0.13 μm. The foundry has combined complementary technologies such as GaAs MESFET and/or pHEMT circuits with indium-gallium-phosphide (InGaP) heterojunction-bipolar-transistors (HBTs) on a single InGaP/GaAs wafer to provide tremendous flexibility for customers. Along with many foundries that started with GaAs, TriQuint now also supports GaN foundry services. For its highest-power devices, TriQuint employs GaN-on-SiC technology to combine the excellent high-frequency, high-power capabilities of GaN material with the excellent thermal conductivity of SiC. GaN is an attractive building material for both high-frequency and high-power use, and a growing number of foundries offer GaN-based foundry services. For example, RFMD is another example of a foundry that started with GaAs and has branched into GaN foundry services (see figure). Such is also the case with Global Communication Semiconductors (GCS), which has long supported GaAs foundry services only to add GaN to its lineup.
One of its customers, Nitronex, recently completed qualification of GCS’s GaN-on-silicon foundry process in support of Nitronex’s discrete and MMIC GaN devices. The qualification process included extensive DC, RF, thermal, reliability, and other parametric testing to ensure that devices fabricated at GCS are every way equal to devices made at Nitronex’s Durham, NC facility. Having the GaN foundry’s output in addition to its own capabilities has Nitronex well positioned for sharp growth in power GaN devices. According to Charlie Shalvoy, the company’s Chief Executive Officer, “The combination of our proprietary 100-mm GaN-on-Si process, and the full suite of production and new process development capabilities at GCS, gives us the ability to be a leader in the rapidly emerging market of GaN RF power devices.”
Silicon-carbide (SiC) substrates were once considered a leading candidate for high-power applications, and a number of foundries provide a full range of services based on SiC wafers, including GSC, Ascatron, Cree, Raytheon UK, and United Silicon Carbide. But many of these foundries have worked with higher-frequency GaAs substrates, including GSC and Raytheon, and the relatively low electron mobility of SiC materials relative to some of the other semiconductor substrates results in relatively low cutoff frequencies for SiC devices. Still for power switching applications and motor drives, or any low-frequency application that requires high power density, SiC is an attractive starting point.
But even foundries that started with SiC, such as Cree, have considered the benefits of higher-frequency materials like GaN, and have consequently expanded their foundry operations to include services based on GaN wafers. Although CREE is well established as a providing of lighting solutions with its light-emitting-diode (LED) devices and modules, the company also supports foundries for high-power SiC processes suitable for lower-frequency power switching and control and a 50-V GaN HEMT process that has resulted in 100-W GaN HEMT devices for LTE cellular communications applications from 1800 to 2200 MHz.
For lower-power, higher-frequency operation, InP-based HEMTs still show the highest cutoff frequencies and lowest noise of all three-terminal devices, and silicon-germanium (SiGe) transistors and diodes have routinely been fabricated for applications well into the millimeter-wave range—including for integrated-circuit (IC) transceivers at 160 and 165 GHz. But silicon CMOS has also shown a great deal of life at higher frequencies, with 90-nm silicon CMOS capable of fabricating ICs with +1-VDC transistors operating at 60 and 77 GHz, and more expensive 65-nm silicon CMOS processes typically delivering transistors operating beyond 100 GHz. However, for the higher-power levels sought for many microwave and millimeter-wave transmit applications, GaN and the foundries that support it have caught the attention of more than a few customers in both commercial and military applications.