Metal pad bonding method based on high repetition rate ultrafast laser thermal effect

Abstract

The present invention discloses a metal pad bonding system and method based on high repetition rate ultrafast laser thermal effect, the bonding system comprises: a laser processing optical path subsystem, which comprises a first laser emitter, a host computer, a beam expander, a first reflector, and a galvanometer and focusing mirror module; a microscopic imaging optical path subsystem, which comprises a second laser emitter, an eyepiece, a CCD image sensor, and a second reflector. The present invention clamps a gold wire between a metal material and a semiconductor material, and then ablate, melt, and sputter the surface of the metal material through the action of a high repetition rate femtosecond laser, and the high-temperature metal molten sputtering material is continuously deposited on the surface of the semiconductor material and gradually cooled, thereby directly forming a metal contact surface, i.e., a pad, on the surface of the semiconductor material. At the same time, due to the continuous action of the high repetition rate femtosecond laser, the semiconductor material, the pad, and the gold wire form heat accumulation and reflect a high thermal effect, and at this time, direct bonding of the metal pad is formed under the action of the high repetition rate femtosecond laser.

Description

Technical Field

[0001] The invention relates to the application field of ultrafast laser welding, and in particular to a metal pad bonding method based on high repetition rate ultrafast laser thermal effect. Background Art

[0002] Wire bonding is one of the important process technologies in semiconductor packaging. Its purpose is to weld the two ends of the metal wire to the chip and the pin respectively to form an electrical connection. During the bonding process, the wire contacts the pad metal under the combined action of ultrasonic energy, pressure or heat, and interatomic diffusion occurs to achieve the purpose of bonding. Wire bonding occupies a mainstream position in electronic packaging because of its simple process, mature technology, low cost, and suitability for a variety of packaging forms. Gold wire bonding is a type of wire bonding process. It uses ultrasonic vibration and bonding force to bond the gold wire leads to the chip pads and substrate pins respectively under the condition of substrate heating or gold wire heating to achieve physical interconnection between the chip and substrate circuits.

[0003] However, due to the high cost of gold wire, in order to reduce the packaging cost, various gold wire substitutes have been launched on the market: copper wire or silver wire and other materials. Among them, bonding copper wire materials have begun to be used because of their better thermal and electrical properties, but due to their high hardness, they are easy to cause chip damage during bonding, and are prone to corrosion in non-airtight packaging, which directly affects the reliability of the device after bonding. Therefore, they can only be used in some mid- and low-end products; bonding silver wire has gradually begun to be used in microelectronic packaging due to its excellent electrothermal properties, good stability and appropriate cost factors, but pure silver wire has low strength, easy electron migration, and difficult to control the growth of intermetallic compounds. The application in the packaging field is also subject to certain restrictions.

[0004] Gold wire bonding is the most commonly used bonding technology in electronic packaging, mainly including gold wire ball bonding and gold wire wedge bonding. The basic process steps of these two technologies are similar, including welding the first solder joint on the chip surface, forming a connecting wire arc, and forming a second solder joint on the lead frame or substrate. However, wire bonding failures often occur in these processes, mainly due to cratering, cracking and warping of bonding points, inconsistent bonding tail wires, cracks or deformation at the root of the lead, and debonding of bonding points. According to statistics, in semiconductor microelectronic packaging, about 1 / 4 to 1 / 3 of semiconductor device failures are caused by chip interconnection. The losses caused by failure and reduced reliability caused by gold wire bonding are far greater than the losses of the process itself. Traditional methods improve processing effects by adjusting parameters, but it is difficult to meet the requirements of high reliability and high efficiency wire bonding. Therefore, further research and development of new methods are needed to solve this problem. Summary of the invention

[0005] In order to solve the above problems, the present invention proposes a metal pad bonding method based on high repetition rate ultrafast laser thermal effect, which provides means and tools for research in the field of ultrafast laser welding to meet the actual needs of scientific research and industrial production.

