Dealing with nanometer-sized particulate contamination is still one of the major challenges during the manufacturing of yielding semiconductor devices. This is especially true for the increasing number of critical processing steps, where residues of particulate matter need to be removed without mechanically damaging sensitive device patterns and, at the same time, achieve the lowest possible substrate loss. If higher substrate loss would be permitted, a more or less pure chemical mechanism could be employed (e.g. particle undercut by substrate etching and lift-off). However, being only allowed to have statistically seen sub-Angstrom material loss, physical forces need to be integrated jointly with the appropriate chemical support. In this paper we describe particle cleaning techniques, which are based on monodisperse droplet impact, controlled bubble cavitation (acoustic and laser induced), moving contact lines as well as normal-directed extensional flflow to meet present and future industry requirements.
Residual, particulate contamination on a silicon wafer is remaining as one of the dominant reasons for yield loss in advanced semiconductor manufacturing.1 As such, the requirements for new technologies and processes to control particulate contamination are becoming more
stringent as smaller device nodes are evolving. As it is stated in the International Technology Roadmap for Semiconductors (ITRS 2012
update), the “killer defect” size (the critical particle diameter) continues to decrease based on the device generation, which renters the critical particle diameter now at less than the MPU (Main Processing Unit) physical gate length.2 And, obviously, high particle removal effifi- ciency (PRE) has to be achieved without creating any structural dam age to ever more mechanically sensitive device structures and with a minimum of material loss (statistically seen in the sub-Angstrom range). The origins of these yield detractors can be fallen-on particles as side products from previous processing steps, or from wafer handling, as well as un-dissolved residues - non-ionic byproducts from wet cleaning process steps, e.g. incomplete photo resist strip residues or residual etch polymers. The industry has therefore defifined very stringent PRE requirements. To comply with these requirements, wet chemical cleaning processes are favored because of unique properties of liquid media to enable transport of any material (dissolved or undissolved) and to function as mediator for physical/mechanical forces to the solid surface, enabling the effective combination of chemical and physical forces. As example the control of zeta-potentials (providing favorable surface charges to keep particles and surfaces sepa
rated) is achieved by choosing the right pH of the solution, while shear stress is controlled through adding physical mechanisms like liquid flflow rates, acoustic agitation or jet-sprays. Pure chemical cleaning is rather rapidly disappearing due to the high material loss required to
under-etch particles and lift them off the surface for which typically 2 nm of substrate etching is required. Subsequently, the particle will be detached when the repulsive electrostatic forces can overcome the attractive van der Waals forces.3
The applied chemistries to achieve PRE are continuing to evolve as new materials are continuing to be introduced into the industry, with a strong focus on highly diluted chemistries. Especially dilute RCA-style cleans4 have shown feasibility and are now mainly used in production. Highly diluted chemistries based on aqueous ammonia and hydrogen peroxide solutions can minimize attack of the oxide and silicon surfaces as well as reduce the chemical cost. Most front end of line (FEoL) chemicals are these days single use only and are applied in terms of single wafer clean processes. Single wafer cleaning equipment is now widely accepted in front end-of-line processing, due to its unmatched on-wafer performance compared to batch systems. Throughput concerns are eliminated due to the availability of multichamber platforms and enablement of throughput matching.
As for physical forces, megasonic (megahertz frequency ultrasonics) particle cleaning was the PRE solution of choice until about the 65 nm node, where jet spray technologies started to take over. At that time, cavitation based processes were not understood well enough and therefore diffificult to control, leading to statistical device damage events on the wafer.
Today’s PRE solutions mainly involve two-flfluid jet technologies, which are in principle multi-disperse droplet sprays being used to increase the shear stress on particles. However, such sprays are rapidly reaching limitations as sensitive device structure damage seems unavoidable. Since the advent of the 2× technology nodes, it became very clear, that novel approaches to achieve the required particle contamination control are required. In this paper, after a short introduction into particle removal fundamentals, we will discuss some promising PRE technologies and highlight some of their fundamentals: monodisperse droplet generation and impact, advanced acoustics/acoustic streaming, moving contact lines, and chemical entanglement.
