Focus on Hybrid Components

MCMs face up to millimetric challenges

The increasing number of commercial systems such as point-to-multipoint radio and automotive radar at millimetre-wave frequencies has brought a new set of challenges to manufacturers, who have to develop techniques for integrating devices and producing large numbers of circuits at these frequencies economically and repeatably. Companies specialising in multi-chip module (MCM) technology at millimetric frequencies include Farran Technology and EADS.

Modules

At the Automated RF and Microwave Measurement Society (ARMMS) meeting in Dunstable, UK, in November, a paper by David Vizard of Farran Technology described the challenges of designing multi-chip modules incorporating MMICs for use at millimetre-wave frequencies. The requirement for manufacturability at low cost is a particular concern in respect of the anticipated growth in both broadband wireless access applications and mm-wave automotive radar.

A variety of different transmission media, substrate materials and modelling tools are used to produce successful designs for these modules. Key design areas that need to be addressed include the external interfaces, which involve a transition from either a waveguide or coaxial connector to a planar circuit environment. The simulation of this transition involves extensive 3D simulation. Because optimisation is generally not feasible at these frequencies, this can be a time consuming process.

Filters

Factors that are particularly critical to the performance of the modules are the provision of filters and low loss interconnects between MMIC stages. These may involve interfaces between the microstrip circuit, the MMIC, and possibly drop in ceramic filters and other elements. The accurate modelling of mm-wave board based filters and transmission lines can be an issue with currently available software. Figure 1(b) demonstrates the discrepancy between modelled and practical test data for a typical microstrip edge-coupled bandpass filter designed for 40 - 42GHz operation, which is shown in Figure 1(a).

Figure1: (a) Outline of a typical 40 - 42GHz microstrip filter, with (b) modelled and actual test data.

Parasitics

When considering the performance of a mm-wave MCM, the parasitics associated with the assembly process must also be factored into the design. These include wire bonding tolerance and mechanical tolerances in chip placement. Because the bond wires have a significant inductance this needs to be accounted for, and where custom MMICs are being used the wire bond effects should ideally be built into the MMIC designs. Figure 2 shows the modelled effects of the bond wires on device performance for a single-stage broadband LNA.

Figure 2: Bond wire effects on S 11 and S 21 performances for a broadband mm-wave LNA

Simulation of an entire mm-wave MCM is not possible with the currently available software. While many MMIC manufacturers will provide small-signal S-parameters for their devices, hardly any provide large signal modelling data. Consequently simulation cannot be performed when using MMICs which are either not operating in their linear region, for example saturated power amplifiers, or when using MMIC components that depend on non-linearity in order to function, for example frequency multipliers.

3D simulation

Although Farran uses a circuit level simulator to perform simulations of devices with linear characteristics, this cannot fully take into account details such as the bond wire connections to the MMIC, as mentioned above, coupling between close-proximity transmission lines, or the external interfaces. The method currently employed for such devices is to simulate them with a 3D electromagnetic simulator and then input the S-parameter data file into the circuit simulator. In this way, small-signal simulation of a chain of components may be carried out using a combination of manufacturers' S-parameter data and that calculated by the EM simulator. 3D EM simulators however cannot model non-linear components.

A major disadvantage of this method becomes apparent when the simulator is required to optimise the circuit, or a portion of it. Unlike circuit simulators, where optimisation is well-defined and very fast, optimisation with 3D EM simulators is comparatively new and time consuming. Combined optimisation of both types of components, circuit simulation and 3D EM simulation, along with non-linear effects is not feasible. The software packages are not generally compatible with each other, therefore an iterative design process has to be employed.

Manufacture

The use of automatic pick-and-place and bonding equipment offers the advantage of higher yields and reliability because of its superior accuracy and repeatability compared to manual assembly. The additional cost is not always justified for prototype production, and automatic placement and connection of MMIC chips is more difficult than for passive components because of their fragility. Farran anticipates introducing automatic assembly techniques for medium-volume pre-production up to volume production. Figure 3 shows a mm-wave hybrid MCM with a wire-bonded MMIC, along with a close-up of the MMIC. Figure 4 shows a 40 - 42GHz upconverter module for broadband wireless access applications.

