Step adjustable attenuator

A step adjustable attenuator comprising a silicon substrate on one face of which are deposited thin film-type resistors and electrical conductors by which the resistors are interconnected. Contact points are arranged at the periphery of the substrate and connected to the resistors. However, the attenuating sections are not interconnected on the substrate. A conductive pattern prepared from a conductive film forms finger-like leads of which inner portions are bonded to the contact points. By its shape, the pattern ensures the interconnections between the sections and forms two rows of outputs, one for the connections on the printed-circuit board on which is implanted the attenuator and the other being associated with displaceable straps enabling attenuation to be adjusted.

This invention relates to step adjustable attenuators comprising an 
integrated circuit intended to be implanted on a printed circuit. 
The attenuators are generally formed by a chain of T or .pi. attenuating 
sections comprising three resistors. Having two modes of connection, those 
attenuating sections have two attenuation values: 0 and .alpha. db, so 
that the total attenuation of the attenuator may assume a certain number 
of discrete values as and when required by the operator. 
Known attenuators having deposited layer-type circuitry are generally made 
with thick conductive films disposed on insulative wafers. The wafer is 
provided with input and output terminals. Thick-film resistors and the 
terminals are interconnected by thick-film conductors in .pi., T or other 
configurations. The wafer is generally implanted on a printed circuit 
provided with straps which make the connections with the other wafers 
implanted with the printed circuit and the interconnections between the 
terminals as to obtain the desired attenuation. The obtained device is not 
very compact because of the thick-film technique and because of the straps 
placed on the printed circuit. Another method is to use a movable wafer 
comprising contact contacts which is combined with the resistor-carrier 
wafer. According to the position of the movable wafer relative to the 
resistor-carrier wafer, the different values of attenuation can be 
obtained. This method has several drawbacks: When a great number of 
attenuating sections is desired, the number of terminals which are to be 
interconnected is very large and several wafers are to be used because of 
the complexity of the combination between the two types of wafers. The 
result is a complicated and non-compact device. Furthermore, it is 
impossible to use the rapid and inexpensive known technique of fabrication 
of integrated circuit. 
The invention uses the advantages of known techniques concerning integrated 
circuits. The attenuator according to the invention is in the form of a 
thin layer and is therefore more compact than a thick layer-type circuit. 
It is formed by deposition onto a silicon substrate. 
An object of the invention is to provide a step adjustable attenuator in 
the form of an integrated circuit with a double row of outputs, the value 
of the attenuation being easily determined by an operator at the 
installation site and this integrated circuit being easily implantable on 
a printed circuit card. 
An object of the invention is to use a conductive pattern produced from a 
metallized film and mounted on the silicon wafer by automatic film 
transfer techniques so as to reduce the costs and to increase the speed of 
production. 
A further object of the invention is a particular arrangement of the 
outputs which is particularly convenient for the operator responsible for 
adjusting attenuation and makes it possible to reduce the overall 
dimensions of the integrated circuit on the printed circuit card. This is 
because the outputs are arranged in two opposite rows, one of which 
comprises all the outputs corresponding to connections intended to be 
established by means of simple straps by the operator, the other row 
comprising the outputs for connection with the printed circuit. 
Another object of the invention is a mode of implantation of the attenuator 
on a printed circuit card, in which the side comprising the second row of 
outputs faces the card, the other side being readily accessible to the 
operator. 
Still another object of the invention is an arrangement of the resistors on 
the silicon and an order of the outputs which minimise the overall 
dimensions, avoid any intersection of the conductive deposits on the 
silicon and make the adjustment of attenuation as easy as possible by 
means of straps all identical with one another which will be welded after 
adjustment, thus providing for good electrical contact.

