Kutz M. Handbook of materials selection

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2 MATERIALS SELECTION FOR SELECT COMPONENTS 1311

Fig. 4 Unit cells of the Si (left) and InP (right, In - dark, P - light) structures.

materials discussed in the scientiﬁc literature for application to telecommuni-

cations is overwhelming. Only innovation will bring some of these exciting

materials into commercial use.

2.1 Packaging

A package can have many different functions: providing protection, distributing

power and signal, and dissipating heat.

The ability of a package to perform

these functions is dependent on the device as well as the materials properties

and design of the package. For example, devices with robust passivation and

protective layers may require a less robust package. Integrated circuits and de-

vices relevant to telecommunications contain layers of conducting, insulating,

and passivating layers, each with different criteria regarding protection from the

environment.

The telecommunications industry continues to grow, and its growth is enabled

by the ability to package its devices. Plastic packaging will play a major role in

its future. Oftentimes, however, packaging is a secondary consideration in the

development of silicon-based devices. Consequently, packaging has received far

less attention than warranted, given its crucial role in reliability, performance,

and cost. The tide is shifting as device fabrication continues to explore ways to

reduce cost and improve performance.

In order for optically interconnected systems to be manufactured realistically,

the packaging issue must be addressed for some telecommunication devices,

reliable packages have electrical and optical interfaces and must also be able to

withstand shock, vibration, and temperature variations. Ideally, these packages

are also of low cost.

Packaging is an area that is common to all forms of telecommunications:

cable, optical, and wireless.

Packaging Materials

A host of packaging materials is used in integrated circuits and thus telecom-

munications. They include metals, ceramics, plastics, and glasses. Packages that

are impervious to the environment (hermetic packages) are usually made from

1312 ADVANCED MATERIALS IN TELECOMMUNICATIONS

metal, ceramic, or glass materials; plastics make up nonhermetic packages. In

this section, we will survey packaging materials and their uses. Sample appli-

cations will also be discussed.

Ceramic Packages. Ceramic packages are laminate sheets of ceramic pow-

der made from a slurry of ceramic powder and a liquid binding mixture of

polymers and solvents. Sheets are cut into the appropriate size and via holes are

punched. These via holes are ﬁled with metal powder and conducting traces of

metal powder are printed on the surface to form circuit patterns. These sheets

(as many as 30) are aligned and laminated. Additional circuit lines are metalli-

zed, and the leads to them are attached. As a whole, ceramic packages have

several beneﬁts, which include hermaticity and the ability to sustain multiple

input/output ports. Alumina-based ceramic packages are ubiquitous in optoelec-

tronic devices due to superior radio frequency (RF) characteristics and high

reliability.

However, these packages have relatively high cost, poor dimensional

precision on ﬁring, and a high dielectric constant.

Refractory Glass Packages. A lower cost alternative to ceramic is refractory

glass. Although glass has less complexity than laminate ceramic sheets, it is a

reliable and hermetic option. Typical glasses used are PbO–ZnO–B

types

with melting points near 400

⬚C.

Although hermetic, glasses can at times trap

moisture and impurities within the package and thus suffer imperfect reliability.

The combination of glass with lead frames made of Kovar (Fe–Ni alloy) has

been found to work well and makes up the technology known as CERDIP, which

denotes the used of ceramics (CER) in dual-in line packages (DIP). CERDIP

packages may use solder glasses which, speciﬁcally, are not glasses, but ceram-

ics.

Metals. Solders and their alloys

are commonly used for die attachment.

They include: tin,

tin–lead,

indium,

and gold eutectic alloys (with tin, silver,

or germanium).

In one approach, stable bonds of gold eutectic alloy solders

have been made with multilayers of alternating gold and tin.

Metals are also employed as diffusion barriers in laser die attachments to

reduce the unwanted development of intermetallic compounds.

Tungsten can

reduce the diffusion of gold from the die into indium solder. Nickel prevents

copper from diffusing into indium as well. To promote reliable bonds, wettable

layers are also used. Nickel–tin improves wetting of barrier layers to gold–tin

alloys. In addition, metals and semiconductor materials are used as heat sinks

in optoelectronic die attachment

and include materials such as copper, copper–

tungsten, silicon and silicon carbide, silver, and variations of each of these.

Metals (Al, Cu, Au, and their alloys) are also used for interconnection.

Plastics. The number of types and uses of polymers in packaging is for-

midable. There are excellent discussions of them in the literature.

Plastics may

be used as dielectric layers or adhesives and in constructing the body of the

moldings. In this section, we will explore them according to their function.

