Microelectromechanical optical switch and method of manufacture thereof

Description of the invention

The present invention relates to a method of forming a mirror on a substrate and a method of making an optical switch on a substrate having an insulating layer covered.

Relatively modern technology now enables the production of microelectromechanical systems (MEMS) on semiconductor substrates, usually silicone substrates. These microelectromechanical systems typically have dimensions in the micrometer range and can be integrated into other electrical circuits on a common substrate. Microelectromechanical systems have numerous applications, such as optical switches, inertial or pressure sensors, and biomedical devices.

MEMS based optical switches are used in a variety of applications to transmit light waves between optical wave guides such as fibers. Current MEMS based optical switches can operate in the plane of the substrate or perpendicular to the substrate. An example of an in-plane optical switch using a vertical mirror is disclosed in C. Marxer et al. Vertical Mirrors Fabricated By Reactive Ion Etching For Fiber Optical Switching Applications, IEEE 1997, pages 49-54. The Marxer optical switch includes a metal coated silicon mirror attached to a double-comb actuator drive member. The dual-comb actuator members operate in opposite directions to push the mirror into the optical path between the optical fibers and to push the mirror out of the optical path. The Marxer optical switch is manufactured in a single step using inductively coupled plasma etching technology with sidewall passivation technology.

The Marxer switch has numerous limitations. For example, its double-comb actuator requires energy in both the extended and retracted positions. Without energy, the mirror is halfway between the fibers, which is undesirable. In addition, while the manufacturing technique of the Marxer switch produces walls with a verticality of 89.3 ° and a surface roughness of 36 nanometers (RMS), there is a need to improve each of these properties. Conventional DRIE and photolithography technologies relying on the use of an oxide mask and ultrasonic mask removal also have negative effects on the structures of the MEMS switches. For example, photolithography techniques usually leave a deposit between structures. Thus, it is desirable to develop a method for producing improved optical switches.

The method of forming a mirror on a substrate according to the invention is characterized in that a pattern mask layer over the substrate is formed over the substrate covering the first area of ​​the substrate and two side areas of the substrate, each adjacent a side of the first area, the uncoated portions of the substrate are then removed using the pattern mask layer to leave a first raised structure in the first area of ​​the substrate and two sacrificial raised structures on each side area of ​​the substrate adjacent to the first uplifted structure. thereafter selectively removing the sacrificial uplifted structures while leaving the first uplifted structure intact and forming a reflecting surface on the sidewall of the first uplifted structure.

Forming the patterned mask layer includes forming an opening between the first region and each side region that is 10 to 30 micrometers wide.

Removing the uncoated portions of the substrate to leave the first uplifted structure includes forming a gap having a width of 10 to 30 micrometers between the first uplifted structure and each of the sacrificial uplifted structures.

Removal of uncoated portions of the substrate involves forming a first raised structure with a surface roughness of 30 nm square average or less.

Etching the substrate involves leaving the sidewall of the first uplifted structure with a verticality of at least 90 ° ± 0.6 °.

Etching the substrate involves forming a groove at least 75 micrometers deep between the first elevated structure and each of the sacrificial elevated structures.

Forming the patterned mask layer involves depositing multiple layers of the same photoresist over the substrate and heating each deposited layer prior to depositing the next layer.

Forming the patterned mask layer includes forming openings in the patterned mask layer exposing areas of substrate surrounding each of the two side regions.

PL 200 407 B1

Preferably, a substrate is used having an insulating layer embedded in the substrate relative to the outer surface of the substrate, wherein removing uncoated portions of the substrate includes removing exposed areas of the substrate surrounding the two side regions to leave sacrificial structures piled on the insulating layer isolated from the first uplifted structure. and selectively removing sacrificial uplifted structures including removing an insulating layer from below sacrificial uplifted structures, thereby releasing the sacrificial uplifted structures from the substrate.

Selectively removing sacrificial uplifted structures includes removing the first uplifted structure within a groove defined by the substrate.

