Electrically trimmable resistor ladder

A resistor ladder (10) includes a plurality of series resistors (16,18,20,22,24,26,28) connected in series with each other between a first terminal (12) and a second terminal (14). A plurality of shunt resistors (60,62,64,66, 68,70) are connected between junctions (48,50,52,54,56) of adjacent series resistors and the second terminal (14). The series resistors (16,18,20,22,24,26,28) and shunt resistors (60,62,64,66,68,70) are formed on a substrate (80) as film resistors which blow open at a predetermined current density. The shunt resistors (60,62,64,66,68,70) have a smaller cross-sectional area than the series resistors (16,18,20,22,24,26,28) such that they successively blow open from the first terminal (12) toward the second terminal (14), while the series resistors (16,18,20,22,24,26,28) do not blow open, as a progressively increasing voltage is applied between the first terminal (12) and the second terminal (14). The resistance of the ladder (10) increases as the shunt resistors (60,62,64,66,68,70) are successively blown. The shunt resistors (60,62,64,66,68,70) preferably have twice the resistance as the series resistors (16,18, 20,22,24,26,28), enabling the resistance of the ladder (10) to be electrically trimmed in increments equal to the resistance of each series resistor (16,18,20,22,24,26,28).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to the art of microelectronics, and 
more specifically to a resistor ladder which can be electrically 
programmed or trimmed even after the circuit in which the resistor ladder 
is incorporated has been encapsulated in a package. 
2. Description of the Related Art 
Resistors can be advantageously formed on microelectronic integrated 
circuit substrates by screen printing using special resistive inks which 
dry to form thick or thin films. The process is economical in that all of 
the resistors can be formed on a substrate simultaneously, and the screen 
printing process is inherently inexpensive. 
A drawback of this process is that it is not especially precise. Deviations 
of 20% for thick film resistors and 10% for thin film resistors from their 
design resistances is not uncommon. 
For precision applications, film resistors are trimmed by selective removal 
of material using a laser or other device to achieve the required 
resistance. A description of laser trimming of film resistors is presented 
in a textbook entitled "HYBRID MICROCIRCUIT TECHNOLOGY HANDBOOK", by J. 
Licari, Noyes Publications, Park Ridge, N.J., 1988, pp. 132-148. 
Laser trimming is an expensive and time consuming process, requiring each 
resistor to be trimmed individually. Another drawback of laser trimming is 
that it cannot be performed after a microelectronic circuit has been 
encapsulated in a protective package. 
A typical microelectronic circuit will include many film resistors having 
different resistances. In the conventional design process, each resistor 
must be individually configured in accordance with its required 
resistance. The layout of the circuit is therefore complicated, time 
consuming and expensive. 
SUMMARY OF THE INVENTION 
A resistor ladder embodying the present invention includes a plurality of 
series resistors connected in series with each other between a first 
terminal and a second terminal. A plurality of shunt resistors are 
connected between the junctions of adjacent series resistors and the 
second terminal. 
The series and shunt resistors are formed on a substrate as film resistors 
which blow open at a predetermined current density. The shunt resistors 
have a smaller cross-sectional area than the series resistors such that 
they successively blow open from the first terminal toward the second 
terminal, while the series resistors do not blow open, as a progressively 
increasing voltage is applied between the first and second terminals. 
The resistance of the ladder increases as the shunt resistors are 
successively blown. The shunt resistors preferably have twice the 
resistance as the series resistors, enabling the resistance of the ladder 
to be electrically trimmed in increments equal to the resistance of each 
series resistor. 
A trim terminal, trim resistor and reverse blocking diode are provided for 
electrostatic discharge (ESD) protection. Two or more resistor ladders can 
be connected in series to provide a wider range of resistance variation in 
combination with smaller incremental changes. 
The present resistor ladder can be electrically trimmed even after the 
microelectronic circuit in which it is incorporated is encapsulated in a 
protective package. Electrical trimming of the present resistor ladder to 
achieve a precise design resistance is substantially simpler, faster less 
expensive than conventional laser trimming. 
