Abstract
A forward looking MEMS based OCT probe (50) is provided that comprises an elongate probe housing (51) having at a first end a probe interface (54) for an optic fibre (56), and at a second opposite end a viewing window (58). The probe housing accommodates a MEMS mirror (10) for sweeping a hght beam (60) through the viewing window and for reflecting light received through the viewing window towards the probe interface, wherein a rotation axis (18) of the MEMS mirror extends transverse to a longitudinal axis (62) defined by the probe housing. The MEMS mirror (10) has a stator (12), a rotor (14), and an actuator (16) with at least one pair of mutually interdigitated comb elements including at least a first comb element fixed to the stator defining a reference plane and at least a second comb element fixed to the rotor and that is further coupled at mutually opposite sides via a respective torsion beam (20A, 20B) to the stator. The rotor has a rotor body (14RB) and a rotor support (14RS), fixed at a first face of the rotor body, that keeps the rotor body at a distance from the stator within said rotation range, the rotor body having a mirror surface (14MS) at a second face opposite the first face, the MEMS mirror comprising the stator (12) and the rotor support (14RS) at mutually opposite sides of the reference plane (RP).
Claims
1. A MEMS mirror having a stator, a rotor, and an actuator with at least one pair of mutually interdigitated comb elements including at least a first comb element fixed to the stator defining a reference plane and at least a second comb element fixed to the rotor, the rotor being coupled at mutually opposite sides via a respective torsion beam to the stator, wherein the respective torsion beams extend along a rotation axis in the reference plane to allow the actuator to rotate the rotor within a rotation range relative to the stator along the rotation axis, the rotor having a rotor body and a rotor support, fixed at a first face of the rotor body, that keeps the rotor body at a distance from the stator within said rotation range, the rotor body having a mirror surface at a second face opposite the first face, the MEMS mirror comprising the stator and the rotor support at mutually opposite sides of the reference plane.
2. The MEMS-mirror according to claim 1, wherein at least one of the following applies: (1) a size of the mirror surface in a direction transverse to the rotation axis is at least 90% of a size of the stator in said transverse direction, or (2) a distance between mutually opposed edges of the mirror surface in that direction is at least equal to a distance between mutually opposed edges of the stator in that direction minus 200 micrometer.
3. The MEMS-mirror according to claim 2, wherein, the rotor body has peripheral portions that extend beyond the stator in transverse direction transverse to the rotation axis and aligned with said reference plane.
4. The MEMS mirror according to claim 3, wherein a respective end of each peripheral portion has a respective extension portion at distance along a respective side face of the stator.
5. The MEMS mirror according to claim 2, wherein the actuator comprises at least one further pair of mutually interdigitated comb elements including at least a third comb element fixed to the stator and a fourth comb element fixed to a peripheral portion of the rotor body.
6. The MEMS mirror according to claim 1, wherein the torsion beams have a T-shaped cross-section in a plane transverse to the rotation axis.
7. The MEMS mirror according to claim 1, wherein the stator further comprises through silicon vias that electrically connect respective comb-elements of the actuator to respective electric contacts at a surface of the stator facing away from the mirror surface.
8. The MEMS mirror according to claim 7, wherein the size of the mirror surface in a direction of the rotation axis is at least 90% of the size of the stator in said direction.
9. A forward looking MEMS based optical coherence tomography (OCT) probe, comprising an elongate probe having at a first end a probe interface for an optic fibre, and at a second opposite end a viewing window, the probe housing accommodating the MEMS mirror of claim 1 for sweeping a light beam through the viewing window and for reflecting light received through the viewing window towards the probe interface, wherein the rotation axis of the MEMS mirror extends transverse to a longitudinal axis of said probe housing extending from said first side to said second side.
10. The OCT probe according to claim 9, wherein the housing further accommodates a driver having an output to provide a drive signal to said actuator.
