The assembly process sometimes can require several process steps to position the structures of the MEMS device. In a conventional assembly process, the MEMS device is assembled by an operator manipulating the MEMS structures, with probe tips attached to hand actuated micromanipulators, while watching the structures through a microscope. This process can be carried out at a conventional probe station. This can be a time consuming process, requiring meticulous attention to ensure proper positioning and application of force during the process. Incorrect positioning of structures or even breakage results without careful attention to probe placement and application of force.

Further, the tolerances of the MEMS assembly tools, such as probe tip definition or even probe station alignment can degrade during assembly causing lower manufacturing yields. This particularly is true where densely packed MEMS devices do not afford a large area in which to assemble each device on the chip.

Moreover, even if the process is relatively simple and does not require precise manipulation, the assembly process can take a long time, limiting manufacturing productivity. This is especially true if there are a very large number of MEMS device on a chip. As MEMS device sizes are reduced, and the number of devices per chip is increased, such probe assembly processes ultimately may not be practical.

There are several drawbacks with conventional probe assembly processes as discussed in U. S. Patent Application No. 09/697036, by Yeh, et al. , filed on 10/25/00, entitled MEMS ASSEMBLY PROCESS USING COMPUTER CONTROLLED PROBE STATION, issued as U. S. Patent No., on, herein incorporated by reference in its entirety. One drawback is that the conventional probe station assembly ultimately will be too time consuming. With conventional array structures having a limited number of devices, manual manipulation is possible. As the number of devices in the array increases, however, manual manipulation by hand will become an overly time consuming and inefficient manufacturing process.

Moreover, manually manipulating the probe tips can result in inconsistent application of force, especially when latching structures during a deployment process. To deploy latched structures, the structures typically are pushed past their eventual latch retained position. If the structures are manipulated too far, breakage of a latch, a hinge, or other associated structure can result. This can result in lower manufacturing yields.

Additionally, if a probe pushes too far, it can cause a latch to engage and retain a structure in a different position from device to device.

This causes non-uniformity in the positioning of structures across the chip.

Furthermore, simultaneous manipulation of multiple probe tips is difficult to accomplish. Unbalanced manipulation of the probes can result in shearing forces which can damage structures.

A further problem with some assembly processes is that manual manipulation of the probe tips along the substrate causes the probe tips to wear and become unusable.

An additional drawback observed by the present inventors is that manipulation of one structure into its latch retained position can result in torque on a second structure. This torque stresses the second structure and its associated structures, and can ultimately lead to breakage, misalignment, and non-uniformities.

Yet another problem occurs if a probe does not manipulate near the center of the structure. Among other problems, this can cause incomplete deployment. If permanent damage does not occur, incomplete deployment requires addition manipulation of the probe by an operator to re-manipulate the structures in an attempt to complete deployment. This results in loss of time and can ultimately result in breakage, misalignment, and non- uniformities.

What is needed is a means to improve manufacturing time. Also, what is needed is a means to improve manufacturing yields. In addition, what is needed is a means to improve the reliability of assembled MEMS devices.

Also, what is needed is a means to lower assembly related costs. Further, what is needed is a means to minimize breakage, misalignments, and non- uniformities, and to improve uniformity of deployment.

SUMMARY In at least one implementation, a method is provided for assembling a MEMS device by retaining a MEMS device in a centrifuge and operating the centrifuge to generate a force sufficient to move at least one structure of at least one MEMS device.

In at least one embodiment, a centrifuge adapted for assembly of MEMS devices having a mount adapted to hold a substrate so that a force generated during operation of the centrifuge is capable of actuating at least one MEMS structure.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a cut away perspective view of embodiments of a centrifuge and a centrifuge mount in accordance with the present invention.

Fig. 2 shows an partial side view of embodiments of a centrifuge and a centrifuge mount in accordance with the present invention.

Fig. 3 shows a cut away top view of embodiments of a centrifuge and a centrifuge mount in accordance with the present invention.

Fig. 4 shows a cut away partial top view of embodiments of a centrifuge and a centrifuge mount in accordance with the present invention.

Fig. 5 shows a side view of an embodiment of a centrifuge mount in accordance with the present invention.

Fig. 6 shows a perspective view of an example of a MEMS device embodied in a MEMS optical switch.

Fig. 7 shows a top view of an example of a MEMS device embodied in a MEMS optical switch prior to assembly.

Fig. 8 shows a side view of the MEMS optical switch of Fig. 7 along the 8-8 line.

Fig. 9, shows the side view of the MEMS optical switch of Fig. 8 illustrating deployment of a mirror structure.

Fig. 10 shows a side view of the MEMS optical switch of Fig. 7 along the 10-10 line with mirror deployed.

Fig. 11 shows the side view of the MEMS optical switch of Fig. 10 illustrating deployment of an actuator arm structure.

