Process method for improving laser cutting machining flatness

Abstract

The invention belongs to the technical field of machine manufacturing, and discloses a technological method for improving laser cutting machining flatness. According to the method, a wafer is immersed in a specific electrolyte, and electrochemical bias voltage is applied while pulse laser irradiation is carried out, so that in-situ finishing of a fusion area is realized under the action of rapid cooling and anode leveling; the electrolyte has high light transmittance, adaptive conductivity and selective dissolving capacity, and can be matched with nanosecond / picosecond laser parameters and synchronous electrochemical control to obtain a high-quality cutting structure of which the side wall roughness Ra is less than 50nm and which is free from obvious burrs and recast layers. The invention provides an integrated process method for synchronously implementing laser cutting and electrochemical polishing in a specific electrolyte environment, and the technical problem that burrs and a recast layer are difficult to restrain in laser cutting of high-thermal-conductivity metal is fundamentally solved by cooperatively regulating and controlling molten pool dynamics and an interface electrochemical reaction through multiple physical fields. And a key process support is provided for high-density three-dimensional packaging and heterogeneous integrated manufacturing.

Description

Technical Field

[0001] This invention belongs to the field of mechanical manufacturing technology and relates to a process method for improving the flatness of laser cutting. Background Technology

[0002] In the fields of advanced semiconductor packaging and microelectronic interconnect manufacturing, high thermal conductivity metals such as copper and gold are widely used in wafer-level interconnect structures due to their excellent electrical conductivity and thermal stability. Laser cutting, with its advantages of non-contact operation, high precision, and programmability, has become one of the key means to achieve patterned separation of high-density interconnect structures.

[0003] For high-reflectivity and high-thermal-conductivity metals such as copper and gold, laser cutting processes in traditional air or inert gas environments still face significant challenges in practical applications. During laser irradiation, the material rapidly melts and partially vaporizes, but due to its high thermal conductivity, heat diffuses quickly to the surrounding area, prolonging the existence time of the molten pool. In addition, the interaction between the metal vapor and the ambient gas easily leads to the formation of irregular burrs and recast layers at the cut edges.

[0004] Such resolidified products are usually attached to the cut sidewalls or bottom in a raised shape, which not only destroys the microscopic flatness of the processed surface, but also induces reliability risks such as local electric field concentration, poor interface bonding or even electrical short circuits in subsequent dielectric layer deposition, electroplating or bonding processes.

[0005] Industry experts have attempted to suppress slag adhesion by optimizing laser parameters (such as pulse width, energy density, and scanning speed) or introducing auxiliary gas purging. While these methods have alleviated surface roughness to some extent, they have failed to fundamentally eliminate the formation mechanism of the recast layer. Some studies have proposed placing laser processing in a liquid environment to utilize the confinement effect and cooling capacity of the liquid medium. However, relying solely on physical cooling and scouring is insufficient for removing highly adhesive molten metal slag, and may introduce new thermo-mechanical coupling instabilities due to liquid disturbances, thereby affecting the consistency of the cutting profile. Summary of the Invention

[0006] To achieve the above-mentioned objectives, this invention provides a process for improving the flatness of laser cutting. The method involves simultaneously applying pulsed laser irradiation and electrochemical anodic dissolution in a specific electrolyte environment to perform in-situ cutting and surface leveling of a high thermal conductivity metal interconnect layer, thereby suppressing the formation of burrs and recast layers at the source and obtaining a cut sidewall with nanoscale surface roughness.

[0007] The core of this invention lies in constructing a dynamic process environment with the synergistic effects of multiple physical fields including light, heat, electricity, and fluid. The wafer to be processed is completely immersed in a transparent, conductive electrolyte with selective anodic dissolution capability. This electrolyte simultaneously serves as the laser transmission medium, cooling medium, and electrochemical reaction medium. While the laser pulse acts on the target area, a controlled DC or pulsed bias voltage is applied between the wafer substrate and the counter electrode, causing the laser-melted metal region to be in an anodic polarization state. Under this combined field, the molten metal is not only constrained by the rapid cooling of the liquid medium, but its surface micro-protrusions also preferentially undergo electrochemical dissolution due to the concentrated current density, thereby achieving in-situ suppression and trimming of the recast layer and burrs.

