Patent Publication Number: US-2023146845-A1

Title: Deposit levelling

Description:
This invention relates to a method for producing a flat feature on a substrate, apparatus for performing such a method, a production line and a printing machine. 
     BACKGROUND AND PRIOR ART 
     Currently, various techniques exist for attaching dies to substrates in semiconductor applications. For example, it is known to stamp a die film onto a substrate, using dedicated equipment. While such a stamping process can produce high-quality results, the equipment cost is high. There is a need within the semiconductor industry to provide alternative techniques using cheaper equipment. One such alternative is, instead of stamping, to use a printing machine to print an Ag (silver) deposit onto a substrate. In more detail, such a known process typically includes the following stages: 
     i) A printing machine is used to print a silver deposit onto a substrate, using a patterned stencil to accurately locate the deposit; 
     ii) The printed deposit is subjected to a drying process, advantageously in an N 2  (nitrogen) environment to prevent oxidation. Typically, the substrate is placed into a smart oven which can provide the N 2  environment and heated to about 120° C., and maintained around this temperature for about 20 to 30 minutes; 
     iii) A “hot die placement” process is used to place the die onto the dried deposit, using bonding temperatures of around 250° C. A dedicated machine is used for this step; and 
     iv) The substrate is subjected to a sintering process, which is usually performed at an increased pressure in a so-called “pressure sintering” process. Typically maximum pressures of around 30 MPa are used, at a temperature up to 300° C. This sintering step is typically performed either in a vacuum or in an N 2  environment to prevent oxidation. This step generally takes place within a dedicated machine, such as the SilverSAM produced by ASM Assembly Systems. 
     Such a process is typically cheaper than a stamping process, since the initial printing step can be performed by a relatively inexpensive printing machine. However, it has been recognised that the printing process is difficult to perform, and is not suitable for all applications. As is well-known in the art per se, a printing process involves placing a patterned stencil over a substrate, placing print medium (in this case silver sintering paste) onto the stencil, then impelling the print medium through apertures in the stencil by scraping an angled squeegee blade across the top of the stencil. The stencil is then lifted off from the substrate, leaving print medium on the substrate in deposits corresponding to the apertures in the stencil. However, as schematically shown in  FIG.  1   , the silver sintering paste deposit  1 , shown here in section with the underlying substrate  2 , may suffer from non-uniform flatness across its horizontal extent in the printing direction, i.e. the direction of squeegee travel. As can be seen, while the central extent of the deposit (between points Y 2  and Y 3 ) may be flat with a consistent upper surface height of Z 1 , the leading edge of the deposit  1  (between points Y 1  and Y 2 ) may be lower than Z 1 , while the trailing edge of the deposit  1  (between points Y 2  and Y 3 ) may be significantly higher than Z 1 , with this characteristic raised feature being known as a “dog-tail” or “dog-ear”  3 . Attaching a die onto a non-flat surface, particularly raised as with a dog-ear, can lead to cracking of the die, which is clearly undesirable. Ideally, to prevent cracking the die should be attached to a surface which has a flatness profile such that height deviations are minimized across the surface. A current target is to achieve a flatness profile with height deviations equal to or less than 10 μm across the surface. As such, a printed deposit such as that shown in  FIG.  1    can only safely be used as a base for a die which is of maximum horizontal extent Y 3 -Y 2 , within normal tolerances. This means that the deposits may be about 25% bigger in the horizontal extent than the die. This both wastes print medium, and imposes a limit on the minimum horizontal separation of adjacent dies on a common substrate. 
     The present invention seeks to overcome these problems, and methodology and apparatus for producing features of printed deposits with improved flatness across their horizontal extent, and in particular which are sufficiently flat to enable dies to be safely attached thereto. 
     In accordance with the present invention, this aim is achieved by using a three-step process including an intermediate surface modification step. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention there is provided a method for producing a flat feature on a substrate, comprising the steps of: 
     i) printing a deposit of print medium onto the substrate, the printed deposit comprising an upper surface, 
     ii) modifying the upper surface of the printed deposit, and 
     iii) levelling the modified deposit to produce a flat feature. 
     In accordance with a second aspect of the present invention there is provided apparatus for performing the method of the first aspect. 
     In accordance with a third aspect of the present invention there is provided a 
     production line comprising a plurality of operation modules, said operation modules comprising a surface modification module, and at least one printing machine, the production line further comprising a transfer mechanism for transferring a workpiece between the operation modules, the production line being arranged such that a workpiece that has been printed by a printing machine may be transferred by the transfer mechanism from the printing machine to the surface modification module to undergo a surface modification process, and subsequently transferred from the surface modification module to a printing machine to undergo a levelling operation. 
