The inaugural CS-MAX show provided a forum to discuss high-volume manufacturing issues across the whole compound semiconductor industry, including GaAs devices, fiber-optic components and LEDs. Tim Whitaker, Richard Dixon and Jon Newey report.

Many technical conferences in the compound semiconductor industry focus on a single material system, or a single epitaxial technique, and tend to have a research-oriented feel. In contrast, the Compound Semiconductor Manufacturing Expo (CS-MAX), held for the first time in Boston on July 911, addressed high-volume manufacturing issues of the type that are becoming increasingly important as the industry expands and matures. Running alongside an exhibition featuring over 100 companies, the technical program at CS-MAX comprised 95 papers and was organized into five tracks microelectronics, fiber-optic components, LEDs, epitaxy and characterization. In addition, a number of short courses and seminars were held on areas such as exhaust gas management, process and device simulation, manufacturing InP ICs, and the roadmap for solid-state lighting. The technical program was composed of talks from industry-leading device manufacturers Honeywell, Vitesse, Nichia, Nortel and Alpha, to name just a few as well as representatives from companies supplying materials and equipment. At the time this provoked some lively debate, for example when Jeff Spiegelman of Aeronex advocated his company s nickel/titania catalyst technology for hydrogen purification, and was questioned closely by those with a vested interest in the incumbent palladium membrane technology. GaAs IC manufacturing Opening the program, Patrick Jenkins described Vitesse Semiconductor s experience in establishing the world s first 6 inch GaAs fab at Colorado Springs, as well as the company s decision to introduce a 4 inch InP process at its facility in Camarillo, CA. Vitesse s manufacturing focus contrasts strongly with Stanford Microdevices, which operates on a fabless model and uses different foundries (including RFMD, TRW, Nortel, GCS and TriQuint) for its various RF process technologies. Bob Van Buskirk, CEO and president, described the advantages of this approach, which include process flexibility, diversity and neutrality; low investment levels and low technical risk; efficient asset utilization; and timely evolution to new technologies. Key lessons are that it is expensive to qualify a new process or foundry; that cycle time is critical and foundries have their own set of priorities (to other external companies and sometimes to internal customers); and that frequent technical contact is critical. Also, designers should be encouraged to be technology-neutral, and intellectual property should be protected. Fiber-optic components Chun Lei of Finisar Corporation discussed uncooled long wavelength Fabry Perot (FP) and DFB lasers manufactured by the company s Demeter Technologies subsidiary. Compared to InGaAsP buried heterojunction devices, said Lei, AlInGaAs FP and DFB lasers offer higher optical gain and speed, improved temperature performance, higher reliability and easier manufacturing. Based on a ridge waveguide design, Finisar s 1.3 m AlInGaAs FP and DFB lasers have a chip size of 250 250 m, with an active stripe width of 3 m (see ). Both DFB and FP designs can be operated at 2.5 Gbit/s from 40 to +85C, and up to 75C at 10 Gbit/s. The AlInGaAs FP laser operates at 10 mW with an MTTF of 8 106 hours at 85C. At 100 mA, the DFB structure is centered on 1283 nm, and offers a side mode suppression ratio of 47 dB at 25C, and 42 dB at 100C. Mark Gottfried described Emcore s PIN photodiodes. Devices operating at 850 nm and 3.125 Gbit/s are grown by MOCVD on 4 inch semi-insulating GaAs substrates, and typically feature 23 m layers of n-GaAs followed by a 1.53.5 m i-GaAs absorber and thin (< 0.4 m) p-type top GaAs layer. A mesa structure design is chosen to reduce parasitic capacitance by placing bond pads directly on the substrate. The 10 Gbit/s design replaces the GaAs epilayers with AlGaAs, making the p- and n-layers more transparent and increasing the responsivity. At both data rates, the diodes featured a responsivity of 0.50 A/W, and dark current (1.6 V) of 0.2 nA. According to Gottfried, 1310 nm InP-based photodiodes are less mature devices than their GaAs counterparts. An n+ InP buffer is grown before the i-InGaAs absorber, which is followed by a p+ InP layer and InGaAs cap. The devices feature a responsivity of over 0.9 A/W, capacitance of 0.6 pF and dark current less than 2 nA. In die form, reliability of 3400 hours has been demonstrated at 85C and 85% humidity. This could lead to moisture-resistant photodiodes that alleviate the cost and yield issues associated with hermetic packaging. Light-emitting diodes In the plenary session on LEDs, Kiyoshi Komatsu of Nichia described his company s manufacturing capabilities for high-brightness LEDs, which now includes a 46 500 m2 building in Tatsumi. Komatsu estimated that the efficiency of white LEDs will increase steadily, reaching 60 lm/W in 2010, at which time costs will fall so that these devices will offer 100 lumens/dollar. Heng Lui of AXT s LED division described the design and manufacturing of AlInGaN-based LEDs. In the area of MOCVD growth, in-wafer uniformity, run-to-run reproducibility and throughput of commercial machines all require improvement. During chip