Patent Publication Number: US-7218125-B2

Title: Apparatus and method for performing a four-point voltage measurement for an integrated circuit

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to integrated circuit testing and specifically to performing a four-point voltage measurement on an integrated circuit. 
   BACKGROUND OF THE INVENTION 
   Disk drives are widely accepted as a cost effective data storage system for a computer, music player or other data processing devices. As shown in  FIG. 1 , a disk drive system  10  comprises a magnetic recording medium, in the form of a disk or platter  12  having a hub  13  and a magnetic read/write transducer  14 , commonly referred to as a read/write head. The read/write head  14  is attached to or formed integrally with a suspension arm  15  suspended over the disk  12  and affixed to a rotary actuator arm  16 . A structural arm  18 , supported by a platform  20 , is pivotably connected to the actuator arm  16  at a pivot joint  22 . A voice coil motor  24  drives the actuator arm  16  to position the head  14  over a selected position on the disk  12 . 
   As a spindle motor (not shown) rotates the disk  12 , the moving air generated by the rotating disk in conjunction with the physical features of the suspension arm  15  lifts the read/write head  14  away from the platter  12 , allowing the head  14  to glide or fly on a cushion of air slightly above an upper surface of the disk  12 . The flying height of the read/write head  14  over the disk surface is typically less than a micron. 
   An arm electronics module  30  may include circuits that switch the head function between read and write operations, a write driver for supplying write current to the head  14  during write operations and an amplifier for amplifying the read signal. The arm electronics module  30  is connected to the head  14  by flexible conductive leads  32   
   The configuration and components of the electronics module  30  vary according to the disk drive design as will be understood by persons familiar with such technology. Although the module  30  may be mounted anywhere in the disk drive  10 , a location proximate the head  14  minimizes signal losses and induced noise in the head signals during a read operation. A preferred mounting location for the module  30  comprises a side surface of the structural arm  18  as shown in  FIG. 1 . 
   The disk  12 , shown in a partial cross-sectional schematic view in  FIG. 2  comprises a substrate  50  and a thin film  52 , disposed thereover. The magnetic transducer or head  14  comprises a write head  14 A for writing data bits to the disk  12  by altering magnetic domains of ferromagnetic material in the film  52 , thereby creating magnetic transitions in the magnetic domains. A read head  14 B reads the magnetic transitions to determine the stored data bit. 
   Data bits and timing information to be written to the disk  12  are supplied by a data processing device  60  (e.g. a computer or music player) in the form of bipolar data pulses in PECL (positive emitter-coupled logic) form. Typically, the PECL bipolar signals representing a logic one and a logic zero differ by about 200 mV or by about 450 mV. The data and timing pulses are supplied to a data write circuit  62  where the data bits are formatted and error detection/correction information appended thereto. 
   To write data bits, the voice coil motor  18  moves the suspension arm  16  to a desired radial position above the surface of the disk  12  while the spindle motor rotates the disk  12  to move a circumferential region to be written under the write head  14 A. A write driver  66 A of a preamplifier  66  (in one embodiment the preamplifier  66  is disposed within the electronics module  30 ) supplies a write current (in certain applications between about 10 mA and 70 mA) to the write head  14 A responsive to the signal from the data write circuit  62 . The write driver  66 A scales up the relatively low voltage levels representing the data bits to a voltage range between about +/−6V and +/−10V. In one embodiment the write driver  66 A converts the PECL signals to logic signals having a larger differential voltage, such as about 3.3V. The write driver  66 A also shapes the write current signal waveform to optimize the data writing process. 
   Write current supplied by the write driver  66 A to the write head  14 A (magnetically coupled to a magnetically permeable core not shown) creates a magnetic field that extends from the core across an air gap between the write head  14 A and the disk  12 . The magnetic field alters a region of ferromagnetic domains in the thin film  52  to store the data bits as magnetic transitions. 
   The direction of the magnetic field generated by the write head  14 A, and thus the direction of the altered ferromagnetic domains, is responsive to the direction of current flow through the write head  14 A. Current supplied in a first direction through the write head  14 A aligns the domains in a first direction (representing a date 0 for example) and current supplied in a second direction (representing a data 1 for example) aligns the domains in a second direction. 
   In the read mode, transitions between adjacent domains are detected to determine the stored data bit. The read head  14 B (comprising either a magneto-resistive (MR) sensor or an inductive sensor) senses the magnetic transitions in the thin film  52  to detect the stored data bits. A differential output signal produced by the MR sensor responsive to the magnetic transitions has a higher magnitude and a higher signal-to-noise ratio than an output signal produced by the inductive sensor. The MR sensor is thus preferred, especially when a higher areal data storage density is desired. State-of-the-art MR read heads include giant magnetoresistive (GMR) heads and tunneling magnetoresistive (TMR) heads. 
   The suspension arm  16  moves the head  14  while the disk  12  rotates to position the read head  14 B above a magnetized region to be read. A DC (direct current) bias voltage of between about 0.025V and about 0.3V is supplied to the read head  14 B by a read circuit  66 B of the preamplifier  66 . Magnetic domains in the thin film  52  passing under the read head  14 B alter a resistance of the magneto-resistive material, imposing an AC (alternating current) component on the DC bias voltage. The AC component representing the read data bits has a relatively small magnitude (e.g., several millivolts) with respect to the DC bias voltage. 
