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Advances in Microelectronics—From Microscale to Nanoscale Devices

Jan Van der Spiegel

Department of Electrical and Systems Engineering

University of Pennsylvania

Philadelphia, PA

9.1. INTRODUCTION

Microelectronic devices have evolved rapidly in terms of size, cost and performance. Scaling of device dimensions has been the engine of the semiconductor industry1 allowing manufacturers to produce consecutive generations of integrated circuits of ever decreasing dimensions and increasing transistor densities. This trend has resulted in feature sizes with nanometer dimensions. Current physical gate lengths of transistors used in high performance integrated circuits are around 50 nm and will go down to 18 and 9 nm by 2010 and 2016, respectively, according to projections made in the 2003 International Technology Roadmap of Semiconductors (ITRS).2 Prototype transistors with gate lengths as small as 15 nm have already been fabricated in research labs around the world.3,4 Clearly, the microelectronics industry has entered the nanotechnology era and is manufacturing millions of nanoscale transistors on a phenomenal scale.

The semiconductor industry has produced one of the most sophisticated manufacturing processes known to mankind. The key productivity drivers have been the scaling of the transistors, the device switching speed, and the reduced cost per function. To get an appreciation of the magnitude it is instructive to mention that the number of transistors produced in 2002 in DRAMs alone exceeds the number of grains of rice produced yearly. In addition, for each grain of rice one can buy 100’s of transistors.5

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Microlectronic devices will play a key role in the future of nanoelectronics. Traditional microelectronic devices (CMOS transistors) are entering the nanoscale regime and are giving rise to extremely inexpensive but extraordinary powerful circuits and systems. Any new (nano) technology will have to reckon with this powerful force in order to become a viable alternative. Also, CMOS circuits and microelectronic technology will be used as a substrate on which to build future nanoelectronic structures. It is not unthinkable that a hybrid between microelectronic devices/technology and nanotechnology will give rise to powerful structures and systems. Certain novel nano devices, such as carbon nanotube transistors for instance, have the same structure as traditional CMOS transistors. Understanding the operation and the limitations of CMOS transistors is important to understand these new device structures.

The goal of this chapter is to review the basics of microelectronic devices (CMOS). We will discuss the structure and operation of CMOS transistors, the concept of scaling and its limitations. The challenges associated with nanoscale CMOS transistors will be reviewed. Finally, we will look at non-classical nanoscale CMOS devices and structures.

9.2. BRIEF HISTORY OF MICROLECTRONIC DEVICES AND TECHNOLOGY

The invention of the bipolar transistor in 1947 by J. Bardeen, W. Brattain, and W. Shockley at Bell Labs, was one of the milestones that made the microelectronics revolution possible. Although the first transistor was made of germanium material, silicon quickly became the material of choice since it could be easily grown as single crystal material. In addition, silicon has a high quality silicon oxide layer that can be used for insulating layers (e.g., gate oxide) and for passivating and masking purposes, which are key steps in the fabrication of a today’s integrated circuit.

Another breakthrough was the introduction of the planar process at Fairchild Semiconductor. This process still forms the basis of the fabrication of current-day integrated circuits. It makes use of the masking properties of to define a region through which impurities can penetrate during the gas phase diffusion, as illustrated in Figure 9.1. Silicon was also superior over germanium in this respect due to its ability to form a stable oxide layer.

The next major invention occurred in 1959 when J. Kilby of Texas Instruments and R. Noyce of Fairchild Semiconductor introduced, independently, the concept of the

integrated circuit. This allowed the fabrication of multiple devices and their interconnection on the same wafer. Subsequent improvements in materials, devices and the planar process have allowed the mass-fabrication of millions of transistors on a single chip. Figure 9.2 summarizes the evolution of the integrated circuit technology since the invention of the transistor.

Another milestone was reached after the successful fabrication of Metal Oxide Semiconductor (MOS) field effect transistors. The idea of the MOS transistor dates back to 1927 when Lilienfeld patented the field effect transistor, but it was not until the early 60’s that one was able to overcome the technical difficulties associated with the fabrication of the MOS transistor.6 One of the main challenges was the quality of the silicon-oxide interface and associated interface states and oxide charges. The advantage of the MOS transistor over a bipolar transistor is its simpler device structure, fewer fabrication steps, the absence of a DC input current and the suitability for mixed-mode circuits.