[0006] The technical solution adopted by the present invention is as follows:

[0007] On the one hand, the present invention proposes a metal pad bonding system based on high-repetition-rate ultrafast laser thermal effect, including a laser processing optical path subsystem and a microscopic imaging optical path subsystem; the laser processing optical path subsystem includes a first laser emitter, a host computer, a beam expander, a first reflector, and a galvanometer and focusing mirror module, the host computer can control the first laser emitter to generate a femtosecond laser with a pulse repetition frequency higher than a set threshold, the femtosecond laser is expanded by the beam expander and then the optical path is adjusted by the first reflector, and then the light beam is focused on the surface of the metal material of the sample to be processed through the galvanometer and focusing mirror module; the sample to be processed includes a metal material, a semiconductor material and a metal wire lead, the metal material is arranged on a moving platform, the semiconductor material is arranged on the other side of the metal material, and the metal wire lead is tightly fixed between the metal material and the semiconductor material by a clamp.

[0008] The microscopic imaging optical path subsystem includes a second laser emitter, an eyepiece, a CCD image sensor and a second reflector. The second laser emitter is configured to generate an illumination laser and form an illumination optical path. The illumination laser focuses the light beam on the sample to be processed through a galvanometer and focusing lens module. The CCD image sensor, the eyepiece, the second reflector and the galvanometer and focusing lens module together constitute a microscopic imaging optical path. The CCD image sensor can detect and locate the metal wire lead in real time through the microscopic imaging optical path, and focus the femtosecond laser on one side of the metal wire lead.

[0009] Furthermore, the laser processing path of the femtosecond laser includes an arcuate processing path and is parallel to the metal wire lead.

[0010] Furthermore, the pulse repetition frequency of the femtosecond laser is higher than a set threshold, and the value of the set threshold includes: 200KHz.

[0011] Furthermore, the wavelength range of the femtosecond laser includes: 266~2000nm.

[0012] Furthermore, the material selection range of the metal material includes: copper, nickel steel alloy, titanium alloy, aluminum alloy or steel.

[0013] Furthermore, the semiconductor material may be made of sapphire, fused quartz, ceramic or silicon.

[0014] Furthermore, the material selection range of the metal wire lead includes: gold wire lead, silver wire lead or copper wire lead.

[0015] Furthermore, the diameter of the metal wire lead ranges from 5 to 30 μm.

[0016] On the other hand, the present invention provides a metal pad bonding method based on high repetition rate ultrafast laser thermal effect, comprising the following steps:

[0017] S1. Using a CCD image sensor to monitor the metal wire lead in real time, positioning the metal wire lead between the metal material and the semiconductor material, and then using a clamp to tightly fit the metal wire lead to the metal material and the semiconductor material surface;

[0018] S2. Focus the femtosecond laser generated by the first laser emitter on the surface of the metal material, and monitor in real time through the CCD image sensor to focus the femtosecond laser on one side of the metal wire lead; set a laser processing path parallel to the metal wire lead, so that the femtosecond laser processes the surface of the metal material, and causes the metal material to ablate, melt and sputter, so that the high-temperature metal melt is continuously deposited on the surface of the semiconductor material and on both sides of the metal wire lead and gradually cools, thereby initially forming an effective bond; at the same time, since the pulse time interval of the femtosecond laser is shorter than the cooling time of the metal material, the repeated ablation of the metal material will form heat accumulation and manifest a high thermal effect, and the metal material, the semiconductor material and the metal wire lead are heated simultaneously under the scanning action of the femtosecond laser, thereby realizing the bonding of the metal pad.

[0019] Furthermore, the laser processing path of the femtosecond laser includes an arcuate processing path.

[0020] The beneficial effects of the present invention are:

[0021] 1. Ultrafast laser welding of gold wire leads to the surface of semiconductor materials is mainly used in chip processing. In the traditional method, a layer of metal contact surface, also called pad, is attached to the surface of semiconductor materials (silicon) before bonding the leads. The metal contact surface is first attached and then spot welded to the surface of the semiconductor material. The processing steps are cumbersome, and different bonding effects will be caused due to different process operations during the processing. The present invention clamps the gold wire with metal materials and semiconductor materials, and then ablates, melts and sputters the surface of the metal material through the action of high-repetition-rate femtosecond laser. The high-temperature metal molten sputtering is continuously deposited on the surface of the semiconductor material and gradually cooled, thereby directly forming a metal contact surface (pad) on the surface of the semiconductor material. At the same time, due to the continuous action of the high-repetition-rate femtosecond laser, the semiconductor material, pad and gold wire form heat accumulation and reflect a high thermal effect. At this time, under the action of the high-repetition-rate femtosecond laser, a direct bonding of the metal pad is formed, which simplifies the processing steps and facilitates production and processing.

