Abstract:
In one aspect, a direct connected silicon control rectifier (DCSCR) includes a substrate having a semiconductor surface, a parasitic PNP bipolar transistor and a parasitic NPN bipolar transistor formed in the semiconductor surface. The parasitic PNP bipolar transistor includes a p+ emitter, an nbase and a pcollector and the parasitic NPN bipolar includes an n+ emitter, a pbase and an ncollector. The DCSCR also includes an electrically conductive line connecting an n+ contact to the nbase to a p+ contact to the pbase so that the nbase and the pbase are shorted.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/940,586 filed Feb. 17, 2014, and entitled “DIRECT CONNECTED SILICON CONTROLLED RECTIFIER (SCR) HAVING INTERNAL TRIGGER,” which is incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    Disclosed embodiments relate to silicon controlled rectifiers (SCRs). 
       BACKGROUND 
       [0003]    Electrostatic Discharge (ESD) induced failure is a major concern for the Metal-Oxide-Semiconductor (MOS) transistor-based integrated circuits (ICs) in main-stream technologies. This reliability issue is further worsened in advanced technology including submicron Complementary-Metal-Oxide-Semiconductor (CMOS) with very low operation voltage (such as an operating voltage &lt;5V) and Bipolar-CMOS-DMOS (BCD) for high voltage applications. There has been a growing demand for the availability of robust ESD protection solutions for advanced and main-stream technologies that are capable of operating in a very narrow ESD design window. A Silicon Controlled Rectifier (SCR) is a common device used for ESD protection. 
         [0004]    A typical on-chip ESD protection scheme for an I/O port (generally accessible at a bond pad on the IC die) involves including an SCR device referred to as an ESD clamp hooked in parallel to the internal circuitry to be protected between the input/output (I/O) pad which interfaces to both the outside world and ground (GND). When an ESD event (e.g., a high power pulse) occurs at the I/O port, the ESD clamp turns on and once on provides a parallel low resistance conduction path that shunts the ESD-induced current (and thus the power) to GND, and thus away from the internal circuitry being protected. 
         [0005]      FIG. 1A  shows a cross-section view of a conventional SCR  100  and  FIG. 1B  the equivalent circuit of the conventional SCR  100 . SCR  100  includes a parasitic PNP bipolar transistor  110  including an nwell  111  as its nbase and a parasitic NPN bipolar transistor  120  including a pwell  121  as its pbase, where the (C) collector of each transistor is coupled to the base (B) of the other transistor. 
         [0006]    SCR  100  is a 4 terminal device having 4 surface terminals formed on a substrate  102  shown having a semiconductor p-type surface (p-sub)  105 . Typical doping concentrations in p-sub  105  are 1×10 15  cm −3 , in nwell  111  from 1×10 17  to 1×10 18  cm −3 , and in pwell  121  from 1×10 17  to 1×10 18  cm −3 . Typical junction depths for nwell  111  and pwell  121  are 1 μm to 1.5 μm. 
         [0007]    The terminals of SCR  100  are identified as “anode gate” (AG)  112 /N+ region formed at the surface of the nwell  111 , “anode”/P+ region  113  formed at the surface of the nwell  111  lateral to the AG  112 , “cathode gate” (CG)/P+ region  122  formed at the surface of the pwell  121 , and “cathode”/N+  123  formed at the surface of the pwell  121  lateral to CG  122 . In SCR  100 , the AG  112  and anode  113  in the nwell  111  are both seen to be connected to I/O PAD (PAD, the PAD to be ESD protected), while the cathode  123  and CG  122  in the pwell  121  are both seen connected to GROUND (GND). Thus, in conventional SCR  100 , the AG  112  and anode  113  are tied together and connected to the I/O PAD, while CG  122  and the cathode  123  are tied together and connected to GND. 
         [0008]    Conventional SCR  100  and its variants use a triggering circuit outside the SCR region to turn on the SCR  100  which takes up area on the chip. Known SCRs generally have a relatively high trigger voltage and leakage current, and process snapback behavior, which make it hard to work in smaller ESD design windows. Moreover, conventional SCR  100  and its variants have a relatively slow turn-on speed which can prove fatal to some relatively EDS-sensitive internal circuitry in the case of a fast-rising ESD stress. 
