Compensation of the Stress Gradient in Physical Vapor Deposited Al1-xScxN Films for Microelectromechanical Systems with Low Out-of-Plane Bending

Abstract

Thin film through-thickness stress gradients produce out-of-plane bending in released microelectromechanical systems (MEMS) structures. We study the stress and stress gradient of Al_0.68Sc_0.32N thin films deposited directly on Si. We show that Al_0.68Sc_0.32N cantilever structures realized in films with low average film stress have significant out-of-plane bending when the Al_1-xSc_xN material is deposited under constant sputtering conditions. We demonstrate a method where the total process gas flow is varied during the deposition to compensate for the native through-thickness stress gradient in sputtered Al_1-xSc_xN thin films. This method is utilized to reduce the out-of-plane bending of 200 µm long, 500 nm thick Al_0.68Sc_0.32N MEMS cantilevers from greater than 128 µm to less than 3 µm.

Keywords: MEMS; aluminum scandium nitride; cantilever beams; fabrication; physical vapor deposition; stress; stress gradient.

Figure 1

Fabrication process for realizing Al_1−xSc_xN cantilevers with (a) p-type (100) Si wafer (b) Al_1−xSc_xN deposition using Evatec CLUSTERLINE^® 200 II PVD system (c) PECVD SiN deposition and patterning using CF₄ RIE (d) KOH in 45% H₂O etch of Al_1−xSc_xN (e) SiN hard mask stripped using CF₄ RIE and Al_1−xSc_xN cantilevers released using isotropic XeF₂ dry etching.

Figure 2

Average stress plots of PVD deposited Al_0.68Sc_0.32N films with (a) Average stress versus thickness plot at a constant 25 sccm N₂ flow and (b) Average stress versus flow for 500 nm Al_0.68Sc_0.32N with pure N₂ flow from 20–30 sccm [3].

Figure 3

Graphics for a 500 nm PVD sputter-deposited Al_0.68Sc_0.32N film where the N₂ process gas flow is held constant through the entire deposition, (a) 45-degree SEM image of center die (C) fabricated from a 100 mm wafer. Each die has 8 released structures which are labeled. Note the high out-of-plane deflection that is clearly visible in the uncompensated structures. (b) Schematic of locations on the 100 mm wafer where a die is pulled 35 mm from the center for imaging and measuring out-of-plane displacements. One die was pull from the north (N), northeast (NE), east (E), southeast (SE), south (S), southwest (SW), west (W), and northwest (NW) locations of the wafer (c) Stack-up of film with constant flow composed of a seed layer, gradient seed layer (Al_1→0.68Sc_0→0.32N) [3] and Al_0.68Sc_0.32N layer.

Figure 4

(a–i) SEM images of cantilevers formed from 500 nm thick PVD Al_0.68Sc_0.32N films deposited under a constant N₂ flow of 25 sccm. Each die was pulled from the north (N), northeast (NE), east (E), southeast (SE), south (S), southwest (SW), west (W), and northwest (NW) locations of the wafer shown in Figure 3b.

Figure 5

Graphics for multi-layer 500 nm PVD sputter-deposited Al_0.68Sc_0.32N film where the N₂ process gas flow is changed between two layers with 30 sccm utilized on the bottom and 25 sccm on the top layer, (a) 45-degree SEM image of the center die (C) fabricated from a 100 mm wafer. Each die has 8 released structures. (b) Stack-up of 2-layer film composed of a seed and gradient layer (Al_1→0.68Sc_0→0.32N) [3] to suppress AOGs and two equal thickness layers with different N₂ process gas flows designed to compensate for the native through-thickness stress gradient.

Figure 6

Graphics for multi-layer 500 nm PVD sputter-deposited Al_0.68Sc_0.32N film, (a) 45-degree SEM image of the center die (C) fabricated from a 100 mm wafer. (b) Stack-up with five equal thickness layers with different N₂ process gas flows to compensate for the through-thickness stress gradient.

Figure 7

Top view SEM images of cantilevers formed from a 5-layer, 500 nm total thickness, PVD deposited Al_0.68Sc_0.32N where the process gas flow for each layer is linearly changed from 30 to 20 sccm through the five layers. Each die was pull from the north (N), northeast (NE), east (E), southeast (SE), south (S), southwest (SW), west (W), and northwest (NW) locations of the wafer shown in Figure 3b.

Figure 8

Z deflection for 500 nm PVD sputter-deposited Al_0.68Sc_0.32N films where the N₂ process gas flow was applied at constant flow throughout the deposition (black) and a 2-layer 30/20 sccm N₂ flow stack (red).