Moulson A.J., Herbert J.M. Electroceramics: Materials, Properties, Applications

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The defect concentrations and their dependence on p

and temperature are

derived from the law of mass action by procedures essentially the same as those

outlined in Section 2.6.2. In the case of polycrystalline high-purity alumina, the

electronic conductivity increases with decreasing grain size and is attributed to

hole transport along grain boundaries.

The conductivity of debased aluminas is dominated by that of the usually

continuous silicate phase, and the ideas concerning charge transport through

glasses (Section 2.6.3) are therefore appropriate. Debased aluminas are typically

more conductive than the high-purity varieties, as illustrated in Fig. 5.21.

The eﬀects of temperature and frequency on the permittivity and dissipation

factor of a high-purity alumina ceramic are shown in Fig. 5.24. The discrepancies

between the permittivity levels in Fig. 5.24 and values given elsewhere are probably

due to diﬀerences in microstructure and measurement technique. Reliable room

temperature values for e

for single-crystal sapphire at 3.4 GHz are 9.39

perpendicular to the c axis and 11.584 parallel to it, which are close to the values

measured optically. The average e

to be expected for a fully dense ceramic form is

therefore 10.12, and values close to this have been determined. Nothwithstanding

the uncertainties there is no doubt that the general behavioural pattern indicated

by Fig. 5.24 is correct and typical of ceramic dielectrics.

As the temperature increases defects and charge carriers become more

responsive to applied ﬁelds, causing both permittivity and loss to increase.

However their inertia prevents them from responding eﬀectively at high

frequencies so that the loss falls, particularly at higher temperatures. An

intergranular phase will clearly have a considerable eﬀect on both the resistivity

and the dissipation factor.

It is an advantage that alumina can be ﬁred in hydrogen without degradation

of the electrical properties since this allows the use of molybdenum heating

elements in the high-temperature furnaces required for sintering the high-purity

282 DIELECTRICS AND INSULATORS

Fig. 5.23 Speculative diagram describing the dependence of charge transport mechanisms in

alumina on temperature and oxygen pressure.

TEAMFLY

Team-Fly

grades. Also, the success of the ‘moly-manganese’ metal–ceramic joining process

rests on resistance to degradation in a hydrogen atmosphere at high temperature

(see Section 3.8).

Applications

Alumina ceramics are used wherever exceptionally good dielectric properties,

high mechanical strength and high thermal conductivity, coupled with a reliable

ceramic–metal joining technology such as the moly-manganese process, are

demanded. Examples of components are illustrated in Fig. 5.25.

LOW-PERMITTIVITY CERAMIC DIELECTRICS AND INSULATORS 283

Fig. 5.24 Dependence of (a) e

and (b) tan d on temperature and frequency for a high-purity

ceramic.

(a)

(b)

The best known use for a 95% alumina ceramic is in spark plugs, mass-

produced components which illustrate well the need for a combination of good

thermomechanical and electrical properties. They must be strong to withstand

not only rough treatment meted out by garage mechanics but also the

thermomechanical shocks experienced each time the engine ﬁres, with

284 DIELECTRICS AND INSULATORS

Fig. 5.25 Alumina-metal components: (A) Components for which a combination of high

mechanical strength, gas-tightness and good electrical insulation are prime requirements: (a)

section of 100 m diameter vacuum-tight ring for an electron synchrotron with internal printed

heater for out-gassing; (b) electrical power lead-throughs; (d) isostatically pressed alumina part

of spark plug together with plug. (B) High power microwave components: (a) Inductive Output

Tube suited to digital TV service and capable of handling 130 kW peak output power, equating

to 30 kW mean power (82 cm long); (b) a 4-cavity klystron generating 20 kW continuous power

at 470–860 MHz for analogue TV transmission (125 cm long); (c) a conical microwave

‘window’ for eﬀecting a vacuum-tight seal between, for example, a microwave generator and

antenna (approx. 20 cm high). ((B) Courtesy of ‘E2V Technologies’ UK.)

temperatures and pressures as high as 900 8C and 10 MPa (100 atm) being

generated. The alumina must also provide adequate insulation against electrical

stresses of the order of 1 MV m

, and the glazed surface can be kept clean so as

to reduce the risk of tracking.

