Localized trigger ESD protection device

The present invention provides an ESD device to reduce the total triggering current without increasing the overshoot voltage. This is achieved by localizing the triggering current, such that the local current density remains high enough to trigger the ESD device. This localized triggering provides a fast and efficient triggering of the ESD device.

FIELD OF THE INVENTION

The present invention generally relates to Electro Static Discharge (ESD) protection circuit. More specifically, the present invention relates to an improved ESD circuit with localized triggering.

BACKGROUND OF THE INVENTION

During ESD, large currents can flow through an Integrated Circuit (IC), potentially causing damage. To avoid this damage, protection circuits are added. One widely used ESD protection device is the Silicon Controlled Rectifier (SCR).

As typical SCR100as known in the art is illustrated inFIG. 1exemplifying both a top view of and a cross section view along line A. The SCR100consists of two bipolar transistors, which are coupled such that during ESD operation they form a positive feedback loop. The first bipolar is a PNP, the bipolar emitter of which is the SCR anode102, the bipolar base is known as the SCR trigger tap G2104. The second bipolar is an NPN, the emitter of which is the SCR cathode106, the bipolar base is the SCR trigger tap G1108. The collector of the PNP and the base of the NPN is the same region. Also, the collector of the NPN and the base of the PNP is the same region.

One of the main advantages of the SCR is its high current handling capability. One of the main disadvantages of the SCR is its limited triggering speed, which might be too slow for very fast ESD events. The SCR can be used in a variety of configurations. In one configuration, it is triggered by the junction breakdown of the PNP base-collector junction, which is the same as the NPN collector-base region. In another configuration, the rising edge of the ESD event might trigger the SCR due to capacitive current flowing through both base-collector junctions.

The most important type of triggering, however, consists in applying a triggering signal at either the G1trigger tap or the G2trigger tap, or both. By increasing the voltage at the G1tap, current will flow through the G1-cathode diode, which will trigger the SCR. Similarly, by decreasing the voltage at the G2trigger tap, current will flow through the anode-G2region, triggering the SCR.

Another widely used ESD device is a diode. In many ESD protection circuits a chain of diodes is used. However, during very fast ESD events, a voltage overshoot is associated with every diode. When placing N diodes in series, this overshoot is approximately multiplied by N. If this overshoot is significant, sensitive nodes in the circuitry (e.g. gate oxides) might be degraded or damaged during the ESD event.

Different implementations of diodes include gated diodes, STI diodes and Non-STI diodes. A top view and a cross section view along line B of a typical gated diode200is shown inFIG. 2. The diodes200include a N-well diode202of N-doped/N+ region201and a P-doped/P+ (anode) region203formed on a N-well region204. The diodes200also include a P-well or P-substrate diode206having the N+ (cathode) region205and the P+ region207in a P-well or P-substrate208. A local gate210, such as a poly-gate, is placed between junction of the N+201and P+203and between the junction of N+205and the P+207. This poly-gate210extends to a full width of the SCR. This gated diode200is fast, but adds more capacitance to the device due to the gated region extending through the full width of the diode. This helps in terms of reducing the overshoot voltage but at the same time adds more capacitance, which makes it difficult to use gated diodes in RF applications.

A typical STI diode300as shown inFIG. 3is similar to the diode200except it does not include a poly-gate. The removal of the poly-gate provides a low capacitance to the device, but it has in most cases lower triggering speed, thus making these STI diodes less efficient for very fast ESD events. The STI diode300instead provides an isolation302between the junction of the N+201and the P+203and between the junction of the N+205and the P+207. A typical Non-STI diode400as shown inFIG. 4is similar to the STI diode300except it does not include the isolation302as described above. However this lack of isolation often leads to high leakage current.

Thus, there is a need in the art to provide an improved ESD protection device that overcomes the disadvantages of the prior art and provides for a fast triggering while reducing the overshoot voltage.

SUMMARY OF THE INVENTION

The present invention provides an electrostatic discharge (ESD) protection device comprising at least one anode formed within a first lightly doped region and at least one cathode formed within a second lightly doped region. The at least one region is formed only in a portion of at least one of the anode and the cathode to provide localized current to trigger the device.

In one embodiment; the region in the anode comprises of at least one extension of the anode and region in the cathode comprises of at least one extension of the cathode.

In another embodiment, the region is a gate region.

In yet another embodiment, the device is an SCR.

In yet another embodiment, the device is a diode.

