Abstract:
According to an example embodiment of the present invention, a semiconductor die having a buried insulator layer between a circuit side and a back side is selectively thinned. During thinning, a selected portion of the bulk silicon layer on the back side is removed and a void created. A circuit is formed in the void and is coupled to the existing circuitry on the circuit side of the die. The new circuit is used to analyze the die during operation, testing, or other conditions. The newly formed circuit enhances the ability to analyze the semiconductor die by adding flexibility to the traditional analysis methods used for integrated circuit dice. The newly formed circuit enables many new ways of interactively using the existing circuitry some of which include replacement of defective circuitry, modification of existing circuit operations, and stimulation of existing circuitry for testing.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor devices and their fabrication and, more particularly, to post manufacturing analysis of semiconductor devices including analyzing and debugging circuitry within an integrated circuit. 
     BACKGROUND OF THE INVENTION 
     Recent technological advances in the semiconductor industry have permitted dramatic increases in circuit density and complexity, and commensurate decreases in power consumption and package sizes for integrated circuit devices. Single-chip microprocessors now include many millions of transistors operating at speeds of hundreds of millions of instructions per second to be packaged in relatively small, air-cooled semiconductor device packages. A byproduct of these technological advances has been an increased demand for semiconductor-based products, as well as increased demand for these products to be fast, reliable, and inexpensive. These and other demands have led to increased pressure to manufacture a large number of semiconductor devices at an efficient pace while increasing the complexity and improving the reliability of the devices. 
     As the manufacturing processes for semiconductor devices and integrated circuits increase in difficulty, methods for manufacturing, testing and debugging these devices become increasingly important. Not only is it important to ensure that individual chips are functional, it is also important to ensure that batches of chips perform consistently. In addition, the ability to detect a defective manufacturing process early is helpful for reducing the possibility of manufacturing a defective device. It is also helpful to be able to perform the manufacture, testing and debugging of integrated circuits in an efficient and timely manner. 
     To increase the number of pad sites available for a die, to reduce the electrical path to the pad sites, and to address other problems, various chip-packaging techniques have been developed. One of these techniques is controlled collapse chip connection or “flip-chip” packaging. With this packaging technology, bonding pads on the die include metal (solder) bumps. Electrical connection to the package is made when the die is “flipped” over and soldered to the package. Each bump connects to a corresponding package inner lead. The resulting packages are low profile and have low electrical resistance and a short electrical path. 
     Once the die is attached to such a 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 an individual die is later singulated. The side of the die including the epitaxial layer and containing the transistors and other circuitry is often referred to as the circuit side, or 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 bulk silicon. In a structural variation, a layer of insulating material is formed on one surface of a single crystal silicon wafer followed by the thin epitaxially grown silicon layer into which the transistors and other circuitry is built. This wafer structure is termed silicon on insulator (SOI). A common insulating material is silicon dioxide. When silicon dioxide is used this layer is commonly called the buried oxide layer (BOX). The transistors formed on an SOI structure show decreased drain capacitance, resulting in a faster switching transistor. 
     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. For example, when a circuit fails, or when it is necessary to modify a particular chip, access to the transistors and circuitry near the circuit side may be possible only from the back side of the chip. This is challenging for SOI circuits since the transistors are in a thin layer of silicon covered by the buried oxide layer and the bulk silicon. Thus, the circuit side of the flip chip die is not visible or accessible. 
     One technique or method for testing and analysis of circuitry in an integrated circuit includes locating defective portions of the circuitry by controlling inputs to the die and monitoring outputs in order to determine if the die is operating as designed. Defective locations may be localized in a number of ways. One method includes the use of testing circuitry located in a die, such as built-in self test (BIST) circuitry including redundant or replacement circuitry. However, concerns about maximizing use of space while minimizing manufacturing costs preclude putting extensive circuitry into a die for the sole purpose of testing. Additionally, constructing a limited number of “test dice” containing testing circuitry is not effective because the test die will have a different design then the standard die and will not therefore necessarily function in the same manner. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method for analyzing a semiconductor die using a circuit formed in a selected portion of a back side of a flip-chip die. The present invention is exemplified in a number of implementations and applications, some of which are summarized below. 
