2.3 The main PCM structures
The scaling of a device is achieved by the improvement of the fabrication techniques, that not always can overcome limitations intrinsic of the lithographic technology avail-able. The other solution is the engineering of the device structure, increasing the possibilities for the scaling, thanks to the changed fabrication procedures involved in the fabrication. Since the beginning of the PCM technology, one of its main bottlenecks has been the high programming current, and all the efforts have been put on the real-ization of a cell structure, featuring the plug area reduction. In the next sections, we show the evolution of the phase-change memory cell architecture, presenting the main structures proposed in the literature and their evolution in the actual PCM industrial products. In this context, we chose some of them for our implementations, in order to perform our analytical tests and the experimental research of this work.
2.3.1 The lance-type structure
The first PCM cell industrial concept was the one described in Fig. 1.4. The structure is vertical, and basically realized by the etching of a via in the dielectric deposited over the bottom electrode. Then, the via is filled with a metal to realize the plug. The chemical-mechanical polishing (CMP) is performed in order to guarantee the planarity of the surface, for the next phase-change material deposition. The vertical structure, in general, results suitable for the back end implementation in the final memory pro-duction. In this structure, the scaling depends on the via diameter φ, and then on the etching and lithography technology resolution available. The lance-type structure, from the first implementations based on GST, demonstrated capability of good resis-tance window (higher than 3 orders of magnitude), high speed (SET time = 40 ns and RESET time = 10 ns), and high cyclability up to 1012 cycles [6, 11].
We realized our lance-type structure, using two different lithography techniques, dependently on the via dimension:
- DUV (deep ultraviolet) lithography, for diameters >250 nm;
- Electronic-beam lithography, for diameters< 250 nm.
In Fig. 2.6 we report the results of one of our lance-type implementation. In the example the plug material is tungsten, with height ≃150 nm. The dielectric material used is SiO2 while the phase-change material integrated in the devices is GeTe. The trend of the RESET and the SET currents, as a function of the plug dimension, shows an initial linear dependency on the area of the plug (slope = 2) at plug diameters higher than 200 nm. When we decrease the size, the trend shifts toward a linear dependency on the plug diameter (slope = 1), as already shown in simulations reported in Fig. 2.5.
2.3.2 The edge structure
The edge structure redesigned the plug geometry, taking advantage of the good control offered by the deposition techniques (physical vapor deposition PVD, or chemical vapor deposition CVD) of the thickness of a metal layer. The structure of the cell becomes horizontal. In Fig. 2.7 we report the design proposed in [83], where the interface area, is
Fig. 2.6. The trend of programming current, in one of our lance-type PCM device implementation.
The material integrated is GeTe. On the right, the programming characteristics of a 50 nm (plug diameter) device, showing the possibility to SET and RESET the cell.
defined by the thickness of the horizontal layer (BE), and its width. The phase-change material is deposited in order to have a lateral contact on the exposed surface of the plug. In the picture, we can see also the estimated capability of scaling of this design, with respect to the previous lance-type structure (in which the contact is at the bottom of the phase-change material). However, the main constraint of this architecture, is the difficulty to guarantee a perfect material lateral contact during the deposition.
2.3.3 The bridge structure
The bridge structure consists of a narrow line of ultrathin phase-change material bridg-ing two underlybridg-ing electrodes [84]. The TiN electrodes are formed very close together to obtain a reasonable threshold voltage, separated by a small insulating gap that de-fines the bridge length L (Fig. 2.8). Then, the phase-change material is deposited by magnetron sputtering, with thickness down to 3 nm (H), creating the “bridge” between the two electrodes. Electron-beam lithography finally defines the bridge widthW down to 20 nm during the patterning step [85]. In this horizontal structure, the rise of the
Fig. 2.7. a) Description of the edge PCM structure. b) Comparison in terms of scaling capabilities, between the lance-type structure and the edge structure [83].
2.3 The main PCM structures 47
Fig. 2.8. Starting from the left: scheme of the bridge structure, integrated with the selector device; the main concept of the bridge structure, showing the phase-change material bridge, with its dimensions, and the two electrodes; the calculated (lines) and the experimental results (symbols) for the RESET current as a function of the cross-sectional area of the phase-change material bridge, for different thicknessH of the phase-change material layer [84, 85].
temperature is inside the phase-change material (self-heating cell), losing the need of a plug element. It means, that the cross-sectional area of the bridge (W × H), de-termines the final density of current. In Fig. 2.8, the trend of the RESET current of the bridge devices, as a function of the bridge area, still approximately confirms the relation in eq. 2.7. The bridge structure demonstrated great scalability, with RESET current lower than 150 µA, but limited endurance up to 30000 cycles [86].
