Structural design and processing technology of high aspect ratio micro gas chromatographic column based on hot reflux technology

Abstract

The invention discloses a structural design and processing technology of a high aspect ratio micro gas chromatographic column based on a hot reflux technology, and belongs to the technical field of glass processing. The silicon-glass high-aspect-ratio micro gas chromatographic column cannot be obtained by anodic bonding; and the problems of difficulty in preparation of a high-aspect-ratio structure, poor surface quality and stress concentration in existing glass micromachining are solved. According to the invention, efficient preparation of the high aspect ratio microstructure of the glass is realized, no obvious defect exists on the surface, and stress and cracks are effectively inhibited; according to the structural design of the miniature gas chromatographic column, the snakelike layout and the optimized depth-to-width ratio design are adopted, so that the separation efficiency is improved, a foundation is laid for high theoretical plate number and excellent separation degree, and the rapid separation requirement is met; the processing technology is stable, the silicon-glass bonding airtightness is good, and the method is suitable for on-site detection and on-line monitoring scenes; and batch preparation can be realized by the wafer-level process, the cost is reduced, and industrial application is facilitated.

Description

Technical Field

[0001] This invention belongs to the field of glass processing technology, specifically relating to the structural design and processing technology of a high aspect ratio micro gas chromatography column based on hot reflux technology. Background Technology

[0002] Gas chromatography is a technique that separates components in a mixture based on the difference in their partition coefficients between the stationary and mobile phases. Since its development in the 20th century, it has been widely used in environmental monitoring, industrial analysis, and safety testing. Although traditional gas chromatographs have strong separation capabilities, they are limited by their large size, heavy weight, long analysis time, and high power consumption, limiting their use to laboratory settings and failing to meet the needs of rapid on-site detection and online monitoring.

[0003] Micro-GC columns are the core component for the miniaturization of gas chromatographs, achieving miniaturization, low power consumption, and rapid separation through microelectromechanical systems (MEMS) technology. Although traditional silicon-based microfabrication processes (such as deep reactive ion etching, DRIE) can fabricate high aspect ratio structures, the fabrication of high aspect ratio structures using glass as an ideal column material (chemical inertness and good insulation) has always faced bottlenecks.

[0004] With the development of microelectromechanical systems (MEMS) technology, miniature gas chromatography columns have achieved improved system integration and instrument miniaturization through microfabrication processes. They can rapidly separate mixed components and are suitable for scenarios such as on-site detection of toxic gases and online industrial monitoring, becoming a research hotspot in the field of chromatography. Among them, miniature gas chromatography columns with rectangular cross-section channels have attracted widespread attention due to their adjustable aspect ratio (reducing the width accelerates component distribution equilibrium, while increasing the depth ensures column capacity). However, despite being an excellent material for fabricating miniature gas chromatography columns, the high aspect ratio microstructure fabrication of glass has always presented technical challenges, limiting further performance improvements.

[0005] The channel layouts of existing micro gas chromatography columns are mainly spiral and serpentine. The serpentine layout can reduce peak broadening and tailing caused by bends, resulting in better separation performance. The internal structure of the column includes packed columns, multi-channel columns, semi-packed columns and open tube columns. Among them, open tube columns are more conducive to improving column efficiency because there is no eddy diffusion. However, the fabrication of glass open tube columns with high aspect ratio structures still presents challenges.

[0006] The micro-machining technology of glass can be divided into mechanical processing, thermal processing, chemical etching, 3D printing and electrochemical processing, etc., and the comparison with the hot reflow process is shown in Table 1 [1]. Mechanical drilling and sandblasting belong to mechanical processing. They are simple, suitable for rapid prototyping, and have low cost. However, the thrust of the mechanical drill bit can easily cause glass deformation and generate conical cracks. The hole diameter is mostly limited to more than 100 μm, and expensive high-strength tools are required to fix the sample. Sandblasting removes material through particle jets without burrs and heat-affected zones, but the surface roughness reaches the micron level. Through holes will form a taper angle, and the maximum depth-to-width ratio is only 2.5.

