Technique images picosecond switching in CMOS circuits
08/01/1997
Technique images picosecond switching in CMOS circuits
A nondestructive technique could help manufacturers debug timing problems in chips, allowing faster CPUs. The technique, hot-electron luminescence, detects the switching of logic gates in large CMOS circuits using images obtained from either the front or back of a chip. The technique was presented at the recent Conference on Lasers and Electro-Optics in Baltimore, MD.
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Figure 1. Three frames from a 34-ns movie showing the psec emission from a ring oscillator with a 90-ps stage-to-stage delay. When the FET in each of the 47 inverters (the left part of the image) switches states, it emits light (colored spots). The emission spots are superimposed on the optically obtained image of the circuit. Each frame of the movie corresponds to a time interval of 34 ps. These time- and space-resolved images let researchers see the operation of the circuit as a function of time. (Photos courtesy of J.A. Kash and J.C. Tsang, IBM Research)
Researchers Jeffrey Kash and James Tsang of IBM`s Watson Research Center, Yorktown Heights, NY, found that when a FET in a CMOS chip switches states, it emits a pulse of infrared light. Because the pulse`s duration is < 100 ps, the luminescence can be used to discover when specific FETs switch. Using off-the-shelf imaging IR detectors, Kash and Tsang obtained simultaneous time- and space-resolved images showing digital circuits in operation.
Figure 1 shows three frames from a movie the researchers made of the operation of a simple CMOS ring oscillator circuit. The optical emission from the front side of the chip is shown in false color, overlaid on a monochrome photograph of the circuit. The detector in this case, a Quantar Technology Mepsicron imaging photomultiplier, allows single-photon counting with sub-nsec time resolution.
The increasing density of wiring and electrical contacts on the front of modern chips makes backside imaging desirable. Because silicon is transparent to light with wavelengths longer than about 1.05 ?m, the hot-electron luminescence can also be imaged through the back of a chip (Fig. 2) using a scientific grade CCD detector.
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Figure 2. Infrared emission from the backside of a normally operating IBM System/390 CMOS microprocessor was captured by a silicon CCD detector to form this image. The chip contains more than 7 million transistors, and measures 18?18 mm. To make the image, the chip was thinned to 200 ?m and polished. (Photo courtesy of J.A. Kash and J.C. Tsang, IBM Research.)
Kash says, "As microprocessors get faster, obtaining information about what`s happening (when gates are switching) is becoming more important." At the moment, diagnostic methods bounce a pulsed electron beam off a wire in a circuit, but this technique is expected to become obsolete within several years, as the density and number of wires in chips increase. Advantages of the hot-electron luminescence method over the pulsed e-beam probes include time resolution of tens of psec. The method is both passive and optical, so it does not interfere with operation of the circuit. It also can acquire data in parallel from large areas of a circuit within a detector`s field-of-view. "There might be millions of transistors on the chip," Kash explains, "and we can look at a large fraction of them at once."
A key application for circuit imaging is learning how to make devices work faster. "If you design a microprocessor to run at 100 MHz and find it only runs at 90 MHz," Kash says, "then the question is: what part of the chip doesn`t run at that speed." Researchers can use time-resolved measurements to find the critical path and fix it. The method requires more development before it can be widely implemented. Kash wants a more sensitive detector to reduce data-gathering time. The researchers also want to extract voltage and current in a circuit from the light intensity.
The future for the technique looks promising, though. As device features get smaller, Kash expects the technique will continue to be useful. "Voltages are decreasing," he says, "but not as fast as feature sizes." Therefore, the electric field inside FETs will increase, providing brighter images.
- Yvonne Carts-Powell