2.1.

DM Design and Wafer Fabrication

For this study, we procured two 2040-actuator DMs from BMC with the characteristics specified in Table 1; the layout is illustrated in Fig. 1. The DMs were manufactured on 1.1 mm thick substrates that are more than four times stiffer than the standard substrate. This thickness was optimized through a finite-element analysis studying the stresses on the mirror caused by random vibrations using conservative RST coronagraph instrument flight acceptance specifications (see blue curve in Fig. 7). It brings about a higher resistance to bending stresses exerted by the thin films. This change also reduces the surface deformation of the unpowered DM and then increases the DM usable stroke for wavefront control (WFC) after flattening. BMC developed custom tooling at the commercial MEMS foundry to work with the new substrate and process these thicker wafers.

Table 1

High-actuator-count MEMS DM properties.

Fig. 1

Diagram of the devices under test. (a) A zoomed-in view of a $2\times 2$ actuator region. Each actuator (0.4 mm across) consists of a flexure anchored at its edges. (b) The full DM layout with 2040 actuators as well as wire traces extending radially to the edges of the 32.8 mm substrate. (c) The full wafer layout as manufactured, which typically has several DMs of different formats to make best use of the available wafer area.

Other than the substrate thickness, no step in the manufacturing process is different from usual BMC DMs. The procedure, illustrated in Fig. 2, is as follows.

Fig. 2

BMC’s MEMS DM fabrication process.

Once the devices were received from the foundry, BMC inspected the DMs using visible and IR optical microscopy and interferometry to identify potential optical, electrical, and subsurface manufacturing defects. Using a custom probe station, each candidate die that passed this initial inspection was tested to determine actuator response. Their electromechanical and optical performance were then characterized, including the responsiveness of each actuator, stroke limits, unpowered surface error, and actuator defects.

We selected two devices for this study, which we refer to as device under test (DUT) 1 and 2. Their initial surface properties are summarized in Table 2. DUT 1 is a 100% functional unit that was coated with an evaporated thin film of aluminum at BMC’s facility. The purpose of DUT 1 is to confirm or reject the hypothesis that a fully functional 2K MEMS DM can survive a launch environment. On the other hand, DUT 2 has some unresponsive actuators. We kept its face sheet uncoated to allow for the post-vibration infrared inspection of the DM surface to help understand any failure mode. This DUT aimed to test the hypothesis that defects causing anomalous actuators can propagate to neighboring actuators during random vibrations.¹⁶

Table 2

Initial surface quality properties of DUT 1 and DUT 2. Unpowered and powered surface errors are domniated by a strong astigmatism, characteristic of MEMS DMs.

2.2.

Electrical Connections

For both DUTs, coated-dies were attached with adhesive to a ceramic package specifically designed for the 2K DMs (see Fig. 3). The DM die and the ceramic package were electrically connected using gold wire-bonds applied with a high-precision automated tool at BMC’s facility. JPL also fabricated flex cables that were connected in the back of the new chip carrier through a pin-grid array (PGA) and terminated in 528-pin MEG-Array connectors (see Fig. 4).

Fig. 3

Front schematic of the ceramic chip carrier, mount, and rigid flex cables designed for the 2K MEMS DMs.

Fig. 4

PGA assembly at the back of the ceramic chip. The MEG-Array connector was not considered part of the random vibe test.

The packaged DMs were tested using high-voltage drivers commercially available from BMC to characterize their electromechanical and optical performance. BMC’s electronics connect to the DM via the MEG-Array connectors.

3.1.

Testing Overview

Both DUT 1 and 2 underwent a battery of tests before and after exposure to random vibrations. The carrier, die, die bonds, PGA joints, and carrier to test mount bonds were inspected in all phases. The actuator functionality and performance were tested using the steps outlined below. The MEG-Array connectors and receptacles are not envisioned as part of the flight system and are therefore not included in our analysis. The workflow of these experiments for both DUTs is illustrated in Fig. 5. Results of these tests are described in Sec. 4.

