Filtronic Exploits The Benefits Of PHEMTs (Cover Story - GaAs Manufacturing)
Filtronic Compound Semiconductor Limited is manufacturing a range of PHEMT-based products using its 6 inch fab, writes Wolfgang Bsch.
Just 18 months ago, Filtronic, a UK-based RF and microwave systems supplier, acquired an empty wafer fabrication facility in County Durham in the north of England. With just over 100 000 square feet of high-quality class 100 clean-rooms available for use, the start-up team installed one of the world s first and largest operational 6 inch GaAs wafer facilities. The equipment was installed and the first processes established within a one-year period. The equipment set was chosen to allow for 0.5 m PHEMT processes to be established with high reliability, high volume and wherever possible complete automation. Batch processing of production lots allows for repeatability not only from substrate to substrate but also across each wafer. The toolset includes steppers, wet and dry etchers, backside processing capability, evaporators, sputtering systems, a host of in-line measurement systems and DC and RF testers, as well as the recently installed high-volume MBE system. This system completes the major first phase toolset purchases and allows for on-site control of the complete wafer processing cycle. This first phase of investment has provided the capacity to process 30 000 wafers per year. Currently, Filtronic Compound Semiconductor Limited (FCSL) has close to 120 employees at the County Durham site, where all aspects of the business from R&D through design and manufacturing to sales, marketing and customer support have been installed. PHEMT technologies State-of-the-art PHEMT technologies are being developed to address the wireless and high-speed fiber-optic markets. Two depletion-mode (D-mode) processes are about to be released; a low noise, single recess process and a high power, double recess process. The low noise process is used for discrete PHEMT product development and low noise amplifier (LNA) development for both handsets and base-stations. The use of the double recess D-mode process for switch products has shown both state-of-the art performance and very high yield on 6 inch wafers. The D-mode process shows typical values for Imax, pinch-off and breakdown of 500 mA/mm, 1.0 V and 15 V, respectively. A 0.5 m enhancement-mode (E-mode) process is in an advanced state of development. The highly selective dry etch used for the gate recess was shown to be critical in controlling the recess depth and therefore threshold voltage of the device. Typical parameters are Imax of 240 mA/ mm, breakdown voltage of greater than 20 V and pinch-off voltage of +0.25 V, ensuring a very low leakage current with zero volts on the gate. Single-supply handset power amplifier development is underway based on this process and shows excellent gain and PAE under 3.5 V operation (see ). AWR s Microwave Office has been selected as the tool of choice for MMIC design; customization of the tool is complete, including automated layout and design rule checking. The first release of the design manual and circuit modeling tools for both active and passive components is due by the end of April 2001. Roughly 60% of all GaAs semiconductor products are utilized in wireless applications (mainly handsets), with fiber optics being the second biggest market. These are the two major market segments being addressed by FCSL with its current product portfolio. A series of switch designs in various packages is available today. Discrete power transistors with overall gate peripheries ranging from less than 1 mm up to 60 mm are in the development phase (see ), together with power amplifier modules. A wideband PA MMIC for optoelectronic applications will be released shortly. Switches In today s handsets, switches are used for a great variety of functions, such as mode selection, band selection, power control and transmit/receive switching functions. State-of-the-art handsets have become more complex, and switches have emerged as key elements which significantly determine the overall handset performance. There are typically three to five switching functions incorporated in a handset. Currently, GaAs-based MMIC switches are replacing PIN-diode-based components. The main reasons for this transition are better performance, wider bandwidth, lower power consumption, higher linearity and smaller size. More complex switching functions, such as those required in 3G phones (e.g. single pole five throw SP5T switches) are much easier to implement in GaAs MMIC technology compared to a traditional MICPIN diode technology. There are several reasons why we believe that PHEMT technology is ideal for switching applications. Compared to traditional MESFET technology, PHEMT semiconductor devices have a lower channel resistance and hence yield lower insertion loss. Due to the very low output capacitance of the 0.5 m PHEMT device, high isolation can be achieved in the switch. The very low knee voltage makes these devices ideal for low-voltage operation, however the power handling capability provides low harmonic generation when operating the switch under high RF input power. Significant effort went into device modeling and circuit optimization to ensure excellent performance and high yield. In this way a first turn-around success was achieved, and within a very short period of time a comprehensive portfolio of switches, ranging from SPDT to DP5T switches, was in fabrication. These highly linear, low loss switches operate up to 38 dBm input RF power with supply voltages lower than 3 V and still meet the operating requirements for E-GSM, DCS, PCS and UMTS systems. A newly developed circuit topology and an optimized switch device design (see ) results in switches that achieve a very low second and third harmonic content of 70 dBc when operated at GSM RF power levels. The typical insertion loss is less than 0.5 dB and the isolation is 24 dB for wireless