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Transistor

MOSFET: Toward the Scaling Limit
A Brief History of MOSFET
Transistor Scaling
MOSFET at the 130nm and 90nm Technology Nodes
Recent Trends in MOSFET Innovation

MOSFET: Toward the Scaling Limit

MOS transistor is the building block of integrated circuits, and is the engine that powers them. Today’s most complex ICs, such as microprocessors, graphics, and DSP chips, pack more than 100 million MOS transistors on a single chip. Integration of one billion transistors into a single chip will become a reality before 2010. This ultra-high level of transistor integration is the result of relentless transistor scaling for the past 40 years.

The most advanced CMOS technology currently in volume production is the 130nm technology. At the 130nm node, the gate length of the high-performance MOSFET is around 60nm. For the upcoming 90nm technology, the gate length will be around 45nm. Beyond that, only a few more transistor scaling may be feasible for the conventional MOSFET as we know it today.

Further scaling may require a different MOSFET architecture, such as non-planar MOSFET built on SOI. However, the current planar MOSFET structure may show resilience with breakthroughs in such critical areas as high-k dielectric and gate electrode engineering, enabling scaling of the conventional MOSFET below a 20nm gate length.

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A Brief History of MOSFET

For a brief history of transistors, visit
http://www.nobel.se/physics/educational/transistor/history/index.html

For a basic understanding of how transistors function, visit
http://www.nobel.se/physics/educational/transistor/function/index.html

The operating principle of the MOSFET transistor was first described in Lilienfield’s historical patent issued in 1926. It took another 34 years before Kahng and Atalla successfully built a working MOSFET in 1960.

For the past 40 years, the semiconductor industry and academia have relentlessly pushed transistor scaling. Along with scaling, the MOSFET transistor evolved from the P-ch MOSFET in the 1960’s to the N-ch MOSFET in the 1970’s. A good understanding of gate oxide quality, such as interface traps, fixed and mobile charges, and a good control of gate oxide quality in a manufacturing environment enabled industry to make the transition from PMOS technology to a higher-performing NMOS technology in 1970’s.

Another important development in the evolution of the MOSFET is the replacement of metal gate with poly-silicon gate. Early MOSFET used aluminum as a gate electrode, hence the name MOSFET. While a metal gate is ideal from a purely transistor-architecture standpoint, it puts a great deal of constraints on process integration. For example, the thermal budget and self-alignment capability of source/drain to the gate had to be compromised with the metal gate. Use of heavily doped poly-silicon as a gate material opened a whole new vista and allowed tremendous improvement in scalability of MOS transistors and technology.

The idea and ability to integrate both P-ch and N-ch MOSFET on a single chip led to the birth of CMOS technology in the mid 1970’s. Since the 1980’s, CMOS technology has become a de facto mainstream IC manufacturing technology. N-ch and P-ch MOS transistors, when connected in CMOS configuration, draw very little DC current during steady state. This low-power characteristic of CMOS makes it possible to integrate over a hundred million transistors on a single chip, as we see in today’s advanced IC chips.

CMOS technology itself has gone through a few important technological innovations. Initially developed as a single-well process, with one of the two MOSFET types built in a well, and the other on silicon substrate, it evolved into a twin-well process where each type of MOSFET sits on its own optimized well. After that, triple-well CMOS technology was developed for certain applications.

Another breakthrough in CMOS technology came with the development of dual-gate CMOS technology. Early on, n+ doped poly-silicon was used as a gate material for both N-ch and P-ch MOSFET. This made P-ch MOSFET operate in the buried-channel mode, while N-ch MOSFET operated in the surface-channel mode. Suppression of short-channel effects such as punch-through is more difficult in buried-channel transistors, leading to a scalability problem in P-ch MOSFET. With the use of implanted poly in a self-aligned CMOS process, dual-gate CMOS was realized that had n+ doped poly-Si gate for N-ch MOSFET, and p+ doped poly-Si gate for P-ch MOSFET. This enabled both N-ch and P-ch MOSFET to operate in surface-channel mode.

