臺灣博碩士論文加值系統

本研究主要可分為兩大部份。第一部份提出結合動力學理論與經過不同藍光雷射脈衝照射之非晶與結晶硫屬相變化記錄膜之電子顯微鏡照片，得到簡單快速研發硫屬相變化記錄膜最佳元素摻雜濃度範圍的方法。我們選用結晶溫度影響可被忽略的膜厚35nm來探討各元素摻雜濃度的最佳化範圍。之後即可以平衡約化活化能(Ec/Em≦1，其中Ec是結晶活化能，Em是熔融活化能)的方式，得到各元素摻雜的最佳濃度範圍。銀、銦、硒摻雜至Sb70Te30記錄膜之濃度分別須稍小於0.8、2.5和2.1原子百分比，而摻雜矽的最佳濃度範圍則介於1.5及4.7原子百分比之間。記錄膜在各元素最佳濃度摻雜範圍內才具有足夠高的記錄敏感度與檔案壽命，同時具有足夠的初始化能力及順利在非晶與結晶相間切換的能力。
在第二部份，我們量測各相變化記錄膜電阻隨溫度變化的數據，在結合熱與電性質之量測結果作為不同相變化記憶晶胞結構的負載與邊界條件後，根據有限元素數值分析法所得結果提出三種新穎之操作模式。第一個模式結合重設電壓(Vreset)與設定電流(Iset)，可大幅降低各相變化記憶胞程式化的臨界條件。此操作模式在不同記憶胞結構及不同製程線寬時，皆具有額外的「直接抹寫」、「自我保護」及「同步時脈」等功能。依此法可再延伸兩個多階操作的模式。第一個多階操作模式利用重設電壓(Vreset)隨後施加不同脈衝振幅、寬度或次數的設定電流(Iset)可得到不同的結晶率，即可在單一記憶晶胞內記錄代表不同數值的多階訊號。第二個多階操作模式利用使記錄區完全結晶之設定電流(Iset)隨後施加不同脈衝振幅、寬度或次數的重設電壓(Vreset)，即可在結晶區中形成不同大小的非晶記錄點。當使用第二個多階操作模式時，因為不需大範圍地熔融記錄膜，故可預期以此模式操作時可提高記憶胞的重複次數。多階記錄之訊號可被適當的讀取電壓以非破壞地方式判別電場引發之電流值。低的臨界程式化條件是手持式內存記憶裝置之必要條件。同時，這幾個操作模式尤其適用於低量產成本的簡單T型垂直記憶胞結構。

This study can be classified into two parts. Firstly, obtaining a simple and quick method for materials development of chalcogenide based phase-change data storage has been testified with combining the kinetics theories and TEM results from both amorphous and crystalline recording films irradiated by blue-ray laser pulses. We selected a crystallization temperature independent thickness of 35 nm to discuss various element doping effects on the fast-growth Sb70Te30 recording films. Afterward, optimum doping concentration ranges of each element can be simply obtained from the balanced reduced activation energy (Ec/Em≦1, where Ec is the activation energy for crystallization, and Em is the activation energy for melting). The optimal doping concentration ranges of Ag, In, and Se should slightly less than 0.8, 2.5, and 2.1 at.%, respectively, and Si within the Sb70Te30 recording film should be located between 1.5 and 4.7 at.% to obtain sufficiently high recording sensitivity and archival stability, while still maintain adequate initialization ability and smooth switch between the amorphous and crystalline states.
Secondly, the temperature dependent resistance variations of different recording films have also been measured. The results from the thermal and electrical analyses were used as the loads and boundary conditions for the numerical simulation on various phase-change random access memory (PRAM) cells conducted with using finite element codes. Base on the simulation results, three innovative operation modes for different PRAM cells have been proposed. The first mode, combining the RESET voltage with SET current, can effectively decrease the programming threshold of the PRAM cells. This operation mode can offer additional functions such as direct- overwrote (DOW), self-protected, and synchronous clock cycle for the PRAM cells (SPRAM) with different structures and various feature sizes. By extending the first mode, two additional multi-level (ML) operation modes can be achieved. The first ML operation mode can be performed by using SET current (Iset) pulses (varying the pulse magnitude, width, or numbers) followed by a suitable RESET voltage (Vreset) pulse to make various crystallization ratios of PRAM cells. The other ML mode can be achieved by using Vreset pulses (varying the pulse magnitude, width, or numbers) followed by a complete Iset pulse to record various sizes of amorphous marks in the crystalline PRAM cells. The repeat cycles can be expected to increase by using the second ML operation mode since it need not melt a large area in the PRAM cells. The ML signals can be achieved by using suitable READ voltage (Vread) pulse and then differentiating by the electric field induced current nondestructively. The low programming threshold is necessary for embedded mobile applications; meanwhile these operation modes are especially suitable for the simple T-shaped vertical cell structure that has the greatest potential for mass production with low cost.

