Abstract:
Analysis of a semiconductor die having silicon-on-insulator (SOI) structure is enhanced by accessing the circuitry within the die from the back side without necessarily breaching or needing to breach the thin insulator layer of the SOI structure. According to an example embodiment of the present invention, a portion of substrate is removed from the back side of a semiconductor die having a SOI structure and a backside opposite circuitry in a circuit side. An exposed region is formed where the substrate has been removed. A detectable response from the exposed region is induced, for example, by an electron beam, as a function of a portion of the active circuitry within the die.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor dies and their fabrication and, more particularly, to analysis of semiconductor dies involving capacitive probing. 
     BACKGROUND OF THE INVENTION 
     The semiconductor industry has recently experienced technological advances that have permitted dramatic increases in integrated 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 hundreds of 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 is an increased demand for products employing these microprocessors and devices for use in numerous applications. As the use of these devices has become more prevalent, the demand for faster operation and better reliability has increased. Such devices often require manufacturing processes that are highly complex and expensive. 
     As the manufacturing processes for semiconductor devices and integrated circuits increase in difficulty, methods for 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 number of defective devices manufactured. 
     Traditionally, integrated circuits have been tested using methods including directly accessing circuitry or devices within the integrated circuit. Directly accessing the circuitry is difficult for several reasons. For instance, in flip-chip type dies, transistors and other circuitry are located in a very thin epitaxially-grown silicon layer in a circuit side of the die. The circuit side of the die is arranged face-down on a package substrate. This orientation provides many operational advantages. However, due to the face-down orientation of the circuit side of the die, the transistors and other circuitry near the circuit side are not readily accessible for testing, modification, or other purposes. Therefore, access to the transistors and circuitry near the circuit side is from the back side of the die. 
     One particular type of semiconductor device structure that presents unique challenges to back side circuit analysis is silicon-on-insulator (SOI) structure. SOI involves forming an insulator, such as an oxide, over bulk silicon in the back side of a semiconductor device. A thin layer of silicon is formed on top of the insulator, and is used to form circuitry over the insulator. The resulting SOI structure exhibits benefits including reduced switch capacitance, which leads to faster operation. Direct access to circuitry for analysis of SOI structure, however, involves milling through the oxide. The milling process can damage circuitry or other structure in the device. Such damage can alter the characteristics of the device and render the analysis inaccurate. In addition, the milling process can be time-consuming, difficult to control, and thus expensive. 
     The difficulty, cost, and destructive aspects of existing methods for testing integrated circuits are impediments to the growth and improvement of semiconductor technologies involving SOI structure. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and system for analyzing a semiconductor die having silicon-on-insulator (SOI) structure in a manner that addresses and can even overcome the above-discussed impediments. The die includes a back side opposite circuitry in a circuit side, and the die analysis involves coupling to the circuitry. 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 SOI structure and a back side opposite circuitry in a circuit side is analyzed. A portion of substrate is removed from the back side of the semiconductor die and an exposed region of the insulator portion of the SOI die is formed. A detectable response is induced from the exposed region as a function of a portion of the circuitry. Detecting a response from the die in this manner expedites and enhances analysis of semiconductor devices that employ SOI construction, thereby improving the manufacture, testing, and technological advancement of such devices. 
     In another example embodiment of the present invention, an electron beam is directed at the exposed region in the semiconductor die using, for example, a scanning electron microscope (SEM). The electron beam generates secondary electrons in the exposed region that are detected and used to analyze the die. Such generation of secondary electrons occurs as a function of an electrical characteristic of a portion of the circuitry in the die. This functional relationship between the generation of secondary electrons and the electrical characteristic of the circuitry is used to analyze the die. 
     According to another example embodiment of the present invention, a system is adapted to analyze a semiconductor die having silicon-on-insulator (SOI) structure and a back side opposite circuitry in a circuit side. The system includes a substrate removal arrangement adapted to remove substrate from the back side of the semiconductor die and form an exposed region in the insulator portion of the SOI structure. A probe arrangement is adapted to induce a detectable response from the exposed region as a function of a portion of the circuitry. A detector is adapted to detect the response and to analyze the die therefrom. 
