Grantee Research Project Results
Final Report: A New Approach for Reducing Global Warming Emissions from Etching by Controlling Ion Energy and Neutral Fluxes
EPA Grant Number: R831459Title: A New Approach for Reducing Global Warming Emissions from Etching by Controlling Ion Energy and Neutral Fluxes
Investigators: Hershkowitz, Noah
Institution: University of Wisconsin - Madison
EPA Project Officer: Aja, Hayley
Project Period: December 1, 2003 through November 30, 2006
Project Amount: $324,812
RFA: Technology for a Sustainable Environment (2003) RFA Text | Recipients Lists
Research Category: Nanotechnology , Pollution Prevention/Sustainable Development , Sustainable and Healthy Communities
Objective:
Semiconductor fabrication is a significant source of global warming gases because of the widespread use of CF4 as well as other related fluorocarbon gases. The goal of the proposal was to develop new methods of plasma etching and deposition using alternative gas sources and RF discharges to achieve acceptable etch selectivity.
The proposal was aimed at finding solutions to the global warming problem based on a detailed understanding of how ion energy distribution functions (IEDF) determine the physics and chemistry and the interaction with substrate surfaces. IEDF control was to be obtained by an appropriate wafer bias frequency combined with plasma density control. In addition, a key requirement of plasma etching is the formation of carbon films. Our experiments were designed to provide fluorine for etching and carbon for thin film formation from separate parent gases.
Varying the wafer bias frequency and adjusting the plasma density by varying the source input power achieved some control of ion energy distribution functions. High-density low-pressure inductive plasma was used to study SiO2 and Si3N4 etching with NF3/C2H4-based feed gas chemistry. Nitrogen trifluoride (NF3), and ethylene (C2H4) were used so that fluorine and carbon, required for selective etching of silicon oxide over silicon nitride, could be supplied from parent gases other than strongly global warming fluorocarbons.
Summary/Accomplishments (Outputs/Outcomes):
Experiments demonstrated that appropriate thin films could be created from a non-fluorocarbon parent gas. It was shown that the etch process established a very thin carbon-based steady-state film, similar to a fluorocarbon chemistry. The film characteristics were determined with ex situ x-ray photoelectron spectroscopy (XPS) and mass spectrometry. It was shown that CHxF, CF2 and CF3 were produced but in small concentrations compared with CHx and CN and that the relative concentrations of CF2 and CF3 on SiO2 were much lower than the concentrations on Si3N4.
Unfortunately, experiments were not successful in achieving selective etching of SiO2 over Si3N4. Etch rates of SiO2 over a wide range of conditions were roughly 0.6 to 0.8 times those of Si3N4 etch rates.
Likely etching mechanisms were identified from ex-situ X-ray photoelectron spectroscopy data. It appears that SiO2 preferentially reacts with CF2 and CF3 but not with C-C or CHxF. Differences in the abilities of SiO2 and Si3N4 to react with C-C structures contributed to higher etch rates of Si3N4. Experiments with acetylene and methyl fluoride (butane) were equally unsuccessful.
It was recognized that RF fluctuations could also be significant in the source plasma potential. A new diagnostic technique was developed for measuring the plasma potential fluctuations in RF plasmas with reactive ions. The goal was to provide a measurement of the variations in ion energy with source RF power. This study was successful. Plasma potential fluctuations were measured with an emissive probe in low-pressure CF4 plasma produced by a 13.56 MHz RF source using the inflection point method in the limit of zero emission. The maximum and minimum values of the plasma potential were observed. The values of the potential fluctuations depend on the RF source power, the amplitude of the applied voltage and the plasma density. The technique had previously been used to study non-reactive rare gas plasmas.
Conclusions:
An unconventional plasma chemistry employing one feed gas for supplying fluorine and another gas to provide carbon film precursor was used in an attempt to develop selective etching of silicon oxide (SiO2) to silicon nitride (Si3N4) that would result in much less global warming gases in the exhaust than conventional fluorocarbon chemistry. The results demonstrated that the desired carbon films could be achieved using a non-fluorocarbon gas ethylene as the film precursor. Unfortunately, SiO2 etch rates were always lower than Si3N4 etch rates. XPS results showed that SiO2 has difficulty reacting with C–C structures in the steady-state layer at its surface but Si3N4 can convert C–C into etch products relatively easily. Silicon nitride etch rates can be slightly decelerated if the steady-state deposited film contains a sufficiently large amount of C–C structures that can impede the removal of CN-containing products from the surface. However, this effect is too small to overcome the lack of selectivity to nitride. The inability to achieve selective etching means that the particular choice of an unconventional etch chemistry for semiconductor device fabrication was unsuccessful. Experiments with acetylene and methyl fluoride (butane) were equally unsuccessful.
CF2 and CF3 on etched silicon nitride samples were found to be significantly larger than those on the oxide sample. The result strongly suggests that CF2 and CF3 are rapidly consumed in etch reactions with an SiO2 surface, thereby causing the steady-state relative concentrations to be lower than nitride. Overall XPS results demonstrated a necessity for a sufficiently large CF2 concentration as the minimum requirement for achieving faster oxide etching than nitride etching. As shown by XPS and mass spectrometry, the lack of high-density CF2 and CF3 is inherent in all low-pressure NF3/hydrocarbon discharges. Therefore, selective etching of SiO2 to Si3N4 could not be achieved in a low pressure (<10mTorr) NF3/hydrocarbon plasma but this chemistry could be applied to nitride-to-oxide selective etching in the future.
Environmental Impact
The experiments provided a first step that might eventually lead to reduction in global warming gas emission by the semiconductor processing industry. They showed that carbon film formation during etching could be produced by parent gases that did not contain fluorine. While small changes in the etch rates were achieved by varying ion energies, selectivity - the relative material etch rates - appropriate for semiconductor device fabrication, was not achieved. Thus the present results will not have any immediate environmental impact.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
| Other project views: | All 7 publications | 2 publications in selected types | All 2 journal articles |
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Lee D, Ting Y-H, Oksuz L, Hershkowitz N. Measurement of plasma potential fluctuations by emissive probes in CF4 radio-frequency plasma. Plasma Sources Science and Technology 2006;15(4):873–878 doi:10.1088/0963-0252/15/4/034. |
R831459 (Final) |
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Machima P, Hershkowitz N. SiO2 and Si3N4 etch mechanisms in NF3/hydrocarbon plasma. Journal of Physics D: Applied Physics 2006;39(4):673-684. |
R831459 (Final) |
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Supplemental Keywords:
Plasma etching, C films, etch selectivity, global warming gas reduction, NF3/C2H4, Sustainable Industry/Business, RFA, Air, Scientific Discipline, Technology for Sustainable Environment, Sustainable Environment, Atmosphere, Environmental Chemistry, Air Pollution Effects, climate change, Environmental Engineering, ion exchange, silicon oxides, Global Climate Change, emission controls, global warming, environmental monitoring, semiconductor manufacturing, adaptive technologies, nitrogen triflouride, fluorocarbon emission controls, cleaner production, fluorochemicals, semi-conductor processing, semiconductor industry, etchingProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.