Abstract:
A semiconductor memory device comprising a plurality of dummy wordlines independently formed with a plurality of normal wordlines, a plurality of dummy wordline drivers for driving the plurality of dummy wordlines, a plurality of control circuits for controlling the plurality of dummy wordline drivers, a plurality of comparing units for comparing a voltage level of the dummy wordline and the predetermined reference voltage level and a plurality of outputting units for outputting signals outputted from the plurality of comparing units.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a semiconductor memory device; and, more particularly, to a semiconductor memory device having a monitoring device capable of measuring the line delay or a model parameter of a wordline or a bitline. 
     DESCRIPTION OF THE PRIOR ART 
     As the integration density of a semiconductor memory device increases, the RC delay and a model parameter of a wordline or a bitline have a significant effect on a semiconductor memory device characteristic. The RC delay and the model parameter are important factors to accurately set timing in an internal operation and to determine whether the goods are commercially competitive. However, there are few methods capable of reliably measuring the RC delay and the model parameter. A method, which is currently in use to measure the line delay, is not a direct measurement, but an indirect measurement so that an accurate measurement cannot be performed. 
     FIG. 1 is a schematic circuit diagram showing a portion of cell block in a DRAM according to the prior art. 
     Referring to FIG. 1, a wordline driver WD is driven in response to a main wordline enable bar signal mwlz outputted from a row decoder (not shown) and a wordline boosting signal Px is applied to a wordline WLn connected to a memory cell  2  by the wordline driver WD. Generally, a dummy wordline and a dummy memory cell, which have the same width and area as the normal wordline and the normal memory cell, are configured at the edge of the normal wordline WLn for stability of a process. 
     The main wordline signal is selected by a row address and one normal wordline boosting signal Px is selected from Px 0  to Px 3  by the address signal and then a voltage level of the normal wordline WLn is changed into a boosting voltage Vpp level, which is higher than a power supply voltage level. One wordline WLn is driven to the boosting voltage Vpp level in response to the main wordline signal. At this time, the dummy wordline is not used so that the voltage level of the dummy wordline is fixed to a ground voltage level. Also, a dummy bitline voltage level is set to a Vblp level, which is a bitline precharge voltage level. 
     In the above configuration, after manufacturing real goods, a characteristic of the goods is determined by how rapidly the voltage level of the wordline WLn or the bitline BL increases to a desired voltage level. It is very important to determine whether the enable time of a bitline sense amplifier, a tRCD_min and a model parameter are matched with those of an actual device. However, an accurate measurement method has not been implemented in the prior art. A conventional measurement method is to measure a data line, which can be measured because the data line is a metal line, and indirectly guess the desired data, so that accurate data cannot be obtained. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a semiconductor memory device having a monitoring circuit capable of measuring the line delay or a model parameter of a wordline or a bitline. 
     In accordance with an aspect of the present invention, there is provided a semiconductor memory device comprising: a plurality of dummy wordlines independently formed with a plurality of normal wordlines; a plurality of dummy wordline drivers for driving the plurality of dummy wordlines; a plurality of control circuits for controlling the plurality of dummy wordline drivers; a plurality of comparing means for comparing a voltage level of the dummy wordline and the predetermined reference voltage level; and a plurality of outputting means for outputting signals outputted from the plurality of comparing means. 
     In accordance with another aspect of the present invention, there is provided a semiconductor memory device comprising: a plurality of dummy bitlines independently formed with a plurality of normal bitlines; a plurality of dummy bitline drivers for driving the plurality of dummy bitlines; a plurality of control circuits for controlling the plurality of dummy bitline drivers; a plurality of comparing means for comparing a voltage level of the dummy bitline and the predetermined reference voltage level; and a plurality of outputting means for outputting signals outputted from the plurality of comparing means. 
     In accordance with a still further aspect of the present invention, there is provided a semiconductor memory device comprising; a plurality of dummy wordlines independently formed with a plurality of normal wordlines; a plurality of normal bitlines independently formed with a plurality of normal bitlines; a monitoring means for measuring voltage on the dummy bitline or the dummy wordline; and a control circuit for controlling the monitoring means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the instant invention will become apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic circuit diagram showing a memory cell of the semiconductor memory device according to the prior art; 
     FIG. 2 is a schematic circuit diagram showing a semiconductor memory device having a monitoring circuit according to the present invention; 
     FIG. 3 is a detailed circuit diagram showing the monitoring circuit of the semiconductor memory device of FIG. 2 according to the present invention; 
     FIG. 4 is a circuit diagram showing a dummy memory cell of the semiconductor memory device of FIG. 2 according to the present invention; 
     FIG. 5 is a circuit diagram showing a dummy bitline sense amplifier of the semiconductor memory device of FIG. 2 according to the present invention; 
     FIG. 6 is a circuit diagram showing a control circuit of the semiconductor memory device of FIG. 2 according to the present invention; and 
     FIG. 7 is a timing diagram of the semiconductor memory device of FIG. 2 according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a semiconductor memory device having a monitoring circuit capable of measuring the line delay and a model parameter of a wordline or a bitline according to the present invention will be described in detail referring to the accompanying drawings. 
