Abstract:
This invention relates to a pressure transmission apparatus for bonding a plurality of chips to a substrate. The pressure transmission apparatus includes a pressure body for applying a bonding force which acts in the bonding direction (B) to the chip. The pressure body has a first pressure side and an opposite second pressure side, both oriented to be transverse to the bonding direction (B). Fixing means are provided to attach to the periphery of the pressure transmission apparatus for fixing of the pressure transmission apparatus on a retaining body in the bonding direction (B). A sliding layer is provided for sliding motion of the pressure body transversely to the bonding direction (B).

Description:
FIELD OF THE INVENTION 
     This invention relates to a pressure transmission apparatus for bonding a plurality of chips to a substrate and a bonding device for bonding a plurality of chips to a substrate. 
     BACKGROUND OF THE INVENTION 
     In the semiconductor industry, there are several possible methods for connecting, i.e., bonding, different functional units to one another. One of these methods is the chip-to-wafer (C2W) method in which individual chips are bonded individually to a substrate. Another important method is the so-called “advanced chip-to-wafer” (AC2W) method in which the chips are first tentatively (temporarily) joined to the substrate and a permanent connection takes place in one bonding step, all chips being permanently bonded to the substrate at the same time. In the bonding step, the chips and the substrate are heated while the chips are being pressed onto the substrate (bonding force). 
     The aforementioned pressure and heating processes create technical problems, especially for structures which are becoming smaller and smaller, for example, due to thermal expansion during heating. 
     Therefore the object of this invention is to devise a pressure transmission apparatus and a bonding device with which the aforementioned method can be optimized and scrap is minimized during bonding. 
     This object is achieved with the features of the claims. Advantageous developments of the invention are given in the dependent claims. All combinations of at least two of the features given in the specification, the claims and/or the figures also fall within the scope of the invention. At the given value ranges, values within the indicated limits will also be considered to be disclosed as boundary values and will be claimed in any combination. 
     SUMMARY OF THE INVENTION 
     The basic idea of this invention is to configure a pressure transmission apparatus for bonding of a plurality of chips to a substrate, wherein the pressure transmission apparatus allows a sliding motion of a pressure body transversely to the bonding direction B. It is advantageous if there are limiting means for limiting the sliding motion transversely to the bonding direction B, the limiting means being in the form of fixing means for fixing of the pressure transmission apparatus on a retaining body in the bonding direction B. The use of this invention is especially advantageous with an AC2W method in which the chips are temporarily joined to a substrate and then bonded thereto using the pressure transmission apparatus in a bonding process. According to the invention, it is advantageous if prior to the pressure transmission, the process of heating to the bonding temperature is at least largely completed. In this way, the pressure body can slide transversely to the bonding direction B more freely than with an adjacent bonding force in the bonding direction B. 
     The substrate is preferably a semiconductor, more preferably a wafer. 
     The invention largely solves the technical problem of thermal stresses which occur during bonding between the pressure transmission apparatus and the chip surfaces since only the stresses of the pressure body itself are relevant since it (i.e., the pressure body) is at least largely decoupled from all components on the other side of the sliding layer transversely to the bonding direction B by the sliding layer as claimed in the invention. The sliding layer thus allows almost completely frictionless relative motion of the components bordering it. As claimed in the invention, the sliding layer is made as a preferably largely carbon-containing solid. Graphite, whose basal planes is aligned at least largely perpendicular to the bonding direction B, is chosen as the material. 
     According to one advantageous embodiment of the invention, it is provided that the fixing means are made elastic transversely to the bonding direction B. Thus, at the same time the fixing means can provide for holding the pressure transmission apparatus together in the bonding direction B while the sliding motion of the pressure body transversely to the bonding direction B is allowed. In addition, the fixing means can act in this way as the above described limiting means and thus can assume several functions as claimed in the invention. 
     Here it is especially advantageous if the fixing means are made as clamps for clamping a pressure body in the direction of the retaining body, especially with the sliding layer located in between. This configuration can be implemented especially easily in a space-saving manner and performs the aforementioned multiple functions. 
