IMEC improves GaN HEMTs with ceramic substrates

Telecommunication providers, the automotive industry and manufacturers of power-conversion systems are all taking increasing interest in GaN-based electronic devices. And although space and military end-users are driving most of today s developments, GaN is widely seen as a promising technology for high-power amplifiers in 3G/4G wireless base stations.

GaN technology offers RF power electronics the combination of high output-power density (even at high frequency), high output impedance, large bandwidth and high linearity that is not possible with existing silicon-LDMOS or GaAs PHEMT technologies. However, the transition from impressive device performance to real market penetration demands the affordable integration of HEMTs into circuits. Future wireless telecommunication systems will also require a very high level of integration. This advance can only be achieved through miniaturization and integration of passive components, and the use of short interconnects to the active device. For RF power systems, combining thermal management and high-level integration is a major challenge. The high current densities inherent to GaN HEMTs require inventive, thermally viable solutions to integrate active and passive devices in RF applications.

One solution is to emulate the GaAs industry s adoption of monolithic microwave integrated circuit (MMIC) technology, with active and passive circuitry formed on the same substrate. Ideally, this substrate must fulfill all the requirements relating to epitaxial growth, high thermal conductivity, high-frequency performance (low RF loss), manufacturability and cost.

GaN native substrates meeting these criteria are unavailable, and although free-standing GaN wafers are under development, prohibitive costs prevent a monolithic approach that demands large-diameter wafers. SiC substrates produce good epitaxial material, and have been used in GaN MMIC circuit demonstrators. However, low-defect-density SiC substrates are not readily available, and processing this hard material is difficult. In addition, the high cost of the semi-insulating SiC substrates needed for high-frequency devices is a major barrier to the widespread use of GaN technology.

Sapphire substrates present another option, but sapphire s low thermal conductivity is a major problem. Silicon wafers are also promising low-cost substrates for GaN epitaxial growth, but the performance of these devices is limited at higher frequencies.

To optimize performance and cost, the Interuniversity Micro Electronics Center (IMEC) in Belgium has investigated an alternative to the monolithic integration approach that offers a greater flexibility of substrate: the system-in-a-package concept, with a multi-chip module (MCM) technology.

The system-in-a-package approach integrates multiple components on a high-density interconnect substrate. MCM allows active devices to be integrated on a low-cost substrate that also features passive circuitry previously formed as part of a three-dimensional stack. This flexible approach optimizes the cost to performance ratio.

IMEC grew GaN devices on sapphire, which is cheaper than SiC, but still capable of producing high-quality, III-nitride heterostructures. Ceramic AlN was chosen ahead of the widely-used glass for the MCM-substrate, because of its high thermal conductivity of 180 W/mK.

Although various methods can be used to mount active die onto the ceramic AlN substrate, including flip-chip technology and wire-bonding, IMEC has pursued a different approach that improves the chip s thermal management (see "The system-in-a-package approach" images). The process involves direct mounting of active die on AlN, followed by sapphire substrate removal. IMEC s method offers new ways to improve hybrid integration of high-power, multi-finger devices that deliver the power densities required for RF devices used in base stations.

Multi-finger devices typically use air bridges to connect isolated contact areas, which are compatible with flip-chip integration. IMEC s method, however, can form large multi-finger devices without air bridges. After devices are diced and bonded onto the AlN carrier and the sapphire substrate is removed by laser lift-off, chlorine-etching exposes the isolated contact areas on the backside of the HEMT. Interconnects are then formed by copper electroplating (see figure 4). IMEC is currently investigating different adhesives to bond the device onto the AlN carrier, because the adhesive s thermal conductivity is crucial to optimizing the HEMT s thermal performance.

The multilayer MCM technology s advantages for RF systems include the selection of materials such as copper and benzocyclobutene that are relatively easy to deposit at low cost; coplanar waveguide interconnects that offer greater performance and flexibility without the need for through holes; and high-quality integrated passive components that can be fabricated with excellent reproducibility and tolerance. The IMEC MCM technology can be used in conjunction with an extensive design library for the passives that is validated up to 50 GHz.

For GaN devices, this RF integration approach delivers flexibility of substrate choice that creates the freedom to optimize the performance to cost ratio. Sapphire substrates are sufficient for epitaxial growth, despite their poor conductivity, because they provide a good platform for producing high-quality material. Different characteristics are required for the MCM substrate, including affordability, good thermal management and low loss at RF frequencies. These needs can be met with AlN ceramic substrates, which are available in sizes up to 10 × 10 cm.

Although the laser lift-off process should not influence HEMT performance, minimal changes to the transistor s characteristics do occur (see "The system-in-a-package approach" images). Despite these minor drawbacks, thermal simulations suggest that the system-in-a-package process produces circuits with better thermal performance than those that use a flip-chip configuration. They show that the operating temperature of a HEMT fabricated using MCM technology, with an improved design working under typical conditions, is far less than 100 °C. Under identical conditions, a HEMT on a sapphire substrate operates at 200 °C, which reduces its reliability. Although these results are still to be confirmed with thermal measurements, experiments have shown that using just the flip-chip process reduces the temperature of a HEMT on a sapphire substrate delivering an output of 5 W/mm from 200 to 100 °C.

The simulations also show that with a flip-chip configuration, the HEMTs can only dissipate power via the solder bumps. However, the laser lift-off process allows heat loss through thermal leakage pads on the interconnects and the thin bonding layer. Thermal management can be further improved after sapphire removal by encapsulating the HEMT with better heat-conducting materials, and by increasing the adhesive layer s thermal conductivity. These refinements produce low device operating temperatures that enable high currents at high drain-source voltages. Some GaN LED manufacturers have shown that the laser lift-off process is compatible with high-volume manufacturing. Consequently, we believe that this approach to removing the substrate is comparable to the classical flip-chip process in terms of cost. However, we think that system-in-a-package technology will be far cheaper than MMICs on SiC, and offer the optimal solution in terms of the cost to performance ratio.

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