Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B81—MICROSTRUCTURAL TECHNOLOGY
  • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
  • B81C1/00—Manufacture or treatment of devices or systems in or on a substrate
  • B81C1/00436—Shaping materials, i.e. techniques for structuring the substrate or the layers on the substrate
  • B81C1/00444—Surface micromachining, i.e. structuring layers on the substrate
  • B81C1/00468—Releasing structures
  • B81C1/00484—Processes for releasing structures not provided for in group B81C1/00476
• • 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
  • G02B26/001—Optical devices or arrangements for the control of light using movable or deformable optical elements based on interference in an adjustable optical cavity
• • 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
  • G02B26/002—Optical devices or arrangements for the control of light using movable or deformable optical elements the movement or the deformation controlling the frequency of light, e.g. by Doppler effect
• • 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
  • G02B26/007—Optical devices or arrangements for the control of light using movable or deformable optical elements the movable or deformable optical element controlling the colour, i.e. a spectral characteristic, of the light
• • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B81—MICROSTRUCTURAL TECHNOLOGY
  • B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
  • B81B2203/00—Basic microelectromechanical structures
  • B81B2203/01—Suspended structures, i.e. structures allowing a movement
  • B81B2203/0118—Cantilevers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B81—MICROSTRUCTURAL TECHNOLOGY
  • B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
  • B81B2203/00—Basic microelectromechanical structures
  • B81B2203/03—Static structures
  • B81B2203/0315—Cavities
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B81—MICROSTRUCTURAL TECHNOLOGY
  • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
  • B81C2201/00—Manufacture or treatment of microstructural devices or systems
  • B81C2201/01—Manufacture or treatment of microstructural devices or systems in or on a substrate
  • B81C2201/0101—Shaping material; Structuring the bulk substrate or layers on the substrate; Film patterning
  • B81C2201/0111—Bulk micromachining
  • B81C2201/0114—Electrochemical etching, anodic oxidation

Definitions

• the present invention generally relates to the methods of manufacturing integrated microstructures from a substrate of semiconductor material by micromachining techniques.
• the invention relates to a process for manufacturing mobile suspended microstructures actuable from monocrystalline silicon, in particular by the techniques of silicon porosification and electropolishing.
• the invention also relates to an integrated microstructure comprising a suspended element made of porous silicon.
• the invention also relates to an adjustable optical element, in particular a dielectric optical filter which can be used for example in spectrophotometers.
• spectrophotometers are used for example for colorimetry, gas detection and analysis of liquids by infrared spectrometry
• microstructures such as pressure sensors, accelerometers, micro-activators, micro-pumps, etc. by the masking and etching techniques of silicon is well known.
• These microstructures generally have complex three-dimensional geometries, for example comprising suspended structural elements.
• the manufacture of these elements generally requires the implementation of a large number of stages, in particular of successive deposits and eliminations of sacrificial layers which involve numerous levels of masks, which makes the manufacture of these microstructures complex. , long and expensive.
• the substrate covered with the layer of structured Si3N4 is subjected to an initial chemical etching in a potassium hydroxide solution to etch the silicon discovered and to form trench primers with a V-shaped cross section.
• An electrochemical etching is then carried out. anisotropic silicon in an electrolytic solution containing HF under illumination to form trenches with rectangular cross section. Once the desired depth is reached, the current density flowing in the solution is increased, which puts the device in an isotropic etching mode and allows the widening of the base of the trenches until the adjacent trenches meet from below and release the beams. These beams thus released form a suspended passive structure which is movable in a plane parallel to the substrate.
• the formation of nanoporous silicon is not sought at any time in the process described in this publication and is even avoided since the formation of porous silicon during the process would damage the vertical surfaces of the trenches.
• the openings of the mask Si3N4 form a matrix network, in order to produce blocks or suspended plates.
• the elements obtained however have a network of perforations.
• this method Although requiring a small number of masks, this method nevertheless has several drawbacks. In fact, this process is limited to the production of passive mobile structures, that is to say structures which cannot be actuated by control means. Furthermore, this method only allows the production of perforated planar structures, which considerably limits its field of application, in particular for optical applications for which it is generally important to have plates having surfaces free of holes. Another disadvantage consists in that this method does not allow plane movable structures to be produced outside the plane of the substrate.
