Patent Publication Number: US-5838043-A

Title: ESD protection circuit located under protected bonding pad

Description:
This is a division of application Ser. No. 08/294,849, filed on Aug. 29, 1994, now U.S. Pat. No. 5,652,689. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to semiconductor devices and more specifically to a circuit for protecting the device from the voltage of an electrostatic discharge (ESD). 
     INTRODUCTION 
     ESD is a familiar problem in the design of semiconductor devices. When the ESD voltage appears at an input-output terminal of a device, it can destroy circuits that are connected to the terminal. These terminals are enlargements of conductive stripes on the surface of the device and are commonly called pads. Field effect transistors are particularly liable to be damaged because the gate oxide is thin and is easily damaged by small voltages that would not damage other components. These circuits are commonly protected by a circuit that is connected to the pad and is turned on by a voltage that is associated with an electrostatic discharge. 
     A problem with these circuits is that they do not contribute to the ordinary operation of the device and they require a silicon area that could otherwise be used for circuits that perform other functions. This lost space is significant because these semiconductor devices have many input/output pads and they commonly have a protection circuit for each pad. It is a common goal in this art to reduce the space required by these protection circuits. 
     THE PRIOR ART 
     U.S. Pat. No. 5,182,220 to M. D. Ker, assigned to the assignee of this invention, shows a protection circuit with silicon controlled rectifiers (SCRs) connected to conduct between the input line and ground and between the input line and V DD . 
     SUMMARY OF THE INVENTION 
     One object of this invention is to avoid using much or all of the space that is otherwise required for ESD protection but could be used for operating circuits. 
     The protection circuit of this invention is adapted to be located under the pad it protects. This space is normally unused, and the space that would otherwise by used for protection circuits can be used for other purposes. 
     Another object is to provide a new ESD protection circuit that has a low capacitance and is faster than conventional protection circuits. Locating the circuit under the pad shortens some of the conductors and thereby decreases their inductance and resistance and also reduces the parasitic capacitance of the protection circuit. 
     The specific embodiment of the invention has features and advantages that will be apparent in the description of the preferred embodiment or will be specifically explained. 
    
    
     THE DRAWING 
     FIG. 1 is a schematic of the protection circuit formed by the preferred semiconductor device of this invention and shows the upper SCR and lower SCR and associated components. 
     FIG. 2 is a plan view of the preferred semiconductor device, showing both the upper SCR and the lower SCR. 
     FIG. 3 is an edge view of the part of the preferred semiconductor device that forms the upper SCR. 
     FIG. 4 is an edge view of the part of the preferred semiconductor device that forms the lower SCR. 
    
