Abstract:
A scale is attached to a carrier by optically contacting. The optically contacting bonds are formed by raised surface regions of the scale set apart from each other. Additional measures, such as the provision of adhesive surfaces, provide a rigid and vibration-resistant joint.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority to Application No. 10 2005 053 088.5, filed in the Federal Republic of German on Nov. 4, 2005, claims priority to Application No. 10 2006 017 708.8, filed in the Federal Republic of German on Apr. 15, 2006 and claims the benefit of U.S. Provisional Application No. 60/737,079, filed on Nov. 15, 2005, each of which is expressly incorporated herein in its entirety by reference thereto. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a method for attaching a scale to a carrier, a scale and a carrier having a scale.  
       BACKGROUND INFORMATION  
       [0003]     To measure the relative position of two machine parts, a scale is attached to one of the machine parts, and a scanning unit is attached to the other of the machine parts movable relative to each other. During the position measuring, a graduation marking of the scale is scanned by the scanning unit.  
         [0004]     A distinction is made between two basic principles when attaching a scale to a carrier. In the case of the first basic principle, the scale is attached to the carrier such that it is able to expand freely with respect to the carrier in response to temperature changes. In this case, fastening elements that are deflectable in the measuring direction, or an elastic adhesive layer are used for the attachment.  
         [0005]     In the case of the second basic principle, the scale is rigidly attached to the carrier. In this instance, the carrier and the scale may be made of a material having the same expansion coefficient. If the carrier and the scale are made of different materials, the thermal characteristic of the carrier is forced on the scale. In the case of the second basic principle, the fastening is accomplished via thin, rigidly curing adhesive layers or by direct contact, such as optical contacting.  
         [0006]     For highly accurate position measuring, scales made of glass or glass ceramic having a negligible expansion coefficient are used. These scales may be effectively machined, so that direct bonding on opposing surfaces is used, as described in German Published Patent Application No. 101 53 147.  
         [0007]     The problem in direct bonding a scale is that the connection can easily be disturbed by impurities or the formation of air bubbles. Moreover, the joining surfaces must be very even, which requires great effort. These problems are amplified in the case of relatively large-area scales. For this reason, the direct bonding of scales has not gained acceptance.  
       SUMMARY  
       [0008]     Example embodiments of the present invention may provide a method that eliminates the foregoing problems, and example embodiments of the present invention may provide a carrier having a scale firmly attached to it.  
         [0009]     Example embodiments of the present invention utilize the attainable advantages of optically contacting, by applying surface forces as large as possible in the form of retaining forces but simultaneously may avoid the disadvantages of optically contacting bond, in that a plurality of optically contacting bond surfaces separate from one another are formed.  
         [0010]     Local separation of the bond due to contamination or scratches is limited by the separation of the optically contacting bond surfaces. Generally, the separation does not propagate due to a broken bond.  
         [0011]     In addition, satisfactory flatness of the scale may be achieved, since disruptive media may escape through the at least one channel leading to the outside.  
         [0012]     According to example embodiment of the present invention, a method for attaching a scale to a carrier includes: producing an optically contacting bond between the scale and the carrier at a plurality of surface regions of the scale spaced apart from each other and separated from each other by at least one channel.  
         [0013]     The optically contacting bond may be produced in the producing step by at least one of (a) direct bonding, (b) low-temperature bonding and (c) anodic bonding.  
         [0014]     The optically contacting bond may be produced in the producing step at surface regions distributed in a two-dimensional grid and set apart from each other.  
         [0015]     The surface regions may include projections having a mutual spacing of less than a thickness of the scale.  
         [0016]     The method may include producing a further connection in addition to the optically contacting bond.  
         [0017]     The further connection may include an adhesive joint, and the further connection producing step may include introducing an adhesive agent between the scale and the carrier.  
         [0018]     According to an example embodiment of the present invention a device includes: a scale; and a carrier, the scale attached to the carrier by an optically contacting bond. The optically contacting bond is provided at a plurality of surface regions of the scale set apart from each other and separated from each other by at least one channel.  