The present invention also provides an electrostatic discharge (ESD) protection device comprising at least one anode formed within a first lightly doped region and at least one cathode formed within a second lightly doped region. The at least one region is partly formed in a portion of one of the first lightly doped region and the second lightly doped region.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a novel means to increase the speed of the SCR. SCR devices typically trigger in one point, after which the current spreads over the full SCR width. Since (dI/dt), instantaneous rate of current change, is one of the limiting factors for SCR triggering speed, triggering the SCR at lower absolute current speeds up triggering. To achieve this, the present invention provides an ESD device to reduce the total triggering current without increasing the resistance (i.e. increasing overshoot voltage) of the trigger path. This is achieved by localizing the trigger current, such that the local current density remains high enough to trigger the SCR, details of which are provided in greater detail below.

Referring toFIG. 5, there is shown a top view of the ESD devices500and the corresponding cross-section views along lines A and B. The ESD devices500include SCRs502formed on a substrate of material of first conductivity type, preferably a p-type doping region (P-sub)504. As shown inFIG. 5, a well506of a second conductivity type, preferably an n-type doping region (N-well) is formed in the P-sub504. Both N-well506and P-sub504may include semi-conducting material, such as, for example, silicon, germanium or combinations of both. P-sub504, as shown inFIG. 5, may preferably be electrically grounded. Even though in the embodiment of the present invention, the first conductivity type is defined as a p-type doping region and the second conductivity type is defined as an n-type doping region, one skilled in the art would appreciate that the first conductivity type may be the n-type doping region and the second conductivity type may be the p-type doping region.

In particular, the SCR502includes an anode503(P+ region in N-well), which is connected to a pad (not shown) and to one side of a resistance of the N-well506or an external resistor. The SCR502also includes a cathode505(N+ region in P-sub) which is connected to a ground (not shown) and to one side of a resistance of the P-well504or an external resistor. A G2tap507is a N+ in the N-well506and a G1tap509is a P+ in the P-well504. The SCR502further includes an extension, P+503′ of the anode503and an extension, N+505′ of the cathode505. As illustrated inFIG. 5, a second LAC, i.e. LAC2508is a spacing between the anode503and the cathode505. A first LAC, i.e. LAC1510is a smallest anode to cathode spacing. Various embodiments of the LAC1510are illustrated inFIG. 5.FIG. 5ashows an extension P+503′ of the anode503and an extension N+505′ of the cathode505. In this example, the extensions503′ and505′ of the anode and the cathode are placed at the same position along the width of the axis of the device to provide for the minimum LAC1So, the LAC,510is between the extensions P+503′ and N+505′.FIG. 5billustrates an embodiment with LAC1510between the extension P+503′ and the cathode505.FIG. 5cillustrates an embodiment with LAC1between the extension N+505′ and the anode503. Although not shown, the extensions503′ and505′ can preferably be placed at other locations. Some examples include extensions505′ and505′ placed centrally along the width axis of the anode and cathode respectively, or alternatively positioned at one of upwards and downwards along the width of the axis of the anode and cathode respectively.

As known in the art, the trigger current flows uniformly spreading over the total width of the SCR device, thus requiring more time for the SCR to trigger. In the embodiment of the present invention, the trigger current flows in the horizontal direction and the width of the device is in the vertical direction as illustrated inFIG. 5a. The smallest anode to cathode spacing, LAC1510is localized at a specific part of the SCR, where the LAC for the rest of the SCR, i.e. LAC2508is larger. This forces the first trigger current to localize at the smallest LAC point, i.e. at LAC1510. By localizing the specific part of the SCR, relatively more trigger current will flow in this LAC1510. As illustrated inFIG. 5a, the LAC1510is spacing between the extensions505′ and505′. The resistance locally at LAC,510becomes lower so more trigger current will flow between the extensions503′ and505′ as shown inFIG. 5a. In this way the trigger path is localized such that the necessary current density will be reached very quickly, while the total absolute current is still relatively low, thus making it trigger faster.

Additionally, it is known in the art to place some type of isolation such as shallow trench isolation (STI) between the active regions. However in the embodiments of the present invention, the isolation is removed to provide a shorter current path between anode and cathode, thus creating a lower resistance. Also, by reducing the spacing between the anode and the cathode with their corresponding extensions creates a bipolar transistor and further lowers the resistance

Note that LACmincan be based either on a minimum design rule or smaller than the rule or even larger than the rule. Even thoughFIG. 5illustrates the extensions P+503′ and the N+505′ which create the reduced LAC1being placed in the middle of the anode and the cathode respectively, it obvious to one skilled in the art that these extensions can be placed anywhere along the anode and/or cathode width. Although the extensions, as illustrated inFIG. 2, are rectangular, the invention is not limited to any particular shape and number of the extensions.