   The differential read output signal, having an amplitude in a range of several millivolts, is input to a signal processing stage  102  followed by an output or converter stage  104 . Typically, both the signal processing stage  102  and the output stage  104  are elements of the preamplifier  66 . The signal processing stage  102  amplifies the signal to increase the signal&#39;s signal-to-noise ratio. The output stage  104  scales up the head signal voltage to a peak voltage value in a range of several hundred millivolts, supplying the scaled-up signal to channel circuits of a channel chip  106  through an interconnect  108 . The channel chip  106  detects the read data bits from the voltage pulses, while applying error detection and correction processes to the read data bits. The read data bits are returned to the processing device  60  via a user interface  110  (e.g., SATA, SCSI, SAS, PCMCIA interfaces). 
   In other data storage systems the head  14  operates with different types of storage media (not shown in the Figures) comprising, for example, a rigid magnetic disk, a flexible magnetic disk, magnetic tape and a magneto-optical disk. 
   To increase storage capacity, a disk drive may comprise a plurality of stacked parallel disks  12 . A read/write head is associated with each disk to write data to and read data from a top and bottom surfaces of each disk. 
   Integrated circuits such as the electronics module  30  described above, typically comprise a silicon semiconductor substrate and semiconductor devices, such as transistors, formed from doped regions within the substrate. Interconnect structures are formed in parallel-like layers overlying the semiconductor substrate to provide electrical connection between devices to form electrical circuits. A conventional interconnect system comprises a plurality of substantially vertical conductive vias or plugs and substantially horizontal conductive interconnect runners or traces, with a dielectric layer disposed between two vertically adjacent interconnect layers. The uppermost interconnect layer comprises a plurality of bond pads for receiving conductive bond wires or solder balls to interconnect the devices of the integrated circuit to off-chip external contacts, such as pins or leads of a package structure. 
   The integrated circuits are fabricated by forming regions of different materials, such as doped regions, dielectric regions and conductive regions, in a semiconductor wafer and on an upper surface thereof. Hundreds of such integrated circuits or die are formed on a single wafer by performing a plurality of sequential fabrication process steps (e.g., implanting and diffusing dopants, depositing materials on the upper surface of the wafer, patterning conductive regions) to form multiple die in the wafer, each die comprising active devices, passive devices (e.g., capacitors and resistors) and the interconnect structures. The manufacturing process steps are carefully developed to maximize the number of integrated circuits that satisfy desired operational specifications. 
   After completion of the fabrication steps, each die undergoes electrical testing (referred to as wafer probe testing) so that only those die that satisfy applicable performance specifications are accepted. After the wafer is singulated, the accepted die are packaged to protect the integrated circuit from contamination and damage. Package leads provide electrical connection of the integrated circuit to other electronic components, such as electronic components mounted on a printed circuit board with the packaged integrated circuit. 
   It is necessary for the preamplifier  66  of the electronics module  30  to provide an accurate read head bias (i.e., a DC voltage bias or a DC current bias) to avoid head degradation and improve data reading accuracy. Each preamplifier is evaluated during the wafer probe test to stage to determine whether the bias is within specifications. During the test, the preamplifier is powered through test probes that supply the necessary externally generated power supply voltages to certain conductive pads of the preamplifier integrated circuit. The bias voltage generated on-chip is then measured by test probes in contact with other conductive pads. 
   It is known that contact resistance between a wafer probe tip and the associated contact region of a bond pad can cause bias measurement errors. Further, contact resistance can increase as testing progresses among die on a wafer as the probe tips collect dirt from the wafer surface. Increased probe tip resistance can cause substantial errors in the measured value of the bias voltage, possibly resulting in misidentification and rejection of otherwise good preamplifiers—or passing unacceptable preamplifiers as acceptable. 
   A similar bias test is performed for preamplifiers that supply a bias current. The bias current is determined by causing the bias current to flow through a know resistance connected to the wafer test probes and measuring the voltage across the resistance. 
   A prior art head bias wafer probe test circuit  199  depicted in  FIG. 3  is one such test probe circuit sensitive to the effects of wafer probe contact resistance when the bias voltage (or current) is measured. A preamplifier on-chip circuit, represented by a voltage source  200 , generates and supplies a bias voltage to input/output pads HRP and HRN during the wafer probe test. When the preamplifier is operational in the disk drive data storage system  10 , the input/output pads HRP and HRN are connected to the read head  14 B. In the circuit  199 , a load resistor R MR , having a resistance equivalent to a nominal head resistance, is connected across the input/output pads HRP and HRN and a voltage measured across these pads indicates the bias voltage. The wafer probe resistances are modeled in  FIG. 3  by resistors R PAR1  and R PAR2 . A voltage measured by the voltmeter  202  is: 
   
