CA1036345A - Dielectric composition for forming electric current regulating junctions - Google Patents

Abstract

TITLE OF THE INVENTION
Dielectric Composition for Forming Electric Current Regulating Junctions ABSTRACT OF THE DISCLOSURE
A dielectric composition for forming electric current regulating junctions that are stable under high temperature conditions during electrical circuit manufacture and operation, which composition comprises a selected high glass transition temperature polymeric binder, for example, a polyimide, and a critical amount, by volume, of dispersed, insulatively coated metal particles, for example, aluminum particles. The junc-tions are insulative when first formed, but when assembled in a circuit and treated with a high frequency, high voltage dis-charge of a Tesla coil, there is sufficient flow of electric current to make the circuit controllably active. Thereafter, the electrically activated Junctions can be switched between low resistance and relatively high resistance values by supplying either an electric current or current pulse to effect transition to relatively high resistance or a high frequency, high voltage discharge to effect transition to low resistance.

Description

BACKGR~UMD OF T}E INVE~TION
: (1) Fleld o~ the In~entlon Thi9 lnvention relates to electric current regula-ting ~unctions that can serve as electrlcally actuated swltches,

(2) Description o~ the Prior Art New developments in the semiconductor ~emory lndustry include altera~le contacts by means of which semiconductor devices arranged in a re~ular array can be 3erlally connected with conductors according to a predeter~lned pattern so that some devices in the array are actl~e and others are not. Such a system i3 called a read-only memory system slnce lt cannot ;
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~ 0363~5 store in~ormation but is u~erul ~or screcning computer input or output. The proce~s of selectlvely connectlng only those ~emiconductor dev~ce~ in ~he array tl~at are lntended to be active is re~erred to as wrlting into the read-only memory.
Generally, there is prepared a cornplete regular array of seml-conductor devices, even though some will not be used in estab-lishing the desired pattern,with an interconnected ~unction composition between each semlconductor device and at least one of its complementary heat~bondable contactlng conductors.
The electrical resistances o~ certain Junctions are altered by simple electrical means to make semiconductor devices active ln the desired pattern. As the connective link between the con-ductor and the 6emiconductor device, the Junction composition -must be highly conductive to avoid exce~sive heat which i~ pro-duced by electrical power d~ssipation i~ the ~unction and which can limit the packaging densities o~ ~emiconductor devices per unit volum~.
In summary of the above, element~ for den~ely packaged ....
read-only memory array~ must contain Junctions that can display low electrical resistance, either when first formed in contact with a ~emiconductor device or when activated electrically to make a semiconductor device controllably active. Junction com- -positions must be thermally stable at temperatures commonly used to form ohmic alloy contacts of metal conductors to semi-conductor device~ in the same and neiehboring elements in a read-only memory array after the Junction has been formed.
The ~unction must be able to function as an electrlcally actuated switch to make a semiconductor deuice controllably active or 1nactive. For example~ it must be able to change ~rom low resi~tance to high re~istance to deactivate the semi-conductor de~ice in order to correct errors made in writing ~ ~ .
., . , ~ 036345 lnto tllc mcmory or to m~ke substantlnl ch~nges in the pattern of the memory.
, Switchllke de~ices which are made wlth mlxtures o~
electrlcally conducting and insulatlng materlals Or the art may :.~
not be sultable rOr use as Junction composltlons becau~e they do not af~ord sufficient range of electrlc current regulation, they pre~ent too much electrical resi6tance, or they are not ;, . .
thermally stable enough to wlthstand processing condltlons commonly u~ed in microcircuit and integrated clrcuit technolo6y-10 For example, such devices may not be able to withstand heating to temperatures at which alloy bonding of contacting conductors to semiconductor devices commonly takes place, or they may in their conductive state show about lO0 ohm~ of re~istance per contact rather than a few mllllohm~ o~ resistance per contact, which low resistance may be necessary to avoid damage to the ~- read-only memory components, including the Juniction com-., j .
posl*lon ltseir, Dy tne cumuiative nea~ prouu~u 'v~ ie ; contacts durlng computer operatlon.
Junction compositlons which are prepared by the vapor . 5 20 deposition o~ pure metal dlrectly onto a semiconductor substrate ~uch as alllcon mQy not be sufficient to establish satisfactorily ~ low contact resistance values because of the potential inter-"~ erence of ~ust a few monolayers of silicon dioxide which may J be present on portions of the semiconductor substrate in a memory array. The metals which generally are employed for vaporization are those that can be heated after vapor deposition j to form a met~l alloy with a sllicon substrate at reasonable i temperatures, such as 350-450C., to in~ure long~lived ohmic ; ~ ' ~, contacts o~ acceptably low conductlvity. Aluminum is an example of such a vaporizable metal~ -'' .

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~U ~ ~Y OF THE INVENTION
It 19 an ob~ect o~ thl~ inventlon to provlde a dl-electrlc ~unctlon composltlon that i9 stable at temperatures commonly used to form cont~cts bebween co~ductors and semicon-ductor devlces in read-only memory systems. It 18 another ob-; ~ect to provide a low reslstance current regulatlng ~unctlon that allows high packaging den~lty of elements ln a memory system. It is an addlti~ al ob~ect to provide an alterable read-only memory ~unctlon composltlon that can be swltched between low and relatlvely hlgh reslstance states by slmple electrlcal means to correct errors or make changes ln a pattern o~ active semiconductor devlces.
In summ~ry, thls inventlon ls directed to a dielec--s trlc compo ition that is userul ln electrlc current regulatlng Junctlons that are st~ble at 200-450C. and that can be actl-vated to a low resi~tance state, typlcally under lO ohms, by exposur~ to a hlgh ~requency, hlgh voltage electrlcal di~charge, ~ for example, a Tesla coll~ sald ~unction in the low resistance ; state being capable of pas~ing a relati~ely low reading current, said ~unctlon being capable of being swltched between low resis-tance and relatlvely hlgh resistance states elther by p~ssine through the ~unction a relatlvely hlgh sel~-extinguishing current greater than the low reading current ( to e~ect transltion to the high reslstance st~te ) or by exposlng the ~unction to said hlgh frequency, hlgh voltage electrical di~charge (to effect transition to the low resistance state~, The composition com~
prises a polymerlc binder havlng a glass transition temperature (Tg) o~ at le~st 100C., preferably at least 150C~, and 40-80%, ; by volume, prefer~bly 45-60~ by volume, o~ metal partlcles 30 having an insulative coating and dispersed in the blnder, m e

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balance to achieve lOO volume ~, thAt i~, 20-60 volume ~, i5 substantially the b~nder. m ls ~nventlon i3 also directed to the electric current regulating Junction which is produced by the electrlcal discharge activatlon of the dielectrlc composltion o~ this lnventlon. By electrical di~charge activation is meant the develo~ment of a capability to assume more than one state of electric resistance ~n respon6e to externally applied electrical slgnals.

DETAILED DESCRIPTION OF THE INVENTION
The invention herein resides in the above-described dielectrlc composition and the useful electric current regu-latlng junctions that are produced therefrom. me polymeric binder in said composition must have a glass transition tem-perature (Tg) of at least 100C., preferably at least 150C., it must be unreactive with said metal filler particles, and lt mu~t be capable of withstanding the thermal stress which is applied during the manufacture Or the ~ystem of which it i~
., a part, for example, during the formation of low resistance ohmic contacts between conductors and semiconductor devices.
In ohmic contact formation, heat is used to form an alloy of a metal, generally deposited by vacuum evaporation, and the particular semiconductor materlal used. Typical in the art are alloy bond~ between aluminum metal and a silicon semicon-ductor or between gold metal and a germanlum semiconductor.
Since the dielectric composition i8 present on at least a por-tion of the semiconductor surface during the heatlng step em-ployed to effect alloying of the conductor component to the semiconductor ~urface, and since the alloying temperatures are normally in the range 200-450C., the filled binder must be stable at such temperatures. For example, gold~germanium con-tact3 are heated at 300C. for about 5-30 minutes during their

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103634s production, while the more common aluminum/silicon contacts require similar times at 450C. durlng thelr productlon.
In general, ~or use with selenium and germanium whlch require a contact-forming temperature of 200-300C. for 5-30 mlnutes, the binder polymer Tg can be as low as 100C.; with the 8ili-con devices which require higher fabrication temperatures the polymer Tg should be at lea~t 150C. The filled binder~s ablllty to withstand degradation and loss of operatlveness during the high temperature ~unction contact-formlng step can be determined by a simple test which lnvolves heating the com-position at the selected alloying temperature for 2 hours and determining the weight 1O8B. Dielectric compositions showing a we~ght loss of not more than ~%, preferably not more ~han 1%, ' are useful as junction-forming compositions. When solvent casting is to be employed to deposit a ~unct~on composltlon ln a contact-forming procedure, the solvent should be removed by heating the composition to lOO~C., under reduced pres~ure if necessary, to achieve constant welght, before heating to the alloying temperatures employed in the test.
Included among the polymeric binders which are u~e-ful in the ~unction-forming compos~tions of this invention are polymers whlch contain small amounts o~ solvents or other ; material~ which may slightly reduce their glass transition temperatures by acting as plasticizers. Normally, such addi-tlves have little ef~ect upon the thermal atability o~ the -i polymers since they are usually low boillng and ea~ily volati-i lized. In microcircuit preparation, such additlves should be ; removed from the polymers by volatilization before v~cuum depo~
~ition o~ the metal conductor~ to avoid out-gassing problems and contaminatlon of pump oll.