1. Large-scale integration requires insulating layers between metals. High-

temperature polymers serve as dielectrics between such layers and have

excellent planarization properties, making them well suited for integra-

2 MATERIALS SELECTION FOR SELECT COMPONENTS 1313

tion. Polyimides represent a class of materials used as interlayer insula-

tors. Usually spun-coated, they form conformal coatings with excellent

thermal and electrical properties. Their ability to form conformal shapes

also renders them useful in creating conformal coating layers as well.

2. Besides polyimides, silicon rubbers serve as conformal coatings. Silicon

rubbers are also used as encapsulate materials that are globbed on top of

devices and modules for protection. Other polymers that serve this func-

tion are Paralenes (poly-p-xylyenes), which are applied at low tempera-

ture through gas-phase polymerization and provide good electrical and

environmental performance.

3. In the manufacture of integrated chips, dies are separated from the wafer

and attached to the lead frame with adhesives. Solder bonding alloys and

organic adhesives are commonly used. Conductive polymers are a low-

cost alternative to gold containing solders and can be used in automated

processes. These compounds contain silver ﬂakes that provide an elec-

trical path. Also, the molding package itself is polymer based. Often

coated with dielectric coating to provide electrical isolation, moldings are

made from epoxy resins, hardeners, ﬁllers, catalysts, and colorant.

Case Studies

Semiconductor Laser and Optical Ampliﬁer Packaging. Packaging has

many functions, as noted above, but in the optical arena, it also provides a means

of alignment of ﬁber arrays and/or laser diodes. Various ways of coupling optical

ﬁber to devices, optical alignment, and packaging approaches are discussed in

the literature

and will not be explored here. It is important to note, however,

that much work remains to be done in developing low-cost, precisely aligned

packages for integrated optical devices.

A number of thermal, mechanical, optical and electrical factors complicate

efforts to package lasers and ampliﬁers. Among these are the following: lasers

have high thermal sensitivity; many devices are sensitive to stress; device per-

formance is often dependent on the ability to suppress Fabry-Perot effects, and

parasitic electrical behavior of packages impairs device response.

For example, the connection of optical ﬁbers to waveguide devices requires

precise ﬁber alignment, with lithographically etched ﬁber guides. Fibers are usu-

ally butted against the waveguide materials that lie adjacent to the active element.

These elements are held in place with solder bumps that provide alignment and

electrical connection. As describe above, many materials interplay in these pack-

aging assemblies underscoring the importance of selecting and designing them.

MEMS Packaging. The general sense of microelectromechanical systems

(MEMS) technologists is that MEMS packaging presents the greatest challenges

faced in the industry. To further complicate matters, packaging issues are appli-

cation speciﬁc and may not translate well from one device to another. As a

result, a few representative cases including telecommunications will be dis-

cussed.

Accelerometers are currently used in airbag deployment devices, but future

applications are on the horizon. In this device, long cantilever beams support a

massive beam. Along the beam are elements of a capacitor that sits between

1314 ADVANCED MATERIALS IN TELECOMMUNICATIONS

two ﬁxed plates. Motion is detected when the spacing between these elements

changes. As one can image, in the case of an airbag, the performance of these

devices is vital. There should be no impediment to sensing deceleration. As a

result, hermaticity is imperative and has been achieved successfully with CER-

DIP hermetic packages.

Analog devices have developed a successful scheme

to precisely orient the package and maintain the sensitivity of the device.

Ink-jet technology, dubbed the ﬁrst MEMS technology, requires a different

set of criteria. In this conﬁguration, a heater element forms microscopic bubbles

that eject ink through a silicon-based nozzle. Here, a package must not block

the jets of ink, yet it must be protected from them. One approach is the creation

of a ﬂexible circuit array bonded to the ink jets with a polymer.

Optical MEMS products add another layer of complexity, as access to the

outside environment is required. In these products a light path is commonly

needed to provide light to the ‘‘eyes’’ of the MEMS device. Although the re-

quirement seems simple, it is a complex task to achieve. As a result of these

rigid constraints, most optical MEMS systems require vacuum hermaticity to

ensure proper operation of the sensitive components.

Although MEMS packages are application speciﬁc, there are some common-

alities among them. For one thing, the device cannot be obstructed and must be

covered by a protective cap. Here, packaging occurs on the wafer level. Another

method of wafer level packaging is a method called ‘‘ﬂip-chip.’’ It entails the

use of a silicon cap with circuitry bonded on top with an encapsulant. The

sealing is done by metallization (diffusion solder) or epoxy. An underﬁll is used

to ﬁll the gap between the chips.

