Comparison of Low-pressure Mercury Lamp and Excimer Lamp

This is a comparison report of the cleaning and reforming performance of a low-pressure mercury lamp and an excimer lamp, and influence of distance.


A comparative investigation of the cleaning and reforming performance of a low-pressure mercury lamp and an excimer lamp was carried out. For light sources, our 200W light (Model name: EUV200GS) was used for the low-pressure mercury lamp, and U company's 4 barrier discharge xenon excimer lamps of 50W were used for the excimer lamp. The performance was evaluated using the contact angle of water drops.
As a result, both types of lamps were almost the same in their performance as shown in Figure 1. It's notable that, the radiation distance largely influenced the performance and there was a great difference between effectively used suitable distances (also referred to as the recommended distance) of both lamps. Short-wavelength UVs of 240 nm and shorter were absorbed by oxygen. However, particularly in case of the absorption of 172nm, since the absorption is strong and attenuation in the air is large, an extremely short distance is recommended for an excimer lamp. Photo-cleaning performance is largely dependent on distance, thus a generally practical limit is set for the distance. Its value is given in Table 1 as a critical distance based on vast experience.

1. Cleaning capacity of both lamps is equal

A low-pressure mercury lamp light source provides no window. On the other hand, the xenon excimer lamp light source often provides a quartz glass window as shown in the figure. Here, the physical distance between the work and the window near the light source was considered as the radiation distance. Test result is shown in the Figure1. In this test, the critical contact angle of the glass was approximately 3 degrees. The time to reach the critical contact angle for the excimer lamp at 1mm of radiation distance and for the low-pressure mercury lamp at 5mm of radiation distance was exactly the same. Even though the low-pressure mercury lamp was at 10mm of radiation distance, there was barely any change, but when the radiation distance for the excimer lamp was changed to 3mm, the reaction time became slightly longer.

2. Influence of distance on lowering the contact angle

Recommended and critical distances for both the light sources are given in Table 1. The other company's data indicates that the excimer lamp is more advantageous. However, setting the radiation distance used for the comparison, i.e., 30mm for the low-pressure mercury lamp and less than 3mm for the excimer lamp is unfair. In our test, regarding the change in contact angle, performances of both lamps at recommended distances are the same, but 30mm is not an advantageous distance for the low-pressure mercury lamp. Since the radiation distance setting range for the low-pressure mercury lamp is relatively wide, its usage is much easier compared to the excimer lamp.

Table1. Recommended distance for photo-cleaning light source

Light source (Recommended)
Suitable distance (mm)
Critical distance (mm)
Xe Excimer Lamp 1-3 8
Low-pressure Mercury Lamp 5-20 200

3. Influence of curing time after cleaning indicated by contact angle: Is cleanliness level different even at the same value?

When the influence of the curing time after photo-cleaning was checked, many interesting results were obtained. There was a phenomenon raising a question whether cleanliness can be determined just by using contact angle analysis. Further research is required regarding the evaluation methods of cleanliness. As shown in Figure 2 and Figure 3, amongst the surfaces reached at the same critical contact angle, during curing after the process, the surfaces with minimum processing time are restored to the contact angle sooner than ones with longer processing time. The phenomenon occurs in the process of removing the organic contaminants layer of the surface up to the monolayer by photo-cleaning, since it shows the critical contact angle value of glass if the base material layer is exposed even slightly. Also, when the organic contaminants layer is reduced, the contact angle does not go below the critical value, but pollution-free glass surface area in! creases. If the process time is shorter, the contact angle is restored sooner by curing. We assume that this is because floating organic compounds tend to be accumulated on an organic contaminated layer rather than on a glass surface.

This phenomenon will cause a problem in the nanometer world.
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