11. The OCT probe according to claim 10, wherein the driver is further provided at least with a feedback input to receive a feedback signal indicative for a rotational state of the rotor, the driver being configured to provide the drive signal at its output based on said feedback signal.
12. The OCT probe according to claim 11, wherein the feedback signal is a zero-crossing (synchronisation, or trigger) signal.
13. The OCT probe according to claim 10, wherein the comb elements include a first and a second, mutually insulated, comb layer of an electrically conducting material, wherein the first comb layer faces the mirror surface, and the second comb layer faces away from the mirror surface, wherein the first and the second comb layer respectively form a first and a second electric pole in each of the comb elements wherein the first electric poles of each of the comb elements of the rotor are electrically interconnected with each other, and are electrically connected to the output of the driver, and wherein at least one of a first and a second electric pole of each of left and right comb elements of the stator are coupled to a respective feedback input of the driver, wherein the left and right comb elements of the stator are arranged at mutually opposite sides of the rotation axis.
14. The OCT probe according to claim 9, having an operational state wherein the MEMS mirror operates in a resonance mode with a resonance frequency between 200 Hz and 4000 Hz.
15. The OCT-probe according to claim 9, wherein the probe comprises in its longitudinal direction a SMF (single mode fibre), a spacer, a GRIN lens bound by tilted faces with respect to said longitudinal direction, a prism, the MEMS mirror, the viewing window.
16. The OCT-probe according to claim 9, wherein the probe housing accommodates a carrier part having a main portion extending in a direction substantially coinciding with a longitudinal direction of the housing and an end portion facing the viewing window, wherein the main portion carries the driver and wherein the end portion carries the MEMS mirror, wherein the carrier at a side carrying the MEMS mirror is tilted with respect to said longitudinal direction.
17. The OCT-probe according to claim 16, provided with the MEMS-mirror, wherein the end portion of the carrier is provided with recesses facing the peripheral portions of the rotor body, or wherein the stator of the MEMS-mirror is arranged on a pedestal on the end portion of the carrier.
18. A forward looking MEMS based OCT system, comprising the OCT probe as defined in claim 9, and further including a coherent light source configured to generate a coherent light beam; a beam splitter/merger to split the coherent light beam into a reference beam and a sense beam; a reference unit to receive and reflect the reference beam; and wherein the probe interface of the OCT probe is to receive the sense beam and the beam splitter/merger is to receive the light reflected from the sample received through the viewing window and reflected by the resonant mirror, to receive the reflected reference beam and to merge the beams received from the reference unit and the probe; a detector to generate a detection signal indicative for the merged beam; a processing unit to process the detection signal.
19. A method of manufacturing a MEMS mirror comprising a stator and a rotor coupled at mutually opposite sides to the stator via a respective torsion beam, the stator having a stator body and at least one stator comb and the rotor having a rotor body and at least one rotor comb, the at least one rotor comb being interdigitated with the at least one stator comb to form an actuator, the at least one stator comb defining a reference plane, wherein the respective torsion beams extend along a rotation axis in the reference plane to allow the actuator to rotate the rotor within a rotation range relative to the stator along the rotation axis, the rotor further having a rotor support, fixed at a first face of the rotor body, that keeps the rotor body at a distance from the stator within said rotation range, and having a mirror surface at a second face opposite the first face, the MEMS mirror comprising the stator and the rotor support at mutually opposite sides of the reference plane, the at least one stator comb and the at least one rotor comb each having at least a first and a second mutually insulated, electrically conductive layer, the method comprising using a first wafer having a first silicon device layer to form the second electrically conductive layers of the combs and at least the stator body, using a second wafer having a second silicon device layer to form the first electrically conductive layers of the combs and using a third wafer to form the rotor part.
