Direct laser patterning of self-assembled monolayer using elliptical laser beams: A theoretical parametric study
Introduction
A self-assembled monolayer (SAM) is a two-dimensional, one-molecule-thick film; it spontaneously forms when certain organic molecules, e.g. thiols and silanes, adsorb on a surface, i.e. noble metals and silica, respectively. SAMs are important for scientific and technological purposes, and they have a variety of applications in bio-technological devices, MEMS, and micro-fluidics [1], [2], [3]. Patterned SAM surfaces, i.e. surfaces having different wettability, charge, or biocompatibility in neighboring regions are useful for producing wall-less micro-fluidic channels, and chemical gradients, which is of interest for controlled liquid delivery on surface [4], [5] and cell separation [6] applications. One of the most studied and used SAM systems is alkaline thiols placed on a gold film that is supported by a glass or silicon substrate, here we also consider such system.
In the context of direct laser patterning (DLP) methodology for SAM patterning using circular beams [7], [8], [9], [10], [11], [12], [13], [14], patterning with elliptical beams (by using cylindrical lenses, for instance), is considered in this study. In DLP, an initially formed homogeneous monolayer, e.g. 1-hexadecanethiol (HDT) SAM (hydrophobic) placed on gold film is irradiated by a laser beam, e.g. 488 nm CW argon ion laser beam [7], [8] to form a bare region (because of thermal desorption of SAMs); then the bare region is backfilled by a second monolayer species, e.g. 16-mercaptohexadecanoic (MHA) acid SAM (hydrophilic) through solution deposition (details of SAM preparation procedure and the experimental set-up can be found elsewhere [8]). The procedure creates hydrophilic patterns on hydrophobic background that can control liquid spreading on a surface. In this way, DLP can be used to manipulate the surface properties. Note that the manipulation of surface properties is not limited to surface wettability; depending on the SAM's tail group, surface charge or surface biocompatibility can be changed as well.
Previously, patterns with feature sizes of 4–170 μm have been produced using laser beams with circular cross sections (various sizes and powers) in a single pass mode [7], [8]. However, making features larger than 170 μm is of interest in many fields. For instance, a potential application is to enhance the efficiency of creating chemical gradient surface [15], [16]. The findings of this study and especially the idea of coupling equations, describing the thermal response of the surface to chemical reactions resulting from heating can also be used for other areas such as welding process using laser beams [17], and oxidation of silicon wafer using laser [12], [13] (a thermo-chemical process). Such an approach, if extended to consider pulsed laser applications, can also be useful for processes that involve photo-polarization and photo-thermal effects, e.g. nano-patterning of indium tin oxide films [18].
Producing large areas using a typical circular beam needs high laser powers or multi-pass processing that (1) may be beyond the capability of a typical research laboratory laser system, or may damage substrate (the beam centerline) due to high laser powers needed and (2) can be a time-consuming process in multi-pass processing. A solution to overcome these problems is to expand the laser beam perpendicular to the scanning direction (making a beam with elliptical cross section). The heat diffusion model and temperature rise by an elliptical laser beam has been discussed in [19], [20], [21]; however, patterning SAMs with elliptical laser beams is not yet reported in the literature. Also, the concept of thermal Mach number (Mth) is introduced in this study for beams with elliptical cross section. In this study, we theoretically examine the feasibility of the idea of using elliptical laser beam in DLP and predict the practical range of working parameters to make largest possible feature sizes (patterning the largest possible area in a single pass); such a patterning enables achieving a desired feature size at minimal laser power, in minimal time. Another novel aspect of this study is the coupling of the thermal equation with a chemical kinetics equation describing the surface changes as a result of heating a surface.
The theoretical predictions and quantitative analyses are made through adopting a previously developed heat diffusion model [19] for elliptical laser beams and coupling it to a kinetics model. There are two methods to find the feature sizes for a pattern: (1) The calculated feature sizes given by heat diffusion model which is associated with the 75–80% [22] SAM desorption; this is a simple and quick approach to quantitatively analyze the correlations between the feature size and processing parameters, i.e. laser power (for a 488 nm CW laser), scanning velocity, and aspect ratio of elliptical beam. (2) More accurate feature sizes defined by full SAM desorption can be found when a coupled thermo-kinetics model as described later in this study is used. The coupled model also sheds light on issues related to the homogeneity of the patterned sample (the edge resolution for patterning), which has application ranging from creating a chemical gradient along the surface, to more uniformly distributed laser welding or annealing zone. However, the second approach requires a more complex and time-consuming calculations. Therefore, on balancing the accuracy of the result and the time, one can choose a proper approach.
We firstly discuss the change of the calculated feature size with change of the laser power, scanning velocity, and beam aspect ratio, to find how they affect feature size, and then determined the optimal combination of these parameters that results in the maximum feature size in a single pass processing. The correlation between the theoretically calculated feature sizes and chemical composition of the surface is demonstrated as well. The width of 0%, 50%, and 100% HDT coverage at different laser powers and aspect ratios are compared, and the edge resolution with respect to aspect ratio is analyzed at the end.
Section snippets
Heat diffusion model for surface temperature
It is assumed that the laser beam is elliptical in cross section and the laser intensity has a Gaussian distribution along both axes. The temperature distribution produced by elliptical laser beam is a function of the following parameters: laser power (P), laser beam size characterized by rx and ry as shown in Fig. 1a, scanning velocity (v), initial sample temperature (Ti) here assumed as 298 K, sample absorptivity (A), thermal conductivity (κ), and thermal diffusivity (D). In this study, a
Heat diffusion approach
According to the first method of finding the feature size i.e. width of the irradiated region that experiences temperatures above 490 K see Fig. 1a, one needs the temperature distribution of the surface. By solving the Eq. (3) numerically (using Maple 8 and values for constant parameters as given in Table 1), the temperature profiles along both axes (along the scanning direction and perpendicular to that) were obtained. Because of the ellipticity of the laser beam, the induced temperature
Summary and conclusions
A quantitative analysis of processing parameter for application of an elliptical beam to achieve the maximum processing area was the purpose of this study. DLP process was used as a case study and a parametric analysis of feature sizes of the patterned SAMs produced by DLP using CW elliptical laser beam is presented, but the findings can be general by just changing the constants. It is shown that scanning an elliptical laser beam along its minor axis has at least two advantages compared with
Acknowledgment
This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Research Chair program.
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- 1
Current address: School of Industrial Engineering, Purdue University, West Lafayette, IN 47907, USA.
- 2
Current address: R&D Incentive Practice, KPMG LLP, 777 Dunsmuir Street, Vancouver, BC, Canada V7Y 1K3.