PROTECTIVE CIRCUIT

Abstract: The present invention relates to a protective circuit for protecting an integrated circuit against electrostatic discharge, so called ESD. The integrated circuit is connected to a supply voltage via a Vcc-pad (2) and an earth pad (3). The protective circuit, which is particularly intended to protect a positively supplied circuit for radio frequency applications against both negative and positive voltage pulses, comprises an input pad (14) and at least one PNP-transistor (20), the input pad being connnected to the integrated circuit, and the PNP-transistor being connected with its collector to the input pad, and its emitter connected either to the Vcc-pad or to the earth pad. The base of the PNP-transistor can be connected to its emitter either directly or via a resistor (24), or not be connected.

Description: Protective circuit Technical field The present invention relates to a protective circuit against electrostatic discharge, so-called ESD. The protective circuit is intended to protect an integrated circuit, especially an integrated circuit for radio frequency applications against both negative and positive voltage pulses. Description of related art An often occurring problem with integrated circuits is the occurrence of electrostatic discharges. Static charges can be built up to high levels and the subsequent discharge can cause great damage. Today diodes are used to protect against such ESD-pulses. The disadvantage of these diodes is that the breakthrough voltage is relatively high. This means that in certain cases it is not possible to protect the circuit sufficiently effectively. US A 5 568 346 reveals as previously known a protective circuit arranged to protect an integrated circuit against ESD-pulses, and this particular circuit comprises diodes. Figure 1 shows an example of such a protective circuit. The integrated circuit 1 , which is fed with an applied voltage between a positive potential at a N cc -pad 2 and an earth pad 3, can be protected against ESD-pulses 6, which can be generated due to static charges at the N cc -pad 2 or at the circuit input pad 4. A positive ESD-pulse between the input pad 4 and the earth pad 3 will be short- circuited via the diode 5b by a main protector 7 to the earth pad. A negative ESD- pulse will be short-curcuited to the earth pad 3 when the amplitude of the pulse exceeds the forward voltage drop for the diode 5 a. If an ESD-pulse would occur between the N cc -pad and the input pad, the circuit is protected with the aid of the main protector. Said main protector can also consist of a reverse biased diode. Usually, however, more sophisticated transistor-based solutions are used. The problem with the above described protective circuit is that it does not permit negative input signals to be applied to the input pad 4 since the signals, if the amplitude exceeds the forward voltage drop for the diode 5a, will be immediately decoupled to the earth pad 3. Positive signals can, however, be applied to the input pad as long as the amplitude is less than the sum of the potential N cc and the forward voltage drop for the diode 5b. In order to also be able to handle negative voltage levels on the input pad, the diode 5a can be removed. A negative ESD-pulse will in this case generate a voltage which is so high that the diode 5b will break down. This breakdown voltage is of substantial size and the integrated circuit 1 can be destroyed even at relatively low voltage levels. Summary of the invention One purpose of the present invention is to provide a protective circuit which, in a flexible and effective manner, protects an integrated circuit against ESD-pulses, and in particular against negative ESD-pulses at an input pad belonging to the protective circuit and connected to the integrated circuit. An additional purpose of the invention is that the input pad included in the protective circuit is to have low resistive and capacitive losses and small coupling to the substrate. These purposes and others are achieved with a protective circuit comprising at least one PNP-transistor with its collector connected to the input pad and its emitter connected to the N cc -pad of the integrated circuit or to its earth pad. The protective circuit can also comprise a main protector connected between the N cc -pad and the earth pad so arranged that protection against ESD-pulses between them is obtained. The base of the transistor can be connected to its emitter, either directly or via a resistor with a resistance. Alternatively it can be left floating, i.e. unconnected. The PΝP-transistor, which is preferably of lateral type with a predetermined distance between the collector and the emitter, can with advantage be integrated with the input pad on a common substrate doped to a first conductivity type. The input pad can include a metal layer, preferably of octagonal shape, at least one underlying doped layer of a second conductivity type, a trench of insulating material which surrounds the underlying doped layer and a low resistive decoupling which surrounds the trench. The low-resistive decoupling can comprise diffused substrate contacts and/or deep contacts with an electrically conducting material, such as tungsten or high-doped polysilicon, substrate contacts filled preferably by means of CND-technology. One advantage of the present invention is that the breakdown voltage of the ESD- protection can be adapted to a particular application by suitable selection of the resistance of the resistor and the distance between the collector and emitter of the PΝP-transistor. Additional advantages of the invention will be evident from the following description. Description of the Figures The invention will be described in more detail below with reference to the accompanying Figures 2-8, which are only shown to illustrate the invention and in no way limit the scope thereof. Fig. 1 shows schematically a protective circuit according to the prior art. Fig. 2 shows schematically a protective circuit according to a first preferred embodiment of the present invention. Fig. 3 shows schematically a protective circuit according to an alternative embodiment of the present invention. Fig. 4 shows schematically