     In accordance with a fourth aspect of the present invention there is provided a printing machine for printing a print medium onto a workpiece in a printing operation, the printing machine comprising a heater for at least partially drying the printed print medium. 
     Other specific aspects and features of the present invention are set out in the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the accompanying drawings (not to scale), in which: 
         FIG.  1    schematically shows, in sectional side view, a deposit of silver sintering paste which has been printed onto a substrate in a conventional print process; 
         FIG.  2    shows a flowchart setting out a process for producing flat features in accordance with an embodiment of the present invention; 
         FIG.  3    schematically shows, in sectional side view, an initial printing process immediately before printing of a deposit; 
         FIG.  4    is similar to  FIG.  3   , showing the initial printing process immediately following printing of the deposit; 
         FIG.  5    schematically shows, in sectional side view, the deposit being partially dried in an oven module; 
         FIG.  6    schematically shows, in sectional side view, a levelling process immediately before levelling of the deposit; 
         FIG.  7    schematically shows, in sectional side view, a levelling process immediately after levelling of the deposit; 
         FIGS.  8  to  11    schematically show production line set-ups suitable for performing the methodology of the present invention; and 
         FIGS.  12  and  13    schematically show, in a sectional side view, interior details of a printing machine which is able to perform both the initial printing step i), the partial drying step ii) and the levelling step iii). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     A flowchart setting out a basic process for producing flat features in accordance with an embodiment of the present invention is shown in  FIG.  2   . In accordance with the present invention, there are three main steps: 
     i) initially, a deposit of print medium is printed onto a substrate, 
     ii) the upper surface of the printed deposit is modified, for example by being partially dried as shown in the embodiment of  FIG.  2   , and 
     iii) the modified deposit is levelled to produce a flat feature. 
       FIG.  2    also shows a fourth step: 
     iv) the flat feature is dried. 
     While this fourth step will be essential for many applications, it is foreseeable that the flat feature produced by the end of the third step above could also be of practical utility without undergoing the drying step. Furthermore, full drying of deposits/features is already carried out with the known processes described in the background section, and there are various ways this could be achieved. The present invention is therefore considered to relate mainly to the first three steps, with the fourth step being an optional but usual follow-on process. 
       FIG.  3    schematically shows, in sectional side view, an initial printing process of the first step, immediately before printing of a deposit  11  of silver sintering paste, with  FIG.  4    being similar to  FIG.  3   , but showing the process immediately following printing of the deposit  11 . 
     In  FIG.  3   , a substantially planar substrate  12  is shown being located within a printing machine. Although not visible in  FIG.  3   , as is well-known in the art per se the substrate  12  is supported from below by tooling located within the printing machine so that it is kept both as flat as possible, and with its upper surface as parallel to the horizontal X-Y plane as possible. At the commencement of a printing operation, the substrate  12  is moved up into contact with a printing stencil  13 , which could comprise a sheet of metal such as stainless steel or nickel, or alternatively a mesh of woven metal filaments. The printing stencil  13  is patterned to include at least one aperture  14  corresponding in shape and position to the intended feature to be formed on the substrate  12 , however of slightly smaller horizontal dimension than the intended feature, as will be described in more detail below. A mass of print medium, in this case silver sintering paste, is supplied to the upper surface of the printing stencil  13 , and then pushed over the surface by an angled squeegee blade  15  travelling in a printing direction P parallel to the Y axis shown. As is well-known per se, the squeegee blade  15  may be carried by a print carriage (not shown) movably suspended from a gantry (not shown) within the printing machine. The print medium will tend to form a roll  16  of material at the leading edge of the squeegee blade  15 . The squeegee blade  15  is typically angled with respect to the printing stencil  13 , for example at an angle of between 50° and 70°, so that movement of the squeegee blade  15  in printing direction P imparts both a horizontal and downward force to the roll  16 , so that the print medium is impelled into the aperture  14  and on to the substrate  12  as the squeegee blade  15  passes over it. As shown in  FIG.  4   , the deposit  11  of print medium has a similar profile to that shown in  FIG.  1   , including the characteristic dog-ear  17  at its trailing edge. In more detail, if the printing stencil  13  has a thickness t 1  as shown, the majority of the deposit  11  will have a thickness at or slightly below t 1 , but with a dog-ear  17  of greater thickness (&gt;t 1 ) and therefore projecting out past the top of the aperture  14 , and in addition the leading edge of the deposit  11  may be of somewhat lower thickness than t 1  as shown. It has been found experimentally that using a printing stencil with a thickness of t 1  between 100 μm and 200 μm, optionally between 125 μm and 175 μm, optionally around 150 μm, may produce optimal results. 