20-36. (canceled)
37. A method of manufacturing a MEMS mirror comprising a stator and a rotor that is coupled at mutually opposite sides to the stator via a respective torsion beam, the stator having a stator body and at least one stator comb and the rotor having a rotor body and having at least one rotor comb, the at least one rotor comb being interdigitated with the at least one stator comb to form an actuator, the at least one stator comb defining a reference plane, wherein the respective torsion beams extend along a rotation axis in the reference plane to allow the actuator to rotate the rotor within a rotation range relative to the stator along the rotation axis, the rotor further having a rotor support, fixed at a first face of the rotor body, that keeps the rotor body at a distance from the stator within said rotation range, and having a mirror surface at a second face opposite the first face, the MEMS mirror comprising the stator and the rotor support at mutually opposite sides of the reference plane, in which the at least one stator comb and the at least one rotor comb each at least have a first and a second mutually insulated, electrically conductive layer, the method comprising using a first wafer having at least a first and a second silicon device layers to form the first and the second electrically conductive layers of the combs and at least the stator body and using a second wafer to form the rotor.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] These and other aspects of the invention are described in more detail with reference to the drawings. Therein:
[0053] FIG. 1 schematically shows a swept source OCT system.
[0054] FIG. 2 schematically shows the an embodiment of the probe described in this invention, that is to be connected with the swept source OCT system.
[0055] FIG. 3A is a perspective view schematically showing an embodiment of the MEMS mirror as claimed.
[0056] FIG. 3B is a side view along the y-axis of the embodiment of FIG. 3A.
[0057] FIG. 3C is a top view along the z-axis of the embodiment of FIG. 3A.
[0058] FIG. 3D is a cross-section according to IIID-IIID in FIG. 3B.
[0059] FIG. 3E shows two rotational states of the MEMS mirror according to the same view as FIG. 3B.
[0060] FIG. 3F is a cross-section according to IIIF-IIIF in FIG. 3C.
[0061] FIG. 3G is a top view along the z-axis of the embodiment of FIG. 3A, showing additional hidden features.
[0062] FIG. 3H is a perspective view of a part of the embodiment of the MEMS-mirror of FIG. 3A.
[0063] FIG. 3I shows 3 versions of the same top view along the z-axis as is represented in FIG. 3H, with 3 different options for possible torsion beam configurations.
[0064] FIG. 4 schematically shows an embodiment of the optical assembly, to transport the light from the swept source system to the tissue and back to the system.
[0065] FIGS. 5A, 5B and 5C schematically shows an embodiment of the carrier part of the housing of the probe. Therein FIG. 5A is a perspective view, FIG. 5B is a side view and FIG. 5C shows a detail according to A in FIG. 5B.
[0066] FIG. 6 schematically shows an embodiment of the top part of the housing of the probe.
[0067] FIG. 7 is a block schematic drawing of an embodiment of a driver for the MEMS mirror.
[0068] FIG. 8A-8U schematically shows subsequent steps of a possible manufacturing method for the described MEMS mirror.
[0069] FIG. 9 schematically shows a possible placement of the MEMS mirror, driver, and other components on a flexible printed circuit board in the probe.
[0070] FIG. 10 schematically shows a possible way to manufacture a catheter tube around the probe tip.
DETAILED DESCRIPTION
[0071] Referring to FIG. 2, an embodiment of a forward looking MEMS based optical coherence tomography probe 50 as claimed herein is pictured. The probe 50 comprises an elongated probe housing 51, consisting of a carrier part 52 and a top part 53, and having at a first end a probe interface 54 for an optic fibre 56. At a second end, opposite to the first end, the probe has a viewing window 58. The viewing window can be open or it can be closed off with a transparent solid, e.g. glass. Inside the probe housing 51 a MEMS mirror 10 is arranged, which, in operation, sweeps a light beam 60 through the viewing window onto a sample, for example tissue in a hollow organ of the human body. The MEMS mirror also guides light reflected from the sample and received through the viewing window back into the optic fibre 56 via the probe interface 54.
