Future Work

As compared to commercial AE sensors, the capacitive MEMS AE sensors hold potential advantages in terms of cost, size, and weight. Via Micro-Electro-Mechanical-Systems (MEMS) technologies, this new type of sensors can be batch fabricated of a large quantity. Numerous MEMS AE sensors can be jointly fabricated on the same chip while having different resonance frequencies. Thus, the acoustic emission signals can be detected at different frequencies across a wider frequency range. Also, the MEMS technology can be integrated with amplifiers and antenna on the same chip.

Although the output signal of the MEMS AE sensor has shown noticeable improvement after packaging, the environment noise still affects the response of the MEMS AE sensor. As one more approach to assess the overall performance, some form of vacuum packaging should be pursued to completely mitigate the effects of the squeeze-film damping and environmental noise. Similarly, the MEMS AE sensors can be tested within a vacuum chamber where the Q-factor can be modified by operation under different vacuum level. Through vacuum packaging, signals with lower intensity from the AE sources can be used as the AE sensors under vacuum should be able to detect weaker AE signals.

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APPENDIX A: COPYRIGHT PERMISSIONS

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APPENDIX B: FABRICATION PROCESS

The fabrication process is designed by one of the author’s group member, Adrian Avila [19] and improved by another group member, Adnan Zaman. The cross-section of the fabrication process is shown from figure 8 to figure 17

 Bottom Electrode:

The substrate of this acoustic emission sensor is a low resistivity silicon wafer. The wafers are cleaned by the RCA cleaning and the solvent cleaning consisting of acetone, methanol, and isopropanol. The silicon wafer is coated with an insulation layer which is 1 μm of thermal silicon oxide or 300 nm of Si3N4. Using acetone, methanol, and DI water to clean wafers after coating the

insulation layer. In order to reduce the humidity, the following steps are drying the wafers by N2

and baking at 110 ᵒC for 5 minutes. The next step is to spin the photoresist and complete the lithography.

In the beginning, the hexamethyldisilazane (HMDS) is spun at 3500 rpm for 1 minute to improve the adhesion on the insulation layer. The bottom electrode is fabricated by a lift-off technique and is a bi-layer processing. The first layer is Polydimthylglutarimide resist, which is also called LOR. The thickness of LOR is decided by the speed of spinning. Typically, the spinning speed is from 2500 to 4500 rpm. The thickness of LOR is from a few hundreds of nanometers to several microns. After spinning is soft bake at 180ᵒC for 510 seconds. The second layer is AZ 1512 photoresist. The first step of the spinning speed is 500 rmp for 10 seconds and the following speed is 2500 rmp for 45 seconds. The soft bake time is 50 seconds. After the soft bake is exposed under

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UV light and post bake at 105ᵒC for 50 seconds. The last step is to dip the wafer into the developer, AZ 726, for 25 seconds. The LOR is not a UV sensitive resist so in order to produce undercuts in LOR layer. It is required to adjust the temperature and soft bake time.

After the lithography step finishes, the wafers are put into Tegal O2 Asher at 50 W and 100 mTorr for 2 minutes to remove the photoresist residual. The next step is to deposit bottom electrode. There are two materials used in this work: platinum for 200nm or ruthenium for 160 nm. The Crown for 20 nm is used to improve the adhesion for the metal layer. The last step is the lift-off process. The wafers are dipped into AZ-400T stripper at 180 °C for 45 to 60 minutes and cleaned with DI water. The bottom layer is finished.

 Insulation Layer:

The insulation layer is deposited by Savannah 200 atomic layer deposition (ALD) system. There are two materials for the insulation layer: Aluminum oxide (Al2O3) or Hafnium oxide (HfO2)

for 45 nm. When choosing the insulation material, it depends on the post-processing method. The HfO2 is better etching by a buffered oxide etch (BOE) than Al2O3.

