JPH0846000A - Method for testing reliability of semiconductor element - Google Patents

Abstract

PURPOSE:To monitor the reliability of oxide for all wafers after finishing the diffusion process by shortening the measuring time required for estimating the lifetime of oxide significantly without lowering the estimation accuracy or increasing the cost. CONSTITUTION:Dependency of the density J of current flowing through an oxide on the field strength E is measured to obtain a Fowler-Nordheim plot. An estimated value of current density J is then determined at the maximum field strength during actual use based on the linear relationship of the plot. Subsequently, a stress field is applied to the oxide and the total quantity of charge Qbd=Jt (C/cm<2>) is measured before breakdown of the oxide. It is then divided by the estimated current density J to obtain the estimated lifetime of oxide at the time of actual use.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art With the progress of higher density, higher integration and miniaturization of semiconductor integrated circuit devices, SiO ₂ used in MOS transistors or MOS capacitors constituting them.
A film (hereinafter referred to as an oxide film) tends to be thin. On the other hand, since the power supply voltage of the semiconductor integrated circuit is usually kept constant, the electric field strength applied to the oxide film tends to increase as the film thickness is reduced. Under these circumstances, the time-dependent dielectric breakdown (TDDB: Time Dependent Dielectric Br
eakdown) has become a serious issue in terms of reliability. This is a phenomenon in which, when an electric field is applied to the oxide film, the oxide film is destroyed at a certain time after the start of the application of the electric field, electrical insulation is lost, and a short circuit occurs. Called oxide film life). Therefore, the time-dependent dielectric breakdown test for estimating the lifetime of the oxide film is an essential item in designing and developing the semiconductor integrated circuit device, and its importance tends to further increase. The conventional method for estimating the lifetime of an oxide film is shown below.

FIG. 4 and FIG. 5 show a conventional method for estimating the life of an oxide film from the time-dependent dielectric breakdown. FIG. 4 shows an example of an oxide film life estimation method, and FIG. 5 shows an example of measurement results of the oxide film life at each stress electric field strength.

In FIG. 4, the horizontal axis represents the electric field strength applied to the oxide film, and the vertical axis represents the oxide film life. In FIG. 4, 1 is the maximum electric field strength during actual use, 2 is the estimated value of the oxide film life during actual use, 3 is the stress electric field strength, 4 is the measured value of 50% cumulative failure time due to oxide film aging breakdown, 5 is an estimated value of 50% cumulative failure time due to dielectric breakdown with time of the oxide film.

In FIG. 5, the horizontal axis represents the stress time for applying an electric field to the oxide film, the right vertical axis represents the cumulative failure rate P due to oxide film aging dielectric breakdown, and the left vertical axis represents ln (calculated from the cumulative failure rate P. -Ln (1-P)) is shown. In FIG. 5, 6 is a measured value of 50% cumulative failure time due to dielectric breakdown with time of the oxide film, and 7 is time dependence of the cumulative failure time under each stress electric field strength. It is generally called a Weibull plot to plot the cumulative failure rate P on a graph in which the horizontal axis is the logarithm of time and the vertical axis is In (-ln (1-P)) as shown in FIG. 5, and the failure follows the Weibull distribution. In this case, the result of the Weibull plot is a straight line. The Weibull plot is widely used as a method of graphing the cumulative failure rate of dielectric breakdown of an oxide film over time.

In order to estimate the oxide film life, a large number of MOS capacitors having the same shape, size, and equivalent oxide film formed by the manufacturing process as a capacitor insulating film are prepared.
Divided into multiple sets. A stress electric field strength 3 higher than the maximum electric field strength 1 in actual use shown in FIG. 4 is applied to each set in the form of constant voltage stress. This stress application causes dielectric breakdown of the oxide films of each set with the lapse of time, and the number of oxide films that become defective increases with time.

The time dependence of the cumulative failure rate under each stress electric field strength is Weibull plotted as shown in FIG. Empirically, the failure due to aging 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 calculated for the time dependence 7 of the cumulative failure rate under each stress electric field strength, and using this, the time when the cumulative failure rate reaches 50% for each stress electric field strength, that is, 50% cumulative failure The actual measurement value 6 of time is obtained.

