Tomoyuki Takeishi

Influence of the watermark in immersion lithography process

In the liquid immersion lithography, uses of the cover material (C/M) films were discussed to reduce elution of resist components to fluid. With fluctuation of exposure tool or resist process, it is possible to remain of waterdrop on the wafer and watermark (W/M) will be made. The investigation of influence of the W/M on resist patterns, formation process of W/M, and reduction of pattern defect due to W/M will be discussed. Resist patterns within and around the intentionally made W/M were observed in three cases, which were without C/M, TOK TSP-3A and alkali-soluble C/M. In all C/M cases, pattern defect were T-topped shapes. Reduction of pattern defects due to waterdrop was examined. It was found that remained waterdrop made defect. It should be required to remove waterdrop before drying, and/or to remove the defect due to waterdrop. But new dry technique and/or unit will be need for making no W/M. It was examined that the observation of waterdrop through the drying step and simulative reproduction of experiment in order to understand the formation mechanism of W/M. If maximum drying time of waterdrop using immersion exposure tool is estimated 90 seconds, the watermark of which volume and diameter are less than 0.02 uL and 350um will be dried and will make pattern defect. The threshold will be large with wafer speed become faster. From result and speculations in this work, it is considered that it will be difficult to development C/M as single film, which makes no pattern defects due to remained waterdrop.

Pattern defect study using cover material film in immersion lithography

In immersion lithography, it is necessary that the surface of wafer has high hydrohybicity in order to prevent the residue of immersion fluid, i.e. pure water, that cause watermark defect. Usage of a cover material film over the resist film is effective to consistent with high hydrohybicity of the surface and high performance of resist film. But it was problem that much pattern deformation defects was observed with the use of an alkali-soluble type cover material film and an immersion exposure tool. As a result of the examination, it was identified that the fraction of film which caused the pattern deformation in the area of several micrometers were the fraction of the cover material. And the fractions of cover coat material were oriented in the coating defects of the cover material film and in the film peeling after scan of the immersion nozzle at the wafer bevel. The coating defects were improved with the chemical of the cover material. An adhesion process was effective to prevent the film peeling of cover material.

A novel post exposure bake technique to improve CD uniformity over product wafers

The impact of wafer warpage on critical dimension (CD) control is getting larger in ArF lithography. The product wafers with stacked films are warped due to the stress caused by the difference in the film stack structure between the top side and the back side of the wafers. A typical warpage of the product wafers is of convex shape, and the amount of the warpage is larger than 50 μm for 200mm wafer. On the other hand, proximity bake method is widely used in the Post Exposure Bake (PEB). When the warped wafer is placed on the hot plate, the gap between the wafer and the hot plate varies across the wafer. That is, the temperature of the wafer center is lower than that of wafer edge. Such a temperature variation affects CD uniformity within wafer. In particular the fact is obvious in ArF chemical amplified resist because PEB sensitivity of ArF resist is larger than 5nm/degree. In this study we optimize PEB zone temperature within wafer to suit the wafer warpage. This method is based on controlling zone temperature of the PEB hot plate with concentrically divided heaters. We carry out that the CD uniformity for the warped wafer is improved by 70% compared with the conventional process.

The effect of gel layer formation during development on critical dimensions

Patterns unfaithful to the aerial images were obtained by the spin-off development method, in which a spin-off step is added to the static development method. The critical dimension (CD) of the isolated line decreased more than that of the dense line as wafer rotation speed increased. The CD difference between the isolated line and the dense line was not due to the alkaline concentration recovery by the stirring effect of the spin-off development method and the aerial image difference between pattern types. It was due to the difference in the ease of removing the semi-dissolving layer, which was observed to cover the sidewall of resist lines, at the boundary between developer and resist during spin-off. The developer flow along the wide space area and the wide area of the layer contacting developer during spin-off are thought to make the CD of the isolated line small. The mechanical removal of the semi-dissolving layer is one of the factors to determine CDs.

Novel development technique using ozonated water

Two kinds of development processes were investigated. One is two-step development in which surface treatment using ozonated water was employed between the two steps of the development. The other is development in which ozonated water and hydrogenated water were employed in the pre-treatment step and the post-treatment step. The above-mentioned processes were applied to KrF resist process of 130nm generation. By pre-treatment using ozonated water and two-step development using ozonated water in inter-treatment, the shot-to-shot CD variation of isolated line (line width = 180nm) and the intra-shot variation were improved from 6.6nm to 4.4nm and from 13.5nm to 8.6nm, respectively. And the total variation was greatly improved from 15.0nm to 8.6nm. Moreover, the number of defects was greatly decreased by post-treatment using ozonated water and hydrogenated water continuously.

The effect of gel layer formation during development on CDs

As fine patterns with small deviation are required, it is necessary to understand the resist development mechanism in order to control critical dimensions (CDs). When the sequence in which wafer rotation process was added to the static development was applied to line patterns, isolated lines and dense lines differed in their CD change behavior. This phenomenon was investigated from the viewpoint of the gel layer.