Sediment leveling

Abstract

A method for producing planar features on a substrate includes the steps of printing a deposit of print media onto the substrate, the printed deposit including an upper surface, modifying the upper surface by, for example, partially drying the printed deposit, and planarizing the modified deposit to produce the planar features.

Description

Technical Field

[0001] The invention relates to a method of producing planar features on a substrate, an apparatus for carrying out the method, a production line and a printing press. Background Art

[0002] Currently, there are various techniques for attaching a die to a substrate in semiconductor applications. For example, it is known to use specialized equipment to stamp a die film onto a substrate. While this stamping process can produce a high-quality product, the equipment costs are high. Therefore, the semiconductor industry needs to provide alternative techniques that use cheaper equipment. One such alternative technique is to print an Ag (silver) deposit onto a substrate using a printer rather than stamping. In more detail, this known process generally includes the following stages:

[0003] i) printing a silver deposit onto a substrate using a printer, using a patterned template to accurately position the deposit;

[0004] ii) drying the printed deposit, advantageously in a N2 (nitrogen) environment to prevent oxidation. Typically, the substrate is placed in a smart oven that can provide a N2 environment and heated to about 120°C and maintained at this temperature for about 20 to 30 minutes;

[0005] iii) placing the die onto the dry deposit using a “hot die assembly” process with a bonding temperature of around 250°C. This step uses a dedicated machine; and

[0006] iv) The substrate is subjected to a sintering process, which is usually carried out under increased pressure in a so-called "pressure sintering" process. Typically a maximum pressure of around 30 MPa is used at temperatures up to 300°C. This sintering step is usually carried out in a vacuum or N2 environment to prevent oxidation. This step is usually carried out in a dedicated machine, such as the SilverSAM produced by ASM Assembly Systems.

[0007] This process is generally less expensive than stamping because the initial printing step can be performed by a relatively inexpensive printer. However, it has been recognized that the printing process is difficult to perform and is not suitable for all applications. As is known in the art, the printing process involves placing a patterned template on a substrate, placing a print medium (in this case, a silver sinter paste) on the template, and then pushing the print medium through the small holes in the template by scraping an angled scraper across the top of the template. The template is then lifted from the substrate, leaving a deposit of the print medium on the substrate corresponding to the small holes in the template. However, as Figure 1As schematically shown, within the horizontal range of the printing direction, i.e., the direction of travel of the scraper, the silver sintering paste deposit 1 (shown here in cross section of the substrate 2 below) may have uneven flatness. As can be seen, although the central range of the deposit (between points Y2 and Y3) may be flat and have a consistent upper surface height Z1, the leading edge of the deposit 1 (between points Y1 and Y2) may be lower than Z1, while the trailing edge of the deposit 1 (between points Y3 and Y4) may be significantly higher than Z1. This typical raised feature is called a "dog-tail" or "dog-ear" 3. Attaching the die to an uneven surface, especially a surface that is raised like a dog-ear, will cause the die to crack, which is obviously undesirable. Ideally, to prevent cracking, the die should be attached to a surface with a flat profile so that the height deviation of the entire surface is minimized. The current goal is to achieve a flat profile with a height deviation equal to or less than 10 μm over the entire surface. Therefore, for safety reasons, printed deposits (e.g. Figure 1 The printed deposit shown can only be used as a base for a die with a maximum horizontal range of Y3-Y2 within normal tolerance. This means that the deposit may be about 25% larger in horizontal range than the die. This wastes printing media and limits the minimum horizontal spacing of adjacent dies on a common substrate. Summary of the invention

[0008] The present invention seeks to overcome these problems by providing methods and apparatus for producing features of printed deposits having improved flatness across their horizontal extent and, in particular, being sufficiently flat that a die can be securely attached thereto.

[0009] According to the invention, this object is achieved by using a three-step process comprising an intermediate surface modification step.

[0010] According to a first aspect of the present invention, there is provided a method for producing a planar feature on a substrate, comprising the following steps:

[0011] i) printing a deposit of print medium onto a substrate, the printed deposit comprising an upper surface,

[0012] ii) modifying the upper surface of the printed deposit, and

[0013] iii) Planarizing the modified deposit to produce flat features.

[0014] According to a second aspect of the present invention, there is provided an apparatus for executing the method of the first aspect.

