Abstract:
A biasing method for and IC with enhanced reverse bias breakdown. A field plate covering the surface PN junction and extending laterally therefrom is biased to partially deplete the island under the field plate and the substrate supporting the island is biased to complete the total depletion of the island under the field plate, establishing a substantially merged vertical field at less than critical for avalanche. Because most of the charge is required to support the vertical component of the field, the rate of change in the horizontal component is small per unit of additional terminal voltage and the lateral extension of the field plate increases the breakdown voltage beyond the plane breakdown for a PN junction of a given doping profile. Vertically isolated conductive material filled trenches laterally abutting the island may be used to decrease the lateral electric field in the corner of the island if the lateral extension of the field plate results in undesirable high field strengths or if proximity to the island edge creates field strength problems with island contacts or interconnect conductors.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to integrated circuits (&#34;ICs&#34;) and more specifically to an IC with increased reverse bias breakdown for a given doping profile. 
     Planar structures with high surface doping are desired for the islands of an IC because of low leakage, ease of manufacture and reproducibility. Where a well of one conductivity type has been created in an island of the other conductivity type to produce a PN junction, the crowding of the field at the PN junction at the surface of the island reduces reverse bias breakdown voltage for a given doping profile. 
     In ICs having such PN junctions, a depletion layer forms in the island when a reverse bias is applied to the PN junction, which layer has a depth directly (a) proportional to the reverse bias applied to the junction and (b) inversely proportional to 1/2 to 1/3 power of the doping concentration of the island. At high voltages the doping concentration of the island is small to avoid avalanche breakdown, and the layer of depletion beneath the PN junction may be quite deep under a reverse bias. To prevent the depletion layer from extending downwardly into the supporting substrate, the island must be thick, which results in an undesirable loss of packing density in the IC. 
     This reduction in island doping concentration also increases the series resistance of the PN junction in the forward direction and limits operating temperature. 
     It is known to use an appropriately biased field plate overlying at least the surface junction of a planar PN junction to increase the reverse bias breakdown voltage of the junction to thereby deplete the area of the island contiguous to and laterally beyond the surface PN junction. Such field plates mitigate, but do not completely eliminate, the degradation of planar breakdown relative to the breakdown of a plane junction having the same doping profile. 
     As is shown, e.g., by the Hartman, et al. U.S. Pat. No. 4,608,590, it is also known in gated diode devices to bias the insulated conductive substrate for the island to induce a field in the island and the insulator underlying the island, to thereby deplete the island directly between the PN junction and the substrate (i.e., the JFET channel), and thereby pinch off the conduction of current through the device. 
     Breakdown occurs in planar PN junctions along the sides of the junction near the island surface or, where there is a field plate, near the island surface around the end of the field plate where the field is crowded. Because breakdown does not generally occur between the PN junction and the bottom of the island, breakdown is not improved by depletion of this area. 
     Hartman, et al. neither disclose nor teach the use of the combination of field plate and substrate bias to totally deplete the island in the area under the field plate, i.e., an area substantially broader than the depletion area directly under the PN junction and encompassing the area where breakdown usually occurs. Inasmuch as an increase in the width of the field plate increases breakdown where total depletion is obtained, the lateral extension of total depletion may be used to increase breakdown to a value in excess of the plane breakdown of a PN junction with the same doping profile. 
     It is accordingly an object of the present invention to provide a novel method and PN junction structure with breakdown greater than breakdown that of the same junction and field plate when the substrate is at island voltage. 
     It is another object of the present invention to provide a novel method and PN junction structure with breakdown greater than breakdown of a plane junction with the same doping profile. 
     It is still another object of the present invention to provide a novel method and PN junction structure in which the island doping can be increased for a given breakdown, thus increasing the maximum operating temperature. 
     These and other objects of the present invention are attained by using the combined bias of an insulated field plate and substrate to totally deplete the entire portion of the island under the field plate before the critical field for avalanche is reached. In one aspect, the present invention combines a biasing which merges the depletion layers from a field plate and a biased substrate over an area sufficiently enlarged by enlargement of the field plate to increase breakdown beyond the breakdown for a plane PN junction with the same doping profile. 
