A high-sensitivity gas sensor based on molecular self-assembly and its preparation method

Abstract

The present invention belongs to the field of semiconductor device technology, specifically a high-sensitivity gas sensor based on molecular self-assembly and a preparation method thereof. The gas sensor of the present invention is prepared using molecular self-assembly technology, including: preparing a heavily doped silicon substrate layer, a lightly doped epitaxial silicon layer, and then a heavily doped layer; depositing a metal layer, performing a molecular self-assembly process on the surface of the metal layer, and spin-coating a block copolymer PS / PMMA; annealing at high temperature to induce the formation of a nanoporous structure; etching away PMMA to obtain a PS nanostructure; transferring the nanoporous structure to a metal electrode layer; etching to obtain a nanoporous structure; removing PS residual glue; and depositing metal under the silicon substrate as a conductive cathode metal electrode contact. The process of the present invention is simple, low-cost, and controllable in operation; the prepared gas sensor has a regular high-density gate channel, has higher gas adsorption capacity, greatly improves the sensitivity of the sensor, and can be used in occasions with smaller size and higher precision requirements.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor devices, and in particular relates to a gas sensor and a preparation method thereof. Background Art

[0002] Gas sensors are devices that convert information such as gas type and concentration into usable signals such as light, electricity, sound, and digital information, which can be used by humans and computers. Gas sensors convert gas signals into electrical signals, which are then transmitted to a microcontroller for processing. Sensors, which can both receive and transmit information, have dual functions and are therefore crucial in everyday life. Gas sensors can quickly detect various flammable, explosive, toxic, and hazardous gases in the air, making them crucial and in high demand in areas such as vehicle exhaust monitoring, coal mine safety, drunk driving, home decoration, and industrial production.

[0003] Current research focuses on toxic and flammable gases, primarily aiming to improve sensor response and sensitivity, reduce response recovery time in harsh environments, and enhance cost and intelligence. A representative semiconductor sensor is the resistive gas sensor. Its principle is that when a semiconductor oxide sensitive material comes into contact with the gas being measured, the material's resistance changes. This resistance reflects the type and concentration of the gas being measured. Semiconductor gas sensors can effectively detect a wide range of gases, including methane, ethane, propane, butane, alcohol, formaldehyde, carbon monoxide, carbon dioxide, ethylene, acetylene, vinyl chloride, styrene, and acrylic acid. These sensors are particularly low-cost and well-suited for civilian gas detection needs. However, traditional semiconductor sensors suffer from poor stability, significant environmental influences, limited selectivity, and insufficient response. The development of new sensitive electrode materials is needed to advance sensors with high sensitivity, selectivity, and fast response speeds.

[0004] Since the 1990s, thin-film sensing technology has developed into a new and important field of sensor research, driven by the application and development of nanomaterials and the in-depth study of porous silicon. Porous silicon gas sensors possess a large surface area, capable of adsorbing large quantities of gas molecules from the external environment, thereby altering some of their optical and electrical properties. The advantages of porous silicon gas sensors include ease of preparation, large surface area, strong chemical sensitivity to the environment, high sensitivity, excellent stability and selectivity, and compatibility with silicon electronic processing technology. However, a major disadvantage of porous silicon gas sensors is their relatively low sensitivity to humidity, which affects their stability, selectivity, and reversibility. Furthermore, the microscopic porous structure makes manipulation and control difficult. Therefore, the characteristics of gas sensors need to be further improved, and their stability needs to be further studied. At the same time, the existing porous silicon gas sensor preparation process has shortcomings such as unevenness, uncontrollable material surface morphology, and large differences in resistance values ​​of samples under the same conditions. Further optimization is needed to control the stability of the process and improve the gas sensitivity and selectivity of the gas sensor. In the future, it will be suitable for collaborative design and integrated manufacturing, further reducing the size of the sensor, realizing sensor wafer-level manufacturing, integrating multiple sensors together to form a sensor array, and integrating data processing and other modules to achieve chip-level packaging manufacturing. Summary of the Invention

[0005] The purpose of the invention is to provide a high-sensitivity gas sensor and a preparation method thereof with simple process, low cost, high controllability and stability.

