Abstract:
A monitor for detecting pass gate leakage in a silicon on insulator device and a method for using the same is described herein. A pulse generator supplies a signal to a set of buffers connected in parallel, which pass on a signal to the source side of a series of NFETs. The plurality of NFETs are ordered by increasing channel widths. The NFETs have grounded gates, and therefore will not pass current due to field effects. Each NFET is connected to a latch, and the latches are originally set to the same state. When the signal supplied to the NFET drops from high to low, pass gate leakage will occur through the channel of each NFET. If pass gate leakage through any given NFET is sufficient, the latch will change states. The latch output signal is sent to a shift register, which can be made to output information. By incorporating the monitor on the chip, pass gate leakage tolerances and specifications can be established in-line during manufacture.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to silicon-on-insulator (SOI) field effect transistors (FETs) and specifically to pass gate leakage in gated devices. 
     In SOI devices such as metal oxide semiconductor (MOS) FETs, the body of the device is disposed on an insulator rather than wafer, and hence is “floating” as compared to conventional bulk devices. This floating body leads to leakage mechanisms that do not occur in bulk devices. One such leakage mechanism that occurs in SOI FETs is referred to as pass gate leakage (PGL). 
     In an n-channel FET (NFET), when the FET is off and has a zero potential at the gate terminal, and the source and the drain are at the supply potential (+V or Vdd), the silicon-insulator interface will be in the accumulation mode. In accumulation mode, the body develops a positive potential after a sufficient period of time. If the source is then suddenly dropped to zero potential, the source-body junction is forward biased, which allows holes to flow to the source and electrons to flow to the body. The electrons that flow to the body (and eventually to the drain) act as a collector current for the lateral NPN bipolar device. 
     In addition to the collector current, the forward bias of the source-body junction temporarily causes the threshold voltage of the device to decrease, which results in increased sub-threshold current. This current, in combination with the collector current, form an undesirable drain current that can impact the correct functioning of the FET. For example, pass gate leakage reduces noise immunity by allowing unwanted signals to pass through the device. What is needed in the art is a method of monitoring pass gate leakage in SOI devices, such as NFETs, that exhibit such unwanted leakage. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the present invention is a wafer processing test circuit comprising a plurality of pass gates having varying channel widths, a plurality of latches each connected to one of said pass gates, each of said latches receiving a leakage signal from its connected pass gate, an input circuit for supplying a test signal to said plurality of pass gates, said test signal having a preselected magnitude, wherein said plurality of pass gates outputs a leakage signal to its connected latch in response to said test signal, thereby causing said latches to assume one of a triggered state and an untriggered state, and storage coupled to said plurality of latches for storing one of said triggered state and untriggered state for each of said latches. 
     Another embodiment of the present invention is a method for testing for pass gate leakage in devices on a wafer comprising applying a test signal of a preselected magnitude to a plurality of pass gates having varying channel widths, outputting a leakage signal from each of said pass gates, receiving said leakage signals at a plurality of latches each connected to one of said pass gates, wherein said leakage signals are not all sufficient to trigger their connected latches, and storing triggered and untriggered latch states in storage coupled to said latches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
     FIG. 1 is a block diagram which illustrates the circuit of one embodiment of the pass gate leakage monitor. 
     FIG. 2 is a schematic diagram of an exemplary circuit that can be used to implement the buffer-pass gate NFET-latch series shown in FIG.  1 . 
     FIG. 3 is a voltage versus time graph for four points on the circuit shown in FIG. 2 for an exemplary pass gate NFET with a 30 micron channel width. 
     FIG. 4 is a voltage versus time graph for four points on the circuit shown in FIG. 2 for an exemplary pass gate NFET with a 40 micron channel width. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A pass gate leakage monitor which is able to test for pass gate leakage in-line during chip fabrication is described herein. The pass gate leakage monitor functions through the application of a test signal pulse to the source side of a device while monitoring leakage current at the drain side of the device. An output or leakage signal on the drain side of the device indicates whether or not a pass gate leakage current above a threshold value occurred. The monitor in its simplest form comprises a device for applying a test signal to pass gates, the pass gates themselves, and a device for sensing the responses of the pass gates to the test signal. Any conventional means for applying a test signal and sensing the responses can be used in the circuit. 
     Referring now to FIG. 1, a block diagram of one embodiment of the pass gate leakage monitor is shown generally at  10 . A voltage supply pad  12  (Vdd) and a ground pad  14  provide contact points for applying potential to the circuit. 
