Abstract:
A method for designing and manufacturing a micromechanical device providing a substrate having an anchoring region forming a sacrificial layer on substrate while leaving bare the anchoring region depositing an adhesion layer ( 30 ) on the sacrificial layer ( 25 ) and the anchoring region ( 20; 120; 220; 320, 325; 420, 425; 620; 755 ); forming a mask on the adhesion layer; depositing an electroplating layer on the unmasked region of the adhesion layer; and removing the mask and the sacrificial layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for designing and manufacturing a micromechanical device, in particular a micromechanical oscillating mirror. 
     Although in principle applicable to arbitrary micromechanical devices, the present invention as well as its underlying problem are explained with respect to a micromechanical oscillating mirror. 
     BACKGROUND INFORMATION 
     Micromechanical oscillating mirrors are used, for example, as switching elements in optical transmission engineering or as scanning element for deflecting a laser beam for bar-code recognition, for room monitoring, or as a marking instrument. 
     The present invention is based on the problem that the micromechanical conventional oscillating mirrors are short-lived and difficult to manufacture. In particular, temperature problems and problems due to mechanical stresses exist with the materials used such as polysilicon. By using low-stress electroplating layers, it is possible, in particular for mirror surfaces to be manufactured without curvature. 
     SUMMARY 
     The designing and manufacturing method according to the present invention has the advantage over conventional design approaches that the resulting micromechanical device is stress- and temperature-compensated so that both freedom in the choice of material and in the selection of the operating temperature, which is typically in the range of −40° C. to +130° C., exists. 
     It is the basic idea of the present invention that the micromechanical device electrodeposited on the unmasked region of the adhesion layer using the “additive technique” is supported in the anchoring region and tiltable about at least one axis or able to execute torsional vibrations subsequent to the removal of the sacrificial layer. In the proposed designs, the advantages of the additive technique can be fully exhausted. 
     The additive technique makes it possible to reduce the size of the previous micromechanical design approaches and, in connection with that, to reduce the price and develop new possibilities for use. The designing and manufacturing method according to the present invention thus provides cost-effective, reliable and long-lived micromechanical devices. In particular, the additive technique allows freely movable metal patterns to be produced on an arbitrary substrate such as a silicon substrate, glass substrate, or a ceramic substrate. 
     In addition, the additive technique allows large, unperforated surfaces to be bared so that massive mirror surfaces having dimensions up to several millimeters can be manufactured. As a single-layer electroplating process, the technique is cost-effective and can be controlled well. A multilayer electroplating process can be carried out, as well, for example, for manufacturing the anchoring regions and the mirror surface or the suspensions separately. Large tilting angles can be attained by correspondingly thick sacrificial layers. 
     According to an example embodiment, a metallic connection pad, e.g. of a circuit integrated in the substrate is provided as anchoring region. Both a manufacture as discrete device and a manufacture in a form that is integrated in a service connection are possible. If the micromechanical device is integrated on an integrated circuit, the metallization of the integrated circuit can advantageously be used for anchoring. 
     According to a further preferred embodiment, a first photoresist layer having a thickness of several microns is formed as sacrificial layer. The photoresist can easily be removed in an isotropic etching process. When using a polymer sacrificial layer, the distance of the mirror element from the substrate can be adjusted very accurately, distances from several microns to approximately 150 μm may be achieved. 
     In another example embodiment, the first photoresist layer is patterned photolithographically for leaving bare the anchoring region. 
     According to a further example embodiment, the adhesion layer is sputtered. 
     In a further example embodiment, the adhesion layer is a conductive layer of Cu—Cr having a thickness of several nanometers. The chromium serves as adhesion layer toward the underlying photoresist; the copper serves as a starting layer for the subsequent electrodeposition. Other adhesion layers, such as Cr—Au, etc., are, of course, also possible. 
     According to a further example embodiment, the mask is formed on the adhesion layer by the following steps: forming a second photoresist layer on the adhesion layer; forming a silicon dioxide layer on the second photoresist layer; patterning a third photoresist layer photolitographically; and plasma etching the silicon dioxide layer for forming a hard mask for the second photoresist layer; and etching the second photoresist layer masked by the patterned silicon dioxide layer down to the adhesion layer. In this context, the second photoresist layer is used as polymer negative matrix for the electrodeposition. 
     In another preferred embodiment, a nickel layer or a nickel-cobalt layer is deposited as electroplating layer. Layers of that kind can be manufactured free of stress, evenly, and with a good reflectivity. 
     According to a further example embodiment, the sacrificial layer in the form of the first photoresist layer, the polymer mold formed by the second photoresist layer, and the adhesion layer are removed subsequent to the deposition of the electroplating layer. 
