Abstract:
According to one aspect of the disclosure, the present invention provides methods and arrangements for testing a flip chip semiconductor device after the back side of the chip has been thinned to expose a selected region in the substrate. For some chips, thinning removes substrate material useful for drawing heat away from the internal circuitry when the circuitry is running at high speeds. To compensate for this material loss, a special test fixture having a heat-dissipating device is arranged to draw heat from the device.

Description:
RELATED PATENT DOCUMENTS 
     The present application is related to co-pending document identified by patent application Ser. No. 09/166,833, Birdsley et al., filed on Oct. 5, 1998, entitled “ENDPOINT DETECTION FOR THINNING OF A FLIP CHIP BONDED INTEGRATED CIRCUIT,” which is assigned to the assignee of the present invention and is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to semiconductor device assemblies, and more particularly to techniques for analyzing and debugging circuitry as may be applied, for example, to a flip-chip bonded integrated circuit. 
     BACKGROUND OF THE INVENTION 
     In recent years, the semiconductor industry has seen tremendous advances in technology which have permitted dramatic increases in circuit density and complexity, and equally dramatic decreases in power consumption and package sizes. Present semiconductor technology now permits single-chip microprocessors with many millions of transistors, operating at speeds of tens (or even hundreds) of MIPS (millions of instructions per second) to be packaged in relatively small, air-cooled semiconductor device packages. A by-product of such high-density and high functionality in semiconductor devices has been the demand for increased numbers of external electrical connections to be present on the exterior of the die and on the exterior of the semiconductor packages which receive the die, for connecting the packaged device to external systems, such as a printed circuit board. 
     In the past, the die and package were first attached and then the electrical connections from the die to the package were made by wire bonding. The wire bonding procedure is simple in concept. A thin (0.7 to 1.0 mil) wire is first bonded to the chip bonding pad and spanned to the inner lead of the package lead frame. The third action was to bond the wire to the inner lead. Lastly, the wire is clipped and the entire process repeated at the next bonding pad. While simple in concept and procedure, wire bonding was critical because of the precise wire placement and electrical contact requirements. In addition to accurate placement, each and every wire must make a good electrical contact at both ends, span between the pad and the inner lead in a prescribed loop without kinks, and be at a safe distance from neighboring lead wires. 
     To increase the number of pad sites available for a die and to address other problems, a different chip packaging technique called controlled collapse chip connection or flip chip packaging is being adopted. In this technology, the bonding pads are provided with metal (solder) bumps. The bonding pads need not be on the periphery of the die and hence are moved to the site nearest the transistors and other circuit devices formed in the die. As a result, the electrical path to the pad is shorter. Electrical connection to the package is made when the die is flipped over the package with corresponding bonding pads and soldered. As a result, the dies are commonly called flip chips in the industry. Each bump connects to a corresponding package inner lead. The packages which result are lower profile and have lower electrical resistance and a shortened electrical path. The output terminals of the package may be ball-shaped conductive bump contacts (usually solder, or other similar conductive material) typically disposed in a rectangular array. These packages are occasionally referred to as “Ball Grid Array” (BGA). Alternatively, the output terminals of the package may be pins, and such a package is commonly known as pin grid array (PGA) package. 
     Once the die is attached to the package, the back side portion of the die remains exposed. The transistors and other circuitry are generally formed in a very thin epitaxially grown silicon layer on a single crystal silicon wafer of which the die is singulated from. The side of the die including the epitaxial layer containing the transistors, and the other active circuitry is often referred to as the circuit side of the die or front side of the die. The circuit side of the die is positioned very near the package. The circuit side opposes the back side of the die. Between the back side and the circuit side of the die is single crystalline silicon. The positioning of the circuit side provides many of the advantages of the flip chip. However, in some instances the orientation of the die with the circuit side face down on a substrate may be a disadvantage or present new challenges. When a circuit fails, when circuit testing is desired, or when it is necessary to modify a particular chip, access to the transistors and circuitry near the circuit side is obtained only from the back side of the chip. This is challenging since the transistors are in a very thin layer (about 10 micrometers) of silicon buried under the bulk silicon (greater than 500 micrometers). Thus, the circuit side of the flip chip die is not visible or accessible for viewing using optical or scanning electron microscopy. 
     Although the circuit of the integrated circuit (IC) is buried under the bulk silicon, infrared (IR) microscopy is capable of imaging the circuit because silicon is relatively transparent in these wavelengths of the radiation. However, because of the absorption losses of IR radiation in silicon, it is generally required to thin the die to less than 100 microns in order to view the circuit using IR microscopy. On a die that is 725 microns thick, this means removing at least 625 microns of silicon before IR microscopy can be used. Thinning the die for failure analysis of a flip chip bonded IC is usually accomplished in two or three steps. First, the die is thinned across the whole die surface. This is also referred to as global thinning. Global thinning is done to allow viewing of the active circuit from the back side of the die using IR microscopy. Mechanical polishing is one method for global thinning. 
