Process for fabricating a micro-electro-mechanical system with movable components

A process for fabricating a micro-electro-mechanical system (MEMS) composed of fixed components fixedly supported on a lower substrate and movable components movably supported on the lower substrate. The process utilizes an upper substrate separate from the lower substrate. The upper substrate is selectively etched in its top layer to form therein a plurality of posts which project commonly from a bottom layer of the upper substrate. The posts include the fixed components to be fixed to the lower substrate and the movable components which are resiliently supported only to one or more of the fixed components to be movable relative to the fixed components. The lower substrate is formed in its top surface with at least one recess. The upper substrate is then bonded to the top of the lower substrate upside down in such a manner as to place the fixed components directly on the lower substrate and to place the movable components upwardly of the recess. Finally, the bottom layer of the upper substrate is removed to release the movable components from the bottom layer for floating the movable components above the recess and allowing them to move relative to the lower substrate, while keeping the fixed components fixed to the top of the lower substrate.

TECHNICAL FIELD

The present invention relates to a process for fabricating a micro-electro-mechanical system (MEMS) composed of fixed components fixedly supported on a base and movable components movably supported on the base.

BACKGROUND ART

Japanese Patent Publication No. 03-230779 discloses a movable micromechanical system fabricated through micro-fabrication technology. The system includes fixed components and movable components both of which are formed from a common silicon substrate and are supported on a base made of a glass or semiconductor material. The common silicon substrate is etched to a limited depth or within a surface layer to form a plurality of posts which project commonly from a remainder layer of the silicon substrate. The posts include the fixed components and the movable components which are resiliently supported to one or more of the fixed components to be movable relative thereto. The silicon substrate is then bonded to the base with the fixed components being placed directly on top of the base and with the movable components being spaced from the top of the base. Thereafter, the silicon substrate is etched to remove the remainder layer or the common platform to release the fixed and movable components from the common platform such the movable components are free to move relative to the fixed components and therefore to the base. In order to assure the movable components free to move without being interfered with the base, it is required to reduce the height of the movable components in relation to the fixed components to give a sufficient gap between the top of the base and the movable components. Due to this structural requirement, the fixed components directly bonded to the base have to be designed to have a height much greater than the movable components. That is, as the gap is required to be greater for assuring the movable components free from being interfered with the base, the height of the fixed components are made to have greater height, which increases the overall height of the micromechanical system and therefore detract from the compactness generally expected to the system.

DISCLOSURE OF THE INVENTION

In view of the above insufficiency, the present invention has been accomplished to provide a unique process for fabricating a micro-electro-mechanical system (MEMS) composed of fixed components fixedly supported on a base and movable components movably supported on the base. The process utilizes an upper semiconductor substrate and a lower substrate which defines the base. A top layer in the upper substrate is selectively etched to form therein a plurality of posts which project commonly from a bottom layer of the upper substrate. The posts include the fixed components to be fixed to the lower substrate and the movable components which are resiliently supported only to one or more of the fixed components to be movable relative to the fixed components and the lower substrate. The lower substrate is also etched in its top surface to form therein at least one recess. The upper substrate is then bonded to the top of the lower substrate upside down in such a manner as to place the fixed components directly on the lower substrate and to place the movable components upwardly of the recess. Finally, the bottom layer of the upper substrate is removed to release the movable components from the bottom layer for floating the movable components above the recess and allowing them to move relative to the lower substrate, while keeping the fixed components fixed to the top of the lower substrate. By provision of the recess in the tops surface of the lower substrate, the fixed components can be sized to have the same height as the movable components and therefore not required to have an extra height for floating the movable components, thereby reducing the overall height of the system to give a low-profile microstructure.

The bottom layer of the upper substrate may be removed firstly by abrasion and subsequently by etching for facilitating to release the fixed and movable components.

