Solid State Phenomena Vols. 145-146

Abstract: The most advanced technology nodes require ultra shallow extension implants (low energy) which are very vulnerable to ash related substrate oxidation, silicon and dopant loss, which can result in a dramatic increase of the source/drain resistance and shifted transistor threshold voltages. A robust post extension ion implant ash process is required in order to meet cleanliness, near zero Si loss and dopant loss specifications. This paper discusses a performance comparison between fluorine-free, reducing and oxidizing, ash chemistries and “as implanted – no strip” process conditions, for both state-of-the-art nMOS and pMOS implanted fin resistors. Fluorine-free processes were chosen since earlier experiments with fluorine containing plasma strips exhibited almost a 10x increase in sheet resistance in the worse case.

Abstract: Photoresist stripping after ion implantation at high dosages (>1E15 atoms/cm2) is the most challenging dry strip process for advanced logic devices. Such high-dose implant stripping (HDIS) frequently leaves residues on the wafers after dry strip, unless fluorine chemistries are employed in the stripping plasma. Silicon loss requirements at sub-45nm nodes generally preclude such aggressive stripping chemistries. Instead, a wet clean is used to remove residues. However, the nature of the residues is not well understood, and are believed to usually contain some of the cross-linked, carbonized organic polymer formed in the implant [1]. In this paper we present chemical and mechanical data on HDIS residues produced from oxidizing and reducing chemistry strip processes.

Abstract: A layer of hardened material (crust) forms on the surface of photo resist (PR) during the implantation. This crust can be described as highly cross-linked polymer [1, 2]. Its thickness and composition depends on the type of PR, implant species, energy, dose, temperature during implantation and other factors. The crust is very resistant against chemical attack. Its chemical resistance tends to increase with the continuous shrink of technology nodes as implant doses increase. Moreover, even small residues of PR, left after cleaning, become more critical with shrinking device geometry. The usual process sequence for stripping a PR after high dose implantation (HDI) is a plasma strip (PS) followed by a wet clean. The drawback of plasma ashing is increased substrate loss and dopant bleach [3]. Plasma strip or plasma ash stand in this paper for the approach of complete PR consumption in the plasma process. Wet stripping alone often is not sufficient for stripping PR after implant doses of ≥ 1x1015 ions/cm2.

Abstract: Stripping high-dose ion-implanted (HDI) photoresists is considered as one of the most challengeable processes in the semiconductor manufacturing due to the difficulty of both removing crust (or carbonized layer) formed during the ion implantation and preventing the silicon recess after subsequent cleaning. The HDI photoresists are conventionally removed by using a two-step process, low-pressure plasma ashing in a single-wafer tool followed by SPM-based wet stripping in a batch immersion tool. Alternative HDI-resist strip methods have been proposed, such as a combination of physical-force pretreatments followed by more traditional wet cleaning steps [1], a SPM-based all-wet process at extremely high temperature (≥ 200°C) [2], and supercritical CO2 combined with chemical additive formulations [3].

Abstract: It is well-known that ion-implant doses greater than 5E14 atoms/cm2 can create an amorphous carbon-like layer “crust”, and also that this crust is extremely difficult to dissolve with wet chemicals. In practice, a combination of dry plasma ashing and wet chemical removal is used to eliminate the photoresist. In this study, a novel photoresist stripping technique is proposed. We have improved wet vapor photoresist stripping technique [1] using the mixture of high-speed steam flow and purified water droplets. Relatively low pressure clean steam is mixed with purified water in the nozzle, and sprayed on a photoresist coated Si wafer. We have also developed a pre-treatment method in a chamber with keeping both temperature and humidity constant, in order to strip post ion-implanted photoresit. The most significant feature of this proposed technique is that we use water only; hence we are able to strip photoresist without chemicals.

Abstract: Photoresist stripping in IC manufacturing has become more challenging as the number of photoresist levels has increased while at the same time allowable material loss and surface damage has decreased. Heavily implanted photoresist is especially challenging due to the dehydrogenated, amorphous carbon layer that forms on the surface [1]. To facilitate implanted photoresist removal, this layer can be attacked by physical processes such as ion bombardment as part of the common dry ashing approach. However, these physical approaches can lead to surface damage and increased material loss. Another approach is to increase the reactivity of the sulfuric acid – hydrogen peroxide mixture (SPM), so that it can penetrate and dissolve the amorphous carbon layer and achieve complete photoresist removal.

Abstract: Ion implantation is one of many critical processes in the fabrication of semiconductor devices. While device geometries have been shrinking, the implant dose has typically been increasing. Historically, photoresist removal has been achieved through a combination of plasma “ashing” and a subsequent wet clean, often using a mixture of sulfuric acid and hydrogen peroxide at elevated temperature. The “piranha” or SPM strip is often followed by an ammonia based clean such as APM to remove particles and sulfate residues from the device. However, device constraints are presently having difficulty accommodating the film loss, surface roughening, high molecular temperatures and hot electron injection which may accompany a plasma ash. [1] The APM clean is also having to undergo modification in order to minimize oxide loss which would adversely affect device performance.

Abstract: The introduction of metal gates and high-k dielectrics in FEOL and porous ULK dielectrics in BEOL presents severe issues [1] and leads to the requirement of new chemistries and processes. A major challenge in cleaning is the removal of photoresist (PR) in both FEOL and BEOL. In current semiconductor device fabrication flow, the photoresist strip process in FEOL is mostly achieved by applying a sequence of plasma ashing followed by a wet-clean step with sulfuric-peroxide mixture (SPM). But in general, ashing leads to strong oxidation or etching of silicon substrate. Hence, several approaches for ashless PR strip have been reported, such as hot SPM [2] and the combination of a pre-treatment using high velocity CO2 aerosol [3].

Abstract: In FEOL processing, doping of active areas like source, drain, and extensions (NMOS and PMOS) is done by ion implantation. Un-doped regions are covered with photoresist to protect them from implantation. Ion implantation modifies the surface of the photoresist to generate a dehydrogenated amorphous carbon layer, the crust [1]. When the implant conditions are more aggressive (higher implant energy and implant dose), the hard crust becomes more and more challenging to be removed [2]. Conventionally, a plasma ashing process followed by a wet cleaning, typically SPM (Sulfuric acid/Hydrogen peroxide mixture) chemistry, can remove the implanted photoresist, but usually leads to damage and strong oxidation of the underlying semiconductor material and hence result in material or dopant loss. As the technology node migrates beyond 45nm, the photoresist removal process should also be compatible with novel materials such as high-k dielectric and metal-gate used in advanced gate stack integration. For these reasons, it is desirable to eliminate the plasma ash and SPM clean chemistry. Wet only PR removal process is studied using new chemistries like solvents that are compatible with the other FEOL process steps, however, the photoresist removal using solvents only still showed lower removal efficiency than conventional processes. It has been demonstrated that the CO2 cryogenic pre-treatment can improve the ion implanted photoresist stripping efficiency of the wet cleaning processes [3], and can also enhance the photoresist removal efficiency by the solvents.