The inventive method for producing an optical switch on a substrate having an insulating layer covered is characterized in that a composite photoresist layer containing multiple layers of the same photoresist material is formed over the substrate, then parts of the photoresist layer are removed with acetone to form a patterned photoresist layer selectively masking areas of the substrate where the mirror wall will be formed, actuator combs, actuator rods, and sacrificial walls, and then etched exposed portions of the substrate into the insulating layer using a patterned masking layer to leave the mirror wall, actuator combs, actuator rods, and sacrificial walls that are positioned at least along the first and second the side wall of the mirror, then parts of the insulation layer below the mirror wall are removed, actuator rods, movable comb and sacrificial walls to free the mirror wall, actuator rods, movable comb and sacrificial walls from the substrate, the sacrificial walls are removed to free the mirror wall positioned in the groove, and a reflecting surface is formed on the side walls of the mirror wall .

The subject matter of the invention is shown in the embodiment in the drawing, in which Fig. 1 shows a top view of an exemplary optical switch shown in an extended position, Fig. 2 - a top view of an exemplary optical switch in a retracted position, Figs. according to the invention, fig. 4 - an exemplary top view of an optical switch during manufacture, fig. 5 - a cross-section of an exemplary mirror made according to the invention, fig. 6 - perspective view of actuator combs formed by the method of the invention, Figure 7 - perspective view of actuator combs made using an oxide mask, Figure 8 - top view of exemplary wave guides, and Figure 9 - perspective view of an exemplary switch package with cutout part.

The method of the invention relates generally to microelectromechanical optical switches using vertical mirrors, and is more particularly suited to MEMS-based optical switches containing vertical components such as mirrors and comb fingers.

Fig. 1 is a top view of an exemplary optical switch made according to the present invention. As will be discussed below, all the features of the optical switch 100 are mainly found in the upper substrate layer. Optical switch 100 includes a mirror 102 coupled to an actuator 104 capable of moving the mirror 102 between an extended position (e.g., Fig. 1) located between the optical wave guides 105 (shown as hatched) and a retracted position (e.g., Fig. 2) located outside the wave guides. In the exemplary embodiment, when the mirror 102 is in the extended position, the light waves are reflected from the mirror 102 to the junction between the wave guides 105A and 105B and 105C and 105D without being transmitted between the opposite wave guides 105A, 105D and 105B, 105C. When the mirror 102 is in the retracted position, switching takes place so that the light waves combine between the wave guides 105A and 105D and the wave guides 105B and 105C without being reflected from the mirror 102. As used herein, the term wave guide is intended to include any medium that transmits the light. including e.g. optical fibers.

The mirror 102 is typically housed in the groove 112. The groove 112 typically has a width sufficient to prevent the mirror 102 from coming into contact with the side walls of the groove 112 during use. Typical groove widths (from one side wall to the other) range from 40 to 50 microns for many applications. Mirror 102 typically includes a narrow wall 114 having a reflective coating on each side, mounted on an elongate base support 116 that connects the narrow wall 114 to the actuator 104. The mirror wall 114 may be about 2-5 micrometers thick or wide for many applications. This leaves an opening between the sides of the narrow wall and the sides of the groove, in many cases about 20 to 25 micrometers. The elongated base support 116 typically is wider than the wall 114 to provide stability to the mirror 102 in use. In this solution4

In addition, the optical switch 100 further comprises a support structure 118 mounted between a base support 116 for a mirror 102 and a base surface 120 of the actuator 104. The support structure 118 is, for example, a mesh structure whose lines extend obliquely with respect to the base surface 120 and base support 116 for mirror 102. The support structure 118 provides additional stability to the mirror 102 during movement between an extended position and a retracted position.

The mirror wall 114 typically includes smooth vertical sidewalls as compared to conventional upright mirrors. For example, the side walls of the mirror wall 114 typically have a surface roughness of 30 nm rms or less and a verticality of 90 ° ± 0.6 ° or greater (e.g., 90 ° ± 0.5 °, 90 ° ± 0.4 °, 90 ° ± 0.6 ° or greater). ° ± 0.3 ° or better). The technology of manufacturing the sidewalls with such properties will be discussed in detail below. It should be noted that the increased verticality and reduced surface roughness of the mirror wall 114 improves the transmission properties of the optical switch 100 compared to conventional optical switches.