Another important application is in klystrons and magnetrons – devices for

the generation of electromagnetic energy, sometimes at very high power levels.

For example, the impressive operating characteristics of a large klystron for a

1.3 GHz radar transmitter are as follows: an electron gun voltage of 278 kV and a

beam current of 324 A providing a pulse peak power of 30 MW, a pulse length of

35 ms and an average power level of 150 kW. At the other end of the power

spectrum are generators for domestic microwave ovens, operating typically at

power levels of 1.2 kW at 2.45 GHz. The high gun voltages used in large

klystrons necessitate very good insulation and, just as for the electron guns in

electron microscopes, it is provided by alumina. A microwave generator,

however, makes a further unique demand. Microwave power is passed from the

vacuum interior of the device to the outside world through a ‘window’ as

transparent as possible to the radiation. Because of the ﬁnite power factor of the

window material heat is developed which must be removed to a suitable sink.

Therefore requirements for a good microwave window material are a low power

factor at the operating frequency, high thermal conductivity, vacuum tightness,

high strength and the capability of being joined eﬀectively to a metal. Alumina

ceramics meet most of the requirements, but beryllia may be used for the highest

power levels because of its unrivalled thermal conductivity. This is so in the case

of a particular commercial ‘gyrotron’ in which a beryllia output window of

diameter 2 in passes 200 kW continuously at 60 GHz.

Where the ceramics form part of a precision engineering structure, as in the

above examples, it is necessary to bring them to the correct dimensions by

diamond machining. They are usually joined to the metal parts by variants of the

moly-manganese process. It is worth noting that the manufacture of metal–

ceramic components has been so successfully developed that it can replace the

metal–glass technology where production costs rather than the technical merits of

the alumina ceramic are the deciding factor. The metal–ceramic technology

dispenses with the highly skilled and expensive glass blower necessary for the

manufacture of devices based on glass envelopes and is suited to mass production.

Alumina ceramics are widely used for thick-ﬁlm circuit substrates and for

integrated circuit packaging. This aspect is discussed in Section 5.5.5.

5.5.3 Beryllia

Beryllia has broadly similar properties to alumina (Table 5.3) but its thermal

conductivity is 5–10 times greater. It is therefore used when thermal dissipation

combined with electrical isolation is of major importance, e.g. in high-power

LOW-PERMITTIVITY CERAMIC DIELECTRICS AND INSULATORS 285

klystrons and power diodes. Because beryllium is one of the rare elements the

oxide is inevitably more expensive than alumina, which limits its use.

The technology developed to produce dense BeO ceramics is similar to that

employed for Al

. Sintering must be carried out at somewhat higher

temperatures since the melting point of BeO is higher at 2570 8C. The

atmosphere during sintering or other high-temperature (41000 8C) treatment

of BeO must be moisture free since otherwise a volatile hydroxide is formed

which is toxic if inhaled as fumes. Sintering and metallization can be carried out

in dry hydrogen. Sintered pieces only give rise to a health hazard when they are

abraded or heated in moist atmospheres without suitable precautions.

5.5.4 Aluminium nitride

AlN (Table 5.3) has recently been introduced as a substrate because it combines

a high thermal conductivity with a thermal expansion coeﬃcient close to that of

silicon. Its resistivity is somewhat lower than that of BeO and Al

but is

suﬃcient for most substrate applications.

AlN dissociates into its elements at 2500 8C. It is formed by the reaction of

aluminium powder with nitrogen and can be sintered in a nitrogen atmosphere.

AlN replaces BeO in many applications since it is competitive in price and does

not present the problems of toxicity in manufacture. It may replace Al

as a

substrate for silicon chips because of its better heat conductivity and its closer

match in thermal expansion.

