Abstract:
A novel device structure and process are described for an SCR ESD protection device used with shallow trench isolation structures. The invention incorporates an SCR device with all SCR elements essentially contained within the same active area without STI elements being interposed between the device anode and cathode elements. This enhances ESD performance by eliminating thermal degradation effects caused by interposing STI structures, and enhances the parasitic bipolar characteristics essential to ESD event turn on. Enabling this unique design is the use of an insulation oxide surface feature which prevents the formation of contact salicides in unwanted areas. This design is especially suited to silicon-on-insulator design, as well as conventional SCR and LVTSCR designs.

Description:
This is a division of patent application Ser. No. 10/189,874, filing date Jul. 5, 2002, now U.S. Pat. No. 6,720,622 Improved SCR-ESD Structures With Shallow 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 process for 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 (MOS) circuit technology is scaled 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. 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 silicide 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, (B). 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  8 , with an N-well  10  and which contains contact regions N+  16  and P+  18 . The N-well  10  contact regions are isolated and bounded by the shallow trench isolation (STI) structures  12 A,  12 B and  12 C. The N-well  10  is also bounded by STI elements  12 A and  12 C. The P substrate  8  also contains N+ contact  20  bounded by STI elements  12 C and  12 D, and P+ contact  22  bounded by STI structures  12 D and  12 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 . The P+ contact  18  is the anode end of the device and is connected to the active circuit input or output pad  4  as well as to the N-well N+ contact  16  by conductor  24 A. 
     The junction between the P+ contact region  18  and the N-well  10  is the first junction of the SCR, and the P+ contact region  18  forms the SCR device anode. The N-well  10  and the P substrate  8  form the second junction. The third device junction is formed by the P substrate  8  and substrate N+ contact  20 , which also is the cathode terminal of the device. N+ contact  20  is connected to a second voltage source, typically ground, and also to substrate P+ contact  22  by conductor element  24 B. 
       FIG. 1B  shows the horizontal topography of the prior art device showing the N-well  10  with associated N+ contact  16 , P+ contact  18 , and related STI structures  12 A,  12 B, and  12 C represented by the dashed lines. Also represented in  FIG. 1B  are the substrate N+ contact  20  and P+ contact  22  as well as the STI structures  12  D and  12  E. The contact elements often use a silicide, or salicide, to improve the silicon to metal contact conductivity. The salicides are typically formed from refractory metals such as titanium (Ti), tungsten (W), tantalum (Ta), or molybdenum (Mo). The typical process is to provide a barrier such as SiO 2  to prevent salicide formation in unwanted areas, deposit the metal followed by a heat process, to form the salicide, and then remove the metal from the unwanted or non-contact areas. 
       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 , which is formed by the N-well  10  and P-substrate  8  junction, again shunting the current to the second voltage source. 
     However, for positive ESD events 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. 2  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 , and the STI structure  12 C formerly straddling the N-well  10  and Substrate  8  boundary has been shifted to the left and a N+ region  28  has been added straddling the lateral boundary. A FET gate  26  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  20  also functions as the SCR cathode. The prior art LVTSCR device operational trigger voltage is reduced by the NFET device breakdown voltage. The remaining STI elements still reduce the desirable ESD protection characteristics as previously discussed. 
     The invention in various embodiments allows the reduced use of STI elements while improving ESD protection by the use of an oxide layer, often called resistor protection oxide, for a silicide block. 
     The following patents describe ESD protection devices. 
     U.S. Pat. No. 6,172,403 (Chen) shows an ESD circuit with a process involving AA, isolation areas, and silicide. 
     U.S. Pat. No. 5,012,317 (Rountree) shows a conventional SCR-ESD circuit protection device with parasitic bipolar transistors. 
     U.S. Pat. No. 5,629,544 (Voldman et al.), U.S. Pat. No. 6,236,087 (Daly et al.), U.S. Pat. No. 5,903,424, (Taillliet), and U.S. Pat. No. 5,530,612 (Maloney) are related ESD patents. 
     The following technical reports discuss ESD protection circuits and STI bound ESD protection networks. 
     “Basic ESD and I/O Design” by S. Dabral et al., 1998 pps 38, 57, 62, and 247. 
     “Designing Power Supply Clamps for Electrostatic Discharge Protection of integrated Circuits” by T. J. Maloney, Microelectronics Reliability 38 (1998) pp. 1691–1703. 