- 6 ---Representative polymerlc binders that have Tg values of at least 100C. include organic polymers, typlcal examples of which can be selected from the well known polybenzlmlda-zoles, aromatic polylmlde~, poly(aml~e-imldes), poly(ester-imides), polysul~ones, polyamldes, polycarbonate~, poly-acrylonitrlles, polymethacrylonitriles, polymethyl methacry-lates, polystyrenes, poly~a-methylstyrenes) and cellulose trlacetates. Representative members of these cla~ses and their Tg values are listed in Table I. Generally, the ; 10 higher the Tg the more thermally stable the polymer i~ as a blnder ln the ~unction composition. This generally may not be true if there ~8 a degradative interaction between the polymer and the metal filler particles, for example, as 18 the case with cobalt particles and polylmides. Exten~ive data on Tg value~ are available in the art.
TAB~E I
Or~anic Polymer~ Tg (C.) ; Aroma~ic polyimide (DAPE-PMDA) 380 Aromatic poly(amide-imide) (MAB/PPD-PMDA)265 Aromatic polysulfone 190 Polycarbonate 150 ;
Aromatic polyamide (70% IP/30~ TP-MPD) 130 Polyacrylonitrile 1~0 ~oly~a-methylstyrene) 130 Polymethacrylonitrile 120 Polymethyl methacrylate 105 Cellulose triacetate 105 Polystyrene 100 ; DAPE ~ di~minodiphenyl ether 3 P~DA ~ pyromellitic dianhydride - MAB C m-aminobenzoic acid :

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PPD - p-phenylenediamine IP - isophthaloyl chloride TP ~ terephthaloyl chloride ; MPD = m-phenylenediamine Aromatic polyimide~ having a Tg of at least 100C., preferably at least 150C., represent a preferred class of polymer~ whlch are useful herein as binders. Such polyimides and their preparation are well known in the prior art, for example, as shown by U.S. Patents ~,179,6~0; ~,179,6~1;
3,179,6~2; ~,179,633; ~,179,634; and 3,287,311. Useful poly-imides can be represented by the formula _ _ ., O O
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- N R / \ ~ - R' - whereln ~C/ \C/
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_ n n i8 an integer sufficiently large to provide the deslred polymer ~g, R is a tetravalent radical derlved from an aromatic tetracarboxyllc acid dlanhydride, the aromatic moiety ha~ing at lea~t one ring of six carbon atoms and characterized by . .
benzenoid unsaturation, and R' is a divalent radical derived from a dia~ine. Aromatic tetracarboxylic acid dianhydrides ~hich are u~eful for preparing operable polyimides include thos~ wherein the four carbonyl groups of the dianhydride are each attached to separate carbon atoms in a ~enzene rlng and wherein the carbon atoms of each pair of carbonyl groups are directly attached to ad~acent carbon atoms in a benzene ring.
Examples of dianhydrides suitable for forming polyimlde bin-ders include pyromellitic dianhydrlde; 2,~,6,7-naphthalenetetra-- . . _ _ .

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1036~45 carboxylic dianhydride; 3,3',4,4'-dlphenyltetracarboxylic dlanhydrlde; 1,2,5,6-n~phth~lenetetracarboxyllc dianhydrlde;
2,2',3,3'-diphenyltetracarboxyllc dlanhydrlde; 2,2-bls(3,4-dl-carboxyphenyl~propane dianhydrlde; bi~(3,4-dlcarboxyphenyl~-sulfone dianhydrlde; and 3,4,3',4'-benzophenonetetracarboxylic dianhydrlde.
Organic diamlnes which are u~eful in the preparation of operable polyimides include those which are represented by the formula ~2N-R'-NH2 wh0rein the divalent radical R' ls selected from aromatic, aliphatic, cycloaliphatlc, comblnations of aromatlc and aliphatic, heterocyclic, and bridged organlc radicals, the latter wherein the bridge atom i9 carbon, oxygen,~
nitrogen, sulrur, silicon or phosphorus. R' can be un~ub3ti-tuted or substltuted, a~ i8 known in the art. Preferred R' ., radicals include tho6c vhich contain ~t least six carbon atoms and are characterized by benzenoid unsaturation, for example~
p-phenylene, m-phenylene, biphenylylene, naphthylene and ~ ~ R ~

; wherein R" is selected from alkylene or alkylidene having 1-3 carbon atoms, 0~ S and S02.
The diamines déscribed above also can be used in the formation of the polyamlde binders. Among the diamines preferred in the formation of polyamide and polyimide binder~
are m-phenylenediamine; p-phenylenediamine; 2,2-bls(4-amino-phenyl)propane; 4,4'-diaminodiphenylmethane; benzldine; 4,4'-.
diaminodiphenyl sulfide; 4~4'-dlaminodiphenyl sulfone; 3,3' diaminodiphenyl sulfone, and 4,4'-diaminodiphenyl ether, .
_g_ 1036;~45 As dlsclosed ln the prior art, some polyimides are not ea~ily fabricatable because of their hlgh meltlng polnts.
With such polylmides, the metal particles which are requlred in the composition of the pre~ent invention are lntroduced during the preparatlon of the polyimide. For example, they can be added to the polyamic acid, a fabricatable intermediate in the formation of the polyimide. As i8 well known, the polyamic acid can be dissolved ln a suitable carrier solvent.
Employlng such techniques, the metal particles can be dis-persed ln a polyamic acid ln a carrier solvent, the amountsof polyamic acid and metal particles being such that upon conversion of at least part of the polyamic acid to polyimide and removal of at least part of the carrier solvent, there will be produced the previously described polyimide-metal particle compo~itlon. Such polyamic acid-carrler solvent- ~
metal particle compositions possess dielectric characteris- ~ -tlcs and can be shaped as desired prior to the conversion of polyamlc acid to polyimide and removal of carrier solvent.
A particularly preferred polyimide binder having a Tg of about 380C (by measurement of electrical dissipation factor) can be prepared from 4,4'-diaminodiphenyl ether and pyromellitic dianhydride by employing the precursor polyamic acid in N-methyl-2-pyrrolidone (available commercially under the trademark PYRE-M.L. Wire Enamel RC-5057). m e polylmide -produced from such a polyamic acid and having aluminu~ parti-cles dispersed in it can withstand a temperature of 450C for sufficient timc to form (by alloying) ohmic low resistance contacts of aluminum conductors to silicon semiconductor de-vices in the same or neighboring elements of an altera~le read-only m¢mory array, and it can withstand continuous use at 220C.

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Aromatic poly~mlde~ havlng the requlslte Tg repre~ent another clasq Or preferred organlc polymers ror use a8 a blnder in thls ln~entlon. Such polymers ~re dlsclosed ln U,S. Patents :- 3 oo6 ,899; 3 og4 511; 3, 232, glo; 3 240, 760; and 3, 354 127. Gne such polymer whlch is u~eful hereln can be represented by mula ~COC6H4CONHC6H4NH tn wherein n i8 an lnteger ~ufflclently large to provide the deslred polymer Tg, Particularly preferred is a polymer of such formula wherein the -COC6H4C0- units are isophthaloy~ and/or terephthaloyl unlts and the -NHC~H4NH- units are m-phenylenediamlnelunlts, one such partlcularly preferred aromatic polyamlde binder can be obtained by reaction of es~entially equimolecular quantities of m-phenyl-enediamine and phthaloyl chloride, the phthaloyl chloride being a mixture of about 70 mole ~ isophthaloyl chloride and 30 mole terephthaloyl chloride. Such a polymer having a Tg of 130C, ls sufflclently therm~lly stable at 300C. to allow formation of a gold/germanlum alloy and t is useful, u~ually a8 a 9 olu-tion of the polyamide containlng dlspersed metal powder, for forming electric current regulatlng ~unction~ for germanium devlce9.
The metal partlcles whlch are used in the composltlon of this lnvention are insulatlvely coated so as to block the passage of electric current from one metal particle to another through the polymer binder. Even though the particles have an electrically conductive metallic interior (106 to 103 reciprocal ohm-cm,~, a thin dielectrlc surface coating i9 sufficlent to lmpart contact resistance, even when the particles are touching in the binder, Consequently, the mere dispersing of metal particles in the polymer binder fall~ to provide a low reslstance conductive path, The dielectric surface that :

