GB2583090A

GB2583090A - Method for joining a first substrate to a second substrate

Info

Publication number: GB2583090A
Application number: GB1905283.6A
Authority: GB
Inventors: Piotr Rosowski Adam; Anthony Capostagno Daniel
Original assignee: SPI Lasers UK Ltd
Current assignee: Trumpf Laser UK Ltd
Priority date: 2019-04-12
Filing date: 2019-04-12
Publication date: 2020-10-21
Also published as: GB201905283D0; GB2583090A8

Abstract

A method of joining a first substrate to a second substrate, comprising; i) providing a first substrate (which may be selected from metals, semiconductors, glasses, ceramics and glass ceramics), a second substrate (which may be selected from glasses, ceramics, glass ceramics and sapphire) and an interface material (which comprises a metal and may be in the form of a metal foil, or, a coating on at least one of the substrates); ii) bringing the first substrate and the second substrate together such that the interface material is between the first substrate and the second substrate, and, iii) directing a laser beam operating at a pre-determined wavelength through the second substrate onto the substrate interface, wherein the metal in the interface material absorbs light from the laser beam such that a join is formed between the first substrate and the second substrate. The metal may be selected from; aluminium, copper, gold, silver, platinum, palladium, molybdenum, nickel, titanium, tin, iron, chromium and stainless steel. The difference in the thermal expansion coefficients of the first and second substrates may be at least 1 ppm/oC. An additional aspect is directed towards an article made according to the method of the first aspect.

Description

Method for joining a first substrate to a second substrate

Field of Invention

This invention relates to a method for joining a first substrate to a second substrate. The invention has particular application for joining transparent materials such as glass or sapphire to ceramics or metal substrates. The invention also has application for making seals in consumer electronics products. **** ** 0 * * *****

Background to the Invention

Joining of glass to ceramic or metal parts is important in the manufacture of electronic goods, where for example, hermetic seals are often desirable. Traditional approaches of applying glues and frits can cause stability problems with parts moving over time and with temperature. Also glues and frits can outgas, which can lead to a reduction in product lifetime.

Laser welding of such parts would be highly desirable. However, laser welding has hitherto typically been limited to materials that share similar properties. This is because, if the parts to be joined together have different melting temperatures and different thermal expansion coefficients, then they are difficult to join, and if they are joined, they often crack and come apart.

Femtosecond and picosecond lasers that emit lasers pulses with pulse widths less than around 5ps have been used to make glass to glass joins, and to join glass to metals. However, femtosecond and picosecond lasers are very expensive, and process speeds are relatively slow. Use of the femtosecond and picosecond laser has thus been limited to the manufacture of precision parts rather than to use in the manufacture of consumer electronic products such as smart phones, watches, tablets and computers.

There is a need for a method for joining a first substrate to a second substrate that reduces or avoids the aforementioned problems. * * *

* * * *** * * * The Invention Accordingly, in one non-limiting embodiment of the present invention there is provided a method for joining a first substrate to a second substrate, which method comprises: * providing the first substrate and the second substrate; * providing an interface material; * bringing the first substrate and the second substrate together such that the interface material is between the first substrate and the second substrate and forms a substrate interface; * directing a laser beam operating at a predetermined wavelength through the second substrate onto the substrate interface; **** * the method being such that: *** * * * the interface material comprises a metal; and * * * the metal absorbs light from the laser beam in an amount sufficient to form a join between the first substrate and the second substrate.

**** The method of the present invention is especially attractive because it is able to join * **** materials that have not previously been able to be joined reliably with lasers, and to do so at * ** speeds consistent with volume commercial electronics manufacturing.

* * *** * * The metal may be in the form of a metal foil, and the method may include the step of inserting the metal foil between the first substrate and the second substrate prior to forming the substrate interface.

The metal may be in the form of a coating on at least one of the first substrate and the second substrate.

The metal may be selected from the group consisting of aluminium, copper, gold, silver, platinum, palladium, molybdenum, nickel, titanium, tin, iron, chromium, and stainless steel.

**** The metal may have a melting point that is at least 100 °C less than a melting point of the second substrate.

S * e The first substrate may be selected from metals, semiconductors, glasses, ceramics, and glass ceramics.

* * * The first substrate may be a metal selected from the group comprising nickel, titanium, tin, iron, chromium, and stainless steel.

* The second substrate may be selected from glasses, ceramics, glass ceramics, and sapphire.

The second substrate may have a transmission at the predetermined wavelength greater than 90%.