[0022] 2. High repetition rate femtosecond laser welding metal pad bonding Due to its unique welding principle, on the one hand, the metal material will be ablated, melted and sputtered under the action of high repetition rate laser, directly forming a pad on the surface of the semiconductor material, and then through the action of high repetition rate femtosecond laser on the semiconductor material, gold wire and pad, heat accumulation is formed and high thermal effect is reflected, thus achieving basic bonding. On the other hand, high repetition rate femtosecond laser acts on the surface of metal material, and the metal material is ablated and sputtered. The high temperature metal molten sputtering will gradually deposit on the edge of the contact between the pad and the gold wire lead, and then cool down and provide higher strength for the bonding of the gold wire lead and the pad, thereby strengthening the strength of the metal pad bonding.

[0023] 3. In addition, with the support of the above advantages, since different metal materials and semiconductor materials have different heat accumulation effects on the action of high repetition rate femtosecond laser, different metal pad bonding effects will be produced. By changing the metal material (copper, invar, titanium alloy, etc.), semiconductor material (silicon, etc.) or lead material (gold wire, copper wire, silver wire, etc.), the above metal pad bonding method can still be completed and different metal pad bonding effects can be obtained, which provides a wider range of experimental production for experimental production. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic structural diagram of a metal pad bonding system based on high repetition rate ultrafast laser thermal effect according to Embodiment 1 of the present invention;

[0025] Figure 2 This is a schematic diagram of the metal wire lead and the ultrafast laser scanning path in Example 1 of the present invention;

[0026] Figure 3 It is a schematic diagram of the ablated sputtered molten material in Example 1 of the present invention, which is deposited, cooled and welded at the edge of the contact point between the gold wire and the pad on the surface of the semiconductor material, and ablated pits are generated on the surface of the metal material.

[0027] Figure markings: 1-second laser emitter; 2-microscope imaging optical path; 3-eyepiece; 4-CCD image sensor; 5-first laser emitter; 6-host computer; 7-femtosecond laser; 8-beam expander; 9-laser processing path; 10-metal material; 11-semiconductor material; 12-metal wire lead; 13-galvanometer and focusing mirror module; 14-1 is the first reflector, 14-2 is the second reflector; 15-illumination optical path; 16-metal melt; 17-metal contact surface, i.e., solder pad. DETAILED DESCRIPTION

[0028] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific implementation methods of the present invention are now described. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the embodiments described are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present invention.

[0029] Example 1

[0030] like Figure 1 As shown, this embodiment provides a metal pad bonding system based on high repetition rate ultrafast laser thermal effect, including a laser processing optical path subsystem and a microscopic imaging optical path subsystem. Among them, the laser processing optical path subsystem includes a first laser emitter 5, a host computer 6, a beam expander 8, a first reflector 14-1, and a galvanometer and focusing mirror module 13. The host computer 6 can control the first laser emitter 5 to generate a femtosecond laser 7 with a pulse repetition frequency higher than a set threshold. The femtosecond laser 7 is expanded by the beam expander 8 and then adjusted by the first reflector 14-1. Then, the beam is focused on the surface of the metal material 10 of the sample to be processed through the galvanometer and focusing mirror module 13.

