Power-transistor packaging is not immune to the cost pressures facing commercial RF and microwave markets. RF power-transistor suppliers have long sought alternative materials to traditional insulating materials such as ceramics, including BeO. But the superior characteristics of BeO continue to make it the material of choice at the highest power levels at frequencies up to 4 GHz, especially in demanding military and aerospace pulsed power applications.

BeO is a key insulating material because of its thermal conductivity (TC) of about 285 W/mK at room temperature. Among insulating materials, this TC rivals all but diamond (as high as 1800 W/mK). The nearest competitor to BeO is aluminum nitride (AlN) at about 185 W/mK, with widely used alumina (Al₂O₃) reaching only 25 W/mK. For applications where thermal management and electrical isolation are critical, it is no wonder that BeO is the primary choice for RF power-transistor packages (Fig. 1).

In addition to thermal conductivity, BeO's thermal coefficient of expansion (TCE) of 6.3 PPM/°C falls between that of silicon (3.3 PPM/°C) and gallium arsenide (5.8 PPM/°C). BeO's TCE is also well matched to other metal matrix composites (MMCs) such as Cu/W, Cu/Mo, and Al/SiC. In other relevant areas, such as electrical resistively and dielectric strength, BeO has better electrical characteristics than AlN and is similar in behavior to alumina. With a low dielectric constant of 6.7 and low loss tangent of 0.0012 at 1 MHz, BeO is also well suited for use at high frequencies. In addition to these characteristics, BeO also is inherently stable in oxidizing environments, in contrast to nitrides such as AlN that tend to decompose over time to their oxide equivalent (see table).

Since beryllia is an oxide ceramic, BeO parts are very stable in oxygen/moisture-containing environments. Ceramic-to-metal joints and metallization coatings are generally very strong and reliable, which is important in the design of high-reliability military systems. In addition, BeO is also about 11 times more resistant to thermal shock than alumina.

Like other ceramic materials, the thermal conductivity of BeO decreases with increases in temperature, in contrast to metals, whose thermal conductivity deteriorates less with increasing temperature. As a result, employing combinations of BeO and metal films has proven very effective for RF power packaging. In the temperature range at which RF power transistors operate, BeO also offers about 30-percent better thermal dissipation than AlN. Even at cryogenic conditions, beryllia remains superior to all other ceramic materials.

The capability of a device or subsystem to rid itself of heat is exceptionally important to product life. For example, the vast majority of failures in base-station electronics are related to thermal issues. Without good package heat dissipation, a device will degrade at an accelerated rate and ultimately fail, and the higher the temperature the sooner this occurs. As a result, device designers consider heat dissipation and thermal management in general an increasingly important concern. Not surprisingly, BeO packaging is very appealing in such dense environments, where even incremental improvements in thermal management can play a major role in extending the operating life of an electronic system.

In applications at higher frequencies, higher current is required near bond interfaces, which necessitates less-resistive interface layers. BeO offers the ability to reduce the area and size of a device while dissipating the same power and reducing capacitance. In small leadless packages, for example, BeO provides an advantage over AlN, which at high frequencies cannot provide the expected electrical or thermal performance that can be achieved when using BeO.

Dielectric constant and dissipation factor are also important parameters in which the characteristics of BeO are desirable. Transmission lines and passive structures are critical components of RF and microwave modules. For many applications, lower dielectric constants are preferred because transmission-line losses are lower and conductor width is increased, which makes circuits more easily produced. Dielectric losses are also reduced with decreasing dielectric constant.

The cutoff frequency of a particular substrate thickness also increases as dielectric constant is lowered for microstrip circuits. As a result, a BeO substrate of a given thickness can be used at frequencies about 20-percent higher than it could with an alumina substrate of the same thickness. Thickfilm lithographically etched, gold-metalized substrates manufactured on BeO exhibit these important characteristics and can operate upwards of 44 GHz. Bonding of copper to BeO enables the conduction of large electrical currents. In addition to conventional metalizing techniques, copper can be applied to BeO via the Direct Bond Copper (DBC) process. DBC on BeO provides conduction paths with lower electrical resistance than thin-film or thickfilm conduction lines. Transistor packages produced via the DBC process perform extremely well through 8 GHz with less than 1 dB of insertion loss.

Several recent developments have further expanded the utility of BeO. Of greatest significance for RF power transistors is a recent increase in thermal conductivity to 325 W/mK achieved by Brush Ceramic Products, (www.brushceramics.com), a division of Brush Wellman, Inc. The enhancement further distances the thermal conductivity of BeO from competing materials, and makes it suited for future RF power devices that deliver greater RF outputs and have correspondingly greater need to dissipate waste heat. The material is currently in beta testing at several customers.

Other enhancements include the ability to produce BeO plates up to 4.5 X 4.5 in., which reduces cost by allowing a greater number of patterns to be created on a single plate. In addition, the new BW1000 substrates from Brush Ceramics increase material strength by up to 25 percent to 38,000 lb./in.² (262 MPa), achieved by adding proprietary additives while maintaining a purity level of 99.5 percent, and by precisely controlling grain growth and grain distribution during sintering.

With the inherent strengths of BeO, an obvious question is why it is not used exclusively (as it once was) in RF power transistor packaging even though it remains extremely popular in optical, automotive, power supply, and many other areas in which its high TC and other desirable characteristics remain unmatched (Fig. 2). The answer is twofold. First, cost is a major driver of every commercial application of RF and microwave technology, and BeO is more costly than alumina and in some applications can be more costly than AlN. If a device's power dissipation and other performance parameters can be accommodated by AlN or alumina, one of these will likely be the insulating material of choice. In addition, laterally diffused metal-oxide-semiconductor (LDMOS) devices, which do not require die attached to a dielectric, eliminate the need for BeO, and are widely used at frequencies as high as 4 GHz.