Fig. 12 shows a cut away partial top view of an embodiment of a centrifuge with a MEMS chip positioned therein in accordance with the present invention.

Fig. 13 shows a cut away partial top view of an embodiment of a centrifuge with a MEMS chip positioned therein in accordance with the present invention.

Fig. 14 shows a cut away partial top view of an embodiment of a centrifuge with a MEMS chip positioned therein in accordance with the present invention.

Fig. 15A shows a cut away partial top view of an embodiment of a centrifuge with a MEMS chip positioned therein in accordance with the present invention.

Fig. 15B shows a perspective view of a simplified embodiment of a centrifuge mount in accordance with the present invention.

Fig. 16 is a perspective view of a MEMS chip from the axis of rotation of a centrifuge showing a simplified illustration of possible opposing torquing moments, generated in certain implementations, during deployment of MEMS structures which are located on opposing lateral sides of the MEMS chip.

Fig. 17 show a perspective view of a non-assembled MEMS device with a retainer structure in accordance with a possible embodiment of the present invention.

Fig. 18 show a perspective view of the MEMS device with the retainer structure of Fig. 17 after assembly.

Figs. 19-22 show perspective views of assembly of a MEMS device with a retainer structure in accordance with a possible implementation of the present invention.

DESCRIPTION In preferred implementations a centrifugal force is utilized to position structures a MEMS device. For this purpose, a centrifuge 200, illustrated in Fig. 1, may be utilized as part of a MEMS assembly process.

With such a method, a MEMS chip (not shown in Fig. 1) is positioned within the centrifuge 200, and rotated by the centrifuge to generate a centrifugal force on the chip. The centrifugal force acts on the structures of the MEMS devices on the chip so as to deploy one or more structures of the MEMS devices into an assembled position. The radius of chip placement and the RPM's of the centrifuge, in view of the mass centers of the structures to be deployed, can be determined by computational or empirical means to control deployment of the MEMS structures.

In a typical implementation, a MEMS chip (not shown in Fig. 1) is retained in position within a centrifuge using a mount 220. A MEMS chip may be mounted, retained, or held directly by the mount 220. Or, it may be mounted, retained, or held indirectly, by the mount 220, such as with a chip carrier, a chip holder, a chip package, or other suitable temporary or permanent chip keeping device.

In the embodiment of Fig. 1, the mount 220 has a cavity 225. The cavity 225 can be constructed to allow the MEMS devices to be situated within the cavity 225 during the assembly process. Locating the MEMS devices within the cavity 225 shields the MEMS devices from air flow which can be generated within the operating centrifuge. This inhibits windage and associated turbulence around the MEMS devices during centrifuge operation.

As such, unwanted windage forces on the structures of the MEMS devices are inhibited.

It is also possible in some implementations to create a vacuum, either complete or partial, within the centrifuge 200 to inhibit windage forces on the MEMS devices. A vacuum may be provided in conjunction with positioning the MEMS devices within the cavity 225, or as an alternative means for inhibiting windage forces on the structures of the MEMS devices.

Turning to Figs. 2 & 3, the substrate of the MEMS chip 310 may be positioned substantially normal (not shown) to a centrifugal force Fc generated by the centrifuge. Or, the substrate 310 may be positioned with a tilt angle that is non-normal to the centrifugal force Fc, as depicted in Figs. 2 & 3.

In Fig. 2, the substrate is shown positioned within the centrifuge 300 so that the plane of the substrate of the MEMS chip 310 is positioned with a tilt angle 21 with respect to normal to the centrifugal force F. The tilt angle 21 shown in Fig. 2 positions the MEMS chip 310 so that a tangential velocity Vt imparted by the centrifuge lies in, or is substantially concurrent with, the plane of the MEMS chip 310. Hence, the tilt angle 21 positions the MEMS chip 310 so that the axis of rotation of the centrifuge 150 does not lie in, or is non-concurrent with, the plane of the MEMS chip 310.

In Fig. 3, the plane of the substrate of the MEMS chip 410 is positioned within the centrifuge 400 with a tilt angle 22 with respect to normal to the centrifugal force Fc. The tilt angle 22 shown in Fig. 3 is substantially concurrent with the axis 450 of the centrifuge, but non- concurrent with a tangential velocity Vt imparted by the centrifuge.

Although not shown, a tilt angle 2in ; which is a combination of tilt angles 21 and 22, may be utilized.

Further, as shown in Fig. 4, the chip 510a may be positioned in the centrifuge with a selected rotation angle 2R to control deployment. The rotation angle refers to the orientation of the chip as the chip is rotated within the plane of the chip about an axis normal to the plane of the chip.

The proper rotation angle 2R may be selected in conjunction with tilt angles 21 and/or 22, with regard to the orientation of the structures on the chip, to govern deployment of one or more structures of the MEMS device.

