David Van Steenwinckel

Lithographic importance of acid diffusion in chemically amplified resists

Since their introduction in the semiconductor industry, chemically amplified resists have proven to offer very valuable benefits to lithography processes, of which improved resist contrast and higher throughput are just two examples. However, the inherent acid diffusion mechanism starts to create some issues. For instance, the reduced chemical contrast due to pronounced acid diffusion during the post-exposure bake will decrease the exposure latitudes and would impact the ultimate resolution. On the other hand, reducing the acid diffusion length will have a negative impact on line edge roughness if one wants to simultaneously keep exposure doses and shot noise effects under control. In this paper, acid diffusion lengths in present-day photoresists at different process conditions are characterized using a lithographic technique. The observed tendencies are correlated with trends in exposure latitude, resolution and the frequency spectrum of line edge roughness. The relationship between acid diffusion length and exposure latitude as well as the relationship between acid diffusion and line edge roughness are addressed in a more fundamental way. The results of this paper highlight the major impact of acid diffusion on important lithographic process characteristics, and the investigated scaling behavior gives guidelines for optimizing exposure latitude and line edge roughness for future technology nodes.

Resist effects at small pitches

The ITRS roadmap and Moore’s law are driving us to print ever smaller features and ever tighter pitches. For these ultrasmall features and pitches, resist effects are expected to play a dominant role in limiting the overall lithographic process capability. To study the impact of resist parameters such as acid diffusion and quencher level on the lithographic performance, we have designed and formulated a matrix of 14 extreme ultraviolet (EUV) resists. In this article, we discuss results that we have obtained from EUV exposures of those resists on the 0.3 numerical aperture EUV Micro Exposure Tool at the Advanced Light Source at Lawrence Berkeley National Laboratories. In addition to exposure latitude and line edge roughness, acid diffusion lengths were characterized using a modification of the extended Nijboer–Zernike theory for each photoresist. A simple theory for the drop in resist contrast as function of the diffusion length is tested and leads to a verified correlation between exposure latitude and di...

Performance of EUV photoresists on the ALS micro exposure tool

The new high NA (0.3) Micro Exposure Tool at the Advanced Light Source (MET@ALS) at Lawrence Berkeley National Laboratories provides the first opportunity to evaluate the ultimate resolution capabilities of chemically amplified resists using EUV lithography. We characterized the imaging capabilities of a well-known tool-test resist (EUV-2D, XP98248B) and a new high resolution resist (MET-1K, XP3454C). Emphasis was placed on evaluating resists for focus and exposure latitude at 50 nm dense and isolated lines. MET-1K is capable of resolving 30 nm lines and shows modulation in 25 nm dense lines. We describe some early process optimization experiments using MET-1K that show further advances in lithographic capability. Another new series of resists (MET-2A, 2B, 2C, 2D) also show great promise for good resolution, LER and sensitivity.

Enhanced processing: sub-50 nm features with 0.8-μm DOF using a binary reticle

With the ever-increasing demand for reducing the size of devices on a chip, one frequently meets the lithographic challenge of printing very small lines on a wafer. The rapidly shrinking process latitudes, especially the depth-of-focus (DOF), really become a burden when trying to print sub-80nm resist lines on a wafer with 193nm lithography. In this paper, we report on a method that is capable of enlarging the process windows for printing small lines, while also reducing a line collapse issue, by using an enhanced resist processing procedure. In this procedure, the PEB time duration, in combination with the exposure dose, is used as a tuning process parameter. It is shown that, by using this procedure, a significant increase in DOF is obtained for printing small (down to 40nm) isolated and semi-dense lines, while the pitch is not scaled down. While using a binary mask and annular 193nm illumination with a NA of 0.63 and s of 0.87/0.57, we show that it is possible to print 50nm lines on a 240nm pitch with 0.8 micron DOF, whereas the standard process, using the vendor recommended PEB, can only print 80nm lines with a comparable DOF. The large process enhancement, among which the reduced curvature in the Bossung plots, is explained by the more efficient use of the acid and quencher present in the photoresist, as well as by the peculiarities of acid diffusion near regions where quencher is remaining. The results obtained are explained by a compact resist model in which acid-quencher reaction-diffusion is incorporated.

Anti-bubble topcoat for immersion lithography

Assessment for introduction of immersion lithography into volume manufacturing has recently started, where one of the key focus areas includes defectivity. Particularly, the possible presence of bubbles in the immersion liquid could act as a defect source. The impact of bubbles strongly depends on their size and distance from the resist. This paper shows that a thick topcoat acts as a pellicle and suppresses the printability of the bubbles. A 1.5 μm thick topcoat has been developed especially for this purpose. A model experiment has been set to validate this approach and leads to a conclusion on the printability of defects depending on their size and distance from the resist. Both simulation and results from the model experiment are shown. In addition, a new method to detect very small bubbles will be introduced.