M Marco Haverlag

Fluorocarbon high‐density plasmas. I. Fluorocarbon film deposition and etching using CF4 and CHF3

Fluorocarbon film deposition in discharges used for oxide etching plays a key role in determining the profile shape of contact holes and the etch selectivity with respect to the mask and the underlayer. For low‐density capacitatively coupled rf discharges this deposition is due to neutral radicals. We report a study of fluorocarbon film deposition and etching phenomena in electron cyclotron resonance (ECR) discharges of CF4 and CHF3. Plasma operation without rf sample bias in the pressure range below 10 mTorr results in the deposition of fluorocarbon films for both gases, with the highest deposition rate in each case at 2 mTorr (≂120 nm/min for a 1000 W CF4 plasma and ≂180 nm/min for CHF3 using the same conditions). For CF4 this behavior differs dramatically from that seen for conventional rf diode plasmas where no deposition occurs. The deposition is due to the more efficient breakdown and ionization of CF4 and CHF3 in the ECR discharge and the lack of energetic ion bombardment of the substrate as compar...

Production and destruction of CFx radicals in radio-frequency fluorocarbon plasmas

Spacially resolved densities of CF, CF2, and CF3  radicals in capacitively coupled 13.56 MHz radio‐frequency (rf) discharges in CF4 and CHF3 were determined by means of infrared absorption spectroscopy employing a tunable diode laser spectrometer. It was established that the stationary CF2 density and density profile in a CF4 plasma depend strongly on the electrode material. This is attributed to different sticking coefficients of CF2 on different surfaces. Furthermore, it was found that the densities of all CFx radicals increase near the electrodes at high gas pressures and rf powers in a CHF3 plasma. This leads to the conclusion that production of CFx radicals takes place in the sheath region close to the electrodes. It is proposed that collisions between ions and source gas molecules are responsible for this production of CFx radicals. In the presence of a destruction process in the plasma glow (e.g., by three‐body recombination with other radicals) and the absence of a fast surface loss process this r...

Measurements of negative ion densities in 13.56‐MHz rf plasmas of CF4, C2F6, CHF3, and C3F8 using microwave resonance and the photodetachment effect

The high‐power density of a frequency quadrupled pulsed Nd‐YAG laser has been used to photodetach electrons from negative ions in rf plasmas generated within a microwave cavity. Negative ion densities have been determined by measuring the frequency shift of the resonance transmission, the shift being caused by the photoelectrons created by irradiating the plasma with the laser pulse. By measurement of the shape of the resonance curve as a function of time and of microwave frequency, and consecutive fitting of a parabola to the top of the resonance curve, the negative ion density has been determined as a function of gas pressure, rf power, and position in the plasma. Measurements were performed in plasmas of CF4, C2F6, CHF3, and C3F8. The results indicate that the negative ion densities are about one order of magnitude larger than the electron density, which is in good agreement with a fluid model calculation. The pressure and power dependence of the electron density and of the negative ion density gives i...

Measurements of radical densities in radio‐frequency fluorocarbon plasmas using infrared absorption spectroscopy

Densities of CF2 radicals, rotational temperatures, and the degree of dissociation in radio‐frequency fluorocarbon plasmas have been measured using Fourier transform infrared absorption spectroscopy and tunable diode laser infrared absorption spectroscopy. The CF2 densities obtained in CF4, CHF3, C2F6, and CF2Cl2 plasmas indicate that the partial pressure of CF2 is around 1%–5% of the total pressure. From the spatial dependence of the CF2 density it was established that at high pressure, CF2 is produced either on the rf electrode or close to the rf electrode. Furthermore, a comparison between measured absorption spectra and a simulation of the rotational distributions has revealed that the rotational temperatures of CF4, CF2, and HF are all close to room temperature. FTIR spectra indicate that in plasmas of gases with a low F/C ratio (due to the presence of H or Cl) the source gas is converted for a significant part into other species.

Temporal behavior of the electron and negative ion densities in a pulsed radio‐frequency CF4 plasma

Electron and negative ion densities in the afterglow and in the plasma initiation phase of a 13.56‐MHz rf discharge in CF4 were measured by using a microwave cavity method and a laser photodetachment technique. Measurements were carried out at low rf powers (≲10 W) and in the pressure range from 100 to 300 mTorr. The electron density in the afterglow showed an enhanced decay rate due to the presence of negative ions. Electrons originating from negative ions through associative collisional detachment with neutral radicals were also detected in the afterglow. Decay curve analysis of the negative ion density gave an ion–ion (presumably CF+3−F−) recombination rate constant of (5±2)×10−13 m3 s−1, and showed that, in the active plasma, the negative ion loss rates by associative detachment and ion–ion recombination are of the same order of magnitude. The behavior of the electron and negative ion densities in the plasma initiation phase indicates that molecules and radicals that slowly accumulate in the plasma do...