     According to an example embodiment of the present invention, a semiconductor die having a buried insulator layer between a circuit side and a back side is selectively thinned. During thinning, a selected portion of a bulk silicon substrate layer on the back side is removed and a void is created. A circuit is formed in the void and is coupled to pre-existing circuitry in the circuit side of the die. The formed circuit is used to analyze or modify the performance of the die during operation and testing. The new circuitry is used in a number of implementations that assist in analyzing the die. Selectively using the formed circuitry to modify the operation of, replace, or test the pre-existing circuitry provides flexibility in die testing and operation. 
     In one particular example embodiment of the present invention, the circuit formed in the back side is formed over an insulator portion of silicon on insulator (SOI) structure in the die. Enough substrate is removed to expose a portion of the insulator. The circuitry includes SOI circuitry and is formed over the insulator. In one implementation, the circuitry is formed directly on the insulator portion in the die, and in another implementation additional insulator material is formed in the back side and the circuitry is formed on the additional insulator. The formed circuitry exhibits benefits including those of SOI circuitry indicated in the Background hereinabove. 
     In another example embodiment of the present invention, a thinning arrangement is adapted to remove a selected portion of a silicon substrate layer from a back side of an integrated circuit die. This localized thinning creates a void into which a formation arrangement is adapted to form circuitry. A coupling arrangement communicatively couples the newly formed circuitry to the pre-existing circuitry of the integrated circuit die in a manner that allows modification, replacement, or testing of the pre-existing circuitry by the newly formed circuitry. An analysis arrangement is then adapted to use the newly formed circuitry and to perform analysis of the die. 
     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and detailed description that follow more particularly exemplify these embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
     FIG. 1 is a flip-chip IC die with circuitry built in the back side, according to an example embodiment of the present invention; 
     FIG. 2 is a flip-chip die with circuitry in the back side, according to another example embodiment of the present invention; 
     FIG. 3 is a flip-chip die with circuitry in the back side coupled to circuitry on the circuit side, according to another example embodiment of the present invention; 
     FIG. 4A is a flip-chip die with circuitry in the back side, according to another example embodiment of the present invention; 
     FIG. 4B is a flip-chip die with circuitry in the back side undergoing coupling to circuitry on the circuit side, according to another example embodiment of the present invention; 
     FIG. 4C is a flip-chip die with circuitry in the back side undergoing coupling to circuitry on the circuit side, according to another example embodiment of the present invention; and 
     FIG. 5 is a system for analysis of a flip-chip die, according to another example embodiment 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 necessarily to limit the invention to the particular embodiments described. On the contrary, the intention 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 for a variety of different types of semiconductor devices, and has been found particularly suited to analysis of a flipchip integrated circuit die having SOI structure. While not necessarily limited to such devices, various aspects of the invention may be appreciated through a discussion of various examples using this context. 
     According to an example embodiment of the present invention, a semiconductor die having a buried insulator layer between a circuit side and a back side is selectively thinned on its back side. A removal process, such as a focused ion beam (FIB) etch, a laser etch or a polishing process, such as mechanical or chemical-mechanical polishing, is used to remove a portion of the back side of the silicon substrate. A selected portion of a bulk silicon substrate layer in the back side is removed and a void is created. A new circuit having silicon-on-insulator (SOI) structure is formed in the back side of the die in the void. In one implementation, construction of the new circuit having SOI structure involves using either the existing buried insulator of the die to form the new circuit with SOI structure, or forming additional insulator, and constructing the new circuit above the new insulator. The newly formed circuit having SOI structure constructed in a back side void of the die is coupled to pre-existing circuitry on the circuit side of the die. The new circuit is used to analyze the die, for example, during operation or testing. 
     The circuit can be formed in various portions of the die and used to effect various results. For instance, in one example embodiment of the present invention a back side of an integrated circuit die having SOI structure with silicon dioxide as the insulator is selectively thinned on the back side and a buried oxide layer (BOX) is exposed. Insulator material is added to the exposed BOX making it thicker and providing additional electrical insulation between the circuitry on the circuit side of the die and circuitry that will be formed on the back side. An epitaxial layer of silicon is formed over the expanded BOX. Circuitry is then formed in the epitaxial layer using a conventional circuit manufacturing process. The newly formed circuitry is electrically coupled to the circuitry located on the circuit side of the die via a conductor. 
