Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2003-03-12
2004-04-13
Cuneo, Kamand (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S071100, C324S1540PB
Reexamination Certificate
active
06720790
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for evaluating the characteristics of an insulating film provided on a semiconductor substrate, particularly an ultra-thin gate insulating film.
As the degree of integration in a semiconductor integrated circuit device has increased significantly in recent years, elements including transistors in a MIS semiconductor device have been required to be smaller in size and higher in performance to correspond to the higher degree of integration. As the elements including the transistors have become smaller in size and higher in performance, the implementation of a MIS (Metal Insulator Semiconductor) structure with high reliability has been in greater demand. To provide a MIS structure with higher reliability, each of the components of the MIS structure which are a gate electrode (Metal), a gate insulating film (Insulator), and a semiconductor substrate (Semiconductor) should have high reliability.
The gate insulating film which is one of the elements composing the MIS structure is expected to be rapidly reduced in thickness to correspond to further miniaturization and higher-speed and lower-voltage operation of the transistors. It is expected that an extremely thin insulating film with a thickness of 2 nm or less will be used actually in the 21st century. The implementation of an excellent gate insulating film is so important an issue as to say that the characteristics of the gate insulating film determine the characteristics of the MIS transistor and therefore the electric characteristic of the semiconductor integrated circuit device.
As a material composing a gate insulating film, a silicon dioxide (SiO
2
) has been used conventionally. However, the limited TDDB (Time Dependent Dielectric Breakdown) reliability of a gate oxide film, i.e., SiO
2
film is expected to be among factors which impair the reliability of an LSI device in the future. If the thickness of the gate oxide film is reduced increasingly to reach 2 nm or less, the problem arises that a tunneling current induced by direct tunneling of carriers through the gate oxide film, i.e., a gate leakage current is further increased. In a system LSI, in particular, such an increased leakage current incurs a significant increase in the power consumption of the LSI device so that a large number of new materials replacing SiO
2
have been proposed as materials composing the gate insulating film in terms of power consumption (Reference: 1999-ITRS Roadmap). Thus, the increased leakage current in the gate oxide film has requested revolutions in the development of a gate insulating film and new materials as well as in quality control at a manufacturing site.
As a test for examining the reliability of a gate insulating film, a so-called TDDB test has been performed conventionally in an accelerated environment. The TDDB test performed under the accelerated environment is a method for examining the lifetime of an insulating film till it reaches a dielectric breakdown by applying a voltage higher than a working voltage, increasing the temperature, examining the current-voltage characteristic (I-V characteristic), and judging that the insulating film breaks down when a current increases abruptly. In the TDDB test, it is general to examine the I-V characteristic and determine, as the lifetime, the time elapsed till an abrupt change is observed in leakage current, while monitoring an amount of leakage current. During the test, measurement using a MIS structure which is a capacitor occupying a large area has been performed normally in terms of examining a defect density in the gate insulating film.
For in-line evaluation, a so-called Hg prober test has also been used widely in which the I-V characteristic and the like are. evaluated by pressing a mercury terminal functioning as a gate electrode against a gate insulating film that has been formed previously to omit the step of forming the gate electrode composing a MIS structure (see, e.g., Japanese Unexamined Patent Publication No. HEI 06-140478). The Hg prober method is performed primarily for the development of an insulating film material, control in the process of fabricating an insulating film, a reliability test for an insulating film, or the like.
In general, the breakdown of a gate insulating film in each of the foregoing tests is judged by an abrupt increase in leakage current. In the TDDB test, a MIS structure (which is also a MIS capacitor) having a gate area of 0.01 mm
2
or more is used widely to provide a sufficient sensitivity with which a current value is detected. In the evaluation method using a Hg prober also, the contact area between the mercury terminal and the gate insulating film is as large as 0.01 mm
2
or more due to the structure of the mercury terminal.
As a result of a further increase in leakage current induced by a recent reduction in the thickness of the gate insulating film, it may be difficult to judge the time at which the gate insulating film breaks down in a test for evaluating the quality of a gate insulating film by observing the I-V characteristic as described above. In addition, a phenomenon termed a false breakdown in a thin film on a 2-nm level, which is different from the conventional breakdown, has made the judgment of a breakdown difficult. A description will be given herein below to the cause of the difficulty by using the TDDB test as an example.
FIG. 1
is a view showing the I-V characteristic of a thermal oxide film (SiO
2
film) with a thickness of 1.5 nm provided on a p-type silicon substrate by using a gate area as a parameter. In the drawing, the horizontal axis represents a gate voltage (V) and a vertical axis represents the absolute value of a gate leakage current (A) As can be seen from the drawing, the gate leakage current value increases as the gate area increases to reach 3 &mgr;m
2
, 30 &mgr;m
2
, and 300 &mgr;m
2
. If a comparison is made between leakage current densities obtained by dividing the gate leakage values by the respective gate areas, however, it will be understood that each of the leakage current densities has a nearly equal value. In
FIG. 1
, there are also shown the I-V characteristics of gate insulating films with respective thicknesses of 1.5 nm and 2.5 nm that have broken down. It will be understood that a substantially uniform characteristic is obtained after the dielectric breakdown irrespective of a difference in film thickness whether the thickness of a gate insulating film is 2.5 nm or 1.5 nm. This may be because the dielectric breakdown has occurred at a certain local leakage spot in the gate insulating film.
However, as the gate area becomes larger to reach 3 &mgr;m
2
, 30 &mgr;m
2
, and 300 &mgr;m
2
, the timing T with which the gate leakage current increases abruptly (the time of breakdown) for the judgment of breakdown becomes less distinct, as shown in FIG.
1
. When the gate insulating film has a large thickness, the timing with which the gate leakage current increases abruptly is distinct as indicated by the broken curve in the drawing. However, the distance between the initial I-V characteristic curve and the post-breakdown I-V characteristic curve is reduced as the thickness of the gate insulating film is reduced, so that it becomes more difficult to recognize the time T at which the gate insulating film breaks down.
FIG. 2
is a view showing the relationship between the gate area and the gate leakage current by using a gate voltage as a parameter, which is for determining the critical value of the gate area over which the breakdown of the gate insulating film cannot be detected any more. In the drawing, the horizontal axis represents the gate area (&mgr;m
2
) and the vertical axis represents the gate leakage current (A). By way of example,
FIG. 2
shows the case where the thicknesses of the gate insulating films are 1.5 nm and 2.5 nm. In the I-V characteristics shown in
FIG. 1
, the gate leakage current Ig is determined uniquely if the gate voltage Vg and the gate area Sg are determined. It may be said that the re
Eriguchi Koji
Hashimoto Yukiko
Watakabe Akio
Nguyen Tung X.
Nixon & Peabody LLP
LandOfFree
Method and apparatus for evaluating insulating film does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for evaluating insulating film, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for evaluating insulating film will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3228027