Referring toFIG. 6, there is shown an ESD device600according to another embodiment of the present invention. In this embodiment instead of the P+ extension503′ and N+ extension505′ in ESD device500ofFIG. 5a, a local gate region602, for example a poly gate is added at a certain area of the active regions of the ESD device. The gate region602preferably comprises of polycrystalline silicon (polysilicon) over an active area as shown inFIG. 6. Active area comprises of an area of high doped regions excluding isolation. Some types of isolations include STI, DTI, PTI, field oxide etc. The gate region602also may preferably comprise a poly over the isolation. In further implementations, the active area may also be needed between the anode503and the cathode505as will be described with reference toFIG. 7below. Referring toFIG. 6a, it illustrates the gate region602added at the anode503.FIG. 6billustrates the gate region602added to the cathode505andFIG. 6cillustrates the local gates602added to only a certain small areas of both the anode603and the cathode605. Thus, in this example, the width of the poly is reduced. As known in the prior art, the poly-gates are generally placed at the full width of the SCR in which case the current flows homogeneously over the whole width. Thus, in order to prevent the spreading of this current over the whole width, the poly-gates602are added only at specific active regions to reduce its width. So, the widths of the poly-gates602are lower than the width of the anode503and the cathode505and widths of the poly-gates602are lower than the length of the G1and G2. This localizes the current within the poly-gates602during turn on the SCR. Thus, the local gates also functions to allow for more trigger current to flow at this localized part of the SCR. Thus these local gates602have similar effect as the extensions discussed above that the current is localized and as such the current density needed for triggering is reached fro a lower absolute total amount of current. Also, the current path is shorter also because it does not have to go under the STI isolation. Even thoughFIG. 6illustrates only one local gate602at each active region, it is obvious to one skilled in the art that more than one local gate can be placed at each active region as long as the current is localized.

FIG. 7illustrates and ESD device700in yet another embodiment by placing a certain potential to the local gates602ofFIG. 6. This is achieved by adding gate contacts702on the local gates602as illustrated inFIG. 6. The gate contacts702may preferably be connected to a biasing circuit (not shown) to provide a certain potential to the local gates602, which in turn aids in triggering of the SCR. As an example,FIG. 7aillustrates the local gate602extends beyond the anode503. Although, not shown, the local gate602may also extend beyond the cathode505.FIG. 7billustrates another example of the local gate602extending between the anode503and part of the P-sub504. Although,FIG. 7illustrates placing contacts702at certain portions of the local gates602, the present invention is not limited in any way with respect to the location of the contacts on the local gates. Additionally, as illustrated in example ofFIG. 7a, the active area is also needed between the anode503and the cathode505, otherwise there is isolation between anode and cathode.] However, it is not necessary to have the active area for the poly gate602which in this example extends beyond the anode503over G2tap507in order to place the contacts702. This is because the current will be conducted by poly itself through the anode503.

In another embodiment (FIG. 8) of the present invention, there is illustrated an ESD devices800similar in accordance with another embodiment of the present invention. In the ESD device800, LAC1is achieved by increasing the length of both the trigger G1509in N-well506and trigger tap G2507in P-sub504. So, the lengths of GI509and G2507are increased more than needed in order to be able to carry the trigger current, such significantly reducing the overshoot voltage. In the example illustrated inFIG. 8a, the length of the G1509is larger than the length of the cathode505and the length of the G2507is larger than the length of the anode505measured along the axis orthogonal onto width axis of the device. Similarly, the length of the anode503and cathode505can be decreased. In another example, inFIG. 8b, only the length of the G2507is increased. Although not shown, alternatively, only the length of the G1509may be increased.

FIG. 9illustrates an ESD device900with various other examples of combinations of the poly gates602and the contacts702.FIG. 9ashows an example with the poly gate602at each of the anode503and G2tap507with the gate contact702at the G2tap507and a poly gate602extending between the anode503and the G2tap507. Also shown is another poly gate602extending between the cathode505and the G1tap509.FIG. 9bshows an example with the poly gate602extending between the anode503and the G2tap507with the gate contact at the N-well506.FIG. 9balso shows another poly gate602extending between the cathode505and the G1tap509. Also, note inFIG. 9b, the length of the G1509is larger than the length of the cathode505and the length of the G2507is larger than the length of the anode503. It is obvious to one skilled in the art that the invention is not limited to the combinations illustrated inFIG. 9.

