Abstract:
The present invention provides a combinded FOX and poly gate structure, for effectively reducing the trigger voltage of a conventional field device, for improving the robustness of a NMOS transistor of a small drive I/O circuit, and for improving the ESD performance of a stack-gate voltage tolerant I/O.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is related to an electrostatic discharge protection device, and more particurly to the electrostatic discharge protection device for the IC pin design.  
         BACKGROUND OF THE INVENTION  
         [0002]    I. Multi-Gate-Fingers NMOSFET and the Issue:  
           [0003]    An NMOSFET is a very effective ESD protection device. In one application, with the gate connected to a gate-driving signal, the NMOSFET is used as the pull down device of a CMOS buffer to drive an output voltage. In a second application, with the gate electrically connected to the ground, the NMOSFET is used to protect an input pin or power bus during an ESD event.  
           [0004]    In a PS (Positive-Voltage-to-VSS) mode ESD event, a positive ESD transient voltage applied to an IC pin while a VSS power pin is at ground potential, the protection of an NMOSFET heavily depends on the snap-back mechanism for conducting large amount of ESD current between the drain and source. To start, the high electric field at the drain junction causes impact ionization, which generates both minority and majority carriers. The minority carriers flow toward the drain contact and the majority carriers flow toward the substrate/pwell contact causing a local potential build up in the current path in pwell. When the local substrate potential is 0.6V higher than the adjacent n+ source potential, the source junction becomes forward biased. The forward biased source junction injects minority carriers (electrons) into the pwell, and those carriers eventually reach the drain junction to further enhance the impact ionization (see Ref.  1 b). And as a continuous loop the MOSFET gets into a low impedance (snap back) state to conduct large amount of ESD current.  
           [0005]    In a multi-finger NMOS structure as shown in FIG. 1A and FIG. 1B, not all gate fingers may turn on during an ESD event. This is because the first few gate finger having turned on quickly get into a snap-back low-impedance condition. It reduces the drain terminal  11  to source terminal  12  voltage to a transient voltage less than the trigger voltage of the NMOS device. This potentially prevents other gate fingers from turning on. Therefore, with only partial number of gate fingers turned on to absorb the ESD energy, the size of the NMOSFET is effectively reduced and the ESD performance degrades.  
           [0006]    When a gate finger is triggered in an ESD event, the entire finger turns on. This is due to a cascading effect that a local source junction in a forward biasing state will inject lots of carriers into the substrate to flow toward the drain junction, which in turn generates more minority carriers (due to impact ionization) flowing back toward the p+ guard ring to raise the adjacent pwell potential. Therefore, the adjacent source region is also turned into a forward bias state. With this cascading effect, the entire gate finger turns on into a snap back condition.  
           [0007]    Prior art MOSFET-based I/O (Input/Output) structures with self-ESD protection typically including a number of NMOSFET and PMOSFET transistors. As shown in FIGS. 2A and 2B, the pull-down NMOSFET may comprises a number of gate elements, with some connected to a first gate signal for the output transistor portion, and some connected to the VSS bus/Ground as the input protection ESD structure. Similarly, the pull-up PMOSFET may comprise a number of gate elements, with some connected to a second gate signal for the output transistor portion, and some connected to the VDD bus as the input protection ESD structure. In the prior-art methods, a gate element formed of a polysilicon element is typically either coupling to a gate signal or to a power bus.  
           [0008]    [0008]FIG. 3A shows the pull-down portion of a well known voltage-tolerant I/O and ESD protection circuit. As an example, VDD is at 3.3V and the NMOS gate oxide can withstand only up to 3.6V. But based on a stack-gate configuration, the voltages across each gate oxides of pull-down NMOS-transistors does not exceed 3.3V, even though a 5-volt signal appears at the pad. This is known as a 5V-tolerant design based on a 3.3V MOS transistors. However, the ESD performance of such 5V tolerant I/O circuit are typically not satisfactory due to shorter contact to gate spacing and larger drain-to-source spacing (less efficient bipolar for ESD snap-back mechanism). FIG. 3B shows an exampled layout for a stack-gate configuration for a (pull-down) I/O circuit.  
           [0009]    [0009]FIG. 4A shows another ESD protection device known as a field device, or a lateral bipolar device. And FIG. 4B shows an exampled layout of a field device, with a channel region formed under a field oxide stripe for separating an n+ source regions. A filed device is a viable option as a primary ESD protection device, a minor drawback is that the trigger voltage is typically a little bit higher than a GGNMOS (Ground-Gate NMOS) or a GCNMOS (Gate-Couple NMOS) transistor.  
