2003 Progress Report: Removal of Photoresist and Post-Plasma Etch Sidewall Films Using Superciritical and Subcritical CO2 with AdditivesEPA Grant Number: R829554
Title: Removal of Photoresist and Post-Plasma Etch Sidewall Films Using Superciritical and Subcritical CO2 with Additives
Investigators: Hess, Dennis W. , Levitin, Galit , Myneni, Satya
Institution: Georgia Institute of Technology
EPA Project Officer: Carleton, James N
Project Period: January 1, 2002 through December 31, 2004
Project Period Covered by this Report: January 1, 2003 through December 31, 2004
Project Amount: $325,000
RFA: Technology for a Sustainable Environment (2001) RFA Text | Recipients Lists
Research Category: Sustainability , Pollution Prevention/Sustainable Development
The overall objective of this research project is to develop a more environmentally benign process for post plasma etch photoresist (PR)/residue removal based on super- or subcritical carbon dioxide (CO2) mixtures. This objective requires the characterization of the composition and bonding structure of the residues, and an understanding of their interactions with CO2 mixtures at both elevated and ambient pressures. In addition, the phase state of the mixture is helpful to permit the design of a suitable process for residue removal.
Surface cleaning and preparation comprises approximately 35 percent of the production steps performed in advanced integrated circuit (IC) manufacturing, and PR stripping accounts for essentially one-half of these cleaning steps. Incomplete removal of residues leads to defects and impurity incorporation into devices, resulting in reduced device yield and reliability. Conventional stripping by plasma and wet processes has serious technological drawbacks. Plasma cleaning can alter the structure and properties of the low dielectric constant (low k) and may create particles. Wet stripping can lead to the oxidation of silicon and metal surfaces, and can encounter difficulty penetrating narrow trenches in modern devices (< 100 nm). In addition, the use of wet chemistry, which is primarily based on hazardous solvents, bases, and acids, generally has a negative environmental impact. Moreover, large amounts of deionized water (DIW) and isopropanol (for drying) are required to complete the stripping process. Super- and subcritical CO2 are being considered as environmentally benign alternatives for PR removal, as indicated by the International Technology Roadmap for Semiconductors (Edition 2001, Interconnect, http://public.itrs.net Exit ). This approach can reduce the use of harmful chemicals and DIW, and thus may serve as a replacement for the current chemically intensive approaches.
Supercritical fluids have densities approaching those of liquids, and have viscosities and diffusivities approaching those of gases. However, supercritical CO2 has little solvating power for PR and most inorganic materials. Thus, the addition of modifiers is required to enhance residue removal. In our work, tetramethylammonium hydroxide (TMAH) was chosen as the base additive (< 5 wt percent in CO2) in the cosolvent mixture because of its ability to attack the fluorinated PR crust, and because of the presence of alkyl groups that contribute to enhanced solubility in CO2. Due to the polar nature of TMAH, its solubility in CO2 is expected to be small. Because methanol and other low molecular weight alcohols have significant solubility in CO2, a 25 percent TMAH in methanol solution was chosen as the cosolvent. Because of the acidic nature of CO2 gas, it participates in acid-base reactions with TMAH to form tetramethylammonium bicarbonate (TMAHCO3), which is the active species in residue and PR removal.
Twelve weight percent of a 4:1 volumetric mixture of 25 percent TMAH in methanol and DIW at 3,000 psi and 70°C removed post-etch PR and residues; more than one phase was present in this CO2-based mixture. The mechanism of removal appeared to be undercutting, and dissolution of the resist beneath the hardened crust; CO2 facilitated the transport of the TMAH/methanol/water droplets to the residue surface.
Variation of pressure and composition of the TMAHCO3 in a CO2 and methanol mixture yielded a single phase. Cleaning with single-phase mixtures was more efficient, and required less time and smaller amounts of chemicals than two-phase mixtures.
Phase behavior studies were performed to understand the chemical equilibrium behavior of the TMAHCO3 chemical systems described above. A thermodynamic equation of state (Peng Robinson) was used to fit the data and define the conditions under which single-phase mixtures could be achieved. Mixtures with up to 70 mole percent CO2 showed single-phase behavior. These studies suggest that effective surface cleaning in IC manufacture can be performed by using high concentrations of recyclable CO2, while minimizing the amount of environmentally harmful chemical additives.
We will investigate the compatibility of our cleaning mixtures with porous low-k films to ensure efficacy and compatibility with such layers. Porous dielectric materials will be used by the IC industry within the next several years. Cleaning solutions must be able to penetrate the nm-scale pores in these films to remove the etch residues. We will also investigate the mechanism of chemical attack/dissolution that is responsible for the removal of fluorocarbon etch residues. This will be accomplished by using infrared spectroscopy and ionic conductivity measurements. Such information will allow us to develop improved models for the prediction of phase behavior.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
|Other project views:||All 14 publications||8 publications in selected types||All 7 journal articles|
||Levitin G, Myneni S, Hess DW. Reactions between CO2 and tetramethylammonium hydroxide in cleaning solutions. Electrochemical and Solid State Letters 2003;6(8):G101-G104.||
||Levitin G, Bush D, Eckert CA, Hess DW. Phase behavior and modeling of CO2/methanol/tetramethylammonium bicarbonate and O2/methanol/tetramethylammonium bicarbonate/water mixtures at high pressures. Journal of Chemical Engineering & Data 2004;49(3):599-606.||
||Levitin G, Myneni S, Hess DW. Post plasma etch residue removal using CO2-TMAHCO(3) mixtures: Comparison of single-phase and two-phase mixtures. Journal of the Electrochemical Society 2004;151(6):G380-G386||