Insulated gate semiconductor device

Abstract: An insulated gate semiconductor device have a gate protec­ tion circuit wherein the breakdown voltage can be arbitrarily selected. In one embodiment, the gate protection circuit in­ cludes first polysilicon layer of a first resistance overlaying the gate insulation layer formed on the semiconductor substrate and laterally spaced from the gate layer, and second polysilicon layer of a second resistance higher than the first resistance contacting the first polysilicon layer to form at least one pair of diodes for protecting the gate insulation layer from electro­ static destruction caused by a sudden voltage surge applied to the gate layer. In a second embodiment, the gate protection circuit includes a pair of Schottky barrier diodes consisting of a gate layer and a separation layer overlaying the gate insulation layer together with a layer contacting the gate layer and separation layer.

Description: INSULATED GATE SEMICONDUCTOR DEVICE The present invention relates to an insulated gate semiconductor device having a gate protection circuit in accordance with the generic clause of claim 1. In power vertical MOSFET or IGBT semiconductor devices, a p-region is formed in the upper surface of an n-type silicon substrate. An n-type source region is then formed inside the p-region. A gate oxide layer is formed on a portion of the p-region and the n-type substrate surface, and a gate layer is then formed on the gate oxide layer. The resulting structure forms a channel through the p-region between the n-type source region and the n-type substrate along the gate oxide layer. A major current flows from a source electrode, connected to the source region and p-region, to a drain electrode disposed on the lower surface of the n-type silicon substrate. The insulated gate semiconductor devices of such structure must be protected from electrostatic destruction caused by a surge in the voltage applied to the gate layer. Particularly, in the field of automotive electronics, it is essential that a gate protection circuit be provided to the device. To satisfy this, a pluarlity of protection diodes connected back to back in series are used. Document IBM TECHNICAL DISCLOSURE BULLETIN, vol. 24, no. 11B, 4/1982, shows an arrangement to provide electrostatic discharge (ESD) protection to chip terminals without the parasitic NPN transistor action which accompanies conventional ESD protection schemes. Japanese Patent Application Laid-Open Publication No. 61-296770 describes a gate protection circuit for insulated gate semiconductor devices in which impurity is introduced into a semiconductor layer of a gate electrode to form an n-p - p-n junction therein, and Zener diodes, which are connected back to back in series are connected across a path formed between source and gate electrodes. Japanese Patent Application Laid-Open Publication No. 58-178566 describes a gate protection circuit in which gate protection diodes are formed by introducing impurity into a specific region in the semiconductor layer. Similarly, Japanese Patent Application Laid-Open Publication No.58-88461 and No. 58-87873 describe a gate protection circuit in which gate protection diodes connected back to back in series are formed by introducing impurity into a semiconductor layer, of a polysilicon layer disposed on a semiconductor substrate, for example. The protection diodes used in prior art gate protection circuits are Zener diodes. These diodes are formed by introducing impurity into the semiconductor layer disposed on a semiconductor substrate or directly into the semiconductor substrate. The breakdown voltage of a Zener diode is 5V or less. The breakdown voltage required for the gate protection circuit is normally 10V or more. To this end, the gate protection circuit constituting Zener diodes comprises a plurality of Zener diodes connected in series in both directions. Further, the breakdown voltage obtained is limited to a multiple of a Zener voltage of a single p-n junction. In such a gate protection circuit, it is very difficult to freely set the breakdown voltage according to the use of the semiconductor device. The present invention overcomes the problems and disadvantages of the prior art by providing an insulated gate semiconductor device with a gate protection circuit whose breakdown voltage may easily be selected. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. These objects are achieved by the characterizing features of claim 1. A semiconductor device not covered by the present invention has at least one pair of diodes, each diode including a first polysilicon layer of the first predetermined resistance, which is made of the same material as the gate layer and overlays the gate insulating layer extending between a region of the semiconductor substrate containing the first region and a gate pad disposed above the gate layer, and a second polysilicon layer of a second predetermined resistance higher than the first predetermined resistance contacting the first polysilicon layer, the conductivity type of the second polysilicon layer being different than that of the first polysilicon layer. The aforesaid pair of diodes are connected back to back in series and disposed between the source region and gate pad, constituting a gate protection circuit. The first polysilicon layer is formed simultaneously with the gate layer, and the extent of impurity concentration of the second polysilicon layer may be arbitrarily selected to give a desired breakdown voltage for a resultant pair of diodes. The resultant pair of diodes structure provides a high breakdown voltage and allow a back to back connection of the diodes in series with each other. An embodiment of the present invention involves an insulated gate semiconductor device having a gate protection circuit comprising a gate layer, a separation region made of the same material as the gate layer and connected to a source region, and a layer means contacting both the gate and separation layers. The gate layer, separation layer, and layer means overlay a gate insulating layer which overlays the upper surface of a semi-conductor substrate. In an embodiment, either the gate or layer means is made of a semiconductor material, and the gate, separation and layer means cooperate to form at least one pair of Schottky barrier diodes. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate the aforesaid embodiments of the invention and together with the description, serve to explain the principles of the invention. Fig. 1 is a sectional view, taken along line A-A in Fig. 2, of an insulated gate semiconductor device not covered by the present invention; Fig. 2 is a plan view of Fig. 1: Fig. 3 is a sectional view of an insulated gate semiconductor device according to an embodiment of the present invention. Fig. 4 is a sectional view of an insulated gate bipolar transistor. Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. Figs. 1 and 2 refer to an embodiment not covered by the present invention and Fig. 3 to an embodiment of the present invention. In Fig. 1, a silicon semiconductor substrate constitutes a drain layer comprising an n - layer 1 and an n + layer 11. A first region or p-well region 2 is diffused into the upper surface of n - layer 1. An n + source region 3 is disposed in the upper surface of n - layer 1 and surrounded by p-well region 2, and a gate layer 4, constituting an n + polysilicon layer, is formed on the surface of a gate insulating layer 5. Gate insulating layer 5 is layered on a portion of the p-well region 2, an end portion of the source region 3 and the n-type substrate so that a channel is formed in the p-well region 2 between the n - layer 1 and the source region 3 under the insulating layer 5 along the gate layer 4. In Fig. 2, the polysilicon layer of gate layer 4 has windows 41. In each window, an Al or aluminum conductor path contacts n + source region 3 and p-well region 2 through a contact hole 61 of an insulating layer 6, thereby to form a source electrode 7. In a vertical MOSFET, the current flowing on a path defined between source electrode 7 and a drain electrode 71, which is in contact with n + drain layer 11, is controlled by a signal input to gate 4. An extension of the polysilicon layer of gate layer 4 is in contact with an Al conductor path 8 through a contact hole 62 of insulating layer 6 as shown in Fig. 2. The Al conductor path 8 is extended to form a gate bonding pad 80. An extension 21 of p-well region 2 is disposed below gate pad 80 to improve the breakdown voltage by extending a depletion layer formed between p-well region 2 and n - layer 1 when a reverse voltage is applied. A first plurality of spaced polysilicon layers of a first predetermined resistance or n + type polysilicon layers 40 are simultaneously formed with gate layer 4 above extension 21 of p-well region 2. A gate insulating layer 51 is interlayed between n + polysilicon layers 40 and extension 21. Each of a second plurality of spaced polysilicon layers of a second predetermined resistance higher than the first predetermined resistance, otherwise referred to as p-type polysilicon layers or p-layers 9, are layered above a respective one of n + polysilicon layers 40, making contact therewith through contact holdes 64 of insulating film 6, and the resultant gate protection circuit structure constituting a pair of p-n diodes is patterned. The n + polysilicon layers 40 and p-layers 9 form two p-n junction diodes, and the p-layers 9 of the two diodes are connected to each other by an aluminium conductor layer 72. One of n + polysilicon layers 40 of the two p-n junction diodes is connected to an extended portion of a source electrode 7 through a contact hole 63, while the other one to a gate pad 80 through a contact hole 65. With such connections, the two diodes are connected back to back in series between the gate layer and source electrode. When the gate voltage applied to the path between the gate layer and source region reaches the breakdown voltage of one of the p-n junction diodes, which is determined by the extent of impurity concentration and thickness of p-layers 9, one of the diodes will break-down according to the polarity of the applied gate voltage, and the breakdown current will flow through the path between the gate layer and source region. Therefore, a voltage higher than the breakdown voltage of the p-n junction diode will not be applied to that path, and consequently, gate insulating layer 5 may be protected from electrostatic breakdown. The gate protection circuit can be similarly constructed in IGBTs, in which a p + -layer is layered under n - drain layer 1 instead of the n + layer 11 shown in Fig. 1. According to an embodiment not covered by the present invention, at least one pair of diodes are formed in an insulated gate semiconductor device, each diode constituting a first polysilicon layer of a first predetermined resistance, which is formed simultaneously with and spaced from the gate layer of the device, and a second polysilicon layer of a second predetermined resistance higher than the first predetermined resistance overlaying the first polysilicon layer. The extent of impurity concentration in the p-n junction of the diodes can be arbitrarily selected. The resultant pair of diodes have a high breakdown voltage, and are connected back to back in series with each other across the path between the gate layer and source region of the semiconductor device. These diodes of a 10V or higher breakdown voltage can protect the device against electrostatic destruction caused by a sudden voltage surge applied to the gate of the semiconductor device. Since these diodes are formed in a semiconductor substrate, it is unlikely that parasitic elements can be formed. Consequently, the gate protection circuit of the present invention can be readily implemented in power vertical MOSFETS and IGBTs. Fig. 3 shows an insulated gate semiconductor device having a gate protection circuit, which is an embodiment of the present invention. In this embodiment, a thermal oxide film 102 serving as a gate insulating layer is formed over an n-type semiconductor substrate 101. An n-type polysilicon layer is deposited over thermal oxide film 102 by chemical vapor deposition or CVD process, and then photoetched to form a gate layer 103 and a separation region 131, spaced from one another. Subsequently, a first region or a p-well region, a part of which is