Abstract:
A method for fusing together, using diffusion bonding, micromachine subassemblies which are separately fabricated is described. A first and second micromachine subassembly are fabricated on a first and second substrate, respectively. The substrates are positioned so that the upper surfaces of the two micromachine subassemblies face each other and are aligned so that the desired assembly results from their fusion. The upper surfaces are then brought into contact, and the assembly is subjected to conditions suited to the desired diffusion bonding.

Description:
BENEFIT OF PROVISIONAL APPLICATION 
     This application claims the benefit of Provisional Application No. 60/176,188, filed Jan. 14, 2000. 
    
    
     FEDERALLY FUNDED RESEARCH 
     This invention was made with Government support under Contract DE-AC04-94AL85000 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     This invention relates to methods of forming micromachines and components. More particularly, it relates to a fabrication process wherein micromachine subassemblies on separate substrates are diffusion bonded together to form a united structure. 
     Micromachines are potentially useful for a wide range of applications requiring robust and complex miniature mechanisms, and particularly for forming an interface between complex microelectronic circuitry and the external world. Many such applications, however, require many discrete levels of components, some of which components are thick compared to the average component thickness, to provide the required functionality. Multiple component levels and thick components, however, are difficult to fabricate based solely on the planar microfabrication techniques known in silicon microelectronics and in such metallic-based fabrication techniques as LIGA (LIGA is a German acronym which stands for lithography, electroplating, and molding). Even when high-aspect-ratio fabrication techniques are used, maintenance of precise intercomponent separations and tolerances is often difficult. In addition, even though suitable techniques exist to planarize the surface of a micromachine subassembly, so that further growth and component definition can take place, problems and costs associated with such serial microfabrication dramatically limit the use of these techniques. 
     There is therefore a need for a method to enable assembly of micromachine subassemblies which require multi-level precision structures. A further need is for a method which can be used to assemble such micromachines in batches, preferably on the scale of entire substrate wafers. Application of such techniques to packaging of micromachines will also be useful to the industry. 
     SUMMARY OF THE INVENTION 
     A method for assembling a pair of micromachine subassemblies, each initially fabricated on its own substrate, and each having a planar upper mating surface. At least one of the substrates also has a release layer located between the substrate proper and the micromachine subassembly. The substrates are oriented so that their upper mating surfaces face each other, and then are pressed together in proper relative orientation. The conditions of temperature, pressure, and time under which the pressing is done are such as to result in diffusion bonding of the upper mating surfaces. After this diffusion bonding step, the release layer is removed, thereby yielding a micromachine assembly atop one of the substrates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows, in schematic form, structures produced while carrying out a specific implementation of the invention. 
     FIG. 1 a  shows a pair of micromachine subassemblies on separate substrates. 
     FIG. 1 b  shows the micromachine subassemblies as they appear during the diffusion bonding step. 
     FIG. 1 c  shows the micromachine assembly after being released from one of the substrates. 
     FIG. 2 shows, in schematic form, structures produced while carrying out a second implementation of the invention. 
     FIG. 2 a  shows a pair of micromachine subassemblies on separate substrates. The upper mating surfaces of the subassemblies belong to a diffusion bonding layer which facilitates the diffusion bonding process. 
     FIG. 2 b  shows the micromachine subassemblies as they appear during the diffusion bonding step. 
     FIG. 2 c  shows the micromachine assembly after being released from one of the substrates. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The instant invention is a group of related methods which serve to assemble two micromachine subassemblies atop each other using diffusion bonding to bond the structures of the two subassemblies together. Specific implementations will be discussed to teach working principles of the invention, but are not intended to limit the scope of the invention to those implementations presented. Rather, the scope of the invention is intended to be set by the claims interpreted in view of the specification. 
     The first implementation to be presented appears in FIG. 1. A first micromachine subassembly is fabricated atop first substrate  100 . The first micromachine subassembly comprises components  101 - 107 . A second micromachine subassembly is fabricated atop release layer  109 , which is grown atop second substrate  108 . The second micromachine subassembly comprises components  110 - 114 . 
     The micromachine subassemblies can be prepared using any suitable technique or combination of techniques. These would include such diverse fabrication approaches as LIGA (an acronym for the German for lithography, electroplating, and molding), deep x-ray lithography, silicon surface micromachining and lithography, electric discharge machining, and direct laser additive fabrication. Substrate can be made of any suitable material, semiconductor, metallic, or ceramic substrates being particularly suitable. The subassembly components are typically made of silicon, polysilicon, or nickel alloys, but can include semiconductors, metals, ceramics, and combinations of these materials. 
     A release layer  109  is formed atop substrate  108 , and the second micromachine subassembly is formed atop release layer  109 . The release layer must be compatible with the materials of substrate  108 , the materials of the second micromachine subassembly, and the conditions under which the diffusion bonding is carried out. This requires that the various materials have similar coefficient of expansion, exhibit good adhesion, do not melt or significantly deform under the diffusion bonding conditions, and similar considerations which are well-known in the art. 
