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.
Silicon AFM probes are ‘carved out’ from 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. The outline of the process is the following: a photosensitive film (photoresist) is deposited on the wafer surface. This layer is structured by selectively exposing only certain parts of it to light and developing it. Then the structures are transferred to the silicon substrate or the layer beneath the photoresist by etching (dissolving) it in a chemically reactive solution.
The exposition (step 4) is done through a mask with a repeatable pattern which allows transferring simultaneously a big 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 from one silicon 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 thus significantly decreasing defect density and greatly improving yield
‘Yield’ is a very important word 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.
It is important to have a clean surface before film deposition, growth and exposition steps
Photoresist is not very 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 top of the silicon surface as a protective 'hardmask' during the silcion etch (step 7).
The wafer is coated with a photosensitive film and then baked in order to harden it.
A photolithographic mask with a certain structure pattern is aligned to the wafer and then the wafer is exposed to light with a certain wavelength. The light softens the exposed photoresist areas and makes them soluble in a developer chemical.
The photoresist in the developed areas is removed, exposing the hard mask layer.
The photoresist pattern is transferred to the mask layer by an etch step.
The pattern is finally transferred to the silicon substrate by another etch step.
Finally the mask layer is removed, leaving the structured silicon surface.
The entire fabrication process usually consists of more than 100 steps and it takes a couple of months to complete. The process flow described below is a simplified version of an actual fabrication process.
Three photolithography steps are needed for producing a silicon AFM probe: one for the chip, a second for the AFM cantilever and a third for the AFM tip.
Some simplifications are made in order to keep the overview more concise and clearer. The photoresist deposition, exposition, development and removal steps are combined in ‘photolithography’ steps and the cleaning steps are omitted.
AFM probe fabrication starts from blanket monocrystalline silicon wafers with crystallographic orientation <100>.
The silicon wafer is oxidized.
The first photolithography is performed on the back of the wafer using the chip mask. After oxide etch, the silicon substrate is patterned by potassium hydroxide (KOH) etch.
The second patterning step defines the lateral dimensions of the AFM cantilevers. The silicon etch determines their thickness.
The last photolithography defines the AFM tip geometry.
The remaining oxide on the top side of the wafer is removed in order to provide a clean surface for the next oxidation.
An additional silicon nitride layer is deposited on top of a thin oxide layer in order to serve as a protective layer for the next step.
The last silicon etch step releases the AFM cantilever and the chip from the rest of the silicon substrate.
After the nitride and oxide layer are removed, the AFM probe fabrication is complete. At this point the AFM probes are attached to the wafer only by several small silicon bridges. The AFM probes can be easily freed by applying a small mechanical force to the holder chip with tweezers.
The actual fabrication process varies for different AFM probe types and for different manufacturers. Different technological processes, process parameters and step sequences are applied. This has a significant impact on the final product quality and yield. At the end, it is up to the customers to find the right AFM probes that satisfy their requirements.
