THE CHALLANGE TOWARDS FASTER  MICROPROCESSOR

 

INTRODUCTION

The short history of digital electronic computers has been an axplosive growth in power, speed, capability, miniaturization, and affordability, all seemingly at the same time.  The first digital computers were laboratary curiosities, fabricated from vacuum tubes taken from other pieces of electronic equipment. These give way to solid-state electronic built from resistors and semiconductor transistors. As the applications for digital electronic broadened, it was found that some simple digital circuit were going to be needed over and over again in almost every application. So integrated circuits, or ICs, were created out of useful combinations of resistors, transistors, and capacitors. These black, multilegged objects are found in almost every electronic device made today. Each IC is designed to perform some specific function, and the types available number in the tens of thousands. The single most complex type of IC is the microprocessor.

 

The first IC to be of any used as a microprocessor was a product from Intel Corporation given the part number of 4004 ,  introduced in 1971 and contained 2,300 transistors. The 4004 could not do much in the way of real work ( a modern pocket calculator can outperform it in mathematical functions),  but it had one vital feature: it could be programmed. That is when fed a steady stream of carefully arranged 4-bit words (opcodes), the 4004 would input each one, perform some small operation, and then would wait for the "instruction" in its program. A skillful programmer could, by carefully sequencing the flow of opcodes to the microprocessor, perform some visible work, such as adding two numbers, turning small indicator lights on and off, and controlling electrical relays connected to motors. To this day the basics of microprocessors and microprocessor programming have not changed. However, the number and complexity of the microprocessors themselves have increased dramatically, with no end in sight.

 

THE  MICROPROCESSOR

A microprocessor is an integrated circuit built on a tiny piece of silicon. It contains  thousands, or even millions, of transistors, which are interconnected via superfine traces of aluminum. The transistors work together to store and manipulate data so that the microprocessor can perform a wide variety of useful functions. The particular functions a microprocessor performs are dictated by software. . Today's Itanium processor, by contrast, contains 25.4 - 60  million transistors.  One of the most common tasks microprocessors perform is to serve as the "brains" inside personal computers, but they deliver "intelligence" to countless other devices as well. For example, they may give your telephone speed‑dial and redial options, automatically turn down  your house's  thermostat at night, and make your car safer and more energy efficient.

 

MICROPROCESSOR COMPARISON

A microprocessor often is identified by its model name or model number. Table 1 summarizes the historical development of the microprocessor and documents the increases in clock speed and number of transistors in chips since 1982.

 

Intel is a leading manufacturer of processors. With their earlier microprocessors, Intel used a model number to identify tha various chips. After learning that CPU numbers could not be trademarked and protected from use by competitors, Intel decided to identify their microprocessors with names, not numbers - thus emerged their series of processors known as Pentium processors. A second

 

brand of Intel processor called the Celeron is designed for less expansive Pcs. Recently there are two more brands, called the Xeon and Itanium processors, are geared towards workstations and servers.

 

Table 1 : COMPARISON OF WIDELY USED MICROPROCESSORS

 

 

Other companies such as Cyrix and AMD currently make Intel_compatible microprocessors. These microprocessors have the same internal design or architecture as Intel processors and perform the same functions, but often are less expansives. Intel and Intel-compatible processors are used in Pcs.

 

An alternative to the Intel-style microprocessor is the Motorola microprocessor, which is found is Apple Macintosh and Power Macintosh systems. The processor used in Apple's PowerPC introduced a new architecture that increased the speed of the processor.

 

The Alpha microprocessor which originally was developed by  Digital Equipment Corporation, is used primarily in workstations and high-end servers. Current models of the Alpha chip run at clock speeds from 300 to 700 Mhz.

 

A  new   type of microprocessor, called an integrated CPU, combines functions of a CPU, memory, and graphics card on a single chip. These chips are designed for lower costing personal computers and smaller in sized.

 

Users of Pentium 4 processor‑based PCs can create professional‑quality movies; deliver TV‑like video via the Internet; communicate with real‑time video and voice; render 3D graphics in real time; quickly encode music for MP3 players; and simultaneously run several multimedia applications while connected to the Internet. The processor debuted with 42 million transistors and circuit lines of 0.18 microns. Intel's first microprocessor, the 4004, ran at 108 kilohertz (108,000 hertz), compared to the Pentium 4 processor's initial  speed of 1.5 gigahertz (1.5 billion  Herzt )

 

 

THE CHALLANGE

To build a powerful microprocessor is to fabricate as many transistors as posibble onto as small space of the silicon surface as posibble .Traditionally interconnections between transistors are made using alluminium, the size of which is of practical importance. The smaller the size of aluminium and the shorter the distance between each component, the greter number of transistor could be pack together. Now, however, the lightweight metal is reaching its limits. It simply cannot conduct electricity fast enough for ultraminiature circuits, and its vulnerability to electromigration will become a real problem as feature sizes are reduced still further. The distance between components in today's aluminum chips, such as the Pentium processor, is limited to 0.25 microns. And that limitation is forcing manufacturers to find alternatives to aluminum. A chip can hold only so many transistors, and at a certain point, transistors lose their ability to conduct effectively.

