Process Parameter Window of High Voltage Power Supply for Magnetron Sputtering Deposition of Aluminum Nitride Piezoelectric Thin Film
Aluminum nitride thin films possess exceptional piezoelectric properties that make them invaluable for microelectromechanical systems, bulk acoustic wave resonators, and surface acoustic wave devices. The reactive magnetron sputtering process used to deposit these films requires precise control of numerous parameters to achieve the stoichiometry, crystal orientation, and microstructure necessary for optimal piezoelectric response. The high voltage power supply powering the magnetron discharge plays a central role in determining the process conditions, with the operating parameters of this supply defining a critical window within which acceptable film properties can be achieved. Understanding the boundaries and dependencies of this parameter window enables reliable production of high quality aluminum nitride films.
The piezoelectric properties of aluminum nitride arise from the non centrosymmetric wurtzite crystal structure, which develops when aluminum and nitrogen atoms occupy specific positions in the hexagonal lattice. The piezoelectric coefficients, particularly the longitudinal coefficient that determines the response in bulk acoustic wave devices, depend critically on the crystal quality, orientation, and stoichiometry of the film. Highly oriented films with the c axis perpendicular to the substrate surface and stoichiometric AlN composition exhibit the strongest piezoelectric response. Deviations from ideal stoichiometry, mixed crystal orientations, or defective microstructures degrade the piezoelectric performance.
Reactive magnetron sputtering deposits aluminum nitride by sputtering an aluminum target in a nitrogen containing atmosphere. The nitrogen reacts with the sputtered aluminum atoms during transport to the substrate or at the substrate surface to form the compound. The process operates in a regime where the target surface is partially covered with a nitride layer, with the degree of coverage depending on the balance between nitride formation from reactive gas and nitride removal by sputtering. This partial target coverage, often called the transition mode, provides the optimal conditions for stoichiometric film deposition with reasonable deposition rates.
The discharge voltage and current from the high voltage power supply directly influence the sputtering rate and the energy of species arriving at the substrate. Higher discharge powers increase the sputtering rate and the flux of aluminum atoms, but also increase the substrate heating from energetic particle bombardment. The discharge voltage in reactive sputtering depends on the target surface condition, with nitride covered surfaces typically exhibiting higher voltages than metallic surfaces due to differences in secondary electron emission. The transition between metallic and poisoned target states involves a voltage change that can serve as an indicator of the target surface condition.
The process parameter window for aluminum nitride deposition is bounded by several constraints related to film quality and process stability. The lower power boundary represents the minimum discharge power necessary to achieve sufficient aluminum flux for stoichiometric film formation with the available nitrogen. Below this power, the films become nitrogen rich with degraded crystalline quality and piezoelectric response. The upper power boundary represents the maximum power compatible with substrate temperature limits and acceptable residual stress in the deposited film. Excessive power causes substrate overheating and may produce films with high compressive stress that can cause cracking or delamination.
The nitrogen partial pressure represents another critical parameter with defined boundaries for acceptable film quality. Insufficient nitrogen leads to aluminum rich films with metallic aluminum inclusions that degrade the piezoelectric and dielectric properties. Excessive nitrogen can cause target poisoning where the target surface becomes fully covered with insulating nitride, leading to process instabilities from arcing and reduced deposition rates. The optimal nitrogen pressure provides sufficient reactive gas for complete reaction of the sputtered aluminum while maintaining the target in the transition regime with stable discharge operation.
The total pressure in the sputtering chamber affects the thermalization of sputtered species and the energy of particles arriving at the substrate. Higher pressures increase the scattering of sputtered aluminum atoms during transport, reducing their energy and potentially affecting the film microstructure. The scattering also reduces the fraction of sputtered atoms that reach the substrate, decreasing the deposition rate. Lower pressures preserve the energy of sputtered species but may provide insufficient scattering for uniform deposition and may increase substrate heating from energetic bombardment. The pressure window balances these competing effects for optimal film properties.
Substrate temperature during deposition influences the crystal quality and stress state of the aluminum nitride film. Higher temperatures provide the atomic mobility necessary for atoms to find optimal lattice sites, improving crystallinity and reducing defect densities. However, excessive temperatures may cause excessive grain growth or undesirable texture changes. Lower temperatures limit atomic mobility and may produce films with high residual stress and poor crystallinity. The substrate temperature depends on the heating from the plasma and energetic particle bombardment in addition to any intentional substrate heating, creating coupling between the power supply parameters and the thermal conditions.
The discharge mode of the high voltage power supply, whether direct current, pulsed direct current, or radio frequency, affects the process characteristics and the achievable parameter window. Direct current sputtering provides continuous deposition but may suffer from arcing when insulating nitride patches form on the target surface. Pulsed direct current operation periodically reverses the voltage to discharge insulating regions, reducing arcing tendency and enabling stable operation in more heavily poisoned target regimes. Radio frequency sputtering can operate with fully insulating targets but typically provides lower deposition rates and may introduce additional process complexity.
The pulse parameters in pulsed direct current sputtering, including pulse frequency, duty cycle, and reverse voltage, provide additional degrees of freedom for process optimization. Higher pulse frequencies provide more frequent discharge of insulating regions, enabling stable operation at higher nitrogen pressures or higher target poisoning levels. The duty cycle determines the ratio of sputtering time to reverse time, affecting the average sputtering rate. The reverse voltage magnitude determines the effectiveness of insulating region discharge. These pulse parameters interact with the discharge power and gas pressure to define the stable operating region for the process.
Process control strategies for aluminum nitride deposition monitor various parameters to maintain operation within the acceptable parameter window. Optical emission spectroscopy can monitor the emission from aluminum atoms in the plasma, providing an indicator of the sputtering rate and target condition. Mass spectrometry or residual gas analysis monitors the partial pressures of reactive gases. Discharge voltage monitoring tracks the target surface condition through the voltage changes associated with metallic and poisoned states. Feedback control systems can adjust the reactive gas flow or discharge power to maintain consistent process conditions despite disturbances or long term drift in system characteristics.