Sidewall passivation during the etching of poly‐Si in an electron cyclotron resonance plasma of HBr

The etching of poly‐Si (n+ and undoped) in a radio‐frequency‐biased electron cyclotron resonance plasma of HBr was studied. Etch rates of Si, oxide, and photoresist were obtained by ellipsometry as a function of the bias voltage and two substrate temperatures (15 and 50 °C) at 5 mTorr pressure. The etch rate of poly‐Si depends on the doping level, with n+ Si etching faster than intrinsic Si. High selectivities of Si over both oxide and photoresist can be achieved at low bias voltages. Using angle‐resolved x‐ray photoelectron spectroscopy, it is shown that a carbon and bromine containing layer is deposited on the sidewall of the poly‐Si during the etching process when a photoresist patterned wafer is used. The thickness of the sidewall film decreases with increasing substrate temperature and increases at the bias voltage is raised. The thickness of this sidewall film influences the anisotropy of the process, with some undercutting occurring at high temperature and low bias voltage.

Speed of streamers in argon over a flat surface of a dielectric

A pin–pin electrode geometry was used to study the velocities of streamers propagating over a flat dielectric surface and in gas close to the dielectric. The experiments were done in an argon atmosphere, at pressures from 0.1 to 1 bar, with repetitive voltage pulses. The dielectric surface played a noticeable role in discharge ignition and propagation. The average speed of the discharge decreased with higher pressure and lower voltage pulse rise rate. It was higher when the conductive channel between the electrodes was formed over the dielectric, rather than through the gas. Space resolved measurements revealed an increase in velocity of the discharge as it travelled towards the grounded electrode.

Measurement of the gas temperature in fluorocarbon radio frequency discharges using infrared absorption spectroscopy

The translational gas temperature was measured in 13.56 MHz radio‐frequency (rf) discharges in CF4 and CHF3. Infrared absorption spectra of CF4 and CF2 were recorded using a tunable diode laser and the gas temperature was deduced from the linewidths of the absorption lines of these molecules. It is shown that linewidth measurements yield a simple and direct method to determine the gas temperature, with an accuracy up to ∼10 K. The results obtained in CF4 and CHF3 plasmas indicate that the translational temperatures of all particles investigated in these plasmas are, at most, 50 K above the room temperature. The temperature increases with increasing gas pressure and rf power, but it is independent of the flow rate. This is attributed to an increased heating rate of the gas. Moreover, it was found that the temperature rise is significantly smaller in CHF3 than in CF4, under the same plasma conditions. This can be attributed to a higher power dissipation by chemical conversion of the parent gas in a CHF3 dis...

Photodetachment effect in a radio frequency plasma in CF4

Experiments to study negative ion densities have been carried out using the photodetachment effect in a rf plasma in CF4. Electrons are detached from the negative ions under the influence of the pulse of a Nd:YAG laser. The induced increase of the electron density is measured as a function of time using the shift of the resonance frequency of a microwave cavity containing the plasma. The negative ion density [F−] is found to be about (4±1)×1015 m−3, a factor 4±1 higher than the electron density.

Amorphous hydrogenated silicon films produced by an expanding argon-silane plasma investigated with spectroscopic IR ellipsometry

Abstract We have produced amorphous hydrogenated silicon (a-Si:H) films from silane with an unconventional deposition technique, a supersonically expanding d.c. arc plasma. The deposited films are analysed mainly by using spectroscopic IR ellipsometry. Further analysis has been performed with scanning electron microscopy, IR absorption spectroscopy and in situ He-Ne ellipsometry. The film structure appears to be strongly linked to the degree of ionization of the expanding beam, the injection location of silane gas, the degree of dissociation and the percentage of the injected hydrogen gas. Deposition at low arc power results in films of polysilane, which are very sensitive to oxidation during air exposure. Without hydrogen injection, films with a high refractive index and low hydrogen content are obtained (below the detection limit of the IR transmission spectrometer). Hydrogen injected in the middle of the plasma arc results in a-Si:H films with a refractive index of 3.75 at 632 nm; this value is close to the indices of the best films obtained with plasma-enhanced chemical vapour deposition (PECVD). In these films, the strength of the vibrational absorption at 2000 cm -1 , which can be assigned to SiH stretch bonds, is equal to the strength of a vibration at 2085 cm -1 . Because the bending absorptions of SiH 2 at 860 and 890 cm -1 are not detected in the films produced, it is concluded that this 2085 cm -1 absorption peak in our films is caused by bond stretching of SiH rather than by that of SiH 2 . As in PECVD, the optimum substrate temperature at which films of good quality are obtained is in the range from 525 to 575 K. The deposition rate is of the order of several nanometers per second.