Abstract:
A method of forming spring structures using a single lithographic operation is described. In particular, a single lithographic operation both defines the spring area and also defines what areas of the spring will be uplifted. By eliminating a second lithographic operation to define a spring release area, processing costs for spring fabrication can be reduced.

Description:
BACKGROUND 
       [0001]    Stressed metal devices have become increasingly important for fabricating interconnects, probes, inductors and the like. However, fabrication of the stressed metal devices is a difficult and expensive process. One reason for the extra expense is the use of multiple lithography steps. 
         [0002]    Prior art spring formation techniques typically use at least two lithography operations. A first lithography operation patterns a stressed or bimorph metal to form a general spring structure. A second lithography operation defines a spring release area (the release area is defined as the region that uplifts from a substrate). The second lithography operation may also be used to plate additional metal onto the stressed metal spring. A detailed description of the entire process is provided in U.S. Pat. No. 6,528,350 which is hereby incorporated by reference in its entirety. 
         [0003]    These two basic lithographic operations have remained the same for about ten years. The cost associated with two lithographic operations has kept spring interconnect technology more expensive then some competing interconnect technologies. Thus a more efficient and thus less expensive way of fabricating a stressed metal device is needed. 
       SUMMARY 
       [0004]    A method of making a spring structure with only a single lithographic operation is described. The method includes the operations of depositing a release layer over a substrate. A resist pattern is formed over the release layer and a spring material deposited in an opening in the resist. The spring material includes an internal stress gradient. After deposition of the spring material, the resist and spring material are exposed to an etchant that penetrates an interface between the resist and spring material. The etchant etches the release layer under a release portion of the spring material to allow a release area of the spring to curl out of the plane of the substrate. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIGS. 1-9  show a side cross sectional view of the operations involved in forming a stressed metal spring using a single lithographic operation. 
           [0006]      FIGS. 10-13  show the use of an optional adhesion and cementation layer underneath the release layer. 
           [0007]      FIG. 14  shows a front cross sectional view of a spring prior to exposure to an etchant. 
           [0008]      FIGS. 15-16  show the front cross sectional view of  FIG. 5  as an etchant penetrates the interface between a spring material and a surrounding mask material. 
           [0009]      FIG. 17  shows the front cross sectional view of  FIG. 5  after the etchant releases the spring from the substrate. 
           [0010]      FIGS. 18-21  show the process used in  FIGS. 14-17  when the rate of etching is enhanced by a gap widening etch that increases the size of a gap between the mask and the spring material. 
           [0011]      FIGS. 22-24  show the use of a negative side profile resist to delay spring uplift. 
           [0012]      FIG. 25  shows a side schematic view of example resulting spring structures. 
           [0013]      FIG. 26  shows a top view of the spring and anchor region with the unreleased portion of the anchor outlined. 
           [0014]      FIGS. 27-28  show perforating the release region to facilitate etching the release layer underneath the spring release region. 
           [0015]      FIGS. 29-30  show alternate spring structure patterns. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A method of creating a stressed metal spring structure using a single lithographic operation will be described. The spring structures are typically used to interconnect circuit devices such as integrated circuits. As used herein, stressed metal is defined as a spring structure with an internal stress gradient typically formed by the deposition of multiple sublayers, each sublayer deposited at a different a different temperature or pressure such that the density in each sublayer is different resulting in an the internal stress gradient. A detailed description of forming a stressed metal spring is provided in U.S. Pat. No. 6,528,350 entitled “Method for Fabricating a Metal Plated Spring Structure” by David Fork which is hereby incorporated by reference. 
         [0017]      FIGS. 1-9  provide a schematic side view of a one lithography operation or an “all-in-one” process for forming a stressed metal spring. In  FIG. 1 , a release layer  104  and a seed layer  108  are deposited over a substrate  100 . Release layer  104  is selected to be a material that can be easily etched to “release” a spring that will be subsequently deposited over the release layer. In one embodiment, release layer  104  is a sputtered titanium (Ti) layer. 
