Abstract:
A novel device structure and process are described for an SCR ESD protection device used with shallow trench isolation structures. The invention incorporates polysilicon gates bridging SCR diode junction elements and also bridging between SCR elements and neighboring STI structures. The presence of the strategically located polysilicon gates effectively counters the detrimental effects of non-planar STI “pull down” regions as well as compensating for the interaction of silicide structures and the effective junction depth of diode elements bounded by STI elements. Connecting the gates to appropriate voltage sources such as the SCR anode input voltage and the SCR cathode voltage, typically ground, reduces normal operation leakage of the ESD protection device.

Description:
This is a division of patent application Ser. No. 09/941,278, filing date Aug. 29, 2001, now issued as U.S. Pat. No. 6,605,493, Silicon Controlled Rectifier ESD Structures With Trench Isolation, assigned to the same assignee as the present invention. 

   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention relates generally to a structure and manufacturing process of a semiconductor device which provides improved ESD protection for internal active semiconductor devices and more particularly to a semiconductor SCR like device which when used with shallow trench isolation, provides improved parasitic bipolar characteristics resulting in improved ESD protection performance. 
   (2) Description of Prior Art 
   The discharge of electrostatic energy from the human body or other sources known as Electrostatic discharge (ESD) into the input or output pads of integrated circuit semiconductor devices has shown to cause catastrophic failures in these same circuits. This is becoming more important as modern metal oxide semiconductor circuit technology (MOS) is sealed down in size and increased in device and circuit density. Prevention of damage from ESD events is provided by protection devices or circuits on the input or output pads of the active logic circuits which shunt the ESD energy to a second voltage source, typically ground, thereby bypassing the active circuits protecting them from damage. Various devices such as silicon controlled rectifiers (SCR) have been utilized to essentially shunt the high ESD energy and therefore the ESD stress away from the active circuits. 
   Isolation is required between these ESD protection devices and the active circuit devices, as well as between the active devices themselves. Originally areas of local thick oxide, often called LOCOS or field oxide, have been used to provide this isolation. While having good isolation properties, this isolation method uses more surface area, or “real estate”, than an alternative isolation method using shallow, relatively narrow trenches filled with a dielectric, typically silicon oxide (SIO 2 ), called shallow trench isolation (STI). 
   While providing good isolation properties, the STI structure has limiting effects on the current triggering and capacity of the SCR ESD protection devices. As discussed in the paper “Semiconductor Process and Structural Optimization of Shallow Trench Isolation-Defined and Polysilicon-Bound Source/Drain Diodes for ESD Networks” by Voldman et al., EOS/ESD Symposium 98-151, pages 151 to 160, during STI formation, the STI region is exposed to the etching process, leading to non-planer STI edges where the silicon region extends above the isolation edge. The non-planer STI edge is called “STI pull-down”. The impact of STI pull-down, and the interaction with the salicide process typically used in current contact technology, as well as junction depth reduction of the diode elements bounded by the STI devices, all degrade ESD protection capabilities by reducing the parasitic bipolar current gain, beta, (β). This increases the holding voltage and trigger current of the lateral SCR, reduces lateral heat transfer capability, and possibly limits the type of ESD networks implemented. Among other things, this can result in device failure before the SCR is fully on, or a high on-resistance for the SCR reducing the ESD failure threshold. 
     FIG. 1A  is a simplified cross section of a typical prior art SCR ESD protection device. Shown is a P substrate  10 , with an N-well  12  and which contains contact regions N+  16  and P+  18 . The N-well  12  contact regions are isolated and bounded by the shallow trench isolation (STI) structures  14 A,  14 B and  14 C. The N-well  12  is also bounded by STI elements  14 A and  14 C. The P substrate also contains N+ contact  20  bounded by STI elements  14 C and  14 D, and P+ contact  22  bounded by STI structures  14 D and  14 E. Also depicted in  FIG. 1A  are parasitic vertical PNP bipolar transistor T 1  and lateral NPN bipolar transistor T 2  with parasitic resistors R 1  and R 2 . As is well recognized, an SCR device is essentially a P-N-P-N structure as depicted in FIG.  1 B. The P+ contact  18  is the anode end of the device and is connected to the active circuit input or output pad  8  as well as to the N+ N-well contact  16 . The junction between the P+ contact region  18  and the N-well  12  is the first junction of the SCR. 
