Tatehito Usui

Measurement of Intensity and Polarization of HeI Forbidden Lines for Diagnostics of Electric Field in a Plasma

Intensity of the HeI 491.1 nm (41P→21P) forbidden line due to the Stark effect, with respect to the HeI 396.5 nm (41P→21S) allowed line, was measured to obtain radial distribution of the electric field in a hollow cathode discharge. The polarization of the lines was also observed to show a capability of detecting the field direction in plasmas. The measured electric field distribution satisfactorily yielded the cathode fall potential, and the polarization was well in agreement with the theoretical prediction. The experiment demonstrated that field strength as low as 250 V/cm could be accurately measured using the present line pair.

Forbidden Transitions in Helium and Lithium Due to Fluctuating Electric Fields for Plasma Diagnostics

Suitable line pairs of the forbidden and the allowed lines in the helium and lithium lines are proposed for measurement of fluctuating electric fields with low frequency in plasmas. The forbidden line intensity with respect to the allowed one is calculated as a function of the electric field strength by using the quasi-static theory. For the electric field of more than several kV/cm, the forbidden line intensities are detectable. The fluctuating electric fields are observed in a small linear theta pinch plasma by using the proposed method. Two line pairs of HeI (HeI 491.1 nm/HeI 396.5 nm and HeI 663.2 nm/HeI 501.6 nm) are used for the observation. Both results obtained almost agree within the experimental uncertainty. The peak intensity of the observed fluctuating field is 5 kV/cm in the early phase of the implosion of the plasma.

Relationship between formation of surface-reaction layers and flux of dissociated species in C4F8/Ar plasma for SiO2 etching using pulsed-microwave plasma

The mechanism of highly selective etching of SiO2 using pulsed-microwave electron-cyclotron-resonance plasma was investigated by analyzing the relationship between plasma dissociations and fluorocarbon layers formed on surfaces during etching with a cyclo-C4F8/Ar gas mixture. Dissociated molecules of CxFy and CFx species were measured without fragmentations using ion attachment mass spectrometry, and both thicknesses and atomic concentrations of reaction layers formed on etched surfaces were analyzed using x-ray photoelectron spectroscopy. Thus, the impact of CxFy molecules on the formation of fluorocarbon layers were analyzed using this measurement system. The authors found that the process window of highly selective etching of SiO2 over Si was enlarged by using pulsed-microwave plasma because a thinner fluorocarbon layer was formed by controlling C4F8 dissociation by changing the duty cycle of the pulsed-microwaves. With conventional continuous plasma, an etch stop occurred at low wafer bias conditions ...

Pattern-size effect on electron shading

The effect of pattern size on electron shading over a line-and-space photoresist pattern on a MOS capacitor was determined by a numerical simulation. It was found that it takes longer for the potential field to attain the steady state as the pattern shrinks and that the steady-state electron shading is independent of pattern size; that is, the potential field is linearly scalable. We also revealed that the gate-voltage change can be easily analyzed by a simple electric circuit model. Moreover the capacitance between the gate electrode and the line-and-space photoresist lines can be estimated by a simple geometrical approximation.

Effect of plasma dissociation on fluorocarbon layers formed under C4F8/Ar pulsed plasma for SiO2 etching

We investigated the effects of gas pressure on the dissociation of C4F8/Ar plasma and the formations of fluorocarbon layers on etched materials by SiO2 etching using pulsed-microwave electron-cyclotron-resonance plasma. Dissociated radicals and molecules of C x F y and CF x species generated from C4F8/Ar plasma were measured by ion attachment mass spectrometry, which is a fragment-free method. The thickness and chemical state of the fluorocarbon layers formed on the etched materials were analyzed by X-ray photoelectron spectroscopy. Higher selective etching of SiO2 relative to Si3N4 was possible at 2.0 Pa but not at 0.5 Pa. Thick fluorocarbon layers, which protect etched materials from ion bombardment, formed on both SiO2 and Si3N4 surfaces; thus, both the SiO2 and Si3N4 etching rates were reduced at a high pressure. However, the CF2 flux ratio increased while the flux ratio of C2F2 decreased as the pressure increased. The increased flux of CF2 enhanced SiO2 etching because CF2 is the main etchant of SiO2. This CF2-rich plasma formed CF2-rich fluorocarbon layers, enhancing the SiO2 etching reaction.

