|Publication number||USRE40282 E1|
|Application number||US 10/145,606|
|Publication date||Apr 29, 2008|
|Filing date||May 9, 2002|
|Priority date||Dec 21, 1996|
|Also published as||US6059450|
|Publication number||10145606, 145606, US RE40282 E1, US RE40282E1, US-E1-RE40282, USRE40282 E1, USRE40282E1|
|Inventors||David Charles McClure|
|Original Assignee||Stmicroelectronics, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (18), Classifications (16), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The subject matter of the present application is related to copending United States application, titled “BURN-IN STRESS TEST MODE”, Docket Number 96-C-53, filed on Dec. 21, 1996, which is assigned to the assignee hereof, and which is herein incorporated by reference.
1. Technical Field
The present invention relates generally to Integrated Circuit (IC) memory devices and more specifically to entry of an integrated circuit memory device into a test mode.
2. Discussion of the Prior Art
Integrated circuit memory devices are routinely subjected to myriad types of test modes. These test modes may be used to subject the integrated circuit device to functional testing, to burn-in testing, and to stress testing, to name just a few examples of testing.
A major concern with integrated circuit devices capable of entering a test mode to be tested is accidental entry into the test mode when the device is not to be tested. Such false entry of an integrated circuit memory device into a test mode is typically caused by a voltage spiking condition of a voltage supplied to the integrated circuit memory device. False entry is exacerbated when the integrated circuit is placed in a noisy environment.
There is thus a need in the art to prevent false entry of an integrated circuit device into a test mode when the device is not to be tested. Any means for preventing false entry of the device into the test mode should protect the device while in a noisy environment in which the device may be subjected to a voltage spiking condition of a supply voltage.
It is an object of the invention to prevent false entry of an integrated circuit device into a test mode when the device is not intended to be in the test mode.
It is further an object of the invention to prevent false entry of an integrated circuit device into a test mode when the device is subjected to a voltage spiking condition of a supply voltage.
In accordance with the present invention, an integrated circuit structure and method provides for the integrated circuit device to respond to an edge transition detection (ETD) pulse in one of two ways. First, in response to the ETD pulse, the integrated circuit device exits a test mode at least temporarily every cycle of the integrated circuit device. Second, a node of the integrated circuit device is reinitialized every cycle if it is not forced by a super voltage indicative of test mode entry. Both of these responses prevent accidental entry of the integrated circuit device into the test mode. If the integrated circuit device is supposed to be in the test mode, it stays in the test mode. If, however, the integrated circuit device is not intended to be in the test mode, the ETD pulse forces the integrated circuit device out of the test mode. Subsequent entry into the test mode of the device is permitted if conditions for entry into the test mode have otherwise been met. The ETD pulse creates a DC current path that exists only for the duration of the ETD pulse and therefore current dissipation of the integrated circuit device is minimized.
The integrated circuit structure has a node that the ETD pulse initializes every cycle of the integrated circuit device. The ETD pulse is triggered by a change in the state of an address pin or other control pin of the integrated circuit device. According to a first embodiment of the invention, the ETD pulse controls an ETD transistor connected to a node of the integrated circuit device. The node is controlled by a diode stack having one or more diodes or by the ETD transistor. The node is capable of being quickly discharged to protect against Vcc noise spikes that can trigger false entry into a test mode.
According to a second embodiment of the invention, the ETD pulse controls the gates of one or more ETD transistors, with each ETD transistor being connected to a bipolar transistor. Each ETD transistor operates to ensure that the emitter of the bipolar transistor to which it is connected is Vbe volts lower than the base voltage Vb of the bipolar transistor in order to counter leakage between the collector and the emitter of the bipolar transistor. The ETD transistor is capable of quickly discharging the node to protect against Vcc noise spikes that can trigger false entry into a test mode.
The novel features believed characteristic of the invention are set forth in the claims. The invention itself, however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
The present invention provides a burn-in stress test mode that is capable of disabling a time-out feature of an IC memory device during a stress test mode of the device in order to facilitate stress testing of the device in a burn-in oven in a timely and economical manner. The time-out feature of an IC memory device is disabled during the burn-in stress test mode by leaving on the wordlines of the device for the duration of the memory cycle for maximum burn-in efficiency.
A major concern of the present invention is the ability to enter into the burn-in stress test mode at the package level of the device. In many IC memory devices there may be no pins available which may be dedicated to enable or enter the burn-in stress test mode. On an asynchronous memory device, for instance, any pin sequence or combination of pin sequences may be needed for device operation and thus the use of device pins to enter the burn-in stress test mode is not a feasible solution. In addition to the dearth of device pins with which to enter the burn-in stress test mode, the use of a greatly elevated voltage such as 15 volts for a CMOS (complementary metal oxide silicon) device on the device pin is likewise an unworkable solution in light of the fact that burn-in ovens do not accommodate the use of greatly elevated voltage levels.