   The N-well  12  and the P substrate  10  form the second junction. The third device junction is formed by the substrate  10  and substrate N+ contact  20 , which also is the cathode terminal of the device. N+ contact  20  is connected to a second voltage source  24 , typically ground, and also to substrate P+ contact  22 .  FIG. 1C  represents the electrical schematic of the prior art device showing the parasitic vertical bipolar PNP transistor T 1  and parasitic lateral NPN bipolar transistor T 2  as well as the resistors R 1  and R 2 . A positive ESD voltage event will cause the T 1  base-collector junction to go into avalanche conduction, turning on T 2  and providing the regenerative conduction action shunting the ESD current to the second voltage source, typically ground. A negative ESD voltage pulse will forward bias the base-collector junction of T 1 , again shunting the current to the second voltage source. 
   However, as indicated above, the STI isolation structures inhibit lateral current conduction near the surface, lower the parasitic bipolar semiconductor current gain, and can interfere with device thermal characteristics. 
     FIG. 2A  represents another prior art protection device, a low voltage trigger SCR (LVTSCR). There is no STI between the N-well N+ contact  16  and SCR P+ anode  18 . The STI structure has essentially been replaced by a N+ region  28  straddling the N-well to P substrate lateral boundary. A FET gate has been inserted between the N+ region  28  and the N+ region  20  which essentially become the drain and source of a NFET respectively. The NFET source region also functions as the SCR cathode. The prior art LVTSCR device operational trigger voltage is reduced by the NFET device breakdown voltage. The STI elements still reduce the desirable ESD protection characteristics as previously discussed. 
     FIG. 2B  represents a prior art modified lateral SCR. This device does not have the NFET of the LVTSCR, but retains the N+ region  28  straddling the N-well  12  and the P substrate  10  lateral boundary and which provides an additional source of current for triggering the SCR thereby enabling a lower trigger voltage than a more conventional SCR. 
   The invention in various embodiments allows selective use of STI elements while improving ESD protection by the strategic use of polysilicon gates 
   The following patents describe ESD protection devices. 
   U.S. Pat. No. 5,465,189 (Polgreen et al.) shows a SCR with isolation. 
   U.S. Pat. No. 5,012,317 (Rountree) shows a conventional SCR protection device. 
   U.S. Pat. No. 4,939,616 (Rountree) sows another SCR type device. 
   U.S. Pat. No. 6,081,002 (Amerasekera et al.), U.S. Pat. No. 5,629,544 (Voldman et al.), U.S. Pat. No. 6,074,899 (Voldman et al.), U.S. Pat. No. 5,945,713 (Voldman), and U.S. Pat. No. 5,923,067 (Voldman) show related SCR protection devices which use STI elements. 
   The following technical report discusses STI bound ESD protection networks 
   “Semiconductor Process and Structural Optimization of Shallow Trench Isolation-Defined and Polysilicon-Bound Source/Drain Diodes for ESD Networks” by Voldman et al., EOS/ESD Symposium 98-151, pages 151 to 160. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is the primary objective of the invention to provide a novel, effective structure and manufacturable method for protecting integrated circuits, in particular field effect transistor devices, from damage caused by electrostatic discharge (ESD) events during normal operation. 
   It is a further objective of the invention to improve ESD protection involving SCR elements employing shallow trench isolation (STI). 
   In addition, it is an objective of this invention to minimize degradation in the SCR diode device characteristics such as diode leakage. 
   It is yet another object of the invention to provide a manufacturable method for forming the SCR ESD protection structure while maintaining the required operating characteristics of the devices being protected. 
   The above objectives are achieved in accordance with the embodiments of the invention that describes a novel structure and process for a SCR like ESD protection device. The device is situated on a semiconductor substrate, typically P doped, and containing a N-well with P+ and N+ contact regions. A STI structure defines one N-well-substrate lateral boundary as well as the N-well N+ contact lateral boundary near the substrate surface. A second STI structure defines the N-well to P substrate lateral boundary near the substrate surface on the N-well opposite the N+ contact region. A third STI structure defines one lateral or horizontal boundary near the surface for the P+ substrate contact. 