Fig. 5

Testing workflow. The tests before exposure to random vibrations consisted of an IR microscope inspection and functional tests using a Fizeau interferometer. DUT 1 was also used for WFC performance testing on a coronagraph testbed. After the random vibe, each DM was functionally testing using an interferometer. DUT 1 was then used for WFC performance testing, and DUT 2 was inspected using the IR microscope.

3.2.

Infrared Inspection

Before applying the face sheet coating to DUT 1, both DUTs were inspected at BMC using transmissive infrared microscopy. The system was automated to translate the DM and image many actuators in sequence. To achieve this, a MLS203 fast $X-Y$ stage was installed on a BX51 Olympus microscope. The microscope was also equipped with a MFC1 Motorized Microscope focus controller to focus either on the wiring or on the mirror layer. The wiring layer was inspected on the entire die of dimension $32.8\times 32.8\mathrm{mm}$ by steps of $400\mu \mathrm{m}$ (size of the actuator pitch) for a total of 6224 images. The mirror layer inspection was restricted to the device area to image each actuator individually, and 4080 images were taken in a serpentine clockwise path. In total, 10,804 images were recorded per DM to be compared before and after the random vibe in case of failure. DUT 2 had three actuators with clearly visible defects just after fabrication (see the infrared image of an anomalous actuator in Fig. 14).

3.3.

Functional Testing

DUT 1 was then coated, and both DMs were sent to the HCIT facility at JPL for testing. Functional testing was done using a Fizeau interferometer (Zygo Verifire). The DMs were placed inside a plastic enclosure that was purged with a continuous flow of dry air to maintain a relative humidity of $<30\%$ during operation and to avoid any electrostatic discharge event.¹⁷ The devices were connected through the MEG-Array connectors to a commercial 14-bit electronics provided by BMC for the 2K-DM to perform a battery of functional spatiotemporal tests. The input voltage was limited to 90 V.

The HCIT team developed a series of functionality tests aimed at highlighting defective actuators before and after the random vibe that were sorted in several categories. A “pinned” actuator is fixed to its unpowered position and is easily noticeable when uniform voltage is applied to the DM. A “free-floating” actuator does not move through electric commands but remains free to move to follow the displacements of its neighbors. “Tied” actuators occur when more than one actuator responds to a command sent to a single actuator index. A “weak” actuator moves significantly less than its neighbors with the same command. Finally, an “anomalous” actuator can be any of the above categories or otherwise defective. The standard functionality test process consists of the routines described below. For each surface measurement recorded by the Zygo interferometer, the uncommanded surface was subtracted, and piston, tip, and tilt aberrations were removed from the data in post-processing.

The functionality test routines are as follows.

3.4.

Performance Testing

DUT 1 was also tested for high-contrast imaging purposes using the In-Air Coronagraph Testbed (IACT)¹⁸ in the HCIT facility at JPL. The IACT optical layout is shown in Fig. 6. A 637 nm monochromatic light source was injected into the enclosed testbed through a single mode fiber to simulate a star. A charge six-Vector Vortex Coronagraph (VVC) was used to limit the sensitivity to low-order wavefront aberrations due to air turbulence.^19–22

Fig. 6

Schematic of the optical layout of IACT in the HCIT facility at JPL. Not to scale.

The injected light was passed through a linear polarizer (LP) and a quarter wave plate (QWP) to circularly polarize the light source upstream of the focal plane mask (FPM). The LP had an extinction ratio of ${10}^5$, and the QWP had a retardance of $0.24\lambda$ at 637 nm. The off-axis parabola (OAP) 1 then collimated the beam and reflected it toward a 18.48 mm pupil, immediately followed by DUT 1. There were 46.2 actuators across the beam at the DM. Similar to the functional test described above, DUT 1 was placed inside a plastic box with a continuous flow of dry air to reduce the humidity. To limit air turbulence, the flow was optimized to reach a relative humidity of 25% to meet the DM specification. A Fluke DewK thermo-hygrometer was inserted in the dry box to actively sense the humidity level through a software watchdog. The box had an opening on the front side to allow the beam to reflect off the DM to the 1524 mm focal length OAP2 that focused the light on the VVC FPM. The FPM was fixed to a three-axis mount. The diffracted light was then reflected on the 762 mm focal plane OAP3 and blocked by a Lyot Stop (LS) of diameter 7.5 mm on a two-axis mount. Considering the magnification of the OAPs, the LS diameter was 81.2% of the pupil image.