[0072] The MEMS mirror 10, an embodiment of which is shown in FIG. 3A-3I has a mirror surface 14MS on a rotor body 14RB with a rotation axis 18 that is arranged transverse to a longitudinal axis 62 that extends in a direction from the first to the second end of the described probe housing 51, as shown in FIG. 2. Therein FIG. 3A-3H illustrate the embodiment of the MEMS mirror in various views and cross-sections as indicated above.
[0073] As can best be seen in FIG. 3F, the MEMS mirror 10, as shown in FIG. 3, comprises a first substrate layer HL1, a second substrate layer HL2, and an actuator layer AL that is sandwiched between the first and second substrate layer. The first substrate layer, the second substrate layer, and the actuator layer together form a stator 12, a rotor 14, and an actuator 16 to enable the rotor to rotate relative to the stator along the rotation axis 18, for example within a rotation range of maximum +/10 degrees, as can best be seen in FIG. 3E. The rotation axis lies in a reference plane RP, being defined by the actuator layer in the neutral state of the rotor.
[0074] As shown in FIG. 3F, the actuator layer may comprise a first and a second mutually insulated device layers DL1 and DL2 respectively of an electrically conducting material. DL1 is arranged at a side of the reference plane facing the mirror surface 14MS, while DL2 is arranged at a side of the reference plane facing the stator. DL1 and DL2 respectively form a first and second electric figpole in each of the comb elements. In the embodiment shown, the first electric poles of each of the comb elements of the rotor are electrically interconnected via an eutectic bond with the first substrate layer HL1, and are electrically connected to an output of a driver, via bond pads 22 on the mirror (FIG. 3C). In an application, such as the probe 50, the at least one of a first and a second electric pole of each of left and right comb elements of the stator may be coupled to a respective feedback input of a driver, wherein the left and right comb elements of the stator are arranged at mutually opposite sides of the rotation axis, such that the MEMS mirror is rotation symmetric. The bipolar construction of the comb elements enables an accurate measurement of the amplitude and the moment of zero-crossing of the rotor.
[0075] As illustrated in FIG. 3F, the rotor comprises a rotor body 14RB and a rotor support 14RS. The rotor support, fixed at a first face of the rotor body, keeps the rotor body at a distance from the stator sufficient to enable the rotor to operate within its specified rotation range, for example the exemplary range from 10 to +10, as is depictured in FIG. 3E. The rotor body 14RB has a mirror surface 14MS at a second face, opposite to its first face. The mirror comprises the stator 12 and the rotor support 14RS at mutually opposite sides of the reference plane RP. In the embodiment shown in FIG. 3F, the respective comb-elements of the actuator are electrically connected to respective bond pads forming electric contacts. The electric contacts denoted as 1: MIR_C, DL1 and 5: MIR_C, DL1 are connected to the device layer DL1 of the rotor comb elements 16RL1 and 16RR1 and further electrically connected via the handle layer HL1 to the rotor comb elements 16RL2 and 16RR2. The electric contacts 2: MIR_L_HND, DL1 and 6: MIR_R_HND, DL1 are connected to the device layer DL1 of the pair of stator comb elements 16SL1, 16SL2 and to the device layer DL1 of the pair of stator comb elements 16SR1, 16SR2 respectively. Furthermore, 3: MIR_L_DEV, DL2 and 7: MIR_R_DEV, DL2 are connected to the device layer DL2 of the pair of stator comb elements 16SL1, 16SL2 and to the device layer DL2 of the pair of stator comb elements 16SR1, 16SR2 respectively. In the embodiment shown, two further contacts 4: NC and 8: NC are shown to the second handle layer HL2. In practice not all electrical contacts 1-8, e.g. the electric contacts 4: NC and 8: NC in this example, need to be connected to an external conductor.