Table of Contents I
List of Figures IV
List of Tables XII
List of Symbols and Abbreviations XIV

Chapter 1 Background and Motivations 1
1.1 Introduction to repeatable memories 1
1.2 The principle of rewritable phase-change storage 3
1.2.1 Phase-change materials 4
1.2.2 Optimum concentration range of each element 8
1.3 Motivations of Study 10
1.3.1 Optimum thickness and concentration ranges of Sb70Te30
recording films 10
1.3.2 Operation thresholds on different PRAM cell structures 11
1.3.3 New operation modes 13

Chapter 2 Literatures Review 16
2.1 Development of rewritable optical data storage 16
2.1.1 Archival life stability and crystallization rate 16
2.1.2 Multi-layered recording technology 19
2.1.3 Thickness dependent crystallization and melting behaviors 21
2.1.4 Isothermal phase change kinetics 25
2.1.5 Non-isothermal phase change kinetics 28
2.1.6 Composition dependent recording behaviors under laser irradiation 29
2.2 The development of PRAMs 31
2.2.1 Operation by current pulses 31
2.2.2 Operation by voltage pulses 39
2.2.3 Thermal cross-talk and operation modes 45
2.3 ANSYS finite element simulation 46

Chapter 3 Experimental Procedures 47
3.1 Sample preparation and measurements 47
3.1.1 Defining the particular thickness and compositions for research 47
3.1.2 Electro-thermal, structure, finite element analyses,
and blue-ray static test 48
3.2 Thermal analyses for phase-change kinetics 50
3.2.1 Non-isothermal kinetics 50
3.2.2 Isothermal crystallization kinetics 50
3.3 ANSYS finite element simulation 52

Chapter 4 Thickness and Element-Doping Effects 51
4.1 Thickness effects on the non-doped Sb70Te30 recording films 51
4.2 Ag-doped effects on Sb70Te30 recording film 63
4.3 In-doped effects on Sb70Te30 recording film 75
4.4 Se-doped effects on Sb70Te30 recording film 85
4.5 Si-doped effects on Sb70Te30 recording film 92
4.6 Short summary for thickness and element-doping effects 99

Chapter 5 Simulation on different PRAM structures 101
5.1 Electro-thermal properties of various recording films 103
5.2 PRAM adopting SiST with additional a-C heating layer 110
5.3 Operation of PRAM (SPRAM) by RESET voltage
and SET current 115
5.4 Peltier effect in the voltage RESET processes 123
5.5 Mult-Level PRAM (ML-PRAM) Operation Modes 125

Chapter 6 Conclusions and Prospects 130
Conclusions of this study 130
Suggestion for future works 132
Publication lists 136

References 139


List of Figures
Chapter 1 Background and Motivations 1
Fig. 1.1 Schematic diagrams of non-volatile phase-change memory 4
Fig. 1.2 Typical cooling schedules of the phase change processes 4
Fig. 1.3 Ternary phase diagram depicting various phase-change alloys and year of discovery of their use in different optical storage products 6
Fig. 1.4 Schematic drawing of the erasure (crystallization) process of two types of
phase change disks: nucleation-driven erasure and growth-driven erasure 7
Fig. 1.5 The maximum user data rate of phase change recording stacks based on nucleation-dominated Ge2Sb2Te5 or a particular doped fast-growth eutectic Sb-Te composition as a function of inverse spot diameter d-1=NA/(1.22λ)
of the focused laser beam 7
Fig. 1.6 Sb－Te Phase-diagram 9
Fig. 1.7 General diagram of DRAM and SDRAM operations 14
Fig. 1.8 Outline diagram of mark radial width modulation 4-level recording on a phase-change optical disk 15
Fig. 1.9 Typical PRAM operational flow chart 15

