Stress Gradient In Films Research Articles

Method for controlling stress gradients in PVD aluminum nitride

In this paper we describe a method for controlling both the residual stress and the through-thickness stress gradient of aluminum nitride (AlN) thin films using a multi-step deposition process that varies the applied radio frequency (RF) substrate bias. The relationship between the applied RF substrate bias and the AlN residual stress is characterized using AlN films grown on oxidized silicon substrates is determined using 100 nm–1.5 μm thick blanket AlN films that are deposited with 60–100 W applied RF biases; the stress-bias relationship is found to be well described using a power law relationship. Using this relationship, we develop a model for varying the RF bias in a series of discrete deposition steps such that each deposition step has zero average stress. The applied RF bias power in these steps is tailored to produce AlN films that have minimized both the residual stress and the film stress gradient. AlN cantilevers were patterned from films deposited using this technique, which show reduced curvature compared to those deposited using a single RF bias setting, indicating a reduction of the stress gradient in the films.

Control of the residual stress gradients in copper films by inert ion implantation

In this work, we studied the effect of inert ion implantation using helium and xenon ions on residual stress gradients in copper films. A copper microcantilever beam array was fabricated on a (1 0 0) silicon wafer by conventional bulk micromachining. The as-fabricated copper microcantilevers showed upward bending deformation as a result of positive residual stress gradient in films. Helium and xenon ions were implanted at various doses and energies into microcantilevers before releasing them from substrates. The magnitudes and signs of residual stress gradients were effectively controlled by both helium and xenon ion implantation and depended on implantation energy and ionic dosages. The mechanism of residual stress control appeared to involve counterbalancing residual stress gradients with compressive residual stresses induced by volume expansions of implanted regions and the direct effect of surface modification by ion bombardment.

Accuracy Correction of the Measuring Method for Film Stress Gradient Based on Curvature Radius

Stress gradient of a thin film along its thickness direction has important influence on performance of MEMS/NEMS devices. The measuring method based on curvature radius is a usual method for extracting film stress gradient. In this paper the error of the method has been analyzed using Finite Element Analysis software Coventorware, and the current theory has been modified according to FEA results to enhance its accuracy. Simulating results indicated that the accuracy of the method has been increased observably after correcting.

Stress and texture in HIPIMS TiN thin films

Titanium nitride (TiN) films in the thickness range of 0.013µm to 0.3µm were grown by high power impulse magnetron sputtering (HIPIMS) on silicon substrates in two deposition modes: a) the substrate was grounded and b) −125V bias was applied to the substrate. On the films we performed microstructure-, film texture- and film stress-analysis. The films deposited under −125V bias experienced a more energetic ion bombardment than the films deposited on grounded substrates. This difference in ion bombardment energy is reflected in the different microstructure. In contrast to previous results for TiN films grown by conventional reactive magnetron sputtering, we observe no major film stress gradient for increasing film thicknesses. We explain this observation from the absence of a 200-to-111 texture crossover during film growth.A moderate ion bombardment leads to TiN films with (111) texture, while an intense ion bombardment leads to films with (001) texture (Greene et al.; Appl. Phys. Lett. 67 (20) 2928–2930 (1995)). At the same time (001) oriented grains are much more susceptible to compressive stress generation by ion bombardment than (111) oriented grains.

Optimization of the refractory x-ray mask fabrication sequence

This article discusses the effects different process flows have on the material and control requirements for refractory (subtractive) x-ray masks. Our investigation of x-ray masks shows that a film’s stress uniformity is more critical (for distortion) than the magnitude of the stress. In particular, we find for the wafer sequence the requirement on the membrane film’s stress uniformity is the most critical specification. For the membrane sequence, the requirement is for stress uniformity for films that are removed from the membrane (after pattern formation). We find that SiC reduces the distortions from the film stress gradient, however, the frame structure determines the distortions from a film’s mean stress. Finally, we conclude the best choice of an x-ray mask flow should be decided on our ability to modify wafer equipment and processes, to control the stress uniformity of the membrane film, to control the stress and uniformity of the resist, hardmask, and absorber films, and finally to handle the lithography required to create predistorted mask patterns.