WO2020125870A1 - Miroir - Google Patents

Miroir Download PDF

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Publication number
WO2020125870A1
WO2020125870A1 PCT/DE2019/101105 DE2019101105W WO2020125870A1 WO 2020125870 A1 WO2020125870 A1 WO 2020125870A1 DE 2019101105 W DE2019101105 W DE 2019101105W WO 2020125870 A1 WO2020125870 A1 WO 2020125870A1
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WO
WIPO (PCT)
Prior art keywords
mirror
etching
structures
etching structures
wafer
Prior art date
Application number
PCT/DE2019/101105
Other languages
German (de)
English (en)
Inventor
Jan Kuypers
Original Assignee
Blickfeld GmbH
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Blickfeld GmbH filed Critical Blickfeld GmbH
Publication of WO2020125870A1 publication Critical patent/WO2020125870A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/481—Constructional features, e.g. arrangements of optical elements
    • G01S7/4811—Constructional features, e.g. arrangements of optical elements common to transmitter and receiver
    • G01S7/4812—Constructional features, e.g. arrangements of optical elements common to transmitter and receiver transmitted and received beams following a coaxial path
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
    • G01S17/06—Systems determining position data of a target
    • G01S17/42—Simultaneous measurement of distance and other co-ordinates
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/481—Constructional features, e.g. arrangements of optical elements
    • G01S7/4817—Constructional features, e.g. arrangements of optical elements relating to scanning
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
    • G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
    • G02B26/0816—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements
    • G02B26/0833—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD

Definitions

  • Various embodiments of the invention relate to techniques for producing a mirror - for example from a semiconductor material.
  • the mirror can, for example, be suitable for use in a scan module.
  • a LIDAR system can be implemented.
  • Various examples of the invention relate in particular to the use of a repetitive arrangement of etching structures on the back of the mirror.
  • Mirrors for directing light are required in various applications.
  • An exemplary application relates to distance measurements using light.
  • pulsed laser light can be used.
  • Primary laser light can be emitted and secondary laser light reflected from an object in the vicinity can be detected.
  • a runtime measurement can be used in various examples.
  • mirrors For example, techniques for generating mirrors are known from: US 7,078,778 B2. Manufacturing techniques that are established in connection with microelectromechanical systems (MEMS) are typically used here.
  • MEMS microelectromechanical systems
  • the mirror is defined by a wafer, for example a silicon wafer.
  • the wafer is then processed using one or more of the following techniques: lithography; Dry etching; Wet etching; Release; etc.
  • DRIE is used to remove large amounts of semiconductor material - as is typically required when manufacturing large-diameter mirrors, for example, to obtain a light mirror that can be moved quickly - the process gas, for example, can be particularly difficult / Apply etching gas homogeneously as a function of the lateral position on the wafer at the required high rates. This can lead to concentration gradients as a function of the lateral position; this in turn can lead to inhomogeneous etching rates. Inhomogeneous etching is undesirable, however, because structures which are arranged at different lateral positions are then processed differently; a parameter spread results. This makes it difficult to implement a reproducible process without significant parameter spread.
  • Another disadvantage of conventional DRIE associated with MEMS techniques is that the heat introduced into the system increases as the volume of material removed increases. The cooling capacity can be difficult to provide especially in the manufacture of large mirrors. A temperature gradient can also lead to parameter scatter.
  • a mirror includes a reflective front and a back.
  • the mirror has a repetitive arrangement of etching structures on the back.
  • the etching structures can have a lateral aspect ratio in the range from 0.3 to 3.
  • the etching structures can have a fill factor with respect to an area of the rear side which is in the range from 10% to 80%, but preferably in the range from 20% to 50%.
  • the lateral aspect ratio thus describes the ratio of length to width of the structure.
  • the mirror could be made of a semiconductor material or another material, such as a ceramic or a plastic or graphite.
  • the front and the back can be arranged parallel to a surface of a wafer made of the semiconductor material.
  • the depth of the etching structures - i.e. perpendicular to the surface of the wafer - not taken into account.
  • a wafer made of the semiconductor material comprises several such mirrors. These mirrors can optionally be connected to a surrounding material of the wafer via intermediate structures.
  • a system includes a mirror as described above.
  • the system comprises at least one spring element, for example one or more torsion spring elements and / or one or more bending spring elements.
  • the at least one spring element extends away from the mirror.
  • the at least one spring element is designed to deform elastically to move the mirror.
  • the elastic deformation can be reversible. This enables smooth scanning of light.
  • One method of forming multiple mirrors on a semiconductor material wafer includes using a DRIE process.
  • the DRIE process is used on one side of a wafer in order to obtain an arrangement of etching structures on the back of the mirrors.
  • the etching structures have a lateral aspect ratio in the range from 0.3 to 3.
  • the etch structures also have a fill factor that is defined with respect to an area of the back of the mirrors. This fill factor is in a range from 10% to 80%, preferably in the range from 20% to 50%.
