MEMS stress isolation mechanism and design method thereof
Technical Field
The invention belongs to the field of MEMS mechanical design, and particularly relates to an MEMS stress isolation mechanism and a design method thereof.
Background
During MEMS device processing or operation, MEMS devices are typically subjected to residual or thermal stress due to coefficient of expansion mismatches or temperature effects of various materials. At present, a common stress isolation mechanism is composed of a rigid outer frame and an external elastic beam, a protected MEMS device is connected with the rigid outer frame, and the rigid outer frame is connected with a substrate anchor point through the elastic beam. Stress on the base plate is absorbed by the elastic beam in the conduction process, and the rigid outer frame ensures that a protected device is not influenced by the stress, and the stress isolation mechanism is mainly designed to have two problems: 1. the elastic beam absorbs almost all the stress, so the rigidity of the elastic beam cannot be too great, thus limiting the mechanical bandwidth of the protected MEMS device; 2. the elastic beams symmetrically distributed outside the rigid outer frame hardly guarantee consistency in the processing process, and asymmetry causes center deviation of the whole outer frame, so that the positioning problem of a protected MEMS device is influenced, and adverse effects are generated on multilayer MEMS devices (such as sandwich type devices). In addition, it has been reported that the rigid outer frame is directly cantilevered without the elastic beam to achieve stress relief and isolation, which introduces cross-coupling effects and increases the process flow.
A ring or regular polygon structure is often used as a sensing element of the MEMS resonant gyroscope, and the degenerate inter-mode energy transfer due to the coriolis force coupling can be used as the angular velocity detection principle. And the free vibration mode of the ring or regular polygon has an N theta standing wave mode, comprises an antinode and a node, and is respectively used as a driving mode and a detection mode of the gyroscope. The invention utilizes the standing wave characteristic of the annular or regular polygon structure to connect the antinode with the substrate anchor point and connect the node with the protected MEMS device anchor point, thereby forming a simple stress isolation mechanism.
Disclosure of Invention
Aiming at the defects of the existing MEMS stress isolation mechanism, the invention provides the MEMS stress isolation mechanism and the design method thereof, and the MEMS stress isolation mechanism mainly comprises an annular or regular polygon structure and anchor points distributed at antinodes and nodes. The invention is based on the standing wave characteristic of the annular or regular polygon structure, connects the antinode with the substrate anchor point, and connects the node with the anchor point of the protected MEMS device, thereby realizing the stress isolation between the MEMS device substrate and the internal protected MEMS device, effectively reducing the stress influence caused by the MEMS processing process and the temperature effect, simultaneously being compatible with the protected MEMS device in the processing technology, and effectively controlling the manufacturing cost.
The technical scheme adopted by the invention is as follows:
one of the purposes of the invention is to provide an MEMS stress isolation mechanism, the MEMS comprises a substrate and an internal protected MEMS device, the stress isolation mechanism is composed of an annular structure or a regular polygon structure which is provided with anchor points, the anchor points are distributed at antinode points and wave nodes, the anchor points distributed at the antinode points are connected with the substrate anchor points, and the anchor points distributed at the wave node points are connected with the internal protected MEMS device anchor points.
Preferably, the number of sides of the regular polygon structure is not less than 8.
Preferably, the MEMS stress isolation mechanism and the internally protected MEMS device have synchronous and compatible processing technology.
As a preferable aspect of the present invention, the free vibration mode of the ring-shaped structure or regular polygon structure includes an N θ standing wave mode, N > 1; an antinode point and a node point which are distributed in central symmetry exist under each standing wave mode.
Preferably, the resonance frequency corresponding to the N θ standing wave mode is three to five times or more of the mechanical bandwidth of the internally protected MEMS device.
Preferably, the substrate anchors are symmetrically distributed on the anti-node points, and the protected MEMS device anchors are symmetrically or asymmetrically distributed on all or part of the anti-node points.
Another objective of the present invention is to provide a method for designing the MEMS stress isolation mechanism, which includes the following steps:
1) selecting a stress isolation mechanism as an annular structure or a regular polygon structure according to the structures and the shapes of the target substrate and the MEMS device protected inside;
2) determining the order of the N theta standing wave mode according to the number m of anchor points needing to be fixed on the target substrate, namely determining the value of N, so that N is m;
3) designing anchor points on the annular structure or the regular polygon structure according to the determined standing wave vibration mode, wherein the anchor points are distributed on anti-node points and node points which are distributed on the annular structure or the regular polygon structure at intervals; for the N theta standing wave vibration mode, two adjacent antinode points and node points are separated by 90 degrees/N;
4) connecting the substrate anchor points with a group of anti-node points on the annular structure or the regular polygon structure, so that the substrate anchor points are symmetrically distributed on the anti-node points; and connecting the internally protected MEMS device anchor points with the wave nodes on the annular structure or the regular polygon structure, so that the protected MEMS device anchor points are distributed on all or part of the wave nodes symmetrically or asymmetrically.