     
       
         
           
             
               
                 
                   V 
                   MEASURED 
                 
                 = 
                 
                   
                     V 
                     BIAS 
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         R 
                         MR 
                       
                       
                         
                           R 
                           MR 
                         
                         + 
                         
                           R 
                           PAR1 
                         
                         + 
                         
                           R 
                           PAR2 
                         
                       
                     
                     ) 
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   When probing many die across the wafer the probe tips become dirty and the contact resistances R PAR1  and R PAR2  increase. As can be seen from equation (1), the increased resistance lower the measured voltage relative to the bias voltage. When the disparity between the measured voltage and the bias voltage is so great as to cause the measured voltage to fall below a minimum acceptable bias voltage value, the preamplifier may be rejected as unacceptable. Thus the probe tip contact resistance can adversely influence yield determinations by identifying as faulty devices that generate a correct bias voltage. 
   The error in the measured voltage is the difference between the actual bias voltage V BIAS  and the measured voltage V MEASURED : 
   
     
       
         
           
             Error 
             ⁢ 
             
                 
             
             [ 
             % 
             ] 
           
           = 
           
             
               ( 
               
                 
                   
                     R 
                     MR 
                   
                   
                     
                       R 
                       MR 
                     
                     + 
                     
                       R 
                       PAR1 
                     
                     + 
                     
                       R 
                       PAR2 
                     
                   
                 
                 - 
                 1 
               
               ) 
             
             * 
             100 
             ⁢ 
             % 
           
         
       
     
   
   For a nominal value of R MR =40Ω (for a GMR read head) and expected values of 0.50Ω for each of R PAR1  and R PAR2 , the error is: 
             Error   ⁢           [   %   ]     =         (         R   MR         R   MR     +     R   PAR1     +     R   PAR2         -   1     )     *   100   ⁢   %     =         (       40     40   +   0.50   +   0.50       -   1     )     *   100   ⁢   %     =     2.4   ⁢   %               
It is known that as the probes become dirty during testing the contact resistance can typically increase to a few ohms, thereby increasing the percentage measurement error.
 