Figure 3: (a) Hybrid module including wire-bonded MMIC, and (b) close-up of manually wire-bonded MMIC chip.

Interconnection and die attach

Depending on the frequency range of interest and the type of packaging being employed, there is a choice between chip-and-wire assembly - bare die interconnected with wire bonds - or flip-chip devices, where the MMIC has connection pads pre-soldered and arranged to allow the inverted device to be flow soldered during production. Both techniques have both merits and disadvantages, but flip-chip devices are a particularly attractive option for mm-wave devices because of the performance advantages offered by the negligible interconnection lengths.

Either epoxy or solder techniques can be used for MMIC die attachment. Epoxy is regarded as the more flexible because it can be easier to re-work and is less expensive, but may not provide the necessary dissipation for power MMICs. This problem can sometimes be overcome by paying particular attention to the thermal environment, for example by mounting the MMICs directly to the module metalwork. Eutectic soldering provides excellent reliability and thermal management, and is generally suited to both high power devices and modules with very high volume throughputs.

Design for test

Test is one of the most important elements of manufacturing cost for MMIC modules, and is estimated to account for more than 40% of the overall cost. This becomes even more important as the functionality and complexity of mm-wave products increases, and hence also the number of parameters that require validation during manufacture. Automatic testing at both wafer and module level is essential for high volume products. It is also important to pay attention to test requirements during the design process. It is often difficult at higher frequencies to make provision for appropriate test points, and Farran is exploring the use of inter-stage test structures that allow on-line probing of a complex assembly by means of customised probe tips interfaced with conventional spectrum or network analysers.

Plastic

EADS Deutschland of Ulm, Germany launched its mm-wave "Microwave Factory" process during the 2000 European Microwave Week in Paris. The Microwave Factory is part of the Defence Electronics and Telecommunications business unit of EADS, which was formerly part of DaimlerChrysler Aerospace. Among its first products is a 28GHz diplexer in metallised plastic technology, and a 3D multi chip module (3DMCM) front end demonstrator at for 38GHz point-to-point applications, shown in Figure 5, that has been fabricated using LTCC technology.

Figure 4: 40 - 42GHz upconverter module for broadband wireless access	Figure 5: 3D multi chip module (3DMCM) front end demonstrator at for 38GHz point-to-point applications, produced by EADS using its "Microwave Factory" process

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Farran Technology 302
EADS 303
ARMMS 304

Wireless takes LTCC modules on board

I ronically, the higher complexity and more rigid performance specifications required of RF front ends for 2.5G and 3G mobile terminals means that there is a shift away from monolithic integration back to hybrid modules that allow different semiconductor modules to be used together. Both National Semiconductors and Murata are using low temperature co-fired ceramic (LTCC) techniques to produce highly integrated radio modules. National has a single RF front-end module solution on the roadmap within the next year, while Alpha Industries has recently introduced a proprietary integration platform to work towards the same end.

Embedded

National Semiconductor was among the pioneers of LTCC modules, introducing synthesisers made with this technology as early as 1996. Now it has introduced both an integrated front-end module for GSM/GPRS and a Bluetooth module that use the same technology, as well as a new synthesiser module, and predicts that LTCC will provide the route forward for higher level integration of wireless components.

LTCC is an integrated multi-layer ceramic packaging technology that provides several advantages over printed circuit boards. These include the support of embedded low loss conductors for microwave circuit designs for both microstrip and stripline applications. LTCC offers greater flexibility compared with conventional thick film, thin film, and high temperature co-fired ceramic technologies.

In addition to a range of application-specific synthesiser modules for AMPS, CDMA, GSM, and PCS cellular applications, NSC offers LTCC foundry services with 0.05mm lines and via capability, and boasting a 5-day prototype turnaround time, along with assembly and test facilities.

Radio

Figure 1: Die photograph of LMX3411 triple-band GSM/DCS1800/PC S1900 radio module with GPRS capability.

The LMX3411, shown in Figure 1, is NSC's triple-band radio module for GSM/DCS1800/PCS1900 with GPRS capability, which is currently being delivered to selected handset manufacturers in Europe. It is compatible with the standard Texas Instruments baseband solution. The company is now involved in designing, together with a reference design partner, a radio module with the same functionality but with a smaller form factor, that will be suitable for use in a 40cm ³ phone. This module will require a proprietary baseband solution that is also under development at NSC.

