Abstract:
An apparatus and method to test components in a semiconductor test structure. On a semiconductor wafer, a test module implemented in one or more scribe lines between a plurality of semiconductor dies is used to test components in the semiconductor test structure. The test module may, for example, test electrical characteristics of chains of vias, transistors, and functional devices, such as oscillators. The test module contains a scan chain control coupled through a plurality of pass gates to each component to be tested. The scan chain control sequentially closes the pass gates to separately test the components in the semiconductor test structure. The test module further interfaces with an external testing device and the results of each test are compared with the expected results to identify faulty components.

Description:
This application is a divisional application of U.S. patent application Ser. No. 11/218,650 filed Sep. 2, 2005 now U.S. Pat. No. 7,365,556. 
   This application claims priority under 35 USC §119(e)(1) of Provisional Application No. 60/606,600, filed Sep. 2, 2004. 
   CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is related to the following co-pending application:
     U.S. patent application Ser. No. 11/218,458 entitled “APPARATUS AND METHOD FOR TESTING CHAINS OF VIAS AND CONTACTS” filed Sep. 2, 2005, incorporated herein by reference.   

   TECHNICAL FIELD OF THE INVENTION 
   The present invention generally relates to testing of an electronic device. More particularly, the present invention relates to testing components in a semiconductor device. Still more particularly, the present invention relates to using active devices to test components in a semiconductor device. 
   BACKGROUND OF THE INVENTION 
   An integrated circuit (IC) is a semiconductor device containing many small, interconnected components such as diodes, transistors, resistors, and capacitors. These components function together to enable the IC to perform a task, such as control an electronic device or perform logic operations. ICs are found in computers, calculators, cellular telephones, and many other electronic devices. 
   ICs and other semiconductor devices are fabricated on small rectangles, known as “dies,” which are filled with multiple layers of the components, such as transistors, resistors, and capacitors, during the fabrication process. The connections between the layers are known as vias. A via is a metal interconnect coupled between two planar conductive layers in a semiconductor die. Multiple vias may be coupled together in what may be referred to as a “via chain” connecting one conductive region in an IC to another conductive region. 
   A manufacturing error in one of the components mentioned above may render an IC or semiconductor device incapable of functioning properly. For example, consider a memory device containing several ICs. If a transistor within one of the ICs fails to function properly, the memory device may produce memory errors. Vias are also subject to manufacturing errors. When a manufacturing error occurs in a via, the via may not conduct properly and thus may prohibit an IC from functioning correctly. For instance, an open via or a partially open via may prohibit a device from functioning as designed. An open via may have a high or infinite resistance, and a partially open via may have a higher than average resistance. 
   Vias in a semiconductor device may be tested by measuring the resistance of via test chains. This technique identifies via chains containing open vias and some partially open vias by their high resistance. A problem with this approach is that tests may fail to identify via chains containing vias that have slightly larger than expected resistances. Consider a via chain in which each via has an expected resistance. For example, a via may have an average resistance of between eight and twelve ohms. Thus, a via chain having one thousand vias has an expected resistance of between eight thousand and twelve thousand ohms. However, one via in the via chain could have a resistance of one thousand ohms, while the other vias have an average resistance of nine ohms. Thus, the resistance of the entire via chain is approximately ten thousand ohms. Even though one via is a partially open via, the resistance of the entire via chain may still fall within the expected resistance range. As a result, measuring the resistance of the entire via chain may fail to identify one or more vias with a higher than average resistance in the via chain. 
   Circuitry to test IC components on a semiconductor device, such as resistors, transistors, vias, and oscillators, may interface with an external testing device. While the test circuitry may test vias and other semiconductor device components so that flawed components may be identified, the test circuitry may take up space on or around the semiconductor device that may otherwise be used to fabricate more IC components. Thus, an apparatus and method that both accurately tests semiconductor devices and takes up little space on or around the semiconductor die would be beneficial. 
   SUMMARY OF THE INVENTION 
   The problems noted above are solved by sequentially testing a plurality of components-under-test (CUT) using a scan chain control. In some embodiments of the invention, a current is sent through the plurality of CUT. A plurality of electrical characteristics of the CUT are measured to determine if the CUT are correctly functioning. The electrical characteristics may be a voltage or a current. The plurality of CUT may be coupled to a plurality of pass gates. Each of the CUT may be a resistive element or a via chain. The via chain may comprise a plurality of vias coupled in series. 
