Abstract:
A method and apparatus for measuring the surface temperatures of wire-bonded semiconductors and the like for preparing thermal maps include a conventional ultrasonic wire bonding machine adapted for mounting a fluorescence-decay temperature sensor in the capillary holder. A trigger box circuit is provided to trigger a temperature measurement based on initiation of an electrical voltage signal from the ultrasonic bonding controller. A computer is provided for coordinating the stage control and temperature measurements, and for collating and plotting the temperature, time and location indications as thermal maps and other displayed/printed correlations.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation of application Ser. No. 09/505,332, filed Feb. 16, 2000, pending, which is a continuation of application Ser. No. 08/943,782, filed Oct. 3, 1997, now U.S. Pat. No. 6,071,009, issued Jun. 6, 2000 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to testing methods and apparatus for semiconductor devices. More particularly, the invention pertains to a method and apparatus for measuring localized temperatures present on semiconductor devices and the like for research and development purposes.  
           [0004]    2. State of the Art  
           [0005]    Modem integrated circuit (IC) devices are commonly formed by joining the electrically active bond pads of a semiconductor die to the conductive lead fingers of a leadframe with metal wires. The wire bonding process may comprise:  
           [0006]    a. thermocompression bonding, which uses pressure and elevated temperature, typically 300-400° C. to bond the wire ends to the bond pads and leadframe;  
           [0007]    b. thermosonic bonding, in which ultrasonic energy is combined with compression at temperatures of about 150° C.; or  
           [0008]    c. ultrasonic bonding, in which ultrasonic energy is typically applied at ambient temperatures. This method is generally limited to some specific metals such as aluminum or aluminum alloy wires on aluminum or gold pads.  
           [0009]    As is well known, the functionality of manufactured electronic devices depends upon successful bonding of the wires to the bond pads of the die and to the lead fingers.  
           [0010]    In each of thermocompression bonding and thermosonic bonding, reliability of the bonding process depends upon the temperatures of the elements being joined.  
           [0011]    It is important for a semiconductor device manufacturer to have the capability for evaluating the quality of conductor bonds, such as wirebonds, leadframe to bump bonds, etc. Evaluation of the bonding process includes, e.g., destructive ball shear tests and wire bond pull tests as well as contaminant tests such as by spectrographic analysis.  
           [0012]    In addition, thermal analysis of the die and leadframe may be done during the conductor bonding operations to yield an indication as to wire bonding quality. Thus, for example, U.S. Pat. No. 5,500,502 of Horita et al. describes a process for bonding a leadframe to a bump using laser irradiation. The state of contact between the leadframe and the bump is then tested using the intensity of the emitted infrared radiation as a measure of the leadframe temperature. Knowing the time lapse between the laser radiation and the measured temperature, the temperature as a function of time may be calculated, particularly a threshold temperature correlated to bond effectiveness and the resulting quality of the wire bond.  
           [0013]    The Horita et al. method does not address the testing of wirebonds. Furthermore, the method depends upon the emission and reflection of infrared radiation, which varies with the surface characteristics of the material whose temperature is being measured. As is well known, both semiconductor dies and leadframes are made of a variety of materials, each of which may have a differing emission/reflection temperature function when laser-irradiated. In addition, a wide variety of materials is used for doping semiconductor dice and for coating dice. For example, U.S. Pat. No. 5,256,566 of Bailey teaches the coating of dice with polysilicon. Thus, the infrared temperature meter must be calibrated for each material, making temperature measurements labor intensive.  
           [0014]    Furthermore, the presence of contaminants on the die or leadframe surfaces will affect the accuracy of the Horita et al. method.  
           [0015]    A method and apparatus for accurately measuring the temperature of very small areas of surfaces, independent of the surface composition, are desirable for research and development purposes in the semiconductor die area.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention is directed to a method and apparatus for accurately measuring the temperature of precisely defined areas of surfaces of materials having a wide variety of compositions, such as a semiconductor die and/or leadframe.  
           [0017]    An apparatus and method for producing a computer-generated thermal map of the surface of a semiconductor die and/or attached leadframe, wafer, or other object are described herein. The apparatus may be used to measure, compile, collate, plot, and display temperatures of a die and its associated leadframe fingers for evaluating a manufacturing process. The apparatus may be configured to back-calculate measured real-time temperatures to a predetermined initial time for preparing thermal maps, e.g., initial or maximum temperatures as a function of location and time.  