         [0019]     The surface regions may include projections provided on at least one of (a) the scale and (b) the carrier.  
         [0020]     The projections may be positioned distributed in a two-dimensional grid.  
         [0021]     The projections may have a mutual spacing of less than a thickness of the scale.  
         [0022]     The scale and the carrier may be connected by a further connection in addition to the optically contacting bond.  
         [0023]     The additional connection may include an adhesive joint, and an adhesive agent may be provided on adhesive surfaces between the scale and the carrier.  
         [0024]     The adhesive surfaces may be separated from projections provided on at least one of (a) the scale and (b) the carrier by grooved depressions.  
         [0025]     The carrier may directly contact the scale at the projections, the adhesive surfaces may be recessed with respect to the projections to provide a gap between the scale and the carrier adapted to receive the adhesive agent, and the grooved depressions may be recessed with respect to the adhesive surfaces.  
         [0026]     The carrier may include at least one opening adapted for introduction of the adhesive agent onto the adhesive surface.  
         [0027]     The adhesive surface may extend to an edge of at least one of (a) the scale and (b) the carrier and may be formed so that the adhesive agent travels by capillary force from the edge to adhesive surfaces arranged away from the edge.  
         [0028]     The carrier may include a taper in a direction of an edge.  
         [0029]     According to an example embodiment of the present invention, a scale includes: an attachment surface adapted for attachment to a carrier, the attachment surface including projections set apart from each other, each projection including an optically contactable surface adapted to produce an optically contacting bond to an opposing surface of the carrier.  
         [0030]     The projections may be positioned distributed in a two-dimensional grid.  
         [0031]     The projections may have a mutual spacing of less than a thickness of the scale.  
         [0032]     Further aspects and features of example embodiments of the present invention are described in more detail below with reference to the appended Figures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIG. 1  illustrates a first scale and a first carrier for attachment of the scale.  
         [0034]      FIG. 2  illustrates the scale illustrated in  FIG. 1  attached to the carrier.  
         [0035]      FIG. 3  is a longitudinal cross-sectional view of the scale and the carrier illustrated in  FIG. 2 .  
         [0036]      FIG. 4  illustrates a second carrier and a second scale.  
         [0037]      FIG. 5  illustrates the scale illustrated in  FIG. 4  attached to the carrier.  
         [0038]      FIG. 6  is a top view of a third carrier and a third scale.  
         [0039]      FIG. 7  is a cross-sectional view of the scale illustrated in  FIG. 6  attached to the carrier.  
         [0040]      FIG. 8  is a cross-sectional view of two alternatives drawn side-by-side for forming the third scale.  
         [0041]      FIG. 9  is a top view of the two alternatives illustrated in  FIG. 8 .  
         [0042]      FIG. 10  illustrates a fourth carrier and a fourth scale.  
         [0043]      FIG. 11  is a top view of the fourth carrier and fourth scale.  
         [0044]      FIG. 12  is a cross-sectional view taken along the line A-A illustrated in  FIG. 11 .  
         [0045]      FIG. 13  is an enlarged view of region B illustrated in  FIG. 12 .  
         [0046]      FIG. 14  is an enlarged view of region C illustrated in  FIG. 11 .  
         [0047]      FIG. 15  illustrates a fifth carrier and a fifth scale.  
         [0048]      FIG. 16  is a top view of the fifth carrier and fifth scale.  
         [0049]      FIG. 17  is a cross-sectional view taken along the line A-A illustrated in  FIG. 16 .  
         [0050]      FIG. 18  is an enlarged view of region B illustrated in  FIG. 17 .  
         [0051]      FIG. 19  is an enlarged view of region C illustrated in  FIG. 16 . 