  • FIG. 1 schematically illustrates a system with a mirror, a spring element and an actuator according to various examples.
  • FIG. 2 schematically illustrates a LIDAR system according to various examples.
  • FIG. 3 schematically illustrates a mirror according to various examples, the mirror having a repetitive arrangement of etching structures on its rear side.
  • FIG. 4 schematically illustrates a lattice structure which is formed by the repetitive arrangement of etching structures in accordance with various examples.
  • FIG. 5 schematically illustrates a lattice structure that is formed by the repetitive arrangement of etching structures according to various examples.
  • FIG. 6 is a cross-sectional view of the mirror of FIG. 3.
  • FIG. 7 illustrates the variation of one or more parameters of the repetitive arrangement of etching structures according to various examples.
  • FIG. 8 illustrates defects in a lattice structure formed by the repetitive arrangement according to various examples.
  • FIG. 9 is a flowchart of an example method for forming multiple mirrors on a semiconductor material wafer.
  • FIG. 10 schematically illustrates a wafer made of semiconductor material with several mirrors defined thereon according to various examples.
  • 11 schematically illustrates the variation of one or more parameters of a repetitive arrangement of etching structures on the rear sides of the mirrors on the wafer as a function of the lateral position on the wafer.
  • 12 schematically illustrates a series of process steps for forming multiple mirrors on a wafer in accordance with various examples.
  • Such mirrors can be used to deflect light, for example laser light.
  • the mirrors can be moved, for example, by an elastic holder. This allows light to be scanned.
  • the mirrors described herein can be used in various application scenarios.
  • the mirrors described herein can be used in the context of: LIDAR with lateral resolution; Spectrometer; Projectors; Endoscopes; etc.
  • mesoscopic mirrors that is, mirrors with a diameter of the respective reflecting front that operate in a transition regime between micromirrors (typically with diameters of the respective reflecting front in millimeters or sub-millimeters).
  • Regime and macroscopic mirrors (typically with diameters of the respective reflective front in the centimeter regime, for example multifaceted polygon mirrors).
  • the reflective faces of the mirrors described herein could have an area in the range of 100 mm 2 to 200 mm 2 .
  • the mirrors described herein could have a circular or elliptical reflective face.
  • the mirrors described here could have, for example, a square or rectangular-shaped reflecting front, or an n-square front, where n is, for example, 5 or 6.
  • Hexagonal mirrors can be used.
  • the techniques described herein can particularly promote the manufacture of mirrors from a semiconductor material using MEMS fabrication techniques.
  • the MEMS fabrication techniques may include one or more of the following processing methods: etching, particularly DRIE; Thin film coating; Lithograph; Liftoff; etc.
  • etching particularly DRIE
  • Thin film coating Lithograph
  • Liftoff etc.
  • techniques related to DRIE are described in: Wu, Banqiu, Ajay Kumar, and Sharma Pamarthy. "High aspect ratio Silicon etch: A review.” Journal of applied physics 108.5 (2010): 051101 or also in Karttunen, Jani, Jyrki Kiihamaki, and Sami Franssila. "Loading effects in deep silicon etching.” Micromachining and microfabrication process technology VI. Vol. 4174. International Society for Optics and Photonics, 2000. Such techniques can also be used in connection with the methods described herein.
  • the techniques described herein can, in particular, enable the production of mesoscopic mirrors that are comparatively lightweight. According to various examples, this is made possible by the provision of a repetitive arrangement of etching structures on the back of the mirror.
  • etching structures semiconductor material can be removed locally, so that the mirror becomes lighter.
  • the fill factor of the etching structures - defined in relation to the area of the back of the mirror - could be in the range from 10% to 80%, preferably in the range from 20% to 50%. In other words, this means that a significant proportion of the semiconductor material on the back can be removed.
  • Typical frequencies of movement are, for example, in the range from 50 Hz to 500 Hz for typical mesoscopic mirrors.
  • the aspect ratio-dependent etching is often caused by the depletion of etching gas at the bottom of an etching structure.
  • the etching rate depends on a lateral aspect ratio of the etching structures. This means, for example, that an elongated trench is etched at a different etching rate than a circle. For example, see Wu et al .: Fig. 6; below.
  • the population-dependent etching shows a dependence of the etching rate on a lateral structure density of the etching structures, that is to say on the area of the semiconductor material exposed to the etching gas. Occupation-dependent etching is observed with etching structures of different sizes and different lateral aspect ratios. Due to the etching dependent on the population, for example, the etching rate for etching structures which are arranged in the center of the rear side of the mirror can be different than for etching structures which are arranged on the edge of the rear side of the mirror. In addition, e.g. the etching rate varies depending on the structure size of the etching structures. Larger etching structures can be etched with a larger etching rate.
  • mirrors can be produced which have a rear side with the repetitive arrangement of etching structures, the etching structures having a lateral aspect ratio in the range from 0.3 to 3.
  • the ratio between the width and length of the etching structures can be in the range from 1: 3 to 3: 1.
