COATINGS FOR MICROIONIC DEVICES

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COATINGS FOR MICROIONIC DEVICES

M. Gouet

To cite this version:

M. Gouet. COATINGS FOR MICROIONIC DEVICES. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-119-C1-129. �10.1051/jphyscol:1986118�. �jpa-00225546�

JOURNAL DE PHYSIQUE

Colloque C1, suppl6ment au n02, Tome 47, fgvrier 1986 page.cl-119

COATINGS FOR MICROIONIC DEVICES

M. GOUET

Laboratoire de Thermodynamique et d'Electrochimie des Oxydes, Universite Paris XII, Avenue d e Gaulle, F-94010 Creteil Cedex, France

R6sum6 - Le concept de microionique d6signe la transposition des procBd6s de fabrication de la micro6lectronique & des dispositifs 6lectro- chimiques. Aprhs quelques exemples de dispositifs, sont expos6es des m6thodes de realisation de couches minces et 6paisses.

Abstract - The concept of microionics designates the transposition t o the field of electrochemistry of t h e methods which made possible the spreading of the microelectronics . Some examples are shown, and methods of making thin and thick films a r e reviewed.

I - INTRODUCTION

In the tremendous advances in integrated electronics one main factor has been the improvement of thin and thick film technology. Miniaturization and high integration have many advantages: low energy consomption, collective manufacturing, reliability, low cost. Velasco created t h e concept of microionics t o qualify t h e methods used in t h e production of microelectronics transposed t o the domain of ionic solid state.

Microionics concern hybrid, thick and thin fiIm technologies, because it uses different materials for different functions. Nevertheless it is conceivable in some cases t o modify t h e characteristics of ionic materials by doping t o make monolithic circuits. For example, ceria doped with some per cent of calcia or yttria is an ionic conductor, and doped with tantalum or niobium oxide it is an electronic conductor, so it is possible t o imagine monolithic cells made only with doped ceria.

The transposition of the methods of the microelectronics t o t h e ionic materials domain is difficult, because ionic materials are generally polycrystalline, with complex structures and compositions, and because interfacial phenomena still poorly understood a r e often limiting kinetically. However, this may sometimes turn in a benefit. Two review papers give details on these topics /1,2/.

[I - EXAMPLES OF MICROIONIC DEVICES.

Isfets and Chemfets sensors

Ion-sensitive field effect transistors (ISFET) and chemically sensitive field effect transistors (CHEMFET) a r e field effect transistors in which the field is created by net electron or ion transfer a t electrolyte-metal and electrolyte-ion exchanger interfaces because of space charge generation /3 ... lo/. The isfet device is generally operated without any reference electrode in t h e liquid, i.e. in t h e floating "virtual

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986118

~ 1 - 1 2 0 JOURNAL DE PHYSIQUE

gate" condition /5,6/. Hybrid technology can lead t o pH and Ca sensitive electrodes in which the Nernst potential generated across a selective membrane (PVC doped with an ion-exchange salt) is applied' t o a source-follower FET. An inner reference material is applied t o the conducting strip before forming the membrane.

Resistive sensors

Several thin film cells are sensitive t o reducible or oxydizing gases, dependant on defect equilibrium processes of t h e electrolyte with the atmosphere, or on electrode kinetics. or example are LaF3, CeF3 or pbF2 cells sensitive t d 0 2 , CO , S02, NO2 and NO. 128, 29, 301.

Potentiometric sensors

Potentiometric sensors with a reference electrode can be made in microionic solid state. As an example, Velasco made an hydrogen microsensor with a planar structure with HUP (Hydrogen Uranyl Phosphate) as an electrolyte, and a metallic hydride as a reference electrode 1311.

Zirconia potentiometric sensors with conventional ceramic structure a r e well known, and attempts a r e made t o make them with thick film 132, 33, 341, thin film or mixed structure 1311. The main use of such sensors is the control of engine exhaust gas, and they work as classical "lambda" sensors. But lambda sensors can only monitor airlfuel ratio a t the stoichiometric value, and a r e poisoned by lead. An other design with a cataly ic diffusion path seems t o clean t h e leaded gas. Somewhere between this diffus~on path and t h e oxygen concentration cell, an electrochemical pump based on a zlrconia membrane can be added, which is used for injecting or removing oxygen from t h e tested gas fraction. Then the tit ration curve is translated quantitatively depending on the electrochemical pump current.

Galvanic cell

The concept of decreasing the cell resistance by using thin film electrolytes has been invoked 1131. However these devices exhibit a high resistance due t o the electrode-electrolyte interfaces. The main advantages of thin film cells a r e the ability t o fabricate large electrode areas easely and t o make miniaturized batteries which could complement other miniaturized electronic devices.

