MOS transistor with enhanced isolation capabilities

Abstract: An isolating transistor (30) is provided to isolate N+ diffused regions (16b, 20b) of active devices (10, 12). The gate (32) of the isolating transistor (30) is of the same conductivity type as the underlying semiconductor surface (28), thereby increasing the threshold voltage of the isolating transistor (30).

Description: TECHNICAL FIELD OF THE INVENTION This invention pertains in general to integrated circuits, and more particularly to an integrated circuit of the type defined in the pre-characterizing part of claim 1. Further it relates to a method of forming isolated MOS transistors. In JP-A-60-66 444 & Patent Abstracts of Japan, vol. 9, no. 201 (E-336)[1924] August 17, 1985 an integrated circuit is described in which an electrode is formed on a field insulating film provided between MOS transistors. The electrode is held at the same potential as the substrate on which said MOS transistors are provided. In their structure the generation of leakage currents can be prevented. BACKGROUND OF THE INVENTION In the field of integrated circuits, it is necessary to provide isolation between active devices, thereby preventing conduction between the diffused regions of different transistors. Normally, isolation is provided by field oxide regions which form a large dielectric area between the diffused regions of the different transistors. However, field oxide regions require a large amount of surface area to provide adequate isolation. As the density of transistors on a semiconductor surface increases, the overhead required by the field oxide regions can no longer be tolerated. Thus, an alternative means of providing isolation is need. In many cases, isolation is provided by an "isolating transistor," which comprises a gate region between the diffused regions of two different transistors. The gate is biased such that the isolating transistor is turned off, thereby preventing conduction between the diffused regions. To provide an isolating transistor between two N type regions, the gate of the isolating transistor is usually grounded to turn off the transistor. However, during operation, noise spikes or ground bounce may cause the voltage threshold of about 2.5 volts to be exceeded, and hence, conduction between the diffused regions will occur. Consequently, the reliability of the device is reduced. While a voltage threshold of 2.5 volts is typical of today's technology, reduction of gate voltages will reduce the voltage threshold, further increasing the problem. Therefore, a need has arisen in the industry for an isolating transistor structure which reliably prevents conduction between diffused regions. SUMMARY OF THE INVENTION In the integrated circuit of the invention which has the features of the characterizing part of claim 1, an isolating transistor is provided which substantially eliminates or prevents the disadvantages and problems associated with prior isolating devices. The isolating transistor of the present invention uses a gate region having an impurity type which is the same as the underlying semiconductor surface with which it is associated. Therefore, if isolating is desired between two N type diffusion regions, a P type polysilicon gate is used, since a P type semiconductor region will be present between the N type diffused regions. Conversely, if isolation is desired between two P+ type diffusion regions, then an N type polysilicon gate is used. The present invention provides the technical advantage of a small isolating device, in which the voltage threshold between the gate region and the diffused region is maximized by taking advantage of different work function between N type and P type polysilicon. This allows the turn-on voltage of the field transistor isolating N channel transistors to be raised by approximately one volt without increasing the isolation oxide thickness. This difference will become increasingly important as devices and operating voltages are scaled smaller. BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings in which: FIGURE 1 illustrates prior art isolating transistor; FIGURE 2 illustrates the isolating transistor of the present invention; and FIGURE 3a-b illustrates a process for forming enhanced isolating transistors between P channel and N channel transistors without additonal masking steps. DETAILED DESCRIPTION OF THE INVENTION The preferred embodiment of the present invention is best understood by referring to FIGUREs 1-3 of the drawings, like numerals being used for like and corresponding parts of the various drawings. FIGURE 1 illustrates a prior art isolating transistor, used to isolate between two N+ diffused regions. As shown, two active N channel transistors 10 and 12 are separated by an isolating transistor 14. The N channel transistor 10 has N+ source/drain regions 16a-b and a polysilicon gate 18, while the N channel transistor 12 has source/drain regions 20a-b and gate 22. The isolating transistor 14 shares a source/drain region 16b with the transistor 10 and a source/drain region 20b with the transistor 12. The gate 24 of the isolating transistor 14 is connected to ground. A thin gate oxide layer 26 separates the gates 18 and 22 from the underlying semiconductor surface 28 and a