processing, there is a lack of pattern recognition systems to identify bad chips, making this a labor-intensive procedure. Due to the nature of the material, dicing losses and costs are high. Test and measurement also needs improvement; for example, there is a weak correlation between chip I-V figures and light output power from the lamp. Degradation is also a major issue that requires better understanding, and possibly the introduction of a better reliability test. Roger Wu described UEC s fourth-generation AlInGaP LED (see p51), while Jim Chi of the Industrial Technology Research Institute gave some informa-tion on Taiwan s burgeoning LED industry. At least 20 companies are now producing LED chips and epiwafers in Taiwan, while many more are packaging the devices. According to Chi, Taiwan s main strengths are the availability of capital and the low cost mindset, while weaknesses include a lack of intellectual property, inadequate R&D and a shortage of engineers. As well as price erosion and the current economic downturn, problems for Taiwan include the strong IP position of companies such as Nichia in the area of GaN technology, and the ongoing national white LED programs in other countries. However, large market growth presents a strong opportunity; Chi predicted that Taiwan s overcapacity situation will be consolidated, low-cost products will open new applications, and manufacturing relationships with mainland China will become increasingly important. Processing and packaging Larry Hanes of Alpha Industries described manufacturing issues associated with shrinking the metallization line dimensions of the company s GaAs MMICs. Alpha chose to scale the source-drain fingers in its FET device from 6 m down to 3 m, with the aim of increasing the chip yield from 6 inch GaAs substrates. The first of the new devices featured severe delamination of the smaller plated metal fingers, which were made from a titanium-gold plated stack. On analysis, these contacts appeared to have undergone undercutting during etching. The undercut was traced to the titanium: in 6 m wide contacts, the undercut had not been sufficient to lead to delamination. Alpha considered several approaches, including wider contact and alternative etches, but settled on a replacement for the Ti layer, choosing to employ a TiW:Au plated base metal instead of Ti:Au. The move to TiW:Au required no mask or design rule changes. MOCVD and MBE The vital importance of accurate temperature measurement and control during MOCVD growth was highlighted by Jeff Bodycomb of Emcore. Real wafer temperatures are known to vary significantly from the readings given by commonly-used thermocouples and pyrometers. Bodycomb explained how Emcore set about correcting for this variation using an emissivity-compensated pyrometer. Known as RealTemp, this device takes readings at high speed enabling its use in multiwafer rotating disk reactors, where it measures the temperature and uses the data to control surface temperature and therefore the grown structure. When growing laser test structures, changes in surface temperature when gas switching occurred could be readily seen. Using RealTemp allows temperature swings to be compensated for, giving much more uniform temperature throughout the growth runs. Laser wavelength variation from wafer to wafer was much reduced as compared to thermocouple-controlled growth. A number of contributions discussed MBE s ascendance from a costly, slow research tool to a true production technology. The advent of large multiwafer reactors with high-capacity sources has lengthened campaign times and increased throughput to make it a cost-competitive epitaxy technique. Applied Epi s Dave Narum discussed the emerging generation of devices and their increasingly stringent demands, including lower noise, higher power and higher frequency. Narum translated these needs into the challenges that device structures are now placing upon MBE. These include maintaining a smooth surface during metamorphic growth, composition control for graded layers, dopant control (< 1016 in collectors, > 1019 in base regions) and abrupt interfaces. The incorporation of nitrogen plasma sources for the emerging class of devices requiring nitride growth was discussed. Characterization of IC wafers Innovative Processing AG (IPAG), a spin-off from the University of Duisburg, Germany, was formed at the end of last year to commercialize InP and GaAs devices, including PIN photodiodes and HBT driver ICs. The company has access to 3000 sq. ft of class 10 and 100 clean room space, MOVPE and MBE growth, circuit design and a processing line. The company s first products, PIN diodes operating at 10 and 4050 Gbit/s, are expected at the end of this year. One of the founders of the company, Peter Velling, described a novel approach to X-ray characterization. Detailed quantitative analysis of epilayers is usually carried out for test structures that are then used to fingerprint and model subsequent data sets from complex layer structures. Extraction of fully quantitative data is often time-consuming. IPAG s method enables fully quantitative measurements to be automatically extracted from X-ray rocking curves without fitting the data to models. This means rapid qualification of structures such as HBTs, enabling the technique to be used in process control. Analysis of an InGaP/GaAs:C device