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A piece of cake

Although no nuptials are in the works, the chefs at Sematech's front-end processes division have cooked up something that is cause for celebration among those struggling with certain electrical measurements.  A specially processed wafer called a "terraced oxide substrate" used to measure work function on a single wafer has taken on the moniker "the wedding cake" because of how the stacked series of four wide, concentric circles or terraces look when viewed and magnified from the side.

The cake features a stairstep-like pattern of oxide thicknesses (produced in a spin-processor etcher), 20–100-Å deep, lodged in those concentric circles.  A high-k/metal stack can be deposited on the entire cake, and then the work function (which relates to the ability to turn a transistor on and off) can be measured on the resulting array of oxide-and-metal stacks.

"You can achieve process uniformity on a single wafer that you can't across several wafers, because with one wafer, the process remains the same across the oxide layers," says the division's associate director, Larry Larson. The "standard work function monitor structure" will ease the search for a high-k gate metal that will ape the ability of silicon to be tuned to two different work functions, which are required to accommodate the negative and positive channels in circuitry, he adds.  Which is another way of saying that engineers might have their cake and eat it too.

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