  • etching structures can be used which are not particularly long (for example no elongated trenches), but in which the width does not deviate from the length by more than a factor of 3.
  • a particularly homogeneous repetitive arrangement can be achieved by using such etching structures: in particular, it may be possible to limit a variation in the etching rate due to the aspect ratio-dependent etching and / or the occupation-dependent etching. This can be achieved, for example, by scaling the size of the etching structures and / or the distances between the etching structures. As a result, a scatter of parameters during the production of mirrors can be limited.
  • System 100 includes a mirror 150 made from a semiconductor material.
  • the mirror includes a front face 151.
  • the front face 151 is reflective.
  • a reflective material - for example gold or silver or aluminum - to be deposited on the front face 151 by means of thin-film coating technology.
  • Light 190 is deflected at the front 151 of the mirror.
  • the mirror 150 also includes a rear face 152.
  • the rear face 152 is located opposite the front face 151.
  • the mirror 150 is e.g. made of silicon or gallium arsenide or another semiconductor material.
  • a spring element 902 is attached to the side of the mirror 150, i.e. between front 151 and back 152, i.e. at the periphery of the mirror 150.
  • multiple spring elements 902 could be connected to the mirror.
  • a plurality of spring elements 902 could be attached to different sides of a circumference of the mirror 150.
  • the spring element 902 is designed to be elastic. By bending and / or torsion etc. of the spring element 902, the mirror 150 is moved and the light 190 is thereby deflected differently.
  • Different spring elements 902 can be used in the various examples described herein.
  • flex spring elements could be used or torsion spring elements could be used.
  • the geometry and / or the material of such spring elements is selected such that a bending mode (for example a horizontal or a vertical bending mode) or a torsion mode have the lowest resonance frequency in each case.
  • the torsion spring element could be bar-shaped; the bending spring element, however, could be plate-shaped.
  • Leaf springs could also be used.
  • the spring element 902 in the example in FIG. 1 extends between the mirror 150 and an actuator 901.
  • the actuator 901 is set up to stimulate a movement of the mirror 150.
  • the actuator 901 it would be possible for the actuator 901 to be configured to resonantly excite a resonant movement of the mass-spring system, which comprises the spring element 902 and the mirror 150.
  • actuator 901 can be used as a general rule.
  • implementation by piezo actuators could be used.
  • a system 100 as shown in connection with FIG. 1 can generally be used in a wide variety of application areas.
  • An exemplary application of the system 100 in connection with a LIDAR system 90 is illustrated in connection with FIG. 2.
  • the LIDAR system 90 comprises in particular the system 100, that is to say the mirror 150, the spring element 902, and the actuator 901.
  • the LIDAR system 90 also includes a computing unit 91, a laser 92, e.g. a laser diode, and a detector 102, e.g. a single-photon avalanche detector array (English single-photon avalanche diode array, SPAD array).
  • a computing unit 91 e.g. a laser 92, e.g. a laser diode
  • a detector 102 e.g. a single-photon avalanche detector array (English single-photon avalanche diode array, SPAD array).
  • the laser 92 and the detector 93 form a coaxial optical arrangement.
  • the coaxial optical arrangement in FIG. 2 can have a transmission aperture (numerical aperture) and / or a detector aperture (numerical aperture), which is limited by the diameter of the reflective front face 151 of the mirror 150.
  • the sensitivity of the LIDAR Systems 90 and thus the range also scales with the area of the detector aperture (a larger detector aperture allows more light to be collected) and typically also with the size of the transmit aperture (taking into account a typically predetermined, limited collimation capacity of the primary laser light 191, due to the emitter area of the Lasers 92). It is therefore particularly desirable to produce a large mirror 150 which is also particularly light at the same time (in order to promote rapid movement of the mirror 150, that is to say to enable large resonance frequencies of the mass-spring system). The techniques described herein make this possible.
  • the computing unit 91 can be implemented by an application-specific integrated circuit (ASIC) and / or a field programmable gate array (FPGA) and / or a general-purpose processor.
  • the computing unit 91 could, for example, comprise an analog-to-digital converter and / or a time-to-digital converter (TDC).
  • ASIC application-specific integrated circuit
  • FPGA field programmable gate array
  • TDC time-to-digital converter
  • the computing unit 91 is set up to control the laser 92 so that it emits the primary laser light 191.
  • the computing unit 91 is set up to control the detector 93 so that it detects the secondary laser light 192.
  • the computing unit 91 is set up to control the actuator 901 so that it causes the mirror 150 to move.
  • the computing unit 91 could be set up to control the actuator 901 so that it resonantly excites the mass-spring system comprising the mirror 150 and the spring element 902, for example in a torsion mode or a bending mode of the spring element 902.
  • mirror 150 Details related to the mirror 150 are described below. In particular, techniques are described which make it possible, on the one hand, to mirror 150 with a to produce a particularly large reflective front face 151; and on the other hand to make the mirror 150 particularly light. Corresponding techniques are described below, for example in connection with FIG. 3.
