Abstract:
The present invention relates to a method for manufacturing a MEMS device, including the actions of: providing a substrate having a back and front surface essentially in parallel with each other, defining in said substrate at least one hidden support by removing material from said substrate, connecting said at least one hidden support onto a wafer with at least one actuation electrode capable to actuate at least a part of said substrate, wherein a rotational axis of said reflective surface is essentially perpendicular to said hidden support. The invention also relates to the MEMS as such.

Description:
This application is a continuation of, and claims priority under 35 U.S.C. § 120 to, application Ser. No. 11/174,568 filed on Jul. 6, 2005 now U.S. Pat. No. 7,372,617. The entire contents of this application is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to techniques for forming an integrated device, e.g. a semiconductor device, and in particular to a method for manufacturing micro mirrors with a hidden hinge and to an SLM comprising such micro mirrors. 
     DESCRIPTION OF THE BACKGROUND ART 
     It is well known in the current art to build spatial light modulators (SLM) of a micro mirror type U.S. Pat. Nos. 4,566,935, 4,710,732, 4,956,619. In general two main principles for building integrated devices, such as micro mirror SLM, have been employed. 
     An integrated circuit (IC) is manufactured to a finished state, and then the micro mirrors are manufactured on said IC. The micro mirrors are built onto the IC wafers. An advantage with this approach is that so called IC foundries can be, used, which presents a very cost efficient manufacturing of the electronics wafers. A disadvantage is that there is a very restricted selection of materials and methods that are usable for the manufacturing of the micro mirrors, because there is an upper temperature limit of about 400° C., above which the electronics will be damaged. This makes the manufacturing of micro mirror having optimal performance more difficult. 
     Another way of building micro mirror SLM&#39;s is at the end of the process for making the IC, micro mirror manufacture may be started on the same wafers. An advantage with this approach may be that there is a greater freedom of selecting materials, methods and temperatures for the manufacture of micro mirrors having good performance. A disadvantage is that the IC wafers cannot be manufactured in standard IC foundries, because they have very strict demands on a process of manufacturing to be standardized in order to be able to maintain the quality in the process. 
     Yet another way of building micro mirror SLM&#39;s may be to manufacture the IC on a first wafer and a micromirror array on a second wafer. Said first and second wafers may be attached to each other by means of bonding. One problem with such a method may be the tight demands on the alignment between said first and second wafers, a misalignment may affect the functionality of one or several pixels. 
     Micromirrors in SLM may be made of aluminum due to its good optical performance. However, there may be some drawbacks by using mirrors made of Aluminum such as: the mirrors may not be perfectly flat, a mirror height may differ between mirrors, the mirrors may bend when tilted, the mirrors may sag when tilted, the mirrors may have a built in predeflection which may be different from mirror to mirror, the hinge may have anelastic behaviour. 
     Therefore, there is a need in the art for an improved method for manufacturing micro electrical/mechanical/optical integrated devices. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing background, the method for manufacturing integrated devices, such as for example micro mirror SLM&#39;s, is critical for the performance of such devices. 
     Accordingly, it is an object of the present invention to provide an improved manufacturing method and/or design for an integrated device which overcomes or at least reduces the above mentioned problems. 
     In an example embodiment, the invention provides a method of for manufacturing a MEMS device, including providing a substrate having a back and a front surface essentially is parallel with each other, defining in said substrate at least one hidden support by removing material from said substrate, connecting said at least one hidden support onto a wafer with at least one actuation electrode capable to actuate at least a part of said substrate, wherein a rotational axis of said reflective surface is essentially perpendicular to said hidden support. 
     In another example embodiment, the invention provides a MEMS device comprising a substrate having at least one reflective surface, at least one hidden support formed out of the same material as said substrate, at least one actuation electrode provided on a wafer capable to actuate said reflective surface, wherein said wafer is connected to said substrate and a rotational axis of said reflective surface is essentially perpendicular to said hidden support. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1-20  shows an example embodiment of process steps in consecutive order according to the present invention for manufacturing a micromirror with a hidden hinge. 
         FIG. 21  illustrates a 3D view of an example embodiment of a micromirror according to the present invention. 
         FIG. 22-32  illustrates another example embodiment of an inventive manufacturing process for the inventive MEMS device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purpose of this application, the terms “wafer” and “substrate” are used interchangeably, the difference between them merely amounting to dimensions thereof. 
