Computing
Which is the Ideal Universal Memory: GST467 Superlattice PCM vs. UltraRAM
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Recently, a research paper published on GST467 by Xiangjin Wu and Asir Intisar Khan of the Department of Electrical Engineering, Stanford University, and AMD’s UltraRAM has baffled tech enthusiasts and experts alike with their promising radical advancement over traditional memory architectures. Both superlattice phase-change memory (PCM) and UltraRAM aim to increase capacity, speed, and efficiency, but their underlying principles are quite different.
Here’s how both of them approach escalating demands for high-performance, energy-efficient computing on face value:
- The novel nanocomposite superlattice PCM increases storage through multi-layered nanostructures, faster operations by nano-scale heating, and ultra-low power.
- UltraRAM increases storage by interconnecting compact memory cells. It also works faster than its traditional counterparts by shortening data paths and intelligent power management.
While we have linked the comprehensive resources that discuss the two in great detail, we didn’t find any credible resource that compares the two. So, let the GST467 Superlattice PCM vs. UltraRAM showdown begin.
Materials and Structure
The innovative superlattice and nanocomposite materials used in each technology fundamentally impact their performance, scalability, and integration capabilities. Comparing the materials and structures provides crucial insight into the fundamental advantages and tradeoffs of these emerging memory solutions.
GST467 Superlattice PCM vs. UltraRAM Parameter: Materials and Structure | |
| Novel Nanocomposite Superlattice Phase-Change Memory | UltraRAM |
| Utilizes superlattice of Ge4Sb6Te7 (GST467) and Sb2Te3/TiTe2 layers. | Based on standard 6T SRAM bitcells in CMOS. |
| GST467 is a nanocomposite with SbTe inclusions for faster switching. | Uses conventional planar or FinFET transistors with no new materials. |
| Ordered low-dimensional structure with atomically sharp interfaces. | Organized in 4Kx72b blocks with specialized interface circuitry. |
| Layer thicknesses ~2-4nm, stack ~60nm using sputtering deposition. | Achieves high density through aggressive layout design rules. |
| Exotic new materials require new manufacturing processes/equipment. | No special materials or steps integrate easily with logic. |
Superlattice Phase-Change Memory utilizes exotic new materials like GST467 nanocomposites in atomically precise superlattice structures. This unique nanoscale design boosts switching speed but demands specialized equipment. In contrast, UltraRAM employs standard CMOS transistors without novel materials, readily integrating into existing manufacturing flows.
The superlattice's ultra-thin 2-4nm layers enable unmatched scaling potential. However, its nanocomposite materials and fabrication complexity pose integration challenges. Conversely, UltraRAM's 4Kx72b SRAM blocks achieve high density through conventional layout techniques compatible with established processes.
While offering performance advantages, the superlattice memory requires new manufacturing investments for its specialized materials and atomic-scale layering. UltraRAM capitalizes on continued CMOS scaling without disruptive changes.
Capacity and Density
Memory density is a critical metric for enabling high-capacity, compact storage solutions demanded by modern computing workloads. Evaluating the maximum achievable density highlights the potential of each technology for enabling future high-density system architectures.
GST467 Superlattice PCM vs. UltraRAM Parameter: Capacity and Density | |
| Novel Nanocomposite Superlattice Phase-Change Memory | UltraRAM |
| Demonstrated functional 40nm devices – smallest PCM reported. | The fundamental unit is a 4Kx72b (288Kb) SRAM block. |
| Excellent characteristics suggest high-density potential with scaling. | Up to 2048 blocks cascaded vertically in one column. |
| Multilayer 3D stacking enables ultra-high density in a small footprint. | Horizontal cascading of columns is also possible with overhead. |
| Low power and heat confinement suit dense 3D stacking. | 3D stacking of cascaded columns for highest densities. |
| Potential to rival or exceed 3D NAND flash density. | 64-high 3D stack provides 36Gb per die stack. |
Novel Nanocomposite Superlattice Phase-Change Memory pushes density scaling frontiers with its 40nm devices – the smallest PCM cells ever achieved. This superlattice technology exhibits excellent high-density characteristics like fast switching, low power, and high endurance, enabling 3D stacking possibilities to achieve NAND flash-rivaling capacities in ultra-compact form factors.
UltraRAM takes a unique cascading approach with its 4Kx72b SRAM building blocks. Vertically interconnecting up to 2048 blocks provides capacities of up to 576Mb per column. Horizontal cascading further extends 2D array densities. Combining cascading with 3D die/wafer stacking using 64 layers yields immense 36Gb capacities within a single compact stack.
