Laser back thinning process based on semiconductor polished wafer

Abstract

The invention discloses a laser back thinning process based on a semiconductor polished wafer. The method comprises the following steps: firstly, forming a layered modified region in a wafer through infrared picosecond laser to realize the improvement and softening of the material absorptivity; then, nanosecond laser is adopted to ablate and strip the modified layer so as to efficiently remove the thickness of the main body, and ultraviolet femtosecond laser is switched to remove the submicron precision material; and finally, amorphous residues are eliminated through chemical etching and chemical mechanical polishing, and an atomic-scale flat surface is obtained. The process integrates laser modification, stepped stripping and collaborative polishing technologies, has the characteristics of efficient processing and low damage, solves the technical problems of low efficiency and poor surface quality in great-depth thinning of silicon carbide, and provides a high-precision and high-reliability solution for manufacturing of ultrathin semiconductor devices.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser-assisted processing, and specifically relates to a laser back thinning process based on semiconductor polished wafers. Background Art

[0002] With the development of miniaturization and high integration of semiconductor devices, wafer back thinning has become a key step in advanced processes such as three-dimensional packaging. For a silicon carbide wafer that has completed front-side patterning, it generally needs to be thinned from an initial thickness of 350 μm to 100 - 150 μm to optimize device performance and packaging density. However, traditional thinning techniques face significant challenges when dealing with large depth and high-precision requirements: mechanical grinding has low efficiency and is prone to causing cracks, wet etching has insufficient precision, and conventional laser processing causes surface damage due to thermal effects, making it difficult to balance efficiency and quality.

[0003] In the prior art, mechanical and chemical methods are prone to introducing subsurface damage or precision deviation during large-scale material removal, while the thermal accumulation effect of direct laser processing deteriorates the surface roughness and requires additional repair steps. These problems are particularly prominent in high-hardness materials such as silicon carbide, resulting in a significant increase in process complexity and cost, and restricting the large-scale application of ultra-thin wafers.

[0004] To address the above technical bottlenecks, the present invention proposes a stepped thinning process that combines laser and chemical synergies. By internally modifying the material through infrared picosecond laser to improve the material absorption rate, and combining dual-band nanosecond laser for efficient peeling and femtosecond laser for precision trimming, rapid thinning with low thermal damage is achieved; subsequently, chemical etching and ultra-precision polishing are used to eliminate residual defects, and finally, while ensuring processing efficiency, an atomic-level flat surface with high precision and low damage is obtained. This process breaks through the limitations of traditional techniques on depth thinning and material properties, providing a new solution for the manufacturing of ultra-thin wafers for high-performance semiconductor devices. Summary of the Invention

[0005] (1) Invention Objectives

[0006] Aiming at the deficiencies of the prior art, the objective of the present invention is to provide a laser back thinning process based on semiconductor polished wafers, aiming to improve the efficiency and quality in the wafer back thinning process.

[0007] (2) Technical Solutions

[0008] The present invention is achieved through the following technical solutions.

[0009] (1) Protect the front side of the semiconductor wafer after front-side patterning by applying a film, and place it on the processing platform after cleaning the back side.

[0010] (2) An infrared picosecond laser is used to focus the laser beam to a predetermined depth inside the wafer through an objective lens, and the laser parameters are set to form a continuous layered modified structure after processing; the predetermined depth is at a position 102 - 103 μm from the patterned surface of the wafer, and then the modification is carried out successively from bottom to top;

[0011] (3) A nanosecond laser source is used, and the processing parameters are set to ablate and remove the modified layer formed in step (2) from top to bottom until the remaining thickness of the wafer is 110 ± 2 μm;

[0012] (4) An ultraviolet femtosecond laser is used to precisely remove the remaining modified material. With the cooperation of a real-time thickness monitoring module, the remaining thickness of the wafer is controlled to 102 - 103 μm;

[0013] (5) A mixed solution with a volume ratio of hydrofluoric acid to nitric acid of 1:3 is used to selectively etch the wafer processed in (4), and ultrasonic vibration is applied synchronously to remove the amorphous phase residues and redeposition products generated by laser processing;

[0014] (6) A chemical mechanical polishing is carried out on the surface of the wafer processed in step (2) using an alkaline polishing solution to remove a margin of 2 - 3 μm until the wafer is thinned to a thickness of 100 μm, obtaining an atomically flat thinned surface.

