Electricity: measuring and testing – A material property using electrostatic phenomenon – For flaw detection
Reexamination Certificate
1998-09-16
2001-12-04
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
A material property using electrostatic phenomenon
For flaw detection
C324S522000, C324S537000, C324S551000, C324S765010
Reexamination Certificate
active
06326792
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for evaluating a dielectric film. In particular, the present invention relates to lifetime prediction of dielectric breakdown.
In accordance with recent improvement in the packing density of semiconductor integrated circuit devices, the feature sizes of device elements have kept on being shrunk. In the field of VLSIs, the thickness of a silicon dioxide (SiO
2
) film used as a gate oxide has become smaller than 10 nm, and lifetime prediction of such a thin oxide film has become more and more important. As an evaluating method for gate oxides, a constant voltage stressing method or a constant current stressing method is widely used.
With reference to
FIG. 18
, a conventional constant voltage stressing method will be first described.
First, a stressing voltage V
0
, a judgement current level I
0
and the number N of samples subjected to measurement are set in step S
50
.
Next, in step S
51
, a measurement probe is moved to a first sample selected among the plural samples. Then, in step S
52
, the stressing voltage V
0
is applied across a dielectric film of the sample. Subsequently in step S
53
, the application of the stressing voltage V
0
across the dielectric film is retained for a time period of t
1
seconds. A leakage current (I) across the dielectric film is measured in step S
54
. In step S
55
, whether dielectric breakdown has occurred is determined on the basis of the measured current level I. For example, it is determined that the dielectric breakdown has occurred when the absolute value of the current level I is larger than the judgement current level I
0
. In the case where it is determined that the dielectric breakdown has not occurred, the measurement procedure returns to step S
53
, so that the steps S
53
, S
54
and S
55
are repeated until the dielectric breakdown is observed. In the case where the breakdown was detected in step S
55
, a time to breakdown is recorded in step S
56
.
When the measurement for all the samples is completed in step S
57
, all of the total stressing times(t
1
to t
N
) for the N samples are used to calculate a time t
BD
in step
58
. Weibull plotting can be adopted for such calculation. The Weibull plotting will be described below.
First, values W are calculated on the basis of a cumulative distribution function F and are plotted with regard to each of the stressing times (t
1
to t
N
) on a log scale. In general, the value W represents a cumulative percent of failure, and the cumulative distribution function F represents the probability that the device will fail at or before time t. The value W is given by the following expression (1):
W=
ln [ln {1/(1−
F
)}] (1)
It is empirically known that a linear relationship can be obtained between the value W and the stressing time t. More specifically, the measurement data plotted in a Weibull plotting paper shows a linear relationship between broken-down oxides and the time t to the breakdown. Fore example, the stressing time t corresponding to W=50% is easily obtained by Weibull plotting. The obtained stressing time t, or t
50
, means a time when 50% of the oxides has broken down. The Weibull plotting is widely used to estimate the lifetime of dielectric films.
When the measurement of all the N samples is not completed in step S
57
, the measurement probe is moved to a subsequent sample (step S
59
), and the measurement procedure returns to step S
52
. The steps S
52
through S
59
are then repeated until the measurement of all the N samples is completed.
The sample number N is generally 20 through 100. This is because the measured time t varies among samples, and hence, the time t
BD
cannot be accurately determined if the sample number N is small.
The time t
BD
obtained in this manner corresponds to the lifetime of oxide breakdown. Therefore, the time t
BD
is used for evaluating the quality and reliability of gate oxides.
Next, with reference to
FIG. 19
, the conventional constant current stressing method will be described.
First, in step S
60
, a current level I
0
for stressing, a critical voltage V
0
and the sample numbers N are set and input a measurement apparatus. In step S
61
, a measurement probe is moved to a first sample.
Next, in step S
62
, the current I
0
is applied to a dielectric film of the first sample. After t
1
seconds from the start of the application of the stressing current I
0
(step S
63
), a gate voltage V is measured in step S
64
. In step S
65
, it is determined whether or not the oxide breakdown has been caused. For example, when the absolute value of the voltage V is smaller than the absolute value of the critical voltage V
0
, it is determined that the oxide breakdown has occurred. When it is determined in step S
65
that the breakdown has not occurred, the measurement procedure returns to step S
62
. Then, the steps S
63
through S
65
are repeated until the oxide breakdown is observed on the first sample.
When the breakdown is detected in step S
65
, a time t from the start of the current stressing to the oxide breakdown is recorded. When the measurement of all the samples is completed (step S
67
), the stressing times t with regard to all the samples are used for calculating a lifetime t
BD
of these samples and a total injected charge Q
BD
(step S
68
). The time t
BD
is determined by using the aforementioned Weibull plotting. Herein, the total injected charge Q
BD
is defined as a value obtained by dividing a product of the time t
BD
and the stressing current I
0
by an gate electrode area S.
When the measurement of all the samples is not completed in step S
67
, the measurement probe is moved to a subsequent sample in step S
69
, so that the procedures in steps S
62
through S
69
can be repeated until the measurement of all the samples is completed. Also in this case, the sample numbers N is approximately 20 through 100.
In these methods for predicting the lifetime of oxide breakdown, it is disadvantageously necessary to prepare a large number of samples and it takes a disadvantageously long time for the measurement. It is generally known that the measurement error is generally in proportion to (N
½
)/N. Therefore, when the sample number N is small, the lifetime prediction cannot be reliable. In order to improve the reliability of the lifetime prediction, it is necessary to increase the sample number, which results in an increases in the measurement time.
The object of the present invention is providing a method and an apparatus for evaluating a dielectric film in which the time and the sample number required for measurement can be reduced without degrading the measurement reliability.
SUMMARY OF THE INVENTION
The dielectric film evaluating method of this invention comprises a stressing step of applying electrical stressing to a dielectric film; and a step of monitoring an A mode stress induced leakage current and measuring a value of the A mode stress induced leakage current flowing when breakdown occurs in the dielectric film.
Alternatively, the dielectric film evaluating method of this invention comprises a stressing step of applying electrical stress to a dielectric film in each of plural samples; a step of monitoring an A mode stress induced leakage current and measuring a value of the A mode stress induced leakage current flowing when breakdown occurs in the dielectric film in each of the plural samples; and a threshold determining step of determining a breakdown threshold of the A mode stress induced leakage current by statistically processing the values of the A mode stress induced leakage current measured in the plural samples.
In one aspect, the dielectric film evaluating method of this invention comprises a stressing step of applying electrical stressing to a dielectric film; a step of measuring an A mode stress induced leakage current; and a lifetime predicting step of predicting a lifetime of the dielectric film on the basis of a relationship between the measured value of the A mode stress
LeRoux Etienne
Matsushita Electronics Corporation
McDermott & Will & Emery
Metjahic Safet
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