JP2024076224A - Processing method - Google Patents

Abstract

To provide a method capable of fine processing on a workpiece.SOLUTION: A workpiece W to be processed is prepared (S100), and a metal thin film 10 is formed on a surface of the workpiece W as preprocessing (S110). The workpiece W formed with the metal thin film 10 is irradiated with a pulse laser beam 20 from a side of the metal thin film 10 (S120), and processing is performed on the workpiece W (S130). Then, the metal thin film 10 is removed as post processing (S140).SELECTED DRAWING: Figure 1

Description

This disclosure relates to laser processing technology.

There is an increasing demand for micromachining technology for semiconductor materials, especially silicon surfaces. Conventionally, near-infrared light used as a light source for laser processing has a wavelength of around 1 μm, and due to restrictions imposed by the diffraction limit, practical processing resolution was limited to a few micrometers. In contrast, extreme ultraviolet light has a wavelength of 30 nm or less, and due to its short wavelength, it is possible to focus light to approximately 1 μm to less than 1 μm, making it a light source suitable for sub-micron resolution laser processing.

However, to process difficult-to-process materials such as silicon with submicron resolution, a high-brightness light source such as an X-ray free electron laser was required, but such light sources were only available in large facilities.

This disclosure has been made in this context, and one of its exemplary purposes is to provide a method capable of micromachining a workpiece.

One aspect of the present disclosure relates to a method for processing a workpiece. The method includes the steps of forming a thin metal film on a surface of the workpiece, irradiating the thin metal film with a laser beam to process the workpiece, and removing the thin metal film.

In addition, any combination of the above components, or mutual substitution of components or expressions between methods, devices, systems, etc., are also valid aspects of the present invention or disclosure. Furthermore, the description in this section (Means for solving the problem) does not explain all essential features of the present invention, and therefore, subcombinations of the described features may also constitute the present invention.

According to certain aspects of the present disclosure, the workpiece can be micromachined.

1A to 1C are diagrams illustrating a processing method according to an embodiment. 2 is a flowchart illustrating the processing method of FIG. 1 . FIG. 1 is a diagram showing a laser processing device. FIG. 2 is a cross-sectional view of Sample 1 after laser irradiation. FIG. 11 is a cross-sectional view of Sample 2 after laser irradiation. FIG. 1 is a diagram showing the processing characteristics of samples 1 to 8.

(Overview of the embodiment)
An aspect of the present disclosure relates to a method for machining a workpiece, the method including the steps of forming a thin metal film on a surface of the workpiece, irradiating the thin metal film with a laser beam to machine the workpiece, and removing the thin metal film.

This laser light cannot process the workpiece when it is directly irradiated on the workpiece, but it can process the workpiece by irradiating a thin metal film formed on the workpiece.

In one embodiment, the workpiece may include Si.

In one embodiment, the material of the metal thin film may include one selected from the group consisting of Au, Ag, Pd, and an alloy containing at least one of them. By using these materials, it is possible to process a Si workpiece to a depth of 50 nm or more.

In one embodiment, the metal thin film may have a penetration length for the laser light that is shorter than 10 nm. Due to the short penetration length, the laser light is absorbed in a thin region of the metal thin film, and the temperature of the metal thin film can be efficiently increased, which is believed to enable efficient processing of the workpiece.

In one embodiment, the metal thin film may have a thermal conductivity of greater than 200 W/m·K. A large thermal conductivity means that heat generated in the metal thin film is efficiently conducted to the workpiece, and therefore the greater the thermal conductivity, the easier it is to process.

In one embodiment, the thickness of the metal thin film may be 5 to 500 nm.

In one embodiment, the laser light may be EUV (Extreme Ultraviolet) light. This allows for focusing of the light to submicron levels, enabling submicron fine processing.

In one embodiment, the laser light may be a higher harmonic of a pulsed laser.

In one embodiment, the diameter of the focused spot of the laser light on the metal thin film may be 1 μm or less.

In one embodiment, the laser light may be focused using a Wolter mirror.

(Embodiment)
The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, parts, and processes shown in each drawing are given the same reference numerals, and duplicated descriptions are omitted as appropriate. In addition, the embodiments are illustrative and do not limit the invention, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

Figure 1 is a diagram illustrating a processing method according to an embodiment.

First, the workpiece W to be processed is prepared and cleaned (S100). The workpiece W may be, for example, a semiconductor substrate such as Si, but is not limited thereto and may be other group IV semiconductors such as Ge. Alternatively, the workpiece W may be a group III-V compound semiconductor such as GaAs or GaN, or a compound semiconductor such as SiGe or SiC. Alternatively, the workpiece W may be an insulating material such as glass.

As a pretreatment, a thin metal film 10 is formed on the surface of the workpiece W to be processed (S110). Vapor deposition can be used to form the thin metal film 10. The thin metal film 10 can be formed in a single layer or multiple layers of about 5 to 500 nm. The thin metal film 10 may also be formed by plating.

