Atmospheric Gas-Phase Catalyst Etching of SiO2 for Deep Microfabrication Using HF Gas and Patterned Photoresist

Micro/nanoscale structure fabrication is an important process for designing miniaturized devices. Recently, three-dimensional (3D) integrated circuits using SiO2 via-holes interlayer filling by copper have attracted attention to extend the lifetime of Moore’s law. However, the fabrication of vertical and smooth-sidewall via-hole structures on SiO2 has not been achieved using the conventional dry etching method due to the limitation of the selective etching ratio of SiO2 and hard mask materials. In this study, we developed a unique method for the deep anisotropic dry etching of SiO2 using atmospheric gas-phase HF and a patterned photoresist. The hydroxyl groups in the photoresist catalyzed the HF gas-phase dry etching of SiO2 at high-temperature conditions. Therefore, fabrication of vertical with smooth-sidewall deep microstructures was demonstrated in the photoresist-covered area on SiO2 at a processing rate of 1.3 μm/min, which is 2–3 times faster than the conventional dry etching method. Additionally, the chemical reaction pathway in the photoresist-covered area on SiO2 with HF gas was revealed via density functional theory (DFT) calculations. This simple and high-speed microfabrication process will expand the commercial application scope of next-generation microfabricated SiO2-based devices.


■ INTRODUCTION
The realization of a microfabrication process for the construction of high-aspect-ratio micro/nanoscale structures on SiO 2 has been a long-standing goal for the development of novel devices in nanotechnology.The "Bosch process"�a deep microfabrication method for Si invented by Robert Bosch GmbH in 1992 1 �is widely used to fabricate advanced Sibased devices such as micro-electromechanical systems 2,3 and through-silicon vias. 4,5In contrast to pure Si, SiO 2 exhibits high chemical resistance, high transparency in the visible range, low thermal conductivity, and extremely low dielectric loss at high frequencies. 6,7By exploiting these unique characteristics, various SiO 2 -based devices that require vertical microstructures with smooth sidewalls have been developed in recent years, including microfluidic chips 8,9 and optical meta-surfaces. 10,11nisotropic microstructures on SiO 2 are typically fabricated via inductively coupled plasma reactive ion etching (ICP-RIE). 12−14 However, the fabrication of high-aspect-ratio vertical and smooth-sidewall microstructures has not been achieved using this method because hard mask materials are etched via ion irradiation from an ICP source.
−17 The chemical reaction of SiO 2 with HF gas can be expressed as follows: In general, the surface treatment of SiO 2 with HF gas is not suitable for anisotropic etching because gas-phase chemical reactions proceed isotropically on SiO 2 .In addition, the dry etching reaction of SiO 2 with HF gas is extremely slow even at high temperatures of >600 °C. 17However, the chemical reaction of SiO 2 can be accelerated by the formation of hydrogen bonds between the HF gas and H 2 O molecules. 15,16,18,19We hypothesized that hydrophilic functional groups in organic materials, such as R−OH or R−NH 2 , also accelerate the dry etching reaction because these functional groups can form hydrogen bonds with the HF gas.A novolac-type photoresist, which contains hydroxyl groups, was examined in this study.The photoresist allows high-resolution fine patterns to be easily formed using conventional lithography.Therefore, we consider that by employing the photoresist on the SiO 2 as the dry etching accelerator, the deep microfabrication of SiO 2 can be achieved through surface treatment with HF gas.−24 The noble-metalcovered area on Si is actively oxidized by the H 2 O 2 .Thus, the Si in the oxidized area is selectively removed by the HF.
According to this mechanism, stable oxide materials such as SiO 2 are not suitable for selective etching via the MacEtch process.
