Ghodssi R., Lin P., MEMS Materials and Processes Handbook

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894 A.G. Darrin and R. Osiander

and epoxy sealing – all but epoxy form hermetic seals [17]. Of these methods,

welding is the most expensive because of the ﬁxturing/tooling needed. Sealing glass

requires higher temperatures in the range of 330–450

◦

C, whereas both epoxy and

seam welding are done at lower temperatures. Kovar

or Alloy 42, electroplated

with nickel and gold, is used for most lids for hermetic MEMS applications. For

AuSn solder sealing, the lids often have a solder preform tacked on, making solder

quantity control and alignment easy. The nickel underlayer serves as a very effective

barrier to corrosion, while the gold surface layer preserves solderability of the nickel

surface. For glass sealing, glass is usually pre-applied onto for the lid for MEMS,

opto-electronic and other devices, and glass is often used in larger array packaging.

Material properties of some solder preform materials are included in Section 12.4.1.

The application of glass lids for optical systems is discussed in t he case study in

Section 12.11.2.

12.3.1 Optical Applications

Optical communications and sensors require hermetically sealed packages that

allow transmission of optical, infrared, and ultraviolet signals. Sapphire, germanium

and special glasses such as BK-7 allow the direct transmission of optical data. For

maximum transmission efﬁciency, anti-reﬂective coatings are applied to the trans-

parent component of the sealing lid. These coatings are nanometer thick layers

of highly refractive ﬂuoride and oxide compounds that are alternately evaporated

or sputtered. These antireﬂective coatings can be tailored to allow speciﬁc wave-

lengths to pass through the lid with less than a 1% r eﬂection loss of signal strength

[18]. Properties of window materials that allow optical and infrared transmission are

shown in Table 12.7.

Table 12.7 Optical and IR transmission of materials as MEMS “windows”

Material CTE, ppm/

◦

C Modulus of elasticity, Gpa Wavelength transmittance, μm

Sapphire 5.3 335 0.25–5.5 >80%

C-1737 3.8 71 0.35–2.6 >90%

BK-7 8.3 82 0.35–1.9 >90%

Ge 6.1 103 1.8–15.0 >50%

Si 4.2 102 1.2–10.0 >50%

12.4 Die Attach Materials and Processes

The methods used to attach a MEMS device to a package are the same as those used

with Integrated Circuit devices. Such “die attach” media serve several functions:

mechanical support, heat dissipation, and possibly electrical contact between the

MEMS device and the package.

12 MEMS Packaging Materials 895

Electrically conductive attachment materials include silver-ﬁlled epoxies, silver-

ﬁlled glasses and low melting solders. The stability and reliability of the attachment

material is largely dictated by the ability of the material to withstand thermome-

chanical stresses created by the differences in the CTE between the MEMS silicon

and the package base material. Silicon has a CTE between 2 and 3 ppm/

◦

C while

most package bases have higher CTE between 6 and 20 ppm/

◦

MEMS packages use solders, adhesives or epoxies for die attach. Each

method has advantages and disadvantages that affect the overall MEMS reliability.

Generally, when a solder is used, the silicon die would have a gold backing. Au-Sn

(80–20) solder generally is used and forms an Au-Sn eutectic when the assembly is

heated to approximately 250

◦

C in the presence of a forming gas. When this method

is applied, a single rigid assembled part with low thermal and electrical resistances

between the MEMS device and the package is obtained. One problem with this

attachment method is that the solder attach is rigid (and brittle) which means it is

critical for the MEMS device and the package CTEs to match since the solder cannot

absorb the stresses.

Adhesives and epoxies are comprised of a polymer bonding material ﬁlled with

metal ﬂakes such as silver since it has good electrical conductivity and has been

shown not to migrate through the die attachment material [19, 20]. These die

attachment materials have the advantage of lower process temperatures. Generally,

temperatures between 100 and 200

◦

C are required to cure the material. They also

have a lower built-in stress from the assembly process as compared to solder attach-

ment. Furthermore, since the die attachment does not create a rigid assembly, shear

stresses caused by thermal cycling and mechanical forces are relieved to some extent

[21, 22]. One particular disadvantage of the soft die attachment materials is that

they have a signiﬁcantly higher electrical resistivity which is 10 to 50 times greater

than solder and a thermal resistivity which is 5 to 10 times greater than solder.