     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). 
     It is yet another object of the invention to provide an ESD protection structure while maintaining the required operating characteristics of the active 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 straddles one N-well to substrate lateral boundary. The same STI abuts the N-well N+ contact region lateral boundary near the substrate surface. A second STI structure defines a device lateral 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+ element 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 is the use an oxide element in strategic locations on the substrate surface, often called resistor protection oxide (RPO), in place of the STI elements, to mask off silicide from areas where it is not desired. This enables the use of fewer STI elements, improving the ESD characteristics of the SCR ESD protection device. 
     In alternative invention embodiments, the RPO is utilized in low voltage trigger SCR (LVTSCR) devices and also in silicon on insulator (SOI) device design. 
    
    
     
       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 horizontal topography of the prior art SCR ESD protection device. 
         FIG. 1C  represents the electrical schematic of the prior art SCR ESD protection device. 
         FIG. 2  represents a prior art low voltage trigger SCR (LVTSCR) protection device cross section. 
         FIG. 3A  is a representation of the cross section of one embodiment of the invention for a SCR ESD protection device. 
         FIG. 3B  is top view of one embodiment of the invention for a SCR ESD protection device. showing the horizontal topography of the invention. 
         FIG. 4  is a simplified cross section of another embodiment of the invention for a LVTSCR. 
         FIG. 5  is a simplified cross sectional representation of another embodiment of the invention for a silicon on insulator device design. 
         FIG. 6  is a flow diagram of the invention process. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 3A  shows a simplified cross section of one embodiment of the invention. A P doped substrate  108  with typical doping concentration of between 1E14 and 1E16 atoms/cm.sup.3 (a/cm.sup.3) contains an N-well  110  with a typical dopant concentration between 1E16 and 1E18 a/cm.sup.3. The N-well  110  is bounded at and near the surface by shallow trench isolation (STI) element  112 A, typically between 0.2 to 1 um wide and 0.4 to 2 um deep. The STI element  112 A is filled with a dielectric, typically silicon oxide (SiO.sup.2). Within the N-well  110  region are a N+  116  and P+  118  contact regions, with typical dopant concentrations of between 1E19 and 1E21 a/cm.sup.3. The N+ region  116  is bounded on the side away from the P+ contact region  118  by the STI  112 A. The substrate  110  has N+ contact  120  and P+ contact  122 , with a typical dopant concentration of between 1E19 and 1E21 a/cm.sup.3 of donor and receptor dopant respectively. The outside edge of substrate P+ contact  122  is bounded by STI element  112 E. A PN junction is formed between N+ contact region  120  and P doped substrate  108  while another PN junction is formed between P+ contact region  118  and the N-well region  110 , as shown in  FIG. 3A . 
     The N-well N+ region  116  and P+ region  118  typically have specific contact areas where the silicon to metallurgy interface contains a salicide to reduce contact resistance. The salicides are typically formed from refractory metals such as titanium (Ti), tungsten (W), tantalum (Ta), or molybdenum (Mo). There is a blanket metal evaporation followed by a thermal annealing process, typically done at a temperature between 450 and 650° C., that forms the salicide. Unwanted unreacted metal is then selectively removed by use of an etchant that does not attack the salicide, the silicon substrate or the SiO 2 . A typical substance used for this etchant is a mixture of deionized water, hydrogen peroxide (H 2 O 2 ), and ammonium hydroxide (NH 4 OH) in a 5:1:1 mixture. Following the removal of unreacted metal, typically a stabilization anneal is performed with a temperature of between 800 and 900° C. to further reduce resistivity. 
     A unique feature of the invention is a protective oxide layer  132 , often called resistor protection oxide (RPO), overlaying the surface in non-contact areas, between the N-well N+ contact  116 , P+ contact  118 , and substrate N+ contact  120  and substrate P+ contact  122 . The oxide  132  is thermally deposited to a thickness between 1000 and 3000 Å. This oxide is a barrier or mask to prevent the salicide used to reduce the contact resistance between the silicon and the metallurgy system, typically aluminum or aluminum doped with silicon, from being formed in or on unwanted areas. 