 

Other then superconductor, which only possible under extremely low temperature, copper is the best conductor of electricity known. It has about 40% less resistance than aluminium, allowing microprocessors based on copper to operate some 15% faster than conventional devices. IBM also claims that its method of depositing the copper wires reduces the amount of wiring in a device by around 30%.

 

The semiconductor industry has recognized copper's potential since the mid‑1960s. Copper offers three key advantages over aluminium, the semiconductor industry's conductor of choice for forty years.

 

1.       Copper conducts electricity more effectively than aluminium.

 

2.       Copper wires are also far less vulnerable to electromigration ‑ a problem caused when high electric currents force individual atoms to move through a wire, creating voids that can eventually cause the wire to break.

 

3.       The width of copper wiring can be squeezed down to 0.2 µm and below far more easily than aluminium interconnects.

 

Despite of all the positives charisteristics, copper also has a significant disadvantage that has, until recently, prevented its use in semiconductor devices. Copper rapidly diffuses into silicon,  fundamentally changing the semiconductor's electrical properties and preventing transistors from working as they should. For that reason, copper was considered to be a killer of semiconductor devices.

 

COPPER OVERCOMES MANUFACTURING CHALLENGE

IBM scientists had devised an early use of copper to extend aluminium's capabilities for chip circuitry. They replaced pure aluminium with an alloy of aluminium combined with a very small amount of copper. The alloy proved more resistant to electromigration than pure aluminium, and its chemical bonds were strong enough to prevent copper atoms from diffusing into the surrounding silicon. Effective though it was, the alloy was merely a stay of execution for aluminium. The industry realized that copper wiring would be essential to continue the miniaturization of transistors, so research teams at IBM and elsewhere set out to solve the three key problems facing copper circuitry: depositing the copper, patterning it, and devising diffusion barriers to prevent the metal from migrating into the silicon.

 

After more than 25 years of  research, IBM had devised a way of connecting the transistors in  computer chips with copper wires, rather than the traditional aluminium  interconnects.

 

THE RESEARCH

IBM succeeded by drawing on earlier fundamental research at the company's laboratories. To deposit copper, the scientists turned to electrolytic plating, which had originally been developed for chip packaging. Patterning relied on the "dual‑damascene" process, devised in the early 1980s. In this technique a deposited layer of oxide is etched twice to form an overlapping pattern of wires and "vias" ‑ the small metal plugs that link separate layers of wiring in chips. The metal is then applied and smoothed by chemical‑mechanical polishing. IBM researchers also exploited work from the early 1980s to develop a diffusion barrier that would stop copper from poisoning the silicon. The IBM barrier is based on tantalum, while other companies have developed silicon nitride films that perform the same preventative function. Meanwhile, Applied Materials has recently introduced what it calls the BLOk (for Barrier Low k) dielectric film, fabricated with chemical vapour deposition. This material, the company claims, "provides an alternative to silicon nitride films, enabling chipmakers to reduce the dielectric constant (k) of their overall copper damascene structures to achieve faster, more powerful devices."

 

Copper technology offers chipmakers four specific advantages over the traditional aluminium approach.

 

1.       Copper's higher conductivity simplifies the interconnect routing: the number of interconnect levels can be reduced from twelve to six, removing more than 200 process steps from device manufacture.

 

2.       Chips containing copper require about 30% less power at any given frequency than aluminium‑based chips, translating into significantly higher performance for mobile applications in particular.

 

3.       Copper operates at faster speeds than aluminium: for 0.13 µm technology, the interconnect delay for devices based on copper and low‑k materials is about half that of components made from aluminium and silicon dioxide.

 

4.       The damascene process reduces the overall manufacturing cost of chips by 30% per interconnect level.

 

DIELECTRICS

New dielectrics propel chips into the fast lane. Novel materials offering lower dielectric constants than silicon dioxide are needed for the manufacture of faster computer chips.

 

Peter Sermon, Knut Beekmann and Simon McClatchie survey the challenges involved in choosing the right candidate. As the semiconductor industry enters the new millennium, scientists and engineers continue to search for the ideal dielectric material for future chip fabrication. This goal is driven by the continued demand for chips that offer faster operating speeds and higher transistor packing densities. As metal widths and line spacings drop below 0.2 µm, the main barriers to higher operating speeds will be delays within the metal interconnects and increases in capacitance between adjacent conducting lines. The latter can lead to cross‑talk.

