Abstract:
A decapsulation apparatus  100  has a laser  8  that removes plastic encapsulant from a device  24.  Chamber  20  is sealed. Exhaust port  9  removes debris and fumes. The device  24  is positioned and scanned using an X,Y table 2. A hinged end  4  rotates the device to an acute angle of incidence with respect to a laser  8 . Endpoint detector  10  senses the exposed integrated circuit and moves or shuts down the laser  8. Docket:    87552.090401 /SE- 1463 TD.A

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional application of U.S. patent application Ser. No.: 09/307,896,filed May 10, 1999 (Attorney Docket No. 87552.99R041). 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to an apparatus and method for removing plastic compounds that encapsulate integrated circuits, and, in particular, to a laser equipped apparatus and method for decapsulating plastic encapsulated integrated circuits.  
           [0003]    The vast majority of integrated circuits are packaged in plastic resins including but not limited to biphenyl, ortho-cresol novolac, and dicyclopentadienyl types. The plastic package seals the enclosed integrated circuit from the external environment, including moisture and dust. The resin contains fillers such as silica or other insulating materials to enhance the physical and mechanical properties of the package. The integrated circuits are encapsulated using a transfer molding process. During that process a solid charge of resin is melted and then forced under pressure into a multi cavity mold that contains a number of integrated circuits. One mold may contain tens or hundreds of integrated circuits. The size of the molded integrated circuits varies in length, width and height. Some devices using the standard dual-in-line package are several millimeters thick. Other small outline packages are a millimeter in thickness.  
           [0004]    There are a number of reasons for removing the plastic encapsulant from finished Integrated circuits. One reason is to monitor the manufacturing process. In most mass manufacturing processes, samples of finished product are often taken and analyzed to check whether or not the finished product is made to the manufacturing specifications. Some times one or more devices fail, it is desirable to analyze those failed devices to detect process flaws so that the flaws can be corrected. Some devices are also reverse engineered in order to discover how the device is constructed.  
           [0005]    Current techniques for removing the plastic are time consuming and environmentally unfriendly. One technique uses fuming nitric or sulfuric acid. That technique can take several hours or more in some cases to remove the plastic, and the spent chemicals must be properly disposed of In addition, these harsh chemicals come in contact with the surface of the integrated chip being exposed, which may chemically remove foreign substances or contaminants residing between the top of the die and the mold compound which will subsequently not be detected in failure analysis. Plasma etching may be used but it is slow and also leaves undesired residues. As such, there is a long felt and unfulfilled need fo r a faster process that is environmentally friendly and less disruptive to the top surface of the integrated circuit chip.  
         SUMMARY  
         [0006]    The invention eliminates the hazardous waste and provides a faster decapsulation process which is less disruptive to the top-of-die surface. The invention provides an apparatus and method for removing plastic encapsulant using a tunable laser. A chamber has a stage for holding the integrated circuit during decapsulation. The stage is an X,Y table that comprises rods so that debris from the removed encapsulant may fall between the rods. Below the stage is a dust bin for collecting the debris. A hinge on the table lets the operator adjust the angle of incidence of the laser beam on the specimen surface.  
           [0007]    A laser outside the chamber shines its beam through a window or other suitable optical opening onto the surface of the device under test. The laser beam is tunable in frequency and amplitude to suitable settings for removing the encapsulant. The laser beam is generated by a YAG or infrared laser or any other laser suitable for breaking the cross linked bonds of the encapsulant without damaging the integrated circuit.  
           [0008]    The decapsulation process is controlled by a computer that includes a microprocessor or digital signal processor, suitable memory, an application program for operating the apparatus and suitable sensors. One sensor is an endpoint detector. It is focused on the integrated circuit to detect reflected light. Where the plastic is removed, the beam strikes the integrated circuit and the amplitude and frequency of the reflected light changes. The endpoint detector senses those changes. In response to a signal indicating that the integrated circuit is exposed, the computer shuts down the laser beam or moves the laser beam to a new location.  
           [0009]    The apparatus has a sealed chamber. Fumes generated by decapsulation are exhausted through a suitable fan or blower-operated exhaust port. A cleaning gas such as nitrogen or compressed air is directed at the surface of the integrated circuit to remove dust. The removed dust is either exhausted of falls to into the dust bin. 