makes the metal particle in~ulatlve can be formed by coating the surface of the particulate material with an ln6ulative chemical compound of the metal being coated, such as an oxide, sulfide or nitride of the metal. Readily obtained metal~
carrying an oxide coating that renders the aggregate of par-ticles in the binder electrically insulative are aluminum, antimony, bismuth, cadmlum, chromium, cobalt, indium, lead~
magnesium, manganese~ molybdenum, niobium, tantalum, titanium and tung~ten~ A preferred metal i~ aluminum with a tarnish film of insulative aluminum oxide which is readily iormed by exposure to ambient atmospheric condltions.
The average size of metal particles useful in this invention is in the range of about 0.01-l,000 microns. The thinner the current regulating ~unction desired, the finer ~hould be the partic}e slze. Particles which are black ln color, that is, have a particle size that is smaller than the vislble wavelength of llght, are preferred. The size of such particles is about 0.01-0.5 micron. The metal particles are present in the composition of thiY invention in an amount which is sufficient to achieve the electrical activation which i9 marked by a sudden initial transition to a state of low resistance, upon activation, but not in such a large amount that the physical strength of the binder is adver~ely af~ected.
The necessary amount of metal partlcles 18 40-8~ volume ~;
this normally includes the amount required for square close packing of the particles in the binder, an arrangement in which the particles are each surrounded by four other particles of the same slze as the nearest neighbors. Particularly preferred ls an arrangement that provides closest partlcle-to-~0 particle approach and, therefore, the state of lowest resistance 1036~4s Apon electrlc~l ~ctlvation. For the preferred alumlnum partlcles about 40-80 volume ~ corresponds to about 60-go weight %, Such a composltlon thus comprlses about 60-~o weight % of alumlnum partlcles and, the balance to achle~e 100 welght ~, about 10-40 welght % of polyme~lc blnder.
Amounts of aluminum below 60% may provlde insuf~lclent range of electrlc current regulatlon and may present too much electrical resistance, Amounts above 90% may make the compositlon crumbly and may make the surface of a Junction layer uneven, Corre~ponding proportlons by welght of other klnd~ of metal partlcles will vary with partlcle density but they are readily determined by one skilled in the art, The normally lnsulative but electrlcally actlvatable composition of thls invention i8 a form-retalnlng solid because of the stif~ness of the blnder material employed, m e composition and the ~unctlon can be formed ln any of many known ways for homogeneously disperslng metal partlcles in a polymeric blnder and for obtaining a current regulating ~unctlon ln the ~orm of a layer o~ desired thicknes~ and ~hape, For example, a coating on 8 semiconductor ~ubstrate can be conveniently obtalned by applylng a ~olution of the blnder ln a sultable volatile 801-vent, with the metal particles belng dispersed therein. m e dl~persion can be applied to the æemiconductor device by palnting, spraying~ dipping or other conventlonal techniques lnvolvlng evaporative drying to form the binder-disper~ed metal partlcle ~unction, Aæ already indicated above, when a high melting polylmide is employed as the blnder, it may be more conveniently handled as lt~ polyamic acld precursor di~solved in a suitable 3o ~olvent, Such a polyamic acld solution can be employed ln the aforesaid Junction-~orming procedure, m e polyamic acid solvent :

should strong-y associate wlth both the polyamic acid and the poly~lide polymer that i8 sub~equently produced and it should be removable by volatllization. Sultable solvents lnclude N,N-dlmethylformamide, N,N-dlmethylacetamlde, dimethyl sulfoxide, N-methyl-2-pyrrolldone and tetramethyl urea. After belng coated on a substrate such as a semlconductor the polyamlc acid can be readily converted to a polyimlde in situ by heating to effect ring closure with eliminatlon of water; at the same time the carrier solvent is volatilized off.
If the polymeric blnder 18 readlly meltable, a ~unction can be made by casting or extruding onto the seml-conductor substrate a polymer melt contain~ng dispersed metal particles. Alternatlvely, a film of the composition can be cast on a support, stripped therefrom, and pressed agalnst the ; sem~conductor contact to make the ~unction.
; The current regulating ~unction usually is a layer whose shape and dlmen~ions depend on the partlcular use.
Those skilled in heater constructlon can readily determine the dimenslons of reslstlve heatlng elements. When in contact wlth N-type or P-type conductive regions of a semiconductor device, layer thickness may be 1-100 mils (25.4-2,540 mlcrons) in microassembly elements or 0.1-25.4 microns ln microcircult elements. Its width i8 usually at least twice its thickness; its length along the semlconductor sub~trate may be considerably greater. Electrlcal resl~-- tance through the layer in the unactivated state is usually at least 108 ohms and i8 typically greater than 101 ohms.
~he electrical resistance of the dielectric compositlon of this lnventlon ln the unactivated state~ for example, in a 3o layered form uaeful a~ a current regulating ~unctlon, results - 14 _ Q~ _ .
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from the dielectric nature of the polymeric blnder and from the lnsulative coating on the metal particles dispersed in the binder. It can be overcome, however, by sub~ecting the composition to an electric potential whlch exceeds a threshold value in that it is ~ufficient to cau~e electrical activation.
It can be postulated that during such electrical ~ctivation, high voltaee electrical discharges break down the insulative coatings on the metal particles which are in the path provided by the electrical dlscharges passing through the ~unction to the semiconductor device. The first discharge appears to form ;
at least one conductive path through the ~unction layer. As a result, electrical activation is marked by a 6udden initial tran3ition to a state of low resistance. Subsequent disch~rges may form additional conductive paths which are substantia}ly parallel and further lower t~e resi~tance. Eventually, a point of diminishing return is reached when the difficulty of forming more paths outweighs the incremental decrease in resistance that ~uch paths provide. To reach this point in a rea~onable time, a means for supplying high voltage discharge~ at a high frequency generally is used, although a succession of individual dis-charges, for example, from a charged condenser, can provide the same result. A convenient means ~or electrical activation of a ~unction utilizes an induction coil, commonly called a Tesla coil. It i~ a special form of transformer used to obtain very high ~econdary voltages from small direct current voltages.
me tran5former primary c~rcuit leads through an automatic ~reak that is opened and closed at-a high frequency of about 500-2~000 cycles per second. Discharges of unequal strength occur in the secondary circuit upon the opening and closlng ~0 of the circuit breaker. me ~unction of this invention i8 placed in the 6econdary circuit. Low ~unction resl~tance is .
' :, ~U36345 prererably obtalned by (1) including an alr gap in the secon-dary circuit~ for exampleJ by brlnging the termlnal point of the Tesla coil to a distance of about one quarter to one half inch away from the ~unction to favor the break dlscharge and make the discharge current ~maller and more undirectional, (2) moving the Tesla coil terminal point from side-to-side to direct the discharges over the surface area of the ~unction to create many breakdown paths, ~d (~) insulating the ~unction and the contacting semiconductor device from electrical ground by placing the element either by itsel~ or as a part of an array on a supporting surface that is an electrical insulator, thuæ reducing the voltage drop and current flow in the ~unction.
Normally~ Tesla coil discharges for about 1-5 seconds are sufficient to reach the point of diminiæhing return in ef~ecting ; transitlon Or the dielectric composition of this invention to a state of low resistance. Once that state i8 reached, the rl resistance value, which generally i9 in the range of a few milliohms to ten ohm~, remain~ essentlally unchanged during the application of a small testing or reading voltage, for example, 5 milllvolts, the reading voltage being ~pplied to cause current ~lo~ ln order to sense that the semiconductor device is active.
` Upon activation the ~unction i8 capable of switching betwe~n low and relatively high resistance states that make the semiconductor device either active or inactive. Switching can be accomplished by alternately connecting a means for supplying an electric current or current pulse, to effect transition to a relatively high resi~tance value at least two orders of magnitude higher than the low resistance value ~ attained by activation, and to a means for supplylng high _ 16 -frequency, high v~ltage electrical discharges, to effect transi-tion to the low resistance value. A two order of magnltude resistance difference between the two states i9 more than adequate for making semlconductor devices controllably actlve or inactive. Digital circuitry requires only about one order of magnltude change, that i9, a change by ~ factor o~ ten.
Value~ of relatively hlgh resistance may range from about 250-500,000 ohms and usually are ln the range 5,000-200, 000 ohms, Occasionally, values of relatively high resistance of thln ~unctlons for microclrcuits may be in the range 10-250 ohms, in whlch case the correspondlng values of low resis-tance will be ln the milliohm range, Varlous means for supplying self-extingui~hing or switchlng-off currents or current pul~es can be used to e~fect `~
transltion to relatively high reslstance value~. A convenlent means i8 a regul~ted voltage source with a limiting series re~lstor whlch together compri~e a current supply capable of about 50 milliamperes output for 0.1-1 second, Durlng thls brie~ time, the current through the ~unctlon ls observed to decrease steadlly to about zero (extlnguishment~, suggesting th~t the metallic conductlve materlal in the conductlve paths is sub~ect to ~witching. m e tranæition can be effected even more suddently (rast decay, that i~, self-extinguishment~
by a capacitor discharge whlch provides 25-50 milli~mperes - peak current or pulse for about one millisecond, suggesting that the conductive paths are broken suddenly by a rapidly - falllng current pulse. About the same switching-of~ current is surficient for relatlvely thick ~unctions in mlcroassemblles a8 for thin ~unctlons in microcircuits. A suitable switching-off current value i~ readily determined for a ~unction by ~ trial using a selected current means, ThiS value, generally :