  • FIG. 3 schematically illustrates the mirror 150.
  • FIG. 3 illustrates details in connection with the rear side 152 of the mirror 150.
  • FIG. 3 is a schematic plan view of the rear side 152 of the mirror 150.
  • a center is in particular 159 of the back 152 and a circumference 158 of the back 152 shown.
  • the area of the rear side 152 (and / or the front side 151 of the mirror 150, not shown in FIG. 3) is in the range from 100 mm 2 to 250 mm 2 .
  • Such sizes of mirrors 150 have a good compromise between the size of the corresponding optical numerical aperture on the one hand and the weight of the mirror 150 on the other.
  • the mirror 150 and in particular the rear side 152 are circular. In general, however, it would be possible for the mirror 150 and thus in particular also the rear side 152 to have a different shape, for example elliptical or square.
  • the rear side 152 has a repetitive arrangement of etching structures 201.
  • the etching structures 201 are circular.
  • FIG. 3 shows a width 261 and a length 262 of the etching structures 201.
  • the width 261 or the length 262 of the etching structures 201 it would be possible for the width 261 or the length 262 of the etching structures 201 to be in the range from 20 pm to 200 pm.
  • the width 261 and the length 262 do not vary as a function of the lateral position x, y on the rear side 152. In other examples, however, a variation would be possible.
  • the etching structures 201 could have a lateral aspect ratio Range from 0.3 to 3.
  • the etching structures 201 correspond to indentations in the rear side 152, in which the semiconductor material from which the mirror 150 is made has been removed.
  • the etching structures 201 form holes in the semiconductor material.
  • the etching structures 201 can in particular have a depth profile (along the Z direction) which is characteristic of the respective etching process - for example DRIE. This could be expressed, for example, in a wall steepness of the walls of the etching structures 201 in the depth direction along the Z axis. Another characteristic property relates to the formation of the bottom of the etching structures 201.
  • the size of the etching structures 201 and the distance between adjacent etching structures 201 define a fill factor.
  • the fill factor determines a ratio between the area of the etching structures 201 (that is, all areas within the small circles that form the etching structures 201 in the example of FIG. 3) and the area of the rear side 152 (that is, in the example of FIG. 3) Area within perimeter 158).
  • the fill factor can be in the range from 10% to 80%, preferably in the range from approximately 20% to approximately 50%.
  • etching structures 201 By using the repetitive arrangement of etching structures 201, a particularly reproducible and easily manageable manufacturing process can be guaranteed. In particular, a significant spread of geometric parameters of the mirror 150 can be avoided; Exemplary parameters that are susceptible to process variations include, for example, the depth of the etching structures 201.
  • Exemplary parameters that are susceptible to process variations include, for example, the depth of the etching structures 201.
  • influences of the aspect ratio-dependent etching and the occupation-dependent etching can be reduced or comparatively appear less prominent. For example, by using the same etching structure 201 in a repetitive manner (in the case of FIG.
  • the influence of the aspect ratio-dependent etching can be reduced - for example in particular in comparison to techniques which use a lightweight construction of the rear side 152 with ribs and recesses in between, compare for example in particular DE 10 2017 222 404.
  • an influence of the Occupancy-dependent etching can be controlled particularly well by using the repetitive arrangement of the etching structures 201: in particular, the exposure of the semiconductor material to the process gas can be predicted or adjusted particularly well locally by using the repetitive arrangement.
  • the repetitive arrangement of the etching structures 201 implements a lattice structure, here in particular with a square unit cell 250.
  • a lattice structure By using a lattice structure, it is particularly easy to create an etching mask that defines the etching structures 201.
  • FIG. 3 While a grid structure with a square unit cell 250 is shown in the example in FIG. 3, it would be possible in other examples to use other types of grid structures, that is to say in particular to use other geometries for the unit cell 250. A few corresponding examples are described below in connection with FIG. 4 and FIG. 5.
  • 4 and 5 illustrate aspects in connection with a lattice structure 299, which is formed by the repetitive arrangement of the etching structures 201. 4 and 5 illustrate aspects in connection with a corresponding unit cell.
  • a square unit cell 250 of the lattice structure 299 is used, which is in particular a parallelogram.
  • the etching structures 261 have a hexagonal shape, but could also have a different shape in other examples.
  • the aspect ratio between width 261 and length 262 is also 1: 1 in the example of FIG. 4.
  • a hexagonal or hexagonal unit cell 250 is used.
  • the etching structures 201 are again circular.
  • a size 251 of the respective unit cell 250 of the lattice structure 299 is also illustrated in FIG. 4 and in FIG. 5.
  • a hexagonal unit cell 250 from FIG. 5 represents an optimum in connection with the use of the circular etching structures 201 with regard to the fill factor to be achieved.
  • the examples shown in FIGS. 4 and 5 can be modified in further examples.
  • other square holes could also be used as etching structures 201.
  • other types of etch structures 201 could be used instead of the circular holes shown in FIG. 5.
