A method for manufacturing a piezoresistor of a piezoresistive pressure sensor

Abstract

The application discloses a manufacturing method of a piezoresistor of a piezoresistive pressure sensor, which mainly comprises a cleaning process, a photoetching process, a diffusion process, an inductively coupled plasma etching process and an annealing process. The application aims at combining the diffusion process and the inductively coupled plasma etching process, and proposes a simplified process scheme, that is, a method of simultaneously manufacturing light and heavy doped areas of the piezoresistive pressure sensor by only once diffusion doping process in cooperation with the etching process, so as to reduce process steps and process cost of the MEMS piezoresistive pressure sensor. The method can be applied to manufacturing of the light and heavy doped areas of the piezoresistive pressure sensor based on a silicon substrate, and has important application value in simplifying process steps and reducing manufacturing cost of the sensor.

Description

Technical Field

[0001] This invention relates to the field of MEMS sensors and MEMS processes, specifically to a method for fabricating a piezoresistive pressure sensor piezoresistive resistor in a MEMS piezoresistive pressure sensor. Background Technology

[0002] In the field of MEMS pressure sensors and processes, piezoresistive pressure sensors hold a leading market share due to their simple structure, simple manufacturing process, significant temperature coefficient, ease of measurement, ease of integration, and good stability. Among these components, the piezoresistive resistor and the ohmic contact area are key components determining the sensor's electromechanical performance, making their fabrication process extremely important.

[0003] In MEMS piezoresistive pressure sensors, the electrical sensing structure consists of three parts: a lightly doped region of the piezoresistive resistor, a heavily doped region of the ohmic contact, and leads. The leads are typically fabricated using magnetron sputtering of metal; the lightly and heavily doped regions are fabricated using doping processes. Doping refers to the process of introducing a certain amount of impurities into a designated area of ​​a wafer to alter its electrical properties. In terms of doping concentration, the heavily doped region has a higher impurity concentration than the lightly doped region; in terms of doping depth, the doped junction depth in the heavily doped region is greater than that in the lightly doped region. Doping processes commonly employ diffusion and ion implantation methods. Compared to ion implantation, diffusion processes do not require expensive equipment and are less costly; therefore, some piezoresistive pressure sensors typically use diffusion for doping.

[0004] Typically, the method for fabricating the lightly doped varistor region and the heavily doped ohmic contact region of a MEMS piezoresistive pressure sensor is as follows: First, the silicon wafer is cleaned using standard methods. Next, a photolithography process is performed to form a pattern for the heavily doped region. Then, this pattern is subjected to a doping process to complete the heavy doping. After forming the heavily doped region, annealing and photoresist removal cleaning processes are performed. Then, another photolithography process is performed to form a pattern for the varistor region. This pattern is then subjected to a doping process to lightly dope it, forming the lightly doped varistor region. After annealing and photoresist removal cleaning processes are performed. Therefore, a detailed analysis of the process reveals that multiple cleaning and doping steps are required. Ion implantation is a costly and complex process, requiring extensive preparatory steps. Furthermore, using a single layer of photoresist as the implantation mask during ion implantation can easily lead to structural deformation and cracking due to the high energy and dosage of prolonged ion implantation. This not only causes the deformed photoresist to adhere to the silicon wafer and become difficult to remove, but also significantly reduces its effectiveness as a barrier layer, resulting in ion doping in unwanted areas. The former requires concentrated sulfuric acid to remove the deformed photoresist, potentially affecting existing structures on the silicon wafer; the latter necessitates using other materials such as silicon oxide as the implantation mask, further increasing the complexity and cost of the process. Therefore, it is essential to develop a diffusion process to simplify the fabrication of doped regions and reduce process difficulty and cost. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of current doping processes, which are relatively cumbersome and costly, by proposing a method for fabricating varistors that requires only one diffusion-based heavy doping process, combined with an etching process to achieve light doping. This method reduces the number of doping processes, simplifies the pre-doping preparation work, shortens the overall process time, and reduces process costs.