     In a further example embodiment, the micromechanical device is an oscillating mirror which is designed in such a manner that it can execute torsional vibrations about at least one axis. The oscillating mirror can be operated as a simple tilting mirror as well as in resonance as scanning mirror when using a thicker sacrificial layer. The oscillating mirrors can be designed in such a manner that they are tiltable in one, two or as many as desired directions. 
     According to another example embodiment, a counter-electrode is provided on the substrate under the mirror surface. 
     According to further example embodiment, the oscillating mirror is designed such that it can execute torsional vibrations about four or more axes. 
     In a further example embodiment, the oscillating mirror is designed such that it is suspended on a surrounding frame which is anchored in the anchoring region. Thus, an uninterrupted or non-cutout mirror region can be achieved. 
     According to another example embodiment, the oscillating mirror is designed such that the anchoring region is provided in a cutout of the mirror surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a-g  show the precess steps of a first specific embodiment of the method according to the present invention for designing and manufacturing a micromechanical device in a cross-sectional representation. 
     FIG. 2 shows a top view of a first example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention. 
     FIG. 3 shows a top view of a second example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention. 
     FIG. 4 shows a top view of a third example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention. 
     FIG. 5 shows a top view of a fourth example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention. 
     FIG. 6 shows a top view of a fifth example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention. 
     FIG. 7 shows a cross-sectional view of a sixth example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the figures, identical reference symbols designate identical or functionally identical elements. 
     FIGS. 1 a-g  show the precess steps of a first specific embodiment of the method according to the present invention for designing and manufacturing a micromechanical device in a cross-sectional representation. 
     In FIG. 1, reference symbol  10  designates a substrate having a ready-processed service connection, the substrate having a passivation layer  15  with open connection pads  20  embedded therein.  25  designates a sacrificial layer in the form of a first photoresist layer,  30  designates an adhesion layer in the form of a sputtered electroplating starting layer (plating base),  40  a second photoresist layer,  50  a silicon dioxide layer,  60  a third photoresist layer, and  35  an electroplating layer in the form of. a nickel plating. 
     The starting point for manufacturing the micromechanical device according to the first specific embodiment of the present invention is the ready-processed service connection including passivation layer  15  and open connection pads  20 . 
     In a first step, as illustrated in FIG. 1 a , a first photoresist layer is applied as sacrificial layer  25  and patterned in such a manner that connection pad  20  lies exposed. This connection pad  20  is used as anchoring region for the micromechanical device to be manufactured. First photoresist layer  25  can expediently be used both for opening connection pad  20  and as sacrificial layer if the opening of connection pad  20  must be carried out in passivation layer  15  first. 
     In a next step, as shown in FIG. 1 b , adhesion layer  30  is sputtered in the form of an electroplating starting layer (plating base) which, in the present example, is a conductive layer of chromium copper. In this context, the chromium provides the adhesion toward underlying first photoresist layer  25 ; the copper serves as starting layer for the subsequent step of electrodeposition. 
     As depicted in FIG. l c , an approximately 15 μm thick second photoresist layer  40  is applied to adhesion layer  30  by spinning and hardened at temperatures of typically 200° C. 
     Using the plasma CVD method (CVD=chemical vapor deposition), an approximately 600 nm thick silicon dioxide layer  50  is deposited on second photoresist layer  40 . Subsequently, silicon dioxide layer  50  is used as a hard mask for patterning underlying second photoresist layer  40  and is patterned for that purpose by a photolithographic process using a third photoresist layer  60 , and by subsequent plasma etching, as shown in FIG. 1 d.    
     Subsequent to overetching silicon dioxide layer  50 , a trench etching of second photoresist layer  40  is carried out using an anisotropic plasma etching process. The pattern resulting therefrom is shown in FIG. 1 e.    
     Deposited into the polymer negative matrix formed by second photoresist layer  40  and resulting in this manner is a nickel plating having a thickness of several microns. Resulting therefrom is the comb pattern shown in FIGS. 1 f  and  1   g . In this context, it should be mentioned that the individual regions of second electroplating layer  35  are interconnected at regions which are not shown in this cross-sectional representation. 
     Subsequently, silicon dioxide layer  50  is removed by wet chemical etching, and the polymer negative matrix in the form of patterned second photoresist layer  40  is removed by dry chemical etching. 
     Subsequently, adhesion layer  30  is wet chemically etched selectively, and the sacrificial layer in the form of first photoresist layer  25  is etched in a plasma, resulting in the pattern shown in FIG. 1 g.    
     The removal of sacrificial layer  25  in the form of first photoresist layer is an isotropic etching process, the photoresist under nickel combs  35  being completely removed. 
     The result is a capacitively operated micromechanical device having free-moving patterns, as can shown in FIG. 1 g.    
     FIG. 2 is a top view of a first example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention. 
     In FIG. 2, reference number  100 , designates generally a first mirror form,  110  a mirror surface,  120  an anchoring region,  130  a torsion spring suspension, and  140 ,  150  counter-electrodes which are provided on the substrate underneath mirror surface  110 . 