[0015] Further, the semiconductor wafer in step (1) is one of single-crystal silicon carbide, single-crystal gallium nitride, or single-crystal diamond wafers;

[0016] Further, in step (2), the pulse width of the infrared picosecond laser is 10 - 100 ps, the wavelength is 1026 - 1064 nm, the numerical aperture of the objective lens is 0.6 - 0.8, the pulse energy is 0.5 - 3.0 mJ, the repetition frequency is 100 - 500 kHz, the scanning speed is 50 - 500 mm / s, the diameter of the focused spot is controlled within 2 - 5 μm, and the Z-axis step amount between layers is 5 - 8 μm;

[0017] Further, in step (3), the wavelength of the nanosecond laser source is 355 - 1064 nm, and the energy density is 3 - 20 J / cm 2 and the repetition frequency is 10 - 100 kHz;

[0018] Further, in step (4), the wavelength of the femtosecond laser is 200 - 400 nm, the pulse width is 100 - 800 fs, the energy density is 0.1 - 3.0 J / cm 2 and the scanning speed is 100 - 1500 mm / s. The thickness monitoring module measures the thickness deviation through a laser interferometer and feedback-controls the removal depth of the femtosecond laser, with an accuracy reaching ±0.5 μm;

[0019] Further, in the step (5), the ultrasonic vibration application frequency is 20 - 80 kHz, the amplitude is 20 - 80 μm, and the treatment time is 30 - 90 s.

[0020] Further, in the step (6), chemical mechanical polishing uses a combination of a polyurethane polishing pad and a silica colloidal polishing solution. The polishing temperature is maintained at 25 ± 1°C, the pH of the polishing solution is 10 - 11, and the polishing pressure is 500 g·cm- 2 .

[0021] (III) Beneficial Effects

[0022] The above technical solutions of the present invention have the following beneficial technical effects:

[0023] (1) The picosecond laser internal softening and modification process proposed by the present invention can achieve material softening and modification inside the wafer, greatly improving the absorption rate of subsequent lasers and the material removal efficiency.

[0024] (2) The nanosecond laser material high-efficiency removal process proposed by the present invention can efficiently remove the target material on the basis of the formation of an internal continuous softening and modification layer by picosecond laser.

[0025] (3) The femtosecond laser low-damage material removal process proposed by the present invention can achieve near-zero damage processing and accurately leave a material removal margin for subsequent CMP.

[0026] (4) The chemical etching surface optimization process proposed by the present invention, combined with chemical action and ultrasonic assistance, can efficiently remove the by-products remaining after laser treatment, improve the surface finish, and prepare the surface shape accuracy for subsequent CMP. Description of the Drawings

[0027] Figure 1 It is a process flow schematic diagram of the SiC wafer treatment of the present invention.

[0028] Figure 2 It is an AFM image of the surface of the silicon carbide thinned after treatment in Example 1. Detailed Embodiments

[0029] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the drawings. It should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present invention.

[0030] Example 1:

[0031] (1) The front side of the patterned silicon carbide wafer is protected by laminating, and after the back side is cleaned, it is placed on the processing platform.

[0032] (2) An infrared picosecond laser with a wavelength of 1064 nm and a pulse width of 10 ps was used to focus the laser beam to a predetermined depth inside the silicon carbide polishing wafer through an objective lens with a numerical aperture of 0.6. A continuous layered modified structure was formed with parameters of pulse energy of 1 mJ, repetition frequency of 100 kHz, and scanning speed of 100 mm / s. The focused spot diameter was controlled at 5 μm, and the interlayer Z-axis step amount was 7 μm. The predetermined depth was 102-103 μm from the patterned surface of the wafer, and then the modification was carried out from bottom to top.

[0033] (3) Using a nanosecond laser light source with a wavelength of 355-1064nm and an energy density of 5J / cm 2 , with a repetition frequency of 50 kHz, the modified layer formed in step (2) is ablated and removed from top to bottom until the remaining thickness of the wafer is 110±2 μm.

[0034] (4) Using ultraviolet femtosecond laser with a wavelength of 343nm, a pulse width of 209fs and an energy density of 0.3J / cm 2 , scanning speed of 300mm / s for high-precision material removal, with real-time thickness monitoring module, measuring thickness deviation through laser interferometer, feedback control of femtosecond laser removal depth, accuracy of ±0.5μm, and controlling the remaining thickness of the wafer to 102-103μm.