     Following this printing operation, the substrate  12  is lowered away from the printing stencil  13 , leaving the deposit  11  on top of the substrate  12 . 
     In the second step, the upper surface of the printed deposit  11  is modified to reduce its tackiness, and in the present embodiment this is achieved by partially drying the deposit  11  in a baking process. This may be achieved in various ways, as outlined further below. However, in one embodiment, schematically shown in  FIG.  5   , the substrate  12  with deposit  11  is transferred out from the printing machine to a separate oven module  17  within the same production line as the printing machine, via a transfer mechanism (not shown in  FIG.  5   ) such as one or more conveyors, which are typically used to transport workpieces between modules of production lines. The oven module  17  shown in  FIG.  5    includes a number, here three, of infra-red lamps  18  which are operative to apply infra-red illumination to the deposit  11  and hence raise its temperature. The aim of step ii) is to partially dry the deposit  11 , i.e. to dry the outer surface of the deposit  11  while leaving the interior of the deposit  11  soft. Experimentally it has been found that this may be achieved by heating the deposit to a temperature in the range 50° C. to 200° C., optionally in the range 100° C. to 200° C., optionally in the range 125° C. to 175° C., optionally in the range 140° C. to 160° C., optionally around 150° C. The deposit may be heated for a time period in the range 30 seconds to 10 minutes, optionally in the range 1 minutes to 5 minutes, optionally around 2 minutes. 
     While in  FIG.  5    the oven module  17  uses infra-red lamps  18  as heating elements, heating of the deposit  11  may of course be performed in various ways, for example using convective heating rather than radiative heating, including using an air impingement process in which heated jets of gas (not necessarily air, but also substantially inert gases such as nitrogen may be used) are directed at the deposit  11 , as is known in the art per se. In fact, in the case of air impingement processes, the upper surface of the printed deposit could be dried to sufficiently reduce its tackiness by applying unheated jets of gas. 
     In step iii), the modified, i.e. in this embodiment partially-dried, deposit is levelled to produce a flat feature. To achieve this, the substrate  12  with its partially-dried deposit  21  is transferred out from the oven module  17  to a printing machine within the same production line as the oven module, via the transfer mechanism. As described in more detail below, this could be a separate printing machine dedicated to performing levelling operations, or alternatively could be the same printing machine which performed step i), if the printing machine is adapted to perform both the printing and levelling operations. The levelling operation is schematically shown in  FIGS.  6  and  7   , with  FIG.  6    showing the partially-dried deposit  21  immediately before levelling, and  FIG.  7    showing the partially-dried deposit  21  immediately following levelling. As in step i), the substrate  12  is supported within the printing machine by tooling (not shown) to maintain flatness and parallelism with the horizontal X-Y plane. The substrate  12  is moved up and into contact with a levelling stencil  22 , which may be of generally similar construction to the printing stencil  13  described previously with respect to  FIG.  3   , however the thickness t 2  of levelling stencil  22  is made thinner than the thickness t 1  of the printing stencil  13 , i.e. t 2 &lt;t 1 . It has been found experimentally that using a levelling stencil with a thickness of t 2  between 50 μm and 150 μm, optionally between 70 μm and 100 μm, optionally around 80 μm, can produce satisfactory results. The levelling stencil  22  is patterned with at least one aperture  24  which corresponds to the final shape, dimension and position of the intended feature  25  (see  FIG.  7   ). As can be seen in  FIG.  6   , the aperture  24  of the levelling stencil  22  is slightly bigger in horizontal extent than the aperture  14  of the printing stencil  13 , which creates air gaps around the partially-dried deposit  21 . To effect levelling, a levelling squeegee  23 , which suitably may be mounted on a print carriage (not shown), is moved across the top surface of the levelling stencil  22  in a levelling direction L, to contact the upper surface of the partially-dried deposit  21 . Since the tackiness of the upper surface is reduced through the previous surface modification through partial-drying step ii), the deposit material does not stick to the levelling squeegee  23 . As shown, the levelling direction L is in the opposite direction to the printing direction P, which arrangement has been found to produce optimal levelling. However, this is not essential, and satisfactory levelling may still be obtained using processes in which L and P have the same direction. Levelling squeegee  23  is shown in the form of a rotatably-mounted bar with a curved-profile edge, which can rotate and therefore roll about a horizontal rotation axis  26  parallel to the X axis, so that it can flatten the partially-dried deposit  21  in a similar manner to a kitchen rolling pin. It should be understood that use of a rotatably-mounted bar as the levelling squeegee is not essential, and indeed a high standard of levelling has been achieved experimentally using a non-rotatable bar with a curved-profile edge, such as the “roll-bar squeegee” produced by ASM. As noted above, since the partially-dried deposit  21  has a dry outer surface, sticking of the print medium to the levelling squeegee  23  is prevented, while the soft interior of the partially-dried deposit  21  allows it to be deformed and flattened without cracking or other damage. As can be seen from  FIG.  7   , the feature  25  formed is substantially flat, and is spread out to fill the aperture  24 . Depending on the relative sizes of the printing aperture  14  and levelling aperture  24 , the feature  25  may have its upper surface at, or, as shown in  FIG.  7   , slightly projecting above the upper surface of the levelling stencil  22 . Following levelling, the substrate  12  is lowered away from the levelling stencil  22 , leaving the feature  25  on the substrate  12 . The substrate  12  may then be transferred or transported where required for the particular application. 