     Where the same or electrically interconnected substrate is used for both vertical and lateral isolation (i.e., at both the bottom and sides of the island), the biasing of the laterally isolating substrate (i.e., at the sides of the island) may present corner breakdown problems in particular island geometries. For example, the potential difference between the biased substrate and either the island contact or a terminal interconnect may add to the field passing through the island adjacent a lateral edge thereof, i.e., the &#34;corner&#34; of the island. This increase in field strength may result in undesirable avalanche at a reduced terminal voltage. A more detailed explanation of corner breakdown may be obtained by reference to my copending application Ser. No. 08/053,343 filed concurrently herewith and assigned to the assignee hereof, the disclosure of said application being herein incorporated by reference. 
     It is accordingly another object of the present invention to provide a novel IC and method for enhancing breakdown by reducing corner breakdown, and more particularly by the separate biasing of the vertical and lateral island isolating substrates. 
     Other objects, advantages and novel features of the present invention will become apparent from the claims and from the following detailed description of preferred embodiments of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation in cross-section of an IC with a PN junction illustrating the extent of the field resulting from the use of a field plate alone; 
     FIG. 2 is an elevation in cross-section of the IC of FIG. 1 illustrating the extent of an additional field resulting from the biasing of the substrate; 
     FIG. 3 is an elevation in cross-section of an IC of FIG. 1 illustrating the merging of the fields from the field plate and the substrate in the area under the field plate; 
     FIG. 4 is an elevation in cross-section of a second embodiment of an IC illustrating the merger of the fields from the field plate and substrate; 
     FIG. 5 is an elevation in cross-section of a third embodiment of an IC illustrating the merger of the fields from the field plate and substrate; 
     FIG. 6 is an elevation in cross-section of a fourth embodiment of an IC in which the vertical and lateral isolation of the island has been electrically separated for independent biasing; 
     FIG. 7 is a plan view of a portion of an IC showing electrically connected lateral trenches; and 
     FIG. 8 is a plan view showing a portion of an IC with electrically isolated trenches. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The IC illustrated in FIG. 1 includes a conventional conductive substrate 10 (e.g., polysilicon) isolated from an N-semiconductor island 12 by a insulating layer 14 (e.g., oxide). A P+ conductivity type well 16 is formed in the surface of the island 12 forming a PN junction. A small N+ well 18 spaced from the well 16 may also be formed in the surface of the island 12 as the surface contact area for the island terminal 20. As a typical example, the depth of the island 12 therein may be in the range 5 to 100 microns, with the depth of the well 16 in the range 1 to 25 microns and desirably at least five microns from the insulation of the substrate. 
     A suitable dielectric layer 22 (e.g., oxide) is formed on the surface of the island. Thereafter, a cathode terminal 20 is connected to the island contact area 18, an anode conductor 24 is connected to the P well 16 and a substrate contact 26 may be formed through the insulator 22 in a conventional manner. A field plate 28 is formed at least over almost all of the surface PN junction and extends laterally in all directions therefrom. 
     To reduce the series resistance of the PN junction, the doping concentration of the island and the well is kept within two orders of magnitude of each other, e.g., in the range of 1×10 13  to 3×10 15  atoms per cubic centimeter for the island 12 and in the range of 5×10 14  to 3×10 17  atoms per cubic centimeter for the P well 16. If this is not a design requirement of the IC, the doping of the well and island need not be so constrained. It should be noted that a doping concentration well in excess of 1×10 14  atoms per cubic centimeter for the island has the effect of increasing the maximum temperature of operation of the PN junction. 
     FIG. 1 does not illustrate the existence of a field or the existence of a depletion layer as a result of the biasing of the substrate from the contact 26. For this no-field condition to exist, the potential of the substrate 10 (as established by the contact 26) must be approximately the potential of the island (as established by the island contact 20). 
     The use of the field plate 28 and the biasing of the substrate 10 allows for an increased impurity concentration in the island 12 as discussed above. 