[0076] In an alternative embodiment, the stator 12 may further comprise through silicon vias that electrically connect respective comb-elements of the actuator 16 to respective electric contacts at a surface 14S of the stator facing away from the mirror surface. Furthermore the torsion beams may serve as an electrical connection to electrically connect comb elements of the rotor via a through silicon via to an electric contact. Therewith the mirror 10 can for example be electrically connected to conductors in a circuit board carrying the mirror. In this way, using through silicon via technology, the bondpads 1: MIR_C, DL1 etc. as shown in FIG. 3F are obviated, which enables an increase of size of the mirror surface 14MS in a direction of the rotation axis 18 to at least 90%, e.g. 100% or even larger of the size of the stator in said direction.
[0077] In the embodiment shown, the rotor body 14RB has peripheral portions 14L, 14R that extend beyond the stator 12 in transverse direction x, x. These peripheral portions are transverse to the rotation axis 18 and are aligned with the reference plane. These peripheral portions 14L and 14R can have a respective extension portion 14L1 and 14R1 respectively at a distance along a respective side face of the stator, as is depictured in FIG. 3B. 14L1 and 14R1 decrease the resonance frequency of the MEMS mirror. A relative low resonance frequency, for example in the range of 200-4.000 Hz, contributes to obtain high quality OCT images. At relatively high resonance frequencies, the lateral resolution will be lower which complicates distinguishing small structures in the image.
[0078] The actuator 16 comprises at least a first comb pair 16SL1 fixed to the stator, and 16RL1 fixed to the rotor support 14RS, and a second comb pair 16SR1 fixed to the stator and 16RR1 fixed to the rotor support 14RS, as is depictured in FIG. 3D. Optionally, the actuator can comprise at least one further pair of mutually interdigitated comb elements, including at least a third comb pair 16SL2 fixed to the stator and 16RL2 fixed to a peripheral portion of the rotor body, and a fourth comb pair 16SR2 fixed to the stator and 16RR2 fixed to the peripheral portion of the rotor body.
[0079] At mutually opposite sides the actuator (at the rotation support RS) is coupled with the stator via a torsion beam 20A, 20B, of which an embodiment is represented in FIG. 311. The torsion beams extend along the rotation axis 18 to the stator. The torsion beams may have a T-shaped cross-section in a plane perpendicular to the rotation axis, i.e. an elongate structure having a first and a second blade like portion both extending in the longitudinal direction of the torsion bar and in cross-section being arranged orthogonal with respect to each other such that a lateral side of the second blade is joined to a centre of a main side of the first blade. In an embodiment thereof the first blade is arranged in a plane parallel to the reference plane defined above and its main side faces the rotor body. The first blade can be in DL2, while the second blade can be in DL1. Other shapes of the torsion beams are also possible, where the key requirement is that the torsion beams should allow a rotation of the rotor body with low stiffness, to allow a low resonance frequency, and have low stress at large tilt angles. It should be stiff in other directions and rotations, to resist other movements. Therefore, the ratio between the height and the thickness of the rotation beams is important. The beams can either be high and thin or broad and low. As a further alternative, the mirror may be suspended at each side to a pair of torsion beams (20A1, 20A2, 20B1, 20B2) that are connected at one end under a slight angle, max. 20 degrees, as shown in FIG. 3I. The triangle shape resists rotations in the horizontal plane, while the beams itself resist movement in the vertical plane.
[0080] In an embodiment a driver 70 is accommodated inside the housing 51 of the probe. The accommodation of the driver close to the MEMS-mirror further improves an accuracy of actuation and control thereof. The driver in FIG. 7 is at least provided with a feedback input to receive a feedback signal Sf1, and an output to provide a drive signal So to the actuator 16 of the MEMS mirror. The feedback signal Sf1 is indicative for a rotational state of the rotor, and relates to the amplitude, frequency or position of the mirror rotation. Information about the mirror rotation can be a zero-crossing signal.