[0006] The method for preparing a high-sensitivity gas sensor proposed in the present invention adopts molecular self-assembly technology, and the specific steps are as follows:

[0007] (1) Prepare a heavily doped silicon substrate layer, a lightly doped epitaxial silicon layer, and then a heavily doped layer with the same doping type and concentration as the silicon substrate layer;

[0008] (2) Depositing a metal layer to serve as a conductive anode metal electrode contact;

[0009] (3) Perform molecular self-assembly process on the surface of the metal layer and spin-coat the block copolymer PS / PMMA;

[0010] (4) After high-temperature annealing, the two monomers with different chemical properties separate into phases, inducing the formation of a nanoporous structure;

[0011] (5) Oxygen selectively etches PS / PMMA, removing PMMA and obtaining PS nanostructures;

[0012] (6) Using the metal conductive layer as a mask, continue etching PS with CHF3 / O2 using reactive ion etching to transfer the nanoporous structure to the metal electrode layer;

[0013] (7) Continue to etch downward silicon with SF6 / O2 to prepare a nanopore structure with a certain depth;

[0014] (8) Dry / wet method to remove PS residual glue;

[0015] (9) Metal is deposited under the silicon substrate to serve as a conductive cathode metal electrode contact.

[0016] The more detailed process steps are described in Example 1.

[0017] In the present invention, the silicon substrate is a single-sided polished single crystal silicon; the silicon substrate is N-type silicon (or P-type silicon), heavily doped, with a doping concentration of 1×10 18 ~1×10 20 cm -3 The epitaxial layer above is made of P-type silicon (or N-type silicon), lightly doped with a doping concentration of 10 12 -10 15 cm -3 The next doped area is doped with N+ (or P+), heavily doped, and the doping concentration and injection type are the same as those of the silicon substrate, that is, N-type silicon (or P-type silicon) is used, and the doping concentration is 1×10 18 ~1×10 20 cm -3 .

[0018] In the present invention, the materials of the surface metal conductive layer (anode) and the metal conductive layer (cathode) under the silicon substrate include but are not limited to metals such as gold, silver, copper, aluminum, and metal silicides such as platinum silicon, iridium silicon, tungsten silicon, and palladium silicon.

[0019] In the present invention, methods for fabricating nanopores of a certain depth include, but are not limited to, reactive ion etching, femtosecond laser etching, nanosecond laser etching, wet etching, and nanoimprinting. The resulting nanopores have diameters ranging from 20-30 nm and controllable depths, from as shallow as 100 nm to as deep as the underlying silicon substrate, connecting the channels; for example, depths range from 100 nm to several microns.

[0020] In the present invention, the conductive metal layer can be prepared by methods including, but not limited to, magnetron sputtering, thermal evaporation, electron beam evaporation, and pulsed laser deposition. Annealing methods include, but are not limited to, conventional thermal annealing, rapid thermal annealing, and laser annealing. The thickness of the conductive metal layer can be between 10 and 50 nm.

[0021] In the present invention, block copolymers used in molecular self-assembly techniques include, but are not limited to, PS-b-PMMA, PS-bP2VP, PS-b-PPC, and PS-b-PAA. Methods for selectively removing PMMA include, but are not limited to, dry etching (i.e., O2 plasma etching) and wet etching (i.e., irradiating the sample with ultraviolet light followed by acetic acid cleaning, which can completely remove PMMA without damaging the PS structure).

[0022] In the present invention, the preparation gas for etching silicon to form a nanoporous structure includes but is not limited to SF6 / O2 mixed gas, halogen elements / O2, sulfide / O2, etc.

[0023] The gas sensor prepared by the present invention has a structural diagram as shown in FIG. Figure 1 As shown, from top to bottom are: a surface metal conductive layer for anode contact, a doped region, an epitaxial layer, a silicon substrate, and a metal conductive layer for cathode contact.

[0024] In the present invention, the silicon substrate is made of N-type silicon (or P-type silicon), heavily doped, with a doping concentration of 1×10 18 ~1×10 20 cm -3 The epitaxial layer above is made of P-type silicon (can also be N-type silicon), lightly doped with a doping concentration of 10 12 -10 15 cm -3 The next doped area is doped with N+ (or P+), and heavily doped. The doping concentration and injection type are the same as those of the substrate, that is, N-type silicon (or P-type silicon) is used, and the doping concentration is 1×10 18 ~1×10 20 cm -3 .

