INPUT PROTECTION DEVICE

Abstract: An input protection device (34) for a semiconductor inte­ grated circuit, comprising a semiconductor substrate (24) of one conductivity type, a first insulator (26) layer formed on the substrate and having an opening (40) extending to the surface of the substrate, a doped region (42) formed in the opening for contacting the substrate, the doped region being of the other conductivity type and forming a p-n junction at its interface with the substrate, an input protection resistor region (17) having a portion contacting the doped region through the opening in the first insulator layer and a portion overlapping the first insulator layer, a second insulator layer (30) having a portion covering the input protection resistor region, the por­ tion of the second insulator layer having an opening (28) extending to the surface of the resistor region, the opening in the second insulator layer being at least in part in registry with the opening in the first insulator layer, and a metallized input wiring layer (32) formed on the second insulator layer and having a region extending through the opening in the second insulator layer to the surface of the protection resistor region. The p-n junciion formed between the substrate and the doped region provides a diode (36) to be reverse biased by the voltage to be normally applied to the input transistor and is operative to pass therethrough a current with a high voltage surge.

Description: FIELD OF THE INVENTION The present invention relates in general to semiconductor integrated circuits and in particular to an input protection device or, more particularly to a surge voltage suppressor for use in a semiconductor integrated circuit. DESCRIPTION OF THE PRIOR ART An input protection device for use in a semiconductor integrated circuit is known which uses an input protective resistor formed by a patterned polysilicon layer over a silicon substrate. Such an input protection device is useful as a surge voltage suppressor for protecting an input transistor of the integrated circuit from a momentary voltage surge which is likely to occur during operation of the integrated circuit. Fig. 1 of the drawings shows the equivalent circuit of a conventional input protection device of this nature. As shown, the prior-art input protection device is provided for a semiconductor integrated circuit including an input pad 10 and an input transistor 12. The input protection device includes a series combination of input protection resistors 16 and 18 connected between the input pad 10 and the input transistor 12. An input protection transistor 20 has its source and drain connected between ground and a circuit node 22 between the resistors 16 and 18 with its gate also connected to ground. Each of the transistors 12 and 20 is assumed to be an n-channel field-effect device. Figs. 2 and 3 of the drawings show a portion of a semiconductor integrated circuit structure including the input protection device thus constructed and arranged. The input protection resistors 16 and 18 are now commonly represented by a patterned polysilicon resistor region 17 which is formed over a silicon substrate 24, as shown in Fig. 3. The substrate 24, which is herein assumed to be of p-type silicon, has a thin layer 26 of silicon oxide having the polysilicon resistor region 17 directly formed thereon. The polysilicon resistor region 17 and the silicon oxide layer 26 are covered with a conformally deposited insulator layer 30 which is formed with a contact hole 28 extending to the surface of the resistor region 17. A metallized input wiring layer 32 is deposited on this insulator layer 30 and has a contact region extending through the contact hole 28 in the insulator layer 30 to the surface of the polysilicon resistor region 17, thereby providing electrical connection between the input pad 10 and the resistor region 17, viz., each of the input protection resistors 16 and 18 in the arrangement shown in Fig. 1. BACKGROUND OF THE INVENTION During operation of the integrated circuit including the input protection device thus constructed in the semiconductor structure, a current caused by a high voltage surge may occur at the input pad 10 of the circuit. As the current is passed through the input protection resistors 16 and 18 to the input transistor 12, the high voltage surge is on one hand attenuated by the input protection resistors 16 and 18 and is on the other hand taken up by means of the input protection transistor 20. In the semiconductor structure shown in Figs. 2 and 3, the current flowing through the metallized input wiring layer 32 as caused by the voltage surge is applied from the input wiring layer 32 to the polysilicon resistor region 17 and may damage the polysilicon at the contact surface of the resistor region with the metallized input wiring layer 32. When this takes place, the patterned polysilicon resistor region 17 is broken and is disabled from attenuating the high voltage surge of the current flowing through the metallized layer. The present invention contemplates the provision of a useful solution to such a problem inherent to this input protection device serving as a voltage surge suppressor. It is, accordingly, a prime object of the present invention to provide a semiconductor integrated circuit having an input protection device providing reliability of operation responsive to a high voltage surge which may occur at an input to an input transistor of the circuit. SUMMARY OF THE INVENTION An input protecting device according to the preamble part of claim 1 is disclosed in GB-A-2 092 826. In this device the cathode of a protection diode is coupled between a polysilicon resistor and an internal circuit consisting of two transistors. The protection diode thus located downstream of the resistor is causativ of serious damage upon application of an abnormally high voltage, and, therefore, the same problem as mentioned above is encountered therein. JP-A-56-83964 discloses a protection device in which the cathode of a protection diode is also coupled between two resistors coupled in series. The series combination of the two resistors are coupled between an input node and an internal circuit, and a protection transistor is coupled between the ground and the internal circuit. The protection diode