       SUMMARY 
       [0009]    Disclosed embodiments include an SCR design referred to herein as a Direct-Connected SCR (hereafter DCSCR), which is configured to use its internal diodes as an internal trigger to establish the triggering path, resulting in a very low trigger voltage (equal to two times of the turn-on voltage of a single diode). Another benefit from disclosed internal triggers is efficient chip area consumption, because no extra trigger circuit is needed, disclosed DCSCRs use about the same semiconductor (e.g., silicon) area as a traditional SCR, such as SCR  100  shown in  FIG. 1A  discussed above. 
         [0010]    After being triggered on, disclosed DCSCRs dominate the current discharge and provide high robustness. This low trigger voltage, together with the DCSCR&#39;s holding voltage generally in the range of about 1V to 3V, provides a snapback-free I-V curve for disclosed SCRs under the standard Transmission Line Pulse (TLP) stressing condition. Such attractive performance characteristics is demonstrated by measurements results from disclosed DCSCRs described below. Disclosed DCSCRs provide a useful, flexible and configurable apparatus for a variety of ESD protection solutions for both low and high voltage IC circuits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1A  shows a cross-section view of a conventional SCR and  FIG. 1B  shows the equivalent circuit of conventional SCR. 
           [0012]      FIG. 2A  show a cross-sectional view of an example DCSCR having dual wells (a pwell and an nwell), according to an example embodiment. 
           [0013]      FIG. 2B  shows a trigger scheme for the DCSCR shown in  FIG. 2A  including the triggered path. 
           [0014]      FIG. 2C  shows the equivalent circuit for the DCSCR shown in  FIG. 2A  hooked between an I/O PAD and GND for protection circuitry on an IC coupled to the I/O PAD (not shown) 
           [0015]      FIG. 3  shows measured TLP I-V curves for the conventional SCR shown in  FIG. 1A  and the disclosed DCSCR shown in  FIG. 2A . 
           [0016]      FIG. 4A  is a single well DCSCR version not showing any dielectric isolation, and  FIG. 4B  is a version showing dielectric isolation between its terminals. 
           [0017]      FIG. 5  shows a twin nwell DC SCR version. 
           [0018]      FIGS. 6A-C  shows a dummy gate versions of disclosed DCSCRs with a single nwell with various isolation schemes. 
           [0019]      FIG. 7  shows a twin well DCSCR with an extra nwell (Nw) in the pwell and an extra pwell Pw in the nwell. 
           [0020]      FIGS. 8A-C  show dummy gate versions of a twin well DC SCR. 
           [0021]      FIGS. 9A and 9B  show a DCSCR realized in triple wells with  FIG. 9B  showing dielectric isolation. Each DCSCR also includes an additional deep nwell (DNW). 
           [0022]      FIG. 10  shows a triple (three) well DCSCR including an additional deep nwell (DNW) centered on the second nwell (Nw). 
           [0023]      FIGS. 11A-C  shows various dummy gate versions of the triple well DCSCR shown in  FIG. 10 . 
           [0024]      FIGS. 12 and 13  show a schematic and cross-section view, respectively, of Type I low-leakage, low-trigger voltage structure (shown as an ESD detector circuit) coupled to a disclosed DCSCR. The ESD Detector Circuit includes an RC network which is coupled to a CMOS inverter. 
           [0025]      FIGS. 14A-C  show a MOS-connected SCR (MCSCR) Type I ESD Detector Circuit coupled to various dummy gate versions of disclosed DCSCRS. 
           [0026]      FIGS. 15 and 16  show the schematic and cross-section view, respectively, of Type II low-leakage and low trigger voltage structure (MCSCR Type II) coupled to a disclosed DCSCR. 
           [0027]      FIGS. 17A-C  shows the MCSCR Type II with various dummy gate versions of disclosed DCSCRs. 
           [0028]      FIG. 18  shows a plurality of series connected disclosed DCSCR for applications in relatively high voltage technologies (Type I). 
           [0029]      FIG. 19  shows a cross-section view of the series stacking of disclosed DCSCR cells for high voltage ESD applications. 
           [0030]      FIG. 20  shows the conceptual I-V characteristics of the series stacking device shown in  FIG. 19 , demonstrating that adjustable trigger and holding voltages can be designed by changing the stacking number N. 
           [0031]      FIG. 21  shows a cross section view of a series stacked structure including a conventional SCR and some disclosed DCSCRs hooked in series. 