The illustrated actuator 104 includes a drive mechanism 122 capable of applying a force that moves mirror 102 to the retracted position and a beam structure 124 that deflects when a force is applied and that returns the mirror to the extended position when no force is applied by drive mechanism 122. The beam structure 124 typically functions as a spring, to deflect under force between the combs and return to its original position in the absence of force. In the illustrated embodiment, the beam structure 124 does not accumulate energy when the mirror is in the extended position. In an exemplary embodiment, the drive mechanism 122 is a single comb drive that includes a stationary comb 108 interleaved with a movable comb 110 to provide a force driving the actuator 104 and thus the mirror 102 between its extended and retracted positions. The longitudinal displacement of the mirror 102 between its extended and retracted positions typically ranges from 40 to 70 microns or greater, and is about 55 microns in the illustrated embodiment.

Each of the fingers of the comb typically has a width in the range of 2 to 4 micrometers and in the example embodiment is about 3 micrometers wide. The two combs 108 and 110 are also tightly spaced. For example, the gap g between adjacent comb fingers typically ranges from 2 to 4 micrometers, and in the example embodiment is about 3 micrometers. In the illustrated embodiment, each of the individual fingers has vertical (at least 90 ° ± 0.6 ° verticality) and smooth (surface roughness of 30 nm rms or less) side walls. The smoothness of the fingers allows the interwoven combs to be tightly packed. This makes it possible to reduce the size of the structure for a given applied force and obtain smaller switches while maintaining or reducing switching speed. The length 1 of each finger, the overlap in the absence of force (shown in Fig. 1) between the two combs 108 and 110, and the number of fingers on each comb 108, 110 are typically selected according to the desired force developed between the two combs 108 and 110 as well as the desired distance travel of the mirror 102 between the extended and retracted positions. In the exemplary embodiment, the fingers have a length 1 ranging from 90 to 110 micrometers and the combs have an overlap of the order of 20 to 30 micrometers. The number of fingers on each comb 108, 110 may vary and may range from 120 to 160 for various applications.

The depicted beam structure 124 includes a double folded bar 126 on each side of the actuating member 104. Since the double folded bars 126 in the exemplary embodiment are symmetrical, only one will be described below. The double-folded wand 126 includes inner wands 128 attached at a first end to the predetermined structure of the substrate 130 and first and second outer wands 132 and 134. The first outer rod 132 connects one end to the ends of the second rods and the other end to the base surface of the actuating member. The second outer rod 134 connects one end to the other rods and the other end to the movable comb 108. Below the predetermined structure of the substrate 130, an insulating layer remains fired, securing the structure to the substrate. The rods 132 and 134 and the tip 136 are stripped of insulation, allowing it to move with the moving comb. In operation, the folded rods 126 function as a spring to deflect as the mirror 102 is moved to its retracted position and to return the mirror 102 to its extended position in the absence of force between the combs 108, 110. Although not shown to scale, the length of each stick 126 (measured from the axis aligned with the mirror 102 to the outer ends of the bars) can range from 700 to 1000 micrometers for many applications.

PL 200 407 B1

Preferably, one or more members of the beam structure 124 (e.g., inner rods 128, outer rods 132 and 134, and / or tip 136) have vertical side walls and smooth surfaces. For example, the verticality of the sidewalls may be 90 ° ± 0.6 ° or greater with a surface roughness of 30 nm rms or less. Techniques for forming vertical and smooth sidewalls of the bar will be discussed below. By increasing the verticality and reducing the roughness of the side walls, the strength of the beam structure 124 can be increased compared to conventional beam structures. This allows, for example, to increase the service life of the beam structure, the deflection distance of the beams and / or to reduce the size of the structure. In the illustrated embodiment, the improved features of the beams allow the formation of a relatively compact optical switch having a single comb drive actuator, relatively large mirror displacement, and low switching speeds.

Actuators having dual comb drives and actuators having single comb drives in an opposing configuration may also be used. For example, a single comb drive actuator may be made such that the comb drive applies a force to extend the mirror and the beam structure returns the mirror to the retracted position. The beam structure may also have various designs and is not limited to the double beam structure shown. For example, other types of structures may be used, such as various double beam structures or single beam structures.

