Conference-ICSFS-16-Effect of Pinpoint-Like Heating in the Drying Process of a Polymer Solution Film on a Substrate

We have proposed and modified a model of drying process of polymer solution coated on a flat substrate for flat polymer film fabrication supposing resist coating process in semiconductor engineering process and so on. And we have clarified dependence of distribution of polymer molecules on a flat substrate after drying on a various parameters based on analysis of many numerical simulations of the model. Then we applied the model to thickness control of a thin film after drying through thermal and evaporative management. In this study, we tried to practice thickness control of a thin film through a simple experiment of thickness control of a thin film after drying through thermal management. And we see that heating a part of a polymer solution film on the substrate from the bottom produces locally higher thickness of a polymer thin film after drying through simple experiments. The results were qualitatively verified through numerical simulation of the model of drying process of polymer solution coated on a flat substrate. [DOI: 10.1380/ejssnt.2012.561]


I. INTRODUCTION
Drying process of polymer solution coated on a flat substrate is very important in various industrial applications, such as fabricating flat polymer film [1,2], inkjet printing [3,4] and so on.Though previous studies about fabrication of flat polymer film discussed mainly time-variation of a polymer liquid film's thickness during drying [1,2], spatio-temporal variation of concentration distribution in a polymer solution liquid film as discussed in our study was not discussed.And previous studies about inkjet printing discussed a droplet of polymer solution [3,4], which is different from a wide polymer solution liquid film as discussed in our study.And Okuzono et al. discussed drying processes of polymer solutions on a solid substrate enclosed by bank [5].But the substrate used by the study was enclosed by bank and is essentially different from that used by our study.
We have proposed and modified a model of drying process of polymer solution coated on a flat substrate for flat polymer film fabrication supposing resist coating process in semiconductor engineering process and so on [6][7][8][9][10][11][12][13][14][15].And we have clarified dependence of distribution of polymer molecules on a flat substrate after drying on a various parameters based on analysis of many numerical simulations of the model.Because above discussion was focused upon general mechanism of drying process of a polymer solution coated on a flat substrate, universal essence of it was clarified.
Then we applied the model to thickness control of a thin film after drying through thermal and evaporative management.In this trial, we cooled or heated specific points of polymer solution coated on the flat substrate at pinpoint-like cooling or heating, or controlled vaporization rates at specific points of the interface between liquid and gas as the need arose.As a result, minute control of vaporization or diffusion could be realized so as to control the thickness profile after drying [16].
In this study, we tried to practice thickness control of a thin film through a simple experiment of thickness control of a thin film after drying through thermal management described above.Water solutions of water-soluble cellulose ether were used as polymer solution applied on a substrate for implementing a simple device.And pinpointlike light was irradiated to a polymer solution film during drying for implementing pinpoint-like heating of it.Then thickness distribution of the polymer film after drying was measured using a surface finish measuring instrument.As a result, we saw that thermal management during drying affected thickness distribution of the polymer film after drying through above-mentioned simple experiment.Concretely, heating a part of a polymer solution film on the substrate from the bottom produces locally higher thickness of a polymer thin film after drying.
Then we compared these results with results of numer-  ical simulation of the above-mentioned model of drying process of polymer solution coated on a flat substrate so as to evaluate the model.On the other hand, the opposite experimental results were found in some situations, for drying completed earlier around the heating point and as a result solutes diffused to locations where drying had not been completed from around the heating point.

II. EXPERIMENTAL
In this experiment water solutions of Metoloses which is water-soluble cellulose ether made by Shin-Etsu Chemical Co., Ltd. were used.Three kinds of Metoloses were prepared in this experiment as shown in the Table I.
These samples are dropped on a flat substrate in this experiment.A flat disk made of acrylic resin is used as a substrate.Its diameter and thickness are 10 cm and 2 mm, respectively.Samples are dropped by a drop on this disk using a pipette.Then each sample is spread evenly to about 30 mm in diameter using a glass stick as shown in Fig. 1.
Then the coated polymer solution is dried under reduced pressure in the device as shown in Fig. 2. The vacuum chamber of the device is made of transparent acrylic acid resin so that irradiated light from the outside can reach the substrate in the chamber.And the substrate is pinpoint-likely heated from the bottom by a halogen lamp during drying as shown in Fig. 3.

