Low-Permittivity Ceramic Dielectric and Insulator

기기연구실김홍정

개 요# 낮은유전율 유전체 (ε<16) - straightforward 절연체로 폭넓게 쓰인다 . ( 기계적 특성이 유전체 특성보다 더 중요하다 . 그러나 Substrate 에 사용될때는 유전체 특 성이 더중요하다 .) - 커패시터 유전체로 응용 ( 고주파수에서 매우작은 커패시던스를 필요로 할때 사용한다 .)

1.Electrical porcelains (a) Clay-based ceramics (b) Talc-based ceramics

2. Alumina3. Beryllia4. Aluminium nitride5. Glasses6. Substrate

Electical procelains(Clay-based ceramics)1. The insulators - 전력계통에서 케이블에 사용된다 .( 예를들어 siliceous clay-based ceramic)

2. Electrical porcelains (a) 원재료 : clay , fluxes and fillers - Clay : ㄱ . Aluminosilicates( 알루미노규산염 ) 의 작은 평평한입자 행태이다 . ㄴ . Kaolinite 는 가장 흔히 쓰이는 점토 광물이다 . - Fluxes : ㄱ . 보통 장석이 쓰인다 . ㄴ . Fluxes 를 녹일때는 유리를 만들때보다 낮은 온도를 사용한다 . - Fillers : 보통 석영이 쓰인다 . - clays 40-60% , flux 15-25%, filler 30-40% (b) 가공 - Forming : ㄱ . 원재료를 물과 섞는다 . ㄴ . A vacuum extrusion pug-mill 에서 body 가 균질화 와 de-aired ㄷ . 도공의 바퀴처럼 유사한 원리로 동작하는 ‘ jolleying’ 방법으로 body 를

만든다 . ㄹ . Body 가 선반과 자신의 무게를 지탱할수 있을때까지 건조한다 .

Electical procelains(Clay-based ceramics) - After the forming stage ㄱ . 절연체를 조심스럽게 건조하고

a glazy slip 을 입힌다 . ㄴ . 그 body and glaze 를 1200 도 정도에서 가열하여 자기화한다 .

(c) microstructure - microstructure comprises the quartz or alu

mina particles embedded in a glass or crystalline matrix(Fig.5.20(c))

( 거기에는 기공이 없어야 되고 기공을 최소화시켜서 유지해야 한다 .)

(d) joining - In case of disc-type transmission line insu

lators the ceramic pieces are joined together via metal parts fixed the ceramic with a hydraulic cement, as illustrated in Fig 5.17

Electical procelains(Clay-based ceramics)3. 특징 (a) discharge - 방전이 a transmission line insulator 근처에서 존재하는 높은 전계 영향으로 일어날지모 른다 . ( 특히 나쁜 날씨나 지리적으로 대기 오염이 심한곳에서 일어날지 모른다 .) (b) Corona discharge - around line-insulator contact 에서 일어날수 있다 . (c) flash-over - There is the risk of the occurrence of flash-over between line and pylon across the insulator surface. - Varistors 를 사용한 적합한 보호장비로 이문제를 해결할수있다 .

4. 응용 - There are many low-voltage applications for porcelain bodies of the siliceous variety ; some examples are switch bases and a variety of fuse holder

Electical procelains(Talc-based ceramics)

1. Talc-based ceramics - Talc-based ceramics are important electrical porcelains that have major

crystalline components of the fired ceramic lying in the ternary phase diagram. (Fig 5.18)

Electical procelains(Talc-based ceramics)1. The principal raw material - 운모는 가장부드러운 광물이다 . ( 모스경도 1) # the various porcelain types fall into three regions of the phase diagram and will be discussed in turn

Electical procelains(Talc-based ceramics)2.Cordierite ceramics - 이것은 매우 낮은 팽창 계수를 가지고 있다 . - Clay and Talc 이 주원료이다 . - 응용 There is a need for good thermal shock resistance combined with high electrical resistivity, for example for high-power electrical fuse holders and supports for high power wire-wound resistors and fan-heater elements

3. Steatite ceramics - 1920 년대에 라디오 산업의 급격한 수요에 따라서 electronic component 로 사용되어졌다 . - 주요특성 a. low dielectric losses which are necessary for higher frequencies. b. 불안정성 * The sintered bodies consist of protoenstatite which is a polymorph of MgSiO3

a small cordierite and a continuous glassy phase surrounding the crystalline phases. * Prolonged heating at 500C converts protoenstatie into clinoenstatite, which is the thermodynamically stable form below 985C.

Electical procelains(Talc-based ceramics) * The dimensional change, badly formulated material, overfired material etc. * Despite this potential instability, steatite ceramics have been widely used and adequate precautions in manufacture have prevented all but a very few case of deterioration during component lifetime of many years in a wide variety of environments - 구성 : talc 85% , clay 15% calcium carbonate2% - 응용 : Many small parts are made for the electronic components industry where low dielectric losses are required , for example for tie-bars and other parts for ganged capacitors, small trimmer capacitors, high-power capacitors, coil formers, lead-throughs and substrates for types of resistor and circuit

4. Forsterite ceramics - 좋은 loss-electric-loss 특성을 가지고 있다 . 그러나 높은 열팽창 계수를 가진다 . - 1960 대에 보다 전문적인 고전압 장치 (titanium-forsterite 로 구조된 ) 의 제조에 사용되었다 . 오늘날 alumina-metal 구조는 완전히 titanium-forsterite 로 대체되었다 . alumina-metal 는 지금 단지 특수한경우만 사용되어지고 있다 .