  • FIG. 6 illustrates aspects related to the mirror 150.
  • FIG. 6 is a sectional view of the mirror 150.
  • FIG. 6 illustrates the mirror 150 for a section along the axis AB from FIG. 3.
  • FIG. 6 is a thickness 272 of the mirror 150 between the front 151 and the rear 152 shown.
  • a reflective thin film 151A is applied to the front face 151.
  • the 6 also shows a depth 271 of the etching structures 201 along the Z axis.
  • the depth 271 is determined by an etching rate of the etching process and the etching time.
  • the etching rate of the etching process can be controlled particularly well or has no particularly large variation due to an influence of the aspect ratio-dependent etching or the occupation-dependent etching. It can therefore be seen in FIG. 6 that the depth 271 of the various etching structures 201 shown varies particularly little, for example less than 20% or for example less than 5% or even less than one percent. This can be in particular based on an average value of the depth 271. This is achieved without the semiconductor material from which the mirror 150 is made or the corresponding wafer having an etching stop layer which would define the depth 271.
  • SOI silicon-on-insulator
  • Etching stop layers can be formed, for example, by an oxide material.
  • the small variation in depths 271 in particular enables well-defined dynamic behavior of mirror 150 to be achieved, for example if the mass-spring system comprising mirror 150 and spring element 902 is driven resonantly by actuator 901 (compare FIGS. 1 and 2 ).
  • a variation in depth 271, on the other hand, would result in that the resonance frequencies of different mirrors 150 scatter or an imbalance is caused when the mirror 150 moves.
  • the thickness 272 of the mirror 150 could be in the range of 100 pm to 300 pm, for example in the range of 200 pm +/- 50 pm.
  • a wafer with an initial thickness of 300 pm or even 725 pm can be thinned and polished starting from the front side of the wafer, in which case the front side 151 of the mirror 150 can be obtained.
  • the initial thickness should not be less than 300 ⁇ m or better 725 ⁇ m.
  • the depth 271 of the etch structures 201 could range, for example, from 50 ⁇ m to 500 ⁇ m from 5 pm to 30 pm. Typically 100 ⁇ m to 250 ⁇ m.
  • a ratio between the depth 271 of the etching structures 201 (approximately an average depth 271) and the thickness 272 of the mirror 150 to be in the range from 0.5 to 0.98.
  • Such a ratio has a good compromise between the lightness of the mirror on the one hand and the rigidity of the mirror on the other.
  • the mirror 150 could have a thickness 272 of 150 pm and the depth 271 of the etching structures 201 could e.g. 135 pm.
  • FIG. 7 illustrates aspects relating to the variation 269 of one or more parameters 500 of the repetitive arrangement of etching structures 201.
  • the one or more parameters 500 are varied along a direction which is parallel to the Y axis; in particular, the variation 269 along the axis AB from FIG. 3 is shown. In general, however, it would be possible for a corresponding variation 269 also or alternatively to be present along a direction which is parallel to the X axis.
  • the variation 269 can be described as a function relative to the distance to the center of the mirror 150.
  • the variation 269 of the one or more parameters can therefore be present as a function of the lateral position on the rear side 152 of the mirror 150.
  • the scenario in FIG. 7 shows that the variation 269 can be formed gradually starting from the center point 159 of the rear side 152 (solid line in FIG. 7) or else step-like (dashed line in FIG. 7).
  • the step of the step-like variation 269 in the example of FIG. 7 is formed close to the circumference 158 of the rear side 152 of the mirror 150, wherein “close to” means that the step is formed at a distance from the circumference 158 that is relevant to the Occupation-dependent etching is.
  • a wide variety of parameters 500 can be varied as a function of the lateral position in the various examples described herein.
  • the length 262 and / or the width 261 of the etching structures 201 (or generally the lateral structure size) to be varied as a parameter 500.
  • the lateral aspect ratio between width 261 and length 262 it would be possible, for example, for the lateral aspect ratio between width 261 and length 262 to be varied.
  • the influence of the population-dependent etching could be compensated for by the opposite influence of the aspect ratio-dependent etching.
  • the unit cell size 251 of a lattice structure implementing the repetitive arrangement could be varied.
  • the density of the semiconductor material exposed to the etching gas can be varied during the etching process, so that the influence of the population-dependent etching can be set.
  • the height 262 and / or the width 251 of the etching structures 201 for example, a continuous layout of the rear side of the mirror 150 can be obtained particularly easily. This simplifies the design process.
  • one or more parameters 500 of the repetitive arrangement of etching structures 201 are varied as a function of the lateral position along the X axis and / or along the Y axis.
  • Another technique for setting the dynamic parameters of the mirror 150 particularly well.
  • the repetitive arrangement of the etching structures 201 itself can be broken or resolved locally, by using defects. A corresponding concept is shown in connection with FIG. 8.
  • FIG. 8 illustrates aspects in connection with the mirror 150.
  • FIG. 8 illustrates a schematic plan view of the rear side 158 of the mirror 150.
