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Originally, both Hybrid and MCM
designs used traditional silicon die, internal construction, and packaging
techniques; most could be classified as subminiature modules. True Hybrids
consisted of passive discretes combined with active, and often passive, silicon
chips. Hybrid and MCM substrates were
primarily component carriers, often double-sided ceramic or small glass-epoxy
boards. The silicon die, which differed
little from the die used in standard microcircuits, gave little performance
improvement, but these designs provided a smaller, space-saving component and a
unique circuit for a specific application. The following describes the modern
trends in accomplishing improved performance and reliability with greater space
saving packaging techniques.
Although there are a variety of Hybrid
configurations, the design involves a substrate with deposited circuitry
(usually thick film), with separately produced active elements that are added to
complete a circuit function. Separately
produced, passive elements may also be used. These added elements might be
produced as silicon chips or as discretes.
The Hybrid package can be found in several design configurations
discussed separately. The deposition
methods used for Hybrids are essentially the same as for MCMs. Deposition technologies are discussed
separately, as the technology used is based more on the choice of metallization,
thick film or thin film. The primary
Hybrid design emphasis is to satisfy a unique circuit function with a minimum
requirement for real estate.
In the true Hybrid, the base material is usually
glass, alumina (Al2O3 ), or beryllia (BeO),
with deposited thick film circuitry of conductors and resistors. Thick film is defined as a deposition greater
than 0.5 mil. Thin film (used
occasionally) is normally less than 10,000 Angstroms, but may only be slightly
less than 0.5 mils. Glass substrates are
desirable for their inherit surface smoothness (important in thin film
technology). Ceramics are preferred when
high thermal conductivity is important, as alumina is 20 times higher than glass
and beryllia is 200 times higher than glass.
Ceramics also have excellent strength, electrical properties, and ease of
The MCM concept is a multi-monolithic,
multi-functional design, consisting entirely of chips (predominantly silicon
die, but GaAS or SiGe may be used when necessary). The variations in MCM design technologies
primarily effect the packaging, metallization deposition, and die/substrate
configuration. Packaging is discussed
separately, and the deposition technologies are essentially the same as for
Hybrids. The classification of modern
MCMs is usually by the die/substrate technology, with the more popular
MCM-C (Co-fired ceramic or glass).
Ceramic substrate-based technologies with a dielectric constant of five
or less. Conductors are usually a fired
material, e.g.: tungsten, molybdenum, or a screenable frit metal thick film
conductor (Au, Ag and Cu). Vias are formed during screening using the same
metallurgy as the metal traces.
MCM-L (Laminated layers of resins, often reinforced). A substrate of laminated printed circuit
board construction, with predominantly copper conductors and
c. MCM-D (Deposited,
un-reinforced dielectric). A deposited
substrate construction of un-reinforced dielectric materials (e.g.: polyimide,
KAPTONtm, Si02, etc.) grown on top of a base
substrate. Stability is achieved from
the base substrate, which may be ceramic, silicon, glass-reinforced laminate or
metal/metal composite materials.
Conductors are sputtered or plated (usually Al, Cu or Au). Vias are formed using the same material with
barrier metals/adhesion promoters as required. Some MCM-D examples include:
(Typical). A typical product of this
technology is one with a ceramic substrate, surface-mount die, polyimide
dielectric, Cu conductors, and is multilayered. The ceramic substrate is usually
Al2O3 , but the interconnect technology may be one (or a
mixture) of chip and wire bond, TAB, and Flip-Chip C4 (Controlled Collapse Chip Connection, an IBM
proprietary process). The Flip-Chip C4
technology provides good thermal dissipation and may be preferable when good
thermal management is required.
2. MCM-Si (Silicon based). The silicon substrate base is the traditional
wafer industry material and therefore keeps costs relatively low. This Si-on-Si process supports a high density
interconnect network, to which multiple varieties of bare die may be
assembled. The metallization is normally
Al and Al-Si, while SiO2 forms the dielectric. The top passivation
layer is usually Si3N4.
This technology provides a uniquely perfect TCE match between the
substrate and die.
3. MCM-HDI (Hi-Density/Embedded chip). The ceramic substrate is usually
Al2O3, however other materials (e.g. AlN) may be
used. The substrate is milled to
accommodate a multiple variety of bare die.
Die pads, accessible at the top of the milled cavity, are attached to
successive layers of interconnect structures. The interconnect structures are
copper traces, with copper vias between laminations of polyimide dielectric
(KAPTONTM). These structures are produced by processes similar to
established laminating and plating techniques. This is an inherently robust
design, offering a very high circuit density and a cost effective technology.
(Chip-on-board), This technology can be classified as an MCM, when only chips on
a substrate are involved. Although
Flip-Chip attachment is possible, typically, the chips are wire bonded using Au
to Al at the die and Au to Au at the substrate. The die is normally protected
with a “glob-type” covering, e.g., silicone.
This design eliminates one tier of interconnects but because of the
die-size footprint, necessitates a more difficult pattern of fine-line
interconnects. The substrate may be
ceramic, polyimide or glass epoxy (standard PCB). COB is a popular technique for
competitive commercial products and may be a cost-effective alternative for
benign environment, short life applications.
GaAs Die. Although MMICs are in GaAs
chip form, they are normally used together in an MCM fabrication process, rather
than as separate entities. Since GaAs devices are used nearly exclusively in
MMICs, they are discussed as follows:
1. GaAs Digital/ Analog
Microcircuits. GaAs digital
microcircuits include gate arrays, memory devices, and various logic
circuits. These digital devices employ a
GaAs substrate and GaAs analog microcircuits, RF devices, and include medium and
high power amplifiers, low noise amplifiers, and oscillators.