     There are a number of uses for the newly formed circuitry in the analysis of the die. In one example embodiment, the new circuitry is used to replace pre-existing circuitry in the die suspected of containing a defect to either positively or negatively confirm the existence of the suspected defect. In a die which contains a defect, a circuit path suspected of containing the defect is replaced with the newly formed circuitry in the back side of the die. The die is operated, and the existence or nonexistence of a defect is detected. Elimination of a recurring failure by the replacement of a specific circuit element indicates the replaced circuit element contains a defect. If the newly formed circuitry eliminates the failure, the replaced circuit is identified as being defective. If the failure still occurs, the replaced circuitry is identified as not necessarily causing the failure. In this way selective replacement of portions of circuitry in the die assists in locating defects. 
     In another example embodiment, circuitry that is known to have been damaged, for instance during earlier analysis, is replaced by circuitry built in the void created in the back side of the die by localized thinning of the die. Circuitry is damaged during analysis, for example, when a buried circuit is accessed by removing circuitry overhead. The removed circuitry is replaced by circuitry in the back side of the die by coupling to the front side circuitry either before or after testing the accessed circuitry. The replacement of the damaged circuitry allows use and analysis of the undamaged portions of the integrated circuit die so that the die is not a loss. The back side circuitry is used to replace the damaged circuitry allowing the rest of the circuitry in the die to be tested instead of having to discard the chip or attempting to repair the damaged circuitry in the front side of the die. 
     In another example embodiment of the present invention, a programmable circuit is formed in the back side of the die and coupled to the front side circuitry. The programmable circuit provides programmability and hence, additional flexibility and uses for the back side circuitry. In one example, the back side programmable circuitry is used to stimulate the circuit side circuitry of the die including such things as biasing a gate or supplying a current or voltage. 
     In another example embodiment, FIG. 1 shows an integrated circuit die having circuitry in a back side of the die. A layer of silicon  10  is epitaxially grown on a buried insulator layer  20  that has been exposed by selective removal of back side silicon substrate. A circuit  30  is formed in the silicon epi-layer  10 . The formed circuit  30  includes one or more circuit structures, such as a transistor containing a source region, a drain region, and a gate. The formed circuit  30  is selectively activatable and is used as a spare circuit, an additional circuit, and/or a replacement circuit. Formation of the circuitry on a thin epitaxial layer above an insulator layer provides additional advantages of an SOI structure including decreased drain capacitance resulting in faster switching times. The faster switching times are particularly useful for operating and analyzing dies at and above the operating speeds demanded in complex applications. 
     In another example embodiment of the present invention illustrated in FIG. 2, a flip-chip die having SOI structure is analyzed. A portion of a bulk silicon layer is removed without exposing the buried insulator layer. The selective removal of a portion of the bulk silicon layer  200  creates a void in the back side of the flip-chip die  210 . The void extends into the bulk silicon layer  200  yet stops short of the buried insulating layer  220 . Circuitry is then formed in the void created by the removal of substrate. First, an insulator is deposited unto the newly exposed surface of the thinned bulk silicon layer, and a thin film dielectric formed. An epitaxial layer is formed over the dielectric layer. As shown in FIG. 2, the resulting structure includes an epitaxial layer  240  containing pre-existing circuitry  230 , a buried insulator layer  220 , a bulk silicon layer  200 ,Ma new buried insulator layer  250 , and a new epitaxial layer  260 . Circuitry  270  is built into the new epitaxial layer  260 . 
     Formation of additional insulator may be performed both when the buried insulating layer has been exposed and when it has not been exposed by the back side substrate removal. The formation of additional insulator is performed in a variety of ways applicable to both instances. One method which encompasses a variety of techniques is chemical vapor deposition (CVD). In one example embodiment, a vapor deposition step is performed followed by an annealing step. In another, a plasma deposition technique is utilized. Still another example method of insulator formation is sputtering amorphous silicon into the void created by substrate removal and oxidizing the silicon by exposure to a wet ambient environment at temperatures between 800°-1000° C. 