A “D” shaped field stop (FS) of size 3 to $10\lambda /D$ was added in the downstream conjugated focal plane. The purpose of the FS was to enhance the contrast in the final focal plane at the camera by minimizing stray light or photoelectrons inside the corrected regions adjacent to saturated regions. The FS was placed on a three-axis mount to optimize focus and can be moved in and out for calibration purposes. The dark images that are later subtracted from the DH images were recorded by fully blocking the light at the FS plane.

After the FS, the beam was then collimated by OAP5 to pass through another set of QWP and LP that minimizes the incoherent leakage caused by the imperfect retardance in the VVC FPM. The rotation angles of the QWP + LP were optimized by minimizing the signal on the science detector with the VVC FPM fully removed from the beam. Finally, the OAP6 directed the light to the science detector where the final image was formed. A neutral density filter wheel can be used for calibration purposes, for instance, to prevent the over-exposure of the unocculted PSF used for calibration, and it has an optical lens to allow for pupil imaging. The science camera Andor Neo sCMOS electrically cooled to $-40°\mathrm{C}$, and the generated heat was removed with a water cooler. The camera was mounted on a single-axis stage to control the focus. The pixel pitch was $6.5\mu \mathrm{m}$, and the resolution of the focal plane images was 24.7 pixels per $\lambda /D$.

Standard WS&C algorithms and calibration procedures dedicated to high-contrast imaging were used to minimize the simulated stellar intensity at the detector plane and improve the raw contrast level (intensity of the attenuated starlight normalized by the maximum of the unocculted PSF).²³ Phase retrieval algorithms based on both Gerchberg–Saxton formalism²⁴ and the fitting of low-order Zernike modes were used to flatten the DM and calibrate its response. At least three images close to the focal plane and three images close to the pupil plane were used to run the algorithm. After flattening the DM, the Strehl ratio of the unocculted PSF was very close to 1.0. The VVC FPM and the LS were automatically centered on the beam iteratively through the acquisition of pupil images. Dark images and off-axis PSFs were then recorded, and the FS was introduced in the beam to allow the desired off-axis DH region to pass through.

Wavefront sensing and WFC were performed, respectively, with pair-wise probing (PWP) and electric field conjugation (EFC)^25,26 through FALCO software.²⁷ Both algorithms require a high-performance DM to achieve contrast levels of $\sim {10}^{-8}$. The DH region where the stellar residuals are attenuated is defined by the FS aperture that goes from 3 to $10\lambda /D$. Although the contrast improves in the DH, the exposure time on the science camera is increased from 100 to 300 s to maintain a sufficient signal-to-noise ratio. The $\beta$-bumping technique²⁸ is regularly used to achieve the best possible coherent contrast in the DH region. The raw contrast in the DH is intricately linked to the DM performance. Performance testing results for DUT1 are presented in Sec. 4.1.

3.5.

Random Vibration Environment

In the previous work, we demonstrated the robustness of actuators that were surrounded by functional actuators under flight-like thermal cycles and vibrations as well as the compatibility of partially functional $50\times 50$ MEMS DMs with vacuum environments.¹⁶ This work expands the previous study in that we specifically test the robustness of actuators at the vicinity of defective actuators as well as the fully functional $50\times 50$ MEMS DMs. The DMs and their respective mounts were shaken at JPL on a 10-in. cube shaker. The flex cable was fixed to the edge of the mount with a flex clamp, whereas the other end of the flex was curled loosely and taped down onto the moving platform of the shaker to avoid any damage. Particle contamination and humidity were controlled during the test to ensure that the DMs were not subject to alternative sources of electric degradation and failure,²⁹ as suspected in previous studies.¹⁵ The applied signal ranged between 20 and 2000 Hz, which corresponds to typical frequency ranges for most launch vehicles. The temporal acceleration spectral density of the vibrations in each of the three spatial axes was between 0.01 and $0.4{\mathrm{g}}^2/\mathrm{Hz}$ and is shown in Fig. 7. The DMs therefore underwent 11.7 gRMS over all frequencies for 2 min per axis. This qualification test is conservative to any potential launch vehicles and surpasses the flight acceptance followed by the RST Coronagraph Instrument specifications.

