JP3239617B2 - Semiconductor device reliability test method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for testing the reliability of a semiconductor device.

[0002]

2. Description of the Related Art With the progress of high density, high integration and miniaturization of a semiconductor integrated circuit device, an SiO ₂ film (hereinafter referred to as an oxide film) used for a MOS transistor or a MOS capacitor constituting the device has been reduced in thickness. Tend to be. On the other hand, since the power supply voltage of the semiconductor integrated circuit device is usually kept constant, the intensity of the electric field applied to the oxide film increases as the thickness decreases. Under this circumstance, the time-dependent dielectric breakdown (TDDB) of the oxide film is performed.
wn) has become a serious problem in reliability. This is a phenomenon in which, when an electric field is applied to an oxide film, the oxide film is destroyed after a certain period of time has elapsed since the start of the application of the electric field, the electrical insulation is lost, and a short circuit occurs. Hereinafter referred to as oxide film life). As described above, a time-dependent dielectric breakdown test for estimating the life of an oxide film is an essential item in the design and development of a semiconductor integrated circuit device, and its importance tends to further increase. A conventional method for estimating the life of an oxide film will be described below.

FIGS. 3 and 4 show a conventional method for estimating the lifetime of an oxide film from dielectric breakdown with time. FIG. 3 shows the measurement results of the oxide film life at each stress electric field strength, and FIG. 4 shows an example of the method of estimating the oxide film life.

In FIG. 3, the horizontal axis represents the stress time during which an electric field is applied to the oxide film, the right vertical axis represents the cumulative failure rate P due to the dielectric breakdown of the oxide film with time, and the left vertical axis represents ln calculated from the cumulative failure rate P. (-Ln (1-P)). In FIG. 3, 1 is the cumulative failure rate at the time when an individual oxide film breakdown occurs under each stress electric field strength, 2 is the time dependency of the cumulative failure rate under each stress electric field strength, and 3 is the oxide film. It is a measured value of 50% cumulative failure time due to aging dielectric breakdown. As shown in FIG. 3, the cumulative failure rate P is represented by the logarithm of time on the horizontal axis and ln (−l
Plotting on the graph of n (1-P)) is generally called Weibull plot. If the fault follows the Weibull distribution, the result of the Weibull plot is a straight line. The Weibull plot is widely used as a method for graphing the cumulative failure rate of dielectric breakdown with time of an oxide film.

In FIG. 4, the horizontal axis represents the electric field intensity applied to the oxide film, and the vertical axis represents the life of the oxide film. In FIG. 4, 4 is an estimated value of the oxide film life, 5 is the maximum value of the oxide film applied electric field strength in actual use, 6 is an estimated value of the oxide film life in actual use, and 7 is the actual measurement of 50% cumulative failure time. The value 8 is the stress field strength.

In order to estimate the life of an oxide film, a number of MOS capacitors are prepared in which an equivalent oxide film formed by the same shape, size and manufacturing process is used as a capacitor insulating film. This is divided into multiple sets. For each set, respectively, a maximum value of 5 by remote high stress field strength 8 of oxide film applied electric field strength in actual use as shown in FIG. 4, it is applied in the form of a constant voltage stress. Due to the application of the stress, dielectric breakdown occurs with time in each set of oxide films, and the number of failure-producing oxide films increases with time.

As shown in FIG. 3, a Weibull plot is made of the cumulative failure rate 1 at the time when individual oxide film breakdown occurs under each stress electric field strength. Empirically, the failure due to the time-dependent dielectric breakdown of the oxide film follows the Weibull distribution, so that the Weibull plot of the time dependence of the cumulative failure rate is usually a straight line. From this, a regression line is obtained for the cumulative failure rate 1 at the time when each oxide film breakdown occurs under each stress electric field strength, and the time dependency 2 of the cumulative failure rate 1 under each stress electric field strength is obtained. Ask for. Using the time dependency 2 of the cumulative failure rate 1, the cumulative failure rate 1
Is 50%, that is, the actual measured value 3 of the 50% cumulative failure time.

