Patterned ALD sidewall metallization on CMOS MEMS and applications

In the trend of Internet of Things (IoT), technologies that integrate CMOS (complementary metal-oxide-semiconductor) electronics with MEMS (microelectromechanical systems) exhibit a promising way to fulfill ever smaller, more power-efficient, and more customized and intelligent devices through system integration and miniaturization. To enable the next-generation micro-sensors and other micro-devices, micromechanical structures made from the CMOS back-end-of-line dielectric and metal layers provide for low-cost monolithic integration of MEMS with circuits. 

One issue with this CMOS MEMS technology is the dielectric sidewalls on the released structures and the resulting charging phenomena that degrade the performance. The dielectric sidewalls also prevent the electrical conduction under the mechanical forces after the MEMS structure is released. To solve these issues, this work develops a novel CMOS MEMS post-processing technique by integrating a selective atomic layer deposition (ALD) coating over the microstructural sidewalls of interest. A conductive ALD layer, such as Pt, is applied to the capacitor sidewalls to eliminate the dielectric charging phenomenon. Moreover, CMOS MEMS metal-metal contact switch is demonstrated by coating ALD metal on contact sidewalls. 

This work successfully demonstrates the implementation of the ALD sidewall patterning technique and validation through multiple CMOS MEMS devices and applications, including resonator oscillators, switches, and accelerometers. A generalized lift-off-based ALD sidewall patterning process is implemented, which can support the high-aspect-ratio MEMS structure (at least > 10:1). The process is able to coat a conformal ALD film on the selected sidewalls of interest without causing a short circuit. 

A conductive ALD film is patterned on the capacitor sidewalls of a resonator oscillator to eliminate dielectric charging phenomena that cause the resonant frequency drift. TiO₂-coated and Pt-coated devices are fabricated, measured, and compared with the uncoated counterparts. The charging time constant is reduced by over three orders of magnitude. Without the drift from charging, the instability at room temperature of 1.3 ppm is demonstrated computed from Allan deviation analysis at the averaging time of 300 s. 

Benefiting from the ALD sidewall patterning process, a lateral metal-metal contact switch is implemented in CMOS MEMS by patterning Pt ALD metal on the sidewalls and interconnecting to metal layers on the structural area of the contact. The lateral motional configuration allows the design of different tips and flexural springs as compliant blocking contacts with an intent to reduce the contact resistance. The results show that the lateral switches with a spring contact design provide the capability to lower the overall contact resistance. 

In this thesis, we develop a generalized ALD sidewall patterning process to eliminate the dielectric charging effect, explore the lateral ohmic contact switch, and study the benefits provided in this post-processing. This technique opens a way in CMOS MEMS for higher performance and wider application by modifying its dielectric sidewalls by coating with a conductive ALD film.