Semiconductor Design Heats Up
Breakthrough in thermal analysis may be key in miniaturization

Photomicrograph
of a transistor gate
on an integrated circuit

-- Richard Mandel

If there is a single theme running through the history of electronics, it is "How do we make it smaller?" The function of several tubes was repackaged into a single envelope, followed by the advent of transistors, then integrated circuits and the persistent development of increasingly complex multi-tasking chips. In the space occupied fifteen years ago by a simple timer device, now resides circuitry that could be the heart of a home entertainment system or a missile's guidance control.

The complexity within the progressively reduced scale in integrated circuits, combined with the use of new semiconducting materials, have combined to produce chips that have better performance, but at the same time produce more heat. The demand for higher speeds -- as in devices for the telecommunications industry that run in pulsed modes at high, constant duty cycles -- also contributes to the likelihood of component failure due to thermal stressing.

To continue research and development for the next generations of submicron integrated circuits, it has become imperative to develop a method of modeling and testing circuit designs with high accuracy. Such a means appears to be at hand in the Department of Mechanical Engineering at Southern Methodist University.

The three-pronged approach

Headed by Professor Peter Raad, the SMU Submicron Electro-Thermal Sciences Laboratory is a government-industry-university cooperative effort, with the goal of enabling electronic designers to simulate thermal effects concurrently with electrical effects. They have developed a program that applies three research paths to accomplish this end.

Raad and a team of researchers initially worked to develop a numerical simulation technique for the rapid modeling of the microcircuit's dynamics, using a finite volume paradigm as the basis. The advantage, according to Raad, is that the simulation can be set to how high a resolution is desired for the test, and the software progressively tests areas of smaller and smaller volume, until it reaches the set point, in the same fashion of a Mandelbrot illustration. The model is unique in that it operates on a standard PC or workstation, and runs much faster than similar software being run on supercomputers. Unique numerical challenges were overcome in developing the software, as the spatial geometric features of many ICs can vary over six orders of magnitude.

The simulation requires as input the thermal properties of the different types of thin-film materials being applied to electronic devices, such as GaAs, titanium, tungsten and others. These materials are typically layered in geometric patters to form a single, three-dimensional block of circuitry. According to Raad, the thin-film's thermal properties are different than those of standard bulk materials, so device-specific measurements become necessary. Also, different fabrication techniques can result in significantly different material structures and boundary resistances.

So Raad and his team assembled a complex, computer-aided probing station using a microscope attached to a series of fiber-optic lasers to measure the properties of thin films. Semiconductor wafers composed of different materials are heated with an Nd:YAG laser pulsed for a few nanoseconds. Then another laser probes the material's surface to capture the process by which the material dissipates heat. The thermal conductivity can be calculated from the rate of diffusion using an inverse mathematical process.

Once data is taken from the simulation model, researchers will validate the results by testing an actual electronic design. Measurements are taken of the temperature field where the heat is generated, while simultaneously measuring electrical performance of the device. Most of the devices requiring this facet of testing will be microwave chips custom-made by a major semiconductor manufacturer who is also a backer of the program. Planned experimental parametric studies include simultaneous temperature and performance measurements over a range of pulse rates, power levels, amplifier gains and thermal boundary conditions. Future additions to the instrumentation will permit vector measurements, such as phase shift. The system is capable of probing a field ten times smaller than a strand of hair and taking a thousand measurements in half a microsecond.

An outgrowth of the research, according to Raad, is that the database would eventually permit scientists to predict the internal thermal dynamics of an IC based on measurements taken of the external temperature fields.

Manufacturers lend a hand

Long term plans for the project are to establish an industry-university collaborative research center at SMU for continued support of electro-thermal-mechanical characterizations of current and evolving integrated circuit technologies. Materials such as barium-strontium-titanate, which exhibits IR sensitivity at room temperatures and does not require cryogenic cooling for performance, can be examined for the development of manufacturing processes and operating systems. There also are high-powered monolithic microwave ICs used in telecommunications that are pulsed and that generate significant heat. Further, there is interest in analyzing electrothermodynamics to gain better management over the effects of dynamic self-heating -- especially important in pulse-modulated systems -- and thermal coupling among devices in multi-chip assemblies.

It is anticipated by Raad that the new computational methodology will have an impact on the fundamentals of numerical simulation of problems, in which the geometries possess features whose spatial dimensions vary widely. The program forwards the development of laser-based measurement systems, particularly with regard to thin-film
materials, and will further the miniaturization of products in commercial, industrial and military applications.

For more information, contact Southern Methodist University, PO Box 75-174, Dallas, TX 75275-0174. 214-768-7650.
Circle 405.