a protective circuit according to an additional alternative embodiment of the present invention. Fig. 5 shows a cross-section of a lateral PNP-transistor which is used in the inventive protective circuit. Fig. 6 shows a lateral PNP-transistor in a view from above. Fig. 7 shows a cross-section of a lateral PNP-transistor and an input pad which is used in the inventive protective circuit. Fig. 8 shows a lateral PNP-transistor, an input pad and a connection between them, in a view from above. Preferred embodiments Fig. 2 shows a protective circuit according to the present invention. An integrated circuit 11 is supplied with a voltage via two conductors 12a, 13 a, which are interconnected to an individual pad, preferably a N cc -pad 12 and an earth pad 13, the potential of the N cc -pad being positive relative to the potential of the earth pad. Bet- ween these conductors, a main protector 17 can be connected, which consists of a reverse biased diode or of more complex transistor-based circuits. The integrated circuit is also connected to an input pad 14. According to the invention, a PΝP-transistor 20 is comprised in the protective cir- cuit, arranged in such a manner that its collector is connected to the input pad 14 and its emitter to the earth pad 13. The base of the PΝP-transistor is connected via a transistor 24 to said earth pad. The PΝP-transistor is intended to constitute a protector against an ESD-pulse 16 at the input pad which is negative relative to the earth pad. The transistor function itself is concentrated to the region between the emitter and the collector. Leakage currents will occur when the voltage increases between the emitter and the collector. The voltage at the base will then increase, which eventu- ally results in the PΝP-transistor beginning to conduct. When the PΝP-transistor has finally begun to conduct current, the voltage will drop rapidly between the emitter and the collector, and the current to the base will be supplied by the high current arising between the emitter and the collector. The voltage at which the PΝP-transistor begins to conduct current depends i.a. on the resistance of the resistor. A properly selected resistance can provide the desired level of protection. If the resistor has a variable resistance, the protective level can be adapted to the application in question. A diode 15b can also be included in the protective circuit. This diode is connected with its cathode to the N cc -pad and its anode to the input pad. Alternatively, the diode can be realized by an ΝPΝ-transistor which is connected with its collector to the higher potential and its base and emitter to the input pad or by a PΝP-transistor which is connected with its base to the higher potential and its collector and emitter to the input pad (not shown in Fig. 2). An additional diode can be included in the protective circuit and be connected with its cathode to the earth pad and its anode to the input pad (not shown in Fig. 2). This diode can be present at the same time as the diode 15b or constitute an alternative thereto. Fig. 3 shows an alternative similar embodiment of the protective circuit according to the invention. The only difference in this case is that the PΝP-transistor 20 is in this case arranged in such a manner that both its emitter and its base (via the resistor 24) are connected to the higher potential N cc instead of to the earth pad. In an additional embodiment of the present invention there are two transistors 20,21 connected with their collectors to the input pad 14 and their emitters to the N cc -pad 12 and the earth pad 13, respectively, as can be seen in Fig. 4. This provides protection from ESD-pulses both between the input pad and the earth pad and between the input pad and the N cc -pad without having to utilize the main protection. One or both of the diodes described in connection with the first embodiment can be used in these two alternative embodiments (one diode 15b is shown in Fig. 3 and none in Fig. 4). In all of the embodiments the base of the PΝP-transistor can be left floating, i.e. unconnected, or be directly connected to the earth pad /N cc -pad without using a resistor (not shown in the Figures). If the base is left floating, the breakdown BN ceo of the transistor is used as the protective level. This level is defined by the distance between the collector and the emitter and the doping of the base. If the resistor 24 is short-circuited, the emitter and the base of the PΝP-transistor will be coupled together and the breakdown voltage will be termed BN ces . The PΝP-transistor is preferably of lateral type and will be described here briefly with reference to Figures 5 and 6. In or on a semiconductor substrate 25 of P-type there lies within a predetermined area a bottom diffusion layer 27 of Ν + -type. On top of this bottom diffusion layer there lies an N-doped epi-layer 29, which is surrounded by an N -doped base contact 31. Over the epi-layer there is grown field oxide 33 within predetermined areas, whereafter P-doped emitter 35 and collector areas 37 are achieved. On top of this there is a passivation layer 39 in which contact holes 41,43,45 are made to estab-lish communication between the active areas and the upper metallic conducting layers 51 ,53,55. Fig. 6 shows the lateral PNP-transistor 20 as seen from above. It shows the lateral extent of the metallic connecting layers 51,53,55 which connect to the base, the collector and the emitter, respectively. The contact holes 41,43,45 are also shown in the Figure. Note however that the contact holes 43 of the collector are shown at a different location in Fig. 6 than in Fig. 5. Fig. 7 shows a cross-section of the input pad 14 and the PNP-transistor 20 according to the invention. The input pad is provided with an upper metal layer 61 and an underlying structure comprising, among other things, an N + -diffusion layer 63 for reduction of the resistive losses which give rise to noise and thus degrade the performance of the protection in radio applications. The underlying structure further comprises an N-doped epi-layer 65, field oxide 67, an insulating layer 69, N -doped contacts 71 and deep trenches 73. The signals arising between the