The first substrate may be characterized by a first thermal expansion coefficient, and the second substrate may be characterized by a second thermal expansion coefficient. The difference between the first thermal expansion coefficient and the second thermal expansion coefficient may be at least 1 ppm / °C. Such a thermal expansion coefficient mismatch makes the rapid joining of the first substrate to the second substrate difficult by prior art methods.

The method may include the steps of: * providing a laser for emitting the laser beam, wherein the laser beam comprises laser pulses having a pulse energy, a pulse width, a pulse repetition frequency; * providing a lens for focussing the laser beam from the laser onto the substrate interface to form a spot having a spot diameter and a pulse fluence; * providing a scanner for scanning the laser beam; * providing a controller for controlling the scanner with a control signal; * writing at least one line on the substrate interface to form the join by scanning the laser beam while pulsing the laser; and * * ** * **** * 410 * * * 0 * * * * * **** * selecting a scan speed, the pulse repetition frequency, and the spot diameter to provide a desired spot to spot separation between the centres of consecutive spots during each scan of the scanner.

* The spot diameter may be between 25pm and 100pm.

* * The spot to spot separation may be between 10% to 95% of the spot diameter. The spot to spot separation may be between 50% to 90% of the spot diameter.

e* * The method may include the step of writing a plurality of the lines, and wherein adjacent ones of the lines are separated by a line spacing at least two times greater than the spot diameter.

The pulse width may be in the range 2Ons to 10,000 ns, The laser may be a fibre laser, a rod laser, a disk laser, a slab laser, a direct diode laser, or a gas laser.

The method may include the step of traversing the laser beam such that the join forms a seal between the first substrate and the second substrate. The seal may form an hermetic enclosure.

The method may include the step of placing at least one of an organic layer, an organic light emitting diode, a semiconductor, a light emitting diode or laser, a cathode, or an anode onto the first substrate prior to forming the substrate interface. Advantageously, this enables hermetic electronic packaging and the production of touch screens, without the use of glues and frits, thus avoiding a reduction in product lifetime owing to outgassing.

The invention also provides a use of a laser to join a first substrate to a second substrate according to the method of the invention.

The invention also provides an article comprising a first substrate joined to a second substrate according to the method of the invention.

The invention also provides an apparatus for joining a first substrate to a second substrate according to the method of the invention. * * * *

* . *** * * * * * **** * * . *0 * * * * ** *** * *

Brief Description of the Drawings

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 shows apparatus for use in the method according to the present invention; Figure 2 shows a pulsed laser waveform; Figure 3 shows a laser beam that has been focussed to for a spot; Figure 4 shows a metal foil; Figure 5 shows a metal foil that has been cut to form a hole in its centre; Figure 6 shows the foil positioned between the first substrate and the second substrate; Figure 7 shows an article that has been packaged between the first substrate and the second substrate, wherein the metal foil is between the first substrate and the second substrate, and which package comprises a plurality of concentric joins; Figure 8 shows a first substrate that has been coated with a metal coating; Figure 9 shows an article that has been packaged between the first substrate and the second substrate, wherein the metal coating is between the first substrate and the second substrate, and which package comprises a plurality of concentric joins; Figure 10 shows a line formed by pulsing the laser while scanning the laser beam; and Figure 11 shows a plurality of lines separated by a line spacing.

Detailed Description of Preferred Embodiments of the Invention Figure 1 shows an apparatus 10 for joining a first substrate 1 to a second substrate 2 with a laser beam 4 at a predetermined wavelength 20. An interface material 5 is between the first substrate 1 and the second substrate 2. The interface material 5 forms a substrate interface 3. The laser beam 4 is directed through the second substrate 2 onto the substrate interface 3. The interface material 5 comprises a metal 6. The metal 6 absorbs light from 4 a * **** the laser beam 4 in an amount sufficient to form a join 7 between the first substrate 1 and the second substrate 2.

A compressive force 18 may be applied to compress the first substrate 1 and the second substrate 2 together while joining them with the laser beam 4.

** * The apparatus 10 is shown as comprising a laser 8, a scanner 9, and an objective lens 19. The scanner 9 moves the laser beam 4 with respect to the interface material 5. The scanner 9 may be a two-dimensional scanner that comprises a first mirror 11 for moving the laser beam 4 in a first direction, and a second mirror 12 for scanning the laser beam 4 in a second direction. The scanner 9 is controlled by a controller 13 which controls the positions of the first and second mirrors 11, 12 by providing at least one control signal 14 to the scanner 9. The controller 13 may also control the laser 8. The first and the second mirrors 11, 12 would typically be attached to galvanometers (not shown). Alternatively, the scanner 9 may be a one-dimensional scanner, such as a polygon scanner, or a resonant mirror scanner configured to scan the laser beam 4 in one direction. Scanning in the orthogonal direction may then be achieved with a translation table (not shown) configured to move the second substrate 2 with respect to the laser beam 4. The controller 13 may control one or both of the one-dimensional scanner and the translation table.