Assuming that the 50% cumulative failure time is the oxide film life, as shown in FIG. 4, the measured value of the 50% cumulative failure time is 6
Is plotted on a semi-logarithmic graph of electric field strength on the horizontal axis and oxide film life on the vertical axis. Empirically, the measured value 6 of the 50% cumulative failure time is a straight line on this graph. Based on this, the regression line of the measured value of the 50% cumulative failure time, that is, the estimated value 5 of the 50% cumulative failure time is obtained, and using this straight line, the oxide film life in the actual use at the maximum electric field strength of 1 in the actual use The estimated value 2 of is calculated.

In order to evaluate the reliability of an oxide film in a short time by using a MOS capacitor on a wafer which has completed the diffusion process, a large number of MOS capacitors having an oxide film to be evaluated as a capacitor insulating film are in a wafer state. I will test it.
In this case, measurement is performed by sequentially probing while moving the measurement point with respect to the MOS capacitor on the wafer using an auto prober.

[0010]

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

Process conditions are frequently changed during manufacturing process development. Since the oxide film life may change due to the change of the process conditions, it is necessary to frequently estimate the oxide film life.

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

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

In the conventional method for testing the life of an oxide film, when the test is performed in the wafer state as it is, the test can be performed simultaneously since the MOS capacitors on the wafer are sequentially probed and measured using an autoprober. The number of such MOS capacitors is limited to one or several that can be electrically connected simultaneously by the probe needle of the probe card. Therefore, it takes a long time to measure a cumulative failure rate by applying a plurality of stress electric field strengths to a large number of MOS capacitors, and this tendency is remarkable especially in the measurement of a low stress electric field strength. This is a major obstacle in estimating the oxide film life.

In order to shorten the measurement time, the stress electric field strength during the test is increased or the MOS used for the measurement is used.
There are methods such as reducing the number of capacitors.

However, when the electric field strength during stress is increased, the time accuracy of the measuring instrument becomes insufficient,
Also, 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-dependent dielectric breakdown time becomes shorter than the original value at the original temperature and electric field strength. There is an error. Therefore, the electric field strength during stress is increased to shorten the measurement time,
The oxide film lifetime of all wafers could not be estimated.

Further, when the number of MOS capacitors used for measurement is reduced, the cumulative failure rate is obtained for a plurality of stress electric field strengths, so that the number of MOS capacitors assigned to each stress electric field strength is further reduced. . Although it is assumed that the MOS capacitors on the same wafer are equivalent to each other, the characteristics actually vary. The cumulative failure rate at each stress electric field strength is obtained from different MOS capacitors. For this reason,
As the number of MOS capacitors assigned to each stress electric field strength decreases, the variation in the characteristics of each MOS capacitor appears as a displacement of the cumulative failure rate distribution at each stress electric field strength. As a result, variations in the characteristics of each MOS capacitor appear as a displacement of the oxide film life at each stress electric field strength, which ultimately increases the error in the estimated value of the oxide film life. In conclusion, reduce the number of MOS capacitors used for measurement and shorten the measurement time,
The oxide film lifetime of all wafers could not be estimated.

After all, in the method of probing the MOS capacitors on the wafer one after another using the autoprober in the wafer state as it is, the estimated value of the life of the oxide film has a sufficiently short measurement time in practice and a sufficiently high precision in practice. Could not be realized, and the oxide film life of all wafers could not be estimated.

An object of the present invention is to provide a low-cost method for estimating the lifetime of an oxide film, which has a sufficiently short measurement time for practical use and a sufficiently high estimation accuracy for practical use. It is possible to measure the estimated lifetime value in a short period of time, which allows changes in the oxide film lifetime due to changes in process conditions during development of the manufacturing process, or processes due to equipment troubles during mass production of products in the factory. This makes it possible to monitor changes in oxide film life due to changes in conditions in a short period of time without overlooking.