[0008] The 50% cumulative failure time is regarded as the oxide film life, and as shown in FIG.
7 is plotted on a semilogarithmic graph of the electric field intensity applied to the oxide film on the horizontal axis and the life of the oxide film on the vertical axis. Empirically, the measured value 7 of the 50% cumulative failure time is a straight line on this graph. Based on this, a regression line of the actually measured value 7 of the 50% cumulative failure time, that is, the estimated value 4 of the oxide film life, is obtained, and this straight line is used to obtain the maximum value 5 of the electric field strength applied to the oxide film in the actual use. The estimated value 6 of the oxide film life of the above is obtained.

In order to evaluate the reliability of an oxide film in a short time using a MOS capacitor on a wafer that has completed a diffusion process, a large number of MOS capacitors using an oxide film to be evaluated as a capacitor insulating film in a wafer state are used. Test as is.
In this case, measurement is performed by sequentially probing the MOS capacitor on the wafer while moving the measurement point using an autoprober.

[0010]

However, the conventional method for estimating the life of an oxide film has the following problems.

Generally, process conditions are frequently changed during development of a manufacturing process. For this reason, the life of the oxide film may change due to a change in the process conditions. Therefore, it is necessary to frequently estimate the life of the oxide film.

Alternatively, when mass-producing a product in a factory, the life of an oxide film varies for each wafer that has completed the diffusion process due to fluctuations in process conditions due to troubles in equipment, etc., and sometimes the life of the oxide film is short, and the reliability standard is short. May not be satisfied. When such an abnormality occurs, it is necessary to immediately measure the estimated value of the oxide film lifetime without overlooking it in order to prevent products with insufficient reliability from being shipped to the market.

In the above case, it is necessary to measure the estimated value of the oxide film lifetime for each wafer in a short period of time for all wafers that have completed the diffusion process.

In the conventional method of testing the life of an oxide film, when performing a test in a wafer state, the MOS capacitors on the wafer are sequentially probed using an auto prober and measured. For this reason, M
The number of OS capacitors is limited to one or several that can be electrically connected at the same time by the probe needle of the probe card. Therefore, it takes a long time to measure the cumulative failure rate 1 by applying a plurality of stress electric field strengths to a large number of MOS capacitors. In particular, this tendency is remarkable in the measurement of the low-stress electric field strength, which is a major obstacle in estimating the lifetime of the oxide film of all the wafers.

In order to shorten the measurement time, there are methods such as reducing the number of MOS capacitors used for the measurement or increasing the stress electric field strength at the time of the test.

However, when the number of MOS capacitors used for measurement is reduced, the cumulative failure rate 1 is obtained for a plurality of stress field strengths. Therefore, the number of MOS capacitors allocated to each stress field strength is further reduced. Become. Although the MOS capacitors on the same wafer are assumed to be equivalent, they actually have variations in characteristics. The cumulative failure rate 1 at each stress electric field strength is obtained from different MOS capacitors. Therefore, as the number of MOS capacitors assigned to each stress electric field intensity decreases, the variation in the characteristics of each MOS capacitor increases as the cumulative failure rate 1 at each stress electric field intensity increases.
It appears as a displacement of the distribution. As a result, variations in the characteristics of each MOS capacitor appear as displacement of the oxide film life at each stress electric field strength, and this eventually increases the error in the estimated value of the oxide film life. In conclusion, it is inappropriate to reduce the number of MOS capacitors used for the measurement, shorten the measurement time, and estimate the oxide film lifetime of all wafers.

Also, when the electric field strength under stress is increased, the time accuracy of the measuring instrument becomes insufficient.
A large amount of heat is generated during the test due to the high electric field, the temperature of the oxide film rises unpredictably, and the time of dielectric breakdown with time becomes shorter than the value at the original temperature and electric field strength. An error occurs. For this reason, the electric field strength during stress is increased to shorten the measurement time,
Estimating the oxide lifetime of an entire wafer is also inappropriate.

After all, in the method of sequentially probing the MOS capacitors on the wafer by using an auto prober in the wafer state and performing measurement, the estimated value of the oxide film life having a sufficiently short measuring time and a sufficiently high accuracy in practical use Cannot be realized, and it is impossible to estimate the oxide film life of all the wafers.

An object of the present invention is to provide a method for estimating the life of an oxide film which has a sufficiently short measurement time for practical use, a sufficiently high estimation accuracy for practical use, and is compatible with the estimated value obtained by the conventional method. It is possible to quickly measure the estimated value of the oxide film life for all wafers that have completed the diffusion process, thereby changing the oxide film life due to changes in the process conditions during the development of the manufacturing process, or at the factory. It is possible to monitor the fluctuation of the oxide film life due to the fluctuation of the process condition due to the trouble of the apparatus and the like in a short period of time without overlooking when mass-producing.