           [0032]      FIG. 22  shows conceptual I-V characteristics of the stacked structure of  FIG. 21 , demonstrating that adjustable trigger and holding voltages can be designed by changing the DCSCR cell number N. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Disclosed embodiments are described with reference to the attached figures, wherein like reference numerals, are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate aspects disclosed herein. Several disclosed aspects are described below with reference to example applications for illustration. 
         [0034]    It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the embodiments disclosed herein. One having ordinary skill in the relevant art, however, will readily recognize that the disclosed embodiments can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring aspects disclosed herein. Disclosed embodiments are not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with this Disclosure. 
         [0035]    Also, the terms “coupled to” or “couples with” (and the like) as used herein without further qualification are intended to describe either an indirect or direct electrical connection. Thus, if a first device “couples” to a second device, that connection can be through a direct electrical connection where there are only parasitics in the pathway, or through an indirect electrical connection via intervening items including other devices and connections. For indirect coupling, the intervening item generally does not modify the information of a signal but may adjust its current level, voltage level, and/or power level. 
         [0036]      FIG. 2A  show a cross-sectional view of an example DCSCR  200  having dual wells, with a pwell and an nwell, according to an example embodiment.  FIG. 2B  shows the equivalent circuit for DCSCR  200  shown in  FIG. 2A . DCSCR  200  can be realized on the chip along with other circuitry (analog or digital, or both analog and digital) in a variety of advanced and main-stream semiconductor technologies, but can also be a standalone (discrete) circuit. 
         [0037]    As with conventional SCR  100 , DCSCR  200  includes a parasitic PNP bipolar shown as  210  and a parasitic NPN bipolar shown as  220 , where the (C) collector of each transistor is tied (common) with the base (B) of the other transistor. One new feature for DCSCR  200  is the anode  213  and anode gate  212  (contacting nbase  111  of the parasitic PNP transistor  210 ) are not tied together, and the cathode  223  and cathode gate  222  (contacting pbase  121  of the parasitic NPN transistor  220 ) are not tied together, as they both are tied together in the conventional SCR shown in  FIG. 1A . Another new feature is the nbase  111  of parasitic PNP transistor  210  is connected to the pbase  121  of the parasitic NPN transistor  220  using an electrically conductive line (e.g., metal line, heavily doped (e.g., n+) polysilicon line, or other highly electrically conductive material available in the process), indicating that both bases  111  and  121  are essentially shorted together. 
         [0038]    The emitters are shown as  213  for the parasitic PNP  210  and  223  for the parasitic NPN  220 . The shadow shaded surface regions between the N+ and P+ regions ( 212 ,  213 ,  222 ,  223 ) represent a field dielectric for device isolation, such as field oxide isolation or trench isolation. 
         [0039]      FIG. 2B  shows a trigger scheme for DCSCR  200  and  FIG. 2C  its equivalent circuit hooked between and I/O PAD and GND. Advantages of disconnecting the nbase  111  of parasitic PNP transistor  210  and the pbase  121  of parasitic NPN transistor  220  from PAD and GND, respectively, and shorting them together, have been found to include reducing the trigger voltage in the SCR cell to about two times of the forward turn-on voltage of a single diode (about 1.3v). DCSCR  200  also increases the SCR&#39;s turn-on speed since the parasitic NPN transistor  220  and parasitic PNP bipolar transistor  210  will be essentially simultaneously switched on. In the conventional SCR  100  in  FIG. 1A , the device is triggered by an avalanche breakdown mechanism between the nwell  111 /pwell  121  junction, which results in a relatively high trigger voltage (e.g., 15 to 40 V, depending largely on the doping level on the more lighter doped side of the junction. The turn-on speed for the conventional SCR  100  is also slower than DCSCR  200  since the parasitic NPN and PNP transistors are not simultaneously triggered. 
         [0040]    With an optional added external trigger diode chain, the trigger voltage can be reduced and the turn-on speed can be increased, but at the expense of a larger chip area. In addition, such a traditional device still possesses a large snapback, as the holding voltage is significantly smaller than the trigger voltage. 