[0007] Laser processing removes material through thermal shock or ablation, producing high aspect ratio microholes (typically ~10⁻⁵⁰) at relatively high speeds. However, the processed surface quality is poor, bumps easily form at the edges of the holes in the glass substrate, and a heat-affected zone ranging from submicron to tens of micrometers exists, affecting subsequent bonding.

[0008] Chemical etching includes wet etching and dry etching (primarily DRIE). Wet etching involves immersing glass in etchants such as HF acid to dissolve the material. It requires a corrosion-resistant mask, but the etching is isotropic, and pinholes and notches are prone to appear at the edges of the structure, with an aspect ratio typically less than 1. DRIE relies on gases such as sulfur hexafluoride and can achieve precise hole structures with diameters as low as 1 μm, a smooth surface (Ra = 2 nm), and an aspect ratio of up to 40. However, the etching rate is extremely low (approximately 0.009 μm / s), and it is limited by the poor thermal conductivity of glass (only 1 / 100th that of silicon), making it difficult to achieve deep etching.

[0009] Electrochemical machining utilizes electrochemical spark processing materials, combining thermal and chemical processing mechanisms. It is suitable for 2.5D microstructure processing of non-conductive materials such as glass, and can achieve high aspect ratios, but it has high surface roughness (second only to sandblasting) and relatively low precision. The application of 3D printing in glass micromachining is still in the exploratory stage.

[0010] Limitations of traditional glass processing techniques: Machining (such as grinding and sandblasting) easily leads to conical cracks in glass, resulting in surface roughness at the micrometer level and a maximum aspect ratio of only 2.5, which cannot meet the requirements of high aspect ratio structures; Laser processing has a heat-affected zone ranging from submicrometer to tens of micrometers, and bumps easily form at the edges of glass holes, affecting the stability of subsequent bonding processes; Wet etching is isotropic, and pinholes and notches easily appear at the edges of structures, with an aspect ratio usually less than 1; Dry etching (DRIE) can achieve a high aspect ratio of up to 40, but the etching rate is extremely low (about 0.009 μm / s), and it is difficult to achieve deep etching due to the low thermal conductivity of glass; Electrochemical processing has high surface roughness (250~350nm) and low precision; 3D printing is still in the exploratory stage, with low processing efficiency and difficulty in large-scale application.

[0011] The shortcomings of the hot reflow process: Although the existing hot reflow process can prepare high aspect ratio structures, the basic theory is incomplete; the influence of process parameters (temperature, time, annealing method) on glass surface quality and reflow depth is unclear, and defects such as surface cracks and internal stress concentration are prone to occur; the characteristics of silicon mold sidewalls and atmospheric environment are not fully considered, resulting in insufficient adhesion between glass and mold and poor reflow uniformity; during wafer-level processing, warping is prone to occur due to loose bonding, increasing the risk of fragmentation in the subsequent grinding and thinning process.

[0012] Defects in the design and fabrication of miniature gas chromatography columns: Channel layouts (such as spiral shapes) are prone to exacerbating peak broadening and tailing due to differences in the inner and outer paths of bends, affecting separation performance; the aspect ratio and theoretical plate height of existing structural designs are not well matched, making it difficult to improve separation efficiency while ensuring column capacity; at the same time, the quality of silicon-glass bonding is affected by factors such as surface flatness and stress, easily leading to insufficient airtightness and affecting column stability. Furthermore, the cost is too high, making mass production difficult.

[0013] Therefore, there is a need for the structural design and fabrication process of high aspect ratio micro gas chromatographs based on thermal reflux technology to solve the problems of current glass processing difficulties, the inability to obtain silicon-glass high aspect ratio micro gas chromatographs by anodic bonding, and the difficulty in preparing high aspect ratio structures, poor surface quality, and stress concentration in existing glass microfabrication. Summary of the Invention

[0014] The purpose of this invention is to provide a structural design and fabrication process for a high aspect ratio micro gas chromatography column based on a hot reflux process, so as to solve the problems mentioned in the background art.