thicker gate oxide 29 separates the isolating gate 24 from the underlying semiconductor surface 28. The gate 24 of the N channel isolating transistor 14 is maintained at ground voltage (zero volts). Normally, this will be sufficient to prevent a flow of electrons between the source/drain regions 16b and 20b associated with the isolating transistor. However, during operation of the active transistors 10 and 12, voltage spikes or ground bounce may cause the voltage to exceed the voltage threshold of the isolating transistor 14. In this instance, the isolating transistor 14 would turn on, thereby allowing current to flow between the source/drain regions 16b and 20b. One way to prevent the inadvertent conduction described above would be to connect the gate 24 of the isolating transistor 14 to a negative voltage. However, in practice, this solution would require that an additional negative voltage would have to be supplied on chip which would add to the complexity and cost making it undesirable. FIGURE 2 illustrates a isolating transistor of the present invention. In this instance, the isolating transistor 30 comprises a P-type polysilicon gate over the channel between the source/drain regions 16b and 20b. The work function of the P type polysilicon s approximately one volt higher than that of the N type polysilicon used in prior art. This advantageously raises the turn-on voltage of the isolation transistor 30 by approximately one volt over that of isolating transistor 14. This higher threshold voltage of the isolating transistor 30 of the present invention allows a greater charge to be present on either of the source/drain regions 16b or 20b before conduction occurs between the source/drain regions. The gate 32 of the isolating transistor 30 of the present invention may be doped in conjunction with the P type source/drain implantation for the P channel transistors, thereby saving an implantation step. To do so, the gates which should remain N type must be masked prior to the implantation. Since a mask is already used for this purpose during source/drain implantation (See FIGUREs 3a-b), no additonal masking steps would be needed. Similarly, the isolating gates formed between adjacent P channel transistors should be doped with an N type dopant in order to increase the threshold voltage of the isolating transistors. The threshold voltage of a transistor is determined primarily by the work function of the gate, the substrate doping, and the thickness of the gate oxide. When transistors are used, as in the case of field plates, to provide isolation between adjacent transistors, the threshold voltage is made as high as possible to prevent the isolation transistor from turning on. In prior art, the oxide is made thick and the substrate doped with a channel stop implant to raise the substrate doping thus raising the threshold voltage. As devices are scaled smaller and higher speed parts are demanded, limits are imposed upon the isolation oxide thickness and the channel stop implant dose. This disclosure teaches the use of the difference in work function between N type and P type polysilicon gates to further improve isolation. The turn-on voltage is directly proportional to the difference between the work function of the gate material and that of the underlying substrate. Vt α  gate -  substrate =  ms For N type polysilicon gate = 4.15 and for P type polysilicon  gate = 5.25. Thus, by using P type polysilicon rather than N type polysilicon for field plate isolation over the P wells, the Vt of the isolation transistors can be raised by at least one volt. A process to dope the isolating gates and diffused regions of the transistors is illustrated in FIGUREs 3a-b. After forming the Nwell region 34 in the P substrate 35 for housing the P channel transistors 36, the gate oxide layer 37a, field oxide regions 37b, and the undoped polysilicon gates 38 are formed using standard processes. A photoresist mask 40 is patterned to expose the regions in which the P type source/drains 42, P channel gates 42a, and the P type isolating gates 43 will be formed. The exposed regions are doped with a P type dopant, typically by implantation and diffusion. In FIGURE 3b, a second photoresist mask 44 covers the P type source/drains 42, and P channel gates 42a are along with the P type isolating gate 43 and an N type dopant is used to dope the remaining MOS transistor gates 45, the N type isolating gate 46 and the N type source/drains 48. Subsequently, the photoresist mask 44 is removed. Since the masks 40 and 44 would normally be used in a CMOS process to define the source/drains 48, additional masks are not necessary to implement the process of the present invention. It should be noted that p-n junctions formed in the polysilicon where the n type and p type regions meet would be a problem if a silicide layer on the polysilicon were not used to short out these junctions. Using current silicide technologies, this problem is avoided. The present invention provides the technical advantage that a reliable isolating transistor is provided without additional masking steps or implantations. The isolating transistor has a high threshold voltage thereby reducing the possibility of inadvertant conduction between its associated source/drain regions.

