Abstract:
According to an example embodiment of the present invention, a defect detection approach involves detecting the existence of defects in an integrated circuit as a function of acoustic energy. Acoustic energy propagating through the device is detected. A parameter including information such as amplitude, frequency, phase, or a spectrum is developed from the detected energy and correlated to a particular defect in the device.

Description:
RELATED APPLICATIONS 
     The present invention is related to U.S. patent application Ser. No. 09/410,147 entitled “ACOUSTIC THREE DIMENSIONAL ANALYSIS OF CIRCUIT STRUCTURES” and filed concurrently herewith, and to U.S. patent application Ser. No. 09/387,182 entitled “MICRO-VOID DETECTION” and filed on Aug. 31, 1999, now U.S. Pat. No. 6,253,621, which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor devices and their fabrication and, more particularly, to semiconductor devices and their manufacture involving testing the devices for defects. 
     BACKGROUND OF THE INVENTION 
     The semiconductor industry has recently experienced technological advances 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 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 has been an increase in the number and complexity of the manufacturing processes, as well as an increase in the difficulties of maintaining satisfactory levels of quality control, testing the devices for defects, and providing a cost-effective product using such processes. 
     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. In addition, many methods require the circuit to be powered. 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 chip. 
     Since access to the transistors and circuitry in flip-chips is generally from the back side of the device, it is often necessary to mill through the back side and probe certain circuit elements in order to test the device. Milling through the back side is often difficult and time consuming. Moreover, circuitry and devices in the integrated circuit may potentially be damaged by milling processes. The difficulty, cost, and destructive aspects of existing methods for testing integrated circuits are impediments to the growth and improvement of semiconductor technologies. 
     SUMMARY OF THE INVENTION 
     The present invention is exemplified in a number of implementations and applications, some of which are summarized below. According to an example embodiment, the present invention is directed to a method for testing an integrated circuit. Acoustic energy propagation in the integrated circuit is detected and circuit defects are detected as a function of the acoustic energy. 
     According to another example embodiment of the present invention, a system is arranged for testing an integrated circuit having circuitry in a circuit side opposite a back side The system includes a substrate removal device and a laser configured and arranged to excite circuitry in the integrated circuit. One or more acoustic energy detectors are used to detect acoustic energy. A computer is configured and arranged to use the detected acoustic energy and generate a parameter that can be used to detect one or more defects in the integrated circuit. For example, the defect can be detected at the computer, or it can be detected via an outside source, such as a human operator viewing a computer-generated parameter. In another example embodiment, the computer, the laser, and the acoustic energy detectors are communicatively coupled. 
     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 which 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 shows an integrated circuit device undergoing testing, according to an example embodiment of the present invention; and 
     FIG. 2 is a flow diagram for a method of testing an integrated circuit device, 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 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 requiring or benefiting from defect analysis. The invention has been found to be particularly suited for post-manufacturing failure analysis of semiconductor devices having target circuitry containing defects such as shorts and opens in the circuit 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, acoustic energy propagation in an integrated circuit is detected. It has been discovered that acoustic energy propagation in integrated circuits varies relative to defects in the circuit. In this manner, the detected acoustic energy propagation is used to detect the existence of defects in the integrated circuit. 
     According to a more particular example embodiment, and referring to FIG. 1, a system  100  is used and a target region  115  of the back side  130  of an integrated circuit die  105  having a back side opposite circuitry  140  in a circuit side  135  is thinned. The device may be thinned, for example, using a laser etching device, a FIB, chemical-mechanical polishing, or other suitable removal device. A laser device  110  is used and a laser beam  120  of a wavelength of about 1064 nanometers is directed at the thinned portion of the back side  130  and optical beam induced current (OBIC) is created, resulting in pn junction activity. OBIC occurs when electron-hole pairs are generated in the die substrate and the photo-generated carriers diffuse to a depletion region. Electron-hole pairs may also be created directly in the depletion region by the laser, contributing to the OBIC. The OBIC generates acoustic energy that varies in relation to the mass of circuitry coupled to the region in which the OBIC is created. The acoustic energy is detected via detector  150  and used for defect analysis at a computer arrangement  160 . The detector  150  may include, for example, an acoustic transducer such as an UltraSonic Transducer available from Sonix, Inc. 
