Depletion controlled isolation stage

Abstract: An input protection device is presented having a depletion controlled isolation stage. In one embodiment of the invention, a depletion controlled isolation resistor (10) is formed between adjacent N+ diffused regions (16 and 18) by N-well diffusion. One N+ diffused region (16) electrically contacts an input bond pad (20) and a primary protective device. The other N+ diffused region (18) electrically contacts a second protective device and the internal circuit (21) it is to protect. The depletion controlled isolation resistor limits the amount of current passing through the resistor to a safe level during an over-voltage condition. In another embodiment of the invention, a depletion controlled isolation stage includes a silicon controlled rectifier (SCR) as the primary protective device in combination with the depletion controlled isolation resistor.

Description: FIELD OF THE INVENTION The present invention relates to the field of input protection circuitry for sensitive electrical devices such as integrated circuitry. More specifically, the present invention relates to the field of protection of over-voltage conditions such as in electrostatic discharge situations. BACKGROUND OF THE INVENTION Input protection circuitry is an essential part of modern integrated circuits, particularly in the area of field effect transistor based devices. Extremely small delicate device structures are very sensitive to high voltages. The threat of high voltage damage begins during fabrication of integrated circuits into products and continues throughout the service life of the integrated circuits. High voltage damage from electrostatic discharge may occur during installation of integrated circuits into products. Sources of electrostatic charge include the human body, within which charge is generated, and installation tools having residual charge. Electrostatic charge may be transferred to integrated circuits through physical contact. Subsequent to installation, high voltage damage is usually the result of a power surge. These high voltages may destroy integrated circuits, thus requiring expensive and tedious repairs on fully manufactured devices which could have been avoided by providing a mechanism for dissipation of the high voltage on the integrated circuit. This problem is particularly acute in complimentary metal oxide semiconductor field effect transistor type integrated circuits. In high density CMOS devices, an extremely high voltage may be developed by electrostatic discharge which easily destroys the very thin gate oxides and very short channel devices of the integrated circuit. One way to protect against these over-voltage conditions is to use a silicon controlled rectifier (SCR) structure compatible with complementary metal oxide semiconductor (CMOS). This structure is shown in US Patent Number 5,012,317, assigned to the assignee of this application. This structure provides excellent protection but the SCR protection device has a very high threshold voltage. This threshold voltage is commonly on the order of 70-80 volts. It has been found in many present day circuits that a 70-80 volt threshold is inadequate to protect some devices on the integrated circuit. Another input protection structure which greatly reduces the threshold voltage of the input protection is shown in U.S. Patent 4,939,616, assigned to the assignee of this application, which shows an input protection device with a low trigger threshold. This structure is a silicon controlled rectifier (SCR) type of device wherein the triggering mechanism is avalanche conduction at the interface between the N-well surrounding a portion of the protection device and the P-type substrate. Therefore, while input protection structures exist, it is highly desirable to provide an input protection structure which greatly increases the over-voltage protection that is available in present input protection structures. SUMMARY OF THE INVENTION Respective aspects of the present invention are set forth in claims 1 and 11. The described embodiments of the present invention provide an input protection device with a depletion controlled isolation stage. In one embodiment of the invention, a depletion controlled isolation stage includes a silicon controlled rectifier (SCR) as a primary protective device in combination with a depletion controlled isolation resistor. The depletion controlled isolation resistor is formed between adjacent N+ diffused regions by N-well diffusion. One N+ diffused region electrically contacts an input bond pad and the primary protective device. The other N+ diffused region electrically contacts a second protective device and the internal circuit it is to protect. The depletion controlled isolation resistor limits the amount of current passing through the resistor to a safe level during an over-voltage condition. An over-voltage condition includes electrostatic discharges and power surges. The triggering mechanism of the SCR is avalanche conduction at the interface between the N+ region at the input surrounding a portion of the protection device and the P-type substrate. Of course, opposite doping types may be effectively used from those disclosed in the described embodiments. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention and to show how the same may be carried into effect reference is made in the following, by way of example to the drawings in which: Figure 1 is a schematic diagram of a prior art input protection circuit. Figure 2 is a side view schematic diagram of another input protection circuit. Figure 3 is a side view schematic diagram of another design of input protection circuit. Figure 4 is a side view schematic diagram of yet another design of input protection circuit. Figure 5 is a side view schematic diagram of still another design of input protection circuit. Figure 6 is a side view schematic diagram of a device similar to that of Figure 2 and including a primary discharge device. Figure 7 is a side view schematic diagram of a device similar to that of Figure 6 and including primary and secondary discharge devices. Figure 8 is the side view schematic diagram of the device of Figure 2 in operation with a low input voltage applied thereto. Figure 9 is a graph depicting the snap back voltage of the device of Figure 2. Figure 10 is the side view schematic diagram of the device of Figure 2 in operation with a high input voltage applied thereto. Figure 11 is a side view schematic diagram of an embodiment of the present invention. Figure 12 is a side view schematic diagram of another embodiment of the present invention. Figure 13 is a plan view diagram of the embodiment of Figure 11. Figure 14 is a plan view diagram of the embodiment of Figure 12. Figure 15 is a graph depicting the triggering voltage of the devices of Figures 11 and 12. Figure 16 is a side view schematic diagram of the device of Figure 11 in operation with a low input voltage applied thereto. Figure 17 is a side view schematic diagram of the device of Figure 12 in operation with a low input voltage applied thereto. Figure 18 is a side view schematic diagram of the device of Figure 11 in operation with a high input voltage applied thereto. Figure 19 is a side view schematic diagram of the device of Figure 12 in operation with a high input voltage applied thereto. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a prior art two stage input protection circuit consisting of a primary discharge device D1, an isolation resistor and a small clamping device D2. In order to protect the internal circuit, device D2 must clamp at voltage V2 which is some value below the gate dielectric rupture threshold. Device D2 must continue to sink current during an electrostatic discharge ESD event until a sufficient voltage is developed across the isolation resistor to cause device D1 to turn on at voltage V1. For advanced processes, however, typical values of V1=25v, V2=15v and isolation resistance=100 ohms result in the isolation resistor conducting up to 100mA for up to 20nS. This may lead to premature failure of the isolation resistor before the full potential of primary protection device D1 is achieved. Figures 2-5 are various designs of input protection circuits having an N-well isolation resistor 10. The devices of Figures 2-5 all include a semiconductor substrate 12 and a lightly doped N-well 14 formed in substrate 12. In the device shown in Figure 2, heavily doped N+ regions 16 and 18 are formed within N-well 14 with region 16 being laterally spaced from region 18. Region 16 is electrically connected to an input bond pad 20. Region 18 is connected to a circuit structure 21 that is to be protected. In the device shown in Figure 3, heavily doped N+ region 16 is formed within N-well 14. Heavily doped N+ region 18 is formed at an intersection of N-well 14 and substrate 12. Region 16 is electrically connected to an input bond pad 20. Region 18 is connected to a circuit structure 21 that is to be protected. In the device shown in Figure 4, heavily doped N+ region 16 is formed at an intersection of N-well 14 and substrate 12. Heavily doped N+ region 18 is formed within N-well 14. Region 16 is electrically connected to an input bond pad 20. Region 18 is connected to a circuit structure 21 that is to be protected In the device shown in Figure 5, heavily doped N+ region 16 is formed at an intersection of N-well 14 and substrate 12. Heavily doped N+ region 18 is also formed at an intersection of N-well 14 and substrate 12. Region 16 is electrically connected to an input bond pad 20. Region 18 is connected to a circuit structure 21 that is to be protected. In the devices of Figures 2-5 resistance is provided by the resistive nature of N-well 14 between heavily doped regions 16 and 18. Each of the devices of Figures 2-5 may also include a primary discharge device D1 electrically connected to input bond pad 20 and to Region 16. As an example, Figure 6 shows a primary discharge device electrically connected to the input bond pad 20 and to Region 16 of the device of Figure 2. A secondary discharge device D2 may also be electrically connected to region 18 and to internal circuit 21. As an example, Figure 7 shows the device of Figure 6 including a secondary discharge device D2. Figure 8 is the device of Figure 2 with primary protection circuit D1 and heavily doped region 16 biased at 5 volts. At 5 volts, an N-well depletion edge 22 is formed within N-well 14 and a P-well depletion edge 30 is formed within substrate 12. Depletion edge 22 defines the boundary of the depletion region. In the range of 0-10 volts, N-well isolation resistor 10 displays a linear slope of voltage to current, as shown in Figure 9. In the range of 15-20 volts, N-well isolation resistor 10 starts to roll over with current I remaining the same as voltage V increases. Current remains the same because the resistance of N-well isolation resistor 10 