The laser 8 can be a fibre laser, a solid-state rod laser, a solid-state disk laser, a slab laser, a direct diode laser, or a gas laser such as a carbon dioxide laser. The laser 8 may be a continuous wave laser or a pulsed laser. For joining substrates, the laser 8 is preferably a pulsed laser. The laser 8 is shown as being connected to the scanner 9 via an optical fibre cable 15 and collimation optics 16.

Referring now to Figure 2, there is shown a series of pulses 21. The series of pulses 21 may be obtained from the laser 8 wherein the laser 8 is a pulsed laser. The series of pulses 21 is characterized by a peak power Ppeak 22, an average power PAv 23, a pulse shape 24, a pulse energy 25, a pulse width 26, and a pulse repetition frequency FR 27 that is related to the period of the waveform 1 / FR as shown. * * **** * * ** * * * *** * * * *

Figure 3 shows a spot 31 formed by focussing the laser beam 4 onto the substrate interface 3. The optical intensity 32 is the power per unit area of the laser beam 4. The optical intensity 32 varies across the diameter of the spot 31 from a peak intensity la 39 at its centre 37, to a 1/e2 intensity 33 of Iv / e2, and to zero. The diameter 34 of the spot 31 is typically taken as the 1/e2 diameter, which is the diameter at which the optical intensity 32 falls to the 1/e2 intensity 33 on either side of the peak intensity 39. The area 35 of the spot 31 is typically taken as the cross-sectional area of the spot 31 within the 1/e2 diameter 34. Figure 3 shows the optical intensity 32 varying with a Gaussian or bell-shaped profile. The optical intensity 32 may have other profiles, including a top hat profile that is substantially uniform within the diameter 34.

Pulse fluence 36 is defined as the energy per unit area of the pulse 21. Pulse fluence is typically measured in J/cm2, and is an important parameter for laser joining because if the pulse fluence 36 is too high, then the first substrate 1 and/or the second substrate 2 may *0 * * crack, and if the pulse fluence 36 is too low, then there may be insufficient energy absorbed 00 * by the metal 6 to form the join 7. * *

0000 The method of the present invention is especially attractive because it is able to join 00.0 materials that have not previously been able to be joined reliably with lasers, and to do so at speeds consistent with volume commercial electronics manufacturing. * ..

* . 000 * Figure 4 shows the metal 6 in the form of a metal foil 40 that has a thickness 41. The method may include the step of inserting the metal foil 40 between the first substrate 1 and the second substrate 2 prior to forming the substrate interface 3.

The metal foil 40 can be formed into a desired shape prior to inserting it between the first substrate 1 and the second substrate 2. For example, the foil 40 may be the foil 50 shown in Figure 5 which has been cut to form a hole 51. Such an arrangement is useful for forming seals in electronic packaging. For example, Figure 6 shows a first substrate 60 that has a side wall 61 and a device 62 fixed onto it. The foil 50 is placed on top of the first substrate 60 prior to placing the second substrate 2 onto the foil 50. The laser beam 4, shown with reference to Figure 1, is then used to join the first substrate 60 to the second substrate 2 as shown with reference to Figure 7. The scanner 9 has moved the laser beam 4 over the foil 50 such that joins 71 are created. The joins 71 are shown as being concentric with each other, with spaces between individual ones of the joins 71. The use of concentric rings is advantageous to forming hermetic seals. Other patterns of joins 71 are also possible. Two dimensional patterns are preferred, which patterns include spirals, concentric circles, hatched lines, meshes, grids, and spots.

The metal 6 may be in the form of a coating 81 as shown with reference to Figure 8. The coating 81 is shown on the second substrate 2. Alternative or additionally, the coating 81 may be formed on the first substrate 1. The laser beam 4, shown with reference to Figure 1, is then used to join the first substrate 60 to the second substrate 2 as shown with reference to Figure 9. The scanner 9 has moved the laser beam 4 over the coating 81 such that joins 91 are created. The joins 91 are shown as being concentric with each other, with spaces between individual ones of the joins 91. The use of concentric rings is advantageous to forming hermetic seals. Other patterns of joins 91 are also possible. Two dimensional patterns are preferred, which patterns include spirals, concentric circles, hatched lines, meshes, grids, and spots.