Lastly, humidity has been shown to increase the aging process of the die-attachment

material [20, 23].

12.4.1 Conductive Die Attach

The eutectic bonding process occurs when the substrate is secured just below the

eutectic melting point on a heated work stage. The die and preform are placed on

the substrate, and a light scrubbing motion is made with modest pressure by the

bond head. This scrub generates a rise in the temperature of the bond to the eutectic

melting point. The melted material solidiﬁes, and creates the bond. Nitrogen is used

as a cover gas in order to prevent oxidation due to the high heat. In some cases,

the die may have a eutectic alloy pre-plated on its back omitting the need for a pre-

form. The perform is a thin foil of a solder alloy of two or more dissimilar metals

placed in the spacet between the die and substrate. The preform has a melting point

that is lower than the melting point of its base materials. Consider a typical preform

composed of gold and silicon. The melting point of gold is 1640

◦

C, and the melt-

ing point of silicon is 1414

◦

C. However, when the materials are combined into a

896 A.G. Darrin and R. Osiander

Table 12.8 Selected eutectic solder preform materials

Materials Melting point,

◦

Au97-Si3 363

Au88-Sn12 350

Au80-Sn20 280

Pb63-35Sn-1.8Sb 230

preform, the melting point becomes 363

◦

C. Table 12.8 shows examples of eutec-

tic preform materials. The actual bonding temperature is about 20

◦

C higher than

the eutectic point. Gold-based eutectics have high ﬂow stresses and offer excellent

fatigue and creep resistance. The high ﬂow stress is a result of lack of early onset

of plastic ﬂow. This lack of plastic ﬂow can lead to high stresses in the MEMS chip

due to the thermal conductive mismatch between the chip and substrate.

12.4.2 Metal-Filled Glasses and Epoxies

A glass bond is formed by adhering the die to the substrate using glass in the form of

a paste. In some cases, such as in silver-glass die attach, the pastes contain silver par-

ticles that enhance thermal and electrical conductivities. A mix of lead borate-based

glass frit with 80 volume% Ag is a typical glass die attach paste.. The glass bonding

process is similar to adhesive bonding, although differences in bonds arise from the

type of material and process temperatures. Glass bonds are heated to 350–450

◦

forming a low viscous liquid. As it cools, the glass hardens to form a bond.

Beneﬁts of using a metal ﬁlled epoxy include relative insensitivity to substrate

metallization, low void content, and low contamination. In addition, the resulting

bonds have good thermal and electrical conductivities and limited stress relax-

ation. However, drawbacks such as higher oxidation rates during high temperature

processing should be noted.

12.4.3 Other Die Attach Materials

Non-conducting epoxy adhesives and insulating polyimides may be ﬁlled with met-

als, solders, or solders ﬁlled with glasses. Epoxies ﬁlled with 70–80% (vol) silver

with improved thermal and electrical conductivities are commonly used in industry.

Non-conductive adhesives may also be used for their electrically insulating proper-

ties, especially for bonds between the die and substrate and other chip to package

connections. In addition to conductive ﬁllers, other agents such as wetting agents

are used to improve manufacturability. Dispensing is conducted at room tempera-

ture with some typical adhesives being acrylic thermoplastic resins, epoxy thermoset

resins, and silicone resins. In the adhesive bonding process, the substrate or package

is secured to an unheated work stage. The wet adhesive material is contained in a

reservoir and a small amount is metered out onto t he substrate, usually in a pattern

12 MEMS Packaging Materials 897

conforming to the shape and size of the die. The die is picked up and placed onto

the adhesive, making the wet bond, bonding is complete once the adhesive dries.

This approach has many advantages: ease of automation, low-curing temperatures,

low cost, wide range of die sizes, and option to rework. The downside includes out-

gassing, contamination/bleed, inferior thermal conductivity, and sensitivity to harsh

environments.