As will be discussed further below with reference to an example case, the chip may be positioned with a rotation angle 2R that provides an assembly force generally orthogonal to a hinge axis of a deploying structure. Also, as discussed further below, in the case of assembly of multiple devices (not shown) spread across a chip 510b, the rotation angle 2R may include an offset rotation angle 2off for mitigating any undesirable effects caused by caused by variations in the centrifugal force across the plane of the chip 510b. Such variations may result from the directional fanning of the centrifugal force Fc laterally across the plane of the chip 510b due to the radial directionality of the centrifugal force Fc.

Positioning the MEMS chip with the proper tilt and rotation angles can be utilized to provide enough force to deploy the structures while governing the centrifugal assembly force Fc imparted to the MEMS structures. This allows the range of movement of the structures to be governed to limit movement of the structures to less than a critical distance necessary to inhibit damage to the MEMS structures. Thus, the mount may retain the MEMS chip so that the centrifugal force Fc exerts enough force to deploy a selected structure of the MEMS devices, without breaking or otherwise damaging the structures.

In some implementations of the present invention, selective deployment of structures of MEMS devices may be accomplished by only partially fabricating, or partially releasing selected structures of the MEMS device prior to placement of the chip, wafer, or other substrate in the centrifuge.

This may allow more effective deployment in situations where the configuration of the structures prevents easy deployment, where sufficient force isolation or force selectivity is not obtainable, where a specific deployment sequence is required, where structures are easily damaged, or in other difficult situations.

In such implementations, for example, a MEMS device may be fabricated with a sacrificial layer which is selectively etched to release one structure of a multi-structure MEMS device for deployment. The released structure may be deployed using a centrifuge as discussed herein, or by some other means, while other structures are retained by the sacrificial layer.

In this way, the movement of selected structures temporarily can be restricted. Thereafter, the restrained structures may be released with a second etch process and allowed to move for deployment, or for operation.

In some implementations a retainer structure (not shown) may be fabricated on the chip to govern the movement of a deploying structure or structures. In one possible implementation, such a retainer structure may restrict the movement of a deploying structure or structures by physically limiting the movement of the deploying structure in response to the centrifugal force.

The retainer may be a temporary structure fabricated to be removed after partial or complete assembly, such as a photoresist structure or other structure easily removed prior to operation of the device. Alternatively, the retainer structure may be fabricated to be part of the assembled device.

In some embodiments, the retainer may be a structure that is fabricated to be used only to restrain the movement of one or more structures during deploying. In other embodiments, the retainer also may be a structure used to restrain the movement of one or more structures after deployment and during operation of the device. Or, the retainer may temporarily restrain the movement of a structure or structures during only a portion of the fabrication and/or assembly. Moreover, the retainer may, but need not be, a stationary structure. Further, it may be deployable to either a temporary, or to a fixed position during assembly. It is also possible that the retainer be a portion of a larger deployable structure. In some implementations, deployment of device structures may be linked so that the structures of the device deploy sequentially. That is, for example, a first structure deploys, allowing, or causing, a second structure to deploy, and so on, in a reverse domino-like effect. In one such implementation, the first structure restrains movement of the second structure until the first structure is fully or partially deployed. In some implementations, the first structure can be used to assist, or to initiate, deployment of a second structure. For example, movement of the first structure to its deployed position urges the second structure into position so that the centrifugal force causes deployment of the second structure. Or, for example, a first structure which is more responsive to the centrifugal force- may be used to urge a second structure which is less responsive to the centrifugal force, for assisting in deployment of the second structure.

Deployment may be linked by physically linking together structures of the device, or by causing physical contact between separate structures, to control the assembly process and govern the movement of the structures during the assembly process.

Further, in some implementations, a hybrid assembly process is performed. In a hybrid assembly process, both a centrifugal force and a conventional deployment technique are used to assemble the MEMS device. For example, a first structure can be completely or partially deployed with a conventional deployment technique, and a second structure can be deployed with a centrifugal assemble force. The hybrid process may be used in conjunction with partial fabrication, partial release, retainers, and/or linked deployment.

Turning to Fig. 5, the MEMS chip 515 may be positioned in the centrifuge with an active mount 505. An active mount 505 adjusts the position of the MEMS chip 515 during operation of the centrifuge. In some situations, active repositing can be utilized to provide a more uniform assembly torque to deploy a MEMS structure. Furthermore, changing the position of the MEMS chip 515 during operation can be used to govern the movement of the structures of the MEMS device to inhibit damage to the structures.

Further, in some situations, active positioning can allow a single step assembly process to deploy more than one structure that would otherwise require a multi-step assembly process. Moreover, repositioning the MEMS chip 515 during operation of the centrifuge may be used to mitigate undesirable effects caused by variations in the centrifugal force across the plane of the MEMS chip 515. This can be utilized to inhibit damage to MEMS device structures.