     In one embodiment of the invention, it is provided that the pressure body, with respect to material and/or dimensions, is chosen such that its thermal expansion properties at least in the transverse direction to the bonding direction B correspond to the thermal expansion properties of the product substrate. The material is preferably the material of the substrate and/or of the chip. It is especially advantageous as claimed in the invention if the dimensions of the pressure body essentially agree with the dimensions of the substrate, especially transversely to the bonding direction B. The thermal expansion properties comprise the coefficient of thermal expansion of the pressure body and/or of the substrate and/or of the chip. In particular, but not exclusively, material classes such as metal, ceramics, plastics or composite materials can be used, in particular Si, CTE matched glass, therefore glass whose coefficient of thermal expansion has been aligned to that of the product substrate, low CTE metals, etc. CTE matched glass and low CTE metals are also known. Mainly materials whose thermal conductivity is very high are preferred. The coefficient of thermal expansion of silicon is for example 2.6×10 −6  K −1 . The absolute amount of the difference of the coefficients of thermal expansion of the product substrate and the embodiment as claimed in the invention is less than 100×10 −6  K −1 , preferably less than 10×10 −6  K −1 , more preferably less than 1×10 −6  K −1 , most preferably less than 0.1×10 −6  K −1 , most preferably of all less than 0.01×10 −6  K −1 , most ideally 0 K −1 . 
     By there being a layer which is elastic at least in the bonding direction B, and which is at least partially interrupted on intermediate spaces of the chip for pressure compensation, the pressure transmission apparatus also compensates for expansions, stresses and height differences in the bonding direction B so that uniform pressure application is enabled. The elastic layer is thus used as a compensation layer. In particular the elastic layer has a modulus of elasticity between 1 MPa and 100 GPa, preferably between 1 MPa and 10 GPa, even more preferably between 1 MPa and 1 GPa. Preferred materials are polymers. Rubber with an effective isotropic modulus of elasticity of roughly 1 MPa is used as the comparison value of the modulus of elasticity. Fiber-reinforced polymers are preferred with an effective modulus of elasticity along the longitudinal axis of the fiber of roughly 100 GPa. They mark the upper limit of the elastic material. 
     In another advantageous embodiment of the invention, it is provided that between the pressure body and the chip there is a hard layer which is at least partially interrupted on intermediate spaces of the chip for direct pressure transmission to the chips. The hard layer has a greater modulus of elasticity (especially twice as great) than the elastic layer, the hard layer being used as a protective layer. In particular the hard layer is also used to form a planar surface, with an average surface roughness of less than 10 μm, preferably less than 1 μm, even more preferably less than 100 nm, most preferably less than 10 nm. The hard layer is preferably made as an ultra hard layer. Preferred materials are metals and ceramics, especially iron materials, nonferrous metals, hard metals, nitride layers, carbide layers, boride layers and oxide ceramics. The thickness of the hard layer in the bonding direction B is less than 10 mm, more preferably less than 5 mm, even more preferably less than 2 mm, most preferably less than 500 μm, most preferably of all less than 100 μm. 
     Advantageously according to another embodiment, it is provided that the pressure body has fixing sites which are located on the periphery of the first pressure side, in the form of a preferably ring-shaped offset. In this way space-saving fixing of the pressure body on the retaining body is possible, due to possible heat transfer from the retaining body to the chips which are to be bonded during bonding a configuration of the pressure transmission apparatus as thin as possible in the bonding direction B being advantageous. 
     The described features of the pressure transmission apparatus apply analogously to the bonding device as claimed in the invention. 
     Other advantages, features and details of the invention will become apparent from the following description of preferred exemplary embodiments and using the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic side view of the bonding device as claimed in the invention; 
         FIG. 2 a    shows a schematic detail view of the bonding device as claimed in the invention in a first embodiment; 
         FIG. 2 b    shows a schematic front view of another embodiment of the pressure transmission apparatus as claimed in the invention; 
         FIG. 2 c    shows a schematic side view of the pressure transmission apparatus as claimed in the invention in another embodiment; 
         FIG. 3 a    shows a schematic detail view of another embodiment of the bonding device as claimed in the invention; 
         FIG. 3 b    shows a schematic side view of the pressure transmission apparatus as claimed in the invention in another embodiment; and 
         FIG. 3 c    shows a schematic side view of the pressure transmission apparatus as claimed in the invention in another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The same or equivalent components are identified with the same reference numbers in the figures. 
