Abstract:
A high-voltage resistor integrated on a semiconductor substrate with opposite sign conductivity, and being of a type with one end connected to the substrate and another end connected to a lower electric potential than the substrate, further comprises at least one thin layer of the field plate type covering at least a section of the resistor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 08/067,664, filed May 26, 1993, now abandoned. 
    
    
     TECHNICAL FIELD 
     This invention relates to a high-voltage resistor integrated on a semiconductor substrate, having an opposite conductivity type, and being of a type with one end connected to the substrate and another end connected to a lower electric potential than the substrate. 
     BACKGROUND OF THE INVENTION 
     In the specific field of application of this invention, it is known to provide high-voltage resistors having a horizontal geometry which is basically of the spiral pattern type and is formed on a semiconductor substrate having a predetermined polarity, opposite in sign from that of the resistor. 
     One end of the spiral is connected to the substrate, and the other end is connected to ground or in any case to a lower electric potential than the substrate. 
     One such construction is known, for instance, from U.S. Pat. No. 4,792,840 and allows the resistor to so distribute the equipotential lines across the substrate surface as to hold the difference of potential at the junction points between the resistor and the substrate surface below the junction breakdown voltage. The potential difference varies linearly toward the spiral center, which allows the resistor to withstand high voltages of up to 400 Volts. 
     While being beneficial from this point of view, said prior approach still has a serious drawback. 
     When any two regions of the resistor are taken which lie close to each other, i.e., regions of two adjacent turns, one of them is bound to be at a lower potential than the other. As a result, the positive charges present in the oxide layer that covers the spiral will be pulled toward the metallic contact associated with the lower potential region. 
     The build-up of positive charges gradually forms a conductive layer proper, referred to in the art as a phantom layer, which extends toward the higher potential region. 
     This phantom layer, being additionally connected to a negative potential present at the metallic contact, establishes conduction between the aforesaid two underlying regions at different potentials. 
     Briefly stated, the phantom layer functions as if it were the gate of a MOS transistor, with the two regions at different potentials corresponding to the source and drain of the transistor. 
     This causes a deep alteration of the spiral resistor characteristics, and deterioration of its reliability. 
     SUMMARY OF THE INVENTION 
     According to principles of the present invention, a high-voltage resistor is provided of the type which has constructional and performance characteristics that overcome the serious drawbacks besetting the prior art. 
     Accordingly, it is an object of this invention to provide a resistor exhibiting superior reliability. 
     Another object of the invention is to arrange for the resistor to always have one end connected to the substrate and the other end to ground, or more generally stated, to have the other end connected to a lower electric potential than the substrate. 
     According to principles of the present invention, the resistor is partially covered with a field-plate layer. 
     Consistent with the inventive solutions, the technical problem is solved by a resistor as indicated and defined in the claims, the invention being determined by the scope of the claims. 
     The features and advantages of a resistor according to this invention will become apparent from the following detailed description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view showing schematically a resistor according to the invention as integrated on a semiconductor substrate; 
     FIG. 2 is a schematic cross-sectional view taken through the resistor in FIG. 1 along line II--II; and 
     FIG. 3 is a top plan view showing schematically a modified embodiment of the inventive resistor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the drawing views, generally and schematically shown at 1 is a resistor formed, in accordance with the invention, on a semiconductor silicon substrate 2 of an integrated circuit or chip. 
     The substrate 2 comprises a first base region 2A having predetermined conductivity, e.g., of the N+ type, and being covered with an epitaxial layer 3 of the N+ type. The latter 3 is conventionally covered with an oxide layer 16. 
     The resistor 1 is obtained by a doping opposite that of the epitaxial layer 3, e.g., of the P+ type. 
     The resistor 1 is formed into an essentially elongate linear pattern, or stated otherwise, is a region characterized by a reduced width with respect to its length to provide a desired resistance value. This region can be formed using conventional diffusion or implantation techniques so as to rise substantially to the surface of the epitaxial layer 3. 
     The epitaxial layer 3 ensures electrical insulation all around the resistor 1. 
     The substrate 2 has an edge region 10 to which one end 8 of the resistor 1 is connected. This connection is accomplished through a pair of metallization contacts 4 and 5. The other end 9 of the resistor 1 is connected to ground or, more generally, to a lower electric potential than the substrate 2. 
     Alternatively, in a different embodiment, the end 8 of the resistor is connected to a first, high voltage potential that is not the substrate voltage and the end 9 is connected to a second, lower voltage potential than that to which the end 8 is connected. 
     The electric potential on the resistor 1 decreases linearly, from the end 8 connected to the substrate 2, to the grounded end 9. 
     Concurrently therewith, the potential difference at the junction 7 between the P+ region of the resistor 1 and the N- epitaxial layer 3 increases. The potential difference between the P+ region of resistor 1 and the N- epitaxial layer 3 is greater near the end 9, than near the end 8. This potential difference could reach the breakdown value for the junction 7. 
     However, the invention advantageously provides for thin metallization layers 6 of the field plate type to be deposited over the resistor 1 which extend through some sections of the resistor 1. 
     Such field-plate layers 6 extend over the P+ region 1 and somewhat over the epitaxial layer 3 beyond the resistor sides. Specifically, the field plate layers 6 also cover the N-P junction 7 on both sides of the resistor 1 and the width of the field plate layers is slightly increased from the edge end 8 to the other end 9. That is, the distances by which the metallization of the field plate layers 6 extend beyond the resistive channel is larger adjacent the end 9 than the end 8. The overlap at the edge nearest the end 9 of each individual field plate layer 6 is thus correspondingly larger than the overlap at its edge nearest the end 8. 
     Shown in FIG. 2 is the effect produced on the resistor 1 by the thin field plate layers 6. 
     The field plate 6, being connected to the P+ region 1 subjected to a lower voltage than that of the N- epitaxial layer 3 containing it, will alter the pattern of equipotential lines 12 in the proximities of the junction 7. This raises considerably the value of the breakdown voltage of the junction 7. 
     On the resistor 1, there are opened vias 11 whereby the various field plates 6 are connected. These vias 11 are arranged centrally of each field plate 6, although the connection could alternatively be established at some off-centered location. Any off-centered connection would not affect the pattern of equipotential lines 12 materially. 
     A modified embodiment of the inventive resistor 1 will now be described with specific reference to the example shown in FIG. 3. 
     In this variant, cooperating parts and details which are constructed and operated in a like manner to the previous embodiment are denoted by the same reference numerals. 
     As shown in FIG. 3, the end 8 of the resistor 1 is connected, in this variant, to the substrate 2 directly at the edge region 10 by via 5 which provides a metallic contact. 
     The other end 9 of the resistor 1 is connected to an insulative region 14 formed centrally of the integrated circuit. 
     This insulative region 14 is also connected to ground in a conventional way. 
     The resistor 1 is set substantially at an inclination angle α to the edge 10 which is a direct function of the voltage gradient through the resistor 1. 
     The resistor inclination to the edge 10 takes into account the fact that the potential difference of the resistor 1 relative to the edge 10 increases linearly toward the end 9. As a result, the distance between the N+ edge 10 and the P+ region of the resistor 1 increases concurrently with increasing potential across the distance from the P+ region of resistor 1 to the edge 10. 
     Concurrently therewith, however, the distance between the resistor 1 and the insulative region 14 decreases as the related potential difference decreases. 
     The resistor 1 of this invention solves, therefore, the technical problem, and affords a number of advantages, of which most important is the increased breakdown voltage across the junction 7 between the resistor 1 and the epitaxial layer 3 on which it is formed. This reflects in improved reliability of the device.