5.5.5 Ceramic ‘packaging’ technology

Electronic circuits are ‘packaged’ using substrates of one form or another

ranging from the plastic-based laminate through to the sophisticated multilayer

ceramic (MLC) substrate. Laminates are dominant in relatively low technologies

such as radio, TV and other consumer products, and thick ﬁlm circuits carried on

ceramic (Al

, BeO, AlN) substrates are extensively used for a wide range of

more demanding applications including communications, automotive control

and military and space technologies.

The substrates carrying the circuits shown in Fig. 4.5 are a 95–96% alumina.

This ceramic has been adopted for its combination of physical and chemical

characteristics and, importantly, low cost. It oﬀers a combination of mechanical,

thermal and electrical properties which meet the in-service requirements, and

compositional and microstructural characteristics suited to thick ﬁlm printing

(see Section 4.2.2). Alumina substrates are manufactured on a very large scale

making the unit costs a small fraction of the total circuit cost.

286 DIELECTRICS AND INSULATORS

Over the past two decades MLC technology has been progressively developed

for advanced packaging, especially by main frame computer manufacturers, and

it is increasingly exploited for microwave communications. Especially signiﬁcant

is the emergence of low temperature co-ﬁred ceramic (LTCC) technology.

The ‘alumina^molybdenum’ IBM thermal conduction module (TCM)

In the 1980s computer circuit designers turned to MLC technology in an eﬀort to

increase component packing density so reducing interconnect lengths. A major

mile-stone was the development of the IBM thermal conduction module (TCM)

used in 308X type computers and illustrated in Fig. 5.26. The following

discussion outlines the general development trends and reasons for them; the

technical details are, presumably, continually changing.

In the manufacture of the multilayer structure for the TCM, square sheets are

stamped from a roll of ‘green’ tape loaded with a debased alumina powder.

Many small (about 150 mm diameter) holes (‘vias’) are punched through the tape

at predetermined positions. Electrical ‘wiring’ patterns with line widths and

LOW-PERMITTIVITY CERAMIC DIELECTRICS AND INSULATORS 287

Fig. 5.26 Multilayer ceramic thermal conduction module: (a) underside showing connector

pads – substrate size 9069066 mm; (b) multilayer construction; (c) silicon chip mounting

positions; (d) schematic diagram illustrating buried interconnections; (e) thermal conduction

module. (Courtesy of IBM Corp., East Fishkill.)

(a) (b)

(c)

(e)

(d)

spacing of about 100 mm are then screen-printed onto each sheet, and the vias

ﬁlled with a molybdenum-based metallizing ink. After the printing stage the

sheets are laminated by warm-pressing and then sintered in a reducing

atmosphere. The ﬁnal structure is an alumina (92% Al

) tile of dimensions

9069065 mm made of up to approximately 33 layers of ceramic with buried

three-dimensional circuitry, as illustrated in Fig. 5.26. The top and bottom

surfaces of the tile are nickel and gold plated with patterns onto which the

semiconductor integrated circuits are subsequently bonded.

In the TCM removal of heat is eﬀected directly from the back of each

semiconductor chip through a spring-loaded piston, assisted by immersing the

multilayer structure in helium gas and by ﬂowing water in the top cover.

In parallel with the development of the TCM intensive eﬀorts were being made

to improve on the technology. The alumina–molybdenum system has the

disadvantage that the requirement to sinter at high temperature (1400 8C) and

in a low oxygen potential atmosphere to maintain the molybdenum in the

metallic state is expensive. A further disadvantage is that the high resistivity of

molybdenum, coupled with the relatively high permittivity of alumina,

signiﬁcantly slows signal transit time. The eﬀorts to lower the sintering

temperature allowing the use of relatively highly conductive copper for the

interconnections led to the adoption of LTCC technology. The following

discussion should be read in conjunction with Section 3.7.4.

