THE MONOLITHIC 3D-IC: Logic + eDRAM on top MonolithIC 3D Inc. Patents Pending 1 How get single crystal silicon layers at less than 400 oC (Required for stacking atop copper/low k) MonolithIC 3D Inc. Patents Pending 2 How are all SOI wafers manufactured today? Oxide Activated n Si Top layer Cleave using 400oC Hydrogen implant Flip top layer and
of top layer bond to bottom layer anneal or sideways mechanical force. Oxide CMP. Activated n Si H Activated n Si Activated n Si Top layer Oxide Oxide H
Silicon Silicon Oxide Silicon Bottom layer Using Ion-Cut (a.k.a. Smart-Cut) technology MonolithIC 3D Inc. Patents Pending 3 Ion-cut (a.k.a Smart-CutTM) Can also give stacked defect-free single crystal Si layers atop Cu/low k Oxide Activated n Si Top layer Cleave using 400oC Hydrogen implant
Flip top layer and of top layer bond to bottom layer Oxide Activated n Si Bottom layer mechanical force. CMP. Activated n Si H Activated n Si Oxide Oxide anneal or sideways H
Oxide Ion-cut vs. other types of stacked Si Poly Si with RTA Ion-cut Si High Perfect single crystal Si. 100cm2/Vs 650cm2/Vs Variability High Low Sub-threshold slope and Leakage High
Low 700-800oC for crystallization <400oC Low See next slide Defect density Mobility Temperature stacked bottom layer exposed to typically Cost Ion-cut is great, but will it be affordable? Arent ion-cut SOI wafers much costlier than bulk Si today? Today: Single supplier SOITEC. Owns basic patent on ion-cut.
Our industry sources + calculations $50 ion-cut cost per $1500-$5000 wafer in a free market scenario (ion cut = implant, bond, anneal). Contents: Hydrogen implant Cleave with anneal SOITEC basic patent expires 2012!!! Free market scenario After 2012 when SOITECs basic patent expires SiGen and Twin Creeks Technologies using ion-cut for solar Monolithic 3D Logic Shorter wires. So, gates driving wires are smaller. MonolithIC 3D Inc. Patents Pending 7
TSV vs. Monolithic 3D 10,000x higher connectivity TSV Processed Top Wafer Align and bond Processed Bottom Wafer TSV Monolithic Layer Thickness ~50m ~50nm Via Diameter ~5m ~50nm
Via Pitch ~10m ~100nm Wafer (Die) to Wafer Alignment ~1m ~1nm TSV size typically >>1um: Limited by alignment accuracy, silicon thickness Monolithic offers 10,000x higher connectivity than TSV MonolithIC 3D Inc. Patents Pending 8 Industry Roadmap for 3D with TSV Technology ITRS 2010
TSV size ~ 1um, on-chip wire size ~ 20nm 50x diameter ratio, 2500x area ratio!!! Cannot move many wires to the 3rd dimension MonolithIC 3D Inc. Patents Pending 9 Monolithic 3D: The Other Option Needs Sub-400oC Transistors Transistor part Process Temperature Crystalline Si for 3D layer Bonding, layer-transfer Sub-400oC Gate oxide ALD high k Sub-400oC
Metal gate ALD Sub-400oC Junctions Implant, RTA for activation >400oC Junction Activation: Key barrier to getting sub-400oC transistors In next few slides, will show 3 solutions to this problem MonolithIC 3D Inc. Patents Pending 10 One path to solving the dopant activation problem: Recessed Channel Transistors with Activation before Layer Transfer Idea 1: Do high temp. steps (eg. Activate) before layer transfer p
n+ Idea 2: Use low-T processes like etch and deposition to define recessed channel transistors, the standard transistor type used in all DRAMs today. STI not shown for simplicity. n+ p Layer transfer n+ Si p Si Oxide p n+ p- Si wafer p- Si wafer H Idea 3: Silicon layer very thin (<100nm), so transparent, can align
perfectly to features on bottom wafer n+ p MonolithIC 3D Inc. Patents Pending Note: All steps after Next Layer is attached to Previous Layer are @ < 400oC! 11 Recessed channel transistors used in manufacturing today easier adoption GATE n+ n+ n+ p
GATE GAT E n+ p V-groove recessed channel transistor: Used in the TFT industry today RCAT recessed channel transistor: Used in DRAM production @ 90nm, 60nm, 50nm nodes Longer channel length low leakage, at same footprint J. Kim, et al. Samsung, VLSI 2003 ITRS MonolithIC 3D Inc. Patents Pending 12 RCATs vs. Planar Transistors: Experimental data from Samsung 88nm devices From [J. Y. Kim, et al. (Samsung), VLSI Symposium, 2003]