thus connected between the two resistors is much liable to cause the resistors to be damaged upon application of an abnormally high voltage. It is therefore the object of the invention to provide a semiconductor integrated circuit having an input protection device providing reliability of operation in responsive to a high voltage surge which may occur at an input for an input transistor of the circuit. This object is achieved by an input protection device for a semiconductor integrated circuit, comprising a) a semiconductor substrate of a first conductivity type; b) a first insulator layer on the substrate, the first insulator layer having an opening extenting to the surface of the substrate; c) a doped region formed in and substantially coextensive with the opening in the first insulator layer, the doped region being of a second conductivity type opposite to the first conductivity type and forming a p-n junction at its interface with the substrate; d) a patterned input polysilicon protection resistor layer having a portion contacting the doped region through the opening in the first insulator layer and a portion overlapping the first insulator layer; e) a second insulator layer having a portion covering the input protection resistor layer, the portion of the second insulator layer having an opening extending to the surface of the resistor region; and f) a metallized layer on the second insulator layer, the metallized input wiring layer having a region extenting through the opening in the second insulator layer to the surface of the input protection resistor region, wherein the opening in the second insulator layer is at least in part in registry with the opening in the first insulator layer. An input protection device thus constructed in accordance with the present invention may further comprise a field-effect transistor having its source and drain connected between the input protection resistor region and ground. In a further development of the invention the input protection device may further comprise an input pad coupled to one and of said metallized input wiring layers which are coupled at the other end to a transistor to be protected by said input protection device, and a protection field-effect transistor having its drain-source current path connected between an intermediate junction provided at a predetermined portion of said patent input protection resistor an a reference voltage terminal. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of an input protection device according to the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding structures, elements and portions and which: Fig. 1 is a schematic circuit diagram view showing the equivalent circuit arrangement of a typical prior-art input protection device serving as a voltage surge suppressor in a semiconductor integrated circuit; Fig. 2 is a fragmentary plan view showing part of the semiconductor integrated circuit structure including the prior-art input protection device illustrated in Fig. 1; Fig. 3 is a fragmentary cross sectional view taken along line III-III in Fig. 2; Fig. 4 is a schematic circuit diagram view showing the equivalent circuit arrangement of a preferred embodiment of an input protection device for use as a voltage surge suppressor in a semiconductor integrated circuit in accordance with the present invention; Fig. 5 is a fragmentary plan view showing part of the semiconductor integrated circuit structure including the input protection device illustrated in Fig. 4; and Fig. 6 is a fragmentary cross sectional view taken along line VI-VI in Fig. 5. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 4 of the drawings, an input protection device embodying the present invention, now represented in its entirety by reference numeral 34, is also provided for a semiconductor integrated circuit including an input pad 10 and an input transistor 12 which is assumed to be an n-channel field-effect device. Similarly to its prior-art counterpart shown in Fig. 1, the input protection device 34 comprises a series combination of input protection resistors 16 and 18 and an input protection transistor 20. The input protection resistors 16 and 18 are connected in series between the input pad 10 and the input transistor 14 while the input protection transistor 20, which is also assumed to be an n-channel field-effect device, has its drain connected to a circuit node 22 between the resistors 16 and 18, the source and gate of the transistor 20 being connected to ground. In accordance with the present invention, the input protection device 34 further comprises a diode 36 having its cathode connected to a circuit node 38 between the input pad 10 and the series combination of the input protection resistors 16 and 18 and its anode connected to ground as shown. In Figs. 5 and 6 of the drawings is illustrated the detailed configuration forming a portion "B" of the circuit arrangement shown in Fig. 4. As shown, the input protection resistors 16 and 18, now commonly represented by a patterned polysilicon resistor region 17 as in Figs. 2 and 3, form part of a semiconductor integrated circuit provided on a silicon substrate 24, as shown in Fig. 6. Each of the transistors 14 and 20 being assumed to be an n-channel transistor, the substrate 24 is assumed to be of p-type silicon. The p-type silicon substrate 24 has a thin layer 26 of silicon oxide formed with an opening 40 on the surface of the substrate 24. A suitable donor-type dopant such as phosphorus is introduced either by diffusion or implant of ions into the silicon substrate 24 through this opening 40. A heavily n-type doped region 42 is thus formed in and coextensively with the opening 40 in the oxide layer 26 with a gate oxide film 44 formed along the perimeter of the region 42 as shown, defining an active surface area of the input protection resistor provided by the polysilicon resistor region 17. The polysilicon resistor region 17 is formed in part on this active surface area of the n-type doped region 42 through the opening 40 in the oxide layer 26 and in part on the oxide layer 26 and the gate oxide film 44. The polysilicon resistor region 17 and the silicon oxide layer 26 are covered with a conformally deposited insulator layer 30 which has a portion covering the input protection resistor region 