[0015] According to a third aspect of the present invention, there is provided a production line comprising a plurality of operating modules, wherein the operating modules comprise a surface modification module and at least one printing press, and the production line further comprises a transfer mechanism for transferring workpieces between the operating modules, wherein the production line is arranged so that the workpieces printed by the printing press can be transferred from the printing press to the surface modification module via the transfer mechanism for surface modification treatment, and then transferred from the surface modification module to the printing press for leveling operation.

[0016] According to a fourth aspect of the present invention, there is provided a printing press for printing a print medium onto a workpiece in a printing operation, the printing press comprising a heater for at least partially drying the printed print medium.

[0017] Further particular aspects and features of the invention are set out in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention will now be described with reference to the accompanying drawings (not to scale), in which:

[0019] Figure 1 schematically illustrates a cross-sectional side view of a deposit of a silver sintering paste printed onto a substrate in a conventional printing process;

[0020] Figure 2 A flow chart of a method for generating a flat feature according to an embodiment of the present invention is shown;

[0021] Figure 3 schematically shows a cross-sectional side view of an initial printing process immediately prior to printing a deposit;

[0022] Figure 4 and Figure 3 Similarly, the initial printing process immediately following the printing of the deposit is shown;

[0023] Figure 5 schematically shows a cross-sectional side view of a deposit partially dried in an oven module;

[0024] Figure 6 schematically illustrates a cross-sectional side view of a planarization process immediately prior to planarizing a deposit;

[0025] Figure 7 schematically shows a cross-sectional side view of a planarization process immediately following a planarization deposit;

[0026] Figures 8 to 11 schematically shows a production line arrangement suitable for carrying out the method of the present invention; and

[0027] Fig.12 and Fig.13Schematically shown is a cross-sectional side view of a detail of the interior of a printing press capable of performing an initial printing step i), a partial drying step ii) and a smoothing step iii).

[0028] Description of reference numerals:

[0029] 1-Silver sinter paste deposit

[0030] 2-Substrate

[0031] 3-Fold ears

[0032] 11- Sediment

[0033] 12,12'-substrate

[0034] 13-Printing template

[0035] 14-Small hole

[0036] 15- Scraper

[0037] 16-Reel

[0038] 17-Oven module

[0039] 18-Infrared lamp

[0040] 21- Partially dried sediment

[0041] 22-Leveling the template

[0042] 23-Smooth scraper

[0043] 24-Small hole

[0044] 25-Features

[0045] 26-Rotation axis

[0046] 30,32,34,37-Printing module

[0047] 31,33,35,40,45-Oven module

[0048] 36,41-Printing and oven combination module

[0049] 38,42-Printing unit

[0050] 39,43-Oven unit

[0051] 44-Leveling unit

[0052] 50,50'-Tools

[0053] 51,51'-Tension frame

[0054] 52,52'-Printing carriage

[0055] 53-Infrared lamp

[0056] P-Printing direction

[0057] L-leveling direction

[0058] t1 - thickness of the printing template

[0059] t2-thickness of the flattened template. DETAILED DESCRIPTION

[0060] Figure 2 A flow chart of a basic process for producing flat features according to an embodiment of the present invention is shown. According to the present invention, there are three main steps:

[0061] i) first, printing a deposit of printing medium onto a substrate,

[0062] ii) Modification of the upper surface of the printed deposit, e.g. Figure 2 Partial drying as shown in the examples, and

[0063] iii) Planarizing the modified deposit to produce flat features.

[0064] Figure 2 A fourth step is also shown:

[0065] iv) Drying the planar feature.

[0066] While the fourth step is essential for many applications, it is contemplated that the flat features produced at the end of the third step described above may also have utility without the drying step. Furthermore, complete drying of the deposit / feature has been performed using known methods described in the background section, and complete drying may be achieved in a variety of ways. Therefore, it is believed that the present invention is primarily concerned with the first three steps, with the fourth step being an optional but typically subsequent step.

[0067] Figure 3 Schematically showing a cross-sectional side view of an initial printing process as a first step immediately before printing a deposit 11 of silver sintering paste, Figure 4 and Figure 3 Similar, but showing the process immediately after printing the deposit 11.

