Loading patent…
AssigneeSociete Francaise de l'Electro-Resistance
InventorsPaul Rene Simon, Bernard LeGrives
Estimating the term…
A process for etching a thin film or foil of an electrically conductive resistive material, preferably an alloy predominantly comprising nickel and chromium, by electrolytic etching under conditions of electrochemical machining above Jacquet's plateau on the I.V. characteristic curve. The process is particularly suitable for manufacturing a planar electrical resistor having a high stability, a low temperature coefficient, and a high ohmic value despite the relatively low resistivity of the film or foil. The anode surface polarization is maintained substantially constant over the surface of the foil or film by removing the by-products of the etching process, i.e., gases, viscous layers and other impurities, by mechanical effects such as an electrolytic flow or mechanical vibration, or both together. These mechanical effects are carefully balanced against the applied potential values such that the rate of formation of the layers and gases at the anode is equal to their rate of removal. Moreover, the anode electrical resistance is kept at a negligible value independent of the progress of the attack, by securing the film or foil to a thick layer of a conductor such as copper, whereby the evolution of potential drop across the foil, due to the electrical current flow, is kept at a negligibly small level. Under these conditions, a quasi-uniform etching is obtained all over the foil or film surface. The operating parameters of the process are also described as well as the advantages of the resistor produced according to the process of the invention.
A process for producing a resistor from a thin film or foil of an electrically conducting resistive alloy comprising the steps of:
Dependencies are inferred from the claim text.
Drawings are temporarily unavailable. Open the PDF at USPTO
No related US applications are recorded in the indexed filing history.
Apparatus or processes specially adapted for manufacturing resistors: using lithography, e.g. photolithography
The resistive element having zig-zag or sinusoidal configuration: lying in one plane
Incorporating printed electric components, e.g. printed resistors, capacitors or inductors: incorporating printed resistors
In which conductive material is applied to the insulating support in such a manner as to form the desired conductive pattern: by affixing…
Term ended 18 Oct 1994 (estimated)
USPTO file history isn’t available for this patent, so only indexed dates are shown.
This invention relates to a process for etching a thin film or foil by electrochemical machining where preferably the foil is an electrically conductive resistive alloy predominantly comprising nickel and chromium. More particularly, this invention relates to a process for manufacturing a planar electrical resistor of the type wherein a thin metallic foil is photo-etched into a pattern of isolated windows and conductive filaments to increase the current path and total resistance value of the foil. Still more particularly, this invention relates to such a process in which the foil is etched from a bonded substrate under conditions of electrochemical machining above Jacquet's plateau on the I-V characteristic curve while maintaining the anode polarization substantially constant over the foil surface to be etched at a value which, for a given flow rate and velocity of electrolyte, or a given amplitude level and frequency of the mechanically vibrated anode, is such that the rate of formation of a viscous layer and oxygen is equal to their rate of removal.
In the prior art, the manufacture of planar electrical resistors is usually accomplished by photo-etching a thin metallic foil into a pattern of isolated windows and conductive filaments in such a way that the current path through the etched foil is significantly increased and the total resistance value of the foil is multiplied accordingly. In the prior art, these resistors may be made with a foil or thin film of a suitable conductive foil, such as one predominantly comprising nickel and chromium, deposited on an isolated substrate and photo-etched with an appropriate acid or, more specifically, with an electrolytic polishing process used in conjunction with an insulating mask. The purpose of this type of etching operation is to form a grid of the conductive foil comprising the largest possible number of conductive filaments for a given surface area, since the resistance final value of the resistor is proportional to the number of filaments and the length-to-width ratio of the filaments.
In practice, the application of the aforementioned techniques is quite difficult and accompanied by a number of significant technical problems. In effect, the use of chemical etching is not entirely satisfactory because of the raggedness and irregularities of the edges of the filament in the grid. Such raggedness or irregularities in the edge contour of the filament significantly and detrimentally affect the subsequent stability of the resistor because of the electric field gradients introduced by these defects if the filaments are too closely adjacent to one another. Consequently, this process of chemical etching is limited to the manufacture of resistors having a relatively low ohmic value wherein the conductive filaments are too narrow or too close to each other. This limited value is typically of the order of 20 k.OMEGA./cm.sup.2 with a foil having a 0.5 .OMEGA./.quadrature. ohmic value.
In the drawings:
FIG. 1 shows an enlarged view, for illustrative purpose and not in detail, in perspective of a planar electrical resistor produced according to the process of the invention;
FIG. 2 shows a magnified view of a portion of the foil configuration produced by the method according to the invention;
In FIG. 1, a typical resistor produced according to the invention is designated generally by the reference numeral 10. The resistor 10 is formed from a metal foil 11 by an electrochemical machining process. The foil 11 is secured to the substrate 12 and the resistor 10 includes a pair of connecting terminals 13 and 14 and a pair of accompanying electrical leads 15 and 16. Typically, a resistor according to the invention has dimensions of about 5.4 by 5.4 mm and a resistance value of about 240K.OMEGA./cm.sup.2 or larger with a foil of 0.5 .OMEGA./.quadrature. resistance. The temperature coefficient of such a typical resistor produced according to the invention is about .+-. 5 parts per million and has a stability of .+-. 20 ppm/year with a load of 0.3W at 125.degree. C ambient temperature.
FIG. 2 is an enlarged view of the metal foil pattern in which foil has been removed from the area forming the slots 18 and leaving foil to form the conductive filament 19. The filaments 19 have smooth parallel edges free from irregularities and the raggedness that is characteristic of conventional chemical etching processes. The filaments 19 are also free from sharp notches. The smoothness of the filaments 19 formed by the electrochemical machining process of this invention permits a substantial decrease in the width of each filament 19 and a significant decrease in the slot width 18 so that the electrical resistance per unit filament length can increase in proportion to the decrease in width thus permitting a smaller overall length of the filament 19. However, according to the invention, the width of the slot 18 is not limited to a distance determined, at least in part, by the zones of preferential attack adjacent the filament 19.
FIGS. 3 through 6 are cross-sectional views of several intermediate steps in the manufacturing process. The resistor according to the invention is constructed of a thin metallic foil 20, on the order of 2.5 to 5.0 microns. The foil or thin film 20 may first be heat treated to adjust its temperature coefficient to a value suitable for further processing. Thereafter, the foil or thin film 20 preferably made from an alloy of nickel and chromium, is bonded to a relatively thick layer of copper 22, for example, 30 to 50 microns thick. One suitable method of bonding the foil to the copper, as is known in the art, is to plate the foil with the copper in a chemical bath. Thereafter, the copper and foil intermediate is glued to a glass substrate 24 by a conductive glue 26 with the foil surface 20 to be etched left exposed at the top thereof. The step of securing the copper to a glasss substrate by a conductive glue is within the skill of the art.