Although both the new module and the LMX3411 require an external PA, the NSC roadmap shows this being integrated into the same module in around a year's time.

A technical paper on the NSC Bluetooth module is planned for publication in two parts in our February and March issues.

Passives

According to Murata, the sizes of ceramic integrated passive components such as band-pass, high-pass and low-pass LC filters, diplexers, couplers and baluns have generally fallen from 2220 to 1206 over the last 10 years, with 0805 or 0603 in prospect, and weight has reduced by 95%. Now the company is using LTCC techniques to integrate active and passive components into a single multi-layer ceramic device, yielding even greater size reductions. Overall electrical performance is said to be as good as or better than the discrete components.

An example of this is the Murata "Switchplexer" for dual-band mobile terminals. This integrates a diplexer functioning as a branching filter along with two RF diode switches, each with a low-pass filter in the RF front-end, into a 6.7 x 5.0 x 2.0mm ³ package. The resulting module can automatically switch between the 900MHz and 1800MHz frequency bands, and also provides the entire antenna and front-end filter network. The surface mount modules are provided in 16mm plastic tape containing 1000 pieces on 180mm diameter reels.

Platform

Alpha Industries has recently introduced its own proprietary MCM technique, the Alpha Integration Platform (aiIP), a manufacturing, packaging and design process that combines various RF components in a single module-based platform.

The new integration platform addresses the demand from the wireless industry for RF solutions that will reduce design complexity and improve time-to-market. The aiIP integrates Alpha's semiconductor processes - HBT, PHEMT and RF discrete semiconductors - with additional RF components from its strategic suppliers.

Products already available using aiIP include complete amplifier solutions and switch/filter modules, and Alpha recently previewed a roadmap leading toward full integration of the RF front-end.

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National Semiconductor 305
Murata 306
Alpha Industries 307

VCOs scale down with chip-scale

The demand for smaller devices for mobile handsets has been a catalyst for dramatic size reductions and the introduction of several innovative packaging technologies. The International Wireless Packaging Consortium (IWPC) in particular has been a champion of developing new techniques for developing packaging that approaches the needs of the wireless industry.

Particular examples of this trend are the introduction of the chip scale package (CSP) and the ball grid array (BGA) package for semiconductor products. Other "die-approximating" package solutions have been available for some time, including flip-chip packages and the traditional die attach via bond wire. CSP technology, however, is uniquely targeted at high volume applications, with dense integration onto printed circuit boards without the need for additional post processing such as the application of underfill. Various companies are already applying CSP techniques to passive devices such as resistor arrays and filter networks, and more recently to discrete passives such as resistors, capacitors and inductors. This brings thin film capacitors and resistors, traditionally the preserve of military electronics due to their cost and performance, into a price bracket that makes them a feasible alternative for high-volume applications such as WLAN modules and mobile terminals.

VCO

Z-Communications, working closely with major handset manufacturers, has combined CSP and BGA technologies to develop a densely integrated, compact VCO platform that it calls the USSP (Ultra Small Scale Package) family. The USSP VCOs are produced with CSP packaged silicon bipolar transistors and silicon hyperabrupt varactor diodes, combined with passive devices with a 0201 outline, and are housed in a surface mount package, measuring a 5 x 5 x 1.5mm ³ .

The oscillators utilise a low cost proprietary thin film technology, along with proprietary chemical vapour deposition processes that produce substrates having high dielectric constant ( r) values of either 5 or greater than 12. This allows a highly integrated substrate to be produced with a small form factor. Inductors and resistors are sputtered on top of successive layers above the circuit traces, and then the few remaining 0201 components are placed using high speed, precision optical robotics. These remaining 0201 components require physical placement since they cannot be integrated due to physical planar area constraints. The active components also require to be automatically placed.

Space

The advantages of this type of process are chiefly in reduced board space: instead of building outwards, multiple layers of components and their respective interconnects are now built up in layers. Since the added geometry of the resistors and inductors is quite small, the additional height incurred is negligible, generally less than 1mm.