     In the case of first mirror form  100 , the anchoring and the suspension are provided in the inner region in mirror surface  110 . Mirror surface  110  is attached to torsion spring suspension  130  for suspension. This suspension is completely stress- and temperature-compensated which is expedient when using metal components on silicon, etc. Otherwise, in fact, the spring rods or mirror elements bend, which generally results in a functional failure. Via the geometry of torsion spring suspension  130 , the resonant frequency of the oscillating mirror including mirror form  100  can be adjusted if a use as a scanner is intended. For example 
     FIG. 3 is a top view of a second example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention. 
     In FIG. 3, number  200  designates a second mirror form,  210  a mirror surface,  220  an anchoring region,  230  a torsion spring suspension, and  240 ,  250 ,  260 ,  270  counter-electrodes which are provided on the substrate underneath mirror surface  210 . 
     In this second mirror form  200 , the anchoring and the suspension are also provided inside in the region of mirror surface  210 ; in contrast to the above first example, however, a deflection about two axes which are perpendicularly to each other is possible here because of two further counter-electrodes. With respect to the second axis, torsion spring suspension  230  does not act as torsion rod but as normal cantilever spring. 
     FIG. 4 is a top view of a third example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention. 
     In FIG. 4, number  300  designates a third mirror form,  305  a frame,  310  a mirror surface,  320 ,  325  an anchoring region,  330 ,  335  a torsion spring suspension, and  340 ,  350 ,  360 ,  370  electrodes which are provided on the substrate underneath mirror surface  310 . 
     In this third example, in contrast to the first and to the second example, torsion spring suspension  330 ,  335  is achieved by two torsion springs which connect mirror surface  310  to frame  305 . Frame  305 , in turn, is joined to the substrate via anchorings  320 ,  325 . With respect to torsion spring suspension  330 ,  335  and mirror surface  310 , this type of construction is expediently stress- and temperature-compensated, as well. 
     FIG. 5 is a top view of a fourth example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention. 
     In FIG. 5,  400  designates a third mirror form,  405  a frame,  410  a mirror surface,  425  an anchoring region,  430 ,  435  a torsion spring suspension, and  440 ,  450 ,  460 ,  470  counter-electrodes which are provided on the substrate underneath mirror surface  410 . 
     In this fourth example, in contrast to the third example according to FIG. 4, the length of torsion springs  435 ,  430  is lengthened in a manner that it extends into mirror surface  410 . This enables adaptation of the desired resonant frequency in the case of a use as scanner. 
     FIG. 6 is a top view of a fifth example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention. 
     In FIG. 6, reference number  600  designates a fifth mirror form,  610  a mirror surface,  620  an anchoring region,  630 ,  631 ,  632 ,  633 ,  634 ,  635 ,  636 ,  637  a torsion spring suspension,  640 ,  641 ,  642 ,  643 ,  644 ,  645 ,  646 ,  647  counter-electrodes which are provided on the substrate underneath mirror surface  610 . 
     In the example shown in FIG. 6, mirror surface  610  may be tilted about eight axes. Anchoring region  620  is circular and is located in the middle of likewise circular concentric mirror surface  610 . Torsion springs  630 - 637  of the torsion spring suspension extend from anchoring region  620  toward annular mirror surface  610 . The direction of tilting is selected by a control (not shown) of counter-electrodes  640 - 647  located below. When working with resonant frequencies of the pattern in the range of 100 Hz to several kHz, images may be projected at a refresh rate in the range of 50-100 Hz with the aid of this mirror form. This example can of course be generalized to more than eight axes for tilting. 
     FIG. 7 is a sixth example of a micromechanical oscillating mirror which can be manufactured using the designing and manufacturing method according to the present invention, FIG. 7 showing the basic construction of the previously described designs in a cross-sectional view. 
     In FIG. 7, reference number  700  designates a sixth mirror form,  710  a substrate,  715  a first insulating layer,  720  a second insulating layer,  730  a mirror surface,  740  a support,  745  a metal layer,  750  a connection pad,  755  an anchoring region,  760  a counter-electrode, and D a deflecting device. 
     In sixth mirror form  700 , in contrast to the specific embodiment according to FIG. 1 a-g , the anchoring is implemented in anchoring region  775  of metal layer  745 , metal layer  745  being a layer which is additionally applied to substrate  710  while interposing first insulating layer  715 . The connection of the oscillating mirror to the service connection (not shown) is carried out by bonding via connection pad  750 . Counter-electrode  760  is used for deflecting this oscillating mirror according to sixth mirror form  700  in deflecting direction D. 
     Although the designing and manufacturing method has been described above on the basis of preferred exemplary embodiments, it is not limited thereto but can be modified in many ways. 
     In particular, the choice of layer materials and the thickness of the applied layers can be selected in an application-specific manner.