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Big chance for technologies mixing

System-in-a-package designs drive developments in advanced MCM technology
Big chance for technologies mixing

Multi-chip modules (MCMs) have been around for many years, and are continually evolving. In an MCM there are multiple bare dies mounted along with signal conditioning or support circuitry such as capacitors, resistors and other parts on a laminate or ceramic base material. Those systems-in-a-package provide reduced size and weight, fewer and shorter interconnects, higher speed and lower parasitics, and grant the opportunity for innovative packaging options. Mixing of thinfilm high-density interconnection (MCM-D) capabilities with build-up laminate board (MCM-L) technologies offers more opportunities.

The simplest definition of an MCM is that of a single package containing more than one IC. It combines high-performance chips with a custom-designed substrate structure that provides mechanical support for the dies and multiple layers of conductors to interconnect them. This arrangement takes better advantage of the IC performance than does interconnecting individually packaged chips, because the dies are smaller and interconnect length is much shorter. The special design of MCMs is the complex substrate structure that is fabricated using multi-layer ceramics, polymers, silicon, metals, glass ceramics and laminates.

Innuendo: placed between ASIC and board level
MCMs introduce a packaging level between the application-specific IC (ASIC) and board level. The utilization of the active silicon area is about 15% for surface-mount ICs on a PCB (SMT), but in an MCM, it can be between 30 to 60% or sometimes even higher. Digital and analog functions can be mixed without serious limitations, and an ASIC can be composed easily with standard processors, devices and memories in one package. For some MCM technologies, passive components can be integrated for decoupling, protection or high-precision passive functions. The next generation of MCMs could even have optical I/Os as an option.
High-speed components can be placed closer to each other, the load on the IC output buffers is lower and signal transmission properties are better. The overall capacitive and inductive loads in a system are lower and easier to control compared to a standard SMT board assembly, and in general it is less susceptible to electromagnetic interference. Its development is in many respects closer to the design of an ASIC than of a system on board level. The design has to start with a detailed specification including function, environmental and mechanical specifications, also with partitioning of the functions into different blocks provided by various chips (standard or ASIC), and with a test strategy.
Testability considerations have to be taken into account in an early design stage, and the test strategies are similar to ASICs, as access to internal test points is limited. Functional verification can be simplified if internal procedures (built-in self-test, boundary scan or special programs) are used. In the electrical design, the netlist of a module is created either from a high-level description or in a more traditional way from an informal specification, and digital, timing and analog simulations are performed. In the physicaldesign stage, the netlist is transformed into alayout.
After manufacturing of the substrate and finished assembly of the components on the substrate, the system can be electrically tested using the internal procedures. Damaged components can be replaced (with highly specialized rework stations) before final packaging or package sealing.
The Institute for Interconnecting and Packaging Electronic Circuits (IPC) has defined three primary categories of MCMs:
• MCM-L (laminate) modules (cross-section figure 1) are made of plastic laminate-based dielectrics and copper conductors using advanced forms of PCB technologies to form the interconnects and vias.
• MCM-D modules (cross-section figure 2) are formed by the deposition of thinfilm metals and dielectrics, which may be polymers or inorganic dielectrics. The thinfilm MCM-D technology is the most advanced and provides the highest inter-connection density and transmitted signal frequencies.
• MCM-C modules are build on co-fired (green) ceramic or glass-ceramic substrates using thickfilm (screen or stencil printing) technologies to form the conductor patterns using fireable metals. The term co-fired implies that conductors and ceramic are heated at the same time.
To summarize, MCM-C is a derivative of traditional hybrid technology, while MCM-L is essentially a highly sophisticated board assembly. MCM-D is the result of manufacturing technologies that draw heavily from semiconductor manufacture. The fundamental benefit of MCM technology is to provide an extremely dense conductor matrix for the interconnection of bare dies and other miniaturized components. This has led some companies to designate their MCM products as high-density-interconnect (HDI) modules.
Combining MCM-D and MCM-L technologies
Much of the emphasis of MCM activities at IMEC is centered around system-in-a-package designs for high-speed digital, mixed analog/digital and microwave circuits. Work is concentrated on the use of new materials and designs for substrates, integration of passive components into the multilayer thinfilm and 3D integration with ultra-thin dies. The focus is on mixing of thinfilm high-density interconnection (MCM-D) capabilities with advanced build-up laminate board (MCM-L) technologies.
MCM-D is processed by using thinfilm material deposited as dielectric and metallization, formed on dimensionally stable materials such as silicon, glass, aluminum, aluminum ceramic or aluminum nitride. The photolithography process for applying and patterning the dielectric and metallization is commonly used in the semiconductor industry. The metallization layers consisting of power planes, signal layers and die bonding pads are created by conventional sputtering, vacuum evaporation methods or electroplating, and is followed by photolithography steps. Aluminum, gold, silver or copper are employed to conductor tracks.
