用于 MEMS 应用的玻璃湿法蚀刻：综述

Abstract 

Glass is one of the most essential materials in various industrial fields. A representative application is as interposers in the semiconductor and display industries and microfluidic devices in the bio-industry. Due to technological advancements, electric devices and various products are becoming lighter and smaller. Consequently, precision processing of the substrate is becoming increasingly important. One significant leading technology among these is through glass via (TGV) technology, which is attracting attention as a future alternative to through silicon via. In particular, TGV should be capable of microfabrication with a large aspect ratio and a consistent small hole size. TGV is typically fabricated using a wet etching process, so wet etching technology is a prominent focus in microfabrication. However, glass has isotropic properties, making achieving a high aspect ratio difficult. Traditionally, research on the wet etching method using hydrofluoric acid (HF) has been conducted. Nevertheless, HF etching has several disadvantages, including air pollution and human harm. Additional studies have been performed to address these issues. This review paper will cover the conventional HF-based wet etching method and alternative HF etching processes (eco-friendly materials). This technology would significantly help overcome the traditional problems associated with HF etching and fabricating precision-processed glass in the semiconductor, display, and bio-device industries.

INTRODUCTION 

Glass is widely used worldwide due to its outstanding properties, such as optical, thermal, and mechanical characteristics.Glass has recently found significant applications in the semiconductor and display fields.For example, it is used as a substrate in various fields. Glass wafers are also used in wafer-level packaging processes for semiconductor devices. The thermal expansion coefficient of glass can be matched to that of silicon, reducing stress during bonding and enhancing device reliability.Additionally, it is well known that glass is used in the display industry as a display substrate. Soda–lime glass or aluminosilicate glass is commonly utilized in liquid crystal displays (LCDs), organic light-emitting diode displays, and touchscreens due to their flatness, transparency, and thermal stability, which are critical for producing highquality images and touch sensitivity.Furthermore, due to its high dielectric strength and low electrical conductivity, it is an insulating layer between conductive elements in semiconductor devices to prevent electrical short circuits and leakage.

With technological advancements, the demand for portable devices has grown, leading to a need for smaller and lighter products. Glass is used extensively as a flatpanel display material in the display industry, providing clear images with excellent resolution, lower power consumption, and thinner display screens.

Various research studies on glass applications have been conducted in the semiconductor and display industries, particularly concerning glass etching technology. One of the primary applications of glass in these industries is as an interposer, known as through glass via (TGV). The formation of TGV is illustrated in Figure 1. Generally, via is fabricated using mechanical machining or laserinduced wet etching (LIE). After that, Ti/Cu seed layers are deposited, and Cu electro/electroless plating is performed. Lastly, the surface is planarized using the CMP process. Due to technical constraints, 3D semiconductors have emerged as a critical technology to overcome performance limitations in existing CMOS processes in these industries.Shorter connection lengths and reduced electrical delays can enhance the electrical performance of 3D semiconductors in high-speed broadband systems. However, traditional organic substrates with thick electrical wiring have limited the number of inputs and outputs. Interposers are inserted between the micro-processed chip and the package, bridging the size difference and increasing the number of system inputs/outputs through fine electrical wiring. The following sections will further demonstrate the use of glass as an interposer.

Fig1

Substrates for microfluidic devices require chemical stability and optical analysis methods, limiting the choice of materials. Typically, transparent polymers like polydimethylsiloxane (PDMS) and transparent ceramic materials like glass are used. Recent research has also actively explored the use of transparent thermoplastic resin and poly (methyl methacrylate).

The technologies mentioned earlier are all based on MEMS. It is a promising technology with applications in various fields. Figure 2 illustrates the applications of TGV and microfluidic channels for MEMS. Another reason MEMS technology is crucial is its effectiveness in terms of productivity, practicality, and economic efficiency. It combines complex electrical, mechanical, chemical, and biological functions required for designing ultrasmall and ultralight smart devices.

Fig2

One alternative method to overcome these issues is LIE.Combining laser ablation with the wet etching process makes creating holes with a high aspect ratio possible. The following chapter will explain the wet etching process more thoroughly. Figure 4 compares laser hole drilling and the LIE process. Laser hole drilling (Figure 4A) involves using a focused laser beam to ablate material from a substrate, creating precise holes or vias. The laser energy heats the material rapidly, causing it to vaporize or melt and be ejected from the substrate. Although it has several advantages, such as high precision and accuracy, burr-free and clean holes, and suitability for a wide range of materials, it also has higher initial equipment costs than traditional drilling methods and limitations regarding materials that can efficiently absorb laser energy. In contrast, the LIE process is more efficient than direct laser hole drilling because the laser’s role is limited to forming a track in the substrate, and the wet etching process can then fabricate appropriate vias. Using a laser as a heat source to induce more active reactions between the material and the etching solution makes it possible to manufacture a clean channel structure with minimal heat impact, in contrast to laser ablation processing. Therefore, not only does LIE offer high precision and resolution with high selectivity, reducing contamination and damage to surrounding areas, but it also provides a competitive advantage in terms of cost compared to processes using expensive femtosecond lasers, as it employs a relatively simple device.

Fig4

As mentioned earlier, wet etching is a conventional etching method from the past. It is challenging to fabricate holes with a high aspect ratio when relying solely on wet etching. The LIE process also incorporates wet etching to create high aspect ratio holes, emphasizing the importance of the wet etching method and the etching solution. Wet etching is a chemical etching process where samples are typically immersed in a bath with a chemical etchant. This isotropic process often results in undercut phenomena, depending on the glass composition. The glass composition strongly influences the wet etching process through chemical reactions. This characteristic weakens acceptable patterning and high aspect ratio etching processes.

