MEMS Vibrating Structures and Metal Resistors

MEMS (fall 2024)

Introduction:

Micro-Electro-Mechanical Systems (MEMS) devices are essential components in a wide range of applications due to their precision and versatility. Achieving optimal mechanical and electrical properties in MEMS designs is critical for enhancing device performance. In this work, we present two design series for MEMS devices: one focused on vibrating microcantilevers and the other on thin-film resistors. Each series includes three designs that explore the limits of key parameters — maximizing, minimizing, and balancing either resonant frequency or resistance. These designs are shaped by both theoretical constraints, such as material properties and stiction, and process limitations, including fabrication resolution and wafer dimensions.

We utilize a silicon-on-insulator (SOI) substrate and a multi-step fabrication process to realize these designs. By leveraging established modeling techniques and considering process constraints, we aim to create robust and efficient MEMS devices suitable for a variety of applications.

Methods:

Design Approach

We developed two series of MEMS device designs to explore a range of mechanical and electrical characteristics.

  • Microcantilever Series: This series focuses on optimizing the resonant frequency of vibrating beams with three variations: maximum, minimum, and balanced resonant frequency.

  • Thin-Film Resistor Series: This series targets the resistance of thin-film resistors, with designs that maximize, minimize, or balance resistance.

Design Constraints

Our designs are constrained by two main factors:

  1. Theoretical Limitations: Physical properties of materials and stiction effects that influence device dimensions and behavior.

  2. Process Limitations: Constraints such as mask resolution (20 μm), wafer area, and fabrication environment cleanliness.

Fabrication Process

We use a silicon-on-insulator (SOI) substrate comprising:

  • A 1.9 μm silicon device layer

  • A 1 μm SiO₂ buried layer

  • A bulk silicon handle (~0.5 mm thick)

The fabrication process involves four key steps:

  1. Oxide Stripping: Remove surface oxides from the wafer’s top and bottom surfaces (thickness ~200 nm).

  2. Metal Deposition and Patterning: Deposit a 70 nm gold layer (for conductivity) and a 10 nm chromium layer (for adhesion). Pattern these layers to form resistor designs.

  3. Silicon Etching: Define the microcantilever structures by patterning and etching the silicon device layer using SF₆ plasma.

  4. Undercutting and Release: Perform a timed HF etch to undercut the buried SiO₂ layer, releasing the microcantilever beams (etch rate ~1 μm/min for 30 minutes).

These steps accommodate the 20 μm minimum feature size imposed by our mask-making process, ensuring accurate and reliable fabrication.

Results:

The fabrication results for the MEMS devices showed successful deposition and etching processes. The deposition rates for chromium and gold were 2.5 Å/s and 8.2 Å/s, respectively, resulting in a combined 80 nm Cr/Au layer. Silicon etching with SF₆ plasma achieved a rate of 893.29 nm/min, while the photoresist SPR220 etched at 150 nm/min. The SiO₂ undercut rate in a 4:1 HF:HCl mixture was approximately 1 μm/min, producing an etch depth slightly above 30 μm after 30 minutes. Microcantilever beams were fabricated with dimensions close to design specifications; however, direct measurement of their resonant frequencies was not possible due to their high frequencies (over 500 kHz) and limitations of the laser vibrometer’s microscope. All beams were successfully released without stiction. For the thin-film resistors, the measured resistances revealed significant contact resistance, which affected the low- and medium-resistance designs, masking the actual values. Despite this, the high-resistance resistor closely matched the theoretical expectation of 1.236 kΩ. To compensate for the inability to measure beam resonances, a MEMS microphone was tested with laser vibrometry, showing a resonance peak at 48 kHz. These results illustrate the balance between theoretical goals and practical limitations, highlighting the importance of precise fabrication and measurement techniques in MEMS design and production​