to serve as a channel, is formed in the upper surface of the semiconductor substrate (not shown) by thermal diffusion process, for example, and an n-type source region (not shown) is subsequently formed in the upper surface of the substrate and surrounded by the p-well region. In this structure, an extension of a gate electrode on the channel becomes gate layer 103. Then a metal such as Molybdenum(Mo), which forms a Schottky barrier at the metal-semiconductor junction, is deposited to form a metal layer by a sputtering process over the n-type polysilicon gate layer 103 and separation region 131. The metal layer is then photoetched to from Schottky barrier metal layer 104 which contacts with the gate layer 103 and the separation region 131. The resultant structure is then covered with an insulating layer 105 made of insulating material such as phosphorous glass (PGS). A gate terminal 161, which contacts gate layer 103 through a respective opening of insulating layer 105, and a source terminal 162, which contacts separation region 131 through an opening of the insulating layer 105 and is connected to a source electrode of the source region through a respective opening, are formed by a conventional manufacturing process of a semiconductor device. The gate and source terminals are made of Al or aluminum. In Fig. 3, gate layer 103 and separation region 131, both being made of n-type polysilicon materials, and schottky barrier metal layer 104 which contacts gate layer 103 and separation layer 131 to form Schottky barriers functionally constitute a pair of Schottky barrier diodes. These diodes are connected back to back in series and are connected between gate and source terminals 161 and 162, thus forming a gate protection circuit. The breakdown voltage of the gate protection circuit may be changed by selecting metal for the Schottky barrier metal layer 104 from among those having respectively different Schottky barrier potential against n-type silicon materials such as Mo or gold (Au). If Molybdenum silicide, or silicide of any other metal having a high melting point is used for gate layer 103 and separation region 131, and n-type polysilicon, for example, for the layer contacting gate layer 103 and separation region 131 instead of Schottky barrier metal layer 104, a gate protection circuit will be formed containing a pair of schottky barrier diodes connected back to back in series the forward bias direction of the diodes being opposite to that in the aforesaid original arrangement. In this embodiment, the breakdown voltage of the gate protection circuit is arbitrarily set by effecting the extent of impurity concentration of the n-type polysilicon of the layer substituted for Schottky barrier metal layer 104. Although the above-mentioned description is related to a MOSFET, the present invention may be applied to an insulated gate bipolar transistor as shown in Fig. 4, in which the structure is the same as that shown in Fig. 1 except that a p + layer 11' instead of an n + layer is formed.

Claims: An insulated gate semiconductor device having a gate protection circuit comprising: a substrate (101) of a first conductivity type having upper and lower surfaces ; a first region of a second conductivity type formed in the upper surface of the substrate ; a source region of the first conductivity type formed in the upper surface of the substrate and surrounded by said first region ; a gate insulating layer overlaying an end portion of said source region, a portion of said first region adjoining to said source region and said upper surface of said substrate adjoining to said portion; gate layer overlaying said gate insulating layer, a channel being formed in said portion of said first region ; and at least one pair of diodes for protecting the gate insulating layer from electrostatic destruction caused by a sudden voltage surge applied to the gate layer ; a first layer (131) being formed on the gate insulation layer (102) and being laterally spaced from the gate layer (103) ; a second layer (104) contacting the first layer, wherein said first layer includes a separation region (131) overlaying the gate insulating layer (102); wherein said gate layer (103), separation region (131), and second layer (104) are formed on the gate insulating layer (102). characterized in that said second layer (104) contacts an extended portion of said gate layer (103) and said separation region (131) to form at least one pair of Schottky barrier diodes for protecting the gate insulating layer (102) from electrostatic destruction caused by a sudden voltage surge applied to the gate layer (103). An insulated gate semiconductor device of claim 1, wherein the material of said gate layer (103) and that of the separation region (131) are identical to each other. An insulated gate semiconductor device of claim 1, wherein either said gate layer (103) and said separation region (131) or said second layer (104) are composed of a semiconductor material. An insulated gate semiconductor device of claim 1, wherein the material of said second layer (104) is composed of a metal having a Schottky barrier potential against n-type silicon materials. An insulated gate semiconductor device of claim 4, wherein said metal is molybdenum. An insulated gate semiconductor device of claim 4, wherein said metal is gold. An insulated gate semiconductor device of claim 4, wherein the material of said gate layer is composed of metal having a melting point corresponding to that of molybdenum silicide. An insulated gate semiconductor device of claim 7, wherein said metal is molybdenum silicide. An insulated gate semiconductor device of claim 7, wherein said metal is silicide. An insulated gate semiconductor device of claim 1, wherein the material of said second layer (104) is n-type polysilicon. An insulated gate semiconductor device of claim 1 wherein a layer of a second conductivity type is formed over said lower surface of said substrate (101) of said first conductivity type.

Inventors: FUJIHIRA TATSUHIKO, FUJIHIRA, TATSUHIKO, FUJIHIRA, TATSUHIKO C/O FUJI ELECTRIC CO.LTD.
Applicants: FUJI ELECTRIC CO LTD, FUJI ELECTRIC CO., LTD.
Application year: 1988
Main classification: H01L27
IPC codes: H01L27, H01L29