         [0018]    Seed layer  108  is deposited over the release layer. Seed layer  108  facilitates growth or deposition of masking materials (typically a resist) and spring materials deposited over seed layer  108 . An example seed layer is a gold (Au) layer deposited by sputtering techniques. 
         [0019]    It is sometimes advantageous to combine release layer  104  and seed layer  108  into a single layer or use a single material for both layers. Combining the two layers reduces the number of deposition operations during fabrication. Examples of a combined seed/release layer are titanium (Ti), copper (Cu) and nickel (Ni) deposited in a single layer over substrate  100 . 
         [0020]    In  FIG. 2 , a lithographic process is used to deposit a mask, typically a hard mask, such as a resist  204 . Resist  204  may be any common commercial photoresist used in semiconductor processing. A method of using this same resist mask for spring patterning, release and overplating will be described. Multiple use of the same mask reduces fabrication cost. Cost reductions arise from both mask count reductions and also elimination of resist spinning, baking developing, exposing and stripping associated with additional maskings. 
         [0021]    In  FIG. 3 , a spring material  304  is deposited in a resist material  204  opening. In one embodiment, spring material  304  is a nickel (Ni) plating deposited in a plurality of sublayers to create an internal stress gradient. Electroless or electroplating techniques may be used to deposit the spring material. In one embodiment, the built in stress gradient is obtained by plating from two baths with different stress characteristics or by varying the current density during plating. A detailed description of forming such stress gradients is provided in Kenichi Kataoka, Shingo Kawamura, Toshihiro Itoh, Tadatomo Suga, “Low contact-force and compliant MEMS probe card utilizing fritting contact,” IEEE Proceedings of Micro Electro Mechanical Systems (MEMS) 2002, pp. 364-367, 2002 which is hereby incorporated by reference. 
         [0022]    Although  FIG. 3  shows a stressed metal spring material, it should be understood that the spring material is not limited to stressed metals. For example, a bimorph or bimetallic material may be used. Temperature or other parameter changes induce stresses in the bimorph or bimetallic material causing the spring release portion to curl out of the plane of the resist. 
         [0023]    After spring material deposition, the entire structure is exposed to a series of interface penetrating etches. The etchant penetrates interface  404 ,  408  between spring material  304  and resist material  204 . The first etchant removes portions of the seed layer near interfaces  404  and  408 . In one example, the seed layer is a gold layer, and a typical etchant is an etchant containing potassium iodide (KI) and iodide (I). 
         [0024]    In  FIG. 5 , a second interface penetrating etchant follows the seed layer etch. The second interface penetrating etch etches release layer  104 . In one example, the release layer is a titanium layer and the second interface penetrating etchant is hydrofluoric acid (HF) or buffered hydrofluoric acid (BHF). The release layer etch starts from the interface region and laterally etches outward from interfaces  404  and  408 . Over time, the etchant removes most or all of the release layer underneath a release portion  504  of the spring material. The release layer removal allows the spring release portion  504  to uplift out of the plane in which it was deposited. 
         [0025]    Although the preceding has been described as a two step operation of first etching a seed layer followed by etching of a release layer, it should be understood that the seed layer and the release layer may be combined into a single layer as previously described. When the seed layer and resist layer are combined, a single etchant solution penetrates the spring material/resist interface and etches the combination seed/release layer. 
         [0026]      FIGS. 6-9  show optional spring material treatments to further enhance spring performance.  FIG. 6 , shows an example of spring overplating with a cladding layer  604 . Example spring overplating materials include NiP plating, NiP+Au plating, or Cu+NiP+Au plating. The particular plating chosen depends on the spring characteristics desired which usually depends on how the spring will be used. Spring characteristics improved by plating include spring conductivity, hardness, wear resistance and stiffness. 