Low temperature co-¢red ceramic technology (LTCC)

Essentially LTCC technology is based on glass-ceramics specially formulated to

meet the operational demands. Substrates must be strong enough to withstand

mechanical stressing during processing and in service. Ideally thermal

conductivity would be high so that heat produced in the components is quickly

dissipated without attendant large temperature gradients in the assembly. This is

signiﬁcant in cases where the component density is high or where power

components (rather than ‘signal’) are involved. As the technology has developed

thermal conductivity has become far less important, especially in the case of the

sophisticated IBM multilayer substrates discussed above in which thermal

management does not rely signiﬁcantly on the ceramic.

As far as electrical properties are concerned most attention is focused on

permittivity and dissipation factor. A high permittivity carries two penalties. One

is dielectric coupling between closely spaced ‘wires’ leading to ‘cross-talk’, and

the other signal transit time, the velocity of an electromagnetic wave being

inversely proportional to the square root of the relative permittivity (see

Eq. (2.120)). Signal transit time is important in so far as it is a factor determining

the speed of a computer.

288 DIELECTRICS AND INSULATORS

LTCC technology starts with a glass powder carried in an organic- (or water)-

based vehicle and cast into a tape. The circuit is printed onto the tape using

highly conducting metals such as silver (m.p. 961 8C), gold (m.p. 1064 8C) or

copper (m.p. 1084 8C) with line widths and separations of 75 mm. Whilst in

the case of silver and gold sintering is carried out in air, in the case of the copper

circuitry, to avoid oxidation of the metal, the co-ﬁring must be in a protective

atmosphere, normally wet nitrogen/hydrogen, adding to processing costs. The

layers are stacked and laminated and ﬁred in the temperature range 850–950 8C

when the glass devitriﬁes to a glass-ceramic. ‘Vias’ punched through the tape

prior to laminating, and ﬁlled with conductor paste eﬀect the electrical

connections between layers. The ﬁnal part is a dense, fully integrated substrate

having optimum mechanical and electrical characteristics onto which other

components, ‘passive’ and ‘active’, can be mounted and interconnected.

For this application the glass-ceramic should completely devitrify during

processing to ensure dimensional and shape stability, have a low relative

permittivity (55), and a thermal expansivity close to that of silicon (3MK

)

so as to avoid the development of temperature-induced stresses in the

semiconductor devices bonded to the ﬁnished substrate. By careful choice of

composition, thermal expansivity and electrical properties can be tailored, as

discussed in Section 3.7.4. Cordierite (see Fig. 5.18) and b-spodumene

(Li

OAl

4ASi

) have suitable properties. The thermal expansivity of

cordierite can be tailored by small modiﬁcations to composition, for example

by adjusting the alumina content and making minor additions of boric oxide and

phosphorus pentoxide. To achieve a fully dense ﬁnal product the glass powder

must completely sinter prior to devitriﬁcation and so the presence of

‘mineralizers’ (crystal nucleating agents) are avoided.

Multilayer, LTCC technology is now exploited in high speed digital circuitry

for computer main frame equipment, the general form of the modules being still

essentially as illustrated in Fig. 5.26. R.R. Tummala [5] and S.H. Knickerbocker

et al. [6] review the glass-ceramic aspects of IBM TCM technology.

The explosive growth in wireless technologies has been accompanied by a

demand for specially tailored ‘microwave dielectrics’, and very recently by the

exploitation of multilayer LTCC-based circuitry (see Section 5.6.5).

5.6 Medium-permittivity Ceramics

Medium-permittivity ceramics are widely used as Class I dielectrics, and in order

to be in this category they need to have low dissipation factors. This precludes

the use of most ferroelectric compounds in their composition since ferroelectrics

have high losses (tan d40.003), particularly when subjected to high a.c. ﬁelds.

Low-loss materials can be obtained with relative permittivities exceeding 500

but accompanied by high negative temperature coeﬃcients, generally exceeding

MEDIUM-PERMITTIVITY CERAMICS 289

71000 MK

. For most purposes medium-permittivity ceramics have e

in the

range 15–100.

There are three principal areas in which these dielectrics are applied.

1. High-power transmitter capacitors for the frequency range 0.5–50 MHz for

which the main requirement is low loss: a negative temperature coeﬃcient of

permittivity is tolerable since it limits the power through the unit when its

temperature increases.

