Large Area Plasma Processing System Research Articles

Applications of electron-beam generated plasmas to materials processing

Nonequilibrium (T/sub e//spl Gt/T/sub ion,gas/) plasma processing often allows for greater process control with reduced environmental impact when compared to other materials processing methods, and therefore presents tremendous opportunities in the areas of thin film development and surface modification. The U.S. Naval Research Laboratory's "Large Area Plasma Processing System" (LAPPS) has been developed based on the high-energy (2 keV) electron-beam ionization process, with the goal of maximizing the benefits of plasma processing over large areas (/spl sim/1 m/sup 2/). This system has been shown to be: 1) efficient at producing plasma in any gas composition; 2) capable of producing low-temperature plasma electrons (<0.5 eV) in high densities (10/sup 9/-10/sup 12/ cm/sup -3/); and 3) scalable to large area (square meters). In this paper, the progress of a number of applications using LAPPS is discussed. Nitride growth in stainless steel was investigated, which demonstrated high rates (over 20 /spl mu/m/h/sup 1/2/) at low temperatures (/spl les/462/spl deg/C). Complementary mass spectrometry showed that the nitriding results correlated to the flux of atomic ions delivered to the substrate. LAPPS was also combined with magnetron sputtering sources to form hybrid systems for thin film deposition of nitrides. The simultaneous operation of LAPPS with a titanium magnetron sputter source increased the growth of the <200> orientation in TiN films, due to the increased atomic nitrogen ion flux. Polymer pretreatment studies were also initiated in these systems; polytetrafluoroethylene substrates pretreated with an oxygen LAPPS exposure demonstrated a significant increase in copper and aluminum film adhesion compared to untreated substrates, with the dominant factor believed to be the changed surface morphology. Similarly dramatic fluorination of polyethylene was demonstrated with plasmas generated in Ar/SF/sub 6/ mixtures.

Plasma enhanced surface treatments using electron beam-generated plasmas

NRL has developed a ‘large area plasma processing system’ (LAPPS) utilizing a high energy (∼2 keV) modulated electron beam to drive the plasma ionization. This system has been shown to be (1) efficient at producing plasma in any gas composition, (2) capable of producing low temperature plasma electrons (<0.5 eV) in high densities (10 9–10 12 cm −3) and (3) scalable to large area (square meters). In this work, the progress of a number of applications using LAPPS is discussed. Nitride growth in stainless steel was investigated, which demonstrated high rates (up to 20 μm/h 1/2) at low temperatures (≤462 °C). Complementary mass spectrometry showed that the nitriding results correlated to the flux of atomic ions delivered to the substrate. LAPPS was also combined with magnetron sputtering sources to form hybrid systems for surface pretreatments of polymers for metallization and thin film deposition of nitrides. In these systems, Teflon® substrates pretreated with an oxygen LAPPS exposure demonstrated a significant increase in copper and aluminum film adhesion compared to untreated substrates, with the dominant factor believed to be the changed surface morphology. The simultaneous operation of LAPPS with a titanium magnetron sputter source increased the growth of the 〈200〉 orientation in TiN films, due to the increased atomic nitrogen ion flux. Additional LAPPS systems are also discussed.

Generation of electron-beam produced plasmas and applications to surface modification

NRL has developed a number of hollow cathodes to generate sheets of electrons culminating in a ‘Large Area Plasma Processing System’ (LAPPS) based on the electron-beam ionization process. Beam ionization is fairly independent of gas composition and produces low temperature plasma electrons (<0.5 eV in molecular gases) in high densities (10 9–10 12 cm −3). The present system consists of a pulsed planar plasma distribution generated by a magnetically collimated sheet of 2 kV electrons (<1 mA/cm 2) injected into a neutral background of processing gases (oxygen, nitrogen, sulfur hexafluoride, argon). Operating pressures range from 2–13 Pa with 150–165 Gauss magnetic fields for a highly localized plasma density of ∼10 11 cm −3. This plasma source demonstrated anisotropic removal rates of polymeric (photoresist) material and silicon with O 2 and Ar/O 2/SF 6 mixtures, respectively. In pure nitrogen, this same source showed a surface nitriding rate of ∼1 μm/h of plasma exposure time on austenitic stainless steel at 400 °C. Time-resolved in situ plasma diagnostics (Langmuir probes and mass spectrometry) of these pulsed plasmas are also shown to illustrate the general plasma characteristics.