2. GaAs Combination (Mixed
Digital/Analog) Microcircuits. These
microcircuits are fabricated on GaAs substrates and employ a mixture of digital
and analog circuits. The combination circuits examined in this manual include
phase shifters and attenuators, which are the predominant
Hybrid and MCM Deposition
Traditionally, aluminum has been the preferred
interconnect material for the primary conductor metallization, but, recent
advances in deposition techniques has brought copper metalization forth
as the current preference. Copper has
the potential to reduce overall resistivity, capacitance, power consumption,
transition times, and metal levels. Each
has a significant advantage for future smaller and faster devices; and, copper
has better resistance to electromigration, a significant problem with
aluminum. The following, however,
pertains to techniques where aluminum is the primary conductor material. Although there are numerous deposition
techniques, there are only three that currently warrant discussion: vacuum,
screened and fired (SAF), and chemical.
A brief discussion of each follows:
a. Vacuums. Two common types of
vacuum deposition are evaporation and cathodic sputtering. Both depend upon the film to condense onto
the substrate, because of heating under a reduced atmospheric pressure. Cathodic sputtering is a combination of a
partial vacuum, a voltage potential applied between the anode (substrate) and
cathode (film material), and in a mixture of gases. Cathodic sputtering provides control over the
characteristic properties of the deposited film, resulting in the formation of
conductors and passive elements, which make up a circuit. The required circuit patterns may be formed
by masking, etching, or when passive elements are involved, a combination of
masking and selective etching.
b. SAF (Screened and fired). SAF
methods are economically popular for high volume production. Inks/pastes (frits or cermets), consisting of
noble metals and metal oxides, are mixed with powdered glass, binders and
solvents, which when screened and fired, leave conductor and passive element
patterns on the substrate surface. The
choice of compatible materials is critical to the end item performance and
c. Chemical. The four most
common types of chemical dispositions used are vapor plating,
electro/electroless plating, chemical-salts reduction, and chemical reactions.
The processes used by fabricators are routinely classified company proprietary;
therefore, little detail is available.
Vapor plating techniques, not as well known as the other methods, may be
used to deposit conductors, semiconductors, resistors, and insulators. Electroplating is limited to conductive
materials, while electroless plating may also be used to deposit non-conductive
materials. Chemical-salt reduction is
used primarily to deposit titanium on alumina, in preparation for further
processing by electroplating and photolithography. Chemical reactions in these processes are not
direct depositing techniques, but are used to remove material or alter materials
Figure 1. Passive Circuit Elements on a GaAs
(Source: Inder Bahl, Microwave Solid State Circuit
Design, John Wiley and Sons, New York,
Passive structures used in microwave device
applications, e.g. GaAs monolithic microwave integrated circuits (MMICs),
include transmission lines, vias, airbridge metallizations, thin film resistors,
metal-insulator-metal (MIM) capacitors, and inductors. Figure 1 is an illustration showing these passive elements. They can also be used as
distributed circuit elements of capacitance and inductance. To be useful in microwave integrated circuits
(MICs) and MMICs, transmission lines must be “planar.” Microstrip and co-planar waveguide (CPW) are
two common types of planar transmission lines. These structures are shown in Figure 2.
“Planar” implies that the characteristics of the element can be determined by
the geometry in a single plane. Planar
transmission lines have the distinct advantage of being lightweight, small,
reproducible, and reliable and most importantly, compatible with solid state
integrated circuit technology.
2. Planar Transmission
The field pattern in micro-strip is commonly referred to as a
quasi TEM pattern because the field lines are not contained entirely in the
substrate. The electric field lines have
a discontinuity in direction at the interface.
Since some of the electric energy is stored in the air and some in the
dielectric, the effective dielectric constant for the waves on the transmission
line will lie somewhere between that of the air and that of the dielectric. Typically, the effective dielectric constant
will be 50-85% of the substrate dielectric constant.
Vias are metallized holes that connect circuit elements on the
top of a GaAs MMIC to the backside metallization of the chip and are used to
provide a low inductance ground path to topside components. Vias are formed by deep etching of GaAs,
using high-density plasmas, including inductively coupled plasmas (ICP) and
parallel plate reactive ion etch (RIE).
Via profiles can range from highly anisotropic to conical.
Air bridges are suspended metallization crossovers, designed to
minimize the parasitic capacitance between signal lines. Figure 3 shows an air bridge
structure. Air bridges are generally electroplated with thick gold plating and
can carry substantial current.
3. Airbridge Interconnect on a GaAs
Nickel-chromium (NiCr) and other resistive materials are used on
GaAs-based circuits to produce thin film resistors. Current ratings vary with respect to the
manufacturer; however, most designs use a maximum current density approximately
1x106 A/cm2. Thin
film sheet resistance depends on the manufacturer’s fabrication process. A typical value for NiCr sheet resistance is
Though various types of capacitors may be used in MMIC circuits,
the most popular is the MIM capacitor, because of the high capacitance per unit
area that can be obtained. This thin film capacitor is made of two metal plates
separated by a dielectric material.
Inductors also have necessary functions in MMIC device designs. They
typically serve as tuning elements and RF chokes in DC bias circuits. They are also one of the easiest passive
elements to fabricate. Lumped element
inductors can be used to provide inductance up to 20nH. Lumped inductors are typically comprised of a
transmission line in a spiral shape.
Spiral inductors can be realized in the form of a single air bridge, air
bridges over an underpass, general air bridges, and use of two metal levels for