Abstract:
An apparatus for monitoring and recording multiple gate dielectric leakage currents during a reliability characterization test. The large number of devices tested allows for longer testing at lower voltages thereby minimizing the need to rely on mathematical models. Solid-state multiplexers (MUX) at multiple levels of test apparatus assembly eliminate excess wiring and allow for constant scan monitoring of the devices under test (DUT) without concern for wearout of electromechanical switches. Data resolution is enhanced with fewer data readings per fixed time period during quiescent periods and multiple readings when the leakage current is changing more rapidly. Thus the resolution of the data is maximized during the critical phase of dielectric breakdown allowing for more precise characterizations of gate dielectrics without the need to increase data storage.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to the field of reliability testing of integrated circuits. More particularly, the invention relates to an apparatus and method for reliability stressing of thin gate and node dielectrics for low voltage technologies. 
     Semiconductor manufacturers who make integrated circuit chips begin by manufacturing semiconductor wafers. Each wafer is typically 100 mm, 125 mm, 150 mm, 200 mm or 300 mm in diameter and contains anywhere from one to several thousand chips or die on the wafer. When manufacture of the wafer is completed, chips or die are cut or “diced” from the wafer and may later be mounted into single chip or multiple chip packages for implementation in a printed circuit board or other applications. 
     When manufacture of a wafer is completed, it is customary practice to test each chip on the wafer to determine whether each chip, as manufactured, electrically matches design criteria, matches performance criteria of the system in which the chip is to be implemented, and will be reliable in operation. 
     Logic circuit manufacturers routinely perform both logic and parametric tests on their products. To test the “logic” of a circuit, a typical tester stimulates various terminals of the circuit with input logic signals while monitoring various output logic signals produced by the tester in response to the input logic signal stimulus to determine if the output signals exhibit expected logic patterns. A “parametric” test measures analog characteristics of the circuit at its terminals. One of the most important parametric characteristics of a logic circuit is its leakage current, the amount of current an input terminal of a logic circuit sources or sinks when the terminal is driven to a high or low logic level. Leakage current is typically measured by connecting the terminal to a voltage source through a precision resistor and measuring the voltage drop across the resistor. Multiple devices are tested in parallel. The voltage drop is proportional to the leakage current. The possibility of excessive leakage creates difficulty in selecting an appropriate resistor value. A small value resistor potentially allows excessive current drain under device short circuit conditions and causes too small of a voltage drop for precise characterization for normal leakage readings. Conversely, a large resistor value will cause in a large voltage drop resulting in excessive impact on the stress voltages being applied to the device under test (DUT). 
     Reliability “logic” testing is used to screen out chips having an undesirably short life span. Typically, a significant percentage of a group of chips will fail early in their lifetime due to marginal conditions during manufacture. Subsequently, a very low percentage of the group will fail during an extended period of use of the chips. Reliability screening of semiconductor chips is typically performed by a process of supplying test signal patterns to chips under test to repeatedly stimulate all devices and wires on a chip, and is typically performed at elevated temperatures and/or voltages to simulate the first six months of operation. Therefore, the screening procedure is known as burn-in. 
     Reliability engineering “parametric” testing of integrated circuits is used to predict product lifetimes and calculate defect densities over the life of the component. In particular, the characterization of gate dielectric leakage currents provides reliability engineers with information critical to life cycle predictions. This process of characterization has historically been time consuming and expensive for semiconductor manufacturers. To accomplish this in a short time, it is necessary to subject product samples to environmental conditions that accelerate the failure modes that cause wearout. 
     Typical methods for creating these stress conditions include the use of temperature and voltage such that the temperature stress used for testing exceeds the operating temperature stress when the circuit is in actual use, and such that the stress voltage used for testing exceeds the operating voltage when the circuit is in actual use. 
     Voltage acceleration has been used as the primary factor for gate and node dielectric stressing throughout CMOS history. Mathematical models were then applied in order to project a characterization of the dielectric from stressed testing conditions. Typically, gate dielectrics are subjected to stress voltage levels for a period of time. The voltage is then lowered to the normal operating levels and the leakage current measured. This is referred to as stress induced leakage current (SILC) testing. SILC testing is often performed at additional voltages close to nominal operating levels for more robust characterizations. 
