Technology with the Environment in Mind

Evaluation Process for Semiconductor Fabrication Materials that are Better for the Environment

INTRODUCTION

Technology Trend for Material Development

For more than four decades the semiconductor industry has been successfully producing one of the smallest and most effective devices that man has made, in step with Moore's Law [1] which basically predicts a doubling of the number of transistors per chip every two years. Today's modern semiconductors have nearly one billion transistors on them.

It takes over 400 individual steps of manufacturing and testing to make such a device. An individual semiconductor chip (with a postage-stamp-size of manufactured silicon inside, called a die) has not substantially changed in size relative to those seen in the mid 1980s, but the circuitry has increased in complexity 1,500 times. The Intel 386 chip had 275,000 transistors on a 1-micron feature size. Today's Intel® Penryn quad-core chip contains 820 million transistors on a 45 nanometer feature size. This increased complexity is attributable to both the continuous reduction in the size of the transistors on the die that allows for more transistors on the die, and the increased number of layers and features created within the external package surrounding the die, that are necessary to dissipate the intense heat away from the die.

Figure 1: Continuum of reduced feature size
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The shrinking feature size of modern semiconductors has created the need to be atomically precise in manufacturing and to have chemicals that are ultra pure. When one examines a 45nm feature size transistor (see Figure 1), the channel (the area where electrons flow from one leg of the transistor to the other) is as little as 20nm, or approximately 100 silicon atoms across. With such a relatively small number of atoms responsible for the overall performance of each transistor, it is critical that they be manufactured without defects or impurities and provide exactly the technical attributes required for the semiconductor to function.

Control of the manufacturing process at the atomic scale has lead to a new generation of materials. Examples of these include material changes driven by the need to reduce the particle size and types of chemical interactions in the chemical mechanical polishing (CMP) of the silicon wafers¹. As the feature size continues to decrease, the size of the mechanical polish particles has to decrease as well in order to prevent destruction of the features. For example, transforming a rough-cut stone into a lustrous gem requires many polishing steps using finer and finer grit. Similarly, the chemicals used in the CMP process will likely change any time the material composition of a feature changes (e.g., using mineral spirits to clean up oil-based paint versus water to clean up latex-based paint). Another example of chemicals that have changed in the past is those used in the etching and deposition² manufacturing process steps. In many of these process steps gas-based chemicals are used in place of liquid-based chemicals, since more control can be achieved with the etch depth and deposition thickness in gas-phase chemical reactions. The last example of materials change is in the wafer patterning area, referred to as lithography³. The challenges for lithography, in light of continually smaller features, are several, and two are described here. First, the feature size is now smaller than the wavelength of the light beam being used to make it, which means either the light source must be changed and/or the engineers must play chemical tricks with the patterning material (called photoresist) that is layered on the silicon wafers. Second, the intensity of the light source and the harshness of subsequent etch process steps both play a role in the composition of the photoresist that is used for any one lithography step. Further complexity arises due to the fact that each step may require a different photoresist.

Future semiconductor devices are critically dependent on the ability of stable and reliable materials to support device operation. As the market continues to demand an increase in scaling (miniaturization) with no penalty in performance, the need for new materials with increased mobility—lower energy and higher speed—is greater. Since devices are comprised of several materials and interfaces, the properties of new materials and their ability to interface with the properties of other materials will require materials with dramatically improved or new properties. All of these changing materials needs have lead to an explosion in the use of differing atoms from the Periodic Table of Elements (see Figure 2).

Figure 2: Increasing complexity of materials
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^[1] Silicon wafers are a thin disc of pure silicon upon which up to a couple of thousand semiconductor die can be manufactured simultaneously. Afterwards, the discs are cut apart into a single die in preparation for the package-assembly and test process steps, before finally being shipped to end users.

^[2] Etching is the process of removing material from predefined areas of the surface of a wafer. Deposition is the building up of material on the wafer surface.

^[3] Lithography is the process of transferring an image from a pattern onto a surface by using light. In the manufacture of semiconductors, it is the process that predefines the device features on the silicon wafer prior to the etch process steps.

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