Low heat loss and small contact area composite electrode for...

Details Low heat loss and small contact area composite electrode for... Low heat loss and small contact area composite electrode for...

2003-08-08

2004-06-08

Tsai, H. Jey (Department: 2812)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S238000, C438S381000

Reexamination Certificate

active

06746892

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to a composite electrode including a low heat loss and small contact area interface with a phase change media. More specifically, the present invention relates to a phase change media memory device in which a composite electrode includes an exposed portion in contact with the phase change media. The exposed portion comprises a small percentage of an overall area of the composite electrode such that there is a small area footprint between the exposed portion and the phase change media and the small area footprint reduces heat transfer from the phase change media to the composite electrode.
BACKGROUND OF THE ART
Memory storage devices based on a phase change material to store information are being considered as an alternative to conventional data storage devices such as hard discs and flash memory, just to name a few. In a phase change material based memory device, data is stored as one of two physical states of the phase change material.
For instance, in an amorphous state, the phase change material can represent a binary zero “0” and the state of the phase change material can be determined by passing a current through two electrodes in contact with the phase change material and sensing a voltage drop across the phase change material. If in the amorphous state, the phase change material has a high resistance, then the voltage drop will be high.
Conversely, the state of the phase change material can be altered to a crystalline state that represents a binary one “1” by passing a current of sufficient magnitude through the electrodes such that the phase change material undergoes Joule heating. The heating transforms the phase change material from the amorphous state to the crystalline state. As mentioned above, a voltage drop across the phase change material can be used to sense the state of the phase change material. Therefore, if in the crystalline state, the phase change material has a low resistance, then the voltage drop will be low.
Another way of expressing the state of the phase change material is that in the amorphous state, the phase change material has a low electrical conductivity and in the crystalline state, the phase change material has a high electrical conductivity.
Ideally, there should be a large enough difference between the high resistance of tne amorphous state and the low resistance of the crystalline state to allow for accurate sensing of the state of the phase change material. Moreover, in a memory device based on an array of phase change material storage cells, some of the storage cells will be in the amorphous state and others will be in the crystalline state. It is desirable to have a minimal variation in the high resistance among the storage cells in the amorphous state and to have a minimal variation in the low resistance among the storage cells in the crystalline state. If either variation is too large, it may be difficult or impossible to accurately sense the state of the phase change material.
In
FIG. 1
, a prior phase change storage cell
100
includes a first electrode
103
, a second electrode
105
, a dielectric
107
, and a phase change material
101
positioned in the dielectric
107
and in electrical communication with the first and second electrodes (
103
,
105
). Typically, the dielectric
107
forms a chamber that surround the phase change material
101
. To alter the state of the phase change material
101
from an amorphous state a (denoted by vertical hash lines) to a crystalline state C (see horizontal hash lines in FIG.
2
), a current I is passed through the first and second electrodes (
103
,
105
). The flow of the current I through the phase change material
101
causes the phase change material
101
to heat up due to Joule heating J.
In
FIG. 2
, a heat H generated by the current I is primarily dissipated through the first and second electrodes (
103
,
105
) because the first and second electrodes (
103
,
105
) are made from a material having a high thermal conductivity, such as an electrically conductive metal, for example. To a lesser extent, a heat h′ is dissipated through the dielectric
107
because the dielectric
107
has a lower thermal conductivity than the first and second electrodes (
103
,
105
). For instance, the dielectric
107
can be a layer of silicon oxide (SiO
2
).
As the heat H flows through the phase change material
101
, a portion of the phase change material
101
undergoes crystallization to a crystalline state C (denoted by horizontal hash lines), while another portion of the phase change material
101
remains in the amorphous state a.
One disadvantage of the prior phase change storage cell
100
is that not all of the energy contained in the Joule heat J is used in transforming the state of the phase change material
101
from the amorphous state a to the crystalline state C. Instead, a significant portion of the Joule heat J is wasted because it is thermally conducted away from the phase change material
101
by the first and second electrodes (
103
,
105
). As a result, more current I is required to generate additional Joule heat J to overcome the heat loss through the first and second electrodes (
103
,
105
).
Increasing the current I is undesirable for the following reasons. First, an increase in the current I results in increased power dissipation and it is desirable to reduce power dissipation in electronic circuits. Second, an increase in the current I requires larger driver circuits to supply the current I and larger circuits consume precious die area. In general, it is usually desirable to conserve die area so that more circuitry can be incorporated into an electronic circuit. Finally, in battery operated devices, an increase in the current I will result in a reduction in battery life. As portable electronic devices comprise an increasingly larger segment of consumer electronic sales, it is desirable to reduce current drain on battery powered electronics so that battery life can be extended.
In
FIG. 3
, a plurality of the prior phase change storage cell
100
are configured into an array to define a prior phase change memory device
111
. Each storage cell
100
is positioned at an intersection of the first and second electrodes (
103
,
105
), a plurality of which are arranged in rows for the second electrode
105
and columns for the first electrode
103
.
In
FIGS. 3 and 4
, one disadvantage of the prior phase change memory device
111
is that during a write operation to a selected phase change storage cell denoted as
100
′, a substantial portion of the heat H generated by the current I dissipates through the first and second electrodes (
103
,
105
) and into adjacent phase change storage cells
100
. Consequently, there is thermal cross-talk between adjacent storage cells
100
. Thermal cross-talk can slow down a switching speed of the phase change memory device
111
and can cause the aforementioned variations in resistance among the storage cells
100
.
Another disadvantage of the prior phase change memory device
111
is that a surface of the phase change material
101
has a large contact area C
A
with the first and second electrodes (
103
,
105
) (only the second electrode
105
is shown) and that large contact area C
A
promotes heat transfer from the phase change material
101
into the first and second electrodes (
103
,
105
).
In
FIGS. 3 and 4
, the contact area C
A
is the result of a large portion of a surface area of the phase change material
101
being in contact with the first and second electrodes (
103
,
105
) such that the heat H transfers easily from the phase change material
101
into the electrodes. The large contact area C
A
also contributes to the aforementioned thermal cross-talk. Moreover, heat loss from any given storage cell
100
, thermal cross-talk from adjacent storage cells
100
, and the contact area C
A
acting individually or in combination can lead to wide variations in resistance among the storage cells
100
. For instance, if one storage cell
100
has its ph

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