     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 semiconductor die having SOI structure for use in connection with the present invention; 
         FIG. 2  is a semiconductor die undergoing analysis, according to an example embodiment of the present invention; and 
         FIG. 3  is a system for analyzing a semiconductor 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 to a variety of different types of semiconductor devices, and has been found to be particularly suited for flip-die and other type devices having silicon-on-insulator (SOI) structure and requiring or benefiting from analysis involving obtaining a response from the insulator portion of the SOI structure. While the present invention is 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 back side opposite circuitry in a circuit side and including SOI structure is analyzed using a response detected from the insulator portion as a function of circuitry in the die. First, a portion of substrate is removed from the back side of the die, and an exposed region of the insulator of the SOI die is formed over circuitry in the circuit side. It has been unexpectedly discovered that, using the insulator, for example, an electron beam on a probe can be used to image a response from the dielectric to analyze the underlying circuitry. In this manner, electrical characteristics are obtained from the circuitry and used to analyze the die. The analysis can be used, for example, to determine die logic states for post-manufacturing analysis. 
       FIG. 1  shows a portion of a flip chip die  100  exemplifying one type of a variety of dies having SOI structure to which the present invention is applicable. The die of  FIG. 1  is shown in an inverted position with the back side facing up, such as would be a flip die bonded to a package substrate. A very thin buried oxide (BOX)  150  is formed over silicon substrate  160 , and a thin layer of silicon  140  is formed on the oxide  150 . Source/drain regions  120  and  130  are formed in the thin layer of silicon  140 . A gate  110 , formed over the thin layer of silicon  140  and an intervening gate insulator layer  142 , is used together with the source/drain regions to create a SOI transistor. 
       FIG. 2  shows a semiconductor die having SOI structure, such as shown in  FIG. 1 , undergoing analysis in accordance with another example embodiment of the present invention. A portion of silicon substrate  260  has been removed from the back side of the die  200 , leaving an exposed probe surface  270  of a BOX layer  250 . The substrate can be removed, for example, using typically-available substrate removal methods and devices, such as using a focused ion beam (FIB), a laser etching device, or an etch chamber having an etch gas and used in combination with a masking step. In one particular example embodiment (not illustrated), the back side silicon substrate is globally thinned using a polishing process, such as chemical-mechanical polishing, and is followed by a locally thinning process. 
     The probe surface  270  is shown on the BOX  250  in this example embodiment. However, the probe surface may be created in any manner sufficient to permit an electrical characteristic of a portion of circuitry in the die to be probed. For example, the probe surface may be formed in the substrate  260 , wherein the amount of the substrate that has been removed creates an opening that is sufficiently deep to facilitate the capacitive coupling. In another example, a portion of the BOX  250  is removed and the probe surface  270  is created in the BOX  250 , with the depth similarly chosen to facilitate the probing. A cold finger (plate) can be applied to one or more selected locations on the die to maintain the die temperature. 
     After the probe surface  270  has been defined, a probing arrangement is used to induce a response that is a function of underlying circuit regions in the die. Transistors  210  and  220 , separated by isolation region  205 , are shown undergoing electron beam probing. First, an electron beam  230  is directed at a portion of the probe surface over a source/drain region  212  of transistor  210 . The beam can be directed at selected regions by imaging the circuitry through the BOX and navigating to the selected region using the image, or using other typically available navigation methods. In one implementation, the electron beam is generated and controlled using a scanning electron microscope (SEM). The BOX  250  acts as a dielectric and facilitates capacitive coupling between the probe region and the source/drain region. Secondary electrons  235  are emitted in response to the electron beam and are modulated by electric fields in the BOX. In this instance, the electric field generated by the source/drain region  212  affects the emission of secondary electrons. If the source/drain region is at a positive voltage, such emission of secondary electrons is inhibited, and if the voltage is zero or negative, the emission of secondary electrons is generally unaffected. This variation in secondary electron emission is used to detect an electrical characteristic from the transistor  210 . 