[0015] To achieve the above objectives, the present invention provides the following technical solution: a structural design of a high aspect ratio micro gas chromatographic column based on hot reflux process, wherein the micro gas chromatographic column is a rectangular cross-section open tube column, including a channel body and auxiliary structures;

[0016] The main body of the channel has a high aspect ratio design, with a depth of 500μm and a width of 30μm, 50μm or 70μm, corresponding to aspect ratios of 16, 10 and 7 respectively.

[0017] The main body of the passage has a serpentine layout, containing 67 straight sections, each 15mm long, with a total length of 1m. Adjacent straight sections are connected by 180° bends, which are designed symmetrically to compensate for path differences.

[0018] The auxiliary structure is a series auxiliary reflow structure set in the non-structured area, used to reduce the height difference of the glass surface after heat reflow.

[0019] This invention also provides the following technical solution: a fabrication process for a high aspect ratio micro gas chromatographic column based on hot reflux technology, comprising the following steps:

[0020] S1. Silicon mold etching: On a 500μm thick silicon wafer, using the intermediate device as a reference, a groove with a depth of 200μm is etched using the DRIE etching process to form a silicon mold. The sidewalls of the silicon mold are pre-oxidized to form a SiO2 layer with a thickness of 50nm to improve the subsequent adhesion to the glass.

[0021] S2, Anodic Bonding: The silicon mold obtained in step S1 is anodicly bonded to 500μm thick BF33 glass to form a sealed chamber;

[0022] S3, Glass hot reflow: The bonding component obtained in step S2 is placed in a nitrogen atmosphere and kept at 1000℃ for 2 hours to allow the glass to flow and fill the groove of the silicon mold.

[0023] S4. Grinding treatment: Grind the upper and lower surfaces of the component after hot reflow in step S3, wherein the glass surface is ground until the surface is flat and the back silicon surface is ground until the glass is exposed.

[0024] S5. Remove silicon mold: Use DRIE etching process to remove the silicon mold in the glass structure to obtain the glass channel;

[0025] S6. Silicon-glass bonding: Align the silicon wafer with a pre-etched 300μm deep channel with the glass channel obtained in step S5 and bond them together to form a closed channel.

[0026] In a preferred embodiment, the SiO2 layer on the sidewall of the silicon mold in step S1 is formed by a thermal oxidation process.

[0027] In a preferred embodiment, the anodic bonding in step S2 is performed in a vacuum environment, with a bonding temperature of 300-400°C and an applied voltage of 500-1000V.

[0028] In a preferred embodiment, the bonding in step S6 is anodic bonding or melt bonding.

[0029] Compared with the prior art, the present invention provides a structural design and fabrication process for a high aspect ratio micro gas chromatography column based on hot reflux technology, which has at least the following beneficial effects:

[0030] (1) This invention achieves efficient preparation of glass microstructures with high aspect ratio (depth 300μm, aspect ratio up to 16), with no significant surface defects and effective suppression of stress and cracks.

[0031] (2) The high aspect ratio micro gas chromatography column based on hot reflux technology provided by the present invention adopts a serpentine layout and optimized aspect ratio design to improve separation efficiency, laying the foundation for high theoretical plate number and excellent resolution, and meeting the needs of rapid separation.

[0032] (3) The processing technology of this invention is stable, the silicon-glass bonding has good airtightness, and it is suitable for on-site testing and online monitoring scenarios; the wafer-level process can realize batch preparation, reduce costs, and facilitate industrial application. Attached Figure Description

[0033] Figure 1 This is a temperature-time curve of the thermal reflux process for a high aspect ratio micro gas chromatography column fabrication process based on thermal reflux technology according to the present invention.

[0034] Figure 2 This is a schematic diagram of the serpentine channel layout of a high aspect ratio micro gas chromatography column based on thermal reflux process according to the present invention.

[0035] Figure 3 This is a flowchart illustrating the fabrication process of a high aspect ratio micro gas chromatography column based on thermal reflux technology according to the present invention.

[0036] Figure 4 This is a schematic diagram showing the glass thermal reflux effect before and after the present invention. Detailed Implementation

[0037] The present invention will be further described below with reference to embodiments.