Polymer Adhesive Bonding. Polymer or adhesive bonding involves curing tem-

peratures of up to 300

◦

C with low forces applied to the substrates in low vac-

uum conditions. Intermediate layers, with thicknesses ranging from a few sub

microns up to tens of microns, may consist of photo-patternable polymers such as

Benzocyclobutene (BCB) or resists like SU-8. Low-k dielectric polymers, like BCB,

are gaining more attention as an adhesive as these allow the creation of electrical

interconnects between different functional modules (system-on-package). Although

adhesive bonding compensates for surface roughness and topographical anomalies,

material vapor pressures make it unsuitable for high vacuum encapsulation (below

–2

torr) within MEMS devices. Low alignment accuracy during bonding poses

another disadvantage to this method. Despite such complications, adhesive bonding

is used in many applications that involve low-temperature wafers.

Anodic Bonding. Anodic bonding uses heat and an electric ﬁeld to join a sili-

con wafer together with an alkali-doped glass wafer. At elevated temperatures, the

alkali oxides in the glass dissociate. The so-formed mobile ions (e.g., sodium) are

driven by the electric ﬁeld toward the cathode, creating an oxygen-rich layer at

the Si-glass interface. The oxygen ions are driven to the Si surface by the electric

ﬁeld, resulting in oxidation of silicon. The bond strength is high and the process is

irreversible. Typical process parameters for Pyrex (borosilicate glass with a sodium

oxide content of ∼3.5% and a closely matching CTE over a wide temperature range)

involve temperatures between 350

◦

to 500

◦

C, high vacuum conditions, and voltages

up to 1000 V. The packages resulting from this process typically are used for her-

metic sealing of MEMS and MOEMS, where elevated bonding temperatures, high

voltages and sodium contamination do not affect on-chip electronics.

Glass-frit Bonding. A paste made from glass powder, solvent, and a tempo-

rary bonder (that ﬁres away) is deposited on a wafer s urface by screen printing. In

general, the glass frit is applied to the wafer cap and is softened by heating to t em-

peratures above the glass softening point. The glass material is then glazed between

300

◦

C and 500

◦

C. Subsequent cooling under high pressure solidiﬁes the glass frit.

Glass frit bonding is used for the caps in Sections 12.11.1 and 12.11.2.

Hermeticity is an important parameter for many MEMS devices. To achieve

high vacuum encapsulation, bonding methods with low out-gassing materials, pre-

cise wafer gap control, and compatible bonding temperatures should be considered.

Table 12.9 provides an overview of bonding technologies [24].

12.4.4 Flip-Chip Bonding

Controlled Collapse Chip Connection (C4) was developed by IBM in the 1960s

as an alternative to manual wire bonding. Often termed “ﬂip-chip,” electrical and

898 A.G. Darrin and R. Osiander

Table 12.9 Overview of different bonding techniques for hermetic sealing of packages

Bonding technology

Suitability for

high vacuum

(<10

torr) Precise gaps

Processing

Temp. (

◦

C) Limitations

Silicon fusion

bonding

√√

>1000 • Good surfaces needed

(micro roughness,

TTV, cleanliness)

Plasma activated

bonding

√√

200–400 • Good surfaces needed

(micro roughness,

TTV, cleanliness)

Anodic bonding

√√

200– 400 • Limited material

choice (Glass – Si)

• Sodium contamination

Eutectic bonding

Au-Si

√√

390

Au-Sn

√√

310

Thermo compression bonding with intermediate layers

Epoxy 150–250 • Out gassing of bond

material

Polymers 150 –250 • Out gassing of bond

material

Glass Frit 350–500 • Out gassing of bond

material

Au-Au Comp

√√

400–450

mechanical interconnects are created by plating solder bumps between bond and

metal pads of the package substrate. Chip-to-substrate misalignments are corrected

by the surface tension of the molten solder, making the ﬂip-chip process self-

aligning. Unlike wire bonding which requires placement of bond pads around the

periphery of the die, the ﬂip-chip process allows the placement of bond pads any-

where on the entire chip, therefore increasing the interconnect density. Having the

ability to unite distinct chips into a single package, ﬂip-chip technology has become

especially attractive in the MEMS industry [25]. Furthermore, the method also pro-

vides the option of rework. Prior to underﬁll chips may be removed and replaced

when needed with minimal risk. For improved reliability, a chip underﬁll may be

injected between the joined chip and the package substrate, although care should be

taken so that the underside is covered entirely by the underﬁll without air pockets

and voids. Complete edge ﬁllets must be formed around all four sides of the chip to

avoid high-stress concentrations [23].