 

The delay problem looks set to be resolved by new interconnect technology that exploits copper instead of standard metal components such as aluminium and tungsten. The capacitance problem will be addressed by reducing the dielectric constant of the inter‑metal dielectric (IMD) that isolates conducting lines from each other. The new materials must have a dielectric constant (k) that is less than 3.9, the value of the standard silicon dioxide dielectrics that are currently in use. This will allow metal lines to be patterned closer together, offering the possibility of faster and more efficient semiconductor devices. However, simply finding a low‑k material is not sufficient. The material must also be integrated into existing chip production lines, which means that potential candidates should possess the same basic properties as silicon dioxide films. These include low moisture absorption and good adhesion to the layers above and below. High thermal stability is also required so that the films do not break down during high‑temperature processing.

 

LITHOGRAPHY

Extreme ultraviolet lithography is one of the most promising methods of creating feature sizes beyond the current limits. Chuck Gwyn of Intel and colleagues from the US Department of Energy review the latest progress in developing the technology needed for the manufacture of production machines.

 

Since the early 1990s the US Semiconductor Industry Association has compiled a roadmap for the semiconductor industry that spells out the technology and equipment requirements for each new generation. It shows that lithography has played an increasingly important role as feature sizes have become smaller. Semiconductor manufacturers have demanded tighter control over the lithography process, which in turn has increased both the complexity and cost of lithography equipment.

 

Conventional optical lithography at wavelengths of 157 nm is expected to support the roadmap into the middle of the next decade, including the fabrication of devices with critical dimensions of around 100 nm. But completely new approaches will be needed for the manufacture of smaller feature sizes ‑ in particular the 70 nm technology generation that is due to be introduced in 2005.

 

One of the most promising candidates is extreme ultraviolet lithography (EUVL). Similar in many ways to conventional optical lithography, this method exploits ultraviolet radiation at wavelengths of 10‑14 nm to achieve higher resolutions and smaller features.

 

ELECTRONS GET READY TO TAKE OVER FROM OPTICS

The high resolution of electron‑beam lithography has been recognized for years, but its progress has been limited by low throughputs.

 

The incredible gains in computing power over the past forty years have largely been enabled by improvements in lithography. This processing technology allows both the size and position of circuit features to be controlled to within just a few nanometres. Achieving such high precision has required advances in all aspects of the technology: the mask, the lithography tool, and the resist and process.

 

One of the most effective  strategies for sustaining the progress of the semiconductor industry has been to shorten the wavelength of the light used to transfer the mask pattern into the resist.

 

But this approach cannot continue indefinitely, partly because feature sizes are reducing faster than new wavelengths can be introduced. Such continued shrinkage has only been made possible by continually improving the performance of lithography tools, which has resulted in optical systems that are comparable in cost and complexity to those used in spy satellites and space telescopes.

 

Resolution enhancement techniques have been introduced to push optical processes to their absolute limits. Even so, however, the smallest feature size that can be printed is still roughly only half the wavelength used. Nor is reducing the wavelength an easy task, since at shorter wavelengths the materials traditionally used in the lithography process start to absorb light.

 

This means that more exotic materials must be introduced, or that the design must be radically changed from refractive to reflective optics.

 

SCALPEL

SCALPEL, for "scattering  with angular limitation projection electron‑beam lithography", is a technology based on electron‑beam lithography that can meet all these requirements.

 

The superior resolution and depth of focus offered by electron‑beam lithography have been apparent for decades. For example, tools based on scanning electron microscopes have been used to pattern small numbers of circuits directly, and to create the masks currently used in optical lithography systems. However, this kind of serial writing can only achieve very low throughputs, and the beam must also trace out the circuit pattern every time a circuit is written.

 

A projection process, in which a mask is used to contain the pattern information, would allow millions of features to be written at the same  time. However, attempts to develop an electron‑beam projection system have failed for two main reasons.

First, conventional stencil masks  absorb energy from the electron beam and generate thermal errors, which limits the electron‑beam energy that can be used.

 

Second,  traditional full‑field optical systems generate significant optical aberrations unless the numerical aperture is extremely small. This in turn leads to so‑called space‑charge effects that destroy the resolution at the beam current needed to provide acceptable throughputs.

 

These major limitations were overcome by Steven Berger and Murray  Gibson at Bell Laboratories in 1989, when they invented the SCALPEL concept. The projection system they devised achieves high throughputs by exploiting a new type of mask and a novel writing strategy.

      

KEY INNOVATIONS

The mask consists of a membrane of material with a low atomic number covered with a layer of high atomic‑number material that contains the  pattern to be printed. The mask is almost completely transparent to  electrons at the energies used (100 keV), but contrast is generated by  exploiting differences in electron scattering. The membrane scatters  electrons weakly and to small angles, while the patterned layer scatters  strongly and to high angles. An aperture in the back focal plane of the  projection optics blocks strongly scattered electrons to form a  high‑contrast image on the wafer . This scheme  means that contrast is generated by the mask, while energy absorption occurs at the aperture. As a result, very little of the incident energy is actually absorbed by the mask, preventing it from being affected by thermal instabilities.