     
    
     DETAILED DESCRIPTION  
       [0052]     Referring to  FIGS. 1 through 3 , a glass or glass-ceramic (e.g., ZERODUR) scale  11  having a measuring graduation marking  21  is illustrated. Measuring graduation marking  21  is an incremental graduation marking able to be scanned for position measuring in measuring direction X. Measuring graduation marking  21  may be a reflecting amplitude or diffraction grating or a phase grating which is used, e.g., in a conventional manner, for highly accurate, interferential position measuring. In the region of its Bessel points, scale  11  has projections  31  which are used as supports for placement onto an opposing surface  41  of a carrier  51 . Carrier  51  may be made of glass or glass ceramic (e.g., ZERODUR), etc.  
         [0053]     Surfaces  61  of projections  31  opposite to opposing surface  41  of carrier  51 , as well as opposing surface  41 , are clean surfaces polished to a high degree. The surface finish required is achieved by mechanical, abrasive polishing, chemical-mechanical polishing, etc.  
         [0054]     Projections  31  on scale  11  may be produced by conventional patterning methods, by covering the regions of projections  31  and etching away the material around projections  31 . Projections  31  are thus formed in one piece on scale  11 .  
         [0055]     Scale  11  is joined to carrier  51  by optically contacting the surfaces  61  of projections  31  to opposing surface  41  of carrier  51 . The basis of the optically contacting is adhesion, as clean, conformable, and polished surfaces adhere to one another when their spacing enters the range of atomic bonding forces. Optically contacting is also referred to as optical bonding, non-adhesive bonding or wringing. Surfaces  61  of the projections are therefore formed such that they have an optically contactable surface  61  for producing an optically contacting bond with opposing surface  41  of carrier  51 .  
         [0056]     This optically contacting may be direct bonding (or direct contacting), which is also referred to as wringing and “Ansprengen” in German. In the case of direct bonding, the bonding action may be increased by the effect of heat, or by applying surface-active agents. Direct bonding using surface-active agents also achieves a good bonding strength at relatively low temperatures. A special type of surface-active agent is the introduction of crystallizing liquid. This optically contacting method is also referred to as low-temperature bonding technique (LTB) and is explained in a treatise from the firm SCHOTT, available over the Internet, having the title: “SCHOTT Low Temperature Bonding for Precision Optics” by Carol Click, Leo Gilroy and Dave Vanderpool, which is expressly incorporated herein in its entirety by reference thereto. When using the LTB method, scale  11  and carrier  51  are each made of glass ceramic having an expansion coefficient close to zero, e.g., ZERODUR.  
         [0057]     The optically contacting may also be anodic bonding, in which on one of surfaces  61 ,  41  of scale  11  or carrier  51  to be joined together, a metallic, electroconductive auxiliary layer, e.g., aluminum, is applied as an intermediate layer between projections  31  and opposing surface  41 . This auxiliary layer may be a vapor-deposited layer. In anodic bonding, a voltage is applied between the auxiliary layer and carrier  51 , so that ions from the auxiliary layer migrate into carrier  51  and/or ions from carrier  51  migrate into the auxiliary layer. The applied voltage generates an electrostatic attractive force which brings about an atomic contact between the scale and the carrier.  
         [0058]     Scales  12  having a two-dimensional measuring graduation marking  22  are increasingly being used for multi-dimensional position measuring. In that case, relatively large-sized scales  12  (e.g., 40 cm×40 cm) are mounted on a surface  42  of a machine part  52 . Example embodiments hereof are suitable for lithographic devices, e.g., in which machine parts  52  on which scale  12  is to be mounted are made of glass ceramic (e.g., ZERODUR) having an expansion coefficient close to zero. Such a machine possessing a scale having a two-dimensional measuring graduation marking is described, for example, in U.S. Patent Application Publication No. 2004/0263846, which is expressly incorporated herein in its entirety by reference thereto.  