 Sacrificial Layer:

The sacrificial layer is deciding the gap distance between the top electrode and the bottom electrode. The plasma-enhanced chemical vapor deposition (PECVD) SiO2 is utilizing for the

sacrificial layer. To deposit SiO2 by PECVD, the RF power is 50 W and operates at 13.56 MHz.

the chamber temperature is 250 °C and the chuck temperature is 60 °C. The pressure is 800mTorr. The flowing of N2O is 500 sccm and SiH4 is 110 sccm. The deposit rate is 2.3 μm per hour.

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 Anchor Patterning:

The next step is the anchor patterning the anchor position is defined by this step. The photoresist AZ 1512 is chosen for the lithography process. The first spinning speed is 500 rpm for 10 seconds and the second spinning speed is 2500 rpm for 45 seconds. The following step is the soft bake at 95 °C for 50 seconds. After exposure, post bake is at 105 °C for 50 seconds. The developer is AZ 726 for 255 seconds. After lithography step finishes, the wafers are put into Tegal O2 Asher at 50 W and 100 mTorr for 2 minutes to remove photoresist residual.

The etch method is selected by materials for the insulation layer and the sacrificial layer. For SiO2 sacrificial layer, the SiO2 layer is etched by BOE. The HfO2 is using reactive ion etching

(RIE) process to remove. This process is using by Alcatel AMS 100 inductively coupled plasma (ICP) etcher. The ICP power is 600 W and RF power is 200 W. the pressure is 2 mTorr. The flowing of Ar is 100 sccm and SF6 is 100sccm. The total time is 8 minutes. For Al2O3 is also using

BOE to etch away.

 Electroplating Seed Layer:

The seed layer is a duo-layer and made by a 20 nm titanium and 200 nm copper layer by AJA Orion 5 sputtering system. The titanium layer is acting as an adhesion layer. The titanium is sputtering at 150 W. The pressure is 3 mTorr for 8 minutes. The copper is at 100 W. The pressure is also 3 mTorr for 20 minutes.

 Top Electrode:

The top electrode decides the resonance frequency of the device and its damping characteristics. In this work, nickel is the metal used to be a top electrode. The resonance frequency from 100 kHz to 1 MHz depends on nickel top electrode layer thickness from 5 to 20 μm. The top electrode starts from the seed layer deposition step. The photoresist AZ 12XT is chosen for the

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lithography process. The first spinning speed is 500 rpm for 10 seconds and the second spinning rate is 1000 rpm for 90 seconds. The following step is the soft bake at 110 °C for 120 seconds. After exposure, the post bake is at 90 °C for 60 seconds. The developer is AZ 300 for 60 seconds. After the lithography step finishes, the wafers are put into Tegal O2 Asher at 50 W and 100 mTorr for 2 minutes to remove the photoresist residual. Before starting the electroplating process, the wafers should hard bake at 110 °C for 5 minutes. To prevent the copper oxidation influence the electroplating process, the wafer should be dipped into DI water and 30% ammonium hydroxide (NH4OH) solution for 30 seconds and wash again with DI water.

The main liquid solution for nickel electroplating process is nickel sulfamate (Ni(SO3NH2)2). The solution recipe is shown in Avila’s dissertation [19]. The equation 41 shows

the thickness for the electroplating process.

 Seed Layer Wet Etch:

After the electroplating for the top electrode, the next step is to remove the seed layer. The etchant is selected by different seed layer materials. The copper layer is removed by 1:1 solution of hydrogen peroxide (H2O2) and 30% Ammonium Hydroxide (NH4OH) and washing the wafer

with DI water. The following step is to etch away the titanium layer by in a 10: 1 solution of DI water and Hydrofluoric acid (HF)

 AE Sensor Release

The final step is to release the gap between the top and bottom electrodes. The sacrificial layer in this work is PECVD SiO2. The etchant for this step is 6:1 Buffered oxide etch (BOE)

solution. After removing away the SiO2, the released sensor should be very careful to wash with