Abstract:
A method for using an organic dielectric as a sacrificial layer for forming suspended or otherwise spaced structures. The use of an organic dielectric has a number of advantages, including allowing use of an organic solvent or etch to remove the sacrificial layer. Organic solvents only remove organic materials, and thus do not affect or otherwise damage non-organic layers such as metal layers. This may reduce or eliminate the need for the rinsing and drying steps often associated with the use of acidic etchants such as Hydrofluoric Acid.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to semiconductor fabrication techniques, and more particularly, to semiconductor fabrication techniques that use a sacrificial layer.  
         BACKGROUND OF THE INVENTION  
         [0002]    Traditionally, the miniaturization of mechanical and/or electromechanical systems has been frustrated by limitations on the manufacture of small lightweight mechanical or electromechanical parts. The intricacy of the parts made their manufacture on a small scale difficult and impractical. In recent years, however, and with the proliferation and increased precision in semiconductor fabrication procedures, many of the mechanical and electromechanical structures in a mechanical system may be replaced by MicroElectroMechanical Structures (MEMS) or other micromachined structures that are fabricated using semiconductor fabrication techniques.  
           [0003]    During the fabrication of many of these structures, a sacrificial layer is provided to provide temporary support for subsequent layers. Once the subsequent layers are fabricated, the sacrificial layer is removed using a selective etch, which releases the upper layers. In many cases, the selective etch must remove the sacrificial layer from a remote location, requiring the etchant to reach deeply into very narrow crevices and channels.  
           [0004]    In many cases, the sacrificial layer is formed from silicon dioxide or some other inorganic oxide or glass. An acid etch such as Hydrofluoric Acid is then used to remove the sacrificial layer. A limitation of using a silicon dioxide or other inorganic oxide or material is that the acidic etchants often require additional processing steps and can be difficult to perform. For example, to get a Hydrofluoric Acid etchant to flow deeply into small crevices and channels occupied by the sacrificial layer, surfactants must often be added. These surfactants can reduce the effectiveness of the Hydrofluoric Acid.  
           [0005]    In addition, the removal of the Hydrofluoric Acid etchant from the small crevices and channels can be difficult. Several rinsing and drying steps are often performed in an attempt to remove the Hydrofluoric Acid from the structure. Even with these additional steps, some residual Hydrofluoric Acid often remains, which can reduce the reliability of the resulting structure.  
           [0006]    Another limitation is that acidic etchants such as Hydrofluoric Acid tend to attack some metals such as aluminum. Accordingly, the sacrificial layer must often be either removed before any metal layers are provided, or additional processing steps must be performed to protect the metal layers from the Hydrofluoric Acid. However, even when the metal layers are provided after the sacrificial layer is removed, some residual Hydrofluoric Acid can remain, as described above. The residual Hydrofluoric Acid can emerge during later processing or testing, and damage the metal layers. This can compromise the reliability of the devices. What would be desirable, therefore, is a method for providing and removing a sacrificial layer without the use of an acidic etchant.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention overcomes many of the disadvantages of the prior art by using an organic oxide as a sacrificial layer. The organic oxide can be selectively removed using an organic solvent or etchant such as acetone, oxygen plasma etch or other organic etchant that is less destructive to other layers of the device. Organic solvents or etchants tend to only remove organic materials. This may reduce or eliminate the need for the above-mentioned rinsing and drying steps associated with the use of acidic etchants such as Hydrofluoric Acid. Further, many organic solvents or etchants can more easily reach deeply into very narrow crevices and channels occupied by the sacrificial layer.  
           [0008]    The use of an organic oxide as a sacrificial layer has many applications. For example, it is known that many MEMS and other micromachined devices have structures such as beams, slabs, combs, and fingers. To form these structures, a sacrificial layer is often used for temporary support. By reducing or eliminating the need for the rinsing and drying steps associated with acidic etchants, many of these structures can be more readily produced. In addition, many MEMS and other micromachined devices include conductive elements such as metal traces, pads, etc. By allowing the conductive elements to be provided before the sacrificial layer is removed, or by reducing or eliminating the processing steps required to protect the conductive elements from an acidic etchant, the fabrication of many of these structures can be simplified.  
           [0009]    The use of an organic dielectric as a sacrificial layer can also be used to produce structures such as low capacitance metal interconnect lines, on-chip inductors, etc. For low capacitance interconnect lines, an organic dielectric may be used as a sacrificial layer between two metal interconnect lines. Later, the sacrificial layer may be removed using an organic solvent, which as described above, does not damage the metal interconnect lines. Since the capacitance between the two metal lines is dependent on the electric pernittivity of the medium between the two conductors, and the electric permittivity of air is typically lower than that of a conventional interlayer dielectric material such as silicon oxide, the capacitance between the two conductors can be significantly reduced. This may significantly improve the performance (e.g., speed and power) of a corresponding circuit or device.  