[0020]

In order to achieve the above object, a method for testing the reliability of a semiconductor device according to the present invention comprises applying a current density having a stepwise waveform which increases with time to an insulating film formed on a wafer. Measuring the electric field strength at each step of the staircase waveform, calculating the total charge Qbd by integrating the current density with respect to time up to the point at which the insulating film breakdown occurs,
The total charge amount Qbd is removed by the current density at each stage of the staircase waveform, an estimated value of the oxide film lifetime at the electric field intensity at each stage of the staircase waveform is obtained, and the estimated value is approximated. Estimate the oxide film life.

[0021]

According to the structure of the method for estimating the life of an oxide film of the present invention, the estimated value of the life of an oxide film is obtained from one MOS capacitor, compared with the conventional method using a large number of MOS capacitors. Can be.

Further, the time of stress application is set so that the time of each step of the staircase waveform is sufficiently shorter than the time at which destruction occurs in the electric field strength of each step. Furthermore, the maximum value of the electric field strength of the staircase waveform applied to the oxide film is determined by the problem that the time accuracy of the measuring instrument is insufficient, and the time of dielectric breakdown due to heat generation at the time of testing due to a high electric field depends on the original temperature and electric field strength. Is set higher within a range that does not cause a problem of becoming shorter than the value in. As an example, the measurement time at the time of applying a low stress electric field in the conventional method of estimating the life of the oxide film is set to be approximately the same as the highest stress electric field strength among the plurality of stress electric field strengths in the conventional method of estimating the life of the oxide film. It is possible to be much shorter than this.

In addition to this, by using the stress of the staircase waveform, it is possible to ensure the time accuracy of the measurement time and to keep it within a practically short range. That is,
Even when the total charge Qbd up to the destruction of the oxide film varies and becomes extremely small, the current density at the initial stage of stress application can be sufficiently reduced. Thus, the time until the oxide film is broken can be made long enough to ensure the measurement accuracy. Conversely, even when Qbd is extremely large due to variation, the current density immediately before breakdown can be sufficiently increased. This can prevent the measurement time from being lengthened.

For the above reasons, the measurement time in the method for estimating the life of an oxide film according to the present invention is reduced from several tenths to several thousandths in comparison with the conventional method for estimating the life of an oxide film.
It is possible to obtain an estimated value of the oxide film life in a measurement time that is sufficiently short for practical use.

On the other hand, in the method for estimating the life of an oxide film according to the present invention, the estimated value of the life of the oxide film at each electric field strength is obtained from the same MOS capacitor, and based on the estimated value of the life of the oxide film at each electric field strength, An estimated value of the life of the oxide film in actual use can be obtained. For this reason, the problem in the conventional method of estimating the life of an oxide film, that is, since the life of the oxide film at each stress electric field strength is obtained from a different MOS capacitor, the variation in the characteristics of each MOS capacitor causes the life of the oxide film at each stress electric field strength to vary. It is possible to eliminate the problem that the displacement appears and the error in the estimation of the lifetime of the oxide film eventually increases.

For the above reasons, the estimation accuracy of the method for estimating the life of an oxide film in the present invention does not decrease even if the measurement time is shortened, and is equal to or higher than that of the conventional method for estimating the life of an oxide film. Can be kept.

Further, the estimated value of the life of the oxide film at each electric field strength in the present invention is based on the total charge amount Qb up to the destruction of the oxide film.
It is determined based on the widely accepted empirical fact that d is a constant value independent of electric field strength or current density. For this reason, it is in good agreement with the measured value of the oxide film life at each electric field strength. As described above, the estimated value of the oxide film life in actual use according to the present invention has high compatibility with the estimated value of the oxide film life in actual use by the conventional method.

[0028]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.

FIGS. 1 and 2 show a method of estimating the life of an oxide film according to the present invention. FIG. 1 shows a measurement result of an electric field intensity applied to an oxide film when a current density having a staircase waveform is applied to the oxide film until the oxide film breakdown occurs, and FIG. 2 shows an example of a method of estimating a lifetime of the oxide film.