           [0018]    The apparatus includes (a) a fiber-optic temperature sensor mounted on the bondhead of a wirebonding machine, and connected to (b) a thermometer apparatus which calculates a temperature based on the sensor output via a (c) signal isolation trigger box having a circuit which is connected to the ultrasonic generator output of the wirebonding machine, whereby a temperature measurement is initiated, and to (d) a computer having software for controlling the wirebonding machine and trigger box and for storing and collating temperature measurements (and other measurements) from the thermometer controller and wirebonding machine.  
           [0019]    The invention may be applied to temperature measurements on a die, wafer, semiconductor device at any stage of construction, or surfaces of other objects of interest. The temperature measurements may be “rastered” over the surface by the stage controller, using any desired increment of movement, because the temperature sensor tip may have a size approximating the size of the area of which the temperature is to be measured. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0020]    The invention is illustrated in the following figures, wherein the elements are not necessarily shown to scale.  
         [0021]    [0021]FIG. 1 is a block diagram of a temperature measurement system illustrating the method and apparatus components of the invention for compiling a thermal map of a semiconductor die and leadframe;  
         [0022]    [0022]FIG. 2 is a partial perspective view of a fiber-optic temperature sensor mounted on a wire bonder head for thermal mapping of a die in accordance with the invention;  
         [0023]    [0023]FIG. 3 is a side view of an optical sensor mount of the invention; and  
         [0024]    [0024]FIG. 4 is a circuit diagram of a signal isolation trigger box of a temperature measurement system of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    A method and apparatus are disclosed herein for measuring temperatures of semiconductor dies and leadframes for producing thermal maps and other representations of the measured temperatures as a function of either location and/or time.  
         [0026]    With reference to the drawings of FIGS.  1 - 4 , and particularly to FIG. 1, a block diagram shows the four major components of the temperature measuring apparatus  10 .  
         [0027]    A wire-bonding machine  12  such as exists in the art is modified as described, infra, for obtaining accurate optical temperature signals at or near surface  40  of a semiconductor device  20  (see FIG. 2), and relaying the signals via a fiber-optic lead or cable  22  to a thermometer controller  14 . The thermometer controller  14  determines the measured temperatures from the signals and transmits the temperature data from the thermometer controller  14  to a computer  18  via a transmission cable  24  such as a standard IEEE488 bus. The computer  18  may be any suitable standard personal computer (PC) having software for storing and manipulating data including temperature, time and position measurements in digital form, controlling other apparatus, and displaying by monitor or printed document the measured readings in a meaningful correlation.  
         [0028]    The invention includes a trigger box  16  which has an electronic trigger circuit connectable to the computer  18  by, e.g., a PC Game Control Adapter Port  28  located thereon. The trigger box  16  provides a signal through computer  18  and transmission cable  24  by which the thermometer controller  14  passes a light pulse through the fiber-optic cable  22  initiating a temperature measurement. Typically, the wire bonding machine  12  has its own software programs for sequentially positioning the semiconductor device  20  and initiating wirebonding by, e.g., ultrasonic generation. The circuit of the trigger box  16  is shown connected via transmission cable  26  to the ultrasonic generator signal V out  of the wire bonding system for coordination of the temperature measurement activation with position control of the wirebonding machine  12 . In the present invention, the ultrasonic generation signal otherwise used for wirebonding is translated into a temperature measurement signal. The positioning and activation of the temperature probe (see FIG. 2) are thus controlled by the software programs of the wirebonding machine  12  and/or the computer  18  to provide the desired location-time response.  
         [0029]    The software of the computer  18  coordinates the timing, recording, and correlation of temperature measurements with position and time.  
         [0030]    In FIG. 2, an exemplary bondhead  32  of a wirebonding machine  12  is depicted as including a bonding arm  34  with a terminal capillary holder  36 . The capillary holder  36  includes a channel  38  through which a wire-dispensing capillary normally passes, dispensing wire for bonding a semiconductor die to a leadframe. As shown in the modification of FIG. 2, an optic temperature sensor  42  with attached fiber-optic lead  22  is mounted in a sensor mount  44  of the invention, and the sensor mount is, in turn, placed in the channel  38 . Thus, the sensor mount  44  replaces the capillary in this configuration. The sensor  42  has a sensor tip  46 , and an opposite, i.e., signal output end  47  of the fiber-optic lead  22  conducts pulses of light from the thermometer controller  14  to the sensor tip  46 , and conducts the resulting fluorescence in the reverse direction to the thermometer controller for calculation of a temperature.  