Operating conditions for the normal operating mode and the burn-in operating mode of an integrated circuit memory device differ with respect to voltage levels and temperature ranges. The temperature of an integrated circuit device tracks changes in operating voltage of the device. Consider again the case of a 1 Meg SRAM memory device. The 1 Meg SRAM memory device has a normal operating range of approximately 2.7 volts to 3.6 volts at a maximum temperature of approximately 85 degrees Celsius. Burn-in of the integrated circuit memory device occurs at significantly higher voltage and temperature conditions in order to accelerate weak bit failures and infant life failures. Thus burn-in of the 1 Meg SRAM device during the burn-in operating mode may occur at 6 volts and 125 degrees Celsius.
The differences in voltage operating conditions between normal operation and burn-in operation are used by the present invention to accomplish entry into the burn-in stress test mode only during the elevated voltage condition consistent with burn-in operation. Entry into the burn-in stress test mode is accomplished internally to the memory device upon sensing the elevated voltage and/or temperature condition characteristic of burn-in operation. Thus, no external control through the device pins is required for entry into the burn-in stress test mode.
Node n1 is formed by the electrical connection of the output of programmable diode stack 20a . . . 20n, the source/drains of transistors 22, 24, and 30, and the gates of transistors 32, 34, 36, 38 of Schmitt trigger 31 as shown in FIG. 1. It should be noted that while diode stack 20a . . . 20n is shown as having a number of diodes, diode stack can be comprised of just one diode or any number of diodes. Transistor 22 is a very weak transistor and thus node n1 is characterized as having a very weak static load on it. Transistor 24 is a strong transistor whose gate is controlled by POR signal 12. Transistor 26 is a weak transistor whose gate is controlled by EDT signal 14.
As mentioned, the gates of the transistors 32, 34, 36, 38 of Schmitt trigger 31 help form node n1. Transistors 32, 34, 36, 38 are serially connected as shown in
Diode stack 20a . . . 20n is made programmable by the presence of fuses 21 which may be connected in parallel with a diode 20 of diode stack 20a . . . 20n as shown. Blowing a fuse 21 causes the diode 20 to which the fuse 21 is connected in parallel to be included in the diode stack 20a . . . 20n, since the diode 20 will no longer be shorted out once the fuse 21 is blown. Thus, in the example shown in
Upon power-up of the device, POR signal 12 pulses high which forces node n1 to a low state initially. Programmable diode stack 20a . . . 20n is connected between node n1 and Vcc as shown. A diode of diode stack 20a . . . 20n is an n+ junction in a p-well, assuming an n-substrate n-p-n device of the type shown in FIG. 2. These p-n diodes 20 provide much greater stability over process variations than that which could be afforded by transistors. The diodes of diode stack 20a . . . 20n should be laid out remotely from other circuits, be well strapped and employ a guarding/dummy collector structure to prevent device latchup. MOSFET connected diodes could be employed in place of the p-n diodes shown. MOSFET connected diodes, however, may be less desirable than p-n diodes because they vary more over process and temperature than do p-n junction diodes which are more tightly controlled over process and temperature variations.
As supply voltage Vcc rises, node n1 will begin rising once Vcc exceeds the diode forward bias voltage drop. At the burn-in temperature of approximately 125 degrees Celsius, the diode forward bias voltage drop of each diode 20 is approximately 0.3 volts. Assuming that the diode stack 20a . . . 20n has thirteen (13) diodes, 20a . . . 20m, connected in series, node n1 will start to rise at 3.9 volts. It should be noted that by prudently choosing the number of diodes in the diode stack 20a . . . 20n no DC (direct current) is consumed at the normal operating range of 3 6 volts or less.
In order to trigger entry of the device into the burn-in stress test mode, supply voltage Vcc must exceed a predetermined voltage level defined as the diode forward bias voltage drop of diode stack 20a . . . 20n plus the trip point of Schmitt trigger 31. Thus, assuming that the diode forward bias voltage drop of diode stack 20a . . . 20n is approximately 3.9 volts and the trip point of Schmitt trigger 31 is approximately 1.6 volts, Vcc must be greater than 5.5 volts to trigger entry into the burn-in stress test mode. At 5.5 volts, node n2 will go to a low state and node n3 will go to a high state.
Schmitt trigger 31, located between nodes n1 and n2, provides hysteresis and noise immunity as voltage supply Vcc slowly ramps up. The diode forward biased voltage drop associated with each diode 20 increases at lower temperatures at the rate of approximately −2.1 mV per degree Celsius. Thus at the lower temperature of approximately 25 degrees Celsius, the diode forward biased voltage drop is approximately 0.5 volts; compare this with the 0.3 volt drop at 125 degrees Celsius noted above. Schmitt trigger 31 therefore provides immunity against mistakenly entering the burn-in stress test mode under normal operating conditions since, using the example discussed above, Vcc must be 13 times 0.5 volts, or 6.5 volts, before node n1 starts rising.