[0038] Please see Figure 2 The present invention provides a structural design for a high aspect ratio micro gas chromatography column based on hot reflux technology. The micro gas chromatography column is a rectangular cross-section open tube column, including a channel body and auxiliary structures.

[0039] The main body of the channel has a high aspect ratio design, with a depth of 500μm and a width of 30μm, 50μm or 70μm, corresponding to aspect ratios of 16, 10 and 7 respectively.

[0040] The main body of the passage has a serpentine layout, containing 67 straight sections, each 15mm long, with a total length of 1m. Adjacent straight sections are connected by 180° bends, which are designed symmetrically to compensate for path differences.

[0041] The auxiliary structure is a series auxiliary reflow structure set in the non-structured area, used to reduce the height difference of the glass surface after heat reflow.

[0042] Specifically, in one embodiment, the micro gas chromatography column channel adopts a rectangular cross-section design with a depth of 500 μm, a width of 30 μm, and a depth-to-width ratio of 16. This design shortens the mass transfer distance to improve separation efficiency through narrow width and ensures column capacity through large depth.

[0043] The column length is designed to be 1m, using Figure 2 The serpentine planar layout shown has a straight section length of 15mm, connected by 180° rounded bends (radius 500μm), containing a total of 67 straight sections. The overall size is controlled within the range of 22mm×19mm, achieving a long separation path in a limited space. The rounded corner design of the bends can reduce peak broadening caused by fluid disturbance and improve separation stability.

[0044] In terms of materials and sealing: the silicon channel (etching depth 300μm) and the glass channel are bonded together by secondary anodic bonding to form a closed structure. The bonding process parameters are 400℃ and 1000V, and the bonding strength is tested to be over 10MPa. A 500μm wide interface area is reserved at the inlet and outlet of the channel for subsequent connection with the quartz capillary.

[0045] Please see Figure 1 and Figure 4 This invention provides a fabrication process for a high aspect ratio micro gas chromatographic column based on a hot reflux process, comprising the following steps:

[0046] S1. Silicon mold etching: On a 500μm thick silicon wafer, using the intermediate device as a reference, a groove with a depth of 200μm is etched using the DRIE etching process to form a silicon mold. The sidewalls of the silicon mold are pre-oxidized to form a SiO2 layer with a thickness of 50nm to improve the subsequent adhesion to the glass.

[0047] S2, Anodic Bonding: The silicon mold obtained in step S1 is anodicly bonded to 500μm thick BF33 glass to form a sealed chamber;

[0048] S3, Glass hot reflow: The bonding component obtained in step S2 is placed in a nitrogen atmosphere and kept at 1000℃ for 2 hours to allow the glass to flow and fill the groove of the silicon mold.

[0049] S4. Grinding treatment: Grind the upper and lower surfaces of the component after hot reflow in step S3, wherein the glass surface is ground until the surface is flat and the back silicon surface is ground until the glass is exposed.

[0050] S5. Remove silicon mold: Use DRIE etching process to remove the silicon mold in the glass structure to obtain the glass channel;

[0051] S6. Silicon-glass bonding: Align the silicon wafer with a pre-etched 300μm deep channel with the glass channel obtained in step S5 and bond them together to form a closed channel.

[0052] Furthermore, it is worth noting that the SiO2 layer on the sidewall of the silicon mold in step S1 is formed by a thermal oxidation process.

[0053] Furthermore, it is worth noting that the anodic bonding in step S2 is carried out in a vacuum environment, with a bonding temperature of 300-400℃ and an applied voltage of 500-1000V.

[0054] Furthermore, it is worth noting that the bonding described in step S6 is either anodic bonding or melt bonding. Anodic bonding has better performance and is suitable for scenarios with high airtightness requirements; melt bonding is a simple process and is suitable for low-cost mass production.