     The method according to the present invention is particularly suited for the manufacturing of micro mirror Spatial Light Modulators. However, it would be applicable to a wide variety of MEMS, thermal and non thermal detector devices, such as, but not limited to, quantum well detectors, pyroelectric detectors, bolometers, etc. It is particularly suitable when for the same reason it is not possible to process/pattern/deposit a structure (e.g. a micro mirror array) directly on a substrate, where another structure (e.g. steering electronics) is present. This can e.g. be the case if the structure provided on said substrate, is temperature sensitive to the process temperature for the processing of the structure to be provided thereon, or when the substrate is poly crystalline and the elements that is grown on top of the substrate have to be monochrystalline. 
       FIG. 1  illustrates schematically a first process step according to an example embodiment of the invention for forming a MEMS device with a hidden hinge. A hidden hinge is a hinge that is hidden by a reflective surface in said MEMS device when viewing said MEMS device from above, i.e., a top view. A starting material is a wafer  130 , which may be made of single crystalline silicon or SOI. On top of said wafer  130  is provided a layer of mask material  120 , for instance silicon oxide. Said mask material  120  may at least partly be covered with a resist film  110 . In said first process step, standard photo-lithography may be used for hinge definition  140  in the mask material  120 . RIE (Reactive Ion Etching), which may be CF 4 , may be used to remove both exposed areas of the resist film  110  and underlying mask material  120 . 
     Definition of hinges in the substrate  130  may be made by using DRIE (Deep RIE),  FIG. 2 . Before said hinges is defined in said substrate  130  the resist film  110  may be removed in a resist remover. Prior to the definition of the hinges in the substrate  130  said substrate may be dipped in 2% HF. The DRIE may be the well known Bosch process. The most simplified process consist of just an anisotropic DRIE etch followed, by an isotropic RIE to form the hinges. Prior to said definition of said hinges in said substrate  130 , a layer of silicon oxide  150  may be provided on the opposite side of said substrate  130  with respect to where said hinges is to be defined, alternatively said layer of silicon oxide  150  may be provided on said opposite side after the definition of said hinges in said substrate  130 . 
     In a next step, a passivation of processed surface may be performed,  FIG. 3 . A dry oxidization may be made in order to relax a stress in the silicon during local oxidation, not illustrated in  FIG. 3 . Said oxidation may be optional to increase accuracy and reduce surface roughness. A PECVD (Plasma Enhanced Chemical Vapour Deposition) of silicon nitride  160  may be made in order to act as an oxidation barrier in a following LOCOS (LOCal Oxidation of Silicon) step. A PECVD of silicon oxide  170  to act as an etch protection may be performed as a protection in a following DRIE step. 
     In  FIG. 4  a removal of passivisation layers (silicon nitride  160  and silicon oxide  170 ) on horizontal surfaces has been performed and a definition of length of hinges in substrate  130  has been made. The passivisation layers may be removed by means of RIE with high directionality (low pressure and high RF power). Revealed surfaces of substrate  130  may be etched by means of DRIE (Bosch process) in order to define said length of said hinges. 
     In a next step, illustrated in  FIG. 5 , a thermal oxidation has been performed to define a width of the hinges. LOCOS may be used to transform part of the substrate  130  in the hinges to silicon oxide  180 . 
     In  FIG. 6  a removal of passivisation layers and mask material and a planarization of substrate have been performed. Passivisation layers  160 ,  170 ,  180  and mask material  120  may be etched away in BOE (Buffered Oxide Etch). Polyimide (PI)  190  may be spun on top of the substrate  130  filling the cavities therein. Reduced pressure or vacuum may be used in order to make sure that said PI will fill said cavities. Said PI may be cured at an elevated temperature. Unwanted PI may be removed with O 2  plasma. 
     A mask material  200  is deposited on top of said substrate  130 ,  FIG. 7 . Said mask material may be aluminum and the deposition may be performed by evaporation. 
     On top of said mask material a film of resist may be provided. Standard photolithography may define areas  220  where the definition of the mirror separation trenches will be in the substrate  130 , see  FIG. 8 . The aluminum beneath the exposed resist may be removed with RIE (Si CL 4 /Cl 2 ). The separation trenches may also be formed as a first step by using a trenched (usually filled by oxide) SOI wafer as used for trench isolation of electronics. 
     Unexposed resist film may be removed with acetone. On top of the mask material  200  and said relieved substrate  130  a layer of silicon oxide is provided, see  FIG. 9 . Said layer may be provided by means of PECVD. 
