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A multigate device or Multigate Field Effect Transistor(MuGFET) refers to a MOSFET which incorporates more than one gate into a single device. The multiple gates may be controlled by a single gate electrode, wherein the multiple gate surfaces act electrically a single gate, or by independent gate electrodes. A multigate device employing independent gate electrodes is sometimes called a Multiple Independent Gate Field Effect Transistor or MIGFET. Multigate transistors are one of several strategies being developed by CMOS semiconductor manufacturers to create ever-smaller microprocessors and memory cells, colloquially referred to as extending Moore's Law.[1] Development efforts into multigate transistors have been reported by AMD, Hitachi, IBM, Infineon, Intel, TSMC, Freescale, UC Berkeley and others and the ITRS predicts that such devices will be the cornerstone of sub-32 nm technologies.[2] The primary roadblock to widespread implementation is manufacturability, as both planar and non-planar designs present significant challenges, especially with respect to lithography and patterning. Other complementary strategies for device scaling include channel strain engineering, silicon-on-insulator-based technologies, and high-k/metal gate materials.

Contents

Industry Need

Planar transistors have been the core of integrated circuits for several decades, during which the size of the individual transistors has steadily decreased. At the current pace of scaling, the industry predicts that planar transistors will reach feasible limits of miniaturization by 2010, concurrent with the widespread adoption of 32 nm technologies. At such sizes, planar transistors are expected to suffer from undesirable short channel effects, especially "off-state" leakage current, which increases the idle power required by the device.

In a multigate device, the channel is surrounded by several gates on multiple surfaces, allowing more effective suppression of "off-state" leakage current. Multiple gates also allow enhanced current in the "on" state, also known as drive current. These advantages translate to lower power consumption and enhanced device performance. Nonplanar devices are also more compact than conventional planar transistors, enabling higher transistor density which translates to smaller overall microelectronics.

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Integration Challenges

The primary challenges to integrating nonplanar multigate devices into conventional semiconductor manufacturing processes include:

  • Fabrication of a thin silicon "fin" tens of nanometers wide
  • Fabrication of matched gates on multiple sides of the fin

Varieties

Dozens of multigate transistor variants may be found in the literature. In general, these variants may be differentiated and classified in terms of architecture (planar vs. non-planar design) and number of channels/gates (2, 3, or 4).

Planar Double Gate Transistors

Planar double-gate transistors employ conventional planar (layer by layer) manufacturing processes to create double-gate devices, avoiding more stringent lithography requirements associated with non-planar, vertical transistor structures. In planar double-gate transistors the channel is sandwiched between two independently fabricated gate/gate oxide stacks. The primary challenge in fabricating such structures is achieving satisfactory self-alignment between the upper and lower gates.[3]

Flexfet

Flexfet is a planar, independently-double-gated transistor with a damascene metal top gate MOSFET and an implanted JFET bottom gate that are self-aligned in a gate trench. This device is highly scalable due to its sub-lithographic channel length; non-implanted ultra-shallow source and drain extensions; non-epi raised source and drain regions; and gate-last flow. Flexfet is a true double-gate transistor in that (1) both the top and bottom gates provide transistor operation, and (2) the operation of the gates is coupled such that the top gate operation affects the bottom gate operation and vice versa.[4] Flexfet was developed, and is manufactured, by American Semiconductor, Inc.

FinFETs

A double-gate FinFET device.

The term FinFET was coined by University of California, Berkeley researchers (Profs. Chenming Hu, Tsu-Jae King-Liu and Jeffrey Bokor) to describe a nonplanar, double-gate transistor built on an SOI substrate,[5] based on the earlier DELTA (single-gate) transistor design. [6] The distinguishing characteristic of the FinFET is that the conducting channel is wrapped around a thin silicon "fin", which forms the body of the device. The dimensions of the fin determine the effective channel length of the device.

In current usage the term FinFET has a less precise definition. Among microprocessor manufacturers, AMD, IBM, and Motorola describe their double-gate development efforts as FinFET development whereas Intel avoids using the term to describe their closely related tri-gate[1] architecture. In the technical literature, FinFET is used somewhat generically to describe any fin-based, multigate transistor architecture regardless of number of gates.

A 25-nm transistor operating on just 0.7 Volt was demonstrated in December 2002 by Taiwan Semiconductor Manufacturing Company. The "Omega FinFET" design, named after the similarity between the Greek letter "Omega" and the shape in which the gate wraps around the source/drain structure, has a gate delay of just 0.39 picosecond (ps) for the N-type transistor and 0.88 ps for the P-type.

Tri-gate transistors (Intel)

Schematic view (L) and SEM view (R) of Intel tri-gate transistors

Tri-gate or 3-D are terms used by Intel Corporation to describe their nonplanar transistor architecture planned for use in future microprocessor technologies. These transistors employ a single gate stacked on top of two vertical gates allowing for essentially three times the surface area for electrons to travel. Intel reports that their tri-gate transistors reduce leakage and consume far less power than current transistors.

In the technical literature, the term tri-gate is sometimes used generically to denote any multigate FET with three effective gates or channels.

Gate-all-around (GAA) FETs

Gate-all-around FETs are similar in concept to FinFETs except that the gate material surrounds the channel region on all sides. Depending on design, gate-all-around FETs can have two or four effective gates. Gate-all-around FETs have been successfully built around silicon nanowire.[7]

See also

References

  1. ^ Risch, L. "Pushing CMOS Beyond the Roadmap", Proceedings of ESSCIRC, 2005, p. 63
  2. ^ http://www.itrs.net/Links/2006Update/FinalToPost/04_PIDS2006Update.pdf Table39b
  3. ^ Wong, H-S. Chan, K. Taur, Y. "Self-Aligned (Top and Bottom) Double-Gate MOSFET with a 25 nm Thick Silicon Channel" IEDM 1997, p.427
  4. ^ Wilson, D.; Hayhurst, R.; Oblea, A.; Parke, S.; Hackler, D. "Flexfet: Independently-Double-Gated SOI Transister With Variable Vt and 0.5V Operation Achieving Near Ideal Subthreshold Slope" SOI Conference, 2007 IEEE International
  5. ^ Huang, X. et al. (1999) "Sub 50-nm FinFET: PMOS" International Electron Devices Meeting Technical Digest, p. 67. December 5–8, 1999.
  6. ^ Hisamoto, D. et al. (1991) "Impact of the vertical SOI 'Delta' Structure on Planar Device Technology" IEEE Trans. Electron. Dev. 41 p. 745.
  7. ^ Singh N et al. (2006). "High-Performance fully depleted Silicon Nanowire Gate-All-Around CMOS devices". IEEE Electron Device Letters 27 (5): 383–386. doi:10.1109/LED.2006.873381.  

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