[0055] For details, please refer to Figure 1 and Figure 4 In one embodiment, a 4-inch Schott BF33 borosilicate glass wafer (400 μm thick) is used as the substrate material, with a softening point of 820°C and an annealing point of 560°C, suitable for the thermoforming requirements of high aspect ratio microstructures; the silicon mold is fabricated based on a 4-inch silicon wafer (500 μm thick), and a trench structure with a width of 105 μm and a depth of 300 μm is processed by deep reactive ion etching (DRIE) as a template for glass reflow. Figure 4 (a represents the silicon-glass bonding state before reflow).

[0056] The bonding stage employs anodic bonding technology: the silicon mold and the glass wafer are aligned in the bonding machine, and a vacuum of 8×10⁻⁻⁻⁶ is applied. 5 The temperature is increased to 400°C and a DC voltage of 1200V is applied. The silicon wafer (anode) and glass (cathode) are tightly bonded by electrostatic attraction, ensuring that there are no air bubbles at the bonding interface and providing a sealed chamber for subsequent reflow.

[0057] The hot reflux process strictly follows Figure 1 The temperature-time curve shown is as follows: the temperature is increased from room temperature to 1000℃ (above the softening point of BF33) at a rate of 5℃ / min, and held at this temperature for 2 hours in a nitrogen atmosphere to fill the grooves of the silicon mold using the fluidity of the softened glass; then the temperature is decreased to 560℃ (annealing point) at a rate of 3℃ / min and held for 1 hour to eliminate internal stress; finally, the temperature is slowly decreased to room temperature at a rate of 1℃ / min to avoid surface cracks caused by excessive temperature difference.

[0058] The process results show that ( Figure 4 b shows the effect after reflow: the glass is completely filled to a depth of 300μm, the surface is flat and without significant defects, and the surface roughness Ra is less than 2nm as measured by atomic force microscopy (AFM), which meets the requirements for subsequent bonding and polishing.

[0059] Reference Figure 3 The fabrication process shown allows for multi-dimensional verification of the device:

[0060] Morphology and dimensions: Observation by scanning electron microscope (SEM) shows that the verticality of the glass channel sidewall is 90°±1° and there are no defects such as cracks or pinholes on the surface; the measured channel width is 30μm±2μm and the depth is 500μm±3μm, with dimensional accuracy controlled within ±5μm and the aspect ratio of 16 consistent with the design value.

[0061] Bonding quality: The silicon-glass bonding interface was observed to be free of bubbles and gaps under an optical microscope, and shear tests showed that the bonding strength was stable at 10~12 MPa; the airtightness test was performed using helium mass spectrometry leak detection, and the leakage rate was <1×10⁻ under a pressure of 10 psi for 30 min. 9 Pa・m³ / s, meeting the sealing requirements for gas chromatography analysis.

[0062] Surface flatness: The height difference between the unstructured area and the structured area on the glass surface after reflow was less than 5% as measured by a profilometer, proving that the auxiliary reflow structure effectively improved the wafer flatness and provided a good foundation for subsequent processes.

[0063] In summary, this invention aims to solve the problem of fabricating high aspect ratio microstructures in glass materials. Traditional gas chromatographs are difficult to meet the needs of on-site detection due to their large size and high power consumption. Although miniature gas chromatography columns can achieve miniaturization, glass processing limitations hinder their performance improvement. Therefore, this study combines MEMS technology with gas chromatography theory and proposes a hot reflux process to prepare high aspect ratio glass structures, providing a new path for optimizing the performance of miniature gas chromatography columns.

[0064] In terms of process exploration, the study systematically compared the limitations of traditional glass micromachining technologies such as machining, laser processing, and chemical etching, and clarified that hot reflow is an effective solution for preparing high aspect ratio structures. Through experiments, it was determined that hot reflow at 1000℃ for 2 hours can achieve microstructure processing with a depth of 300μm and no significant surface defects. The annealing process was optimized by adopting a strategy of cooling to the annealing point at 3℃ / min and then cooling to room temperature at 1℃ / min, which effectively suppressed internal stress and surface cracks in the glass. At the same time, the feasibility of the process was verified by wafer-level fabrication.