[0020]

In order to achieve the above object, a semiconductor device reliability test method according to the present invention applies a plurality of electric field strengths E to an oxide film to determine a current density J flowing in the oxide film. The measured value of the current density J is measured, and a set of logarithmic values of the value J / E ² obtained by dividing the current density J with respect to the reciprocal 1 / E of the electric field strength by the square of the electric field strength E is obtained, Approximate the set of each calculated value by a straight line or a linear expression, estimate the current density flowing in the oxide film at the electric field strength to estimate the oxide film life using the straight line or a linear expression, until the destruction of the oxide film The total charge amount Qbd is measured, and the oxide film life is estimated by dividing the total charge amount Qbd until the oxide film is destroyed by the current density flowing in the oxide film at the electric field strength for estimating the oxide film life.

Further, a constant current is applied to the oxide film at the time of measuring the total charge amount Qbd until the breakdown, and the total charge amount until the breakdown is measured by measuring the time until the breakdown.

Further, when measuring the total amount of charge Qbd until breakdown, the total amount of charge Qbd until breakdown is obtained by sweeping a current through the oxide film in a stepwise wave and increasing it with time.

[0023]

According to the structure of the method for estimating the life of the oxide film of the present invention, the estimated value of the life of the oxide film is obtained from one MOS capacitor, whereas it is obtained from a large number of MOS capacitors in the past. be able to.

Furthermore, the time of stress application is such that the time accuracy of the measuring instrument for the electric field strength applied to the oxide film becomes insufficient, and a large amount of heat is generated during the test due to the high electric field, and the temperature of the oxide film is unpredictable. It is possible to increase it within a range that does not cause the problem that the time-dependent dielectric breakdown time becomes shorter than the original value at the temperature and the electric field strength. By setting the strength to be approximately the same as the highest stress electric field strength, it is possible to make it sufficiently shorter than when a low stress electric field is applied in the conventional oxide film life estimation method.

In addition to this, the current density J flowing in the oxide film
The time for measuring is sufficiently shorter than the time for applying stress.

For the above reasons, the measurement time in the method of estimating the life of an oxide film according to the present invention is the same as that of the conventional method of estimating the life of an oxide film, in which the MOS capacitors on the wafer are sequentially used in the wafer state using an autoprober. It is shortened to several tenths to several thousandths as compared with the method of measuring by probing, and it becomes possible to obtain the estimated value of the oxide film life in a wafer state in a practically short measurement time.

On the other hand, in the method of estimating the lifetime of the oxide film according to the present invention, the measured value of the total charge amount Qbd from the same MOS capacitor to the breakdown and the estimated value of the current density J at each electric field strength are obtained, and these two types of amounts are calculated. From this, the estimated value of the oxide film life at each electric field strength can be obtained. Therefore, since there is a problem in the conventional method of estimating the lifetime of an oxide film, that is, since it is required from the MOS capacitors having different oxide film lifetimes at each stress electric field strength, the variation in the characteristics of each MOS capacitor causes the variation of the oxide film lifetime at each stress electric field strength. The problem that the error in estimating the lifetime of the oxide film finally becomes large can be eliminated.

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

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the drawings by embodiments.

1 to 3 show a method of estimating the life of an oxide film according to the present invention. FIG. 1 is a method for estimating the lifetime of an oxide film, FIG. 2 is a measurement result of electric field strength dependence of a current density J flowing in the oxide film, and FIG. 3 is a voltage of an oxide film applied until the oxide film is destroyed when a constant current density stress is applied. It is an example of the measurement result of time change.

In FIG. 1, the horizontal axis in the upper part of the drawing is the electric field strength E applied to the oxide film, the lower horizontal axis is the reciprocal 1 / E of the electric field strength E applied to the oxide film, and the vertical axis is the oxide film. A value J / E ² obtained by dividing the current density J of the current by the square of the electric field strength E applied to the oxide film is shown.

In FIG. 1, 11 is the value obtained by dividing the measured value of the current density J by the square of the electric field strength E, 12 is the estimated value of J / E ² by the Fowler-Nordheim plot, and 13 is the breakdown. Total charge Qbd and current density J
The estimated value of the oxide film life obtained from the above, 14 is the maximum electric field strength during actual use, and 15 is the estimated value of the oxide film life during actual use.

