Voltage Divider Including a Tapped Resistor Diffused in Semiconductor Substrate

Our present invention relates to a voltage divider comprising a tapped resistor formed by the diffusion of impurities into an elongate surface zone of a semiconductor substrate such as a wafer of monocrystalline silicon.

When such a resistor is traversed by current, predetermined fractions of the overall voltage difference existing between its terminals are available at the several taps. The latter may be constituted by metallic strips placed across the doped zone of the substrate at longitudinally spaced locations. Another possibility is to extend lateral branches from that zone by diffusing impurities--usually identical with those of the zone itself--into adjacent areas and metallizing the extremities of these branches. If the taps thus formed are connected to loads of high resistance (such as, for example, the inputs of operational amplifiers), they will draw only negligible current so as not significantly to affect the overall voltage drop.

In certain instances it is necessary to make the open-circuit potential difference between adjacent taps a very small fraction of that overall voltage drop. Such a situation arises, for example, when a reference potential is to be applied to a resistance bridge requiring compensation of an offset voltage due to an inherent unbalance. It then becomes necessary to choose among several reference voltages differing but little from one another, e.g. by about 1 to 3 mV.

For physical reasons, metal contacts overlying a diffused resistor defined by an elongate zone of predetermined conductivity cannot be spaced close enough to supply voltage differences of this small order of magnitude. The same is true of taps formed by lateral branches extending on one side of the longitudinal median of the zone. If these branches are disposed at opposite sides of the median, their potential difference in the presence of a constant current of a given magnitude could be chosen as small as desired; such positioning, however, is inconvenient in certain instances, as where the selected tap is to be linked to an external load by a lead of predetermined length.

The object of our present invention, accordingly, is to provide a voltage divider of the above-discussed type including a diffused resistor with taps so disposed along one lateral boundary thereof that the potential difference between any two adjoining taps is a small fraction of the overall voltage drop.

We realize this object, in accordance with our present invention, by so shaping the doped elongate zone of the substrate that the diffused resistor defined by that zone has a width which varies from two relatively narrow sections adjacent its metallized ends to a relatively broad intermediate section with an outwardly projecting lateral boundary on one section of the longitudinal median of the zone, that intermediate section defining a region of maximum cross-sectional area and minimum resistance per unit length measured along the median. Within that region, the intermediate section of the doped zone is provided with a plurality of closely juxtaposed branches of substantially the same resistivity extending perpendicularly from the outwardly projecting lateral boundary thereof, these branches terminating in metallized extremities forming taps whose potentials differ by a small fraction of the voltage drop existing between the ends of the zone in the presence of a current flow through the resistor.

It will generally be convenient to make the depth of the doped zone substantially constant throughout its length and width; the branches, if formed in the same diffusion process, will also be of that depth.

The above and other features of our invention will now be described in detail with reference to the accompanying drawing in which:

FIG. 1 is a plan view of a portion of a silicon wafer provided with a diffused resistor according to our invention; and

FIG. 2 is a view similar to FIG. 1, showing a modification.

In FIG. 1 we have shown part of a wafer 1 of monocyrstalline silicon which may be of intrinsic conductivity or only lightly doped so as to have a very high electrical resistance. An elongate zone 2 (not drawn to scale) of predetermined lower resistivity is formed by conventional doping with suitable impurities to constitute a diffused resistor. Opposite ends 3 and 4 of the zone 2 are metallized to serve as terminals to be connected across a current source.

Zone 2 has lateral boundaries B and C on opposite sides of its longitudinal median A. These boundaries extend parallel to each other in the vicinity of terminals 3 and 4 so as to establish two narrow resistor sections of constant width separated by an intermediate resistor section of progressively increasing and then progressively decreasing width. The spacing of opposite points O.sub.2, O.sub.2, and R.sub.1, R.sub.2 at the narrow sections is about half the distance between points P.sub.1 and P.sub.2 which are interconnected by a transverse line D in the region of maximum width. Point P.sub.1 on boundary B forms a sharp corner, here of 90.degree., and lies at the junction of that boundary with a transverse branch y centered on line D and flanked by two other branches x and w which extend perpendicularly to boundary B on opposite sides of corner P.sub.1. The three branches x, y and w, diverging from one another at 45.degree., have metallized extremities X, Y and W at which their voltages can be taken off.

With zone 2 assumed to have a constant depth, its cross-sectional area is largest in the transverse plane coinciding with line D so that the voltage drop per unit length measured along median A is smallest in the immediate vicinity of this line. Furthermore, since boundary B includes an angle of nearly 45.degree. with the median at points close to corner P.sub.1, the distances between taps x, y and between taps y, w are almost .sqroot.2 times the lengths of their projections upon the median A. As a result, only small voltage differences exist along boundary B between the junctions of these branches with zone 2.

In FIG. 2 we have shown the substrate 1 provided with a modified zone of diffusion 2' of generally linear configuration, having metallized terminals 3', 4'; on one side of its median A' it has a straight lateral boundary C' whereas its other lateral boundary B' diverges from boundary C' to form again a broadening intermediate resistor section between two narrow sections of constant width. That width, e.g. as measured between points O.sub.1 ', O.sub.2 ' and R.sub.1 ', R.sub.2 ', is shown to be less than half the maximum width existing between points P.sub.1 ' and P.sub.2 ' that are interconnected by a transverse line D' coinciding with the maximum cross-sectional area of the zone.

In the vicinity of point P.sub.1 ', boundary B' forms an arcuate bulge along the periphery of an imaginary circle z whose diameter is less than half of the maximum width, the radius of curvature of the bulge being thus less than one fourth of that width. Branches x', y', w', u and v extend perpendicularly to boundary B' along the arcuate bulge, the middle branch w' being centered on line D'. The free extremities of these branches are again metallized to form electrodes X', Y', W', U and V.

In this latter embodiment, in which the zone 2' is again assumed to be of constant depth, the width of that zone decreases less sharply in the vicinity of line D' compared with the width of zone 2 of FIG. 1 in the region of line D. This compensates for the fact that boundary B' is substantially parallel to median A' near the midpoint P.sub.1 ' of the bulge and converges only slightly toward that median in the vicinity of the outer branches x' and v. The advantage of the embodiment of FIG. 2 over that of FIG. 1 is, however, that a larger number of taps can be accommodated with small potential differences between them.

If desired, boundary C' of FIG. 2 could be made symmetrical to boundary B' with provision of additional taps in the region of line D'. The median A' would then follow a straight line. It would also be apparent that, with a decrease in the radius of circle z, boundary B' may approach in its intermediate section the angular shape of boundary B shown in FIG. 1.