     As shown in FIG. 2, the biasing of the substrate 10 may create a field with an accompanying depletion layer by creating a potential difference between the substrate 10 (as established by contact 26) and the island (as established by contact 20). While the fields do not meet in FIG. 2, they may be made to do so in the area under the PN junction by increasing the bias of the substrate as taught by Hartman, et al. supra. Merger of these fields under the PN junction will not necessarily improve the reverse bias breakdown because breakdown does not occur there. 
     In FIG. 3, the bias of the substrate 10 has been additionally increased to merge the fields from the substrate 10 and field plate 28 without avalanche, and thus to totally deplete the region under the field plate. For an appropriately large field plate, the breakdown may exceed the breakdown of a plane PN junction with the same doping profile. 
     In order to completely deplete the island 12 between the field plate 28 and the substrate 10, the field plate 28 and the substrate 10 must be reversed biased with respect to the island 12. Typically, the field plate 28 and the substrate 10 are connected to the anode 24 by a conductor. As illustrated in FIG. 3, the field produced by the field plate 28 meets the field produced by the substrate 10 to form substantially vertical field lines. The biasing is selected such that the island 12 between the field plate 28 and the substrate 10 totally depletes before the critical field for avalanche is reached in that region. It should also be noted that the region under the anode 24 can also be completely depleted before the critical field for avalanche is reached. 
     The field plate and substrate are biased to voltages less than the island for N conductivity type islands, and preferably at the voltage of the well therein. As earlier indicated, the difference in the doping concentration at the PN junction should be within two orders of magnitude of each other to reduce the series resistance of the junction. 
     Where total depletion is obtained, the application of additional voltage across the PN junction will increase only slightly the horizontal component of the field in these regions. Further depletion is possibly only in the area of the island laterally beyond the outer edge of the field plate 28. The horizontal rate of change in the field under the field plate 28 will be slower than in a conventional structure because much of the charge in this region is utilized to establish the vertical field lines as described above. Since by application of Poisson&#39;s equation: ##EQU1## 
     where E is the electric field; 
     where ε is the dielectric constant of the semiconductor; and 
     where Nc is the cathode (N island) impurity concentration. Therefore, a laterally extending region of high field can be obtained which is broader than disclosed by Hartman et al, supra, for an IC with the same doping profile, with the voltage V from anode diffusion to cathode diffusion given by the equation: 
     
         V=∫-E.sub.x dx                                        (2) 
    
     Since the horizontal component E x  of the field E is relatively constant under the field plate 28, a higher breakdown may be obtained by increasing the length of the field plate 28 laterally well beyond the PN junction. 
     A common implementation of the biasing of the structure shown in FIG. 3 would be to integrate the field plate 28 with the anode 24, and to bias the substrate 10 by connecting the substrate to the anode 24. Alternatively, and desirably where a high voltage between the substrate 10 and cathode contact 20 may induce breakdown, the substrate 10 may be biased to a value intermediate that of the anode 24 and cathode 20 voltages. This bias may be established by a voltage divider between the anode and cathode, either as part of the IC in a separate island or on the surface of the island as thin film resistors. 
     In an IC having several diodes and transistors at different voltages, it may be desirable to bias the substrate at the most negative voltage on the IC, or at a voltage intermediate between the most negative and most positive voltages on the IC. The bias may be set, e.g., at one half the rated voltage for the junction, or at one-half the voltage across the junction and may be made to vary dynamically therewith. Alternatively, the bias of the trench may be varied along its length and chosen as a function of proximity to an island contact or interconnect. 
     In addition to the junction diode illustrated in FIG. 3, the present invention is also applicable to Schottky barrier diodes as illustrated in FIG. 4. With reference to FIG. 4, the anode contact 24 is selected of an appropriate metal which forms at the island surface a Schottky barrier with the N type island 12. The field plate 30 may be an integral part of the anode contact and extend laterally therefrom. The insulating oxide 22 between the field plate 30 and the island 12 is desirably stepped so as to be the thinnest adjacent to the Schottky barrier region under the anode 24. As in FIG. 3, field plate contact and substrate contact 32 are reverse biased with respect to the cathode contact 20 so as to completely deplete the N type region 12. 
