Means for testing a gate oxide

Abstract: The method of testing a DMOS power transistor (2) comprises the phases of: arranging a switch (4) between the low-voltage circuitry (3) and the gate terminal (G) of the DMOS power transistor (2); maintaining the switch (4) in an open condition; applying a stress voltage to the gate terminal (G); testing the functionality of the DMOS power transistor (2); and, if the test has a positive outcome, short-circuiting the switch (3) through zapping.

Description: The invention relates to a method of testing the gate oxide in integrated DMOS power transistors and an integrated device comprising a DMOS power transistor. As is known, it is desirable to be able to test and "screen" DMOS power transistors in the inspection phase by applying a stress to the gate oxide of the transistors for the purpose of reducing their failures during their operational life, by eliminating the weakest components in the inspection phase. Furthermore, to perform validity tests in integrated power circuits, it is known to equip such circuits with dedicated contact pads to which stress voltages suitable for testing power components are applied; this technique cannot, however, be used directly for DMOS transistors in that the high voltages that are necessary to stress the gate oxide of such components sufficiently (30 V for example) are not compatible with the control circuitry and could lead to breakage. On the other hand, in some applications such as the motor vehicle field, it is desired to guarantee failures of operating devices tending towards 0 p.p.m. and it thus becomes increasingly necessary to have methods of test for DMOS power transistors. US-A-5 502 399 discloses a method of testing MOSFET power transistors having the features of the preambles of claims 1 and 8. The switch element comprises a transistor controlled so as to by-pass a number of protection zener diodes in the testing mode and so allowing a high stress voltage to be applied to the gate of the power transistor. EP-A-0 669 537 discloses a testing method for a power MOS, comprising, between the power MOS and its control circuit, a level shifter or an electronic switch or a fuse, replaced after testing. The object of the invention is therefore to provide an inspection method which enables DMOS power transistors to be tested without damaging the other components of the integrated circuit. The invention provides a method of testing the gate oxide in integrated power transistors, as defined in Claim 1. Furthermore, the invention provides an integrated device comprising a power transistor, as defined in Claim 8. For an understanding of the invention a preferred embodiment will now be described, purely by way of non-exhaustive example, with reference to the accompanying drawings in which: Fig. 1 shows a simplified diagram of an integrated device and an associated test device; and Fig. 2 shows a flow diagram relating to this method. Fig. 1 shows an integrated device 1 comprising a DMOS power transistor 2, a control element 3 of the DMOS power transistor 2 and a switch element formed here by an NPN-type bipolar transistor 4. The illustration also shows four contact pads 7-10 and two metallic regions 11, 12. In detail, the bipolar transistor 4 has the emitter terminal 15 connected to an output of the control element 3 and to the contact pad 7; the base terminal 16 open and the collector terminal 17 connected to the gate terminal G of the DMOS power transistor 2 and to the contact pad 8; the DMOS power transistor 2 has the drain terminal D connected to the contact pad 9 and the source terminal S connected to the contact pad 10. The illustration also shows the gate oxide of the DMOS power transistor 2, denoted by 35. The metallic regions 11, 12, which form part of the metallization line 13 for the contact between the emitter and collector terminals of the bipolar transistor 4, are produced so as to have portions 11a, 11b which face each other, are typically placed at one and the same level of metallization but are electrically insulated by an insulating region (denoted diagrammatically by 36) before the test is carried out. After the testing of the DMOS power transistor 2, these metallic regions 11, 12 are intended to be physically connected to each other to obtain the short-circuit of the bipolar transistor 4 by means of the "zapping" technique as described in greater detail below. A test device 20, which preferably forms part of a more complex test machine, of which only the components necessary for implementing this method are shown, is connected to the contact pads 7-10. The test device 20 comprises a first positive voltage generator 21; a ground line 22; a second positive voltage generator 23; a negative voltage generator 24 and a current measurement element 25. The positive voltage generators 21 and 24 may optionally be implementeed by a single programmable generator. The elements 