     On top of said silicon oxide layer  230  a layer of resist  245  is provided. Standard lithography may define mirror separation trenches  240  and electrode trenches  250  in mask material  230  (silicon oxide). The silicon oxide  230  may be etched by means of RIE, for instance CF 4 . 
     In  FIG. 11 , mirror separation trenches  260  have been formed in the substrate  130 . The resist  245  has been removed by using for instance acetone. Said mirror trenches  260  in said substrate  130  may be made by using the Bosch process. 
     In  FIG. 12 , electrode trenches  255  in the aluminum layer have been defined. Said electrode trenches may be defined by Beans of RIE, for instance SiCl 4 /Cl 2 . 
     PI has been introduced in said mirror trenches  260  in  FIG. 13 . Unwanted PI may have been removed by using O 2  plasma. 
     Electrode trenches  257  in substrate  130  have been made in  FIG. 14 . Said electrode trenches  257  may have been made by using the Bosch process. 
     In  FIG. 15 , silicon oxide  270  may have been PECVD to act as etch protection in the following isotropic DRIE step. 
     In  FIG. 16 , passivation layer  270  on horizontal surfaces have been removed. The removal of said passivation layers may be performed by using RIE. 
     A release of a foot structure has been made in  FIG. 17 . Isotropic RIE (may also be substituted by wet isotropic or anisotropic etching) of the substrate  130  has been made in order to release the foot of he mirror with an under etch of the material between hinges. By removing the material between the hinges, an applied actuation force to deflect the mirror to a certain deflection state may be heavily reduced. The isotropic etch also removes unnecessary material in the mirror, i.e., reduces its weight, which may affect the speed of setting the mirror from one state to another and its self oscillating frequency. 
     In  FIG. 18 , the passivation layers  230 ,  270  and mask layers  200  have been removed. These layers may be removed by means of BOE. 
     In  FIG. 19 , a substrate  300  with actuation electronics  310  has been attached to said substrate  130 . At least one hinge is attached to said substrate  300 . The substrate  300  has en elevated structure  320  for attaching said hinge(s) (alternatively the electrode areas of the substrate  130  may be lowered). Beside said elevated structure  320  actuation electronics  310  is provided. Here one can easily see that there is a big attachment area for the substrate  130  to attach to said substrate  300 . Even if there may be a slightly mismatch between said two substrates, a successful attachment may nevertheless be performed. Said attachment may be a low temperature oxygen plasma assisted bonding, adhesive bonding (gluing), soldering, eutectic bonding, fusion bonding (direct bonding), glass frit bonding, anodic bonding. 
     In  FIG. 20  the buried oxide  280  has been removed from the substrate  130 . This buried oxide may be removed by means of BOE. The mirrors  132  may be released by removing the PI. PI may be removed by using O 2 -plasma. From  FIG. 20  one may see that the mirror structure is relatively stiff. This is due to the vertical part  136 , which will strongly affect the stiffness and planarity of a mirror surface. The hinge  134  may be designed to be as stiff or weak as desired. The mirror may be made of a pure single crystalline material, for instance silicon. Other alternative material of the mirror may be polysilicon, quartz, three-five materials, SiC. In order to improve the electrical conductance, said mirror material may be doped if made of a semiconducting material. A surface facing towards the electronics in substrate  300  may be coated with an electrically conducting material. 
       FIG. 22-32  illustrates an alternative example embodiment of an inventive manufacturing process for the inventive MEMS device. In  FIG. 22  a starting material is a wafer  130 , which may be made of single crystalline silicon or SOI. On top of said wafer  130  is provided a layer of mask material  120 , for instance silicon oxide. Said mask material  120  may at least partly be covered with a resist film  110 . In said first process step, standard photo-lithography may be, used for defining trench separation  300  in the mask material  120 . RIE (Reactive Ion Etching), which may be CF 4  may be used to remove both exposed areas of the resist film  110  and underlying mask material  120 . Definition of trench separation in the substrate  130  may be made by using DRIE (Deep RIE),  FIG. 22 . Before said trench separations  300  are defined in said substrate  130  the resist film  110  may be removed in a resist remover. Prior to the definition of the hinges in the substrate  130  said substrate may be dipped in 2% HF. The DRIE may be the well known Bosch process. The most simplified process consist of just an anisotropic DRIE etch followed by an isotropic RIE to form said trenches. Prior to said definition of said trenches in said substrate  130 , a layer of silicon oxide  150  may be provided on the opposite side of said substrate  130  with respect to where said trenches  300  are to be defined, alternatively said layer of silicon oxide  150  may be provided on said opposite side after the definition of said trenches in said substrate  130 . 