12.4.5 Tape Interconnects

Tape Automated Bonding (TAB) was originally conceived as a rapid and robust

alternative to the slow and manual process of wire bonding [26]. Materials used for

12 MEMS Packaging Materials 899

the tape carrier base include polyimide, polyester, polyethersylfone (PES) and poly-

parabanic acid (PPA). Tape Automated Bonding is applied between MEMS devices

and substrates. Instead of single wires, this process uses a prefabricated carrier tape

with etched and ﬁnish-plated copper leads that can be made for many pad conﬁgu-

rations. Such tapes consist of perforated polyimide ﬁlms, similar to those used for

camera ﬁlms, with stamped openings for device and connection leads. A copper

foil structured by photolithography is then glued on these ﬁlms. This package style

is enjoying resurgence in the MEMS community and many successful applications

exist [27]. Usually, TAB involves cantilevered beam leads connected to IC pads.

Although such a technique is widely used, derivations connect ﬂex inner leads to

the IC. The ﬂexible outer leads are soldered to a PWB. In newer array packages,

the outer leads are replaced with metal posts, connection bumps, or solder balls,

which enable surface mounting. Flex-based BGAs are therefore lightweight, low in

proﬁle, and easily assembled by SMT. The common theme in all of the TAB type

packages is the direct bonding of inner lead ﬂex conductors to chip pads. IBM’s

Tape Ball Grid Array (TBGA) and Tessera’s μBGA are both ﬂex-based packages

and numerous variations of these two TAB concepts exist [28].

12.5 Wire Bonding

Tape Automated Bonding (TAB), Direct Chip Attachment (DCA), and wire bonding

are the options available to interconnect MEMS die. While there are many subsets

and derivatives, all IC interconnections can be classiﬁed into one of these three

basic systems [28]. This section covers the most commonly used method of chip

connection, wire bonding. Most often, this technique uses small gold wires; how-

ever, aluminum or occasionally copper are also used. Wire diameters range from

15 μm to several hundred micrometers. There are two main classes of wire bond-

ing: ball and wedge. Ball bonding is used with gold or copper wires and requires

modestly elevated temperatures. Gold wires are far more common since its does

not oxidize to impede micro-welding. Both gold and aluminum wires are used for

wedge bonding, with heat being required only for gold. In wire bonding, the wire

is attached at both ends using some combination of heat, pressure, and ultrasonic

energy to make a weld. It is generally considered the most cost-effective and ver-

satile interconnect technology. Wire materials properties that affect wire bonding

include yield strength, ultimate tensile strength, elongation, and purity.

12.5.1 Gold Wire Bonding

Gold wires are used extensively for thermocompression and thermosonic bonding.

As such, most common systems involve placing gold wires between a package and

the aluminum bond pads on an IC; with RF devices, gold ribbons are often used to

control parasitic inductance. It is the purity of the gold that controls the mechanical

900 A.G. Darrin and R. Osiander

Table 12.10 Typical mechanical properties

Diameter

Elongation

(%)

Break

strength (g)

minimum

Elongation

(%)

Break

strength (g)

minimum

Elongation

(%)

Break

strength (g)

minimum

Automatic gold wire bonding

0.0007



0.5–2 6 2–5 45.8 5.8 3.5

0.0008



0.5–2 8 2–5 5.5 5–8 5

0.0009



0.5–2 12 2–5 6.5 5–9 6

0.0010



0.5–2 15 2–5 9 5–10 8

0.00125



0.5–2 22 2–5 14 5–11 10

0.0015



0.5–3 32 2–5 19 5–12 16

0.002



0.5–3 42 2–5 37 5–13 33

Manual gold wire bonding

0.0007



0.5–2 5 2–5 3.5 5.8 2.5

0.0008



0.5–2 7 2–5 5 5–8 4

0.0009



0.5–2 11 2–5 5.5 5–8 65

0.0010



0.5–2 12 2–5 7.5 5–8 6.5

0.00125



0.5–2 20 2–5 12.5 5–8 10

0.0015



0.5–3 28 3–7 17.5 7–11 14.5

0.002



0.5–3 40 3–7 35 7–11 30

Gold ribbon (bonded to 99% aluminum 1% silicon pad by thermosonic techniques)