         [0027]    In  FIG. 7  remaining resist is stripped or otherwise removed.  FIG. 8  shows the removal of the seed layer and  FIG. 9  shows the removal of the release layer. A clear-etch containing potassium iodide (KI) and iodide (I) is one common method for removing a gold (Au) seed layer. A clear-etch containing hydrofluoric acid (HF) is one common method for removing a titanium (Ti) release layer. 
         [0028]      FIGS. 10-13  show an alternative spring structure wherein a cementation layer  1004  and adhesion layer  1008  are deposited prior to release layer  104  and substrate  100  deposition. Cementation layer  1004  is typically gold (Au) or nickel (Ni) and the adhesion layer may typically be Mo, MoCr, Ti, or Cr.  FIG. 13  shows cementation layer  1004  enabling selective deposition of metal  1304  under the spring. Metal  1304  enables a stronger anchoring of the spring to the substrate as well as a higher spring constant. 
         [0029]    The process of forming a cementation and adhesion layer under a spring approximately follows the process illustrated in  FIGS. 1-5  except that initially, a cementation layer  1004  and adhesion layer  1008  is deposited between release layer  104  and substrate  100  as shown in  FIG. 10 .  FIG. 11  shows the spring structure that results after a series of processing operations similar to that described in  FIG. 2  through  FIG. 5 . Those processing operations include removal of a portion of release layer  104  thereby exposing the cementation layer and adhesion layers.  FIG. 12  shows the exposed cementation layer  1004  adhering to cladding material in the region immediately underneath the spring.  FIG. 13  shows the final structure after resist stripping and clear etch of the seed and release layers. 
         [0030]      FIGS. 14-18  shows a front cross sectional view of an example spring formation process.  FIG. 14  shows a resist material  1404  deposited over a combination release and seed layer  1408 . Resist material  1404  is typically deposited using a photolithographic process. Once deposited, the resist serves as a mask, usually a hard mask that defines spring material  1412  deposition. As previously described, the spring material is typically deposited such that metal density gradually decreases as distance from substrate  1400  increases. The changing density helps produce the internal stress gradient. 
         [0031]      FIG. 15  shows exposing resist material  1404  and seed layer  1408  to an interface penetrating etch. Arrows  1504 ,  1508  indicate where the etchant passes between resist material  1404  and spring material  1412 . The etchant may penetrate this interface due to the loose contact between resist material  1404  and spring material  1412 . Alternatively, the etchant might overcome the adhesion forces between the resist material and the spring material. In one embodiment, a “natural gap” of less than 20 microns naturally forms between spring material  1412  and resist material  1404  during device fabrication facilitating the flow of etchant between the resist and spring interface. One mechanism for the formation of a gap is through the shrinkage of the resist after plating. This can occur by a variety of means. For example, the resist can undergo a physical change such as drying, the loss of solvent, etc. The resist can also shrink relative to the metal simply by virtue of its comparatively larger temperature coefficient of expansion relative to the substrate and the plated material. If the interface between the plated material and the resist is not strongly bonded, it will not support very much tensile stress, and will open up a gap of nanometer scale dimensions with only minor amounts of shrinkage. This effect can be augmented by depositing the plated material at an elevated temperature relative to the release etch. Further, gap widening can be enhanced by using an additional plasma etching step (e.g. oxygen (O2) plasma) which isotropically etches the photoresist but does not attack metal. 
         [0032]      FIG. 16  shows the beginning stages of etching the combination release and seed layer  1408 . The etching produces gaps  1604  in the release and seed layer  1408  immediately under the resist-spring interface region. The gap in the release layer soon exceeds the size of any natural gap that may exist at the resist spring interface. Over time, the release and seed layer  1408  under spring material  1412  is completely etched away. Upon complete removal of the release and seed layer  1408  underneath spring material  1412 , the internal stress gradient uplifts spring material  1412  as shown in  FIG. 17 . 