operating conditions. For high-volume products in 3G handsets, performance and cost are the key factors in order to maintain competitiveness. In conjunction with novel circuit design, PHEMT technology offers superior switch performance compared to traditional MESFET or MICPIN diode technology. At present, a central feature of the R&D program at FCSL is the size reduction (and hence cost reduction) of the switch MMIC family. The newly optimized semiconductor devices will have an optimized gate layout, which enables a significant reduction in size. FCSL is committed to this technology and its importance for next generation multimode handsets, which will simultaneously require highly linear operation for the W-CDMA/UMTS mode and high-power operation for EGSM and DCS/ PCS operation. At the same time, handset OEMs are clearly moving toward using PHEMT switches and integrating these with filters in the future. Power PHEMTs As mentioned above, FCSL is developing discrete power transistors with overall gate peripheries in the 1 to 60 mm range. Currently these devices are electrically and thermally characterized. Because the junction temperature strongly affects the reliability of GaAs devices (see ), the thermal management is very important. Wafer thinning improves the thermal conductivity and hence allows a more efficient heat removal (as shown in ). FCSL has demonstrated that 6 inch GaAs wafers can be thinned down to 30 m and that devices can successfully be demounted. Integrated modules The level of integration in wireless products is continuously increasing. In combination with the growing functionality of handheld products, there is a clear trend towards integrated modules. Considering that in the future OEMs will outsource a greater portion of their handset manufacturing business, a module approach represents a lower risk and more cost-effective solution. FCSL is currently evaluating the optimum integration technology for a combined switch/filter module. One step further towards a fully integrated front end is a GPS module that combines an integrated antenna, receive filters and an LNA MMIC in one unit. All these components are designed and manufactured within the Filtronic Group. Several customers have already received samples of the first prototypes. On the power amplifier front, dual-band, multi-mode amplifiers with integrated matching networks, power control and monitoring are required for next-generation handsets. FCSL has characterized and measured initial PA MMICs, and the fully integrated PA module is in the development phase. Eventually the market will demand a fully integrated front-end module. Starting from the integrated antenna, to the switches, transmit and receive filters, power amplifiers and low noise amplifiers, and some level of control logic, all functions will be combined in a single module (see ). GaAs PHEMT technology is particularly attractive for the broadband market because of its high gain and excellent microwave frequency performance. D-mode PHEMTs with ft values above 25 GHz are suitable for a number of applications up to the millimeter-wave frequency range. The process offers a low-cost, high-volume solution to the MMIC requirements for the OC-192 optoelectronic driver amplifier market, and a family of devices for this application is being developed. FCSL plans to offer low and medium power distributed amplifiers based on the standard D-mode process. Distributed amplifiers offer ultra-broadband operation by integrating the input and output capacitances of a number of PHEMTs with short inductive microstrip lines into two terminated artificial transmission lines. An input signal propagates through the artificial gate transmission line, exciting the inputs to the PHEMTs and causing modulation of the current flowing between the drain and source terminals. The current from each of the devices adds in phase in the drain transmission line and flows into the load. The MMICs employ both cascode and single common source configured PHEMTs in five- and seven-stage designs depending on the electrical and size specifications. The cascode arrangement is particularly well suited to distributed amplifier design offering high gain, high output resistance, low intrinsic feedback and good gain control via the second gate connection. Typically the low power amplifiers offer small signal gains in the order of 9 dB, with low ripple and better than 10 dB return loss from DC to 20 GHz with DC power requirements of 4 V and 150 mA. A seven-stage low-power design using small gate width PHEMTs in a cascode configuration measures approximately 2.8 1 mm. The medium power amplifier offers a Psat of 25 dBm, a small-signal gain of 7 dB and input and output return losses of greater than 10 dB with DC power requirements of 8 V and 300 mA. A number of custom chip solutions are available with a choice of on- or off-chip terminations, flexibility over pad configuration and chip dimensions. The first prototypes will be available shortly. Summary Through investment in technology and quality, we believe that FCSL is well positioned to become a leading force in GaAs wafer manufacturing specifically for the wireless and the optoelectronic market. The purchase of enabling technology such as MBE systems allows the company greater vertical integration, greater control over the fabrication of transistors, and overall added value offering a comprehensive fab at world-class standard. The current development of PHEMT D-mode and E-mode switches is the first step on a commercial road map that will lead to the introduction of discrete PHEMTs, PAs, LNAs and finally complete RF front-end solutions. Using 6 inch wafer technology has allowed the fabrication of discrete GaAs PHEMT devices and MMICs in a very cost-effective way. FCSL is well positioned to take advantage of these developments which, when combined with other Filtronic group activities, will provide many innovative RF solutions to the wireless market.