Further, the resonance frequency corresponding to the N theta standing wave mode determined in the step 2) is three to five times more than the self mechanical bandwidth of the internally protected MEMS device.
Generally, compared with the prior art, the technical scheme of the invention has the following beneficial effects:
(1) according to the invention, by utilizing the standing wave characteristic of the annular or regular polygon structure, under the action of stress, the antinode point of the structure is deformed, and the position of the node is kept unchanged, so that a protected MEMS device connected to the node is not influenced by the stress, the whole structure is simple, and the isolation effect is obvious;
(2) the stress isolation mechanism comprises various structural designs, can be flexibly adapted to various MEMS devices, and can be in a ring shape, a regular octagon shape, a regular hexadecapegon shape and the like structurally, substrate anchor points can be symmetrically distributed on antinode points of an N theta vibration mode, and connection points of protected devices can be flexibly distributed on wave nodes;
(3) the stress isolation mechanism is compatible with the processing technology of the protected MEMS device, can be synchronously processed, can enable the mechanical bandwidth of the protected MEMS device not to be influenced by adjusting the size of the structure, and effectively reduces the coupling effect brought by the stress isolation mechanism.
Drawings
FIG. 1 is a schematic diagram of two annular stress isolation mechanisms according to an embodiment of the present invention, wherein (a) corresponds to a 2 θ mode and (b) corresponds to a 4 θ mode;
FIG. 2 is a schematic diagram of two regular octagonal stress isolation mechanisms according to an embodiment of the present invention, wherein (a) corresponds to a 2 θ mode and (b) corresponds to a 4 θ mode;
FIG. 3 is a graph of the standing wave mode shape of an annular stress isolation mechanism according to an embodiment of the present invention;
FIG. 4 is a graph of the standing wave mode shape of the ring structure in an embodiment of the present invention;
FIG. 5 is a graph of the standing wave mode shape of a regular octagonal structure in an embodiment of the invention;
in all the figures, the anchor area represents the area of attachment to the substrate anchor point, and the protected MEMS device is equivalently represented as a system of mass and springs.
Detailed Description
In order to more clearly express the objects, technical solutions and advantages of the present invention, the following derivation is further explained with reference to the drawings and formulas. It is to be understood that the principles herein are to be interpreted as illustrative, and not in a limiting sense.
The invention utilizes the standing wave characteristic of the annular or regular polygon structure to connect the antinode with the substrate anchor point and connect the node with the protected MEMS device anchor point, provides an MEMS stress isolation mechanism with simple structure and good stress isolation effect and a design method thereof, and can be applied to the mechanical design of various MEMS devices.
The MEMS comprises a substrate and an MEMS device with protected interior, the stress isolation mechanism is formed by an annular structure or a regular polygon structure provided with anchor points, and the anchor points are distributed at anti-node points and wave nodes;
the annular structure or the regular polygon structure is deformed under the action of stress in the processing or working process, the deformation mainly occurs at an antinode point, and a node keeps unchanged in position, so that the position of the antinode point of the annular or the regular polygon in the stress isolation mechanism is connected with a substrate anchor point, and the position of the node point of the annular or the regular polygon is connected with an internal protected MEMS device anchor point, so that the stress cannot be conducted to a protected MEMS device, and the stress isolation effect is achieved.
As shown in fig. 1 and fig. 2, the MEMS stress isolation mechanism provided by the present invention includes a ring or regular polygon, and the number of sides of the regular polygon is generally not less than 8. Wherein figures 1 and 2 show a circular and regular octagonal configuration, respectively. The annular or regular octagonal group of antinodes is connected with the substrate anchor points to form anchor areas, the number and the positions of the anchor areas determine the standing wave vibration mode, the nodes are connected with the protected MEMS device anchor points, and a spring-mass system is adopted in the figure to represent the protected MEMS device.
In one embodiment of the invention, the ring-shaped structure and the regular polygon structure have the same thickness, and the axial ring width thereof can be uniform or non-uniform according to the crystal orientation of the silicon wafer.
