Tuning Photoresist Adhesion with HMDS: Deep Dive into Photolithography

In the fields of semiconductor manufacturing and microfabrication, photolithography is an essential technique that creates intricate patterns on substrate surfaces. The patterning process is used frequently in electronics, microfluidics, and sensors and creates a protective layer from additional manufacturing processes and mechanical wear during its final application. To create these patterns, a mask and photoresist are applied to the substrate and exposed to light. After exposure, the photoresist is developed using a chemical solution, and the unexposed sections of the photoresist are dissolved, resulting in the desired pattern.

Photoresists are essential in photolithography as they enable precise control over the objects being developed. Not only do they protect areas of the substrate that should not be exposed to light, but they also help create the pattern on the substrate surface. However, photoresists must be paired with a suitable adhesion to stick to substrates and create precise patterns. In some circumstances, photoresist adhesion may need improving due to factors such as cleaning methods, temperature variations, and photoresist quality. This can be conducted by modifying the baking steps during fabrication, while another option is to use an adhesion promoter. Now that we’ve provided insight into photolithography, photoresists, and adhesion, we explain how photoresist adhesion can be improved with a chemical compound known as hexamethyldisilazane (HMDS).

How can HMDS Promote Photoresist Adhesion?

HMDS has several scientific uses, but its primary roles are as a solvent in organic synthesis and an adhesion promoter in photolithography. It is a chemical compound crucial to many processes as it helps protect various materials and improves photoresist adhesion. But how? Let’s look into it.

Using HMDS to improve photoresist adhesion is a two-part chemical process that includes dehydration and an HMDS reaction. A silicon wafer’s surface is hydrophilic as it retains a layer of water in its production stage due to ambient humidity. However, a photoresist cannot stick to a hydrated silicon surface, so a dehydration process must be performed on the surface before adhesion occurs. 

The required process is known as a dehydration bake, during which the layer of water is removed from the wafer’s surface. But, because the natural state of a silicon wafer surface is still hydrophilic, HMDS is required. To conduct the HMDS reaction, the wafer is placed in a high-vacuum chamber filled with vapor HMDS and nitrogen as a carrier gas. The chemical reaction that occurs involves the HMDS bonding with the OH molecules and the three methyl groups (Si(CH3)3) on the wafer’s surface, resulting in a hydrophobic layer that increases adhesion to a photoresist and reduces the risk of it absorbing water.

Platypus Technologies and Photolithography

Platypus Technologies consists of scientists dedicated to advancing surface science solutions. We primarily focus on biological sensors, cell culture, electrochemistry, metal coatings, and nanotechnology, and photolithography is just one of the services we offer.

Contact us today to learn more about HMDS adhesion or photolithography in general.

References and Further Reading

  1. https://www.azom.com/article.aspx?ArticleID=21442