The chemical reactions of photoresists on SiO 2 under HF gas treatments have been investigated.The earliest report, which was published in 1977, introduced the "DryOX" method, 25 wherein a thin oxide film on a Si wafer covered by a negative-tone photoresist was selectively removed using HF gas in vacuum conditions.The interaction of HF molecules with the carbonyl groups of the photosensitizer in the negative photoresist promotes the formation of HF 2 − . 26Selective etching of the thin oxide film under the photoresist was realized by using HF 2 − species.A deep SiO 2 etching method involving a low-temperature HF gas treatment in vacuum conditions and the use of a photoresist was reported in 2003. 27his process actively used H 2 O generated from the reaction between SiO 2 and the HF gas.The defining characteristic of this process is the cooling of the substrate below 20 °C during the HF gas treatment, which enhances the generation of HF 2 − produced by the reaction of H 2 O and HF in the photoresistcovered area on SiO 2 .Therefore, selective etching under the photoresist was performed previously through the formation of HF 2 − , which are employed as active species in the conventional wet-type etching of SiO 2 using liquid HF. 28However, the gasphase HF 2 − or H 2 O molecules likely adsorbed on the etched sidewall surface during this process, accelerating the sideetching reaction at processing temperatures below 100 °C.
In this study, we develop a novel method for hightemperature atmospheric HF gas-phase deep etching of SiO 2 using a novolac-type photoresist as a catalyst.The acceleration of the etching reaction of H 2 O molecules was prevented by employing processing temperatures of >100 °C.Additionally, a deep trench structure with a vertical and smooth sidewall is obtained as the side-etching reactions are suppressed because selective etching occurs only in the catalyst contact area on SiO 2 without producing HF 2 − .Furthermore, high-speed anisotropic etching is realized because the increased process temperature accelerates the chemical reaction on the SiO 2 substrate.Our simple method requires no plasma source, vacuum chamber, or hard mask materials, which are indispensable in conventional processes.Moreover, we examine the chemical reaction pathway in the photoresistcovered area on SiO 2 with HF gas using density functional theory (DFT) calculations.
■ RESULTS AND DISCUSSION HF Gas-Phase Anisotropic Etching Using a Photoresist.The hydrophilic functional groups in the photoresist accelerate the chemical reaction between HF gas and SiO 2 .In addition, the side-etching reaction is suppressed by hightemperature conditions at >100 °C because H 2 O molecules, which serve as the reaction accelerator, cannot be adsorbed onto the substrate.Accordingly, vertical deep microstructures can be fabricated via selective etching due to the HF gas that passes through the photoresist on SiO 2 , as shown in Figure 1a.To verify this hypothesis, HF gas-phase surface treatment was performed at 250 °C for 1000 s using a sample with a patterned photoresist on a silica glass substrate.A vertical deep hole structure with a smooth-sidewall surface was formed in the photoresist-covered area, as shown in Figure 1b.Additionally, the photoresist was observed at the bottom of the hole structure, indicating that the deep microstructure was fabricated in a photoresist-covered area via selective etching with HF gas.Conversely, no structural changes were observed in the areas of bare SiO 2 in the sample after the surface treatment, as shown in Figure 1c.The anisotropic etching rates in the photoresist-covered area at different processing temperatures are shown in Figure 2a.The dry etching rate increased sharply from 100 to 200 °C and reached a maximum at 250 °C.−14 The dry etching depth for each processing time under a surface treatment at 250 °C is plotted in Figure 2b.The etching depth was almost linear with respect to the processing time and reached 76 μm when the surface treatment was performed for 3000 s.This indicated that the etching reaction continued when sufficient HF gas was supplied to the photoresist at the bottom of the trench.Gas-phase molecules can generally diffuse easily through micrometer-scale gaps.Therefore, this HF gas-phase anisotropic etching allows for deep microfabrication without depth limitations.
The resolution of the HF gas-phase anisotropic etching depends on the photoresist patterns on the substrate.In particular, the novolac-type photoresist can be fabricated with a submicrometer-scale pattern in a large area using a general lithography system.Submicron-scale structures with high aspect ratios, such as nanopillar arrays, are highly demanded for biological applications 29−31 or optical devices. 10,11,32A novolac-type photoresist with submicrometer hole patterns was fabricated on SiO 2 , as shown in Figure 2c.The photoresist-covered areas were selectively etched via the HF gas-phase surface treatment, as shown in Figure 2d.Additionally, vertical nanopillar arrays with the same diameter as the photoresist holes were fabricated on SiO 2 , as shown in Figure 2e.Therefore, deep microfabrication at the submicron-scale structure was demonstrated using the HF gas-phase anisotropic etching technique with a novolac-type photoresist.In addition, the vertical nanopillar structure with the same diameter as that of the catalyst pattern indicates that the effect of lateral etching on the sidewalls is negligible in this process.