Fig. 7

Power spectral density of the vibration experienced by the MEMS DMs in the three axes.

4.1.

DUT 1

DUT1 is a 100% yield device intended to validate that a fully functioning MEMS DM passes random vibration testing. The criteria of success are visual inspection of the structure; the actuator responsiveness, stroke, and voltage-to-displacement gain; and the ability to create a DH at relevant contrast levels ($\sim {10}^{-8}$). This section compares the results of functional and performance testing before and after the random vibration at JPL.

Figure 8 represents one of the 16 regular grids of actuators that were poked during the functional test of DUT1 before and after the random vibe test. The image resolution has been slightly decreased after the vibe because we did not ensure same Zygo zoom settings before and after the random vibe test. Such a resolution remains acceptable for direct comparison with pre-vibe data because we have many more than the required 4 pixels per DM actuator. As in the 15 other grids, the behavior of the actuators was identical before and after the random vibe regardless the applied voltage. No anomaly in the influence function shape nor displacement of the DM surface occurred. After DM flattening, the final shape was measured to be 3.41 nm RMS wavefront error before and 3.37 nm RMS after the random vibe. Around the flat setting, the quadratic relationship between the surface displacement amplitude of each actuator and the applied voltage remains identical before and after the vibe. None of the functional tests performed showed any anomalies on DUT1 either before and after the DM was random vibed.

Fig. 8

Optical path difference (in nanometers) that corresponds to a functional test in which a regular grid of actuator was poked (a) before and (b) after the shaking of DUT1.

After functional testing, DUT1 was installed on IACT to test the DM performance in a coronagraph instrument. Figure 9 shows the normalized intensities in the science image before and after the random vibe and after a few dozen WS&C iterations. The mean contrast in the DH is $1.19\times {10}^{-8}$ (before the random vibe) and $9.53\times {10}^{-9}$ (after the random vibe), and the spatial standard deviation is equal to $1.42\times {10}^{-8}$ and $1.28\times {10}^{-8}$, respectively. The mean coherent contrast is equal to $3.80\times {10}^{-9}$ before and $4.34\times {10}^{-9}$ after with a respective standard deviation of $3.71\times {10}^{-9}$ and $3.86\times {10}^{-9}$. The small difference in coherent contrast is explained by a slightly higher internal turbulence on the testbed after the random vibe. We also observe in Fig. 9 some horizontal artifacts that result from diffraction effects caused by a slight misalignment of the FS in the $z$ direction. These highly localized effects did not impact convergence of the PW + EFC algorithm nor the computation of contrast performance. The decrease of flux after the random vibe might induce an underestimation of the mean incoherent intensity leading to a slight overestimation of the contrast performance after the random vibe.

Fig. 9

Post-WS&C contrast maps ($\times {10}^8$) (a)–(c) before and (d)–(f) after the DUT1 underwent flight-like shaking. (a), (d) total; (b), (e) coherent; and (c), (f) incoherent intensities are presented. The exposure time for both total intensity images is 300 s, but the source injection unit has been moved between the experiments, explaining the noise discrepancy.

Figure 10 overlays the radial profile of each total intensity image, the mean contrast of which is calculated in annulus of $\lambda /8D$. On the one hand, both Figs. 9 and 10 emphasize that IACT performance are limited at low separations by an Airy pattern. This pattern is not sensed by PWP. This pattern is known to be an incoherent leakage due to manufacturing defects in the VVC FPM and in the LP and QWP.²³ This leakage could be further reduced by improving the retardance error in the QWP and the extinction of the LP that are currently used. On the other hand, the coherent component in the DH was measured below ${10}^{-8}$ in both cases, and its speckle intensity structure was modified at each iteration. We therefore attribute the remaining coherent component to the internal turbulence in IACT on timescales of a single WS&C iteration. This effect could be reduced on IACT by lowering the dry air-flow or by installing an additional WS&C system to specifically control low-order spatial aberrations at higher temporal frequencies.³⁰ Nonetheless, from the results of both the performance and functional test on DUT1, we can conclude that DUT1, a 100% functional 2K MEMS DM, survived random vibrations similar to a launch vehicle.