     FIG. 2 is a schematic block diagram showing a semiconductor memory device having a monitoring circuit according to the present invention. 
     Referring to FIG. 2, the semiconductor memory device includes a wordline monitoring circuit and a bitline monitoring circuit. The semiconductor memory device can have only the wordline monitoring circuit or the bitline monitoring circuit according to a chip design or the like. The semiconductor memory device having two monitoring circuits will now be described. 
     The semiconductor memory device includes a cell array CA having a plurality of dummy memory cells (not shown), a plurality of normal wordlines, a plurality of dummy wordlines BL, BLb connected to the dummy memory cells, a plurality of normal bitlines, a plurality of dummy bitlines connected to the dummy memory cells, a dummy wordline driver  10 , a dummy bitline sense amplifier  20 , a first comparing unit  30 A, a second comparing unit  30 B, a third comparing unit  30 C and a control circuit  40 . 
     The dummy bitline sense amplifier  20  amplifies data on the dummy bitline and the first comparing unit  30 A compares voltage of the dummy bitline with a first referent voltage Vref 1 . The second comparing unit  30 B compares voltage of the dummy bitline with a second referent voltage Vref 2  and the third comparing unit  30 C compares voltage of the dummy bitline with a third reference voltage Vref 3 . The control circuit  40  generates a plurality of control signals to control that measures voltage of the dummy bitline. 
     The monitoring circuit according to the present invention includes the dummy wordline driver  10 , the dummy bitline sense amplifier  20 , the first, second and third comparing units  30 A,  30 B and  30 C and the control circuit  40  shown in FIG.  2 . The control circuit  40  generates a plurality of control signals in 1 , in 2 , in 4 , in 6 , in 8 , and in 9  to control the dummy wordline driver  10 , the dummy bitline sense amplifier  20  and the first, second and third comparing units  30 A,  30 B and  30 C. 
     Output signals out 1 , out 2  and out 3  of the second, first, and third comparing units  30 B,  30 A, and  30 C, respectively, which are measured values in the monitoring circuit, are transferred into external circuits of the chip through terminals or pads. 
     FIG. 3 is a detailed circuit diagram showing a monitoring circuit to measure the wordline delay in FIG.  2 . 
     Referring to FIG. 3, the monitoring circuit includes a cell array CA, a wordline driver WD, a dummy wordline driver  10 , a first comparing unit  30 A and a driving unit  50 . The wordline driver WD drives a normal wordline WLn in the cell array CA and the dummy wordline driver  10  drives a dummy wordline in the cell array CA in response to a control signal in 1  of the control circuit  40  shown in FIG.  2 . The first comparing unit  30 A compares the voltage level of the dummy wordline with a first reference voltage Vref 1  level and the driving unit  50  drives the first comparing unit  30 A in response to the control signal in 1 . 
     It is preferable that CMOS transistors P 2  and N 3  in the dummy wordline driver  10  are the same size as CMOS transistors P 1  and N 1  in the normal wordline driver WD to obtain accurate data in measuring the normal wordline. 
     The voltage level applied to the dummy word line driver  10  is the boosting voltage Vpp level, which is the same as the voltage level of the normal wordline boosting signal Px. The first comparing unit  30 A includes a differential amplifier  30 A- 1  and a driver  30 A- 2 . The differential amplifier  30 A- 1  receives inputs of the dummy wordline signal and first reference voltage Vref in response to the control signal in 1  and the driver  30 A- 2  amplifies and outputs an output signal of the differential amplifier  30 A- 1 . The differential amplifier  30 A- 1  is a conventional differential amplifier and the driver  30 A- 2  includes three CMOS inverters connected in series. 
     FIG. 4 is a detailed circuit diagram showing a dummy cell  4  for measurement in FIG.  2 . The dummy cell  4  is a conventional dummy cell of DRAM and additionally includes an NMOS transistor N 4  controlled in response to a control signal in 2 , which is an output signal of the control circuit  40  and is activated in a measurement mode. Power supply voltage CVdd is applied to the dummy cell  4  through the NMOS transistor N 4 . When the control signal in 2  is activated, data of a logic ‘high’ level, which is the CVdd level, is written in the dummy cell  4 . 
