Patent Publication Number: US-6709985-B1

Title: Arrangement and method for providing an imaging path using a silicon-crystal damaging laser

Description:
FIELD OF THE INVENTION 
     The invention relates to semiconductor device assemblies, and more particularly to techniques for analyzing and debugging circuitry associated with a flip-chip bonded integrated circuit. 
     BACKGROUND OF THE INVENTION 
     In recent years, the semiconductor industry has seen tremendous advances in technology that 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. 
     There have been a number of semiconductor packaging types used to increase the number of pad sites available for a die and to address other problems. One increasingly popular packaging technique is called controlled collapse chip connection or flip-chip packaging. 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 often referred to as “flip-chip” devices. Each bump connects to a corresponding package inner lead. The packages that result are lower profile and have lower electrical resistance and a shortened electrical path. 
     The output terminals of such packages vary, depending on the package type. For example, some output terminals are ball-shaped conductive bump contacts (usually solder, or other similar conductive material), and they are typically disposed in a rectangular array. These packages are occasionally referred to as “Ball Grid Array” (BGA). Another type of package, commonly known as pin grid array (PGA) package, implements the output terminals as pins. 
     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 from which the die is singulated. The portion of the die that includes the epitaxial layer, which contains the transistors and other active circuitry, is often referred to as the circuit side of the die or the front side of the die. The circuit side of the die is positioned very near the package, and 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 device. 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 (e.g., about 10 micrometers) of silicon buried under the bulk silicon (e.g., 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, hereinafter including both IR and also near-infrared (nIR) where the wavelength is between 800 and 2000 nm) 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 about 200 microns in order to view the circuit using IR microscopy. To illustrate this difficulty, on a die that is 725 microns thick, this means removing (or thinning) at least 625 microns of silicon before IR microscopy can be used. 
     Thinning a flip-chip bonded die for failure analysis can be time consuming and burdensome. According to one common approach, thinning is 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. Mechanical polishing can be done keeping the surface fairly flat across the die surface. This process can also produce very smooth surfaces to obtain clear images using IR (including nIR) microscopy. Too high a surface roughness causes scattering of illumination light and the reflected light from the surface being imaged. 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. 
     A particular problem is encountered attempting to image the circuit beneath the trench surface created by laser chemical etching (LCE). In laser chemical etching, a focussed laser beam is scanned over a small area on the silicon surface of the die in the presence of chlorine gas. The focussed laser beam is absorbed in silicon and the surface gets heated to a very high temperature. The heat caused by the laser beam is generally high enough to cause melting of the silicon surface. The chlorine gas reacts with the hot silicon surface or molten silicon and forms volatile silicon chloride. The unreacted molten silicon re-crystallizes and forms the silicon surface. Although this process produces fairly smooth surfaces, the surface resulting after LCE destroys the imaging process. 
     The surface resulting after laser chemical etching is found to have crystal defects using TEM (Transmission Electron Microscopy) mainly dislocations and threading defects which are not present in the starting material. These crystal defects serve as scattering centers for light and are believed to be the cause for the imaging problems. Additionally the surface formed in the LCE process causes some degree of amorphization (the surface being composed of very small size grains) of silicon. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to imaging circuit regions through a silicon surface that was impaired by thinning. In connection with the present invention, it has been discovered that impairment by crystal defects can be overcome for circuit-viewing by laser-thermal annealing. Such annealing is useful in overcoming crystal-defect impairment as caused by multiple types of thinning processes. 
     According to one example embodiment, the back side of a semiconductor device, such as a flip-chip semiconductor device, is thinned to expose a selected region in the substrate. An imaging path is then provided through the back side of a semiconductor device for subsequently obtaining an image of circuitry as viewed from the back side. The imaging path can be provided using any of various mechanisms including, for example, conventional global and local thinning techniques. Specifically, silicon thinning that uses a laser chemical etching process results in the formation of crystal defects that inhibit the ability to obtain images through the back side of the semiconductor device. This problem is addressed by a combination of a surface smoothing process and an annealing step to reduce the crystal defects in silicon. The present invention overcomes this problem by reducing, e.g., smoothing, the crystal defects. 
