Extendibility Study of Conventional IPA Drying Process From Dynamic Fluid Model for Pattern Collapse
Authors: Otsuji Masayuki , Tanaka Takayoshi , Iwasaki Akihisa , Takahashi Hiroaki , Okuno Yasutoshi 
Published: July 2022

1. Introduction
IPA (iso-Propyl alcohol) has been widely used in semiconductor manufacturing for drying silicon wafer to mitigate water mark and keep the surface clean after wet cleaning process. Then, IPA process has been extended by combining with the other engineering techniques such as surface modification treatment to dry fragile nano-sized Si patterns [1]. As the scaling of advanced semiconductor devices continues, however, the pattern collapse is one of the most challenging issues for IPA drying process today. Capillary force model explains the pattern collapse in the earlier studies, where Laplace pressure originated from meniscus between patterns is a critical factor [2]. And the model has been well referred to and extensively studied in semiconductor industry [3]. Although, the model does not take dynamics of fluid into account, our experimental result suggests that drying rate is also critical, i.e., pattern collapse rate decreases as drying rate increases. This result is not explained by the reported models since the drying rate is not considered as a variable. In this report, a new model is suggested and verified by unique experiments. Our proposed model assumed that osmosis of metastable liquid to nano-size structures occurs from the triple line which is the air-liquid-solid interface, and the region of osmosis is less favorable for pattern collapse. The width of osmosis region would be determined by the drying rate. Also, based on this study, extendibility of IPA drying process to smaller device feature size for advanced node is discussed.
2. Experimental
Wafer processing was conducted in spin process chamber on HVM equipment for drying experiment. IPA was dispensed on rotating wafer then dried at 0 or 1500 rpm. Nano-structured wafer used was with 34 nm-diameter of Si pillars, and aspect ratio (AR) of the pillar was 12:1. Pattern collapse was observed by top-view SEM, and collapse rate was calculated by automated image analysis with obtained SEM pictures. For osmosis observation at the triple line, FPM was prepared by mixing 49% HF, 30% H2O2 and H2O at volume ratio of 1:2:100. A droplet of FPM was dropped on the surface of Si pillar pattern wafer and observed by optical microscopy while the drop was drying out. After drying, pattern collapse of the pillar pattern was observed by X-SEM. To study effect of drying rate and to maximize it, IPA was dispensed on 10×12 mm2 coupon, then vaporized at 0 rpm with flash lamp annealing. AR of the pillar was 16:1. Observation of pattern collapse by top-view SEM was also conducted.
3. Results and Discussion
IPA drying at 0 and 1500 rpm resulted in collapse rate of 91.1% and 46.9%, respectively, on the pillar AR = 12:1 (Fig. 1). IPA was dried out faster when wafer rotation speed was 1500 rpm. Thus, collapse rate decreases as drying speed increases. On osmosis observation, FPM droplet was sucked into between the pillars, causing osmosis at around droplet, and generated colored interference according to liquid film thickness. X-SEM inspection after the droplet drying indicated the most pattern collapse seemed to occur at the osmosis region where liquid thickness nearly equals to the pillar height. We assumed by increasing drying rate the osmosis region would be minimized and the pattern collapse would be suppressed. To verify this model, the following evaluation was conducted. With extremely high drying rate of IPA by flash lamp annealing, we obtained 0% of collapse rate even in the pillar AR = 16:1 (Fig. 2). This result indicates that there is some more process margin for the pattern collapse on IPA drying process.
4. Conclusion
In the IPA drying process, collapse rate decreases as drying rate increases. Our proposed model assumed that osmosis region of metastable liquid to nano-size structures occurs from the triple line, and the pattern collapse happens there. That explains reasoning for lower collapse rate at higher drying speed is that osmosis is minimized when drying rate is maximized. Test result from maximized IPA drying rate by flash lamp annealing indicates that there is some more process margin for the pattern collapse. In our view, by this method IPA drying process might be extended to two more device nodes.
References
[1] T. Koide, et. al., ECS Transactions, 69 (8) 131-138 (2015).
[2] T. Tanaka, et. al., Jpn. J. appl. Phys., 32 6059 (1993).
[3] S. Farshid, et. al., Langmuir Article, 16 (2010).

Figure 1