           [0010]    Likewise, an on-chip inductor can be formed by providing a spiral of metal lines, preferably using two or more metal layers and interconnecting VIA structures. A sacrificial layer can be provided between the two metal layers to support the upper metal layer during the fabrication process. Once the spiral is formed, the sacrificial layer can be removed. By using an organic dielectric, the sacrificial layer can be removed without causing damage to the metal lines that form the inductor.  
           [0011]    In one illustrative method, a suspended structure is formed such as a MEMS or other micromachined structure, low capacitance metal interconnect line structure, onchip inductor, etc. To form such a suspended structure, a first layer is provided. Then, a sacrificial second layer is provided above the first layer. In some embodiments, one or more intervening layers may be provided between the first layer and the sacrificial second layer, if desired. The one or more intervening layers may include, for example, an inorganic dielectric, a polysilicon layer, a metal layer, an etch stop layer, or some other material or material system.  
           [0012]    The sacrificial second layer preferably includes an organic dielectric, and more preferably a low “k” organic dielectric. A third layer is then provided above the sacrificial second layer. Again, and in some embodiments, one or more intervening layers may be provided between the sacrificial second layer and the third layer, if desired. Finally, the sacrificial second layer is removed with an organic solvent or etch, leaving at least a portion of the third layer suspended. Preferably, the first layer, the third layer and selected intervening layers are substantially unaffected by the organic solvent.  
           [0013]    It is contemplated that the first layer and/or one or more of the intervening layers (if provided) may be patterned before the sacrificial second layer is provided. Likewise, it is contemplated that the third layer and/or one or more of the intervening layers (if provided) may be patterned before removing the sacrificial second layer, as desired.  
           [0014]    In another illustrative method, a structure having a first element spaced from a second element is provided. The spacing may be either a vertical spacing or horizontal spacing, or both. In this illustrative method, the first element and the second element are provided with a sacrificial layer therebetween. The sacrificial layer is an organic dielectric, which is subsequently removed using an organic solvent or etch. Preferably, the organic solvent does not substantially affect either the first or second elements, as in some embodiments, the first and/or second elements are exposed to the organic solvent.  
           [0015]    In another illustrative embodiment, a VIA structure is provided for electrically connecting a lower metal layer to an upper metal layer and/or for mechanically supporting the upper metal layer above the lower metal layer. In this illustrative method, a lower metal layer is provided adjacent a substrate and is subsequently patterned. Like above, and in some embodiments, one or more intervening layers may be provided between the lower metal layer and the substrate. A sacrificial layer, including an organic dielectric is then provided over the lower metal layer. Thereafter, an inorganic dielectric such as TEOS (Tetra Ethra Orthor Di Silicate) Oxide is provided above the sacrificial layer. The TEOS Oxide layer is optional, and will eventually provide support to a suspended upper metal layer structure. The TEOS and sacrificial layers are then patterned to provide an opening to the lower metal layer.  
           [0016]    The upper metal layer is then provided over the TEOS layer. The upper metal layer preferably extends down into the opening in the TEOS and sacrificial layers to form a connection with the lower metal layer. Alternatively, the opening may be filled with another material or layer before the upper metal layer is provided. The upper metal layer is then patterned, as desired. Finally, the sacrificial layer is removed using a suitable organic solvent, leaving the TEOS and upper metal layer suspended above the lower metal layer and/or substrate. The above described applications and structures are only illustrative. Those skilled in the art will recognize many other applications or structures that can benefit considerably from the use of an organic dielectric as a sacrificial layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    FIGS.  1 A- 1 C are cross-sectional side views showing an illustrative method for forming a suspended structure;  
         [0018]    FIGS.  2 A- 2 C are cross-sectional side views showing another illustrative method for forming a suspended structure;  
         [0019]    FIGS.  3 A- 3 D are cross-sectional side views showing yet another illustrative method for forming a suspended structure;  
         [0020]    [0020]FIG. 4 is a schematic diagram showing an illustrative inductor formed on a substrate; and  
         [0021]    [0021]FIG. 5 is a cross-sectional side view of the inductor of FIG. 4 taken along line  5 - 5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    FIGS.  1 A- 1 C are cross-sectional side views showing an illustrative method for forming a suspended structure. The illustrative suspended structure may be part of, for example, a beam, a slab, a comb, a finger, etc. of a MEMS or other micromachined device, a low capacitance metal interconnect line, an on-chip inductor, or any other suspended structure. FIG. 1A shows a substrate  10  having a first layer  12 , a sacrificial second layer  14  provided above the first layer  12 , and a third layer  16  provided above the sacrificial second layer  14 .  