[0081] The driver, which may be implemented in an application specific IC (ASIC) may function as a phase-locked loop PLL. This phase-locked loop may comprise a phase frequency detector PFD, a voltage controlled oscillator VCO, a loop filter LF, a high voltage output driver HVD, and a feedback attenuator as schematically shown in FIG. 7.
[0082] The driver may be operable in a sweeping mode, which is the start-up mode, and a normal operating mode succeeding the start-up mode. In the sweeping mode, starting from a standstill of the rotor body a high voltage square wave is applied to the actuator, to start a sweeping state. During this state the VCO frequency is slowly decreased, until the MEMS mirror starts to resonate. Once the amplitude of the mirror is above a certain threshold that indicates that reliable phase detection is possible, the ASIC switches from sweeping mode to the normal operating mode controlled by the PLL, to lock the MEMS mirror at its resonance frequency. The PFD detects the phase difference between the zero-angle crossing of the MEMS mirror, and the falling edge of the actuation signal, which is necessary to be able to operate the mirror in resonant mode.
[0083] At resonance frequency the MEMS mirror rotates stably, with the predefined maximum amplitude. Another benefit of operation at the resonance frequency is that a low voltage is sufficient to sustain the operation.
[0084] FIG. 4 schematically shows an optical assembly 80 for guiding a light beam Bo, received via beam guide, such as a single mode fibre 56 from a light source, such as a tuneable laser source 200 of FIG. 1 via the MEMS mirror 10, towards a tissue T to be examined and to guide light beams Bi, subsequently reflected by the tissue T and by the mirror 10 back via the beam guide 56 for processing and analysis. The optical assembly is part of an optical path that further comprises the MEMS mirror 10 and the viewing window 58. A prism directs the light beam in such a way on the MEMS mirror that the light beam exits the probe via the viewing window in the longitudinal direction of the probe. The emanating light beam can be under a very slight angle of e.g. 10 degrees with the longitudinal axis of the probe.
[0085] The housing 51 of the probe may comprise a top part 53, (see FIG. 6) and a carrier part 52, see FIG. 5A, 5B, 5C, which protect the optical and electrical components and keep them aligned. The carrier part 52 of the housing 51 (FIG. 5A-C), has a main portion 32 extending in a direction substantially coinciding with a longitudinal direction x of the housing, and an end portion 34 facing the viewing window. The main portion carries the driver, while the end portion carries the MEMS mirror. The carrier part at a side carrying the MEMS mirror is tilted with respect to said longitudinal direction. The end portion of the carrier may be provided with recesses 36, see FIG. 5C facing the peripheral portions 14L1, 14R1 of the rotor body 14RB, to enable rotation through the full rotation range. Alternatively, the stator 12 of the MEMS mirror can be placed on an elevated part on the end portion of the carrier, e.g. a pedestal, to enable rotation through the full rotation range.
[0086] The top part 53, as shown in FIG. 6 may protect the electrical components and align the optical assembly, for which it is provided with a slit 122. The top part may allow for positioning of the viewing window 58 under an angle of e.g. 8 degrees 124 to avoid static back reflection.
[0087] The probe is made for use with a swept source OCT system, which may include a tuneable laser as light source. In an embodiment, the tuneable laser has a power of 10-50 mW, a centre frequency of 1060 nm, a bandwidth of around 100 nm, and a spectral sweep rate of at least 100 kHz.
[0088] An exemplary method of manufacturing the MEMS may proceed as illustrated in FIGS. 8A to 8U. In each of FIGS. 8B through 8U three cross-sections are shown from left to right. These are respectively taken according to I-I, II-II in FIG. 3C, and the cross-section on the most right shows the process steps on the alignment marker of the wafer.