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in the milllampere range, will always be greater, generally at least ten times greater, than the readlng or testing value ; of the active operative device. The reading value can be ln the microampere range i~ the reading instrument is ~ufficiently sensitive.
; Subsequent trans1tion from a relatively high resis-tance value to about the same low resistance value obtained by electrical activation of the ~unction can be accomplished in the manner disclosed above for electrical activation.
Although one cyclic transition to a relatively high resls-tance value and a return transition to a low resistance value is enough to establish the switchlike nature of the ~unction compositi~n of this invention, switching can be carried out in at least 10 to as many as lO0 cycles, depending upon the allow-able power dissipation in the low resistance Junction when the semiconductor device is active. Even though this number of cy~le~ is insufficlent for many storage memory application~, ~ it is ample for the repair or conversion of a read-only memory ; in the field.
Elements compriæing a combination of an electric current regulating ~unction of this invention and a semi-conductor device are generally useful in alterable, high packaging density~ memory systems made up of a plurality of such elements in an array with affixed contacting con-- ductors and suitable writlng and reading means. The con-venlence o~ changing the pattern of active semiconductors in an arraY by simple electrical means makes repair or conver-sion of a memory system in the field very simple. Durin~ the manufacture of arrayea elements the pre~erred polyimide-~ased current regulating Junction can withstand the elevated tempera~

, turcs commonly u~ed to form ohmlc low resistance contacts be-tween metal conductors ~nd semlconductor devlces, for example, 450C, the temper~ture which is used ln alloy bonding Or alum-lnum to sillcon. Also, regarding element manu~acture, the ~ormation of a ~unctlon by polymer coatlng technlques may be simpler than by vacuum deposltion techniques. Other demon-strated applications of the composltion and Junctlon of thls invention include heating elements and conductive paints for radlo and televlslon clrcult bo~rds.
In the followlng examples all parts are by weight unless otherwise noted.
ExamPle 1 A. To prepare a ~unction for a microassembly, the fol-lowing procedure was used. Two part~ of a commercially avail-able aluminum powder (having an average particle size of about 20 microns) were di~persed wlth stirring in a solution of one part of a solid polyamlde in N,N-dimethylacetamide. The poly-amide was a high mslecular weight condensation polymer o~
equlmolecular portlons Or m-phenylenediamine and a mixture of 70 parts of lsophthaloyl chlorlde and 30 parts of tereph-- thaloyl chloride. me polyamide had a Tg of 130C. The mix-ture was then poured onto a plate coated with a film of TFE-fluorocarbon reæin available under the trademark TEFLON, - which had been preheated to 50C; it was then heated to 150C
to evaporate off the solvent and form a film. The film was pressed to a thickness of about o.6 mm with an lron coated with TEFLON TFE-fluorocarbon resin, which was heated to 150C.
B. A mechanical ~ig was fabricated bearing a 7 x 7 array of silicon diodes available rrom Motorola under the trade de-signatlon IN4005, each with a spring-loaded contact available from Everett and Charles. The aforesaid film was inserted rL,~ I

?

~ 03~i34S
lnto the Jlg so that the spring-loaded contacts pressed agalnst it. Conductors 1 through 7 were arrAnged in rows contactlng the free diode terminals. Conductors A through G were ar-ranged in column~ contacting the free slde of the fllm oppos-ite the film surface in contact wlth the sprlng-loaded con-tacts. The ~lg lllustrates a æystem for a read-only memory that contains electric current regulating ~unctions and semi-conductor devlces in serles, each combination comprising an element of the memory. With proper alignment of the spring-loaded contacts, each element of the array can be separatelyactlvated to a low resistance value by a 250 volt electrical discharge of a 0.002 microfarad capacitor and can be switched thereafter between low and relatively high resistance value~
by previously described means.
Example ?
A. A ~unctlon-formlng dlelectrlc composltlon was pre-pared as follows. Two parts of black, surface oxidlzed alum-lnum powder having a partlcle size of about 0.01-0.03 micron were dlspersed wlth mortar and pestle ln five parts of commer-cially available PYRE-M.L. Wire Enamel RC-5057. The enamel was a 16.5% solution o~ a polyamic acid (15.2~ converted polymer solids) ln N-methyl-2-pyrrolidone carrier solvent.
me enamel solution was cast onto a smooth clean surface and heated at 135C for 0.5 hour, then at 300C for 1 hour to com-plete the formatlon of the polylmide binder for the dispersed aluminum particles and to evaporate off the carrier solvent and the water of condensation (formed during conversion of polyamic acid to polylmide). The resultlng cured film of the ~unctlon composition was about 0.5 micron thick and its elec-trical reslstance wa~ at least 108 ohms, measured through thefilm between the top surface of the fllm and the bottom sur-face of the conductlve support.

B
. .
:.~

B. The ~ilm was transformed from a high resistance dielectric state into a low resistance conductive state by treatlng the element a~ follows. A conventional Tesla coil capable of providing high frequency, hlgh voltage di~charges was positioned over the film at a height of about 0.5 inch and discharges from the coil were directed over the entire surface of the film while the element was supported on an in~ulative ceramic stand. The coil was turned off after about 2-~ seconds. The electrical resistance was again measured and found to be les~ than 0.1 ohm. Similar ~llms formed on conduc-tive metallic surraces and similarly electrically actlvated show resistance values of about 0.005-0.02 ohm, a few percent of the typical ~orward re~istance of a silicon diode : ' c. me capability of the above ~unction to be switched between low and high resistance states and its capability to retaln such switchability over a period o~ time was demonstrated as ~ollows. The ~unction was connected through contact conduc-i tors to a regulated Lambda power supply ad~usted to provide a current of about 25 milliamperes through a current l~miting resistor. By oscilloscope observation o~ a signal proportional to the current ~low to the element, a steady decrease of the cur-rent ~ abcutæero was ob~erved to occur in about 100 milliseconds The electrical resistance o~ the element was a~ain measured and found to be about 250 ohms, a resistance which is sufficlently high to make a semi~onductor diode inactive in regular computer operation. me ~unction was cycled back to its low resistance state by means o~ the Tesla coil as described above. The cycling was repeated three more times and the element was - stored in the active state. After three months the element 3 ~5 examined and was shown to have retained its activity.
' ~

~ - 21 -:`' :: .

~0363~5 D. me above data and the hlgh degree of thermal stability of the polyimlde binder materlal ~ugge~t that the ~unctlon composition o~ this examplemaybe particularly suited for coating semiconductor subs~rates that are to be sub~ected to relati~rely high temperatures, such as silicon-based seml-conductor devices whlch can be prepared as ~ollows. Semi-conductor diodesof monocry~talllne slllcon can be formed from a melt by conventional growth technlques (using a seed) to form a slngle crystal, doped, slllcon bar which can be sllced into wafers or substrates. Subsequent diffuslon of a P-type dopant such as boron into the ~urface will produce a current-rectifying diode. A coating of the abo~e-de~cribed dispersion can be applied to a portion of the surface of the P-type region of the diode to form a layer which for microcircuit applications can be about 2-5 microns thic~ and several mlcrons long and wide. Then, aluminum conductive layers can be produced on both the ~unction and the neighboring surface of an N-type silicon ~ub-strate 5-10 microns distant from the ~unction u5ing the conven-tlonal photo-aided proce6sing and vacuum evaporation techniques.
After suitable masking, ~hotopolymerization and stripping of photo-resist material to limit the deposit-lon of aluminum to the specified surfaces, the ~unction layer can be heated at ; 135C. for 0.5 hour and then at 300C. for 1 hour to produce a cured ~unction about 0.5 m~cron thick and having a resis-tance, measured in the forward direction o~ the dlode~of at least 108 ohms. In ~uch a ~unction aluminum particles would -be dispersed in a rigid, thermally stable, polymeric polyimide binder. Such a ~unction in combination with the semiconductor device would comprise an element suitable for an alterable ; 3 read-only memor~. The alwninum conductor on the N-type sili-con substrate can then be alloyed to the silicon by further " .