The arrangement of the present invention enables a plurality of resistors 
having different values to be designed and initially formed on a substrate 
as identical resistor ladders, and subsequently trimmed to the required 
individual resistances. This modular design substantially reduces the 
complexity, time and expense of microelectronic circuit layout. 
These and other features and advantages of the present invention will be 
apparent to those skilled in the art from the following detailed 
description, taken together with the accompanying drawings, in which like 
reference numerals refer to like parts.

DETAILED DESCRIPTION OF THE INVENTION 
An electrically trimmable resistor ladder 10 embodying the present 
invention is illustrated in FIGS. 1 and 2 prior and subsequent to trimming 
respectively. The ladder 10 includes an ohmic metal contact pad which 
constitutes a first terminal 12, and a conductor metallization strip which 
constitutes a second terminal 14. 
A plurality of series resistors 16, 18, 20, 22, 24, 26 and 28 are connected 
in series with each other to constitute a series resistor string 30 having 
a first end 32 connected to the first terminal 12 and a second end 34 
connected to the second terminal 14. Adjacent resistors 16, 18, 20, 22, 
24, 26 and 28 are interconnected by ohmic metal contacts 36, 38, 40, 42, 
44 and 46 which constitute junctions 48, 50, 52, 54, 56 and 58 and 
separate the adjacent resistors 16,18, 18,20, 20,22, 22,24 and 24,26 and 
26,28 from each other respectively. 
A plurality of shunt resistors 60, 62, 64, 66, 68 and 70 are formed between 
the ohmic contacts 36, 38, 40, 42, 44 and 46, and the second terminal 14 
respectively. A trim pad or terminal 74 is connected to the first terminal 
12 through a blowable trim resistor 76. 
An NPN transistor 78 has a collector connected to the first terminal 12, a 
base connected to the second terminal 14 and a floating emitter. The 
collector-base junction of the transistor 78 is thereby connected between 
the first and second terminals 12 and 14, and the transistor 78 functions 
electrically as a diode. 
The resistor ladder 10 further includes an electrically insulating 
substrate 80 on which the illustrated components are formed. The series 
resistors 16, 18, 20, 22, 24, 26 and 28, the shunt resistors 60, 62, 64, 
66, 68 and 70 and the trim resistor 76 are preferably formed on the 
substrate 80 as thick or thin film resistors by screen printing or another 
suitable method. Although only one resistor ladder 10 is illustrated, in 
an actual application a large number of ladders 10 will be formed on the 
substrate. 
The number of series and shunt resistors is not limited within the scope of 
the invention, with seven series resistors 16, 18, 20, 22, 24, 26 and 28 
and six shunt resistors 60, 62, 64, 66, 68 and 70 being shown by way of 
example. It will be further noted that the drawings are simplified and not 
drawn to scale. 
In a preferred embodiment of the invention, each series resistor 16, 18, 
20, 22, 24, 26 and 28 will have a first predetermined resistance R, 
whereas each shunt resistor 60, 62, 64, 66, 68 and 70 will have a second 
predetermined resistance of 2R, or twice the resistance of the series 
resistors. This enables the ladder 10 to be programmed or trimmed in 
resistance increments of R. However, the invention is not so limited, and 
the resistances of the series and shunt resistors can have any desired 
values. 
The ladder 10 as illustrated in FIGS. 1 and 2 has a "R/2R" configuration. 
The minimum possible resistance of the ladder 10 is R, whereas the maximum 
possible resistance of the ladder 10 is 7R. The ladder 10 is trimmed by 
successively electrically blowing open the shunt resistors 60, 62, 64, 66, 
68 and 70 from the first terminal 12 toward the second terminal 14. In 
other words, the shunt resistor 60 will be blown open first, followed by 
the resistors 62, 64, 66, 68 and 70 in this order. 
