Memory cells with improved reliability

Details Memory cells with improved reliability Memory cells with improved reliability

2002-09-19

2003-09-16

Dinkins, Anthony (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Fixed capacitor

C361S303000, C257S306000, C439S239000

Reexamination Certificate

active

06621683

ABSTRACT:

BACKGROUND OF INVENTION
Ferroelectric metal oxide ceramic materials such as lead zirconate titanate (PZT) have been investigated for use in ferroelectric semiconductor memory devices. Other ferroelectric materials, for example, strontium bismuth tantalum (SBT) can also be used.
FIG. 1
shows a conventional ferroelectric memory cell
100
having a transistor
130
and a ferroelectric capacitor
140
. An electrode
142
is coupled to a plateline
170
and another electrode
141
is coupled to the transistor which selectively couples or decouples the capacitor from a bitline
160
, depending on the state (active or inactive) of a wordline
150
coupled to the transistor gate.
The ferroelectric memory stores information in the capacitor as remanent polarization. The logic value stored in the memory cell depends on the polarization of the ferroelectric capacitor. To change the polarization of the capacitor, a voltage which is greater than the switching voltage (coercive voltage) needs to be applied across its electrodes. An advantage of the ferroelectric capacitor is that it retains its polarization state after power is removed; resulting in a non-volatile memory cell.
FIG. 2
shows a cross section of a conventional ferroelectric capacitor
140
on a plug
266
. As shown, the capacitor comprises a ferroelectric layer
246
sandwiched between first and second electrodes
141
and
142
. The electrodes typically are formed from a noble metal such as platinum. The lower electrode
141
is coupled to the plug which, for example, is in contact with a diffusion region of the cell transistor. A barrier layer
264
can be provided below the lower electrode to protect the plug from oxidation. An encapsulation layer
283
surrounds the top and side of the capacitor, protecting the capacitor from diffusion of, for example, oxygen.
A contact opening
260
is formed in a dielectric layer
280
, exposing the upper electrode. A contact is formed in the opening to couple the upper electrode to, for example, a plateline. The contact opening is formed by an anisotropic etch such as reactive ion etch (RIE). Etch damage to the upper electrode occurs during the formation of the contact opening. To repair the damage, a recovery anneal is necessary. However, the high temperature of the recovery anneal, typically about 500-700° C., oxidizes the plug. Oxidation increases contact resistance, thereby degrading the plug. Such degradation adversely impacts the reliability of the IC and reduces yields.
From the foregoing discussion, it is desirable to provide a ferroelectric memory IC with improved reliability and yields.
SUMMARY OF INVENTION
The invention relates to memory cells employed in ICs, such as memory ICs or other types of ICs. More particularly, the invention relates to improving the reliability of capacitors. In one embodiment a capacitor includes a bottom electrode, a top electrode, and an intermediate layer therebetween. A contact for coupling to the top electrode is provided. In one embodiment of the invention, at least a portion of the contact is offset from the capacitor. In one embodiment, the capacitor is a ferroelectric capacitor. Other types of capacitors are also useful. By offsetting the contract contact from the top electrode, the etch damage to the top electrode is reduced, thereby reducing or eliminating the need for the anneal to repair the etch damage.


REFERENCES:
patent: 5005102 (1991-04-01), Larson
patent: 5206788 (1993-04-01), Larson et al.
patent: 5369296 (1994-11-01), Kato
patent: 5475248 (1995-12-01), Takenaka
patent: 5495117 (1996-02-01), Larson
patent: 5963466 (1999-10-01), Evans, Jr.
patent: 5973342 (1999-10-01), Nakamura
patent: 6104049 (2000-08-01), Solayappan et al.
patent: 6171871 (2001-01-01), Machida et al.

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