Hideo Eto

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.

Monitoring Reaction Products of Novolac Resists during Puddle Development

We have developed a new technique for monitoring the concentration of reaction products during puddle development in ultralarge-scale integration (ULSI) lithography. Optical measurement has been employed for monitoring the concentration of reaction products by detecting reflected light from a wafer surface, and compared with the sampling method (UV–visible spectroscopy) for reaction products from a developer solution. A correlation coefficient of 0.89 was obtained, indicating a good correlation between this optical method and sampling method. We concluded that the newly developed optical monitoring technique is useful for measuring the concentration of reaction products.

Effect of temperature on photoresist critical dimension during puddle development

We have investigated the effect of a temperature change on photoresist critical dimension (CD) in a wafer during puddle development. The wafer temperature was decreased by the evaporation latent heat of developer solution during puddle development, and the rate of temperature decrease of the peripheral area was higher than that of the central area in the wafer. The temperature of the peripheral area was approximately 1.3 °C lower than that of the central area after 60 s. The temperature distribution was caused by the difference in heat capacitance in the wafer, which was mainly influenced by the wafer chuck. We investigated the influence of temperature on photoresist CD in the wafer, using a diazonaphthoquinone (DNQ)/novolac photoresist. Photoresist CD changed with temperature at a rate of approximately 5 nm/°C, and the CD of the peripheral area became smaller than that of the central area over time. We can improve the CD distribution by controlling the temperature during puddle development or by using a photoresist with a dissolution rate that is less sensitive to temperature.

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.