heating the element and the conductor at 450C. ror about 5 ~0 minutes until localized melting nt the interface of the aluminum and the silicon forms a thin solid solution of aluminum in silicon that freezes out on cooling to form an ohmic low resis-t~nce contact. The junction may discolor because of the thermal instability of a few parts per million of a flow agent added to the polyamic acid enamel to produce a smoother surface, but it remains rigid and suffers less than ~ weight loss at the ; above alloylng conditions. At this stage, the ~unction composi-tion~semiconductor element can be activated by exposure to a Tesla coil discharge under conditions previously described.
In ~his way, the reæistance of the element in the forward direction can be reduced to essentially the forward resis-tance of the diode alone, that i8, about 0.45 ohm. m e ele-ment can be switched between the active low resistance state and the inactive hlgh resiætance state by the cycling techniques previously described.
,. .
Example ~
Readily obtained particles of the metals shown ~n 20 the following Table II and carrying oxide coatings obtained by exposure to ambient atmospheric conditions were dispersed ln an equal volume of each of two different insulating poly-meric binders, namely, the polyamide binder of Example 1 and a commercially available polystyrene binder (as a 20% solution in toluene). One inch square Junction layers of the metal particle-binder compositlons were formed by casting and drying films to thlckne~es of about 0.~ mm. Each film was then transformed from a high resistance state(at least 108 ohms` , through the layer ) to a low resistance state by high frequency, high voltage di~charges of a Tesla coil directed over its surface for about 5 seconds. Table II includes values of Y~

.

~036345 surface resist~vity (in ohms per square) that were measured between conductor electrodes afflxed to opposite edges of the square shaped ~unction layers, Sueh layers are sultable for i service as electrically actuated switches using means previously described to effect transltions to relatively high reslstance values and to effect transitio~ back to essentially the same low reslstlvity value3 shown in Table II.
TABLE I~ .
.. .. ..
Junctlon Reslstivity (ohms/sauare~
Metal Polyamide Binder Polyst~rene Bi~der .
antimony 37.2 ' 44.2 bis~uth ~1.2 18.2 cadmium 13.2 . , .3.7 o~u~ . 2.4 cobalt 2.2 ~.7 , lndium, 2.0 , 1.4 lead ' ,99.2 . 13.2 ma~nesium 1.0 ' 1.0 ' molybdenum . 1.2 1.3 nio~ium 10.2 . 7.2 t~talum ' , ' . 9.2 ` . 8.2 . ' titan~um , 38 . 52 tungster~ ' ' 2,7 , ,,1.7 Example 4 a This example demonstrates the utility of this invention in the production of electrical heaters~
One part by weight of non-leafing bright aluminum j powder was dispersed with stirring in an N,N-dimethylacetamide ,` solution containing 1 part by weight of a solid polyamide ~ 0 condensation polymer of equimolecular portions of .

lU36345 meta-phenylenedlamlne and a mlxture of 70 parts of lso-phthaloyl chlorlde and 30 part~ Or terephthaloyl chlorlde.
The polyamide had a gla~ transltlon temper~ture o~ 130C, A slmple heater wa~ made by coatlng a 4 lnch by 6 lnch porcelaln plate with the solutlon and drying on a hot plate held at 100C, (to form ~ coating about 25 mils thlck), Two copper wires were attached to opposite sides of the coating wlth dots of ~ilver palnt, The he~ter was then electrlcally activated to an electrical resistance of less than 100 ohms between the copper wlres by pa~sing a Tesla coil over its surface at a di~tan~ce of separation of ~bout 0,25 inch, A surface thermometer was placed in the center of the heater and contacted via sillcone grea~e through some insulating lens p~per, The heater showed hlgh stability in tlme wlth recycling between room temper~ture and 150-200C, in air over a period of one month by applying up to 100 volts (alternating current) between the attached leads.
The heater was useful as an electric space heater for home use and as a novelty food tray heater and bottle warmer, . .

103~;345 SUPPLEMENTARY DISCLOSURE

It is an ob~ect of the present lnvention to pro~de a dielectric ~unction composition that can be electrically activated to a low resistance state. It is another ob~ect to provide an activated ~unction that does not revert spontaneously at ordinary temperatures to a high reslstance state.
A need exists for compositions that do not requlre ex-tensive processing in preparing electric current regulating ~unc-tions and heaters. A need also exists for electric heatlng ele-ments that retain their resistance characteristics over relatively long periods of time. It is an ob~ect of the present invention to provide a composition that can be electrically activated to form - a resistance-type heating element It is an additional ob~ect to provide such ~unctions and heating elements that can be formed in a simple manner.
Accordingly the present invention further provides a dielectric composition comprising 10 to 90 volume percent of a unitary polymeric binder, the unitary polymer having a glass tran-sition temperature of at least 100C, preferably 150C, and 10 to 90 volume percent of metal particles having an insulative coating and substantially homogeneously dispersed in the binder, the amounts of binder and metal particles totaling 100 volume percent.
; By unitary polymer is meant an organic polymer which does not depend on the presence of additional reactants to achieve its ultimate stiff (high Tg) character. For example, it does not require crosslinking agents such as the polyamines need to cure epoxy resins.
The present lnvention also provides an electric current ~.
~ regulating ~unction that can be electrically activated to a low v"
`~ resistance state, said ~unction in the low rssistance state being capable of passing a relatively low reading current, said ~unction being capable of being switched between low resistance and rela-tively high resistance states, the current regulating ~unction ' ~:. 2 , . .
~ .,:
., ~` , .'' ' ~, .
:~ , 1(~3f~345 belng further ch~racterlsed as comprl~ing 10 to 90 volume percent of a unltary polymerlc binder, the unltary polymer having a gla~
transition temperature Or at least 100C, and 10 to 90 volume per-cent of metal particles having an insulatlve coatlng and ~ubstan-tially homogeneously dispersed in the blnder, the amounts of blnder and metal particles totallng 100 volume percent. Such ~unctlons have utllity ln memory arrays. For this utility, the ~unction-forming compositlon preferably comprl6es from 75 to 15 volume per-cent of said binder ~nd from 25 to 85 volume percent of said metal particles to attaln a low resistance upon actlvation, as exempll-fled herelnafter.
In addition, the present lnvention provldes an electrl-cal heater, whlch may be in the form of a resistor, comprising 10 to 90 volume percent of a unitary polymeric binder, the unitary polymer having a gla~s transition temperature of at least 100C, and ~0 to 10 volume percent of metal particles having an insulative coating and substantially ho geneously dispersed in the binder, the amounts of ~$nder and metal particles totaling 100 volume per-cent, and two affi~xed electrodes in contact therewith, said com-position being activated by electrlcal discharge. For reslstance-type heating elements which involve activation of relatively large areas and use of edge electrode6, actlvatable ~unction compositions preferably contain 35 to 80 volume percent metal particles, the balance bein~ sub6tantially the binder.
In a preferred embodiment the binder and the metal com-ponent are chosen 80 a~ to provide a ~unctlon that is thermally stable at ~emperatures used to form low resistance contacts between conductors and semi-conductor devices, ~or example, temperatures in the range 200-450C. A preferred method of electrical activa-tion is application across the ~unction of a D.C. (direct current)pulse of short duration, usually between a microsecond and a milli-second. ~180 generally appllcable for electrical activation is -- 3e --.

1~3~;345 brief applicatlon of an A.C. (alternating current) potentlal. If the Junctlon i8 in the low resistance state, lt can be swltched to the high resistance state by applicatlon of a small current-limlted pulse of about 0.1 to lO milllamperes, the lower portion of this range being preferred, the pulse being regulated so that at the end of the pulse the current drops very rapidly to a new iow value, usually zero. If the ~unction ls ln the high resistance state, lt can be switched to the low reslstance state by application of a voltage pulse, typically between about 10 volts and 300 voltsJ
the pulse being regulated so that at the end of the pulse the cur-rent drops comparatively slowly to a low value.
The stiff unitary polymers which are useful herein in-clude sub3tantially linear thermoplastic polymers and polymer, such as the aromatic polyimides, which although theoretically linear, appear to have some degree of crosslinking between the polymer chains (as evidenced by their high degree of insolubility) due to intermolecular reactions between polymer functional groups.
Despite the possibility Or crosslinking between polymer chains, the use of such polymers provides for "one pot" formulations for producing the electrically activatable ~unctions a~d heaters of this invention since there is no dependence on the presence of add-itional crosslinking components. ThusJ the polymeric binders use-ful herein provide for rapid and simple preparation of the ~unc-tions and heaters.
~- m e utility of the composition of this invention is con-siderably dependent upon the glass transition temperature (Tq) of the polymer used. It has been found that for stability of the low reslstance state of the ~unction at room temperature a poly-- mer having a Tg of at least about 60C i8 required (Example 5, Table IV)o However, since memory ~unctions and heaters may be sub-~ect to considerably higher temperaturq~s in use) for the purposes of this invention polymers with a Tg of at least 100C are employed z~
_ ~ _ ~ `
.. .
', ~