In the example embodiment shown in Fig. 5, a spring 525 controls the tilt angle 21 of the MEMS chip 515. The centrifugal force Fc may be increased, or decreased, by changing the RPM of the centrifuge to achieve the desired tilt angle necessary for deployment of one or more structures of a MEMS device while inhibiting damage to the device. Although shown as controlling the tilt angle 21, other mounts employing such means may be utilized to control 22, or to control combination tilt angles 212, or to control an active rotation angle 2R, or even 2, off, in response to a changing application of the centrifugal force Fc.

The mount illustrated in Fig. 5 is an example of one possible type active mount which employs a spring 525 to position the chip in response to the centrifugal force Fc. It is contemplated that other mechanical type variable positioning devices may be used to control the position of the MEMS chip 515 during operation of the centrifuge. Further, the active mount may employ other variable positioning devices such as electro-mechanical, electro-magnetic, hydraulic, fluidic, or the like, to control the position of the MEMS chip 515 during operation of the centrifuge.

An Example : A MEMS Optical Switch In one specific utilization of MEMS devices, MEMS switches are built for optical switching. In such a utilization, structures can be built which have a mirrored surface for reflecting a light beam from a sending optical fiber to a separate receiving fiber, or other device. These types of structures are generally known as optomechanical switches. In one type of MEMS optomechanical switch, a mirrored surface is placed onto a movable structure so that the mirror can be moved in to, or out of, the path of a beam of light. With more than one switch aligned in the beam path, the beam can be directed to one of several receiving fibers. An example is disclosed in U. S. Patent Application Serial No. 09/483,268, by Fan, et al. , filed on 1/13/00, entitled MICROMACHINED OPTOMECHNICAL SWITCHING DEVICES, issued as U. S. Patent No. on, herein incorporated by reference in its entirety.

In one such optomechanical switch, the mirror is positioned in a fixed upright position with respect to an actuator. The mirror is moved vertically into and out of the light beam. An example of such a switch is shown in Fig. 6. The switch 600, positioned on a substrate, has a mirror structure 620 attached to an actuator arm 630 by a hinge 640. With the switch 600 in a lower position, the mirrored portion of the mirror structure 620 is placed into and relects the light beam B1. With the mirror in an upper position, the mirrored portion is out of the path of the light beam B1.

During operation of the switch 600, to move the actuator arm 630 between its raised and lowered positions, typically a surface electrode 612 is used. The electrode 612 is positioned on the surface of the substrate 600 and generally beneath the actuator arm 630. By placing different electrical potentials on the electrode 612 and the actuator arm 630 the actuator arm 630 can be attracted towards the electrode 612. One example of a surface electrode is disclosed in U. S. Patent Application No. 09/697,037, by Li Fan, filed on 10/25/00, entitled MEMS OPTICAL SWITCH WITH SURFACE ELECTRODE PROVIDING REDUCED LIKELIHOOD OF SHORTING AND METHOD OF FABRICATION THEREOF, issued as U. S. Patent No. on, herein incorporated by reference in its entirety.

A torsional spring structure 640 is provided to allow pivoting of the actuator arm 630 with respect to the substrate. The torsional spring structure 640 also may function as a hinge between the actuator arm 630 and a backflap 632. The backflap can be employed to limit upward movement of the actuator arm and inhibit over-torsion of the spring. The torsional hinge 640 may be constructed to exert a biasing force on the actuator arm 630. In this manner, the actuator arm 630 can be moved between a lowered and a raised position.

The backflap 632 is retained in position with respect to the actuator arm 630 by an actuator latch 634. As part of the assembly process, the actuator arm 630 is latched to the backflap 632. The actuator arm 630 is raised until the actuator arm 630 engages an actuator latch 634.

Thereafter, the torsional spring hinge operates to allow the actuator arm 630 and backflap 632 to rotate or pivot about the hinge. An example of a torsional spring hinge is disclosed in U. S. Patent Application Serial No.

09/697,762, by Li Fan, filed on 10/25/00, entitled MEMS OPTICAL SWITCH WITH TORSIONAL HINGE AND METHOD OF FABRICATION THEREOF, issued as Patent No. on, herein incorporated by reference in its entirety.

Also, as part of the assembly process of the switch 600, the mirror structure 620 must be raised from parallel to the actuator arm 630 to an upright position by rotating or pivoting the mirror 620 about a hinge 623.

Latches 624 engage the mirror structure 620 to retain the mirror structure 620 in the upright position. An example of a mirror latch is disclosed in U. S. Patent Application Serial No. 09/697,038, by Li Fan, filed on 10/25/00, entitled MEMS OPTICAL SWITCH WITH A NOTCHED LATCHING APPARATUS FOR IMPROVED MIRROR POSITIONING AND METHOD OF FABRICATION THEREOF, issued as Patent No. on, herein incorporated by reference in its entirety.

