Temperature-Controlled Depth Profiling of Poly(methyl methacrylate) Using Cluster Secondary Ion Mass Spectrometry.  2. Investigation of Sputter-Induced Topography, Chemical Damage, and Depolymerization Effects

• Published in: Analytical chemistry (Washington) 2007-02, Vol.79 (3), p.837-845
• Main Authors: Mahoney, Christine M, Fahey, Albert J, Gillen, Greg, Xu, Chang, Batteas, James D
• Format: Article
• Language: English
• Subjects: Analytical chemistry; Chemistry; Exact sciences and technology; Mass spectrometry; Polymers; Spectrometric and optical methods; Temperature effects; Thin films
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac061357+

Poly(methyl methacrylate) (PMMA) thin films (∼150 nm) on silicon were bombarded with SF5 + polyatomic primary ion projectiles at −75 °C, 25 °C, and 125 °C. The crater bottoms were then characterized using a combination of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). AFM results indicated increased sputter-induced topography formation in the order of −75 °C < 125 °C < 25 °C, consistent with earlier SIMS depth profile results which illustrated optimum depth profile characteristics at low temperatures and less favorable characteristics at room temperature. XPS results indicated that there was a significant amount of C remaining at the crater bottom at 25 °C, suggesting that there is a large amount of organic material remaining despite the loss in characteristic PMMA secondary ion signal in the SIMS depth profile. Specifically, C/Si ratios increased in the following order:  −75 °C < 125 °C < 25 °C, consistent with the trend in topography observed in the AFM results. High-resolution C(1s) spectra of the PMMA film indicated a decrease in the O−CO component with sputtering at both −75 °C and 25 °C. However, there was very little change in the C(1s) spectra in samples sputtered at 125 °C. This was determined to be a result of ion-induced depolymerization which is expected to occur at higher temperatures in PMMA. Residual gas analysis (RGA) gave results that were consistent with this hypothesis, showing increased amounts of PMMA monomer at higher temperatures. Principle components analysis (PCA) of SIMS spectra showed increased PMMA secondary ion intensities coupled with increased O signal in PMMA sputtered at 125 °C. Conversely, SIMS spectra acquired in the sputtered PMMA at −75 °C, and to a smaller degree at 25 °C, showed increased C signals, decreased O intensities, and the appearance of peaks indicative of polycyclic aromatic hydrocarbons, all consistent with increased chemical damage. Overall, these results indicate that while there is increased damage occurring at −75 °C, there is still a significant improvement in the depth profile characteristics. It is concluded that the enhancement in low-temperature depth profiles in PMMA results mainly from the changes in the physical properties of the PMMA at low temperatures, yielding a significant reduction in sputter-induced topgraphy.