Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2001-07-20
2002-10-15
Sherry, Michael (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S658000, C324S765010
Reexamination Certificate
active
06466039
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a ferroelectric film characteristic measuring instrument for measuring the characteristics of a ferroelectric film, particularly the electrical characteristics of a ferroelectric capacitor formed by using a ferroelectric film, a measuring method of the same, and a measuring method for measuring a semiconductor memory device by applying the measuring method of the characteristics of the ferroelectric film.
BACKGROUND ART
In recent years, semiconductor memory devices using ferroelectric film have come under attention as ideal memories since they enable access at write and read speeds comparable to those of a DRAM (dynamic random access memory), enable realization of a number of rewrites greater than that of an EEPROM (electrically erasable and programmable read only memory), and do not lose the written information even when power is not supplied, that is, enable so-called non-volatile storage. For this reason, there has been active research into the electrical characteristic of ferroelectric film in the past few years.
When applying an electric field to a ferroelectric thin film, the phenomenon of electrical polarization where a charge is induced at the surface of the ferroelectric crystal takes place. When the direction of the input electric field is inverted, the direction of the polarization also inverts, that is, polarization inversion takes place. Further, there is a characteristic absent in ordinary dielectrics that the polarization of the ferroelectric thin film remains even if the electric field disappears. A memory which stores digital information of “0” and “1” according to the residual polarization of the ferroelectric film has already been realized. Generally, this kind of memory is called a ferroelectric random access memory (FRAM).
In the development of a ferroelectric memory, it is important to obtain an accurate grasp of the characteristics of the ferroelectric materials. For example, since the reliability, the switching time of the polarization inversion, the residual polarization, and other characteristics of the ferroelectric material directly influence the characteristic of the memory, it is important to accurately measure these characteristics of the ferroelectric material to form a semiconductor memory with a high reliability.
Up to now, different kinds of methods have been proposed for measuring the characteristics of a ferroelectric material. One enables measurement of the characteristic of the polarization inversion of the ferroelectric material by measuring the transient current of a capacitor formed by sandwiching a ferroelectric film with two electrodes laid facing each other when applying a pulse signal having a predetermined amplitude to the electrode of the capacitor.
FIG. 4
shows an example of the measuring instrument for measuring the characteristics of a ferroelectric capacitor. As illustrated, the measuring instrument is constituted by a pulse generator
10
, a signal transmission line
20
, a resistor
30
, and an amplifier (preamplifier)
40
. The capacitor C
FE
is the ferroelectric capacitor to be measured.
In the illustrated measuring instrument, a pulse signal having alternating positive and negative amplitudes (below, referred to as a bipolar pulse) is generated by the pulse generator
10
and applied to the ferroelectric capacitor C
FE
through a transmission line
20
. At the ferroelectric capacitor C
FE
, an electric field in accordance with the applied pulse voltage is generated and polarization occurs accordingly. A current i
FE
accompanying polarization inversion flows to the capacitor C
FE
since polarization inversion of the ferroelectric occurs each time a pulse is applied.
FIGS. 5A and 5B
show the waveform of the bipolar pulse applied to the ferroelectric capacitor C
FE
and the waveform of the transient current i
FE
of the capacitor C
FE
when the pulse is applied. The bipolar pulse shown in
FIG. 5A
is generated by the pulse generator
10
and is applied to the ferroelectric capacitor C
FE
through the transmission line
20
. The transient current i
FE
shown in
FIG. 5B
is generated in the ferroelectric capacitor C
FE
in accordance with this.
Since the current i
FE
is extremely small and cannot be measured as it is, it is amplified for measurement. In the measuring instrument, the small current i
FE
is input to the resistor
30
to generates a voltage drop V
i
. The voltage V
i
is input to the preamplifier
40
, whereby a voltage V
o
amplified by the preamplifier
40
is output. The output voltage V
o
of the preamplifier
40
can be further amplified if necessary. In this way, with the ferroelectric capacitor measuring instrument of the present example, the current i
FE
accompanying the polarization inversion of the ferroelectric capacitor C
FE
to be measured is converted to the voltage signal V
i
and amplified, so the voltage signal V
o
with a large amplitude that is able to be observed or measured can be obtained. By measuring the output voltage V
o
of the preamplifier
40
, the transient current at the time of polarization inversion of the ferroelectric capacitor C
FE
can be measured and the electrical characteristics of the ferroelectric capacitor C
FE
can be studied. Particularly, by measuring the transient current i
FE
of the ferroelectric capacitor C
FE
at the time of repeated polarization inversion, the fatigue life of the ferroelectric material can be estimated, so this measuring method is an important means for estimating the reliability of the ferroelectric memory.
But along with the increasing capacity and density of semiconductor memories, the dimensions of the ferroelectric thin films used in non-volatile memory devices are becoming smaller. For example, in the recently produced non-volatile memory having a storage capacity of 4 Mb (megabytes), the dimensions of the ferroelectric thin film used in the ferroelectric capacitor are on the submicron order, that is, are smaller than 1 &mgr;m (micrometer). With such a small ferroelectric capacitor, when applying a pulse signal having a predetermined amplitude to the electrodes, the polarization current generated along with the polarization inversion of the ferroelectric is extremely weak and measurement of the polarization current becomes difficult.
Further, in the above ferroelectric capacitor measuring instrument, along with the decrease of the capacity of the ferroelectric capacitor C
FE
being measured, the effect of the parasitic capacity of the transmission line
20
can no longer be ignored. In this case, since the current due to the charging and discharging of the parasitic capacity is sufficiently larger than the current due to the polarization inversion of the ferroelectric material, there is the demerit that the current i
FE
generated at the time of polarization inversion of the ferroelectric material cannot be accurately measured based on the output voltage V
o
of the measuring instrument.
DISCLOSURE OF THE INVENTION
The present invention was made in consideration with such a circumstance and has as an object thereof to provide a ferroelectric film characteristic measuring instrument which is able to measure the transient current of a ferroelectric capacitor having very small capacity at the time of polarization inversion with a high precision and a measuring method of the same.
To attain the above object, the ferroelectric film characteristic measuring instrument of the present invention is a ferroelectric film characteristic measuring instrument for measuring the characteristics of a ferroelectric film sandwiched between first and second electrodes arranged facing each other, comprising a pulse generator for generating a pulse signal to be input to the ferroelectric film, a conductive probe having a pointed tip for contacting said first electrode and inputting said pulse signal to said first electrode, and a current measuring circuit connected to said second electrode and measuring a current flowing in said ferroelectric film at the time of application of the pulse signal as an out
Kananen Ronald P.
Nguyen Tung X.
Rader Fishman & Grauer
Sherry Michael
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