Fig. 8 is a top view of an exemplary wave guide and wave guide channel system according to one particular embodiment of the method of the invention. This example relates to optical fiber wave guides.

In an exemplary embodiment, each optical fiber 810 includes an end 820 with a sidewall 830 that tapers into the lens 840. The bevelled sidewall 830 may preferably be aligned with one or more flanges 850 lanes 860 to facilitate alignment of the fibers 810 within. channels 860. The bevelled side walls 830 may further allow the lens 840 to be positioned at the end 820 of each optical fiber 810 closer to the mirror 870. With the bevelled side walls 830 and the lenses 840, the distance from each lens 840 to the mirror 870 can range from 10-30 micrometers and is about 20 micrometers in the exemplary embodiment. Lenses 840 may also focus transmitted light waves. As a result of the focused light and in close proximity to the mirror 870, the light transmission loss can be significantly reduced.

An exemplary method of making a beveled fiber with a lens includes heating the fibers to the melting point, drawing the fiber into the cone, and then cutting the pulled fiber to form bevelled ends. After slitting, the bevelled ends can be heated to thicken the tips and form converging lenses. The thickened ends can also be polished.

As shown in Figures 1 and 2, in operation, a potential difference is applied between the two combs 108 and 110, which creates a force that draws the two combs 108 and 110 together and retracts the mirror 102 from its extended position between the fibers to its retracted position. away from the fibers. The tightly packed and smooth comb fingers apply a force that switches the mirror between its extended and retracted positions within 0.2 to 1 millisecond. Advantageously, the features of the actuating member allow the mirror to be moved a relatively long distance with little deflection in the lateral direction. For example, both the mesh support structure and the curved beam structure serve to reduce the lateral deflection and resonance of the mirror. This serves to further increase the light wave transmission capacity of the optical switch.

Figures 3A-3F and 4 show an exemplary manufacturing method for the optical switch discussed above. The section shown in Figs. 3A-3E corresponds to that of a substrate used to form a vertical mirror wall, such as the narrow wall 114 discussed above.

In this exemplary method, a mask layer of approximately 303 is formed over substrate 301. Substrate 301 is typically formed of a semiconductor material such as silicon and includes a covered insulating layer 302 separating substrate 301 into an upper portion 304 and a lower portion 306. The covered insulation layer 302 may be formed from a semiconductor material such as silicon. for example be an oxide layer such as silicon dioxide. The thickness of the upper substrate 304 may, for example, be about 75 microns. Optical switch structures are formed in the upper portion 304 of the substrate above the insulating layer 302.

Mask layer 303 is used to protect a portion of the substrate during subsequent etching of the substrate. In the illustrated process, mask layer 303 is formed of a double layer.

PL 200 407 B1 of the same photoresist. The photoresist material may be S1818, for example. While it may be preferable to use a dual photoresist layer, the mask layer 303 may be formed of any suitable masking material, including oxide and photoresist material, using known technology. The resulting structure is shown in Fig. 3A.

The dual photoresist layer 303 typically comprises a first photoresist layer 305a formed above the substrate 301 and a second photoresist layer 305b formed from the same material as the first photoresist layer 305a formed over the first photoresist layer 305a. Each photoresist layer 305a, b is typically formed to its maximum thickness. The maximum thickness of a particular photoresist material is usually provided by the manufacturer of that photoresist material and corresponds to the maximum thickness of the photoresist material which provides a specific degree of surface flatness. For S1818, the thickness is approximately 2 micrometers.

Typically, the first photoresist layer 305a is deposited and heated before depositing and heating the second photoresist layer 305b. The use of a double layer with S1818 allows for an intricate reproduction of the relatively thick photoresist layer, and this in turn allows for deep etching of the underlying substrate to form fine grooves in the substrate. S1818 photoresist can also be removed in an advantageous manner. Further details and advantages of creating the double photoresist layer can be found in co-pending US Application No. 09 / 372,428.