     In another example embodiment, a flip-chip die is locally thinned in a selected area of the back side of a die having a buried oxide layer (BOX), exposing the back side of the BOX. In many common SOI applications, the BOX is approximately 400 nanometers thick. Additional insulator is added to the BOX to thicken it to approximately 10-100 microns. A scanning pulsed laser, such as a femto laser, is used to perform the thickening of the BOX through a laser annealing process. A silicon epitaxial layer is formed above the expanded BOX layer and circuitry is formed in the epitaxial layer. The formed circuitry contains one or more of spare gates, spare memory cells, and programmable circuitry circuits. Each piece of formed circuitry may be selectively activated. The formed circuitry is coupled to the pre-existing circuitry and used as desired to modify and/or test the die. 
     In another example embodiment, illustrated in FIG. 3, circuitry  310  formed in a void  320  on the back side of the die is electrically coupled to circuitry  340  on the circuit side  350  of the die  300  by a conductor  360 . The conductor  360 , first connected to an electrical lead on the circuit side  350  of the die, is extended along the outside of the die from the circuit side to the back side where it is contacted to an electrical lead on the circuitry newly formed upon a thickened buried insulator layer  390 . As shown in FIG. 3, multiple connections may be made. Additionally, coupling the new and old circuitry may include coupling them through an analysis device  380  whereby the analysis device makes use of both pieces of circuitry for die analysis. 
     In another example embodiment, the circuitry formed in a back side void is capacitively coupled to the circuitry located on the circuit side of the die. The operation of the back side circuitry creates a capacitance effect in the circuit side circuitry which is manipulated to force the circuit side circuitry to undergo a change in voltage, current, or other electrical power parameter. 
     In another example embodiment, shown in FIGS. 4 a-c,  the coupling of the back side circuitry  410  to the circuit side circuitry  420  is demonstrated. FIG. 4 a  shows the formed integrated circuit die having newly formed back side circuitry  410  formed in a void  430  created by localized thinning of a back side bulk silicon layer  440 . 
     A hole is milled through the die  400  between the two areas of circuitry in FIG. 4 b.  Milling of the die may be performed by an FIB etch, or by other well known means. The hole is milled through a back side epitaxial silicon layer  450 , a buried insulator layer  460  that has been thickened, and into the circuitry  420  on the circuit side of the die. 
     The milled hole is filled with a conductive material  480  that is extended to contact the desired parts of the two regions of circuitry in FIG. 4 c.  A conductive wire lead, or other means of conducting electrical signals, may be required to extend the conductor to the desired portion of circuitry. The connection between the circuits may include contacting a specific portion of a circuit, for example a source/drain region  490 , a gate  495 , an interconnect (not shown), or a bonding pad (not shown). The two regions of circuitry are thereby coupled. 
     One example embodiment of the present invention is a system for flip-chip die analysis. The back side  510  of a flip chip die  500  is selectively thinned by a substrate removal device  590  in FIG. 5. A circuit formation arrangement  580 , including conventional circuitry construction devices, is adapted to form circuitry in a void created by removal of the substrate  520 . Analysis of the die  500  is performed using the newly formed circuitry  510  in the back side void of the die. An analysis device  540  is adapted to send a stimulus through an interface device  550  to the circuit side circuitry  530 . The back side circuitry  510  includes a spare gate, a spare memory cell, and a selectively activatable spare circuit which are selected as needed for analysis. A connection between the back side and circuit side circuitry is formed via a conductive wire  570  between the two. 
     When a defect is determined to exist, yet the location is unknown, the die is operated in a manner such that circuitry suspected of containing a defect is replaced by corresponding circuitry  510  constructed in a back side. The new circuitry is used to replace pre-existing circuitry in the die suspected of containing a defect to either positively or negatively confirm the existence of the suspected defect. In another implementation, the circuitry  510  constructed in the back side is adapted to stimulate existing circuitry in the die, and in another implementation the circuitry is adapted to be used as circuitry in addition to the existing circuitry. 
     While the present invention has been described with reference to several particular example embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention, which is set forth in the following claims.