In the next step S120, the pulsed laser light 20 is focused on the metal thin film 10. The pulsed laser light 20 may be irradiated while fixing the relative positional relationship between the workpiece W and the focusing position, in which case a hole can be dug in the workpiece W. Alternatively, the laser may be irradiated while changing the relative positional relationship between the workpiece W and the focusing position, in which case a pattern can be drawn on the workpiece W.

The wavelength of the pulsed laser light 20 can be determined according to the machining dimensions. For example, when performing submicron (1 μm or less) hole drilling, a wavelength of 100 nm or less can be selected. The fluence of the pulsed laser light 20 may be lower than the machining threshold of the workpiece W.

As a result of the laser irradiation, an opening 30 is formed in the metal thin film 10 and the workpiece W (S130).

In the subsequent post-processing, the metal thin film 10 is removed (S140). This results in a workpiece W with an opening 30 formed therein.

That's the processing method.

Figure 2 is a flowchart explaining the processing method of Figure 1. Each step S100 to 140 of the flowchart corresponds to the steps of Figure 1.

Next, we will explain the laser processing device 100 that can be used for the above-mentioned processing method.

Figure 3 is a diagram showing a laser processing device. The laser processing device 100 includes a laser light source 110, a condensing lens 120, a rare gas 130, a shutter 140, a metal thin film filter 150, and a condensing mirror 160. The laser light source 110 is a near-infrared laser pulse oscillator, and generates near-infrared pulsed light 200 having a femtosecond pulse width. A titanium sapphire laser or the like can be used as the laser light source 110.

The focusing lens 120 focuses the near-infrared pulsed light 200 onto the rare gas 130. The rare gas 130 is a nonlinear medium that generates high-order harmonics, and in this embodiment, Ar is used. In addition to Ar, He, Ne, Kr, and Xe can also be used as the rare gas 130.

The metal thin film filter 150 transmits the EUV pulsed light 202 used for processing among the higher harmonics generated in the rare gas 130, and removes the fundamental wave and other harmonics in the near infrared region.

The collector mirror 160 focuses the EUV pulsed light 202 onto the workpiece W on which a metal thin film is formed. A Wolter mirror can be used as the collector mirror 160, which focuses the EUV pulsed light 202 onto a spot of 1 μm or less on the workpiece W.

The number of pulses of the EUV pulsed light 202 incident on the workpiece W can be controlled by opening and closing the shutter 140. Note that the position of the shutter 140 is not limited to that shown in FIG. 3 and may be located elsewhere.

The above is the configuration of the laser processing device 100.

Next, we will explain the results of processing several samples using the laser processing device 100 in Figure 3.

In the experiment, silicon, which is in high demand for microfabrication on the submicron order, was used as the workpiece W. The following materials were used for the metal thin film 10 formed on the silicon substrate.
Sample 1 Au
Sample 2 Ag
Sample 3 Pd
Sample 4 Pt
Sample 5 Cu
Sample 6 Ni
Sample 7 Cr
Sample 8 No metal thin film

The wavelength of the EUV pulsed light irradiated onto the workpiece is 20 to 50 nm, and the number of shots is 1,000.

Figure 4 is a cross-sectional view of sample 1 after laser irradiation. An atomic force microscope was used to measure the cross-sectional view. Sample 1 has 100 nm thick Ag formed on Si as a metal thin film 10. The interface between Ag and Si is at -0.1 μm in the depth direction y. The machining depth of the Si workpiece W reaches 400 nm (y = -0.5 μm), and the diameter of the machining marks is 0.5 μm or less at a depth of 100 nm, demonstrating that submicron fine machining has been achieved.

Figure 5 is a cross-sectional view of sample 2 after laser irradiation. Sample 2 has 100 nm-thick Au formed on Si as a metal thin film 10. The interface between Au and Si is at -0.1 μm in the depth direction y. The machining depth of the Si workpiece W reaches -100 nm (y = -0.2 μm), and the diameter of the machining marks is about 0.5 μm at a depth of -100 nm, demonstrating that submicron fine machining has been achieved.

In both cases of Figures 4 and 5, the machining depth was measured to be 100 nm or more thicker than the metal thin film layer, indicating that machining has progressed down to the semiconductor sample layer (workpiece W). Note that in the case of a semiconductor sample that does not have a metal layer formed on its surface as a pretreatment, machining does not progress even when an extreme ultraviolet light pulse is irradiated.

The fluence of the EUV pulsed light 202 irradiated to the sample is estimated to be about 130 mJ/cm ^2. On the other hand, the processing threshold of Si is said to be about 270 mJ/cm ^2. Therefore, it should be noted that the Si workpiece can be processed even though the fluence of the EUV pulsed light 202 is half or less than the processing threshold.