2.3.4 The confined structure
As already introduced, scaling of vertical structures is limited by the increase of the current density required to program the cell, due to the increased contribution of the lateral thermal dissipation of the structure at low dimensions. The idea of the con-fined structure, is to “confine” the phase-change material in the same hole realized to implement the plug element, partially or totally replacing it. This enhances the ther-mal confinement of the active volume, since the power delivered to the phase-change material, is confined by the dielectric. Presented in the beginning as highly scalable architecture [87], it showed a big improvement on the final RESET current of the device, reducing up to more than two times the final programming current, and en-durance performance of up to 108 cycles. Like for standard lance-type structure, the main limitation of this structure, is the minimum diameter of the via, achievable by the lithographic node. An evolution of this structure is the “pore” structure [88], where the combination of an accurate etch-back and conformal deposition creates a via with a sub-lithographic critical dimension that is essentially independent of the original via feature size. This technique allowed the reduction of the via (pore) diameter down to 20 nm. The main constraint of this technology is the perfect filling of the via (or of the pore), with the phase-change material, achievable only with more conformal deposition techniques like CVD (with respect to PVD), that depends also on the aspect ratio of the pore to fill (ratio between the pore/via height and its surface).
In recent years, Samsung demonstrated the high scalability of this structure down to sub-20 nm technology node, but abandoning the cylindrical shape of the via, and using a “dash” confined structure [89, 90]. Preserving the main idea of the confined structure, the plug is realized with a dash shape, filled with the metal. Then the recess of the plug,
Fig. 2.9. Implementation of the confined structure. On the left the device basic structure. In the center, the SEM photo of our confined structure after the fabrication. On the right, the programming characteristics of a lance-type device with plug of 300 nm of diameter, and a confined structure realized with the same hole of 300 nm, but decreased by the SiN spacers to a 100 nm via.
allows the deposition by ALD (atomic layer deposition) of the phase-change material, in the “dashed hole” left. The result is a structure with a final plug/phase-change material interface area of 7.5×17 nm, RESET current below 80 µA, and endurance up to 1011 cycles.
The confined structure is interesting for the better thermal confinement (increased power efficiency), and the fact that the current density determined by the plug surface is uniform in the phase-change material, even far from the plug/phase-change material interface, gaining again in power efficiency. But this latter consideration, leads also to the fact, that once reached the critical current density required to melt the phase-change material, all the volume of material is involved in the melting. The consequence, is the difficulty to achieve intermediate resistive states between the full SET and the full RESET of the device. In the structures described before in fact, the current density decreases in the phase-change material moving from the plug/phase-change material interface to the top electrode. It means that a proper control of the current density, can change the total volume of phase-change material involved in the melting (that crosses Tm). It results in the possibility to program the cell in different intermediate resistive states, by controlling just the amplitude of the electrical pulse applied on the cell. In the confined structure, this resistance control can be achieved only by partially recrys-tallizing the melted phase-change material at the end of the programming procedure, requiring a better control of the electrical pulse shape applied on the cell.
In our study, we implemented the confined structure, using a process flow similar to the one implemented for the lance-type, but using the spacer technique. Once created the 300 nm via by DUV, instead to fill it with the plug material, we deposited a uniform layer of silicon nitride (SiN) thick≃100 nm, after etched vertically and anisotropically for a total thickness of 100 nm. The result, is a SiN spacer at the bottom of the via along the dielectric border, with the main function to create a pore of 100 nm of diameter. In Fig. 2.9 we report the scheme of the final confined cell, where we deposited GeTe as phase-change material and where the SiN spacers are evidenced. In the picture are reported also the programming curves of the device, compared with a standard lance-type structure with 300 nm plug. The RESET current results strongly decreased (down to 1.5 mA), and as previewed, the RESET and the SET state are achieved abruptly, without intermediate resistance states.
2.3 The main PCM structures 49
Fig. 2.10. PCMµtrench structure process flow as described in [91] (left). The scheme of theµtrench cell we implemented, showing the trench filled with the phase-change material, and the ring exposed surface (right).
2.3.5 The µtrench structure
The µtrench PCM cell architecture represents a well-tempered merging of all the ap-pealing features of the PCM technology, having been demonstrated to simultaneously achieve low programming currents, small cell size, good dimensional control, and proven multi-megabit manufacturability [91]. Presented for the first time in 2004 by STMi-croelectronics [92], it represents the first real industrial PCM product. This vertical structure combines the control of the thickness of a metal layer to define one plug dimension (like in the edge structure), and the critical dimension of the lithographic node used.
We implemented this structure in order to study the effects of the scaling and at the same time the reliability of this technology. The main process flow is described in Fig. 2.10 [91]:
- deposition of the bottom electrode and of the dielectric (SiO2);
- opening of a squared via in the dielectric by DUV;
- deposition of the metal (WSi) layer by PVD, to create the so called “ring”;
- deposition of the dielectric to fill the via, and final CMP to leave the “ring” surface exposed;
- deposition of the dielectric (SiN);
- opening of the trench by etching of the dielectric;
- deposition of the phase-change material;
- deposition of the top electrode.
