Abstract:
Provided is a method and apparatus for close alignment of two or more electrically conductive wafers which are positioned face-to-face in closely spaced opposition, the wafers having position marks on corresponding portions thereof, the wafers being aligned as to their mating components, as guided by optically comparing the alignment of the respective position marks; deflecting an interior portion of one of the wafers into contact with the other wafer, to partially bond the wafers to each other, then fully contacting and bonding the rest of the wafer pair and then optically checking the resulting wafer alignment to see if same is acceptable.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. 
    
    
     RELATED APPLICATIONS 
     None 
     FIELD OF THE INVENTION 
     This invention relates to wafer alignment and bonding, particularly accurate alignment thereof. 
     BACKGROUND OF THE INVENTION 
     Microsystem technology applications are often composed of two or more patterned substrates that are aligned and then bonded to each other. Wafer-to-wafer or two-substrate bonding is used to form micromechanical structures in such applications. 
     Thus the wafers are aligned in an alignment tool using either image capture, visible or IR alignment methods. Once aligned, the wafers are held in place by a removable clamping fixture that has two more clamps that clamp the edges of the wafers to spacer members to hold the wafers a desired distance apart. That is, the spacers keep the wafers face-to-face but separated by a few microns. 
     After the wafers are aligned, they are locked in a clamping fixture and moved to a separate bonding machine. Once in the bonding machine, a center or interior point contact is made to hold the wafers together at least at the contact point. The clamps are then removed and the spacer retracted and bonding of the wafer pair begins. However, there are drawbacks in the above prior art method. That is, the applying of the clamping fixture invariably allows some movement when clamping force is applied. Also, particles often result from the withdrawal of the mechanical spacers, a chronic problem. 
     Also, during transfer to a bonding chamber, the clamping fixture is subject to mechanical transfer forces and temperature variations which can affect the wafer alignment. 
     Then, in the bonding chamber, a vacuum is usually applied which causes turbulence and can add more particles between wafers before they are sealed together. 
     Accordingly there is need and market for method &amp; apparatus for preserving wafer alignment from the alignment stage to the bonding stage of such wafers while avoiding or minimizing the chance for particles to become embedded between wafers before the bonding step is complete. 
     There has now been discovered method and apparatus for preserving wafer alignment and bonding of such wafers while minimizing particle intrusion therebetween, as further described below. 
     SUMMARY OF THE INVENTION 
     Broadly, the present invention provides a method for alignment of a stack of two or more electrically conductive wafers comprising, 
     placing the wafers in closely spaced face-to-face opposition, which faces have been hydrophilically treated and 
     deflecting one wafer into contact with the other at an interior portion thereof, while holding the remainders of the wafers apart, by exterior pulling means, to partially bond the wafer pair. 
     Also provided is the step of releasing the above wafers so as to permit a contact bonding wave to spread from such interior portion to the wafer edges to adhere the wafers to each other in a full bond. 
     Also per the invention, contact between the spaced wafers can be initiated at opposed, spaced wafer edges causing a contact bonding wave to spread between the wafers to join the remaining portions thereof. However the above initial interior bonding contact method is preferred. 
     Further provided is the method of providing position marks on corresponding portions of the two wafers and optically checking the alignment of the marks before and after full bonding of the wafer pair. 
     Also provided is a 3D bonded microsystem structure of a stack of two or more patterned wafers aligned to within &lt;1 micron or 1μ. 
     Definitions: 
     By wafer alignment, as used herein, is meant alignment of the mating components in a stack of two or more wafers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The Invention will become more apparent from the following detailed specification and drawings in which; 
         FIG. 1  is an elevation schematic view of a prior art wafer alignment apparatus; 
         FIG. 2  is an elevation schematic view of the apparatus of  FIG. 1  in a bonding machine; 
         FIG. 3  is an elevation schematic view of a wafer alignment and bonding apparatus according to the present invention; 
         FIG. 4  is an elevation schematic view of the apparatus of  FIG. 3 , with added components, in operation; 
         FIG. 5  is a fragmentary schematic plan view of an alignment guide system employed by the apparatus of  FIG. 4 ; 
         FIG. 6  is an elevation schematic view of bonded layers as prepared by the prior art apparatus of  FIG. 2  and 
         FIG. 7  is an elevation schematic view of bonded layers embodying the invention prepared by the alignment and bonding methods of the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring in more detail to the drawings, in the prior art, wafer-to-wafer patterned substrates are first aligned and later bonded to each other. Such systems use a mechanical clamping fixture which holds the alignment of wafers at, e.g., 3 points on the wafers&#39; perimeter. Thus two wafers  18  &amp;  20 , are aligned and spaced apart by spacers  20  &amp;  22 , in the clamps  24  &amp;  26 , as shown in  FIG. 1 . 
