MadSci Network: Computer Science |
The micron size referred to is the critical dimension of the fundamental transistors used in the processor. This fundamental transistor is called a MOSFET, which stands for Metal Oxide Semiconductor Field Effect Transistor. A MOSFET is a kind of switch, where the current between two terminals called the source and drain are controlled by a voltage on the third terminal, called the gate. When the gate voltage is high enough, then the transistor turns on. A critical dimension of the MOSFET is its gate length, which is a factor in how close the other two terminals (source and drain) are to each other. The smaller the gate length, the closer these two terminals are to each other, the stronger the electric field and thus the higher the current through the MOSFET when it is "turned on". When the electric field and current is higher, then MOSFET takes less time to turn on. These MOSFETs are connected together in certain patterns to carry out logical operations in the processor, like the boolean algebraic operations of AND, OR, NOT, NAND, NOR, XAND, XOR, etc. These blocks of MOSFETs that perform boolean functions are called logic gates ("gate" is a word used in many ways in electrical engineering). The clock speed of a microprocessor is directly related to how fast these logic gates operate. The higher the current in the fundamental MOSFETs, the lower the delay in switching, the faster the clock speed of the processor. Fabrication facilities are constantly striving to acheive lower and lower gate lengths to get higher and higher microprocessor speeds. Semiconductor manufacturing uses a process called photolithography to pattern the devices that end up in a part like a microprocessor. This patterning is not unlike the photographic developing process, where light is shined through a negative and developed on paper. Photolithography (which in greek means "writing on stone with light";photo=light,litho=stone,graph=write) involves shining ultraviolet light through a similar negative (called a mask) onto a photosensitive layer added to silicon wafer. This photosensitive layer can be selectively removed using an acid that only removes the parts that were exposed to the UV light. Once this photosensitive layer (called photoresist) is patterned, the underlying silicon can also be selectively removed with acid that only eats away the silicon not covered by photoresist. The resolution limits of photolithography are continuously being pushed by processes that tout 0.25um technology, or even 0.15um technology. Photolithography is limited mainly by the wavelength of the light used, diffraction effects that happen at such small dimensions, and the physics behind the acid etching that takes place on both the photoresist and silicon layers. Needless to say, this is a key issue many semiconductor companies deal with on a daily basis. Some useful keywords for further research include: MOSFET, gate delay, photolithography, photoresist, gate length, short channel effect, effective channel length, clock speed, etch, silicon wafer.
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