       
    
    
     DRAWINGS  
       [0010]    [0010]FIG. 1 is a sectional elevation view of the invention showing the integrated circuit oriented at a near normal angle to the laser beam.  
         [0011]    [0011]FIG. 2 is a further view showing the integrated circuit oriented at an acute angle of incidence to the laser beam. 
     
    
     DETAILED DESCRIPTION  
       [0012]    Turning to FIG. 1, there is shown a decapsulation apparatus  100  that comprises a chamber wall  12  that encloses and seals a chamber  20 . The chamber  12  has a clean gas inlet  11  that includes a conduit  22  that directs a stream of clean gas, such as nitrogen or compressed air at a device under test (DUT), i.e., integrated circuit  24 . Chamber  12  also has an exhaust port  9  for removing fumes and dust particles from the chamber. Within the chamber is a stage  2  that is disposed over a dust bin  3 . The dust bin  3  catches large particles that are removed from the DUT  24 .  
         [0013]    A laser  8  is mounted on the outside wall of the chamber  20  and it directs a laser beam  26  toward the DUT  24 . The laser is any suitable laser, such as a YAG or an infrared laser. The laser has its frequency and its amplitude tunable for removing plastic encapsulant from the DUT  24  without causing damage to the encapsulated integrated circuit. The laser beam  26  passes through an optical opening or window (not shown) in the wall of the chamber  12 . The interior of the chamber  20  is illuminated by a suitable light  7 . Operation of the laser on the DUT  24  is observed through a microscope  5 . A microscope  5  has a shutter  6  that may be manually or automatically operated as hereinafter described. Light reflected from the surface of the DUT  24  is detected by endpoint detector  10 . The endpoint detector  10  senses the amplitude or frequency or both of the reflected light.  
         [0014]    The stage  2  is an X,Y positioning table. It is desirable that the stage be made of rods or has a perforated table so that dust and debris removed from the DUT  24  may fall through the stage and the stage mount into the dust bin  3 . Such X,Y positioning tables are well known in the art. They may be operated using piezoelectric operators, linear magnetic motors, or lead screws. As shown in FIG. 1, the DUT  24  is disposed at substantially a normal angle to the laser beam  26 . The stage is hinged at one end  4  so that the DUT  24  may be rotated to a substantially vertical position as shown in FIG. 2. In its vertical position, the laser beam  26  has an acute angle of incidence on the surface of the DUT  24 . That particular position is useful when it is desired to leave a thin layer of encapsulant on the surface of the integrated circuit. Leaving such a thin layer is often desired during failure analysis to detect contaminants on the surface of the encapsulated DUT  24 .  
         [0015]    The apparatus  100  may be manually operated or automatically operated or may be semiautomatically operated. For automatic and semiautomatic operation, the apparatus  100  is provided with a controller  50 . The controller  50  includes a CPU  51  which may be a microprocessor or a digital signal processor. The CPU  51  communicates with a random access memory  53  and a read-only memory  54 . Suitable operating software applications and application software are stored in the RAM  53  or ROM  54 . The CPU  51  controls operations of the various components of the apparatus  100  via the control bus  30  and the control lines  31 - 36  that are respectively connected to the microscope shutter  6 , laser  8 , stage  2 , clean gas inlet  11 , endpoint detector  10 , and the exhaust port  9 . By manual, automatic or semiautomatic operation, the operator may selectively operate any one of the controlled components, move the stage to its desired X,Y position, and rotate the top platform of the stage to its desired Z axis orientation.  
         [0016]    The following wavelengths in the infrared range are especially applicable: 725-900cm−1, 1150-1300cm−1, 1400-1500cm−1 and 1600-1750cm−1. These wavelength ranges were determined from IR chemical analysis of the mold compound resins. A reference if appropriate is “Identifying Plastic Encapsulant Materials by Pyrolysis Infrared Spectrophotometry”, R. K. Lowry, K. L. Hanley, Proceedings, 1998 Intl Symposium for Testing and Failure Analysis, Nov., 1998, pp. 399-401 It is these wavelength ranges in particular where there is significant absorption of IR energy at the molecular level by plastic resins. An incident beam tuned for maximized power in these energy ranges would begin to promote molecular rearrangement and ultimately decompositional breakdown of polymerized resin molecules. Any tuned laser operating in these ranges would promote breakdown not just by thermal heating the material (which almost any incident laser energy with enough power could provide) but also by chemical decomposition. This promotes breakdown in a “material-specific” way, so that excessive heating via straightforward but less-controllable thermal decomposition can be avoided.  