0.0007



0.5–2 5 2–5 3.5 5.8 2.5

0.0008



0.5–2 7 2–5 5 5–8 4

0.0009



0.5–2 11 2–5 5.5 5–8 65

0.0010



0.5–2 12 2–5 7.5 5–8 6.5

0.00125



0.5–2 20 2–5 12.5 5–8 10

0.0015



0.5–3 28 3–7 17.5 7–11 14.5

0.002



0.5–3 40 3–7 35 7–11 30

properties of the wire, such as elongation and break strength. For example, small

amounts, 5–10 ppm by weight, of beryllium or 20–100 ppm by weight of copper are

added. Table 12.10 show typical wire strength values for automated system (HBX),

manual system (HBXL), and ribbon wire bonds, respectively [29].

12.5.1.1 Au-Al System

The gold-aluminum (Au-Al) system is the most common in microelectronics. This

method utilizes the metallization of aluminum to create a bond and does not require

any additional processing steps. Au-Al thermosonic bonding requires elevated tem-

peratures of about 125

◦

C. At this temperature, the needed intermetallics form

quickly between the Au ball and Al bond pad.

Although intermetallic formation is necessary to form the Au to Al weld, a life

limiting factor can be excessive growth of these Au/Al intermetallic compounds.

Over time, Kirkendall voids can form because of differences in diffusion rates

between Al and Au; these voids along the Al-Au interface weaken the brittle inter-

metallic layer, allowing fracturing to occur. As this is a diffusion related process,

12 MEMS Packaging Materials 901

composition of the wire and bond pad can be varied to impede interdiffusion, but

at the cost of making initial bonding more difﬁcult – a clear production versus

reliability tradeoff.

12.5.1.2 Au-Ag System

The gold-silver (Au-Ag) system is a bonding scheme that is primarily used for sec-

ondary bonds (i.e., off-die). Such bonding is found in most leadframe-based package

technologies where a wedge bond is formed between gold wire and a silver-plated

package lead. Bond formation may also be enhanced through thermosonic means at

elevated temperatures of about 250

◦

C. Thermosonic processes sweep aside surface

oxide or sulﬁde ﬁlms to raise the bondability of the silver pad.

12.5.1.3 Au-Au System

Au-Au bonding schemes are often found in MEMS packaging. Risks such as inter-

face corrosion, intermetallic formation, and other mechanisms that degrade the bond

strength are avoided by using this monometallic system. Au-Au bonding is usually

performed at elevated temperature by thermocompression or thermosonic means,

although cold ultrasonic Au-Au wire bonding can also be achieved.

12.5.1.4 Au-Cu System

Gold-copper (Au-Cu) bonding metallurgy is usually employed in bonding gold

wires to bare copper lead frames. Gold wire-copper leadframe bonding produces

three ductile intermetallic phases (Cu

Au, AuCu, and Au

Cu), that at high tem-

peratures, tend to form voids which degrade the bond strength and lower its

reliability. Cleanliness of the bonding surface is therefore imperative in gold-copper

systems to ensure reliable bonding [30].

12.5.2 Aluminum Systems

Although not as common as gold systems, aluminum systems are used through-

out the industry with Al-Al being the most common. Table 12.11 shows typical

mechanical properties for aluminum wires [31].

Table 12.11 Typical mechanical strength for aluminum wires

Al wire

Diameter Elongation % Break strength (min.g.)

0.0007



0.5–3.5 7–10

0.001



1–4 13–16

0.00125



1–4 16–19

902 A.G. Darrin and R. Osiander

12.5.2.1 Al-Al System

For critical end item usage the monometallic bonding scheme of Al to Al is widely

utilized. Al-Al systems are used primarily in hermetic packaging, for bonding alu-

minum wires onto the aluminum bond pads of the die. As this bonding scheme

avoids intermetallic formation and corrosion, it is also a r eliable wire bonding met-

allurgical system. Aluminum wire on aluminum bond pad is achieved ultrasonically

at room temperature. Pure aluminum is too soft to be a ﬁne wire and is often alloyed

with 1% Si or 1% Mg to provide strength.