     For instance, when there is a short circuit in the portion of circuitry in the die coupled to the region in which OBIC is generated, more circuitry is affected by the OBIC via the short. This additional circuitry effectively increases the amount of circuit mass affected by the OBIC as compared to the amount of affected circuitry without the short. As the mass increases, both the frequency and amplitude of the acoustic energy propagation decrease, and the wave propagates slower, delaying the phase. The phase may be detected by synchronizing the acoustic detector with the laser. Using this same approach, open circuits in circuitry within the device reduce the amount of metal mass affected by the OBIC as compared to circuitry not having an open. As the circuit mass decreases, both the frequency and amplitude of the acoustic energy propagation increase, and the phase is advanced. Therefore, a die can be tested using OBIC and, based on a variation in at least one frequency, amplitude, or phase parameter, shorts and opens can be detected. The parameter variations may, for example, be determined by comparing the parameter to a standard parameter developed from a non-defective die. This method includes additional advantages in that it does not require the device to be powered, nor does it require direct access to circuitry within the device. 
     FIG. 2 is a flow diagram showing a method for analyzing a semiconductor device, according to another example embodiment of the present invention. Substrate is removed from a target region at block  210 . A laser is directed at the target region at block  220  and produces OBIC and pn junction activity at block  230 . The OBIC and pn junction activity produces acoustic energy in the device at block  240 . The acoustic energy is detected at block  250  and used for circuit analysis. 
     In another example embodiment, a non-defective integrated circuit is thinned and an exposed region is formed. A laser is directed at the exposed region and OBIC is created, which in turn generates acoustic energy propagation in the device. At least one parameter corresponding to the acoustic propagation is determined and recorded as a standard parameter. The parameter may, for example, include amplitude, frequency, or phase information, and may include a spectrum. Using the recorded standard parameter or parameters of the non-defective die for comparison, an integrated circuit device is analyzed. The integrated circuit device is thinned and an exposed region is formed. A laser is directed at the exposed region of the device under test, and acoustic energy propagation is generated in the device. The propagation is detected, and a parameter corresponding to the propagation is determined. The parameter is then compared to the standard parameter to determine the existence of defects. For example, the standard parameter can be used in pass-fail testing, wherein the parameter from a device under test must about match the standard parameter if it is to pass. 
     In another example embodiment of the present invention, the integrated circuit device is thinned and an exposed region is formed at a predetermined thickness that corresponds to the substrate thickness used in establishing the parameter from a non-defective integrated circuit, as described herein. By thinning the substrate to the predetermined thickness, the laser incident upon the exposed region of the device under test is applied in a similar manner to the application of the laser to the non-defective device when the standard parameter was determined. In doing so, the resulting OBIC in the device under test is more closely related to the OBIC generated during the recordation of the standard parameter. 
     In the above example embodiments, directing a laser at an integrated circuit device may include pulsing the laser. Pulsing the laser can be accomplished by turning the laser on and off, or by otherwise varying the laser&#39;s intensity. By pulsing the laser, the amount of energy transferred to or induced in the device can be controlled. For example, as compared to using and applying a laser having a constant intensity over an uninterrupted period of time, pulsing the laser reduces the amount of energy transferred by the laser to the device. This method is particularly advantageous for exciting circuitry within the device and holding that circuitry at a particular energy level or level of activation, which is useful for testing the device. For instance, holding the circuitry at a particular power level can involve holding a pn junction just below the threshold required to turn the junction on, commonly referred to as the operating voltage. When it is desired to push the pn junction over the threshold an into operation, only a small amount of additional excitation is needed. In addition, pulsing the laser reduces the risk of overheating the device and melting metal in the device. 
     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.