In this structure the interface area is defined by the ring thickness, and the trench width. In Fig. 2.11 this concept is well described in the SEM top view image of the cell, before the phase-change material deposition, where both the trench and the ring thickness are highlighted and describing the final surface area of the plug of the PCM device. In Fig. 2.12 we report the trend of the RESET and the SET current as a function of the trench width, for the same ring thickness (30 nm). The linear trend confirms again the linearity between the programming current and the effective area of the plug/phase-change material interface, predicted in eq. 2.7.
Fig. 2.11. SEM top view image of one of our µtrench devices, before the phase-change mate-rial deposition. The final plug surface is defined by the ring thickness, exposed by the trench, and by the trench width.
Fig. 2.12. Trend of the RESET and SET cur-rent in our µtrench devices, as a function of the trench dimension, for a fixed ring thickness (30 nm).
Fig. 2.13. PCM “Wall” structure process flow as described in [93] (left). The basic scheme of the
“Wall” structure, evidences the structure of the plug (right).
The main resistive contribution of the plug to the programming of the cell (RON), in this structure, is in the regions really close to the plug/phase-change material surface.
In fact, moving from this surface into the ring, the current density decreases rapidly, due to the larger effective area of the ring, below the trench. It means, that the engineering of the plug, to achieve the correctRON to allow SET and RESET operations (without the need of an external selector device to limit the current), becomes difficult.
Nevertheless, a great advantage of theµtrench structure is the really fast programming operations, thanks to the fact that the peak of the current density is always at the plug/phase-change material interface during the programming, and the reduction of the power lost in the plug. However, the co-integration with a MOS selector becomes necessary.
2.3.6 The “Wall” structure
The “Wall” structure represents the direct evolution of the µtrench technology. With respect to theµtrench approach, the “Wall” structure has better lithographic alignment
2.3 The main PCM structures 51
tolerances, still preserving the good programming current controllability, and it allows to save one critical mask, simplifying the storage element process integration [93]. The basics of the process flow are reported in Fig. 2.13. From a functional point of view, this structure enables a better engineering of the cell with respect to the programming operations. In fact, the resistance of the plug (called also wall in this structure) can be simply calculated as
RW =ρW
h
wt (2.12)
where ρW, h, w, and t are respectively the resistivity, the height, the width and the thickness of the wall. The analysis of the structure efficiency (reported in [94]), as already introduced in section 2.1.2, shows that the increase of the heater resistance (obtained by scaling down w and t and increasingh) to achieve a better control of the current in the cell, produces a displacement of the temperature peak inside the plug, with a consequent reduction of the power efficiency of the device. The scaling of cell then, has to be coupled to a proper selection of the properties (thermal and electrical) of the integrated phase-change material. As we will present in the next chapter, we performed our reliability studies on state-of-the-art “Wall” PCM devices fabricated in collaboration with STMicroelectronics.
2.3.7 The PCM structures: a comparison
In table 2.1 we report all the main advantages and disadvantages of the structures analyzed. The evolution of the cell architecture along the years, led to the progressive scaling of the plug/phase-change material interface area. It allowed to reduce the final programming power of the device, opening the road for PCM technology towards em-bedded applications. As we can see from the table, the vertical structures showed more advantages in terms of fabrication, being suitable for the back end of line production.
In terms of thermal confinement of the phase-change material and reduction of pro-gramming current density, the confined structure represents the best solution. On the contrary it requires highly conformal deposition techniques. In order to improve the speed of the device, the current density increase provided strictly at the
plug/phase-Structure Main features Advantages Disadvantages
Lance-type vertical, simple manufacturing, scaling limited by lithography
plug based on a via high endurance (1012) resolution, bad thermal confinement
Edge horizontal, surface defined by difficulty to guarantee
plug based on a metal layer plug thickness good lateral contact Bridge horizontal, no need of a plug element, difficult implementation,
self-heating cell good thermal confinement poor endurance Confined vertical, active material good thermal confinement, need of highly conformal
deposited in the plug hole high scalability deposition techniques µtrench vertical, active area defined by ring temperature peak at the plug/pcm no self-current limitation,
thickness & trench lithography interface, power reduction need of external MOS selector
“Wall” vertical, active area good current control, simple defined by “wall” surface industrial process integration X
Table 2.1. Comparison of the different PCM structures analyzed. The Wall structure results nowa-days the best compromise in terms of fabrication process flow complexity and scaling capability.
change material interface in the µtrench structure represents a great solution, also in terms of programming power reduction. However, the absence of a real current limita-tion requires an external limitalimita-tion provided by an access transistor, not suitable then for cross-bar applications. The best compromise in terms of fabrication process, scaling capability and device engineering opportunity, is the “Wall” structure, that represents nowadays the state-of-the-art of the industrial PCM products.