• the main object of the present invention is to remedy the drawbacks of the above-mentioned prior art by providing a simple and inexpensive process for the manufacture of such mobile suspended microstructures.
• the subject of the invention is a method for manufacturing a mobile suspended microstructure of generally planar or curved shape, from a substrate made of semiconductor material, said substrate having a substantially planar upper surface, characterized in that it includes the following steps:
• the method comprises the additional steps consisting in:
• the structure obtained advantageously forms a bimetallic strip which can be active, for example electrically, to move the mobile microstructure around its rest position.
• the porosification step is carried out with a current profile which makes it possible to create a porosity profile determined according to the direction of the thickness of the suspended microstructure.
• the suspended structure can easily and economically form adjustable optical elements such as Bragg mirrors of the selective band reflector type or bandpass filters (Fabry-Perot filters).
• the angle formed by the suspended structure with the plane of the substrate can be easily adjusted by the activation means described above to adjust the wavelengths that the filters or mirrors respectively transmit and / or reflect.
• the method of the invention makes it possible to open up new applications in the field of micro-optics.
• the subject of the invention is a microstructure comprising a suspended plate, of generally planar or curved shape, said plate being attached to a support structure by at least one of its edges so as to be movable outside the plane. of the support structure, characterized in that said plate is made of porous semiconductor material, oxide or not.
• the invention relates to an optical element, in particular a mirror or a filter, characterized in that it is in the form of a suspended plate of generally planar shape, said plate being attached to a support structure by at least one of its edges to be movable outside the plane of the support structure, said plate being made of porous semiconductor material, oxide or not.
• this optical element it further comprises activation means for adjusting the angle of inclination formed by the plate with the plane of said support structure.
• FIG. 1 shows a perspective view of an activatable suspended optical filter obtained according to the method of the invention
• FIGS. 2 to 8 are sectional views along lines A-A and B-B of FIG. 1 respectively showing a first and a second part of the optical filter at different stages of the manufacturing process according to the invention
• FIGS. 9 and 10 are partial perspective views of the optical filter of FIG. 1, corresponding respectively to the steps illustrated in FIGS. 7 and 8,
• FIG. 11 is a plan view of the optical filter of FIG. 1 corresponding to the process step illustrated in FIG. 5.
• - Figure 12 is a top view of another embodiment of a suspended optical filter obtained according to the method of the invention.
• - Figures 1 and 14 are sectional views along line C-C of Figure 12 respectively showing the optical filter at different stages of the manufacturing process according to the invention.
• an adjustable optical element of generally planar shape such as a band pass filter or a selective band reflector, the inclination of which with respect to the plane of the support structure can be adjusted to adjust the wavelengths that the reflector or the filter respectively transmit and / or reflect.
• an adjustable optical element of generally planar shape, such as a band pass filter or a selective band reflector, the inclination of which with respect to the plane of the support structure can be adjusted to adjust the wavelengths that the reflector or the filter respectively transmit and / or reflect.
• such a method can find many other interesting applications, for example for making adjustable shutters, micromanipulators, etc.
• the filter 1 comprises a plate 2 of generally planar and rectangular shape suspended from a support structure or substrate 4 .
• the plate 2 is suspended by a pair of parallel arms 6, 8 so as to be movable outside the plane P of the support structure.
• a first end 6a, 8a of the arms is attached respectively to two opposite dimensions of the plate 2, while their other end 6b, 8b is attached to the support structure 4 in an edge region.
• the plate 2 and the arms 6, 8 extend over a cavity 10 hollowed out in the support structure 4 and, when at rest, the plate 2 forms an angle inclination A with the upper surface P of the support structure 4.
• the angle A essentially depends on the length and thickness of the arms 6 and 8 as well as on the materials present in them .
• the angle A is of the order of 20 °, but this angle can in other configurations reach much higher values notably exceeding 90 °.
