Creating Patterned Multispectral Filters

Tuesday, July 01 2008 Resist Lift-Off

To address these limitations, Deposition Sciences, Inc. developed the resist lift-off technique for applying patterned multispectral coatings on a single substrate or, for some cases, directly on the surface of a CCD. This technique has been applied successfully at DSI since the early nineties. The coatings can have micron-scale features, consist of as many as 100 coating layers, and meet stringent environmental and durability standards.

Production of multispectral filters using resist lift-off starts with a bare, clean substrate (Figure 1). The substrate is then treated with an adhesion promoter, which helps the photoresist adhere to the substrate. If an adhesion promoter is not used, the photoresist may delaminate during subsequent steps in the process.

After the adhesion promoter, positive photoresist is applied. The amount of photoresist applied is determined by the thickness of the coating to be deposited. As shown in Figure 2, the thickness of the resist should be slightly thicker than the desired coating in order to achieve clean coating edges after the photoresist is removed. The volume of photoresist and the spin speed used to apply the photoresist determines the thickness of the photoresist.

The next step, following proper application of the photoresist, is exposure. Exposure time depends on the thickness of the photoresist; the thicker the photoresist the longer the exposure time. Choosing the correct exposure rate is critical to the edge definition of the coating after photoresist removal. Over exposure causes poor line definition and under exposure hinders deposited coatings and lift-off.

Once the desired area has been exposed, the resist from the exposed area is removed. This is accomplished during the development step of the process. Care must be taken during the development step to avoid damage to the walls of the photoresist. Overdevelopment of the photoresist will cause a rounding of the photoresist edge, which results in damage to the coating during the photoresist removal step. Underdevelopment of the resist will leave resist residue in the patterned areas that are to be coated, resulting in damage to the coating during the stripping step.

The substrates with the patterned photoresist masks are then placed in a vacuum coating chamber where controlled deposition of the desired coating is accomplished (see Figure 3). After deposition, the coated substrate is submerged in solvent, which dissolves the photoresist, allowing the coating on top of the photoresist to be washed away and leaving the desired patterned coating. This procedure is repeated as shown in Figures 4 and 5 to construct multiple filters on the same substrate.