     Many manufacturers have attempted to address low throughput of burn-in and characterization processes by creating burn-in boards onto which many diced chips are placed in chip packages, and then the packages go into sockets on the burn-in boards. Thereafter, each chip on the test board is simultaneously exercised. Thus, many chips are burned-in at once. These systems effectively reduce the time required to burn-in a large volume of chips. 
     However, the increased number of diced chips also increases the physical complexity of taking data. In the case of gate dielectric leakage current characterization, each device requires a read back of voltage and current which is two pairs of wires for every device on the board. These wires multiply quickly to create fragile connections and unwieldy boards. 
     For the forgoing reasons, there is a need for an apparatus and simple method of measuring the leakage current of a large number of gate dielectrics over an extended period of time in a fully automated way. Additionally, there is a need for lowering acceleration of life stress voltages thereby giving longer test times without sacrificing throughput. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus and method for reliability characterization of integrated circuit dice gate dielectrics by monitoring of leakage current. Pursuant to the present invention, provision is made for the continual monitoring of the gate dielectric leakage current on each of a plurality of devices. 
     The teachings of the invention are in three principal areas. The first is an apparatus for monitoring and recording multiple gate dielectric leakage currents during a reliability characterization test. The large number of devices tested allows for longer testing at lower voltages thereby minimizing the need to rely on mathematical models to project dielectric characteristics. The apparatus employs on board solid-state multiplexers (MUX) at multiple levels of test apparatus assembly to eliminate excess wiring and allow for constant scan monitoring of the DUTs without concern for wearout of electro-mechanical switches. The MUXs and DUTs are mounted on circuit cards wherein connections are made by circuit runs on the cards. With appropriate addressing, the MUXs allow for a single output to accommodate all DUT measurements thus eliminating wires from each DUT. 
     The present invention provides a second inventive approach for selectively recording the data resultant from the constant scanning of the leakage currents. In accordance with this aspect of the invention, fewer data readings per fixed time period are stored during quiescent periods and multiple readings are stored when the leakage current is changing more rapidly. Thus the resolution of the data is maximized during the critical phase of dielectric breakdown allowing for more precise characterizations of gate dielectrics without the need to increase data storage. 
     The third inventive facet of the invention is an efficient and expeditious methodology for assuring the accuracy of the readings when switching between voltage stress levels. Constant voltage levels are supplied to the DUTs by circuitry employing operational amplifiers (op-amps) commonly known to those of ordinary skill in the art. The output of an op-amp is affected by inherent characteristics that vary with the input voltage level. The most important of these is DC offset voltage. Typically, potentiometers, or other hardware changes, are employed to “null out” the offset. These potentiometers require resetting whenever the input voltage level is changed (which changes the common mode voltage at the inputs) as is required whenever the test voltage is changed from the stress level to the, possibly multiple, SILC level(s). This is both impracticable and prohibitive. The inventive methodology stores the offset values at each voltage input level required for each DUT channel. The unique combination of voltage level and DUT channel provide an entry into an error correction matrix where these error correction factors are stored. These values are then applied as correction factors to the leakage value calculated and stored. No operator intervention is required between readings and accuracy is maintained at all voltage levels. 