Preferably, the upper substrate is of a SOI (silicon on insulator) structure having a buried oxide layer extending between the top layer and the bottom layer so that the resulting fixed and movable components are be supported on the bottom layer through the buried oxide layer. The bottom layer and the buried oxide layer are removed after the upper substrate is bonded to the lower substrate. In this case, the buried oxide layer can be utilized as a barrier to stop etching the bottom layer with respect to a specific etching method which is effective to remove the bottom layer but not to the oxide layer. This makes it possible to utilize the above specific etching method to remove the bottom layer and to utilize another etching method for removing the oxide layer for optimizing the step of releasing the components. This is particularly advantageous when the bottom layer is preliminary abrade or polished roughly to varying depths for expediting the step of removing the bottom layer, since the subsequent etching with the specific etching method can be stopped at the buried oxide layer irrespective of that the remaining bottom layer suffer from the different thicknesses. The buried oxide layer is preferably removed by the dry etching different from the above specific etching method. In this connection, it is preferred that all of the fixed and movable components are formed to have uniform height standing from the buried oxide layer.

Further, it is preferred that the fixed and movable components projecting on the buried oxide layer are covered with an oxidized coat. With the application of the oxidized coat, the components, which are likely to suffer from serrations or surface irregularities caused at the time of etching the upper layer, can be smoothed. The oxidized coat has a thickness less than the buried oxide layer, and is removed prior to the bonding of the supper substrate to the lower substrate, leaving the buried oxide layer in the upper substrate for use as the barrier.

In order to make the step of bonding the upper substrate to the lower substrate successfully, at least one of the upper and lower substrate is formed at the interface therebetween with a groove that extends to the exterior of the system from within an interior space confined between the upper and lower substrates. The groove acts to escape the air entrapped between the upper and lower substrates at the bonding thereof, enabling to register the upper substrate to the lower substrate successfully and precisely.

The movable components may be formed to have a height shorter than that of the fixed component to give an increased gap in combination with the recess as necessary.

In order to provide the short movable components by etching, the upper substrate may be covered with a mask composed of a first mask covering a portion later formed into the fixed component (30) and a second mask covering a portion later formed into the movable component and also the first mask. The composite mask is etched together with the top layer of the upper semiconductor substrate to such an extent as to reduce the height of the movable component relative to that of the fixed component. Thus, the composite mask acts as an etching depth adjustor to differentiate the height of the fixed and movable components. The first mask is preferably made from a material which is etched at a low etching rate the second mask.

The lower substrate is preferably be covered on its top surface with a dielectric layer for electrically isolating the components from the lower substrate, enabling to electrically insulate particular one or ones of the components from the other. When the lower substrate is made of a semiconductor material, the dielectric layer may be formed by oxidizing the top surface of the substrate.

For protecting the components from being attacked during the step of etching away the bottom layers of the upper semiconductor substrate, it is preferred to cover the parts with an etching-shield prior to bonding the upper substrate to the lower substrate. The etching-shield may be formed by thermally oxidizing the surfaces of the post. In this case, the etching shield is firstly formed on the entire exposed faces of the components and is removed after the components are released from the bottom layer.

The components are in many cases designed to be spaced by different inter-distances so that cavities of different widths are to be left between the adjacent ones of the components after the top layer of the upper substrate is etched. During the etching, the growing cavities of greater width are very likely to be etched to a greater depth, which causes the finished cavities to have different depths. In such case, the bottoms of the cavities are not aligned in the same level, necessitating complicated and cumbersome control in the step of accurately etching away the bottom layer of the upper substrate for releasing release the fixed and movable components from the bottom layer. Therefore, it is practically desirable to align the bottoms of the cavities in the same level for terminating the etching simply at this level when releasing the components, irrespective of the design requirement of spacing the components at varying inter-distances. The present invention gives one approach to align the bottom of the cavities by introducing dummy projections between the components spaced by a large distance from each other. The dummy projections are formed in the top layer of the upper substrate together with the components at such a location as to leave, between the adjacent ones of the dumpy projections and the components, the cavity of which width is generally equal to the that of the remaining cavity or cavities. Thus, the top layer can be etched to the same depth so as to align the bottoms of all the resulting cavities. For this purpose, the dummy projections are selected to have having a width smaller than the components (30,40) and are anchored to the buried oxide layer. The buried oxide layer confined between the dummy projections and the bottom layer is etched away to release the dummy projections, before the upper substrate is bonded to the lower substrate.

