Thermally activated micromirror and fabrication method

A method for fabricating a micromirror in a wafer, including the steps of: depositing and etching layers forming two arms; etching the wafer such that in the back face only a thin portion of the wafer remains in the region of formation of the micromirror and the arms; performing an anisotropic etch, such that the thin portion remains only in the areas of the micromirror and the arms; and performing an isotropic etch to remove the thin portions under the arms, the etching step for forming the arms being performed following their shape and so as to form holes traversing the arms, the holes being positioned at edges of the region separating the micromirror and the wafer on both the side of the micromirror and the side of the portions of the wafer remaining after the anisotropic etching step. The invention also concerns the micromirror.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the national stage application under 35 U.S.C. §371 of the International Application No. PCT/EP2008/058018, and claims the benefit of Int'l Application No. PCT/EP2008/058017, filed Jun. 24, 2008, the entire disclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method for fabricating micromirrors. The present invention also relates to a new thermally activated micromirror structure.

BACKGROUND OF THE INVENTION

Micromirrors formed in semiconductor wafers are used in many devices, such as, for example, medical imaging devices, optical spectrometers and also in barcode readers.

Herein we will consider micromirrors formed from portions of a monocrystalline semiconductor substrate, usually in silicon, fixed to the substrate and capable of being orientated with respect to the substrate by means of thermally deformable arms.

FIGS. 1 and 2illustrate, in plan view and in cross-section taken in the plane A-A′, a micromirror1as described in the article “Thermally actuated micro scanner for bar code reader application”, by F. Khechana, H. Van Lintel, J.-L. Massieu, S. Ackley and P. Renaud, published in Optical MEMS conference 2005.

Micromirror1is fixed to a silicon wafer3by thermally deformable arms5. Arms5extend, in plan view, in a comb shape between wafer3and the micromirror1. The arms5are formed of a portion of continuous conductive track9, which extends between conductive contacts20and21deposited on the wafer3. The conductive track9rests on an underlying insulating layer7. For example, the separation between the wafer3and the micromirror1is between 20 and 40 μm, and the thickness of micromirror1is between 20 and 100 μm. Micromirror1has the shape of a square having sides of one millimetre in length. While a voltage is applied between the two contacts20and21, the conductive line9heats up, and the arms5deform by a bimorph effect which causes the micromirror1to move as illustrated by the arrows inFIG. 2. The arms5are pulled down more when the applied voltage is higher. By regulating the applied voltage, it is thus possible to modify the orientation of the mirror1. The materials of layers7and9are also chosen to provide a desired amplitude of displacement of mirror1. The insulating layer7is for example formed of silicon oxide and the conductive line9of aluminum.

FIGS. 3A to 3Eare partial and schematic cross-section views taken in the plane A-A′ ofFIG. 1and illustrating successive steps in a method of fabricating the micromirror ofFIGS. 1 and 2.

As illustrated inFIG. 3A, we start with a monocrystalline silicon wafer3having a thickness in the order of 300 to 400 μm. A layer7of silicon oxide is formed on the two sides of the wafer3. The layer7has a thickness of around 1 μm.

An aluminum layer9is deposited on the front and back faces of wafer3. The layer9has a thickness of around 1 μm.

In the next steps, the result of which is illustrated inFIG. 3B, on the front face, the stack of layers9and7is partially removed so as to expose the wafer3. The partial removal is performed by means of a mask40illustrated in plan view inFIG. 4. The plane A-A′ of the cross-section ofFIG. 3Bis shown inFIG. 4. The removal is performed such that only the stacks7-9providing the shape of the arms5inFIGS. 1and2remain. The arms5have a width of between 5 and 40 μm, preferably around 30 μm. The contacts20and21can have sides of 100 μm or more in length. Two arms are separated by a gap having the same order of magnitude as the width of the arms. As illustrated inFIG. 3B, on the back face, layer7is removed such that a central part of the wafer3is exposed.

Next, as illustrated inFIG. 3C, a mask50is deposited on the front face.FIG. 5illustrates in plan view the mask50. The plane A-A′ of the cross-section ofFIG. 3Cis shown inFIG. 5. The mask50corresponds to the striped region inFIG. 5. It comprises a central square52linked by arms54to a frame56. The central square corresponds to micromirror1and the arms54to the arms5formed previously.