         [0041]    DCSCR  200  is suitable for ESD protection of I/O pins operating at a low voltage application, especially pins with an operation voltage lower than 1V. As such, disclosed embodiments promise a major improvement for ESD protection device solutions for next-generation low voltage technologies in the future. For operating voltages larger than 1V, the DCSCR  200  may be subject to an undesirable high leakage current. However, circuits are described below in  FIGS. 12 ,  13 ,  14 A-C,  15 ,  16 , and  17 A-C are designed to compress the leakage current, referred to herein as MOS-connected SCR (MCSCR).  FIG. 3  shows the current-voltage characteristics of conventional SCR  100  and disclosed DCSCR  200  measured using the transmission line pulsing (TLP) tester. The pulse width used was 100 ns, with a Pulse rise time of 10 ns. Clearly, DCSCR  200  offers a comparatively very low trigger voltage (1.3V), snapback-free behavior, high ESD robustness, and faster turn-on speed as compared to conventional SCR  100 . 
         [0042]    This Disclosure now discloses several different circuit variations. In between implantation (n+ or p+) there will generally be field oxide or trench isolation formation processing. In a single well version, shown as an nwell in a p-substrate, the DCSCR can be realized as shown in  FIG. 4A  which does not show an dielectric isolation. In  FIG. 4A , the parasitic pnp is shown as  410 , which utilizes the p-sub  105  as its collector, and the parasitic npn is shown as  420  which uses the p-sub  105  as its base. In this embodiment, the p-sub  105  is generally doped to at least 1×10 14  cm −3 , with a typical doping range of 1×10 14  to 1×10 15  cm −3 . 
         [0043]      FIG. 4B  shows the DCSCR of  FIG. 4A  with added dielectric isolation  450 . The single nwell version can add an extra nwell in the p-sub region to increase the pbase resistance in the triggering path.  FIG. 5  shows a twin nwell version. 
         [0044]      FIG. 6A-C  shows a dummy gate versions of disclosed DCSCRs with a single nwell with various isolation schemes. In these dummy gate version, in operation, the dummy gate can be floating, biased to a constant voltage potential, or tied external trigger circuit. The dummy gate(s) themselves will not increase the turn on time. Instead, the dummy gate can block the formation of the field oxide or trench isolation formation. Therefore the current will flow close to surface, but not deep under the field oxide or trench isolation. In such technique the length that triggering current has to flow is reduced, and hence the turn-on time is also reduced. Dummy gates include a gate electrode on a gate dielectric, analogous to the gate stack in a MOS device. 
         [0045]      FIG. 7  shows a DCSCR with an extra nwell (Nw) in the pwell and an extra pwell (Pw) in the nwell.  FIG. 8A-C  show dummy gate versions of a twin well DCSCR with dielectric isolation. As noted above, in the dummy gate version, the gate can be floating, biased to a constant voltage potential, or tied to an external trigger circuit. 
         [0046]    The DCSCR can be realized with triple wells as shown in  FIGS. 9A and 9B  along with an optional deep nwell (DNW). The DNW provides junction isolation between the PWell and the P-sub.  FIG. 9B  is shown including dielectric isolation. 
         [0047]      FIG. 10  shows a triple (three) well (NWell, PWell, and DNW) DCSCR including an additional deep nwell (DNW) centered on a second nwell (Nw).  FIGS. 11A-C  shows the dummy gate version of the triple well DCSCR. As noted above, in this dummy gate version, the gate electrode can be floating, biased to a constant voltage potential, or tied to an external trigger circuit. 
         [0048]    Optional leakage current compression techniques are now disclosed. Disclosed are two different example ways to reduce the leakage current of the DCSCR. In the description below twin well technology is used as an example only for illustration. Disclosed leakage current compression techniques can be also applied to single well, triple well and other technologies. 
         [0049]      FIGS. 12 and 13  show the schematic and cross-section view, respectively, of Type I low-leakage, low-trigger voltage structure that includes an ESD detector circuit coupled to a disclosed DCSCR. The ESD detector circuit includes an RC network which is coupled to a CMOS inverter. Additional novel points of this implementation include the bases of parasitic PNP and NPN transistors are connected to the drain and source of NMOS respectively which is triggered on by RC network, and the NMOS is off during normal operation so that the leakage current will be limited. When the PAD is subject to an ESD event, the RC network will detect the ESD pulse and turn on the NMOS, which feeds the current into the DCSCR and switches it on. The capacitor shown can include a PIP capacitor, MIM capacitor, MOS capacitor, MOM capacitor, or others.  FIGS. 14A-C  show a MOS-connected SCR (MCSCR) Type I ESD detector circuit coupled to various twin well dummy gate DCSCR versions. Disclosed MCSCRs provide optional leakage reduction. 