[0031] The microscopic imaging optical path subsystem includes a second laser emitter 1, an eyepiece 3, a CCD image sensor 4, and a second reflector 14-2. The second laser emitter 1 is configured to generate an illumination laser and form an illumination optical path 15. The illumination laser passes through a galvanometer and focusing lens module 13 to focus the light beam on the sample to be processed. The CCD image sensor 4, the eyepiece 3, the second reflector 14-2, and the galvanometer and focusing lens module 13 together constitute a microscopic imaging optical path 2. Since the diameter of the metal wire lead 12 is very small, there is a large error in positioning and clamping the metal wire lead 12 by the human eye. Therefore, a microscopic imaging optical path 2 is set and the metal wire lead 12 is detected and positioned in real time by the CCD image sensor 4, so that the femtosecond laser 7 is focused on one side of the metal wire lead 12.

[0032] like Figure 2As shown, the sample to be processed includes a metal material 10, a semiconductor material 11 and a metal wire lead 12. The metal material 10 is arranged on a moving platform, the semiconductor material 11 is arranged on the other side of the metal material 10, and the metal wire lead 12 is tightly fixed between the metal material 10 and the semiconductor material 11 by a clamp, and is monitored in real time by a CCD image sensor 4. The femtosecond laser 7 performs laser processing according to the specified laser processing path 9. The metal material 10 undergoes selective ablation under the action of the high-repetition-rate femtosecond laser 7, and the high-temperature metal melt 16 will be continuously deposited on the lower surface of the semiconductor material 11 and on both sides of the metal wire lead 12. At the same time, because the pulse time interval of the femtosecond laser 7 is shorter than the cooling time of the metal, the repeated ablation of the metal will form heat accumulation and reflect a high thermal effect. The metal contact surface 17, the semiconductor material 11, and the metal wire lead 12 can be heated simultaneously under the scanning action of the femtosecond laser 7, thereby achieving high-strength metal pad bonding.

[0033] Among them, the metal pad bonding can be regarded as two parts: the metal material 10 undergoes ablation, melting and sputtering to form a metal contact surface 17, i.e., a pad, on the lower surface of the semiconductor material 11, and the metal wire lead 12, the semiconductor material 11 and the metal contact surface 17 form heat accumulation, which manifests a high thermal effect and is bonded; the metal material 10 undergoes ablation and sputtering, and the high-temperature metal melt 16 is deposited on the lower surface of the semiconductor material 11 and on both sides of the metal wire lead 12, so that the metal pad is bonded.

[0034] like Figure 3 As shown, the metal material 10 is ablated, melted and sputtered under the action of the high-repetition-rate femtosecond laser 7, and the high-temperature molten metal 16 is continuously deposited on the lower surface of the semiconductor material 11 and on both sides of the metal wire lead 12. Then, the molten metal 16 gradually cools down, thereby forming a metal contact surface 17, i.e., a solder pad, on the surface of the semiconductor material 11.

[0035] Preferably, the laser processing path 9 of the femtosecond laser 7 can be an arcuate processing path and is parallel to the metal wire lead 12, such as Figure 2 shown.

[0036] Preferably, the wavelength of the femtosecond laser 7 is in the range of 266-2000 nm, and the pulse repetition frequency is greater than 200 KHz.

[0037] Preferably, the metal material 10 may be made of copper, nickel-steel alloy, titanium alloy, aluminum alloy or steel.

[0038] Preferably, the semiconductor material 11 may be made of sapphire, fused quartz, ceramic or silicon.

[0039] Preferably, the metal wire lead 12 can be a gold wire lead, a silver wire lead, a copper wire lead or other conductive materials.

[0040] Preferably, the diameter of the metal wire 12 may be 5-30 μm.

[0041] In addition, the size of the metal contact surface 17, ie, the pad, is approximately 50 μm×50 μm.

[0042] Accordingly, this embodiment also proposes a metal pad bonding method based on high repetition rate ultrafast laser thermal effect, comprising the following steps:

[0043] S1. The metal wire lead 12 is monitored in real time by the CCD image sensor 4, so that the metal wire lead 12 is positioned between the metal material 10 and the semiconductor material 11, and then the metal wire lead 12 is tightly fitted to the surface of the metal material 10 and the semiconductor material 11 by a fixture;