[0008] The electrolyte is formulated from deionized water, inorganic acid, complexing agent, and surfactant in a specific ratio. The inorganic acid is sulfuric acid, phosphoric acid, or a mixture thereof, with a concentration ranging from 0.1 to 1.0 mol / L; the complexing agent is sodium citrate or potassium sodium tartrate, with a concentration of 0.05 to 0.3 mol / L; and the surfactant is sodium dodecyl sulfate, with a concentration of 50 to 200 mg / L. This formulation ensures that the electrolyte has a transmittance greater than 90% in the 800-1100 nm wavelength range to allow for effective penetration by near-infrared or green light lasers. Simultaneously, its conductivity is maintained between 10 and 50 mS / cm to support stable electrochemical reactions. Furthermore, this system exhibits a significant leveling effect on copper or gold materials under anodic polarization conditions, meaning that the dissolution rate at microscopic protrusions is significantly higher than that in recessed areas.

[0009] The laser source is a nanosecond or picosecond pulsed solid-state laser with a wavelength of 532 nm or 1064 nm, a single pulse energy of 10-200 μJ, a pulse repetition frequency of 1-500 kHz, and a focused spot diameter of 5-30 μm. The laser beam is incident perpendicularly onto the wafer surface through an immersion optical window, and the scanning path is controlled by a high-precision galvanometer system with a linear velocity of 1-50 mm / s. The selection of laser parameters must ensure localized melting on the target material surface while avoiding violent vaporization to reduce the plasma shielding effect and maintain the stability of the electrolyte environment.

[0010] The electrochemical system includes a working electrode, a counter electrode, and a reference electrode. The working electrode is the wafer itself, with its back side connected to the positive terminal of a potentiostat via a conductive clamp. The counter electrode is a platinum sheet or graphite plate, with an area greater than twice the projected area of ​​the wafer, positioned opposite the laser focal region and connected to the negative terminal of the potentiostat. The reference electrode is a saturated calomel electrode or a silver / silver chloride electrode, used for real-time monitoring of the working electrode potential. The applied bias voltage has an amplitude range of 0.2-1.5V (relative to the open-circuit potential) and is applied as a continuous DC pulse or a square wave pulse synchronized with the laser pulse. When using the synchronized pulse mode, the rise time of the electrochemical pulse lags behind the laser pulse peak by less than 500 ns, with a duration of 1-10 μs, to ensure effective anodic polarization during the presence of the molten pool.

[0011] During the process, the laser pulse first forms a transient molten pool on the wafer surface. Due to the high heat capacity of the electrolyte and the forced convection, the lateral thermal diffusion of the molten pool is effectively suppressed, and the solidification time is shortened to the microsecond level. Simultaneously, micron-sized protrusions form on the molten metal surface due to surface tension fluctuations and vapor backpressure. Because these protrusions have smaller radii of curvature, their local current density is significantly higher than that of flat areas under the same electric field strength. According to the principle of electrochemical leveling, the anodic dissolution rate at the protrusions is accelerated. Before the molten pool is completely solidified, these high current density areas are preferentially dissolved and removed, thus preventing the final solidification of the recast layer. Furthermore, the complexing agent in the electrolyte rapidly forms soluble complexes with the dissolved metal ions, preventing their redeposition near the cut and ensuring unobstructed slag removal channels.

[0012] In a preferred embodiment of the present invention, the electrolyte circulation system is equipped with an online filtration and ion concentration monitoring module. The filtration accuracy is 0.1 μm, used to remove particulate impurities generated during processing in real time; the ion-selective electrode is used to monitor the concentration of copper or gold ions, and when the concentration exceeds 50 ppm, it automatically triggers the replenishment of fresh electrolyte to maintain the stability and consistency of the electrochemical reaction.

[0013] In another preferred embodiment of the present invention, the laser scanning strategy employs a bidirectional stepped path. When the depth of a single cut is insufficient, full-thickness penetration is achieved through multiple superimposed scans, with the depth increment of each scan controlled between 2-10 μm, and the spacing between adjacent scan lines being 70%-90% of the spot diameter. During each scan interval, an electrochemical bias voltage is continuously applied to continuously level the sidewalls of the formed shallow groove, thereby accumulating to form a cutting structure with a high aspect ratio (greater than 10:1) and a sidewall roughness Ra less than 50 nm.

