Memory Design I Professor Chris H. Kim University of Minnesota Dept. of ECE chriskim@ece.umn.edu Array-Structured Memory Architecture 2 1
Semiconductor Memory Classification Read-Write Wi Memory Non-Volatile Read-Write Memory Read-Only Memory Random Access Non-Random Access EPROM E 2 PROM Mask-Programmed Programmable (PROM) SRAM FIFO FLASH DRAM LIFO Shift Register CAM 3 Read-Write Memories (RWM) Basic storage elements of semiconductor memory RAM SRAM DRAM SRAM: cell has gain, 6T, FAST, LOW POWER, logic compatible, differential DRAM: cell has no gain, 1T, refresh, slow, DRAM process, single ended, DENSE 4 2
Memory Scaling Trend Itoh, IBM R&D, 2003 High density is the primary design goal for memories Low voltage operation is essential for low power 5 Memory Scaling Trend Long retention time low Ioff High Vt is required Fast access time high Ion High Vgs-Vt is required Vdd cannot be scaled down aggressively for low power consumption Itoh, IBM R&D, 2003 6 3
Why SRAMs are Important Cache Core Logic 0.18μm Cache Core Logic 0.13μm Core Logic Cache 0.09μm Memories have better power efficiency compared to logic ~9.9B out of 10B transistors will be used for SRAMs Company with better SRAM design will dominate 7 σ V t Why SRAMs are Important 1 Area = Taur, Ning 1 WL Normalize ed I ON 1.4 1.2 1.0 0.8 0.6 0.4 NMOS PMOS 0.01 0.1 1 10 100 Normalized I OFF Area is the number one concern minimum sized devices Smaller devices have larger variation Delay variation, stability, leakage is a problem Central limit theorem doesn t hold (σ/μ) 2X 100X 150nm, 110 C 8 4
Positive Feedback: Bi-Stability V i1 V o1 =V i2 V o2 V o1 1 V o1 V i2 V i 2 5Vo1 V o2 =V i1 V i1 V o2 1 5 V o1 V i 2 V i2 = V o1 A C B V i1 =V o2 9 Meta-Stability V o1 A V o1 A V i2 = V i2 = C C B B δ V i1 = V o2 δ V i1 = V o2 Gain should be larger than 1 in the transition region 10 5
WL SRAM Memory Cell 0 1 BL BLB NMOS access transistors Read and write uses the same port: need sufficient margins One wordline to access cell Two bit lines (BL, BLB) to carry the data Almost minimum size transistors for small cell area 11 SRAM Read Operation WL 1 0 1 1 1 BL BLB Both bit lines are precharged to Vdd Wordline is fired for one of the cells on bit line Cell pulls down either BL or BLB Sense amp regenerates the differential signal Data should not flip after read access Driver TR must be stronger than access TR 12 6
SRAM Read Operation Murmann class notes For high density, large number of cells share bitline and wordline Subarray organization for 32Kb: 128 WL s, 256BL s 13 SRAM Read Operation WL bitline dl delay = C bitlineδv bitline I cell BL BLB 50mV SA out C bitline is large due to large number of cells attached I cell is small due to high density cells V bitline has to be minimized for high speed < 100mV bitline voltage difference generated by SRAM cell Let the sense amplifier finish the job Increased noise sensitivity, circuit complexity 14 7
SRAM Read Operation: Precharge 15 SRAM Read Operation: Precharge Option (a) Similar to dynamic logic precharge Balance transistor to equalize bitline voltages Short wordline pulse required to limit bitline swing Option (b) Pseudo-NMOS type circuit Bitline voltage clamped during read Option (c) NMOS pullup instead of PMOS Precharge levels are limited to V dd -V t Can t operate at low V dd 16 8
SRAM Cell Read Margin V dd V dd V dd V dd 0 Vx When cell is not accessed (WL=0) Data is safely kept inside the cell High noise margin When cell is accessed (WL=V dd ) Access transistor acts as a noise source Data 0 is pulled up to V x Cell data can flip if V x rises above V tn 17 Static Noise Margin 1 0.8 V DD V DD V Q V QB V DD V(QB) 0.6 0.4 0.2 1 0.8 Good SNM 0 0 0.2 0.4 0.6 0.8 1 V(Q) E. Seevinck, 1987, JSSC Destructive read problem The size of the largest square enclosed in the butterfly curves = read static noise margin (QB) V( 0.6 0.4 0.2 0 Bad SNM 0 0.2 0.4 0.6 0.8 1 V(Q) 18 9
CMOS SRAM Analysis (Read) WL BL M 4 Q = 0 M 5 Q = 1 M 6 BL V DD M 1 V DD V DD C bit C bit 19 Techniques to Improve Read Margin Cell beta ratio = (W/L) drv / (W/L) access J. Rabaey Increasing the size of the driver NMOS improves read margin But remember, area is the number one constraint in memory design Increasing cell size a not a good trade off 20 10