[0025] The present invention utilizes molecular self-assembly technology to form nanoscale surface pores with small size, high uniformity, and stability. The etch depth is controllable, allowing the desired depth to be achieved, thereby increasing the surface area for small gas molecules to adhere. Small gas molecules, such as reduced (or oxidized) gas molecules, diffuse to the surface, losing (or gaining) electrons, thus carrying a positive (or negative) charge. These electrons (or holes) are induced on the silicon surface. These electrons (or holes) on the nanopore surface form a conductive channel, the gate of the gas sensor, thereby conducting current. As the gas concentration increases, more small gas molecules are adsorbed on the surface, and more electrons are induced in the channel below. The sensitivity of the gas sensor is reflected in the change in the output current of the measured gas, thereby increasing the current flow and improving sensitivity. The high-density nanopore structure produced under these conditions forms a multi-gate structure, similar to a fin field-effect transistor. Furthermore, the regular nanopore structure formed by deep etching of this high-density pore structure significantly increases the contact area for small gas molecules, thereby improving the sensitivity of the gas sensor.

[0026] In the present invention, small molecule gases include but are not limited to NH3, nitrogen oxides (NOx), UHP N2, benzene, petroleum ether, acetonitrile and other gases.

[0027] The characteristics and advantages of the method of the present invention are:

[0028] (1) The nanopore shape prepared by the molecular self-assembly process overcomes the problems of poor stability and strong environmental influence of traditional semiconductor gas sensors, as well as the poor controllability, process non-uniformity, irregular and uniform pore structure of porous silicon gas sensors, and other problems such as high preparation cost and complex process of gas sensors;

[0029] (2) The multi-gate structure gas sensor prepared by molecular self-assembly technology takes advantage of the small diameter and controllability of nanopores to etch a certain depth to the silicon base to form a conductive channel. After the high-density holes on the surface are etched, multiple densely packed nanopore columns with high density are formed, which achieve high absorption of gas molecules in the environment and sense multiple high-density gate channels, thereby increasing the adsorption capacity of the surface area and forming a high-variable current to ultimately achieve high sensitivity of the gas sensor. Compared with traditional porous silicon gas sensors, its sensitivity is about 5 times higher under the same gas and the same temperature, the detection limit is greatly reduced, and it shows a better response to the gas;

[0030] (3) The overall process is simple, low-cost, and has high process stability and strong reliability. The prepared gas sensor multi-gate channel structure has a small nano-column diameter, small period, controllable depth, and strong uniformity.

[0031] (4) The multi-gate, high current, and high sensitivity structure prepared can be applied to gas sensors with smaller size and high precision requirements. In the future, it can be applied to collaborative design and integrated manufacturing, further reducing the size of sensors, realizing sensor wafer-level manufacturing, integrating multiple sensors together to form a sensor array, and integrating modules such as data processing to realize chip-level packaging manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a diagram of the structure of a high-sensitivity gas sensor prepared by the molecular self-assembly method of the present invention.

[0033] Figure 2 The present invention is a flowchart illustrating the preparation of a highly sensitive gas sensor using the molecular self-assembly method.

[0034] Figure 3 This is a structural diagram of Example 2 of a high-sensitivity gas sensor prepared by a molecular self-assembly method according to the present invention.

[0035] Figure 4 It is a schematic diagram of the gas sensor resistance detection circuit of the present invention. DETAILED DESCRIPTION

[0036] Based on the same working principle, the structure of the device can be different, and the specific implementation methods are reflected in different embodiments.

[0037] Example 1 (corresponding to Figure 1 The device structure and Figure 2 Process flow):

[0038] (1) If Figure 2 (a) shows the starting silicon substrate; the substrate is generally doped with heavily N-type doped silicon, with a doping concentration of 10 18 cm -2 to 10 20 cm -2 The thickness of the substrate silicon is 200-800nm; depending on the optical wavelength of the sensor, the substrate can also be made of materials such as silicon germanium, gallium nitride or indium gallium arsenide; the epitaxial layer can be made of P-type or N-type silicon, lightly doped, with a doping concentration of 10 12 to 10 15 cm -3 The upper layer is a doped region, heavily doped with N-type silicon, with a doping concentration of 1×10 18 to 1×10 20 cm -3 ; A layer of metal is deposited on the top layer for ohmic contact. Common metals are aluminum, nickel, titanium or metal silicides such as nickel silicon, titanium silicon, etc. The annealing temperature is between 300 degrees and 900 degrees, and the thickness is 100nm. Figure 2 (b);