   A N-well P+ contact and a N+ substrate contact are also defined. The N-well P+ region forms the anode of the SCR device, and is electrically connected to the N+ N-well contact and to the active logic device input or output pad. The substrate N+ element forms the SCR cathode and is electrically connected to the substrate P+ contact and to a second voltage source, typically ground. A feature of the invention uses gate elements, typically polysilicon with salicides, that overlay the surface regions between the N-well N+ and P+ contact regions and the N-well P+ contact region and adjacent STI. The N-well contacts and associated gate elements are electrically tied together and to the first voltage source, typically the active device input pad. 
   Similar gate elements overlay the surface regions between the substrate P+ and N+ contact regions and the substrate N+ and adjacent STI element. The substrate contacts and associated gate elements are electrically connected together and to the second voltage source, typically ground. The uniqueness of the invention structure is in the insertion of the gate elements and the elimination of the prior art STI structure between the anode N+-P+ contacts and the cathode N+ and P+ contacts. 
   In alternative invention embodiments, the gate structures are utilized in low voltage trigger SCR (LVTSCR) devices and also in the modified lateral SCR device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a cross sectional representation of a prior art SCR ESD protection device structure showing the isolation elements and parasitic bipolar elements. 
       FIG. 1B  is a representation of the junction elements of the prior art SCR ESD protection device. 
       FIG. 1C  represents the electrical schematic of the prior art SCR ESD protection device. 
       FIG. 2A  represents a prior art low voltage trigger SCR (LVTSCR) protection device cross section. 
       FIG. 2B  represents a prior art modified lateral SCR ESD protection device. 
       FIG. 3A  is a representation of the cross section of one embodiment of the invention for a SCR ESD protection device. 
       FIG. 3B  is a representation of an FET gate detail. 
       FIG. 4  is a top view of one embodiment of the invention for a SCR ESD protection device showing the horizontal topography of this embodiment. 
       FIG. 5  is a simplified cross section of another embodiment of the invention for a LVTSCR. 
       FIG. 6  is a simplified cross sectional representation of another embodiment of the invention for a modified lateral SCR ESD protection device. 
       FIGS. 7A through 7F  shows cross section representations of the invention for a SCR ESD protection device at various stages of the manufacturing process. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3A  shows a simplified cross section of one embodiment of the invention. A P doped substrate  110  with typical doping concentration of between 1E14 and 1E16 atoms/cm 3  (a/cm 3 ) contains an N-well  112  with typical dopent concentration between 1E16 and 1E18 a/cm 3 . The N-well  112  is bounded at and near the surface by shallow trench isolation (STI) elements  114 A and  114 B, typically between 0.2 to 1 um wide and 0.4 to 2.5 um deep. The STI elements are filled with a dielectric, typically silicon oxide (SiO 2 ). Within the N-well region  112  are a N+ 116  and P+ 118  contact regions, with typical dopent densities of between 1E19 and 1E21 a/cm 3 . The N+ region is bounded on the side away from the P+ contact  118  by the STI  114 A. Overlaying the surface between the N+ contact  116 , P+ contact  118 , and between P+ contact  118  and STI  114 B, is gate element  126 . As depicted in  FIG. 3B , the gate element  126  is composed of gate oxide insulation  126 C, typically SiO 2  with a thickness between 50 and 180 angstroms (Å), and a doped polysilicon conductor element  126 B with a thickness between 1500 and 3000 Å and with a refractory metal  126 A such as titanium (Ti) or cobalt (Co). The polysilicon (poly) is typically doped with a donor element such as phosphorous to a density of between E17 and E21 a/cm 3  to improve conductivity. The use of this gate element enables the elimination of an STI isolation element between the two contacts, which improves the lateral current conduction and Joule heating capability for an ESD event. 
   The P+ contact  18  is the anode of the SCR device, and is electrically connected to the N+ contact  116 , the gate element  126 , and the active device input or output pad  108 . 