~ 036345 and those wlth a Tg of at least 150C are preferred. Use of such high Tg polymers mlnimlzes the tendency for such memory Junctions to pass spont~neously from the low reslstance state to the hlgh reslstance state (Example 7) and results ln heater resistance stab-lllty even at temperatures whlch exceed the Tg o~ the polymer (Ex-ample 9).
When the ~unctlon is to be employed ln contact wlth a semi-conductor devlce, the metal for the metal particles of the ~unctlon composltion of this invention should be chosen so as to be compatible with the underlylng semi-conductor surface. The metal chosen can be a non-dopant for the semi-conductor host, for example, slll~on, of the contactlng surface reglon of the semi-conductor device. It can also be a dopant for said semi-conductor host provided the type of conductivity it would normally impart ~s the same as the glven type of the contacting surface region of the semi-conductor devlce. In other wordsJ the metal is chosen so that it would not dope said semi-conductor host in the opposite sense to that in which it was already doped ln the formation of the semi-conductor device. For example, it ls well known that al-uminum is a P-dopant and antimony ls an N-dopant for silicon.
Hence, a ~unctlon containing aluminum is compatible with a sill-con surface doped in the P sense and a ~unction containing anti-mony is compatible with a sllicon surface doped in an N sense.
A mixture of metals can also be used provided the mlx-ture is compatible with the semi-conductor device, as discussed - above. Such a mixture may be in the form of particles of an alloy and/or in the form of mixed pure metal particles.
me slze of the metal particles can range from about 0.01 to 250 microns. Preferred is the size range from about 0.05 ; 30 to 20 microns. In general~ the average particle size is no great-er than one tenth (0 1) the junction layer thicknessO

~'' -: - - , . : . :
,. ' ' ' ' ~ .

The metal particle~ may be present in the composltlon of the ~unction layer of the present inventlon in an a~ount which is su~ficient to permit electrical activation. Generally, the amount of metal partlcles is lO-90~ by volume, usually at lea~t 25% by volume and preferably 25-85~ by volume, to attaln low re-sistance upon activation. As mentloned aboveJ for large area heat-er unlts with sdge electrodes, 35 to 80~ by volume of metal part-icles is preferred so that the electrical resistances of such units remain relatively constant for long periods of time at elevated operating temperature. The balance is substantially the polymeric binderO The amount of metal particles should not be so large that the physical strength o~ the binder is seriously deteriorated or the surface of the ~unction is excessively rough.
e electrical resistance of the dielectric ~unction composition in the unactivated state results from the dielectric nature of the polymeric binder and from the insulative coating on the metal particles dispersed ln the binderO It may be overcome, howeverJ by sub~ecting the composition to an electrical potential which exceeds a threshold value in that it is sufficient to cause ~0 electrlcal activation.
A preferred method of electrical activation is application of a D~Co (direct current) pulse of short dùration, usually bet-ween a microsecond and a millisecond, across the elementO The polarity of the pulse is unimportantO When the thickness of the ~unction is less than 25 microns, a potential of 5 to 15 volts is usually suffi~ient; for greater thicknesses, potentials up to sev-eral hundred volts may be required. Preferably, the current is limited to about lOO milliamperes, although llmltatlon to lower currents may be required if sensltive components lie in the current path~
Also generally applicable for electrical activation is the brief application of an AoCo (alternating current) potential~

In this case, the application should not be significantly more ~`,.

1()3~34S
brlef than l/2 cycle. In other respects, the procedure i~ as des-cribed above for D.C. activation.
If the ~unctlon ls ln the low reslstance st~te, it can be switched to the high resist~nce state by appllcatlon of a small current-llmited pulse of about 0.1 to lO milllamperes, the pulse being regulated so that at the end of the pulse the current drops very rapidly to a new low value, usually zero. For the pulse, the lower portlon of the stated range iB preferred, although occa-sionally currents as high as 50 milliamperes may be requiredO By a current~limited pulse is meant a brief flow of current which is kept below a predetermined magnltude by a means such as a resistor in series with the swltchlng device.
If the ~unction is in the hlgh resistance state, it can be switched to the low resistance state by appllcatlon of a vol-tage pulæe, typically between about lO volts and 300 volts, the pulse being regulated so that at the end of the pulse the current drops comparatively slowly to a low value.
Various means for supplylng swltching-off currents or ~ -current pulses can be used to effect transition to relatlvely hiEh resistance values. A ~convenient means is a regulated voltage source coupled with a limiting series resistor, which together comprise a current supply capable of about 50 milliamperes output for 0.l-l second. During this brief time, the current through the junction is observed to decrease steadlly to about zero, sug-gesting that the metallic conductive material in the conductive ~ paths is sub~ect to swltching. m e transition can be effected ; even more suddenly (fast decay) by a capacitor discharge which provides 25-50 mllliamperes peak current or pulse for about one millisecond, suggesting that the conductlve paths are broken sud-denly by a rapidly falling current pulse. About the same switch-ing-off current is sufficient for relatively thick ~unctions ln microassemblies as for thin ~unctions in microcircuits. A suit-, -: _~ _ ' .. ':' ' ' ' . :

able switching-off current value ls readily determlned for a ~unctlon by trial using a selected current means.
The voltage required to switch from the high reslstance state to the low resistance state is less than that required to activate the same ~unction. Otherwise, the procedures for activa-tion and switching from the high resistance to the low re6istance state are essentially equivalent. However, it will be obvious that for very rapid switching the use of A.C. voltage for swltch-ing from the high resistance to the low resistance state is some-times impractical.
An alternate method of electrical activation or switch-ing from the high resistance state involves use of an A~Co dis-charge. Frequently useful for this purpose is the device commonly known as a Tesla coil or vacuum tester (commercially available);
however, with such a device it is difficult to precisely localize the activation. Normally, a very brief application of the vacuum tester discharge is sufficient, the electrode of the vacuum tester being kept out of contact with he ~unction.
The electrical resistance of the ~unction in the low resistance state, as measured with the Simpson ohmmeter, Model 269, ranges from less than one ohm to about l,OOO ohms, usuallg being in the range of 5 to 50 ohms. Said resistance is lowered by use of large size metal particles, activation with a high volt-age (in any manner), or making the current path through the ~unc-tion shortO Higher low resistance state values of resistance oc-cur if the volume proportion of metal is below 25%.
The resistance in the high resistance state (for a ~unc-tion which has never been treated with a Tesla coil) i9 typically greater than 108 ohms. Once a ~unction has been treated with a Tesla coil, the resistance of any subsequently attained high re-sistance state is from 103 to 107 ohms.
The ratio between high resistance and low resistance states for any given ~unction generally is at least 102, suffici-`. 3Z
~ A`i4 : .

1036345ent for wide utility. Dlgltal solld state circultry usually func-tlon~ well wlth one order of magnltude change, although a greater change may be deslrable to allow a margln of safety. Both high and low resistance states are stable for perlod~ of months and are unaffected by appllcation of small reading currents.
Other demonstrated appllcatlons of the compositlon and ~unction of the present invention lnclude heating elements for space heaters, food tray heaters and bottle warmers, and conduc-tive paints for radlo and television circuit boardsO Those skllled in heating constructlon can readily determlne heater dlmensions and resistance obtalnable, for example, by AoC~ discharge activa-tionO
m e followlng examples serve to lllustrate further the features of the present invention. In the examples all parts are by weight unless otherwlse noted.

. ~ .

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rj;
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Example 3 1~6345 A ~unction composition was prepared by mixing 1.0 g of a commercially available aluminum powder with 2.5 g of the polyam~c acid solution of Example 2 and 3 ml of dlmethyl-~oramide. A microscope slide (1 inch x 3 inches) was modified by transversely applying a series of 5 pairs of rectangular strips of adheslve-backed aluminum ~oil. The palrs of ~trlps were arranged BO that they almost met along the center of the slide and each strip extended to the edge of the slide. m e pairs of strips were numbered 1 to 5 and the gap of separation where each pair of strips failed to meet was designated d (given in mils). m e strips were 1/8 inch wide. Each gap and a portion of the correAponding pair of strips was then covered with a layer of the Junction compositlon about 2 or 3 mils thick. The prepared slide was then dired at 135C for 1/2 hour, followed by a 1 hour at 300C, the resultant Junction comprlsing about 18 volume percent aluminum and 82 volume percent polylmide binder. The Junctions were then activated wlth a Tesla coil as previously described and the reslstance i 20 of each was measured to give a value L-l (all resistance mea-surement~ were made with an ohmmeter available as SIMPSON*
Model 269 and are given in ohms). Each Junction was then switched to a high resistance state by momentary application of 18-20 volts. me resistances were again measured giving ; values H-l. The Junctions were switched back to the low re-sistance state with the Tesla coil, giving resistanccs L-2.
Continuing this pattern, the Junctions were further switched back and forth in the same way, glving reslstanceq H-2, L-3 H-3, L-4, H-4~ ... up to and including L-22, H-22 (as shown in Table II).