     Once an area is identified as an area of interest and it is determined that access is needed to a particular area of the circuit, local thinning techniques can be used to thin an area smaller than the die size. Laser microchemical etching of silicon is one method of local thinning. 
     Sometimes it is necessary for failure analysis, or for design debug, after global and/or local thinning, to make electrical contact and probe certain nodes in the circuit. This testing can be performed via the thinned back side or via pads on the circuit side of the chip. When testing via pads on the circuit side of the chip, the chip is typically placed in a test fixture having a circuit connector arranged to connect to these circuit-side pads. The chip is activated through the test fixture, and signals at the nodes are analyzed. For certain applications, it has been discovered in connection with the present invention that the above-described thinning to remove substrate material results in the circuitry over-heating. When there is insufficient substrate material for drawing heat away from the internal circuitry, for example, when the circuitry is running at high speeds, the internal circuitry overheats and becomes inoperative. Consequently, the analysis/debug efforts are destroyed. 
     Accordingly, there is a need for a testing approach that overcomes the above-mentioned shortcomings. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the disclosure, the present invention provides an approach for testing a flip chip semiconductor device after the back side of the chip has been thinned to expose a selected region in the substrate. Thinning removes substrate material useful for drawing heat away from the internal circuitry when the circuitry is running at high speeds. To compensate for this material loss, a test fixture having a heat-dissipating device is arranged to draw heat from the back side of the device. 
     In one example embodiment, the present invention is directed to a method for testing a semiconductor device having a circuit side and a back side. The method includes: thinning the semiconductor device and exposing a selected region in the semiconductor device at its back side; securing the semiconductor device in a test fixture and arranging the semiconductor for testing via its circuit side; and activating the semiconductor device while using the heat-conductive element to dissipate heat generated by the semiconductor device. 
     In another example embodiment, the present invention is directed to an arrangement for testing a semiconductor device having a circuit side and a back side. The test arrangement comprises: a test fixture adapted to secure the semiconductor device; a heat sink coupled to the semiconductor device and adapted  10  to conduct heat generated by the semiconductor device. The test fixture is adapted to test the semiconductor device while the semiconductor device is activated and while the heat sink dissipates heat generated by the semiconductor device. In a more particular embodiment, the semiconductor device includes a circuit die and a package, with the heat sink coupled to the package and drawing heat from the device through the package. 
     The above summary is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description can best be understood when read in conjunction with the following drawings, in which: 
     FIG. 1 shows a side view of a conventional flip chip packaged integrated circuit; 
     FIG. 2 shows a side view of an arrangement for milling and/or testing the integrated circuit of FIG. 1 via a back side of the die, according to one aspect of the present invention; 
     FIG. 3 shows a top view of a portion of a test fixture used to test an IC, according to the present invention; and 
     FIG. 4 shows a cross-sectional view of a flip chip die in a test fixture, according to certain example embodiments of the present invention. 
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiment described. On the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The present invention is believed to be applicable to a variety of different types of semiconductor devices, and the invention has been found to be particularly suited for flip-chip type circuit packages. While the present invention is not limited to flip-chip type circuit packages, an appreciation of various aspects of the invention is best gained through a discussion of various examples using this application. 
     According to a particular embodiment of the present invention, a conventional flip chip type die is tested by first thinning the back side of the die to expose a selected region over suspect circuitry. This suspect circuitry may be analyzed using microscopy techniques or even modified after local thinning. Regardless of how or whether the suspect circuitry is analyzed from the back side of the die, once thinned the die is placed in a test fixture and arranged for testing via its circuit side. The test fixture is adapted to permit coupling of a heat-sink element to the semiconductor device including a circuit die, for example, directly to the die or, alternatively, when a package is part of the semiconductor device, directly to the package. The semiconductor device is then activated while using the heat-conductive element to dissipate heat generated by the semiconductor device, for example, directly from the die or, as the case may be, indirectly through the package. 
     FIG. 1 shows a side view  10  of one type of conventional flip chip type die  12  assembled to a package substrate  14 . Flip chip die  12  has a circuit side  50  and a back side  40 . The circuit side  50  includes a number of circuit devices formed near the circuit side in a portion of the die known as the epitaxial layer  22 . The epitaxial layer  22  has a thickness in the range of 1 to 15 microns. The portion of the die shown above the epitaxial layer is known as the bulk layer  20 . A plurality of solder bumps  26  are made on the circuit side  50  at pads  24 . The solder bumps  26  are the inputs and outputs to the circuitry associated with the die  12 . The flip chip type die  12  is attached to package substrate  14 , such as a package for a flip chip via the solder bumps on the die  12 . The package substrate  14  includes pads  16  which are arranged to correspond to the pattern of solder bumps on the die  12 . The region between integrated circuit  12  and package substrate  14  is filled with an under-fill material  28  to encapsulate the solder bump connections and provide additional mechanical benefits. The pads  16  are coupled via circuitry to pads  18  on the package substrate. Solder bumps  30  are formed on the pads  18 . The solder bumps  30  are the inputs and outputs to the circuitry associated with the package substrate  14 . In another arrangement (not illustrated), the inputs and outputs to the circuitry associated with the package substrate  14  are implemented as pins rather than solder bumps. 