   Current methods for detecting an unacceptably high probe contact resistance include monitoring the measured bias voltage to identify declining voltage values and monitoring the results of any tests that are sensitive to probe contact resistance, such as the bias measurement test of  FIG. 3 . After an unusually large number of integrated circuits are identified as faulty the test sequence may be paused while the probe tips are cleaned (to reduce the contact resistance) and devices previously identified as faulty are retested. The retesting process wastes time and adds cost to the wafer probe tests. Furthermore, some integrated circuits that passed the wafer probe test prior to probe cleaning may have been faulty. 
   The combination of various fabrication and operational anomalies can cause performance parameters of an integrated circuit (such as the bias voltage or current) to vary. These variations may cause the parameter to exceed an acceptable range above or below a nominal value. These variations must be controlled to ensure the performance parameters are within the acceptable range and the testing process must properly identify the devices that are outside the acceptable range. 
   By way of example, the total allowable variation in the bias voltage measurement of a preamplifier may be limited to 7 percent. Factors contributing to this range include fabrication process limitations and operational anomalies that directly affect the bias voltage or current generated on the integrated circuit, and the aforediscussed wafer test anomalies that affect the measured value. The cumulative fabrication and operational anomalies may introduce considerable measurement variability, leaving little head room to accommodate anomalies of the testing process. 
   It is not uncommon to encounter a variation of 1–1.5 percent in a bias reference circuit (e.g., a band-gap voltage reference) that supplies the reference voltage (or current) to the circuit elements that generate the head bias voltage (or current). Operating temperature, semiconductor process variations and mismatches between integrated circuit devices may add another 2–4 percent. Adding variations in wafer probe contact resistance causes the cumulative variation to approach and perhaps exceed the 7 percent limit, especially as the contact resistance increases. Most test processes use an error margin or guard band of about 1 percent lowering the acceptable margin from 7 percent to 6 percent, further increasing the probability that the cumulative error specification will not be satisfied. 
   If the cumulative effect of the several factors contributing to test circuit error exceed a specified limit, e.g., 7 percent, there will be an increase in the number of die that fail the probe test because of error associated with the test circuitry and the measurement process. The number of failed integrated circuits therefore increases. While it is important to reduce the contribution of each anomaly to the cumulative measurement error, such reductions may be costly. For example, reducing the effects of fabrication anomalies may require the development of new process steps or the purchase of new process equipment that permits tighter control over the fabrication process and reduces variations in measurable parameters, such as voltages. Clearly there is a need to reduce the effects of probe contact resistance to minimize error associated with bias voltage measurement. 
   BRIEF SUMMARY OF THE INVENTION 
   According to one embodiment, the invention comprises a test apparatus for measuring a signal generated on an integrated circuit by a source during a testing mode, wherein the signal is supplied to first and second bond pads of the integrated circuit, and wherein during an operational mode an operational component is connected to the first and second bond pads. The test apparatus comprises first and second conductive elements for connecting the first and the second bond pads to a resistive element having a value related to a value of the operational component, a measuring device and third and fourth conductive elements for connecting the measuring device to third and fourth integrated circuit bond pads responsive to the source. 
   According to another embodiment, the present invention comprises a method for measuring a signal generated by a source disposed on an integrated circuit during a test, wherein the signal is supplied to first and second bond pads of the integrated circuit, and wherein during an operating mode an operational component is connected across the first and second bond pads. The method comprises connecting a resistive element having a value related to a value of the operational component across the first and the second bond pads and connecting a measuring device to third and fourth integrated circuit bond pads responsive to the source to determine a measured signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be more easily understood and the advantages and uses thereof more readily apparent when the following detailed description is read in conjunction with the figures wherein: 
       FIG. 1  illustrates a prior art disk drive to which the teachings of the present invention can be applied. 
       FIG. 2  is a partial cross-sectional schematic view of a prior art head and related components of the disk drive of  FIG. 1 . 
       FIG. 3  illustrates a prior art head bias wafer probe test circuit. 
       FIG. 4  illustrates a prior art four-point wafer probe test circuit. 
       FIGS. 5–9  illustrates various embodiments of a wafer probe test circuit according to the teachings of the present invention. 
   