Anisotropic Dry Etching Rate Reduction Using Thermally Decomposed Photoresists.The HF gas-phase anisotropic etching method developed in this study involves a chemical reaction between HF gas and photoresist on the SiO 2 substrate.The gas-phase chemical reaction is typically  accelerated by increasing the processing temperature.However, the etching rate was maximized at a processing temperature of 250 °C.We considered that the photoresist was thermally decomposed at high-temperature conditions.We hypothesized that the hydroxyl groups in the photoresist played two roles in the etching process: catalytic function on the etched surface and incorporation of HF molecules into the photoresist.Therefore, the etching reaction was suppressed because of the reduced number of hydroxyl groups in the photoresist at processing temperatures of >250 °C.To verify the reduction in the etching rate due to the thermal decomposition of the photoresist, the hydroxyl groups in the heated photoresist were analyzed via Fourier transform infrared (FT-IR) spectroscopy, as shown in Figure 3a.The intensity of the O−H stretching vibration peaks in the spectrum of the photoresist decreased with an increase in temperature, confirming that the number of hydroxyl groups in the photoresist decreased upon heat treatment.Samples with fewer hydroxyl groups in the photoresist after the heat treatment were subjected to the HF gas-phase surface treatment, as shown in Figure 3b.The etching rates of the preheated samples were significantly reduced at processing temperatures of 250 and 300 °C owing to the fewer hydroxyl groups in the photoresist.Thus, 250 °C is considered the optimal processing temperature for the HF gas-phase anisotropic etching as it accelerates the chemical reaction and suppresses the thermal decomposition of the photoresist.
Characteristics of Microstructures Fabricated via HF Gas-Phase Anisotropic Etching.The processing rate of the HF gas-phase anisotropic etching of SiO 2 was higher than that achieved by the ICP-RIE process.The verticality and smoothness of the sidewall surface in the microstructures are critical requirements for the microfabrication techniques.Thus, the taper angles and roughness of the sidewalls of the trench structures fabricated via HF gas-phase anisotropic etching and ICP-RIE were evaluated.Table 1 presents the taper angles and arithmetic average roughness (R a ) values of the sidewalls obtained by using different microfabrication methods under various conditions.The microstructures fabricated via the HF gas-phase anisotropic etching exhibited vertical and smoothsidewall structures, even at etching depths of >10 μm.These excellent shapes can be easily fabricated via the HF gas-phase anisotropic etching because side-etching reactions are suppressed by the high temperature, and the shape of the photoresist does not change during the chemical reaction.The trench structures fabricated via ICP-RIE using Cr hard masks with thicknesses of 0.2 and 1.0 μm exhibited taper angles of 9.7 and 2.0°, respectively.In addition, the trench structure with a larger taper angle exhibited a smoother sidewall surface.The ion irradiation of the sidewall surface occurs when the sample has a large taper angle.Therefore, a sidewall surface with a large R a value is smoothened via ion irradiation.Thus, the fabrication of vertical and smooth-sidewall microstructures on SiO 2 was difficult using the ICP-RIE method.In addition, the bottom surface roughness of the microstructure fabricated via HF gas-phase anisotropic etching at 250 °C was evaluated after photoresist stripping.The R a value of the bottom surface was 1.2 nm, which was lower than that of the sidewall surface.Therefore, a vertical and deep microstructure with smooth side and bottom surfaces was obtained on the SiO 2 substrate via HF gas-phase anisotropic etching.
Aspect-ratio-dependent etching (ARDE) 33−36 is a serious problem in the microfabrication using the RIE process.The linear trench structures of different widths on the same SiO 2 substrate were fabricated via HF gas-phase anisotropic etching, as shown in Figure 4.The etching depth is ranked in the order 8 μm > 3 μm > 700 nm, which is a similar trend to the ARDE.We considered that the differences of etching rate occurred due to the thermal decomposition of the photoresist.Thermal decomposed reaction in the photoresist is thought to proceed from the surface layer.In this case, the narrower width photoresist was a high percentage of the thermal decomposed volume because of their large specific surface area.Therefore, the ARDE of the HF gas-phase anisotropic etching occurred due to the differences of the amount of hydroxyl groups on the SiO 2 surface between the narrow and wide-width photoresists.