Further, the present invention gives a control of etching away the bottom layer to release the fixed and movable components successfully, even in the presence of the cavities of different widths or depths. With the presence of the cavities of the different depths, the bottom layer is required to be etched to different depths at portions corresponding to the cavities for successfully releasing the components from the bottom layer. In a preferred embodiment of the present invention, it is contemplated to adjust the etching depth or the etching rate such that the etching advances to the bottoms of all the cavities simultaneously. For this purpose, the upper substrate is etched in its bottom to form a plurality of shelves which project on the bottom of the upper substrate in registration with deep ones of the cavities. The shelf is given a thickness which is proportional to the depth of the associated cavities. The bottom layer is etched away together with the shelves, after the upper substrate is bonded to the lower substrate, to release the fixed and movable components from the bottom layer.

Instead of forming the shelves by etching, it may be possible to utilize a mask which is deposited on the bottom of the upper substrate. The mask covers areas in registration with the cavities and has a thickness which is proportion to the depth of the associated cavity. The mask is etched away together with the bottom layer for releasing the fixed and movable components from the bottom layer of the upper substrate.

In many applications, it is required to electrically Isolate one or more of the components into two zones but to keep the zones mechanically integrated. To give a solution to the requirement, it is preferred to embed a dielectric member in the top layer of the upper substrate at a portion to be formed into one of the components. The dielectric member penetrates through a portion of the top layer so as to electrically divide the resulting component into two zones for electrical insulation therebetween while keeping the zones mechanically integrated with each other.

The present invention also provides a system which is fabricated by the above step. The system includes the lower substrate, and the upper substrate bonded to the lower substrate. The upper substrate is composed of the fixed components fixed to the lower substrate, and the, movable components that are resiliently coupled to one or more of the fixed components to be movable within a plane of the upper substrate relative to the lower base. The movable components are adapted to receive an electric potential relative to the fixed components for developing an electrostatically attracting force by which the movable components are drive to move. The lower substrate is formed in its top surface with at least one recess above which the movable components are located, affording a sufficient gap between the movable components and the lower substrate without critically differentiating the heights of the movable and fixed components. With this arrangement, the fixed components are only required to have the reduced height substantially equal to that of the movable components, which contributes to make the system of a low profiled type.

These and still other advantageous features of the present invention will be apparent from the following detailed description of the preferred embodiments when taken in conjunction with the attached drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring toFIGS. 1 and 2, there is shown an actuator system100, one example of a micro-electro-mechanical system (MEMS) fabricated by the process of the present invention. Basically, the system is composed of fixed components30anchored to a base20and movable components40that are resiliently supported to one or more of the fixed components30in a floating relation to the base20so that the movable components40are movable relative to the base.20. In the illustrated actuator system, the fixed components30define a pair of side effectors130each including a comb-shaped fixed electrode132, and also define anchor studs134. The movable components40define an actuator140having a comb-shaped movable electrode142and springs144by which the actuator140is resiliently supported to the anchor studs134. The actuator140is driven to move along a linear path towards either one of the side effectors130by an electrostatically attracting force developed between the actuator140and one of the side effectors130. For this purpose, the anchor studs134and the side effectors130are formed respectively with terminals136and138that are electrically connected to an external voltage source to develop the electrically attracting force. The base20is formed on its top with a dielectric oxide layer24for electrically isolating the anchor studs134or the actuator140from the side effectors130.

Now, the process of fabricating the system is explained with reference toFIGS. 3 to 7where the system is schematically shown to include the movable components40and the fixed components30which are finally placed on the base20.FIG. 5shows a section of the system.FIGS. 3 and 4illustrate the process step-by-step in the perspective views, whileFIGS. 6 and 7illustrate the same in the sectional views in correspondence toFIG. 5. Prior to explaining the process, it is noted that the process utilizes an upper silicon substrate10and a lower silicon substrate that define the base20, although the substrates may be selected from any other suitable semiconductor material. Further, the above process is applied to a single silicon wafer for each of the substrates to realize a plurality of modules each constituting the system simultaneously. Therefore, the vertical sections shown in the perspective views ofFIGS. 3 and 4are not actually exposed but shown simply only for easy understanding of the process with reference to the single module of the system.

The upper substrate10is provided in the form of a silicon-on-insulator (SOI) structure to have a top layer12and a bottom layer14divided by a buried oxide layer16. The upper substrate10is etched in its top layer12to develop the fixed and movable components30and40, and is subsequently bonded to the lower substrate20to give a consolidated structure in which the components are supported on the lower substrate20.