   The substrate  110  has N+ contact  120  and P+ contact  122 , with a typical dopent concentration of between 1E19 and 1E21 a/cm 3  of donor and receptor dopents respectively. A gate element  132  overlays the surface between the STI  114 B, the N+ contact  120 , and the P+ contact  122 . The P+ contact  122  is bounded by STI  14 C. As depicted in  FIG. 3B , gate elements  126  and  132  are constructed of an insulator, SiO 2    126 C/ 132 C, a doped polysilicon conducting element  126 B/ 132 B, and a silicide  126 A/ 132 A. The presence of the gates  126  and  132  bounding the STI region  114 B have the effect of reducing leakage current when connected to the appropriate voltage sources, that is typically to the anode voltage for gate element  126  and to the cathode voltage for gate element  132 . 
   The N+ contact  120  is the SCR cathode and is electrically connected to the gate  132 , the substrate P+ contact  122 , and a second voltage source  124 , typically ground. The presence of the gate  132  allows for the elimination of an STI isolation structure between the N+ contact  120  and the P+ contact  122 , once again contributing to improved lateral current conduction and improved Joule heating characteristics for an ESD event. The connection of the gates  126  and  132  to the respective voltage sources also has the benefit of reducing device leakage during normal circuit operation. 
   As depicted in  FIG. 4 , the gate elements  126  and  132  have a rectangular horizontal topology that effectively bounds P+ contact region  118  and N+ contact region  120  respectively. The STI structures  114 A and  114 B are indicated by the dotted lines in FIG.  4 . 
   Another embodiment of the invention is shown in FIG.  5 . The unique design of the invention improves the ESD protection of a LVTSCR device. The SCR trigger voltage can be reduced by design by inserting a N+ doped region  128  on the lateral boundary between the N-well  112  and the substrate  110 . This region has a dopent concentration typically between 1E19 and 1E21 a/cm 3  and forms the drain of a N-channel thin oxide field effect transistor (FET). The FET N+ drain  128  connects to the N region N-well  112  base of the SCR and the FET N+ source  120 , which also serves as the SCR cathode, is connected to the second voltage source, typically ground. This arrangement has the effect of lowering the trigger voltage of the SCR by the design of the channel length and/or the gate oxide thickness of the FET to provide a LVSCR element. The unique structure of the invention design places gate  126  between the N-well N+ contact  116  and the SCR P+ anode  118 , and the P+ anode  118  and the FET drain  120 . Again, this eliminates the need for any STI structures between theses elements and at the same time limits leakage by connecting the gate to the SCR anode  118 . Similarly, the invention structure places gate  132  between the FET source  120  and the substrate P+ contact  122 . This gate  12  enables the elimination of STI element  14 D, and serves to reduce the leakage current between N+ region  120  and P+ region  122   
   In yet another embodiment depicted in  FIG. 6 , the invention is applied to the modified lateral SCR, resulting in improved ESD protection capabilities over the prior art design. In the method of the invention, a polysilicon gate  126  with SiO 2  insulator, poly conductor and silicide contact element, bridges the N-well N+ contact  116  and the N-well P+ contact  118 , and also is between the P+ contact  118  and the N+ region  128 . Another gate element  132  bridges the substrate N+ contact  120  and the substrate P+ contact  122 . The N-well N+ contact  116 , gate  126 , and the N-well P+ contact  118 , which also serves as the device anode, are electrically connected together and to the active circuit input pad  108 . 
   The substrate N+ contact  120 , which also serves as the SCR cathode gate  132 , and the substrate P+ contact  122  are electrically connected together and to a second voltage source  124 , typically ground. A STI structure  114 F provides isolation between the N+ contact region  128  and the cathode N+ region  120 . 
   The process to develop an embodiment of the invention is outlined in FIG.  7 A through  FIG. 7F  which shows the structure for a P substrate in various stages of processing.  FIG. 7A  represents a patterned semiconductor substrate  110  with a nominal P doping level of between about 1E14 and 1E16 a/cm 3 . The substrate surface is covered by a thermally grown or chemical vapor deposition (CVD) first oxide layer  140 , sometimes called the pad oxide, which provides thermal stress relief. This layer is typically between 200 and 600 Å in thickness. A CVD layer of silicon nitride (SiN)  142  derived typically from a silane or dichlorosilane source element is placed over the pad oxide as a masking element to a thickness of between 1000 and 2000 Å. This in turn is covered by a conventional photolithographic masking material such as photo resist (PR)  144  with a thickness typically between 4000 and 10,000 Å. The structure as shown in  FIG. 7A  has been patterned in preparation for the N-well  112  doping. A donor dopent, typically phosphorous (P), is implanted with a typical dosage range of between 1E15 and 1E17 a/cm 2 ) and with an energy range of between 30 and 80 KeV. This produces a N-well doping density of between 1E16 and 1E18 a/cm 3 . 
