Introduction to VLSI Technology

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Introduction to VLSI TechnologyAbbreviation of very large-scale integration, the process of placing thousands (or hundreds of thousands) of electronic components on a single chip. Nearly all modern chips employ VLSI architectures, or ULSI (ultra large scale integration). The line between VLSI and ULSI is vague.

Very-large-scale integration (VLSI) is the process of creating integrated circuits by combining thousands of transistor-based circuits into on a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The first semiconductor chips held one transistor each. Subsequent advances added more and more transistors, and as a consequence more individual functions or systems were integrated over time. The microprocessor is a VLSI device. The first "generation" of computers relied on vacuum tubes. Then came discrete semiconductor devices, followed by integrated circuits. The first Small-Scale Integration (SSI) ICs had small numbers of devices on a single chip diodes, transistors, resistors and capacitors (no inductors though), making it possible to fabricate one or more logic gates on a single device. The fourth generation consisted of Large-Scale Integration (LSI), i.e. systems with at least a thousand logic gates. The natural successor to LSI was VLSI (many tens of thousands of gates on a single chip). Current technology has moved far past this mark and today's microprocessors have many millions of gates and hundreds of millions of individual transistors. As of mid-2004, billion-transistor processors are not yet economically feasible for most uses, but they are achievable in laboratory settings, and they are clearly on the horizon as semiconductor fabrication moves from

the current generation of 90 nanometer (90 nm) processes to the next 65 nm and 45 nm generations. At one time, there was an effort to name and calibrate various levels of large-scale integration above VLSI. Terms like Ultra-large-scale Integration (ULSI) were used. But the huge number of gates and transistors available on common devices has rendered such fine distinctions moot. Terms suggesting more-than-VLSI levels of integration are no longer in widespread use. Even VLSI is now somewhat quaint, given the common assumption that all microprocessors are VLSI or better

Why VLSI? Integration improves the design Lower parasitics = higher speed Lower power consumption Physically smaller Integration reduces manufacturing cost - (almost) no manual assembly

Laboratory Microwind layout and simulation package Dedicated to training in sub-micron CMOS VLSI design Layout editor, electrical circuit extractor and on-line analogue simulator

The beginningMicroprocessors are essential to many of the products we use every day such as TVs, cars, radios, home appliances and of course, computers. Transistors are the main components of microprocessors.

At their most basic level, transistors may seem simple. But their development actually required many years of painstaking research. Before transistors, computers relied on slow, inefficient vacuum tubes and mechanical switches to

process information. In 1958, engineers managed to put two transistors onto a Silicon crystal and create the first integrated circuit, which subsequently led to the first microprocessor.

Major Design Challenges

Microscopic issues o ultra-high speeds o power dissipation and supply rail drop o growing importance of interconnect o noise, crosstalk o reliability, manufacturability o clock distribution Macroscopic issues o time-to-market o design complexity (millions of gates) o high levels of abstractions o design for test o reuse and IP, portability

o systems on a chip (SoC) o tool interoperability

Integrated Circuits Trends Digital logic is implemented using transistors in integrated circuits containing many gates. o small-scale integrated circuits (SSI) contain 10 gates or less o medium-scale integrated circuits (MSI) contain 10-100 gates o large-scale integrated circuits (LSI) contain up to 104 gates o very large-scale integrated circuits (VLSI) contain >104 gates Improvements in manufacturing lead to ever smaller transistors allowing more per chip. o >107 gates/chip now possible; doubles every 18 months or so

Variety of logic families

o o o o

TTL - transistor-transistor logic CMOS - complementary metal-oxide semiconductor ECL - emitter-coupled logic GaAs - gallium arsenide

