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Besides airborne base contamination, can anything else cause T-toping in chemically amplifies resists?
What is Kรถhler illumination?
A T-topping in positive chemically amplified
and ten years ago for microscope illuminators by August Kรถhler, is the type of illumination used by all modern projection lithography tools (such as steppers and scanners). In technical terms, a Kรถhler illuminator is an imaging system where a source (a conventional disk, an annulus of light, or a quadrupole of four disks, for example) is imaged into the entrance pupil of the main objective lens of the stepper. In addition, the reticle is placed at the exit pupil of the illuminator lens (called the condenser lens). With this arrangement comes some very advantageous properties. Any given point on the mask is illuminated by the entire source. As a result, the uniformity of the intensity at the mask is independent of how uniform the actual source is. In addition, each point on the mask sees the same range of angles of light illuminating it (i.e., the same partial coherence). But the most important property of this illumination is that each point on the mask sees a set of illuminating angles such that the diffraction pattern enters the objective lens at the same place. In other words, every point on the mask (every point in the field) will exhibit the same basic imaging performance.
resists is caused by a reduced level of deblocking (the post-exposure bake induced chemical reaction that enhances resist solubility) at the top of the resist. This reduced deblocking, in turn, is caused by unnaturally low levels of acid, the photochemical product of exposure. So the question is, what can cause acid loss at the top of the resist? Airborne base contaminants, which land on the resist and diffuse into the top portion, can neutralize acid during the delay time between exposure and PEB. The effects of airborne base contamination can be reduced by reducing the levels of base contaminants, reducing the delay time between exposure and PEB, and lowering the diffusivity of the contaminant within the resist. In addition, base additives in the resist can somewhat reduce the magnitude of T-topping whenever the base additive is a stronger base than the contaminant. Besides airborne contamination, acid evaporation from the top of the resist can produce the same type of T-topping. This occurs when the acid molecule is too small so that its rate of evaporation is not negligible. Most modern resists have large enough acid molecules to avoid this problem. In addition, solvent evaporation during post-apply bake leads to a drier resist film at the top surface. Since solvent content can affect acid diffusivity and therefore the deblocking reaction rate, the result can be a reduced level of deblocking at the top of the resist. While this effect only rarely leads to T-topping, it can enhance the magnitude of T-topping coming from another source.
Chris A. Mack, KLA-Tencor
A Kรถhler illumination, developed one hundred
Do you have a lithography question? Just e-mail lithocolumn@kla-tencor.com and have your questions answered by Chris Mack or another of our experts. 22
Spring 2004
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