2.2 Solid-State Semiconductor Lasers

Advances in material properties have led to ubiquitous deployment of solid-state

lasers in CD players, barcode scanners, and optical communications. Speciﬁ-

cally, lasers with output centered around 1.3 or 1.55

␮

m have been prepared

using molecular beam epitaxy (MBE) and metal–organic chemical vapor dep-

osition (MOCVD) to deposit thin ﬁlms of (In

⫺

)(As

⫺

) solid solutions,

hereafter referred to as InGaAsP.

The wavelength of the laser light can be

selected by choosing a desired lattice constant and then locating the proper

composition (Fig. 5). This allows the materials scientist to lattice match to a

particular substrate and tune to the operating wavelength, which is related to the

bandgap energy of the semiconductor. InGaAsP has been selected as the ideal

material for telecommunication lasers because of the following properties: (a) a

small direct bandgap that allows for efﬁcient conversion of electrical energy to

light at the desired wavelengths, namely 1.28–1.62

␮

m, (b) substrates of InP

grown by the Czochralski technique are commercially available, and (c) tech-

nology to form waveguides has been developed.

The microelectronics industry has migrated toward silicon-based devices,

even though GaAs demonstrates a higher electron mobility. However, neither of

these candidates possess the qualities of InGaAsP. Speciﬁcally, silicon possesses

an indirect bandgap that prohibits the radiative recombination of electron-hole

pairs. Further, GaAs with superior electrical performance and a direct bandgap

emits light in the visible spectrum when doped with aluminum. For this reason,

(Al,Ga)As devices are found everywhere as red light-emitting diodes and lasers;

2 MATERIALS SELECTION FOR SELECT COMPONENTS 1315

0.5

1.5

2.5

5.6 5.65 5.7 5.75 5.8 5.85 5.9 5.95 6 6.05 6.1

lattice constant (Å)

bandgap (eV)

InAs

InP

AlAs

GaAs

Fig. 5 Bandgap and lattice parameter as a function of composition

for (In

1⫺x

)(As

1⫺y

) solid solutions.

however, the bandgap energies required for telecommunication are much lower.

Therefore, all of the processing steps that were developed for Si and GaAs

devices were duplicated for InP chemistry.

State-of-the-art telecommunications systems use 80 wavelengths separated by

50-GHz spacings. Control of the MOCVD growth conditions for solid-state la-

sers is paramount for a reliable process where layer thicknesses can deviate by

only a few angstroms. The typical InGaAsP thin-ﬁlm stack to fabricate a laser

cavity are illustrated in Fig. 6. Transmission electron microscopy (TEM) of an

InP semiconductor laser clearly shows layer thickness and grain structure and

also indicates quality of epitaxial ﬁlms as seen in Ref. 20.

Many functionalities have been incorporated into solid-state lasers, which has

been reviewed in detail for telecommunication applications.

The materials used

in these laser devices are all similar, yet the device designs are radically different.

The goal is to integrate as many functionalities as possible while keeping reli-

ability high and cost low. For example, distributed feedback (DFB) lasers can

produce continuous-wave light at a very speciﬁc wavelength, which is required

for DWDM since the channel spacing is only 50 GHz. Further, due to the chro-

matic dispersion of optical ﬁber, the spectral width of the laser emission is

minimized to avoid broadening of the light pulse during propagation in the ﬁber.

A narrow spectral width is accomplished by fabricating a Bragg grating beneath

the laser cavity (Fig. 7). The laser cavity is a multilayer quantum well (MQW)

structure that provides higher gain, a lower threshold, and lower losses.

Electroabsorption modulated lasers (EML) are an integrated laser and mod-

ulator on an InP substrate. Current devices can operate at 10 GHz, and research

is pushing the modulation rate to 40 GHz. Further integration is achieved in

wavelength-selectable lasers with gain, and modulated intensity are being de-

1316 ADVANCED MATERIALS IN TELECOMMUNICATIONS

Fig. 6 Semiconductor laser cross section with emitted light from

the buried channel active region.

Fig. 7 Cross section of a distributed feedback (DFB) laser.

veloped (Fig. 8). For this device, each DFB laser produces continuous-wave light

at a speciﬁc wavelength. The system selects which laser to activate; then that

light passes through the combiner and into a semiconductor optical ampliﬁer

(SOA), which boosts the intensity, after which the light is modulated by the

electronic data stream. High-performance laser packages, which monitor output

2 MATERIALS SELECTION FOR SELECT COMPONENTS 1317

Fig. 8 Schematic top view representation of a wavelength selectable laser.