One of the earliest thin film galvanic cells to draw significant currents reported by Yamamoto and Takahashi /15,201 was of the type Ag/Ag3SI/12,C. Berger has patented other silver-based thin film cells for use in microelectronic integrated circuits, with RbAg 415, Ag3S1, Ag714P04 and Ag4SHg14 as electrolytes /16,17,18/). The thick film technique was used by Ervin t o prepare cells of the type Ag/RbAg415 /Rb2Teb 1191. Other examples a r e lithium iodide 121, 221 or lead fluoride 1231 cells.

Coulometers

Coulometers and timers do not require extremely high conductivity or energy storage.

Films offer t h e possibility of cell arrays in which multiple timing or integrations can be accomplished. An electrolytic coulometer tabulates coulombs of charge by plating a metal (or chemical compound) on an inert indicator electrode. A typical example is a cell consisting of a gold indicator electrode and a silver counterelectrode separated by a solid electrolyte containing silver ions. Solid electrolytes investigated include AgBr 1241, RbAg41g 125,261, Ag19t15P207 1271.

Electrochromic displays

Electrochromism is a colour change induced in a material by an applied electric field or current. It was proposed for application in information displays by Dreyer 1371.

Many electrochromic cells which consist of thin or thick films have been patented.

The most widely studied material is W 0 3 which is sandwiched between two electrodes 138, 39, 401 or between electrode and electrolyte 1341. Similar effects were found for MOO 3 I4 1.

Another electrochromic cell uses as a substrate t h e ceramic fast-ion conductor cationic beta-alumina, ( t h e cation being lithiurn, sodium or potassium), all t h e other components a r e deposited in thin film form 1431.

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SUSSTR4TE EVAP CONTACTS

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F4. 4 : A p- thin dien hyhogen-gas n m o h and A e l e c t h i e d h w p o u e a d t u U&co & c o U /31/, and a a c k 6 hz(nconia oxygen 4eMOh inoeuding heating d e m e n t adten Uiddm Mot. Cy /35/.

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111 - PREPARATION OF THIN AND THICK FILMS Chemical vaDour d e ~ o s i t i o n (C.V.D.).

A material may be plated from a vapour if it forms a volatile compound which can readily be dissociated below t h e melting point of the material. C.V.D. is one of t h e most important techniques for the semi-conductor industry and corrosion protection.

The kinetics of reactive film preparation is a complex multi-barrier problem. However reactive film preparation does work reliably, with better epitaxial growth than with purely physi a1 methods. This is explained by the activating effect of chemical reactions: t h e alteration of chemical bonds a t t h e surface region causes sufficient mobility for proper readjustment of t h e particles in the growing film.

For zirconia t h e chemical vapour deposition is obtained by feeding ZrClq vapour into an 0 2 /H2 flame 1441. Hf02 and ZrO2 films can b e deposited by the oxygen-assisted decomposition of hafnium p-diketonates a t temperatures in t h e range 400-500 ^OC. Films obtained a r e fine-grained and close t o stoichiometry 145,461.

Parameters as gas flow, gas composition, geometry of t h e reaction chamber, substrate temperature,. a r e adjusted empirically.

Spraying and Pyrolysis

Another method for metallic oxides is based on t h e thermal decomposition of an aerosol obtained by conventional spraying /SO/ or by ultrasonic spraying 148,491 of a solution of a metallic salt or of an organo-metallic compound (tetrachloride, acetylacetonate, ...I on a substrate a t a temperature around 500 or 600 C. This method only differs from classical C.V.D. by t h e transport mode of the reactive but can be applied t o blendings . It is a low cost process which can be applied t o prepare a wide variety of complex compounds.

Vacuum evaporation

The material is vaporized and condensed on the substrate surface in a vacuum sufficient t o prevent collision between t h e film-forming atoms and residual gas atoms. Thus t h e vacuum deposition process involves atoms having thermal energies (0.1 ... 0.5 eV) and line-of -sight deposition. To avoid contamination of the deposit, suitable support materials for evaporation sources a r e refractory metals and oxides.

Further selection is made by considering t h e possibilities of alloying and chemical reactions between the support and evaporant materials. Resistively heated vaporization sources a r e normally used for low temperature materials, and electron- beam heated sources a r e used t o vaporize refractory materials without pollution.