Silicon dioxide or polymers such as polyimide or benzocyclobutene (BCB) are normally used as dielectric to separate the metal layers and to provide the low dielectric constant (2.7 to 3.5) needed to achieve thin dielectric layers, narrow track widths and adequate track impedances. Wet etching, plasma etching or lasers are normally applied for processing vias. Additional layers may be added to include thinfilm integrated resistors and thinfilm capacitors as an option.
Another possible option is to incorporate additional circuitry such as memory, module input/output protection (ESD, EMC) etc. in the bulk substrate if silicon is used. This implies a better utilization of the active silicon area because the I/Os on a chip can be considerably simplified and thereby require less space. The main features of IMEC’s MCM-D technology are the use of copper for interconnection lines, photo-sensitive BCB for dielectric layers and electroless Ni:P/Au for the final contact metallization layer (up to 5). A resistor (20 to 200ohm/square) and a capacitor (0.6 to 1nF/mm2) layer may be integrated into the structure. The low processing temperatures also enable employing a large variety of substrates. These thinfilm structures are generally produced on silicon, glass, ceramic or even metallic substrates, which only serve as carrier for the layers.
After assembly of the dies on such a substrate, the base material itself needs to be packaged, increasing the overall system cost. This is a significant disadvantage compared to laminate or ceramic high-density interconnect substrates, which can be considered as ball-grid-array ‘interposer’ substrates, therefore not requiring any additional packaging, except for over-molding and solder-ball attachment.
MCM-SL/Dtechnology
The IMEC alternative combines thinfilm high-density interconnection capabilities with that of advanced build-up laminate board technologies. This MCM-SL/D technology implements thinfilm on top of a sequentially laminated PCB. We have made progress in processing thinfilm copper and BCB layers on high Tg (glass transition temperature) laminate PCB substrates, produced using a sequential lamination of a double-sided PCB and resin-coated copper foils (RCC).
Thinfilm processing equipment must be capable of handling PCBs as ‘wafers’. This means that the substrate must be flat and rigid to allow automated handling (figure 3). We have developed a method for achieving a flat starting surface for thinfilm layers. This basically resulted in filling the gaps between the copper conductors and microvias with a resin material, while achieving local substrate flatness within ±5µm. By using first a thinfilm BCB layer on this substrate, a very smooth surface is obtained, well suited for applying fine-line, thinfilm metallization. With such a method, electrical connections through the substrate can be realized and the substrate be used as part of an MCM package, which can then be used as a ball-grid-array (BGA) style component.
In the MCM-SL/D technique, the SL core has to be fairly basic so that it does not produce additional problems with yield or tolerance. The functionality of this core is limited mainly to power and ground distribution as well as providing front-to-back electrical connections. On one side of this substrate, components are attached preferably flip-chipped, wire-bonded or as chip-scale packages (CSP). The other side of the substrate features solder balls or fine-pitch connectors.
IMEC has used a four-layer core with drilled holes of 200 to 300ìm diameter and large via areas with a minimum diameter of 500ìm. The holes in the substrate are later filled by laminating resin coated copper (RCC) foils on each side of the board. Selection of the basic core material is difficult. The materials traditionally used for PCBs such as FR4 will not remain stable during the MCM-D process flow, because it is subject to warpage and stretching after the core manufacture.
Either flexible or rigid core material can be used. Flexible material provides handling problems, while the very rigid ones introduce difficulties in obtaining sufficiently starting flatness. IMEC realized that a compromise between a very flexible and rigid substrate has to be used, and selected a polyimide/glass core while looking at other materials for suitability. Polyimide glass provides high thermal stability but absorbs a significant amount of moisture. This absorption results in significant wafer bowing which is however reversible upon drying. Other high Tg laminate materials, such as Rogers RO4003, have proven to be better suited for this application.
Sputtering of thick copper layers significantly increases substrate bowing due to high internal stress and process temperatures. This bowing is not reversible. This means that it is preferable to produce the metallization layers by a sputter-plating process, where first a thin seed layer is deposited before the actual, thick metal layer is applied selectively by electroplating, using a photoresist mask layer.
With test structure for the assembly of components (figure 4) on the MCM-SL/D, substrates investigation has taken place, looking at both wire bonding and flip-chip interconnection. Excellent wire bondability was observed on the smooth electroless Ni:P/Au surface finish. For the flip-chip assembly a solder mask defined pad is preferred as it lowers the stress on the BCB dielectric layers.
Integration of RF frontends in one package
Mixed-signal circuits have digital and analog circuitry placed in the same package, increasing density and functionality. MCM technology provides numerous advantages for mixed-signal ICs, since its concept of interconnecting multiple chips inside a single package incorporates inherently the mixed-signal principle. This is especially true for the thinfilm version of MCM technology (MCM-D), where accuracy and process tolerances are so well controlled that high-quality RF and microwave circuits may be realized within the interconnection substrate. RF and microwave circuits can consist of lumped passive components (resistors, capacitors, inductors), combinations of such passives creating filter functions, distributed passive structures (transmission lines, couplers, baluns) and antennas.