Wet etching has several disadvantages, such as low etching accuracy and contamination during the etching process. The most significant challenge is the difficulty in controlling manufacturing parameters. However, there are numerous advantages to using wet etching. First, it is a cost-effective process and easy to set up in the etching environment. Therefore, the wet etching process is advantageous for mass production.Furthermore, it offers a high etch rate, superior selectivity, and cost-effectiveness compared to other methods.

There are two types of wet etching processes: dip and spray etching. Dip etching is typically used on a laboratory scale, where samples are immersed in a bath for a specific duration. It is straightforward to set up the etching tool. In this process, etching time and temperature are the main factors affecting etching uniformity. This method offers high selectivity and is cost-effective. In the case of spray etching, not only are etching time and temperature essential, but pressure and the degree of spray dispersion are also significant factors. While it may be more challenging to control etching parameters than dip etching, spray etching offers high productivity and can reduce etchant usage. Moreover, spray etching lowers the risk of sample or etchant contamination due to residual particles generated during etching. Figure 5 illustrates representative spray etching methods. The choice between dip and spray etching depends on the application’s specific requirements and the company’s preferences.

Fig5

Glass is typically etched using hydrofluoric acid (HF) and various additional mineral acids, such as HCl, HNO3, H2SO4, and more.The wet etching process using HF solutions has been extensively studied. However, due to the side effects associated with HF, there has been ongoing research to find alternatives. HF is known for its high oxidation potential, leading to glass over-corrosion, the release of harmful gases, facility corrosion, wastewater treatment challenges, and adverse environmental impacts.As a result, the demand for developing an HF replacement etching solution has grown significantly. In general, fluoride-based salts have been explored as etchants to replace HF, and numerous non-fluoric acid etching solutions based on these fluoride salts have been proposed. However, many studies involving non-fluoric acid etching solutions are primarily applicable at the laboratory scale.The challenge arises when attempting to scale up these solutions for large production lines, where issues like oxidation reduction and reduced cleaning power can lead to process defects. Furthermore, the etching process often operates at temperatures between 50 and 70℃, higher than room temperature, to maintain productivity. Therefore, developing new glass etchants without HF remains a critical need.

Section 3 discussed the development of an alternative etching solution to replace HF due to various issues associated with wet etching using HF. However, a small amount of HF is still used in some cases due to concerns related to etch rate problems despite its potentially harmful effects on human health. The significant challenge remains the formation of sludge, which has not been fully resolved. Another major drawback of wet etching is its isotropic characteristic, where etching proceeds uniformly in all directions. For these reasons, there is a growing interest in alkaline etching, which is environmentally friendly and offers high selectivity. Typically, alkaline etching is performed at elevated temperatures exceeding 50℃, using solutions of NaOH or KOH. Therefore, additional research needs to be conducted to overcome the above problems. The chemical reactions of glass etching using alkali solutions follow Equation (5).

One possibility to consider is the femtosecond laserinduced material modification of SiO2, leading to the formation of SiOx (where x is less than 2). This alteration may enhance the reactivity of the irradiated area, especially toward OH−. The formation of Si–Si bonds is particularly relevant, as these bonds are readily susceptible to attack by OH−, considering that KOH is a widely used etching agent for silicon.

Although the chemical reactions between Si wafers and various alkali etching solutions have been extensively studied and well-documented in numerous papers,the mechanisms of glass etching using these solutions are still poorly understood because of the complex compositions of glass. As a result, this review does not provide an in-depth explanation of the chemical reactions involved in glass etching. However, some papers have compared NaOH and KOH in glass etching. The main difference between NaOH and KOH etching for glass lies in their chemical properties and etching characteristics, outlined in Table 5. Distinctions between the two alkaline solutions are highlighted, with high and low standards defined for NaOH and KOH solutions. Typically, KOH etches glass faster than NaOH, which can be advantageous for applications where speed is crucial. Additionally, KOH tends to exhibit more excellent selectivity than NaOH. However, NaOH may demonstrate a more uniform etching behavior across different types of glass. Although the optimal etching temperature may vary for NaOH and KOH, KOH etching is generally performed at higher temperatures than NaOH. Differences in etching behavior can result in varying surface roughness, clarity, and smoothness, influenced by factors such as concentration, temperature, and etching time. It is important to note that both NaOH and KOH are corrosive and require careful handling. However, KOH may pose slightly more significant hazards due to its higher reactivity and ability to generate more heat upon dissolution in water. Therefore, the choice between NaOH and KOH etching for glass depends on the application’s requirements, including the desired etching rate, selectivity, and surface characteristics.

This paper reviewed various wet etching processes and their applications using a glass substrate. First, wet etching using HF, a representative etching solution for glass, was described. HF etching solution is commonly used worldwide, and numerous research studies have been conducted. However, alternative glass etching solutions are needed due to various disadvantages, such as being harmful to the human body and producing toxic byproducts. One alternative etching solution is the NH4HF2 etching solution. It is less dangerous than HF to the human body and the environment but has a slow etch rate and imposes limitations on the glass surface after etching. Additionally, it still exhibits isotropic characteristics. Therefore, alkaline etching has become a hot topic worldwide due to its anisotropic characteristics, leaving glass surfaces smooth after etching. Although alkaline etching has several disadvantages, such as a high-temperature process and a slow etch rate, it is an up-and-coming etching solution for glass. This paper not only reviews the etching solutions and their applications but also attempts to demonstrate the etching mechanisms of each etching solution.