         [0059]     It may be necessary to mount a plurality of scales  12  in two-dimensional fashion side-by-side like a mosaic on a machine surface  52  of 1 m×2 m, for example, in order to cover the requisite measuring region of approximately 1 m×2 m. This is because scales  12  having, for example, a measuring graduation marking  22  able to be scanned photoelectrically are only able to be produced with the necessary precision in sizes of, e.g., approximately 40 cm×40 cm. Each of these scales  12  may be attached to carrier  52  as illustrated in the Figures described below.  
         [0060]     The optically contacting methods explained above are used for this attachment.  
         [0061]     In  FIGS. 4 and 5 , such a scale  12  having a two-dimensional measuring graduation marking  22 , also referred to as a cross grating, is illustrated as an example. Projections  32  having optically contactable surfaces  62  are formed on the surface of scale  12  facing carrier  52 . These projections  32  may be spatially distributed two-dimensionally, either in a geometrically uniform manner in a normal grid, or in a statistical distribution. Projections  32  may each be circular, having a diameter of, e.g., less than 30 mm, e.g., 200 μm to 4 mm, and having a mutual spacing, e.g., less than the thickness of scale  12 , the mutual spacing being the edge spacing, i.e., 4 mm in  FIGS. 11 and 16 . The height of projections  32  may be greater than, e.g., 10 nm, for example, 20 nm to 50 μm. The flatness (waviness) of surfaces  62  of projection  32  may be in the range of less than, e.g., 500 nm on a diameter of approximately 10 mm, e.g., 30 nm per 0.10 mm. Surfaces  62  of projections  32  formed as optically contacting surfaces are arranged in a common plane. Typical values of the thickness of scale  12  are, e.g., 1 mm to 15 mm. The lower the diameter of projections  32 , and the lower the mutual spacing, the lower the height of projections  32  may also be.  
         [0062]     The two-dimensional, spatial distribution of projections  32  may be implemented such that, between projections  32 , opening channels  200  are formed which extend, relative to the X-Y plane, to the edge of scale  12 . This measure permits surface-active agents to escape easily from the space between scale  12  and carrier  52  after the optically contacting process. In addition, trapped air over the entire surface of scale  12  is able to escape easily via opening channels  200 , thus increasing the bonding strength and providing good planarity of scale  12 .  
         [0063]     Projections  32  constitute a type of nub and are formed so that the edges, which are transitions to the depressions next to them that form opening channels  200 , are rounded off. In this manner, surfaces  62  to be optically contacting may be more effectively cleaned and, if desired, surface-activated. An additional aspect is that contact points for separation may be prevented and the risk of material splintering off may be substantially reduced.  
         [0064]     For maintenance, the optically contacting bond may be broken by introducing a medium, e.g., compressed air, through at least one bore in carrier  52  or in scale  12 , into the gap of scale  12  and carrier  52 , thereby generating a pressure that forces scale  12  and carrier  52  apart.  
         [0065]     For example, for scales  13  jutting out past carrier  53  (illustrated, for example, in  FIGS. 6 and 7 ), there is the risk that induced vibration may cause the edge regions of scale  13  to alternately peel off and come together again. This event leads to unpredictable change in the short-period variation in length of the projecting scale region. Additional measures may be provided for preventing this.  
         [0066]     Thus, an additional safety mechanism may be provided for supporting scale  11  to  15  at carrier  51  to  55 . This additional safety mechanism may include retaining elements in the form of springs, retaining clips, magnetic retaining elements, electrostatic clamp circuit, vacuum holding devices, etc., or adhesive holding devices such as oil films, etc., or adhesive bonding methods, etc., may be used. This additional safety mechanism may be implemented at least at the edge region of the optically contacting joint, i.e., at the edge region of scale  13  and/or carrier  53 , e.g., at the edge region of the overlap of scale  13  and carrier  53 .  
         [0067]     Particularly suitable adhesive joints for supplementing the optically contacting are explained below with reference to FIGS.  6  to  19 . In this context, the surface pressure between connection partners  13  and  53  is increased with the aid of adhesive agent  7 , by prestressing discrete optically contacting surfaces  63 , e.g., in the edge zone of the connection of scale  13  and carrier  53 .  
