CPU Chipset - Published on Monday, 28 January 2008 00:30 Written by Robert Kihlberg
There are a number of reasons for a manufacturer of integrated circuits to migrate to a smaller process technology. They can mainly be summarized to less consumption of silicon, higher frequencies and lower power consumption. First we should explain what 65nm and 45nm stands for. 45nm stands for 45 nanometers, which is 0.000045mm. The same as 1/20,000 of a millimeter. This distance defines the shortest length of a gate in a transistor. With this in mind, we'll discuss the different benefits of reducing this distance.
A direct and easy to understand effect of decreasing the size in process technology is that every transistor needs less space on the chip. Theoretically, a 45nm transistor is 30% slimmer than a corresponding 65nm transistor. A transistor shrinks both in length and in width, which means that a 45nm transistor needs less than half the space of a 65nm transistor. One could wonder why this makes any difference, a few nanometers back or forth. The answer is that it makes a significant difference. Once a processor is designed it will be made out of silicon and it's not unrealistic that a certain type of processor will be made in quantities of several hundred millions. If the cost for silicon can be halved the manufacturing cost is more or less halved.
Sadly, due to physical limitations an ideal scaling isn't possible. Exact numbers are not official, but a good guess is a size reduction of somewhere around 40%. Another important aspect concerning this it what you call yield, which is how many working circuits you get from a silicon wafer. Since the silicon wafer is round and a transistor is rectangular you can't use all the available space. The lesser size of the processor the more processors can fit on inside the rim of the circular wafer.
The pictures above are sketches of a silicon wafer and how the rectangular processor cores are placed on them. The images are out of proportion but illustrate the manufacturing profit you can make with a new technology. The left picture shows only 8 whole processor cores while the right shows 30 whole cores. The rectangles are about 36% smaller, which means almost 4 times as many whole cores.
To make it more realistic we introduce some defective spots. In both cases we have three defective cores, which means that there is only 5 fully working cores in the left picture while in the right we get 27. These numbers are exaggerated to show the difference with a smaller manufacturing technology, and to show the effects of high quality silicon and what the size of the wafers means for these companies.
For the performance cores they have chosen to use some of the advantage in size to add more memory. To the left you see a highly magnified picture of the new Penryn core with 6MB L2 cache, and to the right the Conroe die with 4MB L2 cache. The L2 cache is the large areas to the left. Today this takes a big portion of the total space of the silicon surface of the processor. Even with 50% more cache, Penryn is still smaller than Conroe. Here is a comparison between different models and how much space the L2 cache takes of the whole circuit.
|Core||Process||Cache||Logic||Total||Normalized silicon surface|
The silicon surface is normalized to Conroe and the calculations assume that Penryn uses half of its space for cache. This results an estimated amount of transistors for the logic, which is the same for the 65nm and the 45nm models, which means that all 65nm circuits has the same logic, and in the same way for all 45nm circuits. The reasoning is based on that 3 different models will be made of every processor. In some cases it can be an economical benefit to make fewer models, but afterwards deactivate different amount of cache. Fact is that Wolfdale only represents 67% of Conroe's surface, even with larger cache. If we speculate further we see a probable future 1MB model of Wolfdale would be less than half the size of Conroe.
These outputs will obviously affect the prize of the processors, but there are more benefits with a finer process technology.
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