   A control that may be the scan chain control couples to the plurality of pass gates. The control is capable of closing and opening the pass gates to allow determination of a voltage across each of the CUT. The control may sequentially close and open each pass gate to allow determination of the voltage across each of the CUT. The voltage across each of the CUT may be compared with an expected voltage. 
   A source pad and a sink pad couple to the plurality of CUT. An output pad couples to the plurality of pass gates. The voltage across each of the CUT is determined at the output pad. A testing device may couple to the source pad, sink pad, and output pad. The testing device is capable of providing a current to the source pad and a connection to ground to the sink pad. The testing device is further capable of measuring the voltage across each of the CUT at the output pad. 
   The CUT may be located on one or more of a plurality of semiconductor dies on a semiconductor wafer. The semiconductor dies may be separated by a plurality of scribe lines on the semiconductor wafer. In some other embodiments, the CUT, pass gates, and the control may be on one or more of the plurality of scribe lines. 
   In some embodiments of the invention, each of a plurality of CUT couples to one of a plurality of transmission switches. Each of the CUT may be a transistor, such as a p-channel field effect transistor (PFET) or an n-channel field effect transistor (NFET). The CUT couple in parallel between a source pad and a sink pad. A current from the source pad is sent through the plurality of CUT one CUT at a time. Current is received from one of the CUT by the sink pad. A control coupled to the plurality of transmission switches is capable of sequentially closing and opening the transmission switches to determine and characterize a current-voltage relationship of each of the CUT. The current-voltage relationship from each of the CUT may be compared with an expected current-voltage relationship for each of the CUT. 
   An input pad couples to the plurality of transmission switches. A testing device may couple to the input pad, source pad, sink pad, and control. The testing device is capable of providing current to the source pad and receiving current from the sink pad. The testing device is also capable of transmitting the signal to the input pad. The testing device further measures the current flowing through each of the CUT at the sink pad. 
   Each transmission switch may further comprise a pass gate. The pass gate further comprises an input, an enable input, and an output. The input receives a signal from the input pad, and the enable input receives an enable signal from the control. Each transmission gate may further include a disable transistor. The disable transistor may include a gate connection, source connection, and drain connection. The gate connection couples to the enable input, the drain connection couples to the output, and a voltage source input couples to the source connection. The output is coupled to one of the CUT. The output sends the signal from the input to the CUT when the enable input receives an enable signal from the control. As described above, the CUT may be a transistor. The CUT transistor comprises a source connection, a gate connection coupled to the output of the transmission switch, and a drain connection coupled to the sink pad. 
   The voltage source input may receive a logic high or logic low signal. The disable transistor switches to a closed state to pass the logic high or logic low signal to the output when the enable input receives a disable signal from the control. The disable transistor may be a PFET or an NFET. 
   In some other embodiments of the invention, a plurality of CUT couples in parallel between a source pad and a ground pad. The source pad applies a voltage to the CUT and the ground pad provides a ground to the CUT. Each of the CUT is an oscillator. The oscillator further comprises an input coupled to a plurality of inverting logic. A divider couples to one of the plurality of inverting logic. The divider is capable of receiving a signal from one of the plurality of inverting logic and reducing frequency of the signal. An output couples to the divider. 
   As described above, a voltage is applied to the CUT and at least one electrical characteristic of the CUT is measured one CUT at a time. A control that may be a scan chain control coupled to each of the plurality of CUT is capable of providing an input signal to test each CUT to determine if the output of each of the CUT matches an expected output for each CUT, thus determining if the CUT are correctly functioning. A multiplexer couples to each CUT. The multiplexer receives an output signal from each CUT. A divider may couple to the multiplexer. The divider may be capable of reducing the frequency of the signal from the multiplexer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a semiconductor wafer containing multiple semiconductor dies; 
       FIG. 2  shows a portion of the semiconductor wafer shown in  FIG. 1 ; 
       FIG. 3   a  shows a via chain configured between two different planar levels in a semiconductor die; 
       FIG. 3   b  shows a via chain configured between multiple planar levels in a semiconductor die; 
       FIG. 4 , in accordance with some embodiments of the invention, shows a test module capable of testing via chains; 
       FIG. 5 , in accordance with some embodiments of the invention, shows a scan chain control within the test module; 
       FIG. 6  shows a sample input and output of the scan chain control illustrated in  FIG. 5 ; 
       FIG. 7 , in accordance with some embodiments of the invention, shows a test module capable of testing transistors; 
       FIG. 8   a , in accordance with some embodiments of the invention, shows a PFET transmission switch for use in testing PFETs; 
       FIG. 8   b , in accordance with some embodiments of the invention, shows an NFET transmission switch for use in testing NFETs; 
       FIG. 9 , in accordance with some embodiments of the invention, shows a test module capable of testing oscillators; and 
       FIG. 10 , in accordance with some embodiments of the invention, shows an oscillator including a number of logic gates. 