   In accordance with common practice, the various described device features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Like reference characters denote like elements throughout the figures and text. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Before describing in detail an exemplary method and apparatus, it should be observed that the present invention resides primarily in a novel and non-obvious combination of elements and process steps. So as not to obscure the disclosure with details that will be readily apparent to those skilled in the art, certain conventional elements and steps have been presented with lesser detail, while the drawings and the specification describe in greater detail other elements and steps pertinent to understanding the invention. 
   The following embodiments are not intended to define limits as to the structure or method of the invention, but only to provide exemplary constructions. The illustrated embodiments are permissive rather than mandatory and are not exhaustive. Use of the present invention is not limited to a disk drive storage system and the invention can be used in applications other than those described herein. 
   Four-point probe tests are used for measuring the resistivity (i.e., sheet resistance) of semiconductor regions in order to eliminate errors caused by wafer probe contact resistance. Components of a four-point probe test circuit, depicted in  FIG. 4  include four collinear tungsten probes  220 ,  222 ,  224  and  226  in contact with a sample  230  under test. Current I supplied by a current source  234  flows between the outer probes  220  and  226  and through the sample  230 . A voltage V is measured across the two inner probes  222  and  224  by a voltmeter  236 . Ideally, the voltmeter  236  draws an insubstantial current so as not to jeopardize the accuracy of the results. If the sample  230  is assumed to be of semi-infinite volume and the interprobe spacings are equal, i.e., s 1 =s 2 =s 3 =s, then it can be shown that the resistivity of the sample  230  is given by  =2πsV/I, where V is the measured voltage and I is the supplied current. 
   According to the invention, a four-point probe assembly measures the read head bias voltage, while limiting the effects of probe contact resistance on the measured value. The assembly can also measure other on-chip voltages that are sensitive to the wafer probe contact resistance, such as an output voltage of an analog-to-digital converter or an on-chip band gap reference voltage (a master reference voltage that controls other voltages generated on-chip). 
   In one embodiment of the invention, bond pads used for other purposes during operation are employed during the wafer probe testing process to measure the bias voltage. During the test, these pads, including either or both output pads and input pads, are disconnected from their associated on-chip output or input circuits and connected to a four-point measurement circuit through controlled multiplexers. In addition to measuring the head bias voltage, the wafer probe contact resistance can be determined. 
   An embodiment of a wafer probe test circuit  250  of the present invention is illustrated in  FIG. 5 . The test circuit  250  comprises four input/output pads designated HRP, HRN, HRP SENSE  and HRN SENSE . During the test, a resistance R MR , representing a nominal head resistance, is connected across the terminals or pads HRP and HRN through test probes  254  and  256 . A contact resistance of the test probes  254  and  256  is identified by resistances R PAR1  and R PAR2 . The voltmeter  202  is connected across the terminals or pads HRP SENSE  and HRN SENSE  through test probes  258  and  260 , each having a respective contact resistance R PAR3  and R PAR4 . The input impedance of the voltmeter  202  (Zin METER ) is very large compared to the contact resistance of the test probes  258  and  260  and is therefore assumed to be infinite, i.e., no current is drawn by the voltmeter  202 . The measurement for the head bias, i.e., the voltage measured across HRP SENSE  and HRN SENSE  in the wafer probe test circuit  250  is: 
             V   MEASURED     =       V   BIAS     ⁡     (       Zin   METER         Zin   METER     +     R   PAR3     +     R   PAR4         )             
Using a nominal probe contact resistance of 0.5Ω, the associated error is:
 
             Error   ⁢           [   %   ]     =         (         Zin   METER         Zin   METER     +     R   PAR3     +     R   PAR4         -   1     )     *   100   ⁢   %     =       (         10   6         10   6     +     10   3     +   0.50       -   1     )     *   100   ⁢   %             
The error approaches zero as Zin METER  becomes very large relative to the contact resistances. Thus the measured voltage for the test circuit  250  approaches the bias voltage irrespective of the nominal contact resistance values. By way of example, when Zin METER  has a one megaohm internal impedance, assuming each probe has a contact resistance of 0.5 ohm, the error between the measured voltage and the actual bias voltage is less than 1 percent.
 
   After determining V MEASURED  as above, the voltmeter  202  may be used to measure a voltage across the load resistor R MR  (having a known resistance) and a measured voltage V MR  is compared to the measured voltage V MEASURED  to calculate the total contact resistance of the test probes  254  and  256  from: 
   
     
       
         
           
             
               
                 
                   
                     R 
                     PAR1 
                   
                   + 
                   
                     R 
                     PAR2 
                   
                 
                 = 
                 
                   
                     ( 
                     
                       
                         
                           V 
                           MEASURED 
                         
                         
                           V 
                           MR 
                         
                       
                       - 
                       1 
                     
                     ) 
                   