[0006] This invention includes the following steps:

[0007] 1) Silicon wafer cleaning;

[0008] In step 1), the specific steps for cleaning the silicon wafer can be as follows: according to the cleaning process, the silicon wafer is cleaned using standard cleaning solution, in the order of cleaning solution No. 3, cleaning solution No. 1, and cleaning solution No. 2; after cleaning solution No. 2 is completed, the silicon wafer is placed in a vacuum drying oven and dried thoroughly by vacuuming.

[0009] 2) Photolithography forms the lightly doped and heavily doped regions.

[0010] In step 2), the specific steps for photolithography to form the lightly doped region pattern of the varistor can be as follows:

[0011] (1) Spin-coat photoresist AZ5214-E and pre-bake;

[0012] (2) Expose the image in a lithography machine, then remove it;

[0013] (3) After development and microscopic inspection, harden the film.

[0014] In step 2), the pattern formed by photolithography is the lightly doped region of the varistor and the heavily doped region of the ohmic contact. Photolithography simultaneously creates the two regions of the sensor, the lightly doped region of the varistor and the heavily doped region of the ohmic contact, so that the two regions can be simultaneously heavily doped with boron ions in step 3).

[0015] 3) Simultaneously perform boron ion diffusion heavy doping process on both lightly and heavily doped regions to form heavily doped regions;

[0016] In step 3), the specific steps for forming the heavily doped region can be as follows: place it in a diffusion furnace and perform boron ion heavy doping on the region obtained by photolithography in step 2), with the heavy doping concentration value being the doping concentration value of the ohmic contact region.

[0017] In step 3), the diffusion temperature, time, and boron surface gas concentration in the boron ion diffusion doping process are determined as needed.

[0018] 4) Photolithography to form the lightly doped region pattern of the varistor:

[0019] In step 4), the specific steps for photolithography to form the lightly doped region pattern of the varistor can be as follows:

[0020] (1) Spin-coat photoresist AZ5214-E and pre-bake;

[0021] (2) Expose the image in a lithography machine, then remove it;

[0022] (3) After development and microscopic inspection, the film is hardened.

[0023] 5) Perform silicon etching on the lightly doped region to form the lightly doped region;

[0024] In step 5), the pattern formed by photolithography is only a lightly doped region. At this point, this region has been heavily doped and etched in step 6). This step is the most critical step of the invention. By controlling the etching depth and using a four-probe tester, the required doping concentration of the varistor can be precisely etched directly. Existing processes require repeating the diffusion doping process in step 3) on the lightly doped region, which is time-consuming and difficult to control the doping concentration. The steps of this invention omit the process of repeating the diffusion on the lightly doped region, greatly reducing the process difficulty and time; the specific steps are as follows:

[0025] (1) Place it in an inductively coupled plasma etching apparatus and etch the area obtained by photolithography in step 5) in stages;

[0026] (2) After etching, remove the sample and use a four-probe tester to test the doping concentration of the etched area. If the doping concentration does not meet the theoretical doping concentration of the lightly doped area, continue etching.

[0027] In step 5), the etching process includes, but is not limited to, silicon etching processes such as reactive ion etching and inductively coupled plasma etching. The lightly doped region pattern obtained by photolithography in step 5) is etched. The etching depth is determined as needed. The etching removes some of the heavily doped, low-resistivity regions containing high concentrations of boron ions, and the remaining region is the lightly doped, high-resistivity region required by the varistor.

[0028] 6) Annealing and cleaning the silicon wafers.

[0029] In step 6), the specific steps of annealing and cleaning the silicon wafer can be as follows: place it in an annealing furnace and perform high-temperature annealing on the silicon wafer, so that boron ions can diffuse in steps again, making the doping concentration closer to the design value.

[0030] The advantages of this invention are:

[0031] 1) The traditional method of heavy doping in lightly doped and heavily doped regions of piezoresistive pressure sensors is optimized by using only a single diffusion heavy doping process combined with etching to complete the transformation from heavily doped to lightly doped regions. This eliminates the need for a single light doping process, significantly reducing process cost and complexity. Currently, when fabricating lightly doped piezoresistive pressure sensors and heavily doped ohmic contact regions, the traditional method requires separate doping processes with different concentrations for the two regions due to the different required resistances. For example, in Chinese patent CN104062059A (A MEMS Piezoresistive Pressure Sensor and Manufacturing Method), the piezoresistive region requires light boron diffusion followed by concentrated boron diffusion, which inevitably increases process complexity and cost.

