1.5 Technologies for Building processors and Memory
Processors and memory have improved at an incredible rate, because computer designers have long embraced the lastest in electronic technology to try to win the race to design a better computer. Figure 1.10 shows the technologies that have been used over time, with an estimate of the relative performance per unit cost for each technology. Since this technology shapes what computers will be able to do and how quickly they will evolve, we believe all computer professionals should be familiar with the basics of integrated circuits.
A transistor is simply an On/Off switch controlled by electricity. The integrated circuit (IC) combined dozens to hundreds of transistors into a single chip. When Gordon Moore predicted the continuous doubling of resources, he was predicting the growth rate of the number of transistors per chip. To describe the tremendous increase in the number of transistors from hundreds to millions, the adjective very large scale is added to the term, creating the abbreviation VLSI, for very large-scale integrated circuit.
This rate of increasing integration has been remarkably stable. Figure 1.11 shows the growth in DRAM capacity since 1977. For decades, the industry has consistently quadrupled capacity every 3years, resulting in an increase in excess of 16,000 times!
To understand how manufature integrated circuits, we start at the beginning. The manufacture of a chip begins with silicon, a substance found in sand. Because silicon does not conduct electricity well, it is called a semiconductor. With a special chemical process, it is possible to add materials to silicon that allow tiny areas to transform into one of three devices:
- Excellent conductors of electricity (using either microscopic copper or aluminum wire)
- Excellent insulators from electricity (like plastic sheathing or glass)
- Areas that can conduct or insulate under special conditions (as a switch)
Transistors fall in the last category. A VLSI circuit, then, is just billions of combinations of conductors, insulators, and switches manufactured in a single small package.
The manufacturing process for integrated cirtical to the cost of the ships and hence important to computer designers. Figure 1.12 shows that process. The process starts with a silicon crystal ingot, which looks like a giant sausage. Today, ingots are 8-12 inches in diameter and about 12-24 inches long. An ingot is finely sliced into wafers no more than 0.1 inches thick. These wafers then go through a series of processing steps, during which patterns of chemicals are placed on each wafer, creating the transistors, conductors, and insulators discussed earlier. Today's integrated circuits contain only one layer of transistors but may have from two to eight levels of metal conductor, separated by layers of insulators.
A single microscopic flaw in the wafer itself on in one of the dozens of patterning steps can result in that area of the water failing. These defects, as they are called, make it virtually impossible to manufacture a perfect wafer. The simplest way to cope with imperfection is to place many independent components on a single wafer. The patterned wafer is then chopped up, or diced, into these components, called dies and more informally known as chips. Figure 1.13 shows a photograph of a water containing microprocessors before they have been diced; earlier, Figure 1.9 shows an individual microprocessor die.
Dicing enables you to discard only those dies that were unlucky enough to contain the flaws, rather than the whole wafer. This concept is quantified by the yield of a process, which is defined as the percentage of good dies from the total number of dies on the wafer.
The cost of an integrated circuit rises quickly as the die size increases, due both to the lower yield and the smaller number of dies that fit on a water. To reduce the cost, using the next generation process shrinks a large die as it uses smaller sizes for both transistors and wires. This improves the yield and the die count per wafer. A 32-nanometer (nm) process was typical in 2012, which means essentially that the smallest feature on the die is 32 nm.
Once you've found good dies, they are connected to the input/output pins of a package, using a process called bonding. These packaged parts are tested a final time, since mistakes can occur in packaging, and then they are shipped to customers.
The first equation is straightforward to derive. The second is an approximation, since it does not subtract the area near the border of the round wafer that can not accommodate the rectangular dies (see Figure 1.13). The final equation is based on empirical observations of yields at integrated circuit factories, with the exponent related generally not linear in the die area.
check yourself:
A key factor in determining the cost of an integrated circuit is volume. Which of the following are reasons why a chip made in high volume should cost less?
1. With high volumes, the manufacturing process can be tuned to a particular design, increasing the yield.
2. It is less work to design a high-volume part than a low-volume part.
3. The masks used to make the chip are expensive, so the cost per chip is lower for higher volumes.
4. Engineering development costs are high and largely independent of volume; thus, the development cost per die is lower with high-volume parts.
5. High-volume parts usually have smaller die sizes than low-volume parts and therefore have higher yield per wafer.