We choose thermal treatment as part of a methodology to remove chemical residue on the surface of a mask. This new step of thermal treatment is inserted into our standard cleaning process for embedded attenuate phase shift masks (EAPSMs). The treatment is carried out in a modified hot plate system at various temperatures and times. After thermal treatment, ion chromatography measures the residual ions on a given surface. The thermal treatment is found to considerably reduce residual sulfate ions on the mask surface. The remaining sulfate ions on the mask are <0.18 ng/cm2 using thermal treatment.
In recent years, organic compounds have been clarified as one of major root causes of the haze, and carbon and amine components are major of them to organize. Therefore, both two should be controlled simultaneously for preventing haze defects on photomask. It is well known that UV/O3 treatment has a strong efficiency of removing organic matters1. For that reason, we have inserted it into our cleaning process, especially for EA.PSMS. And the surface variation of mask, after UV/O3 treatment, has been confirmed by the change of surface free energy on it. And organic matters remaining on mask surface have been identified by Gas Chromatography Mass Spectrometry (GC MS) with two different sample preparation methods: Thermal Desorption (TD) and direct extraction. As a result of UV/O3 treatment, we confirmed that it has an excellent removing efficiency of aromatic compounds and semi-volatile organics on mask surface. Finally, through the haze accelerating tests, we have known that conventional SPM/SC-1 cleaning with UV/ O3 treatment has been having a much higher threshold energy value in terms of causing haze defects on masks about 20 times higher than that of the cleaning with just SPM/SC-1.
We have chosen a combination of thermal treatment and hot D.I water rinsing as a part of methodologies to remove chemical residuals on mask surface. A new step of thermal treatment has been inserted in our standard manufacturing procedure for EAPSM. After thermal treatment, Ion Chromatography (IC) methods are used to confirm the surface cleanliness. As a result of our study, thermal treatment can considerably reduce residuals (e.g. ammonium, sulfuric and others) on mask surface. So, it could be suggested that the thermal treatment is an effective way to minimize residual ions. Also, in order to understand on haze source and mechanism, we investigated on artificial acceleration method for haze formation. We preceded haze acceleration test by using modified haze acceleration system (UV 172nm light). From the acceleration test, we found that humidity, irradiation energy, concentration of chemical residuals are dominant factors of haze formation.
Various sources contribute to mask haze formation including: chemical residuals from mask cleaning, out-gassing from pellicle glue/materials, and contaminants from the scanner ambient. This joint work examines cleaning techniques for haze minimization and whether or not there is haze formation after continuous laser irradiation. Masks with various designs and different cleaning techniques were tested in an ideal environment, isolated from out-gassing or other possible contaminants from the fab environment. Masks with and without patterns were subjected to 40kJ, accumulated dose, of laser radiation to simulate a wafer fab environment. Ion Chromatography (IC) and other surface analytical techniques were used to check the surface condition of masks before and after laser exposure. No haze was found on masks through transmission and IC measurements, when the test chamber was N2 purged. This may suggest that new cleaning techniques have helped reduce chemical residuals on masks. It is less likely for haze to grow when masks are clean to an ionic level and when laser exposure occurs in an uncontaminated, purged environment.
We investigated the control of residual ions on the mask surface and the phase/transmission change rate by using thermal treatment after a conventional cleaning process. We hypothesized that the remaining sulfuric ions on the mask surface could combine with other ions and produce compounds during the thermal treatment. These compounds are easily removed by a hot D.I water rinse. Our study shows that the amount of remaining sulfuric ions is 250ng/mask when the mask has been thermally treated. The amount of sulfuric ions is substantially reduced compared to the results of other cleaning processes. Additionally we have found that the thermal treatment can be reduced varying the phase/trans value according to the cleaning cycle and the variation was stable even with a higher concentration of SC-1 solution.