       FIG. 1  shows a schematic and highly simplified representation of a bonding device with a housing  11  which surrounds a bonding chamber  16 . Bonding chamber  16  can be loaded and unloaded via an opening which is not shown and which can be closed with a door which is not shown. In the bonding chamber  16  a defined atmosphere and temperature can be set, for which there can be corresponding feed lines and discharge lines or heating means. A retaining body  9  and/or a receiver  10  are used as heating means. 
     On the floor of the bonding chamber  16  receiver  10  is provided for accommodating a product substrate  14 . Fixing of the product substrate  14  can take place by vacuum paths, mechanical clamping or by electrostatics. 
     The product substrate  14  is comprised of a substrate  12  which rests on the receiver  10  and is fixed on the latter, and chips  13  which are temporarily fixed on the substrate  12  and for whose permanent fixing the bonding device is intended. 
     Above the receiver  10  there is a retaining body  9 . The retaining body  9  is able to move relative to the receiver  10 , specifically in one X, one Y and one Z direction. The retaining body  9  can preferably also be turned around, i.e., adjusted, relative to the X, Y and/or Z axis in order to compensate for wedge faults or to improve the orientation of the surfaces to one another. The relative motion takes place preferably by movement of the retaining body  9 , while the receiver  10  is rigid or does not have any degrees of freedom. 
     After loading of the product substrate  14  into the bonding chamber  16 , a pressure transmission apparatus  1  is attached to one lower side  9   u  of the retaining body  9 . The pressure transmission apparatus  1  is aligned relative to the product substrate  14  and lowered until the pressure transmission apparatus  1  adjoins its side  1   u  which is opposite the product substrate  14 . A defined bonding force is applied to the chip surface  13   o  of the chip  13  by a corresponding control of the movement in the bonding direction B. At the same time or subsequently, the product substrate  14  is heated to a bonding temperature. The process is controlled by a control apparatus. 
       FIG. 2 a    details the pressure transmission apparatus  1  and the product substrate  14 . The relations of the illustrated parts are made highly schematic for purposes of representation. In this respect, the chips  13  do not correspond to true circumstances. 
     In the first illustrated embodiment according to  FIG. 2 a   , the pressure transmission apparatus  1  in addition to a pressure body  2  which all embodiments have, additionally has a retaining body  4  which is clamped to the pressure body  2  via a fixing means  8  in the form of several clamps which have been clipped on the periphery of the pressure body  2  and of the retaining body  4 . 
     Between the pressure body  2  and the retaining body  4 , a sliding layer  3  is provided over the entire surface, by which the sliding motion of the pressure body  2  transversely to the bonding direction B is enabled. 
     The pressure body  2  and the retaining body  4  are made especially mirror-imaged (reflected on the sliding layer  3 ). 
     The pressure transmission apparatus  1  is fixed on the retaining body  9  of the bonding device by the fixing means  8 , especially arms  8   a  molded on the clamps. Alternatively or in addition the retaining body  4  can be fixed on the retaining body  9  by vacuum paths. 
     The fixing means  8  have a U-shaped section  8   u  between whose arms the clamping of the pressure body  2  to the retaining body  4  takes place. The U-shaped section  8   u  thus surrounds one part of the periphery of the pressure body  2  and of the retaining body  4 . For engaging the U-shaped section  8   u  there are fixing sites  17  on the pressure body  2  on a first pressure side  2   o . In the embodiment shown, fixing sites  17  are in the form of a ring-shaped offset on the periphery of the pressure body  2 . Transversely to the bonding direction B at the fixing site there is play  18  (i.e., a space) so that a sliding motion of the pressure body  2  transversely to the bonding direction B is enabled. 
     The opposite arm of the U-shaped section  8   u  engages a corresponding fixing site of the retaining body  4 , preferably there being fixing without play here. 