21-25 are connected selectively to the contact pads 7-10 during the test and zapping phase, as shown diagrammatically by means of switches 27-30. In particular, the switch 27 selectively connects the pad 7 to the second positive voltage generator 23, to ground line 22 or to nothing; the switch 28 selectively connects the pad 8 to the first positive voltage generator 21, to the negative voltage generator 24 or to nothing; and the switches 29 and 30 open and close the connection between the pads 9, 10 and the current measurement element 25. The method for testing the gate oxide of the DMOS transistor is implemented preferably during the final inspection phase (EWS-Electrical Wafer Sort phase) and will now be described with reference to Fig. 2. First of all, block 40, the switches 27 and 28 are controlled so as to connect the contact pad 8 to the first positive voltage generator 21 and the contact pad 7 to ground (line 22). In this phase, the contact pads 9 and 10 remain disconnected. The first positive voltage generator 21 supplies a high stress voltage of value such as not to degrade the gate oxide 35 of the DMOS power transistor 2 but greater than the maximum operating voltage of the transistor 2 (typically equal to 5 MV/cm), 30 V for example. Furthermore, this stress voltage must be lower than the BV CEO value of the bipolar transistor 4. In this way, the bipolar transistor 4 is open and electrically isolates the control element 3 (just like the other low-voltage components of the device 1) from the gate terminal G of the DMOS power transistor 2 and from the high stress voltage. Then, block 41, the switches 29 and 30 are closed to connect the two contact pads 9 and 10 to the two heads of the current measurement element 25 and the current loss of the DMOS power transistor 2 is measured. Alternatively, the parameters of the DMOS power transistor 2 may be measured to assess its correct functionality. At the end of the test phase, if the DMOS power transistor 2 has passed the test (output YES from block 42), the "zapping" of the bipolar transistor 4 is carried out, block 43. For example, the switches 27 and 28 are controlled so as to connect the contact pad 7 to the second positive voltage generator 23 and the contact pad 8 to the negative voltage generator 24. The switches 29 and 30 are open. The second positive voltage generator 23 supplies a voltage lower than the critical one for the control element 3 (less than 10 V for example) and the negative voltage generator supplies a negative voltage of a few volts, so that the voltage drop between the emitter and collector terminals of the bipolar transistor 4 is greater than the inverse breakdown voltage BV CEOi of the bipolar transistor. Consequently, a high current begins to flow through the latter, which causes the heating and a micro-fusion of the metallic regions 11, 12 which become joined by virtue of a migration of the metallic material through the insulating region 36 interposed between them, such as to cause the short-circuit between the two said metallic regions 11, 12 and hence the formation of a single by-pass region which short-circuits the bipolar transistor 4. In this way, in operating condition, the DMOS power transistor 2 is directly connected to the respective control element 3. Vice-versa, if the DMOS power transistor 2 has not passed the test, the entire integrated device 1 is discarded (block 44). The advantages of the method and the device described emerge clearly from the description above. In particular, the fact is emphasized that the integrated device 1 permits a complete test of the functionality of the DMOS power transistor, without damaging the integrated device 1. Furthermore, the integrated device requires only the additional presence of the bipolar transistor 4 and of the contact pads 7 and 8, and hence the manufacturing costs and the complexity of the device are comparable with those of current devices of the same type. The test and short-circuit procedure for the bipolar transistor 4 make use of the asymmetry properties of the breakdown voltages of the transistor 4 in open base condition and are therefore simple, can be carried out with process steps and apparatus which are per se known in the field of semi-conductor manufacture, and are completely reliable. Finally it will be clear that numerous modifications and variants, all of which come within the scope of the inventive concept, may be introduced to the method and the device described and illustrated here. In particular, the bipolar transistor 4 may be replaced by other components, also of other technologies, such as CMOS transistors, provided that they are capable of guaranteeing, on their own or combined with other devices, the withstanding of the stress voltage across them and the steps for testing the functionality of the DMOS power transistor after the stress may be carried out according to the desired procedures.