,., ~ * denotes trademark .

,.~ ,,~l , N ~ K o~ ~ K
~1 ~I N N
: m L~ O
.~ 'p4 '~4 ~P4 ~ 4 N Lr~
~ 2 N 2 N Lr~
N -- .
* * * * ** *
~*****
~ 0~ 0~

H O t~
~3 ~ ~ ;
~ a)c~ Q)m * * * * * ~ ~-. C~ I o. o o o o ~ .
~ ~ m N N N N N ~ ,~
.~ 0 0 L ~ :;1-O~ `
o o 1~ o ~ ~ ~ ~ . ~

2 2 cu 2 2 X o 2 2 2 A
~I O 11~ 0 o o ,~ o~ u~ u~ ~ N
:~
* * * ~ *
o o o u~ u~ ~m ~ :
:"l h d~ ~ ~ N N N N ~1 Ir\ ~ F~ .~
.: ~ Cl5 ~ l N N ~ N ~ 0 .:~, U~ 1~ h :! ~ a a~ ~
-` h ~ h 0 .. 1 c~
- ~ ~ H N ~) ~ 15~ N ~) :~ In . * *

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1()363~5 Example 4 Junctlon compositlons were prepared uslng the alum-inum powder (varying weights) and the polyamic acid solut1on ~10.0 g) of Example 3. Also as in that example, microscope slldes were prepared, but wlth gaps approximately 1/8 inch.
: The gaps were covered with the Junction composition to a depth o~ about 6-10 mils and the slides were baked at 135C
and 300C as in Example 3. Each Junction (after reading the resistance, H-l, wlth a Simp60n ohmmeter, Model 269) was then ; 10 actlvated by the application of 600 volts through a 104 ohm : reslstor and the reslstance was measured (L-l). Twenty volts was applied momentarlly across the resistor and the resistance was measured again (H-2). Then, 50 volts was applied through a 104 ohm resistor and again the resistance was measured (L~2). m e last two treatments were repeated (H-3 and L-3 ~ were obtained). The results are shown in Table III.

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. ~ ... .

103634S ~ ~
~ ou~oo aooo ooou~
U~ ~ o ~I 0~ ~o .... .... ....
C; ~; C C: d ~ ~ ~ C; ~
HHHH HHHH HHHH
~q O N ¦ O O O O O O O O O In O O
--~¦ N 2 ~ 2 --l o u~cu o~
, ~ :. :
~, .... .... .... ..
~i N ~ ~ ~ ~ q~
m . c c ~ H H C H H C C C H
. cs ,',, ~ ,~ oooo oooo ooo~
l ~ U~ O 0~ 0 0 0 0 ~ O 0~ N
H~ tU rl C~J t~ C~l ~ N ~C) r l ~I r~l :- ~ . ...
H :~ . -~3 ~1*..... ' .... .... ''~': '~.'.
~,~ ~q~ 6-, d d ~ ~:: C d 1: C C C C ~
H H H H H H H H H H H H
.;
~,' ~ .
o o o o o o o o ~q~I r~ l N N N N ~ ~1'') ~f) ::
_ O
_ 1~ = - = N = - = ~ -1 . . . q-~
., ~ O O
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~ ~ ~ N ~ N ~)::t H N t~
~ ,,,, 7,,, ,,,, ~ --cB ~1 ~1 ~ Cll N N ~1~ (~ (~) H
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, Example 5 103634s Compositlons were prepared wlth equal weights of a com-mercially available aluminum powder and various binders. Each composltlon was coated onto a microscope slide to form a layer about 20 to 50 microns thick (after drying). Fllms were drled 24 hours at room temperature and then 4 hours at 40-50Co Volume percentages of aluminum metal ranged from about 25 to 40% in the drled compositions. Two dots of a commercially avallable silver palnt were applied on the surface approximately 1 cm apart to serve as electrodes. Each coating was then actlvated by the application of 400 volts D.C. (direct current) through a 10,000 ohm reslstor, contact belng made by touchlng the palnted dots. m ereàfter, re-slstances between the dots were measured periodically wlth a Slmpson ohmmeter Model 269, as shown in Table IV.

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Example 6 1~634S
The loadlng of a commerclally available aluminum powder in a commerclally available polystyrene and ln varlous polylmlde blnders was studied as a function of resistance after electrical ~ctlvatlon. The samples were prepared by dispersion (via hand stirring) of the aluminum powder in solutions (20 g polystyrene/
100 ml of toluene) of the polymer and coating these on 1 inch x 3 inch glass mlcroscope slideæ (about 4-5 mils thick)~ The poly-styrene samples were simply allowed to air dry for several days at 50C, since the polystyrene had been dissolved in tolueneO
The polyimide samples were treated in accordance with the follow-ing instructions for each of the three different types:
A. 5057 Polyimide - Alr drled at 80C overnight, then held for , , 30 minutes at 135C and 60 mlnutes at 300C. Used was a 1501%
solution of polyamic acid in N-methyl-2-pyrrolido~e.
B. Polyimide 10301-91 - Air dried at 80-90C overnight, then cured at 250C for 30 minutes. Used was a 22.3% solution of polyamic acid in cresol.
C. PYRE ML*-lll - Air dried at 50C overnight, then cured at 250C for 30 mlnutes. Used was a 31.3~ solution of polyamic acid ln acetone.
After the fllms were drled and/or cured, sllver palnt strlps were painted (uslng a commercially avallable silver palnt) ; one lnch apart on each slide; the slides were allowed to dry over-nl~ht. The resistances were then measured with a Simpson 269 Serles 2 VOM. The slides were activated with a commerclally avail-able Tesla coil and the resistances were measured again. The values obtained are in ohms/square since the dried films were about 1 mil thick.

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~36345 The data are li~ted ln Table V. The data show that the ~unctlon is always non-conductlve (initlal resistance 101 ohm~/square) be~ore electrlcal activation. The data also show that a non-conductlve/conductlve threshold exists somewhere between 7 and lO volume percent alumlnum. Flnally, an intermediate region seem~ to exlst between 10 and 25 vol- ~ .
ume percent where the reslstance after activation i8 depend- - -ent on loadlng, especlally between lO and 15 volume percent.
Above thl~ intermedlate region the reslstance after actlvation is independent of loading.

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Example 7 103~;345 A comparlson was made of the stabillties of the low re-slstance states of two ~unctions, one made with a polyimide blnder havlng a Tg suitable for this lnvention and the other made wlth a commercially available model hobby cement (AMBROID* Liquid Cement) having a Tg of about 45C. A commercially available aluminum pow-der was employed in both ,~unctionsO In ~ manner essentially as described in Example 3, test specimens were prepared as 1/8 lnch wide strips on a 1 inch x 3 inch glass microscope slide which had two 1/8 inch wide aluminum foil electrodes pre-glued to the slide to form a 1/4 inch gap.
me initial resistances were greater than 101 ohmsO
me slldes were placed on a hot plate and maintained at a temper-ature of 125C in air overnightO The slides were then removed and activated with a Tesla coil. The resistances~ were then mea-sured and the slides were returned to the hot plate and maintained at the 125C accelerated aglng temperature. The resistances were then measured (cold) with the Simpson VOM (on the R x 100 scale) from time to time. These results are llsted in Table VI.
me results listed in Table VI show that whereas the AMBROID cement ,~unctions became infinitely resistive after 24-36 hours at 125C, the polyimide ~unctions remained relatively con-stant even after 132 hours at 125Co This confirms that high Tg materials, such as a polyimide, are capable of producing ~unc-tions having a longer life in the "on" state at elevated tempera-tures and are, therefore, superior to low Tg polymeric binders All the ~unctions of this invention disclosed in Ex-amples 3 through 7 are suitable for use in direct contact with a compatible semi-conductor device as illustrated in Example 2 or as heating elementsO