                   * 
                   
                     R 
                     MR 
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   The wafer probe test circuit  250  can be used to more accurately determine an on-chip voltage (such as the read head bias voltage supplied by a preamplifier within the electronics module  30  of  FIG. 1 ) than the prior art wafer probe test circuit  199  depicted in  FIG. 3 . However, since each test probe  254 ,  256 ,  258  and  260  must contact a bond pad to measure the bias voltage, the  FIG. 5  embodiment requires two additional bond pads, i.e., HRP SENSE  and HRN SENSE  as the bond pads HRP and HRN are conventionally fabricated for connecting the read head  14 B to the integrated circuit for operation in the disk drive data storage system  10 . As is known, provision of additional bond pads is often undesirable as each pad occupies a significant die area and can be costly to fabricate. 
   According to another embodiment, one or more bond pads are multiplexed between functioning as the bond pads HRP SENSE  and HRN SENSE  during wafer probe testing and operating normally with the associated circuitry during integrated circuit operation. An on-chip multiplexing device disconnects the bond pads from their operational circuits for connection to the wafer probe test circuit and reconnects the bond pads to their operational circuits during operation. 
   Each integrated circuit may include several bond pads suitable for multiplexed operation and therefore suitable for use as the bond pads HRP SENSE  and HRN SENSE . Certain on-chip fault conditions and off-chip fault conditions detected on-chip generate fault signals Vin 1  and Vin 2  that are supplied to output drivers  301  and  302  (see  FIG. 6 ) for generating output signals Vout 1  and Vout 2  suitable for driving off-chip fault indicating elements via respective bond pads  308  and  310 . Exemplary fault conditions that generate a fault signal Vin 1  or Vin 2  include on-chip detection of a low voltage level supplied from an off-chip source, detection of an open write head and detection of an over-temperature condition on the integrated circuit. As illustrated in  FIG. 6 , an enable signal controls each of the output drivers  301  and  302 . 
   To measure the on-chip bias voltage represented by the voltage source  200 , the resistor R MR  representing the read head is connected to the output pads HRP and HRN through test probes  254  and  256 . The bias voltage is connected directly to the output pads HRP and HRN and to the bond pads  308  and  310  by dosing (enabling) the transmission gates  314  and  316  in response to a signal VsenseEnable applied to a control terminal G of each transmission gate  314  and  316 . 
   An appropriate enable signal is supplied to the output drivers  301  and  302  to disable the drivers such that they present a high output impedance at the bond pads  308  and  310 . The voltmeters  302 A and  302 B measure the voltages V MEASURED1  and V MEASURED2 , respectively, with respect to ground, at the bond pads  308  and  310  through the test probes  258  and  260 . 
   An “on” resistance of the transmission gates  314  and  315 , designated R ON  in the equations below, is preferably small compared with an input impedance of the voltmeters  302 A and  302 B. In an embodiment where the input impedance of each voltmeter  302  is 1 MΩ, and the value of R ON  is assumed to be 1 kΩ, the error between the bias voltage  200  and the single ended voltage measurements with respect to ground is: 
   
     
       
         
           
             Error 
             ⁢ 
             
                 
             
             [ 
             % 
             ] 
           
           = 
           
             
               
                 ( 
                 
                   
                     
                       Zin 
                       METER 
                     
                     
                       
                         Zin 
                         METER 
                       
                       + 
                       
                         R 
                         ON 
                       
                       + 
                       
                         R 
                         PAR3 
                       
                     
                   
                   - 
                   1 
                 
                 ) 
               
               * 
               100 
               ⁢ 
               % 
             
             = 
             
               
                 
                   ( 
                   
                     
                       
                         10 
                         6 
                       
                       
                         
                           10 
                           6 
                         
                         + 
                         
                           10 
                           3 
                         
                         + 
                         0.50 
                       
                     
                     - 
                     1 
                   
                   ) 
                 
                 * 
                 100 
                 ⁢ 
                 % 
               
               ≈ 
               
                 
                   - 
                   0.1 
                 
                 ⁢ 
                 % 
               
             
           
         
       
     
   
   An error of 0.1% may be safely ignored and does not significantly affect the measurement nor substantially contribute to the 6 percent or 7 percent margin. The measurements conducted by the wafer probe test circuit  300  of  FIG. 6  are thus substantially insensitive to the wafer probe contact resistance R PAR3  and R PAR4  and the transmission gate resistance R ON . Preferably, the output drivers  301  and  302  exhibit a high output impedance in an off or disabled state to minimize effects on the wafer probe test circuit  300 . 
   The two single ended measured voltages may be summed to obtain the measured bias voltage:
 
 V   BIAS   ≈V   MEASURED   =V   MEASURED1   +V   MEASURED2   (3)
 
