The Next-Generation Wafer

The thickness of silicon wafers for use as semiconductors has increased in line with wafer diameter, as show in the graph below. There is industry-wide standardization in place for diameters of 200mm upwards.
As a result, 200mm wafers are standardized at a thickness of 725μm (0.725mm) and 300mm wafers are standardized at 775μm(0.775mm).

However, these thicknesses were determined without due consideration from neither scientific nor industrial standpoints, thereby giving rise to a number of issues during device processing.
Examples of affected processes include lithography, impacted by issues stemming from slight irregularities in chuck shapes, and rapid thermal annealing, challenged by thermal distortion, resulting slips and potential wafer breakage during FLA (flash lamp anneal) and other forms of quick annealing. Increased wafer thickness would result in a more robust solution mitigating the aforementioned problems.

So, how should the thickness of 450mm wafers be determined? SUMCO is in the process of examining wafer thickness and mechanical properties through various simulations and experiments on prototype wafers.

It is absolutely critical that wafers are kept flat when being handled, which is an integral part of processes such as wafer shaping and device production. The main issue is sagging caused by the weight of the wafer (gravity).

Using a variety of support techniques, gravitational sag was measured on prototype 450mm wafers with thicknesses of between 825 and 1,800μm. Whereas the weight of a 300mm wafer with a thickness of 775µm is 128g, it is around 450-670g for a 450mm wafer, meaning that sagging could be expected to have a considerable effect.

The graph shows measurements for gravitational sag on a peripherally supported wafer.
Wafer sag varies significantly depending on the thickness (and consequently weight) of the wafer.

To maintain the same level of wafer sag as a 300mm wafer (thickness: 775µm), a 450mm wafer would need to have a thickness of 1,800µm(1.8mm).

Plotting measurements for gravitational sag against overall wafer thickness based on an assumed means of support equivalent to a front-opening shipping box (FOSB), as currently used to ship 300mm wafers, minimum gravitational sag occurred at a thickness of 1,470µm.

With an interval of 400mm between forks, minimum gravitational sag occurred at a wafer thickness of 1,330µm, and at a 1,060μm with an interval of 300mm. As no minimum gravitational sag was observe within the range of prepared wafer thicknesses with forks at an interval of 200mm, it is assumed that minimum sag would occur at a thickness of 825µm or less.