In recent years the explosion in demand for multispectral imaging has coupled with the industry’s insatiable need for weight reduction, there-by greatly increasing the demand for more sophisticated approaches to producing optical filters that are used in these systems. One method to meet the challenge of reducing the weight of a multispectral system is to eliminate beam-splitting optics and multiple detectors by patterning a filter array on a single substrate, or directly on the CCD itself.
Metal contact masking is the traditional method for applying one or more coatings in patterned form on a single substrate. For many applications, metal masks made from materials such as stainless steel are relatively inexpensive to fabricate, easy to use, and are capable of withstanding the in-vacuum process conditions associated with the vacuum deposition of stable and durable optical coatings. However, for more sophisticated applications, such as multispectral patterning on a single substrate or directly on the CCD, metal masks are expensive to manufacture and down-grade the substrate if allowed to come in contact with it during coating. Metal masks are also difficult to align with the substrate. Alignment is especially challenging when producing a deposited pattern that can be cleanly aligned so its edges interface with the edges of existing patterns without gaps or overlaps.
Other methods for producing multi-spectral filters are similarly limiting. Coating individual substrates, dicing these substrates to required dimensions, and then bonding them to a substrate to form a multispectral array is time-consuming, costly, and limited by the size constraints of the processes involved. Similarly, multispectral filters produced using colored glass or gels are not very durable and they limit the designer to the catalog of available color glasses and gels. The semiconductor industry has achieved finely detailed patterns using direct etch photolithography and ion etching. While these techniques work exceedingly well for silicon or silicon-based materials, they are not effective in patterning thick optical coatings consisting of multilayer stacks of two or more all-dielectric coating materials.
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.
Optical coatings as thick as 20 μm with features as small as 5 μm have been successfully and repeatedly produced using the resist lift-off process. As many as four patterns have been successfully applied to an individual substrate or CCD using resist lift-off.
An example of a 100-mm wafer containing hundreds of patterned filters that are 10 μm thick and produced using the lift-off method is shown in Figure 6. The wafer contains 232 four-color chips and 52 large color pads on the wafer. The large color pads are used to verify the performance of each of the four coatings over the entire surface of the wafer. The actual spectral performance of each of the color filters is shown in Figure 7.
Durability & Stability
The patterned multispectral filters discussed and shown in Figure 6 are coated at DSI using their patented MicroDyn® activated sputtering process. In MicroDyn, reactive sputtering of oxides is augmented by a microwave plasma that forms a wider range of oxygen species (ozone, etc.) to enhance the reaction process.
In all sputtering techniques, the sputtered target atoms come out with a large amount of energy, leading to inherently hard coatings. By hard coatings, we mean coatings that have greater mechanical and thermal stability than similar coatings produced by evaporation or PICVD. Due to the low stresses associated with sputtered coatings, multilayer coatings as thick as 100 micrometers have successfully been produced by this deposition technology.
MicroDyn sputtering can deposit a wide variety of materials including alloys and graded index of refraction materials. The process is very stable and allows the thickness of the vast majority of coatings deposited by this method to be controlled by time.