showed that the observed X-ray intensities resulted from tensile strain in the base layer caused by carbon on arsenic sites, and surface roughness resulting from the formation of islands at the p+-GaAs:C interface of the InGaP/GaAs (emitter/ base) structure. Overall, no degradation in the performance of GaAs HBTs was noted, and these devices were reported to offer a robust process with ft and fmax values in excess of 120 GHz. James Greene of Corning Tropel, a manufacturer of substrate flatness measurement equipment capable of assessing 120 wafers per hour, described how different measurement techniques impact the lithography process. Capacitance, normal and grazing incidence interferometry, and air gauging each have benefits and limitations and can be matched to different compound semiconductor device families. Peter Hiesinger from the Fraunhofer Institute described the development of the EL2 Absorption Topography System, which uses IR absorption to measure the uniformity of the EL2 intrinsic deep donor across a GaAs wafer. This mid-gap donor occurs when an As atom sits on a Ga site. The concentration and ionization degree of this donor determines the semi-insulating properties of a wafer and is controlled by the Ga/As ratio during growth. The uniformity of this donor is crucial to the operation of RF devices made on semi-insulating GaAs substrates. Data from the tool showed a lateral resolution down to 30 mm and reproducibly gave variations as low as 1%. Differences in EL2 uniformity in LEC- and VGF-grown wafers were shown before and after annealing. Though annealing improved EL2 uniformity, it also reduced resistivity. Fab management Several sessions looked at the issues of productivity, cycle times and efficiency from a number of different perspectives. Mary Young of Nortel Networks discussed how the goal of doubling fab capacity and halving cycle time had been meet by working with a consultant, MAX International Engineering Group, to implement an Overall Resource Effectiveness scheme. This monitored how all resources were being used, not just equipment, to identify and quantify bottlenecks. It also monitored how implementing a structured improvement process eventually lead to the goals being achieved. Lisa Aucoin of Raytheon RF Components described the challenges of running a fab dedicated to a number of low-volume processes at different levels of maturity, for military and commercial customers. This limits the usefulness of a silicon-like model of a dedicated facility for each process (see p81). In the fiber-optic arena, Tom Dearmin of Lasertel discussed the role of statistical process control when establishing a new company (see Compound Semiconductor August 2001, p61). Seppo Orsila described how Modulight is using database-centered manufacturing to reduce operator involvement in routine tasks and simple decision-making, thereby reducing human error and increasing throughput. UEC unveils new wafer-bonded AlInGaP LED structure United Epitaxy Company (UEC) used the CS-MAX stage to announce its fourth-generation AlInGaP LED structure, which it hopes will challenge the highest performance devices developed by LumiLeds Lighting (see Compound Semiconductor April 2001, p59). A key feature of AlInGaP devices, which are grown on absorbing GaAs substrates, is the need to extract light efficiently from the device. LumiLeds uses a wafer-bonding approach to replace the GaAs wafer with a transparent GaP substrate, and also employs relatively complex chip-shaping techniques. An alternative approach, which was used by UEC in its first AlInGaP devices, is to incorporate a DBR structure above the absorbing GaAs substrate to reflect light back towards the top surface. UEC s second-generation device included surface roughening to reduce the effect of total internal reflection, while a further iteration also incorporated a current-confining layer (see Compound Semiconductor Dec 2000/Jan 2001, p13). For its fourth-generation device, UEC has also moved to a wafer-bonded approach. However, rather than using direct bonding of a semiconductor (GaP) wafer, UEC has developed an indirect bonding process to join the wafer to a transparent substrate such as sapphire or glass. The process sequence, shown in the , was described by Roger Wu, manager of UEC s Epitaxy Department. The AlInGaP LED epitaxial structure is grown on an n+-GaAs substrate, and includes a transparent InGaP contact layer adjacent to the substrate. The structure is coated with an adhesive polymer and wafer-bonded with unaxial pressure at elevated temperature. Next, the GaAs substrate is removed, and a mesa structure is etched to allow p- and n-contact formation. The adhesive has to meet some stringent criteria, in particular good chemical resistance, thermal stability and optical clarity. A low cure temperature and small volume shrinkage during curing are also required, as is the ability to bond to various substrates. Reporting that the adhesive polymer successfully passed various tape lift-off, high temperature and chemical stability tests, Roger Wu showed photos of LED wafers bonded to glass and sapphire substrates. Encouraging performance results similar to those obtained from UEC s third-generation devices (> 40 lm/W at 625 nm) were reported. Flip-chip bonding to a suitable substrate (see , final step) is predicted to give rise to further performance increases.