[0068] Figure 3 The substantially planar substrate 12 shown in FIG. is located within the printing press. Figure 3, but as is well known in the art, the substrate 12 is supported from below by a tool located within the printer so as to remain as flat as possible and its upper surface as parallel to the horizontal XY plane as possible. At the start of the printing operation, the substrate 12 moves upward to contact a printing template 13, which may include a sheet of metal (e.g., stainless steel or nickel), or alternatively a woven metal mesh. The printing template 13 is patterned to include at least one small hole 14, the shape and position of the small hole corresponding to the expected feature to be formed on the substrate 12, but its horizontal dimension is slightly smaller than the horizontal dimension of the expected feature, which will be described in more detail below. A large amount of printing medium (in this embodiment, silver sintering paste) is supplied to the upper surface of the printing template 13 and then pushed across the surface by an angled scraper 15 traveling in a printing direction P parallel to the Y axis shown. As is well known, the scraper 15 can be carried by a printing carriage (not shown) that is movably suspended above a stage (not shown) within the printer. The printing medium will tend to form a web 16 at the leading edge of the scraper 15. The scraper 15 is usually angled with the printing template 13, for example, between 50° and 70°, so that the movement of the scraper 15 in the printing direction P exerts a horizontal and downward force on the web 16, so that when passing the print medium, the scraper 15 pushes the print medium into the small holes 14 and onto the substrate 12. Figure 4 As shown, the deposit 11 of the printing medium has Figure 1 The profile shown is similar to the profile shown, including a unique fold 17 at its trailing edge. In more detail, if the printing template 13 has a thickness t1 as shown, the majority of the deposit 11 will have a thickness equal to or slightly lower than t1, but its fold 17 is thicker (> t1) and thus protrudes beyond the top of the small hole 14. In addition, the thickness of the leading edge of the deposit 11 can be slightly lower than t1 shown. Experiments have found that using a printing template with a thickness t1 between 100 μm and 200 μm, optionally between 125 μm and 175 μm, optionally around 150 μm can produce the best results.

[0069] After this printing operation, the substrate 12 is lowered away from the printing template 13 , leaving the deposit 11 on top of the substrate 12 .

[0070] In a second step, the upper surface of the printed deposit 11 is modified to reduce its stickiness, and in the present embodiment this is achieved by partially drying the deposit 11 in a baking process. This can be achieved in a number of ways, as further outlined below. However, in one embodiment, Figure 5 The schematic diagram shows a substrate 12 with a deposit 11 being transported via a transfer mechanism such as one or more conveyor belts ( Figure 5The workpieces are transferred from the printing press to a separate oven module 17 located in the same production line as the printing press (not shown), and a conveyor belt is usually used to transfer the workpieces between modules of the production line. Figure 5 The oven module 17 shown in the figure comprises a plurality (here three) of infrared lamps 18, which are operable to apply infrared illumination to the deposit 11 and thereby increase its temperature. The purpose of step ii) is to partially dry the deposit 11, i.e. to dry the outer surface of the deposit 11, while softening the interior of the deposit 11. It has been found through experiments that this can be achieved by heating the deposit to a temperature in the range of 50°C to 200°C, optionally in the range of 100°C to 200°C, optionally in the range of 125°C to 175°C, optionally in the range of 140°C to 160°C, optionally around 150°C. The deposit may be heated for a period of time in the range of 30 seconds to 10 minutes, optionally in the range of 1 minute to 5 minutes, optionally around 2 minutes.

[0071] exist Figure 5 In the embodiment, the oven module 17 uses infrared lamps 18 as heating elements, of course, the heating of the deposit 11 can be carried out in various ways, such as using convection heating instead of radiation heating, including using an air impingement process, in which a heated gas jet (not necessarily air, but also a substantially inert gas such as nitrogen) is directed to the deposit 11, as is known in the art. In fact, in the case of an air impingement process, the upper surface of the printed deposit can be dried by applying an unheated gas jet to substantially reduce its viscosity.