A stopper structure 616 may be employed to restrain the movement of the actuator arm 630, so that it is inhibited from contacting the electrode 612 located below the actuator arm 630. This inhibits shorting between the actuator arm 630 and the actuator surface electrode 612. Such a stopper structure 616 is disclosed in U. S. Patent Application No. 09/697,767, by Li Fan, filed on 10/25/00, entitled MEMS MICROSTRUCTURE POSITIONER AND METHOD OF FABRICATION THEREOF, issued as Patent No. on herein incorporated by reference in its entirety.

In a conventional probe assembly process discussed in the above referenced application 09/697,036 by Yeh et al. , the device is assembled by manipulating the probe tips onto the substrate and sliding them simultaneously under the actuator arm 630 to raise the actuator arm 630 and latch it to the back flap 632. After the actuator arm 630 is latched to the back flap 632, the stopper structure 616 may be locked in place, and the mirror manipulated to latch it in an upright position.

In a possible implementation of the present invention, the structures of the MEMS optical switch 600 of Fig. 6 may be assembled using a centrifuge.

Fig. 7 shows an illustration of the MEMS optical switch 700 prior to assembly. In this example, the mirror structure 720 will be retained by latches 724 at about a 90 degree angle with respect to the actuator arm 730.

The actuator arm 730 will be retained by latches 734 with about a 150 angle with respect to the back flap 732.

Because of the varied angles of deployment and the relative masses of the structures of the MEMS device of Fig. 7, positioning the substrate normal to the centrifugal force will not cause deployment of both the mirror 720 and the actuator arm 730/back flap 732. One reason is because the mirror structure 720 must be pivoted past its eventual latch retained position to set the mirror latches 724. Further, as the mirror structure 720 is pivoted about the hinge 723, an opposing bias force is exerted by the hinge 723 which acts as a torsional spring. The opposing bias force increases in a non-linear fashion as the mirror structure is pivoted. As a result, the force necessary to latch the mirror structure 720 can be significantly greater than the force necessary to latch the actuator arm 730 to the back flap 732.

In addition, with the substrate normal to the centrifugal force, the torque generated by the centrifugal force that acts on the actuator arm 730 is greater than that acting on the mirror. This is due to the greater combined mass of the actuator arm 730 and mirror structure 720 as compared to the mirror structure 720 alone, and to the larger moment arm about the actuator arm hinge 740 than about the mirror hinge 723. As a result, the actuator arm 730 deploys with less centrifugal acceleration than the mirror structure 720.

Increasing the centrifugal acceleration by increasing the RPMs of the centrifuge can damage the actuator arm hinge 730, or the actuator arm latches 734, without deploying the mirror structure. Thus, in this case, positioning the substrate normal to the force would not latch the mirror structure 720, and could cause breakage of the actuator arm hinge 740 and/or the backflap latches 734.

By selectively positioning the MEMS chip in the centrifuge, however, it is possible to use multiple or a single centrifuge operation to assemble the MEMS optical switch 700. In a multi-step assembly, the MEMS chip is positioned within the centrifuge and the centrifuge is operated to deploy a first MEMS structure. Thereafter, the MEMS chip is repositioned within the centrifuge and the centrifuge operated again to deploy a second MEMS structure. Although not necessary in the present example, additional repositioning and centrifuge operation may be employed to deploy additional structures if desired. Further, as discussed above, it is possible to utilize an active mount to reposition the MEMS chip.

Also as discussed above, the tilt angle of the MEMS chip is selected so that sufficient force is applied to deploy a structure or structures without causing damage to hinges or other MEMS structures. As such, the tilt angle can be selected to govern the movement of a structure to inhibit damage to the MEMS devices structures.

Turning to Figs. 8 & 9, in this example, a tilt angle 2M is selected so that the mirror structure 720 is allowed to move a few degrees past its eventual deployed angle NM with respect to the substrate 710. This is to allow the latches 724 to extend into position to retain the mirror structure 720. Thus, the substrate 710 is positioned within the centrifuge with a tilt angle 2M that allows a component of the centrifugal force Fc to be provided through the range of movement of the mirror structure 720 which is sufficient to pivot the mirror structure 720 from the plane of the substrate 710 to beyond its eventual latched angle NM. For example, a tilt angle 2M of about 60 degrees, with the centrifugal force Fc being about 4,000 G's (or within a range of about 3,000 to 10,000 G's), is sufficient to deploy the mirror structure 720.

Turning to Figs. 10 & 11, with the multi-step centrifuge assembly process, after the mirror structure 720 has been deployed, the MEMS chip is repositioned within the centrifuge to deploy the actuator arm 730. Actuator arm latches 734 retain the actuator arm 730 to the back flap 732 at about a 150 degree angle with respect to the backflap. Thus, the actuator arm 730 needs only to travel slightly more than an angle NA of about 30 degrees with respect to the substrate 710.