• the suspended parts of this microstructure that is to say the plate 2 and the arms 6 and 8, are made of a porous semiconductor material, for example porous silicon by the method according to the invention which will be described in detail below.
• the porosity of the plate 2 in the direction of its thickness is produced according to a determined profile which adapts to the nature of the filter desired.
• the optical filter 1 further comprises activation means 12 for adjusting the angle of inclination A around its rest position and thus forming an orientable filter.
• the activation means 12 comprise a metallization layer forming on the one hand contact pads 14 and on the other hand conductive tracks 16 connected together.
• the contact pads 14 are preferably formed outside the contour of the suspended elements, for example on the surface of the support structure 4, while the conductive tracks 16 partially overlap above suspended portions of the structure .
• the conductive tracks 16 extend in a sinuous trace above the arms 6 and 8, almost over their entire length. These tracks are typically made of gold but can be made of any electrically conductive material.
• the material forming the contact pads 14 and the conductive tracks 16 will be chosen from the group comprising gold, platinum, copper and nickel. It can also be seen in FIG. 1 that an intermediate layer 18 extends under the entire surface of the contact pads 14 and of the conductive tracks 16.
• the layer 18 results from a particular embodiment of the manufacturing method of the invention which will be described below and has no particular function in the operation of the filter!
• the arms 6 and 8 with the metallization layer and, where appropriate, the overlying intermediate layer 18 form, because of their difference in coefficient of thermal expansion, a bimetallic strip which, under the influence of the temperature, generates a curvature of the arms 6 and 8 and causes a corresponding displacement of the plate 2.
• the plate 2 can be actuated by applying voltages of the order of 5 volts to the connection pads. It will be noted that this activation can lead the plate to mechanical resonance, that is to say to oscillate from its rest position. Depending on the geometry of the suspended microstructure, the resonance frequency can typically vary between 100 and 3000 Hz and in the resonant state the amplitude of variation of the angular position of the plate 2 can reach 30 °.
• the suspended microstructure which has just been described can undergo various modifications without departing from the scope of the present invention.
• the plate 2 can be suspended from the support structure 4 by a portion of its peripheral edge, the conductive tracks 16 then extending, in a sinuous manner, along said edge portion in question, while encroaching on a suspended edge area of the plate 2.
• the plate 2 can also be curved. It will also be noted that it is possible to provide several conductive tracks which can be activated independently, for example to twist the plate 2.
• the thickness of the plate 2 is of the order of 30 ⁇ m, its surface is of the order of 0.5 mm 2 and the length of the arms of the order of 500 ⁇ m.
• the angle A can vary between 0 ° and 180 °.
• the plates (not shown) defining the substrate from which the optical filters are manufactured are made of a semiconductor material, typically of monocrystalline silicon doped p.
• FIG 2 there is shown the substrate 4 previously polished on at least its upper face 30, on which has been deposited over its entire surface a layer 32 which will serve as a mask or masking layer for subsequent porosification steps and electropolishing of the substrate 4.
• Silicon nitride (Si3N4) will preferably be used to make the layer 32 insofar as the Si3N4 resists hydrofluoric acid (HF) well with which the porosification and electropolishing steps will be carried out .
• the layer 32 was deposited by a chemical vapor deposition (CVD) process over a thickness of approximately 200 nm on both sides of the substrate, then the Si3N4 deposited on the underside 34 was removed , for example by a dry attack with SF6.
• the next step shown in FIG. 3 consists in depositing a metallization layer 36 over the entire surface of the layer 32. This layer is for example deposited by evaporation under vacuum.
• an adhesion layer 38 has conventionally been previously deposited, the nature of which naturally depends on that of the metallization layer 36.
• the metallization layer 36 is a layer of gold (Au) and the adhesion layer 38 is a layer of Nickel-Chromium (Ni-Cr). Typically layers 38 and 36 are deposited respectively on thicknesses of 10nm and 800nm.
• the metal chosen to form the metallization layer 36 advantageously forms part of the assembly comprising gold, platinum, copper and nickel.