         [0033]      FIGS. 18-21  show a process similar to the process of  FIGS. 14-17  except that a gap widening etch facilitates the interface penetrating etch. In  FIG. 19 , a gap widening etch such as oxygen (O2) plasma is used to create or widen a gap  1904 ,  1908  between the spring and the resist material. In an alternate implementation, exposure to rapid temperature changes produces different expansion rates in different materials. In particular, rapid temperature changes induce different expansions of the mask and the spring material resulting in expanding of the gap between the mask and the spring material. Larger mask/spring material gaps facilitate etchant flow to the release and seed layer  2004 . Eventually the release and seed layers underneath the spring are etched away allowing spring release in  FIG. 21 . 
         [0034]    During device fabrication, it is sometimes preferable to delay spring uplift or “pop-up” until a later time in device processing. For example, when springs are formed as interconnects on a wafer, handling a smooth wafer substrate is simpler then handling a wafer substrate with uplifted spring surfaces. In such cases,  FIGS. 22-24  show a structure that delays spring uplift using a negative side resist profile at the resist and spring material interface.  FIG. 22  shows depositing a stressed metal spring material  2204  in resist gap  2208 . Resist side walls  2216  form a negative profile, such a negative side profile may be achieved by various techniques such as the use of negative resist, or through a resist image reversal process. Spring material  2204  forms a complimentary positive profile interface that matches the negative side profile where spring material  2204  is wider at a base and narrows toward a top layer of the spring material. 
         [0035]    In  FIG. 23 , an interface penetrating etch penetrates spring material  2204 /resist  2212  interface removing release and seed layer material  2216  under spring material  2204 . After release layer removal, an internal stress gradient provides an uplift force that tends to lift spring material  2204 . The negative profile interface along resist  2212  edge counters the uplift force and keeps down spring material  2204 . When uplift is desired, the resist is removed in  FIG. 24  allowing the internal stress gradient to uplift spring material  2204 . 
         [0036]      FIG. 25  shows an example array of spring structures  2504 ,  2508  formed by the described methods. Anchor region  2512  of each spring formed by the described single step lithography method is typically larger than traditional stressed metal spring anchors. Larger anchors prevent the etch that undercuts and releases the uplift portion of the spring from undercutting the entire anchor region. 
         [0037]      FIG. 26  shows a schematic view of an example spring  2604  including an anchor region  2608  and a release or uplift region  2612 . In order to allow complete undercutting of the release region while not completely undercutting the anchor region, the distance from the anchor region center to the nearest anchor region edge should be substantially greater than the distance from any point in the release region to the nearest release region edge. Typically, after release of the uplift portion, only a subset region, attached anchor release layer  2616  of spring anchor  2608 , remains bonded to the underlying substrate. Thus, when distance “d” represents the widest portion of release region  2612  and when a minimal interface penetrating etch releases the release region  2612 , the outer perimeter of attached spring anchor  2608  is typically at least a distance d/2 from the resist-spring interface. Another way to look at it is that the spring anchor  2608  perimeter extends approximately d/2 beyond the anchor release layer  2616  perimeter. 
         [0038]    Although the spring dimensions may vary considerably, one typical use for the spring structure is to interconnect integrated circuit elements. Thus the springs are typically quite small. Typical dimensions for “d” are often less than 200 microns. Typical spring lengths are less than 1000 microns. 
         [0039]    When smaller anchors are desired, (or faster release times needed), perforations incorporated into the spring release portion facilitates the etch process.  FIG. 27  shows a rectangular perforation  2704  in a spring release portion while  FIG. 28  shows circular perforations  2804  in a similar spring release portion. 
         [0040]      FIGS. 29 and 30  show alternate spring structures although many other shapes will come to those of ordinary skill in the art. The one common criterion of the various shapes is that a larger wider region of the structure serves as a spring anchor and one or more narrower and longer regions of the structure serve as springs. 
         [0041]    The preceding specification includes numerous examples and details such as geometries, materials used and the like. Such examples and details are provided to facilitate understand of the invention and its various embodiments and should not be interpreted to limit the invention. Instead, the invention should only be limited by the claims, as originally presented and as they may be amended, to encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.