[0044] S2. Focus the femtosecond laser 7 generated by the first laser emitter 5 on the surface of the metal material 10, and monitor in real time through the CCD image sensor 4 so that the femtosecond laser 7 is focused on one side of the metal wire lead 12; set a laser processing path 9 parallel to the metal wire lead 12, so that the femtosecond laser 7 is processed on the surface of the metal material 10, and the metal material 10 is ablated, melted and sputtered, so that the high-temperature metal melt is continuously deposited on the surface of the semiconductor material 11 and on both sides of the metal wire lead 12 and gradually cooled, thereby initially forming an effective bond; at the same time, since the pulse time interval of the femtosecond laser 7 is shorter than the cooling time of the metal material 10, the repeated ablation of the metal material 10 will form heat accumulation and reflect a high thermal effect, and the metal material 10, the semiconductor material 11 and the metal wire lead 12 are heated simultaneously under the scanning action of the femtosecond laser 7, thereby realizing metal pad bonding, effectively avoiding the failure of metal pad bonding and improving the reliability of bonding.

[0045] Example 2

[0046] This embodiment is based on embodiment 1:

[0047] In this embodiment, the metal material 10 is brass, the semiconductor material 11 is quartz, and the metal wire lead 12 is a gold wire lead. The sizes of brass and quartz are selected to be approximately 10mm×20mm×2mm, the diameter of the gold wire lead is approximately 25μm, and the crystal surface used needs to be polished to ensure high-quality contact. It is clamped with a clamp and detected in real time by the CCD image sensor 4 to ensure that the gold wire lead is in tight contact with the brass and quartz surfaces according to the experimental requirements, and the high-repetition-rate femtosecond laser 7 is focused on one side of the gold wire lead. Subsequently, laser processing is performed according to the steps of the metal pad bonding method based on high-repetition-rate ultrafast laser thermal effect provided in Example 1.

[0048] Among them, the host computer 6 (such as the main control computer) makes the femtosecond laser 7 at the laser emission point reflect through the first reflector 14-1 and then enter the galvanometer and focusing mirror module 13, so that the femtosecond laser 7 is focused on the upper surface of the brass. The pulse width of the femtosecond laser 7 is set to 250fs, the laser power is 6W, the repetition frequency is 1MHz, the output wavelength is 1030nm, and the scanning speed is 100mm / s. The main control computer controls the first laser emitter 5 and the three-dimensional motion platform to perform processing according to the set laser processing path 9, and the processing spacing is 50μm. The brass undergoes selective ablation under the action of the high-repetition-rate femtosecond laser 7, and the high-temperature brass molten sputtering will continuously deposit on the lower surface of the quartz and on both sides of the gold wire. At the same time, because the pulse time interval of the high-repetition-rate femtosecond laser 7 is shorter than the cooling time of the brass, the repeated ablation of the brass will form heat accumulation and reflect the high thermal effect. Under the scanning action of the femtosecond laser 7, the brass, quartz, and gold wire are heated at the same time, thereby achieving high-strength metal pad bonding.

[0049] The above is only a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the form disclosed herein, and should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications and environments, and can be modified within the scope of the concept described herein through the above teachings or the technology or knowledge of the relevant field. The changes and modifications made by those skilled in the art shall not deviate from the spirit and scope of the present invention, and shall be within the scope of protection of the claims attached to the present invention.

[0050] In the description of the present invention, it should be noted that the directions or positional relationships indicated by the terms "center", "upper", "lower", "left", "right", "inner", "outer", etc. are based on the directions or positional relationships shown in the accompanying drawings, or are the directions or positional relationships usually placed when the present invention is used, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", "third", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