  • the example in FIG. 8 basically corresponds to the example in FIG. 3.
  • the repetitive arrangement of the etching structures 201 is again designed as a lattice structure 299 with a square unit cell 250 (again, as already explained in connection with FIG. 3, other types of unit cells 250 and other types of etching structures 201 would be possible).
  • the lattice structure 299 has two defects 259.
  • the defects 259 of the lattice structure denote lattice locations of the lattice structure 299 at which an etching structure 201 would be provided, but no etching structure 201 is placed.
  • Defects 259 can also denote those lattice points of the lattice structure in which an etching structure 201 is provided, but which differs significantly from the other etching structures 201 of the lattice structure, at least with regard to one or more geometric parameters, such as lateral aspect ratio, width and / or length. E.g. flaws can be used in which individual grid positions are misoccupied, i.e. the next neighbors are properly occupied.
  • the dynamic parameters of the mirror 150 can be influenced. For example, it would be possible to create or reduce an unbalance of the mirror 150. It would also be possible to suitably set the natural frequencies of the various modes of the mass-spring system, including the mirror 150 and the spring element 902. A corresponding adaptation can alternatively or additionally be achieved, for example, by the suitable choice of the geometry of the unit cells 250 when using a lattice structure 299 of the etching structures 201.
  • the natural torsional frequency may be approximately equal to the natural bending frequency (a condition also referred to as degeneracy). This can result in a complicated dynamic of the mirror 150 with resonant excitation of the mass-spring system; Coupling effects and / or non-linearities can result. Therefore, by providing one or more defects 259, the frequency distance between the torsional natural frequency and the bending natural frequency can be set so that it is not less than 30 Hz. With typical quality factors of mass-spring systems, this dimensioning of the frequency spacing causes a separation of the resonance peaks and a reduced coupling between the different eigenmodes. This makes the dynamics of the mirror 150 easier to control.
  • a multiple (for example, the 2x multiple or the 3x multiple, etc.) of the torsional natural frequency is equal to the natural bending frequency; in such a case there may also be a coupling. Therefore, if one or more defects 259 are provided, the frequency spacing between an n times (n is an integer) multiples of the natural torsional frequency and the natural bending frequency may not be less than 30 Hz. The same applies if the bending natural frequency is the lowest natural frequency of the mass-spring system. Then, by providing one or more defects 259, it can be achieved that a frequency distance between an n-fold multiple of the bending natural frequency and the torsional natural frequency is not less than 30 Hz.
  • FIG. 9 is a flowchart of an exemplary method for forming mirrors on a semiconductor material wafer. In Fig. 9, optional blocks are shown with broken lines.
  • the method of FIG. 9 can be used to fabricate one or more mirrors 150, as discussed above in connection with FIGS. 1-8.
  • the etching mask can be produced, for example, by a lithography process, for example optical lithography using a suitable photoresist and photomask.
  • the etch mask can define multiple mirrors on the wafer, e.g. by defining the scope of the mirror by means of suitable further etching structures.
  • the etching mask can optionally also define intermediate structures via which the mirrors are connected to a surrounding area of the wafer, for example webs or bridges.
  • the etch mask could define an array of mirrors. Several mirrors can therefore be present at the wafer level.
  • the etching mask can in turn define a repetitive arrangement of etching structures.
  • the etching mask can have a lateral structure that defines the repetitive arrangement of etching structures.
  • more than one etching mask and more than one etching process can be used to define the multiple mirrors on the wafer and to define the repetitive arrangement of etching structures at the mirror level.
  • At least one etching process takes place in block 1001, for example a DRIE etching process in the example of FIG. 9.
  • Semiconductor material is removed by the at least one etching process.
  • the mirror and the repetitive arrangement of the etching structures on each of the mirrors is formed in this way.
  • the influence of aspect ratio-dependent etching and occupation-dependent etching can be reduced during the etching in block 1001.
  • the etching mask could define a lateral variation of one or more parameters of the repetitive arrangement of the etching structures (cf. FIG. 7). Then, in block 1002, the wafer is thinned and / or polished.
  • thinning could e.g. done mechanically.
  • a wet chemical thinning e.g. in KOH, TMAH, EDP, HNA, etc
  • RIE dry etching process
  • the determination in block 1003 can be made in various ways. For example, in one variant it would be possible to measure the weight of the wafer and to draw conclusions from this about the weights of the mirrors. For this purpose, the thinning and / or polishing can be temporarily interrupted. The weight could also be estimated, for example by indirectly observing the material removal during thinning and / or polishing in block 1002. In particular, the weight could be estimated taking into account a calculated depth of the etching structures.
  • Such an estimation of the weight can be made possible in particular because the variation 269 of the depth of the etching structures by using the repetitive arrangement is comparatively small (as for example above in connection with the depth 271 of the etching structures 201 in connection with the example of FIG. 6 described). Then, depending on the check as to whether the weight is within the specification, the thinning and / or polishing can be continued or ended in block 1002.