Claims: An integrated circuit comprising an isolating MOS transistor (30) for providing isolation between two N channel MOS devices (10, 12), said isolation MOS transistor (30) comprising: a P type semiconductor region (28); two N type source/drain regions (16b, 20b) formed in said P type semiconductor region (28) , one of said source/drain regions (16b) associated with one (10) of the N channel devices and the other source/drain region (20b) associated with the other N channel device (12); and a gate region (32) separated from said P type semiconductor region (28) by a thick insulating layer (29) and defining a channel between said source/drain regions (16b, 20b), said gate region (32) being connected to a predetermined voltage to render said channel non-conductive, characterized in that said gate region (32) is of P type conductivity to provide an increased threshold voltage and thereby reduce the possibility of inadvertant conduction between said source/drain regions (16b, 20b). The integrated circuit of claim 1, wherein said source/drain regions (16b, 20b) comprise N+ type diffusion regions. The integrated circuit of claim 1, wherein said predetermined voltage is ground voltage. The integrated circuit of claim 1 further comprising a second isolation MOS transistor for providing electrical isolation between P-channel transistors having P type source/drain regions formed in an N type semiconductor region, said second isolation MOS transistor comprising: N type gate regions separated from the N type semiconductor region between one of the P-channel and N-channel transistors. A method of providing isolation between two N type diffused regions (16b, 20b) disposed in a P type semiconductor region (28), comprising the steps of: providing a P type polysilicon gate region (32) separated from the portion of the semiconductor region (28) between the diffused regions (16b, 20b) by a thick insulating layer (29); and applying a predetermined voltage to the polysilicon gate region (32). The method of claim 5 wherein said step of appyling a predetermined voltage comprises the step of applying ground voltage to said polysilicon gate (32). A method of forming isolated MOS transistors (36) in an integrated circuit, ones of the transistors (36) having source/drain regions (42) of a first conductivity type formed in a semiconductor region (34) of a second conductivity type and other ones of the transistors have a source/drain regions (48) of the second conductivity type formed in a semiconductor region (35) of the first conductivity type, comprising the steps of: forming thick oxide regions (37b) on the semiconductor region (35) of the first conductivity type between adjacent transistors having source/drain regions (48) of the second conductivity type and forming isolating gates (43) of a first conductivity type on said oxide regions (37b); forming thick oxide regions (37b) on the semiconductor region (34) of the second conductivity type between adjacent transistors (36) having source/drain regions (42) of the first conductivity type and forming isolating gates (46) of a second conductivity type on said oxide regions (37b); and forming interconnects between said isolating gates and a predetermined voltage. The method of claim 7 wherein step of forming said isolating gate comprises the steps of: forming a polysilicon layer over the semiconductor region (34,35) of the first and second conductivity types; removing portions of said polysilicon layer to define gates for the MOS transistors and said isolating gates; doping the isolating gates (46) between source/drain regions (42) of the first conductivity type with a dopant of a second conductivity type; and doping the isolating gates (43) between source/drain regions (48) of the second conductivity type with a dopant of the first conductivity type. The method of claim 8 wherein said step of doping said isolating gates (46) between source/drain regions (42) of the first conductivity type comprises the steps of: forming a first mask exposing the source/drain regions (42) of the first conductivity type and isolating gates (43) between source/drain regions (48) of the second conductivity type; and doping the source/drain regions and isolating gate regions exposed by said first mask with a dopant of the first conductivity type. The method of claim 9 wherein said step of doping the isolating gates (43) between source/drain regions (48) of the second conductivity type comprises the steps of: forming a second mask exposing the source/drain regions (48) of the second conductivity type and isolating gates (46) between source/drain regions (42) of the first conductivity type; and doping the source/drain regions and isolating gate regions exposed by said second mask with a dopant of the second conductivity type. The method of claim 7 wherein said forming interconnects step comprises the step of forming interconnects between said isolating gates and ground voltage.

Inventors: PATERSON JAMES L, TIGELAAR HOWARD L, PATERSON, JAMES L., TIGELAAR, HOWARD L.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1988
Main classification: H01L29
IPC codes: H01L29, H01L21, H01L27