     Then the so aligned wafers are transported, in the prior art, from an alignment stage to a bonding stage, such as bonding apparatus  10 , shown in  FIG. 2 . 
     Here, the so clamped and aligned wafer pair are placed in the bonding apparatus  10 , between a pair of spaced chucks  12  &amp;  14 , as indicated in  FIG. 2 , except that the plunger  32  is retracted and wafer  16  is still spaced from wafer  18 , per  FIG. 1  and such wafer  16  is yet undeflected and in close proximity or in contact with chuck  14  of bonding apparatus  10 , as indicated in  FIGS. 1 &amp; 2 . Subsequently, the plunger  32  descends, contacting the wafer  16  and deflecting an interior portion thereof toward and into contact with wafer  18  below, as shown or indicated in  FIG. 2 . Upon such contact, a bonding wave spreads radially outwardly from the contact point between the two wafers and, at about the same time, the clamped spacers  20  and  22  are withdrawn from between such wafers, which allows the contact wave to propagate to the wafers&#39; edge. Heat and pressure are then applied to create a strong and permanent bond between the two wafers  16  &amp;  18 . 
     However, misaligned wafers and entrained particles therebetween can result with the above two-stage prior art method, as discussed above and as indicated in  FIG. 6 . 
     That is, two wafers  70  &amp;  72 , with circuitry therein, have been bonded together at layers  74  and  76  but with at least one particle  84  captured therebetween, which creates a void  86  between such layers, resulting in incomplete bonding therebetween and deformation of such layers, resulting in a faulty bonded structure  66 , with impaired conductivity and performance, as shown or indicated in  FIG. 6 . 
     A new procedure and improved apparatus is required to overcome the above prior art shortcomings. The present invention provides a way out of the above difficulty, as shown or indicated in  FIGS. 3 and 4 . 
     Thus, wafers  46  &amp;  48  are held in vacuum contact with chucks  42  and  44  while maintaining e.g., a 5-10 micron (μ) separation. No physical spacers are needed for separating the two wafers, thus eliminating potential particle incursions. 
     The wafers are then precision aligned, to less than 1μ offset, using visible and/or IR illumination. That is, as shown or indicated in  FIGS. 4 &amp; 5 , viewing apertures  58  &amp;  59  are provided in the respective vacuum chucks  44  &amp;  42 , for IR alignment, lamp  53  is mounted under aperture  59  and IR light detector  54 , in combination with focusing lens  56 , being mounted above aperture  58 , as shown in  FIG. 4 . The IR light detector is electrically connected to light monitor  55  to display the light received from IR lamp  53 . 
     To complete the alignment system, the upper wafer  48  is marked with crosshairs  60  while the lower wafer  46  is marked with crosshairs  62 , as indicated in  FIG. 5 , such crosshairs being in at least partial alignment with the viewing apertures  58  and  59 , which crosshairs are illuminated by IR lamp  53  and protected upwardly to lens  56  (or just to the eye of the observer, not shown), through IR receptor  54  and thence to monitor  55  (via conductor  57 ), which displays the relative position of the crosshairs  60  &amp;  62 , on the monitor  55 , as shown or indicated in  FIG. 4 . Thus the respective wafers  46  and  48  can be readily positioned in accurate alignment when the crosshairs  60  &amp;  62  line up, as shown or indicated in  FIGS. 4 &amp; 5 , before the plunger  52  is activated to deform or press wafer  48  toward wafer  46 . That is, the alignment of the crosshairs  60  &amp;  62  can be viewed at the monitor  55 , at no magnification or various magnifications, such as low magnification, 50-500× or high magnification 500-1000×, to achieve highly precise alignment of the wafers  46  &amp;  48 , as indicated in  FIGS. 4 &amp; 5 . Once satisfactory alignment is achieved, the plunger  52  can descend, through aperture  45 , in chuck  44 , pushing an inner portion of wafer  48  toward wafer  46  and into contact therewith in a central or interior portion of each wafer. That is, at this point in which interior contact between two wafers is made, separation of the rest of the wafer pair is maintained by the vacuums applied by vacuum chucks  44  &amp;  42 , as indicated in  FIG. 4 . Thus, the wafer pair are now bonded at an interior area with no shift in wafer alignment to contend with. To this end the plunger  52  can be round in cross-section, including circular, oval or other round shape or a combination thereof, as can its aperture  45  per  FIG. 4  hereof. 