[0014] The described process is applicable to interconnect layers made of copper, gold, silver, and their alloys, and is particularly suitable for wafer-level packaging structures with a thickness of 10-100 μm. For copper materials, the preferred electrolyte composition is: 0.5 mol / L sulfuric acid, 0.2 mol / L sodium citrate, and 100 mg / L sodium dodecyl sulfate; the applied bias voltage is 0.8 V (relative to open circuit potential); and the laser parameters are: 532 nm wavelength, 50 μJ single pulse energy, 100 kHz repetition rate, 20 μm spot diameter, and 10 mm / s scanning speed.

[0015] For gold materials, the preferred electrolyte composition is: 0.3 mol / L phosphoric acid, 0.15 mol / L potassium sodium tartrate, and 150 mg / L sodium dodecyl sulfate; the applied bias voltage is 1.2 V; and the laser parameters are: 1064 nm wavelength, 120 μJ single pulse energy, 50 kHz repetition frequency, 25 μm spot diameter, and 5 mm / s scanning speed. Under these conditions, crack-free and hole-free cutting of gold interconnect structures can be achieved, with smooth and continuous sidewalls, meeting the interface flatness requirements for subsequent low-temperature eutectic bonding.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1. The process method described in this invention achieves three technical effects by precisely coupling liquid-phase laser cutting and electrochemical polishing in space and time: First, by utilizing the physical constraint and rapid cooling effect of the electrolyte, the life of the molten pool is significantly shortened and the expansion of the heat-affected zone is suppressed; Second, through the anodic leveling mechanism induced by the applied electric field, surface defects are repaired in situ in the molten state, eliminating the recast layer from the source; Third, the electrolyte composition design takes into account optical transparency, electrochemical activity and product solubility, ensuring the stability and repeatability of the synergistic effect of multiple physical fields.

[0018] 2. An integrated process method for simultaneously performing laser cutting and electrochemical polishing in a specific electrolyte environment is provided. By coordinating the control of molten pool dynamics and interfacial electrochemical reactions through multiple physical fields, the technical problem of suppressing burrs and recast layers in laser cutting of high thermal conductivity metals is solved. Detailed Implementation

[0019] This invention provides a process method to improve the flatness of laser cutting. By simultaneously implementing pulsed laser irradiation and electrochemical anodic dissolution in a specific electrolyte environment, in-situ cutting and surface flattening of high thermal conductivity metal interconnect layers are achieved. This suppresses the formation of burrs and recast layers at the source and obtains cut sidewalls with nanoscale surface roughness. The core of this method lies in constructing a dynamic process environment with synergistic effects of optical, thermal, electrical, and fluid multi-physics fields. This allows for precise coupling of laser melting, liquid cooling confinement, and electrochemical selective dissolution within a microsecond timescale, achieving ultra-flat separation of interconnect structures made of high thermal conductivity metal materials such as copper and gold.

[0020] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples, so as to ensure that those skilled in the art can fully understand and implement the present invention.

[0021] Example 1: The metal interconnect layer is copper (50 μm thick); the electrolyte contains 0.5 mol / L sulfuric acid + 0.2 mol / L sodium citrate + 100 mg / L sodium dodecyl sulfate; laser parameters are 532 nm wavelength, 50 μJ single pulse energy, 100 kHz repetition frequency, 20 μm spot diameter, and 10 mm / s scanning speed; bias voltage is 0.8 V (DC).

[0022] Fabrication process: Wafer immersion in electrolyte → simultaneous application of laser irradiation and bias voltage → multiple rounds of superimposed scanning → dicing completed.

[0023] Example 2: The metal interconnect layer is gold (50 μm thick); the electrolyte contains 0.3 mol / L phosphoric acid + 0.15 mol / L potassium sodium tartrate + 150 mg / L sodium dodecyl sulfate; laser parameters are 1064 nm wavelength, 120 μJ single pulse energy, 50 kHz repetition frequency, 25 μm spot diameter, and 5 mm / s scanning speed; bias voltage is 1.2 V (DC).

[0024] Preparation process: Same as in Example 1.

[0025] Example 3: Bias voltage 0.5V, other formulations and processes are the same as in Example 1;

[0026] Preparation process: Same as in Example 1.

[0027] Example 4: Bias voltage 1.5V, other formulations and processes are the same as in Example 1;

[0028] Preparation process: Same as in Example 1.

[0029] Example 5: Laser single pulse energy 30μJ, other formulations and processes are the same as in Example 1;

[0030] Preparation process: Same as in Example 1.