increases with the increase in voltage and the N-well depletion region retracts toward heavily doped region 16. The resulting current and voltage characteristic, shown in Figure 9, is a velocity saturation effect. The velocity saturation effect causes a constant current region similar to the constant current regions in MOS devices. At some point, however, the electric field between heavily doped regions 16 and 18 will reach a critical point where the resistance value of N-well isolation resistor 10 snaps back to a constant value. This is also shown in Figure 9 where the snap back voltage V SB is reached at about 45 volts. Thereafter, N-well isolation resistor 10 will act like an NPN device. The resulting N-well depletion edge 22, P-well depletion edge 30 and current to voltage characteristics of the devices of Figures 3-5 (not shown) are similar to those for the device of Figure 2. The snap back voltage V SB is a function of the distance between regions 16 and 18. Closing the distance between the two heavily doped regions lowers the snap back voltage. Conversely, increasing the distance between the two heavily doped regions raises the snap back voltage. For example, a snap back voltage of 10 volts is realized with a distance of 2 microns between regions 16 and 18. A snap back voltage of 45 volts is realized with a distance of 8 microns between the heavily doped regions. In any event, the snap back must not occur before primary protective device D1 is activated or damage may result to the internal circuit 21. Figure 10 shows the circuit of Figure 7 experiencing an electrostatic discharge voltage of 25 volts. N-well depletion edge 22 retracts around heavily doped regions 16 and 18. In one embodiment of the invention, an N-well depletion resistor is utilized in a two stage input protection circuit in combination with a silicon controlled rectifier SCR as the primary protection device D1, as shown in Figures 11 and 12. N-well isolation resistor 10 is formed in a semiconductor substrate 12 which is lightly doped P-type. Lightly doped N-well 14 is formed in substrate 12. A heavily doped N-type region 16 is formed within N-well 14 in Figure 11 and at the intersection of N-well 14 and substrate 12 in Figure 12. In Figures 11 and 12, a heavily doped N-type region 18 is formed in N-well 14, region 18 being laterally spaced from region 16. A heavily doped P-type region 24 is formed in N-well 14 adjacent region 18 opposite region 16. A heavily doped N-type region 26 is formed adjacent region 24 at the interface between N-well 14 and P substrate 12. A heavily doped N-type region 28 is formed in P substrate 12, region 28 being spaced from region 26. Regions 18 and 24 are electrically connected to an input bond pad 20. Region 16 is electrically connected to secondary protective device D2. Region 28 is electrically connected to Vss. Figure 13 is a layout diagram of one method of laying out the over-voltage protection device of Figure 11 and Figure 14 is a layout diagram of one method of laying out the over-voltage protection device of Figure 12. In Figures 13 and 14, bond pad 20 is connected through the vias (represented as dots) to N+ region 18 and P+ region 24. The resistance of the depletion controlled resistor is provided by the resistive nature of lightly doped N-well between N+ regions 16 and 18. Doped regions of the over-voltage protection device may be advantageously formed in substrate 12 using masking and ion implantation techniques which are well known in the art. On the other hand, other doping techniques may be used, such as diffusion from a solid source. N-well 14 is formed at the same time the N-wells for P type field effect transistors are formed on the integrated circuit containing the over-voltage protection device. N+ regions 16, 18, 26 and 28 are formed at the time N-channel source/drain formation is performed for forming N-channel transistors on the integrated circuit. P+ region 24 is formed at the time the source/drain formation is conducted for P-channel transistors for the other components of the integrated circuit. P+ region 24 is the anode of the SCR and N+ region 28 is the cathode. Therefore, the devices of Figures 2-7 and 11-12 are completely compatible with CMOS fabrication processes. Figure 15 is a graph showing the current to voltage characteristic between bond pad 20 and ground for the over-voltage protection devices shown in Figure 11 and 12. As can clearly be seen from the graph, the over-voltage protection device begins to conduct current within the range of 20-30 volts. Figure 16 is the device of Figure 11, and Figure 17 is the device of Figure 12, showing N-well depletion edge 22 and P-well depletion edge 30 at an input voltage of 5 volts. Figures 18 and 19 are the devices of Figures 16 and 17, respectively, showing N-well depletion edge 22 and P-well depletion edge 30 at an input voltage of 25 volts. While specific embodiments of the present invention are disclosed herein, they are not to be construed in a limiting sense. For example, the described embodiments provide over-voltage protection for positive over-voltage situations. It is clear that changed doping polarities will provide over-voltage protection for opposite polarities. In addition, the described embodiments are utilized in conjunction with integrated circuitry. For example, embodiments which do not include integrated circuitry may advantageously provide over-voltage protection for discrete type devices.

