Electrical resistors and methods of making same

An electrical resistor which is fabricated from traces of resistive material on a substrate of insulating material. The traces are interconnected electrically in series by first links and in parallel by second alternating links, which are connected to different terminals on the substrate. The second links are cut, preferably by laser trimming, so as to select the value of resistance of the resistor by reducing the number of traces connected in parallel and increasing the number of traces connected in series. Where the resistance of each trace is "R", the value of the resistance is adjustable by severing the second links from R/n to nR, where n is the number of traces.

FIELD OF THE INVENTION 
The present invention relates to electrical resistors and resistor networks 
which present selectable values of resistance and also to methods of 
making same. The invention is especially suitable for providing resistors 
in thin film or foil form which are laser trimmable to adjust or select 
the resistance thereof. 
BACKGROUND OF THE INVENTION 
Laser trimmable planar resistors and resistor networks are well known and 
are commercially available, from the various vendors including Vishay 
Intertechnology, Inc. (its Ohmtek subsidiary being located in Niagara 
Falls, N.Y.) in a wide variety of configurations. In the patent literature 
there are various patents relating to resistors and resistive networks of 
this general type. See the following U.S. Pat. Nos.: 4,859,981; 4,782,320; 
4,785,277; 4,772,774; 4,582,976; 4,565,000; 4,563,564; 4,386,460; 
4,375,056; 4,362,737; 4,298,856; 4,146,867; 3,983,528; 3,657,692; and 
2,261,667. 
In the manufacture of precision resistors great importance is attached to 
the means of adjusting their ohmic resistance value to a specific, 
targeted FIGURE and to do so with precision and consistency. It is 
desirable to do this over as wide a range of resistance values as 
possible. Such capability permits the manufacture of otherwise-identical 
resistors to a semi-finished state in large quantities, with attendant 
economies of scale. Small quantities of these semi-finished resistors can 
then be adjusted to a final specific resistance value, as required. The 
greater the range of adjustability, the fewer the number of semi-finished 
types which need to be "stocked" to cover the entire range of resistance 
values which may be required in all possible situations. 
This need for adjustability over a wide range is especially acute in the 
case of resistor networks. These consist of a multiplicity of resistive 
elements, generally of different value, which are usually employed as 
voltage dividers. In such cases the effectiveness of the network is highly 
dependent upon all the individual elements possessing nearly identical 
performance characteristics. Performance characteristic generally refer to 
the degree of stability exhibited by the resistor under a variety of 
adverse physical or chemical stresses either externally or internally 
generated. Uniformity in this respect can be assured if the resistors in a 
network are all manufactured in a common production lot. They can then be 
differentiated solely by resistance value in the adjustment operation. The 
greater the range of adjustability of a given type, the less the 
dependence upon different production lots with potentially different 
performance characteristics. 
SUMMARY OF THE INVENTION 
It is the principal object of present invention to provide improved 
adjustable or selectable resistors and resistor networks of extremely high 
precision and very wide resistance range, and to afford methods for making 
such resistors and networks. 
Briefly described, the invention provides a planar resistor having an 
insulative substrate with first and second electrical terminals on the 
substrate. By thin film or foil deposition, a pattern of traces of 
resistive material are deposited on the substrate and forms a multiplicity 
of resistive paths interconnected in series by first links which may be of 
the same resistive material as the traces. A plurality of selectively 
removable connections (second links) interconnect the traces in parallel 
between the first and second electrical terminals. These second links 
extend from the first links, which connect one of the opposite ends of the 
traces in series, to the first terminal and from others of the first 
links, which connect the other of the opposite ends of the traces, to the 
second terminal. To obtain a precise resistance of value between Rn and 
R/n, where R is the resistance of the traces and n is the number of 
traces, the second links, are selectively removed thus removing selected 
parallel connections to provide a desired resistance between the first and 
second electrical terminals. The multiplicity of resistance traces are 
arranged in a generally parallel, uniformly wide and mutually spaced 
relationship. Additionally, in accordance with one embodiment, of the 
invention, the multiplicity of resistances are of generally identical 
length. In accordance with another and presently preferred embodiment of 
the invention, the multiplicity of resistances are of generally differing 
lengths. In accordance still with another preferred embodiment of the 
invention, the selectively removable connections are laser removable. 
Alternatively, connections which are electrically or chemically or 
mechanically fusible may be employed.

DETAILED DESCRIPTION 
Reference is now made to FIG. 1, which illustrates a planar resistor 10. An 
insulative substrate is used. It is typically formed of silicon, glass, 
ceramic or any other suitable dielectric material. Defined on a surface of 
substrate are first and second electrical terminals 12 and 14, which are 
preferably formed of a highly conductive material such as aluminum, gold, 
nickel or platinum. 
Disposed between terminals 12 and 14 is a resistive array 16, made from a 
thin film or foil of a suitable material of precisely known resistance, 
such as Nichrome or Tantalum Nitride or any other suitable material having 
good stability over ranges of temperature and time. The resistive array 
16, if realized in a thin film, is preferably formed by known technologies 
of vacuum deposition, such as Joule effect evaporation or cathodic 
sputtering and photolithographic engraving techniques. If a foil is used, 
conventional techniques for foil patterning may be employed. 
The resistive array 16 has a multiplicity of parallel resistive units 
(traces or paths), each in the form of a strip 18 and each being of 
uniform and identical width, thickness, length and separation from its 
neighbors. 
The strips 18 are connected in series one to another between terminals 12 
and 14, by means of series connections or links 20 which are typically 
continuations of the strips 18 and extend from alternate strips between 
opposite ends thereof. The strips 18 are also each connected in parallel 
between terminals 12 and 14 by means of selectively fusible parallel 
connections (second links) 22, which are also typically defined as 
continuations of strips 18 and extend from connections 20. Connections 22 
are preferably laser fusible in accordance with conventional laser fusing 
techniques described in the prior art mentioned hereinabove, which are 
incorporated herein by reference, and using apparatus of the general type 
commercially available from Chicago Laser and ESI Corporation of Portland, 
OR, USA. 
In accordance with the present invention, selective fusing of one or more 
respective parallel connections 22 produces an open circuit thereat, 
enabling the resistance of the array to be increased in a step-wise 
fashion, while maintaining other characteristics of the resistor. 
Additionally, in accordance with an embodiment of the present invention, 
an additional resistive top hat element 24 may be provided as part of the 
resistor pattern and which may be cut by conventional laser trimming 
techniques in such a manner as to provide continuous, and thereby more 
precise, adjustment of the resistance. The cut may be made along the 
length of the element 24 through the connecting link 22 starting at the 
end of the element 24 at the left hand side of the FIG. 
For the configuration of FIG. 1, including n resistive strips of individual 
resistance R, there are a large number of different combinations of fusing 
patterns, which can provide a multiplicity of discrete different 
resistance values. When none of the parallel connections (second links) 
are cut the overall resistance is minimal, R min=R/n. When all of the 
parallel connections 22 are cut, this resistance is maximal at R max=nR. 
There are 2.sup.2n different series and parallel combinations, which can 
provide theoretically 2.sup.2n different resistance values between R max 
and R min. In practice, less than 2.sup.2n different resistance values are 
provided due to redundancy or impracticality. Typically, the number of 
strips or resistance elements n is between 5 and 30, although n may be 
between 2 and the number of resistance elements (strips) which can be 
accommodated on a substrate. To obtain values intermediate between the 
discrete values obtained by fusing links, additional variations in 
resistance can be obtained by trimming the top hat element 24. This can be 
done by making and extending a length wise cut therethrough so as to 
provide a continuous increase in resistance value. 
Reference is now made to FIG. 2 which illustrates a resistor 30 constructed 
and operative in accordance with the presently preferred embodiment of the 
present invention. It may be made in a manner similar to the embodiment of 
FIG. 1. Disposed between terminals 32 and 34 is a resistive array 36 with 
series connections (first links) 40 and parallel connections (second 
links) 42 to terminals 32 and 34. The array 36 is made up of a plurality 
of parallel resistive units (path or traces), each in the form of a strip 
38 and each being of precisely uniform and identical width, thickness and 
separation from its neighbor, but of different length. The series 
connections 40 have links 42 to the terminals 32 and 34 which are 
selectively fused (cut) to incrementally change the resistance value. Top 
hat 44 is for the same function as top hat 24 FIG. 1. The top hat 44 is 
cut lengthwise from the right hand end connection to make an analog 
adjustment in the incremental value selected by cutting the links 42. 
As compared with the embodiment of FIG. 1, the embodiment of FIG. 2, in 
which the lengths of resistors elements 38 differ from each other, 
provides a greater amount of redundancy for each given adjusted resistance 
value. This increased redundancy enables connection fusing patterns to be 
selected having relatively high ratios of heat disspation surface to 
substrate surface, while limiting temperature gradients between parts of 
the resistive array. 
Reference is now made to FIG. 3 which illustrates a resistor network 
including five generally identical resistors of the type illustrated in 
FIG. 2, where n=21 and the resistance value of the strips is a nominal 
2,000 ohms. The resistors are each formed, each with a different fused 
connection pattern, so as to have five different final resistance values. 
It is seen that in this example, the resistance realized ranged from 95 
ohms, when all of the connections 42 are left intact, to 42,000 ohms, when 
all of the connections 42 are fused except for the links at either end of 
the chain. In FIG. 3 the five resistors are shown interconnected and 
having output terminals 46. 
While five resistors are shown, a greater or lesser number of resistors of 
any suitable configuration and connection fusing pattern, may be combined 
into a resistor network either by being formed integrally on a single 
substrate, such as a wafer, or by wire bonding between physically 
independent elements. Without the availability of a single resistor 
pattern which is adjustable over a very wide resistance range individual 
resistors would have to be selected from, different production lots 
conventionally made with individual patterns of very limited resistance 
range. This would adversely affect both the economics of the network 
fabrication process and the operational performance of the network. In 
many applications the circuit function is dependent upon precisely fixing, 
and maintaining, the ratio of various resistance values, one to another. 
To accomplish this it is important that any changes in resistance value 
which take place subsequent to initial fixing be as uniform as possible 
among all the elements. This can most easily be achieved by arranging that 
all the resistors in a given network are derived from the same production 
lot. 
In the foregoing description it will be apparent that improved resistance 
elements which are of selectably adjustable value of resistance or 
individual resistors or in networks have been described. Variations and 
modifications thereof within the scope of the invention will undoubtedly 
become apparent to those skilled in the art. Accordingly, the foregoing 
description should be taken as illustrative and not in a limiting sense.