Referring to Figures 1 and Figures 4 to 9, the metal 6 may be selected from the group consisting of aluminium, copper, gold, silver, platinum, palladium, molybdenum, nickel, titanium, tin, iron, chromium, and stainless steel.

The metal 6 may have a melting point that is at least 100 °C less than a melting point of the second substrate 2.

The first substrate 1 may be selected from metals, semiconductors, glasses, ceramics, and glass ceramics.

The first substrate 1 may be a metal selected from the group comprising nickel, titanium, tin, iron, chromium, and stainless steel.

The second substrate 2 may be selected from glasses, ceramics, glass ceramics, and sapphire. * * **** * **** * **

* * * *** 0 * * * 0000 The second substrate 2 may have a transmission at the predetermined wavelength 20 greater than 90%.

* * * The first substrate 1 may be characterized by a first thermal expansion coefficient, and the second substrate 2 may be characterized by a second thermal expansion coefficient. The difference between the first thermal expansion coefficient and the second thermal expansion coefficient may be at least 1 ppm / °C. Such a thermal expansion coefficient mismatch makes the rapid joining of the first substrate 1 to the second substrate 2 difficult by prior art methods.

* * * The method may include the steps of: * providing the laser 8 for emitting the laser beam 4, wherein the laser beam 4 comprises the laser pulses 21 shown with reference to Figure 2 having the pulse energy 25, the pulse width 26, the pulse repetition frequency 27; * providing the lens 19 shown with reference to Figure 1 for focussing the laser beam 4 from the laser 8 onto the substrate interface 3 to form the spot 31 having the spot diameter 34 and the pulse fluence 36; * providing the scanner 9 for scanning the laser beam 4; * providing the controller 13 for controlling the scanner 9 with the control signal 14; * writing at least one line 105 shown with reference to Figure 10 on the substrate interface 3 to form the join 7 by scanning the laser beam 4 while pulsing the laser 8; and * selecting a scan speed 101 shown with reference to Figure 10, the pulse repetition frequency 27, and the spot diameter 34 to provide a desired spot to spot separation 102 between consecutive spots during each scan of the scanner 9.

The spot diameter 33 may be between 25pm and 100pm.

The spot to spot separation 102 may be between 10% to 95% of the spot diameter 33. The spot to spot separation 102 may be between 50% to 90% of the spot diameter 33.

* * * * * * * * * * es * * a.** * * 0 Ogle.

* * * *0 * * * **** The method may include the step of writing a plurality of the lines 105 as shown in Figure 11. Adjacent ones of the lines 105 may be separated by a line spacing 111 at least two times greater than the spot diameter 33.

The pulse width 26 may be in the range 2Ons to 10,000 ns, The laser 8 may be a fibre laser, a rod laser, a disk laser, a slab laser, a direct diode laser, or a gas laser.

As shown with reference to Figures 4 to 9, the method may include the step of traversing the laser beam 4 such that the join 7 forms a seal between the first substrate 1 and the second substrate 2. The seal may form an hermetic enclosure as shown in Figures 7 and 9.

The device 62 shown with reference to Figures 7 and 9 may comprise an organic layer, an organic light emitting diode, a semiconductor, a light emitting diode or laser, a cathode, or an anode. Advantageously, this enables hermetic electronic packaging and the production of touch screens, without the use of glues and frits, thus avoiding a reduction in product lifetime owing to outgassing.

The invention also provides a use of a laser 8 to join a first substrate 1 to a second substrate 2 according to the method of the invention.

The invention also provides an article 70 shown with reference to Figure 7, or an article 90 shown with reference to Figure 9. The articles 70 and 90 each comprise a first substrate 1 joined to a second substrate 2 according to the method of the invention.

The invention also provides an apparatus 10 for joining a first substrate 1 to a second substrate 2 according to the method of the invention.

The laser 8 may be an optical fibre laser having a single mode or a multi mode rare-earth doped fibre. The laser beam 4 may have a beam quality defined by an M2 value less than 6, preferably less than 4, and more preferably less than 1.3.

