Non-Aqueous/Dry Cleaning Technology without Causing Damage to Fragile Nano-Structures

Abstract

With semiconductor-device geometry shrinking and becoming more complex, conventional aqueous cleaning/drying tends to collapse free-standing MEMS and high-aspect-ratio fragile LSI structures due to the high surface tension of aqueous chemicals and water. The use of physical cleaning aids such as megasonics with dilute chemistry or high-pressure atomizing jet sprays can also cause damage to nano-structures. The process window for damage-free cleaning is also becoming narrower as device geometry shrinks. This makes the development of new damagefree cleaning methods a high priority. In this paper, various alternative damage-free non-aqueous/dry cleaning techniques are described and discussed including low pressure, elevated temperature HF vapor cleaning, cryogenic aerosol cleaning, supercritical CO 2 cleaning, and pinpoint dry cleaning that employs lasers, nano-probes, or nano-tweezers. There will be more research challenges and opportunities in these environmentally-benign damage-free cleaning technologies in the future. IntroductionAs semiconductor devices become ever more highly integrated and their geometry continues to shrink, even slight silicon and oxide etching loss during wafer cleaning can have a negative impact on the characteristics of metal-oxide-semiconductor (MOS) transistors (1). Therefore, highly diluted chemicals are used during aqueous silicon-wafer cleaning (2, 3), such as conventional RCA cleaning, to minimize the material loss. Dilute chemistry is also preferable from the viewpoints of both micro-roughness control of the silicon surface and environmental control of the chemical consumption.However, it is difficult to remove particles to the extent desirable with highly diluted chemicals. In order to enhance the particle-removal efficiency, physical aids such as megasonic agitation are generally employed (4), but it tends to cause structural damage to fragile LSI circuit patterns. Reducing the megasonic power can reduce the megasonic damage to the fragile structures, but it also reduces the particle-removal efficiency. In order to reduce the device damage by the megasonic energy, some modifications such as an addition of lower-ionization-potential gas and/or liquids to DI water have been proposed (5). In general, conventional aqueous cleaning methods using any kind of sonification become less effective as the particle diameters get smaller.An alternative physical technique, a DI-water/gas-mixture jet spray cleaning technique, has been reported (6). This technique has an advantage over megasonics because it can remove smaller particles due to the greater impact of droplets at near-sonic speed. Thus, the water/gas jet spray at a high flow rate or at high speed of the carrier gas without any addition of chemicals can remove particles on unpatterned wafer surfaces without material loss caused by chemical etching. But we must be mindful of the possibility of structural damage on patterned wafers that can be caused by this type of physically assisted treatment as well as megasonic agitation. In fact,...

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