Silicon Nitride etch has been a building block of Semiconductor manufacturing for many years. The overall Si etch rate is dominated by the combination of process temperature and %H2O. Selectivity is controlled by Si level. Water content can be monitored through conductivity, refractive index, or the most preferred method, non-contact Near Infrared (NIR) spectroscopy. There is a variety of commercial analyzers designed for this purpose.
The main challenge is measurement of Si. We have previously described an automated method for analysis of Si in traditional Si3N4 etching solution. However, high selectivity processes require new solutions.
H3PO4 and H2O Measurement
H3PO4 and its counterpart H2O are measured by both NIR spectroscopy and conductivity methods. Table 1 summarizes the performance of the two methods.
Comparison of H3PO4 results between on-line automated NIR method and off-line ICP-MS:
The on-line results are comparable to those of ICP-MS, but with much better time response (<5min) and automated sampling/feedback (lab analysis by ICP-MS can take several weeks with fab logistics.)
Conductivity represents mobility of the ions when under the driving force of an electrical field and is highly sensitive to temperature. Modern temperature control devices enable efficient temperature control so that the effect of temperature is greatly suppressed. The figure above shows a typical conductivity calibration curve with temperature correction, which has a good correlation with H3PO4 concentration.
Silicon is measured by adding predetermined concentrations of Flouride ions to a predetermined amount of etchant solution, and measuring the potential of a Flouride Ion Specific Electrode (FISE) in this test solution. Under ideal conditions, the potential (E) of a FISE is given by the well known Nernst equation:
E = E0 – (2.303 RT/F) log [F–]
Si Measurement in Low Temperature Etch
Si is measured in a wet bench low temperature hot phosphoric etch process.
- Reagent with Carboxylic acid added improves the sensitivity.
- Sensitivity is further studied with various fractions of Acetic acid in the reagent
- The accuracy of this method with Carboxylic acid in the reagent is evaluated by off-line ICP-MS method. The results from this improved Flouride method match those from ICP-MS.
- Good stability of Organosilicate results by the same method with Carboxylic acid in the reagent.
- All measured results have an accuracy of <2%.
A variety of methods have been developed to measure the Silicon Nitride etch process bath in realtime. Results from these analyses can be used for tight process control to achieve high selectivity for Silicon Nitride removal.
S.J. Baffat, M.S. Lucey, M.R. Yalamanchilli “Hot Phosphoric Acid APC for Silicon Nitride Etch”, Semiconductor International, 8/1/2002
Hong et al. “Compositions for Etching and Methods of Forming a Semiconductor Device Using the Same”, US Patent 9,136,120
Cho et al. “Etching Composition and Method for Fabricating Semiconductor Device Using the Same”, US Patent 8,821,752
Nowling et al. “Low Temperature Etching of Silicon Nitride Structures Using Phosphoric Acid Solutions”, US Patent 8,716,146
Shalyt et al. “Analysis of Silicon Concentration in Phosphoric Acid Etchant Solutions”, US Patent 8,008,087
E. Shalyt, G. Liang, P. Bratin, C. Lin “Real-Time Monitoring for Control of Cleaning and Etching Solutions” Proceeds of SPWCC Conference, USA, 2007
Shalyt et al. “Analysis of Silicon Concentration in Phosphoric Acid Etchant Solutions” US Patent Application 20160018358