JPS58147899A - Measuring method of semiconductor memory - Google Patents

Abstract

PURPOSE:To take a measurement of the threshold value of a complete LSI which is conventionally impossible by analysing the operation of a flip-flop from the inversion of a storage state caused by a temporary drop in power voltage. CONSTITUTION:A voltage VCC is lowered approximately to the threshold voltage of a driving transistor (TR) in a memory cell at xV/sec (procedure 1) and then raised to its original level again (procedure 2). Then, the stored information in the memory cell 1 is read out to check whether inversion to ''1'' is caused or not (procedure 3). The minimum value of the lowered voltage VCC is set to lower values gradually at ZmV steps and the procedures 1-3 are followed repeatedly. In the procedure 3, when the stored information is inverted to ''1'', the minimum value of the lowered VCC in the preceded procedure 1 is regarded as the critical voltage of the TR which was conducted just before the power voltage is lowered between two driving TRs in the memory cell. On the basis of this critical voltage, the threshold voltage of the TR is found.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for indirectly measuring the threshold voltage of a transistor in a memory cell in a finished semiconductor memory.

In a finished semiconductor memory product, changes over time in the threshold voltages of internal elements cause deterioration of the electrical characteristics of the memory IJLSI, and as a result, the memory LSI no longer operates normally. For this reason, it is important to measure the threshold voltage of internal elements and observe its fluctuations, but until now it has not been possible to
In I, it was considered impossible to measure the threshold voltage of internal elements.

The present invention provides a complete LSI, which was previously considered impossible.
In flip-flop type static random access memories using insulated gate field effect transistors or Schottky gate field effect transistors, the memory generated by temporarily lowering the supply voltage can be used to measure the threshold voltage of A method for measuring semiconductor memory that makes it possible to determine the threshold voltage of the transistor that was in a conductive state before lowering the power supply voltage among the two drive transistors in a memory cell, based on state reversal and analysis of flip-flop operation. It provides:

The present invention will be described in detail below using the drawings.

Figure 1 shows a static N-channel/channel M to which the present invention is applied.
O8RAM (Metal Oxide Semiconductor
ndductorRandom Access Memo
ry) shows a 6-Tr type memory cell.

In the figure, VCC represents the power supply voltage, 'r, +
T2 is an enforcement type drive transistor, 'r
, + 'r, represents a depletion type load transistor. Further, the threshold voltages of 'rl+'r2 are respectively set to Vt+ + vT2. For example, V,, > V,
If no, T! After setting vcc to vl
,〉vcc>vT2,
T1 becomes blocked at a certain power supply voltage that is in the middle of decreasing at
T2 becomes conductive. In other words, the memory state is reversed.

This state is maintained even if VCC is returned to its original voltage. The present invention is based on this phenomenon.

An embodiment of the first invention of the present application will be explained based on the above basic idea. Although the storage state of the memory is 1'' and '0#, the following explanation assumes that the initial state is "0".

VCCt' is lowered at xV/see to near the threshold voltage of the drive transistor in the memory cell (procedure 1), and raised again to the original voltage at yV/see (procedure 2). Subsequently, the stored information of the memory cell is read and it is checked whether it has been inverted to '1'' (procedure 3).The minimum value for lowering VCC is gradually set to a lower value in ZmV steps, and procedures 1 to 3 are repeated. If the stored information is reversed to 1'' in step 3', the transistor that was in a conductive state before lowering the power supply voltage among the two drive transistors of that memory cell is This is called the critical voltage of Based on this critical voltage, the threshold voltage of the transistor can be determined. In the case of this embodiment, if the critical voltage when v1□>VT2 is VCR, then vTl=vCR-%f/ItI (1). Here, ID is the drain current of T3, β1 is T1
represents the gain constant of

The bar graph in Figure 2 shows the results of measuring the critical voltage of the transistor with the higher threshold voltage of the two drive transistors in each memory cell for all memory cells of a certain semiconductor memory using the measurement method described above. This is the relative frequency distribution. Furthermore, the curve -'(X) in Figure 2 shows the distribution of critical voltages of the drive transistors of all memory cells, including drive transistors with low threshold voltages among the drive transistors in the memory cells, based on the measurement results. This was calculated assuming that the distribution is normal. Furthermore, V in FIG. RO is the value of the critical voltage of the test transistor on the same chip, which is determined based on equation (1) from the measured value of the threshold voltage of the transistor for test in the manufacturing process. Its value lies between the mean value of the distribution and (mean value - standard deviation). From the results of this measurement method shown in FIG. 2, it is possible to determine the threshold voltage of the drive transistor with the higher threshold voltage among the two drive transistors in the memory cell of the semiconductor memory with a high lateral variation. be proven.