Portions of the double photoresist layer 303 are removed to form a photoresist mask layer 309 as shown in Fig. 3b. Removal of a portion of the photoresist mask layer 303 may be accomplished using photoresist techniques. In particular, when using the S1818 photoresist material, it is possible for example to remove parts of the photoresist layer with acetone without the aid of ultrasounds. The exposed portions of the substrate 301 will be removed in the next production. The patterned mask layer 309 generally covers portions of substrate 301 that are to remain after substrate removal. The remaining parts of the substrate 301 typically form the components of the optical switch obtained (mirror walls, groove side walls, wave guide channels, actuator combs, spars, etc.).

As stated above, the section of Figs. 3A-3F shows the steps of manufacturing a mirror wall. In this case, the pattern mask layer 309 includes a first mask portion 311 covering a first region 311a of substrate 301 and two side mask portions 313 covering side regions 313a of the substrate adjacent each side of the first mask portion 311. The side walls 315 of the photoresist mask layer 309 are used to define the edge of a groove in which the mirror is formed. The mask portion 311 is provided to form a mirror wall in the first region 311a. The side mask portions 313 cover the side areas 313a in which sacrificial walls are formed.

The side mask portions 313 serve to limit the exposed portion of the substrate 301 during etching and to increase the verticality of the walls of the mirror structure in the first region 311a. The size of the gap between the mask portion 311 and each side mask portion 313 is selected to optimize the verticality of the resulting mirror structure in the first region 311a. Gap widths of 10 - 30 micrometers are suitable for many applications. A 20 micrometer gap performs particularly well in the removal process described below. A more detailed consideration of the benefits of using this type of sacrificial wall can be found in co-pending US Application No. 09,372,700.

Figure 4 is a top view of the optical switch after the mapping of the masking layer. The shaded area represents the patterned mask layer 402 and the open areas indicate the exposed portions of the underlying substrate 404. The patterned mask layer 402 includes a mask portion 406 that is provided to form a sacrificial wall around optical switch components such as a mirror wall and, for example, the outer bars. The areas of substrate underneath mask portion 406 are removed after etching of open areas of substrate 404 as discussed below. The use of sacrificial wall mask portion 406 facilitates vertical etching of adjacent structures, such as the mirror wall and the bars, as set forth below.

After the patterned mask layer 309 is in place, the exposed portions of the substrate 301 are removed as shown in Figure 3C. This removal process can be performed using deep reactive ion etch (DRIE). In the first solution it is used

PL 200 407 B1 standard BOSCH DRIE process. This process is typically a three-step process under the following conditions:

<td>Pressure</td><td>15 m track</td>

<td>Flow of He (Helium used in the BOSCH DRIE standard process for cooling)</td><td>7.45 (normal cubic centimeters per minute)</td>

3 3

In step 1, a C4F8200 flow (70 cm³/ min), SF6200 (0.5 cm³/ min) and argon (40 cm³/ min) for 4 seconds. In step 2, a C4F8200 flow (0.5 cm³/ min), SF6200 (50 cm³/ min) and argon (40 cm³/ min) for 3 seconds. In step 3, a C4F8200 flow (0.5 cm³/ min), SF6200 (100 cm³/ min) and argon (40 cm³/ min) for 5 seconds. In an alternate embodiment, the flow times for the first and second stages are increased (e.g., 5 seconds and 4 seconds, respectively) and the flow times for the third stage are reduced (e.g., to 3 seconds). This alternative solution advantageously provides a greater number of vertical sidewalls than the standard BOSCH DRIE process.

The stripping process typically uses an etchant that is selective for the covered insulating layer 302, thereby stopping the etching process on that layer. As a result of the structures of the side walls 321 and side mask portions 313, a raised structure 319 is formed underneath mask portion 311 with vertical side walls 320. In the illustrated embodiment, the sidewalls 320 typically have a verticality (with respect to the horizontal plane of the substrate) of 90 ° ± 0.6 ° or greater (90 ° ± 0.5 °, 90 ° ± 0.4 °, 90 ° ± 0, 3 °). This procedure also results in a raised structure 319 with relatively smooth side walls. For example, using this process, the surface roughness of the sidewalls may be 30 nm rms or less.