   If needed, the total contact resistance R PAR1 +R PAR2  can be calculated according to equation (2) above. 
   In another embodiment bond pads other than pads  308  and  310  and their associated drivers  301  and  302  can be used for the wafer probe test if the on-chip devices connected to the bond pads can be controlled to an off state and if the devices exhibit a relatively high output impedance when in the off state. 
   In the  FIG. 6  embodiment the transmission gates  314  and  316  are depicted as NMOSFET devices with the control terminal comprising a gate terminal. In other embodiments the transmission gates comprise PMOSFET devices responsive to an opposite-polarity control signal or a complementary pair of a parallel PMOSFET and an NMOSFET device controlled by a VsenseEnable signal and its complement signal. 
   In an alternative embodiment illustrated in  FIG. 7 , a wafer probe test circuit  340  replaces the two single-ended voltage measurements V MEASURED1  and V MEASURED2  of  FIG. 6  with a single voltage measurement V MEASURED  across the bond pads  308  and  310 . In this embodiment,
 
V BIAS ≈V MEASURED .
 
   A wafer probe test circuit  360  illustrated in  FIG. 8  utilizes a single multiplexed bond pad  362  to determine the bias voltage. Single-ended (i.e., measured with respect to ground) bias voltages V BIAS + and V BIAS − are connected to the bond pad  362  by disabling an output driver  364  and alternately enabling one of the transmission gates  368  and  370  and disabling the other by applying appropriate VsenseEnable1 and VsenseEnable2 signals to control terminals G. The two resulting single-ended voltage measurements determined by the voltmeter  302  are combined according to equation (3) above to determine the bias voltage supplied by the voltage source  200 . 
   Another embodiment of a wafer probe test circuit  400  of  FIG. 9  may be suitable when an output impedance of output drivers  402  and  404  is not sufficiently large to allow the impedance to be neglected determining the bias voltage measurement. To remove the effects of the output impedance, transmission gates  410  and  412  are disposed in series between output terminals  422  and  424  of the output drivers  402  and  404  and the bond pads  308  and  310 . 
   To measure the bias voltage, the VsenseEnable signals drive the transmission gates  314  and  316  to a closed state, supplying the bias voltage from the voltage source  200  to the bond pads  308  and  310 . The transmission gates  410  and  412  are driven to an open state by applying a complementary signal &lt;VsenseEnable&gt; at a gate G of each transmission gate  410  and  412 , creating an open circuit between the bond pads  308  and  310  and the respective output terminals  422  and  424 . The voltmeters  302 A and  302 B measure the voltages V MEASURED1  and V MEASURED2  with respect to ground through the test probes  258  and  260 . The sum of the two measured voltages equals the bias voltage. 
   To operate the chip (during operation the wafer probe test circuit  400  is absent), the output drivers  402  and  404  are enabled by an appropriate enable signal applied thereto, the VsenseEnable signals drive the transmission gates  314  and  316  to an open state and the complementary signals &lt;VsenseEnable&gt; drive the transmission gates  410  and  412  to a closed state. The output terminals  422  and  424  of the drivers  402  and  404  are thereby connected to the bond pads  308  and  310 . 
   In yet another embodiment, the enable signal controlling the output drivers  402  and  404  may not be required as use of the transmission gates  410  and  412  may be sufficient to limit the effects of the output drivers  402  and  404  on the testing process. 
   According to the present invention, a method and apparatus are taught for removing the effects of wafer test probe contact resistance while measuring an on-chip voltage or current, such as an MR head bias voltage. The method and apparatus teach a four-point voltage or current measurement method and apparatus using existing integrated circuit bond pads, i.e., without the need to add bond pads to the integrated circuit as it is known such pads can be costly in terms of both fabrication costs and consumed circuit area. 
   A method and apparatus according to the present invention can also detect onset of an excessive probe contact resistance, at which time the probes can be cleaned, instead of waiting for multiple die failures to suggest that probe cleaning may be warranted and retesting failed die after probe cleaning. Further, once the contact resistance is known, the voltage or current measurements can be mathematically corrected to remove the contact resistance effects. The testing can continue, even with a high probe contact resistance. This process can reduce the probe cleaning frequency and the wafer test time and cost. 
   The teachings of the present invention can be applied to measuring other on-chip voltages and currents by including multiple controlled multiplexers on-chip and controlling the multiplexers to present the desired voltage for measuring across the bond pads. 
   Although the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for the elements thereof without departing from the scope of the invention. The scope of the present invention further includes any combination of elements from the various embodiments set forth herein. In addition, modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its essential scope. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.