[0039] (2) Preparation of molecular self-assembly technology, first spin-coating PS / PMMA block copolymer (PS-b-PMMA concentration 37-b-16.8 kg / mol), such as Figure 2 (c) Then, annealing treatment at 220 °C is performed to accelerate the movement of polymer molecular chains and induce controllable microphase separation of polymer chains, forming a well-defined and highly ordered nanostructure, e.g. Figure 2 (d) The block copolymer contains homopolymers with different chemical properties. Because of the differences between them, PS-b-PMMA is treated with O2 plasma RIE, where the etching selectivity ratio of PS to PMMA is about 1:2, so PMMA can be removed first, as shown in Figure 2. Figure 2 (e), and the remaining PS can be used as a mask for the next step of etching; then the fine nanostructure after removing PMMA is transferred to the underlying metal layer with high fidelity by reactive ion etching, and CHF3 / O2 mixed gas is used to RIE the PS as a mask to the metal layer, as shown in Figure 2 (f);

[0040] (3) Use SF6 / O2 reactive ions to continue etching deeply into the silicon substrate, and control the morphology, depth, period size, etc. of the holes transferred to the silicon base by etching gas, time, power, pressure and other conditions, and reach a certain etching depth to the silicon base to actually form a nano-pillar structure; dry etching can also use fluorine-based or halogen element gases, such as SF6, CHF3, HBr or Cl2; while wet etching generally uses TMAH, KOH and other solutions; SF6 / O2 etching for 5-10 minutes can transfer holes to the silicon base with high fidelity to form high-density conductive channels in gas sensors, such as Figure 2 (g);

[0041] (4) Remove the residual PS glue by O2 dry etching for 2 minutes or wet acetone and isopropyl alcohol washing; finally, deposit metal at the other end of the channel under the substrate and anneal to form a N-type heavily doped region as shown in the figure. Figure 2 The electrode shown in (h) is usually made of aluminum, nickel, titanium or metal silicide, such as nickel silicon, titanium silicon, etc., and the annealing temperature is between 300 degrees and 900 degrees.

[0042] Comparative Example 1: Preparation of conventional porous silicon gas sensor (electrochemical method)

[0043] (1) Clean the initial silicon substrate and place the treated silicon wafer in anhydrous ethanol for later use; take out the silicon wafer and wash it several times with hot and cold deionized water alternately;

[0044] (2) Using the vacuum coating method, aluminum electrodes were deposited on the unpolished surface of the silicon wafer under a vacuum of 10 Pa. The thickness of the aluminum film was about 1-2 μm. During the electrochemical corrosion process, the aluminum film was easily corroded by HF. In the experiment, two materials, black glue and paraffin, were used to coat the aluminum electrodes to protect the aluminum film.

[0045] (3) Use a glass knife to cut the silicon wafer into small pieces and clean them for later use; prepare the etching solution, HF (40%): ethanol (99.7%): deionized water = 1:2:1; add anhydrous ethanol and use a magnetic stirrer to allow the hydrogen generated during the etching process to escape smoothly, so that the silicon wafer and the etching solution can fully contact and react, and relatively uniform holes can be formed on the surface of the silicon wafer;

[0046] (4) Prepare PS. Place the prepared etching solution in a sealed etching tank. Place the silicon wafer covered with paraffin in the etching solution in the etching tank. Turn on the etching power supply and pass an appropriate current. After etching for a period of time, take out the silicon wafer and clean it. Finally, deposit a nano-metal film with catalytic properties on the surface of the porous silicon as an electrode for gas-sensitive detection.

[0047] Scanning electron microscopy (SEM) analysis of the surface morphology of porous silicon and molecular self-assembly revealed that the PS sample produced by chemical etching had a relatively uniform surface and a relatively large thickness. The porous silicon surface was "sponge-like," consisting of numerous randomly distributed, interconnected pores with pore sizes ranging from approximately 100 to 600 nm, which is nanoscale, and the sample surface was relatively smooth. The high-density pores produced by molecular assembly technology in Example 1 exhibited a uniform, regular, and controllable arrangement, with a pore size of approximately 30 nm and a very smooth surface, making them very suitable for adsorbing gas molecules.

[0048] The gas sensitivity test was conducted at room temperature. The porous silicon gas sensor in the comparative example and the high-density silicon gas sensor in Example 1 were placed in a sealed gas chamber with a certain gas concentration. This facilitated the control of the gas concentration inside. The electrodes on both sides of the gas sensor were connected to a test circuit. Under a constant bias voltage, as the gas concentration in the gas chamber changed, the resistance of the porous silicon gas sensor changed. The change in the gas sensor resistance was calculated based on the change in the voltage across the load resistor. The test circuit schematic is shown in the figure below. Figure 4 The data-related characteristic parameters are calculated based on the electrical quantities read out during testing, thereby enabling evaluation of gas-sensing performance. The sensor responds to formaldehyde, ammonia, and alcohols to varying degrees, with ammonia being particularly sensitive. At 1 kppm to 3 kppm, the S values ​​for the comparative porous silicon range from 3 to 11, while those for the high-density silicon prepared by molecular self-assembly in Example 1 range from 12 to 53. Sensitivity increases with increasing gas concentration.

