Positve-Tone vs Negative-Tone Photoresist - Hanna Naquines
Photoresists are light-sensitive polymers that are used to transfer patterns from a photomask to a substrate. This process is called photolithography and it can be summarized by Figure 1. The grey represents a silicon wafer, the typical substrate used in photolithography. The purple signifies photoresist, and black and white represents the dark and transparent parts of a photomask respectively. The patterns created can be used for a variety of applications, the application will determine which type of photoresist used, either positive-tone or negative-tone. These two types of resists have different properties and employ different mechanisms for pattern transfer.
The use of photoresists is a critical part of the photolithographic process. By coating a surface, called a substrate, the photoresist can be formed into a pattern which can be utilized in a myriad of different ways. Photoresists can be applied to substrates, usually silicon wafers or glass slides, in a few ways as seen in Figure 2. Spray-coating can be used for irregularly shaped or textured objects, though it will not coat a substrate uniformally. There is also the issue of introducing the photoresist into the air. Another way is using dip-coating to coat larger objects2. This is also utilized for irregularly shaped objects. This method involves physically immersing the substrate into a volume of resist. This method has the disadvantage of requiring a larger volume of photoresist to fully immerse the substrate, where reuse is hindered due to possible cross-contamination. Spin-coating, the most common, involves depositing the resist directly to the center of the substrate and then spinning the substrate. The photoresist is spread all over the substrate via centrifugal force, with the speed and duration determining the height of the resist. Examples of how the speed effects the height can be seen in Figures 3 and 4. This process leads to a very smooth and uniform layer.
A positive-tone photoresist typically contains three main components: a photoactive compound, a base resin, and an organic solvent system5.This photoresist, when exposed to UV light, is soluble in an aqueous developer. This is due to its photoactive compound. When the photoactive compound is not present, the resin film is very soluble in its alkaline developer and will be washed away when submerged in it6. But when the photoactive compound is present, the solubility of the resin is decreased dramatically, so the photoactive compound can be considered to inhibit the removal of photoresist. Positive-tone photoresist will naturally contain the photoactive compound, this compound is destroyed with UV radiation. As seen in Figure 1, photoresist unexposed to UV light will remain when developed, while the exposed will be washed away. So, the pattern made by the photoresist will match the dark areas of a photomask.
For negative-tone photoresist, exposure to UV light has the opposite effect of positive-tone photoresist7. This photoresist begins as a very soluble polymer. When initially deposited, this photoresist will wash away easily within its developer. Then, the UV light will crosslink the negative-tone photoresist, increasing its resistance to developer. When developing, this portion will remain while the pattern unexposed to the UV light will develop away. The pattern remaining will the opposite pattern of the mask. As seen in Figure 1, the pattern made by negative-tone photoresist will be the opposite pattern of the dark parts of the photomask, effectively making a negative image of the photomask. Generally, a negative-tone photoresist will utilize an organic developer.
Positive-tone and negative-tone photoresist contain many differences other their reaction to UV light. There are many advantages and disadvantages for both, do choosing which to use depends greatly on the application. Compared to negative-tone resists, positive-tone resists have a much higher resolution and contrast, meaning the much smaller features can be achieved using a positive-tone photoresist5. However, negative-tone can attain a higher aspect ratio, the ratio of feature height to width can be greater7. Furthermore, using negative-tone resist can give taller features, so if the microfluidic device required involve tall and narrow structures, it would be best to use negative-tone resists. A study using both resists to create creating tall, narrow channels showed that positive-tone photoresist walls were more prone to collapse than negative-tone resist walls8.
Another difference between the two photoresists is their resistance to organic solvents5. A positive-tone photoresist will not be resistant while a negative-tone photoresist will be resistant. This makes sense because the developers used for negative-tone resists are organic solvents, they wash away cross-linked polymer and cannot dissolve the cross-linked portions. Another difference is the ability to remove the photoresists after exposure and development. Negative-tone resists show greater adhesion to the substrate surface than positive-tone photoresist7. This is good for negative-tone resists because you can have stronger bonding, but it is also good for positive-tone resists because some processes utilizing photolithography only require the photoresist to pattern other materials onto a substrate, which requires easy removal of photoresist when finished. Negative-tone resists also tend to be cheaper than positive-tone photoresists.
Photoresists are used in a wide variety of applications, including electronics and microfluidics. After the coating and development of the resists, the patterns can either be left as is or they can be used to further modify the substrate. A processes like wet and dry etching will remove some of the substrate, allowing the formation of microchannels within the substrate. Deposition can allow for the addition of thin metal layers which can used to create electrodes or metal lines. These processes can be repeated creating multiple layers of differently patterned materials which is useful for creating semiconductors. A photoresist pattern can also be used as a master mold, where a polymer can be poured over it and removed, creating an opposite patterned substrate. This is useful for replica molding and microcontact printing.
Using Sidewall Modifications for Waveguides
Using a positive-tone photoresist, wave guides of varying angles can be made9. This is due to the sidewall modification ability of the positive-tone photoresist. A waveguide is a special kind of structure used to slightly alter a wave like radio, sound or light. For creating an optical waveguide, a simplified process is shown in Figure 5. The buffer and core are two layers of glass, one as the bottom, base layer and the other as the waveguide itself. The top layer is the photoresist, which can be modified in the height and curvature which will determine the angle of the walls of the waveguide. First, the photoresist pattern is transferred from the mask and its initial rectangular shape is formed. By flowing the resist during hard baking, the circular shape can be formed. Then RIE is used to etch away at the core layer, creating the final waveguide shape. Shown in Figure 6 are the resultant angles that can be achieved when modifying the top photoresist layer. As shown, a vertical wall and an angled wall can be made.
Using Sidewall Modifications for Rounded Channels
Another group utilized the ability of positive-tone photoresist to modify its sidewalls to created rounded channels for use in microfluidic valving10. For this, one fluidic channel and multiple orthogonal air channels on top are created using soft lithography. A representation of the device can be seen in Figure 7. Here, the bottom channel contains fluid flowing through and top air channels can either have air or nothing flowing through. When air is flowing through, the top channel expands and presses down on the bottom channel, effectively stopping the flow of liquid at that point, this is considered a pneumatic valve. With multiple air channels opening and closing, a peristaltic pump can be created which can control the flow of the liquid. To create these rounded channels, first the Shipley SJR 5740 photoresist is spun and a mask pattern is transferred. While baking, the photoresist is reflowed which creates the rounded channel.
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