A novel 3-D MEMS structure design concept is proposed to implement vertically standing micromirrors using SOI and SU-8. The operation principle and design are discussed in this chapter. The finite-element simulations of the structures using ANSYS are also presented. Finally, the micromachined mirrors without etch holes are investigated to improve the optical quality.
2-1 Introduction
Structures with large vertical dimensions can be implanted by integrating the hinge elements and hinged microstructures. However, there are some problems with conventional hinge design. As shown in Figure 2-1, the microfabricated hinge pin has a rectangular rather than round cross section and the mask alignment restrictions result in hinge play [21]. The resulting spacing allows the hinge pin to move horizontally and vertically. Although the play in general is not detrimental to the functionality of the hinge, the angle accuracy of the assembled structures will be greatly affected due to this play. For some applications, the angle positioning error result form this play is acceptable. But there are applications that require much greater accuracy, especially in optics. Therefore, methods of improving the accuracy of the assembled components are necessary.
In addition to the hinge design, the latch mechanism is also very important for 3-D structure assembly. In general, mechanical locks are used to control the rotation of the hinged plate. Figure 2-2 illustrates a basic mechanical lock design that is commonly used in micro-optics [8]. This mechanical locking mechanism consists of two hinged side plates with V-shaped openings, followed by a long, narrow groove.
The assembly process of this design is demonstrated in Figure 2-3. After the hinged plate is rotated out of plane by a microprobe and becomes upright to the substrate, the side plates are folded onto the plate. Therefore, the upright plate is firmly locked into the groove. The disadvantage of this structure is that it involves too many operations to assemble the structure by using microprobes. There are totally three operations.
First of all, the microprobe is used to lift the hinged structure to the upright position.
Secondly, one of the two side plates is folded and engaged one side of the plate by a microprobe. Finally, a microprobe is used again to fold the other one onto the plate.
This is a time consuming process and the probability of damaging the assembled structure is high. Another problem of this structure is that the device is held in place by friction forces between the hinged plate and the side plates. This leads to an uncertain position when the system undergoes a shock. The side plates may be
Play
Hinge pin Play
Hinge pin
Figure 2-1 (a) Cross section view of the conventional hinge, (b) SEM image exhibits a large amount of play in the hinge [21].
(a) (b)
displaced away from the hinged plate, allowing the hinged plate to collapse from its vertical position and thereby destroy the 3-D structure.
Side latch plates
V-shaped opening Hinged plate
Figure 2-2 Side latch plates with V-shaped lock slots to fix the hinged plate [8].
Figure 2-3 Multiple assembly processes are applied to complete the 3-D structure.
The final assembled structure is fixed in position by friction force.
Friction force
In traditional microprobe-assembled devices discussed above, the probes must be manipulated in different directions with precise and complex motion control.
Sometimes they must be inserted into the gaps formed by the release of sacrificial layers or aligned with the thin flat structures on the surface before the assembly process. To eliminate the need for precise probe control, a novel assembly process and micro structures are proposed. In this approach, all probe motion is reduced to a simple one-push operation. Therefore, the probes only need to be aligned with a relatively large push pad with large alignment tolerance in the lateral directions. Since the probes move vertically in the push operation, the vertical alignment is not critical.
Furthermore, the structures can be over pushed in the assembly process and thus the motion control is much eased compared to traditional techniques. In addition to the simplified assembly process, a novel V shaped hinge is used to eliminate the play in traditional hinge designs in the proposed devices. Therefore, the positioning accuracy can be improved.
2-2 Integration of SOI wafer and SU-8 structural layer
The technology of combining SOI wafer with SU-8 negative photoresist was proposed to fabricate the microoptical devices [22]. The optical frame structures are made of the SCS layer of the SOI wafer. The reasons to use SOI wafers instead of conventional silicon wafers are:
1. They do not have the stress problems in polysilicon micromaching, 2. They have better surface smoothness,
3. A broad range of made-to-order SCS layer thicknesses are available.
The SU-8 photoresist is used as another structural layer. Traditional surface micromachining usually uses thin polysilicon as the structural material. The reasons
for replacing it by SU-8 are:
1. Polysilicon deposition is a high temperature process. If circuits are integrated into the same substrate, the high temperature condition will affect the performance.
The SU-8 process is a spin coating, low temperature process, and thereby suitable for circuit integration.