In today’s advanced CMOS technology, dual-gate CMOS is the standard feature, along with twin-well or triple-well architecture.

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Transistor Scaling

Up until now, MOSFET scaling has proceeded based on the scaling theory without serious roadblocks. MOS transistors with a gate length as short as 10nm, although experimental, have been demonstrated.

The scaling theory, based on a constant E-field, requires supply voltage, threshold voltage, gate length, and gate oxide thickness to be scaled down by a scaling factor. The doping level in the channel must be scaled up by the same scale factor. The junction depth of source and drain also needs to be scaled down to suppress the short-channel effect.

Another important aspect of transistor scaling is the scaling of parasitic resistances and capacitances. These parasitic components do not necessarily scale with transistor scaling. Therefore, it becomes increasingly critical to minimize parasitic components in order to get the best return-on-scaling on transistor performance. A good example to address this issue is the silicidation of drain, source, and gate. Cobalt silicide used in advanced CMOS technology dramatically reduces parasitic resistances in the device.

Transistor scaling, in practice, has not followed exactly the constant E-field scenario. For performance reasons and due to product requirements, scaling of supply voltage did not happen as fast as geometrical scaling, such as gate length and gate oxide thickness. Because of this, the electric field in the device increased with scaling, resulting in aggravation of short-channel effect (SCE). In addition, it also increased reliability concerns such as hot carrier effect (HCE) and gate oxide reliability. Various transistor design techniques have been proposed and investigated to deal with SCE and HCE.

One of the most important developments in transistor design to deal with SCE and HCE is the use of lightly doped drain (LDD) in conjunction with poly-Si gate sidewall spacer. This technique, introduced in the industry at late 1970’s, has become a standard feature for sub-micron transistors, typically having gate lengths of 0.50um and below. Various other ideas have been proposed and adopted in transistor design. These include retrograde channel doping, super-steep retrograde channel, halo or pocket implant with a large tilt angle, pre-amorphization implant (PAI), notched gate, L-shaped spacer, offset spacers, MDD (medium doped drain) and source/drain extension.

A good overview of MOSFET scaling and issues on transistor scaling below 50nm gate length can be found in “CMOS Design Near the Limit of Scaling,” by Y. Taur, pp. 213-221, March/April 2002, IBM Journal of Research and Development.
In today’s advanced CMOS technology, MOS transistors are typically implemented in a dual gate CMOS configuration: n+ poly gate for NMOS, and p+ poly gate for PMOS. Dual-gate CMOS allows both N-ch and P-ch transistors to operate in a surface-channel mode. However, it presents process integration issues such as boron penetration in the p+ poly gate, which causes device instability and gate oxide reliability problems in the P-ch MOSFET. Reduction of the thermal budget to minimize boron penetration can cause insufficient dopant activation in the gate poly, leading to poly depletion problems. These are some of the challenges in further scaling of MOSFET below a 100nm gate length.

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MOSFET at the 130nm and 90nm Technology Nodes

The most advanced MOS transistors used in volume production today are probably those of Intel’s used in their 130nm logic technology. For generations, Intel has displayed its masterful skills of transistor design, placing its transistors at the top of the competition for transistor performance.

According to an Intel paper delivered at IEDM 2001, transistor gate length and gate oxide thickness at the 130nm node is 60nm and 1.5nm, respectively. Transistor drive current is 1300uA/um for N-ch and 660uA/um for P-ch at Vdd=1.4V with off-state leakage current of 100nA/um. These are the best performance reported in the industry at the 130nm node.

For 90nm technology, Intel presented a paper at IEDM in 2002 for its initial version of 90nm technology, followed by a production-ready 90nm process presented at IEDM 2003. Intel’s production version 90nm technology is quite different from the previous version a year ago, in terms of integrating strained silicon into the CMOS process.

