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While overclocking is often regarded as a rogue process, the premise behind it is well documented within the computer industry. Once a particular processor design has been finalized and taped out for silicon production, the manufacturer moves into the production and marketing phase of development. The manufacturing of processors or of any circuit device, is known as fabrication.
The fabrication of an integrated circuit device begins with the selection of a substrate material. The substrate provides a base for the layers of electrical circuits that create transistor pathways. Both the type and quality of the substrate are important in determining maximum operating speed for a given processor design.
All commercial processors currently on the market are built atop a silicon substrate. Silicon is a readily available element with good electrical isolation properties. It can be harvested from a variety of sources, and can even be obtained from common sand. The use of silicon minimizes production costs for processor manufacturers.
Silicon substrates in today's processors contain impurities left during the extraction process. These limit the substrate's electrical insulation efficiency, and lead to lower yield rates and slower core operating speeds.
CMOS fabrication techniques will likely change to accommodate upcoming generations of processors. Processors are currently manufactured using aluminum or copper metal layers within the transistor gate array. Copper offers less resistance and better conductivity than its aluminum counterpart. Nearly all newer processor designs therefore incorporate copper trace-route technologies, though an evolution in substrate technologies will be required to consolidate the gains in speed and efficiency.
Silicon-on-insulator (SOI) is primed to be the next substrate manufacturing standard. It differs from CMOS in that it places the transistor silicon junction atop an electrically insulated layer, commonly of glass or silicon oxide. Capacitance, a measure of ability to store an electrical charge, can be minimized in the gate area using the SOI switching technique.
Any transfer medium that can conduct electricity will exhibit capacitance to some degree. A MOS transistor is regarded as a capacitance circuit, implying that the MOS circuit must actually charge to full capacitance before it can activate its switching capability. The process of discharging and recharging a transistor requires a relatively long time compared to the time needed to switch the voltage state of a metal layer within a traditional transistor architecture. SOI is an attempt to eliminate this capacitance boundary: a low capacitance circuit will allow faster transistor operation. Accordingly, the ability to process more instructions in a given timeframe increases as latency in the transistor array decreases.
IBM has pioneered research into SIMOX, a silicon purification method that uses a high-temperature injection to introduce oxygen into a silicon wafer and thus purify the substrate material. Oxygen bonds with silicon at high temperatures; thus a thin layer of silicon oxide film is formed. This nearly perfect layer allows for direct bonding of a pure crystalline silicon substrate. The greatest advantage of SIMOX is its significantly lower production costs compared to crystalline-based SOI methods that use expensive ruby or sapphire design materials.
Silicon-on-insulator is not the only upcoming technology to revolutionize the substrate production process. Perhaps the most promising future technology involves the compression and purification of nitrogen. In this process, purified nitrogen gas is compressed and tempered into a solid form. Once depressurized, the nitrogen remains in a solid state. Substrates produced from this technique are expected to be almost perfectly pure, while the abundant supply of nitrogen within our atmosphere could lower production costs.
Light lithography is used to etch specific circuit pathways within a processor core. A shadow mask is created from a scaled blueprint of the processor's core circuitry. This shadow mask is then used in conjunction with a light etching process that literally burns the circuit pathways into the processor substrate. Additional shadow masks are then applied to create the complex multilayer circuitry found within a processor. Figure 3-7 shows a silicon wafer being tested after etching.
Etching can lower production costs by producing multiple processors at once. A large wafer of silicon is placed within the light masking system, which produces a "batch" of processors during a single pass. Each processor shares a common circuit design, with certain fail-safe and redundancy features embedded into the core architecture. Variation in quality among processors is due to the physical limitations involved in production.
AMD is scheduled to release a silicon-on-insulator processor based on its popular Athlon architecture before the end of the year 2002. This new Athlon design should arrive under a development project codenamed Thoroughbred, the first introduction of SOI technologies into the mainstream computing market for x86 architectures. Assuming the Thoroughbred design proves successful, other manufacturers, including Intel, will move quickly to adopt similar production techniques to extend the operating speed of current processor designs.
Laboratory testing shows that SOI-based processors can achieve up to a 25% improvement in transistor cycle time compared to the same architecture manufactured with more traditional CMOS fabrication techniques. Performance gains can average 25 to 35% when SOI is employed. Considering the efficient scalability of such an advanced design, the upcoming Athlon Thoroughbred could rapidly emerge as the dominant choice for overclocking enthusiasts.
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