Assuming that all of the shunt resistors 60, 62, 64, 66, 68 and 70 are 
blown open (although not explicitly illustrated), the ladder 10 will 
consist of only the series resistors 16, 18, 20, 22, 24, 26 and 28 which 
have a combined resistance of 7R. If the shunt resistors 60, 62, 64, 66 
and 68 are blown open and the shunt resistor 70 is not blown open, the 
resistors 70, 26 and 28 will have a combined resistance of R. 
More specifically, the resistors 26 and 28 are connected in series and have 
a combined resistance of 2R. The combined resistance (2R) of the resistors 
26 and 28 is connected in parallel with resistance (2R) of the resistor 
70. The combined resistance of the resistors 70, 26 and 28 is therefore R, 
and is connected in series with the five series resistors 16, 18, 20, 22 
and 24 such that the resistance of the ladder 10 is 6R. 
By extrapolation, blowing open each shunt resistor 60, 62, 64, 66, 68 and 
70 increases the resistance of the ladder 10 by an increment of R. FIG. 2 
illustrates the shunt resistors 16, 18 and 20 as being blown open such 
that the ladder 10 has a resistance of 4R. 
The series and shunt resistors are designed such that the shunt resistors 
60, 62, 64, 66, 68 and 70 will successively blow open as a progressively 
increasing voltage is applied between the terminals 12 and 14. A higher 
voltage is required to blow open each successive shunt resistor 60, 62, 
64, 66, 68 and 70 because the resistance of the ladder 10 increases as 
each shunt resistor is blown, and a progressively higher voltage is 
required to create the same current flow through the next shunt resistor 
which is to be blown. 
Film resistors will blow open at a known current density. Assuming that all 
of the resistors are made of the same material, it is necessary that the 
shunt resistors have a smaller cross-sectional area than the series 
resistors so that the current density in the shunt resistors will exceed 
the current density in the series resistors at any voltage applied across 
the terminals 12 and 14. 
This enables the shunt resistors to be blown open without the series 
resistors blowing open. In the R/2R arrangement, the cross-sectional area 
of each shunt resistor 60, 62, 64, 66, 68 and 70 is approximately one-half 
that of each series resistor 16, 18, 20, 22, 24, 26 and 28. However, the 
resistances and cross-sectional areas of the series and shunt resistors 
are not limited to any particular values within the scope of the 
invention. It is merely necessary that the shunt resistors blow open and 
the series resistors do not blow open. 
The first terminal 12 is interconnected to other elements in the 
microelectronic circuit (not shown), and can be enclosed by a cover or 
other protective encapsulation. The second terminal 14 is typically a 
ground plane or other reference level, and can be accessed from outside 
the encapsulation. The trim terminal 74 is provided external of the 
encapsulation, and enables the ladder 10 to be trimmed after the 
encapsulation is added to the structure while providing electrostatic 
discharge (ESD) protection for the circuitry. 
For trimming the ladder 10, a voltage is applied to the trim terminal 74 
which is positive with reference to the second terminal 14. This reverse 
biases the collector-base junction of the transistor 78, such that the 
transistor 78 is effectively disconnected from between terminals 12 and 
14. Thus, the ladder 10 can be trimmed vias the trim terminal 74 and the 
trim resistor 76. 
After trimming, a voltage is applied to the trim terminal 74 which is 
negative with reference to the second terminal 14. This forward biases the 
collector-base junction of the transistor 78 which provides a low 
resistance path between the terminals 12 and 14. The negative voltage, 
which effectively appears across the trim resistor 76, is selected to be 
high enough to blow open the resistor 76 and thereby disconnect the trim 
terminal 74 from the first terminal 12. Thus, even if an electrostatic 
charge is applied to the trim terminal 74 upon subsequent handling and 
operation of the structure, the charge will not be applied to the ladder 
10. 
EXAMPLE 
The series, shunt and trim resistors are formed of thin film material which 
blows open at a typical current density of 200 microamperes per 
micrometer. R=2K ohms, whereas 2R=4K ohms. It will be assumed that the 
film thickness of all of the resistors is the same. Since cross-sectional 
area is equal to width times thickness, the current densities in the 
resistors are proportional to their widths. 