12.5.2.2 Al-Ag System

The aluminum-silver (Al-Ag) system is most commonly used in lower-cost thick-

ﬁlm hybrid technologies to bond an aluminum wire onto a thick-ﬁlm silver alloy

(with Pt or Pd) bonding pad of a ceramic substrate. The Al-Ag phase diagram

has several different intermetallic phases that may cause degradation as a result of

excessive interdiffusion between the Al and Ag. Al-Ag bonds exhibit accelerated

corrosion in the presence of moisture, which most often is the result of an ionic con-

taminant such as chlorine. The risk of corrosion in the ﬁeld is usually mitigated by

using large diameter aluminum wires and thick silver alloy bonding sites.

12.5.2.3 Al-Ni System

For power device and high temperature applications, aluminum-nickel systems are

often used. This metallurgical system usually consists of a large diameter alu-

minum wire and nickel bonding pads or package posts plated by electroless plating

methods.

Al-Ni wire bonding systems may present some bondability problems because of

the tendency of the nickel surface to oxidize quickly. Wire bonding is performed

in an inert atmosphere after the pads are nickel-plated. To improve bondability,

abrasive cleaning may be required. Some substrate manufacturers deposit a very

thin layer of gold over the nickel surface to prevent it from oxidizing.

12.5.3 Copper Systems

The Cu-Au system is vulnerable to void formation at high temperatures, and requires

bonding surface cleanliness. In the Cu-Au system, void formation may be a partic-

ular issue due to intermetallic growth. In addition, an intermetallic CuAl

phase

can form which is brittle and can cause failure of the bond. These systems are also

vulnerable to corrosion in the presence of chlorine and moisture.

12.6 Electrical Connection Processes

Both power and signal connections must be made from the MEMS chip to out-

side electronics. The main types are cable connectors, board connectors, chip

connectors and probe testers. Cable connectors make a (normally demountable)

12 MEMS Packaging Materials 903

connection between two electrical cables. They are referred to as in-line connec-

tors, if the connection is made by insertion of a male part into a female part, in a

direction parallel to the cable axis. The cable may have many conductors. If these lie

parallel in a plane, the connector is often known as a ribbon connector. In a ribbon

connector, the connecting elements (often known as tongues) are normally arranged

on a substrate. The tongues may then deﬂect in plane (i.e., parallel to the substrate

plane) or out-of-plane (perpendicular to the substrate). Thus, it is possible to have an

in-line connector with in-plane deﬂection of the connector tongues [32]. A typical

device package is composed of a ﬂat lead frame with numerous leads at the interior

die position. The MEMS device is positioned and aligned with the interior opening

of the lead frame, after which it is electrically connected to the interior of the lead

frame by small wires.

12.7 Encapsulation

Encapsulants come in two physical forms, solids and liquids, but with similar com-

positions. Solid types are epoxy molding compounds (EMC) and are blends of solid

epoxy resin, hardener, ﬂame retardant, ﬁller, and several additives. EMC preforms

or “hockey pucks” are used in transfer-molding machines. Similar in composi-

tion, liquid encapsulants are formed from liquid forms of resin and hardeners. In

this form, the material may be dispensed and applied directly instead of molded

onto the chip and interconnect. Polyimides, polyamide-imides, silicones, acrylics,

polyurethanes, ﬂuoropolymers, parylenes and epoxies are typical materials that

could be used for encapsulation or conformal coatings. Except for parylenes, which

are vapor deposited, the coatings are usually applied in liquid form and are cured

using infrared or ultraviolet radiation. This section will discuss both encapsulants

and conformal coatings.

12.7.1 Polyurethane

Polyurethanes are available as one or two component resins for conformal coat-

ings. Electronic components are often protected from environmental inﬂuence and

mechanical shock by enclosing them in polyurethane. Typically, polyurethanes

are selected for excellent abrasion resistances, good electrical properties, excellent

adhesion and impact strength, and low temperature ﬂexibility. The disadvantage of

polyurethanes is the limited upper service temperature (typically 250

◦

F (121

◦

C)).

In production, the manufacturer would purchase a two part urethane (resin and cata-

lyst) that would be mixed and sprayed or poured onto the circuit. In most cases, the

ﬁnal circuit board assembly would be unrepairable after the urethane has cured.

12.7.2 Polyimide

Polyimides are a family of thermoset and thermoplastic resins characterized by

repeating imide linkages. There are four types of aromatic polyimides: (1) con-

densation products formed by the reaction of pyromellitic dianhydride (PMDA)