     A trigger pad  16  provides a contact point for applying voltage from the voltage supply pad  12  to the circuit. The trigger pad  16  is connected to a buffer  18 , which is used to supply a clean signal to a pulse generator  20 . The buffer  18  can be any buffer conventionally used for such an application. For example, the buffer  18  can be an NFET with a gate electrode connected to the trigger pad  16 , the drain connected to Vdd, the source connected to the pulse generator  20 , and a resistor to ground. 
     The pulse generator  20  applies a test signal to the pass gates, and can be any conventional pulse generator that can provide the required test signal pulse to the circuit at a predetermined magnitude. For example, as shown in FIG. 1, the pulse generator can be a parallel set of delay chains coupled to an exclusive OR gate. In this embodiment, a delay inverter chain  22  is connected in parallel with an inverter  24 . The signals from the delay inverter chain  22  and the inverter  24  are fed into an exclusive OR gate  26 , which creates a signal that starts low, becomes high, and then drops back to low. Since hole flow occurs rapidly, but accumulation of charge in the body occurs relatively slowly, it is preferable for the pulse generator  20  to create a test signal pulse with a sufficiently long initial low to allow for sufficient accumulation in the body of the pass gate FETs. The pulse created is sufficient to supply each of the remaining buffers (see below) with a signal surpassing the threshold voltage of the remaining buffers. The pulse can be reversed, of course, if a signal inverter/buffer is added or removed later in the circuit. 
     The test signal created by the pulse generator  20  is sent in parallel to multiple sets of buffers in series with pass gate NFETs. For example, the signal is sent to buffer  28  and pass gate NFET  30 , buffer  28 ′ and pass gate NFET  30 ′, and buffer  28 ″ and pass gate NFET  30 ″. For clarity, only three complete sets of buffers and pass gate NFETs are shown, but any number of sets can be used, depending upon the application. The pass gate NFETs  30 ,  30 ′,  30 ″ are, in this embodiment, the device being tested for pass gate leakage, and therefore each gate is grounded. Since each of the pass gate NFETs  30 ,  30 ′,  30 ″ are grounded, any current passing through the pass gate NFETs will be due to leakage currents rather than current passing through the pass gate NFET due to a field effect produced by the gate. That is, pass gate NFETs  30 ,  30 ′, 30 ″ will be “off” during testing, and will not pass any current other than leakage current. 
     The pass gate NFETs  30 ,  30 ′,  30 ″ can be organized in any manner within the test circuit. Preferably, the pass gate NFETs  30 ,  30 ′,  30 ″ are ordered according to channel width to produce a series of pass gate NFETs with broadening channel width. For example, pass gate NFET  30  can have a relatively narrow channel width, pass gate NFET  30 ′ can have a channel width slightly larger than pass gate NFET  30 , and so on, until pass gate NFET  30 ″, which will have the widest channel of all of the pass gate NFETs. In this example, NFET  30  and  30 ″ have channel widths of 30 μm and 40 μm respectively. It is preferable that at least ten pass gate and latch circuit combinations be used. However, if the process variation is known, one can design the pass gate width and the latch sensitivity to meet the needs of monitoring and therefore determine how many combinations are needed. This configuration of pass gate NFETs is preferable, since pass gate leakage is dependent upon channel width. Together, the pass gate NFETs form a graded pass gate NFET series  31  comprising pass gate NFETs of gradually increasing channel width. 
     Connected to each pass gate NFET is a latch which functions as a means for sensing the leakage signal produced by the pass gate NFETs. For example, latch  32  is connected to pass gate NFET  30 , latch  32 ′ is connected to pass gate NFET  30 ′, and latch  32 ″ is connected to pass gate NFET  30 ″. The latches  32 ,  32 ′,  32 ″ can be any conventional switch that is capable of changing states in response to pass gate leakage in excess of a threshold value. A pad  36  is connected to the latches  32 ,  32 ′,  32 ″, and can be used to preset the latches  32 ,  32 ′,  32 ″ to a uniform initial state. 
     Each latch  32 ,  32 ′,  32 ″ will send a signal to a storage device such as a shift register  34 . The shift register  34  can be any conventional register or storage means that can store the signal information from the latches, and then output that signal information, including, but not limited to, conventional multiplexers. A toggle pad  38  is connected to the shift register  34 , and functions to initiate transfer of shift register  34  information out of the circuit through another pad  40 . The toggle pad  38 , shift register  34 , and output pad  40  are one possible configuration for storage and output of the latch signals, and one skilled in the art will readily see that many other configurations of storage and output devices are within the scope of this invention. 