     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the features and advantages of the present invention will become better understood with regard to the following detailed description, appended claims, and the accompanying drawings where: 
     FIG. 1 shows a functional block diagram of the apparatus of the present invention; 
     FIG. 2 shows a detailed drawing of the preferred embodiment of the channel monitors of the present invention; 
     FIG. 3 shows a detailed drawing of the preferred embodiment of the module level of the present invention incorporating eight channel monitors; 
     FIG. 4 shows a detailed drawing of the preferred embodiment of the card level of the present invention incorporating three modules; and 
     FIG. 5 shows a detailed drawing of the preferred embodiment of the box level of the present invention incorporating eight cards, a total of 192 DUTs. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to FIG. 1, there is shown an apparatus in accordance with an embodiment of the invention. Generally, the electrical characterization of the semiconductor devices is performed with the chips mounted in carriers and thereafter tested in parallel. A channel  1  monitor  112  mounted on a module  1   114  supplies a constant voltage to DUT  1  (not shown) and is arranged to provide a voltage and current measurement from the DUT. These measurements are provided to module multiplexer (MUX) M 1   116 . Appropriate addressing will cause module MUX M 1   116  to provide specific DUT measurements on a line  120 . This arrangement is replicated through channel monitor n  122 . Module MUX Mn  126  provides measurements from each of its n DUTs on a line  128 . Each voltage and current measurement of each channel monitor is similarly multiplexed at module MUX M 1   116 . Appropriate addressing will cause each of the module level MUXs to provide a specific channel monitor measurement. Card MUX C 1   130  mounted on card C 1   124  is arranged to receive each of the module MUX output signals from all modules  1  through n. Appropriate addressing will cause card MUX C 1   130  to select the signal from a specific module MUX and provide this on a line  132 . The arrangement of modules on card C 1   130  is replicated x times through card Cx  134 . Card MUX Cx  136  mounted on card Cx  124  is arranged to receive the module MUX output signals from all modules  1  through n on card Cx. Appropriate addressing will cause card MUX Cx  136  to select the signal from a specific module MUX and provide this on a line  138 . Cards  1  through x are mounted in box or housing  144  wherein box MUX B  140  is arranged to receive the card MUX signals from each and every card level MUX, MUX C 1  to Cx. Appropriate addressing of card MUX B  140  will select a specific card level MUX signal to provide to a voltmeter (DVM) or equivalent measuring device on a line  142 . Hence addressing of card MUX B causes a selection of a specific card MUX Ci which is providing the signal from a specific module level MUX Mi which is further selecting only one channel monitor measurement. Thus, it will be seen that constant scan monitoring is achieved through a single output wire. 
     In an exemplary embodiment, 192 dice are monitored simultaneously. This allows for greatly increased throughput and, therefore, an ability to characterize the DUTs gate dielectrics by subjecting them to stress voltages for longer periods of time thereby not relying on mathematical models to extrapolate as far. Four levels of assembly, channel monitor, module, card, and box are shown respectively in FIGS. 2 through 5. 
     A channel monitor  200  generally shown in FIG. 2 wherein detailed line matching, filtering, etc. has been eliminated to reveal the essential circuitry. The purpose of the channel monitor is threefold: (1) to supply a constant voltage to a designated DUT, ( 2 ) to report the leakage current drawn by the DUT, and ( 3 ) to report the voltage at which the leakage current is measured. Regulated input voltage is supplied on a line  202  to a non-inverting terminal of a first operational amplifier (op-amp)  204  arranged as a voltage follower. First op-amp  204  provides an output on a line  206  to a non-inverting terminal of a first instrumentation amplifier  208  and also to one of a plurality of current sensitivity resistors  210 ,  212 , and  214 . The current sensitivity resistors are precision components having a tolerance of 0.1% (or otherwise based upon the needed precision of the readings) and having a resistance based upon the expected leakage current. The selected resistor is then connected to the DUT whereby the constant stress voltage is applied and leakage current allowed to flow to the DUT on a line  216 . Line  216  is also connected to an inverting terminal of the first instrumentation amplifier  208  whereby the second instrumentation amplifier detects the voltage drop across the resistor and provides an output on a line  218  which is a voltage representing the leakage current flowing through the resistor. Line  216  is also connected as a feedback resistor to an inverting terminal of the first op-amp  204  by line  216  whereby first op-amp output  206  follows the voltage at the input line  202 . Line  216  is connected to a non-inverting terminal of a second instrumentation amplifier  220 . An inverting terminal of the second instrumentation amplifier  220  is connected to circuit ground by a line  222  so that an output of the second instrumentation amplifier on line  224  reports the voltage at which the leakage current is measured. 