Figure 6 shows the processing characteristics of samples 1 to 8. It was confirmed that processing had progressed to the Si portion for Ag, Au, and Pd. For Ag and Au, as shown in Figures 4 and 5, processing had progressed to a depth of 400 nm and 100 nm in the Si portion, respectively. For Pd, processing had progressed to a depth of 50 nm in the Si portion, and although the processing depth was shallower than for Ag and Au, it was confirmed that the Si had been processed. Note that while the bottom of the hole was sharp for Ag and Au, the bottom of the hole was flat for Pd.

On the other hand, for the Pt, Cu, Ni, Cr and Si (without metal thin film) samples, no progress of processing was observed in the Si part.

Figure 6 shows the optical penetration depth of EUV light with an energy of 42 eV for each material. The optical penetration depth of EUV light with a photon energy of 42 eV for Au and Ag is 7 nm and 8 nm, respectively, which is shorter than the other materials. From this, it is believed that the input pulse energy is absorbed near the incident surface of the metal thin film 10, which tends to cause a local temperature rise, which makes it possible to process Si. In contrast, Pt, Cu, Ni, Cr and Si have a long optical penetration depth of 10 nm, which does not result in a local rise in the temperature of the metal thin film 10, which is believed to be the reason why processing does not progress to the Si portion.

Figure 6 also shows the thermal conductivity of each material. Comparing the thermal conductivities of Au, Ag, and Pd, which were confirmed to be used in processing the Si part, the results are 318, 429, and 72 (W/m·K), respectively. Inferring from the result that Au and Ag have deeper processing depths than Pd, a material with a short light penetration length and high thermal conductivity is considered desirable for processing the Si part at a fluence below the processing threshold of silicon. This is thought to be because a material with high thermal conductivity can efficiently transfer heat generated in the metal thin film to the silicon part, thereby raising the temperature of the Si.

The following findings can be obtained from the above experimental results.
(1) It is preferable that the material of the thin metal film 10 has an optical penetration depth of 10 nm or less. It is believed that the short optical penetration depth contributes to localized heating of the thin metal film 10.

(2) The thermal conductivity of the metal thin film 10 is preferably 200 W/m·K or more, and more preferably 300 W/m·K or more. This allows the localized heat generated by the laser irradiation to be conducted with high efficiency to the Si portion to be processed, which is thought to assist in the processing of the Si.

According to this embodiment, it is possible to microfabricate semiconductor materials with submicron resolution using extreme ultraviolet light pulses obtained from a tabletop device, without using a high-brightness light source in a large facility such as an X-ray free electron laser.

In addition, by making it possible to process using a tabletop extreme ultraviolet laser light source, the cost of microfabrication has been significantly reduced, making it possible to inexpensively develop elements with functionality achieved through microfabrication.

In addition, the pretreatment required for processing is simple, so the hurdle for implementing this technology in microfabrication processes for semiconductor materials is low. The global laser processing market is growing rapidly at around 10% per year, and the invention has a wide range of application, including the creation of functional surfaces through surface microfabrication and the miniaturization of through-silicon vias, which are essential for semiconductor implementation.

The present invention has been described using specific terms based on the embodiments, but the embodiments merely show the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments as long as they do not deviate from the concept of the present invention as defined in the claims.

Reference Signs List W Work 10 Metal thin film 20 Pulsed laser light 30 Aperture 100 Laser processing device 110 Laser light source 120 Condenser lens 130 Rare gas 140 Shutter 150 Metal thin film filter 160 Condenser mirror 200 Near-infrared pulsed light 202 EUV pulsed light

Claims (11)

A method for machining a workpiece, comprising the steps of:
forming a thin metal film on a surface of the workpiece;
irradiating the metal thin film with a laser beam to process the workpiece;
removing the thin metal film;
A processing method comprising: The processing method according to claim 1, characterized in that the workpiece is a semiconductor material. The processing method according to claim 1, characterized in that the workpiece contains Si. The processing method according to claim 2, characterized in that the material of the metal thin film includes one selected from the group consisting of Au, Ag, Pd, and an alloy containing at least one of them. The processing method according to any one of claims 1 to 4, characterized in that the metal thin film has a penetration length for the laser light that is shorter than 10 nm. The processing method according to any one of claims 1 to 4, characterized in that the metal thin film has a thermal conductivity of greater than 200 W/m·K. The processing method according to any one of claims 1 to 4, characterized in that the metal thin film has a thickness of 5 to 500 nm. The processing method according to any one of claims 1 to 4, characterized in that the laser light is EUV (Extreme Ultraviolet) light. The processing method according to claim 8, characterized in that the laser light is a high-order harmonic of a pulsed laser. The processing method according to claim 8, characterized in that the diameter of the focused spot of the laser light on the metal thin film is 1 μm or less. The processing method according to claim 10, characterized in that the laser light is focused using a Wolter mirror.

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