Because most of the resistance to deployment is encountered near the latching position, a tilt angle 2A of about 30 degrees is selected so that latching occurs when the actuator arm 730 is at around 90 degrees with respect to the centrifugal force. At 90 degrees, the greatest amount of centrifugal force acts on the actuator arm 730 to generate an assembly moment about the hinge 740. As such, the generated centrifugal force may be minimized to limit the overall forces applied to the MEMS device. Hence, lower RPM's may be used to deploy the actuator arm 730 than otherwise would be required. Limiting the overall applied centrifugal force in this manner allows better control of deployment and inhibits damage to the associated structures. In one example, about 600 G's (or within a range of about 400 to 900 G's) of centrifugal force Fc is sufficient, with a tilt angle 2A of 30 degrees, to latch the actuator arm 730 to the back flap 732.

In the above example, one will realize that the orthogonal orientation of the axes of the mirror hinge 723 and the actuator arm hinge 740 facilitates a multi-step assembly process. Also, one will further realize that in this selected example, the selected tilt angles are complementary nature and would allow repositioning MEMS chip in the centrifuge by removing the chip from the mount, rotating the chip 90 degrees about an axis normal to the MEMS chip, and placing back in the same mount. In other implementations of the MEMS assembly process, different centrifuge mounts, having different and non-complimentary tilt angles may be utilized. For example, even in the above example, the mirror structure 720 in the present example will satisfactorily deploy from tilt angles ranging from about 10 degrees to about 85 degrees and beyond.

It should be noted that the tilt angle 2M in this example not only governs the component of centrifugal force Fc on the mirror structure 720, but also governs it for the actuator arm 730 as well. The tilt angle 2M reduces the component of the centrifugal force Fc urging the actuator arm 730. In this example, the tilt angle 2M is selected so that the actuator arm 730 does not deploy. Thus, the tilt angle 2M not only inhibits damage to the mirror hinge 723 and the mirror latches 724, it also inhibits damage resulting from forces on, and movement of, the actuator arm 730.

A multi-step centrifuge assembly process may be performed with a partial release of selected structures. For example, after the MEMS device 700 is fabricated as shown in Fig. 7, a sacrificial layer of oxide (not shown) typically is deposited thereover. A portion of the sacrificial oxide, such as a portion covering the mirror 720 and mirror latches 724 is removed to allow deployment of the mirror 720. Thereafter the remaining portion of the sacrificial oxide is removed to allow deployment of the actuator arm 730 to the back flap 732.

It is contemplated that the oxide layer can be removed with liquid, or even gas phase acid etching. Liquid acid etch may be performed in accordance with U. S. Patent Application Serial No. 09/697,035, filed on <BR> <BR> 10/25/00, by Gritters et al. , entitled METHOD FOR IMPROVING POLYSILICON STRUCTURES OF A MEMS DEVICE BY MASKING TO INHIBIT ANODIC ETCHING OF THE MEM <BR> <BR> POLYSILICON STRUCTURES, issued as U. S. Patent No. , on , herein incorporated by reference in its entirety. Thus, during the liquid etch process exposed metallic surfaces would be masked to inhibit degradation of the polysilicon structures caused by anodic etching of the polysilicon structures during oxide removal.

In some implementations of the present invention, a hybrid assembly process, one that utilizes both a conventional deployment technique and a centrifugal force can be used. As such, one structure may be deployed using a conventional technique while another structure is deployed using a centrifugal force. For example, the mirror 720 may be deployed using a probe tip assembly technique, and thereafter, a centrifugal force is used to deploy the actuator arm 730. Or, the mirror structure 720 may be deployed using a centrifugal assembly force, and thereafter, the MEMS chip removed from the centrifuge to complete assembly utilizing a conventional technique, such as probe tip assembly. The conventional technique may be utilized to assist with, or to complete the assembly of the MEMS devices. Moreover, after removal of the MEMS chip from the centrifuge, if one or more structures fail to properly deploy, a conventional technique, such as for example probe tip assembly may be utilized to rework, or to complete assembly of such structures. The hybrid assembly process may utilize centrifugal assembly forces and any conventional technique such as for example: frictional force actuation processes, such as scratch drive actuation processes; stress actuation processes, such as with bimorphic/polymorphic structures; mechanical shock assembly processes; electrostatic assembly processes; magnetic and/or electromagnetic assembly processes; fluidic assembly processes such as surface tension assembly processes, for example solder reflow, and/or such as turbulence flow assembly processes; gas flow assembly processes such as air flow assembly; or other known technique.

In some implementations of the present invention, it is possible to select a tilt angle that allows deployment of multiple structures in a single centrifuge operation without having to reposition the MEMS chip. In the above example, because of the differing hinge spring constants, masses, and moment arms for each structure, a composite tilt angle is selected so that force vector components of the centrifugal force Fc urge each individual structure enough for deployment without damaging the MEMS device.