  • the intensity of thinning and / or polishing can be adjusted based on measuring and / or estimating the weight.
  • the intensity can be adjusted by duration, as shown in FIG. 9.
  • the intensity could, for example, also be varied by applying pressure to a corresponding material or by a speed at which the material is moved relative to the wafer.
  • Such a variable adjustment of the process parameters in connection with the thinning and / or polishing in block 1002 enables the weight of the mirrors to be set particularly precisely. This means that the dynamic parameters of the mirror can be set particularly precisely.
  • a thin reflective layer can be applied to the front of the wafer, which was previously thinned and / or polished (see also thin layer 151A in the example in FIG. 6).
  • the mirrors are then separated in block 1005. This means that individual mirrors are exempted. This means that the mirrors are separated from the surrounding wafer material, for example by cutting through the intermediate structures 320 and / or by detaching them from a carrier wafer (handling wafer).
  • the sequence of the process steps in FIG. 9 can be varied in other examples.
  • thinning and / or polishing is performed after the DRIE process is applied. This enables the adaptive adaptation of the process parameters for the thinning and / or polishing in block 1003. In other examples, however, it would also be possible for the thinning and / or polishing to be carried out before the etching process.
  • FIG. 10 illustrates aspects related to a wafer 300.
  • the wafer 300 could be obtained by a method according to the example of FIG. 9.
  • the wafer 300 has a front side 301, which is shown in FIG. 10.
  • 10 also shows a center point 309 and an outer circumference 308 of the wafer.
  • FIG. 10 illustrates the coordinate system C ', V and Z' at the wafer level.
  • the coordinate system at the wafer level (see FIG. 10) can be aligned or rotated with respect to the coordinate system X, Y and Z at the mirror level (see FIG. 3).
  • a diameter of the wafer 300 is typically in the range between 100 mm and 300 mm. This means that the diameter of the wafer 300 in FIG. 10 is much larger than the typical diameter of a mirror 150. Therefore, a plurality of mirrors 150 on the wafer 300 can also be processed in parallel.
  • FIG. 10 illustrates the wafer 300 before the separation (cf. FIG. 9: block 1005) of the various mirrors 150.
  • the mirrors 150 are still connected to the surrounding semiconductor material via intermediate structures 320 (here: webs) Wafers 300 connected.
  • the intermediate structures 320 are basically optional. It is possible that the different mirrors 150 on the wafer 300 are all of identical design. In other examples, however, it would also be possible for a certain variation from mirror to mirror 150 to be provided on the wafer 300. In particular, it would be possible, for example, for the repetitive arrangement of the etching structures 201 to be adapted from mirror to mirror 150, that is to say an adaptation as a function of the lateral position on the wafer 300. Corresponding techniques are illustrated in connection with FIG. 11.
  • FIG. 11 illustrates aspects relating to the repetitive arrangement of etching structures.
  • FIG. 11 illustrates the variation 269 'of one or more parameters 500 of the repetitive arrangement of etching structures as a function of the lateral position at the wafer level, that is to say in the example of FIG. 11 as a function of the lateral position along the Y' axis .
  • 11 shows in particular the variation of one or more parameters 500 along the line A'-B 'from FIG. 10.
  • the variation 269 'along the Y' axis and / or along the X 'axis would be possible, i.e. as a function of the lateral position on the wafer 300.
  • the example in FIG. 11 basically corresponds to the example in FIG. 7.
  • the variation 269 'of one or more parameters 500 is shown at the wafer level; 7 shows the variation 269 of one or more parameters 500 at the mirror level.
  • variation 269 it is possible to use variation 269 'in addition or as an alternative to variation 269.
  • one or more parameters 500 of the repetitive arrangement of the etching structures can be varied in the example of FIG. 11.
  • Examples include: the lateral structure size, such as the length 262 and / or the width 261 of the etching structures 201, the aspect ratio of the etching structures 201 and the unit cell size 251 (provided the repetitive arrangement of etching structures forms a lattice structure). This in turn can reduce or compensate for an influence of etch ratio-dependent etching and / or an influence of occupation-dependent etching. The influence of an inhomogeneous process gas distribution can also be reduced.
  • FIG. 12 illustrates aspects related to an exemplary method for forming multiple mirrors 150 on a wafer 300.
  • FIG. 12 illustrates various process steps to manufacture mirrors 150 using MEMS techniques. The process steps from FIG. 12 could be implemented, for example, by the method according to FIG. 9.
  • FIG. 12 Only a single mirror 150 is shown in FIG. 12. However, the method according to FIG. 12 can be carried out in parallel for an array of mirrors 150 on the wafer 300 (compare FIG. 10).
  • wafer 300 is first obtained.
  • the wafer 300 has a front side 391 and a rear side 392.
  • Wafer 300 has no etch stop layer.
  • the wafer 300 could, for example, consist of silicon as a semiconductor material, in particular of single-crystal silicon.
  • the wafer 300 is oxidized.