 

The other key innovation was the introduction of a new writing strategy. Rather than illuminating the entire area to be printed ‑ as happens in full‑field optical systems ‑ the 0.25 x 0.25 mm electron  beam is scanned to create a field only 3 mm wide on the wafer surface. The mask and wafer must then be scanned mechanically through the field, forming a series of 3 mm strips that cover the printable area. These strips can be "stitched together" at the wafer to produce a continuous pattern. This so‑called subfield scanning writing strategy ‑ combined with the new mask design ‑ overcomes the limitations that have hindered attempts to create a projection electron‑beam lithography system.

 

Research at Bell Labs has been directed towards developing the three main components of a lithography system: the mask, the resist and process, and the exposure tool.

The fabrication of the masks is in fact relatively simple, since the SCALPEL system exploits a linear  imaging process. Optical systems, in contrast, are based on nonlinear imaging, which means that tiny imperfections in the mask features can be dramatically magnified at the wafer. Not only is this undesirable, but it also results in very tight specifications for the mask and hence a higher cost.

 

The patterned layer on a SCALPEL mask is also two to three times thinner than for an optical mask, which makes it easier to control feature sizes during mask patterning and etching.

 

NANO TUBE

Nanotubes is a rolled up sheets of graphite of only few armstrongs in diameter.  It can also be made into electronic devices like diodes and transistors.  Researchers had shown that nanotubes should indeed be useable for diodes and other electronic components, once the fabrication techniques improve.

 

A recent report from IBM justified that  their scientist have invented a computer circuit base on a single molecul which is expected to initiate the production of smaller and faster chips.  They fabricate logic gates base on micro cylindrical carbon atoms.  Nanotubes can be the best substitute of silicon since the  present microprocessor cannot shrink further.  

 

WHAT IS THE LIMIT  ?

Today copper chips are down to 0.13 microns  and system with speed up to 1.8 Gigaherzt has been in the market for quite some time .  In august 2000, it was reported that Intel cranked a Pentium 4 processor up to  2GHz in a technology demonstration designed to show off the power of its forthcoming desktop PC chip.  The Pentium 4, due in PCs soon, is based on an entirely new chip design  or microarchitecture, that was designed from the ground up for computing on the Internet, Albert Yu, senior vice president in charge of Intel's microprocessor products group, said at the start of the company's developer conference in San Jose California.  With room for achieving higher speeds in the future. An analysts say the new chip should be able to reach speeds of 10 gigahertz or so in five years.

 

What will be the speed limit ? Many people considered the 1 Ghz Pentium III is already far more than satisfactory for their purpose. May be software companies together with manufacturing companies are thinking of creating something to make sure that there will always be a demand for faster microprocessor .

 

 

 

References

 

1.       Turley, James L. Advanced 80386 Programming Techniques: Osborne McGraw-Hill 1988

2.       H.J.Mitchell. 32-Bit microprocessors: John Wiley & Sons(SEA) Pte.Ltd.-Singapore 1986

3.       Shelly,Cashman,Vermaat: Discovering Computers 2001, Course Technology, USA, 2000

4.       http://www.chips.ibm.com/products/powerpc/

5.       http://www.intel.com/intel/product/processors.htm

6.       http://developer.intel.com/design/pentium4/

7.       http://developer.intel.com/design/pentium4/papers/249438.htm

8.       http://www.microprocessor.sscc.ru

 

 

Appendage

 

Table 2 describe s guidelines for selecting an Intel processor. One thing to remember is that, the higher the clock speed, the faster the proessor and also more expansive .

 

Table 2 : Guidlines for processor selection

Intel Processor	Clock Speed	Use
Xeon	800-1000 MHZ	Power users with workstation; servers on a network
Pentium Family	Above 800	Users that design professional graphics and drawings, produce and edit videos, record and edit music, participate in video conference calls, create professional web sites, play graphic-intensive multiplayer Internet games.
	700-800	Users that design professional documents containing graphics such as newsletters or number intensive spreadsheets; produce multimedia presentations; use the internet as intensive research tool ; edit photographs; send documents and graphics via the web; watch video; play graphics-intensive games on CD or DVD; create personal web sites
	Below 700	Home users that manage personal finances, create basic documents with word processing and spreadsheet software  communicate with others on the web via e-mail, chats, and discussion; shop on the web; create basic web pages.
Celeron	400-633	Home users that manage personal finances, create basic documents with word processing and spreadsheet software edit photographs; make greeting cards and calenders; use educational or entertainment CD-ROMs, communicate with others on the web via e-mail, chats, and discussion