The free vibration mode of the ring or regular polygon in the stress isolation mechanism comprises an N theta standing wave vibration mode (N >1), an anti-node and a node exist under each vibration mode, and the anti-node and the node are symmetrically distributed, so that the anti-node can be freely deformed in the processing or working process, and the position of the node is always kept unchanged. The ring structure in fig. 1 and 2 has two anchoring modes corresponding to different standing wave vibration modes: FIGS. 1(a) and 2(a) correspond to a 2 θ mode, with nodes 45 ° from antinodes; fig. 1(b) and 2(b) correspond to a 4 θ mode, with the node 22.5 ° from the anti-node.
In one embodiment of the present invention, the stress isolation mechanism is a ring structure (mode shape is a 2 θ standing wave mode shape). Fig. 3 is a stress isolation mechanism with a 2 θ standing wave mode according to the present invention, and it can be seen that the node connected to the anchor point of the MEMS device to be protected is not affected by the tensile stress or the compressive stress of the substrate.
In one embodiment of the present invention, the distribution of anchor points of the ring structure in the stress isolation mechanism can be designed in various ways, and the design range covers all standing wave vibration modes of the ring structure. FIG. 4 shows the simulation results of the 2 theta-8 theta standing wave mode of the ring structure provided by the present invention, wherein the N theta standing wave mode corresponds to the deformation caused by the residual stress or thermal stress. In the result graph, white circles indicate a ring structure to which no stress is applied, and dark patterns indicate simulation cases of different standing wave patterns, and it is seen that the positions of nodes do not change at all times in all standing wave patterns. Connecting an antinode corresponding to each standing wave vibration mode with a substrate anchor point for realizing stress release and ensuring free deformation, and enabling the anchor points on the substrate to be symmetrically distributed on the antinode, so as to ensure uniform annular stress and no influence on the node due to deformation; and the anchor point of the protected MEMS device can be selectively connected with the wave node without being connected completely, so that the protected MEMS device is not influenced by stress deformation.
In one embodiment of the present invention, the regular polygon in the stress isolation mechanism may be any regular polygon, such as regular octagon, regular hexadecimal, etc., and the anchor point design also covers all standing wave vibration modes. Fig. 5 shows the results of simulation of several typical standing wave modes (2 θ, 4 θ, and 8 θ) of the regular octagonal structure, and as a result, in the graph, white regular polygons represent regular polygonal structures to which no stress is applied, and dark mode represents simulation of different standing wave modes, and similarly, the positions of nodes do not change at all times in all standing wave modes. The antinode points corresponding to each standing wave vibration mode are connected with the substrate anchor points, and the anchor points on the substrate are symmetrically distributed on the antinode points; while the anchor point of the protected MEMS device may be selectively connected to the wave node, and need not be connected in its entirety.
The annular structure or the regular polygon structure and the MEMS device protected inside have synchronous and compatible processing technology, synchronous processing can be achieved, meanwhile, the mechanical bandwidth of the protected MEMS device can be unaffected by adjusting the size of the structure, and the coupling effect brought by the stress isolation mechanism is effectively reduced.
The design method of the stress isolation mechanism comprises the following steps:
1) selecting a stress isolation mechanism as an annular structure or a regular polygon structure according to the structures and the shapes of the target substrate and the MEMS device protected inside;
2) determining the order of the N theta standing wave mode according to the number m of anchor areas needing to be fixed on the target substrate, namely determining the value of N, so that N is m;
3) designing anchor points on the annular structure or the regular polygon structure according to the determined standing wave vibration mode, wherein the anchor points are distributed on anti-node points and node points which are distributed on the annular structure or the regular polygon structure at intervals; for the N theta standing wave vibration mode, two adjacent antinode points and node points are separated by 90 degrees/N;
4) connecting the substrate anchor area with a group of anti-node points on the annular structure or the regular polygon structure, so that the substrate anchor area is symmetrically distributed on the anti-node points, thereby ensuring that the annular or regular polygon is uniformly stressed and the deformation of the annular or regular polygon does not influence the node; and connecting the internally protected MEMS device anchor region with the wave nodes on the annular structure or the regular polygon structure, so that the protected MEMS device anchor region is distributed on all or part of the wave nodes symmetrically or asymmetrically, and the specific layout depends on the mechanical design requirement of the protected MEMS device.
In one specific implementation of the invention, the resonance frequency corresponding to the N θ standing wave mode depends on the size of the dimensional parameter of the annular or regular polygon structure, and in order not to affect the mechanical bandwidth of the internally protected MEMS device, the resonance frequency corresponding to the N θ standing wave mode determined in step 2) is three to five times or more of the self mechanical bandwidth of the internally protected MEMS device.
It should be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.