Fig. 9 Basic schematic of a high performance laser package.

from the laser cavity, can actively control the wavelength of the device by chang-

ing the temperature of the InGaAsP laser chip. The typical materials used in a

solid-state laser package are illustrated in Fig. 9. The major assemblies include

an InP substrate with InGaAsP laser, a thermoelectric cooler with feedback loop

to control temperature, an isolator made by a Faraday rotator and polarizers (see

Section 2.8), spinel and glass lenses, an etalon to monitor the wavelength of the

laser output, and a photodetector to monitor light output from the back facet,

and heat sink subassemblies. The subassemblies are attached by soldering and

wire bonding. The assembly is mounted in an alumina-based hermetic ceramic

package. Finally, the ﬁber is aligned and the steel ferrule is laser welded. The

future challenges for higher performance solid-state laser devices are immense.

2.3 Photodetectors

Two main types of photodetectors are used in telecommunications: pin (p-

intrinsic-n) and APD (avalance photodiodes).

APDs are a high-performance

ultrafast photodetector that provide gain for weak signals. APD devices are ex-

pensive; therefore, if use of pin photodiodes is possible, it is the preferred choice.

In addition, APD detectors require large biases on the order of 50 V, whereas

pin devices usually are biased about 5 V. Again as in solid-state semiconductor

lasers, the materials are chosen based on bandgap and ultimately on quantum

efﬁciency. For telecommunications, the chosen materials are based on solid so-

1318 ADVANCED MATERIALS IN TELECOMMUNICATIONS

Fig. 10 Light guided down a water stream demonstrating principle of internal reﬂection of

light.

lutions of InGaAsP on InP. To improve device performance, a lattice match with

the substrate is desired in order to minimize the dark current and thus improves

the signal-to-noise ratio of the device.

Additional improvements can be made by appropriate metal contacts. The

choice for metal contacts on semiconductors can be an art. For example, AuGe/

Ni has been used on InP and TiAu on In(As, P).

The selection of a metal

contact is initially made depending on the doping state of the semiconductor.

The work function (

␾

) of the metal needs to be less than the work function for

n-type and greater than the work function for p-type semiconductors in order to

make an ohmic contact. The work function is deﬁned as the difference between

the Fermi energy and the ionization energy.

2.4 Optical Fiber

The principle of guiding light within a medium is not new. In 1854, John Tyndall

demonstrated that a stream of water could guide light.

In his experiment, a

light shines through a stream of water jetting out from the side of a tank of

water would and bends along the stream’s path (see Fig. 10). The stream of

water successfully guides light until the turbulence of the stream’s surface made

the beam more diffuse. The water guides the light by internally reﬂecting it.

That is, the light at the water/air boundary is reﬂected back into the water.

Internal reﬂection occurs only when going from a material of high refractive

index (water) to a material of lower refractive index (air).

Today we have more sophisticated means of guiding light, one of which is

through the use of optical ﬁbers. Optical ﬁbers are narrow strands of glass,

polymers, or a composite of both. One of the early identiﬁed challenges of

guiding light through a single medium, such as glass, is that light leaks whenever

the medium contacts materials other than air, that is, material with a refractive

index greater than air. The solution to this problem was addressed by making a

ﬁber ‘‘sandwich,’’ placing an outer cladding layer around the ﬁber core. In this

manner, light travels along the ﬁber core conﬁned by the lower refractive outer

layer. This arrangement not only conﬁnes the light but also prevents leaks should

the ﬁber touch materials that are more highly refractive (see Fig. 11).

2 MATERIALS SELECTION FOR SELECT COMPONENTS 1319

Fig. 11 Light guided in an optical ﬁber by total reﬂection in the high refractive core bounce

back by the low refractive cladding.

The majority of ﬁbers are made from glass consisting of silica (SiO

)or

silicate. Glass ﬁber properties have a range that suits both short- and long-haul

transmission. Plastic ﬁbers are used less widely and are limited to short distances

but can withstand more abusive environments due to greater mechanical tough-

ness compared with glass. In the design of optical ﬁbers it is important to con-

sider the loss or reduction of signal as it travels in the material. Sources of loss

in silica ﬁbers are absorption from impurities, absorption due to molecular vi-

brations, and scattering by ﬁber atoms.

These losses can be resolved by re-

ducing the number of impurities, as well as by optimizing the material’s

dispersion and tailoring the chemical composition to create graded refractive

indexes. For the silica ﬁber, the chromatic dispersion is near zero at 1.33

␮

and has its lowest attenuation at 1.55

␮

In the next following sections we will examine the materials used in optical

ﬁber and their fabrication.