     FIG. 5 is a detailed circuit diagram showing the dummy bitline sense amplifier  20  shown in FIG.  2 . The bitline sense amplifier  20  is operated in the same manner as a normal bitline sense amplifier and is controlled in response to control signals outputted from the control circuit  40  so that an accurate measurement of a delay of the dummy bitline sense amplifier  20  is carried out in the same way as that of the normal bitline sense amplifier. The dummy bitline sense amplifier  20  includes a sense amplifying unit  22 , a precharging unit  24  and isolation transistors  26 A and  26 B. The sense amplifying unit  22  amplifies data on a pair of dummy bitlines BL and BLb and the precharging unit  24  precharges and equalizes the pair of dummy bitlines. The isolation transistors  26 A and  26 B isolate the pair of dummy bitlines BL and BLb connected to the dummy cell  4  from the pair of dummy bitlines BL and BLb connected to the dummy sense amplifier  22  in a sensing operation. 
     Referring to FIG. 5, the dummy bitline sense amplifier  22  is operated in response to the control signal in 8 . The dummy bitline sense amplifier  22  is operated in the same manner as a common bitline sense amplifier in its sensing and precharging operations. 
     FIG. 6 is a detailed circuit diagram showing the control circuit  40 , that is, a timing signal generating circuit, shown in FIG. 2. A signal in_test is enabled in a specific mode, such as a special test mode or the like, and then disabled after tRAS. 
     Referring to FIG. 6, the control circuit  40  generates control signals in 1 , in 2 , in 4 , in 6 , in 8  and in 9  in a test mode. 
     FIG. 7 is a timing diagram of FIG.  2 . 
     Referring to FIG. 7, when the control signal in 1  is generated in response to the in_test signal, which is activated in the test mode, the dummy wordline driver  10  shown in FIG. 3 is driven so that the dummy wordline WL is driven to a level Vpp. At this time, the fist comparing unit  30 A compares the voltage level of the dummy wordline WD with a first reference voltage Vref 1  level. When the voltage level of the dummy wordline WL becomes higher than the first reference voltage Vref 1  level, the voltage level of an output node  38  of the differential amplifier  30 A- 1  moves from a logic ‘high’ level to a logic ‘low’ level. Before the control signal in 1  is enabled, the first reference voltage Vref 1  is higher than the voltage level of the dummy wordline WD so that the voltage level of the output node  38  is maintained with a logic ‘high’ level. 
     The operation of the differential amplifier  30 A- 1  will now be described in detail. 
     When an output node n 50  is set at a logic ‘high’ level by the driving unit  50  of the differential amplifier  30 A- 1 , a current on node  36  starts to flow into a ground. Two PMOS transistors T 1  and T 2 , which are in the differential amplifier  30 A- 1 , are the same size and two NMOS transistors T 3  and T 4 , which are also in the differential amplifier  30 A- 1 , and are also the same, so that the two PMOS transistors T 1  and T 2  drive a uniform current independent of voltage applied to nodes  32  and  38 . 
     When the dummy wordline driver  10  is operated in response to the control signal in 1 , a current flowing through the NMOS transistor T 4  is greater than that through the NMOS transistor T 3  in the initial operation, which means that the voltage level of the dummy wordline is less than the first reference voltage Vref 1  level, because the NMOS transistor T 4  is more highly biased than the NMOS transistor T 3 . The node  38  reaches a logic ‘low’ level more quickly than the node  32 . Since the voltage level of the node  32  is high, the gate voltage level of the PMOS transistors T 1  and T 2  is high so that the current flowing through the PMOS transistors T 1  and T 2  is reduced. Accordingly, the voltage level of the node  38 , which is the output node of the differential amplifier  30 A- 1 , becomes a logic ‘low’ level. 
     Subsequently, when the voltage level of the dummy wordline increases and becomes higher than the first reference voltage Vref 1  level, the NMOS transistor T 3  is more highly biased than the NMOS transistor T 4  so that the voltage level of the node  32  moves to a logic ‘low’ level. Namely, the drivability of the PMOS transistor T 2  becomes higher than that of the NMOS transistor T 4  so that the voltage level of the output node  38  moves to a logic ‘high’ level. 