         SUMMARY OF THE INVENTION  
         [0010]    The objective of the present invention is to provide a combined FOX and poly gate structure for effectively reducing the trigger voltage of a conventional field device, for improving the robustness of a NMOS transistor of a small drive I/O circuit, and for improving the ESD performance of a stack-gate voltage tolerant I/O.  
           [0011]    According to the present invention, an electrostatic discharge (ESD) protection device comprises:  
           [0012]    a semiconductor bulk of a first conductivity type;  
           [0013]    a first doped region of a second conductivity type formed in the semiconductor bulk;  
           [0014]    a second doped region of a second conductivity type formed in the semiconductor bulk;  
           [0015]    a channel region formed between the first doped region and the second doped region;  
           [0016]    a first gate segment formed over a first part of the channel region; and  
           [0017]    a first field-oxide segment formed over a third part of the channel region; wherein a first part of the first gate segment at least partially overlaps the first field-oxide segment.  
           [0018]    In accordance with one aspect of the present invention, wherein the first and third parts form a first continuous portion of the channel.  
           [0019]    In accordance with one aspect of the present invention, wherein the first gate segment and the first field-oxide segment are substantially collinear.  
           [0020]    In accordance with one aspect of the present invention, wherein the first gate segment comprises a polysilicon element over an oxide layer.  
           [0021]    In accordance with one aspect of the present invention, further comprises a plurality of islands formed over the bulk and being enclosed by the first doped region.  
           [0022]    In accordance with one aspect of the present invention, wherein the plurality of islands comprises a first and second arrays of islands; the first array of islands comprises polysilicon-over-oxide islands; and the second array of islands comprises field-oxide islands.  
           [0023]    In accordance with one aspect of the present invention, wherein the first array of islands is closer to the channel region than the second array of islands.  
           [0024]    In accordance with one aspect of the present invention, further comprises a second gate segment formed over a second part of the channel region; and a first part of the second gate segment overlaps at least partially the first field-oxide segment.  
           [0025]    In accordance with one aspect of the present invention, wherein the second and third parts form a second continuous portion of the channel.  
           [0026]    In accordance with one aspect of the present invention, wherein the first doped region couples to a pad.  
           [0027]    In accordance with one aspect of the present invention, wherein the second doped region couples to a power bus.  
           [0028]    According to the present invention, an electrostatic discharge (ESD) protection device comprises:  
           [0029]    a semiconductor bulk of a first conductivity type;  
           [0030]    a first doped region of a second conductivity type formed in the semiconductor bulk;  
           [0031]    a second doped region of a second conductivity type formed in the semiconductor bulk;  
           [0032]    a channel region formed between the first and the second doped regions;  
           [0033]    the channel region comprising a split-channel region and a non-split-channel region;  
           [0034]    the split-channel region including a first and a second sub-channel regions spaced apart from each other; wherein the first sub-channel region being adjacent to the first doped region and the second sub-channel region being adjacent to the second doped region;  
           [0035]    a first gate segment formed over the first sub-channel region;  
           [0036]    a second gate segment formed over the second sub-channel region;  
           [0037]    a first field-oxide segment formed over the non-split-channel region; and the first and the second gate segments form a stack-gate structure.  
           [0038]    In accordance with one aspect of the present invention, wherein the first and the second gate segments are substantially parallel to each other.  
           [0039]    In accordance with one aspect of the present invention, wherein the first gate segment, the second gate segment and the first field-oxide segment are substantially parallel to each other.  
           [0040]    In accordance with one aspect of the present invention, wherein the split channel region is connected to the non-split channel region to form a continuous channel region.  
           [0041]    In accordance with one aspect of the present invention, wherein the first gate segment comprises a polysilicon element over an oxide layer.  
           [0042]    In accordance with one aspect of the present invention, wherein the second gate segment comprises a polysilicon element over an oxide layer.  
           [0043]    In accordance with one aspect of the present invention, wherein the first gate segment have a first part overlapping a field-oxide extension segment; and the second gate segment have a second end overlapping the field-oxide extension segment.  
           [0044]    In accordance with one aspect of the present invention, further comprises a plurality of islands formed over the bulk and being enclosed by the first doped region.  
           [0045]    In accordance with one aspect of the present invention, wherein the plurality of islands comprises a plurality arrays of islands.  
