The metrology of the highly selective Si-nitride etching

The etching of silicon nitride has been one of the foundations of semiconductor manufacture for several years. The total etching rate for silicon (Si) is determined by the combination of process temperature and H2O, while the selectivity is defined by the Si content.

It is possible to monitor water content by refractive index, conductivity or non-contact near infrared (NIR) spectroscopy – the latter is the most preferred method. A number of commercial analyzers have been developed for this purpose.

The measurement of Si is the main challenge in this process, and an automated method for analyzing Si in a conventional Si3N4 etching solution was previously described. However, processes with high selectivity require the use of new solutions.

A number of methods have been developed for real-time measurement of the silicon nitride etch process bath. The results of these analyzes can be used for stringent process control in order to achieve a high level of selectivity for silicon nitride removal.

H3PO4 and H2O measurement

Both H3PO4 and H2O can be measured with both NIR spectroscopy and conductivity methods. The performance of the two methods is summarized in Table 1.

Source: ECI technology

A comparison of the H3PO4 results between an automated online NIR method and an offline ICP-MS shows that online results are comparable to ICP-MS results, but with a much better response time (<5 minutes) and the additional one Benefit from automated sampling and feedback. It should be noted that the laboratory analysis via ICP-MS for Fab Logistics can take several weeks.

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Conductivity represents ion mobility under the driving force of an electric field. The conductivity is very temperature sensitive and requires the use of modern temperature control devices to enable efficient temperature control and greatly suppress the effects of temperature changes.

The figure above shows a characteristic conductivity calibration curve including temperature correction. This confirms a good correlation with the H3PO4 concentration.

Si measurement

The measurement of silicon takes place via the addition of predetermined concentrations of fluoride ions to a predetermined amount of etching solution. The potential of a fluoride-ion-specific electrode (FISE) is measured in this test solution.

In the ideal case, the potential (E) of a FISE results from the Nernst equation:

E = E0 – (2.303 RT / F) log [F–]

Si measurement in low temperature etching

The Si measurement is performed using a wet bench low temperature hot phosphor etching process.

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Organosilicate measurement

Adding carboxylic acid to the reagent improves sensitivity. The sensitivity can also be examined by adding various acetic acid fractions contained in the reagent.

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The accuracy of this method with carboxylic acid in the reagent can be assessed using an offline ICP-MS method. The results of this improved fluoride method were found to be similar to ICP-MS.

The results of the same procedure with carboxylic acid in the reagent also showed good stability of the organosilicate, all measured results being confirmed with an accuracy of <2%.

Image source: ECI technology


A number of methods have been developed to provide accurate real-time measurement of the silicon nitride etch process bath. The results of these analyzes can be used in a strict process control in order to achieve a high selectivity for the removal of silicon nitride.


  1. SJ Baffat, MS Lucey, MR Yalamanchilli “Hot Phosphoric Acid APC for Silicon Nitride Etch”, Semiconductor International, 08/01/2002
  2. Hong et al. “Etching compositions and methods of forming a semiconductor device using the same,” US Patent 9,136,120
  3. Cho et al. “Etching Composition and Method of Making Semiconductor Devices Using Same,” US Patent 8,821,752
  4. Nowling et al. “Low temperature etching of silicon nitride structures using phosphoric acid solutions”, US patent 8,716,146
  5. Shalyt et al. “Analysis of Silicon Concentration in Phosphoric Acid Etching Solutions”, US Patent 8,008,087
  6. E. Shalyt, G. Liang, P. Bratin, C. Lin “Real-time monitoring for the control of cleaning and etching solutions” Proceeds from the SPWCC conference, USA, 2007
  7. Shalyt et al. “Analysis of the silicon concentration in phosphoric acid etching solutions” US patent application 20160018358

This information has been obtained, reviewed and adapted from materials from ECI Technology.

For more information on this source, see ECI Technology.

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