What are shown on previous diagrams cover only the so called front-end processing - fabrication steps that go towards forming the devices and inter-connections between these devices to produce the functioning IC's. end result are wafers each containing a regular array of the same IC chip or die. The wafer then has to be tested and the chips diced up and the good chips mounted and wire-bonded in different types of IC package and tested again before being shipped out. An entire circuit is manufactured in a single piece of silicon, first appeared around 1960 At that time only a few simple gates offering primitive logic functions such as not, nand, nor etc. could be accommodated (SSI) By 1970 MSI circuits with about a thousand transistors appeared By 1980 LSI circuits of approximately one hundred thousand devices were possible Moores Law Gordon Moore cofounder of Intel Corporation visualized in the 1970s that chip building technology would improve very quickly He projected that the number of transistors on a chip would double about every 18 months

Choice of Technology Two distinct types of technology are fabricated in silicon based upon o BJT (Bipolar Junction Transistor) o MOS (Metallic Oxide Semiconductor) Since processing of these technologies is very different, it is impractical to mix them up within a chip MOS logic occupies much smaller area of silicon than the equivalent BJT logic MOS technology has a much higher potential packing density A MOS logic circuit requires appreciably less current and hence less power than its bipolar counter part However, bipolar circuits operate faster than MOS circuits Even so, the speed-power product for MOS logic compares favorably with that of BJT logic The structure of an MOS transistor is much simpler than that of bipolar devices and this makes manufacturing process easier This in turn should result in fewer faults occurring in fabrication (high yield) Dynamic logic circuits cannot be implemented in bipolar technology Thus in terms of area, power dissipated, yield and flexibility MOS technology is superior to BJT

Moores Law Gordon Moore: co-founder of Intel Predicted that the number of transistors per chip would grow exponentially (double every 18 months) Exponential improvement in technology is a natural trend: The Cost of Fabrication

Current cost $2 - 3 billion Typical fab line occupies 1 city block, employees a few hundred employees Most profitable period is first 18 months to 2 years For large volume ICs packaging and testing is largest cost For low volume ICs, design costs may swamp manufacturing costs

Relative sizes of ICs in graph

Limits of Moores Law? Growth expected until 30 nm gate length (currently: 180 nm) o size halved every 18 mos. - reached in o 2001 + 1.5 log2((180/30)2) = 2009 o what then? Paradigm shift needed in fabrication process

Switches We can view MOS transistors as electrically controlled switches Voltage at gate controls path from source to draing=0 d nMOS g s s d ON s s s d OFF s d OFF g=1 d ON

d pMOS g

Digital equipment is largely composed of switches Switches can be built from many technologies relays (from which the earliest computers were built) thermionic valves transistors The perfect digital switch would have the following: switch instantly use no power have an infinite resistance when off and zero resistance when on Real switches are not like this!

Semiconductors and Doping Adding trace amounts of certain materials to semiconductors alters the crystal structure and can change their electrical properties in particular it can change the number of free electrons or holes N-Type semiconductor has free electrons dopant is (typically) phosphorus, arsenic, antimony

P-Type semiconductor has free holes dopant is (typically) boron, indium, gallium Dopants are usually implanted into the semiconductor using Implant Technology, followed by thermal process to diffuse the dopants IC Technology Speed / Power performance of available technologies The microelectronics evolution SIA Roadmap Semiconductor Manufacturers 2001 Ranking

Metal-oxide-semiconductor (MOS) and related VLSI technology pMOSSource Polysilicon SiO2 Gate Drain


p+ n bulk Si

nMOSSource Gate

Drain Polysilicon SiO2


n+ p bulk Si


Basic MOS Transistors Minimum line width Transistor cross section Charge inversion channel

Source connected to substrate Enhancement vs Depletion mode devices pMOS are 2.5 time slower than nMOS due to electron and hole mobilities

What is a Silicon Chip? A pattern of interconnected switches and gates on the surface of a crystal of semiconductor (typically Si) These switches and gates are made of areas of n-type silicon areas of p-type silicon areas of insulator lines of conductor (interconnects) joining areas together Aluminium, Copper, Titanium, Molybdenum, polysilicon, tungsten The geometryof these areas is known as the layout of the chip Connections from the chip to the outside world are made around the edge of the chip to facilitate connections to other devices

Fabrication Technology Silicon of extremely high purity chemically purified then grown into large crystals Wafers crystals are sliced into wafers

wafer diameter