The wavelength 20 is preferably in the range 1000nm to 1100nm. Such wavelengths are emitted by ytterbium-doped fibre lasers. * es * * * Ooe * *

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications and additional steps and components may be provided to enhance performance. Individual components shown in the drawings are not limited to use in their drawings and may be used in other drawings and in all aspects of the invention. The present invention extends to the above mentioned features taken singly or in any combination. * *** * * **** * **** * * *

* * * *** * * * *

Claims

Claims 1. A method for joining a first substrate to a second substrate, which method comprises: * providing the first substrate and the second substrate; * providing an interface material; * bringing the first substrate and the second substrate together such that the interface material is between the first substrate and the second substrate and forms a substrate interface; * directing a laser beam operating at a predetermined wavelength through the second substrate onto the substrate interface; the method being such that: * the interface material comprises a metal; and * the metal absorbs light from the laser beam in an amount sufficient to form a join between the first substrate and the second substrate.
* * * * * * * *** * * 2. A method according to claim 1, wherein the metal is the form of a metal foil, and the * * method includes the step of inserting the metal foil between the first substrate and the * * * * * **** second substrate prior to forming the substrate interface.
3. A method according to claim 1, wherein the metal is in the form of a coating on at least * ** * * * *** * * one of the first substrate and the second substrate. *
* * 4. A method according to any one of the preceding claims, wherein the metal is selected from the group consisting of aluminium, copper, gold, silver, platinum, palladium, molybdenum, nickel, titanium, tin, iron, chromium, and stainless steel.
5. A method according to any one of the preceding claims, wherein the metal has a melting point that is at least 100 °C less than a melting point of the second substrate.
6. A method according to any one of the preceding claims, wherein the first substrate is selected from metals, semiconductors, glasses, ceramics, and glass ceramics.
**** 7. A method according to any one of claims 1 to 6, wherein the first substrate is a metal selected from the group comprising nickel, titanium, tin, iron, chromium, and stainless steel.
* * * ** * 8. A method according to any one of the preceding claims, wherein the second substrate is selected from glasses, ceramics, glass ceramics, and sapphire.
9. A method according to any one of the preceding claims, wherein the second substrate has a transmission at the predetermined wavelength greater than 90%.
10. A method according to any one of the preceding claims, wherein the first substrate is characterized by a first thermal expansion coefficient, and the second substrate is characterized by a second thermal expansion coefficient, and the difference between the first thermal expansion coefficient and the second thermal expansion coefficient is at least 1 ppm / °C.
11. A method according to any one of the preceding claims, wherein the method includes the steps of: * * providing a laser for emitting the laser beam, wherein the laser beam comprises * **** laser pulses having a pulse energy, a pulse width, a pulse repetition frequency; **-* * providing a lens for focussing the laser beam from the laser onto the substrate interface to form a spot having a spot diameter and a pulse fluence; * ** *** * * providing a scanner for scanning the laser beam; * a * providing a controller for controlling the scanner with a control signal; * writing at least one line on the substrate interface to form the join by scanning the laser beam while pulsing the laser; and * selecting a scan speed, the pulse repetition frequency, and the spot diameter to provide a desired spot to spot separation between the centres of consecutive spots during each scan of the scanner.
12, A method according to claim 11, wherein the spot diameter is between 25pm and 100pm. * *Y
* * * *** * * * * 13. A method according to claim 11 or claim 12, wherein the spot to spot separation is between 10% to 95% of the spot diameter.
14. A method according to claim 13, wherein the spot to spot separation is between 50% to 90% of the spot diameter.
15. A method according to any one of claims 11 to 14, wherein the method includes the step of writing a plurality of the lines, and wherein adjacent ones of the lines are separated by a line spacing at least two times greater than the spot diameter.
16. A method according to any one of claims 11 to 15, wherein the pulse width is the range 2Ons to 10,000 ns,
17. A method according to any one of claims 11 to 16 wherein the laser is a fibre laser, a rod laser, a disk laser, a slab laser, a direct diode laser, or a gas laser.
18. A method according to any one of the preceding claims, wherein the method includes the step of traversing the laser beam such that the join forms a seal between the first substrate and the second substrate.
19. A method according to claim 18, wherein the seal forms an hermetic enclosure.
20. A method according to any one of the preceding claims, wherein the method includes the step of placing at least one of an organic layer, an organic light emitting diode, a semiconductor, a light emitting diode or laser, a cathode, or an anode onto the first substrate prior to forming the substrate interface.
21. A use of a laser to join a first substrate to a second substrate according to the method of any one of claims 1 to 20.
22. An article comprising a first substrate joined to a second substrate according to the method of any one of claims 1 to 20.
23. An apparatus for joining a first substrate to a second substrate according to the method of any one of claims 1 to 20. ***** II * ** * * * 1.* * ****