Mechanisms of HF Gas-Phase Anisotropic Etching of SiO 2 using Novolac-Type Photoresists.We examined the mechanism of the HF gas-phase anisotropic etching of SiO 2 via the generation of gas-phase SiF 4 and H 2 O.According to the previously reported models in which the reaction of SiO 2 with HF was studied using computational simulations, 18,19,37−40 the reaction pathway of SiO 2 with HF gas using the novolac-type photoresists was investigated via DFT calculations.
The initial state of SiO 2 and HF with phenol, which is the catalytic group of the novolac-type photoresist, is shown in Figure 5a.The initial state was carefully chosen so as to be the most stable among possible geometries.The hydroxyl group of phenol interacts with HF and Si−OH, which increases the H− F bond length by 0.03 Å relative to that in the initial state in  the absence of phenol (Figure 5b), indicating that the H−F bond is weakened by the interaction.The F atom in the HF molecule attacks the Si atom on the SiO 2 surface, which leads to the transition state, as shown in Figure 5c.The H−F bond length in the transition state is increased by 0.17 Å, relative to that in the transition state in the absence of phenol, as shown in Figure 5d.Therefore, the nucleophilic substitution of the F atom with the Si atom proceeds readily in the presence of phenol because the aforementioned interaction with the phenol weakens the H−F bond in the transition state.Finally, the hydroxyl group on SiO 2 is replaced by the F atom via nucleophilic substitution (Figure 5e), after which the H atom of the HF molecule is transferred to the hydroxyl group of the phenol.Additionally, an H 2 O molecule is generated by the reaction of Si−OH with a H atom derived from the phenol.Subsequently, the hydroxyl group of the phenol is restored via that proton transfer mechanism.The energy barrier to Si−F bond formation in this reaction pathway is calculated to be 0.86 eV, which is 0.30 eV lower than that in the reaction pathway without phenol (Figure 5f), as shown in Figure 5g.These results indicate that the novolac-type photoresist catalyzes the dry etching of SiO 2 using HF gas.
The dry etching characteristics of SiO 2 in the absence of catalysts were then experimentally investigated by using thermogravimetry analysis (TGA) under HF gas-phase conditions, as shown in Figure 6a.The weight loss due to the etching reaction in the temperature range of 100−300 °C was less than 0.1%.On the other hand, significant weight loss was observed when the treatment temperature exceeded 400 °C.The SiO 2 surface after HF gas-phase treatment at each processing temperature without catalysts is shown in Figure 6b.A smooth surface was observed under the 200 °C condition.However, surfaces with large roughness due to the etching reactions were observed at above the 500 °C conditions.These  results indicate that high-temperature conditions above 400 °C are required for the dry etching of pure SiO 2 with HF gas.Therefore, the etching reaction on the SiO 2 surface not covered by the photoresist and etched sidewalls did not proceed under the catalytically active conditions because of the high reaction barrier energy in the catalyst-free reaction pathway.Thus, the vertical microstructures with smoothsidewall surfaces were fabricated without using hard mask materials because of the excellent catalytic activity of hydroxyl groups in the novolac-type photoresist.

■ CONCLUSIONS
Inspired by the chemical reaction of HF gas-phase surface treatment in the presence of H 2 O molecules, deep anisotropic dry etching of SiO 2 was realized under atmospheric conditions by utilizing the hydroxyl groups in a novolac-type photoresist as catalysts.The method imparts catalytic functions to the photoresist-covered area on the substrate at high temperatures, which accelerates anisotropic etching and prevents the adsorption of H 2 O molecules on the etched sidewall.Therefore, vertical and smooth-sidewall microstructures were easily fabricated on the SiO 2 at the processing rate of 1.3 μm/ min without using plasma sources, a vacuum chamber, or hard mask materials.In addition, fabrication of nanopillar arrays which have critical roles in next-generation biological or optical applications was demonstrated.This process enables the submicron-scale deep microfabrication without depth limitations.Therefore, microstructures with a high aspect ratio of 100 or more can be formed on the SiO 2 substrate via HF gasphase anisotropic etching.Moreover, according to the anisotropic etching mechanisms established in this study, this process may be applicable to materials other than SiO 2 (such as SiN or SiC).This simple atmospheric gas-phase catalyst etching process may facilitate the development of newconceptual microfabricated applications in various fields.
