ALD/ALE 2021 Session AA16: Emerging: Others (Protective Coatings, Hardness, MEMs...)

By the means of Atomic Layer Deposition (ALD), we developed piezotronic strain sensitive sensors integrated in polyimide cantilevers, where a zinc oxide (ZnO) thin film is deposited on top of patterned interdigitated platinum electrodes (Figure 1(a)). The rapidly spreading Internet-of-Things is accelerating MEMS (Micro-ElectroMechanical Systems) industry to deliver highly sensitive and miniaturized self-sensors with low consumption and cost-effective production process. Due to its high film conformality, low temperature processing, self-limiting nature and thickness control at the nanoscale level, ALD technique has emerged as an ideal technique to add new functionalities in MEMS.

We propose to rationalize the ALD processing deposition parameters on the sensor’s electrical properties and the sensitivity of detection for mechanical strain and light. We report on the evidence of negative capacitance phenomena occurring at the interface of Schottky junctions over a wide frequency range (between 20 Hz and 100 kHz). We demonstrate an original way to modulate the sensors capacitance characteristics in the presence of a light source as well as by applying a mechanical strain to the devices (Figures 1(b), (c) and (d)). The rationale behind these observations will be discussed. The ALD thin film is made of wurtzite polycrystalline zinc oxide with a privileged (002) orientation. We optimized the deposition temperature to be compatible with microfabrication processing on polymer and photoresists by thin film growth below 100 °C. Hence, Schottky junctions are realized by microstructuring interdigitated micro-combs at the interface of high work function platinum metal electrodes and a semiconducting piezoelectric ZnO thin film. The obtained piezotronic junction has the particularity of an exponential dependence of the flowing diode current as a function of the applied mechanical strain. The sensitivity is thus greatly improved with gauge factor higher than 100.

In the last stage of this work, we will present the strain sensors size miniaturization for integration in microcantilevers in a full polymer body, compatible with AFM (Atomic Force Microscopy) scanning probe operations to highlight the very high sensitivity of detection. These results open up new perspectives and applications towards the miniaturization of highly sensitive and low power consumption environmental sensors, as well as for broadband impedance matching in radio frequency applications by the means of negative capacitance devices.

Atomic-scale processing has had a major impact in the fields of microelectronics and CMOS fabrication technology, building upon the significant advances in academia. Now the field of micro-electomechanical systems (MEMS) is poised to reap the benefits of atomic-scale fabrication, as gains achieved with standard process technologies remain limited to low-cost devices. To achieve this breakthrough, ALD atomic-scale techniques will play a vital role, driving MEMS deep into the nanoscale regime by engineering innovative designs to take advantage of the scaled piezo- and ferroelectric properties encountered therein. Using a combination of ALD, ALE, and pre- and post- treatments for area-selective growth of function-enhancing features and layers, ALD can hermetically seal off devices from harsh in-process or working environments, eliminate stiction, tailor conformal multilayer geometries to provide new functionalities such as phonon crystals and metamaterials, control stress, enhance polarization and structural integrity, lower operating voltage, increase chip work density by an order of magnitude, and meet thermal, mechanical, geometrical, barrier, interface, and ferroic materials design requirements for competitive commercial MEMS technologies and devices. These breakthroughs will be enabled by a wide array of viable precursors, providing new metal oxides and metals of increasing diversity and complexity from throughout the periodic table, including the recently developed nitrides, sulfides, and tellurides [1-3]. The resulting new materials, along with rapidly occurring growth and integration of computer modeling for precursor-surface chemical reactions [4] and hardware improvements [3] will enhance the scale and pace of MEMS modernization. ALD commercial infrastructure and equipment sales, currently predicted to increase to about USD $2 billion in 2026 at a compound annual growth rate from 2020 of 26.3% can leverage off the larger MEMS global market. Impacts of the key atomic-scale processes that can fuel this exciting expansion of the MEMS technology arena will be reviewed, emphasizing the benefits for the future of prototyping and scaled fabrication in commercial, industrial, and defense applications.