Numerous electronic systems, particularly telephone channel circuits, 
comprise attenuators formed by a chain attenuating sections comprising 
resistors and having a T-shaped or .pi.-shaped structure. In the case of 
application to telephony for example, it is desirable to be able to adjust 
the attenuation of the attenuator at the installation sites, i.e. after 
production of the circuit. FIG. 1 is an electrical circuit diagram of an 
attenuator formed by two .pi.-shaped sections each comprising three 
resistors: r.sub.1, r.sub.2 and r.sub.3 for the first and r.sub.4, r.sub.5 
and r.sub.6 for the second. When it is closed, a switch 21 enables the 
resistors r.sub.1 and r.sub.2 to be connected to earth. When it is closed, 
a switch 20 enables the resistor r.sub.3 to be short-circuited. There are 
two principal modes of operation: 
21 open, 20 closed: the output voltage .sqroot.2 is equal to the input 
voltage .sqroot.1 and the transmission factor a.sub.1 =.sqroot.1/.sqroot.2 
of the section is equal to 1. 
21 closed, 20 open: the factor a.sub.1 depends or r.sub.1, r.sub.2, r.sub.3 
and upon the matching conditions. If the section is matched at the input 
and at the output, r.sub.c being its characteristic resistance, and if 
r.sub.1 =r.sub.2, then 
EQU a.sub.1 =y.sub.1 +1+Y.sub.1 (Y.sub.1 +2) 
where y.sub.1 =r.sub.3 /r.sub.1 =r.sub.3 /r.sub.2 and r.sub.c =r.sub.1 
.sqroot.y.sub.1 +2/y.sub.1. 
The second section of which the input voltage is .sqroot.2 and the output 
voltage .sqroot.3, operates in the same way with two switches 22 and 23. 
If r.sub.c is its characteristic resistance, its attenuation a.sub.2 may 
assume two values: 1 or y.sub.2 =1 .sqroot.y.sub.2 (y.sub.2 +2) where 
y.sub.2 =r.sub.6 /r.sub.4 =r.sub.6 /r.sub.5. The total attenuation of the 
attenuator: a=a.sub.1 a.sub.2 may therefore assume 4 discrete values. More 
generally, an attenuator comprising n sections contains 2 n switches and 
its attenuation may assume at the most 2.sup.n discrete values which may 
be graduated for example with a scale of 0.5 dB. 
The operation of the T sections is similar to that of the .pi. sections. 
FIG. 2 is an electrical circuit diagram of an attenuator formed by 5 T 
sections. They are respectively formed by groups of resistors (R.sub.1, 
R.sub.2, R.sub.3), (R.sub.4, R.sub.5, R.sub.6), (R.sub.7, R.sub.8, 
R.sub.9), (R.sub.10, R.sub.11, R.sub.12), (R.sub.13, R.sub.14, R.sub.15) 
and have attenuations of, respectively, A.sub.1, A.sub.2, . . . A.sub.5. 
The attenuator is intended to be formed by thin layer techniques on a 
silicon substrate. It is desired to minimize the production costs and 
overall dimensions and, hence, the number of outputs. It is therefore 
preferable to avoid the intersections between the zones where the 
resistors and the metallic connections are deposited and of course between 
the metallic connections themselves. This means that the cells are not 
interconnected to one another inside the circuit. Thus, refering to FIG. 
2, the points 3, 7, 11, 15 corresponding to the outputs of the first four 
sections are not connected respectively to the points 5, 9, 13, 17 
corresponding to the inputs of the last four sections. The points 1 and 19 
respectively corresponding to the input and to the output of the 
attenuator also have to be connected to the electronic circuit in which 
the component is to be inserted. It can also be seen on FIG. 2 that four 
points corresponding to the earth of the circuit are provided, namely 4, 
8, 12, 16, which the operator responsible for adjustment will be able to 
connect to the points 2, 6, 10, 14, 18 which are the ends of the resistors 
R.sub.3, R.sub.6, R.sub.9, R.sub.12, R.sub.15. 
FIG. 3 is a block diagram showing the implantation of the resistors 
deposited onto a silicon substrate 100. The various points 1 to 19 of FIG. 
