Bonding pad structure for testing integrated circuits during manufacture

Abstract: A method of fabricating a bond and the bond. The process includes providing a lower level (M1,13, 15) of electrically conductive metal disposed on a substrate having a pair of spaced apart sections. An electrically insulating layer (11) is then disposed over the lower level and vias (23) are formed in the electrically insulating layer, individual ones of the vias extending to one of the spaced apart section of the lower level. An upper level of electrically conductive metal (M2, 17, 19) is disposed on the electrically insulating layer, the upper level having a pair of spaced apart sections, each coupled to one of the sections of the lower level through a via. One of the pair of spaced apart sections of the lower level is preferably essentially U-shaped (13) and the other section (15) of the lower level is essentially rectangular shaped and extends into the open end of the "U". One of the pair of spaced apart section of the upper level is essentially rectangular (17) with a rectangular aperture at its central region and the other section of the upper level (19) is essentially rectangular and disposed within the rectangular aperture. A bond (23) is formed completely enclosing the spaced apart sections of the upper level.

Description: FIELD OF THE INVENTION This invention relates generally to integrated circuits, and more particularly, to a method and structure which permits the measurement of individual circuits and/or circuit components which could otherwise not be individually measured when connected within the integrated circuit. BACKGROUND OF THE INVENTION It is often desirable to test individual circuit portions and/or individual components of an integrated circuit prior to completion of fabrication. Such testing can avoid completion of fabrication of devices which already have a defect, thereby providing an economic saving and removing potentially inferior or non-working devices from the line. This type of testing has importance and several solutions have been provided by the prior art. For example, in prior art procedure shown in FIGURE 1, in order to improve the safe operating area (SOA) of a power transistor, a clamp in the form of a diode stack having a standard diode and a zener diode in back to back relation is placed across the gate and drain of the transistor. The diode stack will break down prior to the breakdown of the power transistor. This places the transistor in a forward biased SOA which is much stronger than placing the transistor in a reverse biased SOA (RBSOA). A problem with this arrangement is that, if the breakdown voltage of the stack is close to or above that of the transistor, the transistor may go into an RBSOA mode and will break down first with the stack having essentially no effect. It is therefore apparent that, in order for such an arrangement to be successful, it is imperative that the breakdown voltage of the transistor be much greater than the breakdown voltage of the diode stack. However, there is no good way to determine that this relation exists, so it cannot be accurately determined upon testing whether a breakdown is being caused by the diode stack or the power transistor. Disadvantage of this type of arrangement are that (1) the BV dss of the transistor cannot be tested, (2) the BV dss of the transistor cannot be distinguished from the BV of the diode stack, (3) leakage in the transistor cannot be distinguished from leakage in the diode stack and (4) the unclamped energy capability of the transistor cannot be tested because the BV of the diode stack is less than BV dss . In a second prior art arrangement shown in FIGURE 2, the diode stack and the transistor are separated from each other at the output by providing a split bond pad having two separated sections, one section attached to the diode stack and the other section attached to the transistor. In this way, testing of the stack can take place separately at probe with the split bond pad later being connected together during bonding by having the bond extend across both bond pad sections. Problems that arise with this procedure are that the bond pad must be large, thereby decreasing the number of components that can be placed in a given area, there is an inefficient use of high current bonding since only half of the bond area is used for high current switching, there is the possibility of contaminant ingress due to the nitride opening at the bond pad metal edge of the field oxide and there is the possibility that an undesirable hump may be formed during the bonding procedure rather than a conformal bond to the pads. The use of split hand pads is also known from each of US-A-5 381 105, GB-A-2 004 417 and JP-A-01 217 976. SUMMARY OF THE INVENTION In accordance with the present invention, the above described problems of the prior art are minimized if not eliminated by leaving discrete portions of the circuit disconnected for the purpose of testing and properly connected after probe test by normal implementation of wire bonding in the assembly process. Individual circuit elements can be tested individually without interfering with each other. This is accommodated by use of a normally-sized bond pad with the entire periphery of the bondwire-to-metallization bonding area available to supply a high current path with the bond sealing the opening between the metal conductors to prevent ingress of contaminants into the opening. The bond pad includes plural (preferably two) region (preferably levels) of electrically conductive metal, preferably TiW/Al, Al, Al-Cu or any metal with a compatible bonding process (usually the metals are the same), one alongside or atop the other and spaced from each other with a standard electrical insulator (generally silicon dioxide or silicon nitride). The second region or bottom level includes two section which are spaced from each other (e.g. a U-shaped section with a rectangular section disposed within the "U" and spaced from the "U"). The first region or top level also includes two sections, one section (e.g. a first rectangular section although it could also be circular or polygonal or any other twc dimensional shape with a hollow interior) coupled to one of the bottom section (e.g. the U-shaped section) through a via in the insulator and the other section (e.g. a second rectangular section within the hollow rectangular interior and spaced from the first rectangular section) coupled to the other bottom section (e.g. the rectangular section) through a via in the electrical insulator. One of the sections of the first region or top level is preferably located in the center of the bond pad area to insure that the bond later made is positioned in and over the space between the two top level sections. In operation, one of the bottom sections is coupled to the zener stack and the other bottom section is coupled to the power transistor. Probe testing is accomplished by contacting the top section coupled through a via to the appropriate bottom section with a probe in standard manner. When bonding to the bond pad later takes place, the bond will enter the space between the two upper level sections and cover the centrally located space between the two top sections. In this way, the bond area is available, the space between top sections will be likely to be covered by a bond during bonding due to its central location and the bond pad area is not changed to accommodate testing. Also, no contaminants can enter the bond region as in the prior art discussed above. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be further described, by way of example, with reference to the accompanying drawings in which: FIGURE 1 is a first prior art circuit for making test measurements on circuitry; FIGURE 2 is a second prior art circuit for making test measurements on circuitry; FIGURE 3 is a view in elevation of a bond pad in accordance with the present invention; FIGURE 4 is a top view of the bonding pad of FIGURE 3; and FIGURE 5 is a cross sectional view taken along the line 5-5 of FIGURE 4. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGURES 3 to 5, there is shown a bond pad that includes two levels of electrically conductive metal designated as M1 and M2, one atop the other. The metal level M1 is preferably a good electrical conductor compatible with the second level metal and the metal level M2 is preferably a good electrical conductor compatible with the bonding process in the assembly area. The bottom metal level M1 is spaced from the top metal level M2 by a standard electrically insulating layer 11, preferably of silicon dioxide or nitride. The bottom level M1 includes two sections 13 and 15 which are spaced from each other by a space 27 and includes a U-shaped section 13 with a rectangular section 15 disposed within the "U" and spaced from the "U" and having a portion thereof extending outwardly beyond the "U". The top level M2 also includes two sections 17 and 19, one section 17 being a rectangular section with a hollow rectangular interior coupled to one of the bottom section 13 through a via 21 in the insulator 11 containing the metal of upper level M2 therein. The other section 19 is a second rectangular section within the hollow rectangular interior of section 17 and is spaced from the first rectangular section 17 by space 25. The section 19 is coupled to the other bottom section 15 through a via 23 in the electrical insulator 11 which contains the metal of the upper level M2 therein also. The section 19 of the upper level M2 is preferably located in the center of the bond pad area to insure that the bond (shown in FIGURE 4) later made is positioned in and over the space 25 between the two top level sections 17 and 19. In operation, one of the bottom sections 15 is coupled to the zener stack and the other bottom section 13 is coupled to the power transistor. Probe testing of the transistor is accomplished by contacting the top section 17 coupled through a via 21 to the appropriate bottom section 13 with a probe in standard manner. When bonding to the bond pad later takes place, the bond will enter the space 25 between the two upper level sections 17 and 19 and cover the centrally located space 25 between the two top sections. In this way, the bond area is available, the space 25 between top sections will be likely to be covered by a bond during bonding due to its central location and the bond pad area is not changed to accommodate testing. Also, no contaminants can enter the bond region as in the prior art discussed above.