     For a flip chip type die such as die  12  of FIG. 1, failure analysis of a flip chip bonded IC can be accomplished using a global and/or local thinning process, such as mechanical polishing and laser microchemical etching, as previously discussed. 
     FIG. 2 shows a schematic view of an example thinning/test system  100  in which a substrate  20  of a flip chip type die  12 , as attached to a package substrate or surface  14 , is thinned. The system includes a focused ion beam apparatus  130  for removing portions of the die. The focused ion beam apparatus  130  produces a focused ion beam  132  that, in the presence of a gas that is reactive with the substrate, can be a used to locally thin the die. This is accomplished, for example, by removing a portion of silicon across the back side surface  40 . The reactive gas may be, for example, xenon difluoride or chlorine. The focused ion beam  132  can also be used to locally thin a portion of the die  12 . Localized thinning is done on an area of the back side  40  which is less than the entire area of the back side  40 . In one embodiment, the focused ion beam  132  achieves global thinning and localized thinning of the die  12 . In another embodiment, a polisher (chemically-based and/or mechanically-based) achieves global thinning of the die  12 . 
     After thinning, an electrical signal measuring apparatus  140  can be coupled to pins  110  and  120  to measure a response to the current induced by focused ion beam  132 . The apparatus  140  may be a conventional pico-ammeter, for example. The measured response to the induced current changes as the thickness of the bulk layer  20  of the die is reduced by milling in the location where the focused ion beam  132  is directed. This is performed to permit end point detection of the milling process, as described in connection with the above-referenced patent document. 
     After thinning, other electrical signal measurements can be made through the use of a test fixture, an example of which is manufactured and sold in accordance with a standard Schlumberger ATE (Automatic Test Equipment) Interface and specifications provided for the particular test fixture needed. Test fixtures of this type are adapted to permit a test connector to be connected directly to the test board, that connects to test pins of the package. As discussed above, however, the thinning results in the removal of substrate material that is otherwise used for drawing heat away from the internal circuitry when the circuitry is running at high speeds. Thus, when the device under test is activated via the test fixture, the thinning process may result in there being insufficient material that would otherwise be used for drawing heat away from the internal circuitry while operating at high speeds. 
     FIGS. 3 and 4 illustrate an example test fixture  200  useful for testing the internal circuitry of the thinned die when the circuitry is running at speeds up to its highest speeds. As can be recognized from FIG. 4, the example test fixture  200  is set up to test a semiconductor device that includes both a die  202  and a package  204 . The test fixture  200  includes an insulative vacuum seal plate  210  for vacuum-drawing and vacuum-maintaining the test fixture  200  against the test chamber. 
     This example test fixture  200  also includes a test connector  220  electrically connected to the die  202 . Wires  220   a  and  220   b  emanating from the test connector  220  are fed through apertures in the seal plate  210  and connect to a test board  222 . Using conductors on the test board  222 , electrical communication from the wires  220   a  and  220   b  continues on, through sets of socket pins  214   a  and  214   b , to provide power and signal access to the die  202 . A conventional computer-based analyzer (not shown) is then used to analyze the internal circuitry of the thinned die while activated. 
     To compensate for the loss of heat-sink ability due to the thinning process, the test fixture  200  includes a heat-dissipating device (or “heat sink”)  240  to draw heat from the back side of the device. In one particular embodiment, the heat-dissipating device  240  is an active device, such as is available from Pellitier, Inc. In another particular embodiment, the heat-dissipating device  240  is a passive device, made from a highly thermal-conductive metal, such as copper. 
     The heat-dissipating device  240  is constructed and arranged to fit in an aperture of the seal plate  210 , with a vacuum seal  250  surrounding the heat-dissipating device  240  and providing an air-tight vacuum seal for a vacuum draw. The vacuum seal  250  may be manufactured from an expandable material, such as rubber or a pliable poly-based material. Because the heat may cause a slight outward expansion, the vacuum seal  250  has a thickness and a pliability factor adequate to tolerate the expansion without losing the air-tight seal. 
     The test fixture  200  of FIGS. 3 and 4 can be implemented by modifying one of the above-referenced types of test fixtures to include the aperture arrangement for a heat sink, as shown. 
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiment described. On the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.