1. V. Pore et al., J. Am. Chem. Soc. 2009, 131, 10, 3478-3480. DOI 10.1021/ja8090388

2. N. P. Dasgupta et al., Acc. Chem. Res. 2015, 48, 2, 341-348. DOI 10.1021/ar500360d

3. G. B. Rayner Jr. et al., J. Vac. Sci. & Technol. A 38, 062408 (2020). DOI 10.1116/6.0000454

3D microelectromechanical systems (3D-MEMS) are an emerging application space for piezoelectric films grown using ALD. ALD provides an ideal solution for the deposition of piezoelectric materials on trench sidewall or 3D fin structures which may be used to improve the size, weight, power and frequency tunability of MEMS devices. Aluminum nitride is a technologically relevant material for piezoelectric MEMS due to its significant piezoelectric response, high breakdown voltage and large mechanical quality factor. AlN is amenable to film growth by ALD, although there are relatively few reports of ALD AlN used for MEMS due to the need for additional process development to meet stringent crystal structure, film purity and grain orientation requirements for device performance. We explore several strategies for controlling the grain orientation of ALD-grown AlN on planar substrates, which includes the use of {111}-textured Pt as a growth template, precursor chemistry and dose variation, stress-engineered substrates, inductively-coupled plasma conditions for film bombardment during growth, and ALD equipment modifications. For select cases, we report the mechanical Q, determined from measurements of MEMS resonator structures, and piezoelectric coefficients, determined from measurements on MEMS cantilevers, of ALD deposited AlN . We analyze the Pt-AlN interface properties primarily by using TEM with EDS. The baseline ALD AlN process yielded completely c-axis oriented aluminum nitride as determined by x-ray diffraction, and a rocking curve full-width half max of 2.9° was achieved. The relative dielectric constant was measured to be 8.1 < K < 8.6 and an average dielectric loss of < 1% was observed within the an applied electric field range of +/- 3350 kV/cm (+/- 35 V across 104 nm thick AlN) at 10 kHz. The leakage current of the textured AlN was quite low at 1.5 x 10^-6 A/cm² over the applied field range of +/- 1820 kV/cm (+/- 19 V across 104 nm thick AlN).

The semiconductor industry has considerable experience with thermal spray protective coatings of parts in the etch and deposition areas to modify surface chemistry, provide corrosion resistance, or provide a barrier layer.These coatings are effective but are generally thick (150 - 200 μm) and may not provide sufficiently low porosity for parts exposed to reactive gases or plasmas.The ALD process creates high purity thin-films that are dense, highly conformal and defect free. Materials such as Al₂O₃ and other metal oxides are resistant to the reactive halogens that part surfaces see during semiconductor processing.

To address these challenges, ALD coatings have been developed to uniformly coat showerheads, pedestals, and other parts with high aspect ratios with 100 – 500 nm thin-films such as Al₂O₃, SiO₂, or other metal oxides.These films protect the part from the reactive halogen radicals and extend green to green time. Besides protecting the parts from reactive gases, the high aspect ratio ALD coatings on chamber components can serve as a diffusion barrier to avoid metal migration from the part itself and reduce conditioning times.

Additionally, parts with these ALD coatings can be recycled. Using Selective Coating Removal, the ALD protective coating is removed with minimum damage to the substrate, including the high aspect ratio features, before the part is prepared for ALD recoating. The loop essentially extends part life almost indefinitely which decreases cost of ownership.

This paper addresses the recent advances in the use of ALD thin films as a functional, protective coating that enhances part performance and reduces process costs.It will also cover the technology to selectively remove the deposition layer and ALD coating without damage to the part.Surface preparation, final cleaning, metrology and analytical testing for validation will also be discussed.

Keywords: ALD, Al₂O₃, corrosion, diffusion barrier, SiO₂, aspect ratio, etch, CVD