FIG. 10illustrates some other embodiments of adding poly-gates602.FIG. 10aillustrates an implementation in which the poly-gate602is added at a junction between the anode503and the G2507and a junction between the G1509and the cathode505. A second implementation is depicted inFIG. 10bin which the poly-gate602is added only at the cathode-G1(505-509) junction. A third implementation is depicted inFIG. 10cin which the poly-gate602is added only at the anode-G2(503-507). In these embodiments, the poly-gate602is small and does not extend over the full width of the SCR502.

In yet another embodiment, as illustrated inFIG. 11a, there is shown a poly-gate602added over the total width of the device at the junction between the anode503and the G2507and another poly-gate602added over the total width of the device at the junction between the cathode505and the G1509. Alternatively, as shown inFIGS. 11aand11b, only one poly gate602is added between the cathode-G1(505-509) junction and between the anode-G2(503-507) junctions respectively. As noted above, in these examples also the width of the poly602can be reduced and can be placed anywhere along the width axis of the device. Even thoughFIGS. 11a,11band11cillustrate the use of only one poly gate602between the junctions, it is obvious to one skilled in the art that more than one poly gate602can be placed at these junctions as long as the current is localized.

FIGS. 12athrough12iillustrate combinations of various implementations of the SCR devices with poly-gates over the total device width.FIG. 12aillustrates a poly-gate602of the increased width placed both at the cathode-G1(505-509) junction and the anode-G2(503-507) junction in which both the lengths of G1509and G2507are increased. In other words, the length of the G1509is larger than the length of the cathode509and the length of the G2507is larger than the length of the anode503.FIG. 12billustrates a poly-gate602of an increased width placed both at the cathode-G1(505-509) junction and the anode-G2(503-507) junction in which only the length of G2507is increased.FIG. 12cillustrates a poly-gate602of an increased width placed both at the cathode-G1(505-509) junction and the anode-G2(503-507) junction in which only the length of G1509is increased.

FIG. 12dillustrates a poly-gate602of an increased width placed only at the anode-G2(503-507) junction in which both the lengths of G1509and G2507are increased.FIG. 12eillustrates a poly-gate602of an increased width placed only at the anode-G2(503-507) junction in which only the length of G2507is increased.FIG. 12fillustrates a poly-gate602of an increased width placed only at the anode-G2(503-507) junction in which only the length of G1509is increased. Alternatively,FIG. 12gillustrates a poly-gate602of an increased width placed at the cathode-G1(505-509) junction in which both the lengths of G1509and G2507are increased.FIG. 12hillustrates a poly-gate602of an increased width placed at the cathode-G1(505-509) junction in which only length of G2507is increased.FIG. 12iillustrates a poly-gate602of an increased width placed at the cathode-G1(505-509) junction in which only the length of the G1509is increased.

FIGS. 13athrough13iillustrate combinations of the various implementations of the SCR devices of the poly-gates of the reduced width.FIG. 13aillustrates a poly-gate602of the reduced width placed both at the cathode-G1(505-509) junction and the anode-G2(503-507) junction in which both the lengths of G1509and G2507are increased.FIG. 13billustrates a poly-gate602of a reduced width placed both at the cathode-G1(505-509) junction and the anode-G2(503-507) junction in which only the length of G2507is increased.FIG. 13cillustrates a poly-gate602of the reduced width placed both at the cathode-G1(505-509) junction and the anode-G2(503-507) junction in which only the length of G1509is increased.

FIG. 13dillustrates a poly-gate602of the reduced width placed only at the cathode-G1(505-509) junction in which both the lengths of G1509and G2507are increased.FIG. 13eillustrates a poly-gate602of an increased width placed only at the cathode-G1(505-509) junction in which only the length of G2507is increased.FIG. 13fillustrates a poly-gate602of an increased width placed only at the cathode-G1(505-509) junction in which only the length of G1509is increased. Alternatively,FIG. 13gillustrates a poly-gate602of a reduced width placed at the anode-G2(503-507) junction in which both the lengths of G1509and G2407are increased.FIG. 13hillustrates a poly-gate602of a reduced width placed at the anode-G2(503-507) junction in which only length of G2507is increased.FIG. 13iillustrates a poly-gate602of a reduced width placed at the anode-G2(503-507) junction in which only the length of the G1509is increased.