The first MOS transistors were p-channel devices (PMOS) in which the current consists of positive charge carriers. The introduction of the NMOS transistor was another step forward since electrons in a NMOS transistor move faster than the holes in a PMOS transistor. The combination of both NMOS and PMOS transistors on one substrate gave rise to the Complementary MOS or CMOS. The main advantage of CMOS over NMOS is that CMOS gates do not consume standby power, except for a small leakage current (which can become significant in sub-micron and nanoscale transistors). Advanced CMOS provides high performance (speed) at relatively low power, which has made it the dominant technology of choice for the fabrication of large-scale integrated circuits since the early 80’s. Bipolar transistors are mainly used for high-speed, low-noise analog and microwave applications. The integration density of bipolar ICs is considerably lower than that of CMOS ICs.

Many subsequent improvements in the fabrication have allowed the realization of ultrasmall feature size transistor. Shrinking transistor dimensions (scaling) combined with larger

chip sizes has given rise to ever more complicated integrated circuits. Over the last 40 years, the transistor count per chip has doubled about every 18 months, as is illustrated in Figure 9.3. This trend was originally observed by G. Moore of Intel Corp and has been called Moore’s law.7,8 This has brought us from the SSI (small scale integration), to MSI (medium scale), to VLSI (very large scale), to ULSI (ultra large scale) integrated circuits. Current integrated circuits contain up to tens of millions of transistors, giving rise to sophisticated systems on a chip.

A different way to look at Moore’s law is to plot the minimum feature size being used to fabricate integrated circuits. Figure 9.4 illustrates the trend over the last 40 years.

Dimensions have been shrinking by 12–14% per year. As can be seen, continued scaling at this pace will eventually lead to devices on a molecular and atomic scale that operate at very different principles than those we are using today.

The reduced dimensions, increased chip and wafer size, and improvement in process technology have allowed the cost per function to go down by 25–32% per year. Figure 9.5 illustrates this trend for the average selling price per bit of DRAM memory which has been decreasing at a cumulative annual rate of 32%. This remarkable trend has been an important driving force behind the semiconductor industry.

Figure 9.6 shows schematically the breakdown of the factors contributing to the cost reduction. About 12–14% of the cost reduction has become possible due to shrinking of minimum feature size, 4% is due to increased wafer size, 2% is the result of yield improvement, and another 7–19% is due to other innovations in technology and design efficiency. Feature size reduction accounts for almost 50% of the cost reduction.

At the same time the cost per function has been decreasing, the performance of devices has increased exponentially. This is illustrated by the clock frequency of microprocessors that have been increasing at an average rate of 29% per year, shown in Fig. 7.

9.3.BASICS OF SEMICONDUCTORS

9.3.1.Semiconductor Model and Energy Band Structure

A majority of microelectronic devices and circuits are fabricated with silicon (Si). Devices for specialized applications are sometimes built with germanium (Ge), gallium arsenide (GaAs) and other III–V or II–VI compounds. As can be seen from the periodic table in Figure 9.8, these atoms belong either to column IV (elemental semiconductors of Si, and Ge), or columns III and IV (compound semiconductors of Ga and As). The atoms in column IV have the common property that they have 4 electronics in their outer shell, while the atoms in columns III and V have 3 and 5 electrons, respectively. The electrons in the outer orbit are the valence electrons and determine to a large extent the chemical and electrical properties of the material.

We will use silicon as an example to discuss the properties of semiconductors. Silicon has a total of 14 electrons, as illustrated schematically in Figure 9.9.10,11,12 These electrons

circle around the atom nucleus. The closer the electrons are to the nucleus of the atom the larger the attractive force will be and the tighter they are bound to the nucleus. This

can be conveniently expressed by the amount of energy of each electron. In contrast to the macroscopic word, only certain energy levels are allowed. It was Niels Bohr who predicted in 1913 that the electrons are restricted to certain orbits or that the energy of the electrons is quantized. This is the result of the quantum mechanical nature encountered in systems of atomic scale.

We notice that in Figure 9.9 only two electrons occupy the same energy levels. This is the result of Pauli’s exclusion principle which states that each energy level can accommodate only two electrons, corresponding to two energy states. The outer shell (valence shell) has 4 energy levels of which the lowest ones are occupied by two valence electrons each. A similar picture is valid for the other atoms in column IV of the periodic table since all these atoms have 4 valence electrons. The energy quantization and Pauli’s exclusion principle have important consequences for the electronic structure of materials as will become clear shortly.

Let us now consider a silicon crystal that consists of many silicon atoms. Silicon material can be in the form a single crystal, poly-crystalline or amorphous. For the fabrication of semiconductor devices, single crystal, high purity semiconductors are used in which each atom occupies a precise location. Silicon crystal has a diamond lattice in which each silicon atom has four neighbors as shown in Figure 9.10. In this configuration each silicon atom shares a valence electron with its four neighbors. As a result of the electron sharing, the eight allowed energy states in the outer shell of the silicon atom are filled with electrons. The neighboring atoms form covalent bonds through the sharing of its valence electrons which gives rise to a stable structure. By repeating the structure of Figure 9.10a, one can build up the silicon crystal. This is schematically illustrated by the two-dimensional bond model of Figure 9.10b.