[0032] 2) Reduce one doping process, eliminate the preliminary preparation work for this step, and significantly reduce the overall workload of the process;

[0033] 3) Reducing the number of doping processes lowers the unavoidable damage to the silicon wafer and improves process yield. Chinese patent CN114999903A (A method for manufacturing borosilicate films to reduce the thickness of the diffused borosilicate glass layer on silicon wafers) and related literature (Application of MEMS pressure sensors in the study of dental mechanical properties) both mention that during the diffusion process, oxygen reacts with boron-containing gases to form a thin layer of borosilicate glass on the silicon wafer surface. Subsequent removal methods typically employ mechanical methods (sandblasting or grinding), which can easily increase the stress on the silicon wafer and may also damage the surface morphology, leading to wafer breakage or even scrap. The method proposed in this invention reduces the number of diffusion doping processes, thus lowering the potential damage to the silicon wafer.

[0034] 4) By controlling the etching depth, the boron ion concentration in the lightly doped region can be controlled relatively precisely, resulting in good process matching. In the diffusion doping process, the final ion concentration distribution function of the doped region is:

[0035]

[0036] In the formula, Q is the ion concentration; D is the diffusion coefficient, which is related to both diffusion temperature and time; t is the diffusion time; and exp is an exponential function with base e. It is evident that the ion concentration distribution through diffusion doping is greatly affected by temperature and time. Based on the electrical sensing principle of piezoresistive pressure sensors, the boron ion concentration in the lightly doped (piezoresistive) region must be as close as possible to the design value. Even slight deviations or external interference during fabrication can lead to discrepancies between the boron ion concentration and the expected value, resulting in performance deviations. The method proposed in this invention controls the etching depth and uses a four-probe tester to ensure the doping concentration in the lightly doped region. If the boron ion concentration in the etched region is too high, etching can continue until the design requirements are met. Attached Figure Description

[0037] Figure 1 This is a process flow diagram of the one-time fabrication of lightly and heavily doped regions proposed in this invention.

[0038] Figure 2 This is a surface diagram of the electrical structure of a MEMS piezoresistive pressure sensor fabricated using traditional manufacturing processes.

[0039] Figure 3 This is a graph showing the light and heavy doping curves of a MEMS piezoresistive pressure sensor fabricated using traditional processes.

[0040] Figure 4 This is the theoretical curve for etching a heavily doped region into a lightly doped region.

[0041] Figure 5 These are the ion concentration curves of the lightly and heavily doped regions produced by this invention.

[0042] Figure 6 This is a surface view of the electrical structure of the MEMS piezoresistive pressure sensor fabricated according to the present invention.

[0043] exist Figure 1-6 In the middle, each is marked as:

[0044] 1-Silicon wafer, 2-Photoresist, 3-Heavily doped region, 4-Varistor region, 5-Ohmic contact region, 6-Step formed after etching of the lightly doped region of the varistor, 7-Lightly doped curve of conventional preparation process, 8-Heavily doped curve, 9-Lightly doped curve obtained by the present invention. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the following embodiments will be used in conjunction with the accompanying drawings to further illustrate the invention. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0046] The embodiments of the present invention include the following steps:

[0047] 1) Silicon wafer cleaning;

[0048] 2) Photolithography forms the lightly doped region of the varistor and the heavily doped region of the ohmic contact; the pattern formed by photolithography is the lightly doped region and the heavily doped region. Photolithography simultaneously creates the lightly doped region and the heavily doped region of the ohmic contact of the sensor, so that the two regions can be heavily doped with boron ions in step 3).

[0049] 3) Simultaneously perform boron ion diffusion heavy doping process on the light and heavy doped regions to form a heavily doped region; the diffusion temperature, time and boron surface gas concentration of the boron ion diffusion doping process are determined as needed.

[0050] 4) Annealing and cleaning the silicon wafers;

[0051] 5) Photolithography forms a lightly doped region pattern for the varistor; the pattern formed by photolithography is only a lightly doped region, which has now been heavily doped and is etched in step 6).