     Removal of the release layer without damaging the functionality of the assembled micromachine subassemblies must also be possible. Removal is typically accomplished using wet chemical or plasma etching techniques. Examples would include use of a silicon oxide release layer between a silicon substrate and polysilicon micromachine components, and use of a copper release layer between an alumina substrate and nickel micromachine components. The silicon oxide release layer can be removed using a hydrofluoric acid etchant, and the copper release layer can be removed using an acid treatment. 
     FIG. 1 a  shows a variety of micromachine components which have been fabricated atop first substrate  100 , and also atop release layer  109 . Pillar  101  is to form a base for alignment guide  110 , which has the form of a hollow cylinder. Alignment mount  102  also has the form of a hollow cylinder, within which guide rod  103  is press fitted. Guide rod  103  slides within alignment guide  111  when the two subassemblies are fitted together, thereby insuring that the mutual alignment and orientation is correct. We have found that alignment of the two subassemblies of better than one micron when assembled can be obtained using such alignment guides. 
     Disk mount  104  is a larger hollow cylinder, to which disk  112  will be attached in the assembly process. Axle  105  forms an axle for wheel  106 , which can be fabricated in place, or fabricated separately, and then inserted onto axle  105 . Wheel  106  will then be held in place by attaching axle cap  113 . Finally an axle on a higher level is formed by bonding axle mount  107  to shaft  114 . 
     A small number of specific component types are illustrated in this example. However, there are a huge range of micromechanical components available for use in such subassemblies. Use of specific components to illustrate aspects of the instant invention should not be read as limiting the scope of the invention to subassemblies comprising those specific components. 
     We also point out that the simple structures of FIG. 1 can be duplicated a large number of times over the surface of a substrate, so that many micromachine assemblies can be bonded in a single operation. This capacity for wafer-scale assembly is an important practical aspect of the instant invention. 
     The upper surfaces (meaning the surfaces farthest from the supporting substrate) of the various components forms the planar mating surfaces of their respective micromachine subassemblies. Usually, however, the initial fabrication process will not produce mating surfaces which are sufficiently flat for reliable diffusion bonding, which requires intimate contact of the surfaces to be bonded. 
     Accordingly, to produce planar mating surfaces it is usually necessary to apply a fabrication technique suited to producing a sufficiently flat surface. Such techniques include, but are not limited to, diamond lapping and chemical-mechanical polishing. Such techniques and their application are well-known in the art. 
     FIG. 1 b  shows the two substrates and their respective micromechanical subassemblies positioned as they are during the diffusion bonding process. The strength of the diffusion bond can, in some cases, be improved by cleaning foreign materials, in particular oxide layers, from the planar mating surfaces before pressing them together. Such cleaning is not a necessary part of the instant invention, but at times will result in a superior product. 
     Techniques suited for such cleaning are generally known in the art, and often include removing foreign materials with an oxygen plasma, or other means of exposure to oxygen, followed by etching away the resulting oxygen layer. For example, a nickel surface can be cleaned by exposure to an oxygen plasma followed by an ammonium hydroxide treatment to remove the nickel oxide layer which results. 
     Diffusion bonding conditions are well known for a wide range of materials, and are accessible to one skilled in the art. For the specific case of electrodeposited nickel mating surfaces, it is beneficial to place the surfaces in a high vacuum (order of 10 −6  Torr), and then to backfill with argon prior to carrying out diffusion bonding. Suitable conditions for the diffusion bonding process are temperatures near 500° C. and pressures in the 10-100 megapascal region. These conditions, in the case of these electrodeposited nickel mating surfaces, must be maintained for a period typically of several hours to obtain a robust diffusion bond. Typical diffusion bond shear strengths in this case are similar to the yield strength of the nickel material. 
     FIG. 1 c  shows the assembled micromachine components after the release layer has been removed and the second substrate removed. Pillar  101  and alignment guide  110  have been bonded to form raised alignment guide  115 , which can serve to guide the addition of an additional micromechanical subassembly. Alignment mount  102  and alignment guide  111  have been bonded to form alignment channel  116 , which is filled by guide rod  103 . 
     Disk mount  104  and disk  112  have bonded to form raised disk  117 . Note that a structure similar to  117  can be used to package other micromechanical components. 
     Axle  105  and axle cap  113  have bonded to form axle assembly  118 , around which wheel  106  can still rotate. Axle mount  107  and shaft  114  have bonded to form raised axle  119 . 
     An additional implementation of the instant invention is shown in FIG.  2 . FIG. 2 a  shows first substrate  200 , on which are fabricated support pillars  201  and  202 , on top of which are formed bonding layers  203  and  204 . The bonding layers are often lapped or polished to form planar mating surfaces. 
     On top of second substrate  205  is formed release layer  206 , on top of which is formed plate  207 . On plate  207  is formed bonding layer  208 , which again is often lapped or polished to form planar mating surfaces. Note that if the material of the bonding layers is sufficiently soft, material deformation at the interface can allow adequate diffusion bonding for many purposes. 
     FIG. 2 b  shows the substrates and subassemblies in position for diffusion bonding. As in the earlier implementation, cleaning of the mating surfaces prior to pressing them together can be beneficial. 
     FIG. 2 c  shows the assembled micromachine after diffusion bonding and after the release layer and second substrate are removed. Bonding layers  203 ,  204 , and  207  have merged in the diffusion bonding process to form bonding pads  209  and  210 .