         [0068]     Fastening with the aid of adhesive agent  7  prevents the breaking-off and loss of scales  13 , for example, from inadvertent contact by an installer.  
         [0069]     In this context, the adhesive layer produces deformations of scale  13 , which are, at most, locally minimal. Position and flatness are still extremely precise and largely drift-free due to the optically contacting joint.  
         [0070]      FIG. 6  illustrates a scale  13  protruding from carrier  53  at edge regions. Some of annular projections  33  of scale  13  are additionally provided with a cementing point, of which a cross-section of one is illustrated in  FIG. 7 . To differentiate the projections  32  that are only optically contacted and the projections  33  that are additionally secured by adhesive agent  7 , these are provided with different reference numerals, and projections  33  secured by adhesive agent  7  are represented in black in  FIG. 6 . For projections  33  additionally fastened by adhesive agent  7 , a circular adhesive surface  73 , which is separated from optically contacting surface  63  by a grooved depression  83  in the form of an adhesive stop, is arranged inside annular optically contacting surface  63 . This prevents adhesive agent  7  from reaching optically contacting surface  63  when it is introduced.  
         [0071]     For clarity, the measuring graduation marking is no longer illustrated.  
         [0072]     The regions lying deeper than optically contacting surface  63 , i.e., adhesive surfaces  73  and depressions  83 , are produced, for example, in a lithographic manner. Possible alternatives include mechanical machining, e.g., milling, or, for a suitable material, laser machining.  
         [0073]     Adhesive surface  73  and the E-module of adhesive agent  7  should only kept as large as absolutely necessary, in order to keep the bending deformation of scale  13  due to tensile forces after the curing of the adhesive agent only as large as necessary, but as small as possible. Tensile forces are caused by shrinkage of adhesive agent  7 .  
         [0074]     Given the same size of adhesive surface  73 , negligible, short-period deflection of scale  13  may also be attained using an oval shape of optically contacting surface  63  and adhesive surface  73 , illustrated, in each instance, on the right side. Regardless of the structural arrangement, the goal is to absorb the forces applied by adhesive agent  7  upon curing, as all-around as possible, and at a support distance as small as possible, which is provided by the projection or optically contacting surface  63  surrounding adhesive surface  73 .  
         [0075]     A method for optically contacting and adhesive fastening includes: bringing scale  13  into contact with carrier  53 ; aligning scale  13  on carrier  53 , the alignment being able to be facilitated by, for example, introducing a gas, e.g., air, through bore  93  into the gap of scale  13  and carrier  53  in order to prevent optically contacting in this state; pressing scale  13  against carrier  53 , and therefore optically contacting scale  13 , in the aligned state, the pressing being able to be generated by producing a vacuum (evacuation) in the gap of scale  13  and carrier  53 ; and introducing adhesive agent  7  to adhesive surface  73  via bores  93  in carrier  53 .  
         [0076]     In order to prevent deformation of scale  13  during measuring operation, due to shrinkage or swelling of adhesive agent  7 , e.g., caused by a change in air humidity, bore  93  may be sealed air-tight after introduction of adhesive agent  7 . As an alternative, after optically contacting has occurred, a gas having a defined humidity (e.g., nitrogen, helium, etc.) may be directed through bore  93  into the gap of scale  13  and carrier  53 , and therefore to adhesive surfaces  73 , in order to prevent deterioration of adhesive agent  7 .  
         [0077]     When a suitable adhesive agent  7  is used, the adhesive-secured optically contacting joint may be separated, e.g., for maintenance, by, for example, heating the adhesive agent  7  or cracking it with the aid of light of a defined wavelength, or using chemical agents. For separation by heating, a heating rod may be inserted into bore  93  in order to locally heat adhesive surface  73 . For separation by use of a chemical solvent, this may also be introduced through bore  93 .  
         [0078]     Alternatively, or in addition, a pressure may be generated in the gap of scale  13  and carrier  53 , via bore  93 , in order to separate the optically contacting joint.  