Claims: A semiconductor device having a depletion controlled isolation stage device connected between an input node (20) and a circuit structure of the semiconductor device so as to protect the circuit structure against over-voltage conditions, the isolation stage device comprising: a lightly doped region (14) having a first conductivity type formed in a lightly doped substrate (12) having a second conductivity type; a first heavily doped region (16) having said first conductivity type and formed at least partially in said lightly doped region (14), said first heavily doped region (16) being electrically connected to said circuit structure; a second heavily doped region (18) formed in said lightly doped region (14) spaced from said first heavily doped region (16) and having said first conductivity type, said second heavily doped region (18) being electrically connected to said input node (20); a third heavily doped region (26) having said first conductivity type and formed at the intersection of said lightly doped region (14) and said substrate (12); a fourth heavily doped region (24) having said second conductivity type and, formed in said lightly doped region (14), said fourth heavily doped region (24) positioned between said second and third heavily doped regions (18, 26), said fourth heavily doped region (24) being electrically connected to said input node (20); and a fifth heavily doped region (28) having said first conductivity type and formed in said substrate (12), said fifth heavily doped region (28) being spaced from said lightly doped region (14), said doped regions being formed in said substrate (12) so as to provide resistive means electrically connected between said first heavily doped region (16) and said second heavily doped region (18), and having a resistance responsive to the voltage between said first heavily doped region (16) and said second heavily doped region (18). The semiconductor device of claim 1 wherein said first heavily doped region (16) is completely formed in said lightly doped region (14). The semiconductor device of claim 1 wherein said first heavily doped region (16) is formed at the intersection of said lightly doped region (14) and said substrate (12). The semiconductor device of any preceding claim including a primary discharge device, said primary discharge device being electrically connected to said second heavily doped region (18). The semiconductor device of any preceding claim including a secondary discharge device (D2), said secondary discharge device (D2) being electrically connected to said first heavily doped region (16). The semiconductor device of any preceding claim comprising a reference voltage supply (Vss) electrically connected to said fifth heavily doped region (28). The semiconductor device of claim 6, wherein said third, fourth and fifth heavily doped regions (26, 24, 28) form a silicon controlled rectifier (SCR). The semiconductor device of any preceding claim formed on an integrated circuit. The semiconductor device of any preceding claim, wherein said first conductivity type is N and said second conductivity type is P. The semiconductor device of any preceding claim, wherein said input node (20) is a bond pad of an integrated circuit. A method of manufacturing a semiconductor device having a depletion controlled isolation stage device connected between an input node and a circuit structure of the semiconductor device so as to protect the circuit structure against over-voltage conditions, the method comprising the steps of: forming a lightly doped region (14) having a first conductivity type in a lightly doped substrate (12) having a second conductivity type; forming a first heavily doped region (16), having a first conductivity type, at least partially formed in said lightly doped region (14), said first heavily doped region (16) being electrically connected to said circuit structure; forming a second heavily doped region (18), having a first conductivity type, in said lightly doped region (14) spaced from said first heavily doped region (16) so that said second heavily doped region (18) is electrically connected to said input node; forming a third heavily doped region (26), having said first conductivity type, at the intersection of said lightly doped region (14) and said substrate (12); forming a fourth heavily doped region (24) having said second conductivity type in said lightly doped region (14), with said fourth heavily doped region (24) positioned between said second and third heavily doped regions (18, 26), and with said fourth heavily doped region (24) electrically connected to said input node; and forming a fifth heavily doped region (28) having said first conductivity type in said substrate (12) and spaced from said lightly doped region (14). The method of claim 11 wherein said first heavily doped region (16) is completely formed in said lightly doped region (14). The method of claim 11 wherein said first heavily doped region (16) is formed at the intersection of said lightly doped region (14) and said substrate (12). The method of any one of claims 11 to 13, including the step of forming a primary discharge device electrically connected to said second heavily doped region (18). The method of any one of claims 11 to 14, including the step of forming a secondary discharge device (D2), said secondary discharge device (D2) being electrically connected to said first heavily doped region (16). The method of any one of claims 11 to 15, wherein said fifth heavily doped region (28) is electrically connected to a reference voltage (Vss). The method of claim 16 wherein said third, fourth and fifth heavily doped regions (26, 24, 28) form a silicon controlled rectifier (SCR). The method of any one of claims 11 to 17 wherein the semiconductor is formed on an integrated circuit. The method of any one of claims 11 to 18, wherein said first conductivity type is N and said second conductivity type is P. The method of any one of claims 11 to 19, wherein said input node (20) is a bond pad of an integrated circuit.

Inventors: DUVVURY CHARVAKA, MAKI TATSUROH, ROUNTREE ROBERT, DUVVURY, CHARVAKA, MAKI, TATSUROH, ROUNTREE, ROBERT
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1992
Main classification: H01L27
IPC codes: H01L27, H01L21