         [0023]    Although not shown in FIG. IA, one or more intervening layers may be provided between the substrate  10  and the first layer  12 , the first layer  12  and the sacrificial second layer  14 , and/or the sacrificial second layer  14  and the third layer  16 , as desired. The one or more intervening layers may include, for example, an inorganic dielectric, a metal layer, a polysilicon layer, an optical layer, an etch stop layer, or any other material or material system that is desirable for a desired application. In some embodiments, the first layer  12  and/or selected intervening layers (if provided) may be patterned before the sacrificial second layer is provided.  
         [0024]    The sacrificial second layer  14  preferably is an organic dielectric, and more preferably a low “k” organic dielectric. Example commercially available low “k” organic dielectrics include FLARE(L, which is available from Honeywell International, FOX®, which is available from Dow Corning, and SILK®, which is available from Dupont Corporation. The sacrificial second layer  14  is preferably spun on to a desired thickness.  
         [0025]    As shown in FIG. 1B, the third layer  16 , and any intervening layers (if provided) may be patterned using conventional photolithographic techniques. In one embodiment, the third layer  16  is patterned to define the desired suspended structure, such as a beam, a slab, a comb, a finger, etc. The etchant used to pattern the third layer  16  may etch some of the sacrificial second layer  14  through opening  20 , as shown. However, this is not required or even desired in some applications. The illustrative opening  20  provides access to the sacrificial second layer  14 .  
         [0026]    Referring to FIG. 1C, the sacrificial second layer  14  is then removed with an organic solvent, preferably through opening  20 , leaving at least a portion of the third layer  16  suspended over the first layer  12 . The organic solvent may be, for example, an acetone, an oxygen plasma etch or any other organic solvent.  
         [0027]    As indicated above, using an organic dielectric as a sacrificial second layer has a number of advantages. One advantage is that organic solvents or etchants can be used to remove the sacrificial layer. As indicated above, many organic solvents can more easily reach deeply into very narrow crevices and channels occupied by the sacrificial layer. Another advantage of using an organic solvent is that it typically only removes organic materials. Thus, when the first layer  12  and the third layer  16  are metal layers, the organic solvent may not affect or otherwise damage these layers. In some cases, the organic solvent may actually clean these materials, which may potentially improve the performance or reliability of the resulting device. As a result, the first layer  12 , the third layer  16  and any other layers that are exposed to the organic solvent need not be provided before the sacrificial layer  14  is removed, or separately protected from the organic solvent or etchant when provided after the sacrificial layer  14  is removed. This may simplify the fabrication of such structures.  
         [0028]    FIGS.  2 A- 2 C are cross-sectional side views showing another illustrative method for forming a suspended structure. This method is similar to that described above with respect to FIGS.  1 A- 1 C, but an intervening layer  46  is provided between the sacrificial second layer  44  and the third layer  49 . The intervening layer  46  may be, for example, an inorganic dielectric, a metal layer, a polysilicon layer, an optical layer, an etch stop layer, or any other material or material system that is desirable for a particular application.  
         [0029]    [0029]FIG. 2A shows a substrate  40  having a first layer  42 , a sacrificial second layer  44  provided above the first layer  42 , an intervening layer  46  provided above the sacrificial second layer  44 , and a third layer  48  provided above the intervening layer  46 . Although not shown in FIG. 2A, one or more other intervening layers may be provided between the various layers, if desired. In the illustrative embodiment, the intervening layer  46  is an inorganic dielectric that will eventually provide support for the third layer  48 .  
         [0030]    As shown in FIG. 2B, the third layer  48  and intervening layer  46  are patterned using conventional photolithographic techniques. In one embodiment, the third layer  48  is patterned to define the desired suspended structure, such as a beam, a slab, a comb, a finger, etc. The etchant used to pattern the third layer  48  may etch the intervening layer  46  and the sacrificial second layer  44 , as shown, to provide an opening  50 . The opening  50  may provide access to the sacrificial second layer  44 .  
         [0031]    Referring to FIG. 1C, the sacrificial second layer  44  is then removed with an organic solvent, preferably through opening  50 , leaving at least a portion of the third layer  48  and the intervening layer  46  suspended over the first layer  42 . As indicated above, the intervening layer  46  may help provide support to the third layer  48  once the third layer  48  is released.  
         [0032]    FIGS.  3 A- 3 D are cross-sectional side views showing yet another illustrative method for forming a suspended structure. In this embodiment, a VIA or support structure for electrically connecting a lower metal layer to an upper metal layer and/or for mechanically supporting an upper layer above a lower layer is provided. A lower layer  60  is provided adjacent a substrate  62 , and is subsequently patterned leaving lower layer elements  60   a  and  60   b.  Like above, and in some embodiments, one or more intervening layers may be provided between the lower layer  60  and the substrate  62 , if desired.  