Some problems arise from incongruent evaporation of compounds: the constituents present in t h e solid or liquid s t a t e differ in vapour pressure, so t h e composition of t h e condensate is not the same as that of the source material. In particular, many oxides ( ~ 1 2 0 3 , BaO, Bi203, NiO, Sn02, Ti02, Zr02, C ~ O 2...) when heated in vacuum or sometimes even in air dissociate t o their lower oxides. Moreover, solid solutions a r e modified in composition. Compositional changes can be predicted from thermodynamics, but in practice, empirical information is t h e most reliable guide in determining t h e experimental conditions necessary t o produce films of t h e desired composition.

Some other special techniques as reactive, multi-source, and flash evaporation have been developed for compounds:

In reactive evaporation relatively high oxygen pressures (10 -5 t o torrs) a r e deliberately maintained t o produce fully oxidized metal films 1561. For example, Biz03 (yellow in bulk), is in vacuum deposited film found t o be shiny and blackish, in contrast t o the transparent films obtained by evaporation with oxygen pressure of 10-4 torrs, or by oxidizing bismuth films in air /53,54/.

C I - 1 2 4 JOURNAL DE PHYSIQUE

The setting of two or more sources in the same evaporation system is widely practiced t o produce multilayer films. By operating two sources simultaneously multiconstituent films can be deposited. To avoid cross contamination,the sources must be separated by shields. It is possible t o codeposit materials which form neither compounds nor solid solutions. An important example is cermets, whose multiphase nature becomes apparent only after anne ling and recristallization. The main problem in this technique is t o control t h e condensation rates ratio.

An other method use an electron beam which jumps between two or more crucibles.

Flash evaporation is a fast volatilization of finely divided particles falling on t h e overheated evaporator . Then t h e less volatile material evaporates without fractionation.

Vacuum evaporation has been used for RbAgq15 122,341, Agl9115 P207 123,241 Li3N 191, AgI 172,731, AgBr 121,731, PbF2 /20,74,75/, LaF3 1251, PbSnF4 1761.

Ion implantation

Recently a method called "ion implantationt' has been developed based on the study of ion collisions. The condensation probability of ions depends on their energy. It is close t o unity in t h e range of thermal energies but decreases with increasing energy values. Moreover it can acquire negative values (sputtering effects). Ions with a sufficiently high energy can penetrate into the specimen, and a r e implanted.

An other method is the field emission deposition (ioncote process, or FED). 1521. A blunt wetted needle is used to generate a coating spray of ions and droplets under t h e action of a high electric field (H.T. typically 12 kV). Coatings of a variety of elements and alloys can be produced in this way, with droplet diameter about 2 pm.

Sputtering

In the sputtering process high energy particles (generally positive gas ions) striking a surface cause t h e ejection of atoms by momentum transfer. Due t o t h e high energy of t h e sputtered atoms (1-100 eV), sputter-deposited films have generally better adhesion than vacuum-deposited ones.

The more simple way t o generate t h e positive ions for sputtering is t o establish a glow discharge by settin8 up a high potential between two flat parallel electrodes in a low pressure gas (10 - t o 1 torr).

When the pressure is lowered, the glow goes out and sputtering stops. But sputtering a t lower pressures should be interesting, to low gas trapped in the film, and to improve the adhesion because of the higher energy of t h e sputtered atoms when they reach the substrate.

In order t o sustain the discharge, t h e ionization can be increased by injecting electrons from a source other than the target cathode. The simplest method is t h e use of a hot filament (triode process).

High frequency electromagnetic radiation can excite a discharge in a gas even without any electrodes in the system. The radio-frequency power can be applied by a coil but is generally fed directly t o the cathode, superposed on the D.C. bias.

When the target material is an insulator, the charge on t h e surface cannot be neutralized, and most of the field is concentrated across the insulator. If the dielectric target is mounted onto a conducting plate t o which a high frequency power is applied, t h e target positive charge is neutralized by plasma electrons during t h e half of the cycle when t h e target is positively charged.

An other method is the neutralization of the ion beam. Neutralization in this context means that enough electrons are added t o obtain electrical neutrality, and does not imply a beam of ground s t a t e atoms. The neutralization removes all macroscopic eiectric fields from t h e substrate 1831.

An important point in t h e sputtering process is that t h e chemical composition of a condensed film will often be the same that the one of the cathode from which it was sputtered.

Reactive gas may be deliberately added t o the sputtering system t o produce compounds in thin film form.

Sputtering was used for AglyI 1 5 P Z 9 1771, Na G a 2 0 3 1781, NapA1203 1791, Z r 0 2 /80,r)l/,

CeO 1841. p

Further improvements may be realized by t h e combination of two or more methods.

Ion evaporation combines low energy ion implantation and thermal evaporation.

