Abstract:
An improved restraining clip for holding fragile substrates has been invented. The clip can be used to provide a force on the backside of substrates during processing. The invention has a compound spring mechanism, effectively uncoupling the force used to hold the substrate from that applied by an operator to move the clip. The preferred configuration incorporates a multiple leaf spring for providing force to the substrate, a coil spring to hold the clip in place, and a lever arm for the operator to provide lifting and turning forces on the clip. This combination of features allows the clip to be used on fragile substrates, such as Ga—As, which are easily damaged by other clips.

Description:
BACKGROUND OF THE INVENTION 
     This invention generally relates to support structures for restraining substrates, such as semiconductor wafers, during processing. The invention further relates to a compound spring assembly that can be easily manipulated by an operator with forces that exceed the strength of the substrate while imparting forces for restraining the substrate without damage. 
     DESCRIPTION OF THE PRIOR ART 
     The deposition, or coating, of materials is a process that is widely used in the manufacturing of many semiconductor and optical components. An example of a batch processor  111  that can perform this type of process is shown schematically in FIG.  1 . Batch processor  111  includes a vacuum chamber  101 , an evaporative source  103  of coating material  105 , and a framework  107  generally positioned to uniformly coat a plurality of optical components, flat panel display panels, semiconductor wafers or other substrates  109  that are usually located approximately equidistant from source  103 . Using an evaporative source, coating materials may include many metals, semiconductors and refractory materials. Vacuum chamber  101  is opened, and substrates  109  are manually loaded onto framework  107  one at a time. The number of substrates that can be held by a framework varies with the substrate, chamber and framework size, with frameworks typically adapted to hold substrates of 25 mm to 200 mm in diameter or larger. Framework  107  is connected to a motor  113 , rotating the framework within the chamber to increase the uniformity of coating material  105  being deposited. Other systems, not shown, may have fixed frameworks, while others may incorporate multiple frameworks and planetary or other rotational systems for moving substrates about to produce a specific coating effect. 
     At the low pressures under which evaporative coating operations occur, typically 10 −6  to 10 −7  Torr, material  105  travels in a straight line from source  103 , coating surfaces with a direct line-of-sight to the source. In the configuration of FIG. 1, a framework front side  115  and substrate front side  119  both face source  103  and are coated, while a framework back side  117  and substrate back side  121  face away from source  103  and are generally not coated. Details of one of the many substrates which may be held in the prior art framework is shown in FIGS. 2 and 3. Substrate  109  is placed over one of the apertures  201 , which are located over much of the framework surface and have a shape roughly the same as the substrates they are meant to hold. Substrate  109  is supported on substrate front side  119  by a plurality of aperture tabs or an aperture lip  203 , which supports some or all of the edge of substrate  109 , and on substrate back side  121  by a prior art clip mechanism  205 . Some systems have more than one clip for each substrate, especially for larger substrates, with a 150 mm substrate having 2 to 4 clips holding it in place. A substrate restraining force  217  is transferred from a deflected coil spring  211  to a substrate contact point  209  by a clip  219 , which is rigid and metallic. Typical prior art clips are made of alloys such as stainless steel or Inconel, with a thickness and shape that allow them to be rigid. Clip  219  can be pulled away from substrate  109  by pinching a clip handle  207  between thumb  301  and forefinger  303 , and rotated clear of aperture  201  about a pivot pin  215 , as shown in FIG.  3 . 
     A second prior art clip mechanism  401  is shown in side and top view in FIGS. 4 and 5, respectively. In clip mechanism  205 , a combined clip spring  401  is made of a mildly flexible metal, producing less of a restraining force than prior art clip mechanism  205 . In addition, coil spring  211  of the first prior art clip has been incorporated directly into the clip through a bend  403 . For this configuration, clip materials and shapes are chosen to allow some spring force to be imparted by the clip. 