[0089] In the exemplary method a MEMS mirror is manufactured from a first wafer 150, a second wafer 160 and a third wafer 170 as shown in FIG. 8A. Therein the first wafer 150 is used to form a base layer and a first comb layer (DL2), the second wafer 160 is used to form a second comb layer (DL1). The third wafer 170 is used to form the rotor body and rotor support layer. The wafers 150, 160, 170, e.g. silicon-on-insulator (SOI) wafers, may have a thickness, but not necessarily mutually the same thickness, in the order of a hundred micrometers to a few hundreds of micrometer. In the embodiment shown the wafers each have a thickness of 300 m. The first and the second wafer 150, 160 are SOI. The first wafer 150 comprises a first and a second silicon layer 153, 155 insulated by an insulator layer 154. The second wafer 160 comprises a first and a second silicon layer 163, 165 insulated by an insulator layer 164. Alternatively, the first SOI wafer 150 and second SOI wafer 160 can be replaced with one DSOI wafer, which comprises 3 insulator layers and 4 silicon layers.
[0090] The insulator layers 154, 164, for example siliconoxide layers, may for example have a thickness of a few hundred nm to a few micrometer, for example a thickness of 1 m. The exposed surface 151, 152 of the first wafer 150 may be protected with thermal oxidation, after which frontand/or backside alignment markers 151m, 152m can be etched (FIG. 8B). After that, the pattern of the comb drives and the torsion beams to be formed can be translated to the wafer via a mask lithography step, and this pattern can be etched in the first silicon layer 153 via DRIE etching (FIG. 8C). After this, via a partial-oxygen-etch holes 156 for the future bond pad locations can be opened (FIG. 8D).
[0091] In a next step the second wafer 160, that is to form the second comb layer may be bond to the first wafer 150 via fusion bonding (FIG. 8E). The second wafer 160 can then be ground back to the siliconoxide layer 164 of 0.2 m (FIG. 8F). Next, the assembly of the first and the second wafer can be turned around and the backside 152 can be etched (FIG. 8G). The second wafer 160 may now be etched to expose the alignment mark 151m on top of the first wafer 150 (FIG. 8H). The 0.2 m siliconoxide layer 164 of the second wafer 160 may then be etched FIG. 8I, after which polysilicon 164ps may be deposited in etch holes 164h therein (FIG. 83). After this, the second wafer 160 can be etched such that the contacts in the first wafer 150 are exposed again and be provided with an electrically conductive layer. For example a metal or an alloy of metals, e.g. AlCu may be deposited at these exposed locations, to create the bondpads 22 (FIG. 8K). Next, via DRIE etching the pattern of the comb drive and the torsion beams to be formed may also be etched in the second silicon layer (FIG. 8L), in this embodiment having a thickness of 27 micrometer. The siliconoxide layer 164 in the area assigned to the comb drive may now be removed via etching (FIG. 8M).
[0092] When a DSOI wafer is used instead of wafers 150 and 160, the start assembly is analogous to the one shown in FIG. 8E, but without any etched structures. First, side 152 can be protected with thermal oxidation, and marker 152m can be etched. Next, the top silicon layer can be grounded back to the top siliconoxide layer 164, analogously as shown FIG. 8F. Further, analogous to what is shown in FIG. 8G, after turning of the wafer, the backside 152 can be etched. Layer 164 and 165 can be etched to create alignment mark 151m. Next, the siliconoxide layer 164 can be etched, analogous to FIG. 8I, and analogous to FIG. 8J polysilicon 164ps may be deposited in etch holes 164h formed in the etching step of FIG. 8I. Next, the top silicon layer 160 can be etched to expose the locations 160e, as shown in dashed lines in FIG. 8F. After exposure, bondpads 156 can be created in the layers below, analogously as shown in and described with reference to FIG. 8C and FIG. 8D, via partial-oxygen-etch holes. The created etch holes may be provided with an electrical conductive layer. For example a metal or metal alloy, e.g. AlCu may be deposited at these exposed locations 160e to create the bondpads 22, as analogously shown in and described with reference to FIG. 8K. Next, via DRIE etching the pattern of the comb drives end torsion beams can be etched, possibly in two or more steps with different etching masks to provide all the detail, in the top two silicon wafers, analogous to FIG. 8L and FIG. 8C. The siliconoxide layer 164 in the area assigned to the comb drive may now be removed via etching, analogous to FIG. 8M. The manufacturing process may then proceed with the step below described with reference to FIG. 8N, regardless whether the process started with separate SOI wafers 150 and 160, or one combined DSOI wafer.