As illustrated inFIG. 3D, etching is then performed on the back face, of the silicon forming the wafer3. The etch is a wet etch performed with the help of potash (KOH). Etching occurs on back face everywhere where the back face is not protected by the mask provided by insulating layer7. The etch is continued until, with regard to the opening of layer7, only a thin layer of wafer3remains, in the order of 30 to 40 μm, on the front face of wafer3. This thin layer corresponds more or less to the desired thickness of the micromirror.

As illustrated inFIG. 3E, we then proceed with an isotropic etch from the front face. The surfaces of wafer3not protected by the mask50are etched. The micromirror1is thus delimited and only remains linked to the rest of the wafer3by arms5. While not visible in the cross-sections, it should be noted that, during these steps, the regions between the arms5are removed by etching. Simultaneously, the thin portions of the silicon wafer under the arms5are etched, laterally, and are totally removed while the thin portion of wafer which corresponds to the micromirror1is only lightly etched on its large sides. The etch is for example a dry etch performed by means of a plasma containing sulfur hexafluoride (SF6).

FIG. 6Aillustrates the same structure in plan view. The plane A-A′ ofFIG. 6Ais the axis of the cross-section ofFIG. 3E.FIG. 6Bis an enlarged view of a portion ofFIG. 6Aat the level of frame60, in other words at the level of the edge between wafer3and the region separating wafer3from the micromirror. The elements drawn by dashed line inFIGS. 6A and 6Bwill be described below.

The micromirrors obtained according to the above known method present problems. In particular, it has been found that after a certain number of uses or following shock, cracks in the arms result.

A further problem with these micromirrors is the appearance of errors in the orientation of the mirror during the lifetime of the device.

A further problem with these micromirrors is the appearance of defects in the form of cracks in the arms at their fulcrums on the side of the mirror and on the side of the wafer.

A further problem with these micromirrors is the fact that the previous problems are not detectable during test phases but appear during use of the device containing the micromirrors, causing breakdown of the device and imposing particularly difficult maintenance operations on the user.

SUMMARY OF THE INVENTION

The present invention aims at a method of fabricating micromirrors that do not present the problems of the micromirrors obtained by known methods.

According to one aspect of the present invention, there is provided a method of fabricating, in a monocrystalline silicon wafer, a micromirror linked by at least two thermally deformable arms, comprising the following steps:

depositing and etching, on the front face of the wafer, layers forming the arms;

etching the wafer in a region of the back face such that only a thin portion of the wafer remains in the region of formation of the micromirror and the arms;

performing an anisotropic etch to delimit the micromirror and the wafer, such that the thin portion of the wafer remains only in the areas of the micromirror and the arms; and

performing an isotropic etch to remove the thin portion of the wafer under the arms; the etching step for forming the arms being performed following the shape of the arms and so as to form holes traversing the arms, the holes having a diameter less than the width of the arms and being positioned at edges of the region separating the micromirror and the wafer on both the side of the micromirror and the side of the portions of the wafer remaining after the anisotropic etching step.

According to an embodiment of the present invention, before the step of forming the arms, before the anisotropic etching step or before the isotropic etching step, parallel metallic lines are formed in the area of the formation of the micromirror, outside the region for fixing the arms.

According to another embodiment of the present invention, the metallic lines are formed by pulverization or vaporization of at least an insulating and/or metallic material using a hard mask.

According to another embodiment of the present invention, the anisotropic etching is a dry etch performed by means of a plasma comprising octafluorobutene and sulfur hexafluoride.

According to another embodiment of the present invention, the isotropic etch is a dry etch preformed using a plasma comprising sulfur hexafluoride.

According to an aspect of the present invention, there is provided a micromirror linked by at least two thermally deformable arms to a monocrystalline silicon wafer, each arm having traversing holes at locations at the edges of the region separating the micromirror and the wafer on both the side of the micromirror and the side of the wafer, the traversing holes having a diameter less than the width of the arms.

According to an embodiment of the present invention, each arm comprises an insulating layer and an overlying conductive layer.