• the layer 36 is structured to form the activation means 12 of the microstructure to be manufactured, in this case of the plate 2, already described in connection with the figure. 1. In other words, the contact pads 14 and the conductive tracks 16 are defined (FIG. 1).
• a photoresist layer (not shown) is deposited on the metallization layer 36, for example using a spinner, the photoresist is exposed through a mask (also not shown), the opening of which defines the contour of the contact pads. 14 and conductive tracks 16 desired, the exposed parts of the photoresist are conventionally developed by the wet method in order to discover the parts of the metallization layer 36 to be eliminated, the exposed parts of the layer 36 are etched, the parts of the layer d membership 38 having been discovered by the previous etching, then the remaining parts of the photoresist layer are eliminated.
• the etching operation of the gold layer is carried out in a potassium iodide solution
• the etching operation of the NiCr layer is carried out, after rinsing and prior drying, in a solution wet attack, for example a solution of Ce (NH4) 2 (NO) 3 and CH3COOH
• a solution wet attack for example a solution of Ce (NH4) 2 (NO) 3 and CH3COOH
• the removal of the photoresist layer is carried out conventionally, for example, by a solvent.
• the structuring of the layer 36 is carried out so that part of it, in this case the conductive tracks 16, at least partially encroaches on the surface which will later form the surface of the suspended microstructure, the arms 6 and 8 in the example described.
• the structuring will be carried out so that the conductive tracks 16 (FIG. 1) are connected to the contact pads 14 situated outside the surface which will later form the surface of the suspended microstructure.
• openings 40 and 42 which together substantially define the extent of the desired suspended microstructure. These openings 40, 42 thus uncover the parts of the substrate 4 corresponding to this extent. It will be noted that openings 42 are at least arranged around the conductive tracks 16.
• the method of structuring this layer 32 is similar in principle to that described in connection with FIG. 4. It will nevertheless be specified that this structuring consists in eliminating by a dry attack with SF6 the masking layer 32 at the places not protected by the photoresist layer previously exposed through a mask of suitable shape to make the openings 40 and 42.
• the covered parts of the structured layer 36 are protected, that is to say the conductive tracks 16 and the contact pads 14.
• the next step illustrated in FIG. 6 consists in electrochemically porosifying the parts of the substrate 4 discovered in the previous step, that is to say the parts appearing through the openings 40 and 42.
• the porosification of the substrate is carried out on a thickness corresponding substantially to the desired thickness of the plate 2 to be manufactured.
• the substrate is mounted on a support structure and then has between the electrodes, a double electrochemical cell containing a solution of electrolyte containing 50% by weight of hydrofluoric acid and ethanol in a volume ratio 1: 1.
• the substrate is then subjected for a few minutes to anodic polarization with a current density of the order of 100mA / cm 2 to porosify the silicon appearing through the openings 40 and 42, to the desired thickness.
• the porosified silicon is designated by the reference 44 in Figures 6 and following.
• the pore size, the pore density and the thickness of the porosified layer depend on the nature of the substrate 4 and on the process parameters used.
• the size of the pores is of the order of the pressure gauge and their density is of the order of 50%.
• the thickness of the plate 2 is typically of the order of 30 ⁇ m.
• this porosification step it is possible during this porosification step to modulate, as a function of time, the density of the porosification current in order to define successive layers of different porosity according to the direction of the thickness of the porosified layer.
• this variant is of particular interest in the context of the production of optical filters formed by a stack of layers of different refractive indices such as for example Bragg filters.
• the technique of modulating current densities to obtain such stacks of porous layers of different refractive indices are already described in patents DE 4,319,413 and US 5,696,629 which are incorporated in the present description by reference.
• a masking layer (not shown) can be formed on the rear face of the substrate in order to homogenize the distribution of the current in the porous silicon during the porosification stage.
• This masking layer is preferably made of silicon nitride (Si 3 N 4 ) and advantageously but without limitation, has a configuration which is the mirror image of the mask 32 of Si 3 N 4 which has been previously deposited on the front of the substrate.
• the next step illustrated by FIGS. 7 and 9 consists in electropolishing, that is to say electrochemically eliminating, the silicon underlying the porosified layer 44 to form the cavity 6 which surrounds the plate 2 and the arms 6, 8 below the level of layer 32 and thus achieve the desired suspended structure.