           [0046]    In accordance with one aspect of the present invention, wherein the first doped region coupling to a pad.  
           [0047]    In accordance with one aspect of the present invention, wherein the second doped region coupling to a power bus.  
           [0048]    According to the present invention, an electrostatic discharge (ESD) protection device, comprises:  
           [0049]    a semiconductor bulk of a first conductivity type;  
           [0050]    a first doped region of a second conductivity type formed in the semiconductor bulk;  
           [0051]    a second doped region of a second conductivity type formed in the semiconductor bulk;  
           [0052]    a channel region formed between the first and the second doped regions;  
           [0053]    a first and a second arrays of islands formed over the bulk and being enclosed by the first doped region; wherein  
           [0054]    the first array of islands comprising polysilicon-over-oxide islands;  
           [0055]    the second array of islands comprising field-oxide islands; and  
           [0056]    the first array of islands being closer to the channel region than the second array of islands.  
           [0057]    The present invention may best be understood through the following description with reference to the accompanying drawings, in which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0058]    [0058]FIG. 1A and FIG. 1B show a conventional multi-finger NMOS structure;  
         [0059]    [0059]FIG. 2A and FIG. 2B show a conventional pull-down NMOSFET comprising a number of gate elements;  
         [0060]    [0060]FIG. 3A shows a conventional pull-down portion of a well known voltage-tolerant I/O and ESD protection circuit;  
         [0061]    [0061]FIG. 3B shows an exampled layout for a stack-gate configuration for a pull-down I/O circuit;  
         [0062]    [0062]FIG. 4A shows another ESD protection device known as a field device or a lateral bipolar device;  
         [0063]    [0063]FIG. 4B shows an exampled layout of a field device;  
         [0064]    [0064]FIG. 5A shows the poly-silicon gate element(s) according to the present invention;  
         [0065]    FIGS.  5 B, 5 C, 5 D, and  5 E show the cross-section view of FIG. 5A;  
         [0066]    [0066]FIG. 6A show the small (short) NMOS gate elements (for the small-drive NMOS transistor) according to the present invention;  
         [0067]    [0067]FIGS. 6B and 6C show the cross-section view of FIG. 6A;  
         [0068]    [0068]FIG. 7A shows another optimized island structure for NMOS transistor I/O or ESD protection device according to the present invention;  
         [0069]    [0069]FIGS. 7B and 7C show the cross-section view of FIG. 7A;  
         [0070]    [0070]FIG. 8 shows the stacked-gate elements and a FOX stripe form a continuous stripe of gate structure according to the present invention; and  
         [0071]    [0071]FIG. 9 shows islands can be placed in the drain region of the structure of FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0072]    1. Improved Field-Device ESD Protection Element With Reduced Trigger Voltage:  
         [0073]    As illustrated in FIG. 5A, poly-silicon gate element(s)  52  being formed as an extension at one or two ends of a field oxide stripe  51 . A channel region is formed under the continuous stripe of overlapping filed-oxide and poly-silicon-gate elements  52 , and sandwitched between a continuous drain diffusion edge and a continuous source diffusion edge.  
         [0074]    The combined FOX stripe  51  and poly-gate element  52  form an “effective gate finger” between the drain  53  and source diffusion regions  54 . When the poly-gate-element is triggered in an ESD event, the entire FOX/poly-gate-element finger turns on. This is due to a cascading effect that a local source junction in a forward biasing state will inject lots of carriers into the substrate to flow toward the drain junction, which in turn generates more minority carriers (due to impact ionization) flowing back toward the p+ guard ring to raise the adjacent pwell potential. Therefore, the adjacent source region is also turned into a forward-bias state. With this cascading effect, the snap back occurring at the poly-gate element  52  quickly cascading down along the FOX stripe and thus the entire FOX/poly-gate-element finger turns on into a snap back condition.  
         [0075]    As shown in FIG. 5A, the two gate elements  52  can be the same node connecting to the same gate voltage or gate signal. Alternatively, they can be different nodes connecting to different gate voltages or signals.  
         [0076]    [0076]FIGS. 5B, 5C,  5 D and  5 E show the cross-section view of FIG. 5A device along the dotted lines A-A′,B-B′,C-C′ and D-D′.  