For example, referring to Figs. 1,2, and 8-11, a composite tilt angle 212 having a tilt angle with a 21 providing a 2M of about 80 degrees and a 22 providing a 2A of about 30 degrees, with about 4,000 G's of centrifugal force Fc, may be possible to deploy both structures.

In the above described example, the RPM's of the centrifuge may be selected by computational or empirical means. The selected RPM's will depend on the radial distance of the MEMS chip from the axis of the centrifuge, the centers of mass of the structures, the corresponding force components during deployment, and the resistance to deployment (in this example determined by the spring constants of the torsional hinge structures and by the biasing of the latches).

Turning to Fig. 12, placing the MEMS chip 1210 in the centrifuge 1200 with a tilt angle 21 (as discussed with reference to Fig. 2 above) as illustrated, can cause MEMS devices (not shown) on the MEMS chip 1210 to be located at differing radial distances from the axis 1250 of rotation of the centrifuge 1200. As such, MEMS devices located closer to the axis 1250 of rotation at near R1 for example, will experience less centrifugal force than MEMS device located farther from the axis 1250 of rotation near R2.

Accordingly, there will be a variation of force radially across the MEMS chip. Depending on the radial distance (about R2 minus R1, for example) of the MEMS devices across the MEMS chip 1210, the centrifugal force exerted on the MEMS devices near R2 can be 25% or more than the force exerted near R1. It has been found that in some cases, such a differential in the applied centrifugal force can cause damage to MEMS devices near R2, while devices near Ri remained unassembled.

Thus, to inhibit damage to the MEMS devices and ensure proper deployment in certain implementations, the variation in the centrifugal force imparted to the MEMS devices across the MEMS chip should be less than about 25%. Based on empirical results, in order to ensure high yield, the force variation would be controlled to less than about 10% in deploying the mirror in the above example.

To accomplish this, the distance of the MEMS chip 1210 from the axis of rotation 1250 is selected for a given tilt angle 21 and/or 22 to limit the force differential across the MEMS chip 1210. Maximizing the radius of placement of the MEMS chip 1210, minimizes the force differential. Thus the radius from the axis of rotation 1250 of the centrifuge is selected so as to limit the variation in assembly force radially across the MEMS chip 1210 to less than a threshold necessary to inhibit damage to susceptible structures of the MEMS devices. This may be particularly desirable in some implementations, such as for example in embodiments without supplemental restraining structures, or in situations where the necessary governing means are not possible to protect susceptible structures.

In one example, the mirror 720 (shown in Fig. 7) of a completely released MEMS device, is assembled without damaging hinges 723 or 740 (shown in Fig. 7) of a 4x4 MEMS optical switch array having a radial distance across the MEMS chip (R2-R1) close to about 25 millimeters. This is accomplished by mounting the MEMS chip at about 100 millimeters from the axis of rotation to the center of the MEMS chip, with a tilt angle 21 of about 60 degrees.

Turning to Fig. 13, maximizing the radius of placement of the MEMS chip 1310 from the axis of rotation 1350 also can minimize centrifugal force variations laterally across the MEMS chip 1310. Because the centrifugal forces act radially, centrifugal forces F1 and FC2 acting on MEMS devices near opposite lateral sides of the MEM chip experience lateral components Xi and x2 acting in opposite directions. Increasing the radius of placement, reduces the magnitude of the lateral components Xi and x2.

Turning to Figs. 14, providing a tilt angle 22 (as discussed with reference to Fig. 3), also can be used to minimize the variation in the lateral component of the centrifugal force. As shown in Fig. 14, by positioning the MEMS chip 1410 with a tilt angle 22, the lateral components X3 and X4 of the centrifugal forces are reduced as compared to Xi and x2 of Fig. 13.

Although not shown in Fig. 14, it should be noted that in some implementations, it may be possible to utilize a tilt angle 2z approaching, or even at, 90 degrees. A 90 degree 22 tilt angle will eliminate the lateral x-component variation across the MEM chip 1410. It is contemplated that a 90 degree 2z tilt angle could be utilized along with an offset rotation angle 2Roff incorporated into the rotation angle 2R, shown in Fig. 4, to actuate and even deploy, structures of MEMS devices. For example, after a previous deployment of the actuator arm 730 above the substrate, as shown in Fig. 7, a 90 degree 22 tilt angle along with a 10 degree offset rotation angle 2Roff may be utilized to actuate the mirror 720.

Fig. 15A is a simplified illustration of a MEMS chip 1510 at a 90 degree 22 tilt angle within the centrifuge 1500 as discussed above. Fig.

15B shows a perspective view of a simplified mount 1520 illustrating about a 10 degree offset rotation angle 2Roff incorporated into the rotation angle 2R of the the MEMS chip 1510.

