High-speed, high-precision processing of glass substrates in semiconductors

A research group led by Associate Professor Yusuke Ito of the Graduate School of Engineering at the University of Tokyo has developed a laser technology that enables extremely high-speed and high-precision processing of glass and other transparent materials that are notoriously difficult to process, outspeeding conventional methods by one million times.

By precisely controlling laser light, the group showed that the physical properties of materials can be altered in just a picosecond (1 ps = 10^-12 s), enabling instantaneous processing of only the necessary areas. As a result, precision micro hole drilling on glass substrates, which is needed for next-generation semiconductors, has been realized at practical levels of speed and precision (Fig. 1). In addition, since the laser is significantly less intense than with conventional methods, this approach can also help lower the power consumption and reduce the cost of semiconductor manufacturing equipment.

Processing of the glass substrates needed in next-generation semiconductors

From resin substrates to glass substrates

A substrate for mounting semiconductor devices is essential for electronic equipment such as smartphones and automobiles. Conventionally, resin substrates have been widely used, but in recent years, the adoption of glass substrates has been advancing in pursuit of higher-performance mounting and support for high-speed communication.

Glass substrates have high electrical insulation, enabling the suppression of signal crosstalk and leakage current, thereby reducing interference between adjacent circuits. In addition, because glass can be machined to an extremely flat surface, fine wiring can be assembled more easily with high precision. Furthermore, it exhibits minimal warp due to temperature fluctuations, making it a suitable material for semiconductor devices with precise structures.

Glass substrates are difficult to process by laser

Unfortunately, glass also has its drawbacks. Although hard, it is also brittle and weak to impact and pressure, which makes it prone to cracking and chipping during processing and complicates condition adjustment. The process of forming microvias (via holes) that penetrate the substrate is particularly problematic.

For general package substrates, it is necessary to form a dense volume of extremely high-aspect-ratio holes with a depth of over 1 mm and a diameter of less than 100 µm. Etching techniques have conventionally been attempted for processing, but the many process steps and the environmental burden have become issues. Therefore, laser processing has gained some attention, but conventional laser processing technology requires more than 10 seconds per hole. When drilling tens of thousands of holes, the total time is enormous, making it unsuitable for mass production. For these reasons, the development of high-speed and high-precision processing technology is essential for the practical application of glass substrates.

Enabling ultrafast processing by controlling the spatial and temporal waveforms of laser light

The research group developed a new processing method that simultaneously controls the “spatial waveform” and “temporal waveform” of laser light to successfully form an extremely high-aspect-ratio hole with a depth of 1 millimeter and a diameter of 3 micrometers in only 20 µs (1 µs = 10^-6 s) (Fig. 2). This represents a 1,000,000-fold improvement over conventional processing speeds.

Spatial waveform control: Bessel beams

The spatial waveform of the laser utilized a special beam profile known as a “Bessel beam.” The spatial waveform is the brightness distribution observed when a laser beam is cross-sectioned. It indicates the spatial distribution of the light, such as whether the intensity is concentrated at the center or uniformly distributed across the beam. Unlike ordinary laser light, a Bessel beam has a narrow, intense central core and resists spreading, even as it travels. Additionally, it has the property of re-forming its beam shape even after encountering obstacles. By utilizing these characteristics, the energy of the laser beam was shaped into a long, slender columnar focus, enabling straight-through processing from the front surface to the back surface of the glass substrate.

Temporal waveform control: Two types of laser light

The temporal waveform indicates how the laser light is altered over time. In this study, a temporal waveform combining intense picosecond pulses with longer-duration, lower-intensity light lasting microseconds was used. Irradiation with these two types of light temporarily generates free electrons in the glass substrate and then induces light absorption in the normally transparent glass. The absorbed light energy is converted into kinetic energy, which leads to rapid heating, melting, and evaporation, forming a hole (Fig. 3).

Significantly reduced power consumption

Through this method, equivalent or superior processing performance is achieved at significantly lower light intensity than in conventional femtosecond laser processing (1 femtosecond = 10^-15 s). A femtosecond laser is a laser that concentrates high energy in an extremely short period of time, but the high cost of equipment and large power consumption have posed issues. The significance of this achievement lies in realizing ultrafast processing not by increasing laser output, but by ingeniously modifying the laser light itself. Furthermore, almost no cracking or shape distortion damage, issues commonly associated with processing, was observed, demonstrating that the technique enables processing suitable for advanced semiconductors.

Setting the basis for transparent material processing technology

This technology can be applied not only to glass, but also to the processing of other transparent materials such as sapphire, silicon carbide, and diamond. Therefore, its applications are not limited to the semiconductor industry but hold potential across a broad range of fields. Furthermore, the concept of “instantaneously altering material properties to enable processing” is expected to trigger a new technological revolution throughout the manufacturing sector.