2 are materialized by studs having the same references. Implantation 
complies with the following requirements: the ends of the resistors are 
connected by film-type metallic conductors to the output contact studs 
numbered 1, 2, 3, 5, 6, 7, 9, 10, 11, 13, 14, 15, 17, 18, 19. Similarly, 
the various earth points are interconnected and connected to the output 
contacts studs numbered 4, 8, 12, 16 and to an additional contact M, by 
other conductors. The various contact studs 1, 2, . . . 19, M follow one 
another in this order at the periphery of the substrate 100. There is no 
intersection between the various elements deposited on the substrate 
(resistors and connecting paths). The techniques of thin-layer 
implantation are known. The formation of a thin layer on a silicon 
substrate is somewhat special in that silicon is conductive. One of the 
possible techniques is the following: to form an electrical insulation, 
the first step is to oxidize the silicon to obtain a layer of silicon 
oxide approximately 2 .mu.m thick, followed by the deposition from above 
of a 2.5 .mu.m thick layer of tantalum which is also oxidized. The inert 
tantalum oxide will not be attached during etching operations through the 
successive masks. A layer of tantalum nitride approximately 500 A thick, a 
layer of nichrome 500 to 1000 A thick and a layer of gold 1.5 .mu.m thick 
are then successively deposited, the layer of tantalum nitride being 
deposited by cathode sputtering and the other two layers being deposited 
by vacuum evaporation. The three layers are selectively etched through 
masks to obtain resistors formed by the layer of tantalum nitride and 
conductors and output contact studs formed by the three superposed layers. 
The values of the resistances are adjusted by laser by forming notches in 
the deposited layers. For measuring each resistance, one of the contact 
studs 1 to 19 is used together with test contact studs (not shown in the 
Figure) which are respectively connected to the common points of the three 
resistor forming each attenuating section. 
The values of the resistances depend upon the geometry of the diffused 
zones and upon the thickness of the deposit. FIG. 4 illustrates an example 
of geometry which is suitable for resistors of low resistance value 
(typically from 10 to 500 ohms), for example the resistor R.sub.1. It is 
formed by a rectangular deposited layer 63 of width 1 situated between two 
paths 61 and 62 which are separated by a distance h and of which one 
ensures connection with the output contact stud 1 whilst the other ensures 
connection with the resistors R.sub.2 and R.sub.3. If .phi. is the 
resistivity per unit area, the resistance obtained is equal to R=.phi.h/1. 
A notch 64 formed by a laser beam enables the value of the resistance to 
be precisely adjusted in dependence upon its length. 
FIG. 5 illustrates another example of geometry suitable for resistances of 
higher value and which allows easily carried out dimensions, for example 
the resistor R.sub.3. The resistor R.sub.3 is formed by a deposit 73 
connected by two tags 77 and 78 to two paths 71 and 72 of which one 
ensures connection with the output contact stud 2 and the other ensures 
connection with the resistors R.sub.1 and R.sub.2. An odd number (3 in the 
Figure) of notches 74, 75, 76 enables the resistance to be increased by 
lengthening the path h followed by the current. By acting on the length of 
at least one of these notches, it is possible to adjust the value of the 
resistance. 
The wafer 100 described above referring to FIG. 3 has to be associated with 
a conductive pattern in order to forms the external outputs of the 
attenuator. FIG. 6 is a plan view of the silicon wafer 100 carrying the 
circuit of the attenuator and of the associated pattern. This pattern is a 
set of finger-like conductive leads of which the inner portions extend 
within the periphery of the wafer and the other ends are arranged in two 
opposite rows forming the outputs. The various leads are denoted by the 
same numbers as the contact points of the wafer to which they will be 
bonded. The output ends of the leads corresponding to the contacts 1 to 
19, i.e. to the various points of the circuit which may have to be 
interconnected, are oriented on the same side which will be called the 
side C. The lead corresponding to the contact 1, i.e. to the input at the 
attenuator, is divided in two to form an additional output E. Similarly, 
the lead corresponding to the contact 19, i.e. to the output of the 
attenuator, is divided in two to form the output S. The earth of the 
attenuator is connected to two outputs M and N. The outputs E, S, M, N are 
oriented on the side B of the circuit opposite the side C. Various 
techniques may be used for transferring the wafer onto the pattern. One of 
the most interesting methods so far as speed of production and low cost in 
large numbers are concerned is the method of automatic transfer onto film. 
This method consists in automatically and simultaneously carrying out the 
following operations for a large number of identical circuits: 
cutting a perforated copper-lined film to form the patterns adapted to the 
wafers, 
forming gold contacts on the wafers, 
positioning the wafers relative to the perforated film and fixing them to a 
support with wax, 
simultaneously welding all the connections to the contact studs of each 
wafer: after the alignment of the contacts relative to the inner ends of 
the leads has been verified, a welding tool is lowered and directly heats 
the copper ends while pressing them against the gold contacts of the 
wafers, which forms a gold-tin eutectic. The heat given off melts the wax 
by which the wafers are held on the support so that they are only integral 
with the film, 
encapsulation of each circuit in resin which coats the wafer and part of 
the film, which forms cases, 
separation of the circuits by cutting the film between the cases. 
FIG. 6 also shows the limits of the case 200 obtained with its two opposite 
sides C and B and the ends of the leads forming the outputs of each side 
of the case. The connections between the attenuating cells are formed by 
the pattern itself which is designed to form bridges 35, 79, 1113 and 1517 
respectively between the outputs 3 and 5, 7 and 9, 11 and 13, 15 and 17. 
Since these various outputs have to remain accessible for the adjustment 
of attenuation, the outputs 3, 5, 7, 9, 11, 13 are widened and slotted in 
the longitudinal direction. Thus, the output 3 is divided into two parts 
which are separated by bending: the branch 30 intended to be optionally 
connected to the output 1, and the part 35 which forms a bridge with the 
output 5. Attenuation is determined by placing straps of the jumper type 
or by any displaceable short-circuiting means. Thus, for the first 
attenuating section, an operator will have to place a strap either between 
the outputs 1 and 30, which gives a transmission factor for this section 
equal to 1, or between the outputs 2 and 4 which gives a transmission 
factor equal to: 
##EQU1## 
Since the output 4 is situated between the outputs 3 and 5 which are 
joined by the bridge 35, it has to be shorter so that its end is situated 
below the bridge. The outputs 2, 4, 6, 8, 10, 12, 14, 16, 18 are thus 
shorter than the outputs 1, 3, 5, 7, 9, 11, 13, 15, 17, 19. The former are 
bent back on one side or the other so that they are separate from the 
latter and enable short-circuiting jumpers to be moved readily into 
position. In addition, since there are only four outputs for the earth, 
although five are necessary, one of them, namely the output 16, is widened 
and slotted to form two branches 161 and 162 which are separated by 
bending. It is preferable for the pattern to be produced in such a way 
that the distances between two outputs or branches intended to be 
connected by a strap are all equal, the same applying to their width. In 
this way, it is possible to use straps of one and the same type. FIGS. 7 
and 8 show two detailed views of certain outputs of the pattern: 
Referring to FIG. 7, the outputs 2 and 4, which are bent back and separated 
by a distance d, may be connected by a strap 24. The outputs 3 and 5 are 
divided respectively into (30, 35) and (50, 35), the branch 35 forming a 
bridge between these two outputs for permanently connecting them. The 
branch 30 may be connected to the output 1 from which it is separated by 
the distance d. The branch 50 may be connected to the branch 70 of the 
output 7 from which it is also separated by the distance d. The widths of 
the outputs 2 and 4 and of the branches 30 and 50 are equal to e. 
Referring to FIG. 8, the output 16 is divided into two branches 161 and 162 
bent in two different directions. The branch 162 is bent in the same 
direction at the output 18 so that it may be connected thereto by a strap 
1618. The branch 170 of the output 17 and the output 19, which are bent in 
the same direction, may be connected by a strap 1719, these two 
connections being mutually exclusive. The straps may be of different 
types. They have to be readily displaceable to enable attenuation to be 
rapidly adjusted and must be capable of being welded once the adjustment 
has been made, thus ensuring permanent contacts. In cases where it is 
intended to use commercially available jumpers, a standard value must be 
adopted for the distance d: for example d=5.6 mm. It is also possible to 
use specially produced straps comprising a handle or any other means 
enabling them to be readily gripped and intended to be removed once the 
adjustment has been made to be replaced by connecting means fixed by 
welding to the selected outputs. 