In a paper delivered at IEDM 2002, Intel described implementation of strained-silicon channel MOSFET in its 90nm bulk CMOS technology. Intel claims a transistor performance gain of 10%-20% for both N-ch and P-ch MOSFET by using strained-silicon channel epitaxially grown on relaxed SiGe layer. With transistor gate length and gate oxide thickness of 50nm and 1.2nm, transistor drive current of 1260uA/um for N-ch and 630uA/um for P-ch is achieved at Vdd=1.2V with off-state leakage current of 40nA/um. Judging by the published data, Intel’s MOSFET DC performance is indeed the best in the industry at the 90nm node.

Intel’s 2002 result seems quite remarkable in the sense that it demonstrated for the first time integration of strained-silicon MOSFET into CMOS technology with performance enhancement for both N-ch and P-ch. However, Intel did not disclose the ring oscillator speed of its 90nm technology, suggesting that the N-ch and P-ch transistor drive currents reported in IEDM 2002 were probably obtained from two different processes, each optimized for N-ch and P-ch separately. So, the strained-silicon channel MOSFET with a relaxed SiGe layer did not seem ready for full CMOS integration at Intel in 2002.

At IEDM 2003, Intel presented a manufacturing-ready 90nm technology, in which it described a completely different approach to implementing strained silicon in CMOS. In effect, Intel abandoned the strained-silicon channels formed over a relaxed SiGe layer, the method it had used in the 2002 version 90nm technology, illustrating the difficulty in integrating this approach into the CMOS process.

In the manufacturing-ready 90nm technology, Intel took a much less aggressive approach in implementing strained silicon. For PMOS, source and drain regions are etched, and then embedded with compressively strained SiGe film, which exerts a uniaxial compressive strain to the channel. The resulting P-ch drive current with a gate length of 50nm is an astounding 700uA/um, ~10% higher than the already impressive 2002 value. This P-ch drive current is a record high at 90nm node. The N-ch transistor does not incorporate SiGe layer at all. Instead, tensile stress in the channel comes from a highly tensile silicon-nitride capping layer deposited over the silicide. The resulting N-ch drive current is the same as the 2002 value at 1260uA/um, but Intel had to scale down the N-ch gate length to 45nm to maintain the same drive current as in 2002.

For more on MOSFET at 130nm and 90nm node, visit Logic section in the Technology area.

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Recent Trends in MOSFET Innovation

Up until now, MOSFET scaling has progressed without running into a serious roadblock. Engineering ingenuity always overcame challenges at every generation of scaling, improving density and performance along the way. Nevertheless, complex and performance-hungry IC products demand the highest transistor performance at each generation, making chip manufacturers scramble to find the best solution possible. In addition, as we approach the physical limits of MOSFET scaling, research on other alternatives to conventional device and process is accelerating. Some of the topics under intense investigation are SOI, strained silicon, high-k gate dielectric and gate stack engineering, and non-planar MOSFET.

Bulk vs. SOI
Bulk CMOS technology has been the mainstream IC manufacturing technology since the early 1980’s. In recent years, however, several major companies, such as IBM, Motorola, and AMD, committed to SOI technology for their high-performance logic products. IBM, the industry leader in SOI technology, started a serious SOI development effort in the late 1980’s, and has been shipping products on SOI since 1998. IBM claims a SOI performance benefit of 25%-30% at the chip level, when compared with comparable bulk technology.

The SOI push by IBM got a boost from the 65nm SOI technology development alliance with IBM and AMD, announced in early 2003. AMD was the first company to use SOI technology for products selling into the multi billion-dollar merchant market. So its impact on the proliferation of SOI technology could be significant. However, how quickly SOI will appeal to a broader spectrum of IC manufacturers remains to be seen.

For more on bulk versus SOI, go to the SOI section in the Technology area.

Strained-Silicon Transistor
Because of its potential to make a dramatic improvement in MOSFET performance, strained-silicon transistor has been a hot topic in recent years, and is now beginning to make inroads into manufacturing technology. Reflecting this trend, a whole session was dedicated for the first time to strained-silicon MOSFET papers at IEDM 2003.

While the underlying physics of strained silicon and its effect on carrier mobility is well understood, incorporating it in a manufacturing process and delivering the promised performance for both NMOS and PMOS in CMOS technology is not an easy task. As mentioned previously, Intel’s original attempt to integrate strained silicon in 2002 took a major detour in 2003, and it settled with a much less aggressive method of implementing strained silicon in its volume-manufacturing 90nm technology.