[0049] Example 2 (corresponding to Figure 3Device structure diagram):

[0050] Example 2 is similar in structure to Example 1, except that the substrate is heavily doped with P-type, the epitaxial silicon can also be N-type or P-type, and the upper doped layer is heavily doped with P-type rather than N-type. Therefore, the process flow of this example is similar to that of Example 1, except that only the substrate and doped layer types in step (1) need to be changed. The product morphology and gas sensitivity test results are similar to those of Example 1 and will not be repeated here.

Claims

1. A method for preparing a gas sensor based on molecular self-assembly, characterized in that: It consists of the following parts: silicon substrate layer, epitaxial silicon layer, doped silicon layer, and metal conductive layer. The specific preparation steps are as follows: (1) Prepare a heavily doped silicon substrate layer, a lightly doped epitaxial silicon layer, and then a doped silicon layer with the same doping type and concentration as the silicon substrate layer; (2) Depositing a metal conductive layer to serve as a conductive anode metal electrode contact; (3) Perform molecular self-assembly process on the surface of the metal conductive layer and spin-coat the block copolymer PS / PMMA; (4) After high-temperature annealing, the two monomers with different chemical properties separate into phases, inducing the formation of a nanoporous structure; (5) Oxygen selectively etches PS / PMMA, removing PMMA and obtaining PS nanostructures; (6) Using the PS nanostructure as a mask, continue etching with CHF3 / O2 using reactive ion etching to transfer the nanoporous structure to the metal conductive layer; (7) Continue to etch downward silicon with SF6 / O2 to prepare a nanopore structure with a certain depth; (8) Dry / wet method to remove PS residual glue; (9) A metal conductive layer is deposited under the silicon substrate layer to serve as a conductive cathode metal electrode contact.

2. The preparation method according to claim 1, characterized in that The silicon substrate layer is single-sided polished single crystal silicon; the silicon substrate layer is N-type silicon, heavily doped, and the doping concentration is 1×10 18 ~1×10 20 cm -3 ; The epitaxial silicon layer is made of P-type silicon, lightly doped with a doping concentration of 10 12 -10 15 cm -3 ; The next layer of doped silicon is doped with N+, heavily doped, and the doping concentration is 1×10 18 ~1×10 20 cm -3 .

3. The preparation method according to claim 1, characterized in that The materials of the surface metal conductive layer and the metal conductive layer under the silicon substrate layer are selected from gold, silver, copper, aluminum, platinum silicon, iridium silicon, tungsten silicon, and palladium silicon.

4. The preparation method according to claim 1, characterized in that The method for preparing the nanopore described in step (7) adopts reactive ion etching, femtosecond laser etching, nanosecond laser etching, wet etching or nanoimprinting.

5. The preparation method according to claim 1, characterized in that The diameter of the nanopore in step (7) is 20-30 nm, and the depth is controllable, from as shallow as 100 nm to as deep as the underlying silicon substrate layer, connecting the channel.

6. The preparation method according to claim 1, characterized in that The preparation method of the metal conductive layer adopts magnetron sputtering, thermal evaporation or electron beam evaporation pulse laser deposition; the annealing method is conventional thermal annealing, rapid thermal annealing or laser annealing; the thickness of the metal conductive layer is 10-50nm.

7. The preparation method according to claim 1, wherein: The block copolymer in step (3) is selected from PS-b-PMMA, PS-b-P2VP, PS-b-PPC, and PS-b-PAA; In step (5), oxygen selectively etches PS / PMMA to remove PMMA, using dry etching or wet etching.

8. A gas sensor prepared by the method according to any one of claims 1 to 7, wherein the structure thereof comprises, from top to bottom, a surface metal conductive layer for anode contact, a doped silicon layer, an epitaxial silicon layer, a silicon substrate layer, and a metal conductive layer for cathode contact; wherein: The silicon substrate layer is made of N-type silicon and is heavily doped with a doping concentration of 1×10 18 ~1×10 20 cm -3 The epitaxial silicon layer above is made of P-type silicon, lightly doped with a doping concentration of 10 12 -10 15 cm -3 ; The next layer of doped silicon is doped with N+, heavily doped, and the doping concentration is 1×10 18 ~1×10 20 cm -3 .

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