[0035] (5) selectively etching the wafer processed in (4) with a mixed solution of hydrofluoric acid and nitric acid in a volume ratio of 1:3, and simultaneously applying ultrasonic vibration, with an ultrasonic vibration application frequency of 40 kHz, an amplitude controlled to 40 μm, and a processing time of 60 s, to remove the amorphous phase residue and redeposition products generated by laser processing;

[0036] (6) Chemical mechanical polishing was performed using an alkaline polishing solution with a pH value of 10 and a combination of a polyurethane polishing pad and a silica colloidal polishing solution. The polishing temperature was maintained at 25°C and the pressure was 500 g·cm- 2 The pressure removes 2-3μm of excess until the wafer is thinned to 100μm thickness, obtaining an atomically flat surface.

[0037] Test results: Figure 2 This is an AFM image of the thinned surface of silicon carbide obtained after the treatment in Example 1. The surface has small surface undulations and high surface quality.

[0038] It should be understood that the above specific embodiments of the present invention are only used for exemplary illustration or explanation of the principles of the present invention, and do not constitute a limitation to the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention shall be included within the protection scope of the present invention. In addition, the appended claims of the present invention are intended to cover all variations and modifications that fall within the scope and boundaries of the appended claims, or equivalent forms of such scope and boundaries.

Claims

1. A laser back thinning process based on a semiconductor polished wafer, characterized in that It includes the following steps: (1) Protect the front of the semiconductor wafer after front patterning by laminating, and place it on the processing platform after back cleaning; (2) Use infrared picosecond laser, focus the laser beam to a predetermined depth inside the wafer through an objective lens, set the laser parameters, and form a continuous layered modified structure after processing; the predetermined depth is at a position 102 - 103 μm from the patterned surface of the wafer, and then modify it successively from bottom to top; (3) Use a nanosecond laser source, set the processing parameters, and ablate and remove the modified layer formed in step (2) from top to bottom until the remaining thickness of the wafer is 110 ± 2 μm; (4) Use ultraviolet femtosecond laser to precisely remove the remaining modified material, and cooperate with the real-time thickness monitoring module to control the remaining thickness of the wafer to 102 - 103 μm; (5) Use a mixed solution with a volume ratio of hydrofluoric acid to nitric acid of 1:3 to perform selective etching on the wafer processed in (4), and apply ultrasonic vibration synchronously to remove the amorphous phase residues and redeposition products generated by laser processing; (6) Use an alkaline polishing solution to perform chemical mechanical polishing on the surface of the wafer processed in step (2), remove 2 - 3 μm of the remaining amount until the wafer is thinned to a thickness of 100 μm, and obtain an atomically flat thinned surface.

2. The laser back thinning process based on a semiconductor polishing wafer according to claim 1, wherein The semiconductor wafer in step (1) is one of single-crystal silicon carbide, single-crystal gallium nitride, or single-crystal diamond wafers.

3. A laser back thinning process based on a semiconductor polishing wafer according to claim 1, wherein, In step (2), the pulse width of the infrared picosecond laser is 10 - 100 ps, the wavelength is 1026 - 1064 nm, the numerical aperture of the objective lens is 0.6 - 0.8, the pulse energy is 0.5 - 3.0 mJ, the repetition frequency is 100 - 500 kHz, the scanning speed is 50 - 500 mm / s, the diameter of the focused spot is controlled within 2 - 5 μm, and the Z-axis step amount between layers is 5 - 8 μm.

4. A laser back thinning process based on a semiconductor polishing wafer according to claim 1, characterized in that, In the step (3), the wavelength of the nanosecond laser source is 355 - 1064 nm, the energy density is 3 - 20 J / cm 2 , and the repetition frequency is 10 - 100 kHz.

5. A laser back thinning process based on a semiconductor polishing wafer according to claim 1, characterized in that, In the step (4), the wavelength of the femtosecond laser is 200 - 400 nm, the pulse width is 100 - 800 fs, the energy density is 0.1 - 3.0 J / cm 2 , the scanning speed is 100 - 1500 mm / s. The thickness monitoring module measures the thickness deviation through a laser interferometer, and feeds back to control the removal depth of the femtosecond laser, with the accuracy reaching ±0.5 μm.

6. A laser back thinning process based on a semiconductor polishing wafer according to claim 1, characterized in that, In step (5), the application frequency of ultrasonic vibration is 20 - 80 kHz, the amplitude is 20 - 80 μm, and the processing time is 30 - 90 s.

7. A laser back thinning process based on a semiconductor polishing wafer according to claim 1, wherein, In the step (6), chemical mechanical polishing is carried out by using a combination of a polyurethane polishing pad and a silica colloidal polishing liquid. The polishing temperature is maintained at 25 ± 1 °C, the pH of the polishing liquid is 10 - 11, and the polishing pressure is 500 g·cm -2 .

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