[0072] In step iii), the modified (i.e., partially dried in this embodiment) deposit is flattened to produce flat features. To achieve this, the substrate 12 with the partially dried deposit 21 is transferred from the oven module 17 via a transfer mechanism to a printer located in the same production line as the oven module. As will be described in more detail below, the printer can be a separate printer dedicated to performing the flattening operation, or alternatively can be the same printer that performs step i), if the printer is suitable for performing both the printing operation and the flattening operation. Figure 6 and Figure 7 The leveling operation is schematically shown, wherein Figure 6 shows a partially dried deposit 21 immediately before leveling, Figure 7 A partially dried deposit 21 is shown immediately after flattening. As in step i), the substrate 12 is supported within the printer by a tool (not shown) to maintain flatness and parallelism with the horizontal XY plane. The substrate 12 moves upward to contact the flattening template 22, which may have the same shape as previously described. Figure 3The structure of the printing template 13 described above is substantially similar, but the thickness t2 of the flattening template 22 is made smaller than the thickness t1 of the printing template 13, that is, t2 < t1. It has been found through experiments that using a flattening template with a thickness t2 between 50 μm and 150 μm, optionally between 70 μm and 100 μm, and optionally around 80 μm can produce satisfactory results. The flattening template 22 is patterned to have at least one small hole 24, which corresponds to the final shape, size and position of the desired feature 25 (see Figure 7 ). Figure 6 As can be seen in the figure, the apertures 24 of the leveling template 22 are slightly larger than the apertures 14 of the printing template 13 in horizontal extent, which can create air gaps around the partially dried deposit 21. In order to achieve leveling, a leveling blade 23, which can be appropriately mounted on a printing carriage (not shown), moves along a leveling direction L through the top surface of the leveling template 22 to contact the upper surface of the partially dried deposit 21. Since the viscosity of the upper surface is reduced by the previous surface modification treatment of the partial drying step ii), the deposit material will not stick to the leveling blade 23. As shown, the leveling direction L is opposite to the printing direction P, and it has been found that this arrangement can produce optimal leveling. However, this is not necessary, and a process with L and P having the same direction can still obtain satisfactory leveling. The leveling blade 23 shown is in the form of a rotatable mounting rod with a curved profile edge, which can be rotated and thereby rolled around a horizontal rotation axis 26 parallel to the X axis, so that the rotatable mounting rod can flatten the partially dried deposit 21 in a manner similar to a kitchen rolling pin. It should be understood that the use of a rotatably mounted rod as a flattening blade is not essential, and in fact, a high standard of flattening has been achieved experimentally using a non-rotatable rod with a curved profile edge (e.g., a "roller rod blade" produced by ASM). As described above, since the partially dried deposit 21 has a dry outer surface, the printed medium is prevented from sticking to the flattening blade 23, while the soft interior of the partially dried deposit 21 allows it to deform and flatten without causing cracking or other damage. Figure 7 It can be seen that the formed feature 25 is substantially flat and is expanded to fill the aperture 24. Depending on the relative sizes of the printed aperture 14 and the flattened aperture 24, the upper surface of the feature 25 can be located on the upper surface of the flattened template 22 or as shown in FIG. Figure 7 It is shown protruding slightly from the upper surface of the planarization template 22. After planarization, the substrate 12 is lowered off the planarization template 22, leaving the features 25 on the substrate 12. The substrate 12 can then be transferred or transported to where needed for a particular application.

[0073] In most cases, it is envisioned that the substrate 12 and its features 25 will proceed to a final step iv) of baking the features 25 to completely dry them. This step can be conveniently performed in a conventional production line oven module, optionally in a conventional production line oven module, such as by using radiant heating techniques such as infrared lamps or convection heating systems. Figure 5 The process of heating the features to a temperature in the range of 100°C to 200°C, optionally in the range of 125°C to 175°C, optionally in the range of 140°C to 160°C, optionally around 150°C has been found experimentally to produce satisfactory results. Furthermore, it has been found that heating the flat features for a period of time in the range of 4 minutes to 15 minutes, optionally in the range of 6 minutes to 10 minutes, optionally around 8 minutes, can produce satisfactory results. The resulting fully dried features can then be used as required for a particular application. For example, a die can be attached to the feature and then a pressure sintering step performed, which steps are known per se in the art.

[0074] Figures 8 to 11 An exemplary production line arrangement suitable for carrying out the method of the present invention is schematically shown. In these figures, each rectangular block represents a different module of the production line, and arrows represent the transfer of workpieces (i.e., substrates, substrates with deposits printed thereon, or substrates with features formed thereon) throughout the production line.