     FIG. 2 shows one possible circuit that can be used to test for pass gate leakage in a pass gate NFET. The schematic in FIG. 2 represents a buffer, pass gate NFET, and latch combination as represented in FIG. 1 by, for example, buffer  28 , pass gate NFET  30 , and latch  32 . The schematic can represent any or all of the buffer-NFET-latch combinations in the circuit, with all buffer-NFET-latch combinations preferably comprising the same components. Transistors T 1  and T 4  represent a buffer and inverter (e.g. buffer  28  in FIG.  1 ), transistor T 5  represents the pass gate NFET that is being tested for pass gate leakage (e.g. pass gate NFET  30  in FIG.  1 ), and transistors T 2 , T 3 , and T 6  represent a latch (e.g. latch  32  in FIG.  1 ). The transistors have channel width and length dimensions in microns given immediately after the transistor number. Although one embodiment is shown in FIG. 2, one skilled in the art will realize that transistor dimensions can be readily be altered depending on the application. T 5  will have varying channel widths, depending upon which pass gate NFET in the series  31  in FIG. 1 is represented, and what the needs of the application are. In FIG. 2, T 5  is shown as, alternatively, a 40 micron pass gate NFET and a 30 micron pass gate NFET, and the test results for both in a circuit trial will be detailed below. These two widths are exemplary only, and any width NFET channel can be incorporated into the series  31  of pass gate NFETs. 
     The buffer/inverter comprising T 1  and T 4  can be, for example, an enhancement mode PFET and NFET, respectively. Together T 1  and T 4  form a CMOS buffer/inverter that is used in the circuit to apply or remove a potential from the source side of the T 5 . An input test signal  42  “IN” is received from the pulse generator  20  and delivered to the gate electrodes of T 1  and T 4 . The source of T 1  is connected to Vdd and the drain for T 1  is connected to the source for T 5  and the drain for T 4 . The source for T 4  is connected to ground. 
     T 5  is an enhancement mode pass gate NFET, and represents one of the varying width pass gate NFETs ( 30 ,  30 ′,  30 ″) shown in FIG.  1 . The gate for T 5  is grounded, and the source for T 5  is connected to the drain of T 1  and the drain of T 4 . The drain of T 5  is connected to the drain of T 2  and the gate electrodes of T 3  and T 6 . As described above, only leakage current will flow through the T 5  channel during operation of the circuit. 
     T 2  is an enhancement mode PFET. The source for T 2  is connected to Vdd. This transistor functions as part of the latch for the circuit. T 3  is also an enhancement mode PFET, with a source connected to Vdd and a drain connected to the T 2  gate electrode, the shift register  34 , and the drain of T 6 . T 6  is an enhancement mode NFET and the source for T 6  is connected to ground. An output signal  44  is sent to the shift register  34  from the latch. 
     The functioning of the circuit will now be described in detail, with reference to FIGS. 3 and 4 where appropriate. FIG. 3 represents voltage-time plots of points A, B, C, and D as shown in FIG. 2 for a 30 micron wide channel pass gate NFET, and FIG. 4 represents voltage-time plots of the same points for a 40 micron wide channel pass gate NFET. 
     In the initial state, shown by the vertical line labeled “E” in FIGS. 3 and 4, the input test signal  42  from the pulse generator  20  is low. In FIGS. 3 and 4 this low initial input is shown in plot C. The low input signal  42  closes T 1  and opens T 4 , thereby causing point A to go high. This inversion is represented by plot A in FIGS. 3 and 4. The source of T 5 , therefore has a potential of Vdd in the initial state. 
     In the initial state, point “D” has been set to low, thereby closing T 2 . Since T 2  is closed, the gate electrodes of T 3  and T 6  are high at Vdd, and T 3  is therefore open. T 6 , on the other hand, is closed by the high gate voltage. Since T 3  is open and T 6  is closed, D is grounded and thus stays low, and the latch is set with a low output signal  44 . Since T 2  is closed, however, the drain side—point “B”—of T 5  is high at Vdd. FIGS. 3 and 4 show that B is initially high and D is initially low, and the initial state for the two different pass gate NFETs is therefore the same. In fact, the initial state of the circuit described above will generally be the same for every pass gate NFET in the circuit. 
     During the period when both points A and B are at high voltage, the body of the pass gate NFET, T 5 , will be in accumulation mode. The input signal should be held low long enough to allow for the desired accumulation. Once accumulation has reached the desired level, the pulse generator  20  changes the input test signal  42  to high. This change is represented in FIGS. 3 and 4 by point “F” on the time axis, and the change to high is seen in plot C at time F. 