Plasma diagnostics in large area plasma processing system

A series of plasma diagnostic have been carried out in our large area plasma processing system which is based on a modulated electron-beam produced plasma. These discharges were created in both electrically conducting and insulated vacuum chambers operated in 30–120 mTorr of pure gases (argon, oxygen, and nitrogen). Langmuir probes, microwave transmission, mass spectrometry, and external current sensors show robust discharges were made over fairly wide parameter ranges resulting in plasma densities of 1–4×1011 cm−3 and temperature ranging from 0.2 eV for the molecular gases and 1.4 eV for argon. The effects of various experimental techniques, parameters, and contamination issues are discussed in detail.

Probe diagnostic development for electron beam produced plasmas

A variety of diagnostics have been fielded in different plasma sources to provide new information about the large area plasma processing system (LAPPS) at NRL. Specifically, a high resolution digital Langmuir probe system and ion flux/energy diagnostics have been employed in pulsed and dc electron beam produced plasmas as well as in an inductively coupled source as a baseline experiment. rf induced dc bias, ion flux, electron and ion temperature, and plasma potential in the proximity of rf powered electrodes placed into these plasmas were measured. It is found that the LAPPS plasma channel is essentially unaffected by the presence of the rf voltage, with the net ion flux unchanging in the presence of rf. These findings are consistent with earlier theoretical models of LAPPS [Manheimer et al., Plasma Sources Sci. Technol. 9, 370 (2000)].

Beam-generated plasmas for processing applications

The use of moderate energy electron beams (e-beams) to generate plasma can provide greater control and larger area than existing techniques for processing applications. Kilovolt energy electrons have the ability to efficiently ionize low pressure neutral gas nearly independent of composition. This results in a low-temperature, high-density plasma of nearly controllable composition generated in the beam channel. By confining the electron beam magnetically the plasma generation region can be designated independent of surrounding structures. Particle fluxes to surfaces can then be controlled by the beam and gas parameters, system geometry, and the externally applied rf bias. The Large Area Plasma Processing System (LAPPS) utilizes a 1–5 kV, 2–10 mA/cm2 sheet beam of electrons to generate a 1011–1012 cm−3 density, 1 eV electron temperature plasma. Plasma sheets of up to 60×60 cm2 area have been generated in a variety of molecular and atomic gases using both pulsed and cw e-beam sources. The theoretical basis for the plasma production and decay is presented along with experiments measuring the plasma density, temperature, and potential. Particle fluxes to nearby surfaces are measured along with the effects of radio frequency biasing. The LAPPS source is found to generate large-area plasmas suitable for materials processing.

Use of radio frequency bias in the large area plasma processing system

In the large-area plasma processing system (LAPPS) scheme, rf bias can be used for either of two purposes: to provide energetic ion bombardment of a substrate, or to controllably raise the electron temperature (which is intrinsically very cool) to a desired value. The physics of rf bias in the LAPPS differs from the situation in conventional processing reactors for several reasons: (1) The plasma density adjacent to the substrate can be so high that the ion plasma frequency exceeds the microwave frequency. (2) Plasma transport to the substrate is across a magnetic field. (3) Ionization occurs only in a thin, well-defined planar sheet, and thus the volume occupied by plasma is very broad in two dimensions but thin in the third dimension. (4) The surface area of the substrate is comparable to that of the containment vessel. We discuss the modifications to the theory of rf bias that are needed to account for these factors in a LAPPS plasma.

Theoretical overview of the large-area plasma processing system(LAPPS)

A large-area plasma processing system (LAPPS) is under developmentat the Naval Research Laboratory. In the LAPPS, the plasma is generated by asheet electron beam with voltages and current densities of the order ofkilovolts and tens of milliamps per cm2. The plasma dimensions are ametre square by a few centimetres thick. The beam is guided by a magneticfield of 50-300 G. Since an electron beam of this type efficiently ionizesany gas, high electron densities of n~1012-1013 cm-3are easily generated at 30-100 mTorr background pressure. In addition tolarge area and high electron density, the LAPPS has advantages for plasmaprocessing. These include independent control of ion and free radical fluxesto the surface, very high uniformity, very low electron temperature (Te,{<}1 eV, but can be controllably increased to a desired value) and ageometry that is well suited for many applications. This paper sketches aninitial theoretical overview of issues in the LAPPS and compares aspects of thetheory to a preliminary experiment.