by Tony McKie
CEO, memsstar
Self-assembled monolayers (SAMs) are a material that has come to the fore in a number of applications, such as biological and consumer electronics, as they have a wide usage scope. This article reviews SAM materials, their deposition methods, and the potential applications of these films. At its core, a self-assembled monolayer is a molecule of two halves: a head group and a tail group. The head group is typically the functional side of the molecule, whose function is dictated by the head group itself. In some cases, this could be a molecule that allows for the binding of biological material in a lab-on-chip type of application. Another use could be to modify the surface energy that is present on the device surface itself in such a way as to reject the buildup of moisture on the sample surface, thus making the surface hydrophobic.
The tail group is the side of the molecule that will bond to the substrate. Similar to the head group, there are a number of options, depending on the surface to which this material will bond. Were someone looking to bond to a silicon surface, it is reasonable to use a chlorosilane head termination, while a gold surface would use a thiol-based tail group. Knowing in advance the surface to which these materials are to be deposited will inform the choice of materials a user can consider.
Deposition methods
Currently, there are two commonly used approaches: wet deposition and vapor phase deposition. Wet deposition is a low-cost method whereby a sample is submerged in a solution of the SAM material to allow the SAM to bond to the surface and correctly align. Time frames typically range from a few hours to a whole day per sample. In an academic setting, this time frame isn’t necessarily an issue, but in a manufacturing environment, this would be wholly unsuitable for volume production. Additionally, due to its interaction with atmospheric conditions, the SAM material will slowly begin to polymerize, and as such, the lifetime use of the material in this deposition method is very limited.
In comparison, the vapor phase method results in an organized film after a five-minute deposition stage. This method benefits from such a quick deposition time due to the use of a surface plasma treatment prior to deposition. This results in the substrate becoming highly reactive with the incoming SAM material, resulting in a much quicker reaction with the sample’s surface and aiding in the formation of the chemical bond itself. The coating material is typically contained within a pressure vessel and kept under vacuum, which significantly increases the useable lifetime of the coating material, allowing users to utilize all the material in the vessel.
Applications
The current most common volume application for SAM coatings is creating hydrophobic surfaces for consumer MEMS devices. In a MEMS pressure sensor, microphone, accelerometer or gyroscope, which are often considered the big four of MEMS, a hydrophobic coating will be utilized to avoid the risk of static friction (stiction).
Stiction is the permanent adhesion of a movable device layer to another layer by one of a number of mechanisms: van der Waals forces, electrostatic charging, coulombic attractions or hydrogen bonding. Should any of these main effects occur and cause a charge or attraction between two layers to form, if the stuck layer does not have a restorative force greater than that being applied by this formed charge, then this layer remains adhered to the other. By changing the surface energy to reject the bonding of water to the surface of a functioning device, the lifetime of these devices will significantly increase.
Another developing application for SAM films is their use in medical devices, such as drug coatings or DNA immobilization for biological analysis. By making the functional group react with only specific materials, a user can create a film coating on drugs that targets specific locations, which can be used for cancer treatments. By tailoring the drug coating to bond only to a cancer cell, this allows for the highly localized treatment of harmful cells, thus reducing the ravaging effects of cancer treatments on a body.
Combinations of MEMS devices and the utilization of a SAM coating have started to appear in recent research. The application of a SAM to a simple MEMS cantilever or bridge structure whose deflection in either an upward or downward direction will impact the device’s capacitance, and as such, its ability to be used as a sensor has been discussed. By using a SAM coating for this bridge that binds with specific biological materials, a measurement sensor specific to a single fluid, enzyme, etc., can be discerned. This also has applications in medical diagnostic testing, such as with glucose levels to aid diabetics in their insulin dosage.
Microfluidics is the final and major application of SAM coatings. A hydrophilic coating applied to microfluidic channels can pull liquids flat, which can then draw fluids through filtering systems for purification. This further supports medical applications, and can also be used in any scenario where fluid filtration may be required.
Analysis methods
The function of SAMs is dictated by the chemical used. The head group could make the sample hydrophobic, bio-active, hydrophilic or have another function altogether. The tail group—i.e., the end that bonds to the device’s surface—will also be selected based on the surface to which it is to bond. This could be a chlorosilane group (SiCl3), which ultimately forms Si-Si bonds on a substrate and releases extremely low levels of chlorines gas. This would typically bond to silicon, silicon-oxide and silicon-nitride surfaces, along with a range of metal surfaces. The other most common group, thiols, utilizes a sulphur bond to react with gold surfaces.
As the name of these materials suggests, their thickness is exceptionally thin, making optical confirmation of a deposited film incredibly difficult. There are some tests that can be applied to samples, depending on the functionality of the head group. For example, a hydrophobic surface will cause water to “ball up” on the top layer, which can be imaged. Bio-active coatings will ultimately bind to their target material when exposed to a hydrophobic surface, and hydrophilic surfaces will pull liquids flat.
All of these tests, however, require that a sample be exposed to a testing medium. Due to the thin nature of the films, only a few testing methods are easily available to a user to determine the presence of the film, including AFM, ellipsometry and X-ray reflectometry (XRR). These methods are all feasible testing systems, but each carries a condition to their use as a test method. AFM will destroy the coating where the AFM tip has been dragged along the surface. Ellipsometry relies on a model existing for that specific material, and XRR is not a commonly available method.
What do these testing methods look like, and can they be used in partnership to confirm the presence of any of these films?
One of the most commonly used SAM coatings is perfluorodecyltrichlorosilane (FDTS). This is a long-chain fluorocarbon material that provides devices with a hydrophobic termination. The contact angle of a surface coated with this FDTS film is expected to be in the region of 110° Celsius or greater, and has an estimated film thickness of 10-15Å, or 1-1.5nm.
AFM analysis at increments of 30 seconds into the deposition process shows the deposition and organization of a SAM layer. The mean roughness (Ra) continues to increase until all binding sites have been filled by the material and the Ra suddenly drops as the film becomes organized and smooth. Further depositing on top of this, however, is not advised.
Continuing to analyze the deposition beyond a normal five-minute deposition, it is clear that the roughness begins to increase again. Additionally, the SAM material is not bonding to the substrate as there are no free bonding sites. In this instance, the FDTS begins to bind to itself in small localised clusters, thus creating particles on the sample surface.
The use of ellipsometry and XRR was performed by Dr. Jan Uwe Schmidt at the Fraunhofer Group, IPMS, Dresden, when analyzing the FDTS deposition method. By utilising the X-ray analysis method, a film thickness was accurately measured and compared against a proposed model within their ellipsometry system. This allowed the team at IPMS to confirm and develop a model via ellipsometry that can accurately determine the thickness of this particular SAM coating. This, of course, would need to be done for each coating under analysis to confirm the thickness. However, with access to this kind of equipment, it is very much possible that models could now be generated for a wide range of SAM coatings, making nondestructive analysis of the SAM films a possibility.
Conclusion
Self-assembled monolayers have become a significant means of adding functionality to systems where this functionality could not originally be designed into the device. By tailoring the functional and binding groups of a SAM material, users can treat a wide range of devices and surfaces with an equally wide range of functionalities, thus increasing the range of applications in which a device can be used.
Analyzing SAM coatings is not a simple task. Determination of the coatings' presence requires that you either expose the sample to the target material to which your specific SAM coating is sensitive, or you work on a bespoke metrology method that, in itself, requires some specific knowledge of the coating. Here, we have highlighted some of the methods that could be utilized and how to clarify the material presence via droplets where applicable. By utilizing some of these approaches, we can better understand the quality of SAM coatings.
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