     The RPO layer enables the proper device salicide processing without having to use the STI elements of prior art. As previously discussed, the STI elements can be detrimental to the ESD protection capability of the device by reducing the parasitic bipolar current gain, beta, (.beta.), and can also reduce lateral heat transfer capability. Because of the RPO layer is formed on N+ contact region  120  and P+ contact region  118 , the top surfaces of the two related PN junction have no salicide thereon, thereby avoiding reduction of the parasitic bipolar current gain. 
     Processing is continued in a conventional manner to complete the devices on the substrate. The P+ contact  18  is the anode of the SCR device, and is electrically connected to the. N+ contact  116 , and the active device input or output pad  104  by conductor  124 A. The N+ contact  120  is the SCR cathode and is electrically connected to the substrate P+ contact  122 , and a second voltage source, typically ground, by conductor  124 B. Not shown in the figure for clarity, but typically the device surface is covered by a passivation layer, either SiO 2  or silicon nitride (Si 3 N 4 ), or a borophosphorus silicate glass with a thickness of between 3000 and 7000 Å for protection against scratching, moisture or other damage. 
       FIG. 3B  shows the device horizontal topography with the RPO depicted by the area  132 . It is noted that the invention does not require the prior art STI elements  12 B,  12 B,  12 C and  12 D, as depicted by the dotted lines in the plan view of prior art  FIG. 1B . 
     Another embodiment of the invention is shown in  FIG. 4 . The unique design of the invention further improves the ESD protection of a LVTSCR device by enabling the elimination of STI elements  12 C and  12 D shown in  FIG. 2  for prior art. The N+ doped region  128  straddling the lateral boundary between the N-well  110  and the substrate  108  has a dopent concentration typically between 1E19 and 1E21 a/cm 3 . This N+ difussion region  120  serves as the drain of a N-channel thin oxide field effect transistor (FET) with associated gate  126 . The FET N+ drain  128  connects internally to the N region N-well  110  base of the SCR and the FET N+ source  120 , which also serves as the SCR cathode. The cathode  120 , FET gate  126 , and substrate P+ contact  122  are connected to a second voltage source, typically ground, by conductor  124 B. 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 N-well P+ contact  118  is the device anode and is connected to the N-well N+ contact  116  and the active circuit I/O pad by conductor  124 A. 
     The unique structure of the invention design places a protective oxide layer  132 , or RPO layer, over the device surface except for the specific contact areas, to prevent the formation of salicide in areas not required. This eliminates the need for the prior art STI structures shown in  FIG. 2  as STI elements  12 B and  12 C between the SCR anode and cathode. Again, the elimination of these STI improves the ESD performance of the SCR device. 
     Again, not shown in the figure for clarity, but typically the device surface is covered by a passivation layer, either SiO 2  or silicon nitride (Si 3 N 4 ), or a borophosphorus silicate glass with a thickness of between 3000 and 7000 Å to provide device protection. 
     In yet another embodiment depicted in  FIG. 5 , the invention is applied to a silicon on insulation (SOI) SCR protection device. There are several techniques in achieving an SOI structure well known in the art such as using sapphire as the insulator or using oxide as the insulator by using a heavy oxygen implant to create the oxide layer. As depicted in  FIG. 5 , a silicon wafer  108  has received an implant of oxygen to form a buried layer of SiO 2    134  as the insulation layer below the wafer surface. Typically, a high dosage of oxygen ions (O + ), between 1E18 and 5E18 a/cm 2  with an implant energy between 150 and 180 KeV is used to create the insulation layer. The insulation layer is typically between 0.3 and 0.5 microns below the surface. The wafer is typically heated between 350 and 450° C. during the implant process to insure that the surface maintains its crystallinity. A post implant anneal is performed at a temperature between 1050 and 1200° C. for 3 to 5 hours to form the buried layer of SiO 2 . The anneal step also allows excess oxygen in the surface silicon to out-diffuse, increasing the dielectric strength of the buried oxide layer. After the anneal, an additional layer of epitaxial silicon is deposited to assure that a single crystal active device region  136  of at least 0.5 um or greater in depth exists for the fabrication of active devices. 
     The use of trench isolation with SOI technology can be restrictive as the trench can contact the insulation element. When STI elements are used for isolation between SCR elements this can completely block device current flow around STI regions located between the SCR anode and cathode. 