     The first and second prior are clips of FIGS. 2 through 5 were designed primarily for substrates that can withstand the forces imparted by those clips, and the spring force acts to both restrain the clip and hold the substrate. These clips have wide utility, though not without problems, with silicon substrates. As galium-arsenide (Ga—As) substrates have become more common, several problems in using prior art clips to restrain the more fragile Ga—As substrates have become evident, mainly due to their imparting a large force over a small substrate area. Consider the first prior art clip of FIGS. 2 and 3. Rigid clip  219  can rotate about pivot pin  215  and is held in place by stiff coil spring  211 . This clip design has several drawbacks. Since the prior art clip is held in place against the substrate by a stiff spring, the force imparted by the clip has a limited lower value—it is difficult to design such a clip with a small force. If the operator accidentally releases clip handle  207  before placing it on substrate  109 , clip  219  is forced by stiff spring  211  onto substrate  109 , damaging more fragile substrate materials such as Ga—As. Another drawback of this design is that since stiff spring  211  also keeps the clip rigid, slight deformities due to mishandling the clip may result in a clip contact surface  209  smaller than the design area, with an increase in the substrate contact forces. Yet another drawback is that clip handle  207  is close to substrate back side  121 , and thus the operator must be very careful not to touch the substrate. The second prior art clip of FIGS. 4 and 5 incorporates a less rigid, and slightly elastic material for a combined clip and stiff spring  403 . This design is slightly better at maintaining a maximum contact surface with substrate  109 , but suffers from many of the same limitations as the first clip design. 
     The limitations of prior art clips with fragile substrates has been noted, and have been partially addressed by the use of slotted restraining devices that hold the substrate by the application of forces primarily on the edge of the substrate. In U.S. Pat. No. 4,971,676, Doue et al. describe a framework in which a substrate is held in place by one edge spring and a plurality of edge abutments. Although that invention places less stress on a substrate once it is in place, other problems result from the sliding required in placing or removing the substrate. For example, inserting the substrate into the narrow slots defined by the edge abutments can result in breakage of fragile substrates. In addition the sliding required by that invention can scratch or create and deposit an unacceptable level of particulate matter on the substrate surface. 
     SUMMARY OF THE INVENTION 
     It is an advantage of the present invention that the substrate restraining force imparted by the clip is uncoupled from the force that holds the clip in place. 
     It is a further advantage of the present invention to impart restraining forces on a fragile substrate that will result in a lower incidence of substrate damage. 
     It is another advantage of the present invention that forces used by the operator in positioning the present invention on a fragile substrate are not imparted onto the substrate. 
     It is a further advantage of the present invention that the forces used by the operator with the present invention provide positive and firm control while imparting a much smaller force on a substrate. 
     It is yet a further advantage of the present invention that the force imparted onto the substrate occurs over a larger contact area and at multiple points by the use of multiple leaves. 
     It is a further advantage of the present invention that the operators fingers are kept way from the wafer during operation. 
     It is yet another advantage that the present invention be simple and inexpensive to manufacture and be easy to use. 
     Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1, previously described, is a schematic of prior art vacuum deposition systems. 
     FIG. 2, previously described, is a side view detail of a first prior art clip shown restraining a substrate. 
     FIG. 3, previously described, is a top view of a first prior art clip. 
     FIG. 4, previously described, is a side view of a second prior art clip. 
     FIG. 5, previously described, is a top view of a second prior art clip. 
     FIG. 6 is a side view of the first clip embodiment shown restraining a substrate. 
     FIG. 7 is a end view of the first clip embodiment. 
     FIG. 8 is a side view of the first clip embodiment showing the application of a lifting force by an operator. 
     FIG. 9 is a top view of the first clip embodiment. 
     FIG. 10 is a detail of the framework apparatus. 
     FIG. 11 shows a flat pattern of the first clip embodiment before bending. 
     FIG. 12 is a section view of the first clip embodiment through one prong. 
     FIG. 13 is a top view of a second clip embodiment. 
     FIG. 14 is a side view of a second clip embodiment showing lifting motion. 
     FIG. 15 is an end view of a second clip embodiment 
     FIG. 16 is a top view of a third clip embodiment. 
     FIG. 17 is a side view of a third clip embodiment. 
     FIG. 18 is an end view of a third clip embodiment. 
     FIG. 19 shows a first alternative multiple prong designs. 
     FIG. 20 shows a second alternative multiple prong designs. 
     FIG. 21 shows a third alternative multiple prong designs. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An Embodiment of the Present Invention 
     FIGS. 6 to  12  show a first embodiment of the present invention. The figures include static and moving side views (FIGS.  6  and  8 ), an end view (FIG.  7 ), two top views (FIGS.  9  and  10 ), and clip pattern drawings (FIGS.  11  and  12 ). As shown in FIGS. 6 to  9 , the first clip embodiment has a compound spring clip assembly  601  that includes a combined spring clip and lever arm  619  that can rotate about a pivot pin  603 . Components of spring and lever  619  include a leaf spring  605  and a lever arm  607  with a clip handle  609 , and a clip supporting member  611  that has a pivot hole  613  through which pivot pin  603  passes. Supporting member  611  and pivot hole  613  are more clearly seen on the clip assembly drawing, FIG.  11 . Also part of spring clip assembly  601 , a second coil or other separate spring  615  is held under compression between a pivot pin end  617  and supporting member  611  to force clip  601  onto framework  107 . 
