*4.2. Out-of-Plane Cantilever Deflection in Uncompensated Al1*−*xScxN Materials*

*4.2. Out-of-Plane Cantilever Deflection in Uncompensated Al1−xScxN Materials*  Low average stress (membranes) and low through-thickness stress gradient (cantilevers) are critical for realizing high yield MEMS structures with low out-of-plane bending. The multilayer stress and stress gradient compensation approach reported in this paper can be utilized to simultaneously achieve low average stress and low through-thickness stress gradient. Table 2 provides a summary of the various flow conditions and the resulting stress and out-of-plane cantilever deflections. While low average stress is achievable in all films, only a multi-layer Al1−xScxN achieves the low through-thickness stress gradient required to realize cantilevers with low out-of-plane bending. The compressive-to-tensile stress gradient through the Al1−xScxN film thickness results in a high degree of out-ofplane bending in uncompensated Al1−xScxN films. A *Z*-axis microscope is used to measure Low average stress (membranes) and low through-thickness stress gradient (cantilevers) are critical for realizing high yield MEMS structures with low out-of-plane bending. The multilayer stress and stress gradient compensation approach reported in this paper can be utilized to simultaneously achieve low average stress and low through-thickness stress gradient. Table 2 provides a summary of the various flow conditions and the resulting stress and out-of-plane cantilever deflections. While low average stress is achievable in all films, only a multi-layer Al1−xScxN achieves the low through-thickness stress gradient required to realize cantilevers with low out-of-plane bending. The compressive-to-tensile stress gradient through the Al1−xScxN film thickness results in a high degree of out-of-plane bending in uncompensated Al1−xScxN films. A *Z*-axis microscope is used to measure the tip deflection with the anchor as the z = 0 reference. Figure 3 provides SEM images of the high out-of-plane bending in Al0.68Sc0.32N cantilevers deposited with a constant flow of 25 sccm.

the tip deflection with the anchor as the z = 0 reference. Figure 3 provides SEM images of the high out-of-plane bending in Al0.68Sc0.32N cantilevers deposited with a constant flow of sation.

Al0.68Sc0.32N layer.

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**Table 2.** Cantilever tip deflection for 500 nm Al0.68Sc0.32N with and without stress gradient compen-


**Table 2.** Cantilever tip deflection for 500 nm Al0.68Sc0.32N with and without stress gradient compensation.

**Figure 3.** Graphics for a 500 nm PVD sputter-deposited Al0.68Sc0.32N 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 (Al1→0.68Sc0→0.32N) [3] and **Figure 3.** Graphics for a 500 nm PVD sputter-deposited Al0.68Sc0.32N film where the N<sup>2</sup> 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 (Al1→0.68Sc0→0.32N) [3] and Al0.68Sc0.32N layer.

Figure 4 shows top view SEM images of cantilevers fabricated from Al0.68Sc0.32N films from across a 100 mm wafer demonstrating that the residual stress gradient within the film not only induces bending but also generates twisting and rotations within released structures. The center of the wafer produces structures with minimal twisting while the edge of the wafer produces significant twisting depending on the location of the die. The twisting is due to the interaction of the through-thickness stress gradient with the radial variation of the average stress across the 100 mm diameter wafer and is consistent with previous studies [3] which exhibit more compressive average stresses at the wafer edge and more tensile average stresses near the wafer center. If no stress compensation is established, depending on the performance requirements for a MEMS device, the location of the die on the wafer and the orientation of the released structures can lead to differences in out-of-plane bending and twisting. Figure 4 shows top view SEM images of cantilevers fabricated from Al0.68Sc0.32N films from across a 100 mm wafer demonstrating that the residual stress gradient within the film not only induces bending but also generates twisting and rotations within released structures. The center of the wafer produces structures with minimal twisting while the edge of the wafer produces significant twisting depending on the location of the die. The twisting is due to the interaction of the through-thickness stress gradient with the radial variation of the average stress across the 100 mm diameter wafer and is consistent with previous studies [3] which exhibit more compressive average stresses at the wafer edge and more tensile average stresses near the wafer center. If no stress compensation is established, depending on the performance requirements for a MEMS device, the location of the die on the wafer and the orientation of the released structures can lead to differences in out-of-plane bending and twisting.

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**Figure 4.** SEM images of cantilevers formed from 500 nm thick PVD Al0.68Sc0.32N 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 4.** (**a**–**i**) SEM images of cantilevers formed from 500 nm thick PVD Al0.68Sc0.32N films deposited under a constant N<sup>2</sup> 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.