2. SU-8 can be used to create structures as thick as 2 mm and with aspect ratios up to 25 with standard UV-lithography [23].
3. SU8 has good mechanical property and chemical resistance. These features are highly favorable for mechanical microstructures. Furthermore, SU-8 is a soft material compared to other commonly used materials in MEMS. The Young’s modulus of SU-8 is in the range from 2 to 7.5 GPa [24], which is about 40 times lower than the Young’s modulus of silicon [25].
2-3 Device design
The proposed device to verify the concept is a vertically standing mirror. The device must be assembled more easily, positioned more accurately, maintained more robustly, and have excellent optical properties.
2-3-1 Assembly process
The layout view and the 3-D solid model of the proposed device are shown in Figure 2-4. It consists of a mirror plate, V-shaped hinges and flexible side support latches. The push pad is the place where the microprobe pushes. The backside cavity is used as a room for the assembly procedure. The mirror plate is made of single
The assembly procedure of the device is shown in Figure 2-5. A microprobe station is utilized as the assembly equipment. The microprobe is first moved to align with the push pad (Figure 2-5 (a)). When the push pad is pushed down by the microprobe, the mirror plate rotates about the pin axis. Once the mirror plate moves out of the plane, the wing of the mirror plate contacts the bottom of the side latch.
This upward force will simultaneously drive the flexible side latches to rotate out of the plane (Figure 2-5 (b)). As the mirror plate is rotated to the upright position, it will slide into the V-shaped slot of the flexible side latches (Figure 2-5 (c)). The V-shaped lock slot and the downward restoring force of the spring will firmly lock the mirror plate in place. At this time, the microprobe is moved away, and the whole assembly process is complete (Figure 2-5 (d)). The complete 3-D microstructure is assembled in just one push operation.
Figure 2-4 (a) Layout of the proposed micromirror device, (b) 3-D solid model of the micromirror device.
Back-side cavity region Mirror plate
V-shaped hinges Flexible side latches
Back-side cavity Push pad
(a) (b)
Wing
2-3-2 Locking mechanism
In addition to the simplified one-push assembly process, the proposed device also employs a novel V-shaped hinge for better angle positioning compared to traditional micromachined hinges. Besides, flexible side latch is used to reduce the assembly process.
As discussed above, the hinge play of the conventional microhinge design results in angular deviation of the assembled structure. The novel V-shaped hinge element is proposed to address this issue. The layout view and 3-D solid model of this element
Figure 2-5 Assembly process with only one push operation.
(a) (b)
(c) (d)
Side latch move out of the plane
Standing mirror V-shaped slot
Probe
(single crystal silicon) layer, and the sacrificial layer in the SOI/SU-8 process. The assembly process of the V-shaped hinge is demonstrated in Figure 2-7. When the mirror plate is in the horizontal position (Figure 2-7 (a)), the wide hinge pin and the V-shaped hinge are not in contact. The mirror plate can rotate freely without bending the V-shaped hinge until the hinge pin touches the bottom of the V-shaped hinge (Figure 2-7 (b)). When the mirror plate is rotated further, the hinge pin causes the V-shaped hinge to bend. The hinge pin will be locked between the two sides of the V-shaped hinge when the mirror plate is in the upright position (Figure 2-7 (c)).
(a) (b) (c)
Wide hinge pin
Contact
Bent V-shaped hinge
Figure 2-7 3-D view of the V-shaped hinge during assembly process.
Figure 2-6 (a) Layout of the V-shaped hinge, (b) 3-D solid model.
(b) (a)
V-shaped hinge
V-shaped hinge Traditional hinge Wide hinge pin
Figure 2-8 shows the positioning principle of the V-shaped hinge. As shown in Figure 2-8 (b), the vertical play can be eliminated due to the downward restoring force of the flexible V-shaped hinge. This downward force locks the hinge pin in place. Furthermore, the hinge pin will be locked between the two sides of the V-shaped hinge when the mirror is in the upright position, as shown in Figure 2-8 (c).
Hence the x-direction play and y-direction play can also be eliminated.
Figure 2-8 (a) 3-D solid model of the V-shaped hinge after assembly, (b) cross-section view of the restricted z-direction movement, (c) top view of the positioning mechanism.