<Processing of the Upper Substrate>

At the first step, the upper substrate10is thermally oxidized or treated with chemical-vapordeposition (CVD) to form an oxide layers50of uniform thickness on its top and bottom, as shown inFIG. 3A. Then, a photo-resist film60is applied on the entire top oxide layer50, as shown inFIG. 3B, followed by being selectively removed to leave a resist pattern62on the oxide layer50, as shown inFIG. 3C. Subsequently, the top oxide layer50not covered by the resist pattern62is etched away by the known CHF3 etching plasma, as shown inFIG. 3D, after which the resist pattern62is removed by the oxygen plasma to leave a mask52of the oxide layer on top of the substrate10, as shown inFIG. 3EandFIG. 6AWith the mask52on the upper substrate10, the top layer12of the upper substrate10is dry-etched by the deep reactive ion etching (DRIE) by a depth of about 100 μm down to the buried oxide layer16to form the fixed components30and the movable components40commonly projecting and supported on the bottom layer14through the buried oxide layer16, as shown inFIGS. 3F and 6B. At this time, the side faces of the components suffer from serrations13of about 0.2 μm depth which are inevitably accompanied by the deep reactive ion etching. In order to remove the serrations13or the surface irregularities, the components are thermally oxidized be covered with an oxidized coat18, as shown inFIGS. 3G and 6C, which is thereafter etched away together with portions of the serrations13by exposing the top of the upper substrate10to a wet etching medium, for example, a hydrofluoric acid solution. In this wet etching, the mask52is also etched away to give the structure, as shown inFIGS. 4A and 6D, in which the fixed and movable components30and40project on the bottom layer14with the side faces of the components being finished smooth.

<Processing of the Lower Substrate>

By this time, the lower substrate20is etched to give a recess22in its top surface in registration with the movable components40of the upper substrate10through the steps ofFIGS. 7A to 7C. Prior to being etched, the lower substrate20is thermally oxidized or treated with the CVD treatment to form on its top and bottom oxide layer, and is then masked with a resist pattern and removed of the oxide layer not covered by the resist pattern, after which the resist pattern is etched away by the CHF3 etching plasma to leave a mask72of the top oxide layer as shown inFIG. 7A. The above preliminary treatments are made in the like manner as is made for the upper substrate explained with reference toFIGS. 3A to 3E. With the mask72on its top surface, the lower substrate20is etched to form the recesses22of about 5 μm to 10 μm depth, as shown inFIG. 7B. The etching is made either by wet-etching using a potassium hydroxide or by dry-etching with the deep reactive ion etching (DRIE). hereafter, the lower substrate20is removed of the mask72by an etching medium, for example, a hydrofluoric acid solution, followed by being thermally oxidized to form dielectric oxide layers24of dielectric nature on its top and bottom, as shown inFIGS. 7C and 4B.

Although not shown inFIG. 4, the lower substrate20is additionally formed in its top surface with grooves26. The grooves26are formed simultaneously with the recesses22in order to release the air entrapped between the upper substrate10and the lower substrate20at the time of bonding the substrates, facilitating the bonding procedure. For this purpose, the grooves26is designed to extend at the interface between the upper substrate10and the lower substrate20from within an interior space of the system to the exterior of the system. That is, the grooves26is formed in the wafer forming the lower substrate20to run from portions to be bonded to the fixed components30to a point outside of the portion mating with the upper substrate10through any portion giving a confined interior spaces with the upper substrate. When the upper substrate10is so designed as to leave an open space at portions other than the fixed components30, the grooves26are suffice to run from the portions mating with the fixed components30to the point outside of the portion mating with the upper substrate10.