[0031] Example 6: Laser single pulse energy 180μJ, other formulations and processes are the same as in Example 1;

[0032] Preparation process: Same as in Example 1.

[0033] Example 7: The bias voltage is a square wave pulse (hysteresis laser peak value 300ns, duration 5μs), and the rest of the formulation and process are the same as in Example 1;

[0034] Preparation process: Same as in Example 1.

[0035] Example 8: Copper interconnect layer thickness 100μm, laser scanning depth increment 8μm, other formulations and processes are the same as in Example 1;

[0036] Preparation process: Same as in Example 1.

[0037] Comparative Example 1: Laser cutting only, no bias voltage applied; the rest of the formulation and process are the same as in Example 1;

[0038] Preparation process: Same as in Example 1 (without electrochemical system startup).

[0039] Comparative Example 2: Electrolyte-free laser cutting in an air environment; other laser parameters are the same as in Example 1;

[0040] Preparation process: Same as in Example 1 (without electrolyte immersion step).

[0041] Test method:

[0042] Machining accuracy testing: Atomic force microscopy was used to measure the sidewall roughness Ra; scanning electron microscopy was used to observe the burr height and recast layer thickness; and the depth-to-width ratio of the cut was measured.

[0043] Structural integrity test: Check the regularity of the cut contour; observe for cracks, holes or excessive corrosion; verify the full thickness penetration of the metal layer.

[0044] Process stability test: 10 groups of samples were processed continuously to evaluate the roughness variation coefficient between batches; the effect of changes in electrolyte ion concentration on performance was monitored.

[0045] The test data comparisons are shown in Table 1 and Table 2.

[0046] Table 1 Comparison of Sidewall Roughness and Burr Height

[0047] Test Project Sidewall roughness Ra (nm) Burr height (μm) Example 1 30 none Example 2 35 none Example 3 42 0.3 Example 4 28 none Example 5 38 0.2 Example 6 29 none Example 7 27 none Example 8 33 none Comparative Example 1 220 2.5 Comparative Example 2 250 3

[0048] Table 2 Comparison of Recast Layer Thickness, Aspect Ratio, and Number of Scan Rounds Required for Penetration

[0049] Test Project Recast layer thickness (μm) Aspect Ratio Number of scan rounds required for penetration Example 1 none 12:01 3 Example 2 none 10:01 3 Example 3 0.15 11:01 3 Example 4 none 13:01 2 Example 5 0.1 10:01 4 Example 6 none 14:01 2 Example 7 none 15:01 2 Example 8 none 12:01 5 Comparative Example 1 1.2 8:01 3 Comparative Example 2 1.5 7:01 3

[0050] Examples 1-8 have a sidewall roughness Ra≤42nm, with no obvious burrs or recast layer, which is far superior to the comparative examples. Comparative examples 1-2 have serious surface defects due to the lack of electrolyte and electrochemical leveling, which confirms that multi-physics field synergy is the key to leveling.

[0051] Increasing the bias voltage (Example 3→1→4) optimizes the leveling effect and reduces roughness; increasing the laser energy (Example 5→1→6) improves the cutting efficiency and increases the aspect ratio; pulse synchronous bias (Example 7) further optimizes the performance.

[0052] Both copper and gold interconnect layers can achieve high-quality cutting; the thick metal layer (Example 8) maintains excellent flatness by adjusting the scan increment, adapting to different packaging requirements.

[0053] Compared to traditional vapor phase cutting (Comparative Example 2), the roughness of the embodiment is reduced by more than 87%, burrs and recast layers are basically eliminated, and the aspect ratio is improved by 70%, meeting the requirements of advanced packaging for nanoscale flatness.

[0054] The method described in this invention solves the industry problem of surface defects in high thermal conductivity metal cutting by combining laser cutting and electrochemical leveling. Different parameter combinations can achieve ultra-flat processing, which is suitable for semiconductor packaging and microelectronic interconnect manufacturing scenarios.