Alumina1. 구조 , 제작 , 특성 (a) Alumina - siliceous 광물중에서 가장 널리 퍼진것이 alumina 이다 . - 단일결정에서 사파이어나 루비로 나타나고 큰침전물 에서는 보크사이트로 존재한다 . (b) 제작 - α-alumina : 알루미늄생산의 부산물로 powder 형태이다 . - 보크사이트 : Bauxite, which is naturally contaminated with other oxides, principally SiO2 and Fe2O3, is purified by the Bayer process. - The hydroxide is washed and then calcined at temperatures in excess of 1000C to produce α-Al2O3. - Practically all the powder for production of alumina ceramics is produced in this way but a purer product, for instance for growing artificial sapphire crystals, can be made by preparing ammonium alum by dissolving 99.999% aluminium metal in sulphuric and neutralizing the excess acid with ammonia. - The alum is allowed to crystallize, which helps to purify it, and is then calcined below 1000C to yield γ-Al2O3, which is preferred for some methods of sapphire preparation or at higher temperatures to give α form - Crystallite in alumina ceramics 광물학적으로 corundum( 사피이어와 동의어 , 모스경도 9) 으로 알려져있다

Alumina( 특성 )- The Less pure aluminas are blended with silicates so that they can be sintered at 1350C or less- The highest-purity materials require a temperature of 1750C at atmospheric pressure or hot-pressing.- As cost increases with sintering temperature, the grade used in practice is usually the least pure that has adequate properties.- Table 5.4 gives the properties of a range of aluminas and shows the improvement in properties with purity.

Alumina( 특성 )- There is considerable uncertainty about the room temperature values of the electrical resistivity of good insulators ; the best estimates are probably derived by extrapolation of the linear log ρ-1/T plots of the type (Fig.5.21)

- The reliable measurement of the resistance of very high resistance specimens is made difficult because of the relatively low resistance of the ceramic surface due to adsorbed impurities, and even that of the surrounding air or other gas.

- This can be circumvented by using a properly guarded measurement method.

Alumina( 특성 )(b) A guard ring- When the effects of gas conduction have to be guarded against the ring must be extended into a Cylinder.

- The guard ring and one of the centre electrodes are connected to the voltage supply but only the current through the central electrodes is measured

-Satisfactory measurement can be made in this way at 200C and above, but whit high resistivies and lower temperatures the polarization and reordering of defects and impurities result in an initially high current that takes many hours to fall to a steady state

Alumina( 특성 )-The electrical conductivity of sapphire is the sum of ionic and electronic components, with the relative contributions being a function of sintering temperature, atmosphere and dopants.

-In the case of ceramic form the situation is further complicated by charge transport along gain boundaries

-Figure 5.23 illustrates the general pattern of behaviour for sapphire

-The precise position of the boundaries defining the ‘fields’ for the various mechanisms are by no means certain and depend strongly on impurity content

Alumina( 특성 )-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 value for εr 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.

- Notwithstanding the uncertainties there is no doubt that the general behavioural pattern Indicated by fig.5.24 is correct and typical of insulating dielectrics.

Fig.5.24 Dependence of (a) ε r and (b) tan δ on temperaturs and frequency for a high-purity Al2O3 cermaic

Alumina( 특성 )- 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 grades.

- The success of the ‘moly-manganese’ metal-ceramic joining process rests on resistance to degradation in a hydrogen atmosphere at high temperature

Alumina( 활용 )- 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.

- 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

- Another important application is in klystrons and magnetrons-devices for the generation of electromagnetic energy, sometimes at very high power levels

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

Beryllia- Beryllia has broadly similar properties to alumina but its thermal conductivity is 5-10 times greater.

- It is therefore used as a substrate when thermal dissipation combined with electrical isolation is of major importance.

- In high-power klystrons and power diodes. Because beryllium is one of the rarer elements the oxide is inevitably more expensive than alumina, which limits its use

Aluminum nitride- AIN 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 Al2O3 but is sufficient for most substrate applications

- AIN may compete BeO in some applications since it could be competitive in price

- It may replace Al2O3 as a substrate for silicon chips because of its better heat conductivity and its closer match in thermal expansion.

Glasses- Like single crystals, glasses can be formed into homogeneous bodies that have certain advantages as dielectrics

- They can be extruded directly, at temperatures above their softening point, as thin ribbons down to thicknesses of 25um

- Glasses have high dielectric strengths : approximately 50MV/m when formed into multilayer units, compared with about 10MV/m for ceramics in similar structure

- The disadvantages of glass are a low permittivity and a low thermal conductivity.

- The latter shortcoming limits the use of glasses as substrates despite their ideal smooth surface

Substrates- Substrate require a suitable combination of mechanical, thermal, chemical and electrical properties.

- They must be strong enough to survive the necessary processing to bond components to them and , in turn, to attach them to equipment

- Surface finish is important, particularly if thin layers are to be deposited by

evaporation or sputtering in the manufacture of thin-film components and circuits

- Substrate ceramic are usually required to have low dissipation factors in order to maintain high circuit Q levels

- For most purpose they should also have a low permittivity to minimize cross

coupling between conductor

Substrates- The requirement for microwave substrates are particularly rigorous

- One of their main function is to carry conductive tracks of metal that serve to guide microwave radiation in stripline devices.

- In this case a major need is for an extremely smooth surface to reduce losses in the metal

- There are applications, particularly at the lower microwave frequencies, in which size can be advantageously reduced by the use of higher- permittivity materials.