Power Consumption
Minimizing power consumption is essential for energy-efficient computing, particularly in mobile and embedded applications. Comparing active and standby power illustrates the energy profiles best suited for different use cases.
GST467 Superlattice PCM vs. UltraRAM Parameter: Power Consumption | |
| Novel Nanocomposite Superlattice Phase-Change Memory | UltraRAM |
| Record-low 85μA reset current in 40nm devices. | 60-100μW active power for 4Kx72b block in 40nm. |
| ≈5 MW/cm2 reset power density, over 10X lower than PCM. | Sleep mode cuts standby power to a few milliwatts. |
| Superlattice structure confines heat, minimizing thermal losses. | Power gating reduces idle leakage to near zero. |
| SbTe nanoclusters lower the crystallization barrier and switching power. | Drowsy mode allows moderate leakage reduction with fast wake-up. |
| Ultra-scaled size reduces absolute power for phase change. | Configurable modes optimize power vs. performance tradeoffs. |
| 10nW read power with 0.1V, leveraging high resistance ratio. | Leverages intrinsic low-voltage CMOS efficiency. |
| Near-zero standby power due to its nonvolatile nature. | Highly power efficient for low-power and high-perf uses. |
The novel GST467 superlattice PCM achieves unparalleled low-power operation due to its unique nanostructured materials. At the 40nm scale, it demonstrated record-low 85µA reset currents corresponding to a minuscule 60µW write power and an incredible 5 MW/cm2 power density—over 10x better than standard PCM. This superlattice precisely confines heat with embedded SbTe nanoclusters, lowering the crystallization barrier.
UltraRAM leverages standard low-voltage CMOS circuitry, with its compact 4Kx72b blocks optimized for 60-100µW active power. Its true strength lies in configurable standby modes – Sleep cuts idle leakage to milliwatts, while Power Gating eliminates it completely. The Drowsy mode balances moderate leakage reduction with fast wake-up for higher performance. These flexible modes enable optimal energy/performance tradeoffs for diverse applications.
Voltage Scaling
Operating at low voltages is crucial for integrating with advanced CMOS logic nodes. Evaluating voltage scaling potential reveals compatibility with modern semiconductor manufacturing and design flows.
GST467 Superlattice PCM vs. UltraRAM Parameter: Voltage Scaling | |
| Novel Nanocomposite Superlattice Phase-Change Memory | UltraRAM |
| Record low 0.7V operation in 40nm devices demonstrated. | Operates down to ~0.9V in a 40nm CMOS process. |
| Crucial for compatibility with advanced CMOS logic voltages. | Designed for the robust low-voltage operation of bitcells/peripherals. |
| Superlattice structure confines fields for low-voltage switching. | Can use high-density logic process with ultra-low Vt. |
| SbTe nanoclusters enable uniform nucleation at low voltages. | Small bitcell arrays maintain performance at low voltages. |
| Sharp transitions minimize set/reset voltage margin for low Vread. | The cascaded architecture enables localized voltage domains. |
| Favorable for further scaling, but the ON/OFF ratio is limiter. | Potential to scale to 0.65V or lower at 7nm. |
| Access transistor design is critical for adequate write current. | Assist techniques like negative bitline writing, boosting, etc. |
The GST467 superlattice PCM achieves record-low 0.7V operation in 40nm devices. This ultralow voltage operation is vital for compatibility with the latest CMOS logic operating around 0.8V. The superlattice's nanoscale confinement enables switching with minimal voltages, further assisted by SbTe nanoclusters promoting uniform crystallization. Its sharp set/reset transitions minimize voltage margins for low-voltage reads.
UltraRAM's CMOS underpinnings allow operation down to 0.9V in 40nm nodes. Its SRAM bitcells and peripherals are carefully designed for reliable low-voltage operation using techniques like transistor optimization and specialized circuit topologies. The small bitcell arrays and localized voltage domains mitigate low-voltage challenges. UltraRAM can leverage bleeding-edge low-Vt logic processes and potentially scale to 0.65V or lower using assist techniques.
Access Speed and Latency
Memory performance directly impacts overall system responsiveness and throughput. Benchmarking access times and latencies exposes the performance envelopes best aligned with different application requirements.