         [0033]    A sacrificial layer  64 , including an organic dielectric, is provided over the lower layer  62  as shown. Thereafter, and in the illustrative embodiment, an inorganic dielectric layer  66  such as TEOS (Tetra Ethra Orthor Di Silicate) Oxide is provided above the sacrificial layer  64 . The TEOS Oxide layer  66  will eventually provide support for a suspended upper layer structure, as more fully described below. As shown in FIG. 3B, the TEOS layer  66  and the sacrificial layer  64  may be patterned using a patterned mask  70 , preferably resulting in an opening  72  to the lower layer element  60   a.    
         [0034]    Referring now to FIG. 3C, once the patterned mask  70  is removed, an upper layer  74  is provided above the TEOS layer, as shown. The upper layer preferably extends down into the opening  72  to form a connection with the lower layer element  60   a,  as shown. Alternatively, however, the opening  72  may be filled with another material or layer (not shown) before the upper layer  74  is provided. The upper layer  74  is then patterned, as desired. Finally, and as shown in FIG. 3D, the sacrificial layer is removed using a suitable organic solvent or etchant, leaving the patterned TEOS layer  66  and upper layer  74  suspended above the lower layer  60  and/or substrate  62 .  
         [0035]    In one embodiment, the first layer  60  and the third layer  74  are conductive elements having, for example, an aluminum or copper concentration. When so provided, the first layer  60  and the third layer  74  may be electrically connected at interface  80 . Alternatively, opening  72  may be filled with a plug, such as a tungsten plug, before the third layer  74  is provided to provide the electrical interconnection. Either way, such a structure can provide a VIA structure that provides the electrical connection and mechanical support for the third layer  74 .  
         [0036]    In this embodiment, the first layer  60  and the third layer  74  are exposed to the solvent or etchant used to remove the sacrificial second layer  64 . By using an organic dielectric for the sacrificial second layer  64 , an organic solvent or etchant can be used to remove the sacrificial second layer  64 . Organic solvents tend to only remove organic materials, and thus will not affect or otherwise damage the conductive first layer  60  and third layer  74 . In some cases, the organic solvent may actually clean these layers, which may improving the performance or reliability of the resulting device.  
         [0037]    As can be seen in FIGS.  3 A- 3 D, the first layer  60  may include first layer element  60   b.  When the sacrificial second layer  64  is removed, the first layer elements  60   a  and  60   b  are separated by an air gap. This illustrates that the present invention may be used to form a spacing between elements that is either vertical or horizontal, or both.  
         [0038]    [0038]FIG. 4 is a schematic diagram showing an illustrative inductor formed on a substrate. The inductor is generally shown at 90, and has a number of metal lines interconnected into a spiral shape, preferably using two or more metal layers and interconnecting VIA structures. In FIG. 4, the metal lines that are formed on a first metal layer are shown in solid lines, and the metal lines that are formed on a second metal layer are shown in dashed lines. The VIA structures are shown as black boxes, and are used to provide an electrical interconnection between the metal lines on the first metal layer with appropriate metal lines on the second metal layer. As indicated above, the metal lines on the first metal layer, the metal lines on the second metal layer, and the VIA structures are preferably combined to form a spiral structure, as shown.  
         [0039]    To fabricate the on-chip inductor, the metal lines formed on the first metal layer are preferably provided and patterned, followed by a sacrificial layer formed of an organic dielectric. Then, the VIA structures and the metal lines on the second metal layer are provided and patterned, preferably in a manner similar to that described above with respect to FIGS.  3 A- 3 D. Once the metal lines on the first metal layer and the metal lines on the second metal layer are connected in a spiral pattern, the sacrificial layer is removed. By using an organic dielectric, the sacrificial layer can be removed with an organic solvent or etchant. As indicated above, an organic solvent or etchant typically only removes organic material, and thus may not damage the metal lines that form the inductor. Once the sacrificial layer is removed, each of the metal lines is preferably separated from adjacent metal lines by an air gap. This is shown in FIG. 5, which shows a cross-sectional side view of the inductor of FIG. 4 taken along line  5 - 5 . By providing an air gap between adjacent metal lines, the parasitic capacitance between metal lines can be reduced, thereby providing a more ideal inductor.  
         [0040]    By using an organic dielectric, and thus an organic solvent, the metal lines on the first metal layer and the metal lines on the second metal layer can be exposed to the organic solvent. Thus, these metal lines need not all be fabricated before the sacrificial layer  14  is removed, or separately protected from the etchant when provided after the sacrificial layer  14  is removed. This may simplify the fabrication of such structures.  
         [0041]    The above described applications and structures are only illustrative. Those skilled in the art will recognize many other applications or structures that can benefit considerably from the use of an organic dielectric as a sacrificial layer.