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United States Patent [151 3,642,527 Purdes Et Al United States Patent [151 3,642,527 Purdes et al. v [45] Feb. 15, 1972 [54] METHOD OF MODIFYING va dielectric substrate, such as barium titanate, may be ELECTRICAL RESISTIVITY modi?ed by ?rst forming a relatively porous substrate which CHARACTERISTICS OF DIELECTRIC may be handled without breaking, as by pre?ring the sub SUBSTRATES strate, masking selected portions of the substrate with a material such as a photoresist material which will vaporize [72] Inventors: Andrew J. Purdes, Pawtucket, R.I.; Ernest during ?nal ?ring of the substrate, contacting the substrate ' M. Just, Plainville, Mass. with a solution of a ?rst reactant, immersing at least a portion [ 73] Assignee: Texas Instruments Incorporated, Dallas, of the substrate in a solution of a second reactant which will Tex. react with the ?rst reactant to precipitate in situ in a portion of the substrate a compound which is insoluble in the solutions [22] Filed: Dec. 30, 1968 and which is adapted to modify the electrical resistivity characteristics of the substrate, and thereafter ?ring the sub [21] Appl. No.: 787,989 strate at a temperature on the order of l,400°-l,450° C. to reduce the porosity of the substrate and to incorporate the in {52] us. c1 ....................................... ..117/212, 117/223 soluble compound into the lattice of selected portions of the [511 1m. (:1 ...................................... ..B44d l/l8 substrate. Where it is desired to dope selected portions of an [58] Field 6: Search .................. ..106/39; 117/212, 213, 123, undoped substrate to the desired thickness and form thick ?lm positive temperature coef?cient (PTC) thermistors, the ‘ 117/223 starting material may be an undoped barium titanate, for ex~ [56] References Cited ample, the solution of the ?rst reactant may be an aqueous solution of a compound such as ammonium hydroxide, and UNITED STATES PATENTS the ‘solution of the second reactant may be an aqueous solu tion of a compound such as lanthanum acetate which reacts 3,296,359 1/1967 Ramsey, Jr. et al .............. ..117/212 x with the ammonium hydroxide to precipitate lanthanum 3,362,851 l/l968 Dunster .................. .. hydroxide in situ and thereby dope selected portions of the 3,4l5,679 12/1968 Chuss ..................... .. substrate. Where it is desired to produce areas of high~electri 3,450,581 6/1969 Shortes ............................ ..117/212 x cal resistivity in selected portions of the substrate, the starting material is a doped barium titanate, for example, the solution Primary Examiner—Alfred L. Leavitt of the ?rst reactant may be an aqueous solution of a com Assistant Examiner-Wayne F. Cyron pound such as ammonium hydroxide, and the solution of the Attomey—Harold Levine, Edward J. Connors, Jr., John A. second reactant may be an aqueous solution of a ferric com Haug and James P. McAndrews pound such as ferric chloride which reacts with the ammoni um hydroxide to precipitate in situ ferric hydroxide which, [57] ABSTRACT when ?red, produces a high-resistivity area. The electrical resistivity characteristics of selected portions of 12 Claims, N0 Drawings 3,642,527 1 2 METHOD OF MODIFYING ELECTRICAL RESISTIVITY CHARACTERISTICS OF DIELECTRIC SUBSTRATES pre?red at temperatures within the range l,lO0°- 1 ,300° C. for approximately I hour to produce a relatively porous substrate This invention lies in the ?eld of dielectric substrates and which may be easily handled without breakage and which does not soften in aqueous media. more particularly relates to improved methods of modifying 5 the electrical resistivity characteristics of selected portions of The selected portions of the pre?red substrate which are dielectric substrates. not to be doped are next masked by means of tape, plastic Heretofore, efforts to produce thick-?lm positive tempera coating, photosensitive resist compositions or other suitable ture coefficient (PTC) thermistors have involved the applica? materials known to those in the art. Among suitable photosen tion of ?lms of. doped thermistor material to a substrate by sitive resist compositions which may be utilized in the practice methods such as silk screening and subsequent ?ring of the of the invention may be mentioned thosemarketed by East resulting ?lm-substrate combination. Such ?lm application man Kodak Company under the trade designations KPR techniques suffer from certain shortcomings. Thus, it has been (KODAK Photo Resist), KMER (KODAK Metal~Etch Resist) difficult to achieve reproducibility of the mechanical and elec and KTFR (KODAK Thin-Film Resist). It is highly preferable trical characteristics of thermistors produced in this manner. that the masking material be one which is adapted to be Films which must be ?red in order to achieve the desired elec completely vaporized or burnt off during the ?nal ?ring step trical characteristics often crack due to nonuniform shrink described hereinafter. age. Cracking may also occur due to variations in thermal ex Following the masking step, the pre?red substrate is placed pansion between the ?lm and the