     Another embodiment of the present invention is shown in FIG. 5 where a bipolar transistor is illustrated. With reference to FIG. 5, an N+ emitter region 34 is formed in the P well anode region 36 which forms the base of the transistor. A base metal contact 38 is provided for the base region 36 and an emitter contact 40 is made to the emitter region 34. The closely spaced emitter and base metalization 38 and 40, respectively, jointly act as a field plate for the base-collector junction and cover substantially all portions thereof as well as a substantial portion of the island 12 laterally beyond the surface junction. 
     With continued reference to FIG. 5, the collector-to-emitter breakdown is increased over that of a conventional structure of the same doping profile by the fact that the field under the base 36 and the emitter 40 is limited to a value less than the collector-emitter avalanche field by the total depletion of the N region 12 there. Further, it is increased by the screening effect afforded by the total depletion from the field plate and the substrate bias of the adjacent N collector region 12. 
     Where the geometry of the IC dictates that the field therefrom passes through the edge of the island between the island contact or its connector and the biased substrate, the field may cause avalanche in the corner and thus reduce the breakdown of the device. As shown for example in the upper right hand corner of FIG. 3, a field may exist between the substrate 10 and the island contact area 18 (the heavy straight arrow) and/or between the substrate 10 and the conductor 20 (the heavy curved arrow). Such fields may be additive to the field resulting from the potential difference between the substrate 10 and the island 12 generally, and because of the short distance, cause avalanche. 
     These fields may be reduced by reducing the bias of the substrate 10, but such reduction in the areas under the field plate 28 will defeat the merging of the fields. In one aspect of the present invention, the bias of the vertical and lateral isolation of the island may be made independent so that (a) the fields can be made to merge under the field plate 28 while (b) the potential difference between the substrate (10) and the island contact 20 or its connector is kept small. 
     One embodiment of an IC with an independent vertical and lateral bias is illustrated in FIG. 6. With reference to FIG. 6 where like elements have been accorded like numerical designations, lateral isolation is provided by a trench 42 filled with a conductive substance 44 such as polysilicon. Where the vertical and lateral substrate bias voltages are independent, the lateral bias voltage as applied to contact 26 may be made to approximate the potential of the island contact 20 or its connector thereby minimizing the field therebetween and reducing corner breakdown. 
     As shown in FIG. 7, the trench 50 may be equipotential all of the way around the island 12. If so, the trenches of adjacent islands 12&#39;, 12&#34; etc., may be integrated to save space on the IC. However, as shown in FIG. 8, it may be desirable to separate the trenches 50, 50&#39;, 50&#34;, etc. so that they may be differently biased as a function of (a) the devices contained within the respective islands isolated thereby and/or (b) the potentials at which such devices are operated. 
     It may also be desirable to isolate different portions of the trench along its length so that critical areas such as the areas of a trench under the island interconnects or immediately adjacent the island contacts may be biased differently as needed. 
     Although the present invention has been described using as illustrative embodiments, particular conductivity type islands and wells, it is to be understood that the present invention is also applicable to ICs using the opposite conductivity type. As is known in the art, the island insulation may be tapered in thickness, and the doping concentration may have a gradient. 
     The depth of the island which is totally depleted may be greater than the depth which can be depleted by the bias on the field plate alone, thus increasing the design flexibility for the depth and impurity concentration of the island. 
     As discussed above, the biasing of the field plate and substrate totally deplete the island between them and increases breakdown for a given doping profile. This means that increased doping concentrations may be used for the same breakdown, thereby increasing the maximum operating temperature. 
     Where the geometry of the IC dictates the location of contacts or conductors adjacent the lateral edge of the island, the separate biasing of vertical and lateral substrates provide greater design flexibility, as does biasing the lateral isolation differently along its length. 
     While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those skilled in the art from a perusal hereof.