Dutch lithography equipment maker ASML has revealed a wafer handling stage for “double patterning”, one route to 32nm chip production and beyond.
“Thanks to a new concept and materials, the wafer stage is considerably lighter than previous generations,” says ASML. “This, in combination with a design that reduces overhead, enables high acceleration for shorter positioning times. As a result, the platform will initially improve productivity by more than 30%.”
In a lithography scanner, the wafer being patterned is clamped to a “stage” that moves beneath the all-important lens system, while the mask containing the pattern being projected moves above.
This co-ordinated ballet results in the image remaining stationary with respect to the wafer, and requires incredible velocity and position control.
Accuracy increase
According to ASML, positioning to an accuracy of better than 8nm is possible with its existing technology, which is sufficient to produce chips down to just below 40nm, the point where 193nm single patterning runs out of steam.
Below this, double patterning is required where adjacent features are printed using two separate masks.
The advantage of double patterning is that each mask has more room for optical proximity correction (OPC), adding computer-calculated shapes around features which compensate for the blurring that occurs when the features are smaller than one wavelength. OPC is used in single-exposure lithography at 90nm
and below.
A disadvantage of double patterning is that it requires more than twice the positioning accuracy. ASML calculates that 32nm double patterning requires 2nm positioning, even though single patterning at 22nm using EUV lithography will only require 4nm accuracy.
A second disadvantage of double patterning is that it doubles the number of photolithography steps needed to make a chip, so wafers have to be positioned twice as fast to maintain fab wafer throughput.
Calculations showed the firm’s existing stage could not meet both the forthcoming accuracy and throughput requirements.
Weight reduction
Increased throughput meant more acceleration: up to 150m/s sq (15g compared with the 3g dragster drivers experience when racing) so the new stage had to be lighter than the stainless steel and aluminium original. “We have a 68% weight reduction by making the stage smaller and moving to carbon fibre and ceramics,” says mechatronics designer Harry van der Schoot.
The smaller stage is also stiffer, and well damped, with vibrations from acceleration settling “within a few milliseconds”, says van der Schoot.
An H-drive used to push the stage around, similar to the way an X-Y plotter moves its pen, with two motors sliding a bar side-to-side and a third pushing the stage along the bar. The new stage is moved horizontally in two dimensions by non-contact force.
Van der Schoot would not say how this force is applied, but told EW that it is not inductively driven, which leaves magnetic positioning as the likely candidate, particularly as he reveals the stage is magnetically levitated and floats above a plain surface.
Laser interferometry continues to provide positional feedback for the stage. ASML’s lithography scanners have two square wafer-sized stages moving around each other in a square. Current designs use interferometer lasers at the edge of the square, which means beams up to 300m are required.
Air turbulence and temperature pollution from the rapidly moving stages made this a non-starter for 2nm. Instead, interferometers are now mounted above the stages, shortening the paths to a few millimetres and meeting the accuracy requirement.