Assessing the Oxidative Degradation of N-Methylpyrrolidone (NMP) in Microelectronic Fabrication Processes by Using a Multiplatform Analytical Approach.

Gavin Lennon,Matthew Klun,Robert Pieh,Ragini Ramdas,Scott J. Funston,Sara Dobbin,Shannon Willox,Diego F. Cobice,Stewart Fairlie

doi:10.1155/2020/8265054

Abstract

During the construction of recording head devices, corrosion of metal features and subsequent deposition of corrosion by-products have been observed. Previous studies have determined that the use of N-methylpyrrolidone (NMP) may be a contributing factor. In this study, we report the use of a novel multiplatform analytical approach comprising of pH, liquid chromatography/UV detection (LC/UV), inductively coupled plasma optical emission spectroscopy (ICP-OES), and LC/mass spectrometry (LC/MS) to demonstrate that reaction conditions mimicking those of general photoresist removal processes can invoke the oxidation of NMP during the photolithography lift-off process. For the first time, we have confirmed that the oxidation of NMP lowers the pH, facilitating the dissolution of transition metals deposited on wafer substrates during post-mask and pre-lift-off processes in microelectronic fabrication. This negatively impacts upon the performance of the microelectronic device. Furthermore, it was shown that, by performing the process in an inert atmosphere, the oxidation of NMP was suppressed and the pH was stabilized, suggesting an affordable modification of the photolithography lift-off stage to enhance the quality of recording heads. This novel study has provided key data that may have a significant impact on current and future fabrication process design, optimization, and control. Results here suggest the inclusion of pH as a key process input variable (KPIV) during the design of new photoresist removal processes.

Highlights

Semiconductor device fabrication is the process used to create integrated circuits that are present in electrical and electronic devices [1]. e fabrication process shown in Figure 1 begins with a wafer of semiconductor material and includes a sequence of photographic and chemical processing steps during which electronic circuits are gradually created on the wafer substrate [2]
Assessment of NMP pH Variation under Standard Operating Conditions. e stability of the pH of NMP used in the photolithography lift-off process was first observed over a period of time under standard operating conditions in which the chemical is exposed to air. e pH remained nearly constant, ranging between pH 10.39 and 9.69 over a period of 144 h
Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) Analysis of NMP under Inert Operating Conditions. e dissolution of elements from the surface of WIP wafers during the photolithography lift-off process was verified by using ICP-OES analysis to observe the variation in elemental content within the NMP over its lifetime in the fabrication process (Figure 2(b))

Summary

Introduction

Semiconductor device fabrication is the process used to create integrated circuits that are present in electrical and electronic devices [1]. e fabrication process shown in Figure 1 begins with a wafer of semiconductor material and includes a sequence of photographic and chemical processing steps during which electronic circuits are gradually created on the wafer substrate [2]. Analysis of the photolithography engineering process indicated that the issue was the result of transition metal feature corrosion at the contact reader stack occurring during the photoresist lift-off process (unpublished data). In this instance, the chemical in question was NMP which is known to dissolve certain transition metals [13]. Is exposes the underlying copper metal allowing further oxidation-dissolution process to proceed Based on this and other work [15] relating to the impact of system pH on metal dissolution, we hypothesize that NMP itself has a considerably basic pH, exposure of the chemical to work-in-progress (WIP) wafers may initiate a chemical transformation which acts to solvate transition metals on the surface of the wafer, whilst simultaneously lowering the pH of the system— accelerating the dissolution process and/or the NMP degradation process. We report the development and application of a novel multiplatform analytical approach which combines the use of pH, LC/UV, ICP-OES, and LC/MS methods to assess NMP oxidative degradation pathways and monitor its progression within photolithography lift-off processes

Materials and Methods

Results and Discussion

Conflicts of Interest