by Katherine Derbyshire, Contributing Editor, Solid State Technology

Mar. 24, 2008 – It’s convenient to visualize the image created by a photomask as a binary array of bright and dark regions. Yet in the era of subwavelength lithography, scattering around mask features actually creates a grayscale image. Intensity at the center of a bright area is near maximum, intensity at the center of a dark area is near zero, but the areas in between are likely to have a smoothly varying intensity profile, with Gaussian distributions replacing sharp-edged delta functions. As a result, resist contrast defines the features actually transferred to the wafer. Above a given exposure threshold, photoacids in positive tone resist remove protection molecules, rendering the resist soluble in developer. The cleared, “bright” areas of the resist pattern are those in which exposure intensity exceeds this threshold.

Lithographers use a variety of mask enhancement features to control optical scattering and maximize image resolution. Generally speaking, though, bright areas of the mask tend to become wider due to diffraction effects, while dark areas tend to become narrower. Attempts to compensate for this behavior tend to reduce the contrast between light and dark areas, and thus the available dose margin.

When positive imaging is used to print narrow trenches, most of the mask area is dark, while the trenches appear as bright lines. Diffraction at the edges of the trench regions tends to degrade the resolution of these features. In negative imaging of narrow trenches, on the other hand, most of the mask area is bright, improving contrast at the edges of the dark trench regions.

Traditionally, Shinji Tarutani and other researchers at Fujifilm Electronic Materials Research Laboratories explained in a presentation at the recent SPIE Advanced Lithography Meeting, achieving high resolution with negative imaging has been difficult. To achieve a high dissolution rate in alkaline developer, negative tone resists incorporate polar and hydrophilic groups. Yet the cross-linking reaction which captures the exposed image may not destroy these groups. Rather, their presence can allow developer to penetrate the resist film, leading to pattern swelling, bridging, and other defects.¹

Instead, Tarutani explained, the Fujifilm group focused on negative development of conventional positive-tone resist. They found that an appropriate organic solvent can attack the acid-labile groups found in unexposed areas of positive tone resist. Under these conditions, the contrast in dissolution rates between exposed and under-exposed regions depends on the molecular weight and structure of the resist. Increasing the molecular weight slows the penetration of organic solvent, reducing the dissolution rate. By using a bright mask with negative developer, the group achieved linewidth roughness (LWR) of 4.2nm in 32nm trenches (128nm pitch). Positive developer was unable to resolve these features (see Figure 1).

Figure 1: Linewidth roughness in 32nm trenches at 128nm pitch, imaged with (a) positive tone and (b) negative tone development. LWR is 4.2nm in (b). (Image courtesy Fujifilm Electronic Materials)

Though high-resolution patterning of narrow trenches is helpful, the ability to use both negative and positive development for the same resist raises even more interesting possibilities. If negative development clears the “dark” areas of the areal image, while positive development clears the “bright” areas, what about the gray regions in between?

As the Fujifilm group showed, it is possible to tune the resist imaging threshold so that some regions lie below the exposure threshold for positive development, but above the threshold for negative development. These gray areas remain intact after both positive and negative development. As shown in Figure 2, such regions occur at one-half the photomask pitch, placing an untouched resist column on either side of each mask feature. The effect is to double the pitch frequency, and thus the density of lines and spaces.

Figure 2: Pitch frequency doubling with double development. (a) regions are cleared by positive development, (b) regions by negative development, leaving (c) regions as the final resist pattern. (d) illustrates the mask pattern, while (e) and (f) are the exposure thresholds for positive and negative development, respectively. (Image courtesy Fujifilm Electronic Materials)

Double patterning is often proposed as a way to improve resolution without investing in more expensive exposure tools or more complex photomasks. For example, one proposed pitch frequency doubling scheme places a spacer on either side of a patterned hard mask. When the hard mask is etched away, the remaining spacer captures the desired pattern.

The key challenge for most double patterning approaches is cost. The spacer-based approach adds additional deposition and etch steps. Other schemes can achieve a wider variety of patterns by using two different photomasks, but the additional exposure adds even more expense. From a cost perspective, Fujifilm’s use of double development looks very promising indeed.

So far, however, the work remains at the feasibility study stage. Initial results obtained 120nm pitch line and space patterns, using 0.75 NA ArF dry exposure. Extrapolating this result to 1.35 NA immersion exposure would give 33nm half-pitch resolution, but such fine patterns have not yet been printed. — K.D.

Reference

¹ Shinji Tarutani, Hideaki Tsubaki, and Shinichi Kanna, “Development of materials and processes for double patterning toward 32nm node 193nm immersion lithography process,” Proc. SPIE vol. 6923 paper no. 14 (2008; in press).