[0093] In this subsequent step, thermal oxidation can be applied to outer surfaces 171, 172 of the third wafer 170 (FIG. 8N). Alignment marker 172m can be etched. As further shown in FIG. 8N, one of these surface 171 can be etched to create deposition locations 171d for deposition of electric contacts 171c. In the embodiment shown these contacts 171c are obtained by deposition of a gold layer in these locations (FIG. 8O).
[0094] Next, a pattern of the main part of the rotor 14RB and the rotor support 14RS may be etched in the third wafer 170 with DRIE etching (FIG. 8P). Subsequently, the third wafer 170 can be mechanically and electrically connected to the assembly of the first wafer 150 and the second wafer 160 via an eutectic bond (FIG. 8Q) with the exposed surface of the second wafer 160. The semi-finished product comprising the first, second and third wafer may now be released at the side of the first wafer, and may be turned to etch the second silicon layer 155 of the first wafer with DRIE etching to open up the area defined for the comb drive and the torsion beams (FIG. 8R). Next, the intermediate insulator layer 154 of siliconoxide may be etched in the same area at the base side of the comb drive structure, to fully open up the comb drives (FIG. 8S). The MEMS assembly can now be turned around, such that the third wafer 170 is at the top side, after which via DRIE etching the third wafer 170 may be cut free (FIG. 8T). A reflective layer, for example an Al or Au layer having a thickness of some tens of nm, e.g. 50 nm may then be deposited and patterned on top of the third wafer surface to form the mirror surface 14MS (FIG. 8U).
[0095] The top part 53 and carrier part 52 of the housing can be 3D printed in medical grade plastic. Passive electric components including resistors, capacitors and an inductor for example may be connected to both mutually opposite faces of a flexible printed circuit board 110 (FIG. 9). An ASIC 70, serving as the driver for the MEMS 10, can be ground back to a naked die, and be stud-bumped. This stud-bumped ASIC can then be connected to one side of the flexible printed circuit board. Electrically conductive wires can be soldered to pads 114 on the back side of the flexible printed circuit board to enable receiving and sending of signals from and to the ASIC 70, e.g. a signal that provides the angular position of the mirror. The flexible printed circuit board can be glued to the carrier part 52, in such a way that an inductor 112 and the ASIC are on the surface of the flexible printed circuit board that faces the carrier part. The MEMS mirror 10 can now be glued to the end portion 34 of the flexible printed circuit board 110. The assembly of the carrier part 52 and the flexible printed circuit board can be clamped inside an assembly holder, such that wire bonding of the six bond pads 22 of the MEMS mirror can take place. The top part 53 can now be glued in place on top of the assembly of the carrier part 52 and the printed circuit board 110, after which the optical fibre assembly can be placed on top of this support in the groove that is made for that purpose. Next, the fibre assembly can be aligned with the MEMS mirror. To turn this probe into a watertight catheter, the viewing window 58 can be glued to the front side of the top support, under an angle of 8 to avoid static back reflection. Next, a heat shrink tube 59 will be pulled around the support structure, which can be at least 1 cm longer than the support structure itself (FIG. 10), extending at the first (optical interface) side of the probe. At the second (viewing window) side the heat shrink tube will be shrunk around the edges of the window 58. A longer heat shrink tube can be pulled around the wires, and over the 1 cm extension of the other heat shrink tube, where both tubes can be glued together. At the opposite end of this long tube a split can be made such that the tube is split in mutually separate tubes, one for the optical fibre and one for electrical conductors to the probe. Both tubes end with a corresponding connector.
