When it comes to patterning a silicon wafer or some other kind of substrate the first process that usually comes to mind is deposition/lithography using a photomask or direct-write lithography — develop, etch, strip and wash/clean. This “etch-back” process is widely used and usually delivers outstanding high-resolution features, low defect rates and excellent layer-to-layer pattern registration. But the “etch-back” process isn’t always the answer.
A shadow mask or the ‘lift-off’ process, two options to consider when patterning a silicon wafer.
Here the pros and cons when considering one or both of these procedures:
Shadow Mask: A shadow mask is a thin sheet of metal with pattern features etched completely through the material. The shadow mask is placed into intimate contact with the substrate and placed into a thin film deposition chamber. The thin film material is deposited through the substrate and then directly onto the substrate.
Pros: This simplified process eliminates several steps associated with the “etch-back” process; including:
- You don’t have to apply photo resist (spin on, and bake).
- You don’t have to supply a photomask.
- You don’t need lithography, as no photomask exposure, or direct write laser processes are required.
- You don’t have to develop the image in photo-resist.
- You don’t have to etch the thin film layer.
- You don’t have to strip the residual photo resist, and
- You don’t have to clean the substrate.
- Not for high resolution applications.
- Feature edges on the substrate will appear more ragged than those made using an “etch-back” process.
- Alignment to existing patterns is more difficult and not as accurate.
- Lifespan – after repeated uses the holes in the shadow mask will begin to close up as layer after layer of material builds up.
When to use: Shadow masks are ideal for lower resolution applications where alignment to previous layers is not critical and the elimination of multiple processing steps adds value by reducing costs and reducing processing times.
Lift-Off: The lift-off process eliminates the etching step of the “etch-back” process. In this process the substrate is coated with photo resist and imaged with a photomask or direct write lithography and the image is developed prior to the substrate being placed in the deposition chamber. In the deposition chamber, the thin film material is deposited onto the substrate, covering both the photo resist image and the regions that are not protected by photo resist. After deposition the photo resist (and the metal on top of it) is lifted off of the substrate with the assistance of chemicals such as acetone, leaving thin film material in the areas of the wafer that were unprotected by the photo resist.
Pros: The most common reason for using a ‘lift-off’ process is for those circumstances in which the etchant used in an “etch-back” process would also attack the substrate or the previous pattern layers, resulting in unwanted removal of material or damage.
Cons: The feature edges will be more ragged than those produced in an “etch-back” process, due to a tearing effect that happens as the thin film material breaks away while being lifted off.
When to use: The cost reduction, due to the elimination of process steps when compared to “etch-back” processing, is insignificant, since only the etching step is eliminated. Therefore, the only time lift-off processing makes sense, is when the etchant necessary to remove the thin film coating would damage the substrate or previously patterned layers.
The etch-back process yields consistent results, and will continue to be the method of choice, for wafer and substrate patterning. However, it is beneficial to have options for those circumstances when the “etch-back” process is impractical, or expensive. In particular, using a shadow mask with its inherent cost savings should be seriously considered when specifications and circumstances permit.