The photoresist material is removed as shown in Fig. 3D. This can be done using acetone as stated above. By using acetone without the assistance of ultrasound, the photoresist material can be removed without damaging brittle structures such as actuator combs, mirror and folded rods. The use of acetone in this way also allows for more efficient removal of contaminants from the substrate. By removing the photoresist, portions of the covered insulating layer 302 are removed. The insulating layer 302 is typically removed using a buffered insulating etchant (e.g., a 10: 1 solution of hydrochloric acid versus water). During this process, the etching agent removes the exposed portions of the insulating layer 302 as well as portions of the insulating layer 302 below the silicone structures formed above the insulating layer 302. It should be noted that below relatively narrow silicone structures (e.g., mirror walls, actuator rods, comb fingers, etc.), the underlying insulating layer 302 is sufficiently removed to separate these structures from the substrate 301. with the positioned supports 130 for the rods, the base portion 109 of the stationary comb 110), the insulating layer 302 remains intact, thereby securing these elements to the substrate 301. This allows the movement of structures such as the mirror, rods and the movable comb.

The stripping process is typically accomplished by dipping substrate 301 into etchant 322 as shown in Figure 3E. During this process, the insulating layer 302 beneath the sacrificial walls 321 is removed and the sacrificial walls 321 sink into the etching solution 322. This leaves a first uplifted structure 319 (mirror wall) supported by an insulating layer / substrate lying under another portion of the switch (e.g., restrained supports). bar 130). A raised structure 319 is formed between the two side walls of the groove 323 to define the mirror wall. The resulting structure is shown in Fig. 3F.

The use of a double photoresist layer of the same material in conjunction with the formation of sacrificial side wall masks enables the production of relatively deep, thin and vertical structures with smooth surfaces. These structures can be used, for example, as mirrors, the fingers of the actuator comb and / or the rods of the beam structure. Using such technology, verticality of the uplifted structures of at least 90 ° ± 0.6 ° can be achieved with a surface roughness of 30 nm rms or less.

It should be noted that during post-treatment, the mirror wall is usually coated with reflective metal to form a reflecting surface. As a result of the improved verticality and reduced surface roughness of the mirror wall, the reflecting surface has increased verticality and reduced roughness, which reduces scattering and improves the switch properties. In post-treatment, the metal is also deposited on the two combs to form electrodes for the combs. Such metal deposition may be performed using known technology. The plate is there

The PL 200 407 B1 is typically doped with boron prior to treatment to impart conductivity to the substrate and allow the application of a potential difference between the combs.

Fig. 5 shows an exemplary vertical structure formed by the method described above. The section may represent the section of a vertical element such as a mirror or a rod of a beam structure. The vertical structure 500 has sidewalls 502 having verticality (represented by the angle λ, between the horizontal plane 504 of the substrate and the plane 506 of the sidewall 502 (on the order of 90 ° ± 0.6 ° or greater) and a surface roughness of 30 nanometers averaged or less.

Figures 6 and 7 show the combs of an actuator formed using two different technologies. Fig. 6 shows combs formed using the double photoresist of S1818 and a removal process using acetone rather than ultrasound as discussed above. In contrast, Figure 7 illustrates the manufacture of similar items using a patterned oxide mask layer. As can be seen, the fingers of the comb formed using this process have reduced surface roughness and more defined features. The fingers of the combs of Fig. 6 also contain less debris between the fingers. Since debris can shorten the fingers of the actuator and degrade the performance of the device, this results in further improvements in product quality and device performance.

Fig. 9 shows a switch packet that includes a MEMS optical switch obtained by the method of the present invention. Exemplary package 900 includes a housing 910 containing a 2 x 2 optical switch 920. Switch 920 may, for example, be similar to the switch shown in Figures 1 and 2 above. From switch 920, four optical fibers 930 extend outwardly relative to housing 910. Fibers 930 may, for example, interconnect switch 930 with other network components. Although not shown, the fibers 930 may pass through channels formed in the substrate body. Wires 940 extend from switch 930 combs, typically leading to a power source. It should be noted that this package is given as an example and not as limiting the invention.