         [0050]      FIGS. 15 and 16  show a schematic and cross-section view, respectively, of Type II low-leakage and low trigger voltage structure (MCSCR Type II) ESD detector circuit. This ESD Detector Circuit includes an RC network which is coupled to an NMOS transistor. The operation of this structure is similar to the Type I ESD detector circuit described above.  FIG. 17A-C  shows the MCSCR Type II ESD detector circuit with various twin well dummy gate DCSCR versions. 
         [0051]    The above described DCSCR can also be used for ESD protection solutions for high voltage ICs. Two types of ESD clamps for high voltage ESD protection are disclosed.  FIG. 18  shows a plurality (N) of series connected disclosed DCSCR for applications in relatively high voltage technologies (Type I). This approach stacks several DCSCR cells in series to achieve a desirable high trigger and high holding voltages. The stacking number depends on the required trigger voltage, or the ESD design window. Since a single DCSCR cell does not possess a snapback behavior, the stacking of multiple DCSCR cells also lacks snapback, thus providing an excellent ESD design window with adequate trigger and holding voltages for immunity of potential latch-up and core circuit damage. In order to realize this stacking arrangement, an n-type buried layer (NBL) or Deep N-Well (DNW) may be used to isolate the pwell from P-type substrate (P-sub). 
         [0052]      FIG. 19  shows a cross-section view of the series stacking of DCSCR cells for high voltage ESD applications with 4 DCSCR cells shown.  FIG. 20  shows the conceptual I-V characteristics of the stacked device of  FIG. 19 , demonstrating that adjustable trigger and holding voltages can be designed by changing the stacking number N. 
         [0053]    If a large trigger voltage is needed and at the same time limiting the number of DCSCR cells is desirable, then the stacking structure can include a typical SCR in combination with several DCSCR in series). Such typical SCR will dominate the trigger voltage of the series stacked circuit.  FIG. 21  shows a cross section view of a stacking structure including a conventional SCR and three disclosed DCSCRs hooked in series. 
         [0054]    Defining the holding voltages of DCSCR and typical SCRs as V H,novel  and V H,typical  respectively, and the trigger voltages of DCSCR and typical SCRs as V T1,novel  and V T1,typical  respectively. N is defined as the number of DCSCR in the stacked ESD circuit. For a specific ESD design window, one can design the trigger and holding voltages according to the following condition so as to reduce or eliminate snapback behavior: 
         [0000]    
       
      
       N×V 
       H,novel 
       +V 
       H,typical 
       ≧N×V 
       T1,novel 
       +V 
       T1,typical  
      
     
         [0055]      FIG. 22  shows I-V characteristics of the stacked device of  FIG. 21 , demonstrating that adjustable trigger and holding voltages can be designed by changing the DCSCR cell number N (i.e., the case of N=0 indicates the structure includes 1 typical SCR or DCSCR and zero DCSCR, the case of N=1 consists of 1 typical or DCSCR and 1 DCSCR, etc.). 
         [0056]    As described above, known SCRs have relatively high trigger voltage and leakage current, and process snapback behavior, which make it hard to work in small ESD design windows. Moreover known SCRs have slower turn-on speed which can be fatal to fast-rising ESD stress if the SCRs do not turn on in time to provide a current shunt. Disclosed DCSCRs in contrast provide a much lower trigger voltage (about two forward diode drops, or about 1.3V), non-snapback behavior and faster turn-on speed without extra semiconductor area because the triggering circuit can be all internal. Disclosed DCSCRs can thus be fit in various ESD design windows and provide effective ESD protection on chip in both low and high voltage semiconductor technologies, as well as next generation very-low voltage semiconductor technologies. Uses for disclosed DCSCRs include on-chip ESD protection (I/O or power clamp) for ICs for both low and high voltage semiconductor technologies, as well as next generation very-low voltage semiconductor technologies. 
         [0057]    While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not as a limitation. Numerous changes to the disclosed embodiments can be made in accordance with the Disclosure herein without departing from the spirit or scope of this Disclosure. Thus, the breadth and scope of this Disclosure should not be limited by any of the above-described embodiments. Rather, the scope of this Disclosure should be defined in accordance with the following claims and their equivalents.