Single substrate equipment for removing resist used in the manufacture of LCDs was developed using Mitsubishi Electric's high concentration clean ozone generation technology, technology for high speed removal of resist using ozone, and SPC's washing equipment technology. It is the first resist removal equipment in the world that does not use expensive chemicals (resist removal liquid^*4).

This equipment does not use the expensive resist removal liquid. Consequently, its running cost can be reduced to 1/5 of that of conventional equipment. Furthermore, there is no need to treat waste water including removal liquid, thus enabling environmental impact to be reduced to 1/10 or less. In addition, the use of ozone permits the resist removal process to be integrated with the substrate washing process that was previously necessary after the resist was removed, thus enabling installation space to be reduced by about 1/2. Furthermore, single substrate process equipment is used by the line conveying method, enabling the equipment to easily handle large LCD substrates for television sets, and so on. It is also possible to use this equipment instead of a conventional types, which use removal liquid, or to use it in combination with a conventional types.

Development Background

In the manufacture of LCDs, the processes of pattern fabrication (film deposition, resist coating, exposure, development, etching, and so on), resist removal, and substrate washing are repeated several times, then transistors are formed on the glass substrate. Previously, removal liquid was used in the resist removal process, and also a combination of ultraviolet light irradiation and water washing was used for cleaning the substrate (Fig.1). However, the removal liquid was expensive (at least several hundred yen per liter), and was used in large quantities (at least a several hundred cc per 1m² of the substrate). Consequently, the cost reduction of removal liquid and the minimization of environmental impact of waste liquid were important issues.

Ozone is a powerful oxidizing agent. After taking part in a chemical reaction, it disassociates and reverts to oxygen, hence it leaves no residue and causes no environmental impact. Experiments aimed at realizing a resist removal process have been carried out for some time, however the removal rate was too slow to be practical.

Development Details

Since 1997, Mitsubishi Electric has been marketing clean type ozonizers that generate ozone at high concentrations (200 g/Nm^3*5) or higher) with high efficiency. Now, the company has developed a new resist removal method employing high concentration ozone. This method can remove resist about 10 times faster and is more practical than previous ozone methods.

1. Development of high-speed resist removal technology
   Up to now, research has been carried out on a method of removing resist by causing it to react with ozone gas at high temperature over 250°C and also a method of removal by immersing the substrate in ozone water made by dissolving ozone in pure water. However when ozone gas is used, the rate of removal of the resist is about 0.01 µm/minute at room temperature, and even when ozonated water is used, the removal rate is about 0.1 µm/minute. In other words, the removal rates of these methods are much lower than that of existing processes using removal liquid (several µm/minute); hence these methods are impractical.
   
   Mitsubishi Electric has succeeded in raising the resist removal speed by using ozone gas that has a concentration of 200 g/Nm³ or more. In addition, as a result of performing a detailed study of the resist removal mechanism, it was found that the resist undergoes hydrolysis after being oxidized by the ozone. Mitsubishi Electric therefore investigated several ways of supplying ozone and water. As a result, they were able to realize a resist removal rate of about 1 µm/minute or more, which is about 10 times as high as that obtained using conventional ozone methods.
   
   
2. Realization of the line conveying method single substrate process equipment
   The developed equipment is a single substrate equipment that can perform line conveying (Fig.2). Consequently, it can handle large substrates, enabling it to help reduce the cost of manufacturing large substrates used for television screens, and so on. It is also possible to use this equipment in place of conventional types, which uses removal liquid, or to use it in combination with conventional types.
   
   
3. Reduction of running costs, environmental impact and equipment floor area
   Resist can be removed by using ozone gas generated only from the raw gas (oxygen gas). Consequently, no costly removal liquid is needed, and the running cost can be reduced to about 1/5 of that of a conventional method.
   In the previous method, organic compounds in waste water mainly consisted of removal liquid. Since the resist removal process using ozone requires no removal liquid, the amount of organic compounds in waste water can be reduced to less than 1/10.
   Also, the oxidizing power of ozone enables organic residue on the substrate to be dissociated by oxidation, and removed. Consequently, the previous substrate washing process followed by the resist removal process is no longer needed. This in turn eliminates the need for washing equipment that uses ultraviolet light, for example, after the resist has been removed. Consequently, the area of the equipment can be reduced to about 1/2 of that of previous equipment.
   
   
4. Resist removal and washing processes that do not involve the application of physical force
   This equipment makes use of the chemical reaction of ozone. Consequently, the resist removal and substrate washing processes do not involve the use of physical forces, such as brush cleaning or ultrasonic power, that are likely to chip the substrate or damage the wiring. The overall process is therefore highly reliable and safe.

Future Developments

From November 2000, SPC will start to test samples from LCD manufacturers. It will also exhibit panels showing the new resist removal equipment for LCD manufacturing at the LCD/PDP International 2000 Exhibition between October 25th and 27th, and will commence marketing the new equipment in January 2001.

In addition to LCD resist removal, Mitsubishi Electric intends to apply the new technology to semiconductor resist removal and wafer washing as well.