Plasma Processing of Materials Scientific Opportunities and Technological Challenges (1991) / Chapter Skim
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4 SCIENTIFIC FOUNDATION OF PLASMA PROCESSING
4 SCIENTIFIC FOUNDATION OF PLASMA PROCESSING
Pages 37-64
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From page 37... ...
on rate, uniformity, anisotropy, and selectivity during etching; · Measurement of appropriate rate constants and cross sections that are used to model technologically important discharges; Development and use of diagnostic techniques such as mass spectrometry, optical actinometry, laser-induced fluorescence (LIF) , and Raman spectroscopy to measure species concentrations in the plasma; Use of light scattering to show that plasmas produce particles that reduce cievice yield.
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From page 38... ...
The recalibration and reoptimization of manufacturing process steps by empirical means alone is inefficient and costly. Unfortunately, the complexity of plasma processes and the lack of fundamental understanding make detailed, quantitative process simulation based on first principles seem unlikely in the near future.
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From page 39... ...
Similarly, we are not able to control plasma processes because there are no guarantees that machines operate at the intended internal conditions: diagnostic techniques are needed to characterize both the plasma and wafer states in situ and in real time. SURFACE PROCESSES Surfaces exposed to plasmas experience bombardment by energetic ions, electrons, neutrals, and photons.
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From page 40... ...
With the advent of massively parallel computing facilities in the next 5 to 10 years, simulations of surface processes like those discussed above should become increasingly easy to TO PUMPOUT ~C12; i ~ ~ ~ ~! ~ Si ding oxygen (3 T -- INDUCED ~ ~ \\N VOLATILIZATION ~ OXYGEN I \\ Of SiCI'~l, I S CI H ADSO PTION OF ~ SiCl H ON I j Sl WALL AH I (hi)
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From page 41... ...
BEAM-SURFACE EXPERIMENTS Much of our fundamental understanding of surface processes occurring during etching and deposition comes from well-controlled plasma simulation experiments in which beams of ions, electrons, neutrals, and photons are directed either together or alternately at well-characterized surfaces (Figure 4.3~. The beams are typically analyzed using mass spectrometry.
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From page 42... ...
Figure 4.4 shows how the etching rate with both ion and reactive flux to the surface exceeds the sum of the individual etch rates for ion sputtering and neutral reaction. Without experiments of this type, we cannot hope to understand surface reactions that dominate the outcome of industrial plasma processes.
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From page 43... ...
Although the advent of low-pressure, high-density plasmas is helping to narrow this "flux gap," we still need scaling relationships to connect beam-surface studies to plasma processes. Secondly, ion energies have generally been too large for comparison to the newest generations of plasma reactors.
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From page 44... ...
Such work requires collaboration between surface scientists and plasma process engineers and is an opportunity to exploit synergies between national, industrial, and university laboratories. PLASMA GENERATION AND TRANSPORT Large fluxes of energetic reactive particles ions, electrons, neutrals, and photons over large surface areas make plasmas useful in materials processing.
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From page 45... ...
Fluid and kinetic models are able to represent complex geometries and address the details of individual reactors. Only kinetic models can provide accurate particle energy distributions that are so vital for plasma reactor design and process simulation.
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From page 46... ...
The theoretical capability has yet to be developed and exploited in plasma processing, primarily because very little basic research has been devoted to processing. FEUID SIMULATIONS One approach to dealing with the complexity of processing discharges is by means of fluid simulations, in which each of the charged particle species (electrons, and positive and negative ions)
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From page 47... ...
Thermal Plasma Modeling Numerical models of thermal plasmas have been developed for free-burning arcs, wallstabilized arcs, and convection-stabilized arcs. More recently, arcs with turbulent boundary layers in high-velocity nozzle flows have been extensively characterized through models and diagnostics.
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From page 48... ...
Thermal plasmas used for materials processing are usually inhomogeneous with regard to substance and phase, i.e., particulates or clroplets in a vapor environment, and gas mixtures whose compositions vary strongly with position. Energy exchange and momentum exchange in multiphase mixtures have been described; however, the results are strongly dependent on the simplifying assumptions made.
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From page 49... ...