     In most cases, it is envisaged that the substrate  12  and its feature  25  will proceed to a final step iv) of baking the feature  25  to fully dry it. This step may conveniently be performed in a conventional production line oven module, optionally in the same oven module  17  as shown in  FIG.  5   , for example using radiative heating techniques with, for example, infra-red lamps, or a convective heating system. It has been found experimentally that heating the feature to a temperature in the range 100° C. to 200° C., optionally in the range 125° C. to 175° C., optionally in the range 140° C. to 160° C., optionally around 150° C. produces satisfactory results. Furthermore, it has been found that heating the flat feature for a time period in the range 4 minutes to 15 minutes, optionally in the range 6 minutes to 10 minutes, optionally around 8 minutes produces satisfactory results. The fully-dried feature this produced may then be used as required for the particular application. For example, a die may be attached to the feature, and then a pressure sintering step performed, which steps are known in the art per se. 
       FIGS.  8  to  11    schematically show exemplary production line set-ups suitable for performing the methodology of the present invention. In these figures, each rectangular block represents a different module of the production line, with arrows showing the transfer of workpieces (i.e. substrates, substrates with a deposit printed thereon or substrates with a feature formed thereon) through the production line. 
       FIG.  8    shows a simple linear production line set-up. Module  30  is a printing machine set-up to perform the initial printing step i). Module  31  is a surface modification module, in this embodiment an oven module adapted to perform the surface modification through partial-drying step ii). Module  32  is a printing machine set-up to perform the levelling step iii). Finally, module  33  is an oven module adapted to perform the full-drying step iv). 
       FIG.  9    shows a set-up with reduced footprint. Module  34  is a printing machine which is able to perform both the initial printing step i) and the levelling step iii). Module  35  is an oven module which is operative to perform both the surface modification through partial-drying step ii) and the full-drying step iv). In practice, a workpiece is shuttled between these modules to performs all the steps, i.e. a deposit is printed in module  34 , transferred to module  35  for partial-drying, then transferred back to module  34  for levelling, followed by a final transfer to module  35  for full-drying. 
       FIG.  10    shows a set-up with a small footprint and reduced shuttling as compared to that shown in  FIG.  9   . Module  36  is a combined print and oven module, with a print unit  38  and oven unit  39 . The print unit  38  is adapted to perform the initial print step i), while the oven unit  39  is adapted to perform the surface modification through partial-drying step ii). This may be realised by, for example, retrofitting at least one infra-red lamp to an existing printing machine, which may be illuminated to heat the substrate as soon as the printing step i) has completed, while the substrate remains supported on the tooling. Module  37  is a printing machine adapted to perform the levelling step iii). Module  40  is an oven module adapted to perform the full-drying step iv). 
       FIG.  11    shows a set-up with a module  41  which is combined print and oven module with print unit  42  adapted to perform the initial print step i), an oven unit  43  for performing the surface modification through partial-drying step ii), and a levelling unit  44  operative to perform the levelling step iii). Module  41  will be described in more detail below with respect to  FIG.  12   . Module  45  is an oven module adapted to perform the full-drying step iv). 