The remainder of the assembly process involves the use of state-of-the-art surface mount placement equipment. By collaborating with the manufacturers of optics-based surface mount assembly equipment, Z-Communications has optimised the automated assembly process for with 0201 components.

Placement

Figure 1: The USSP series of VCOs from Z Communications is produced using chip-scale packaged semiconductor devices combined with passive devices with a 0201 outline mounted in a package measuring a 5 x 5 x 1.5mm 3

Z-Communications is also ready to realise component placement accuracy with 0107 components for CSP solutions for Si bipolar transistors. From this technology base, the company will incorporate sputtered capacitors for further component count reduction and potential future reduction in packaging.

Once the assembly process is complete, the substrates are reflowed in a convection oven, separated, trimmed, tested and packaged in conventional tape/reel format for integration into the handset.

The USSP2330L is an example of a VCO produced using this process, which was developed for subscriber terminals for IEEE 802.11b WLAN cards. The operating bias was required to be less than 3V DC to minimise the presence of additional voltage rails, subsequent cost and additional PCB space. The design also had to have minimum current consumption, as the application is battery powered. In fact the design runs from 2.7V bias and draws 8mA.

Under these bias conditions, the device was characterised over the entire operating temperature range from -40ý to 85ýC, and generated 0dBm nominal output power. It is capable of tuning across 2.300-2.360GHz with a control voltage range of 0.5V to 2.5V. This enables it to support existing charge pump outputs from off-the-shelf phase locked loop manufacturers. Figure 1 shows a typical tuning curve over temperature.

Phase noise

Phase noise performance was a major area of concern. The tendency of oscillators is to degrade in phase noise performance with reduction in size. This is natural because the reduction in cavity size leads to a subsequent increase to parasitic noise. Several techniques can be employed to offset this degradation: some of the more popular approaches include the utilisation of high dielectric constant substrates, high ^f T bipolar transistors, utilisation of noise cancellation/ minimisation techniques and generally tailoring the circuit topology to minimise the effect of parasitics. The implementation of noise cancellation/minimisation techniques is incompatible with the design goal of package reduction, since extra components are needed to return an out of phase signal back to the oscillator tank circuit.

Figure 2: Typical tuning curve of USSP VCO over temperature

However, by working closely with leading bipolar and substrate manufacturers, Z-Communications has managed to implement the other three approaches mentioned above, to resulting in an oscillator with single side band (SSB) phase noise performance as depicted in Figure 3. The SSB phase noise at 10kHz from the carrier is -85dBc/Hz typical, and further research is planned to improve this performance while at the same time driving the frequency of operation higher. There is a requirement for 1.8 - 1.9GHz products for 3G subscriber applications with -95dBc/Hz SSB phase noise at 10kHz: Z-Communications expects to be able to meet this development goal within the next few months with the use of higher dielectric constant substrates.

Figure 3: SSB noise performance of Z-Communications 2.3GHz VCO using chip-scale packaging

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International Wireless Packaging Association 308
Z-Communications 309

Cheaper chirp with DDS

Direct digital synthesis (DDS) has now made the transition from leading-edge to a relatively mature technology that enables a diverse range of synthesiser products to be developed. Two recent examples from ITT Industries are a digital chirp synthesiser hybrid, that makes chirp waveform technology available for commercial wireless communications applications, and a range of single-channel modular synthesiser subsystems for frequencies up to 18GHz.

Phase locked loop (PLL) frequency synthesisers also occupy an important place in the industry, and new models from Fujitsu, Murata and National Semiconductor have recently entered volume production for cellular handset applications.

Chirp

Figure 1: The STEL-2375A from ITT Industries is a hybrid direct digital synthesiser with a 1GHz clock rate and an output bandwidth of 400MHz

The STEL-2375A from ITT Industries, shown in Figure 1, is a hybrid direct digital synthesiser with a 1GHz clock rate and an output bandwidth of 400MHz. In order to create an output waveform with a frequency that changes continuously with time (chirp), the GaAs oscillator includes both a phase accumulator and frequency accumulator, both with 32bit resolution. The frequency accumulator is a particular feature of this device, since most DDS sources utilise only a single phase accumulator. A DAC and a digital synthesiser are also included in the hybrid package.

The module operates on standard ECL supply voltages, and features double-buffered input latches to allow the simple generation of custom chirp or CW waveforms. An evaluation board is also available, that requires only power, a RF clock source and a PC compatible controller for an RS 232 serial interface.

Subsystem

Figure 2: The ITT Industries STEL-9941 is a single channel S-band synthesiser subsystem with a frequency resolution of 0.7Hz and a switching speed of 150ns

ITT Industries has also introduced the STEL-9941, a single channel S-band synthesiser subsystem, which is shown in Figure 2. The device combines DDS with microwave mix/divide circuits to achieve performance specifications that include a frequency resolution of 0.7Hz and a switching speed of 150ns.

Standard models operate from 1.6GHz to 2.8GHz, but the modular design allows ITT to provide performance options up to 18GHz with only slightly longer lead times. These generic modules are attached to a common base plate with blind mate connectors. Readjustment is not necessary when changing modules: this allows for easier maintenance and set-up flexibility. The internal oscillator can be either free running or phase-locked to an external 10MHz reference. The synthesiser has an output power of 0dBm ý1.5dB and an operating temperature range of 0 to 70ýC.

The unit weighs approximately 5.9kg and measures 251 x 152 x 135mm ³ .

PLL

The UL Series of PLL synthesisers from Fujitsu is intended for mobile communications applications in the frequency range 50MHz to 2.6GHz. Developed in co-operation with Kyushu Fujitsu Electronics Ltd., the UL Series comprises three models that have commenced volume shipment from November, with target sales of 8 million units per month.

Using Fujitsu's U-ESBIC4 BiCMOS process, the UL Series features low power consumption: the 2.6GHz/1.2GHz MB15F78UL consumes 4.5mA at 2.7V, 20% less than the previous model (the MB15F78SP). In addition, newly designed independent circuits have allowed Fujitsu to reduce the lock-up time by approximately 20%.

Current

The other two models are the MB15F72UL, which operates at 1.3GHz/0.35GHz and draws 2.5mA, and the 2.0GHz/0.6GHz MB15F73UL that has 3mA current consumption. All the models feature a stable charge pump circuit capable of dual current switching through serial data transfer between ý1.5mA and ý6.0mA. Input voltage sensitivity is -15dBm minimum.

A thin package bump chip carrier (BCC) package, the TSSOP-20 has a thickness of 0.60mm, and offers a reduction of 25% in the total area compared with the earlier version. The UL series is pin-compatible with the earlier SP series.

VCO

The LMX9402 is a LTCC PLL/VCO module from National Semiconductor, designed for use in GSM basestations. It features a phase noise of -150dBc/Hz at 1MHz, with a typical RMS phase error of 0.7ý.

The module, shown in Figure 3, consists of a 2.4GHz frequency synthesiser (the LMX2330A "PLLatinum"), loop filter, and VCO. The frequency synthesiser is fabricated using National's ABiC BiCMOS process (f _T = 14GHz), while the loop filter and VCO are fabricated on LTCC. Data transfer is via a three-wire serial interface bus.

Figure 3: The LMX9402 PLL/VCO module from National Semiconductor is fabricated using LTCC technology

W-CDMA

Murata's integrated synthesiser module, HFQD08Q2330M380A02, for W-CDMA handsets contains RF and IF VCOs, RF and IF loop filters and a dual PLL-IC. RF output frequency range is 2.30GHz to 2.36GHz, with an IF of 380MHz. Lock-up time is 300ýs between 2.30GHz and 2.36GHz ý1kHz. Output levels are -3dBm ý3dB in both cases, and output impedance is 50 . The comparison frequency for both functions is 200kHz. Spurious response to reference leakage ý200kHz is -50dBc, and to 2nd and 3rd harmonics is -15dBc.

Maximum current consumption of the module is 26mA and nominal power consumption is 72mW. The module size is 12.6 x 8.0x 2.0mm ³ , and packaging is in embossed plastic tape.

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ITT Industries 310
Fujitsu 311
National Semiconductor 312
Murata 317