According to another embodiment of the present invention, each arm has a width of between 10 and 100 μm and the holes have a diameter of between 2 and 50 μm.

According to an embodiment of the present invention, the micromirror comprises a network of parallel lines.

According to another aspect of the present invention, there is provided a bar code reader having at least one micromirror according to any of the preceding embodiments.

According to another aspect of the present invention, there is provided a spectrometer comprising at least one micromirror according to any of the preceding embodiments.

DETAILED DESCRIPTION OF THE INVENTION

For clarity, as is generally the case in representation of microsystems, the various figures are not drawn to scale.

The present invention is based on studies performed by the inventors into the origins of defects observed during operation. In particular, the inventors have studied photographs of the structure obtained by the known method described above.

As illustrated by the dashed lines inFIG. 6Aand the enlargement ofFIG. 6B, after the isotropic etch of the silicon performed during the step described in relation toFIG. 3E, the inventors have found that a point of silicon65remains under each portion of arms5at the fulcrum of the arms, on the side of the micromirror as well as on the side of the wafer.

The inventors have determined that these points65are the origin of the problems observed during operation because they cause wearing and/or detachment of arms5and provoke defects in arms5both on the side of the micromirror1and on the side of the wafer3.

It is believed by the inventors that the formation of points65is inherent in the isotropic etching process of the silicon under the arms5.

To avoid the formation of points65, an embodiment of the present invention provides the method described in relation to the cross-section views ofFIG. 7A-E, the plan views ofFIGS. 8 and 9and the plan view ofFIG. 10A, of which a part100is enlarged inFIG. 10B.

As illustrated inFIG. 7A, we start with a monocrystalline semiconductor wafer3. Wafer3is for example a silicon wafer having a thickness of between 300 and 400 μm. The wafer3is covered on its back face by a layer7and on its front face by a stack of the layer7and a conductive layer9. The layers7and9are made of materials suitable for forming a bimorph. The layer7is for example an insulating layer. The layer7is the result for example of a thermal oxidation of the surfaces of wafer3. The layer7can equally result from the deposition of a silicon oxide (SiO2) layer or an alternative insulator such as silicon nitride (Si3N4). Alternatively, layer7can have a multi-layer structure comprising several insulating materials such as silicon oxide (SiO2) or silicon nitride (Si3N4). Layer7has a thickness in the order of 1 to 3 μm, for example 1 μm. The conductor9is a material of low resistance, preferably metallic, suitable for forming a bimorph actuator with the underlying insulating layer7. The layer9is for example a layer of aluminum having a thickness of between 1 and 3 μm, for example around 1 μm.

In the next steps illustrated inFIG. 7B, on the front face, the stack of the conductor9and insulator7is etched such that only a shape80, shown in plan view inFIG. 8, remains. The shape80is the desired shape of the arms5. The arms5have holes70, which completely traverse the stack of layers7and9. The top surface of wafer3is visible at the end of the holes70. The holes70have a diameter less than the width of the arms5. The diameter of the holes70is chosen such that the resistance of the arms5is not compromised. For example, for arms5having a width of between 10 and 100 μm, preferably around 20 μm, the diameter of the holes70is in order of 2 to 50 μm, preferably 6 μm. The holes70are formed at places that correspond, on both the side of micromirror1and also the side of the wafer3, to the edges of a region formed later in the process, which separates the wafer3from the micromirror1.

Next, as illustrated inFIG. 7C, a mask90illustrated in plan view inFIG. 9is deposited on the front side. Mask90corresponds to the striped regions inFIG. 9. Mask90includes a central square92linked by arms94to a frame96. The central square92corresponds to the micromirror1and the arms94to the arms5formed previously. Mask90further has holes98at points between the arms94and, on one side, the central portion92, and on the other side, the frame96. The holes98are aligned with the holes70.

As illustrated inFIG. 7D, an etch is then performed, for example a wet etch using potash, such that the wafer3is deep etched from the back face such that only a thin portion of silicon having a thickness of between 30 and 40 μm, for example 40 μm, remains on the front face.