GST467 Superlattice PCM vs. UltraRAM Parameter: Access Speed and Latency | |
| Novel Nanocomposite Superlattice Phase-Change Memory | UltraRAM |
| 40ns write time in 40nm devices, >10X faster than PCM. | ~1ns read/write time for a 72-bit word in 40nm. |
| <10ns read time demonstrated, approaching DRAM performance. | Single cycle read/write for many operations. |
| SbTe nanoclusters seed rapid, uniform crystal nucleation. | Cascaded blocks allow parallel accesses across arrays. |
| Confined heating enables fast temperature rise/fall times. | Pipelined registers break critical paths for high clock speeds. |
| A high resistance ratio enables fast, reliable sensing. | Read slightly faster than write due to simpler circuitry. |
| Array-level write latency is still higher than DRAM. | A small 72b word size requires more operations for large transfers. |
| Non-destructive reading can reduce effective read latency. | High clock speeds and advanced interfaces mitigate small words. |
The GST467 superlattice PCM breaks new speed barriers with 40ns write times—over 10x faster than standard PCM at 40nm scale. Its nanoclusters facilitate rapid, uniform crystallization, while confined heating allows quick temperature transitions, yielding these blazing performance levels. Furthermore, read times under 10ns are nearing those of DRAM, attributed to the high resistance ratio enabling reliable sensing.
While ultra-fast for a single cell, achieving low PCM array write latency requires overcoming the innate constraints of the phase-change mechanism. Parallelizing operations using advanced architectures and interfaces helps mitigate this bottleneck. Significantly, its non-destructive reads eliminate write-back overheads for reduced effective latency.
UltraRAM leverages its SRAM-like design, achieving astonishing ~1ns access times for 72-bit words, thanks to compact bitcell arrays and advanced circuits. Parallel accesses across cascaded blocks minimize latency impacts, aided by pipeline registers. While its narrow word width necessitates more operations for large transfers, high clock speeds and advanced interfaces compensate. Overall, UltraRAM delivers exceptional performance, rivaling conventional SRAM.
Endurance and Retention
Robust data endurance and retention are critical for non-volatile storage, enabling reliable long-term operation. Comparing these metrics identifies suitability for applications ranging from code storage to database caching.
GST467 Superlattice PCM vs. UltraRAM Parameter: Endurance and Retention | |
| Novel Nanocomposite Superlattice Phase-Change Memory | UltraRAM |
| Over 2×10^8 write cycles in 40nm devices demonstrated. | Not designed for long-term retention – volatile SRAM technology. |
| Superlattice structure minimizes defects and distributes stress evenly. | “Shadow storage” provides short-term backup using embedded NVM. |
| Retention over 10^5 hours at 85°C, ~10 years projected. | Backup time is limited by the endurance of shadow NVM (e.g., ReRAM). |
| Regular layers inhibit diffusion/segregation over long periods. | Endurance >10^15 cycles, limited by transistor wear-out. |
| Higher crystallization temperature improves amorphous phase stability. | SRAM cells are designed for lower stress than logic transistors. |
| Fundamentally limited by atomic rearrangement at long timescales. | Allows much higher write traffic than surrounding logic. |
| Error correction can extend effective retention time further. | Not intended for data archiving, but it is robust for active use. |
The GST467 superlattice PCM demonstrates exceptional endurance, exceeding 2×10^8 write cycles in 40nm devices, significantly improving upon conventional PCM. Its unique superlattice structure minimizes defects while distributing strain evenly, contributing to this robustness. Data retention is also impressive, with demonstrated 10^5 hour capabilities at 85°C, projecting to over ten years at room temperature.
UltraRAM, being a volatile SRAM technology, is not designed for long-term, non-volatile storage. However, it innovatively integrates “shadow storage” using embedded non-volatile memories like ReRAM to provide short-term data backup during power failures. Its true strength lies in phenomenal endurance beyond 10^15 write cycles, limited only by transistor wear-out mechanisms. Optimized for low stress, UltraRAM cells can withstand much higher write traffic than surrounding logic components.
While not intended for archival storage, both technologies showcase compelling endurance and retention traits tailored to their target application spaces – GST467 PCM for non-volatile storage and UltraRAM for reliable active in-memory computing.
Click here to learn about the 3d processors that will shape the future of data transfers.
Error Correction
Ensuring data integrity through error correction is paramount for any memory subsystem. Assessing ECC capabilities and overhead illuminates real-world reliability over the lifetime of the technology.
GST467 Superlattice PCM vs. UltraRAM Parameter: Error Correction | |
| Novel Nanocomposite Superlattice Phase-Change Memory | UltraRAM |
| Can leverage existing ECC schemes like Hamming and BCH codes. | Leverages the same ECC as surrounding CMOS logic/SRAM. |
| Need to handle asymmetric “stuck-at” errors common in PCM. | May require less frequent ECC due to SRAM reliability. |
| Write-verify-write is used to mitigate stuck-at errors. | Differential sensing enhances noise immunity. |
| Advanced stuck-at-aware ECC codes provide better correction. | Must handle multi-bit errors across 72b data width. |
| Tradeoff between ECC complexity and density/performance. | Hierarchical/interleaved schemes reduce ECC overhead. |
| ECC is essential for long-term reliability over many cycles. | Integration with existing logic ECC engines/IP. |
Error correction is vital for ensuring data integrity over the lifetime of any memory technology. The GST467 superlattice PCM can readily leverage well-established ECC schemes like Hamming and BCH codes. However, it must account for asymmetric “stuck-at” errors through techniques like write-verify-write or advanced stuck-at-aware ECC algorithms trading off complexity and overhead.
For UltraRAM, one key advantage is its seamless integration with the same ECC engines and IP, which also protects the surrounding CMOS logic and SRAM. Due to its inherent SRAM-like reliability, UltraRAM may necessitate less frequent ECC interventions. However, managing multi-bit errors across its 72-bit data path requires careful ECC design. Employing techniques like hierarchical and interleaved ECC can reduce overhead while maintaining robust correction capabilities.
Ultimately, deploying comprehensive error correction is indispensable for the reliable long-term operation of both memory technologies despite their differing architectures and error profiles. The optimal ECC implementation balances detection strength, area cost, and performance impact.
On-die Variation and Drift
Process variations and operational drift can degrade memory performance and reliability. However, by evaluating memory's susceptibility to these factors, we gain insight into manufacturing complexities and its operating stability.
GST467 Superlattice PCM vs. UltraRAM Parameter: On-die Variation and Drift | |
| Novel Nanocomposite Superlattice Phase-Change Memory | UltraRAM |
| Highly ordered superlattice minimizes cell-to-cell variation. | Subject to transistor variation like Vt, leakage currents. |
| Small confined cell geometry reduces edge effects. | Adaptive biasing/calibration compensates for bitcell variations. |
| Adaptive write/MLC schemes handle set/reset voltage spreads. | Redundancy and repair improve yield and reliability. |
| Much lower resistance drift than conventional PCM. | HKMG transistors and statistical design reduce variation. |
| Regular layers and high crystallization temp stabilize amorphous. | Wear-leveling and refresh manage aging/wear-out effects. |
| Drift compensation tracks per-cell changes over time. | Overall reliable operation through circuit/system techniques. |
| Stronger ECC tolerates increased variation from drift. | Leverages mature CMOS variation-aware design methodologies. |
On-die variation and drift present reliability challenges for non-volatile memories, including novel superlattice PCM technologies. This technology's highly ordered nanostructure minimizes cell-to-cell variability when compared to conventional PCM, thanks to its stable superlattice layers and high crystallization temperature, which intrinsically reduces resistance drift over time.
Nonetheless, to counteract these challenges, employing techniques like adaptive write/MLC programming, drift compensation algorithms, and stronger error correction is essential. These circuit-level approaches counteract residual variation in switching voltages and resistance levels. Through meticulous management via monitoring and adjustment, a synergistic approach ensures consistent long-term operation and performance.
Well-established variation-aware design methodologies are already in use for CMOS-based UltraRAM. Notably, adaptive biasing, along with redundancy, mitigates impacts from transistor mismatches. Careful process optimizations minimize variability from wear-out mechanisms. Also, statistical simulations are being used to guide implementations tolerating expected spreads. All in all, these measures pave the way to create a proven toolkit that addresses manufacture-time and operational instabilities.
In essence, both technologies fundamentally manage variability through architectural advantages combined with circuit techniques. This approach helps tackle drift/mismatch while upholding long-term reliability for robust real-world deployments.
3D Integration Potential
3D stacking can drastically enhance memory density and bandwidth, especially in devices with limited space. By analyzing its feasibility, we can gain better insights into its scalability for future high-performance and high-capacity system architectures.
GST467 Superlattice PCM vs. UltraRAM Parameter: 3D Integration Potential | |
| Novel Nanocomposite Superlattice Phase-Change Memory | UltraRAM |
| A highly uniform superlattice structure enables vertical stacking. | Compatible with standard CMOS, it enables 3D monolithic integration. |
| Multiple superlattice layers with vertical interconnects. | Stacked dies or sequential layers using standard processes. |
| Enables much higher density by stacking multiple arrays. | Highly scalable density and power with CMOS scaling. |
| Shorter vertical connections reduce latency and power. | Tightly coupled logic/memory for in-memory computing. |
| Thermal management challenges from 3D power density. | Power delivery, testing, reliability challenges in 3D stacks. |
| Vertical interconnect yield and reliability are critical. | Techniques like regulators, DVFS, and redundancy for 3D viability. |
| Pioneering work on 64-layer 3D PCM arrays demonstrated. | Enables ultra-high capacities for big data/ML applications. |
The innovative superlattice structure of GST467 PCM inherently enables vertical stacking for 3D integration. Multiple atomically uniform layers are connected through vertical interconnects like TSVs, enabling dramatically higher densities by stacking numerous arrays. However, this density comes with thermal challenges from concentrated 3D power dissipation.
On the other hand, UltraRAM leverages CMOS compatibility for straightforward 3D integration – either monolithically fabricating sequential layers or stacking individual dies. Its scalability with logic nodes supports the ever-increasing 3D densities ideal for applications like in-memory computing. However, it is noteworthy that tailoring power delivery, testing, and redundancy is critical for reliable 3D UltraRAM operation.
While both technologies exhibit strong 3D potential with unique tradeoffs, extensive research is needed to optimize these designs. Having said that, the milestone 64-layer GST467 prototypes and petabyte-scale 3D UltraRAM projections display promising density and bandwidth improvements. Mastering 3D fabrication will open the doors to the true high-capacity, high-performance promise of these innovations.
Integration and Manufacturing
Ultimately, seamless integration with existing semiconductor processes and design flows is essential for commercial viability. Considering integration complexity exposes practical manufacturing adoption challenges.
GST467 Superlattice PCM vs. UltraRAM Parameter: Integration and Manufacturing | |
| Novel Nanocomposite Superlattice Phase-Change Memory | UltraRAM |
| Fabricated using standard thin-film deposition/patterning. | Highly compatible with standard CMOS logic processes. |
| Superlattice layers require precise thickness control. | Based on conventional 6T SRAM bitcell design. |
| Modified etch/clean processes to prevent superlattice damage. | Minor modifications to bitcell for UltraRAM features. |
| Thermal budget management is needed for superlattice integration. | Seamless integration with logic transistors and interconnects. |
| Barrier layers prevent interdiffusion with CMOS materials. | Scaling challenges at 7nm and beyond for dense bitcells. |
| Promising results for high-performance, low-power devices. | Leverages advanced patterning and FinFET transistors for scaling. |
| Novel 3D integration and packaging are also being explored. | Rigorous test/characterization for new modes and packaging. |
The innovative GST467 superlattice PCM, fabricated using standard deposition and patterning processes, is set to push integration frontiers. However, its atomically precise superlattice layers demand stringent thickness control.
Modified etch chemistries are employed to prevent damage, while thermal budgeting becomes critical for CMOS co-integration without interdiffusion – aided by barrier layers. As corrective measures, barrier layers help with thermal budgeting, which is essential for CMOS co-integration without interdiffusion. They also provide protection against damage.
In contrast, UltraRAM seamlessly integrates by modifying conventional 6T SRAM bitcells within standard logic process flows. Such design tweaks also enable features like sleep modes while interfacing with transistors and interconnects. Yet, scaling to 7nm and beyond necessitates advanced patterning and novel transistor architectures. Hence, comprehensive test methodologies and advanced packaging schemes will be needed to characterize new functions.
Wrap Up
Both Superlattice PCM and UltraRAM have unique value propositions and limitations, but instead of trying to achieve an omnipotent winner, i.e., universal memory, system architects need to synergistically co-optimize these disruptive technologies.
Advanced Micro Devices, Inc. (AMD -0.36%)
With an annual revenue of $22.68 billion and net income of $854 million, Advanced Micro Devices aims to challenge the hegemony of bigger players like Nvidia and may foray into more premium categories with innovations like UltraRAM. Intel has, however, publicly displayed interest in PCM, thereby adding to its relevance in the future.