     In a more particular embodiment, the imaging path is provided by thinning via laser-chemical etching the back side of the semiconductor device under a pressure exceeding a threshold level, and then reducing the pressure to a level below the threshold level and scanning the back side of the semiconductor device using a laser. IR microscopy is then used to capture an image of a circuit in the circuit side of the semiconductor device through the back side of the semiconductor device. 
     In another example embodiment, the present invention is directed to a system for providing an imaging path for imaging through the back side of a semiconductor device to its circuit side. The system includes: a laser-chemical etching arrangement including a laser-chemical etcher configured and arranged to apply a high temperature to the back side of the semiconductor device and to thin the back side at a first energy level, the laser-chemical etching arrangement further including an adjustment to use the laser-chemical etcher for scanning the back side of the semiconductor device in the imaging path and at an energy level that is less than the first energy level and for reducing crystal defects in the imaging path; and a microscopy arrangement configured and arranged to apply light through the imaging path and to obtain images of circuitry in the circuit side of the semiconductor device. In a more particular example embodiment, the system further includes a gas chamber and the adjustment is configured and arranged to change pressures within the chamber from a first pressure in excess of about one milli-Torr to a second pressure that is less than about one milli-Torr. 
     According to another example embodiment, the back side of a die is globally thinned via plasma etching, and crystal defects resulting therefrom are overcome for viewing underlying circuit material by laser-thermal annealing. 
     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 integrated circuit packaged as a flip-chip device; 
     FIG. 2 shows a side view of an arrangement for thinning the integrated circuit of FIG.  1  and for providing an imaging path via a back side of the flip-chip die, according to the present invention; 
     FIG. 3 shows a side view of an arrangement for capturing images using the imaging path referred to in connection with FIG. 2, also according to 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 example embodiment of the present invention, a conventional die, such as the die of a flip-chip type device or a normally-oriented packaged die, is tested after creating an imaging path through the back side of a semiconductor device. The back side of a semiconductor device (also referred to as a die or integrated circuit) is first thinned to expose a selected region over suspect circuitry. For many applications, thinning results in the formation of crystal defects that inhibit the ability to obtain images through the back side of the semiconductor device. One example embodiment overcomes this problem by thinning via laser-chemical etching the back side of the semiconductor device under a pressure exceeding a threshold level, and then reducing the pressure to a level below the threshold level and scanning the back side of the semiconductor device using a laser. IR microscopy is then used to capture an image of a circuit in the circuit side of the semiconductor device through the back side of the semiconductor device. The suspect circuitry may be then further analyzed using microscopy techniques and may even be modified via the imaging path. 
     FIG. 1 shows a side view  110  of one type of conventional flip-chip type die  112  assembled to a package substrate  114 . Flip-chip die  112  has a back side  120  and a circuit side in a portion of the die known as the epitaxial layer  122 . The epitaxial layer  122  includes a number of circuit devices and has a thickness in the range of one to fifteen microns. Bulk polysilicon fills the back side  120  and is referred to as the bulk polysilicon layer. A plurality of solder bumps  126  are made on the circuit side at pads  124 . The solder bumps  126  are the inputs and outputs to the circuitry associated with the flip-chip type die  112 . The flip-chip type die  112  is attached to the package substrate  114  via the solder bumps on the die  112 . The package substrate  114  includes pads  116  that are arranged to correspond to the pattern of solder bumps  126  on the die  112 . The region between the die  112  and package substrate  114  is filled with an under-fill material  128  that encapsulates the solder bump connections and provides additional mechanical benefits. The pads  116  are coupled via circuitry to pads  118  on the package substrate, and solder bumps  130  are formed on the pads  118 . The solder bumps  130  are the inputs and outputs to the circuitry associated with the package substrate  114 . 
     For a flip-chip type die such as die  112  of FIG. 1, failure analysis of a flip-chip bonded IC is usually accomplished using a global and/or local thinning process involving, for example, a process that mechanically polishes and laser-microchemically etches with the thinning process resulting in the formation of crystal defects that inhibit the ability to obtain images through the back side, as previously discussed. 