<Bonding the Upper Substrate to the Lower Substrate>

As shown inFIGS. 4B and 7D, the upper substrate10thus prepared is then placed on the lower substrate20upside down, as shown inFIGS. 4C and 4D, andFIG. 7D, with the fixed components30bonded to the top of the lower substrate20and with the movable components40floated above the recesses22. The bonding is accomplished by heat-pressing the upper substrate10at the fixed components to the top of the lower substrate20. Finally, the bottom layer14of the upper substrate10is removed to leave only the fixed components30and the movable components40on and above the top of the lower substrate20, as shown inFIG. 4G. The removal of the bottom layer14is made firstly by abrasion with the chemical-mechanical polishing (CMP) treatment to a depth short of the oxide layer16, as shown inFIGS. 4E and 7E, and then by dry-etching with the inductively coupled plasma (ICP) down to the oxide layer16, as shown inFIGS. 4F and 7F. Finally, the oxide layer16is removed by use of the CHF3 etching plasma to reveal only the fixed and movable components30and40supported on the lower substrate20, as shown inFIGS. 4G and 5. Thus, the fixed components and the movable components are supported on the lower substrate20through the dielectric oxide layer24so that the separate ones of the components can be electrically isolated by way of the dielectric oxide layer24from each other. It is noted in this connection that the oxide layer16is best utilized as a barrier to stop etching the bottom layer14with inductively coupled plasma (ICP), thereby leaving only the oxide layer16of uniform thickness. In other words, even when the preliminary abrasion or polishing causes the remaining bottom layer14to suffer from different thicknesses from portions to portions, the etching away of the remaining bottom layer can be stopped at the oxide layer16. With this result, the subsequent etching step of removing away the oxide layer16by use of CHF3 etching plasma can be easily controlled in order to release the fixed and movable components from the oxide layer16successfully without causing over-etching and under-etching. Although not illustrated in the figures, the wafer is thereafter divided into the individual modules each constituting the actuator system.

FIGS. 8 and 9illustrate an optical switch, as one application of the present invention. The optical switch200incorporates a like actuator system100composed of a fixed effector130and an actuator140, and is designed to be of cross-connect type having two input light guides202and two output light guides204respectively for connection with input and output optical fibers210and220in order to pass an incoming light signal through each of the input optical fibers to the selected one of the output optical fibers. A mirror230is carried by the actuator140to be driven thereby to shift between a projected position and a retracted position along a linear path. In the projected position, which is shown inFIG. 8, the mirror230is enabled to reflect the light signal from each of the input light guides202at an angle to each of the output light guides204arranged in an angled relation with the input light guides202. In the retracted position, the mirror230is retracted away from a cross yard208, allowing the light signal from each of the input light guides202to proceed straight to each of the aligned output light guides204. The light guides202and204and the mirror230are also formed together with the components of the actuator system commonly from the upper substrate10, and is bonded on to the lower substrate20, in accordance with the process as discussed in the above.

Second Embodiment

FIGS. 10 and 11illustrate a process for fabricating the like a micro-electro-mechanical system (MEMS) in accordance with the second preferred embodiment which is similar to the above embodiment except for the use of the upper semiconductor substrate10A made of a bare silicon monocrystal. The upper substrate10A is firstly coated on its top with an oxide layer50A formed by the thermal oxidation or the CVD process, and is masked With a resist pattern62A, as shown inFIG. 10A. Then, the oxide layer50is etched to leave a mask52A on top of the upper substrate10A (FIG. 10B), after which the resist pattern62A is removed off (FIG. 10C). Then, the upper substrate10A is treated with the deep reactive ion etching (DRIE) to form the fixed components30A and the movable components40A commonly projecting on the bottom layer14A of the upper substrate10A, as shown inFIG. 10D. Next, the exposed faces of the components30A and40A are thermally oxidized in order to eliminate serrations13A appearing on the faces of the components as a result of the deep etching. The resulting oxide coat18A left on the faces of the components (FIG. 10E) is removed together with the mask52A also of made of silicon oxide by use of the hydrofluoric acid solution to give a structure ofFIG. 11A, in which the faces of the components are smoothed.

Subsequently, the upper substrate10A is again thermally oxidized or treated with the CVD process to give an etching-shield74covering the exposed surfaces including the faces of the components30and40, as shown inFIG. 11B. Then, in the like manner as explained in the first embodiment, the upper substrate10A is placed upside down upon the lower substrate20A and is bonded thereto with the fixed components30A fixed to the top of the lower substrate20A and with the movable components40A disposed respectively above the recesses22A, as shown inFIG. 11C. Then, the bottom layer14A of the upper substrate10A is etched away by applying the inductively coupled plasma (ICP) in order to release the components30A and40A, as shown inFIG. 11D. Since the ICP etching proceeds at a high etching rate, it is likely to attack the components at the last stage of nearly completing to etch away the bottom layer14A. However, the etching-shield74protects the components from being attached and keeps them intact after the components are completely released. Finally, the etching shield74is etched away by use of the CHF3 plasma, thereby realizing the structure, as shown inFIG. 11E.

