The path to InP production

A third commercially viable technology - gallium arsenide - emerged in the late 1980s from the world of compound materials, and we have yet to see the emergence of a compound semiconductor technology that can displace GaAs as the material of choice for high-performance, high-volume commercial applications. However, devices based on a new technology just beginning to emerge from the laboratory and based on indium phosphide and its derivatives constitute the fourth wave in semiconductor materials. InP clearly offers compelling performance advantages over GaAs in fiber-optic, millimeter-wave and even wireless applications. We believe that these advantages will provide the differentiation for InP to eventually displace GaAs as the chosen technology in the compound semiconductor industry.

Market forces

All new semiconductor technology begins in the research lab, where success is measured by the ability to produce a handful of devices that set new performance standards. However, before the technology can be adopted into higher volume consumer applications, huge challenges must be overcome. The fourth wave of semiconductor technology, InP, faces the same barriers to adoption that all new technologies encounter. Attempting to force a technology into the commercial market will inevitably lead to failure - but where there are compelling advantages for the customer, a successful market launch is virtually unstoppable.

For InP-based devices, customer pull in several target markets is undeniable. In the fiber-optic arena, InP is the only semiconductor technology that allows photodetectors and lasers to be integrated on the same substrate with other analog and mixed signal functionality, providing advances in integration and cost reduction that could spur significant paradigm shifts in this market. In the wireless industry, InP-based amplifiers provide significant performance improvements, such as lower power consumption, high linearity and low temperature sensitivity, that significantly enhance the battery life and reception in current handset designs. In millimeter-wave applications that are beyond the capabilities of GaAs or Si, InP devices can easily be fabricated for passive imaging and other applications that are just emerging in the marketplace.

New technologies need coordinated accomplishments on a number of fronts to make the leap from the research lab to the production line. This was true for Si, and it was also true for GaAs. Fortunately, most of the equipment and process innovations that took GaAs into production just a decade ago can also be applied to InP production. Although there are specific alloy differences, advances in epitaxial equipment, including multiwafer MOCVD and MBE systems (figure 1), are used in both InP and GaAs production, as are technologies such as dry etching and substrate thinning. Since the manufacturing infrastructure is the same for both technologies, there is no need for large investment in new fabrication line equipment to switch to InP. Furthermore, substrate vendors are now making great strides in the development of large-diameter InP substrates, just as they did for GaAs a decade ago.

Up the learning curve

When GaAs technology was initially funded for defense and aerospace applications, it took more than a decade to make the transition into commercial applications. InP is making that same transition today, going down the path initiated for GaAs in the early 1990s. In 1993, TRW announced plans to produce GaAs HBTs for the commercial marketplace. This was initially received with skepticism, as the HBT was perceived to be neither reliable nor manufacturable in high production quantities. Today, the GaAs-based HBT is used in many high-volume consumer applications and is the technology of choice for several commercial telecommunications applications. This includes products from an industry leader, which licensed TRW s technology and holds close to 20% of the worldwide GaAs MMIC market for wireless applications. We are now seeing InP follow the same ramp up in terms of substrate size and quality, generating a concomitant reduction in price and the same acceptance by customers who see a performance differentiator that they can use to win new sockets.

Still, the advantages of InP process technology are often overshadowed by concerns regarding the cost, yield and reliability of a process technology in its formative years. The lack of InP processing experience that is rife in the market today is perhaps the greatest barrier, as companies attempt to move the InP process from the research lab into the production line. In other words, regardless of InP s superior performance, a cost-effective manufacturing process is required in order to allow this technology to compete effectively in the marketplace.

One approach has been to model the InP process development on proven, high-volume GaAs HBT production processes. Lessons learned from the GaAs transition are applicable to the InP scenario. For example, the process flow in the GaAs HBT process is directly transferable to the InP HBT process.

Challenges for volume production

For any new technology to gain acceptance and long-term success in the market, it must have not only the ability to be produced in volume, but also must have proven reliability - the key to the initial application. InP HEMT MMICs are already proving themselves in challenging in-orbit applications. In this way, the stringent reliability requirements of satellite components are actually paving the way for commercial, terrestrial applications of InP.

A key facet of technology adoption, the ability to produce any technology in volume, must be designed into the semiconductor process during the development phase. However, this capability only comes into its own when the fabrication line is fully exercised. As volume production increases, the cost for any semiconductor device - including those made using InP - will come down. As processing experience improves and yields rise (figure 2), prices for a wide range of components decrease by factor of two or more through the life cycle of a typical device.

Initial production snags are to be expected when new technologies transition from R&D to prototype to volume production. Ge, Si and GaAs all experienced growing pains, and InP can benefit by examining the types of problems encountered by GaAs. When initial attempts to commercialize GaAs were made several years ago, the availability, cost and reproducibility of GaAs substrates were a challenge, as was establishing a reproducible GaAs HBT production process. A backside process for thinning and fabricating GaAs substrates was also critical. The result was a high-volume, cost-efficient GaAs production line. When applied to the InP production process, these lessons expedite volume production.

Production challenges aside, the cost of materials is another factor, and one already addressed through the GaAs experience. Wafer processing requires some of the most expensive materials, and HEMT and HBT devices are only as good as the materials with which they are built. Extensive material characterization and a complete understanding of the material quality before processing is the key to success for InP devices (figure 3). Establishing correlations between material characteristics and device performance for both GaAs and InP is a prerequisite for stable and successful fabrication, and fundamental physical modeling enables theoretical correlation to actual results.

At present, InP lithography is several generations behind Si, yet InP still yields performance that far exceeds that of any production Si process. As InP moves to smaller geometries, this performance differentiation will increase significantly. SiGe HBT devices with 0.13 µm emitters cannot compete with InP HBT devices with emitters 10 times that size. Just imagine what will happen to circuit performance as InP HBTs are reduced to sizes approaching that of Si today. The InP HEMT has the highest performance of any transistor, with published results for integrated circuits operating at 220 GHz, far in excess of the results of any other substrates. Figure 4 shows an example of InP HBT production capability.

Conclusion

New technologies such as InP first generate excitement in university and government laboratories, as well as corporate research and development centers. Reputations made as "firsts" are published and presented at key conferences, but, as the technology matures, researchers often lose interest and move on to other firsts. However, this phase of technology development is actually the most important, and in many ways the most difficult. The companies that are successful in transitioning these technologies into production are not necessarily the first to publish. Success will be found by the company that can transition an exciting technology into a viable commercial application, long after the researchers have moved on to more exciting projects.

A final key to success is to ensure that the research engineers - the people who developed the technology - participate in the transition from research to prototype to production, only moving on to new research endeavors after they have achieved full production. Rather than merely throwing the technology over the fence to the production people, the correct approach should be a team effort involving all who participated in the birth and nurturing of the new technology.