Ion plating combines evaporation with cathode sputtering /51,57,58,69/: using an evaporation source with an electron beam gun, r.f. ion plating is carried out in t h e material discharge atmosphere without introducing any gas.

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JOURNAL DE PHYSIQUE

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Screen printing

Thick film circuitry became a significant part of t h e market for microelectronics.

Thick film circuits a r e made by successive operations of screen printing, drying and firing. A wide range of conductive dispersions is now commercially available, including platinum and palladium based inks, s o we should be concerned here mainly by electrolytical inks and specially zirconia inks.

The dispersions have t o b e formulated t o give good screen printing properties and a r e generally a mixture of a glass powder, an active powder (pigment), and a vehicle.

The glass gives cohesive properties, t h e pigment controls t h e conductive or dielec- tric properties, and t h e vehicle is used t o control t h e viscosity during printing.

About t h e s a m e processing steps a r e used t o produce all t h e thick films. The dispersion is forced by t h e action of a squeegee through a stainless s t e e l screen, which bears t h e p a t t e r n t o be generated, t o a substrate. The printed p a t t e r n is dried t o remove t h e vehicle volatile solvents and then fired in a n oxidizing atmosphere .

The residual organic binders a r e burnt off and t h e glassy powder melted t o a coherent film, binding t h e pigment t o t h e suostrate.

F i g . 10 : The phinoipee 06 .the &meen-phinting ulith a a i x o t n o p i c pante.

The organic part of t h e p a s t e has a complex rheological role: Without any mechanical sollicitation t h e viscosity must be high t o avoid spontaneous pouring through t h e screen, or t o avoid spreading on t h e substrate a f t e r printing. But, during t h e printing operation, it must be fluid t o go through t h e screen. On t h e substrate, t h e viscosity must increase but not too quickly, because a lateral limited flowing due t o surface tension a t t e n u a t e s t h e t r a c e s of t h e l a t t i c e of the screen.

The requirements for zirconia films in oxygen sensors besides a good adherence, a r e pure ionic conductivity and lack of open porosity and cracks: physical or electrochemical diffusion of gases through t h e zirconia membrane would quickly deteriorate t h e reference electrode.

On polished alumina t h e adherence of zirconia is good. On platinum electrodes t h e binding is weaker, but sufficient if electrodes and electrolyte a r e co-fired.

Pure ionic conductivity prevents additions of glass powder or other binding materials. The presence of metallic electrodes (platinum)) limits t h e firing temperature and duration.

Chlorides have a n influence o n t h e microstructure of t h e powder and o n t h e sintering /62,63,64/. Commercial powders a r e composed of agglomerates (0.5 ... ² microns) of particles whose dimensions a r e less than 0.2 microns. The highest density agglomerates induce earlier local compaction and porosity when fired. Therefore a dispersion of agglomerates in t h e screen printable paste is a condition for low porosity 1851.

Another method proposed by Nissan 1361 consists in filling up t h e pores of a first film by laying on one or several coatings of ink with smaller particles.

T h e shrinkage during t h e elimination of t h e organic vehicle and during firing c a n reach from 10 t o 20 per c e n t in e a c h dimension. For a screen-printed film on a substrate this retraction must occur only in one direction, perpendicular t o t h e substrate. Prints from t h e screen c a n remain on t h e green film, and initiate fissuring during firing-The solution t o this problem lies in optimization of t h e rheology of t h e paste.

In spite of these difficulties, it seems t o be possible t o prepare tight films of zirconia by screen printing.

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Plasma spraying

Plasma spraying is a well-established process for thermal-barrier coatings. The powder particles injected into a plasma jet a r e quickly melted and propelled onto a substrate where they a r e quenched. The quality of a coating depends mainly on the velocity and temperature of t h e particles prior t o their impact. For successful coatings the feed material must be a free-flowing powder with a particle size range and distribution carefully controlled. Spray powders a r e usually made by crushing of a fused ceramic followed by selective milling and sizing, Gel process uses as feed material a concentrated dispersion of t h e ceramic oxide or oxide mixture, which is gelled t o the required particle size and shape and then calcined t o give t h e free- flowing oxide particles /59,60,61,83/.

IV - PROVISIONAL CONCLUSION

Microionic technology is only now beginning t o be explored. Up t o this point we transposed t o t h e microionic domain t h e methods which were valuable for microelectronics. Materials which a r e potentially useful for solid s t a t e electrochemistry a r e far more numerous than materials for electronics. For each preparation procedures must be adjusted, taking into account compatibilities with the other materials and requirements for technological homogeneity.

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