Chapter 2 Literatures Review 16
Fig. 2.1 Schematic drawing of a four-layer IPIM stack with one recording layer 21
Fig. 2.2 Cross-sectional view of the rewritable dual-layer phase-change disk 21
Fig. 2.3 Dependence of the crystallization time of Ge4-xSb2Te7 films on film
thickness and composition 22
Fig. 2.4 Complete erasure time versus the Ge1Sb2Te4 recording layer thickness in
both IPIM and II’PI’IM stacks 22
Fig. 2.5 Complete erasure time as a function of the radius of the recorded mark for different values of fast-growth phase-change layer-thickness 23
Fig. 2.6 (a) Dependence on phase-change layer thickness of the minimum complete erasure time of a mark of radius ~60 nm (b) Maximum crystal growth
velocity in doped SbTe alloy as a function of phase-change layer thickness 24
Fig. 2.7 Typical vertical T-shaped PRAM structure 32
Fig. 2.8 Current-voltage characteristics for PRAM cell element in both the RESET
and SET state showing key device parameters: READ/SET/RESET
regimes, SET and RESET states, holding voltage (Vh) and switching
threshold voltage (Vth) 32
Fig. 2.9 Programming cycles of the vertical T-shaped PRAM 33
Fig. 2.10 Schematic cross section of the vertical memory cell when an
additional heating layer (a-C) is inserted between the TiN heater and
Ge2Sb2Te5 layers 34
Fig. 2.11 Structures of PRAM (OUM) (a) Bottom (b) Edge contact type cell 35
Fig. 2.12 (a) μ-trench scheme top view; Schematic and STEM-TEM cross section
along (b) X and (c) Y directions 35
Fig. 2.13 (a) Schematic view and (b) process sequences of ring-type BEC 36
Fig. 2.14 Schematic views of (a) conventional and (b) PCM-U devices 37
Fig. 2.15 SEM picture of an ultra-thin PCB memory cell test structure fabricated
for proof-of-principle evaluation. Phase-Change lines, patterned using
e-beam in widths ranging from 20 to 200nm, bridge the TiN electrodes 37
Fig. 2.16 SEM cross-section of (a) μTrench , and (b) Lance heaters 38
Fig. 2.17 Pore memory element process modules 38
Fig. 2.18 I-V characteristics measured on the PRAM cell after various SET pulses 39
Fig. 2.19 Phase-change memory cells of line concept. (a) Schematic cross-section
of a line concept memory cell with TiN contacts and Al bondpads
processed on a slilicion wafer. (b) Scanning electron micrograph of
such a cell (length~500 nm, width~50 nm) made after strcutring of the phase-change layer, which is done by blectron-beam lithography.
TiN contacts are structured by contact lithography. Inset, detail of
similar cell with approximate length ~100nm and width ~50nm 40
Fig. 2.20 I-V curves of GeSbTe PRAM cells at different temperatures 41
Fig. 2.21 Measured current increases drastically at threshold voltage ~139 V 41
Fig. 2.22 The I-V characteristics of the PRAM cells (a) with amorphous SiGexNy
layer, (b) without upper heating layer, and (c) with crystalline SiGexNy
layer. Cross-section structure of the PRAM cell is shown in the insets 43
Fig. 2.23 Schematic of PCM memory arrays with nano-wire diodes as memory
cell selection devices 43
Fig. 2.24 Cross-sectional TEM images for the PRAM cells programmed by
(a) a short reset pulse of 1.5 V, 50ns, and (b) a long set pulse of 1.5 V,
1 ms. Both cells are pre-reset with the same pulse of 4 V, 50ns to form
the initial amorphous volume 44

Chapter 3 Experimental Procedures 47
Fig. 3.1 Experimental flow chart of the study 48
Fig. 3.2 Sketch of the transient reflectivity/resistance-temperature-time
measuring system 50