     As depicted in  FIG. 5 , the invention embodiment for SOI technology, the SCR structure is composed of an N-well  110  with N+ contacts  116  and SCR anode P+ contact  118 . Adjacent to the N-well  110  is a P-well  114  with N+ contact  120  forming the device cathode and a P+ contact  122 . The heavily doped electrical contact areas typically contain a silicide or salicide between the silicon surface and the aluminum metallurgy conductor elements  124 A and  124 B. The SCR device is bounded on one side by STI element  112 A and on the other side by STI element  112 E, and there are no STI elements within the SCR active device area. The device anode  118  is electrically connected to the N-well  110  P+ contact  116  and the I/O node  104  by a metallurgical conductor element  124 A, typically aluminum, or silicon doped aluminum. The device cathode  120  is electrically connected to the P-well  114  P+ contact  122  and a second voltage source, typically ground, by a similar metallurgical conductor element  124 B. The electrical contact at the silicon surface typically contains a refractory metal salicide, such as TiSi 2 , to reduce electrical contact resistivity and prevent unwanted metallurgical annealing with the silicon. 
     As shown in  FIG. 5 , this invention embodiment provides an insulating RPO layer  132  on the device surface in non-contact areas to prevent salicide formation in those areas. This feature enables the reduction in the use of STI structures improving device ESD performance, or enabling an SCR ESD protection structure in situations not possible before. As in other embodiments, the N-well N+ contact  116 , N-well  110 , P-well  114  and P-well P+ contact  122 , effectively form a PN diode that is useful for shunting negative ESD energy occurring at the input terminal  104  away from the active devices. 
     Not shown in the figure for clarity, but the device surface is typically covered by a passivation layer, either SiO 2  or silicon nitride (Si 3 N 4 ), or a borophosphorus silicate glass with a thickness of between 3000 and 7000 Å 
     The process to develop an embodiment of the invention for an SCR on a P doped silicon substrate is outlined in the flow diagram of  FIG. 6 . Starting with a P doped substrate, a N-well  50  is created, typically by doping with an implant of phosphorous (P) with a dosage between 1E15 and 1E18 atoms/cm 2  and with an energy of between 30 and 80 KeV to produce an N-well with a dopant concentration of between 1E16 and 1E18 a/cm 3 . The creation of STI elements  52  is typically performed using an etching process such as a dry anisotropic plasma etch to form the trenches to a depth between 0.4 and 2 um deep and between 0.2 and 1 um wide. The trenches are then filled with a dielectric, typically SiO 2 , by LPCVD, or by an APCVD, or by 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. 
     As indicated in  FIG. 6 , the creation of the N+ contact regions  54  is done by using a donor element such as arsenic (As), with a dosage level between 1E13 and 1E15 a/cm 2 , and with an energy between 20 and 40 KeV. This results in contact dopent regions with a concentration of between 1E19 and 1E21 a/cm 3 . The P+ contact regions in the N-well and substrate are similarly created by an implant process but using an acceptor element, typically boron (B), with a dosage of between 1E12 and 1E13 a/cm 2 , and an implant energy of between 40 and 80 KeV resulting in a dopent concentration of between 1E19 and 1E21 a/cm 3 . 
     As indicated by the flow element  58  in  FIG. 5 , the creation of the thermal oxide layer  58  is typically done with a thermal process at a temperature of between 700 and 1100° C. The oxide, frequently called resistor protection oxide, is etched at the appropriate contact areas to open the oxide to the contact regions. The refractory metal evaporation  60 , is performed to produce a blanket of the metal, typically metal such as titanium (Ti), tungsten (W), tantalum (Ta), or molybdenum (Mo). 
     After the evaporation, an anneal is done at temperatures between 450 and 650° C. to form the salicide in the contact areas. Removing unwanted unreacted metal  62  from the device is typically done by an etch such as a mixture of DI H 2 O, 30% H 2 O 2 , and NH 4 OH in a 5:1:1 mixture. A stabilization anneal is performed at a temperature between 800 and 900° C. Continued device processing  64  includes creating conductor elements by evaporating metal, typically aluminum doped with 1% silicon, patterning and etching to remove metal from unwanted areas, and providing a passivation layer such as SiO 2 , or silicon nitride (SiN), or borophosphorus silicate glass (BPSG). For BPSG, deposited SiO 2  is doped with boron from a diborane source and doped with phosphorous from a phosphine source at a temperature between 400 and 500° C. followed by a densification between about 700 and 900° C. to form 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 chances in form and details may be made without departing from the spirit and scope of the invention.