         [0079]     The following examples described with reference to FIGS.  10  to  19  illustrate alternatives that facilitate the introduction of adhesive agent  7 .  
         [0080]     As illustrated in FIGS.  10  to  14 , adhesive agent  7  is dosed from the edge of scale  14  and carrier  54  and is drawn to adhesive surface  74  by capillary forces. Grooved or groove-shaped depressions  84  between surfaces  64  of nub-shaped projections  34  and adhesive surfaces  74  prevent the adhesive agent from contacting optically contacting surface  64 .  
         [0081]     Support is provided by optically contacting surfaces  64  in direct proximity to the dosing channel and inside the adhesive region, formed by adhesive surfaces  74 . Depressions  84  prevent adhesive agent  7  from contacting optically contacting surfaces  64  (detachment due to drawn-in adhesive agent  7  is prevented).  
         [0082]     As illustrated in FIGS.  15  to  19 , a slot  95 , which is used for introducing adhesive agent  7  to adhesive surface  75 , is introduced into carrier  55 . Adhesive agent  7  is drawn by capillary action from slot  95  to adhesive surface  75 . In this manner, a shrinking adhesive point on the protruding region of scale  15  is prevented, and adhesive agent  7  cannot pull protruding scale  15  down.  
         [0083]     Carrier  55  may have a taper  100  in the direction of the edge. This renders carrier  55  more flexible, and it undergoes the deformation of protruding scale  15  along with it. The risk of separation in the edge region may thereby be reduced. An exemplary embodiment is illustrated in  FIG. 15 .  
         [0084]     A taper  100  of the edge region of carrier  51  to  55  may be used, with or without adhesive fixing, for improving the optically contacting stability.  
         [0085]     Contact surfaces  63  to  65 , which are formed by projections  33  to  35  and surround adhesive surface(s)  73  to  75 , may be positioned about adhesive surface  73  to  75  as symmetrically as possible. This keeps the deformation of the scale graduation marking surface small as well.  
         [0086]     Channels  200  leading to the outside separate nub-shaped projections  31  to  35  from each other in an otherwise planar optically contacting surface (providing, e.g., escape of the air from the gap, improvement of the optically contacting behavior). Several combinations of surfaces/nubs/grooves having, or not having, adhesive-stop depressions  83 ,  84 ,  85  are possible.  
         [0087]     In order to protect the optically contacting joints from external effects and creeping-under, the gap between scale  11  to  15  and carrier  51  to  55  may be sealed, after generation of the optically contacting joint, by sealing the edge at the periphery of scale  11  to  15 . Varnishes or adhesive agents may be used for this purpose. Protection may also be achieved by flooding the gap with a medium, for which purpose a gas having defined properties, for example, is introduced into the space between projections  31  to  35 , i.e., into channels  200 , and flows through it.  
         [0088]     In the above-mentioned examples, projections  31  to  35  set apart from one another are formed in one piece on scale  11  to  15  in the form of nubs. Alternatively or additionally, projections  31 ,  35  may also be formed on carrier  51  to  55 . Projections  31  to  35  may also be formed by a layer deposited on scale  11  to  15  or carrier  51  to  55  and patterned.  
         [0089]     The form and arrangement of projections  31 ,  35  are not limited to the arrangements shown.  
         [0090]     When working with at least approximately square or round scales, the projections may form a kinematically determined support, in that only three projections are provided, distributed in one plane.  
         [0091]     The optically contacting methods have in common that surfaces  61  to  65 ,  41  to  45  to be joined are brought toward each other in close contact until they are a few interatomic distances apart, in order to either be able to be attracted due to the power of the van der Waals forces (direct bonding), or else to be able to produce an atomic bond by the formation of a few atomic layers in the form of an intermediate bond (LTB, anodic bonding).  
         [0092]     The dimensions specified in the drawings are indicated in mm and only show the orders of magnitude schematically.