Die Deprocessing; Reactive Ion Etching; Plasma Etching; Wet Etching

Die Deprocessing

Die delayering or deprocessing is the semiconductor failure analysis technique of stripping off the upper layers of the die to expose a defect site that is buried underneath these layers.

Die delayering is usually done as a sequence of steps, removing a layer or two at a time. Since each layer is chemically and physically different from the others, the delayering steps are different from each other as well.

Many die delayering techniques exist, e.g., plasma etching, reactive ion etching, wet chemical etching, etc. A typical die delayering sequence starts with either plasma or reactive ion etching to remove the nitride passivation on top of the die surface, followed by a series of wet chemical etching steps to remove the rest of the die layers.

Plasma etching is a dry and anisotropic (in one direction only) etching process that consists of the following steps: 1) a glow discharge produces chemically reactive species from a relatively inert gas; 2) the species diffuse to and get adsorbed by the surface to be etched; 3) the species reacts with the surface, producing a volatile byproduct; 4) the byproduct is desorbed from the surface, completing the etch process; and 5) the byproduct is released to the bulk gas.

Reactive ion etching is similar to plasma etching, except that it involves bombardment of the surface being etched with accelerated reactive ions. These accelerated ions sputter material off the substrate as they hit its surface, as well as react with the substrate material. Thus, with RIE, etching is accomplished by two processes: sputtering and chemical reaction.

Wet etching involves the application of liquid solutions to the die surface to remove one or more layers of materials or to highlight defects. The chemicals used during wet etching depends on the etching selectivity desired. For example, HF is not selective, i.e., it can be used to etch out almost all the layers on the die surface. On the other hand, hydrogen peroxide is highly selective and would etch out only the TiW layer on the die surface. Wet etching is always followed by a an acetone or alcohol rinse, which in turn is followed by a D.I. water rinse.

Figure 1. Examples of Reactive Ion/Plasma Etchers

Table 1 shows examples of die deprocessing steps. Note that the amounts of etch time were not included in this table because these will depend on the thickness of the layer to be etched.

Silicon defects may be highlighted with either a Wright etch or a SIRTL etch.

Table 1. Examples of Die Deprocessing Steps

Layer to be Etched	Example of Useable Etch
Nitride Passivation	RIE using 90%/10% CF₄/O₂ gas mixture
Silicon Dioxide	Silox Etch at 25 deg C
Aluminum Metal Line	Phosphoric/Acetic/Nitric Acid (PAN) Etch at 60 deg C
Ti-W Barrier Metal	Hydrogen Peroxide at 60 deg C
SiCr Thin Film Resistor	RIE using 90%/10% CF₄/O₂ gas mixture
Polysilicon Layer	Polysilicon Wet Etch at 25 deg C

See also: Wet Etching Recipes

Sectioning; Focused Ion Beam; SEM/TEM; Wet Etching Recipes

FA Lab Equipment; Basic FA Flows; Package Failures; Die Failures

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