Lawrence J. Overzet

Modulated discharges: Effect on plasma parameters and deposition

The initial motivation for using amplitude modulated (kHz) rf discharges was as an aid for ascertaining the temporal evolution of various transient species which are formed from the feedstock commonly used for the deposition or etching of semiconductors. Such gases are easily dissociated and hence the density of some of the transient molecules may easily exceed the remaining fraction of the donor gas. From a diagnostic point of view, the use of ‘‘time’’ will be shown to be a cheap and a most valuable aid in ascertaining some of the kinetics of such glows. Most surprising was the fact that the use of a modulated discharge can have a profound effect on the processing goal itself. For instance, it has been shown that the bandgap of amorphous silicon deposited from SiH4 is lower with a modulated discharge than that deposited from a continuous wave (cw) one; simultaneously, the amount of ‘‘dust,’’ as measured by Mie scattering, is also much lower. These experiments and others using etching gases point to a rol...

Enhancement of the plasma density and deposition rate in rf discharges

The peak and time averaged electron density in rf excited silane‐helium mixtures increased significantly above the cw value by square wave modulating the source. The deposition rate of amorphous hydrogenated silicon films is also enhanced and apparently follows the electron density. Attachment to the discharge products appears to be responsible.

Enhancement of the negative ion flux to surfaces from radio‐frequency processing discharges

The relative flux of negative ions from rf discharges through CF4 and F2‐He has been studied using mass spectrometry. The negative ion flux impinging on a surface in contact with a glow discharge is usually quite small because of the sheaths formed to contain the more mobile electrons. In the afterglow, however, these sheaths decay on the time scale of the electron density loss rate while negative ions can remain in the discharge region significantly longer. As a consequence, the negative ion flux to a surface in contact with the glow discharge can rise by a very large amount in the afterglow when the sheaths have diminished. It is shown here that the flux of the negative ions can be enhanced by factors of 50 to greater than 1000 by square wave amplitude modulating the rf discharge. This enhancement depends upon the dc self‐bias voltage and the modulation frequency as well as the particular negative ion. Data are presented for both CF4 and F2‐helium discharges, along with the correlation to simple models.

Comparison of electron-density measurements made using a Langmuir probe and microwave interferometer in the gaseous electronics conference reference reactor

A Langmuir probe and a microwave interferometer have been combined to measure the electron density of argon glow discharges in the Gaseous Electronics Conference reference reactor [Bull. Am. Phys. Soc. 36, 207 (1991)]. The two techniques indicate the same charge density at 100 mTorr to within 30%. This 30% difference is easily explained by the experimental peculiarities. While the predicted charge densities obtained from the two techniques track one another as the applied rf voltage is varied at constant pressure, they do not track one another as the pressure is varied at constant rf voltage. In fact, the charge densities predicted from the Langmuir probe using Laframboise’s theory (Tech. Rep. 100, Univ. Toronto Inst. Aerospace Study, 1966) are factors of 2 and 4 times lower than those from the interferometer at 250 and 500 mTorr, respectively. It appears that the probe alters the charge density in its vicinity when the probe radius becomes greater than the ion mean free path. The interferometer has also ...

Modeling and measurements of the negative ion flux from amplitude modulated rf discharges

The time dependent as well as amplitude modulation frequency dependent ion flux from parallel plate radio frequency discharges have been measured by mass spectrometry and modeled using the fluid equations. The negative ion flux to the grounded electrode becomes large after the rf excitation is turned off and has complex temporal features which depend upon the ion, gas mixture, applied dc bias voltage and other externally controlled parameters. Some of these features can be explained with the assistance of the fluid equation model. In short, negative ions can be formed during the active glow as well as after the glow excitation has been turned off. The resulting negative ion flux to the grounded electrode during the afterglow is determined in part by when the negative ion is formed. For example, if the amplitude modulation frequency at which the flux of a negative ion goes to zero is larger than 100 kHz, it indicates that ion is formed in the afterglow. (Of course, it may exist during the active glow as we...

Spatial variations in the charge density of argon discharges in the Gaseous Electronics Conference reference reactor

The spatially dependent ion and electron concentrations of argon discharges in the gaseous electronics conference reference reactor reach a saddle point maximum in the center of the glow. The concentrations were measured using a Langmuir probe. The charge density decreases in the axial direction from the glow center to the electrode sheath edges as expected. In contrast, the charge density increases in the radial direction from the glow center to the radial electrode edge. The maximum occurs approximately 1 cm inside of the actual electrode edge. The most plausible explanation for this increase is enhanced ionization at the electrode edges due to field enhancement there. The electric field is enhanced by the close proximity of the ground shield and driven electrode at the radial edge of the electrode. We estimate that the ionization rate in the plasma near the electrode edge must be almost double of that in the glow center.

Improvements to the floating double probe for time-resolved measurements in pulsed rf plasmas

A double probe is particularly well suited as a pulsed radio-frequency discharge diagnostic because it imposes minimal perturbation on the plasma, can follow the variations in the plasma potential easily, and requires no fixed potential reference from the plasma reactor walls. As pulsed discharges gain more attention in the semiconductor processing industry the double probe may be a more widely applicable diagnostic than the single probe, especially in plasma reactors which have insulating or semi-insulating walls. In order to use a double probe to make fast time-resolved measurements of electron temperature and ion density in a pulsed discharge, care must be taken to eliminate stray capacitance in the measurement circuitry. This difficulty was overcome by using optically isolated circuitry to achieve an ultimate time resolution of better than 500 ns. There must also be a reliable method for cleaning the probe. This was achieved by adding a third, reference electrode to serve as a needed ground reference....

Negative Ion Extraction from Pulsed Discharges

Time-resolved measurements of pulsed discharges can provide information on how negative ions can be used for surface processing. Negative ions are ordinarily trapped inside the plasma volume, but pulsed plasmas allow for efficient negative ion extraction during the afterglow period because the negative ion to electron concentration ratio can increase dramatically. In addition, high-density sources can facilitate negative ion extraction because of their thin sheaths and remote position with respect to the processing wafer. In either case, the first negative ions to reach a processing surface are likely to have crossed the bulk of the sheath region as electrons and attached near the surface.

Negative and positive ions from radio frequency plasmas in boron trichloride

The negative and positive ion mass spectra from radio frequency glow discharges in boron trichloride and 90% argon‐10% boron trichloride mixtures have been investigated by amplitude modulation of the discharge excitation. The negative ion spectrum from pure boron trichloride was found to extend beyond 200 amu under most conditions even though positive ions above 120 amu were not detected under any conditions. Surprisingly, only trace signals of BCl−3 were found from the discharges, while the signals from Cl−, BCl−4, B2Cl−4, and B2Cl−5 were much larger. We were unable to detect either positive or negative ions above 120 amu from 10% boron trichloride discharges.

Ion energy control in an insulating inductively coupled discharge reactor

An electrically insulating plasma reactor with a Faraday shielded inductive source antenna permits direct control of the dc plasma potential of the discharge. This control may be used to provide a tailored ion energy distribution at a substrate of fixed potential or to fix the plasma potential at a chosen value. With a reactor constructed from Pyrex tubing and a Faraday shielded inductive source coil, all that is required to control the plasma potential is a small electrostatic probe in contact with the plasma. By applying a stairstep potential to the probe, it is possible to create an ion energy distribution possessing virtually any desired shape. Insulating reactors could provide a novel way to control ion production and ion energy separately without the need for direct substrate biasing. They may also provide a way to maintain a consistent plasma potential in a processing reactor in situations where the reactor walls are easily contaminated by the process.