[0052] 6) Perform silicon etching on the lightly doped region to form a lightly doped region; the etching process includes, but is not limited to, reactive ion etching, inductively coupled plasma etching, and other silicon etching processes. The lightly doped region pattern obtained by photolithography in step 5) is etched, and the etching depth is determined as needed. By etching, some of the heavily doped, low-resistivity regions containing high concentrations of boron ions are removed, and the remaining region is the lightly doped, high-resistivity region required by the varistor.

[0053] 7) Annealing and cleaning the silicon wafers.

[0054] The following combination Figure 1 Provide a specific example.

[0055] 1-Silicon wafer, 2-Photoresist, 3-Heavily doped region, 4-Varistor region, 5-Ohmic contact region, 6-Step formed after etching of the lightly doped region of the varistor, 7-Lightly doped curve of conventional preparation process, 8-Heavily doped curve, 9-Lightly doped curve obtained by the present invention.

[0056] This embodiment includes the following steps:

[0057] 1) Standard cleaning of silicon wafer 1. The standard cleaning process sequence is: No. 3 solution cleaning, No. 1 solution cleaning, and No. 2 solution cleaning.

[0058] 2) Spin-coat photoresist 2 onto silicon wafer 1. Photoresist 2 is AZ5214-E2. The spin-coating speed is 2000 r / min and the spin-coating time is 20 s. Expose the image on the MA6 lithography machine for 15 s. The pattern is the heavily doped region 3 that will undergo the boron ion heavy doping process, which includes the varistor region 4 and the ohmic contact region 5.

[0059] 3) Place it in the diffusion process equipment and perform a boron ion heavy doping process on the heavily doped region 3. The diffusion temperature is 1000℃, the time is 100min, and the surface concentration of boron gas is 8e19.

[0060] 4) Remove the silicon wafer from the diffusion equipment and clean it using a No. 3 solution.

[0061] 5) Spin-coat silicon wafer with photoresist AZ5214E2 at a spin speed of 2000 r / min for 20 s; expose on lithography machine MA6 for 15 s, and the lithographic pattern is the varistor region 4.

[0062] 6) Place it in an inductively coupled plasma etching apparatus to etch the varistor region 4 to a depth of 700 nm to obtain a lightly doped region with a low boron ion concentration.

[0063] 7) Remove it from the etching equipment and place it in an annealing furnace for high-temperature annealing at 1000℃ for 30 minutes;

[0064] 8) Remove the silicon wafer from the annealing furnace and clean it using a No. 3 liquid cleaning process;

[0065] Figure 3These are the doping curves for the heavily and lightly doped regions formed using a traditional two-stage boron diffusion process. The doping concentration curves for the two regions show that the heavily doped region has a high doping concentration, low sheet resistance, and large junction depth; while the lightly doped region has a low doping concentration, high sheet resistance, and small junction depth. Therefore, theoretically, etching away the high-dose (concentration), low-sheet-resistance boron ion region in the heavily doped region will yield a low-dose (concentration), high-sheet-resistance boron ion region, leaving the remaining region equivalent to the lightly doped region. Figure 4 As shown. Figure 5 The figures show the doping curves of the heavily and lightly doped regions of the MEMS piezoresistive pressure sensor fabricated by this invention. Curve 8 is the heavy doping concentration curve of the ohmic contact region, and curve 9 is the light doping concentration curve of the piezoresistive region obtained by etching. It can be seen that the sheet resistance and joint depth are very close to the lightly doped curve 7 of the traditional fabrication process, indicating that the method proposed in this invention can replace the traditional fabrication process and reduce the process difficulty and cost.

[0066] Figure 6 This is a schematic diagram of the electrical structure surface of the piezoresistive pressure sensor fabricated according to the present invention. Since the piezoresistive region 4 is obtained by etching heavily doped regions, this is compared with the schematic diagram of the electrical structure surface fabricated using traditional processes. Figure 2 Step 6 is formed after the lightly doped region of the piezoresistive resistor is etched. The height of this step is in the range of several hundred nanometers, depending on different needs, and has no impact on the performance of the piezoresistive pressure sensor.