substrate during the normal in a solution of a ?rst reactant which may be ammonium conditions of temperature variations encountered in use. hydroxide, for example, and allowed to become saturated by Thus, there has been a need for a practical method of produc the solutionJFor most substrates, saturation requires approxi ing thick-?lm positive temperature coefficient thermistors mately one-half hour. In order to effect doping of the un which permits effective control of thethickness of the doped masked portions of the substrate, the substrate is then placed layer and ?nal thermistor thickness. in a solution of a second reactant, lanthanum acetate for ex There have also been problems heretofore associated with 25 ample, which is adapted to react with the ?rst reactant and electrically insulating surfaces on selected areas of a semicon precipitate in situ in the substrate and insoluble dopant com ducting substrate, particularly in providing thin insulating re pound, e.g., lanthanum hydroxide. Upon placing a substrate gions in the substrate. saturated with hydroxyl ions in a solution of a soluble Among the objects of the invention, therefore, may be men lanthanum salt such as lanthanum acetate, the lanthanum ions tioned the provision of an improved method of modifying the diffuse into the substrate and are precipitated as lanthanum electrical resistivity characteristics of selected portions of a hydroxide. In this manner, the thickness of the doped layer dielectric substrate; the provision of such a method which per (and the ?nal thermistor thickness) can be readily controlled, . mits doping of selected portions of an undoped ceramic sub being dependent upon the concentration of the two reactant strate to a desired thickness; the provision of a method of this solutions containing the hydroxyl and lanthanum ions and the type which also permits the production of areas of relatively period during which the substrate is contacted therewith. To high electrical resistivity in selected portions of a doped sub determine the desired concentrations, a pre?red substrate is strate; and the provision of such methods which may be readi weighed before and after saturation with water. Knowing the ly and conveniently carried out and which permit controlled weight of water absorbed per weight of substrate, the proper modi?cation of the electrical resistivity characteristics of concentration of the ?rst reactant (e.g., ammonium hydrox selected portions of a dielectric substrate. Other objects and ide) may be readily computed for any desired dopant level. features will be in part apparent and in part pointed out _ The lanthanum acetate solution may then be prepared so that hereinafter. stoichiometric proportions of lanthanum ions are available to The present invention is thus directed to the method of react with the hydroxyl ions, i.e., one La+3 ion is available to modifying the electrical resistivity characteristics of selected react with 3 (OI-l)‘ ions. ' 45 portions of a dielectric substrate which comprises ?rst provid By precipitating an insoluble dopant compound in the sub‘ ing a dielectric substrate which may be handled without strate in this‘manner, ?lms with the desired thickness and breakage, contacting the substrate with a solution of a ?rst homogeneity may beobtained. On the other hand, if the sub reactant, contacting at least a portion of the substratev with a strate were introduced into a dopant solution without previ solution of a second reactant which reacts with the ?rst reac 50 ously having been saturated with a solution of a ?rst reactant tant to precipitate in situ in a portion of the substrate a com ‘such as ammonium hydroxide, the dopant compound would pound which is insoluble in the aforesaid solutionsand which have entered the substrate by capillary action. This type of do~ is adapted to modify theelectrical resistivity characteristics of pant uptake is extremely rapid and very dif?cult to control. the substrate, and causing the substrate to incorporate the in Moreover, with this type of uptake, the doped substrate would soluble compound into the lattice of selected portions of the 55 redistribute the lanthanum ions during drying or ?ring thereby substrate and thereby modifying the electrical resistivity resulting in a ?lm of nonhomogeneous resistivity or semicon characteristics of the selected portions of the substrate. ductivity. With the method of the present invention, the In one embodiment, the invention is more speci?cally lanthanum is precipitated as lanthanum hydroxide and the directed to the method of doping selected portions of an un position of the lanthanum atoms remains ?xed during drying. doped barium titanate substrate to a desired thickness by ?rst To incorporate the insoluble dopant compound into the lat~ forming a relatively porous barium titanate substrate which tice of the surface layer of the substrate and produce semicon may be handled without breakage, contacting the substrate duction, the substrate is ?red at a temperature of approxi with a solution of a ?rst reactant, introducing into at least a mately l,400°~1,450° C.
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