     The module level  300  of the present apparatus is generally shown in FIG.  3 . At the module level eight (8) channel monitors,  200 ,  304  through  316  are mounted and their outputs multiplexed together. Each monitor provides three (3) outputs, as described above. The constant voltage output from channel monitor  0   200  is passed on to its designated DUT on line  216 . The channel  7   316  outputs representing the leakage current and the measuring voltage on lines  218  and  224  respectively are each connected to an input of a 16 to 1 module multiplexer (MUX)  322 . The outputs of the remaining seven (7) modules are connected in a corresponding manner. Module MUX  322  is arranged to receive address signals on lines  324 ,  326 ,  328 , and  330 . The address signals designate which one of the 16 module MUX  322  input signals to provide to a first module MUX output on a line  332 . Therefore, there are nine (9) outputs from the module: eight (8) constant voltage outputs and one (1) module MUX output. 
     The card level  400  of the present apparatus is generally shown in FIG.  4 . Three (3) modules,  300 ,  404 , and  406 , are mounted on each card. The eight (8) constant voltage outputs from each module are passed through for connection to their designated DUTs with constant voltage from channel  0  to module  0  on line  216  shown as an explicit example. The module MUX output from module  0   300  is connected to a card MUX  410  via line  332 . In a like manner, module MUX outputs from module  1   404  and module  2   406  are also connected to the card MUX  410  via lines  408  and  412 , respectively. Card MUX  410  is arranged to receive address lines  414  and  416  that will designate which of the three (3) module MUX outputs is provided to a card MUX output on a line  418 . 
     The box level  500  of the present apparatus is generally shown in FIG.  5 . Eight (8) cards  400 , and  502  through  514 , are in the box  500 . The card MUX output from card  0   400  is connected to a box MUX  516  via line  418 . In a like manner, card MUX outputs from cards  1   502  through card  7   514  are connected to the box MUX  516  via lines  518  through  530  respectively. Box MUX  516  is arranged to receive address lines  532 ,  534 , and  536  that will designate which of the eight (8) card MUX outputs is provided to a box MUX output on a line  538 . Line  538 , in turn, is connected to a voltage measuring device (not shown) for measuring DUT leakage current and measuring voltage levels and thus providing data for characterizing the gate dielectric. 
     Channel selection address lines  324 ,  326 ,  330 , and  328 ; module selection address lines  414  and  416 ; and card selection address lines  534  and  536 ; are all made available to an addressing device such as a computer through an electrical signal connection means such as a PC parallel port connector. Thus a specific signal (either leakage current or measurement voltage) from a particular DUT is made available to the voltage measuring device on line  538  by providing a specific card, module, and channel address. Further, automatic means for addressing is employed to constantly scan all the DUTs with the output of the results from the voltage measuring device recorded within a computer memory. 
     A feature of the invention is embodied within the methodology of recording the resultant data measurements. Greater resolution of the characterization data is achieved by taking more measurements during intervals of larger changes in leakage current. The criterion for recordation of data in the computer memory is either time or magnitude of change in leakage current. Hence, if a scanned leakage current exceeds a predetermined amount, the computer will request recordation of a reading otherwise a reading will be recorded only after passage of a designated time interval. Increasing the designated time interval reduces the amount of data storage required for each device multiplied by the number of devices over the length of the test. For devices that are failing, the computer will be requesting multiple readings as the leakage increases (or decreases) thus providing greater resolution of the dielectric breakdown characteristics during the critical failure phase. 
     A further feature of the present invention is the novel methodology for nulling out the error factors associated with the DC offset voltage. Leakage Testing is typically performed for at least two voltage levels: leakage current at stress level voltage, and stress induced leakage current (SILC) performed at or near operating voltages. Since DC offset voltage varies with op-amp input (common mode) voltage, whenever the test voltage is changed from the stress level to the, possibly multiple; SILC level(s) the nulling of the DC offset must be changed as well. This is both impracticable and prohibitive. The inventive methodology stores the offset values at each voltage input level required for each DUT channel as an error correction factor. The unique combination of voltage level and DUT channel provide an entry into an error correction matrix where these error correction factors are stored. These values are then applied as correction factors to the leakage value calculated and stored. No operator intervention is required between readings. 
     The present invention can be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. The present invention can also be embodied in the form of computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. The present invention can also be embodied in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by a computer, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits. 
     While a preferred embodiment has been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.