Features of the receiving of piezoelectric thin films by plasma spraying of powdery AlN

At a Glance

Metadata	Details
Publication Date	2020-03-03
Journal	Russian Technological Journal
Authors	В. С. Фещенко, K. N. Zyablyuk, Э. А. Сенокосов, В. И. Чукита, Д. А. Киселев
Institutions	Institute of Radio-Engineering and Electronics, University of Science and Technology
Citations	8
Analysis	Full AI Review Included

Executive Summary

This research investigates a novel, low-temperature method for depositing piezoelectric Aluminum Nitride (AlN) thin films using plasma spraying (RF sputtering) from powdered targets.

Core Value Proposition: The developed process maintains the substrate temperature at or below 300 °C, making AlN deposition fully compatible with existing silicon semiconductor (CMOS) manufacturing technologies.
Methodology: A highly modified VUP-5M vacuum post with a custom RF magnetron (13.56 MHz) was used to sputter AlN from a compacted powder target.
Structural Achievement: Films (200 nm thick) were confirmed via IR spectroscopy to be polycrystalline AlN with a strong (002) crystallographic orientation, crucial for high piezoelectric response.
Piezoelectric Performance: Films deposited on diamond substrates achieved a piezoelectric coefficient (d₃₃) of 2.2 pm/V, which is 60% of the value reported for single-domain bulk AlN.
Surface Quality: Films deposited on single-crystal substrates (diamond/Pt and silicon) exhibited excellent surface quality, with Root Mean Square (Rms) roughness values less than 0.5 nm.
Application Potential: The resulting films show significant piezoelectric effect, enabling their use in integrated piezoelectric sensors and ultrasonic emitters.

Technical Specifications

Parameter	Value	Unit	Context
Film Thickness	200	nm	All samples investigated
Max Substrate Temperature	300	°C	Key process advantage (CMOS compatibility)
RF Generator Frequency	13.56	MHz	Plasma excitation frequency
RF Generator Max Power	1.5	kW	Maximum output power
Sputtering Gas Pressure (Ar)	0.5-0.7	Pa	Optimal deposition regime
Auto-Bias Voltage	-120 to -180	V	Applied during sputtering
Deposition Rate	300	nm/hour	Achieved speed
Piezoelectric Coefficient (d₃₃)	2.2	pm/V	AlN film on Diamond substrate (60% of bulk)
Piezoelectric Coefficient (d₃₃)	7.2	pm/V	AlN film on Silicon substrate (High-oriented multi-domain structure)
Roughness (Rms) - Diamond	0.45	nm	Least roughness achieved
Roughness (Rms) - Sitall	6.26	nm	Worst roughness achieved
IR Absorption Peak	672	cm^-1	Corresponds to E1(TO) phonon, confirming 002 orientation
AlN Powder SSA (A100 grade)	1.2-2.0	m²/g	Specific Surface Area
AlN Powder D50 (A100 grade)	7.0-10.0	µm	Median particle size

Key Methodologies

The thin AlN films were obtained using a modified RF plasma spraying technique on a VUP-5M vacuum post.

Equipment Modification:
- The VUP-5M vacuum post was fitted with a custom RF magnetron attachment (13.56 MHz generator).
- A custom anode ring and a grounded screen were installed to minimize parasitic sputtering of the magnetron components, which typically occurs in large RF plasma zones.
Target Preparation (Powder Sputtering):
- AlN powder (A100 grade) was mixed with deionized water (5 g AlN in 50 ml H₂O) to form a slurry.
- The slurry was deposited onto a high thermal conductivity AlN ceramic base (48 mm octagon, 0.5 mm thick).
- After drying, the resulting AlN powder layer thickness was 0.3-0.4 mm.
Deposition Parameters:
- The process utilized an Argon (Ar) plasma at a pressure of 0.5-0.7 Pa.
- The substrate holder was heated to a maximum of 300 °C using an incandescent lamp heater.
- RF power was controlled via a matching network, minimizing reflected wave amplitude (0.58-0.64 V) and maintaining an auto-bias voltage between -120 V and -180 V.
Process Limitation:
- The AlN target surface rapidly contaminated (turning dark gray, likely due to metallic aluminum formation from AlN decomposition) after approximately 1 hour of operation.
- Deposition of films thicker than 300 nm requires multi-step processing, including cleaning and replacing the contaminated powder layer between stages.
Characterization Techniques:
- Surface Morphology: Scanning Probe Microscopy (MFP-3D) was used to measure roughness (Rms and Ra).
- Crystallographic Structure: Fourier-transform IR spectroscopy (FSM-1201) was used to confirm the presence of the E1(TO) phonon peak at 672 cm^-1, indicating wurtzite structure and preferred 002 orientation.
- Piezoelectric Properties: Scanning Probe Microscopy was employed to measure the local piezoelectric coefficient (d₃₃).

Commercial Applications

The ability to deposit highly oriented, piezoelectric AlN films at low temperatures (300 °C) opens up significant opportunities for integration with conventional microelectronics.

Integrated MEMS/NEMS: Enables the fabrication of high-performance micro- and nano-electromechanical systems directly onto silicon wafers without damaging existing circuitry.
RF Filters and Resonators: Production of high-frequency Surface Acoustic Wave (SAW) and Bulk Acoustic Wave (BAW) devices for 5G/6G communication systems, leveraging AlN’s excellent acoustic properties.
Piezoelectric Sensors and Transducers: Manufacturing of integrated pressure sensors, accelerometers, and ultrasonic emitters (e.g., for medical imaging or non-destructive testing).
Optoelectronic Integration: Allows for the integration of AlN-based deep-UV LEDs and photodetectors onto complex substrates where high-temperature processing is prohibitive.
Energy Harvesting: Development of micro-scale piezoelectric energy harvesting devices that convert ambient vibrations into electrical energy for low-power electronics.

View Original Abstract

Оne of the promising materials in solid state electronics is the AlN compound. A wide range of semiconductor devices are produced from it, such as photodetectors, LEDs, piezoelectric converters, etc. But the widespread use of products based on AlN prevents low manufacturability designs based on it. In this regard, the development of new technologies for the production of devices based on AlN is relevant.The work is devoted to the study of thin AlN films obtained by plasma spraying from AlN powder. The review of existing technologies of production of thin films AlN is carried out.Their advantages and disadvantages are discussed. Information on the modernization of the VUP-5 installation, which allowed to spray the AlN film from the powdered state, is given.One of the significant advantages of the process developed in this work is that the substrate is heated to temperatures no higher than 300 oC, which in turn allows to combine this technology with the technology of silicon semiconductor devices.As a result, films with a thickness of 200 nm on various substrates were obtained and their surface structure was studied. It is shown that AlN films deposited on single crystal substrates such as diamond and silicon have the least roughness, while films on sitall have the worst roughness.The transmission spectra of the obtained AlN films were investigated by IR spectroscopy. With their help, it was shown that a polycrystalline AlN layer oriented in the crystallographic direction 002 is formed on the substrate. The piezoelectric properties of the obtained films were investigated by scanning probe microscopy. It is shown that their piezoelectric coefficient d33 is 60% of the value for a single-domain single-crystal sample for a diamond substrate, which indicates a sufficiently high quality of the resulting film.It is concluded that, although the quality of the layers strongly depends on the substrate, nevertheless, they exhibit a significant piezoelectric effect, which allows the use of this method for the manufacture of piezoelectric sensors, ultrasonic emitters, etc.

Tech Support

Original Source

DOI: https://doi.org/10.32362/2500-316x-2020-8-1-67-79