Techniques to Improve Read Margin High V t transistors Internal node on low side needs to rise to V t or more Virtually never happens when V t is larger than half V dd Cell is extremely stable at ultra-low power design point Beta ratio constraint is relaxed smaller driver and larger access TR can be used for faster read and write SNM low V t SNM high V t 21 Techniques to Improve Read Margin Boosted cell supply Supply voltage of SRAM cell is higher h than outside Makes driver stronger than access, suppressing the rise in the low side Effectively improves the beta ratio Driver NMOS can be downsized, decreasing cell size V dd V dd V dd + V dd 0 V dd 22 11
WL SRAM Write Operation 1 0 1 1 BL 0 BLB Launch the write data on BL and BLB Word line signal is fired Low bit line value flips cell data Access TR must be stronger than PMOS load 23 CMOS SRAM Analysis (Write) WL V DD M 4 Q = 0 M 6 M 5 Q = 1 PR= ( W / L) ( 4 W /LL ) 6 M 1 V DD BL = 1 BL = 0 24 12
SRAM Cell Write Margin V dd J. Rabaey 0 V dd V dd 0 0 V dd = (W/L) pmos / (W/L) access Access transistor must be stronger than PMOS to pull the below the trip point (typical pull-up ratio ~ 1.5) To avoid cell size increase, correct pull-up ratio achieved by controlling V tn and V tp 25 Techniques to Improve Write Margin Sizing: access TR vs. PMOS in latch Higher WL voltage for access TR Virtual VDD Higher voltage Sizing 26 13
6T-SRAM Layout Until 90nm BL BLB V DD GND WL Compact cell Bitlines: M2 Wordline: strapped in M3 27 6T-SRAM Layout From 65nm 28 14
6T versus 4T SRAM 6T SRAM Cell Supply current is limited to the leakage current of transistors in the stable state 4T SRAM Cell High degree of compactness High power consumption 29 RAM Variations Many variations to the basic 6T SRAM cell More functionality, smaller cells Dual read or single write cell True multi-ported cell Content addressable memory (CAM) 4T memory cell 3T memory cell 2T memory cell 1T DRAM cell 30 15
Dual Read or Single Write Cell WL0 WL1 BL BLB Two wordlines, one for each access transistor Small increase in cell size Can either read two different cells in one cycle or write to one cell 31 Multi Ported Cell WL0 WL1 BL1 BL0 BLB0 BLB1 Each port has separate address Memory access bandwidth is twice (ideally) Write through : data written can be read by another port in the very same cycle 32 16
Content Addressable Memory (CAM) Some application need to find out if anything in the memory matches a certain key value (e.g. tag) Special memory with XOR gate that compares cell value with the data on the bilines Precharged match line shared across word will stay high only when the entire word matches Needs a encoder that will output the matching address 33 Smaller RAM Cells Internal nodes don t go to Need 2 wordlines, read WL Vdd and write WL Cell won t work at low Vdd Can have 1 or 2 bitlines High value stored is (Read/Write) degraded Not very small, since it has Effective strength of NMOS more wires driver is reduced Refresh needed 34 17
WL BL 1-T DRAM Cell WL Write 1 Read 1 M 1 C S X GND V DD 2 V T V DD BL V DD /2 V sensing DD /2 C BL Write: C S is charged or discharged by asserting WL and BL. Read: Charge redistribution takes places between bit line and storage capacitance C S ΔV = VBL V PRE = V BIT V PRE ------------ C S + C BL Voltage swing is small; typically around 250 mv. 35 DRAM Cell Observations 1T DRAM requires a sense amplifier for each bit line, due to charge redistribution read-out. DRAM memory cells are single ended in contrast to SRAM cells. The read-out of the 1T DRAM cell is destructive; read and refresh operations are necessary for correct operation. When writing a 1 into a DRAM cell, a threshold voltage is lost. This charge loss can be circumvented by bootstrapping the word lines to a higher value than V DD 36 18
Sense Amp Operation V BL V(1) V PRE DV(1) Sense amp activated Word line activated V(0) t 37 1-T DRAM Cell Capacitor Metal word line Poly n + n + Inversion layer Poly induced by plate bias Cross-section SiO 2 Field Oxide Diffused bit line Polysilicon gate Layout Polysilicon plate M 1 word line Uses Polysilicon-Diffusion Capacitance Expensive in Area 38 19
Advanced 1-T DRAM Cells Word line Insulating Layer Cell plate Capacitor dielectric layer Cell Plate Si Capacitor Insulator Refilling Poly Transfer gate Isolation Storage electrode Storage Node Poly 2nd Field Oxide Si Substrate Trench Cell Stacked-capacitor Cell 39 Good References on RAM K. Itoh, VLSI Memory Chip Design, Springer-Verlag New York, LLC Y. Nakagome, M. Horiguchi, T. Kawahara, and K. Itoh, Review and future prospects of low-voltage RAM circuits, Vol. 47, No. 5/6, 2003, IBM J R&D R. W. Mann, W. W. Abadeer, M. J. Breitwisch, O. Bula, et al, Ultralow-power SRAM technology, Vol. 47, No. 5/6, 2003, IBM J R&D 40 20