     The clip of the present compound spring assembly has one or more spring elements providing the substrate restraining force and another, stiffer element providing the higher forces necessary to restrain the clip and for human manipulation of the clip. FIG. 6 shows the clip positioned to restrain a substrate. In this position spring and lever  619  is held firmly in place by the clip assembly holding force provided by a coil spring  615 . A substrate restraining force  623  near the end of the cantilevered end of leaf spring or springs  605  results only from the bending action of leaf spring  605  and is spread out over a contact area  621 . Coil spring  615  provides a clip assembly holding force to keep the restrained end of leaf spring  605  in place. Uncoupling of the substrate restraining force from the clip assembly holding force is provided by having a clip supporting member that is held in place by the coil spring against the action of the leaf spring, and by having a coil spring force that is greater than the leaf spring force, preferably by a factor of at least three. FIG. 8 shows the clip raised for removal or placement of the substrate in the framework. The operator has pulled handle  609  towards a pivot pin end  617  against the action of coil spring  615 . Coil spring  615  is chosen to have a restraining force chosen for easy and positive manipulation by the operator, and is entirely uncoupled from substrate restraining force  623 . The forces imparted on the substrate by the clip of the present invention is less than 20 grams, preferably in the range of 3 to 5 grams. In addition to uncoupling the spring forces, this configuration has the added benefit of limiting possible damage from operator error. It has been shown that for even very fragile Ga—As substrates, releasing the handle from a raised position will not transfer enough force to the substrate to damage it. Likewise, if the operator inadvertently pushes handle  609  towards substrate  109  with enough force to overcome the force of spring  615 , only the relatively light force of leaf spring  605  will be transmitted to the substrate, limiting the possibility of substrate damage from the clip. Another aspect of this clip is that the handle be placed far from the substrate to reduce the likelihood of an operator inadvertently touching the wafer. 
     Further details of leaf spring  605  are shown in FIG. 9 top view. The leaf spring has two spring leaves  901  that are loosely coupled through a cross bar  903 . The clip of the present invention is not limited to the leaf configuration of the first clip embodiment, and other embodiments with multiple leafs and cross bars will be discussed subsequently. The spreading out of contact force is an important aspect of this invention. In contrast, prior art clips have a point, or at best a small line of contact, resulting in large, localized forces on the substrate. The leaf spring spreads out the substrate restraining force over a larger area, thus reducing the maximum localized stress within the substrate and minimize the chances for substrate damage. Because of the multiple leaves  901 , the restraining force from leaf  605  is shared between the pair of substrate contact areas  621 , reducing the maximum substrate contact stress. The flexibility of leaves  901  allows the leaves to bend in response to contact with substrate  109 , providing a larger contact area than is possible using rigid, prior art materials. Substrate restraining force  623  can be tailored to very low values, down to several grams of force, by altering the thickness, shape and material of leaf spring  605 . Additional control of the force on substrate  109  is affected by the curvature in leaf spring  605  near substrate contact area  621 . The purpose of cross bar  903  is to provide some linkage between leaves  901  so that they move to limit damage to leaf spring  605  in the event that one of leaves  901  is accidentally deformed by the operator. 
     FIG. 10 shows a view of the back side  117  of framework  107 . Each aperture  201  is associated with at least one clip assembly  601 , with more clips used for larger substrates and apertures. Also shown in FIG. 10 is the range of movement of clip assembly  601  from an aperture clear position  1003  to a substrate restraining position  1001 . Aperture clear position  1003  is representative of the position of clip  601  when there is no substrate in its corresponding aperture, while the substrate restraining position  1001  shows the placement of clip  601  for holding substrate  109  against framework  107 . In moving the clip between positions, clip  601  is grasped by the operator, providing the force to lift and rotate the clip. The lifting of clip  601  as shown in FIG. 8. A lifting force on clip  601  is generated by pinching the clip as shown in FIG. 8, or by grasping handle  609  between thumb and forefinger and pulling in the direction shown. The clip is then rotated about pivot pin  603  with between positions  1001  and  1003  by applying a slight sideways force to the end of lever arm  609 . 