In the above invention, as the layer state before lowering the power supply voltage, if the transistor with the higher threshold voltage of the two drive transistors of the memory cell is in a conductive state, the threshold voltage of the transistor can be measured. , when the transistor is in a closed state, no reversal of the memory state occurs and the threshold voltage of the transistor cannot be determined. The second invention of the present application provides a method in which reversal of the memory state always occurs and the threshold voltage can be reliably determined. That is, if memory information is written in advance so that the transistor with the higher threshold voltage of the two drive transistors of the memory cell becomes conductive, and then the method of the above embodiment is executed, a certain voltage becomes conductive. If the value corresponds to the critical voltage or less of the drive transistor in the conductive state, the memory state will always be reversed, so the threshold voltage of the drive transistor set in the conductive state can always be determined. One way to find out which of the two drive transistors has a higher threshold voltage is to find out which one of the two drive transistors has a higher threshold voltage. There is a method that utilizes the fact that the threshold voltage is higher than other threshold voltages.

When determining the threshold voltage of the transistor with the higher threshold voltage among the two drive transistors of the memory cell for all the memory cells of the semiconductor memory, if the second invention of the present application is applied, the two drive transistors for all the memory cells of the semiconductor memory are It is necessary to turn on the transistor with a higher threshold voltage among the drive transistors, making writing of storage information complicated.

The third invention of the present application provides a method for determining the threshold voltage of the transistor with the higher threshold voltage among the two drive transistors of the memory cell for all memory cells of a semiconductor memory by a simple write operation. be. FIG. 3 shows a time chart showing the measurement procedure of an embodiment of the third invention. First, write memory information ``O'' to all memory cells (step 11).Next, V
. 0 is lowered at xV/sec to near the threshold voltage of the drive transistor in the memory cell (step 12), and raised again at the original voltage level at yV/seo (step 13). continuation,
The stored information of the memory cells is read out and all memory cells are checked to see if there is a memory cell inverted to '1'' (step 14).The lowest value for lowering the vco is gradually set to a lower value in ZmV steps, and steps 12 to Repeat step 14. In step 14, if there is a memory cell inverted to '1', lower the power supply voltage of the two drive transistors of the memory cell to the lowest value that VCC was lowered in the previous step 12. Let be the critical voltage of the transistor that was previously conducting. Based on this critical voltage, the threshold voltage of the transistor is determined from equation (1) (step 15). By repeating step 11 and steps 12 to 11iii15 described above, the threshold voltage of the higher threshold voltage of the two drive transistors can be found for approximately half of the memory cells with probability. Next, write the memory information "B" to all memory cells (step 16), and step 1
2゜V according to step 13. 0 and return VCC to its original voltage. Continue reading memory information,
Next, check whether there is a memory cell that is inverted to ``On'' (step 17). Set the lowest value for lowering vcc to a gradually lower value using the ZmV stenog, and repeat step 129 and step 17. Step 17 If there is an inverted memory cell in @Q#I, V in the previous step 12.

The lowest value obtained by lowering C is taken as the critical voltage of the transistor that was in a conductive state before lowering the power supply voltage among the two drive transistors of the memory cell. Based on this critical voltage, calculate the threshold voltage of the transistor from equation (1) (
Step 15). The above-mentioned move j@16 and procedure 12. Step 1
31 Step 17, Step 15 are repeated (Step ①) to find the threshold voltage of the transistor with the higher threshold voltage among the two drive transistors of each memory cell that could not be found in Process ②. I can do it.

The threshold voltage of the drive transistor on the higher threshold voltage side among the 2f drive transistors of the memory cell can be determined for all memory cells of the semiconductor memory by the above-mentioned process E and process II.

