Kurt L. Haller

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.

Double-beam laser absorption spectroscopy: shot noise-limited performance at baseband with a novel electronic noise canceler

In a high J-value scheme (photo-excitation sequence), the authors investigate the characteristics of three-step photo-ionization, through an autoionizing level, of a complex atom using three single-mode pulsed dye lasers. The report covers (1) ion yield dependence on the balance of three laser intensities; (2) AC Stark effect, observed in intermediate excitation; and (3) multi-photon-resonance effect in a stepwise near-resonant excitation. The experimental results are discussed through comparison with the theoretical analyses, that include the effects of magnetic sublevel degeneracy.

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.

In-situ Particulate Contamination Studies In Process Plasmas

Laser light scattering measurements show that a variety of processing plasmas used during semiconductor fabrication produce significant amounts of in-situ, particulate contamination. The particles are produced by chemical and/or mechanical means during plasma exposure. In etching plasmas, simultaneous measurement of particulates by laser light scattering and of plasma negative ions by two-photon laser-induced fluorescence indicates the particles are negatively charged and are electrostatically trapped at the sheath boundaries. Similar observations have been obtained in sputtering plasmas. Mechanisms for particle formation are suggested. In some cases, nucleation and growth from plasma negative ions and etch products is indicated. In other cases, stress-inducing processes may fracture thin films on chamber surfaces thereby injecting particles into the plasma. These particles become negatively charged by acquiring electrons from the plasma. In each case, the particles are suspended at the sheath boundaries and drop onto the wafer when the if power is turned off, thereby contaminating critical product surfaces. The use of an inexpensive HeNe laser for monitoring particle contamination levels during processing is discussed along with implications of this work for dry process equipment technology.