As illustrated inFIG. 7E, an anisotropic silicon etch is then performed, for example, a dry etch performed by means of a plasma containing sulfur hexafluoride (SF6) and octafluorobutene (C4F8). On the front face, the silicon is etched according to the shape of mask90. The etch thus extends to the region separating the central square92from the frame96and in the regions separating the arms5and the holes98. The silicon is thus etched around the micromirror1and in the holes70traversing the arms5. The holes70are thus extended in the silicon to traverse the wafer3. The holes70traverse the silicon at the edges of the region separating the wafer3and the micromirror1. The regions separating two adjacent arms are formed during this step. Between the wafer1and the micromirror1only the portions of silicon adjacent and underlying the arms5and the portions penetrated by holes70remain.

An isotropic etch is then performed, the result of which, after etching mask90on the front face and layer7on the back face, is represented in cross-section view inFIG. 7Fand in plan view inFIG. 10A, of which a portion100is enlarged inFIG. 10B. The etch is for example a dry etch performed by means of a plasma comprising sulfur hexafluoride (SF6). The etch extends under the arms5simultaneously laterally from each side, from below and from the edges72of holes70. At holes70, the etch thus follows the perimeter72and confers a rounded contour74to the silicon at the level of the fulcrums of arms5at the edge of wafer3and at the edge of the micromirror.

The extension of the etch to the thin portion of the wafer3via the holes70allows the known formation of points65ofFIGS. 3E,6A and6B to be avoided.

The removal of points65allows problems observed in the known structures to be avoided.

According to a further embodiment of the invention illustrated in plan view inFIG. 11, a network107of parallel lines is formed on the top surface of micromirror1. The network107is formed outside the region in which the arms5are fixed. The parallel lines of network107are of a number, a width and a spacing chosen to form a diffraction grating adapted for diffracting a light beam received by the micromirror to a beam having one or a number of predetermined wavelengths. The network107can be formed on the micromirror1such that the wavelength of the light beam reflected by the micromirror depends on the orientation provided by arms5.

According to an embodiment of the invention, the network107results from a modification of mask90such that the network is formed at the same time as arms5. The parallel lines forming the network thus comprise the stack of the layers9and7.

According to a further embodiment of the invention, the network107is formed separately from the formation of arms5. The network107results thus from a specific sequence of deposition of a metallic or an insulating material.

According to a further embodiment, the network107is deposited by means of a shadow mask method wherein at least one material is sprayed or vaporized through holes of a hard mask deposited on the structure.

The parallel lines of the network107are for example made of a metal such as gold. Alternatively, they are made of an insulator such as silicon oxide.

According to an embodiment, the sequence for forming network107is implemented before the formation of arms5, in other words before depositing and etching the layers7and9.

According to a further embodiment, the network107is formed after the formation of arms5, but before depositing mask90.

A number of particular embodiments of the present invention have been described. Various alternatives and modifications will appear to those skilled in the art. Furthermore, those skilled in the art will understand that the shape of different parts, in particular the arms and the micromirror, are not limited to the shapes represented. Furthermore, inFIGS. 1 to 11, it is considered that the isolated line7-9that forms arms5has alternate fulcrums on the micromirror1and the wafer3. However, as illustrated in the plan view ofFIG. 12, the line7-9can present more alternations in the interval separating the micromirror1and the wafer3than fulcrums on the micromirror and/or on the wafer. Each fulcrum of the arms5on the micromirror1or the wafer3is associated with a traversing hole70.

Those skilled in the art will also understand that the hereabove described materials can be modified. The conductive line9can thus be made of any metal different from aluminum as far as it is suitable for being deposited by PVD, such as copper, tungsten, molybdenum, or other metals. The layer7is described as an insulating layer such as silicon oxide, silicon nitride or a multi-layer structure comprising multiple insulating layers. However, layer7can be any insulating or conductive layer or a multi-layer structure comprising multiple insulating and/or conductive layers as long as layer7forms a bimorph together with layer9.

Similarly, when formed independently from arms5, the network107is described as being made of parallel lines of a metal or an insulator. However, the lines can comprise a multi-layer structure comprising several conductive or insulating layers. The multi-layer structure can also comprise conductive and insulating layers.

Such alterations, modifications and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalent thereto.