(a)
(b) (c) Bent V-shaped hinge
Restoring force of V-shaped hinge Bent V-shaped hinge
Hinge pin Hinge pin locked between the two sides of the V-shaped hinge
Flexible side latch is another feature in the proposed device. The conventional side latch is susceptible to shock. In order to address this problem, the side latch is designed to exert a force on the hinged plate by a flexure mechanism. The conventional side plate is attached to the substrate by staple hinges. It can freely rotate about the pin axis. The locking mechanism is by the friction force. If the side plate is attached to the substrate by spring elements, it will be able to create a force due to the restoring force of the spring elements. This concept is illustrated in Figure 2-9.
The operation of this side latch design is shown in Figure 2-10. Before the mirror plate is lifted, the wing plate and the side latch are not in contact (Figure 2-10 (a)). As the mirror plate is elevated, the wing plate touches the bottom of the side latch and causes it to rotate out of plane about the spring axis (Figure 2-10 (b)). The wing plate continuously drives the side latch to rotate until the mirror plate slides into the V-shaped opening. Once the mirror plate is rotated further to the upright position, it will slide into the lock slot in the side latch and lock firmly in place (Figure 2-10 (c)).
The side plates now provide force upon the mirror plate. This will hold the mirror plate in position more robustly and enhance the shock resistance.
Figure 2-9 (a) Layout of the side plate structure, (b) 3-D solid model.
(a)
2-4 V-shaped hinge
J. E. Sader [26] proposed an analytical model for determining the spring constant of the V-shaped spring structure. Figure 2-11 illustrates the geometry of the V-shaped spring whose spring constant is:
,
where k and k△L are the spring constants when the load is applied to the end of tip and when it is applied a finite distance ∆L from the end, respectively, E is Young’s modulus of the spring material, t is the thickness of the structure, and d, b, L,θare the geometric parameters as shown in Figure 2-11.
Figure 2-10 Flexible side latch assembly process
(a) (b) (c) The mirror is
at rest
The mirror is
lifting The mirror is
upright
Figure 2-12 shows the bending of the V-shaped spring before and after assembly.
The SCS layer of the SOI wafer is 5 μm thick. The two sacrificial layers are 2-μm-thick BOX and 3-μm-thick PECVD oxide. The SU-8 spring layer is 13 μm thick. The hinge pin is 11 μm wide. The structure of the V-shaped hinge before release is shown in Figure 2-12 (a). The V-shaped hinge will bend upward by 1 μm after assembly, as shown in Figure 2-12 (b). Hence, the V-shaped hinge must be designed to bend by 1 μm without breaking.
According to Equation 2-1, the spring constant can be determined from the required bending of 1 μm at the tip. In this thesis, the V-shaped hinge is designed in the space shown in Figure 2-13, determined by the dimension of the mirror plate.
Since this is a preliminary feasibility study of the V-shaped hinge mechanism, further optimization of the spring design is in progress. The geometric parameters are listed in the Table 2-1. The spring constant calculated by substituting these values into Equation 2-1 is 23.6 F/m. By Hooke's law, the downward restoring force due to the 1 μm bending is then 23.6 μN. ANSYS is used to simulate the stress distribution in the structure. The requirement is to have the stress well below the yield strength of the SU-8. Figure 2-14 shows the ANSYS simulation results. The x-component stress,
1μm
(a) (b) Figure 2-12 Cross-section view of the V-shaped hinge, (a) before release oxide, (b)
after release and assembly.
3 μm5 μm
When mirror plate is upright
y-component stress, z-component stress, and Von stress are 6.8 MPa, 1.9 MPa, 1.92 MPa and 6.31 MPa, respectively. All stresses are below the yield strength of SU-8, which is about 50 MPa. The simulated value of the spring constant is 22.55 F/m and the simulated force to bend the tip by 1 μm is 22.55 μN.
Parameter Dimension Unit
Young’s modulus E 4.02 GPa
Poisson’s ratio 0.22
Yield strength 50-70 MPa
Geometry of V-shaped hinge Dimension Unit
Thickness t 13 μm
Skewed rectangular width d 10 μm
V-shaped spring width b 88 μm
Table 2-1 SU-8 material properties and spring geometric parameters Figure 2-13 Design space for the V-shaped element.
125 μm
135 μm
The comparison of the analytical and simulated spring constants is listed in Table 2-2. The relative error is 4.5﹪. The difference of these two structures is that the proposed V-shaped hinge is not exactly the V-shaped structure in [26], as shown in Table 2-2. There is also a 2﹪difference between the analytical equation and original FEM simulation [26]. Therefore, Equation 2-1 can still be used as a guideline for the V-shaped hinge design.