         [0031]    The optic sensor  42  has a sensing tip  46  which may comprise a phosphorescent material which, following activation by a pulse of light radiation, emits fluorescent light at a decaying rate dependent upon temperature. For some applications, the phosphor may be applied as a coating to the measured surface, and the emitted light signal detected remotely, i.e., without contact of the sensor with the surface. Temperature measurement sensors and thermometer controllers using the above-described principles are commercially available from Luxtron Corporation, Santa Clara, Calif., for example, as embodied in a Luxtron Model  790  Fluoroptic™ Thermometer. A suitable available diameter of a sensor tip  46  is approximately 25 microns. Thus, temperatures of small areas on a semiconductor die, lead finger, etc. (as small as about 25 microns) may be accurately determined in about 500 milliseconds using such a sensor tip  46 .  
         [0032]    As shown in FIG. 2, the temperatures of the active surface  40  of a semiconductor device  20  may be rapidly determined at each of an array of closely spaced measurement locations  48 . These locations  48  may be on the semiconductor device surface  40 , leadframe surface, or other surface. The apparatus may be used for measuring temperatures of both inner leads and outer leads of a semiconductor device. As is known in the art, the stage or platform, not shown, upon which device  20  is mounted, may be moved, i.e., “rastered” along small directional increments in both an X-axis  52  and a Y-axis  54 , controlled by a stage or platform movement program within the wirebond machine  12  or in computer  18 . In addition, the stage or the bonding arm  34  may be moved in a vertical Z-axis  55  to control the proximity of the sensor tip  46  to the surface  40  being measured. Temperature measurements may be obtained in rapid succession at the desired locations and times, enabling creation of thermal maps indicating surface temperatures as a function of location and/or time. If desired, the temperatures prior to the first measurement, e.g., a maximum initial temperature, may be estimated by rearward extrapolation of a subsequently-measured time-temperature curve.  
         [0033]    Referring to drawing FIG. 3, shown is a sensor mount  44  of the invention, placed in a channel  38  of a capillary holder  36 , the latter shown with an annular cross-section. The sensor mount  44  comprises a series of tubing members  56 ,  58 ,  60 ,  62  and  64  which are concentrically, coaxially fitted together to form a rigid mount through which a fiber-optic lead or cable  22  passes. The mount  44  is shown as including an outer housing  56  into which an upper housing  58  and a lower housing  60  are fitted. The lower housing  60  is shown with a bend  50  having an angle  66  of about 15 degrees to about 60 degrees (15° to 60°). Thus, the outer housing  56  will be at an angle  66  with respect to the axis  68  of the sensor  42 . An upper cable support  62  is fitted into the upper end of the upper housing  58 , and a lower sensor support  64  is fitted into the lower end of the lower housing  60  before it is bent. The fiber-optic cable or lead  22  fits within the sensor mount  44 . The sensor  42  may be press-fitted or cemented in the lower sensor support  64  to prevent any movement therein. It is important that the sensor  42  is uniformly positioned in the capillary holder  36  for uniformly precise contact with, or distance from, the surface  40  whose temperature is to be measured. Thus, the distance  72  between the center of the bonding arm  34  and the sensor tip  46  is maintained at a constant value.  
         [0034]    An exemplary sensor mount  44  may be formed using the following elements for an optical sensor of approximately 0.026 inch diameter:  
         [0035]    An Outer Housing  56 : approximately 1.50 inches long stainless steel tubing, having an outside diameter equal to approximately 0.109 inch, an inside diameter equal to approximately 0.085 inch.  
         [0036]    An Upper Housing  58 : approximately 1.80 inches long stainless steel tubing, having an outside diameter equal to approximately 0.083 inch and an inside diameter equal to approximately 0.063 inch.  
         [0037]    A Lower Housing  60 : approximately 0.80 inch long stainless steel tubing, having an outside diameter equal to approximately 0.083 inch and an inside diameter equal to approximately 0.063 inch.  
         [0038]    An Upper Cable Support  62 : approximately 0.25 inch long stainless steel tubing, having an outside diameter equal to approximately 0.065 inch reduced to 0.061 inch and an inside diameter equal to approximately 0.047 inch.  
         [0039]    A Lower Sensor Support  64 : approximately 0.65 inch long stainless steel tubing, having an outside diameter equal to approximately 0.063 inch and an inside diameter of approximately 0.023 inch drilled out to a 0.026 inch diameter.  
         [0040]    The sensor  42  with fiber-optic cable  22  is strung through the upper cable support  62 , upper housing  58 , outer housing  56 , lower housing  60 , and lower sensor support  64 . The sensor  42  is fixed within the lower sensor support  64  so that the arm-to-sensor tip distance  72  conforms to that programmed into the wirebonder software. A useful distance  72  for a particular wirebond machine may be 0.36 inch.  
