Kayoko Omiya

Effect of Gas Addition on Ozone Ashing

The effect of gas addition on ozone ashing was investigated for the purpose of increasing the ashing rate and decreasing the ashing temperature. Ashing rate increased with the addition of alcohol gas or ammonia vapor to ozone gas. The higher the flow rate and ozone concentration, the greater was the effect of increasing the ashing rate, however, no effect was observed when the ashing rate was low due to the insufficient ozone concentration. Infrared spectra indicated that C-D bonds exist in the ashing-treated resist, when deuterated methanol is used as an additive. In the case of alcohol gas addition, hydrogen atoms of alcohol reacted with the double bonds of the resist to generate radicals in the resist, leading to an increase in the rate of oxidative decomposition of the resist. On the other hand, in the case of ozone ashing in the presence of ammonia vapor, the results of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy showed that N-H bonds were formed on the resist surface by the ashing. It is considered that species containing the N-H of ammonia were added to the resist structure to accelerate the oxidative decomposition of resist, leading to an increase in the ashing rate.

A Process for Photoresist Removal after Aluminum Etching Using Plasma Treatment in a Gas Containing Hydrogen

A photoresist removal process with plasma treatment after aluminum film etching was developed. During a previous reactive ion etching step, reaction products are deposited on the sidewalls of the patterns etched in the Al film and the photoresist on top of it. The deposited material is analyzed by X-ray photoelectron spectroscopy and found to consist of chlorides or Al. It is demonstrated that the deposited film can be removed by treatment in inductively coupled plasma using hydrogen-containing gas (methyl alcohol in the present study) with a radio frequency bias applied to the substrate. The optical spectra indicate that the reactive species are hydrogen and other reactive species (e.g., CH and OH) generated by dissociation of methyl alcohol.

Effect of CF4 Addition on Downflow Ashing under Atmospheric Pressure

We studied the rate increase of ozone ashing under atmospheric pressure. It was verified that when an ozonizer discharge was used under atmospheric pressure, the ashing rate increased to up to one and a half times the original rate with the addtion of CF4. F element and C–F bonds, which might appear as a result of the addition of CF4, were not detected on the resist surface by Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analyses. On the other hand, the formation of HF and an increase in CO and CO2 were observed by FT-IR analysis of the excited gas. Therefore, the increased ashing rate with the addition of CF4 is likely due to the active decomposition product which contains fluorine radicals produced in the discharge. These active species are transported under atmospheric pressure and cause H extraction from –OH groups which exist in the resist and result in improved oxidative decomposition.

Enhancement of Etching Rate of SiN Films by Addition of Gases Containing Hydrogen to CF 4 / O 2

We studied a method for increasing the etching rate of SiN films in CF 4 /O 2 downflow plasma. It was confirmed that the etching rate of SiN films deposited by plasma-enhanced chemical vapor deposition (PE-CVD) was 50-200 times higher than that of the films deposited by low-pressure chemical vapor deposition (LP-CVD). The activation energy for PE-CVD SiN (PE-SiN) was 0.62 kcal/mol and that for LP-CVD SiN (LP-SiN) was large, 5.3 kcal/mol, indicating that different etching reactions occurred. Assuming that the difference is caused by the many H atoms contained in PE-SiN, we compared the etching rates by adding gaseous H 2 , water, and methanol, which include H atoms, to CF 4 /O 2 . The results show that the etching rate increased by a maximum factor of 10 for LP-SiN and approximately a factor of 1.5 times for PE-SiN. From these results, we propose the following etching mechanism for the enhancement in the etching rate. Hydrogen added to CF 4 /O 2 reacts with N atoms on the SiN films to form NH 3 , which is then removed as a volatile component. With the removal of these N atoms, the SiN surface becomes Si rich; therefore, the etching reaction of Si atoms and the F radicals is accelerated, thereby increasing the etching rate.

A highly selective photoresist ashing process for silicon nitride films by addition of trifluoromethane

A highly selective photoresist ashing process was developed for the fabrication of thin-film transistor liquid-crystal displays (TFT-LCDs). This ashing process utilizes downflow plasma consisting of a carbon trifluoromethane/oxygen (CHF3/O2) gas mixture at a low temperature. The etching selectivity of photoresist films to silicon nitride (SiN) film increased when using the CHF3/O2 gas mixture plasma, as compared to that when using the carbon tetrafluoride/oxygen (CF4/O2) gas mixture plasma. At the CHF3 gas flow rate of 30 sccm, a high etching selectivity ratio of about 1080 for the photoresist films to the SiN films was achieved at room temperature. On the basis of surface analysis results for SiN films and plasma analysis results for the CHF3/O2 gas mixture, a mechanism for the high etching selectivity of the photoresist films was proposed. Reaction products that were formed on SiN films by the CHF3/O2 gas mixture plasma obstructed the etching of SiN films by fluorine (F) radicals, resulting in the high selectivity. It was found that the CHF3/O2 gas mixture plasma reacted with SiN, resulting in the formation of a protective reaction product that is considered to be an ammonium salt such as (NH4)2SiF6.