RCATs Less junction leakage RCATs Less DIBL i.e. short-channel effects MonolithIC 3D Inc. Patents Pending 13 RCATs vs. Planar Transistors (contd.): Experimental data from Samsung 88nm devices From [J. Y. Kim, et al. (Samsung), VLSI Symposium, 2003] RCATs Similar drive current to standard MOSFETs Mobility improvement (lower doping) compensates for longer Leff RCATs Higher I/P capacitance MonolithIC 3D Inc. Patents Pending 14 Step 1. Donor Layer Processing Step 1 - Implant and activate unpatterned N+ and P- layer regions in standard
donor wafer at high temp. (~900oC) before layer transfer. Oxidize (or CVD oxide) top surface. SiO22 Oxide layer (~100nm) for oxide -to-oxide bonding with device wafer. P- PN+N+ P- P- Step 2 - Implant H+ to form cleave plane for the ion cut P- PN+N+ P- P- MonolithIC 3D Inc. Patents Pending H+ Implant Cleave Line in N+ or below 15 Step 3 - Bond and Cleave: Flip Donor Wafer and Bond to Processed Device Wafer Cleave along H+ implant line using 400oC
anneal or sideways mechanical force. Polish with CMP. - Silicon N+ <200nm P- SiO2 bond layers on base and donor wafers (alignment not an issue with blanket wafers) Processed Base IC MonolithIC 3D Inc. Patents Pending
16 Step 4 - Etch and Form Isolation and RCAT Gate Litho patterning with features aligned to bottom layer Etch shallow trench isolation (STI) and gate structures Deposit SiO2 in STI Grow gate with ALD, etc. at low temp Gate (<350 C oxide or high-K metal gate) Oxide +N Advantage: Thinned donor wafer is transparent to litho, enabling direct alignment to device wafer alignment marks: no indirect alignment. Ox Isolation Gate Ox P-
Processed Base IC MonolithIC 3D Inc. Patents Pending 17 Step 5 Etch Contacts/Vias to Contact the RCAT Complete transistors, interconnect wires on donor wafer layers Etch and fill connecting contacts and vias from top layer aligned to bottom layer +N P- Processed ProcessedBase BaseICIC MonolithIC 3D Inc. Patents Pending 18 Compare 2D and 3D-IC versions of the same logic core with IntSim 22nm node 600MHz logic core 2D-IC
3D-IC 2 Device Layers Comments Metal Levels 10 10 Average Wire Length 6um 3.1um Av. Gate Size 6 W/L 3 W/L Since less wire cap. to drive
Die Size (active silicon area) 50mm2 24mm2 3D-IC Shorter wires smaller gates lower die area wires even shorter 3D-IC footprint = 12mm2 Power Logic = 0.21W Logic = 0.1W Due to smaller Gate Size Reps. = 0.17W Reps. = 0.04W Due to shorter wires
Wires = 0.87W Wires = 0.44W Due to shorter wires Clock = 0.33W Clock = 0.19W Due to less wire cap. to drive Total = 1.6W Total = Pending 0.8W MonolithIC 3D Inc. Patents 19 SoC Device Architecture Pull out the memory to the second layer 50% of SoC is embedded memory, 50% of the logic area is due to gate sizing buffers and repeaters.
=> Base layer 25%, just the logic => 2nd layer eDRAM with stack capacitor 25% of the area of eDRAM (1T) needs to replace 50% of the equivalent SRAM 1T vs. of 6T ~ 1:3, could be used for: Use older node for the eDRAM, with optional additional port for independent refresh Additional advantage for dedicated layer of eDRAM Optimized process Only 3 metal layers, no die area wasted on loigic 10 metal layers Repetitive memory structure easy for litho and fab 2D SoC to Monolithic 3D (eDRAM on top of Logic) 2D SoC Logic + Memory 14mm Footprint = 196mm2 14mm 3D SoC Memory Footprint = 49mm2
7mm 7mm Logic Monolithic 3D SoC Side View Stack Capacitors (for eDRAM) RCAT transistors (eDRAM + Decoders) Logic circuits Base wafer with Logic circuits eDRAM Use RCAT for bit cell and decoders Vdd WL Bit Line eDRAM with independent port for refresh Bit Line
Vdd WL WL-Refresh eDRAM vs SRAM on top Smaller area and shorter lines should result in competitive performance Independent port for refresh should allow reduced voltage and therefore comparable power Summary First use of MonolithIC 3D technology for SoC could be pulling out the embedded memory to a 2nd layer 2nd Layer embedded memory could use RCAT + Stack Capacitor EDA may need to be adjusted but existing EDA could be used by modifying the memory library and other software shortcuts Estimated benefits: ~1/3 Device cost (first layer size is ~1/4 and second layer is low cost using older process node, repetitive layout, and only 3 metal layers) power Comparable or better performance
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