In one variant of the invention, one of the earth outputs, for example the 
output 12, is left out and the resistors R.sub.8 and R.sub.10 are joined 
so that the outputs 11 and 13 are merged into a single output. In this 
case, the output 8 is divided into two branches, like the output 16, so 
that it may be connected by a strap to the outputs 6 and 10, the output 4 
remaining intended to be connected to the single output 2. Production of 
the pattern will be carried out in such a way that the distances between 
the outputs or branches intended to be connected are all equal. This 
variant enables two outputs to be dispensed with, thus enabling space to 
be saved. 
The described embodiment and its variant are only given by way of 
non-limiting example. The number of attenuating sections is determined 
according to requirements. According to the invention, implantation of the 
resistors must permit the use of a dual-in-line pattern, one row of 
outputs, optionally divided into several branches, enabling short circuits 
to be established by means of straps. In addition, the implantation plan 
must avoid intersections of conductors and the superposition of conductive 
layers on the substrate. 
FIG. 9 shows the mode of implantation of the attenuator on a printed board 
201 containing the circuit into which the attenuator is inserted. The 
attenuator 200 is disposed on the slice. Its outputs E, M, N, S, facing 
the printed board 201, side B, are connected to the external circuit. The 
outputs 1 to 19, side C, are thus readily accessible to the operator 
responsible for adjustment. In addition, this arrangement reduces clutter 
on the printed board. 
In one embodiment of the invention, the values of resistances were selected 
in such a way that the successive attenuating sections had the following 
respective attenuations: A.sub.1 =0.5 dB, A.sub.2 =1 dB, A.sub.3 =2 dB, 
A.sub.4 =4 dB, A.sub.5 =8 dB, the input and output impedances being equal 
to 600 ohms. 
The following Table shows the attenuation values obtained in dependence 
upon the contacts established between the various output branches. The 
symbol X indicates that the contact is established. 
______________________________________ 
1- 50- 90- 130- 170- 2- 6- 10- 14- 162- 
30 70 110 150 19 4 8 12 161 18 A (dB) 
______________________________________ 
X X X X X 0 
X X X X X 0.5 
X X X X X 1 
X X X X X 1.5 
X X X X X 2 
X X X X X 2.5 
X X X X X 3 
X X X X X 3.5 
X X X X X 4 
X X X X 4.5 
X X X X X 5 
X X X X X 5.5 
X X X X X 6 
X X X X X 6.5 
X X X X X 7 
X X X X X 7.5 
X X X X X 8 
X X X X X 8.5 
X X X X X 9 
X X X X X 9.5 
X X X X X 10 
X X X X X 10.5 
X X X X X 11 
X X X X X 11.5 
X X X X X 12 
X X X X X 12.5 
X X X X X 13 
X X X X X 13.5 
X X X X X 14 
X X X X X 14.5 
X X X X X 15 
X X X X X 15.5 
______________________________________ 
A set of component values is as follows: 
______________________________________ 
R.sub.1 = R.sub.2 = 17.26 ohms 
R.sub.3 = 10.417 k ohms 
R.sub.4 = R.sub.5 = 34.5 ohms 
R.sub.6 = 5.002 k ohms 
R.sub.7 = R.sub.8 = 68.77 ohms 
R.sub.9 = 2.583 k ohms 
R.sub.10 = R.sub.11 = 135.8 ohms 
R.sub.12 = 1.258 k ohms 
R.sub.13 = R.sub.14 = 258.3 ohms 
R.sub.15 = 567.7 ohms 
______________________________________ 
According to the position of the short-circuiting jumpers shown in FIG. 9: 
31, 57, 1012, 1315, 1618, the attenuation is A=0+0+2+0+8=10 dB. 
The characteristic dimensions of this attenuator for the case illustrated 
in FIG. 9 are: L.sub.1 =46 mm, L.sub.2 =15 mm and L.sub.3 =2 mm, the 
dimensions of the wafer being 3 mm.times.3 mm. The width of the outputs or 
output branches is e=0.4 mm and the distance d is d=5.6 mm.