     FIGS. 2 and 3 show a schematic view of an example thinning/test system in which a substrate  220  of a flip-chip type die  212 , as attached to a package substrate  214 , is secured in a fixture  208  within a chamber  210 . The system includes a laser-chemical etching apparatus  230  for thinning the die  212  by removing portions of the back side  220  of the die  212 . The laser-chemical etching apparatus  230  produces a laser beam  232  which, in the presence of a gas that is reactive with the substrate, can be used to locally or globally thin the die, such as to remove silicon across a selected portion of, or the entire back side of, surface  240 . The reactive gas may be, for example, xenon difluoride or chlorine. In the example system of FIG. 2, the reactive gas is chlorine provided by way of gas-input port  237  to the chamber  210  and is scanned to create an aperture or trench  235 . The trench  235  in this illustrated example application provides a locally-thinned portion of the die  12 . In another application, the back side  220  is globally thinned to remove the surface  240  entirely.  20  In another embodiment, a polisher (chemically-based and/or mechanically-based) achieves global thinning and a trench, such as trench  235 , is formed using the laser-chemical etching apparatus  230  of FIG.  2 . An example laser-chemical etching apparatus that can be used in this application is a laser chemical etcher (LCE), manufactured and sold by Revise, Inc. 
     After the back side  220  of the die  212  is thinned over a selected region or certain circuitry (or circuit material) within the substrate, the laser-chemical etching apparatus  230  is used with the reactive gas to smooth the newly formed surface at the thinned region. The laser beam  232  produced by the laser-chemical etching apparatus  230  produces a localized high temperature that induces the formation of crystal defects. As discussed above, these crystal defects inhibit the ability to obtain images, such as images normally obtained using IR microscopy, through the back side of the semiconductor device. The newly formed surface at the thinned region is smoothed by the laser-chemical etching apparatus  230  to reduce, or even substantially remove, these defects. 
     In connection with the present invention, it has been discovered that one way to realize this smoothing after laser-chemically etching the back side  220  of the semiconductor device under a first chamber pressure (e.g., exceeding a threshold level) and at a first power level (e.g., about 4 watts), involves reducing the pressure (or even evacuating the chamber pressure) and then laser scanning the back side of the semiconductor device in vacuum (e.g., at about the first pressure) and at a reduced power level (e.g., about 3 watts). This reduced power scan clears or smooths the target surface. Then, to laser anneal this region, another laser scan is applied in vacuum at about the first power level (e.g., about 4 watts). 
     According to a more particular embodiment, the system of FIG. 2 provides the imaging path by first thinning the back side  220  of the semiconductor device at a chamber pressure greater than about 1 milli-Torr. The chamber pressure is removed and then the back side of the semiconductor device scanned to smooth the trenched surface bottom. The laser-chemical etching apparatus  230  of FIG. 2 includes adjustment controls  238  and  239  for respectively adjusting the power and scan speed of the laser-chemical etching apparatus  230 . The gas-input port  237  to the chamber  210  of FIG. 2 includes a valve  233  for reducing the chamber pressure. 
     FIG. 3 illustrates the die  212 , after the thinned region has been smoothed as described above and with an unimpeded imaging path through the back side of the semiconductor device for IR microscopy. An IR microscopy apparatus, including an infrared transmitter/receiver  310  and a monitor  312 , is used to capture an image of a circuit in the circuit side of the die  212  through its back side  220 . An example IR microscopy apparatus that can be used in this application is a confocal laser scanning microscopy, manufactured and sold by Zeiss, Inc. 
     In an alternative embodiment, an apparatus within a chamber can include both a laser-chemical etcher (such as apparatus  230 ) and an infrared transmitter/receiver so that a die secured beneath the laser-chemical etcher can be thinned, smoothed and then imaged without removing the die from the fixture securing the die within the chamber. 
     After thinning and smoothing, images are obtained to further evaluate the circuit side of the die. Using these images, other electrical signal measurements can be made through the use of a conventional test fixture. 
     In another example embodiment, plasma etching is used rather than laser-chemical etching. The back side of the die is first globally thinned by a plasma etching process, and collision-induced damage at the target surface is cleared or smoothed by locally or globally laser-thermal annealing in vacuum at a substantial power level,. e.g., about 4 watts. 
     The present invention is amenable to various modifications and alternative forms that depart from the exact specifics shown by way of example in the drawings and the particular embodiments described above. The invention is to cover modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.