Chapter 4 Thickness and Element-Doping Effects 51
Fig. 4.1.1 Reflectivity variation with temperature for the as-deposited Sb69.9Te30.1 recording films with thickness of 10, 35, and 70 nm at a heating rate of
10 oC/min 52
Fig. 4.1.2 TEM images and diffraction patterns of pure Sb69.9Te30.1 recording film
(a) before and after annealed at (b) 200, (c) 300, and (d) 400 oC 52
Fig. 4.1.3 (a) DSC curve for as-deposited Sb69.9Te30.1 film at a heating rate of
10 oC/min, and (b) TEM images and diffraction patterns of pure
Sb69.9Te30.1 recording film after annealed at 150oC 54
Fig. 4.1.4 Kissinger’s plots for Sb69.9Te30.1 recording films with various thicknesses 59
Fig. 4.1.5 Fraction of crystallization as a function of time for the 35 nm-thick Sb69.9Te30.1 recording film isothermally annealed at temperatures of
5, 10, 15, and 20 oC below Tc 60
Fig. 4.1.6 ln [-ln(1-X)] versus ln (t) plots for the 35 nm-thick Sb69.9Te30.1
recording film isothermally annealed at temperatures of
5, 10, 15, and 20 oC below Tc 60
Fig. 4.1.7 Fraction of crystallization as a function of time plots for Sb69.9Te30.1
recording film with thicknesses of 10, 35, and 70 nm isothermally
annealed at temperatures of 10 oC below Tc 61
Fig. 4.1.8 ln [-ln (1-X)] vs. ln (t) plots for Sb69.9Te30.1 recording film with
thicknesses of 10, 35, and 70 nm isothermally annealed at
temperatures of 10 oC below Tc. 61
Fig. 4.1.9 ln (K) versus (1/T) plots for Sb69.9Te30.1 recording films with
thicknesses of 10, 35, and 70 nm. 62

Fig. 4.2.1 Reflectivity variation with temperature for the Ag-doped Sb70Te30
recording films at a heating rate of 10oC/min 63
Fig. 4.2.2 TEM images and diffraction patterns of Ag0.8Sb70.5Te28.7 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 64
Fig. 4.2.3 TEM images and diffraction patterns of Ag4.0Sb68.4Te27.6 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 65
Fig. 4.2.4 TEM images and diffraction patterns of Ag10.8Sb63.1Te26.1 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 66
Fig. 4.2.5 Kissinger’s plots for various Ag-doped Sb70Te30 recording films 69
Fig. 4.2.6 The fraction of crystallization as a function of time for the Ag-doped
Sb70Te30 recording films 70
Fig. 4.2.7 The ln[-ln(1-X)] versus ln(t) plots in the acceleration regime
(0.25 ≤ X ≤ 0.75) for Ag-doped Sb70Te30 recording films 71
Fig. 4.2.8 The ln(K) versus (1/T) plots for Ag-doped Sb70Te30 recording films 71
Fig. 4.2.9 TEM images of as-deposited (a) Sb69.9Te30.1 (b) Ag0.8Sb70.5Te28.7
(c) Ag4.0Sb68.4Te27.6 (d) Ag10.8Sb63.1Te26.1 recording films after static test 73
Fig. 4.2.10 TEM images of annealed (a) Sb69.9Te30.1 (b) Ag0.8Sb70.5Te28.7
(c) Ag4.0Sb68.4Te27.6 (d) Ag10.8Sb63.1Te26.1 recording films after static test 74