The width of each series resistor 16, 18, 20, 22, 24, 26 and 28 and the 
trim resistor 76 is selected to be 12 micrometers, whereas the width of 
each shunt resistor 60, 62, 64, 66, 68 and 70 is 5 micrometers. With the 
trim resistor 76 included, the resistance of the ladder 10 is initially 
2R. All of the current into the ladder 10 will flow through the trim 
resistor 76, and will then divide equally through the shunt resistor 60 
(which has a resistance of 2R) and all of the other resistors (which have 
a combined resistance of 2R). 
The current I required to blow open the resistor 60 is I=200 milliamperes 
per micrometer .times.5 micrometers=one milliampere, and the current into 
the ladder 10 must be 2 milliamperes. The voltage V which must be applied 
between the trim terminal 74 and the second terminal 14 to cause one 
milliampere of current to flow through the resistor 60 is therefore V=2 
milliamperes .times.4K ohms=8 volts. 
Blowing open each shunt resistor 60, 62, 64, 66, 68 and 70 adds an 
increment of R to the resistance of the ladder 10. The voltage must 
therefore be increased in increments of .DELTA.V=2 milliamperes .times.2K 
ohms=4 volts to blow open each successive shunt resistor 60, 62, 64, 66, 
68 and 70. The voltage required to blow open the last shunt resistor 70 is 
32 volts. 
The current I required to blow open the trim resistor 76 is I=200 
milliamperes per micrometer .times.12 micrometers=2.4 milliamperes, and 
the voltage V required to blow open the trim resistor 76 is V=2.4 
milliamperes .times.2K ohms=4.8 volts. 
Electrical trimming of the present resistor ladder 10 to achieve a precise 
design resistance is substantially simpler, faster less expensive than 
conventional laser trimming, since all that is required is to apply a 
predetermined voltage between the trim terminal 74 and the second terminal 
14 to achieve a desired resistance. 
In addition, the arrangement of the present invention enables a plurality 
of resistors having different values to be designed and initially formed 
on a substrate as identical resistor ladders 10, and subsequently trimmed 
to the required individual resistances. This modular design substantially 
reduces the complexity, time and expense of microelectronic circuit 
layout. 
FIGS. 3 and 4 illustrate another resistor ladder 10' embodying the present 
invention in which like elements are designated by the same reference 
numerals used in FIGS. 1 and 2, and corresponding but modified elements 
are designated by the same reference numerals primed. 
The ladder 10' differs from the ladder 10 in that the ohmic metal contacts 
36, 38, 40, 42, 44 and 46 are omitted, and series resistors 16', 18', 20', 
22', 24', 26' and 28', are connected directly together at junctions 48', 
50', 52', 54', 56' and 58' respectively The cross-sectional width of each 
shunt resistor 60, 62, 64, 66, 68 and 70 is one-third the cross-sectional 
width of each series resistor 16', 18', 20', 22', 24'26' and 28'. 
In this embodiment, the series resistors 16', 18', 20', 22', 24', 26' and 
28' are preferably formed integrally as one continuous, longitudinally 
elongated resistor, and the shunt resistors 60, 62, 64, 66, 68 and 70 are 
connected thereto at longitudinally spaced locations which constitute the 
junctions 48', 50', 52', 54', 56', and 58' respectively. 
The ladder 10; is advantageous in that it is easier to fabricate and takes 
up less space on the substrate 80 than the ladder 10. However, the ladder 
10' is not as precise as the ladder 10, in that the shunt resistors 60, 
62, 64, 66, 68 and 70 will be blown, in actual practice, at slightly 
different locations along their lengths. 
The portions of the shunt resistors 60, 62, 64, 66, 68 and 70 which remain 
connected to the respective series resistors 16', 18', 20', 22', 24', 26' 
and 28' locally increase the widths of the series resistors in an 
unpredictable manner, thus introducing a small element of uncertainty into 
the final resistance of the ladder 10'. 
