Magnification correction by changing wafer temperature in proximity x-ray lithography

Abstract

Magnification errors arising from wafer processing cause significant overlay errors in lithography. We have devised a new, simple method that involves changing the wafer temperature to correct magnification errors on an x-ray stepper. A wafer is first heated in the wafer orientation unit, and then it is allowed to cool as it is transferred to the wafer stage due to the temperature difference between the wafer and the air in the chamber. For a given cooling time, the reproducibility of the wafer temperature is less than 0.3 °C; and the maximum available temperature change is 1.5 °C. The key point is that the placement of patterns on a wafer immediately after the chucking remains fixed regardless of further changes in the wafer temperature, provided that the force holding the wafer to the stage is strong enough. The maximum magnification error that can be corrected by this method is about 4 ppm. This is determined by the maximum available temperature change (1.5 °C) and the coefficient of linear expansion for Si. The results of overlay tests revealed the correction controllability to be below 0.2 ppm. When this correction method was used, the total overlay accuracy was found to be 38 nm (3σ), which includes mask placement error, etching process error, and so on.

X-ray imaging: applications to patterning and lithography

“…Helium has an excellent transparency to the x-rays, and also provides a good thermal coupling (cooling) between mask and wafer. Interestingly enough, it is possible to adjust the helium flow temperature to slightly change the mask expansion and thus the magnification, if necessary [23].…”

Overlay compatibility between two synchroton radiation steppers

“…The total overlay accuracy obtained using a single stepper in the same way was less than 28 nm. 6 In order to clarify the origin of the overlay error, it is broken down into four factors: an alignment ͑ALG͒ component, a magnification ͑MAG͒ component, common in-plane ͑CIP͒ deformation, and the residual error ͑OTH͒, meaning ''other.'' Figure 2 shows histograms of the error factors for each stepper in-a͒ Electronic mail: fukuda@aecl.ntt.co.jp dividually and for the two when used together.…”

Printing characteristics of proximity x-ray lithography and comparison with optical lithography at 100 and 70 nm technology nodes

“…In fact, it has already been found to provide sufficient critical dimension ͑CD͒ control for 130 and 100 nm line patterns both theoretically and experimentally. 1,2 The overlay accuracy has also been improved through a novel magnification correction technique 3 to meet the requirements of the 100 nm technology node. The extendibility of this technology to the sub-100 nm region has been investigated by carrying out exposure tests for line-and-space patterns, isolated lines, and dynamic random access memory ͑DRAM͒-like and static random access memory-like patterns.…”