Proposed V-shaped hinge V-shaped spring [26]
∆L1=24 μm, ∆L2=10 μm, La=130 μm, W =7μm d =10 μm, b =88 μm, t =13 μm, θ =15.8° ,
L=154 μm, ∆L=34 μm
d=10 μm, b =88 μm, t =13 μm, θ=15.8°
simulated value using ANSYS:
→ spring constant = 22.55 F/m
analytical value using Equation 2-1:
→ spring constant = 23.6 F/m
Table 2-2 Comparison of two V-shaped structures
2-5 Flexible side latch
The side plates are attached to substrate by spring elements. Four types of springs are used in this thesis, namely the serpentine-type, box-type, meander-type, and skewed box-type, as shown in Figure 2-15. The characteristics of each spring design during in assembly process will be discussed in Chapter 4.
The box type spring is used as an example and shown in Figure 2-16. The left part of the spring can be viewed as two springs with length L2 and L4 connected with a rigid truss. If the required twist angle is 10°, we can design beam L4 and beam L2 to
Figure 2-14 Simulations of the V-shaped spring (a) x-component of stress, (b) y-component of stress, (c) z-component of stress, (d) Von stress.
(a) (b)
(c) (d)
,
where θ is the twist angle, Γ is the twist moment, L is the beam length, G is the shear modulus, a is half of the side of the square cross section and K is equal to 2.25a4.
In this thesis, the thickness of SU-8 is 13 μm. Therefore the width of each spring element is also 13 μm and a is equal to 6.5 μm. The maximum shear stress of SU-8 is about 7 MPa [28]. Thus the maximum shear stress τmax in the structure must be smaller than this value and 6 MPa was adopted in the design. By substituting these values into Equation 2-2, K and Γ were calculated as 4×10-21 m4 and 2.7×10-9 N·m.
From , L4 and L2 can be calculated as 167 μm and 272 μm, respectively.
The short truss is assumed as a rigid structure, therefore its torsion and bending are neglected. The four types of springs were designed according to the above concepts.
The geometry parameters of the four spring types is also listed in Figure 2-15.
Several locking height are also designed. As shown in Figure 2-17, locking height is the vertical distance between the lock point and the substrate. Obviously, a larger locking height means a more robust structure after assembly. The locking heights include 38 μm, 76 μm, 114 μm, 152 μm, which are 5%, 10%, 15%, and 20%, respectively, of the mirror plate height 760 μm.
max 3
L1=39 μm L2=196 μm
Figure 2-15 Four types of springs, (a) serpentine spring, (b) box spring, (c) meander spring, (d) skewed box spring.
L2
2-6 Micromachined mirrors without etch holes
In order to free large microstructure, etch holes are inevitable in the surface micromachining. However, etch holes cause two major effects on the optical properties of micromachined mirrors, namely, diffraction and reduction of reflectivity [29]. These are undesirable in free-space optical systems. The etching holes can be eliminated if backside etch is employed, as shown in Figure 2-18. The big hole in the backside of the substrate was used for HF release without etch holes in the micromirror surface. However, the large open hole in the backside will cause the silicon device layer and buried oxide to break due to the residual stresses [30, 31].
Hence, another approach for backside etching was used to increase the yield of fabrication. As shown in Figure 2-19, the backside pattern was changed to an array of rectangles. The goal of this design is to make the exposed area of oxide after DRIE as small as possible, and therefore reducing the effect of the residual stresses. After the backside etch, the oxide layer could be etched by HF. The array of silicon pillars would drop during the release process. Hence the final pattern after release is identical to Figure 2-18 (b).
Locking height
Side support plate
Standing mirror plate
Hinge
Figure 2-17 Cross-section view of the side latch.
2-7 Summary
The V-shaped hinge is used to address the problems of play. Flexible side latch can provide a force on the hinged structure for better robustness. One-push operation reduces the assembly time and complexity. The mirror without etch holes is used to improve the optical characteristics. In the next Chapter, the fabrication process will be
Micromirror without etch holes Backside hole for HF flowing
Figure 2-18 Backside pattern with a large open hole.
Large open hole
Silicon pillars
Figure 2-19 Backside pattern with a pillar array.
(a) (b) (a) (b)