       FIG.  12    schematically shows, in a sectional side view, interior details of a printing machine which set up to perform both the initial printing step i), the surface modification through partial drying step ii) and the levelling step iii). As shown, the printing machine has a large interior, so that it can house two separate printing areas—effectively the interior workings of two separate printing machines are combined into a single printing machine frame. On the left hand side as shown is a printing set-up for performing step i). A substrate  12  is supported on tooling  50 , which as is well-known in the art per se is supported on a rising table (not shown) which can move vertically to bring the substrate  12  towards or away from a printing stencil  13 . The printing stencil  13  is held in a tensioned state within a tensioning frame  51 , such as the well-known VectorGuard® tensioning frame for example. Squeegee blade  15  is mounted to a printing carriage  52  for movement therewith. Positioned underneath the printing stencil  13  are infra-red lamps  53 , which are shown switched off. On the right hand side as shown is a levelling set-up for performing step iii). A substrate  12 ′ is supported on tooling  50 ′, which again is supported on a rising table (not shown). The levelling stencil  22  is held in a tensioned state within a tensioning frame  51 ′. Levelling squeegee  23  is mounted to a printing carriage  52 ′ for movement therewith. 
       FIG.  13    schematically shows the printing machine of  FIG.  12    during the surface modification through partial drying step ii). Tooling  50  and the substrate  12  are lowered away from the printing stencil  13 . This action brings the freshly-printed deposit  11  into line of sight with the infra-red lamps  53 , which are shown switched on. Partial drying of the deposit  11  can occur in situ. 
     Following completion of the partial drying step ii), and once any substrate  12 ′ with a feature  25  has been transferred away from the tooling  50 ′, the substrate  12  with its partially-dried deposit can be transferred to the tooling  50 ′ on conveyors (not shown), which may conveniently be constructionally similar to those commonly used in the art to effect input and output of workpieces to and from printing machines. 
     Of course, there are various ways of implementing such a printing machine, for example the infra-red lamps could be placed in a variety of locations. In particular, they may be placed in association with the levelling set-up, in which case a freshly-printed substrate  12  with deposit  11  could be transferred to tooling  51 ′ subsequent to completion of step i), then the surface modification through partial drying step ii) could take place while the substrate is supported by tooling  51 ′, prior to levelling. In a yet further possibility, an oven module could be provided between the printing and levelling set-ups, so effectively the interior workings of two separate printing machines and an oven module are combined into a single printing machine frame. Other types or arrangements of heater are also possible, such as convective heating means. 
     Similarly, the combined print and oven module  36 , as shown in  FIG.  10   , could comprise a printing set-up with integrated heating elements as shown in the left side of  FIGS.  12  and  13   , while the levelling set-up shown in the right hand side could be located in a separate printing module  37 . 
     Of course, it is also possible to provide a printing machine module similar to that shown in  FIG.  12    but omitting the heating elements. In this case, a workpiece may be transferred to a separate oven module following the initial printing step i), undergo the surface modification through partial drying step ii) then be transferred to the levelling set-up of the printing machine module to undergo levelling step iii). 
     The above-described embodiments are exemplary only, and other possibilities and alternatives within the scope of the invention will be apparent to those skilled in the art. For example, while the above-described embodiments have focused on surface modification of the printed deposit through air-drying, other techniques may be used to modify the surface to reduce its tackiness. For example, a photochemical treatment process may be applied to the printed deposit to alter the surface chemistry and so reduce tackiness. It may also be possible to combine the aforementioned surface modification processes, for example a combination of two or more of infra-red heating, convection heating, air impingement and photochemical treatment processes could be employed in step ii). 
     REFERENCE NUMERALS USED 
       1 —Silver sintering paste deposit 
       2 —Substrate 
       3 —Dog-ear 
       11 —Deposit 
       12 ,  12 ′—Substrate 
       13 —Printing stencil 
       14 —Aperture 
       15 —Squeegee blade 
       16 —Roll 
       17 —Oven module 
       18 —Infra red lamps 
       21 —Partially-dried deposit 
       22 —Levelling stencil 
       23 —Levelling squeegee 
       24 —Aperture 
       25 —Feature 
       26 —Rotation axis 
       30 ,  32 ,  34 ,  37 —Printing modules 
       31 ,  33 ,  35 ,  40 ,  45 —Oven modules 
       36 ,  41 —Combined print and oven modules 
       38 ,  42 —Print unit 
       39 ,  43 —Oven unit 
       44 —Levelling unit 
       50 ,  50 ′—Tooling 
       51 ,  51 ′—Tensioning frame 
       52 ,  52 ′—Print carriage 
       53 —Infra-red lamps 
     P—Printing direction 
     L—Levelling direction 
     t 1 —Thickness of printing stencil 
     t 2 —Thickness of levelling stencil