Claims: A method of testing the gate oxide of an integrated power transistor (2) having a gate terminal (G) connected to a control element (3), comprising the steps of: arranging a switch element (4, 11, 12) between said control element (3) and said gate terminal (G) of said power transistor (2), said switch element (4, 11, 12) including an electronic switch device (4) having a first (17) and a second (15) terminal; maintaining said switch element in an open condition; applying a stress voltage to said gate terminal (G); and testing said power transistor by means of a test condition, characterized in that said power transistor is a DMOS power transistor, and in that said first (17) and second (15) terminals of said electronic switch device (4) are connected to electrically conductive regions (11, 12) which face each other and are electrically insulated from each other by means of an insulating portion (36), the method further comprising a step of short-circuiting said electronic switch device (4) in case said test condition being passed; said step of short-circuiting comprising the step of supplying a fusion current to said first and second electrically conductive regions (11, 12) such as to cause an at least partial fusion of said electrically conductive regions and a permanent physical connection between said first and second electrically conductive regions. A method according to Claim 1, characterized in that said electronic switch device (4) has a control terminal (26) and in that , said phase of maintaining said electronic switch device (4) in an open condition comprises the step of leaving said control terminal floating. A method according to Claim 2, characterized in that said first terminal (17) is connected to said gate terminal (G) of said DMOS power transistor (2) and said second terminal (15) is connected to said control element (3) and in that said step of applying a stress voltage comprises applying a first voltage drop between said first and second terminals of a lower value than the breakdown voltage of said electronic switch device. A method according to Claim 3, characterized in that said step of applying a first voltage drop comprises applying a positive voltage to said first terminal (17) with respect to said second terminal (15). A method according to one of the preceding Claims, characterized in that said step of testing comprises measuring the current loss of said DMOS power transistor (2). Method according to Method according to claim 1, wherein said electronic switch device (4) comprises a control terminal (16), characterized in that said step of supplying a fusion current comprises applying a second voltage drop between said first (17) and second (15) terminals which is greater than the breakdown voltage of said electronic switch device (4) and leaving said control terminal (16) floating. Method according to Claim 6, characterized in that said step of applying a second voltage drop comprises applying a positive voltage to said second terminal (15) with respect to said first terminal (17). An integrated device (1) comprising a power transistor (2) having a gate terminal (G) connected to a control element (3), switch means (4, 11, 12) having a first (17) and a second terminal (15) connected respectively to said gate terminal (G) and to said control element (3), said switch means comprising an electronic switch device (4) and by-pass means (11, 12) disposed in parallel to said electronic switch device, characterized in that said power transistor (2) is a DMOS power transistor, and in that said by-pass means comprise a first and a second electrically conductive region (11, 12) having mutually facing portions which are electrically separated by an insulating region (36) and are intended to form a single by-pass region after applying a fusion current to said first and second electrically conductive regions (11, 12), such as to cause an as least partial fusion of said electrically conductive regions. A device according to Claim 8, characterized in that said electrically conductive regions (11, 12) belong to a contact metallization line (13) of said electronic switch device (4). A device according to Claim 9, characterized in that it comprises a first and a second contact pad (8, 7) connected to said first (17) and, respectively, second terminal (15) through said second (12) and, respectively, first (11) electrically conductive region. A device according to one of Claims 8-10, characterized in that said electronic switch device (4) comprises a bipolar transistor having base (16), emitter (15) and collector (17) terminals, said base terminal (16) being floating, said collector terminal (17) being connected to said first terminal and said emitter terminal (15) being connected'to said second terminal of said switch means (2, 4).

Inventors: BERTOTTI FRANCO, MURARI BRUNO, NOVARINI ENRICO, BERTOTTI, FRANCO, MURARI, BRUNO, NOVARINI, ENRICO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: G01R31
IPC codes: G01R31