#### *4.3. Stress Gradient Compensated Al1−xScxN Films and Cantilevers 4.3. Stress Gradient Compensated Al1*−*xScxN Films and Cantilevers*

The through-thickness stress gradient is the primary source of out-of-plane bending in released Al1−xScxN structures. To find the additional flow needed to compensate the stress gradient, Equation (3) is used to estimate the local stress. At 25 sccm N2 flow, the local stress within the first 15 nm after the seed layer has been deposited is approximated to be −424 MPa using Equation (3). At 500 nm, the local stress is 276 MPa. A compensating 424 MPa in the initial layers, −276 MPa at the top of the film, and the appropriate opposing stress gradient is required to compensate for the through-thickness stress gradient. Using Figure 2b, at 20 and 30 sccm N2 flow, a 500 nm Al1−xScxN film will yield an average stress The through-thickness stress gradient is the primary source of out-of-plane bending in released Al1−xScxN structures. To find the additional flow needed to compensate the stress gradient, Equation (3) is used to estimate the local stress. At 25 sccm N<sup>2</sup> flow, the local stress within the first 15 nm after the seed layer has been deposited is approximated to be −424 MPa using Equation (3). At 500 nm, the local stress is 276 MPa. A compensating 424 MPa in the initial layers, −276 MPa at the top of the film, and the appropriate opposing stress gradient is required to compensate for the through-thickness stress gradient. Using Figure 2b, at 20 and 30 sccm N<sup>2</sup> flow, a 500 nm Al1−xScxN film will yield an average stress of −450 and 317 MPa, respectively. We utilized Equation (3) to design a 2-layer stack

deposited at 30 sccm (lower) and 20 sccm (upper) to compensate for the through-thickness stress gradient. Figure 5 provides an SEM image of a 2-layer Al1−xScxN material where the N<sup>2</sup> flow is varied between layers to suppress the stress gradient and the resulting out-ofplane bending in cantilevers. Here, 30 sccm N<sup>2</sup> flow is utilized during the deposition of the AlN seed, Al1−xScxN gradient layer, and the lower 225 nm of the bulk film while 20 sccm N<sup>2</sup> flow is utilized when depositing the upper 225 nm of the film stack. The approach successfully compensates for the through-thickness stress gradient and reduces the out-ofplane cantilever bending in the center of the wafer from 109 µm for the uncompensated materials to less than 3 µm for cantilevers realized in the stress gradient compensated 2-layer material. posited at 30 sccm (lower) and 20 sccm (upper) to compensate for the through-thickness stress gradient. Figure 5 provides an SEM image of a 2-layer Al1−xScxN material where the N2 flow is varied between layers to suppress the stress gradient and the resulting out-ofplane bending in cantilevers. Here, 30 sccm N2 flow is utilized during the deposition of the AlN seed, Al1−xScxN gradient layer, and the lower 225 nm of the bulk film while 20 sccm N2 flow is utilized when depositing the upper 225 nm of the film stack. The approach successfully compensates for the through-thickness stress gradient and reduces the outof-plane cantilever bending in the center of the wafer from 109 µm for the uncompensated materials to less than 3 µm for cantilevers realized in the stress gradient compensated 2 layer material.

of −450 and 317 MPa, respectively. We utilized Equation (3) to design a 2-layer stack de-

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**Figure 5.** Graphics for multi-layer 500 nm PVD sputter-deposited Al0.68Sc0.32N 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 (Al1→0.68Sc0→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 5.** Graphics for multi-layer 500 nm PVD sputter-deposited Al0.68Sc0.32N film where the N<sup>2</sup> 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 (Al1→0.68Sc0→0.32N) [3] to suppress AOGs and two equal thickness layers with different N<sup>2</sup> process gas flows designed to compensate for the native through-thickness stress gradient.