Claims

1. A metal pad bonding method based on high repetition rate ultrafast laser thermal effect, applied to a metal pad bonding system, characterized in that: The metal pad bonding system includes a laser processing optical path subsystem and a microscopic imaging optical path subsystem; The laser processing optical path subsystem comprises a first laser emitter (5), a host computer (6), a beam expander (8), a first reflector (14-1), and a galvanometer and focusing mirror module (13); the host computer (6) can control the first laser emitter (5) to generate a femtosecond laser (7) with a pulse repetition frequency higher than a set threshold; the femtosecond laser (7) is expanded by the beam expander (8) and then adjusted to an optical path by the first reflector (14-1); and then the beam is focused on a surface of a metal material (10) of a sample to be processed by the galvanometer and focusing mirror module (13); the sample to be processed comprises a metal material (10), a semiconductor material (11), and a metal wire lead (12); the metal material (10) is arranged on a moving platform, the semiconductor material (11) is arranged on the other side of the metal material (10), and the metal wire lead (12) is tightly fixed between the metal material (10) and the semiconductor material (11) by a clamp; The microscopic imaging optical path subsystem comprises a second laser emitter (1), an eyepiece (3), a CCD image sensor (4) and a second reflector (14-2); the second laser emitter (1) is configured to generate an illumination laser and form an illumination optical path (15); the illumination laser passes through a galvanometer and focusing mirror module (13) to focus the light beam on a sample to be processed; the CCD image sensor (4), the eyepiece (3), the second reflector (14-2) and the galvanometer and focusing mirror module (13) together constitute a microscopic imaging optical path (2); the CCD image sensor (4) can detect and locate the metal wire lead (12) in real time through the microscopic imaging optical path (2), and focus the femtosecond laser (7) on one side of the metal wire lead (12); The metal pad bonding method comprises the following steps: S1. The metal wire lead (12) is monitored in real time by a CCD image sensor (4), so that the metal wire lead (12) is positioned between the metal material (10) and the semiconductor material (11), and then the metal wire lead (12) is tightly attached to the surface of the metal material (10) and the semiconductor material (11) by a clamp; S2. The femtosecond laser (7) generated by the first laser emitter (5) is focused on the surface of the metal material (10), and the femtosecond laser (7) is focused on one side of the metal wire lead (12) through real-time monitoring by a CCD image sensor (4); a laser processing path (9) parallel to the metal wire lead (12) is set so that the femtosecond laser (7) is processed on the surface of the metal material (10), and the metal material (10) is ablated, melted and sputtered, so that the metal melt is continuously deposited on the surface of the semiconductor material (11) and on both sides of the metal wire lead (12) and gradually cooled, thereby initially forming an effective bond; at the same time, since the pulse time interval of the femtosecond laser (7) is shorter than the cooling time of the metal material (10), the repeated ablation of the metal material (10) will form heat accumulation and reflect a high thermal effect, and the metal material (10), the semiconductor material (11) and the metal wire lead (12) are heated simultaneously under the scanning action of the femtosecond laser (7), thereby achieving metal pad bonding.

2. The metal pad bonding method based on high repetition rate ultrafast laser thermal effect according to claim 1, characterized in that: The laser processing path (9) of the femtosecond laser (7) comprises an arcuate processing path and is parallel to the metal wire lead (12).

3. The metal pad bonding method based on high repetition rate ultrafast laser thermal effect according to claim 1, characterized in that: The pulse repetition frequency of the femtosecond laser (7) is higher than a set threshold value, and the value of the set threshold value includes: 200KHz.

4. The metal pad bonding method based on high repetition rate ultrafast laser thermal effect according to claim 1, characterized in that: The wavelength of the femtosecond laser (7) ranges from 266 to 2000 nm.

5. The metal pad bonding method based on high repetition rate ultrafast laser thermal effect according to claim 1, characterized in that: The material selection range of the metal material (10) includes: copper, nickel-steel alloy, titanium alloy, aluminum alloy or steel.

6. The metal pad bonding method based on high repetition rate ultrafast laser thermal effect according to claim 1, characterized in that: The material selection range of the semiconductor material (11) includes: sapphire, fused quartz, ceramic or silicon.

7. The metal pad bonding method based on high repetition rate ultrafast laser thermal effect according to claim 1, characterized in that: The material selection range of the metal wire lead (12) includes: gold wire lead, silver wire lead or copper wire lead.

8. The metal pad bonding method based on high repetition rate ultrafast laser thermal effect according to claim 1, characterized in that: The diameter of the metal wire lead (12) ranges from 5 to 30 μm.

9. The metal pad bonding method based on high repetition rate ultrafast laser thermal effect according to claim 1, characterized in that: The laser processing path (9) of the femtosecond laser (7) comprises an arcuate processing path.

Images