[0067] In summary, this invention mainly consists of a cleaning process, a photolithography process, a diffusion process, an inductively coupled plasma etching process, and an annealing process. This invention combines the diffusion process and the inductively coupled plasma etching process to propose a simplified process scheme. Specifically, it allows for the simultaneous fabrication of lightly and heavily doped regions of a piezoresistive pressure sensor through a single diffusion doping process, reducing the number of process steps and the cost of MEMS piezoresistive pressure sensors. This method is applicable to the fabrication of lightly and heavily doped regions in piezoresistive pressure sensors, including but not limited to silicon substrate-based piezoresistive pressure sensors, and has significant application value in simplifying sensor fabrication processes and reducing manufacturing costs.

[0068] The above embodiments are merely preferred embodiments of the present invention and should not be considered as limiting the scope of the present invention. All equivalent variations and improvements made within the scope of the present invention should still fall within the patent coverage of the present invention.

Claims

1. A method for manufacturing a piezoresistive pressure sensor piezoresistive resistor, characterized in that... Includes the following steps: 1) Silicon wafer cleaning; 2) Photolithography forms patterns of lightly doped and heavily doped regions; The photolithography process creates two regions for the sensor: a lightly doped region of the piezoresistive sensor and a heavily doped region of the ohmic contact, so that both regions can be simultaneously heavily doped with boron ions in step 3). 3) Simultaneously perform boron ion diffusion heavy doping process on both lightly and heavily doped regions to form heavily doped regions; The specific steps for forming the heavily doped region are as follows: placing it in a diffusion furnace, and performing boron ion heavy doping on the region obtained by photolithography in step 2), with the heavy doping concentration value being the doping concentration value of the ohmic contact region. 4) Photolithography to form the lightly doped region pattern of the varistor: 5) Perform silicon etching on the lightly doped region to form the lightly doped region; The pattern formed by photolithography in step 4) is only a lightly doped region. At this point, the region has been heavily doped and is etched in step 5). By controlling the etching depth and using a four-probe tester, the required doping concentration of the varistor can be precisely etched out directly. The specific steps are as follows: (1) Place it in an inductively coupled plasma etching apparatus and etch the area obtained by photolithography in step 4) in stages; (2) After etching, remove the sample and use a four-probe tester to test the doping concentration of the etched area. If the doping concentration does not meet the theoretical doping concentration of the lightly doped area, continue etching. The etching process includes, but is not limited to, reactive ion etching, inductively coupled plasma etching, and other silicon etching processes. The lightly doped region pattern obtained by photolithography in step 5) is etched. The etching depth is determined as needed. The etching removes some of the heavily doped, low-resistivity regions containing high concentrations of boron ions, and the remaining region is the lightly doped, high-resistivity region required by the varistor. 6) Anneal and clean the silicon wafer to complete the fabrication of the piezoresistive pressure sensor piezoresistive resistor; The specific steps for annealing and cleaning the silicon wafer are as follows: the wafer is placed in an annealing furnace and subjected to high-temperature annealing, and boron ions are further diffused in steps to make the doping concentration closer to the design value.

2. The method for manufacturing a piezoresistive pressure sensor piezoresistive resistor as described in claim 1, characterized in that... In step 1), the specific steps for cleaning the silicon wafer are as follows: according to the cleaning process, the silicon wafer is cleaned using standard cleaning solution, in the order of cleaning solution No. 3, cleaning solution No. 1, and cleaning solution No. 2; after cleaning solution No. 2 is completed, the silicon wafer is placed in a vacuum drying oven and thoroughly dried under vacuum.

3. The method for manufacturing a piezoresistive pressure sensor piezoresistive resistor as described in claim 1, characterized in that... In step 2), the specific steps for forming the lightly doped and heavily doped regions using photolithography are as follows: (1) Spin-coating photoresist and pre-baking; (2) Expose in a lithography machine and remove; (3) After development and microscopic examination, harden the film.

4. The method for manufacturing a piezoresistive pressure sensor piezoresistive resistor as described in claim 1, characterized in that... In step 4), the specific steps for photolithography to form the lightly doped region pattern of the varistor are as follows: (1) Spin-coating photoresist and pre-baking; (2) Expose in a lithography machine and remove; (3) After development and microscopic examination, harden the film.

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