Semiconductor Memory: DRAM and SRAM
Outline Introduction Random Access Memory (RAM) DRAM SRAM Non-volatile memory UV EPROM EEPROM Flash memory SONOS memory QD memory
Introduction Slow memories Magnetic and optical disk Magnetic tape Slow and high capacity Semiconductor memories DRAM SRAM Non-volatile memory High cost, fast, and use of MOSFET
Introduction (2)
Introduction (3)
DRAM (1) Introduction The highest density semiconductor product Used as the indicator of the technology level Forerunner of the most advanced technology Consist of 1T1C (1 transistor and 1 capacitor) Charge stored in capacitor When capacitor is charged to a high voltage, the DRAM cell stores a 1 state; when C is charged to a low voltage, the DRAM cell stores a 0 state.
DRAM (2) Operation principle (I) Write and read operation - Write: When WL is selected, the data stored on BL will be stored in the capacitor -Read: BL is pre-charged to V DD /2, when WL is selected, the potential at X will be higher than V DD /2 or lower than V DD /2 depends on the stored data.
DRAM (3) Operation principle (I) Charge sharing sensing -C B /C S (bit line capacitance over memory cell capacitance, normally larger than 10) is one of most important parameter in DRAM design - when read, the potential will be charged. The sense amplifier must be sensitive enough to read this signal - the change of potential in memory cell node can be given as below: - in practice, the minimum voltage detected by the sense amplifier is 100mV. For higher density, the sensitivity of sense amplifier should be much increased.
DRAM (4) Operation principle (I) Refresh - The voltage stored on 1 memory cell will slowly decades to a 0 voltage state through various leakage mechanisms - Periodically reading the data and rewriting the same data is necessary which is called refresh. - Leakage mechanisms include - Junction leakage - Pass transistor sub-threshold leakage - Leakage through capacitor dielectrics - Other parasitic leakage paths
Operation principle (I) DRAM (5) Soft errors - alpha particle double charged helium nuclei that originates from the decay of radioactive elements in the package or metallization material. only determined by the total amount of charge collected by the cell node. - cosmic rays a very small percentage of high-energy neutrons will interact by colliding with silicon nucleus, which generate charges.
DRAM (6) Operation principle (I) Cell scaling Approaches to cell scaling reduce thickness increase dielectric constant increase capacitance area (i) stacked capacitor DRAM cell (ii) trench capacitor DRAM cell
DRAM (7) Operation principle (I) Cell scaling Reducing dielectric thickness t - this approach is mainly limited by Fowler-Nordheim tunneling current - primarily constrains in reducing the dielectric thickness originate from dielectric reliability, leakage current, etc. Increasing dielectric constant - O (oxide) ONO (oxide nitride oxide) NO (oxide nitride) Ta2O5 higher dielectric constant material (BST, PZT, etc.) - combination of oxide and nitride can provide at best 4 nm in effective oxide thickness - Ta2O5 film has a higher dielectric constant, and demonstrated with 3 nm effective oxide thickness, but the process feasibility is still not proven yet. - for PZT, BST, a lot of process issues exist.
DRAM (8) Operation principle (I) Cell scaling Increasing area A Stacked capacitor DRAM cell the capacitor dielectric film is formed between the two heavily doped poly-si (or metal) and is stacked over the pass transistor. the device structure is simple process is less sensitive
DRAM (9) Operation principle (I) Cell scaling Increasing area A Trench capacitor DRAM cell the capacitor dielectric film is formed inside a trenched etched into the silicon substrate and between two heavily doped silicon layers or metals has advantage in super planarity process is complicated and difficult
DRAM (10) Comments Cell scaling New memory architecture and circuit technique to reduce cell size On-chip integration of DRAM and logic circuits (embedded DRAM) Increasing difficult to meet soft-error requirement
DRAM (11)
1G DRAM
SRAM (1) Introduction An SRAM cell is a bistable transistor flip-flop, or two inverters connected back to back. Type of SRAM Depletion load Resistor load Full CMOS TFT load
SRAM (2) Example TFT load SRAM cell structure P-channel TFT is fabricated on top of an n+ poly-si gate The n+ poly-si gate is shared with bottom n-channel bulk MOSFET and p-channel TFT The drain of TFT is offset from the gate to reduce the leakage current
SRAM (3) Comparison of different SRAM structures Depletion load Use depletion-mode n-mos transistor as load Oldest type before CMOS technology Resistor load Use high-resistance polysilicon resistor as load Small cell area and high density Full CMOS load Use bulk p-mos transistor as load Cell size tend to be larger than R-load cell Very low standby current Currently the dominant technology due to good stability at low power supply TFT load Use p-channel TFT as load Combines the low-power advantages of the full CMOS load cell and the high density advantages of R-load cell Has been popular for a while, but received less attention recently due to poor stability at low power supply and process complexity
SRAM (4) Operation principles Write operation (writing 1 as example) - Word line of the cell is selected (raised to VDD, 5V) T5 and T6 turn ON - Bit-bar line must be forced low T1 turns OFF and T3 turns ON - The drain voltage at C5 rises due to the current flowing through T5 and T3 - When T2 has been turned ON, the bit line can be returned to its steady level, leaving the cell in the state of storing a 1.
Operation principles SRAM (5) Read operation (reading 1 as example) - The word line of the cell is selected (raised to VDD) T5 and T6 are turned ON. - Both the Bit line and Bit-bar line must be biased at some voltage (for example, 3V) - When the cell is selected, currents flow through T6 and T2 to Vss and through T3 and T5 to the bit line. - Since T2 remaining ON, the voltage of bit-bar line is reduced to less than 3V - While the voltage of bit line is pulled up above 3V since T1 is OFF but T3 is ON - The differential output signal between the bit and bit-bar lines is fed into the sense amplifier, which in SRAMs is a differential amplifier capable of providing rapid sensing.
SRAM (6) Cell stability analysis Cell stability determines the soft-error rate and the sensibility of the memory cell to variation in process and operating condition Cell ratio (β ratio) Recall, β=µc ox (W/L) The ratio between the β of the driver transistor and the β of the access transistor One important parameter that determines the SRAM cell stability If the ratio is too low, it can bring the cross-coupled low side too high and can flip the cell Higher ratio result in better stability, however cause larger cell size.
SRAM (7)
SRAM (8) Soft errors Alpha particles When an alpha-particle hits the high side of cell, the collected charge lowers the potential of the high node and can flip the cell, thus cause a soft error Cosmic rays Same as DRAM
SRAM (9) Comments SRAM memory cell size trends Full CMOS memory cell becoming dominate due to low-voltage advantage Increasing difficulty in meeting soft-error requirement.
References S. Shichijo, DRAM and SRAM, Chapter 7, ULSI Devices (Editors: CY Chang and SM Sze), John Wiley & Sons, 2000. J. A. Mandelman, et al, Challenges and future directions for the scaling of dynamic random-access memory (DRAM), IBM J of Res & Dev, Vol. 46, No. 2/3, pp. 187, 2002. P. W. Diodato, Embedded DRAM: More than just a memory, IEEE Communication Magazine, July, 2000. Proceedings of ISSCC Proceedings of IEDM Proceedings of VLSI Symp on Technology IEEE Trans on ED, IEEE EDL