Orlando Escorcia

Damage of Low-k and Ultralow-k Dielectrics from Reductive Plasma Discharges Used for Photoresist Removal

Hydrogen-containing plasma discharges, used to remove photoresist in integrated circuit manufacturing, are compared for their damaging effect on porous ultralow-k dielectrics. Such reductive ash processes studied are (A) a low- pressure N 2 /H 2 chemistry, reactive ion etch (RIE)-type discharge; (B) a high-pressure NH 3 chemistry, RIE-type discharge; and (C) a high-pressure H 5 /He remotely generated discharge. Two porous SiOCH dielectrics of differing porosity and k values 2.5 and 2.2 are used in this comparative study. Diagnostic methods used for film analysis include Fourier transform infrared spectroscopy, thermal desorption spectroscopy, density, and k-value measurements. Both RIE ash processes (A) and (B) were found to cause significant dielectric damage through film densification, -CH 3 loss, water gain, and dielectric constant increases. Film damage is noted to be more severe for the higher porosity k = 2.2 film. In contrast, all measured parameters for both dielectric films showed low damage with use of the remotely generated discharge. Positronium annihilation lifetime spectroscopy is employed in this work to show pore collapse and surface densification with use of either RIE reductive ash.

Plasma Impacts to an O-SiC Low-k Barrier Film

An oxygen-doped silicon carbide (O-SiC) barrier film with a dielectric constant of 3.3 was tested using a downstream ashing plasma tool. We investigated the interactions of three plasma chemistries, O 2 /H 2 /N 2 , H 2 /N 2 , and H 2 /He plasmas, with the O-SiC films. The plasma damage to the films were evaluated and chemical structure changes were examined. While the O 2 /H 2 /N 2 plasma changed the O-SiC film into a SiO 2 -like film, the H 2 /He plasma caused minimum damage to the film, and material removal can be controlled within 5%. The infrared spectra indicate no noticeable chemical structure changes after the H 2 /He plasma exposures. Further, the electrical properties, including dielectric constant, leakage current, and dielectric breakdown voltage, were measured after the films were exposed to these plasmas. These electrical characteristics are preserved after the films were exposed to the H 2 /He plasma. The results indicate that the H 2 /He plasma ashing chemistry can be effectively applied to the O-SiC films without generating degradation of the key film characteristics.

Activated He:H2 Strip of Photoresist over Porous Low-k Materials

Introduction As design rules go beyond 90nm the introduction of porous low-k materials in Cu/low-k integration has forced dramatic changes in photoresist stripping and residue removal in BEOL applications. The major challenge is to remove photoresist and composite residues without corroding the copper or changing the dielectric constant of the porous ILD materials. This requires that the dry strip process not only can effectively remove photoresist and organic BARC materials but also can preserve the physical, chemical, and electrical properties of the ILD materials. Further, the dry strip process needs to be compatible with subsequent wet cleans. Currently there are two approaches of dry strip in Cu\low-k integration applications. One approach is the popular low pressure, low temperature anisotropic strip, which use O2 or N2H2 plasma chemistry with physical and ion enhancement to achieve lateral selectivity. The problems associated with the directional O2 strip are sidewall carbon depletion due to backscattered ions/electrons, cap rounding and etch stop loss due to sputtering, and CD shift due to etch stop redeposition. The other approach is to use downstream plasma isotropic strip with reducing chemistries. The key to success for the isotropic strip is to correctly choose the gas chemistry and process conditions. At Axcelis we have developed an activated He:H2 strip process excited by a downstream microwave plasma that has shown excellent performance on multiple Cu\low-k applications [1-4]. The He:H2 strip process substantially minimizes the damages to the ILD materials and enables post-strip wet cleans.

Evaluation of Plasma Strip Induced Substrate Damage

High dose, ultra shallow junction implant resist strip requires minimal substrate loss and dopant loss. Silicon recess (silicon loss) under the source/drain (S/D) extensions increases the S/D extension resistance and decreases drive currents by changing the junction profile. ITRS surface preparation technology roadmap [1] targets silicon loss to be 0.4Å per cleaning step for 45nm and 0.3Å for 32nm generation. Fluorine-containing chemistries which are often used to enhance implanted resist strip and residue removal result in unacceptable substrate loss. A non-fluorine plasma strip was developed in earlier work and is qualified for 45nm logic production [2]. The objective of this work is to study the substrate damage that is induced by the resist strip plasma process. Silicon surface oxidation and silicon loss of different plasma strip chemistries were evaluated with various metrologies such as optical ellipsometry, electrical oxide measurement, XPS, TEM and mass measurement. The impact of different strip chemistries on dopant retention and distribution is also discussed.

Study of Controlled Oxygen Diffusion Approaches for Advanced Photoresist Strip

Two alternative plasma strip processes were developed to meet the photoresist (PR) removal requirements of future technology nodes. Compared to traditional oxidizing chemistries, the new plasma strip approaches showed significantly lower silicon oxidation and substrate loss, while achieving good residue removal capabilities. Plasma strip-induced dopant loss and profile changes were also evaluated for gate-first and gate-last high-k/metal gate applications.

Photoresist strip challenges for advanced lithography at 20nm technology node and beyond

Photoresist strip has traditionally been a low technology process step, but is becoming increasingly more complex with the migration to ultra-shallow junctions, 3D structures, double patterning, and high-mobility channels. At junction depths of a few tens of nanometers, surface effects become increasingly important. Small changes to surface conditions can affect junction resistivity, junction depth, and dopant activation. Advanced high-resolution chemically amplified resist can be problematic when used as an implant mask. Ion beam induced chain scission and photoacid generation can lead to thermal instabilities during the resist strip process. Multilevel resist structures can be difficult to remove and rework and high aspect ratio 3D structures can require near infinite selectivity during the strip processes. This paper will summarize the issues and offer options for solutions.

Effluent Management for Non-Oxidizing Plasma Strip Processes

This paper reports on the techniques employed to control the redeposition of the partially dissociated organic photoresist (PR) byproducts in advanced non-oxidizing strip processes developed to meet the PR removal requirements of future technology nodes. System features, such as the design of heated process chamber walls and an on-board, RF-based oxygen plasma effluent abatement system are described in detail. The performance of these features to prevent or eliminate hydrocarbon buildup and manage effluent with non-oxidizing strip processes is also presented and discussed.