Fluctuations in the threshold voltage cause deterioration of the electrical characteristics of the memory IJLSI. The present invention can measure the pre-driving phenomenon of electrical characteristic deterioration of the memo IJLsI as a variation in the threshold voltage, and can be applied to reliability evaluation of the memo IJLsI. One embodiment is shown in FIG. FIG. 4 shows the change in threshold voltage after performing an accelerated test for a certain period of time, and shows it in a bar graph based on relative frequency. As shown in the figure, an increase in the threshold voltage is observed, which is a pre-driving phenomenon of deterioration of the electrical characteristics of the memory IJLsI. From this, the present invention provides Memo I
It is clear that this method is effective for evaluating the reliability of JLSI.

As explained above, the present invention makes it possible to easily observe the change in threshold voltage over time and detect the deterioration of the electrical characteristics of a memory LSI at the stage of a prodromal symptom. The effectiveness of the present invention is remarkable when evaluating the influence of pot electrons on the LSI characteristics of short channel M'08LSI.

[Brief explanation of drawings]

Figure 1 shows a static N-channel M to which the present invention is applied.
Figure 2 is a 6-Tr type upper Mori cell circuit diagram of O8RAM, and a bar graph of the relative frequency distribution showing the experimental results of an embodiment of the present invention. Actual measurement of the threshold voltage of a transistor for manufacturing process inspection on the same chip as the curve 1IIf (, c) obtained by assuming that the critical voltage distribution of the drive transistors of all memory cells including low drive transistors is a normal distribution. A characteristic diagram that also shows the critical voltage VcBo of the test transistor determined from the value based on equation (1), FIG. 3 is a time chart diagram showing the measurement procedure of an embodiment of the present invention, and FIG. FIG. 7 is a relative frequency distribution diagram showing an example of the results of the amount of change over time of the threshold voltage obtained by applying the invention. Patent applicant: Nippon Telegraph and Telephone Public Corporation Representative: Tsune Hakusui and one other person

Claims (3)

[Claims] (1) In a 79-nogphronop type static random access memory using an insulated gate field effect transistor or a Schottky gate field effect transistor, the power supply voltage is lowered from the rated value to a certain voltage and then returned to the original voltage. The operation to read the memory state of
The maximum value of the certain voltage when the certain voltage is changed and the storage state of the memory is always set to the same state before the power supply voltage is lowered, and the storage state is different from before the power supply voltage is lowered. , 2 in the memory cell
A method for measuring a semiconductor memory, comprising determining the threshold voltage of a transistor that was in a conductive state before lowering a power supply voltage among drive transistors. (2) In a flip-flop type static random access memory using an insulated gate field effect transistor or a Yotsuki gate field effect transistor, the transistor with a lower threshold voltage is closed in advance, and the transistor with a higher threshold voltage is closed. After writing the stored information in the memory so that the voltage becomes conductive, the power supply voltage is lowered from the rated value to a certain voltage, and the voltage is returned to the original voltage, and the stored state of the memory is read. The storage state of the memory before lowering the power supply voltage is always set to the same state, and the memory state is different from before lowering the power supply voltage. A method for measuring a semiconductor memory, comprising determining the threshold voltage of a transistor that was in a conductive state before lowering a power supply voltage among drive transistors in 21 directions. (3) In a flip-flop type static random access memory using an insulated gate field effect transistor or an insulated gate field effect transistor, storage information "O" (t or '1') and then lowering the power supply voltage from the rated value to a certain voltage and returning it to the original voltage again to read the memory state of the memory. Always set the memory state to @On (still "al") and repeat, until the memory state becomes '1' (or 'o')
Determine the threshold voltage of the transistor that was in a conductive state before lowering the power supply voltage among the two drive transistors in the memory cell from the maximum value of the certain voltage when the voltage is inverted; After writing the memory information "'l#" (or "o") to all memory cells of the memory, the power supply voltage is lowered from the rated value to a desired voltage and returned to the original voltage to change the memory state of the memory. The read operation is
By changing the desired voltage and always setting the memory state of the memory to 1" (or 10#) before lowering the power supply voltage, the memory state becomes 0# (or 'l').
), from the maximum value of the desired voltages when the voltage is inverted to A method for measuring semiconductor memory characterized by: