Ultra-High Density Phase-Change Storage and Memory

Transcription

1 Ultra-High Density Phase-Change Storage and Memory by Egill Skúlason Heated AFM Probe used to Change the Phase Presentation for Oral Examination 30 th of May 2006 Modern Physics, DTU Phase-Change Material

2 Outline Introduction Memory vs Storage Changes in Data Storage Phase-Change Memory and Storage Resent Developments in the PC Concepts Ultra-High-Density PC Memory (IBM, Nat. Mat. 2006) Phase-Change Line-Cell Memory (Philips, Nat. Mat. 2005) Summary 2

3 Memory vs Storage Computer memory and storage Components, devices and recording media that retain data for some interval of time (e.g. operation, text, picture or audio) Represents the information in the form of binary digits 1s and 0s Memory Usually solid state storage known as RAM Or other forms of fast but temporary storage (primary storage) Storage Slower than primary storage but of more permanent nature Optical disks or magnetic storage (hard disks) So far, no practical universal storage medium exists All forms of storage have some drawbacks Computer systems usually contain several kinds of storage, each with an individual purpose 3

4 Changes in Data Storage (1) Paper tape and punch cards (1890s) Recorded by punching holes into paper or cardboard medium Read electrically or optically (solid or hole?) Magnetic storage (1950s) Different patterns of magnetization (non-volatile) Accessed using one or more read/write heads (sequential access) e.g. floppy disk, hard disk and magnetic tape 4

5 Changes in Data Storage (2) Semiconductor (SC) storage (~1960) Integrated circuits used to store information A chip with millions of tiny transistors Modern computers: primary storage consists of dynamic volatile SC memory or dynamic RAM Recently, non-volatile SC memory (flash) for offline storage in home computers Optical disk storage (patent1960s, later commercial) Tiny pits etched on the surface of a circular disk Read by illuminating the surface with a laser diode and observing the reflection Non-volatile and sequential accessed e.g. CD s and DVD s 5

6 Changes in Data Storage (3) Magneto-optical disk storage (late 1980s) The magnetic state on a ferromagnetic surface stores information Read optically and written by combining magnetic and optical methods Non-volatile, sequential access, slow write and fast read Recently proposed methods Molecular memory: stores information in organic molecules that can store electric charge (primary storage) Phase-change memory: different mechanical phases of materials used to store information Read information by observing varying electric resistance Non-volatile, random access, read/write storage Might be used as primary, secondary and off-line storage 6

7 Phase-Change Memory and Storage Phase-change memory from atomic order (crystalline) to disorder (amorphous) was found by S.R. Ovshinsky in the 1960s, called Ovonic memory The material is a chalcogenide alloy of Ge, Sb and Te (GST), already widely used in optical disks and SC memories Million overwrite PC technology enabled rewritable DVD When the structure changes from amorphous to crystalline, the optical absorption edge of the material shifts to a longer wavelength The two phases also have dramatically different electrical resistivity and these properties form the basis by which data is stored The amorphous, high resistance state is used to represent a binary 1, and the crystalline, low resistance state represents a binary 0 7

8 Amorphizing and Crystallizing the GST Phase-change materials (GST), are technologically very important for read-write optical and electrical storage, because they can be switched rapidly back and forth between amorphous and crystalline phases by applying appropriate heat pulses (optical or electrical) GST can be amorphized by a short pulsed heating (~10 ns) above the melting temperature (~600 C) with subsequent rapid cooling (~10 9 K/s) Recrystallization is achieved by a slightly longer heat pulse (~100 ns) below the melting temperature but above the glass-transition temperature (~200 C) 8

9 Outline Introduction Memory vs Storage Changes in Data Storage Phase-Change Memory and Storage Resent Developments in the PC Concepts Ultra-High-Density PC Memory (IBM, Nat. Mat. 2006) Phase-Change Line-Cell Memory (Philips, Nat. Mat. 2005) Summary 9

10 Writing Nanoscale Bits Scanned probe and other methods have showed that it is technically difficult to provide and control the required heating (~50 MW m -2 K -1 ) at densities above ~1 Tb/inch 2 In many cases, adjacent bit erasure, limits the achievable storage densities, whereas other methods, such as near-field optical techniques, suffer from low throughput and insufficient heating at small bit sizes The research team from IBM have managed to write ultrahigh-density bit pattern by locally heating the GST material with a tip of an AFM They have also designed a nanoheater as an alternative to the heated AFM tip 10

11 Atomic Force Microscope AFM images 11

12 Thermal Recording of Ultra-High-Density Phase-Change Bit Patterns Experimental setup: Laser pulsed heated AFM tip -> ultra small (d: < 5 nm) heat source Crystalline bits visible as little valleys because their density is higher than that of the amorphous phase 0.4 Tb/inch 2 40 nm pitch between bits Line profile of the PC bits: 7 A height difference Hamann et al, Nature Materials, 5 (2006) 12

13 Ultra-High-Density PC Bit Patterns 0.4 Tb/inch2 1.6 Tb/inch2 Part of a 1.6 Tb/inch2 bit pattern erased crystalline Previously erased bit pattern rewritten at 3.3 Tb/inch2 3 orders of magnitude denser than with commercial optical storage technology amorphous Hamann et al, Nature Materials, 5 (2006) 13

14 The Nanoheater This thin-film resistive nanoheater can reliably generate hot-spots with dimensions of less than 50 nm -> might be technically viable alternative to the heated AFM tip AFM image of the nanoheater Design: Advanced electron-beam lithography used to fabricate thinfilm (~25 nm thick) Pt substrates The total heater size is 1 x 3 µm 2 Hamann et al, Nature Materials, 5 (2006) 14

15 Nanoheater Thermal Properties Temperature image Characterizing the temperature distribution of the nanoheater: They raster-scanned a cold AFM tip (in a tapping mode) over the powered-up heater, while monitoring its temperature-dependent resistance Finite-Element Calculations Evidently the resistance-change drops laterally away from the heater quite sharply (hot-spot < 50 nm) -> Consistent with standard FE calculations Hamann et al, Nature Materials, 5 (2006) 15

16 Nanoheater Thermal Properties Heat transfer to the nanoheater: By applying more power to the nanoheater, its temperature increases -> determine a thermal resistance of ~ 1.1 K/µW for a typical nanoheater -> In excellent agreement with finite element (FE) calculations Hamann et al, Nature Materials, 5 (2006) 16

17 An All-Thermal Memory/Storage Concept The heater is directly patterned on a PC film Recording by applying appropriate current pulses Amorphizing: 10 ns 1.5 V Crystallizing: 100 ns 0.7 V 2 ohms Reading by measuring the phase-dependent thermal resistance of the heater with low bias-current (5.1 ma) Hamann et al, Nature Materials, 5 (2006) 10 4 cycles doable but the baseline drifts ca. 30% -> better heater design and drive electronics 17

18 Outline Introduction Memory vs Storage Changes in Data Storage Phase-Change Memory and Storage Resent Developments in the PC Concepts Ultra-High-Density PC Memory (IBM, Nat. Mat. 2006) Phase-Change Line-Cell Memory (Philips, Nat. Mat. 2005) Summary 18

19 Design of PC-RAM Cell An individual PC-RAM memory cell cannot be based on PC volume alone Current designs have transistors and a resistor associated with each cell The PC volume is normally contacted top and bottom, but the relatively large cross-section of such cells leads to high power consumption It is therefore of interest to try geometries in which the current is lateral rather than vertical The research team from Philips have designed a new type of cell to write the PC bit pattern, which they call the phase-change line cell memory 19

20 Concepts for PC Non-Volatile Memories The part of the PC layer undergoing the reversible PC transition is shown in red The dielectric surrounding are denoted in blue Expected that PC in the LC will appear within the line -> there the current density and temperature rise will by largest Advances of line cells over OUM: - No special electrodes are needed. - Only dielectric material (SiO 2 ) surrounds the portion of the film undergoing PC -> lower programming power and current Lankhorst et al, Nature Materials, 4 (2005) 20

21 PC Memory Cells used to test Feasibility of Line Concept Cross-section of a line-concept memory cell with TiN contacts and Al bondpads processed on a silicon wafer Scanning electron micrograph of such a cell (length 500 nm, width 50 nm) made after structuring of the PC layer, which is done by electron-beam lithography Inset, detail of similar cell with dimensions ca. 100 nm by 50 nm Lankhorst et al, Nature Materials, 4 (2005) 21

22 Phase-Change Materials used in Optical and Electrical Memories In this PC line memory study, they use a Sb-Te composition, doped with Ge, In, Ag and/or Ga Usually, OUM device studies use the Ge 2 Sb 2 Te 5 composition Lankhorst et al, Nature Materials, 4 (2005) 22

23 Temperature Dependent Resistivity of Phase- Change Films with the Line Cell Concept Electrical and structural properties of Ge 2 Sb 2 Te 5 and doped SbTe materials Both materials show large difference in resistivity between the amorphous and the crystalline states Amorphous Ge 2 Sb 2 Te 5 film crystallizes first into a cubic state and above 300 C into more stable hexagonal state Doped SbTe film crystallizes directly into hexagonal state -> better suited in the line cell concept Lankhorst et al, Nature Materials, 4 (2005) 23

24 Typical Line-Cell Electrical Characteristics d.c. Cell Resistance with 50 ns Pulses Crystalline state: Large increase in resistance observed when applying voltage pulses above the RESET threshold voltage -> becomes amorphous Amorphous state: At voltage pulses in the SET window the phase is crystallized and after the SET window it is amorphized again The difference between the incompletely recrystallized state and the amorphous state is large enough for reading the cell Lankhorst et al, Nature Materials, 4 (2005) 24

25 Feasibility of Fast and Reversible Programming of PC-LC with Low Power Real time oscilloscope traces between low ohmic crystalline and high ohmic amorphous state Similar experiments on cells with various line length have shown that the required SET pulse time for complete recrystallization of the amorphous state decreases with decreasing line length -> The speed of the memory thus increases by decreasing the cell PC-LC: 200 nm, cross section ~20 2 nm 2 Read pulse: 0.3 V and 30 ns Black: voltage applied over the cell Red: resulting current Programming time (crystallizaiton) : Real LCM doped-sbte: ns Real OUM GST: ns 25 Lankhorst et al, Nature Materials, 4 (2005)

26 Cycle Endurance of the LC PC-LC: 100 nm, cross section ~25 2 nm 2 Lankhorst et al, Nature Materials, 4 (2005) d.c. cell- resistance are measured after applying a single RESET and a single SET pulse Typically 10 6 cycles are achieved At 10 7 cycles or more, the RESET pulse no longer leads to amorphization and in addition d.c. cell resistance has decreased At cycles the LC is dead The desired value is but 10 6 cycles are similar to flash memory For OUM, typically cycles are reported Might be possible to improve the LC by optimizing materials and cell design 26

27 Scaling of Line-Cell Length Measured RESET voltages (circles) & maximum threshold voltage (triangles) as a function length of lines The RESET voltage decreases with decreasing line length The shortest lines begin to show a reversal of this trend, caused by a relatively large heat loss at the pads at the end of the line Maximum threshold voltages are determined for cells that have been programmed into the maximal possible RESET resistance -> V T increases with increased line length Low enough for cells shorter than 200 nm Lankhorst et al, Nature Materials, 4 (2005) 27

28 Scaling of Cross-Section & Line-Cell Length RESET current can be reduced by scaling-down the cross-section of the line The measured currents agree with the trend obtained from numerical calculations (except the 80 and 100 nm lines) Dashed lines: 2D numerical calculations on cylindrical lines (home made program to solve differential equations) Lankhorst et al, Nature Materials, 4 (2005) Further reduction of programming current is possible by using better thermal insulation material 28

29 Summary Similar idea to store information for more then 100 years, from the paper tape to the phase-change materials Using AFM, it is possible to write 3.3 Tb/inch2 pattern which is 3 orders of magnitude denser than commercially available today and 3 times denser than previously published for PC memory Designing a nanoheater as an alternative to the AFM tip, it is possible to read, write and erase GST film many times 29

30 Designing the PC cell where the current is laterally instead of vertically and doping the SbTe material gives promising results Summary The PC material can easily been distinguished between amorphous and crystalline phases and this LC can run for million cycles Conclusion PC materials have the potential to become universal memory devices in near future technology 30