Preferential etching techniques in combination with light optical microscopy are still the workhorse for the evaluation of defect types and area densities in engineered silicon substrates such as silicon‐on‐insulator (SOI) wafers. Most etching recipes are based on chromium (VI) compounds as oxidizing agent. Since in the meantime the use of carcinogenic Cr (VI) compounds is restricted by law there is a need for Cr‐free recipes with adequate or superior defect delineation performance. We present novel Cr‐free etching chemistries based on organic peracids or on organic oxidizing agents (OOE) such as benzoquinone and derivatives in organic solvents. Some of these have very low removal rates (nm/min at 25 °C), are very defect sensitive and are especially suitable for the application on thin (<50 nm) SOI films. Preferential etching solutions can be characterized by parameters like removal rate, activation energy for the etching process and selectivity. The selectivity describes the ratio of the removal at the crystal defect and at the perfect crystal. In this work the selectivity was determined experimentally on dislocations for various defect etching chemistries. Dislocations were generated by damaging silicon substrates by controlled indentation with a diamond tip and subsequent annealing at 1000 °C. After preferential etching the depth of etch pits was measured by an atomic force microscope. The activation energy was evaluated via an Arrhenius plot of the temperature dependent etch rates. At the dislocations the activation energies are reduced by about 5%. For the various defect etching chemistries selectivities in the range of 1.5–2.5 were found.
Successful implementation of novel materials like SOI and sSOI into the mass production of advanced devices requires monitoring of their crystal quality and defect densities. Preferential etching techniques in combination with light optical microscopy are still the workhorse for a quick and simple evaluation of defect types and area densities.Classical etching recipes applied to SOI and sSOI substrates reveal certain limitations and the most common recipes are based on toxic Cr(VI). Therefore there is a need for alternative Cr-free recipes with adequate or superior performance for application to SOI and sSOI. We discuss some basic aspects of preferential etching and present two different types of novel Cr-free preferential etching chemistries which are suitable for the application on SOI. Organic peracid etches consist of hydrofluoric acid, an organic acid and hydrogen peroxide which reacts with the organic acid forming the corresponding organic peracid which then acts as the oxidizing agent. These etch solutions have very low removal rates (0.4 -1.5 nm/min at 25{degree sign}C), are very defect sensitive and are suitable for the application on SOI films as well as on CZ bulk material where they reveal vacancy agglomerates (D defects, COPs) with high sensitivity.
Defect etching is a well-established method used to reveal different kinds of crystalline defects in semiconducting materials. Most of the etch solutions used today have two disadvantages. They contain hexavalent chromium which is highly toxic and they are not suitable for application on thin films. There is a demand for environmentally friendly etch solutions which can also be used for new materials like SOI. Due to their properties Organic Peracid Etches (OPE), mixtures which contain a short-chain alkanoic acid like acetic or propionic acid, hydrogen peroxide and hydrofluoric acid are suitable for defect delineation in thin and very thin (<50 nm) films. Such solutions were also tested on CZ and FZ silicon substrates. In this case characteristic square-shaped etch figures caused by D-defects (COPs) were found after etching.
Chromium-free preferential etching techniques in combination with light optical microscopy were compared with the non-destructive Laser Scattering Tomography (LST) for the evaluation of crystal defect densities in Czochralski substrates grown under different conditions. Dichromate containing etching solutions (original Secco etch and dilute Secco etch) were included into the study as reference. The chromium-free etching solutions with high etch rates comprised mixtures of nitric, hydrofluoric and acetic acid with water (JEITA 1, MEMC). Those with low etch rates consisted of mixtures of nitric and acetic or propanoic acid with hydrogen peroxide which form peracetic or perpropanoic acid (Organic Peracid Etches). OPE solutions provide improved discrimination of different types of defects and work also on highly doped substrates. As a general result, it turned out that the defect densities determined by the preferential etching solutions applied were significantly higher than those evaluated by LST. Relatively close to the LST defect densities are those determined by original Secco etch for larger etch pits.
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