     On a first pressure side  2   o  of the pressure body  2  which is facing away from the sliding layer  3 , pressure elements  5  are attached, specifically at the positions which correspond to the chips  13  on the substrate  12  along the X and Y direction, therefore along the first pressure side  2   o . The pressure elements  5  are comprised of an elastic layer  6 , which is interrupted corresponding to the intermediate spaces  15  defined between the chips  13 , and of a hard layer  7 , which has been applied to the elastic layer  6 . The hard layer  7  is likewise interrupted analogously to the elastic layer  6  in the region of the intermediate spaces  15  so that any side forces which run transversely to the bonding direction are minimized when pressure is applied to the chips  13 . 
     When pressure is applied to the chips  13  with the pressure surface  1   u , here on one surface  7   o  of the hard layer  7 , the centers of gravity  5   s  of the pressure elements  5  are aligned with and centered to the centers of gravity  13   s  of the individual chips  13 . The pressure elements  5  exactly follow the chips  13  due to the similar or identical coefficients of thermal expansion of the pressure body  2  and of the substrate  12 , and due to the largely frictionless relative motion of the pressure body  2  transversely to the bonding direction B so that no transverse displacements and transverse forces are caused. 
     In  FIG. 2 b    there is a simplified fixing means  8 ′ in which the pressure body  2  and the retaining body  4  are fixed by a modified fixing means  8 ′ which attaches exclusively at fixing sites of the pressure body  2 , but not on the retaining body  4 . In this embodiment, the fixing takes place in the bonding direction B between the pressure body  2  and the retaining body  9 . The kinematics of the pressure body  2  and of the sliding layer  3  with the play  18  is that in  FIG. 2 a   . Thus an altered, simpler pressure transmission apparatus  1 ′ in a simpler design is formed since the fixing means  8 ′ and the retaining body  4 ′ are made simpler. 
     In the embodiment according to  FIG. 2 c   , the retaining body  4  is omitted and the sliding layer  3  is provided directly between the pressure body  2  and the retaining body  9 . An altered pressure transmission apparatus  1 ″ is formed which is made simpler than the above described pressure transmission apparatus  1 ,  1 ′ since a retaining body  4 ,  4 ′ can be omitted and the pressure transmission apparatus  1 ″ can be made thinner and better transfers the heat, to the extent heat is produced by the retaining body  9 . 
     Conversely, in the embodiment shown in  FIG. 3 a   , the pressure elements  5  shown in  FIGS. 2 a  to 2 c    are omitted so that the first pressure side  2   o  of the pressure body  2  assumes the function of the hard layer  7 . There is an elastic layer  6 ′ between the pressure body  2  and the retaining body  4 . On the interface between the elastic layer  6 ′ and the pressure body  2  there is the sliding layer  3  while the fixing means  8  corresponds to the fixing means  8  from  FIG. 2 a   . Thus an altered embodiment of the pressure transmission apparatus  1 ′″ is provided. 
     The first pressure side  2   o  in this embodiment acts as a pressure surface  1   u ′ with which pressure is applied to the surface  13   o  of the chip  13 . 
     In the embodiment shown in  FIG. 3 b   , according to the embodiment shown in  2   b  there are fixing means  8 ′ which omit engagement with the retaining body  4 ′. Thus an altered embodiment of the pressure transmission apparatus  1 ″″ is provided. 
     In the execution of the pressure transmission apparatus  1 ′″″ shown in  FIG. 3 c    the retaining body  4 ,  4 ′ is omitted according to the execution described in  FIG. 2 c   . Accordingly fixing means analogously to the fixing means  8 ″ are used. 
     REFERENCE NUMBER LIST 
     
         
           1 ,  1 ′,  1 ″,  1 ′″,  1 ″″ pressure transmission apparatus 
           1   u ,  1   u ′ pressure surfaces 
           2  pressure body 
           2   o  first pressure side 
           2   f  second pressure side 
           3  sliding layer 
           4 ,  4 ′ retaining body 
           5  pressure element 
           6 ,  6 ′ elastic layer 
           7  hard layer 
           8 ,  8 ′  8 ″ fixing means 
           8   a  arms 
           8   u  u-shaped section 
           9  retaining body 
           9   u  lower side 
           10  receiver 
           11  housing 
           12  substrate 
           13  chips 
           13   o  chip surface 
           13   s  center of gravity 
           14  product substrate 
           15  intermediate spaces 
           16  bonding chamber 
           17  fixing sites 
           18  play 
         Bonding direction B