[0075] Figure 8 A simple linear production line arrangement is shown. Module 30 is a printer arrangement for performing the initial printing step i). Module 31 is a surface modification module, in this embodiment an oven module suitable for surface modification by a partial drying step ii). Module 32 is a printer arrangement for performing a flattening step iii). Finally, module 33 is an oven module suitable for performing a complete drying step iv).

[0076] Fig. 9 An apparatus with reduced footprint is shown. Module 34 is a printer capable of performing the initial printing step i) and the leveling step iii). Module 35 is an oven module operatively used for surface modification by a partial drying step ii) and a complete drying step iv). In practice, the workpiece is shuttled between these modules to perform all the steps, i.e. the deposit is printed in module 34, transferred to module 35 for partial drying, then transferred back to module 34 for leveling and finally transferred to module 35 for complete drying.

[0077] and Fig. 9 compared to, Fig.10An arrangement with a small footprint and reduced round trips is shown. Module 36 is a combined printing and oven module having a printing unit 38 and an oven unit 39. Printing unit 38 is adapted to perform the initial printing step i) and oven unit 39 is adapted to perform the surface modification by a partial drying step ii). This can be achieved, for example, by retrofitting at least one infrared lamp to an existing printing press, which can be illuminated to heat the substrate once the printing step i) is completed while keeping the substrate supported on the tool. Module 37 is a printing press adapted to perform a leveling step iii). Module 40 is an oven module adapted to perform a complete drying step iv).

[0078] Fig.11 A device is shown with a module 41 which is a combined printing and oven module, wherein a printing unit 42 is adapted to perform an initial printing step i), an oven unit 43 is used to perform a surface modification by a partial drying step ii), and a flattening unit 44 is operative to perform a flattening step iii). Reference will be made to Fig.12 Module 41 is described in more detail. Module 45 is an oven module suitable for performing the complete drying step iv).

[0079] Fig.12 Schematically shown is a cross-sectional side view of the internal details of a printing press arranged to perform an initial printing step i), surface modification by a partial drying step ii) and a smoothing step iii). As shown, the interior of the printing press is larger so that it can accommodate two independent printing areas - in fact, the internal workings of two independent printing presses are combined into a single printing press frame. On the left side shown is the printing apparatus for performing step i). The substrate 12 is supported on a tool 50, which is supported on a lifting platform (not shown) that can be moved vertically to move the substrate 12 closer to or away from the printing template 13 as is known in the art. The printing template 13 is held in tension in a tensioning frame 51, such as the well-known VectorGuard (RTM) tensioning frame. The scraper 15 is mounted on the printing carriage 52 and moves therewith. Located below the printing template 13 is an infrared lamp 53, which is shown as being turned off. On the right side shown is the smoothing apparatus for performing step iii). The substrate 12' is supported on a tool 50' which in turn is supported on a lifting platform (not shown). The flattening template 22 is held in tension in a tensioning frame 51'. The flattening blade 23 is mounted on a printing carriage 52' and moves therewith.

[0080] Fig.13 Schematic representation of the surface modification by partial drying step ii). Fig.12The tool 50 and substrate 12 are lowered away from the printing template 13. This action brings the newly printed deposit 11 into the view of the infrared lamp 53 which is shown turned on. Partial drying of the deposit 11 can occur in situ.

[0081] After completion of the partial drying step ii), once any substrate 12' having features 25 has been transferred from the tool 50', the substrate 12 having the partially dried deposit can be transferred to the tool 50' on a conveyor belt (not shown), which can conveniently be similar in structure to the conveyor belts commonly used in the art to enable workpieces to enter and exit a printing press.

[0082] Of course, there are many ways to implement this printer, for example the infrared lamps can be placed in different positions. In particular, the infrared lamps can be placed in association with the leveling device, in which case the newly printed substrate 12 with the deposit 11 can be transferred to the tool 51' after step i) is completed, and then the surface modification by partial drying step ii) can be carried out while the substrate is supported by the tool 51' before leveling. In another possibility, an oven module can be arranged between the printing device and the leveling device, effectively combining the internal working devices and oven modules of two independent printers into a single printer frame. Other types or arrangements of heaters (such as convection heating devices) are also possible.

