AFM Probe Fabrication

This note is a brief guide to silicon AFM probe fabrication. It is intended for those who want to learn about how AFM probes are produced and have little or no experience in the field of semiconductor technology.

General Micro-Patterning Process

Silicon AFM probes are fabricated on single crystal silicon wafers. The wafers are patterned in a micromachining process in order to obtain the desired structures. The general step sequence for one structuring step is shown below.

1. Blanket silicon waferIt is important to start with a high-quality monocrystalline silicon wafer with few defects and a smooth, clean surface.

2. Hardmask layer deposition / growthPhotoresist is not resistant against silicon-etching chemicals such as potassium hydroxide (KOH). This is why an additional layer of silicon oxide or silicon nitride is deposited or grown on the silicon surface as a protective 'hardmask' during the silicon etch.

3. Photoresist depositionThe wafer is coated with a photosensitive film (photoresist) and then baked in order to harden it. A positive photoresist, like the one used in our example, becomes soluble in photoresist developer after exposure to ultraviolet light. Negative photoresists, which become insoluble after UV exposure, are also available.

4. ExpositionAn optical mask with a certain structure pattern is positioned over the wafer and then the wafer is exposed to ultraviolet light. The light softens the exposed photoresist areas.

5. Photoresist developmentThe photoresist in the developed areas is dissolved in a photoresist developer, exposing the hardmask layer. The photoresist is baked again to harden it further.

6. Oxide / nitride hardmask patterningThe photoresist pattern is transferred to the hardmask layer by etching in acid, typically hydrofluoric acid (HF) for silicon oxide and phosphoric acid (H3PO4) for silicon nitride.

7. Photoresist removalThe remaining photoresist is stripped from the wafer in acetone.

8. Substrate patterningThe pattern is transferred to the silicon substrate by a KOH etch.

9. Oxide / nitride removalFinally the mask layer is removed, leaving the structured silicon surface.

Photolithography is the general term for the technique of transferring geometric patterns from the mask to the photoresist covering the substrate surface. It includes the photoresist deposition, baking, exposition, development and stripping, as well as the related cleaning steps. Broader definitions include the thin film or substrate etching steps as well.

The photolithography mask features a repeatable pattern which allows transferring simultaneously a large number of identical structures on to the wafer. This makes the process much more efficient than single unit production. Usually, several hundred AFM probes are produced on a wafer.

Various cleaning steps are performed at different stages of processing. They are of immense importance to the fabrication process. Cleaning removes residual particles and chemicals from the wafer surface. As a result, defect density decreases significantly, while photoresist and layer adhesion improve significantly, both of which increase the yield.

Yield is very important in the world of microfabrication. In order to achieve high overall yields, the yields of individual steps must be orders of magnitude higher. Therefore, every single processing step must be carried out with great care and under strict control over process parameters.

General AFM Probe Fabrication Steps

The AFM probe fabrication processes vary for different AFM probe types and for different manufacturers. Different technological processes, process parameters and step sequences are applied. They have a significant impact on the final product quality and yield.

The process flow described below is a simplified version of an actual fabrication process, which consists of over 100 steps and takes a couple of months to complete.