Control bar signal 18 controls the gate of pull-down transistor 28 and pull-up transistor 30. Control bar signal 18 may be a function of a Chip Enable signal or other suitable control signal of the memory device. Thus, Control bar signal 18 is a low true signal that goes low when in a stress test mode or when the memory device is deselected. During the burn-in stress test mode, Control bar signal 18 goes low to disable the DC (direct current) current path through diode stack 20a . . . 20n, through transistors 22, 24, down through transistor 28 to ground that exists while the device is being stressed at an elevated voltage of 6 volts or more. NC signal 16 is an optional signal that may be used to externally inhibit entry into the burn-in stress test mode if desired. A No Connect pin which provides NC signal 16 may be any device pin specified as a no-connect in the pinout of the datasheet of the device or any device pin which does not have to be exercised. For instance, an output enable (OE) pin of the device could be readily used in place of the No Connect pin. The No Connect pin would recognize the time-out feature of the device, thereby causing the output of NAND gate 46, Burn-In Mode bar signal 58, to ignore the output of Schmitt trigger 31. The signal at node n3 and NC signal 16 are input signals to NAND logic gate 46. If NC signal 16 is a low state, then Burn-in mode bar signal 58 would be forced high, thereby allowing for the time-out circuit of the device to operate so that operational life (op-life) studies of the device may be conducted. Op-life studies predict how long the device may be expected to last in normal operation.
Burn-In Mode bar signal 58 indicates when the device has successfully entered the burn-in stress test mode. The status of Burn-In Mode bar signal 58 may be monitored either through Burn In Flag 54 or through a Device Pin 48. Burn-In Flag 54 is useful in indicating when the device has entered the burn-in stress test mode since it is not readily apparent when the device has or has not timed out. Burn-in flag 54 may be a test pad at wafer level which accommodates testing of the device. Device Pin 48 may be an input pin or an output pin of the device. At the device package level, a weak leakage transistor 50 is connected to Device Pin 48; transistor 50, as a weak leakage transistor, causes approximately 10 to 100 μA of pin leakage. No Connect pin 16 is a package pin of the IC memory device that similarly may be monitored.
Burn-in stress test mode circuit 10 of
The ETD capabilities of
As an example, consider the consequences of a voltage spike without the benefit of ETD pulse 14 controlled weak ETD transistor 26. If supply voltage Vcc were to spike up to 5.5 volts temporarily, then node n1 would correspondingly be pulled-up and then be very slowly discharged by very weak transistor 22. The addition of ETD transistor 26 to the circuitry of
The diodes of diode stack, rather than being capable of overcoming ETD transistor 26, can be sized so that there are incapable of overcoming ETD transistor 26. ETD transistor 26 would not be a weak transistor, so that every cycle the memory device would be at least temporarily taken out of the test mode for the duration of the test mode in order to prevent false triggering, or entry, of the memory device into the test mode. This occurs regardless of whether the memory device is supposed to be in the test mode. After the ETD pulse, the memory device enters the test mode if it is intended to be in the test mode and if the time-out feature of the memory device has not commenced.
Test mode circuit 60 of
The ETD transistors 64a . . . 64n could be replaced with static loads wherein ETD pulse 14 could be tied to supply voltage Vcc. Alternately, a transistor could be placed in parallel with each ETD transistor 64a . . . 64n, wherein each such transistor being placed in parallel with an ETD transistor 64 has its gate controlled by supply voltage Vcc.
Weak transistors having gates tied to supply voltage Vcc can be placed in parallel with ETD transistors 64a . . . 64n, but this arrangement would provide a current path and therefore standby current introduced by the load of the weak transistors. Inclusion of Control bar signal 18 that is a function of a Chip Enable signal in
Control bar signal 18 controls the gate of pull-down transistor 68 and pull-up transistor 70. During a test mode, Control bar signal 18 goes low to disable the DC (direct current) current path through bipolar transistors 62a . . . 62n, ETD transistors 64a . . . 64n, and transistor 68 to ground. Control bar signal 18 is also an input signal to gate 46 that goes high during normal operation of the memory device to inhibit entry into the test mode or to cause the test mode to be exited. Similarly, NC signal 16 is an optional signal that may be used to externally inhibit entry into the burn-in stress test mode if desired or to force the memory device to exit the test mode. An active low signal of either Control bar signal 18 and NC signal 16 will cause the output of NAND gate 46, Test Mode bar signal 74 to ignore the output of Schmitt trigger 31.
Similarly, a p-channel transistor could be placed in series with the collectors of bipolar transistors 62a . . . 62n to power supply Vcc and transistor 70 would be replaced by an n-channel transistor connected between node n1 to ground. The gates of the p-channel transistor and the n-channel transistor would be controlled by Control bar signal 18. The source of the p-channel transistor would be connected to supply voltage Vcc. In this implementation, NAND gate 46 would be a two input gate having only two input signals provided to it: the signal at node n3 and NC signal 16.
While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.
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|U.S. Classification||714/36, 714/724, 365/201|
|International Classification||G06F11/00, G11C29/46, H03K5/1534, G11C8/18, G11C7/10|
|Cooperative Classification||G11C7/1045, G11C8/18, H03K5/1534, G11C29/46|
|European Classification||G11C29/46, G11C7/10M7, H03K5/1534, G11C8/18|