Advanced Monitoring of TMAH Solution

Authored by Michael MacEwan, Jingjing Wang, Eugene Shalyt, Chuannan Bai, Guang Liang, Vishal Parekh
Presented at the 12th Annual Symposium on Ultra Clean Processing of Semiconductor Surfaces in Brussels, Belgium, 2014.

ECI Technology, Totowa, NJ 07512, USA

Outline

• Project Goal
• Importance of TMAH in Semiconductor Processing
• Analysis Method Development
  • TMAH
  • Carbonate
  • Surfactant
• Field Process Data
• Conclusion

Project Goals

Develop Metrology to Facilitate Process Improvement and Knowledge for TMAH Solutions.

Chemical Structure of TMAH

Tetra Methyl Ammonium Hydroxide – a non-metallic source of the Hydroxyde ion.

Advantages of TMAH

• No alkali metals {Li, Na, K} used as potential contaminant
• Used in positive photoresist developers
• Anisotropy (111):(100) ~ 1:10 to 1:35
• Does not significantly etch SiO₂ or Al (bond wire safe)

TMAH Applications – Lithography

It desolves acidic photoresist after exposure

Rate is concentration dependent and slows over time.

TMAH Applications – Etch

Etch rate is effected by TMAH: rate increases at lower concentration.

Methods of Analysis for TMAH

Application requirements are 0.002 wt% accuracy.

Analysis of TMAH – pH

• Examples of concentration change on pH:
• 0.01 wt% change in TMAH ~ 0.1 pH change
• 0.001 wt% change in TMAH ~ 0.01 pH change

Current on-line techniques (ISFET, Optical) have performance to 0.1 pH units – not enough sensitivity

Analysis of TMAH – Titration

Titration is effected by the presence of carbonate
40ppm CO₃^2- will cause 0.006 wt% error

Analysis of TMAH – NIR

TMAH concentration range from 0-3 wt%

Error is within ±0.015 wt% for all samples – still not enough accuracy to achieve the goal.

Analysis of TMAH – Conductivity

Excellent linearity within process operating range

±0.001 wt% reproducibility and accuracy obtained by temperature correction.

Analysis of Carbonate – Spectroscopy

• Surfactant does not interfere with CO₃^{2- analysis}
• ^{Test sample contains trace levels of CO₃2-}
• Range of 0-40ppm CO₃^2- is linear

Carbonate absorption in TMAH

CO₃^2- increase significantly in unprotected sample over 14 hours

Analysis of Surfactant – Surface Tension

Surfactant in TMAH solution decreases the ST
ST measurement is not sensitive in 172-175 ppm range.

Analysis of Surfactant – CVS

CVS of TMAH-Surfactant sample – Method is sensitive to low level of surfactant change.

CVS Response for Surfactant Concentration

Increasing surfactant concentration has a stronger effect.

Proposed configuration of Analyzer

TMAH Etch Field Process Data

Conclusion

1. A comprehensive suite of automated analytical methods has been developed to meet the challenges of modern semiconductor TMAH applications for Sigma Etch and PR development:
2. TMAH/DI
3. Carbonate
4. Surfactant

Methods have been successfully field-validated.

Thank you for your attention.