Chemical Engineering: Oxidation Furnace

The Photolithography Process

• The photolithography process involves several critical steps, including inspect, spin, coating, softbake, alignment, expose, develop, and hardbake.

Spin Coating

• The wafer is placed on a vacuum chuck and a vacuum holds the wafer in place.
• Resist is applied, and the chuck accelerates to achieve the desired resist thickness.
• The chuck continues to spin to dry the film.

Photoresist

• Photoresist is a mixture of organic compounds in a solvent solution.
• There are two types of resist: positive resist and negative resist.
• Positive resist becomes more soluble when exposed to UV light, and a positive mask is left after development.
• Negative resist hardens when exposed to UV light, and a negative mask is left after development.

Softbake

• After the photoresist is applied to the desired thickness, a softbake is used to remove the residual solvents of the photoresist.
• After the softbake, the wafer is cooled to room temperature.

Alignment

• Alignment is one of the most critical steps in the entire microsystems fabrication process.
• A misalignment of one micron or smaller can destroy the device and all the devices on the wafer.
• Each layer must be aligned properly and within specifications to the previous layers and subsequent layers.

Expose

• The wafer is exposed by UV light from a light source traveling through the mask to the resist.
• A chemical reaction occurs between the resist and the light, and only those areas not protected by the mask undergo a chemical reaction.

Develop

• Portions of the photoresist are dissolved by a chemical developer.
• With positive resist, the exposed resist is dissolved while the unexposed resist remains on the wafer.
• With negative resist, the unexposed resist is dissolved while the exposed resist remains.

Hardbake

• The hardbake hardens the photoresist for the next process.
• The temperature of the hardbake is higher than that of the softbake after coat.

The Role of Photoresist

• The choice of photoresist and its properties, such as sensitivity to specific wavelengths of light, resolution, and resistance to subsequent process steps, are crucial in determining the accuracy and quality of the pattern transfer.

Surface Conditioning

• Surface conditioning prepares the wafer to accept the photoresist by providing a clean surface.
• It coats the wafer with a chemical that boosts adhesion of the photoresist to the wafer's surface.
• The process involves baking the wafer to remove water molecules, applying HMDS to create a hydrophobic surface, and cooling the wafer to room temperature.