Gary S. Selwyn

In situ laser diagnostic studies of plasma-generated particulate contamination

Laser light scattering measurements show that certain silicon etching plasmas produce a significant amount of in situ, particulate contamination. The particles are suspended at the sheath boundaries. Simultaneous measurement of plasma negative ions by the use of two‐photon laser‐induced fluorescence technique suggests that the particles are negatively charged and so are electrostatically trapped at the sheath boundaries. The parametric conditions for particle formation and growth in the plasma are identified. A mechanism for nucleation and growth is suggested involving formation of plasma negative ions from etch products, ion clustering with plasma species, and cluster growth into particles with electrostatic suspension and trapping. The particles drop onto the wafer when the rf is turned off. Implications for dry process technology are discussed.

Particle trapping phenomena in radio frequency plasmas

Particles generated in an argon plasma and suspended at the plasma/sheath boundary are localized by lateral trapping fields. In the commercial rf etching reactor used in this work, the particles and their motion in real time are observed by laser light scattering with the laser beam rapidly rastered in a plane parallel to the rf electrode. Repulsion between individual, relatively large particles is observed, verifying that there is significant negative charge on the particles. Two types of trapping regions are commonly seen: rings of particles around the outside edge of silicon wafers, and domes of particles over the centers of the wafers. It is shown that these effects are influenced by the topography of the electrode. In addition, particle densities >107 cm−3 for particles of diameter 0.2 μm are inferred from transmission studies for certain plasma conditions.

Insitu plasma contamination measurements by HeNe laser light scattering: A case study

Using a simple, inexpensive HeNe laser and a video camera to measure light scattering intensity and location, particulate contamination in an SiO2 sputtering process used in semiconductor manufacturing has been studied under actual process conditions in a class 10 cleanroom. Particulates were observed during all aspects of the sputtering process. It was seen that portions of the process which resulted in mechanical stress on the tool walls produced the greatest flux of particles inside the tool. However, the sputtering step was the major contributor to contamination in this chemically simple process, because of its long duration and the stress‐inducing nature of the plasma. The contamination level in this plasma is estimated to exceed the cleanroom ambient by three orders of magnitude. As in other process plasmas, the particles were suspended at the sheath/plasma boundary. It is argued that a relatively weak electrostatic field is required for gravitational counterbalance of these highly charged particles...

Rastered laser light scattering studies during plasma processing: Particle contamination trapping phenomena

The distribution and transport of particles in materials processing plasmas has been studied with a rastered laser light scattering technique. Contrary to expectation, the distribution of particles in a plasma processing tool is rarely random. Instead, structured clouds of particles form at the plasma/sheath boundary. The effect is attributed to trapping of the particles by weak electric field nonuniformities and the characteristic negative charge of isolated particles in a plasma. Field nonuniformities appear to be influenced by the topography and material design of the tool. For example, the presence of a Si wafer often induces significant particle trapping. Examples of particle trapping in a laboratory system are given, and similar phenomena are also verified in a manufacturing sputter deposition tool operating in a class 100 cleanroom. The implications of particle trapping in plasma processing are discussed.

Plasma particulate contamination control. II. Self‐cleaning tool design

Macroscopic particles are often observed in etching, sputtering, and deposition plasmas, in laboratory and manufacturing tools, in concentrations far exceeding ambient cleanroom conditions and acceptable process specifications. Controlling particle contamination is an important goal in modern fabrication lines. Controlling plasma‐generated particulates is especially important as these tools and processes are a major source of wafer contamination. Plasma particles acquire negative charge by electron attachment and are influenced by electric fields and plasma inhomogeneities. Improper tool design exacerbates tool contamination problems by trapping particles near wafers or other sensitive surfaces. Conversely, proper tool and electrode design can provide greatly improved contamination results. The concept of a self‐cleaning plasma tool is presented for the first time, along with evidence of the effectiveness and usefulness of this approach in laboratory and manufacturing studies. The method does not reduce t...

Spatially resolved detection of O atoms in etching plasmas by two‐photon laser‐induced fluorescence

Spatially resolved concentration profiles of ground‐state oxygen atoms in O2/Ar plasmas have been obtained under loaded etching conditions through the use of a two‐photon laser excitation process. These results provide a quantitative measure of the reactive atom concentration gradient during etching of kapton or graphite on the rf‐driven electrode. The effects of load, ion bombardment, and diffusion on the reactive atom concentration may be directly monitored by this in situ, unobtrusive, three‐dimensional probe technique.

Laser detection of diatomic products of plasma sputtering and etching

We report on in situ detection of diatomic products of plasma sputtering and reactive ion etching using the technique of laser‐induced fluorescence. The diatomic molecules SiN, SiO, and SiF are observed in the gas phase when a silicon surface is subjected to ion bombardment in plasmas containing N2, O2, and CF4, respectively. Information about the production mechanisms is obtained from the measured product concentrations under varying plasma conditions.We report on in situ detection of diatomic products of plasma sputtering and reactive ion etching using the technique of laser‐induced fluorescence. The diatomic molecules SiN, SiO, and SiF are observed in the gas phase when a silicon surface is subjected to ion bombardment in plasmas containing N2, O2, and CF4, respectively. Information about the production mechanisms is obtained from the measured product concentrations under varying plasma conditions.

Detection of Cl and chlorine‐containing negative ions in rf plasmas by two‐photon laser‐induced fluorescence

Chlorine atoms have been detected in rf etching plasmas of CClF3 and CCl2F2 with three‐dimensional spatial resolution using a two‐photon laser‐induced fluorescence technique. The spin‐forbidden 4p(4S0)–3p(2P0) transition is pumped by absorption of two 233.3‐nm laser photons. Decay of the excited state results in 725–775 nm emission. Spatially resolved plasma concentration measurements under certain etching conditions indicate an anomalously large signal spike at the plasma/sheath boundary, an effect attributed to an aggregation of chlorine‐containing negative ions which are also detected by this technique. The negative ions are detected by laser‐induced photodetachment followed by two‐photon excitation of atomic Cl.

Self-consistent fluid modeling of radio frequency discharges in two dimensions

Results from a two‐dimensional (2D) fluid simulation of a parallel plate, capacitively coupled radio frequency discharge bounded by a cylindrical insulator with a grounded exterior surface are presented. We find that the radial sheath at the insulator focuses current into the plasma region adjacent to the sheath. This 2D effect has important ramifications for the ionization rate, which peaks sharply in the metal‐insulator corners. We have experimentally observed the enhancement of the emission rate in a corner using spatially resolved optical emission spectroscopy. A ‘‘thick’’ insulator yields radial profiles for the time‐averaged plasma density and potential that are essentially uniform. A ‘‘thin’’ insulator, however, results in an off‐axis maximum in the plasma density and potential due to the corner ionization.

Trapping and behavior of particulates in a radio frequency magnetron plasma etching tool

Particle contamination has been studied in a commercial, magnetron reactive ion etching tool using rastered laser light scattering. Particles present in the plasma were imaged in real time, showing their spatial position and motion. The effect of the rotating magnetic field was also studied. In the absence of the magnetic field, particles were trapped near the inside circumference of the wafer clamp ring directly over the edge of the wafer, near the protruding ‘‘fingers’’ contacting the wafer. With the rotating magnetic field, particles moved in a wave like motion within the confines of the clamp ring and near the interior tool walls. Magnetron operation also resulted in a decrease of the sheath thickness, suspending particles closer to the wafer. Several process changes were shown to promote particle removal from the plasma.