5. Conclusion

This chapter has reviewed the mechanisms behind electrostatic tuning of MEMS resonators through modifications to the force-engagement profile of comb-drive actuators. Variable-height comb-finger tunable resonators were designed, simulated, fabricated, and tested for the first time. Such devices can provide similar tuning to variable-gap comb-finger designs, however without the penalty of increasing the device footprint. Electrostatic springs as high as 1.19 N/m (using 70 V) or 1.66 N/m (using 120 V) were demonstrated, with a maximum frequency tuning of 17% of the original f₀. Although most designs discussed in this work utilized a single gray-level, simulations were able to show that finer control of the force-engagement profile is possible by using the many intermediate heights available through gray-scale technology.

As a direct result of the development and integration of gray-scale technology presented in the first 3 chapters of this dissertation, all of the above tuning and frequency response control is provided without increasing the overall resonator footprint. While the resonant frequency and Q-factor of the devices discussed were kept low, the design and simulation principles developed can be applied to virtually any of the resonator applications mentioned previously [65, 67-69, 138-144].

CHAPTER 5: GR AY-SCALE FIBER ALIGNER I: Conce pt, Design, an d Fabrication

1. Introduction

Alignment of an optical fiber within an optoelectronic module is a continuing challenge in optoelectronic packaging, and often dominates module cost [76]. Ultimately, passive alignment and packaging techniques would be preferred for their simplicity. Passive systems utilizing silicon waferboards and flip-chip bonding have reported alignment accuracies of 1-2pm [149-151], mostly through attempts to improve process and dimensional control (and in turn increasing processing cost). Common sources of error that make passive sub-micron alignment difficult include fiber core eccentricity, fiber diameter, v-groove width and placement, or variation in etch angle [88]. Particular difficulty in configurations using flip-chip bonding has been encountered with non-uniform solder ball volume distribution, which can cause vertical shifts in alignment [152-154].

Even if high-accuracy fabrication and flip-chip bonding can be accomplished, such tight tolerances increase the cost of processing and assembly, and severely limit throughput. For example, relaxing placement tolerances from the 1pm to 20pm level can increase throughput of a pick-and-place machine by an order of magnitude [75]. Further complicating the drive for passive techniques, groups now report up to 3dB of loss from only 1-2pm of axial misalignment [155]. Thus, as current alignment requirements approach 0.2pm [85], passive alignment becomes unrealistic regardless of the amount of process control. Multi-axis on-chip methods for final alignment of the optical fiber are therefore attractive replacements for the expensive and slow macro actuators currently required to achieve sub-micron alignment.

The primary challenge for on-chip fiber alignment systems is realizing both horizontal and vertical actuation of the fiber to compensate for shifts in either direction, such as vertical shifts from solder ball irregularities [152-154]. Previous MEMS fiber actuators have demonstrated multi-axis on-chip alignment [90, 95]. However, these systems typically require specialized fiber preparation (attachment of permanent magnets to the fiber tip [90]) or rare fabrication techniques (LIGA [95]). Such requirements limit their feasibility as a packaging option. In contrast, the 2-axis fiber actuator developed in this research requires no special fiber preparation and is realized using gray-scale technology - a batch technique using standard MEMS equipment. This gray-scale fiber aligner exploits the coupled motion of opposing in-plane actuators with integrated 3-D wedges). The device creates a dynamic v-groove (controlled via MEMS in-plane actuators) to modify the horizontal and vertical position of the optical fiber [156, 157].

The developed optical fiber alignment system can act as a platform for integrated packaging of optoelectronics devices, addressing one of the most costly and time­consuming aspects of mass-producing such components. Integrated packaging platforms using the chosen fabrication techniques are inherently mass-producible and compatible with electronics integration, promoting dense integration of optical and electronic systems in a single component.

Section 5.2 will discuss the concept of operation and layout of the developed MEMS gray-scale fiber aligner. Section 5.3 will discuss the competing optical loss mechanisms in the device which serve as guidelines for system design. The layout and dimensions of each actuator component are described in Section 5.4, while the fabrication and assembly of the device are detailed in Section 5.5 and 5.6. Finally, a brief demonstration of the actuation mechanism using an optical profiler is provided in Section 5.7. The following chapter will discuss optical testing results.