     Further detail of spring and lever  619  is presented in FIGS. 11 and 12 which show the plan view, before being bent into shape, of the sheet which will form the clip. Spring and lever  619  is formed from one piece of material by bending the sheet along the dashed lines in FIG. 11 to form the shape shown in FIGS. 6 to  9 . Clip materials are chosen to be suitable for forming both a leaf spring and a rigid portion, and may be stainless steel, Inconel or other suitable spring materials, with 17-7ph stainless steel being preferred. The leaf spring thickness depends on the spring material and shape as well as the required restraining force. A thickness of 0.002 to 0.010 inch, with a preferred value of 0.004 inch will produce the low leaf spring force of the present invention. Lever arm  607 , handle  609 , and supporting member  611  are of a thickness and shape which allows for rigid manipulation by an operator under the force of spring  615 . A thickness greater than 0.010 inch will produce the required rigidity, with 0.015 inch being the preferred value. The variation of thickness through spring and lever  619  as shown in FIG. 12 is accomplished by photochemical milling, which is also used to form the details of leaf spring  605 . Other methods can also be used to manufacture the clip, including mechanical milling, shearing and stamping. The spring and lever  619  is then formed by bending the handle portion at the dotted lines indicated on FIG. 12, and by bending leaf spring  605  into the shape shown on FIG. 8, resulting in the final spring and lever shape. 
     Many embodiments of a compound spring mechanisms for application in this invention that would be obvious to one skilled in the art. Other obvious variations, some of which are included in further discussions of other embodiments, include incorporating the action of the coil spring into the pivot connection of the clip, various taperings of the leaf spring to produce different substrate restraining forces, increasing the number of contact points, and modifying the lever arm angle and length, to name a few variations. 
     Alternate Embodiments 
     A second embodiment of the present invention is shown in FIGS. 13 to  15 . This embodiment differs from the first embodiment in three ways. First, the coil spring of the first embodiment is replaced with an integrated second spring  1301  formed as part of supporting member  611 . Eliminating the coil spring of the first embodiment results in a simpler clip assembly. While this simplifies the clip design and possibly reduces the cost, there is a smaller range of second spring constants than in the first embodiment, which has a separate spring. Secondly, lever arm  607  is comprised of two portions that emanate from supporting member  611 , meeting near the same location as clip handle  609  of the first embodiment. Third and lastly, spring leaves  901  are tapered as a method for producing the required leaf spring constant, and do not have the cross bar of the first embodiment. 
     A third embodiment of the present invention, as shown in FIGS. 16 to  18  is bimetallic. The clip assembly is comprised of two connected materials chosen to perform the separate functions of providing the substrate restraining force and being the supporting member. The two materials and thicknesses are chosen for their mechanical properties, and so may range from being different materials and thicknesses to having some properties in common. Specifically, leaf spring  605  is made of a material and with a thickness to provide the desired substrate restraining force. Lever arm  607  and supporting member  611  is of a material and thickness to provide the rigidity for manipulation and holding leaf spring  605 . A joint  1601  of the two materials is made using TIG, spot welding or any joining method appropriate for the two materials. Appropriate materials for the leaf spring include 301 stainless steel and Inconel, with 17-7ph stainless steel preferred, with thicknesses of 0.002 to 0.010 inch. The lever arm and connecting member materials can be made from 301, 303, 304, 316 stainless steel or other materials compatible with the vacuum environment, with thicknesses of 0.010 to 0.040 inch. 
     Several alternate leaf configurations are shown in FIGS. 19 to  20 . These FIGs demonstrate a small number of the large variety and combination of spring leaves  901  and cross bars  903  possible. The choice of leaf spring design depends on the required total spring force, individual leaf force and placement of the force on the substrate. FIG. 19 shows a leaf configuration similar to the second and third embodiments, in that there is no cross bar, but where spring leaves  901  are straight rather than tapered. Damage to the substrate is governed by the maximum pressure on the substrate, which is in term determined by the total substrate restraining force and the total contact area. The maximum pressure will be determined by the leaf spring constant, which is a function of the leaf thickness and width, and the distance that the spring is bent while holding the substrate. Variations in the leaf spring geometry can be used to control both the force and pressure on the substrate. Additional combinations of leaves and cross bars are shown in FIGS. 20 and 21, including a single leaf that bifurcates to two leaves in FIG. 20, and a two leaf version that has a larger cross bar. Additional variations in leaf design, not shown, may include more than two leaves with more than two contact locations for spreading the substrate restraining force over a larger area. 
     Hence, although this invention has been described with respect to the embodiment discussed here, those embodiments are illustrative only. No limitation with respect to these embodiments is intended or should be inferred. It will be observed that numerous variations and modifications may be effected without departing from the true spirit and scope of the novel concept of the invention, and it is intended that the scope of the invention be defined by the claims appended hereto.