In FIG. 2, the horizontal axis shows the electric field strength E applied to the oxide film, and the vertical axis shows the current density J of the current flowing through the oxide film. In FIG. 2, 16 is the current density J and 17 is the measured value of the current density J.

In FIG. 3, the horizontal axis represents the stress time t for applying the constant current density stress to the oxide film, and the vertical axis represents the oxide film applied voltage Vox during the application of the constant current stress.

The oxide film life of the wafer after the diffusion process is measured by using a MOS capacitor formed on the wafer, and the estimated value 15 of the oxide film life in actual use shown in FIG. 1 is obtained from the measured value. Desired. The thickness of the oxide film this time is 12 nm, and the dimensions of the oxide film of the MOS capacitor used for estimating the lifetime of the oxide film are 100 μm × 50 μm.

The MOS capacitors on the wafer are sequentially probed while moving the measurement points on the wafer by using a measuring device linked to an auto-prober in the wafer state. This measuring instrument is equipped with a voltage source, current source, voltmeter, ammeter, and capacitance meter.

First, with respect to the MOS capacitor, the voltage at which the capacitance of the oxide film appears as the capacitance of the MOS capacitor is M.
Applied to the OS capacitor, the capacitance of the MOS capacitor is measured. The oxide film thickness is calculated from the capacitance and the oxide film size.

Next, by applying a voltage to the same MOS capacitor, an electric field having an electric field strength of about 6 to 9 MV / cm is applied to the oxide film, and as shown in FIG. Is obtained. Here, the electric field strength applied to the oxide film is obtained by dividing the oxide film applied voltage by the oxide film thickness obtained previously.

From the measured value 17 of the current density J16 thus obtained, as shown in FIG. 1, the measured value J / E ² obtained by dividing the measured value 17 of the current density J17 by the square of the electric field strength E is measured. A so-called Fowler-Nordheim plot is performed in which the logarithm of the value is plotted against the reciprocal 1 / E of the electric field strength E. This is because the current flowing through the oxide film when an electric field is applied is mainly the Fowler-Nordheim tunnel current,
The current density J is J with respect to the electric field strength applied to the oxide film.
= AE ² exp (-b / E) (A and b are constants). From this dependence, the value J / E obtained by dividing the current density J of the current flowing in the oxide film mainly composed of the Fowler-Nordheim tunnel current by the square of the electric field strength E
^The result of plotting the logarithm of ² against the reciprocal 1 / E of the electric field strength E is a straight line. This plot is called a Fowler-Nordheim plot, and from this value obtained by dividing the measured value of the current density J by the square of the electric field intensity E using this linear relationship, J / Estimate 12 of E ² is determined. By multiplying the estimated value 12 of J / E ² by the Fowler-Nordheim plot by the square of the electric field strength E, E ² , it is impossible to obtain an actually measured value because the arbitrary electric field strength E, especially the current density J is minute. It is possible to obtain an estimated value of the current density J at a low electric field strength near the maximum electric field strength 4 during possible actual use.

Then, a current density J = 1.0 A / cm ² constant current stress is applied to the oxide film of the same MOS capacitor, and the total charge amount Qbd until breakdown is measured. The use of the constant current stress has an advantage that the variation in the measurement time due to the variation in the film thickness of the oxide film can be reduced as compared with the case of the constant voltage stress. FIG. 3 shows the time variation of the oxide film applied voltage at this time. MO for constant current stress
When applied to the oxide film of the S-capacitor, the absolute value of the oxide film applied voltage gradually changes over time due to the trapping of electrons into the oxide film as shown in time change 18 of the oxide film applied voltage during constant current application. To increase. After that, the applied voltage to the oxide film suddenly and largely decreases after a certain period of time, 18 seconds in this example. The time point when the oxide film applied voltage suddenly and drastically decreases is the time point 19 when the oxide film breakdown occurs. The time t from the start of the constant current stress application to the time 19 when the oxide film is destroyed is multiplied by the stress current density J to obtain the total charge amount Qbd = Jt until the breakdown. In this example,
The total amount of charge Qbd up to destruction is 18 C / cm ² . The total charge amount Qbd until the breakdown has a constant value that does not depend on the electric field strength or the current density J.