This effect is not present in the ladder 10 because the series resistors 
16, 18, 20, 22, 24, 26 and 28 are interconnected by the ohmic contacts 36, 
38, 40, 42, 44 and 46 which have much lower resistance than the shunt 
resistors 60, 62, 64, 66, 68 and 70. 
FIG. 5 illustrates another resistor ladder 100 embodying the present 
invention which essentially consists of two ladders 10' connected in 
series. Although not specifically illustrated, the concept of FIG. 5 is 
equally applicable to the ladder 10. 
The arrangement of two resistor ladders connected in series enables the 
ladder 100 to have a larger range of resistance variation than the ladders 
10 and 10', and/or a smaller incremental change in resistance. In the 
example illustrated, the resistance of the ladder 100 is variable from 11R 
to 99R in increments of R. 
The ladder 100 includes a first section 101 including 10 series resistors 
102, 104, 106, 108, 110, 112, 114, 116, 117 and 118 which are connected in 
series with each other between a first terminal 120 and a second terminal 
122. Each of these series resistors has a resistance of R. 
Nine shunt resistors 123, 124, 126, 128, 130, 132, 134, 136 and 138 are 
connected between the first terminal 120 and the junctions (not 
designated) of the respective series resistors 102, 104, 106, 108, 110, 
112, 114, 116, 117 and 118, and the second terminal 122 in the manner 
described above with reference to FIGS. 3 and 4. Each of the shunt 
resistors has a resistance of 2R. 
The ladder 100 further includes a second section 140 including nine series 
resistors 142, 144, 146, 148, 150, 152, 154, 156 and 158 which are 
connected in series with each other between the second terminal 122 and a 
third terminal 160. Each of these series resistors has a resistance of 
10R. Eight shunt resistors 163, 164, 166, 168, 170, 172, 174 and 176 are 
connected between the second terminal 122 and junctions of the respective 
series resistors, and the third terminal 160. Each of the shunt resistors 
has a resistance of 20R. 
Further illustrated are a first ESD protection circuit including a first 
trim terminal 180 which is connected to the first terminal 120 through a 
first blowable trim resistor 182 having a resistance of R, and a floating 
emitter NPN transistor 184 whose collector is connected to the first 
terminal 120 and whose base is connected a second trim terminal 186. 
A second ESD protection circuit includes the second trim terminal 186 which 
is connected to the second terminal 122 by a second blowable trim resistor 
188 having a floating emitter NPN resistance of 10R, and a transistor 190 
whose collector is connected to the second terminal 122 and whose base is 
connected to the third terminal 160. 
The first terminal 120 and the third terminal 160 constitute the ends of 
the ladder 100 for operation after trimming or programming. The ladder 100 
has a resistance of 11R with no resistors blown. The resistance of the 
first section 101 is R, the resistance of the second section 140 is 10R, 
and the sections 101 and 140 are connected in series with each other such 
that the resistances of the two sections 101 and 140 are added together. 
The sections 101 and 140 can both be trimmed to provide any resistance 
between 11R and 99R in increments of R. The second section 140 is trimmed 
to provide the upper decade increment (10R to 90R in increments of 10R), 
and the first section 101 is trimmed to provide the lower decade increment 
(1R to 10R in increments of R). 
Since the sections 101 and 140 are connected in series with each other, the 
lower decade increment is added to the upper decade increment. For 
example, if the resistors 123, 124 and 126 are blown open such that the 
resistance of the first section 101 is 4R, and the resistors 163, 164, 
166, 168, 170, 172 and 174 are blown open such that the resistance of the 
second section 140 is 80R, the resistance of the ladder 100 taken between 
the terminals 120 and 160 will be 4R+80R=84R. 
While several illustrative embodiments of the invention have been shown and 
described, numerous variations and alternate embodiments will occur to 
those skilled in the art, without departing from the spirit and scope of 
the invention. Accordingly, it is intended that the present invention not 
be limited solely to the specifically described illustrative embodiments. 
Various modifications are contemplated and can be made without departing 
from the spirit and scope of the invention as defined by the appended 
claims.