[0055] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A process for improving the flatness of laser cutting, characterized in that, Includes the following steps: The wafer to be processed is completely immersed in a transparent, conductive electrolyte with selective anodic dissolution capability; In the electrolyte environment, pulsed laser irradiation and electrochemical anodic dissolution are simultaneously performed to cut and level the surface of the high thermal conductivity metal interconnect layer on the wafer in situ. The electrolyte serves simultaneously as a laser transmission medium, a cooling medium, and an electrochemical reaction medium. The pulsed laser is incident perpendicularly onto the wafer surface through an immersion optical window, forming a transient molten pool; Simultaneously with the laser pulse, a controlled bias voltage is applied between the wafer as the working electrode and the counter electrode, causing the molten metal region to be in an anodic polarization state. The rapid cooling of the electrolyte constrains the lateral thermal diffusion of the molten pool, and the electrochemical leveling effect preferentially dissolves the microscopic protrusions on the surface of the molten pool, thereby suppressing the formation of burrs and recast layers and obtaining a cutting sidewall with nanoscale surface roughness.

2. The process method for improving the flatness of laser cutting according to claim 1, characterized in that, The electrolyte is prepared from deionized water, inorganic acid, complexing agent, and surfactant; the inorganic acid is sulfuric acid, phosphoric acid, or a mixture thereof, with a concentration of 0.1-1.0 mol / L; the complexing agent is sodium citrate or potassium sodium tartrate, with a concentration of 0.05-0.3 mol / L; the surfactant is sodium dodecyl sulfate, with a concentration of 50-200 mg / L; the electrolyte has a transmittance greater than 90% in the wavelength range of 800-1100 nm and a conductivity of 10-50 mS / cm.

3. The process method for improving the flatness of laser cutting according to claim 2, characterized in that, The electrolyte circulation system is equipped with an online filtration module and an ion concentration monitoring module; the filtration accuracy of the filtration module is 0.1 μm; the ion concentration monitoring module includes an ion-selective electrode, which automatically triggers the replenishment of fresh electrolyte when the copper or gold ion concentration exceeds 50 ppm.

4. The process method for improving the flatness of laser cutting according to claim 1, characterized in that, The pulsed laser is generated by a nanosecond or picosecond solid-state laser with a wavelength of 532nm or 1064nm, a single pulse energy of 10-200μJ, a pulse repetition frequency of 1-500kHz, and a focused spot diameter of 5-30μm; the laser scanning path is controlled by a galvanometer system with a linear velocity of 1-50mm / s.

5. The process method for improving the flatness of laser cutting according to claim 4, characterized in that, The laser scanning strategy adopts a bidirectional stepped path; when the single cutting depth is insufficient, full thickness penetration is achieved by multiple superimposed scans, with each scan depth increment being 2-10μm, and the spacing between adjacent scan lines being 70%-90% of the spot diameter.

6. The process method for improving the flatness of laser cutting according to claim 5, characterized in that, During each scan interval, the bias voltage is maintained to continuously electrochemically level the sidewalls of the formed shallow trench, ultimately forming a cut structure with an aspect ratio greater than 10:1 and a sidewall surface roughness Ra less than 50 nm.

7. The process method for improving the flatness of laser cutting according to claim 1, characterized in that, The electrochemical system includes a working electrode, a counter electrode, and a reference electrode. The working electrode is a wafer, the back of which is connected to the positive electrode of a potentiostat via a conductive clamp. The counter electrode is a platinum sheet or graphite plate, with an area greater than twice the projected area of ​​the wafer, placed on the opposite side of the laser focal area, and connected to the negative electrode of the potentiostat. The reference electrode is a saturated calomel electrode or a silver / silver chloride electrode, used to monitor the working electrode potential in real time.

8. The process method for improving the flatness of laser cutting according to claim 7, characterized in that, The bias voltage amplitude is 0.2-1.5V, applied relative to the open circuit potential; The application method is either continuous DC or a square wave pulse synchronized with the laser pulse; when the synchronous pulse mode is used, the rising edge of the electrochemical pulse lags behind the peak value of the laser pulse by less than 500 ns, and the duration is 1-10 μs.

9. The process method for improving the flatness of laser cutting according to claim 1, characterized in that, The high thermal conductivity metal interconnect layer material is copper, gold, or silver and their alloys, with a thickness of 10-100 μm.

10. The process method for improving the flatness of laser cutting according to claim 9, characterized in that, When the metal interconnect layer is copper, the electrolyte composition is 0.5 mol / L sulfuric acid, 0.2 mol / L sodium citrate, and 100 mg / L sodium dodecyl sulfate; the bias voltage is 0.8 V; and the laser parameters are 532 nm wavelength, 50 μJ single pulse energy, 100 kHz repetition frequency, 20 μm spot diameter, and 10 mm / s scanning speed.