[0083] Similarly, if Fig.10 As shown, the printing and oven combination module 36 may include a Fig.12 and Fig.13 The printing device with integrated heating element shown on the left, while the flattening device shown on the right can be located in a separate printing module 37.

[0084] Of course, you can also set Fig.12 A similar printer module but omitting the heating elements. In this case, the workpiece can be transferred to a separate oven module after the initial printing step i), surface modified by a partial drying step ii), and then transferred to the flattening device of the printer module for a flattening step iii).

[0085] The above 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 embodiments focus on surface modification of printed deposits by air drying, other techniques may be used to perform surface modification to reduce its viscosity. For example, a photochemical treatment process may be applied to the printed deposit to change the surface chemistry, thereby reducing viscosity. The above surface modification processes may also be combined, for example, a combination of two or more of infrared heating, convection heating, air impingement, and photochemical treatment processes may be used in step ii).

Claims

1. A method for producing planar features on a substrate, comprising the steps of: i) printing a deposit of a printing medium onto a substrate, the printed deposit comprising an upper surface, ii) modifying the upper surface of the printed deposit, and iii) leveling of linear sediments to produce flat features, wherein said step i) comprises placing a printing template on said substrate, said printing template having a thickness t1 and at least one printing aperture, placing said printing medium on said printing template, and moving a scraper through said printing template to push said printing medium through said at least one printing aperture; and Wherein step iii) comprises performing a flattening operation in which the substrate is moved into contact with a flattening template comprising holes having a larger horizontal extent than the printing apertures, and a flattening blade is moved across the flattening template into contact with the modified upper surface of the printed deposit.

2. The method according to claim 1, wherein step ii) comprises partially drying the printed deposit.

3. The method according to claim 2, wherein step ii) comprises heating the deposit to a temperature in the range of 50°C to 200°C.

4. The method according to claim 3, wherein step ii) comprises heating the deposit to a temperature of around 150°C.

5. The method of claim 2, wherein step ii) comprises heating the deposit for a period ranging from 30 seconds to 10 minutes.

6. The method of claim 5, wherein step ii) comprises heating the deposit for a period of about 2 minutes.

7. The method of claim 2, wherein step ii) comprises heating the deposit by applying infrared lighting to the deposit, using convection heating, or using an air impingement process.

8. The method of claim 1, wherein step ii) comprises treating the printed deposit using an air impingement process or a photochemical treatment process.

9. The method of claim 1, wherein the planar template has a thickness t2 that is less than t1.

10. The method of claim 1, wherein the planar blade comprises a curved contour edge.

11. The method of claim 10, wherein the smoothing blade comprises a rotatably mounted rod.

12. The method according to claim 1, comprising the subsequent steps of: iv) baking the flat features to dry them.

13. The method of claim 12, wherein step iv) comprises heating the planar feature to a temperature in the range of 100°C to 200°C.

14. The method of claim 13, wherein step iv) comprises heating the planar feature to a temperature of about 150°C.

15. The method of claim 12, wherein step iv) comprises heating the planar feature for a period ranging from 4 minutes to 15 minutes.

16. The method of claim 15, wherein step iv) comprises heating the planar feature for a period of time of about 8 minutes.

17. The method of claim 1, wherein the print medium comprises a silver sintering paste.

18. An apparatus for performing the method according to any one of claims 1 to 17.

19. A production line, comprising a plurality of operating modules, wherein the operating modules include a surface modification module and at least one printing machine, and the production line also includes a transfer mechanism for transferring workpieces between the operating modules, wherein the production line is configured such that a workpiece is printed by placing a printing template on a substrate through a printing machine, wherein the printing template has a thickness t1 and at least one printing hole, a printing medium is placed on the printing template, and a scraper is moved through the printing template to push the printing medium through the at least one printing hole, and the workpiece can be transferred from the printing machine to the surface modification module through the transfer mechanism for surface modification treatment, and then transferred from the surface modification module to the printing machine for a flattening operation, in which the substrate is moved to contact the flattening template, the flattening template includes holes with a horizontal range larger than the printing holes, and a flattening scraper is moved through the flattening template to contact the modified upper surface of the workpiece.

20. The production line of claim 19, wherein the surface modification module comprises an oven.

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