A novel wireless on-wafer plasma sensor using light-emitting diodes

We developed a novel wireless, on-wafer plasma sensor that uses light-emitting diodes (LEDs). This sensor measures the potential differences during plasma exposure that are a factor in charging damage occurring in semiconductor devices. The sensors measurement principle is based on the diode characteristics and the measurement is very easy; we need only to expose the sensor to plasma and count the in-series LEDs that are bright. If they are bright, it means the applied voltage exceeds a known critical value. If they are dark, it means the voltage is below the value. Thus the sensor uses light for data acquisition, and no wire connections are necessary. Furthermore, it can be handled like a standard wafer. Its fundamentally quick and inexpensive measurement feature makes it highly advantageous for application to conventional measurements. This novel sensor makes it possible to measure plasma properties and minimize the potential of charging damage in real time.

A new current-balance model for predicting charging damage during high-temperature plasma processes

We formulated a simple circuit model to investigate the mechanism of the charging damage during high-temperature plasma processes. This model only takes into account the balance of the current that comes into and goes out of the silicon substrate during PECVD. The current paths are plasma to substrate (and vice versa) and electrodes of the electrostatic chuck or susceptor to substrate (and vice versa). We determined the substrate potential with this circuit model and evaluated the degree of charging damage over a wafer. This simple circuit model shows that the low resistance between the substrate and the electrodes of the chuck or the susceptor drastically increases the charging damage of the gate oxides, but neither the clamp voltage of the dipole-type electrostatic chuck nor the leakage current influences the charging damage. These results agree fairly well with experimental measurements. The model also explains the experimental observation that a 100 nm thick oxide on the backside of the wafer significantly reduces the damage, but a thicker oxide only slightly reduces it.

Spectroscopic Investigation of Recombination Process in a Pulsed Discharge Plasma Using Laser Induced Fluorescence Technique

The temporal behavior of the population density of the first excited level of the hydrogen atom in a pulsed discharge plasma of a helium and hydrogen mixture has been measured using the laser induced fluorescence technique, while the population densities of the higher excited levels have been determined from the emission measurements. The population density of each level shows a rapid enhancement in the early stage of afterglow. The experimental results are compared with the collisional-radiative model, including various dissociation processes of hydrogen molecules. The experimental results can be explained well with the model. The population mechanism of the first excited level in the afterglow is found to be sensitive to the optical escape factor of the Lα line

Temporal Behaviors of Hα Fluorescence Induced by High-Intensity Dye-Laser in Plasmas

The time-dependent collisional-radiative model including the effect of resonant absorption of the Hα line is applied to a hydrogen plasma in order to investigated the temporal behavior of the Hα fluorescence, which is used to determine the population density of the lower level. Four typical temporal behaviors of the fluorescence in plasmas are discussed. One of them has been found to useful for accurate determination of the population density. An experiment has also been made in a pulsed discharge plasma to demonstrate that a useful type of fluorescence can be achieved in a laboratory plasma.

An Application of Dye-Laser-Induced Fluorescence Technique to Absolute Calibration of a Vacuum Monochromator

An application is decribed of the dye-laser-induced fluorescence technique to improve the atomic branching ratio method of the absolute calibration of a vacuum monochromator and detector system. A model experiment shows that a new type of line pair, such as the Hα and the Lα line, of which the VUV line originates from the lower energy level in a transition relating to the visible or the near ultra-violet line, can be used for calibration by using the optical pumping technique.