Claims: A bond pad for a semiconductor device which comprises: a first electrically conductive region (M1) disposed on a semiconductor chip, said first region having a pair of spaced apart sections (13, 15); a second electrically conductive region (M2), said second region having a pair of spaced apart sections (17, 19), each of said spaced apart sections of said second region being coupled to a different one of said sections of said first region; an electrically insulating layer (11) disposed between said first and second regions; and a bond region completely enclosing said spaced apart sections of said first region. The bond pad of Claim 1, wherein said first region forms an upper level and said second region forms a lower level, said electrically insulating layer being disposed substantially over said second region and having vias therein, each said via extending substantially from said first section to a said second section. The bond pad of Claim 2, wherein at least one of said spaced apart sections of said upper level is completely surrounded by a further section of said upper level. The bond pad of Claim 2, or Claim 3 wherein one of said pair of spaced apart sections of said lower level is substantially U-shaped and the other section of said lower level is substantially rectangular shaped and extends into an open end of the "U". The pad region of Claim 2 or Claim 3 wherein at least one of said pair of spaced apart sections of said upper level is substantially rectangular and includes a substantially rectangular aperture at a central region thereof, a further section of said upper level being substantially rectangular and disposed within said rectangular aperture. , A method of fabricating a bond pad which comprises the steps of: forming a first electrically conductive region (M1) on a semiconductor chip, said first region having a pair of spaced apart sections (13, 15); forming a second electrically conductive region (M2), having a pair of spaced apart sections (17, 19), each of said spaced apart sections of said second region being coupled to a different one of said sections of said first region; forming an electrically insulating layer (11) disposed between said first and second regions; and forming a bond region completely enclosing said spaced apart sections of said first region. The method of Claim 6, wherein said step of forming said first region comprises forming an upper level and said step of forming said second region comprises forming a lower level, and forming said electrically insulating layer substantially over said second region. The method of Claim 7 further comprising providing vias in said electrically insulating layer, each via extending from said region of said first section to said region of said second section. The method of Claim 7 or Claim 8, wherein at least one of said spaced apart sections of said upper level is completely surrounded by a further section of said upper level. The method of any of Claims 7 to 9, wherein at least one of said spaced apart sections of said lower level is substantially U-shaped and a further section of said lower level is substantially rectangular shaped and extends into the open end of the "U". The method of any of Claim 7 to 10, wherein at least one of said spaced apart sections of said upper level is substantially rectangular having a rectangular aperture at a central region thereof and a further section of said upper level is substantially rectangular and disposed within said rectangular aperture.

Inventors: BALDWIN DAVID JOHN, TEGGATZ ROSS E, BALDWIN, DAVID JOHN, TEGGATZ, ROSS E.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1997
Main classification: H01L21
IPC codes: H01L21, H01L23