Figure 6b displays a 5-layer Al0.68Sc0.32N stack used to compensate for the throughthickness stress gradient. Since lower flows produce more compressive films while higher flows produce more tensile films, a layer stack is utilized where for each consecutive 100 nm layer, an additional 2.5 sccm of flow provides a tensile-to-compressive transition to cancel the original compressive-to-tensile stress gradient through the thickness. For the 5 layer material the N2 flow is varied over the range from 30 to 20 sccm to yield a low average stress in addition to a low through-thickness stress gradient. After release, the cantilevers remain consistently flat, as shown in Figure 6a and Figure 7, especially when compared to the cantilevers formed in the uncompensated films. The cantilevers in Figures 4– 7 use the same naming conventions depicted in Figure 3a,b. In Figure 6a the maximum tip deflection is approximately 5.8 +/− 0.4 µm, −7.6+/− 0.4 µm, and −4.0+/− 0.4 µm when measured from the 1, 2, and 3 positions, respectively. The 1, 2, and 3 structures exhibited the same behavior and deflection as those directly across from them, namely cantilevers 5, 6, and 7. Table 2 compares the average wafer stress and cantilever tip deflection for the uncompensated and stress gradient compensated Al0.68Sc0.32N materials. The out-of-plane cantilever displacement for the 5-layer, low stress material at position 2 is reduced by more than 14-fold for the center die and 19-fold for the east die 35 mm from the wafer center. Overall, substantial reductions in out-of-plane tip displacement are observed for all cantilevers fabricated in the stress gradient compensated films. The 5-layer film does not have an AlN/gradient seed layer and possess a higher tip deflection than the 2-layer Figure 6b displays a 5-layer Al0.68Sc0.32N stack used to compensate for the throughthickness stress gradient. Since lower flows produce more compressive films while higher flows produce more tensile films, a layer stack is utilized where for each consecutive 100 nm layer, an additional 2.5 sccm of flow provides a tensile-to-compressive transition to cancel the original compressive-to-tensile stress gradient through the thickness. For the 5-layer material the N<sup>2</sup> flow is varied over the range from 30 to 20 sccm to yield a low average stress in addition to a low through-thickness stress gradient. After release, the cantilevers remain consistently flat, as shown in Figures 6a and 7, especially when compared to the cantilevers formed in the uncompensated films. The cantilevers in Figures 4–7 use the same naming conventions depicted in Figure 3a,b. In Figure 6a the maximum tip deflection is approximately 5.8 +/− 0.4 µm, −7.6 +/− 0.4 µm, and −4.0 +/− 0.4 µm when measured from the 1, 2, and 3 positions, respectively. The 1, 2, and 3 structures exhibited the same behavior and deflection as those directly across from them, namely cantilevers 5, 6, and 7. Table 2 compares the average wafer stress and cantilever tip deflection for the uncompensated and stress gradient compensated Al0.68Sc0.32N materials. The out-of-plane cantilever displacement for the 5-layer, low stress material at position 2 is reduced by more than 14-fold for the center die and 19-fold for the east die 35 mm from the wafer center. Overall, substantial reductions in out-of-plane tip displacement are observed for all cantilevers fabricated in the stress gradient compensated films. The 5-layer film does not have an AlN/gradient seed layer and possess a higher tip deflection than the 2-layer film with seed layer. This is because the gradient in the N<sup>2</sup> process gas flow was designed

using the data from Figure 2 where all the films have the seed layer. Table 3 confirms the precise control of the tip deflection by varying the range of N<sup>2</sup> gas flow for a 2-layer material stack. The range of gas flows controls the stress gradient while the mean N<sup>2</sup> gas flow controls the average stress within the film. In Table 3, a 2.5 sccm increase of the range from 27.5–22.5 sccm to 30–22.5 sccm reduces the deflection 2-fold. using the data from Figure 2 where all the films have the seed layer. Table 3 confirms the precise control of the tip deflection by varying the range of N2 gas flow for a 2-layer material stack. The range of gas flows controls the stress gradient while the mean N2 gas flow controls the average stress within the film. In Table 3, a 2.5 sccm increase of the range from 27.5-22.5 sccm to 30-22.5 sccm reduces the deflection 2-fold.

film with seed layer. This is because the gradient in the N2 process gas flow was designed

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**Figure 6.** Graphics for multi-layer 500 nm PVD sputter-deposited Al0.68Sc0.32N 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 6.** Graphics for multi-layer 500 nm PVD sputter-deposited Al0.68Sc0.32N 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 N<sup>2</sup> process gas flows to compensate for the through-thickness stress gradient. *Micromachines* **2022**, *13*, x FOR PEER REVIEW 11 of 13

**Figure 7.** Top view SEM images of cantilevers formed from a 5-layer, 500 nm total thickness, PVD deposited Al0.68Sc0.32N 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), south-**Figure 7.** Top view SEM images of cantilevers formed from a 5-layer, 500 nm total thickness, PVD deposited Al0.68Sc0.32N where the process gas flow for each layer is linearly changed from 30 to 20 sccm

This work provides, for the first-time, methods to individually control the stress and stress gradient in Al1−xScxN films while maintaining film quality. Figure 8 shows the outof-plane displacement along the length of the cantilever for the uncompensated and 2 layer compensated Al0.68Sc0.32N materials while Table 3 summarizes the out-of-plane tip displacement. The compensated cantilever tip bending confirms that the radius of curvature can be controlled in the released Al1−xScxN structures. The multilayer gas gradient method can be utilized to simultaneously and independently control both the average stress, via the average N2 flow, and through-thickness stress gradient, via the throughthickness variation of the N2 flow, in Al1−xScxN thin films. Previously reported methods to control Al1−xScxN average film stress do not provide control of the through-thickness stress gradients within the film. While the previous RF substrate bias method reported by Knisley [14] achieved independent control of stress and through-thickness stress gradient in AlN films and demonstrated reduced radius of curvature and tip displacement in AlN cantilevers, use of an RF substrate bias is less suitable for Al1−xScxN. Addition of an RF substrate bias results in more compressive film stress and is a good technique for stress control of AlN where the films are highly tensile without an RF substrate bias [14]. Al0.68Sc0.32N, by contrast, is highly compressive when deposited at the low process pressures that suppress formation of anomalously oriented grains (AOGs) without using an RF substrate bias [3]. Thus, addition of an RF substrate bias to an Al0.68Sc0.32N growth will require even higher process pressures to achieve near-neutral average film stress, and un-

east (SE), south (S), southwest (SW), west (W), and northwest (NW) locations of the wafer shown in

der such process conditions a large number of AOGs would be expected.

