Concerns about thermal crosstalk
A further processing complication was the inclusion in one orthogonal direction of thermal barriers as a contribution to both reducing reset current and reducing thermal crosstalk. To explore thermal crosstalk, the Samsung authors used a metric that considers the ratio of the total programming current applied to the array over the reset current (i.e. Ipgm
). This will have a high value when all nearest neighbors in the array are being reset and have a low value when those same devices are being set. Based only on calculation, the authors concluded that as a cumulative effect, it would take 107
write cycles with the ratio at three before the data in an adjacent reset state cell would be compromised. This was based on a measured activation energy of 3.97 eV, which also gave the data retention time of 15 months at 85°
C, if accurate suitable for a PCM RAM as a PRAM.
It was this work that prompted Samsung’s dire warning with respect to the future of PCM, that thermal crosstalk is now one of the greatest concerns for PCM scaling: They stated it is “inevitable” that beyond the 20-nm node PCM memory will face thermal disturbance.
The lowest value of reset current reported was 90 μA; for a 22 nm × 7.5 nm structure; this calculates to a current density of 5.4 × 107
. It is interesting to note that a Samsung paper at IEDM10 presented data on a similar, but smaller, 17 nm × 7.5 nm cell with a calculated current density of 6.2 × 107
. If that trend of increasing current density with scaling continues for the dash structure, considerations of thermal crosstalk might be the least of their problems.
Finding the golden composition
One of the highlights of one of the IBM PCM papers at IEDM11 was their reporting of the discovery of a PCM material with what they termed a “golden composition.” 2
On the GST phase diagram, this composition lies approximately at the junction of the Sb-GeTe and Ge-Sb2
tie lines in the direction of germanium enrichment. This new germanium-enriched Ge2
(GST212) composition is considered golden because it has four significant attributes:
- A crystallization temperature of 250° C compared with the 150° C of the most-used GST
- A higher resistance in the crystallized state
- A more rapid crystallization
- A transition into the hexagonal crystal structure at temperatures greater than 500° C.
All else being equal, these characteristics would translate at the PCM device level into higher elevated temperature data retention, lower reset current, faster switching and a lesser tendency for voids to form during processing. Another very positive aspect of the IBM work was the inclusion of more than one device in the write/erase lifetime results.
While IBM results indicated a write/erase lifetime in the range 107
cycles, they did not address the subject of element separation. The devices were operating with a best value of reset current (500 μA) at a contact current density of about 2.5 × 107
. The devices were monolithically integrated in a 128 M-bit array.
To explore how element separation might impact the golden composition, let's consider the approximate location of the golden composition in the crystallization plane relative to GST (see figure 3). Although the starting composition of GST, shown on the lower plane phase diagram of Figure 3, is homogeneous, with accumulated write/erase cycles, element separation causes small volumes within a PCM to change in composition. The composition changes are represented by the colored dots on the second plane in figure 3. For illustrative purposes, a small circle is used to indicate regions of different compositions, although in reality the mixed compositions are spread over a larger, irregularly shaped area. What is of greatest interest is the positioning of GST at the bottom of crystallization temperature valley in the crystallization temperature surface, the third plane. It could be argued that some of the success of GST is caused by the fact that because of its location at the bottom of the crystallization temperature valley, most of the composition changes caused by element separation result in materials with a higher crystallization temperature.
Figure 3: Linking the phase diagram to write/erase element separation and upper plane crystallization temperature, highlighting the location of the germanium-enriched Ge2Sb1Te2 golden composition.