Third Embodiment

FIG. 12illustrates a process for fabricating the like a micro-electro-mechanical system (MEMS) in accordance with the third preferred embodiment which is similar to the above embodiment except that it is contemplated to make the movable components40A shorter in its height than the fixed components30A. The movable components40A may be required to be given a height shorter than the fixed components30A for reason of leaving a large gap on the lower substrate20A and/or adjusting the mechanical characteristics. In order to differentiate the height of the fixed and movable components, the present embodiment utilizes a composite mask composed of a first mask52and a second mask54which is etched at a higher rate than the first mask52but at a lower rate than the upper substrate10A, i.e., silicon, when subjected to the same etching treatment.FIG. 12Ashows the first mask52which is formed by selectively removing or etching away portions of the oxide layer formed on top of the upper substrate10A in the same manner as discussed in the first embodiment with reference toFIGS. 3A to 3E. Then, the upper substrate10A is thermally oxidized or treated with the CVD process to form an additional oxide layer50of uniform thickness covering the entire top surface of the upper substrate including the first mask52, as shown inFIG. 12B. Subsequently, with a resist pattern84deposited on the additional oxide layer50, as shown inFIG. 12C, the upper substrate10A is etched to selectively remove the portions of the additional oxide layer50to develop the second mask54on the upper substrate10and also on the first mask52, as shown inFIG. 12D, after which the resist pattern84is removed to realize the composite mask, as shown inFIG. 12E. Then, the upper substrate10thus covered with the composite mask is treated with the deep reactive ion etching (DRIE) until the second mask54is completely etched away, as shown inFIG. 12F. At this time, the unmasked portions of the upper substrate10is etched deep to form the movable components40, while keeping the fixed components30covered by the first mask52which has been etched only to some extent. Finally, the first mask52is removed from the top of the upper substrate10by use of the hydrofluoric acid solution, revealing the fixed components, as shown inFIG. 12G. Thus, the upper substrate10A is processed to give different heights within a range of 5 to 10 μm to the fixed and movable components by use of the composite mask. Although the present embodiment is explained with the use of the upper substrate made of the bare silicon, the above process can be equally applied to the SOI structure as utilized in the first embodiment.

Fourth Embodiment

FIG. 13illustrates a useful scheme of etching away the bottom layer14A of the upper substrate10A in accordance with a fourth embodiment of the present invention. The present embodiment is particularly useful in case where the components are designed to be spaced laterally by largely varying widths, and where the upper substrate10A is devoid of the buried oxide layer. In such case, cavities15of greatly different widths are to be left between the adjacent ones of the posts or the components30A and40A after the top layer12A of the upper substrate10A is etched away. It is true that, during the etching of the top layer, the growing cavities of greater widths are likely to be etched to a greater depth, which causes the finished cavities to have different depths, failing to align the bottoms of the cavities in the same level. Irrespective of the misaligned bottoms of the cavities, the present embodiment gives an easy etching control for releasing the components30A and40A from the bottom layer14A of the upper substrate10A.

For this purpose, the bottom surface of the upper substrate10A is formed with shelves17which are in exact registration with the cavities15, and each of which has a thickness is proportional to the depth of the associated cavity. The shelves17are obtained by selectively etching portions of the bottom surface of the upper substrate10A. The thickness of the shelves17is controlled by varying the etching depth portions by portions. With the addition of the shelves17, the etching of the bottom layer14can advance to the bottoms of all the cavities by a uniform rate, thereby releasing all of the components from the bottom layer14A successfully. This means that, as indicated by the dotted lines in the figure, the etching depth is made uniform throughout the bottom layer14A so that the etching can be easily controlled simply by the etching time.

FIG. 12illustrates a modification of the above process which is similar to the above embodiment except that the shelves17B is formed through the steps of firstly forming a field oxide layer (SiO2) on the bottom of the upper substrate10A and then etching away portions of the oxide layer to leave the shelves or mask17B on the bottom of the upper substrate. In this modification, the thickness of the masks17B is controlled by differentiating the thickness of the field oxide layer portions by portions, or by repeating the formation of the field oxide layer and etching portions thereof on the areas to be provided with the masks17B of greater thickness. The masks17B thus made of the silicon oxide is etched at a rate lower than the rest of the upper substrate of silicon, which guarantees the effect of successfully removing the bottom layer14A of the upper substrate10A for releasing the components at a dotted line shown in the figure.