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- Example 8 1~6345 Readlly obtalned particles of the metsls shown ln the followlng Table VII and carrying oxide coatings (obtalned by ex-posure to amblent atmospherlc condltions) were dlspersed ln an equal volume of eaeh of two dlff,erent lnsulating polymerlc binders, namely, the polyamlde binder of'Example 1 and a commerclally avail-able polystyrene binder (a8 a 20~ solutlon ln toluene). one lnc~
square Junction layers of the metal partlcle-binder composltions were formed by casting and drying fllms to thlcknesses Or about o o.6 mm. Each fllm was then transformed from a hlgh reslstance state (at least 108 ohms through the layer) to a low resistance state by hlgh frequency, high voltage dlscharges of a Tesla coil directed over lts surface for about 5 seconds. Table VII includes yAlue8 of re81sti~ity ( in ohms ~quare) that werQ measured betwe~
conductor electrodes affixed to opposite edges Or the square shaped Junctlon layers. Such layers are sultable for service as electrlcally actuated swltches in combination with semi-conductor devices using means previously descrlbed to effect transltlons to relatively hlgh reslstance values and to effect transitlons back to essentlally the same low reslstivity values shown in Table VII.
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Junctlon Resistivity (ohms/~quare) Metal Polyamlae B~naer Polystyrene Blnder antlmony 37.2 44.2 blsmuth 31.2 18.2 cadmlum 13.2 3.7 chromlum 4.0 2.4 cobalt 2.2 3.7 l~dlum 2.0 1.4 lead 99.2 13.2 magnesiu~ 1.0 1.0 molybdenum 1.2 1.3 nlobium 10.2 7.2 t~talum 9.2 8.2 titanium 38 52 tungsten 2.7 1.7 ~ . ~ , r.~ . "; ' I

Example 9 1~6345 Th18 example demonstrate~ the utllity of thl~
lnvention in the productlon of electrlcal heater~, One part by weight o~ a commercially avallable, non-leaflng, brlght a~uminu~ powder was dlspersed with stlrring in an N,N-dimethylacetamlde solution containing 1 part by weight of a solid polyamide condensation polymer of equimolecular portions of meta-phenylenedlamine and a mixture of 70 parts of isophthaloyl chloride and 30 parts of tere-phthaloyl chloride, The polyamide had a glass transltiontemperature of 130C, A simple heater was made by coating a 4 inch by 6 inch porcelain plate wlth the solutlon and drying on a hot plate held at 100C to form a coatlng about 25 mils thlck.
Along each 4-inch e~ge a copper wire electrode wa~ -bonded to the surface with silver paint. m e heater was then electrically activated to an electrical resistance of about 60 ohms between the copper wlres by means of a discharging Te~la coll passed over its surface at a distance of separation ~' of about 6 mm, A sur~ace thermometer was placed in the center ; o~ the heater and contacted via sllicone grease through some insulating lens paper, The heater showed high stabillty in tlme with cycling between room temperature (at nlght) and 150-200C tdurlng the day) ln air o~er a period of one month by applylng up to 100 volts (alternating current) between the attached lead6. m e heater was useful as an electric ' space heater for home use and as a food tray heater and bottle warmer, .
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Claims (35)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A dielectric composition that is stable at 200-450°C., said composition comprising 20-60 volume % of a polymeric binder having a glass transition temperature of at least 100°C. and 40-80 volume %, to achieve 100 volume %, of metal particles having a dielectric coating comprised of an insulative chemical compound of the metal and dispersed in the binder.

2. The composition of Claim 1 wherein the polymeric binder is a polyimide.

3. The composition of Claim 1 wherein the poly-meric binder is a polyamide.

4. The composition of Claim 1 wherein the poly-meric binder is a polystyrene.

5. The composition of Claim 1 wherein the poly-meric binder has a glass transition temperature of at least 150°C.

6. The composition of Claim 1 wherein the metal particles having an insulative coating have an average size of 0.01-1,000 microns.

7. The composition of Claim 1 wherein the metal particles having an insulative coating are aluminum particles.

8. The composition of Claim 7 comprising 10-40 weight % of binder and 60-90 weight % of aluminum particles having an insulative coating, the amounts of binder and aluminum particles totaling 100 weight %.

9. The composition of Claim 1 in a volatile carrier solvent for the binder.

10. A composition comprising a polyamic acid, a carrier solvent and metal particles having an insulative coating and dispersed therein, the amounts of polyamic acid and metal particles being such that conversion of at least part of the polyamic acid to polyimide and removal of at least part of the carrier solvent provides the dielectric composition of Claim 1.

11. An electric current regulating junction that can be activated to a low resistance state by exposure to a high frequency, high voltage electrical discharge, said junction in the low resistance state being capable of passing a relatively low reading current, said junction being capable of being switched between low resistance and relatively high resistance states, the low resistance state being switched to the high resistance state by passing through the junction a relatively high self-extinguishing current greater than the low reading current, the high resistance state being switched to the low resistance state by exposing the junction to said high frequency, high voltage electrical discharge, said junction comprising the composition of Claim 1.

12. An electric current regulating junction disposed as a layer that is characterized in having low and high resistance states which can be switched from one to the other, the high resistance state having a resistance which is 10-500,000 ohms and at least ten times the resistance of the low resistance state, the low resistance state being capable of passing a low reading current and being switchable to the high resistance state by passing through the junction a relatively high self-extinguishing current greater than the low reading current, the high resistance state being switch-able to the low resistance state by exposing the junction to a high frequency, high voltage electrical discharge, the current regulating junction being further characterized as being stable to 200-450°C. and comprising 20-60 volume % of e polymeric binder having a glass transition temperature of at least 100°C. and 40-80 volume % to achieve 100 volume %, of metal particles having a dielectric coating comprised of an insulative chemical compound of the metal, and dispersed in the binder.

13. The junction of Claim 12 wherein the polymeric binder has a glass transition temperature of at least 150°C.
and the metal particles are aluminum particles

14. The junction of Claim 13 wherein the layer thickness is 0.1-25.4 microns and the aluminum particles have an average size of 0.01-0.5 microns.

CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE

15. Dielectric composition comprising 10-90 volume % of a unitary polymeric binder, the unitary polymer having a glass transition temperature of at least 100°C, and 10-90 volume % of metal particles having a dielectric coating comprised of an insulative chemical compound of the metal and sub-stantially homogeneously dispersed in the binder, the amounts of binder and metal particles totaling 100 volume %.

16. The composition of Claim 15 wherein the polymer is a polyimide.

17. The composition of Claim 15 wherein the poly-meric binder is a polyamide.

18. The composition of Claim 15 wherein the poly-meric binder is a polystyrene.

19. The composition of Claim 15 wherein the poly-mer has a glass transition temperature of at least 150°C.

20. The composition of Claim 15 wherein the metal particles having an insulative coating have an average size of 0.01-250 microns.

21. The composition of Claim 20 wherein the average size is 0.05-20 microns.

22. The composition of Claim 15 comprising 15-75 volume % of binder and 25-85 volume % of metal particles having an insulative coating.

23. The composition of Claim 22 wherein the particles are aluminum particles having an insulative coating.

24. The composition of Claim 15 in a volatile carrier solvent for the binder.

25. A composition comprising a polyamic acid, a carrier solvent and metal particles having an insulative coating and substantially homogeneously dispersed therein, the amounts of polyamic acid and metal particles being such that removal of the carrier solvent and conversion of at least part of the polyamic acid to polyimide provides the dielectric composition of Claim 15.

26. Electric current regulating junction that can be electrically activated to a low resistance state, said junction in the low resistance state being capable of passing a relatively low reading current, said junction being capable of being switched between low resistance and relatively high resistance states, said junction comprising the composition of Claim 15.

27. Electric current regulating junction disposed as a layer that is characterized in having low and high resistance states which can be switched from one to the other, the high resistance state having at least ten times the resistance of the low resistance state, the low resistance state being capable of passing a low reading current and being switchable to the high resistance state, the high resistance state being switchable to the low resistance state, the current regulating junction being further charact-erized as comprising 10-90 volume % of a unitary polymeric binder, the unitary polymer having a glass transition tem-perature of at least 100°C, and 10-90 volume % of metal particles having a dielectric coating comprised of an in-sulative chemical compound of the metal and substantially homogeneously dispersed in the binder, the amounts of binder and metal particles totaling 100 volume %.

28. The junction of Claim 27 wherein the binder and metal are so chosen that the junction is stable at 200-450°C.

29. The junction of Claim 27 wherein the polymer has a glass transition temperature of at least 150°C and the particles are aluminum particles having an insulative coating.

30. The junction of Claim 29 wherein the layer thick-ness is 0.1-25.4 microns and the particles have an average size no greater than 0.1 the junction layer thickness.

31. The junction of Claim 29 wherein the polymer is a polyimide.

32. The junction of Claim 29 wherein the polymer is a polyamide.

33. The junction of Claim 27 wherein the polymer is a polystyrene

34. The junction of Claim 26 wherein the junction is activated by application of a direct or alternating current pulse.

35. An electrical heater, comprising 10-90 volume %
of a unitary polymeric binder, the unitary polymer having a glass transition temperature of at least 100°C, and 90-10 volume % of metal particles having a dielectric coating comprised of an insulative chemical compound of the metal and substantially homogeneously dispersed in the binder, the amounts of binder and metal particles totaling 100 volume percent, and two affixed elec-trodes in contact therewith, said composition being activated by electrical discharge.