     The beam  230 ′ is then caused to scan the transistor  220  and probe source/drain region  222  in a similar manner to which source/drain region  212  was probed. Secondary electrons  235 ′ are emitted from source drain region  222  and used to detect an electrical characteristic from the region  222 . As shown, the emission of secondary electrons  235 ′ is inhibited as compared to secondary electrons  235 . This inhibited emission is detected and used to detect that the two transistors exhibit different characteristics. In this instance, source/drain region  222  has a higher positive voltage than source/drain region  212 . The inhibited secondary electron response from source/drain region  222  is detected and used to determine that it is at a higher positive voltage. If desired, various other circuit portions of the die can be analyzed using the same technique, and responses can be used to detect variations in voltage across the die. The responses can then be used to create an image representing the voltage states of the die. For more information regarding capacitive coupling, probing and the detection of secondary electrons, reference may be made to Christopher G. Talbot,  Probing Technology for IC Diagnosis , in FAILURE ANALYSIS OF INTEGRATED CIRCUITS  113 , (Lawrence C. Wagner ed., 1999). 
     In another example embodiment of the present invention, a non-defective die is analyzed in the same manner as described hereinabove. Response data from the die can be obtained for one or several circuit elements to be subsequently analyzed in a defective die. The data from the non-defective die is then stored and compared with data obtained from the defective die. In one particular implementation, a series of test vectors are input to the non-defective die. The same signals are input to the defective die, and the response is compared. Variations in the response can be used to detect that the defective die is not operating properly. 
     According to another example embodiment of the present invention,  FIG. 3  shows a system  300  arranged to probe a semiconductor die  301  having SOI structure, such as the die shown in  FIGS. 1 and 2 . The die  301  is located on a die holder  310 . A electron beam generator  320  is adapted to direct an electron beam  322  at an exposed probe region in the die  301 . The beam  322  induces a response from the die via the BOX that is a function of underlying circuitry in the die. A detector  330  is adapted to detect secondary electrons  332  emitted from the die  301  in response to the electron beam  322 . The detected secondary electrons are used to detect an electrical characteristic of the die  301 , such as described hereinabove. In one implementation, the die holder  310 , beam generator  320  and detector  330  are part of a single SEM arrangement, shown by dashed lines  370 . 
     The system  300  optionally includes a controller adapted to control the probe, such as a computer arrangement  340 . In one implementation, the computer arrangement is adapted to receive electrical characteristics of the die  301  via the detector  330  and interpret those characteristics to analyze the die. The computer arrangement  340  can also be programmed to control the beam generator  320  for obtaining various results. For instance, the beam can be pulsed rapidly to obtain a waveform response from the detector. The pulsing can be generated in sequences having a duration in the microsecond, picosecond or even shorter range. This is particularly useful for obtaining waveforms from high frequency applications, and for obtaining a capacitive measurement in response to a particular input signal. 
     The computer  340  is further optionally coupled to a display  350  adapted to provide information indicative of the response from the detector  330 . For example, the display can be used to display a waveform response from the die in the pulsed application described above. In addition, the display can be used to display an image of the die having variations in contrast related to the secondary electrons detected. Portions of the die emitting few secondary electrons (e.g., circuitry having a positive voltage) show up as dark spots, and portions emitting greater numbers of secondary electrons show up as bright spots. 
     In another example embodiment of the present invention, the die  301  is powered for analysis using supply  360 . The power supply is coupled to the die  301  via the holder  310  and is used to input signals to circuit elements in the die. A response to the input signals is obtained by detecting the secondary electrons  332 . The input signals may, for example, include normal operating signals supplied to the die, or may include signals known to cause a failure in the die. In one particular implementation, the signals are input in a continuous loop that generates a failure in the die. The response is capacitively monitored and used for analyzing the die. 
     In another example embodiment of the present invention (not shown), the system  300  includes a substrate removal device adapted to remove substrate from the back side of the die  301  and form an exposed region having a probe area. In one implementation, the substrate removal device includes a FIB, and in another implementation, the substrate removal device includes a laser etching device. The exposed region is formed having a depth that is sufficient to facilitate inducing a response related to a source/drain region or other circuitry portion in the die, such as by exposing a BOX layer in a SOI structure. 
     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.