    
     THE PREFERRED EMBODIMENT 
     FIG. 1--Introduction 
     FIG. 1 shows an I/O pad 22, the internal circuits 23 that are to be protected, a conductive line 24 that connects the pad to the internal circuits&#39; and the upper SCR circuit 18 and the lower SCR circuit 19 that protect the internal circuits from ESD voltages at the pad. The two SCR circuits are similar to the corresponding transistors of Ker, but the associated components differ because the semiconductor structures differ. Some of the reference characters identify nodes in FIG. 1 that correspond to semiconductor structures in FIGS. 2-4. 
     FIG. 1--The Upper SCR Circuit 
     PNP transistor Q3 and NPN transistor Q4 are interconnected to form an SCR. When either Q3 or Q4 conducts in its emitter-collector circuit, it provides base current to turn on the other transistor. When both transistors have been turned on, they stay latched on until their base current has been interrupted or has been reduced to a value that will not sustain the latching operation. 
     These transistors and the associated components conduct between ground and line 24 when a positive ESD voltage appears at pad 22. The circuits conductor with a sufficiently low resistance to prevent the voltage on line 24 from damaging the internal circuits. 
     The transistors can be turned on in several ways. If the voltage on line 24 becomes sufficiently positive, the reverse biased base-collector junctions can break down, for example by impact ionization, and provide base current for the other transistor. 
     The transistors can also turn on in response to a fast rising positive going voltage on line 24. Note the capacitors C1, C2 and C3. If the voltage at the I/O pad rises fast enough, a charging current will flow to ground in the circuit path of resistor RW1, the base-emitter junction of Q3, and the series circuit of capacitors C2 and C3. Similarly, a circuit to ground is formed by RW1, C1, C2 and the base-emitter junction of Q4. Thereafter, Q3 conducts in circuit with the base-emitter junction of Q4, and Q4 conducts in circuit with the base-emitter junction of Q3 to establish the latching operation. 
     A diode D2 is formed between the line 24 and VDD in a series circuit with resistors R1 and RW1. This diode is forward biased by normal positive voltages on line 24 but it operates in the high resistance part of its volt-ampere characteristic because these voltages appear mostly across the two series resistors R1 and RW1. Diode D2 conducts in its reverse direction in response to a higher negative voltage on line 24. 
     Resistor RW1 is shown twice in FIG. 1 (at the anode of D2 and the emitter of Q3) to simplify the circuit diagram and simplify its relation to FIGS. 2-4. 
     The Lower SCR Circuit 
     In the lower circuit 19, transistors Q2 and Q1 are connected to form an SCR that conducts between line 24 and VDD. The lower SCR turns on in response to a predetermined high voltage of a negative polarity on line 24 and from a fast rising negative going pulse on line 24, as will be understood from the description of the analogous operation of the upper SCR. The structure that forms this SCR also forms a diode D1 which conducts in its reverse direction in the way described for diode D2. These devices and their associated components will be understood from the detailed description of the upper SCR circuit and from the description of the semiconductor structure that forms the lower circuit. 
     The SCR Structure--Introduction to FIGS. 2-4 
     The device has an N -  substrate 27, circuit devices (to be described later) formed in the upper surface, a layer 28 of an insulating material such as glass, a metal layer 26 and a bonding pad 22. The substrate 27 and its overlying insulation extend beyond the sides shown in the edge views of FIGS. 3 and 4 to include the internal circuits 23 of FIG. 1 and ordinarily to include other circuits and other pads (preferably with individual protection circuits of this invention). The pad metal layer 26 extends to form conductor 24 which connects the pad 22 to the internal circuits 23. The bonding pad has approximately the size shown in relation to the structure that forms the protection device. These structures are well known and the invention will be useful with other semiconductor arrangements. 
     The Upper SCR Structure--FIGS. 2 and 3 
     This protection device is formed in three p-wells; two n+ regions provide for connection to the V DD  terminal of a power supply. These five regions are insulated at the surface of the substrate by suitable insulation such as the field oxide represented by the elongated hexagonal shapes. The structure is symmetrical about a vertical centerline in the drawing and two upper circuits as shown in FIG. 1 are formed, one on each side of this dividing line. 
     An opening is provided in the general insulation layer 28 to permit a single downward extension 29 of the pad metal 26 to contact the center well. A p+ region in the well forms node 2 and makes this connection. 
     N+ diffusions 31 and 32 form node 8, a connection between the substrate and VDD. The path from these diffusions to the other components forms resistor R 1 . 
     The center p-well 34 cooperates with each outside p-well 33 and 35 and the intervening substrate to form a transistor Q3 as a lateral parasitic bipolar structure. The center p-well forms node 3, the emitter of PNP transistor Q3 and p-wells 33 and 35 each form the collector of this transistor and the intervening substrate forms node 4, the base. The path through the center well forms resistor RW1. 
     Transistor Q4 is formed as a vertical parasitic bipolar transistor in each outer well 33 and 35. N+ diffusion in the outer wells form node 6, the emitter of NPN transistor Q4. The underlying substrate forms node 4&#39; the collector of Q4, and the intervening part of the outer p-well forms the base. 
     The junction of the center well 34 and the substrate form the diode D2. 
     The capacitance formed by the junction of the center p-well and the substrate is represented by a schematic capacitor C1. All paths through C1 include RW1. These capacitor electrodes are node 3 and node 4 respectively. 
     Capacitor C1 forms a circuit between node 3 (one end of resistor RW1, just described) and node 4 in the upper SCR circuit, the common connection point of the base of the PNP transistor and the collector of the NPN. 
     The Lower Circuit--FIGS. 2 and 4 
     The lower circuit is formed in three p-wells. The center p-well 10 has a P+ diffusion 15 for connection to ground. This connection is shown schematically in the drawing and is shown in detail in FIG. 2. 
     N+ diffusions symmetrically arranged in the center p-well with respect to ground diffusion 15 are connected to the pad metal by extensions that are similar to conductor 29 in FIG. 3. An N+ diffusion adjacent to the outside wells forms a connection from VDD to the substrate 4. 
     The center well 10, the outside wells 12, and the intervening substrate cooperate to form transistor Q1 as a lateral parasitic bipolar PNP transistor. Well 12 forms the emitter, well 10 forms the collector, and the substrate forms the base. A P+ diffusion connects the outer well 12 to VDD and the path from this diffusion to the emitter region forms resistor RW3. 
     Q2 is a vertical parasitic transistor formed in the center well and the underlying substrate. An N+ diffusion in the center well which forms node 9, the emitter of Q2. The substrate 4 forms the collector&#39; and the intervening region of the center well 10 forms the base. The capacitors and resistors will be understood from the drawing and the more detailed description of FIG. 2. 
     OTHER EMBODIMENTS 
     From the description of the preferred embodiment of the invention, those skilled in the art will recognize variations within the spirit of the invention and the intended scope of the claims.