As a result of the close proximity of silicon atoms, the energy states of the atoms in a crystal will change slightly from those of a single atom. Let us assume that there are N silicon atoms, which will give rise to 4N valence electrons, since each atom in the crystal contributes four valence electrons. According to Pauli’s exclusion principle, only two electrons can occupy the same energy level. In order to accommodate the 4N electrons, the energy levels of the single atom have to change slightly so that there will be a total of 4N energy levels spread out over two energy bands, illustrated in Figure 9.11. The bottom energy band contains 2N closely spaced energy levels and the top band contains another 2N energy levels. Since there are 4N valence electrons, the bottom energy band will be able to accommodate all the electrons. As a result, at a temperature of 0 K, the lower band will be completely filled and the top one will be empty. We call the lower energy band the valence band and the upper one the conduction band. The energy difference between the two bands is called the band gap The band gap in silicon is 1.12 eV at room temperature.

9.3.2. Charge Carriers in Semiconductors

The model of Figure 9.10b indicates that all the valence electrons are used to form the covalent bonds between neighboring atoms and as a result, there are no free electrons

available that can give rise to conduction. This is confirmed by the energy band diagram of Figure 9.11b in which the valence band is completely filled with electrons, leaving no room for the electrons to move around. One can compare the situation of a filled valence band to a bottle completely filled with grains of salt. It is going to be very hard to move the grains around, even after shaking it, since there is no space for the grains to move. For the electrons to move there should be some empty energy levels to which they can jump. The above situation is true only at zero absolute temperature. However, at room temperature some electrons will have sufficient thermal energy to break away from the silicon atom. This occurs when the electron jumps across the band gap, as illustrated in Figure 9.12. Once these electrons are in the conduction band, they encounter plenty of empty energy states which make it easy to move around. The higher the temperature the more electrons will have crossed the band gap. We call these mobile electrons since they give rise to conduction in the semiconductor. What is interesting is that for each electron that jumps from the valence band to the conduction band, there will be an empty state created in the valence band. Thus, the electrons in the valence band will be able to move around as well. Since

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the conduction in the valence band is the result of the removal of an electron, which leaves a positive charge behind, we call these carriers conveniently holes. Thus, the conduction in a pure semiconductor is the result of both electrons and holes. Since each electron that jumps across the band gap creates a hole, the concentration of the mobile electrons, n, and holes, called p, is equal. We call this concentration the intrinsic carriers concentration

It is clear that the width of the band gap is an important characteristic of a semiconductor and of materials in general. For instance, it allows us to explain the difference between insulators, conductors and semiconductors. An insulator is a material whose valence band is completely filled and whose energy gap is so large that no electrons can jump across it. An example is whose large energy gap of 8–9 eV makes it an excellent insulator. On the other hand, a conductor is a material whose valence band is only partially filled so the electrons can easily jump to the empty states in the valence band and move around freely. A semiconductor acts as an insulator at zero temperature and becomes a poor conductor at higher temperature since the energy gap is small enough for some electrons to free themselves by jumping across the energy gap at room temperature.

For silicon, the fraction of valence electrons that jump across the energy gap is less than

temperature (T = 300 K). Since the thermal energy allows the electrons to jump across the

in which T is the absolute temperature and k the Boltzmann constant. For pure semiconductors one can write that

The last expression will be proven later on.

9.3.3. Intrinsic and Extrinsic Semiconductors

The semiconductor material we discussed in the previous section is called intrinsic since it consists of pure semiconductor material without added dopants. One of the key properties of semiconductors is that one can modify its characteristics by adding dopants or impurities. Adding relatively tiny amounts of dopants has a very strong effect on the electrical properties of semiconductors. These impurities have one more or one fewer electron than silicon in their outermost shell. Looking at the periodic table in Figure 9.8 one notices that phosphorous P sits next to silicon and is located in column V, indicating that it has five valence electrons. One can now dope a silicon crystal with phosphorous atoms using a process called ion implantation or diffusion. Since P has a similar size as silicon, it is relatively easy to replace some of the silicon atoms with phosphorous ones, as shown in Figure 9.13.

Since only four of the five valence electrons are needed for the valence bonds to fill the outer shell around the silicon atom, the fifth electron supplied by the phosphorous atom is very weakly bound to its atom core. Actually, at room temperature it has enough energy