In FIG. 1, the horizontal axis represents the stress time for applying the current density to the oxide film, the left vertical axis represents the current density J applied to the oxide film, and the right vertical axis represents the electric field intensity E applied to the oxide film. In FIG. 1, reference numeral 11 denotes an oxide film applied current density having a staircase waveform, 12 denotes an oxide film applied current density at each stage of the staircase waveform, 13 denotes a time change of an oxide film applied electric field strength, and 14 denotes an oxide film applied current at each current density. The electric field strength 15 is the time when the oxide film breakdown occurred.

In FIG. 2, the horizontal axis represents the intensity of the electric field applied to the oxide film, and the vertical axis represents the life of the oxide film. In FIG. 2, reference numeral 14 denotes an oxide film applied electric field strength at each current density,
6 is the estimated value of the oxide film lifetime at each electric field strength, 17 is the estimated value of the oxide film lifetime, 18 is the maximum value of the oxide film applied electric field intensity in actual use, and 19 is the estimated value of the oxide film life in actual use. is there.

In order to evaluate the reliability of the oxide film in a short time by using the MOS capacitor on the wafer after the diffusion step, a number of MOS capacitors in which the oxide film to be evaluated is a capacitor insulating film and the MOS capacitor in the wafer state is used. Test as is.
In this case, the measurement is performed by sequentially probing while moving the measurement point with respect to the MOS capacitor on the wafer using an auto prober.

The oxide film life of the wafer after the diffusion step is measured using a MOS capacitor formed on the wafer, and an estimated value 19 of the oxide film life in actual use shown in FIG. 2 is obtained from the measured value. Can be The thickness of the oxide film this time is 12 nm, and the size of the oxide film of the MOS capacitor used for estimating the life of the oxide film is 100 μm × 50 μm.

The MOS capacitors on the wafer are sequentially probed and measured while moving the measurement points on the wafer while using the measuring device in conjunction with the autoprober in the wafer state. The instrument has a voltage source, a current source, a voltmeter, an ammeter, and a capacity meter.

First, a voltage is applied to the MOS capacitor so that the MOS capacitor is in a storage state. In the case of a MOS capacitor formed on a P-type substrate, a negative voltage such as -5 V is applied to the upper electrode with respect to the substrate.
Is applied. Conversely, in the case of a MOS capacitor formed on an N-type substrate, a positive voltage is applied to the upper electrode with respect to the substrate,
For example, +5 V is applied. When the MOS capacitor is in the accumulation state, the capacitance of the oxide film appears as the capacitance of the MOS capacitor. So we measured the capacitance of the MOS capacitor,
The oxide film thickness is calculated from the capacitance and the oxide film size. This oxide film thickness is used when obtaining the electric field intensity applied to the oxide film from the voltage applied to the oxide film.

Subsequently, as shown in FIG. 1, an oxide film applied current density 11 having a stepwise waveform which increases with time until the time when oxide film breakdown occurs is applied. In this embodiment, as the oxide film applied current density 12 in each step of the staircase waveform, from the start of stress application to the stress application time 10 seconds, the oxide film applied current density J = 0.1 A / cm ² , and the stress application time 10 seconds J = 0.2 A / cm ² up to 20 seconds, J = 0.5 A / cm from stress application time 20 seconds to 30 seconds
cm ^2, time 15 stressing time oxide breakdown than 30 seconds has occurred, up to 36 seconds in this example J = 1.0A / c
Apply m ² . In this process, the time change 13 of the voltage applied to the oxide film is measured by measuring the time change of the voltage applied to the oxide film and dividing the measured value by the oxide film thickness obtained previously. From the measured values, the electric field intensity 14 applied to the oxide film at each stage of the staircase waveform is obtained. In this example, the electric field strength applied to the oxide film in each step of the staircase waveform is 11.8 MV / cm, 12.2 MV / cm, 1
2.7 MV / cm and 13.1 MV / cm.

At a stress time of 36 seconds, the voltage applied to the oxide film suddenly decreases greatly. The point at which the sudden large decrease in the oxide film applied voltage occurs is the point at which the oxide film breakdown occurs.
It is. The current density applied to the oxide film up to the time point 15 at which the oxide film breakdown occurs is integrated with respect to time, and the total charge amount Qbd (C / cm ² ) until the oxide film breakdown is calculated. In this example, the total charge Qbd until the oxide film is destroyed is 14 C / cm ² . The total charge amount Qbd up to the breakdown is a constant value independent of the electric field strength or the current density J.