The RF and microwave MCM-D technology consists of alternating thin layers of BCB dielectric and copper metallizations. In this flexible multi-layer configuration a wide variety of high frequency passive structures may be realized. Figure 5 shows a typical cross-section of this build-up structure.
Passives components and MCM-D technology
The high frequency MCM-D structures developed at IMEC include lumped passive components, distributed passive structures, integrated antennas and HF system integration. Passive components such as resistors, capacitors and inductors can form an array that may either be integrated into the substrate or assembled on the substrate .The decision which to use depends, for example, on the number and value of the passive components, availability of substrate area and the stability and tolerance needed.
Thinfilm technique is ideal for the fabrication of resistors in MCMs, providing miniaturization and high component density. Resistor materials are typically SiCr, NiCr or TaN. Thinfilm resistors can give excellent matching, longer-term stability and high reliability. Thickfilm resistors have higher temperature coefficient and noise than thinfilm versions but a much larger range of values. Screen-printing of capacitors and inductors on hybrid substrates takes a considerable area, and the capacitors are limited to small values. IMEC has shown that all basic grouped passive components may be integrated in MCM-D technology. Their characteristics remain excellent as long as their electrical size remains small compared to the effective wavelength of the signal in the structure. Integrated spiral inductors are circular and use a coplanar topology to provide an inductance range of 0.5 to 75nH. The frequency range isup to 20GHz, depending on inductance value while the quality factor is in the range of 30 up to 150, depending on frequency and inductancevalue. The inductors measure from 500ìm to2 mm in diameter, with a minimum conductor width of 20ìm and minimum conductor spacing of 10ìm.
Two types of capacitors can be produced. Parallel-plate capacitors are coplanar with two metal plates on top of each other. The dielectric they use is 5pF/mm2 BCB and 0.8nF/mm2 tantalumpentoxide. Small value capacitors are accurately realized using interdigitated structures, using a minimal gap of 10mm.
Integrated resistors are manufactured using a TaN alloy producing 25ohm/sq. The temperature coefficient of resistance (TCR) is less than -200ppm/K. For designs of up to 50GHz, a microwave design library is available. This allows a designer to include passive subsystems together with microwave functions. For the integration of resistors and capacitors, a three-mask process uses TaN as resistive material and Ta2O5 as capacitor dielectric. These passive components are deposited on a glass substrate prior to the deposition of the multilayer build-up based on BCB dielectric layers and electroplated Cu metallization. The finishing top layer is based on AuNi, allowing different assembly methods, including wire bonding or flipped chips, to be made externally. The design library is coupled to an automatic layout generator that allows interactive mask production.
ZUSAMMENFASSUNG
Multichip-Module (MCM) gibt es seit vielen Jahren als Alternative für Applikationen, in denen sich ASICs aufgrund zu geringer Stückzahlen nicht lohnen, aber kunden- oder anwendungsspezifische Lösungen nötig sind. Auch unter der Bezeichnung System-in-a-Package (SiP) ist diese Technik aufgrund zahlreicher Vorteile aktueller denn je. Weiteren hohen Anwendungsnutzen verspricht man sich nun vom Mix unterschiedlicher Fertigungsmehtoden, beispielsweise MCM-D (High-density Dünnfilm-Technik) mit MCM-L (Schaltungsaufbau auf Laminat vergleichbar SMT-Boards).
RÉSUMÉ
Les modules multichip (MCM) représentent depuis de nombreuses années une alternative pour lesapplications dans lesquelles les ASIC ne sont pasintéressants en raison des quantités trop réduites mais qui exigent des solutions spécifiques au client et à l’application. Egalement appelée system-in-a-package (SIP), cette technique est, grâce à de nombreux atouts, plus d’actualité que jamais. On attend maintenant d’autres avantages importants du mélange de différentes méthodes de fabrication comme MCM-D (technique du film fin haute densité) avec MCM-L (montage sur stratifié comparable aux circuits SMT).
SOMMARIO
I moduli multichip (MCM) esistono già da moltianni come alternativa per applicazioni in cui gli ASICs non risultano convenienti per via dei quantitativi ridotti, ma che rendono necessarie soluzioni specifiche per le applicazioni e i clienti. Questa tecnica è talmente attuale come non lo è mai stataanche nella denominazione System-in-a-Package (SiP) grazie ai suoi innumerevoli vantaggi. Adesso la mistura di diversi metodi di produzione, quali ad esempio MDM-D (tecnologia a film sottili High-densitiy) combinata con MCM-L (struttura dicollegamento su laminato paragonabile ai SMT-Boards) promette ulteriori e notevoli vantaggi d’applicazione.
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Titelbild EPP EUROPE Electronics Production and Test 11
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11.2023
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