2. Stable capacitors for general electronic use: a stability better than +1% is

needed over the operational temperature and voltage ranges, and the

frequency lies mainly in the 1 kHz to 100 MHz range.

3. Microwave resonant devices: these operate between 0.5 and 50 GHz and

require stabilities of better than +0.05% over the operational temperature

range with dissipation factors better than 2610

The applications have been listed in order of increasingly stringent

speciﬁcations so that a material satisfactory for 3 would also satisfy 1 and 2,

although it might be unnecessarily expensive.

Medium-permittivity dielectrics are based on interlinked MO

groups, where

M is either a quadrivalent ion such as Ti, Zr or Sn or a mixture of divalent,

trivalent and pentavalent ions with an average charge of 4+. The oxygen

octahedra share corners, edges or faces (see Fig. 2.2) in such a way that the O

ions form a close-packed structure. Sites in the O

lattice may be occupied by

divalent cations that lie in interstices between the MO

octahedra, as in

perovskite-type materials. TiO

octahedra are the most commonly found groups

in medium- and high-permittivity dielectrics. The behaviour of rutile (TiO

)

ceramic is typical of these classes of dielectric in several respects.

5.6.1 Rutile ceramic

Titania occurs in three crystalline modiﬁcations: anatase, brookite and rutile.

Because above approximately 800 8C both anatase and brookite have

transformed to rutile it is the only form of signiﬁcance in the ceramics context

and attention here is limited to it.

The rutile structure (Fig. 5.27) is based on nearly close-packed oxygen ions

with Ti

ions occupying half the octahedral sites. The tetragonal unit cell

contains two formula units, and the Ti

is at the centre of a distorted oxygen

octahedron.

Rutile is anisotropic, with the values of e

at room temperature being

approximately 170 and 90 in the c and a directions respectively. In the

polycrystalline ceramic form e

averages to intermediate values with a

290 DIELECTRICS AND INSULATORS

temperature coeﬃcient of approximately 7750 MK

. The e

ðo,TÞ and

tan dðo,TÞ characteristics of a titania-based ceramic are shown in Fig. 5.28.

Pure rutile is an excellent insulator at room temperature with an optical band

gap between the ﬁlled O 2p valence band and the empty Ti 3d conduction band

probably in the range 3.5–4.0 eV. A thermal energy of approximately 1.7–2.0 eV

can transfer electrons from the valence to the conduction band leading to

semiconductivity. Figure 5.29 shows typical conductivity data for a high-purity

titania ceramic (499.95 wt% TiO

) measured in oxygen at 1 atm.

An important feature of TiO

is the extent to which it can be chemically

reduced above approximately 900 8C with accompanying signiﬁcant changes in

electrical conductivity, as shown in Fig. 5.30. The fall in resistivity is

accompanied by a loss of oxygen and the movement of Ti ions onto interstitial

sites, probably the empty octahedral sites in the rutile structure:

þ Ti

! O

ðgÞþTi

....

þ 4e

ð5:18Þ

The law of mass action leads to

½Ti

....

n

¼ K

1

ð5:19Þ

and, since n  4½Ti

....

,

n ¼ð4K

1=5

1=5

ð5:20Þ

Measurements of conductivity at 1000 8C and oxygen pressures below 101 mPa

(10

710

atm) have conﬁrmed Eq. (5.20) (see Section 2.6.2).

The presence of interstitial Ti ions is conﬁrmed by other research and is

consistent with the structural change that occurs with larger oxygen deﬁciencies

(e.g. in TiO

1.95

). This involves a process of crystallographic shear that

accommodates the oxygen loss and results in the Ti ions on one side of the

shear plane being in interstitial positions relative to those on the other side.

Therefore, in reality, a grossly non-stoichiometric phase is made up of

stoichiometric regions separated by a series of regularly spaced crystal shear

MEDIUM-PERMITTIVITY CERAMICS 291

Fig. 5.27 The crystal structure of rutile.