   

   NOTATION AND NOMENCLATURE 
   Certain terms are used throughout the following description and claims to refer to particular system components and configurations. As one skilled in the art will appreciate, the companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ” Also, the terms “couple,” “couples,” or “coupling” are intended to mean either an indirect or direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection or though an indirect electrical connection through other devices and connections. 
   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   A semiconductor wafer is a slice of semiconductor material that is divided into small sections known as dies. Semiconductor devices, such as logic gates, are fabricated onto the dies. These devices consist of diodes, transistors, resistors, and capacitors that operate to perform a designed function once the dies are separated from the wafer. 
   In a semiconductor wafer, a test module tests vias, transistors, and logic gates, such as oscillators, in semiconductor test structures that are semiconductor devices on a semiconductor die or in a scribe line surrounding the semiconductor die. In some embodiments, the semiconductor test structures may be semiconductor devices fabricated to validate the process for manufacturing integrated circuits. In some other embodiments of the invention, the semiconductor test structure may be semiconductor devices on the die that are part of a functional integrated circuit, such as a digital signal processor (DSP), memory device, transmitter/receiver device, and so on. 
   The test module may be located on the semiconductor die or in the scribe line surrounding the semiconductor die. The test module tests components in semiconductor test structures on the semiconductor die or the scribe line; thus, resistors, transistors, vias, and functional devices, such as oscillators, may be tested by the test module. The test module interfaces with an external test device and sequentially tests the components on the semiconductor test structure using a scan chain control and a plurality of pass gates. 
   Referring to  FIG. 1 , a semiconductor wafer  100  contains several semiconductor dies  110   a ,  110   b , and  110   c .  FIG. 2  shows an enlarged portion of semiconductor wafer  100 . Semiconductor dies  110  include semiconductor devices that may contain layers of components, such as resistors, capacitors, transistors, diodes, and vias. A plurality of scribe lines  210  separate the semiconductor dies  110 . The scribe lines  210  represent areas where semiconductor wafer  100  will be cut to separate individual semiconductor dies when the fabrication process is complete. 
   As described above, semiconductor test structures in semiconductor dies  110  or scribe line  210  contain a plurality of vias. Vias may be connected together to form a via chain. The via chain connects layers of components in the semiconductor test structure. Referring to  FIG. 3   a , a cross-sectional view is shown of the via chain located in a semiconductor test structure. The via chain connects layer  300   a  to  300   b . Each layer  300  contains a plurality of conductive plates  302   a - 302   h . Each plate  302   a - 302   h  couples to at least one other plate  302   a - 302   h  by one of the vias  304   a - 304   g . Thus, for example, plate  302   a  couples to plate  302   b  through via  304   a . Plates  302   a - 302   h  may be formed from any conductive material or combination of conductive materials including, for example, copper, aluminum, tungsten, and/or doped polysilicon. Plates  302  may also contain one or multiple layers. An isolating layer  305 , which may be composed of silicon dioxide, may isolate plates  302   a - 302   a.    
   In some embodiments of the invention, via chain  301  shown in  FIG. 3   b  may be located in a semiconductor test structure on semiconductor die  110  or in scribe line  210 . Via chain  301  includes a plurality of layers  350   a  . . .  350   n . Each layer  350   a  . . .  350   n  is coupled to at least one other layer  350   a  . . .  350   n  by one of the vias  352   a  . . .  352   m . Layers  350   a  . . .  350   n  may be formed from any conductive material or combination of conductive materials, such as copper, aluminum, tungsten, and/or doped polysilicon. A number of isolating layers  306   a - 306   m , which may be composed of silicon dioxide, may isolate layers  350   a  . . .  350   n.    
