This note is a comprehensive 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 the wafer. This makes the process much more efficient than single unit production. Usually, several hundred AFM probes are produced from one silicon wafer.
It is important to have a clean surface before film deposition, growth and exposition steps
In the case of silicon-etching chemicals, i.e. potassium hydroxide (KOH), the photo resist is not very resistant and therefore an additional layer (silicon oxide or silicon nitride) is deposited before it (step 2), structured after development (step 6) and used as a protective layer during the 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 photo resist areas and makes them soluble in a developer chemical.
The photo resist in the developed areas is removed, exposing the hard mask layer.
The photo resist 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 patterning modules (or photolithography steps) are needed for producing a silicon AFM probe: one for the chip, a second for the cantilever and a third for the tip.
Some simplifications are made in order to keep the overview more concise and clearer. The photoresist deposition, exposition and development steps are combined in ‘photolithography’ steps and the photoresist removal steps are skipped.
There is also a number of cleaning steps that are not shown in the process flow. 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.
Probe fabrication starts from blanket monocrystalline silicon wafers with crystallographic orientation . The silicon is usually highly doped for higher conductivity in order to improve static charge dissipation.
The silicon wafer is oxidized on both sides. The silicon oxide acts as a hard mask for successive etch steps.
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. KOH etches monocrystalline silicon along the plane much faster that along the plane. This explains the 54.7° degree slopes of the etched trenches.
The second patterning step defines the lateral dimensions of the cantilevers. The silicon etch determines thier thickness.
The last lithography is responsible for defining the tip geometry. Different probe makers use different masks that lead to different tip shapes. The mask is critical for the end quality of the probe.
The remaining oxide on top of the wafer is removed in order to provide a clean surface for the next oxidation.
Thermal oxidation occurs as a result of the bonding of oxygen from the atmosphere with silicon from the wafer surface. As a result, during oxidation silicon is consumed from the wafer. The tip becomes smaller and sharper. An additional silicon nitride layer is deposited on top of the thin oxide layer in order to serve as a protective layer for the next step.
The last silicon etch step releases the cantilever and the chip from the rest of the silicon substrate.
After the nitride and oxide layer are removed, the probe fabrication is complete. Usually the probes are held to the wafer by several small silicon holders that can be easily broken by applying a small mechanical force on the chip.
The actual fabrication process varies for different AFM probe types and for different manufacturers in terms of step sequence, process parameters, materials and technology used, etc. This has a significant impact on final product quality and yield. At the end, it is up to the customers to find the right AFM probes that satisfy their requirements.