Glass Materials

Glass optical ﬁbers are nearly always vitreous silica doped with small amounts

of germanium oxide (GeO

), phosphorus oxide (P

), or ﬂuorine, but can be

mixtures of oxides, sulﬁdes, and selenides

that create a random covalent net-

work. Unlike ordered crystalline materials, there is no well-deﬁned pattern and

accordingly no well-deﬁned melting point.

The largest class of optical ﬁbers

is made from oxide glasses, in particular silica (SiO

), with a refractive index

of 1.458 at 850 nm.

The refractive index of a material may be changed by

controllably adding impurities or dopants. Such dopants include ﬂuorine and

metal oxides. Inclusion of GeO

or P

increases the index, whereas the inclu-

sion of ﬂuorine or B

decreases the refractive index.

Silica’s properties make it a formidable candidate for optical ﬁber due to its

low thermal expansion, high transparency, and high resistance to thermal shock.

However, the high melting point needed for its fabrication from the molten form

gives rise to the need to explore alternative low-melting-point silicates. These

materials include soda-lime silicates, borosilicates, and germanosilicates. Soda-

lime silicate glasses are combinations of silica, alkaline oxides (such as Na

or LiO

) and other oxides (MgO, ZnO, or BaO).

As a result, these materials

are easy to melt and fabricate into ﬁber.

In some cases it is necessary to boost the signal periodically due to attenuation

and losses. The signal can be ampliﬁed by doping the ﬁber with erbium. The

EDFA (erbium-doped ﬁber ampliﬁer) has proven to be an enabling technology

1320 ADVANCED MATERIALS IN TELECOMMUNICATIONS

for optical communication.

A pump laser at 0.98 or 1.48

␮

m excites the erbium

atoms, which transfer their energy an additional 1.55

␮

m light waves by laser

action to amplify the weakened signal.

The ƒ-orbital transition in Er

⫹

yields

the ampliﬁcation around the 1.5-

␮

m band used for long-haul telecommunication.

The promise of a low-cost alternative to the EDFA is the EDWA (erbium-doped

waveguide ampliﬁer). An advantage of EDWAs is the ability to be integrated

with other optical components due to the fabrication of the waveguide on a

silicon optical bench.

To increase the channels, it is necessary to extend the bandwidth beyond 1.5

␮

m. Presently, a combination approach of rare-earth doped ﬁber ampliﬁcation

and Raman ampliﬁcation is employed.

Another variation of the structure of optical ﬁber is the development of pe-

riodically varying index in the ﬁber core, called a ﬁber grating.

By selectively

transmitting wavelengths and reﬂecting others depending on the pattern spacing,

these ﬁbers can be modiﬁed to act as mirrors, sensors, ﬁlters, or resonant cavi-

ties.

Plastic-Clad Fibers

For short data transmission ranges (several hundred meters) where high losses

are tolerable, plastic-clad ﬁbers are an inexpensive alternative. The ﬁbers are

composed of silica cores with lower refracting plastic cladding made of silicone

resin (with an index of 1.405 at 850 nm) or perﬂuoronated ethylene propylene

(Teﬂon) (with a refractive index of 1.338). Only used for step-indexed ﬁbers,

the core diameters are larger (150–600

␮

m) than conventional all-glass cores

and cladding systems and permit low-cost interfaces with light sources and de-

tectors.

Plastic Fiber

For short distances (maximum 100 m) an all-plastic multimode step-index al-

ternative may be used.

Although they have signiﬁcant signal losses, they are

tough and durable and thus do not require special handling. High refractive

differences between core and cladding make this an attractive alternative for

short haul applications, but costs are still high. Core and clad combination of a

polystyrene (index 1.6) core and methyl methacrylate (index 1.49) cladding have

been demonstrated.

Fabrication of Optical Fibers

All-glass optical ﬁbers are made of extremely pure silica glass. The cladding is

made of the purest material, while the core material includes deliberately and

precisely controlled amounts of impurities (which increase its refractive index).

Two common methods are used in the manufacture of all-glass optical ﬁbers

They are direct-melt processes and vapor-phase oxidation processes.

In the direct-melt method, the optical ﬁber is made directly from molten

silicate glass. Two concentric platinum crucibles, one for the core and one for

the cladding, are ﬁlled with the appropriate molten glass. A ﬁber is then drawn

from the oriﬁces at the bottom of the crucibles, rendering a continuous drawing

process.

31,34

Despite the relative ease of this manufacturing technique, careful