US3469226A - Thin film resistor - Google Patents

Description

Sept. 23, 1969 B. sOLOw 3,469,226

THIN FILM RESISTOR Filed Oct. 26. 1967 .6 I INVENTOR. BENJAMIN SOLOW ATTORNEYS United States Patent 3,469,226 THIN FILM RESISTOR Benjamin Solow, North Hollywood, Calif., assignor to Angstrohm Precision Incorporated, Van Nuys, Callfi, a corporation of Delaware Filed Oct. 26, 1967, Ser. No. 678,330 Int. Cl. H01c 1/02 U.S. Cl. 338-252 9 Claims ABSTRACT OF THE DISCLOSURE A film resistor which is trimmable to a predetermined value and which, once adjusted, is not materially affected by its environment. A resistive film is deposited in a groove formed in an appropriate substrate and a pair of leads secured to this substrate are connected to respective opposite ends of the film. An encapsulating material is formed around the substrate and over the filmfilled groove. The value of resistance is adjustable by grinding or otherwise removing a portion of the film at the upper edges of the groove, until the desired value is achieved.

Background of the invention This invention relates to electrical resistors and more particularly to thin film resistors adjustable to predetermined values.

It is advantageous in many instances to have a resistor which can be adjusted or trimmed to a particular resistance value to suit operating requirements. For example, in an R-C network, it would often be desirable to trim the resistance to select a particular time constant. However, adjustable resistors of conventional design have been found wanting for one reason or another. For example, one known adjustable resistor of conventional design employs a zig-zag or sinuous resistive pattern formed on a flat substrate, with portions of the resistive pattern being manually fracturable to select particular resistance values. This type of component is adjustable only in discrete steps, and, in addition, requires considerable care to achieve the intended adjustment without chipping or breaking the device. Another known adjustable resistor is the potentiometer, which generally includes a wire wound or film resistive path and a contact slidably movable along this path. Such continuously adjustable resistors are, however, subject to the influence of external effects which can afiect the resistance value. For example, the resistance determined by the adjustment of the potentiometer can be aiTected by vibration, which can cause the slidable contact to move, and is also susceptible to disturbance from dust or other contaminants which may affect the electrical connection between the slidable contact and the resistive path. In addition, continuously adjustable resistors are unnecessary and too expensive for many purposes. Further, such devices are not commensurate in size with modern microcircuits since the mechanical structure required in a potentiometer to provide adjustment limits the achievable miniaturization.

Various techniques are known for adjusting the value of certain conventional resistors during a manufacturing process, but such adjustment requires complex equipment and cannot be practically carried out by a user of the device without elaborate apparatus.

In accordance with the present invention, a resistor is provided which is of a size suitable for use in microcircuits and which is either useable as fabricated or which is easily adjustable, for example by a user of the device, after it is fabricated. Once adjusted to an intended value, the resistor is substantially immune to changes in value which would otherwise be caused by external effects.

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Summary of the invention Briefly, a resistor according to the invention comprises an electrically insulating body having an elongated groove formed in a surface thereof, with a film of resistive material disposed within the groove. A pair of terminals are located on the insulating body and in electrical contact with the resistive film. For example, lead wires can be secured within respective opposite ends of the insulating body and in electrical contact with opposite ends of the resistive path formed by the resistive film contained Within the groove. This structure is encapsulated within a suitable protective material. To adjust the value of the resistance, a portion of the resistive film is removed to increase its resistance, for example, by grinding through a portion of the protective material covering the groove and abrading away a portion of the resistive film in the groove. In practice, the device can be connected to an ohmmeter or other resistance measuring instrument while the resistive material is being removed, so that the adjustment can be stopped when the desired value of resistance is achieved. Alternatively, the resistor can be adjusted while it is in circuit to thereby trim a particular circuit parameter dependent upon the resistance value. Once the desired value is attained, a protective material is coated or otherwise applied to the surface of the film exposed by the adjustment operation to complete its encapsulation.

Brief description of the drawings The invention will be more fully understood from the following detailed description, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a greatly enlarged pictorial view of a resistor embodying the invention;

FIG. 2 is a greatly enlarged pictorial view of a resistor according to the invention, with the casing removed;

FIG. 3 is a greatly enlarged elevation view, partly in section, of a resistor according to the invention;

FIG. 4 is a greatly enlarged sectional end view of the invention taken along line 44 of FIG. 3;

FIG. 5 is a greatly enlarged sectional end view of the invention after adjustment of the resistance value; and

FIG. 6 is a greatly enlarged bottom view of the invention.

Referring to FIGS. 1, 2 and 3, there is shown, to a greatly exaggerated scale, one embodiment of a resistor constructed according to the invention. A generally rectangular body 10 of electrically insulating material, such as ceramic or glass, has a pair of leads 12 and 14 secured in respective opposite end portions 16 and 18, and a pedestal or raised portion 20 disposed on one surface of body 10 between end portions 16 and 18. The ends of pedestal portion 20 confronting end portions 16 and 18 can have chamfered surfaces, such as surfaces 23 and 25 visible in FIG. 2, for reasons to be discussed hereinbelow, or the ends of the pedestal portion can be unbeveled. A groove 22 of V-shaped cross-section is formed in pedestal portion 20, extending between end portions 16 and 18. The slope of the groove is selected to provide a predetermined degree of resistance change for a given removal of resistive film, as will be explained hereinafter. Although shown as being V-shaped, the groove need not be of this particular shape but can also be of arcuate or other cross-section.

The resistive path is composed of a film 24 of resistive material deposited onto the surface of groove 22, onto chamfered surfaces 23 and 25, and at least partially onto the upper surfaces of end portions 16 and 18, over which a conductive film has been formed. The outermost edges of resistive film 24 are substantially flush with the outermost edge of pedestal portion 20. As seen in FIGS. 2

and 3, a conductive layer 26 of, typically, gold is formed on the upper surfaces of endportions 16 and 18, and the ends of resistive film 24 are formed over a portion of layer 26. Lead wires 12 and 14 are electrically connected to the resistive path by means of respective lead wire heads 28 and 30 which are secured to conductive layers 26, for example, by conductive cement.

In the illustrated embodiment, the resistive film 24 is formed over a portion of conductive film 26 and the lead wire heads are connected directly to this conductive film. Alternatively, the resistive film can be deposited completely over the conductive film with the lead wire heads connected to the resistive film. Good electrical contact is stilled achieved in this latter instance since th resistive film is of microscopic thickness and, in the region of electrical contact, offers negligible resistance. Chamfered surfaces 23 and 25 provide a suitably configured substrate to accommodate deposition of resistive film 24 between groove 22 and end portions 16 and 18. The transitional surfaces offered by the chamfered ends allow deposition of the resistive film without cracking due to sharp corners, and also minimize chipping of the ceramic body during abrasion of the resistive film during adjustment of the resistance value.

Body 10 and a portion of leads 12 and 14 are encapsulated in a casing 32 of suitable insulating material, such as epoxy, and as seen in FIGS. 3 and 4, the upper edges of the resistive film 24 is covered with a small thickness of encapsulating material. If the resistor is of correct value for a particular use, the resistor can be employed in this form.

In order to adjust the resistance value of the device, the outermost edges of film 24 are removed, for example by abrasion, thereby increasing the resistance of the resistive path by reducing the cross-sectional area of the resistive path. This removal is easily accomplished by running, for example, a grinding wheel or ultrasonic grinder across the top surface of the device to remove a selected amount of resistive material. As illustrated in FIG. 5, edges 34 and 36 of film 24 have been abraded to selectively increase the resistance. The slope of the groove is chosen to provide an intended degree of resistance change for a given removal of substrate material. For example, a relatively shallow groove permits a rather large variation in resistance for a rather small removal of material, while a steep groove permits a relatively small change in resistance for a given removal of material. To expeditiously determine the resistance value, the device can be connected to an ohmmeter via leads 12 and 14 to measure the resistance as the resistive material is being removed. It is evident that only a thin edge of film 24 is exposed to the environment after the abrasion operation. Thus, substantially all of the resistive film remains protected from the environment by casing 32, and the edges of film 24 exposed by the abrasion can be coated or otherwise covered with an epoxy, paint or other suitable material 38 to fully protect the trimmed resistor.

The casing 32 serves not only to protect the device from its environment but also functions as a support to allow grinding of the resistive film without chipping the ceramic body on which the resistive path is formed. Lead heads 28 and 30 can be dimensioned to act as stops to prevent grinding too deeply into resistive film 24. As depicted in FIGS. 1 and 6, the casing 32 is formed on its bottom side with a pair of depressions 40 and 42 which define a centrally disposed ridge 44 which tapers into respective ends 46 and 48. This casing configuration is produced by the mold design which accurately positions the resistor for uniform encapsulation and is useful to denote the bottom of the device; that is, the side opposite to the adjusting surface. The adjusting surface is also identified by the position of the lead wires, which extend from the device near the bottom surface than the top, and could also be marked by a suitable color or other code.

Fabrication of the resistor is accomplished using well known film deposition techniques, and its method of manufacture need not, therefore, be discussed in detail. In brief, layers 26 are sprayed and fired onto end portions 16 and 18 via suitable masks, and the resistive film 24 is evaporated or otherwise applied through a suitably configured mask onto a portion of conductive layers 26 and onto the surfaces of groove 22. The protective casing 32 is then molded or otherwise formed around the device. For some purpose, a paint or glaze coating is sufficient protection.

As a typical example of a resistor embodying the invention, a film of ohms per square formed in a ceramic body with an active area (one square) of 50 mils x 50 mils has an initial resistance of 100 ohms. This resistance is variable to 500 ohms by removal of 40 mils of film width 24. Thus, a variation in resistance of 5:1 is easily achieved by removal of only a relatively minute quantity of resistive material.

The invention is not to be limited by what has been particularly shown and described except as indicated in the appended claims.

What is claimed is:

1. A resistor comprising:

a body of electrically insulating material having first and second end portions and a raised pedestal portion disposed between said first and second end portions, and having a groove formed only in the outermost surface of said raised pedestal portion;

a film of resistive material deposited in said groove to provide a resistive path;

first and second terminals formed on respective opposite ends of said body, each including a lead wire secured in a respective end portion and electrically connected to said film of resistive material; and

a casing of insulating material formed around said body.

2. The resistor according to claim 1 wherein the respective ends of said raised portion confronting said end portions are joined with respective end portions by respective transitional surfaces adapted to support a deposited resistive material, said film of resistive material being deposited on said transitional surfaces.

3. The resistor according to claim 1 wherein the respective ends of said raised portion confronting said end portions are joined with respective end portions by respective transitional surfaces adapted to support a deposited resistive material; and further including a film of conductive material formed on respective end portions;

said film of resistive material being deposited on said transitional surfaces and in electrical contact with said film of conductive material; and

said lead wires being respectively connected to said film of conductive material.

4. The resistor according to claim 1 wherein said first and second terminals each include a film of conductive material formed on each of said end portions and electrically connected to a respective end of said resistive path, said lead wires being electrically connected to said conductive film.

5. The resistor according to claim 1 wherein said groove is of V-shaped cross-section, and said resistive film is deposited on the walls of said groove.

6. The resistor according to claim 1 wherein said V- shaped groove and said resistive film have outer edge disposed substantially flush with the outermost edges of said raised portion.

7. The resistor according to claim 1 wherein the respective ends of said raised portion confronting said end portions are chamfered downward and outward from said groove to respective end portions.

8. The resistor according to claim 1 wherein said first and second terminals are electrically connected to respective ends of said resistive path by means of respective lead heads, said heads being dimensioned to lie below the outermost surface of said raised portion and operative to 5 6 prevent grinding of the resistive path below the plane of 2,700,719 1/1955 Coler 338333 X said heads. 2,883,502 4/1959 Rudner 338333 X 9. The resistor according to claim 1 wherem said cas- 3,012,216 12/1961 Coper 333 322 ing of insulating material has a first thickness over said outermost surface and a second greater thickness over the surface of said body opposite said outermost surface, said second thickness being formed around a portion of said lead wires thereby to indicate the adjustable surface of 3,319,210 5/1967 Sandone 338322 5 E. A. GOLDBERG, Primary Examiner said resistor.

References Cited 10 338-195, 311, 322, 333 UNITED STATES PATENTS 1,767,715 6/1930 StOekle 338-495 1,889,379 11/1932 Ruben 338333

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