Manufacturing Issues of BEOL CMOS-MEMS Devices

Abstract

In this paper we present a comprehensive report on the issues found during the manufacturing of high-yield CMOS-MEMS sensors based on vapor-phase hydrogen fluoride (vapor-HF ) oxide etching.During the study we have identified the main issues affecting CMOS-MEMS high-yield manufacturing regarding the silicon oxide as a sacrificial material, the passivation as a release mask, the BEOL as structural material for MEMS design and the aluminum-sputtering as a sealing layer for the MEMS cavity. This study has been carried out by systematically analyzing over 100 full wafers in 10 different runs on four different foundries using 0.5 µm, 0.18 µm and 0.15 µm CMOS processes, containing both test structures and fullsensor designs.

Monolithic Sensor Integration in CMOS Technologies

“…Despite that the CMOS process is not a MEMS process, our analysis of more than 100 full wafers in 10 different runs on four different foundries using 0.5 µm, 0.25 µm, 0.18 µm and 0.15 µm CMOS-process nodes [8], shows that there is an excellent mechanical repeatability among wafers [16], [35], demonstrating that volume-production is feasible, reaching a 95% yield even after standard plastic-mold packaging.…”

“…The design of MEMS sensors and actuators using the CMOS manufacturing line is very process-dependent [8]. Special design techniques must be used to overcome the inherent CMOS process limitations and uncontrolled mechanical characteristics [16].…”

“…There are several techniques for achieving true monolithic integration (see [7] and [8] for a comprehensive list), but on this paper we focus on what is called CMOS-MEMS BEOL (Back-End Of Line) micromachining, that is, the use of the metallization interconnection layers of the CMOS process as structural layers for the MEMS device [9]- [14]. In its simplest form [11], by just performing a release step, the silicon oxide surrounding the metallization interconnection layers is selectively removed, allowing the structures to move in response to external stimuli.…”

“…In its simplest form [11], by just performing a release step, the silicon oxide surrounding the metallization interconnection layers is selectively removed, allowing the structures to move in response to external stimuli. This technique has the lowest cost and complexity among all options and has already shown its potential in high-yield, full-wafer scale, production environments [8], [15].…”

CMOS-MEMS Oscillator Architecture and Phase Noise: A Mini-Review

“…In addition to traditional quartz resonator-based reference oscillators (Vittoz et al, 1988) and quartz crystal microbalance (QCM) sensors (Ferrari et al, 2006), miniaturized oscillators using microelectromechanical systems (MEMS) technology have been intensively studied in both academia and industry over the past two decades (Lavasani et al, 2011;Chance et al, 2014;Zaliasl et al, 2015;Naing et al, 2020;Kalia et al, 2021;Chang et al, 2022) to be used in demanding applications with size constraints. Among the existing MEMS oscillator fabrication approaches, the CMOS-MEMS technology is one of the most promising approaches to achieving the monolithic integration of mechanical resonators and electronic circuits (Xie et al, 2002;Dai et al, 2005;Chen et al, 2011;Li C.-S. et al, 2012;Chen et al, 2019;Valle et al, 2021). In this approach, MEMS mechanical structures can be formed from the back-end of line (BEOL) metal/dielectric (Verd et al, 2006;Li et al, 2012c;Li et al, 2015b;Liu et al, 2018) or the front-end of line (FEOL) polysilicon layers (Verd et al, 2005;Lopez et al, 2009) by removing predefined sacrificial layers, thereby eliminating the thermal budget constraints that are often encountered in other custom IC-MEMS integration processes (Fedder et al, 2008).…”