[0065] In terms of structural design, a model was established based on gas chromatography theory to establish the relationship between the channel depth-to-width ratio and the theoretical plate height. The parameters of the rectangular cross-section were determined (depth 500μm, width 30-70μm, depth-to-width ratio 7-16). A column length of 1m was selected to meet the requirements of rapid separation, and a serpentine planar layout was adopted to reduce peak broadening. In process optimization, an auxiliary reflux structure, a SiO2 layer on the mold sidewall, and a nitrogen atmosphere were added to further improve the thermal reflux effect. Finally, a high depth-to-width ratio micro gas chromatography column was successfully prepared.

[0066] Performance tests show that the column has excellent airtightness. At the optimal column inlet pressure of 8 psi, the resolution of p-toluene and p-xylene reaches 8.5, with theoretical plate numbers of 6038 plates / m and 8136 plates / m, respectively, which fully meet the requirements for rapid separation of mixed components. The research results provide a reliable process for the fabrication of high aspect ratio microstructures in glass and lay the foundation for the miniaturization and high performance of micro gas chromatography columns.

[0067] The above embodiments fully verify the feasibility of the structural design and processing technology of the present invention. The prepared high aspect ratio micro gas chromatography column meets the design requirements in terms of dimensional accuracy, surface quality and airtightness.

[0068] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several equivalent substitutions or obvious modifications can be made without departing from the concept of the present invention, and all such modifications, achieving the same performance or purpose, should be considered within the scope of protection of the present invention.

Claims

1. The structural design of a high aspect ratio micro gas chromatographic column based on hot reflux technology, characterized by: The micro gas chromatography column is a rectangular cross-section open tube column, including the channel body and auxiliary structures; The main body of the channel has a high aspect ratio design, with a depth of 500μm and a width of 30μm, 50μm or 70μm, corresponding to aspect ratios of 16, 10 and 7 respectively. The main body of the passage has a serpentine layout, containing 67 straight sections, each 15mm long, with a total length of 1m. Adjacent straight sections are connected by 180° bends, which are designed symmetrically to compensate for path differences. The auxiliary structure is a series auxiliary reflow structure set in the non-structured area, used to reduce the height difference of the glass surface after heat reflow.

2. The fabrication process of a high aspect ratio micro gas chromatographic column based on hot reflux technology, characterized in that: Includes the following steps: S1. Silicon mold etching: On a 500μm thick silicon wafer, using the intermediate device as a reference, a groove with a depth of 200μm is etched using the DRIE etching process to form a silicon mold. The sidewalls of the silicon mold are pre-oxidized to form a SiO2 layer with a thickness of 50nm to improve the subsequent adhesion to the glass. S2, Anodic Bonding: The silicon mold obtained in step S1 is anodicly bonded to 500μm thick BF33 glass to form a sealed chamber; S3, Glass hot reflow: The bonding component obtained in step S2 is placed in a nitrogen atmosphere and kept at 1000℃ for 2 hours to allow the glass to flow and fill the groove of the silicon mold. S4. Grinding treatment: Grind the upper and lower surfaces of the component after hot reflow in step S3, wherein the glass surface is ground until the surface is flat and the back silicon surface is ground until the glass is exposed. S5. Remove silicon mold: Use DRIE etching process to remove the silicon mold in the glass structure to obtain the glass channel; S6. Silicon-glass bonding: Align the silicon wafer with a pre-etched 300μm deep channel with the glass channel obtained in step S5 and bond them together to form a closed channel.

3. The fabrication process of a high aspect ratio micro gas chromatographic column based on hot reflux technology according to claim 2, characterized in that: The SiO2 layer on the sidewall of the silicon mold in step S1 is formed by a thermal oxidation process.

4. The fabrication process of a high aspect ratio micro gas chromatographic column based on hot reflux technology according to claim 2, characterized in that: The anodic bonding in step S2 is performed in a vacuum environment, with a bonding temperature of 300-400℃ and an applied voltage of 500-1000V.

5. The fabrication process of a high aspect ratio micro gas chromatographic column based on hot reflux technology according to claim 2, characterized in that: The bonding described in step S6 is anodic bonding or melt bonding.

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