         [0017]    In operation, a DUT  24  is placed or otherwise mounted on the top of the stage  2 . While the embodiment shown in FIG. 1 includes only a single device, those skilled in the art will appreciate that multiple devices may be mounted on the stage. Either manually or with the assistance of controller  50 , the stage  2  is positioned in an X,Y plane relative to the laser beam  26 . of laser  8 . Laser  8  is under control of the controller  50 . Laser  8  may be any suitable laser that has its amplitude and frequency tuned and controlled by controller  50 . Such suitable lasers include YAG lasers, as well as infrared lasers. It is desired to use a laser with a suitable power and frequency for breaking the cross-linked polymeric bonds of the plastic resin that encapsulates the DUT  24 . The laser may be operated at a relatively low level to provide a beam that strikes the stage. The operator may then position the stage by using a joy stick device  40  and the microscope  5 . In the preferred embodiment, the stage is moved to initially focus the laser or one of the comers of the DUT  24 . Once the starting point for the laser has been selected, then the stage moves in a raster pattern along a first axis, steps transverse to the first axis at least the width of the beam, and then reverses direction and travels back along the first axis. In this manner, the beam  26  raster-scans across the DUT  24 . Of course, if desired, the laser beam  26  may be raster-scanned using optical methods, including prisms and/or mirrors that are selectively moved to sweep the beam across the surface of the DUT  24 .  
         [0018]    When the beam  26  is operated at its effective frequency and amplitude, the plastic encapsulant is removed from the DUT  24 . The removal process creates a cloud of debris and fumes. The fumes and some of the debris are withdrawn from the chamber  20  via the exhaust port  9 . The heavier debris particles fall through the rods or holes in the stage  2  and are captured in the dust bin  3 . Some of the dust may settle onto the DUT  24 . Clean gas  22  drives the dust away from the DUT  24 . The clean gas  22  includes any suitable gas, such as nitrogen or air for dispersing the dust particles away from the surface of the DUT  24 . Such dispersal permits the operator to view the DUT in process and removes particles from the immediate path of the laser so that the DUT  24  has its encapsulant more effectively removed.  
         [0019]    Decapsulation may be carried out automatically. During automatic decapsulation, the laser is operated until the endpoint detector  10  detects a change in the amplitude and/or frequency of light reflected from the DUT  24 . When the integrated circuit is uncovered, the reflected light changes its frequency. The amount of reflected light may also change. The DUT detects these changes and provides a signal via signal and control line  35  to the controller  50 .  
         [0020]    Controller  50  receives and sends signals on control and sensor bus  30  via an A to D and D to A converter  52 . As is often the case, the control and sensing signals are analog signals. Thus, it is necessary to convert the analog signals to digital signals so that they can be understood by the CPU  51 . If the CPU  51  is a DSP, the DSP has a built-in A to D and D to A converter.  
         [0021]    Controller  50  receives the signal from the endpoint detector  10 . When the endpoint detector  10  signals that the integrated circuit is uncovered, the controller  50  advances the stage to the next position to continue removing encapsulant. As such, for a given beam width, the laser is focused on the DUT  24  until the underlying integrated circuit is exposed. Upon detection of the exposed integrated circuit, the stage is moved in a continuous or stepwise pattern to subsequent positions.  
         [0022]    As indicated above, it is also possible to manually operate the apparatus or to semi-automatically operate the apparatus. For example, it is often desired to provide one or more pinholes down to the surface of the integrated circuit. Those holes can be provided by selectively removing encapsulant using the laser and the endpoint detector.  
         [0023]    Having thus described the preferred embodiment of the invention, those skilled in the art will appreciate that further changes, modifications, additions and omissions may be made to that embodiment without departing from the spirit and scope of the appended claims.