■ METHODS Sample Preparation.Optical-grade synthetic silica glass substrates (square; length: 25 mm; thickness: 0.5 mm) were cleaned via ultraviolet (UV) ozone treatment for 15 min, treated with hexamethyldisilazane, and coated with photoresists.Positive-tone novolac-type photoresists (THMR-iP 3100, Tokyo Ohka Kogyo, Japan) were spin-coated on the substrate at 2500 rpm for 25 s.Periodic 10-μm-diameter dots were patterned on the substrate over an area of 20 mm 2 using a maskless photolithography system (DL-1000A2, Nano System Solutions, Japan) and an alkaline developer solution (NMD-3, Tokyo Ohka Kogyo, Japan).Finally, organic contaminants were removed from the parts of the substrate surface that were not covered by the photoresist pattern via UV-ozone cleaning.
Surface Treatment with HF Gas.The high-temperature HF gasphase surface treatment of the samples was performed using a rapid thermal annealing system (VHC-P610, ULVAC, Japan).The etched samples were then placed on a carbon plate heated to >100 °C via IR heating.The gas-phase conditions of the samples were controlled via local gas injection.The chamber of the rapid thermal annealing system was evacuated to be <20 Pa using a rotary pump and subsequently returned to the atmospheric pressure by supplying dry N 2 gas.The amount of H 2 O in the chamber was reduced by replacing the air with low-dew-point N 2 gas.N 2 gas was then passed through the exhaust valve at a rate of 5.0 standard liters per minute (SLM) under atmospheric conditions.The sample was then heated under surface treatment conditions for 2 min, and the temperature was maintained for 1 min to stabilize the sample surface temperature.Finally, HF gas-phase surface treatment of the sample was performed by switching the gas line in front of the local injection tube.The HF and N 2 gases were used as the surface treatment gases at flow rates of 1.0 and 4.0 SLM, respectively.
Characteristics of the HF gas-phase dry etching of SiO 2 in the absence of catalysts were investigated using a thermogravimetric analyzer (STA2500 Regulus, NETZSCH, Germany).An alumina container with 50 mg of SiO 2 powder was used as the etching sample.The amount of H 2 O and organic contaminants in the samples was reduced by annealing to 500 °C under N 2 conditions before the etching tests.The temperature conditions were increased from 100 to 800 °C at 10 °C/min.The gas-phase condition was HF/N 2 = 10 vol% flowing at 50 standard cubic centimeters per minute under atmospheric pressure conditions.
Anisotropic Dry Etching with ICP-RIE.Samples fabricated via the ICP-RIE method were prepared by using two types of Cr films with thicknesses of 0.2 and 1.0 μm deposited on the silica glass substrate, which served as the hard mask material for the RIE.The Cr films were patterned into linear shapes using a wet-type chemical etching liquid (Cr201, Kanto Kagaku, Japan).The trench structure at a depth of 6−7 μm was fabricated using ICP-RIE with CHF 3 and CF 4 gases.
Characterization.The etched structures were examined via fieldemission SEM (SU8000, Hitachi, Japan).The R a of the etched sidewall was measured by using inline three-dimensional (3D) atomic force microscopy (NX-3DM, Park Systems, Korea).The functional groups in the photoresist were analyzed via FT-IR spectroscopy (NICOLET iS50 FT-IR, Thermo Fisher Scientific) to investigate the thermal decomposition that occurred at various processing temperatures with the photoresist on a silica glass substrate, which was heated at each temperature for 2 min.