• the openings 42 are arranged so that the dimensions D of the portions of the masking layer 32 separating two consecutive openings are less than twice the thickness Z silicon to be porosified, which corresponds to the thickness of the plate to be manufactured.
• the plate 2 and the arms 6, 8 are still held only by a bridge formed by the masking layer 32 along the respective contour of the plate 2 and of the arms 6, 8 which connect them to the substrate 4 as well as, where appropriate, by porous silicon which has not been electropolished due to the low current density which can be achieved with a particular arrangement of the openings 42 in the layer 32 in the connection or suspension zone arms to the support structure.
• the next step illustrated in FIGS. 8 and 10 consists in releasing the suspended three-dimensional structure formed from the plate 2 and the arms 6 and 8 from the substrate 4, in order to give the plate mobility outside the plane of the substrate. This step therefore consists in eliminating the portions of the masking layer 32 lying along the contour of the plate 2 and of the arms 6, 8.
• the plate 2 is suspended from the substrate by means of the arms 6, 8 which are themselves partly connected to the substrate 4 by the masking 32 and metallization layers 14, 16 which extend both above the arms 6, 8 and of the substrate 4.
• the masking 32 and metallization layers 14, 16 which extend both above the arms 6, 8 and of the substrate 4.
• a portion of porous silicon being under these parts of 32 and 36 and this to strengthen the mechanical strength of the suspended structure.
• the substrate 4 is maintained in the electrolyte of the double electrochemical cell while reversing the polarity of the cell with respect to that used during the porosification and electropolishing steps, this in order to protect the already porosified structure.
• the suspended structure namely the plate 2 and the arms 6,8 straighten themselves out of the plane P of the substrate 4 due to the internal stresses of the different materials forming the structure.
• the release step may be followed by a step of oxidizing the surface of the suspended microstructure obtained, for example by placing the substrate in an oven at around 900 ° C under an oxidizing atmosphere for a few hours.
• This optional oxidation step notably makes it possible to reduce the refractive index of the plate 2, to eliminate the absorption of visible light and to induce additional stresses which tend to increase the angle A (FIG. 1) at rest. Furthermore, this oxidation step advantageously makes it possible to prevent the aging of the porous silicon.
• the metallization layer 36 can be deposited before the masking layer 32. In this case, care will however be taken to structure the metallization layer 36 before the masking layer is deposited. It is also possible to replace the thermal actuation means with electrostatic actuation means. In this case, a conductive layer forming a first frame of a capacitor, the second frame being formed by the substrate, will be deposited on at least part of the suspended structure.
• FIGS. 12 to 14 show the case where the desired suspended microstructure has the shape of a square or rectangular plate 2 suspended from the support structure 4 by a portion of one of its edges 50.
• the elements identical to those described in connection with Figures 1 to 11 have been designated by the same reference numerals.
• connection of the plate 2 to the substrate 4 connection normally provided by portions of the masking layer 32 surmounted by the conductive tracks 16, has been reinforced.
• This can be obtained by locally decreasing the current density during the porosification step by providing openings or windows 42 in parts of the masking layer 32 located in the connection area 52 of the plate 2 to the substrate 4, which here has the form of a strip extending above the edge 50 of the plate 2. If the openings 42 in this connection zone 52 are spaced apart by a width D, the underlying silicon lying between the openings 42 will be completely porosified as soon as the lateral porosification reaches DI2.
• the conductive tracks 16 forming the activation means preferably have a sinuous trace delimiting intermediate zones in which the openings 42 are advantageously formed.

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• Manufacturing & Machinery (AREA)
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• Astronomy & Astrophysics (AREA)
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• 1999
  • 1999-09-10 EP EP99117852A patent/EP1088785A1/de not_active Withdrawn
• 2000
  • 2000-09-05 EP EP00960604A patent/EP1263675A1/de not_active Withdrawn
  • 2000-09-05 WO PCT/EP2000/008655 patent/WO2001019723A1/fr active Application Filing

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