Wright etch
Encyclopedia
The Wright Etch is a preferential etch for revealing defects in (100) and (111) oriented, p- and n-type silicon
. It was developed by Margaret Wright Jenkins in 1976 while working in research and development
at Motorola Inc. It was published in 1977 . This etchant reveals clearly defined oxidation-induced stacking faults, dislocations, swirls and striations with minimum surface roughness or extraneous pitting. These defects are known causes of shorts and current leakage in finished semiconductor
devices (such as transistor
s) should they fall across isolated junctions. A relatively slow etch rate (~1 micrometre per minute) at room temperature provides etch control. The long shelf life of this etchant allows the solution to be stored in large quantities. The present report summarizes the highlights of the Wright etch. For a detailed discussion of observation and findings, please consult the original publication .
60 ml conc. HF (hydrofluoric acid
)
30 ml conc. HNO3 (nitric acid
)
30 ml of 5 mole CrO3 (mix 1 gram of chromium trioxide
per 2 ml of water)
2 grams Cu (NO3)2 . 3H2O (copper nitrate)
60 ml conc. CH3COOH (acetic acid
)
60 ml H2O (deionized water)
In mixing the solution, the best results are obtained by first dissolving the copper nitrate in the given amount of water; otherwise the order of mixing is not critical.
by a suitable oxidizing agent(s). Then the resulting oxide layer is removed from the surface by dissolution in a suitable solvent
, usually HF. This is a continuous process during the etch cycle. In order to delineate a crystal defect, the defect area must be oxidized at a slower or faster rate than the surrounding area thereby forming a mound or pit.
In the present system, the silicon is oxidized with HNO3, CrO3 solution (Cr2O7=) and Cu (NO3)2. The Cr2O7=, a strong oxidizing agent, is considered to be the principal oxidizing agent
. The ratio of HNO3 to CrO3 solution stated in the formula produces a superior etched surface. Other ratios produce less desirable finishes. With the addition of a small amount of Cu (NO3)2, the definition of the defect was enhanced. Therefore it is believed that the Cu (NO3)2 affects the localized differential oxidation rate at the defect site. The addition of the acetic acid gave the background surface of the etched silicon a smooth finish. It is theorized that this effect is attributed to the wetting action of the acetic acid which prevents the formation of bubbles during etching.
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
. It was developed by Margaret Wright Jenkins in 1976 while working in research and development
Research and development
The phrase research and development , according to the Organization for Economic Co-operation and Development, refers to "creative work undertaken on a systematic basis in order to increase the stock of knowledge, including knowledge of man, culture and society, and the use of this stock of...
at Motorola Inc. It was published in 1977 . This etchant reveals clearly defined oxidation-induced stacking faults, dislocations, swirls and striations with minimum surface roughness or extraneous pitting. These defects are known causes of shorts and current leakage in finished semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
devices (such as transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s) should they fall across isolated junctions. A relatively slow etch rate (~1 micrometre per minute) at room temperature provides etch control. The long shelf life of this etchant allows the solution to be stored in large quantities. The present report summarizes the highlights of the Wright etch. For a detailed discussion of observation and findings, please consult the original publication .
Etch formula
The composition of the Wright etch is as follows:60 ml conc. HF (hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
)
30 ml conc. HNO3 (nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
)
30 ml of 5 mole CrO3 (mix 1 gram of chromium trioxide
Chromium trioxide
Chromium trioxide is the inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.This compound is a dark-red/orange brown solid, which dissolves in water concomitant with hydrolysis...
per 2 ml of water)
2 grams Cu (NO3)2 . 3H2O (copper nitrate)
60 ml conc. CH3COOH (acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
)
60 ml H2O (deionized water)
In mixing the solution, the best results are obtained by first dissolving the copper nitrate in the given amount of water; otherwise the order of mixing is not critical.
Etch mechanism
The Wright etch consistently produces well-defined etch figures of common defects on silicon surfaces. This attribute is attributed to the interactions of the selected chemicals in the formula. Robbins and Schwartz described chemical etching of silicon in detail using an HF, HNO3 and H2O system; and an HF, HNO3, H2O and HC2H3O2 (Acetic acid) system. Briefly, the etching of silicon is a two-step process. First, the top surface of the silicon is converted into a soluble oxideOxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
by a suitable oxidizing agent(s). Then the resulting oxide layer is removed from the surface by dissolution in a suitable solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
, usually HF. This is a continuous process during the etch cycle. In order to delineate a crystal defect, the defect area must be oxidized at a slower or faster rate than the surrounding area thereby forming a mound or pit.
In the present system, the silicon is oxidized with HNO3, CrO3 solution (Cr2O7=) and Cu (NO3)2. The Cr2O7=, a strong oxidizing agent, is considered to be the principal oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
. The ratio of HNO3 to CrO3 solution stated in the formula produces a superior etched surface. Other ratios produce less desirable finishes. With the addition of a small amount of Cu (NO3)2, the definition of the defect was enhanced. Therefore it is believed that the Cu (NO3)2 affects the localized differential oxidation rate at the defect site. The addition of the acetic acid gave the background surface of the etched silicon a smooth finish. It is theorized that this effect is attributed to the wetting action of the acetic acid which prevents the formation of bubbles during etching.
Summary
This etch process is a quick and reliable method of determining the integrity of pre-processed polished silicon wafers or to reveal defects that may be induced at any point during wafer processing. It has been demonstrated that Wright etch is superior in revealing stacking faults and dislocation etch figures when compared with those revealed by Sirtl and Secco etchings. Please see references for comparison micrographs. This etch is widely used in failure analysis of electrical devices at various wafer processing stages as seen in articles: "Pipeline Defects in Flash Devices Associated with Rings OSF" * and "Defect Etching in Silicon" * http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_6/advanced/t6_1_2.html . In these publications, by comparison, the Wright etch was the preferred etchant to reveal defects in silicon crystals.External links
- http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_6/advanced/t6_1_2.html