         [0077]    2. Small-Drive NMOS Transistor:  
         [0078]    As was shown in FIGS. 2A and 3B prior art, for small-drive NMOS transistors, typically additional dummy (ground-gate) NMOS fingers are used in conjunction with the NMOS output transistor to assure adequate total NMOS size for ESD protection. The draw back is that not all NMOS are turned on simultaneously and thus the ESD performance is less consistent.  
         [0079]    As shown in FIG. 6A, the small (short) NMOS gate elements (for the small-drive NMOS transistor) can be placed at one or two ends of a FOX/poly-gate-element stripe. Once the poly-gate element  52  is triggered, the entire FOX/poly-gate-element stripe, as an effective gate finger, quickly turns into snap back.  
         [0080]    Since during an ESD event, most of the heat is generated at the center portion of a gate finger, preferably but not as a required condition, by placing the FOX stripe substantially at the center position of the “FOX/poly-gate-element” finger reduces the chances of gate-oxide (of the poly-gate element) rupture during the ESD event.  
         [0081]    A preferred “islands” placement was shown in the drain region  53 , in that, an array of FOX islands  61  and an array of floating poly-islands  62  are placed along a stripe of drain region. As a preferred option, the array of poly islands are closer to the gate and channel than the array of the FOX islands. In this arrangement, the contact  55  to gate spacing can be reduced from a traditional 5 um down to 3 to 4 um.  
         [0082]    The combined arrays of FOX and poly island  62  structure enjoys the advantage that the trigger voltage is the lower of either with the FOX or poly islands  62 , regardless of what process technology is used. And with FOX closer to the contact where the high ESD voltage comes in, less chance for damaging the gate oxide of a poly island.  
         [0083]    [0083]FIGS. 6B and 6C show the cross-section view of FIG. 6A device along the dotted lines E-E′ and F-F′.  
         [0084]    3. NMOS Gate Fingers With Preferred Island Structures.  
         [0085]    [0085]FIG. 7A shows another optimized island structure for NMOS transistor I/O or ESD protection device. An array of FOX islands  61  and a first array of floating poly-islands are placed along a stripe of drain region  53 . As a preferred option, the first array of poly islands  62  are closer to the gate  521  and channel than the array of the FOX islands  62 . As another option, a second array of poly islands  62  can be disposed between the array of FOX islands  61  and the drain contacts  55 .  
         [0086]    [0086]FIGS. 7B and 7C show the cross-section view of FIG. 7A device along the dotted lines G-G′ and H-H′.  
         [0087]    4. Voltage Tolerant I/O for ESD Protection:  
         [0088]    As was shown in FIG. 8, stacked-gate elements ( 81 , 82 ) and a FOX stripe  51  form a continuous stripe of gate structure. The ESD trigger will initially taking place at the poly-gate to drain edge, and cascading along the entire FOX/stacked-gate-element finger. The ESD improvement is due to FOX stripe functions as the effective gate finger, and also depending on the process technology, the bipolar gain can be larger across the drain/FOX/source structure than across the drain/poly-gate/source structure. As we know, the ESD protection mechanism is largely bipolar action among the drain, substrate and source regions, and larger bipolar gain improves ESD performance.  
         [0089]    [0089]FIG. 9 shows islands can be placed in the drain region  53  of the structure of FIG. 8, for more uniform ESD current dispite the discontinuity of the poly-gate element to the FOX gate stripe.  
         [0090]    5. Islands:  
         [0091]    For a reference of the formation of the island structure, please refer to U.S. Pat. No. 5,721,439 patent.  
         [0092]    While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.  
         [0093]    Prior Arts Listed:  
         [0094]    1. “ESD in silicon Integrated Circuits,” Amerasekera and Duvvury, John, Wiley and Sons, 1995.  
         [0095]    2. U.S. Pat. No. 5,721,439 MOS transistor structure for electro-static discharge protection circuitry.  
         [0096]    3. U.S. Pat. No. 5,248,892 Semiconductor device provided with a protection circuit.  
         [0097]    4. U.S. Pat. No. 6,046,087 Fabrication of ESD protection device using a gate as a silicide blocking mask for a drain region.  
         [0098]    5. U.S. Pat. No. 6,064,095 Layout design of electrostatic discharge protection device.  
         [0099]    6. U.S. Pat. No. 6,153,913 Electrostatic discharge protection circuit.  
         [0100]    7. U.S. Pat. No. 6,157,065 Electrostatic discharge protective circuit under conductive pad.  
         [0101]    8. U.S. Pat. No. 6,236,073 Electrostatic discharge device.