17. This portion of the insulator layer 30 is formed with a contact hole 28 which extends to the surface of the resistor region 17 and which is at least in part in registry with the opening 40 in the oxide layer 26 which provides a first insulator layer in an input protection device according to the present invention. A metallized input wiring layer 32 of, for example, aluminum is deposited on this insulator layer 30 and has a contact region extending through the contact hole 28 in the insulator layer 30 to the surface of the polysilicon resistor 17. As shown in Fig. 5, the metallized input wiring layer 32 has an extension 10' forming the pad 10 illustrated in Fig. 4 and provides electrical connection between the input pad 10 and the resistor region 17, viz., each of the input protection resistors 16 and 18 in the arrangement shown in Fig. 4. The polysilicon resistor region 17 and the doped region 42 in the structure illustrated in Figs. 5 and 6 are provided in association with the input transistor 12 and input protection transistor 20 in the circuit arrangement shown in Fig. 4. These transistors 14 and 20 are however not shown in Figs. 5 and 6 for brevity of illustration and because of the fact that the configuration of each of such devices is apparent to those skilled in the art. In the input protection device embodying the present invention, there is thus formed a p-n junction at the interface between the p-type silicon substrate 24 and the n-type doped region 42, thereby providing the diode 36 in the arrangement shown in Fig. 4. This p-n junction or diode 36 is reverse connected between the input pad 10 (Fig. 4) and the substrate 24 with its cathode grounded through the p-type silicon substrate 24. The diode 36 is thus reverse biased by the current which normally flows through the metallized input wiring layer 32 during operation of the integrated circuit. In the event a high voltage surge occurs at the input pad 10 and the voltage applied to the diode 36 reaches the Zener breakdown level of the p-n junction, the reverse connected diode 36 becomes conductive and provides a current path in addition to the current path established through the input protection resistors 16 and 18. The current with the high voltage surge is thus on one hand attenuated by the input protection resistors 16 and 18 and taken up by means of the input protection transistor 20 and is on the other hand passed through the diode 36 and the substrate 24 to ground. All these effects contribute to relieving the input transistor 14 from the direct attack of the high voltage surge and provide reliability of projection from surge voltages which may be produced during operation of the integrated circuit. While it has been assumed that each of the input protection transistor 20 of the input protection device 34 embodying the present invention is an n-channel field-effect transistor, the input protection transistor which form part of an input protection device according to the present invention may be constituted by a p-channel field-effect transistor or a p-n-p or n-p-n bipolar transistor. While, furthermore, the input transistor 14 of the semiconductor integrated circuit to which the input protection device 34 embodying the present invention is applied has also been assumed to be an n-channel field-effect transistor, an input protection device according to the present invention may also be applied to a semiconductor integrated circuit using a p-channel field-effect transistor or a p-n-p or n-p-n bipolar transistor as the input transistor thereof. Where a p-channel field-effect transistor is used as each of the input transistor 14 and the input protection transistor, n-type silicon is used as the substrate 24 so that the doped region 42 should be formed by introduction of an acceptor-type dopant such as boron. In this instance, the diode or p-n junction which forms part of the modified embodiment of an input protection device according to the present invention has its anode connected to the input pad 10 and its cathode connected to ground through the n-type silicon substrate, though not shown in the drawings.

Claims: An input protection device for a semiconductor integrated circuit, comprising a) a semiconductor substrate (24) of a first conductivity type; b) a first insulator layer (26) on said substrate, the first insulator layer having an opening (40) extending to the surface of said substrate; c) a doped region (42) formed in and substantially coextensive with said opening, the doped region being of a second conductivity type opposite to said first conductivity type and forming a p-n junction at its interface with said substrate; d) a patterned input polysilicon protection resistor layer (17) having a portion contacting said doped region through said opening and a portion overlapping said first insulator layer; e) a second insulator layer (30) having a portion covering said input protection resistor layer, said portion of the second insulator layer having an opening (28) extending to the surface of the resistor; and f) a metallized input wiring layer (32) on said second insulator layer, the metallized input wiring layer having a region extending through said opening (28)in said second insulator layer to the surface of said input protection resistor characterized in that said opening (28) in the second insulator layer (30) is at least in part in registry with the opening (40) in said first insulator layer (26). An input protection device as set forth in claim 1, further comprising a field-effect transistor (20) having its source and drain connected between said input protection resistor (17) layer and ground. An input protection device as set forth in claim 1, in which said input protection device further comprises an input pad (10) coupled to one end of said metallized input wiring layer which is coupled at the other end to a transistor (12) to be protected by said input protection device, and a protection field-effect transistor (20) having its drain-source current path connected between an intermediate junction provided at a predetermined portion of said patterned input protection resistor and a reference voltage terminal.

Inventors: SATO, YOSHINORI
Applicants: NEC CORPORATION
Application year: 1986
Main classification: H01L27
IPC codes: H01L27, H01L21