Sample Preparation Techniques (Theory & Applications)- Deprocessing (i) Wet Chemical Etching

Wet Chemical Etching- Procedures for Removing Layers of the Chip Structure

The preparatory analysis of semiconductor circuits requires techniques for the step-by-step removal of the layers of the chip structure down to the substrate, which remove the individual levels as selectively as possible and do not attack adjacent material (‘reverse engineering).

Wet chemical etching procedures always act isotropically (uniformly in all directions) but are not always sufficiently selective as regards material.

In simple semiconductor structures the layers can usually be removed in a sufficiently selective way by wet chemical etching.

Refined chip manufacturing techniques, e.g. the application of barriers of metals and their silicides or nitrides, of levelling glasses, of silicon nitride spacers (sidewall protection) or of organic dielectrics between multiple wiring, are increasingly forcing the transition to dry etching procedures in the plasma.

In addition to the removal of layers, chemical preparation can also be used to emphasize faults, in that a defect position may be for example magnified, under-etched or marked with colour (‘decoration’) and thus made to stand out more clearly.

Removal of Silicon Nitride Like silicon oxide, silicon nitride is used as surface passivation for the final semiconductor circuit. It usually occurs as a double layer with oxide, in which it lies on top.

The standard method for removing nitride is dry etching in the plasma. It can be removed by wet chemical methods (selectively against oxide) by boiling to C in commercial 85% phosphoric acid.

Other methods include etching in 1– 10% hydrofluoric acid and treatment in a hydrofluoric acid/glycerine mixture (1- 3 molar solution of 48% hydrofluoric acid in glycerine) at temperatures between C.

The disadvantage of all wet chemical procedures, however, is the simultaneous removal of exposed aluminium; therefore, the procedures can only be used in particular cases.

Silicon Oxide, Formation & Etching Pure or doped with foreign substances, silicon oxide (SiO 2 ) is a component of all silicon planar technology. Oxide layers are used for; the electrical separation of diffusion areas (thick or field oxide); the isolation of superposed conducting layers; the levelling of steps and mechanical protection of a chip; As a gate oxide;

The composition and position of the oxide layers in a sample may point the way for the preparation method.

Formation & Properties of Silicon Oxides Common procedures for the generation of SiO 2 layers as follows; Thermal oxides CVD oxides Pyrolytically generated oxides Sputter oxides

Etching of Silicon Oxide The removal of silicon oxide is necessary in almost any preparatory analysis. Currently, the standard method is etching in the plasma, but wet chemical methods are still commonly used.

The fact that oxides with different etch rates are superposed on one another, while the fact that the wet chemical etchants attack aluminium and even silicon (very slowly) is often problematic.

To ensure a uniform attack on the SiO 2 without excessive damage to the underlying layers, one needs a precise knowledge of the thickness of the SiO 2 layer and of the etch rate in the chosen etchant.

Problems; Not manage to control the corrosion process frequently under the microscope and terminate it at the appropriate time; the lower of SiO 2 layer is attacked more rapidly than the layers above, under-etching of edges, contact holes and defect position is unavoidable.

Therefore, careful choice of the etchant is essential to success. The only practicable etchant for SiO 2 layer is hydrogen fluoride (HF), which, in the absence of water, is transformed into SiF 4 (Tetrafluorosilane/Silicon tetrafluoride);

SiO 2 + 4HF SiF 4 + 2H 2 0 While in the aqueous medium the reaction with storage of HF to form H 2 SiF 6 (Fluorosilicic Acid/Hexafluorosilicic acid) can be written as: SiO 2 + 6HF H 2 SiF 6 + 2H 2 0

In practice, concentrated (40-48%) or dilute hydrofluoric acid together with mixtures of hydrofluoric acid and ammonium fluoride NH 4 F (‘buffered hydrofluoric acid’) are used for the SiO 2 etching.

Removal of Aluminium As a base, aluminium dissolves in strong and weak acids or bases. Suitable etchants for aluminium include:

1) Hydrochloric acid, sulphuric acid - approximately 30% each, application at room temperature up to 50 0 C. - Concentration and temperature are not critical, since oxides and silicon are not attacked even for longer etching times.

2) Phosphoric acid - 65%, application at 50 0 C. - Etch rate 0.2 µm/min. - Very uniform attack, also very gentle to oxides and silicon.

In all procedures, the start of the etching is delayed by the natural aluminium hydroxide layer, which is normally up to 5 nm thick.

The start and the end of the reaction are recognizable by the start and the end of the generation of gas. It is not critical if the etching time is exceeded, since the next layer is not usually attacked.

For aftertreatment it is sufficient to rinse well in deionized water and isopropanol or acetone, possibly under ultrasound, and blow dry with filtered compressed air or warm air under the hot-air drier.

Etching of Silicon The etching processes described below are suitable for polysilicon and mono- crystalline substrate silicon, for which, however, different etch rates are found in most cases.

Acid Etching In all non-oxidizing acids, including hydrofluoric acid, silicon is practically insoluble. However, it can be dissolved by oxidizing acids, e.g. nitric acid, according to the following scheme :

3Si + 4HNO 3 3SiO 2 + 4NO + 2H 2 0 The addition of hydrofluoric acid converts the SiO 2 formed into a soluble aggregate: SiO 2 + 6HF H 2 SiF 6 + 2H 2 0

Thus, the etching reaction follows the summation equation 3Si + 4HNO HF 3H 2 SiF 6 + 4NO + 8H 2 0

Cleaning The Chip Surface- Removal of Surface Impurities - Impurities on the surface of a part are disruptive when they obstruct a visual observation or act as etching masks in a subsequent etching process. - Thus, it is vital to remove them before each further treatment of the chip. - Of course, a cleaning may only proceed to the extent that does not alter the state of the part, i.e. its electrical function and indications of faults are preserved.

- Impurities can be divided into the following groups according to their nature and composition: i) residues of moulding materials or protective coverings such as silicone or imide, possibly also adhesive; ii) corrosive materials and corrosion products already present in the part; iii) salts, electrolytic residues, moisture, which, for example, remain after inadequate rinsing and drying following the opening of plastic packages;

iv) oily or fatty layers, high-boiling organic solvents; v) dust particles from the air, fingerprints, etc.