Instead of measuring the total amount of charge Qbd until breakdown by applying a constant current stress shown in this example, the stress current density is swept stepwise and increased with the passage of time.
Integrating the time and current density J at each stage until breakdown,
The method of obtaining the total charge amount Qbd until the breakdown is also a practical method because the value of the total charge amount Qbd until the breakdown can be measured over a wide range.

The total amount of electric charge Qbd until the oxide film is broken, which is obtained in this way, is calculated by measuring the current density J at an arbitrary electric field strength obtained previously, and particularly the electric current density J is very small because it is a low electric field. It is divided by the estimated value of the maximum electric field strength 14 at the time of actual use, which is impossible to obtain, and the estimated value 13 of the oxide film life obtained from the total charge amount Qbd to the breakdown and the current density J at each electric field strength, among them, An estimated value 15 of the oxide film life in actual use at the maximum electric field strength 14 in actual use is obtained.

The test time in the embodiment of the present invention is such that when there are 5 measurement points on each wafer for about 50 wafers in one lot after the diffusion process, all the measurements are completed in a short time of about 1.5 hours. After that, an estimated value of the oxide film lifetime at each measurement point on each wafer is obtained. This measurement time requires only one MOS capacitor to obtain an estimated value of one oxide film life, and the measurement time per one can be shortened. This is a few tenths of the case described above, and a dramatic improvement in measurement efficiency is realized.

On the other hand, in the oxide film life estimation method of this embodiment, the estimated value of the oxide film life at each electric field strength is
It can be obtained from the total amount of charge Qbd measured from one MOS capacitor until destruction and the estimated value of the current density J at each electric field strength. For this reason, the estimation accuracy of the oxide film life does not decrease even though the measurement time is shortened.

[0045]

According to the present invention, the measurement time for obtaining the estimated value of the oxide film life can be reduced without degrading the estimation accuracy.
It can be greatly shortened. As a result, it is possible to measure the estimated lifetime of the oxide film in a short period for all the wafers that have undergone the diffusion process.

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

On the other hand, when the products are mass-produced in the factory, it is possible to monitor the fluctuation of the oxide film life due to the fluctuation of the process conditions due to the trouble of the apparatus or the like for all the 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 drawings]

FIG. 1 is a diagram showing a method of estimating an oxide film lifetime according to an embodiment of the present invention.

FIG. 2 is a diagram showing measurement results of electric field strength dependence of a current density J flowing in an oxide film according to an embodiment of the present invention.

FIG. 3 is a diagram showing a time change of an oxide film applied voltage up to oxide film breakdown when a constant current density stress is applied according to an embodiment of the present invention.

FIG. 4 is a diagram showing a conventional method for estimating the lifetime of an oxide film.

FIG. 5 is a diagram showing the measurement results of the oxide film lifetime at each conventional stress electric field strength.

[Explanation of symbols]

 11 value 12 estimated value 13 estimated value 14 maximum electric field strength 15 estimated value

Claims (3)

[Claims] 1. A plurality of electric field intensities E are applied to an oxide film to measure a current density J flowing in the oxide film, and the current density J is a reciprocal of the electric field intensity with respect to the measured value of the current density J. J is divided by the square of the electric field strength E to find a set of logarithmic values of J / E ² , and each set of the calculated values is approximated by a straight line or a linear expression, and oxidation is performed using the straight line or the linear expression. The current density flowing in the oxide film at the electric field strength for estimating the film life is estimated, and the total charge amount Qb until the oxide film is destroyed.
The oxide film life is estimated by measuring d and dividing the total charge amount Qbd until the oxide film is destroyed by the current density flowing in the oxide film at the electric field strength for estimating the oxide film life. Reliability test method for semiconductor devices. 2. The total amount of charge until breakdown is measured by applying a constant current to the oxide film at the time of measuring the total amount of charge Qbd until breakdown, and measuring the time until breakdown. A method for testing reliability of a semiconductor device as described above. 3. The total charge amount Qbd until breakdown is obtained by sweeping a current through the oxide film in a stepwise wave to increase with time when the total charge amount Qbd before breakdown is measured. Reliability test method for semiconductor devices.