     Since the output signal of the output node  38  in the differential amplifier  30 A- 1  is relatively weak, the output signal has to be amplified for measurement through a measurement pad out 2 . The driver  30 A- 2  functions to the output signal of the differential amplifier  30 A- 1 . Also, if the first comparing unit  30 A of FIG. 3 is successively operated in a normal mode, the stand-by current increases. In order that the first comparing unit  30 A is turned off during normal operation and is turned on while the dummy wordline is being driven to prevent the above current consumption, the comparing unit  30 A is enabled in response to the control signal in 1 . The driving transistor T 5  of the differential amplifier  30 A is enabled in response to the control signal in 1 . 
     The present invention is not limited solely to the object of measuring when a voltage level of the dummy wordline reaches a predetermined voltage level. Namely, when the first reference voltage Vref 1  is variable, it is possible to determine when the voltage level of the dummy wordline reaches the first reference voltage Vref 1  level. Accordingly, if a user applies a desired reference voltage Vref level, such as a 1V, 3V or Vext level, when the voltage level of the dummy wordline is higher than the reference voltage Vref level, the comparing unit  30 A outputs a signal. If an analog operation of the wordline is transformed to a digital operation and a delay time from the control signal in 1  to the measurement pad out 2  is measured, a RC delay of the dummy wordline can be detected. 
     Before the dummy wordline driver is driven, data of a logic ‘high’ level have to be written in the dummy cell for measurement, which is carried out by the control signal in 2 . Namely, referring to the timing diagram of FIG. 7, when the in_test signal is at a logic ‘high’ level, the level of the control signal in 2  is maintained at a logic ‘high’ level so that the NMOS transistor N 4  of FIG. 4 is turned on. Accordingly, the data of a CVdd level are written in the dummy cell. When the test operation starts in response to the in_test signal, the control signal in 2  is decreased to a logic ‘low’ level so that the NOMS transistor N 4  of FIG. 4 is turned off. If the dummy wordline is enabled in response to the control signal in 1 , a charge sharing operation of the dummy bitline is carried out only using the capacitance of a cell, such as a normal cell. 
     Referring to FIG.  2  and FIG. 5, when the dummy wordline is enabled, the voltage level of the dummy bitline becomes Vblp (bitline precharge voltage)+dv (voltage added by the charge sharing) by the charge sharing operation. On the other hand, the voltage level of the dummy bitline bar is fixed at a level Vblp. When an applied second reference voltage Vref 2  level is higher than that of the dummy bitline, an output signal out 1  is generated. When the output signal out 1  of the second comparing unit  30 B reaches a logic ‘low’ level, the dummy bitline sense amplifier  20  is driven in response to the signal out 1 . 
     Referring to FIG.  5  and FIG. 7, the driving transistors  22 A and  22 B of the dummy bitline sense amplifier  20  are driven in response to the control signal in 8  and a sensing operation of the dummy bitlines BL and BLb is carried out. The control signal in 4  of the precharging unit  24 , which maintains the voltage level of the dummy bitlines BL and BLb at a precharge voltage Vblp level, has to reach the logic ‘low’ level more quickly than the control signal. 
     When the sensing operation starts, the dummy bitline BL increases to a logic ‘high’ level and the dummy bitline bar BLb decreases to a logic ‘low’ level in order that data of a logic ‘high’ level can be written in the dummy cell. The third comparing unit  30 C compares a dummy bitline BL voltage level with a third reference voltage Vref 3  level applied from an external circuit and, when the dummy bitline BL voltage level is higher than the third reference voltage Vref 3  level, the comparing unit  30 C outputs signal out 3 . 
     Referring to FIG. 7, the disable time of the control signal in 4  outputted from the control circuit  40  has to be later than that of the control signal in 1 . When the dummy bitlines BL and BLb are precharged to the precharge voltage Vblp level and the dummy wordline WL is enabled, a current path between Vblp connected by the control signal in 4  and CVdd connected by the control signal in 2  is generated. When the control signal in 4  is disabled, the signal in_test is delayed so that the above problem is solved. 
     It is preferable that the first comparing unit  30 A, the second comparing unit  30 B and the third comparing unit  30 C are configured with the same structure for sensing under identical surrounding conditions according to the present invention. The isolation transistors  26 A and  26 B of dummy bitline sense amplifier  20  are inserted to make the surrounding conditions identical to those of the normal bitline sense amplifier. Accordingly, the surrounding conditions between the normal bitline and the dummy bitline or the normal wordline and the dummy wordline are identified so that accurate measurement data are expected. 
     When the monitoring circuit is used according to the present invention, an accurate wordline or bitline RC delay and a model parameter can be measured so that a semiconductor memory device having accurate timing of its internal operation can be fabricated. 
     While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.