     The next step is to complete the bonding of the wafer pair. That is, the vacuum holding the rest or peripheral portions of wafer  48  separate from the rest of the wafer  46 , can be suitably reduced to permit a bonding wave to spread radially outwardly from the bonded interior portion of the pair to the wafers&#39; edges to achieve full (or peelable) bonding of the wafer pair. 
     The alignment of the above wafer pair can then be verified by the above IR optical system. Thereafter heat and/or pressure can be applied to the so bonded wafer pair either in situ (or after transfer to a bonding apparatus or bonder), to create a stronger more permanent bond, with continuing precise alignment of the wafer pair, while avoiding voids and/or trapped particles between the so bonded wafers. 
     The inventive product provided according to the method and apparatus of the invention, is shown as 3D bonded microsystem structure  68  in  FIG. 7 , wherein the respective wafer layers  90  &amp;  92  are precisely bonded in close contact with little or no intrusive particles or voids disrupting the continuity of contact therebetween, permitting excellent electrical properties of the so bonded and aligned structure made possible by the present Invention. 
     Recall that in prior art systems, e.g., per  FIGS. 1 &amp; 2 , there is much uncertainty of the alignment integrity in the transport of a wafer pair from an alignment fixture to a bonder, due to the actual separation of the wafers by spacers  20  and  22  of the prior art. Only after the bonded wafer pair is removed from the bonder can one check the alignment between the so bonded pair. Typically, there is a shift between prior alignment and final bond of the wafer pair. However, the inventive method and apparatus greatly reduces the potential for such alignment shift, as indicated above. 
     Accordingly, the method and apparatus of the present invention provides safeguards against misaligned bonded wafers, incursions of particles and voids between wafers, to provide precise wafer structures with properly aligned circuitry and high electrical conductivity and also can greatly lower the reject rate of malformed wafer stacks. 
     With vacuum chucks per the invention, the wafer pair can be separated by minimum distance of &lt;5-10μ, with the vacuum chucks preferably specified flat to within ±1 micron. With such close proximity of the wafer pairs, the depth of focus for the optical objectives will be minimized in order to obtain under (&lt;) 1μ alignment accuracy in high-power optical viewing of the wafer alignment crosshairs or other alignment marks. 
     The vacuum chucks that hold the wafers are attached to electrically controlled precision motor stages that use piezoelectric motor interferometers or other means, to position the movement of the stages to within less than (&lt;) 1μ alignment accuracy. This technology is readily available in the semiconductor industry and is used in wafer steppers for the exposure of photoresist patterns on wafers. 
     The above process is dependent upon the wafers to be aligned, having hydrophilic surfaces that have been chemically pretreated e.g., with hot hydrogen peroxide, sulfuric acid or an rca clean typically used in semiconductor processes. 
     That is, two wafers with hydrophilic surfaces are attracted to each other and can form a reasonable room temperature bond (i.e., a peelable bond) of sufficient strength so as to maintain prior alignment while the wafer pair is transferred to a bonder for heat treatment and further bond strengthening, as desired. 
     It should be noted that if the wafer pair are brought into initial contact and a contact wave spreads to bond the wafer pair at ambient temperature the wafers will now be bonded with no shift in alignment, as confirmed by the IR optical system described above. If however, the wafer pair does bond out of alignment, as determined by the above IR optical tests, the wafers can yet be peeled apart released and the above alignment and bonding process repeated, the alignment of the realigned and re-bonded pair confirmed optically and then the now accurately bonded pair can be bonded in situ or transferred to a bonder with heat and/or pressure applied thereto for a more permanently bonded wafer structure. 
     Also as indicated previously, more than two wafer layers can be stacked, aligned and bonded in the manner described above to form a bonded multi-wafer structure, as desired, per the invention.