   In some other embodiments of the invention, each layer  350   a  . . .  350   n  in via chain  301  may consist of a plurality of conductive plates (not shown in  FIG. 3   b ) similar to layers  300   a  or  300   b  in  FIG. 3   a . The plurality of conductive plates (not shown in  FIG. 3   b ) in each layer  350   a  . . .  350   n  may connect to other layers  350   a  . . .  350   n  through vias not shown in  FIG. 3   b.    
   Returning to  FIG. 2 , a test module  200  is fabricated on semiconductor die  205 . Test module  200  may test the functionality of semiconductor test devices on wafer  100 . In some embodiments of the invention, a test module  215  may also be placed in scribe line  210 . By placing the test module in scribe line  210  instead of on semiconductor die  205 , more components, such as resistors, transistors, diodes, and capacitors, may be fabricated on semiconductor die  205  (not shown in  FIG. 2 ). In accordance with some embodiments of the invention, multiple test modules may be coupled in scribe line  210  or on semiconductor die  205 . Test modules may also be coupled in multiple scribe lines  210 . 
   As described above, semiconductor test structures on semiconductor die  205  or scribe line  210  contain a plurality of vias coupled together in long chains described as via chains. A test point is a contact point that couples test module  200  to a via chain. In some embodiments of the invention, test module  200  couples to a via chain through multiple test points. Test module  200  determines the resistance of sections of the via chain between any two test points. Each section of the via chain between any two test points may be designated a via subchain. Thus, test module  200  may determine the resistance of each via subchain. As the number of vias in a via subchain decreases, more vias with above average resistances may be identified. Therefore, it is desirable for test module  200  to utilize as many test points as possible along the via chain. 
   Referring to  FIG. 4 , test module  200  tests via chains for vias with high resistances. Test module  200  couples to a via chain  406 . The via chain  406  is divided into a first via subchain  415 , second via subchain  416 , and third via subchain  417 . The via subchains are connected in series between a current source pad  400  and a current sink pad  405 . First via subchain  415  connects to current source pad  400  through test point  410  and second via subchain  416  through test point  411 . Third via subchain  417  connects to second via subchain  416  through test point  412  and current sink pad  405  through test point  413 . 
   Test point  410  couples to an output pad  401  through a first pass gate  470 . Output pad  401  connects to test point  411  through a second pass gate  471 , test point  412  through a third pass gate  472 , and test point  413  through a fourth pass gate  473 . A scan chain control  450  couples to each pass gate through separate lines. 
   Current source pad  400 , current sink pad  405 , and output pad  401  are conductive contacts capable of interfacing with an external testing device (not shown), such as a probe device. The external test device may pass a test current through current source pad  400  to current sink pad  405  through first via subchain  415 , second via subchain  416 , and third via subchain  417 . The external testing device may provide a ground connection to current sink pad  405  (not shown in  FIG. 4 ) in order to facilitate current flow through the via chain  406 . Scan chain control  450  sequentially and individually closes each pass gate. When each pass gate is closed, the pass gate connects one of the test points to output pad  401 . Thus, the voltage across the via subchain(s) in between the test point connected to output pad  401  and a constant voltage reference, such as current source pad  400  or current sink pad  405 , may be measured by an external testing device coupled to test module  200 . 
   As described above, scan chain control  450  sequentially and individually closes each pass gate, and the external testing device measures the voltage across via subchain(s) between each test point and a constant voltage reference. The voltage across each via subchain is recorded by an electronic test device, such as a computer or embedded controller, coupled to the external testing device. The resistance (R) for each via subchain may be calculated using the voltage across the via subchain (ΔV) and the test current (I test ) through the via subchain. Ohm&#39;s law states: 
                 R   =       Δ   ⁢           ⁢   V       I   test               Equation   ⁢           ⁢   1               
Thus, for example, the voltage (ΔV) across the first via subchain  415  is the difference between the voltage at test point  410  and the voltage at test point  411 . Thus, the resistance of the via subchain may be calculated using Equation 1 and stored in the electronic test device. If the resistance of the via subchain is outside of an expected distribution, which indicates that the via subchain contains one or more vias with high resistance values, the semiconductor test structure may be marked as flawed. Via subchains containing low resistance values may also be identified.
 
   In accordance with some embodiments of the invention, the via chain  406  shown in  FIG. 4  may be divided into more or less than three via subchains. For example, the via chain  406  may be divided into 2, 10, 20, or more via subchains. The number of pass gates corresponds to the number of test points along the via chain. Further, multiple input pads similar to current source pad  400  and current sink pad  405  may also couple to via chain  406 . 
   Turning now to  FIG. 5 , in accordance with some embodiments of the invention, scan chain control  450  is shown in more detail. Scan chain control  450  contains a first flip-flop  505 , second flip-flop  510 , third flip-flop  515 , and fourth flip-flop  520  coupled together in series between a data input pad  500  and a data output pad  530 . Each flip-flop contains a data input connection, a data output connection, and a clock input. The data input connection of each flip-flop connects to the data output connection of the previous flip-flop with the exception of first flip-flop  505 . The data input connection  550  of first flip-flop  505  connects to data input pad  500 . The data output connection  590  of the fourth flip-flop  520  connects to data output pad  530 . A clock input pad  501  connects to the clock input of each flip-flop. 
   Data input pad  500 , data output pad  530 , and clock input pad  501  are capable of interfacing with the external testing device (not shown), such as a probe device. The external testing device may transmit a data input signal through data input pad  500  to the data input connection  550  of first flip-flop  505 . Data output pad  530  may also couple to the external testing device and may transmit information from the data output connection  590  of fourth flip-flop  520  to the external testing device. Further, the external testing device may connect to clock input pad  501  and transmit a clock signal to each flip-flop. In some embodiments of the invention, circuitry connected between clock input pad  501  and the clock input connection of each flip-flop (not shown) may ensure a proper clock signal to each flip-flop. 
   The data output connection of each flip-flop further connects to the pass gates in test module  200  in  FIG. 4 . Thus, the output of each flip-flop is transmitted to one of the pass gates. For example, line one  461  connects the data output connection  555  of first flip-flop  505  to first pass gate  470 . Thus, data output connection  555  transmits the output of first flip-flop  505  to first pass gate  470  through line one  461 . 
   As described above, the external testing device connects to data input pad  500 , clock input pad  501 , and data output pad  530  of scan chain control  450  and further to current source pad  400 , current sink pad  405 , and output pad  401  of test module  200 . When the external testing device allows test current through the via chain  406  in  FIG. 4 , the external testing device also generates a data input signal through data input pad  500  to the data input connection  550  of first flip-flop  505 . First flip-flop  505  outputs the data input signal through data output connection  555  at the next rising edge of the clock signal received through the clock input  560 . The remaining flip-flops receive the data input signal from the previous flip-flop and pass the data input signal through their respective data output connections at the next rising edge of the clock signal. 
   Referring to  FIG. 6 , a timing diagram for scan chain control  450  shows the clock input  601 , data input  602 , line one  661 , line two  662 , line three  663 , and line four  664 . As described above, the external testing device passes a clock signal through clock input pad  501 . The clock signal is a periodic square wave signal to synchronize operation of scan chain control  450 . The external testing device also passes a data input signal  600  through data input pad  500  at time to. In accordance with some embodiments of the invention, as shown in  FIG. 6 , data input signal  600  from t 0  to t 2  is logical high for one cycle of clock input  601 . However, data input signal from t 0  to t 2    600  may be any signal capable of activating the pass gates in test module  200 . Thus, in some other embodiments of the invention, data input signal  600  may be logically low for activating pass gates. Additionally, data input signal  600  may last for more than one clock cycle. 
   First flip-flop  505  receives data input signal  600  from time t 0  to t 2  and outputs the data input signal for one clock cycle at the rising edge of clock input at time t 1 . Thus, the data input signal is passed from data output connection  555  through line one  461  to first pass gate  470  from time t 1  to t 3 . First pass gate  470  may be configured to close and connect test point  410  to output pad  401  when a logical high is received. Thus, scan chain control  450  may close first pass gate  470  when the data input signal is output from first flip-flop  505 . First pass gate  470  is opened at time t 3  when the signal transmitted through line one returns to a logical low. 
   Second flip-flop  510  receives the data input signal from first flip-flop  505  from time t 1  to t 3 . Second flip-flop  510  transmits the data input signal to third flip-flop  515  from time t 3  to t 4 . Second flip-flop  510  also transmits the data input signal to second pass gate  471  from time t 3  to t 4 . Thus, scan chain control  450  closes second pass gate  471  after first pass gate  470  is in an open state, connecting test point  411  to output pad  401 . Scan chain control  450  places second pass gate  471  into an open state at time t 4  when a logical low signal is transmitted through line two  462 . 
   Third pass gate  472  and fourth pass gate  473  are closed and opened in a similar order through line  3   463  and line  4   464 , respectively. As shown in  FIG. 6 , from time t 5  to t 6 , fourth flip-flop  520  transmits the data input signal to fourth pass gate  473  and data output pad  530 . Fourth pass gate  473  closes when the data input signal is received through line  464 . Data output pad  530  also couples to the external testing device. The external testing device may compare the data input signal from fourth flip-flop  520  to the original data input signal  600  in order to verify proper operation of the scan chain control  450 . 
   In accordance with some embodiments of the invention, scan chain control  450  may contain more or less than four flip-flops. For example, scan chain control  450  may contain nine flip-flops if test module  200  contains nine pass gates. Thus, scan chain control  450  may also consists of nine lines connecting scan chain control  450  to the nine pass gates. 
   Returning to  FIG. 4 , pass gate  470  in test module  200  consists of a p-channel field effect transistor (PFET)  480 , n-channel field effect transistor (NFET)  481 , and an inverter  482 . PFET  480  functions as an open switch when a voltage representing a logical high is applied to gate connection  451  of the PFET. PFET  480  functions as a closed switch when a voltage representing a logical low is applied to the gate connection  451  of the PFET. Conversely, NFET  481  functions as an open switch when a voltage representing a logical low is applied to gate connection  452  of the NFET, and NFET  481  functions as a closed switch when a voltage representing a logical high is applied to the gate connection  452  of the NFET. Inverter  482  provides an output that is the inverse of the input. Thus, if a logical high is passed to the inverter, the inverter outputs a logical low and vice versa. 
   For first pass gate  470 , the drain connection of PFET  480  connects to the drain connection of NFET  481  at test point  410 . The source connection of PFET  480  connects to the source connection of NFET  481  at junction  453 . Junction  453  further connects to output pad  401 . Scan chain control  450  connects to the gate connection of NFET  481  gate through line one  461 . Scan chain control  450  also connects to the gate connection of PFET  480  through line one  461  and further through inverter  482 . Inverter  482  inverts the signal transmitted to the gate connection of PFET  480 . 
   Turning now to  FIG. 7 , test module  200 , in accordance with some other embodiments of the invention, may contain a plurality of transistors coupled between a current source pad  400  and a current sink pad  700 . The transistors may be a collection of PFETs, such as PFET  725 , a collection of NFETs, such as NFET  710 , a collection of PFETs and NFETs, or other types of transistors. Each transistor consists of a source connection, gate connection, and drain connection. For NFET  710 , current may flow from drain connection  715  to source connection  720  when an appropriate voltage is applied to gate connection  717 . The remaining transistors shown in  FIG. 7 , such as PFET  725 , operate similarly. 
   The drain connection of each transistor couples to current source pad  400 , and the source connection of each transistor connects to current sink pad  700 . The gate connection of each transistor couples to a corresponding transmission switch. The transmission switches shown in  FIG. 7  are described below and shown in  FIG. 8 . NFET transmission switch  800   a,  NFET transmission switch  800   b , PFET transmission switch  801   a , and PFET transmission switch  801  couple to a switch input pad  705 . 
   Current source pad  400 , current sink pad  700 , and switch input pad  705  are conductive contacts capable of interfacing with an external testing device, such as a probe device. A voltage source may connect to switch input pad  705  through the external testing device. Further, a scan chain control  450  couples to each transmission switch. As described above, scan chain control  450  is capable of sequentially activating each transmission switch and may be similar to scan chain control  450  detailed in  FIG. 5 . 
   The external testing device sends a test current to current source pad  400 . When scan chain control  450  closes NFET transmission switch  800   a , switch input pad  705  connects to gate  717  of NFET  710  and applies a voltage to gate  717 . The voltage applied to gate  717  through switch input pad  705  may be varied, thus varying the amount of test current allowed to flow from current source pad  400  to current sink pad  700  through NFET  710 . By observing the varying voltages and currents at the current source pad, current sink pad, and switch input pad, test module  200  may determine the current-voltage relationship for each transistor. Thus, the current-voltage relationship may be analyzed and transistors with undesirable current-voltage relationships may be identified. 
   In accordance with some embodiments of the invention, test module  200  shown in  FIG. 7  may contain more or less than four transistors. The number of transmission switches corresponds to the number of transistors coupled between current source pad  400  and current sink pad  700 . 
   Turning now to  FIGS. 8   a  and  8   b , PFET transmission switch  801  and NFET transmission switch  800  shown in  FIG. 7  are illustrated in more detail. PFET transmission switch  801  represents PFET transmission switch  801   a  and  801   b . NFET transmission switch  800  represents NFET transmission switch  800   a  and  800   b . Both transmission switch  801  and  800  contain a PFET  480 , NFET  481 , and an inverter  482 . PFET  480  functions as an open switch when a logical high is passed to gate connection  451  of the PFET  480 . PFET  480  functions as a closed switch when a logical low is passed to the gate connection  451  of the PFET  480 . Conversely, NFET  481  functions as an open switch when a logical low is passed to gate connection  452  of the NFET  481 . NFET  481  functions as a closed switch when a logical high is passed to the gate connection  452  of the NFET  481 . Inverter  482  provides an output that is the inverse of the input. Thus, if a logical high is passed to inverter  482 , the inverter outputs a logical low and vice versa. 
   For the transmission switches shown in  FIGS. 8   a  and  8   b , the drain connection of PFET  480  connects to the drain connection of NFET  481  at switch input  810 . As shown in  FIG. 7 , switch input  810  connects to switch input pad  705 , which may connect to an external testing device that provides a voltage. The source connection of PFET  480  connects to the source connection of NFET  481  at switch output  820 . Switch output  820  connects to the gate connection of a transistor, such as NFET  710  or PFET  725  in  FIG. 7 . An enable input  825  connects the gate connection of NFET  481  to scan chain control  450  shown in  FIG. 7 . Enable input  825  also connects to the gate connection of PFET  480  through inverter  482 . Inverter  482  inverts the signal transmitted to the gate connection of PFET  480 . 
   As shown in  FIG. 8   a  for PFET transmission switch  801   a  and  801   b , enable input  825  connects to gate connection  831  of a PFET  830 . PFET  830  further connects switch output  820  to voltage VDD  835 . VDD  835  represents a positive voltage corresponding to a logical high. Table 1 shows the switch output for different switch and enable inputs for PFET transmission switch  801 . 
                                       TABLE 1                       Enable Input   Switch Input   VDD   Switch Output                           0   0   1   1 (strong)           0   1   1   1 (strong)           1   0   1   0           1   1   1   1                        
As shown in Table 1, when PFET transmission switch  801  receives a logical high (1) through enable input  825 , PFET transmission switch  801  is closed and connects switch input  810  to switch output  820 . When PFET transmission switch  801  receives a logical low (0) through enable input  825 , PFET transmission switch  801  is opened and connects voltage VDD  835  to switch output  820 . The strong logical high signal from switch output  820  ensures that a PFET is not activated in  FIG. 7 .
 
   Returning to  FIG. 7 , PFET transmission switch  801   a  may thus connect switch input pad  705  to PFET  725  if a logical high is received by enable input  825  of PFET transmission switch  801   a . PFET  725  may be activated if a logical low is transmitted from switch input pad to gate  727 . Gate  727  of PFET  725  may be connected to voltage VDD  835  if a logical low is received by enable input  825  of PFET transmission switch  800   a.    
   As shown in  FIG. 8   b  for NFET transmission switch  800   a  and  800   b , enable input  825  connects to gate connection  841  of an NFET  840  through inverter  482 . NFET  840  further connects switch output  820  to voltage VSS  836 . VSS  836  represents a negative voltage corresponding to a logical low. Table 2 shows the switch output for different switch and enable inputs for NFET transmission switch  800 . 
                                       TABLE 2                       Enable Input   Switch Input   VSS   Switch Output                           0   0   0   0 (strong)           0   1   0   0 (strong)           1   0   0   0           1   1   0   1                        
As shown in Table 2, when NFET transmission switch  800  receives a logical low (0) through enable input  825 , NFET transmission switch  800  is opened and connects switch output  820  to voltage VSS  836 . The strong logical low signal sent from switch output  820  ensures that an NFET in  FIG. 7  is not activated.
 
   When NFET transmission switch  800  receives a logical high (1) through enable input  825 , NFET transmission switch  800  is closed and connects switch input  810  to switch output  820 . Returning to  FIG. 7 , NFET  710  may be activated if a logical high is transmitted from switch input pad to gate  717  of NFET  710 . 
   Turning now to  FIG. 9 , test module  200  in accordance with some other embodiments of the invention is shown. Test module  200  contains a plurality of oscillators  910   a  . . .  910   n  coupled in parallel between a voltage source pad  920  and a voltage ground pad  925 . An oscillator  910   a  in test module  200  is shown in more detail in  FIG. 10 . Oscillator  910   a  consists of an oscillator enable input  1000  connected to an oscillator output  1010  through a number of logic gates. Oscillator enable input  1000  couples to scan chain control  450  in  FIG. 9 . In some embodiments of the invention, oscillate enable input  1000  connects to a first input of a NAND gate  1015 . The NAND gate connects to a string of inverters  1020   a  . . .  1020   m , where the number of inverters is even. Each inverter  1020  provides an output that is the inverse of the input. 
   Inverter  1020   d  connects to pre-output divider  1065 . Pre-output divider  1065  contains a string of dividers  1055   a - 1055   n  capable of reducing the frequency of inverter  1020   d  output signal. Pre-output divider  1065  outputs the resulting signal to oscillator output  1010  if output enable input  1050  is active. Divider  1065  contains a NAND gate  1060 . NAND gate  1060  outputs inverter  1020   d  output to oscillator output  1010  if a logic high is received from an output enable input  1050 . 
   Inverter  1020   m  connects to a second input of NAND gate  1015 . Table 3 shows the NAND gate output for different inputs. 
                           TABLE 3               First Input   Second Input   NAND Gate Output                   0   0   1       0   1   1       1   0   1       1   1   0                    
As shown in Table 3, NAND gate  1015  outputs a one for every input combination except a logic high (1) for the first input and second input. In some other embodiments of the invention, the logic gates shown in  FIG. 10  may be replaced with circuits arranged to provide a periodically varying output signal at oscillator output  1010 .
 
   Oscillator  910   a  is designed to periodically generate a square wave at oscillator output  1010 . When scan chain control  450  sends a logic high to the first input  1000  of NAND gate  1015  and the second input of NAND gate  1015  receives a logic high from inverter  1020   m,  NAND gate  1015  outputs a logic low. Thus, oscillator  910  begins to oscillate, and oscillator output  1010  produces a periodically varying square wave if output enable input  1050  receives a logic high. In some embodiments of the invention, output enable input  1050  may also couple to scan chain control  450  and oscillate enable input  1000  (not shown in  FIG. 10 ). Thus, oscillator output  1010  produces a periodically varying square wave if output enable input  1050  and oscillate enable input  1000  receive a logic high from scan chain control  450 . Oscillator  910  ceases oscillation when output enable input  1050  and oscillate enable input  1000  receive a logic low from scan chain control  450 . 
   Returning to  FIG. 9 , scan chain control  450 , which is shown in  FIG. 5 , sequentially sends a logic high signal to each oscillator  910  through oscillate enable input  1000  and output enable input  1050 . The oscillator output  1010  of each oscillator connects to a multiplexer  905 . Thus, multiplexer  905  receives signals from each oscillator  910 . Multiplexer  905  connects oscillators  910   a  . . .  910   n  to a divider  910  and further to an output pad  901 . As described above, scan chain control  450  enables one oscillator  910  at a time. Oscillators  910  output a logic high from oscillator output  1010  when output enable input  1050  is not enabled. Multiplexer  905  may be configured to pass oscillating signals to divider  910 . Divider  910  slows down the signal from the selected oscillator and passes the signal to output pad  901 . 
   Voltage source pad  920 , voltage ground pad  925 , and output pad  901  are conductive contacts capable of interfacing with the external testing device. The external testing device may power logic gates in each oscillator through voltage source pad  920 , and the external testing device may further provide a ground connection through voltage ground pad  925 . 
   In some embodiments of the invention, the external testing device compares output received at output pad  901  from each oscillator  910   a  . . .  910   n  with an expected output for each oscillator. The external testing device may identify unreliable oscillators with incorrect outputs, incorrect frequency, and so on. In accordance with some embodiments of the invention, device test module  200  shown in  FIG. 9  may contain more or less than four oscillators. 
   As described above, test modules  200  shown in  FIGS. 4 ,  7 , and  9  may test chains of vias, transistors, functional devices such as oscillators, or other integrated circuit components. The test modules and semiconductor test structures may be placed on scribe line  210  in  FIG. 2 , thus allowing components on semiconductor wafer  100  to be tested without taking up space on semiconductor die  110 . 
   While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.