metal layer 61 and the underlying structure can be decoupled via the N -doped contacts 71 so that the signals will not come down into the substrate 74 where they could disturb the surroundings by e.g. cross-talk. The trenches 73, which are filled with an insulating material and surround the rest of the underlying structure, further limit the above-mentioned couplings. The trenches prevent the coupling in the upper adjacent surface layers from occurring with the same strength. Note that the metal layer must lie within the area defined by the trenches. The portion of the signal which still reaches down to the substrate is decoupled with substrate contacts 75 which can consist of a series of dopings of the same type as the substrate or of holes or grooves filled with a low-resistive filling, e.g. high-doped polysilicon or tungsten, which has been deposited using CND-technology (see e.g. Swedish Patent Applications 9601119-2 and 9601444-4). The lateral PΝP-transistor, which is identical to the transistor shown in Figures 5 and 6, is also surrounded by deep trenches 77. The collector 37 of the transistor and its resistor (not shown in Fig. 7) are connected to the metal layer 61 of the input pad as close thereto as possible. Note that Fig. 7 is schematic and does not show a number of layers included in the semiconductor structure. For example, there is not shown the electrical connection between the input pad and the lateral transistor. Preferably, the connecting layer of the transistor is localized to the first (lowermost) metal layer of the structure while the metal layer of the input pad is localized to the last (topmost) metal layer of the structure. Fig. 8 shows a preferred layout for the metal layer 61 of the input pad, the lateral PNP-transistor 20, and a connection 79 between them, as seen from above. Note that the input pad is not square but has been made octagonal to minimize the capa-citive losses. The protective circuit according to the invention is particularly suitable for protecting integrated circuits intended for radio and other high-frequency applications. An advantage of the protective circuit according to the invention is that the breakdown voltage for the PNP-transistor 20 can be programmed within certain limits with the aid of the resistor and the distance between the collector and the emitter of the PNP-transistor. The invention is, of course, not limited to the embodiments described above and shown in the drawings. Rather, it can be modified within the scope of the accom- panying patent claims. The invention is obviously not limited as regards layout, dimensions or geometries of the input pad, the PNP-transistor or the protective circuit.

Claims: Claims 1. Protective circuit for protecting an integrated circuit (11) against electrostatic discharge, so-called ESD, said integrated circuit being connected to a supply voltage via a N cc -pad (2) and an earth pad (3), characterized in that it comprises an input pad (14), and at least one PΝP-transistor (20), the input pad being connected to the integrated circuit and the PΝP-transistor being connected with its collector to the input pad and its emitter to either the N cc -pad or to the earth pad. 2. Protective circuit according to Claim 1 , characterized in that it comprises a main protector (17) connected between the N cc -pad and the earth pad and disposed to protect against ESD-pulses therebetween. 3. Protective circuit according to Claim 1, characterized in that the base of the PΝP-transistor is connected to its emitter. 4. Protective circuit according to Claim 2, characterized in that the base of the PΝP-transistor is connected to its emitter via a resistor (24) with a predetermined resistance. 5. Protective circuit according to Claim 2, characterized in that the emitter of the PΝP-transistor is connected to the N cc -pad and that it further comprises a second PΝP-transistor with its collector connected to the input pad and its emitter connec- ted to the earth pad. 6. Protective circuit according to one of Claims 1-5, characterized in that it comprises at least a first diode (15b) with its anode connected to the input pad and its cathode connected to the N cc -pad. 7. Protective circuit according to one of Claims 1-6, characterized in that it comprises at least a second diode with its anode connected to the input pad and its cathode connected to the earth pad. 8. Protective circuit according to one of Claims 1-7, characterized in that the PNP- transistor is of lateral type with a predetermined distance between the the collector and the emitter. 9. Protective circuit according to one of Claims 1-8, characterized in that the PNP- transistor and the input pad are made integrated on a common substrate (25) doped to a first conductivity type. 10. Protective circuit according to one of Claims 1-9, characterized in that the in- put pad comprises a metal layer (61), at least one underlying doped layer (63,65) of a second conductivity type, and a contact (71) of said second conductivity type connected to the doped layer. 11. Protective circuit according to Claim 10, characterized in that the metal layer of the input pad is octagonal in shape. 12. Protective circuit according to Claim 10 or 11, characterized in that the input pad comprises a trench (73) of insulating material which surrounds the underlying doped layer. 13. Protective circuit according to Claim 12, characterized in that the input pad comprises a low-resistive decoupling (75) which surrounds the trench. 14. Protective circuit according to Claim 13, characterized in that the low-resistive decoupling comprises diffused substrate contacts. 15. Protective circuit according to Claim 13 or 14, characterized in that the low- resistive decoupling comprises deep substrate contacts filled with an electrically conducting material, in particular tungsten or high-doped polysilicon. 16. Protective circuit according to Claim 15, characterized in that the substrate contacts are filled by means of CND-technology.

Inventors: JOENSSON JONAS KONRAD, NOSTROEM HANS, JOENSSON, JONAS, KONRAD, NOSTROEM, HANS
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1998
Main classification: H01L27
IPC codes: H01L21, H01L27, H03K17