Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 CHAPTER 6 Semiconductor Memories

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 INTRODUCTION Memory circuits provide the means of storing information (data) on a temporary or permanent basis and for future recalls. Magnetic memory Generally is capable of storing large amount of data at very low cost, but the access time (the time it takes to locate and then read or write) is usually very long. Semiconductor memories use electrical signals to identify memory location and its content. The access time in several orders of magnitude faster that magnetic memory. MOS and bipolar technologies can be used to implement various types of semiconductor memories.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 CLASSIFICATION OF SEMICONDUCTOR MEMORIES Semiconductor memories volatileNon-volatile loose their data once the power supply is turned off. SRAM, DRAM ROM, EPROM can retain their data even after power is removed.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 STATIC RAM (SRAM) Random Access Memory (RAM) is a readable and write-able volatile memory. The term random access means that the user can access any location of the entire memory and in any order. RAM is further divided into static RAM (SRAM) and dynamic RAM (DRAM). Static RAM is a simple latch circuit (flip-flop) that remembers its state until it is toggled.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 STATIC RAM (SRAM) The simplest SRAM would be a simple data latch with pass transistors for selection and isolation. The actual implementation is usually carried out by the 6-transistor cell.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 STATIC RAM (SRAM)

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 STATIC RAM (SRAM)

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 DYNAMIC RAM (DRAM) Fabricated using MOS technology and are noted for their high capacity, low power requirement, and moderate operating speed (when compared to SRAM). DRAMs make use of MOS capacitors to store the data as electronic charges. The capacitors can be switched in and out of the bit lines via a pass transistor. The storage capacitor will loose its charge over time. Therefore, DRAMs must be refreshed in a regular basis.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 DYNAMIC RAM (DRAM) The read operation for DRAM is a destructive process. Therefore, extra peripheral circuits must be used to rewrite the DRAM cells as soon as it is read. These operations are incorporated as part of the DRAM chip and are transparent to the users. The memory organization can be similar to the SRAM array. However, the memory size for DRAMs is usually much larger. Currently DRAM in 1G-bit size are available while SRAM sizes of under a 1M-bit are more common.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 DYNAMIC RAM (DRAM) The most important difference of the DRAM fabrication process from other technology is the storage capacitor. The single-transistor DRAM cell requires a capacitor that can store sufficient charge to allow the cell state to state true between refresh cycles. The most significant development in the DRAM devices has been the advance in the capacitor design.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 DYNAMIC RAM (DRAM) For a minimum Tox, the remaining adjustable parameters are ε i and A. The DRAM capacitors have been improved in two ways: increasing the surface area and increasing the capacitor dielectric constant

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Trenched DRAM Cell One area of progress in making a capacitor with a larger surface area is to form the capacitor in a trench. This technique make use of a deep trench (> 7μm into the silicon). It has the advantage of allowing the transistors to be formed nearly planar on the surface with the trench extending below the device active area.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Trenched DRAM Cell Fabrication results of Trenched DRAM Cell The effect of the large surface area and the ability to thin the gate dielectric by improving the reliability of the thin oxides has resulted in a DRAM that can store more charge in smaller top surface area.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Stacked DRAM Cell Another area of tremendous progress in DRAM technology goes in the other direction and forms the capacitors in geometries that extend above the silicon to create a large capacitor area. The large surface areas can be created using a large planar capacitor shaped like a dome or a crown. These structures also take advantage of higher-dielectric constant materials for the inter-level dielectric for the capacitors — Ta2O5 (Ba, Sr)TiO3, etc.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Stacked DRAM Cell

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Trench Cell vs. Stacked Cell

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Trench Cell vs. Stacked Cell In the trench technology, the cell process is completed before the gate oxidation. Therefore there is no thermal process due to cell capacitor formation after the MOSFET formation. Another advantage in the trench cell is that there is no height difference between cell array region and peripheral circuit region. In the stacked cell, the height difference require high aspect ratio contact holes and difficulty in the planarization process after the cell formation. The MOSFET formation steps are followed by the stacked capacitor formation steps.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Trench Cell vs. Stacked Cell These include high temperature processing steps such as storage node insulator (SiO 2 /SiN) formation, SiN deposition for the self-aligned contact formation, etc. Salicide (Self Aligned siLICIDE) process for the source and drain of the MOSFETs should be carefully designed to endure the high temperature process steps. For applications requiring Embedded DRAM, trenched cell is more attractive because the DRAM cells are formed before the MOSFETs and because there is little height difference between the cell array and other regions on the chip.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 READ ONLY MEMORY(ROM) Certain applications may require the memory to hold data that are either permanent or will not be changed frequently.In this case, nonvolatile memory is the candidate. As the name implies, Read Only Memory (ROM) has no provision to write or update its memory contents. The programming is usually done during the manufacturing process or by a burning procedure prior to field use.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 READ ONLY MEMORY

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 READ ONLY MEMORY

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 READ ONLY MEMORY

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 READ ONLY MEMORY One obvious disadvantage of the mask ROM is the fact that a new photomask must be prepared if the stored data is to be changed. This will also be accompanied by a sizable turn-around time when manufacturing the new ROMs. An alternate method of implementing the ROM is with a programmable technology such as fuse or anti-fuse. In this case, the ROM becomes a programmable parts, hence the name PROM. One advantage of the PROM is the fact that all ROMs, regardless of data content can be manufactured using the same set of photomask and fabrication procedures.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 READ ONLY MEMORY However, a one-time-only programming procedure must be applied prior to field use. After the PROM is programmed, its contents cannot be changed anymore.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 READ ONLY MEMORY Why EEPROMs? Field programmable capability to wireless portable telecommunication equipment True 5V or lower operation Compatible with CMOS/BiCMOS processes High operation speeds and high density Key to embedded systems Solid-state nonvolatile memories Multi-level Encoding

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Conventional Flash E2PROM cell Structures Similar to an ordinary MOSFET except for an extra gate buried in the silicon dioxide. Charges are stored in the floating gate to alter the threshold voltage of the E 2 PROM cell. Simple construction and fabrication steps Very high packing density Flash E 2 PROM and E 2 PROMs share the same technology

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Flash E 2 PROM Cell Operations For programming, hot electrons are created by the large drain bias current. These electron tunnels through the thin gate oxide and become trapped in the floating gate.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Limitations of Existing Flash E 2 PROM Cells Most cells suffer from hole trapping in the thin gate oxide during erasing. Reduction in VTH window after several cycles of erase and programming.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Memory Circuits Semiconductor memories are comprised of a main storage array surrounded by peripheral circuits for read and write operations. These peripheral circuits includes row and column address decoders, data buffers/registers, sense amplifiers, and charge pumps circuits. The decoders and buffer/registers are basically digital circuits and can be implemented using conventional VLSI design methodology. The sense amplifier is basically a high gain circuit that is used to differentiate the stored information with a reference voltage.

Norhayati Soin 06 KEEE 4426 WEEK 14/2 31/03/2006 Memory Circuits This is especially critical if the storage element is aDRAM cell. Most memory arrays require separate high and low voltage supply to operate. In order to eliminate the need for multiple external power supplies, more memory chips have built-in charge pump circuits to generate multiple voltage levels from a single supply (usually 5V or 3.3V).