Polishing of a rough silicon surface can be achieved easily with a wet chemical process. Applications of chemical polishing of silicon are:

• Preparation of sample surfaces for X-ray crystallography
• Rounding of edges and corners in order to optimize conformity during subsequent coating processes
• Smoothing of workpiece surfaces, i. e. for micro-embossing or galvanic molding

For the chemical polishing of silicon, the following facilities and supplies are needed:

• Wet bench with disposal facility and offtake
• Protective equipment for 2 people
• Hot plate for heating of the mixed solutions
• Process resistant tools for handling of samples
• Measuring microscope with differential interference contrast mode (DIC) to analyse surface quality
• Profilometer for roughness measurements

• Hydrofluoric acid, nitric acid and acetic acid for the chemical polishing
• Hydrofluoric acid to remove the thin oxide film
• Sulfuric acid and hydrogen peroxide for the piranha solution

The chemical polishing follows machining of the silicon workpiece, which can be carried out by wet chemical or dry chemical etching processes as well as by milling or laser ablation. Due to the utilized etching mixtures the procedure is not compatible with existing coatings (metals , functional oxides) and must be carried out prior to their application. Figure 1 shows a micrograph and Figure 2 is a profilometer measurement of the initial state of a rough silicon surface, as it can be found on the backside of single-side polished wafers.

Figure 1: DIC-mode micrograph of a rough silicon surface. Click on the image for an enlarged view.

Figure 2: Profile scan of the rough silicon surface with a measured roughness of Ra= 400 nm before chemical polishing

The chemical polishing is carried out with a mixture of hydrofluoric acid, nitric acid and acetic acid. The components perform various functions that intertwine: the nitric acid oxidizes the exposed silicon to silica, which then converts the hydrofluoric acid into silicon tetrafluoride and thus detaches it from the surface. The acetic acid improves the wetting characteristics and thus enables polishing of microstructures.

For successful chemical polishing, the correct pretreatment of the sample surface is important. Previous processes can form residues that adversely affect the chemical polishing of the surfaces. Organic residues such as photoresist or adhesive film locally influence the etching rate, and the achievable surface quality deteriorates thereby. The particle residues shown in Figure 3 are also clearly visible in the corresponding profilometer measurement of Figure 4. Such a surface does not meet the requirements of the aforementioned applications.

Figure 3: Micrograph of the polished surface without a pretreatment. Click on the image for an enlarged view.

Figure 4: Profile scan of the polished silicon surface without preatreatment with a measured roughness of Ra = 53 nm

Therefore, organic residues are removed with the so-called piranha solution or Caro’s acid. Because the piranha solution has a strong oxidizing power, a thin oxide film is formed on the silicon surface, which delays the etching attack of the chemical polishing process. Therefore, the silicon workpiece is briefly placed in a low-concentration hydrofluoric acid solution to remove the oxide layer. Due to the highly exothermic reaction of the chemical polishing it is important to match the volume of the bath and the agitation to achieve a uniform etching progress. As a result, this process chain creates a virtually residue-free surface, as can be seen in Figure 5. The profilometer measurement in Figure 6 shows that the surface roughness is improved compared to a processing without the pretreatment.

Figure 5: Micrograph of the polished surface with pretreatment. Click on the image for an enlarged view.

Figure 6: Profile scan of the polished silicon surface with preatreatment with a measured roughness of Ra = 48 nm