Many companies have been investigating different ways to create strained-channel MOSFETs, and reported performance gains to a different degree for NMOS or for PMOS or for both. IBM, the leader in strained-silicon research, for years has demonstrated performance benefit of strained silicon in various publications and conferences. In a recent paper presented at IEDM 2003, IBM described a process that integrates strained silicon into fully depleted ultra-thin body SOI MOSFET without an SiGe layer in the SOI film. Dubbed by IBM “strained silicon directly on insulator (SSDOI),” this process involves a layer transfer of strained silicon grown on a relaxed SiGe layer to a handle wafer by wafer bonding after the strained silicon is capped with SiO2. After separation of the host and handle wafers, the SiGe film is selectively removed by polishing and etching, leaving ultra-thin strained silicon over the buried oxide. With this technique, high field mobility, extracted from long-channel transistors, showed improvement of 125% for electrons and 21% for holes. These improvements are comparable to those IBM achieved on bulk MOSFET with strained silicon.

Whether with bulk or SOI MOSFET, the full potential of strained silicon has yet to be realized in a fully integrated CMOS process that offers performance enhancement for short-channel transistors.

High-k Dielectrics and Gate Stack Engineering
Conventional silicon dioxide has already reached its scaling limit because of the high leakage current for the thickness required for today’s deep sub-100nm gate transistors. Although heavily nitrided SiO2 improves gate leakage current, and thus extends the SiO2 lifetime possibly down to the 65nm generation, it also has a downside of mobility degradation. A breakthrough in gate dielectric is necessary to enable 45nm technology. A great deal of work has been done in the past and is currently being undertaken to develop manufacturable high-k gate dielectrics. However, it is uncertain at this time if high-k gate dielectric and the associated gate stack process will be ready in time for the 45nm generation. This is a huge task, and it poses the biggest challenge to transistor scaling that the semiconductor industry has ever faced.

High-k dielectric material, such as HfO2, ZrO2, and Al2O3, received attention as promising material and results were demonstrated with these materials, HfO2 in particular. However, there are still issues that need to be resolved before high-k material can be integrated into manufacturing technology: mobility degradation, interface traps, fixed charge, thermal stability, interfacial layer property, gate leakage current, dielectric reliability, interaction with gate electrodes, high-k film deposition and etch, surface preparation, and thermal processing.

Gate electrode is also an important part of gate stack engineering, whether with high-k dielectric or with SiO2. For ultra-short channel transistors, traditional problems associated with poly gate such as poly depletion and boron penetration become critical issues for transistor performance and scaling. To alleviate these problems, metal gate electrode is being actively researched. One method, proposed by IBM, AMD, and STMicro, uses full silicidation of gate poly using nickel or cobalt, which can be easily integrated into a conventional CMOS process. In another approach called replacement metal gate, the transistor is formed using poly-Si gate, and then the poly-Si is removed and replaced by a metal electrode. This method maintains the self-alignment capability without having to etch the metal gate, and avoids high temperature processing after the metal gate is formed. It is as if history repeats itself, as we see the MOSFET gate electrode, originally made of aluminum, go to a design of poly-silicon, and then return to metal again, although a different metal instead of aluminum this time.

Non-planar Transistors
The non-planar CMOS transistor has also been actively pursued recently as a means of easing scalability and improving MOSFET performance. These transistors are typically built on an SOI substrate and take various different forms and names. Some examples include FinFET, originally developed by a group at the University of California at Berkeley, double-gated SOI, tri-gated SOI, depleted-substrate transistor (DST), and Omega FET –a variant of FinFET investigated by TSMC. At present, most of the non-planar transistor work is at the device level, and is at an experimental stage. Many process-integration issues must be resolved before this technology can be used in a manufacturing process.

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Monthly Forum
April 2005

Flash Memory Technology:

Promises and Challenges

May 2005  

Managing Stress in the sub-100nm Technology Era

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