Fig. 10

Post-WS&C radial profiles of the raw contrast on the science detector before (blue) and after (red) the DUT1 underwent random vibe testing.

4.2.

DUT 2

DUT2 had a few defective actuators and no metallic coating with the intention of testing the hypothesis that anomalous actuators can propagate to neighbors during rocket launch. DUT2 is not coated to allow for IR inspection after the random vibe. DUT2 also underwent the battery of functional tests described above, but it was not used for performance testing.

In pre-vibe function testing, DUT2 was found to be $\sim 99.3\%$ functional (see in Fig. 11). It had three pinned actuators, two couples of tied actuators, and two couples and one triplet of weak and tied actuators (the voltage to amplitude gain of which is divided by the number of associated actuators). One result of the functional test is shown in Fig. 12, in which the same grid of actuators was poked with respect to the grid in Fig. 8. The poke grid measurements show that DUT2 did not change behavior at the actuator level. As an example, Fig. 13 shows the deflection for neighbors of one defective actuator (index 1283) while applying individual voltage of 0.025 BMC unit on top of a flat bias of 0.05 BMC unit. Their influence functions were fitted with a Gaussian with the maximum amplitude being reported in this plot. Error bars are computed as the standard deviation of the measured amplitude for the eight neighbors. The deflection of 1283 actuator’s neighbors remains identical before and after random vibration: failure did not propagate during rocket launch simulation. These results were confirmed by the remaining functional tests, described in Sec. 3.3.

Fig. 11

Grid of DUT2 actuators. Yellow: tied actuators (60–89, 129–164). Green: tied and weak actuators (502–549, 593–641–690, 1337–1338). Red: pre-vibe pinned actuators (1283, 1701, 1999). Orange: post-vibe pinned actuators due to poor connections at the MEG-Array connectors level (1392, 1616).

Fig. 12

Optical path difference (in nanometers) that corresponds to a stage of functional test, in which a regular grid of actuator was poked, before (left) and after (right) random vibe testing of DUT2.

Fig. 13

Deflection of actuator 1283s neighbors. We applied 0.025 BMC unit on these individual actuators on top of 0.05 BMC unit applied to all DUT 2 actuators.

Preliminary tests with post-vibe DUT2 presented new anomalous actuators with respect to pre-vibe, particularly tied characteristics. After further investigation, we realized that these anomalies were due to poor connections at the MEG-Array connectors level rather than the DM. Indeed, the connectors were disconnected before the random vibe and then reconnected for the functional tests. This process is sensitive because the pins can be easily bent if the connectors are not carefully handled. The defective connectors were fixed either by disconnecting and then reconnecting the faulty MEG-Array connector or by replacing the connector savers if the initial reconnection appeared unsuccessful. The state of the MEG-Array connectors was carefully inspected throughout the whole process. Given the challenges faced by our team related to connectors, we advocate for the development of more robust high-density connector technology and more practical DM driver electronics.⁸

We also imaged the initial defective actuators and their neighbors with the infrared microscope to confirm that the DM was not affected by the simulated rocket launch. No changes to the anomalous actuators nor their neighbors were notice during the post-vibe infrared inspection. Figure 14 shows the infrared image of one tied actuator as well as the neighbor of another, focusing either on the wiring layer or on the mirror layer, before and after the random vibe. The anomaly shown on the pinned actuator is apparent on both layers. The comparison of the images before and after the random vibe shows that the damage has not propagated from the initial defect. The second set of images shows that none of the neighbor carrier, die, die bonds, PGA joints, nor actuators were affected by the random vibe test. From these results, we saw no evidence that anomalous actuators propagate to neighbors during the random vibe.

Fig. 14

(a), (c) Pre-vibe and (b), (d) post-vibe infrared images of both (a), (c) a pinned actuator and (b), (d) the direct neighbor of a pinned actuator. (a) and (c) are focused on the wiring layer. (b) and (d) are focused on the mirror layer in reflection due to the DM package.