  • Corresponding oxide layers 701, 702 are shown on the surfaces of the wafer 300.
  • the oxide layers 701, 702 serve as masking material for the subsequent DRIE.
  • oxide layers 701, 702 other materials could be used for masking, for example lacquers etc.
  • a lithographic structuring of the oxide layer 701 then takes place in process step 3003, as an etching mask 705.
  • a photoresist can be used in combination with an exposure step through a photomask and then a wet chemical etching step (not shown in FIG. 12).
  • the etching mask 705 of the oxide layer 701 later defines the etching structures 201 and the mirrors 150, as well as further etching structures 280.
  • DRIE is then used in process step 3004.
  • the etch rate through aspect ratio dependent etch and population dependent etch.
  • the etching rate also varies depending on a structure size of the structuring 705.
  • Such effects can also be exploited in order to achieve a particularly simple manufacture of the mirrors 150 - with only one etching mask 705. This can reduce rejects. Manufacturing costs can be reduced.
  • the etching structures 201 and the further etching structures 280 are defined by the etching mask 705.
  • the etching structures 201 are located within the circumference 158 of the mirrors 150 (still to be formed) (compare, for example, FIG. 8); while the further etching structures 280 define the circumference 158.
  • the etching structures 201 and the further etching structures 280 are formed in one and the same DRIE process step 3004. Only one photo mask is used, or only one structuring 705.
  • the width 291 of the further etching structures 280 is larger than the width 261 of the etching structures 201.
  • the lateral structure size and / or the lateral aspect ratio of the further etching structures 280 it would be possible for the lateral structure size and / or the lateral aspect ratio of the further etching structures 280 to be different from the lateral structure size and / or the lateral aspect ratio of the etching structures 201.
  • the etching rate of the DRIE is different for the etching structures 201 and the further etching structures 280.
  • the lateral structure size and / or the lateral aspect ratio of the further etching structures 280 it would be possible for the lateral structure size and / or the lateral aspect ratio of the further etching structures 280 to be changed in this way compared to the lateral structure size and / or the lateral aspect ratio of the etching structures 201 that the etching rate of the DRIE is greater for the further etching structures 280 than for the etching structures 201.
  • the lateral structure size of the etching structures 201 would be in a range in which the etching rate is strongly dependent on the lateral structure size. This would typically be the case for widths 261 and / or lengths 262 in the range from 20 pm to 200 pm.
  • the width 291 of the further etching structures 280 can e.g. are in the range of greater than 200 pm. If intermediate structures 320 are present, these can be constricted by this width 291 e.g. correspond to 50 pm or 20 pm.
  • the depth 281 of the further etching structures 280 is greater than the depth 271 of the etching structures 201 (as is also shown in FIG. 12 in connection with process step 3004).
  • the further etching structures 280 are not formed uniformly along the entire circumference 158 of the mirrors 150.
  • the lateral width 291 of the further etching structures 280 could have a constriction at certain points - the etching rate is then reduced there.
  • the intermediate structures 320 are filled with full or partial interruption of the further etching structures 280 are formed.
  • the intermediate structures 320 are basically optional.
  • the oxide layers 701, 702, or generally the masking material and the etching mask 705, are removed. Wet chemical etching can be used.
  • connection layer 710 can be used for this.
  • process step 3007 the thinning and polishing of the wafer 300 is carried out starting from the front side 391 of the wafer 300. It can be seen from FIG. 12 that the thinning and polishing is carried out to a depth position 399 which is arranged between the depth 281 of the further etching structures 280 and the depth 271 of the etching structures 201, cf. Process step 3008 after completion of thinning and polishing.
  • a further etching process then takes place in process step 3009, typically a wet chemical etching process. This serves to remove the connection layer 710 in the region of the further etching structures 280.
  • the reflective layer 151A is applied, e.g. by steaming.
  • connection layer 710 is dissolved by a suitable etching process. Possibly. the intermediate structures 320 would still have to be removed or severed.
  • DRIE can be used, but due to the thinning, the corresponding time is limited. Only a mask can be used, so that the lithography can be carried out easily and quickly. SOI wafer with Etch stops are not required. DRIE can be used from the back of the wafer - rather than from the front, as with SOI wafers.
  • the mass moment of inertia of the mirror can be set in a targeted manner by varying the etching time and / or the intensity of the thinning / polishing. Fluctuations in the thickness of the wafer can be compensated for. Fluctuations in etching rates can be compensated. The wafer can be weighed before and / or during and / or after the etching.

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  • Engineering & Computer Science (AREA)
  • General Physics & Mathematics (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
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  • Optics & Photonics (AREA)
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Abstract

Selon divers exemples, l'invention concerne un miroir de fabrication particulièrement légère et robuste. Le miroir est adapté pour être balayé par résonance dans un système correspondant, par exemple dans une application LIDAR. Le miroir (150) présente, sur sa face arrière (152), un agencement répétitif de motifs gravés (201). Le miroir peut par exemple être fabriqué en un matériau semi-conducteur.
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