DFT Calculations.The mechanism of HF gas-phase anisotropic etching of SiO 2 using the novolac-type photoresist was investigated using DFT calculations at the B3LYP 41,42 /6-311 + G(d, p) level of theory using the Gaussian 16 program package. 43The default conditions for all geometric optimization calculations were adopted. 44n the DFT model, a β-cristobalite (100) cluster structure with dangling Si bonds terminated with −OH was used as an initial geometry instead of silica glass.A cluster model was used for DFT calculations of amorphous solids such as glasses in previous studies. 18,37This cluster structure was fully geometry optimized and exhibited no imaginary vibrations during vibrational analysis.The novolac-type photoresist was modeled as a single phenol molecule that plays a key catalytic role.In the first step of the reaction pathway calculation, the phenol interacted with both an HF molecule and Si− OH in an initial structure.The interaction energy at which the phenol forms hydrogen bonds with HF molecule and Si−OH was calculated to be −0.932eV.The potential energy was then explored with respect to the attack of the HF molecule to the Si atom in Si−OH.The optimized transition state had an imaginary vibration mode along the direction of the etching reaction.Finally, the stable final and initial state structures along the reaction path from the transition state were determined via intrinsic reaction coordinate (IRC) 45 calculations.The reaction pathways to the final and initial states were explored by using the IRC calculation along the corresponding imaginary vibration modes and reverse imaginary modes.The geometries of the final steps of the IRC calculations in both directions were used for determining the final and initial states.More details on the IRC calculations are given in ref 46.

Figure 1 .
Figure 1.HF gas-phase anisotropic etching of silica glass substrates in photoresist-covered areas.(a) Concept of HF gas-phase anisotropic etching during processing (left) and after HF gas-phase treatments (right).(b) Cross-sectional scanning electron microscopy (SEM) images of the photoresist on SiO 2 before (left) and after (right) HF gas-phase treatment.(c) Top surface SEM images of photoresists on SiO 2 before (left) and after (right) HF gas-phase treatment.

Figure 2 .
Figure 2. Characteristics of HF gas-phase anisotropic etching for silica glass substrates in photoresist-covered areas.(a) Etching rates at different processing temperatures.The error bars show the 3σ standard deviations (n = 5).(b) Etching depth under 250 °C surface treatment for different processing times.The error bars show the 3σ standard deviations (n = 5).(c) Top surface SEM image of photoresists with submicrometer hole patterns on SiO 2 .(d) Top surface SEM image of nanopillar arrays on SiO 2 fabricated via HF gas-phase surface treatment using a photoresist with submicron hole patterns.(e) 30°-tilted SEM image of nanopillar arrays on SiO 2 fabricated via HF gas-phase surface treatment with submicron hole patterns.

Figure 3 .
Figure 3. Thermal decomposition of novolac-type photoresists.(a) O−H stretching vibrations in the FT-IR spectra of the heated photoresists.(b) HF gas-phase anisotropic etching rate of silica glass substrates at 250 and 300 °C using preheated photoresists.The error bars show the 3σ standard deviations (n = 5).

Figure 4 .
Figure 4. Cross-sectional SEM image of the SiO 2 substrate microfabricated via HF gas-phase anisotropic etching.The surface treatment was performed at 300 °C for 2000 s using a novolac-type photoresist with different widths as a catalyst.

Figure 5 .
Figure 5.Chemical reaction pathway of the HF gas-phase etching of SiO 2 .(a) Initial state of SiO 2 dry etching in the presence of phenol.(b) Initial state of SiO 2 dry etching in the absence of phenol.(c) Transition state of dry etching when the HF molecule approaches the Si atom in the presence of phenol.(d) Transition state of dry etching in the absence of phenol.(e) Final state after replacement of Si−OH with a F atom on SiO 2 when a phenol interacts with SiO 2 .(f) Final state after replacement of Si−OH with a F atom on SiO 2 in the absence of phenol.(g) Relative energy diagram (total energy of the initial state in each pathway set to zero) from the initial state to the final state along the transition state in the presence of phenol and in the absence of phenol.

Figure 6 .
Figure 6.Characteristics of HF gas-phase dry etching of SiO 2 in the absence of catalysts.(a) TGA curve of SiO 2 in HF gas-phase condition.(b) Top surface SEM images of SiO 2 after HF gas-phase treatment at high-temperature conditions.

Table 1 .
Etching Depths, Taper Angles, and R a Values of the Sidewall of Trench Structures Obtained by Using HF Gas-Phase Anisotropic Etching and ICP-RIE under Different Conditions