Structural Changes in a Resist Resulting from Plasma Exposure during the Reactive Ion Etching Process

Structural changes in a resist resulting from plasma exposure during the reactive ion etching (RIE) process were studied using 13C-NMR analysis and Fourier transform IR spectroscopy (FT-IR). Two types of resist were chosen containing different amounts of diazonaphthoquinone derivative as an initiator with the same type of base polymer, cresol novolak resin. It was found that a slightly soluble layer was formed by the cross-linking reaction of the main chain of novolak resin as a result of plasma exposure in the case of the resist with a low amount of photosensitizer. The molecular weight concurrently decreased, probably due to the chain-scission of the main chain of novolak resin. On the other hand, in the resist containing a larger amount of photosensitizer, the molecular weight of the material increased, probably due to the occurrence of some type of cross-linking reaction such as thermal condensation. From these results, we obtained some guidelines for developing a resist stripping process without a residue.

Photoresist ashing process using carbon tetrafluoride gas plasma with ammonia gas addition

A low-damage photoresist ashing process was developed for the fabrication of thin-film transistor liquid-crystal displays (TFT-LCDs). This process utilizes a downflow plasma using a carbon tetrafluoride/oxygen (CF4/O2) gas mixture at room temperature. Although this process simultaneously achieves a high ashing rate and a low etching rate for an underlying amorphous silicon (a-Si:H) film containing hydrogen (H), contact resistance increases. We achieved contact resistances of less than 2 kΩ by the addition of ammonia (NH3) gas into the CF4/O2 gas mixture plasma. The ratio of reactive fluorine radicals (F) to argon atoms (Ar) decreased with increasing NH3 gas flow rate and became less than 0.7 at the NH3 gas flow rate higher than 15 sccm. Reaction products formed on a-Si:H films by the addition of NH3 gas to the CF4/O2 gas mixture plasma obstructed the etching of the a-Si:H films by F. On the basis of plasma analysis results for the CF4/O2/NH3 gas mixture, a possible mechanism for low damage to a-Si:H films was proposed.

Highly Selective Photoresist Ashing by Addition of Ammonia to Plasma Containing Carbon Tetrafluoride

A highly selective photoresist ashing process performed at low temperature using a downflow plasma consisting of a carbon tetrafluoride/oxygen (CF 4 /O 2 ) gas mixture was developed for the fabrication of thin film transistor liquid crystal displays (TFT-LCDs). Although the ashing rate was increased by using the CF 4 /O 2 gas mixture plasma, the etching selectivity for underlying amorphous silicon (a-Si:H) films containing hydrogen decreased. The etching rate of a-Si:H films was decreased by the addition of ammonia (NH 3 ). Since the etching rate of a-Si:H films decreased to zero at NH, flow rates higher than 15 standard cubic centimeters per minute, an infinitely high etching selectivity for the photoresist films was achieved at room temperature, On the basis of the surface analysis results for a-Si:H films, a mechanism for the high etching selectivity of the photoresist films was proposed. Reaction products that were formed on a-Si:H films by the addition of NH 3 gas to CF 4 /O 2 gas mixture plasma obstructed the etching of a-Si:H films by fluorine (F) radicals, resulting in the high selectivity. It was found that the NH 3 gas that was added to CF 4 /O 2 gas mixture plasma reacted with a-Si:H, resulting in the formation of a protective reaction product which is considered to be an ammonium salt such as (NH 4 ) 2 SiF 6 .

38.3: High Selectivity Photoresist Ashing by the Addition of NH3 to CF4/O2 or CHF3/O2

A low temperature and high selectivity ashing process using CF4/O2 downfiow plasma in TFT-LCD fabrication was developed. Reaction products were formed on the underlying a-Si:H with the addition of NH3 gas to CF4/O2plasma. These products obstructed the etching of a-Si:H films by F radicals, therefore the selectivity increased.