By dividing the value of the total charge amount Qbd up to the breakdown of the oxide film by the current density applied to the oxide film in each step of the previously measured staircase waveform, the oxidation at each electric field intensity in each step of the staircase waveform is obtained. An estimated value 16 of the film life is obtained. In this example, the estimated value of the oxide film lifetime is 140 seconds, 70 seconds, 28
Seconds, 14 seconds.

As shown in FIG. 2, the estimated value 16 of the oxide film life at each electric field strength is plotted on a semi-logarithmic graph of the electric field strength applied to the oxide film on the horizontal axis and the oxide film life on the vertical axis. Based on this, the regression line of the estimated value 16 of the oxide film life at each electric field strength, that is, the estimated value 1 of the oxide film life,
Then, using this straight line, an estimated value 19 of the life of the oxide film in actual use at the maximum value 18 of the electric field strength applied to the oxide film in actual use is obtained.

In the embodiment of the present invention, when there are five measurement points for each wafer for about 50 wafers in one lot after the diffusion process is completed, the entire measurement is completed in a short time of about 2.5 hours. Then, an estimated value of the oxide film lifetime at each measurement point on each wafer is obtained. This measurement time is necessary because only one MOS capacitor is required to obtain an estimated value of the lifetime of one oxide film, and the measurement time per one capacitor can be reduced.
This is several tenths of the conventional method for estimating the lifetime of an oxide film, and a dramatic improvement in measurement efficiency is realized.

On the other hand, in the method for estimating the life of an oxide film according to the present invention, the estimated value of the life of the oxide film at each electric field strength is obtained from the same MOS capacitor, and based on the estimated value of the life of the oxide film at each electric field strength, The estimated value of the oxide film life in actual use is obtained. Therefore, the accuracy of estimating the life of the oxide film does not decrease even though the measurement time is shortened.

Further, since the estimated value of the oxide film life at each electric field strength in the present invention agrees well with the actually measured value, the estimated value of the oxide film life in the actual use according to the present invention is It has high compatibility with the estimated value of the oxide film lifetime.

[0043]

According to the present invention, the measurement time for obtaining the estimated value of the oxide film lifetime can be greatly reduced without lowering the estimation accuracy while maintaining the compatibility of the estimated value with the conventional method. Can be shortened. As a result, the estimated value of the oxide film lifetime can be measured in a short period of time for all the wafers that have completed the diffusion process.

As a result, in the development of the manufacturing process, the change in the reliability of the oxide film due to the change in the process conditions can be evaluated in a short period of time, and the effect of greatly improving the development efficiency and reducing the development cost can be obtained.

On the other hand, when products are mass-produced in a factory, it is possible to monitor fluctuations in the oxide film life due to fluctuations in process conditions due to equipment troubles and the like for all wafers at low cost and in a short period of time. Therefore, the effect that the reliability of the product can be improved at low cost can be obtained.

[Brief description of the drawings]

FIG. 1 is a view showing a measurement result of an electric field intensity applied to an oxide film when a current density having a staircase waveform is applied according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a method for estimating the life of an oxide film according to an embodiment of the present invention.

FIG. 3 is a view showing a measurement result of an oxide film life at each stress electric field strength according to a conventional method.

FIG. 4 is a diagram illustrating a method for estimating the life of an oxide film according to a conventional method.

[Explanation of symbols]

 11, 12 Current density 13 Time change 14 Electric field strength 15 Time point 16, 17 Estimated value 18 Maximum value 19 Estimated value

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 21/66 G01N 27/92 G01R 31/26

Claims (1)

(57) [Claims] 1. A method according to claim 1, further comprising: applying a current density having a stepwise waveform increasing with time to an insulating film formed on the wafer, measuring an electric field intensity at each step of the stepwise waveform, and measuring the electric field intensity until a point at which an insulating film breakdown occurs. The current density is integrated with respect to time to calculate the total charge Qbd, the total charge Qbd is removed by the current density at each stage of the staircase waveform, and the estimation of the oxide film lifetime at the electric field intensity at each stage of the staircase waveform A reliability test method for a semiconductor device, comprising: obtaining a value, approximating the estimated value, and estimating an oxide film life at an arbitrary electric field intensity.