     When the input test signal  42  changes to high, T 1  opens and T 4  closes. When T 4  closes, point A drops to ground voltage as charge flows through T 4  to ground. The change from high to low at point A is shown in FIGS. 3 and 4 in plot A at time F. 
     In both the 30 micron and 40 micron cases, the sudden voltage drop on the source side of T 5  results in pass gate leakage through T 5  and a corresponding leakage signal to the latch. The leakage current discharges through T 4 . As a result of the sudden drop in voltage on the source side of T 5 , the drain side of T 5  undergoes a sudden voltage drop caused by the pass gate leakage. This voltage drop on the drain side of T 5  can be seen in FIGS. 3 and 4 in plot B at time F. 
     In the case of the 30 micron T 5 , the sudden voltage drop at B causes the voltage at the gate electrodes of T 3  and T 6  to drop sufficiently low to partially close T 3  and partially open T 6 . As a result, point D goes high, the output signal  44  goes high, and the gate electrode at T 2  is partially opened, further lowering the potential at point B. This is depicted in plot D of FIG. 3, in which a sudden spike in voltage is seen just after time F. In this case, however, the pass gate leakage is not sufficiently large to trigger the latch, and change its state. That is, the potential at B is not reduced enough to cause T 3  to close enough and T 6  to open enough to result in a complete opening of T 2 . Consequently, as shown in plot B of FIG. 3, T 2  remains closed enough to rapidly increase the voltage at B. As a result, T 3  is opened, T 6  is closed, and the voltage at point D, as shown in plot D of FIG. 3, returns to ground voltage. The final result is an output signal  44  to storage that remains low because the latch is untriggered. 
     In the 40 micron pass gate NFET, conversely, the pass gate voltage is sufficient to lower the voltage at B enough to cause the latch to trigger and change states. Specifically, the change to low at B causes T 3  to close and T 6  to open, which results in a change to high at point D. The change to high at D is enough to fully open T 2 . With T 2  open, the voltage at B remains low, and T 3  remains open. The latch has been triggered, the state of the latch has been changed, and the output signal  44  has been shifted from low to high. With the latch state changed, the output signal  44  will remain high, regardless of further cycling of the input signal  42 , because the drain side of T 5  will remain in a low state until the latch is reset. The voltage changes of the circuit for the 40 micron pass gate NFET are shown in FIG.  4 . At time F the output signal in plot D changes from low to high, while the potential on the drain side of T 5  goes from high to low, as shown in plot B of FIG.  4 . 
     Although a latch that has triggered and changed states cannot be changed back to its original state by further input signals, a latch that has not changed states can do so. In other words, due to minor variances in the input signal  42  or the circuit itself, a latch that has not been changed in state during the first period of the input test signal could be triggered in a subsequent period. At some pass gate NFET width, however, the minor variations will not overcome the insufficiency of the pass gate leakage, and the latch will not be changed in state even if the input signal  42  is allowed to cycle repeatedly. 
     After the input signal  42  returns to its original low state, the various latches will either have been changed in state or not changed in state, and the shift register  34  will receive either a high or low signal from each latch as a result. By activating the toggle pad  38 , the information in the shift register  34  can be read out through the output pad  40 . The output of the shift register (e.g. the last stage of a J-K flip-flop shift register) can be measured and recorded after each pulse transition applied to the toggle pad. The data will generally be represented as a series of data bits, with each type of bit representing one of the latch states. Typically, the NFETs with very narrow channels below the threshold pass gate leakage width will all have the same latch state (low output), with some mixing of latch state as the critical channel width for pass gate leakage is reached. Thereafter, the pass gate NFETs with broader channels will generally all have a latch state opposite to that of the narrow channels (that is, a high output). The pass gate NFET channel width at which the latch output signal  44  changes states represents the threshold pass gate leakage channel width. 
     To retest for pass gate leakage, the latches can be reset by grounding the latches at the pad  36 . When the latches are grounded in this manner, point D drops to low voltage, and T 2  is closed as a result. As before, T 3  is then opened and T 6  is closed, and a steady state is reached wherein B is at a high potential and D is at ground. The latches will all now be in a reset state, and repeat testing of the pass gates can be performed. 
     The pass gate monitor described above provides a tool for monitoring pass gate leakage of SOI devices on a chip. Since no external equipment is needed to test for pass gate leakage, and since all of the required circuitry can be incorporated directly into the integrated circuits on the chip, pass gate leakage monitoring can be performed in-line. Such in-line monitoring of pass gate leakage allows for efficient pass gate leakage tolerances as well as greater quality control. 
     While preferred embodiments have 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 only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the claims.