Fifth Embodiment

FIG. 15illustrates a process in accordance with a fifth embodiment of the present invention which is similar to the first embodiment except that dummy projections19are formed integrally with the upper substrate10in order to give generally uniform etching depth in forming the components30and40. The dummy projections19are positioned at portions where the components are spaced by a greater width so as to leave a generally uniform width between the adjacent ones of the components30and40and the dummy projections19. As shown inFIGS. 15A and 15B, the upper substrate10is etched with a mask52on its top to form the dummy projections19in addition to the components30and40in the top layer12above the buried oxide layer16, followed by being removed of the mask52. The dummy projections19are selected to have a width smaller than any one of the components and are supported on the buried oxide layer16together with the components30and40. Then, the upper substrate10is subject to the wet-etching to remove portions of the oxide layer16anchoring the dummy projections19, thereby releasing them from the upper substrate10, as shown inFIG. 15C. The etching proceeds firstly to remove the portions of the oxide layer16corresponding to the bottoms of the cavities15between the adjacent ones of the components and the dummy projections, and then proceeds laterally to remove the portions of the oxide layer16to such an extent as to completely remove the portions below the dummy projections19. Since the components have a width greater than that of the dummy projections19, the corresponding portions of the oxide layer16supporting the components are left adhered to the bottom layer14of the upper substrate10, although they are etched to some extent. Thus, the components30and40are kept anchored to the bottom layer14of the upper substrate10. Thereafter, the upper substrate10is placed on the lower substrate20upside down and bonded thereto, shown inFIG. 15D, and is subjected to the etching for removing the bottom layer14as well as the oxide layer16for releasing the components30and40from the bottom layer, as shown inFIG. 15E.

Sixth Embodiment

FIG. 16illustrates a process in accordance with the sixth embodiment of the present invention which is basically similar to the first and second embodiments but is further contemplated to electrically isolate at least one of the components into two mechanically integrated zones in match with a need for applying different electric potentials to the two integrated zones. The electrical isolation is achieved by embedding a dielectric material into a portion or portions of the upper substrate10which are finally formed into the components to be divided into the two mechanically integrated zones. The dielectric material is made by oxidization of the upper substrate, i.e., silicon dioxide (SiO2) integrally formed in the top surface of the upper substrate10, as explained below in details.

Firstly, the upper substrate10is covered with a mask90of the oxide layer developed by thermal oxidization or by the CVD treatment made to the top of the upper substrate, and is then etched in its top layer by the deep reactive ion etching (DRIE) to form therein caves11, as shown inFIG. 16A. Then, mask90is removed by use of the hydrofluoric acid solution, as shown inFIG. 16B, after which the upper substrate10is thermally oxidized to form on its top a fresh oxide layer50of silicon dioxide (SiO2) which also fills the caves11to define the dielectric members53embedded in the top layer of the upper substrate10, as shown inFIG. 16C. Subsequently, the oxide layer50is selectively etched to leave a mask52on top of the substrate10, as shown inFIG. 16D. The etching is made such that the resulting mask52has a thickness greater at portions later formed into the fixed components30than at portions later formed into the movable components40. Then, the upper substrate10is treated with the deep reactive ion etching (DRIE) to form the fixed and movable components30and40, during which the mask52is etched to such an extent as to be left only on the fixed components30but cleared from the top of the movable components40, as shown inFIG. 16E. Subsequently, the mask52remaining on top of the upper substrate10is removed by exposure to the hydrofluoric acid solution, leaving the dielectric members53kept embedded within the respective caves11, as shown inFIG. 16F. Thereafter, in the like manner as is made in the previous embodiment, the upper substrate10is placed on the lower substrate20upside down and bonded thereto (FIG. 16G), after which the bottom layer14is etched away to release components30and40to give the structure ofFIG. 16H.

The oxide layer50forming the dielectric members53may be developed by any other treatment other than the above thermal oxidation, for example, by the CVD, the SOG (Spin On Glass) method, pyro-oxidation, or TEOS (Tetraethoxysilan, Tetraethylorthosilicate) deposition. Although the lower substrate is made of the semiconductor material in the above embodiments, it may be made of a glass or the like dielectric material.