     FIG. 7B  represents the partially completed device after the N-well masking elements have been removed, shallow trench isolation (STI) elements have been inserted into the structure, and the substrate patterned in preparation for the doping of the N+ contracts regions,  116 ,  120 . The STI elements are created with an etching process, typically a dry anisotropic plasma etch to form the trenches, typically to a depth of between 0.4 and 2.5 um in depth and between 0.2 and 1 um in width. The trenches are subsequently filled with a silicon oxide (SiO 2 ) by a low-pressure chemical vapor deposition (LPCVD), or a sub-atmospheric CVD (SACVD), or a high-density plasma process. After filling, the STI elements are planarized by either an etch process or, more typically, a chemical mechanical polish (CMP) process. The SiN layer  142  is removed, typically using a hot phosphoric acid (H 3 PO 4 ) with a temperature between about 150 and 180 degrees centigrade (°C.), and the pad oxide has been removed typically using dilute hydrofluoric acid (HF). 
   A gate oxide layer  146  is then thermally grown to a thickness of between about 50 and 180 Å, and a layer of polysilicon (poly)  148  has been deposited by LPCVD to a thickness of between 1500 and 3000 Å to serve as part of the gate conductor system. The LPCVD poly process uses a 100% silane source, or, alternatively, a gas stream containing N 2  or H 2 . The poly is typically doped with a donor element such as As to produce a dopent concentration of between 1E17 and 1E21 a/cm 3  to improve conductivity. The N+ contact regions  116  and  120  are doped with a donor element as indicated in  FIG. 7B , typically arsenic (As), with a dosage level between about 1E13 and 1E15 a/cm 2 , and with an energy between 20 and 40 KeV. This results in N+ contact regions with a donor concentration of between about 1E19 and 1E21 a/cm 3 . 
     FIG. 7C  shows the partially completed device patterned and prepared for the P+ contact region implant. This is done with an acceptor element, typically boron (B) with a dosage of between about 1E12 and 1E13 a/cm 2 , and an implant energy of between 40 and 80 KeV resulting in a P+ contact area with a dopent concentration of between 1E19 and 1E21 a/cm 3 . The device is then patterned to remove the gate oxide and polysilicon from regions where not required, and an oxide layer  150 , is deposited, typically by LPCVD as shown in FIG.  7 D. 
   This oxide  150  is patterned in preparation for etching metal contact opening, typically using a RIE anisotropic etch process, to the N+ regions  116  and  120 , and the P+ regions  118  and  122 , and the contact regions of gates  126  and  132 . This is followed by a blanket evaporation of metal, typically using aluminum or 1% silicon doped aluminum, but could be other alloys such as titanium platinum. The main metallurgy system could also be used in conjunction with refractory type “barrier” metals such as titanium-tungsten (TiW) or titanium nitride (TiN). Most commonly used methods for developing the metallurgy system on the wafer are vacuum evaporation using either filament, electron beam or flash hot plate as sources, or physical vapor deposition (PVD) commonly known as sputtering. Common sputtering methods would be RF sputtering or magnetron sputtering. With any method, the wafer is blanketed with the metal, the patterned and unwanted metal removed by etching resulting in a structure shown in FIG.  7 E. 
   The metal element  152 A electrically connects the SCR anode P+ region  118  with gate  126  and the N-well N+ contact  116 . Not shown is the completion of the conductor to the logic circuit input pad. Similarly, as depicted in  FIG. 7E , the metal element  152 B electrically connects the SCR cathode N+ region  120  with the gate  132  and the substrate P+ contact  122 . Not shown is the electrical connection of the conductor to a second voltage source, typically ground. 
   As represented in  FIG. 7F , after selective removal of unwanted metal, a final passivation covering layer  154  is deposited, typically SiO 2 , or silicon nitride (SiN), or borophosphorus silicate glass (BPSG). 
   While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.