The chemistry is then modeled, either using zerodimensional rate equations for the various neutral species, with rate constants based on the "known" plasma conditions, or using one-dimensional or multidimensional diffusive, free-flow, or Monte Cario transport models for the generation and loss of neutral species. Closing this open loop to solve simultaneously for both charged and neutral particle concentrations and energy distributions is a prerequisite for successful process simulation.
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From page 50... ...
The issue of turbulence in thermal plasma processing has already been discussed above ("Thermal Plasma Modeling") and identified as a major issue.
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From page 51... ...
But comparable efforts have also been established in Europe in France, Germany, England, and Italy primarily- and more in Japan. While the efforts in the United States have been largely uncoordinated and lack the participation of plasma reactor vendors, both Japan and France have national programs in reactive plasma diagnostics and simulations (discussed below in the section titled "Funding for Plasma Processing Researched.
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From page 52... ...
to estimate relative, ground-state densities and qualitative changes in the electron energy distribution function. Emission can also be used in measuring the internal energy distribution functions of molecules electronic, vibrational, and rotational.
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From page 53... ...
In high-density, thermal plasmas, Thompson scattering can be used to make both electron density and energy distribution measurements; but, for the lower-density discharges most often used in electronics materials processing, the electrostatic probe is the only method available. When properly applied, probes offer valuable insights into discharge phenomena.
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From page 54... ...
But it is clear that negative ions have dramatic effects on discharge properties, on interpretation of probe diagnostics, and on material properties. For example, large concentrations of negative ions result in large bulk electric fields that modify both positive-ion and electron energy distribution functions so that they differ from what they would be without negative ions.
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From page 55... ...
However, such programs would appear to have high merit. Data Base for Plasma Generation and Transport Although many of the necessary experimental and theoretical tools exist now or will exist in the near future, the basic data needed both to model and to diagnose plasma processes are generally lacking or are at best difficult to access.
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From page 56... ...
The coming of age of massively parallel computers can revolutionize the calculation of electronic, ionic, and heavy-atom cross sections for complicated, chemically reactive systems. The development of standard cross-section codes, along with a judicious program of measurements to verify the calculation, would have an enormous impact on the understanding and design of systems for the plasma processing of materials.
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From page 57... ...
This information is available in the scientific literature and is summarized in numerous textbooks and research monographs. However, complete cross-section sets for a given material system are generally not easily found or generated; yet, such sets are essential input to a plasma process simulation or reactor design.
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From page 58... ...
Surface properties determine boundary conditions that directly determine plasma transport. The emission of charged and neutral particles affects rates of ionization and dissociation and the shapes of energy distribution functions.
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From page 59... ...
in plasma spraying, it is the momentum and energy exchange between the plasma and the spray powder that largely determines the quality of the coating, and considerable effort has gone into describing this interaction; (2) in plasma synthesis, i.e., the plasma generation of particles from a vapor-phase chemical reaction, nucleation and particle growth processes are still poorly
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From page 60... ...
A new set of calibration measurements must be performed whenever process variables-such as discharge power, pressure, gas mix, film doping, and photoresist composition-are changed. Reliable models for plasma generation and transport could have a dramatic impact on profile simulation, providing that adequate models and data for etching and deposition surface chemistry also exist.
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From page 61... ...
Wisconsin again has a strong background in fusion research, and Minnesota has a lone history of then plasma processing research. V J V By and large, current funding has been inadequate and insufficiently coordinated to support the generation of plasma diagnostic data, surface interaction studies, development of new in situ surface diagnostic techniques, simulation of plasma generation and transport, simulation of surface processes, and compilation of a minimal basic data set.
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From page 62... ...
There are, however, significant differences. The Japanese program targets a broader range of fundamental science topics (e.g., measurements of electron-impact cross sections and excited-state excitation transfer coefficients)
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From page 63... ...
Although basic studies have had significant impacts on previous plasma process and reactor development, the basic science needed to build design tools and expert systems is much more extensive. Three areas are recognized as needing concerted, coordinated experimental and theoretical research: surface processes, plasma generation and transport, and plasma-surface interactions.
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From page 64... ...
Because plasma generation and transport are the primary focus of new reactor design, the simulation tools already developed could be developed further and directly implemented in CAD tools. The advent of new computer technology will enable plasma simulators to meet the challenges of calculating particle energy distribution functions for multidimensional, magnetized systems.
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