III. RESULTS AND DISCUSSIONS
Thickness distribution of thin film of 60SH-03 formed on a substrate after drying is shown in Fig. 4.This is measured by noncontact 3-dimension measuring technology.The position to which light was irradiated is shown by a light mark drawn in the bottom of the X-axis.We can see that thickness of the thin film on the heated position of the substrate after drying is slightly higher than that on the other positions.The experimental results are similar to former results of numerical simulation of the model.
Then the experimental results were verified through numerical simulation of the above-mentioned model.Concretely locally circular heating as shown in Fig. 3 was added to the polymer solution film from the bottom in numerical simulation of the model.The results of the numerical simulation are shown in Fig. 5, where [N] represents average number density of polymer molecules in the central region after drying.Comparing Fig. 4 and Fig. 5, we can see that the results of the numerical simulation reproduce well the experimental results.
But, the opposite experimental results that thickness of the thin film on the heated position of the substrate after drying is not slightly higher than that on the other positions were found in some situations.Concretely, in case of larger molecular weight of solutes or thinner thickness solution film or stronger irradiated light, locally lower thickness distribution of a polymer thin film after drying In case a coated solution film is thinner.In case a coated solution film is thicker.
FIG. 6: Dependence of thickness distribution of a thin film of 60SH-03 formed on a substrate after drying on thickness of a coated solution film.
In case light irradiation is stronger.
In case light irradiation is weaker.is produced around the heating point.
Dependence of thickness distribution of a thin film of 60SH-03 formed on a substrate after drying on thickness of a coated solution film is shown in Fig. 6.We can see locally lower thickness distribution of a polymer thin film is produced around the heating point when a coated solution film is too thin.
Dependence of thickness distribution of a thin film of 60SH-03 formed on a substrate after drying on strength of irradiated light (strength of heating) is shown in Fig. 7.We can also see locally lower thickness distribution of a polymer thin film is produced around the heating point when strength of irradiated light (strength of heating) is too strong.
And thickness distribution after drying in case of larger molecular weight of solutes is shown in Fig. 8.We can see locally lower thickness distribution of a polymer thin film is produced around the heating point when molecular weight of solutes is too large.
The reason why the above opposite results appear is common to three cases, that is, in case of larger molecular weight of solutes or thinner thickness solution film or stronger irradiated light.In these three cases, drying completes earlier around the heating point and as a result solutes diffuse to locations where drying has not been completed from around the heating point as shown in Fig. 9.In addition, in case of the last case, the phenomena that the location where drying completes later rises up because of larger cohesive force due to larger molecular weight of solutes causes rise of a thin film there after drying.If organic solvent should be used, above phenomena might not appear because of faster vaporization rate.
To verify the reasons mentioned above, numerical simulation was carried out as follows.In the numerical simulation, in order to realize the heated position of the solution film completes drying earlier, vaporization rate on the position to which light is irradiated is made smaller after a certain time.Concretely, when the residual solvent amount becomes to 103.35% of the amount of residual solvent in the end time of the numerical simulation, the vaporization rate on the position to which light is irradiated is artificially reduced to 98%.The results of the numerical simulation are shown in Fig. 10, where [N] represents average number density of polymer molecules in the central region after drying.We can see that locally lower thickness distribution of a polymer thin film after drying is produced around the heating point in the numerical simulation from Fig. 10.
This study is still immature but paves the way for the development of thickness control of a thin film after drying through thermal management shown above.

IV. CONCLUSIONS
We see that heating a part of a polymer solution film on the substrate from the bottom produces locally higher thickness of a polymer thin film after drying through simple experiments.The results were qualitatively verified through numerical simulation of the model of drying process of polymer solution coated on a flat substrate.
In case of larger molecular weight solutes or thinner thickness solution film or stronger irradiated light, locally lower thickness distribution of a polymer thin film after drying is produced around the heating point because so-lutes diffuse to locations where drying has not been completed from around the heating point.
This study paves the way for the development of thickness control of a thin film after drying through thermal management.

FIG. 1 :
FIG. 1: An image of samples dropped and extended on a flat substrate used in this experiment.

FIG. 3 :
FIG.3:The substrate is pinpoint-likely heated from the bottom by a halogen lamp during drying.

FIG. 5 :
FIG. 5: Results of numerical simulation of the model when locally circular heating is added.

FIG. 7 :FIG. 8 :FIG. 9 :
FIG. 7: Dependence of thickness distribution of a thin film of 60SH-03 formed on a substrate after drying on strength of irradiated light (strength of heating).

FIG. 10 :
FIG. 10: Results of numerical simulation of the model in case vaporization rate on the position to which light is irradiated is made smaller after a certain time.

TABLE I :
Three kinds of Metoloses used as solute in this experiment.
FIG.2:An image of the device in which the coated polymer solution is dried under reduced pressure.