FIGS. 14athrough14hillustrate combinations of the various implementations of the SCR devices of the poly-gates of both the increased and the reduced widths.FIG. 14aillustrates a combination of a poly gate602of a reduced width placed between the anode-G2(503-507) junction and a poly gate602of an increased width placed between the cathode-G1(505-509) junction. Alternatively,FIG. 14billustrates a combination of a poly gate602of a reduced width placed between the cathode-G1(505-509) junction and a poly gate602of an increased width placed between the anode-G2(503-507) junction.

FIG. 14cillustrates a combination of a poly-gate602of the increased width placed between the anode-G2(503-507) junction and the poly-gate602of the reduced width placed between the cathode-G1(505-509) junction in which lengths of both the G1509and the G2507are increased.FIG. 14dillustrates a poly-gate602of the increased width placed between the anode-G2(503-507) junction and the poly-gate602of the reduced width placed between the cathode-G1(505-509) junction in which only the length of the G2507is increased. Alternatively,FIG. 14eillustrates a poly-gate602of the increased width placed between the anode-G2(503-507) junction and the poly-gate602of the reduced width placed between the cathode-G1(505-509) junction in which only the length of the G1509is increased.

FIG. 14fillustrates a combination of a poly-gate602of the reduced width placed between the anode-G2(503-507) junction and the poly-gate602of the increased width placed between the cathode-G1(505-509) junction in which lengths of both the G1509and the G2507are increased.FIG. 14gillustrates a combination of a poly-gate602of the reduced width placed between the anode-G2(503-507) junction and the poly-gate602of the increased width placed between the cathode-G1(505-509) junction in which only the length of G2507is increased.FIG. 14hillustrates a combination of a poly-gate602of the reduced width placed between the anode-G2(503-507) junction and the poly-gate602of the increased width placed between the cathode-G1(505-509) junction in which only the length of G1509is increased.

Even though figures in the present invention illustrate a one finger, one sided SCR, it is known to one skilled in the art that the present invention is not limited to any number of SCR fingers, nor to any specific SCR configuration.

In further embodiments of the present invention there is provided a novel means to increase the speed of the diode.FIG. 15illustrates a top view of the ESD devices1500and the corresponding cross-section views along lines A and B. The ESD devices1500includes a N-well diode1502having N+ region1501and P+ region (anode)1503formed in a N-well region1504. The device1500also includes a P-well diode1506having N+ region1505(cathode) and P+ region1507formed in a P-substrate1508. Both the N-well1504and the P-sub1508may include semi-conducting material, such as, for example, silicon, germanium or combinations of both. P-sub1508, as shown inFIG. 15, may preferably be electrically grounded. As illustrated inFIG. 15a gate1510is added to only at certain small areas of both the anode1503and the cathode1505of each of the diodes1502and1506This reduces the width of the gate1510along the width axis of the device. So, by reducing the width of the gates1510, only very limited amount of capacitance is added to the device. Furthermore, similar to the concept of the SCR as described above, the current is localized within the gates1510during turn on of the diode which increases the triggering speed of the diodes and limits the overshoot voltage.

FIG. 16illustrates a top view of the ESD devices1600and the corresponding cross-section views along lines A and B. The ESD device1600is similar to the ESD device1500as described above, however the ESD device1600also includes a very short NON-STI (blocking region for STI) region1602placed between the active regions, preferably between the anode and the cathode of the diodes. Due to the out-diffusion, the spacing between the anode503and the cathode505is reduced, providing a localized triggering path between the active regions, while only adding a limited amount of leakage current.

Although figures illustrated in the present invention do not show any triggering circuit, it is known to one skilled in the art that a triggering circuit can be connected to the G2tap, the G1tap or both. Also, the triggering circuit can consist of any combination of, but not limited to, diodes, resistors, capacitors, transistors, inductors etc. Thus, the present invention is not limited to any specific triggering circuit, nor is it limited to how this triggering circuit or the ESD clamp itself is connected to a sensitive circuit.

The above embodiments of the present invention as described above have common features of improving the triggering speed of the SCR by improving the triggering speed of either Anode-G2diode, G1-cathode diode or both or triggering diodes and reducing the overshoot voltage.

Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings without departing from the spirit and the scope of the invention.