         [0041]    Following assembly of the lower sensor support  64  within the lower housing  60 , both are bent at a bend radius of, e.g., 0.25 inch. The upper housing  58  and lower housing  60  (containing a part of the lower sensor support  64  and the fiber-optic cable  22 ) are partially inserted and fitted within the outer housing  56 . The upper cable support  62  is fixed in the upper housing  58 .  
         [0042]    The completed sensor mount  44  is inserted in the capillary channel  38  or into another channel, not shown, in an extrinsic or intrinsic part of the “bonding” arm  34 , maintaining the desired arm-to-sensor tip distance  72 . The sensor mount  44  may be permanently bonded to a capillary holder  36  which is removable from the bonding arm  34 . Sensors  42  of other sizes or types are easily interchanged. For example, specific sensors are available for contact and non-contact applications.  
         [0043]    While sensors  42  having tips  46  having a diameter of approximately 25 microns are available, sensors of any other suitable sizes and types may be used, generally requiring a modification in the tubing sizes used to form the sensor mount  44 .  
         [0044]    Materials other than stainless steel may be used, and, of course, members of other dimensions may be used, depending upon the dimensions of the fiber-optic sensor  42  and capillary channel  38 .  
         [0045]    Referring to drawing FIG. 4, shown is a trigger box circuit  70  by which a signal for initiating a temperature measurement is generated and transmitted to the thermometer controller  14 .  
         [0046]    The trigger box circuit  70  includes a primary circuit  74  activated by the voltage V out    76  across the wirebonder ultrasonic generator  78 . The primary circuit  74  includes a voltage source  80  which applies a constant voltage  82  across a series-connected rectifier diode  84  and an NPN bipolar transistor  86 . The positive output terminal  88  of the wirebonder ultrasonic generator  78  is connected to the base  92  of the NPN bipolar transistor  86 , and the negative output terminal  90  is connected to the collector  94  of the transistor  86 . Thus, a voltage signal  76  from the ultrasonic generator  78  results in a significant current gain or amplification.  
         [0047]    The primary circuit  74  includes resistors  96  and  98  to control the circuit current.  
         [0048]    A secondary circuit  100  includes a transistor  102  comprising a bilateral trigger diode having a grounded collector  101 . The transistor  102  is triggered by current flow through the rectifier diode  84  to provide a voltage signal to the computer  18 , and thence to the thermometer controller  14 . The secondary circuit  100  is preferably connected to a computer  18  by a PC Game Control Adapter Port  104 , controlled by an IBM standard PC Game Port Card and providing a ground lead  106 , a constant voltage lead  108  having a series resistor  112 , and a trigger lead  110 . The transistor  102  and rectifier diode  84  together comprise a trigger  77 .  
         [0049]    In one example of a trigger box circuit  70  of the invention, the following specifications may be used:  
         [0050]    transistor  86 : N2221  
         [0051]    transistor  102 : 4N2G  
         [0052]    rectifier diode  84 : any suitable type  
         [0053]    resistor  96 : 200 ohms  
         [0054]    resistor  98 : 2000 ohms  
         [0055]    resistor  112 : 5100 ohms  
         [0056]    voltage source  80 : +6.0 volts  
         [0057]    constant voltage lead  108 : +5.0 volts  
         [0058]    When making temperature measurements without bonding wires, the ultrasonic generator is disconnected from its ultrasonic generator controller  78 , and the voltage output  76  is used only to trigger the pulse of light in the thermometer controller  14  for temperature measurement.  
         [0059]    As described herein, the temperature measuring/plotting apparatus may be used to plot temperatures on thermal maps of various configurations, as determined by the programs in the computer  18 , wirebond machine  12  and/or thermometer controller  14 . Thus, temperatures may be presented as time functions or location functions, or both, on a “map” or other numerical or graphical display format. For example, a series of thermal maps, each representing a different time interval from a given event, may be prepared to depict isothermal lines on the measured surface. Such will be useful in research and development studies related to semiconductor device manufacturing.  
         [0060]    Use of a temperature measuring apparatus whose accuracy is not dependent upon surface characteristics is of great advantage, eliminating the repeated calibrations otherwise required.  
         [0061]    Exemplary signal transmission cables and connections are indicated as connecting the major elements of the invention. However, other signal transmission apparatus may be used, including wireless infrared transmission, for example.  
         [0062]    While the present method and apparatus have been described with respect to the modification of a conventional wire bonding apparatus, any suitable apparatus may be used which can provide the necessary parameters for the operation and control of the temperature measurement method and apparatus.  
         [0063]    It may be evident to persons skilled in the art that various changes and modifications may be made in the temperature measuring method and apparatus of the invention as disclosed herein without departing from the spirit and scope of the invention as defined in the following claims.