Figure 3b.

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.

**Table 3.** Cantilever tip deflection for a die near the center of a 500 nm Al0.68Sc0.32N film with AlN and Sc gradient (Al1→0.68Sc0→0.32N) seed layers.


*4.4. Discussion of Stress Gradient Cancellation Trends*

This work provides, for the first-time, methods to individually control the stress and stress gradient in Al1−xScxN films while maintaining film quality. Figure 8 shows the out-of-plane displacement along the length of the cantilever for the uncompensated and 2-layer compensated Al0.68Sc0.32N materials while Table 3 summarizes the out-of-plane tip displacement. The compensated cantilever tip bending confirms that the radius of curvature can be controlled in the released Al1−xScxN structures. The multilayer gas gradient method can be utilized to simultaneously and independently control both the average stress, via the average N<sup>2</sup> flow, and through-thickness stress gradient, via the through-thickness variation of the N<sup>2</sup> flow, in Al1−xScxN thin films. Previously reported methods to control Al1−xScxN average film stress do not provide control of the through-thickness stress gradients within the film. While the previous RF substrate bias method reported by Knisley [14] achieved independent control of stress and through-thickness stress gradient in AlN films and demonstrated reduced radius of curvature and tip displacement in AlN cantilevers, use of an RF substrate bias is less suitable for Al1−xScxN. Addition of an RF substrate bias results in more compressive film stress and is a good technique for stress control of AlN where the films are highly tensile without an RF substrate bias [14]. Al0.68Sc0.32N, by contrast, is highly compressive when deposited at the low process pressures that suppress formation of anomalously oriented grains (AOGs) without using an RF substrate bias [3]. Thus, addition of an RF substrate bias to an Al0.68Sc0.32N growth will require even higher process pressures to achieve near-neutral average film stress, and under such process conditions a large number of AOGs would be expected. *Micromachines* **2022**, *13*, x FOR PEER REVIEW 12 of 13

flow stack (red).

**5. Conclusions** 

lished version of the manuscript.

corresponding author upon reasonable request.

**Conflicts of Interest:** The authors have no conflicts to disclose.

**Figure 8.** Z deflection for 500 nm PVD sputter-deposited Al0.68Sc0.32N films where the N2 process gas flow was applied at constant flow throughout the deposition (black) and a 2-layer 30/20 sccm N2 **Figure 8.** Z deflection for 500 nm PVD sputter-deposited Al0.68Sc0.32N films where the N<sup>2</sup> process gas flow was applied at constant flow throughout the deposition (black) and a 2-layer 30/20 sccm N<sup>2</sup> flow stack (red).

timized to control stress for N2 flows between 20 to 30 sccm. The average stress within the films ranged from 78.6 MPa to 349.6 MPa. The out-of-plane tip deflection for 100 µm long cantilevers fabricated in 500 nm thick Al0.68Sc0.32N films was reduced from >109 µm for films without stress gradient compensation to less than 3 µm and 8 µm for 2- and 5-layer compensated film stacks for dies studied in the wafer center. The resulting deposition parameters provide methods to control stress and through-thickness stress gradients in highly Sc alloyed AlN materials and are promising for next-generation MEMS devices.

**Author Contributions:** Conceptualization, R.B. and R.H.O III; methodology, R.B.; validation, R.B., K.K., and M.D; formal analysis, R.B., K.K., and M.D; investigation, R.B. and K.K.; resources, R.H.O III; data curation, R.B., K.K., and M.D.; writing—original draft preparation, R.B.; writing—review and editing, R.B., M.D., and R.H.O III; visualization, R.B.; supervision, R.H.O III.; project administration, R.H.O III; funding acquisition, R.H.O III. All authors have read and agreed to the pub-

**Funding:** This work was funded in part by the NSF CAREER Award (1944248) and in part The Defense Advanced Research Projects Agency (DARPA) Small Business Innovation Research (SBIR) under award HR0011-21-9-0004. This work was carried out in part at the Singh Center for Nanotechnology at the University of Pennsylvania, a member of the National Nanotechnology Coordinated Infrastructure (NNCI) network, which is supported by the National Science Foundation (Grant No. HR0011-21-9-0004). This research was, in part, funded by the U.S. Government. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Government. **Data Availability Statement:** The data that support the findings of this study are available from the
