Introduction
At the beginning of February engineers and designers came together for the International Solid-State Circuit Conference (ISSCC) at the Marriott Hotel in San Francisco to present papers on the latest advances in solid-state circuits. One of the highlights of this year’s conference were the sessions on high-frequency microprocessors. Three companies, Compaq, IBM and Intel introduced papers on CPUs that cross the 1 GHz barrier; AMD did not give a talk but actually showed a working system with a 1.1 GHz Athlon in one of the hotel suites.
The landmark of 1 GHz is reached only 8 years after achieving 100 MHz. Microprocessor designs are now sufficiently complex that it often takes the second- or third-generation technology before all critical paths in a design are optimized. This explains why the highest frequency processors presented at the conference are all incremental improvements to existing micro-architectures. The other keys to achieving higher frequencies are the advancements in process technology and optimization of interconnects.
Compaq introduced a 6-way out-of-order issue custom VLSI implementation of the Alpha architecture that runs at 1 GHz. With a size of 13.1×14.7 mmІ, the die contains 15.2 million transistors and is manufactured in 0.18 µ CMOS with 7 aluminum interconnect layers and flip-chip packaging. The processor features two on-chip cache arrays: a 64 KByte 2-way set-associative instruction cache and a 64 KByte 2-way associative dual-ported data cache. The microprocessor runs with a nominal internal voltage of 1.65 V and supports two voltage levels at the chip interface. For the packaging Compaq chose a 587-pin ceramic PGA using flip-chip technology, but also supports wirebond packaging.
Even though the Alpha processors have always been known for their speed, they never gained a serious market share. Compaq is the only significant OEM using the chips in their servers. Some analysts believe that this might change, however, predicting that Compaq will switch to Intel’s 64-bit architecture as soon as the processors become available.
IBM presented a paper on a 1 GHz, single-issue, 64-bit PowerPC processor. The chip contains 19 million transistors and utilizes a 4-stage design in a 0.22 µ process with a six-layer copper interconnect. Usually very long pipelines are necessary to generate high frequencies. IBM managed to achieve 1 GHz with a very short pipeline, which is the more efficient way to do it because longer pipelines add to the complexity of the chip.
Copper vs. Aluminum
Over the last few years there has been a debate on the merits of copper versus aluminum interconnects. Copper offers lower resistance and better electro-migration properties but adds complexity to the fabrication process. It is also more difficult to use low-K dielectrics with copper.
These are some of the reasons why Intel decided to implement their 1 GHz version of the Coppermine processor in 0.18 µ CMOS with aluminum interconnects: aluminum metal lines combined with SiOF low-K dielectric and decreased gate dimensions enable higher speed transistors. The architectural enhancements include a 256 KByte Advanced Transfer Cache (ATC). It offers a bandwidth of 16 GByte/s to the L2 cache, lower latency (4x improvement in L2 latency over the Pentium III processor) and higher cache associativity (8-way set associative, 1024 sets).
As Intel elaborated at the press conference, making changes to the architecture was less of a risk than switching to copper and potentially decreasing the yield for a high volume product. However, for the next shrink to 0.13 µ the company will utilize copper interconnects.
AMD did not present a paper at the ISSCC but demonstrated a system running on a 1.1 GHz Athlon. The chip was manufactured at the Fab 30 facility in Dresden, Germany, using AMD’s HiP6L 0.18 µ process with copper interconnects. The system did not require any special cooling techniques. According AMD the speedy Athlon will hit the streets in the fourth quarter of this year. It is most likely that Intel will time the launch of its 1 GHz Coppermine processor accordingly. The chip is going to ship before their next processor named Willamette.
It seems that the 1 GHz barrier is not a roadblock anymore. The latest news from the Intel Developer Forum going on in Palm Springs this week report that Intel Chairman Andy Grove showed off a 1.5 GHz Willamette processor operating at room temperature. The race continues…