As with other the implementations discussed herein, it may be desirable to utilize a retainer structure (not shown) in addition to appropriate tilt angles 21 and/or 22, rotation angle 2R, and radius of placement. For example, a retainer structure (not shown) may be utilized to limit the movement of the deployed actuator arm 730 when deploying the mirror 720 using the 90 degree 22 tilt angle discussed above.

Turning to Fig. 16, in some situations, the lateral x-component variation across the MEMS chip 1610 can cause moments in opposing directions. Fig. 16 is a simplified illustration of a MEMS chip from the perspective of the axis of rotation of the centrifuge. In the simplified illustration of Fig. 16, lateral x-component forces will produce moments ma and mb in opposite directions around deployed actuator arms 1630a and 1630b.

Thus, moment mb is shown acting on the back side of mirror 1620b which will facilitate its deployment. At the same time, moment ma is shown acting on the front side of the mirror 1620a, which will inhibit mirror 1620a deployment. Consequently, in accordance with observations by the present inventors, in such cases the mirror 1620a will not assemble even at extremely high centrifuge RPM's.

To reduce the effects of the opposing directions of the lateral x- components, an offset rotation angle 2Roff may be incorporated into the rotation angle 2R of the MEMS chip 1610. In this example, with a 80 degree 21 tilt angle and a zero degree 22 tilt angle, incorporating an offset rotation angle 2Roff of about 5 degrees can sufficiently reduce the opposing moment, illustrated as moment ma in Fig. 16, to allow deployment of mirrors structures spread across of the MEMS chip. In this example, a sufficient offset rotation angle 2Roff can be achieved to causes the moment ma to act on the backs of all mirrors across the MEMS chip to allow their deployment.

Turning to Figs. 17-18, shown is a simplified example of one possible type retainer 1700 used to govern the movement a structure of a MEMS device.

In this example a limiter 1726 deploys, as shown in Fig. 18, to a latch 1724 retained position and limits over actuation of arm structure 1730. In this way, the movement of the arm structure 1730 is governed by the retainer 1700 to inhibit damage that could otherwise occur during centrifuge operation.

In this example, the retainer 1700 does not actuate with the arm 1730 so it can be made with more mass for easy deployment with a centrifuge process, and to allow the arm 1730 to withstand greater assembly force. As such, it improves the process window of MEMS centrifuge assembly. Further, in some implementations, a retainer can allow a single operation of the centrifuge to deploy multiple MEMS structures without otherwise causing damage to susceptible structures of the MEMS device.

Turning to Figs. 19-22, shown is simplified example of use of another type retainer 1900. In this example, the retainer 1900 restrains a first structure 1930, while a second structure 1920 is being deployed. This improves the process window for centrifuge assembly of the second structure and inhibits damage that could otherwise occur as a result of ungoverned movement of the first structure 1930. After deployment of the second structure 1920, the retainer 1900 is removed from over the first structure 1930 as shown in Fig. 21, allowing the first structure 1930 to move. The first. structure 1930 is then deployed as depicted in Fig. 22.

In the specific example of Figs. 19-22, the retainer 1900 is used during assembly of the MEMS device and does not operate with the MEMS device after the assembly process has been completed. As such, in this example, the retainer 1900 temporarily governs the movement of the first structure 1930 during a portion of the assembly process. The retainer 1900 is subsequently repositioned as part of the assembly process to allow further assembly, and eventual operation, of the MEMS device. It is possible to reposition the retainer 1900 using the centrifuge.

Certain implementations and embodiments of the present invention allow significant advantages over conventional assembly techniques. Certain implementations and embodiments can allow significant time savings over conventional assembly processes. Moreover, certain implementations and embodiments can allow improved manufacturing yield. In addition, certain implementations and embodiments can allow improved reliability of assembled MEMS devices. Furthermore, certain implementations and embodiments can allow reduced costs otherwise associated with conventional assembly processes. Additionally, certain implementations and embodiments can minimize breakage, misalignments, and non-uniformities, and can improve deployment uniformity.

In the above example, certain implementations and embodiments of the present invention have been described with regard to the surface micro- machining of a particular MEMS device. The invention of the present application, however, is not limited to the above specific example. It is intended by the present inventors that various embodiments and implementations of the present invention be applied to a wide range of other MEMS devices. Also, application of the present invention to a wider range of micro devices is contemplated.

Moreover, other centrifuge derived forces, such as those generated by starting or stopping of the centrifuge may be employed to assist in, or to cause deployment of MEMS structures. In addition, it is not required that application of the centrifugal force be generated by a centrifuge. Any means capable imparting a centrifugal force to the MEMS structures may be utilized.

Therefore, while illustrative implementations and embodiments of the present invention have been described in detail above, many changes to these implementations and embodiments may be made without departing from the true scope of the present invention. The present invention, therefore, is limited only as claimed below and the equivalents thereof.