Fig. 4.3.1 Reflectivity variation with temperature for the In-doped Sb70Te30
recording films at a heating rate of 10oC/min 75
Fig. 4.3.2 TEM images and diffraction patterns of In2.5Sb70.0Te27.5 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 76
Fig. 4.3.3 TEM images and diffraction patterns of In5.3Sb70.4Te24.3 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 77
Fig. 4.3.4 TEM images and diffraction patterns of In7.7Sb71.3Te21.0 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 77
Fig. 4.3.5 ln(α/Tx2) versus 1/Tx for both crystallization and melting of the
In-doped Sb70Te30 recording films 79
Fig. 4.3.6 The fraction of crystallization as a function of time for the
In-doped Sb70Te30 recording films 80
Fig. 4.3.7 The ln[-ln(1-X)] versus ln(t) plots for In-doped Sb70Te30 recording films 81
Fig. 4.3.8 The ln(K) versus (1/T) plots for In-doped Sb70Te30 recording films 81
Fig. 4.3.9 TEM images of as-deposited (a) Sb69.9Te30.1 (b) In2.5Sb70.0Te27.5
(c) In5.3Sb70.4Te24.3 (d) In7.7Sb71.3Te21.0 recording films after static test 83
Fig. 4.3.10 TEM images of annealed (a) Sb69.9Te30.1 (b) In2.5Sb70.0Te27.5
(c) In5.3Sb70.4Te24.3 (d) In7.7Sb71.3Te21.0 recording films after static test 84

Fig. 4.4.1 Reflectivity variation with temperature for the Se-doped Sb70Te30
recording films at a heating rate of 10oC/min 85
Fig. 4.4.2 TEM images and diffraction patterns of Se2.1Sb70.4Te27.6 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 86
Fig. 4.4.3 TEM images and diffraction patterns of Se4.8Sb70.3Te24.9 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 87
Fig. 4.4.4 TEM images and diffraction patterns of Se6.6Sb71.3Te22.1 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 87
Fig. 4.4.5 ln(α/Tx2) vs 1/Tx for both crystallization and melting of recording films 89
Fig. 4.4.6 The fraction of crystallization as a function of time for the
Se-doped Sb70Te30 recording films 90
Fig. 4.4.7 The ln[-ln(1-X)] vs ln(t) plots for Se-doped Sb70Te30 recording films 91
Fig. 4.4.8 The ln(K) vs (1/T) plots for Se-doped Sb70Te30 recording films 91

Fig. 4.5.1 Reflectivity variation with temperature for the Si-doped Sb70Te30
recording films at a heating rate of 10oC/min 92
Fig. 4.5.2 TEM images and diffraction patterns of Si1.5Sb74.0Te24.6 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 93
Fig. 4.5.3 TEM images and diffraction patterns of Si4.7Sb71.1Te24.2 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 94
Fig. 4.5.4 TEM images and diffraction patterns of Si13.1Sb64.2Te22.7 recording film
(a) before and after annealed at (b) 200oC, (c) 300oC, and (d) 400oC 94
Fig. 4.5.5 Kissinger’s plots for both crystallization and melting of the
Si-doped Sb70Te30 recording films 96
Fig. 4.5.6 The fraction of crystallization as a function of time for the
Si-doped Sb70Te30 recording films 97
Fig. 4.5.7 The ln[-ln(1-X)] vs ln(t) plots for Si-doped Sb70Te30 recording films 98
Fig. 4.5.8 The ln(K) vs (1/T) plots for Si-doped Sb70Te30 recording films 98

Chapter 5 Simulation on different PRAM structures 101
Fig. 5.1.1 Temperature dependent sheet resistance variations for the Ge2Sb2Te5
and Sb70Te30 recording films at a heating rate of 10oC/min 105
Fig. 5.1.2 Temperature dependent sheet resistance variations for the Ag-doped
Sb70Te30 (AST) recording films at a heating rate of 10oC/min 106
Fig. 5.1.3 Temperature dependent sheet resistance variations for the In-doped
Sb70Te30 (IST) recording films at a heating rate of 10oC/min 106
Fig. 5.1.4 Temperature dependent sheet resistance variations for various Si-doped
Sb70Te30 (SiST) recording films at a heating rate of 10oC/min 107
Fig. 5.1.5 Temperature dependent sheet resistance variations for the Se-doped
Sb70Te30 (SeST) recording films at a heating rate of 10oC/min 107
Fig. 5.2.1 The cross-section geometry of the 1T/1R PRAM cell with an a-C
additional heating layer 110
Fig. 5.2.2 The temperature profile of the PRAM cell; the inset is the temperature variation with time of the hottest node 111
Fig. 5.2.3 Maximum temperature versus SET current for various SiST recording
films and feature sizes of the PRAM cells 112
Fig. 5.2.4 Maximum temperature versus RESET current for various SiST recording films, Seebeck coefficients, and feature sizes of the PRAM cells 113
Fig. 5.2.5 The manufacturing inaccuracy of ±1 nm of the a-C layer of the PRAM cell adopted in F=150nm can cause a huge temperature variation as high as 545oC at the hottest node under the RESET operation 114