     Said trenches  300  may be filled by first spinning Polyimide (PI)  310  on top of the substrate  130  filling the cavities therein. Reduced pressure or vacuum may be used in order to make sure that said PI will fill said cavities. Said PI may be cured at an elevated temperature. Unwanted PI may be removed with O 2  plasma, see  FIG. 24 . 
       FIG. 25-29  illustrates the process steps for defining the buried or hidden hinges. In  FIG. 25 , standard photo-lithography may be used for defining entrance holes  310  in the mask material  120 . RIE (Reactive Ion Etching), which may be CF 4 , may be used to remove both exposed areas of the resist film  110  and underlying mask material  120 . 
     In  FIG. 26  a dry etch may be used for defining holes  320  in the substrate  130 . After said holes  320  have been defined in said substrate  130  a stripping of said resist  110  may be performed. After stripping the resist a layer of oxide may be deposited in order to arrange a layer of oxide in said holes  320 . 
     In  FIG. 27  a dry etch may be used to etch horizontal surfaces of said layer of oxide. 
     In  FIG. 28  an isotropic dry etch may be used to create a cavity  330  and buried hinges  340  in the substrate  130 . In  FIG. 29  the oxide layer has been removed in BOE. In  FIG. 30  an alternative cross section of the structure is illustrated, the cross section is illustrates to the left to  FIG. 30 . In  FIG. 31  the substrate  130  may be bonded onto a wafer  400  with actuation electrodes  410 . The oxide layer  150  may be removed by means of BOE, and the polyimide by dry etching in O 2  plasma, see  FIG. 32   
       FIG. 21  illustrates a perspective view of an example embodiment of a mirror structure  132  according to the present invention. Said mirror structure comprising a mirror surface  135 , supports  134 , cavity  131 , base element  136 , a first leg  142  and a second leg  144 . The mirror structure  132  may have at least one cross section which is as thick as the original substrate  130 , which in this particular embodiment may be the distance from the mirror surface  135  to an electrostatically attraction surface  145 ,  147 . This may give the mirror structure good mechanical properties, such as high stiffness, i.e., the mirror surface is essentially rigid while being in a deflected position. The supports  134  may be thin pillars. The supports may support the mirror structure  132  and at the same time function as a hinge. In the illustrated example embodiment in  FIG. 21  said support is arranged so that the rotational axis is essentially in the middle of the structure. In an alternative example embodiment said rotational axis may be arranged off center, which may be achieved by displacing the supports from a center position. An axis of rotation of the mirror surface  135  may be parallel to the mirror surface and perpendicular to the support  134 . 
     The base element  136  and the support  134  may be denoted a hidden hinge. In another embodiment the base element  136  is minimized so that the support  134  only may be denoted the hidden hinge (hidden support). The cross section of said pillars may be polygonal, for instance triangular or rectangular. The base element  136  may be attached to the supports  134 . A bottom surface of the base element  143  may be attachable to another surface, such as a wafer with steering-electronics. The legs  142 ,  144  may have surfaces  146 ,  148  essentially perpendicular to the mirror surface  135 . The cavity  131  may be formed by means of an isotropic etching process according to the example embodiment above. The mirror structure  132  may be doped. The doping is preferably made prior to defining the cavity  131  and supports  134 , i.e., the substrate to be used for defining said mirror structure may be doped. In this embodiment the electrostatically attraction surface  147  may be used to rotate the mirror structure  132  clockwise. The electrostatically attraction surface  145  may be used to rotate the mirror structure  132  counter clockwise, i.e., said structure may be rotated clockwise or anti clockwise from non actuated state. The surface  143  of the base element  136  may be at another level compared to the electrostatically attraction surfaces  145 ,  147 . 
     In the embodiments disclosed hereinabove the actuation of the mirror element has been electrostatic. However, other means of actuating the mirror element is possible such as thermal, piezoelectric or magnetic, which is well known for a skilled person is the art. 
     Thus, although there has been disclosed to this point particular embodiments of the method of combining components to form an integrated device, it is not intended that such specific references be considered as limitations upon the scope of this invention except in-so-far as set forth in the following claims. Furthermore, having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may suggest themselves to those skilled in the art, it is intended to cover all such modifications as fall within the scope of the appended claims.