Fig. 5.3.1 (a) The cross-section diagram of the PRAM cell with vertical
structure, and (b) The cross-section diagram of the PRAM cell
with line structure (F=65nm) 116
Fig. 5.3.2 Maximum temperature as a function of RESET voltages for the PRAM vertical and line cells with c-SeST or c-GST recording layer 117
Fig. 5.3.3 Maximum temperature as a function of RESET currents for the PRAM vertical and line cells with c-GST or c-SeST recording layer 118
Fig. 5.3.4 Maximum temperature as a function of SET voltages for the PRAM
vertical and line cells with a-GST or a-SeST recording layer 119
Fig. 5.3.5 Maximum temperature as a function of SET currents for the PRAM
vertical and line cells with a-GST or a-SeST recording layer 120
Fig. 5.3.6 (a) Maximum temperature as a function of RESET voltages for the
PRAM vertical cells with c-SeST recording layer at various
feature sizes, and (b) Maximum temperature as a function of
SET currents for the PRAM vertical cells with c-SeST
recording layer at various feature sizes 122

Fig. 5.4.1 Applying a positive reset voltage pulse to the BEC as (a) S=+6.0E-5,
and (b) S=+1.2E-4V/K of the recording layer 124
Fig. 5.4.2 Applying a negative reset voltage pulse to the BEC as (a) S=+6.0E-5,
and (b) S=+1.2E-4 V/K of the recording layer 124

Fig. 5.5.1 Various amorphous marks in the crystalline background of PRAM cells 127
Fig. 5.5.2 Current density profiles of PRAM cells applying a small voltage pulse 128
Fig. 5.5.3 The current density variations with time on the top surface at the center
of the top electrode 129

Chapter 6 Conclusions and Prospects 130
Fig. 6.1 Sheet resistance and reflectivity variation with temperature of the as-deposited
Ag10.8Sb63.1Te26.1 recording films at heating and subsequent cooling processes. 133

List of Tables
Chapter 1 Background and Motivations 1
Table 1.1 Memory technology benchmark 3

Chapter 2 Literatures Review 16
Table 2.1 Development of phase-change materials 16
Table 2.2 Overview of the crystallization temperatures, activation energies for crystallization, and archival life stabilities at 50℃ of various
fast-growth materials 18
Table 2.3 Overview of the minimum crystallization times of an amorphous mark
of 125 nm radius of various fast-growth compositions 19
Table 2.4 Values of m in the Johnson–Mehl–Avrami (JMA) kinetic law 27

Chapter 4 Thickness and Element-Doping Effects 51
Table 4.2.1 The crystalline phase transitions of various Ag-doped Sb70Te30
recording films at various temperatures 66
Table 4.3.1 The crystalline phase transitions of various In-doped Sb70Te30
recording films at various temperatures 78
Table 4.4.1 The crystalline phase transitions of various Se-doped Sb70Te30
recording films at various temperatures 88
Table 4.5.1 The crystalline phase transitions of various Si-doped Sb70Te30
recording films at various temperatures 95
Table 4.6.1 Crystallization and melting temperatures, activation energies,
and reduced values thereof 99
Table 4.6.2 JMA kinetic constants 99

Chapter 5 Simulation on different PRAM structures 101
Table 5.1 Sheet resistance values of various element-doped Sb70Te30 and Ge2Sb2Te5 recording films in both amorphous and crystalline states 104
Table 5.2 Resistivity values of various element-doped Sb70Te30 and Ge2Sb2Te5
recording films in both amorphous and crystalline states 108
Table 5.3 Physical properties of the GST, SiST, W, SiO2, TiN, and a-C 110
Table 5.4 The minimum operation conditions required for various PRAM cells 121

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